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Pulsed corona-induced degradation of organic materials
in water
Hoeben, W.F.L.M.
DOI:
10.6100/IR535691
Published: 01/01/2000
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Hoeben, W. F. L. M. (2000). Pulsed corona-induced degradation of organic materials in water Eindhoven:
Technische Universiteit Eindhoven DOI: 10.6100/IR535691
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Pulsed corona-induced degradation of
organic materials in water
PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Technische Universiteit Eindhoven,
op gezag van de Rector Magnificus, prof.dr. M. Rem,
voor een commissie aangewezen door het College
voor Promoties in het openbaar te verdedigen
op donderdag 15 juni 2000 om 16.00 uur
door
Wilhelmus Frederik Laurens Maria Hoeben
geboren te Geldrop
Dit proefschrift is goedgekeurd door de promotoren:
prof.dr. W.R. Rutgers
en
prof.dr.ir. C.A.M.G. Cramers
Copromotor:
dr.ir. E.M. van Veldhuizen
CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN
Hoeben, Wilhelmus Frederik Laurens Maria
Pulsed corona-induced degradation of organic materials in water / by
Wilhelmus Frederik Laurens Maria Hoeben
Eindhoven: Technische Universiteit Eindhoven, 2000. -Proefschrift.ISBN 90-386-1549-3
NUGI 812
Trefw: gepulste corona / elektrische ontladingen / AOP / fenol / afbraak /
oxidatie / conversie / efficiëntie / vloeistofchromatografie
Subject headings: pulsed corona / electrical discharges / AOP / phenol / degradation /
oxidation / conversion / efficiency / liquid chromatography
This project has been financially supported by “Technologie voor Duurzame
Ontwikkeling (TDO)”, Technische Universiteit Eindhoven.
Ontwerp omslag: B. Mobach / TUE
Drukwerk: Universiteitsdrukkerij Technische Universiteit Eindhoven
“A company has control of its production only if
it also knows the make-up of its waste water”
[Ullmann, Encyclopedia of Industrial Chemistry 1994, 5th edn.
Vol. B6, Weinheim: Verlag Chemie, ISBN 3-527-20136-X, 474]
Aan mijn ouders, broer en zus
Contents
1.
Introduction .......................................................................................... 1
2.
Theory ..................................................................................................11
3.
Experimental setup .............................................................................37
4.
Results .................................................................................................49
1.1
1.2
1.3
1.4
2.1
2.2
2.3
2.4
Advanced oxidation processes ..................................................................... 2
Electrical discharges ................................................................................... 6
Model compounds ...................................................................................... 7
Thesis scope ............................................................................................. 9
Corona discharges.....................................................................................11
Oxidizers..................................................................................................13
Degradation of organic compounds ..............................................................15
Oxidation of model compounds ...................................................................20
2.4.1 Phenol ..........................................................................................20
2.4.2 Atrazine ........................................................................................23
2.4.3 Malachite green..............................................................................23
2.4.4 Dimethyl sulfide .............................................................................24
2.5 Diagnostics ..............................................................................................24
2.5.1 Chemical diagnostics ......................................................................25
2.5.2 Electrical diagnostics.......................................................................31
2.5.3 Optical diagnostics .........................................................................32
3.1
3.2
3.3
3.4
Reagents and reactors ...............................................................................37
Chemical diagnostics .................................................................................40
Electrical diagnostics .................................................................................45
Optical diagnostics ....................................................................................46
4.1 Pulsed corona discharges ...........................................................................49
4.1.1 Hydroxyl radicals............................................................................49
4.1.2 Ozone...........................................................................................55
4.1.3 Corona pulse energy .......................................................................59
4.1.4 Corona treatment of deionized water.................................................63
4.2 Oxidation of phenol ...................................................................................65
4.2.1 Chromatography.............................................................................65
4.2.2 Mass spectrometry .........................................................................87
4.2.3 Spectroscopy.................................................................................92
4.2.4 Electrical conductometry ...............................................................102
4.2.5 Microtox ecotoxicity .....................................................................105
4.2.6 Total organic carbon .....................................................................108
4.3 Oxidation of other model compounds .........................................................109
4.3.1 Atrazine ......................................................................................109
4.3.2 Malachite green............................................................................110
4.3.3 Dimethyl sulfide ...........................................................................112
5. Discussion ............................................................................................113
5.1
5.2
5.3
5.4
5.5
Pulsed corona discharges .........................................................................113
Corona-induced phenol oxidation ...............................................................116
Phenol oxidation pathways .......................................................................122
Analysis techniques.................................................................................137
AOP comparison .....................................................................................141
6. Conclusions ..........................................................................................145
6.1
6.2
6.3
6.4
Pulsed corona discharges .........................................................................145
Oxidation of model compounds .................................................................146
Analytical techniques...............................................................................147
Outlook .................................................................................................148
7. References ...........................................................................................149
Summary ...................................................................................................159
Samenvatting ............................................................................................161
Dankwoord / Acknowledgements .........................................................163
Curriculum Vitae ......................................................................................164
1. Introduction
Since a long time, natural processes have not been able anymore, to rectify the
environmental load caused by the ever-increasing world population. Our water reserves
are a main issue of interest, because pollution from both the atmosphere and soil will
eventually enter the aqueous phase by deposition and percolation respectively.
Sources of pollution are both nature and mankind. Examples of natural pollution are
volcanic activity, forest fires and decomposition of vegetation. Pollution by mankind is
caused by e.g. nutrition, transportation, accommodation, synthesis and energy
exploitation. Although probably not always acknowledged, chemical activity is
indispensable to sustain life; also it is needed to comply with a high standard of living.
Examples are medicaments, cleaning and disinfecting products, cosmetics, stabilizers,
artificial fertilizers, pesticides, fuel, batteries, polymers (thermoplastics, thermosetting
resins, elastomers, fibers), paint and dyes.
Both synthesis and application of these product classes inevitably yield pollution. In
addition to biological waste like carbohydrates, proteins, urea, fats, food & vegetation
residues and carbon dioxide, we also encounter priority compounds. These materials
exhibit carcinogenic, mutagenic and/or teratogenic properties, which implies that a noeffect-level in fact does not apply for these compounds. In addition, priority compounds
can be highly persistent.
Some organic priority compounds are for instance [1]: halogenated dioxins/
benzofurans/xanthenes from the incineration of halogenated phenols, polychlorinated
biphenyls (PCB’s) used as dielectric media, fire retardants; polycyclic aromatic
hydrocarbons (benzo[a]pyrene, dibenzo[a:h]anthracene) in soot and coal tar/pitch from
the incomplete combustion of hydrocarbons and from coal gasification; simple aromatic
hydrocarbons (benzene, nitrobenzene, p-dichlorobenzene, o-phenylenediamine) used as
precursors in organic chemical synthesis; chlorinated aliphatics (chloroform,
tetrachloromethane, trichloroethylene) applied as solvent and/or stain remover;
pesticides (DDT, kepone, lindane) for crop protection and pest control; ammunition
(TNT, picric acid, nitroanilines); monomers (acrylonitrile, vinylchloride, urethane) from
the synthesis, processing and incomplete combustion of polymers, dyes (benzidinebased) for the colorization of e.g. textile, leather and polymers.
Inorganic priority compounds are for instance heavy metals & salts (Cd, Ni, Cr),
asbestos, arsenic/compounds, beryllium/compounds and radioactive materials.
Although we left the ages of unscrupulous operation long ago, we inherit innumerous
highly polluted waste sites of former gasworks, ammunition and pesticide plants, oil/gas
drill and refinery locations, mining sites, fuel stations, dry-cleaning facilities, waste
dump and incineration sites. Conventional microbiological degradation desperately needs
the assistance of new technologies, like for instance advanced oxidation processes, to
degrade hazardous persistent materials by chemical oxidation.
2
Chapter 1.
1.1. Advanced oxidation processes
Advanced Oxidation Processes (AOP’s) aim at the in-situ production of strong oxidizers.
The oxidizing power is reflected by the standard reduction potential E0. Table 1.1 shows
some oxidizers in decreasing power order and E0 values, expressed for reduction halfcell reactions [2,3]. The potential is defined relative to the standard hydrogen electrode
potential [4]. The Gibbs free energy change ∆G of the redox-reaction is calculated from
the resulting electromotive force of both half-cell reactions corrected for activity
dependence (E), the number of electrons involved (n) and the Faraday constant
(F=96485 C/mol), see Equation 1.1.
Table 1.1
Standard reduction potential values for some oxidizers at T=298.15 K,
for acidic conditions pH=0 applies.
Reduction half-cell reaction
XeF+ e- → Xe + F2OF2 (g) + 4H+ + 4e- → O2 (g) + 4HF
OH + H+ + e- → H2O
O (g) + 2H+ + 2e- → H2O
O3 + 2H+ + 2e- → O2 + H2O
H2O2 + 2H+ + 2e- → 2H2O
HClO2 + 2H+ + 2e- → HClO+H2O
HO2 + H+ + e- → H2O2
Cl2 + 2e- → 2 Cl∆G = − n ⋅ F ⋅E
E0 (V)
3.4
3.29
2.56
2.43
2.08
1.76
1.67
1.44
1.40
(1.1)
The strongest oxidizers known are xenonfluoride (XeF) and possibly H4RnO6, but these
oxidizers are not commercially attractive for water treatment because of both extreme
reactivity and remaining toxicity in reduced form. Also, halogen-based oxidizers are not
acceptable as oxidizer, because they halogenate organic materials to e.g.
trihalomethanes [5] which are very harmful compounds; in addition their reaction leads
to salt formation. It is obvious, that metal-based oxidizers like permanganate (MnO4-)
and dichromate (Cr2O72-) also are not desirable. Of interest are thus oxygen-based
halogen/metal-free oxidizers like the hydroxyl radical (OH), atomic oxygen (O), ozone
(O3) and hydrogen peroxide (H2O2).
Next, a concise description is presented for major AOP’s with regard to the generation
of oxygen-based halogen-free oxidizers, particularly hydroxyl radicals. A comparison of
AOP’s is discussed in section 5.5.
Introduction
3
Ozone-UV oxidation
In the ozone-UV technology [6,7], hydroxyl radicals are produced from ozone, water
and UV photons; high-pressure mercury or xenon lamps deliver the photons, see
Equation 1.2.
O3 + H2O + hν → 2OH + O2
λ≤310 nm
(1.2)
Ozone is produced on location by an ozonizer, which converts atmospheric or pure
oxygen into ozone by corona discharges [8,9]. These electrical discharges are produced
in a barrier discharge electrode setup, where the electrodes are separated by a dielectric
material e.g. glass or ceramic at a thickness of about 0.5-3 mm. The applied voltage is
8-30 kV and the frequency range is 60-2000 Hz. The energy efficiency is about 60
g/kWh for air or 120 g/kWh for oxygen [10]. The theoretical maximum efficiency is
calculated from the standard formation enthalpy change ∆Hf0=144.8 kJ/mol for the
reaction 3O2→2O3 and is about G=1193 g O3/kWh. Commercial ozone generators are
based on different electrode configurations, e.g. fluid-cooled shell & tube type
generators for generation of large ozone amounts and air-cooled plate type generators
for small amounts. Cooling is very important, to prevent decomposition of ozone.
Hydrogen peroxide-UV and Fenton oxidation
Hydrogen peroxide is decomposed by UV photons into hydroxyl radicals [11], see
Eq.1.3a. Also, the reaction of hydrogen peroxide with iron (II) ions produces hydroxyl
radicals; this reaction is known as the Fenton reaction (Eq.1.3b) [12]. In addition, Fe(III)
ions contribute to hydroxyl radical formation by Eq.1.3c/d (Fenton like reaction) and
indirectly by regeneration of Fe(II).
H2O2 + hν → 2OH
Fe2+ + H2O2 → OH + OH- + Fe3+
Fe3+ + OH- Fe(OH)2+
Fe(OH)2+ + hν → OH + Fe2+
250 nm<λ<300 nm
λ=350 nm
(1.3a)
(1.3b)
(1.3c)
(1.3d)
The advantage of photo-Fenton/Fenton like reactions over hydrogen peroxide-UV is
mainly explained by the efficient use of light quanta, because the absorption of Fe(III)
chelates (hydroxo, carboxyl) extends to the near UV-visible region and their molar
absorption coefficient is relatively high compared to the molar absorption coefficient of
hydrogen peroxide. Synthesis of hydrogen peroxide is mainly performed according to
the following processes [13,14]:
*Anthraquinone (AO) process: Reduction of a 2-alkyl-9,10-anthraquinone to the
corresponding hydroquinone by hydrogen, followed by the oxidation of the
hydroquinone by oxygen to hydrogen peroxide and the anthraquinone.
*2-Propanol process: Oxidation of 2-propanol by oxygen produces 2-propanol-2hydroperoxide, which decomposes into hydrogen peroxide and acetone.
*Electrochemical processes: Anodic oxidative coupling of sulfate ions to persulfate ions,
followed by hydrolysis of the persulfate via the peroxomonosulfate to hydrogen
peroxide and bisulfate ions.
The theoretical maximum efficiency, calculated from the standard formation enthalpy
change ∆Hf0=98.3 kJ/mol for the reaction H2O (l) +½O2 (g) → H2O2 (l) is about
G=1246 g H2O2/kWh.
4
Chapter 1.
Photocatalytic oxidation
Photocatalytic oxidation produces hydroxyl and hydroperoxyl radicals at an irradiated
semiconductor surface in contact with water [15,16]. Excitation of electrons in the
semiconductor surface layer by UV photons will promote electrons from the valence
band to the conductivity band. In this way electron-deficient holes (h+) are created in
the valence band and free electrons (e-) will be available in the conductivity band.
Equations 1.4a-f are the main reactions, that take place at the irradiated semiconductor
surface. Water is absorbed onto the surface, resulting in the formation of H+ and OHions, see Eq.1.4a/b. Hydroxyl radicals are produced by oxidation of water (Eq.1.4c) or
oxidation of hydroxyl ions (Eq.1.4d), while hydroperoxyl radicals are obtained from the
superoxide anion (O2-), see Eq.1.4e/f.
2H2O + 4h+ → 4H+ + O2
2H2O + 2e- → 2OH- + H2
H2O + h+ → OH + H+
OH- + h+ → OH
O2 + e- → O2O2- + H+ → HO2
(1.4a)
(1.4b)
(1.4c)
(1.4d)
(1.4e)
(1.4f)
Some applied semiconductors are titanium oxide (TiO2), zinc oxide (ZnO) and cadmium
sulfide (CdS). The most well-known is the anatase crystal structure of TiO2. Its bandgap energy is 3.2 eV; the irradiation wavelength λ<385 nm. TiO2 has favourable
photochemical stability and photocatalytic activity.
Wet oxidation
In wet oxidation, water with dissolved oxygen is used to oxidize the target compound
[17,18]. The process can be performed at e.g. subcritical (4 MPa<p<20 MPa,
513K<T<593K) or supercritical conditions (p>22.1 MPa, T>647K). These conditions
enable optimal solubility of oxygen and organic compounds in water. Metal ions can be
added to catalyze the oxidation. Equations 1.5a-h are the main reactions. Hydroxyl
radicals are produced from the dissociation and oxidation of water (Eq.1.5a/b).
Hydroperoxyl radicals are formed from the oxidation of water (Eq.1.5b) and the target
compound RH (Eq.1.5f). Hydroxyl radicals are also produced from hydrogen peroxide
(Eq.1.5d) and from the reaction of atomic oxygen with the target compound (Eq.1.5h).
Hydrogen peroxide is produced by recombination of hydroperoxyl radicals (Eq.1.5c) or
by reaction of hydroperoxyl radicals with the target compound (Eq.1.5g). Atomic
oxygen is produced from the dissociation of oxygen (Eq.1.5e). Although the
hydroperoxyl radical is less reactive than the hydroxyl radical, it plays an important role
because of its relative abundance.
H2O → OH + H
H2O + O2 → OH + HO2
2HO2 → H2O2 + O2
H2O2 → 2OH
O2 → 2O
RH+O2 → R + HO2
RH + HO2 → R + H2O2
RH + O → R + OH
(1.5a)
(1.5b)
(1.5c)
(1.5d)
(1.5e)
(1.5f)
(1.5g)
(1.5h)
Introduction
5
Radiolysis
Irradiation of water by high-energy photons or electrons dissociates water molecules
into hydroxyl radicals and hydrogen atoms or ionizes water molecules, see Eq.1.6a/b
[19,20]. Ionized water molecules react with water to produce hydroxyl radicals, see
Eq.1.6c. By saturation of the water with nitrous oxide (N2O), solvated electrons
(Eq.1.6d) are converted into hydroxyl radicals (Eq.1.6e). Also the target compound is
dissociated or ionized. Halogenated target compounds RXn react rapidly with solvated
electrons, see Eq.1.6f.
H2O → OH + H
H2O → H2O+ + eH2O+ + H2O → H3O+ + OH
e- + H2O → eaqN2O + eaq- + H2O → N2 + OH + OHRXn + eaq- → RXn-1 + X-
(1.6a)
(1.6b)
(1.6c)
(1.6d)
(1.6e)
(1.6f)
High-energy photons are obtained from a radioactive source (60Co-γ) and electrons are
produced by an electron beam accelerator or a Van de Graaff generator.
Ultrasonic irradiation
The introduction of ultrasonic energy into a liquid causes electrohydraulic cavitation
[21,22]. The applied frequency range is from 15 kHz up to 1 MHz. The generation of
ultrasound energy can be performed by electromechanical (piezoelectric or magnetostrictive) or liquid-driven (liquid whistle = low intensity) transducers.
The cavitation process involves the oscillation of the radii of pre-existing gas cavities by
the periodically changing pressure field of the ultrasonic waves. The rapid implosion of
the eventually instable gas bubbles causes adiabatic heating of the bubble vapour
phase. In this way, localized and transient high temperatures and pressures are reached,
e.g. p>300 bar and T>3300 K in aqueous solution. These vigorous conditions invoke
dissociation and pyrolysis of the liquid phase molecules and present target compounds.
Water will be dissociated into hydroxyl radicals and hydrogen atoms, see Eq.1.7a.
Organic compounds are dissociated into radicals (Eq.1.7b/c) and functional groups like
carboxyl and nitro groups are removed, see Eq.1.7d/e.
H2O → OH + H
AB → A + B
RXn → RXn-1 + X
RCOOH → RH + CO2
RNO2 → RO + NO
(1.7a)
(1.7b)
(1.7c)
(1.7d)
(1.7e)
6
Chapter 1.
1.2. Electrical discharges
The discharge of electric energy into a dielectric medium may cause dissociation,
ionization and excitation of the dielectric molecules or atoms [23]. Depending on the
energy input, the produced plasma is non-thermal or thermal. In thermal plasmas the
ionization level is high, about 10-2. Examples of thermal electrical discharges are
lightning and arc discharges. Typical numbers of electron density (ne) and electron
energy (Te) for lightning discharges are about ne=1⋅1017-5⋅1017 cm-3 and Te=2.2 eV
(corresponding to 25000 K). Corona and glow discharges are non-thermal plasmas.
Their ionization level is very low, about 10-6. The electron density of a corona plasma is
about ne=1013 cm-3. The chemical reactivity of corona discharges is based on the fact,
that the electric field strength at the discharge streamer heads is extremely high viz.
about 200 kV/cm, corresponding to 1000 Td. This implies an average electron energy
of about Te=10 eV, which reaches beyond the dissociation energy of water (5.16 eV),
oxygen (5.17 eV) and nitrogen (9.80) [24]. Within the energy distribution of electrons
in the streamer head, even higher energetic electrons exist that cause ionization [25] of
oxygen (12.07 eV), water (12.62 eV) and nitrogen (15.58 eV).
A very particular advantage of pulsed corona discharges is the fact, that a highly
reactive streamer discharge medium is created, while the bulk gas is at ambient
temperature and pressure [26,27]. Therefore, pulsed corona promises higher efficiency
than other advanced oxidation processes.
Corona discharges in water produce hydroxyl radicals and hydrogen atoms from the
dissociation and ionization of water molecules, see Eq. 1.8a-c. In a humid gas phase,
corona discharges additionally create radicals, ions and metastables from the
dissociation and ionization of the gas phase molecules or atoms. In humid air, the
following main oxidizer species are produced: hydroxyl radicals, ozone, atomic oxygen,
singlet oxygen and hydroperoxyl radicals, see Eq. 1.8a-n. Also, small amounts of
nitrogen oxides like NOx and N2O are formed according to Eq. 1.8o-r.
H2O + e- → OH + H + eH2O + e- → H2O+ + 2eH2O+ + H2O → H3O+ + OH
N2 + e- → N2* + eO2 + e- → O2* + eN2 + e- → 2N + eO2 + e- → 2O + eN2 + e- → N2+ + 2eO2 + e- → O2+ + 2eO2 + e- → O2O2 + e- → O- + O
O2 + O → O3
H + O2 → HO2
H + O3 → HO3
N + O → NO
NO + O → NO2
N2+ + O2- → 2NO
N2 + O → N2O
dissociation
ionization
dissociation
excitation
excitation
dissociation
dissociation
ionization
ionization
attachment
dissociative attachment
association
association
association
association
association
recombination
association
Next to these reactions, many others exist [28].
(1.8a)
(1.8b)
(1.8c)
(1.8d)
(1.8e)
(1.8f)
(1.8g)
(1.8h)
(1.8i)
(1.8j)
(1.8k)
(1.8l)
(1.8m)
(1.8n)
(1.8o)
(1.8p)
(1.8q)
(1.8r)
Introduction
7
Figure 1.1 shows a typical CCD image of pulsed positive corona discharges in air over a
water surface. The bright areas are a superposition of about 100 streamer discharge
channels. The streamer channels are thin; their diameter is 1 mm or less. The streamer
discharges start from a 30 pins anode and then propagate towards the gas-liquid
interface. The discharges do not enter the aqueous phase due to the high relative
permittivity of water. The cathode plate is situated directly outside and underneath the
glass reactor vessel.
Figure 1.1
CCD image of pulsed positive corona discharges in air over a water
surface. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm. The CCD camera settings are: diaphragm f/5.6, exposure time
1 s. The image has been taken by A.H.F.M. Baede.
The first applications of electrical discharges in water date from Russian experiments in
the seventies on pre-breakdown phenomena in liquid dielectrics by Klimkin [29] and
Alkhimov [30] and also for the destruction of bacteria in water. In the early eighties
water ozonation was introduced for waste water treatment, while from about 1987 the
pulsed corona discharge technology was introduced for the degradation of water
pollutants by Clements and Sato [31].
Till now, pulsed corona discharge technology has been in an experimental stage, mainly
for the removal of nitrogen oxides and sulfur dioxide from flue gas and the destruction
of hydrocarbons and odor components in waste gas [27].
1.3. Model compounds
In this thesis, the oxidizing power of pulsed corona discharges is investigated with
regard to organic compounds. The choice has been made to study some well-known
compound classes viz. the bulk chemical phenol, the herbicide atrazine, the dye
malachite green and the odor component dimethyl sulfide, see Figure 1.2. Phenol,
atrazine and malachite green have been degraded in aqueous solution, dimethyl sulfide
has been degraded in the gas phase. A summary of particular compound properties and
applications is discussed now.
8
Chapter 1.
Cl
OH
N
HN
N
N
+
N
N
S
NH
X-
phenol
Figure 1.2
atrazine
malachite green
dimethyl sulfide
Model compounds applied for corona-induced oxidation.
Phenol (hydroxybenzene) [32,33] is a moderately toxic crystalline solid, readily
absorbed by the skin. Investigations and carcinogenic or co-carcinogenic properties give
ambiguous indications. The human lethal dose value is about 140 mg/kg. Phenol is a
major precursor in synthesis of e.g. polymers (phenol-formaldehyde, polycarbonate and
epoxy resins) including light stabilizers, pharmaceuticals (acetylsalicylic acid, vitamin E,
antioxidants), microbicides/fungicides (chlorinated phenols, hydroxybiphenyls), dyes
(azo, nitro, triarylmethane, from aniline raw material), photochemicals (diazocoupling,
developers) surfactants (alkylphenols), fragances (phenolic ethers); therefore phenol
occurs in waste flows released from the synthesis of these compounds.
Also during oil refinery and cokes production phenol is set free. Lignins, the
biopolymeric construction materials of plant cell walls and wood tissue, consist of e.g.
hydroxybenzene-functional monomeric units [34]. Lignins occur in waste flows from
paper mills. The proper water solubility of phenol makes it ideally suitable for coronainduced aqueous phase oxidation.
Atrazine (6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine) is a harmful
and persistent herbicide [35,36]. It is widely used for selective weed control (broadleaf
and grassy weeds) in corn, sugar cane and asparagus but is also used for nonselective
weed control on noncropped land. Atrazine is mobile in sand and loam and low to
intermediately mobile in clay loam; irrigation induced leaching and dilution has led to
atrazine residues persisting for 3 years in soil of irrigation ditches at depths certainly up
to 90 cm. Atrazine is a possible human carcinogen [1]. The maximum permissible
concentration of atrazine in drinking water is 0.1 ppb.
Malachite green [37,38] is a triphenylmethane cationic dye. These dye types favour a
high color strength and brilliance, but their light fastness is generally poor. Its green
color is due to the absorption of the wavelengths λ=427.5 nm (FWHM=40 nm, log
(ε)=4.30) and λ=621 nm (FWHM=60 nm log (ε)=5.02). Malachite green is applied for
coloring paper, leather, inks, waxes and polyacrylonitrile fibers. Malachite green exhibits
acute oral toxicity and is a suspected human carcinogen due to its photosensitizing
properties. It causes acute lethal toxicity in fish, inhibits algae growth and also inhibits
activated sludge.
Introduction
9
Dimethyl sulfide [39,40] is a highly volatile liquid spreading a very unpleasant odor. The
odor detection limit is about 0.75 mg/m3. The boiling point is 37°C and partial pressure
is p293K=55.5 kPa. On industrial scale, dimethyl sulfide is emitted from e.g. Kraft
pulping paper mills, fish processing and from incineration of cattle cadavers. Dimethyl
sulfide is used as marker for odourless gases. Organic sulfides also occur in abundance
from natural sources. Inhalation of dimethyl sulfide may cause narcosis and paralysis of
the nerve system controlling the respiration and circulation. The maximum allowable
concentration is suggested to equal the odor detection limit, about 0.75 mg/m3 equally
to 0.29 ppm.
1.4. Thesis scope
The scope of this thesis is an investigation of the applicability and technical feasibility
of pulsed positive corona discharges for the degradation of organic materials at low
concentration in aqueous solution. Although research on aqueous phase remediation by
AOP’s is a topic of growing interest since about 20 years, corona research for these
purposes is still rather exotic. For this reason a multidisciplinary project has been
performed in which the following knowledge sources have participated: the physics of
electrical discharges, physical-chemical analysis and organic chemistry. In this way an
adequate fundamental comparison can be made between corona discharge technology
and other more known AOP’s; a financial consideration of corona with regard to other
AOP’s and conventional waste water treatment has not been part of this project. The
study is based on the model compounds phenol, atrazine, malachite green and dimethyl
sulfide; the corona-induced degradation of phenol has been studied in detail. Key parts
are the conversion efficiency and identity of the oxidation products.
In chapter 2, a concise description is presented with regard to corona discharges,
oxidizer properties, degradation of organic compounds and applied chemical/electrical/
optical diagnostics. Chapter 3 describes the applied reagents, reactors, diagnostics and
configurational settings.
Chapter 4 represents the experimental results of corona-induced oxidizer production and
model compound degradation. The corona-induced production of the oxidizers hydroxyl
radicals and ozone is discussed in section 4.1. In-situ electron spin resonance and exsitu molecular probe fluorescence spectrometry have been applied for the detection of
hydroxyl radicals, while the formation of ozone has been quantified using in-situ
absorption spectrometry.
Section 4.2 describes the corona-induced degradation of phenol. The conversion
efficiency has been determined from the conversion and required energy input. The
conversion has been determined by liquid chromatography, namely ion-exclusion
chromatography and reversed-phase high performance liquid chromatography. The
energy input has been determined from corona pulse voltage and current waveform
measurements. In addition to ex-situ conversion determination, oxidation progress has
also been monitored by application of in-situ laser-induced fluorescence spectroscopy.
The obtained phenol oxidation product mixtures have been analyzed by several
analytical techniques. Primary product identification has been performed by IonSpray
and electron-impact mass spectrometry. The environmental impact has been studied by
Microtox ecotoxicity tests and total organic carbon measurements.
10
Chapter 1.
Electrical conductometry has been applied to monitor phenol oxidation progress by the
formation of carboxylic acids. The analysis of gaseous phenol oxidation products has
been studied by Fourier transform infrared spectroscopy and an aldehyde screening
test. Oxidation pathway models have been constructed in order to account for the
composition of the phenol oxidation product mixture.
In section 4.3 the corona-induced degradation of atrazine, malachite green and dimethyl
sulfide is described. Atrazine conversion has been determined by reversed-phase HPLC,
malachite green degradation by several reactor geometries has been measured by
decolorization using absorption spectrometry. The oxidation of dimethyl sulfide has
been measured by gas chromatography.
The influence of different corona parameters and reactor configurations has been
determined from phenol conversion and malachite green decolorization measurements;
this has resulted in a preferred corona configuration.
In chapter 5, a discussion is presented about pulsed corona discharges, corona-induced
phenol degradation, phenol oxidation pathways, analysis techniques and a fundamental
comparison of different AOP’s. The conclusions are summarized in chapter 6.
2. Theory
This chapter starts with a fundamental description of corona discharges in air. Oxidizer
properties and general degradation pathways of organic compounds are discussed,
followed by specific model compound oxidation products reported from literature.
Finally the applied chemical, electrical and optical diagnostics are concisely summarized.
2.1. Corona discharges
The requirements for the formation of corona discharges in air at atmospheric
conditions are a sharply non-uniform electrical field and a starting condition [23]. The
sharply non-uniform E-field is achieved by applying a high voltage to e.g. a point-toplane electrode configuration in air. Corona triggering is enabled, if an ion-electron pair
is produced within the inception region of the high voltage electrode, where the
electron can gain enough energy to ionize molecules of the dielectric. The ion-electron
pair is usually produced by cosmic rays or natural radioactivity, which cause ionization
of air at a rate of about 109 m-3s-1. The polarity of a corona can be either positive or
negative. The positive corona consists of cathode-directed streamer discharge channels,
while the negative polarity corona is anode-directed.
Positive polarity corona
Once the ion-electron pair is formed within the inception area, the electron is
accelerated in the electric field, see Figure 2.1. On its way to the anode the electron
collides with other molecules from the dielectric; if the applied field is high enough,
primary avalanche electrons are produced. The molecules excited in this primary
avalanche cause photoionization by emitting high energy photons and secondary
avalanche electrons are produced.
The primary avalanche electrons sink in the anode and leave behind a primary avalanche
of positive ions having low mobility. The secondary avalanche of electrons runs into the
primary avalanche positive ions and a quasineutral plasma channel is produced. The
remaining secondary avalanche positive ions form a positive space charge at the head
of this plasma channel. If the electical field induced by the space charge reaches a
value of the order of the external field, a streamer can be produced. Then, the number
of positive ions in the head should reach a value of at least 108, according to Meek
[41]. By recurrence of the described processes, the streamer channel grows from the
anode tip into the direction of the cathode (cathode directed streamer), see Figure 2.2.
At the streamer head, an intense electrical field strength of about E=200 kV/cm is
reached that accounts for chemical reactivity viz. radical formation. The streamer grows
from the anode at a speed of about 108 cm/s, but stops where the E-field drops below
the critical value.
Corona discharges have been applied in pulsed form, to prevent transport of ions, that
is Ohmic dissipation.
12
Chapter 2.
A
+-
A
-
+
A
eIiI+
A
- eI
+i +
I
-
+
eIIiII+
+
+
-
+
A
iI+/eIIiII+
+
+
-
+ N ≥108
i
C
C
C
C
C
1
2
3
4
5
Figure 2.1
The formation of a cathode-directed streamer in air; A = anode tip, C =
cathode plate. (1) The starting condition: an ion-electron pair within the
inception area. (2) The production of a primary avalanche of electrons eIleaving behind a primary avalanche of ions iI+; the electron avalanche
causes photo-ionization. (3) The primary avalanche electrons sink into the
anode, while secondary avalanche electrons eII- produced by photoionization, run into the primary avalanche ions. (4) Recombination of
secondary avalanche electrons eII- and primary avalanche ions iI+ produces
a quasineutral plasma channel; at the end of this channel a positive space
charge is formed by the secondary avalanche ions iII+. (5) If the number of
positive ions in the channel head is at least 108, then a streamer can be
produced.
Figure 2.2
The E-field of a cathode-directed streamer in air. At the streamer head the
field strength is about 200 kV/cm. The breakdown field strength of air at
ambient conditions is Ec≈30 kV/cm.
Negative polarity corona
Positive ions produced by the primary avalanche of electrons decrease the field strength
at the negatively charged point-shaped electrode. Therefore, the electrons lose energy
on their way to the anode and get attached to electronegative oxygen. These negative
ions drift slowly towards the anode, while the primary avalanche positive ions sink into
the cathode. Then the field at the cathode recovers and the processes restart at the
formation of a new ion-electron pair within the inception area. For the case of nonpulsed DC operation, this recurrent process is known as the Trichel pulse regime.
Opposite to positive polarity corona, the electrons can only gain kinetic energy within
the inception area and therefore the formation of radicals is limited to this region.
Theory
13
2.2. Oxidizers
Some characteristic properties are described for the following oxidizers produced by
advanced oxidation processes: the hydroxyl radical, the ozone radical ion, ozone,
atomic oxygen, hydrogen peroxide and the hydroperoxyl radical.
Hydroxyl radical
The hydroxyl radical (OH) is one of the strongest oxidizers among the oxygen-based
oxidizers [42]; its standard reduction potential is E0=2.56 V in acidic environment, see
also Table 1.1. The hydroxyl radical is extremely reactive: its life in water is about 2 ns
and the radius of diffusion is about 20 Å [43]. In its reaction with inorganic ions,
electrons are transferred from the ion to the hydroxyl radical, via an intermediate adduct
consisting of the ion, the hydroxyl radical and -depending on the coordinating properties
of the ion- a solvent molecule. With regard to organic molecules, the hydroxyl radical
reacts electrophilic and adds to unsaturated bonds of e.g. alkenes and aromatic rings.
The hydroxyl radical also abstracts hydrogen atoms from organic molecules. In a
strongly alkaline environment, the hydroxyl radical exists in its conjugated form: the
oxygen radical ion O-, according to Equation 2.1. The acid dissociation constant of the
hydroxyl radical is about pKa=11.9.
OH + OH- O- + H2O
(2.1)
The oxygen radical ion is a nucleophilic particle that preferentially abstracts hydrogen
atoms from organic molecules. It reacts much more slowly than the hydroxyl radical.
Ozone radical ion
Although the hydroxyl radical is generally assumed to be the strongest oxygen-based
halogen-free oxidizer, the ozone radical ion (O3-) is reported to be an even more
powerful oxidizer in acidic solution [2]. It has a standard reduction potential E0=3.3 V.
The reduction half-cell reaction is given by Equation 2.2a. The O3- ion is produced from
the reaction of the oxygen radical ion and oxygen, according to [42], see Equation
2.2b. In aqueous solution, the O3- ion will oxidize water by which a hydroxyl radical, a
hydroxide ion and oxygen are produced, see Equation 2.2c.
O3- + 2H+ + e- → O2 (g) + H2O
O - + O 2 → O 3O3- + H2O → OH + OH- + O2
E0=3.3V at T=298.15 K and pH=0
(2.2a)
(2.2b)
(2.2c)
14
Chapter 2.
Ozone
Ozone (O3) [8,9] is a strong oxidizer, as is indicated by E0 = 2.08 V. It oxidizes water
to hydrogen peroxide. Therefore the bulk solubility of ozone in water is rather low viz.
about 0.1 mM at T= 293 K [44]. By irradiation with photons of wavelength λ≤310 nm,
ozone is decomposed into a singlet oxygen atom and a singlet oxygen molecule
(Eq.2.3a) [6]. In humid air, the singlet oxygen atom reacts with water to hydroxyl
radicals (Eq.2.3b); in the aqueous phase initially hydrogen peroxide can be produced
due to recombination of hydroxyl radicals that cannot escape from the solvent cage,
see Eq.2.3.c. The singlet oxygen molecule is also very reactive, its life in water is about
4.4 µs [43].
O3 + hν → O (1D) + O2 (1∆g)
O (1D) + H2O(g) → 2OH
O (1D) + H2O(l) → H2O2
λ≤310 nm
(2.3a)
(2.3b)
(2.3c)
In acidic environment at normal temperatures, ozone reacts selectively with organic
compounds as an electrophilic molecule [45]. The electrophilic behaviour of ozone is
explained by the positively charged oxygen atom in the possible resonance structures,
which are mainly represented by (A) and to a small extent by (B), see Figure 2.3.
O
O
+
O
O
O
O
(A)
Figure 2.3
+
O
O
O
O
+
O
+
(B)
O
Ozone resonance structures; the positively charged oxygen is electrophile.
Ozone is destroyed by hydroxyl radicals, according to the Equations 2.4ab. The net
reaction is the conversion of ozone into oxygen.
O3 + OH → O2 + HO2
HO2 + O3 → OH + 2O2
(2.4a)
(2.4b)
Ozone mass transfer from the gas phase into water is diffusion controlled; the Henry
coefficient KH, expressing the equilibrium partitioning of a compound between the gas
and liquid phase, is large viz. KH≈3.76⋅103 at 20°C [46] which implies a negligible
resistance to mass transfer in the gas film compared to the liquid film. The mass
transfer rate is influenced by e.g. the gas phase ozone concentration, temperature,
pressure, gas dispersion, solution ionic strength, solution acidity and presence of
reactive compounds in the liquid phase.
Atomic oxygen
Atomic oxygen (O) is produced by dissociation of molecular oxygen, which requires an
energy of about 498.4 kJ/mol [24] corresponding to 5.2 eV. In acidic environment the
oxygen atom is a stronger oxidizer than ozone, E0 =2.43 V. Its stability is however
very limited. In the gas phase, atomic oxygen directly reacts with molecular oxygen to
ozone, where the activation energy of this reaction is only Ea=16.7 kJ/mol [47].
Atomic oxygen oxidizes water to hydrogen peroxide.
Theory
15
Hydrogen peroxide
Hydrogen peroxide (H2O2) [13,14], the dimerization product of hydroxyl radicals, is less
reactive than the hydroxyl radical. Its standard reduction potential is E0=1.76 V in
acidic environment. By photolysis, hydrogen peroxide decomposes into hydroxyl
radicals; the HO-OH bond strength is only 213 ±4 kJ/mol [24], which corresponds to
2.2 eV. Concentrated hydrogen peroxide (>90%) is extremely instable; the
decomposition into water and oxygen is strongly exothermic viz. 98.3 kJ/mol. It is due
to the ability of hydrogen peroxide to simultaneously oxidize and reduce itself.
Hydrogen peroxide is a weak acid: its acid dissociation constant is about pKa=11.75 at
T=293 K; however in 50% aqueous solution pKa~9 [47], see Equation 2.5.
H2O2 + H2O H3O+ + HO2-
(2.5)
Hydroperoxyl radical
The hydroperoxyl radical (HO2) is a much less strong oxidizer than the hydroxyl radical,
ozone or hydrogen peroxide; its standard reduction potential is E0=1.44 V in acidic
environment and thus just excels chlorine as oxidizer, see Table 1.1. The HO2 radical is
produced in oxygen enriched water from hydrogen atoms that are formed by
dissociation of water molecules, see Equation 2.6ab. In alkaline environment, the
hydroperoxyl radical exists as the superoxide radical ion O2-, see Equation 2.6c. The
acid dissociation constant of the hydroperoxyl radical is about pKa=4.4. Hydroperoxyl
radicals often react with each other to hydrogen peroxide and oxygen, see Equation
2.6d [48].
H2O → H + OH
H + O2 → HO2
HO2 + OH- O2- + H2O
2HO2 → H2O2 + O2
(2.6a)
(2.6b)
(2.6c)
(2.6d)
2.3. Degradation of organic compounds
Chemical oxidation is an important method to degrade organic compounds. The
objective of degradation is mineralization i.e. conversion of the target compound to
carbon dioxide, water and -depending on the nature of the compound- inorganic ions
like e.g. chloride, nitrate, phosphate and sulfate; the inherent toxicity of possibly
obtained fluoride and bromate ions cannot be overcome by the oxidation process. In
practice, complete mineralization is normally not requested, except for extremely
dangerous materials. In many cases it is both justified and efficient to partially degrade
the target compound in order to enable further degradation by microbiological
treatment. For that case, the chemical oxidation step is needed to destroy persistent
molecular structures, to remove high ecotoxicity and enhance water solubility.
Examples of degradation pathways of organic compounds are discussed now. The
degradation of unsaturated bonds by ozone, hydroxyl radicals and oxygen is discussed.
In addition to chemical oxidation, reduction and pyrolysis are briefly mentioned.
16
Chapter 2.
Chemical oxidation
Ozone can react both directly and indirectly. The indirect way takes place under neutral
or alkaline conditions via hydroxyl radicals. The direct way in acidic environment is the
electrophilic addition of ozone to unsaturated bonds of alkenes and aromatic
compounds. This addition reaction initially produces a molozonide, which rearranges
immediately to an ozonide, see Figure 2.4. The ozonide decomposes by ring- cleavage
and a zwitterion and an aldehyde or a ketone are produced. In water, the zwitterion
hydrolyzes to a hydroxyalkyl hydroperoxide. Depending on the substituent groups, the
hydroxyalkyl hydroperoxide decomposes into an aldehyde or a ketone by elimination of
hydrogen peroxide or a rearrangement to carboxylic acids occurs [49].
R4
R3
R1
+
R2
O
O
R43
+
C OO
R3
O
R2
R4
O
R3
O
molozonide
+
R1
O
O
R2
R12
R43
ozonide
R12
R43 C OOH
OH
H2 O
zwitterion
Figure 2.4
R1
O
O
alkene
R12
R4
+
+
C OO
zwitterion
R21
R34
O
aldehyde/ketone
RCOOH, RCHO, R2CO
hydroxyalkyl
hydroperoxide
The reaction of ozone with an unsaturated bond of an alkene or an
aromatic compound yields bond cleavage. Products are carboxylic acids,
aldehydes or ketones. R is a substituent group.
Hydroxyl radicals attack regions of high electron density and therefore add to
unsaturated bonds of aromatic compounds and alkenes. Attack of a hydroxyl radical on
an aromatic compound produces hydroxycyclohexadienyl radicals; attack of oxygen on
these radicals yields endoperoxyalkyl and endoperoxyl radicals; the endoperoxyl radicals
yield endoperoxides [19,50], see Figure 2.5. The very instable endoperoxides
decompose by ring-cleavage to unsaturated aliphatic hydrocarbons with polyfunctional
groups like carboxyl, aldehyde, carbonyl or alkanol groups. Also carbon monoxide may
be eliminated.
X
X
HO
HO
aromatic
compound
Figure 2.5
⋅
O2
O
O
HO
X
X
X
⋅
O2
hydroxy
endoperoxy
cyclohexadienyl alkyl radical
radical
O
O
HO
⋅ HO
OO
endoperoxyl
radical
O
O
-OH,=O
polyfunctional
aliphatic
hydrocarbons
endoperoxides
The attack of a hydroxyl radical and oxygen on an aromatic compound
produces endoperoxides, which decompose to unsaturated aliphatic
hydrocarbons with polyfunctional groups.
Theory
17
Attack of the hydroxyl radical and oxygen on an alkene produces hydroxyalkylperoxyl
radicals. These radicals dimerize to a tetraoxide intermediate. The tetraoxide may
decompose in many ways. However, an important pathway is a fragmentation reaction
that yields α-hydroxyalkyl radicals, aldehydes/ketones and oxygen. The α-hydroxyalkyl
radical scavenges oxygen and produces an α-hydroxyalkylperoxyl radical that yields an
aldehyde or a ketone by elimination of hydroperoxyl radicals, see Figure 2.6 [50,51].
R4
R3
R1
R43
HO
R2
R34
alkene
R43
OH
⋅
OH
⋅
R12
O2
R34
R21
hydroxyalkyl
radical
R12
+
α-hydroxyalkyl
radical
Figure 2.6
OH
R12
⋅
OO
2x
OH
R21
⋅
R43
OH
⋅
R12
+
2
R12
OO
O
α-hydroxyalkyl
peroxyl radical
aldehyde
or ketone
+
R34
R21
+ O2
O
α-hydroxyalkyl
radical
R43
R12
2
tetraoxide
hydroxyalkyl
peroxyl radical
R43
O2
R43
aldehyde
or ketone
⋅
HO2
hydroperoxyl
radical
The attack of a hydroxyl radical and oxygen on an alkene produces a
hydroxyalkylperoxyl radical; dimerization of hydroxyalkylperoxyl radicals
yields a tetraoxide intermediate. The tetraoxide decomposes into αhydroxyalkyl radicals, aldehydes/ketones and oxygen. The attack of
oxygen on an α-hydroxyalkyl radical yields an aldehyde or a ketone by
elimination of a hydroperoxyl radical.
Hydroxyl radicals also abstract hydrogen atoms from a saturated hydrocarbon chain, by
which radical sites are created on the hydrocarbon chain where oxygen can attack. This
results in the formation of unsaturated bonds and hydroperoxyl radicals, see Figure 2.7.
The produced unsaturated hydrocarbon will be cleaved by ozone attack, see Figure 2.4.
H
HO
H
-H2O
saturated
hydrocarbon
Figure 2.7
H
•
radical
hydrocarbon
O2
H
H
H
O
O
peroxy radical
hydrocarbon
+
HO2•
unsaturated hydroperoxyl
hydrocarbon
radical
Hydrogen abstraction from a saturated hydrocarbon chain by a hydroxyl
radical, followed by oxygen attack produces an unsaturated hydrocarbon
and a hydroperoxyl radical.
During oxidation, covalently bonded halogens, nitrogen, phosphorous and sulfur -if
present- are removed from the target molecule and converted to inorganic ions like
halides, nitrates, phosphates and sulphates. Figure 2.8 shows the oxidation of
dichloromethane by hydroxyl radicals and oxygen, eventually yielding carbon monoxide,
carbon dioxide and hydrogen chloride; the intermediate phosgene is highly toxic, but it
rapidly hydrolyzes to carbon dioxide and hydrogen chloride [50,51].
18
Chapter 2.
Cl
H C H
Cl
dichloromethane
Cl
H C
Cl
O2
H
Cl C
H
O2
-H2O
+
Cl
H C OO•
- HO
Cl
HO
-ClOH
H
Cl C OO•
H
O
H2O
CO2 + 2H+ + 2Cl-
Cl
Cl
phosgene
CO
+
HO
+ H+ + Cl-
The oxidation of dichloromethane by hydroxyl radicals and oxygen
eventually yields carbon monoxide, carbon dioxide and hydrogen chloride;
phosgene is a highly toxic intermediate, which rapidly hydrolyzes to
carbon dioxide and hydrogen chloride.
Figure 2.8
Reduction
Reduction of unsaturated hydrocarbons by hydrogenation does not invoke bond/ringcleavage but only saturation takes place [52]. Nevertheless aromaticity is destroyed in
this way. Figure 2.9 shows the hydrogenation of benzene to cyclohexane. In contrast
to benzene, cyclohexane is less harmful [1].
H
benzene
H
⋅
H
hyd
H
hydroxycyclo
cyclohexadienes
hexadienyl radicals
hyd
cyclohexene
cyclohexane
Reduction of benzene to cyclohexane by hydrogenation (hyd).
Figure 2.9
Reduction of azo dyes invokes cleavage of the azo bond (-N=N-), which implies
fragmentation of the azo dye molecule into two amino (RNH2) compounds, see Figure
2.10. This reaction explains reductive fading (decolorization) of the dye, induced by
ketyl or carboxy radicals [53]. However, this degradation reaction produces e.g. highly
harmful aniline.
O
O
NH
OH
NH OH
N
SO3Na
N
SO3Na
NH2
reduction
4H+
+
4e-
SO3Na
NH2
+
SO3Na
aniline
Acid Red 1
Figure 2.10
The reduction of the azo dye Acid Red 1; this reaction is an example of
reductive fading (dye decolorization).
Theory
19
Pyrolysis
Pyrolysis is thermal decomposition of an organic compound in the absence of oxygen.
By pyrolysis, molecules are dissociated into radicals and elimination of functional groups
may take place like e.g. decarboxylation of carboxylic acids, dehydration of alkanols
and esters, dehalogenation, loss of nitro and sulfone groups, nitrogen, carbon
monoxide, see Figure 2.11. Thermal cracking of alkanes yields lower molecular weight
alkenes, but pyrolysis of simple aromatics leads to polymerization viz. polycyclic
aromatic hydrocarbons [54].
1. CCl4
CCl3
+
•
NO2
Cl
•
O
+
2.
OH
3. O
OH
OH
OH O
4.
Figure 2.11
NO
O
OH
+
CO2
+
Pyrolysis of organic compounds: 1. Dissociation of tetrachloromethane; 2.
Nitric monoxide release from p-nitrophenol; 3. Decarboxylation of malonic
acid yields acetic acid; 4. Cracking of n-butane to ethane and ethylene.
Knowledge of the target compound conversion level and/or conversion efficiency is not
sufficient to qualify an advanced oxidation process. It is also very important to identify
the intermediate and final oxidation products. During the oxidation of organic
compounds intermediates may be produced, which exhibit higher toxicity than the
target compound. Examples are dibenzofurans and dioxins, produced from supercritical
water oxidation of phenol, as will be discussed in section 2.4.1. The oxidation of
halogenated hydrocarbons yields highly harmful halogenated aldehydes/carboxylic acids
[51,55]. Therefore oxidation progress must have proceeded, until these harmful
intermediate products have been converted.
20
Chapter 2.
2.4. Oxidation of model compounds
A survey from literature is presented about the general oxidation pathways of the
applied model compounds phenol, atrazine, malachite green and dimethyl sulfide.
2.4.1. Phenol
The oxidation of phenol produces a wide oxidation product range, consisting of
polyhydroxybenzenes/quinones, ring-cleavage products and polymerization products.
Literature data originate from radiolysis [50,56,20], oxidation by hydrogen peroxide
[11,57,58], ozone [7] and other chemical oxidizers [34,52], photocatalytic oxidation
[15], photolysis [59] and oxidation by supercritical water [18,60,17].
Polyhydroxybenzenes and quinones
Among the polyhydroxybenzenes [61] are the dihydroxybenzenes (DHB’s): catechol
(1,2-DHB), resorcinol (1,3-DHB), hydroquinone (1,4-DHB) and the trihydroxybenzenes
(THB’s): pyrogallol (1,2,3-THB), hydroxyhydroquinone (1,2,4-THB) and phloroglucinol
(1,3,5-THB), see Figure 2.12. These compounds are produced by attack of the hydroxyl
radical on the benzene ring. With increasing amount of hydroxyl groups attached to the
benzene ring, the stability of the hydroxybenzene towards oxidation strongly decreases
[52]. Therefore higher hydroxylated benzenes are not likely to be found during vigorous
oxidizing conditions.
Quinones are produced by oxidation of polyhydroxybenzenes. The following quinones
are reported: 1,4-benzoquinone, 1,2-benzoquinone and hydroxybenzoquinone. 1,4benzoquinone is produced by oxidation of hydroquinone. 1,2-benzoquinone is a very
unstable oxidation product of catechol. Hydroxybenzoquinone is produced by
hydroxylation of 1,4-benzoquinone or partial oxidation of hydroxyhydroquinone; it is
reported to undergo polycondensation in aqueous solutions. 1,3-benzoquinone does not
exist, because the structure would be nonplanar and highly strained [62].
OH
OH
OH
OH
OH
OH
OH
OH
OH
catechol
resorcinol
OH
hydroquinone
O
O
O
O
pyrogallol
OH
OH
OH
HO
OH
hydroxy- phloroglucinol
hydroquinone
OH
O
1,4-benzoquinone
1,2-benzoquinone
Figure 2.12
Polyhydroxybenzenes and quinones produced by the oxidation of phenol.
O
hydroxybenzoquinone
Theory
21
Ring-cleavage products
The oxidation of polyhydroxybenzenes and quinones produces ring-cleavage products.
Observed products are unsaturated and saturated C1-C6 hydrocarbons with
polyfunctional groups like carboxyl-, aldehyde-, ketone- or alkanol- groups, see Figure
2.13. Alkanol-functional groups are oxidized to aldehyde groups, while aldehydes are
oxidized to carboxylic acids. The following classes can be mentioned:
Saturated monocarboxylic acids: formic, acetic, propionic and glyoxylic acid. Saturated
dicarboxylic acids: oxalic, malonic, ketomalonic, D,L-malic, succinic, glutaric and adipic
acid. Unsaturated monocarboxylic acids: acrylic acid. Unsaturated dicarboxylic acids:
maleic, fumaric and cis,cis-muconic acid. Saturated aldehydes: formaldehyde,
acetaldehyde and glyoxal. Unsaturated hydrocarbons: acetylene and butadiene.
Acetylene is reported to be produced under supercritical conditions by addition of
oxygen to catechol [17]. Butadiene is reported as a decomposition product of
hydroxyhydroquinone [16].
O
O
H
O
H
H
H
H
O
formaldehyde acetaldehyde
O
O
H
glyoxyal
OH
OH
formic acid
O
O
oxalic acid
O
malonic acid
OH
O
adipic acid
OH
O
ketomalonic acid
OH
D,L-malic acid
O
O
O
Figure 2.13
O
HO
O
HO
OH
acrylic acid
O
OH
O
OH
O
succinic acid
glutaric acid
OH
O
OH
OH
HO
OH O
O
OH
O
propionic acid glyoxylic acid
OH O
OH
H
OH
acetic acid
O
HO
O
O
HO
O
OH
maleic acid
HO
OH
O
fumaric acid
O
OH
O
OH
cis,cis-muconic acid
Aldehydes and carboxylic acids produced by the oxidation of phenol.
22
Chapter 2.
Polymerization products
The radical-induced oxidation of phenol also invokes molecular coupling, see Figure
2.14. Reported dimerization products are 4,4’/2,4’/2,2’-dihydroxybiphenyl and 4/2hydroxydiphenylether. These products are formed by dimerization of phenoxy radicals.
Purpurogallin is produced from the dipolar dimerization of the ortho-quinone of
pyrogallol. Supercritical water oxidation of phenol yields the following multiring
condensation products: dibenzofuran, dibenzofuranol, dibenzo-p-dioxin, 9H-xanthene-9one, 2,3-dihydro-1H-indene-1-one. Polymerization products are the so-called “synthetic
humic acids” consisting of hydroquinone and (hydroxy)benzoquinone monomeric units;
these amorphous products exhibit a dark-brown color. The toxicity of the coupling
products is higher to much higher than the toxicity of phenol. Especially the
benzofurans and dioxins are highly unwanted, but these compounds are avoided or
destroyed by supercritical conditions over T=600°C.
OH
OH
OH
O
OH
OH
OH
OH
O
OH
4,4'-DHBP
2,4'-DHBP
2,2'-DHBP
dihydroxybiphenyl
4-HDE
2-HDE
hydroxydiphenylether
O
O
O
O
dibenzofuran
dibenzofuranol
OH O
O
O
dibenzo-p-dioxin
9H-xanthene-9-one
OH
OH
HO
HO
purpurogallin
O
OH
OH
O
O
O
O
OH
n
OH
O
n
synthetic humic acids
Figure 2.14
O
Polymerization products formed during the oxidation of phenol.
2,3-dihydro-1HIndene-1-one
Theory
23
2.4.2. Atrazine
The oxidation of atrazine involves deaminoalkylation, dechlorination and hydroxylation
of the s-triazine ring. Ring-cleavage has not been reported. Literature data have been
obtained from photocatalytic oxidation [36] and photo-Fenton oxidation [63,64]. Some
important oxidation products are shown by Figure 2.15.
By oxidation of the aminoalkyl groups the following products are formed: 4-acetamido2-chloro-6-(isopropylamino/ethylamino)-s-triazine. Partial dealkylation products are
deethylatrazine and deisopropylatrazine. Complete oxidation of the alkyl groups yields
diaminoatrazine. The dechlorination preferentially occurs after considerable degradation.
Due to deaminoalkylation and dechlorination nitrate and chloride ions are produced.
Also ethane has been detected. The final oxidation product is cyanuric acid (2,4,6trihydroxy-1,3,5-triazine).
Cl
N
Cl
N
N
HN
N
H 2N
N
NH
N
NH
deethylatrazine
O
Cl
N
HN
N
N
Cl
4-acetamido-2-chloro6-(isopropylamino)-s-triazine
NH
HN
N
N
NH
NH2
deisopropylatrazine
HO
N
N
OH
cyanuric acid
Cl
O
4-acetamido-2-chloro6-(ethylamino)-s-triazine
N
N
N
HN
Cl
N
atrazine
N
OH
N
H2 N
N
N
NH2
diaminoatrazine
Figure 2.15
Atrazine and some major oxidation products.
2.4.3. Malachite green
The oxidation of malachite green is described in literature by the lowering of light
fastness [53]. Two degradation mechanisms may apply viz. dealkylation (methyl groups
attached to nitrogen) and molecular fragmentation, starting from the carbinol base.
Reported products are N,N-di- and N-monomethyl-4-aminobenzophenone and N,Ndimethyl-4-aminophenol, see Figure 2.16.
24
Chapter 2.
N
N
+
N
N
OH
[ox]
OH
H+
malachite green
MG carbinol base
O
O
N
N
N
N,N-di- and N-monomethyl4-aminobenzophenone
Figure 2.16
OH
N,N-dimethyl4-aminophenol
Malachite green and some of its oxidation products.
2.4.4. Dimethyl sulfide
Literature data on the oxidation of dimethyl sulfide have been obtained from
[39,40,65]. Dimethyl sulfide is initially oxidized to dimethyl sulfoxide, which can be
further oxidized to dimethyl sulfone, methanesulfonic acid and finally sulfuric acid, see
Figure 2.17.
S
S
O
O
S
O
dimethyl
sulfide
dimethyl
sulfoxide
dimethyl
sulfone
[ox]
Figure 2.17
[ox]
[ox]
O
S OH
O
methanesulfonic acid
[ox]
O
HO S OH
O
sulfuric acid
Dimethyl sulfide and some of its oxidation products.
2.5. Diagnostics
An overview is presented of major chemical, electrical and optical diagnostics, which
have been applied to study the formation of oxidizers and the oxidation of model
compounds by pulsed corona discharges. Applied chemical diagnostics are liquid
chromatography, mass spectrometry, aldehyde screening, electron spin resonance,
Microtox ecotoxicity, total organic carbon content and acidity. Electrical diagnostics are
corona pulse voltage & current measurements and conductometry. Applied optical
diagnostics are UV absorbance spectrometry and fluorescence & infrared spectroscopy.
Theory
25
2.5.1. Chemical diagnostics
Liquid chromatography
Separation of the liquid-phase oxidation product mixture into its components is
necessary for determination of the conversion of the target compound and identification
of the oxidation products. In this way every product can be detected separately with
maximum sensitivity, because mutual influence is not possible. In this thesis the liquidphase oxidation product mixture has been separated by reversed-phase high
performance liquid chromatography (rp-HPLC) and ion-exclusion chromatography (ICE)
[66,67,68].
In liquid chromatography, a sample in a carrier flow i.e. eluent or mobile phase is
introduced into the separation column containing a stationary phase. The sample
components will partition between the stationary phase and the mobile phase due to
component-specific physical interaction mechanisms. In this way component-specific
retention thus separation is established. Detection of the eluting components is
commonly performed by a UV absorbance detector but the detection by e.g. mass
spectrometry, fluorescence, electrical conductivity or refractive index are also optional.
Rp-HPLC has been initially applied for exploration of the complex oxidation product
mixture. It is the most applied and versatile liquid chromatography technique, suitable
for separation of a wide group of organic compound classes including phenols,
polycyclic aromatic hydrocarbons, alkanols and alkanes. The retention mechanism is
based on non-specific hydrophobic interaction (dispersive forces) but also on dipoledipole and proton donor/acceptor interaction.
In rp-HPLC the non-polar stationary phase is commonly an alkyl-bonded silica packing
e.g. a C8- or C18-alkane grafted on silica; the polar mobile phase is a mixture of water
and an organic modifier. The retention behaviour of the components, thus the
separation, can be adjusted by changing the eluent composition i.e. eluent strength.
The eluent strength is the power of the eluent to displace components interacting with
the stationary phase. The used eluents often consist of mixtures of water and
acetonitrile or methanol, which do not absorb the UV light of the absorbance detector
at analytically-important wavelengths (cutoff wavelength: acetonitrile: 190 nm,
methanol: 210 nm, water: 191 nm [69]). The eluent dosage is performed at constant
composition (isocratic conditions) or by means of a gradient (increasing eluent
strength). The gradient dosage is applied to force the elution of compounds that
strongly interact with the stationary phase. Unfortunately rp-HPLC is not suitable for
the separation of carboxylic acids, which are important oxidation products.
In order to resolve the carboxylic acids, ion-exclusion chromatography (ICE) has been
applied. The retention mechanism is mainly based on the exclusion of anions from a
cationic-exchange resin. The anions cannot penetrate the resin, because they encounter
the Donnan potential, which guarantees the electrical neutrality within the resin. The
applied ICE column has a polystyrene-divinylbenzene partially crosslinked resin with
sulfonate (RSO3-H+)-functional groups. An aqueous solution of a strong acid, here
trifluoroacetic acid, is used as eluent. Strong carboxylic acids exist in ionic form (H+A-)
and will be excluded from the stationary phase. On the contrary, weak carboxylic acids
in molecular form will be able to diffuse into the resin pores.
26
Chapter 2.
The carboxylic acids are thus separated by their acidic strength, which is reflected by
the acid dissociation constant pKa. Other ICE retention mechanisms are hydrophobic
interaction and molecular size, both originating from the partially cross-linked resin. The
separation can be influenced by the eluent acidity, because the eluent acidity
determines the dissociation behaviour of the carboxylic acids according to the
Henderson-Hasselbalch relationship, see Equation 2.7. α Equals the dissociation
fraction.
 α 
pK a = pH − log 

1− α 
and
α=
[ A− ]
[HA] + [ A− ]
HA + H2O H3O + + A−
(2.7)
Although the eluent strength will also influence the retention behaviour of ion-exclusion
chromatography, the addition of organic modifiers to the acidic aqueous mobile phase
has not been applied for the used column. The partially cross-linked polymeric resin is
expected to swell due to the absorption of organic modifier, which may result in
cracking of the resin.
Next to UV absorbance detection, the carboxylic acids have been detected using a
conductivity detector. However, the acidic eluent necessary for ICE separation causes a
high background conductivity, which makes the detection of separated carboxylic acids
impossible. Therefore the eluent conductivity has to be decreased by removal of the
highly conductive hydronium ions, which is accomplished by a suppressor. The applied
suppressor is a micromembrane suppressor, which consists of cation-exchange
membranes and is supplied with an aqueous ammonia solution. According to the
Donnan equilibrium, these membranes exclude the trifluoroacetate anions of the
trifluoroacetic acid eluent but allow hydrogen ions to pass by exchange with ammonium
ions, while electrical neutrality is maintained. The highly conductive hydronium ion is
thus replaced by the less conductive ammonium ion.
Liquid chromatography / mass spectrometry
In order to identify the oxidation products after separation by the liquid chromatograph,
a mass spectrometer is connected to the LC system by means of a special interface.
This LC-MS coupling has to introduce a huge eluent flow (typically about F=1 ml/min)
containing tiny amounts of separated components, into the high vacuum of the
quadrupole mass spectrometer. In this work an IonSpray interface has been utilized
[70,71].
The IonSpray interface is a pneumatical and electrostatical nebulizer. A splitted fraction
of the eluent carrying the separated components from the LC system is introduced into
a hollow needle, which is energized at high voltage. Together with the eluent a
nebulizer gas flow is introduced; if necessary in combination with an organic solventbased make-up liquid to improve the sprayability of high water content eluents. A mist
of highly charged droplets is produced in the direction of the mass spectrometer. Due to
evaporation the droplets decrease in size and the electrical field at the surface of the
droplet increases. When a critical field has been reached, ions are emitted from the
surface of the droplets. These ionization conditions are very mild: no fragmentation
takes place and thus molecular ions are produced. These ions are transferred to the
orifice, where they can enter the quadrupole mass-spectrometer. Eluent molecules are
prevented from entering the mass spectrometer by a gas curtain interface.
Theory
27
IonSpray is particularly suitable for the analysis of thermolabile and ionic components.
For the identification of carboxylic acids and hydroxybenzenes, the IonSpray interface
has been operated in the negative ion mode: the applied needle voltage is negative with
respect to the grounded wall. Also ammonia has been added to the aqueous oxidation
product mixture, to facilitate the production of anions (e.g. phenolate, carboxylates).
Next to the IonSpray interface, the Atmospheric Pressure Chemical Ionization (APCI)
interface has been tested for applicability. Here, the splitted flow from the LC system is
nebulized and evaporated by heating. The evaporated flow is directed to a corona
discharge needle, where chemical ionization of sample and solvent takes place. APCI is
particularly suitable for the processing of high eluent flow rates containing high
concentration of electrolytes. However, the sample components have to be
thermostable.
Electron-impact mass spectrometry
In addition to IonSpray-MS, electron-impact mass spectrometry (EI-MS) has been
performed for the identification of phenol oxidation products. After evaporation in a prevacuum compartment, the mixture components are introduced into the main vacuum
compartment by differential pumping. Here the components are ionized by an electron
beam. Produced are molecular fragments but also molecular ions.
Solid phase extraction
Solid phase extraction (SPE) [66] is a sample preparation technique implying a trace
enrichment procedure and solvent transfer step, in order to obtain samples with
detectable amounts of components in a suitable solvent.
The SPE column contains a sorbent bed material that is similar to the stationary phases
used high performance liquid chromatography. The procedure involves four steps. First
the SPE sorbent bed is conditioned, where an organic solvent is used to increase the
interaction surface area and displace contaminants; excess solvent is then removed by
rinsing with a liquid similar to the sample solvent. The second step is the sorption of the
mixture components onto the sorbent bed. The third step is the removal of undesired
sample matrix with a weak solvent. In the last step, the components are desorbed into
a small volume by means of a solvent with sufficient strength. Adsorption and
desorption are performed by vacuum suction.
Aldehyde screening test
The production of volatile aldehydes during the oxidation of phenol has been verified by
a gas sampling aldehyde screening test according to NIOSH [72]. This test is suitable to
identify C1-C7 aliphatic saturated aldehydes (e.g. formaldehyde, acetaldehyde) but also
unsaturated aldehydes like acrolein and crotonaldehyde. The test involves the collection
of gas phase components in a sampling tube, filled with the derivatization agent 2(hydroxymethyl)-piperidine (HMP) on a carrier phase. HMP specifically reacts with
aldehydes to an oxazolidine compound, see Figure 2.18.
28
Chapter 2.
+
N
H
R
O
N
H
OH
O
+
H2 O
R
HMP
aldehyde
Figure 2.18
oxazolidine
The chemical derivatization of an aldehyde with HMP.
The oxazolidine can be identified by gas chromatography-mass spectrometry. The
molecular ion of a specific aldehyde equals the molecular weight of the original
aldehyde plus 97. For example formaldehyde (HCHO) is recognizable by a base peak at
m/z=97 and other characteristic ions have m/z values 126 and 127 (molecular ion
C7H13NO).
Electron spin resonance
Electron spin resonance (ESR) has been applied in-situ, to identify oxidizer radicals
produced by the corona discharges and radical intermediate phenol oxidation products.
Of interest are inorganic oxidizer radicals e.g. the hydroxyl radical (HO•) / oxygen anion
(O-•) and hydroperoxyl radical (HO2•) / superoxide anion (O2-•); organic radical
intermediates that are produced by oxidation of phenol are e.g. the
dihydroxycyclohexadienyl[peroxyl] radicals C6H5(OH)2[OO]•. Next to the mentioned
radicals, hydrogen atoms may be detectable.
A spin trap has been applied to trap the short living radicals and form a stable
measurable adduct. Literature references originate from in-situ glow discharge
electrolysis [73], in-situ radiolysis [74] and ex-situ sonolysis [22] of aqueous spin trap
solutions. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap has been used. The
formation of the DMPO-OH adduct from DMPO and the hydroxyl radical is shown by
Figure 2.19. The DMPO-OH adduct exhibits a half-life of about 2 hours [75]. The
DMPO-OOH adduct, however, is reported to be highly unstable [76].
N
O
H
+
DMPO
Figure 2.19
HO
N
O
H
OH
DMPO-OH
The trapping of a hydroxyl radical by the spin trap DMPO.
The ESR spectrum of the DMPO-OH adduct consists of a 1:2:2:1 quartet, due to the
equivalence of the 14N and 1H hyperfine coupling constants viz. aN=aH=1.49 mT.
DMPO-H has a nine-line spectrum due to coupling with one 14N nucleus (aN=1.66 mT)
and 2 identical protons (aH=2.25 mT). However, no hyperfine coupling constants are
known for the adduct of DMPO and the organic DHCHD(P) radicals.
Theory
29
Microtox ecotoxicity test
In order to investigate the detoxification progress during the degradation of phenol,
Microtox ecotoxicity tests have been performed [77]. In the Microtox test, the
ecotoxicological effect of a chemical substance or a mixture is determined from the
bioluminescence behaviour of the marine bacterium Vibrio fischeri before and after
exposure to the medium. The bioluminescence output decreases with increasing toxicity
of the chemical compound. The ecotoxicity is expressed as an effect concentration (EC)
at a particular effect level, in this thesis as a 20 % bioluminescence decrease (E20). Low
EC values imply high ecotoxicity. The EC value is determined by interpolation of the
dose-effect relationship, see Equation 2.8. The effect is expressed by the gamma value
(Γt), that is related to the inhibitory effect (Ht) according to Equation 2.9. The inhibitory
effect is calculated from the bioluminescence intensity after the exposure (ITt) and a
corrected background intensity (Ict), according to Equation 2.10.
log(ct ) = b ⋅ log(Γt ) + log(a)
Γt =
Ht
100 − Ht
Ht =
(2.8)
(2.9)
Ict − ITt
⋅100
Ict
(2.10)
Evidence for acute ecotoxicity is obtained, by measuring the effect level at different
exposure times. Table 2.1 shows EC505min values for phenol and some of its oxidation
products [78]. Although 50% effect levels are more commonly reported in literature,
the measurement of 20% effect levels implies higher sensitivity of the Microtox test. It
should be noted, that a comparison of effect concentrations determined at different
levels i.e. 20% or 50% and different exposure times like 5 min, 15 min or 30 minutes is
not possible. The deviation of the reported EC50 values is probably caused by the origin
of the applied bacteria cultures viz. the supplier and supplied state (freeze-dried, liquiddried or fresh).
Table 2.1
EC505min effect concentrations of phenol and some of its oxidation
products. The phenol EC505min value is an average value determined from a
set of 22 literature references [78].
Compound
Phenol
Catechol
Resorcinol
Hydroquinone
1,4-Benzoquinone
Formaldehyde
Glyoxal
Glyoxylic acid
Oxalic acid
Formic acid
Acetic acid
EC505min (mg/l)
28.6 ±7.9
32.0
310; 375
0.042; 0.079
0.0085; 0.020; 0.08; 1.4
3.0; 8.7; 9.0; 10; 10.1; 904.17
754 ±55.1
11.2 ±0.16
11.3 ±0.22
7.91 ±0.22
9.24 ±0.38
30
Chapter 2.
Total organic carbon
A different way to measure oxidation progress of an organic compound is the
determination of the carbon content of the oxidation product mixture. The total carbon
content TC is defined as the sum of the total organic carbon TOC (hydrocarbons) and
total inorganic carbon TIC (carbonate, bicarbonate, carbon dioxide) [79].
Due to oxidation, the carbon skeleton of an organic compound is gradually chopped into
shorter carbon chain molecules containing oxygen-based functional groups viz. aliphatic
aldehydes and carboxylic acids. The last member in the oxidation sequence is formic
acid, which upon oxidation yields the unstable carbonic acid. The TOC level of the
oxidation product mixture decreases by release of carbon dioxide (mineralization) and
volatile or gaseous intermediates.
The measurement of total organic carbon does not reveal information about the
chemical or toxicological properties of the sample. However, if the total carbon content
equals the total inorganic carbon content, the organic carbon has been completely
mineralized and the remaining toxicity is only due to inorganic ions originating from
covalently bonded elements.
Acidity
The acidity of the oxidation product mixture is an indication for oxidation progress,
because the oxidation of an organic compound yields carboxylic acids. Unfortunately, it
is impossible to deconvolute the overall acidity into concentrations of all carboxylic
acids present in the oxidation product mixture. The produced carboxylic acids are
generally weak acids, compared to mineral acids like hydrochloric acid and nitric acid.
The strongest acid, produced by oxidation of phenol is oxalic acid. The acidic strength
is expressed by the acid equilibrium constant Ka, according to Equation 2.11. The acid
dissociation constant pKa=-10log(Ka) of phenol and some of its acidic oxidation products
is shown by Table 2.2 [80].
Ka =
[H3O + ] ⋅ [ A− ]
[HA]
Table 2.2
HA + H2O H3O + + A−
2.11
Acid dissociation constants of phenol and some of its acidic oxidation
products; dibasic acids (H2A) show a two-step dissociation.
Compound
Oxalic acid
Maleic acid
Glyoxylic acid
Formic acid
Phenol
pKa,I
1.23
1.83
3.18
3.75
9.89
pKa,II
4.19
6.07
-
Theory
31
2.5.2. Electrical diagnostics
Corona pulse energy & conversion efficiency
The energy of a single corona discharge pulse (Ep) is determined from the pulse voltage
V(t) and corona current Icor(t). The corona discharge current Icor(t) is calculated from the
difference of total current I(t) and capacitive current Icap(t), see Equation 2.12. The
capacitive current is determined by measuring the capacitance of the reactor geometry
Cg at voltages below the corona onset, where I(t)=Icap(t), see Equation 2.13. The
corona pulse energy Ep is then calculated by integration of pulse voltage times corona
current over the pulse time, see Equation 2.14 [81].
Icor (t) = I(t) − Icap (t)
(2.12)
dV (t)
∧
dt
E p = ∫ V (t) ⋅ Icor (t) dt
Icap (t) = Cg ⋅
V < Vonset
(2.13)
(2.14)
pulse
The conversion efficiency is defined according to the G yield value. G expresses the
number of target compound molecules converted with regard to the required energy
input, illustrated by Equation 2.15. X is the conversion of the target compound, C0 is
the initial target compound concentration (mol/l), Vol is the solution volume (l), Ep is the
pulse energy (J), f is the pulse repetition rate (Hz) and t is the oxidation time (s). In
literature G units are expressed as mol/J, “molecules per 100 eV” or g/kWh. Table 2.3
shows the interconversion of the efficiency units.
G=
X ⋅ C0 ⋅ Vol
Ep ⋅ f ⋅ t
Table 2.3
(2.15)
The interconversion of G yield efficiency units.
mol/J
(100eV)-1
g/kWh
mol/J=
1
1
100N Ae
1
3.6 ⋅ 106 FW
(100eV)-1=
100NAe
1
100N Ae
3.6 ⋅ 106 FW
3.6 ⋅ 106 FW
100N Ae
1
g/kWh =
NA
e
FW
6
3.6⋅10 FW
= Avogadro’s constant: 6.022⋅1023 mol-1
= 1.6022 ⋅10-19 J/eV
= target compound molecular weight (g/mol)
32
Chapter 2.
Electrical conductometry
Electrical conductometry has been applied to measure the conductivity of the oxidation
product mixture in order to monitor oxidation progress. The oxidation of an organic
compound yields carboxylic acids, which are partially dissociated in aqueous solution
depending on their acid strength and therefore contribute to the electrical conductivity.
The conductance G (unit Siemens) is the reciprocal value of the electrical resistance R
according to Ohm’s law, see Equation 2.16. ρ Equals the resistivity, A and l are the
conductor’s area and length.
G=
1
=
R
1
l
(ρ ⋅ )
A
=
1 A
1
⋅ =σ ⋅
ρ l
K
(2.16)
The conductivity σ equals the product of the conductance G and the cell constant K.
With regard to the phenol oxidation product mixture, conductivity is nearly exclusively
due to protons. The carboxylic acid anions and other anions like chloride and nitrate
contribute to a limited extent to conductivity, as is illustrated by the molar conductivity
at infinite dilution λ0=(σ/c)c→0, see Table 2.4 [82].
Table 2.4 Molar conductivity at infinite dilution for some ions observed during
oxidation of an organic compound.
Ion
Proton
Chloride
Oxalate
Nitrate
Formate
Acetate
H-Oxalate
*)
Formula
H+
Cl(½)C2O42- *)
NO3HCOOCH3COOHC2O4-
λ0 (m2Smol-1)⋅104
349.7
76.3
74.1
71.4
54.6
40.9
40.2
Double charge
2.5.3. Optical diagnostics
UV absorbance spectrometry
Quantitative ozone measurements have been performed by UV absorbance
spectrometry. By absorption of energy, a molecule is transferred from its ground state
to an excited state. The relaxation of the molecule to the ground state may involve
radiationless transfer, fluorescence or phosphorescence, depending on the electronic
structure of the molecule. According to Lambert-Beer’s law, the intensity of the
absorbed radiation (I) is related to the number density of the absorbing molecule (N), the
absorption cross section (σ(λ)) and the optical path length (x) see Equation 2.17. This
relation is valid for monochromatic light and diluted solutions viz. concentration ≤10-2
mol/l [83]. The cross section for ozone at λ=260 nm is σ260=1.14⋅10-21 m2 [9,84].
I = I0 exp(− Nσ x)
(2.17)
Theory
33
Laser-induced fluorescence spectroscopy
Laser-induced fluorescence (LIF) spectroscopy has been applied for in-situ conversion
measurements of phenol in aqueous solution.
The fluorescent properties of a molecule are determined by both electronic and
structural requirements [85]. Electronic requirements are that the molecule absorbs
energy frequencies lower than the strength of the weakest bond, the molecule’s first
excited singlet state S1 has a life-time of about 10-8 s and the first excited singlet state
(S1) and lowest triplet state (T1) are well separated. In all media, except for lowpressure gasses, organic molecules emit fluorescence from the lowest vibrational level
of the first excited singlet state to the singlet ground state (S0), see Equation 2.18.
S1 → S0 + hνfl
(2.18)
Structural requirements are that the molecule contains a planar rigid conjugated system
of double bonds, preferentially in a cyclic structure and especially in linear polycyclic
molecules where the π electrons can readily circulate inside the molecule; groups
substituted to the conjugated system shall be electron donating like e.g. hydroxyl and
(dimethyl)amino groups and polysubstitution shall not influence the π electron mobility.
The application of LIF spectroscopy for monitoring the phenol conversion during
oxidation in aqueous solution is not straightforward, because of following reasons. The
fluorescence radiation from phenol excited states is quenched by other phenol
molecules, oxidation products and water. The excitation laser beam is absorbed by
phenol and the oxidation products in aqueous solution. Some oxidation products also
show fluorescence by laser excitation. The different processes that occur after
excitation of a phenol molecule by the laser beam are described by Figure 2.20. Four
energy levels are described viz. the ground state (0), the initial phenol excited state (2),
a lower excited phenol state (1) and the electronic state of a quenching molecule Q (3).
2
n2/τ
1
B02 n0 ρ
kQnQn2
3
kQnQn1
A20n2
A10n1
0
Phenol
Figure 2.20
Q
A Jablonski diagram for different energy states of phenol and a
quenching compound Q.
34
Chapter 2.
ni equals the population density of state i. A20 and A10 are the Einstein coefficients for
spontaneous emission from state 2 to 0 and state 1 to 0 respectively. kQ is the
quenching coefficient. τ is the characteristic time constant for decay from state 2 to
state 1. B02 is the Einstein coefficient for induced absorption for the transition from
state 0 to 2. ρ equals the laser photon density, which can be expressed according to
Lambert-Beer’s law by Equation 2.19. ρ0 equals the laser intensity before entering the
aqueous solution, σ02 is the absorption cross section of phenol in aqueous solution and
n0 is the population density of the phenol ground state 0.
ρ = ρ 0 exp(−σ 02Ln0 )
(2.19)
The population density of excited state 1 grows by decay of excited state 2 to the
excited state 1 according to n2/τ. The population density of excited state 1 decreases
by decay to the groundstate 0 by A10n1 and by quenching due to molecule Q according
to an amount kQnQn1, see Equation 2.20.
The population density of excited state 2 grows by laser excitation by an amount
B02n0ρ, The population density of the excited state 2 decreases by decay to the excited
state 1 by n2/τ, by decay to the ground state 0 by A20n2 and by quenching due to
molecule Q according to an amount kQnQn2, see Equation 2.21.
dn1 n2
=
− A10 n1 − kQ nQ n1
dt τ
(2.20)
dn2
n
= B02 n0 ρ − 2 − A20 n2 − kQ nQ n2
τ
dt
(2.21)
The assumption of steady-state conditions is valid for collision-dominated conditions,
which is justified for the liquid phase. Then, the derivates of population density with
respect to time are equal to zero and the fluorescence intensity can be expressed by
Equation 2.22.
ILIF = ηA10 n1 = η
B 02 ρ 0 n0 exp (− σ 02 Ln0 )
(kQ nQ + A10 )(kQ nQτ + 1 + A20τ )
(2.22)
η is a proportionality constant. From Equation 2.22 it can be derived that the
fluorescence intensity versus the population density n0 reaches a maximum value at
n0=(σ0L)-1. At higher phenol concentrations absorption and quenching overrule the
fluorescence.
Phenol complies with the requirements for fluorescence. When excited at a wavelength
of about λex=270 nm in aqueous solution, phenol emits a maximum fluorescence near
300 nm<λfl<310 nm [85-90]. The excitation wavelength of the dihydroxybenzenes is
in same range i.e. 265 nm<λex<285 nm and these compounds show a somewhat
weaker fluorescence at the wavelengths 315 nm<λfl<340 nm.
Theory
35
The trihydroxybenzenes are weakly to non-fluorescent. Phloroglucinol shows the
weakest fluorescence, possibly because it also reacts in a tautomeric keto-form [61]
which is non-aromatic, see Figure 2.21.
O
OH
O
OH
HO
phloroglucinol
O
1,3,5-cyclohexanetrione
Keto-enol tautomerism of phloroglucinol (1,3,5-trihydroxybenzene).
Figure 2.21
1,4-Benzoquinone is not a hydroxybenzene but forms a redox couple with
hydroquinone; the compound is non-aromatic viz. a cyclic diene and also shows weak
fluorescent properties. Saturated hydrocarbons do not comply with the requirements for
fluorescence and therefore major literature references on fluorescence involve solely
aromatic compounds.
Fluorescent molecular probe
A fluorescent molecular probe has been applied to identify the hydroxyl radical in
corona-exposed aqueous solution. This molecule is a non-fluorescent compound, that
specifically reacts with hydroxyl radicals in a very sensitive way to produce a strongly
fluorescent molecule. In this thesis the molecular probe coumarin-3-carboxylic acid,
abbreviated as CCA, has been utilized [91,92]. CCA reacts with the hydroxyl radical to
7-hydroxycoumarin-3-carboxylic acid, abbreviated as 7-OHCCA, see Figure 2.22.
O
O
HO
O
O
OH
OH
O
CCA
Figure 2.22
O
7-OHCCA
The non-fluorescent CCA and the strongly fluorescent product of OH and
CCA: 7-OHCCA.
The excitation wavelength of 7-OHCCA is λex=396 nm. The fluorescence maximum is
near λfl=450 nm. Using CCA, time resolved measurement of the hydroxyl radical
production rate kinetics is possible.
However, a problem is the very limited solubility of CCA in water. The choice for a
better solvent or solvent addition are no option: the hydroxyl radical will react with the
solvent molecule and quantitative measurements of the hydroxyl radical concentration
are not accurate anymore.
36
Chapter 2.
Infrared spectroscopy
For identification of gaseous phenol oxidation products, infrared spectroscopy has been
applied [93-95]. The gross selection rule for IR spectroscopic activity is that a
molecule’s dipole moment shall change during the normal mode of vibration. The
change of the dipole moment establishes an electrical field that can interact with the
electrical vector of the radiation. Normal modes of vibration are mutually-independent
synchronous motions of atom groups. Some typical molecular vibrations are stretch
vibrations (symmetric and anti-symmetric) and deformations (scissoring, rocking,
wagging, twisting). Linear molecules consisting of N atoms vibrate in 3N-5 different
ways while non-linear molecules show 3N-6 different vibrations.
The wavelength range of interest to organic compounds is 4000-600 cm-1
corresponding to 2.5-17 µm. This region can be divided into two parts viz. the
absorption range due to characteristic functional groups (4000-1200 cm-1) and the
finger-print area, where absorptions provide information about the overall constitution
of the molecule (1200-600 cm-1).
With regard to the oxidation of phenol, the following compound classes are of major
interest: carbon oxides, aldehydes and carboxylic acids. Indirectly involved are ozone
and nitrogen oxides produced by corona in air.
Carbon dioxide shows two stretch vibrations and two perpendicular bending motions.
The stretch vibrations are located at 2349 cm-1 (anti-symmetric) and 1333 cm-1
(symmetric). The bending modes occur at 667 cm-1.
Carbon monoxide shows one stretch vibration at 2143 cm-1.
Aldehydes, ketones and carboxylic acids are recognizable by carbonyl stretch vibrations
in the range 1760-1690 cm-1.
Carboxylic acids show a broad absorption at 3000-2500 cm-1 due to O-H stretch
vibrations. At 1300-1080 cm-1 C-O stretch vibrations occur.
Alkanols show O-H stretch vibrations at 3640-3610 cm-1 and vibrations of hydrogen
bonds at 3600-3200 cm-1. C-O stretch vibrations occur at 1300-1080 cm-1.
Nitrogen dioxide has two stretch vibrations viz. at 1618 cm-1 (anti-symmetric) and at
1318 cm-1 (symmetric) and a bending mode at 750 cm-1.
Nitrous oxide has two stretch vibrations viz. an XY stretch at 2224 cm-1 and YZ stretch
at 1285 cm-1 and two bending modes at 589 cm-1.
Ozone has two stretch vibrations viz. at 1103 cm-1 (symmetric) and 1042 cm-1 (antisymmetric) and a bending mode at 701 cm-1.
Finally water vapour, produced by application of pulsed corona discharges over the
aqueous solution, causes absorptions due to stretch vibrations at 3657 cm-1
(symmetric) and 3756 cm-1 (anti-symmetric) and a bending mode at 1595 cm-1.
3. Experimental setup
A description of the applied reagents, experimental configurations and chemical,
electrical and optical analysis techniques is presented.
3.1. Reagents & reactors
Table 3.1 shows a list of applied model compounds and oxidation products,
chromatographic eluents and other reagents.
Table 3.1
Applied reagents: name, Chemical Abstract Service registry number,
molecular weight, density of liquids, purity and origin.
Compound
Acetic acid
Acetonitrile
Acrylic acid
Ammonia
Argon
Atrazine
1,4-Benzoquinone
Catechol
CCA
Dimethyl sulfide
DMPO
Formic acid
Glyoxal
Glyoxylic acid
Helium
Hydrogen peroxide
Hydroquinone
Hydroxyhydroquinone
Maleic acid
Malonic acid
Methanol
Nitrogen
Oxalic acid
Phenol
Phloroglucinol
Propionic acid
Pyrogallol
Resorcinol
Succinic acid
Toluene
Trifluoroacetic acid
1)
sg=HPLC supra gradient
CAS no.
64-19-7
75-05-8
79-10-7
7664-41-7
7440-37-1
1912-24-9
106-51-4
120-80-9
531-81-7
75-18-3
3317-61-1
64-18-6
107-22-2
298-12-4
7440-59-7
7722-84-1
123-31-9
533-73-3
110-16-7
141-82-2
67-56-1
7727-37-9
144-62-7
108-95-2
6099-90-7
79-09-4
87-66-1
108-46-3
110-15-6
108-88-3
76-05-1
FW (g/mol)
60.05
41.05
72.06
17.03
39.95
215.69
108.10
110.11
190.15
62.13
113.16
46.03
58.04
74.04
4.00
34.01
110.11
126.11
116.07
104.06
32.04
28.01
90.04
94.11
126.11
74.08
126.11
110.11
118.09
92.14
114.02
ρ (g/cm3)
1.05
0.782
1.051
0.91
0.846
1.015
1.22
1.265
1.342
1.11
0.79
0.992
0.865
1.490
Grade(w%)1)
100
sg
99
25
99.999
99.9
98
99+
99
97
98-100
40
50
99.995
30
99+
99
99
99
sg
99.9
99+
99+
>99
>99
99
99+
99+
99.7
>98
Manufacturer
Merck
Biosolve
Aldrich
Merck
Messer
Riedel-de Haën
Aldrich
Aldrich
Aldrich
PolyScience
Aldrich
Merck
Aldrich
Aldrich
Hoekloos
Merck
Aldrich
Aldrich
Aldrich
Aldrich
Biosolve
Hoekloos
Aldrich
Aldrich
Merck
Fluka
Aldrich
Aldrich
Aldrich
Merck
Merck
38
Chapter 3.
Malachite green has been obtained as a gift from the faculty of Chemistry/department
of Instrumental Analysis/TUE. No accurate information about the malachite green anion
is known, therefore data have not been further specified. It may regard e.g. malachite
green carbinol hydrochloride CAS no. [123333-61-9] FW=382.94 g/mol or malachite
green oxalate CAS no. [2437-29-8] FW=927.03 g/mol.
General remarks
All corona experiments have been performed at ambient conditions. The compound
solutions have been prepared from deionized water (Millipore, resistivity 10 MΩcm),
except for a few exploratory experiments for which tap water has been applied (as
indicated). The reactor contents is continuously homogenized by a magnetic stirring bar.
The sample volume is 1 ml or less. Due to the different analytical approaches, several
reactor types have been applied. Table 3.2 shows the used reactor types and
dimensions.
Table 3.2
R.
no.
Applied reactor+anode types, listed by reactor configuration number
R. no.; VL=liquid phase volume, Vtot=total reactor contents, ∅=internal
diameter, L=length, h=height.
Experiment
1a DMPO ESR
1b DMPO oxidation
2a CCA probe
2b CCA oxidation
3a Ozone
3b Phenol /EI-MS
4
5a
5b
5c
6a
6b
6c
7
Calorimetry
Conducto-water
Conducto-HB
Microtox test
Phenol
Atrazine
Phenol/LC-MS
Aldehyde
screening
8a LIF
8b
9 FTIR
10 TOC
11 Malachite green
12 Dimethyl sulfide
configuration
Anode1)
VL (ml)
Cyl+cir2)
Beaker
Cube
Vessel
Vessel
Cube
Reactor
Dimensions
(mm)
∅=45, h=305
∅=85, h=122
70x80x100
∅≤95, h~200
∅≤95, h~200
57x62x100
Fe
Fe
Fe
Fe
Fe
Fe
500
249
242
500
500
100
640
500
560
1000
1000
353
Cube3)
Beaker
75.5x80x99
∅=85, h=122
variable
250
598
500
Vessel
∅≤95, h~200
500
1000
Cylinder
∅=45, h=305
1p W
1pFe/W/Pt
1p W
30p Fe
31p Fe
31p Fe
30p Fe
30p Fe
500
570
Vessel
Cube
Vessel
Beaker
Bar
Cylinder
∅=90, h~100
57x62x100
∅=70, h=121
∅=100, h=142
30x80x100
∅=37, L=310
30p Fe
30p Fe
30p Fe
30p Fe
various
wire
300
100
250
498
125
gas phase
600
353
500
1000
240
330
Type
30p
30p
30p
30p
30p
30p
Vtot (ml)
Experimental setup
1)
39
Anode
•30,31p Fe
The generally applied anode consists of an aluminum plate: diameter 30 mm, thickness
1.5 mm. 30 or 31 steel pins are separated at 5 mm mutual distance; the global pin
dimensions are: length=14-15 mm, thickness=0.6 mm, 60 µm tip.
•various
Different electrode configurations have been tested: single pin/multipin/wire anode in
the liquid phase or gas phase at several distances from the liquid/gas interface, the
cathode is situated inside/outside the reactor. Details are given in the appropriate
section.
•wire
The applied reactor is a cylindrical gas phase reactor, equipped with a 128 mm
effective length wire anode situated in the centre. A cathode wire mesh surrounds the
glass tube on the outside.
•1p Fe/W/Pt
The steel tip equals the generally used anode tips. The tungsten tip has been obtained
from a welding rod by grinding and polishing: the material is an alloy of tungsten and a
small amount of thorium; global dimensions are: length=15 mm, thickness=1 mm, 30
µm tip. The platinum tip has been obtained from platinum wire; global dimensions:
length=15 mm, thickness=0.5 mm, 70-100 µm tip.
In all configurations except for reactor configuration 11 and some setups of reactor
configuration 4, the cathode is situated directly outside and underneath the reactor
glass vessel bottom and therefore is dielectrically separated from the anode.
2)
In-situ ESR
The reactor volume is about 570 ml; teflon tubing (L~5 m, ∅=4 mm) and viton pump
tubing (L~20 cm, ∅~5 mm) have been applied, the circuit volume is about 70 ml. Due
to the pulsating pump flow, the anode-to-liquid distance d is somewhat fluctuating.
3)
Calorimetry
The reactor heat capacity Creactor has been determined from the construction materials
(glass and quartz) and the contents (water) [96].
Quartz: (2.3x80x99+2.3x75.5x99)mm3 ≡ 34.6 cm3; cpquartz=0.17 Jg-1K-1; Cquartz=13.0
J/K. Glass: (3.0x80x99+ 3.0x75.5x99 +3.0x85.6x75.6)mm3 ≡ 65.6 cm3; cpglass=0.48
Jg-1K-1; Cglass=78.1 J/K. Water: cpwater=4.184Jg-1K-1; CH2O=4.18⋅volume; therefore
Creactor=(91.1+4.18⋅volume) J/K. The heat loss due to imperfect insulation is small,
about 0.2 Kelvin per 15 minutes.
40
Chapter 3.
3.2. Chemical diagnostics
Liquid chromatography
The applied column types, liquid chromatography systems and conditions are
summarized by Table 3.3.
Table 3.3
LC
no.
Applied LC setups, listed by the LC configuration number; Col=column
no., Sys=system no., F=eluent flow, T=column temperature,
con=conductivity, ACN=acetonitrile, TFA=trifluoroacetic acid.
Experiment1)
1 DMPO 1.0 mM
2 CCA 1.0 mM
3 Water
1)
2)
3)
4)
Col Sys
Eluent (v/v%)
2b
1
1
3 H2O/ACN=90/10
3 TFA 1 mM
4 TFA 1 mM
4 Phenol 0.05 mM
5a Phenol 1.0 mM
5b
6 Phenol 1.0 mM air/Ar
3 1b H2O/ACN=80/20
2a 2 H2O/ACN grad3)
12) 1a TFA 1 mM
1 4 TFA 1 mM
7 TOC 1.0 mM phenol
8 CC 1.0 mM HB
9 MT phenol ≤0.4 mM
10 Atrazine 0.12 mM
1 3
1 3
2b 3
3 1b
TFA 1 mM
TFA 1 mM
H2O/ACN=70/30
H2O/ACN=55/45
Settings
F
Detector λ (nm) T (°C)
(ml/min)
1.0 230
20
0.8 210,270
50
0.8 200,210,220,
35
255,270,con
1.0 254, 2804)
20
1.0 210
20
0.8 210
35
0.8 200,210,220,
35
255,270,con
0.8 270
35
0.8 225, 270, 278
35
1.0 270
35
1.0 216
20
TOC=total organic carbon, CC=conductometry & conversion, HB=hydroxybenzenes,
MT=Microtox
Including OA-HY guard column L=20 mm, ∅=3 mm
Grad= gradient: 4 min 0→5%, 3 min 5→20 min, 8 min 20→50%, 1 min 50→75% ACN
0-5 min: 254 nm, 5-10 min: 280 nm
Columns:
1 Merck Polyspher OA-HY (ICE)
Dimensions: column L=300 mm, ∅=6.5 mm;
Stationary phase: Polystyrene-divinylbenzene with -SO3-H+ functional groups,
dp=8 µm, crosslink ratio 8%; Column no. 150095
2 Zorbax Rx-C18 (rp-HPLC)
Dimensions L=150 mm, ∅=4.6 mm, dp=5µm,
Stationary phase: octadecyl grafted silica
a Column no. PN883967.902 - DU5023
b Column no. PN883967.902 - DU4041
3 Zorbax SB-C18 (rp-HPLC)
Dimensions L= 150 mm, ∅= 4.6 mm, dp=5 µm
Stationary phase: octadecyl-grafted silica; Column no. -
Experimental setup
41
LC-systems:
The injection volume is 20 µl for all analyses.
1a. Philips PU4100 Liquid Chromatograph and Philips PU4110 UV/VIS detector
1b. System 1 including autosampler: Marathon ser.no. 0106, Spark Holland
2. Merck Hitachi L-6200A Liquid Chromatograph and Merck Hitachi L-4250
UV/VIS detector
3.
Hewlett-Packard HP1100 series Liquid Chromatograph, HP-G1315A diode array
detector, G1311A quaternary pump, G1322A solvent degasser, G1313A
autosampler, G1316A column compartment.
4. System 3 in series with a micromembrane suppressor (Dionex AMMS-ICE II) and
conductivity detector (Dionex CDM-2). The conductivity detector has been set to
output range=300 µS and 1 Volt full scale; software: 106 units per Volt. The
suppressor is supplied with a 5 mM aqueous ammonia solution at a flow rate
F=2.0 ml/min.
Mass spectrometry
Configuration 1: Perkin Elmer Sciex API 300
The oxidation product sample is introduced into the IonSpray compartment by means of
a liquid chromatograph (Shimadzu LC-10AT); the acetonitrile flow is F=1.0 ml/min; the
flow split ratio is 1:10 because the maximum allowable IonSpray flow is 0.2 ml/min.
The settings are: NC=-3.5 kV, step=0.3 amu, dwell time=3.0 ms, pause=2.0 ms.
The scanned mass-range=40-300 amu. 0.1 v/v% ammonia has been added to the
oxidation product mixture to promote the ionizability.
Configuration 2: Balzers PPM 421
By means of a PEEK capillary (L=3 m, ∅=0.17 mm), the oxidized solution is
introduced into the prevacuum compartment where p<10-2 mbar; this compartment is
separated from the mass spectrometer by a 100 µm orifice. The pressure at the
detector is p<3⋅10-5 mbar. Ionization is performed by an electron beam of 100 eV. The
ions are detected in a count mode. The scanned mass-range is 1-500 amu.
Solid phase extraction
The following SPE column types have been used: 500 mg C18 and 500 mg C6H5 (Baker).
50 ml of the oxidized phenol solution has been extracted. Desorption has been
performed using 0.5 ml methanol.
42
Chapter 3.
Gas chromatography
Configuration 1: Aldehyde screening
The toluene extract from the sorbent tubes has been analyzed by gas chromatographymass spectrometry (GC-MS), using following configurations:
Instrument:
Column:
Detector:
Temperature:
Shimadzu QP5000
CP-Sil5; L=25 m, dc=220 µm
Quadrupole
T0=50°C, dT/dt=10°C/min, T1=275°C, F=2 ml/min
Instrument:
Column:
Detector:
Temperature:
Hewlett-Packard 6890+Leco Pegasus IIA
CP-Sil5; L=8 m, dc=50 µm
Time of flight
T0=60°C, dT/dt=20°C/min, T1=275°C, F=4 ml/min
Configuration 2: Conversion of dimethyl sulfide
Instrument:
Column:
Detector:
Temperature:
Injection:
Hewlett-Packard 5890
HP Ultra-1; WCOT MeSil.; L=20m, dc=320 µm, df=0.52 µm,
He 100 bar
Sulfur Chemiluminescence, T=800°C, pcell=234 mBar
Isotherm T=40°C
PTV: cold introduction, splitless mode, injection volume=100 µl
T0=40°C, dT/dt=12.5°C/s, T1=320 °C, flow 250 ml/min
10 ppm DMS standards have been prepared in methanol, because DMS is insoluble in
water. The retention time of DMS for the specified conditions is tR= 0.492 ± 0.009
minutes.
Aldehyde screening test
The presence of volatile aldehydes during the oxidation of phenol has been investigated
using aldehyde-specific gas sampling tubes (Supelco ORBO-23, no. 2-0257-U). The
solid sorbent tubes contain 10% 2-(hydroxymethyl)piperidine (HMP) on Supelpak 20N
(20/40) carrier divided over two sections: the main section contains 120 mg and the
backup section contains 60 mg. The required gas sampling volume is 5 liter. By means
of a flow controller (Brooks 5850) and controller unit (Gossen 5875) the reactor has
been purged with an argon 5.0 flow at a rate F=200 ml/min, see reactor configuration
7. Figure 3.1 shows the applied setup. After sampling, HMP and possible oxazolidines
are extracted from the sorbent tube contents using 1 ml toluene by ultrasonic agitation
during 60 minutes. The extract is analyzed by GC-MS.
Experimental setup
43
Ar
ST
FC
R
Figure 3.1
Setup for sampling of volatile aldehydes from an oxidized phenol solution;
ST=sampling tube; Ar=argon 5.0 flow, FC= flow controller, R=corona
reactor.
Electron spin resonance
The identification of oxidizer radicals and phenol oxidation intermediate radicals has
been studied by in-situ Electron Spin Resonance (ESR). The applied reactor is
constructed from a glass tube (QVF) and two teflon covers with o-rings, see reactor
configuration 1a. By means of chemically-inert tubing and a peristaltic pump, a
continuous flow system has been created between the corona reactor and an ESR
quartz flat cell (flow area: 1x10 mm), see Figure 3.2. The flow rate is about 8 cm3/s
(N~2 s-1). The applied spin trap is 5,5-dimethyl-1-pyrroline N-oxide. Measurements
have been performed using a Bruker ESP300 ESR spectrometer. The settings are
microwave frequency: 9.75 GHz; modulation frequency: 100 kHz, modulation
amplitude: 5 G. The centre field has been set to 3355 G/width 100 G and to 3450
G/width 500 G. The temperature is T=295 K.
P
R
Figure 3.2
ESR
In-situ ESR setup for the detection of radicals; R=reactor, P=peristaltic
pump.
44
Chapter 3.
Microtox test
For determination of the reference bioluminescence intensity, two Vibrio fischeri
solutions have been prepared using freeze-dried bacteria: a commercial Microtox seawater diluent and the Millipore water used to prepare the standard phenol solutions with
NaCl added (20g/l). Before the Microtox test is performed, the degree of acidity of
every solution has been adjusted to 5.0<pH<5.5 using a diluted NaOH solution. Also
the oxygen content of the solutions has been verified.
A series of 250 ml phenol solutions at concentrations 0.02 mM, 0.05 mM, 0.1 mM,
0.2 mM and 0.4 mM have been exposed to pulsed corona discharges during 30
minutes, see reactor configuration 5c. To investigate the possible ecotoxicity increase,
caused by corona treatment of Millipore water i.e. the formation of hydrogen peroxide
and nitric acid, the 0 mM sample has been included in the test. The 0.4 mM
concentration is the upper limit for observation of a 20% effect. All samples have been
stored away under argon to bridge the time before analysis.
The bioluminescence intensity of untreated and oxidized phenol solutions has been
determined in duplicate, with regard to the reference bioluminescence intensity. The
EC20 effect value has been reported at the Vibrio fischeri exposure times tVF=5 min,
t=15 min and t=30 minutes.
Total Organic Carbon
The carbon content of phenol solutions has been determined versus the oxidation time
(reactor configuration 10) using a Dohrmann TC190 analyzer. Prior to analysis the
samples have been diluted by a factor 4 with deionized water. The total organic carbon
(TOC) content is determined from the difference of total carbon content (TC) and total
inorganic carbon content (TIC). The TC content is calculated from the amount of carbon
dioxide that is released from catalytic combustion of the sample. The carbon dioxide
concentration is measured by infrared spectroscopy. The TIC content is determined
from the carbon dioxide release that occurs after acidification of the sample with
phosphoric acid.
Acidity
A pH measuring device (Metrohm 691) has been used to measure the solution acidity of
untreated and oxidized phenol solutions. Before the measurements, the instrument is
calibrated using calibration buffer solutions (Merck).
Experimental setup
45
3.3. Electrical diagnostics
Electrical circuit
The electrical circuit for generation of pulsed corona discharges is shown by Figure 3.3.
By means of a high voltage power supply of negative polarity (Wallis), a capacitor
(Thomson CSF-LCC) is charged through a 10 MΩ load resistor (Metallux). By triggering
the spark gap, the energy stored in the capacitor is discharged into a glass reactor
vessel, by means of a capacitive electrode configuration. In this way, conductive
currents are prevented. Positive corona is produced at the anode, situated at a distance
d over the aqueous solution. The anode is single-pin or multipin consisting of 30 or 31
steel pins at 5 mm mutual distance. The cathode plate is located outside and
underneath the reactor vessel. A 50 MΩ tail resistor (Philips) is installed parallel to the
reactor, to rezero the voltage after every discharge pulse. The spark gap is triggered by
frequency-adjustable 9 kV pulses. The reactor contents is stirred by a 1 cm magnetic
stirring rod. The circuit is settled in welded-alumina EMC compartments.
C =1 nF
R =10 MΩ
L
+
anode
high
voltage
source
triggered
sparkgap
reactor
I-probe
Figure 3.3
R =50 MΩ
V-probe
cathode
The high voltage circuit to generate pulsed corona discharges.
The pulse voltage is measured at the anode by means of a high voltage probe
(Tektronix P6015A: 1000x, 3.0 pF, 100 MΩ; compensation box 015-049). The current
is determined at the cathode by means of a coil probe (Pearson 2877: 1 V/A). Data
acquisition is performed by a 400 MHz 2 Gs/s digital oscilloscope (Tektronix TDS380).
Data signals are obtained in eightfold averaging mode and 20 MHz filtering enabled. The
probe cables are shielded with copper non-woven jackets to avoid interception of high
frequency noise, emitted by the spark gap. A coil (L) is inserted in series with the
capacitor to damp parasitic high frequency oscillations.
For reasons of discrete sampling times, the assumption has been made that the pulse
energy is constant in between two measurements: Ep(ti-1<t≤ti) = Ep(ti). With regard to
the observed small changes in the pulse energy during oxidation runs of several hours,
this assumption is justified.
46
Chapter 3.
Probes calibration
For calibration of the voltage probe, a 20 V 100 kHz square waveform from a function
generator (Textronics CFG250) is presented to the probe. The voltage probe calibration
factor is determined from a set of successive acquisitions of function generator voltage
and probe-indicated voltage. The observed calibration factor is about 909x.
The current probe is calibrated by application of the 20 V 100 kHz square waveform
across a 1 kΩ resistor. The current probe calibration factor is determined from a set of
successive acquisitions of voltage across the resistor and current indicated by the probe
in series with the resistor. The observed calibration factor is about 1.0 V/A. The current
probe cable is terminated with a 50 Ω impedance to avoid signal reflection; then 1 volt
corresponds to 2 A.
Due to a difference in probes cable length, a time lag exists between the voltage and
current signal. Assuming that electric charge travels through the probe cable with the
speed of light, the time lag is about 3.3 ns/m. The time lag correction is very important
for accurate pulse energy determination. The actual time lag has been determined from
the voltage and current signals observed during the current probe calibration. The
observed time lag is about 8 ns.
Electrical conductometry
The electrical conductivity of corona-exposed deionized water and aqueous
hydroxybenzene solutions has been measured using a conductivity meter (Cole Parmer
01481-92) equipped with a gold-plated electrodes dip-cell (CP 01481-93). The cell
constant of the used dip cell equals K=10 cm-1. This conductivity meter is temperature
compensated: dG/dT=2%/°C. Before every experiment, the device is calibrated using a
445 µS/cm NaCl standard solution (CP 01489-93) at 25°C.
3.4. Optical diagnostics
UV absorption spectrometry
The ozone production by pulsed corona discharges in air has been measured by UV
absorption spectrometry. The applied setup is shown by Figure 3.4, see also reactor
configuration 3a. A high-pressure mercury lamp (Philips 3110E) has been used as
source. By means of quartz optics, the light beam is transmitted through a quartz wall
reactor (57x62x100 mm, optical path=57.6 mm) and then imaged on the slit of a
0.5 meter monochromator (Jarrell Ash 82025), equipped with a 1200 mm-1 grating and
photomultiplier (Hamamatsu R636). The absorption at 260 nm is measured. After
amplification (Textronics AM502) of the photomultiplier signal, data acquisition is
performed by and A/D converter (TSC500).
Experimental setup
47
+
+
R
PM
M-JA
AM
A/D
Hg
Figure 3.4
UV absorbance setup for quantitative ozone measurements. Hg=highpressure Hg lamp, +=positive quartz lens, R=reactor, M-JA=
monochromator, PM=photomultiplier, AM=amplifier, A/D=analog-todigital converter.
Laser-induced fluorescence
A Nd:YAG solid-state pulsed laser (Continuum 9030) has been applied as excitation
source for LIF measurements on aqueous phenol solutions. The applied excitation
wavelength λ=266 nm is produced from a twofold frequency doubling of the
fundamental wavelength λ=1064 nm. The laser pulse width is about 6 ns and the
linewidth is 1.0 cm-1. The beam repetition frequency is set to 10 Hz. The measured
laser pulse energy is about 0.8-1.1 mJ. The beam waist at the detection volume is
about 3 mm. Figure 3.5 shows a schematic drawing of the setup.
By means of a Pellin-Broca prism, the excitation wavelength is separated from the other
harmonics (355 nm and 532 nm). The applied reactors are equipped with quartz
observation windows, see reactor configuration 8. The fluorescence intensity is
measured at a 90° angle relative to the laser beam. By means of a quartz lens and
quartz fibre the fluorescence radiation is introduced into a monochromator (Jobin-Yvon
H25) equipped with a 150 mm-1 grating. Detection is performed by an ICCD camera
(Andor ICCD-452/DH534-18) operated in the gated mode: the ICCD is externally
triggered by the Nd:YAG laser. The exposure time is 5 seconds, which yields the
fluorescence intensity due to 50 laser pulses. The laser triggers the ICCD camera via a
pulse delay generator (Stanford DG535). A low-pressure mercury lamp (Cathodeon
93109) has been used for wavelength calibration.
D
R
+
Nd:YAG
PB
ex.tr.
QF
M-JY
Figure 3.5
ICCD
PC
DG
LIF setup for measuring phenol degradation by pulsed corona.
Nd:YAG=laser, PB=Pellin-Broca prism, R=reactor, +=positive quartz
lens,
QF=quartz
fibre,
M-JY=monochromator,
ICCD=detector,
DG=delay generator; PC=computer, D=laser dump.
48
Chapter 3.
Fluorescent molecular probe
The detection of hydroxyl radicals has been performed by means of the molecular probe
coumarin-3-carboxylic acid (CCA). 1 mM aqueous solutions have been prepared. Before
oxidation, a sample is taken from the corona reactor, see reactor configuration 2a. This
non-fluorescent CCA sample is used for background determination. The fluorescence
intensity is measured by a Perkin Elmer LS50B fluorescence spectrometer. The
excitation wavelength is λex=396 nm. The applied quartz containers (Hellma QS111)
have an optical path L=10 mm and the volume is 3500 µl. The scanspeed is 100
nm/min. The excitation and emission slitwidth are 5 nm.
For the hydroxyl radical concentration determination, 100 ml standards have been
prepared consisting of CCA and unstabilized hydrogen peroxide. In order to have equal
initial CCA concentrations, all standards have been prepared from the same 1.0 mM
CCA stock solution by a 10-fold dilution. The standard solutions contain 0.1 mM CCA
and increasing amounts of hydrogen peroxide i.e. 4.7⋅10-5 M, 6.6⋅10-5 M, 7.9⋅10-5 M,
2.0⋅10-4 M, 3.6⋅10-4 M and 9.7⋅10-4 M.
Infrared spectroscopy
The gas phase over oxidized phenol solutions has been analyzed by Fourier-transform
infrared spectroscopy (FTIR, Bruker IFS 66). A DTGS detector has been applied. The
scanned wavelength range is 3000-1500 cm-1. The resolution is 4 cm-1.
The volume of the applied reactor is 500 ml and the optical path is 120 mm, see
reactor configuration 9; the reactor is equipped with sapphire windows. The
spectrometer is purged with nitrogen at a rate F=500 l/h to exclude carbon dioxide and
water vapour. After corona-exposure, the sealed reactor is immediately transferred from
the high voltage setup to the spectrometer. Spectra have been recorded 15 minutes
after installation of the reactor into the spectrometer, in order to recover a carbon
dioxide-free measurement compartment. Background spectra have been recorded from
the empty reactor, the reactor filled with the phenol solution at t=0 and the empty
spectrometer compartment in between the measurements.
4. Results
In this chapter the results will be presented concerning the production of oxidizers by
pulsed corona discharges in humid air. A detailed analysis of the oxidation of the model
compound phenol is given regarding conversion efficiency, oxidation products and
analysis techniques. In addition, the oxidation of the model compounds atrazine,
malachite green and dimethyl sulfide is described.
4.1. Pulsed corona discharges
This section describes the results of experiments on the effects of pulsed corona
discharges in air over deionized water. The production of hydroxyl radicals and ozone is
described, reactor geometrical capacitances and pulse energies are reported and the
analysis of corona-exposed deionized water is discussed.
4.1.1. Hydroxyl radicals
In order to detect hydroxyl radicals in corona-treated water two approaches have been
applied. The first approach is trapping of the hydroxyl radical by the spin trap DMPO,
followed by in-situ ESR detection of the produced DMPO-OH adduct. The second
approach is the reaction of the hydroxyl radical with the fluorescent molecular probe
coumarin-3-carboxylic acid (CCA), followed by ex-situ fluorescence spectrometry on
produced 7-hydroxy CCA (7OHCCA).
The spin trap approach
A 500 ml 1.0 mM aqueous DMPO solution has been exposed to pulsed corona
discharges in a continuous flow system, see reactor configuration 1a. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. During the oxidation for
70 minutes, several ESR scans have been made in order to detect the DMPO-OH
adduct. No ESR-absorption signals have been observed. Hereafter, an extra 2.5 mmol
DMPO amount has been added to the continuous flow system in order to be sure, that
enough fresh spin trap is available for the trapping of radicals. Also this extra addition
has not resulted in any absorption signals after 50 minutes of continuation.
A second experiment has been performed, in which a solution containing 1.0 mM
phenol and 1.0 mM DMPO spin trap has been oxidized by pulsed corona discharges.
Now the spin trap is intended to trap radical intermediates produced by the attack of
hydroxyl radicals on phenol i.e. dihydroxycyclohexadienyl(peroxyl) radicals. These
organic radicals are more stable in aqueous solution than hydroxyl radicals. Again, no
ESR absorption signals have been detected within 75 minutes of corona-exposure time.
The possibility may exist that DMPO is destroyed by the pulsed corona discharges. This
has been verified by application of liquid chromatography to untreated and oxidized
DMPO solutions. A reversed-phase HPLC column and UV absorbance detector have
been used according to LC configuration 1. Figure 4.1 shows the conversion of DMPO
as a function of time.
50
Chapter 4.
Conversion (%)
40
30
20
10
0
0
Figure 4.1
15
30
45
60
Time (min)
75
90
The conversion of a 250 ml 1 mM DMPO solution by pulsed corona
discharges. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm.
The observed conversion course will mainly imply the degradation of DMPO by
oxidizers, but the production of the DMPO-OH adduct will also be included. Figure 4.2
shows a possible degradation pathway of DMPO by ozone.
- H2O
O3
N
O
H
DMPO
Figure 4.2
H
O
N
O
O O
molozonide
O
N
O
H
O
O
ozonide
+
N
O
H2O
C
O
H O
O
zwitterion
H
NO2
OH
OOH
hydroperoxy
alkanol
- H2O2
NO2
NO2
O
OH
O
H
Possible degradation pathway of the spin trap DMPO by ozone.
Degradation of DMPO by ozone causes ring-cleavage of the 5-membered ring and 4nitro-4-methyl valeric acid or -valeraldehyde may be produced. Also singlet oxygen in
known to degrade DMPO [97]. Identification of oxidation products has not been
performed.
Although conversion increases significantly with the exposure time, within short times
the conversion is still low (X10min<5%) and enough spin trap is available to trap
hydroxyl radicals. In addition, the half-life of the DMPO-OH adduct is reported to be
about 2 hours [75].
Results
51
The fluorescent molecular probe approach
Intensity (arb.u.)
A 242 ml 1.0 mM CCA solution has been exposed to pulsed corona discharges for 40
minutes, see reactor configuration 2a. The corona parameters are V=25 kV, C=1 nF,
f=100 Hz, d=1.0 cm. The fluorescence intensity of the non-treated and oxidized
solutions as a function of time is illustrated by Figure 4.3.
10
9
8
7
6
5
4
3
2
1
0
400
Figure 4.3
40 min
30 min
20 min
10 min
0 min
450
500
550
Wavelength (nm)
600
Fluorescence spectra of a 1 mM CCA solution after different coronaexposure times (0-40 min). The excitation wavelength λex=396 nm. The
corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Intensity (arb.u.)
The fluorescence from non-treated CCA is marked by t=0 min and is very weak
compared to the fluorescence from the reaction product of CCA and the hydroxyl
radical: 7-OHCCA. The fluorescence intensity of the oxidized solutions with regard to
the non-treated CCA solution (background) is illustrated by Figure 4.4.
10
9
8
7
6
5
4
3
2
1
0
400
Figure 4.4
40 min
30 min
20 min
10 min
450
500
550
Wavelength (nm)
600
Fluorescence spectra of a 1 mM CCA solution after different coronaexposure times (10-40 min). Subtracted background is the untreated CCA
solution. The excitation wavelength λex=396 nm. The corona parameters
are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
52
Chapter 4.
The increase of the fluorescence intensity with the exposure time is almost linear. This
implies that the production rate of the hydroxyl radicals is constant. This experiment
has been repeated applying a longer exposure time, see Figure 4.5.
Intensity (arb.u.)
25
90 min
20
15
60 min
45 min
10
30 min
5
15 min
0
400
450
500
550
600
Wavelength (nm)
Figure 4.5
Fluorescence spectra of a 1 mM CCA solution after different coronaexposure times (15-90 min). Subtracted background is the untreated CCA
solution. The excitation wavelength λex=396 nm. The corona parameters
are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Here, the increase of the fluorescence intensity is still reasonably proportional to the
exposure time, although it may be expected, that CCA will be degraded after longer
exposure times. The stability of CCA towards pulsed corona discharges has been
determined by ion-exclusion chromatography using the LC configuration no. 2b. A
500 ml 1.0 mM CCA solution has been oxidized by pulsed corona discharges for 3
hours. The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The
conversion of CCA versus the oxidation time is shown by Figure 4.6.
Conversion (%)
25
210 nm
20
270 nm
15
10
5
0
0
30
60
90
120
150
180
Time (min)
Figure 4.6
The conversion of a 500 ml 1 mM CCA solution by pulsed corona
discharges. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm.
Results
53
CCA appears to be fairly stable towards pulsed corona discharges. After 3 hours only
20% conversion occurs. Possible degradation of CCA may involve multifold
hydroxylation and ring-cleavage of the benzene ring, hydroxylation of or ozone attack
on the double bond present in the pyran ring followed by ring-cleavage, see Figure 4.7.
Identification of oxidation products has not been applied.
O
a)
O
O
O
OH
O
OH
CCA
O
HO
O
b)
OH
O
O
d)
O
O O O
OH O
Figure 4.7
O
O
OH
O
O
O
O
c)
O
OH
Possible degradation of CCA: a) hydroxylation of the benzene ring, b) ringcleavage of the benzene ring by ozone, c) hydroxylation of the pyran ring
double bond, d) ring-cleavage of the pyran ring by ozone.
Attempts have been made to quantify the production of hydroxyl radicals. The
fluorescence intensity of 7-OHCCA has been calibrated versus the amount of hydroxyl
radicals produced per solution volume. Fluorescence standard solutions have been
prepared from CCA and hydrogen peroxide, where hydrogen peroxide has been used as
source for hydroxyl radicals. With regard to the quantitative approach, unstabilized
hydrogen peroxide has been utilized, to avoid scavenging of hydroxyl radicals by the
stabilizer normally present in commercial hydrogen peroxide solutions. Different
amounts of hydrogen peroxide i.e. known amounts of hydroxyl radicals have been
added to 100 ml 0.1 mM CCA solutions. A pure 0.1 mM CCA solution has been
exposed to corona discharges. The corona parameters are V=25 kV, C=1 nF, f=100
Hz, d=1.0 cm. The fluorescence intensity of both the oxidized CCA solution and the
standards is shown by Figure 4.8.
Intensity (arb.u.)
5
30 min
20 min
10 min
0 min
9.7E-4 M
3.6E-4 M
2.0E-4 M
7.9E-5 M
6.6E-5 M
4.7E-5 M
4
3
2
1
0
400
450
500
550
OH
OH
OH
OH
OH
OH
600
Wavelength (nm)
Figure 4.8
Fluorescence spectra of an 0.1 mM CCA solution after different coronaexposure times (0-40 min) The corona parameters are V=25 kV, f=100
Hz, C=1 nF, d=1.0 cm. Also shown are the fluorescence spectra of the
CCA/H2O2 fluorescence standards, indicated by a virtual hydroxyl radical
concentration. The excitation wavelength λex=396 nm.
54
Chapter 4.
Unfortunately, the fluorescence standards appear to show no correlation between the
fluorescence intensity and the amounts of hydroxyl radicals added (as hydrogen
peroxide). Also, the fluorescence intensity of the untreated 0.1 mM CCA solution i.e.
background is even higher than the intensity of several fluorescence standards.
These contradictory results are explained by the fact that the preparation of the
fluorescence standards is critical. The addition of hydrogen peroxide to the CCA
solution should be in stoichiometric proportions, here equal molarities. However, the
solubility of CCA in water is poor: higher concentrations than 1 mM are not feasible.
This implies, that hydrogen peroxide should be added in very diluted form to the CCA
solution. Very dilute hydrogen peroxide solutions are not stable, because hydrogen
peroxide will oxidize the water, according to the redox reactions shown by Equations
4.1a-c. The addition of hydrogen peroxide to the 0.1 mM CCA solutions is thus
unverifiable.
H2O2 + 2e- → 2 OHH2O2 + 2H+ + 2e- → 2 H2O
2H2O → O2 + 4H+ + 4e-
reduction of hydrogen peroxide
reduction of hydrogen peroxide
oxidation of water
(4.1a)
(4.1b)
(4.1c)
Summary
The formation of hydroxyl radicals during exposure of water by pulsed corona
discharges has been demonstrated by fluorescence spectroscopy using the OH-specific
fluorescent molecular probe CCA. The production of hydroxyl radicals appears to be
rather constant during 90 minutes of corona exposure. It has not been possible to
attribute the observed fluorescence intensity to certain amounts of hydroxyl radicals. Insitu Electron Spin Resonance using the spin trap DMPO has not been able to identify
the hydroxyl radical under the same conditions.
Results
55
4.1.2. Ozone
The production of ozone by pulsed corona discharges in air has been determined for
several reactor conditions using UV absorbance spectrometry, see reactor configuration
3a. First, ozone measurements have been performed for corona discharges in an
ambient air-filled reactor at different load voltages viz. -25 kV, -20 kV, 15 kV, 20 kV,
25 kV and 30 kV. The ozone concentration versus time is shown by Figure 4.9. It has
been observed, that the maximum ozone concentration increases with increasing
absolute load voltage. At a given voltage, the negative polarity corona produces less
ozone than the positive polarity corona. After about 16 minutes the corona has been
switched off: a decrease in ozone concentration follows as a result of the backward
reaction of ozone into oxygen by wall recombination.
-3
[Ozone] (m )
6.0E+22
30 kV
25 kV
4.0E+22
-25 kV
20 kV
2.0E+22
-20 kV
15 kV
0.0E+00
0
5
10
15
20
Time (min)
Figure 4.9
Ozone concentration versus time in a reactor filled with ambient air,
energized at different load voltages. The corona parameters are
V (indicated), C=1 nF, f=100 Hz, d=1.0 cm. After about 16 min the
corona discharges have been stopped.
Hereafter, ozone concentrations have been measured for the case of a reactor filled
with ambient air and 100 ml deionized water. The load voltages have been 20 kV,
25 kV and 30 kV, see Figure 4.10. The ozone concentration over the deionized water
increases with the load voltage just as for the case of the ambient air-filled reactor, but
the maximum ozone concentration over water is lower than the ozone concentration in
air. Also it has been observed, that the initial ozone production rate i.e. initial curve
slope is higher for the water-filled reactor compared to the ambient air-filled reactor.
The difference in maximum ozone concentration can be explained by the fact that
ozone is destroyed by hydroxyl and hydroperoxyl radicals produced in humid air over
water, according to Equations 2.4ab. Also ozone reacts with water, see Equations
2.3a-c. The water-filled reactor has a smaller gas phase volume (253 ml) than the
ambient air-filled reactor (353 ml), therefore initially the ozone production rate is higher.
56
Chapter 4.
6.0E+22
30 kV air
-3
[Ozone] (m )
30 kV water
4.0E+22
25 kV air
25 kV water
20 kV air
2.0E+22
20 kV water
0.0E+00
0
5
10
15
Time (min)
Figure 4.10
Ozone concentration versus time in a reactor filled with ambient air and
100 ml deionized water or solely ambient air, energized at different load
voltages. The corona parameters are V (indicated), C=1 nF, f=100 Hz,
d=1.0 cm.
Next, the influence of phenol in water on the gas phase ozone concentration has been
measured. 100 ml 1 mM phenol solutions have been oxidized by corona in ambient air
at voltages 25 kV and 30 kV. The ozone concentration versus time for deionized water
and phenol solutions is shown by Figure 4.11. The presence of phenol in the deionized
water additionally decreases the ozone concentration, so it is clear that phenol in
aqueous solution consumes ozone produced over the aqueous solution. The reaction
products of phenol oxidation will be discussed in sections 4.2 and 5.3.
-3
[Ozone] (m )
6.0E+22
30 kV water
30 kV 1 mM phenol
4.0E+22
25 kV water
2.0E+22
25 kV 1 mM phenol
0.0E+00
0
5
10
15
Time (min)
Figure 4.11
Ozone concentration versus time in a reactor filled with ambient air and
100 ml deionized water or 100 ml 1 mM phenol solution, energized at
different load voltages. The corona parameters are V (indicated), C=1 nF,
f=100 Hz, d=1.0 cm.
Results
57
Application of pulsed corona discharges in pure oxygen may produce higher ozone
levels. This has been verified by purging the reactor with oxygen at the flow rates
F=100 ml/min and F=200 ml/min. The load voltages are 20 kV, 25 kV and 30 kV, see
Figure 4.12. With increasing load voltage and constant oxygen flow rate the ozone
concentration increases. Increasing the oxygen flow rate appears to have a minor
positive effect on the ozone production at 20 kV, but has a strongly negative effect on
the ozone production at the load voltages 25 kV and 30 kV.
This is explained by the fact, that at higher flow rates the produced ozone is removed
from the reactor by the oxygen purge. This effect can also be illustrated by comparison
of the ozone production at V=30 kV & F=200 ml/min and the ozone production at
V=25 kV in an oxygen saturated reactor (F=0 ml/min). The positive effect of the
higher voltage is overruled by the negative effect of a high flow rate.
1.0E+23
30 kV 100 ml/min
25 kV 0 ml/min
-3
[Ozone] (m )
8.0E+22
6.0E+22
30 kV 200 ml/min
25 kV 100 ml/min
4.0E+22
25 kV 200 ml/min
2.0E+22
20 kV 200 ml/min
20 kV 100 ml/min
0.0E+00
0
Figure 4.12
2
4
6
Time (min)
8
10
Ozone concentration versus time in a reactor purged with oxygen at
different flow rates, energized at different load voltages. The corona
parameters are V (indicated), C=1 nF, f=100 Hz, d=1.0 cm.
Finally the ozone production has been monitored versus the integrated corona pulse
energy for the reactor filled with ambient air and 100 ml deionized water. The load
voltages are 15 kV, 20 kV, 25 kV and 30 kV, see Figure 4.13. From the initial slope of
the ozone concentration versus corona energy curves, the following ozone production
efficiency has been calculated: 5.45⋅1020 ±0.05 m-3J-1. This value equals about
40 g/kWh for the applied reactor and is quite high, because this efficiency has been
obtained in humid air. Nowadays commercial ozone generators reach efficiencies of
50-60 g/kWh in dry air [98].
58
Chapter 4.
-3
[Ozone] (m )
6.0E+22
30 kV
4.0E+22
25 kV
2.0E+22
20 kV
15 kV
0.0E+00
0
50
100
150
200
E (J)
Figure 4.13
Ozone concentration versus the integrated corona pulse energy in a
reactor filled with ambient air and 100 ml deionized water, energized at
different load voltages. The corona parameters are V (indicated), C=1 nF,
f=100 Hz, d=1.0 cm.
Summary
The production of ozone by pulsed corona discharges in air and oxygen has been
demonstrated. The ozone concentration increases with the corona load voltage.
Negative corona produces less ozone than positive corona at equal absolute load
voltage. Ozone produced by corona over water is destroyed by hydroxyl and
hydroperoxyl radicals, which are also formed by the corona in humid air. Ozone
produced in the gas phase over an aqueous phenol solution is consumed by phenol.
Pulsed corona discharges in oxygen produce higher ozone concentrations than
discharges in air, but high oxygen purge flow rates remove ozone from the reactor. The
achieved ozone production efficiency for humid air is about 40 g/kWh.
Results
59
4.1.3. Corona pulse energy
In order to calculate the pulse energy of corona discharges applied in the gas phase,
first the capacitance of the applied setup has been determined. The several
configurations mainly differ by liquid/gas phase volume and anode type. Figure 4.14
shows a selection of best fits of I(t) and Cg⋅dV(t)/dt for estimation of the geometry
capacitances. Table 4.1 shows the measured average geometry capacitances for all
applied setup configurations.
Table 4.1
Reactor
no.
6ab
6c
6c
3b
3b
5ab
Average geometry capacitances (Cg) and 95% confidence intervals.
Anode
(pins)
31
30
30
30
30
1
liquid vol (ml)
500
500
500
100
100
250
Reactor
liquid and gas type
tap water, air
deionized water, air
phenol 1 mM, air
phenol 1 mM, air
phenol 1 mM, Ar purge
deionized water, air
Cg
(pF)
4.3 ±0.1
2.5 ±0.1
2.6 ±0.1
2.3 ±0.2
2.4 ±0.0
1.4 ±0.1
All configurations have the anode situated d=1.0 cm over the aqueous solution, while
the cathode plate is situated outside and directly underneath the glass reactor vessel.
Although the determined capacitances are rather small, the capacitive current for these
configurations is not negligible, because the slope of the voltage is very steep, that is
dV(t)/dt≈1011-1012 V/s. The observed oscillations are due to parasitic impedances from
the circuit in combination with fast voltage rise times created by the triggered spark
gap.
The presence of phenol in the water does not influence the capacitance, see reactor
configuration 6c; the relative permittivity of phenol is about εR,phenol≈12.4 while water
has a relative permittivity εR,H2O≈80.1. In addition, the applied molar fraction of phenol to
water is negligible viz. 10-3 M:55.4 M=1.82⋅10-5. With regard to the oxidation of
aqueous phenol solutions, neither formic acid (εR≈51.1) nor hydrogen peroxide (εR≈74.6)
will contribute to the permittivity of water, because of their low concentration. Data
have been obtained from [99]. From the equal capacitances of reactor configuration 3b
with air or argon, it can be derived that the capacitance is not affected by vigorously
purging of the reactor. Although the relative permittivities of argon and air are nearly
equal, the argon purge at a flow rate F=100 ml/min. results in a gas-dispersed phenol
solution that may have a different capacitance than the immobile phenol solution in the
air-filled reactor. Finally, the capacitances cannot be related to the indicated liquid
phase volumes, because the reactors have different dimensions.
From the measured capacitances, the capacitive current has been calculated that is part
of the total current recorded during the corona experiments. The pulse energy has been
estimated from the pulse voltage and corona current. Table 4.2 shows pulse energies,
averaged over the different observation times, during the applied pulsed corona-induced
oxidation experiments discussed in chapter 4. Characteristic voltage and current
waveforms are presented together with the efficiency values in the appropriate
sections.
60
Chapter 4.
0.8
0.20
Current (A)
0.4
Cg=4.2 pF
Vload=9 kV
0.2
Config. 5ab
I(t)
Cg⋅dV(t)/dt
0.15
Current (A)
I(t)
Cg⋅dV(t)/dt
0.6
Config. 6ab
0.10
Cg=1.6 pF
Vload=7 kV
0.05
0.0
0.00
-200
-0.2
0
200
400
-200
-0.05
0
Time (ns)
Time (ns)
Config. 6c
water
Cg=2.5 pF
Vload=7 kV
0.1
0.0
-200
0
200
Config. 6c
I(t)
Cg⋅dV(t)/dt 1 mM phenol
0.2
Current (A)
I(t)
Cg⋅dV(t)/dt
0.2
Current (A)
400
0.3
0.3
Cg=2.5 pF
Vload=7 kV
0.1
0.0
-200
400
0
200
400
-0.1
-0.1
Time (ns)
Time (ns)
0.40
0.40
I(t)
Cg⋅dV(t)/dt
0.20
Config. 3b
air
Cg=2.3 pF
Vload=7 kV
0.10
0.30
Current (A)
0.30
Current (A)
200
I(t)
Cg⋅dV(t)/dt
0.20
Config. 3b
Ar purge
Cg=2.4 pF
Vload=7 kV
0.10
0.00
0.00
-200
-0.10
0
200
400
-200
-0.10
Time (ns)
0
200
400
Time (ns)
Figure 4.14
A selection of best fits of I(t) and Cg⋅dV(t)/dt; the result is Cg. Indicated
are reactor configuration numbers.
Table 4.2
Time-averaged pulse energies and 95% confidence intervals for the
performed experiments.
reactor
Experiment
no.
corona in gas phase over aqueous solution
3a
Ozone measurements in a reactor, filled with
air and 100 ml deionized water
6a
6b
6c
3b
3b
5a
5b
Phenol oxidation, 500 ml 0.05 mM; air
Atrazine oxidation, 500 ml 0.12 mM; air
Phenol oxidation, 500 ml 1 mM; air
Phenol oxidation, 100 ml 1 mM; air
Phenol oxidation, 100 ml 1 mM; argon flow
Deionized water oxidation, 250 ml; air
Hydroxybenzenes oxidation, 250 ml 1 mM; air
Corona parameters
V,C,f
15 kV, 1 nF, 100 Hz
20 kV, 1 nF, 100 Hz
25 kV, 1 nF, 100 Hz
30 kV, 1 nF, 100 Hz
30 kV, 100 pF, 50 Hz
30 kV, 100 pF, 50 Hz
25 kV, 1 nF, 100 Hz
25 kV, 1 nF, 100 Hz
25 kV, 1 nF, 100 Hz
25 kV, 1 nF, 100 Hz
25 kV, 1 nF, 100 Hz
Ep
(mJ)
0.6 ±0.2
1.4 ±0.4
3.5 ±0.3
5.9 ±1.1
13.4 ±1.4
10.7 ±0.5
10.0 ±0.1
5.8 ±0.2
10.6 ±0.3
4.3 ±0.2
5.6 ±0.3
Results
61
Although this thesis deals with the application of pulsed corona discharges in the gas
phase over aqueous solutions of the target compound, also some pulse energy
calculations are shown related to the application of corona in the liquid phase. The
reason for this approach is to compare pulse energies of both applications. Much
environmental corona research is performed as liquid phase corona [100-102].
Both a deionized water and tap water liquid phase have been regarded. Reactor
configuration 4 has been used. The anode is made from a W/Th alloy (welding rod) and
has a 30 µm tip. Both cathode plate and anode tip are situated in the liquid phase at a
mutual distance ∆=2.0 cm. It has not been possible to calculate the pulse energy
according to the procedure described by section 2.5.2, because the capacitive current
is high as a result of the high capacitance of the water-filled reactor with immersed
electrodes. Therefore the pulse energy has been estimated by two different methods
viz. a calorimetric determination and a calculation based on the average direct current
delivered by the high voltage power supply.
At a voltage V=19 kV, a pulse repetition rate f=100 Hz and electrode distance ∆=2.0
cm the pulse energy has been calculated for the reactor, filled with different amounts of
deionized water i.e. Vol=200 ml, 250 ml, 300 ml, 400 ml. Table 4.3 shows the energy
per pulse calculated by the two methods.
Table 4.3
Volume
(ml)
200
250
300
400
Pulse energy measured in deionized water at different volumes. The
corona parameters are V=19 kV, C=1 nF, f=100 Hz, ∆=2.0 cm.
Pulse energy (mJ)
Calorimetry
Power supply
61 ±9
90 ±8
56 ±8
88 ±7
55 ±8
85 ±7
58 ±9
87 ±7
With regard to both methods the following remarks can be made. A possible error made
by the calorimetric method is due to imperfect thermal insulation. However, the
observed temperature decrease rate after stopping the experiment is only about 0.2 K
per 15 minutes, while the duration of the experiment is comparable, viz. 10-20
minutes. The error involved in the pulse energy obtained from the power source average
direct current is due to the assumption, that power dissipation only takes place in the
external circuit by the 10 MΩ load resistor.
Calorimetric- and average direct current-based measurements on 250 ml tap water
under equal corona conditions have yielded pulse energies of 130 ±30 mJ and 260 ±30
mJ respectively. The pulse energy in tap water is much higher than the pulse energy in
deionized water, because of the higher electrical conductivity of tap water. The applied
deionized water exhibits a conductivity of about 30-100 µS/cm, while the used tap
water has a conductivity of about 2580 µS/cm. The reported pulse energies for
deionized water and tap water have been determined at a voltage V=19 kV and are yet
much higher than the pulse energies in air, determined at V=25-30 kV.
62
Chapter 4.
The formation of corona discharges in water requires evaporation of water at the anode
tip, thus a high pulse energy. Figure 4.15 shows a CCD image of corona discharges at
an anode tip immersed in deionized water. Stroboscopic illumination has been applied
using a He/Ne laser chopped at 10 Hz. The discharges appear as bright irregular-shaped
channels. Vapour bubbles appear as rows of dots. The size of the vapour bubbles is
approximately several tenths of a millimeter.
Figure 4.15
CCD image of corona discharges at an anode tip (top) immersed in
deionized water. Stroboscopic illumination has been applied. The
discharges appear as bright irregular-shaped channels. The vapour bubbles
appear as rows of dots. The corona settings are V=20 kV, f=0.1 Hz,
d=2.6 cm. The exposure time is 1 s. The image has been taken by
A.H.F.M. Baede.
The reason for the application of corona in the liquid phase is to produce the oxidizers
i.e. hydroxyl radicals directly at the location where they are needed to avoid loss due to
recombination. However, in section 4.1.1 it has been shown that by application of
pulsed corona discharges over water, yet the action of hydroxyl radicals in aqueous
solution can be demonstrated, by means of the hydroxyl radical specific molecular
probe CCA. The hydroxyl radicals are produced directly by dissociation of water
molecules and indirectly from ozone, water and UV photons.
Summary
Two different methods have been applied to estimate the pulse energy for application of
corona in water viz. a calorimetric determination and a method based on the average
direct current from the power supply. By comparison of the pulse energies of corona
discharges applied over and in water, it has been shown that the application of corona
over water is much more favourable. This is caused by the fact that the production of
corona in water requires evaporation of water at the anode tip. The pulse energy
measured for corona in tap water is higher than the pulse energy for corona in deionized
water, due to a higher electrical conductivity of tap water compared to deionized water.
Results
63
4.1.4. Corona treatment of deionized water
The application of pulsed corona discharges in an air gas phase over deionized water
changes the water in several ways. Corona discharges in air produce ozone and small
amounts of nitrogen oxides. In water, the ozone is converted into hydrogen peroxide
while the nitrogen oxides are converted into nitric acid. The corona discharges strike the
water surface and dissociate water molecules into hydroxyl radicals and hydrogen
atoms. Hydroxyl radicals recombine to hydrogen peroxide. Hydrogen atoms react with
dissolved oxygen to hydroperoxyl radicals, which recombine to yield hydrogen peroxide
and oxygen. Recombination of hydrogen atoms produces hydrogen. Also, the metal of
the anode tip may be sputtered due to the high electric field strength, so elementary
metal or metal oxides may be found in the water. The chemical change of coronaexposed deionized water has been investigated by application of electrical
conductometry, spectrochemical ICP analysis and ion-exclusion chromatography.
250 ml deionized water samples have been exposed to pulsed corona discharges, see
reactor configuration 5a. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm. Three different anode tips have been tested: steel, tungsten and platinum.
The following relative temperature increase (∆T/T0) values have been measured after 60
minutes of corona exposure: Fe: 0.14, W: 0.15, Pt: 0.08. Figure 4.16 shows the
electrical conductivity of the deionized water as a function of the corona-exposure time.
Conductivity (µS/cm)
500
Fe
400
W
300
Pt
200
100
0
0
Figure 4.16
20
40
Time (minutes)
60
The electrical conductivity of 250 ml deionized water samples versus the
oxidation time. Used anode tip materials are steel, tungsten and platinum.
The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Measured pulse energies are: EpFe=4.5 ±0.3 mJ, EpW=4.1 ±0.2 mJ, EpPt=4.2 ±0.3 mJ.
The conductivity of the deionized water samples increases with the corona-exposure
time. The increase is highest when corona is produced using the steel anode tip, while
the platinum tip brings about the lowest conductivity increase. The conductivity
differences using different anode tip materials may imply the effect of anode material
thus the sputtering of anode material into the water. However, the anode tip geometries
are far from identical, because this is very difficult to achieve. Therefore the amounts of
sputtered material may be different. Spectrochemical ICP analysis has been applied to
the deionized water samples oxidized by the steel and tungsten tips, but no metals have
been identified. Therefore it is likely, that the conductivity increase arises from nitric
acid. The mutually different anode tips may have produced different amounts of
nitrogen oxides versus the oxidation time.
64
Chapter 4.
The 30 minutes corona-exposed deionized water samples have been analyzed by ionexclusion chromatography using a diode array UV absorbance detector and conductivity
detector in series, see LC configuration 3. Figure 4.17 shows the chromatograms of
oxidized and untreated deionized water obtained by using steel, tungsten and platinum
anode tips.
4.40E+05
Conductivity (arb.u.)
UV absorbance (mAU)
100
80
60
Fe: 3.06 min
W: 3.05 min
Pt: 3.06 min
40
20
Fe: 8.72 min
W: 8.74 min
Pt: 8.75 min
4.35E+05
Fe
W
Pt
4.30E+05
4.25E+05
4.20E+05
0
0
5
10
Retention time (min)
Figure 4.17
15
20
0
5
10
15
Retention time (min)
20
UV absorbance (λ=210 nm, left) and conductivity (right) chromatograms
of 250 ml 30 minutes exposed deionized water samples. Used anode tip
materials are steel, tungsten and platinum. The parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
The UV absorbance chromatogram shows three similar strong absorptions at about
tR=3.06 minutes. The UV absorbance intensity decreases according to the order
200 nm>210 nm>220 nm>>255 nm>270 nm. This first peak represents excluded
anions and is only present in corona-exposed aqueous solutions. Nitrate ions are likely
to account for this peak. Nitrite is less probable, because it is oxidized to nitrate by the
pulsed corona discharges.
The conductivity chromatograms reveal a very strong negative peak at about tR=3.5
min and a weak conductivity signal at tR=8.7 minutes, for all solutions. The strong
negative peak represents the elution of water from the sample. This peak overrules a
possible conductivity signal due to the components eluting at tR=3.06 min, considering
the time difference of about 0.24 minutes between the signals from the UV absorbance
and conductivity detector. The components eluting at tR=8.7 min are retained by the
ICE column, thus their identity equals either an organic molecule or a cation. However,
the fresh deionized water is, except for traces, free from any organic or inorganic
compounds. Metal ions are likely to be retarded by the ICE mechanism but have not
been identified by spectrochemical ICP analysis.
Summary
By exposure of deionized water to pulsed corona discharges, the electrical conductivity
significantly increases. This is likely due to nitrate ions, originating from nitrogen oxides,
which are produced by corona in air. Corona-induced anode metal sputtering is
theoretically possible, but metals have not been identified.
Results
65
4.2. Oxidation of phenol
This section gives a detailed analysis of the oxidation of the model compound phenol
regarding conversion, energy efficiency, oxidation products and analysis techniques.
4.2.1. Chromatography
Reversed-phase HPLC
100
100
80
80
Conversion (%)
Conversion (%)
The initial measurements of phenol conversion have been performed using a standard
reversed-phase HPLC column according to LC configuration 4. This column is suitable
for the separation of phenol from its oxidation product components e.g.
polyhydroxybenzenes and carboxylic acids. Phenol conversion is calculated from the UV
absorbance detector peak area as a function of time. 500 ml 5 mg/l (0.05 mM) phenol
solutions have been prepared using tap water. The pulsed corona discharges take place
in air over the phenol solution, see reactor configuration 6a. The influence of the
parameters voltage (V), pulse repetition rate (f), anode-tip-to-water distance (d) and
solution acidity (pH) on the phenol conversion are shown by Figure 4.18. During these
experiments, only one parameter is varied at a time, while the others are kept constant
at the standard values V=30 kV, f=50 Hz, d=1.0 cm, pH=n.a. (not adjusted), t=30
min (except V series: t=45 min). The plots show individual trends. The absolute values
cannot be compared mutually, because singular data acquisition has been applied.
60
t=45 min
f=50 Hz
d=1.0 cm
pH=n.a.
40
20
t=30 min
V=30 kV
d=1.0 cm
pH=n.a.
40
20
0
0
15
20
25
30
Voltage (kV)
35
0
40
100
100
80
80
Conversion (%)
Conversion (%)
60
60
t=30 min
V=30 kV
f=50 Hz
pH=n.a.
40
20
0
50
100
150
Frequency (Hz)
200
250
60
t=30 min
V=30 kV
f=50 Hz
d=1.0 cm
40
20
0
0.0
Figure 4.18
0.5
1.0
1.5
Distance (cm)
2.0
2.5
2
4
6
pH (-)
8
The conversion of phenol as a function of the indicated parameters.
500 ml 5 mg/l (0.05 mM) phenol solutions have been oxidized.
10
66
Chapter 4.
The conversion of phenol appears to increase non-linear with the applied corona load
voltage. This is likely due to the radical formation processes by the corona plasma. By
increasing the voltage the radical production indeed grows, but the probability of
recombination also increases. Therefore the radical production at higher voltages is
likely to be less efficient.
The conversion increases with the pulse repetition rate for the range 0 Hz<f<100 Hz.
The unexpected decrease of the conversion at 200 Hz might be related to corona
instability at higher repetition frequencies as a result of the application of a pressurized
triggered spark gap.
Conversion is considerably higher in an alkaline solution than in an acidic solution. In
alkaline solution ozone reacts by hydroxyl radicals which are far more reactive than
ozone. In addition, at high pH the existence of the phenolate anion (C6H5O-) is extra
favourable with regard to the electrophilic nature of hydroxyl radicals [7].
Adjusting the anode-tip-to-water distance at about d=1.0 cm yields the highest
conversion. Application of pulsed corona discharges at the gas-liquid interface limits the
production of oxidizers e.g. ozone, therefore a certain gas phase volume i.e. distance is
favourable. This effect has also been observed by experiments on the influence of
electrode configurations on the decolorization of malachite green dye, which will be
described in section 4.3.2. The application of pulsed corona discharges in an aqueous
solution is energetically unfavourable, because this causes evaporation of water at the
anode tip [103].
In addition to this experiment, the influence of the solution volume on the conversion of
phenol has been determined in threefold, according to the parameters V=30 kV,
C=100 pF, f=50 Hz, d=1.0 cm, t=30 min. The conversion of the 500 ml 5 mg/l
solution is 60.7% ±7.0% while the conversion of the 250 ml 10 mg/l solution is
73.5% ±4.0%. The oxidizers are more efficiently consumed for the case of the 250 ml
solution, compared to the 500 ml solution. Application of pulsed corona discharges to
thin liquid films will be favourable to conversion.
In the subsequent experiments, pulsed corona discharges have been applied in the gas
phase over the aqueous solution of the target compound [104], considering an anodetip-to-water distance d=1.0 cm. Although alkaline conditions favour the conversion of
phenol, no such pH adjustments have been made, because the applicability of the
pulsed corona technology should be qualified in an intrinsic way.
The oxidation of a 5 mg/l (0.05 mM) phenol solution in tap water has been performed in
threefold. The corona parameters are V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm. The
conversion and 95% confidence interval are plotted as a function of time, see Figure
4.19.
After an oxidation time of 1 hour, a conversion X=92% ±7.9% has been reached. The
relationship between ln(C/C0) and the oxidation time t is linear, implying first order
reaction kinetics with a rate constant k1≈4.1⋅10-2 min-1. Characteristic pulse voltage,
current and power are shown by Figure 4.20. The efficiency of phenol conversion,
expressed by the G yield value is shown by Table 4.4.
Results
67
Conversion (%)
100
80
60
40
20
0
0
Figure 4.19
20
40
Time (min)
60
80
The conversion of a 500 ml 5 mg/l (0.05 mM) phenol solution (tap waterbased). The corona parameters are V=30 kV, C=100 pF, f=50 Hz,
d=1.0 cm.
8.0E+05
30
2.0E+04
20
I(t), Icor(t)
1.0E+04
10
0.0E+00
-100
6.0E+05
0
0
100
200
-1.0E+04
300
400
-10
Power (W)
V(t)
Current (A)
Voltage (V)
3.0E+04
4.0E+05
2.0E+05
0.0E+00
-100
-2.0E+05
Time (ns)
0
100
200
300
400
Time (ns)
Figure 4.20
Typical pulse voltage, current and power waveforms for corona in air,
recorded after 1 hour of oxidation. The corona parameters are V=30 kV,
C=100 pF, f=50 Hz, d=1.0 cm.
Table 4.4
Conversion (X), pulse energy (Ep) and efficiency (G) during oxidation of a
500 ml 5 mg/l (0.05 mM) phenol solution (tap water-based); average
values and 95% confidence intervals are presented. The corona
parameters are V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm.
t (min)
15
30
45
60
X (%)
24.5 ±18.3
60.7 ±7.0
85.8 ±35.6
92.3 ±7.9
Ep (mJ)
13.7 ±4.2
13.1 ±5.1
14.3 ±25.4
12.4 ±7.4
G (mol/J)⋅108
1.08 ±1.14
1.39 ±0.54
1.20 ±2.63
1.10 ±2.08
G (100eV)-1
0.10 ±0.11
0.13 ±0.05
0.12 ±0.25
0.11 ±0.20
G (g/kWh)
3.7 ±3.9
4.7 ±1.8
4.1 ±8.9
2.5 ±5.5
The waveforms show rather much parasitic high frequency oscillations. These have
been largely suppressed in the subsequent experiments. The 95% confidence levels of
conversion and efficiency are rather large. The reason for the poor reproducibility is
likely due to corona instability during these introductory experiments.
68
Chapter 4.
The first quantitative measurements have been applied to phenol, hydroquinone, 1.4benzoquinone and resorcinol. 500 ml 25 mg/l (0.27 mM) phenol solutions (tap waterbased) have been exposed to pulsed corona discharges in air for 5 hours. The corona
parameters are V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm. The concentration
determination has been performed in threefold, every 30 minutes.
The first analysis showed the production of hydroquinone and resorcinol. The duplicate
and triplicate measurements only showed hydroquinone and the conversion rate of
phenol and hydroquinone was somewhat lower than for the case of the first
measurement, see Figure 4.21. It has not been possible to distinguish hydroquinone
from 1.4-benzoquinone, because these compounds show co-elution using the reversedphase HPLC column according to LC configuration 4. Hydroquinone is initially produced
and may be oxidized to 1.4-benzoquinone. The carboxylic acids formic acid and acetic
acid have been qualitatively identified by Capillary Zone Electrophoresis.
Concentration (mol/l)
3.0E-04
phenol
hydroquinone
2.0E-04
resorcinol
1.0E-04
0.0E+00
0
60
120
180
240
300
Time (min)
Figure 4.21
The oxidation of a 500 ml 25 mg/l (0.27 mM) phenol solution (tap waterbased). The corona parameters are V=30 kV, C=100 pF, f=50 Hz,
d=1.0 cm.
Results
69
Ion-exclusion chromatography
The applicability of a reversed-phase HPLC column for separation of the phenol
oxidation product mixture has appeared to be limited. Although polyhydroxybenzenes
can be properly separated, the carboxylic acid-functional ring-cleavage products cannot
be retained. In order to separate both of these mutually different product groups, an
ion-exclusion column has been applied.
The conversion of phenol has been measured simultaneously using two different LC
configurations with a reversed-phase HPLC column (5a) and an ion-exclusion column
(5b). 500 ml 1.0 mM (94 mg/l) phenol solutions have been oxidized by pulsed corona
discharges in air for 3 hours, see reactor configuration 6c. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The conversion determination has been
performed in threefold, every 15 minutes during 3 hours. The samples have been
directly analyzed after sampling. At startup and at the end of the experiment the
solution acidity, conductivity and temperature have been recorded. Figure 4.22 shows
the observed conversion as a function of the oxidation time measured by both columns.
Characteristic pulse voltage and current waveforms are shown by Figure 4.23. The
efficiency, conversion and pulse energy are listed by Table 4.5.
There appear to be no significant differences between the conversion measurements
using a reversed-phase HPLC column or an ion-exclusion column. Therefore the more
powerful ICE column has been used for subsequent separations. The conversion time
relationship seems to obey first order kinetics with a rate constant k1≈2.9⋅10-3 min-1.
After three hours of oxidation, the conversion is about 39%. The efficiency is in the
range of 1.9⋅10-8-2.7⋅10-8 mol/J and slowly decreases as a function of the oxidation
time, because during oxidation progress less phenol molecules are available.
50
rp-HPLC
Conversion (%)
40
ICE
30
20
10
0
0
60
120
180
Time (min)
Figure 4.22
Phenol conversion measured simultaneously by rp-HPLC and ICE
chromatography. Oxidation of a 500 ml 1.0 mM (94 mg/l) phenol
solution. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm.
70
Chapter 4.
3.0E+05
30
V(t)
2.0E+04
1.0E+04
20
10
I(t)
Icor(t)
0.0E+00
0
-100
0
100
200
Time (ns)
300
Current (A)
Power (W)
Voltage (V)
3.0E+04
2.0E+05
1.0E+05
0.0E+00
400
-100
0
100
200
Time (ns)
300
400
Figure 4.23
Typical pulse voltage, current and power waveforms for corona in air,
recorded after 1 hour of oxidation. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
Table 4.5
Phenol conversion (X), pulse energy (Ep) and efficiency (G) during the
oxidation of a 500 ml 1.0 mM (94 mg/l) phenol solution; average values
and 95% confidence intervals are presented. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The conversion has been
determined by ICE chromatography.
t (min)
0
15
30
45
60
75
90
120
150
180
X (%)
0.0 ±0.0
4.6 ±1.1
9.5 ±4.4
13.3 ±0.5
17.5 ±3.3
20.7 ±2.3
24.1 ±2.8
30.0 ±3.2
34.3 ±1.9
39.3 ±1.9
Ep (mJ)
9.9 ±0.5
10.2 ±0.1
10.1 ±0.8
10.0 ±0.3
10.0 ±0.7
9.9 ±0.3
G (mol/J)⋅108
2.67 ±1.10
2.41 ±0.44
2.21 ±0.20
2.10 ±0.22
1.92 ±0.07
1.85 ±0.04
G (100eV)-1
0.26 ±0.11
0.23 ±0.04
0.21 ±0.02
0.20 ±0.02
0.19 ±0.01
0.18 ±0.01
G (g/kWh)
9.06 ±3.74
8.15 ±1.50
7.50 ±0.67
7.10 ±0.75
6.50 ±0.24
6.27 ±0.13
The change in solution acidity, conductivity and temperature are respectively
∆pH=-3.1 ±0.1, ∆σ=+1773 ±106 µS/cm and ∆T/T0=+0.1 ±0.1. The production of
carboxylic acids is evidently shown by both the drastic pH decrease and conductivity
increase. The energy dissipation, illustrated by the small solution temperature increase
is very favourable, compared to the necessarily forced cooling that is applied for pulsed
corona discharges in water.
An approach has been made to identify a number of important oxidation products of
phenol, by comparison of the retention times of unknown components in the ionexclusion chromatogram by the retention times of pure possible candidate oxidation
products. The following components have been verified: the polyhydroxybenzenes:
catechol, resorcinol, hydroquinone, pyrogallol and hydroxyhydroquinone; the quinone:
1,4-benzoquinone; the carboxylic acids: succinic acid, maleic acid, malonic acid,
propionic acid, oxalic acid, acrylic acid, acetic acid and formic acid. Figure 4.24 shows
a representative ICE chromatogram, recorded after 3 hours of oxidation time.
71
0
Figure 4.24
20.48
21.27
14.99
15.67
3.43
4.19
4.81
5.47
5.94
6.88
7.14
8.50
9.33
9.72
10.23
31.86
Absorbance (arb.u.)
3.24
Results
10
20
Retention time (min)
30
A representative ion-exclusion chromatogram of a 500 ml 1.0 mM
(94 mg/l) phenol solution, oxidized for three hours. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The UV
absorbance detector is set at 210 nm.
Table 4.6 shows a survey of retention times. The retention times observed from the
oxidation product mixture are average values, recorded in threefold after 2 h, 2.5 h and
3 hours of oxidation time. Also shown are pure candidate oxidation products.
Table 4.6
Retention times observed in the chromatograms of oxidized phenol
solutions and retention times of pure candidate oxidation products;
average values and 95% confidence intervals are presented. Unidentified
peaks are labelled with a question mark.
Oxidation product mixture
peak no
tR (min)
1
3.24 ±0.00
2
3.45 ±0.01
3
4.20 ±0.00
4
4.81 ±0.00
5
5.47 ±0.01
6
5.94 ±0.00
7
6.88 ±0.01
8
7.15 ±0.01
9
8.49 ±0.01
10
9.34 ±0.01
11
9.72 ±0.01
12
10.23 ±0.01
13
14.94 ±0.03
14
15.72 ±0.03
15
20.53 ±0.04
16
21.31 ±0.04
17
31.88 ±0.03
tR (min)
3.61
4.75
5.99
7.21
8.04, 8.69
10.25
14.36
16.22
20.49
21.26
31.86
Pure compound
name
nitrate, hydrogen peroxide
(as peak 1), oxalic acid
?
maleic acid
?
malonic acid
?
succinic acid
formic acid, acetic acid
?
?
propionic acid
pyrogallol
hydroxyhydroquinone
hydroquinone / 1,4-benzoquinone
catechol / resorcinol
phenol
72
Chapter 4.
The first peak in the ion-exclusion chromatogram represents a component, which leaves
the column without retention. The peak may also represent different co-eluting
unretarded components. It may be assumed that this peak represents negative ions,
because the ion-exclusion column excludes these ions, see section 4.1.4. An
asymmetric second peak directly follows the first peak.
The candidate anion is nitrate (NO3-) that is formed in aqueous solution by nitrogen
oxides (NOx), produced by corona discharges in air. It may also be possible, that this
peak is caused by hydrogen peroxide or dissolved ozone. An experiment has been
performed, in which an oxidized phenol solution has been purged by helium. The peak
area of the first peak remains unchanged by the helium purge; thus ozone cannot
explain this peak, because it has been removed by the purge. However, by continuation
of the corona discharges in helium, the peak area of the first peak increases. Hydrogen
peroxide might thus also explain the existence of this peak, because it is produced from
hydroxyl radicals, which are formed by the dissociation of water molecules. The ionexclusion chromatogram of a diluted hydrogen peroxide solution reveals the same
typical set of two adjacent peaks, as observed in the chromatogram of oxidized phenol
solutions.
The asymmetric second peak may also represent oxalic acid, the strongest carboxylic
acid present in the phenol oxidation product mixture: its first dissociation constant is
pKa,I=1.23 [80].
It has been observed that the elution regions of the carboxylic acids and the different
polyhydroxybenzenes are distinct. With regard to the used setup and conditions, the
carboxylic acids elute between 3-10 minutes, the trihydroxybenzenes between 14-17
minutes, the dihydroxybenzenes between 20-22 minutes and monohydroxybenzene
phenol at about 32 minutes.
However, the identity of every single peak cannot be completely guaranteed without
the definite proof of mass spectrometry. As yet, the production and conversion of the
oxidation products have been reported by plotting the total peak area of the mentioned
product classes as a function of the oxidation time, see Figures 4.25 and 4.26.
1.5E+06
UV absorbance (counts·s)
UV absorbance (counts·s)
3.0E+06
2.0E+06
1.0E+06
1.0E+06
5.0E+05
0.0E+00
0.0E+00
0
30
60
90
120
Time (min)
Figure 4.25
150
180
0
30
60
90
120
150
180
Time (min)
The production of dihydroxybenzenes (left) and trihydroxybenzenes (right)
as a function of time during the oxidation of a 500 ml 1.0 mM (94 mg/l)
phenol solution. The corona parameters are V=25 kV, C=1 nF, f=100
Hz, d=1.0 cm.
Results
73
The dihydroxybenzenes catechol, resorcinol and hydroquinone seem to reach a
maximum concentration after about 75 minutes. They may be transformed into
trihydroxybenzenes, quinones or may undergo ring-cleavage by oxygen or ozone attack.
The trihydroxybenzenes pyrogallol/hydroxyhydroquinone will definitely undergo ringcleavage, because they are stronger reducing agents than the dihydroxybenzenes. The
trihydroxybenzene phloroglucinol (1,3,5-THB) is an exception, because it also reacts in
a tautomeric keto-form, see section 2.5.3 Figure 2.21. By ring-cleavage of the
hydroxybenzenes, a large variety of carboxylic acids is produced.
UV absorbance (counts·s)
1.5E+07
1.0E+07
5.0E+06
0.0E+00
0
30
60
90
120
150
180
Time (min)
Figure 4.26
The production of carboxylic acids as a function of the time during the
oxidation of a 500 ml 1.0 mM (94 mg/l) phenol solution. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
From corona experiments in helium, also phenol conversion has been reported. A
comparison of conversion values by corona in helium and air is reported in threefold
after 30 minutes and 60 minutes of oxidation time, see Table 4.7.
Table 4.7
Phenol conversion (X) by corona in helium and air. Oxidation of a 500 ml
1.0 mM (94 mg/l) phenol solution; presented are average values and 95%
confidence intervals. The parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm.
Time (min)
0
30
60
X (%) helium
0
9.6 ±6.4
13.4 ±3.4
X (%) air
0
9.0 ±2.1
17.5 ±2.5
The conversion by corona discharges in helium cannot be caused by ozone, because
oxygen has been removed from the reactor by the helium purge. Therefore it is likely,
that hydroxyl radicals are produced by helium ions/metastables bombardment of water
molecules.
74
Chapter 4.
Quantitative ion-exclusion chromatography
Six major phenol oxidation products have been selected for quantitative analysis i.e.
hydroquinone, hydroxyhydroquinone, formic acid, oxalic acid, glyoxylic acid and
glyoxal. The analysis has been performed using the ion-exclusion column and a series
connection of a UV absorbance and conductivity detector, according to LC
configuration 6. The UV absorbance detector has been applied to detect the
hydroxybenzenes, while the conductivity detector has been utilized for detection of the
carboxylic acids.
Pulsed corona discharges have been applied in both an air and argon gas phase over
100 ml 1.0⋅10-3 M phenol solutions, see reactor configuration 3b. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. By application of the corona
in both air and argon, important aspects of the degradation mechanism can be resolved.
The question is, whether either the hydroxyl radical or ozone accounts for degradation,
or both oxidizer species contribute together to the chemical conversion of phenol in
aqueous solution. The concentration of phenol and the selected oxidation products has
been measured as a function of the oxidation time [105].
The calibration lines of the standard solutions are shown by Figures 4.27 and 4.28. It
has been observed that the calibration lines of phenol, hydroquinone and formic acid
have high correlation coefficients (r2). The standard series of oxalic acid and glyoxylic
acid show moderate correlation. The correlation of the hydroxyhydroquinone standards
is poor and the glyoxal data show no correlation.
20000
200 nm
8000
270 nm
y = 1E+07x + 58.69
R2 = 0.9996
4000
y = 2E+06x + 15.101
R2 = 0.9994
Absorbance (mAUs)
Absorbance (mAUs)
12000
15000
5000
8.0E-04
0.0E+00
1.2E-03
200 nm
Absorbance (mAUs)
Absorbance (mAUs)
8.0E-04
1.2E-03
25
270 nm
3000
2000
1000
200 nm
20
210 nm
15
10
5
0
0
4.0E-04
8.0E-04
[Hydroxyhydroquinone] (mol/l)
Figure 4.27
4.0E-04
[Hydroquinone] (mol/l)
5000
0.0E+00
y = 1E+06x + 11.995
R2 = 0.9984
0
4.0E-04
[phenol] (mol/l)
4000
270 nm
10000
0
0.0E+00
y = 2E+07x + 244.39
R2 = 0.9978
200 nm
1.2E-03
0.0E+00
4.0E-04
8.0E-04
1.2E-03
[Glyoxal] (mol/l)
UV absorbance detector calibration lines of phenol, hydroquinone,
hydroxyhydroquinone and glyoxal. Indicated are UV absorbance detector
wavelengths.
Results
75
6.0E+05
4.0E+05
y = 6E+08x - 3180.4
2
R = 0.9983
2.0E+05
8.0E+05
0.0E+00
0.0E+00
4.0E-04
8.0E-04
1.2E-03
[Formic acid] (mol/l)
Conductivity (arb.u.)
6.0E+05
y = 5E+08x - 13546
R2 = 0.9625
Conductivity (arb.u.)
Conductivity (arb.u.)
8.0E+05
6.0E+05
y = 8E+08x - 91872
R2 = 0.8966
4.0E+05
2.0E+05
0.0E+00
0.0E+00
4.0E+05
4.0E-04
8.0E-04
1.2E-03
[Oxalic acid] (mol/l)
2.0E+05
0.0E+00
0.0E+00
4.0E-04
8.0E-04
1.2E-03
Glyoxylic acid (mol/l)
Figure 4.28
Conductivity detector calibration lines of formic acid, oxalic acid and
glyoxylic acid.
The correlation quality of the standard is dependent on the chemical stability of the
standard compound. Hydroxyhydroquinone is susceptible to oxidation, even by oxygen
present in water. Glyoxal hydrolyzes and consequently exists in several oligomerized
forms in aqueous solution [106]. Due to the large number of standards and samples in
combination with long oxidation and analysis times, ageing of the samples is
unavoidable. Nevertheless, ageing has been minimized by storage of both the standards
and samples before analysis in dark vials under nitrogen at 0°C.
Characteristic UV absorbance and conductivity chromatograms, obtained from phenol
solutions oxidized by corona in air and argon during 2 hours, are shown by Figures 4.29
and 4.30. It has been observed, that the oxidized solutions have different colors: the
solution oxidized by corona in air is pale yellow, while the solution oxidized by corona in
argon is pale beige.
The observed retention times of the standards are shown by Table 4.8. Both the
hydroxybenzenes and glyoxal do not exist in ionic form in aqueous solution, therefore
these compounds are not detectable by the conductivity detector. The time lag
between the signals of conductivity and UV absorbance detector is caused by the
intermediate suppressor.
76
Chapter 4.
Figure 4.29
UV absorbance chromatograms of 100 ml 1.0 mM phenol solutions,
oxidized by corona in air (left) and argon (right) during 2 hours. The
corona parameters are: V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Indicated are UV absorption wavelengths.
Figure 4.30
Conductivity chromatograms of 100 ml 1.0 mM phenol solutions, oxidized
by corona in air and argon during 2 hours. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Table 4.8
Absorbance (UV) and conductivity (CON) detector average retention times
of the standard compounds. Also shown are 95% confidence intervals.
UV tR
3.18
4.84
7.26
10.03
17.25
20.83
32.30
(min)
±0.09
±0.01
±0.01
±0.01
±0.01
±0.03
±0.07
CON
3.44
5.07
7.49
tR (min)
±0.06
±0.02
±0.01
-
Standard compound
Oxalic acid
Glyoxylic acid
Formic acid
Glyoxal
Hydroxyhydroquinone
Hydroquinone
Phenol
Results
77
The conversion of phenol in aqueous solution by pulsed corona discharges in both an air
and argon atmosphere is shown by Figure 4.31. The conversion by corona in argon
appears to be higher than the conversion in air. After 2 hours, the corona in air has
converted 59% of the initial present phenol amount, while the corona in argon has
converted 88%. The conversion seems to obey first order kinetics: the rate contants
are k1air ≈8.2⋅10-3 min-1 and k1argon ≈1.8⋅10-2 min-1.
Conversion (%)
100
200
270
200
270
80
60
nm
nm
nm
nm
air
air
Ar
Ar
40
20
0
0
30
60
90
120
Time (min)
Phenol conversion during oxidation of 100 ml 1.0 mM phenol solutions by
corona in air and argon. The corona parameters are V=25 kV, C=1 nF,
f=100 Hz, d=1.0 cm. Indicated are UV absorbance wavelengths.
Figure 4.31
The DHB hydroquinone elutes just before its isomers resorcinol (1,3-DHB) and catechol
(1,2-DHB), which cannot be separated by the ion-exclusion column, see Figure 4.29.
An estimation of the order of magnitude of the total DHB concentration has been
performed by relating the total DHB peak area to the calibration line of hydroquinone,
assuming similar extinction coefficients for the three DHB isomers. Figures 4.32 and
4.33 show the hydroquinone concentration and the total DHB concentration versus the
oxidation time, respectively. During the oxidation the maximum hydroquinone
concentration is 8.4⋅10-6 M in air and 3.1⋅10-5 M in argon, after about 45 minutes.
[Hydroquinone] (mol/l)
5.0E-05
270 nm air
270 nm argon
4.0E-05
3.0E-05
2.0E-05
1.0E-05
0.0E+00
0
30
60
90
120
Time (min)
Figure 4.32
The production of hydroquinone during the oxidation of 100 ml 1.0 mM
phenol solutions by corona in air and argon. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
78
Chapter 4.
The total DHB concentration reaches a maximum value of 1.3⋅10-4 M in air and
3.4 ⋅10-4 M in argon after about 60 minutes. It is remarkable, that the DHB amounts
produced by corona in argon are considerably higher than the DHB amounts produced
by corona in air.
5.0E-04
270 nm air
[DHB] (mol/l)
4.0E-04
270 nm argon
3.0E-04
2.0E-04
1.0E-04
0.0E+00
0
30
60
90
120
Time (min)
An estimation of the production of total dihydroxybenzenes during the
oxidation of 100 ml 1.0 mM phenol solutions by corona in air and argon.
The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Figure 4.33
The THB hydroxyhydroquinone has not been identified in the phenol oxidation product
mixture. A possible explanation for the absence of this compound is the limited stability
of this strong reducing agent. Nevertheless, it has been observed by ICE analysis
immediately after corona treatment in air, see Figure 4.24. With regard to the ringcleavage products of phenol, the following observations have been made. The
production of formic acid by corona in air is much higher than the production by corona
in argon. After 2 hours of corona discharges, a concentration of 2.3⋅10-4 M is reached
by corona in air and a concentration of 8.9⋅10-5 M is reached by corona in argon. For
the observed time span, the production rate increases almost linearly with time, see
Figure 4.34.
[Formic acid] (mol/l)
2.5E-04
CON air
CON argon
2.0E-04
1.5E-04
1.0E-04
5.0E-05
0.0E+00
0
30
60
90
120
Time (min)
Figure 4.34
The production of formic acid during the oxidation of 100 ml 1.0 mM
phenol solutions by corona in air and argon. The corona parameters are:
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Results
79
The estimation of the oxalic acid concentration involves an error margin, because oxalic
acid appears to co-elute with a component leaving the column unretarded, as can be
observed from Figures 4.29 and 4.30. This component is likely to be the nitrate ion,
according to the discussion in section 4.1.4. Oxalic acid elutes rapidly, because it is the
strongest organic acid observed in the phenol oxidation product mixture. Oxalic acid has
only been detected during phenol oxidation by corona in air: after two hours of
oxidation a concentration of about 3.9⋅10-4 M has been reached. Figure 4.35 shows the
production of oxalic acid versus the oxidation time for corona discharges in air.
[Oxalic acid] (mol/l)
5.0E-04
CON air
4.0E-04
3.0E-04
2.0E-04
1.0E-04
0.0E+00
0
30
60
90
120
Time (min)
The production of oxalic acid during the oxidation of a 100 ml 1.0 mM
phenol solution by corona in air. Oxalic acid has not been detected during
oxidation by corona in argon. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
Figure 4.35
The dialdehyde glyoxal has not been found in traceable amounts. Glyoxylic acid, an
oxidation product of glyoxal, appears in small amounts in both the phenol solution
oxidized by corona in air viz. 4⋅10-5 M to 5⋅10-5 M and the phenol solution oxidized by
corona in argon viz. 3⋅10-5 M, see Figure 4.36.
[Glyoxylic acid] (mol/l)
6.0E-05
4.0E-05
2.0E-05
CON air
CON argon
0.0E+00
0
30
60
90
120
Time (min)
Figure 4.36
The production of glyoxylic acid during the oxidation of 100 ml 1.0 mM
phenol solutions by corona in air and argon. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
80
Chapter 4.
A very strong absorption signal has been found in the UV absorbance chromatograms
of the phenol solution oxidized by corona in air, at the retention time tR=13.73 ±0.01
min and only pronounced at the wavelengths λ=255 nm and λ=270 nm. There is a
corresponding conductivity signal at the retention time tR=13.97 ±0.03 min. The
signal appears to be almost absent in the UV absorbance and conduction
chromatograms of the phenol solution oxidized by corona in argon. Figure 4.37 shows
the observed peak areas of both detectors versus the oxidation time.
6.0E+04
255 nm
270 nm
CON
800
4.0E+04
400
2.0E+04
0
0.0E+00
0
Figure 4.37
30
60
90
Time (min)
Conductivity (arb. u)
Absorbance (mAU)
1200
120
The production of an unknown compound during the oxidation of a
100 ml 1.0 mM phenol solution by corona in air. The corona parameters
are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
The identity of this component might be revealed in the following way. The component
appears at the end of the carboxylic acid retention time range (3<tR<15 min) of the
UV absorption chromatogram. It exists in partially dissociated form in aqueous solution,
because it is detectable by the conductivity detector. Its strong UV absorption at
wavelengths λ=255 nm and λ=270 nm indicates the presence of unsaturated carboncarbon bonds. The compound is formed under ring-cleavage conditions.
It is known from literature that the oxidation of phenol, benzene or catechol yields the
ring-cleavage product cis,cis-muconic acid (cis,cis-1,3-butadiene-1,4-dicarboxylic acid)
[11,107,108]. According to the observations, the unknown compound possibly is
cis,cis-muconic acid. Cis,cis-muconic acid has not been introduced in the standard
series, because of its presumed but ambiguous limited stability [7].
Typical pulse voltage, current and power waveforms for corona in air and argon are
shown by Figure 4.38. The corona current in argon is higher than the current in air. This
is explained by the electronegative character of oxygen, that tends to inhibit the corona
discharges in air compared to the discharges in argon.
Results
81
1.0E+04
20
10
I(t)
Icor(t)
0.0E+00
-100
0
100
Voltage (V)
2.0E+04
Current (A)
Voltage (V)
V(t)
300
2.0E+04
20
1.0E+04
10
I(t)
Icor(t)
-100
400
0
corona in air
0
200
300
400
corona in argon
6.0E+05
Power (W)
Power (W)
100
Time (ns)
Time (ns)
6.0E+05
30
V(t)
0.0E+00
0
200
corona in argon
3.0E+04
30
Current (A)
corona in air
3.0E+04
4.0E+05
2.0E+05
4.0E+05
2.0E+05
0.0E+00
0.0E+00
-100
0
100
200
300
400
-100
Time (ns)
0
100
200
300
400
Time (ns)
Typical pulse voltage, current and power waveforms for corona in air (left)
and argon (right), recorded after 1 hour of oxidation. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Figure 4.38
From the pulse energy and conversion, the efficiency of phenol oxidation by corona in
air and argon has been calculated, see Table 4.9 and Figure 4.39.
Table 4.9
Efficiency (G), pulse energy (Ep) and phenol conversion at λ=270 nm (X270)
during oxidation of 100 ml 1.0 mM phenol solutions by corona in air and by
corona in argon. The corona parameters are V=25 kV, C=1 nF,
f=100 Hz, d=1.0 cm.
Time
(min)
15
Ep
(mJ)
6.3
30
45
60
75
90
105
120
6.2
5.8
5.7
5.8
5.4
5.7
5.6
corona in air
X270
G
G
Ep
G ⋅108
(%)
(mol/J) p100eV (g/kWh) (mJ)
14.4
2.60
0.25
8.8
10.2
26.1
2.35
0.23
8.0
10.3
35.0
2.23
0.21
7.5
10.8
42.1
2.01
0.19
6.8
10.4
48.0
1.79
0.17
6.1
10.7
52.6
1.74
0.17
5.9
10.8
56.2
1.48
0.14
5.0
10.9
59.2
1.38
0.13
4.7
11.0
corona in argon
X270
G
G
G ⋅108
(%)
(mol/J) p100eV (g/kWh)
18.8
2.09
0.20
7.1
43.1
2.35
0.23
8.0
50.2
1.72
0.17
5.8
65.5
1.72
0.17
5.8
75.5
1.53
0.15
5.2
82.2
1.36
0.13
4.6
85.8
1.20
0.12
4.1
88.5
1.05
0.10
3.6
82
Chapter 4.
Only minor differences in efficiency appear to exist for phenol conversion by corona in
air and argon. With increasing conversion, less phenol molecules are available for
oxidation and a competition with intermediate oxidation products exists, thus the
efficiency decreases.
G (mol/J)
3.0E-08
2.0E-08
1.0E-08
air
argon
0.0E+00
0
Figure 4.39
20
40
60
80
Conversion X270 (%)
100
Efficiency of phenol conversion during oxidation of 100 ml 1.0 mM phenol
solutions by corona in air and argon. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The conversion has been
measured by a UV absorbance detector set at λ= 270 nm.
Nevertheless, the difference between the phenol oxidation pathways for corona in air
and argon is distinct. This is explained by simplified degradation pathways of phenol by
hydroxyl radicals, oxygen, ozone and argon ion bombardment. A detailed phenol
oxidation mechanism is presented in section 5.3.
The attack of the hydroxyl radical on phenol will initially produce para- and orthodihydroxycyclohexadienyl (DHCHD) radicals, see Figure 4.40. The meta-DHCHD radical
is not considered here, because it is likely to be less stable than the others.
OH
OH
HO
•
HO
phenol
Figure 4.40
OH
•
H
OH
H
para-DHCHD
radical
ortho-DHCHD
radical
The formation of dihydroxycyclohexadienyl (DHCHD) radicals from the
attack of the hydroxyl radical on phenol.
In an air atmosphere, oxygen will attack these radicals to produce dihydroxycyclohexadienylperoxyl (DHCHDP) radicals, see Figure 4.41. The ortho-DHCHDP radical
may produce catechol by elimination of the hydroperoxyl radical (HO2). The paraDHCHDP radical probably yields hydroquinone by dimerization to a tetraoxide followed
by decomposition; hydroperoxyl elimination is not likely here, because of the larger -H
to •OO- distance of the para-DHCHDP radical compared to the ortho-DHCHDP radical.
By action of oxygen, the DHCHDP radicals are eventually converted to endoperoxides.
These very instable intermediates decompose by ring-cleavage.
Results
83
Direct ozone attack on phenol will also invoke ring-cleavage by addition, see Figure
4.42. In this way, a complex mixture of aliphatic unsaturated and saturated C1-C6
hydrocarbons will be produced, having polyfunctional groups like carboxyl, aldehyde,
ketone or hydroxyl groups. Glyoxal, glyoxylic acid and oxalic acid may result from
multifold attack of ozone on phenol. Formic acid may be produced by carbon monoxide
loss from glyoxylic acid or by decarboxylation of oxalic acid.
OH
hydroquinone
OH
T
OH
HO
OO •
HO
H
O2
•
O2
HO
H
OH
HO
H
O2
OH
•
O
O
DHCHDP
DHCHD
OO •
H
OH
OH
O2
COOH, CHO, CO, OH
OH
OO•
C1-C6 ring cleavage products
endo
peroxides
- HO2
OH
OH
catechol
Figure 4.41
A simplified mechanism of the oxidation of DHCHD and DHCHDP radicals
to endoperoxides, followed by aromatic ring-cleavage.
OH
OH
O
O
OH
O
O3
molozonides
phenol
O
O3
OH
Figure 4.42
ozonides
OH
O
O
O
O
O
zwitterions
O
O
HO
H
+
C O O H2O
HO
C H
O
O
H
OOH
HO
OH
-H2O
H
O
hydroperoxy
alkanols
A simplified mechanism of phenol oxidation by ozone.
O
OH
H
O
unsaturated
polyfunctional
aliphatic hydrocarbons
84
Chapter 4.
On the contrary, when pulsed corona discharges take place in an argon atmosphere
over an oxygen-free phenol solution, the argon ions/metastables created by the corona
discharges will dissociate water molecules to produce hydroxyl radicals (and hydrogen
atoms) but no ozone can be formed. Hydroxylation of phenol will be the main
degradation pathway and hydroxybenzenes will be found in much higher amounts than
for the case of corona oxidation in air, see Figure 4.43.
OH
HO
•
HO
H
HO
HO
OH
-H2O
H
hydroquinone
H
HO
DHCHD
OH
isomerization
OH
•
OH
O
H
OH
Figure 4.43
HO
HO
O
OH
H
OH
-H2O
OH
H
OH
OH
catechol
A simplified mechanism of phenol oxidation by hydroxyl radicals under
oxygen-free conditions.
The fact, that still some ring-cleavage products are found during corona oxidation in
argon, can be explained by ring fragmentation by the argon ions/metastables
bombardment. Ring-cleavage also takes place by small amounts of oxygen, that cannot
be removed by argon purging. The differences in amounts of polyhydroxybenzenes
bring about the color difference between the oxidized solutions.
As can be derived from both the chromatograms and a carbon mass balance, a certain
number of unknown products remains, whose identity is difficult to resolve. They are
likely to be carboxylic acids, because the conductivity detector is able to observe these
partially ionic compounds. Candidates are C1-C6 mono- and dicarboxylic acids with
alkanol-, aldehyde- or ketone-functional groups.
Finally, by comparison of the conductivity chromatograms of oxidized phenol solutions
and deionized water exposed to corona in air (section 4.1.4), the following observation
has been made: the conductivity signal at tR=8.73 min from deionized water exposed
to corona in air is present in the chromatogram of the phenol solution oxidized by
corona in argon, but is absent in the chromatogram of the air-oxidized phenol solution.
This might be explained by anode metal sputtering differences between corona in argon
and air. Also it should be considered, that the regarding peak may be due to an
oxidation product with equal retention time.
Results
85
Gas chromatography
By oxidation of phenol in aqueous solution, the following oxidation products may be
introduced in the gas phase over the oxidized solution: carbon oxides, unsaturated
hydrocarbons and volatile aldehydes. A preliminary direct gas chromatography analysis
of a helium flow reactor purge did not reveal any information. Therefore a specific
procedure has been chosen according to NIOSH for screening the presence of
aldehydes. The identification of other candidate oxidation products has been performed
by infrared spectroscopy and is discussed in section 4.2.3.
The aldehyde screening test involves a chemical derivatization reaction of aldehydes.
Gas sampling tubes containing the derivatization agent HMP are exposed to an argon
5.0 purge, originating from the reactor.
A 500 ml 1.0 mM phenol solution has been oxidized for 3 hours, see reactor
configuration 7. The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Before oxidation, the reactor and phenol solution are purged with argon 5.0 and the
purge gas is directed through a gas sampling tube; this is the background sample. Due
to this purge the corona discharges take place in argon 5.0. After the oxidation the
reactor is purged again and the collected purge gas is directed through a different fresh
tube; this is the corona sample. The background and corona sample tube contents are
extracted with toluene and the extract is analyzed by gas chromatography-mass
spectrometry, see GC configuration 1. The obtained chromatograms are shown by
Figure 4.44.
FID signal (arb.u. )
4.0E+06
corona
sample
3.0E+06
2.0E+06
1.0E+06
background
sample
0.0E+00
130
150
170
190
210
230
Retention time (s)
Figure 4.44
GC chromatograms of toluene-extracted sampling tube contents. The
tubes have been exposed to an argon 5.0 purge originating from the
reactor, before oxidation (background sample) and after oxidation (corona
sample). The reactor contains 500 ml 1.0 mM phenol solution. The
corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
86
Chapter 4.
The chromatograms reveal non-derivatized HMP present in both samples, eluting
between about 170 and 210 seconds. HMP-aldehyde derivatization products should
occur just before and after the elution of non-derivatized HMP. Although many small
peaks are visible, there appear to be no differences between the corona sample and the
background sample. All peaks below 145 seconds definitely originate from the
extraction solvent, viz. toluene together with xylene impurities.
It is concluded that no volatile aldehydes have been detected during oxidation of a
phenol solution by corona in argon, within the lower detection limit of this procedure.
This limit is not specified, because this test is an overall screening technique. A specific
acetaldehyde test (method 2538) using the same derivatization technique but different
parameters has a lower detection limit of 0.74 ppm ≡ 1.3 mg/m3.
Summary
The conversion of phenol in aqueous solution by pulsed corona discharges increases by
increasing the corona load voltage, corona pulse repetition rate and solution alkalinity.
The location of the discharges is best at some distance from the liquid-gas interface.
Ion-exclusion chromatography has proven to be considerably more powerful for
separation of the complex phenol oxidation product mixture than reversed-phase HPLC.
Identified phenol oxidation products are di- and trihydroxybenzenes, mono- and
dicarboxylic acids. The phenol oxidation pathways strongly depend on the composition
of the gas phase, where the corona discharges are produced. Corona in argon or helium
also invokes oxidation by the formation of hydroxyl radicals due to ions/metastables
bombardment of water. The maximum obtained phenol conversion efficiency is about
G=2.7⋅10-8 mol/J ≡ 0.26 (100eV)-1 ≡ 9.1 g/kWh at X=9.5% conversion by corona in
air. According to an aldehyde screening test, no volatile aldehydes have been detected
during oxidation of a phenol solution by corona in argon.
Results
87
4.2.2. Mass spectrometry
The first attempts to identify phenol oxidation products have been performed by liquid
chromatography coupled mass spectrometry (LC-MS) analyses on samples obtained by
Solid Phase Extraction (SPE), see MS configuration 1. Phenol degradation by pulsed
corona discharges involves oxidation by highly reactive, thus non-specific, hydroxyl
radicals. This means, that a wide variety of oxidation products is formed. However, the
oxidation of low content phenol solutions, according to the scope of this thesis, implies
that the produced amounts of oxidation products are very low. The oxidation of high
content phenol solutions may produce a different oxidation product mixture as a result
of polymerization. Also these solutions require a long treatment time to achieve
reasonable conversion. Pre-concentration of the oxidation product mixture of low
content phenol solutions by SPE is attractive. Unfortunately, LC-MS analyses of SPEprocessed oxidation product samples have not revealed the identity of any of the
components. Also it has been observed, that certain oxidation products will be lost to
the SPE matrix. This has been concluded from chromatograms taken from an oxidized
phenol solution before and after SPE processing. Therefore, SPE has not been applied to
subsequent analyses of phenol oxidation product mixtures despite its advantages.
Different LC-MS approaches have been made, but all of them have been performed offline i.e. without ICE column. The acidic eluent, required for separation according to the
ion-exclusion mechanism, causes a high background noise that complicates the analysis
of the weak signals of the oxidation product components. Also the spraying of the
aqueous eluent has appeared to be problematic, due to the high surface tension of
water. Only sparingly results have been achieved by application of negative IonSpray of
a phenol oxidation product sample in a 100% acetonitrile flow. Prior to the MS analysis
0.1 v/v % concentrated ammonia has been added to the sample, to promote the ion
formation by converting carboxylic acids and phenols into their anionic form. The
sample has been prepared by oxidation of a 500 ml 1.0 mM (94 mg/l) phenol solution
for 3 hours, see reactor configuration 6c. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm. With regard to the interpretation of the mass-spectrum
shown by Figure 4.45, it should be remarked that the indicated masses are equal to
Mass-1, because [M-H]- ions are produced.
69
Intensity (arb.u.)
6.0E+05
62
113
4.0E+05
93 115
97
175
89 99 141
2.0E+05
0.0E+00
0
Figure 4.45
50
100
150
200
m/z (amu)
250
300
IonSpray mass spectrum of a 500 ml 1.0 mM phenol solution, oxidized
during 3 hours. The corona parameters are V=25 kV, C=1 nF,
f=100 Hz, d=1.0 cm.
88
Chapter 4.
An extensive list of probable phenol oxidation products has been mainly derived from
literature references (section 2.4.1), see Table 4.10. This list is used to identify the
observed masses (FW). m/z=62 might be a (CO2)(H2O) cluster. m/z=69 has not been
identified. If the peak at m/z=89 is no noise peak, it may be due to singly deprotonated
oxalic acid or deprotonated lactic acid (90). m/z=93 is due to phenol (FW=94 g/mol,
76.6%C, 6.4%H, 17.0%O). m/z=97 and 99 have not been identified. m/z=113 is
likely to be trifluoroacetic acid (114), that has been left behind in the IonSpray
compartment from previous attempts. m/z=115 may originate from singly
deprotonated maleic/fumaric acid or deprotonated dioxobutyric acid (116). m/z=141 is
probably a singly deprotonated muconic acid enantiomer (142). The peak at m/z=175
may contain tetrahydroxybenzoquinone (172). Higher masses are likely to be
polymerized benzoquinones/hydroquinones [34], although polymerization is not likely to
be important during the oxidation of 1 mM phenol solutions.
Other attempts have included the application of APCI mass spectrometry in combination
with the TFA eluent and addition of acetonitrile to favour the sprayability. Also the
sample preparatory options i.e. fractional collection and freeze drying have been
applied. No spectra could be obtained from these experiments. There are several
reasons for the fact, that identification by LC-MS has been problematic. Although the
range of oxidation products is broad, the individual component concentration is low.
Negative IonSpray, especially suitable for ionizing phenols and carboxylic acids is less
sensitive than positive ion spray. The available interfaces IonSpray and APCI appear to
be unsuitable for analyzing masses below 200 amu.
Table 4.10a A list of possible phenol oxidation products, sorted by increasing
molecular weight FW (g/mol). FW<FWphenol. Also shown are abbreviated
compound structure and w/w-% carbon, hydrogen and oxygen.
Compound name
Acetylene
Carbon monoxide
Ethylene
Formaldehyde
Carbon dioxide
Acetaldehyde
Formic acid
Butadiene
Glyoxal
Acetic acid
Acrylic acid
Malonaldehyde
Glyoxylic acid
Propionic acid
Maleic aldehyde
Pyruvic acid
Butyric acid
Oxalic acid
Lactic acid
Compound structure
CH≡CH
CO
CH2=CH2
HCHO
CO2
CH3-CHO
HCOOH
CH2=CH-CH=CH2
HC(O)-CHO
CH3-COOH
CH2=C(H)COOH
HC(O)-CH2-CHO
HC(O)-COOH
C2H5-COOH
HC(O)-CH=CH-CHO
CH3-C(O)-COOH
C3H7-COOH
HOOC-COOH
CH3-CH(OH)-COOH
%C
92.3
42.9
85.7
40.0
27.3
54.5
26.1
88.9
41.4
40.0
50.0
50.0
32.4
48.6
57.1
40.9
54.5
26.7
40.0
%H
7.7
0.0
14.3
6.7
0.0
9.1
4.3
11.1
3.4
6.7
5.6
5.6
2.7
8.1
4.8
4.5
9.1
2.2
6.7
%O
0
57.1
0
53.3
72.7
36.4
69.6
0
55.2
53.3
44.4
44.4
64.9
43.2
38.1
54.5
36.4
71.1
53.3
g/mol
26
28
28
30
44
44
46
54
58
60
72
72
74
74
84
88
88
90
90
Results
89
Table 4.10b A list of possible phenol oxidation products, sorted by increasing
molecular weight FW (g/mol). FW> FWphenol. Also shown are abbreviated
compound structure and w/w-% carbon, hydrogen and oxygen.
Compound name
Oxobutyric acid
Valeric acid
Malonic acid
o,p-Benzoquinone
Muconaldehyde
Dihydroxybenzenes
Cyclohexadienediol
Cyclohexanedione
Dioxosuccinic aldehyde
Maleic / fumaric acid
Dioxobutyric acid
Caproic acid
Ketomalonic acid
Succinic acid
Hydroxybenzoquinone
Trihydroxybenzenes
Oxalacetic acid
Glutaric acid
Malic acid
Dihydroxybenzoquinones
Muconic acid
Dioxosuccinic acid
Adipic acid
Tartaric acid
Trihydroxybenzoquinone
Dibenzofuran
Tetrahydroxybenzoquinone
Dibenzo-p-dioxin
Dihydroxybiphenyl
Dimer BQ-BQ
Dimer HQ-BQ
Purpurogallin
Compound structure
HC(O)-(CH2)2-COOH
CH3-(CH2)3-COOH
HOOC-CH2-COOH
O=C6H4=O
HC(O)-CH=CH-CH=CH-CHO
C6H4(OH)2
HO(H)-C6H4-(H)OH
C6H8(O)2
HC(O)-C(O)-C(O)-CHO
HOOC-CH=CH-COOH
HC(O)-C(O)-CH2-COOH
CH3-(CH2)4-COOH
HOOC-C(O)-COOH
HOOC-(CH2)2-COOH
O=C6H3(OH)=O
C6H3(OH)3
HOOC-CH2-C(O)-COOH
HOOC-(CH2)3-COOH
HOOC-CH2-CH(OH)-COOH
C6H2(OH)2(O)2
HOOC-CH=CH-CH=CH-COOH
HOOC-C(O)-C(O)-COOH
HOOC-(CH2)4-COOH
HOOC-CH(OH)-CH(OH)-COOH
C6H(O)2(OH)3
C6H4(O)C6H4
C6(O)2(OH)4
C6H4 (O,O)C6H4
HOC6H4-C6H4OH
C6H3(O)2-C6H3(O)2
C6H3(O)2-C6H3(OH)2
C6H(OH)3C5H3(OH)(O)
%C
47.1
58.8
34.6
66.7
65.5
65.5
64.3
64.3
42.1
41.4
41.4
62.1
30.5
40.7
58.1
57.1
36.4
45.5
35.8
51.4
50.7
32.9
49.3
32.0
46.2
85.7
41.9
78.3
77.4
67.3
66.7
60.0
%H
5.9
9.8
3.8
3.7
5.5
5.5
7.1
7.1
1.8
3.4
3.4
10.3
1.7
5.1
3.2
4.8
3.0
6.1
4.5
2.9
4.2
1.4
6.8
4.0
2.6
4.8
2.3
4.3
5.4
2.8
3.7
3.6
%O
47.1
31.4
61.5
29.6
29.1
29.1
28.6
28.6
56.1
55.2
55.2
27.6
67.8
54.2
38.7
38.1
60.6
48.5
59.7
45.7
45.1
65.8
43.8
64.0
51.3
9.5
55.8
17.4
17.2
29.9
29.6
36.4
g/mol
102
102
104
108
110
110
112
112
114
116
116
116
118
118
124
126
132
132
134
140
142
146
146
150
156
168
172
184
186
214
216
220
With regard to hydroxylated phenols and C1-C6 ring-cleavage products, the phenol
oxidation product mixture exhibits a mass range of about 30 amu (formaldehyde) till
172 amu (tetrahydroxybenzoquinone).
Next to off-line LC-MS, mass analyses have been performed by electron-impact mass
spectrometry (EI-MS), see MS configuration 2. In order to have sufficient detector
signal from the oxidation products, a 100 ml 5.0⋅10-1 M (concentrated) phenol solution
has been oxidized for 3 hours, see reactor configuration 3b. The corona parameters are
V=30 kV, C=1 nF, f=300 Hz, d=1.0 cm. By oxidation the initial colorless phenol
solution turns deep brown. Mass spectra have been recorded for the mass range 1-500
amu, see Figure 4.46.
90
Figure 4.46
Chapter 4.
EI-mass spectra of a phenol oxidation product mixture, obtained by
oxidation of a 100 ml 0.5 M phenol solution by corona in air. The corona
parameters are: V=30 kV, C=1 nF, f=300 Hz, d=1.0 cm.
The most evident masses found are m/z=94 (phenol) and m/z=95 (protonated phenol).
Protonation is caused by the ionization of water molecules by the 100 eV electron
beam. m/z=110 may originate from the hydroxybenzene isomers, but muconic
aldehyde is also possible. m/z=105 might be protonated malonic acid. m/z=106
cannot be identified. The observed masses below 94 amu may contain phenol oxidation
products, but are mainly due to EI-fragmentation of phenol.
Results
91
The mass analyses from 110-500 amu sometimes reveal traces of higher molecular
weight species, but these results are not reproducible when the detector integration
time is increased. This is remarkable, because the deep brown color of the oxidized
solution clearly implies the presence of polymerized benzoquinones/hydroquinones i.e.
synthetic humic acids [34]. It is possible that the observed suspended polymeric
particles cannot enter the mass spectrometer by means of the applied capillary tubing.
Summary
Several IonSpray/APCI-MS and EI-MS measurements have not confirmed the presence
of both theoretically possible and literature-reported oxidation products, in spite of the
fact that significant phenol conversion has been demonstrated. This has limited the
quantitative measurements, because the identification of compounds from complex ICE
chromatograms by comparison of retention times is problematic. As main reason for the
negative results can be mentioned that, although the product range is broad, the
concentration of individual oxidation products is low.
92
Chapter 4.
4.2.3. Spectroscopy
Laser-Induced Fluorescence spectroscopy
Next to chromatography, the conversion of phenol has also been measured by LaserInduced Fluorescence (LIF) spectroscopy [109,110]. Regarding the fact, that LIF
literature only regards gas phase studies, the application of LIF for liquid phase analysis
is considered to be a new approach. LIF spectroscopy enables in-situ and time-resolved
measurements, which is very favourable to batch-wise sampling and analysis time
inherent to liquid chromatography. In addition to phenol conversion measurements, the
in-situ monitoring of total hydroxybenzenes fluorescence is an approach to oxidation or
detoxification progress.
A typical LIF spectrum of phenol in aqueous solution at 1.0⋅10-5 M concentration is
shown by Figure 4.47. The excitation is performed by the fourth harmonic wavelength
of a Nd:YAG laser i.e. λ=266 nm. No fine structure of rotational lines can be observed,
because a low resolution grating (150 mm-1) has been applied to scan the broad
wavelength range.
Figure 4.47
A typical LIF spectrum of an 1.0⋅10-5 M aqueous phenol solution, excited
at λ=266 nm.
The fluorescence of phenol excited states in aqueous solution will be quenched by
water, phenol and oxidation product molecules. The measured dependence of the LIF
peak intensity at 298 nm on the phenol concentration is shown by Figure 4.48. The
intensity appears to be linear for the range 1.0⋅10-6 M to 1.0⋅10-5 M.
The extent of quenching by oxidation products has been investigated as follows. The
fluorescence peak intensity of phenol-resorcinol and phenol-pyrogallol mixtures has
been measured at constant total concentration of 1.0⋅10-5 M while the partial
concentrations have been mutually varied, see Figure 4.48. Resorcinol and pyrogallol
are the polyhydroxybenzenes that are both most stable and most fluorescent. By
increasing the concentration of resorcinol or pyrogallol thus decreasing the phenol
concentration, the LIF peak intensity at 298 nm decreases linearly. Collisional
quenching of phenol molecules by the oxidation products resorcinol and pyrogallol thus
appears not to influence linearity at these concentration levels. Also, the tested
concentration range of oxidation products is far higher than actually observed during
phenol oxidation.
Results
93
1.0
LIF signal (arb.u.)
0.8
0.6
0.4
0.2
0.0
0.0
Figure 4.48
phenol
phenol-resorcinol
phenol-pyrogallol
0.2
0.4
0.6
0.8
-5
Phenol concentration (10 mol/l)
1.0
The LIF peak intensity of phenol versus the concentration. Also shown is
the LIF peak intensity of several phenol-resorcinol and phenol-pyrogallol
aqueous mixtures at a total concentration of 1.0⋅10-5 M.
A 100 ml 1.0⋅10-5 M phenol solution has been oxidized for 20 minutes, see reactor
configuration 8b. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm. The LIF spectrum of the phenol solution after 4 minutes of oxidation is
shown by Figure 4.49. Also indicated is the LIF spectrum due to the remaining amount
of phenol after 4 minutes of oxidation. This decomposed spectrum has been derived by
fitting the LIF spectrum of the unoxidized phenol solution at t=0 minutes to the LIF
spectrum of the oxidized solution, over the wavelength range 250-280 nm.
Figure 4.49
The LIF spectrum of a 100 ml 1.0⋅10-5 M phenol solution, oxidized for 4
minutes. Also shown is the decomposed spectrum of non-oxidized phenol.
The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
With regard to Figure 4.48, it is justified to assume that the decomposed phenol
fluorescence intensity is equal to the phenol concentration. Figure 4.50 shows the
phenol concentration versus the oxidation time.
94
Chapter 4.
The phenol concentration seems to decrease according to two different decay time
constants. For the first 10 minutes the decay time constant is about 25 minutes, from
10 to 17 minutes the decay time constant is about 9.8 minutes.
-5
Concentration (10 mol/l)
1
0.1
0
5
10
15
20
Time (min)
Figure 4.50
The phenol concentration versus time during oxidation of a 100 ml
1.0⋅10-5 M phenol solution. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
Next, the oxidation of 300 ml 1.0⋅10-4 M phenol solutions has been studied under
similar conditions, see reactor configuration 8a; the oxidation time is 3 hours. The
effect of the laser on phenol degradation has been determined by measuring the LIF
peak intensity at λ=298 nm of a 300 ml 1.0⋅10-4 M phenol solution versus the laser
exposure time at a laser beam pulse rate of 10 Hz, see Figure 4.51.
LIF signal (arb.u.)
1
0.1
0
50
100
150
200
Time (min)
Figure 4.51
Decrease of the LIF peak intensity of a 300 ml 1.0⋅10-4 M phenol solution
due to degradation by exposure to a Nd:YAG laser beam at wavelength
λ=266 nm and 10 Hz pulse rate.
The degradation effect is substantial for long exposure times. The 266 nm photons
have an energy of 4.7 eV, which is very close to the bond strength of a hydrogen atom
attached to a benzene ring (465 ±3 kJ/mol [24]). Therefore, for all experiments the
exposure time has been kept very short viz. 5 seconds and the laser pulse energy has
been minimized to 0.8-1.1 mJ.
Results
95
The influence of quenching effects with regard to the 1.0⋅10-4 M phenol solution has
been tested for the range 10-6-10-4 M, see Figure 4.52. The LIF intensity is linear to the
phenol concentration for the range 1⋅10-6-4⋅10-5 M. At higher concentrations deviation
from linearity occurs due to absorption of the excitation photons; the phenol data points
fit well to Equation 2.22 where kQ=0, as is shown by the dotted line. Both resorcinol
and pyrogallol quench the fluorescence of phenol excited states significantly; their
degree of quenching is mutually comparable.
1.0
LIF signal (arb.u.)
0.8
0.6
0.4
phenol
phenol-resorcinol
phenol-pyrogallol
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
-4
Phenol concentration (10 mol/l)
Figure 4.52
The LIF peak intensity of phenol versus the concentration. The dotted line
shows the absorption effect of the laser intensity. Also shown is the LIF
intensity of several phenol-resorcinol and phenol-pyrogallol aqueous
mixtures at a total concentration of 1.0⋅10-4 M.
The LIF peak intensity of the 300 ml 1.0⋅10-4 M phenol solution is plotted versus the
oxidation time by Figure 4.53. The solution fluorescence has been decomposed into the
fluorescence of phenol and its oxidation products for the time range 0-150 minutes.
With regard to this plot, no corrections have been performed for either laser absorption
by the solution or quenching due to oxidation products.
1
LIF signal (arb.u.)
Phenol
Oxidation products
0.1
0.01
0
50
100
150
Time (min)
Figure 4.53
The decomposed LIF peak intensity due to phenol and oxidation products,
during oxidation of a 300 ml 1.0⋅10-4 M phenol solution. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
96
Chapter 4.
It is observed, that the oxidation products reach a maximum concentration after about
20 minutes of oxidation. After this time these intermediate products are further
oxidized. The intermediate products comply -next to phenol- with the requirements for
fluorescence and are thus likely to be polyhydroxybenzenes.
Application of the absorbance correction to the decomposed phenol LIF intensity versus
time course according to Equation 2.22, yields the absolute phenol concentration, see
Figure 4.54. The error bars mark the deviation in the concentration due to quenching
effects. Characteristic decay time constants τ apply for three intervals, viz. τ0-50min=19
minutes, τ50-120min=25 minutes and τ120-150min=100 minutes. These different decay rates
originate from the production and consumption of intermediate polyhydroxybenzenes;
after disappearance of phenol non-fluorescent saturated carboxylic acids remain that
eventually mineralize.
-4
Phenol concentration (10 mol/l)
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80
100 120 140 160
Time (min)
Figure 4.54
The absolute phenol concentration versus time during oxidation of a 300
ml 1.0⋅10-4 M phenol solution. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
Figure 4.55 shows the LIF spectra of 1.0⋅10-4 M solutions of the dihydroxybenzenes
and trihydroxybenzenes. Also included is the quinone 1,4-benzoquinone. The LIF
intensity decreases by the following order: phenol > hydroquinone > resorcinol >
catechol > hydroxyhydroquinone > pyrogallol > 1,4-benzoquinone, phloroglucinol.
The trihydroxybenzenes are much weaker fluorescent than the dihydroxybenzenes and
their maximum fluorescence intensity is at about 350 nm. Especially hydroquinone and
hydroxyhydroquinone seem to account for the observed increased LIF intensity at
higher wavelengths.
Results
97
1200
30000
Hydroquinone
Hydroxyhydroquinone
Resorcinol
Pyrogallol
LIF signal (arb.u.)
LIF signal (arb.u.)
40000
Catechol
20000
1,4-Benzoquinone
10000
0
250
300
350
400
450
800
400
0
250
500
Wavelength (nm)
Figure 4.55
Phloroglucinol
300
350
400
450
500
Wavelength (nm)
LIF spectra of 1.0⋅10 M dihydroxybenzene (left) and trihydroxybenzene
(right) solutions.
-4
air t=40 min
1
LIF signal (arb.u.)
LIF signal (arb.u.)
Figure 4.56 shows LIF spectra due to the phenol oxidation products, recorded after
40 minutes and 100 minutes of exposure time for corona in air and argon. These
spectra have been derived by subtraction of the decomposed phenol spectrum from the
observed solution LIF spectrum.
Ar t=40 min
1
Ar t=100 min
air t=100 min
0
250
300
350
400
Wavelength (nm)
Figure 4.56
450
500
0
250
300
350
400
Wavelength (nm)
450
500
LIF spectra due to phenol degradation products, recorded after 40 min and
100 min, during degradation of 1.0⋅10-4 M phenol solutions by pulsed
corona discharges in an air (left) and argon (right) atmosphere. The corona
parameters are V=25 kV, V=1 nF, f=100 Hz, d=1.0 cm.
The LIF spectra of oxidized phenol solutions reveal fluorescence at higher wavelengths
compared to the LIF spectrum of phenol. This is especially the case for phenol degraded
by corona in argon. From the ICE measurements (section 4.2.1) it has been concluded,
that the degradation of phenol by corona in argon produces considerably higher
amounts of dihydroxybenzenes. According to Figure 4.55 the fluorescence at higher
wavelengths is likely due to the dihydroxybenzene hydroquinone. The contribution of
the trihydroxybenzenes to fluorescence is negligible.
98
Chapter 4.
300 ml 1.0⋅10-4 M polyhydroxybenzene solutions and a 1,4-benzoquinone solution have
been oxidized by pulsed corona discharges in air. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm. Figure 4.57 shows the LIF peak intensity of the
solution versus the oxidation time. Peak wavelengths are according to Figure 4.55.
Although no spectrum decomposition has been performed here, the LIF intensity decay
can be related to the disappearance of the hydroxybenzenes by oxidation. The oxidation
stability of the DHB’s decreases by the order resorcinol > 1,4-benzoquinone,
hydroquinone > catechol while the stability of the THB’s decreases according to the
order phloroglucinol > pyrogallol > hydroxyhydroquinone. The mutual stability within
the DHB and THB classes may be explained by the resonance structures and by ring
strain due to sterical hindrance of hydroxyl groups. It is remarkable that the THB
stability is rather similar to the DHB stability, because THB’s are much stronger
reducing agents than DHB’s [61]. This contradiction may arise from the fact that the
oxidation of the hydroxybenzenes in aqueous solution by ozone is mass transfer limited.
0
10
resorcinol
catechol
hydroquinone
1,4-benzoquinone
-1
10
-2
10
0
30
60
Time (min)
Figure 4.57
90
0
pyrogallol
hydroxyhydroquinone
phloroglucinol
LIF signal (arb.u.)
LIF signal (arb.u.)
10
120
10
-1
10
-2
0
30
60
90
120
Time (min)
The solution LIF peak intensity versus time during the oxidation of 300 ml
1.0⋅10-4 M dihydroxybenzene (left) and trihydroxybenzene (right)
solutions. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,
d=1.0 cm.
Summary
Although LIF spectroscopy is generally applied for gas phase studies, it has been
demonstrated that it is also applicable to in-situ conversion measurements of phenol in
aqueous solution at concentrations up to 1.0⋅10-4 M. For that case, the phenol LIF peak
intensity has been obtained from the solution LIF peak intensity by spectrum
decomposition, as a correction with regard to quenching effects. The LIF spectrum due
to the phenol oxidation products indicates the presence of polyhydroxybenzenes, which
is consistent with the results from ICE measurements. The polyhydroxybenzenes are
less resistant to oxidation than phenol, but the decrease of the LIF intensity of DHB’s
and THB’s has appeared to be comparable, although these compounds differ in reducing
strength. This may be due to mass transfer limitation of the oxidation reaction by
ozone.
Results
99
Infrared spectroscopy
corona in air
corona in argon
0.06
0.04
0.02
Absorbance (arb.u.)
In addition to the aldehyde screening test, the gas phase over oxidized phenol solutions
has also been analyzed by Fourier-transform infrared spectroscopy (FTIR). 250 ml
1.0 mM phenol solutions have been oxidized by pulsed corona discharges in both an air
and argon gas phase, see reactor configuration 9. Figure 4.58 shows the FTIR spectra
recorded after three hours of oxidation.
0
3000
2500
2000
-1
Wavenumber (cm )
1500
Figure 4.58
FTIR spectra of the gas phase over 250 ml 1.0 mM phenol solutions,
oxidized by pulsed corona discharges in air or argon during 3 hours. The
corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
The strong absorption at 2349 cm-1 is due to carbon dioxide (anti-symmetric stretch
vibration). Carbon monoxide (2143 cm-1) has not been detected. The very weak
absorption at 2224 cm-1, observed in the spectrum from corona in air, may originate
from traces of nitrous oxide (N2O). Within the scanned wavenumber range, no other
compounds have been identified. Table 4.11 shows carbon dioxide concentrations.
Table 4.11
Time
(min)
0
30
60
120
180
The production of carbon dioxide during oxidation of 250 ml 1.0 mM
phenol solutions by pulsed corona discharges in air or argon. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
[CO2] (ppm v/v)
air
argon
330
0
160
590
230
330
1100
410
The mineralization of phenol by corona in air is faster than the mineralization of phenol
by corona in argon.
100
Chapter 4.
For the case of air the oxidizers ozone, hydroxyl radicals and oxygen can produce
carbon oxides from phenol. In argon, oxidizers can only be produced from the
dissociation of water by argon ions and from trace amounts of oxygen dissolved in the
water.
Although carbon monoxide has not been detected, the production of this compound is
possible from the decomposition of endoperoxides [19] produced by phenol oxidation,
see Figure 4.59. However carbon monoxide is oxidized to carbon dioxide by ozone or
by hydroxyl radicals and oxygen, see Equations 4.2a-d.
OH
HO
O2
HO
O
H
O
O
H
O
OH
phenol
Figure 4.59
OH
H H
O
+
CO
OH
endoperoxide
The production of carbon monoxide from the decomposition of an
endoperoxide produced during the oxidation of phenol by hydroxyl radicals
and oxygen.
CO + O3 → CO2 + O2
CO + HO• → HCO2•
HCO2• + O2 → CO2 + HO2•
HCO2• + HO• → CO2 + H2O
(4.2a)
(4.2b)
(4.2c)
(4.2d)
The production of carbon dioxide has also been measured during the oxidation of
250 ml 0.1 mM phenol solutions, under the same conditions. The observed carbon
dioxide concentrations are shown by Table 4.12 and Figure 4.60.
Table 4.12
Time
(min)
0
30
60
90
120
180
The production of carbon dioxide during oxidation of 250 ml 0.1 mM
phenol solutions by pulsed corona discharges in air or argon. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm
[CO2] (ppm v/v)
air
argon
330
18
467
132
591
173
687
240
787
287
922
350
It is clear, that the carbon dioxide production depends on the initial phenol
concentration. The concentration increase versus the oxidation time is non-linear.
Results
101
[CO2] (ppm v/v)
1000
800
corona in air
600
400
200
corona in argon
0
0
30
60
90
120
150
180
Time (min)
Figure 4.60
The production of carbon dioxide during oxidation of 250 ml 0.1 mM
phenol solutions by pulsed corona discharges in air or argon. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
The theoretical amount of carbon dioxide that can be produced from phenol is
calculated according to Equation 4.3.
C6H5OH + 7O2 → 6CO2 + 3H2O
(4.3)
1 mol phenol can produce a maximum amount of 6 moles carbon dioxide. The molar
gas volume of carbon dioxide is about Vm=24.3 liter at p=p0 and T=293.15 K [111].
The 250 ml 1.0 mM phenol solution contains 2.5⋅10-4 mol phenol that corresponds to
36.5 ml carbon dioxide, while the 0.1 mM phenol solution can produce only one tenth
of this amount. The gas phase volume is also equal to 250 ml. The percentage of
carbon converted by oxidation of both 0.1 mM and 1 mM phenol solutions during 180
minutes in air and argon is shown by Table 4.13.
Table 4.13
Percentage of carbon converted into carbon dioxide during oxidation of
250 ml 0.1 mM and 1.0 mM phenol solutions by corona in air or argon.
The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
t=180 min
air
argon
0.1 mM
922 / 14600 ≡ 6.3 %
350 / 14600 ≡ 2.4 %
1.0 mM
1100 / 146000 ≡ 0.8 %
410 / 146000 ≡ 0.3 %
However, if the solubility of carbon dioxide in water [44] is taken into account viz.
7.07⋅10-4 mol CO2/mol H2O ≡ 3.92⋅10-2 mol/l ≡ 1.72 g/l ≡ 952.0 ml/l at T=293.15 K
and p=p0, the liquid phase may also contain large amounts of dissolved carbon dioxide.
Then the percentage of carbon dioxide converted is much higher. The 1.0 mM phenol
solution is clearly less mineralized than the 0.1 mM solution. The converted carbon ratio
air-to-argon is about 2.7 for both phenol concentrations.
Summary
Only CO2 has been detected in the gas phase over oxidized phenol solutions. The
amounts produced by corona in argon are distinctly lower than the amounts produced
by corona in air. The observed CO2 levels possibly imply a low level of mineralization.
102
Chapter 4.
4.2.4. Electrical conductometry
Electrical conductometry has been applied in order to monitor oxidation progress of
phenol and some of its intermediate oxidation products, by the increase of solution
conductivity due to the production of carboxylic acids.
The electrical conductivity of the hydroxybenzene solutions has been determined as a
function of the oxidation time. Simultaneously, the hydroxybenzene conversion has
been determined by ion-exclusion chromatography, see LC configuration 8.
Experiments have been performed using 250 ml 1 mM aqueous hydroxybenzene
solutions, see reactor configuration 5b. Tested compounds are phenol (PHE), catechol
(CAT), hydroquinone (HQ), pyrogallol (PG) and hydroxyhydroquinone (HHQ). The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm and the maximum oxidation
time is 40 minutes. A single tungsten anode tip has been used. The following relative
temperature increase (∆T/T0) values have been measured after 40 minutes of corona
exposure: PHE: 0.10, CAT: 0.08, HYD: 0.09, PYR: 0.10, HHQ: 0.11.
As reported in section 4.1.4, the solution conductivity also increases by the formation
of nitrate ions, due to the application of corona discharges in air. Therefore the
conductivity measurements on oxidized hydroxybenzene solutions have been reported
with regard to the following background: the conductivity of deionized water as a
function of the oxidation time, under the same conditions the hydroxybenzenes have
been oxidized. The conductivity measurements as a function of the oxidation time are
shown by Figure 4.61.
Conductivity (µS/cm)
500
PHE
CAT
HYD
PYR
HHQ
400
300
200
100
0
0
10
20
30
40
Time (min)
Figure 4.61
The conductivity of 250 ml 1.0 mM hydroxybenzene solutions versus the
oxidation time. Tested hydroxybenzenes are phenol (PHE), catechol
(CAT), hydroquinone (HYD), pyrogallol (PYR) and hydroxyhydroquinone
(HHQ). The conductivity is reported with regard to the conductivity of
deionized water oxidized under the same conditions. The corona
parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Results
103
It has been observed that the conductivity increases with the oxidation time for all
tested compounds. Also, the conductivity at a fixed oxidation time increases according
to the order phenol, catechol/hydroquinone, pyrogallol, hydroxyhydroquinone. This order
resembles the increase in reducing properties of the regarded hydroxybenzenes.
Hydroxyhydroquinone is the strongest reducing agent of the tested hydroxybenzenes
and the oxidation of this compound i.e. production of carboxylic acids will thus be
fastest. The untreated hydroxyhydroquinone solution already shows conductivity of
about 35 µS/cm, which is caused by oxidation of this compound by oxygen dissolved in
the water. The conversion of the hydroxybenzenes is shown by Table 4.14.
Table 4.14
Time (min)
0
10
20
30
40
Conversion X of 250 ml 1.0 mM hydroxybenzene solutions. A tungsten
single-pin anode has been used. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
XPHE (%)
0.0
1.7
3.3
5.0
49.3
XCAT (%)
0.0
3.0
6.1
9.4
12.2
XHYD (%)
0.0
3.1
6.2
9.2
12.4
Unfortunately the conversion determination of hydroxyhydroquinone and pyrogallol
failed. The phenol conversion at 40 minutes is unlikely. The pulse energy has been
rather constant during all oxidation experiments, so the oxidation conditions are
comparable. Average pulse energies with 95% confidence interval are as follows: for
phenol oxidation 5.1 ±0.3 mJ, for catechol oxidation 5.5 ±0.0 mJ and for
hydroquinone oxidation 6.1 ±0.1 mJ. Figure 4.62 shows the hydroxybenzene
conversion versus the solution conductivity observed during oxidation.
15
Conversion (%)
PHE
CAT
10
HYD
5
0
0
Figure 4.62
100
200
Conductivity (µS/cm)
300
The hydroxybenzene conversion versus the solution conductivity.
PHE=phenol, CAT=catechol and HYD=hydroquinone. The conductivity
is reported with regard to the conductivity of deionized water oxidized
under the same conditions. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
104
Chapter 4.
There appears to be a relationship between conversion and conductivity. The
monohydroxybenzene phenol shows a different course than the dihydroxybenzene
isomers catechol or hydroquinone with mutually comparable course. Theorizing,
because of the missing conversion data, the slope of the trihydroxybenzene conversion
versus conductivity relationship will be much smaller than the slope of the
dihydroxybenzenes conversion course.
The relationship is applicable for a component-specific oxidation progress range where
the conversion increases with conductivity, knowing the initial conductivity. Due to
mineralization, the carboxylic acids will eventually disappear thus the conductivity will
decrease from that time. Also it should be noted, that after 100 % conversion of the
target compound, still carboxylic acids may be present that contribute to conductivity.
Finally the conversion efficiency values are reported for phenol, catechol and
hydroquinone by the G yield, see Table 4.15.
Table 4.15
The conversion efficiency of phenol, catechol and hydroquinone; average
values and 95% confidence intervals are presented.
Compound
Phenol
Catechol
Hydroquinone
G mol/J ⋅108
1.4 ±0.07
2.3 ±0.07
2.2 ±0.06
G (100 eV)-1
0.13 ±0.01
0.22 ±0.01
0.21 ±0.01
G (g/kWh)
4.6 ±0.3
9.1 ±0.3
8.6 ±0.2
From these data it is confirmed, that the dihydroxybenzenes catechol and hydroquinone
are stronger reducing agents than phenol. The observed efficiency values are generally
less favourable than the efficiency values observed in section 4.2.1; this may be
explained by application of a single pin anode.
Summary
Electrical conductometry has been tested as an alternative conversion measurement.
There appears to be a relationship between solution conductivity and target compound
conversion for a certain oxidation progress range. This relationship is based on the
formation of carboxylic acids providing conductivity by deprotonation in aqueous
solution.
Results
105
4.2.5. Microtox ecotoxicity
The degree of detoxification during oxidation of phenol by pulsed corona discharges has
been investigated by Microtox ecotoxicity tests. The ecotoxicity is expressed as an
effect concentration EC20, defining the concentration at which 20% inhibitory effect
takes place. The EC20 value is determined from a concentration series by interpolation
of the dose-effect relationship, see Equation 2.8. In order to quantify the ecotoxicity of
phenol during oxidation, an EC20 value has been determined before and after oxidation
of a series of different phenol concentrations. 250 ml solutions at 0.02 mM, 0.05 mM,
0.1 mM, 0.2 mM and 0.4 mM initial phenol concentration [PHE]0 have been oxidized for
30 minutes, see reactor configuration 5c. The corona parameters are V=25 kV, C=1
nF, f=100 Hz, d=1.0 cm. Table 4.16 shows the phenol conversion X and absolute
converted phenol amount mX, see LC configuration 9.
Table 4.16
[PHE]0 (mM)
0.02
0.05
0.10
0.20
0.40
Phenol conversion (X) and absolute converted amount (mX) after 30
minutes of oxidation of 250 ml solutions. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
X (%)
64.6
72.8
53.0
20.2
25.3
mX (mg)
1.4
3.5
5.0
3.8
9.6
The untreated phenol solutions show a dose-effect relationship according to Equation
2.8. The EC20 values, dose-effect parameters {b,a} and correlation coefficient r2 are
shown by Table 4.17. As the EC20 values do not change with the Vibrio fischeri
exposure time tVF, the ecotoxicity effect is acute.
Table 4.17
tVF (min)
5
15
30
EC20 values of phenol in deionized water versus the Vibrio fischeri
exposure time. Also shown are the dose effect parameters {b,a} and
correlation coefficient r2.
EC20 (mM)
0.06
0.07
0.07
b (-)
0.70
0.90
0.91
a (-)
-0.79
-0.62
-0.62
r2 (-)
0.9999
0.9825
0.9784
On the contrary, no EC20 value can be determined for the oxidized phenol solutions,
because there appears to be no dose-effect relationship. All samples except for the
0.1 mM sample show very low bioluminescence intensity, implying high ecotoxicity of
these oxidized phenol solutions. Table 4.18 shows the observed effects, indicated by
the gamma value, for the different phenol solutions and Vibrio fischeri exposure times.
It should be noted that the effect increases with the exposure time tVF. This means that
the ecotoxicity effect of the oxidized samples is not acute, which may be explained by
mass transfer-limited diffusion of the oxidation products within the Vibrio fischeri
bacterium.
106
Table 4.18
[PHE]0
(mM)
0.02
0.05
0.10
0.20
0.40
Chapter 4.
Ecotoxicity effects of oxidized phenol solutions, indicated by the gamma
value, for different Vibrio fischeri exposure times; average values (AVG)
and standard deviations (σn-1) are presented. 250 ml solutions have been
oxidized for 30 minutes. The corona parameters are V=25 kV, C=1 nF,
f=100 Hz, d=1.0 cm.
Γ 5 min (-)
AVG
σn-1
24.9
13.1
85.6
59.6
1.1
0.0
6.2
0.1
30.9
8.0
Γ 15 min (-)
AVG
σn-1
31.3
16.7
105.2
57.0
1.4
0.0
15.9
0.1
52.5
20.3
Γ 30 min (-)
AVG
σn-1
49.3
29.9
151.4
91.2
2.1
0.1
51.0
1.8
94.5
50.3
An explanation for the absence of the dose-effect relationship after oxidation is the
fact, that all oxidized samples contain a complex range of oxidation products. Especially
hydroquinone and 1,4-benzoquinone are highly toxic towards Vibrio fischeri, see Table
2.1. By oxidation progress, these compounds disappear but the oxidation product
mixture increases in acidity which is also unfavourable to these bacteria.
As part of the Microtox test, the solution acidity and oxygen content have been
measured for both untreated and oxidized samples, see Table 4.19. If the sample
acidity exceeds the range 6<pH<8.5 and/or the sample has a low oxygen content
(<0.5 mg/l), these effects are inhibitory and bias the effect of the target compound.
It appears, that oxidation increases the acidity of both the Millipore water and the
phenol solutions. For the case of Millipore water, the production of nitrogen oxides by
corona discharges in air results in the formation of nitric acid. The acidity increase by
oxidation of phenol is caused by the formation of carboxylic acids.
Table 4.19
[PHE]0
(mM)
0
0.02
0.05
0.10
0.20
0.40
Solution acidity and oxygen content (expressed as percentage of the
saturation concentration) for untreated and oxidized samples. 250 ml
solutions have been oxidized for 30 minutes. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Untreated
pH (-)
[O2] (%)
5.7
65
5.9
63
5.9
62
6.2
61
6.2
51
6.2
60
Oxidized
pH (-)
[O2] (%)
4.2
71
4.0
60
4.0
61
3.9
48
3.8
65
3.8
51
The small differences in oxygen content between the oxidized and untreated phenol
solutions, if significant, might be caused by different amounts of polyhydroxybenzenes
produced during the oxidation. These compounds are strong reducing agents and thus
have a high affinity for oxygen. The oxygen content of oxidized Millipore water is
somewhat higher than the oxygen content of untreated Millipore water. This might be
due to the presence of hydrogen peroxide.
Results
107
The influence of the solution acidity on the Microtox test has been verified by
determination of the gamma values of non-pH adjusted oxidized phenol solutions at an
exposure time tVF =5 minutes. The acidity of these solutions is about pH=4, see Table
4.19. The gamma values of oxidized phenol solutions that have not been pH-adjusted,
are shown by Table 4.20.
Table 4.20
Gamma values of non-pH adjusted oxidized phenol solutions, after 5
minutes exposure time; average values (AVG) and standard deviations
(σn-1) are presented. 250 ml solutions have been oxidized for 30 minutes.
The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
Γ 5 min
[PHE]0 (mM)
0.02
0.05
0.10
0.20
0.40
AVG
13.7
1.4
5.9
395.6
193.1
σn-1
6.1
0.1
1.2
127.5
25.5
The gamma values are highest for the 0.2 mM and 0.4 mM samples, which are more
acidic than the other samples. Also, these gamma values are very different from the
gamma values of pH-adjusted oxidized solutions, illustrated by Table 4.18. The pH
effect is thus very relevant.
Finally, it has been observed, that the oxidation of Millipore water also induces
ecotoxicity. The bioluminescence intensity at tVF=5 minutes of fresh Millipore water is
It5=100.8 / σn-1=0.9. The bioluminescence intensity of pH-adjusted oxidized Millipore
water is It5=100.1 / σn-1=2.5. However, if oxidized Millipore water is not pH-adjusted,
the bioluminescence intensity is It5 = 67.8 / σn-1=5.2.
This effect is also caused by acidity. The corona discharges produce nitrogen oxides in
air, which dissolve in the water and form nitric acid. By adjusting the acidity of the
solution the ecotoxicity vanishes, so the effect is not caused by the nitrate ion.
Although hydrogen peroxide is produced by the corona discharges and hydrogen
peroxide exhibits ecotoxicity (EC5015min=16 mg/l) [78], this effect has not been
observed: the bioluminescence intensities of untreated Millipore water and pH-adjusted
oxidized Millipore water are equal. Hydrogen peroxide at low concentrations
decomposes rapidly into oxygen and water.
Summary
After oxidation during a fixed time (30 min) of a series of different concentration phenol
solutions, no dose-effect relationship appears to exist anymore. Although mutually
different conversions have been measured, the Microtox ecotoxicity effect is high for
both low and high initial phenol concentration. This effect is especially caused by
hydroquinone and 1,4-benzoquinone, that are highly toxic to Vibrio fischeri bacteria.
Although these compounds disappear by oxidation progress, carboxylic acids are
produced that cause pH-toxicity.
108
Chapter 4.
4.2.6. Total organic carbon
During the oxidation of a 498 ml 1.0 mM phenol solution, see reactor configuration 10,
the total organic carbon level (TOC) has been measured every 15 minutes during
2 hours. The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. In
addition, the conversion has been measured by ICE chromatography, according to LC
configuration 7. Table 4.21 shows the TOC and conversion (X) values versus the
oxidation time. X2 is a repeated conversion measurement, 21 hours after the first
measurement X1.
Table 4.21
t(min)
0
15
30
45
60
75
90
105
120
Total organic carbon (TOC) level and conversion (X) during the oxidation
of a 498 ml 1.0 mM phenol solution. The time span between the two
conversion measurements is 21 hours. The corona parameters are
V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.
TOC (mg/l)
52.0
58.4
62.8
57.2
59.2
53.6
67.6
62.4
56.8
X1 (%)
0.0
3.3
7.0
10.8
13.9
17.0
19.9
21.3
25.2
X2 (%)
0.0
3.2
7.1
11.0
14.4
17.2
20.3
22.3
25.7
It should be remarked, that the theoretical TOC level of an 1.0 mM phenol solution
equals 72.1 mg/l. The measured value at t=0 however is somewhat lower. This may
be due to an incidental sample preparation error, because the samples had to be diluted
before analysis.
The TOC level appears to be rather constant during oxidation, while the conversion
increases. This means, that the oxidation products are likely to remain in the liquid
phase for the observed conversion range. This is in accordance with the results from
the infrared analyses of the gas phase over oxidized phenol solutions (section 4.2.3)
and the aldehyde screening test (section 4.2.1).
From the conversion measurements (X1 and X2) it is clear, that after stopping of the
oxidation the conversion of phenol does not significantly proceed within 21 hours.
Possible conversion progress after stopping the corona discharges might be due to
remaining amounts of ozone and hydrogen peroxide. In between these two conversion
measurements the TOC measurements have been performed.
Summary
At a conversion up to 25%, the TOC level of an oxidized 1 mM phenol solution appears
to be rather constant. This implies that the oxidation products are likely to remain in the
liquid phase for the observed conversion range.
Results
109
4.3. Oxidation of other model compounds
In this section a global survey is given of the oxidation of atrazine herbicide, malachite
green dye and dimethyl sulfide odor component. The conversion of atrazine in aqueous
solution has been measured by reversed-phase HPLC, while the oxidation progress of
malachite green in aqueous solution has been determined by in-situ absorption
spectrometry. Dimethyl sulfide has been oxidized in the gas phase and the conversion
has been measured by gas chromatography.
4.3.1. Atrazine
A 500 ml 25 mg/l (0.12 mM) atrazine solution (tap water-based) has been oxidized for
5 hours, see reactor configuration 6b. The corona parameters are V=30 kV, C=100
pF, f=50 Hz, d=1.0 cm. Every 15 minutes (0-3 hours) or 30 minutes (3-5 hours) the
conversion has been determined in threefold using a rp-HPLC column according to LC
configuration 10. Figure 4.63 shows the conversion versus the oxidation time. The
conversion efficiency versus time is shown by Table 4.22.
Conversion (%)
100
80
60
40
20
0
0
60
120
180
240
300
Time (min)
Figure 4.63
The conversion of a 500 ml 25 mg/l (0.12 mM) atrazine solution (tap
water-based). The corona parameters are V=30 kV, C=100 pF,
f=50 Hz, d=1.0 cm.
Table 4.22
Conversion (X), pulse energy (Ep) and efficiency (G) during oxidation of a
500 ml 25 mg/l (0.12 mM) atrazine solution (tap water-based); average
values and 95% confidence levels are presented. The corona parameters
are V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm.
t (min)
60
120
180
240
300
X (%)
12.7 ±5.5
27.2 ±15.3
38.5 ±21.0
47.8 ±28.8
56.5 ±32.5
Ep (mJ)
10.8 ±2.1
10.7 ±0.5
10.4 ±1.2
11.2 ±1.9
10.2 ±0.6
G (mol/J)⋅1010
7.67 ±5.14
8.22 ±5.17
7.89 ±3.29
6.83 ±1.93
7.21 ±4.86
G (100eV)-1⋅103
7.40 ±4.96
7.93 ±4.99
7.62 ±3.18
6.59 ±1.86
6.96 ±4.69
G (g/kWh)
0.60 ±0.40
0.64 ±0.40
0.61 ±0.26
0.53 ±0.15
0.56 ±0.38
110
Chapter 4.
As a function of the oxidation time, the error bars grow to considerable size. The
deviation increase cannot be due to sample inhomogenity because the reactor content
is continuously mixed by a magnetic stirring bar. The time dependent effect may be a
related to a UV absorbance detector stability problem.
The conversion time relationship seems to obey first order kinetics with a rate constant
k1≈2.7⋅10-3 min-1. After 5 hours of oxidation the atrazine conversion is about 57%. The
efficiency values clearly show that atrazine is a very stable compound. At least seven
oxidation products have been found in the chromatogram, but the identification of these
products has not been possible. Pelizzetti [36] has concluded from detailed GC-MS
experiments, that photocatalytic oxidation of aqueous atrazine will not destroy the
triazine ring and that the final oxidation product is cyanuric acid. Cyanuric acid (2,4,6trihydroxy-1,3,5-triazine) is a questionable carcinogenic compound [112].
Summary
Atrazine is a rather stable herbicide. The conversion efficiency is about one order of
magnitude lower than the conversion efficiency of phenol. It has not been possible, to
identify the variety of oxidation products.
4.3.2. Malachite green
The influence of different electrode configurations on the decolorization of malachite
green dye has been determined by absorption spectrometry. From a LED light source, a
photodiode detector, an amplifier and a recorder a simple setup has been constructed
that may be a reasonable alternative to a precious and laboratorybound liquid
chromatograph or absorption spectrometer.
125 ml 1.94 mg/l malachite green solutions have been exposed to pulsed corona
discharges during 1.5 hours, see reactor configuration 11. The load voltage has been
set to V=20-30 kV, depending on the configuration. The time span t24 has been
determined, after which 24% decrease in absorption at wavelength λ=590 nm has
been measured. This percentage is based on the configuration which showed slowest
decolorization.
The configuration variables are as follows: the anode has been located at a distance
d=-4 mm, +1 mm, +5 mm, +10 mm relative to the solution surface; the used anode
types are single pin, 4 pins, 8 pins and a wire anode; in one case an air flow has been
supplied at a single pin anode tip (configuration no. VII); the cathode location is in the
water on the bottom of the reactor or directly outside/underneath the reactor. Table
4.23 shows the power input (P), decolorization time span (t24) and efficiency (G).
It can be observed that capacitive configurations, where the cathode plate is situated
outside/underneath the glass reactor vessel, are generally more favourable to the
efficiency than configurations where the cathode is inside the aqueous phase. This is
caused by the fact that for the case of a capacitive configuration, conductive current is
not possible.
Results
111
Table 4.23
Conf.
nr.
I
II
III
IV
V
VI
VII
VIII
IX
Power input (P), decolorization time span (t24) and efficiency (G) for the
oxidation of malachite green in aqueous solution. The corona parameters
are V (configuration dependent), C=5 pF, f=150 Hz, d (indicated).
Cathode
Anode
position
type
d (mm)
outside
4 pins
+1
outside
8 pins
+1
outside
wire
+1
outside
1 pin
+1
outside
1 pin
-4
inside
1 pin
+1
inside
1 pin+air
-4
inside
1 pin
+10
inside
1 pin
+5
P
(mW)
395
366
493
383
432
1789
6944
3963
6301
t24
(min)
12.1
15.9
32.2
57.9
94.0
26.4
35.0
61.9
66.7
G
(mg/kWh)
412
337
123
89
48
67
16
13
7
Immersion of the anode in the aqueous solution is less favourable than situating the
anode in the gas phase over the solution. The production of corona discharges at an
anode tip immersed in an aqueous phase, demands extra energy for evaporation of the
liquid, which is less efficient than the formation of the corona discharge in the gas
phase. A multipin anode is more efficient than a single pin anode. The multipin anode
seems to produce a more efficiently-dimensioned reactive volume (plasma) for the
production of oxidizers than a single pin, while the energy input differences are
comparable. The anode wire efficiency is in between the performances of multipin and
single pin; although it spans a favourable space, it lacks the pin shape necessary for the
creation of high electric field strengths. Although the addition of air to the discharge is
likely to favour the production of ozone by provision of oxygen, configuration VII has
appeared to show low performance. In this way, large amounts of oxygen-based ions
may have been produced, that have caused high conduction currents within this noncapacitive configuration.
It should be noted, that the decolorization of malachite green does not imply that
detoxification takes place. Decolorization is caused by the destruction of chromophoric
groups; at that time, mineralization of the large dye molecule is certainly not obvious.
The observed decolorization efficiency is about one to two orders of magnitude lower
than the conversion efficiency of phenol.
The degradation of malachite green is complex, because the number of probable attack
sites for oxidizers like the hydroxyl radical, ozone and oxygen is very large. The
methyne central carbon is prone to attack by the hydroxyl radical or ozone yielding
bond cleavage, thus removal of the main chromophoric part i.e. C=C6H4=N+(CH3)2
from the dye molecule. Also, the aromatic rings can be hydroxylated or cleaved. In
addition to the oxidation products mentioned in section 2.4.3, the following products
may be formed: nitro/hydroxybenzophenones, nitrophenols, benzoic acid derivatives,
aliphatic aldehydes/carboxylic acids and nitrate ions.
Summary
A multipin anode over the aqueous phase, combined with a cathode directly outside and
underneath the reactor is the most favourable configuration for the decolorization of
malachite green.
112
Chapter 4.
4.3.3. Dimethyl sulfide
This experiment differs from the others, because the target compound dimethyl sulfide
(DMS) has been oxidized in the gas phase using a concentric electrode tubular reactor
according to reactor configuration 12. A gas chromatograph equipped with sulfur
chemiluminescence detector has been applied for analysis, see GC configuration 2.
10 ppm and 100 ppm standard solutions of DMS in methanol have been prepared. The
corona parameters are V=30 kV, C=100 pF, f=50 Hz. Before every corona
experiment, the reactor contains about 2.5 ppm DMS in ambient air. Three oxidation
times have been investigated. Every experiment has been performed in threefold. Table
4.24 shows the results.
Table 4.24
t (s)
15
30
60
Conversion (X) of dimethyl sulfide in air versus the oxidation time. Also
shown are initial and final average concentrations [DMS] and 95%
confidence intervals. The corona parameters are V=30 kV, C=100 pF,
f=50 Hz.
[DMS]0 (ppm)
2.33 ±0.60
2.59 ±1.05
2.54 ±0.22
[DMS]t (ppm)
1.28 ±0.25
0.77 ±0.25
0.14 ±0.03
X (%)
45
70
94
Next to the oxidation products mentioned in section 2.4.4, oxidative demethylation
likely yields formaldehyde and formic acid. The aim of DMS removal from industrial
waste gas flows is odor destruction. Although destruction of dimethyl sulfide has been
demonstrated, thorough oxidation progress is necessary, because the oxidation
products are still harmful. Dimethyl sulfoxide is an odourless, strongly hygroscopic
liquid that is readily absorbed by the skin; it causes especially irritation of the mucous
membranes; dimethyl sulfoxide is reported to exhibit mutagenic and teratogenic
properties [113]. Formaldehyde is strongly irritating and has mutagenic, teratogenic and
probably carcinogenic properties [1]. Formaldehyde is oxidized to formic acid and formic
acid will eventually disappear by mineralization, but sulfuric acid has to be removed by
a gas scrubber.
Summary
2.5 ppm Dimethyl sulfide in 330 ml ambient air has been converted for about 94 %
after 60 seconds using gas phase corona discharges at V=30 kV and f=50 Hz. The
destruction of this organosulfur compound initially yields harmful intermediates;
although qualitative product analysis has not been performed, likely final oxidation
products are sulfuric acid, carbon dioxide and water.
5. Discussion
A survey of the results is presented in combination with points of special interest and
new approaches. The discussed topics are general pulsed corona discharge application,
corona-induced phenol oxidation, the oxidation mechanism of phenol, a presentation of
the applied analysis techniques and a comparison of some advanced oxidation
processes.
5.1. Pulsed corona discharges
Oxidation by corona in air
For the case of corona in air, possibly nitrate ions have been identified in the oxidation
product mixture, see section 4.1.4. The amounts of nitrogen oxides produced by corona
in air are small viz. a few ppm’s [114]. Nitrate seems to exhibit no ecotoxicity,
according to the Microtox tests described in section 4.2.5 on corona exposed deionized
water; the observed toxicity appearing from corona-exposure of deionized water is due
to H3O+ from nitric acid. The question arises, whether nitrogen might be introduced into
the oxidation products by means of nitro groups, resulting in a very significant increase
of toxicity. This should be accomplished by the attack of radical species like nitrogen
oxide radicals, produced by oxidation of nitrite or nitrate ions. However, the electron
affinities of NO2 and NO3 are very high viz. 2.27 eV and 3.94 eV respectively [115],
therefore the production of these radicals is highly unlikely. In addition, nitrite ions are
oxidized to nitrate ions by oxidizers like the hydroxyl radical and ozone. NO has a low
electron affinity (0.03 eV), but will also be oxidized to nitrate and exhibits poor
solubility in water. Compared to corona in oxygen, the application of corona in air, thus
has no extra negative impact on the toxicity of the oxidation products. The production
of nitrate can be avoided by application of corona in an oxygen atmosphere, of course
for the case of nitrogen-free target compounds.
Liquid/gas phase corona oxidation products
By application of corona in aqueous solution or in the gas phase (air, oxygen) over the
solution it is reasonable to argue, whether the way of application may give rise to
oxidation product mixtures with different chemical composition. For the case of
aqueous phase corona, the discharge channels in the solution will locally create
transient extreme temperatures and pressures; in addition to oxidation by hydroxyl
radicals, the target compound may undergo pyrolytical decomposition, comparable to
the supercritical conditions favoured by ultrasonic irradiation or wet oxidation.
For the case of corona in air or oxygen, in addition to hydroxyl radicals and ions, the
corona discharges produce ozone that diffuses into the liquid phase and oxidizes the
target compound. Due to their high reactivity, hydroxyl radicals produced in the gas
phase cannot reach the liquid, but may produce hydrogen peroxide over the solution.
114
Chapter 5.
Hydroxylation in aqueous solution is explained by the reaction of ozone, water and UV
photons or hydrogen peroxide and UV photons. At the gas-liquid interface, the
discharge streamers produce limited amounts of reactive species by ions/metastables
bombardment; pyrolytical decomposition of the target compound will not be relevant,
due to the small contact area between the discharge channels and the target compound
solution.
Additives
The conversion efficiency of pulsed corona discharges can likely be improved by the
application of additives. Fe(II,III) salts invoke the decomposition of produced hydrogen
peroxide into hydroxyl radicals according to Fenton chemistry, see Equations 1.3b-d.
Extra hydrogen peroxide can be added, which is decomposed into hydroxyl radicals by
UV photons produced by the corona. Application of corona in an oxygen atmosphere
increases the production of ozone, atomic oxygen and oxygen ions. In alkaline solution,
ozone reacts by hydroxyl radical chemistry and weakly acidic compounds (e.g. phenols)
then exist in anionic form that is preferentially attacked by the electrophilic oxidizer
species. Nitrous oxide addition, as applied in radiolysis, may also yield extra hydroxyl
radicals for the case of corona treatment, according to Equation 1.6e. By turbulent
mixing using a compressed gas, the gas-liquid interface area is increased, which is likely
to be favourable to conversion.
Corona-induced synthesis
It may be suggested that pulsed corona discharges can also be utilized for the synthesis
of organic compounds. Application of pulsed corona discharges in an argon atmosphere
produces polyhydroxybenzenes in considerably higher amounts than corona in air, as
has been described in section 4.2.1. Although unwanted ring-cleavage takes place, this
may be suppressed by removal of remaining oxygen from the reactor by degassing or
additional purging with inert gas and minimizing the corona energy input. Ring-cleavage
also inevitably occurs during hydrogen peroxide-based commercial production of
polyhydroxybenzenes from benzene or phenol. The advantages of pulsed corona
discharges over ex-situ hydrogen peroxide addition are a fine-tunable hydroxyl radical
dosage and the in-situ production of hydrogen peroxide which is very safe and
convenient.
By application of pulsed corona discharges to non-aqueous media under oxygen-free
conditions, the medium is dissociated into radical species, while the absence of
oxidizers prevents mineralization. For instance the exposure of a monomer to pulsed
corona discharges under oxygen-free conditions is likely to invoke polymerization; also
coupling reactions may be performed between different reactants. The advantages of
corona-induced synthesis to conventional synthesis may be simplified synthesis
pathways and reduced waste.
Of course, radical formation can also be achieved by pulse radiolysis or electron-beam
treatment. However these technologies are expensive, complex and demand shielding
of radiation, while corona application is straightforward and comparatively inexpensive.
Discussion
115
The synthesis of several organic compounds has been applied already for several years
by electrochemical reactions viz. by electrolysis. Pathways include cathodic coupling
(adiponitrile from acrylonitrile), cathodic hydrogenation (aniline from nitrobenzene) and
anodic functionalization by hydroxylation (hydroquinone from benzene), halogenation
(prefluorinated dialkyl ethers from dialkylethers) or oxygenation (dimethyl sulfoxide from
dimethyl sulfide) [116].
Corona discharges versus electrolysis
A major application of electrical energy for chemical purposes is electrolysis. The
differences between electrolysis and corona are very distinct:
Electrolysis deals with low voltage (a few volts) and high current densities: typical
values for inorganic electrolysis are j=1-10 kA/m2 and for organic electrosynthesis
j=0.1-1 kA/m2 [116]. Pulsed corona features high voltage and low time-averaged
currents: typical values are a voltage of 10-100 kV and a time-averaged current of a
few milliamperes in gaseous dielectrics. For the case of electrolysis both oxidation and
reduction take place at the anode and cathode respectively; for the case of corona
either oxidizing or reducing species can be produced, depending on the constitution of
the dielectric medium where the discharges take place.
Electrolysis is applied in molten salts, aqueous solution with electrolytes or for some
cases in pure organic liquid phases; corona is applied either in the gas phase or in
aqueous solution.
The location where reactions occur is the electrode surface for the case of electrolysis;
the corona discharge channel tips and -to a less extent- the discharge channels form a
variable volume where reactive species are produced.
For electrolysis, ion migration in solution and electron transfer at the electrodes take
place; the application of pulsed corona discharges in the gas phase minimizes ion
migration, electron transfer occurs from the streamer head by the discharge channel
towards the electrode.
Electrolysis is operated at DC voltage, while corona can be applied as either fast
positive polarity pulses (gas and water remediation) or at AC voltage (ozonizers).
An unconventional form of electrolysis is contact glow discharge electrolysis (CGDE)
[117]. This phenomenon represents DC glow discharges across a gaseous envelope
between anode or cathode and the surrounding aqueous, non-aqueous or molten phase.
CDGE is initiated from normal electrolysis by increasing the voltage to a level, where a
distinct current drop and growth of a gaseous sheath over an electrode are observed.
The applied voltage is much higher than the voltage of normal electrolysis viz. some
hundreds of volts, while the current is several milliamperes. By application of CGDE to
aqueous solutions of e.g. ammonia [118] or ethanol [119], products are formed, which
are explained by the dissociation and ionization of liquid phase molecules by electron
and ion bombardment.
As far as known, CGDE has not been qualified as AOP, probably because of the
requirement of a high solution conductivity and an unfavourable conversion efficiency
due to solvent evaporation by resistive heating.
116
Chapter 5.
5.2. Corona-induced phenol oxidation
Ozone to phenol ratio
An estimation has been made of the ratio R of ozone molecules consumed by phenol to
the amount of converted phenol molecules. Data have been obtained from ozone
measurements over 100 ml deionized water and over a 100 ml 1.0 mM phenol solution
and a phenol oxidation experiment using a 100 ml 1.0 mM phenol solution. All
experiments have been performed under equal conditions: V=25 kV, C=1 nF,
f=100 Hz, d=1.0 cm.
The amount of ozone molecules consumed by phenol is calculated from the difference
of the ozone production efficiency for the case of 100 ml deionized water (GO2water) and
the ozone production efficiency for the case of a 100 ml 1.0 mM phenol solution
(GO21mM phenol) . The phenol conversion efficiency (Gphenol1mM) is known from Table 4.9. The
ratio calculation is shown by Equation 5.1. Regarding this estimation, the assumption is
made that phenol is exclusively converted by ozone.
R=
GO3
water
mM phenol
− GO3 1
Gphenol
1mM
(5.1)
The ozone production rate is determined from the initial slope (S) of the ozone density
versus time plots given by Figure 4.11. The determined slopes are about
Swater=1.54⋅1020 m-3s-1 and S1mM phenol=1.22⋅1020 m-3s-1. The ozone production efficiency
is calculated from the initial slope S (m-3/min), the pulse energy (Epwater=3.5 mJ,
Ep1mM phenol=5.6 mJ), the pulse repetition rate (f=100 Hz) and the gas phase volume
(Volg=253 cm3).
The ozone production efficiency for the case of 100 ml deionized water is about
GO3water=1.12⋅1017 J-1≡1.85⋅10-7 mol/J≡1.79 (100eV)-1, while the efficiency regarding
the 100 ml 1.0 mM phenol solution is about GO31mM phenol=5.50⋅1016 J-1≡9.13⋅10-8 mol/J≡
0.88 (100 eV)-1. The amount of ozone molecules consumed by phenol equals ∆GO3=
5.67⋅1016 J-1≡9.42⋅10-8 mol/J≡0.91 (100 eV)-1. The initial phenol conversion efficiency
is about Gphenol1mM=1.44⋅1016 J-1≡2.39⋅10-8 mol/J≡0.23 (100 eV)-1. From these values it
has been calculated that about 4 ozone molecules are used to convert 1 phenol
molecule: R=4. A theoretical stoichiometric ratio is impossible to determine. The reason
for this is the complex phenol oxidation mechanism, as will be discussed in section 5.3.
Hydroxyl radical production efficiency
An estimation of the order of magnitude of the hydroxyl radical production efficiency
can be made as follows. The initial slope of the phenol concentration versus time
course indicates the rate at which phenol is converted at the start of corona oxidation.
At that time, produced oxidizers are exclusively consumed by phenol and by
recombination but not yet by oxidation products.
Discussion
117
For the case of oxidation in argon, it may be assumed that phenol is mainly converted
by hydroxylation. The exact stoichiometric ratio is unknown but ranges theoretically
from OH:phenol=1-5, although ratios higher than 1 are unlikely.
Then, including radical loss processes and polyfold OH attack on phenol, a lower limit
estimation of the hydroxyl radical production rate equals 1/5th of the phenol decrease
rate in argon at t=0. This production rate and the measured energy input, which is
assumed to be completely used for radical production, yield an approximation of the
hydroxyl radical production efficiency.
The first order exponential fit of the phenol conversion versus time has been determined
from Figure 4.31, while the initial phenol concentration C(0)=1.0 mM, see Equation
5.2. The pulse energy at time t=0 is about 10 mJ, see Table 4.9. Therefore at a pulse
repetition rate f=100 Hz, 60 Joules are consumed every minute. The reactor volume is
100 ml.
C(t) = C(0) exp(−k1 ⋅ t)
k1 ≈ 1.8 ⋅ 10−2 min−1
 dC(t) 
−5
−3
−1
 dt  = − k1 ⋅ C(0) ≈ − 1.8 ⋅ 10 mol dm min

t =0
(5.2)
Then, the lower limit of the hydroxyl radical production efficiency GOH is estimated to be
1/5x1.8⋅10-5 mol dm-3 min-1x0.1 dm3x1 min/60 J ≈ 6⋅10-9 mol OH/J ≡ 0.06 OH /100 eV.
For the case of OH/phenol=1 stoichiometry, the efficiency is about 3⋅10-8 mol/J ≡
0.29 OH/100 eV and is equal to about 345 eV/OH. By comparison of this value to the
phenol conversion efficiency values listed by Tables 4.4, 4.5 and 4.9 and assuming that
phenol exclusively reacts with hydroxyl radicals, the OH-to-phenol ratio is globally 1-2.
Phenol conversion efficiency
A comparison of the observed efficiency values for the conversion of phenol is given
below. The relevant data are given by Tables 4.4, 4.5 and 4.9. It has appeared, that
the efficiency of oxidation of a 500 ml 0.05 mM phenol solution at V=30 kV and
f=50 Hz is considerably less favourable than the efficiency of oxidation of a 500 ml
1 mM phenol solution at V=25 kV and f=100 Hz. The removal of phenol from diluted
aqueous solutions is probably less efficient, because here the highly reactive oxidizers
have less probability to encounter a phenol molecule before they disappear by
recombination or decomposition. Ozone may decompose into oxygen by wall
recombination, by homogeneous catalysis by light and by nitrogen oxides produced
from corona in air. In humid air, hydroxyl radicals react with nitrogen and oxygen to
produce nitrogen oxides. Hydroxyl radicals oxidize water to hydrogen peroxide.
Within this context it is also important to state, that the removal of target compounds
is likely to be more efficient, when pulsed corona discharges are applied to thin films of
the aqueous target solution. This has been demonstrated by the conversion
measurements on two oxidized phenol solutions having equal initial amounts of phenol
but a different solution volume and phenol concentration, see section 4.2.1. The
influence of the gas-liquid interface area on the mass transfer of oxidizers may
contribute to the differences between the efficiency values of Table 4.5 and 4.9.
118
Chapter 5.
Phenol conversion efficiency in corona literature
The phenol efficiency values, determined from this thesis, have been compared to some
literature references on phenol oxidation by liquid phase corona and ozonation. The
efficiency has been determined at 50% phenol conversion (G50). Table 5.1 shows the
literature references, an overview of the applied conditions and the phenol conversion
efficiency [104].
Table 5.1
A comparison of phenol conversion efficiency G50 values, obtained from
liquid phase corona literature and from this thesis. Symbol description:
C0=initial phenol concentration Vol=phenol solution volume, Ep=pulse
energy, f=pulse repetition rate, t50=oxidation time to reach 50% phenol
conversion.
corona in solution
corona in air over solution
ozone
References
[100]1)
[101]
[102]
[120]
Thesis
[45]
C0 (mM)
0.03
0.53
0.53
0.02
0.05
1.0
1.0
0.5
Vol (cm3)
550
1000
2502)
500
500
100
500
4000
Ep (mJ)
1750
800
8802)
30
13
6
10
f (Hz)
60
50
48
50
50
100
100
t50 (min)
180
260
7
180
26
83
237
8
G50 (mol/J)
7.3·10-12 4.2·10-10 3.7·10-9 3.1·10-10 1.2⋅10-8 1.7⋅10-8 1.8·10-8 5.2·10-8
G50 (100eV)-1 7.0·10-5 4.1·10-3 3.6·10-2 3.0·10-3 1.2⋅10-1 1.6⋅10-1 1.7·10-1 5.0·10-1
1)
2)
The energy consumption inside the power supply is included in the pulse energy.
Additional information [121]. In this reference the yield is given as G = 1.0·10-2 (100 eV)-1.
The phenol conversion efficiency by ozonation has been calculated using an ozone
consumption of 61.5 mg/min x 8 minutes and assuming an ozone production efficiency
of 100 g/kWh.
It has been observed that the efficiency of phenol conversion by application of corona
in air over the solution is one to two orders of magnitude higher than the efficiency of
liquid phase corona. In references [100-102] it is mentioned, that the liquid phase is
externally cooled during application of the corona discharges. The temperature increase
of the aqueous solution due to exposure to gas phase corona, has been measured to be
unsignificant, see section 4.1.4, 4.2.1 and 4.2.4.
It is expected, that further optimization of the corona reactor will lead to a comparable
or even higher efficiency than that of ozonation. First trial runs by corona in oxygen
have already yielded higher phenol conversion efficiency values than ozonation [104].
This is explained by the broad range of reactive species that are produced by corona
discharges in air viz. hydroxyl radicals, ozone, atomic oxygen, UV photons, ions and
metastables.
Discussion
119
LIF spectra of oxidized phenol solutions
Oxidized phenol solutions, especially those exposed to corona in an argon atmosphere,
show an increased fluorescence intensity at wavelengths in the range 400-500 nm, see
Figure 4.56. This fluorescence is not due to phenol, see Figure 4.47. This fluorescence
is partially explained by the presence of polyhydroxybenzenes. The strongest
fluorescent polyhydroxybenzene is hydroquinone, which shows a fluorescence
maximum at about 335 nm but does not show fluorescence at wavelengths higher than
450 nm. The trihydroxybenzene hydroxyhydroquinone shows a similar fluorescence
course but at a much lower intensity.
Other molecules that can explain for the fluorescence have to be in accordance with the
requirements for fluorescence, see section 2.5.3. Muconic acid may likely be no
candidate fluorescent product, although it has a conjugated system of π electrons (it is
a diene) and the carbon chain is cyclic and flat, because the carbon chain is not linkedup. Regarding the fluorescence range a remaining possibility is, that synthetic humic
acids account for this fluorescence behaviour. These polymeric molecules viz. contain
hydroquinone monomeric units, see section 2.4.1. On the contrary the occurrence of
these compounds in oxidized 1 mM phenol solutions is likely to be limited.
Microtox toxicity units
Although it has not been possible to determine the toxicity of the phenol oxidation
product mixture by an effect concentration EC20 value, yet a quantitative estimation of
ecotoxicity can be made by means of a toxicity units (TU) calculation [122]. The TUx
value describes the amount of dilution that is needed for a sample to reach a defined
effect percentage x. High TU values imply high toxicity. The TUx value is calculated by
summation of the quotients (tux,i) of component concentration and component ECx value
for each product mixture component i, see Equation 5.3. The general assumption is
made that the toxicity of the individual components is additional.
TUx =
∑
components
 C(t) 

 = ∑ tux, i
 ECx  i components
(5.3)
A TU50 calculation has been performed with regard to the quantitative ICE analysis of
1.0 mM phenol solutions exposed to pulsed corona discharges in air and argon,
described in section 4.2.1. However, this calculation is a rather rough estimation,
because some oxidation products have not been identified. Also the actual catechol and
resorcinol concentrations are not known, because these components could not be
separated; the same is true for hydroquinone and 1,4-benzoquinone. For a worst case
approach it has been assumed that only hydroquinone and catechol represent the
dihydroxybenzenes; resorcinol viz. has the lowest ecotoxicitity among the
dihydroxybenzenes. 1,4-benzoquinone and hydroquinone have comparable ecotoxicity.
Muconic acid is likely to be present but neither Microtox ecotoxicity nor concentrations
are known. Finally, the addition of individual toxicity units is only justified, if the
components show comparable toxicity effects. Although this assumption is
questionable, the essence of ecotoxicity change during oxidation is demonstrated.
120
Chapter 5.
A global ecotoxicity estimation has been performed on the phenol oxidation product
mixture, obtained after 1 hour and 2 hours of exposure to corona in air and argon, using
the EC50 values from Table 2.1. The following compounds are concerned: phenol,
hydroquinone, catechol, formic acid, oxalic acid and glyoxylic acid. From the different
hydroquinone effect values reported in literature [78], an average value EC505min ≈ 0.061
mg/l has been used for the calculations. Table 5.2 shows the individual component
toxicity contributions (tu50) and the total mixture ecotoxicity (TU50).
Table 5.2
An estimation of the toxicity unit value TU50 of 100 ml 1.0 mM phenol
solutions, after exposure to pulsed corona discharges in air or argon for
t=60 min and t=120 min. The corona parameters are V=25 kV,
C=1 nF, f=100 Hz, d=1.0 cm.
Corona in air
Phenol
Hydroquinone
Catechol
Formic acid
Oxalic acid
Glyoxylic acid
Corona in Ar
Phenol
Hydroquinone
Catechol
Formic acid
Oxalic acid
Glyoxylic acid
t=60 min
C (mg/l)
tu50 (-)
58.0
2.0
0.4
6.6
13.6
0.4
5.9
0.7
31.2
2.8
3.1
0.3
TU50=
12.8
t=120 min
C (mg/l)
tu50 (-)
40.8
1.4
0
0
8.6
0.3
10.8
1.4
34.9
3.1
4.0
0.4
TU50=
6.6
t=60 min
C (mg/l)
tu50 (-)
34.0
1.2
2.1
34.4
35.2
1.1
1.7
0.2
0
0
0
0
TU50=
36.9
t=120 min
C (mg/l)
tu50 (-)
11.4
0.4
1.5
24.6
29.1
0.9
4.0
0.5
0
0
2.2
0.2
TU50=
26.6
The total ecotoxicity of the phenol oxidation product mixture seems to be mainly
caused by hydroquinone or benzoquinone, due to their very low EC value. The oxidation
of phenol by corona in argon produces much higher amounts of dihydroxybenzenes than
the oxidation of phenol by corona in air, which results in high ecotoxicity as is reflected
by the high TU values. Regarding the fact, that at t=0 minutes TU50=3.5, the total
ecotoxicity initially increases considerably but decreases again during oxidation
progress.
Finally, the ecotoxicity is discussed for some hypothetical situations, where a 1.0 mM
phenol solution is completely converted into one of the described oxidation products,
according to stoichiometry, see Table 5.3. By this approach, a comparison of phenol
oxidation products ecotoxicity is illustrated.
Discussion
Table 5.3
121
Ecotoxicity of hypothetical situations, where a 1 mM phenol solution is
completely converted into one of the described oxidation products,
according to stoichiometry.
Oxidation product
from 1 mM phenol
Hydroquinone
Catechol
Resorcinol
Formic acid
Oxalic acid
Glyoxylic acid
Stoichiometry
(mM)
1
1
1
6
3
3
TU50
(-)
1805.1
3.4
0.3
34.9
23.9
19.8
The stoichiometric conversion of a 1 mM phenol solution (TU50=3.3) into a 1 mM
hydroquinone solution thus creates the most dramatic ecotoxicity, while the conversion
of a 1 mM phenol solution into a 1 mM resorcinol solution should result in a decrease of
ecotoxicity according to literature EC50 values [78]. The ecotoxicity increase by
conversion of a 1 mM phenol solution into a solution of the mentioned carboxylic acids
at stoichiometric concentration, is maximum 2 % of the ecotoxicity increase due to the
formation of a 1 mM hydroquinone solution.
Final oxidation products
Phenol oxidation has only been performed during a limited time range in order to ensure
stable oxidation conditions, which is important for accurate efficiency measurements.
By continued oxidation of the investigated product mixtures by corona in air, the
observed polyhydroxybenzenes disappear by ring-cleavage, which is favourable to
ecotoxicity. Also the possibly present quinones will be converted. The produced ringcleavage products, viz. polyfunctional aliphatic hydrocarbons, gradually degrade and
finally all organic carbon has been converted into carbon dioxide. If all carbon dioxide
has left the water, ecotoxicity has disappeared. Theoretically, remaining minor
ecotoxicity may appear from carbonic acid i.e. pH toxicity. However, carbonic acid is a
weak acid: pKa,I=6.46 and pKa,II=10.22; only about 1 % of the dissolved carbon
dioxide appears to react with water [123].
Continued degradation by corona in argon will also eventually result in mineralization,
due to hydroxylation and argon ions & metastables bombardment. By absence of
oxygen, oxygenation now mainly proceeds by hydroxyl radicals, originating from the
dissociation of water. Without detailed information about the composition of the phenol
oxidation product mixtures, it is not justified to state that oxidation of phenol by corona
in argon results in oxidation products with a different oxygen content than the products
obtained from oxidation of phenol by corona in air. On the contrary it is clear, that the
mineralization rate due to corona in argon is different from the mineralization rate due to
corona in air.
Previous to complete mineralization, a large variety of organic oxidation products exists,
which will be discussed in section 5.3.
122
Chapter 5.
5.3. Phenol oxidation pathways
A complete description of the oxidation mechanism of phenol, containing all possible
intermediate products is probably unattainable, because of the huge amount of likely
reactions involved in radical-induced oxidation. Simplification of the oxidation
mechanism is only possible, when the most relevant intermediates have been identified
in the oxidation product mixture. Although a considerable number of oxidation products
has been found by ICE chromatography, the revelation of the identity by mass
spectrometry has appeared to be very complicated.
Therefore, from a theoretical point of view the oxidation of phenol by hydroxyl
radicals/oxygen and ozone is described in this section. The deduction has been
performed by analogy with the studies of Pan and Von Sonntag on the oxidation of
benzene & alkenes [19,50] and fundamental ozone chemistry according to Bailey [49].
The attack of the hydroxyl radical and oxygen on phenol
The action of the hydroxyl radical is always reported in combination with oxygen. The
hydroxyl radical concentration is very low due to its high reactivity, while oxygen is
present in high excess amounts: at T=293K the solubility of oxygen in water is about
1.4 mM [44].
Attack of the hydroxyl radical on the benzene ring of phenol, produces 1,2- and 1,4dihydroxycyclohexadienyl (DHCHD) radicals, see Figure 5.1. The formation of the 1,3DHCHD radical is not discussed in literature, probably because this radical is less stable.
Oxygen adds to these radicals to form dihydroxycyclohexadienylperoxyl (DHCHDP)
radicals. For easy reference, the intermediates and products are coded with a number
according to the attack position of the hydroxyl radical and oxygen.
OH
O2
•
HO
OH
1
5
4
HO
H
1,4-DHCHD
radical
2
3
4
OH
HO
•
H
OH
1,2-DHCHD
radical
Figure 5.1
HO
OO •
HO
H
DHCHDP-14
radical
O2
HO
OO•
H
OH
DHCHDP-12
radical
The production of 1,2- and 1,4-dihydroxycyclohexadienyl (DHCHD)
radicals from the attack of the hydroxyl radical on phenol. Also shown is
the production of dihydroxycyclohexadienylperoxyl (DHCHDP) radicals
from the attack of oxygen on the DHCHD radicals.
Discussion
123
The DHCHDP radical may react in different ways, see Figure 5.2. Catechol can be
produced from the DHCHDP-12 radical by elimination of a hydroperoxyl radical. For the
case of the DHCHDP-14 radical, hydroperoxyl elimination is less probable due to the
larger -H to •OO- distance compared to the DHCHDP-12 radical; hydroquinone is likely
to be formed from the decomposition of a tetraoxide (T) that is produced from the
dimerization of two DHCHDP-14 radicals.
Also possible is the formation of α,α’-endoperoxyalkyl (EPA) radicals. This reaction is
reversible, because of the very weak endoperoxyl bond. The EPA radicals may scavenge
oxygen again and then endoperoxyalkylperoxyl (EPAP) radicals are produced according
to an irreversible reaction.
OH
T
2x
HO
OO •
HO
H
OH
hydroquinone
OH
H
O
O
DHCHDP-14
HO
•
H
HO
O2
O
O
H
H
OO •
H
OH
EPA-14 radical
EPAP-14 radical
OH
OH
HO
+
HO2•
catechol
OO•
H
OH
OH
DHCHD-12
H
•
O
O
OH
H
OH
EPA-12 radical
Figure 5.2
O2
OO
•
H
H
O
O
H
OH
EPAP-12 radical
The production of dihydroxybenzenes or α,α’-endoperoxyalkyl (EPA)
radicals from dihydroxycyclohexadienyl (DHCHD) radicals. Also illustrated
is the formation of endoperoxyalkylperoxyl (EPAP) radicals from the attack
of oxygen on EPA radicals.
The produced dihydroxybenzenes hydroquinone and catechol will also undergo oxidation
and yield trihydroxybenzenes and ring-cleavage products. These pathways are not
discussed, because they are comparable to the described mechanisms.
124
Chapter 5.
The dimerization of EPAP radicals produces a tetroxide (T), that decomposes into two
endoperoxides (EP) and oxygen, see Figure 5.3. The unstable endoperoxides undergo
ring-cleavage; regarding EP-14b/12b, ring-cleavage occurs together with the elimination
of carbon monoxide. In this way, aliphatic monounsaturated C5 & C6 hydrocarbons (P14/12) with carboxyl-, aldehyde-, ketone- or alkanol-functional groups are produced.
H
HO
H
HO
O
O
HO
H
H
O
O
2
H
OO •
OH
H
HO
O
P-14d
O
H
O
H
O
O H
H OH
OH
P-14e
O2
H
H
HO
EP-14a
T
-H2O
P-14c
OH
HO
O
H
O
H
OH
OH
EPAP-14
OH
H
HO
O
O
-CO
O
OH
P-14f
P-14g
OH
EP-14b
HO
HO
H
OO
2
H
H
O
O
H
OH
T
HO O
H
O
O
O
EP-12b
OH
H
O
H
OH
-H2O
O
H
H
O
H
OH
P-12d
P-12c
OH
H
H
OH
P-12e
OH
OH
O
Figure 5.3
H
OH
O2
EPAP-12
OH
O
H
O
H
EP-12a
OH
•
O
O
H
H
O
OH
O
OH
O
OH
H
OH
-CO
H
O
H
OH
O
P-12f
The dimerization of endoperoxyalkylperoxyl (EPAP) radicals produces a
tetroxide (T) that decomposes into endoperoxides and oxygen. The
endoperoxides undergo ring-cleavage and aliphatic monounsaturated
C5 & C6 hydrocarbons (P-14/12) with polyfunctional groups are produced.
Discussion
125
The monounsaturated products P-14/12 can also be attacked by a hydroxyl radical and
oxygen. Also ozone attack is very likely but is not discussed here. Every product P can
produce two hydroxyperoxyl C5 & C6 radicals HP-14/12, because the hydroxyl radical
and oxygen can attack on both sides of the alkene bond, see Figures 5.4 and 5.5.
H
HO
O
O
H
O
H
HO
H
•OO
H
HO
HO
+O2
H
O
H
O
H
O
2x
T
2
P-14d
H
O
O
H
O
H
H
HO
O
O H
H OH
OH
HO
+O2
H
HO
H
HO
H
•OO
O
O H
H OH
OH
HP-14e2
H
OO
H
HO
•
H
O
O H
H OH
OH
HP-14e1
P-14e
O
H
OH
H
HO
O
OH
+O2
OH
H
+
2x
T
H
+
2
T
2
H
HO H
O
T
O
H
+
H
T
2
H
HO
H
OH
saturated
hydroxyperoxyl
C5,C6 radicals
2x
OH
+
T
O
O
O2
OH +
H
O2
H
OH
H
OH
2
O
•
H
H
H
2
+
O
αHA-14e2
OH
αHA-14g1
P-14g1-I
OH
H
H
•
O
O H
O2
αHA-14e1
+
O
H
+
HO
P-14e2-I
2
O2
O
•
OH
2x
+
H
OH
2
H
O
O2
O
αHA-14d2
H
2
+
HO H
H
HO
OH
P-14e1-I
2x
O2
•
O
P-14d2-I
2x
+
H
O
αHA-14d1
O
H
O
•
HO
H
O
2
H
2
O
O
OH
HP-14g2
monounsaturated
polyfunctional
C5,C6 hydrocarbons
Figure 5.4
H
O
OH
HP-14g1
H
HO
H
•OO
P-14g
O
O
P-14d1-I
HP-14d2
HO
H
•OO
H
HO
H
HO
O
HP-14d1
HO
H
HO
H
•OO
H
+
OH
P-14-g2-I
saturated
polyfunctional
C2,C3,C4 hydrocarbons
O
2
H
•
HO
H
αHA-14g2
α-hydroxyalkyl
C2,C3,C4 radicals
The attack of the hydroxyl radical and oxygen on the monounsaturated
ring-cleavage products (P-14) produces hydroxyperoxyl C5 & C6 radicals
(HP-14). Dimerization of HP-14 produces a tetraoxide intermediate (T)
that decomposes into polyfunctional saturated hydrocarbons (P-14-I), αhydroxyalkyl radicals (αHA-14) and oxygen.
126
Chapter 5.
The HP-14/12 radicals dimerize to a tetraoxide, that decomposes into saturated
aliphatic polyfunctional C2-C4 hydrocarbons (P-14/12-I), saturated aliphatic αhydroxyalkyl-functional C2-C4 radicals (αHA-14/12) and oxygen.
O
O
H
O
H
O
H
O
OH
H
O
H
H
OH
•OO H
H
OH HO +O HP-12d1
2
O
H
O
H
P-12d
O
2x
T
H
O
OH
H
H
OH HO
+O2
2
H
OH
O
H
O
H
OH HO
2x
T
H
T
monounsaturated
polyfunctional
C5,C6 hydrocarbons
saturated
hydroxyperoxyl
C5,C6 radicals
H
OH
H
O
O
2
T
2
+
H
O2
H
OH
+
O2
+
O2
+
O2
•
H
αHA-12e2
OH
O
O
H
OH
O
H
P-12f2-I
O
OH
O
+
2
H
2
+
H
•
2
H
OH
H
O
HO
•
αHA-12f1
H
OH
T
O2
αHA-12e1
P-12f1-I
2x
H
OH
O
H
2x
+
•
HO
P-12e2-I
HP-12f1
H
O
OH
H
H
O
OO •
HO H
HP-12f2
+
OH
2
O2
H
αHA-12d2
H
P-12e1-I
2x
+
OH
O
O
OH
O
O
OH
H
2
H
2
P-12d2-I
OH
P-12f
Figure 5.5
+O2
H
•
H
OH
H
αHA-12d1
H
OH+
O
OH
H
O
OH
H
H
O
OH
•OO
H
H
O
HO
O
HP-12e1
HO O
H
H
OH
H
H
O
OO •
HO H
HP-12e2
O
2
P-12d1-I
OH
P-12e
H
H
HO H
HP-12d2
HO O
+
O
H
OH 2x
H
T
OO •
OH
O
HO
H
H
OH
H
H
O
H OH
•OO
O
2
+
saturated
polyfunctional
C2,C3,C4 hydrocarbons
2
O
OH
•
H
αHA-12f2
α-hydroxyalkyl
C2,C3,C4 radicals
The attack of the hydroxyl radical and oxygen on the monounsaturated
ring-cleavage products (P-12) produces hydroxyperoxyl C5 & C6 radicals
(HP-12). Dimerization of HP-12 produces a tetraoxide intermediate (T)
that decomposes into polyfunctional saturated hydrocarbons (P-12-I), αhydroxyalkyl radicals (αHA-12) and oxygen.
Discussion
127
Finally, by scavenging oxygen, the α-hydroxyalkyl radicals (αHA-14/12) are converted
into α-hydroxyalkylperoxyl radicals (αHAP-14/12), see Figure 5.6.
H
O
O2
•
H
HO
O
αHA-14d1
O2
O
•
HO
H
O2
O
HO
αHA-14e1/14g1
H
HO
H
•
O2
O
H
αHA-14e2
OH
H
•
O2
HO
H
αHA-14g2/12e2
O
H
O
HO
•
H
OH
H
O2
OH
O
H
OH
O
HO
•
H
αHA-12e1/12f1
α-hydroxyalkyl
radicals
Figure 5.6
O
H
•OO
HO
HO2•
+
HO2•
+
HO2•
+
HO2•
+
HO2•
+
HO2•
+
HO2•
H
O
HO
P-14e1/14g1-II
H
HO
H
O
O
H
OH
H
P-14e2-II
H
H
O
H
OH
H
P-14g2/12e2-II
O
O
H
OH
O
HO
H
OO•
αHAP-12d1
H
O
H
OH
H
O
P-12d1-II
H
H
OH
OO•
H
αHAP-12d2/12f2
O
O
H
P-12d2/12f2-II
OH
OH
O
+
O
OH
H
H
O2
HO2•
P-14d2-II
H
OH
OH
O
H
αHA-12d2/12f2
OH
O
O
O2
•
O
O
αHAP-14g2/12e2
αHA-12d1
H
H
+
HO H
H
H
O
HO
H
O
P-14d1-II
H
αHAP-14e2
O
H
H
•OO
O
O
OH
OO
H
•
HO
αHAP-14e1/14g1
H
OH
OH
H
αHAP-14d2
OH
H
H
HO H
H
O
•OO
HO
H
αHA-14d2
•
O
O
αHAP-14d1
HO H
H
H
OO
HO
•
H
OH
HO
H
OO •
αHAP-12e1/12f1
α-hydroxyalkylperoxyl
radicals
H
OH
O
O
H
P-12e1/12f1-II
saturated polyfunctional
C2,C3,C4 hydrocarbons
The attack of oxygen on α-hydroxyalkyl radicals produces α-hydroxyalkylperoxyl radicals that decompose into saturated polyfunctional C2-C4
hydrocarbons by elimination of a hydroperoxyl radical.
128
Chapter 5.
The α-hydroxyalkylperoxyl radicals (αHAP-14/12) decompose into saturated aliphatic
C2-C4 hydrocarbons with carboxyl-, aldehyde-, ketone- or alkanol-functional groups by
elimination of hydroperoxyl radicals. A summary of all presented saturated
polyfunctional oxidation products of hydroxyl radical & oxygen - induced phenol
oxidation is given by Figure 5.7. The occurrence frequency of the compounds is
indicated by the compound code used in the oxidation pathways.
O
H
H
P-12d1-I, P-12f1-I, P-12d2/12f2-II
glyoxal
(58 g/mol)
P-14e2-I, P-14g2-I, P-14e1/14g1-II
glyoxylic acid
(74 g/mol)
H
P-14d2-I, P-14d1-II
ketomalonaldehyde
(86 g/mol)
O
P-14d1-I, P-14d2-II
hydroxymalonaldehyde
(88 g/mol)
OH
P-14g1-I, P-12e1-I, P-14g2/12e2-II
3-hydroxy-1,2-propanedione
(88 g/mol)
P-12e2-I, P-12f2-I, P-12e1/12f1-II
2-hydroxy-3-oxopropionic acid
(104 g/mol)
P-12d2-I, P-12d1-II
2-hydroxy-1,3,4-butanetrione
(116 g/mol)
P-14e1-I, P-14e2-II
2,4-dihydroxy-1,3-butanedione
(118 g/mol)
O
H
O
O
OH
O
O
H
O
H
H
HO
O
H
O
H
H
O H
OH
H
OH
H
O
O
O
H
O
H
H
OH
H
O
HO H
O
H
O
H
OH
Figure 5.7
Saturated polyfunctional C2, C3 & C4 hydrocarbons produced from the
oxidation of phenol by hydroxyl radicals and oxygen.
Discussion
129
The attack of ozone on phenol
Next, the electrophilic addition of ozone on phenol is described. Although the phenolate
anion (C6H5O-) reacts considerably faster with ozone than phenol [7], these reactions
have not been described here, because this anion only exists in an alkaline environment.
The two most likely ozone attack positions of the phenol molecule are the 2,3- and 3,4position. The 1,2- position is not considered here, because the hydroxyl group is likely
to cause sterical hindrance. For easy reference, now all intermediates and products are
coded with a number according to the attack position of ozone.
The attack of ozone on phenol produces the molozonides M-23 and M-34, see Figure
5.8. These molozonides rearrange immediately to the ozonides O-23 and O-34. The
unstable ozonides decompose by ring-cleavage to zwitterion-functional products. The
zwitterion group can be formed on both sides of the former attack position, resulting in
four compounds viz. Z-23a/b and Z-34a/b. In water, the zwitterion groups hydrolyze to
hydroxyalkylhydroperoxyde-functional ring-cleavage products (H-23a/b and H34a/b).
OH
OH
O
+ O
C H
OH
O
O
O
O3
M-23
H O
Z-23a
OH
O
O
O
OH
O-23
OH
1
5
4
H
H-23a
OH
H
O
+
C H
O
Z-23b O
2
OOH
OH
O
H
H
H
H-23b
O
OH
OOH
3
4
OH
OH
+
O3
OH
O
O
H
OH
O
M-34
O
Z-34a
O
O
C
O
H O
O
H
OH
O-34
H
+
C HO
H
O
OOH
OH
O
H-34a
OH
O
Z-34b
Figure 5.8
H
HOO
H
O
OH
H-34b
The attack of ozone on the 2,3- or 3,4-position of phenol initially
produces molozonides (M-23/34), which rearrange immediately to
ozonides (O-23/34). The unstable ozonides decompose to zwitterionfunctional ring-cleavage products (Z-23/34). Hydrolysis of the zwitterionfunctional products yields hydroxyalkylhydroperoxide-functional ringcleavage products (H-23/34).
130
Chapter 5.
The first generation hydroxyalkylhydroperoxides (H-23ab and H-34a/b) decomposes into
monounsaturated C6 ring-cleavage products (P-23c/d/e and P-34c/d/e) by release of
water or hydrogen peroxide, see Figure 5.9. Every set of hydroperoxides produces three
different products P, because P-23d and P-34d can be produced from both of the
hydroperoxides. The markers c/d/e identify the six possible products by the position of
a carboxyl or aldehyde endgroup.
O
O
OH
H
O
OH
H
O
H-23a
OH
H
H-23b
OOH
OH
H
O
H
OH
OOH
+
H2O
+
H2O2
+
H2O
+
H2O
+
H2O2
+
H2O
P-23c
O
H
O
H
O
P-23d
O
O
H
OH
O
P-23e
O
O
H OH
OH
H
H
O
H-34a
OOH
OH
O
P-34c
O
H
O
H
OH
O
H
HOO OH
H-34b
H
Figure 5.9
O
P-34d
O
O
H
O OH
P-34e
The decomposition of the first generation hydroxyalkylhydroperoxides
(H23 and H34) into monounsaturated C6 ring-cleavage products (P23 and
P34) with carboxylic acid- or aldehyde-functional endgroups by release of
water or hydrogen peroxide respectively.
Discussion
131
The monounsaturated C6 ring-cleavage products P23c/d/e and P34c/d/e can also be
attacked by ozone. Attack by a hydroxyl radical and oxygen is also possible, but is not
discussed here. Then, a second generation molozonides M-45c/d/e, M-56c/d/e and
ozonides O-45c/d/e, O-45c/d/e is produced, see Figures 5.10 and 5.11. Decomposition
of the second generation of ozonides splits the former phenol molecule into two parts.
A second generation zwitterions (Z-45f/g/h/i and Z-56f/g/h/i) is produced, of which the
Z-45g/56g and Z-45h/56h zwitterions can be formed in two ways. Together with the
zwitterions a first generation of saturated polyfunctional C2 & C4 hydrocarbons is
produced (P-45c1/c2/d1/d2/e1/e2-I and P-56c1/c2/d1/d2/e1/e2-I).
O
O
O
O
OH
H
O
O
OH
H
O
O3
O
O
P-23c
H C
O
O
O
O
OH
H
O
O
O
H
+
O
P-45c1-I
Z-45f
H
O
HO
O
H C
O
+
O
Z-45g
O
O
+
O
H
H
O
O
H
O3
O
O
O
P-23d
O
H
O
H
O
O
M-45d
O
O
O
H
O
H
C
+
Z-45h
H
O
+
O O
O-45d
O
H
P-45d1-I
O
O
+
O
H
O
O O
H C
H
P-45c2-I
O
O
O
H
O O
+
O-45c
M-45c
O
OH
+
H
O
H
P-45d2-I
Z-45g
O
O
H
+
O
O
H
OH
O
O
O3
O
O
P-23e
O
M-45e
O
H
OH
O
O
O
H C
O O
Z-45h
O
O
O
O-45e
H
OH
O
H
OH
O
+
C
O
O
Z-45i
Figure 5.10
OH
+
O
O
H
P-45e1-I
O
O
+
O
H
H
P-45e2-I
Ozone attack on the 4,5-position of the monounsaturated C6 ring-cleavage
products (P-23) yielding a second generation molozonides (M-45) and
ozonides (O-45). The ring-cleavage of the 4,5-ozonides produces a second
generation zwitterions (Z-45) and a first generation saturated C2 & C4
hydrocarbons (P-45-I)
132
Chapter 5.
H
O
O
O
O
H OH
O3
O
O
O
H OH
O
O
O
M-56c
P-34c
O
H
C
O
O
O
O
O
O
+
O
OH
+
H
+
C
O
O-56c
H
O
H
P-56c1-I
Z-56f
O
H OH
O
O
O
O
O
+
H
OH
O
Z-56g
P-56c2-I
H
O
O
O3
H
O
H
O
O
O
O
O
P-34d
H
O H
O
O
O
M-56d
O
C
O
H
H
O
O
+
O
H
H
+
C
O
O-56d
H
O
O
H
Z-56h
O
H
O
+
P-56d1-I
O
O
O
O
+
O
Z-56g
H
H
P-56d2-I
H
O
O3
O
H
OH
O
P-34e
O
O
O
O
H
O OH
M-56e
O
O
O
O
H
O OH
O-56e
OH
+
C
O
H
O
O
Z-56h
O
O
O
O
H
+
C
O
OH
Z-56i
Figure 5.11
O
+
+
O
O
O
H
P-56e1-I
O
O
H
H
P-56e2-I
Ozone attack on the 5,6-position of the monounsaturated C6 ring-cleavage
products (P-34), yielding a second generation molozonides (M-56) and
ozonides (O-56). The ring-cleavage of the 5,6-ozonides produces a second
generation zwitterions (Z-56) and a first generation saturated C2 & C4
hydrocarbons (P-56-I)
The second generation zwitterions (Z-45 and Z-56) hydrolyzes to a second generation
hydroxyalkylhydroperoxides (H-45f/h, H-56f/h, H-4556g/i), which decompose into a
second generation saturated polyfunctional C2 & C4 hydrocarbons (P-45f1/f2/h1/h2-II,
P-56f1/f2/h1/h2-II, P-4556g1/g2/i1/i2-II) by elimination of water or hydrogen peroxide,
see Figure 5.12.
Discussion
133
O
OH
O
H
C
O
O
+
OH
O
O
OH
O
OH
HO
O O
H
P-45f1-II
+
H2O
P-45f2-II
+
H2O2
P-56f1-II
+
H2O
P-56f2-II
+
H2O2
P-45g/56g1-II
+
H2O
P-45g/56g2-II
+
H2O2
P-45h1-II
+
H2O
P-45h2-II
+
H2O2
P-56h1-II
+
H2O
P-56h2-II
+
H2O2
P-45i/56i1-II
+
H2O
P-45i/56i2-II
+
H2O2
OH
OOH
H
O
O
H-45f
Z-45f
O
O
OH
H
O
O
HOO
H
+
C
O
O
OH
O
O
O
OH
OH
Z-56f
H-56f
H
H
HO
H
H C+ O
O O
Z-45g=Z-56g
OH
O
O
O
H
OH
H
O
O
O
O
OH
H
OOH
H-45g=H56g
O
O
H O
O
O
H
H
C
O
O
H
+
O O
O
HO
H
OOH
Z-45h
H
H-45h
O
O
+
C
O
H
O
O
HOO
H
O
H
O
O
O
OH
H
HO
H
O
O
O
O
H
H-56h
Z-56h
H
OH
O
O
H
O
H
OH
H
+
C
O
OH
O
O
Z-45i=Z-56i
OH
HO
H
O
O
O
OH
OOH
H-45i=H-56i
OH
O
O
H
Figure 5.12
Hydrolysis of the second generation zwitterions (Z-45 and Z-56). Also
illustrated
is
the
decomposition
of
the
second
generation
hydroxyalkylhydroperoxides (H-45 and H-56) into a second generation
saturated C2 & C4 ring-cleavage products (P-45/56-II) with carboxylic acidor aldehyde-functional endgroups, by release of water or hydrogen
peroxide respectively.
134
Chapter 5.
A summary of all presented saturated polyfunctional oxidation products of ozoneinduced phenol oxidation is given by Figure 5.13. The occurrence frequency of the
compounds is indicated by the compound code used in the oxidation pathways.
H
O
O
H
P-45c1-I, P-45d1-I, P-56c1-I,
P-56d1-I,P-45g/56g2_II
glyoxal
(58 g/mol)
P-45e1-I, P-56e1-I, P-45g/56g1-II,
P-45i/56i2-II
glyoxylic acid
(74 g/mol)
P-45i/56i1-II
oxalic acid
(90 g/mol)
P-45d2-I, P45-e2-I, P-56d2-I,
P-56e2-I, P-45h2-II, P-56h2-II
1,2,4-butanetrione
(100 g/mol)
P-45c2-I, P-45f2-II, P-56h1-II
2,4-dioxobutyric acid
(116 g/mol)
P-56c2-I, P-56f2-II, P-45h1-II
3,4-dioxobutyric acid
(116 g/mol)
P-45f1-II, P56f1-II
oxo-succinic acid
(132 g/mol)
OH
O
O
H
OH
O
O
OH
O
O
H
O
H
O
HO
O
H
O
O
O
O
H
OH
O
OH
O
OH
O
Figure 5.13
Saturated polyfunctional C2 & C4 hydrocarbons produced from the
oxidation of phenol by ozone.
Discussion
135
The described reaction chemistry of the oxidation of phenol by hydroxyl radicals,
oxygen and ozone accounts for the formation of a broad range of polyfunctional
oxidation products, but is still incomplete. Many of the described products are not
stable and will further degrade due to the rigorous oxidizing conditions. With regard to
the possible oxidation products, a following summary can be made:
The oxidation of phenol by hydroxyl radicals and oxygen initially yields
dihydroxybenzenes and monounsaturated aliphatic C5 and C6 hydrocarbons. The
dihydroxybenzenes are either oxidized to trihydroxybenzenes or undergo ring-cleavage
like phenol. Repeated attacks of the hydroxyl radical and oxygen result in the formation
of saturated aliphatic C2, C3 and C4 hydrocarbons with carboxyl-, aldehyde-, ketoneand/or alkanol-functional groups.
The oxidation of phenol by ozone initially yields di- and monounsaturated aliphatic C6
hydrocarbons. Attack of ozone on these ring-cleavage products yields saturated
aliphatic C2 and C4 hydrocarbons with carboxyl-, aldehyde- and/or ketone-functional
groups.
Glyoxal, glyoxylic acid and oxalic acid are well-known stable oxidation products of
phenol, see section 2.4.1. In addition these products form an oxidation decay series to
carbon dioxide. Ketomalonaldehyde and hydroxymalonaldehyde are precursors to
ketomalonic acid, which is described in literature, see section 2.4.1. The products with
aldehyde-functional endgroups will be oxidized to carboxylic acids. Carboxyaldehydes
are mentioned in literature [124] as products from the cleavage of aromatic compounds
by ozone. With regard to the produced oxocarboxylic acids (α: RC(O)COOH, β:
RC(O)CH2COOH, γ: RC(O)-(CH2)2COOH) the α-form is relatively stable, the β-form
decomposes by decarboxylation to corresponding ketones and the γ-form is again stable
and does not decarboxylate [125]. β-hydroxy aldehydes (R-C(OH)-CH2-CHO) generally
dimerize or polymerize [126].
Oxalic acid, glyoxylic acid and polyhydroxybenzenes have been identified in this study,
see section 4.2.1. Although the other observed products viz. formic, acetic, propionic,
malonic, succinic and maleic acid cannot be directly traced in the presented oxidation
pathways, these compounds are eventually explained from continued degradation of the
products described in the models or from combined OH/O3 oxidation reactions.
The phenol degradation pathways have been described for either oxidation by the
hydroxyl radical or ozone, in order to present a clear model. For the case of practical
conditions for corona oxidation in humid air, all described reactions occur mixed up and
also other reactive species will be present like e.g. singlet oxygen, metastables, ions
and UV photons.
Modeling of the reaction kinetics of phenol oxidation by pulsed corona discharges is
only meaningful, if the relevant reaction pathways are known from experimental
observations. This has been the main problem, because both separation and
identification of the theoretically possible intermediates/products is analytically
considered to be very difficult. Detailed clarification of the phenol oxidation mechanism
by e.g. using labelled compounds, blocking of reactive molecular sites and chemical
derivatization reaches beyond the scope of this thesis.
136
Chapter 5.
Toxicological aspects of phenol degradation
The discharge of oxidized phenol solutions into surface waters may lead to oxygen
depletion, due to the presence of polyhydroxybenzenes, which are strong reducing
agents. Hydroquinone and 1,4-benzoquinone show very high ecotoxicity according to
the Microtox test. Although several carboxylic acids produced during mineralization of
phenol occur in nature, a concentrated discharge flow causes ecotoxicity due to acidity.
Although direct human intoxification by phenol oxidation products is not likely, indirect
exposure via the food chain may eventually occur. Therefore a short discussion about
human toxicity is presented [1,61,127-129]:
All hydroxybenzenes cause irritation or etching of skin and mucous membranes and are
skin poisons. After resorption respirational paralysis takes place. 1,4-benzoquinone
causes severe eye injury. Global fatal doses are: phenol 5-10 g, resorcinol 12 g,
hydroquinone 5-12 g, pyrogallol 2-3 g. Hydroxybenzenes exhibit mutagenic properties,
this means that these compounds cause chromosomic and genetic abberations;
mutagenic activity is reported according to the order hydroquinone >
hydroxyhydroquinone > pyrogallol.
For the dimerization products 2,2’- and 4,4’-dihydroxybiphenyl human mutation data
are reported. The multiring condensation product dibenzo-p-dioxin is a questionable
carcinogenic compound.
Although endoperoxides cannot be isolated from the oxidation product mixture because
of their instability thus reactivity, these compounds probably are the most harmful
intermediates. Endoperoxides may also release singlet oxygen which is highly reactive
towards physiological materials [43].
Unsaturated carboxylic acids like muconic, maleic, fumaric and acrylic acid are harmful
due to their reactivity of the alkene bond. By metabolysis, the alkene bond may be
converted into a carcinogenic epoxide, as has been explicitly described for benzene and
polyaromatic hydrocarbons. The epoxide causes DNA damage, because it reacts with
nucleic acids.
Aldehydes exhibit mutagenic properties, cause strong irritation of the mucous
membranes and skin and act on the central nervous system; examples are
formaldehyde, acetaldehyde, acrolein and glyoxal. Unsaturated aldehydes (enals) are
reported to be particularly reactive with some biological molecules [124].
Formic and acetic acid are entitled as etching protoplasma poisons due to their acidity,
but their salts show low toxicity. Skin poisoning is possible for the case of formic acid,
because of fat-dissolving properties. Acetic acid exhibits a lethal dose value of about
20-50 g. In nature, formic acid occurs in stinging-nettle and ants, while acetic acid is an
important ingredient of vinegar.
Oxalic acid shows a very strong etching effect (low pKa), combines calcium and causes
kidney damage or cardial paralysis; the lethal dose value is about 5-15 g. Oxalic acid
salts are toxic. In nature it occurs in small amounts in rhubarb and spinach.
Glyoxylic acid is a metabolite in mammalian biochemical pathways. It also occurs in in
plants e.g. in unripe food e.g gooseberries. It is oxidized in the human body to oxalic
acid and thus causes comparable effects. The lethal dose LD50=2500 mg/kg (oral,rat).
Glyoxylic acid has possibly mutagenic properties.
Discussion
137
5.4. Analysis techniques
Liquid chromatography
The identification of corona-induced liquid phase oxidation products of organic
compounds is best performed by ion-exclusion chromatography (ICE). The oxidation
mechanism induced by pulsed corona discharges results in a diverse product range,
consisting of several aliphatic carboxylic acids, which can be properly separated by ionexclusion chromatography. The polyhydroxybenzene intermediate oxidation products do
not interfere with the carboxylic acids.
The application of a conductivity detector in series with a UV absorbance detector
offers specific sensitivity for the carboxylic acids. For the case of a target compound
with fluorescent properties like e.g. polycyclic aromatic hydrocarbons, benzene, phenol
and laser dyes, a fluorescence detector offers conversion measurements with much
higher sensitivity, although the non-fluorescent degradation products require yet
identification by a UV absorbance or conductivity detector.
The required analysis time of phenol-containing oxidation products is long viz. about
30 minutes, due to strong interaction of phenol with the partially-crosslinked
polystyrene-divinylbenzene stationary phase resin of the ICE column. Depending on the
column type, it may be possible to introduce organic modifier into the acidic aqueous
eluent by means of a gradient elution program. In this way the retention of phenol
and/or polyhydroxybenzenes may be forced after elution of the complex carboxylic acid
range, in order to shorten the analysis time. However, this option has not been
investigated, because identical column performance cannot be guaranteed anymore
after operation with organic modifier-based acidic eluents.
Mass spectrometry
The product range complexity rapidly increases with the complexity of the target
compound molecular structure. The fact, that the number of possible oxidation products
from a very simple molecular structure like a benzene ring is already extensive,
inevitably demands the application of mass spectrometry for identifcation after
separation. Mass spectrometry has been applied both directly by electron-impact MS
and via IonSpray and APCI interfaces with the liquid chromatograph (off-line MS).
Direct mass analysis of oxidized high initial concentration phenol solutions has not
revealed significant information, although the appearance of the oxidized solution
implies thorough chemical changes. Concerning off-line MS, the signals of separated
components are overruled by the high background signal due to the acidic ICE eluent.
The available interfaces are not suitable for handling the separated low molecular
weight components.
138
Chapter 5.
Gas chromatography
Although GC-MS technology is more straightforward than LC-MS technology, the GC
analysis of high water content samples is problematic. Therefore the oxidation products
have to be transferred from the aqueous phase to an organic phase, by solid-phase
extraction or freeze-drying. Several attempts have failed, because certain compounds
within the complex mixture have been lost during this sample preparation step. For
application of GC-MS, also chemical derivatization of the oxidation product compounds
may be necessary, for the production of thermally stable compounds and for improved
separation and detectability.
Aldehyde screening test
For gas phase analysis, both gas chromatography-mass spectrometry (GC-MS) and
Fourier transform Infrared spectroscopy (FTIR) have been applied. Chemical
derivatization has been applied to trap possible aldehydes present in the gas phase. Gas
sampling can be performed in-field; the storage life of tubes after sampling depends on
the analyte and the sampling/derivatization technique: sampled aldehyde screening
tubes are stable for at least one week at 25°C. Nevertheless, here the performed tests
have been directly processed. Direct gas sampling may be disadvantageous, due to the
presence of water vapour that is produced by the corona discharges over the aqueous
solution.
Hydroxyl radical identification
The fact that in-situ ESR has not revealed hydroxyl radicals can be explained by the low
sensitivity of ESR compared to fluorescence spectrometry. Also it is reported by Sun
[130], that the spin trapping efficiency η of DMPO for hydroxyl radicals is only η=33
±3 % for OH generated by photolysis of hydrogen peroxide and η=28 ±3 % for OH
generated by photocatalytic oxidation of water. The trapping of hydroxyl radicals by
DMPO is thus likely to be far from effective, although DMPO is a well-known and
widely applied spin trap.
Fourier transform infrared spectroscopy
Gas phase analysis by FTIR can be performed in-situ and is highly sensitive. Simple
molecules like oxides of carbon, nitrogen and sulfur are completely identified.
Unfortunately, full revelation of the identity of a complex hydrocarbon, especially in a
mixture, is nearly impossible. On the contrary, the presence of functional groups like
e.g. hydroxyl (carboxylic acids, alkanols), carbonyl (aldehydes, carboxylic acids, esters,
ketones), nitrogen-based groups (nitriles, amines, amides, nitro compounds), sulfurbased groups (thiols) and the presence of aromaticity (mono, polycyclic) or aliphatic
unsaturated hydrocarbons (alkenes, alkynes) can be monitored accurately. It should be
noted, that in-situ FTIR measurements may encounter disturbance by water vapour.
Discussion
139
Liquid chromatography versus LIF-spectroscopy
Reversed-phase HPLC features a very powerful separation of the oxidation product
mixture components combined with identification by a wide range of detectors. On the
contrary, samples have to be analyzed off-line and the analysis times are always longer
than that of in-situ spectroscopic measurements. Sample ageing is relevant for the case
of e.g. the phenol oxidation product mixture, because after stopping the corona
discharge experiment i.e. the oxidation time, residual generated oxidizers continue the
degradation in the time lag before analysis. Especially the trihydroxybenzenes are
susceptible to further degradation after sampling. In addition, all hydroxybenzenes are
light-sensitive.
LIF spectroscopy features a high sensitivity, time- and space-resolved analysis of
fluorescent priority compounds like e.g. polyaromatic hydrocarbons, benzene and
aniline. Although LIF spectroscopy is actually applied for the study of gas phase
reactions, its application for the study of fluorescent molecules in aqueous solution is
possible, but requires some points of attention. Only low concentration solutions can be
studied, because laser absorption by a high concentration solution results in conversiondependent excitation. Despite fluorescence quenching, the conversion determination
can be justified by decomposition of the solution fluorescence spectrum into the
spectrum of the target compound and oxidation products. Analyte photolysis due to the
excitation photons can be minimized, by application of short exposure times and low
laser power.
By electronic excitation, the acid dissociation constant pKa of phenol is reported to
change considerably that is, phenol becomes a much stronger acid [131]. In the ground
state S0 pKa=10, while for the first excited singlet state S1 pKa=3.6-4.0. For the
lowest triplet state T1 pKa=8.5. By laser excitation, excited phenol molecules may be
produced that will dissociate into phenolate anions according to the Equations 5.4ab.
{C6H5OH}* + H2O H3O+ + {C6H5O-}*
Ka =
[H3O + ][{C6 H5O − }*]
[{C6 H5OH }*]
(5.4a)
(5.4b)
The phenolate anions are preferentially attacked by electrophilic oxidizer species. In
liquid chromatography, the detection of separated components by UV absorbance or
fluorescence indeed also creates excited phenol molecules, but this is an ex-situ
measurement.
If phenol conversion measurements by in-situ LIF spectroscopy are biassed by the
mentioned effect, the excitation volume should be sufficiently high and also oxidizers
should be present in that volume. It is likely, that oxidation mainly takes place at the
gas-liquid interface; the laser beam enters the reactor vessel some centimeters below
the water surface. The excitation volume is estimated from a 3 mm laser beam waist
and 10 cm optical path length to be about 0.7 ml which is only 0.2 v/v% or 0.7 v/v %
of the total reactor volume (300 or 100 ml). Therefore this effect can be ignored.
140
Chapter 5.
Conductometry
Conductometry measurements in general may be applied as very cheap and simple insitu tests to globally monitor oxidation progress of C/H/O-based target compounds infield, by the increase of the carboxylic acid concentration. For the case of halogen/
nonmetal-based organic target compounds (viz. chlorobenzene and aniline), conductivity
increase during oxidation progress is also due to the formation of inorganic ions
(chloride and nitrate respectively) and then specific pH conductometry is to be preferred
for monitoring the disappearance of total carbon. For that case, a certain conductivity
thus remains after complete mineralization of the target compound.
Microtox ecotoxicity
The toxicity change due to degradation of phenol by pulsed corona discharges has been
monitored by a bacteria test viz. the Microtox ecotoxicity test. Actually, the
determination of environmental toxicity should also take place for other trophic levels
like for fish, crustaceans, algae and activated sludge, however this approach reaches
beyond the scope of this thesis. The toxicity effects revealed by the Microtox test
cannot be related to human toxicity, because these are based on bacteria viz. Vibrio
fischeri. This can be illustrated by the EC505min effect values [78] and human lethal dose
values [127] of some compounds with very high human toxicity: potassium cyanide:
EC505min= 4.77 mg/l CN- while 0.15-0.25 g is an average lethal dose for adults;
aflatoxin B1: EC505min=21.97 mg/l while 5 µg/kg is an upper exposure limit; arsenic (III)
oxide: EC505 min=73.73 mg/l while 0.12-0.3 g is a lethal dose for adults.
General remarks
It can be summarized that all applied analysis techniques have generally yielded very
consistent results. Hydroxyl radicals account for the formation of polyhydroxybenzenes
from phenol and have been demonstrated by fluorescence spectrometry using the
molecular probe CCA. Ozone is consumed by phenol in aqueous solution according to
UV absorbance spectrometry and accounts for the formation of carboxylic acids
together with hydroxyl radicals and oxygen. Polyhydroxybenzenes have been identified
by rp-HPLC, ICE, LIF and the Microtox test. Carboxylic acids have been demonstrated
by ICE and conductometry. No other gaseous phenol oxidation products than carbon
dioxide have been identified according to FTIR, the aldehyde test and the TOC
measurements. The less consistent results, viz. radical identification by ESR and the
identification of oxidation products by MS, are explained by sensitivity problems and/or
non-optimal conditions.
Of particular interest for global in-field monitoring of oxidation progress are in-situ UV
absorbance or fluorescence spectrometry and conductometry. UV absorbance is
generally applicable to organic compounds, while fluorescence spectroscopy has a very
high dynamic range but is only applicable to fluorescent molecules; on the contrary
several aromatic compounds exhibit fluorescent properties [85-87,131]. Unfortunately,
spectroscopic analysis of turbid waste water is not possible; TOC measurements then
are more favourable but these measurements are not in-situ and are more complicated.
Conductometry is cheap, simple and suitable for monitoring oxidation progress of any
organic compound, but the results are rather emperical.
Discussion
141
5.5. AOP comparison
Fundamental merits and problems are described for the advanced oxidation processes
[132,12] discussed in section 1.1 together with corona discharge technology. A
financial comparison of these technologies has not been part of this study. General
problems concerning oxidation are the conversion of nitrogen from air into nitrogen
oxides and nitrous oxide. Nitrogen oxides contribute to environmental acidification;
nitrogen dioxide is suspected to exhibit human reproductive toxicity [133]. Nitrous
oxide is a greenhouse gas. Oxidation of halide-containing waste water may result in
halogenation of organic target compounds [124]; also problematic is the oxidation of
bromide to carcinogenic bromate (BrO3-).
Ozone/UV
Pro: Ozone is a powerful oxidizer that can be produced from a simple ozonizer setup
and air. Also, hydrogen peroxide is produced from the oxidation of water by ozone.
Contra: The reactions of ozone in aqueous solution are mass transfer limited. Molecular
ozone reacts much more slowly than hydroxyl radicals. Unreacted/undecomposed ozone
leaving the reactor has to be detoxified e.g. by chemical reduction. The UV-lamp power
efficiency and lamp life are limited; the penetration depth of UV radiation in turbid
aqueous solutions is low.
Hydrogen peroxide/UV
Pro: Hydrogen peroxide is a pure source of hydroxyl radicals. Activation can be applied
by UV photons and/or iron (II,III) salts. The quantum yield for generation of hydroxyl
radicals from photolysis of hydrogen peroxide is about 1.0.
Contra: The transport, storage and handling of hydrogen peroxide require special safety
precautions. Hydrogen peroxide shows only weak absorption in the range 200-300 nm
and also absorption at wavelengths higher than 300 nm is not significant; by addition of
iron salts the hydroxyl radical production efficiency is greatly enhanced, but a high iron
salt concentration is required according to stoichiometry. The application of other salts
than iron hydroxo/carboxyl chelates causes unnecessary release of anions like e.g.
sulfate, chloride or nitrate. Lamp power efficiency and lamp life are limited. Turbid
waste water is problematic.
Photocatalytic oxidation
Pro: A simple setup is required.
Contra: The quantum yield for the generation of hydroxyl radicals on the surface of the
most generally applied photocatalyst titanium dioxide is low. Indicated numbers are only
4-8% for TiO2 slurries or even lower for immobilized photocatalyst particles. Also the
quantum yield is dependent on the light intensity. Mainly for these reasons, the scale-up
procedure from laboratory setup to industrial application is problematic. Mass transfer
limitation occurs for immobilized photocatalysts. After reaction, suspended
photocatalyst particles have to be separated from the oxidation product mixture. Again,
lamp power efficiency and lamp life are limited. Turbid waste water is problematic.
142
Chapter 5.
Wet oxidation
Pro: Supercritical water conditions favour high solubility of organic compounds plus
oxygen and extreme chemical reactivity.
Contra: The setup requires autoclave-proof housing including e.g. high-pressure pumps,
(pre)heaters, compressors thus only has large scale applicability. Oxygen addition is
necessary. After oxidation, vapour/liquid/solids separation and cooling of the product
mixture are necessary. Due to depressurization, only batch flow operation is possible.
The vigorous conditions may enable polymerization and multiring condensation of
organic compounds.
Electron beam treatment
Pro: The generation efficiency of hydroxyl radicals by high energy electrons is about
2.7 OH per 100 eV. The E-beam technology is especially suitable for the degradation of
halogenated hydrocarbons, because these compounds react rapidly with solvated
electrons. Waste water containing solid matter up to 5 % is accepted without pretreatment. Continuous flow operation is possible.
Contra: The setup involves complex equipment and the technology only has large scale
applicability. The shielding of β - radiation is necessary. The appearance of electrons from
the setup through the titanium foil high vacuum separator causes energy dissipation.
Idle E-beam setups require power due to vacuum maintenance and cathode heating. The
application of nitrous oxide for improved hydroxyl radical production from aqueous
electrons requires additional facilities.
Ultrasonic irradiation
Pro: With a simple setup, vigorous conditions can be created in aqueous solution.
Continuous flow operation is possible.
Contra: A main issue is the fact that the scale-up procedure is complex. The shielding
of ultrasound is important, because the intense first subharmonic of the applied driving
frequency and white noise cause hearing impairment; also, a potential hazard is the
formation of aerosols from the harmful solution by surface wave activity [134]. The
transducer device undergoes erosion by intense cavitation.
Pulsed corona
Pro: The pulsed corona technology features a simple setup and produces a broad range
of oxidizers. Oxidizers are produced by highly efficient processes. Target compounds
can be oxidized in both the liquid phase and gas phase. Continuous flow operation is
possible.
Contra: Shielding of electromagnetic radiation is necessary to avoid interference with
adjacent electronic devices. The life of pulsed-corona circuits is limited, because longterm generation of steep pulses will eventually strain the electrical components. Anode
corrosion may occur.
Discussion
143
Electric arc
Pro: The thermal plasma offers the highest destruction power. Conversion of gas, liquid
and solid phase target compounds is possible.
Contra: The energy consumption is very high. After decomposition, cooling and
stripping of the gaseous reaction products is necessary. The required facilities require
large scale operation. The process is batch-wise. The electrodes life is limited.
General remarks
It is sensible to state, that one particular ideal AOP does not exist. The applicability
depends on e.g. the nature of the target compound(s), the pollution magnitude and
concentration, geographical location of the pollution and AOP performance stability.
Extremely dangerous materials like 2,3,7,8-tetrachlorodibenzo-p-dioxin [1] have to be
destructed regardless of the costs. With regard to high quantities, continuous-flow
operation is preferable to batch-wise processing. Treatment of low concentration
intermediate toxic waste flows should be performed with maximum efficiency. The AOP
setup complexity and pollution magnitude determine whether it is affordable to build or
position the setup at/near the location where the pollution is situated. The AOP
performance stability is dependent on e.g. the process-technological complexity but also
on the input flow composition stability; therefore every AOP should be continuously
monitored by chemical-analytical measurement techniques on the processed waste flow
to guard performance stability.
With regard to the obtained results described in this thesis and mentioned corona
literature references, pulsed corona technology can be considered as a true AOP. Its
strongest feature is the efficient conversion of low concentration target compounds
from both the liquid or gas phase, using simple technology. It is worth-wile to study
scale-up possibilities. Then also a cost analysis can be derived. At this experimental
stage of investigation, it is not possible to determine the process costs for a specific
detoxification: unknown parameters are viz. the size of power supply, power charges,
reactor design, target compound(s), destruction level, flow rate, pollution magnitude,
maintenance costs, setup depreciation and analytical monitoring costs.
144
Chapter 5.
6. Conclusions
6.1. Pulsed corona discharges
It has been demonstrated that the application of pulsed corona discharges in the gas
phase over aqueous target compound solutions is able to degrade the target compound
by oxidation. This application of corona in air has a more powerful effect than an ozone
generator, because apart from ozone a wide variety of highly reactive species are
produced like hydroxyl radicals, singlet oxygen, metastables, ions and UV photons.
Hydroxyl radicals are produced in the gas phase by ionization and dissociation of water
molecules at the corona discharge streamer tips. Also, hydroxyl radicals are produced
indirectly, in the aqueous phase from ozone and UV photons, where ozone has been
produced by the dissociation of oxygen molecules.
The production of hydroxyl radicals in water has been investigated by ex-situ
fluorescence spectrometry using a molecular probe and in-situ electron spin resonance.
The molecular probe coumarin-3-carboxylic acid (CCA) has revealed a linear
fluorescence increase with the corona exposure time, implying a constant hydroxyl
radical production rate for the observed time span. Quantitative hydroxyl radical
measurements using CCA-hydrogen peroxide standards have failed, due to instability of
dilute hydrogen peroxide solutions and poor CCA solubility in water. In-situ electron
spin resonance using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has not
revealed hydroxyl radicals, which has been attributed to an insufficient sensitivity
compared to fluorescence spectrometry.
The production rate of ozone has been determined by UV absorbance spectrometry.
The ozone concentration increases with the load voltage, while negative corona always
yields less ozone than positive corona at the same absolute value of the load voltage.
The ozone concentration over aqueous phenol solutions has appeared to be lower than
the ozone concentration over deionized water, which has been explained by the
reaction of ozone with phenol. In the applied setups, ozone concentrations up to about
5.5⋅1022 m-3 in air and about 8.0⋅1022 m-3 in oxygen have been measured. The obtained
ozone production efficiency is about 40 gO3/kWh which is quite favourable, because it
has been achieved in humid air.
The action of radicals, metastables and ions has been illustrated by the application of
corona in argon and helium for the degradation of phenol in aqueous solution. Although
under inert gas conditions no ozone can be produced, considerable conversion has been
measured. This conversion is caused by the production of hydroxyl radicals from the
dissociation of water by ions and metastables bombardment.
From analyses of corona-exposed deionized water by electrical conductometry, ionexclusion chromatography and spectrochemical ICP analysis it has been derived, that
nitric acid is formed from nitrogen oxides, produced by the corona discharges in air.
From experiments on the effect of different electrode configurations on the conversion
of phenol and decolorization of malachite green, it has been concluded, that a
favourable electrode configuration consists of a capacitive configuration with a multipin
anode. The dielectrical separation of cathode and anode avoids conductive currents in
the water, which dissipate energy at the expense of contributing to radical production.
The multipin anode produces a more efficiently-dimensioned reactive volume for the
production of oxidizers than a single pin.
146
Chapter 6.
With regard to the generally applied corona conditions, these are a voltage V=25 kV, a
pulse repetition rate f=100 Hz, an anode-tip-to-water distance d=1.0 cm and a
solution volume Vol≤500 ml, the measured pulse energy range is about 5 -15 mJ.
The application of corona in the gas phase over aqueous target compound solutions has
appeared to be more efficient than application of corona in the aqueous phase. This has
been demonstrated by both a comparison of phenol efficiency values from literature
with experimentally obtained values and pulse energy measurements in aqueous
solution. Calorimetry-based pulse energy measurements in deionized water have yielded
values of about 55-61 ±9 mJ at a voltage V=19 kV and pulse repetition rate f=100
Hz. The application of corona discharges in the liquid phase indeed produces radicals in
the direct vicinity of the target compound. However, for creation of the discharges the
liquid phase at the anode tip has to be evaporated, which is less efficient than the
production of an oxidative environment in the gas phase over the aqueous target
compound solution. The order of magnitude difference in pulse energy between liquid
phase and gas phase corona is in accordance with the observed efficiency differences,
see Table 5.1.
6.2. Oxidation of model compounds
The oxidation of phenol in aqueous solution has been studied in detail. The phenol
oxidation product mixture has a complex composition. Observed oxidation products are
dihydroxybenzenes, trihydroxybenzenes, carboxylic acids and carbon dioxide. The
following polyhydroxybenzenes have been identified: catechol, resorcinol,
hydroquinone, pyrogallol and hydroxyhydroquinone. The possible existence of quinonoid
compounds could not be confirmed. Several aliphatic carboxylic acids have been
observed: formic, acetic, propionic, oxalic, malonic, maleic, succinic and glyoxylic acid;
possibly also a muconic acid enantiomer. Except for the carboxyaldehyde glyoxylic acid,
no aldehydes have been detected.
Pathways have been constructed for the oxidation of phenol by the hydroxyl radical and
oxygen and for phenol oxidation by ozone. The described mechanisms account for the
production of polyhydroxybenzenes and a broad range of aliphatic carboxylic acids with
polyfunctional groups.
The measured conversion efficiency for a 500 ml 1.0 mM phenol solution by corona in
air at a conversion X=24% is G=2.2⋅10-8 J/mol ≡ 0.21 (100eV)-1 ≡ 7.5 g/kWh.
After 2 hours of oxidation of a 100 ml 1.0 mM phenol solution by corona in air and
argon using the generally applied corona parameters, a quantitative product analysis has
yielded the following approximate concentrations (air/argon values in mmol/l): phenol:
0.43/0.12 mM; hydroquinone: 0/0.01 mM; total dihydroxybenzenes: 0.08/0.28 mM;
formic acid: 0.23/0.09 mM; oxalic acid: 0.39/0 mM; glyoxylic acid: 0.05/0.03 mM;
glyoxal: 0/0 mM.
The oxidation mechanism of phenol by corona in argon appears to be very different
from the oxidation of phenol by corona in air. Degradation of phenol by corona in argon
mainly yields polyhydroxybenzenes due to hydroxyl radicals produced from the
dissociation of water by ion/metastables bombardment; in argon, ring-cleavage is of
minor importance because of the absence of oxygen and ozone.
Conclusions
147
Application of a Microtox ecotoxicity test on a series of 1 mM phenol solutions exposed
to corona in air for different times, has yielded a substantial toxicity increase. This
effect is mainly due to the presence of polyhydroxybenzene- and possibly also quinoneintermediate products; especially hydroquinone and 1.4-benzoquinone show very high
ecotoxicity. The formation of carboxylic acids also accounts for the toxicity increase,
but to a far less extent. The toxicity increase is only temporal, because the
polyhydroxybenzenes, quinones and eventually also the carboxylic acids will be
mineralized. The temporal toxicity increase is not a specific problem of corona
technology, but is inherent to oxidation.
Carbon dioxide has been detected in the gas phase over oxidized phenol solutions, but
carbon monoxide and volatile aldehydes have not been identified. After 3 hours of
oxidation using the standard corona conditions, about 0.8% carbon from a 1.0 mM
phenol solution and about 6.3% carbon from a 0.1 mM phenol solution have been
converted into CO2. The amount of CO2 produced by phenol oxidation by corona in air
is about 2.7 times higher than the amount produced by corona in argon. The total
organic carbon content appears to be rather constant during oxidation, which implies
that for the observed oxidation time span the oxidation products mainly remain in the
liquid phase, except for the small amounts of CO2 produced.
The degradation of the atrazine herbicide, malachite green dye and dimethyl sulfide odor
component by pulsed corona discharges has resulted as follows. The conversion
efficiency of a 500 ml 0.12 mM atrazine solution oxidized by corona in air at a
conversion level X=13 % is about 7.7⋅10-10 mol/J ≡ 7.4⋅10-3 (100eV)-1 ≡ 0.6 g/kWh.
This low efficiency is inherent to persistent triazine herbicides. In order to achieve a
24% decolorization of malachite green at λ=590 nm, the degradation efficiency using
several electrode configurations has been determined. An immersed electrode
configuration with a single pin anode has yielded the lowest efficiency viz. about
7 mg/kWh; a capacitive electrode configuration with a 4-pin anode has yielded the
highest efficiency: about 412 mg/kWh. Dimethyl sulfide has been degraded in the gas
phase. The conversion of air containing 2.5 ppm dimethyl sulfide has been measured to
be 45% after 15 seconds, 70% after 30 seconds and 94% after 1 minute. Corona
treatment appears to be very effective for stench abatement.
6.3. Analytical techniques
Ion-exclusion chromatography (ICE) has appeared to be much more powerful than
reversed-phase HPLC for separation of the phenol oxidation product mixture. Although
ICE has been designed for the separation of complex mixtures of carboxylic acids, also
the polyhydroxybenzenes are well-separated mutually and do not interfere with the
acids. On the contrary, the separation of individual polyhydroxybenzene isomers is best
performed by reversed-phase HPLC.
Identification of the complex phenol oxidation product mixture by mass spectrometry is
necessary before quantitative analyses can be performed. However, both off-line LC-MS
and electron-impact MS have yielded very limited information about the composition of
the oxidation product mixture. The applied IonSpray and APCI LC-MS interfaces appear
to be not applicable to the relatively low molecular weight phenol oxidation products.
Also, the individual product concentrations are low.
148
Chapter 6.
The applied solid phase extraction technique has appeared to be not successful.
Therefore, based on literature, major oxidation products have been identified by
comparison of retention times.
For global in-field monitoring of oxidation progress of an organic compound, acidity
increase by electrical conductometry is a very unpretentious option. In addition, simple
devices based on detection by UV absorbance or fluorescence (when applicable) can be
applied to monitor the conversion of organic materials. However for the case of turbid
waste water, spectroscopic analysis techniques fail; then, TOC measurements are to be
preferred.
Due to sensitivity differences, in-situ ESR has not been able to resolve hydroxyl
radicals, while fluorescence spectrometry using the CCA molecular probe has distinctly
demonstrated hydroxyl radical production in aqueous solution.
6.4. Outlook
The very favourable conversion efficiency justifies the continuation of pulsed coronainduced water treatment; topics of interest may be described as follows. Optimization
of the reactor/electrode configuration can be achieved by increasing the contact area
between the target compound solution and the corona discharges. Also an investigation
of corona pulse duration and pulse shape may result in an even higher oxidizer
production efficiency. Continuous flow application, scale-up possibilities and process
stability should be studied. With regard to commercially applied organic
electrosynthesis, it is interesting to investigate the possibilities of corona-induced
synthesis. As electric discharge source for waste water treatment, the application of a
flat dielectric barrier lamp may be promising; these xenon-excimer-based lamps produce
172 nm radiation at a very high efficiency of about 60%. High resolution LIF
spectroscopy should be studied, in order to investigate the possibility to resolve the
oxidation products by their rotational spectra. The identification of unresolved oxidation
products might be improved by application of chemical derivatization prior to liquid- or
gaschromatography / mass spectrometry.
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Summary
Since recently, some advanced oxidation processes (AOP’s) have been applied for the
degradation of harmful materials, as alternative to microbiological waste water
treatment or as add-on technology. The AOP focuses on the degradation of persistent
toxic materials by radical-induced oxidation; for this purpose, halogen- and metal-free
oxygen-based oxidizers are utilized, especially hydroxyl radicals. By advanced oxidation
a persistent toxic compound is converted into microbiologically degradable products, or
is even mineralized to carbon dioxide, water and -depending on the nature of the target
compound- inorganic ions like e.g. nitrate, sulfate and phosphate. Hydrogen
peroxide/UV, ozone/UV and wet oxidation AOP’s have already been applied on modest
scale; the status of electrical discharge, electron-beam and photocatalytic AOP’s is still
experimental.
This thesis describes the degradation of organic materials in aqueous solution by pulsed
corona discharges. These electrical discharges have been applied in the gas phase over
the target compound solution. As a result of the extremely high electric field strength
(about 200 kV/cm) at the head of the discharge channels, locally present molecules of
the dielectric are dissociated, excited or ionized. In humid air the following reactive
species are produced: hydroxyl radicals, oxygen atoms, ozone, nitrogen metastables,
ions and UV photons. The pulsed corona technology has been studied in detail using the
model compound phenol (hydroxybenzene), which is an important precursor in organic
chemical synthesis. Also the degradation of some other model compounds has been
studied viz. atrazine (herbicide), malachite green (dye) and dimethyl sulfide (odor
component). A favourable electrode configuration has been derived from the
experiments.
In addition to the oxidation of model compounds, also the production of oxidizers has
been investigated. The production of hydroxyl radicals in aqueous solution has been
studied by fluorescence spectrometry in combination with the fluorescent molecular
probe coumarin-3-carboxylic acid (CCA); also in-situ electron spin resonance has been
applied using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). UV absorption
spectrometry has been applied for the quantitative analysis of ozone.
The conversion level, conversion efficiency and oxidation mechanisms of the model
compound phenol have been investigated. Conversion and oxidation pathways have
been determined by means of the liquid chromatographic techniques reversed-phase
HPLC and ion-exclusion chromatography; in addition, IonSpray and electron-impact
mass spectrometry have been applied. Also, phenol oxidation has been studied in-situ
using laser-induced fluorescence spectroscopy. The gas phase over the oxidized
aqueous phenol solution has been analyzed by infrared spectroscopy and an aldehyde
screening test. From an environmental point of view, Microtox ecotoxicity tests and
total organic carbon measurements have been performed on fresh and oxidized phenol
solutions. Reaction mechanisms have been derived for phenol oxidation by hydroxyl
radicals, oxygen and ozone.
160
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The conversion of atrazine has been performed by reversed-phase HPLC. Dimethyl
sulfide has been oxidized in the gas phase and the conversion has been determined by
gas chromatography. The decolorization of malachite green due to oxidation by corona
has been determined for several electrode configurations using absorption spectrometry.
The most efficient electrode configuration consists of a multipin anode situated in the
gas phase over the aqueous solution of the target compound, while the cathode is
dielectrically separated from the anode. In this way an effectively dimensioned plasma
is produced and energy dissipation due to vapour formation is avoided.
During oxidation of aqueous solutions of the fluorescent molecular probe CCA, the
fluorescence intensity appears to increase linearly with the oxidation time, which
implies a constant hydroxyl radical production rate in water. Calibration of the
fluorescence intensity as a function of the amount of hydroxyl radicals added using
hydrogen peroxide and CCA has appeared to be impossible. Identification of hydroxyl
radicals by in-situ ESR has been unsuccessful, although the spin trap DMPO has been
utilized to produce a stable adduct.
The ozone concentration in air over water increases with the applied load voltage, while
positive corona produces more ozone than negative corona. The measured ozone
production efficiency is about 40 g/kWh which is high, because the ozone has been
produced in humid air. During oxidation the ozone concentration over aqueous phenol
solutions is significantly lower than the ozone concentration over deionized water,
which is explained by the reaction of ozone with phenol.
The oxidation of phenol in aqueous solution by pulsed corona discharges in air yields a
complex mixture of oxidation products. Polyhydroxybenzenes are produced, which yield
aliphatic aldehydes and carboxylic acids by ring-cleavage. The derived oxidation
pathways explain the formation of polyhydroxybenzenes and aliphatic polyfunctional
hydrocarbons. The intermediate oxidation products are more harmful than phenol,
therefore thorough oxidation progress is required; the temporal toxicity increase is no
specific problem occurring from corona technology, but is inherent to oxidation. Except
for small amounts of carbon dioxide, no other gaseous phenol oxidation products have
been identified. This is in accordance with an observed nearly constant total organic
carbon level of the aqueous solution during oxidation. There appear to be large
differences between oxidation products obtained by corona in air and corona in argon.
By corona in argon mainly hydroxylation of phenol takes place, while in the presence of
air also ring-cleavage takes place. The measured efficiency at 24% phenol conversion
by corona in air is about 22 nanomol/J, which corresponds to 7.5 g/kWh.
Atrazine and malachite green are very stable compounds, which appears from the
conversion efficiency: the efficiency of atrazine is 0.6 g/kWh at 27% conversion, while
the efficiency of malachite green is 0.4 g/kWh at 24% absorption decrease at λabs=590
nm with regard to the most favourable electrode configuration. On the contrary,
dimethyl sulfide is readily converted.
Samenvatting
Ten behoeve van de afbraak van schadelijke chemische verbindingen worden sinds
enige tijd enkele geavanceerde oxidatieprocessen (AOP’s) toegepast als vervanger voor
-of in combinatie met- conventionele microbiologische afvalwaterbehandeling. Een AOP
beoogt de degradatie van persistente toxische verbindingen via radicalaire oxidatie;
hierbij wordt gebruik gemaakt van halogeen- en metaalvrije, op zuurstof gebaseerde
oxidatoren, met name hydroxylradicalen. Door geavanceerde oxidatie wordt een
persistente toxische verbinding gedegradeerd tot microbiologisch afbreekbare
verbindingen, of eventueel gemineraliseerd tot koolstofdioxide, water en -afhankelijk
van de aard van de verbinding- anorganische ionen zoals bijvoorbeeld nitraat, sulfaat en
fosfaat. Van de AOP’s worden waterstofperoxide/UV, ozon/UV technologie en
nattelucht oxidatie reeds op beperkte schaal toegepast; elektrische ontladingen,
elektronenbundel technologie en fotokatalytische oxidatie bevinden zich nog in de
experimentele fase.
In dit proefschrift wordt de afbraak beschreven van organische verbindingen in waterige
oplossing met behulp van gepulste corona-ontladingen. Deze elektrische ontladingen zijn
toegepast in de gasfase boven the oplossing van de doelcomponent. Ten gevolge van
de extreem hoge elektrische veldsterkte (ongeveer 200 kV/cm) aan de kop van de
corona-ontladingskanalen, worden aldaar aanwezige moleculen van het diëlectricum
gedissocieerd, geëxciteerd of geïoniseerd. In vochtige lucht worden aldus de volgende
reactieve
deeltjes
geproduceerd:
hydroxylradicalen,
zuurstofatomen,
ozon,
stikstofmetastabielen, ionen en UV fotonen. De gepulste corona technologie is
gedetailleerd getoetst op de modelstof fenol (hydroxybenzeen), een belangrijke
precursor in de organisch-chemische synthese. Tevens is de degradatie van enkele
andere modelcomponenten bestudeerd, te weten atrazine (herbicide), malachiet groen
(kleurstof) en dimethyl sulfide (geurcomponent). Uit de experimenten is een gunstige
elektrodenconfiguratie naar voren gekomen.
Naast de oxidatie van doelcomponenten is ook de produktie van oxidatoren onderzocht.
De produktie van hydroxylradicalen in waterige oplossing is bestudeerd met
fluorescentie spectrometrie in combinatie met de fluorescerende moleculaire probe
coumarine-3-carboxylzuur (CCA); tevens is in-situ elektron spin resonantie toegepast
met behulp van de spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). UV absorptiespectrometrie is toegepast voor de kwantitatieve analyse van ozon.
Van de modelstof fenol zijn omzettingsgraad en efficiëntie alsmede de
oxidatiemechanismen onderzocht. De conversie en oxidatieroutes zijn bepaald met
behulp van de vloeistofchromatografische technieken reversed-phase HPLC en ionexclusie chromatografie; ook zijn IonSpray en electron-impact massaspectrometrie
toegepast. Tevens is in-situ de oxidatie van fenol bestudeerd met laser-geïnduceerde
fluorescentiespectroscopie. De gasfase boven de geoxideerde fenoloplossing is
geanalyseerd met infrarood spectroscopie en een aldehyde test. Vanuit milieutechnisch
oogpunt zijn Microtox ecotoxiciteitstests en totaal organisch koolstof analyses
uitgevoerd op onbehandelde en geoxideerde fenoloplossingen. Reactiemechanismen zijn
opgesteld voor de oxidatie van fenol door hydroxylradicalen, zuurstof en ozon.
162
__
De conversie van atrazine is bepaald met reversed-phase HPLC. Dimethylsulfide is
geoxideerd in de gasfase en de conversie is gemeten met gaschromatografie. De
ontkleuring van malachietgroen ten gevolge van oxidatie door corona ontladingen is
gemeten voor diverse elektrodenconfiguraties door middel van absorptiespectrometrie.
De meest efficiënte elektrodenconfiguratie bestaat uit een meerpunts anode in de
gasfase boven de waterige oplossing van de doelcomponent, waarbij de kathode
diëlektrisch gescheiden is van de anode. Aldus wordt een effectief gedimensioneerd
plasma gevormd en wordt energieverlies door dampvorming voorkomen.
Bij oxidatie van waterige oplossingen van de fluorescerende moleculaire probe CCA,
blijkt de fluorescentie intensiteit lineair toe te nemen met de oxidatietijd, waaruit een
constante produktiesnelheid van hydroxylradicalen in water is gebleken. Calibratie van
de fluorescentie-intensiteit als functie van de hoeveelheid toegevoegde hydroxylradicalen met behulp van waterstofperoxide en CCA is niet mogelijk gebleken. Het is
niet gelukt, om hydroxylradicalen aan te tonen met behulp van in-situ ESR, ondanks dat
een spin trap is toegepast voor de vorming van een stabiel adduct.
De ozonconcentratie in lucht boven water neemt toe met de aangelegde spanning,
terwijl positieve corona meer ozon produceert dan negatieve corona. De gemeten
produktie efficiëntie van ozon bedraagt ongeveer 40 g/kWh en is hoog, temeer daar de
ozon in vochtige lucht geproduceerd is. Tijdens oxidatie blijkt de ozonconcentratie
boven fenoloplossingen significant lager dan de ozonconcentratie boven gedeïoniseerd
water, hetgeen verklaard wordt door de reactie van ozon met fenol.
De oxidatie van fenol in waterige oplossing door middel van gepulste corona
ontladingen in lucht levert een complex mengsel van oxidatieprodukten. Geproduceerd
worden polyhydroxybenzenen, waaruit door ringopening alifatische aldehyden en
carboxylzuren ontstaan. De opgestelde modellen verklaren de produktie van
polyhydroxybenzenen en alifatische polyfunctionele koolwaterstoffen. De intermediaire
oxidatieprodukten blijken schadelijker dan fenol, waardoor grondige oxidatie dus vereist
is. De aanvankelijke toxiciteitstoename is geen specifiek probleem van corona
technologie, doch is inherent aan oxidatie. Behalve kleine hoeveelheden koolstofdioxide,
zijn geen andere gasvormige oxidatieprodukten van fenol aangetroffen. Dit stemt
overeen met een waargenomen nagenoeg constante waarde van het totaal organisch
koolstofgehalte van de vloeistoffase tijdens oxidatie. Er blijken grote verschillen te
bestaan tussen de oxidatieprodukten van fenol, die ontstaan zijn door behandeling met
corona in lucht en corona in argon. Voor het geval van corona in argon treedt
hoofdzakelijk hydroxylering van fenol op, terwijl in aanwezigheid van zuurstof ook
ringopening optreedt. De gemeten conversie efficiëntie bij 24% fenol conversie door
corona in lucht bedraagt ongeveer 22 nanomol/J, wat overeenkomt met 7.5 g/kWh.
Atrazine en malachietgroen zijn zeer stabiele verbindingen, hetgeen blijkt uit de
conversie efficiëntie: deze bedraagt voor atrazine 0.6 g/kWh bij 27% conversie en voor
malachietgroen 0.4 g/kWh bij 24% absorptievermindering bij λabs=590 nm voor de
meest gunstige elektrodenconfiguratie. Dimethylsulfide wordt daarentegen snel
omgezet.
Dankwoord / Acknowledgements
Dit multidisciplinaire proefschrift is tot stand gekomen mede dankzij de inzet van een
groot aantal personen.
Allereerst dank ik eerste promotor Wijnand Rutgers en co-promotor Eddie van
Veldhuizen bijzonder voor toekenning van de promotieplaats, intensieve begeleiding,
enthousiasme en een zeer prettige samenwerking. Tevens dank ik Gerrit Kroesen, Frits
de Hoog en Bram Veefkind voor hun belangrijke bijdrage aan mijn promotieonderzoek.
Tweede promotor Carel Cramers en begeleider Henk Claessens (beiden faculteit
Scheikundige Technologie / capaciteitsgroep Instrumentele Analyse) dank ik voor hun
ondersteuning op chemisch-analytisch gebied en het beschikbaar stellen van
laboratoriumfaciliteiten.
René Janssen (faculteit Scheikundige Technologie / capaciteitsgroep Macromoleculaire
en Organische chemie) dank ik voor de diverse discussies en voor de leiding bij de ESR
experimenten.
Voor chemisch-analytische assistentie ben ik verder dank verschuldigd aan Marion van
Straten, Eric Vonk, Jan Jiskra, Ber Vermeer, Marc van Lieshout, Joost van Dongen,
Gius Rongen, Henri Snijders en Hans Damen. Ook dank aan Denise Tjallema.
Het was geweldig vertoeven bij de capaciteitsgroep Elementaire Processen in
Gasontladingen; bedankt voor de leuke tijd en samenwerking: Loek Baede (speciaal voor
de technische realisatie van corona en voor fotografie), Lambert Bisschops, Leon
Bakker, Rina Boom, Jean-Charles Cigal, Jurgen van Eck, Marjan van de Elshout, Hans
Freriks, Marc van de Grift, Charlotte Groothuis, Gerjan Hagelaar, Daiyu Hayashi (special
thanks for his important contribution to LIF spectroscopy), Marcel Hemerik, Carole
Maurice, Gabriela Paeva, Koen Robben, Eva Stoffels, Winfred Stoffels en Geert
Swinkels. En niet te vergeten mijn oud-collega’s van Elektrotechniek (EG): Vadim
Banine, Hub Bonné, Frank Commissaris, Ad Holten, Ad van Iersel, Herman Koolmees,
Roel Moors en Mariet van Rixtel.
Dank voor jullie bijdrage: afstudeerders Roy Janssen, Hens Renierkens, Ralph van Eijk;
stagiairs Geert Dooms, Coen van de Vin, Chris Peters, Marco van Steen, Martijn Siffels.
Verder dank ik: Marius Bogers en medewerkers van de faculteitswerkplaats N voor de
vervaardiging van onderdelen ten behoeve van experimentele opstellingen. Marco van
de Sande (N/ETP) voor uitvoering van de spectrochemische ICP metingen. Gerard
Harmsen (Directoraat-Generaal RWS-RIZA, Lelystad) voor advies over ecotoxicologische
tests. KEMA KPG/CET Arnhem voor uitvoering van de TOC metingen.
Thanks to Pavel Šunka (Institute of Plasma Physics of the Czech Academy of Sciences,
Prague CR) and Bruce Locke (Florida State University, Tallahassee USA) for received
hospitality and pleasant discussions.
Tenslotte dank ik speciaal mijn ouders, broer en zus voor hun grote belangstelling en
ondersteuning.
Curriculum Vitae:
Daar op de basisschool al behoefte bestond aan een eigen chemisch laboratorium, is de
auteur (geboortedatum 02-07-1966) na afronding van het VWO Peelland College te
Deurne in 1985 Scheikundige Technologie gaan studeren aan de toenmalige TH
Eindhoven. Na zijn afstuderen in 1990 als polymeertechnoloog is hij werkzaam geweest
bij NKF Kabel in Delft als chemisch technoloog en ook als plaatsvervangend chef van
het materialen laboratorium energie aldaar. In november 1995 is hij gestart met dit
promotieonderzoek bij de faculteit Elektrotechniek / vakgroep Elektrische Energiesystemen, in samenwerking met de faculteit Scheikundige Technologie / capaciteitsgroep Instrumentele Analyse, waarna het onderzoek is voortgezet en afgerond bij de
faculteit Technische Natuurkunde / capaciteitsgroep Elementaire Processen in
Gasontladingen (EPG). Het promotieonderzoek heeft geleid tot dit proefschrift.
A youthful passion to own a private chemical laboratory convinced the author (date of
birth 02-07-1966) to apply for the chemical engineering curriculum at Eindhoven
University of Technology, after finishing his pre-university education at Peelland College
Deurne in 1985. After his MSc degree in polymer engineering in 1990, he joined NKF
Kabel in Delft as chemical engineer and also as deputy supervisor of the materials
laboratory. In November 1995 he started this PhD project at the faculty of Electrical
Engineering / Electrical Energy Systems group in cooperation with the faculty of
Chemical Engineering / Instrumental Analysis group; he continued and finished the
project at the faculty of Applied Physics / department of Elementary Processes in Gas
Discharges (EPG). The PhD project has resulted in this dissertation.
Stellingen
behorende bij het proefschrift
“Pulsed corona-induced degradation of organic materials in water”
door W.F.L.M. Hoeben
1. De gepulste corona-ontladingen technologie omvat meer dan alleen de produktie
van ozon uit zuurstof. [dit proefschrift]
2. Ten behoeve van waterreiniging is het toepassen van gepulste corona-ontladingen
in de gasfase efficiënter dan toepassing in de vloeistoffase. [dit proefschrift]
3. Bij toepassen van gepulste corona-ontladingen in de gasfase boven een waterige
oplossing van de doelverbinding, is de chemische samenstelling van de gasfase
van grote invloed op het degradatiemechanisme van de doelverbinding.
[dit proefschrift]
4. Een ion-exclusie kolom is vanwege het carboxylzuur-specifieke retentie
mechanisme zeer geschikt voor de scheiding van oxidatieprodukten mengsels
van organische verbindingen. [dit proefschrift]
5. Bij degradatie van een organische verbinding door oxidatie kunnen intermediaire
oxidatieprodukten ontstaan, die schadelijker zijn dan de doelverbinding.
[dit proefschrift]
6. Simulatie beperkt milieuvervuiling.
7. Kunstmatige mineralisatie is energetisch gezien inefficiënt.
8. Het potentieel van geavanceerde oxidatie technologie is hoofdzakelijk gelegen in
het onschadelijk maken van organische probleemstoffen.
9. Milieubewustheid komt nog steeds niet altijd gelegen.
10. Het is niet mogelijk, om milieukundig onderzoek te verrichten zonder enig
chemisch afval te produceren.
11. World Wide Web adressen zijn vanwege het vluchtige karakter geen bruikbare
literatuurreferenties.
12. Het weggooien van een proefschrift is een vorm van energiedissipatie.
13. Hoge schoorstenen verlangen veel wind.
14. Allesreinigers reinigen niet alles.
15. Een goed tandarts is zeker zo belangrijk als een goed gebit.