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Plasma Science and Technology, Vol.18, No.3, Mar. 2016
Yield of Ozone, Nitrite Nitrogen and Hydrogen Peroxide Versus
Discharge Parameter Using APPJ Under Water∗
CHEN Bingyan (陈秉岩)1,2,3,4 , ZHU Changping (朱昌平)2,5 , FEI Juntao (费峻涛)4,5 ,
HE Xiang (何湘)3 , YIN Cheng (殷澄)5 , WANG Yuan (王媛)2 , GAO Ying (高莹)2 ,
JIANG Yongfeng (蒋永锋)6 , WEN Wen (文文)1 , CHEN Longwei (陈龙威)7
1
Department of Mathematics and Physics, Hohai University, Changzhou 213022, China
Jiangsu Province Key Laboratory of Environmental Engineering, Nanjing 210036, China
3
Hohai University Nantong Institute of Marine and Offshore Engineering, Nantong 226300,
China
4
College of Energy and Electrical Engineering, Hohai University, Nanjing 210098, China
5
College of Internet of Things, Hohai University, Changzhou 213022, China
6
College of Mechanical and Electrical Engineering, Hohai University, Changzhou 213022,
China
7
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
2
Abstract
Discharge plasma in and in contact with water can be accompanied with ultraviolet
radiation and electron impact, thus can generate hydroxyl radicals, ozone, nitrite nitrogen and
hydrogen peroxide. In this paper, a non-equilibrium plasma processing system was established
by means of an atmospheric pressure plasma jet immersed in water. The hydroxyl intensities
and discharge energy waveforms were tested. The results show that the positive and negative
discharge energy peaks were asymmetric, where the positive discharge energy peak was greater
than the negative one. Meanwhile, the yield of ozone and nitrite nitrogen was enhanced with
the increase of both the treatment time and the discharge energy. Moreover, the pH value of
treated water was reduced rapidly and maintained at a lower level. The residual concentration of
hydrogen peroxide in APPJ treated water was kept at a low level. Additionally, both the efficiency
energy ratio of the yield of ozone and nitrite nitrogen and that of the removal of p-nitrophenol
increased as a function of discharge energy and discharge voltage. The experimental results were
fully analyzed and the chemical reaction equations and the physical processes of discharges in
water were given.
Keywords: atmospheric pressure plasma jet, underwater discharge, ozone, nitrite nitrogen
PACS: 52.77.−j, 52.80.−s, 52.80.Wq
DOI: 10.1088/1009-0630/18/3/11
(Some figures may appear in colour only in the online journal)
1
Introduction
plasmas are accompanied with electrons impact (110 eV), ultraviolet (UV) light, and shock waves; thus
they can lead to the generation of active species such
as hydroxyl radical (OH) and oxygen radical (O), hydroperoxyl radical (HO2 ), peroxide hydrogen (H2 O2 )
and ozone (O3 ) as well as other substances where OH
and O radicals are the most important oxidants in the
wastewater treatment [1,3−5]
In recent years, discharge plasma in and in contact
with water has become an increasingly hot research
area [1−5] . Discharge plasma is one of the advanced oxidation processes (AOPs), which has more significant
features with high efficiency and fast speed for water
treatment than the process of sorption and biodegradation [5−8] . As a new technology of AOPs, atmospheric pressure plasmas (APPs) have a good application prospect in the wastewater treatment [1−5,9,10] .
The different discharge parameters (such as discharge
voltage, discharge energy and reduced electric fields)
lead to the differences of both the chemical kinetic
and the reaction rate coefficients of the generation of
nitrogen-oxygen, ozone and hydrogen peroxide by discharge in gas-liquid mixture [11] . Underwater discharge
There are many kinds of gas-liquid two-phase discharge reactors [3,12,13] . The characteristics of discharge reactors working in the gas-liquid mixed phase
have some differences with discharge in both the pure
gas phase and the pure liquid phase [3,14,15] . However, it has the advantage that includes the characteristic of both the gas discharges and liquid discharges
in the water treatment field. According to the dispersion characteristic of the gas and liquid phase, the dis-
∗ supported by National Natural Science Foundation of China (Nos. 11274092, 11404092, 61401146), the Nantong Science and Technology Project, Nantong, China (No. BK2014024), the Open Project of Jiangsu Province Key Laboratory of Environmental Engineering,
Nanjing, China (No. KF2014001), and the Fundamental Research Funds for the Central Universities of China (No. 2014B11414)
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CHEN Bingyan et al.: Yield of Ozone, Nitrite Nitrogen and Hydrogen Peroxide Versus Discharge Parameter
plasma acid water instead sulfuric acid and nitric acid,
is needed to produce more of the acid substance (nitric acid and nitrous acid) during the discharge plasma
interacted with water. Hence, it is urgently needed to
confirm both the yield and concentration for the formation of ozone, hydrogen peroxide and nitrite nitrogen in
the heterogeneous reaction of plasma in and in contact
with water. In this investigation, the yield of ozone,
nitrite nitrogen and hydrogen peroxide were discussed
under different reaction conditions, such as discharge
energy, treatment time, and voltage. Meanwhile, the
emission spectrum and discharge parameter were investigated. Furthermore, the efficiency energy ratio (EER)
for the yield of byproducts (ozone and nitrite nitrogen)
and the degradation efficiency of an organic contaminant (p-nitrophenol) was tested to verify the reaction
efficiency of the present system.
charge reactors working in the gas-liquid mixed phase
can be divided into three main categories: the discharges in the continuous liquid phase and dispersed
gas phase [3,14−17] , the discharges in the continuous
gas phase and dispersed liquid phase [13,18−20] , and the
discharges in the continuous phase of both the liquid
and gas [10−13,21−23] . In this paper, a non-equilibrium
plasma processing system was established by means of
an atmospheric pressure plasma jet (APPJ) immersed
in flowing water, which was classified as the discharge
in the continuous phase of both the liquid and gas.
The reaction rate of water treatment can be enhanced by the concerted reactions among ultraviolet,
ozone and peroxide hydrogen [1,3,5] . On the other hand,
the concentration of nitrite nitrogen in APP treated
water must be concerned. Since the presence of nitrate
and nitrite, novel application of plasma acid water as
acid catalyst (such as instead of sulfuric acid and nitric acid) in organic reactions is explored [24,25] . The
characteristics of the chemical efficiency, parameters of
discharge electrodes, leakage current and Joule heating
of water have been extensively studied in spark breakdown [16,26−30] . The yield of peroxide hydrogen has
been examined in the gas-liquid phase with pulsed dielectric barrier discharge (DBD) [19] . Meanwhile, the
degradation characteristics of the organic compound
have been explored with APPJ in a flowing aerated solution [10] . However, the yield of ozone, nitrite nitrogen
and hydrogen peroxide has not been investigated under different discharge parameters using APPJ under
water.
2
2.1
Experimental setup and
method
Apparatus
In this study, an APPJ immerged in flowing water is
used to examine the yield of ozone and nitrite nitrogen.
Fig. 1(A) shows the schematic chart of the experimental setup. Similar to the literature [10] , the apparatus
consists of a stirred reactor (SR), an APPJ, an air compressor, and a series of measurement instruments. The
SR consists of a three-neck quartz flask, a rotational
stirrer connected with an adjustable speed motor, and
a nozzle of APPJ immersed into flowing water within
the flask. The used gas is exhausted from the gas outlet
on the flask.
As this study considered the use of hydrogen peroxide and ozone, and nitrite nitrogen is undesirable
in water treatment. On the other hand, the use of
Fig.1 Schematic diagram of the experimental setup. (A) Chart of experimental system, (B) Atmospheric pressure plasma
jet source [31,32] , (C) Draft of water crossing the region of plasma jet [10]
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Plasma Science and Technology, Vol.18, No.3, Mar. 2016
The schematic diagram of the APPJ is shown in
Fig. 1(B). Similar to the literature [31,32] , the plasma
device is an APPJ (Plasmatreat, Open Air PFW10),
which consists of a conical inner copper electrode and a
nozzle of grounded outer electrode. The atmospheric
air provided by the compressor is the working substance, and a stable discharge plasma plume is obtained.
The rotational stirrer made by polytetrafluoroethylene (PTFE) can adjust the flow velocities of the aqueous solution from 0 m/s to 3.200 m/s precisely via a
variable speed motor. Fig. 1(C) shows a draft of the
fluid flowing velocity. The gas flow velocity is about
22.0 m/s. The expression of the revolutions per second
(RPS) and the solution flow velocity (the velocity of
circular motion) is given as:
v = ωR = 2πnR ,
Z
E(t) =
u(t)i(t)dt,
(2)
0
where E(t) refers to the plasma discharge energy in a
duration of one discharge period, u(t) is the discharge
voltage, and i(t) is the discharge current.
The efficiency energy ratio (EER) of the degradation
of PNP or the yield of production (ozone and nitrite
nitrogen) is defined as the numerical ratio of the degradation efficiency or the yield of the production of unit
plasma discharge energy in units of millijoule (mJ). It
is calculated as the following formula:
Eer =
|c0 − c1 |Vl
,
E(t)
(3)
where Eer is the EER in units of mg/mJ, while c0 and
c1 are the concentrations for the feeding and the treated
solutions in units of mg/L. V1 is the volume of APPJ
treated solution in units of milliliter (mL), E(t) is defined by Eq. (2).
In this paper, with depending on the conditions
as the following, the discharge frequency is fixed at
about 20 kHz, the solution volume in each experiment
is 100 milliliters (mL), the aerated fluid flow velocity
is 1.884 m/s, and the gas flow rate is 25.0 LPM (gas
source is air).
(1)
where n is the RPS of the stirrer, which is measured
by a photoelectric tachometer. d is the size of the region of the plasma plume. In this setup, the series size
with d ≈10 mm, R ≈30 mm, and n=8 to 16 RPS are
known. The fluid flow velocity can be calculated, which
is approximately from 1.508 m/s to 3.016 m/s.
2.2
t
Analytical methods
As experimental reagents, a water-soluble organic
substance of analytical grade p-nitrophenol (PNP) provided by Amresco Co., was used in the experiment.
Distilled and de-ionized water is used as the solvent,
and the pH value and conductivity are 7.1 µS/cm and
11 µS/cm, respectively. The initial concentration of
PNP is 100 mg/L. In this paper, each experiment used
100 milliliter of aqueous solution.
The optical emission spectrometry (OES) is used for
detecting spectra emission intensities of active species,
such as OH, O and NO from the APPJ plume via a
deep UV spectrometer (Ocean Optics, Maya2000 Pro.,
the resolution with 0.1 nm). The absorption spectrum
was used to follow the concentrations of ozone, nitrite
nitrogen, hydrogen peroxide, and PNP. Therefore, reaction intermediates can have some interference in the accurate determination. Concretely, PNP concentration
was followed by an ultraviolet-visible spectrophotometer (Pgeneral, T6S), H2 O2 concentration was measured
by a hydrogen peroxide detector (DPM-H2 O2 and the
reagent with WAK-H2 O2 ), nitrite nitrogen was measured by a nitrite nitrogen detector (GDYS-101SX3),
and ozone in the treated water was measured by an
ozone detector (GDYS-101SC2). Additionally, the pH
value of the treated water was measured by a pH meter
(Mettler Toledo, SG78-B).
The electric parameter diagnoses have been used to
obtain the discharge parameter. The plasma discharge
voltage and current were measured by a digital oscilloscope (TDS2012B) equipped with a voltage probe
(P6015A) and a Rogowski current probe (PT-7802).
The energy per discharge period is calculated by equation:
3
3.1
Results and discussion
Spectra of discharge plasma plume
Fig. 2 shows the comparison of emission spectra intensities of APPJ working in air (red line) and under
water (blue line). Meanwhile, the intensity spectrum of
N2 (337.1 nm) was used as a reference, and the intensity of APPJ in air was adjusted to be approximately
equal to the one in water. With the same discharge
energy the intensity of the OH spectra band (306.2 nm
to 320 nm, A2 Σ → X 2 Π) [33−35] , Hα (656.1 nm), O
(777.2 nm), and O (844.6 nm) of APPJ working in water are stronger than in air. This phenomenon can be
explained in that the yield of active species has been
enhanced by the plasma plume contacting with more
water molecules. Hence, the relative intensity increased
from OH spectra band, Hα and O with APPJ immersed
in flowing water. Additionally, the spectra of ON (AX) are used to determine the vibrational and rotational
temperatures of the discharge plasma [36] . We got the
simulated spectra at Tvib = 4100 K and Trot = 330 K
by using the software Lifbase.
Other OH spectra are observed on 282.0 nm of
A2 Σ+ (ν = 1) → X 2 Π(ν=0), 306.8 nm and 309.2 nm
of A2 Σ+ (ν = 0) → X 2 Π(ν = 0) [37,38] . A band
of O atoms spectra (370 nm to 400 nm) were presented, which are the ionized oxygen atoms (O+ )
for 3p(4 S0 ) → 3s(4 P) with 371.5 nm, 372.9 nm and
375.1 nm; 3d(4 D)→3p(4 D0 ) with 385.6 nm, 386.6 nm
and 388.5 nm; 3p(2 P0 ) → 3s(4 P) with 394.7 nm,
395.7 nm and 397.6 nm [39] . It clearly indicates that
excited atomic O (777.2 nm and 844.6 nm), N2
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CHEN Bingyan et al.: Yield of Ozone, Nitrite Nitrogen and Hydrogen Peroxide Versus Discharge Parameter
2 +
(337.1 nm) and N+
2 (388.4 nm and 424.1 nm, B Σu →
2 +
[40]
X Σg ) exist in the plasma plume
. In the band from
306 nm to 320 nm, another line of N2 (315.7 nm) [37]
was observed.
(EMF), and the additive effect from the high voltage
source and EMF which produced the second voltage
peak.
Fig.2 Intensities comparison of APPJ emission spectra
working in air and in water. The plasma discharge energy is
38.0 mJ per period discharge, the gas flow rate is 25.0 LPM
The highest intensity of N2 (337.1 nm) is the emission
from the N2 second positive (N2 -SPS), which excited a
band of NO-β emissions (297.6 nm, 315.7 nm, 357.7 nm
and 378.9 nm) [40] . The spectra bands of NO (200 nm
to 290 nm) [37] and NO-β (297.6 nm to 357.7 nm) [41] ,
the copper electrode emission spectra of CuI (296.1 nm
and 324.8 nm) [42] , Cu atoms (510.6 nm, 521.8 nm and
515.3 nm) [43] , and Cu+ ions (3d10 →3d9 4S, 377 nm
and 379 nm) [44] were observed in this investigation.
Moreover, the copper lines are observed in the case of
APPJ in water and not in air. This experimental phenomena should be explained as the copper lines coming
from both the erosion and ablated copper electrode.
3.2
Fig.3 Waveforms of discharge voltage and current under
different discharge energy, (a) With about 33.0 mJ, (b)
With about 38.0 mJ, (c) With about 43.0 mJ
Discharge waveforms
Additionally, two stairs have been observed both on
the discharge voltage and current waveforms. Further,
the appearance positions of stairs on the voltage waveforms are asymmetric within a completely discharged
period. This can be explained as the following: the
resistance of the discharge region is a good conductor
during the discharge state, and it changes to an insulator during the un-discharged state. The symmetrical
stairs appear on the waveforms of discharge currents,
which are the transition state of the discharge current
from the conduction state to the cut-off state. The
asymmetrical stairs appear on the waveforms of discharge voltage during zero discharge current, which are
the voltage changing delay by suddenly increasing the
resistance of the discharge region. Meanwhile, the stairs
would be created by the additive effect of the voltage
from self-inductance and the high voltage source.
In Fig. 4, the amplitudes of plasma discharge power
increased with increasing discharge energy. Meanwhile,
the peak power of positive discharge was larger than
negative discharge. Additionally, the absolute value of
positive discharge voltage is higher than the negative
The voltage and current waveforms of APPJ under different discharge energies are shown in Fig. 3.
Meanwhile, the discharge power curves are shown in
Fig. 4. With discharge waveforms in Fig. 3(a)-(c) corresponding to the power curves in Fig. 4(a)-(c), the energy per discharge period was 33.45 mJ, 37.71 mJ, and
42.81 mJ, which can be calculated by Eq. (3) or the full
width at half maximum (FHMW).
In this study, the plasma plume of the used APPJ
is produced by the type of arc discharge. The power of
the high voltage supply is limited, so the discharge characteristics would be affected by the discharge parameters [4,10] . In Fig. 3, as the discharge energy was increased, the amplitudes of discharge voltage decreased
and the discharge current increased. Meanwhile, there
is a second voltage peak on the time of the peak point
of the discharge current waveform. The reason is a
parasitic inductance and capacitance (about 2.47 µH
and 68.51 pF, respectively) existing between the inner
and outer electrodes, the sudden changing of discharge
current leading to a self-induced electromotive force
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Plasma Science and Technology, Vol.18, No.3, Mar. 2016
[45−47]
asymmetrical voltage stairs in Fig. 3.
Moreover, the difference between the positive discharge and the negative discharge decreased with increasing discharge energy in Fig. 4. The reason can
be explained by the charged particle density in the region of discharge gap. The discharge current increased
with increasing discharge power and accompanied by
decreasing discharge voltage (see also Fig. 3). Further,
the discharge current trended to the continuously conducting state. Hence, the reduced interruption time of
discharge current led to the increase of the number of
ions in the discharge gap, and the breakdown in the
region with sufficient ions became easier than the case
in the fewer ions. So, the discharge energy asymmetry
decreases with increasing discharge energy.
one
. Thus phenomena could be explained as the
following.
3.3
Concentration effected by treatment time
The concentrations of the product (ozone, nitrite nitrogen and hydrogen peroxide) in treated water would
be changed with increasing the processed time by using the discharge in and in contact with water. The
influences of generated ozone and nitrite nitrogen are
shown in Fig. 5. The result indicates that the concentrations increased both ozone and nitrite nitrogen with
increasing treatment time from 1 min to 2 min. The
concentrations of ozone kept as approximately a constant from 2 min to 9 min, and the concentrations of
nitrite nitrogen decreased firstly then leveled off from
2 min to 9 min. The concentration of nitrite nitrogen
is higher by one order than ozone.
Fig.4 Discharge power curves under different energy per
discharge duration, (a) With about 33.0 mJ, (b) With about
38.0 mJ, (c) With about 43.0 mJ
Since the asymmetry of the electrode structure, two
asymmetry peaks have also been observed on the waveforms of discharge power curves within a complete discharge period. The size of the inner electrode is smaller
than the outer one in this investigation. The smaller
inner electrode has a higher current and electron density, and the bigger outer electrode has a smaller current and electron density. This leads to the maximum
electron density not being located on the boundary of
the cathode sheath, and it appearing in the vicinity
of the instantaneous anode. Since the outer electrode
is connected to the ground, the outer electrode is the
instantaneous cathode during the positive half cycle
of the applied voltage, the ionization region vicinity
of the sheath is comparatively bigger than the inner
electrode, and the discharge current is comparatively
greater. During the negative half cycle of the applied
voltage, the inner electrode is the instantaneous cathode, the ionization region vicinity of the sheath is comparatively smaller than the outer electrode, and the discharge current is comparatively lower. Consequently,
the asymmetry discharge power corresponds with the
Fig.5 Concentrations of ozone and nitrite nitrogen in water versus treatment time. With the aqueous solution is
de-ionization water, the plasma discharge energy is 38.0 mJ
per period discharge
Meanwhile, Fig. 6 shows how the treated solution pH
value and nitrite nitrogen concentration changed with
treatment time. Additionally, the residual concentrations of H2 O2 are shown in Fig. 7. The absolute value
residual concentration was used to confirm the generation of H2 O2 in treated water, and the percentage
residual concentrations for initial concentrations with
125 mg/L, 12.5 mg/L, and 1.25 mg/L were used to analyze the decomposition of H2 O2 in the treated solution. The mechanisms of ozone, nitrite nitrogen, peroxide hydrogen, and becoming acidic treated water are
described below.
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CHEN Bingyan et al.: Yield of Ozone, Nitrite Nitrogen and Hydrogen Peroxide Versus Discharge Parameter
NO + O3 → NO2 + O2
(11)
NO2 + H ↔ NO + OH
(12)
+
3NO2 + H2 O → 2NO−
3 + 2H + NO
(13)
The combined reaction of the OH radical from reaction (5) produced H2 O2 in Fig. 7 is as follows
OH + OH → H2 O2
(14)
Due to the presence of UV light (see Fig. 2), the
photolysis reaction is accompanied with the whole process. During the treatment time from 2 min to 9 min
in Fig. 5, the concentrations of ozone and nitrite nitrogen can be kept by photolysis and reduced lightly by
thermal volatilization. The photolysis reactions can be
expressed as follows [43,44,50] :
Fig.6 Treated solution pH value and nitrite nitrogen concentration changed with treatment time. With the plasma
discharge energy is 38.0 mJ per period discharge
O3 + hν → O2 + O(1 D) (λ ≤ 320 nm)
(15)
In the range of 300-350 nm, the photolysis of nitrate
in aerated aqueous solutions at pH <6 proceeds in two
pathways [52−54] :
e + H2 O → e + H + OH (Te = 1 − 2 eV)
(5)
O2 + hν → O + O(1 D) (λ = 200 − 220 nm)
(6)
H2 O + hν → OH + H (λ = 145 − 246 nm)
3.4
(10)
(18)
+
+ 2H
−
+ hν → O + NO (λ = 200 − 400 nm)
(19)
(20)
(21)
Yield of ozone and nitrite nitrogen
With the same treatment time, the yields of both
ozone and nitrite nitrogen in treated water depend on
the changing of both the discharge voltage and energy. The concentrations of ozone and nitrite nitrogen
in APPJ treated water were examined under the discharge energy with 33.0 mJ, 35.5 mJ, 38.0 mJ, 40.5 mJ
and 43.0 mJ. The yields of ozone and nitrite nitrogen
are shown in Fig. 8 and the corresponding pH value
of the treated solution is shown in Fig. 9. The results
Finally, any O radicals from reactions (4) and (6) can
react with N2 . Furthermore, the generation mechanism
of nitrite nitrogen in Fig. 5 and plasma acidic water in
Fig. 6 can be explained as the reaction among NO, H
and H2 O. The main reactions are as follows [11,50,51] :
N + OH → NO + H
+
NO−
2
H2 O2 + hν → 2OH (λ = 190 − 350 nm)
(8)
(9)
NO−
3
Part of NO2 and N2 O2 is exhausted from the gas
outlet of the reactor. Meanwhile, it is continuously
produced and exhausted of the NOx in APPJ treated
water. Hence, the concentration and the pH value of
the treated solution remain stable from 2 min to 9 min
in Fig. 6 during the reaction balance of generation and
decomposition of the nitrite nitrogen.
The photolysis of H2 O2 is an important factor, which
led to the decomposition of H2 O2 quickly and maintained a low level of the residual concentration of H2 O2
in Fig. 7. The reaction is as follows [55] :
(7)
O(1 D) + N2 → NO + N
(17)
N2 O4 + H2 O →
NO−
2
Then, with the third molecule M (N2 or H2 O) as a
heat energy carrier, any O atoms can react with O2 ,
and produce ozone in Fig. 5 [40,48,49] :
O + O2 + M → O3 + M
−
3
NO−
3 + hν → O( P) + NO2
2NO2 ↔ N2 O4
First, since the electron mean energy is about
1-10 eV in the plasma plume [31] , and the irradiation of
UV is strong (see Fig. 2), the radicals of excited oxygen atoms, hydroxyl radical and hydrogen radical can
be generated by photolysis and electron impact [10,19] :
(4)
(16)
The OH radicals and NO2 gas quickly diffused in water. A combination reaction of the other part of NO2
could produce N2 O4 , and the reactions of N2 O4 and
H2 O and photolysis are as follows [51,53] :
Fig.7 Residual concentration of hydrogen peroxide in
treated water versus treatment time. With the solvent
and reagent are de-ionization water and H2 O2 (provided
by Amresco Co.), the initial concentrations of H2 O2 are
125 mg/L, 12.5 mg/L, 1.25 mg/L, and 0 mg/L, respectively. The plasma discharge energy is 38.0 mJ per period
discharge
e + O2 → O + O + e (Te = 0 − 5 eV)
+
NO−
3 + H + hν → OH + NO2
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Plasma Science and Technology, Vol.18, No.3, Mar. 2016
indicated that concentrations of ozone and nitrite nitrogen increased with increasing discharge energy, and
the concentration of nitrite nitrogen was higher by one
order than ozone. Meanwhile, it becomes more acidic
with higher discharge energy, and the pH value of the
treated aqueous solution was linked with the concentration of nitrite nitrogen.
This led to the concentrations of ozone and nitrite
nitrogen increasing with the increasing discharge energy in Fig. 8. Even further, with the reactions (13),
(17), (19), (22), and (24), the nitrite ion (NO−
x ) was
produced, which leads to the formation of the acid liquid [52] , and the pH value became smaller with increasing discharge energy in Fig. 9.
3.5
Efficiency energy ratio of yield and
degradation
Batch experiments were carried out for the performance evaluation of EER for the yield of byproducts
(ozone and nitrite nitrogen) and the degradation efficiency of PNP. Depending on the conditions, the aerated solution flow velocity is kept with 1.571 m/s, the
discharge energy is set on 33.0 mJ, 35.0 mJ, 38.0 mJ,
40.5 mJ, and 43.0 mJ, and the discharge voltages correspond with 8.75 kV, 8.25 kV, 7.00 kV, 6.30 kV and
5.50 kV, respectively.
The EERs of the yield of ozone and nitrite nitrogen were evaluated with Eq. (3) under different discharge energies and voltages, and the results are shown in Fig. 10. The EER of the yield of
ozone was obtained as 0.026 mg/mJ, 0.037 mg/mJ,
0.045 mg/mJ, 0.049 mg/mJ and 0.048 mg/mJ, respectively. Meanwhile, the one of nitrite nitrogen was obtained as 0.348 mg/mJ, 0.428 mg/mJ, 0.468 mg/mJ,
0.494 mg/mJ and 0.493 mg/mJ, respectively. Additionally, the EER of the PNP removal is shown
in Fig. 11. The degradation EERs of PNP were obtained as 0.095 mg/mJ, 0.122 mg/mJ, 0.158 mg/mJ,
0.166 mg/mJ and 0.157 mg/mJ, respectively.
Fig.8 The concentration of ozone and nitrite nitrogen under different energy per discharge duration. The treatment
time was 1.0 min
Fig.9 The comparison of pH value and the nitrite nitrogen concentration of treated aqueous solution under different energy per discharge duration. The treatment time was
1.0 min
The mechanisms of yield increasing of ozone and nitrite nitrogen with increasing discharge energy are described below. The discharge electron density is high
under high levels of discharge energy. More of O and
ON radicals can be generated by more of an electron
impact with Eqs. (4) and (5), and UV with Eqs. (6)
and (7) under high levels of discharge energy.
Another path that produces nitrite ion (NO−
x ) is
where hydrated electron (eaq ) and OH involved in the
reaction. The NO2 radical is produced by the reaction between nitrite ion and OH, and the nitrite ion is
produced by the reactions of hydrated electron. The
possible reactions involved are as follows [56] :
+
OH + NO → NO−
2 +H
−y
−(y+1)
e−
aq + NOx → (NOx )
(NOx )−(y+1) + H2 O → 2OH−(y+1) + NOx
(22)
Fig.10 Yield of ozone and nitrite nitrogen under different
discharge voltages and discharge energies (treatment time is
1.0 min, gas flow rate is 25.0 LPM, with de-ionized water,
the volume is 100 mL. (a) Yield of ozone concentration, (b)
Yield of nitrite nitrogen (NOx ) concentration)
(23)
(24)
where x = 1 or 2, y = 0 or 1.
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CHEN Bingyan et al.: Yield of Ozone, Nitrite Nitrogen and Hydrogen Peroxide Versus Discharge Parameter
of ozone and nitric acid in discharge process water by
using APPJ.
Acknowledgments
The authors would like to thank Prof. Wan Jinglin,
Prof. Liu Kefu, Prof. Ren Zhaoxing, Prof. Ma Tengcai,
Prof. Li Jie, Prof. Sun Bing, Prof. Liu Dongping, Prof.
Zhu Tuo, Prof. Tang Hongwu, Prof. Kong Gangyu,
Prof. Liu Dingxin, Prof. Chen Hua, Dr. Jiang Song,
Dr. Gao Yuan, Dr. Zhou Yan and Dr. Chen Yuanyuan
for their valuable analysis and discussions. Finally, we
would like to thank undergraduates Li Yanping, Gan
Yuling, Wang Lei, Wu Yeqian, Wang Jiankun, Wang
Jingyi, Li Mei, et al. for their valuable assistance in
the experiments.
Fig.11 Degradation efficiency versus different discharge
energies (treatment time is 5.0 min, gas flow rate is
25.0 LPM, with de-ionized water, the volume is 100 mL,
the PNP initial concentration is 100 mg/L)
Within one period of discharge duration, the level
of the EERs of the yield of ozone and nitrite nitrogen increased significantly from 33.0 mJ to 40.5 mJ,
decreased slightly from 40.5 mJ to 43.0 mJ and obtained maximum at the discharge voltage approximately 6.30 kV. Meanwhile, the level of EER of the
degradation efficiency of PNP has the same tendency.
This phenomenon can be explained as follows: the high
voltage supply has limited power. When the discharge
voltage decreased as a constant with increasing discharge energy (see Fig. 3), the electron density could be
increased. Moreover, the decreased voltage leads to the
reduction of the electric field strength, and the kinetic
energy of the electron would be decreased. Further,
the lower levels of discharge voltage led to electrons
with lower energy levels in the plasma plume. The efficient production of O and OH radicals with reactions
(4) and (5) was decreased due to the collision between
water molecules and low energy level electrons. Hence,
the EERs of the yield of ozone and nitrite nitrogen decreased with decreasing of discharge voltage.
4
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In this paper, the yield of ozone, nitrite nitrogen and
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(Manuscript received 8 September 2015)
(Manuscript accepted 23 October 2015)
E-mail address of CHEN Bingyan: [email protected]
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