Download MODULE 7.5 Chemical methods of treatment of hazardous

Environmental Chemistry and Analysis
Prof. M.S.Subramanian
MODULE 7.5
Chemical methods of treatment of hazardous wastes
Chemical Oxidation and reduction
1
Ozonolysis
2
Acid-base neutralization
3
Chemical precipitation
4
Hydrolysis
5
Ion exchange
5
Thermal treatment methods
6
Performance of hazardous wastes incinerators
7
Advantages of incineration
7
Disadvantages of incineration
8
Wet air oxidation
8
Photolysis
9
Biological treatment of hazardous wastes
9
Land treatment
10
Preparation of wastes for disposal
12
Land disposal
12
Land fills
12
Surface impoundments
14
Underground injection
15
References
Indian Institute of Technology Madras
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Environmental Chemistry and Analysis
Prof. M.S.Subramanian
MODULE 7.5
Chemical and other methods of treatment of
hazardous wastes
The selection of a treatment process for a waste stream depends on
among other factors the nature of the waste, desired characteristics of the output
stream. Most of the times the chemical property of the waste constituents
determine its applicability in waste treatment. In this chapter the major chemical
treatment processes applicable to hazardous waste such as chemical
oxidation-reduction, acid-base neutralisation, precipitation,
hydrolysis, ion
exchange, thermal treatment methods, wet air oxidation photolysis and
biodegradation are discussed.
Chemical Oxidation and reduction:
(I) Oxidation reduction methods provide another important chemical
treatment alternative for hazardous wastes. One important chemical redox
treatment involves the oxidation of cyanide wastes from metal finishing industry,
using chlorine in alkali solution. In this reaction CN- is first converted to a less
toxic cyanate. Further chlorination oxidises the cyanate to simple carbondioxide
and nitrogen gas.
NaCN + Cl2 + 2NaOH → NaCNO + 2NaCl + H2O ………………………….(1)
2NaCNO + 3Cl2 + 4NaOH → 2CO2 + N2 + 6NaCl + 2H2O …………….......(2)
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Prof. M.S.Subramanian
(II) Another important redox treatment process is the reduction of hexavalent
chromium Cr(VI) to trivalent chromium Cr(III) in large electroplating operations.
Sulphur dioxide is used as the reducing agent and the reactions are as follows.
3SO2 + 3H2O → 3H2SO3 ...............................(3)
2CrO3 + 3H2SO3 → Cr2 (SO 4 )3 + 3H2O..........(4)
(III) Iron (II) in solution can be precipitated as ferric hydroxide by oxidation.
4Fe2 + + O2 + 10H2O → 4Fe(OH)3 + 8H+ ………………………………….……(5)
(IV) In a similar way sulphurdioxide is oxidised to sulphuric acid
2SO2 + O2 + 2H2O → 2H2SO4 …………………………………………..…..(6)
(V) A large variety of oxidisable contaminants in waste water and sludges are
oxidised by ozone which can be generated on site by an electrical discharge
through dry air or oxygen.
(CH2O) + 2[O] → CO2 + H2O ………………………………………………(7)
CH3CHO + [O] → CH3COOH ………………………………………….…...(8)
Ozonolysis:
Ozone is a very powerful oxidising agent. Although this process has not
been demonstrated in any full-scale facility, its application to TCDD and PCBs is
quite promising. With respect to TCDD it was demonstrated that if the dioxins
were suspended as an aerosol combined with CCl4, 97% degradation of TCDD
was possible. Ozone in conjunction with UV radiation has been shown effective
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for the destruction of polychlorinated phenols and pesticides. In both the cases
the key requirements were to concentrate the TCDD in a medium where they
were susceptible to attack and provide a free radical for reaction with dioxin
molecule.
Acid-base neutralisation:
Hazardous wastes are categorised as corrosive when their solution pH is
less than 2 or more than 12.5. Such wastes can be chemically neutralised .
Generally acidic wastes are neutralised with slaked lime [Ca(OH)2] in a
continuoulsy stirred chemical reactor. The rate of addition of lime is controlled by
feed back control system which monitors pH during addition. Lime is least
expensive and is widely used for treating acidic wastes. Since the solubility of
lime in water is limited, solution of excess lime do not reach extremely high pH
values.
Alkaline wastes may be neutralised by adding sulphuric acid. It is a
relatively inexpensive acid. For some applications acetic acid is preferable since
it is non toxic and biodegradable. Alkaline wastes can also be neutralised by
bubbling gaseous carbondioxide forming carbonic acid. The advantage of CO2 is
that it is often readily available in the exhaust gas from any combustion process
at the treatment site.
Many waste treatment processes like oxidation/reduction, adsorption, wet
air oxidation, ion-exchange, stripping and biochemical treatment require prior pH
adjustment.
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Chemical precipitation:
This technique can be applied to almost any liquid waste stream
containing a precipitable hazardous constituent. By properly adjusting pH, the
solubility of toxic metals can be decreased, leading to the formation of a
precipitate that can be removed by settling and filtration.
Quite often lime [Ca(OH)2] or caustic soda is used for precipitation of the
metal ions as metal hydroxides. For example the following reaction suggests the
use of lime to precipitate the metal as hydroxide.
⎯⎯
→ M(OH)2 + Ca2 + …………………………..(9)
M2 + + Ca(OH)2 ←⎯
⎯
Chromium is precipitated as hydroxide.
Cr 3 + + 3(OH− ) ⎯⎯→ Cr(OH)3 ……………………………………(10)
Sodium carbonate also has been used to precipitate metals as hydroxides
(Fe(OH)3•XH2O), carbonates (CdCO3), basic carbonate salts (2PbCO3•Pb(OH)2).
Carbonate ion hydrolyses in water to give hydroxide ion
CO32 − + H2O → HCO3− + OH− ……………………………………(11)
Even lower concentrations of metals in the effluent can be removed by
precipitating them as sulphides. Ferrous sulphide can be used as a safe source
of sulphide ion to produce sulphide precipitates with other metals that are less
soluble than ferrous sulphide. Reducing agents such as sodium borohydride can
be used to precipitate the metal ions from solution in the elemental form.
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4Cu2 + + NaBH4 + 2H2O → 4Cu + NaBO2 + 8H+ ………….(12)
Hydrolysis:
Hydrolysis treatment can be given to those hazardous waste constituents
which are very reactive with water. Examples of those substances are halides,
carbide, hydride, alkoxide, and active metal.
SiCl4 + 2H2O → SiO2 + 4HCl …………………….……………....(13)
CaC2 + 2H2O → Ca(OH)2 + C2H2 ……………………………..…(14)
NaAlH4 + 4H2O → 4H2 + NaOH + Al(OH)3 ………………………(15)
NaOC2H5 + H2O → NaOH + C2H5OH …………………………....(16)
Ca + 2H2O → Ca(OH)2 + H2 ………………………………………(17)
Ion exchange:
Ion exchange is judged to have some potential for the application of
interest in situations where it is necessary to remove dissolved inorganic species.
However
other
competing
processes
-
precipitation,
flocculation
and
sedimentation - are broadly applicable to mixed waste streams containing
suspended solids and a spectrum of organic and inorganic species. These
competing processes also usually are more economical. The use of ion
exchange is therefore limited to situations where polishing step was required to
remove an inorganic constituent that could not be reduced to satisfactory levels
by preceding treatment processes.
One example for this is the use of anion exchanges for the removal of
anionic nickel cyanide complex and chromate ions from waste solutions.
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2Re s+ OH− + [Ni(CN)4 ]2 − → (Re s+ )2 [Ni(CN)4 ]2 − + 2OH− …………….(18)
2Re s+ OH− + CrO 42 − → (Re s+ )2 (CrO24− ) + 2OH− ………………….…(19)
Ion -exchange resins have also been used in the removal of radionuclides from
radioactive wastes.
Thermal treatment methods:
Thermal incineration is a process that uses high-temperature thermal
oxidation to convert a waste to a less bulky, less toxic or less noxious material. It
can be considered as a volume-reduction process in that many of the component
elements of organic materials, including the most common ones (carbon,
hydrogen and oxygen) are converted wholly or partially to gaseous form, leaving
only the non combustible inorganic volume.
The principal products of incineration are carbondioxide water vapour and ash.
C(organic) + O2 → CO2 + heat …………………………………………....(20)
4H(organic) + O2 → 2H2O + heat ……………………………………….…(21)
Reaction such as (20) and (21) destroy organic matter and generate heat
required for the cleavage of C-Cl bonds in organochlorine compounds. In case of
non inflammable organochloride wastes supplemental fuel such as methane or
petroleum-liquid is necessary to incinerate them.
The hazardous products of incineration are compounds of sulphur, nitrogen,
halogen and heavy metals (mercury, arsenic, selenium, lead and cadmium). If
the
gaseous
combustion
products
of
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incineration
contain
undesirable
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Prof. M.S.Subramanian
compounds,air pollution control equipment is required. The solid and liquid
effluents may require treatment prior to ultimate disposal or discharge.
The most critical factors that determine complete combustion of hazardous
wastes are (1) high combustion temperatures above about 900oC to ensure that
the thermally resistant compounds react (ii) availability of sufficient amount of
oxygen for combustion. (iii) sufficient residence time to allow the reactions to
occur.
Performance of hazardous wastes incinerators:
Performance of hazardous-waste incinerators can be measured in terms of
destruction removal efficiency (DRE). DRE accounts for both the destruction
in the combustion chambers(s) and the removal of organics in any air-pollution
control equipment. DRE can be calculated as the percentage of mass difference
of input(feed) and output (stack emission) waste constituents through the
incinerator. DRE has been defined on a compound-specific basis and thus must
be calculated for each constituent of interest separately. According to RCRA
(Resource conservation and Recovery Act) requirement, a minimum DRE of
99.99 percent for most of organic compounds and a DRE of 99.9999 percent for
dioxins and dibenzo furans should be achieved for efficient combustion.
Advantages of incineration:
The basic process technology is available and reasonably well developed.
Incineration can be scaled to handle large volumes of liquid waste. It is the best
known method for the disposal of "mixed wastes". It is an excellent disposal
method for biologically hazardous wastes. Large areas of land are not required.
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Disadvantages of incineration:
The equipment is costly. The ash may or may not be toxic , but must be
disposed of properly. The gaseous and particulate products of combustion may
be hazardous and should be controlled by air pollution control technology.
Wet air oxidation:
It is the aqueous phase oxidation of dissolved or suspended organic
substances
at
elevated
temperatures
(150-325oC)
and
pressures
(2000 kPa to 20,000 kPa) water. Which makes up the bulk of the aqueous phase,
serves to catalyse the oxidation reactions so they proceed at relatively low
temperature, and at the same time serves to moderate the oxidation rates
removing excess heat by evaporation. It also acts as excellent heat transfer
medium, which enables the wet air oxidation process to be thermally selfsustaining with relatively low organic feed concentrations. The high pressures
allow high concentration of oxygen to be dissolved in water and the high
temperature assist the reaction to occur.
In wet air oxidation, the waste is pumped into the system with high-pressure
pump and mixed with air from an air compressor. The waste is passed through a
heat exchanger and then to a reactor where atmospheric oxygen reacts with the
organic matter waste, sometimes in the presence of catalysts. The oxidation is
accomplished by a temperature increase. The gas and liquid phases are
separated. System pressure is controlled to maintain the reaction temperature.
The process can be used for the removal of cyanide from electroplating waste
solutions.
2Na+ + 2CN− + O2 + 4H2O → 2Na+ + 2HCO3− + 2NH3 ……………………………(22)
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Photolysis:
In photolysis, chemical bonds are broken under the influence of light. In
primary photochemical process, the target species is converted to an
electronically excited state, usually a diradical, which is sufficiently energetic to
undergo chemical reaction. The fate of the excited molecule and therefore the
effectiveness of a photolysis treatment process, depends on its chemical
structure and on the medium in which it is carried out. For the photolysis process
to be effective in the treatment of hazardous wastes stream, the radiation source
must be sufficiently energetic, must be absorbed by the target species and the
final photochemical products must be less toxic. To date much of the research
work on the treatment of highly toxic wastes has centered on two types of
constituents:
polychlorinated
biphenyls
(PCBs)
and
chlorinated
dibenzo-p-dioxins(CDDs) eg: tetrachloro dibenzo-p-dioxin (TCDD). The three
requirements of photolysis of TCDD are 1) Dissolution in a light transmitting film
2) presence of organic hydrogen odour and 3) ultraviolet light. In such photolysis
reactions initially a reactive intermediate such as HO• is formed which
participitate in chain reactions that lead to the destruction of the compound.
Biological treatment of hazardous wastes:
Biological processes are in general, the most cost effective techniques for
treating aqueous waste streams containing organic constituents. The physical
and chemical properties of the compound influence its biodegradability. With
appropriate organisms and under right conditions, even phenol which is
considered to be biocidal can be degraded. The microorganism must be allowed
to acclimate to the waste to be treated prior to routine operation of the process.
Even some compounds which were considered as biorefractory may be
degraded by microorganisms adapted to their biodegradation. DDT for example
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is degraded by properly acclimated pseudomonas. The relatively highly
chlorinated PCBs are degraded by anaerobic bacteria under less anaerobic
condition. These products can be further decomposed.
To increase the biodegradability of the hazardous wastes, the pH of the
medium should be adjusted to an optimum value of 6-9 and the oxygen level
should be high. Concentrations of soluble inorganics in the hazardous wastes
should be kept low so that enzymatic activity is not inhibited. Trace
concentrations of inorganics may be partially removed from the liquid waste
stream during the biological treatment, because of adsorption onto the microbial
cell coating.
In aerobic waste treatment, hazardous wastes such as chemical processes
wastes and land fill leachates can be degraded by aerobic microorganisms such
as bacteria and fungi in the presence of oxygen.
In anaerobic waste treatment, microorganisms degrade different organic
wastes in the absence of oxygen. During the process H2S is generated which
precipitates toxic heavy metal ions as their sulphides. The overall degradation of
the hypothetical organic compound (CH2O) can be written as follows.
2(CH2O) → CO2 + H2O …………………….(23)
Land treatment
Land treatment of hazardous wastes involves controlled application of the
waste onto the soil surface. The objectives of land treatment are the biological
and chemical degradation of organic waste constituents and the immobilisation of
inorganic waste constituents. Land treatment differs from land fills in that with
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land treatment, the assimilative capacity of the soil is used to detoxify,
immobilise, and degrade all or a portion of the applied waste. Land fills are
containments that store hazardous wastes and control the migration of wastes or
by-products from the land fill sites. Liners are not required with land treatment.
Hazardous wastes should not be placed in a land treatment site unless the
waste can be made less hazardous or nonhazardous by the reactions occuring in
the soil.
Hazardous-waste land treatment is the controlled application of hazardous
waste on the aerobic soil horizon, accompanied by continued monitoring and
management in order to alter the physical, chemical and biological characteristics
of the waste and to render the waste less hazardous.
Land treatment of wastes is accomplished by mixing the wastes with soil
under appropriate conditions. Important microorganisms like bacteria, including
those from the genera, agrobacterium, anthrobacteri, bacillus, flavobacterium,
pseudomonas are involved in biodegradation. In addition actinomycites and fungi
are all involved in the biodegradation of wastes. Bacterial cultures may develop
through acclimation over long periods of time and which are able to degrade
these normally recalcitrant compounds.
Land treatment is applicable to petroleum refining wastes, biodegradable
organic chemical wastes including organochlorine compounds. However it is not
suited to the treatment of waste containing acids, bases, toxic inorganic
compounds, salts, heavy metals and excessively soluble volatile and flammable
organic compounds.
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Indian Institute of Technology Madras
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Preparation of wastes for disposal:
The hazardous wastes after treatment are converted into a suitable form
before they are sent for ultimate long term disposal. The techniques that were
used for this purpose are immobilisation, stabilisation, fixation, and solidification.
These techniques involve different physical and chemical processes which
ultimately convert the hazardous wastes into different acceptable forms for land
disposal.
Land disposal:
Land disposal techniques include Land fills, surface impoundments and
underground injection wells.
Land fills:
In order to protect public health and environment the design of hazardous
waste land fills should be adequate. Fig.1 shows the three levels of safeguard
that has been incorporated into the system.
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Control ControlMonitoring
well
well
well
Control
well
Monitoring
well
Monitoring
well
Cover
Waste
Level 1
Level 3
Level 2
Level1: Liner plus
leachate
collection/treatment
Level 2: Back up liner
plusleachate
collection/treatment
Level3: Wells to monitor
and if needed,
control leachate
plume
Impervious layer
Fig 1: Three levels of safeguard in hazardous waste land fills
The primary system consists of an impermeable liner, either of clay or
synthetic material, coupled with a leachate collection and treatment system. The
land fill is covered with impervious material and sloped to permit adequate run off
so that infiltration is minimised and thus preventing the rain water or snow melt
from entering the soil. To reduce the stress on the liner material, side slopes of
the land fill are kept at a maximum of 3:1.
To promote movement of waste to pumps for extraction to the surface and
subsequent treatment, a leachate collection system has been designed by
contours. To channel the leachate to a pumping station below the land fill, plastic
pipes are used, the collected leachates are brought to surface using pumps and
they are given waste specific treatment which includes (1) passing the leachate
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through a column consisting of sorbent material such as carbon or flyash. (2) the
leachate is also subjected to suitable physical-chemical units such as chemical
addition, flocculation, sedimentation, pressure filtration, pH adjustment, and
reverse osmosis to remove the dissolved waste. To provide a back up leachate
collection system, a secondary system of another barrier is contoured. The
secondary collection system in the event of failure of primary system conveys the
leachate to pumping station, which in turn relays the waste water to the surface
for treatment.
The third safeguard system consists of a series of discharge wells up-gradient
and down-gradient to monitor ground water quality area and to control leachate
plumes if primary and secondary systems fail. Upgradient wells gives the
background levels of selected chemicals in the ground water, which can be
compared with the concentrations of these chemicals in the discharge from that
of the down-gradient wells. Thus this system provides an alarm mechanism if the
primary and secondary systems fails.
Land fills are allowed with sufficient vent points so that if methane is
generated, it may be burned off continuously.
Surface impoundments:
A surface impoundment is a man-made excavation, diked area, or natural
topographic depression designed to hold an accumulation of liquid wastes. The
construction is similar to that described for land fills in that bottom walls should
be impermeable to liquids and provision must be made for leachate collection.
Proper geological siting and construction with floors and walls composed of
low-permeability soil and clay are important in preventing the pollution, since the
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chemical and mechanical challenges to the liner materials in surface impounds
are severe.
Underground injection:
Underground injection or deep-well disposal consists of injecting the hazardous
liquid wastes under pressure to the underground strata isolated by impermeable
rock strata from the acquifiers. The following factors must be taken into
consideration before discharge of hazardous wastes into the deep wells. Since
the wastes are injected into a region of elevated temperature and pressure, some
chemical reactions may occur involving the waste constituents and the mineral
strata. Corrosion may be severe. Problems such as clogging may occur if the
liquid wastes contains oils, soils, and dissolved gases. The main concern with
underground injection is the potential for contaminating underground drinking
water supplies, if the disposal well is not properly cased or if it is damaged.
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References
1. Aland Wild., Soils and the environment, Cambridge University Press,
New York, 1993.
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Limited, New Delhi 2001.
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global perspective, Oxford university press, New York, 2000.
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MA,1998.
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and V. Ramachandran., Extraction and Processing for the treatment and
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Pennsylvania, 1994.
9. Loconto, Paul R, Trace environmental quantitative analysis, Taylor and
Francis, 2006.
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Hazardous Waste Management, Mc Graw-Hill, inc. New York, 1994.
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(Publishers) Ltd, London, UK, 1985.
12. Pradyot Patnaik., Handbook of Environmental Analysis, CRC Press,
Boca Raton, Florida, 1997.
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International (P) Limited, New Delhi, 1991.
14. Roger N. Reeve., and John D. Barnes., Environmental Analysis, John
Wiley & sons, Chichester, UK, 1994.
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Environmental Chemistry and Analysis
Prof. M.S.Subramanian
15. Stanley E. Manahan., Environmental Chemistry, 8th Ed., CRC Press LLC,
Boca Raton, Florida, 2005.
16. Thomas G. Spiro., and William M. Stigliani., 2nd ed., Prentice Hall of India
(P) Ltd., New Delhi, 2003.
17. Vladmir N. Bashkin., Environmental Chemistry: Asian Lessons, Kluwer
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