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MODULE 7.5 Chemical methods of treatment of hazardous wastes
Chemical Oxidation and reduction Ozonolysis Acid-base neutralization Chemical precipitation Hydrolysis Ion exchange Thermal treatment methods Performance of hazardous wastes incinerators Advantages of incineration Disadvantages of incineration Wet air oxidation Photolysis Biological treatment of hazardous wastes Land treatment Preparation of wastes for disposal Land disposal Land fills Surface impoundments Underground injection References 1 2 3 4 5 5 6 7 7 8 8 9 9 10 12 12 12 14 15 16

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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(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

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. ⎯⎯ → M2 + + Ca(OH)2 ←⎯ M(OH)2 + Ca2 + …………………………..(9) ⎯ 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
2 − CO3 − + 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.

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.

2Re s+ OH− + [Ni(CN)4 ]2 − → (Re s+ )2 [Ni(CN)4 ]2 − + 2OH− …………….(18) 2Re s+ OH− + CrO 42 − → (Re s+ )2 (CrO2 − ) + 2OH− ………………….…(19) 4 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 incineration contain undesirable

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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)

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 9
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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

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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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 Cover Waste

Monitoring well

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

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

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.