Electrochemical processes for the remediation of ...

Journal of Scientific & Industrial Research Vol. 63, January 2004, pp 11-19

Electrochemical processes for the remediation of wastewater and contaminated soil: emerging technology P Chandra Mouli, S Venkata Mohanb and S Jayarama Reddy* Electrochemical Research Laboratories, Department of Chemistry, Sri Venkateswara University, Tirupati 517 502 The review aims at providing an overview of electrochemical processes used for accomplishing the remediation of industrial wastewater, particularly elecrooxidation processes and electrocoagulation. The applicability of these technologies for the remediation of contaminated soil is also discussed. The use of electrochemical process as a pre-treatment step to enhance the biodegradability of wastewater containing recalcitrant or inhibitory compounds can potentially be justified due to the formation of intermediate, which is easily degraded in subsequent biological treatment. The enhanced degradation of wastewater may be attributed to the electrochemical oxidation of the organic compounds present in the aqueous phase to simple molecules, which may be easily biodegraded. The principle of electrooxidation process, involved in the remediation process was described along with specific applications. The application of electrocoagulation process for wastewater treatment is also reviewed. Keywords:

Electrochemical processes, Contaminated soils.

Electrooxidation,

Introduction The ecosphere continues to be challenged by the increasing amount and variability of toxic contaminants that are emitted by industrial activities like manufacture of chemicals, including many pesticides, herbicides, chemicals and pharmaceuticals, dye stuff units, and meat packing units1-8; which are more saline9-11. Such environmental contaminants are becoming more wide spread as the pace of industrial activity accelerates, especially in the developing countries, and also as a consequence of the difficulties in restricting the emissions of industrial activity. Particularly, rivers, canals, estuaries, and other water bodies are being constantly polluted due to indiscriminate discharge of industrial effluents12-14. Water has acquired increasing importance due to its two unique characteristics: high specific heat capacity and high solvent power with respect to many inorganic and some organic substances. As a consequence of these two factors; water can serve as a heat reservoir, cooling agent, steam sources, etc., and __________ *Author for correspondence Tel: +91-877-2249962/2248418, Fax: +91-877- 2248592/2248499, E-mail: [email protected] b Biochemical and Environmental Engineering Centre, Indian Institute of Chemical Technology, Hyderabad 500 007

Biodegradability,

Electrocoagulation,

Wastewater,

is subsequently returned to the environment at a high temperature. On the other hand, water in its function as a reaction medium, or even a reaction partner, has now developed into the most significant source of wastewater in any industry, in particular, chemical and pharmaceutical processes15. Improper disposal of these aqueous wastes may increase the probability of contamination of water resources and is main health and environmental concern due to its high toxicity. There is, therefore, an urgent need to develop innovative, more effective, and inexpensive technologies for treatment of wastewater. In this review, we wish to address an overview of electrochemical processes such as: (i) Electrooxidation processes and (ii) Electrocoagulation to accomplish treatment of industrial wastewater. Industrial Wastewater Treatment Treatment plants intended for the purification of any industrial wastewater utilize a combination of processing techniques such as: — Intended for the elimination of undissolved substances, including screening, clarification, and filtration. Physical Treatment

— Intended to treat dissolved and suspended pollutants by means of Physico-chemical/Chemical Treatment

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physico-chemical processes which include neutralization, coagulation, ion-exchange, chemical precipitation, chemical oxidation, carbon adsorption, ultra filtration, reverse osmosis, electro-dialysis, volatilization or gas stripping. This treatment demands transportation, storing, and handling of hazardous chemicals and leads to the generation of toxic sludge16. — Advanced oxidation processes involve the chemical, photochemical or photocatalytic production of hydroxyl radical (•OH) [Eq. (1and 2)], which acts as a strong oxidizing agent able to react with organics, yielding dehydrogenated or hydroxylated derivatives17-21. Advanced Oxidation Processes

H2O2 → •OH + •OH

...(1)

H2O → •H + •OH

...(2)

Recently the use of Fenton reaction to generate hydroxyl radicals to degrade organic contaminants has also received lot of attention22,23. But these processes are not desirable, because of the high cost of equipment and operating costs and also the secondary pollution arising from the residual chemicals and finally the low efficiency. — Intended for the removal of dissolved substances, in the course of which colloidally dispersed materials are also destabilized and flocculated. Biological treatment involves the biotechnological application of processes that occur in nature, which include aerobic and anaerobic processes. In aerobic process, organic component of effluent is oxidized to carbon dioxide (mineralization) or some other metabolic intermediate products in the presence of dissolved oxygen, whereas in anaerobic metabolism, organic substances are first converted to carboxylic acids (acidification) and then disproportionate to carbonic acid, methane and hydrogen (methanogenic fermentation) in the absence of oxygen15. The advantage of biological treatment is the enormous adaptability of microorganisms to a wide variety of substrate media7,24, but this is a long term treatment in large physical areas and leads to the generation of non-biodegradable, soluble, and cellular residues and also the high salinity of the effluent inhibit the microbial growth. Furthermore, high molecular weight fractions present in some types of aqueous effluents found in several industries tend to be resistant to biodegradation and inhibition25-27. Hence the overall efficiency of the biological

Biological treatment

treatment, even after employing well-acclimatized microorganisms is far from satisfactory28. It is essential to enhance the biodegradability of the effluent by breaking the refractory chemicals prior to the biological process to enhance the biodegradability which can be achieved by proper application of physico-chemical or chemical or advanced oxidation methods. Also, recent literature indicates the applicability of electrochemical process as an attractive alternative for application prior to biological methods. Electrochemical Processes Recent progress in effluent treatment has lead to the development of electrochemical techniques29-37, which may offer an attractive alternative and more environmentally friendly process for treating aqueous streams containing organic compounds. Electrochemical processes are probably the most adequate tools in the aqueous effluent treatment, which are ideally suited to the present age. The process will not require chemical additions and indeed electrons are the only reactants added to the process to simulate reaction. These processes include electrooxidation and electrocoagulation. (i) Electrooxidation  Electrooxidation may offer an attractive alternative38-43, for treating aqueous streams containing the organic compounds, via simultaneous evolution of oxygen at an anode surface, which is probably the most adequate tool in the aqueous effluent treatment, ideally suited to present age where environmental considerations are always to the fore44. Electrooxidation is a mediated reaction and occurs via oxygen atom’s transfer from water in the solvent phase to the oxidation product. The overall process of anodic oxygen transfer may be represented by the generic reaction45 [Eq. (3); where R is organic reactant and ROX is oxidation product]. On the other hand, R may mineralize, according to Eq. (4); known as combustion reaction. R + XH2O → ROX + 2XH+ + 2Xe-

...(3)

R + XH2O → X/2 CO2 + 2XH+ +2Xe-

...(4)

These processes were investigated for different organic compounds46,47, in which, it is impossible to suppress the most side reaction, i.e. the anodic oxidation of water to give O2, and the consequence is that only low current yields can be achieved. Further disadvantage of direct electrochemical oxidation is

MOULI et al.: ELECTROCHEMICAL PROCESSES FOR REMEDIATION OF WASTEWATER

the low miscibility of most organics with water. As a result, mass transfer from the bulk solution to the anode is hindered and the achievable space-time yields are low. Also, electrolysis often consumes much energy, especially in dilute wastewater treatment processes48; For this reason the efficient electrochemical methods for water purification based on the indirect electrooxidation of contaminants involving electrogeneration of strong oxidants is now in progress which is a more reliable technology for the degradation of toxic organic wastes and provides better results over the direct electrooxidation. In this process the waste is oxidized in the bulk solution by a mediator, mostly a transition metal in a higher oxidation state49-51. After waste oxidation in the bulk solution the reduced mediator is reoxidized at the anode, which is capable of oxidizing further organic molecules. The mechanism proposed for this process of organic compounds in aqueous solutions presents, as the first step the disadvantage of water which leads to the •OH adsorbed on the electrode surface52 [Eq. (5)], then MOX+1 species can be formed from hydroxyl radicals in the active sites of the MOX coating [Eq. (6)], finally, this physiosorbed species oxidises the organic molecules [Eq. (7)]. MOX + H2O → MOX(•OH) + H+ + e-

...(5)

MOX(•OH) → MOX+1 + H+ + e-

...(6)

MOX+1 + R → MOX + RO

...(7)

A second possible reaction path is also considered, in which the organic compound is mineralized [Eq. (8)], which is more likely to occur on higher oxygen overpotential surfaces that contain PbO2, SnO2 or Sb2O5. [MOX(•OH)]Y + R → 2YCO2 + 2YH+ + 2Ye- + YMOX ...(8) Thus the pollutants can be treated with electrogenerated H2O2 in a two-compartment cell, in which the hydrogen peroxide is continuously supplied to the cathodic compartment from the two-electron reduction of sparged oxygen on graphite or reticulated vitreous carbon cathodes [Eq. (9)]. +

O2 + 2H +2e

-

→ H2O2

...(9)

Also the wastewater can be treated using an undivided electrolytic cell containing a Pt anode and a carbon-polytetraflouroethylene (PTFE) O2 fed cathode where H2O2 is electrogenerated [Eq. (9)]. In

13

these processes the pollutants can be mainly destroyed by the combined action of electrogenerated •OH [Eq. (10)] at the anode53. Therefore the processes are called advanced electrochemical oxidation processes (AEOPs). H2O → •OHads + H+ + e-

...(10)

On the other hand the oxidizing power can be enhanced in an acidic medium by the addition of Fe2+ to produce •OH, which can be produced by a well known Fenton’s reaction between ferrous ion and hydrogen peroxide in the medium [Eq. (11)], and the process can be treated as electrochemical Fenton treatment54. Fe2+ + H2O2 → Fe(OH)2+ + •OH

...(11)

This catalytic reaction is propagated from Fe2+ regeneration, which takes place by the reduction of Fe3+ species with H2O2 [Eq. (12)] and also from the reaction of Fe3+ at the cathode [Eq. (13)]. -H+ 3+

Fe +H2O2 ↔ [Fe-O2H]2+ → Fe2+ + HO2•

...(12)

Fe3+ + e- → Fe2+

...(13)

+H+

Under an UV irradiation the mineralization process can be accelerated by the photolysis of Fe3+ complexes and by the enhancement of Fe2+ regeneration due to the additional photoreduction of Fe3+ species via photo-Fenton reaction [Eq. (14)], which can takes place within the wavelength range 320-480 nm. hν

Fe(OH)2+ →

Fe2+ + •OH

…(14)

These reactions lead to a constant catalytic Fe2+ concentration lower than 0.05 mM in the solution after 15 min of electrolysis. Consequently the electrogenerated H2O2 can also be oxidized to O2 at the anode55 [Eq. (15 and 16)]. H2O2 → HO2• + H+ + e-

...(15)

HO2• → O2 + H+ + e-

...(16)

Usually the electrooxidation process can be carried out in an electrochemical cell. The schematic diagram of the experimental set up is shown in Figure 1. The rate of degradation of any organic compound can be assessed by measuring the per cent

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(b) Destabilization of the contaminants, particulate suspension, and breaking of emulsions; (c) Aggregation of the destabilized phases to form flocs. The mechanism of EC is highly dependent on the chemistry of the aqueous medium, especially conductivity. In addition, other characteristics such as, pH, particle size, and chemical constituent concentrations also influence the EC process. This can be explained with two specific examples involving aluminium and iron as coagulant aids. — The electrolytic dissolution of the aluminium anode produces the cationic monomeric species62 such as, Al+3 and Al(OH)2+ at low pH, which are transformed initially into Al(OH)3 at appropriate pH values and finally polymerized to Aln(OH)3n (Scheme 1). However, depending on the pH of the aqueous medium, other ionic species such as, Al(OH)2+, Al2(OH)24+, and Al(OH)4- may also be present in the system. These charged species could effectively remove the pollutants by adsorption to produce charge neutralization. (a) Aluminium

Figure 1  The schematic diagram of the experimental set up: (A) Power supply, (B) Digital ammeter, (C) Power integrator, (D) Electrocatalysis reactor, (E) Circulated pump, (F) Water bath, (G) Reservoir, (H) Magnetic stirrer, (I) To biological process, and (J) Sampling port

of COD reduction or by determining the intensive current efficiency (ICE)56 through the measurement of COD by the standard method (closed reflux/photometry), while the organic compound present in effluent and its oxidation products can be carried out for each run or for certain time interval with high performance liquid chromatography (HPLC). (ii) Electrocoagulation (EC)  Coagulation is a phenomenon in which the charged particles (a colloidal suspension) are neutralized by neutral collision with counter ions present in the coagulant added, and are agglomerated, followed by the process of sedimentation. Alum [Al2 (SO4)3.18H2O] is a coagulant which has been widely used for ages for the wastewater treatment. In the EC process the coagulant is generated in situ by electrolytic oxidation of an appropriate anode material. In this process, charged ionic species are removed from effluent by allowing it to react: (i) With an ion having opposite charge, or (ii) With floc of metallic hydroxides generated within the effluent. The EC technology offers an alternative to the use of metal salts or polymers and polyelectrolyte addition for breaking stable emissions and suspensions57. The technology removes metals, colloidal and solid particles, and soluble inorganic pollutants from aqueous media by introducing highly charged polymeric metal hydroxide species58-61. These species neutralize the electrolytic charges on suspended solids and oil droplets to facilitate coagulation. This process involves three successive steps: (a) Formation of coagulants by electrolytic oxidation of the ‘sacrificial electrode’;

 Iron produces iron hydroxide, Fe(OH)n (where n is 2 or 3), in an electrolytic system by oxidation63 (Scheme 2 and 3). The formed Fe(OH)n remains in the aqueous stream as a gelatinous suspension, which can remove the pollutants from effluent either by complexation or by electrostatic attraction, followed by coagulation. Wastewater containing Cr6+ (CrO42-) ions can be removed by the EC technique using iron as the sacrificial anode. Also the use of Fe3+ as flocculation agent in wastewater treatment has considerable advantages because of its innocuity compared to Al3+ ions, which shows some toxic effects64.

(b) Iron

Generally the electrocoagulation can be carried out in a simple electrochemical cell, with single number of both the electrodes, but this is not suitable for effluent treatment since, for a workable rate of metal dissolution the use of electrodes with large surface area is required. This has been achieved by using the cells with monopolar electrodes, either in parallel or series connections. During the electrolysis the +ve side undergoes anodic reactions, while on –ve

Scheme 1

MOULI et al.: ELECTROCHEMICAL PROCESSES FOR REMEDIATION OF WASTEWATER

15

Mechanism 1

Scheme-2 Mechanism 2

Scheme-3

Figure 2  Typical representation of the treatment flow diagram with electrochemical processes

side, cathodic reaction is encountered. Consumable metal plates such as, iron and aluminium are usually used as sacrificial electrodes to continuously produce ions in the system. The released ions neutralize the charges of the particles and there by initiate coagulation. Applicability of Electrochemical Processes for Remediation In industry the economic factors usually decide the inclusion of a wastewater treatment system. Cooling water, which is not in need for treatment, is separated from that which does require treatment. If the effluent is requiring treatment containing nonbiocompatible complex organic matter and fails to meet the quality specifications for electrochemical treatment, it must first be subjected to physical/physico-chemical pre-treatment, followed by primary sedimentation to enhance the degradability, after which it can be fed to electrochemical system. The typical representation of the flow diagram is shown in Figure 2.

The data acquired from the literature65 (Table 1), shows the efficiency of the different electrooxidation processes for treatment. The degradability (TOC removal) of 2,4-dichlorophenoxyacetic acid (2,4-D) with an initial concentration of 230 ppm, is 10 per cent, and 11 per cent, 52 per cent, and 83 per cent by anodicoxidation and in the presence of electrogenerated H2O2, electro-Fenton and photoelectroFenton processes respectively, at pH value close to 2.9 which reveals that the degradation rate is very slow for the substrate and its oxidation derivatives by anodicoxidation and slightly higher in the presence of electrogenerated H2O2 due to the reaction with H2O2 and HO•2 produced [Eq. (15)], whereas the destruction of these contaminants is notably accelerated when the solution contains Fe+2. In fact the main oxidizing species in both processes is the absorbed •OH generated in very small concentration by the anodic decomposition of water [Eq. (10)]. The increase in TOC removal observed for the electroFenton process can then be ascribed to fast reactions of pollutants with •OH produced in the medium [Eq.

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Table1 — Percentage of TOC removal and apparent current efficiency obtained for the mineralization of 100 ml of a 230 ppm 2,4-D solution in 0.05 M Na2SO4 + H2SO4 at pH 3.0 under different experimental conditions at 250 C (ref. 65) Method

Anodic oxidation

Anodic oxidation in the presence of electrogeneration of H2O2 Electro-Fenton process

Photoelectro-Fenton process

Applied current (mA)

Per cent TOC removal

ACEa

100

10

4.1

300

15

2.1

450

18

1.6

100

11

4.7

300

23

3.1

450

26

2.5

100

52

7.4

300

63

9.4

450

73

7.7

100

83

37.3

300

88

12.8

450

82

8.9

a

Apparent current efficiency

(11)], while in photoelectro-Fenton process the highest mineralization is due to the higher production rate of •OH [Eq. (14)] and/or additional photodecomposition of several intermediates. In fact the complete oxidation of complex organic compound to final product CO2, by electrochemical treatment is uneconomical due to high-energy consumption. It may be worthwhile to treat the industrial wastewater to the biodegradable stage (degrade to aliphatic carboxylic acids) followed by an economical biological process. The use of electrochemical process as a pre-treatment step to enhance the biodegradability of wastewater containing recalcitrant or inhibitory compounds can potentially be justified if the resulting intermediate is easily degraded in subsequent biological treatment. Thus the partial oxidation of organic biorefractry pollutants provides a valuable pre-treatment technique for subsequent cheap biological treatment (Figure 2). The electrochemical techniques such as electrooxidation processes and electrocoagulation were became as an advanced technology for treating the industrial wastewater. Boudenne et al.66, studied the elctrochemical oxidation of an aqueous phenol via

carbon black catalyst, which demonstrates the complete degradation of phenol and the process could be used by industry as further treatment of aqueous phenolic wastes. Lin et al.28, explored the effects of wastewater salinity, pH and current density, temperature, and initial concentration on treatment efficiency of electrochemical method. It was reported that at low or medium salinity, phenol removal is dominated by direct oxidation at the electrode surface but at high salinity indirect oxidation by hypochlorous acid is a major one. Johnson et al.67, reported the discovery of optimal electrocatalytic materials and conditions for rapid degradation of organic compounds to CO2 (g). It shows that the rates of anodic degradation for carboxylic acids can be increased by increasing the anode temperature in acidic media, and achieved the successful anodic degradation of cynuric acid at Pt anode, which is known to be a stable end product in the photocatalytic degradations of s-triazines. Saracco et al.68, carried the anodic oxidation of coumaric acid on a Pt-Ti electrode. The study indicated that the kinetics are enhanced by; increasing the temperature, enhancing current density, alternating electrode potential, lowering the initial pH, and dissolving Fe+3 ions. Galla et al.69, reported the mediated electrooxidation for the degradation of real pesticides, simulates for warfare agents, and industrial wastes, which succeeds with yields near to 100 per cent. Torres et al.70, studied the electrochemical oxidation of industrial wastewater containing 5amino-6-methyl-2-benzimidazolone (AMBI) employing Pt anode due to electrolysis. This nonbiodegradable effluent produces simultaneous oxidation of AMBI and chloride ions. Highly oxidative chlorine intermediate species enhanced the degradation of AMBI. It was reported that, solution temperature, pH, and current density affect the degradation process. Complete mineralization after electrochemical treatment was readily achieved in the coupled fixed bed biological reactor. The activated carbon fibre (ACF) electrode electrolysis process can successfully decolorize the number of dyes despite wide variety of the chemical structure and type71. The colour removal ratio of solution is uniformly above 85 per cent and the TOC removal ratio ranged widely from 30-70 per cent. In the last decade, elctrocoagulation technolgy has been increasingly used in south America and Europe for treatment of industrial wastewater

MOULI et al.: ELECTROCHEMICAL PROCESSES FOR REMEDIATION OF WASTEWATER

containing metals and also used primarily to treat wastewater from pulp and paper industries, mining and metal-processing industries61. Koparal and Ogutveren72, reported the complete removal of nitrate from water using electrocoagulation, which has been accomplished accompanying with the precipitation of Fe(OH)3 produced in water by soluble anode. In addition EC, has been applied to treat wastewater containing chemical and mechanical polishing waste60, organic matter from landfill leachates58, and deflourination of water59. Electrokinetics procedure is the application of direct electric current to soil, which is an emerging engineering technique for the remediation of contaminated soil for both metal and organic contamination73. An electric current between electrodes inserted into the soil generates hydrogen ion at the anode and hydroxyl ions at the cathode, which moves into the soil by electromigration, forming a pH gradient74 facilitates movement of metals towards the cathode, where they are removed and collected. Anionic metal complexes, inorganic anions and negatively charged organics will migrate towards the anode while dipolar interactions between water molecules and soil surface lead to an electroosmotic flow of water towards the cathode, which is able to transport uncharged organics. The application of electrokinetics to the remediation of organic-contaminated soil has focused upon electroosmotic approaches for uncharged molecules such as cola tar constituents, including PAHs75. The concept of moving the organic contaminant into an area containing degradative microorganisms is attractive because of the potential for movement into an inoculated degradative zone, dispersion from areas of high contaminations and release from sequestration in soil micropores. The coupling of electro kinetic movement of an organic contaminant, 2,4-D acid through soil and its biodegradation in situ has been demonstrated by Jackman and coworkers76. Under an applied current density of 0.89 A/m2, 2,4-D moved towards the bacteria and its concentration decreased. James et al.77, reported the partial reoxidation at a Ti/IrO2 as the most satisfactory method for the remediation of nitrotoulene explosives. Conclusions The following conclusions are drawn from the study:

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• The electrochemical methods such as electrooxidation processes and electrocoagulation form an attractive alternative, and are successfully applied for industrial wastewater treatment. They neither produce any undesired reaction co-products nor use toxic or hazardous materials. • The chemical change in both methods, is brought about by the ability to add or remove electrons, a non-polluting reagent, from the chemical treatment or from a chemical species to be treated. This eliminates the use of redox reagents to carry out oxidation or reduction. • The high degradation potential of the electrooxidation processes is ascribed to the production of oxidizing •OH in solution from electrogenerated H2O2, and is reported to be improved in comparison to that of chemical and photochemical methods. • Electrooxidation processes as a pre-treatment step, enhance the biodegradability of wastewater containing recalcitrant or inhibitory compounds, by resulting intermediate is easily degraded in subsequent biological treatment. • Electrochemical technique can be effectively used for the remediation of contaminated soil matrix. • These techniques were also suitable for on-site treatment, especially in small-scale use. References 1

2

3

4

5

6

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