6 Electrochemical Methods for Degradation of Organic Pollutants in Aqueous Media Enric Brillas and Pere-Lluı ´ s Cabot Universitat de Barcelona, Barcelona, Spain Juan Casado Carburos Meta ´ licos S.A., Barcelona, Spain I. INTRODUCTION In the last 30 years, a large variety of electrochemical techniques for the destruction of toxic and refractory (i.e., nonbiodegradable) organic pollu- tants for wastewater treatment have been proposed and developed. Only a few conventional methods are related to the direct electrolysis of pollutants at the electrode. This process can occur either by a direct electron transfer reaction to (reduction) or from (oxidation) the undesired organic, or by a chemical reaction of the pollutant with previously electrogenerated species, which remain adsorbed at the electrode surface. Most electrochemical methods are based on indirect (or mediated) electrolysis in which the target pollutant is removed in the solution by active species produced reversibly or irreversibly at the electrode. The two types of procedures are contrasted in Fig. 1. The use of electrochemical techniques offers the following distinctive advantages for wastewater treatment [1]: 1. Environmental compatibility. The main reactant is the electron, which is a clean reagent. 2. Versatility. Electrolytic treatments can deal with solid, liquid, or gaseous pollutants to generate neutral, positively, or negatively TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. charged inorganic or organic products, also inducing the pro- duction of precipitates, gaseous species, pH changes, etc. In ad- dition, a plethora of reactors and electrode materials, shapes, and configurations can be utilized [2]. It is noteworthy that the same reactor can be used frequently for different electrochemical re- actions with only minor changes, and that electrolytic processes can be scaled easily from the laboratory to the plant, allowing treatment volumes ranging from milliliters to millions of liters, respectively. 3. Safety. Electrochemical methods are generally safe because of the mild conditions usually employed and the small amount and in- nocuous nature of the added chemicals. 4. Energy efficiency. Electrochemical processes are amenable to work at low temperatures and pressures, usually below ambient con- ditions. Electrodes and cells can also be designed to minimize power losses due to poor current distribution and voltage drops. In some instances, the required equipment and operations are simple and, if properly designed, can be made relatively inexpensively. Figure 1 Schemes for different electrochemical treatments of organic pollutants. (a) Direct electrolysis by anodic oxidation in which the pollutant reacts at the electrode surface with adsorbed OH . produced from water oxidation at a high O 2 - overpotential anode. (b) Indirect electrolysis where the pollutant reacts in the solution with an irreversibly electrogenerated reagent B + produced from the oxidation of inactive B at the anode. Brillas et al.236 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. The electrochemical methods described in this chapter for the destruction of organics in wastewaters are classified in Fig. 2. The direct electrolytic processes include conventional procedures of cathodic reduc- tion and anodic oxidation. The indirect methods deal with the use of redox mediators as reversibly electrogenerated reagents, as well as oxidants as irreversibly electrogenerated reagents at the anode (e.g., O 3 , ClO À ,Cl 2 , and ClO 2 ) or the cathode (e.g., H 2 O 2 ). Emerging processes related to electrogenerated Fenton reagent and other electrochemical oxidation processes based on the combined use of iron ions and electrogenerated H 2 O 2 are also described. Other indirect electrolytic processes include conventional methods of phase separation, such as electrocoagulation, electroflotation, and electroflocculation. Fundamen- tals, laboratory experiments, scale-up studies, and environmental/indus- trial applications for the different electrochemical techniques are discussed in this chapter. Figure 2 Classification of electrochemical methods for the destruction of organics in aqueous wastes. Electrochemical Methods for Degradation 237 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. II. CATHODIC REDUCTION The direct electroreduction of organics on suitable cathodes is a method that can be utilized for the dechlorination of pollutants in wastewaters. Many chlorinated organic compounds are produced in the industry and are used as solvents, refrigerants, pesticides, transformer oils, etc. Because most of such compounds are nonbiodegradable, they cause major environmental problems. Often they are found at small concentrations in a wide variety of wastewaters, and are usually decontaminated by concentration techniques such as adsorption on activated carbon or extraction by organic solvents. In fact, the products thus separated have to be further destroyed to avoid increasing pollution in the environment. Some of the existing methods are expensive (e.g., Na treatment), or can produce very dangerous by-products such as dioxins obtained by incineration. The use of cathodic dehalogena- tion as an alternative method has the following advantages [1,3–5]: 1. The treatment can be performed at ambient temperature. 2. No additional reagents are required. 3. As the chlorine atoms are selectively removed, the resulting dechlo- rinated compounds can be degraded by a cheaper method such as a biological treatment. It has to be taken into account that during the cathodic reduction, H 2 evolution is a common side reaction in aqueous media and, therefore, a cathode with high hydrogen overpotential is usually selected to obtain suitable electrodegradation efficiencies [1,6,7]. Moreover, dissolved oxygen can also be reduced (see Sec. VI). Different materials have been utilized as cathodes including carbon electrodes, Pb, Hg, Pt, Cu, Ni, Ni alloys, Ni composites, Ti, TiO 2 , and metal hydrides. Several problems have been detected in the use of carbon electrodes. Graphite develops fractures along its basal planes due to the intercalation of ions or organic molecules that migrate under the electrical field through them. In addition, carbon elec- trodes can suffer from degradation by radicals formed during the electro- reduction of dissolved oxygen. Fortunately, these problems are solved using three-dimensional carbonaceous materials made of partially graphitized amorphous carbon and graphite felts [8,9]. Problems of stability have also been found with Pb during the electroreductive dehalogenation of several chlorinated organic compounds [9,10]. Mercury has several drawbacks, including the limitation in current densities, probable metal leaks to the electrolyte, and difficulties in scaling-up with liquid metals. This section is devoted to the application of cathodic reduction for treating aliphatic and aromatic pollutants at low concentrations, and also to the dechlorination of chlorofluorocarbons (CFCs) in aqueous media. The Brillas et al.238 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. aromatic pollutants are typically chlorinated compounds that contaminate wastewaters, whereas CFCs are volatile hydrocarbons with chlorine and fluorine atoms, which were primarily used as refrigerants and gas propel- lants. CFCs destroy the stratospheric ozone layer and contribute to the greenhouse effect. The Montreal Protocol provided an international agree- ment to stop CFC production beginning in 1996. However, about 2 Â 10 6 tons of CFCs were still stored in freezing devices in 1999. To avoid probable leaks to the atmosphere, they should be degraded or converted to useful chemicals. Because destructive methods such as incineration, UV photolysis, catalytic decomposition at high temperatures, and solar thermal technology are expensive and/or can produce dangerous by-products, the partial conversion of CFCs to useful and harmless compounds is much more attractive. A. Aliphatic and Aromatic Compounds Direct electrolysis has been applied successfully to the degradation of chloroalkanes such as CHCl 3 [11] and CCl 4 [12]. As these compounds react scarcely with radicals such as OH Á and as their toxicity is directly related to their chlorine content, the reductive dechlorination is especially attractive. The electrochemical reduction enables the removal of substituents, espe- cially halogen atoms, along with the hydrogenation of the molecule. The products are less toxic and more biodegradable and/or sensitive to electro- chemical oxidation [4,5]. However, an important problem in electrochemical dehalogenation is the low current efficiency. These reactions take place at room temperature at potentials of about À1 V vs. saturated calomel electrode (SCE). The general reaction per chlorine atom can be written as follows: R À Cl þ 2H þ þ 2e À !R À H þ HCl ð1Þ Sonoyama et al. [11] have studied the electroreductive decomposi- tion of chloroform on 15 kinds of metal electrodes using a Pyrex cell di- vided by a glass filter with a cathode area of 16–18 cm 2 . Experiments performed by electrolyzing 200 mL of deaerated aqueous 0.1 M K 2 SO 4 solution with 6.20 mM CHCl 3 at 1 mA cm À2 up to a total charge of 50 C showed a strong dependence of the decomposition efficiency and the main product formed on the metal tested. The hydrogenation of chloroform on Ag, Zn, Pd, and Cu cathodes proceeded at near 100% efficiency and the main product was CH 4 . In contrast, 88% dichloromethane was selectively generated on Pb. Scherer et al. [12] studied the kinetics of CCl 4 dechlorination on an oxide-free iron rotating disk electrode in borate buffer (pH 8.4) at a potential such that an oxide film would not form. The rate of CCl 4 reduction Electrochemical Methods for Degradation 239 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. was dominated by reactions at the metal–solution interface, as is the case in oxide-covered granular iron. The electrolysis of chlorinated aromatics and aliphatics at concentra- tions ranging from 40 to 560 ppm on carbon fiber gives effective dechlo- rination [3,13]. Several trials were performed at 10 A using 1 L of aqueous NaOH+Na 2 SO 4 with small additions of methanol or acetonitrile to enhance the solubility of the pollutant. The cell utilized is illustrated in Fig. 3. The fiber bundles, containing 50,000 single fibers of 8 Am diameter, are clam ped at the entrance side. The Pt mesh anode is separated from the cathode by a cation- permeable NafionR membrane. The current efficiencies obtained were low, although a value of 75% was found for dichlorvos (Table 1). Despite the low current efficiencies, the process is feasible at reasonable costs and yields a high degree of detoxification. The treatment of wastewaters with 50 ppm pentachlorophenol by electrochemical reduction using C fiber electrodes for 30 min decreased its concentration to below the detection limit of 0.5 ppm [13]. During the treatment, the toxicity decreased by a factor of 20. The final reaction products were phenol and, possibly, monochlorophenols. Many substituted phenols have been electroreduced at Pt electrodes in ethanol–water mixtures [14] or in acid (0.05 M H 2 SO 4 or 2 M HClO 4 ) solutions [15], leading to cyclohexanols with a current efficiency close to 100%. The reaction appears to proceed via a surface process involving the adsorption of phenols and hydrogen atoms formed at the cathode. As cyclo- hexanols are biocompatible, these substituted phenols can be degraded by Figure 3 Scheme of the flow-through multifiber cell for the electrodechlorination of organic compounds in wastewaters. (From Ref. 3.) Brillas et al.240 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. electroreduction coupled to biological degradation. However, p-nitrophenol and 4-chlorophenol did not yield cyclohexanols. Phenol can be obtained by the electroreduction of 4-chlorophenol on a palladized carbon cloth or palladized graphite cathodes in a divided cell containing acetate buffer [16]. The suggested mechanism involves the adsorption of 4-chlorophenol on the carbon surface near the carbon/Pd interface, followed by its hydrogenation with the hydrogen atoms adsorbed on the Pd surface. A complete dechlori- nation of 25 mL of 153 ppm 4-chlorophenol in 0.05 M sodium acetate acetic acid buffer at À0.7 V vs. SCE on 2-cm 2 palladized carbon cloth cathode required 15 hr, during which time the current decreased from 2.2 to 0.8 mA. Poorer results were obtained when Pt was used instead of Pd. Chlorinated hydrocarbons have also been reduced on Cu cathodes in aqueous solutions [7]. In this case, a fixed-bed, flow-through reactor filled with Cu balls, 0.2–0.6 mm in diameter, supported on a Pt gauze was employed. Hexachlorocyclohexane was dechlorinated rapidly and com- pletely. Tetrachloroethylene, trichloroethane, and chlorobenzene were less reactive. However, unsatisfactory results were obtained with a polychlori- nated biphenyl (PCB). The electroreduction of trichloroethylene (0.4 g L À1 )onCuin0.05M NaOH was found to be more efficient than on Ag or Cd cathodes [4], with the current efficiency increasing when the applied current density decreased. At a current density of 4 mA cm À2 , the current efficiencies for the dehalogenation of monochloroacetic acid, dichloroacetic acid, chloro- form, and trichloroethylene were 2%, 10%, 87%, and 29%, respectively. 5-Chlorosalicylic acid could not be dechlorinated on Cu. Nagaoka et al. [17] Table 1 Results Obtained for the Electrochemical Dehalogenation in the Cell Shown in Fig. 3 at 10 A in 1 L of 0.1 M NaOH+0.1 M Na 2 SO 4 Compound Initial concentration [ppm] Number of Cl removed CE [%] Energy cost [kW hr m À3 ] 2-NH 2 -4-Cl-phenol 100 1 0.7 60 4-Cl-C 6 H 4 NO 2 48 1 0.4 55 DDVP a 560 1(2) 75 0.8 C 2 Cl 6 50 6 1 77 Pentachlorophenol 50 5 2 36 2,4,5-T b 100 3 1 70 C 2 Cl 4 74 4 3 70 1,2,4-Cl 3 C 6 H 3 40 3 0.7 70 a Dichlorvos or dimethyl-2,2-dichlorovinyl phosphate. b 2,4,5-Trichlorophenoxyacetic acid. Source: Ref. 3. Electrochemical Methods for Degradation 241 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. reported the quantitative electroreduction of trichloroethylene in water– acetonitrile at composite electrodes consisting of metal particles (stainless steel, Ni, and Cr) and oxidatively treated glassy carbon particles. Trichloro- ethylene was first reduced to chloroacetylene on the glassy carbon pa rticles, and the latter was further reduced to acetylene on the metal. o-Chlorobenzoic acid, which is resistant to anodic oxidation, can be cathodically reduced on a Pb cathode to o-chlorobenzyl alcohol [18]. This alcohol was further oxidized to o-chlorobenzaldehyde at a PbO 2 anode. The extent of its degradation was about 90%. In the presence of MnSO 4 , both the alcohol and the aldehyde suffer oxidative degradation at the anode during the electrolysis to yield mainly aliphatic acids. Funabashi et al. [19] electroreduced iodine-containing organic com- pounds such as iodotyrosine from medical waste solutions to separate iodide, which was adsorbed by Ag-coated Al 2 O 3 for effective separation. Another procedure for the electrochemical dechlorination of pollutants in aqueous media consists of the use of a corrodible metal or a bimetallic system without the application of external current. The reaction proceeds as in corrosion—with the anodic regions being dissolved and with reduction taking place at cathodic regions. The rate of reduction is lower than in the case of the cathodic reduction with imposed DC voltage because the potential of the local cathodes is less negative. However, the dechlorination rate can be increased with the metal surface area exposed to the wastewater. A full-scale column reactor has been described by Sweeny [20,21], and this device has been tested for the treatment of industrial wastewaters using various combinations of catalyzed Zn, Al, or Fe mixed with sand. The detoxification of hexachlorocyclopentadiene, trihalomethanes, chloroethy- lenes, chlorobenzene, chlordane, atrazine, and nitrophenols was reported. Bachmann et al. [7] employed a suspension of steel grit, covered partially with Cu by cementation. In this case, Fe was oxidized to Fe(II) whereas the chlorinated compound was reduced on Cu releasing Cl À . Matheson and Tratnyek [22] sequentially dehalogenated carbon tetrachloride via chloro- form to methylene chloride on fine-grained iron metal. Trichloroethylene was also dechlorinated by iron, although more slowly than carbon tetra- chloride. Grittini et al. [23] have shown the complete dechlorination of PCBs to biphenyl in an aqueous methanol solution in a few minutes by contacting the solution with a Pd/Fe system. In this case, the reduction was assumed to be due to hydrogen adsorbed by Pd during Fe corrosion [16]. B. Chlorofluorocarbons Electrochemical reduction processes of CFCs leading to partially or com- pletely dehalogenated compounds for synthetic purposes have been Brillas et al.242 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. described in the literature. Many examples in which the CFCs are converted to hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and/ or fluorocarbons (FCs) have been reported. HCFCs are not as destructive to stratospheric ozone; nevertheless, their production will be gradually reduced to zero in 2020. HFCs and FCs are harmless to stratospheric ozone and there is currently no limitation for their production. Edison [24] disclosed the conversion of CFCs to HCFCs, HFCs, and FCs using a divided cell with a Hg pool cathode in ethanol (60 vol.%)–water containing potassium acetate. In one example, the conversion of 1,1,2- trichloro-1,2,2-trifluoroethane (CFC 113) to chlorotrifluoroethene (CTFE), an industrial monomer, at 20 mA after passing a total charge of 18,700 C was 64 mol%. The cathodic reaction is: Cl 2 FC À CF 2 Cl þ 2e À !ClFC¼CF 2 þ 2Cl À ð2Þ Cabot et al. [25,26] reported CFC 113 electroreduction in Pb and Cd cathodes, combined with a hydrogen diffusion anode in MeOH (50– 80 vol.%)–water mixtures with 0.75 M NH 4 Cl and 50 ppm PdCl 2 ,with the MeOH content allowing significant CFC solubility. The current effi- ciency at 80 and 200 mA cm À2 was 98%, and difluoroethene and trifluo- roethene were the main products in the gas phase. H 2 can be selectively oxidized at the gas diffusion electrode (GDE), and so there is no need for separators, reducing the energy cost. The process has also been extended to CFC 11, and derivatives including fluoromethane have been obtained [27,28]. Inaba et al. [29] have introduced a different cell to work with gaseous compounds (Fig. 4). A metal-plated solid polymer electrolyte (SPE) com- posite electrode faces the gas to be reduced. On the other side, the SPE is in contact with 0.1 M NaOH in which a Pt wire and an Ag/AgCl ref- erence electrode are immersed. This system permits the electroreduction of insoluble reactants in water without employing organic solvents. For ex- ample, 2-chloro-1,1,1,2-tetrafluoroethane (HCFC 124) is transformed into 1,1,1,2-tetrafluoroethane (HFC 134a). The cathodic reaction can be written as follows: CF 3 À CHFCl þ H 2 O þ 2e À !CF 3 À CFH 2 þ Cl À þ OH À ð3Þ This reaction is considered to be catalyzed by active hydrogen atoms formed on Pd. The product is recovered as a gas mixture, and Cl À and OH À ions move to the electrolyte as the counterions of the anion ex- change membrane. Delli et al. [30] have studied the electroreduction of dichlorodifluo- romethane (CFC 12) in aqueous solutions on Pd, Au, Cu, and Ag, chemi- Electrochemical Methods for Degradation 243 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. cally deposited on NafionR 117 membranes using a cell similar to that of Fig. 4. CFC 12 circulates over the metal on one side of the membrane, while on the other side, there is a 2-M NaOH solution with the Pt anode and the Ag/AgCl reference electrode. CH 2 F 2 (HFC 32, a new refrigerant) and CH 4 were obtained on Ag at À1.4 V vs. Ag/AgCl, with current effi- ciencies of 60% and 30%, respectively. CH 4 was the main product gen- erated on Au, Pd, and Cu at À1.0 V, with current efficiencies of 14%, 15%, and 47%, respectively. Wetproofed porous electrodes, applied previously in fuel cells, have also been tested as cathodes for electrosynthesis from gaseous and liquid starting materials with limited solubility in water. The reagent is supplied through the hydrophobic electrode, which is in contact with an aqueous electrolyte. They present some attractive advantages over conventional electrodes because they: 1. Accelerate electrode reactions 2. Lower diffusion limitations significantly 3. Simplify product isolation. As an example, Table 2 compares the results obtained using a smooth Cd cathode and a hydrophobicized Cd electrode prepared from powdered Cd, carbon, acetylene black A-437E, and polytetrafluoroethylene (PTFE) [31]. The current efficiency and current density increased for smooth Cd Figure 4 Schematic diagram of the electrolytic cell with a solid polymer electrolyte composite electrode. SPE=Neosepta AM-1; CE=Pt wire; RE=Ag/AgCl; WEC= cathode compartment; CEC=anode compartment. (From Ref. 29.) Brillas et al.244 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... applying 64 mA cmÀ2, Cu-, In-, and Pb-supported GDEs gave almost 100% efficiency without producing H2 Zn-, Ag-, Cu-, and In-supported GDEs produced mainly CH4 The Pb-supported GDE induced only dechlorination, and 93% HFC 32 is selectively obtained with 74% faradaic efficiency The evolution of faradaic efficiency of products with current density for Cuand Pb-supported GDEs is depicted in Fig 6a and b, respectively... resins, hydraulic and lubricating oils, and waste process solvents [63 ] The method has also been applied to ethylene glycol [68 ], benzene [68 ], kerosene [63 ], organic acids [69 ], isopropanol [70], acetone [71] and organophosphorous, organosulphur, and chlorinated aliphatic and aromatic compounds, including PCBs [63 ] For example, Choi et al [69 ] have obtained 94– 96% destruction efficiencies for ethylendiaminetetraacetic... Reaction pathway for the electrochemical incineration of p-benzoquinone at a Pt anode covered with a quaternary metal oxide film (From Ref 54.) E Other Organic Pollutants Aqueous solutions containing pollutants such as 1,2-dichloroethane [58], dyestuffs [59], benzene [49 ,60 ], cyclohexane [60 ], ethanol [60 ], methanol [60 ], carboxylate anions in nuclear wastes [61 ], and glucose [62 ] have been treated successfully... chlorodifluoromethane, and ( w ) H2 (From Ref 33.) TM Copyright © 2003 by Marcel Dekker, Inc All Rights Reserved Electrochemical Methods for Degradation 247 of metal-supported porous carbon GDEs—the best metals being Cu and Ag CFC 13 is dechlorinated and defluorinated on the Cu-supported GDE, where CH4 and CHF3 (HFC 23) were primarily formed Dechlorination proceeds selectively on the Ag-supported GDE, giving... (bottom) and a cross-section of the fixed packed bed electrochemical reactor (top) containing a PbO2 anode, a stainless steel cathode, and a Nafion 427 cationic membrane as a separator between anolyte and catholyte The oxidation of aniline yielded p-benzoquinone (C6H4O2) and maleic acid (C4H4O4) as intermediates, from the following reactions: C6 H5 NH2 þ 2H2 O ! C6 H4 O2 þ 3Hþ þ 4eÀ þ NH4 þ C6 H4 O2 þ 6H2... products, and the initial reagent is recovered For example, for pollutant degradation by the oxidized form, the electrolysis is performed in the presence of the reduced form R of the mediator couple, which is anodically oxidized: R ! O þ eÀ ð22Þ and the oxidized form O reacts with the pollutant in the solution to recover the redox reagent R: O þ pollutant ! R þ productsðsÞ ð23Þ Degradation can also be performed... Inc All Rights Reserved Electrochemical Methods for Degradation 261 Figure 14 Schematic diagram of the packed bed cell and the flow circuit utilized for the treatment of human wastes: (1) reservoir; (2) pump; (3) valve; (4) flow meter; (5) anode current collector; (6) packed bed anode; (7) cathode; (8) water condenser; (9) water inlet; (10) water outlet; and (11) outlet for gases (From Ref 57.) TM Copyright... Reserved Electrochemical Methods for Degradation 267 generally soluble and the precipitation of chloride salts is thus avoided In addition, the rate for the reaction of Co(III) with water is slow at room temperatures, and the economical and environmental costs of cobalt as a mediator are smaller than for other metals, particularly silver Zawodzinski et al [70] have compared the mediated and the direct... of oxidants for the degradation of organic pollutants by indirect electrolysis A Electrogeneration Reactions for Ex Situ Applications The anodic synthesis of ozone from anodic oxidation of water in sulfuric acid media has been known since 1840 The reaction and its standard potential are: 3H2 O ! O3 þ 6Hþ þ 6eÀ E 0 ¼ 1:51V ð 36 Oxygen evolution occurs at a much lower potential and is therefore favored... by electrolysis in the presence of the oxidized form Thus, the reduced species formed at the cathode reacts with the organic pollutants, whereas the oxidized form is regenerated: O þ eÀ ! R R þ pollutant ! O þ productðsÞ TM Copyright © 2003 by Marcel Dekker, Inc All Rights Reserved ð24Þ ð25Þ Electrochemical Methods for Degradation 263 Note that pollutant degradation will cease when the current is switched . m À3 ] 2-NH 2 -4 -Cl-phenol 100 1 0.7 60 4-Cl-C 6 H 4 NO 2 48 1 0.4 55 DDVP a 560 1(2) 75 0.8 C 2 Cl 6 50 6 1 77 Pentachlorophenol 50 5 2 36 2,4,5-T b 100 3 1 70 C 2 Cl 4 74 4 3 70 1,2,4-Cl 3 C 6 H 3 40. HF. By applying 64 mA cm À2 , Cu-, In-, and Pb-supported GDEs gave almost 100% efficiency without producing H 2 . Zn-, Ag-, Cu-, and In-supported GDEs produced mainly CH 4 . The Pb-supported GDE. 6 Electrochemical Methods for Degradation of Organic Pollutants in Aqueous Media Enric Brillas and Pere-Lluı ´ s Cabot Universitat de Barcelona, Barcelona,