Electrochemical Treatment of 4-Nitrophenol-Containing Aqueous

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Ind. Eng. Chem. Res. 2004, 43, 1944-1951

Electrochemical Treatment of 4-Nitrophenol-Containing Aqueous Wastes Using Boron-Doped Diamond Anodes P. Can ˜ izares, C. Sa´ ez, J. Lobato, and M. A. Rodrigo* Departamento de Ingenierıá Quı´mica, Facultad de Ciencias Quı´micas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain

The electrochemical oxidation of aqueous wastes containing 4-nitrophenol (4-NP) using a borondoped diamond thin-film electrode has been studied. Within the parameter ranges used (1504000 mg of 4-NP dm-3, pH 2-12, 30-60 mA cm-2, 25-60 °C), the complete treatment of the organic waste was achieved. The maximum current efficiencies were obtained under kinetic control. On the basis of the results of voltammetric and galvanostatic electrolysis studies, a simple mechanistic model was proposed. The first stage in the treatment of 4-NP-containing aqueous wastes is the release of the nitro group from the aromatic ring. As a consequence, phenol or quinones are formed. These organic compounds are oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. On the cathode, the reduction of the 4-NP to 4-aminophenol takes place. In alkaline media, this compound can be polymerized and transformed into a dark brown solid. Introduction The use of direct and mediated electrochemical oxidations for the treatment of aqueous wastes has undergone rapid development in recent years.1-6 This technology can be successfully applied to the treatment of wastewater containing nonbiodegradable organics such as phenol, chlorophenol, nitrophenol, and aniline.7-14 In most cases, total mineralization of the organic compounds can be achieved. This fact confirms electrochemical oxidation to be one of the most promising techniques for the treatment of wastewater containing small amounts of aromatic compounds. Nevertheless, this technology is not currently being applied commercially, mainly because of its presumed high energy consumption. The resulting high operating costs are due to the large number of electrons required to completely oxidize organic matter to carbon dioxide and to the small efficiencies obtained with some electrodes. To decrease the energy requirements, an alternative treatment could be the partial oxidation of pollutants to obtain wastes that can be biologically treated (in a combined process). This option requires less energy, but it is important to consider that some intermediates could also be toxic and such materials must be completely removed.15 Thus, in the search for less-expensive applications, it is important to have a good mechanistic understanding of the process. On the other hand, the development of new anodic materials has allowed the current efficiencies of electrochemical processes to be increased. One of these new materials is boron-doped diamond (BDD), which is able to achieve high current efficiencies in electrochemical wastewater treatment (because of its high oxygen overpotential) and exhibits outstanding chemical stability (chemically inert, high hardness). Nitrophenols are among the most common organic pollutants in industrial and agricultural wastewaters.16-21 These compounds are involved in the * To whom correspondence should be addressed. Fax: 926295318. E-mail: [email protected].

synthesis of many chemicals, particularly in the field of pesticides, and some of their derivatives can be used as insecticides or herbicides. They are present in the industrial effluents of chemical plants that manufacture explosives, dyestuffs, and products for leather treatment, and they are also present in agricultural irrigation effluents. Nitrophenols are considered to be hazardous wastes and priority toxic pollutants by the U.S. Environmental Protection Agency.22 It is therefore important to assess the fate of these compounds in the environment. In recent years, only a few papers have addressed the electrochemical treatment of nitrophenols,13,14,23 and they have focused only on mediated oxidation processes. The mechanisms described are complex, and the oxidation leads to the formation of nitrogenated and nonnitrogenated organic intermediates. It is assumed that the process starts with the formation of dihydroxycyclohexadienyl radicals. These radicals can be transformed into nitrocatechol and hydroquinone (which lead to nitrite ion liberation). In a later stage, nitrite ions are also released from nitrocatechol, resulting in polyhydroxybenzenes. The aromatic intermediates undergo further oxidation and are transformed into non-nitrogenated ring-opening products and later into carbon dioxide. Nitrite ions are also oxidized to nitrate ions. These reaction mechanisms are also observed for other oxidative treatments (e.g., wet air oxidation24), but unfortunately, no works describing the direct electrochemical oxidation of these wastes have been published. The goal of the work described here was to increase our understanding of the mechanisms involved in the electrochemical oxidation of 4-nitrophenol (4-NP) on boron-doped diamond electrodes and to elucidate the influence of the waste characteristics (initial concentration, supporting electrolyte, and pH) and the operating conditions (current density and temperature) on the process. Experimental Section Analytical Procedure. The carbon concentration was monitored using a Shimadzu TOC-5050 analyzer.

10.1021/ie034025t CCC: $27.50 © 2004 American Chemical Society Published on Web 03/31/2004

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 1945

Figure 1. Arrangement of the pilot plant. Detail of the electrochemical cell section.

Chemical oxygen demand (COD) was determined using a HACH DR200 analyzer. Carboxylic acids were monitored by HPLC using a Supelcogel H column (mobile phase, 0.15% phosphoric acid solution; flow rate, 0.15 mL min-1). The UV detector was set at 210 nm. Aromatics were also monitored by HPLC using a Nucleosil C18 column (mobile phase, 65% water/33% methanol/2% acetic acid; flow rate, 0.50 mL min-1). In this case, the UV detector was set to 280 nm. Nitrogen inorganic ions (NO3-, NO2-, and NH4+) were measured by ion chromatography (Metrosep Anion Dual 2 column; mobile phase, 1.3 mM Na2CO3 and 2.0 mM NaHCO3; flow rate, 0.80 mL min-1). Total nitrogen was measured by chemiluminiscence using a Shimadzu TN3000 instrument. To measure electrogenerated oxidants, I-/I2 assays were performed. By titration with thiosulfate in the presence of starch, this technique can quantify all of the oxidants capable of oxidizing I- to I2, including peroxodisulfate and hydrogen peroxide. Determination of the Instantaneous Current Efficiency (ICE). The chemical oxygen demand method was used for the determination of the current efficiency for the oxidation of 4-nitrophenol. In this method, the COD was measured during electrolysis, and the instantaneous current efficiency was calculated using the relation

ICE )

(CODt+∆t - CODt)FV 8I∆t

(1)

where CODt and CODt+∆t are the chemical oxygen demands (in grams of O2 per cubic decimeter) at times t and t + ∆t (in seconds), respectively; I is the current intensity (in amperes), F is the Faraday constant (96 487 C mol-1), V is the volume of the electrolyte (in cubic decimeters), and 8 is a dimensional factor for unit consistency {32 g of O2 (mol of O2)-1/[4 mol of e- (mol of O2)-1]}. Electrochemical Cell. The oxidation of 4-nitrophenol was carried out in a single-compartment electrochemical flow cell (Figure 1). A diamond-based material was used as the anode, and stainless steel (AISI 304) was used as the cathode. Both electrodes were circular (100 mm in diameter) with a geometric area of 78 cm2 each and an electrode gap of 9 mm. The electrolyte was stored in a glass tank (500 mL) and circulated through

the electrolytic cell by means of a centrifugal pump. A heat exchanger was used to maintain the temperature at the desired set point. The experimental setup also contained a cyclone for gas-liquid separation, as well as a gas absorber to collect the carbon dioxide contained in the gases evolved from the reactor into sodium hydroxide. Preparation of the Diamond Electrode. Borondoped diamond films were provided by CSEM (Neuchaˆtel, Switzerland) and synthesized by the hot filament chemical vapor deposition technique (HF CVD) on single-crystal p-type Si(100) wafers (0.1 Ω cm, Siltronix). The temperature range of the filament was 2440-2560 °C, and the temperature of the substrate was 830 °C. The reactive gas was methane in excess dihydrogen (1% CH4 in H2). The dopant gas was trimethylboron with a concentration of 3 mg dm-3. The gas mixture was supplied to the reaction chamber at a flow rate of 5 dm3 min-1, giving a growth rate of 0.24 µm h-1 for the diamond layer. The resulting diamond film thickness was about 1 µm. This HF CVD process produces columnar, random texture, and polycrystalline films with an average resistivity of 0.01 Ω cm. Prior to use in galvanostatic electrolysis assays, the electrode was polarized for 30 min with a 1 M H2SO4 solution at 50 mA cm-2 to remove any kind of impurity from its surface. Voltammetry Experiments. Electrochemical measurements were obtained using a conventional threeelectrode cell in conjunction with a computer-controlled potentiostat/galvanostat (Autolab model PGSTAT 30, Eco Chemie B.V., Utrecht, The Netherlands). Diamond was used as the working electrode, Hg/Hg2Cl2‚KCl (saturated) as a reference electrode, and stainless steel (AISI 304) as a counter electrode. All electrodes were circular (10 mm diameter) with a geometric area of 7.8 cm2 each. Voltammetry experiments were performed in unstirred solutions (200 mL). The anode was anodically polarized for 5 min with a 1 M H2SO4 solution at 0.1 A prior to each experiment. Galvanostatic Electrolysis. Galvanostatic electrolyses were carried out to determine the main intermediates formed in the process. The synthetic wastewaters used in the experiments contained different concentrations of 4-nitrophenol (4-NP), 5000 mg of Na2SO4 dm-3, and H2SO4 in suitable amounts to give a pH of 2 (or

1946 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004

Figure 2. Cyclic voltammograms on BDD anodes of 4-NP solutions on sodium sulfate media (5000 mg dm-3) at pH (a) 2 and (b) 12. Scan rate ) 100 mV s-1. (1) no organic matter, first cycle; (2) 500 mg dm-3 4-NP, first cycle; (3) 500 mg dm-3 4-NP, second cycle; and (4) 500 mg dm-3 4-NP, third cycle. Auxiliary electrode, stainless steel AISI 304; reference electrode, SCE.

Figure 3. Linear sweep voltammograms on BDD anodes of 4-NP solutions in sodium sulfate media (5000 mg dm-3) at pH 2. Auxiliary electrode, stainless steel AISI 304; reference electrode, SCE. (a) Scan rates (mV s-1): (1) 50, (2) 100, and (3) 250, . (b) Concentrations of 4-NP (mg dm-3): (1) 150, (2) 250, (3) 500, and (4) 1000 mg dm-3.

Table 1. Experimental Conditions Studied in This Work

phenols (1.4-1.6 V vs SCE), and the second peak (p2) is very close to the oxygen-evolution region (2.2-2.4 V vs SCE) and is partially overlapped by this side reaction. Peak p1 decreases in size and shifts toward the right in the second scan. The shift is significant in acid media and almost negligible in alkaline media. Peak p2 is overlapped by oxygen evolution in the second and third scans. No peaks were found in the reverse scan. The presence of 4-NP shifted the oxygen-evolution process toward higher potentials. Parts a and b of Figure 3 show the effects on the anodic oxidation of the scan rate and the concentration of 4-NP, respectively. It can be observed that both peaks increase in size and shift toward higher potentials with increasing scan rate. Peak p1 increases in size with increasing concentration. Peak p2 overlaps with oxygen evolution, and as a consequence, this process seems to begin at lower potentials. The increase in the size of the peaks and the shift toward the oxygen-evolution region with increasing scan rate indicate that these peaks correspond to irreversible reactions. Likewise, the decrease in the size of the reverse peaks corresponding to peaks p1 and p2 (Figure 2) suggests the occurrence of a later chemical reaction involving the electrochemically formed products (EC mechanism). In fact, the voltammetric behavior observed is characteristic of the anodic oxidation of psubstituted phenols on BDD electrodes, and it has been previously reported for 4-chlorophenol.11,25 According to these earlier reports, it seems reasonable that both peaks (p1 and p2) correspond to the oxidation of 4-NP to the 4-nitrophenoxy radical and the subsequent oxida-

run

C0 (mg of C dm-3)

supporting media

1 2 3 4 5 6 7 8 9

150 150 150 1500 1500 1500 1500 1500 4000

Na2SO4/H2SO4 H2SO4/NaOH Na3PO4/H3PO4 Na2SO4/H2SO4 H2SO4/NaOH Na2SO4/H2SO4 Na2SO4/H2SO4 Na3PO4/H3PO4 Na2SO4/H2SO4

pH

T (°C)

j (mA cm-2)

2 12 2 2 12 2 2 2 2

25 25 25 25 25 25 60 25 25

30 30 30 30 30 60 30 30 30

NaOH to reach a pH of 12). The pH was kept constant by the continuous introduction of sulfuric acid (or sodium hydroxide) into the electrolyte reservoir. Table 1 lists the conditions applied in each experimental run. The cell potential was constant during each electrolysis reaction, indicating that appreciable deterioration of the electrode or passivation phenomena did not take place. The electrolyte flow rate through the cell was 1250 cm3 min-1. The linear velocity of the fluid was 2.31 cm s-1, and the space velocity was 2.08 min-1. Results and Discussion Voltammetric Study. Figure 2 shows cyclic voltammograms of acid and alkaline aqueous solutions containing 4-NP and Na2SO4. Curves obtained under the same experimental conditions but without 4-NP are also shown. The presence of 4-NP leads to the appearance of two anodic oxidation peaks: the first peak (p1) is in the typical potential region for the oxidation of


Figure 4. (a) Cyclic voltammograms on BDD anodes of 4-NP solutions (150 mg dm-3) in sodium sulfate/sulfuric acid media (pH 2, 5000 mg dm-3): (1) first cycle and (2) second cycle. Auxiliary electrode, stainless steel AISI 304; reference electrode, SCE. Scan rate ) 100 mV s-1. A, anodic start of the CV. (b) Detail of the oxidation-reduction peaks, p3 and p4.

tion of this nitrophenoxy radical to the corresponding nitrophenoxy cation (eq 2).

Both electrochemically formed compounds are very reactive and can couple to form polymers or undergo other chemical transformations such as the release of the nitro group (or substitution with hydroxyl groups) to form non-nitrogenated phenolic or quinonic intermediates. The oxidation of nitrophenols to form nonnitrogenated compounds is also reported in the literature for mediated electrochemical processes13,14 and for other oxidation techniques such as wet oxidation and Fenton oxidation.24,26 The shift of the oxygen-evolution process toward the right might be due to polymer formation on the electrode surface. On the other hand, to confirm the release of the nitro group, voltammograms were obtained in a wider range of potentials (Figure 4). The appearance of a new anodic oxidation peak (p3) and its corresponding reduction peak (p4) can be observed. The former peak is in the potential region of quinones. When hydroquinone is added to the solution, the peak increases in size, indicating that both peaks (p3 and p4) might be related to the oxidation of p-benzoquinone to hydroquinone.24 This observation supports a mechanism involving the release of the nitro group to form p-benzoquinone. In Figure 4, two new reduction peaks (p5 and p6) can also be observed that can be identified as the reduction of 4-NP to 4-hydroxy-

Figure 5. Variations of (a) TOC and (b) COD with the specific electrical charge passed in the electrochemical oxidation of wastes containing 4-NP (pH 2, T ) 25 °C, j ) 30 mA cm-2). b 150 mg dm-3, 0 1500 mg dm-3, and 2 4000 mg dm-3.

Figure 6. Variation of the instantaneous current efficiency (ICE) with COD in the electrochemical oxidation of wastes containing 4-NP (pH 2, T ) 25 °C, j ) 30 mA cm-2). 0 150 mg dm-3, 9 1500 mg dm-3, and 2 4000 mg dm-3.

laminophenol and the transformation of this compound to 4-aminophenol. Galvanostatic Electrolysis. Figure 5 shows the variation of the total organic carbon (TOC) concentration and the chemical oxygen demand (COD) in the electrolyte with the specific electrical charge passed (in ampere hours per cubic decimeter) as a function of the initial 4-NP concentration (pH 2, 5000 mg of Na2SO4 dm-3, j ) 30 mA cm-2). As can be observed, the complete disappearance of the organic compounds contained in the waste is achieved. Figure 6 shows the ICE vs COD graph for the same experiments. It can be observed that the efficiency in


the oxidation is 1 for COD values higher than 1750 mg dm-3 and that the ICE decreases linearly to zero for lower concentrations. Thus, the obtained data match with models proposed in the literature11,27,28 in explaining the electrochemical oxidation of organic matter using BDD electrodes. These electrodes can oxidize 4-NP with maximum efficiency if the reaction is kinetically controlled, and the efficiency decreases only when the process becomes diffusion-controlled. Thus, this anode material can reduce the specific energy requirements of the electrochemical oxidation of organics in aqueous wastes. The specific energy consumptions obtained in the experiments of this work range from 35 to 50 kWh per kilogram of COD removed. These values can still be decreased with an optimized cell design, although they are lower than those obtained in other works for the electrochemical treatment of aqueous wastes with other electrodes.29 These values are also lower than those obtained with other technologies such as ozonation.30 This fact confirms electrochemical oxidation to be one of the most promising techniques for the treatment of wastewaters containing nitro-aromatic compounds. However, great efforts must still be made to achieve wide use of this technology. Figure 7a shows the variation with the specific electrical charge passed of the main carbon compounds involved in the oxidation process (pH 2, 5000 mg of Na2SO4 dm-3, j ) 30 mA cm-2). The electrochemical oxidation of 4-NP on BDD electrodes leads to the formation of carbon dioxide and solid compounds as the final product. The solid compounds were brown-colored and were located onto the cathode surface during the treatment. This material had a very low electric resistance, as the voltage of the cell remained constant during all the experiment. After several chemical assays, this compound was identified as a polymer resulting of the condensation of the p-aminophenol formed cathodically in alkaline media.24,31,32 The polymer was not formed in acidic media, but it was stable under acidic conditions. This might explain the formation of polymer on the cathode surface in the treatment of acidic wastes given that, close to the cathode, some hydroxyl anions are formed during hydrogen evolution, causing a local increase in the value of the pH. This solid compound can easily be removed from the cathode surface at the end of the treatment and has a very low resistance. Thus. its appearance does not exclude the possible industrial application of this technology. In Figure 7a, it can also be observed that the main organic intermediates formed in every case (measured by HPLC) are hydroquinone and oxalic acid. Phenol, p-aminophenol, p-nitrocatechol, and maleic acid are also found in high concentrations. Some quantities of catechol and resorcine and other carboxylic acids were also detected, but in very low concentrations. Figure 7b shows the variation of the nitrogen species with the specific electrical charge passed. It can be seen that the organic nitrogen was transformed into oxidized nitrogen (nitrates and nitrites), ammonia, and polymeric nitrogen. The ammonia was formed by the reduction of the oxidized nitrogen. By mass balance, it was concluded that no volatile species of nitrogen were formed during the treatment of 4-NP. Figure 7c shows the variation with the specific electrical charge passed of oxidants measured at the sample collection time and several times thereafter. It can be observed that the oxidant concentration in-

Figure 7. Variations of the concentrations with the specific electrical charge passed. Experimental conditions: C0 ) 1500 mg dm-3, pH 2, T ) 25 °C, j ) 30 mA cm-2. (a) Organic intermediates and products obtained: 2 inorganic carbon, 4 polymeric carbon, b hydroquinone, 9 4-nitrocatechol, O 4-aminophenol, 0 phenol, ( oxalic acid and ) maleic acid. (b) Nitrogen species: 2 organicsoluble nitrogen, 4 polymeric nitrogen, 9 N (NO2- + NO3-), and ) N (NH4+). (c) Oxidants (( 0, 9 1, 2 2, ) 3, 0 4, and 4 24 h).

creased initially with the charge applied until a constant value was achieved. The oxidants can continue oxidizing in the sample probe with time, and a decrease in the oxidant power was obtained. Figure 8 shows the effects of the current density and temperature on the process. As can be seen, an increase in the temperature leads to a more efficient process. Because direct oxidation processes remain almost unaffected by temperature, this fact must be explained in terms of the presence of inorganic electrogenerated reagents. In the literature, it has been proposed28,33,34 that, in the electrochemical oxidation on BDD electrodes of wastes containing sulfates, some peroxodisulfates can be formed (reaction 3). These compounds are known to be very powerful oxidants that can attack organic matter. Peroxodisulfates can decompose and form hy-


Figure 10. Simple mechanistic model proposed to explain the main processes occurring in the electrochemical treatment of 4-NP wastes using BDD anodes. Figure 8. Variations of TOC and COD with the specific electrical charge passed (C0 ) 1500 mg dm-3, pH 2, sulfate media). 0 25 °C, 30 mA cm-2; 2 25 °C, 60 mA cm-2; and b 60°C, 30 mA cm-2.

Figure 9. Variations of TOC and COD with the specific electrical charge passed (C0 ) 1500 mg dm-3, 25 °C, 30 mA cm-2). 0 pH 2, sulfate media; b pH 12, sulfate media; and 2 pH 2, phosphate media.

drogen peroxide and other oxidants. As a result, several oxidant reagents might be involved in the mediated processes.

2SO42- f S2O82- + 2e-

(3)

In Figure 8, it can also be observed that an increase in the current density leads to a less efficient process. This behavior is characteristic of diffusion-controlled processes.34,35 In such systems, an increase in current density is not able to increase the rate of oxidation of the organics at the electrode but merely favors the anodic side reactions. Because none of these reactions generate oxidized redox reagents, a decrease in the efficiency is observed. No significant differences were found in the intermediates formed or in their concentrations, indicating that the operating conditions do not influence the reaction mechanisms. Figure 9 shows the effects of the pH and the presence of other salts in the electrolyte. Alkaline pH seems to lead to more efficient processes. This fact can easily be explained taking into account that the extent of polymerization is greater under alkaline conditions than in acidic media. On the other hand, it can be seen that the oxidation rate is higher in media containing sulfates than in media containing phosphates. This fact can easily be explained by taking into account the fact that, in sulfate-containing media, some peroxodisulfates can

be formed whereas, in phosphate media, no inorganic reagents coming from the oxidation of these anions can be formed. Nevertheless, no significant differences were found in the intermediates formed or in their concentrations. Thus, the presence of reversible redox reagents, which can be oxidized at the anode surface and later act as intermediaries for shuttling electrons between the pollutant substrate and the electrode, plays an important role in the global oxidation rate and complements direct oxidation processes in the overall treatment. The results of the galvanostatic electrolysis together with information obtained from the voltammetric study allow for the development of a simple mechanistic model (Figure 10) that describes the intermediates formed during the treatment. According to this model, the first stage in the treatment of 4-NP-containing aqueous wastes is the release of the nitro group from the aromatic ring. As a consequence, phenol or quinones are formed. These organic compounds are oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide.28 In the cathode, the reduction of 4-NP to 4-aminophenol and the transformation of nitrate into ammonia take place. In alkaline media, 4-aminophenol is polymerized and transformed into a dark brown solid that remains on the cathode surface up to the end of the treatment. The oxidation processes in this model either can occur directly on the electrode surface or can be mediated by peroxodisulfate and other inorganic reagents electrogenerated at the anode surface. This mechanistic model is in agreement with other models proposed in the literature to explain the chemical oxidation and reduction of 4-NP.13,14,18,23,24 Likewise, this simple mechanistic model not only explains the presence of particular compounds but is also consistent with the time evolution of the different intermediates. Thus, Figure 11 shows the results of a simulation using the mechanistic model proposed in this work and an electrochemical cell model previously published by our group.28,34 The cell model relies upon the existence of two types of zones in the electrochemical reactor: the zones close to the electrode surface with a thickness equivalent to the Nernst diffusion layer (reaction zones), where electrochemical and some strong mediated oxidation/reduction processes develop, and the remaining reactor volume (bulk zone), where some mediated oxidation/reduction reactions can develop. Within each zone, the concentration of each compound is assumed to depend only on time and not on position. Mass-transport processes between zones were quanti-


Both direct and mediated electrochemical oxidation processes are involved in the electrochemical treatment of 4-NP with BDD anodes. Carbon dioxide and an easily removable solid material are the final products in the electrochemical treatment of 4-NP with BDD anodes. The main intermediates are hydroquinone, phenol, p-aminophenol, and maleic and oxalic acids. Taking into account the results of the voltammetric and galvanostatic studies, a simple mechanistic model was proposed. According to this model, the first stage in the treatment of 4-NP-containing aqueous wastes is the release of the nitro group from the aromatic ring. As a consequence, phenol or quinones are formed. These organic compounds are oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. Simultaneously, at the cathode, the reduction of the 4-NP to 4-aminophenol (and the reduction of oxidized nitrogen to ammonia) takes place. In alkaline media, 4-aminophenol is polymerized and transformed into a dark brown solid that remains on the cathode surface up to the end of the treatment. Acknowledgment

Figure 11. Variations of the concentrations of 4-NP, intermediates and final products with time. Points represent the experimental data, and the solid lines represent the results of the simulation. (Experimental conditions: C0 ) 1500 mg of 4-NP dm-3, pH 2, T ) 25 °C, j ) 30 mA cm-2.) (a) 2 4-NP, ( quinones, 0 phenol, 9 4-aminophenol, O maleic acid, 4 oxalic acid, b CO2, ) polymeric carbon. (b) 2 organic-soluble nitrogen, ( NO2-, 0 NO3-, 9 NH4+, O polymeric nitrogen.

fied by assuming that the local rate of exchange is proportional to the concentration difference between the two zones. This description allows the mathematical complexity of the reaction system to be simplified significantly and can yield, together with an appropriate kinetic model, good agreement between experimental and simulation results. As can be observed, the combination of both the electrochemical cell and mechanistic models yields good agreement between the simulation results and the experimental data. The model was applied to all of the data obtained, and the coefficient of variation was always found to be under 10% (with an average value of 7%). Conclusions The following conclusions can be drawn from the work described here: Electrochemical oxidation using a diamond thin-film anode can be successfully used for treating aqueous 4-NP wastes. The total removal of the organic carbon in the waste is achieved, regardless of the current intensity, initial 4-NP concentration (within the range studied), and temperature. The current efficiency of the process is 1 if it is kinetically controlled and decreases linearly to 0 from the COD limiting value if it is diffusion-controlled.

This work was supported by the MCT (Ministerio de Ciencia y Tecnologıá, Spain) and by the EU (European Union) through Project REN2001-0560. The Contribution of Junta de Comunidades de Castilla-La Mancha (Consejerıá de Ciencia y Tecnologıá) is also acknowledged. Literature Cited (1) Papouchado, L.; Sandford, R. W.; Petrie, G.; Adams, R. N. Anodic Oxidation Pathways of Phenolic Compounds. J. Electroanal. Chem. 1975, 65, 275. (2) Sharifian, H.; Kirk, D. W. Electrochemical Oxidation of Phenol. J. Electrochem. Soc. 1986, 133, 921. (3) Comninellis, Ch.; Pulgarin, C. Anodic Oxidation of Phenol for Waste Water Treatment. J. Appl. Electrochem. 1991, 21, 703. (4) Leffrang, U.; Ebert, K.; Flory, K.; Galla, U.; Schmieder, H. Organic Waste Destruction by Indirect Electrooxidation. Sep. Sci. Technol. 1995, 30, 1883. (5) Boudenne, J.-L.; Cerclier, O. Performance of Carbon BlackSlurry Electrodes for 4-Chlorophenol Oxidation. Water Res. 1999, 33, 494. (6) Can˜izares, P.; Domıńguez, J. A.; Rodrigo, M. A.; Villasenõr, J.; Rodrı´guez, J. Effect of the Current Intensity in the Electrochemical Oxidation of Aqueous Phenol Wastes at an Activated Carbon and Steel Anode. Ind. Eng. Chem. Res. 1999, 38, 3779. (7) Brillas, E.; Bastida, R. M.; Llosa, E.; Casado, J. Electrochemical Destruction of Aniline and 4-Chloroaniline for Wastewater Treatment using a Carbon-PTFE O2-fed Cathode. J. Electrochem. Soc, 1995, 142, 1733. (8) Polcaro, A. M.; Palmas, S. Electrochemical Oxidation of Chlorophenols. Ind. Eng. Chem. Res. 1997, 36, 1791. (9) Brillas, E.; Mur, E.; Sauleda, R.; Sańchez, L.; Peral, J.; Domeńech, X.; Casado, J. Aniline Mineralization by AOP’s: Anodic Oxidation, Photocatalysis, Electro-Fenton and Photoelectro-Fenton Processes. Appl. Catal. B: Environ. 1998, 16, 31. (10) Azzam, M. O.; Al-Tarazi, M.; Tahboub, Y. Anodic Destruction of 4-Chlorophenol Solution. J. Haz. Mater. 2000, B75, 99. (11) Rodrigo, M. A.; Michaud, P. A.; Duo, I.; Panizza, M.; Cerisola, G.; Comninellis, Ch. Oxidation of 4-Chlorophenol at Boron-Doped Diamond Electrodes for Wastewater Treatment. J. Electrochem. Soc. 2001, 148 (5), D60. (12) Ureta-Zanãrtu, M. S.; Bustos, P.; Berrıós, C.; Diez, M. C.; Mora, M. L.; Gutie´rrez, C. Electrooxidation of 2,4-Dichlorophenol and Other Polychlorinated Phenols at a Glassy Carbon Electrode. Electrochim. Acta 2002, 47, 2399.

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Received for review July 28, 2003 Revised manuscript received February 3, 2004 Accepted February 3, 2004 IE034025T

Electrochemical Treatment of 4-Nitrophenol-Containing Aqueous

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