Electrochemical Degradation of Perchloroethylene in Aqueous Media

Ind. Eng. Chem. Res. 2010, 49, 4123–4131

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Electrochemical Degradation of Perchloroethylene in Aqueous Media: Influence of the Electrochemical Operational Variables in the Viability of the Process Verońica Saéz, Maria Deseada Esclapez, Ignacio Tudela, Pedro Bonete, and Jose´ Gonza´lez-Garcıá* Grupo de NueVos Desarrollos Tecnolo´gicos en Electroquı´mica: Sonoelectroquı´mica y Bioelectroquı´mica, Departamento de Quı´mica Fı´sica e Instituto UniVersitario de Electroquı´mica, UniVersidad de Alicante, Ap. Correos 99. 03080 Alicante, Spain

An extensive study of the influence of the operational variables in the electrochemical degradation of perchloroethylene in water has been carried out using an electrochemical filter-press reactor. The influences of the initial concentration, volumetric flow, and electrode geometry and nature on the kinetics, degradation efficiency, and mechanism have been analyzed by determining the values of process performance parameters such as fractional conversion, degradation efficiency, mass balance error, and selectivity. The best results obtained were a fractional conversion higher than 85%, degradation efficiencies of 65%, selectivities close to 0.8, and energetic consumption around 3 kWh m-3 obtained with two-dimensional electrodes. Threedimensional carbon electrodes did not provide a more competitive option. Introduction Halogenated compounds receive continuous attention from an environmental point of view because they have been detected in all environmental media, including air, water, soil, and organic matter.1 We can find them as natural degradation byproduct and also as man-made manufactured products (pesticides, organic solvents, and industrial organics). Among them, chlorinated solvents are paid significant attention due to their worldwide use and dissemination, and especially because they are contaminating our groundwater and other drinking water resources. The resistance of these compounds to transformation via mainstream biochemical processes has encouraged the development not only of the current methods for pollutant treatments2–4 but also of new emerging technologies5,6 in order to shed light on this unsolved problem. Bearing in mind that phase-transfer technologies, such as air stripping, carbon adsorption, and membrane separation, are not a real solution for this problem,7 procedures which degrade the toxic nature of the molecule must receive a large amount of attention. Due to the electronegative character of the chlorine groups in the molecules concerned, reduction appears as a valid method. Reductive dechlorination can be achieved by chemical routes such as using the zerovalent iron, Fe0,8 and also electrochemical degradation.9 In front of the popular relevance of the zero metal treatment,10 the electrochemical route presents benefits such as enhanced reduction kinetics and controllable electrode passivation or corrosion.11 In the literature, we can find electrochemical treatments for chlorinated compounds. Despite the advantages in the electrochemical treatment of using reduction, oxidation, and both electrode reactions to degrade the molecule,12 it has routinely been reported that chlorinated compounds present only acceptable results by electroreduction13,14 and, normally, the effective process performance requires the use of nonaqueous reaction media15–17 or unpopular cathodic materials such as lead and mercury.18–20 However, due to the fact that polluted water is the real problem requiring a solution, it is necessary to focus the research on improving the efficiency of the electrochemical treatments directly on aqueous systems.21 Therefore, several * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +34965903855.

approaches are being developed in order to enhance the efficiency of the electrochemical alternative for aqueous effluents: (i) the use of unmodified22 and modified23 threedimensional electrodes, (ii) the use of new electrode materials,24 (iii) the use of new electrochemical reactor designs,25 and (iv) the use of combined technologies.26 A lot of the work found in the literature about the electrochemical degradation of chlorinated organocompounds is based on the use of three-dimensional electrodes. This is because, from a practical point of view, a greater decrease in the level of contaminant in solution is achieved if we increase the mass transport conditions by increasing km with higher flow rates and/ or by increasing the electrode surface area through the use of electrodes with high specific surface area, i.e. three-dimensional electrodes.27 With this geometry, the nature of the cathodic material has routinely been the subject of study in the literature. Sonoyama and Sakata28 studied bare, Cu-, Zn-, and Ag-carbon fiber electrodes and presented the Ag-carbon fiber electrode as the best cathodic material. This material provided fractional conversion (FC) of 100% for different chlorinated organocompounds such PCE, 1,1,1-trichloroethane, 1,1,2-trichloroethane, and 78% for TCE in the presence of background electrolyte and with low flow rates (0.06 L/h). Li and Farrel23 used Pdand pure Fe porous electrodes for the degradation of TCE and carbon tetrachloride in a divided flow cell, and they pointed out corrosion problems when a pure-Fe porous electrode was used. He et al.29 studied copper porous electrodes, which provided FC close to 80% at the lowest flow rates analyzed. The influence of the flow rate using three-dimensional electrodes has also been reported in the literature. Sonoyama and Sakata,28 using a three-dimensional Ag-carbon fiber electrode, correlated the degradation efficiency (DE) with the presence of background electrolyte (K2SO4) and the volumetric flow. In that work with [PCE]0 ) 200 µM, FC values close to 100% were obtained at low volumetric flow (0.06 L/h) independently of the concentration of K2SO4. A higher volumetric flow needed a higher concentration of K2SO4 to maintain the total fractional conversion. Li and Farrel,23 using Pd- and pure Fe porous electrodes, analyzed the degradation of [TCE]0 ) 1.0 mM and carbon tetrachloride in a divided flow cell. They pointed out that, under their experimental conditions, the process was under mass transport conditions at low flow rates, moving

10.1021/ie100134t  2010 American Chemical Society Published on Web 04/07/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010

to a poor influence of the flow rate at higher values. He et al.29 have also analyzed the influence of the volumetric flow using a porous cathode under mass transport control for the treatment of [CCl4]0 ) 0.650 µM. They reported a decrease in the FC from 72% to 44% when the volumetric flow was increased from 1.00 to 1.88 L/h, attributing this change to a shortened retention time. In spite of the fact that PCE is a widely used solvent in many areas of industry and has been reported as a major intermediate in the degradation of other chlorinated compounds, to our knowledge, there has not been an extensive study into the electrochemical degradation of this compound. The reason for choosing PCE as a target molecule is based on the fact that PCE is actually the solvent for dry-cleaning services and also the solvent for the shoe industry. Both of these industries produce water saturated with PCE, the concentration of which is around 900 µM. Trying to analyze the performance of the process and taking into account the fact that PCE is a volatile compound and the real concentration is normally lower than saturation, concentrations at the micromolar level were chosen as a starting point. Our group has initiated an extensive study of the electrochemical degradation of perchloroethylene from an initial stage with spectroelectrochemical30 and extensive voltammetric31 studies toward bulk electrolyses32 studies. The present work is already focused on the scale-up of the process using a filter-press electrochemical reactor. With this experimental setup, the technical, economical, and environmental viability of the electrochemical treatment has been analyzed. Thus, the influence of the initial concentration, volumetric flow, and electrode materials on the kinetics, mechanism, and efficiency of the process has been addressed. Experimental Section Materials. Perchloroethylene, C2Cl4 99.9% (Aldrich) was used without any further purification. Aqueous solutions were prepared with purified water obtained from a Milli-Q UV Plus system (18.2 MΩ cm resistivity), Ar-purged, where previously Na2SO4 had been dissolved to give a concentration of 0.05 M. Perchloroethylene (PCE) was then added and the solution stirred in a closed volumetric flask with a glass-covered magnetic bar, and the solutions were left overnight with slow stirring to dissolve and equilibrate. Care was taken to minimize the effects of evaporation losses by using nearly full volumetric flasks with tight-fitting ground-glass stoppers that were further sealed with Teflon tape. The initial concentration of PCE lies in the range 60-452 µM. Different electrode materials were used in this study. As twodimensional cathodes, plates of copper (13 × 9 × 0.1 cm) provided by Goodfellow, of lead (13 × 9 × 0.3 cm) provided by Goodfellow, and graphite JP845 (13 × 9 × 0.5 cm) provided by Le Carbone-Lorraine were used as received. As anodes, plates of commercial DSA-O2 (13 × 9 × 0.1 cm) and DSAO2-Cl2 (13 × 9 × 0.1 cm) provided by I.D. Electroquı´mica S. L were used as received. The PbO2 anode (13 × 9 × 0.6 cm) was homemade, prepared on a carbon substrate by electrodeposition from acidic Pb(II) solutions.33 A carbon felt (RVC 4002) provided by Le Carbone Lorraine was used as three-dimensional cathode. A carbon felt slide (7 × 9 × 0.4 cm) was implemented by pressing in the compartment, using the previous graphite plate as a current collector. Experimental Setup. Figure 1A depicts the UA63.03 filterpress electrochemical reactor used. According to previous results,32 an undivided configuration with turbulent promoters type B34 or a three-dimensional electrode35 has been used, with

Figure 1. (A) Electrochemical filter-press reactor: (1) backplate, (2) backplate joint, (3) polypropylene block with flow channels, (4) electrode joint, (5) cathode, (6) anode, (7) compartment joint, (8) compartment. (B) Experimental setup: (1) electrochemical filter-press reactor, (2) electrolyte reservoir, (3) gasmeter, (4) pump, (5) flowmeter, (6) temperature control system, (7) cooling system, (8) power supply, (9) voltmeter, (10) probes holder, (11) pHmeter, (12) conductimeter.

current and fluid flow in perpendicular configuration. This reactor UA63.03 is a homemade cell which has been previously fully characterized by the authors36 and used in a wide range of applications,37–40 the results of which were routinely confirmed at large scale. This reactor presents an active area of 63 cm2 with an interelectrode gap close to 3 mm. Figure 1B shows the global experimental setup. A Micro pH 2000 Crison pHmeter and a Crison conductimeter model 525 were used to measure the initial and final pH and conductivity of the solution. Analysis. PCE and byproduct were monitored using gas chromatography and high performance liquid chromatography. Aqueous and gaseous phases were routinely analyzed according to the analytical procedures explained in detail elsewhere.32 Chloride anions and other anionic species were monitored by ionic chromatography. Samples were filtered through a 0.45 µm nylon filter and were injected into a 792 basic ion-exchange chromatograph (IC), with a suppressor module, Metrohm, equipped with a conductivity detector and a Metrosep A Supp 4 column, stable for any pH. The eluent was a 1.7 mM sodium


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hydrogen carbonate/1.8 mM sodium carbonate solution, with a flow of 1 mL/min. Results and Discussion The previous work of the authors32 concluded that the dual treatment (undivided configuration) was the best option for carrying out the electrochemical degradation of PCE, according to the chloride level results obtained at the end of the process. The voltammetric study provided little information31 about the electrocatalysis of the process due to the slow kinetics of the electron transfer, with low current efficiencies. Therefore, it can be advised that, in order to avoid wasting charge, a working current density higher than 4 mA cm-2 (used in the previous work32) must not be used. In order to favor the electron transfer of the electroactive species (PCE) over the background reaction, enhanced mass transport conditions and higher initial concentrations (within the low range of solubility of this compound) and higher active electrode area should be provided, if possible. So, these operational variables (initial concentration, volumetric flow and electrode dimension and nature) have been systematically studied in the electrochemical filter-press reactor, providing the first step for a further scale-up of the process. Apart from the variable under study, the standard values for the operational variables were initial concentration 150 µM, Qv 150 or 50 L/h, cathode lead, anode lead dioxide, solution volume 1.5 L, working current density 4 mA cm-2, and temperature 20 C. In order to quantify the performance of the process, the following performance parameters were determined at the end of the process: (i) fractional conversion (FC), defined as FC )

[PCE]t)0 - [PCE]t)tf [PCE]t)0

× 100

(1)

(ii) degradation efficiency (DE), defined as DE )

[Cl-]t)tf - [Cl-]t)0 4[PCE]t)0

× 100

(2)

(iii) the Cl- mass balance error (Cl-MBE), defined as

(

P)chlorinated compound

∑

Cl-MBE ) 1 -

(mg of Cl in P)t)tf

(mg of Cl in PCE)t)0

)

× 100

(3)

(iv) current efficiency, CE, determines the percentage of the charge used in the desired reaction (the C-Cl bond cleavage). This was routinely calculated by following the chloride ion formed, (CECl-). Other authors determine the CE following the formed organic compounds (CEorg). In the present work, carried out at constant current, the current efficiency based on chloride ion is defined as follows: CECl- )

[Cl-]t)tfV2F It

× 100

(4)

where V is the electrolyte volume (L), F is the Faraday constant (C mol-1), I is the current (A), and t is the time (s); (v) selectivity or speciation, S, defined as Si )

mol of the desired product i

∑ mol of all compounds

(5)

Figure 2. Normalized PCE concentration decay for the different initial concentrations of PCE. [PCE]0 ) b 190, 2 210, and 9 230 µM. Cathode Pb, anode PbO2, Qv ) 50 L/h.

and other economical parameters such as (vi) energetic consumption, EC, (kW h m-3 treated) and (vii) degradation, D, (m3 treated m-2 day-1)) are determined in function of the operational variables. Influence of the Initial Concentration and Flow Conditions. The first task was to check the reproducibility of the experimental procedure and the influence of the initial concentration. Once the reproducibility had been checked (figures not shown), the systematic analysis was carried out. Figure 2 shows several normalized concentration vs time plots for different initial concentrations, showing that the concentration decay is independent of the initial concentration. It must be highlighted that the dispersion of the superposition in Figure 2 was lower than (18 µM. Studying a wide range of concentrations, (figure not shown) it could be seen that there is not a strong influence of the initial concentration on the kinetic degradation of PCE. This behavior is proof that the process presents first-order kinetics, which is compatible with mass transport control. Working with concentrations ranging at the micromolar level, Tsyganok and Otsuka,41 using Pd-C felt cathodes in divided flow cells, reported that an increase in the concentration toward a certain value implied a rise in the conversion rate during the early stages of electrolysis. However, some retardation effect on the dechlorination reaction was observed at higher concentrations by the same authors. The authors ascribed this effect to the saturation of the active sites of palladium particles with the starting material molecules at higher concentration of the substrate, which retards the generation of active hydride species on Pd. Li and Farrel23 used Pd and pure Fe porous electrodes for the degradation of TCE and carbon tetrachloride in a divided flow cell, and they pointed out an increase of the current efficiency as the concentration of TCE increased from 0.1 to 1.2 mM. They also pointed out mass transport effects only when low fluid flows were used. These authors also suggested that the primary pathway for TCE reduction by iron and palladized iron electrodes is indirect and involves atomic hydrogen as the reducing agent. In our case, after 5 h of electrolysis, the final concentration of the starting material decreased to about 18 µM for any initial concentration within the range studied [150-452 µM]. The concentration decay seems to follow the typical exponential decrease for a process under full mass transport control. In fact, the mass transport control is favored at the imposed experimental conditions: low concentrations and high overpotentials (in

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Figure 3. Normalized PCE concentration decay for the different volumetric flows 9 100, 2 150, b 200, and ( 250 L/h. Cathode Pb, anode PbO2, [PCE]0) 150 µM.

absolute values). Therefore, the next step was the analysis of the influence of the volumetric flow. Figure 3 shows the normalized PCE concentration decay for the different volumetric flows studied. In this case, the PCE concentration decay increases with an increase in the volumetric flow, according to the mass transport effects. It is important to highlight that, together with the forced convection associated with the fluid flow, extra agitation close to the electrodes surface is developed due to hydrogen (in the cathode) and oxygen (in the anode) evolution. Due to the low values of the PCE concentration and the galvanostatic conditions of the experiments, the averaged influence of this extra agitation does not vary over the experiments, and it can be considered that the mass transport is under steady state conditions. In order to check if the electroreduction process of PCE is under full mass transport control for these experimental conditions, the variation of the current efficiency with the electrolysis time for the different volumetric flows used in the process was analyzed; see Figure S1 in the Supporting Information. It has been suggested that a current efficiency exponential decay with time could be expected for a fully mass-transport-controlled process42 and our experimental results follow this tendency. From these results, a mass transport correlation of the type Sh ) aRebScc was derived using reactor modeling equations.42 The gas evolving during these experiments affects the values of the coefficient a ) 0.025 and b ) 1.095, (taking DPCE ) 9.08 × 10-10 m2 s-1 43) in comparison with the values a ) 0.17 and b ) 0.82 reported for the same reactor36 using the copper(II) electrodeposition as a test reaction. All of these results indicate that the process is, at least, nearly under full mass transport control. Figure 4A shows the normalized TCE concentration transient and 4B shows the normalized DCE concentration transient as main byproduct for the different volumetric flows studied at a lead cathode. At low flow rates, the appearance of DCE is a little delayed from the appearance of TCE. However, at large volumetric flow, this delay disappears. Both byproduct concentrations increase until a steady-sate concentration is reached. Even for longer electrolysis time, the concentration of both intermediates does not decrease. In our previous work of batch electrolyses at controlled potential,30 TCE and DCE were detected at low potentials with a TCE/DCE relationship close to 2, in the case of a carbon electrode, and with no clear relationship for the other cathodes. However, those results were obtained in a divided configuration in contrast to this present

Figure 4. Evolution of the normalized concentrations of (A) TCE and (B) DCE for the different volumetric flows 9 100, 2 150, b 200, and ( 250 L/h. Cathode Pb, anode PbO2, [PCE]0 ) 150 µM.

work, which has been carried out in undivided configuration. With galvanostatic conditions for batch electrolyses,32 similar tendencies to the present results were obtained, but no information related to the possible improvement of the degradation efficiency by the optimization of practical variables such as volumetric flow or electrode geometry could be analyzed. Unfortunately, there is little work in the literature, if any, on this subject using 2D electrodes in flow cells, so we are not able to make critical comparisons with other results. Influence of Electrode Materials. In the previous work,31 the influence of the nature of the cathodic material was analyzed from a voltammetric point of view and, even though lead showed better results, it is clear that lead metal is not acceptable from an environmental point of view. Therefore, different cathodic materials have again been checked to be used in this practical approach. Figure 5 shows the evolution of (A) PCE, (B) TCE, and (C) DCE as the main intermediates detected with the electrolysis time. The volumetric flow used for this comparison was 150 L/h. The normalized PCE concentration decay for the different cathodic materials studied (lead, copper, and carbon) shows a fast decay for copper and carbon in comparison with lead at short times, but the influence from the nature of the cathodic material disappears at longer times. The fast appearance of TCE and DCE for copper and carbon is in agreement with the PCE decay noted before. In addition, there is a delay for the appearance of TCE and DCE when lead is used as a cathode. However, this delay disappears at high flow rates (Figure 4B).


Figure 5. (A) PCE, (B) TCE, and (C) DCE normalized concentrations evolution for the three cathodic materials studied: 9 carbon; b copper; 2 lead. Anode PbO2. Qv ) 150 L/h. [PCE]0 ) 150 µM.

Due to the fact that an undivided configuration is used and that some previous results32 noted the influence of the anodic process in the degradation of PCE, different practical anodic materials have also been used in this study. Figure 6 shows the normalized PCE concentration decay and the evolution of the main byproduct such TCE and DCE with electrolysis time for (A) DSA-O2, (B) DSA-O2-Cl2, and (C) PbO2 anodes. This comparison has been performed at a volumetric flow of 50 L/h. It must be stressed that there is not a strong influence of the anodic material in the direct degradation of PCE (comparative

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Figure 6. Evolution of the normalized concentrations of 9 PCE, 2 TCE, and b DCE with the electrolysis time for (A) DSA-O2, (B) DSA-O2-Cl2, and (C) PbO2 anodes. Cathode Pb, [PCE]0 ) 150 µM, Qv ) 50 L/h.

figure not shown). It seems that the behavior is a little worse as the anodic material becomes more electrocatalytic for the chlorine evolution, but the differences are not clearly significant. However, some difference can be envisaged with the intermediates. Only when lead dioxide is used as anode, a delay in the appearance of TCE and DCE is observed. As a consequence of this situation, the influence of the working electrode structure has also been analyzed in this work, using a carbon felt electrode as a three-dimensional cathode and comparing with two-dimensional electrodes (carbon plate). Carbon felt has previously been characterized44 by the authors, and it has presented an excellent performance in high added-

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Figure 7. Evolution of the normalized PCE, TCE, and DCE concentrations for 2D (0, ∆, O) and 3D (9, 2, b) carbon electrodes. [PCE]0 ) 150 µM, Qv ) 150 L/h, anode PbO2.

value organic product electrosynthesis, where fractional conversion must be close to 100%.45 Figure 7 shows the normalized PCE concentration decay and the normalized TCE and DCE concentration evolutions for two- and three-dimensional carbon electrodes. It is clear that an increase in the total area (220 cm2 cm-3 for the 3D electrode in comparison with the projected area of the 2D electrodes), keeping the applied current constant, will permit work with lower current densities. In this case, it has produced a faster decrease in the PCE concentration and also a more rapid appearance of higher concentrations of TCE and DCE. However, at longer times, the level of TCE and DCE is quite similar using both cathodic geometries, and only a lower concentration of PCE is achieved using 3D electrodes. The influence of the flow rate using carbon felt as cathode was analyzed to complete our work with 3D electrodes. The normalized PCE concentration decay overlapped independently of the flow rate studied within the range [50-250 L/h]. This behavior may be attributed to a process which is not under mass transport control and mainly depends on the electron transfer step. Other results yield FC changing from 92 to 84% and DE ranging from 41 to 15% when the volumetric flow changes from 50 to 250 L/h. As we have pointed out in the introduction section, low flow rates have a poor influence on the fractional conversion of solutions with low conductivities, but higher flow rates require higher conductivities to maintain the performance28 influence. He et al.29 have reported a similar FC tendency to that reported in this work. Table 1 shows the main results for the performance parameters obtained during our experimental work; for their comparison with those found in the literature under similar experimental conditions, see Table S1, Supporting Information. First of all, the Cl-MBE is extremely low, providing proof that all the formed products are detected, and allowing confident conclusions related to the performance of the process to be obtained. With our system, the electrochemical reduction in an undivided configuration with two-dimensional electrodes has shown a high fractional conversion of the starting material, PCE, (FC > 82%) and degradation efficiency (DE > 50%) at a competitive EC (including the electrolysis costs) (∼ 3 kW h m-3) and D (∼ 1.1 m3 m-2 day-1). These values obtained with traditional electrodes are close to those obtained with sintered electrodes (DE around 80% with CE around 0.03%) using organic solvent in batch mode,46 or even with three-dimensional electrodes such as copper foam in a flow cell (FC around 80%,

at low flow rates).29 The implementation of carbon felt electrodes, again in undivided configuration of our filter-press reactor, has provided FC > 90%, DE 50%, and D ∼ 1.1 m3 m-2 day-1 but at noncompetitive EC (including the electrolysis costs) ∼ 65 kW h m-3. Apart from the energetic cost, our results are better than those reported previously for simple carbon fibers (FC 44%).28 Only when these carbon fibers are activated with silver or zinc,28 with Pd,41 or pure metal (activated or not with palladium)23 have better results been reported (FC 100% and DE 80-99%) in a divided flow cell. Controversial results using three-dimensional electrodes come from the fact that the behavior of a three-dimensional electrode is a complex subject for several reasons: (i) nonuniform potential and current density distribution,42 (ii) the influence of the apparent conductivity of the electrolyte on the potential distribution,35,47 and (iii) different behavior between reactors with current and fluid flows in parallel (flow-through) or perpendicular (flow-by) configurations.27 A more uniform potential distribution is achieved if favorable conditions, such as low values of KmAe, thin thickness of the electrode in the current flow direction, a high value of effective electrolytic conductivity, and low inlet concentration of the electroactive species, are applied. All these items were previously checked in our filter-press reactor so our high EC was more a consequence of the necessity to increase our working current when the carbon felt electrode was used as a threedimensional electrode. The working current used with the twodimensional electrodes (250 mA) did not provide any FC when three-dimensional electrodes were used and a higher current (3000 mA) was needed to obtain similar results. Assuming a specific area for carbon felt of 220 cm2 cm-3 and a uniform current distribution (which cannot be a reasonable assumption), the current density used is around 0.8 mA cm-2, i.e., lower than that used with the two-dimensional electrode. If we keep in mind the low current efficiency accounted with our carbon felt (Table 1), it seems that the reduction of PCE only takes place simultaneously, at least, with the proton discharge. The use of the carbon felt electrode provides a fast decrease of the PCE concentration and slightly lower concentrations of pollutant at the end of the process, but an unreasonably high EC. In addition, we have obtained low speciation (high selectivity) of byproduct with a very low mass balance error. These results are in agreement with those found in the literature (see Table S1, Supporting Information). Sonoyama et al.48 tried to improve the electrochemical flow reactor design for water purification uses at levels lower than parts per billion standards. This was a practical study analyzing the influence of the added electrolyte and of the length of the column electrode in the decomposition of parts per billion levels of chloroform and other trihalomethanes, as well as trichlorothylene and tetrachloroethylene in tap water. Despite the relevant level of NO2- produced during the treatment, they showed that almost all the toxic halocarbons in tap water can be continuously decomposed by the electrochemical method. This is a very attractive feature of the electrochemical technology in comparison with other technological approaches.49 Besides, all formed products are soluble, so the scale-up to industrial size can be developed using an electrochemical filter-press reactor, with all its advantageous features.34,50 However, from the analysis of our results it seems that, under these experimental conditions, the electroreduction of PCE is under full mass transport control using 2D electrodes, and, therefore, the influence of convection is important, especially at short reaction times. However, using 3D electrodes in our galvanostatic conditions, the process does not seem to take place under mass transport conditions due to the high active


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-2

Table 1. Experimental Results for the Controlled Current Density Electrolyses ([PCE]0 ) 150 µM, T ) 20 °C, jw ) 4 mA cm ) WE/CE

Qv/L h-1

χf/mS cm-1

pHf

CECl-/%

FC/%

DE/%

S

Cl-MBE/%

Pb/PbO2 Pb/PbO2 Pb/PbO2 Pb/PbO2 Pb/DSA-O2 Pb/DSA-O2-Cl2 Pb/PbO2 Cu/PbO2 C/PbO2 C felt/PbO2

100 150 200 250 50 50 50 150 150 150

9.2

6.1 7.5 7.0 6.3 9.0 4.5 6.1 6.2 6.0 6.2

2 2.5 1 1 1.0 0.5 2.1 0.9 1.5 0.1

84 87 86 84 84 84 87 82 84 85

51 65 42 68 53 18 36 51 50 52

0.749 0.872 0.707 0.836 0.767 0.546 0.860 0.754 0.733 0.792

4 2 5 13 3 33 37 5 10 2

8.9 8.7 8.6 8.8 9.1 8.4 9.1 9.0

surface area. It is clear that, even at high convection conditions and high active surface area (achieved with the threedimensional electrodes), the electrochemical reduction presents two main drawbacks under these experimental conditions. First, and most importantly, are the steady-state concentrations of the final products which, though low, do not decrease to undetectable values for longer times, and second, the presence of a background electrolyte is necessary to ensure enough electrical conductivity and avoid EC penalty. This last aspect might not be a serious drawback when natural water, with its own salinity, should be treated, but the real problem is related to the secondary effects apart from the EC penalty. Sonoyama and Sakata28 pointed out the necessity of improving the structure of the reactor (tubular cell) to impose a homogeneous potential distribution. He et al.29 have studied and simulated the behavior of three-dimensional electrodes not only in high-conductivity (2.2 S/m) but also in low-conductivity (0.05 S/m) conditions and at different flow rates in a parallel fluid-current flow configuration. Another aspect to be analyzed with the technical viability is the economical aspect. In the literature, there are not many economical viability studies about electrochemical treatments at this scale. Sonoyama et al.28 have considered the electrochemical decomposition of chlorinated compounds as treatment of tap water, keeping in mind that the compounds produced in the process should be harmless to the environment and the cost of the treatment of water should not be too high. They have provided an electricity cost for the electrochemical decomposition of CHCl3 (not including the electricity for driving the electrolysis) which is close to 0.2 kWh m-3. In addition, the authors highlighted two important targets in order to develop practical systems: (i) water hardly containing a supporting electrolyte and (ii) low flow rates. They pointed out that the efficiency for decomposition of CHCl3 depended largely on the flow rate in a solution of dilute supporting electrolyte. At a flow rate of 0.06 L/h, the efficiency for the decomposition of chloroform was almost independent of the concentration of K2SO4. In our case, the economical viability analysis has been carried out for those reactions which have shown an optimal performance and the results are shown in Table 1. Conclusions Electrochemical degradation presents a low speciation for the treatment of PCE, which lends simplicity to the process. However, even working with high volumetric flows or high electrode areas, the concentration of chlorinated compounds is not decreased at longer times. This tendency is shown for all the studied electrodes, indicating that there is no specific influence of the electrode material. Therefore, more environmentally friendly materials, such as carbon or copper (instead of lead) can be used as the cathode for this application. Even though there are not conclusive results, the better performance of the three-dimensional electrode at shorter times,

final products/other details TCE, TCE, TCE, TCE, TCE, TCE, TCE, TCE, TCE, TCE,

DCE/2.9 kW h m-3 DCE/2.7 kW h m-3 DCE/2.8 kW h m-3 DCE/3.8 kW h m-3 DCE/4.2 kW h m-3 DCE/4.3 kW h m-3 DCE/2.9 kW h m-3 DCE/3.8 kW h m-3 DCE/3.0 kW h m-3 DCE/66.7 kW h m-3 (jw ) 0.8 mA cm-2)

when the process seems to be under kinetic control, could support the relevance of the indirect electron transfer by the adsorbed hydrogen. From our results, it can not be concluded that the use of carbon felt is a good option for applied approaches (despite the shorter time needed) due to the higher EC. Two-dimensional electrodes, with an averaged cost close to 3 kW h m-3 using the undivided configuration (which prevents any complication with the separation component (membranes)), provide the best option. Further work with carbon felt (metallically activated or not) is planned. Acknowledgment J.G.-G. and M.D.E. thank Caja de Ahorros del Mediterrańeo. J.G.-G. thanks the Ministry of Industry, Tourism and Trade for its financial support under contract DEX-560620-2008-135. J.G.G., M.D.E., and I.T. thank Generalidad Valenciana for its financial support under projects AORG09/051 and ACOMP09/ 128. J.G.-G. and I.T. also thank Generalidad Valenciana for its financial support under grant FPA/2009/024. The authors thank Dr. Mary Thompson for her help with editing the English of the manuscript. List of symbols and Abbreviations 2D ) two-dimensional 3D ) three-dimensional a, b, c ) mass transport coefficient correlation CE ) counter electrode acting as anode, also current efficiency CECl- ) current efficiency based on chloride ion analysis Cl-MBE ) mass balance error following chlorine atoms, % D ) degradation, m3 treated m-2 day-1 DPCE ) PCE diffusion coefficient DCE ) dichloroethylene DE ) degradation efficiency, % DSA ) dimensionally stable anode EC ) energetic consumption, kW h m-3 F ) Faraday constant, C mol-1 FC ) fractional conversion, % IC ) ion-exchange chromatography jw ) working current density in galvanostatic electrolyses km ) mass transport coefficient, m s-1 Qv ) volumetric flow, L h-1 PCE ) perchloroethylene Re ) Reynolds number Sc ) Schmidt number Sh ) Sherwood number S ) selectivity or speciation TCE ) trichloroethylene WE ) working electrode acting as cathode χ ) conductivity, mS cm-1

Supporting Information Available: Comparison of our experimental results ([PCE]0 ) 150 µM, T ) 20 °C, jw ) 4

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ReceiVed for reView January 20, 2010 ReVised manuscript receiVed March 18, 2010 Accepted March 25, 2010 IE100134T

Electrochemical Degradation of Perchloroethylene in Aqueous Media

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