Electrochemical Regeneration of Activated Carbon Cloth Exhausted

May 7, 2008 - The electrochemical regeneration of an activated carbon cloth exhausted with a common herbicide (bentazone) was investigated under ...
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Environ. Sci. Technol. 2008, 42, 4500–4506

Electrochemical Regeneration of Activated Carbon Cloth Exhausted with Bentazone CONCHI O. ANIA AND ´ GUIN* FRANC ¸ OIS BE ´ CRMD, CNRS-University, 1B rue de la Ferollerie, ´ 45071 Orleans, France

Received December 20, 2007. Revised manuscript received February 24, 2008. Accepted March 17, 2008.

The electrochemical regeneration of an activated carbon cloth exhausted with a common herbicide (bentazone) was investigated under different operating conditions. The reversibility of the desorption process was confirmed by monitoring the UV spectra of the solution while cathodic polarization is being applied. Neither nanotextural nor chemical changes are produced in the carbon cloth upon polarization in the absence of the adsorbate. Upon cathodic polarization of a carbon cloth working electrode preloaded with bentazone, negative charges appear on the surface. A partial bentazone desorption results from repulsive electrostatic interactions between the negative charges on the carbon cloth and bentazone. When the electrode potential is below the thermodynamic value for cathodic decomposition of water, hydroxyl ions are liberated. Such ions provoke local pH changes that are responsible of the dissociation of bentazone and carbon surface groups to their anionic form. As a consequence of the pH increase, an almost reversible desorption of bentazone is observed. The effects of several operating parameters on the regeneration efficiency were evaluated. Higher regeneration efficiencies were attained under potentiostatic as compared to galvanostatic conditions, as OH- production strongly depends on the applied potential.

Introduction Dependence on chemical pesticides and fertilizers is one of the most adverse aspects of intensive agriculture and industrial activities, with negative environmental, health, and economic aspects. World annual consumption of pesticides has followed an upward trend for the past decades; the amount used exceeded 2 million tons in 2001, herbicides accounting for the largest fraction of the total use (1). Furthermore, the use in home gardening of herbicides, insecticides, and fungicides has become very popular. These compounds provide easy-to-use, cost-effective solutions to protect ornamental plants and lawns or to control weeds that damage paths and drives. Such domestic use engenders a further problem: most users are not aware of either the environmental or health risks of the use/abuse of pesticides. Although the environmental concerns on pesticide use in developing countries were raised 30 years ago, hazardous pesticides are still used with little or no protection. Pesticides are ubiquitous and mobile, and since water resources are * Corresponding author phone: +33 238255375; fax: +33 238255376; e-mail: [email protected]. 4500

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interconnected, pollution at one point can extend widely. Even if a pesticide appears relatively environmentally benign, its breakdown products can exhibit much higher toxicity than the parent compound (2, 3). Hence preventing and conserving water resources (ground and public drinking water) from pesticide contamination has become an important environmental issue (4). Significant progress has been achieved in the past decades in treating wastewater, resulting in a measurable improvement in water quality. Several treatment technologies have been applied, the biological treatment being the most widely used in domestic water streams. However, most of the pollutants found in wastewater coming from agricultural and industrial activities are either inhibitors of the biological processes or nonbiodegradable compounds. Consequently, these water streams must be treated before being released to the common water, and very often these compounds have to be eliminated by high cost and irreversible methods, which make the whole process not affordable from an economical point of view. Adsorption has become a well-established technique to remove pollutants, activated carbon being the prevailing adsorbent for the purification of water with low pollutant concentration. Generally, the loaded carbon is regenerated ex situ by heating or steaming, which is a high-energyconsuming process and a costly procedure, and although the efficiency is relatively high, there is a considerable loss of activated carbon (5). An alternative, not so well studied, is the use of electrosorptive techniques for the regeneration of exhausted activated carbons. Until now, extensive research has been carried out on the electrosorption of toxic inorganic ions (6–8) and a few polarizable organic molecules (9–13). Although the results have shown good efficiencies of removal of ions, scarce works report the use of electrochemical techniques for the removal of pesticides or more complicated polycyclic aromatic compounds (14–18). Moreover, these techniques have not been so far explored for the regeneration of the exhausted adsorbents. The main objective of this research was to explore the application of electro-desorptive techniques for the regeneration of an activated carbon cloth loaded with a common herbicide. The targeted pollutant being a valuable chemical, its recovery and/or degradation upon polarization of carbon cloth will also be investigated. The efficiency and reversibility of the electrosorption process upon several cycles were carefully investigated. Special attention was paid to the likely role of polarization conditions (i.e., galvanostatic and potentiostatic) on the regeneration efficiency. The nanotextural changes induced in the adsorbent after several cycles, which affect the complex process of adsorption and electrosorption from diluted solutions, were also investigated. As probe, we have used a postemergence herbicide (bentazone), which has become very popular for the control of broad-leaved weeds and crops since 2003 after the ban of atrazine in the EU. In association, the herbicides alachlor and bentazone can be substitutes for atrazine to provide the same action spectrum on weeds in cereals grains crops (mainly maize and rice), and its impact on environment has already been established (19).

Experimental Section Materials. A commercial activated carbon cloth (AX), obtained from physical activation of rayon and supplied by ACTITEX, France, was chosen for this study. Before usage, the sample was washed in distilled water at 60 °C, dried at 10.1021/es703192x CCC: $40.75

 2008 American Chemical Society

Published on Web 05/07/2008

FIGURE 1. Scheme of the electrochemical cell used for the electrodesorption experiments. 110 °C overnight, and kept in a desiccator. Bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2, 2-dioxide] with the highest purity specification was obtained from Aldrich. Electrochemical Cell. A conventional three-electrode system was used in the electrodesorption studies (Figure 1). The working electrode was composed of the carbon cloth (cut in the desired dimensions and accurately weighted, ca. 11 mg) attached to a gold plate acting as current collector. A platinum grid served as an auxiliary and Hg/Hg2SO4 as reference electrode. All experiments were performed with an electrochemical potentiostat-galvanostat (VMP-2 from Biologic, France), with a computer interfaced to the electrochemical workstation to control the experimental parameters. A 20 mL volume of bentazone aqueous solution (initial concentration 20 ppm) was used. Due to the low electrical conductivity of the pesticide solution, experiments were carried out in an inert supporting electrolyte solution (0.01 mol L-1 Na2SO4). Regeneration of the Carbon Cloth. Details on the experimental procedure have been reported elsewhere (16, 17). Briefly, the carbon cloth was previously loaded at open-circuit conditions (OC). Once loaded, the sample is polarized at different regimes (both galvanostatic and potentiostatic) for 3 h, to provoke desorption of the pollutant. Small samples of the solution (∼1.5 mL) were taken out at predetermined time intervals to measure the evolution of pH and pollutant concentration during sorption/desorption, using a UV spectrometer (Uvikon Xs, Bio-TEk Instruments). The extracted samples were reintroduced in the cell in order to avoid changes in the total volume of solution. After each desorption cycle, the cloth sample was allowed to rest at OC (i.e., no polarization) in the same cell until complete bentazone uptake is again attained. The regeneration efficiency (RE) was evaluated as the ratio between the amount of pesticide adsorbed after the regeneration treatment and the amount adsorbed in the fresh carbon cloth. Nanotextural and Chemical Characterization of the Carbon Samples. The nanotexture of the carbon cloths was characterized by N2 adsorption at -196 °C in an automatic apparatus (Autosorb-1, Quantachrome). Before the experiments, the samples were outgassed under vacuum at 120 °C overnight. The isotherms were used to calculate the specific surface area, SBET, total pore volume, VT, and pore size distribution. The pore size distribution (PSD) was evaluated using the density functional theory (DFT). Additionally, the distribution of pores smaller than 0.7 nm (ultramicropores) was assessed from CO2 adsorption isotherms at 0 °C with DR

TABLE 1. Nanotextural and Chemical Characteristics of the Pristine Carbon Cloth (AX), after Bentazone Exposure (AX exh), after Electrochemical Polarization in the Absence of Bentazone (AX pol), and after a Few Cycles of Cathodic Regeneration (AX reg) at Constant Potential

sample AX AX AX AX AX

SBET [m2 g-1]

Vtotala [cm3 g-1]

1018 0.432 pol 1076 0.457 exh 484 0.237 reg (1) 1003 0.495 reg (6) 823 0.365

Vmicropores [cm3 g-1]

Vmesopores [cm3 g-1]

VCO2c [cm3 g-1]

0.318 0.321 0.148 0.291 0.245

0.033 0.034 0.047 0.031 0.029

0.353 0.340 0.170 0.320 0.286

b

b

pHPZC 5.7 5.5 -

a Evaluated at a relative pressure of 0.95 in the N2 adsorption isotherms at -196 °C. b Evaluated from the DFT method applied to the N2 adsorption isotherms at -196 °C. c Evaluated from the DR method applied to the CO2 adsorption isotherms at 0 °C.

formulism. The samples were further characterized by the determination of the point of zero charge (pHPZC) by a modification of the mass-titration procedure as described elsewhere (20). Thermal analysis was performed before and after bentazone adsorption (“exh” series), using a Setaram thermogravimetric analyzer. The instrument settings were as follows: heating rate of 10 °C min-1, and argon atmosphere with 100 mL min-1 flow rate.

Results and Discussion Characterization of the Carbon Cloths. The adsorption capacity of an adsorbent toward the removal of pollutants is straightforwardly related to its physicochemical properties. Therefore, it becomes basic to determine the porous texture and the chemical structure of the carbon cloth. Detailed characteristics of nanotexture of the as-received cloth are presented in Table 1. The carbon cloth presents a large surface area and porosity, as evaluated by gas adsorption data. A further analysis of the PSD combining the information of N2 and CO2 adsorption isotherms indicated that the pore volume determined by CO2 data is larger than the corresponding volume of micropores “seen” by N2, indicating that the carbon cloth is mainly composed of ultramicropores. Moreover, the volume of mesopores accounts for less than 10% of the overall porosity. In order to explore the eventual changes in nanotexture and surface chemistry of the carbon cloth after electroVOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Series of successive cyclic voltammograms of the carbon cloth at 2 mV s-1 sweep rate taken to progressively more negative cutoff potentials. The dotted line represents the first voltammogram, whereas the red curve corresponds to the one recorded after the last cycle with cathodic polarization down to -1200 mV and anodic polarization up to +200 mV vs Hg/Hg2SO4. chemical treatment, a blank experiment (AX pol) was carried out by exposing the cloth to polarization (between -1200 and +200 mV vs Hg/Hg2SO4) in the absence of the pollutant. It has to be mentioned that cathodic polarization was chosen based on the results reported in earlier studies, where it was observed that anodic polarization of the carbon cloth promotes the electroadsorption of bentazone (16). Figure 2 shows cyclic voltammograms of the cloth electrode using a three-electrode cell in 0.01 M Na2SO4 as supporting electrolyte. Initially the carbon cloth was polarized between -600 and +200 mV, and it was progressively submitted down to more negative cutoff potentials (down to -1200 mV). The rectangular shape of the voltammograms down to a potential cutoff of -600 and -700 mV proves a pure capacitive and reversible behavior of the carbon cloth in this potential range. When the potential cutoff reached -800 mV, a small current leap due to hydrogen insertion is observed together with a slight increase in anodic current attributed to the reversible electrochemical oxidation of adsorbed hydrogen (21). This effect is more evident when the potential cutoff is decreased to -1200 mV. At successive series of CV taken to progressively more negative potentials, a fast current increase occurs, due to an enhancement in the kinetics of hydrogen production, and the anodic response also increases. Afterward, no changes are observed in the electrochemical behavior of the carbon cloth, as shown by the CV down to -600 mV which superimposes with the initial voltammogram of the as-received sample. Hence the process of hydrogen insertion can be considered as fully reversible. As expected, neither the nanotexture (Table 1) nor the surface chemistry (inferred from the pHPZC and thermal analysis in Figure 3) of the carbon cloth were altered during the cathodic treatment. This is reasonable, taking into account that (i) anodic polarization was restricted up to +200 mV vs Hg/Hg2SO4, since higher values cause oxidation of the carbon cloth (22), and (ii) upon cathodic polarization, H2 and OH- are liberated at the working electrode according to the following: Working electrode: 2H2O + 2e- f H2 + 2OH+

Counter electrode: 2H2O f O2 + 4H + 4e

-

(1) (2)

In summary, it can be inferred that cathodic polarization of the carbon cloth does not alter either its physicochemical 4502

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properties or its electrochemical performance. Thus, changes induced in bentazone desorption during cathodic polarization (see discussion below) should be attributed to the presence of charges on the carbon surface (ca. electrostatic interactions), rather than to nanotextural changes. This absence of changes in the carbon nanostructure after cathodic polarization is important from the point of view of the regeneration efficiency in multiple steps. The nanotextural characteristics of the cloth after bentazone adsorption (AX exh) and after subsequent cycles of regeneration were also investigated (Table 1). As expected, the surface area and pore volume of the loaded sample decrease significantly. The fall was particularly important in the volume of micropores evaluated by both nitrogen and CO2 adsorption (∼52%), suggesting that adsorption takes place preferentially in the narrow microporosity of the carbon cloth (16). The changes after electrochemical regeneration will be further addressed. Cathodic Regeneration of the Carbon Cloth. As mentioned previously, cathodic polarization was chosen for the regeneration of the carbon cloth based on the results reported in earlier studies. Anodic polarization of the cloth was found to promote bentazone electroadsorption (16). On the other hand, the adsorption mechanism revealed a key role of the solution pH on bentazone uptake. Both findings lead us to explore the reversibility of bentazone removal by reversal of polarization (i.e., cathodic). Initially, the electrochemical stability window of the herbicide was determined. It is indeed important to make a distinction between the effects of polarization on bentazone in absence and in presence of the carbon cloth. To verify that bentazone was not oxidized or reduced as a consequence of the polarization itself, the electrochemical stability window was determined over the gold disk current collector as electrode. Thus, cyclic voltammetry of the aqueous 0.01 mol L-1 Na2SO4 solution, with and without bentazone, was carried out at a scan rate of 20 mV s-1. Initially cyclic voltammograms were recorded in the potential range between -800 and +200 mV vs Hg/Hg2SO4. The absence of reduction or oxidation peaks in the cyclic voltammogram of bentazone confirmed the electrochemical stability of the herbicide in the potential range applied. Further polarization of the solution was carried out at potential values more negative than -850 mV vs Hg/ Hg2SO4, since it was found that desorption occurred at more negative cutoff potentials, as it will be further addressed below. In this case, the rapid increase in current due to cathodic decomposition of water disabled to observe any contribution from a breakdown of the pesticide, so that UV absorption spectra of the solution were recorded in order to detect possible changes in the adsorbate. No changes were found in the UV spectra, confirming that bentazone is electrochemically stable under the cathodic polarization regime used. Bentazone electrochemical stability in the presence of the carbon cloth was implicitly evaluated during the regeneration studies. As it will be later discussed, monitoring the absorption spectra of the solution proved that (i) no structural changes occurred on bentazone due to polarization in the presence of the carbon cloth and (ii) no other species are released to the solution. Moreover, the possibility of bentazone decomposition and adsorption of the degradation compounds on the carbon cloth does not seem to be happening, as indicated by the TGA and porosity analyses. In a previous study, the adsorption of bentazone was investigated at open circuit on activated carbon cloths, and the effects of carbon surface heterogeneity and solution pH on the adsorption process were thereby addressed (16, 17). Briefly, bentazone adsorption depends on two factors: the solution pH (ca. ionization of the adsorbate) and the degree of functionalization of the carbon surface, the uptake being

FIGURE 3. TG and DTG profiles in argon of the pristine carbon cloth (AX), after bentazone adsorption (AX exh), and after cathodic potentiostatic regeneration (AX reg) at -1500 mV at different times (90 and 180 min). For comparison purposes the sample submitted to polarization before adsorption (AX pol) is also included.

FIGURE 4. Changes in the UV spectra of the solution (A) while cathodic galvanostatic polarization is being applied (current I ) -5 mA) to the preloaded carbon cloth. (B) Evidence of the hypsochromic shift between the spectra of the parent bentazone solution (pH 5.5) and of the aliquots extracted after cathodic polarization for 120 and 180 min. improved in acidic media and in carbons with low oxygen content. The most likely forces between the carbon cloth and bentazone seemed to be (i) dispersive interactions between the aromatic ring of bentazone and the π electrons of the graphene layers and (ii) electrostatic attraction and repulsion when ions are present. Moreover, anodic polarization of the carbon cloth led to an enhancement of bentazone adsorption rate. This was explained in terms of structure of the pesticide and charge on the carbon surface while polarized, and a mechanism was proposed. The present work aims at investigating the effect of cathodic polarization for the regeneration of the spent adsorbent. For this purpose, the carbon cloth loaded with the herbicide was introduced into the electrochemical cell and was subjected to cathodic polarization for 3 h in 0.01 mol L-1 Na2SO4. Both galvanostatic and potentiostatic conditions were investigated. Initially, galvanostatic cathodic polarization was conducted, with current intensities ranging from -1 up to -5 mA. It was observed that the working electrode potential reached values of -0.9 and -1.7 V vs Hg/Hg2SO4, for applied

currents of -1 mA and -5 mA, respectively. During galvanostatic treatment, gas evolution was observed from the counter and working electrodes, due to the decomposition of water; this is expected given the electrode potential values recorded. The UV absorption spectra of the pesticide solution continuously recorded upon cathodic polarization (Figure 4) confirmed the electrochemical stability of bentazone. The only remarkable change was the increase in the characteristic peak of bentazone with the polarization time. Figure 5 illustrates the kinetics curves obtained when the previously loaded carbon cloth is polarized at cathodicspotentiostatic and galvanostaticsconditions, subsequently loaded under open circuit (OC) conditions, and so on. The results are expressed in terms of relative concentration (C/ C0), taking as initial concentration (C0) that of the solution used in preloading the carbon cloth. During electrolysis, the amount of bentazone in solution increased with time, indicating that it is being reversibly desorbed from the activated carbon (Figures 4 and 5). The amount desorbed is lower than the initial concentration of bentazone (C/C0 < 1); VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Comparison of kinetics curves for bentazone desorption upon cathodic polarization (open symbols) at different operating conditions on the preloaded carbon cloth electrode, followed by adsorption under open circuit (OC) conditions (solid symbols).

FIGURE 6. Desorption rate of bentazone at decreasing potentials of the carbon cloth electrode preloaded under open circuit conditions (OC). The potentials are expressed vs Hg/ Hg2SO4. this might be a consequence of the limited polarization time which was only maintained for 3 h. The results show that bentazone molecules are weakly adsorbed (no strong or irreversible interactions occur between the adsorbate and the carbon) on the carbon cloth and that the adsorbate does not undergo decomposition and/ or polymerization. This observation is in good agreement with the mechanism of bentazone adsorption proposed in earlier works (16, 17). After electrodesorption, bentazone was again adsorbed at open circuit conditions on the carbon cloth. In a systematic analysis of the operating conditions we have also applied decreasing electrode potential values to explore the extent of desorption in a potentiostatic mode (Figure 6). The yield of the regeneration appears to be strongly related to the electrode potential, as at values higher than -900 mV vs Hg/Hg2SO4 very little desorption occurred. According to the Nernst equation, the equilibrium potential for reaction 1 is E ) -0.38 V vs NHE (-1.07 V vs Hg/Hg2SO4). As shown by Figure 6, electrodesorption mostly occurs at potentials lower than the thermodynamic value for cathodic decomposition of water (reaction 1), indicating that hydroxyl anions are liberated in the pores of the working electrode. 4504

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As a result, local pH in the confined pore space of the carbon cloth is significantly increased upon cathodic polarization, suggesting that local pH changes are the key to promote desorption. Simultaneous monitoring of the cell pH during cathodic polarization showed a slight decrease of up to 1 unit (from 5.5 to 4.5). This was corroborated by a hypsochromic shift in the UV spectra (Figure 4). This behavior is attributed to the liberation of protons at the counter electrodesaccording to reaction 2swhich are not neutralized by the hydroxyl ions formed at the carbon cloth. In this case the pH balance does not apply because the cathodic process includes charging of the electrical double layer of the porous carbon cloth, and simultaneous trapping of OH- in the micropores. It should be kept in mind that pH changes affect both the dissociation of bentazone (pKa ∼ 3.3) and the surface groups on the carbon cloth. Therefore, the generation of hydroxyl ions in the confined pore space of the working electrode according to reaction 1 during cathodic polarization promotes the conversion of surface groups and of adsorbed bentazone molecules to their anionic form at local pH higher than pHPZC and pKa, respectively. As a consequence, repulsive electrostatic interactions between the negative charges created at the edge of the graphene layers and the bentazone anions favor desorption. The substitution of bentazone anions by water molecules on the adsorption sites might facilitate the desorption step. This mechanism is supported by our previous work on bentazone removal under open-circuit conditions (16). On the other hand, it should be kept in mind that another type of interaction is possible. Because of the π-system interaction, it is expected that solute molecules are adsorbed with their aromatic ring parallel to the graphene layers to maximize the dispersive interactions (that arise from the random charge fluctuations) (23, 24). Such orientation has been reported to be favored in most organic aromatics at small coverage conditions. From this orientation, charge transfer might take place easily between the polarized carbon surface and the π-electrons. Since “net” charges appear on the carbon cloth upon cathodic polarization, charge-density is changed. Such changes control the dipole-charge interactions and are able to (partly) govern both the rates and extents of desorption (25). It is likely that the negatively charged carbon cloth might

FIGURE 7. Regeneration efficiency (RE) and stripping efficiency (SE) of the carbon cloth preloaded with bentazone over successive cycles. give rise to repulsive forces at the interface with adsorbed bentazone molecules, thus promoting desorption. This mechanism would explain the slight desorption observed at -900 mV vs Hg/Hg2SO4 (Figure 6) which, although occurring to a small extent, cannot be neglected. In this case, the electrode potential is slightly higher (lower in absolute value) than the thermodynamic value for water decomposition, suggesting that some extent of desorption is also promoted by the polarization of the carbon cloth and not only by the dissociation of surface groups. Figure 3 compares TG and DTG profiles of the loaded carbon cloth (AX exh) and of the cloth after cathodic regeneration (AX reg). The first peak at temperatures lower than 100 °C is assigned to the removal of physisorbed water, whereas the second peak centered at around 280 °C, which was not detected before the uptake, is assigned to the bentazone physisorbed in the porous network of the carbon cloth. It can be concluded that the cathodic treatment produces an almost quantitative desorption of bentazone from the carbon cloth after 3 h. Comparatively, by decreasing the polarization time (i.e., 90 min) a small desorption peak appeared in the temperature range 200-400 °C. This has been attributed to bentazone remaining inside the carbon matrix, pointing out that the extent of desorption depends on the time of cathodic polarization. The carbon cloth was subjected to adsorption of bentazone and electrochemical regeneration cycles at different regimes to follow the changes occurring in the adsorption capacity of the regenerated samples. The electrochemical regeneration of the activated carbon cloth was evaluated by means of the regeneration efficiency (RE) and the stripping efficiency (SE) for the samples subjected to multiple regeneration steps. The RE factor was calculated as RE ) Qi/Qo × 100, where Qi is the adsorptive capacity of the regenerated carbon at a given cycle i, and Qo the adsorptive capacity of bentazone in the fresh carbon (both expressed in grams). The SE was defined as the ratio between the adsorption capacities of two subsequent cycles (SE ) Qi/Qi-1). The SE parameter allows to obtain the efficiency of pollutant desorption in the different cycles, enabling to study the evolution of the overall regeneration yield. It can be observed in Figure 7 that the regeneration efficiency is about 90-80% for the first cycle, although it gradually decreases during the subsequent cycles. Regarding the operating conditions, higher RE was achieved at -1700 mV as compared to -1500 mV, supporting that desorption

is related to the electrode potential. Conversely, the stripping efficiency SE is very high, it even slightly increased after the first cycle, and it remains above 70% during the following cycles. This indicates that the amount desorbed in every single cycle is somewhat constant during the first cycles. Under potentiostatic conditions the SE fall with the number of cycles is more regular (particularly for -1500 mV), exceeding the values attained at galvanostatic conditions. After six cycles, the fall in the yield of regeneration becomes more abrupt when severe conditions are used, such as -1700 mV or -5 mA, pointing out either an incomplete desorption of the pollutant or a likely damage in the carbon cloth. This result was corroborated by analyzing the nanotextural properties of the carbon cloth after several regeneration cycles. As it can be seen in Table 1, there is a slight decrease in the micropore volume of the regenerated carbon in the first cycle, this effect being more evident after six successive cycles. This behavior explains the regeneration efficiency being lower than 100% in the first cycle. The decrease in the volume of CO2 adsorbed indicates that the blockage of the porosity is more important in the ultramicropores, confirming that the removal of bentazone is not complete. This may be attributed to either an inadequate duration of polarization and/or to a partial damage of the carbon cloth with charging/ discharging also possibly occurring.

Acknowledgments The authors thank Actitex, France, for kindly supplying the activated carbon cloth. C.O.A. thanks T.V.S. for her help with the thermal analysis and the Spanish MEC for a Ramo´n y Cajal research contract. Dr. Khomenko is also acknowledged for fruitful discussions.

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(15) Niu, J.; Conway, B. E. Adsorptive and electrosorptive removal of aniline and bipyridyls from waste-waters. J. Electroanal. Chem. 2002, 536, 83–92. (16) Ania, C. O.; Be´guin, F. Mechanism of adsorption and electrosorption of bentazone on activated carbon cloth in aqueous solutions. Wat. Res. 2007, 41, 3372–3380. (17) Ania, C. O.; Be´guin, F. Electrochemically assisted adsorption/ desorption of bentazone on activated carbon cloth. Adsorption 2007, 13, 579–586. (18) Ban, A.; Chafer, A.; Wendt, H. Fundamentals of electrosorption on activated carbon for wastewater treatment of industrial effluents. J. Appl. Electrochem. 1998, 28, 227–236. (19) Dousset, M.; Babut, F.; Andreux, Schiavon, M. Alachlor and bentazone losses from subsurface drainage of two soils. J. Environ. Qual. 2004, 33, 294–301. (20) Ania, C. O.; Cabal, B.; Parra, J. B.; Pis, J. J. Importance of hydrophobic character of activated carbons on the removal of naphthalene from aqueous phase. Adsorpt.: Sci. Technol. 2007, 25, 155–168. (21) Jurewicz, J.; Frackowiak, E.; Be´guin, F. Towards the mechanism of electrochemical hydrogen storage in nanostructured carbon materials. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 981–987. (22) Kinoshita, K. In Carbon: electrochemical and physicochemical properties; John Wiley & Sons: New York, 1988; p 293. (23) Niu, J.; Conway, B. E. States of orientation of pyridine and 1,4pyrazine as a function of electrode potential and surface charge at a high-area, porous C-electrode. J. Electroanal. Chem. 2004, 564, 53–63. (24) Coughlin, R. W.; Ezra, F. S. Role of Surface Acidity in the Adsorption of Organic Pollutants on the Surface of Carbon. Environm. Sci. Technol. 1968, 2, 291–297. (25) Damaskin, B. B.; Petri, O. A.; Batrakov, V. V. Adsorption of Organic Compounds at Electrodes. Plenum, New York 1971.

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