Highly Efficient and Mild Electrochemical Incineration: Mechanism and

Sep 15, 2010 - ... of methyl groups, which promotes the electrophilic attack of ·OH. .... Xiaofeng Xu , Peng Liao , Songhu Yuan , Man Tong , Minghen ...
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Environ. Sci. Technol. 2010, 44, 7921–7927

Highly Efficient and Mild Electrochemical Incineration: Mechanism and Kinetic Process of Refractory Aromatic Hydrocarbon Pollutants on Superhydrophobic PbO2 Anode YANZHU LEI, GUOHUA ZHAO,* YONGGANG ZHANG, MEICHUAN LIU, LEI LIU, BAOYING LV, AND JUNXIA GAO Department of Chemistry, Tongji University, 1239 Siping Road, 200092 Shanghai, China

Received May 19, 2010. Revised manuscript received September 3, 2010. Accepted September 7, 2010.

Aqueous aromatic hydrocarbons are chemically stable, high toxic refractory pollutants that can only be oxidized to phenols and quinone on either Pt or traditional PbO2 electrodes. In this study, a novel method for the electrochemical incineration of benzene homologues on superhydrophobic PbO2 electrode (hydrophobic-PbO2) was proposed under mild conditions. Hydrophobic-PbO2 can achieve the complete mineralization of aromatic hydrocarbons and exhibit high removal effect, rapid oxidation rate, and low energy consumption. The kinetics of the electrochemical incineration was also investigated, and the results revealed that the cleavage of the benzene ring is a key factor affecting the incineration efficiency. Moreover, on hydrophobic-PbO2, the decay of intermediates was rapid, and low concentrations of aromatics were accumulated during the reaction. The removal of the initial pollutants and the effects of oxidative cleavage were related to the number of methyl groups on the benzene ring. Specifically, the results of physical experiments and quantum calculations revealed that the charge density of carbon atoms increases with an increase in the number of methyl groups, which promotes the electrophilic attack of · OH.

Introduction Benzene, toluene, and xylene (BTX) are hazardous aromatic hydrocarbons produced and used in industry. High levels of exposure to BTX can affect the central nervous system, causing drowsiness, dizziness, headaches, and nausea. Longterm, low-level exposure may also cause chronic effects, including central nervous system damage, leukemia, and aplastic anemia. In addition, BTX have proved highly carcinogenic (1-4). Therefore, BTX are categorized as environmental priority pollutants by the EPA. Effective treatments of BTX are highly requested (5). Various methods for the removal of BTX have been proposed, including catalytic combustion (6), extraction (5), physicochemical adsorption (7), and biological strategies (8). However, many of the aforementioned techniques are complicated, time-consuming and are likely to produce * Corresponding author e-mail: [email protected]. 10.1021/es101693h

 2010 American Chemical Society

Published on Web 09/15/2010

secondary pollution. Advanced oxidation processes (AOPs) are considered alternative approaches. Among AOPs, electrochemical anodic oxidation is simple, effective and environmentally compatible (9-12). BTX are chemically stable (13) and possess relatively high oxidation potentials, for example 2.81 V vs SCE for benzene. Thus, the complete mineralization of BTX is difficult to achieve with most conventional anodes, such as Pt and SnO2/Ti electrodes. In addition, the direct electrochemical oxidation of benzene produces benzoquinone, which is easily reduced to hydroquinone, causing passivation of the active electrode surface (14). Hence, benzene can only be oxidized to phenolic compounds on the Pt electrode (1). This electrode is therefore applied in industry for the synthesis of quinone and phenolic compounds derived from benzene. Alternatively, borondoped diamond electrode (BDD) is nontoxic and environmentally friendly with a higher oxidation capacity for benzene than Pt (15). Although PbO2 is another alternative anode material, the incineration of BTX cannot be achieved on traditional PbO2 electrodes and the mechanism of the reaction is seldom reported in the literature. Recently, a superhydrophobic PbO2 electrode (hydrophobic-PbO2) with high electrochemical performance was developed (16). It is an easy-to-prepare, low-cost material that exhibits high oxygen overvoltage and can produce large quantities of hydroxyl radicals ( · OH). Thus, hydrophobic-PbO2 is considered a promising anode for environmental treatment process. The present study was focused on the effective and complete removal of stable and refractory benzene (PhH), toluene (PhMe) and p-xylene (PX) by mild electrochemical incineration on hydrophobic-PbO2 electrode. To better understand the mechanism of incineration, and to discern the relationship between electrode performance and oxidative cleavage of BTX, intensive investigations were conducted on the formation and decay rates of intermediates, the ringopening ratio of the benzene ring and the overall reaction kinetics. For the first time, the relationship between the removal rate and the molecular structures of BTX was verified both experimentally and via quantum chemistry calculations.

Experimental Section Preparation of the Anodes. Traditional-PbO2 and hydrophobic-PbO2 were prepared according to the procedure described in our previous study (16). The details are listed in S1 of the Supporting Information (SI). Briefly, a layer of TiO2 nanotubes (NTs) was introduced on a Ti substrate, and a small amount of Cu was predeposited on the bottom of the TiO2-NTs layer. Fluorine resin was added during the electrodeposition of PbO2 to achieve a superhydrophobic surface with a contact angle of approximately 140°. Hydrophobic-PbO2 exhibited a high oxygen evolution potential, (approximately 2.5 V vs SCE), favorable conductivity, high stability and an extended service life of 335 h at 1 mA cm-2 in 1 M H2SO4 solution (SI S2). Electrochemical Degradation of BTX. Electrochemical incineration experiments were conducted at room temperature (25 ( 2 °C) in a cylindrical single-compartment cell equipped with a magnetic stirrer and a reflux condenser. To prevent volatilization, ice water (0 °C) was circulated through the reflux condenser, which remained open to the atmosphere. The area of the working anodes was equal to 4 cm2, and a 4 cm2 Ti sheet was used as the cathode. The gap between the electrodes was set to 1 cm. 70 mL of a solution of 0.1 M Na2SO4 containing 100 mg L-1 PhH, PhMe or PX (with solubility of approximately 1750, 535, and 198 mg L-1, VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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respectively, (SI S3) was electrolyzed, and the current density was controlled at 20 mA cm-2. Analytical Methods. The evolution of BTX and related intermediates was monitored via high performance liquid chromatography (HPLC) (Agilent HP 1100) (SI S4). By assuming that the formation and decay of each intermediate were simplified overall sequential reactions, regardless that they might be formed via complicated and multistep processes, an approximate reaction model was postulated. A numeric genetic algorithm (17, 18) was applied to simulate the reaction model and determine the total apparent formation rate constant (kf) and the decay rate constant (kd) by fitting the experimental data (SI S5). The total organic carbon (TOC) content was measured with a TOC analyzer (TOC-Vcpn, Shimadzu, Japan), and the chemical oxygen demand (COD) was measured using a standard dichromate method. The instantaneous current efficiency (ICE) was calculated from the COD values (19): ICE )

FV d(COD) 8000I dt

(1)

where d(COD) is the change in COD over time (mg L-1), t is the electrolysis time (s), I is the current (A), F is the Faraday constant (96 487 C mol-1), and V is the volume of the electrolyte (L). The fraction of benzene that underwent oxidative cleavage (%ring-opening) was obtained from the following equation (20): %(ring-opening) ) 1 - %aromatics ) 1-

∑ [aromatic] (C0 - Ct)

t

× 100

(2)

where ∑[aromatics]t is the total concentration of detected aromatic intermediates (in mM) and C0 and Ct are the concentrations of PhH at time 0 and time t, respectively (in mM). %Aromatics was defined as the ratio of detectable, stable aromatic intermediates to the amount of converted PhH (17, 21). Thus, because the concentrations of CO2, H2O and detectable carboxylic acids can be quantified, (%ringopening) is equal to 1 - aromatics. The average electrochemical energy consumption (AE) in Wh mgCOD-1 was defined as the average amount of electrochemical oxidation energy required to remove 1 mg of COD (22): AE )

UcellI∆t (COD0 - CODt)V

(3)

where Ucell is the average cell voltage (V), I is the current (A), ∆t is the electrolysis time (h), V is the volume of the treated solution (L) and (COD0 - CODt) is the change in COD.

Results and Discussion Effective Electrochemical Incineration of PhH on Hydrophobic-PbO2. The electrochemical incineration of PhH on hydrophobic-PbO2 was studied, and the results were compared to those for Pt, BDD, and traditional-PbO2 (Figure 1). At 240 min, the removal of PhH on a Pt electrode was 17.1% and the COD removal was 9.4%. The results are in accordance with those in the literature; thus, complete mineralization is hard to achieve on an active Pt electrode (20). Similarly, the removal of PhH and COD on traditional-PbO2 were only 38.3% and 29.7%, respectively. Remarkably, the removal of PhH on hydrophobic-PbO2 reached up to 82.8%, approximately 2.2 times greater than that on traditional-PbO2 and 1.2 times 7922

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FIGURE 1. (A) Removal of PhH as a function of the oxidation time on hydrophobic-PbO2, BDD, traditional-PbO2 and Pt electrodes at a current density of 20 mA cm-2; (B) The relationship between ln(C0/C) and the oxidation time; (C) COD removal as a function of the oxidation time. that of BDD, a nonactive anode with a high oxidation capacity (23, 24). Moreover, the COD removal of hydrophobic-PbO2 was 75.4%. The aforementioned results were attributed to the quantity of · OH physically adsorbed on the surface of the electrodes and freely dissociating in the solution surrounding the electrode region (SI S6). Hydrophobic-PbO2 exhibits a strong oxidation capacity due to the large quantity of · OH generated during the reaction (e.g., 20.1 µM at 180 min). Moreover, the adsorption capacity of its superhydrophobic surface is extremely weak, which allows high concentrations of free · OH in solution. As a surface-active and hydrophilic material, the ability of the Pt electrode to generate · OH is low (20, 25), with only 4.5 µM detected. Compared to Pt, traditional-PbO2 generates more · OH (approximately 15.2 µM at 180 min) than BDD (14.4 µM at 180 min). However, with traditionalPbO2, · OH is chemically adsorbed on the hydrophilic surface and is less active. As a result, the oxidation capacity of traditional-PbO2 is inhibited (19, 26). To evaluate the reaction rate of PhH and the contribution of · OH to the oxidation, the applied potential was varied,

TABLE 1. Mechanism of Electrochemical Degradation of Aromatic Pollutants on Pt, Traditional-PbO2, Hydrophobic-PbO2, and BDD electrode Pt traditional-PbO2 hydrophobic-PbO2 BDD

pollutant

concentration removala (%)

COD removala (%)

PhH PhMe PX PhH PhMe PX PhH PhMe PX PhH PhMe PX

17.1 22.6 25.6 38.3 40.9 43.0 82.8 90.5 94.8 68.9 78.9 84.4

9.4 17.8 19.9 29.7 32.4 36.5 75.4 82.9 88.4 61.5 69.1 76.8

tincinerationb (min)

ks (× 10-3 min)

ICE (%)c

AEd (Wh mgCOD-1)

600 540 480 750 660 600

0.78 1.07 1.23 2.01 2.19 2.34 7.34 9.81 12.3 4.87 6.49 7.73

10.7 13.9 16.5 39.2 41.8 44.6 100 100 100 96.6 98.9 100

0.66 0.65 0.61 0.68 0.64 0.58 0.16 0.15 0.15 0.41 0.39 0.36

a Detected at 240 min. b tincineration is the time required to achieve greater than 95% removal of TOC. initial stage. d The AE was calculated for 1 mg of COD removal.

c

Calculated for the

FIGURE 2. Evolution of the intermediates during the incineration of PhH on Pt, traditional-PbO2,, hydrophobic-PbO2, and BDD as a function of time. and the oxidation current densities were determined (SI S7). When a potential of 1.2 V was applied, the oxidation current density of 100 mg L-1 of PhH on the Pt and traditional-PbO2 electrodes increased by 3% and 4%, respectively, only slightly higher than the background current density. At the same potential, the oxidation current density of PhH on hydrophobic-PbO2 increased by 12% from the background current density, which is twice that of BDD (6%). When the potential was set to 3.0 V, the oxidation current density on Pt remained low (5%), whereas, there was a much greater increase on both the BDD and hydrophobic-PbO2 electrodes, 34% and 58% respectively. As reported in the literature (27), when the electrode potential is greater than the oxygen overpotential, organic compounds are oxidized by radicals ( · OH, etc.) that are generated on electrodes with strong oxidation capacities. When the electrode potential is below the oxygen overpotential, the effect of · OH is weakened. Thus, the increase in current density at 1.2 V is primarily due to the oxidation of organic pollutants on the electrode surface. Moreover, the current density on hydrophobic-PbO2 is greater than that of Pt and traditional-PbO2 because electrocatalysis on hydrophobic-PbO2 is superior. The distinct enhancement in current density at 3.0 V, which is greater than the oxygen overpotential (2.5 V for the hydrophobic-PbO2), can be ascribed to the participation of · OH in the oxidation process. The indirect oxidation of PhH by · OH is dominant on the electrode surface; thus, there is a notable increase in current density on hydrophobic-PbO2 since more effective · OH is available.

The kinetics for the removal of PhH was investigated by determining the apparent rate constant (ks), which corresponds to the pseudofirst-order reaction. The calculated ks values for PhH oxidation on Pt and traditional-PbO2 were 0.78 × 10-3 and 2.01 × 10-3 min-1, respectively. On hydrophobic-PbO2, the ks increased to 7.34 × 10-3 min-1, which is 1.5 times that of BDD (4.87 × 10-3 min-1) and almost 10 times that of Pt, indicating a faster removal rate. The ICE and AE of each degradation stage were calculated according to eqs 1 and 3, respectively. Hydrophobic-PbO2 exhibited a higher ICE than BDD during the entire electrolysis process. As shown in Table 1, the ICE of hydrophobic-PbO2 at 0 min was almost 100%, slightly greater than that of BDD (96.6%). For the Pt and traditional-PbO2 electrodes, the ICEs were only 10.7% and 39.2% respectively. The AE of Pt (0.66 Wh mgCOD-1) was similar to that of traditional-PbO2 (0.68 Wh mgCOD-1) but was much lower than that of hydrophobicPbO2 (only 0.16 Wh mgCOD-1). It is due to the strong oxidation capacity for PhH and the low cell voltage resulting from the superior conductivity of hydrophobic-PbO2. The aforementioned results indicate that the direct and rapid electrochemical incineration of PhH could be realized on hydrophobic-PbO2 and that high mineralization rates could be obtained. Mechanism and Kinetics of PhH Incineration on Hydrophobic-PbO2. The concentration of the intermediates is plotted in Figure 2 as a function of time. The maximum accumulated concentration (Cmax) and the evolution of the VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Intermediate Conversion Rates and Kinetic Parameters of PhH on Pt, Traditional-PbO2, Hydrophobic-PbO2 and BDD

intermediates are dependent on the essential properties of the electrodes, that is, the ability to generate · OH. To better understand the kinetics of the process, the concentration evolution of each intermediate was expressed as an overall sequential reaction model, and undetected and unstable intermediates were ignored, regardless of the complexity of the multiple process. The evolved concentrations of the intermediates obtained by HPLC were simulated with a genetic algorithm (18, 24) to determine the formation rate constant (kf) and the decay rate constant (kd) of the overall reaction. Based on the evolution of intermediates over time, a reaction pathway for PhH oxidation was proposed. Oxidative cleavage of the benzene ring was a convenient separation point; thus, the reaction was divided into two stages. The formation of aromatics prior to the cleavage of benzene ring was regarded as the first stage of the reaction, and the conversion of carboxylic acids was considered as the second. The first stage of oxidation occurs on the electrode surface and in the double layer region. For the active Pt electrode, PhH is primarily oxidized on the electrode surface by chemically adsorbed · OH, whereas with nonactive BDD and PbO2 electrodes, physically adsorbed · OH, along with that in the double layer are the primary oxidants. As previously mentioned, the surface of hydrophobic-PbO2 is superhydrophobic and possesses a weak adsorption capacity. Thus, the oxidation of PhH is dominated by free · OH in the solution surrounding the electrode. During this stage, the substrate first transfers from the solution to the double layer region, where it can diffuse to the electrode surface. The benzene ring of the substrate is then attacked by · OH physically absorbed on the electrode surface or dissociated in the double layer region to form phenol and dihydroxybenzenes. Subsequently, phenolic compounds are oxidized to benzoquinone. On Pt and traditional-PbO2, the detected main aromatic intermediates were phenol, p-dihydroxybenzene, o-dihydroxybenzene, and p-quinone (Figure 2 (A-E)). On hydrophobic-PbO2 and BDD, m-dihydroxybenzene was also detected. It is well-known that aromatics tend to accumulate on Pt electrodes (14, 28). Thus, the Cmax values for phenol at 60 min and p-dihydroxybenzene at 120 min reached up to 19.4 and 14.3 mg L-1 respectively. The concentrations were 7924

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still greater than 12.0 mg L-1 even after 240 min of oxidation. However, the accumulated concentration of phenolic compounds on hydrophobic-PbO2 was much lower. For example, the Cmax of phenol at 60 min was only 11.7 mg L-1, which was significantly lower than that of BDD (13.1 mg L-1). It is notable that the Cmax values of p-quinone on the Pt and traditionalPbO2 electrodes were 10.6 and 6.6 mg L-1 at 60 min, respectively, and high concentrations were maintained after 240 min. However, on hydrophobic-PbO2, the Cmax of p-quinone was only 3.3 mg L-1, and the concentration quickly decreased below 0.3 mg L-1. The kf and kd of each aromatic intermediate is provided in Table 2. Pt displayed the fastest rate for the formation of phenol and the lowest kd. Thus, significant amounts of phenol accumulated on the electrode surface. Contrastively, the highest kd of the aromatic intermediates was observed on hydrophobic-PbO2. For example, the kd of phenol was 3.45 × 10-2 min-1, approximately 1.4 times greater than that of the Pt and traditional-PbO2 electrodes. The kd of p-quinone rose to 6.88 × 10-1 min-1 on hydrophobic-PbO2, more than 9 times that of the Pt electrode and 1.4 times that of BDD. These results suggest that the rates of oxidation and decay of aromatics on hydrophobic-PbO2 were higher than those on the Pt and traditional-PbO2 electrodes. As a result, the accumulation of toxic compounds was reduced when using hydrophobic-PbO2. Thus, the passivation of the electrode surface by fouling can be effectively avoided, along with a fouling-induced reduction in the current efficiency. Figure 2 F and G depict the evolution of carboxylic acids during the second stage of the reaction. On hydrophobicPbO2, · OH attacks benzoquinone, which is converted into maleic acid and then to oxalic acid. Finally, the carboxylic acids are completely mineralized to CO2 and H2O. On traditional-PbO2 and Pt electrodes, the cleavage of phenolic compounds and p-quinone is difficult to achieve, and therefore maleic acid and oxalic acid seldom form. Moreover, if any carboxylic acids form, they cannot be effectively oxidized, and the concentrations increase slowly due to the passivation of the electrode surface by phenolic compounds. Both maleic acid and oxalic acid were produced on hydrophobic-PbO2 and BDD. The concentrations of the two carboxylic acid intermediates over time displayed similar

FIGURE 3. Charge density of C atoms in the benzene rings of PhH, PhMe and PX. trends to those of the aromatic intermediates. The Cmax values for maleic and oxalic acids on hydrophobic-PbO2 were 12.2 and 24.3 mg L-1, respectively, and finally decreased to 1.1 and 4.3 mg L-1 at 240 min. The kd values for maleic acid and oxalic acid were approximately 0.11 and 2.99 × 10-2 min-1, respectively. Thus, the degradation of maleic and oxalic acids on hydrophobic-PbO2 was faster and more effective. On Pt and traditional-PbO2 electrodes, the ratios of phenol and p-dihydroxybenzene to the total amount of stable, detectable intermediates at 240 min were greater than the corresponding ratios on hydrophobic-PbO2 and BDD, indicating that a higher concentration of phenolic compounds had accumulated on Pt and traditional-PbO2. The final aromatic product of phenols is benzoquinone, which is extremely reactive toward · OH. Thus, the percentage of p-quinone is also indicative of the oxidative cleavage capacity and degradation ability of electrodes. On the Pt electrode, high percentages of p-quinone were observed (11.8%) at 240 min due to the weak ability of Pt to generate · OH (29). Traditional-PbO2 had a 6.0% accumulation of p-quinone, indicating a poor ring-opening efficiency. However, on hydrophobic-PbO2, most of the p-quinone was oxidized to carboxylic acids, and the relative percentage was only 0.8%, suggesting that the oxidative cleavage of PhH was highly efficient on hydrophobic-PbO2. The total percentages of aromatics on the Pt and traditional-PbO2 electrodes were 80.5% and 88.2%, respectively, more than 4 times that of carboxylic acids. These results demonstrate that aromatics tend to accumulate on the surface of hydrophilic anodes. The total percentage of carboxylic acids was up to 41.9% on hydrophobic-PbO2, because it exhibited a strong ringopening capacity for aromatic compounds and quickly oxidized them to short-chain acids. The above analysis indicates that the ring-opening of aromatic intermediates plays a key role in electrochemical incineration of BTX and may also reflect the oxidation capacity of the electrodes. To further investigate this hypothesis, the %ring-opening of PhH was calculated according to eq 2. On Pt, traditional-PbO2, hydrophobic-PbO2 and BDD, the %ring-opening values were 10.7%, 25.8%, 92.7%, and 65.4% at 240 min, respectively (SI S8). Obviously, the ratio was rather low on both Pt and traditional-PbO2 electrodes, indicating that the oxidation reaction nearly stopped at the aromatic stage. The accumulation of aromatics results in the poisoning and passivation of the electrode surfaces. Passivation occurs mainly before ring-opening; thus, the aromatics cannot be effectively degraded to carboxylic acids. As a result, the concentration of carboxylic acids on Pt and traditionalPbO2 was low. Due to its superior ring-opening ability, the %ring-opening on hydrophobic-PbO2 was 92.7% at 240 min, and minor amounts of aromatics accumulated on the electrode surface. The electrochemical adsorption (Γ) of the intermediates on the electrode surface was calculated according to a chronocoulometric method (30), and the results reinforced the aforementioned conclusions. On Pt and traditional-PbO2 electrodes, the Γ values were 6.89 × 10-10 and 9.75 × 10-11

mol cm-2, respectively, indicating that the adsorption of intermediates on hydrophilic electrode surfaces was substantial (14, 31, 32). As a result, the electrodes are prone to fouling and deactivation. On hydrophobic-PbO2, the Γ was only 5.76 × 10-12 mol cm-2, lower than that of BDD (1.61 × 10-11 mol cm-2). The results further suggest that hydrophobicPbO2 exhibits a weaker adsorption capacity than BDD due to the strong hydrophobicity. The above mechanism study well disclosed the relationship between electrode performance and the oxidative cleavage ability. Benzene rings were quickly cleaved on hydrophobic-PbO2, resulting in a high oxidation rate and low accumulation of intermediates, making the complete incineration of PhH possible. Oxidation of Benzene Homologues with Increasing Numbers of Methyl Groups. Because PhH can be completely incinerated on hydrophobic-PbO2, the incineration of PhMe and PX, which differ from PhH with respect to the number of methyl groups, was investigated (SI S9). The effective removal of PhMe and PX can also be achieved on hydrophobic-PbO2. However, PX displayed a higher removal efficiency (94.8% at 240 min) than PhMe (90.5%) and PhH (82.8%). The ks of PX was 12.3 × 10-3 min-1, also higher than that of PhMe (9.81 × 10-3 min-1) and PhH (7.34 × 10-3 min-1). This phenomenon can be explained by the charge density of carbon atoms on the benzene rings, which directly affects the electrophilic attack of · OH. Using density functional theory (DFT) (33) in Gaussian 2003 software package, the charge densities were calculated after minimizing the energy. As shown in Figure 3, the charge densities of each C atom on the benzene ring of PhH were identical (-0.2347), and the total charge density (∑C) was -1.4082. PhMe and PX both possess methyl groups, which are known to be electrondonating groups. PhMe has one methyl group, and the ∑C was -1.6834. The charge densities of the Cs at the para and ortho positions with respect to the methyl were -0.2446 and -0.2436 respectively, more negative than that the C atoms in PhH. PX possesses two methyl groups. Because of the electron-donor ability of the methyl groups, the charge density of the ortho C was -0.2466 and the ∑C was -1.9912, more negative than that of PhMe and PhH. Thus, as the number of methyl groups increases, the total charge density of carbon becomes more negative. · OH is a well-known electron-deficient radical and an electrophilic reagent. It is prone to attack C atoms with more negative charge densities, making them easier to oxidize. As a result, CsC bond rupture occurs easily. PhH, PhMe, and PX exhibit different oxidation behavior on hydrophobic-PbO2 due to differences in their charge densities. Among them, PX was the easiest to cleave because the C atoms of PX possess the most negative charge density. The %ring-opening of PX was 95.6% at 240 min, higher than that of PhMe (94.2%) and PhH (92.7%). · OH tends to attack the methyl C of PX first, followed by the phenyl C at the ortho position, resulting in rapid cleavage of benzene rings. Although the theoretical calculation clarified the effects of methyl groups on the removal efficiency of BTX, the VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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complete mineralization depends not only on the removal of the substrates and the oxidative cleavage of the benzene ring by · OH, but also on the formation and decay rates of the intermediates. Thus, the proposed theory does not yet provide a sound explanation for the observed rates of mineralization. Nevertheless, our experiments indicate that the electrophilic attack of · OH on benzene rings with a greater negative charge density favors ring-opening, which plays a key role in the removal of the substrates and the oxidation of intermediates. The removal of the substrates may affect the mineralization efficiency, especially when the intermediates obtained from different pollutants are similar. Thus, as expected, PX was easier to mineralize than PhMe and PhH. The COD removal was 88.4% for PX at 240 min, higher than that of PhMe (82.9%) and PhH (82.8%). The ICE of PX was also higher (83.1% at 90 min) than that of PhMe (80.8%) and PhH (71.7%). These results verify the assumption that the removal of substrates and the cleavage of benzene rings may affect the ultimate mineralization efficiency to a certain degree. Table 1 also shows that all the four tested electrodes demonstrated this correlation between increasing removal efficiency and the number of attached methyl groups, with PX being much easier to oxidize than PhH and PhMe in each case. However, PX still could not be completely cleaved and mineralized on either Pt or traditional-PbO2 electrodes. For example, the COD removal on Pt was only 19.9% at 240 min. Complete mineralization was not achieved even when the electrolysis time was increased to more than 20 h. Hydrophobic-PbO2 always exhibited the highest mineralization efficiency for BTX because large quantities of · OH were generated, allowing the attack and oxidative cleavage of benzene rings. For instance, the time required to achieve the complete mineralization of PhH and PX (with TOC removal g98%) on hydrophobic-PbO2 was only10 and 8 h, respectively. If the current density is increased to 30-50 mA cm-2, the complete TOC removal of PhH can be achieved in as little as 6 h.

Acknowledgments This work was supported by the National Natural Science Foundation of China (20877058 and 21077077) and the 863 Program of the Ministry of Science (2008AA06Z329). We thank Professor Dongming Li from Tongji University for her helpful discussion and kind assistance in technique writing.

Supporting Information Available Preparation of the anodes (S1); Stability and service life of the electrodes (S2); References on the saturated solubility of BTX (S3); Detection of BTX and related intermediates (S4); Kinetic model and mathematic simulation (S5); Determination of the concentration evolution of · OH (S6); Increases of the oxidation current density of PhH on Pt, traditionalPbO2, hydrophobic-PbO2, and BDD as a function of the applied potentials (S7); Ring-opening ratio of PhH (S8); Concentration and COD removal of PhH, PhMe and PX on hydrophobic-PbO2 (S9). This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Montilla, F.; Huerta, F.; Morallon, E.; Vazquez, J. L. Electrochemical behaviour of benzene on platinum electrodes. Electrochim. Acta 2000, 45, 4271–4277. (2) Sun, M.; Li, D.; Zheng, Y.; Zhang, W.; Shao, Y.; Chen, Y.; Li, W.; Fu, X. Microwave hydrothermal synthesis of calcium antimony oxide hydroxide with high photocatalytic activity toward benzene. Environ. Sci. Technol. 2009, 43, 7877–7882. (3) Kazumi, J.; Caldwell, M. E.; Suflita, J. M.; Lovley, D. R.; Young, L. Y. Anaerobic degradation of benzene in diverse anoxic environments. Environ. Sci. Technol. 1997, 31, 813–818. 7926

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