Efficient Electrochemical Oxidation of Perfluorooctanoate Using a Ti

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Efficient Electrochemical Oxidation of Perfluorooctanoate Using a Ti/SnO2-Sb-Bi Anode Qiongfang Zhuo,† Shubo Deng,† Bo Yang,‡ Jun Huang,† and Gang Yu*,† † ‡

POPs Research Centre, School of Environment, Tsinghua University, Beijing 100084, China School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, China

bS Supporting Information ABSTRACT: The electrochemical decomposition of persistent perfluorooctanoate (PFOA) with a Ti/SnO2-Sb-Bi electrode was demonstrated in this study. After 2 h electrolysis, over 99% of PFOA (25 mL of 50 mg 3 L-1) was degraded with a first-order kinetic constant of 1.93 h-1. The intermediate products including short-chain perfluorocarboxyl anions (CF3COO-, C2F5COO-, C3F7COO-, C4F9COO-, C5F11COO-, and C6F13COO-) and F- were detected in the aqueous solution. The electrochemical oxidation mechanism was revealed, that PFOA decomposition first occurred through a direct one electron transfer from the carboxyl group in PFOA to the anode at the potential of 3.37 V (vs saturated calomel electrode, SCE). After that, the PFOA radical was decarboxylated to form perfluoroheptyl radical which allowed a defluorination reaction between perfluoroheptyl radical and hydroxyl radical/O2. Electrospray ionization (ESI) mass spectrum further confirmed that the oxidation of PFOA on the Ti/SnO2-Sb-Bi electrode proceeded from the carboxyl group in PFOA rather than C-C cleavage, and the decomposition processes followed the CF2 unzipping cycle. The electrochemical technique with the Ti/SnO2-Sb-Bi electrode provided a potential method for PFOA degradation in the aqueous solution.

’ INTRODUCTION PFOA is environmental persistent, endocrine-disrupting, and resistant to most conventional treatment technologies.1 Due to its hydrophobic and oleophobic properties, PFOA has been produced in large amounts and widely applied in nonstick polymers, oxidative protection coating on metals, and fire retardants.1 The extensive use of PFOA has resulted in its detection in human sera,2 livers of birds,3 and water.4 PFOA is very stable due to the high bond energy of the C-F bond (127 kcal 3 mol-1)1 and a high reduction potential (F þ e- f F , Eo = 3.6 V).5 The environmental distribution and bioaccumulation4 of PFOA have promoted the development of decomposition methods. Some special methods such as oxidative, reductive, and thermolysis methods have been developed to remove PFOA.1 For example, persulfate (S2O82-)6 and phosphotungstic acid (H3PW12O40)7 were used as photocatalysts to oxidize PFOA. With the reductive decomposition methods, the special reducing agents with low standard reduction potentials and high electrondonating strengths were applied, such as UV-aqueous iodide solution.8 In contrast, thermolysis methods can degrade PFOA faster than the oxidative and reductive methods. It was reported that the half-life of PFOA was 22 min for sonolysis9 and 1.3 h for the incineration process.10 r 2011 American Chemical Society

Compared with the above methods, electrochemical oxidation has become a promising method for wastewater treatment because of its strong oxidation performance, low-volume application, and environmental compatibility.11 The anodic oxidation ability was dependent mainly on the properties of anodes and organic compounds.12 Typical anodes were graphite,13 activated carbon fiber,14 platinum,15 dimensionally stable anode (DSA) (e.g., Ti/RuO2, Ti/IrO2, Ti/SnO2 and Ti/PbO2),16-19 and boron-doped diamond (BDD).20 We have tested the Pt, Ti/IrO2, Ti/RuO2, and Ti/IrO-RuO2 to degrade PFOA, but poor performances were obtained. These were because the oxygen evolution potential (OEP) were not high for these anodes, and that the hydroxyl radical produced by water electrolysis was not effective for perfluorinated surfactants.1,9,21 Ti/SnO2-Sb showed complete destruction of compounds,12 but the life span of the SnO2-based electrodes was relatively short. Previous research has indicated that the incorporation of additive metals (Pt, Ce) into SnO2-Sb coating can enhance the electrode durability.22,23 Carter et al. reported that BDD anode had the ability to degrade perfluorooctane sulfonate,24 Received: July 28, 2010 Accepted: January 21, 2011 Revised: January 7, 2011 Published: March 01, 2011 2973

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Environmental Science & Technology but the mechanism about fluorine replacement by the oxygen of the hydroxyl radical was controversial. In contrast, most researchers reported that hydroxyl radical had little effect on C-F bond.1,9,21 Based on the above considerations, PFOA decomposition performances in electrochemical system with a Ti/SnO2-Sb-Bi anode were studied. The intermediate products of PFOA, the oxygen source of the intermediate products, and the oxidation potential of PFOA were investigated to reveal the pathways and the reaction mechanism. To our knowledge, this is the first report that elucidated the decomposition mechanism of PFOA by the electrochemical oxidation method.

’ EXPERIMENTAL SECTION Electrode Preparation and Characterization. Titanium sheets (42  42 mm and 1  1 mm) were first polished with dry grinding cloth (180 mesh) and waterproof abrasive paper (300 mesh and 800 mesh) and then kept in NaOH solution (40%, m/m) at 80 °C for 2 h to remove grease. The sheets were etched in boiling hydrochloric acid (18%, v/v) at 98 °C for 2 h to produce a gray surface with uniform roughness. The coating solution for Ti/SnO2-Sb-Bi electrode was prepared by the Pechini method.25 Briefly, citric acid and ethylene glycol were mixed and agitated at 60 °C until full dissolution, and then the solution was heated to 90 °C with stirring. The mixture of SnCl3 and SbCl4 dissolved in isopropanol was added at the molar ratio of 1:3:10 for Sb:citric acid:ethylene glycol. Finally, BiCl3 dissolved in the isopropanol was added in the above solution, to achieve a molar ratio of 89:3:8 for Sn:Sb:Bi, respectively. The solution was maintained at 90 °C for 2 h to obtain sol-gel. The same method was used to prepare the Ti/SnO2-Sb electrode except for the bismuth component. Ten μL sol-gel was dripped onto the both sides of the Ti sheets with finnpipette and then dispersed uniformly by a brush. After that, the Ti sheets were dried at 120 °C for 10 min in an oven and sintered at 600 °C for 10 min in a muffle furnace. The above loading procedures were repeated 16 times. The last baking was maintained at 600 °C for 1 h. Electrochemical Experiments. Electrochemical measurements were performed using a CHI 636B electrochemical workstation with a conventional three-electrode cell (Shanghai Chenhua instrument Co. Ltd., China). The OEP of Ti/SnO2Sb-Bi electrode was measured with the linear sweep voltammetry technique in 1.4 g 3 L-1 NaClO4 electrolyte. The Ti/SnO2-Sb-Bi (1  1 mm) was used as the work electrode. A platinum sheet (1  1 mm) and a SCE served as the counter electrode and the reference electrode, respectively. Electrode stability was investigated using the method of the accelerated life test, which was performed by the anodic polarization at 100 mA 3 cm-2 in 0.5 M H2SO4 solution. The anode potential was measured as a function of time, considering that the electrode was deactivated when the potential increased 5 V from its initial value. In the batch experiments, PFOA degradation in solution was investigated using two reactors. Reactor I was illustrated in Figure S1 of the Supporting Information (SI). The electrochemical oxidation of PFOA was carried out in a cylindrical container (19 mm radius and 40 mm height) made of organic glass and equipped with a magnetic stirrer at a speed of 1500 r 3 min-1. The work electrode of Ti/SnO2-Sb-Bi had the effective area of 11.33 cm2 in PFOA solution. The Ti sheet drilled with holes of 0.3 mm and 0.6 mm diameter was used as the counter electrode

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and SCE served as the reference electrode. The distance between the work and counter electrode was 10 mm. The reactor II (Figure S2 of the SI) was made of organic glass, where the Ti/ SnO2-Sb-Bi (42  42 mm) was adopted as the work electrode. The Ti plate with an equivalent area was used as the counter electrode, and SCE served as the reference electrode. The distance between the work and counter electrode was 10 mm. A constant current or potential on the anode was controlled using the chronopotentiometry or amperometric i-t technique with the CHI 636B electrochemical workstation. The experiments were carried out in duplicates. Analytical Methods. An ion-chromatograph system (Dionex ICS-1000, USA) consisting of an automatic sample injector (sample injection volume: 25 μL), a degasser, a pump, a guard column (Dionex AG22, 4  50 mm, USA), a separation column (Dionex AS22, 4  250 mm, USA), a column oven (30 °C), and a conductivity detector with a suppressor device was used to determine the concentration of F-. The mobile phase was composed of 4.5 mM Na2CO3/1.4 mM NaHCO3, and the flow rate was set at 1 mL 3 min-1. The calibration condition was shown in Table S1 of the SI. Concentrations of CF3COO- and C2F5COO- were determined with an ion-chromatograph system (Dionex ICS-2000, USA) using an manual sample injector (sample injection volume: 25 μL), a degasser, a pump, a guard column (Dionex AG22, 4  50 mm, USA), a separation column (Dionex AS22, 4  250 mm, USA), a column oven (30 °C), and a conductivity detector with a suppressor device. The mobile phase was 30 mM KOH solution and the flow rate was 1.5 mL 3 min-1. The calibration condition was specified in Table S2 of the SI. Concentrations of C3F7COO-, C4F9COO-, C5F11COO-, and C6F13COO- were analyzed using ultraperformance liquid chromatography coupled with a tripe-stage quadrupole mass spectrometer (UPLC-MS/MS, Quattro Premier XE, Waters Corp., USA) equipped with Acquity UPLC BEH C18 column (2.1  50 mm, 1.7 μm). The column oven was kept at 50 °C. The mobile phase A was 2 mM ammonium acetate in 100% methanol, and the mobile phase B was 2 mM ammonium acetate in methanol/H2O (5/95, v/v). The flow rate was set at 0.3 mL 3 min-1. The gradient condition was listed in Table S3 of the SI. The sample volume injected was 10 μL with an automatic sampler. An electrospray negative ionization mode was used to identify the products in the liquid phase. The analysis was carried out in multiple reaction monitoring (MRM) mode. The pressure of sheath gas (N2) was 0.4 MPa. The capillary potential was 2.8 kV, and the cone voltage was 35 V. The source temperature was 120 °C and the desolvation temperature was 400 °C. The calibration condition was specified in Table S4 of the SI. ESI mass spectrometry was used to identify the oxygen source in the intermediate products of PFOA after electrolysis for 45 min in H2(18O). The full scan (m/z 120-600) mass spectrum was obtained using a triple-stage quadrupole mass spectrometer (Quattro Premier XE, Waters Corp., USA) with an electrospray negative ionization mode. The mobile phase A was 100% methanol, and the mobile phase B was 100% H2O. The flow rate of A/B mixture (1/1, v/v) was 0.2 mL 3 min-1. The capillary potential was 2.2 kV, and the cone voltage was 10 V. The source temperature was 120 °C, and the desolvation temperature was 400 °C. Quantitative analysis of C7F15COO- was performed by a HPLC-system (LC-10ADvp, Shimadzu) equipped with a TC-C18 column (Agilent, 4.6  250 mm, 5 μm) and a CDD-6A 2974

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Environmental Science & Technology conductivity detector. A column oven was set at 40 °C. A mixture of methanol/0.02 M NaH2PO4 (65/35, v/v) was used with the flow rate of 1.2 mL 3 min-1. The sample volume injected was 20 μL. The calibration condition was shown in Table S5 of the SI.

’ RESULTS AND DISCUSSION Electrochemical Stability of Ti/SnO2-Sb-Bi Electrode. The stability of the Ti/SnO2-Sb-Bi electrode was tested by the accelerated lifetime experiments under high current density conditions, which was a common method to evaluate the life span of electrode. The accelerated lifetime for the Ti/SnO2-Sb electrode was only 0.4 h, while this value was doubled (0.8 h) for the Ti/SnO2-Sb-Bi electrode under the same conditions. The reason for the longer life service on the Ti/SnO2-Sb-Bi was explained in Section S2 of the SI. This value was also higher than those reported for the Ti/SnO2-Sb2O5 electrode (10 min, 0.1 A 3 cm-2, 0.5 M H2SO426 and 0.68 h, 20 mA 3 cm-2, 1 M H2SO427). PFOA Decomposition on Ti/SnO2-Sb-Bi Electrode. The electrochemical degradation of PFOA was evaluated on the two electrodes in reactor II. The removal ratio [(C0-Ct)/C0] of PFOA was 93.3% for the Ti/SnO2-Sb electrode within 3 h electrolysis, slightly higher than 89.8% for the Ti/SnO2-Sb-Bi electrode. Due to the longer service lifetime of the Ti/SnO2-Sb-Bi

Figure 1. Changes of C7F15COO- and F- concentrations as a function of electrolysis time in reactor I. Reaction condition: 25 mL of 50 mg 3 L-1 PFOA, 1.4 g 3 L-1 NaClO4, constant current = 0.25 A, T = 305 K.

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electrode, it was selected to investigate the electrochemical degradation performance of PFOA. The effect of electrolysis time on the PFOA degradation was presented in Figure 1. With longer electrolysis time, the concentration of PFOA decreased, while the concentration of Fincreased. PFOA could not be detected by HPLC after electrolysis of 2 h. The concentration of F- continued to increase even after the disappearance of PFOA, indicating that some intermediate products were degraded and- resulted in the increase of F-. The defluorination ratio (i.e., CF produced, t/CFin PFOA, initial) reached 63.8% after electrolysis of 3 h. Alternatively, the-F- yield can also be evaluated using the F index (i.e., - nF produced/ nPFOAdegraded8). The F index was 10 after electrolysis of 3 h, implying approximately 10 defluorinations per PFOA molecule. Incomplete defluorination suggested that some intermediates were fluorinated compounds. In addition to F-, the shorterchain perfluorocarboxyl anions including CF3COO-, C2F5COO-, C3F7COO-, C4F9COO-, C5F11COO-, and C6F13COO- were also detected (Figure 2). It was observed that the concentration of C6F13COO- and C5F11COO- increased during the first hour electrolysis but decreased gradually after reaching the maximum at 1 h (Figure 2a). C4F9COOreached the maximum value at 1.5 h. Further electrolysis decreased the concentrations of C6F13COO-, C5F11COO-, and C4F9COO-, leading to the increase of C3F7COO(Figure 2a), C2F5COO- and CF3COO- (Figure 2b). Therefore, the degradation of PFOA followed a stepwise CF2 flake-off manner. As depicted in Figure 2b, the F mass basically maintained balance within the first half of the three-hour electrolysis period, while it gradually decreased with further increasing electrolysis time. The imbalance of F mass during the second half of the three-hour electrolysis was possibly due to undetectable fluorocarbon products. For example, 1-C fluorocarbons accumulated more with the increase of electrolysis time. After electrolysis of 3 h, the pH decreased from 4.71 to 3.24. Since the pKa value of HF was 3.2, HF was also present in the solution during the electrolysis of PFOA. The degradation of PFOA can be expressed as a pseudofirst-order kinetic reaction, namely, d[PFOA]/dt = -k[PFOA]. Where, k was the first-order kinetic rate constant for PFOA decomposition with the Ti/SnO2-Sb-Bi electrode. A linear fit of kinetic plot gave k = 1.93 h-1, which was higher than the k = 0.174 h-1 in the reduction system of UV/KI,8 and lower than the k = 2.46 h-1 with sonochemistry method28 (Table S6 of the SI). These results indicated that the electrochemical oxidation with Ti/SnO2-Sb-Bi electrode was also an efficient method to decompose PFOA.

Figure 2. The intermediate products concentrations and F mass balance as a function of electrolysis time: (a) the concentrations of C6F13COO-, C5F11COO-, C4F9COO-, and C3F7COO- and (b) the concentrations of C2F5COO-and CF3COO-; F mass balance calculated according to the eq S1 of the SI. Reaction condition was the same with that in Figure 1. 2975

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Environmental Science & Technology Electrochemical Oxidation Mechanism of PFOA. The firstorder kinetic constants were determined at the different potentials, as shown in Figure 3. At the potential of 2.0 or 3.0 V (vs SCE), PFOA was not degraded during electrolysis of 2 h. The hydroxyl radical had already been produced from water electrolysis at the potential of 2.0 or 3.0 V (vs SCE), because the oxygen evolution potential was 1.7 V (vs SCE) for Ti/SnO2-Sb-Bi anode (HO 3 þ HO 3 f O2 þ 2Hþ þ 2e-, Figure S3 of the SI). Therefore, the hydroxyl radical was ineffective for PFOA decomposition in our study. Previous studies also showed that hydroxyl radical could not degrade perfluorinated surfactants.1,9,21 While,

Figure 3. The first-order kinetic constants (r) at the different electrode potentials in reactor II. Reaction condition: 200 mL of 100 mg 3 L-1 PFOA, 1.4 g 3 L-1 NaClO4, T = 305 K, reaction time = 2 h.

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hydroxyl radical was the strong oxidative species, and it could decompose most aliphatic and aromatic organics through extracting the H-atom to form water.1 PFOA, however, does not contain a H-atom, thus the hydroxyl radical act through a direct electron transfer to form the less thermodynamically favored hydroxyl ion (HO 3 þ e-f HO-, E0 = 1.9 V).1 When the potential was adjusted to 3.5 V (vs SCE), it was found that the first-order kinetic constant was increased to 0.0145 h-1. The electrochemical oxidation mechanism of organic compounds involved the electron transfer from organic compounds to the anode or the oxidation of the hydroxyl radical produced from water electrolysis,29 namely, direct or indirect electrochemical oxidation. Although hydroxyl radical could not effectively decompose PFOA, the electrochemical oxidation method has a superior advantage compared with conventional advanced oxidation processes (AOPs) in which the hydroxyl radical was a kind of strong oxidative species. For example, the potential on the anode can be adjusted to a certain potential value, which has higher oxidation ability than some oxidative species (e.g., HO 3 and Cl 3 ) and can decarboxylate PFOA. As a result, the degradation of PFOA at the potential of 3.5 V (vs SCE) was attributed to the direct electron transfer from PFOA to the anode, namely, direct electrochemical oxidation process. When the potential continuously increased from 3.5 to 4 V (vs SCE), the first-order kinetic constant jumped from 0.0145 to 0.384 h-1. However, the first-order kinetic constant showed a little increase with further increase of the potential from 4 to 4.5 V (vs SCE). Therefore, the decomposition reaction of PFOA was a diffusioncontrolled process in the range of 4-4.5 V (vs SCE). According to the tangent surveying method similar to the determination method for the OEP, the oxidation potential of PFOA was 3.37 V (vs SCE) on the Ti/SnO2-Sb-Bi electrode (Figure 3).

Figure 4. ESI mass spectrum of PFOA dissolved in the H2(18O) after 45 min electrolysis in reactor I. Reaction condition: 100 mg 3 L-1 PFOA in 12 mL H2(18O), 1.4 g 3 L-1 NaClO4, constant current = 0.25 A, T = 305 K. 2976

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Environmental Science & Technology

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To elucidate the source of oxygen atom in the intermediate products, the electrochemical oxidation of PFOA in the H2(18O) was investigated. The ESI mass spectrum of PFOA dissolved in the H2(18O) after electrolysis of 45 min was illustrated in Figure 4. The oxygen atom in [C7F15C(16O)(16O)]- belonged to 16O. After electrolysis, 18O was incorporated into the intermediate products. The large peaks at m/z = 367, 317, 267, 217, and 167 corresponded to [C6F13C(18O)(18O)]-, [C5F11C(18O)(18O)]-, [C4F9C(18O)(18O)]-, [C3F7C(18O)(18O)]-, and [C2F5C(18O)(18O)]-, respectively. These observations clearly indicated that water acted as oxygen source for the short-chain perfluorocarboxyl species. The degradation pathways of PFOA by oxidation, reduction, and sonolysis have been reported in the literature,1 but the pathways by electrochemical oxidation have not been reported. Based on the discussion about the potential dependence of the first-order kinetic constant and the results shown in Figure 4, the electrochemical oxidation mechanism of PFOA was proposed as follows. PFOA decomposition first occurred through a direct one electron transfer from the carboxyl group of PFOA to the anode, and the PFOA radical formed at the potentials of 3.37 V (vs SCE) in Step (1)

Scheme 1. Oxidation Mechanism of PFOA on the Ti/SnO2Sb-Bi Electrodea

a

radicals34 in Step (7) C7 F15 ð18 OÞð18 OÞ 3 þ RFð18 OÞð18 OÞ 3

M þ C7 F15 Cð16 OÞð16 OÞ- f M þ C7 F15 Cð16 OÞð16 OÞ 3 ð1Þ The PFOA radical was subsequently decarboxylated to form the perfluoroheptyl radical in Step (2)30 C7 F15 Cð16 OÞð16 OÞ 3 f C7 F15 3 þ Cð16 OÞ2

ð2Þ

The produced perfluoroheptyl radical then followed two reaction pathways. One way was that the perfluoroheptyl radical combined the hydroxyl radical produced from water electrolysis to form the thermally unstable alcohol C7F15(18O)H, shown in Step (3).7 The role of the hydroxyl radical was confirmed in Section S5 of the SI. C7F15(18O)H underwent HF elimination to form C6F13C(18O)F in Step (4)31 C7 F15 3 þ Hð18 OÞ 3 f C7 F15 ð18 OÞH

ð3Þ

C7 F15 ð18 OÞH f C6 F13 Cð18 OÞF þ Hþ þ F-

ð4Þ

Hydrolysis of C6F13C(18O)F yielded C6F13C(18O)(18O)32,33 with HF loss in Step (5). So far, one CF2 unit was flaked off -

C6 F13 Cð18 OÞF þ H2 ð18 OÞ f C6 F13 Cð18 OÞð18 OÞ þ 2Hþ þ F-

ð5Þ C6F13C(18O)(18O)- produced in Step (5) transferred one electron to the anode, repeated Step (1), and resumed the CF2 unzipping cycle, as shown the cycle I (Steps 1-5) in Scheme 1. The other reaction pathway was proposed that perfluoroheptyl radical reacted with oxygen produced from water electrolysis,30 shown in Step (6) C7 F15 3 þ ð18 OÞ2 f C7 F15 ð18 OÞð18 OÞ 3

ð6Þ

The produced perfluoroheptylperoxy radical then combined with another perfluoroheptylperoxy radical to form perfluoroalkoxy

Circle (I) represented Steps 1-5, and circle (II) included Steps 6-8.

f C7 F15 ð18 OÞ 3 þ RFð18 OÞ 3 þ ð18 OÞ2

ð7Þ

After that, the perfluoroheptyoxy radical decomposed, producing perfluorohexyl radical and carbonyl fluoride30 in Step (8). The obtained carbonyl fluoride would hydrolyze to produce carbon dioxide and HF30 in Step (9) C7 F15 ð18 OÞ 3 f C6 F13 3 þ Cð18 OÞF2

ð8Þ

Cð18 OÞF2 þ H2 18 O f Cð18 O 2 Þ þ 2HF

ð9Þ

The perfluorohexyl radical produced in Step (8) combined with O2, repeated Step (6), and followed the CF2 unzipping cycle in the same manner, as shown in the cycle II (Steps 6-8) in Scheme 1. In Figure 4, the peak at m/z = 237 corresponding to [C4F9(18O)]-confirmed the occurrence of the reaction in Step (10), which was similar to Step (3) C4 F9 3 þ Hð18 OÞ 3 f C4 F9 ð18 OÞH

ð10Þ

C4F9 (18O)H was more stable than other short-chain perfluoroalkyl alcohol, thus it can be detected by the ESI mass spectrometry. It was notable that there was no 16O atom in perfluorocarboxyl anions bearing C2-C6 perfluoroalkyl groups in Figure 4, further confirming that the oxidation of PFOA with the Ti/SnO2-Sb-Bi electrode was initiated from carboxyl group of PFOA rather than C-C cleavage. Future Technical Considerations. Electrochemical oxidation using the Ti/SnO2-Sb-Bi anode in this work provided a promising method for treatment of PFOA wastewater. To apply this technology to industrial wastewater treatment, the key issue is to increase the current efficiency and reduce the energy consumption. Thus, the fabrication of anode, the electrolysis conditions, and the reactor structure should be optimized. The life span of anode can be improved by changing the fabrication method, incorporating the interlayer,11,35 and introducing additive metals into the coating.22,23 For the electrolysis conditions, 2977

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Environmental Science & Technology the current efficiency can be enhanced by increasing the mass transfer and using the modulated current electrolysis.36 In this study, the power density was calculated to be 132.5 W 3 L-1 for the reactor I, and 13.04 W 3 L-1 for the reactor II (see Table S8 in SI), which were lower than 250 W 3 L-1 in the sonochemistry system.28 To treat a large amount of wastewater, several parallel anodes with dual sides coated should be situated in a batch reactor to enhance the removal efficiency for PFOA. In addition, the decomposition efficiency can also be increased by reducing the interelectrode gap and increasing the ratio of surface area to water volume. As far as the matrix effect concerned, some inorganic or organic compounds may influence the oxidation rate. For example, the electrochemical oxidation rate of N-nitrosodimethylamine was not affected by dissolved organic carbon, Cl-, and HCO3- at low concentration. But the effects were appreciable at higher concentration of 100 mM Cl- or HCO3-.37 Considering some coexisting components in actual wastewaters, electrochemical oxidation system should be equipped with pretreatment processes to remove some organic compounds. The wastewater from the semiconductor plants may be treated directly using electrochemical oxidation with the Ti/ SnO2-Sb-Bi electrode, since it contains a high concentration of PFOA but less other components.

’ ASSOCIATED CONTENT

bS Supporting Information. Regents; reactors schemes; calibration conditions for the perfluorocarboxyl anions bearing C1-C7 perfluoroalkyl groups, linear sweep voltammetry of Ti/ SnO2-Sb and Ti/SnO2-Sb-Bi electrodes; characteristics of Ti/ SnO2-Sb and Ti/SnO2-Sb-Bi electrodes including SEM and XRD; the electrochemical oxidation performance of 200 mL of 100 mg 3 L-1 PFOA. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*Phone: (þ86-10)62787137. Fax: (þ86-10)62794006. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (grant no. 20707011 and 50625823), the National High-Tech Research and Development Program of China (863 Program, grant no. 2009AA063902), and the Science and Technology Project of Beijing for the Distinguished Doctor Degree Dissertation (grant no. YB20081000304) for the financial support. ’ REFERENCES (1) Vecitis, C. D.; Park, H.; Cheng, J.; Mader, B. T.; Hoffmann, M. R. Treatment technologies for aqueous perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA). Front. Environ. Sci. Eng. China 2009, 3, 129–151. (2) Hansen, K. J.; Clemen, L. A.; Ellefson, M. E.; Johnson, H. O. Compound-specific, quantitative characterization of organic: fluorochemicals in biological matrices. Environ. Sci. Technol. 2001, 35, 766–770. (3) Kannan, K.; Choi, J. W.; Iseki, N.; Senthilkumar, K.; Kim, D. H.; Masunaga, S.; Giesy, J. P. Concentrations of perfluorinated acids in livers of birds from Japan and Korea. Chemosphere 2002, 49, 225–231. (4) Moody, C. A.; Martin, J. W.; Kwan, W. C.; Muir, D. C. G.; Mabury, S. C. Monitoring perfluorinated surfactants in biota and surface

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water samples following an accidental release of fire-fighting foam into etohicoke creek. Environ. Sci. Technol. 2002, 36, 545–551. (5) Wardman, P. Reduction potentials of one-electron couples involving free-radicals in aqueous-solution. J. Phys. Chem. Ref. Data 1989, 18, 1637–1755. (6) Hori, H.; Yamamoto, A.; Hayakawa, E.; Taniyasu, S.; Yamashita, N.; Kutsuna, S. Efficient decomposition of environmentally persistent perfluorocarboxylic acids by use of persulfate as a photochemical oxidant. Environ. Sci. Technol. 2005, 39, 2383–2388. (7) Hori, H.; Hayakawa, E.; Einaga, H.; Kutsuna, S.; Koike, K.; Ibusuki, T.; Kiatagawa, H.; Arakawa, R. Decomposition of environmentally persistent perfluorooctanoic acid in water by photochemical approaches. Environ. Sci. Technol. 2004, 38, 6118–6124. (8) Park, H.; Vecitis, C. D.; Cheng, J.; Choi, W.; Mader, B. T.; Hoffmann, M. R. Reductive defluorination of aqueous perfluorinated alkyl surfactants: effects of ionic headgroup and chain length. J. Phys. Chem. A 2009, 113, 690–696. (9) Moriwaki, H.; Takagi, Y.; Tanaka, M.; Tsuruho, K.; Okitsu, K.; Maeda, Y. Sonochemical decomposition of perfluorooctane sulfonate and perfluorooctanoic acid. Environ. Sci. Technol. 2005, 39, 3388–3392. (10) Krusic, P. J.; Marchione, A. A.; Roe, D. C. Gas-phase NMR studies of the thermolysis of perfluorooctanoic acid. J. Fluorine Chem. 2005, 126, 1510–1516. (11) Zhao, G. H.; Cui, X.; Liu, M. C.; Li, P. Q.; Zhang, Y. G.; Cao, T. C.; Li, H. X.; Lei, Y. Z.; Liu, L.; Li, D. M. Electrochemical degradation of refractory pollutant using a novel microstructured TiO2 nanotubes/ Sb-doped SnO2 electrode. Environ. Sci. Technol. 2009, 43, 1480–1486. (12) Li, X. Y.; Cui, Y. H.; Feng, Y. J.; Xie, Z. M.; Gu, J. D. Reaction pathways and mechanisms of the electrochemical degradation of phenol on different electrodes. Water Res. 2005, 39, 1972–1981. (13) Cameselle, C.; Pazos, M.; Sanroman, M. A. Selection of an electrolyte to enhance the electrochemical decolourisation of indigo. optimization and scale-up. Chemosphere 2005, 60, 1080–1086. (14) Fan, L.; Zhou, Y. W.; Yang, W. S.; Chen, G. H.; Yang, F. L. Electrochemical degradation of amaranth aqueous solution on ACF. J. Hazard. Mater. 2006, 137, 1182–1188. (15) Liu, L.; Zhao, G. H.; Wu, M. F.; Lei, Y. Z.; Geng, R. Electrochemical degradation of chlorobenzene on boron-doped diamond and platinum electrodes. J. Hazard. Mater. 2009, 168, 179–186. (16) Yunus, R. F.; Zheng, Y. M.; Nanayakkara, K. G. N.; Chen, J. P. Electrochemical removal of rhodamine 6G by using RuO2 coated Ti DSA. Ind. Eng. Chem. Res. 2009, 48, 7466–7473. (17) Makgae, M. E.; Klink, M. J.; Crouch, A. M. Performance of solgel titanium mixed metal oxide electrodes for electro-catalytic oxidation of phenol. Appl. Catal., B 2008, 84, 659–666. (18) Chen, X. M.; Gao, F. R.; Chen, G. H. Comparison of Ti/BDD and Ti/SnO2-Sb2O5 electrodes for pollutant oxidation. J. Appl. Electrochem. 2005, 35, 185–191. (19) Quiroz, M. A.; Reyna, S.; Martínez-Huitle, C. A.; Ferro, S.; Battisti, A. D. Electrocatalytic oxidation of p-nitrophenol from aqueous solutions at Pb/PbO2 anodes. Appl. Catal., B 2005, 59, 259–266. (20) Martínez-Huitle, C. A.; Quiroz, M. A.; Comninellis, C.; Ferro, S.; Battisti, A. D. Electrochemical incineration of chloranilic acid using Ti/IrO2, Pb/PbO2 and Si/BDD electrodes. Electrochim. Acta 2004, 50, 949–956. (21) Schr€oder, H. Fr.; Meesters, R. J. W. Stability of fluorinated surfactants in advanced oxidation processes-a follow up of degradation products using flow injection-mass spectrometry, liquid chromatography-mass spectrometry and liquid chromatography-multiple stage mass spectrometry. J. Chromatogr., A 2005, 1082, 110–119. (22) Montilla, F.; Morallon, E.; Battisti, A. D.; Vazquez, J. L. Preparation and characterization of antimony-doped tin dioxide electrodes. part 1. electrochemical characterization. J. Phy. Chem. B 2004, 108, 5036–5043. (23) Chen, X. M.; Yao, P. D.; Wang, D. H.; Wu, X. Z. Antimony and cerium co-doped tin oxide electrodes for pollutant degradation. Chem. Eng. J. 2009, 147, 412–415. (24) Carter, K. E.; Farrell, J. Oxidative destruction of perfluorooctane sulfonate using boron-doped diamond film electrodes. Environ. Sci. Technol. 2008, 42, 6111–6115. 2978

dx.doi.org/10.1021/es1024542 |Environ. Sci. Technol. 2011, 45, 2973–2979

Environmental Science & Technology

ARTICLE

(25) Xu, L. K.; Xin, Y. L.; Wang, J. T. A comparative study on IrO2Ta2O5 coated titanium electrodes prepared with different methods. Electrochim. Acta 2009, 54, 1820–1825. (26) Ding, H. Y.; Feng, Y. J.; Liu, J. F. Preparation and properties of Ti/SnO2-Sb2O5 electrodes by electrodeposition. Mater. Lett. 2007, 61, 4920–4923. (27) Chen, A. C.; Nigro, S. Influence of a nanoscale gold thin layer on Ti/SnO2-Sb2O5 electrodes. J. Phy. Chem. B 2003, 107, 13341–13348. (28) Vecitis, C. D.; Park, H.; Cheng, J.; Mader, B. T.; Hoffmann, M. R. Kinetics and mechanism of the sonolytic conversion of the aqueous perfluorinated surfactants, perfluorooctanoate (PFOA), and perfluorooctane sulfonate (PFOS) into inorganic products. J. Phy. Chem. A 2008, 112, 4261–4270. (29) Martínez-Huitle, C. A.; Ferro, S. Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes. Chem. Soc. Rev. 2006, 35, 1324–1340. (30) Kutsuna, S.; Hori, H. Rate constants for aqueous-phase reactions of SO4- with C2F5C(O)O- and C3F7C(O)O- at 298 K. Int. J. Chem. Kinet. 2007, 39, 276–288. (31) Nohara, K.; Toma, M.; Kutsuna, S.; Takeuchi, K.; Ibusuki, T. Cl atom-initiated oxidation of three homologous methyl perfluoroalkyl ethers. Environ. Sci. Technol. 2001, 35, 114–120. (32) Wallington, T. J.; Hurley, M. D.; Fracheboud, J. M.; Orlando, J. J.; Tyndall, G. S.; Sehested, J.; Møgelberg, T. E.; Nielsen, O. J. Role of excited CF3CFHO radicals in the atmospheric chemistry of HFC-134a. J. Phy. Chem. 1996, 100, 18116–18122. (33) Debruyn, W. J.; Shorter, J. A.; Davidovits, P.; Worsnop, D. R.; Zahniser, M. S.; Kolb, C. E. Uptake of haloacetyl and carbonyl halides by water surfaces. Environ. Sci. Technol. 1995, 29, 1179–1185. (34) Chambers, R. D. Fluorine in organic chemistry; Durham, UK, 1973. (35) Zhao, G. H.; Zhang, Y. G.; Lei, Y. Z.; Lv, B. Y.; Gao, J. X.; Zhang, Y. N.; Li, D. M. Fabrication and electrochemical treatment application of a novel lead dioxide anode with superhydrophobic surfaces, high oxygen evolution potential, and oxidation capability. Environ. Sci. Technol. 2010, 44, 1754–1759. (36) Panizza, M.; Kapalka, A. Comninellis, Ch. Oxidation of organic pollutants on BDD anodes using modulated current electrolysis. Electrochim. Acta 2008, 53, 2289–2295. (37) Chaplin, B. P.; Schrader, G.; Farrell, J. Electrochemical destruction of N-nitrosodimethylamine in reverse osmosis concentrates using boron-doped diamond film electrodes. Environ. Sci. Technol. 2010, 44, 4264–426.

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dx.doi.org/10.1021/es1024542 |Environ. Sci. Technol. 2011, 45, 2973–2979