Enhanced Adsorption of PFOA and PFOS on Multiwalled Carbon

Aug 23, 2011 - Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technol...
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Enhanced Adsorption of PFOA and PFOS on Multiwalled Carbon Nanotubes under Electrochemical Assistance Xiaona Li, Shuo Chen, Xie Quan,* and Yaobin Zhang Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, People's Republic of China

bS Supporting Information ABSTRACT: Removal of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) from aqueous solution has attracted wide attention in light of their environmental persistence, bioaccumulation, and potential toxicity. Although various destructive technologies were developed, removal of PFOX (X = A and S) under mild conditions are still desirable. In this work, multiwalled carbon nanotubes (MWNTs) were applied to remove PFOX in electrochemically assistant adsorption. Electrosorption kinetics and isotherms were investigated relative to open circuit (OC) adsorption and adsorption on powder MWNTs. Compared with powder MWNTs adsorption, the initial adsorption rate (υ0) of 100 μg/L PFOX at 0.6 V increased 60-fold (PFOA) and 41-fold (PFOS) according to pseudosecondorder kinetics model and the maximum electrosorption capacity (qm) of PFOX (50 μg/L to 10 mg/L) increased 150-fold (PFOA) and 94-fold (PFOS) simulated with Langmuir model. These significant improvements were assumed to benefit from enhanced electrostatic attraction under electrochemical assistance. Furthermore, the used MWNTs were found to be regenerative and reusable. This work provides not only a new approach to effective removal of perfluorochemicals from aqueous solution but also a low energy-consumption and environmentally-friendly strategy for application of carbon nanotubes in water treatment.

’ INTRODUCTION Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) possess unique properties of chemical stability, thermal resistance, and high surface activity, and therefore have been used widely in industry and consumer products in the past decades. Although the major producer has ceased most perfluorochemicals (PFCs) production, PFOS is still used as a necessary surface treatment agent for photolithography process in semiconductor industry.1 Direct or indirect emissions of PFOX (X = A and S) during manufacture, usage, and disposal have resulted in their widespread distribution in the environment. By far, PFOX has been detected in air, water, sediment, human blood sera, livers, and breast milk as well as a wide range of wildlife all over the world,2 even higher concentration PFOX was found in special point sources, industry wastewater and the blood of occupationally exposed people. For example, about 6.5 mg/L of PFOA and 2.3 mg/L of PFOS were detected in groundwater collected from military bases using PFCs as film-forming foams for fire training 3 and PFOS concentration up to 1650 mg/L was found from the wastewater caused by the photolithographic process.4 Also, about 10 mg/L of PFOX was detected from workers’ blood sera of the 3 M Company.5 In addition, the potential toxicity of PFOX to both humans and ecosystems has been demonstrated by toxicological research.6,7 Therefore, effective removal of PFOX was considered to be of environmental concern and has promoted wide interest. However, because of strong C F bonds, 5 PFOX is chemically r 2011 American Chemical Society

stable and resistant to traditional biodegradation. Although various destructive technologies including photochemical oxidation and sonochemical decomposition were developed for PFOX degradation,1,3,4,8 10 harsh experimental conditions and production of recalcitrant pollutants are still a great challenge to effective PFOX removal. Adsorption was proven to be an effective approach in removing trace pollutants from aqueous solution by virtue of low energy cost, high adsorption capacity, and environmental friendliness. Common adsorbents such as activated carbon and charcoal have been used in community and household water treatment.11 As a promising adsorbent, carbon nanotubes (CNTs) were applied to remove pollutants in recent research.12 The advantage of CNTs is attributed to their unusual one-dimensional hollow nanostructures, high surface area, and adsorption capacity.12 Superior adsorption characteristics of CNTs over activated carbon were found in the removal of endocrine-disrupting chemicals, natural dissolved organic compounds, trihalomethanes, and so on.12 14 However, limited production scales and recycling technology restrict its practical application in water purification and wastewater treatment. Received: June 15, 2011 Accepted: August 23, 2011 Revised: August 13, 2011 Published: August 23, 2011 8498

dx.doi.org/10.1021/es202026v | Environ. Sci. Technol. 2011, 45, 8498–8505

Environmental Science & Technology Electrosorption is a novel approach used for pollutant removal, 15 18 which is generally considered as a potential-induced adsorption of contaminants on surface of charged electrodes involving a nonfaradic process. Electrosorption has been found to be able to improve the adsorption rates and adsorption capacity by imposing an electrical field. In literature, conventional porous carbon materials including activated carbon and carbon aerogel were applied as electrodes to adsorb contaminants such as activated dyes, phenol, aniline, bentazone, and pyridyls.17,18 However, either the electron transfer resistance of activated carbon or the micropores structure of carbon aerogel is disadvantageous to good electrosorption performance. 19 As a new carbon nanomaterial, MWNTs possess superior characteristics of high conductivity and electrochemical stability.19 However, their application in electrosorption was less reported.20 We report here, for the first time, the adsorption of PFOX on MWNTs under electrochemical assistance. The objectives of this work were to realize the effective removal of PFOX by electrosorption on MWNTs and to demonstrate the promising application of MWNTs for pollutant removal from aqueous solution. Correspondingly, MWNT electrodes were prepared by electrophoretic deposition (EPD). The electrosorption kinetics and isotherms of PFOX under different polarization potentials were investigated. Meanwhile, OC adsorption and adsorption on powder MWNTs were performed for comparison. In addition, the effect of electrolyte concentration toward the adsorption capacity was discussed. The experiments on regeneration and repeated availability of MWNTs were also carried out.

’ EXPERIMENTAL SECTION Materials. The pristine MWNTs were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. The MWNTs was purified with the method described in our previous work21 (see details in Supporting Information (SI)) and the physicochemical characteristics of purified MWNTs were listed in Table S1 of the SI. PFOA (acid, 96%) and PFOS (potassium salt, 98%) used in this research were purchased from Acros organic Co. and Sigma-Aldrich Co., respectively (see Table S2 of the SI for physicochemical properties). Standard stock solutions of PFOX were prepared by dissolution of solid standards in methanol, which were stored at 4 °C. The volume ratio of methanol in aqueous solution of electrosorption experiments was controlled to be less than 0.1% to minimize cosolvent effect. Fabrication of MWNT-Based Electrodes. The direct current EPD was used to prepare MWNT electrodes according to the method developed by Wu et al. 22 Approximately 75 mg of MWNTs from desiccator was added into 300 mL anhydrous isopropanol and sonicated for 60 min to disperse MWNTs. Then, Mg(NO3)2 3 6H2O (65 μM) was added into MWNTs suspension and sonication continued for another 15 min to get Mg2+-adsorbed MWNTs suspension as shown in Figure 1(a). It is worthy of mention that the presence of Mg2+ salt plays an important role in the deposition process, which could affect the deposition rate and the thickness of MWNTs films.22 A typical TEM image of the MWNTs was shown in Figure 1(b). A diagram of EPD process used to fabricate MWNTs electrodes was shown in Figure 1(c). About 10 mL suspension was added into a quartz cell (30 mm long 12 mm wide 40 mm deep), where a platinum foil (20 mm 35 mm) was used as anode and a polished titanium sheet (0.5 mm thick, 20 mm  40 mm, from Tianjin Gerui Co., Ltd.) was used as cathode

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Figure 1. (a) Optical image of stable MWNTs suspension (0.25 mg/mL) used for EPD. (b) The TEM picture of MWNTs. (c) Equipment of EPD process used to prepare MWNTs electrode. (d) Low-magnification and (e) high-magnification SEM images of MWNTs film prepared by EPD. (f) The profile image of MWNTs electrode.

keeping a distance of 10 mm between two electrodes. The Titanium plate polished with different abrasive papers was cleaned in turn in an ultrasonic bath of ethanol and deionized water for 5 min, and then dried at room temperature. The weight of the Ti plate was determined by an analytical balance with the accuracy of 0.1 mg. EPD was performed at a potential of 160 V and sustained for 3 min, during this process, a current flow of 2 mA was observed. The resulting MWNT electrode was dried in air to remove the residual isopropanol and the weight of MWNTs electrode was measured again. The amount of MWNTs was determined to be 1 mg as the difference between MWNTs electrode and blank Ti plate. The photo of the obtained electrode with uniform and compact morphology was given in Figure S1 of the SI. The apparent surface area was measured to be 6 cm2 for one side. The front and profile scanning electron microscopy (SEM) images of MWNTs electrode are shown in Figure 1(d) (f), respectively. The thickness of MWNTs film is about 15 μm as observed from SEM image in Figure 1(f). Electrosorption and Adsorption Experiments. PFOX was dissolved in methanol as a stock solution, which was diluted to aqueous solution with different concentrations in isotherm experiments. To minimize the cosolvent effects, the volume of methanol in diluted solution was less than 0.1%. All of the adsorption experiments were performed at room temperature (25 °C) and conventional three-dimensional electrodes quartz unit cell (Figure S2, details for the reactor was given in SI) was employed in electrosorption and OC adsorption experiments. The electrosorption kinetics of PFOX (100 μg/L) were carried out in 20 mL aqueous solution containing 1 mM Na2SO4 with 0.6 and 0.6 V potentials. For comparison, adsorption kinetics of 100 μg/L PFOX on powder MWNTs was performed in 20 mL 8499

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Figure 2. (a) C V curves of solution system with and without PFOX in three-dimensional electrodes unit cell (working electrode: MWNTs, counter electrode: Pt, reference electrode: saturated calomel electrode, 0.1 mM Na2SO4). (b) EIS of MWNTs electrode prepared by EPD method. The insert is the EIS of bare Ti electrode.

aqueous solution containing 1 mM Na2SO4 in a quartz cell, where 1 mg powder MWNTs was used as adsorbents. The isotherm experiment condition of electrosorption (0.6 V), OC adsorption, and powder MWNTs adsorption was the same as that applied for kinetics, and PFOX concentrations were in the range of 50 μg/L to 10 mg/L. The pH values of PFOX solution before adsorption is 6.5 (PFOA) and nearly neutral (PFOS), respectively. The concentrations of PFOX were quantified by a high-performance liquid chromatography with an electrospray tandem mass spectrometer (HPLC-ESI-MS/MS). The analysis method was described in SI. To desorb PFOX from MWNTs electrode, the electrode, after reaching adsorption saturation in a 50 mg/L PFOX solution (100 mL), was moved to 90 °C deionized water without adsorbents and stirred for a long time. The concentration of PFOX at different time was analyzed to determine the desorption equilibrium time. Recovery experiments of PFOX were conducted in quartz cells containing 20 mL PFOX solution in the concentration range from 50 μg/L to 10 mg/L to investigate the experimental uncertainty resulted from the adsorption of PFOX on reactor cell. The solutions were stirred at the same rate as the adsorption batch experiments for 3 h (PFOA) and 2 h (PFOS), respectively. The concentrations of PFOX before and after stirring were measured and compared, and the recovery of PFOX was determined to be 88.6 ( 3.4% for PFOA and 105.1 ( 12.7% for PFOS, respectively. Therefore, the adsorption of PFOX was directly calculated according to the mass loss.

’ RESULTS AND DISCUSSION Characterization of Electrodes. The specific surface area of MWNTs was measured to be 519.7 m2/g by N2 adsorption desorption isotherm (Figure S3 of the SI). This large surface area is postulated to be crucial to high adsorption capacity. The average pore diameter, calculated from the pore distribution curve (shown in Figure S3 of the SI) is 4.5 nm, indicating that the MWNTs mainly consist of mesopores. According to the electrical double-layer model developed by Yang et al., both micropores and macropores will decrease electrosorption capacity since micropores could increase electrical double-layer overlapping effect and macropores could reduce surface area of adsorbent. 19,23 By contrast, mesopores are advantageous for improving the electrosorption capacity.

The potential of zero charge (Epzc) of MWNTs electrode, namely the potential of electrode surface with zero net charge, was measured by immersion potential method to be 46 mV in 1 mM Na2SO4 electrolyte.24 Zeta potential (ζ), the potential difference between the dispersion medium and the stationary layer of fluid attached to MWNT particles,25 was determined to be 23.7 mV and 24 mV under the kinetics experimental conditions at pH 6.5 (PFOA) and pH 7.0 (PFOS), respectively. The details for Epzc and ζ measurements are described in the SI and the ζ potential curve in pH range of 2 11 is shown in Figure S4 of the SI. It was found that different surface potentials of powder MWNTs and MWNTs electrodes could affect their adsorption performance as will be discussed in the next subsection. The point of zero charge (pHpzc) of MWNTs electrode, indicating pH of the solution at which the net charge on MWNTs is zero, is 7.3 as determined by batch equilibrium experiments according to reported method 26 (details in the SI). This value is irrespective of the electrolyte concentration as indicated by Figure S5 of the SI. According to the curve in Figure S5 of the SI, when pH < pHpzc, the surface charge of the electrode is positive; when pH > pHpzc, the surface charge of the electrode is negative. To explore the electrochemical characteristics of the electrosorption system, the cyclic voltammetry (C V), and electrochemical impedance spectroscopy (EIS) measurements were performed. C V of the solution, with and without PFOX, was recorded between 1.2 and 1.2 V at a scan rate of 10 mV/s (Figure 2a). Our results indicate that no redox events occur within the scan range, indicating that the MWNTs have a sufficient electrochemical potential window and PFOX is chemically stable in the scan range of 1.2 to 1.2 V. The EIS of MWNTs electrode was scanned in aqueous solution containing 2.5 mM Fe(CN)63-/4- and 0.1 M KCl with a signal amplitude of 5 mV and a frequency range from 1 Hz to 100 kHz. As shown in Nyquist diagrams (Figure 2b), the surface electron transfer resistance (Ret) of bare Ti electrode is 1701 Ω, which sharply dropped to 4.2 Ω after MWNTs films deposition on Ti plate, implying MWNTs coating can facilitate the electron transfer. Electrosorption Kinetics. In our initial experiments, different polarization potentials of 0.6 V, 0.3 V, 0.6 V, and OC condition were applied to investigate the adsorption performance. It was found that removal efficiency of PFOX was sequenced in the 8500

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Figure 3. Parts (a), (b), and (c) show the adsorption kinetics curves of PFOA and PFOS at 0.6 V, OC adsorption, 0.6 V and powder MWNTs. Each curve was simulated with pseudosecond-order model, respectively. (d) The removal efficiency of PFOX reaching adsorption equilibrium at 0.6 V, OC adsorption, 0.6 V and powder MWNTs, respectively.

Table 1. Adsorption Kinetics Parameters of PFOA and PFOS under Different Experimental Conditions pseudosecond-order parameter (PFOA) experimental conditions 0.6 V OC 0.6 V powder MWNTs

qe  10

3

υ0  10

3

pseudosecond-order parameter (PFOS) qe  10

k2

3

υ0  10

3

k2

(mmol/g)

(mmol/h/g)

(g/mmol/h)

R

(mmol/g)

(mmol/h/g)

(g/mmol/h)

R2

8.2 5.9

54.5 23.7

810 680

0.999 0.995

6.5 6.5

53.8 21.5

1290 520

0.999 0.993

5.6

15.1

480

0.998

6.2

19.2

500

0.991

2.2

0.9

191

0.977

3.2

1.3

132

0.949

order of: 0.6 V > 0.3 V > OC > 0.6 V. The adsorption kinetics were performed at 0.6 V, OC and 0.6 V to investigate the influence of different polarization toward adsorption. As shown in Figure 3(a),(b), a fast initial transfer of PFOX from aqueous solution to surface boundary layer of MWNTs electrode was observed within the first 30 min applying either negative or positive potential. This process was followed by a slow diffusion to internal sites of MWNTs. Finally, only 3 and 2 h were required to reach electrosorption equilibrium for PFOA and PFOS, respectively. As shown in Figure 3(c), at least 8 h was required to reach adsorption equilibrium for both PFOA and PFOS by powder MWNTs adsorption. The pseudofirst-order equation and pseudosecond-order equation were popularly used to simulate the electrosorption kinetics of carbon nanomaterials.27 29 In our experiments, it was found that the pseudosecond-order model could well simulate the adsorption kinetics under electrochemical assistance and that

2

for powder MWNTs with higher correlation coefficient (R2), the related equation is described in the SI and the parameters are listed in Table 1. According to Table 1, initial adsorption rate υ0 at 0.6 V is 54.5 μmol/h/g (PFOA) and 53.8 μmol/h/g (PFOS), respectively. This value was found to be 2.3 times (PFOA) and 2.5 times (PFOS) of that for OC adsorption, indicating that the initial adsorption rate could be improved under a low positive potential. Furthermore, the initial adsorption rates υ0 at 0.6 V improved 60-fold (PFOA) and 41-fold (PFOS) compared to those of powder MWNTs regarding the υ0 of 1.4 μmol/h/g (PFOA) and 2.3 μmol/h/g (PFOS) by powder MWNTs adsorption, which indicates that the initial adsorption rate of PFOX could enhance dramatically under the electrochemical assistance. It is found that bias potential has an important influence on the adsorption of PFOA on MWNTs electrode (Figure 3(d)). After reaching adsorption equilibrium, 61% removal efficiency (RE) 8501

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Figure 4. (a) Electrosorption isotherms of PFOX on MWNTs electrode at 0.6 V and OC adsorption. (b) Adsorption isotherms of PFOX on powder MWNTs. Simulation by Langmuir model (- - -) and Freundlich model (—).

Table 2. Langmuir and Freundlich Model Parameters under Different Experimental Conditions Langmuir constants adsorbate PFOA

experimental conditions

b (L/mmol)

R2

KF (mmol(1-n) Ln/g)

n

R2

0.6 V

0.98 ( 0.06

286 ( 57

0.993

6.50 ( 1.50

0.48 ( 0.05

0.987

OC

0.70 ( 0.12

167 ( 83

0.975

4.54 ( 0.79

0.52 ( 0.04

0.992

powder MWNTs PFOS

qm (mmol/g)

Freundlich constants

0.6 V OC powder MWNTs

0.0065 ( 0.0007

526 ( 249

0.853

0.018 ( 0.003

0.27 ( 0.04

0.951

0.94 ( 0.13 0.57 ( 0.02

1000 ( 300 1429 ( 163

0.980 0.997

19.44 ( 12.30 4.16 ( 1.95

0.56 ( 0.10 0.39 ( 0.08

0.956 0.930

0.010 ( 0.001

2941 ( 1471

0.907

0.029 ( 0.010

0.24 ( 0.07

0.835

was observed at 0.6 V. When the potential rise to 0.6 V, RE enhanced to 92%, higher than those obtained by either OC adsorption (68%) or adsorption on powder MWNTs (37%). Considering the negative ζ value of powder MWNTs at pH 6.5 and dissociated PFOA in aqueous solution due to low pKa, electrostatic repulse between MWNTs and PFOA anion is presumed to impede PFOA adsorption. For MWNTs electrode, the electrostatic attraction between PFOX and MWNTs is favorable to improve the adsorption capacity at pH nearly neutral since pHpzc of MWNTs electrode is 7.3 and the surface charge is positive when pH < pHpzc.17 Therefore, it is understandable that the adsorption capacity of PFOA on MWNTs electrode is higher than that of powder MWNTs and the RE improved when bias potential increased from OC to 0.6 V. Surprisingly, higher removal efficiency and initial adsorption rate than those of powder MWNTs adsorption were also obtained at 0.6 V, which seems contrary to the anticipated electrostatic repulsion between MWNTs electrode and the anions of PFOX. Considering the high deionization capacity of carbon nanotubes toward Na+ ions under electrical field, 29 a possible explanation to this observation is that Na+ ions are attracted by the negative polarized MWNTs surface and provides bridging with the PFOX. A similar argument was found in the works by Niu et al. and Ji et al.30,31 But additional studies need to be done to uncover the whole adsorption mechanism. As for PFOS, 95% RE was observed at 0.6 V at an equilibrium concentration according to Figure 3(d), which is higher than that afforded by powder MWNTs (71% RE), but there is only slight enhancement of RE at 0.6 V compared with adsorption at 0.6 V (RE, 89%) or OC adsorption (RE, 91%). Increased electrostatic

Figure 5. The effects of electrolyte concentration on electrosorption capacity of PFOA (pH 6.5) and PFOS (pH 7) (100 μg/L PFOX).

attraction should be useful for improved RE compared with powder MWNTs adsorption, which is similar with that of PFOA. However, PFOX is aliphatic hydrocarbons and increasing alkyl length can improve their hydrophobicity.32 Since PFOS has longer perfluoroalkyl chain than that of PFOA, the stronger hydrophobic interaction should be considered in the process of PFOS adsorption at OC or 0.6 V potential. Therefore, it is reasonable that RE of PFOS at 0.6 V is slightly higher than those of OC adsorption and 0.6 V adsorption under the influence of hydrophobicity and electrostatic interaction. Electrosorption Isotherms. The adsorption isotherms of PFOX are illustrated in Figure 4. All isotherms exhibit nonlinear 8502

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Figure 6. (a) Adsorption kinetic curves of PFOX (50 mg/L) at 0.6 V and desorption kinetics curves after reaching adsorption saturation. (b) The electrosorption capacity of MWNTs toward PFOX (50 mg/L) using for 5 times (the data of each point was collected at the adsorption equilibrium concentration in 1 mM Na2SO4 solution). The inset (1) is the SEM image of MWNTs before electrosorption. The inset (2) is the SEM image of MWNTs after PFOS electrosorption for 5 times.

characters and are described by Freundlich and Langmuir models, two well-known isotherms33 (see SI for details). The model parameters are listed in Table 2. At a potential of 0.6 V, Langmuir model exhibits a higher R2 than that of Freundlich model, indicating that the monolayer adsorption is dominant (see SI for details). The qm at 0.6 V was calculated by Langmuir model to be 0.98 mmol/g (PFOA) and 0.94 mmol/g (PFOS), improving 1.4-fold (PFOA) and 1.6-fold (PFOS) compared to that of OC adsorption, respectively. In spite of the possibility of a reduced effective surface area of MWNTs coated on electrode in comparison with that of powder MWNTs, the increased qm at 0.6 V, 150fold (PFOA) and 94-fold (PFOS) of those for powder MWNTs adsorption, were still found in Table 2. In view of surface charge of MWNTs electrodes is positive when pH < pHpzc,17 electrostatic attraction between PFOX anion and the electrode would strongly increase at a positive bias as described in the subsection of kinetics. The electrical double-layer capacity of electrode could in turn be improved, leading to a superior adsorption capability. Effect of Electrolyte. Na2SO4 was used frequently as supporting electrolyte of electrosorption process since it was hardly affected by environmental factors.17,18,34 Although the hydration of SO42- ion limits its adsorption on carbon materials electrodes,34 adsorption of SO42- ion on carbon aerogels electrodes was observed.35 In order to investigate the effect of electrolyte on electrosorption, the adsorption of PFOX on MWNTs were conducted with different Na2SO4 ion strengths (0, 1, 5, and 10 mM) under electrochemical assistance. The corresponding adsorption capacities were shown in Figure 5, demonstrating that the adsorption capacity decreases along with increasing electrolyte concentration from 1 to 10 mM. For example, electrosorption capacities of 7.6 μmol/g for PFOA and 7.4 μmol/g for PFOS were observed in the presence of 1 mM Na2SO4. In comparison, lower electrosorption capacities of 6.3 μmol/g for PFOA and 5.2 μmol/g for PFOS were obtained in the presence of 10 mM Na2SO4. We proposed that although higher electrolyte concentration accelerated the transfer of pollutant ions under imposed electric field, competitive adsorption of SO42- ion on electrode could occupy the adsorption sites of MWNTs, resulting in lower electrosorption capacity. However, control experiments without electrolyte showed that electrosorption capacities are 6.7 μmol/g for PFOA and 5.6 μmol/g for PFOS, lower than those obtained

in the presence of 1 mM electrolyte, indicating the important role of proper electrolyte concentration in electrosorption. Regeneration of MWNTs and Their Reusability. To examine the regeneration and reusability of MWNTs for electrosorption, desorption experiments were performed after approaching adsorption saturation in either 50 mg/L PFOS (0.09 mmol/L) or PFOA (0.12 mmol/L) aqueous solution at 0.6 V. The MWNTs electrode was cleaned with stirring in aqueous solution at 90 °C. The amounts of PFOS on MWNTs over the entire period are shown in Figure 6(a). Both PFOS and PFOA were found to desorb from MWNTs rapidly and the desorption equilibriums reached within 1 h. To the end, about 85% of PFOS or PFOA was released into water without exfoliation of MWNTs from Ti plate. The electrosorption capacity of PFOX on desorbed MWNTs was investigated by repeating adsorption desorption experiments for five times using 50 mg/L PFOX. The adsorption capacities were found to be 0.95 mmol/g (PFOA) and 0.83 mmol/g (PFOS) after using for five times, which are comparable to those of 0.98 mmol/g (PFOA) and 0.94 mmol/g (PFOS) for use at the first time. The SEM image of MWNTs after repeating electrosorption of PFOS as well as that of new MWNTs are shown in Figure 6(b). According to the images, no obvious change on MWNTs morphology characteristics is observed. In addition, in order to prove that the adsorption but not destruction is the primal mechanism of PFOX removal in electrosorption process, LC-MS qualitative analysis of PFOX solution and infrared spectra (FTIR) analysis of MWNTs coated on the electrode were performed before and after electrosorption experiments (see details in SI and Figures S6, S7). In summary, MWNTs with relatively high surface area and good electrochemical stability were employed to fabricate the electrode for removing PFOS and PFOA by electrochemical assistant adsorption. Our research shows that the adsorption rate and adsorption capacity could be improved at a low polarization potential compared with OC adsorption. Furthermore, compared with powder MWNTs adsorption, the initial adsorption rates at 0.6 V increase 60-fold for PFOA and 41-fold for PFOS and the maximum adsorption capacities increase 150-fold for PFOA and 94-fold for PFOS. The electrosorptive performance of MWNTs toward PFOX shows a great potential for removal of PFCs from practical water environments. However, perfluorochemicals are a family of aliphatic hydrocarbons with 8503

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Environmental Science & Technology hydrophobicity and olephobicity, it is likely that distinctive adsorption characteristics may be obtained if the compounds of different structure were used as aiming pollutants. This work provides a promising approach by virtue of its low energy consumption and environmental friendliness for contaminants removal, which will facilitate the practical application of carbon nanotubes in water treatment.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of purification and characterization of MWNTs, electrosorption experimental reactor, analysis method of PFOX, measurement of the Epzc and pHpzc of MWNTs electrodes and ζ potential of powder MWNTs, pseudosecond-order kinetics model, Langmuir model, Freundlich model, LC-MS qualitative analysis of PFOX solution, FTIR spectra analysis, Tables S1 S2 and Figures S1 S7. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-411-84706140; fax: +86-411-84706263; e-mail: [email protected].

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