ARTICLE pubs.acs.org/JPCA
Hydrothermally Enhanced Electrochemical Oxidation of High Concentration Refractory Perfluorooctanoic Acid Hanshuang Xiao, Baoying Lv, Guohua Zhao,* Yujing Wang, Mingfang Li, and Dongming Li Department of Chemistry, Tongji University, 1239 Siping Road, 200092 Shanghai, People's Republic of China ABSTRACT: A green hydrothermally enhanced electrochemical oxidation (HTEO) technique is developed to treat the high concentration refractory perfluorooctanoic acid (PFOA) wastewater on boron-doped diamond (BDD) film electrode. Results show that HTEO can demonstrate higher degradation efficiency for PFOA than the normal electrochemical oxidation (EO) process, with the removal of PFOA, total organic carbon (TOC), and organic fluorine in the HTEO process increasing by 1.1, 1.8, and 2.1 times, respectively. The kinetics study indicates that the degradation of PFOA follows a first-order reaction in the HTEO process with the apparent reaction rate constant 3.1 times higher than that in the EO process. The higher degradation efficiency of PFOA is due to the hydrothermal enhancement in electrochemical properties of the electrode and solution. Compared with EO, during the HTEO process, the conductivity and ionic migration rate of the solution is improved by 540% and 60%, respectively. In addition, the Tafel slope is increased to 343 from 279 mV dec 1, indicating an inhibition effect of oxygen evolution reaction and a more effective oxidation of PFOA. In particular, the hydrothermal condition promotes a high formation rate of hydroxyl radical with the concentration almost 2 times of that in EO, which is considered the inner factor leading to the higher degradation efficiency. The density functional theory simulations demonstrate that the nonterminal C C bonds in the main carbon chain can be easily destructed in the hydrothermal condition, as confirmed by the experimental detection of intermediates of C5F11COOH, C4F9COOH, C3F7COOH, C2F5COOH, CF3COOH, and some dicarboxylic acids. As a result, a reaction pathway is tentatively proposed.
’ INTRODUCTION Perfluorooctanoate acid (PFOA), the metabolic end product of fluorinated organic compounds, is highly toxic.1 It presents not only in urban2,3 but also in remote areas,4 and has been frequently detected in plants, marine life, and even the bodies of human beings.1,5 7 PFOA is highly chemically stable due to the high bond energy of C F bond.1 The unique chemical and biological stability precludes them from most conventional degradations or metabolisms and contributes to their persistence and bioaccumulation.8,9 Recently, some efforts, such as photocatalytic oxidation,10,11 sonochemical degradation,12 adsorption,13 and mechanical chemical degradation14 have been made to develop effective technologies to remove them. For example, perfluorooctanoate acids were decomposed in a sealed reactor with S2O82 as oxidant.11 PFOA under UV vis light illumination also degraded in the presence of H2O2.15 The methods above indicate that the fluorinated organic compounds can be decomposed by strong oxidants. However, these approaches have some drawbacks such as the low reaction rate and high cost. Electrochemical oxidation (EO) is a promising method for wastewater treatment, and it is environmentally compatible with clean oxidant and without secondary pollutants. Some studies showed that the PFOA can be decomposed by the EO method on the electrodes such as BDD and Ti/SnO2 Sb-Bi anodes.16 18 However, these processes were restricted in degrading the low r 2011 American Chemical Society
concentration PFOA because of its high oxidation difficulty, thus led to a low electrode reaction rate, i.e., a low current density.16 18 Moreover, the passivation of the electrode has negative effect on the decomposition rate in the EO process.19,20 With increase of the pollution concentration, the adverse impact will be aggravated. Thus the degradation of the high concentration PFOA by EO is a great challenge for the electrochemist. To obtain a fast mineralization of PFOA, even with the high concentration, the normal EO process is expected to be further enhanced to achieve an increased oxidation ability and reaction rate. It is well-known that hydrothermal oxidation (HTO) can be used to treat the refractory organic pollution substances. The hydrothermal process can also effectively heighten the electrolysis efficiency by avoiding the foul of the electrode.21 Moreover, lower dielectric constant of the solution and larger amount of active radicals produced from water could be obtained in a high temperature and pressure environment than that in normal one.22 Once combining the HTO with EO method, the formed HTEO is expected to obtain a remarkably enhanced performance for electrochemical oxidation with the degradation efficiency Received: August 5, 2011 Revised: October 20, 2011 Published: October 20, 2011 13836
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The Journal of Physical Chemistry A higher than that either of HTO or EO. On the one hand, the conductivity and mass transfer efficiency of the solutions might be increased by the hydrothermal condition, which can lead to a more effective mineralization of the pollutions. And the elevated temperature in the hydrothermal system is likely to activate the electrode surface and maintain more active sites on the electrode.21,23 This is similar to the traditional heating, microwave, ultrasound, photoelectric-synergistic catalytic oxidation, and some other physical methods used in electrochemical oxidation processes which can improve the treatment efficiency for pollutants in these reaction systems.24 26 And the hydrothermal condition might also stimulate the enhancement of the electrochemical properties of the electrode, such as lower electrochemical impedance and higher oxidation ability. On the other hand, with the synergic effect of EO, the high operation temperature and pressure required in the HTO process is expected to be decreased. Moreover, the electrode which can retain stable electrochemical properties at high temperature and pressure is very important in a HTEO process. Here, the BDD electrode is selected due to its excellent properties, especially its superior stability than other typical electrodes.25 In this study, highly refractory PFOA is used as target pollutant to be degraded by using the HTEO method. The degradation efficiency is evaluated by the removal of PFOA, TOC, and organic fluorine. The possible enhancement mechanism is studied through investigating the electrochemical properties of the electrode and solution, the 3 OH concentration evolvement and the decay kinetics in the HTEO process. Furthermore, the main intermediates are analyzed systematically by HPLC and GC-MS and the quantum simulations with density functional theory (DFT) are calculated for the potential energy barriers, which help us to understand the degradation mechanism for PFOA oxidation in this HTEO degradation process. This study will provide a new approach for the degradation of high concentration refractory wastewaters.
’ EXPERIMENTAL SECTION Chemical Reagents. PFOA was reagent grade from Sigma. Other chemicals used were analytical grade. All solutions were prepared with deionized water. The BDD electrode used in this work was made by chemical vapor deposition on a conductive monocrystalline silicon substrate with the boron doping of 1300 ppm. The thickness of the diamond film is about 1 μm, the thickness of the obtained BDD electrode is about 1 mm and its oxygen evolution potential (OEP) is 2.44 V vs SHE. Degradation Reaction of PFOA. The reaction setup used in the HTEO process is shown in Figure 1. In brief, the experiments were carried out in a 0.55-L Teflon-lined stainless steel autoclave electrochemical cell equipped with a magnetic stirrer, deposited in an oil bath to obtain a target temperature. A 400-mL PFOA solution (200 mg L 1) was degraded in an aqueous medium containing 0.05 M Na2SO4 as supporting electrolyte. Before reaction, nitrogen was supplied to purge the air and obtain a reaction pressure of 0.3 MPa. The BDD film and Pt electrodes were used as the anode and cathode, respectively, with a 2 cm gap and a working area of 5.5 cm 2. As the temperature reached needed values, the degradation of PFOA was executed at constant current density of 20 mA cm 2 and a stirring rate about 1000 r min 1. Liquid samples were taken out for analysis at an interval of 30 min through the sampling pipe. The HTEO studies were performed at the temperature ranging from 80 to 120 °C
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Figure 1. Schematic diagram of experiment setup.
and the data obtained at 100 °C were discussed carefully in this work for a profound analysis. The individual EO process was carried out at the same conditions as the HTEO process but without heating. Considering that a small sealed reactor was used, the liquid concentration variation affected by water evaporation at higher temperature was negligible in the study. Analytical Procedure. The intermediate compounds were analyzed by HPLC (Varian Prostar 210, Ultimate TM AQ-C18, 4.6 150 mm) with the mobile phase of methanol/a buffer (pH 2.3) of 1:2 (v/v) 50 mmol NaH2PO4/50 mmol H3PO4 = 30:70 at 0.8 mL min 1. GC/MS (Agilent 6890/5973N, Hp-1, L = 30 m, Ø = 0.25 mm, e = 0.5um) was also used for the volatile intermediates analysis. The column temperature program was: 323 K (5 min), 323 553 K (10 K min 1, hold time: 5 min). The concentrations of F in the solution produced during the reactions were detected by a F selective electrode. TOC was measured by TOC analyzer (TOC-Vcpn, Shimadzu, Japan).The hydroxyl radical concentration was determined according to the literatures.27,28 Specially, formaldehyde was generated quantitatively by the reaction between hydroxyl radicals and dimethyl sulfoxide (DMSO), and then reacted with 2,4-dinitrophenylhydrazine (DNPH) to form the corresponding hydrazone (HCHO DNPH). The formed hydrazone was analyzed by HPLC (Varian Prostar 210, Agilent Zorbax Eclipse XDB-C18 column (150 4.6 mm, 5 μm)), with the mobile phase of methanol/H2O of 60:40 (v/v) at a rate of 1.0 mL min 1 and the UV detection wavelength selected at λ = 355 nm. Quantum Mechanics Simulations. DFT simulations were used to investigate possible PFOA reaction mechanism using the DMol3 package29 in the Accelrys Materials Studio (Accelrys Corporation: San Diego, CA.) modeling suite. All simulations used the double numerical basis set with polarization (DNP)30 and the gradient-corrected Vosko Wilkes Nair Becke Parr (VWN-BP) functionals for exchange and correlation.31 The nuclei and core electrons were described by DFT-optimized semilocal pseudopotentials.32 Implicit solvation was incorporated by the COSMO33 polarized continuum model. Transition state searches were performed by a quadratic synchronous transit method34 and refined by an eigenvector following method.35 The energyoptimized structures and transition states were verified by frequency calculations.
’ RESULTS AND DISCUSSION Enhancement of PFOA Degradation in the HTEO Process. The degradation of PFOA was studied in the HTO, EO, and HTEO processes, respectively. Figure 2 shows the removals of 13837
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Figure 3. Polarization curves of BDD film anode in the EO process (a) and in the HTEO process (b); the Tafel plot in the EO process (inset of a) and in the HTEO process (inset of b).
Figure 2. Removal of PFOA (a), TOC (b) and organic fluorine (c) for 400 mL solution of 200 mg L 1 PFOA in different processes.
the PFOA (a), TOC (b) and organic fluorine (c). It can obviously be observed that, in the HTO process, the removals of PFOA and TOC within 6 h are only 6.7% and 6.1%, respectively, indicating that the mild hydrothermal environment (100 °C, 0.3 MPa) used has little effect on the degradation of the refractory PFOA. In the EO process, the removals of PFOA and TOC are 44.7% and 31.5%, respectively, while the removals of PFOA and TOC during HTEO reach as high as 93.6 and 89.2%,
respectively, 2 2.8 times that in the EO process and 13 14.6 times that in the HTO process, respectively. It is easy to see that the married effect of EO and HTO on the degradation of PFOA is emphatically higher than for either of the single one. On the basis of the fact that the PFOA removal results, the treatment capacity of 12.48 mg h 1 in the HTEO process can be obtained, much higher than that of 5.96 mg h 1 in the EO process, and also far higher than that in ref 16. The defluorination ratio, i.e., CFproduced/CFinitial, also reflects the degradation efficiency of PFOA. From Figure 2c, we can see that the defluorination ratios in the HTEO process are 42.1% and 90.2% at 3 h and 6 h, respectively. Its corresponding F indexes,36 PFOA F i.e., nFproduced/nPFOA degraded, (ndegraded and nproduced are the mole numbers of the degraded PFOA and produced F , respectively, in the system) are 6.3 and 13.5, respectively, indicating 6.3 and 13.5 fluorides lost per PFOA molecule at 3 h and 6 h. While the defluorination ratios in the EO process are only 22.2% and 48.2% with the F indexes of 3.3 and 7.2 at 3 h and 6 h, respectively. The defluorinations are incomplete in both EO and HTEO processes, suggesting the formation of some fluorinated intermediates, while a faster defluorination ratio corresponding to a better oxidative rate can be achieved in HTEO, implying that the electrochemical oxidation process was well promoted by the hydrothermal oxidation process. The electrochemical properties of the electrode and solution are studied to explain the high degradation efficiency of PFOA in the HTEO process. As is well-known, the Tafel slope (b-coefficient) is an excellent parameter to estimate the possibility for oxygen evolution reaction on the anodes in an electrochemical 13838
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Table 1. Physical and Electrochemical Properties of the Electrode and Solutiona liquid conductivity μS cm EO process HTEO process a
1
electrode
ion mobility rate m2 s
1
physical resistance Ω
electrochemical impedance Ω
b-coefficient mV dec
6400
1.66 10
5
100
62 000
279
41 000
2.65 10
5
15
15 000
343
1
b-coefficient: Tafel slope.
process.28,37,38 Using a steady-state anodic polarization method,39 the polarization curves and Tafel plots of the BDD film anode in the HTEO and EO processes are obtained and shown in Figure 3. In the HTEO process, the Tafel slope is 343 mV dec 1, higher than that in the EO process (279 mV dec 1). Studies indicate that the high slope means the increase of the difficulty for water electrolysis to produce oxygen especially at the high potential region, thus the side reaction of oxygen formation was suppressed and a more effective oxidation of the pollution was obtained.38 Other physical properties of the electrode and the liquid in the HTEO process should also account for the higher PFOA removal comparing to that in the EO process. Table 1 shows that the physical resistance of the BDD electrode decreases dramatically from 100 to 15 Ω under hydrothermal conditions. This is because the semiconductor conductivity is increased at higher temperatures. The electrochemical impedance of BDD is also decreased from 62 000 to 15 000 Ω, and at the same time, it obtains a higher Tafel slope, showing enhanced physical properties under hydrothermal conditions. The mass transfer process is generally considered another key factor of the degradation of organic pollutions. In the HTEO process, the high temperature environment used can dissolve the diffusion-controlled problem quite well, which is believed to be an important rate-limiting factor in the decomposition of PFOS.18 Simultaneously, the conductivity of the solution is also 6.4 times that in the EO process, accompanying with the prominent increase of the ion migration, which also benefits for the promotion of the current efficiency. The 3 OH formed in oxidation process also plays key role in PFOA degradation. Figure 4a shows the accumulative 3 OH concentration in these three processes as the function of time. Obviously, the hydroxyl radicals are barely produced in the pure HTO process, while they are formed continuously in the EO and HTEO process and increase with the reaction time. In the EO process, the concentration of 3 OH increases from 7.15 μM at 0.5 h to 22.96 μM at 3 h. While in the HTEO process, it increases from 11.94 μM at 0.5 h to 44.65 μM at 3 h, almost twice that in the EO process. These results indicate that the generation of hydroxyl radicals on the BDD surface can be greatly promoted by the hydrothermal condition. As is known, BDD has an excellent ability to produce 3 OH because of its weak adsorption to it.20 Also the hydroxyl radical is a direct and highly efficient oxidant, which can react with most refractory organics at near diffusioncontrolled rates.40 Moreover, the formed 3 OHs can effectively avoid electrode fouling.41 So the prominently increased 3 OHs are believed to be responsible for the high degradation efficiency of PFOA. When increasing the temperature, the 3 OH concentration also increases (Figure 4b). This might be explained by the fact that the elevated temperature can reduce the passivation of the electrode making and more active sites can be obtained. Compared with Figures 2 and 4, the PFOA decomposition result
Figure 4. Concentration of hydroxyl radical at various times in different processes.
matches very well with the formation of 3 OHs. The role of 3 OHs is thus confirmed. It should be noted that the side reaction of forming oxygen is also faster under HTEO conditions. However the wet air oxidation process might make even less contribution on PFOA degradation due to the great decrease of the oxygen solubility at high temperature.42 The degradation of PFOA was attributed to the electron transition oxidation by the study of Yu et al.16 However, this process was studied in a three-electrode cell where the reaction proceeds at a low electrode reaction rate, so the PFOA degradation prefers the direct electron oxidation mechanism. When a higher current density is used, the hydroxyl radical-mediated 13839
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Table 2. Calculated Activation Barriers for Bond Breaking: (8) CF3 (7)CF3 (6)CF3 (5)CF3 (4)CF3 (3)CF3 (2)CF3 (1) COOH activation barriers for site for 3 OH attacked C F
bond breaking (kJ mol 1) 254.3
(1)
(2)
265.9
(2)
(3)
143.4
(3)
C (4) C
(4)
C (5) C
139.3 135.3
(5)
(6)
135.3
(6)
(7)
131.3
(7)
(8)
157.2
C C
C C C
C C
C C C
Figure 5. Kinetic analysis for the first-order reaction of PFOA in different processes.
oxidation is generally the main mechanism for the degradation of organics.19 In the PFOA decomposition process, Fujishuma et al. found that when the current density is higher than 0.6 mA cm 2, the direct electron oxidation is no longer the main mechanism.17 In this work, the degradation of PFOA is performed at a higher current density of 20 mA cm 2. Moreover, the degradation efficiency of PFOA is consistent with the concentration of the hydroxyl radicals. Therefore, we can reasonably assume that the degradation process of PFOA in HTEO is the hydroxyl radicalmediated oxidation. The kinetics for PFOA degradation in the HTEO process is also studied. The degradation of PFOA follows a first-order reaction according to the evolution of the ln(C/C0) of PFOA, which linearly varies with time in both of the EO and HTEO processes (Figure 5). The apparent reaction rate constant is 0.108 h 1 at 20 °C in the EO process, while it increases with the elevated hydrothermal temperature and reaches 0.444 h 1at 100 °C in the HTEO process. According to the Arrhenius equation, the Ea of PFOA degradation is also calculated, which is 18.9 kJ mol 1 in the EO process (the reaction temperatures ranging from 20 to 40 °C), while it is only 13.8 kJ mol 1 in the HTEO process (the reaction temperatures ranging from 80 to 120 °C). The lower Ea in the HTEO process can be attributed to the hydrothermal condition. It is interesting to note that, although a mild hydrothermal condition is used, the PFOA, TOC, organic fluorine removal ratios in the HTEO process are even much higher than the summation of those in the HTO and EO processes, which increase 1.11, 1.56, and 1.12 times, respectively (Figure 2). Thus the HTEO process is believed not to be a simple summation of the HTO and EO processes, but the synergy effect of them. In this process, not only the mass transportation rate of the solution is greatly enhanced due to the higher temperature, but also the physical and electrochemical properties of anode and liquid are improved under hydrothermal conditions, which results in the highly efficient degradation of PFOA. Reaction Mechanisms of PFOA in the HTEO Process. For further understanding the degradation of PFOA, the DFT simulation, which is considered to be a good method to study the reaction mechanism, has also been studied in this work.18,32,33 The corresponding activation barriers calculated for the reactions
Figure 6. Mechanism of PFOA degradation in the HTEO process.
are shown in Table 2. The activation barrier for the C F bond break is 254.3 kJ mol 1, similar to the value calculated by Carter et al.,18 and those for breaking the C C bond in the end of carbon chain with and without the carboxyl group are 265.9 and 157.2 kJ mol 1, respectively. However, the activation barriers for breaking the inner C C bonds in the main carbon chain are only 135.3 ( 8 kJ mol 1. The results indicate that nonterminal C C bonds in the main carbon chain are the most possible ones attacked by hydroxyl radicals with almost equal probabilities in the HTEO process. The intermediates formed in the HTEO process are also analyzed by HPLC and GCMS. In the first 0.5 h, there are mainly C5F11COOH, C4F9COOH, C3F7COOH, C2F5COOH, CF3COOH, and dicarboxylic acids (including oxalic acid and some unidentified acids) detected in the system. As the reaction time reaching 2 h, the concentration of C5F11COOH and C4F9COOH decrease gradually in this system and those of C3F7COOH, C2F5COOH, and CF3COOH increase gradually. Subsequently, oxalic acid is detected to be the main product in the later reaction process. Compared with the PFOA decomposition studied in previous references,16,19 the intermediates of C6F13COOH is absent in the HTEO process. It is likely that the concentration of C6F13COOH is too low to be detected or it is further decomposed into short chain perfluorinated acids due to the high oxidation ability in the HTEO process. However, the BDD used is a typical nonactive electrode which approves the electrochemical oxidation mediated by hydroxyl radicals,20 especially at a higher current, while based on the calculated results, the 13840
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The Journal of Physical Chemistry A energy barrier for breaking the C C bond at the end of the carbon chain including the carboxyl group is very high, which is not favorable to form C6F13COOH. Combining the detected intermediate products with the calculated results, it is deduced that the PFOA decay follows the pathway shown in Figure 6. In the first step, the nonterminal C C bonds of PFOA are attacked by the hydroxyl radicals, and the main intermediates of C5F11COOH, C4F9COOH, C3F7COOH, C2F5COOH, and CF3COOH produced with a nearly equal probability. At the same time, some dicarboxyl acids are correspondingly formed. Subsequently, C5F11COOH and C4F9COOH can be destructed further, converting into C3F7COOH, C2F5COOH, and CF3COOH. Then, CF3COOH can be oxidized further to form oxalic acid, and finally change into CO2 and H2O. In addition, the formed CF3COOH might also be oxidized directly into CO2 and H2O.
’ CONCLUSIONS In this work, a green and efficient HTEO method is developed for the treatment of high concentration refractory organic wastewater. Using this method, highly refractory PFOA was effectively mineralized. The PFOA and TOC removal are 93.6% and 89.2%, respectively, after 6 h degradation, far higher than that in the EO one. The treatment capacity is 12.48 mg h 1, which is 2.1 times that in the EO process. PFOA decay follows a first-order reaction in both of the HTEO and EO processes, the rate of the apparent reaction rate constant (kHTEO/kEO) is 4.1. High efficiency in the HTEO process is attributed, on the one hand, to the enhanced electrochemical properties of the electrode and solution, on the other hand, to the evidently increased 3 OH concentration due to the hydrothermal effect. The DFT simulations indicate that the PFOA degrades by destructing the nonterminal C C bonds in the main carbon chain in the HTEO process. This is supported by the intermediates detection by HPLC and GCMS, in which C5F11COOH, C4F9COOH, C3F7COOH, C2F5COOH, CF3COOH, and some dicarboxylic acids are produced at first. Subsequently, they further convert into the shorter chain perfluorinated acids and oxalic acid, and finally change into CO2 and H2O. ’ AUTHOR INFORMATION Corresponding Author
*Phone: (86)-21-65981180; Fax: (86)-21-65982287; E-mail: g.zhao@ tongji.edu.cn.
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (nos. 20877058; 21077077), 863 Program (nos. 2008AA06Z329) from the Ministry of Science and the Program for Young Excellent Talents in Tongji University (no. 1380219089). ’ REFERENCES (1) Li, F.; Zhang, C. J.; Qu, Y.; Chen, J.; Chen, L.; Liu, Y.; Zhou, Q. Sci. Total Environ. 2010, 408, 617–623. (2) Murakami, M.; Shinohara, H.; Takada, H. Chemosphere 2009, 74, 487–493. (3) Quinones, O.; Snyder, S. A. Environ. Sci. Technol. 2009, 43, 9089–9095. (4) Skutlarek, D.; Exner, M.; Farber, H. Environ. Sci. Pollut. Res. 2006, 13, 299–307.
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