Theoretical and Experimental Insights into the Electrochemical

Article pubs.acs.org/est

Theoretical and Experimental Insights into the Electrochemical Mineralization Mechanism of Perfluorooctanoic Acid Junfeng Niu,*,† Hui Lin,† Chen Gong,‡ and Xiaomin Sun‡ †

State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, P.R. China ‡ Environment Research Institute, Shandong University, Jinan 250100, P.R. China S Supporting Information *

ABSTRACT: The electrochemical mineralization mechanism of environmentally persistent perfluorooctanoic acid (PFOA) at a Ce-doped modified porous nanocrystalline PbO2 film anode was investigated using density functional theory (DFT) simulation and further validated experimentally. The potential energy surface was mapped out for all possible reactions during electrochemical mineralization reaction of PFOA. The hydroxyl radical (·OH), O2 and H2O took part in the mineralization process and played different roles. The ·OH-initiated process was found to be the main degradation pathway, and the existence of O2 obviously accelerated the degradation process of PFOA in aqueous solution. On the basis of the DFT calculations, an optimal electrochemical mineralization mechanism of PFOA was proposed, which involved the electronic migration, decarboxylation, radical reaction, hydrogen abstraction reaction, and radical fragmentation reaction. The proposed mechanism was verified by the dynamics and intermediate determination experiments. Furthermore, the observed ·OH concentration showed that the electrolysis system could produce enough ·OH for PFOA mineralization process, indicating that the proposed ·OH-initiated process derived from DFT calculations was feasible. These insightful findings are instrumental for a comprehensive understanding of the mineralization of PFOA in the electrolysis system.

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INTRODUCTION As a new environmentally persistent toxic and bioaccumulative pollutant, perfluorooctanoic acid (C7F15COOH, PFOA) has drawn increasing attention due to its potential risk to environment and human health.1−3 PFOA with its hydrophobic and oleophobic properties has been widely used in many applications including surfactants, surface treatment agents, metal coating, and fire retardants.4−6 The extensive use of PFOA has resulted in widespread contamination of various environmental and biological matrices such as waters, sediments, human blood, and wildlife.7−10 However, PFOA is difficult to be treated using most conventional technologies even Fenton’s reagent due to the relatively slow reaction rates between PFOA molecules and aqueous hydroxyl radical (·OH) .11,12 Therefore, new techniques to mineralize PFOA are in immediate need. Oxidative electrochemical technologies are appropriate to degrade toxic or biorefractory organic pollutants because of its high oxidation efficiency, fast reaction rate, easy operation, © 2013 American Chemical Society

amenability to automation, environmental compatibility, and cost effectiveness. Electrochemical oxidation process can be operated at room temperature and atmospheric pressure without adding other reagents.13−16 Research into the electrochemical purification of wastewater containing organic pollutants has primarily focused on “non-active” anodes such as tin oxide (SnO2), lead dioxide (PbO2), and boron-doped diamond (BDD), which have high oxygen evolution overpotential and can produce a larger amount of hydroxyl radicals (·OH).14 Previous studies have found that PFOA and perfluorooctane sulfonate (PFOS) can be electrochemically decomposed by BDD film electrode.17−19 Zhuo et al.20 reported that PFOA could also be degraded by the dimensionally stable anode (DSA), Ti/SnO2−Sn-Bi. Recently, we have Received: Revised: Accepted: Published: 14341

July 9, 2013 November 12, 2013 November 22, 2013 November 22, 2013 dx.doi.org/10.1021/es402987t | Environ. Sci. Technol. 2013, 47, 14341−14349

Environmental Science & Technology

Article

verified to connect the designated reactants and products by performing an intrinsic reaction coordinate (IRC) analysis.39 All the work was performed using the Gaussian 03 programs.40 The polarized continuum model (PCM) was used to calculate the solution-phase energy.41,42 All the energies calculated at the MPWB1K/6-31+G (d, p) level (for details, see Text S1 of the Supporting Information (SI)). In this study, TS, IM, and P represent the transition state, the intermediates, and products, respectively. For example, “TS1” represents the “transition state 1” and IM1 represents “intermediate 1”. Materials. All chemicals in the experiments were reagent grade or higher and used as received. Trifluoroacetic acid (TFA, 98%), perfluoropropanoic acid (PFPrA, 98%), perfluorobutanoic acid (PFBA, 98%), perfluopentanoic acid (PFPeA, 98%), perfluorohexanoic acid (PFHxA, 98%), perfluoheptanoic acid (PFHpA, 98%), and PFOA (98%) were supplied by SigmaAldrich Chemical Co., Ltd. (St. Louis, MO, China). Sodium hydroxide (NaOH) and ammonium acetate (CH3COONH4) were obtained from Sinopharm (Beijing, China). Milli-Q (deionized, DI) water was prepared by Millipore with conductance of 18.2 MΩ cm at 25 ± 1 °C and used in all the experiments. Electrochemical Experiments. The electrochemical cells were made of organic glass (for details, see Figure S1 of the SI). Fifty milliliters of PFOA solution (0.25 mmol L−1) were treated with 10 mmol L−1 Na2SO4 as supporting electrolyte. The Cedoped modified porous nanocrystalline PbO2 film electrode and a Ti electrode (dimension: 5 × 5 cm; thickness: 1 mm) were used as the anode and cathode, respectively, with a 20 mm gap and a working area of 25 cm2. The mineralization experiments of PFOA were performed in galvanostatic mode with a current density of 10 mA cm−2 and stirring rate of 800 r min−1. The reaction solution was sampled for analysis every 15 or 30 min during the experiments. During each sampling, the electrolysis was stopped and the solution was sufficiently stirred to ensure a homogeneous solution. All experiments were triplicated and carried out at room temperature. The corresponding results were averaged with the standard deviation 2.70 V vs standard hydrogen electrode (SHE). Although the activation barrier for direct oxidation of PFOA at the anode was 108.88 kcal mol−1, the experiment was performed at sufficiently high overpotential (the average voltage across the electrodes was 12.55 V) in which the reactions could proceed with little or no activation barrier. Figure 1 presents the possible reaction pathways of C7F15COO− in electrochemical degradation process. As shown in Figure 1, in subsequent reactions, the C7F15· could react with ·OH, O2, and H2O, which were abundant in the electrolyte. The geometries for the optimized structures of the intermediates and transition state in degradation reactions with the bond lengths are shown in SI Figure S5 and Figure S6. (ii). The Reaction of C7F15· with ·OH. The degradation mechanism initiated by ·OH was mainly considered since the ·OH was abundant and highly active in the electrolysis process. The profiles of the potential energy surface including the

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RESULTS AND DISCUSSION Computation of the Electrochemical Mineralization Process. (i). Decarboxylation Reaction. Direct defluorination of perfluorocarboxylic acids (PFCAs) through nucleophilic substitution is very difficult due to the strong C−F bond. Previous studies indicated that perfluoroalkyl acids (PFAAs) such as the oxidative degradation of PFOA and PFOS began in the elimination of the end group.16−18,25,39 The electron transfer process of carboxylic acid occurred on anode first, which belongs to electrochemical process and generates carboxyl radical IM1. Then the decarboxylation reaction generated perfluoroalkyl radical IM2 and CO2 via the transition state TS1. The potential barrier and exothermic energy were 1.96 and 19.96 kcal mol−1, respectively. The parameter ΔEb stands for potential barrier, which is the difference between the energy of transition state and those of reactants. The parameter ΔEr represents reaction heat, which is the difference value between the energies of the products and those of the reactants. The reaction channels are described as follows: 14343

dx.doi.org/10.1021/es402987t | Environ. Sci. Technol. 2013, 47, 14341−14349


Article

Figure 2. The profile of the potential energy surface for the reaction of C7F15COO·.

reactant, IM, and TS are shown in Figure 2. And the structures of the stationary points are drawn in SI Figure S7. The reaction of the C7F15· with ·OH was quite easy because the association reaction between the two free radicals was a barrierless process and the perfluoroheptanol IM3 was generated. The exothermic energy was 111.85 kcal mol−1. Our previous experiments proposed that HF might be removed from IM3 to generate acyl fluoride IM4, the reaction barrier was 53.43 kcal mol−1, and the reaction heat was 13.04 kcal mol−1. Obviously, it is difficult for the reaction to take place due to the high potential barrier. It could be found that the hydrogen atom of IM3 was abstracted by ·OH via the transition state TS3 with a low potential barrier of 6.62 kcal mol−1, and a new free radical IM5 was generated. Then the IM5 could lose a COF2 group to form perfluoroalkyl radical C6F13· (IM7) with a potential barrier of 8.32 kcal mol−1. As for the other pathway, F atom could also be removed to generate acyl fluoride IM4 with a reaction barrier of 33.02 kcal mol−1. With a high potential barrier of 53.43 kcal mol−1, the latter pathway was easier to occur thermodynamically than that of the HF desorption process for the four-membered ring. The above reaction channels are described as follows:

IM5 → TS8 → IM7 + COF2 ΔE b = 8.32 kcal mol−1 ΔEr = −5.51 kcal mol−1

Acyl fluoride IM4 generated in the subsequent reactions of IM3 and IM5 could react with ·OH to produce adduct IM6 via the transition state TS4, and the potential barrier was 4.70 kcal mol−1. Then the HF in IM6 could be removed through the concerted reaction to generate carboxyl radical P1 via the transition state TS5. The potential barrier and endothermic energy were 43.90 and 8.47 kcal mol −1, respectively. Calculation results showed that the fluorine atom in IM6 could be removed alone via the transition state TS6, which had a lower reaction barrier, 26.34 kcal mol−1. The reaction channels are described as follows: IM4 + · OH → TS4 → IM6 ΔE b = 4.70 kcal mol−1 ΔEr = −19.68 kcal mol−1

ΔE b = 43.90 kcal mol−1 ΔEr = 8.47 kcal mol−1

(3)

ΔE b = 26.34 kcal mol−1

ΔE b = 53.43 kcal mol−1

ΔEr = 24.76 kcal mol−1

(4)

ΔE b = 6.62 kcal mol−1 (5)

IM5 → TS7 → IM4 + F· ΔE b = 33.02kcal mol−1 ΔEr = 27.68kcal mol−1

(10)

From the profile of the potential energy surface for the reaction of C7F15COO· (see Figure 2), it could be seen that perfluoroheptanol, perfluoroacyl fluoride, perfluoroheptanoic acid and perfluorohexyl could be generated in the reaction of the C7F15· with ·OH. There are two carbon chain shortening ways in the degradation mechanism. One is the degradation from PFOA ion to PFHpA radical. In the channels (1), (2), (3), (4), (8), (9) or (10), the PFOA ion could be transformed into PFOA radical (IM1), IM3, IM4, P2 or P1. In the above channels, the group CF2 was decomposed and the 8-carbon chain was shortened to 7-carbon chain. It should be pointed out that

IM3 + · OH → TS3 → IM5 + H 2O ΔEr = −3.30 kcal mol−1

(9)

IM6 → TS6 → P2 + F·

IM3 → TS2 → IM4 + HF ΔEr = 13.04 kcal mol−1

(8)

IM6 → TS5 → P1 + HF

IM2 + ·OH → IM3 ΔEr = −111.85 kcal mol−1

(7)

(6) 14344



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there are two HF unzipping processes with high potential barriers (above 40 kcal mol−1) which are the rate controlling steps, that is, channels (4) and (9). Then the similar degradation pathways could continue from PFHpA to PFHxA, to PFPeA, PFBA, to PFPrA, to TFA, and finally to CO2 and HF. The other way is the degradation process from perfluoroheptane radical (IM2) to perfluorohexane radical (IM7). In channels (3), (5), and (7), the perfluoroheptane radical could be transformed into perfluorinated heptanol (IM3), perfluorinated heptane oxygen radicals (IM6) and perfluorohexane radical (IM7). In the above channels, the group CF2 was also decomposed and the 7-carbon chain was shortened to 6-carbon chain. The similar degradation mechanism could continue from the C6F13·, to C5F11·, to C4F9·, to C3F7·, to C2F5·, and finally to CF3·. Then CO2 and HF were formed eventually and PFOA was degraded completely. The second CF2 unzipping process occurred more easily because the potential barriers in the transformation of perfluoroheptane radical was about 8 kcal mol−1 and was lower than the two HF unzipping processes. Thus the second degradation process was the major pathway in thermodynamics. The optimal degradation pathway should be channels (1), (2), (3), (5), and (7), which are labeled as red in Figure 2. By repeating this CF2-unzipping cycle (channels (3), (5) and (7)), C7F15· could be finally decomposed to CO2 and HF. (iii). The Reaction of C7F15· With O2. Apart from ·OH, the C7F15· could also react with other substances in the electrolyte due to itself high activity. A large number of O2 would be generated at the anode in electrolysis process. The C7F15· could react with O2 to generate peroxide IM8; the energy of IM8 was 74.86 kcal mol−1 which was lower than the total energy of reactants. The oxygen atom of IM8 could be abstracted by another O2 molecule to generate IM5 and O3 via the transition state TS9, and the potential barrier was 38.60 kcal mol−1. The IM8 could also abstract the hydrogen atom from H2O to form IM9 and ·OH via the transition state TS10, and then the IM9 could be decomposed into IM5 and ·OH. The subsequent reactions of IM5 have been discussed above. The profile of the potential energy surface is shown in Figure 3 and the reaction channels are described as follows:

IM8 + O2 → TS9 → IM5 + O3 ΔE b = 38.60 kcal mol−1 ΔEr = 13.57 kcal mol−1

IM8 + H 2O → TS10 → IM9 + OH · ΔE b = 30.99 kcal mol−1 ΔEr = 23.37 kcal mol−1

(13)

IM9 → IM5 + ·OH ΔEr = 46.19 kcal mol−1

(14)

Obviously, the presence of O2 accelerated the degradation reaction since it exerts a promotion effect on the generation of · OH. As for other degradation pathways proposed in previous studies and in Figure 1, such as, the H abstracted reaction by O2 to form HO2·, the OF desorption process in the fourmembered ring, the formation IM5 and O2 between the two peroxide IM8, could not be confirmed using the quantum chemisty method.46,47 (iv). The Reaction of C7F15· with H2O. As a solvent, H2O is the most abundant species in the electrolyte solution. Does it play a role or need not to be considered? The following experiments were carried out to answer this question. First, H2O could be electrolyzed into ·OH which is the main oxidizing species. Second, the hydrogen atom of H2O could be abstracted by the C7F15· to generate fluoro heptanoic hydrocarbon IM10 and ·OH via the transition state TS11. The potential barrier and endothermic energy were 13.23 and 3.62 kcal mol−1, respectively. IM2 + H 2O → TS11 → IM10 + OH· ΔE b = 13.23 kcal mol−1 ΔEr = 3.62 kcal mol−1

(15)

Subsequently, the fluoro heptanoic hydrocarbon IM10 might have two reaction pathways. One pathway is that H atom in IM10 is abstracted by ·OH to produce C7F15· and H2O. This process was the reversible as C7F15· + H2O → TS11 → IM10 + ·OH, which had no effect on the degradation of C7F15· or IM10. The other pathway is the nucleophilic substitution reaction of IM10 with ·OH in the subsequent process, which involved in the inversion of configuration. It is difficult to occur because the steric hindrance is high. The reaction with other stronger nucleophilic solvent would be discussed in the following studies. In a word, H2O molecules had an important effect on the degradation of PFOA. The structures of the stationary points of C7F15· with O2 and H2O are drawn in SI Figure S8. On the basis of the DFT calculations presented above, an optimal electrochemical mineralization mechanism of PFOA was proposed by Scheme 1. First, PFOA molecular transferred an electron to the anode (eq 1), and then the activated PFOA molecule (IM1) under went decarboxylation reaction to form C7F15· (IM2) and CO2 via the transition state TS1 (eq 2). The IM2 could react with ·OH to produce adducts C7F15OH (IM3). Subsequently, a hydrogen atom of IM3 would be abstracted by ·OH via the transition state TS3 and generated C7F15O· (IM5). Then, IM5 could be decomposed into C6F13· (IM7) and COF2, while the COF2 could easily hydrolyze to CO2 and HF. By repeating this CF2-unzipping cycle (C7F15· to C6F13·), C6F13·

IM2 + O2 → IM8 ΔEr = −74.86 kcal mol−1

(12)

(11)

Figure 3. The profile of the potential energy surface for the reaction of C7F15· with O2 molecule. 14345



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Scheme 1. Proposed Optimal Electrochemical Mineralization Mechanism of PFOA

could be successively degraded to C5F11·, to C4F9·, to C3F7·, to C2F5·, to CF3·, and finally be completely mineralized to CO2. According to Scheme 1, PFOA molecule was directly mineralized to CO2 and HF nearly immediately after decarboxylation reaction to form C7F15· (eq 2), over the anode surface. Therefore, the short-chain PFCAs accumulation could not be observed during the PFOA mineralization process in electrochemical system. PFOA Mineralization on PbO2 Anode Surface. In order to verify the theoretical results, we conducted the additional experiments on the electrochemical mineralization of PFOA by Ce-doped modified porous nanocrystalline PbO2 electrode. Figure 4 shows the PFOA, TOC, and fluoride concentrations in aqueous solution as a function of electrolysis time at a constant current density of 10 mA cm−2 with a 10 mmol L−1 Na2SO4 supporting electrolyte solution in 120 min. The PFOA degradation ratio, TOC removal ratio, and defluorination ratio were >98%, 94.3%, and 90.4%, respectively, at 90 min, indicating that the Ce-doped modified porous nanocrystalline PbO2 electrode can nearly completely mineralize PFOA to CO2 and HF. A partial order kinetics was used to simulate the PFOA and TOC decay kinetics. The partial order kinetics expression is given by eq 16: [Ct /C0]0.5 = −kt + 1

Figure 4. PFOA removal ratio, TOC removal ratio, and defluorination ratio as a function of electrolysis time operated at a constant current density of 10 mA cm−2 with a 10 mmol L−1 Na2SO4 supporting electrolyte.

Table 1. Electrochemical Degradation Kinetics of PFOA in Aqueous Solution

(16) PFOA TOC

The kinetics parameters such as apparent rate constants (k) and half-lives (t1/2) are summarized in Table 1. As shown in Table 1, the k values for PFOA decay and TOC decay were 0.013 and 0.011 min−1, respectively. Note that the TOC removal rate was only slightly lower than the PFOA degradation rate during the treatment, indicating that only a

rate constants (k, min−1)

half-lives (t1/2, min)

time range (min)

R2

0.013 0.011

22.5 26.6

0−90 0−90

0.989 0.993

small amount of the intermediates could be accumulated in bulk solution. As shown in Figure 5a, short-chain PFCAs bearing C4∼C7 was not be detected by HPLC, indicating that 14346



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series of reactions. The generated PFHpA can also undergo this process and be degraded to PFHxA. By repeating this CF2unzipping cycle, PFOA could be finally decomposed to CO2 and HF. Thus, the accumulation of short-chain PFCAs and high molar ratios of the generated PFCA/initial PFOA were observed during the PFOA degradation process. These phenomena were observed in many PFOA oxidative degradation techniques such as homogeneous photochemistry with H3PW12O40·6H2O12 and K2S2O8,48 and TiO2 photocatalysis.25 Obviously, the optimal PFOA degradation pathway obtained by DFT calculations was inconsistent with these previous studies. This study showed that PFOA could be readily mineralized to CO2 over Ce-doped modified porous nanocrystalline PbO2 film anode. Compared with the previous reported results, the microdegradation mechanism of PFOA by electrochemical mineralization was confirmed using the quantum chemistry method. The obtained results in this study provide some insightful information which should be instrumental for a comprehensive understanding of the mineralization of PFOA in the electrolysis system.

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ASSOCIATED CONTENT

S Supporting Information *

Texts S1−S4, Tables S1−S3, and Figures S1−S9 include schematic diagram of the experimental apparatus, the details of the analytical methods, standard chromatograms and calibration conditions for various products, geometries for the optimized structures of the intermediates and transition state structures in degradation reaction with the bond lengths (Å), the diagram of reaction routes of C7F15· with ·OH radicals, the diagram of reaction routes of C7F15· with O2 and H2O, and concentration of hydroxyl radical vs reaction time in electrolysis system. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. (a) HPLC chromatograms of the sample solutions collected at different electrolysis time; (b) the concentrations of intermediates, that is, PFCAs with shorter chain length than PFOA, as a function of electrolysis time operated at a constant current density of 10 mA cm−2 with a 10 mmol L−1 Na2SO4 supporting electrolyte.

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the concentrations of these short-chain PFCAs were lower than 1.0 mg/L−1. To further determine the short-chain PFCA concentrations and their changes during the electrochemical degradation process of PFOA, we also conducted the quantitative analyses of short-chain PFCAs by using the LCMS/MS method. As shown in Figure 5b, the concentrations of all the short-chain PFCAs were very low, and the highest concentrations were only approximately 2 μmol L−1, less than 1% of the initial molar concentration of PFOA (0.25 mmol L−1). The two main conclusions were derived from these experiments. One is that almost no short-chain PFCAs generate during the electrochemical mineralization process. The second conclusion is that PFOA is directly mineralized to CO2 and HF over the PbO2 anode surface with less intermediates re-enter to the bulk solution. Evidently, these results were consistent with the conclusions from the DFT calculations; well verified that the proposed mechanism as shown in Scheme 1 was the main mineralization process of PFOA in electrochemical system. In the proposed mechanism, ·OH plays a key role in the PFOA mineralization process. In order to evaluate the production ability of ·OH by the Ce-doped modified porous nanocrystalline PbO2 film anode, the concentration of ·OH was determined under a current density of 10 mA cm−2 (see Figure S9 of the SI). The results showed that the electrolysis system can supply enough ·OH for PFOA mineralization process. Presently, the main oxidative degradation mechanism of PFOA was considered that PFOA was degraded to short-chain PFCA with less a perfluorinated carbon, PFHpA, through a

AUTHOR INFORMATION

Corresponding Author

*(J. F. Niu) Phone: +86-10-5880-7612; fax: 86-10-5880-7612; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation for Innovative Research Group of China (No. 51121003), the National Natural Science Foundation of China (No. 51378065, 21277082, 21337001) and the Fundamental Research Funds for the Central Universities of China (No. 2012LZD03).

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