Correspondence/Rebuttal pubs.acs.org/est
Comment on Electrolytic Manipulation of Persulfate Reactivity by Iron Electrodes for TCE Degradation in Groundwater contribution of SO•− 4 on TCE degradation in persulfate/iron 1 electrodes system. In order to scavenge SO•− 4 , the authors should improve the concentration of methanol in their further studies. However, owing to the higher rate constant with SO•− 4 7 −1 −1 4 (kSO•− = 1.6−7.7 × 10 M s ), ethanol might be a more 4 suitable quencher than methanol to evaluate the relative 5,9 Meanwhile, although their contribution of SO•− 4 and •OH. 1 ESR studies showed that the intensity of DMPO−OH adducts signals is much stronger than that of DMPO−SO4 adducts signals, it does not mean that the concentration of •OH was higher than that of SO•− 4 in persulfate/iron electrodes system due to the fast transformation from DMPO−SO4 adducts to DMPO−OH adducts via nucleophilic substitution.10−13 In conclusion, their radical scavenging and ESR studies could only demonstrate that SO•− 4 and •OH cocontribute to TCE degradation, but were not enough to demonstrate that •OH contribution is more significant than SO4•−. Meanwhile, although the contribution of •OH in activated persulfate is commonly oxidation has been recognized,9,11,14−16 SO•− 4 assumed as the dominant radical for the oxidation of organic contaminations by Fe2+ activated persulfate.2,9,17 Additionally, the authors1 thought that there might be other degradation pathways to TCE degradation which are not identified except • •− the contributions of SO•− 4 and •OH. Although HO2 /O2 have been reported to be generated in the activation of persulfate,11 18 it is known that HO•2 /O•− 2 are inert toward TCE. Was •OH Produced from the Reaction of SO•− 4 with Water? In this work,1 the authors thought that •OH was produced from the reaction of the generated SO•− 4 with water in persulfate/Fe anode system. Indeed, it has been demonstrated that SO•− 4 could react with water at all pHs to produce •OH in the persulfate activation system.4,5,16,19 Unfortunately, the authors1 neglected the huge difference of the reaction rate constants of SO•− 4 with TCE and water. The reaction rate 1 −1 −1 4 s ) is much constant of SO•− 4 with water (k1 < 6 × 10 M •− •− lower than that of SO4 with TCE (kSO4 = 1.8 × 109 M−1 s−1). Thus, although the concentration of water is as high as 55.56 M, the ck of water ( 8.5) could induce the mechanism of SO•− 4 interconversion to •OH in the persulfate activation system,19−23 •OH produced from
I
n a recent study,1 Yuan et al. developed an effective approach to manipulate the reactivity of persulfate in situ for trichloroethylene (TCE) degradation with iron electrodes. Their study is interesting and provides options to resolve the intrinsic drawbacks of persulfate in applications. Their paper attempted to elucidate the mechanism of TCE degradation in persulfate/iron electrodes system. As interested readers of their paper, we would like to raise our doubts on their discussion about the mechanism of TCE degradation in persulfate/iron electrodes system. Was •OH Contribution to TCE Degradation More Significant than That of SO•− 4 ? Activation of persulfate has attracted a great deal of attention not only because persulfate is relatively stable for its slow reaction kinetics with organics but 2 •− also because of the generation of SO•− 4 and/or •OH. SO4 is an more selective radical for electron transfer reactions than •OH which can undergo reaction rapidly by hydrogen abstract and/or addition.3−5 Additionally, SO•− 4 is a more stable radical than •OH for its longer half-life.6,7 Owing to the difference of SO•− 4 and •OH, it is important for researchers to identify the primary reactive oxidants and to differentiate the contribution 1 of SO•− 4 and •OH in the activation of persulfate. In this work, the authors stated that •OH contribution to TCE degradation in persulfate/iron was more significant than that of SO•− 4 electrodes system. However, this statement is not sufficiently supported by their experimental results. = 4.0 × 105 In their radical scavenging studies,1 TBA (kSO•− 4 M−1 s−1, k•OH = 6.0 × 108 M−1 s−1) and methanol (kSO•− = 3.2 4 × 106 M−1 s−1, k•OH = 9.7 × 108 M−1 s−1) were selected to evaluate the relative contribution of SO•− 4 and •OH. However, it should be noted that the reaction rate constants of TCE with 9 −1 −1 •− s (obtained SO•− 4 and •OH are about kSO4 = 1.7 × 10 M by means of competition kinetics (mixtures of benzoic acid and TCE) in UV/PDS system) and k•OH = 4.0−4.3 × 109 M−1 s−1,8 respectively. Here, the value of ck (c is the concentration of the probes, k is the reaction rate constant of the probes with SO•− 4 or •OH) is used to compare the competitive capacity of the probes for SO•− 4 and •OH. Because the ck of 60 mM TBA for •OH is 10−22 times greater than that of 0.4 mM TCE, 60 mM TBA is enough to scavenge •OH in persulfate/iron electrodes system. However, as shown in their Figure 4a,1 the inhibition of TCE degradation increased slightly with the addition of TBA from 60 mM to 300 mM. The increased inhibition of TCE degradation could be attributed to the quenching of SO•− 4 because the ck of 300 mM TBA for SO•− 4 is about one-sixth of that of 0.4 mM TCE. Therefore, the relative contribution of •OH to TCE degradation in persulfate/iron electrodes system should be less than that of the calculated value by them. However, since the ck of 300 mM methanol for SO•− 4 is only about 1.5 times as much as that of 0.4 mM TCE, 300 mM methanol is not sufficient to scavenge SO4•−. Thus, the difference between the inhibition by the addition of 300 mM TBA and 300 mM methanol is not enough to represent the © 2014 American Chemical Society
Published: March 28, 2014 4630
dx.doi.org/10.1021/es501061n | Environ. Sci. Technol. 2014, 48, 4630−4631
Environmental Science & Technology
Correspondence/Rebuttal
− the reaction of SO•− 4 with OH should also be very little in 1 their work due to the low solution pH (less than 7.0).
(15) Fang, G. D.; Gao, J.; Dionysiou, D. D.; Liu, C.; Zhou, D. M. Activation of persulfate by quinones: Free radical reactions and implications for the degradation of PCBs. Environ. Sci. Technol. 2013, 47 (9), 4605−4611. (16) Liang, C.; Su, H. W. Identification of sulfate and hydroxyl radicals in thermally activated persulfate. Ind. Eng. Chem. Res. 2009, 48 (11), 5558−5562. (17) Kolthoff, I.; Medalia, A.; Raaen, H. P. The reaction between ferrous Iron and peroxides. IV. Reaction with potassium persulfate. J. Am. Chem. Soc. 1951, 73 (4), 1733−1739. (18) Gehringer, P.; Proksch, E.; Szinovatz, W.; Eschweiler, H. Radiation-induced decomposition of aqueous trichloroethylene solutions. Int. J. Radiat. Appl. Instrum. A: Appl. Radiat. Isotopes 1988, 39 (12), 1227−1231. (19) Pennington, D. E.; Haim, A. Stoichiometry and mechanism of the chromium (II)-peroxydisulfate reaction. J. Am. Chem. Soc. 1968, 90 (14), 3700−3704. (20) House, D. A. Kinetics and mechanism of oxidations by peroxydisulfate. Chem. Rev. 1962, 62 (3), 185−203. (21) Norman, R.; Storey, P.; West, P. Electron spin resonance studies. Part XXV. Reactions of the sulphate radical anion with organic compounds. J. Chem. Soc. B 1970, 1087−1095. (22) Hayon, E.; Treinin, A.; Wilf, J. Electronic spectra, photochemistry, and autoxidation mechanism of the sulfite-bisulfitepyrosulfite systems. SO−2 , SO−3 , SO−4 , and SO−5 radicals. J. Am. Chem. Soc. 1972, 94 (1), 47−57. (23) Peyton, G. R. The free-radical chemistry of persulfate-based total organic carbon analyzers. Mar. Chem. 1993, 41 (1), 91−103.
Jing Zou† Jun Ma*,†,‡ Jianqiao Zhang† †
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State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, P.R. China ‡ National Engineering Research Center of Urban Water Resources, Harbin 150090, P.R. China
AUTHOR INFORMATION
Corresponding Author
*Phone: (+86)-0451-8628-2292; fax: (+86)-0451-8628-3010; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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