Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Rapid Selective Circumneutral Degradation of Phenolic Pollutants Using Peroxymonosulfate− Iodide Metal-Free Oxidation: Role of Iodine Atoms”
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e appreciate the interest of Zhou et al.1 in our recent article and welcome the opportunity to respond. Our work2 investigated the interaction of iodide ions (I−) and peroxymonosulfate (PMS) and primarily emphasized the strong selective degradation of phenolic pollutants. Although the nature of the active species and their interaction with pollutants in this system deserves further investigation, we would like to take this opportunity to clarify some of the points raised.
system, and it is possible that the product hydroxymethyl (peroxy) radicals react further with I−. The production of iodophenols was found to be low under the condition investigated in Figure S102. Zhou et al.1 claim that “the negligible formation of iodophenols is attributed to their further transformation by excess HOI.” However, HOI reacts with iodophenols at second-order rate constants less than 102 M−1 s−1 (pH 4−5).9 Meanwhile, HOI can be rapidly reduced by I− (5 × 109 M−1 s−1)9 to I−3 or oxidized by PMS to IO−3 (1.08 × 102 M−1 s−1, pH 5).10 In a study by the commenting authors,10 HOI was not detected in a PMS-I− system with an I−/PMS ratio of 0.1. In our study,2 the ratio of I−/PMS was more than 0.7 (Figure 1a2 and Figure S102). On the basis of the rapid interaction between I− and HOI and the relatively modest reactivity of HOI toward phenol (2.1 ± 0.2 × 103 M−1 s−1), it is difficult to satisfactorily explain the rapid degradation of phenol and the limited production of iodophenols by HOI (Figure S102).
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MAJOR ACTIVE SPECIES Although the reaction of PMS with reductants via a twoelectron mechanism has been reported,3 under most circumstances the reaction proceeds via one-electron transfer to 4 − produce sulfate radicals (SO•− 4 ). The reactions of I with common peroxides, including hydrogen peroxide and peroxydisulfate, have been investigated, and the involvement of iodine atoms (I•) in these redox processes has been proposed.5,6 Zhou et al. claim that “Lente et al.3 have thermodynamically demonstrated that halide ions (I−, Br−) are oxidized by PMS in a formal two-electron process.” However, this conclusion is applicable only under alkaline conditions (pH 8, where the formal potential of the redox 3 couple HSO−5 /SO•− 4 is 0.72 V). Because this value is pHdependent, the formal potential of the redox couple HSO−5 / SO•− at pH 4.5 (our experimental condition) is 0.92 V, 4 corresponding to a free energy of 39.6 kJ mol−1 ((1.33−0.92) × 96.485). This free energy is much lower than that measured (55 kJ mol−1)3 for I− oxidation by PMS, suggesting that I− oxidation via one-electron transfer is thermodynamically favorable. In our study, low levels of iodophenols were generated during the degradation of phenol by PMS-I− (Figure S102). However, contrary to the claim of Zhou et al.,1 we did not rely on this low production of iodophenols to deduce the electron transfer mechanism of I•. Although the mechanisms of I• attack deserve further investigation, studies suggest that such mechanisms will result in iodinated products through the addition of I•.7,8 In the scavenging experiments, in which methanol was used to probe the involvement of radicals, the methanol did not significantly inhibit the overall (final) degradation of phenols. Zhou et al.1 claim that “the authors interpreted this result as the complete trap of SO•− by I− to generate I• which was 4 unreactive towards methanol.” However, we would like to clarify that in our study we did not state that methanol acted as a complete trap of SO•− 4 . In our article, we indicated that the inhibitory effect, although slight, of methanol during the degradation of phenols might suggest the involvement of SO•− 4 . Zhou et al.1 assume that the effect of methanol should be much more significant. Considering the complexity of the potential reactions, we currently lack a complete model of this reaction © XXXX American Chemical Society
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IODOPHENOL PRODUCTS Iodophenols are produced during the treatment of phenols by PMS in the presence of I−, as noted by Zhou et al.1 However, like in most oxidative processes, the accumulation of products depends on the relative doses of oxidants, activators (catalysts), and substrates. Zhou et al.1 claim that “appreciable amounts of iodinated products can be generated in the PMS-I− system containing natural organic matter (NOM).” However, as shown in their study, the production of iodinated products also depends on the substrate properties, and the amount of total iodinated organics produced by PMS-I− was significantly lower than that produced by HOCl-I−.10 However, the production of iodophenols may be greater under specific reaction conditions. Certain iodophenols, such as 2,4,6-triiodophenol, have very limited solubility, and the production of these intermediates may contribute to the reduced amount of total organic carbon in the solution. Yong Feng Kaimin Shih* Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong SAR, China
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AUTHOR INFORMATION
Corresponding Author
*Phone: +852-2859-1973. Fax: +852-2559-5337. E-mail:
[email protected] (K. Shih). ORCID
Kaimin Shih: 0000-0002-6461-3207 Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.est.7b03417 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
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Correspondence/Rebuttal
REFERENCES
(1) Zhou, Y.; Li, J.; Jiang, J.; Gao, Y.; Yang, Y.; Pang, S. Y.; Ma, J.; Guan, C.; Wang, L. Iodine atom or hypoiodous acid? Comment on “Rapid selective circumneutral degradation of phenolic pollutants using peroxymonosulfate-iodide metal-free oxidation: Role of iodine atoms”. Environ. Sci. Technol. 2017, DOI: 10.1021/acs.est.7b02888. (2) Feng, Y.; Lee, P. H.; Wu, D.; Shih, K. Rapid selective circumneutral degradation of phenolic pollutants using peroxymonosulfate-iodide metal-free oxidation: Role of iodine atoms. Environ. Sci. Technol. 2017, 51 (4), 2312−2320. (3) Lente, G.; Kalmár, J.; Baranyai, Z.; Kun, A.; Kék, I.; Bajusz, D.; Takács, M.; Veres, L.; Fábián, I. One- versus two-electron oxidation with peroxomonosulfate ion: Reactions with iron(II), vanadium(IV), halide ions, and photoreaction with cerium(III). Inorg. Chem. 2009, 48 (4), 1763−1773. (4) Oh, W. D.; Dong, Z. L.; Lim, T. T. Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects. Appl. Catal., B 2016, 194, 169−201. (5) Milenković, M. C.; Stanisavljev, D. R. Role of free radicals in modeling the iodide-peroxide reaction mechanism. J. Phys. Chem. A 2012, 116 (23), 5541−5548. (6) Taube, H. The production of atomic iodine in the reaction of peroxides with iodide ion. J. Am. Chem. Soc. 1942, 64 (1), 161−165. (7) Popolan-Vaida, D. M.; Wilson, K. R.; Leone, S. R. Reaction of iodine atoms with submicrometer squalane and squalene droplets: Mechanistic insights into heterogeneous reactions. J. Phys. Chem. A 2014, 118 (45), 10688−10698. (8) Enami, S.; Hoffmann, M. R.; Colussi, A. J. Halogen Radical Chemistry at Aqueous Interfaces. J. Phys. Chem. A 2016, 120 (31), 6242−6248. (9) Bichsel, Y.; Von Gunten, U. Formation of iodo-trihalomethanes during disinfection and oxidation of iodide-containing waters. Environ. Sci. Technol. 2000, 34 (13), 2784−2791. (10) Li, J.; Jiang, J.; Zhou, Y.; Pang, S. Y.; Gao, Y.; Jiang, C.; Ma, J.; Jin, Y.; Yang, Y.; Liu, G.; Wang, L.; Guan, C. Kinetics of oxidation of iodide (I−) and hypoiodous acid (HOI) by peroxymonosulfate (PMS) and formation of iodinated products in the PMS/I−/NOM system. Environ. Sci. Technol. Lett. 2017, 4 (2), 76−82.
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DOI: 10.1021/acs.est.7b03417 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
