Correspondence/Rebuttal pubs.acs.org/est
Iodine Atom or Hypoiodous Acid? Comment on “Rapid Selective Circumneutral Degradation of Phenolic Pollutants Using Peroxymonosulfate−Iodide Metal-Free Oxidation: Role of Iodine Atoms” phenol requires at least 854 μM PMS, which is much higher than the concentration of PMS (325 μM) used by Feng et al.
V
ery recently, Feng et al.1 proposed a novel and green oxidation process by the combination of peroxymonosulfate (PMS) and iodide ion (I−) for rapid and selective degradation of phenolic contaminants such as phenol and bisphenol A (BPA). These authors conclude that iodine atom (I•) rather than hypoiodous acid (HOI) formed in situ (eq 1 vs 2) plays a dominant role, and I• reacts with these phenols through electron transfer rather than an electrophilic substitution pathway leading to the negligible formation of iodinated products. • − HSO−5 + I− → SO•− 4 + I + OH
(1)
HSO−5 + I− → HOI + SO24 −
(2)
I−
14HSO−5 + C6H6O → 14SO24 − + 14H+ + 6CO2 + 3H 2O (3)
III. ROLE OF SO4•− According to the mechanism (eq 1) proposed by Feng et al., SO4•− should be produced from the reaction of PMS with I− accompanying the generation of I•. Then, SO4•− can react with I− again to generate I• (eq 4) or competitively oxidize phenols (eq 5).
This seems likely to be in contrast to our recent finding that appreciable amounts of iodinated products can be generated in the PMS/I− system containing natural organic matters (NOM), where HOI is considered as the dominant reactive species.2 We believe that the experimental evidence presented by Feng et al. to rule out the involvement of HOI in the PMS/I−/phenols system is not convincing.
(4)
SO•− 4
(5)
+ phenols → products
Feng et al. found that the addition of methanol in great excess (up to 2.5 M) negligibly inhibits the degradation of phenol or BPA. The authors interpreted this result as the complete trap of SO4•− by I− to generate I• which was unreactive toward methanol.9 However, this explanation seems somewhat strange by considering the scavenging capacities of various constituents for SO4•− in the PMS/I−/phenol/ methanol system. For example, the scavenging capacity of methanol at 2.5 M for SO4•− is calculated to be 0.8−2.43 × 107 s−1 (the second-order rate constant for SO4•− with methanol of 3.2−9.7 × 106 M−1 s−1 given in ref 1 is adopted),10 which is nearly 12−36 times higher than the sum of the scavenging capacity of phenol at 20 μM (1.76 × 105 s−1, the second-order rate constant for SO4•− with phenol is 8.8 × 109 M−1 s−1)11 and I− at 50 μM (5 × 105 s−1, the second-order rate constant for SO4•− with I− is not available and assumed to be as high as 1010 M−1 s−1). This means that methanol in excess (2.5 M) can outcompete I− and phenol for SO4•− and thus should significantly inhibit the degradation of phenol even if PMS reacting with I− experiences I• rather than HOI pathway. However, this prediction is inconsistent with the experimental results presented by Feng et al.1 In fact, the negligible effect of methanol provides another evidence to support the involvement of HOI since HOI is unreactive toward methanol.12
I. EFFECT OF AMMONIUM Feng et al. stated that ammonium (NH4+) was a potential scavenger of HOI. To examine the role of HOI, the effect of NH4+ (up to 10 mM) on the degradation rate of phenol (25 μM) by the combination of PMS (65 μM) and I− (50 μM) at pH 6 was investigated by these authors. On the basis of the finding of the negligible effect of NH4+, they suggested that other active species, instead of HOI, existed in the PMS-I− oxidation. However, it has been already demonstrated that NH4+ is unreactive toward HOI, in contrast to the cases of chlorine and bromine which can be rapidly transformed by NH4+ to haloamines (i.e., chloramines and bromamines) of relatively low reactivity.3−6 So, the negligible influence of NH4+ cannot serve as a reliable line of experimental evidence to exclude the involvement of HOI in the PMS/I− system. II. TOTAL ORGANIC CARBON (TOC) REMOVAL Feng et al. found that about 61% of TOC was mineralized when 100 μM phenol was treated by 325 μM PMS in the presence of 325 μM I− at pH 6.0 for 10 min. This result was interpreted as another evidence against the involvement of HOI because HOI is only capable of transferring phenol to iodophenols, which cannot cause rapid TOC removal. However, we do not believe these data on TOC removal are reliable by considering the stoichiometry of the mineralization of phenol by PMS catalyzed by I− (eq 3) even if I• is involved and mineralization occurs.7,8 According to eq 3, the mineralization of 61 μM © XXXX American Chemical Society
2− − • SO•− 4 + I → SO4 + I
IV. FORMATION OF IODINATED AROMATIC PRODUCTS Feng et al. examined the formation of iodinated aromatic products (i.e., 2-iodophenol, 3-iodophenol, and 2,4,6-triiodophenol) after the complete degradation of 10 μM phenol in the presence of 65 μM PMS and 50 μM I− at pH 6.0 (i.e., at a specific reaction time of 4 min), and they found that the yield
A
DOI: 10.1021/acs.est.7b02888 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
and Foundation for the Author of National Excellent Doctoral Dissertation of China (201346).
of total iodophenols was less than 2.0%. The authors stated that the production of halogenated byproducts should not be a serious concern when using PMS-I− oxidation for phenol degradation. However, we believe that the negligible formation of iodophenols is attributed to their further transformation by excess HOI (eqs 6 and 7) when considering the molar ratio of [PMS]: [I−]: [phenol] = 6.5:5:1 and the relatively high reactivity of HOI with iodophenols.12 HOI + phenol → iodophenols
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(1) Feng, Y.; Lee, P.; 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. (2) Li, J.; Jiang, J.; Zhou, Y.; Pang, S.; 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. (3) Greyshock, A. E.; Vikesland, P. J. Triclosan reactivity in chloraminated waters. Environ. Sci. Technol. 2006, 40 (8), 2615−2622. (4) Johnson, J. D.; Overby, R. Bromine and bromamine disinfection chemistry. J. Sanit. Eng. Div. 1971, 97 (5), 617−628. (5) Wolfe, R. L.; Ward, N. R.; Olson, B. H. Inorganic chloramines as drinking water disinfectants: A review. J. Am. Water Works Assoc. 1984, 76 (5), 74−88. (6) Bichsel, Y.; von Gunten, U. Oxidation of iodide and hypoiodous acid in the disinfection of natural waters. Environ. Sci. Technol. 1999, 33 (22), 4040−4045. (7) Luo, W.; Zhu, L.; Wang, N.; Tang, H.; Cao, M.; She, Y. Efficient removal of organic pollutants with magnetic nanoscaled BiFeO3 as a reusable heterogeneous Fenton-like catalyst. Environ. Sci. Technol. 2010, 44 (5), 1786−1791. (8) Lyu, L.; Zhang, L.; Wang, Q.; Nie, Y.; Hu, C. Enhanced fenton catalytic efficiency of γ-Cu-Al2O3 by σ-Cu2+-ligand complexes from aromatic pollutant degradation. Environ. Sci. Technol. 2015, 49 (14), 8639−8647. (9) Alfassi, Z. B.; Huie, R. E.; Marguet, S.; Natarajan, E.; Neta, P. Rate constants for reactions of iodine atoms in solution. Int. J. Chem. Kinet. 1995, 27 (2), 181−188. (10) Neta, P.; Huie, R. E.; Ross, A. B. Rate constants for reactions of inorganic radicals in aqueous solution. J. Phys. Chem. Ref. Data 1988, 17 (3), 1027−1284. (11) Ziajka, J.; Pasiuk-Bronikowska, W. Rate constants for atmospheric trace organics scavenging SO4•‑ in the Fe-catalysed autoxidation of S(IV). Atmos. Environ. 2005, 39 (8), 1431−1438. (12) 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. (13) Liu, K.; Lu, J.; Ji, Y. Formation of brominated disinfection byproducts and bromate in cobalt catalyzed peroxymonosulfate oxidation of phenol. Water Res. 2015, 84, 1−7. (14) 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.
(6)
HOI + iodophenols → small molecule iodinated products (7)
Similarly, a recent study has demonstrated the formation of bromophenols and their further transformation to small molecular brominated byproducts (e.g., bromoforms and bromoacetic acids) in the PMS/Br−/phenol system.13 In summary, the experimental data provided by Feng et al. cannot sufficiently support the presence of I•, but otherwise suggest that HOI is the primary oxidizing species in the PMS/ I− system. Actually, Lente et al.14 have thermodynamically demonstrated that halide ions (I− and Br−) are oxidized by PMS in a formal two-electron process, which most likely includes oxygen-atom transfer. Accordingly, the formation of halogenated byproducts of concern during water treatment by the combination of PMS and halide ions (I− and Br−) or during treatment of I− and Br− containing water by PMS should receive great attentions.
Yang Zhou† Juan Li† Jin Jiang*,† Yuan Gao† Yi Yang† Su-Yan Pang‡ Jun Ma† Chaoting Guan† Lihong Wang† †
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REFERENCES
State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China ‡ College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China
AUTHOR INFORMATION
Corresponding Author
*Phone: 86-451-86283010. Fax: 86-451-86283010. E-mails:
[email protected],
[email protected] (J. Jiang). ORCID
Jin Jiang: 0000-0003-2355-4244 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledged the financial support by the National Key Research and Development Program (2016YFC0401107), National Natural Science Foundation of China (51578203), Funds of the State Key Laboratory of Urban Water Resource and Environment (HIT, 2016DX13), B
DOI: 10.1021/acs.est.7b02888 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
