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Response to Comment on “Oxidation of Sulfoxides and Arsenic(III) in Corrosion of Nanoscale Zero Valent Iron by Oxygen: Evidence against Ferryl Ions (Fe(IV)) as Active Intermediates in Fenton Reaction” n a recent article,1 we have developed a method using several sulfoxides as indicators to distinguish between HO• and Fe(IV), based on the finding that Fe(IV) species could react with these compounds through a 2-electron transfer step producing corresponding sulfones, which markedly differed from their HO•involved products. By measuring the oxidation products of these sulfoxides, we tested the hypothesis that the nature of the oxidants produced during the corrosion of nanoscale zerovalent iron (nZVI or Fe0) by oxygen (O2) via the Fenton reaction changed with pH from HO• at low pH to a less reactive species Fe(IV) at neutral pH values. However, the finding that no sulfone products were generated in the nZVI/O2 system over a wide pH range allowed us definitely ruling out Fe(IV) as an active Fenton intermediate. We gratefully appreciate the critical comments of Remucal et al.2 on our article and welcome the opportunity to respond. The major point of their arguments was the discrepancy between their data and measured rate constants for Fe(IV) or HO•. For the following reasons, however, we believe that this issue held by these authors as the evidence for a “third” oxidant is likely related to the complex reactions of HO•. In other words, their data may be reasonably explained by taking peroxyl radical chemistry into account but without assuming a different oxidant rather than Fe(IV) or HO•.
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’ PRODUCTION OF METHANESULFINIC/METHANESULFONIC ACID FROM OXIDATION OF DMSO BY FENTON’S REAGENT In Fenton control experiments, Remucal et al.2 observed that formaldehyde production from dimethyl sulfoxide (DMSO) was negligibly affected at pH 3 but significantly increased at pH 7 by the addition of methanol ([DMSO]/[methanol] = 1). This result was interpreted as: the oxidant produced at pH 3 (i.e., HO•) was scavenged efficiently by DMSO, whereas methanol outcompeted DMSO at pH 7, contrary to expectations if FeO2þ or HO• were the main oxidants at circumneutral pH values.2 HO• reacts with DMSO mainly by abstracting methyl from sulfur producing methanesulfinic acid and methyl radical. Further reactions of methylperoxyl radical formed by the reaction of methyl radical with O2 at diffusion-controlled rate can produce formaldehyde.1 It is worth noting that partial conversion of methanesulfinic acid to methanesulfonic acid may occur in advanced oxidation processes, possibly due to the autoxidation induced by methylperoxyl radicals.3 Reconducting the Fenton control experiments, we found that MSA production (i.e., [methanesulfinic acid]þ[methanesulfonic acid]) from DMSO was not significantly affected by the presence of methanol at both pH values (Figure 1), consistent with their relative reactivity toward HO•. The exact reasons for the discrepancy between two studies are not clear, which may involve the interference of methanol with the mechanism of formaldehyde production between HO• and DMSO. In addition, it seems likely that the negligence of the important case by Sedlak et al. that the reaction of HO• with DMSO also produces formaldehyde can r 2011 American Chemical Society
Figure 1. Production of MSA from oxidation of DMSO by Fenton’s reagent ([Fe(II)]0 = 100 μM; [H2O2]0 = 1 mM; [DMSO] = [Methanol(MeOH)] = 200 mM; reaction time = 60 min).
account for the inconsistency of their data with the relative reactivity of DMSO and methanol toward HO•(ref 6 in ref 2).
’ COMPLEX REACTIONS OF METHYLPEROXYL RADICALS IN THE PRESENCE OF METHANOL IN THE NZVI/ O2 SYSTEM Important reactions of methylperoxyl radicals and their derived active species in the nZVI/O2 system include: (i) bimolecular decay of methylperoxyl radicals through a tetroxide intermediate, (ii) reaction with HO2•/O2•- or Fe2þ of methylperoxyl radicals, (iii) Fenton-like reaction with Fe2þ or selfdecomposition of methylhydroperoxide, and (iv) rearrangement, reaction with Fe2þ, or H-abstraction from methanol of methoxyl radical (e.g., refs 3,4). In the presence of O2, hydroxymethyl radicals formed in Case (iv) (probably mainly from methanol) are rapidly converted into the corresponding peroxyl radicals, which follow a similar pathway to that of methylperoxyl radicals finally forming various stable products including formaldehyde.5 In these regards, methanol-enhanced production of formaldehyde2 may suggest that the yield of formaldehyde per mole of hydroxymethyl peroxyl radical is much higher than that of methylperoxyl radical at neutral pH in the presence of iron, consistent with our conclusion.1 However, this proposition or other possible mechanisms (e.g., the cross reaction between primary methylperoxyl radicals and secondary hydroxymethylperoxyl radicals4) warrant further investigations. ’ SOME MINOR POINTS NEED TO BE ADDRESSED Since the acid-base property of ferryl(IV) ion is unknown so far, we use the formula FeIVO2þ (i.e., Fe(IV)) to represent all forms of this species in solution, similar to the case of ferrate(VI) Published: March 04, 2011 3179
dx.doi.org/10.1021/es2004304 | Environ. Sci. Technol. 2011, 45, 3179–3180
Environmental Science & Technology
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(i.e., [Fe(VI)] = [H2FeO4] þ [HFeO4-] þ [FeO42-]). One possible explanation for the out-competition of Fe(II) with sulfoxides for Fe(IV) at pH 8-9 (contrary to the trend at acidic pH values) is that the increase of pH increases the reaction rate of Fe(II) with Fe(IV) in relative to neutral sulfoxide molecules probably due to the formation of Fe(II) hydroxo complexes (like other oxidants such as O2, H2O2, and O3). It is argued that the pH-dependence of product formation between the compounds and HO• under oxic condition is strongly dependent on the nature of peroxyl radical and solution chemistry (e.g., presence or absence of iron redox species) (e.g., refs 4,5). The validity of the proposition of Vermilyea and Voelker (ref 9 in ref 1) that Fe(II) and/or reducing moieties of fulvic acid decreased the yield of phenol from the reaction of HO• with benzene by reacting with organic radical intermediate I needs more studies. Su-Yan Pang, Jin Jiang,* and Jun Ma* State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, China
’ AUTHOR INFORMATION Corresponding Author
*Phone: 86-451-86283010; fax: 86-451-82368074; e-mail: majun@ hit.edu.cn. (J. M.);
[email protected] (J. J.).
’ ACKNOWLEDGMENT This study was funded by the Open Project of Key State Laboratory of Urban Water Resource and Environment, HIT (QA201012, 2010DX10), and the National Natural Science Foundation of China (51008104). ’ REFERENCES (1) Pang, S.-Y.; Jiang, J.; Ma, J. Oxidation of sulfoxides and arsenic(III) in corrosion of nanoscale zero valent iron by oxygen: Evidence against ferryl ions (Fe(IV)) as active intermediates in Fenton reaction. Environ. Sci. Technol. 2011, 45, 307–312. (2) Remucal, C. K.; Lee, C.; Sedlak, D. L. Comment on “Oxidation of sulfoxides and arsenic(III) in corrosion of nanoscale zero valent iron by oxygen: Evidence against ferryl ions (Fe(IV)) as active intermediates in Fenton reaction. Environ. Sci. Technol. 2011, 10.1021/es104399p. (3) Flyunt, R.; Leitzke, A.; Mark, G.; Mvula, E.; Reisz, E.; Schick, R.; von Sonntag, C. Determination of •OH, O2•-, and hydroperoxide yields in ozone reactions in aqueous solution. J. Phys. Chem. B 2003, 107, 7242–7253. (4) Schuchmann, H.-P.; von Sonntag, C. Methylperoxyl radicals: a study of the γ-radiolysis of methane in oxygenated aqueous solutions. Z. Naturforsch. 1984, 39b, 217–221. (5) Monod, A.; Chebbi, A.; Durand-Jolibois, R.; Carlier, P. Oxidation of methanol by hydroxyl radicals in aqueous solution under simulated cloud droplet conditions. Atmos. Environ. 2000, 34, 5283–5294.
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