Environ. Sci. Technol. 2008, 42, 5378
Response to Comment on “Factors Affecting the Yield of Oxidants from the Reaction of Nanoparticulate Zero-Valent Iron and Oxygen” We appreciate the interest of Jiang et al. (1) in our recent article (2) and welcome the opportunity to respond. Although their comments provide insight into the complex chemistry of iron and reactive oxygen species, they do not change the interpretation of our results or our conclusions. Our study addressed the production of oxidants by the reaction of nanoparticulate zero-valent iron (nZVI, Fe0(s)) with oxygen and the potential for using the reaction for the transformation of organic pollutants. By measuring the oxidation products of four probe compounds, we tested the hypothesis that the nature of the oxidants produced in this system changed with pH from hydroxyl radical at low pH to another less reactive species at neutral pH values. The comments of Jiang et al. (1) mainly focused on the lower-than-expected production of oxidation products at low pH in a Fenton control experiment and apparent discrepancies between our data and measured rate constants for Fe(IV) (e.g., ref 2 in ref 1, ref 18 in ref 2). We address these issues in the following paragraphs. Probe Compound Reactivity at pH 3. In a series of experiments in which alcohols or organic acids were present at high concentrations, we reported product yields that varied by approximately an order of magnitude at acidic pH values (Figure 5 in ref 2), even though the published rate constants for the reaction of the parent compounds with hydroxyl radical (OH•) are similar (∼109 M-1 s-1; Table 1 in ref 2). In particular, the yield of formaldehyde (HCHO; the product of methanol oxidation) was noticeably lower than the yields of acetone (the product of 2-propanol oxidation) and parahydroxybenzoic acid (pHBA; one of the three main products of benzoic acid oxidation) at pH 3. In an effort to explain the difference in yields and determine if the presence of nZVI was responsible, we conducted experiments using Fenton’s reagent as a source of oxidant (Figure S1 in ref 2). We found that the ratios of acetone and pHBA to HCHO were qualitatively similar in the two experiments (i.e., acetone/ HCHO was 11.9 in the nZVI experiment and 11.3 in the Fenton’s reagent experiment, while pHBA/HCHO was 2.4 in the nZVI experiment and 1.4 in the Fenton’s reagent experiment). The main purpose of this comparison was to confirm that the lower-than-expected HCHO yield was related to the reaction of methanol with reactive oxygen species and not a reaction occurring on the nZVI surface. Jiang et al. (1) objected to our use of the qualitative description “similar” to compare the product ratios presented in Figure S1 of ref 2. Their comment seems to reflect a misunderstanding of our intended point. We agree with Jiang et al. (1) that the modest difference in the relative product yields may be related to the behavior of the intermediate species and that further research is needed to elucidate the effect of Fe2+ and Fe0(s) on the reaction mechanisms. However,
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those issues were beyond the scope of this paper, which focused on characterizing the effect of pH on oxidants produced during the corrosion of zero-valent iron and the potential for using the reactions for contaminant remediation. The oxidation of all four probe compounds under acidic conditions supports our hypothesis that OH• is produced at low pH, whereas as the decreased oxidation of benzoic acid and 2-propanol at elevated pH values supports our hypothesis that a more selective oxidant is produced as pH increases. Jiang et al. (1) make a valid point that additional H2O2 will be produced during the oxidation process and that the fate of H2O2 is likely to be different in these two experimental systems, with more OH• production in the Fenton system. However, it is worth noting that nZVI was oxidized to Fe(II) very quickly at pH 3 (i.e., ∼90% of nZVI was oxidized after 5 min, the earliest time point; Figure 1 in ref 2) and very little Fe0(s) would be present to react with H2O2 formed from oxidation of the probe compounds. HCHO production lagged behind slightly and was 90% complete after ∼7 min. Fe(IV) Reactivity. The ferryl ion (Fe[IV]) has been proposed as an alternate oxidant in the Fenton reaction (e.g., ref 43 in ref 2). However, the reactivity of Fe(IV) with organic compounds has not been studied in detail for many compounds. To the best of our knowledge, rate constants for Fe(IV) have been determined by reacting Fe2+ and ozone at pH 0-3 (e.g., ref 2 in ref 1, ref 18 in ref 2). It may be more appropriate to only compare our system to qualitative observations in more relevant studies conducted near neutral pH (e.g., refs 11 and 14 in ref 2) where no rate constants are available, rather than extrapolating known rate constants at extremely acidic pH values. We agree with Jiang et al. (1) that deprotonation of benzoic acid could affect its reactivity, but do not believe that it is appropriate to extrapolate these changes on the basis of experiments with formic acid. In the absence of more data on Fe(IV) reactivity at neutral pH values, we are unable to resolve the interesting issues raised by Jiang et al. (1). Nevertheless, their comments do not change our conclusion that the formation of oxidation products from the four probe compounds under neutral pH conditions cannot be explained by the presence of OH• and that an alternate oxidant, such as Fe(IV), may be responsible.
Literature Cited (1) Jiang, J.; Pang, S.; Ma, J. Comment on “Factors affecting the yield of oxidants from the reaction of nanoparticulate zerovalent iron and oxygen”. Environ. Sci. Technol. 2008, (14), 5377. (2) Keenan, C. R.; Sedlak, D. L. Factors affecting the yield of oxidants from the reaction of nanoparticulate zero-valent iron and oxygen. Environ. Sci. Technol. 2008, 42, 1262–1267.
Christina R. Keenan and David L. Sedlak Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, California 94720 ES801387S
10.1021/es801387s CCC: $40.75
2008 American Chemical Society
Published on Web 06/07/2008
