Environ. Sci. Technol. 2008, 42, 8169
Response to Comment on “PolyoxometalateEnhanced Oxidation of Organic Compounds by Nanoparticulate Zero-Valent Iron and Ferrous Ion in the Presence of Oxygen” We appreciate the opportunity to respond to the comments of Jiang et al. (1) on our recent article (2). We believe that their first two comments are of minor importance and reflect a lack of understanding of our paper. Their third comment about the nature of the oxidant is more significant and we have provided some additional data to address the issue. The three issues are addressed in the sequence presented by Jiang et al. (1). Difference in the Oxidant Yield between the Fe(II)/O2 and nZVI/O2 Systems in the Presence of POM. We believe that the first comment originated from a misreading of our paper. On page 4925 (2), we stated, “the higher yield of HCHO in the nZVI/O2 system is probably due to the reaction of nZVI with oxygen (reactions 1-3)” to explain the slight difference in formaldehyde yield between the two systems at neutral pH values. For the reasons that are not apparent, Jiang et al. (1) erroneously assumed that we referred to acidic pH conditions. As explained in our paper, the higher yields of oxidants in the nZVI/O2 system under acidic pH conditions result from the fact that nZVI can transfer three electrons to POM, whereas Fe(II) has only one electron to transfer to POM. We already have shown that the nZVI/O2 system produces approximately 3 times higher concentrations of POM- than the Fe(II)/O2 system (Figure 3 in ref 2). While the rate constant for the Fenton reaction of Fe(II)-POM complexes may indeed be higher than that of uncomplexed Fe(II), it is not the main reason why higher yields are observed in the nZVI/O2 system. Loss of Oxidants by Coprecipitation of Fe(II) and Fe(III). Again, we believe that this comment reflects a misunderstanding of our paper. We agree with the statement from Jiang et al. (1) that the loss of oxidants rather than the loss of dissolved Fe(II) may be responsible for the low product yield observed in the Fe(II)/O2 system at neutral pH in the absence of POM. However, this is essentially the explanation that was given in our paper when we stated that precipitation of Fe(II) limited iron availability. It is our belief that the ferryl ion (Fe[IV]) produced in the vicinity of coprecipitates of Fe(II) and Fe(III) may be more easily consumed by reaction with Fe(II) than the Fe(IV) produced in the bulk solution. Nature of the Oxidant Produced from the Fenton reaction in the Presence of POM. The nature of the reactive oxidant produced by the Fenton reaction (•OH vs Fe[IV]) has been a controversial issue for decades (3). We agree with Jiang et al. (1) that oxidation of benzoic acid or 2-propanol is not incontrovertible evidence for the formation of •OH, and a more reactive Fe(IV) species may be generated in the presence of POM (ref 4 of ref 1). To provide more insight into the nature of the oxidant, we conducted additional experiments using Fenton’s reagent with an equimolar mixture of methanol and benzoic acid to compare the ratios of the oxidation products (formaldehyde (HCHO) and hydroxybenzoic acids (HBA)) under three different conditions: pH 2, pH 7, and pH 7 with POM (Figure 1). At pH 2, the ratio of HBA to HCHO (γHBA/HCHO ) 4.84 ( 0.79) was consistent with the ratio calculated from the reported rate constants for the reactions of benzoic acid and methanol with •OH (kbenzoic acid ) 4.3 × 109 M-1s-1, kbenzoate ) 5.9 × 109 M-1s-1, kmethanol ) 9.7 × 108 M-1s-1, kbenzoic acid/kmethanol ) 4.4; ref 32 in ref 2). The γHBA/HCHO value at pH 7 was only 0.47 ( 0.070 because Fe(IV) has a lower reactivity with benzoic acid
10.1021/es802169g CCC: $40.75
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2008 American Chemical Society
FIGURE 1. Production of HBA and HCHO, and ratio of HBA to HCHO ([Fe(II)]0 ) 20 µM; [H2O2]0 ) 50 µM; [benzoic acid]0 ) [methanol]0 ) 20 mM; [POM]0 ) 0.1 mM; [PIPES]0 ) 1 mM; reaction time ) 30 min; Total HBA yields ([HBA]t) were calculated from the yields of p-HBA and the reported product ratio for the three isomers of hydroxybenzoic acid (ortho:meta:para ) 36:34:30; ref 5). (or benzoate; pKa ) 4.2) than methanol. The addition of POM at pH 7 increased the γHBA/HCHO value to 1.35 ( 0.15, but this value was still much lower than the expected value of 6.1 () kbenzoate/kmethanol), which suggests that the reactive oxidant is not free •OH. Possible explanations for this discrepancy include the simultaneous formation of Fe(IV) and •OH (ref 36 in ref 2), production of caged •OH (4), or formation of a more reactive Fe(IV)-POM species as suggested by Jiang et al. (1). The lack of information on the reactivity of Fe(IV) at neutral pH as noted by Jiang et al. (1) prevents us from reaching any conclusion on this issue. Despite our inability to definitely identify the oxidant(s), our study (2) has important implications for contaminant remediation because the introduction of POM to the nZVI/O2 or the Fe(II)/O2 system under neutral pH conditions converts a relatively unreactive oxidant (most likely Fe[IV]) into a species that is better suited for oxidation of recalcitrant aromatic compounds.
Literature Cited (1) Jiang, J.; Pang, S.; Ma, J. Comment on “Polyoxometalateenhanced oxidation of organic compounds by nanoparticulate zero-valent iron and ferrous ion in the presence of oxygen”. Environ. Sci. Technol. 2008, 42, 8170. (2) Lee, C.; Keenan, C. R.; Sedlak, D. L. Polyoxometalate-enhanced oxidation of organic compounds by nanoparticulate zerovalent iron and ferrous ion in the presence of oxygen. Environ. Sci. Technol. 2008, 42, 4921–4926. (3) Goldstein, S.; Meyerstein, D.; Czapski, G. The Fenton reagents. Free Radical Biol. Med. 1993, 15, 435–445. (4) Walling, C.; Amarnath, K. Oxidation of mandelic acid by Fenton’s reagent. J. Am. Chem. Soc. 1982, 104, 1185–1189. (5) Zhou, X. L.; Mopper, K. Determination of photochemically produced hydroxyl radicals in seawater and freshwater. Mar. Chem. 1990, 30, 71–88.
Changha Lee, Christina R. Keenan, and David L. Sedlak Department of Civil and Environmental Engineering, 657 Davis Hall, University of California, Berkeley, California 94720 ES802169G
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