Environ. Sci. Technol. 2008, 42, 5377
Comment on “Factors Affecting the Yield of Oxidants from the Reaction of Nanoparticulate Zero-Valent Iron and Oxygen” In a recent study (1), Keenan and Sedlak investigated the oxidation of four probe compounds (i.e., methanol, ethanol, 2-propanol, and benzoic acid) by the reaction of nanoparticulate zero-valent iron (Fe0(S)) and oxygen (O2) over a wide range of pH. Their findings are very interesting and advance our understanding on the reaction mechanism and the nature of the oxidants formed during the corrosion of Fe0(S) by O2. However, there are several questions related to their research. In discussion, the authors stated that “similar ratios of products were found in Fenton control experiments (Figure S1 of the original manuscript), suggesting that the different yields are not due to the presence of Fe0(S) surfaces.” For the following reason we think that this statement is not sufficiently supported by their experimental results. Reanalyzing the data presented in that paper, we found that in pH 3 Fe0(S)/O2 system, the product yields in the decreasing order were 2-propanol (acetone, R ) 11.9) > benzoic acid (parahydroxybenzoic acid, R ) 2.4) > methanol (formaldehyde, R ) 1.0) > ethanol (acetaldehyde, R ) 0.65; in parentheses were the primary product from the reaction of a probe compound with HO• and the molar ratio (R) of the product yield to the formaldehyde yield); whereas, the product yields in pH 3 Fenton control were 2-propanol (R ) 11.3) > ethanol (R ≈ 2.2) > benzoic acid (R ) 1.4) > methanol (R ) 1.0). Clearly, the R values (especially the orders of these values) obtained in the Fe0(S)/O2 system and the Fenton control were not “similar” but different. At pH 3, the Fenton reaction exclusively produces HO• with an equivalent yield per hydrogen peroxide (H2O2), which can be efficiently scavenged (>98%) by the probe compounds employed in the Fenton control experiments (1). The reactions of HO• with the probe compounds in the presence of oxygen usually proceed via peroxyl radicals, and the further reactions of these radicals can result in the formation of primary products but also H2O2 (2, 3). The yield of H2O2 per mole of HO• (
