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

Previous studies have shown that the corrosion of zerovalent iron (ZVI) by oxygen (O₂) via the Fenton reaction can lead to the oxidation of various organic and inorganic compounds. However, the nature of the oxidants involved (i.e., ferryl ion (Fe(IV)) versus hydroxyl radical (HO ^̇)) is still a controversial issue. In this work, we reevaluated the relative importance of these oxidants and their role in As(III) oxidation during the corrosion of nanoscale ZVI (nZVI) in air-saturated water. It was shown that Fe(IV) species could react with sulfoxides (e.g., dimethyl sulfoxide, methyl phenyl sulfoxide, and methyl p-tolyl sulfoxide) through a 2-electron transfer step producing corresponding sulfones, which markedly differed from their HO^̇-involved products. When using these sulfoxides as probe compounds, the formation of oxidation products indicative of HO^̇ but no generation of sulfone products supporting Fe(IV) participation were observed in the nZVI/O₂ system over a wide pH range. As(III) could be completely or partially oxidized by nZVI in air-saturated water. Addition of scavengers for solution-phase HO^̇ and/or Fe(IV) quenched As(III) oxidation at acidic pH but had little effect as solution pH increased, highlighting the importance of the heterogeneous iron surface reactions for As(III) oxidation at circumneutral pH.