DFT Study on the Mechanism of As(III) Oxidation in the Presence of Fe(II) and O₂

In natural aquatic environments, the fate of arsenic (As) is significantly influenced by redox processes involving iron (Fe) species. Understanding the mechanisms governing As transformation in the presence of Fe species is crucial for comprehending its environmental impact and advancing remediation strategies. In this work, the oxidation of As(III) in oxygenated Fe(II) solutions was investigated. Density functional theory (DFT) methods were employed to explore the reaction of Fe(II) with ³O₂ and subsequent As(III) oxidation by reactive species generated from Fe(II) oxidation. Electron paramagnetic resonance analysis was utilized to confirm the formation of reactive species in the solution. Based on these results, it is concluded that ¹O₂, ·O₂H, and Fe(IV) are the critical oxidants responsible for As(III) oxidation in oxygenated Fe(II) solutions under circumneutral conditions. ¹O₂ readily oxidizes As(III) by forming an arsenic superoxide AsO₅H₃. Interaction of As(III) with ·O₂H or Fe(IV) leads to As(IV), which is further oxidized to As(V) by ³O₂, Fe(III), and Fe(IV).