Co-oxidation of As(III) and Fe(II) by oxygen through complexation between As(III) and Fe(II)/Fe(III) species

Previous studies have shown that the oxidation of Fe(II) by molecular oxygen can lead to the co-oxidation of As(III) at neutral pH. However, the mechanism of As(III) oxidation in the presence of Fe(II) with respect to the interaction between As(III) and Fe(II) is still unclear. In this work, we examined the oxidation of As(III) in the presence of Fe(II) in air-saturated water, which is affected by pH, co-existing phosphate, and scavengers (tert-butanol, superoxide dismutase, catalase and dimethylsulfoxide). Results confirm that the formation of the Fe(II)-As(III) complex (formation constant, K_{Fe(II)-As(III)} = 10^3.86 M^-1) plays an extremely important role in the initial stage of As(III) oxidation at pH 7.25. The oxidation of Fe(II)-As(III) promotes the production of H₂O₂ and colloidal ferric hydroxide. H₂O₂ reacts with the Fe(II)-As(III) complex through oxidizing Fe(II) to Fe(IV), which then causes the partial oxidation (ca. 50%) of As(III). The other part of As(III) oxidation by H₂O₂ occurs, in the form of Fe(III)-As(III) complex, through direct electron transfer from As(III) to H₂O₂ but not through Fe(IV). This work provides new mechanistic insight into arsenic and iron redox chemistry in the environment and furthers our understanding of Fenton reactions at neutral pH.