Important contribution of Cu(III)-peroxo species to As(III) oxidation in the Cu(II)-S(IV) system

Abstract

Sulfate radical (SO4•−) is always thought to be an important oxidizing species in the metal-sulfite [S(IV)] systems, however, the oxidizing species correlated with the metal species have rarely been mentioned. In this study, the oxidizing species that oxidize arsenite [As(III)] in the Cu(II)-S(IV) system were investigated under alkaline conditions. Kinetic results indicate that complexation between Cu(II) and S(IV) is the pre-requisite to initiate various oxidizing species, followed by internal electron transfer from S(IV) to Cu(II) with the formation of SO3•− and the CuI(SO3)n−2n+1 complexes. Quenching experiments rule out contributions of SO5•−, peroxymonosulfate (HSO5−, PMS), hydroxyl radical (•OH), superoxide radical (O2•−) and singlet oxygen (1O2) to As(III) oxidation in the Cu(II)-S(IV) system and indicate that some kinds of peroxo species contribute. On the other side, apart from the oxidizing species such as SO5•−, PMS and SO4•− generated via a series of reactions between SO3•− and O2, the CuI(SO3)n−2n+1 complexes also react with O2 to form CuII-O2•−-(SO32−)n superoxo species and CuIII-O22−-(SO32−)n peroxo species, as revealed by Raman analysis. UV–vis spectrum further confirms the presence of Cu(III) in the Cu(II)-S(IV) system. We therefore suggest that both SO4•− and the Cu(III)-peroxo species play important roles in the oxidation of As(III) in the Cu(II)-S(IV) system. This study enriches our understanding on the diversity of oxidizing species in the metal-S(IV) systems under alkaline conditions.