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

Sulfate radical (SO 4 •− ) 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 SO 3 •− and the Cu I (SO 3 ) n −2n+1 complexes. Quenching experiments rule out contributions of SO 5 •− , peroxymonosulfate (HSO 5 − , PMS), hydroxyl radical ( • OH), superoxide radical (O 2 •− ) and singlet oxygen ( 1 O 2 ) 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 SO 5 •− , PMS and SO 4 •− generated via a series of reactions between SO 3 •− and O 2 , the Cu I (SO 3 ) n −2n+1 complexes also react with O 2 to form Cu II -O 2 •− -(SO 3 2− ) n superoxo species and Cu III -O 2 2− -(SO 3 2− ) 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 SO 4 •− 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.