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

• Trace Cu(II) activates S(IV) to oxidize As(III) under alkaline condition. • The produced Cu(I) complexes with S(IV) to form CuI(SO 3) n −2n+1 complexes. • The CuI(SO 3) n −2n+1 complexes react with O 2 to form Cu(III)-peroxo species. • Cu(III)-peroxo species and SO 4 •− are important oxidants in As(III) oxidation. 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 CuI(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 (1O 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 CuI(SO 3) n −2n+1 complexes also react with O 2 to form CuII-O 2 •−-(SO 3 2−) n superoxo species and CuIII-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. [ABSTRACT FROM AUTHOR]