The fast redox cycle of Cu(II)–Cu(I)–Cu(II) in the reduction of Cr(VI) by the Cu(II)-thiosulfate system

Thiosulfate (S 2 O 3 2− ) is an important ligand to complex metal cations, however, the reactivity of metal-thiosulfate complexes has barely been mentioned. In this study, the reactivity of the Cu(II)–S 2 O 3 2− system in the reduction of Cr(VI) was investigated. Kinetic results show that the reduction rates of Cr(VI) decrease with increasing pH values from 3.0 to 5.0, and 94.3% and 97.5% of 10 mg L −1 Cr(VI) was rapidly reduced within 1 min at pH 3.0 and within 30 min at pH 5.0, respectively at the molar ratio of Cu(II):S 2 O 3 2− of 0.05. We rule out the contributions of S species of tetrathionate (S 4 O 6 2− ) and sulfite (SO 3 2− ) to Cr(VI) reduction and point out that the produced Cu(I) in the Cu(II)–S 2 O 3 2− system is the key reductant that mediates the reduction of Cr(VI). We suggest that complexation between Cu(II) and S 2 O 3 2− with the formation of Cu II (S 2 O 3 ) 2 2− is the pre-requisite for the formation of Cu I (S 2 O 3 ) n 1−2n , which plays an important role in Cr(VI) reduction, accompanied by the re-oxidation of Cu(I) to Cu(II) by Cr(VI), achieving the rapid redox cycling of Cu(II)–Cu(I)–Cu(II). Such a redox cycle also mediates the denitrification process of NO 2 − to NH 3 /NH 4 + under weakly acidic conditions. This study enriches our understanding on the reducing reactivity of the Cu(II)–S 2 O 3 2− system and the importance of the Cu(II)–Cu(I)–Cu(II) redox cycle towards environmental oxidizing contaminants.