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.
(Copyright © 2022 Elsevier Ltd. All rights reserved.)