Catalytic role of Cu(II) in the reduction of Cr(VI) by citric acid under an irradiation of simulated solar light

• Cu(II) can markedly improve the reduction of Cr(VI) by citric acid with simulated solar light.
• The optimal removal of Cr(VI) by citric acid with simulated solar light is realized at pH 4.
• The catalytic role of Cu(II) in the reduction of Cr(VI) is ascribed to the formation of Cu(II)–citric acid complex.
• α-OH group in organic acids was the key factor for the Cu(II) photocatalytic reduction of Cr(VI).

The catalytic role of Cu(II) in the reduction of Cr(VI) by citric acid with simulated solar light was investigated. The results demonstrated that Cu(II) could significantly accelerate Cr(VI) reduction and the reaction obeyed to pseudo zero-order kinetics with respect to Cr(VI). The removal of Cr(VI) was related to the initial concentrations of Cu(II), citric acid, and the types of organic acids. The optimal removal of Cr(VI) was achieved at pH 4, and the rates of Cu(II) photocatalytic reduction of Cr(VI) by organic acids were in the order: tartaric acid (two α-OH groups, two –COOH groups) > citric acid (one α-OH group, three –COOH groups) > malic acid (one α-OH group, two –COOH groups) > lactic acid (one α-OH group, one –COOH group) ≫ succinic acid (two –COOH groups), suggesting that the number of α-OH was the key factor for the reaction, followed by the number of –COOH. The formation of Cu(II)–citric acid complex could generate Cu(I) and radicals through a pathway of metal–ligand–electron transfer, promoting the reduction of Cr(VI). This study is helpful to fully understanding the conversion of Cr(VI) in the existence of both organic acids and Cu(II) with solar light in aquatic environments.