Assessing the Photocatalytic Reduction of Cr(VI) by CuO in Combination with Different Organic Acids

Cupric oxide (CuO) photocatalytic reduction of chromium(VI) [Cr(VI)] by organic acids including tartaric acid, citric acid, malic acid, lactic acid, and succinic acid was investigated at pH 3 and 25 °C through batch experiments. The results demonstrate that the removal of Cr(VI) by organic acids assisted with CuO could markedly be improved under the irradiation of simulated solar light. The removal rates of Cr(VI) were in the order: tartaric acid (with two α-OH groups and two -COOH groups) > citric acid (with one α-OH group and three -COOH groups) > malic acid (with one α-OH group and two -COOH groups) > lactic acid (with one α-OH group and one -COOH group) > succinic acid (with no α-OH group and two -COOH groups), suggesting that the reductive efficiency of Cr(VI) strongly depends on the number of α-OH groups in organic acids. A possible mechanism of CuO photocatalytic reduction of Cr(VI) by organic acids is proposed. CuO is firstly dissolved to release Cu(II), and then form Cu(II)-organic acid complexes. Furthermore, Cu(I) and active intermediates are produced through a ligand-to-metal charge-transfer pathway under irradiation, responsible for the rapid reduction of Cr(VI). In addition, the impact factors for CuO photocatalytic reduction of Cr(VI) by tartaric acid were further examined. High CuO loading and tartaric acid concentration, and low pH in the reaction system were conducive to the reduction of Cr(VI), and the removal of Cr(VI) obeyed to a pseudo zero-order kinetic model.