Photoredox of Cr(Ⅲ)-Malate Complex and Its Impacting Factors

摘要

The transformation of less toxic Cr(Ⅲ) species to harmful Cr(Ⅵ) is worth concerning. Compared with free Cr(Ⅲ), however, the photo-oxidation of Cr(Ⅲ)-organic acid complexes is seldom reported. In this study, Cr(Ⅲ)-malate complex was synthesized and purified, and its photo-oxidation was investigated to reveal the potential conversion pathway of Cr(Ⅲ) to Cr(Ⅵ). The results indicated that Cr(Ⅲ)-malate complex could be gradually photo-oxidized to Cr(Ⅵ) through a ligand-metal charge transfer path. Higher pH and stronger light intensity promoted the conversion process. A 50-μM Cr(Ⅲ)-maIate complex was almost completely oxidized to Cr(Ⅵ) within 420-min irradiation of 500 W medium-pressure mercury lamp at pH 12. The introduction of H_2O_2, considered as a direct source of hydroxyl radicals (OH) in the presence of Cr(Ⅱ), markedly enhanced the yield of Cr(Ⅵ), and a complete oxidation of Cr(Ⅲ)-malate complex (50 μM ) was realized within 20 min. Under a weak acidic condition, the production of Cr(Ⅵ) was coupled with the reduction of Cr(Ⅵ) by malic acid and its free radical generated from Cr(Ⅲ)-malate complex, leading a gradual decrease in Cr(Ⅵ) concentration with the reaction.