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

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-mu M Cr(III)-malate complex was almost completely oxidized to Cr(VI) within 420-min irradiation of 500W medium-pressure mercury lamp at pH 12. The introduction of H2O2, considered as a direct source of hydroxyl radicals (center dot OH) in the presence of Cr(II), markedly enhanced the yield of Cr(VI), and a complete oxidation of Cr(III)-malate complex (50 mu M) 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.