Photo-oxidation of Cr(III)-citrate complexes forms harmful Cr(VI).
Photo-oxidation is a potential pathway for the transformation of Cr(III) to Cr(VI) in natural environments. In this study, the Cr(III)-citrate complex (Cr(III)-cit) was prepared and its speciation was determined by high performance liquid chromatography (HPLC). Results showed that Cr(III)-cit existed in [Cr(III)-H-cit](+) and [Cr(III)-cit] species in a pH range of 3-5, in [Cr(III)-cit] only from pH 6-8, in [Cr(III)-cit] and [Cr(III)-OH-cit](-) from pH 9-11, and only in [Cr(III)-OH-cit](-) at pH 12. Additional experiments were conducted in batch systems with pHs of 5 to 12 at 25 °C, where aqueous Cr(III) and Cr(III)-cit were fully exposed to light from medium pressure mercury lamps and a xenon lamp mimicking solar light irradiation. Results demonstrated that oxidation of Cr(III) in Cr(III)-cit was much faster than that in aqueous Cr(III). Rates of Cr(III) photo-oxidation were not sensitive to pH in the range from 7 to 9 but increased significantly with further increases in pH, which was consistent with the distribution of Cr(III) forms. It appeared that [Cr(III)-cit-OH](-) was the most photochemically active form and Cr(II), resulting from a ligand-to-metal charge-transfer (LMCT) pathway after light absorption, was a precursor of the oxidation of Cr(III) to Cr(VI). Both dissolved oxygen and the hydroxyl radical ((•)OH), an intermediate, served as oxidants and facilitated the oxidation of Cr(II) to Cr(VI) via a multiple step pathway. The photoproduction of (•)OH was detected by HPLC using benzene as a probe, supporting the proposed reaction mechanism.
Dai
Runan
R
College of Sciences, Nanjing Agricultural University, Nanjing 210095, PR China.
Yu
Changyuan
C
Liu
Jing
J
Lan
Yeqing
Y
Deng
Baolin
B
D001554:Benzene / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D002857:Chromium / Q000737:chemistry*; D019343:Citric Acid / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration / Q000528:radiation effects; D017665:Hydroxyl Radical / Q000737:chemistry; D008027:Light; D010084:Oxidation-Reduction / Q000528:radiation effects; D012995:Solubility / Q000528:radiation effects
D002857:Chromium; D019343:Citric Acid; D017665:Hydroxyl Radical; D001554:Benzene