Photo-Oxidation of Cr(III)−Citrate Complexes Forms Harmful Cr(VI)

详细信息    查看全文

• 作者：Runan Dai ; Changyuan Yu ; Jing Liu ; Yeqing Lan ; Baolin Deng
• 刊名：Environmental Science & Technology
• 出版年：2010
• 出版时间：September 15, 2010
• 年：2010
• 卷：44
• 期：18
• 页码：6959-6964
• 全文大小：240K
• 年卷期：v.44,no.18(September 15, 2010)
• ISSN：1520-5851

文摘

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.