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

Abstract

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.

Toxicity and biosorption of chromium from aqueous solutions by the diatom Planothidium lanceolatum (Brébisson) Lange-Bertalot

“…Cr(III) used in some tanneries and other manufacturing industries, is released and photo-oxidized in the more toxic Cr(VI) form. Both dissolved oxygen and the hydroxyl radical ( • OH) trigger the oxidation of Cr(III) to Cr(VI) via a multiple step pathway (Dai et al, 2010). This oxidation has been reported by many other authors (Bartlett and James, 1979;Stromberg et al, 1984).…”

In situ ATR-FTIR investigation and theoretical calculation of the interactions of chromate and citrate on the surface of haematite (α-Fe2O3)

“…Earlier studies have conrmed that mineral surfaces can act as catalysts for redox reactions between Cr(VI) and citric acids to form nontoxic Cr(III). [8][9][10][11] But the molecular mechanisms of their interactions at mineral-water interfaces remain unclear.…”

Mobilization and isotope fractionation of chromium during water-rock interaction in presence of siderophores

“…After 24 hours, the samples were removed from the shaker table and the solution was filtered using an acid-cleaned filter tower with a 0.2µm cellulose acetate membrane filter (both from Sartorius). The reaction vessels were not aired during the leaching experiments and the experiments were run in the dark in order to avoid the unlikely decomposition of organic matter by UV radiation and to avoid Cr photooxidation (e.g., Dai et al, 2010). The leachates were measured for pH and were then acidified with supra-pure hydrochloric acid to pH <2 and stored for later analysis of Cr isotopes (see 2.2) and for REY analysis using a low-resolution quadrupole ICP-MS Perkin-Elmer NexION 300.…”