Surface and solution abiotic processes in the redox transformations of chromium

There are two major Cr oxidation states: Cr(III) and Cr(VI). Chromium(III) is of less environmental concern because of its low solubility and mobility in soil and aquifer systems. Chromium(VI), on the other hand, is more mobile and more toxic to plants and toxic and carcinogenic to animals. Thus, development of an efficient remediation method for the conversion of Cr(VI) to Cr(III) is an important issue.; Although Cr(VI) reduction by chemical agents such as Fe(II) is rapid, the redox products, Fe(III)/Cr(III) hydroxides, must then be appropriately disposed and managed to prevent reoxidation. The results of the current study indicate that excess Cr(VI) can be adsorbed on the hydroxide surface, thus resulting in possible contamination when the Fe/Cr hydroxides are disposed.; Organic matter is widely distributed in soil and aquatic systems. Low molecular weight organic acids, such as citrate and oxalate and artificial organic ligands such as HEDTA not only can accelerate Cr(VI) reduction by Fe(II) but also can donate electrons to Cr(VI) under illumination.; Although TiO₂ shows low Cr(VI) adsorption ability, the retention of Cr(VI), due to reduction to Cr(III), was significantly increased in the presence of light. The catalysis of Cr(VI) reduction was also observed with goethite, ferrihydrite, and smectite in the presence of light. Light, in these reactions, is an initiative factor in catalyzing Cr(VI) redox reactions on the particle/water interfaces.; Inorganic cations, such as Fe(III), can inhibit Cr(III) oxidation by birnessite. Inorganic ligands such as phosphate can also affect Cr(III) oxidation, but through a different mechanism, by the formation of Cr(III)-P phases. In general, organic ligands decrease Cr(III) oxidation by birnessite, either by dissolving Mn oxides or by forming stable Cr(III)-organic complexes.; The possibility of Cr(VI) reduction by naturally occurring materials is important because it would reduce the costs of remediation. This laboratory study indicates that Cr(VI) reduction can be rapidly and efficiently achieved by the utilization of catalysts, e.g., TiO₂, and organic compounds under illumination. Systems could potentially be developed for use in industry which utilize artificial ponds under sunlight to catalytically reclaim Cr(VI) waste streams.