Chromate removal by dithionite-reduced clays: Evidence from direct X-ray adsorption near edge spectroscopy (XANES) of chromate reduction at clay surfaces

Chromium(VI) in the environment is of particular concern because it is toxi c to both plants and animals, even at low concentrations. As a redox-sensit ive element, the fate and toxicity of chromium is controlled by soil reduct ion-oxidation (redox) reactions. In-situ remediation of chromium combines r eduction of Cr(VI) to Cr(III) and immobilization of chromium on mineral sur faces. In this study, Fe-rich smectite, montmorillonite, illite, vermiculit e, and kaolinite were examined to determine reactivity in sorption-reductio n of Cr(VI). The clays were compared to forms that were reduced by sodium d ithionite. Clays containing Fe(Il) efficiently removed soluble Cr(VI) from solution. Chromium K-edge X-ray absorption near edge structure (XANES) sugg ested that clays containing Fe(II) reduced Cr(VI) to Cr(III), immobilizing Cr at the clay/water interface. Adsorption of Cr(VI) by the Fe(II)-containi ng clay was a prerequisite for the coupled sorption-reduction reaction. Sod ium dithionite added directly to aqueous suspensions of non-reduced clays r educed Cr(VI) to Cr(III), but did not immobilize Cr on clay surfaces. The c apacity of clays to reduce Cr(VI) is correlated with the ferrous iron conte nt of the clays. For dithionite-reduced smectite, the exchangeable cation i nfluenced the sorption reaction, and thus it also influenced the coupled so rption-reduction reaction of Cr(VI). The pH of the aqueous system affected both the amount of Cr(VI) reduced to Cr(III) and the partition of Cr(III) b etween aqueous and adsorbed species. A plot of pH vs. amount (adsorption en velope) adsorbed for the coupled sorption-reduction reaction of Cr by reduc ed smectite exhibited a similar pattern to that of typical anion-sorption.