Adsorption and Desorption of Hexavalent Chromium in an Alluvial Aquifer Near Telluride, Colorado

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Abstract
A laboratory investigation of reactions between hexavalent chromium [Cr(VI)] and alluvium was conducted to evaluate reactions of Cr(VI) contaminating an alluvial aquifer near Telluride, CO and to determine the mechanisms responsible for these reactions. Uncontaminated alluvium and groundwater (spiked with CrO42−) from the study site were used in batch and column experiments. Results of these experiments show that Cr(VI) was adsorbed by the alluvium. Distribution coefficients from batch experiments ranged from 52 L/kg at an equilibrium CrO42− concentration of 0.4 µmol/L to 1.7 L/kg at an equilibrium concentration of 1400 µmol/L. The zero point of charge for the alluvium was approximately 8.3, and the alluvium had a positive net charge at the groundwater pH of 6.8. Visual and chemical evidence indicated that Fe oxide and hydroxide coatings on the alluvial particles principally were responsible for the adsorption of Cr(VI). During column experiments, Cr(VI) initially was desorbed easily from the alluvium by Cr‐free groundwater; however, the rate of desorption decreased rapidly, and > 60 pore volumes of groundwater were required to decrease the effluent concentration of Cr(VI) to 3 µmol/L [drinking water standard for Cr(VI) = 1 µmol/L]. The quantity of Cr(VI) adsorbed varied with the type and concentration of other anions in solution.

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