Unregulated disposal of Cr-containing industrial solid waste is a serious environmental concern as it leads to hexavalent chromium [Cr(VI)] contamination of subsurface through leaching and subsequent run-off. To safeguard ecosystems downgradient of the contaminant plume, accurate prediction of Cr(VI) fate and transport is desired. While Cr(VI) is highly soluble and mobile it may not be the only form of Cr(VI) transported. Secondarily precipitated low solubility Cr(VI)-colloids could also be controlling Cr(VI) fate. A recent field study conducted in the tannery hub of India (Kanpur) found groundwater from the shallow aquifers to be severely contaminated with Cr(VI). At a few locations, dissolved Cr(VI) were found to be at saturation with respect to BaCrO4(s) when saturation indices were computed for relevant Cr(VI)-containing minerals, using a carefully selected and updated thermodynamic database. Substantial difference in total chromium concentrations between unfiltered and filtered groundwater samples coupled with SEM-EDX on filtered residues suggested the possible presence of colloidal chromium. To understand the mechanism as well as the kinetics of formation of BaCrO4 colloids under approximate in situ conditions, two batch experiments were performed respectively with ultrapure water and Cr-free groundwater, where both Cr(VI) and barium (Ba) were spiked externally. Significant difference in Cr(VI) and Ba concentrations between unfiltered and filtered samples were observed within a minute, indicative of rapid kinetics of formation of Cr(VI) colloids. Saturation indices suggested presence of BaCrO4 at equilibrium in both the batch reactors. X-ray diffraction (XRD) analyses of the solids collected confirmed the presence of BaCrO4(s) as one of the major mineralogical phases, apart from barite (BaSO4(s)). To estimate the rate of precipitation of BaCrO4, flow-through reactor experiments were performed. Results from aqueous and soil-phase characterization of reaction byproducts and preliminary rate estimates through modeling would be presented, that would likely enhance the understanding of colloidal Cr(VI) formation and transport in groundwater.

Colloidal transport of hexavalent chromium in groundwater

2019 Spring National Meeting

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