Unregulated disposal of chromium ore processing residue (COPR), a Cr-bearing highly alkaline solid waste generated during the production of basic chromium sulfate used for tanning in leather industries, is one of the major sources of hexavalent chromium [Cr(VI)] contamination in groundwater, especially in the developing countries. COPR is a predominantly crystalline complex mixture of Cr-bearing phases such as magnesiochromite, Cr(VI)-hydrogarnet, Cr(VI)-hydrocalumite and Cr(VI)-ettringite, and non-Cr phases such as calcite, brownmillerite and dolomite. The abundance of host Cr(VI)-phases makes the identification of Cr(VI) leaching mechanisms and measurement of leaching rates in water challenging. Weathered COPR collected from a dumping site in Kanpur, India, was thoroughly characterized using X-ray diffraction (XRD) and X-ray fluorescence (XRF), to identify its mineralogical and elemental compositions, respectively. A five-step sequential extraction of COPR was performed to estimate the exchangeable and non-exchangeable solid-associated Cr fractions. To investigate the impact of predominant divalent cations in rain water on Cr(VI) leaching from COPR, batch equilibrations of the sampled COPR were performed in triplicates with three types of eluents (0.01 M CaCl<sub>2, </sub>0.01 M MgCl<sub>2</sub> and synthetic rain water) at varying liquid-to-solid ratios. Dissolved Cr(VI) along with co-ions for each reactor were measured at regular intervals until these systems reached equilibrium. Results were interpreted by comparing variation of saturation indices of candidate phases for the different conditions using an updated thermodynamic database for all the relevant Cr- and non-Cr containing solids and species. Furthermore, to investigate the kinetics of Cr-release from weathered COPR, flow cell experiments were performed at varying solid-solution ratios for the three eluent conditions. Flow-through reactive transport modeling was performed to estimate rates of Cr(VI) release from COPR. Aqueous and solid-phase characterization results from batch and flow-cell experiments will be presented in a conceptual framework that identifies the drivers for Cr(VI) release from COPR under natural weathering conditions.

Extent and rates of chromium (VI) leaching from weathered chromium ore processing residue (COPR)-impacted soils

2019 Spring National Meeting

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