| Chromium ore processing residue (COPR) has been disposed at hundreds of sites in the US mid-Atlantic Region, including major transportation corridors, harbors, and facilities. Because chromium compounds persist in COPR, the long-term leaching of toxic hexavalent chromium, Cr(VI), is a continuing legacy. The objectives of this research effort were to apply and integrate analytical and experimental studies on COPR to characterize the physical, mineralogical, and chemical composition of COPR and how changes in pH and Eh influence the leaching of Cr(VI).; COPR from two disposal sites in the mid-Atlantic region of the US has been characterized by a combination of mineralogical, chemical, and physical analyses. In addition, experimental studies (batch and column) on the release and uptake of hexavalent chromium, Cr(VI), from/to COPR with treatments of acid and/or reductants have been performed to understand the leaching behavior of COPR under potential remedial scenarios. COPR is a complex assemblage of Cr-bearing, alkaline minerals and Cr(VI) leaching is dependent on competing mechanisms of precipitation/dissolution of Cr(VI)-bearing compounds (minerals and poorly-ordered phases), anion-exchange, and adsorption/desorption to charged surfaces. Batch studies suggest that leaching of Cr(VI) can be enhanced by acidification; however, recalcitrant solid-phase Cr(VI) remains after single additions of acid, Cr(VI) release is temporal, and pH rebound occurs. The effectiveness of in situ treatment of COPR by permeation with FeSO4H2SO4 is potentially limited because the treatment impacts the pore structure of COPR, requires large volumes of treatment solution, and recalcitrant Cr(VI) remains in the solid-phase.{09} Ex situ treatment of COPR with multi-cationic polysulfide, CaS x, reduced solid-phase concentrations of Cr(VI) (< 10 mg kg -1) without pH adjustment and Cr(VI) leaching was < 0.33 mg L -1 for 23.5 pore volumes of flow (PVF). In the absence of source treatment and/or containment, mobilization of Cr(VI) is likely to continue because of the slow dissolution of Cr(VI)-bearing compounds, solid-phase transport limitations, and continued mobilization of Cr(VI) from anion exchange and desorption mechanisms. |
