Heterogeneous reactions control Cr(VI) release and sequestration in complex chemical mixtures of Cr, Fe, Cu, and organics.

Waters contaminated with toxic hexavalent chromium [Cr(VI)] can contain co-occurring metals and organic compounds, promoting simultaneous redox, complexation, and precipitation reactions. This study systematically investigates how Fe minerals, aqueous Fe(II) and Cu(II), and various low molecular weight organic compounds induce Cr(VI) adsorption, reduction, and precipitation reactions. Through batch and column experiments and microscopic and spectroscopic analyses, we determine how aqueous metals and organic compounds interact with Cr(VI) to promote sequestration or release of Cr adsorbed to iron (oxyhydr)oxides. Aqueous Cu precipitates from solution, which removes Cr(VI) from the water column, but may prevent some Cr(VI) from being reduced. With the addition of ascorbic acid, Cu can be mobilized as Cu(0) colloids with Cr also being released. Aqueous Fe(II) promotes Cr(VI) reduction, but also may mobilize Cr associated with reacted Fe (oxyhydr)oxides. The results identified in our study provide insights about overlooked reactions that can control the sequestration and release of Cr in contaminated waters containing complex mixtures of inorganic and organic chemicals which have relevant implications for remediation strategies and recovery of critical minerals.