Immobilization of Chromium by Iron Oxides in Nickel–Cobalt Laterite Mine Tailings

Mine tailings generated from hydrometallurgical processing of nickel-cobalt laterite deposits contain high levels of chromium (Cr), with the hexavalent species being a toxic pollutant and carcinogen. However, the partitioning, speciation, and local bonding environment of Cr in the mine tailings remain largely unknown, hindering our ability to predict its toxicity and long-term behavior. Coupling detailed mineralogical, spectroscopic, and geochemical characterization with sequential extraction of tailings from active and rehabilitated dams, we show that Cr is present in its least toxic form, Cr(III), and largely immobilized by recalcitrant minerals. This immobilization also regulates dissolved Cr concentrations in the interacting waters to levels up to five times lower than the global regulatory limit (50 μg L^-1). Solid-phase Cr concentrations were ≤1.5 wt % with 39-61% of Cr incorporated into hematite, and to a lesser extent, alunite, both of which formed early in the hydrometallurgical extraction process of mined laterite ores. The remaining Cr was present as recalcitrant chromite residues from the primary source laterites. We highlight that, although hydrometallurgical extractions liberate Cr from laterite ores during processing, they also provide ideal chemical pathways for the formation of highly stable, crystalline hematite that successfully sequesters Cr, while restricting its environmental mobility.