PNNL

Abstract

Incorporation of pollutants e.g., heavy metals, or critical elements e.g., lithium, as impurities in mineral phases can significantly affect their mobility or sequestration in the environment. Even when present at low concentrations, impurities can alter the solubility and reactivity of the host mineral. In this study, we investigate the incorporation of trace amounts of iron (Fe3+) and chromium (Cr3+) during the crystal growth of the aluminum (Al3+) hydroxide, gibbsite, a major component of bauxite ores, an important soil mineral, and also a dominant mineral phase in stored nuclear wastes. Using a comprehensive suite of analytical techniques, we show that both Cr3+ and Fe3+ can be incorporated into the gibbsite lattice during co-precipitation by replacing Al3+ in octahedral sites, up to ~ 0.4 mol%. These small amounts are consistent with limited to no structural isomorphism shared between Al3+ and Cr3+/Fe3+ hydroxide precipitates, nor room temperature miscibility of their isostructural M2O3 oxide forms, in contrast with oxyhydroxide forms where Al3+ and Fe3+ share similar structural topologies. The observed limited uptake of Cr3+/Fe3+ in gibbsite could nonetheless have important impacts on the distributions of these metals in soils and sediments, as well as the subsequent reactivity of gibbsite in complex environments such as in legacy radioactive wastes.