Understanding Trace Iron and Chromium Incorporation During Gibbsite Crystallization and Effects on Mineral Dissolution

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 a dominant mineral phase in stored radioactive wastes. Using a comprehensive suite of analytical techniques, we show that both Cr³⁺ and Fe³⁺ can be incorporated into the gibbsite lattice during coprecipitation by replacing Al³⁺ in octahedral sites. 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. Despite the limited uptake of Cr³⁺/Fe³⁺, we show that these impurities have significant implications for gibbsite dissolution behavior. The limited uptake of Cr³⁺/Fe³⁺ (e.g. 0.43% Cr³⁺ and 0.4% Fe³⁺), we show that these impurities have significant implications for gibbsite dissolution behavior and subsequent reactivity in complex environments.