Understanding\nTrace\nIron and Chromium Incorporation\nDuring Gibbsite Crystallization and Effects on Mineral Dissolution

Incorporation\nof pollutants, e.g., heavy metals, or critical\nelements,\ne.g., lithium, as impurities in mineral phases can significantly affect\ntheir mobility or sequestration in the environment. Even when present\nat low concentrations, impurities can alter the solubility and reactivity\nof the host mineral. In this study, we investigate the incorporation\nof trace amounts of iron (Fe<sup>3+</sup>) and chromium (Cr<sup>3+</sup>) during the crystal growth of the aluminum (Al<sup>3+</sup>) hydroxide,\ngibbsite, a major component of bauxite ores, an important soil mineral,\nand a dominant mineral phase in stored radioactive wastes. Using a\ncomprehensive suite of analytical techniques, we show that both Cr<sup>3+</sup> and Fe<sup>3+</sup> can be incorporated into the gibbsite\nlattice during coprecipitation by replacing Al<sup>3+</sup> in octahedral\nsites. These small amounts are consistent with limited to no structural\nisomorphism shared between Al<sup>3+</sup> and Cr<sup>3+</sup>/Fe<sup>3+</sup> hydroxide precipitates, nor room temperature miscibility\nof their isostructural M<sub>2</sub>O<sub>3</sub> oxide forms, in\ncontrast with oxyhydroxide forms where Al<sup>3+</sup> and Fe<sup>3+</sup> share similar structural topologies. Despite the limited\nuptake of Cr<sup>3+</sup>/Fe<sup>3+</sup>, we show that these impurities\nhave significant implications for gibbsite dissolution behavior. The\nlimited uptake of Cr<sup>3+</sup>/Fe<sup>3+</sup> (e.g. 0.43% Cr<sup>3+</sup> and 0.4% Fe<sup>3+</sup>), we show that these impurities\nhave significant implications for gibbsite dissolution behavior and\nsubsequent reactivity in complex environments.