Facet-dependent adsorption of aluminum(iii) on hematite nanocrystals and the influence on mineral transformation

Abstract

Iron (Fe) (oxyhydr)oxides, which are among the most reactive iron-bearing minerals commonly found in nature, are enriched in various dopants, such as aluminum (Al). This substitution can cause important changes in the physicochemical properties of the minerals that determine their reactivity towards nutrients and pollutants. For instance, the morphology changes of hematite from rhombohedral crystals to lamellar disks induced by Al incorporation are strongly influenced by the adsorption behaviors of Al(III) on the exposed faces of the initial hematite nuclei. However, the intrinsic mechanism remains unclear. In this study, hematite nanoplates (HNPs) with mainly exposed {001} facets, nanorods (HNRs) with exposed {001} and {110} facets, and nanocubes (HNCs) with exposed {012} facets were synthesized and characterized, and the facet-dependent adsorption behaviors towards Al(III) and the mineral transformation behaviors, as well as Al(III) adsorption on natural hematite single crystals were systematically investigated. We find that Al(III) adsorption densities on these synthetic hematite nanocrystals decrease in the order HNRs > HNCs > HNPs under different environmental conditions. At both pH 4 and 12, the Kelvin potential difference (ΔV) measurements show that the adsorption densities of Al(III) on natural hematite single crystals decrease in the order {110} > {012} > {001}. Transformation of synthetic hematite samples in the presence of Al(III) and Fe(III) results in the formation of goethite and new hematite crystals, with the contents of retained Al decreasing in the order HNRs > HNCs > HNPs. These results help us understand the intrinsic mechanisms of hematite morphology changes with substituents, and the Fe (oxyhydr)oxide mineral reactivity towards various nutrients and metal pollutants.