Density functional calculations of La(H₂O)₁₀³⁺ and Mg(H₂O)₆²⁺ adsorption on kaolinite surfaces

Ion-type rare earth ore is a crucial category of strategic mineral resources in China. Rare earth elements in these ores are adsorbed on the surface of clay minerals (such as kaolinite) in the form of ion hydrate. Given the structural properties of rare earth ions and impurity ion hydrates, this study explores the mechanism of their adsorption on the surface of clay minerals. By drawing on the density functional theory, our calculations show the stable configurations of La³⁺ and Mg²⁺ hydrated ions are La(H₂O)₁₀³⁺ and Mg(H₂O)₆²⁺. We then calculated the adsorption energy, density of states, and electron transfer of La(H₂O)₁₀³⁺ and Mg(H₂O)₆²⁺ on the aluminum hydroxyl face (0 0 1) and the silica-oxygen face (0 0 1). Results show that the adsorption energies were -221.19 and -142.51 kJ/mol for La(H₂O)₁₀³⁺ and Mg(H₂O)₆²⁺ on the aluminum hydroxyl face (0 0 1) of kaolinite, while -96.65 and -71.10 kJ/mol on the silica-oxygen face (0 0 1) of kaolinite. The adsorption of La(H₂O)₁₀³⁺ was more stable on the kaolinite surface. We found less number of charge transfer between hydrated ions and kaolinite surface, where hydrated ions are adsorbed mainly through the formation of hydrogen bonding between H_W and O_W in the coordination water and H_S and O_S on the kaolinite surface.