Theoretical investigation of hydrated [Lu(OH)2]+ adsorption on kaolinite(0 0 1) surface with DFT calculations

Abstract Kaolinite is a major mineral present in ion-adsorption rare earth ores, which are the primary source of lutetium (Lu). In those ores, Lu is mainly found to be adsorbed on the kaolinite surface in the form of hydrated or hydroxyl-hydrated ions, and can therefore be desorbed by ion-exchange leaching. This study used density functional theory calculations to investigate the adsorption mechanisms and bonding nature of hydrated [Lu(OH)2]+ ions on the kaolinite (0 0 1) surface. Lu3+ exhibited directed tendency geometry with high coordination number of nine. [Lu(OH)2(H2O)6]+, the most stable ion in aqueous system, was considered as the initial structure for the adsorption models. To investigate bonding between hydrated [Lu(OH)2]+ and the kaolinite surface, outer-layer (related to hydrogen bonding) and inner-layer (related to coordination bonding) adsorption models were applied. The surface charge and HOMO distribution of the deprotonated Al-OH surface were also investigated. Owing to the high reactivity of the deprotonated hydroxyls, inner-layer adsorption was dominated by coordination bonds between Lu and surface oxygen, with bidentate complexation at Ol site giving the largest adsorption energy (-663.86 kJ/mol). Moreover, partial density of state projections and Mulliken bond populations showed that the antibonding combination of Lu 5d and Os 2p is the dominant orbital contribution of Lu and surface oxygen. These insights into the Lu adsorption mechanism can aid in developing improved ion-exchange leaching techniques.