Adsorption of hydrated Al3+ on the kaolinite (001) surface: A density functional theory study

In order to reveal the microscopic occurrence of Al3+ impurity in ionic rare earth ore on kaolinite surface, density functional theory (DFT) was used to construct the stable hydrate model of Al3+ in the water system. The structure and bonding mechanism of the complexes of outer layer adsorption and inner layer (monodentate/bidentate) adsorption of hydrate Al3+ on kaolinite (001) surface were studied. The DFT simulation results were verified by molecular dynamics (MD) stimulation and adsorption experiments. The results showed that the stable hydrate form of Al3+ is [Al(H2O)(6)](3+). When hydrated Al3+ is adsorbed on the outer layer of the kaolinite (001) surface, Al3+ is more inclined to be adsorbed on the Si-O surface. When hydrated Al3+ is adsorbed on the inner layer of the kaolinite (001) surface, Al tends to form a bond with Ou atoms formed by the deprotonation of the "upright " hydroxyl group on the surface to form monodentate adsorption complexes, and maintains coordination with the surrounding 5 H2O; Al tends to form bonds with the Ou and Ol atoms formed by the deprotonation of the "upright " and "lying " hydroxyl groups on the surface to form bidentate adsorption complexes, due to steric hindrance effect, Al only maintains coordination with the surrounding 3 H2O. The MD simulation results verified the rationality of the adsorption configuration calculated by DFT. Combined with Mulliken population, charge density and partial density of state (PDOS) analysis, it is shown that the Al-Os bond in the monodentate/ bidentate adsorption complex of hydrated Al3+ has the characteristics of strong ionicity and strong bond filling. The hydrated Al3+ preferentially forms a more stable bidentate adsorption complex on kaolinite surface. Both the calculation of adsorption energy and the results of adsorption experiments support this conclusion.