Adsorption of Al(OH)(3-+ (n = 2–4) on Kaolinite (001) Surfaces: A DFT study

To investigate the microscopic adsorption mechanism of aluminum ion on kaolinite surfaces, we studied the adsorption of different monomeric hydroxyl aluminum on two types of neutral kaolinite (001) surfaces by means of density functional theory (DFT). This paper constructed three forms of monomeric hydrolysis models of aluminum ion, these being termed Al(OH)2+, Al(OH)3 and Al(OH)4−. Meanwhile, the most stable adsorption configurations were selected for the analysis of the bonding, atomic population, charge transfer and density of states. Calculated results demonstrated that the three monomeric hydrolysis components are primarily adsorbed on the kaolinite (001) surface, i.e., the aluminum oxide octahedron surface. The order of adsorption stability is Al(OH)3 > Al(OH)2+ > Al(OH)4−, and the optimal adsorption sites are all located above the oxygen atoms in the hydroxyl groups which are parallel to the surface. When Al(OH)2+ ions and Al(OH)3 molecules are adsorbed on the kaolinite (001) surface, they depend on the interaction between the aluminum atoms in the adsorbate and the surface oxygen atoms, followed by hydrogen bonding. The adsorption of Al(OH)4− ion on the kaolinite (001) surface is dominated by hydrogen bonding. Following the adsorption of Al(OH)4− ion, the positive charge of the two kaolinite surfaces increased and the electronegativity decreased.