DFT Study on the Adsorption of Monomeric Hydroxyl Aluminum on Fe(II)/Mg Replacement Kaolinite (001) Surfaces

In the natural environment, Al and Si in the kaolinite crystal structure are likely to form lattice defects or be replaced by low-valence positive ions so that the base surfaces have permanent negatively charged sites. It is therefore very important to investigate the adsorption process and adsorption mechanism of adsorbates on the replaced surfaces. In this paper, two types of surface models formed by replacing Al atoms in the alumina octahedron of kaolinite (001) surface with Fe(II) and Mg were selected as the adsorption surfaces, these being the kaolinite Fe(II)_Al-(001) and Mg_Al-(001) surfaces, respectively. Then, we used density functional theory (DFT) to simulate the adsorption of three monomeric hydroxy aluminum models (i.e., Al(OH)₂ ⁺, Al(OH)₃, and Al(OH)₄ ^-) on the two replaced surfaces. Results show that, when compared to the adsorption on the ideal kaolinite (001) surface, the adsorption energies of the three adsorbates adsorbed on the replaced surfaces are lower and the adsorption is more stable. When the three adsorbates are adsorbed on the kaolinite Fe(II)_Al-(001) surface, adsorption stability increases with the number of hydroxyl groups, and hydrogen bonding and electrostatic adsorption play a major role. Conversely, when they were adsorbed on the kaolinite Mg_Al-(001) surface, the stability of the adsorption deteriorated as the number of hydroxyl groups increased. Moreover, the decisive roles are the interaction between the aluminum atoms in the adsorbates and the oxygen atoms on the replaced surface and the electrostatic adsorption.