Adsorption mechanism of hydroxyl polytridecaaluminum on kaolinite surface: A DFT study

The purpose of this study is to investigate the adsorption mechanism of hydroxyl polytridecaaluminum (Al₁₃) on kaolinite (0 0 1) and (00 $\overline{\text{1}}$) surfaces during the flocculation of polyaluminum chloride. Active sites of hydroxyl polytridecaaluminum and adsorption sites on the kaolinite surface were determined by frontier orbital and Mulliken charge, and different adsorption models were constructed. Density functional theory was used to calculate these models. The results demonstrate that the adsorption of Al₁₃ on kaolinite (0 0 1) and (00 $\overline{\text{1}}$) surfaces all occurs spontaneously, and the adsorption models with the lowest energy on (0 0 1) and (00 $\overline{\text{1}}$) surfaces are formed by five and four hydrogen bonds, respectively. Mulliken charge distribution analysis shows that significant electron transfer occurs in both models, with transfer amounts of 1.16 e and 0.54 e, respectively. The analysis of the partial density of states shows that two Al atoms in the two Al–O octahedra in the outer layer of Al₁₃ have certain bonding interactions with the O atoms of the kaolinite surfaces. The calculations indicate that the interactions of Al₁₃ with kaolinite (0 0 1) and (00 $\overline{\text{1}}$) surfaces are mainly electrostatic interactions and hydrogen bonding, and the adsorption strength of Al₁₃ on the (0 0 1) surface is higher than that on the (00 $\overline{\text{1}}$) surface.