First principles investigation on adsorption mechanism of As(OH)3 on the kaolinite (001) surface

The environmental fate of arsenic (As) and arsenic-containing compounds are garnering more attention as a result of the global As contamination. Arsenous acid (As(OH)3) exists as a common arsenic-containing compound in the aqueous phase. Natural clay minerals like kaolinite have the potential to be adsorbents. First-principles calculations based on DFT were used to investigate the adsorption mechanism between As(OH)3 molecules and the kaolinite (001) surface. The two-fold bridge sites of the kaolinite (001) surface were found to be most stable for As(OH)3 adsorption. Three representative stable adsorption configurations, B2, H6, and T3, were selected to analyze. Strong hydrogen bonds formed between As(OH)3 molecules and the kaolinite (001) surfaces made the main contribution to the adsorption. The coverage (Theta) dependence to the adsorption stability was methodically investigated with a wide coverage range of 0 < Theta < 1.0 monolayers (ML). The adsorption energy of As(OH)3 increased in the coverage range of 0 < Theta < 0.5 ML and then decreased in the coverage range of 0.5 <Theta < 1.0 ML. Other properties of the As(OH)3/kaolinite (001) system, including the bonding interaction, atom population/charge, electronic density of states, and lattice relaxation were also discussed and analyzed in detail.