Atomic-scale understanding of Se(IV) surface complexation on gibbsite: Insights from first-principles molecular dynamics

Reactions of elements at minerals-water interface are critical for their environmental fate in aquatic environment. Aiming at a mechanistic understanding of Se(IV) interaction with metal hydroxide, we carried out first-principles molecular dynamics (FPMD) simulations to quantitatively explore the microscopic structural characteristics and thermodynamic properties of Se(IV) complexation on different surfaces of gibbsite (Al(OH)3). The complexing structures in different ways of HSeO3− adsorbed on basal and edge surfaces are characterized in detail. On basal surface, outer-sphere complexation of HSeO3− through hydrogen bonding and ternary inner-sphere complexation by cation bridging are formed. On edge surface, HSeO3− can be complexed as monodentate, mononuclear bidentate and binuclear bidentate inner-sphere ways onto different binding sites, and the complexing bond lengths agree well with available data by spectroscopic experiments. Desorption free energies indicate that inner-sphere edge complexes are favorable and spontaneous, and the binuclear bidentate complex on (010) edge is the most favored. The calculated acidity constants (pKa) of edge complexes indicate that these HSeO3− complexes can get deprotonated at ambient conditions. Accordingly, reaction pathways of the ligand exchange and thermodynamic stabilities of the different inner-sphere complexes are clarified. Consequently, the crystal facet- and pH-dependent complexation reaction mechanism of Se(IV) is disclosed based on the acquired surface reactive sites, geometric structures and coordination characteristics, electron transfer and bonding analysis, and thermodynamics, which is beneficial to inspiring an improved understanding of the aqueous environmental processes of inorganic contaminants.