Elucidating the Underestimated Role and Microscopic Mechanisms of Kaolinite in Oxyanion Retention: Insights from DFT Calculations and Surface Complexation Modeling

Clay minerals are among the most abundant interfaces in soils, yet their role in oxyanion adsorption has long been underestimated, which hinders accurate assessments of oxyanion mobility and environmental risks in natural soil systems. In this study, after removing soil organic matter and Fe oxides from soil samples, it was found that As(V) adsorption on clay soils accounted for 18-56% of that in original soils. Furthermore, adsorption experiments involving P(V), As(V), and Sb(V) on kaolinite, a typical soil clay mineral, under various hydrochemical conditions, combined with density functional theory calculations and surface complexation modeling, were conducted to explore the molecular-level mechanisms. The results revealed that P(V), As(V), and Sb(V)─despite being electrostatically repelled by basal surfaces─were able to form inner-sphere bidentate (B) and outer-sphere (OS) complexes on the edge surfaces of kaolinite. Moreover, the differences in hydrogen bonding intensity and steric repulsion account for their surface species discrepancy, with the quantity of OS complexes following the order P(V) > As(V) > Sb(V), which relates well to their hydrated ionic radii and explains their adsorption capacity order on kaolinite. This study advances our understanding of the active interfaces in soils and provides mechanistic insights for improving predictive geochemical models.