Bridging the Gap between Molecular Simulations and Surface Complexation Modeling for Heterogeneous Surfaces: A Case Study with Uranium and Arsenic Adsorption on Clay Minerals

Among all natural submicrosized phases, clay minerals are ubiquitous in soils and sedimentary rocks in nature as well as in engineered environments, and while clay minerals' adsorption properties have been studied extensively, their unique level of surface reactivity heterogeneities necessitates further investigation at the molecular level to understand and predict the influence of these heterogeneities on their macroscopic properties. In this study, we investigated the surface structures and desorption-free energies of U(VI) species (UO₂²⁺) and As(V) species (H₂AsO₄^- and HAsO₄^2-) complexed at different edge surface reactive sites of a cis-vacant montmorillonite layer using first-principles molecular dynamics (FPMD). We show that U(VI) forms bidentate and tridentate complexes on montmorillonite edge surfaces, whereas As(V) monodentate complexes are the most stable. Then, we constrained a state-of-the-art surface complexation model (SCM) with surface acid-base chemistry and surface complexation properties obtained from the molecular-level simulation results. While our results highlighted the complexity of the interfacial chemistry controlling the complexation of inorganic contaminants on clay mineral surfaces, they also evidenced the reliability of an integrated workflow using modern multiscale simulation techniques to address the challenge of predicting heterogeneities in mineral surface reactivity.