Variation in Cation Adsorption Mechanism Controlled\nby Chemical and Structural Heterogeneities at the Quartz (101)–Water\nInterface

Mineral–water interfacial\nreactions are central to chemical\nprocesses that control the fate of nutrients and contaminants in natural\nenvironments. Mineral surfaces commonly have complex structures and\ncompositions whose impact on interfacial reactivity is poorly understood.\nHere, we investigated the effects of surface heterogeneities on Rb<sup>+</sup> sorption at the quartz (101)–10 mM RbCl solution interface\nat pH 9.8 using in situ high-resolution X-ray reflectivity. Two surface\nlocales (i.e., Spots A and B) having distinct interfacial structures\nwere chosen: Spot A was characterized by its low defect density (≤20%\ntopmost Si vacancies) and Rb<sup>+</sup> adsorption occurred predominantly\nas an inner-sphere complex. In comparison, Spot B had a higher defect\ndensity (∼50% vacancies) and was covered with poorly crystalline\nSiO<sub>2</sub>. A substantially larger Rb<sup>+</sup> uptake (i.e.,\n7-times higher coverage) was observed on this defective surface where\nRb<sup>+</sup> incorporated in the vacancy sites (confirmed by density\nfunctional tight binding-based molecular dynamics simulations) or\nadsorbed directly on the disordered film. These results provide a\ndirect quantification of how surface heterogeneity influences the\ngeochemical behavior of mineral–water interfaces, in particular\nhighlighting the important role of chemical and structural defects\non the sorbate speciation and coverage at silicate mineral surfaces.