Variation in Cation Adsorption Mechanism Controlled by Chemical and Structural Heterogeneities at the Quartz (101)-Water Interface

Mineral-water interfacial reactions are central to chemical processes that control the fate of nutrients and contaminants in natural environments. Mineral surfaces commonly have complex structures and compositions whose impact on interfacial reactivity is poorly understood. Here, we investigated the effects of surface heterogeneities on Rb⁺ sorption at the quartz (101)-10 mM RbCl solution interface at pH 9.8 using in situ high-resolution X-ray reflectivity. Two surface locales (i.e., Spots A and B) having distinct interfacial structures were chosen: Spot A was characterized by its low defect density (≤20% topmost Si vacancies) and Rb⁺ adsorption occurred predominantly as an inner-sphere complex. In comparison, Spot B had a higher defect density (∼50% vacancies) and was covered with poorly crystalline SiO₂. A substantially larger Rb⁺ uptake (i.e., 7-times higher coverage) was observed on this defective surface where Rb⁺ incorporated in the vacancy sites (confirmed by density functional tight binding-based molecular dynamics simulations) or adsorbed directly on the disordered film. These results provide a direct quantification of how surface heterogeneity influences the geochemical behavior of mineral-water interfaces, in particular highlighting the important role of chemical and structural defects on the sorbate speciation and coverage at silicate mineral surfaces.