Mineral-water interfacial reactions control the fate and transport of nutrients and contaminants in natural environment. Natural mineral surfaces can contain various types of defects, imposing additional complexities on surface reactivities. In this study, we interrogated the types of molecular defects on the quartz (101)-water interface and their impacts on the sorption of Rb<sup>+</sup> using in situ specular X-ray reflectivity. The structural defects of the idealized surfaces were expressed using the Robinson roughness model that describes the surface roughness using statistical variations in the coverage of stoichiometric surface planes with different heights. The structural defects of nonstoichiometric surfaces were modelled by considering partial occupancies of the constituent elements (i.e., Si and O) at the surface. The results reveal a strong correlation between these surface heterogeneities and Rb uptake, where the Rb coverages vary drastically (by a factor of ~three) on a single crystal surface. At a surface spot with a low heterogeneity (i.e., with little or no Si vacancies), Rb adsorbs mostly as an inner-sphere complex at ~2 Å from the surface. In contrast, at a surface spot with a higher heterogeneity (i.e., a higher defect density for the top Si and O atoms), additional Rb adsorbs both closer to the surface (i.e., at &lt;1 Å) and farther from the surface (up to ~10 Å). This study provides a quantitative demonstration of the effects of surface heterogeneity on the geochemical behavior of minerals that strongly affects the fate of nutrients and contaminants in aqueous conditions.

Effects of surface structural defects on Rb<sup>+</sup> sorption on the quartz (101) surface

ACS Spring 2022

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Effects of surface structural defects on Rb+ sorption on the quartz (101) surface

Effects of surface structural defects on Rb⁺ sorption on the quartz (101) surface