Y(III) sorption at the orthoclase (001) surface measured by X-ray reflectivity

Interactions of heavy metals with charged mineral surfaces control their mobility in the environment. Here, we investigate the adsorption of Y(III) onto the orthoclase (001) basal plane, the former as a representative of rare earth elements and an analogue of trivalent actinides and the latter as a representative of naturally abundant K-feldspar minerals. We apply in-situ high-resolution X-ray reflectivity to determine the sorption capacity and molecular distribution of adsorbed Y species as a function of Y3+ concentration and pH. We observe an inner-sphere (IS) sorption complex at a distance of ~1.5 Å from the surface and an outer-sphere (OS) complex at 3–4 Å. Based on the adsorption height of the IS complex a bidentate, binuclear binding mode, in which Y3+ binds to two terminal oxygens seems most plausible. The total Y coverages of IS and OS species are max. ~1.3 Y3+/AUC for all Y concentrations (AUC: area of the unit cell = 111.4 Ų), which is in the expected range based on the estimated surface charge of orthoclase (001).