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

Neumann, J.; Lessing, J.; Lee, S. S.; Stubbs, J. E.; Eng, P. J.; Demnitz, M.; Fenter, P.; Schmidt, M.


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).

Keywords: solid liquid interface; rare earth elements; trivalent actinides; crystal truncation rod; resonant anomalous X-ray reflectivity; feldspars

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