Y(III) Sorption at the Orthoclase (001) Surface Measured by X-ray Reflectivity

Abstract

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 highresolution X-ray reflectivity to determine the sorption capacity and molecular distribution of adsorbed Y species as a function of the Y 3+ concentration, [Y 3+ ], at pH 7 and 5. With [Y 3+ ] ≥ 1 mM at pH 7, 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 Y 3+ binds to two terminal oxygens, is proposed. In contrast, mostly OS sorption is observed at pH 5. The observed maximum Y coverage is ∼1.3 Y 3+ /A UC (A UC : area of the unit cell = 111.4 Å 2 ) for all the investigated pH values and Y concentrations, which is in the expected range based on the estimated surface charge of orthoclase (001).

Unraveling pH-Dependent Changes in Adsorption Structure of Uranyl on Alumina (012)

“…Because of this large sorption height, we conclude that this species is not a simple adsorbate ion that binds directly to specific surface sites. Instead, its presence may indicate the onset of surface precipitation and/or adsorption of polynuclear species on the surface, consistent with higher mole fractions of U VI dimers and trimers in solution expected under the pH condition and also consistent with observations reported for uranyl adsorption on alumina powders …”