Trivalent minor actinides, An(III), pose a high environmental risk in the long-term storage of spent nuclear fuel in deep geological repositories. Their environmental fate can be controlled by interactions with minerals of repository’s host rocks. One host rock considered in the US is a plutonic igneous rock enriched with feldspars. Therefore, a fundamental understanding of retention of An(III) by feldspar surfaces contributes to a reliable risk assessment of repository sites. Typically, sorption behavior is studied using mineral powders, in which information about crystallographic effects on sorption processes are not accessible. Here, we study adsorption of Y(III), a rare earth analogue for An(III), on two different crystal orientations, (001) and (010), of orthoclase, the K-endmember feldspar mineral, using surface X-ray diffraction (SXRD). We identify a bidentate inner-sphere species at a distance of ~1-2 Å from the top surface oxygens with a coverage of &lt; 0.2 Y3+/AUC (AUC: area of the unit cell; AUC,(001) = 55.53 Å2, AUC,(010) = 30.82 Å2). Outer-sphere and extended outer-sphere sorption complexes are found at distances of &gt; 4 Å with coverages of &gt; 0.2 Y3+/AUC, depending on pH and concentration of Y3+. Uptake of Y(III) increases generally with increasing [Y(III)] and pH, in good agreement with results reported for mineral powders. The comparison of the reactivity of the two crystal facets shows that the molecular structure of the adsorbed complexes on the two crystal orientations appears similar. However, the quantitative contributions of the species differ presumably because of a different surface charge density of the two investigated crystallographic planes. These observations emphasize the importance of understanding facet-dependent reactivities to enable the development of more reliable predictive thermodynamic models of the mobility of contaminants in the environment.

Impact of crystal orientation on sorption uptake and surface speciation of Y(III) on orthoclase

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