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Structural Investigation of the Adsorption von Y(III) on Orthoclase (001) Single Crystals using Resonant Surface X ray Diffraction

Neumann, J.; Lessing, J.; Demnitz, M.; Eng, P.; Stubbs, J.; Stumpf, T.; Schmidt, M.


Transport of radionuclides (RNs), from deep geological repositories for radioactive waste, such as the highly toxic trivalent minor actinides (An(III)) Am and Cm, will be controlled by their interactions with charged mineral phases. Many countries such as Finland, Sweden, and Germany consider a repository in crystalline rock, which contains large amounts of feldspars, e.g. orthoclase (K-feldspar). Hence, reliable risk assessments of potential repository sites depend on a fundamental understanding of sorption quantity and structure of An(III) on feldspars. Typically, those interactions are investigated using mineral powder samples [1], which depict an idealization of the natural system due to the small grain size of the mineral. In those studies, information about macroscopic effects on sorption processes, like crystal orientation or surface roughness, are not accessible. Therefore, in this work we study the adsorption of Y(III), as an inactive rare earth analogue for An(III), on natural single crystal orthoclase samples of the (001) crystal orientation using the modern synchrotron-based, surface X-ray diffraction technique.


Natural single crystal orthoclase samples were freshly cleaved along their (001) orientation and reacted overnight in a solution of [Y3+] = 0.01 M at pH = 5.0 or 6.9. After the reaction was finished, surface X-ray diffraction (SXRD) was measured in situ at beamline 13 ID-C (GeoSoilEnviroCARS) of the Advanced Photon Source at Argonne National Laboratory. SXRD yields the total electron density profile of the mineral/water interface by measuring crystal truncation rods (CTR). For the first time, resonant anomalous X-ray reflectivity (RAXR) is applied on orthoclase for identification and quantification of sorption species, in our case Y3+. Coverage of adsorbed Y3+ is given in units of Y/AUC (area of the orthoclase (001) unit cell = 55.57 Å2).


The study investigates the adsorption of Y3+ on orthoclase (001) at two different pH values. RAXR spectra of both samples show strong modulations at the Y X-ray absorption edge (17.038 keV), indicating that Y3+ has been adsorbed to the orthoclase surface. Analysis of amplitudes and phases of the RAXR spectra yield information about coverage and distance of the adsorbed species from the surface.
At pH 5.0, two sorption species at a distance of 2.47 (Species A) and 8.35 Å (Species B1) from the uppermost oxygen-atoms (Osurf) of the mineral surface are identified. At higher pH (6.9), the adsorbed Y is located at a distance of 1.50 (Species C) and 4.38 Å (B2) from Osurf. The Y3+ aquo ion has hydration shells in a distance of 2.36 and 4.40 Å. Therefore, Species A can be attributed to an outer-sphere (OS) and species B1 and B2 to extended outer-sphere (EOS) sorption complexes. In contrast, Species C is closer to the surface than any other sorption species observed in this study. At the investigated pH of 6.9, more sites of the orthoclase surface are deprotonated, obviously leading to the release of parts of the hydration shell of Y. Therefore, Species C is interpreted as an inner-sphere (IS) sorption complex. A plausible, bidentate binding motif for Species C is suggested based on the obtained results, where Y3+ is bound to two nearest Osurf resulting in a Y-O bond length of 2.46 Å in an angle of 39.0°.
While the interfacial speciation between the two samples is different, the total Y coverage is found to be similar for both samples (~0.6 Y/AUC). At pH 6.9 more than 70 % of the adsorbed Y3+ is bound as IS complex (Species C, 0.43 Y/AUC). The obtained coverage of the IS complex corresponds to ~2/3 of an adsorbed Y3+ monolayer, assuming bidentate coordination to two Osurf. Overall, the obtained sorption quantity and interfacial speciation are in good agreement with the powder studies, supporting the applicability of the previously developed SCMs to simulate retention of An(III) by K-feldspar for macroscopic systems.
However, we also identify reasonable amounts of adsorbed EOS complexes that are typically not found in studies using mineral powders and therefore not considered thermodynamic models. This result points out the need of studies working on macroscopic mineral samples to assess the impact of those species, and more general the controlling parameters relevant for natural systems, such as crystal orientation, surface roughness, and a realistic solid-liquid ratio. In conclusion, the results of this study contribute to a more realistic and reliable prediction of the mobility of trivalent actinides in the environment, and will enable a better risk assessment for deep geological repositories for radioactive waste.

  • Lecture (Conference)
    TransRet Workshop: Processes Influencing Radionuclide Transport and Retention, 12.-13.10.2021, Karlsruhe, Deutschland

Publ.-Id: 33393