Spatially-resolved sorption of Cm(III)/Eu(III) on heterogeneous crystalline rocks

Many countries will use deep geological repositories (DGR) to store their heat generating high-level radioactive waste. Crystalline rock is one of the potential host rocks, but possesses high inherent mineralogical heterogeneity. Since the molecular retardation reactions of radionuclides at water-mineral interfaces depend mainly on the availability of reactive sites, heterogeneity is expected to play a major role for contaminant transport in a DGR. The fundamental understanding and transferrability of this heterogeneity into modeling different transport scenarios is of urgent need for a reliable safety assessment of a repository. Through correlation of spectroscopic information with spatial resolution we characterized the nanostructure of crystalline rock surfaces and the surface speciation of selected radionuclides, namely Eu(III) and Cm(III) thereon. We utilized vertical scanning interferometry, autoradiography, and Raman microscopy in combination with µTRLFS – microfocus time-resolved laser-induced fluorescence spectrsocopy.[1] Using these novel techniques the surface speciation of Eu(III) and Cm(III) can be qualified and quantified. Moreover, we were able to correlate mineralogy, topography, and grain boundary effects with radionuclide speciation, allowing us to draw conclusions about radionuclide retention mechanisms on mineral surfaces.
Our work focussed on granite from Eibenstock (Germany) and migmatised gneiss from the Bukov URL (Czech Republic). We characterized the sorption of Cm(III) and Eu(III) on feldspar, mica, quartz and accessory mineral areas on both rocks. [1-3] Using autoradiography and µTRLFS we linked the sorption uptake on the heterogeneous surface with the mineralogy and the surface roughness, showing that surface roughness within the same mineral phase has an impact not only on the amount of sorption uptake, but also the radionuclide surface speciation and thus bond strengths and reversibility.
Using µTRLFS we identified how the speciation correlates to mineral phases and surface roughness. A higher surface roughness induces more binding sites available to Eu(III) and Cm(III) resulting in strongly bound trivalent radionuclide surface complexes and a higher sorption uptake. On quartz and feldspar high surface roughness leads to ternary Cm(III) complex formation on the surface presumably with silicate and carbonate ions avaliable in solution.
In comparison to Eibenstock granite, Bukov gneiss inherently contained a greater number of accessory minerals. We observed that some of them seem to dominate the sorption process, lowering the sorption of Eu(III) on the major components feldspar and quartz in comparison to Eibenstock granite.[2] The leftover Cm(III)/Eu(III) sorb on stronger and preferential sorption sites, which are located in regions exhibiting a high surface roughness.[3] This could be clearly proven for Cm(III)/Eu(III) surface complexes being stronger on feldspar. With this work we demonstrated a successful upscaling approach to derive molecular understanding of radionuclide retention processes from the nm to the cm sacle.

References
[1] Molodtsov, Sorption of Eu(III) on Eibenstock granite studied by µTRLFS: A novel spatially-resolved luminescence-spectroscopic technique, Scientific Reports, 9, 6287 (2019).
[2] Molodtsov, A µTRLFS investigation on the sorption of Eu3+ on Bukov migmatised gneiss on the molecular level, Environmental Science & Technology, submitted.
[3] Demnitz, A spatially-resolved study on the sorption of Cm(III) on different crystalline rocks using surface investigation techniques, in preparation.