Influence of surface roughness on the sorption of Cm(III) on crystalline-water interfaces

Many countries plan to use deep geological repositories to dispose of their highly radioactive nuclear waste. Internationally, crystalline rock is a potential host rock because of its strong geotechnical stability, low permeability and low solubility. After a potential water ingress into a nuclear waste repository, radionuclides might be mobilized in aqueous solution. The topographical surface features affect the speciation and therefore retention potential of radionuclides on the host rock. Therefore, there is a need for using sophisticated techniques that allow to characterize the nanostructure of such crystalline rock with spatial resolution and the molecular speciation of the actinides thereon. As a representative for trivalent actinides, such as Am(III), Np(III), and Pu(III), we have chosen the actinide Cm(III). It possesses excellent luminescence properties, allowing us to determine Cm(III) sorption uptake and molecular speciation.

Our investigations focused on cleaved orthoclase (K-feldspar) single crystals and thin slices of different crystalline rocks stemming from the Grimsel test site (GTS) in Switzerland. Cleaved pieces of orthoclase or thin sections of the crystalline rocks were immersed in a sorption solution containing Cm(III). The experiments were undertaken at selected pH values (5.5, 6.9 and 7.3) and different inherent mineralogical complexity of the systems. Subsequently, we applied correlated spectroscopy to analyze the samples. Thus, we were able to correlate mineralogy, topography, and grain boundary effects with radionuclide speciation, allowing us to identify important radionuclide retention processes and parameters. [1, 2].

We observed that Cm(III) sorption uptake and speciation on orthoclase single crystals depends not exclusively on the mineral phase and the solution conditions, such as the pH, but also on the surface roughness. At pH 5.5 the sorption uptake differed between low and high surface roughness areas, while the speciation on both areas remained largely similar. Increasing the pH to 6.9 not only increased the overall sorption uptake, also the speciation between smoother and rougher surfaces differed. Using luminescence peak and lifetime analysis we could determine that the speciation is highly dependent on the availability on strong sorption sites. Based on our results we proposed a simple model for Cm(III) sorption on an orthoclase surface.

Further we compared the Cm(III) speciation on orthoclase singe crystals with orthoclase mineral grains on a thin section prepared from Grimsel specimen rock. The sorption was now influenced additionally by the presence of a heterogeneous surface, affecting the strength and form of surface sorbed Cm(III) species.

We conclude that in addition to mineral composition, surface roughness needs to be considered adequately by reactive transport models to describe interfacial speciation of contaminants and respective retention patterns for the safety assessments of nuclear waste repositories.