In this study a novel technique, micro-focus time-resolved laser-induced luminescence spectroscopy (µTRLFS) is presented to investigate heterogeneous systems like granite (mainly consisting of quartz, feldspar, and mica), regarding their sorption behavior. µTRLFS is a spatially-resolved upgrade of conventional TRLFS, which allows point-by-point analysis of single minerals by reducing the beam size of the analytic laser beam to below the size of mineral grains. This provides visualization of sorption capacity as well as speciation of a luminescent probe, here Eu 3+ . A thin-section of granitic rock from Eibenstock, Saxony, Germany was analyzed regarding its mineralogy with microprobe X-ray fluorescence (µXRF) and electron probe microanalysis (EPMA). Afterwards, it was reacted with 5.0 × 10 −5 mol/L Eu 3+ at pH 8.0 and uptake was quantified by autoradiography. Finally, the µTRLFS studies were conducted. The results clearly show that the materials interact differently with Eu 3+ , and often even on one mineral grain different speciations can be found. Alkali-feldspar shows very high uptake, with an inhomogeneous distribution, and intermediate sorption strength. On quartz uptake is almost 10-fold lower, while the complexation strength is higher than on feldspar. This may be indicative of adsorption only at surface defect sites, in accordance with low hydration of the observed species.

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Reactive transport modeling (RTM) is an essential tool for the prediction of contaminants' behavior in the bio-and geosphere. However, RTM of sorption reactions is constrained by the reactive surface site assessment. The reactive site density variability of the crystal surface nanotopography provides an "energetic landscape", responsible for heterogeneous sorption efficiency, not covered in current RTM approaches. Here, we study the spatially heterogeneous sorption behavior of Eu(III), as an analogue to trivalent actinides, on a polycrystalline nanotopographic calcite surface and quantify the sorption efficiency as a function of surface nanoroughness. Based on experimental data from micro-focus time-resolved laser-induced luminescence spectroscopy (μTRLFS), vertical scanning interferometry, and electron backscattering diffraction (EBSD), we parameterize a surface complexation model (SCM) using surface nanotopography data. The validation of the quantitatively predicted spatial sorption heterogeneity suggests that retention reactions can be considerably influenced by nanotopographic surface features. Our study presents a way to implement heterogeneous surface reactivity into a SCM for enhanced prediction of radionuclide retention.

The interaction of Eu­(III) with thin sections of migmatized gneiss from the Bukov Underground Research Facility (URF), Czech Republic, was characterized by microfocus time-resolved laser-induced luminescence spectroscopy (μTRLFS) with a spatial resolution of ∼20 μm, well below typical grain sizes of the material. By this approach, sorption processes can be characterized on the molecular level while maintaining the relationship of the speciation with mineralogy and topography. The sample mineralogy was characterized by powder X-ray diffraction and Raman microscopy, and the sorption was independently quantified by autoradiography using 152Eu. Representative μTRLFS studies over large areas of multiple mm2 reveal that sorption on the heterogeneous material is not dominated by any of the typical major constituent minerals (quartz, feldspar, and mica). Instead, minor phases such as chlorite and prehnite control the Eu­(III) distribution, despite their low contribution to the overall composition of the material, as well as common but less studied phases like Mg–hornblende. In particular, prehnite shows high a sorption uptake as well as strong binding of Eu to the mineral surface. Sorption on prehnite and hornblende happens at the expense of feldspar, which showed the highest sorption uptake in a previous spatially resolved study on granitic rock. Similarly, sorption on quartz is reduced, even though only low quantities of strongly bound Eu­(III) were found here previously. Our results illustrate how competition of mineral surfaces for adsorbing cations drives the metal distribution in heterogeneous systems.

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Abstract. Many countries will use deep geological repositories to dispose of highly active nuclear waste. Crystalline rock is a potential host rock because of its strong geotechnical stability, low permeability and low solubility; however, its inherent mineralogy is heterogeneous, consisting of a wide set of minerals in varying amounts. Therefore, there is a need for using sophisticated techniques that allow spatial resolution to characterize the nanostructure of such crystalline rock surfaces and the speciation of the actinides therein. As a representative for trivalent actinides, such as Am(III), Np(III), and Pu(III), which are expected to be present due to the reducing conditions encountered in a deep geological repository, we have chosen the actinide Cm(III). Cm(III) possesses excellent luminescence properties, which allows us to not only examine the sorption uptake but also the speciation of Cm(III) on the surface. We combined spatially resolved investigation techniques, such as vertical scanning interferometry, calibrated autoradiography, and Raman microscopy coupled to micro-focus time-resolved laser-induced luminescence spectroscopy (µTRLFS) (Molodtsov et al., 2019). Thus, we were able to correlate mineralogy, surface roughness, and grain boundary effects with radionuclide speciation, allowing us to identify important radionuclide retention processes and parameters (see Fig. 1). Investigations focused on granite from Eibenstock (Germany) and migmatised gneiss from Bukov (Czech Republic). Cm(III) sorption on the rock's constituting minerals – primarily feldspar, mica and quartz – was analyzed quantitatively and qualitatively. We observed that Cm(III) sorption uptake and speciation depends not only on the mineral phase but also the surface roughness (Demnitz et al., 2021). An increasing surface roughness leads to higher sorption uptake and a stronger coordination of the sorbed Cm(III). On the same mineral grains sorption differed significantly depending if an area exhibits a low or high surface roughness. In the case that one mineral phase dominates the sorption process, sorption of Cm(III) on other mineral phases will only occur at strong binding sites, typically where surface roughness is high. Areas of feldspar and quartz with high surface roughness additionally showed the formation of sorption species with particularly high sorption strength that could either be interpreted as Cm(III) incorporation species or ternary complexes on the mineral surface (Demnitz et al., 2021). We conclude that in addition to mineral composition, surface roughness needs to be adequately considered to describe interfacial speciation of contaminants and respective retention patterns for the safety assessments of nuclear waste repositories.