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.
Spatially resolved sorption of Cm(III) on crystalline rock: influence of surface roughness and mineralogy
