Influence of surface roughness on the sorption of Cm(III)

INTRODUCTION

Many countries will use deep geological repositories to dispose of their highly radioactive nuclear waste. With an international perspective, 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 to characterize the nanostructure of such crystalline rock surfaces 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), which are expected to be present due to the reducing conditions encountered in a deep geological repository, we have chosen the actinide Cm(III). It possesses excellent luminescence properties, allowing us to determine sorption uptake and molecular speciation of Cm(III) on the surface.

DESCRIPTION OF THE WORK

Investigations focused on cleaved orthoclase (K-feldspar) single crystals as well as different crystalline rocks stemming from various regions in Europe. Cleaved pieces of orthoclase or thin sections of the crystalline rocks were immersed in a sorption solution containing [Cm(III)] = 5∙10-7 – 10-5 M. The experiments were undertaken at selected pH values (5.0 and 6.9) and different inherent mineralogical complexity of the systems. Subsequently, we applied correlated spectroscopy to analyze the samples. We combined vertical scanning interferometry, calibrated autoradiography, and Raman microscopy coupled to µTRLFS. 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. For experimental details, see [1].

RESULTS AND DISCUSSION

We observed that Cm(III) sorption uptake and speciation depends not only on the mineral phase, but also the surface roughness. Already on single crystal orthoclase an increasing surface roughness leads to higher sorption uptake (see Fig. 1) and to a stronger coordination of the sorbed Cm(III).
Increasing the mineral complexity of the system further, we used thin sections of various crystalline rocks, consisting mainly of feldspar, quartz, and mica. We observed significant differences in sorption uptake on individual mineral phases as well as the resulting speciation. In case that one mineral phase dominates the sorption process, e.g. amphibole, 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 adsorbed ternary complexes on the mineral surface.[2]
At pH values of around 6.8 Cm(III) generally sorbs more weakly to the surface, while preferentially targeting mineral phases such as mica instead of feldspar or quartz. At a higher pH of 8.0 the sorption uptake on other mineral phases increases, with a general trend towards more strongly bound Cm(III) surface 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.

Acknowledgements:

This work has been developed in the frame of the iCross project. We gratefully acknowledge funding provided by the German Federal Ministry of Education and Research (BMBF, Grant 02NUK053) and the Helmholtz Association (Grant SO-093).

REFERENCES

1. Molodtsov et al., “Sorption of Eu(III) on Eibenstock granite studied by µTRLFS: A novel spatially-resolved luminescence-spectroscopic technique” Scientific Reports, 9, Article Number 6287 (2019), https://doi.org/10.1038/s41598-019-42664-2.

2. Demnitz et al., “Effects of surface roughness and mineralogy on the sorption of Cm(III) on crystalline rock” Journal of Hazardous Materials, (2021), https://doi.org/10.1016/j.jhazmat.2021.127006.