MR012-08
Uranium Surface Speciation in Clay-Rich Barrier Systems

Tuesday, 15 December 2020: 17:58
Virtual
Ruth M Tinnacher, California State University East Bay, Department of Chemistry & Biochemistry, Hayward, CA, United States, Michael S Massey, California State University East Bay, Hayward, CA, United States, Amrita Bhattacharyya, Lawrence Berkeley National Laboratory, Berkeley, CA, United States, Rinat I Gabitov, Mississippi State University, Mississippi State, MS, United States and Christophe Tournassat, BRGM, French Geological Survey, Orléans, France
Recorded Presentation
Abstract:
Clay-rich engineered barrier systems at future nuclear waste repositories are expected to limit radionuclide mobility due to their low permeability and high sorption affinity. However, for long-term predictions of contaminant mobility in these systems, a mechanistic understanding of radionuclide sorption reactions and radionuclide speciation on montmorillonite clay surfaces is needed.

In this study, we characterized uranium surface speciation on montmorillonite with the primary goal to examine the formation of U(VI)-carbonato surface complexes on Na‑montmorillonite. This type of surface complex has been invoked in a number of previously-published U(VI)-montmorillonite sorption models that do not specifically account for the “spill-over” effect, the influence of the electrostatic potential of basal surface sites on the potential of edge surface sites. However, our recently developed surface complexation model including the spill-over effect was able to predict U(VI) sorption onto Na-montmorillonite over a wide range of pH and pCO2 conditions without involving any U(VI)-carbonato surface complexes.

We have now characterized uranium surface speciation on montmorillonite by EXAFS spectroscopy over a range of pH conditions (pH 5, 7 and 8), total U and Ca concentrations, and in the presence and absence of atmospheric CO2. We also analyzed spectral data for natural and synthetic U-rich calcite samples for comparison. For montmorillonite samples, uranium L3-edge EXAFS spectra were surprisingly similar, even though chemical solution conditions had been varied. These preliminary results suggest that despite the fact that U(VI) solution speciation is substantially different under various chemical conditions, U surface speciation on Na‑montmorillonite may remain quite similar. These results support the current parametrization of our surface complexation model, which does not include any U(VI)‑carbonato surface complexes.

This research is funded by DOE’s Nuclear Energy University Program.

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