Influence of carbonate complexation on the sorption of U(VI) on montmorillonite

Clay minerals play an important role in the retention/retardation of radio-contaminants in the near- and far-fields of a radioactive waste repository. Identifying and quantifying the radionuclide sorption processes occurring at clay/solution interface over a representative range of relevant conditions is indispensable for performance assessment.
Next to hydroxide ions, carbonate is the predominant inorganic ligand in most natural groundwaters and clay porewaters. Actinides are known to form very strong carbonate complexes in solution, which could potentially decrease the metal ion sorption and thus increase the migration rates of actinides. The aim of this study is:
To apply Extended X-Ray Absorbtion Spectroscopy to verify whether or not U(VI)-carbonato ternary complexes form at the montmorillonite surface

The macroscopic sorption data show a significant influence of dissolved carbonate on the sorption of U(VI)) on Na-montmorillonite.
Structural data obtained by EXAFS show unambiguously that U(VI) forms inner-sphere complexes at the montmorillonite surface (splitting of the Oeq shell, Si and Fe shell).
No difference is observed in the absence and the presence of carbonate (no C shell ~ 2.90 Å [1], no Odist shell), indicating that no U(VI)-carbonato ternary complexes
form at the montmorillonite surface.