U(VI) sorption by Ca-bentoniteunder alkaline and saline conditions as a function of pH and carbonate content

Bentonite is considered as buffer and backfill material within the geo-technical barrier of such a repository. Therefore, profound understanding of the uranium retention processes in bentonite under environmentally relevant conditions is essential for long-term safety assessment. As the pore water chemistry of North German clay formations is characterized by high ionic strengths, corrosion of concrete present in the repository, is promoted. Upon corrosion, hyperalkaline (10 < pH < 13) cement pore waters evolve [3], which can alter the retention potential of bentonite towards radionuclides. Batch sorption experiments as a function of pH, c(U), SLR and carbonate content were conducted in combination with spectroscopic techniques such as time-resolved laser-induced fluorescence spectroscopy (TRLFS) and attenuated total reflectance Fourier-transform infrared (ATR FT-IR) spectroscopy in order to gain insight into the underlying processes on the macroscopic as well as on the molecular level. Lower U(VI) sorption in the presence of carbonate in the solution up to pH 9.5 can be explained with the formation of only weakly adsorbing uranyl carbonate species. TRLFS spectra verify the prevalence of these complexes already at low carbonate concentrations in the solution. In the pH region 10 – 12, an almost complete retention of U(VI) was observed in the absence as well as at low carbonate concentrations. TRLFS measurements reveal an abrupt change in U(VI) speciation at pH 10.5, to uranyl hydroxo complexes even at low carbonate concentrations (experiments at ambient atmosphere). TRLFS spectra suggest that anionic uranyl hydroxo complexes prevail in the pH region 10.5 – 12. Beside the adsorption of those, also precipitation processes are considered as a possible cause for the removal. pH-dependent U(VI) solubility tests have shown that the U(VI) retention in this pH region is apparently a combination of adsorption and precipitation processes. By contrast, at higher carbonate concentrations the U(VI) retention in the pH region 9.5 – 11 is very low . Under these conditions uranyl-carbonate complexes dominate the speciation up to pH 11.5. Consequently, the U(VI) speciation and the resulting sorption behavior in the (hyper)alkaline environment is highly dependent on the amount of carbonate in the solution.