Influence of calcium on uranium and neptunium sorption on clay minerals at (hyper)alkaline conditions


Influence of calcium on uranium and neptunium sorption on clay minerals at (hyper)alkaline conditions

Philipp, T.; Schmeide, K.; Stumpf, T.

Our previous experiments have shown that U(VI) is retained very strongly by Ca-bentonite in the pH range 10-12. Different spectroscopic techniques (site-selective TRLFS, EXAFS) have proven that the underlying retention mechanism under the given conditions was adsorption (not precipitation) despite the negative mineral surface charge and the anionic character of prevailing aqueous U(VI) species. It was hypothesized that attachment is facilitated by mediating calcium cations which are present in the solution. Therefore, the influence of calcium on the sorption of U(VI), Np(V) and Np(VI) at alkaline conditions was systematically studied. These radionuclides were selected as their aqueous speciation at high pH values is characterized by the predominance of anionic hydroxide species. Furthermore the adsorption of 45Ca on clay minerals and the resulting effect on the mineral surface charge was examined.
It was found that 45Ca adsorbed almost completely on Ca-bentonite between pH 8 and 13. Zeta potential measurements showed a partial compensation of the strongly negative surface charge of Ca-bentonite upon introduction of calcium. Hence, calcium is present at the surface, offering possible sorption sites for anionic uranium and neptunium species. Batch sorption experiments at different calcium concentrations revealed enormous effects on the retention of U(VI), Np(V) and Np(VI) between pH 10 and 13. Exemplarily shown for U(VI) in the figure below, the strong retention could not be observed in the experiment with kaolinite, where calcium was completely absent. Consequently, calcium is a crucial factor for the safety assessment of deep geological radioactive waste repositories, where (hyper)alkaline conditions evolve due to cement degradation.

Keywords: Calcium; U(VI); Np(V); Np(VI); adsorption; surface charge

  • Lecture (Conference)
    2nd International Conference on Radioanalytical and Nuclear Chemistry, 05.-10.05.2019, Budapest, Ungarn

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Publ.-Id: 28936