Influence of pH, carbonate and calcium concentration on U(VI) retention by clay minerals at (hyper)alkaline conditions – A batch sorption and spectroscopy study

Clays are considered as potential host rocks and backfill material for deep geological repositories for radioactive waste. Therefore, profound understanding of radionuclide retention processes at clay mineral surfaces is essential for a long-term safety assessment. As a result of the degradation of concrete within such a repository, hyperalkaline cement pore waters can evolve. Since the U(VI) sorption behavior at alkaline conditions is still poorly understood, batch experiments were combined with spectroscopic investigations in order to gain insight into the underlying retention processes on the molecular level.
U(VI) batch sorption experiments (pH 8-13) with various clay minerals at different carbonate concentrations (absence, 0.5 and 100 mM) showed a decreased U(VI) retention in the presence of carbonate up until a certain pH (pH 9.5 or pH 11, depending on [CO32-]) due to the formation of weakly sorbing uranyl carbonate complexes in aqueous solution, confirmed by time-resolved laser-induced fluorescence spectroscopy (TRLFS). This is in accordance with previous studies. However, also in the presence of carbonate, U(VI) retention is increased in even stronger alkaline solutions, which is attributed to the preferred formation of hydrolyzed U(VI) species.
In order to clarify the mechanisms responsible for the very strong U(VI) retention in the pH range 10-12 (absence and 0.5 mM CO32-), uranyl complexes on Ca-bentonite surfaces were examined directly, using site-selective TRLFS and extended X-ray absorption fine structure (EXAFS) spectroscopy (ESRF, Grenoble). Both techniques showed the presence of two different U(VI) surface complexes and no indication for U(VI) precipitation. Consequently, under the given conditions, adsorption is the dominant retention process despite the negative mineral surface charge and the anionic character of prevailing aqueous U(VI) species (i.e. UO2(OH)3-). The retention could be realized by mediating cations, which adsorb to the mineral surface in the first place, leading to a local compensation of negative surface charge. Experiments with different calcium concentrations confirmed that the presence of calcium significantly enhances the U(VI) retention between pH 10 an 12.