Uranium(VI) Uptake by Synthetic Calcium Silicate Hydrates
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Uranium(VI) Uptake by Synthetic Calcium Silicate Hydrates J. Tits1, T. Fujita2, M. Tsukamoto2 and E. Wieland1 1 Paul Scherrer Institute, Villigen PSI, Switzerland 2 Central Research Institute of the Electric Power Industry (CRIEPI), Tokyo, Japan ABSTRACT The immobilization of U(VI) by C-S-H phases under conditions relevant for the cementitious near field of a repository for radioactive waste has been investigated. C-S-H phases have been synthesized using two different procedures: the “direct reaction” method and the “solution reaction” method. The stabilities of alkaline solutions of U(VI) (presence of precipitates or colloidal material) were studied prior to sorption and co-precipitation tests in order to determine the experimental U(VI) solubility limits. These U(VI) solubility limits were compared with the U(VI) solubilities obtained from thermodynamic speciation calculations assuming the presence of combinations of different solid U(VI) phases. The solid phase controlling U(VI) solubility in the present experiments was found to be CaUO4(s). The U(VI) uptake kinetics and sorption isotherms on C-S-H phases with different C:S ratios were determined under various chemical conditions; e.g., sorption and co-precipitation experiments and different pH’s. U(VI) was found to sorb fast and very strongly on C-S-H phases with distribution ratios (Rd values) ranging in value between 103 L kg-1 and 106 L kg-1. Both sorption and co-precipitation experiments resulted in Rd values which were very similar, thus indicating that no additional sorption sites for U(VI) were generated in the co-precipitation process. Furthermore, C-S-H synthesis procedures did not have a significant influence on U(VI) uptake. The U(VI) sorption isotherms were found to be non-linear, and further, increasing Ca concentrations resulted in increasing U(VI) uptake. The latter observation suggests that U(VI) uptake is controlled by a solubility-limiting process, while the former observation further indicates that pure Ca-uranate is not the solubility-limiting phase. It is proposed that a solid solution containing Ca and UO 22+ could control U(VI) uptake by C-S-H phases. INTRODUCTION For disposal options involving the isolation of low- and intermediate-level radioactive waste in the cementitious near field of a deep underground geologic repository, uptake by calcium silicate hydrate (C-S-H) phases is expected to play an important role in retarding the migration of radionuclides in both the near field and the altered far field. In the near field, C-S-H phases are major components of hardened cement paste (e.g., [1]), and the interaction of hyperalkaline fluids from the repository with the mineral components of sedimentary rocks produces a range of C-SH-type secondary minerals in the far field (pH plume) [2]. Studies concerning the retention of trace concentrations of radionuclides by C-S-H phases have mainly focused on adsorption as the dominant uptake mechanism. However, for some radionuclides other uptake mechanisms such as co-precipitation (solid-solution f
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