Aqueous–solid solution thermodynamic model of U(VI) uptake in C–S–H phases

Abstract

Reliable thermodynamic models assessing the interaction of radionuclides with cementitious materials are important in connection with long-term predictions of the safe disposal of radioactive waste in cement-based repositories. In this study, a geochemical model of U(VI) interaction with calcium silicate hydrates (C–S–H phases), the main component of hardened cement paste (HCP), has been developed. Uranium(VI) sorption isotherms on C–S–H phases of different Ca:Si ratios (C:S) and structural data from spectroscopic studies provided the indispensable set of experimental data required for the model development. This information suggested that U(VI) is neither adsorbed nor incorporated in the Ca–O octahedral layers of the C–S–H structure, but rather is located in the interlayer, similar to Ca2+ and other cations. With a view to the high recrystallisation rates and the cryptocrystalline ‘gel-like’ structure of the C–S–H phases, these observations indicated a U(VI) uptake driven by the formation of a solid solution. The aqueous–ideal solid solution thermodynamic model of U(VI) uptake in C–S–H was developed using the Compound Energy Formalism (CEF) as an extension of the recently developed model for ‘pure’ C–S–H solubility. The sub-lattices proposed in the CSH3T model were occupied with U-bearing species defined in accordance with spectroscopic observations. This led to an initial set of nine C–S–H-U(VI) end members. Parameterization of the model was done using the GEM-Selektor code (http://gems.web.psi.ch/) and experimental sorption isotherms of U(VI) in C–S–H phases with 0.6 ⩽ C:S ⩽ 1.6 in the absence of alkalis, which allowed the end members [(CaO)2(UO3)(SiO2)2.5(H2O)5, (CaO)2(UO3)1.5(SiO2)2(H2O)5 and (CaO)3(UO3)1.5(SiO2)2(H2O)5.5] to be identified. The resulting CSH3T-U model with six end members was found to predict trends in U(VI) uptake by cementitious materials, even without introduction of ‘energetic’ non-ideality. Furthermore, reasonable agreement between modelling and U(VI) sorption data obtained from the cement-type zone of the Maqarin natural analogue was observed, suggesting that C–S–H phases might be participating in the U(VI) control at the site. The CSH3T-U model was further used to predict the effect of carbonate on the retention of U(VI) by cementitious materials. The presence of calcite (ubiquitous in conventional cement formulations) has no influence on the retention of U(VI) as the concentration of carbonate in solution is too low (<2 × 10−4 M) to influence U complexation under hyperalkaline conditions. However, the addition of free carbonate to the system accelerates the degradation of C–S–H by draining Ca2+ from the interlayer. At low C:S ratios, this effect can significantly reduce the retention of U by cementitious materials.