Aqueous–solid solution thermodynamic model of U(VI) uptake in C–S–H phases Xavier Gaona ⇑ , Dmitrii A. Kulik, Nathalie Macé 1 , Erich Wieland Laboratory for Waste Management, Paul Scherrer Institut, Villigen PSI, Switzerland article info Article history: Received 22 February 2011 Accepted 10 September 2011 Available online 16 September 2011 Editorial handling by R. Fuge 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 devel- opment. This information suggested that U(VI) is neither adsorbed nor incorporated in the Ca–O octahe- dral layers of the C–S–H structure, but rather is located in the interlayer, similar to Ca 2+ 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 mem- bers. 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 6 C:S 6 1.6 in the absence of alkalis, which allowed the end members [(CaO) 2 (UO 3 )(SiO 2 ) 2.5 (H 2 O) 5 , (CaO) 2 (UO 3 ) 1.5 (SiO 2 ) 2 (H 2 O) 5 and (CaO) 3 (UO 3 ) 1.5 (SiO 2 ) 2 (H 2 O) 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 Ca 2+ from the interlayer. At low C:S ratios, this effect can significantly reduce the retention of U by cementitious materials. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Uranium plays a central role in the nuclear fuel cycle and in the production of nuclear weapons. It is a significant component and long-term dose contributor in high and intermediate level waste forms. Furthermore, it has become a widespread subsurface con- taminant mostly due to mining, milling and isotopic enrichment activities (Abdelouas et al., 1999), and as a result of its military application in DU-based (depleted uranium) munitions (Alvarez et al., 2006; Oliver et al., 2008). Under oxidizing conditions, the chemistry of U is controlled by its +VI redox state, which shows a higher mobility than the +IV re- dox state dominant under reducing conditions. In the alkaline to hyperalkaline pH range, U(VI) also remains stable under anoxic and slightly reducing conditions. The mobility of U(VI) in subsur- face environments is mainly controlled by dissolution/precipita- tion of U solids and adsorption to minerals. For both mechanisms, silicates seem to play an important role as they strongly interact with U under a wide variety of environmental conditions (Reich et al., 1998; Allard et al., 1999; Hudson et al., 1999; Waite et al., 2000; Gabriel et al., 2001; Froideval et al., 2003; Arai et al., 2006; Chardon et al., 2008). In the framework of radioactive waste management, major efforts have been under- 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.09.005 ⇑ Corresponding author. Present address: Institut für Nukleare Entsorgung, Karlsruhe Institute of Technology, Karlsruhe, Germany. E-mail address: xavier.gaona@kit.edu (X. Gaona). 1 Present address: CEA-Saclay, DEN/DANS/DPC/SECR/L3MR, 91191 Gif-Sur-Yvette Cedex, France. Applied Geochemistry 27 (2012) 81–95 Contents lists available at SciVerse ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem