Thermodynamics of AFm-(I2, SO4) solid solution and of its end-members in aqueous media

Abstract

► Solid solution formation between AFm-SO 4 and AFm-I 2 . ► AFm-SO 4 as a good candidate for retarding 129 I. ► Gibbs energy of AFm-(I 2 , SO 4 ) solid solution obtained from solubility experiments. ► Sub-regular solid solution model with end-members defined using Vanselow convention. Cementitious materials are foreseen as being used for the immobilization of radionuclides in deep-storage facilities for nuclear waste. The 129 I contained in the waste is of major concern due to its long half-life and its anionic speciation in groundwater (I − ), which hinders the retention by most minerals due to their negatively charged structural surfaces at near-neutral pH. AFm-SO 4 is one of the phases present in cementitious materials. Experimental data suggest that the formation of solid solution between AFm-SO 4 and AFm-I 2 has the potential to retard 129 I mobility. In order to improve the present geochemical models, thermodynamic properties of AFm-I 2 and of its solid solution with AFm-SO 4 were evaluated based on precipitation and dissolution experiments in aqueous solutions between 4 °C and 60 °C. The solubility product of AFm-I 2 was determined at 23 °C ± 2 °C to be log K s 0 = - 13.57 ± 0.28 (2 σ ) for the dissolution reaction Ca 2 Al ( OH ) 6 I · 2 H 2 O ↔ 2 Ca 2 + + Al ( OH ) 4 - + 2 OH - + I - + 2 H 2 O . The Gibbs energy of formation of AFm-I 2 at standard state (1 bar, 25 °C) was assessed to be −3325.0 ± 2.4 (2 σ ) kJ mol −1 . The experimental data for AFm-(I 2 , SO 4 ) phases were consistent with the formation of a continuous solid solution without miscibility gaps. A good fit to these data was obtained using a sub-regular solid solution model with negative Margules interaction parameters W SO4 = −1.0 kJ mol −1 and W I = −8.0 kJ mol −1 , and end-members (Ca 2 Al(OH) 6 I and Ca 4 Al 2 (OH) 12 SO 4 ) defined according to the Vanselow convention for ion exchange. The results show the potential of AFm-SO 4 to act as a sink for 129 I, notably in carbonate-free systems. Small amounts of calcite can hamper the formation of AFm-(I 2 , SO 4 ) solid solution. The study provides relevant thermodynamic data for geochemical modeling of 129 I in the cementitious near-field of a nuclear waste repository.