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

Cementitious materials are foreseen as being used for the immobilization of radionuclides in deep-storage facilities for nuclear waste. The 129I 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-SO4 is one of the phases present in cementitious materials. Experimental data suggest that the formation of solid solution between AFm-SO4 and AFm-I2 has the potential to retard 129I mobility. In order to improve the present geochemical models, thermodynamic properties of AFm-I2 and of its solid solution with AFm-SO4 were evaluated based on precipitation and dissolution experiments in aqueous solutions between 4 °C and 60 °C. The solubility product of AFm-I2 was determined at 23 °C ± 2 °C to be log K s 0 = - 13.57 ± 0.28 (2σ) 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-I2 at standard state (1 bar, 25 °C) was assessed to be −3325.0 ± 2.4 (2σ) kJ mol−1. The experimental data for AFm-(I2, SO4) 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 WSO4 = −1.0 kJ mol−1 and WI = −8.0 kJ mol−1, and end-members (Ca2Al(OH)6I and Ca4Al2(OH)12SO4) defined according to the Vanselow convention for ion exchange. The results show the potential of AFm-SO4 to act as a sink for 129I, notably in carbonate-free systems. Small amounts of calcite can hamper the formation of AFm-(I2, SO4) solid solution. The study provides relevant thermodynamic data for geochemical modeling of 129I in the cementitious near-field of a nuclear waste repository.