Evaluation of thermodynamic data for aqueous Ca-U(VI)-CO3 species under conditions characteristic of geological clay formation

Uranium(VI) forms stable Ca-U(VI)-CO3 ternary complexes, which are highly mobile in the environment. This characteristic can be critical as regards radioactive waste disposal deep geological formations. While formation of aqueous complexes has been clearly established and identified under room temperature conditions, effect on their barely investigated. An experimental study was thus designed to evaluate stability these with (20–80 °C) well-controlled physicochemical conditions typical clay rock (especially CO2 pressure). method is based a competition between processes complex uranium retention an anion-exchange resin. A modelling approach PhreeqC code, using Davies equation for calculation activity coefficients, carried out derive thermodynamic data complexes. The Van't Hoff Equation applied explain variation constant values function temperature. work suggests: log10β113o= 27.27 ± 0.04 log10β213o= 29.8 0.2 CaUO2(CO3)32- Ca2UO2(CO3)3(aq) respectively, hence good agreement literature data. enthalpy reaction determined here from experiments suggests that does not facilitate (ΔrHmo = −24.4 2.2 kJ mol−1), whereas it very little −3 10 mol−1).