Co-precipitation of radium in high ionic strength systems: 1. Thermodynamic properties of the Na–Ra–Cl–SO4–H2O system – Estimating Pitzer parameters for RaCl2

The fate of divalent radium (Ra2+) in high ionic strength systems is an important issue with relevant aspects to environmental concerns. As a consequence, the description of the thermodynamic properties of Ra2+ in such systems is a critical matter which was not addressed yet. This can be done using the Pitzer formalism, which is regarded as the most accurate approach to describe activity coefficients of aquatic species in high ionic strength solutions. Isopiestic measurements, measurements of the electromotive force or the solubility of RaSO4,S in NaCl solutions, which are essential for the evaluation of the Pitzer ion interaction parameters, were not experimentally tested, due to the high radioactivity concentration which involves such experiments. Therefore, the present study examined the possibility to extrapolate the RaCl2 cation–anion interaction parameters (β(0), β(1) and Cϕ) by linear regression of other MCl2 cation–anion interaction parameters with their respective hydrated ionic radii (M2+ = Mg2+, Ca2+, Sr2+ and Ba2+).γRa2+γRa2+ calculated with the estimated RaCl2 parameters revealed that the ratios of activity coefficients γMg2+/γBa2+>γCa2+/γBa2+>γSr2+/γBa2+>1>γRa2+/γBa2+γMg2+/γBa2+>γCa2+/γBa2+>γSr2+/γBa2+>1>γRa2+/γBa2+ follow the opposite order of the ratios of hydrated ionic radii RMg2+/RBa2+