Development of accurate chemical thermodynamic database for geochemical storage of nuclear waste. Part III: Models for predicting solution properties and solid-liquid equilibrium in cesium binary and mixed systems

Abstract

The models described in this study are of high importance in the development of thermodynamic database needed for nuclear waste geochemical storage as well as for technology for extracting cesium resources from saline waters. In this study we developed new not concentration restricted thermodynamic models for solution behavior and solid-liquid equilibrium in CsF-H2O CsOH-H2O and Cs2SO4-H2O systems at 25 °C. To parameterize models we used all available experimental osmotic coefficients data for whole concentration range of solutions and up to saturation point. The new models are developed on the basis of Pitzer ion interactions approach. The predictions of new developed here models are in excellent agreement with experimental osmotic coefficients data (ϕ) in binary solutions from low to extremely high concentration (up to 21.8 mol.kg-1 for CsOH-H2O and up to 35.6 mol.kg-1 for CsF-H2O). The previously developed by Christov by Christov and co-authors and by other authors Pitzer approach based thermodynamic models for five (5) cesium binary systems (CsCl-H2O CsBr- H2O CsI-H2O CsNO3-H2O and Cs2SeO4- H2O) are tested by comparison with experimental osmotic coefficients data and with recommendations on activity coefficients (γ±) in binary solutions. The models which give the best agreement with (ϕ)- and (γ±) -data from low to high concentration up to m(sat) are accepted as correct models which can be used for solubility calculations in binary and mixed systems and determination of thermodynamic properties of precipitating cesium solid phases. The thermodynamic solubility products (ln Kosp) and the Deliquescence Relative Humidity (DRH) of solid phases precipitating from saturated cesium binary solutions (CsF(cr) CsCl(cr) CsBr(cr) CsI(cr) CsOH(cr) CsNO3(cr) Cs2SO4(cr) and Cs2SeO4(cr)) have been determined on the basis of evaluated and accepted binary parameters and using experimental solubility data. The reported mixing parameters [θ(Cs M2+) and ψ(Cs M2+ X)] evaluated by solubility approach for 15 cesium mixed ternary systems (CsCl-MgCl2-H2O CsBr-MgBr2-H2O CsCl-NiCl2-H2O CsBr-NiBr2-H2O CsCl-MnCl2-H2O CsCl-CoCl2-H2O CsCl-CuCl2-H2O CsCl-CsBr-H2O CsCl-RbCl-H2O Cs2SO4-CoSO4-H2O Cs2SeO4-CoSeO4-H2O Cs2SO4-NiSO4-H2O Cs2SeO4-NiSeO4-H2O Cs2SO4-ZnSO4-H2O and Cs2SeO4-ZnSeO4-H2O) are tabulated.