A thermodynamic model for the solubility of barite and celestite in electrolyte solutions and seawater to 200°C and to 1 kbar

Abstract

This paper describes a model for barite and celestite solubilities in the Na–K–Ca–Mg–Ba–Sr–Cl–SO 4–H 2O system to 200°C and to 1 kbar. It is based on Pitzer's ion interaction model for the thermodynamic properties of the aqueous phase and on values of the solubility products of the solids revised in this work. It is shown how barite and celestite solubilities in electrolyte solutions can be accurately predicted as a function of temperature and pressure from previously determined Pitzer's parameters. The equilibrium constant for the BaSO 4(aq) ion pair dissociation reaction is calculated from recently reported barite solubility in Na 2SO 4 solutions from 0 to 80°C. Pressure corrections are evaluated through partial molal volume calculations and are partially validated by comparing model predictions to measured barite and celestite solubilities in pure water to 1 kbar and in NaCl solutions to 500 bars. The model is then used to investigate the tendency of ion pairing of Ca, Sr and Ba with sulfate in seawater. Finally, the activity coefficient of aqueous barium sulfate in seawater is calculated for temperature, pressure and salinity values found in the ocean and compared to published values.