Thermodynamic model for the solubility of BaSeO4(cr) in the aqueous Ba2+-SeO42--Na+-H+-OH--H2O system: Extending to high selenate concentrations

The solubility of Ba(SeO4, SO4) precipitates was determined as a function of the BaSeO4 mole fractions, ranging from 0.0015 to 0.3830, and time with an equilibration period extending to as long as 302 days. Equilibrium/steady state conditions in this system are reached in ≤ 65 days. Pitzer’s ion interaction model was used to calculate solid and aqueous phase activity coefficients. Thermodynamic analyses showed that the data do not satisfy Gibbs-Duhem equation, thereby demonstrating that a single-solid solution phase does not control both the selenate and sulfate concentrations. Our extensive data with log10 [Ba]) ranging from -3.6 to -5.9 mol.kg-1, log10 [SeO4]) ranging from -3.6 to -5.2 mol.kg-1, and log10 [SO4] ranging from -4.0 to -5.3 mol.kg-1 can be explained with the formation of an ideal BaSeO4 solid solution phase that controls the selenium concentrations and a slightly disordered/less-crystalline BaSO4(s) (log10 K0sp = -9.5 instead of -10.05 for barite) that controls the sulfate concentrations. In these experiments the BaSO4 component of the solid solution phase never reaches thermodynamic equilibrium with the aqueous phase. Thermodynamic interpretations of the data show that both the ideal BaSeO4 solid solution phase and less-crystalline BaSO4(s) phase are in equilibrium with each other in the entire rangemore » of BaSeO4 mole fractions investigated in this study.« less