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

Abstract

The aqueous solubility of BaSeO₄(cr) was studied at 23 ± 2 ℃ as a function of Na₂SeO₄ concentrations (0.0001 to 4.1 mol kg^–1) and equilibration periods (3 to 596 d). The equilibrium, approached from both the under- and over-saturation directions, in this system was reached rather rapidly (≤3 d). The SIT and Pitzer's ion-interaction models were used to interpret these data and the predictions based on both of these models agreed closely with the experimental data. Thermodynamic analyses of the data show that BaSeO₄(cr) is the solubility-controlling phase for Na₂SeO₄ concentrations <0.59 mol kg^–1. The log ₁₀K⁰ value for the BaSeO₄(cr) solubility product (BaSeO₄(cr) ⇌ Ba²⁺+ SeO₄^2–) calculated by the SIT and Pitzer models were very similar (− 7.32 ± 0.07 with Pitzer and − 7.25 ± 0.11 with SIT). Although the BaSeO₄(cr) solubility product and Ba concentrations as a function of Na₂SeO₄ concentrations predicted by both the SIT and Pitzer models are similar, the models required different sets of fitting parameters. For examples, 1) interpretations using the SIT model required the inclusion of Ba(SeO₄)₂^2– species with log ₁₀K⁰ = 3.44 ± 0.12 for the reaction (Ba²⁺+ 2SeO₄^2– ⇌ Ba(SeO₄)₂^2–), whereas these species are not needed for Pitzer model, and 2) at Na₂SeO₄ concentrations >0.59 mol kg^–1 it was also possible to calculate the value for log ₁₀K⁰ for the solubility product of a proposed double salt (Na₂Ba(SeO₄)₂(s) ⇌ 2Na⁺+ Ba²⁺+ 2SeO₄^2–) which for the SIT model is − (8.70 ± 0.29) whereas for the Pitzer model it is − (9.19 ± 0.19). The ion-interaction/ion-association parameters hitherto unavailable for both the SIT and Pitzer models required to fit these extensive data extending to as high ionic strengths as 12.3 mol kg^–1 were determined. The model developed in this study is consistent with all of the reliable literature data, which was also used to extend the model to barium concentrations as high as 0.22 mol kg^–1 and pH ranging from 1.4 to 13.8, in addition to selenium concentrations as high as 4.1 mol kg^–1.