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

The aqueous solubility of BaSeO4 (cr) was studied at 23 +/- 2 degrees C 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(10) K-0 value for the BaSeO4 (cr) solubility product (BaSeO4 (cr) Ba2+ + SeO42- 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(10) K-0 = 3.44 +/- 0.12 for the reaction (Ba2+ + 2SeO(4)(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(10) K-0 for the solubility product of a proposed double salt (Na2Ba(SeO4)(2)(s) 2Na(+) + Ba2+ + 2SeO(4)(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).