Thermodynamic model for the solubility of BaSeO_4(cr) in the aqueous Ba~(2+)-SeO_4~(2?)-Na~+-H~+-OH~?-H_2O system: Extending to high selenate concentrations

摘要

The aqueous solubility of BaSeO_4(cr) was studied at 23 ± 2 °C as a function of Na_2SeO_4 concentrations (0.0001 to 4.1mol kg~(?1)) and equilibration periods (3 to 596 d). The equilibrium, approached fromboth the underand over-saturation directions, in this systemwas 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 BaSeO_4(cr) is the solubility-controlling phase for Na_2SeO_4 concentrations <0.59mol kg~(?1). The log_(10)K~0 value for the BaSeO_4(cr) solubility product (BaSeO_4(cr) Ba~(2+) + SeO_4~(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 BaSeO_4(cr) solubility product and Ba concentrations as a function of Na_2SeO_4 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 SITmodel required the inclusion of Ba(SeO_4)_2~(2?) species with log_(10) K~0 = 3.44±0.12 for the reaction (Ba~(2+) + 2SeO_4~(2?) Ba(SeO_4)_2~(2?)), whereas these species are not needed for Pitzer model, and 2) at Na_2SeO_4 concentrations >0.59mol kg~(?1) it was also possible to calculate the value for log_(10) K~0 for the solubility product of a proposed double salt (Na_2Ba(SeO_4)_2(s) 2Na~+ +Ba~(2+) +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/ionassociation parameters hitherto unavailable for both the SIT and Pitzer models required to fit these extensive data extending to as high ionic strengths as 12.3mol 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 themodel to bariumconcentrations 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 4.1mol kg~(?1).