Thermodynamic Model for the Solubility of Ba(SeO₄, SO₄) Precipitates

Journal Article · Fri Aug 15 00:00:00 EDT 2014 · Radiochimica Acta, 102(8):711-721

DOI:https://doi.org/10.1515/ract-2013-2207· OSTI ID:1170480

Rai, D.; Felmy, Andrew R.; Moore, Dean A.; Kitamura, Akira; Yoshikawa, Hideki; Doi, Reisuke; Yoshida, Yasushi

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 log ₁₀ [Ba] ranging from -3.6 to -5.9 mol kg^-1, log ₁₀ [SeO₄] rangingfrom-3.6 to -5.2 mol kg^-1, and log 10 [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) (log ₁₀Κ°_sp = -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 range of BaSeO₄ mole fractions investigated in this study.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1170480
• Report Number(s): PNNL-SA-104137
• Journal Information: Radiochimica Acta, 102(8):711-721, Journal Name: Radiochimica Acta, 102(8):711-721
• Country of Publication: United States
• Language: English