Journal Article
Radiochimica Acta, vol. 102, iss. 8, pp. 711-721, 2014
Authors
Dhanpat Rai, Andrew R. Felmy, Dean A. Moore, Akira Kitamura, Hideki Yoshikawa, Reisuke Doi, Yasushi Yoshida
Abstract
Abstract
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] ranging from − 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 10
K
○
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.
