Journal Article
Radiochimica Acta, vol. 93, iss. 8, pp. 443-455, 2009
Authors
Dhanpat Rai, Mikazu Yui, Nancy J. Hess, Andrew R. Felmy, Dean A. Moore
Abstract
SummaryStudies were conducted on the dissolution of Th-doped borosilicate glass, a complex assemblage of 24 different elements in various proportions, in a wide range of pH values (0.5 to 12) and carbonate (as high as 6.2 m) and bicarbonate (as high as 1.0 m) concentrations, and as a function of time to determine whether the observed thorium concentrations exhibit an equilibrium phenomenon and to ascertain whether existing thermodynamic data can be used to interpret these results. Measurable Th concentrations were observed in either the very acidic (pH 0.5 to 4) solutions or alkaline solutions containing relatively high carbonate/bicarbonate concentrations. Steady state Th concentrations were reached over time from both the oversaturation and undersaturation directions, indicating that Th concentrations are controlled by an equilibrium solubility phenomenon. The XRD, EXAFS, and thermodynamic analyses of solubility data failed to definitively identify the nature of the solubility-controlling solid in this complex system; however, the data suggests that the solubility-controlling solid most likely involves the Th-silicate phase and that the observed Th concentrations are up to many orders of magnitude lower than in equilibrium with ThO2(am). The solubility study also showed that existing thermodynamic data for carbonato complexes of Th can be used to reliably predict Th concentrations in relatively dilute to concentrated carbonate/bicarbonate solutions.
