THERMODYNAMIC MODELS FOR HIGHLY-CHARGED AQUEOUS SPECIES - SOLUBILITY OF TH(IV) HYDROUS OXIDE IN CONCENTRATED NAHCO3 AND NA2CO3 SOLUTIONS

An aqueous thermodynamic model is proposed to describe the solubility of Th(IV) hydrous oxide in the aqueous Na+-HCO3--CO32--OH--ClO4--H2O s ystem extending to high concentration at 25 degrees C. This model is r elatively simple in that only two aqueous species are included: Th(OH) (3)CO3- and Th(CO3)(5)(6-). Fitter ion interaction parameters, beta((0 )) and beta((1)) for Na+ with Th(CO3)(5)(6-), are also determined (1.3 1 and 30, respectively). Reconciliation of all of the experimental sol ubility data for Th(IV) hydrous oxide in NaClO4 media required the int roduction of a large mixing parameter for the highly charged Th(CO3)(4 )(-). The relatively large values required for the ion interaction par ameters beta((0)) and beta((1)), together with commensurately large mi xing terms with the bulk anionic species, resulted in considerable unc ertainty in determining standard state equilibrium constants for the f ormation of the highly charged Th(CO3)(5)(6-) species. This uncertaint y is a result of the large contributions from beta((0)) and beta((1)) to the excess solution free energy at the concentrations (0.1m) where this species becomes important. The magnitude of the mixing term impli es that formation of this species depends strongly upon the bulk ionic media. X-ray absorption results, confirming the presence of the thori um pentacarbonate species in concentrated bicarbonate and carbonate so lutions, are also included.