An aqueous thermodynamic model is proposed to describe the solubility of Th(IV) hydrous oxide in the aqueous Na+-HCO3--CO32--OH--ClO4--H2O system extending to high concentration at 25 degrees C. This model is relatively 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 (1.31 and 30, respectively). Reconciliation of all of the experimental solubility data for Th(IV) hydrous oxide in NaClO4 media required the introduction of a large mixing parameter for the highly charged Th(CO3)(4)(-). The relatively large values required for the ion interaction parameters beta((0)) and beta((1)), together with commensurately large mixing terms with the bulk anionic species, resulted in considerable uncertainty in determining standard state equilibrium constants for the formation of the highly charged Th(CO3)(5)(6-) species. This uncertainty 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 implies that formation of this species depends strongly upon the bulk ionic media. X-ray absorption results, confirming the presence of the thorium pentacarbonate species in concentrated bicarbonate and carbonate solutions, are also included.
