The aqueous complexation of thorium with citrate under neutral to basic conditions

Summary

The aqueous complexation of thorium with citrate was investigated under neutral to basic conditions and over a broad range of ionic strengths. The solubility data for ThO₂(am) as a function of citrate concentration indicate the presence of stable species with citrate-to-metal ratios of between two to three. The dependence of the ThO₂(am) solubilities on hydrogen ion concentration can also be readily explained by the classical assumption of hydrolysis of the central Th(IV) ion to form mixed thorium-hydroxide-citrate complexes. ¹³C NMR spectra of the species in solution confirm that the citrate-to-metal ratio of the species in solution is between two and three and show that the citrate attaches to the metal in a bidentate fashion through oxygens on the α-carboxylate and α-alkoxyl groups, rather than through the carboxylate groups. The ¹³C NMR spectra, as well as a density functional theory (DFT) electronic structure study of the presumptive complexes, suggests that the associated α-hydroxyl proton can be displaced during complex formation. These findings indicate an alternative explanation for the observed changes in solubility as a function of hydrogen ion concentration, the displacement of protons from the citrate alkoxyl groups via metal binding. Removal of protons from the alkoxyl groups or hydrolysis of the central Th(IV) cannot be distinguished by thermodynamic measurements, however the species with the α-hydroxyl proton removed (i.e. ThOH(Cit)₂^5- and Th(Cit)₃^8-) would appear to better represent the microscopic binding. Apparent equilibrium constants for the solution phase reactions of these species and the hydrous thorium oxide have been calculated as a function of ionic strength.