Characterization of Th carbonate solutions using XAS and implications for thermodynamic modeling

The chemical behavior of actinide elements in tank solutions, in soil, and in groundwater is dependent upon the chemical species that form when aqueous solutions come in contact with the actinide compounds. In particular the chemical speciation of the reduced actinide oxidation states (III and IV) are important, for example, to DOE waste tank processing and, more generally, to nuclear waste disposal issues. Predicting the solubility of the actinides in these solutions requires identification of the strong aqueous complexes, such as carbonates and organic chelating agents, that can form in aqueous solution. Previous speciation work has often relied on indirect techniques such as potentiometric titrations or solubility measurements. Recent XAS experiments determine directly the speciation of the Th carbonato species of seven solutions under a range of carbonate concentrations and pH conditions. The presence of the pentacarbonato complex is confirmed and the complex`s stability at low carbonate concentrations is determined. These experimental results support a proposed thermodynamic model that describes the solubility of Th(IV) hydrous oxide in the aqueous Na{sup +}-HCO{sub 3}{sup {minus}}-CO{sub 3}{sup 2{minus}}-OH{sup {minus}}-ClO{sub 4}{sup {minus}}-H{sub 2}O system extending to high concentrations at 25 C. This model is relatively simple in that only two aqueous species are includedmore » Th(OH){sub 3}CO{sub 3}{sup {minus}} and Th(CO{sub 3}){sub 5}{sup 6{minus}}.« less