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Yuanxian Xia, Andrew R. Felmy, Linfeng Rao, Zheming Wang, Nancy J. Hess

Thermodynamic model for the solubility of ThO₂(am) in the aqueous Na⁺-H⁺-OH^--NO₃^--H₂O-EDTA system

The solubility of ThO₂(am) in the aqueous Na⁺-H⁺-OH^--NO₃^--H₂O-EDTA system as a function of pC_H⁺ (= - log [H⁺]) and variable NaNO₃ (0.5M to 6.0M) has been determined. The experimental observations show that between pC_H⁺ values 4.2 to 8.2, a stoichiometric 1:1 Th-EDTA complex forms that completely saturates the added chelate concentration. Th concentrations then decrease linearly with increasing pC_H⁺ at pC_H⁺>9.0. These changes in solubility are not predicted by currently available thermodynamic models. The ion-interaction model of Pitzer was used to interpret these solubility data. Thermodynamic analysis indicates that the speciation under basic conditions is dominated by monomeric mixed Th-OH-EDTA complexes. X-ray absorption near-edge spectroscopy confirmed the absence of the higher order ( e.g. dimeric) species. The equilibrium constants for the following reactions were determined from analysis of the solubility data: ThO₂(am) + EDTA^4- + 2H₂O ↔ Th(OH)₂EDTA^2- + 2OH^-, log K = -6.0; and ThO₂(am)+EDTA^4-+2H₂O ↔ Th(OH)₃EDTA^3-+ OH^-, log K∼-7.5. Pitzer ion-interaction parameters for the Th(OH)₂EDTA^2- and the Th(OH)₃EDTA^3- species were calculated. It was also determined that the solubility method for examining the complexation of tetravalent actinides with EDTA is limited to relatively high aqueous EDTA concentrations, relative to the amount of ThO₂(am) precipitate, owing to the adsorption of the EDTA chelator by the solid phase.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 91, 12/2003
Pages: 751

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