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

Abstract

Abstract The solubility of ThO2(am) in the aqueous Na+-H+-OH--NO3 --H2O-EDTA system as a function of pCH⁺ (= -log[H+]) and variable NaNO3 (0.5M to 6.0M) has been determined. The experimental observations show that between pCH⁺ 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 pCH⁺ at pCH⁺>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) + EDTA4- + 2H2O ↔ Th(OH)2EDTA2- + 2OH-, log K = -6.0; and ThO2(am) + EDTA4- + 2H2O ↔ Th(OH)3EDTA3- + OH-, log K∼-7.5. Pitzer ion-interaction parameters for the Th(OH)2EDTA2- and the Th(OH)3EDTA3- 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.