(242f) Modeling Partition Equilibria for Uranyl Nitrate, Nitric Acid and Water Extracted by Tributyl Phosphate in Dodecane

A new model for the extraction of uranyl nitrate, nitric acid and water from aqueous solution to organic phases containing tributyl phosphate (TBP) was recently developed. It is based on the model developed by Naganawa and Tachimori (H. Naganawa, S. Tachimori, Bull. Chem. Soc. Jap. 70, 809, (1997)), which describes the extraction of nitric acid and water by assuming the formation of 10 complexes involving TBP molecules, nitric acid and water in the organic phase. It is demonstrated that the introduction of 6 additional organic phase complexes allows to accurately describe extraction of water, nitric acid and uranyl nitrate into organic phase composed of 30% in volume of TBP (PUREX process) at 25°C. The thermodynamic activities of the components in the aqueous phase are modeled by the extended Pitzer expression, which gives a good representation of the osmotic coefficient for pure binary aqueous solutions of uranyl nitrate and nitric acid up to 4 mol L^-1 and 10 mol L^-1, respectively. The stoichiometries of the 6 additional complexes were obtained one by one by the differential evolution meta heuristic optimization algorithm. The results suggest the formation of complexes containing 2 and 3 TBPs for 1 uranyl nitrate with additional water molecules.