Modeling Extraction of Uranyl Nitrate, Nitric Acid and Water by Tributyl Phosphate

A model of the extraction of uranyl nitrate, nitric acid and water from aqueous solution to the organic phase with tributyl phosphate (TBP) was developed. It is based on the model developed by Naganawa and Tachimori (H. Naganawa, S. Tachimori, Bull. Chem. Soc. Jap. 70, 809, (1997)), which successfully describes extraction of nitric acid and water with less than 10 TBP complexes. The introduction of 6 additional organic phase complexes allowed us to accurately describe extraction of water, nitric acid and uranyl nitrate into organic phase composed of 30% in volume of TBP at 25°C. The thermodynamic activities of the components in the aqueous phase were modeled by the extended Pitzer expression. This method 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 a meta heuristic optimization procedure, which suggests the formation of complexes containing 2 and 3 TBPs for 1 uranyl nitrate.