Correlation between Extraction Equilibrium of Uranium(VI) and Density of CO2 Medium in a HNO3/Supercritical CO2−Tributyl Phosphate System

The extraction equilibrium of U(VI) between a nitric acid solution and a supercritical CO2 phase containing tributyl phosphate (TBP) is formulated taking into account that (i) a distribution ratio of a metal extracted is a function of a distribution constant of each component involved in the extraction reaction, (ii) the distribution constant is defined as a ratio of solubilities of the component in both phases, and (iii) the solubility in the CO2 phase is correlated with density of CO2. A simple linear relationship between the distribution ratio, D U, of U(VI) and density, ρ, of CO2 is derived; log D U = a log ρ + A + B, in which a is a proportional constant implying the solvation characteristic of the solute in supercritical CO2, A is a pressure-independent constant, and B is a variable determined by the distribution equilibrium of HNO3. The equation derived was verified experimentally by the measurement of the distribution ratio of U(VI) and HNO3 under various conditions of pressure and temperature. A novel concept of selective supercritical fluid extraction of metals by means of pressure-tuning or CO2 density-tuning was proposed.