Pressure dependence of extraction behavior of plutonium and uranium(VI) from nitric acid solution to supercritical carbon dioxide containing tributylphosphate

Plutonium(IV) and uranium(VI) were extracted into supercritical CO2 fluid phase (SF-CO2) containing tributylphosphate (TBP) with equilibrium distribution ratios, D, e. g., D-Pu(IV) = 3.1 and D-U(VI) = 2.0, for the extraction of 2 x 10(-3) M Pu(IV) and U(VI) from 3 M HNO3 into SF-CO2 containing0.3 M TBP at 60 degreesC and 15 MPa A simple linear relation between D and density rho of CO2; log D a log rho + b ( a,b; constants), was observed, which was explained theoretically by the formulation of the extraction equilibrium taking into account the phase distribution behavior of extractant TBP and extracted species, i. e. Pu(IV)- and U(VI)-TBP complexes involved in the el;traction reaction. The slopes a of the log D vs. log rho plots were -(1.6 +/- 0.1) and -(2.7 +/- 0.5) for the extraction of Pu(IV) and U(VI), respectively. The differences in D as well as the slope a between Pu(IV) and U(VI) make it possible to design the U and Pu separation method by which one can achieve an enhancement of the extraction efficiency and selectivity by tuning the operation pressure. (C) 2000 Elsevier Science Ltd. All rights reserved.