Highly Efficient Plutonium Extraction from Nitric Acid Feeds Using a Phosphonium Phosphinate Task-Specific Ionic Liquid

A phosphonium phosphinate-functionalized ionic liquid, Cyphos IL 104, has been evaluated for the highly efficient extraction of Pu(IV) from aqueous feeds containing nitric acid, showing various extraction mechanisms dependent on the acid concentration. At 1 M HNO3, the extraction was found to proceed via a “cation exchange” mechanism involving the Pu(NO3)2[(R4P+)(R2POO–)]22+species, whereas in 7 M HNO3, it proceeded via an “ion-pair” mechanism involving neutral species Pu(NO3)6(R4P+)2. At 1 M HNO3, the separation factor for Pu over U (SFPu–U = DPu/DU) was found to be ∼2, while that for Pu over Am (SFPu–Am = DPu/DAm) was ∼30. In the case of 7 M HNO3, the SFPu–U value increased to ∼14, while the SFPu–Am value was found to have increased significantly to 8400. The extraction of the UO22+ and Am3+ ions followed a “cation exchange” mechanism at both acidities, albeit with different speciations. The extraction processes were found to be spontaneous, with the ΔG values following the trend ΔGPu > ΔGU > ΔGAm. The plutonium extraction was found to be exothermic with a higher entropy change of complexation, indicating the formation of an inner-sphere complex. The sluggish mass transfer during the extraction was attributed to the relatively high viscosity of the ionic liquid. The solvent systems exhibit good radiolytic stability of up to 1000 kGy of γ ray exposure. Three contacts with complexing agents, viz. oxalic acid for Pu4+, NaHCO3 for UO22+, and EDTA for Am3+, were successfully employed for quantitative (more than 99.99%) back extraction from the loaded ionic liquid phase. The solvent system also exhibited a high degree of recyclability over five consecutive cycles.