The Structure of tributyl phosphate solutions: Understanding extractant aggregation in applied solvent extraction separations

Abstract:  Tributyl phosphate (TBP) is an important industrial extractant used in the Plutonium Uranium Reduction Extraction (PUREX) process to recover hexavalent uranium and tetravalent plutonium from irradiated nuclear fuel. Metal extraction by TBP was traditionally considered to proceed through the formation of discrete coordination complexes. However, recent work has suggested that colloidal, self-assembled reverse micellar structures may be responsible for metal recovery under conditions of high metal and acid loading. In this work, the existence of such mesoscale structures in solution is evaluated using a combination of experimental and computational chemistry techniques. Organic phase samples containing TBP-extracted hexavalent uranium and tetravalent zirconium (a plutonium surrogate) in a nonpolar solvent were characterized using diffusion NMR spectroscopy, rheology, and small angle neutron scattering. These experiments yielded contradictory results when interpreted using models applicable to colloidal systems, suggesting that reverse micelles are not present in TBP extraction systems. TBP aggregation in classical molecular dynamics simulations of samples containing nitric acid is consistent with this conclusion, and suggest that TBP forms small, discrete species in solution.