(372c) Molecular Dynamics Simulations and Vapor Pressure Osmometry Studies of Tri-n-Butyl Phosphate for Liquid-Liquid Extraction

Solvent extraction (SX) or liquid-liquid extraction is a technique frequently used for the recovery and purification of metal ions from used nuclear fuel by phase separation. One of the most commonly used SX techniques is called PUREX (Plutonium URanium EXtraction), which employs tri-n-butyl phosphate (TBP) as an extracting agent to selectively complex Uranium and Plutonium from the constituents of the used fuel. However, our understanding of the metal extraction mechanism and phase separation involve many molecular level events are far from complete. Specifically, TBP tends to self-assemble into reverse micelles, interfering extraction kinetics and phase behavior. To intensify the problem, these reverse micelles can aggregate and form an undesired third phase, which interrupts the centrifugal contactor operation and can be potentially hazardous when metal ions are concentrated in a reduced volume. To gain insights into the thermodynamics and kinetics of interactions of TBP molecules with other molecules in extraction systems, both molecular dynamics (MD) simulations and experiments are performed. Force field parameters for TBP have been determined by conducting a characterization study of TBP bulk liquid behavior using atomistic models with different sets of van der Waals and atomic charge parameters. Various properties of TBP in the liquid phase including the mass density, electric dipole moment, and self-diffusion coefficient are obtained and compared favorably with available experimental results. The self-association of TBPs in the presence of n-dodecane is examined by using Vapor Pressure Osmometry (VPO) technique at different temperatures, producing experiment results that are in good agreement with previous works for low concentrations of TBP/n-dodecane mixtures. However, there is a discrepancy at higher concentrations of TBP due to the assumptions of previous studies that TBP is ideal and only dimerizes in solutions. An improved model for TBP self-association is being developed by including TBP trimers and possibly higher order aggregates as observed in MD simulations. The deep understanding of TBP’s conformation and associations in solvent gained from this study will further elucidate the role of TBP in liquid-liquid extraction process and enhance the interpretation of the phenomena observed (third phase, metal phase transfer).