(393d) Elucidation of Molecular Processes in Liquid-Liquid Extraction of Metal Ions: A Molecular Dynamics Study

Aqueous processes are the current state-of-the-art for separation and recovery of various elements or element groups from spent nuclear fuel. The key feature of all aqueous-based reprocessing technologies is the selective transfer of elements between aqueous and organic solution phases via solvent extraction in liquid-liquid contactor devices. The current reprocessing separation models are highly empirical in nature as they are based on limited experimental data. As a result, their utility is limited to predicting the transfer of major components in fairly well developed chemical systems. Clearly, a fundamental understanding of interfacial transport in presence of specific extracting agents is critical to developing effective solvent extraction processes. Current generation liquid-liquid extraction systems for reprocessing of spent nuclear fuel generally consist of an aqueous phase which contains the metal ions to be extracted, an organic phase, and a specific extracting agent that mostly resides in the organic phase. The separation steps are envisioned to proceed sequentially as follows. First, the extractant molecules bind to the cations and form a complex at the aqueous-organic interface. This complex then migrates to the organic phase due to its high solubility in the organic phase effectively carrying the metal ion from the interface region into the bulk of the organic phase. This complex migration process involves many molecular level events including, solvation of the ions and the extractant molecule in bulk and at the interface, binding kinetics of the extractant and the cation, phase equilibria between the complex at the interface and in bulk phases, diffusion of the extractant-cation complex from the interface to the organic phase, etc. As part of a multiscale modeling effort, we aim to elucidate specific molecular level events in the overall process and evaluate their impact on the metal ion extraction, we have conducted a series of molecular dynamics simulations to examine the complex formation process, and migration of extractant molecules. Specifically, we have considered a prototypical multi-component system consisting of an aqueous phase and an organic phase. The aqueous phase contains the dissolved metal ions (UO₂)²⁺, H₃O⁺ and nitrate NO₃^- and the organic phase consists of n-dodecane, and an extractant, tributyl phosphate (TBP). In this presentation molecular dynamics simulation results, including complex formation (UO₂(NO₃)₂?2(TBP)) in the aqueous phase, extraction activity of TBP at the aqueous/organic interface as well as migration rate will be discussed