Liquid-liquid extraction: from macroscale mathematical modeling to interfacial studies

Christina Louie

doi:10.7273/000005057

This dissertation focuses on advancing two general areas in liquid-liquid extraction (LLE) where specific studies have an emphasis on systems relevant to the nuclear industry, namely the Plutonium Uranium Redox Extraction (PUREX) process. The first is the macroscale mathematical modeling used for the analysis of experimental LLE data. One of the challenges associated with this area is that the models often conflate the interfacial intrinsic kinetics and mass transfer effects into lumped parameters. This makes comparisons across studies difficult because the mass transfer effects will vary depending on the experimental apparatus used by each investigator. In an endeavor to overcome this challenge, an alternative approach is taken where the interfacial chemical kinetics and mass transfer coefficients are maintained as separate parameters throughout the model development. While this yields a more complex mathematical model it allows for the direct determination of the interfacial chemical kinetics free from mass transfer effects. Direct comparisons between experimental studies can then be made through the interfacial chemical kinetics since they should remain constant across studies of the same extraction. This approach is applied to the specific case of the extraction of HNO3 by the surfactant tributyl phosphate (TBP). The second area is focused on studies investigating interfaces of systems relevant to the PUREX process using vibrational sum frequency generation (vSFG) spectroscopy. Direct experimental information at the liquid-liquid interface is eventually required to advance many areas of LLE because inferences based on bulk phase information is limiting. In recent years vSFG has proved to be a very powerful surface selective technique that can obtain molecular level information of hydrophobic-aqueous interfaces. This work focuses on utilizing vSFG spectroscopy to probe the partitioning of the surfactant dibutyl phosphate (DBP) to air-aqueous interfaces as changes are made to the bulk aqueous phase. The first section focuses on establishing fundamental qualitative relationships for the partitioning of DBP relative to bulk aqueous phase concentrations of HDBP, HNO3, and cation ratios of H+:Na+. The second section focuses on establishing fundamental qualitative relationships for the partitioning of DBP in relation to specific ion effects. Specifically, the Hofmeister cations Cs+, Na+, Li+, and Mg2+ were investigated. These studies are some of the first in-depth investigations of DBP at hydrophobic-aqueous interfaces using vSFG, providing a foundation for future more complex studies of air-liquid or liquid-liquid interfaces involving heavy metals.

Liquid-liquid extraction: from macroscale mathematical modeling to interfacial studies

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