Probing Molecular Interactions in the Organic Phase of TALSPEAK-like Systems using NMR Spectroscopy
Krahn, Elizabeth O'Leary
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Solvent extraction is the most common separation technique employed to partition lanthanides and actinides between an organic phase, containing an organophosphorus extractant, and an aqueous phase containing concentrated buffers and actinide-selective complexing agents. Understanding interactions between molecules and metals in the organic phase, aqueous phase, and at the liquid-liquid interface of these TALSPEAK-like systems can aid in advancing optimization of existing extraction processes as well as the development of new ones. Of interest in this dissertation are the rates and mechanisms of selected features of phase transfer reactions in solvent extraction. Little information is found in the present literature that focuses on the dynamics of metal-extractant interactions in the organic phase of such systems. Nuclear Magnetic Resonance (NMR) spectroscopy is recognized as a technique that could provide useful insights into ligand exchange reaction dynamics in the apolar organic phase; application of NMR spectroscopy to provide such insights is the principal focus of this work. The following investigation focuses on the study of ligand exchange kinetics of HEH[EHP] and HDEHP interactions with lanthanide (Ln) and actinide (An) ions. HEH[EHP] is an organophosphorus cation exchanger extractant used in advanced TALSPEAK processes whose chemistry can be easily probed using 31P NMR spectroscopy. By collecting NMR spectra at variable temperatures, exchange rate constants and activation parameters can be calculated, to aid in determining the rate and mechanism of exchange. By creating small changes in the organic phase, that are relevant to solvent extraction, further knowledge is gained concerning the HEH[EHP] exchange system. These alterations include varying the concentration of free HEH[EHP], switching diluents (from an aliphatic to an aromatic), creating a more polar environment, and the addition of a second extractant molecule. The final portion of this work extends the investigation to HEH[EHP] and HDEHP exchange on An cations; thus, probing interactions with trivalent and tetravalent An. The results of this investigation provide further information on complexation in the organic phase of TALSPEAK-specific systems, as well as help advance the development of NMR spectroscopy as a valuable tool in understanding complex formation in solvent extraction systems.