UTILIZATION OF TRADITIONAL AND NOVEL NMR SPECTROSCOPIC TECHNIQUES TO PROBE INTERACTIONS IN THE ORGANIC PHASE OF LIQUID-LIQUID SOLVENT EXTRACTION SYSTEMS
MetadataShow full item record
Solvent extraction is a technique commonly used is the reprocessing of used nuclear fuel elements. It utilizes the partitioning of a metal ion between two immiscible phases, typically an acidic aqueous phase and a light nonpolar organic phase, to provide separation. The extractant is an amphiphilic molecule that contains a polar “head” that will interact with the aqueous phase and metal ions and a long chain aliphatic “tail” that provides solubility in the nonpolar organic media. While the aqueous chemistry of solvent extraction systems has been well characterized, the organic phase is still challenging to model well. Part of the challenge is that many of the techniques used to characterize the aqueous phase, such as electrochemistry and spectrophotometric titrations, are difficult to implement in nonpolar media. The techniques that are available for the organic phase typically probe interactions indirectly through changes in the aqueous phase or vapor pressure. Nuclear magnetic resonance (NMR) has the potential to directly probe interactions in the organic phase. A new 2D NMR technique called Diffusion Ordered Spectroscopy (DOSY) can be applied to organic phases in solvent extraction systems. In this work, typical characterization techniques such as slope analysis and Eu luminescence are used to explore the apparent change in stoichiometry of the metal complex in extractions by (2-ethylhexyl) phosphonic acid mono (2-ethylhexyl) ester (HEH[EHP]). NMR based techniques, such as analyzing the changes in chemical shift, are used to probe the speciation of the HEH[EHP] extractant. DOSY is finally used as an analogue to small angle neutron scattering to determine the molecular weight of diffusing species. This is used to probe the interactions present in the organic phase of HEH[EHP] based extractions. The solvating unsymmetrical diglycolamide (UDGA) extractants are also investigated using DOSY. UDGAs will interact with the octanol phase modifier needed for extraction, as well as extracted water and nitric acid. The stoichiometry of the metal complex formed by UDGAs with lueticium was also determined using DOSY. This work shows the utility of DOSY for the determination of the fundamental chemistry driving extractions and of changes in solution structure as the extraction progresses.