PROFILING DILUENT/EXTRACTANT/LANTHANIDE COMPLEX INTERACTIONS IN ADVANCED TALSPEAK SEPARATION SYSTEMS
Dodd, Devon Edward
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Solvent extraction has generally been the method of choice for the reprocessing of used nuclear fuel. Fundamentally, solvent extraction is based on the differences in solubility of chemical species dissolved in a biphasic (liquid-liquid) system. In conventional solvent extraction systems one of these phases is aqueous while the other is an immiscible organic phase. A metal ion is generally the solute species-of–interest that is partitioned between the two phases. Phase transfer can occur if the organic phase is modified by adding an organic surfactant molecule/complexing agent/extractant that binds the metal ion at the aqueous/organic interface converting the charged metal ion into an electroneutral and lipophilic complex. Most modern extractants are amphiphilic in nature, i.e., they combine a cation-seeking polar head with a “greasy” tail. Extractants create a layer of molecules at the interfacial region between organic and aqueous phases from which the hydrophilic head extends into the aqueous phase where it can come into contact with a hydrated cation or a hydrophilic metal complex. A “diluent”, or inert solvent, is selected to dissolve the extractant to improve its physical or extractive properties while maximizing the solubility of the electroneutral metal-extractant complex. The impact of the diluent on the behavior of cation exchanging ligands relevant to Advanced TALSPEAK have been investigated in this work. Specifically, the research was focused on assessing the impact on the apparent stoichiometry of the extracted complex, the extraction efficiency, the energetics of extraction, and the interfacial aggregation of extractant. The interaction between extractant and an additional co-solvent, or phase modifier, in the bulk phase and at the liquid-liquid interface also will be discussed.