Synthesis and Characterization of Aqueous Pyridine Ligands for Actinide Lanthanide Separation
Uhnak, Nicolas Edwin F
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Responsible stewardship of used nuclear fuel requires the operation of a closed nuclear fuel cycle and proper management of the actinide materials through transmutation. The separation and isolation of the minor actinides (MA), particularly Am and Cm, from the fission product lanthanides (Ln) remains a considerable challenge towards the goal of a closed fuel cycle. Solvent extraction processes aimed at this difficult separation have shown some success, in the US, these efforts have largely focused on the TALSPEAK (Trivalent Actinide Lanthanide Separation by Phosphorus-reagent Extractant from Aqueous Komplexes) or TALSPEAK-like extraction systems. In TALSPEAK and TALSPEAK-like systems the separation of the MA from the Ln is facilitated by a polyaminopolycarboxylate (PAPC) ligand that selectively binds the MA while an cation exchanging extractant (typically di-(2-ethylhexyl)phosphoric acid (HDEHP)) binds the Ln. Traditional PAPC ligands suffer from slow phase transfer kinetics, and require strict buffering to control pH and to improve kinetics (particularly true for lactic acid buffering in traditional TALSPEAK). Aromatic nitrogen donor (pyridine based) ligands have shown improved binding and dissociation kinetics over their aliphatic counterparts, and competitive complexation strength that make them a suitable replacement for traditional PAPC ligands in TALSPEAK and TALSPEAK-like extraction systems. To this end new pyridine and bipyridine based ligands have been synthesized to be used as actinide selective complexants when paired with a TALSPEAK or TALSPEAK-like solvent. Aqueous solubility limitations inherent to pyridine based ligands have been overcome by the addition of solubilizing functional groups such as 1-methylpiperazine, and iminodiacetic acid. Newly synthesized pyridine ligands have been characterized and evaluated in TALSPEAK or TALSPEAK-like combined extractant systems TALSPEAK-MME (Mixed Monofunctional Extractant) and ALSEP (Actinide Lanthanide SEParation) using radiotracer and stable metal solvent extraction studies. The findings of these studies and their implications to MA/Ln separation techniques are discussed in this dissertation.