The separation of americium from curium in spent nuclear fuel (SNF) has important implications for spent nuclear fuel reprocessing. Removal of curium from SNF reduces the heat load in newly refabricated fuel elements and minimizes the buildup of heavier actinides (Cf-252) caused by repeated recycle of curium in reactors. Unfortunately, this separation has historically been difficult to accomplish due to their similar chemistries without resorting to chromatographic or precipitation techniques. However, a size-based approach to separating these two elements has been proposed that could easily be incorporated into solvent extraction-based separations schemes like the TALSPEAK process. In this approach, the formation of aqueous-phase ternary complexes (i.e., complexes of a metal with a large primary and smaller secondary ligand) with Am(III) - but not with Cm(III) - would increase the thermodynamic stability of Am(III) relative to Cm(III), making Am(III) less extractable. As there are few reports of ternary complexes in the literature, the objective of this work was to investigate the factors that influence their formation.

Factors such as ligand size, basicity, and steric constraints influence whether or not ternary complexes form and how strong such complexes will be. Spectroscopic, calorimetric, and thermometric techniques were used to investigate how these factors affect the formation of ternary complexes containing a large polyaminocarboxylate and a smaller dicarboxylate ligand bound to americium, neodymium, samarium, holmium, terbium, or erbium. Results from investigations with diethylenetriamine-N,N,N',N',N"-pentaacetic acid (DTPA), which is the octadentate ligand used in the TALSPEAK process, and lactate as the secondary ligand indicated that inner sphere complexes were not formed under the conditions used because the DTPA ligand is too large to accommodate lactate. Outer-sphere ternary complexes may form but they were estimated to be too weak to significantly affect the TALSPEAK extraction thermodynamics.

Ternary complexes were readily formed with the septadentate DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) and hexadentate CDTA (trans-1,2-diaminocyclohexanetetraacetic acid) ligands, whose binary complexes (e.g., Ln(CDTA)^-) have at least one residual water of hydration that can be displaced by a small secondary ligand. The results of thermodynamic investigations of Ln(CDTA)^- complexes with the secondary ligands oxalate, malonate, and iminodiacetate revealed that the strength of the ternary complexes with CDTA generally increased with decreasing ionic radius when steric hindrance was minimal, indicating that the bonding with these ligands was primarily ionic. In addition, secondary ligands that formed five-membered ring complexes were more stable than those that formed six-membered rings, and more basic ligands formed stronger complexes. Similar ternary complexes with DO3A showed little increase in thermodynamic stability compared to analogous CDTA complexes, which is likely due to increased steric hindrance in the DO3A complexes.