Seaborg and coworkers established in the 1950s that the most effective approach to accomplishing a Ln(III)/An(III) group separation was to introduce donor atoms softer than oxygen, specifically Cl-, into a low selectivity separation platform like Dowex 50 cation exchange resin. This soft-donor effect was attributed to the slightly stronger covalent contribution in the bonding of actinides to such ligands. A decade later, Weaver and Kappelmann developed the TALSPEAK concept, based on the introduction of multiple N-donor holdback reagents, like DTPA (diethylenetriamine pentaacetic acid) to accomplish a similar separation. Though the chemistry of TALSPEAK is more complex than the soft donor-cation exchange separation, it did enable a potentially high throughput countercurrent solvent extraction approach to An(III)/Ln(III) separations with high separation factors. Because the covalency effect is relatively weak (7-8% increased bonding affinity for An(III) over similarly-sized Ln(III)), the adoption of reagents with more N-donor atoms increases separation efficiency. However, the octadentate DTPA suffers from some issues arising from the comparatively slow complexation/decomplexation kinetics of complex molecules. In the Advanced TALSPEAK system, the primary aqueous holdback reagent is HEDTA ((2-hyroxyethyl)ethylenediaminetriacetic acid), a pentadentate aminopolycarboxylate ligand, with three water molecules completing the cation coordination environment. The Ln(HEDTA)(OH2)3 aqueous complex leaves room for the addition of a secondary soft donor ligand, for example iminodiacetic acid. In this report, selected results of an investigation of the thermodynamics and kinetics of such ternary complexes and their impact on Advanced TALSPEAK separations will be described.

Characterizing mixed ligand f-element complexes for better An(III)/Ln(III) separations

253rd National Meeting (2017)

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