As part of a broader effort to develop new approaches to separations in an advanced nuclear fuel cycle, a set of hydrophilic Schiff base ligands are being explored as possible masking agents. Currently, the separation of actinide and lanthanide cations frequently employs organophosphorus reagents. Yet, these extractants do not often display large selectivity between trivalent actinides and lanthanides, due to the similarities in the chemical properties of these elements. Consequently, organophosphorus extractants are frequently applied in combination or in sequence with anionic extractants under acidic conditions, and may require addition of soft donors. On the other hand, several of the actinides have higher oxidations states (i.e., V and VI) available under conditions in which they exist as linear dioxo cations. Under this configuration the actinides behave significantly different from trivalent lanthanides. The stability of these oxidation states can be a challenge for some transuranic actinides under acidic conditions. Studies suggest, however, that these oxidation states may be stabilized by coordination of ligands to the equatorial plane of the actinyl ion. For example, recent studies indicate that Schiff bases may stabilize U(V). Considering these reports, we are investigating ligands for the selective complexation of An(V/VI)-cations. Preliminary data from the solvent extraction of U(VI) and Eu(III) by a dialkylphosphate extractant in the presence of a water soluble Schiff base have indicated highly selective holdback of the U(VI) ion. These results represent a reversal in Ln/An selectivity. Spectroscopic studies have been undertaken to investigate the aqueous complexation chemistry and ligand speciation. Possible improvements in Ln/An separation factors may afford the design of separation systems under conditions not commonly considered for nuclear fuel and waste effluent processing.

Hydrophilic Schiff base ligands for actinyl/lanthanide separations

248th National Meeting (2014)

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