Investigations of Hydrophilic Schiff Base Ligands for the Separation of Actinyl and Lanthanide Cations

The water soluble tetradentate Schiff base, N,N'-bis(5-sulfonato-salicylidene)-ethylenediamine (H₂salen-SO₃) and water soluble penta-dentate Schiff base, N,N'-bis(5-sulfonato-salicylidene)- diethylenetriamine (saldien-SO₃), have been evaluated for their ability to form aqueous complexes with uranyl(VI) cation and trivalent lanthanide cations. Using spectrophotometric and ¹H NMR titrations, both ligands were demonstrated to readily coordinate with the uranyl(VI) cation, but not to the same extent with trivalent lanthanide cations. Solvent extraction systems employing HDEHP in toluene and these Schiff bases in nitrate media have resulted in holdback factors for uranyl(VI) of up to 3-orders of magnitude. Extraction of europium(III) under similar conditions resulted in holdback factors of no more than 3 and D_Eu of less than 1 by H₂salen-SO₃. However, the system with saldien-SO₃ resulted in Eu/U separation factors of approximately 1000 (D_Eu ∼800, DU < 1) between 0.5 hours and 5 hours. Although Schiff base hydrolysis is prevalent under the studied conditions, these results indicate that the mixed-donor chemistry of these ligands may be a useful scaffold for actinyl holdback reagents to accomplish Ln(III)/An(V,VI) solvent extraction selectivity. (authors)