Actinide binding by Kläui ligands: REDOX speciation and sorption on an extraction chromatography resin

Abstract

AbstractThe sorption of Eu(III) and actinide ions in various oxidation states from nitric acid solutions by an extraction chromatography resin containing 1 wt. % of the Kläui ligand Cp*Co[P(O)(OR)2]3−[Cp*=pentamethylcyclopentadienyl, R=–CH2CH2CH3] on Amberlite®XAD-7HP was examined. At 0.3 M HNO3and a metal-to-ligand ratio of 0.07, the relative affinity of the resin for the ions investigated followed the order: tetravalent»hexavalent>trivalent>pentavalent; however, the relative affinity for the trivalent and hexavalent ions can be reversed, depending on the extent of ligand loading and the nitric acid concentration. The sorption of the tetravalent ions was exceptionally strong in the entire range of nitric acid concentration examined (0.2 to 8 M HNO3). The large difference in the affinity of the resin with the Kläui ligand for tetravalent actinides compared to hexavalent actinides is unusual for ligands with the P=O donor moiety.Resin samples loaded with various actinide ions were examined spectrophotometrically. No Np(V) and Pu(III) species were identified on the resin; rather, reduction–oxidation (REDOX) reactions occurred during equilibration, resulting in their complete conversion to M(IV) species bound by the Kläui ligand. Similarly, the sorption behavior of Pu(VI) and Np(VI) was complicated by their reduction to M(IV) upon sorption. The observed REDOX processes were apparently driven by the extremely high affinity of the Kläui ligand for the tetravalent ions. The acid–base properties of the methyl derivative of the Kläui ligand were investigated in aqueous solution, and its pKawas found to be highly dependent upon the solution ionic strength. The binding constants of this ligand with various actinide ions measured in a mixed methanol/carbon tetrachloride solvent exhibited qualitative agreement with the sorption selectivity trends.