SOLVENT EXTRACTION SEPARATION OF TRIVALENT AMERICIUM FROM CURIUM AND THE LANTHANIDES

The sterically constrained, macrocyclic, aqueous soluble ligand N,N-bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H(2)BP18C6) was investigated for separating americium from curium and all the lanthanides by solvent extraction. Pairing H(2)BP18C6, which favors complexation of larger f-element cations, with acidic organophosphorus extractants that favor extraction of smaller f-element cations, such as bis-(2-ethylhexyl)phosphoric acid (HDEHP) or (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (HEH[EHP]), created solvent extraction systems with good Cm/Am selectivity, excellent trans-lanthanide selectivity (K-ex,K-Lu/K-ex,K-La = 10(8)), but poor selectivity for Am against the lightest lanthanides. However, using an organic phase containing both a neutral extractant, N,N,N',N'-tetra(2-ethylhexyl)diglycolamide (TEHDGA), and HEH[EHP] enabled rejection of the lightest lanthanides during loading of the organic phase from aqueous nitric acid, eliminating their interference in the americium stripping stages. In addition, although it is a macrocyclic ligand, H(2)BP18C6 does not significantly impede the mass transfer kinetics of the HDEHP solvent extraction system.