*WINNER* Investigation of N-Donor Organic Ligands for Selective Minor Actinide Separations

Abstract

The development of improved extractants for metal ion separations will aid in the closure of nuclear fuel cycles. New nitrogen-donor compounds are being designed, synthesized, and characterized as ligands for f-element separations, in a collaboration between synthetic organic (TTU), radioanalytical (TTU) and computational (U. Alabama) teams. The radioanalytical component encompasses a two-stage screening process that has been devised to test the viability of the ligands. The first phase is focused on screening of ligand solubility and solution stability. The second is to measure trivalent actinide/lanthanide (An(III)/Ln(III)) solvent extraction selectivity. The ligands were first screened for solubility in toluene, 1-octanol, trifluoromethylphenyl sulfone (FS-13), and tert-butyl benzene. When a compound is reasonably soluble, it is then screened for stability and metal ion solvent extraction, using 154Eu and 241Am radiotracer solutions to determine distribution ratios (DM) and Am/Eu selectivity. Compounds that meet reasonable criteria move on to systematic evaluation, in which solvent extraction slope analysis and spectroscopic experiments are carried out to understand the two-phase equilibrium chemistry. Conditions for the first phase screening process, using 3,3ʹ-dimethoxyphenyl-bis-1,2,4-triazinyl-2,6-pyridine (MOB-BTP) as a benchmark ligand, have been optimized. Thus far, seventeen compounds have been screened. A new tridentate ligand 4,4ʹ-dimethylphenyl-bis-1,2,4-triazinyl-2,6-pyridine (SC-B-31) provided 30% greater Am/Eu selectivity and a 10-fold larger DAm (4.7) than those of MOB-BTP in FS-13. Although its structure only differs by the distal ring substituents, SC-B-31 is approximately 10-fold less soluble in FS-13 than MOB-BTP. Results from these experiments are informing the design of structures with improved solubility and separation efficacy.