Highly selective separation of tetravalent plutonium from complex system with novel phenylpyridine diamide ligands

Abstract

In this study, three phenylpyridine diamide ligands, namely, 2,2′-((pyridine-2,6-diylbis(3,1-phenylene))bis(oxy))bis(N,N-diethylacetamide) (PPEA, L1), 2,2′-((pyridine-2,6-diylbis(3,1-phenylene))bis(oxy))bis(N-ethyl-N-phenylacetamide) (PEPA, L2), and 2,2′-(((4-phenylpyridine-2,6-diyl)bis(3,1-phenylene))bis(oxy))bis(N,N-dioctylacetamide) (PPOA, L3), were synthesized and explored for the solvent extraction of Pu(IV) in a HNO₃ medium using 1-(trifluoromethyl)-3-nitrobenzene as the diluent. The effects of HNO₃ concentration, extractant concentration, and temperature on the Pu(IV) extraction efficiency were studied. All three extractants displayed high selectivity for Pu(IV) over other metals such as U(VI), Np(V), Am(III), and various fission elements. At 3 M HNO₃, the distribution ratio for Pu(IV) reached 27.18, in contrast to 1.11, 0.3, and 0.03 for U(VI), Np(V), Am(III), respectively. Slope analysis and UV titration revealed the formation of 1 : 1 Pu(NO₃)₄/ligand complexes during extraction. The extraction reactions had negative Gibbs free energies, indicating the spontaneous nature of Pu(IV) extraction at room temperature. Furthermore, the extractants demonstrated good stripping ability and reusability, and their radiolytic stability was reasonable up to an absorbed dose of 100 kGy, underscoring their potential for practical applications. Overall, this study broadens our understanding of actinide–diamide ligand coordination and actinide chemistry during coordination, paving the way for the design and synthesis of new extractants.