Selective extraction of Pu(iv) by novel benzodioxodiamide ligands in nitric acid: experimental and theoretical study

To address the need for efficient and selective separation of plutonium (Pu) in nuclear fuel reprocessing, four ortho-phenoxy diamide extractants were synthesized based on the proven extraction performance of diamide ligands. The extraction behavior of Pu(iv) in nitric acid media and coordination mechanisms was systematically investigated through combined experimental and theoretical approaches. By rationally tuning steric and electronic effects within the ligand structures, the role of cooperative coordination by multi-oxygen donor in governing Pu(iv) extraction performance was elucidated. The extraction efficiency of Pu(iv) follows the order L2 (ButoBenzoDODA) > L1 (n-OctDODA) > L3 (ClBenzoDODA) > L4 (BenzoDODA). The distribution coefficients of Pu(iv) increase markedly with increasing HNO₃ concentration and ligand concentration, and stable 1 : 2 metal-to-ligand complexes are formed during extraction. Compared with various competing metal ions, all ligand systems exhibit a pronounced extraction preference toward Pu(iv), among which L2 shows the best overall performance, achieving a maximum distribution ratio of 12.37 and maintaining good recyclability over multiple extraction-stripping cycles. Density functional theory (DFT) calculations with experimental study reveal that the Pu-O(C[double bond, length as m-dash]O) interactions are mainly electrostatic in nature, with amide oxygen atoms serving as the primary coordination sites and ether oxygen atoms providing cooperative stabilization. The superior extraction performance of L2 is attributed to its more favorable electronic structure, consistent with the experimental observations. Overall, this study clarifies the synergistic roles of steric and electronic effects in Pu(iv) extraction and provides a theoretical and experimental basis for the rational design of high-performance plutonium extractants.