Remarkable separation of trace amount plutonium using a hydrophilic multiamide ligand: synthesis, extraction, spectroscopic, crystal structure and density functional theory studies

The removal of trace plutonium (Pu) from uranium products and organic wastes during spent nuclear fuel reprocessing remains a critical challenge, resulting in excessive plutonium content in uranium products and waste organic liquid. Currently, most organic ligands with selective separation functions are lipophilic, while research on water-soluble, highly selective ligands is relatively scarce, and there are also few reports on the single crystal of these ligands coordinating with plutonium. Herein, a hydrophilic multiamide ligand, N,N,N′,N′′,N′′-hexaethyl-nitrilotriacetamide (NTAamideC2), was synthesized and evaluated for its Pu(IV) back-extraction efficiency under harsh conditions. Systematic experiments revealed that NTAamideC2 achieved >99% Pu(IV) back-extraction rate within 15 min across a wide nitric acid concentration range (0–5 M), even with elevated dibutyl phosphate (DBP ≤20000 ppm). Remarkably, the separation factor (SFPu/U) reached 767 at 1.5 M HNO3, demonstrating exceptional selectivity over uranium(VI). Spectrophotometric titration and DFT calculations confirmed the formation of 1:1 and 1:2 Pu(IV)-NTAamideC2 complexes, with logβ values of 7.42 ± 0.01 and 13.23 ± 0.02, respectively. Single-crystal X-ray diffraction analysis of {[Pu2(H2O)2(NTAamideC2)4](H2O)2(NO3)(ClO4)7} revealed a nine-coordinated PuO7N2 geometry, where two NTAamideC2 molecules bind via six O and two N atoms. Compared to conventional agents (AHA/HSC), NTAamideC2 exhibited superior acid tolerance and selectivity, aligning with the CHON principle for sustainable nuclear waste management. This work provides a robust strategy for Pu(IV) removal in uranium purification cycles and advances fundamental insights into Pu coordination chemistry, offering significant potential for industrial nuclear fuel reprocessing.