Superior Anion-Selective Amine-Amide Hybrid Extractants for Effective Separation of ⁹⁹TcO₄^-/ReO₄

With the rapid development of nuclear energy, achieving efficient and highly selective separation of TcO₄^- from acidic nuclear wastewater has become an urgent challenge. In this context, three amine-triamide ligands were selected to investigate the effect of ligand structure on the extraction performance toward TcO₄^-/ReO₄^-. Among them, the short-chain ethyl-substituted ligand N,N-diethyl-N',N',N″,N″-tetraoctylnitrilotriacetamide (L_I) exhibited superior extractability and selectivity for TcO₄^-/ReO₄^- anions in dilute nitric acid solution compared to the long-chain butyl-substituted (L_II) and octyl-substituted (L_III), using n-dodecane as the diluent. The study of the relationship between ligand structure and extraction performance revealed that the overall extraction performance of amine-triamide ligands─including loading capacity, extraction efficiency, and selectivity─is governed by a combination of steric hindrance and lipophilicity. Remarkably, even at a NO₃^-/ReO₄^- molar ratio as high as 1000:1, ligand L_I maintained a D_Re value of 3.5 with negligible coextraction of coexisting cations, outperforming most functional adsorbents reported to date. Slope analysis and loading capacity tests indicated the formation of a 1:1 extracted complex between the amine-triamide ligand and TcO₄^-/ReO₄^-. Furthermore, characterization by electrospray ionization mass spectrometry (ESI-MS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) confirmed that the extraction proceeds via an anion-exchange mechanism.