Extraction and Complexation of Fission Products with Unsymmetrical Phenanthroline Diamide Ligands from Simulated High-Level Liquid Waste

The extraction performance and complexation properties between some typical fission products (FPs) with phenanthroline-based ligands <i>N</i>²,<i>N</i>,²<i>N</i>⁹-triethyl-<i>N</i>⁹-tolyl-1,10-phenanthroline-2,9-dicarboxamide (DE-ET-DAPhen) and <i>N</i>²-ethyl-<i>N</i>,⁹<i>N</i>⁹-dioctyl-<i>N</i>²-tolyl-1,10-phenanthroline-2,9-dicarboxamide (DO-ET-DAPhen) were described in this work. The low distributions of Ln(III) observed in the solvent extraction study showed the potential of the ligands for the separation of actinides and lanthanides in high-level liquid waste (HLLW). Further extraction studies on other FPs showed that the ligands could efficiently extract Pd(II) and Cd(II) using <i>n</i>-octanol as a diluent. Various methods including slope analysis, ¹H NMR titration, UV-vis spectroscopic titration, and single-crystal X-ray diffraction were adopted to explore the complexation behavior of the ligands with target metal ions. The complexation study results indicated that the ligands formed 1:1 complexes with Ln(III) in single-phase solutions, while 1:1 and 2:1 complexes with Cd(II) and Pd(II) were detected. Meanwhile, the stability constants (log β) of the complexes measured through UV-vis spectroscopic titration were reported. According to the diffraction data of the single crystals, the complex structures of [Ln(DE-ET-DAPhen)(NO₃)₃] (Ln = La, Tb, and Lu) and [Cd₂(DE-ET-DAPhen)₂(NO₃)₄] were proposed. The systematic study of the extraction and interaction between some typical FPs with the ligands provides a comprehensive understanding of the ligands' properties, which contribute to the development of the application process of the ligands in the reprocessing of HLLW.