Efficient Coextraction of U(VI), Np(V), and Pu(IV) Using Tetradentate N‑Heterocyclic Amide-Triazine Extractants

Abstract

The spent nuclear fuel contains significant amounts of actinides in various oxidation states and trivalent lanthanides with strong radioactivity. The development of extractants with strong affinity for different actinides is conducive to improving the economy of nuclear fuels and reducing radioactive hazards. The unsymmetrical N-heterocyclic-derived tetradentate extractants, Et-Tol-CyMe₄-ATBP (L_I) and Et-Tol-CyMe₄-ATPhen (L_II), contain N,O-hybrid donor atoms and large-sized coordination cavities, which may have a stronger affinity for actinides than lanthanides. In this work, U­(VI), Np­(V), and Pu­(IV) were selected to explore the selective extraction potential of high-valent actinides over Ln­(III). The results indicated that both L_I and L_II exhibit stronger extraction capacity for U­(VI), Np­(V), and Pu­(IV) than Ln­(III) (3 M HNO₃, D_{U(VI) and Pu(IV)} > 40 and D_Np(V) > 200, SF_{U(VI) and Pu(IV)/Ln(III)} > 400 and SF_{Np(V)/Ln(III)} > 2000). The introduction of the phenanthroline skeleton in L_II significantly enhances the extraction and separation ability for actinides over lanthanides under highly acidic conditions. Continuous three-stage stripping showed that U­(VI), Np­(V), and Pu­(IV) could be completely stripped by 0.1 M HCl+HF (S_U(VI) = 99.80%, S_Np(V) = 100.35%, and S_Pu(IV) = 100.95%). The slope analysis indicated that L_I and L_II mainly formed complexes with a stoichiometric ratio of 1:1 with U­(VI) and Np­(IV), and they formed complexes with a stoichiometric ratio of 1:1 and 1:2 with Pu­(IV). The structural and coordination properties of the complexes formed between the two extractants and U­(VI) or Th­(IV) (substituted for Pu­(IV)) were qualitatively and quantitatively studied using NMR titration, ESI-MS, UV–vis titration, ITC titration, and single-crystal X-ray diffraction (log β_U(VI) = 6.26 ± 0.01, log β_Th(IV) = 5.81 ± 0.02 and 10.67 ± 0.09). Overall, L_I and L_II demonstrate excellent separation ability for U­(VI), Np­(V), and Pu­(IV) over Ln­(III) and highlight their potential for the grouped actinide extraction process.