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

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 (LI) and Et-Tol-CyMe4-ATPhen (LII), 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 LI and LII exhibit stronger extraction capacity for U(VI), Np(V), and Pu(IV)
than Ln(III) (3 M HNO3, DU(VI) and Pu(IV) > 40 and DNp(V) > 200, SFU(VI) and Pu(IV)/Ln(III) > 400
and SFNp(V)/Ln(III) > 2000). The introduction
of the phenanthroline
skeleton in LII 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 (SU(VI) = 99.80%, SNp(V) = 100.35%, and SPu(IV) = 100.95%). The slope analysis indicated that LI and LII 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 βU(VI) = 6.26 ± 0.01, log βTh(IV) = 5.81 ± 0.02
and 10.67 ± 0.09). Overall, LI and LII 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.