Selective Extraction of Uranium(VI) from Thorium(IV) Using New Unsymmetrical Acidic Phenanthroline Carboxamide Ligands

In the thorium–uranium fuel
cycle, 232Th will
produce fissional 233U and radioactive 232U
simultaneously after irradiation. The selective separation of uranium
from thorium-based spent fuel can enhance the utilization efficiency
of nuclear fuel and promote the safety of nuclear energy utilization.
In this work, a new type of unsymmetrical acidic phenanthroline carboxamide
ligand represented as 9-(N,N-dialkylcarbamoyl)-1,10-phenanthroline-2-carboxylic
acid (DEAPA and DOAPA) was synthesized for selective separation of
U(VI) over Th(IV). The separation of small amounts of U(VI) from large
amounts of Th(IV) could be achieved by using DOAPA to extract at high
acidity (4 M HNO3) and then using 1 M HNO3 as
the stripping agent. The complexation mechanism of these two ligands
with U(VI) and Th(IV) was investigated through slope analysis, NMR
spectrometric titration, UV–vis spectrophotometric titration,
and single-crystal X-ray diffraction methods. Results from slope analysis
and NMR spectrometric titration showed that DOAPA formed a 1:1 complex
with U(VI) and formed 1:1 and 1:2 complexes with Th(IV). The larger
stability constants (log β) of 1:1 complexes of DEAPA/DOAPA
with U(VI) than those with Th(IV) implied the stronger affinity of
the ligands with U(VI), which was in good coincidence with the solvent
extraction results. The crystal structures of the 1:1 complex of U(VI)
and the two species of 1:1 and 1:2 complexes of Th(IV) with DEAPA
were successfully analyzed by X-ray diffraction. This work provided
a new type of unsymmetrical acidic phenanthroline carboxamide extractant
for the efficient separation of U(VI) from Th(IV). The systematic
study of the extraction and complexation behavior would be helpful
to explore the effect of functional groups as well as design extractants
with excellent performance.