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.