The effects of structural variations in neutral phosphorus extractants on UO22+ extraction behavior and radiation stability: Experimental and theoretical insights

In the present work, the effects of four neutral phosphorus extractants (TBP, TAP, THP, TEHP) with different molecular structures on the extraction behaviors towards U(Ⅵ) and radiation stability were systematically studied. The results of the batch extraction and the γ-radiolysis experiments manifested that the extraction performance and radiation stability of the four extractants followed the order of TEHP > THP > TAP > TBP. Moreover, DFT calculations and Multiwfn software were utilized to analyze the properties of the extractants. The electrostatic potentials (ESP) and frontier molecular orbital of extractants indicated that the nucleophilic ability was ranked as TEHP > THP > TAP > TBP. The analyses of quantum theory of atoms in molecules (QTAIM) and Wiberg indices (WBIs) revealed that the UO22+ ions exhibited stronger complexation ability with O atoms in TEHP compared to the other three extractants. The bonds between UO22+ ions and extractants were primarily ionic in nature. From the energetic perspective, the thermodynamic energy demonstrated that the Gibbs free energy of TEHP in aqueous solution was the most negative, implying that it should be easier to combine with UO22+. This work elucidates that the increase of alkyl chain length and the presence of branched chains can enhance the extraction performance and radiation stability of neutral phosphorus extractants. It not only lays the theoretical foundation for designing novel organic phosphorous extractants with superior extraction performance and high radiation stability, but also provides scientific basis for the extraction process of uranium in spent nuclear fuel reprocessing.