Abstract
The dithiophosphinic acids (HS2PR2) have been used for the selective separation of trivalent actinides (AnIII) from lanthanides (LnIII) over the past decades. The substituents on the dithiophosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two dithiophosphinic acids with significantly different AnIII/LnIII separation performance, i.e. diphenyl dithiophosphinic acid (HS2PPh2) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS2P(o-CF3C6H4)2], are employed to understand the substituent effect on the bonding covalency between the S2PR2- anions (R = Ph and o-CF3C6H4) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO2(S2PR2)(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of \% S 3p character inthese orbitals is essentially identical for the UO2(S2PPh2)2(EtOH) and UO2[S2P(o-CF3C6H4)2]2(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U-S bonds for the two UO2(S2PR2)2(EtOH) complexes are contradictory to the significantly different AnIII/LnIII separation performance of the two dithiophosphinic acids, thus the covalency seems to be unable to account for substituent effects in the AnIII/LnIII separation by the dithiophosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids.
Highlights
The selective separation of trivalent actinides (AnIII) and lanthanides (LnIII) is one of the most urgent issues for the implementation of the partitioning and transmutation strategy within advanced nuclear fuel cycles, and it is recognized as a critical challenge in separation science due to the almost identical ionic radii as well as the similar chemical and physical properties between AnIII and LnIII
Several factors emerged in the previous studies that are responsible for the substituent effect in the AnIII/LnIII separation by the dithiophosphinic acids, including (1) the deprotonation properties of the dithiophosphinic acids (Wang et al, 2019; Pu et al, 2020), (2) the chemical stoichiometry and structure of the extracted complexes (Pu et al, 2020; Xu & Rao, 2014; Greer et al, 2020; Tian et al, 2002, 2003; Weigl et al, 2005), and (3) the difference in the binding affinity of the ligands to the metals (Keith & Batista, 2012; Lan et al, 2012; Cao et al, 2010; Bhattacharyya et al, 2011)
Et al examined the electronic structure of several S2PR2À anions and found that the S2P(oCF3C6H4)2À anion was a ‘softer’ extractant as compared with the S2PPh2À anion, which promoted the selectivity towards actinides (Daly, Keith, Batista, Boland, Clark et al, 2012)
Summary
The selective separation of trivalent actinides (AnIII) and lanthanides (LnIII) is one of the most urgent issues for the implementation of the partitioning and transmutation strategy within advanced nuclear fuel cycles, and it is recognized as a critical challenge in separation science due to the almost identical ionic radii as well as the similar chemical and physical properties between AnIII and LnIII. As the direct examination of complexes of the trivalent actinide such as AmIII by synchrotron XAS is not feasible due to high radioactivity, in this work we examined the UO2(S2PR2)2(EtOH) (R = Ph, o-CF3C6H4) complexes by sulfur K-edge XAS to detect the substituent effect on the bonding covalency between the dithiophosphinate anions and the uranyl ion. Results in this work will be informative in terms of the substituent effect on the bonding covalency between the dithiophosphinate anions and trivalent actinides, and helpful for the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids
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