Role of sulphur and oxygen in phosphine ligands in the extraction process of Am3+ over Eu3+ ions in water medium – A theoretical study

Abstract

Spent nuclear fuel management, particularly the separation of minor actinides, is critical to the continued growth of the nuclear industry and to address the global energy crisis. However, due to the identical chemical properties of trivalent An3+ and Ln3+, it is difficult to distinguish them. Therefore, theoretical research on the mechanism of selective extraction of An3+ is needed. Two phosphine oxide ligands and two phosphine sulphide ligands with similar structures but different bridging frameworks namely phenanthroline and biphenyl, were theoretically explored and compared using relativistic density functional theory in the solvent phase. Here, the M-O bonds are stronger than the M-S bonds. QTAIM shows that the M-O/N bonds of the phosphine oxide ligands have a small amount of covalent character. Gibbs free energy shows that the formation of the complexes is spontaneous while energy decomposition analysis deciphers the binding of M(NO3)3 to these ligands. All the ligands studied had greater affinity for Am3+ than for Eu3+, which contributes to the selectivity of Am3+ over Eu3+ in the water medium. This study provides some important information for the An/Ln separation of phosphine oxide and phosphine sulphide ligands, which may help in the development of more effective phosphine based ligands for the separation of An3+/Ln3+ ions.