Research Projects

The rare earth elements (REE), which include the lanthanides and actinides, are the major components of nuclear discharge. Although nuclear energy is green, the treatment of radioactive waste disposal is one of the major concerns. The nuclear wastes often contain the long-lived actinides such as Neptunium (Np), Americium (Am), Curium (Cm) and traces of Uranium(U) and Plutonium (Pu). The late actinides, i.e. U and Pu, prefer the trivalent oxidation state that faces a significant separation challenge due to their chemical similarity with trivalent lanthanides (Ln3+), arising from the contraction and core-like behaviour of 5f orbitals. This chemical similarity emphasizes the need for highly selective ligands capable of distinguishing between lanthanides and actinides. Among different extraction processes, the solvent extraction process is one of the most widely adopted techniques. In the solvent extraction process, a non-polar organic solvent is used to extract the Ln3+ from an aqueous solution. The extraction efficiency is often quantified by the distribution ratio, D = [M3+]org/ [M3+]aq, where high D values indicate strong metal–ligand complexation. Following the hard-soft acid base (HSAB) principle, the Bis(2,4,4-trimethylpentyl)dithiophosphinic acid, Bis(p-chlorophenyl)dithiophosphinic acid, and 2,9-bis-lactam-1,10-phenanthroline (BLPhen) are the suitable choices that exhibit promising selectivity in the separation of REEs. While the strong attraction between the ligand with the Ln3+ ion is dominant, the ligand-to-metal charge transfer via ligand p-orbital and metal d-orbitals, with the limited involvement of f-orbital, governs this selectivity, favouring softer actinides. Although quantum mechanical treatments, such as density functional theory (DFT), have been utilised, they often fall short of accuracy due to their large self-interaction error. Further, the accuracy of DFT depends on the functionals used. In addition, for heavy metals, as with REEs, DFT addresses relativistic effects through the incorporation of pseudopotentials. These factors often lead to large errors in binding energies of the [Ln-L]n+ complexes (n is the charge of the complex). In the current proposal, we aim to employ the coupled-cluster (CC) method to achieve a highly accurate description of rare earth element (REE) complexes. Given that REEs are involved with strong spin–orbit coupling (SOC), it becomes essential to incorporate these effects explicitly in theoretical calculations. Therefore, our computational framework will integrate within the relativistic coupled-cluster formalism. This approach is expected to provide more reliable ideas and will guide experimental chemists in the separation of the REEs.