Redox assisted solvent extraction to enable highly efficient separation of cerium from other lanthanides: Experimental studies and DFT calculations

Cerium (Ce) has become an important strategic resource in high-tech fields owing to its excellent optical, magnetic, acoustic, and electrical properties. The recovery of non-natural Ce derived from nuclear fission through spent fuel reprocessing is of potential significance for the reservation of extremely precious resources. In the present work, Ce³⁺ was oxidized to Ce⁴⁺ by NaBiO₃ and then extracted by alkyl phosphate ligands. The extraction efficiency can be enhanced by increasing acidity, raising extractant concentration and decreasing temperature. By taking the advantage of the intrinsic selectivity to tetravalent lanthanides (Ln) by alkyl phosphates, highly efficient and selective extraction of Ce⁴⁺ from other trivalent lanthanides with remarkable separation factor (SF) over 10³ was achieved. Meanwhile, the back extraction of Ce could be accomplished through reducing Ce⁴⁺ to Ce³⁺ by H₂O₂. Further studies on structure-property relationship by spectroscopic methods and DFT calculations revealed that alkyl substituent groups with more C atoms enhanced the complexation between Ce⁴⁺ and the ligand, while steric hindrance effect could weaken the extractability of Ce⁴⁺. Moreover, a shorter CeO bond and a stronger covalent interaction in the Ce⁴⁺ complex, as compared to the Ln³⁺ complex, contributed to the excellent selectivity to Ce⁴⁺ over Ln³⁺.