The efficient separation of thorium (Th) from uranium (U) and rare earth elements (REEs) is crucial for the Th-based nuclear energy. To address the coextraction limitation of light REEs by NTAamide(n-Oct), two novel tetraamide derivatives of EDTA─EDTAamide(n-Bu) and EDTAamide(n-Oct)─were synthesized and characterized. Their extraction behaviors toward Th4+, UO22+, and RE3+ in HNO3 media were systematically investigated. The extraction performance of both ligands decreased monotonically with increasing acidity. Both ligands achieved exceptional Th/RE separation, with separation factor (SFTh/RE) ranging from 64 to 8.3 × 102 at 0.50–2.0 mol/L HNO3. Notably, EDTAamide(n-Bu) exhibited superior Th/U selectivity (SFTh/U = 78–1.3 × 102) over EDTAamide(n-Oct) (SFTh/U = 14–25), with optimal performance at 1.0 mol/L HNO3. A 1:1 Th4+–EDTAamide(n-Bu) complex was confirmed, where the ligand coordinates as a hexadentate chelate via two central N atoms and four amide O atoms, forming a deca-coordinate Th4+ center with four inner-sphere H2O molecules. The extraction process follows a spontaneous and endothermic neutral complexation mechanism, accompanied by an entropy increase. In a simulated monazite leachate, EDTAamide(n-Bu) achieved 98.1% Th4+ recovery with high selectivity, underscoring its strong potential for practical application.
Zhou et al. (2026) studied this question.