EDTA-Derived Tetraamide Ligands for Highly Selective Separation of Thorium over Uranium and Rare Earths

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(<i>n</i>-Oct), two novel tetraamide derivatives of EDTA─EDTAamide(<i>n</i>-Bu) and EDTAamide(<i>n</i>-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 (SF_Th/RE) ranging from 64 to 8.3 × 10² at 0.50-2.0 mol/L HNO₃. Notably, EDTAamide(<i>n</i>-Bu) exhibited superior Th/U selectivity (SF_Th/U = 78-1.3 × 10²) over EDTAamide(<i>n</i>-Oct) (SF_Th/U = 14-25), with optimal performance at 1.0 mol/L HNO₃. A 1:1 Th⁴⁺-EDTAamide(<i>n</i>-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(<i>n</i>-Bu) achieved 98.1% Th⁴⁺ recovery with high selectivity, underscoring its strong potential for practical application.