Selective extraction of Am(III) from Cm(III) and Eu(III) using a novel phenanthrolinamide ligand: Thermodynamics, species, and structure

Abstract The separation of tridentate actinides from lanthanides and that between Am(III) and Cm(III) is one of the most challenging tasks in the process of partitioning–transmutation of nuclear waste. Herein, we report a novel tridentate N–N–O hybrid ligand, i.e., N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (ETPhenAm), for the selective extraction toward Am(III)/Cm(III) over Eu(III) from HNO3 solutions. ETPhenAm in 3-nitrotrifluorotoluene has excellent Am(III) extraction ability and Am(III)/Eu(III) separation performance. Notably, the good selectivity of ETPhenAm toward Am(III) over Cm(III) suggests that ETPhenAm exhibits good ionic radius selectivity in the interaction with the intraseries cations such as Am(III) and Cm(III). Extractive complexes of formula M(NO3)3L and M(NO3)3L2 (M = Am, Cm, and Eu) are identified in the extraction system. HNO3 concentration in the aqueous phase heavily affects the composition of the extractive species in the organic phase because of the strong basicity of ETPhenAm. Extraction observations are validated by determining the protonation constants of the ligand and the reaction equilibrium constants for the generation of complexes M(NO3)3L and M(NO3)3L2 (M = Nd and Eu) in CH3OH/10%(v)H2O solution. The structures of complexes M(NO3)3L and M(NO3)3L2 are confirmed through X-ray diffraction of compounds [La(NO3)3(ETPhenAm)2] (I), [Nd(NO3)2(ETPhenAm)2]NO3 (II), and [Eu(NO3)2(ETPhenAm)2]NO3∙[Eu(NO3)3(ETPhenAm)(CH3OH)]∙2CH3OH (III).