Efficient Extraction in High-Acidity HNO3 Solution for Selective Separation of Am3+ Over Eu3+ by Amine-Diamide Ligands

Five amine-diamide ligands (DAMIA) with varying alkyl chain lengths and steric hindrances are developed for selective extraction of Am3+ over Eu3+ in high-acidity HNO3 solution with the aim of understanding the structure–activity relationship. The ability of these ligands to extract metal ions is closely related to their basicity. The weaker the basicity is, the stronger the extraction ability is. The substituent on the aminic N has more significant effect on the extraction ability than that on the amidic N. Among the five ligands, the DAMIA containing five branched 2-ethylhexyl substituents demonstrates the highest extraction ability and selectivity at molar acidity level. Slope analysis suggests the formation of a 1:1 metal/ligand extracted species, which is also supported by the results obtained from the UV–vis spectrophotometric titration. Furthermore, the analyses of Fourier transform infrared (FT-IR), time-resolved laser-induced fluorescence spectroscopy (TRLFS), and X-ray photoelectron spectroscopy (XPS) for the extracted complex indicate that one Eu3+ ion may be nine-coordinated by one ligand molecule in a tridentate manner via two amide O atoms and one tertiary amine N atom and three bidentate NO3– ions. There is no H2O molecule in the inner coordination sphere of central Eu3+. Additionally, based on the studies of solvent extraction and complexation, the extraction of metal ions follows the neutral complexation extraction model, and the corresponding thermodynamic parameters (ΔH, ΔS, and ΔG) are also presented.