Solvent extraction of americium(III) and europium(III) with tridentate N,N-dialkyl-1,10-phenanthroline-2-amide-derived ligands: extraction, complexation and theoretical study

The extraction and complexation behavior of soft–hard combined N,N-dialkyl-1,10-phenanthroline-2-amide (PTA) ligands with Am(III) and Eu(III) in HNO3 solution was investigated. The effects of acidity of the aqueous solution, shaking time, concentration of the extractant and temperature on the distribution ratios were studied. The extraction ability for both Am(III) and Eu(III) was found to decrease with an increase in the length of the substituent alkyl chains, and the highest extractability and selectivity for Am(III) over Eu(III) were found for the diethyl-substituted ligand. The separation factor for Am(III) over Eu(III) reached around 7.6 at low acidity and high salinity. UV-vis titrations revealed that the dialkyl-substituted tridentate PTAs all predominantly formed 1 : 1 complexes with Eu(III), which agreed well with the results of slope analyses in the extraction experiments. The stability constants (KEuL) as well as the protonation constants (KH) were also determined by UV-vis titration. Computational chemistry gave a good explanation of the relationship between the alkalinity and the protonation energy of the proposed PTA ligands. Density functional theory (DFT) calculations on the optimized structures of Am(III) and Eu(III) complexes with C2-PTA showed that the selectivity may originate from differences in the degree of covalency of the bonds between the metal ions and the donor N atoms, which fits well with the experimental results.