Evolution of Acid-Dependent Am³⁺ and Eu³⁺ Organic Coordination Environment: Effects on the Extraction Efficiency

Coordination of trivalent lanthanide and actinide metal ions by lipophilic diglycolamides and phosphonic acids has been proposed for their separation through extraction from aqueous nitric acid solutions. However, the nature of M³⁺ coordination complexes in these combined solvent systems is not well understood, resulting in low predictability of their behavior. This work demonstrates that a combination of N,N,N',N'-tetrakis(2-ethylhexyl)diglycolamide (T2EHDGA) and weakly acidic 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) in n-dodecane exhibits a complicated extraction mechanism for Eu³⁺ and Am³⁺, which continuously evolves as a function of the aqueous phase acidity. At low aqueous phase nitric acid concentrations, M³⁺ ions are primarily extracted via exchange of the phosphonic acid proton and coordination with HEH[EHP]. At high aqueous phase nitric acid concentrations, HEH[EHP] remains protonated, and M³⁺ ions are transported to the organic phase by the coextraction of nitrate anions from the aqueous phase, thus forming complex species with T2EHDGA. At moderate acid regimes, both ligands participate in the coordination of M³⁺ ions and show a synergistic relationship resulting in considerable enhancement of M³⁺ transport into the combined solvent system over the simple sum of the individual extractants. The observed synergism is caused by differences in organic phase M³⁺ speciation and has a significant impact on the performance of the organic solvent. Distribution studies with Eu³⁺ indicate that nominally two or three T2EHDGA ligands participate in metal extraction in the presence of phosphonic acid, while nominally three diglycolamide ligands participate in the presence or absence of phosphonic acid. While synergistic behavior has been observed in many solvent-extraction processes, this system demonstrates a clear correlation between the continuously changing organic speciation of M³⁺ and its transport into the organic solvent. This paper reports the spectroscopic characterization of the organic phase M³⁺ species by IR, X-ray absorption, and visible spectroscopies. Spectroscopic evidence indicates a mixed-ligand complex, i.e., a ternary complex at the moderate acid regime, where the greatest degree of synergism is observed. Differences in synergistic extraction of Am³⁺ and Eu³⁺ at the low acid regime were observed, indicating their dissimilar extraction behavior.