Effect of ligand alkyl chain length on lanthanide extraction using liposomes incorporating diglycolamic acid

This study introduces an environmentally benign liposome-based extraction system that selectively separates and recovers lanthanides (Ln) without requiring an organic solvent. Recovery of Ln, particularly from high-level radioactive liquid waste streams, is critical for resource utilization and reducing long-term radiotoxicity. The proposed system incorporates highly Ln-selective diglycolamic acid (DGAA) ligands into phospholipid liposomes. Fluorescence spectroscopy demonstrated that europium (Eu) adsorption, mediated by the hydrophilic head group of the incorporated DGAA ligands, reached equilibrium almost immediately upon mixing. Inductively coupled plasma-mass spectrometry revealed that the adsorption efficiency increased with increasing concentration of incorporated DGAA. Approximately three DGAA ligands are required to adsorb one Eu(III) ion. The selectivity for heavy Ln was comparable to that of conventional DGAA-based solvent-extraction systems and was found to be independent of DGAA alkyl chain length. In contrast, the incorporation stability depended on alkyl chain length; for example, liposomes containing N,N-didodecyldiglycolamic acid retained the ligand without significant precipitation or fluorescence decrease for 7 days at concentrations up to 237 µM/g-egg lecithin. These findings establish liposome-based extraction as a flexible platform in which the ligand head group governs the adsorption characteristics while the alkyl chain length dictates the incorporation stability, providing a quantitative foundation for future solvent-free separation technologies.