Metal Alkyl Ether Carboxylates as Amphiphilic Metal Salts for Tunable Liquid–Liquid Extraction of Critical Metals

Abstract Efficient separation of critical metals from complex secondary resources remains a key challenge in hydrometallurgical recycling and resource recovery. Conventional liquid–liquid extraction typically relies on hydrophobic extractants dissolved in organic diluents, resulting in chemically complex systems that often raise environmental concerns. Here, we introduce an alternative extraction platform based on metal alkyl ether carboxylates (MAECs) derived from the COMPLET (COncept of Melting Point Lowering due to EThoxylation) approach. In these systems, the metal complexes themselves constitute amphiphilic ionic species that partition between aqueous and organic phases without the need for externally added extractants. As such species may arise directly during leaching with alkyl ether carboxylic acids, MAECs offer the potential to integrate leaching and separation within a simplified resource-recovery process. Using mixed MAEC systems representative of battery-recycling and electronic-waste streams, we systematically investigated the liquid–liquid extraction behavior of representative transition metals (Mn, Co, Ni, and Cu). Solvent screening, together with systematic variation of temperature, pH, ionic strength, ligand structure, and concentration, revealed robust and tunable extraction behavior governed primarily by metal-carboxylate interactions and anion hydrophobicity. Consistent extraction sequences (Mn < Co < Ni < Cu) were observed across diverse solvents, enabling selective metal partitioning under mild conditions. Sequential extraction experiments further demonstrated the concept's compatibility with repeated extraction cycles, while additional studies on alkali metals (Li, Na) and rare earth elements (Y, La, Eu) illustrated the broader applicability of the MAEC platform. These results establish MAEC-based systems as a versatile and tunable extraction concept in which amphiphilic metal complexes themselves act as the extractable species, providing a simplified platform for critical-metal recovery with the potential to reduce process complexity and the number of extraction chemicals compared to conventional solvent extraction.