The increasing demand for critical minerals and the environmental impact of heavy metal pollution necessitate the development of sustainable and selective separation technologies. Traditional thermal and chemical recycling methods suffer from low selectivity, high energy consumption, and the generation of secondary pollutants. Electrochemical separations using ionic liquids (ILs) offer a promising, energy-efficient, non-thermal alternative for selective metal recovery. However, a key knowledge gap remains in understanding how interfacial solvent structuring and ion solvation dynamics influence selectivity at electrified interfaces. We hypothesize that the selective separation of heavy metals and rare earth elements (REEs) can be enhanced by modulating ion structuring at electrified interfaces using co-solvent ionic liquid (CSIL) systems. Specifically, we propose that electric fields induce the self-assembly of IL molecules, forming structured interphases that facilitate selective ion desolvation and reduction, based on ionic size, charge density, and solvation behavior. To test this hypothesis, we employed 1-ethyl-3-methylimidazolium chloride (C₂mimCl) as a CSIL in aqueous electrolytes for the electrochemical separation of toxic heavy metals and REEs. Using electrochemical impedance spectroscopy, Raman and infrared spectroscopy, and atomic force microscopy, we characterized electric-field-induced interfacial structuring of IL–water mixtures. These structured interphases created tunable solvation environments that promoted selective ion desolvation and reduction. In mixed-metal systems containing Pb²⁺ and Cd²⁺, we observed significantly enhanced Pb²⁺ recovery, driven by preferential complexation with the CSIL and the formation of ordered interfacial layers under applied electric fields. Extending this approach to REEs, we studied La³⁺ and Yb³⁺ separation from chloride-based solutions. Operando infrared spectroscopy, electrochemical quartz crystal microbalance (EQCM), and theoretical modeling revealed a direct relationship between interfacial organization and selective ion binding. Notably, Yb³⁺ was enriched over La³⁺ due to differences in solvation dynamics and IL interactions. Our findings demonstrate a novel, non-thermal solvent extraction strategy that leverages interfacial ion structuring for selective metal recovery. This approach provides a sustainable pathway for reclaiming critical elements from complex waste streams using minimal amounts of ionic liquids and offers foundational insight into ion behavior in mixed solvent systems.
