Synergistic Deep Eutectic Solvents for Lithium Extraction

Hydrophobic deep eutectic solvents (DESs) have attracted much attention as sustainable extraction media for various metals. However, in previously developed DESs, only one DES component is involved in metal complex formation, which has limited their extraction performance and selectivity. Here, we propose a novel synergistic DES concept for improving the performance of environmentally benign liquid-liquid extraction of lithium (Li). Two different conventional extractants, a beta-diketone and a neutral extractant, were effective for creating the synergistic DESs. When the two extractants were mixed, they formed a liquid DES. DESs have much lower viscosity than conventional hydrophobic ionic liquids. The Conductor-like Screening Model for Real Solvents method was used to estimate the formation of DESs with the melting point depressed by the intermolecular interaction between their components. The extraction performance of Li was greatly enhanced with the synergistic DESs. The extraction capacity of Li reached 4.4 g/L using the optimum DES combination because it was solvent-free and highly concentrated. Using the optimum DES, highly selective recovery of Li over Na and K was demonstrated with a model brine solution. This is the first report of efficient and selective extraction of Li with DES-based liquid-liquid extraction.