Selective lithium recovery from dilute conditions and sea-bittern using a hydrophobic eutectic solvent

Abstract

The accelerating demand for lithium, driven by the growth of renewable energy systems and electric vehicles, has intensified the need for sustainable recovery methods from unconventional resources such as seawater and sea brine. Conventional precipitation-based processes are inefficient for direct lithium separation due to the low concentration of lithium and the overwhelming presence of competing ions like sodium, magnesium, and calcium. In this study, a hydrophobic eutectic solvent (HES) composed of acetylacetone and trioctylphosphine oxide was developed and applied for lithium extraction from both synthetic dilute solutions and real sea-bittern. A multi-stage liquid–liquid extraction strategy was implemented to enhance selectivity and maximize recovery. Under the optimized conditions, the process achieved ∼95% lithium extraction efficiency in single-ion systems and 90.4% in multi-ion environments, demonstrating strong discrimination against interfering cations. Mechanistic insights were gained through NMR and ATR–FTIR spectroscopy, and kinetic modeling, which revealed the complexation sites and stoichiometric pathways involved in lithium transfer. The HES exhibited remarkable reusability, with efficient regeneration through acid stripping and consistent extraction performance across multiple cycles. Application to actual sea-bittern validated the method's robustness, scalability, and industrial relevance. Overall, this work establishes a multi-stage HES-based extraction process as a green, selective, and efficient alternative for lithium recovery from natural brine resources.