Tailored low-melting mixture solvents with high selectivity and efficiency for boron and lithium extraction

Boron (B) and lithium (Li) are two crucial elements in salt lake brine. The increasing demand for B, coupled with its low natural abundance, has prompted people to pay increasing attention to its extraction. Conventional B extraction methods often suffer from environmental hazards and process complexity, limiting their industrial applicability. The purpose of this study is to develop a greener solvent that is thermally stable, low volatile, low-cost, easy to synthesize, non-toxic, and biodegradable. Deep eutectic solvents (DESs), particularly low-melting mixture solvents (LoMMSs), are promising alternatives due to their tunable properties and environmental sustainability. This article reports the development of a novel LoMMSs system that uses trioctylphosphine oxide (TOPO) as the hydrogen bond acceptor (HBA) and 2-ethyl-1,3-hexanediol (EHD) as the hydrogen bond donor (HBD) for selective extraction of boric acid (H₃BO₃) in salt lake brine. Extraction experiments demonstrated that the TOPO:EHD (1:2) system achieved a B extraction rate of 99 % under optimized condition. Mechanistic investigations using Fourier-transform infrared (FT-IR) spectra and quantum chemical calculations (reaction Gibbs free energy change of -13.99 kJ/mol) confirmed the formation of stable B-LoMMSs complexes through hydrogen bonding and esterification reactions. After the removal of B, Li was synergistically extracted using the novel LoMMSs composed of menthol (ML) and tributyl phosphate (TBP) in combination with FeCl₃, achieving an extraction rate of 82 %, demonstrating the potential of LoMMSs based comprehensive resource extraction strategy for salt lake brine.