Tailoring hydrophobic deep eutectic solvent for selective lithium recovery from dilute aqueous solutions

The efficient recovery of lithium-ion (Li+) from seawater is expected to be a promising solution to the shortage of lithium resources. This work demonstrated an application of a hydrophobic deep eutectic solvent (HDES) based system for Li+ recovery from dilute aqueous solutions (≤1 ppm). Methyltrioctyl ammonium chloride (N8881Cl) and decanoic acid (DecA) with a molar ratio of 1:2 were used as the extractant and bis(2-ethylhexyl) phosphate (DEHPA) as the co-extractant. N8881Cl/2DecA-DEHPA showed the highest extraction efficiency (80%) in a single extraction with the initial pH of the aqueous phase, the volume ratio of organic to the aqueous phase (O/A), and the volume fraction of HDES to be 11.10, 1/3 and 50%, respectively. FTIR analysis revealed the existence of cation exchange between Li+ and DEHPA. A total of ten regeneration cycles of extraction processes demonstrated that the extraction system was stable for selective extraction of Li+. The separation factor of Li/Na (βLi/Na) in the mixed ion solution was 9.76, indicating that the N8881Cl/2DecA-DEHPA system has the capability to selectively extract Li+ with a high Li/Na ratio. This will help design possible green HDES for lithium recovery from dilute aqueous solutions, e.g., raw and concentrated seawater.