Tailoring hydrophobic deep eutectic solvent for selective lithium recovery from the mother liquor of Li2CO3

•HDES synthesized by oleic acid and tetrabutyl ammonium chloride are more environmentally benign co-extractant.•The green synergic system TBP-HDES showed high lithium selectivity extraction.•The extraction mechanism were mainly cation exchange and Li+ coordination. Recovering Li+ from the mother liquor obtained during the process of Li2CO3 production has been considered as a promising solution to the shortage of lithium resources. However, as the mother liquor is a complex saline system with high concentration of Li+ (1.42 g/L) and a large amount of Na+ (55.62 g/L), there is no competent method to carry out for the effective utilization of Li+. This work reports a class of hydrophobic deep eutectic solvents (HDESs) formed by tetrabutylammonium chloride (TBAC) and oleic acid (OA), which is a novel co-extraction reagent for Li+ extraction. In addition, tributyl phosphate (TBP) is used as extractant and ammonia is applied to regulate pH. TBAC/3OA showed the highest extraction efficiency in single extraction (76.8%) with the ammonia concentration, the volume ratio of organic phase and aqueous phase (O/A phase ratio), and volume fraction of HDES as 1.5 mol/L, 1, and 30%, respectively. Mechanism studies through the UV–vis and FT-IR proved that cation exchange between Li+ and HDES, and Li+ coordination was the dominant force in the separation process. The complexing ability of P = O in TBP led to the selective extraction of Li+. The reusability experiments of organic phase demonstrated that the extraction efficiency was still above 60% after five cycles, indicating the favorable stability of the extraction system. Finally, the separation factor (βLi/Na) of Li/Na in the mother liquor was 20.5, indicating that the TBP-HDES system has the capability to selectively extract Li+ with high Na/Li ratio. This will help to design possible green HDES for lithium recovery.