Application of Response Surface Methodology for Selective Extraction of Lithium Using a Hydrophobic Deep Eutectic Solvent

Lithium is a strategic element for energy storage. Extraction from surface water is a promising method for the purification of lithium. Synthetic model brines containing lithium and sodium ions were prepared for extraction experiments. A hydrophobic deep eutectic solvent (HDES) was synthesized using tetrabutylammonium chloride (TBAC) as the hydrogen bond acceptor (HBA) and decanoic acid (DA) as the hydrogen bond donor (HBD). Trioctylphosphine oxide (TOPO) was chosen as the coextractant. Fourier transform infrared (FT-IR) and 1H NMR analyses confirmed the physical interactions between the HBD and HBA. Response surface methodology was used to optimize the operational parameters for lithium extraction percentage and selectivity. The organic phase containing TBAC(1)/DA(2) and 10 wt % of TOPO at T = 23.0 °C and pH = 10.4 demonstrated the highest extraction performance. The optimized point yielded a selectivity of 4.41 and an extraction efficiency of 48.57% for one stage of extraction. The ion exchange reaction between lithium and hydrogen in the deep eutectic solvent (DES) phase was confirmed by FT-IR spectroscopy. Five regeneration cycles revealed that the organic phase remained stable after the third cycle. The extraction efficiency of lithium ions from a model brine with the same composition as that of Iran’s sources was 51.63% compared to those of sodium, potassium, magnesium, and calcium ions, which were 9.97, 3.11, 4.38, and 2.29%, respectively. The HDES synthesized in this study is cost-effective and biodegradable compared to other HDESs reported in the literature.