A two-step green liquid-liquid extraction strategy for selective lithium recovery and application to seawater

Efficiently harvesting lithium from seawater remains a challenge due to the extremely dilute concentration of lithium ions (Li+) and the existence of many foreign metal ions that are far more abundant. Here, a hydrophobic deep eutectic solvent (HDES)-based two-step liquid-liquid extraction (LLE) strategy is developed for the first time that realizes the separation of dilute Li+ (<1 mg/L) from solutions with a high concentration of sodium, potassium, magnesium and calcium ions. This strategy contains the removal of divalent cations and selective recovery of Li+, which minimizes the influence of coexisting metal ions. In the first LLE step, the unwanted divalent cations are removed with a negligible loss of Li+ by the bis(2-ethylhexyl)phosphate/kerosene (DEHPA/KERO) system. Then, the DEHPA/HDES system can selectively extract Li+ from monovalent cations. All solvents used are readily available and low cost with methyltrioctylammonium chloride and decanoic acid at a 1:2 mol ratio (N8881Cl/2DecA) as the main extractant. Extraction equilibrium is reached within 10 s. The single extraction efficiency of Li+ is 66.4%, and the separation coefficient of lithium and sodium (βLi/Na) reaches 11.6 at the optimized extraction conditions. Finally, the proposed two-step LLE strategy is applied to the simulated seawater. The extraction performance of N8881Cl/2DecA-DEHPA was not significantly weakened after being reused ten times.