Liquid-liquid extraction of lithium from aqueous solution using novel ionic liquid extractants via COSMO-RS and experiments

Ionic liquids (ILs) have been proposed as promising extractants to broaden the potential utility of liquid–liquid extraction process for lithium, but developing an attractive IL from numerous available ILs remains challenging. In this work, a rapid preliminary predicting and screening method based on COSMO-RS was proposed. The design strategy was to collocate anions that had intensive affinity to lithium ion with high hydrophobic cations. Based on it, a series of candidate ILs were prepared and characterized by 1H and 13C NMR spectroscopy, including [P4444]-based and [N4444]-based ILs with phosphate, phosphinate and long-chain carboxylate anions. The further extraction experiments indicated that the tetrabutylammonium mono-2-ethylhexyl(2-ethylhexyl)phosphate ([N4444][EHPMEH]) had the highest Li+ extraction efficiency. The extraction mechanism was investigated by slope analysis method and ATR-IR spectroscopy. It was determined that every Li+ associated with 1.33 molecule [N4444][EHPMEH] to form the extracted species. The stripping study showed that the Li+ was able to be stripped by only 0.5 mol/L HCl without additional agents. The extraction experiments of Li+ from multi-metal-ion solution were also carried out, and the distribution ratio of Li+ was nearly twice those of Na+, K+, and Rb+, indicating that [N4444][EHPMEH] was available to separate lithium ion from the other alkali metal ions.