Liquid-liquid extraction of lithium using lipophilic dibenzo-14-crown-4 ether carboxylic acid in hydrophobic room temperature ionic liquid

A green liquid-liquid extraction (LLE) system was developed for the recovery of lithium (Li+) from sodium and potassium ions, which are typically present at high concentrations in seawater. Dibenzo-14-crown-4ether (DB14C4) was functionalized with a long lipophilic alkyl C18 chain and a pendent proton ionizable carboxylic acid group to obtain a lithium (Li+) carrier system (DB14C4-C18-COOH) with high Li+ extraction performance and good stability in the room temperature ionic liquid diluent, CYPHOSIL 109. The Li+ extraction efficiency of DB14C4-C18-COOH/CYPHOSIL 109 can be enhanced by increasing the solution pH and DB14C4-C18-COOH concentration. Further examination of extraction results reveal 1:1 coordination between DB14C4-C18-COOH and Li+ which was also supported by density functional theory calculations. At room temperature, the developed LLE system effectively extracted dilute Li+ from Na+ (selectivity αLi+/Na+ = 1954) and K+ (αLi+/K+ = 138). Kinetic and thermodynamic parameters were evaluated for optimum Li+ extraction conditions. Sequestered Li+ can be easily recovered from the LLE system using dilute hydrochloric acid. Results from recycling tests showed stable Li+ extraction performance hence it can be used for long term application. Overall results indicate the potential application of DB14C4-C18-COOH/CYPHOSIL 109 as a treatment step to recover Li+ from brine or seawater.