Selective recovery of lithium from mother liquor of Li2CO3 by synergistic hydrophobic deep eutectic solvents: Performance and mechanistic insight

Efficient recovery of lithium from mother liquor of Li2CO3 is considered as an effective way to mitigate the rapidly growing demand of lithium but poses a daunting challenge. Herein, a series of synergistic hydrophobic deep eutectic solvents (HDESs) coupling β-diketones and neutral extractants were developed as novel extraction media for recovery of Li+ from aqueous solution. The newly developed HDES exhibited very low viscosity and high hydrophobicity, enabling their direct use in liquid–liquid extraction. The extraction performance of the systems could be easily manipulated by simply altering the composition of HDESs, with the thenoyltrifluoroacetone-tributyl phosphate (HTTA-TBP) and thenoyltrifluoroacetone-trioctylphosphine (HTTA-TOPO) HDES displaying excellent extraction capacity and selectivity towards Li+. The selective extraction mechanism was exemplified by HTTA-TBP HDES and revealed by molecular dynamics simulation for the first time, whereby Li+ was driven into HDES phase mainly through its strong electrostatic interaction with deprotonated HTTA, while TBP interacted with Li+ via the coordination effect. The stoichiometry of extracted complex was 1:1:1, with the form of Li+·TTA-·TBP. The cooperation of HTTA and TBP in HDES prohibited the participation of water molecule in the formation of extracted complex, thus preventing the emulsification during extraction. Interaction energy analysis demonstrated that Li+ interacted with both TTA- and TBP much more strongly than Na+ and K+, explicitly elucidating the high selectivity of HDES towards Li+ over Na+ and K+. Finally, both of the HTTA-TBP and HTTA-TOPO HDES were utilized to recover Li+ from the mother liquor of Li2CO3, with recovery rates of Li+ over 80 % through single-stage extraction, scrubbing and stripping. Moreover, the stripped HDESs could be directly used in subsequent extraction cycles without regeneration, and their extraction performances remained virtually unchanged, suggesting that the developed HEDSs are highly promising and feasible.