High positively charged composite nanofiltration membranes modified by a novel bis-quaternary ammonium monomer for Li+ extraction from high Mg2+/Li+ ratio salt lakes

The demand for lithium resources is increasing globally. Salt lakes have been an important source of lithium exploitation. However, Mg 2+ /Li + separation is a challenge mainly for lithium extraction from salt lakes with high Mg 2+ /Li + ratio. In this paper, a novel monomer QTHEED with bis-quaternary ammonium was designed and synthesized. Composite nanofiltration membranes were also prepared by surface modification of polyamide membranes. The permeance (23.08 L m −2 h −1  MPa −1 ) and Li + /Mg 2+ selectivity (21.7) of modified membranes are both ∼3 times higher than those of the pristine membrane. Increased positive charge of membranes enhances the rejection of Mg 2+ . Furthermore, QTHEED modification brings hydrophilic groups, Turing structure, and larger membrane pores. Simulation results confirmed that the incorporation of the QTHEED enabled the membranes to effectively separate Mg 2+ and Li + and to be more conducive to the passage of water molecules. In addition, the nanofiltration performance of the modified membrane was stable after 40 h of continuous nanofiltration tests at 4 bar pressure. This study presented a novel and efficient approach for separating Mg 2+ and Li + in industrial settings.