Enhanced Mg2+/Li+ separation by nanofiltration membrane through surface modification using spirocyclic diamine

The efficient extraction of lithium from salt-lake brines has drawn significant attention due to the increasing demand for lithium. In this study, we modified a polyethylenimine (PEI)-trimesoyl chloride (TMC) nanofiltration membrane by grafting a rigid-contorted spirocyclic diamine monomer (SBI) containing quaternary ammonium onto its surface. The incorporation of the rigid spirocyclic structure loosened the selective layer and increased the molecular weight cut-off of the membrane to 394 Da. The SBI modified membrane exhibited a pure water permeance of 16.0 L & sdot;m- 2 & sdot;h- 1 & sdot;bar- 1, 3.5 times higher than that of the PEI-TMC membrane. Furthermore, the existence of quaternary ammonium moieties enhanced the positive charge on the membrane surface, thereby strengthening the Donnan exclusion against Mg2+. The proposed membrane had a high rejection for Mg2+ while exhibiting a relatively low rejection for Li+, leading to a Mg2+/Li+ ratio of 6.9 in the permeate from a feed solution with a Mg2+/Li+ ratio of 100. This study highlights the potential of membrane surface modification using the SBI monomer to enhance lithium extraction through nanofiltration.