Bifunctional Group Synergy in PEI-TMC NF Membrane for Balanced High Permeability and Mg/Li Separation

The nanofiltration (NF) membrane, as an emerging lithium resource extraction technology, precisely screens Mg2+ and Li+ through the Donnan effect and steric hindrance. However, most NF membranes still struggle to break through the “trade-off effect”, and how to effectively overcome this effect to achieve a balance between high ion rejection and high permeability remains a great challenge. In this study, sodium alginate (SA) was added to the aqueous phase during the preparation of polyethylenimine-trimesoyl chloride (PEI-TMC) NF membranes. The bifunctional groups of SA can achieve synergistic effects: −COOH can produce electrostatic interaction with PEI to reduce amine sites, while −OH can form a competitive relationship with PEI to reduce the reactivity of amine groups, thereby forming a loose and thin separation layer. The permeability was significantly improved, and its water flux increased from 34.68 L·m–2·h–1 to 92.04 L·m–2·h–1, representing a nearly 3-fold increase compared to the PEI-TMC NF membrane. At the same time, the reduction of amine groups participating in the reaction under the optimal conditions increases the positive charge on the membrane surface, maintaining the original high separation performance SLi+/Mg2+=21.18). Through a three-stage nanofiltration process, the Li+/Mg2+ ratio in real salt-lake brine can be reduced to 0.86, and the improved adsorption-membrane coupled process can further reduce the Li+/Mg2+ ratio to 0.04, significantly reducing the interference of high-concentration Mg2+ in the lithium extraction process. Finally, this study systematically compared the cost and environmental impact of the modifier used in this work with those of other common modifiers. The results show that the proposed modifier is cost-effective and environmentally friendly, with higher feasibility for practical industrial applications. Therefore, this work exhibits promising application prospects in the field of lithium extraction from salt lakes.