Facile surface charge tuning of nanofiltration membrane with dual functional betaine to improve the permselectivity and CaSO4 scaling resistance

Long-term stable operation of nanofiltration membranes is critical to ensure safe and efficient water treatment processes, requiring membranes that exhibit both high permselectivity and robust scaling resistance. This study presents a novel surface functionalization strategy based on the dual-functional monomer betaine hydrazide hydrochloride for tailoring the surface charge of nanofiltration membranes. The functionalized membranes demonstrated a combination of high water flux, efficient monovalent/divalent salt separation, and good resistance to CaSO 4 scaling – a prevalent and persistent challenge in membrane processes. The optimal functionalized nanofiltration membranes displayed a water flux of 59.8 L·m −2 ·h −1 and a Na + /Mg 2+ separation factor of 48.6, corresponding to improvements of 32.3 % and 120.9 %, respectively, over the control membrane. In addition, the membrane achieved superior Li + /Mg 2+ separation, reducing the Mg 2+ /Li + ration from 80 in the feed to 1.328 in the permeate with a Li + /Mg 2+ separation factor of 60.6. This separation factor represents a 112.6 % improvement over that of the control membrane. Long-term scaling tests revealed improved anti-scaling performance, with the functionalized membrane retaining 91 % of its initial flux after 96 h of Ca 2+ and SO 4 2− exposure, compared to only 83 % for the control membrane. Real-time optical coherence tomography in situ monitoring further confirmed the enhanced scaling resistance of the functionalized membrane. These results highlight the great potential of functionalized nanofiltration membranes for high-performance desalination and ion separation applications. • Membrane surface charge was tuned by dual functional betaine. • Water flux and separation factors of monovalent/divalent cations were improved. • The functionalized membrane exhibited improved CaSO 4 scaling resistance. • OCT was applied to monitor the CaSO 4 scaling formation in the process of operation.