Active-passive synergistic alleviation of membrane scaling from groundwater by grafting charged positive monomers on the surface of nanofiltration membranes

Groundwater in rural areas often contains hardness ions that contribute to scaling on polyamide nanofiltration (NF) membranes used in drinking water treatment. To mitigate this issue, we chemically modified the residual functional groups (acyl chlorides and primary amines) on the surface of a hybridized polyamide layer by reacting them with three small-molecule monomers featuring quaternary ammonium salts: choline chloride, 2,3-dihydroxy- N , N , N -trimethylpropan-1-aminium chloride, and 2,3-epoxypropyltrimethylammonium chloride. This modification created a weakly charged, positive electric field on the underlying negatively charged polyamide surface, enhancing the membrane resistance to scaling and fouling. The chemical grafting altered the membranes' physicochemical properties, including wettability, charge characteristics, surface carboxyl group density, and pore structure. Notably, the modified NF membranes exhibited higher LiCl rejection rates compared to unmodified membranes, demonstrating the effectiveness of electrostatic repulsion against positively charged ions. This finding was supported by changes in zeta potential measurements. Furthermore, the sulfate/chloride separation factor for the modified NF membrane increased from 13.6 to 34.0, indicating improved selectivity while maintaining the desalination mechanism of typical negatively charged membranes. The extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) theory calculations revealed that reducing polar interactions on the NF membrane surface was crucial for enhancing anti-fouling performance. The weakly charged positive electric field actively repelled Ca 2+ from groundwater contacting the membrane surface, while the negatively charged polyamide layer passively inhibited Ca 2+ interaction with the membrane surface through electrostatic repulsion of CO₃ 2− . This dual mechanism resulted in excellent anti-CaCO₃ scaling behavior for the modified NF membranes.