Engineering spatial charge distribution in polyamide nanofiltration membranes for enhanced treatment of oilfield-produced water

Oilfield-produced water (OFPW), a major byproduct of hydrocarbon extraction, is highly saline and contains elevated concentrations of divalent ions, such as calcium (Ca 2+ ), posing significant challenges for its treatment, transportation, and reuse. Nanofiltration (NF) membranes offer an energy-efficient solution for OFPW treatment by selectively rejecting multivalent ions while allowing monovalent ion transport; however, conventional NF membranes often suffer from limited Ca 2+ rejection and severe fouling. In this study, a polyamide NF membrane with an engineered spatial charge distribution was fabricated via mixed-monomer interfacial polymerization using piperazine and polyethyleneimine, assisted by a dodecyl phosphate (DDP) surfactant. The resulting membrane features a negatively charged front surface and a positively charged rear surface, forming a dual-charged configuration. This structure integrates Donnan repulsion from the positive sublayer and size-sieving effects, leading to enhanced ion selectivity and antifouling performance. Compared with commercial NF270 and DL membranes, the fabricated membrane achieved significantly higher Ca 2+ rejection (98.95%) and Na + /Ca 2+ selectivity (21.44), while maintaining competitive water permeability and reduced fouling during treatment of both simulated and real OFPW. These results demonstrate that tailoring spatial charge distribution in NF membranes is an effective strategy for improving ion selectivity and fouling resistance in high-salinity water treatment applications.