Positively Charged Hollow Fiber Nanofiltration Membrane With an SPSf / PEI Bilayer Prepared via Dip‐Coating

ABSTRACT Nanofiltration (NF) has attracted considerable interest in water purification and resource recovery. Hollow fiber membranes offer high packing density, large surface‐area‐to‐volume ratio, and low‐pressure high‐flux operation. However, the fabrication of hollow fiber NF membranes often suffers from non‐uniform and discontinuous separation layers, while most conventional NF membranes are negatively charged and inclined to fouling caused by the adsorption of positively charged ions. In this work, a positively charged hollow fiber NF membrane was fabricated by constructing a sulfonated polysulfone (SPSf)/polyethyleneimine (PEI) electrostatic assembly bilayer via dip‐coating. The SPSf interlayer was first coated onto the polyethersulfone (PES) surface to enhance interfacial adhesion, facilitate PEI assembly, and prevent excessive PEI penetration into the membrane pores, thereby forming a dense, defect‐free selective layer via electrostatic interactions. The PES/SPSf‐PEI membrane showed high rejection for MgCl 2 (90.96%), crystal violet (99.56%), and rhodamine B (99.81%). Moreover, a flux recovery ratio of 86.77% was achieved after three fouling‐cleaning cycles using rhodamine B as a model foulant. This work provides a facile and controllable approach for preparing high‐performance positively charged hollow fiber NF membranes, showing great potential for divalent cation rejection and dye treatment.