Preparation of positively charged acid-resistant hollow fiber nanofiltration membrane with excellent separation performance by surface modification

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There is a significant requirement for acid-resistant nanofiltration (NF) membranes with positively charged surface in order to effectively treat and recycle acidic industrial wastewater that contains heavy metal ions. Currently, acid-resistant nanofiltration membranes still exhibit low water permeance and typically display only weak positively charged or even negatively charged surface, which presents a vast challenge for the effective removal of heavy metal ions. In this work, we successfully produced a positively charged hollow fiber (HF) NF membrane with outstanding water permeability by combining interfacial polymerization and surface modification approaches. The interfacial polymerization reaction was conducted on the inner surface of an HF polysulfone substrate using polyethyleneimine as the aqueous monomer and cyanuric chloride as the organic monomer. This process results in the formation of a positively charged polyamine selective layer. Subsequently, the surface of the selective layer was modified with an aqueous solution of (3-bromopropyl) trimethylammonium bromide (BPTAB) via a Hoffman alkylation process, which increases the positive charge density and hydrophilicity of the HF NF membrane. Consequently, the BPTAB-modified HF NF membrane achieves a water permeance of 44.5 L m−2 h−1 MPa−1, which is a 130 % increase compared to the unmodified baseline HF NF membrane. Meanwhile, it exhibits excellent desalination performance, corresponding to a rejection of 97.3 %, 99.3 % and 99.1 %, for MgCl2, CoCl2 and MnCl2, respectively. Additionally, it demonstrates excellent resistance to acids, which remains a 93.3 % MgCl2 rejection after being submerged in pH = 1 HNO3 solution for 60 d.