Abstract: Positively charged composite nanofiltration (NF) membranes with good stability were prepared by dopamine (DA) assisted poly(ethylene imine) (PEI) deposition on a polysulfone ultrafiltration (UF) substrate followed by a cross-linking step. Attenuated total reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electronic microscopy, and atom force microscopy were employed to characterize the surface chemistry and morphology of the obtained composite membranes. The DA and PEI co-deposition conditions were optimized based on knowledge of the co-deposition mechanism. The effects of the cross-linker concentration, cross-linking time, and reaction temperature on the permeation and separation properties of the prepared composite membranes were investigated in detail. Under optimized conditions, the MgCl₂ rejection and permeation flux of the composite membrane reached 80.4% and 19.6 L/(m²·h), respectively (the feed was 0.01 mol/L of MgCl₂ solution under a test pressure of 0.4 MPa). The rejection of various salts followed the order MgCl₂≈CaCl₂> MgSO₄>NaCl>Na₂SO₄, suggesting the membranes were positively charged. The composite membranes showed good durability under alkaline aqueous conditions. This study provided new insights into the fabrication of mussel-inspired thin-film composite nanofiltration membranes.

Key words: Dopamine (DA), Poly(ethylene imine) (PEI), Co-deposition, Nanofiltration (NF) membranes, Cross-linking

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