Fabrication of composite nanofiltration membranes by dopamine-assisted poly(ethylene imine) deposition and cross-linking

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 MgCl 2 rejection and permeation flux of the composite membrane reached 80.4% and 19.6 L/(m 2 ·h), respectively (the feed was 0.01 mol/L of MgCl 2 solution under a test pressure of 0.4 MPa). The rejection of various salts followed the order MgCl 2 ≈CaCl 2 >MgSO 4 >NaCl>Na 2 SO 4 , 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.