Thin film composite nanofiltration membranes assembled layer-by-layer via interfacial polymerization from polyethylenimine and trimesoyl chloride

Thin film composite (TFC) nanofiltration membranes were formed by interfacial polymerization from polyethylenimine (PEI) and trimesoyl chloride (TMC) on a microporous polyethersulfone (PES) substrate. Membranes with a layer-by-layer structure were prepared by repeated cycles of sequential reactant depositions and reactions to improve the salt rejection. The resulting membranes were characterized for chemical composition, surface charge and morphology of the polyamide skin layer. The effects of the sequence of reactant depositions, the number of cycles of reactant deposition and reaction, the concentrations of the reactant solutions and temperature of heat treatment on the membrane performance were investigated. The resulting membrane formed by a single cycle of interfacial polymerization with 3.5 wt% PEI and 0.7 wt% TMC had a positively charged surface and showed a good nanofiltration performance; salt rejections of 95.1% for MgCl2, 94.4% for MgSO4, 80.5% for Na2SO4 and 85.1% for NaCl with a pure water permeation flux of 24.5 L/(m2 h) were obtained at a feed pressure of 0.8 MPa gauge. The membranes formed with the PEI-TMC deposition sequence exhibited valley-ridge structures evenly distributed on the membrane surface, while the membranes made with a reversed deposition sequence (i.e., TMC-PEI) showed irregularly distributed nodular-like structures. In general, the PEI-TMC membranes were much more permeable than the TMC-PEI membranes. The salt rejections of the interfacially formed composite membranes were improved by proper heat treatment.