Optimizing the surface properties of nanofiltration membrane by tailoring the diffusion coefficient of amine monomer

High-performance hollow fiber NF membranes with network structure were successfully prepared by introducing macromolecular polyvinyl alcohol (PVA) into aqueous phase to change the diffusion coefficient of amine monomer (piperazine, PIP) in organic phase. Based on Wilke-Change equation, a theoretical model (DAO) dealing with viscosity of aqueous phase, molecular weight of PVA, and diffusion coefficient of amine monomer in organic phase was proposed. The applicability of the DAO model was verified by ultraviolet spectrophotometry (UV) monitoring and one-dimensional diffusion model. Subsequently, the structure of NF membranes was analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results suggested that the diffusion coefficient of amine monomer was negatively correlated with aqueous phase's viscosity and polymer's molecular weight. With PVA205 (Molecular weight is 205000 Da) used to change the viscosity of aqueous phase, the diffusion coefficient of PIP (in n-hexane) was close to 2.70 × 10−6 cm2/s, and the resultant NF membrane (NF-PVA205-1) showed excellent performance; specifically, NF-PVA205-1 achieved a flux as 107 L/m2·h (4 bar), which is 3.5-times higher than that of pure polyamide NF membrane, and the rejection rate for sodium sulfate can reach 97.9%. Such a facile fabrication method proposed in this study can balance the "trade-off" effect of traditional NF membrane.