Tailoring PES nanofiltration membranes through systematic investigations of prominent design, fabrication and operational parameters
Design and fabrication of nanofiltration (NF) membranes with the desired characteristics and separation performance is of paramount importance. In this study various asymmetric nanofiltration membranes were designed and fabricated using poly(ethersulfone) (PES) via phase inversion technique. The effects of variation in polymer concentration, solvent type, additives in the dope solution and composition of coagulating agent were studied as the selected design parameters. The effects of variation in solvent evaporation time, coagulation bath temperature, casting speed (shear rate) and membrane thickness were also investigated as the selected fabrication parameters. The results obtained reveal that increasing the polymer concentration, promoting a delay in demixing through a change of solvent and composition of coagulating agent as well as decreasing the coagulation bath temperature, increasing the solvent evaporation time and membrane thickness all result in NF membranes with less overall porosity and mean pore size, lower water flux and higher salt rejections. Furthermore, addition of hydrophilic organic acids, (e.g., ascorbic and citric acids) in the dope solution and incrementally increasing the casting shear rate promote overall porosity of the membrane, water flux and salt rejection. Membranes derived from PES/N-methyl-2-pyrrolidone (30/70 wt%) could offer maximum salt rejections of 47.35% and 99.84% for sodium chloride and magnesium sulfate, respectively. The pure water flux in the membranes could be enhanced up to 54.88 l m−2 h−1 by addition of 1 wt% citric acid into the dope solution. In terms of operational parameters, increase in both feed pressure and pH could enhance the membrane flux and salt rejections. The findings in this study provide useful guidelines and methods for the design and fabrication of high performance asymmetric NF membranes with the desired microstructure, productivity and separation performance.