Composite forward osmosis hollow fiber membranes: Integration of RO- and NF-like selective layers to enhance membrane properties of anti-scaling and anti-internal concentration polarization

One of the major barriers that hinder the application of forward osmosis (FO) processes is the lack of optimized membranes that possess high water flux, low salt leakage and good anti-internal concentration polarization (ICP) and anti-fouling/anti-scaling properties during practical FO operations. In this study, novel composite FO hollow fiber membranes with an RO-like selective skin and an NF-like secondary selective skin were successfully developed for the first time. The fabrication procedures involved making ultrafiltration hollow fiber substrate using poly(amide–imide) (PAI) polymer material via phase inversion method, followed by interfacial polymerization (IP) and chemical modification to yield a polyamide RO-like inner skin and a positively charged NF-like outer skin, respectively. It was found that the sequence of conducting IP on the substrate inner surface prior to the chemical modification of the outer skin was preferred in order to achieve better performance for the resultant double-skinned hollow fibers. The newly developed FO hollow fibers exhibited high water permeability of 2.05 L/m2 h bar and 85% rejection to NaCl at 1 bar pressure as well as superior FO water flux of 41.3 L/m2 h and low ratio of salt flux over water flux of 0.126 g/L when using DI water and 2.0 M NaCl as feed and draw solutions, respectively, in the active layer facing draw solution (AL-DS) orientation. Furthermore, the experimental results showed that the composite FO hollow fiber membrane was able to outperform single-skinned membrane when the feed contains divalent ions or the feed exhibits high scaling tendency to membrane. This suggested that the integration of RO- and NF-like two selective skins in FO membrane is an effective way to minimize the ICP effect, mitigate the membrane scaling and thus enhance the feasibility of FO processes for practical applications