Covalent organic framework incorporated outer-selective hollow fiber thin-film nanocomposite membranes for osmotically driven desalination

In this study, new outer-selective hollow fiber (OSHF) thin film nanocomposite (TFN) forward osmosis (FO) membranes incorporated with amine-rich Schiff-based network (SNW-1) nanoparticles (NPs), a type of covalent organic frameworks (COFs), were developed by vacuum-assisted interfacial polymerization (VAIP). The SNW-1 NPs possess hydrophilic nature and porous internal structures, which are desirable for producing highly permeable and efficient FO membranes. SNW-1 NPs were conformally packed across the outer surface of the HF substrate by vacuum pressure applied during the VAIP process leading to defect-free coatings. Furthermore, covalent bonding between secondary amine (NH2−) groups across the SNW-1 network and remanent carboxylic groups present from the monomer used for the interfacial polymerization step supported the adhesion of SNW-1 NPs to the native poly(amide) layer present across the TFN membrane surface. As a result, SNW-1 loading at 0.001 wt% exhibited the best FO performance with enhanced water flux more than 23% of the pristine one (31.5 L m−2 h−1) and relatively low specific reverse solute flux (SRSF, Js/Jw) at 0.18 g L−1 using 1 M NaCl and DI water in series of TFN composite membranes. The optimum loading at 0.001 wt% was much lower than those in other home-made TFN membranes (normally above 0.05 wt%) due to the minimum loss of SNW-1 in the VAIP. Also, the TFN membrane offered excellent FO operation stability tested for 72 h. Such novel approach is promising to optimize FO processes for desalination and water treatment in the future.