Effect of the supporting layer structures on antifouling properties of forward osmosis membranes in AL-DS mode

Antifouling Double-Skinned Forward Osmosis Membranes by Constructing Zwitterionic Brush-Decorated MWCNT Ultrathin Films

Pressure retarded osmosis (PRO) process is hindered by severe fouling occurring within the porous support of the forward osmosis (FO) membranes. We designed a novel double-skinned FO membrane containing a polyamide salt-rejecting layer and a zwitterionic brush-decorated, multiwalled carbon nanotube (MWCNT/PSBMA) foulant-resisting layer on the back side. Our results demonstrated that the coating of the MWCNT/PSBMA layer on the porous polyketone (PK) support imparted enhanced hydrophilicity and smaller membrane pore size, thereby providing excellent resistance toward both protein adhesion and bacterial adsorption. We also further evaluated this resultant double-skinned membrane (i.e., TFC-MWCNT/PSBMA) in dynamic PRO fouling experiments using protein and alginate as model organic foulants. Compared to the pristine TFC-PK and hydrophobic TFC-MWCNT membranes, the TFC-MWCNT/PSBMA membrane exhibited not only the lowest water flux decline but also the highest water flux recovery after simple physical flushing. These results shed light on fabrication of antifouling PRO membranes for water purification purposes.

In Situ Surface Modification of Thin-Film Composite Polyamide Membrane with Zwitterions for Enhanced Chlorine Resistance and Transport Properties

High-performance chlorine-resistant thin-film composite (TFC) membranes with zwitterions were fabricated by in situ surface modification of polyamide with 2,6-diaminopyridine and the subsequential quaternization with 3-bromopropionic. The successful modification of the TFC polyamide surface with zwitterions was confirmed by various characterizations including surface chemistry, surface hydrophilicity, and surface charge. The transport performance of the membrane was measured in both of the cross-flow reverse osmosis (RO) and forward osmosis processes, and the results showed that the modified TFC membrane improved both of its water permeability and perm-selectivity with the increased A and A/ B ratios upon modification with zwitterions. The chlorination challenging experiments were performed to demonstrate that the modified membrane enhanced its chlorine resistance without affecting its salt rejection upon 16 000 ppm·h chlorination exposure. A chlorination mechanism study illustrated that the modified membrane with zwitterions could prevent the Orton rearrangement of the benzene ring of the polyamide layer. Importantly and excitingly, the optimal chlorinated TFC membrane with zwitterions achieved a very high water flux of 72.15 ± 2.55 LMH with 99.67 ± 0.09% of salt rejection in the cross-flow RO process under 15 bar.

Concentration and Recovery of Dyes from Textile Wastewater Using a Self-Standing, Support-Free Forward Osmosis Membrane

Forward osmosis (FO) can potentially treat textile wastewaters with less fouling than pressure-driven membrane processes such as reverse osmosis and nanofiltration. However, conventional FO membranes with asymmetric architecture experience severe flux decline caused by internal concentration polarization and fouling as dye molecules accumulate on the membrane surface. In this study, we present a new strategy for concentrating dye by using a self-standing, support-free FO membrane with a symmetric structure. The membrane was fabricated by a facile solution-casting approach based on a poly(triazole- co-oxadiazole- co-hydrazine) (PTAODH) skeleton. Due to its dense architecture, ultrasmooth surface, and high negative surface charge, the PTAODH membrane exhibits excellent FO performance with minimal fouling, low reverse salt flux, and negligible dye passage to the draw solution side. Cleaning with a 40% alcohol solution, after achieving a concentration factor of ∼10, resulted in high flux recovery ratio (98.7%) for the PTAODH membrane, whereas significant damage to the active layers of two commercial FO membranes was observed. Moreover, due to the existence of cytotoxic oxadiazole and triazole moieties in the polymer structure, our PTAODH membrane exhibited an outstanding antibacterial property with two model bacteria. Our results demonstrate the promising application of the symmetric PTAODH membrane for the concentration of textile wastewaters and its superior antifouling performance compared to state-of-the-art commercial FO membranes.