Concentration and Recovery of Dyes from Textile Wastewater using a Self-standing, Support-free Forward Osmosis Membrane.

Forward osmosis (FO), an emerging membrane separation process, 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, ultra-smooth surface, and high negative surface charge, the PTAODH membrane exhibits excellent FO performance with minimal fouling, low reverse salt flux of 27.1±0.5 mmol m-2 h-1 (1.5 M Na2SO4 as draw solution), and negligible dye passage to the draw solution side. During operation cycles with this membrane, the dye concentration progressed to a concentration factor of ~10 before the membrane was cleaned with solutions of varying ethanol concentration. Physical cleaning with a 40% alcohol solution resulted in a flux recovery ratio as high as 98.7% for the PTAODH membrane, whereas similar cleaning of two commercial FO membranes resulted in a significant increase in the reverse salt flux, suggesting damage to the membrane's active layer by the cleaning solution. 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, E. coli and S. aureus. The results overall demonstrate the promising application of the fabricated symmetric PTAODH membrane for the treatment and concentration of textile wastewaters and its superior antifouling performance compared to state-of-the-art commercial FO membranes.