Effect of polymer-entwined reduced graphene oxide laminates on the performance and stability of forward osmosis membranes for water desalination

Layered graphene oxide (GO) membranes have been widely employed for water desalination in forward osmosis (FO) systems, however, the balance between performance and long-term stability of membranes is still a trade-off in the practical application. A newly developed laminar GO-based FO membrane was prepared by a two-step protocol, including acid treatment of GO before crosslinking and reduction of GO by hyperbranched polyethylenimine (HPEI), and followed by coating HPEI-reduced graphene oxide (HPEI:rGO) laminates by polyethylene glycol (PEG). It is suggested that the long-chain and amino-enrichment HPEI polymer can be covalently bonded with the acid-treated GO at elevated temperatures. This chemical process partially removed oxygen-containing functional groups of GO and formed a nanocomposite structure with tuned interlaminar spacing for better selectivity of NaCl ions. Subsequently, the infiltration of PEG molecules through the HPEI-rGO structure improved the hydrophilicity, and hence the water permeability across the PEI60k:rGO/PEG membrane. In this study, the resultant membrane exhibited water flux of 7.7 LMH, salt rejection efficiency of 97.9 %, and low reverse solute flux of 0.56 gMH in an FO filtration process. Also, synergistic effects of the double-polymer modification reinforced enhancement of dimensional integrity and durability of the membrane structure in both ionic and harsh conditions.