A freestanding graphene oxide framework membrane for forward osmosis: Separation performance and transport mechanistic insights

Graphene oxide (GO)-based membranes have been explored for application in forward osmosis (FO). However, the mass transfer inside GO framework membranes is far from sufficiently understood, and designing freestanding GO framework membranes with further enhanced performance remains a challenge. In this study, freestanding GO framework membranes were synthesized and investigated in a FO system. The membrane water flux, reverse salt flux, and forward salt flux were systematically investigated using different feed and draw solutions. The membrane synthesized using GO nanosheets and polyelectrolyte spacers (poly(sodium 4-styrenesulfonate) (PSS) (GO-PSS membrane) possesses high water flux, low reverse and forward salt flux, and low specific reverse salt flux (i.e., high water/salt selectivity). We found that both the size and surface charge density of the membrane pores influenced the salt transport inside the membrane in FO. By combining results from experiments and numerical modeling, the influence of the electrical double layer (EDL) extension inside the membrane pores on salt transport was quantitatively analyzed. Additionally, it was found that the EDL extension can influence the forward and reverse solute transport simultaneously (i.e., bidirectional transport). Furthermore, viable strategies toward designing GO framework membranes with enhanced performance for application in FO as well as their potential application niche were discussed.