Graphene oxide nanofiltration membranes with confined Na+ in two-dimensional nanochannels

The interactions between metal cations and graphene derivatives may bring about significant changes to the properties and structures of graphene-based devices, which further affect their performance. Under practical pressure-driven separation conditions, the presence of metal cations in the feeding solutions is universal. They not only play the role of target solutes but also act as the modifier to tune the structure and transmembrane resistance of graphene oxide (GO) membranes. Herein, the influences of hydrated Na+/Mg2+ on the two-dimensional nanochannels of GO membranes under pressure driven conditions are revealed. Hydrated Na+ tend to be confined in between GO sheets, leading to the expansion of the nanochannels which facilitates the passage of water molecules. The accumulation of hydrated Mg2+ at the edges of GO sheets reduces both the cross-sectional area and the number of the transport pathways, increasing the transmembrane resistance of water molecules. On this basis, high flux Na+-GO nanofiltration membranes were designed, which exhibited similar to 6 times increase in pure water flux compared with GO membranes. The water flux in dye solutions was increased by 4- 5 times without scarifying the rejection for methyl orange and direct yellow. The Na+-GO membranes showed good structural stability both in water and salt/dye solutions, confirming that the treatment with dynamic Na+ flow is a reliable strategy to improve the nanofiltration efficiency of GO membranes. It was demonstrated that hot water filtration was a simple but efficient way to recover the initial structure and performance of GO membranes.