Competition of co-existing cations to eliminate negative effect of Na+ on graphene oxide membrane structure and stabilize the separation performance

Abstract

The impact of inorganic ions present in aqueous environment on the structural stability of graphene oxide (GO) membrane has raised increasing attention. This study investigated how the competition of co-existing cations with Na+ affected GO membrane structure and separation performance. Results showed that the presence of Na+ alone (NaCl solution) led to significantly increased membrane flux (400% of the ultrapure water flux) and decreased rejection (from 17.2% to 3.5%). However, the flux and rejection of GO membrane were more stable when the solutions contained co-existing K+ or Ca2+. The separation performance variation was attributed to the structural variation of the GO membrane. Characterized by X-ray diffraction, the orderly laminar structure of GO membrane completely disappeared in NaCl solution but was well maintained in NaCl/KCl or NaCl/CaCl2 solution. The concentration required for stabilization was 1 mM for co-existing Ca2+, which was lower than that of K+ (5 mM). The ability of co-existing K+ or Ca2+ to eliminate the negative effect of Na+ was ascribed to their priority to enter nano-capillary channels via stronger attractive interactions with GO nanosheets. Owing to the small size of K+ or bridging effect of Ca2+ with GO nanosheets, interlayer interactions within GO membrane were enhanced, and thus, its structure was stabilized.