Cation-dependent structural instability of graphene oxide membranes and its effect on membrane separation performance

Graphene oxide (GO) membranes formed by stacked GO nanosheets can separate contaminants from water via nanosized capillaries, the properties of which, however, might be altered by intercalation of solutes. This study first reports the impacts of common inorganic ions on the structure and separation performance of non-covalently bonded GO membranes. When filtering NaCl, KCl, MgCl₂, CaCl₂, Na₂SO₄, and K₂SO₄ solutions, the flux of GO membranes, fabricated by vacuum filtration, exhibited significant increase over the filtration time in Na⁺ solutions, but maintained stable in the other electrolyte solutions. Rejection decline was observed for all the electrolytes with an order of Na⁺ > Mg^2 + ≈ Ca^2 + > K⁺ for the chlorides. X-ray diffraction indicated the GO membrane structure in K⁺ solutions was well maintained and slightly expanded in Mg^2 +/Ca^2 + solutions over time. However, the ordered structure of GO membranes disappeared after long exposure to Na⁺, explaining the significantly increased flux and reduced rejection. The effect of Na⁺ on the GO membrane structure was ascribed to its relatively large size but weak attraction with GO nanosheets. These findings highlight the importance of introducing strong covalent bonding between GO nanosheets to stabilize the structure and separation performance of GO membranes for water purification.