Effect of cations on stabilizing graphene oxide membranes in aqueous solutions

Owing to the unique layer structures and efficient water transport, graphene oxide membranes (GOMs) have shown great promise in molecular and ion separation. In the above application fields, the stability of GOMs in aqueous solutions is the most essential requirement. In this paper, vortex shaking experiments and XRD characterization were carried out to test and illustrate the stability of GOMs in different solutions. Normally, when dry GOMs were immersed in aqueous solutions, the water molecules could rapidly enter layer space and enlarge the interlayer spacings (d-spacings), thus reducing the stability of GOMs in aqueous solutions. However, it was also found that the ions in aqueous solutions could play a positive role in restraining the d-spacings and stabilizing the GOMs. For monovalent cations (for example, H+ and Na+), high concentrations were required to guarantee the good stability of GOMs, which could be explained by the theory of electrostatic double layer. In contrast, low concentrations of divalent or trivalent cations (for example, Sr2+, UO22+ and Eu3+) could also efficiently crosslink the graphene oxide sheets, thus enhancing the stability of GOMs. Based on these results, to ensure the stability in practical application, a method of modifying original GOMs by 1 mM Eu3+ was proposed and verified by sucrose and metal ions permeation experiments. The results indicated that the modified GOMs could keep stable and be qualified to achieve selective separation.