NaCl, KCl, MgCl 2 and CaCl 2 powders with the number of moles shown in Table were mixed with 5 mL, ~0.1 mg/mL GO solution to prepare cation intercalated GO solutions. The dispersibility of the mixture was ensured by incubating the suspension for 24 hours. Then, the suspension was filtered through PVDF substrate under ~0.9 bar pressure. Another set of intercalated GOMs was prepared from 5 mL, 0.1 mg/mL of GO solution containing 0.2 µmol of K + , Na + , Ca 2+ , Mg 2+ and Fe 3+ . Characterization of GO sheets and GOMs. Transmission electron microscopy (TEM, JEOL, JEM-2100F, Japan) was used to investigate the dimension of GO sheets on lacey carbon grid.