Effect of Oxide Content of Graphene Oxide Membrane on Remarkable Adsorption for Calcium Ions

Graphene oxide membranes (GOMs), as one of the most promising novel materials, have gained great interest in the field of adsorption. However, the oxygen content of graphene oxide is directly related to its adsorption properties, such as suspension stability, adsorption capacity, and reusability of GOMs. Here, a series of reduced GOMs with oxygen content from 28% to 12% were conveniently prepared by the thermally reduced and the corresponding interlayer spacing of these membranes changed from 8.0 Å to 3.7 Å. These prepared GOMs have remarkable Ca^2+ adsorption capacity, which increases with the oxygen content or interlayer spacing of GOMs. Importantly, the max adsorption capacity of the mass ratio between adsorbed Ca^2+ and pristine GOMs can reach up to 0.481 g/g, which is about one order of magnitude higher than the adsorption capacity of activated sludge, magnetic Fe_3O_4, functionalized silica, zeolite molecular sieve, and other reported previously. Moreover, GOMs show excellent stability and the Ca^2+ can be easily desorbed by water, so that the GOMs can be reused. Our previous theoretical analysis suggests that this remarkable adsorption is attributable to the strong interactions between Ca^2+ and GO sheets, including the ion-\pi interactions between Ca^2+ and aromatic graphitic rings as well as the electrostatic interaction between Ca^2+ and oxygen-containing groups.