Tailoring permeation channels of graphene oxide membranes for precise ion separation

The design and tailoring of membrane permeation channels is crucial for precise separation of ions or molecules. Herein, a method for tuning the permeation channels of graphene oxide (GO) membranes is developed, in which the basal planes and edges of GO flakes are simultaneously crosslinked by dicarboxylic acid and diamine. By altering the chain length of the crosslinkers, the size, structure and properties of the permeation channels are successfully tuned. In the permeation of single metal salt solution, the fluxes of metal ions are related to the swelling degree of GO membranes. In the separation of mixed salt solutions, the fluxes are determined by the radii of hydrated cations, and the crosslinked GO membranes display outstanding size-selectivity. This is the first report about the distinct orders of permeation fluxes of single salt solution and mixed salt solution. With the increasing hydrophobicity of diamines, the permeation fluxes of aqueous solution decrease. The optimized crosslinked GO membranes display excellent fluxes and separation factor, which are more than 2 times and 3 times that of the pristine membranes, respectively. The elastic modulus of the crosslinked GO membranes is double that of the pristine one, and the swelling degree in water is 75 times lower.