Water Treatment Of Graphene Oxide Membranes Based On Ion Control

Because they can provide ultrathin,high-flux,and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution,GO membranes?partially oxidized,stacked sheets of graphene?have shown great potential for use in water desalination and purification,gas and ion separation,biosensors,proton conductors,lithium-based batteries,and super-capacitors.Unlike carbon nanotube?CNT?membranes,in which the nanotube pores have fixed sizes,the pores of GO membranes--the interlayer spacing between GO sheets--are of variable size.This presents a challenge for using GO membranes for filtration.Despite the great efforts to tune and fix the interlayer spacing,it remains difficult to reduce the interlayer spacing sufficiently to exclude small ions while still maintain this separation constant against the tendency of GO membranes to swell when immersed in aqueous solution,which greatly affects the applications of GO membranes.Here,we experimentally demonstrate the achievement of highly efficient and selective ion rejection with GO membranes by controlling the interlayer spacing using cations themselves,including K⁺,Na⁺,Ca²⁺,Li⁺,and Mg²⁺.The interspacing can be controlled with precision as small as 1?.Moreover,GO membranes controlled by one type of cation can exclude other cations having larger hydrated volumes,which can only be accommodated with a larger interlayer spacing.First-principles calculations and ultraviolet?UV?absorption spectral experiments reveal that this unexpected behaviour is due to hydrated cation adsorption at the coexistence regions of oxidized groups and aromatic rings binding the GO sheets through strong noncovalent cation-?interactions,together with a stable hydrogen bond network.These findings open up new avenues for GO membrane use in water desalination and purification,lithium-based batteries and super-capacitors,molecular sieves for separating ions or molecules,and other applications.Except the strong adsorption based on cation-?interaction,a strong adsorption between?rings and the graphene flake surface were obtained by using the density functional theory,which markedly increased with increasing size of the electron-rich?rings,showing the particular importance of?-?interactions in the adsorption process.From the experimental adsorption for organic matter with various number of?rings by graphene and other carbon based materials,it reveal that the adsorption rates of organic matter were direct related to the?rings structure.And a fast fluorescence quenching test of fluorescent agent is driven by adsorption of graphene by?-?interactions.These findings of?-?interactions reveal the mechanism of organic matter adsorbed by carbon based materials,which provides a theoretical basis for treatment of organic pollutants by carbon based materials.