Ion Transport Behavior through Thermally Reduced Graphene Oxide Membrane for Precise Ion Separation

The cation transport behavior of thermally treated reduced graphene oxide membranes (GOMs) is reported. The GOMs were reduced by heat treatment at 25, 80, and 120 degrees C and then characterized by Fourier transform infrared spectroscopy, X-ray powder diffraction, and X-ray photoelectron spectroscopy to determine oxygen group content, C/O ratio, and layer spacing. The permeation rates of various cations with different sizes and charge numbers through these membranes were measured to understand the effect of the cations on transport behavior. The results indicated that the cation transport through the membranes depended on the layer spacing of the membrane and ion size and charge. Cations of the same valence permeating through the same GOM could be differentiated by their hydration radius, whereas the same type of cation passing through different GOMs could be determined by the spacing of the GOM layers. The cation valence strongly affected permeation behavior. The GOM that was prepared at 120 degrees C exhibited a narrow layer spacing and high separation factors for Mg/Ca, Mg/Sr, K/Ca, and K/Fe. The cations moving through the membrane could insert into the membrane lamellas, which neutralized the negative charge of the membrane, enlarged the layer spacing of the GOMs, and affected cation permeation.