Effect of competition between swelling and dye adsorption on the performance and selectivity of graphene oxide membranes

The performance of graphene oxide (GO) based nanofiltration membranes is strongly influenced by their interlayer spacing, governed by two competing factors: GO swelling in the liquid phase and penetrant adsorption. In this work, the structure of GO membranes was optimized <i>via</i> H₂O₂ treatment, and their performance was evaluated during nanofiltration of cationic and anionic dyes. For anionic dyes, the permeate flux remained stable, whereas cationic dyes caused a significant and irreversible flux decline. To clarify this behavior, dye adsorption experiments and <i>in situ</i> diffraction analyses of the GO interlayer distance were performed. The highest adsorption capacity was recorded for cationic methylene blue (MnB) at 643 mg g^-1 (GO), compared to 97 mg g^-1 (GO) for anionic methyl orange (MO). In the case of MO, the occupation of the interlayer space by dye molecules was compensated by slight structural expansion, maintaining flux stability. Conversely, the filtration of even a small amount of MnB solution caused a reduction in <i>d</i>-spacing from 12.1 ± 0.1 Å to 11.7 ± 0.1 Å, followed by further shrinkage to 11.4 ± 0.1 Å due to the electrostatic compression of the negatively charged GO with the positively charged dye molecules. Combined with physical blockage by dye molecules, this led to a rapid decline in membrane permeance described well by Poiseuille-based permeance trends. These results show that adsorption, particularly at low penetrant concentrations, can affect measured rejection rates and that adsorption can substantially alter the membrane permeance. Considering the roles of adsorption and electrostatic interactions, charged dyes are unsuitable for permeation tests intended to assess the intrinsic size-exclusion properties of two-dimensional lamellar membranes.