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

Abstract

Graphene oxide (GO) based nanofiltration membrane performance are strongly influenced by the interlayer spacing, governed by two competing factors: GO swelling in liquid phases and penetrant adsorption. In this work, the structure of GO membranes was optimized via 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 in situ 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 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 d-spacing from 12.1±0.1 Å to 11.7±0.1 Å, followed by further shrinkage to 11.4±0.1Å due to electrostatic compression negatively charged GO with positively charged dyes. Combined with physical blockage by dye molecules, this led to a rapid decline in membrane permeance describing well by Poiseuille-based permeance trends. These results show that adsorption, particularly at low penetrant concentrations, can distort measured rejection and that adsorption can substantially alter membrane permeance. Considering the roles of adsorption and electrostatic interactions, charged dyes are unsuitable for permeation tests intended to assess intrinsic size-exclusion properties of two-dimensional lamellar membranes.