Cationic and anionic azo-dye removal from water by sulfonated graphene oxide nanosheets in Nafion membranes

Graphene oxide flakes functionalized with 3-amino-1-propanesulfonic acid (denoted as GO(SULF)) as a powder or incorporated into an ionomer membrane such as Nafion (DuPont) were studied for water purification applications. The adsorption and the photocatalytic activity of the GO(SULF) powder itself or confined as a nano-additive in the membrane (Nafion-GO(SULF)) were investigated by measuring the degradation of a cationic dye, methylene blue (MB), in the dark and under UV/Visible light illumination. The results were compared with the ability of these systems to degrade an anionic dye, methyl orange (MO), in order to evaluate the role of the polymer-dye interaction. The degradation of the azo dyes depends on the mutual interaction between GO(SULF) flakes, the polymeric matrix and the dye: Nafion-GO(SULF) strongly reduces MB both under dark and illumination conditions in the same way as the GO(SULF) powder, while, for MO degradation, the composite membrane is more efficient than GO(SULF) alone. Finally, the possibility of reusing the same photocatalytic material several times, which is the main advantage of embedding active nanomaterials in a polymer matrix, is demonstrated by the effective regeneration of the nanocomposite membranes.