Tailoring the specific crosslinking sites of graphene oxide framework nanosheets for controlled nanofiltration of salts and dyes

Graphene oxide (GO), a material that is proven to have favorable attributes in different applications is continuously being researched and employed in membrane-based separations including nanofiltration, but challenges including instability in an aqueous environment, resulting in redispersion and swelling, as well as the difficulty in controlling the ion transport are still matters for concern. In this research, we addressed these problems by covalently crosslinking GO with different monomers of the same chain lengths, including ethylenediamine (EDA), ethylene glycol (EGL), and oxalic acid (OXA) to tailor specific crosslinking site, in able to subsequently improve the stability of the membranes while tuning the d-spacing for controllable transport of ions and molecules. These membranes produced via pressure-assisted filtration were subjected to physicochemical investigations where it revealed that surface properties including hydrophilicity and charge, as well as the membrane free volume were altered and each played a crucial role in their separation performance. The nature of the crosslinker dictated the site where crosslinking happened (GO's edges or basal planes) and influenced the passage of species, especially the fast water permeation. The membrane crosslinked with OXA has displayed an excellent pure water permeability that could reach up to ∼39.6 L m⁻² h⁻¹ at a pressure of 6 bar, a rejection for Na₂SO₄ of around 93%, and rejection of organic dyes, methylene blue and methyl orange of >99%. Moreover, the membranes were observed to have a stable performance in various operating conditions and exhibited an outstanding antifouling property and chlorine resistance that are essential qualities needed for desalination in realistic conditions.