Ultrafast and reversible adsorption of methylene blue on functionalized graphene for water purification

Carbon-based nanomaterials are increasingly investigated for water purification due to their high surface area, tunable reactivity, and versatile adsorption mechanisms. Among them, graphene and its derivatives have demonstrated superior performance over activated carbon and carbon nanotubes, although their use is often hindered by sheet aggregation and limited dispersibility. In this work, we report the synthesis and characterization of a sulfonated reduced graphene oxide derivative (rGO-S), obtained through a diazotization reaction with sodium sulfanilate. The covalent introduction of benzenesulfonate groups was confirmed by TEM, EDX, XPS, FTIR, and TGA, revealing a homogeneous functionalization and enhanced water dispersibility (0.49 mg/mL). rGO-S exhibited a highly negative zeta potential (−45.3 mV), preventing aggregation and increasing the number of accessible adsorption sites. Using methylene blue (MB) as a model cationic pollutant, adsorption studies demonstrated remarkable performance: equilibrium was reached in less than one minute, with kinetics best described by a pseudo-second-order model and a two-fold increase in adsorption capacity (326 mg/g) compared to pristine rGO. Moreover, rGO-S displayed efficient regeneration through ion-exchange with NaCl solutions, achieving up to 92% desorption in multiple cycles, in combination with ethanol, while maintaining stable adsorption capacity upon reuse. Selectivity tests against a mixture of dyes confirmed the preferential uptake of MB, highlighting the role of electrostatic interactions with sulfonate groups. Overall, the sulfonated graphene derivative combines fast kinetics, good efficiency, selectivity, and reusability, providing promising perspectives for scalable water treatment applications and pollutant recovery within a circular economy framework.