Ultrafast and reversible adsorption of methylene blue on covalently sulfonated reduced graphene oxide

Carbon-based nanomaterials are widely studied for water purification owing to their high surface area and tunable surface chemistry. Among them, graphene derivatives often outperform activated carbon and carbon nanotubes, though their application is limited by aggregation and poor dispersibility. Here, we report the synthesis of sulfonated reduced graphene oxide (rGO-S) via diazotization with sodium sulfanilate. Covalent grafting of benzenesulfonate groups was confirmed by TEM, EDX, XPS, FTIR, and TGA, showing homogeneous functionalization and improved dispersibility (0.49 mg/mL). The highly negative zeta potential (−45.3 mV) prevents aggregation and enhances accessible adsorption sites. The covalently sulfonated graphene exhibited ultrafast adsorption kinetics for methylene blue, reaching equilibrium within less than one minute, which is considerably faster than previously reported graphene-based adsorbents. Kinetics followed a pseudo-second-order model, with an adsorption capacity of 326 mg/g − twice that of pristine rGO. The material also demonstrated efficient regeneration through ion exchange with NaCl (combined with ethanol), achieving up to 92% desorption over multiple cycles while maintaining stable performance. Selectivity tests in dye mixtures confirmed preferential methylene blue uptake driven by electrostatic interactions with sulfonate groups. Overall, rGO-S combines ultrafast kinetics, high efficiency, selectivity, and reusability, highlighting its potential for water treatment and pollutant recovery applications.