Sulfuric acid-induced magnetic chitosan/graphene oxide composite hydrogel beads for pH-dependent adsorption of anionic and cationic dyes: Mechanisms, optimization, and reusability

This study reports the synthesis of magnetic chitosan/graphene oxide hydrogel beads (Cht-H⁺/GO/Fe₃O₄) via a facile coprecipitation method using sulfuric acid as a protonation-inducing medium that promotes electrostatic stabilization within the chitosan matrix. The composite composition was systematically optimized, with the best performance achieved at a Cht:GO:Fe₃O₄ mass ratio of 50:10:40. Adsorption performance was evaluated as a function of pH, initial dye concentration (20–100 mg L⁻¹), adsorbent dosage (0.02 g), contact time (60 min for Metanil Yellow (MY) and 80 min for Methylene Blue (MB)), temperature (25 ± 2 °C), agitation speed (150 rpm), and solution volume (30 mL). The results revealed strong pH-dependent behavior, with optimal removal of MY at pH 4.0 and MB at pH 10.0, consistent with amphoteric surface properties (zeta potentials of +9.59 mV and − 26.04 mV, respectively). Adsorption is governed by a combination of electrostatic attraction, hydrogen bonding, and π–π interactions involving chitosan functional groups and graphene oxide domains. Adsorption equilibrium was reached at approximately 60 min for MY and 80 min for MB. Kinetic and isotherm analyses showed that adsorption process is well described by the pseudo-second-order model and Langmuir/Sips isotherms, with maximum adsorption capacities of 134.16 mg g⁻¹ for MY and 104.81 mg g⁻¹ for MB. In binary dye systems at pH 7.0, reduced adsorption efficiencies were observed due to competition for active sites, demonstrating pH-dependent selectivity with preferential MY uptake. The composite retained more than 75% of its initial adsorption capacity after four adsorption–desorption cycles, and its magnetic recoverability, reusability, and efficient performance against both anionic and cationic dyes demonstrate its practical potential for treating complex dye-contaminated wastewater.