Electrochemically Exfoliated Graphene/Iron Oxide and Graphene/Iron Oxide/Bismuth Oxybromide Composites for Methylene Blue Adsorption: Synthesis, Characterization, Kinetic, and Isotherm Studies

Abstract

Methylene blue (MB) is a cationic dye whose removal from contaminated aqueous media is important for wastewater treatment. This study evaluated graphene/iron oxide (EG/Fe₃O₄) and graphene/iron oxide/bismuth oxybromide (EG/Fe₂O₃/BiOBr) composites for MB adsorption. The binary composite was prepared by electrochemical exfoliation; BiOBr was synthesised hydrothermally, and the ternary composite was subsequently assembled through hydrothermal treatment. The materials were characterised by scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. XRD indicated Fe₃O₄ as the dominant iron oxide phase in EG/Fe₃O₄ and the presence of Fe₂O₃ and BiOBr in the ternary composite. Batch adsorption experiments examined contact time, initial MB concentration, and adsorbent dose. Equilibrium was reached after approximately 60 min in the contact-time experiments. The pseudo-second-order model provided the better kinetic fit for both composites. Equilibrium data were best described by the Langmuir model for EG/Fe₃O₄ and by the Freundlich model for EG/Fe₂O₃/BiOBr. The maximum adsorption capacities were 21.41 mg g⁻¹ and 27.10 mg g⁻¹, respectively. Maximum removal efficiencies of 99.8% for EG/Fe₃O₄ and 99.6% for EG/Fe₂O₃/BiOBr were obtained in the adsorbent-dose experiments. These findings indicate that both composites are effective MB adsorbents under the tested batch conditions, with differences in removal efficiency and adsorption capacity.