Advanced methylene blue adsorption with a tailored biochar/graphene oxide/magnetite nanocomposite: characterization, optimization, and reusability

This study aimed to develop a novel biochar/graphene oxide/magnetite (BC/GO/Fe₃O₄) nanocomposite for the effective adsorption of methylene blue (MB) from aqueous solutions and real wastewater. The biochar was synthesized from plant materials such as Rumex abyssinicus, Parthenium hysterophorus, and Prosopis juliflora. Comprehensive characterization techniques including proximate analysis, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) surface area analysis, and point of zero charge (pHPZC) were employed. The study utilized a central composite design within a response surface methodology framework to optimize the adsorption process, considering factors such as pH (5, 7, 9), contact time (35, 60, 85 min), adsorbent dose (1.5, 2.25, 3 g/200 mL), and initial MB concentration (25, 45, 65 mg/L). The optimum conditions (pH of 8.99, contact time of 15 min, adsorbent dose of 1.24 g, and initial MB concentration of 64.99 mg/L) achieved MB removal efficiency of 87.978%. The Langmuir isotherm and pseudo-second-order models provided excellent fits (R² = 0.99 and 0.98, respectively). The Langmuir showed favorable adsorption conditions with qmax value of 136.89 mg g⁻¹, and the intraparticle diffusion alone did not control the rate-limiting step. The nanocomposite demonstrated consistent removal efficiencies (98.4%) over six regeneration cycles, indicating its potential for effective wastewater treatment, though further industrial-scale studies are needed.