Synergistic Impact of Pyrolysis Temperature and Fe₃O₄ Modification on Citrus Aurantium Biochar for Congo Red Removal

This research investigates the synergistic impact of pyrolysis temperature and magnetic modification on the adsorption performance of Citrus aurantium-derived biochar for Congo Red (CR) removal. Biochars were produced via KOH activation at 600°C and 700°C in both powdered and granular forms. The 700°C powdered biochar was further functionalized with Fe₃O₄ nanoparticles using a co-precipitation method to synthesize a magnetic composite (MBCP700). Characterization through SEM, XRD, and EDX confirmed a heterogeneous porous structure and successful crystallization of phase-pure magnetite with a high iron loading of 36.44%. FTIR analysis revealed unique nitrogen-containing functionalities (C≡N) and specific hydroxyl environments in Citrus aurantium that differentiate it from other citrus species.Adsorption experiments demonstrated that powdered biochars achieved significantly higher removal efficiency than granular variants. While biochar activated at 600 °C exhibited faster initial kinetics due to an oxygen-rich surface, the 700 °C variant provided a higher ultimate capacity. The magnetic composite (MBCP700) reached dynamic equilibrium within 30–60 minutes, with the pseudo-second-order model and Langmuir isotherm providing the best fit, suggesting monolayer chemisorption. Thermodynamic analysis indicated a spontaneous and endothermic process (ΔH° = 15.9 kJ·mol⁻¹). Central Composite Design (CCD) optimization identified the ideal operating conditions as a 50 min contact time, 0.75 gL-1 MBCP700 dosage, and 60 mgL-1 initial CR concentration. Under these optimum conditions, the material achieved a predicted adsorption capacity of 53.56 mgg-1 and a maximum removal efficiency of 92.00%. These results highlight MBCP700 as a highly efficient and magnetically recoverable adsorbent for textile wastewater treatment.