Synergistic enhancement of adsorption capacity and separability in magnetic rice husk biochar for Methyl Orange remediation

• Fe 3+ modification boosted rice husk biochar's surface area by 187 times to 319.3 m 2 ·g -1 . • The magnetic adsorbent achieved a 51.91 mg·g -1 capacity for Methyl Orange, optimal at pH 3. • Superparamagnetism (2.56 emu·g⁻¹) enables rapid magnetic separation post-adsorption. • Adsorption is monolayer and physisorption-driven, fitting pseudo-first-order and Langmuir models. Methyl Orange (MO) is a widely used organic dye that poses significant environmental risks due to its toxicity, persistence, and difficulty in degradation. To address this issue, this study aimed to develop a low-cost, magnetically separable adsorbent for the efficient removal of MO from aqueous solutions. Magnetic biochar (Fe 3+ -BC) was synthesized via pyrolysis of rice husk impregnated with FeCl 3 ▪6H 2 O. The BC and Fe 3+ -BC were systematically characterized using SEM, BET, XRD, XPS, and VSM, and their adsorption performance toward MO were evaluated under varying conditions. The results showed that Fe 3+ -BC exhibited a dramatically increased specific surface area (from 1.71 to 319.3 m 2 ▪g -1 ) and a saturation magnetization of 2.56 emu▪g -1 , enabling facile magnetic separation. The maximum MO adsorption capacity reached 51.91 mg▪g -1 under optimal conditions (pH=3, adsorbent dosage=40 mg). Adsorption kinetics followed the pseudo-first-order model (R 2 =0.984), and the isotherm data were best described by the Langmuir model (R 2 =0.967), indicating a monolayer physisorption mechanism. Although the adsorption capacity decreased after regeneration cycles, retaining 43.88% after two cycles, the study demonstrates that Fe 3+ -BC is a promising adsorbent for dye wastewater treatment, combining high adsorption performance with easy recovery.