Magnetic waste-derived Fe₃O₄/activated carbon–red mud composite for enhanced adsorption and photocatalytic removal of methyl orange

Abstract

This study presents the development of a multifunctional magnetite-AC-RM composite for efficient methyl orange (MO) removal through combined adsorption and photocatalytic processes. The composite was prepared at a 2:2:1 wt ratio (AC:RM:Fe₃O₄) via co-precipitation. XRD and FTIR analyses confirmed the coexistence of Fe–O, Al–O, and Si–O functional groups, while BET results showed a surface area of 558.58 m² g⁻¹, higher than AC-RM (442.780 m²/g), indicating partial pore reopening after Fe₃O₄ incorporation. SEM–EDS analysis revealed a uniform nanoparticle distribution, and VSM confirmed strong ferromagnetic behavior, allowing rapid magnetic recovery. The magnetite-AC-RM composite exhibited superior adsorption performance with over 90% MO removal within 50 min at pH 2. The adsorption equilibrium followed the Langmuir isotherm, suggesting monolayer adsorption on a homogeneous surface, while kinetic data fitted well to the pseudo-second-order model (R² > 0.99), indicating chemisorption as the dominant mechanism. Thermodynamic parameters (ΔG° = −3954 to −1499 J/mol, ΔH° = −41.15 kJ/mol, ΔS° = −122.75 J/mol K) confirmed that the process was spontaneous and exothermic. Overall, the synergistic integration of activated carbon, red mud, and Fe₃O₄ produced a highly reactive, magnetically recoverable, and thermodynamically favorable adsorbent. The magnetite-AC-RM composite offers a low-cost, efficient, and sustainable solution for removing anionic dyes such as methyl orange from aqueous solution.

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