Adsorptive removal of lead (Pb2+) ions using carbon–based magnetic nanocomposite

Lead (Pb ²⁺ ) contamination in aquatic environments poses a serious risk to human health and ecosystems, necessitating the development of efficient, sustainable, and regenerable adsorbent materials. In this study, a carbon-based magnetic nanocomposite (AC/MnFe ₂ O ₄ ) derived from orange peel waste was synthesized and systematically evaluated for Pb ²⁺ removal from aqueous solutions. The structural, morphological, elemental, and magnetic properties of the synthesized materials were characterized using SEM, EDX, XRD, and VSM, confirming successful immobilization of MnFe ₂ O ₄ nanoparticles onto the porous activated carbon matrix and sufficient magnetic separability. The effects of key operational parameters, including solution pH (2–10), initial Pb ²⁺ concentration, adsorbent dosage, temperature (25–60°C), and contact time, were systematically investigated. Adsorption kinetics were analyzed using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models. Equilibrium data were evaluated using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) isotherm models. The Langmuir model exhibited excellent agreement with experimental data, yielding a high maximum adsorption capacity of 129.87 mg/g and a favorable separation factor (R _L =0.023). The low mean adsorption energy obtained from the D–R model (E = 3.79 kJ/mol) suggests that Pb ²⁺ adsorption is predominantly governed by physisorption mechanisms. Thermodynamic analysis revealed negative Gibbs free energy values (ΔG°=−6.72 to − 0.45 kJ/mol) at lower temperatures, confirming the spontaneous nature of Pb ²⁺ adsorption, while the negative enthalpy change (ΔH°=−60.75 kJ/mol) indicates an exothermic process. Reusability studies demonstrated that the AC/MnFe ₂ O ₄ nanocomposite retained over 66% of its initial adsorption efficiency after nine adsorption–desorption cycles. The combination of high adsorption capacity, favorable kinetics, magnetic recoverability, and good regeneration performance highlights the AC/MnFe ₂ O ₄ nanocomposite as a promising and sustainable adsorbent for Pb ²⁺ remediation in water treatment applications.