Papaya peel valorization for the green synthesis of magnetic Fe₃O₄ nanoparticles for Pb(II) removal from aqueous solutions and mining wastewater

Lead (Pb) contamination in aquatic environments poses serious environmental and public health risks due to its persistence, toxicity, and bioaccumulation. In this study, a sustainable magnetic nanoadsorbent, Fe₃O₄ functionalized with papaya peel extract (Fe₃O₄@PPE), was synthesized via a green route using papaya peel extract as a natural reducing and stabilizing agent for Pb(II) removal from aqueous solutions. FTIR, UV–Vis, and SEM analyses confirmed the formation of phytochemically functionalized magnetite. Batch adsorption experiments showed that Pb(II) removal was strongly influenced by pH, adsorbent dosage, contact time, temperature, and initial metal concentration. Under optimal conditions (pH 7, adsorbent dosage 0.1 g, contact time 45 min, and 25 °C), Fe₃O₄@PPE achieved Pb(II) removal efficiencies close to 98%. The point of zero charge was approximately 5.5, indicating favorable adsorption at near-neutral pH. Equilibrium data were best fitted by the Langmuir model (R² = 0.9874), with a maximum adsorption capacity of 18.87 mg g⁻¹, suggesting monolayer adsorption on a finite number of active sites. Kinetic results were best described by the pseudo-second-order model (R² = 0.9921), indicating that adsorption was mainly governed by specific surface interactions. Thermodynamic analysis confirmed that the process was spontaneous and exothermic. The applicability of Fe₃O₄@PPE was further assessed in Pb(II)-fortified real mining water, where removal efficiencies of 94.64%, 73.06%, and 66.87% were obtained at initial concentrations of 10, 20, and 30 mg L⁻¹, respectively. These results demonstrate that Fe₃O₄@PPE is a promising low-cost and eco-friendly adsorbent for Pb(II) remediation in complex aqueous matrices. • Green synthesis of Fe₃O₄ magnetic nanoparticles using papaya peel extract. • Sustainable nanoadsorbent developed for efficient Pb(II) removal from water. • Maximum adsorption capacity of 18.87 mg g⁻¹ following the Langmuir model. • Pseudo-second-order kinetics indicates chemisorption-controlled adsorption. • Efficient Pb(II) removal demonstrated in real mining wastewater samples.