Mechanistic insights into selective Pb(II) removal by magnetic sludge-derived biochar: A combined experimental and...

Transforming sewage sludge into functional materials offers a dual approach to waste management and water remediation. In this study, a magnetic biochar (Fe₃O₄-BC-ZnCl₂) was prepared via facile ZnCl₂ activation and in situ magnetization strategy. The resulting composite exhibited a high specific surface area (291.49 m² g^-1) with uniformly dispersed magnetic sites, thereby overcoming the separation limitations of powdered adsorbents. Adsorption experiments showed that Fe₃O₄-BC-ZnCl₂ had a maximum Pb(II) adsorption capacity of 59.96 mg g^-1 and a removal efficiency of up to 96.21%. Its Pb(II) adsorption capacity fell within the range reported for representative sludge-based biochar adsorbents in the literature. Notably, the material maintained good selectivity for Pb(II) in a ternary competitive system (Pb-Cu-Cd). The adsorption process followed monolayer chemisorption, and the material exhibited satisfactory regenerability, retaining more than 68% of its removal efficiency preserved after four cycles. Combined FTIR, XPS, and Density Functional Theory (DFT) analyses revealed a synergistic removal mechanism in which pore filling and electrostatic attraction contributed to the initial uptake, while coordination with Fe-O sites and oxygen-containing functional groups governed the strong chemical binding of Pb(II). These results demonstrate that Fe₃O₄-BC-ZnCl₂ is a recyclable adsorbent for Pb(II) removal and provides a feasible route for the high-value utilization of municipal sewage sludge.