Pyrolysis of heavy metal-enriched plant residues: stabilization of Pb/Zn/Cd and MnFe2O4-modified biochar for Congo red adsorption.

Phytoremediation of heavy metal-contaminated soils generates large quantities of metal-enriched plant residues that require safe disposal. Herein, we report an integrated valorization strategy for Sedum alfredii Hance residues: pyrolysis for heavy metal stabilization, followed by magnetic modification on the resulting biochar for Congo red (CR) adsorption. Pyrolysis was conducted at 400-900 °C, and the transformation behavior of Pb, Zn, and Cd was evaluated. The results demonstrated that pyrolysis effectively converted the acid-soluble/exchangeable fractions of Pb, Zn, and Cd into more stable residual forms, and their leaching concentrations decreased significantly with increasing pyrolysis temperature. Biochar produced at ≥ 700 °C exhibited no phytotoxicity in wheat seed germination tests. To enhance its adsorptive performance, the biochar pyrolyzed at 700 °C was magnetically modified with MnFe2O4. The modification improved the pore structure and surface function groups, facilitating CR adsorption. The resulting MnFe2O4-SBC biochar possessed a specific surface area of 91.68 m2/g and a maximum CR adsorption capacity of 199.89 mg/g, following the Langmuir isotherm and pseudo-second-order kinetic models. After five adsorption-desorption cycles, MnFe2O4-SBC retained high CR adsorption capacity, with minimal metal leaching. Mechanistic analysis revealed that adsorption was governed by electrostatic attraction, ion exchange, hydrogen bonding, π-π interactions, and surface complexation. Collectively, this study demonstrates that pyrolysis is a viable route for the safe disposal and value-added utilization of hyperaccumulator residues, turning a disposal burden into a functional biochar material.