Abstract
Magnetic biochar is widely used for wastewater treatment, but conventional modification routes often suffer from progressive pore blockage and severe particle aggregation. Emerging Fe/Mn co-modification offers a promising solution, where Fe provides magnetic separability while Mn optimizes surface morphology and enriches oxygen-containing groups for enhanced metal binding. To overcome these structural trade-offs in conventional synthesis, this study developed a simplified one-pot Mn/Fe co-modification route to synthesize a magnetically separable walnut-branch biochar composite (SO-BC). Compared to two-step and single-metal modifications, SO-BC exhibited an optimized mesoporous network (pore volume increased by 327.6% vs. pristine biochar) and an enriched presence of oxygen-containing functional groups as verified by FTIR. Notably, SO-BC possessed superparamagnetic-like behavior with a high saturation magnetization of 48.2 emu·g−1, ensuring facile recovery from solutions. Kinetic and isotherm data confirmed that Pb(II) adsorption on SO-BC was a monolayer chemisorption process. The removal mechanism was primarily attributed to surface complexation with oxygen/metal-oxide sites, as well as pore filling and electrostatic interactions. Ultimately, SO-BC reached a maximum adsorption capacity (Qm) of 157.7 mg·g−1. These results suggest that the integrated Mn/Fe co-modification route effectively capitalizes on bimetallic synergy while preserving biochar porosity, offering a streamlined approach for valorizing biomass waste into robust heavy metal adsorbents.