Simplified one-pot Mn/Fe co-modification of walnut branch biochar for enhanced Pb(II) adsorption and reusability

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.