Study on the adsorption effect and mechanism of biochar from forest surface fuels on heavy metals

1, Use forest fuels to make biochar, reducing wildfires and heavy metal pollution. • 2, Broadleaf biochar had superior Pb(II)/Cd(II) adsorption (81 and 39 mg/g). • 3, Electrochemical properties and specific surface area drive adsorption efficiency. Producing biochar from forest fuels represents a sustainable approach to mitigating wildfires, though its potential for addressing heavy metal contamination remains poorly understood. This study employed slow pyrolysis with restricted oxygen to produce biochar from the surface fuels of four typical tree species in North Asia ( Larix gmelinii , Pinus sylvestris var. mongolica , Quercus mongolica , and Fraxinus mandshurica ) at varying temperatures (300–700°C). Adsorption kinetics and isotherms for Cd(II), Cr(VI), Pb(II), and Zn(II) ions were systematically investigated. Characterization was performed using Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). Machine learning (Random Forest RF) and Structural Equation Modeling (SEM) were innovatively applied for in-depth analysis. Results indicate that Broadleaf tree biochar ( Quercus mongolica , Fraxinus mandshurica ) demonstrates superior adsorption for Pb (II) and Cd (II) (81 mg/g and 39 mg/g, respectively), while conifer biochar shows enhanced adsorption for Cr (VI) (11.84 mg/g). Mechanistic analysis indicates that adsorption of these four metal ions primarily follows monolayer adsorption accompanied by chemical interactions, including: chelation by oxygen-containing functional groups, M-π electron interactions, and CO 3 2− precipitation. SHapley Additive exPlanations (SHAP) (based on models with R 2  > 0.94) and Partial Least Squares Structural Equation Modeling (PLS-SEM) revealed that Electrochemical Properties (Ep) and specific surface area (SSA) are key physicochemical properties influencing adsorption efficiency, directly affecting biochar's ability to adsorb heavy metal ions. Collectively, these findings demonstrate that biochar produced from forest surface fuels not only offers a strategic solution for wildfire management, but also effectively remediates heavy metal contamination, thereby contributing to sustainable ecosystem development. Download: Download high-res image (584KB) Download: Download full-size image