Mechanism and Application of Biochar as an Electron Shuttle in the Remediation of Soil Pollutants

Biochar has attracted increasing attention in soil remediation due to its porous structure, abundant surface functional groups, and environmental compatibility. Beyond its conventional roles as an adsorbent and soil amendment, biochar has increasingly been recognized as an electron shuttle that mediates electron transfer between electron donors and acceptors, thereby promoting redox reactions involved in pollutant transformation, immobilization, and toxicity mitigation. However, the structural basis, influencing factors, and underlying electron-transfer mechanisms of biochar-mediated processes remain insufficiently integrated. This review summarizes biochar-mediated electron transfer in soil remediation. Three pathways are discussed: direct electron transfer through conductive carbon matrices, indirect electron transfer mediated by redox-active surface functional groups, and composite interfacial electron transfer involving microorganisms, metal oxides, and nanomaterials. By introducing the synergistic effects between biochar and microorganisms, metal oxides, and nanomaterials, the functions of biochar in immobilizing heavy metals and degrading organic pollutants are emphasized. Biochar's electron-shuttling ability is closely related to aromatic carbon structures, redox-active functional groups, and persistent free radicals, while key factors affecting electron-shuttling performance, including feedstock properties, pyrolysis temperature, pH, coexisting ions, pollutant concentration, and modification strategies, are also discussed. This review offers guidance for the design of biochar-based soil remediation strategies.