Engineering Multifunctional Biochars for Integrated Environmental Systems: Multi-Medium Performance, Challenges, and Research Priorities

The valorization of agricultural and other waste residues into biochar represents a promising strategy for sustainable waste management and environmental remediation within a circular economy framework. Engineering multifunctional biochars like agricultural waste-derived biochars (AWDBs) exhibit tunable physicochemical properties governed by feedstock characteristics and thermochemical conversion conditions, enabling their application across water, soil, and sediment systems. While extensive research has demonstrated the effectiveness of biochar in isolated environmental compartments, natural systems function as interconnected water–soil–sediment continua, where pollutants, nutrients, and organic matter dynamically interact. This review critically synthesizes recent advances in the production, properties, and environmental applications of biochars, with a particular focus on their multifunctional performance in coupled environmental systems. Mechanistic insights into contaminant sequestration, nutrient cycling, and microbial interactions across media are discussed, alongside evidence of synergistic and antagonistic effects arising from cross-media processes. Despite significant progress, major knowledge gaps persist, including limited integrated multi-medium studies, lack of standardized assessment methodologies, insufficient understanding of long-term biochar stability, and challenges associated with field-scale implementation. Future research directions are proposed to address these limitations through standardized protocols, engineered multifunctional biochars, long-term monitoring, and policy integration. Advancing a system-based perspective is essential to unlock the full potential of agricultural waste-derived biochars for sustainable and scalable environmental remediation.