Single- and multi-metal engineered, functionalized hybrid biochars for heavy metal adsorption: synthesis, structure-function relationships, and coordination mechanisms

Heavy metals pose a serious threat to global water systems due to their toxicity, persistence, and strong bioaccumulative potential in ecosystems and humans. Among various remediation strategies, adsorption is regarded one of the most effective and versatile approaches. Biochar (BC) has gained significant attention as a sustainable and low-cost adsorbent owing to its simple preparation, tunable porosity, high surface area, and intrinsic functional groups. However, pristine BC often exhibits limited adsorption efficiency toward diverse metal ions, necessitating targeted engineering and surface modification. To address this, two modification approaches, namely metal-based nano-architected BC (NA-BC) and surface-functionalized BC (f-BC), have emerged as leading platforms. NA-BCs are synthesized from either single metals (e.g., oxides, hydroxides, and metal-organic frameworks) or multi-metal systems (e.g., layered double hydroxides, spinel oxides), with magnesium, manganese, calcium, aluminum, zinc, and iron commonly used as precursors for their low toxicity, cost-effectiveness, and chemical versatility. Meanwhile, surface functionalization introduces hydroxyl, amino, sulfonic, thiol, carboxylic, and phosphate groups via inorganic or organic modifications, yielding f-BCs and f-NA-BCs with tunable porosity, enriched surface functionalities, and enhanced stability. These features collectively improve adsorption capacity, selectivity, and reusability. The heavy metal removal by BC composites involves physisorption and chemisorption, including pore diffusion, electrostatic attraction, chelation/coordination, ion exchange, precipitation, and redox reactions, which can be validated through XRD, FTIR, XPS, and DFT analyses. Looking ahead, advancing scalability, environmental safety, and machine-learning-guided material design will be crucial for developing next-generation engineered BCs capable of practical implementation in fixed-bed filters in disaster-affected zones and industrial wastewater treatment.