Engineered Biochar for Gaseous Air Pollutant Control: Modification Strategies, Synergistic Adsorption Mechanisms, and Comparative Evaluation with Conventional Adsorbents

Biochar is a carbon-rich material derived from the pyrolysis of biomass. It is drawing more attention for air pollutant control because of its low cost, abundant feedstocks, and environmental sustainability. This review examines the application of engineered biochar to control gaseous air pollutants. Meanwhile, the preparation and modification methods for engineered biochar, including pyrolysis, activation, acid–base treatments, amine functionalization, and metal loading, are also systematically summarized. These approaches are used for tailoring pore structural features and the distribution of surface functional groups. Moreover, the review clarifies the synergistic mechanisms of physisorption–chemisorption for CO2, SO2, NOx, and VOCs, emphasizing the cooperative regulation resulting from nitrogen-containing functional groups and metal sites. For example, nitrogen-modified biochar increased SO2 adsorption capacity from 78.9 mg/g for pristine biochar to 135 mg/g under mixed gas conditions, while Na2CO3-activated cattail-derived biochar achieved an n-hexane dynamic adsorption capacity of 1.00 g/g. Subsequently, this study comprehensively compared engineered biochar with conventional adsorbents, including activated carbon, zeolites, carbon molecular sieves, and metal–organic frameworks, considering gas-phase pollutant removal efficiency, removal cost, and operational difficulty of supporting equipment. Although biochar is slightly inferior to high-end commercial activated carbon in terms of adsorption performance, it can be modified to achieve a level comparable to that of activated carbon. As the raw materials for biochar are derived from waste, the cost is virtually zero, and the production cost is approximately 50–80% lower than that of commercial activated carbon. These results demonstrate the potential of engineered biochar as a low-cost, sustainable alternative for controlling industrial gaseous pollutants.