Advances in Biochar-Based Adsorbents and Performance Assessment for the Removal of Gaseous Pollutants

This review critically examines physical and chemical strategies used to enhance the adsorption of CO2, volatile organic compounds (VOCs), SO2 and H2S by biochar-based sorbents. The literature indicates that adsorption performance is governed by the combined effects of feedstock properties, thermochemical conversion conditions, pore structure, surface area and surface chemistry. Physical activation, particularly with steam or CO2, primarily promotes pore development and increases accessible surface area, whereas chemical treatments with KOH, H3PO4, H2SO4, amine-based compounds, and biopolymers can additionally modify surface functionality and gas–surface interactions. Across the reviewed studies, KOH activation and nitrogen functionalization are frequently associated with enhanced CO2 uptake, while acidic surface modification can improve interactions with selected polar VOCs. For sulfur-containing gases, pore development together with appropriate surface functional groups can facilitate both physical adsorption and surface reactions. Rather than identifying a universally optimal modification route, the available evidence shows that sorbent design should be tailored to the physicochemical properties of the target gas. The review also examines biomass residues as precursors and key technical factors affecting the scalability of biochar-based gas purification systems. Overall, the literature highlights biochar’s potential for gas adsorption and the need for evaluation of modification strategies and validation under realistic conditions.