Modification of biomass-derived biochar: A practical approach towards development of sustainable CO₂ adsorbent

The persistent increase in the atmospheric concentration of carbon dioxide (CO₂), the primary anthropogenic greenhouse gas contributing to global warming, makes research directed towards carbon capture and storage (CCS) imperative. In the past few years, among the available adsorbents, biochar has drawn significant interest as a promising carbon-based material for low-temperature CO₂ capture from flue/fuel gas (such as biogas or gasification-derived syngas) owing to its environmentally friendly nature, cost-effective and facile preparation method, and sustainable adsorption performance. This work provides a review of recent studies on the development of biochar from biomass feedstocks and its subsequent modification through various approaches, including physical, chemical and physicochemical activations for post-combustion CO₂ capture. An overview of the factors, including pyrolysis temperature, heating rate and time, and different modification methods, affecting the physicochemical attributes of biochar such as surface area, microporosity, surface properties and functional groups is presented. Biochar with a large micropore volume, a narrow microporosity (0.3–0.8 nm) and basic surface characteristics would be effective in adsorbing CO₂ molecules. In this regard, physical modification of biochar is closely related to pore formation, whereas chemical modification emphasizes the creation of oxygen and nitrogen-containing functional groups; hence, they contribute to the enhanced CO₂ capture through porosity development and surface chemistry alteration, respectively. Biochar has presented a strong selectivity towards CO₂ compared to other gasses and has revealed a sustainable performance in multi-cycles of CO₂ adsorption–desorption; these are crucial features to ensure the large-scale application of biochar for CO₂ capture.