A decade of advances in agricultural waste-derived biochar for direct air CO2 capture: Activation, modification, and performance trends

Global warming caused by the increasing concentration of CO2 in the atmosphere is a critical global issue. Although reducing CO2 emissions or capturing CO2 from stationary sources has been extensively studied and implemented, removal of CO2 from dispersed or mobile sources remains challenging. Consequently, direct air capture (DAC) using various adsorption methods has gained significant attention in recent years due to advantages such as low environmental impact and facile adsorbent regeneration. Numerous adsorbents—including zeolites, metal-organic frameworks, clays, and activated carbons—have been studied. Biochar adsorbents produced from agricultural wastes offer several advantages, such as low cost, ease of synthesis, and adsorption capacities comparable to other adsorbents. This review focuses on biochar adsorbents derived from different agricultural wastes over the past decade. It discusses the effects of key factors like activation and modification strategies, pore size distribution, and pyrolysis temperature on CO2 adsorption performance. Reported adsorption capacities range from 7.3 mmol/g for biochar produced from yellow mombin at pH 2.8 to 0.11 mmol/g for biochar from Nauclea diderrichii at pH 8.0. Biomass pH, pyrolysis temperature, and activating/modifying agents significantly influence biochar's adsorption capacity. Additionally, economic and environmental assessments indicate that biochar production costs are considerably lower than those of conventional adsorbents, and the associated environmental impact is negligible relative to the amount of CO₂ captured.