Renewable biochar derived from mixed sewage sludge and pine sawdust for carbon dioxide capture

Carbon dioxide (CO₂) is the main anthropogenic greenhouse gas contributing to global warming. In this study, a series of KOH-modified biochars derived from feedstock mixtures (i.e., S3W7 biomass consisting of 70% pine sawdust and 30% sewage sludge; S5W5 biomass consisting of 50% pine sawdust and 50% sewage sludge) at different temperature (i.e., 600-800 °C) were prepared for evaluating CO₂ adsorption performance. The KOH-activated biochars prepared with S3W7 biomass displayed larger surface areas and micropore volumes compared to those of S5W5 biochars. In particular, the highest CO₂ adsorption capacity (177.1 mg/g) was observed on S3W7 biomass at 700 °C (S3W7-700K), due to the largest surface area (2623 m²/g) and the highest micropore volume (0.68 cm³/g). Furthermore, surface functional groups, hydrophobicity, and aromaticity of biochar and presence of hetero atoms (N) also were actively involved in CO₂ adsorption of biochar. In addition, in situ DRIFTS analysis advanced current understanding for the chemical sorption mechanisms by identifying the transformation composites of CO₂ on biochars, and characterizing the weakly adsorbed and newly formed mineral species (e.g., carbonates) during the CO₂ sorption process. This study may provide an insight into the research of CO₂ capture by identifying physical and chemical adsorption, and expand the effective utilization of natural biomass-based biochar for mitigation greenhouse gas emission.