New insights into CO₂ sorption on biochar/Fe oxyhydroxide composites: Kinetics, mechanisms, and in situ characterization

Despite its importance, chemical process has been often overlooked in CO₂ sorption on carbon based oxyhydroxide composites. In this study, pristine and ball-milled biochar/Fe oxyhydroxide composites were fabricated for CO₂ sorption at 25 °C. The composites, particularly the ones with high Fe content, were effective sorbents for CO₂ with the capacities of up to 160 mg g⁻¹. The primary mechanism of CO₂ sorption on biochar composites with low Fe content was physical adsorption. When the Fe content increased, biochar/Fe oxyhydroxide composites showed enhanced CO₂ sorption capacities, but the sorption kinetics became slower. This is because the governing CO₂ sorption mechanism was shifted from physical adsorption to chemical reaction between Fe oxyhydroxides and CO₂. The formed (oxy)hydroxycarbonate could be decomposed at a temperature between 50 and 125 °C. Furthermore, ball milling could speed up CO₂ mineralization rate on the composites, especially for those with high Fe content, to favor the relative significance of chemical sorption. Both physical and chemical CO₂ sorption mechanisms were verified by different characterization methods including in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Findings of this study not only demonstrate the importance of chemical sorption, but also provide new insights on CO₂ capture by low-cost and environmentally benign biochar/Fe oxyhydroxide composites. Besides, the low regeneration temperature of chemically-sorbed CO₂ gives biochar/Fe oxyhydroxide composite a competitive edge over other CO₂ sorbents, which often need a high regeneration temperature or are not regenerable.