Effect of thermal treatment on the structure and CO2 adsorption capacity of biochar from chicken manure and onion peels

This study investigates the thermal conversion of chicken manure and onion peels into biochars with tailored properties for CO 2 adsorption. The process was conducted at 700, 800 and 900 °C under inert atmosphere. The resulting biochars were characterised using Fourier-Transform Infrared Spectroscopy, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, nitrogen sorption analysis, and CO 2 adsorption measurements. Increasing pyrolysis temperature enhanced carbonisation, reduced heteroatom content, and promoted pore development, particularly in onion peels-derived in biochars. Nitrogen sorption analysis revealed the development of mesoporous and microporous structures, with the biochar derived from onion peels at 900 °C exhibiting the highest specific surface area (109.8 m 2 /g). However, the CO 2 adsorption capacity showed divergent trends: it correlated with surface area development in chicken manure biochars, whereas onion-peel-derived biochars showed a non-monotonic relationship, with the 700 °C sample having the highest adsorption capacity (26.6 cm 3 /g at 100 kPa). In contrast, chicken manure biochars exhibited increased CO 2 adsorption capacity with increasing pyrolysis temperature, which was associated with the development of micropores and the presence of calcium-rich mineral phases, identified by XRD and XRF analyses. Raman and XPS analyses indicated progressive structural ordering and deoxygenation with increasing pyrolysis temperature. The results demonstrate that CO 2 adsorption in biochars derived from onion peels is mainly governed by pore-filling mechanisms, while mineral-assisted interactions significantly contribute to CO 2 capture in chicken manure biochars.