Insights into CO₂ adsorption on KOH-activated biochars derived from the mixed sewage sludge and pine sawdust

The environment issues associated with global warming and climate change caused by continuous increase in greenhouse gas emissions have attracted worldwide concerns. As renewable resources with good adsorption property, biochar is an efficient and environmental friendly adsorbsent for CO₂ capture. In this study, the CO₂ adsorption behavior of biochars derived from feedstock mixtures of 70% pine sawdust and 30% sewage sludge by KOH modification was investigated. The textual properties and functional groups of the pristine biochars have been significantly enhanced after KOH activation. With highly developed microporosity, the specific surface area (SSA) of the KOH-modified biochars increased by 3.9-14.5 times. Furthermore, higher CO₂ adsorption capacities of 136.7-182.0 mg/g were observed for the modified biochars, compared to pristine ones (35.5-42.9 mg/g). The development of micropores by KOH activation significantly increased the CO₂ adsorption capacity. Meanwhile, the presence of hetero atoms (O and K) also positively influenced CO₂ adsorption capacity of biochar. Noticeably, both physical and chemical adsorption played a crucial role in CO₂ capture, which was verified by different characterization methods including high resolution scanning electron microscope, X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The Findings of this study demonstrate the -significance of chemical sorption by identifying the transformation of CO₂ by biochar composites and in situ characterization of weakly adsorbed and newly formed mineral species during the CO₂ sorption process. Moreover, BC700K showed 97% recyclability during 10 consecutive adsorption-desorption cycles at 25 °C, 1 bar. The results obtained in the present study may inspire new research interest and provide a comprehensive insight into the research subject to biochars derived from feedstock mixtures for CO₂ capture.