Alkali-modified biochar for CO2 capture

Alkali activation is normally performed by soaking and stirring biomass or biochar in an alkaline solution for several hours. Alkaline metals and alkaline-earthy metals may help to produce base sites with a high affinity for acid CO2, then enhance the acid-base chemistry between CO2 and biochar. As a result, alkaline metals and alkaline earth metals may improve CO2 adsorption ability (Xu et al., 2016).

The activation of biochar using potassium hydroxide or sodium hydroxide increases the oxygen content and surface basicity of the biochar (Fan et al., 2010Li et al., 2014). Compared to KOH, NaOH, as a more economical option, is less corrosive to the biochar, resulting in smaller micro-pores and specific surface area of the resulting biochar. Kaya (Kaya and Uzun, 2021) et al. impregnated the pyrolyzed pine cone carbon using a concentrated KOH solution and obtained biochar with a BET surface area of 1714.5 m2/g, which showed a significant increase in surface area compared to the unactivated biochar (259.63 m2/g) and a maximum adsorption capacity of 3.64 mmol/g for CO2.This may be attributed to the fact that when activated with KOH, the carbon and the KOH solution and the formation of new functional groups, such as K2CO3 (Basta et al., 2009), which promoted the generation of pores. Moreover, the author mentioned that the pine cone char activated with KOH had a larger surface area and a higher adsorption capacity than the char activated with HCl under the same conditions. The higher BET surface area and ultra-micropore and super-micropore volume of KOH-activated biochars were larger than those of CO2 and steam-activated biochars, resulting in a considerable improvement in CO2 adsorption capacity. (Shahkarami et al., 2015). Similar phenomenon has been observed by Igalavithana et al. (Igalavithana et al., 2020) that the KOH activation accelerated the development of micropores and thus the significant increase of CO2 adsorption capacity. To enhance basicity, the surface area of biochar could also be increased by pre- and, or post-treatment, where KOH is the most commonly used activation agent (Serafin et al., 2021).”

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