https://doi.org/10.1016/j.ccst.2022.100059
“The N-functional groups of biochar are closely related to its performance in CO2 capture, as discussed in Section 4.2.2. Biochar is usually capable of capturing CO2 by physical uptake, and incorporation of amine species reduces the surface area and pore volume of biochar and leads to a reduction in CO2 capture capacity (Chen et al., 2014). In some cases, the CO2 capture capacity from the introduced amine species does not compensate for the CO2 capture capacity through the loss of physical uptake due to the reduction in active surface area. On the other hand, the introduction of amines in biochar changes the adsorption mechanism from physisorption to chemisorption or combined physisorption (depending on the amine loading), which can enhance the selectivity for CO2 at relatively high temperatures. Solid sorbents ornamented with basic amine groups have been shown to have a greater CO2 adsorption capability in previous investigations (Shafawi et al., 2021). Surface treatment with amine reagents can attach particular functional groups to the solid surface, such as alkyl-amines, to improve the CO2 adsorption capability. Urea is frequently used to introduce nitrogen into biochar materials, because urea is a nontoxic and inexpensive amine group (Qiao and Wu, 2022; Rouzitalab et al., 2018; Qiao et al., 2020).Qiao et al. (Qiao et al., 2020). prepare biochar by a one-step method of modifying AD digestate by urea and obtained a high CO2 adsorption capacity (1.22 mmol/g).
For instance, Bamdad et al. (Bamdad et al., 2018) employed methods for amine functionalization of biochar, i.e. nitration and condensation of aminopropyl triethoxysilane. They found the nitrogen-doped biochar exhibited a CO2 adsorption capacity of 165.04 mg/g, which was more than double that of the original biochar (79.22 mg/g). In addition, the cycle performance of the modified biochar was rather stable, with the CO2 adsorption capacity decreasing by around 20% after ten cycles. Riya (Chatterjee et al., 2018) described a novel biochar activation way that combines physicochemical activation for CO2 capture. The amine-modified biochar that had been ultrasound-treated was employed for CO2 adsorption. Raw biochar has a relatively poor adsorption capacity of 0.3 mmol/g but following physical modification with ultrasound and chemical activation with amine, its capacity was improved to 2.79 mmol/g at 70 °C. The modified biochar retained 56% of the initial adsorption capacity after 15 cycles, and amine functionalization of biochar was accomplished at 35 °C with almost no temperature rise. Biochar was first treated with ultrasonic irradiation for 30 s, then chemical amination at ambient temperature. From Fig. 8, it was discovered that ultrasound exfoliated charcoal graphene clusters, cleansed, and opened clogged micropores, and enhanced surface area. CO2 adsorption was enhanced by the formation of covalent bonds between the nucleophilic active sites on the modified biochar surface and the electrophilic CO2 molecules. However, excessive concentrations of N functional groups, on the other hand, may block the micropore entrance and lower the surface area (Zhang et al., 2014). Therefore, it can be concluded that the combination of physical and chemical activation can lead to biochar adsorbents with more micropores and better adsorption capacity.”