# Sewage Sludge Derived Materials for CO2 Adsorption
## 3 Results and Discussion
Nitrogen (N\({}_{2}\)) adsorption-desorption isotherms (Figure 4) revealed a small, specific surface area for the sewage sludge, about 2.97 m\({}^{2}\)/g, which increased to 15.61 m\({}^{2}\)/g after the pyrolysis process (Table 4), and the same tendency was observed for the pore volume but not for the pores diameter, which slightly decreased for all mixtures (Table 4). Figure 3: SEM microphotographs of dried sewage sludge (**A**), SSPYCHAR (**B**), I_1 (C), I_2 (D), I_3 (E) and I_4 (F). The highest surface area and pore volume were obtained for I_4, while the pore's diameter remained almost constant, and only in the case of I_3 the diameter slightly decreased. In comparison with other studies [40; 44; 45; 46], the surface area of I_4 significantly increased and the pore diameter slightly increased [46]. Regarding the shape of isotherms and hysteresis loops (Figures 4-6), these were Type II with H3 hysteresis according to IUPAC [46; 47]. FTIR analysis (Figure 7) was used to identify the typical functional groups of the raw sewage sludge (SS), its char (SSP\({}_{\text{VCHAR}}\)) and activated chars (group I). The peaks at 2918 cm\({}^{-1}\) and 2851 cm\({}^{-1}\) were assigned to CH\({}_{2}\) asymmetrical and symmetrical stretching vibration [48], indicating the presence of aliphatic structures in the SS sample. It can be observed that these bands disappeared after pyrolysis, leaving only an imperceptible trace, highlighting total decomposition of aliphatic C-H bonds [49]. The peaks at 1625 cm\({}^{-1}\) from SS and 1572 cm\({}^{-1}\) from SSP\({}_{\text{VCHAR}}\) correspond to C=O vibration, indicating the existence of aldehydes and acids from proteins in the raw sewage sludge [25]. The peaks at 3246 cm\({}^{-1}\), 1400 cm\({}^{-1}\), 1010 cm\({}^{-1}\) and 529 cm\({}^{-1}\) revealed the surface carbon structure, including chain hydrocarbons and functional groups such as hydroxyl and aromatic rings [50; 51]. The peak at 3246 cm\({}^{-1}\) was attributed to O-H stretching vibration, related to water, alcohols or carboxylic acids. The peak from 1400 cm\({}^{-1}\) corresponded to C-H stretching vibration of CH\({}_{2}\) and CH\({}_{3}\) groups. The band from 1010 cm\({}^{-1}\) was attributed \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material Type** & **S\({}_{\text{BET}}\)** & **V** & **d\({}_{\text{P}}\)** \\  & **m\({}^{2}\)/g** & **cm\({}^{3}\)/g** & **Å** \\ \hline SS * & 2.972 & 0.030 & 41.666 \\ SSP\({}_{\text{VCHAR}}\) & 15.608 & 0.042 & 39.250 \\ I\_1 & 3.305 & 0.008 & 39.256 \\ I\_2 & 6.447 & 0.017 & 39.252 \\ I\_3 & 3.618 & 0.014 & 31.308 \\ I\_4 & 921.013 & 0.657 & 39.306 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Textural properties of the materials based on sewage sludge (SS). Figure 4: Adsorption isotherms and pore distribution for sewage sludge (SS) and SSP\({}_{\text{VCHAR}}\). to C-O and Si-O bonds, indicating the presence of quartz and stable C-O bond [25; 49]. The peak from 529 cm\({}^{-1}\) corresponded to C-H vibration of alkenes, which almost disappeared in the \(\mathrm{SS}_{\mathrm{PYCHAR}}\). Figure 5: Adsorption isotherms and pore distribution of I_1, I_2 and I_3 samples. Figure 6: Adsorption isotherms and pore distribution of I_4 sample. CH\({}_{2}\) and CH\({}_{3}\) groups. The intensity of these peaks for the activated chars highlighted the increased distribution rate of the OH and CH\({}_{2}\) structure. The peaks from 540-570 cm\({}^{-1}\) (group I), attributed to C-H vibration of alkenes, almost disappeared in the samples I.4, indicating that these samples did not contain primary alkenes. The material treated with HCl was used for CO\({}_{2}\) adsorption test, due to its high specific surface area. The results are presented in Table 5. Additionally, the adsorption capacity, separation efficiency and CO\({}_{2}\) recovery were calculated [52,53]. Prior to adsorption test, the gas mixtures were confirmed through GC analysis.