# Sewage Sludge Derived Materials for CO2 Adsorption
## 3 Results and Discussion  
Table 2 presents the elemental composition from sewage sludge to the obtained materials. Thus, a similar C content for the sewage sludge and for the obtained char treated with KOH and HCl can be observed. High C content was also noticed in the sewage sludge char (29.46 wt.%), which slightly decreased, with about 3 wt.%, after the oxidation process. Additionally, the C content decreased for the materials treated with KOH. The highest O content was observed in the sewage sludge, about 17 wt.%, which drastically decreased for all obtained materials. The N content slightly decreased. Sulfur element was observed only in sewage sludge and its char. The H content decreased five times for char and oxidized char and about two times for the obtained materials. Table 3 reveals the evolution of metals content. Enrichment in metals for the char can be observed, with an exception being noticed for Fe. Comparing the \(\text{SS}_{\text{PYCHAR}}\) with the I_1, I_2, I_3 and I_4, a notable increase for K and Fe can be seen. Additionally, a slight increase was observed for Na in the case of materials treated with KOH, while Cr, Mn, Ni, Cu, Zn, Pb and Si decreased. The scanning electron microscopy (SEM) revealed a homogenous surface for the sewage sludge material (Figure 3A) and a porous surface for the char (Figure 3B). The pyrolysis process was conducted to the formation of different pore sizes, due to the carbonization of the organic matter. Analysing the micrographs of the obtained mixtures (Figure 3C-E), irregular shapes and erosion surfaces can be observed that probably formed after the KOH treatment [42]. The erosion degree increased with the KOH content. Additionally, the HCl contributed to the mixture erosion (Figure 3E). The surface of I_2, Figure 3D, is similar to a \"broccoli bouquet\", with porous convolutions, being strongly physically modified under the influence of alkaline hydroxide. The formation of pores on the surface of materials can be associated with the evaporation of some chemicals that were activated during the thermal process [43], developing a \"cav-errous\" surface. Additionally, the KOH reacts with the mineral, forming CO\({}_{2}\) and H\({}_{2}\)O, among others. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Element (wt\%)** & **SS** & \(\text{SS}_{\text{PYCHAR}}\) & **I\_1** & **I\_2** & **I\_3** & **I\_4** \\ \hline C & 34.51 & 29.46 & 10.85 & 8.87 & 6.02 & 31.93 \\ O & 17.11 & 0.38 & 0.37 & 0.41 & 0.44 & 0.5 \\ N & 6.26 & 4.13 & 1.67 & 1.28 & 0.66 & 0.65 \\ S & 0.86 & 0.47 & 0 & 0 & 0 & 0 \\ H & 5.01 & 1.17 & 2.07 & 2.76 & 2.95 & 2.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The elemental composition. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{8}{c}{**Metal (mg/g)**} \\ \cline{2-11}  & **Cr** & **Fe** & **Mn** & **Ni** & **Cu** & **Zn** & **K** & **Na** & **Pb** & **Si** \\ \hline SS & 0.19 & 28.95 & 0.14 & 0.02 & 0.36 & 0.11 & - & - & 0.02 & 144.75 \\ \(\text{SS}_{\text{PYCHAR}}\) & 0.45 & 0.09 & 0.26 & 0.23 & 1.64 & 1.52 & 4.70 & 2.65 & 0.06 & 323.32 \\ I\_1 & \textless{}0.01 & 150.95 & 0.19 & \textless{}0.01 & 0.66 & 0.36 & 197.93 & 9.5 & \textless{}0.02 & 147.80 \\ I\_2 & \textless{}0.01 & 9.69 & 0  
The special inorganic compound in the sewage sludge, SiO\({}_{2}\), acts as a built-in template, which prevents agglomeration, resulting in the formation of porous structure. From Figure 3C-E, some sheets can be can be observed, representing a mixture of carbon, Cu, Zn, Fe and Si. In comparison with the sewage sludge or SSPYCHAR, the I_4 (treated with HCl) displayed hierarchical structure with pores, Figure 3F. The sheets were destroyed by the addition of HCl, which led to the transformation of the material surface by increasing the porosity.