## 3 Results and Discussion
### Characterization of the Activated Carbons
The order according to development of the specific surface area for the different synthesized carbons was as follows: AC-ZnCl\({}_{2}\) > AC-FeCl\({}_{3}\) > AC-Fe(NO\({}_{3}\))\({}_{3}\) > AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\). Regarding the pore volume, AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) carbon showed a practically negligible micropore volume, whereas with the AC-ZnCl\({}_{2}\) sample, the opposite was observed, i.e., the micropore volume corresponded to practically half of the total pore volume. This behavior was corroborated by the N\({}_{2}\) adsorption-desorption isotherms, as discussed before. The FT-IR spectra of the activated carbons are depicted in Figure 2a. A broad band located at \(\sim\)3400 cm\({}^{-1}\) can be attributed to O-H stretching vibration due to the presence of water in the carbon samples. The band close to 2900 cm\({}^{-1}\), very strong for the AC-ZnCl\({}_{2}\) and AC-FeCl\({}_{3}\) samples, is associated with stretching vibration of aliphatic C-H bonds, whereas the peak at 2331 cm\({}^{-1}\), observed only in the AC-Fe(NO\({}_{3}\))\({}_{3}\) and AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) spectra, indicates the presence of ketone functional groups. The band close to 1560 cm\({}^{-1}\) corresponds to C=C stretching vibration of the aromatic rings [33]. Thus, the bands associated with alcohols, phenols, esters, acids, and ethers were observed at a wavelength of 1100 cm\({}^{-1}\). Finally, the bands attributed to out-of-plane bending vibrations of O-H and C-H were observed between 500 and 750 cm\({}^{-1}\)[34]. Figure 1: N\({}_{2}\) adsorption–desorption isotherms (**a**) and pore size distributions (**b**) of the activated carbon (AC) materials. Figure 2b shows the thermogravimetric (TG) analysis results of the synthesized activated carbons, which evaluated the thermal stability of the solids. In all the samples, a weight loss was observed at temperatures around 100 \({}^{\circ}\)C, which is attributed to the evaporation of the adsorbed water. The weight loss observed between 200 and 400 \({}^{\circ}\)C may be due to the decomposition of volatile organic and inorganic compounds, and that in the range between 400 and 600 \({}^{\circ}\)C may be attributed to the decomposition of high-molecular-weight compounds. At temperatures around 600 \({}^{\circ}\)C, a more abrupt weight loss was observed for activated carbons synthesized with Fe(NO\({}_{3}\))\({}_{3}\) and Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\), whereas this drop was more stable for activated carbons synthesized from impregnating agents with chloride, i.e., ZnCl\({}_{2}\) and FeCl\({}_{3}\). Therefore, the weight loss observed at 600 \({}^{\circ}\)C can be mainly attributed to the thermal decomposition of the used activating agent. The elemental analysis results of the samples and the corresponding isoelectric point (pH\({}_{\text{PE}}\)) values are collected in Table 3. Generally, activated carbons are mainly composed of fixed carbon and inorganic compounds. As can be seen in Table 3, the fixed carbon contents (C, wt.%) were 55.60, 59.77, 33.51, and 31.71% for AC-ZnCl\({}_{2}\), AC-FeCl\({}_{3}\), AC-Fe(NO\({}_{3}\))\({}_{3}\), and AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\), respectively--values significantly lower than those found in activated carbons obtained from fossil fuels [25]. Even when the material used as a precursor has a high organic matter content, at a high carbonization temperature, the organic matter decomposes; this causes a decrease in the fixed carbon percentage, transforming it into ash. Therefore, the higher the carbonization temperature, the lower the percentage of fixed carbon found in the final material, indicating greater carbonization of organic matter. For this reason, the carbonization temperature of sludge in industry usually varies between 400 and 600 \({}^{\circ}\)C, since in this temperature range, the product obtained is often suitable for use as an alternative fuel due to its high organic content [35].