## 3 Results and Discussion
### Characterization of the Activated Carbons  
The textural properties of the synthesized activated carbons were studied through N\({}_{2}\) adsorption-desorption analysis. The isotherms and pore size distributions of these materials are shown in Figure 1a,b. The textural parameters, specific surface area (S\({}_{\text{BET}}\)), external surface area (S\({}_{\text{ext}}\)), total pore volume (V\({}_{\text{Total}}\)), and micropore volume (V\({}_{\text{Micro}}\)) are reported in Table 2.  
According to the International Union of Pure and Applied Chemistry (IUPAC) classification, all the samples exhibited type IV isotherms, characteristic of mainly mesoporous materials. At medium pressure, due to capillary condensation, the desorption branch did not match with the adsorption isotherm, causing a hysteresis loop, which demonstrates the presence of wider pores (meso- and macropores) in the carbon structure [30]. In addition, the hysteresis loops presented by the activated carbons showed two parallel and \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & & & \multicolumn{6}{c}{**Elemental Analysis (\%)**} & \\ C & O & H & N & P & S & Cl \\
47.89 & 6.84 & & 6.62 & 3.38 & 1.18 & 0.49 & 1.13 \\ \hline \multicolumn{10}{c}{**Metal Content (\%)**} \\ Na & Mg & Al & Si & K & Ca & Ti & Mn & Fe & Cu & Zn & Ni \\
0.15 & 0.02 & 0.38 & 0.17 & 0.49 & 7.25 & 0.02 & 0.01 & 0.19 & 0.01 & 0.02 & 0.03 \\ \hline \multicolumn{10}{c}{**Total solids**} \\  & & **(g/L)** & & & & & & & & & \\ \hline \multicolumn{10}{c}{**Volatile solids**} \\  & & **(g/L)** & & & & & & & & & \\ \hline \multicolumn{10}{c}{**Fixed solids**} \\  & & **(g/L)** & & & & & & & & & \\ \hline \multicolumn{10}{c}{**Chemical oxygen demand (COD)**} \\  & & **(g O\({}_{2}\)/L)** & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characterization results of the sludge. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Adsorbent**} & **S\({}_{\text{BET}}\)** & **S\({}_{\text{ext}}\)** & **V\({}_{\text{Total}}\)** & **V\({}_{\text{Micro}}\)** & **V\({}_{\text{Micro}}\)/V\({}_{\text{Total}}\)** \\  & **(m\({}^{2}\)/g)** & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** & **(cm\({}^{2}\)/g)** & **(cm\({}^{2}\)/g)** & **V\({}_{\text{Micro}}\)/V\({}_{\text{Total}}\)** \\ \hline
**AC-ZnCl\({}_{2}\)** & 558 & 145 & 0.35 & 0.15 & 0.43 \\
**AC-FeCl\({}_{3}\)** & 468 & 170 & 0.56 & 0.16 & 0.29 \\
**AC-Fe(NO\({}_{3}\))\({}_{3}\)** & 240 & 158 & 0.32 & 0.04 & 0.13 \\
**AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)** & 233 & 203 & 0.48 & 0.01 & 0.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Textural properties of the carbon materials. almost horizontal branches (except for the case of AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)), which can be classified as H4-type hysteresis loops, according to the IUPAC classification. Sludge is a heterogeneous waste and, therefore, generates activated carbons with heterogeneous textural properties--that is, with meso-microporous character. This phenomenon was previously observed in activated carbons synthesized from biomass [31; 32]. In the case of AC-ZnCl\({}_{2}\), it can be observed that part of the pore filling occurs at low pressure, since it could be considered a mainly microporous activated carbon. In contrast, in the case of AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) carbon, a significant contribution of macroporosity was observed, showing capillary condensation at high pressure values. The pore size distributions of the materials were evaluated by the Barrett-Joyner-Halenda (BJH) method. It can be seen that all the samples showed a pore distribution mainly centered in the micro-mesoporosity range, except for the AC-Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) sample, which showed wider pores (\(\sim\)50 nm). Table 2 shows that the AC-ZnCl\({}_{2}\) carbon showed the highest specific surface area.