## 3 Results and Discussion
### Porous Carbons Characterization  
#### 3.1.1 N\({}_{2}\) Adsorption-Desorption Isotherms and Pore Size Distribution  
The textural parameters obtained from the N\({}_{2}\) adsorption-desorption isotherms are shown in Table 2.  
As expected, the biochar sample presented an extremely low specific surface area (<5 m\({}^{2}\) g\({}^{-1}\)) caused by the blocking of tarry substances produced in the pyrolysis process [31; 32]. The strategy of producing first the biochar and then impregnating it with the activation agent has the proven advantage of improving the surface area and pore size distribution in the resulting activated carbon with less consumption of the chemical agent, since KOH diffuses more easily into the char matrix to react with the carbon atoms [33; 34]. Although the two-stage approach increases the energetic consumption of the process, it is balanced with the higher surface areas of the resulting carbons, as well as a fine development of the porosity. Thus, the textural properties of the activated biochar were significantly improved after KOH activation. The mechanism of KOH \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Name** & **S\({}_{\text{BET}}\) (m\({}^{2}\) g\({}^{-1}\))** & **V\({}_{\text{T}}\) (cm\({}^{3}\) g\({}^{-1}\))** & **V\({}_{\text{Mic}}\) (cm\({}^{3}\) g\({}^{-1}\))** & **V\({}_{\text{Mes}}\) (cm\({}^{3}\) g\({}^{-1}\))** \\ \hline Biochar & \textless{}5.0 & - & - & - \\ CC 1  & 1030 & 0.56 & 0.30 & 0.26 \\ ACK700\_1\_1:1.5 & 1063 & 0.51 & 0.25 & 0.26 \\ ACK800\_1\_1:1.5 & 1542 & 0.76 & 0.33 & 0.43 \\ ACK800\_1\_1:1 & 1214 & 0.45 & 0.37 & 0.08 \\ ACK800\_1\_1.2 & 1624 & 0.74 & 0.40 & 0.34 \\ ACK800\_2\_1:2 & 678 & 0.35 & n.d. & n.d. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Textural parameters of carbons obtained from the N\({}_{2}\) adsorption–desorption isotherms. activation for porosity formation was already described: a redox reaction catalyzed by alkali metal occurs, where the carbon atoms are oxidized to CO or CO\({}_{2}\); then, the reduced alkali metal is intercalated between the graphene layers, expanding the carbon structure [35; 36]. The produced carbons presented surface areas higher than the commercial carbon, which constitutes a very interesting result. However, sample ACK800\(2\)1:2 presented a much lower surface area (678 m\({}^{2}\) g\({}^{-1}\)), indicating that 2 h of activation was excessive and promoted some collapse of the pores. As presented in Table 2, the activation temperature has a significant impact on the surface area, allowing the development of more micropores and mesopores at 800 \({}^{\circ}\)C. Additionally, the KOH impregnation ratio has an impact on the textural properties, since the higher Biochar:KOH mass ratio (1:2) provided an activated carbon with the highest surface area. According to these results, the sample impregnated with Biochar:KOH in a 1:2 ratio and activated at 800 \({}^{\circ}\)C for 1 h was selected for further characterization and application in the adsorption assays, given its advanced porosity and higher surface area. From this point onwards, this sample will simply be called ACK. A comparison between ACK carbon and CC commercial carbon was investigated. Figure S2 (Supplementary Materials) gives the N\({}_{2}\) adsorption isotherms of the ACK and CC samples. ACK carbon presents a type I (b) isotherm, while CC carbon shows mixed type I and IV isotherms [29]. The higher volume of adsorbed N\({}_{2}\) of the ACK sample agrees with the higher surface area of this carbon. The type I (b) isotherm is associated with microporous materials presenting micropore size distributions over a broad range, including wider micropores and narrow mesopores (<2.5 nm), whereas the type IV isotherm indicates both micro- and mesoporous structures, according to the IUPAC classification [37]. The larger hysteresis loop of CC isotherm indicates larger mesopores in this sample. Figure 1 displays the PSD of ACK and CC carbons.