## 3 Results and Discussion  
### Characterization of the Char-Based Materials  
Table 1 summarizes the effect of Na\({}_{2}\)CO\({}_{3}\) activation and citric acid modification at different dosages on the key properties of the char materials, including surface area, microporosity, and CO\({}_{2}\) adsorption capacity at 100 kPa. The untreated char displays a low specific surface, no microporosity, and null CO\({}_{2}\) adsorption, while the char submitted to activation with Na\({}_{2}\)CO\({}_{3}\), without citric acid modification, shows the highest specific surface area (S\({}_{\text{BET}}\) = 417.6 m\({}^{2}\) g\({}^{-1}\)) and a significant micropore contribution (S\({}_{\text{MP}}\) = 247.2 m\({}^{2}\) g\({}^{-1}\), V\({}_{\text{MP}}\)/V\({}_{\text{T}}\) = 28.9%). This material also led to the highest CO\({}_{2}\) adsorption capacity (q\({}_{\text{CO2}}\) = 73.7 mg g\({}^{-1}\)), suggesting that Na\({}_{2}\)CO\({}_{3}\) activation alone significantly enhances the char's adsorption capacity. As reported in the activation of other plastics such as polyacrylamide, the thermal decomposition of Na\({}_{2}\)CO\({}_{3}\) can develop porosity in the structure [50]. However, as observed in the N\({}_{2}\) isotherms illustrated in Figure 2A, a type IV isotherm was observed according to the IUPAC guidelines [51], with an important rise at low pressures, which suggests that the material compromises a mixed contribution of mesopores and micropores. The char modified with CA 1 M led to a minimal rise in the textural properties, S\({}_{\text{BET}}\) = 19.4 m\({}^{2}\) g\({}^{-1}\), and V\({}_{\text{T}}\) = 0.074 cm\({}^{3}\) g\({}^{-1}\), probably due to the reaction of labile carbon \begin{table}
\begin{tabular}{l c c c c c} \hline \multirow{2}{*}{**Sample**} & **S\({}_{\text{MP}}\)** & **V\({}_{\text{T}}\)** & **V\({}_{\text{MP}}\)** & **V\({}_{\text{MP}}\)/V\({}_{\text{T}}\)** & **q\({}_{\text{CO2}}\) at 100 kPa** \\  & **(m\({}^{2}\) g\({}^{-1}\))** & **(m\({}^{2}\) g\({}^{-1}\))** & **(cm\({}^{3}\) g\({}^{-1}\))** & **(cm\({}^{3}\) g\({}^{-1}\))** & **(\%)** & **(mg g\({}^{-1}\))** \\ \hline char & 2.6 & 0.0 & 0.009 & 0.000 & 0.0 & 16.8 \\ char-CA-1.0 & 19.4 & 0.0 & 0.074 & 0.000 & 0.0 & 22.1 \\ AC & 417.6 & 247.2 & 0.436 & 0.126 & 28.9 & 73.8 \\ AC-CA-0.5 & 324.9 & 175.5 & 0.371 & 0.093 & 25.1 & 57.5 \\ AC-CA-1.0 & 341.0 & 141.6 & 0.424 & 0.072 & 16.9 & 55.0 \\ AC-CA-1.5 & 356.4 & 186.0 & 0.392 & 0.098 & 25.0 & 59.3 \\ \hline \end{tabular}
\end{table} 
Table 1: Textural properties after Na\({}_{2}\)CO\({}_{3}\) activation and citric acid modification by N\({}_{2}\) adsorption isotherms at 77 K and CO\({}_{2}\) adsorption at 273 K.  
Figure 2: N\({}_{2}\) adsorption–desorption isotherms at 77 K (**A**) and micropore size distribution (**B**) by the HK method obtained from the CO\({}_{2}\) adsorption isotherm at 273 K of the activated carbon after citric acid modification. during the acid treatment. However, microporosity was completely absent both in the char before and after CA modification. The CA treatment of the activated char with Na\({}_{2}\)CO\({}_{3}\) produced a decrease in the total surface area and pore volume in all cases. The surface interactions of CA with the original AC display the occlusion of micropores and partial reaction with the most reactive surface groups, indicating lower micropore contribution with lower V\({}_{\rm MP}\)/V\({}_{\rm T}\) ratios. Similar findings have been reported during the CA modification of commercial activated carbon formulas, in which 34% of the surface area was registered [52]. From all the CA-modified samples, the AC-CA-1.0 outlines the highest mesoporous reached after CA treatment, and consequently the lowest CO\({}_{2}\) uptake due to the preferred microporosity contribution required by the adsorption of this molecule.