## 3 Results
### Pore Properties of Spent GAC and Its Regenerated Products  
The pore properties, such as the BET surface area, total pore volume, micropore surface area, and micropore volume of the spent GAC and regenerated products, are summarized in Table 2. The nitrogen adsorption-desorption isotherms (at \(-\)196 \({}^{\circ}\)C) and the corresponding mesopore size distributions of the regenerated samples are presented in Figures 2 and 3.  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Property a** & **Value** \\ \hline Moisture (wt\%) & 9.99 \(\pm\) 0.50 \\ Ash (wt\%) & 13.58 \(\pm\) 0.66 \\ Volatile matter (wt\%) & 13.34 \(\pm\) 0.95 \\ Fixed carbon b (wt\%) & 73.08 \\ \hline \hline \end{tabular} a
[FOOTNOTE:a]Footnote a: footnotetext: Mean \(\pm\) standard deviation for three determinations of an as-received spent GAC sample. b  
\end{table}
Table 1: Proximate analysis of spent GAC (denoted as KN).  
Figure 3: Pore size distributions of some regenerated GAC products.  
Figure 2: N\({}_{2}\) adsorption–desorption isotherms of some regenerated GAC products.  
The key findings of the results presented in Table 2 and Figures 2 and 3 are summarized in the following points:  
1. The regenerated samples exhibited significantly improved pore properties compared to the spent GAC (e.g., BET surface area of 425.17 m\({}^{2}\)/g and total pore volume of 0.184 cm\({}^{3}\)/g). The variations in pore property trends were not consistent with pyrolysis temperature and holding time. Interestingly, the maximum pore properties (BET surface area of 605.38 m\({}^{2}\)/g and total pore volume of 0.272 cm\({}^{3}\)/g) were obtained under the minimal process conditions (800 \({}^{\circ}\)C and 0 min holding time).
2. The CO\({}_{2}\)-activated products generally displayed even higher pore properties than both the spent GAC and the pyrolysis-regenerated products except in the cases of KN-800-00 and KNC-800-00. It can be attributed to further activation by the interaction of the CO\({}_{2}\) gas with carbon (AC or char) due to the following chemical reaction [1]: \[\text{C}_{\text{(solid)}}+\text{CO}_{\text{2(gas)}}\to 2\text{CO}_{\text{(gas)}}\] Above 800 \({}^{\circ}\)C, CO\({}_{2}\) gas reacted with the carbon matrix, thus opening the pores, which resulted in higher pore properties. The maximal pore properties (BET surface area 723.23 m\({}^{2}\)/g, and total pore volume 0.327 cm\({}^{3}\)/g) of the CO\({}_{2}\)-activated GAC products were produced at 900 \({}^{\circ}\)C and a 30 min holding time. These results show that, under optimized pyrolysis-activation conditions, approximately 70% of the pore properties can be achieved, making these regenerated GACs highly promising for water treatment applications.
3. As shown in Figure 2, the nitrogen adsorption-desorption isotherms of spent GAC (KN) and the regenerated products (KN-900-30 and KNC-900-30) displayed both Type I (major) and Type VI (minor) patterns, typically indicating the presence of both micropores and mesopores [29]. Type I curves exhibited steep uptake at low  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Spent GAC (KN) and** & \(\text{S}_{\text{BET}}\)\({}^{\text{b}}\) & \(\text{S}_{\text{micro}}\)\({}^{\text{c}}\) & \(\text{V}_{\text{t}}\)\({}^{\text{d}}\) & \(\text{V}_{\text{micro}}\)\({}^{\text{c}}\) \\
**Regenerated GAC Product \({}^{\text{a}}\)** & **(m\({}^{2}\)/g)** & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** & **(cm\({}^{3}\)/g)** \\ \hline KN & 425.17 & 324.04 & 0.184 & 0.129 \\ KN-800-00 & 605.38 & 385.99 & 0.272 &  
relative pressures, corresponding to high micropore adsorption potential. On the other hand, the Type VI isotherms featured slight hysteresis loops, starting around a relative pressure of approximately 0.30. Figure 3 further confirms the presence of mesopores with the recorded peak at approximately 4.0 nm, while an upward trend below 2.0 nm on the left of the curve indicates a microporous structure. For more precise micropore distribution analysis, models such as Dubinin-Astakhov (DA) and Horvath-Kawazoe (HK) are recommended [29; 30].