## 3 Results and Discussion
### HCl Gas Adsorption Test  
Breakthrough curves (\(C_{t}/C_{0}\) vs. time) were drawn to analyze the adsorption of HCl in the upflow fixed-bed column; the data were evaluated using the following equations [28]: Effluent volume:  
\[V_{eff}=Qt_{total} \tag{1}\]  
where \(t_{total}\) represents the total operation time (min) and \(Q\) represents the flow rate (cc/min). The total uptake capacity (\(q_{total}\)) is given by Equation (2) [29]:  
\[q_{total}=\frac{Q}{1000}\int_{t=0}^{t=t_{total}}C_{0}dt \tag{2}\]  
where \(C_{ad}\) is the concentration of HCl gas adsorbed in the AC pellets. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **S\({}_{\text{bet}}\) (m\({}^{2}\)/g)** & **Total Pore Volume (cm\({}^{3}\)/g)** & **Micropore Volume (cm\({}^{3}\)/g)** & **Microporeity (\%)** \\ \hline AC Pellet & 2073 & 1.15 & 0.98 & 85.2 \\
600 W 1 M 1 min & 2106 & 1.38 & 1.23 & 89.1 \\
600 W 1 M 3 min & 2236 & 1.48 & 1.24 & 83.8 \\
600 W 1 M 5 min & 2086 & 1.35 & 1.17 & 86.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Surface properties after microwave treatment of AC pellet samples. Figure 7: A BET of the microwave-processed AC pellet. The total amount of HCl flowing into the column (\(m_{total}\), mg) is defined as follows [30]:  
\[m_{total}=\frac{C_{0}V_{eff}}{1000} \tag{3}\]  
The total HCl removal (\(R_{total}\), %) was calculated as follows [30]:  
\[R_{total}=\frac{q_{total}}{m_{total}}\times 100 \tag{4}\]  
The mass transfer zone (\(MTZ\)), \(Z_{m}\), is defined as follows [31]:  
\[Z_{m}=Z\bigg{(}1-\frac{t_{0.05}}{t_{0.95}}\bigg{)} \tag{5}\]  
where \(Z_{m}\) is the length of the \(MTZ\) (cm), \(Z\) is the bed height (cm), \(t_{0.05}\) is the time at the breakthrough (min), and \(t_{0.95}\) is the time at saturation (min). The column usage of the fixed-bed column adsorption device (\(f\)) is defined as follows [31]:  
\[f=\bigg{(}1-\frac{0.5\times MTZ}{bed\;height}\bigg{)}\times 100 \tag{6}\]  
In this study, we conducted an experiment to evaluate the HCl adsorption characteristics of samples treated with AC pellets and microwaves for 1, 3, and 5 min; the breakthrough curve is shown in Figure 8. Using the formula above, we obtained variables such as the adsorption and removal values, the results of which are shown in Table 7, which indicate that the HCl adsorption characteristics of the AC pellet increased in the following order: \(<\) 5 min \(<\) 3 min \(<\) 1 min. These results are attributed to the effect of the oxygen functional groups on the pellets after the microwave-nitric acid treatment. Depending on the length of the \(MTZ\), the bed utilization factor (\(f\)) is also displayed in Table 7. The 5-min sample demonstrated the highest bed utilization at 83.33%, and the AC pellets showed a similar bed utilization at 81.19%. Consequently, the factors affecting the gas adsorption not only include the oxygen functional groups but also the specific surface area and microporosity. While \(MTZ\), \(q_{total}\), and \(m_{total}\) are influenced by C\({}_{0}\), the consistent experimental conditions, including the weight of the adsorbent and flow rate, allow for meaningful comparisons between the different samples. Figure 8: The effect of microwave treatment on the HCl gas breakthrough curve of pellets.