## 4 Discussion  
### Model Validation  
Figure 2 shows the experimental results obtained by increasing the inlet concentration of H\({}_{2}\)S from 500 to 2000 ppm, while maintaining the NH\({}_{3}\) concentration at 500 ppm. At 500 ppm of H\({}_{2}\)S, the outlet concentration was 0 ppm because all of the introduced H\({}_{2}\)S \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Symbol** & **Value** & **Unit** \\ \hline Gas-phase flow rate & Q\({}_{\rm G}\) & 2 1  & dm\({}^{3}\) min\({}^{-1}\) \\ Reactor volume & V\({}_{\rm L}\) & 2 1  & dm\({}^{3}\) \\ Bubble volume & V\({}_{\rm G}\) & 0.08 2  & dm\({}^{3}\) \\ Mass transfer & K\({}_{\rm L}\)a\({}_{\rm S}\) & 0.4 2  & min\({}^{-1}\) \\ coefficient of H\({}_{2}\)S & K\({}_{\rm L}\)a\({}_{\rm N}\) & 0.0007 2  & min\({}^{-1}\) \\ Zero-order kinetic & k\({}_{\rm S}\) & 1.0 2  & mg dm\({}^{-3}\) min\({}^{-1}\) \\ constant of NH\({}_{3}\) & k\({}_{\rm N}\) & 0.3 2  & mg dm\({}^{-3}\) min\({}^{-1}\) \\ Retardation constant & K\({}_{\rm I}\) & 0.001 2  & mg dm\({}^{-3}\) \\ Henry’s law constant & H\({}_{\rm S}\) & 0.4 3  & dimensionless \\ Henry’s law constant & H\({}_{\rm N}\) & 0.00695 3  & dimensionless \\ of NH\({}_{3}\) & & \multicolumn{1}{c}{} & \\ \end{tabular}
\end{table} 
Table 1: Model parameters used in this study. was absorbed and removed in 60 min. However, when the concentration was increased to 1000 ppm, H\({}_{2}\)S was not completely eliminated, with 200 ppm being detected at the outlet. When 2000 ppm H\({}_{2}\)S was introduced, the outlet concentration increased to approximately 1000 ppm. At a H\({}_{2}\)S concentration of 500 ppm, the elimination capacity was approximately 42 g m\({}^{-3}\) h\({}^{-1}\), resulting in a complete absorption and removal. For a H\({}_{2}\)S concentration of 1000 ppm, the removal efficiency was 80%, corresponding to an elimination capacity of 42 g m\({}^{-3}\) h\({}^{-1}\). Similarly, the removal efficiency was 60% at a 2000 ppm inlet concentration, corresponding to a removal rate of 36 g m\({}^{-3}\) h\({}^{-1}\). Combined, the removal rate of H\({}_{2}\)S at a current density of 40 mA cm\({}^{-2}\) was approximately 40 g m\({}^{-3}\) h\({}^{-1}\). The outlet concentration of NH\({}_{3}\) was less than 5 ppm, regardless of the inlet H\({}_{2}\)S concentration. The removal rate of NH\({}_{3}\) was higher than that of H\({}_{2}\)S due to its water solubility. In particular, the removal rate of NH\({}_{3}\) remained unaffected by changes in the H\({}_{2}\)S concentration, indicating that the design factors for the EWS must be derived by focusing on the H\({}_{2}\)S loading rate. As shown in Figure 2c, the concentration of aqueous H\({}_{2}\)S is 0 mg-S dm\({}^{-3}\) for 500 ppm introduced, indicating the complete oxidization of H\({}_{2}\)S. However, the H\({}_{2}\)S absorbed in the liquid phase was not completely oxidized with increasing the inlet concentration. This indicates that a current density of 40 mA cm\({}^{-2}\) was insufficient in oxidizing H\({}_{2}\)S at a concentration \(\geq\) 1000 ppm. A concentration of aqueous NH\({}_{3}\) exhibited similar values as observed for H\({}_{2}\)S inlet at 1000 and 2000 ppm, while NH\({}_{3}\) decreased at a concentration of 500 ppm H\({}_{2}\)S. In other words, some of the aqueous NH\({}_{3}\) was oxidized when the aqueous H\({}_{2}\)S concentration was 0 mg S/L. However, no aqueous NH\({}_{3}\) was removed if aqueous H\({}_{2}\)S was present in the solution. In the electrochemical treatment of the solution with coexisting H\({}_{2}\)S and NH\({}_{3}\), H\({}_{2}\)S was dominantly oxidized by the RCS. After complete elimination of H\({}_{2}\)S, the oxidation of NH\({}_{3}\) was initiated by residual RCS. Therefore, the removal of both absorbed H\({}_{2}\)S and NH\({}_{3}\) through oxidation requires a higher current density.