## 3 Results and Discussion
### Effect of Potassium Ferrate (K\({}_{2}\)FeO\({}_{4}\)) Dose on Removal Efficiency  
The optimized K\({}_{2}\)FeO\({}_{4}\) dose for effective dye removal was examined at pH 3, which was determined to be the ideal pH condition in Section 3.1. Figure 3 shows the removal efficiency as a function of dosage at different dye (a: OD and b: LGB) concentrations at pH 3. Overall, the removal efficiencies of both dyes increased with increasing K\({}_{2}\)FeO\({}_{4}\) doses. In particular, the OD and LGB concentrations (initial concentration 25 mg/L) decreased rapidly when the K\({}_{2}\)FeO\({}_{4}\) dose was increased from 28 to 42 mg/L. As the K\({}_{2}\)FeO\({}_{4}\) dose was increased to 56.40 mg/L and 70.50 mg/L, the highest OD removal of 99.68% was achieved in 15 min, and the highest LGB removal of 99.68% was achieved in 30 min. This result was better than that observed in previous studies, in which ozonation coupled with a biological aerated filter removed 100% of the azo dye from industrial wastewater in 120 min [16] and the advanced oxidation process degraded 100% of the reactive azo dye in 40 min [25]. The performance of K\({}_{2}\)FeO\({}_{4}\) was compared with that of NaOCl and FeCl\({}_{3}\)-6H\({}_{2}\)O. As shown in Figure 4a, K\({}_{2}\)FeO\({}_{4}\) exhibited superior removal efficiency at 25 mg/L of OD. Furthermore, as its dose increased to 60 mg/L, complete removal was achieved. In contrast, FeCl\({}_{3}\)\(\cdot\)6H\({}_{2}\)O had the lowest removal efficiency (20%), and its performance was not enhanced by an increase in the dose. The removal efficiency of NaOCl increased up to 80% when the OD dose was greater than 80 mg/L; however, it was still lower than that of K\({}_{2}\)FeO\({}_{4}\). Moreover, its performance was reduced when treated with 50 mg/L of OD (Figure 4b). Additionally, 92% of the OD was removed, even though its dose was increased to over 80 mg/L. In contrast, the removal efficiency of NaOCl was reduced to half, even when the dose was increased to 80 mg/L. FeCl\({}_{3}\)\(\cdot\)6H\({}_{2}\)O showed a marginal increase in the removal of 50 mg/L of OD; however, its removal efficiency was still less than 40%. The superior efficiency of K\({}_{2}\)FeO\({}_{4}\) for dye oxidation was mainly due to the high reactivity and oxidizing capacity of Fe (VI). Based on the oxidizing power, the reduction potential of Fe (VI) (2.20 to 0.70 V) was estimated to be higher than that of ozone (2.076 V), hydrogen peroxide (1.776 V), chlorine (1.358 V), and perchlorate (1.389 V). In addition, ferrate (VI) is quick to oxidize the dye, as compared to permanganate and chromate. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Reaction Rate (fmCJ/H)**} \\ \hline \multicolumn{4}{c}{**Orange II Azo Dye**} & \multicolumn{4}{c}{**Lissamine Green B Dye**} \\ \hline  & **Ferrate** & **NaOCl** & **Ferrate** & **NaOCl** \\ \hline pH 3 & \(-1.1894\) & \(-0.6330\) & \(-1.2110\) & \(-2.4569\) \\ pH 6 & \(-0.9997\) & \(-0.4907\) & \(-0.9899\) & \(-0.7396\) \\ pH 9 & \(-0.9058\) & \(-0.5722\) & \(-0.8723\) & \(-0.9314\) \\ pH 12 & \(-0.8489\) & \(-1.1426\) & \(-1.0671\) & \(-0.9036\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the removal rates of K\({}_{2}\)FeO\({}_{4}\) vs. NaOCl, and FeCl\({}_{3}\)-H\({}_{2}\)O for OD and LGB azo dyes. Figure 3: Effect of potassium ferrate dose on the removal efficiency of (**a**) orange II azo dye (**b**) and lissamine green B dye. Furthermore, oxidation by K\({}_{2}\)FeO\({}_{4}\) generated subsequent coagulation through its reduction. It synergistically helped in the removal of organic dyes. However, FeCl\({}_{3}\cdot\)6H\({}_{2}\)O had a single coagulation effect, resulting in poor removal efficiency. Similar to OD removal, the combination of oxidation and coagulation exhibited the highest removal efficiency (100%) for LGB. At 25 mg/L, LGB was almost completely removed by K\({}_{2}\)FeO\({}_{4}\) (99.97%) (Figure 5a).