## 3 Results and Discussion
### Effect of Potassium Ferrate (K\({}_{2}\)FeO\({}_{4}\)) Dose on Removal Efficiency
The removal efficiency of NaOCl was comparable to that of K\({}_{2}\)FeO\({}_{4}\); that is, it reached a removal efficiency of 96.21% at an LGB dose of 88.81 mg/L. A marginal decrease in removal efficiency was observed for K\({}_{2}\)FeO\({}_{4}\) and NaOCl when the LGB concentration was increased to 50 mg/L, corresponding to 93.26%  
Figure 4: Comparison of potassium ferrite with NaOCl and FeCl\({}_{3}\cdot\)6H\({}_{2}\)O with respect to the removal efficiency of **(a)** 25 mg/L and **(b)** 50 mg/L of orange II azo dye at pH 3.  
and 88.57%, respectively. Overall, K\({}_{2}\)FeO\({}_{4}\) and NaOCI exhibited a similar performance for LGB removal. Similar to the OD treatment, FeCl\({}_{3}\) 6H\({}_{2}\)O had the poorest removal efficiency (approximately 20%) when treating 25 mg/L and 50 mg/L of LGB (Figure 5b). At the dose of 30 mg/L, K\({}_{2}\)FeO\({}_{4}\) reacted more actively with 25 mg/L of LGB than with 25 mg/L of OD, corresponding to 80% of LGB and 61% of OD removal efficiencies. However, 90% removal of OD (25 mg/L) was achieved at a K\({}_{2}\)FeO\({}_{4}\) dose of 40 mg/L, while  
Figure 5: Comparison of potassium ferrite with NaOCI and FeCl\({}_{3}\) 6H\({}_{2}\)O with respect to the removal efficiency of (**a**) 25 mg/L and (**b**) 50 mg/L of lissamine green B dye at pH 3. 90% of the LGB (25 mg/L) was removed at a K\({}_{2}\)FeO\({}_{4}\) dose of approximately 50 mg/L. A similar trend was observed at 50 mg/L of K\({}_{2}\)FeO\({}_{4}\), indicating that the reaction rate with LGB was faster at the low dose of K\({}_{2}\)FeO\({}_{4}\) (<30 mg/L), while at the higher dose (>30 mg/L) the reaction rate with OD was faster. This is because the relatively large LGB with a higher molecular weight (576.6 Da) reacted more strongly to K\({}_{2}\)FeO\({}_{4}\) at low dosage, as compared to OD (350.32 Da). Meanwhile, the OD removal rate was slowly saturated as a function of the K\({}_{2}\)FeO\({}_{4}\) dose, compared to LGB. In comparison to OD removal, single oxidation treatment with NaOCl effectively removed LGB, reaching 100% (25 mg/L LGB) and 88.6% (50 mg/L LGB) removal efficiencies as a function of the NaOCl dose, while OD removal efficiency reached 88% (25 mg/L OD) and 40% (50 mg/L OD). Single coagulation treatment with FeCl\({}_{3}\)-6H\({}_{2}\)O exhibited the poorest efficiency, removing only 20% of LGB. The combination of oxidation and coagulation by K\({}_{2}\)FeO\({}_{4}\) occurred in multiple steps, particularly at pH 3, where the highest amount of OD was removed. In contrast, single coagulation by FeCl\({}_{3}\)\(\cdot\)H\({}_{2}\)O treatment occurred in two steps. The first step of nucleation was completed in 5 min, and most of the OD was removed by coagulation during this period. During the remaining period of nucleation growth, no further OD degradation occurred. Thus, a single coagulation process is not an efficient method for azo dye removal because it takes a longer time and a higher amount of coagulation, as compared to the combination of oxidation and coagulation by ferrate. Oxidation by NaOCl was completed in 5 min at all pH conditions (3-12), and there was no further OD degradation during the remaining period (up to 30 min). In addition, a single coagulation process generates colored coagulated solid wastes, which may result in the toxicity of the sludge produced and an increase in total dissolved solids in the treated wastewater [26].