## 3 Results and Discussion
### Reactive Azo Dye Oxidation
#### 3.2.3 Effects of Photo-Fenton Parameters
An inspection of the 3-D surface graph and the 2-D contour plot in Figure 9b illustrated that the removal rate of the Levafix dye was enhanced with the increase in the concentrations of both H\({}_{2}\)O\({}_{2}\) and Fuller's earth. However, the curvature displayed in Figure 9a indicates that there is a significant interaction effect between the [PERSON]'s earth and hydrogen peroxide doses. This interaction encourages the generation of hydroxyl radical species that have a positive effect on the dye removal rate. Nevertheless, a further increase in the concentrations of both reagents resulted in a reduction in the dye oxidation rate. Thus, an optimal ratio of [PERSON]'s earth/H\({}_{2}\)O\({}_{2}\) is essential to increase the yield of (OH) radicals [27]. Figure 9: The 3-D (**a–c**) and 2-D (**d–f**) surface and contour plots of the coded independent variables and their responses in Levafix Dye removal: (**a**) H\({}_{2}\)O\({}_{2}\) and [PERSON]’s earth catalyst concentrations; (**b**) H\({}_{2}\)O\({}_{2}\) concentration and pH; (**c**) [PERSON]’s earth catalyst concentrations and pH. Figure 9b explains the influence of the [PERSON]'s earth catalyst dose and the significant parameter of pH in the augmented 3-D and 2-D response surface and contour plots. The graph confirms that the oxidation efficiency enhanced with an increasing catalyst dose. However, an alkaline environment is not favorable for Levafix oxidation. The optimum pH is near the acidic pH 3.0. The existence of scavenger species is apparent in the reaction medium rather than the reactive hydroxyl radicals in the reaction media. So, the result is a reduction in the overall reaction of oxidation, clarifying this finding. The combined 3-D and 2-D response surface and contour plots displayed in Figure 9c demonstrate the influences of the independent hydrogen peroxide and pH values, respectively, on the Levafix dye as azo dye removal rates. An investigation was carried out on the figure results of Levafix Dye oxidation. In this set of experiments, the pH varied over the range of 2.5-3.5. The surface and contour plots of the RSM show that the highest percentage of dye removal was obtained at the intersection near the origin of the two variables. For an extra examination of the proposed model, the statistically optimized predicted parameters investigated using Mathematica software were examined. The optimal conditions revealed from the predicted model were 818 mg/L, 1.02 mg/L, and 3.0 for H\({}_{2}\)O\({}_{2}\), [PERSON]'s earth catalyst, and pH, respectively. Then, an extra three replicates of experiments were carried out to confirm the predicted values' adequacy. After a 2 min reaction time, the measured percentage of dye removal (71%) was close to the predicted value (72%), which was obtained by applying the suggestive factorial design. Moreover, the overall oxidation reaction reached a 99% dye removal rate within 15 min of the reaction time. This investigation verifies that the response surface methodology is a satisfactory approach for optimizing the operational parameters' influence using the [PERSON]'s earth-based Fenton system.