## 3 Results and Discussion
### Reactive Azo Dye Oxidation
#### 3.2.3 Effects of Photo-Fenton Parameters
This demonstrates its importance since the pH influences H\({}_{2}\)O\({}_{2}\) decomposition and the hydrolytic speciation of metal ions. In this regard, to evaluate its influence on the Levafix Dye oxidation through the modified photo-Fenton system, the initial pH values were altered from an acidic value to an alkaline value to evaluate their effects on the system. The pH values varied over the range of 3.0 to 8.0 according to the results displayed in Figure 8; an alkaline pH value is not favorable for the oxidation reaction using the photo-Fenton system. However, further oxidation was achieved when the acidic pH was used. Notably, it was obvious that the acidic pH (3.0) value of wastewater attained the highest removal efficiency. The removal efficiency reached 97% within 20 min of the illumination time, at which point the Levafix dye oxidized into other intermediates. However, increasing the aqueous solution pH to an alkaline pH value results  
Figure 7: Effect of hydrogen peroxide concentration on photo-Fenton oxidation system (experimental conditions: azo dye Levafix Dark Blue 50 ppm; Fuller’s earth. 1.0 g/L; and pH 3.0). in the presence of excess unfavorable ions. Such ions react with superoxide species rather than the metals in the catalyst. This hinders the overall reaction. These metals are mostly accountable for provoking H\({}_{2}\)O\({}_{2}\) to produce (OH) radicals [27]. The result is the formation of the hydroperoxide (HO\({}_{2}\)) inactive radical, which reduces the oxidation efficacy [28]. Hence, the Levafix oxidation yield further declined at an alkaline pH. The findings of this investigation of the low efficiency at a high pH are in accordance with the previous findings of [PERSON] et al. [15], who treated a nitrobenzene-contaminated aqueous stream using a Fenton-based reaction. Box-Behnken Regression Design Fitting  
RSM, response surface regression methodology, was applied to explore the operational parameters' optimal values for the modified photo-Fenton system based on the [PERSON]'s earth clay system. These parameters include the [PERSON]'s earth clay and hydrogen peroxide concentrations, as well as the pH value, in order to maximize the Levafix dye efficacy removal. The used outlined matrix is tabulated in Table 1, as well as the experimental dye response after the oxidation reaction, with the predictive model values presented using Equation (10). This equation exploring the second-order polynomial regression model exhibits the response surface in terms of the coded variables for the Levafix dye removal response. \[ \begin{split}\mathrm{Y}(\%)=90.47+2.28\ \varepsilon_{1}-4.68\ 5 \varepsilon_{2}-5.08\ \varepsilon_{3}-4.75\ 6\varepsilon_{1}^{2}+0.23\ \varepsilon_{1} \varepsilon_{2}-\\ 12.57\ \varepsilon_{1}\varepsilon_{3}-4.98\ \varepsilon_{2}^{2}+6.3 \ \varepsilon_{2}\varepsilon_{3}-18.41\ \varepsilon_{3}^{2}\end{split}  \tag{10}\]  
An ANOVA test based on [PERSON]'s statistical analysis was performed for the assessment of the statistical consequence and the adequacy of a quadratic model. With a minimum deviation, a small probability value (<0.005), and a high \(R^{2}\) (the regression coefficient value), the model was shown to be the best fit. The \(R^{2}\) value of the Levafix Dark dye oxidation response was 94.3%. Figure 8: Effect of pH value on photo-Fenton oxidation system (experimental conditions: azo dye Levafix Dark Blue 50 ppm; Fuller’s earth 1.0 g/L; and H\({}_{2}\)O\({}_{2}\) 800 mg/L). A graphical representation of the abovementioned equation (Equation (10)) demonstrates the influences of the experimental parameters on the response. The 3-D (three-dimensional) surface and the 2-D (two-dimensional) contour plots of the operational parameters were designed using Matlab software, and the results are displayed in Figure 9a-c. The data displayed in the figure revealed the response of each experimental influencing variable and the major interactions between those variables.