Effect of pH Value on the Treatment Efficiency

The pH of the aqueous environment is categorized as a vital influencing variable in Fenton oxidation. This demonstrates its importance since the pH influences H2O2 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

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 H2O2 to produce (OH) radicals [27]. The result is the formation of the hydroperoxide (HO2) 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 Nichela et al. [15], who treated a nitrobenzene-contaminated aqueous stream using a Fenton-based reaction.

**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 H2O2 800 mg/L).

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 Fuller's earth clay system. These parameters include the Fuller'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.

$$\begin{aligned} \Upsilon(\%) &= 90.47 + 2.28 \ \varepsilon\_1 - 4.68 \ 5 \varepsilon\_2 - 5.08 \ \varepsilon\_3 - 4.75 \ 6 \varepsilon\_1^2 + 0.23 \ \varepsilon\_1 \varepsilon\_2 - \\ &12.57 \ \varepsilon\_1 \varepsilon\_3 - 4.98 \ \varepsilon\_2^2 + 6.3 \ \varepsilon\_2 \varepsilon\_3 - 18.41 \ \varepsilon\_3^2 \end{aligned} \tag{10}$$

An ANOVA test based on Fisher'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*<sup>2</sup> (the regression coefficient value), the model was shown to be the best fit. The *R*<sup>2</sup> value of the Levafix Dark dye oxidation response was 94.3%.

A graphical representation of the abovementioned equation (Equation (10)) demonstrates the influences of the experimental parameters on the response. The 3-D (threedimensional) 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. 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 H2O2 and Fuller's earth. However, the curvature displayed in Figure 9a indicates that there is a significant interaction effect between the Fuller'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 Fuller's earth/H2O2 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**) H2O2 and Fuller's earth catalyst concentrations; (**b**) H2O2 concentration and pH; (**c**) Fuller's earth catalyst concentrations and pH.

Figure 9b explains the influence of the Fuller'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 H2O2, Fuller'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 Fuller's earth-based Fenton system.
