# Optimization of Organic Content Removal from Aqueous Solutions by Fenton-Ozonation
## 3 Results
### Fenton Process
#### 3.1.2 ANOVA Analysis of Significant Terms
As concerns the interaction factors, only that between the H\({}_{2}\)O\({}_{2}\) dose and solution pH is statistically significant after 10 min of treatment. Independent from the treatment duration, the squared term of solution pH has an important effect on the removal efficiency, and only at the end of the Fenton process, after 60 min, the squared term of Fe-AC dose becomes significant. Figure 4 shows normal probability and residual plots as well as the agreement between predicted and experimental values of the responses after 60 min of Fenton process. Figure 4: Normal probability plot (**a**), residual plot (**b**), and predicted vs. experimental plot (**c**) for the removal efficiency in terms of TOC data, after 60 min of treatment. Diagnostic plots show that the studentized residuals align on a straight line in the normal plot and present no trends and a random distribution in the residuals versus run number plot. The goodness of fit can be visualized in the data correlation of predicted versus actual plot. Figure 5 presents the effects of significant factors on the removal efficiency. The increase in the L27-Fe dose factor is reflected in a strong increase in the performance of the treatment process, the steeper slope stressing a stronger influence on the removal efficiency response. The H2O2 dose factor in the considered experimental range shows that the optimal ratio in relation to iron grafted on L27 AC has been reached towards the inferior value of 100 mg L\({}^{-1}\) and below. As expected, the strongest influence in the degradation process of 2,4-D molecule is given by the solution pH factor. Thus, a decrease of solution pH below the value of 5 results in a steep increase in removal efficiency, with a local maximum a pH value of 2.9 (run 13, \(Y_{\text{TOC}}^{60 min}=59\%\)). Figure 6 shows the influence of H2O2 dose on the apparent constant rate determined based on the first 10 min of the Fenton process. Seemingly, at the beginning of the Fenton process, H2O2 dose has an optimal value where the degradation of 2,4-D molecule reaches a maximum value for the apparent constant rate in close dependence to the concentration of iron grafted onto the L27 AC. Figure 5: Plots of marginal means of removal efficiency in relation to TOC after 60 min of treatment as influenced by (**a**) L27-Fe dose, (**b**) H2O2 dose, and (**c**) solution pH; solid line—suggested model; dashed line—confidence interval; \(\blacksquare\)—predicted responses for each factor level. Design Expert software was also employed to determine the local optimum values for the removal efficiency in relation to TOC, after 60 min of treatment. The optimization method used by the software is described in reference [31]. Table 7 presents the optimization criteria for the considered response. Among the three different cases considered in the optimization study, case#1 maximized the objective function for the entire experimental range. The removal efficiency in relation to TOC achieves a value of 78.5% with a maximal desirability corresponding to a L27-Fe dose of 730 mg L\({}^{-1}\), a H\({}_{2}\)O\({}_{2}\) concentration of 224 mg L\({}^{-1}\), and at the known optimal solution pH of 2.98 [30]. The other two cases show optimal values of removal efficiency at a pH of 5.5 (45.4%) and at a pH value of 8.1 (49.3%).