# Optimization of Organic Content Removal from Aqueous Solutions by Fenton-Ozonation
## 3 Results
### Fenton Process
#### 3.1.1 Initial State of ANOVA Analysis  
In order to evaluate the statistical significance of the main and interaction factors over the considered responses, an ANOVA analysis was carried out (Tables 4 and A2). The independent variables were the dose of Fe-grafted AC (x\({}_{1}\)), dose of H\({}_{2}\)O\({}_{2}\) (x\({}_{2}\)), and the solution pH (x\({}_{3}\)), whereas the process performance was evaluated based on the responses: 2,4-D removal efficiency (Y, %) after 10, 20, 30, 40, and 60 min of treatment. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Source**} & \multicolumn{4}{c}{**Sum of Square; Degrees of Freedom; F-Value; \(p\)-Value \(\times\) 10\({}^{2}\)**} \\ \cline{2-7}  & \(\mathbf{Y_{TOC}^{2}}\) **,** & \(\mathbf{Y_{TOC}^{2}}\) **,** & \(\mathbf{Y_{TOC}^{2}}\) **,** & \(\mathbf{Y_{TOC}^{2}}\) **,** & \(\mathbf{Y_{TOC}^{2}}\) **,** & \(\mathbf{Y_{TOC}^{2}}\) **,** & \(\mathbf{Y_{TOC}^{2}}\) **,** \\ \hline Model & 2700; 9; 6.59; 106 ** & 2810; 9; 6.66; 1.03 ** & 6870; 9; 7.22; 0.81 * & 3020; 9; 15.3; 0.08 * & 3410; 9; 20.6; 0.31 * \\ x\({}_{1}\) & 398; 1; 8.72; 2.13 ** & 328; 1; 7.00; 3.32 ** & 826; 1; 7.81; 2.67 ** & 536; 1; 24.5; 1.66 ** & 522; 1; 28.5; 0.11 * \\ x\({}_{2}\) & 24,9; 1; 0.55; 48.4 & 102; 1; 2.18; 18.4 & 126; 1; 1.19; 31.2 & 95.5; 1; 4.36; 7.51 ** & 198; 1; 10.8; 1.34 ** \\ x\({}_{3}\) & 1080; 1; 23.6; 0.18 * & 1560; 1; 33.2; 0.07 * & 3240; 1; 30.7; 0.09 * & 1320; 1; 60.4; 0.01 * & 1730; 1; 94.3; \textless{}0.01 * \\ x\({}_{1}\)x\({}_{2}\) & 17.3; 1; 0.38; 55.7 & 0.22; 1; 0.005; 94.7 & 1.14; 1; 0.01; 92 & 2.74; 1; 0.13; 73.4 & 0.24; 1; 0.01; 91.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: ANOVA analysis for Y\({}_{TOC}\) responses after different treatment durations. Figure 3: HPLC removal efficiency by the Fenton process. Experimental data shown in circles (\(\Circle\)), and trend line shown in a dotted line (- -). (**a**) runs 1–4; (**b**) runs 5–8; (**c**) runs 9–12; (**d**) runs 13–17. According to the experimental design detailed in Table 3.  
Design Expert software (State-Ease Inc., Minneapolis, MN, USA, Version 7.1.5) was used to fit two second-order polynomial models to the spherical central composite design data. In this phase of the initial ANOVA analysis, the unprocessed statistical models of \(Y_{TOC}\) had a freedom degree value of 9, whereas the freedom degree of residuals was 7, corresponding to a value for F tabulated/critic of 3.29. In order for the models to be statistically significant, the F-value should be higher than the F tabulated. Commonly, this condition is satisfied after removing the insignificant terms from the models. However, all five models meet the F-value > F tabulated condition. An interesting observation is that with the increase in treatment time, the F value increases significantly, strengthening the significance of the overall model describing the Fenton process. The same observation is also valid in the case of the \(p\)-value (low probabilities indicate high effects of the models; on the contrary, high probabilities that the null hypothesis is true, i.e., no factor effects, imply no significant effect). Based on the models of removal efficiencies at 10, 20, 30, and 60 min, the \(p\)-value decreases when treatment time increases in the order 0.0106 > 0.0103 > 0.0081 > 0.0031. However, the lowest \(p\)-value is reached after 40 min of treatment, 0.0008. While the first two models, at 10 and 20 min, are significant for a confidence interval of 95%, the subsequent three models have a more accurate confidence interval of 99%. As already stated, the main goal of the present study was to screen the performance of the Fenton process over a wide and unbalanced range of values for the three considered independent values: Fe-grafted AC dose, H\({}_{2}\)O\({}_{2}\) dose, and solution pH. However, it was quite difficult to ascertain why the H\({}_{2}\)O\({}_{2}\) dose parameter has a relatively low effect on the overall performance of the Fenton process, especially in its early phase.