## 3 Results and Discussion
### CAF Degradation by Fenton Process
This allows for effective analysis of the independent variables for caffeine degradation through response surfaces and contour curves, as demonstrated in Figure 2.  
Both response surfaces exhibited similar behaviors, indicating that the simultaneous increase in the dosage of H\({}_{2}\)O\({}_{2}\) and FeSO\({}_{4}\).7H\({}_{2}\)O results in greater caffeine degradation, favoring the formation of hydroxyl radicals. In that regard, it is important to emphasize that in the proposed data model for 5 min of Fenton reactions, the interaction term for H\({}_{2}\)O\({}_{2}\) and FeSO\({}_{4}\).7H\({}_{2}\)O was statistically significant, and in the data model for 60 min of reactions, the H\({}_{2}\)O\({}_{2}\) concentration term was significant. These variables obtained \(p\)-values  
Figure 2: Response surfaces for D\({}_{\text{CAF}}\) and C\({}_{\text{F}}\) as function of dosage of H\({}_{2}\)O\({}_{2}\) and FeSO\({}_{4}\).7H\({}_{2}\)O after 5 min of reaction (**a**,**c**) and after 60 min of reaction (**b**,**d**). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Factor** & **Observed Minimum** & **Critical Values** & **Observed Maximum** \\ \hline H\({}_{2}\)O\({}_{2}\) (mg L\({}^{-1}\)) & 25.0 & 129.7 & 150.0 \\ Fe\({}^{+2}\) (mg L\({}^{-1}\)) & 0.8 & 2.3 & 4.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Critical values of Fenton reaction experimental DOE. very close to 0.05 in the two proposed models and therefore were considered significant in both cases. The high percentage of degradation in 5 min is due to the predominance of caffeine oxidation, but the subsequent drop indicates a decrease in the degradation rate. The rapid degradation kinetics are consistent with studies on the Fenton process. The concentration ranges chosen were satisfactory, not harming the degradation reaction. Table 9 shows the critical values of the experimental design, indicating the optimal points for the independent parameters studied. The optimized concentrations were used in the combined process experiments for times of 5 and 60 min. The present work achieved satisfactory degradation efficiencies compared to previous studies, particularly due to the optimization of parameters. For instance, [PERSON] et al. [52] achieved 95% degradation of the caffeine molecule using [PERSON]'s reagent, [PERSON] et al. [53] obtained a maximum degradation efficiency of approximately 80%, while [PERSON] [47] achieved only 9.3% degradation under optimal conditions. Similarly to what is found in the literature, the conducted tests showed inefficiency in mineralization, with rates not exceeding 5%. In AOPs, contaminant degradation involves sequential reactions initiated (Equations (20)-(22)) by radicals attacking organic compounds [40]. The hydroxyl radical operates in three stages, eliminating hydrogen, reacting with the compound, and creating intermediates until complete mineralization. Therefore, AOP efficiency encompasses total mineralization, preventing critical residue by destroying contaminants and their byproducts [40]. An evaluation of its effectiveness involves complete oxidation, yielding CO\({}_{2}\), water, and minerals. Partial mineralization might produce harmful byproducts, requiring post-treatment before discharge into water bodies [39]. Since AOPs can transform pollutants into simpler species easily degraded by common technologies [54], there is an imperative to upgrade water treatment plants with additional tertiary treatment technologies to address pollutants resistant to conventional methods. \[\text{RH}+\text{-OH}\rightarrow\text{H}_{2}\text{O}+\text{-R} \tag{20}\]  
\[\text{H}_{2}\text{O}_{2}+\text{-R}\rightarrow\text{Byproduct} \tag{21}\]  
\[\text{-OH}+\text{Byproduct}\rightarrow\text{H}_{2}\text{O}+\text{CO}_{2}+ \text{Minerals} \tag{22}\]  
Hence, the specific investigation of the Fenton reaction in this study revealed the incomplete degradation of caffeine and its intermediate byproducts, resulting in the production of secondary residues that require characterization to comprehend their potential risks.