## 3 Results and Discussion
### Analysis of the Mineralization System by Sulfate and Nitrate Markers
Additionally, at higher iron concentrations,partial oxidation pathways may prevail, leading to the formation of less oxidized nitrogen products, such as ammonium ion, to the detriment of nitrate. \[\text{Fe}^{2+}+\text{HO}^{\bullet}\rightarrow\text{Fe}^{3+}+\text{HO}^{-} \tag{21}\]  
Therefore, these results show that there is an optimal concentration of ferrous catalyst [Fe(II)]0 = 0.5 mg/L corresponding to 22 mol SMX: 1 mol Fe(II) that maximizes the efficiency of the photo-Fenton process in terms of conversion of SMX nitrogen to oxidized species such as nitrate. Figure 5: (**a**) Sulfate; (**b**) nitrate; (**c**) aromaticity; (**d**) color measured in the SMX aqueous solutions treated by photo-Fenton. Experimental conditions: [SMX]0 = 50.0 mg/L; [H\({}_{2}\)O\({}_{2}\)]0 = 2.0 mM; [UV] = 150 W; [pH]0 = 3.0; [T] = 25 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c} \hline
**[Fe(II)]0** & \(\left[\text{SO}_{4}^{2-}\right]_{\text{total}}\) & \(\left[\text{SO}_{4}^{2-}\right]_{\text{cataly}}\) & \(\left[\text{SO}_{4}^{2-}\right]_{\text{lostration}}\) & \(\left[\text{NO}_{3}^{2-}\right]\) \\ **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** \\ \hline
0.0 & 19 & 0.0 & 19 & 2.3 \\
0.5 & 19 & 0.9 & 18 & 3.8 \\
1.0 & 19 & 1.7 & 17 & 2.5 \\
2.0 & 21 & 3.4 & 18 & 2.3 \\
3.0 & 23 & 5.2 & 18 & 0.9 \\
4.0 & 25 & 6.9 & 18 & 0.6 \\
5.0 & 29 & 8.6 & 20 & 0 \\ \hline \end{tabular}
\end{table} 
Table 3: Sulfate and nitrate concentrations measured in the treated water. Experimental conditions: [SMX]0 = 50.0 mg/L; [H\({}_{2}\)O\({}_{2}\)]0 = 2.0 mM; [UV] = 150 W; [pH]0 = 3.0; [T] = 25 \({}^{\circ}\)C.  
Another relevant factor that may contribute to the decrease in nitrate formation at high catalyst concentrations is the interaction of iron with the phenolic by-products [39] generated during SMX oxidation. Specifically, the cleavage of the isoxazole ring and the oxidation of the sulfonamide group can lead to the formation of aminophenol-like structures, which have an affinity for forming stable complexes with Fe(III) [40]. This complexation limits the availability of iron to participate in the catalytic cycle of the photo-Fenton process, decreasing the generation of hydroxyl radicals and, consequently, inhibiting the complete oxidation of organic nitrogen to nitrate. Furthermore, Fe-aminophen complexes can stabilize amino groups, preventing their transformation into oxidized species such as NO\({}_{3}^{-}\). This phenomenon reinforces the need to optimize iron concentration to avoid catalytic deactivation effects associated with the formation of complexes with oxidation intermediates. When there is a higher concentration of Fe(II) in the medium, part of this iron is oxidized to Fe(III), which can form stable complexes with the phenolic by-products (Equation (22)) [41]:  
\[\mathrm{Fe^{3+}+aminophen\leftrightarrow[Fe(aminophen)]^{2+}complex} \tag{22}\]  
These complexes have two relevant effects on the process:  
_Iron sequestration_: Iron is trapped in stable complexes that do not participate in the photo-Fenton cycle (they are not regenerated to Fe(II)). This reduces the availability of Fe(III) for the further generation of hydroxyl radicals. _Inhibition of the oxidation of nitrogenous intermediates_: Fe-aminophen complexes can mask nitrogenous functional groups (such as amino), which protects nitrogen from being completely oxidized to NO\({}_{3}^{-}\). This favors the accumulation of partially oxidized products, such as NH\({}_{4}\)\({}^{+}\). The decrease in nitrate concentration observed when using [Fe(II)]\({}_{0}\) > 0.5 mg/L can be explained, in part, by the increased formation of Fe-aminophen complexes, which reduce the fraction of active iron in the photo-Fenton cycle, interfere with the mineralization of organic nitrogen to nitrate, and increase the stability of intermediate byproducts, reducing their reactivity toward hydroxyl radicals.