## 3 Results and Discussion
### Analysis of the Mineralization System by Sulfate and Nitrate Markers  
The molecular structure of SMX contains a benzene ring, a sulfonamide group (-SO2 NH2), and an isoxazole ring (heterocyclic with oxygen and nitrogen). In the photo-Fenton process, highly oxidizing hydroxyl radicals are generated that non-selectively attack SMX, breaking bonds and decomposing the SMX molecule. The general reaction that represents this process is shown in Equation (19) [26]. Based on this global reaction, the sulfate and nitrate species have been analyzed because they have been considered markers for monitoring the degradation of sulfur and nitrogen atoms, thus allowing the degree of mineralization achieved with the treatment to be established. \[\text{SMX}+\text{HO}^{\bullet}\rightarrow\text{oxidized products}+\text{NH}_{4}^{+}+\text{NO}_{3}^{-}+\text{SO}_{4}^{2-}+ \text{CO}_{2}+\text{H}_{2}\text{O} \tag{19}\]  
The process of sulfate formation during SMX oxidation involves several chemical reactions, mainly related to the degradation of the SMX heterocyclic ring and the oxidation of the sulfonamide group, which releases sulfate ions as the final product upon complete oxidation. Hydroxyl radicals attack the SMX molecule at different positions, including the sulfonamide group, and oxidized intermediate species such as sulfinic and sulfonic acids, and finally, sulfate ions, are produced. Figure 5a shows the sulfate concentration analyzed in the treated water samples once a steady state was reached (see Table 3). It is worth noting that the stoichiometric concentration of sulfate generated in the water from 50.0 mg/L of SMX corresponds to 19.0 mg/L. This measurement is the one obtained by performing the oxidation without using an iron catalyst (UV/H2O2). However, by enhancing the treatment by adding the catalyst in the form of ferrous sulfate, sulfate measurements higher than the stoichiometric value are obtained. This is because it is necessary to subtract the sulfate concentration added in the form of a catalyst ([SO2\({}^{-}_{4}\)]total, mg/L) from the sulfate concentration measured in the water ([SO2\({}^{-}_{4}\)]cataly, mg/L) (Equation (20)). The results obtained confirm that in all the tests performed, SMX is oxidized to the stoichiometric concentration of sulfate. Analyzing the general oxidation mechanism of SMX shown in Figure 6[37; 38], this result confirms that under the tested operating conditions, SMX degrades to aniline and sulfate. \[\left[\text{SO}_{4}^{2-}\right]_{\text{total}}=\left[\text{SO}_{4}^{2-}\right] _{\text{cataly}}+\left[\text{SO}_{4}^{2-}\right]_{\text{oxidation}} \tag{20}\]  
Figure 5b shows the nitrate concentration analyzed in the treated water samples. During the oxidation of SMX by the photo-Fenton process, a non-linear measurement between the ferrous ion concentration and the formation of nitrate as an oxidation product of the nitrogen present in the contaminant molecule will be observed. The results show that the nitrate concentration in the aqueous samples increases with increasing catalyst concentration until reaching a maximum value at a concentration of [Fe(II)]0 = 0.5 mg/L. From this point, a further increase in the iron concentration resulted in a progressive decrease in the amount of nitrate generated. This behavior can be attributed to the kinetics of the photo-Fenton process, where, operating at low catalyst concentrations, the generation of hydroxyl radicals is efficient, favoring the oxidation of the organic nitrogen contained in the SMX to its most oxidized form, which is the nitrate ion. However, when the concentration of [Fe(II)]0 > 0.5 mg/L increases, a self-competition phenomenon occurs, in which the excess ferrous ion acts as a hydroxyl radical scavenger, according to the following reaction (Equation (21)). This consumption of radicals reduces their availability to attack the pollutant, limiting the complete conversion of nitrogen to NO\({}^{-}_{3}\).