## 3 Results and Discussion
### Kinetic Model for Water Color Changes  
Figure 4 presents the evolution of the color parameter (in absorbance units, AU) as a function of time during the degradation of SMX by the photo-Fenton process. Together with Figures 2 and 3, it provides a comprehensive view of the contaminant removal and the transformation of its intermediate products, since it evaluates the evolution of water color, which is related to the presence of organic intermediates and conjugated structures. Although SMX disappears quickly, the color persists for longer, indicating the formation of colored by-products. At the beginning of the process (0-10 min), a rapid initial color formation is observed in the water, until reaching a maximum value, suggesting the formation of colored intermediates derived from the degradation of SMX, of highly conjugated nature (quinones, oxidized aromatic compounds, etc.) [35; 36]. Subsequently (10-40 min), the color gradually decreases over time, indicating the degradation and transformation of these intermediates as the radicals fragment the colored intermediates. Finally (>40 min), almost complete elimination of the color occurs, indicating advanced mineralization of the organic compounds to carboxylic acids, CO\({}_{2}\), and H\({}_{2}\)O.  
Analyzing the effect of iron catalyst dosage, it is observed that operating with lower Fe(II) concentrations (0.5-2.0 mg/L), color removal is slower, since the generation of hydroxyl radicals is limited. Operating at higher Fe(II) concentrations (4.0-5.0 mg/L), color degradation occurs more rapidly, indicating a greater availability of oxidizing species to attack the organic intermediates. SMX disappears within a few minutes, but the intermediates still contain aromatic and chromophore structures. The color reduction follows kinetics similar to the loss of aromaticity, suggesting that the compounds responsible for the color are aromatic intermediates. As hydroxyl radicals attack the aromatic rings, they open and lose their ability to absorb visible light. The complete elimination of color indicates that the system not only transforms SMX into intermediates (carboxylic acids) but eventually converts them to CO\({}_{2}\) and water. Combining photo-Fenton treatment with biological bioremediation techniques can offer a more comprehensive solution for the removal of emerging contaminants. Initially, chemical processes can degrade contaminants into simpler compounds, which are then fully mineralized by microorganisms in subsequent biological processes. The kinetics follow first-order behavior, as the color reduction occurs exponentially. Based on this approach, a series reaction model (Equation (12)) has been proposed, in which SMX degrades to colored reaction intermediates according to a first-order kinetic constant  
Figure 4: Predictions of first-order series kinetic model for water color changes during the SMX oxidation 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.  
\(\mathrm{k}_{\mathrm{f,color}}\) (1/min). Then, once the maximum concentration of colored intermediates has been formed, the color of the water begins to fade according to a first-order kinetic constant \(\mathrm{k}_{\mathrm{d,color}}\) (1/min). \[\mathrm{SMX}+\mathrm{HO}^{\bullet}\stackrel{{\mathrm{k}_{ \mathrm{color}}}}{{\rightarrow}}\mathrm{Colored~{}species}+\mathrm{HO}^{ \bullet}\stackrel{{\mathrm{k}_{\mathrm{color}}}}{{\rightarrow}} \mathrm{CO}_{2}+\mathrm{H}_{2}\mathrm{O} \tag{12}\]  
The mass balance proposed for the water color changes is shown in Equation (13). It is worth noting that a parameter \(\alpha\) has been incorporated, which is the conversion factor between concentration and absorbance units (AU L/mg).