## 3 Results and Discussion
\begin{split}\text{DyeH}+\text{HO}\bullet\overset{h\
u}{=}& \text{Dye}\bullet+\text{H}_{2}\text{O}\\ \text{DyeH}+\text{SO}_{4}^{-}\bullet\overset{h\
u}{=}& \text{Dye}\bullet+\text{H}\text{SO}_{4}^{-}\\ \text{Dye}\bullet+\text{Fe}^{3+}\overset{h\
u}{=}& \text{Dye}^{+}+\text{Fe}^{2+}\end{split}  \tag{6}\]  
In the second step, these organic intermediates can be degraded at a higher rate to smaller molecular compounds with a rate constant of k2. The values of k1 and k2 for each amount of H2O2 and S2O82- added are presented in Tables 1 and 2 (the R2 correlation coefficients are given in parentheses after the actual values of k1 and k2). For both dyes, the results indicate that the initial peroxide or persulfate ion concentrations play a crucial role in the kinetic of photodecoloration. For hydrogen peroxide (Table 1), there is a linear increase in the reaction rates for both steps until 999 \(\mu\)g/L (29.38 \(\mu\)mol/L), which seems to represent the optimal concentration for the setup presented in the paper. After this optimal hydrogen peroxide concentration value, there is a leveling off in the reaction rate. Excess amounts of H2O2 could determine the trapping of the generated hydroxyl radicals to form hydroperoxyl radicals (with lower oxidation potential) and water, according to the Mechanism (7). Additionally, a higher amount of generated Fe2+ with the increase in H2O2 addition could determine the decrease in the concentration of the hydroxyl radicals [33; 37]. For persulfate (Table 2), the optimum concentration for photodecoloration is maintained at 999 \(\mu\)g/L (5.20 \(\mu\)mol/L), after which the generated iron ions can similarly quench both sulfate and hydroxyl radicals through the following Mechanism (7):\[ \begin{split}\text{HO}\bullet&+\text{H}_{2}\text{O}_{2} \overset{h\
u}{=}\text{HO}_{2}\bullet+\text{H}_{2}\text{O}\\ \text{HO}\bullet&+\text{Fe}^{2}\overset{h\
u}{=}\text{HO}^ {-}+\text{Fe}^{3+}\\ \text{SO}_{4}^{-}\bullet&+\text{Fe}^{2+}\overset{h\
u}{=} \text{SO}_{4}^{2-}+\text{Fe}^{3+}\end{split}  \tag{7}\]  
In general, the methylorange-related rate constants are up to 42% lower than those corresponding to methylene blue due to the higher molecular mass of the former. A similar trend was also observed for semiconductor-aided photocatalysis [38]. Tables 1 and 2 also indicate that the total amount of iron generated in the solution during the photo-Fenton process is below the recommended values for groundwater (0.3 mg/L, as recommended by the World Health Organization) in the case of using hydrogen peroxide. A higher total iron amount is generated for potassium persulfate, which may need additional precipitation and filtering steps for the wastewater. A comparison of the overall efficiency of photo-Fenton over the widely available systems reported in the reference literature must be viewed with some caution, due to the different experimental conditions, reagent concentrations, model pollutant concentrations, and catalyst specific surfaces used, but it is widely agreed that fast degradation rate constants (k), coupled with high decoloration and mineralization rates, constitute important candidate parameters. Additionally, the use of the smallest amount of catalyst, iron ions, and oxidant possible is desired for economic efficiency reasons and to limit additional wastewater purification steps and avoid the excessive degradation of the catalyst [7; 39; 40]. In some studies, the importance of the photo-Fenton reaction medium's pH value being as close to neutral as possible is also stressed [7; 41]. For the sake of comparison, to highlight the benefits of using the proposed system with other photo-Fenton setups, Table 3 lists the photo-Fenton process parameters and efficiencies in other studies extracted from the reference literature.