## 3 Results
### Effect of Oxidant Concentration  
With the aim of studying the effect of different oxidant concentrations on the photo-Fenton and photo-Fenton-like process, experiments were conducted at four different concentrations of hydrogen peroxide and S\({}_{2}\)O\({}_{8}\)\({}^{2-}\), respectively (100, 200, 300, and 500 mg L\({}^{-1}\)). The selected concentrations were based on values commonly reported in the literature for similar photo-Fenton systems, ensuring both relevance and comparability with previous studies [32; 33; 34]. Furthermore, all experiments involving the classical and modified photo-Fenton process were conducted at pH 3, where Fe\({}^{2+}\) is most soluble and active. Experiments at a neutral pH were not performed, as the process is known to be inefficient under such conditions due to Fe\({}^{3+}\) precipitation and poor hydroxyl radical formation [35]. The photodegradation rates of the dye were calculated over a period of one hour and the results are depicted in Figure 4.  
Figure 4: **(A)** Photocatalytic degradation of Nile Blue and **(B)** % degradation achieved after 60 min, in the presence of Fe\({}^{+2}\) and different H\({}_{2}\)O\({}_{2}\) concentrations, **(C)** photocatalytic degradation of Nile Blue and **(D)** % degradation achieved after 60 min, in the presence of Fe\({}^{+2}\) and different concentrations of S\({}_{2}\)O\({}_{8}\)\({}^{2-}\) (C\({}_{0}\) = 15 mg L\({}^{-1}\), C\({}_{\mathrm{Fe}}\)\({}^{42}\) = 2 mg L\({}^{-1}\), C\({}_{\mathrm{oxidant}}\) = 100, 200, 300, 500 mg L\({}^{-1}\)). From Figure 4A,B, it can be observed that increasing the concentration of hydrogen peroxide up to 300 mg L\({}^{-1}\) increases the degradation rate of the dye. However, above 300 mg L\({}^{-1}\), a significant decrease is observed due to the scavenging effect of the oxidant according to the following reactions (Equations (6)-(8)):  
\[\mathrm{H_{2}O_{2}+HO^{\bullet}\to H_{2}O+HO_{2}}^{\bullet} \tag{6}\]  
\[\mathrm{HO_{2}}^{\bullet}+\mathrm{HO^{\bullet}\to H_{2}O+O_{2}} \tag{7}\]  
\[\mathrm{HO^{\bullet}+HO^{\bullet}\to H_{2}O_{2}} \tag{8}\]  
The calculated rate constants for 100, 200, 300, and 500 mg L\({}^{-1}\) of oxidant were 0.037, 0.064, 0.159, and 0.012 min\({}^{-1}\), respectively. Additionally, the calculated R\({}^{2}\) values (0.90-0.96) confirm that the degradation kinetics under varying H\({}_{2}\)O\({}_{2}\) concentrations followed a pseudo-first-order model. On the contrary, Figure 4C,D illustrate that an increase in the concentration of persulfate ions leads to an accelerated degradation rate of the target compound. This is attributed to the enhanced generation of sulfate radicals (SO\({}_{4}\)\({}^{\bullet-}\)) as the concentration of S\({}_{2}\)O\({}_{8}\)\({}^{2-}\) increases. However, the variation in the degradation achieved is not substantial, as a portion of the additional radicals produced may undergo self-recombination, as demonstrated in (Equation (9)) [36], as follows:  
\[\mathrm{SO_{4}}^{\bullet-}+\mathrm{SO_{4}}^{\bullet-}\to S_{2}\mathrm{O_{8}}^{ 2-} \tag{9}\]  
Photo-Fenton: Effect of Fe\({}^{+2}\) Concentration  
In order to study the effect of the iron concentration on the photo-Fenton process, experiments were carried out at four different Fe\({}^{+2}\) concentrations (1, 2, 3, and 5 mg L\({}^{-1}\)). The H\({}_{2}\)O\({}_{2}\) concentration was set at 200 mg L\({}^{-1}\) and the dye concentration was set at 15 mg L\({}^{-1}\). The results of these experiments are presented in Figure 5A,B. Increasing the iron concentration from 1 to 5 mg\(\cdot\)L\({}^  
Effect of Ultraviolet Radiation  
In order to study the effect of UV radiation on the Fenton and modified Fenton reaction, a catalytic and photocatalytic treatment experiment was carried out for a total duration of 90 min (Figure 6). The first treatment took place for 60 min in the dark, while for the next 30 min, the UV lamp was turned on. In Figure 6A, it is evident that the presence of light significantly influences the degradation rate of the dye by accelerating the reaction.