## 3 Results
### Heterogeneous Photocatalysis
#### 3.3.2 Goethite  
Preliminary Study  
In order to evaluate the photocatalytic efficiency of the goethite, a photocatalytic experiment was conducted without the presence of oxidizing agents. The results are demonstrated in Figure 9. Blank experiments in the dark, prior to illumination, revealed that 16% of the target dye was adsorbed on the mineral's surface. As can be observed, the degradation rate of Nile Blue dye reaches 11.4% after one-hour irradiation. Obviously, the mineral demonstrates low photocatalytic activity that leads to high e\({}^{-}\)/h\({}^{+}\) pair recombination due to its positive conduction band potential (2.2 eV) and inability to reduce molecular oxygen [(39; 40)]. Due to the slow rate of dye decomposition by simple photocatalysis in the presence of the goethite catalyst, experiments were carried out with the addition of an oxidant for the reasons mentioned in the section Effect of H\({}_{2}\)O\({}_{2}\) Concentration in the presence of MOF catalyst. Experiments at four different oxidant concentrations (100, 200, 300, and 500 mg L\({}^{-1}\)) were conducted and the results are presented in Figure 9. In this specific system (UV/ Goethite/H\({}_{2}\)O\({}_{2}\)), there are three possible sources of iron, including Fe\({}^{+2}\), Fe\({}^{+3}\), and FeOOH, while two possible mechanisms for hydroxyl radical generation can occur:  
The first mechanism is similar to that occurring in homogeneous Fenton reactions, due to the presence of iron ions leached from the catalyst through its reductive dissolution. These ions react with hydrogen peroxide, resulting in the formation of hydroxyl radicals [(41)]. Figure 9: (**A**) Photocatalytic degradation of Nile Blue and (**B**) % degradation achieved in 60 min, in the presence of Goethite catalyst and different H\({}_{2}\)O\({}_{2}\) concentrations (C\({}_{0}\) = 15 mg L\({}^{-1}\), C\({}_{cataly}\) = 100 mg L\({}^{-1}\), Coxidant = 100, 200, 300, 500 mg L\({}^{-1}\)). The second mechanism refers to the surface adsorption and redox reaction with hydrogen peroxide. The main process of hydroxyl radical formation takes place at the goethite-water interface and is described by the following reactions (Equations (12)-(14)) [41]:  
\[\equiv \text{Fe}^{+3}+\text{H}_{2}\text{O}_{2}\rightarrow\equiv \text{Fe}(\text{HO}_{2})^{2+}+\text{H}^{+} \tag{12}\] \[\text{Fe}(\text{HO}_{2})^{2+}\rightarrow\equiv \text{Fe}^{+2}+\text{}^{\bullet}\text{HO}_{2}\] (13) \[\equiv \text{Fe}^{+2}+\text{H}_{2}\text{O}_{2}\rightarrow\text{Fe}^{+3} +\text{}^{\bullet}\text{OH}+\text{OH}^{-} \tag{14}\]  
In Figure 9, we observe that increasing the concentration of hydrogen peroxide increases the degradation of the dye, which is due to the increased production of hydroxyl radicals by the mechanisms mentioned above. The increased degradation rates are indicative of catalyst activation, assisted by H\({}_{2}\)O\({}_{2}\), and the possibility of a heterogeneous photo-Fenton reaction taking place between the Fe atoms contained in the catalyst and H\({}_{2}\)O\({}_{2}\). Finally, the experimental results indicate a synergistic effect in the applied process. This is evidenced by the following calculation:  
\[\frac{\text{\%\emph{removal by Goethite}}/\text{H}_{2}\text{O}_{2}}{\text{ \%\emph{removal by Goethite}}+\text{\%\emph{removal by H}}_{2}\text{O}_{2}}= \frac{46.21}{11.38+20.69}\%=1.44>1\]  
A ratio greater than one confirms that the combined use of goethite and hydrogen peroxide leads to a more efficient degradation of the dye than when each component is applied individually. This synergistic interaction suggests that goethite may play a catalytic role. Accordingly, in order to evaluate the effect of persulfate, experiments were carried out at four different concentrations of S\({}_{2}\)O\({}_{8}\)\({}^{2-}\) (100, 200, 300, 300, and 500 mg L\({}^{-1}\)). The results are presented in Figure 10. It is obvious that the rate of dye degradation increases with a rising concentration of persulfate ions.