## 5 Conclusions  
The photocatalytic degradation of Nile Blue dye was comprehensively investigated using various homogeneous and heterogeneous Fenton-based photocatalytic techniques. The experimental results demonstrated that UV radiation alone led to a relatively slow degradation rate of the dye. However, the introduction of oxidants--particularly hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) and persulfate ions (S\({}_{2}\)O\({}_{8}\)\({}^{2-}\))--significantly enhanced the degradation process, with persulfate proving to be the most effective in facilitating dye removal.  
In homogeneous systems, the conventional photo-Fenton process exhibited the highest degradation efficiency. The reaction rate was strongly influenced by both the iron and oxidant concentrations. An increase in H\({}_{2}\)O\({}_{2}\) concentration improved the degradation up to an optimal value of 300 mg L\({}^{-1}\), beyond which efficiency declined, likely due to scavenging effects. Conversely, higher concentrations of Fe\({}^{2+}\) ions consistently enhanced the degradation rate, with no inhibitory effects observed within the tested range. A similar trend was noted in the photo-Fenton-like process, where increasing the oxidant concentration further accelerated the reaction. Importantly, UV irradiation played a critical role in enhancing the degradation efficiency in both homogeneous systems.  
In heterogeneous photocatalytic systems, the catalyst alone showed limited activity in degrading the dye. However, the addition of oxidants--particularly hydrogen peroxide--significantly improved the process performance, with an optimal concentration at approximately 200 mg L\({}^{-1}\). Beyond this threshold, a decrease in degradation efficiency was observed. When persulfate ions were used, a lower degree of synergism between the catalyst and oxidant was noted, yet effective dye degradation was still achieved.  
Comparative analysis of the investigated techniques revealed the superior performance of homogeneous systems, especially the classical photo-Fenton process, which also resulted in complete mineralization of the target compound. Nevertheless, the potential of heterogeneous techniques should not be overlooked. These methods provide efficient dye removal without the need for rigorous pH adjustments--commonly required in homogeneous processes--and eliminate the generation of iron-containing sludge. Moreover, solid catalysts offer significant practical advantages, including a high photocatalytic performance, reusability, and easy separation from the treated solution, making them promising candidates for sustainable wastewater treatment applications.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15147917/s1](https://www.mdpi.com/article/10.3390/app15147917/s1)]([https://www.mdpi.com/article/10.3390/app15147917/s1](https://www.mdpi.com/article/10.3390/app15147917/s1)), Figure S1: Discoloration of Nile Blue dye solution in the presence of H\({}_{2}\)O\({}_{2}\) and Goethite at time t = 0, t = 45 and t = 60 min respectively (C\({}_{0}\) = 15 mg L\({}^{-1}\), C\({}_{\text{oxidant}}\) = 200 mg L\({}^{-1}\), C\({}_{\text{cataly}}\) = 500 mg L\({}^{-1}\)), Figure S2: Nile Blue dye solution in the presence of Goethite at (A) pH = 3, (B) pH = 7, (C) pH = 10 (C\({}_{0}\) = 15 mg L\({}^{-1}\), C\({}_{\text{oxidant}}\) = 200 mg L\({}^{-1}\), C\({}_{\text{cataly}}\) = 100 mg L\({}^{-1}\)).  
G.P.: investigation, formal analysis, data curation, writing--original draft; E.E.: formal analysis, methodology, visualization, writing--review and editing; D.L.: conceptualization; supervision, project administration; funding acquisition, writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflicts of interest.