## 3 Results and Discussion
### Screening of Different AOPs
#### 3.1.3 Heterogeneous Photocatalysis Processes  
In heterogeneous photocatalysis, the hydroxyl radicals are generated through light-induced reactions of water molecules on the surface of a solid catalyst [39]. TiO\({}_{2}\) was the photocatalyst selected for the process, given its superior properties, in comparison to other materials. Specifically, TiO\({}_{2}\) is considered to be the most prominent catalyst for this type of AOP, as it combines chemical stability with enhanced photocatalytic activity, while it remains non-toxic and relatively cheap, as it is widely abundant [40].  
Before evaluating the degradation of SDX, using a process with this principle of radical generation, it was important to perform a series of reference experiments to assess the adsorption of SDX molecules on TiO\({}_{2}\) nanoparticles and their potential degradation by UV-A irradiation. For the adsorption tests, 100 mg/L TiO\({}_{2}\) in suspension with SDX was left for 30 min in the dark and under stirring. To ensure the effective suspension of the catalyst, TiO\({}_{2}\) was diluted in 1 L of UW and left under stirring for at least 5 h, prior to being added to the spiked solution and diluted to a final volume of 2.3 L. The results showed 8% SDX removal, indicating low removal by adsorption. The type of TiO\({}_{2}\) that was employed has a point of zero charge (pH\({}_{\text{PZC}}\)) of approximately 6.7 [32; 41], while SDX has a pKa of 6.17 [42]. Thus, the low levels of absorbance can be explained by the repulsive forces that develop, as both the pollutant and the catalyst are positively charged at the specific conditions (C). Similar to the adsorption tests, UV-A irradiation at 365 nm resulted in a moderate degradation of the compound (14%). This was expected based on the UV spectrum of the molecule (Supplementary Materials, Figure S1). It should be noted that the pH value of the initial solutions was approx. 5.2, and no further adjustments were made.  
The photocatalytic experiments were performed next with 100 mg/L TiO\({}_{2}\) in suspension and a UV-A dose per unit volume of 4.26 W/L. As shown in Figure 1, despite the relatively slow degradation rate of this process compared to the other AOPs studied, SDX was completely removed after 30 min of operation, while the mineralization efficiency showed a considerable improvement, with a TOC removal of 54% being achieved.  
The bar chart in Figure 2 provides a comparative analysis of the tested AOPs, focusing on their respective performance in terms of degradation and mineralization efficiency. These results highlight the effectiveness of heterogeneous photocatalysis as a promising approach for the combined SDX degradation/mineralization. Considering the presence of an aromatic and a heterocyclic ring in the target contaminant, heterogeneous photocatalysis was expected to outperform photolytic processes. These ring structures are known for their robustness and stability, making them typically difficult to degrade in the absence of photocatalysts [43; 44; 45; 46]. Given the promising potential of heterogeneous photocatalysis, a further parametric investigation of this process was undertaken.  
Figure 2: AOP degradation and mineralization performance.