## 3 Results and Discussion
### Photocatalytic Tests
#### 3.2.2 Reusability Study of Photo-Fenton-like Treatment  
In full-scale wastewater treatment, catalyst recovery and reuse are very important to reduce the cost and operability of the process. A stable catalyst should preserve its activity during several consecutive treatment cycles. The study of the stability and reusability of the composites was evaluated by performing four consecutive cycles of the photo-Fenton-like treatment, as shown in Figure 6e. These tests were conducted starting from a catalyst-H\({}_{2}\)O\({}_{2}\) molar ratio of 1:15, since it was the one that presented the best results in the previous section. Only one injection of H\({}_{2}\)O\({}_{2}\) was performed at the beginning of the first cycle. At the end of each cycle, a more concentrated dye injection was made to start from the same conditions, minimizing the catalyst losses and maintaining the amount of H\({}_{2}\)O\({}_{2}\) left over from the previous cycle because this was similar to what would occur in a larger scale process. Figure 6e shows the behavior of the degradation capacity of the ZnO/g-C\({}_{3}\)N\({}_{4}\) composites throughout the four cycles. The first two cycles achieved approximately the same level of degradation within the same time, so the catalytic activity of the ZnO/g-C\({}_{3}\)N\({}_{4}\) composites remained stable. After 30 min of treatment, the degradation achieved in each of them was 95.91% and 92.23%, respectively, and in 45 min, the dye degradation was total in both cases. As for the third and fourth cycles, it can be observed that there was a decrease in activity. The degradation after 30 min was 77.12% in the third cycle and 74.44% in the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & \begin{tabular}{c} **Initial Rhodamine B** \\ **Concentration (mg/L)** \\ \end{tabular} & **Treatment** & **Degradation (\%)** & **Reference** \\ \hline ZnO/g-C\({}_{3}\)N\({}_{4}\) nanocomposites & 10 & \begin{tabular}{c} Photocatalysis \\ (200W Xe Lamp, 380 nm) \\ \end{tabular} & 100 (80 min) & [17] \\ K-doped g-C\({}_{3}\)N\({}_{4}\)/ZnO & 20 & \begin{tabular}{c} Photocatalysis \\ (2 sun intensity, 420 nm) \\ \end{tabular} & 92.00 (120 min) & [55] \\ ZnO/g-C\({}_{3}\)N\({}_{4}\) composites & 10 & \begin{tabular}{c} Photocatalysis \\ (0.5 g/L) \\ \end{tabular} & 10 & \begin{tabular}{c} This article \\ Photo-Fenton-like \\ (6W UV Lamp, 365 nm) \\ \end{tabular} & 98.12 (120 min) & [32] \\ Cu-Cu\({}_{2}\)O/g-C\({}_{3}\)N\({}_{4}\) & 10 & \begin{tabular}{c} Photo-Fenton-like \\ (300 W Xe Lamp, 400 nm) \\ \end{tabular} & 90.00 (50 min) & [30] \\ Magnetic nanocubes (2 g/L) & 10 & \begin{tabular}{c} Photo-Fenton-like \\ (48W UV Lamp, 365 nm) \\ Photo-Fenton-like \\ (500W Xe Lamp) \\ \end{tabular} & 94.04 (180 min) & [34] \\ NH\({}_{2}\)-MIL-53(Fe) (0.08 g/L) & 40 & \
\begin{tabular}{c} Photo-Fenton-like \\ (500W Xe Lamp) \\ \end{tabular} & 95.00 (120 min) & [64] \\ ZnO/g-C\({}_{3}\)N\({}_{4}\) composites & 10 &
\begin{tabular}{c} Photo-Fenton-like \\ (30 W UV Lamp, 395 nm) \\ \end{tabular} & 100 (45 min) & This study \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Compilation of different recent works in which Rhodamine B was degraded by photocatalysis and photo-Fenton-like using different materials as catalysts. fourth cycle and reached total degradation after 60 min of treatment. This decrease was also reflected in the value of the kinetic constants, where it is observed that as the cycles progressed, their value decreased (Figure 6f). This could be due in part to the possible loss of composites in the sample collection and the possible erosion at their interfaces and solutions [60]. On the other hand, it could be attributed to the fact that H\({}_{2}\)O\({}_{2}\), as seen above, plays an important role in degradation, so since only a single injection has been made, even though it is being generated, it may not be sufficient. This would affect the activity, and degradation would be lower and slower.