## 3 Results and Discussion
### Combined Process
#### 3.3.2 Total Organic Carbon Monitoring  
This study evaluated the TOC content in different phases of the CP, focusing on the ability of GAC to reduce TOC in the solution. Given the limited mineralization (5%) achieved in the individual Fenton study and considering the potential criticality of the byproducts formed, it was sought to integrate the Fenton reaction into the adsorption step with GAC. Figure 7 illustrates the reduction in TOC throughout the four combined processes, considering three Fenton reaction times: 0 min (end of adsorption), 5 min, and 60 min. The reduction was calculated in relation to the initial value of the caffeine solution (TOC\({}_{0}\)), which started with a concentration of 20 mg L\({}^{-1}\) in each process. Average error bars are identified in each column. Figure 7 reveals that at the end of adsorption, the TOC was reduced by more than 50% in the four CPs. However, this value suggests the desor  
Figure 6: C\({}_{\rm F}\) of caffeine for Fenton reaction times of 0 (end of adsorption), 5, and 60 min. Figure 7: The reduction in the TOC of the caffeine solution for the Fenton reaction times of 0 (end of adsorption), 5, and 60 min in relation to the TOC of the initial solution of 20 mg L\({}^{-1}\). from the surface of the material into the solution, since the adsorption tests obtained more than 89% caffeine removal, and consequently, the TOC reduction should be greater. For the CPs with 5 min of the Fenton reaction, an increase in the TOC reduction is observed, except for the first, reaching a final reduction of more than 60%. The second combined process achieved a 96% reduction in TOC. These reductions do not necessarily indicate mineralization, but rather the adsorption of organic compounds onto the granular activated carbon. When extending the Fenton reaction time to 60 min, there was an increase in the TOC in the solution after 55 min of the reaction, associated with a decrease in oxidants and the depletion of active sites in the GAC. The results show greater effectiveness within 5 min of the Fenton reaction, followed by an accumulation of organic intermediates up to 60 min, reflecting a pattern of rapid initial degradation kinetics, in accordance with the literature [38; 47; 48; 61]. At the end of the fourth CP, a reduction of 87% in TOC was achieved in 5 min of the reaction and 54% in 60 min, satisfactory values compared to the results in the literature for caffeine mineralization. [PERSON] et al. [48] and [PERSON] et al. [47] concluded that the application of the Fenton process was not effective for the mineralization of caffeine and the authors chose to couple UV radiation with the Fenton reaction. After combining UV radiation with the same previous experimental conditions of the Fenton reaction, they obtained mineralization efficiencies of 78% and 32%, respectively. The results of the Fenton reactions performed by the authors corroborated the behavior obtained in the present work when studying the Fenton reaction individually. [PERSON] et al. [62] applied the ozonation process to study the mineralization of caffeine and obtained a maximum efficiency of 30.8%. The result obtained by the authors was very close to that previously obtained by [PERSON] et al. [47] when coupling UV radiation with the Fenton reaction. In other words, the ozonation process for caffeine is more efficient than the Fenton technique alone, but still requires further research focusing on maximizing the mineralization of the pollutant. On the other hand, [PERSON] et al. [57] studied the application of ultrasound (US) and UV radiation in combination with H\({}_{2}\)O\({}_{2}\) and TiO\({}_{2}\) according to the following four sequences: UV-H\({}_{2}\)O\({}_{2}\), US/UV-H\({}_{2}\)O\({}_{2}\), UV/TiO\({}_{2}\), and US/UV/TiO\({}_{2}\). The authors obtained mineralization efficiencies of 21.7%, 35.4%, 60%, and 66%, respectively.