## 3 Results and Discussion
### Combined Process
#### 3.3.3 Regeneration Efficiency of Granular Activated Carbon  
The regeneration capacity of GAC was evaluated by analyzing caffeine removal in the adsorption stages of the four CPs conducted in triplicate. The same GAC was used, undergoing filtration and drying after the Fenton reactions. Figure 8 shows the adsorption efficiency and final caffeine concentration in each combined process, with average error bars indicated for each response function.  
Figure 8 reveals consistently high efficiencies (>89%) in caffeine removal by activated carbon during the four adsorption cycles, resulting in low final caffeine concentrations (<2.1 mg L\({}^{-1}\)) in the solution. In the first adsorption step (first combined process, R\({}_{0}\)), the removal was approximately 97.5%, reducing to ~97.1% (second combined process) after the first regeneration. The second regeneration (second combined process) resulted in a reduction of only 7.8%, reaching an average of ~89.5% (third combined process). After the third regeneration, the solid adsorbent was able to adsorb ~91.5% of the 20 mg L\({}^{-1}\) caffeine solution in the fourth combined process, an increase of 2% compared to the previous adsorption step. The adsorbent regeneration efficiency (E\({}_{\text{R}}\)) was calculated using the adsorption of the first combined process (97.5%) as reference (R\({}_{0}\)) and the values of the  
three subsequent processes as variables (R\({}_{1}\), R\({}_{2}\), and R\({}_{3}\)). Figure 9 illustrates the regeneration efficiencies for each cycle, with average error bars indicated for each curve.  
Figure 9 shows the effectiveness of the combined process in the regeneration of GAC over three experimental cycles, reaching regeneration percentages of 99.6%, 91.8%, and 93.8%. The efficient regeneration is attributed to the ability of the Fenton reaction to degrade organic molecules on the GAC surface, keeping its textural characteristics unchanged. Studies in the literature, including [PERSON] et al. [35], highlight the effectiveness of the oxidative mechanism of the Fenton reaction in the regeneration of GAC without significant structural changes, corroborating the results obtained in this work. The authors investigated the GAC regeneration in the adsorption of a model organic pollutant (methylene blue) using the Fenton reaction. The results revealed a maximum regeneration efficiency of 71% under the established conditions, which included a reaction period of 2 h, as well as concentrations of 500 mmol L\({}^{-1}\) of H\({}_{2}\)O\({}_{2}\) and 0.5 mmol L\({}^{-1}\) of Fe\({}^{+2}\). In the present study, a higher maximum regeneration efficiency (99.6%) was achieved compared to the study by [PERSON] et al. [35] with a shorter Fenton reaction time (1 h), a 57.5% lower Fe\({}^{+2}\) concentration (0.04 mmol L\({}^{-1}\)), and no significant alteration in pore diameter. The concentration of  
Figure 8: Caffeine removal efficiency on GAC in adsorption stage for four combined processes carried out in sequence.  
Figure 9: Regeneration efficiencies for each cycle of CP.  
H\({}_{2}\)O\({}_{2}\) was 574 mmol L\({}^{-1}\), similar to that found by the authors. Although the pollutants compared here are different, the efficiency and potential of using the Fenton reaction as a regenerant for caffeine-saturated activated carbon are demonstrated by the high efficiency, short reaction time, and low reagent concentrations. An analysis of the works of [PERSON] et al. [33], [PERSON] et al. [34], and [PERSON] et al. [32; 63] reveals that the process proposed in this study achieves higher regeneration efficiencies in a shorter time, with low concentrations of reagents, highlighting the efficiency of the Fenton reaction in the regeneration of GAC saturated with caffeine.