## 1 Introduction  
Caffeine is extensively used in the food, beverage, and pharmaceutical industries due to its stimulating effects [1; 2; 3; 4]. On account of its diverse applications and global consumption, caffeine stands as the most widely consumed psychoactive drug worldwide [5]. Consequently, it is frequently identified in various environmental matrices due to improper disposal practices. Since its presence in water is closely related to human activity, caffeine has been used as a chemical indicator of water quality in relation to the presence of pollutants, thus being associated with the sanitation conditions of a region [6; 7; 8]. Furthermore, caffeine is amidst the emerging contaminants (ECs) that present an environmental challenge [6; 8]. These contaminants are characterized by their resistance to conventional wastewater treatments, environmental persistence, potential for bioaccumulation, and toxic, mutagenic, and carcinogenic effects [8; 9; 10; 11; 12]. As a result, effective removal of caffeine from wastewater is essential to ensure the safe discharge of treated effluents into watercourses in order to protect human health and preserve ecosystems [13; 14]. Conventional wastewater treatment processes, such as biological [15] and physico-chemical [16; 17] methods, have been found to be ineffective in the removal of caffeine. Given that ECs exhibit high persistence in aqueous environments, advanced technologies are required in addition to conventional treatments for the complete removal of these pollutants [9]. Some of the techniques reported include bioremediation [18], membrane filtration [19], adsorption [20], and advanced oxidative processes (AOPs) [21]. Among these techniques, adsorption stands out for its ability to separate unwanted components in a solution by reducing their concentrations through mass transfer between the aqueous solution and the surface of the adsorbent [22; 23; 24; 25]. The process is simple and cost-effective, with low waste generation [26]. However, adsorption has some limitations such as saturation of the adsorbent, which requires its replacement over time [27]. To identify when the solid will be saturated, some factors must be considered, including the physicochemical properties of the solid, concentration of the adsorbent, and contact time between the contaminated water and the adsorbent material [23; 24; 28]. However, these disadvantages can be overcome through regeneration of the adsorbent, which makes the process more sustainable and generates less final solid waste [27]. In addition, GAC is a frequent choice for removing various pollutants in the environmental field due to its high surface area, porosity, and the wide range of functional groups on its surface [23; 25; 29], besides lowering the cost of the process. For better results, AOPs such as the Fenton reaction can be employed in combination with adsorption. The Fenton reaction involves the generation of hydroxyl radicals (\(\bullet\)OH), a highly reactive species, which can rapidly oxidize and break down the caffeine molecule, promoting the degradation of the pollutant in water [30; 31]. The combination of the Fenton reaction and adsorption allows for the degradation of the residual contaminant in the remaining solution while simultaneously promoting the regeneration of the saturated adsorbent through the degradation of the adsorbed organic molecules on the adsorption sites [32; 33; 34; 35]. Caffe degradation can generate persistent intermediates [36; 37]; therefore, combining it with adsorption allows the adsorbent to also adsorb the intermediate reaction products and thus reduce the concentration of oxidation byproducts in the solution [38]. The decrease in reaction intermediates in the solution is important because these molecules can be harmful, sometimes more so than the original molecule, and therefore, careful control of the reaction conditions is necessary [39; 40].