# Catalytic Advanced Oxidation Processes for Sulfamethoxazole Degradation
## 1 Introduction
Also, the presence of CNT contributes to a rise in the surface area of the composite, thereby increasing the available active centers when compared with pure TiO\({}_{2}\)[19; 20]. In spite of photocatalysis being mainly used in organic pollutant degradation, removal of heavy metals was also investigated [21]. The sequence of two solar photocatalytic processes (TiO\({}_{2}\) photocatalysis in the presence of O\({}_{2}\) followed by the presence of N\({}_{2}\)) was proven to be efficient in the removal of copper, iron, and zinc presented in real soil washing effluents. The results indicate that wastewaters can be successfully treated under a natural sunlight source, although it requires more time to reach significant pollutant removal efficiencies than simulated irradiation. When photolysis, ozonation, and an appropriate catalyst are combined, the catalyst is photo-excited through radiation and O\({}_{3}\) molecules can adsorb on the catalyst surface, resulting in the production of active oxygen radicals (O\({}^{\bullet}\)). Another advantage is that the presence of O\({}_{3}\) decreases the recombination of the pair e\({}^{-}\)/h\({}^{+}\)[22]. Catalytic reactions involving hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) are mainly promoted by the addition of transition metals with high valence, such as Ti, V, Mo, W, and Re [23]. The formation of hydroxyl radicals by H\({}_{2}\)O\({}_{2}\) can also be promoted by Fenton reaction or photo-Fenton, whose catalytic process is based on electron transfer between a cationic metal and H\({}_{2}\)O\({}_{2}\). In this process, Fe\({}^{2+}\) acts as a catalyst and combined with H\({}_{2}\)O\({}_{2}\) under acid conditions produces reactive species, especially OH\({}^{\bullet}\) radicals. Furthermore, the photo-Fenton process presents improved efficiency due to the formation of additional amounts of OH\({}^{\bullet}\) radicals through photoreduction of the iron ions formed during the Fenton reaction, since the photoreduction leads to regeneration of these ions to complete the catalytic cycle [24]. This improvement was confirmed by [PERSON] et al. during the degradation of three pesticides (imidazol, methomyl, and formetanate) under solar radiation [25]. The homogenous photo-Fenton process revealed to be more efficient than photocatalysis in the presence of TiO\({}_{2}\) in terms of pollutant removal, mineralization degree, and toxicity reduction. This work aims to study the degradation of sulfamethoxazole (SMX) by catalytic advanced oxidation processes. SMX is a synthetic antibiotic of the sulfonamides group commonly used to treat respiratory and urinary diseases caused by bacteria [26]. Additionally, SMX is also used in agriculture as herbicide and in preventive care in veterinary medicine [27]. This antibiotic is found in waterbodies in small concentrations, but its presence in the environment can cause extensive damage to aquatic organisms, especially the emergence of resistant bacteria [28; 29]. There is no doubt about the occurrence of sulfamethoxazole (SMX) as a pollutant in several aquatic instances around Europe. According to [PERSON] et al., SMX was detected in Germany in the range 300-1500 ng L\({}^{-1}\) and 0.4 ug L\({}^{-1}\) in wastewater treatment plants, 30-85 ng L\({}^{-1}\) and 0.03 ug L\({}^{-1}\) in surface waters, and 410 ng L\({}^{-1}\) in underground waters [30]. [PERSON] et al. reported the presence of SMX in drinking waters (approximately 22 ng L\({}^{-1}\)) in the Netherlands and USA [31]. In Portugal, the presence of SMX was also confirmed. According to the report performed by [PERSON] et al., SMX was detected in concentrations around 53.3 ng L\({}^{-1}\) in the Douro river, 0.27 ng L\({}^{-1}\) in drinking waters, and 8.0 ng L\({}^{-1}\) at the sampling point in the Tagus river [32]. As SMX is a persistent contaminant, it is not easily degraded by conventional treatment processes, requiring the application of advanced techniques.