# Catalytic Advanced Oxidation Processes for Sulfamethoxazole Degradation
## 1 Introduction  
Advanced oxidation processes (AOPs) are based on the oxidation of organic pollutants by strong oxidizing species, notably hydroxyl radicals (HO\({}^{\blacktriangleright}\)), which react rapidly and indiscriminately with several organic compounds. Their main objective is to mineralize the pollutants into carbon dioxide and water, or convert them into by-products, sometimes less harmful to human health and the aquatic environment [1]. These processes have been shown as an effective alternative to the removal of persistent compounds, such as hormones, drugs, and pesticides. In the presence of catalysts, it is expected that the oxidation capacity of the processes will increase and, consequently, the degradation of the main pollutant and intermediates would be easily attained, leading to higher levels of mineralization. Catalytic reactions can be homogeneous or heterogeneous, wherein solution and catalyst are in the same phase, or in distinct phases, respectively. Carbon-based catalysts are widely studied and presented good results in catalytic ozonation reactions [2; 3; 4; 5; 6; 7]. There are different suggestions of reaction mechanisms for ozonation catalyzed by carbon materials. Some studies have suggested that carbon material acts as the initiator of Odecomposition, producing H\({}_{2}\)O\({}_{2}\) and, consequently OH\({}^{\bullet}\) in solution [2; 5; 8; 9]. On the other hand, some researchers suggested that O\({}_{3}\) adsorption and reaction on the carbon surface led to the formation of free radicals which can react with organic species [3; 5; 10]. Furthermore, OH\({}^{\bullet}\) present in solution and those on the surface of the material may contribute to the mineralization of the compounds [11]. The efficiency of the catalytic ozonation process depends on the chemical and textural properties of the catalyst, as well as on the temperature and pH of the solution. Usually, the higher surface area of the carbon material leads to better O\({}_{3}\) decomposition [11]. Furthermore, delocalized \(\pi\) electrons on the carbon nanotube (CNT) surface are known to be active sites for the decomposition of O\({}_{3}\) and radical formation [4]. On the other hand, acidic CNTs present lower catalytic activity, because the electron density is more moderate, resulting in fewer electrons on the surface to react with O\({}_{3}\)--despite exhibiting high content of oxygenated surface groups and delocalized \(\pi\) electrons [4]. The decomposition of O\({}_{3}\) in water also strongly depends on the pH of the solution, because the pH increase enhances the decomposition of O\({}_{3}\) and, also, the surface properties of the catalysts can be modified [12]. Photocatalytic reactions arise from the combination of photolysis with a catalyst, in this case, named as photo-catalyst. The photo-catalyst should be photoactive, capable of responding to visible and/or UV light, biologically and chemically inert, photo-stable, and non-toxic [13; 14]. Photocatalysts are typically materials that have a band gap energy between the valence band (VB) and conduction band (BC). However, in some conditions, the electrons are promoted from VB to BC generating an electron-hole pair (e\({}^{-}\)/h\({}^{+}\)), thus presenting electrical conductivity, and can act as an electron donor or acceptor [15; 16]. The most used photo-catalyst is TiO\({}_{2}\) because it is not toxic, chemically stable, and inexpensive [15]. Composites with TiO\({}_{2}\) and CNT are an interesting alternative in photocatalytic reactions since it is expected that the use of TiO\({}_{2}\)/CNT composites reduces the recombination of the pair e\({}^{-}\)/h\({}^{+}\), because the excited electron of the conduction band of TiO\({}_{2}\) can migrate to the CNT [17; 18]. Another aspect is that the presence of Ti-O-C linkages increases light absorption, leading to an increase in the photocatalytic activity.