# Catalytic Advanced Oxidation Processes for Sulfamethoxazole Degradation
## 3 Results
### SMX and TOC Removal  
SMX and TOC removals achieved during catalytic ozonation, photocatalysis, and photocatalytic ozonation are presented in Figure 2.  
In ozonation catalyzed by CNT and Fe/CNT, SMX was removed faster in the early stages; however, after 10 min of reaction, SMX present in solution was removed entirely in all ozonation systems. Regarding the TOC removal, ozonation catalyzed by TiO\({}_{2}\) promotes a removal of 41%, which is lower than the amount obtained by single ozonation; TiO\({}_{2}\) is a photoactive catalyst and poorly reacts in the presence of O\({}_{3}\). The catalytic ozonation in the presence of Fe/CNT led to the highest mineralization level; 75% of removal after 3 h of reaction. Iron, like other transition metals, shows significant catalytic activity in ozonation, once it promotes the decomposition of dissolved O\({}_{3}\) in hydroxyl radicals, especially in acidic conditions, as shown in Equation (3). In this case, the presence of CNT avoids hydrolysis and iron precipitation, while iron promotes the increase of the number of active centers on the catalyst surface; thus, the formation of OH\({}^{\bullet}\) radicals increases and, consequently, the oxidation of intermediates formed during the reaction is promoted, leading to a higher mineralization [38; 39]. Additionally, O\({}_{3}\) adsorption and reactions on the surface of the CNT can occur, and generation of surface oxygen radicals is promoted, justifying the importance of the textural properties in these processes [3; 5; 26]. \[\mathrm{Fe^{3+}}+\mathrm{O_{3}}+\mathrm{H_{2}O}\rightarrow\mathrm{FeO}^{2+}+\mathrm{OH}^{\bullet}+\mathrm{O_{2}}+\mathrm{H^{+}} \tag{3}\]  
Photocatalysis in the presence of CNT and Fe/CNT ensured higher SMX removal in the early stages, but after 120 min of reaction no differences were observed and SMX present in solution, as  
Figure 1: TEM image from the Fe/CNT catalyst. shown in Figure 2\(c\), was degraded in all systems. On the other hand, a catalyst is required to achieve exceptional levels of mineralization, as can be seen in Figure 2d. The Fe/CNT led to the highest TOC removal reaching mineralization of 78%, while only 14% is removed by the non-catalytic process. The combination of CNT and TiO\({}_{2}\) increases the catalytic activity in terms of TOC removal, probably due to the formation of more oxidizing radicals. CNT material is a photosensitizer, has a reducer character, and high electronic conductivity; thus, it can increase the mobility of electrons and hence the formation of radicals is improved, increasing the degradation, especially of the intermediaries, and consequently more significant mineralization was attained. The results of SMX removal by catalytic oxidation with H\({}_{2}\)O\({}_{2}\) are depicted in Figure 3, together with the adsorption results on TiO\({}_{2}\) and CNT. Figure 2: Dimensionless SMX (**a**,**c**,**e**) and total organic carbon (TOC) (**b**,**d**,**f**) concentration during catalytic ozonation (O\({}_{3}\)), photocatalysis (L), and photocatalytic ozonation (L/O\({}_{3}\)) in the presence of CNT, Fe/CNT, TiO\({}_{2}\), and TiO\({}_{2}\)/CNT. SMX adsorption was not observed on TiO\({}_{2}\), which was expected since TiO\({}_{2}\) is highly stable and it has a small surface area and pore volume. [PERSON] et al. [40], [PERSON] et al. [41], and [PERSON] et al. [42] reported similar adsorption results to those obtained in this work. Moreover, adsorption on CNT removed 61% of SMX and reached a TOC removal of 52% after 3 h. The adsorption of SMX is more pronounced in the early stages of the contact time, remaining constant after the initial stage due to surface saturation. The high surface area and volume of pores of the CNT may justify the high adsorption values. [PERSON] et al. [5; 26] presented a similar profile; however, the adsorption degree obtained was lower (between 30% and 50%), probably due to the lower catalyst concentration (0.143 g L\({}^{-1}\)) used.