# Catalytic Advanced Oxidation Processes for Sulfamethoxazole Degradation
## 3 Results
### By-Products and Intermediates Analysis  
AMI is one of the first products of SMX degradation. In O\({}_{3}\)-based processes, the detection of AMI occurs during the first 30 min of reaction and, in most cases, AMI concentration is lower in the presence of CNT and Fe/CNT. However, in photocatalysis and photocatalysis with H\({}_{2}\)O\({}_{2}\), AMI was not degraded after 180 min of reaction (results not shown). BZQ was detected in most of the reactions, but in concentrations lower than the quantification limit (0.10 mg L\({}^{-1}\)); thus, it was not possible to determine the amount of BZQ released. Nevertheless, BZQ was quantified in photocatalysis in the presence of CNT and Fe/CNT, probably due to the low degradation kinetics which leads to a high BZQ concentration in solution. BZQ concentration was lower in photocatalysis than in photolysis, showing that the presence of catalysts led to a lower accumulation of this intermediary in solution (results not shown). Carboxylic acids formed are hard to degrade and usually remain in solution because they are recalcitrant compounds [16; 47]. The degradation of many pollutants by the different AOPs resulted in the presence of carboxylic acids. Maleic, fumaric, succininic, glyoxylic, oxalic, and formic acids were identified as end products of the oxidation of phenol in aqueous media by the electro-Fenton process [48]. Maleic, glyoxylic, succininic, and fumaric acids were the predominant carboxylic acids formed in the earlier stages of the treatment. Glyoxylic and succininic acids started to build in the first minutes, reaching a maximum concentration at 2 h. Glyoxylic and succininic acids disappeared, followed by the appearance of formic and oxalic acids as end products before total mineralization. During the degradation of salicylic acid by different electrochemical advanced oxidation processes, the formation of several intermediates was verified, including short-chain carboxylic acids [49]. In this study, it was suggested that \(\alpha\)-ketoglutaric and glycolic acids are formed by a different pathway of malic, maleic, fumaric, and tartronic acids. Glycolic acid further transformed into glyoxylic acid. All these acids independently oxidized to oxalic acid as the ultimate by-product, which directly mineralized to CO\({}_{2}\). The degradation of beta-blockers by electro-Fenton and solar photoelectro-Fenton treatments also confirmed the destruction of aromatic intermediates by hydroxyl radicals and, consequently, the formation of carboxylic acids [50]. The presence of oxalic and oxalic acids as end by-products was confirmed, contributing to the TOC in the treated final solution. In most of the reactions, the concentration of carboxylic acids was lower in the presence of a catalyst than in the non-catalytic processes. For instance, in photocatalytic ozonation with H\({}_{2}\)O\({}_{2}\), as shown in Figure 5d, the formation of OXM was always lower in the presence of any one catalyst than in the non-catalytic process, and in the presence of Fe/CNT and CNT, a total removal was observed. On the other hand, OXL concentration was higher in catalytic processes, probably due to the reaction mechanism which favored the formation of OXL during the degradation of SMX and primary intermediates. The identification and quantification of SO\({}_{4}\)\({}^{2-}\) and NH\({}_{4}\)\({}^{+}\) ions were carried out in the processes without H\({}_{2}\)O\({}_{2}\) due to the interference of Na\({}_{2}\)SO\({}_{3}\) in the detection method. Ion concentrations increased along time; however, in most cases, their concentration in the non-catalytic processes was higher than in the catalytic treatments. Probably, it occurred due to adsorption of ionic species on the catalyst surface or, in the case of N-based ions, gas phase transfer could occur and ions transformed into N\({}_{2}\) or NH\({}_{3}\) forms (results not shown) [51].