## 3 Results and Discussions
### Degradation of Amoxicillin by Solar Photocatalysis  
For solar photocatalysis with a 0.1 m\({}^{2}\) FPR surface, the analysis showed, for the best-observed experiment, a significant AMX degradation when working with a pH 4.15, without the addition of H\({}_{2}\)O\({}_{2}\), obtaining a percentage of 27% of COD removal, 60 min after  
Figure 5: Band-gap calculation, according to the absorbance spectrum of TiO\({}_{2}\) P25, by means of the graphic representation [F(R)\(\hbar\)\(\
u\)]\({}^{1/2}\) against \(\hbar\)\(\
u\) of the Kubelka–Munk method. the start of the reaction, and showing, besides, under these same experimental conditions, a 46.9 % decrease in COD, in an extended reaction time of 80 min. In this case, due to the acidic pH of the solution being close to the pH level of maximum stability of the drug, which is around 5 [48; 50], there is the possibility of a better performance in the degradation of this drug with the use of a catalyst in the system. At pH 3.5, 7, and 9, no degradation was observed on this surface (0.1 m\({}^{2}\)). [PERSON] and [PERSON] in 2020 [13], explain that at a lower magnitude of pH (very acidic), the adsorption of AMX with TiO\({}_{2}\) is affected by the positive charges on their surfaces, where the point of zero charges of TiO\({}_{2}\) (p\(\simeq\) 6.4), as well as that of amoxicillin, become positive in acidic media, repelling each other. In the case of the removal of AMX with the addition of H\({}_{2}\)O\({}_{2}\), at these pH values (3.5, 7, and 9), no decrease in the total or partial removal of the contaminant is observed. The range of solar radiation received during the experiments was close to 1000 W/m\({}^{2}\), the wavelength (\(\lambda<\) 387 nm) was in the UV-A region of the electromagnetic spectrum, and the band-gap energy of the photocatalyst (3.2 eV), provided the necessary activation energy of the photocatalyst (TiO\({}_{2}\) P25); however, due to the size of the reactor surface (0.1 m\({}^{2}\)), the best percentage of COD removal obtained was only 46.9%, as already mentioned. [PERSON] et al. in 2012 [51], determined that the speed of the degradation reactions given by the solar photocatalysis process is observed as a set of different operational parameters, such as the intensity of the solar radiation received during the experiments, the concentration of the contaminant and by the reaction surface and the concentration of the catalyst used. Therefore, experiments were carried out using a FPR with a surface area of 1 m\({}^{2}\), to find out if this factor influenced the degradation of amoxicillin, under the same conditions as in the degradation reaction of the previous experiments. Figure 6 shows the results obtained by COD for the degradation of AMX by solar photocatalysis, using TiO\({}_{2}\) P25 as the photocatalyst, on a 1 m\({}^{2}\) surface FPR under the reaction conditions mentioned in Section 2.3. The results (Figure 6), are similar to what was observed in the experiments carried out with a 0.1 m\({}^{2}\) FPR for solar photocatalysis in that the only reaction conditions that showed significant degradation of AMX were at pH 4.15, not having observed any degradation at pH 3.5, 7 and 9. A possible explanation for such results is AMX speciation: when in aqueous solution, AMX can present different forms according to the solution pH; at pH 4.15, a zwitterionic form is mainly present [52]. This being said, better results were to be expected at this pH, as the AMX molecule is negatively charged while TiO\({}_{2}\), is positively  
Figure 6: COD/COD\({}_{0}\) analysis with respect to time for the degradation of AMX by solar photocatalysis, using TiO\({}_{2}\) P25 as photocatalyst, on 1 m\({}^{2}\) surface FPR, at pH 4.15; the green curve, with the addition of H\({}_{2}\)O\({}_{2}\) (4 mM), and the curve in blue, without the addition of H\({}_{2}\)O\({}_{2}\). charged as the pH is below its isoelectric point, so attraction between AMX and catalyst molecules facilitates interaction ad thus, degradation.