*2.2. Photocatalytic Degradation*

Before proceeding to the CFX and LFX photodegradation, several preliminary studies were carried out to establish the optimal reaction conditions. To do this, a study of the optimal pH was initially carried out, and it was established that the most appropriate pH for both antibiotics was to use solutions at pH = 7, which guaranteed the maximum solubility of both contaminants in water. Another parameter that was evaluated was the initial antibiotic concentration. For this, the three catalytic systems studied were used, using, in all cases, the highest concentration of Au (10%). The results for CFX and LFX are shown in Figure S1. As can be seen, there is a clear relationship between the percentage of degradation and the initial concentration of the antibiotic. In the case of CFX (Figure S1a), the highest activity in the three catalytic systems studied was obtained at a concentration of 20 mM. In the case of LFX (see Figure S1b), the optimal concentration was half (10 mM), possibly because LFX is slightly less soluble in water than CFX. Another of the preliminary studies that were carried out established the optimal amount of catalyst in the reaction medium. As can be seen in Figure S2, there is a clear correlation between the amount of catalyst and the percentage of degradation. For both antibiotics, a sustained increase in the efficiency of the process was observed, until reaching a load of 1.1 gL−<sup>1</sup> in CFX (Figure S2a), and 1 gL−<sup>1</sup> in LFX (Figure S2b). From these values, a drop in efficiency was observed, which has been justified on the basis of possible radiation scattering processes that potentially occur from a certain catalyst load. Taking into account the above considerations, the optimal reaction conditions for each antibiotic were established. In the case of CFX, the optimized conditions were: pH = 7, [CFX]<sup>i</sup> = 20 mM, and catalyst loading of 1.1 gL−<sup>1</sup> , while for LFX, the reaction conditions were: pH = 7, [CFX]<sup>i</sup> = 10 mM, and catalyst loading of 1 gL−<sup>1</sup> .

Figures 7 and 8 show the results of photodegradation of CFX and LFX, respectively, for each of the catalysts studied. The study was carried out for a total time of 180 min and, for both contaminants, the behavior was very different depending on the catalyst. In the case of CFX (Figure 7), it was observed that after 3 h, the degradation ranged between 73% and 99%. The highest efficiency was obtained with 10%Au@ZnONPs-3%rGO-3%gC3N4, while the lowest degradation was obtained with 1%Au@ZnONPs. In this last catalyst, the percentage of Au on the surface clearly affects the reaction, facilitating absorption and formation of electron-hole pairs, showing greater efficiency with 5%Au@ZnONPs, and a maximum efficiency with 10%Au@ZnONPs. In the case of LFX, the behavior is similar, showing the highest degradation with 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> (96%), and the lowest with 1%Au@ZnONPs (51%). As seen with CFX, the addition of a higher gold load greatly increased the efficiency of the process. For both CFX and LFX, the incorporation of rGO considerably improved the efficiency of photodegradation, which was maximized with the addition of gC3N4, especially in catalysts with 10%Au. Another observation to consider when comparing the degradation profiles of CFX and LFX is the different rate at which the process is carried out. The degradation of CFX is certainly faster than that of LFX, indicating that CFX is more recalcitrant to degradation. The greater degradation observed when incorporating rGO and, additionally, gC3N<sup>4</sup> is due to a synergistic effect of improvement of the electrical and conductive properties of the material, and to an increase in absorption in the visible region, as evidenced by the determination of the bandgap (see Figure 5).

In order to investigate the kinetic behavior of the photodegradation of CFX and LFX, the pseudo-first-order kinetics were studied by representing −ln (C/C0) vs. the irradiation time (see Figure S3). The results obtained are shown in Table S1, indicating that the apparent rate constant of the catalysts increases with an increasing amount of Au on the surface, and when incorporating rGO and gC3N4. The correlation coefficients R<sup>2</sup> (see Table S1) are higher for the linear fits of LFX than CFX although, as derived from the results obtained, for both contaminants the degradation clearly follows pseudo-first-order kinetics. This behavior can also be correlated with the BET surface area of these catalysts. Thus, as shown in Table 1, the materials that show the highest BET area values are those that have better catalytic behavior, with higher apparent rate constants.

**Figure 7.** Photodegradation rate of CFX as a function of time, using the different catalysts evaluated.

**Figure 8.** Photodegradation rate of LFX as a function of time, using the different catalysts evaluated.

To further study the photocatalytic activity of 10%Au@ZnONPs-3%rGO-3%gC3N4, control experiments and recyclability tests were carried out. The control experiments for both contaminants were similar (Figure S4). After 3 h of reaction, it was found that CFX and LFX were highly recalcitrant. Under photolysis conditions, that is, in the absence of the catalyst with the rest of the components present, a very slight reduction in the concentration of both CFX and LFX was observed, which indicates that radiation absorption is the only pathway responsible for this change and, as seen in Figure S4, it is clearly inefficient. In the absence of oxygen, a greater degradation of CFX than LFX was observed but, in both cases, the percentage of degradation after 3 h of reaction is not relevant. From control experiments, photocatalysis is shown to be the primary degradation route, indicating that catalysis, or even photolysis, is totally insufficient to degrade antibiotics. The results of recyclability of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> for the degradation of CFX and LFX during five cycles are shown in Figure S5. The catalyst was recovered after each cycle by centrifugation (3000 rpm for 20 min). After the supernatant was decanted, the catalyst was washed three times with deionized water and once with ethanol (using centrifugation–sonication cycles), and dried for at least 5 h at 60 ◦C. The parameters used for the degradation of each antibiotic were set to optimal values, as previously described. In CFX (Figure S5a) it was found that the degradation remained practically constant after each cycle, going from 99% after the first cycle to 97% after the fifth cycle. In the case of LFX (Figure S5b), the recyclability was not as good, going from a degradation of 97% in the first cycle to 90% after the fifth cycle. In both cases (CFX and LFX) the photocatalyst recovered after the fifth cycle was analyzed by XRD (not shown) showing a diffraction pattern without notable differences with respect to the XRD of the catalyst before use.
