**4. Conclusions**

The photocatalytic degradation of two quinolone-type antibiotics (CFX and LFX) in aqueous solution was studied, using catalysts based on ZnONPs, which were synthesized by means of a thermal procedure. Subsequently, the efficiency of ZnONPs was optimized by incorporating different cocatalysts of gold, rGO, and gC3N4, obtaining a total of nine different catalysts that were used in the photodegradation reaction of CFX and LFX. The most efficient catalyst was 10%Au@ZnONPs-3%rGO-3%gC3N4, allowing degradations of both pollutants above 96%. This catalyst has the largest specific area, and its activity has been related to a synergistic effect, involving factors as relevant as the surface of the material and the ability to absorb radiation in the visible region, mainly produced by the incorporation of rGO and gC3N<sup>4</sup> to the semiconductor. The use of different scavengers during the catalytic process, together with the determination of bandgaps of the different components of the photocatalyst, has made it possible to establish a possible photodegradation mechanism of CFX and LFX in which superoxide radicals (·O2−) are the main reactive species involved in the process. The results obtained are certainly relevant because, in less than 3 h, almost complete photodegradation of CFX and LFX occurs, with conversions above 96%. Some catalysts based on TiO<sup>2</sup> doped with boron have shown high degradation percentages (ca. 88%) [68]. In other cases, copper tungstate (CuWO4) catalysts doped with graphene have made it possible to obtain CFX degradations of ca. 97% [69]. Other

catalysts based on ZnO doped with silver allowed degradation to 99% of CFX [49], although preliminary studies showed many difficulties in photodegrading LFX. Considering the results published so far, the catalyst developed in this research is cost effective, easy to synthesize, and highly effective to for the degradation of both CFX and LFX, opening up a wide field of possibilities in environmental decontamination processes. In addition, the developed catalyst could have relevant uses for facing environmental problems generated by other pollutants, from an applied and global point of view.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/catal12020166/s1, Figure S1: Evaluation of the initial concentration of (a) CFX and (b) LFX on the catalytic efficiency of 10%Au@ZnONPs, 10%Au@ZnONPs-3%rGO, and 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> in the photodegradation reaction; Figure S2: Evaluation of the initial concentration of 10%Au@ZnONPs, 10%Au@ZnONPs-3%rGO, and 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> on the efficiency of the photodegradation reaction of (a) CFX and (b) LFX; Figure S3: Pseudo-first order kinetics of photodegradation of (a) CFX and (b) LFX using different catalysts; Figure S4: Control experiments for 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> with (a) CFX and (b) LFX, under visible radiation; Figure S5: Recyclability of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> after five consecutive catalytic cycles of photodegradation of (a) CFX and (b) LFX under visible radiation; Figure S6: Photocatalytic activity of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> on the degradation of (a) CFX and (b) LFX in the presence of various scavengers under visible radiation; Table S1: The pseudo-first-order kinetics constants for the photodegradation of CFX and LFX.

**Author Contributions:** Conceptualization, A.M., F.M.; methodology, F.M., A.M.; formal analysis, F.M., M.C.C., J.D.; investigation, A.M., K.F., C.M.; resources, F.M., C.M., F.I.P.; writing—original draft preparation, F.M.; writing—review and editing, F.M., A.M., K.F., C.M., M.C.C., J.D.; supervision, A.M., F.M.; project administration, A.M., F.M.; funding acquisition, A.M., F.M., M.C.C., J.D., C.M., F.I.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** Financial support from the NSF Center for the Advancement of Wearable Technologies-CAWT (Grant 1849243), and from the framework of the UE M-ERA.NET 2018 program under the StressLIC Project (Grant PCI2019-103594) are gratefully acknowledged.

**Data Availability Statement:** The data is contained in the article and is available from the corresponding authors on reasonable request.

**Acknowledgments:** The authors thank Raúl S. García for his arduous technical help in the procedures for the synthesis and characterization of materials. Technical assistance of I. Poveda from "Servicio Interdepartamental de Investigacion, SIdI" at Autonomous University of Madrid (Spain), is gratefully acknowledged. The facilities provided by the National Center for Electron Microscopy at Complutense University of Madrid (Spain) is gratefully acknowledged. K.F. thanks PR NASA Space Grant Consortium for a graduate fellowship (#80NSSC20M0052).

**Conflicts of Interest:** The authors declare no conflict of interest.
