## 4 Conclusions  
The present study proves that TiO\({}_{2}\)-assisted photocatalysis effectively removes various GCs present in freshwaters at \(\upmu\)g per liter levels. Degradations occurred within 15 min for all GCs, and they were found to be from one to two orders of magnitude higher than direct photolysis, also in the case of CORT and HCORT that scarcely absorb solar light. The photoproducts formed during the first steps of the photocatalytic process, with a lifetime comparable to that of their parent compounds, were identified and allowed to elucidate the general mechanistic path. As indicated above, two main paths, different from those observed in the direct photolysis experiments, dominate the GCs photooxidation, involving either the unsaturated ketone moiety in ring A, especially for those GCs having a second conjugated double bond, or the C-H bonds on the steroid ring, with a limited number of isomers, which ultimately demonstrate that the process initiates from the TiO\({}_{2}\)-drug complex.  
In contrast to what was seen from photolytic experiments, we found that GCs photodegradation in the presence of TiO\({}_{2}\) proceeds slightly faster in tap water than in river water due to natural interferents such as HAs and inorganic anions more abundant in the latter. Nonetheless, TiO\({}_{2}\) photodegradation remained effective and faster than direct photolysis, also in the presence of matrix constituents, such as natural organic matter and salts, ubiquitous species in freshwaters. Furthermore, the reaction was shown to be efficient under natural solar light thanks to the photocatalyst ability to absorb in a wide range of wavelengths. The degradation mechanism is dominated by OH radicals generated on the catalyst surface, which then react with adsorbed GCs following different paths based on the particular chemical structure of each drug, while products from direct photolysis are still present but as a minority. Furthermore, even if the kinetic reaction constants are comparable across all the studied GCs, from an environmental point of view, it is important to underline that those compounds bearing a second conjugated double bond on ring A have access to a wider range of degradation paths, leading in the end to the formation of more degraded byproducts which mostly lost the pharmacologically active steroid ring.  
Once again, photocatalysis was shown to be the method of choice for water depollution. The mechanism may change somewhat, but the great stability of the titania crystals under irradiation, coupled with the good adsorption properties and catalytic activity, overcome any other method.  
The following are available online at [[https://www.mdpi.com/2076](https://www.mdpi.com/2076)]([https://www.mdpi.com/2076](https://www.mdpi.com/2076)) -3417/11/6/2486/s1, Table S1: Optimized MRM conditions for the HPLC-ESI-MS/MS analysis.  
Table S2: Fragmentation of photolytic products of CORT ([M + 1]\({}^{+}\) = 361), Table S3: Fragmentation of photolytic products of HCORT ([M + 1]\({}^{+}\) = 363), Table S4: Fragmentation of photolytic products of BETA ([M + 1]\({}^{+}\) = 393). Table S5: Fragmentation of photolytic products of DEXA ([M + 1]\({}^{+}\) = 393), Table S6: Fragmentation of photolytic products of PRED ([M + 1]\({}^{+}\) = 359), Table S7: Fragmentation of photolytic products of PREDLO ([M + 1]\({}^{+}\) = 361), Table S8: Fragmentation of photolytic products of TRIAM ([M + 1]\({}^{+}\) = 435).  
Conceptualization, M.S., A.-A., L.P. and F.M.; formal analysis, M.S.; investigation, A.C. and S.N.; writing--original draft preparation, M.S., L.P. and A.A.; writing--review and editing, M.S., L.P., F.M. and S.N.; visualization, M.S., L.P., A.C. and F.M. All authors have read and agreed to the published version of the manuscript.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author.