## 3 Results and Discussion
### Identification of Photoproducts  
As mentioned above, all the studied GCs underwent a fast decomposition both in tap and river water. HPLC-ESI-MS/MS qualitative analysis revealed many photoproducts (see Supplementary Materials), and some trends can be identified based on the structure of the starting GCs. Six main reaction pathways can be described, involving either the hydroxyacetyl side chain or the steroid ring (see Scheme 4) and two of them, viz. (iv), cyclohexadienone isomerization, only available for those GCs containing a second conjugated double bond, and (v), the photooxidation of C17 side chain, have been already described as the main reaction paths in the direct photolysis of glucocorticoids ([23] and references therein). Nonetheless, the products coming from these two routes are now a minority among the identified degradation mechanisms, and we were able to characterize byproducts from path (iv) only for TRIAM (see Supplementary Materials), while the importance of path (v) is highly reduced with respect to direct photolysis, especially for those GCs having a second conjugated double bond on ring A. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Compound** & **CORT** & **PREDLO** & **TRIAM** \\ \hline
**Matrix** & & \(k_{deg}\) **(min\({}^{-1}\))** & \\ \hline tw & 0.35 (3) & 0.51 (1) & 0.355 (9) \\ tw + SO\({}_{4}\)\({}^{2-}\) (50 mg L\({}^{-1}\)) & 0.41 (3) & 0.539 (5) & 0.31 (1) \\ tw + NO\({}_{3}\)\({}^{-}\) (20 mg L\({}^{-1}\)) & 0.28 (1) & 0.45 (1) & 0.37 (1) \\ tw + HAs (10 mg L\({}^{-1}\)) & 0.10 (1) & 0.10 (1) & 0.08 (1) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Photodegradation constants (\(k_{deg}\)) determined in tap water under simulated sunlight in the presence of inorganic salts and humic acids (HAs). Among the remaining four main pathways, route (i) is a minority process only observed for CORT, HCORT, and PRED, and it comes from the formal reduction of the carbonyl at C17. The same is true for path (iii), which led to the reductive dehalogenation of DEXA. The latter reaction has been previously described for DEXA-phosphate photochemistry in the presence of a reducing agent [42]. Both path (i) and (ii) can be related to the free electrons generated on the catalyst surface upon irradiation reacting with adsorbed GCs at the catalyst surface. Nevertheless, most of the identified photoproducts are peculiar of the photocatalytic process, not produced under direct photolysis [23], and can be directly linked to OH radicals photo-generated from the irradiated TiO\({}_{2}\) catalyst. This powerful oxidant can then attack the steroid backbone either on C4=C5 double bond on ring A, path (vi), leading to double bond hydration followed by ring-opening, as already reported in the literature for PREDLO [29], then starting a cascade reaction (path vii, viii, and ix), ultimately yielding highly degraded products, or via OH insertion on the internal rings, (ii) producing hydroxylated byproducts [27]. The preference toward one or more of these degradation paths for the seven studied GCs can be rationalized by looking at their molecular structure, where the not-conjugated CORT and HCORT prefer to undergo path (ii) or (v), so reacting either at the C17 hydroxayectiy side chain or via OH insertion on the inner steroid rings, while the GCs bearing a second double bond on ring A shift the preferred OH attack to this site (path vi), resulting in a C5-OH pivotal intermediate which then further reacts (path vii, viii, and ix) forming several different oxidized byproducts. Nonetheless, path (ii) and (v) do not completely disappear but contribute to the overall photodegradation process to a lesser extent.