## 3 Results and Discussion
### Kinetic Degradation in Actual Samples  
The degradation profiles of the investigated GCs (50 \(\upmu\)g L\({}^{-1}\)) in tap (a) and river (b) water samples under simulated solar light are shown in Figure 1.  
A pseudo-first-order equation (see Equation (1)) satisfactorily fitted all of the experimental data:  
\[\frac{C_{t}}{C_{0}}=\ e^{-k_{deg}t} \tag{1}\]  
\(C_{0}\) is the initial GC concentration, \(C_{t}\) is GC concentration at time \(t\), and \(k_{deg}\) is the kinetic degradation constant. Table 2 shows the photolytic and photocatalytic degradation constants observed under the different experimental conditions described above. The most relevant result is that GCs direct photolysis occurred slowly, particularly for those GCs that scarcely absorb solar light (CORT and HCORT), with no significant difference between tap and river waters. On the contrary, TiO\({}_{2}\)-assisted photocatalysis appeared to be independent of the chemical structure of the GCs. It was much faster than the direct photolysis, one order of magnitude for the less persistent studied GCs (PRED and PREDLO) and up to two orders of magnitude for the more persistent ones (CORT and HCORT). Both observations are well in accord with the drug initial adsorption on the TiO\({}_{2}\) granule surface, which is known to happen mostly through the carbonyl function (see Scheme 3), with no considerable effect on the rest of the molecule. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Photolysis**} & \multicolumn{2}{c}{**Photocatalysis**} \\  & \multicolumn{2}{c}{**Simulated Solar Light**} & \multicolumn{2}{c}{**Simulated Solar Light**} & \multicolumn{2}{c}{**Natural Sunlight**} \\ \hline \multirow{2}{*}{**Compound**} & **Tap Water** & **River Water** & **Tap Water** & **River Water** & **River Water** \\  & \(k_{deg}\) **(min\({}^{-1}\))** & \(k_{deg}\) **(min\({}^{-1}\))** & \(k_{deg}\) **(min\({}^{-1}\))** & \(k_{deg}\) **(min\({}^{-1}\))** & \(k_{deg}\) **(min\({}^{-1}\))** \\ \hline CORT & 0.00106(5) & 0.00128(3) & 0.35(3) & 0.300(8) & 0.39(1) \\ HCORT & 0.00246(9) & 0.0033(2) & 0.38(1) & 0.315(8) & 0.38(2) \\ BETA & 0.0115(6) & 0.0114(1) & 0.336(9) & 0.32(1) & 0.35(2) \\ DEXA & 0.0117(1) & 0.0097(6) & 0.386(9) & 0.314(7) & 0.61(1) \\ PRED & 0.0185(9) & 0.0186(6) & 0.431(9) & 0.33(1) & 0.474(9) \\ PREDLO & 0.024(1) & 0.0199(4) & 0.51(1) & 0.336(4) & 0.52(1) \\ TRIAM & 0.0139(3) & 0.0099(4) & 0.355(9) & 0.29(1) & 0.38(1) \\ \hline \hline \end{tabular}  
* In brackets, the standard deviation values (sd), R\({}^{2}\) values in the range 0.992–0.999.  
\end{table} 
Table 2: Observed GCs degradation constants (\(k_{deg}\)) (50 ug L\({}^{-1}\) GCs, 0.5 g L\({}^{-1}\) TiO\({}_{2}\), tap and river water, simulated, and natural sunlight). Figure 1: Evolution profiles of each GC (50 ug L\({}^{-1}\)) in tap (**a**) and river water (**b**) under simulated sunlight: CORT (\(\Diamond\)), HCORT (\(\ast\)), BETA ( \(\times\) ), DEXA (\(\triangle\)), PRED (\(\square\)), PREDLO (\(+\)), and TRIAM (\(\Circuit\)) (see Section 2.3 for the irradiation conditions). **Scheme 3. ** Initial adsorption of the drugs on the TiO\({}_{2}\) granule surface through the carbonyl function. As it is generally assumed, within TiO\({}_{2}\) crystals the titanium nuclei are bound to six oxygen anions in a bipyramid. However, the outermost titanium ions are not fully compensated by the oxygen charge and bear some positive charge. Complexation then involves a Ti\({}^{3+}\) ion on the surface of the TiO\({}_{2}\) granule and the n orbital in the carbonyl function, as it happens in the acid catalysis of the addition to \(\alpha\), \(\beta\)-carbonyl onto the \(\beta\) (a) or carbonyl (b) functions [37; 38]. As previously reported in the literature [29], GCs degradation is then expected to proceed mainly via the photocatalytic oxidation of the adsorbed molecules on the catalyst surface.