## 3 Results and Discussion
### Kinetic Degradation in Actual Samples
This reaction is initiated on the surface of the catalyst, but the generated reactive intermediates are susceptible to a cascade of oxidative processes, which may happen both at the catalyst surface and in the nearby solution where photoproduced OH radicals also play a role. Likewise, the decrease in the decomposition rate, observed when a fluorine atom is introduced on the GC scaffold, well fits with the proposed mechanistic scheme (Scheme 3). No significant difference in the order of reactivity was observed in the photocatalytic GCs degradation at mg per liter concentration; a quantitative GCs removal (\(\geq\)95%) under actual conditions (natural pH, untreated water sample, and simulated sunlight) occurred in 15 min for all GCs. The observed kinetic degradation constants (\(k_{deg}\)) calculated for each GC are shown in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**GC** & **AOP** & **Matrix** & \begin{tabular}{c} **Kinetic Constant, \% Removal**, \\ **Degradation Time** \\ \end{tabular} & **Ref** \\ \hline DEXA 2-50 mg L\({}^{-1}\) & \begin{tabular}{c} Gamma ray, \\ gamma ray with H\({}_{2}\)O\({}_{2}\) or Fenton \\ \end{tabular} & Pure water, pH 7.2 & \begin{tabular}{c} 5 \(\times\) 10\({}^{-4}\) - 4.7 \(\times\) 10\({}^{-3}\) Gy\({}^{-1}\), \\ 8 \(\times\) 10\({}^{-4}\) - 1.6 \(\times\) 10\({}^{-3}\) Gy\({}^{-1}\) \\ \end{tabular} & [35] \\ \hline PRED 50 mg L\({}^{-1}\) & \begin{tabular}{c} Electrochemical \\ oxidation process \\ 20 mA cm\({}^{-2}\) \\ \end{tabular} & \begin{tabular}{c} Pure water, pH 11, \\ Na\({}_{2}\)SO\({}_{4}\) 1 g L\({}^{-1}\) \\ \end{tabular} & \begin{tabular}{c} 0.1052 mg h\({}^{-1}\), 78\% removal \\ 4 h \\ \end{tabular} & [33] \\ \hline PRED acetate 5 mg L\({}^{-1}\) & \begin{tabular}{c} O\({}_{3}\) 50 mg min\({}^{-1}\) \\ \end{tabular} & Pure water, pH 3 & \begin{tabular}{c} 0.0595 min\({}^{-1}\), 90\% removal \\ 30 min \\ \end{tabular} & [32] \\ \hline PREDLO 3.6 mg L\({}^{-1}\) & UV/chlorine, 254 nm & \begin{tabular}{c} Phosphate buffer pH 7, \\ artificial fresh water \\ pH 6 \\ \end{tabular} & \begin{tabular}{c} 5.53 \(\times\) 10\({}^{-3}\) s\({}^{-1}\) \\ \end{tabular} & [31] \\ \hline PREDLO 100 mg L\({}^{-1}\) & UV-Fenton, 360 nm & Pure water, pH 3 & Quantitative removal 15 min & [30] \\ \hline DEXA 5 mg L\({}^{-1}\) & \
\begin{tabular}{c} ZrO\({}_{2}\) 1.5 g L\({}^{-1}\), 365 nm \\ WO\({}_{3}\) 0.5 g L\({}^{-1}\), \\ \end{tabular} & Pure water, pH 3 &
\begin{tabular}{c} ZrO\({}_{2}\) 0.0078 min\({}^{-1}\), 76\% \\ removal 180 min \\ WO\({}_{3}\) 0.0277 min\({}^{-1}\), 100\% \\ removal 80 min \\ \end{tabular} & [34] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Degradation constants (\(k_{deg}\)) for each GC (10 mg L\({}^{-1}\)) in tap water under simulated sunlight (see Paragraph 2.3 for the irradiation conditions) and comparison with other AOPs. These results are particularly informative for evaluating the validity of TiO\({}_{2}\) remediation in real matrices under UV-A or solar light compared with other advanced oxidation processes, such as ZnO [36] WO\({}_{3}\) and ZrO\({}_{2}\)[34] semiconductors, ozone [32], gamma irradiation [35], UV/chlorine process [31], photo-Fenton [30] and electrochemical oxidation process [33]. For example, ZnO has a low stability in aqueous solutions at natural pH [36]; WO\({}_{3}\) and ZrO\({}_{2}\) require strong acidic conditions to work [34]. Other AOPs, such radiolysis [35] or photo-Fenton [30], need the addition of a sacrificial oxidizing agent (H\({}_{2}\)O\({}_{2}\), S\({}_{2}\)O\({}_{8}\)\({}^{2-}\), etc.) to improve their photocatalytic performance; ozonization [32] and electrochemical oxidation process [33] require acidic and alkaline solutions, respectively, and UV/chlorine process [31] occurs under UV-C irradiation. None of the above-mentioned AOPs is competitive in terms of GCs degradation rate if compared to TiO\({}_{2}\) and, importantly, they have never been applied to GCs degradation at \(\upmu\) L\({}^{-1}\) levels in environmental conditions.