## 3 Results and Discussion
### Matrix Effects: Salts and Humic Acids  
As opposed to photolysis, GCs TiO\({}_{2}\)-mediated photodegradation took place slightly slower in river samples than in tap water samples. It was faster under natural sunlight in river water than simulated light (see Table 2), proving the process efficiency despite changeable environmental factors, such as incident power [39]. Despite the similar qualitative composition of the two considered matrices (see Table 1), a decrease in \(k_{deg}\) was observed for all GCs in river water samples than in tap water samples under the same experimental conditions (see Table 2). We attempted to evaluate the potential matrix contribution to this effect, although, in general, this is a challenging task, especially in real matrices where the many species present may interact with each other and also with several other xenobiotics [40]. Among the various macro-constituents, nitrate and sulfate were investigated because they are more abundant than others in the considered river water sample (see Table 1) and, at the same time, are known to affect drug photodegradation [40], including GCs. Indeed, some previous works reported the influence of SO\({}_{4}\)\({}^{2-}\) and NO\({}_{3}\)\({}^{-}\) on the AOPs degradation rate of GCs. In particular, SO\({}_{4}\)\({}^{2-}\) was found to reduce the photodegradation rate of BETA [36], PREDLO [31], and PRED [32], acting directly as hydroxyl radical scavenger to form the respective anion radicals; NO\({}_{3}\)\({}^{-}\) was reported to slow down the decomposition rate of DEXA decreasing OH radical concentration indirectly via NO\({}_{2}\)\({}^{-}\)[35]. Other important interferents naturally present in river water are humic acids (HAs); these ubiquitous \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 \begin{tabular}{c} BETA \\ phosphate 30 mg L\({}^{-1}\) \\ \end{tabular} & \begin{tabular}{c} ZnO 0.44 g L\({}^{-1}\), 254 nm \\ ZnO 0.44 g L\({}^{-1}\), \\ persulfate, 254 nm \\ \end{tabular} & Pure water, pH 9 & \begin{tabular}{c} Removal 63\%, 180 min \\ Removal 98\%, 180 min \\ \end{tabular} & [36] \\ \hline PREDLO 25 mg L\({}^{-1}\) & \begin{tabular}{c} TiO\({}_{2}\) P25 1 g L\({}^{-1}\), 365 \\ nm or solar light \\ \end{tabular} & Pure water, pH 6.7 & \begin{tabular}{c} Removal 94\%, 1 h solar light \\ Removal 73\%, 1 h 365 nm \\ \end{tabular} & [29] \\ \hline \begin{tabular}{c} CORT acetate 10 mg \\ L\({}^{-1}\) \\ \end{tabular} & \begin{tabular}{c} TiO\({}_{2}\) P25, 375 nm, air \\ \end{tabular} & \begin{tabular}{c} Different buffer \\ solution, air saturated \\ persulfate air saturated \\ \end{tabular} & \begin{tabular}{c} 0.040 min\({}^{-1}\), quantitative \\ removal 100 min, 0.071 min\({}^{-1}\), \\ quantitative removal 30 min \\ \end{tabular} & [28] \\ \hline DEXA 10 mg L\({}^{-1}\) & \begin{tabular}{c} TiO\({}_{2}\) P25 0.2 g L\({}^{-1}\), \\ simulated solar light \\ \end{tabular} & \begin{tabular}{c} Pure water, air \\ saturated \\ \end{tabular} & Quantitative removal 15 min & [27] \\ \hline CORT, HCORT, BETA, & \begin{tabular}{c} \\ DEXA, PRED, \\ PREDLO, \\ TRIAM \\ \end{tabular} & \
\begin{tabular}{c} TiO\({}_{2}\) P25 0.5 g L\({}^{-1}\), \\ simulated solar light \\ \end{tabular} & Tap water, natural pH &
\begin{tabular}{c} 0.184(5), 0.230(7), 0.19(1), \\ 0.206(5), 0.177(7), 0.224(3), \\ 0.24(1) min\({}^{-1}\) \\ Removal \textgreater{}95\% 15 min \\ \end{tabular} & [This work] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._species can act both as photo-sensitizers or compete in absorbing solar light [40; 41]. Recent studies examined the inhibition effect of natural organic matter (NOM) on the degradation rate of PREDLO [31] and showed that increasing the NOM concentration, \(k_{deg}\) decreased. Based on the \(k_{deg}\) values reported in Table 2 and on the GCs structures, three model GCs, viz.