## 4 Discussion  
### Influence of pH of the Working Solution  
The pH of the aqueous solution containing the organic substrate to be degraded has an important effect on the degradation efficiency, both by modifying the activity of the TiO\({}_{2}\) photocatalyst and by favoring the hydroxyl radical formation reactions. It is considered that the pH value influences the position of the transition band between the conduction band and the valence band of TiO\({}_{2}\) which determines its amphoteric character. It has been determined that around a pH of 7, the catalyst surface is electrically neutral. Below and above this value, the surface of the TiO\({}_{2}\) photocatalyst is charged positively and negatively according to the following equilibria [21]:  
\[\text{TiO}^{+}_{2}\rightleftarrows\text{TiO}+\text{H}^{+} \tag{20}\]  
\[\text{TiO}\rightleftarrows\text{TiO}^{-}+\text{H}^{+} \tag{21}\]  
Chlorophenols and their intermediates formed during the photocatalytic degradation process are electronegatively charged compounds or are electrically neutral. This results in better adsorption of organic molecules on the catalyst surface and higher degradation efficiencies at acidic pH values of the solution. The overall photocatalytic reaction can be described by a reaction of the following form [31]:  
\[\text{TiO}_{2}\left(\text{photocatalyst}\right)+\text{OH}^{-}+2\text{H}^{+}+ \text{O}^{-}_{2}\rightarrow\text{3}\cdot\text{OH}+\text{TiO}_{2}\left( \text{photocatalyst}\right) \tag{22}\]  
The equilibrium constant (K\({}_{\text{e}}\)) for the above reaction has the following form:  
\[\text{K}_{\text{e}}=\frac{\left[\cdot\text{OH}\right]^{3}}{\left[\text{OH}^{- }\right]\left[\text{H}^{+}\right]^{2}\left[\cdot\text{O}_{2}^{-}\right]} \tag{23}\]  
Considering the ionic product of water:  
\[\left[\text{OH}^{-}\right]\left[\text{H}^{+}\right]=\text{K}_{\text{w}}=1 \times 10^{-14} \tag{24}\]  
Equation (23) can be written as follows:  
\[\left[\cdot\text{OH}\right]^{3}=\text{K}_{\text{e}}\text{K}_{\text{w}}\left[ \text{H}^{+}\right]\left[\cdot\text{O}_{2}^{-}\right] \tag{25}\]  
Figure 9: Influence of initial concentration of the organic substrate on its photocatalytic degradation (pH 3 of working solution; hydrogen peroxide/chlorophenol molar ratio N of 1); (**a,b**) photocatalytic degradation kinetics; (**c,d**) linear fitting of pseudo-first-order kinetics. From the relation presented above, it is observed that between the concentration of hydrogen ions [H\({}^{+}\)] and the concentration of hydroxyl radicals [-OH] there is a direct dependence. Thus, the more acidic the conditions under which the experiment takes place, the more hydroxyl radicals are produced that will degrade the organic substrate faster. This can be highlighted by comparing the kinetic parameters obtained for various pH values of the working solution. However, at pH less than 3, the efficiency of the process decreases due to the protonation effect of hydrogen peroxide with the formation of the oxonium ion (H\({}_{3}\)O\({}_{2}\)\({}^{+}\)), which prevents the decomposition of H\({}_{2}\)O\({}_{2}\) and thus the formation of -OH radicals. The aspects presented above are the basis for the identification of the mechanisms of photocatalytic degradation of 2,4-dichlorophenol. At least two of them could be confirmed by the results obtained in this work. The first one involves the formation of nascent hydrogen (-H) from protons (H\({}^{+}\)) adsorbed on the surface of the photocatalyst, which can successively displace chlorine from the benzene ring. Subsequently, the formed phenols can be attacked by hydroxyl radicals (-OH) which can open the benzene ring with the formation of aliphatic compounds. The second one involves a direct attack of the hydroxyl radicals which would lead to the displacement of chlorine from the para position with the formation of 2-chloro-4-hydroxyphenol.