## 3 Results and Discussion
### Influence of Ph
This can be due to the more selective and less reactive nature of SO4\({}^{\bullet-}\) radicals compared to hydroxyl radicals HO\({}^{\bullet}\). Moreover, at pH < 7, SO4\({}^{\bullet-}\) radicals are consumed by S\({}_{2}\)O8\({}^{2-}\) to form SO4\({}^{2-}\) and S\({}_{2}\)O8\({}^{\bullet-}\), as shown in (Equation (9)). Thus, SO4\({}^{\bullet-}\) anions easily degrade TA but cannot complete its mineralization under the acidic conditions. Also, along with radical SO4\({}^{\bullet-}\), there are several other sulfur intermediates such as HS\({}_{2}\)O8\({}^{-}\), HSO4\({}^{-}\), SO4, and H\({}_{2}\)SO5, which are generated and more dominant in the reaction medium according to the following equations (Equations (10)-(13)) [56]:  
\[\mathrm{SO_{4}}^{\bullet-}+\mathrm{S_{2}O_{8}}^{2-}\rightarrow\mathrm{SO_{4} }^{2-}+\mathrm{S_{2}O_{8}}^{\bullet-} \tag{9}\]  
\[\mathrm{S_{2}O_{8}}^{2-}+\mathrm{H^{+}}\rightarrow\mathrm{HS_{2}O_{8}}^{-} \tag{10}\]  
\[\mathrm{HS_{2}O_{8}}^{-}\rightarrow\mathrm{SO_{4}}^{\bullet-}+\mathrm{SO_{4} }^{2-}+\mathrm{H^{+}} \tag{11}\]  
\[\mathrm{S_{2}O_{8}}^{2-}+\mathrm{H^{+}}\rightarrow\mathrm{HSO_{4}}^{-}+ \mathrm{SO_{4}} \tag{12}\]  
\[\mathrm{SO_{4}}+\mathrm{H_{2}O}\rightarrow\mathrm{H_{2}SO_{5}} \tag{13}\]  
In alkaline medium, hydroxyl radicals HO\({}^{\bullet}\) are formed (Equation (3)). Being very powerful and non-selective, hydroxyl radicals are capable to mineralize TA and its aromatic intermediates. Hence, the maximum aromatics and TOC removals occurred at pH = 9. For photo-Fenton (UV/H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\)) process, removal efficiency reaches a maximum at initial pH of 3. This is similar to a previous investigation [42; 48]. Several studies reported that optimal pH for the production of hydroxyl radicals by the photo-Fenton must be in the range three-five [66; 67]. It was also demonstrated that at low acid medium (pH > 5), the Fe\({}^{2+}\) catalyst regeneration becomes very difficult because of Fe(OH)\({}_{3}\) precipitation. However, in high acid conditions (pH < 3) protons can function as scavengers for hydroxyl radicals [42]. Additionally, there may be inhibition for the radical forming activity of iron. In this study, the pH is varied from three to 10. UV/H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\) process performance was significantly influenced by varying the pH as shown in Figure 4. From Figure 4a, it is obvious that the change in pH had no significant effect on aromatics removal since aromatics removals of 99.3 and 90% at pH values three and 10 after 2 h photo-Fenton treatment of TA. However, TOC removal during UV/H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\) process decreased as pH increased (see Figure 4b). As shown in Figure 4b, TOC removal remains almost constant with the increasing of initial pH from pH = 3 to pH = 5. In contrast, when the pH of the reaction was higher than five (neutral or alkaline pH), the TOC removal efficiency rapidly decreased as pH increased. After 2 h of treatment, the obtained percentage of TOC removal was 94.27, 92.29, 80.9, 65.28, and 55.2% at pH 3, 5, 7, 9, and 10, respectively. The findings demonstrated that aromatics were completely degraded into aliphatic intermediates by the photo-Fenton process for acid and neutral mediums, but the degradation was incomplete in the alkaline medium. These results can be interpreted the easier and rapid decomposition of H\({}_{2}\)O\({}_{2}\) hydroxyl radical in acidic medium. In addition, the competitive reactions to H\({}_{2}\)O\({}_{2}\) photo-reduction such as the consumption of hydroxyl radicals by the protons, the dismutation of hydrogen peroxide and the duplication of hydroxyl radicals are negligible under these conditions. This indicated that the majority of H\({}_{2}\)O\({}_{2}\) is decomposed into an important amount of hydroxyl radicals. Consequently, this condition is the most favorable for complete treatment of TA by the photo-Fenton process.