## 3 Results and Discussion
### Influence of Hydrogen Peroxide and Ferrous Iron Concentrations
In this case, a small content of hydroxyl radicals results mainly from the photodecomposition of hydrogen peroxide by UV light and it causes a slow removal in the mineralization. Besides, a gradual increase in the concentration of ferrous ions to 0.18 mM increases the concentration of hydroxyl radicals HO\({}^{\bullet}\) in the aqueous medium through the photo Fenton process, under the Fenton reaction, to form ferric ions (Equation (5)) and by the photo-reduction of Fe(OH)\({}^{2+}\) complex (Equation (7)). Moreover,  
Figure 6: Effect of Fe\({}^{2+}\) amount on (**a**) Aromatics removal and (**b**) TOC removal by photo-Fenton process (UV/H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\)). Experimental conditions: [TA] = 0.1 mM; [Fe\({}^{2+}\)] = 0–0.27 mM; [H\({}_{2}\)O\({}_{2}\)] = 29.4 mM; T = 25 \({}^{\circ}\)C; and pH = 3.  
when high amounts of Fe\({}^{2+}\) are added to the wastewater, the regeneration of the catalyst (Fe\({}^{2+}\) ions) from Fe\({}^{3+}\) ions becomes slow due to the precipitation of ferric hydroxide Fe(OH)\({}_{3}\), which prevents the penetration of UV light into the reaction medium and therefore it reduces the quantum efficiency of the UV lamp. Consequently, this inhibits the generation of hydroxyl radicals by ferrous iron. In addition, the use of a much higher concentration of Fe\({}^{2+}\) could lead to the self-scavenging of hydroxyl radical HO\({}^{\bullet}\) by converting it to hydroxyl ions HO\({}^{-}\) during the oxidation of Fe\({}^{2+}\) (Equation (16)). From these results, we can deduce that a 0.18 mM dose of ferrous ions is optimal for a better treatment of aqueous solutions containing 0.1 mM of TA at pH 3.0 and 29.4 mM of H\({}_{2}\)O\({}_{2}\), using the photo-Fenton process. \[\mathrm{Fe}^{2+}+\mathrm{HO}^{\bullet}\rightarrow\mathrm{Fe}^{3+}+\mathrm{HO} ^{-} \tag{16}\]  
Similar behavior is noticed in the case of hydrogen peroxide dosage (Figure 7b). The addition of initial hydrogen peroxide concentration from 14.7 to 29.4 mM resulted in an increase in mineralization extents from 84.22% to 94.27% of TOC removal after 120 min of treatment, as seen in Figure 7b. A low concentration of H\({}_{2}\)O\({}_{2}\) did not allow achieving complete mineralization of TA in the photo-Fenton treatment. This can be explained by Equation (6). Increasing the amount of H\({}_{2}\)O\({}_{2}\) up to 29.4 mM caused considerable efficiency improvement because of more reactive radical generation (Equations (5) and (6)), which strongly enhanced the efficiency of TA degradation. Further increase of the H\({}_{2}\)O\({}_{2}\) dose, greater than 29.4 mM, decreased the mineralization efficiency from 94.27% to 80.19% after 120 min of the treatment of TA. After reaching an optimal value, a further boost of H\({}_{2}\)O\({}_{2}\) concentration negatively influences the efficiency of the process due to the OH\({}^{\bullet}\) scavenging effect of H\({}_{2}\)O\({}_{2}\) and recombination of hydroxyl radicals (Equation (17)). In addition, when hydrogen peroxide concentration is elevated, the competitive reactions of hydroxyl radicals with H\({}_{2}\)O\({}_{2}\) excess in order to produce HO\({}_{2}\)\({}^{\bullet}\) (Equations (18) and (19)) consume a considerable amount of HO\({}^{\bullet}\) radicals, which leads to a decline in the process efficiency. \[\mathrm{HO}^{\bullet}+\mathrm{HO}^{\bullet}\rightarrow\mathrm{H}_{2}\mathrm{O }_{2} \tag{17}\]  
\[\mathrm{H}_{2}\mathrm{O}_{2}+\mathrm{HO}^{\bullet}\rightarrow\mathrm{H}_{2} \mathrm{O}+\mathrm{HO}_{2}\mathrm{{}^{\bullet}} \tag{18}\]  
\[\mathrm{HO}_{2}\mathrm{{}^{\bullet}}+\mathrm{HO}^{\bullet}\rightarrow\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2} \tag{19}\]  
Figure 7: _Cont._  
Therefore, a H\({}_{2}\)O\({}_{2}\) dose of 29.4 mM was found to be optimal for a complete disappearance of TA in 60 min and total TOC removal in 120 min in the photo-Fenton oxidation for treating TA wastewater. It was found that both parameters Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\) had a positive effect on the percentage of TOC removal over the studied range.