## 3 Results and Discussion
### Effect of Fe\({}^{2+}\) Dose and Iron Source  
Fe\({}^{2+}\) ions play the role of reactant/catalyst in [PERSON]'s reaction (Equation (1)) capable of decomposing H\({}_{2}\)O\({}_{2}\) into hydroxyl radicals and forming ferric ions. Firstly, the influence of Fe\({}^{2+}\) dose on the degradation of DAP was evaluated for doses between 0 and 100 mg/L during Fenton oxidation of 100 mg/L DAP using 1000 mg/L of H\({}_{2}\)O\({}_{2}\) at pH 3.2. Figure 3 demonstrates the influence of the Fe\({}^{2+}\) dose on DAP decay during the Fenton process. This figure shows that the degradation of DAP was remarkably affected by the Fe\({}^{2+}\) concentration. In the absence of Fe\({}^{2+}\), the percentage of the DAP degradation yield was less than 5%, while it increased markedly in the presence of Fe\({}^{2+}\). Furthermore, the DAP degradation yield increased with augmenting Fe\({}^{2+}\) concentration from 0 to 50 mg/L; however, it remained unchanged or decreased with a further increase in Fe\({}^{2+}\) higher than 50 mg/L. Indeed, DAP degradation yields (after 180 min) were 69.96%, 84.98%, 100%, 93.19 and 87.18% for Fe\({}^{2+}\) doses of 10, 25, 50, 75 and 100 mg Fe\({}^{2+}\)/L, respectively. Maximum DAP degradation efficiency (100 %) was achieved by 50 mg/L of Fe\({}^{2+}\). The DAP degradation yield showed progressive improvement with the increase in the initial Fe\({}^{2+}\) dose within the range 10-50 mg/L and could be due to the greater generation of hydroxyl radical by reaction (Equation (1)). Moreover, higher Fe\({}^{2+}\) doses than 50 mg/L can lead to consumption of the hydroxyl radical \({}^{\bullet}\)OH by excessive Fe\({}^{2+}\) ions by reaction (Equation (2)) inducing the decrease in DAP degradation yield [24; 31].  
\[\mathrm{Fe}^{2+}+{{}^{\bullet}}\mathrm{OH}\rightarrow\mathrm{Fe}^{3+}+ \mathrm{OH}^{-} \tag{2}\]  
Figure 2: Influence of the initial pH on the DAP degradation kinetics during the Fenton process (H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\)). Experimental conditions: [DAP]\({}_{0}\) = 100 mg/L; [H\({}_{2}\)O\({}_{2}\)] = 1000 mg/L; [Fe\({}^{2+}\)] = 50 mg/L; pH = 3.2–7.  
Thus, the optimum [H\({}_{2}\)O\({}_{2}\)]/[Fe\({}^{2+}\)] mass ratio for Fenton oxidation of DAP at pH 3.2 was 20 confirming the need for Fe\({}^{2+}\) doses, which was much too less than the stoichiometric dose as reported by several studies [24,31]. To compare the effect of the catalyst source on degradation of DAP by the Fenton process, a series of experiments were performed using a fixed concentration of DAP of 100 mg/L and a dose of H\({}_{2}\)O\({}_{2}\) of 1000 mg/L at pH 3.2, and 50 mg/L Fe\({}^{2+}\) obtained from different sources. Four iron sources (ferrous sulfate, ferric oxide, pyrite and FeF\({}_{2}\)) were supplied during DAP degradation by Fenton reaction. The results of these experiments are presented in Figure 4. This figure shows that the DAP concentration decreased from the beginning of treatment by the Fenton process for all iron sources. However, in the presence of ferrous sulfate, the DAP degradation yield was higher than that obtained with the addition of ferric oxide, pyrite or FeF\({}_{2}\). The complete DAP degradation was observed only when ferrous sulfate was used as a source of iron after 180 min. DAP degradation yields were 100%, 91.49%, 85.98% and 77.17% for ferrous sulfate, pyrite, FeF\({}_{2}\) and ferric oxide, respectively. The results confirm that DAP degradation is affected by the iron source. Consequently, DAP degradation is significantly favored by homogeneous catalysis when Fe\({}^{2+}\) is available in a soluble form than heterogeneous catalysis when an insoluble source of iron is used.  
Figure 3: Effect of Fe\({}^{2+}\) content on the DAP degradation kinetics during the Fenton process (H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\)). Experimental conditions: [DAP]\({}_{0}\) = 100 mg/L; [H\({}_{2}\)O\({}_{2}\)] = 1000 mg/L; [Fe\({}^{2+}\)] = 0–100 mg/L; pH = 3.2.