## 3 Results and Discussion
### Catalytic Degradation of MO  
The synthesized FeNP was used to catalyze MO degradation in the presence of H\({}_{2}\)O\({}_{2}\) as an oxidizing agent according to Table 1. MO degradation using a different amount of H\({}_{2}\)O\({}_{2}\), different contact time and different initial MO concentration was observed by UV V is spectroscopy and the results are shown in Figure 6.  
Figure 5: X-Ray diffraction (XRD) pattern of FeNP. Figure 6: (**A**)The degradation of MO as a function of time; (**B**) Pseudo first-order kinetic model of the degradation reaction; (**C**) The degradation of MO as a function of initial MO concentration; and (**D**) The degradation of MO as a function of a volume of H\({}_{2}\)O\({}_{2}\). In the absence of FeNP, degradation of MO was not noticed even after 24 h, indicating that the oxidation pathway of MO by peroxide does not occur. However, degradation of MO occurred only when FeNP was introduced into the reaction mixtures indicating essentiality of the FeNP for promoting the degradation, which probably occurred through free radicals pathway [28]. The introduction of the FeNP may facilitate the formation of OH\({}^{\bullet}\) radical through which the degradation of the MO proceeds. Degradation was relatively slow, 90% removal was achieved within the first two hours. Almost complete removal of the dye can be reached after three hours within the studied conditions. The kinetic data were found to follow a pseudo first order kinetic model given in Equation (1):  
\[ln\ (C_{t}-C_{e})\ =\ ln\ (C_{e})\ -\ K_{p1}t \tag{1}\]  
where \(C_{e}\), \(C_{t}\), \(t\), and \(K_{p1}\) are MO concentration at equilibrium and at time t (mg/L), time (min), and Pseudo first-order constant (mg/L\(\cdot\)min). The R\({}^{2}\) and the rate constants \(K_{p1}\) were calculated by plotting \(ln(C_{t}-C_{e})\) vs. \(t\), as shown in Figure 6B. The rate constant \(K_{p1}\) for MO degradation was found to be 0.0219 mg\(\cdot\)L\({}^{-1}\cdot\)min\({}^{-1}\). To examine the effect of MO initial concentration on the degradation efficiency a set of experiments were conducted under the same conditions with only MO concentration varied within the range 5-40 ppm according to Table 1. It can be seen from Figure 6C that increasing MO concentration from 5 ppm to 30 ppm increased the degradation efficiency gradually from 93.7% to 98.7%. When the concentration was 40 ppm, the degradation activity decreased to 98.1. This could be explained based on the saturation of all active sites on the surface. Thus, increase in MO concentration will not increase the rate, as the active sites of the FeNB become the limiting factor for the reaction. Interestingly, Figure 6D shows the negative effect of the volume of H\({}_{2}\)O\({}_{2}\) added to the reaction mixture. Increasing H\({}_{2}\)O\({}_{2}\) from 1 mL to 5 mL resulted in decreasing degradation from 99.5% to 98.1%. This could be related to some competition effect between H\({}_{2}\)O\({}_{2}\) and MO for the active adsorption sites on the FeNP, and thus affects as an inhibitor for the catalyst at high concentrations. Figure 7A shows the effect of 0.1 g of FeNP on the pH of 20 mL of 0.1 M NaCl solution with an initial pH within 2-12. The Figure clearly shows the buffering effect of the FeNP within this pH range. Accordingly, for any solution pH\({}_{i}\) within this range, the pH\({}_{e}\) will be constant and equal to pH\({}_{\text{ZC}}\). Figure 7A shows that the FeNP has a pH\({}_{\text{ZC}}\) of 4.8, which indicates that this FeNP would be a good candidate for removing MO by adsorption only if the solution pH was set at a value less than 4.8 since the FeNP surface is expected to have a positive charge in any solution with pH value lower than 4.8 and since MO is an anionic dye, it is expected to be attracted to the positive surface. Figure 7B shows the dependence of FeNP adsorption performance on the solution pH. Both MO adsorption percentage (R%) and capacity (q) decrease by increasing solution pH from 2 to 12.