## 3 Results and Discussion
### Contribution of the Fenton/Fenton-like Processes to the Removal of Dyes
However, when both procedures are combined, the contaminant can be eliminated by up to 96.4% (Experiment 12 for chrysoidine) or 97.5% (Experiment 9 for methylene blue), which represents almost complete removal. Therefore, it can be affirmed that under appropriate conditions, a synergy can be created between radical oxidation processes and dye degradation mediated by the presence of iron nanoparticles. For methylene blue, as indicated above, adsorption of the dye onto the surface of nZVI also contributes to the removal efficiency [54; 103]. The use of Fe\({}^{0}\) nanoparticles as heterogeneous catalysts in Fenton-like processes for contaminant degradation significantly reduces the problems encountered in homogeneous Fenton processes (where the iron catalyzing hydrogen peroxide decomposition is in the form of Fe\({}^{2+}\) and/or Fe\({}^{3+}\) salts in solution). These drawbacks include generation of large amounts of iron sludge after treatment that needs to be disposed of; consumption of significant quantities of Fe\({}^{2+}\) and hydrogen peroxide, with the molar ratio of H\({}_{2}\)O\({}_{2}\) to Fe\({}^{2+}\) reaching 9:1; the requirement to operate under acidic conditions (pH \(<\) 3) to prevent the precipitation of iron salts as hydroxides or oxohydroides; and the need for the neutralization of initially acidified solutions before disposal as waste [104; 105]. In contrast to these drawbacks, the use of Fe\({}^{0}\) nanoparticles as heterogeneous catalysts offers significant advantages, such as virtually no precipitation of iron hydroxides or oxyhydroides since the concentration of Fe\({}^{2+}\) or Fe\({}^{3+}\) ions in solution is much lower; easy separation of the catalyst after application; possibility of catalyst reuse in certain cases; and an extended pH range for operation [106]. However, the use of nanoparticulate Fe\({}^{0}\) as a heterogeneous catalyst in Fenton and Fenton-like processes does have some disadvantages. One of the most important is that the contaminant molecules need to transfer from the solution to the surface of the material to reach the active sites for degradation. Additionally, the reactions involved in these heterogeneous processes are usually slower compared to conventional homogeneous processes at the same concentrations. On the other hand, heterogeneous reactions are generally more efficient in terms of H\({}_{2}\)O\({}_{2}\) consumption. Regarding the oxidation mechanism, according to the literature [106], it can be proposed that oxidation occurs through two different mechanisms: (i) a process catalyzed homogeneously by iron ions released into the solution; and (ii) heterogeneous reactions between the solutes and the surface groups of iron nanoparticles. The reactions involved in the heterogeneous catalytic Fenton process by Fe\({}^{0}\) nanoparticles are schematically represented in Figure 11.  
On the right side of Figure 11, the oxidation of Fe\({}^{0}\) by O\({}_{2}\) (under aerobic conditions) in the presence of an acidic medium is schematically depicted, resulting in the generation of Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\). On the left side of the Figure, the reduction of Fe\({}^{3+}\) to Fe\({}^{2+}\) mediated by Fe\({}^{0}\) is illustrated by the reaction:  
\[2 Fe^{3+}+Fe^{0}\to 3 Fe^{2+} \tag{17}\]  
Once the Fe\({}^{2+}\) ion is formed, either through one pathway or the other, the cascade of radical reactions known as the Fenton process begins, with its first stage being the catalytic decomposition of hydrogen peroxide, as mentioned earlier:  
\[Fe^{2+}+H_{2}O_{2}\to HO\cdot~{}+OH^{-}+Fe^{3+} \tag{18}\]  
The hydroxyl radicals (OH) contribute to the oxidation of the dye molecule. In other words, the Fe\({}^{0}\)/H\({}_{2}\)O/O\({}_{2}\) system is capable of generating oxidizing species such as H\({}_{2}\)O\({}_{2}\) and -OH. Subsequently, the reactions would follow the typical mechanisms of the Fenton process to produce intermediate oxidized products or, ideally, complete mineralization of the organic compound.