## 3 Results and Discussion
### Contribution of the Fenton/Fenton-like Processes to the Removal of Dyes  
In the previous section, the contribution of Fe\({}^{0}\) nanoparticles to the elimination process of the dye in aqueous solution was analyzed. As mentioned before, the results of the experimental design are presented in Table 1. Experiment number 10 was conducted in the absence of tea extract, and therefore, the total elimination of chrysodine G or methylene blue (74.4% and 85.5%, respectively) can be attributed to the oxidation of the molecule through Fenton and Fenton-like processes, considering that the reducing action of the polyphenols present in the extract is essential for nanoparticle formation. In the case of chrysodine G, there are numerous studies on the possible degradation mechanisms of azoic dyes in general using Fenton-like processes (see, for example, [101], and the references cited therein). However, for chrysodine G in particular, information is much more limited. Firstly, it is worth noting that in the case of the \"Fenton-like\" process, the initial reactions that occur are:  
\[Fe^{3+}+H_{2}O_{2}\to FeOOH^{2+}+H^{+} \tag{14}\]  
\[FeOOH^{2+}\to HOO\cdot\ +Fe^{2+} \tag{15}\]  
Fe\({}^{2+}\) can then react with hydrogen peroxide to generate hydroxyl radicals:  
\[Fe^{2+}+H_{2}O_{2}\to HO\cdot\ +OH^{-}+Fe^{3+} \tag{16}\]  
This latter reaction constitutes the first stage of the conventional Fenton process. Therefore, according to the literature, the Fenton-like process can be considered analogous to Fenton but with a certain delay in its initial stages. Schematically, the main reactions leading to the generation of radicals from the catalytic decomposition of hydrogen peroxide in the presence of Fe\({}^{2+}\) are shown in Figure S4 (Supplementary Material). However, proposing a general mechanism for the degradation of azoic dyes through attack by hydroxyl radicals is far from a simple task. In any case, it seems that there is a commonly accepted agreement that these radicals attack the azo group's double bond (-N=N-), either through hydrogenation or hydroxylation of the double bond. The first stage of both processes is illustrated in Scheme 2. Next, the hydroxylated or hydrogenated molecule undergoes successive bond cleavages, leading to the formation of various reaction intermediates that, in the case of chrysodine, contain oxygenated benzene rings. These intermediate compounds can undergofurther oxidation with ring opening, ideally leading to complete mineralization into CO2 and H2O [101]. For methylene blue, the earliest stage of degradation by -OH radicals can proceed through ring- or sulfur-hydroxylation, methyl-oxidation, or N-demethylation (see Scheme 3), with probably concurrent participation of the three processes [102]. Elucidating the mechanism and determining the reaction intermediates is a laborious and not always feasible task. In this particular case, where iron nanoparticles have been generated in situ, that is, in the presence of an extremely complex mixture of polyphenols and other molecules initially present in the green tea leaf extract, amd can also undergo bond cleavage, trying to propose a degradation pathway would involve a remarkably speculative procedure. Synergistic Effect of Nanoparticle Formation and Advanced Oxidation Processes on the Removal of Dyes  
The experimental results presented in Table 2 demonstrate that the elimination of dyes using only Fe\({}^{0}\) nanoparticle formation (Experiment 21) reaches 61.5% for chrysodine and 53.0% for methylene blue. In contrast, when exclusively applying radical oxidation  
Figure 3: Plausible initial stage of the -OH-mediated degradation of methylene blue in the Fenton process. (**1**) ring-hydroxylation; (**2**) sulfur-hydroxylation; (**3**) methyl-oxidation; (**4**) N-demethylation. through Fenton and/or Fenton-like processes (Experiment 10), the elimination achieves results of 74.4% for chrysoidine and 85.5% for methylene blue.