## 3 Results and Discussion
### Contribution of Fe\({}^{0}\) Nanoparticles to the Elimination Process of Dyes  
As indicated in the previous section, it can be observed that experiment number 21 in Table 1, which is carried out in the total absence of hydrogen peroxide (coded value equal to \(-2\)), results in a removal efficiency equal to 61.5% and 53.0% for chrysoidine and methylene blue, respectively. Therefore, in this experiment, dye removal is solely attributed to the effect of iron nanoparticles. In the literature, numerous references can be found regarding the elimination of dyes in general, and azo dyes in particular, through treatment with iron nanoparticles. However, at the time of writing this work, to the best of the authors' knowledge, there is no existing literature on the use of iron nanoparticles for the elimination of chrysoidine in solution. Several of the consulted studies suggest that the removal of the dye in solution is mediated by adsorption processes. For example, [PERSON] et al. [90] investigated the removal of the azo dye Reactive Black 5 through adsorption by iron nanoparticles and biomass of the seaweed _Macrocystis pyrifera_. According to these authors, elimination efficiencies between 69% and 80% can be achieved by the combined use of biomass and nanoparticles, and they attribute this high removal capacity to the immobilizing effect of the biomass on the nanoparticles. Additionally, [PERSON] et al. [91] also attribute the high removal rates of another azo dye, Acid Black 24, to adsorption phenomena by micro- and nano-sized Fe\({}^{0}\) particles. According to these authors, an increase in particle size leads to lower efficiency in removing the contaminant due to the subsequent decrease in specific surface area. However, many more authors justify the high removal efficiency of azo dyes by iron nanoparticles based on their high reactivity, specifically their ability to break the azo group's double bond (N=N). This bond cleavage, according to some authors [92; 93], would be conditioned by a two-stage process. The first stage, which is reversible, involves the oxidation of Fe\({}^{0}\) within the nanoparticle to Fe\({}^{2+}\) and/or Fe\({}^{3+}\) and the addition of hydrogen to the azo group's double bond (N=N), resulting in the formation of an unstable reaction intermediate. The second stage involves the cleavage of the bond and the formation of two aminated compounds. Schematically, the process can be represented as in Scheme 1:Other authors propose a combined mechanism in which the dye removal occurs through both adsorption phenomena and the cleavage of the azo group's double bond mediated by iron nanoparticles [94; 95; 96]. Thus, the process can be described in five stages: (i) transfer of a dye molecule from the bulk solution to the vicinity of the iron nanoparticle surface; (ii) adsorption of the dye onto the nanoparticle surface; (iii) cleavage of the azo group's double bond (N=N) through the chemical reaction described above; (iv) desorption of the reaction products from the nanoparticle surface; and (v) transfer of the reaction products from the vicinity of the nanoparticle back into the bulk solution. This overall process is schematically represented in Figure 9.  
To verify whether the elimination process of chrysodine by Fe\({}^{0}\) nanoparticles was solely due to adsorption or if there was also a contribution from the catalytic cleavage of the azo group, a sample of nanoparticles was prepared and then brought into contact with the dye solution without the addition of hydrogen peroxide, thus excluding the contribution of the Fenton process to chrysodine degradation. The prepared nanoparticles were filtered and dried. Subsequently, the infrared spectrum of a portion of these nanoparticles alone and in contact with the dye was recorded. For comparative purposes, the spectrum of the dye itself was also recorded, along with the spectrum of a sample of nanoparticles that were not in contact with chrysodine. Figure S3 (Supplementary Material) represents the FT-IR spectrum of the dyes.