## 3 Results and Discussion
### Contribution of Fe\({}^{0}\) Nanoparticles to the Elimination Process of Dyes
Various bands can be observed in the spectrum, and their assignment is provided below. Some of these bands are common to both dyes. For instance, the bands centered around 1600 and 1390 cm\({}^{-1}\) correspond to the stretching vibration of the -C=C- bonds in the aromatic ring. Additionally, the characteristic bands of primary amino groups can be seen, centered around 3400 and 3300 cm\({}^{-1}\), which can be attributed to the symmetric and asymmetric stretching vibrations of the N-H bonds. Finally, the bands centered around 1240-1250 cm\({}^{-1}\) correspond to the vibrations of C-N bonds in aryl amines. Regarding chrysodine G in particular, the most characteristic spectral features of azo dyes are the bands located at 1509 and 1158 cm\({}^{-1}\), which are typical of the azo (-N=N-) chromophore. In the case of methylene blue, the band centered at 1600 cm\({}^{-1}\) can also be due to CH=N stretching. -C-N and N-N stretching is the cause of the peaks that appear at 1252 cm\({}^{-1}\) and 1224 cm\({}^{-1}\), respectively. A typical spectral feature of methylene blue is the band centered  
Figure 9: Stages in the degradation of an azo dye by Fe\({}^{0}\) nanoparticle-catalyzed double bond cleavage. at 1064 cm\({}^{-1}\), which is attributable to the C-S-C bonds. Finally, the out-of-plane bending vibrations of C-H bonds give rise to the bands located around 890 cm\({}^{-1}\) and 840 cm\({}^{-1}\). On the other hand, Figure 10 displays the FT-IR spectra corresponding to the nanoparticles both before and after their contact with the dyes. The spectra of the nzVI sample in contact with chrysodine G show a remarkable similarity. The spectrum of the nanoparticles without the dye displays distinct bands at 3305, 2330, 1613, and 1018 cm\({}^{-1}\), which are known to be characteristic of iron nanoparticles according to sources in the literature [97; 98]. Notably, the two spectra are almost identical, and there are no noticeable peaks related to the dye. If the dye had been adsorbed onto the iron nanoparticles, the main bands seen in the spectrum of the dye (Figure S3) would have been observed in the nanoparticles' spectrum after contact. However, this is not the case, which unequivocally rules out the existence of adsorption processes between the iron nanoparticles and chrysodine. These findings align with previously published studies on the removal of other azoic dyes using Fe\({}^{0}\) nanoparticles [96; 99; 100]. In contrast, the FTIR spectrum of the nzVI sample exposed to methylene blue indicates that adsorption plays a significant role in the dye removal process. The spectrum of nzVI after contact with the dye solution exhibits several spectral features corresponding to methylene blue. Specifically, the bands originally observed in the methylene blue spectrum at 1392, 1335, 1252, 1174, 1139, 889, and 837 cm\({}^{-1}\) are also evident in the spectrum of nzVI used for treating the methylene blue solution. However, in this latter case, these bands appear at slightly lower wavenumbers, ranging between 2 and 8 cm\({}^{-1}\) lower. This indicates a weakening of the bonds in the respective organic moieties, as a consequence of the presence of interactions between the adsorbed dye molecules and the active sites of nzVI, which behave as a sorbent. Figure 10: FT-IR of nzVI before and after contact with chrysodine-G (C) and methylene blue (MB).