## 3 Results and Discussion
### FTIR Analysis  
An FTIR analysis for inorganic materials provides information on the molecular structure and chemical composition of the material. This analysis can provide information to identify functional groups and study the molecular interactions, among other determinations [66].  
For the present study, Figure 4 shows the spectra obtained by FTIR for IJ, decomposed IJ, IJ with As, IJ with Cr, and IJ with As + Cr particles.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Element (\% wt.)** & **IJ** & **IJ-D** & **IJ-As** & **IJ-Cr** & **IJ-As + Cr** \\ \hline O & 46.42 & 47.83 & 46.87 & 47.32 & 45.76 \\ Fe & 25.47 & 31.25 & 26.58 & 27.85 & 27.52 \\ S & 12.12 & 4.58 & 12.39 & 11.39 & 10.62 \\ Na & 3.79 & 1.39 & 3.88 & 5.23 & 4.25 \\ Al & 0.98 & 0.86 & 0.84 & 1.17 & 2.35 \\ Zn & 3.12 & 11.34 & 2.45 & 3.96 & 1.89 \\ Si & 1.02 & 1.09 & 1.59 & 1.22 & 3.94 \\ K & 1.16 & 1.01 & — & 0.29 & — \\ As & 0.44 & — & 1.01 & — & 1.04 \\ Cr & — & — & — & 0.08 & 0.28 \\ Cu & 1.78 & 0.65 & 1.54 & 1.49 & 2.35 \\ Pb & 3.69 & — & 2.85 & — & — \\ \hline \end{tabular}
\end{table}
Table 1: Results of the analysis performed using SEM-EDS, showing the average semi-quantitative and punctual composition results.  
In the FTIR spectrum observed in Figure 4, the results obtained for the samples of industrial jarosite (A), decomposed industrial jarosite (B), industrial jarosite with adsorbed As (C), industrial jarosite with adsorbed Cr (D), and industrial jarosite with adsorbed As + Cr (E) are shown. In the case of spectra A, C, D, and E, absorption bands in the 3400-3700 cm\({}^{-1}\) region are observed, which correspond to the OH\({}^{-}\) groups of the jarosite. Similarly, adsorption bands in the 1000-1200 cm\({}^{-1}\) region (1005, 1084, and 1190 cm\({}^{-1}\)) are observed, which correspond to regions representing the stretching vibration of the sulfate (\(SO_{4}^{-2}\)) groups. Absorption bands in the 500-700 cm\({}^{-1}\) region are also observed, which represent the stretching vibration of the Fe-O bonds. Finally, to fully confirm the existence of jarosite in each sample, the 400-500 cm\({}^{-1}\) region is examined, which reflect the absorption bands due to the bending vibration of the hydroxyl (OH\({}^{-}\)) groups.  
For the case of As(V), in spectra C and E, small absorption bands are observed in the 850-950 cm\({}^{-1}\) region, where the As-O stretching vibration is located. Additionally, absorption bands can be observed in the 500-600 cm\({}^{-1}\) region, which correspond to the bending vibration of the As-O bond. Similarly, arsenate can be localized in the absorption region corresponding to 870-920 cm\({}^{-1}\), which is attributed to the As-O stretching vibration of the arsenate group [56; 67]. Likewise, the presence of bonds in the 2050-2150 cm\({}^{-1}\) region, which may correspond to the As=O, As-O, Cr=O, and Cr-O bonds [56].  
On the other hand, the identification of Cr(III) in spectra D and E can be obtained by finding the most common absorption bands. These can be found in the 600-700 cm\({}^{-1}\) region, attributed to the Cr-O stretching vibration, while for the Cr-O bending vibration, they can be found in the 400-500 cm\({}^{-1}\) region. However, it should be taken into account that the exact positions of each absorption band can vary depending on the chemical form of both As and Cr, as well as on the presence of other ions or molecules in the sample.  
Finally, spectrum B shows practically no observable absorption bands, since the crystalline structure of the compounds was destroyed and few bonds remained, and those that did were too weak at the end of said decomposition.  
Figure 4: FTIR spectra obtained for (A) JJ; (B) decomposed JJ; (C) JJ with As adsorbed; (D) JJ with Cr adsorbed, and (E) JJ with As-Cr adsorbed.