## 3 Results and Discussion
### SEM-EDS Analysis  
Figure 3 shows the images obtained by SEM, using secondary electrons (SE), of the I) samples, decomposed I] (IJ-D), and I] after the adsorption of As(IJ-As), Cr (IJ-Cr), and As-Cr (IJ-As + Cr). Images were obtained at magnifications of 15,000 (left column) and 5000 (right column). A difference between I] and IJ-D can be observed; the former shows the crystalline structure of jarosite, while in the latter, this structure has practically disappeared. The jarosite crystallizes in the hexagonal crystal system, with a crystal structure typical of the hexagonal scalenohedral type [60,61]. More specifically, jarosite has a crystal structure composed of layers of iron (Fe) octahedra and sulfate (SO\({}_{4}\)) tetrahedra, which alternate in a hexagonal arrangement. In this type of structure, the adsorption of As on the decomposed jarosite particles, which are mainly composed of iron hydroxides (Fe(OH)\({}_{3}\)), is basically due to the formation of As and Fe complexes, while the adsorption of Cr is basically due to the surface adsorption phenomena and the formation of complexes between Cr and Fe [62]. On the other hand, in the images corresponding to I] after the adsorption of As, Cr, and As-Cr, the crystalline structure is recovered once again after undergoing decomposition, adsorption, and recrystallization processes by adjusting the pH to 1.1. This means that the waste is capable of opening its structure during decomposition, allowing the selective and simultaneous adsorption of As(V) and Cr(III) ions, and, finally, recovering its crystalline structure once these elements have been encapsulated. Figure 2: XRD spectra obtained for **(A)** IJ; **(B)** decomposed IJ; **(C)** IJ with As adsorbed; **(D)** IJ with Cr adsorbed; and **(E)** IJ with As-Cr adsorbed. The crystallization of industrial jarosite after the adsorption of As(V) and Cr(III) ions suggests that the adsorbent material may have undergone structural and chemical changes during the adsorption process. These may be beneficial for the stabilization and immobilization of As(V) and Cr(III) ion in the matrix of the adsorbent material [14; 63]. In a broader context, the results of this study have important implications for the remediation of soils and waters contaminated with As(V) and Cr(III) ions. Industrial jarosite may be a viable and cost-effective option for the adsorption of these ions compared with other more expensive adsorbent materials [27; 30; 31; 64]. Similarly, SEM-EDS point microanalyses were performed on the various particles found using this technique. The microanalysis is point-based (approximately 1 \(\upmu\)m\({}^{3}\)) and semi-quantitative, and it was only performed to verify the adsorption of As(V) and Cr(III) ions on the IJ particles. However, this technique could have some challenges and limitations, such as low sensibility and detection limits, spectral interferences, homogeneity of the sample, sample preparation, calibration and normalization of the spatial resolution  
Figure 3: SEM (SE) images obtained for (**A**) IJ; (**B**) decomposed IJ; (**C**) IJ with As adsorbed; (**D**) IJ with Cr adsorbed; and (**E**) IJ with As-Cr adsorbed. limitations, and sample interactions with the electron beam. Table 1 shows the average composition obtained for the samples shown in Figure 3.  
From the results observed in Table 1, it can be seen that the IJ initially contains As (0.44%), which originates from the minerals treated in the early stages of concentration and that may have been part of arsenopyrite or arsenocopper pyrite, among others. In the case of decomposed IJ, it is observed that the contents of S, Na, and Cu decrease, while Pb and As no longer appear, which is the result of the dissolution of sulfate and associated metals, leaving a residue consisting of FeOH and franklinite, as seen in the DRX results. Finally, for the solids IJ-As, IJ-Cr, and IJ-As + Cr, the presence of As (1.01%), Cr (0.08), and As + Cr (1.04 and 0.28%) can be observed.