## 3 Results and Discussion
### Characterization of the Bean Peel in the Sorption Process  
Figure 4 shows the SEM images of the outside (a) and inside (b) surfaces of the white bean peel with and without pre-treatment and after 6 h of contact time with the aqueous solution containing the mixture of PTEs (500 \(\mathrm{\SIUnitSymbolMicro g}\)/L, the highest concentration evaluated) at pH 4.0, 5.6, and 7.0. As shown in Figure 4a, the outside surface of the bean peel before pre-treatment presented a rough and irregular texture, with slight visible elevations and depressions. After pre-treatment, the outside surface of the bean peel remained rough, but more compact and with an appearance more uniform, possibly due to the removal of water-soluble particles of organic and inorganic species. After the sorption experiments, the outer surface of the pre-treated bean peel showed a more irregular morphology, more notorious at pH 4.0 and 7.0, but without obvious signs of loss of structural integrity. Unlike the outside surface of the bean peel, as shown in Figure 4b, the inner surface of the untreated white bean peel showed a very rough morphology, a flake-like morphology, and after the pre-treatment, the flakes appeared smaller. After the sorption experiments, the inner surface was brighter and more organized. Moreover, the inner surface of the peel became less organized and polished with the pH increase. This fact was coincident with the higher percentages of removal for Cu(II) at a lower pH, namely at pH 4.0 (Figure 1). Nevertheless, no significant changes were observed in the morphology of both sides of the bean peel after 6 h of contact time with the mixture solution. In addition, EDX spectroscopy was used in this work to provide information about the adsorption of Zn(II), Cu(II) and Pb(II) in the surface of the biomaterial, which could confirm the removal of the elements from water. However, the presence of Zn(II), Cu(II) and Pb(II) was not clearly observed in the EDX spectra after the biosorption process (see Supplementary Materials Figure S4), possibly due to the low concentrations of the elements in the experiments. In order to evaluate structural changes during the sorption process, FTIR-ATR spectra of bean peels were acquired before and after being in contact with a mixed solution of PTEs. Figure 5 shows the FTIR-ATR spectra of outside and inside the bean peel before and after contact with the mixed solution with an initial concentration of 500 \(\mu\)g/L during 2 h, 4 h, and 6 h.  
Figure 4: SEM images of the outside (**a**) and inside (**b**) surface of the white bean peel without and with pre-treatment (top images) and with pre-treatment after 6 h of contact with the PTE solution (500 \(\mu\)g/L) at pH 4.0, 5.6, and 7.0 (bottom images of (**a,b**)). The FTIR-ATR spectra of peels before and after use in adsorption tests show the presence of various bands in the region of 3600 and 400 cm\({}^{-1}\). Such bands were assigned accordingly their wavenumbers [43,44]. The broad band between 3650-3000 cm\({}^{-1}\) can be attributed to O-H stretching from carboxyl and hydroxyl groups and to N-H stretching of amides. The peaks observed between 3000 cm\({}^{-1}\) and 2800 cm\({}^{-1}\) are associated with C-H stretching of methyl and methylene groups in aliphatic chains. It highlighted a distinct peak at about 1740 cm\({}^{-1}\) for the inner white bean peels characteristic of the stretching of C=O of carboxylic groups. Typically, the spectral region spanning between 1700 cm\({}^{-1}\) and 1500 cm\({}^{-1}\) reflects the presence of functional groups containing C=O and C=C bonds. Furthermore, within this interval, it was observed two additional bands centered at 1650 cm\({}^{-1}\) and 1530 cm\({}^{-1}\), which are attributed to N-H bending and C-N stretching, respectively. The bands present between 1500 cm\({}^{-1}\) and 1200 cm\({}^{-1}\) are typically attributed to compounds with carboxylic groups.