## 3 Results
### Characterization of OP
The peak at 2928 cm\({}^{-1}\) is attributed to the vibrations of the asymmetric and symmetric stretching of the C-H groups in aliphatic acids and the peak at 2850 cm\({}^{-1}\) is due to the symmetric vibrations of the -CH\({}_{2}\)- group of aliphatic acids. The presence of free and esterified carboxylic groups are found with the peaks at 1736 and 1632 cm\({}^{-1}\), respectively. The band at 1419 cm\({}^{-1}\) can be attributed to the plane strain vibration of the methyl, methylene and methoxy groups in aliphatic and aromatic compounds. The symmetric stretching of -COO- is due to the peak at 1380 cm\({}^{-1}\) and at 1330 cm\({}^{-1}\) it corresponds to the stretching vibration of the C-O groups of carboxylic acids and alcohols. The peak at 1241 cm\({}^{-1}\) is attributed to the vibrations of the COO-group. At 1008 cm\({}^{-1}\) it is due to the stretching vibrations of the C-H, C-O-H or C-O-R groups of the alcohols and carboxylic acids [16; 17; 18; 45; 46; 47; 48]. The intensity of the peaks in the samples decreases in the following order: OP > WTOP > MTOP > OPTmW, this means that there is less presence of the functional groups of the different compounds present in OP (cellulose, pectin, aliphatics, etc.). Figure 6: Isoelectric point of the different samples. Figure 7: FTIR spectra of OP in different unused treatments. On the other hand, the FTIR spectra of the samples were taken after the Cr (III) adsorption process at different pH values, which are presented in Figure S4. It can be observed that the intensity of the functional groups present in the OP was significantly modified since the intensity decreased as the pH of the solution increased, or, in some cases, the peak disappeared, this is indicative of the participation of the functional groups in the capture of Cr ions. In the case of WTOP, it is observed that the intensity of the peaks at 3290, 2896, 1721, 1630, 1414, 1367, and 1008 cm\({}^{-1}\) decreased, which indicates that there are several functional groups participating in the Cr adsorption process (III) in the orange peel; for example, the OH groups of cellulose, hemicellulose, pectin, as well as the CH, CH\({}_{2}\) groups of aliphatic compounds and carboxylic groups and esters. The peak at 1322 cm\({}^{-1}\) did not present a significant change, which indicated that the functional group COO- has no participation in the removal of Cr (III), there are also bands that disappear in WTOP at pH 2.7, such as 1367, 865 and 665 cm\({}^{-1}\) that are attributed to the C-O groups of carboxylic acids and alcohols. With MTOP, a similar behavior to WTOP is observed, although the decrease in the intensity of the peaks is less than that of the shell treated with water; however, in the MTOP samples, it is observed that the following bands disappear: 1420, 1147, 863, and 665 cm\({}^{-1}\), which indicates that in addition to the previously mentioned functional groups, the CH\({}_{2}\), CH\({}_{3}\), and OCH\({}_{3}\) groups belonging to the aliphatic compounds of the biomaterial also participate in the adsorption. This is probably because the functional groups CH\({}_{2}\), CH\({}_{3}\), and OCH\({}_{3}\), present in the pretreated orange peel can be in various combinations, which particularly influences the properties of the material, including the structure and reactivity. This is reflected in the hydrophobic or hydrophilic character of the adsorbent material; additionally, the degree of dissociation of chromium in solution at different pH allows the electrostatic bonding of CR (III) and the functional groups of the shell to be established. OPTmW presented a similar behavior compared with the other two treatments; however, the decrease in the intensity of the peaks was proportionally less compared to that obtained by the other two treatments, in the bands at 1420, 1147, 863, and 665 cm\({}^{-1}\). The peak at 1067 cm\({}^{-1}\) attributed to the C-O groups of the carboxylic acids and alcohols present in the orange peel, disappeared in this treatment.