## 3 Results
### Characterization of OP  
In Figure S3, the adsorption-desorption isotherm of WTOP is observed and in the box is a broad image of the hysteresis that is formed in the isotherm which is type IV (UIPAC classification), which indicates that it has a porous structure [41; 42; 43; 44], with an area of 2.85 m\({}^{2}\)/g, and an average pore size of 5.49 nm; thus, the bioadsorbent is classified as a mesoporous material (2-50 nm). OP is a mesoporous adsorbent as it has been indicated in some works [42; 43], depending on the pretreatment used in the biomaterial, a specific area between 1.19 and 10 m\({}^{2}\)/g [42; 44] is obtained, which contributes favorably to the good adsorption capacity of OP. The isoelectric points of the biadsorbents including untreated OP were determined as shown in Figure 6, obtaining values of 3.32, 4.10, 5.08, and 6.04 that correspond to OP, WTOP, OPTmW, and MTOP, respectively. Taking into consideration the obtained values, it is inferred that the adsorption of Cr (III) on the surface is not carried out by electrostatic forces because the pH of the solutions is less than 2.72 and the surface of the bioadsorbents is positively charged, which implies that an exchange of species, such as OH (in basic medium), can be carried out on the surface of the orange peel, although there is no electrostatic attraction, which depends directly on the characteristics of the bioadsorbent [12; 22; 45; 46]. Figure 5: Cr (III) desorption at different pH and treatment: (**a**) WTOP, (**b**) MTOP and (**c**) OPTmW. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**WTOP**} & \multicolumn{3}{c}{**MTOP**} & \multicolumn{3}{c}{**OPTmW**} \\ \hline  & & \multicolumn{3}{c}{pH = 0.91} & & & & \\ C\({}_{\text{adv}}\) & \(k_{1}\), mg/g & \(k_{2}\), mg/g & \(k_{1}\), mg/g & \(k_{2}\), mg/g & \(k_{1}\), mg/g & \(k_{1}\), mg/g & \(k_{2}\), mg/g & \(k_{1}\), \(k_{2}\), mg/g & \(k_{1}\), \(k_{2}\), mg/g & \(k_{1}\), \(k_{2}\), mg/g & \(k_{1}\), \(k_{2}\) \\ g/L & h\({}^{\text{S}}\) & h\({}^{\text{S}}\) & h\({}^{\text{S}}\) & h\({}^{\text{S}}\) & h\({}^{\text{S}}\) & h\({}^{\text{S}}\) & h\({}^{\text{S}}\) & \\ \hline
4 & 20.55 & 9.232 & 2.226 & 7.936 & 8.846 & 0.897 & 7.192 & 4.141 & 1.737 \\
8 & 9.709 & 5.312 & 1.828 & 4.622 & 11.94 & 0.387 & 4.761 & 3.073 & 1.549 \\
12 & 5.735 & 4.133 & 1.388 & 4.118 & 8.156 & 0.505 & 3.884 & 2.860 & 1.358 \\
16 & 4.584 & 2.427 & 1.891 & 0.357 & 3.898 & 0.784 & 3.289 & 2.772 & 1.187 \\
20 & 4.618 & 1.372 & 3.367 & 2.757 & 2.891 & 0.954 & 3.157 & 2.173 & 1.453 \\ \multicolumn{8}{c}{pH = 1.78} & & & & & \\
4 & 20.39 & 7.321 & 2.785 & 8.496 & 13.258 & 0.641 & 8.7282 & 6.627 & 1.317 \\
8 & 13.42 & 7.808 & 1.712 & 7.142 & 10.827 & 0.661 & 6.5951 & 5.769 & 1.143 \\
12 & 7.771 & 6.771 & 1.147 & 4.656 & 8.606 & 0.541 & 5.8969 & 5.357 & 1.101 \\
16 & 7.798 & 5.672 & 1.375 & 4.959 & 1.766 & 2.809 & 5.2646 & 2.681 & 1.964 \\
20 & 7.289 & 3.285 & 2.221 & 3.829 & 5.649 & 0.678 & 5.0513 & 2.484 & 2.034 \\ \multicolumn{8}{c}{pH = 2.72} & & & & & \\
4 & 9.545 & 22.98 & 0.415 & 17.33 & 12.52 & 1.384 & 11.5779 & 9.309 & 1.244 \\
8 & 11.62 & 15.31 & 0.759 & 17.72 & 7.292 & 2.431 & 13.4812 & 6.256 & 2.155 \\
12 & 8.034 & 10.63 & 0.755 & 6.906 & 5.096 & 1.355 & 5.5757 & 5.219 & 1.068 \\
16 & 7.391 & 8.126 & 0.909 & 10.03 & 3.061 & 3.277 & 7.4106 & 3.131 & 2.367 \\
20 & 2.091 & 6.497 & 0.322 & 7.957 & 2.303 & 3.456 & 6.3875 & 2.781 & 2.297 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Kinetic constants for external and internal diffusion processes. The orange peel has compounds with functional groups that can participate in the pollutant removal processes. Figure 7 shows the IR spectra of OP before being in contact with the Cr (III) solution, which is why the pH is not specified; the following peaks were observed: at 3295 cm\({}^{-1}\) corresponds to the vibrations of the O-H group of cellulose, hemicellulose, lignin, pectin, or adsorbed water.