## 3 Results  
### Cr (III) Adsorption Isotherms  
Figure S1 shows the experimental data obtained from the Cr (III) removal process at equilibrium with different treatments applied to OP; additionally, the adjustments obtained with the different models can be observed. Table 3 shows the parameters of each of the adsorption isotherms. The reported data are the average values of the replications that \begin{table}
\begin{tabular}{c c} \hline \hline
**Model** & **Equation** \\ \hline Pseudo first order (PFO) & \(\mathrm{q}\,=\,\mathrm{q}_{\mathrm{max}}[1\,-\,\mathrm{exp}(-\mathrm{k}_{1}t)]\) \\ Pseudo second order (PSO) & \(\mathrm{q}\,=\,\frac{t}{\mathrm{k}_{2}+\mathrm{delta}_{\mathrm{min}}}+\frac{ \mathrm{t}}{\mathrm{delta}_{\mathrm{max}}}\) \\ Elovich & \(\mathrm{q}\,=\,\frac{1}{\mathrm{p}}\,\mathrm{ln}(\mathrm{q}\mathrm{g})+\frac{1}{ \mathrm{p}}\,\mathrm{ln}t\) \\ Intraparticle diffusion (ID) & \(\mathrm{q}\,=\,\mathrm{k}_{\mathrm{d}}\mathrm{d}^{\mathrm{d}}\mathrm{e}^{5}\) \\ External diffusion (ED) & \(\mathrm{q}\,=\,\frac{\mathrm{C}\mathrm{d}V}{\mathrm{m}}\,[1\,-\,\mathrm{exp}(- \mathrm{k}_{\mathrm{exp}}t)]\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Kinetic models of adsorption [22; 24]. were carried out, in all cases, the standard deviation values were less than 5.0%, and it can be seen that the best adjustment, regardless of the pH and treatment used in the OP, is that obtained using the Freundlich model. This indicates that the surface of the adsorbent presents heterogeneous active sites [28; 29], the value of \(n\) for WTOP was less than 1 (indicating that the adsorption process of Cr (III) in the bioadsorbent was chemical regardless of pH), while for MTOP and OPTmW the value of \(n\) was greater than 1, this implied that the ion removal process is physical [24; 30; 31; 32]. The use of OP for the adsorption of Cr (III) has already been analyzed; however, there is no consensus regarding the adsorption model since the adjustment of the experimental data varies considerably, as well as in the different treatments that may be presented. [PERSON] et al. [18], using OP treated with formaldehyde, obtained a better fit with the Langmuir model and an adsorption capacity of 9.43 mg/g. For their part, [PERSON] et al. [19] obtained an adsorption capacity of 72.73 mg/g with the same model. Additionally, other authors used different adsorbents, such as yucca [10], fortail [7], _Chorella sorokiniana_[33], _Hylocomium splendens_[15], _Spirogyna_ spp. [34], and _Sargassum filipendula_[35], obtaining adsorption capacities of 41.6, 11.7, 56.68, 25, 30.21, and 33.02 mg/g, respectively. Likewise, in the adsorption of Cr (III) with adsorbents, such as Hydrotalcites [28], _Nannochloris microalgae_[29], lichens [32], and carrot residues [30], adsorption capacities were reached at equilibrium of 17.0, 34.7, 52.1 and 45.09 mg/g respectively, adjusting with the Freundlich model. The adsorption capacity achieved with OP treated in the present study is within the adsorption range (9.43 and 72.73 mg/g) reported by other authors, who used hexane, acetone, and formaldehyde in the pretreatment of OP. The results obtained suggest that the three treatments applied (water, methanol and methanol-water) to the orange peel at the three pH levels studied (0.91, 1.78 and 2.72) are within the range of the adsorption capacity reported in the literature [26]. It should be noted that based on the results of the simulation of the predominance of species, where it was observed that at a pH greater than 3.0 the Cr (III) solution precipitates into Cr\({}_{2}\)O, and for solutions at a pH greater than 5.0 the species diagram shows that, in addition to Cr (III), the solution presents species such as CrOH\({}^{2+}\), Cr(OH)\({}^{4+}\), Cr(OH)\({}^{5+}\)4; in addition to the above, and taking into consideration that, in leather tantaning, the pH values used with chromium salts are around 3.5 [31], it was decided to use a pH lower than 3.0 to avoid the precipitation of the Cr salt and the presence of other species.