## 4 Conclusions  
The results obtained show that the best conditions for the removal of Cr (III) present in the aqueous solution are at the highest pH, that is, 2.72, with WTOP being the best treatment with an adsorption capacity of 53.15 mg/g. The best adjustment in the Cr (III) adsorption was presented with the Freundlich model, while in the kinetic process a better adjustment was had with the Elovich model. Both models mention that both the surface and the active sites have heterogeneous energy, which allows for better adsorption. On the other hand, values between 0 and 1 of the separation factor (R\({}_{\rm L}\)) were determined, which indicates that the elimination of Cr (III) is favorable, in addition, with the D-R isotherm it was found that the process of capturing the ions carried out through the ion exchange mechanism with the surface and that the removal is spontaneous (\(\Delta\)G \(<\) 0). In the study of mass transfer in the adsorption of Cr (III), it was observed that in the case of WTOP at pH 2.72 the internal mass transfer dominates, while in the other tests there is influence of the external mass transfer, which causes it to have a lower adsorption capacity compared to WTOP. The desorption study showed that for the WTOP and MTOP cases there is a recovery percentage greater than 40%, which implies that Cr (III) can be recovered to be reused in various industrial processes, such as in a tannery. The characterization of the bioadsorbents revealed that there is no presence of electrostatic forces that are involved in the removal process of the Cr ions, while the analysis of the surface by FTIR showed that the functional groups of OP are modified with the applied treatment. Furthermore, its presence significantly affects the adsorption process on the surface of the adsorbent, with WTOP being the one with the highest presence of these functional groups, which is why it has a better adsorption at a pH of 2.72. Based on the percentages of desorption obtained, it is considered necessary to analyze the removal of Cr (III) from the orange peel using different operating conditions to achieve high recovery of Cr (III) and thus be able to reuse both the tanning salts and the orange peel in adsorption processes, to improve the profitability and sustainability of this process. The contribution of this project regarding sustainability lies in the fact that the use of orange peel, discarded from various processes in the food industry, is an alternative to avoid contamination of soils and the environment due to the disposal without treatment of these easily fermentable residues. Considering that this material has adsorption capacity within the range of other materials contemplated in the literature. The following are available online at [[https://www.mdpi.com/article/10.3390/app11146346/s1](https://www.mdpi.com/article/10.3390/app11146346/s1)]([https://www.mdpi.com/article/10.3390/app11146346/s1](https://www.mdpi.com/article/10.3390/app11146346/s1)), Figure S1: Fitting the experimental data with the adsorption isotherms of WTOP, MTOP and OPTmA with different pH, (a) 0.91, (b) 1.78 and (c) 2.72; Figure S2: Analysis of internal mass transport at different pH. (a) WTOP, (b) MTOP and (c) OPTmW; Figure S3: FT-IR spectra of OP in the adsorption of Cr (III) with different treatments: (a) WTOP, (b) MTOP and (c) OPTmW; Figure S4: FT-IR spectra of OP in Cr (III) adsorption with different treatments: (a) WTOP, (b) MTOP and (c) OPTmW; Table S1: Kinetic data of the Cr (III) removal using WTOP; Table S2: Kinetic data of Cr (III) removal using MTOP; Table S3: Kinetic data of Cr (III) removal using OPTmW; Table S4: Linear regression of the fit of the data that is part of the external transport process. Conceptualization, L.P.-S. and J.A.H. ; Formal analysis, J.A.H. and R.H.-S.; Investigation, L.P.-S. and J.A.H. ; Methodology, A.A., M.S.-H. and A.T.; Writing--original draft, L.P.-S. and J.A.H. ; Writing--review and editing, R.H.-S. All authors have read and agreed to the published version of the manuscript.