## 3 Results
### Kinetics of Removal of Cr (III) Ions Using OP
The removal percentages of Cr (III) with WTOP at pH 0.91, 1.78, and 2.72 were 0.84, 1.4, and 1.43 times higher compared to those obtained with OPTmW. In Figures 1-3 it can be seen that the equilibrium stage for the adsorption of Cr (III) is achieved with different contact times. The time in which equilibrium is reached is affected by the type of pretreatment applied, in the case of WTOP, the time it reaches saturation on the surface decreases with increasing pH, (at 0.91 the time is 9 h, at 1.72 the time is 8 h) and at 2.72 the time is 6 h). In the case of WTOP and OPTmA, concerning pH with contact times of 7.0 and 7.5 h, no apparent change is observed in the time in which saturation is reached. Likewise, in Figures 1-3, it can be observed that the amount of eliminated Cr (III) depends directly on the adsorbent concentration, decreasing with the increase in the OP mass for any treatment and/or pH of the solution, this effect is attributed to the clogging of the active sites in the adsorbent [22]. Figure 3: OPTmW adsorption capacity at different pH: (**a**) 0.91, (**b**) 1.78, and (**c**) 2.72. Figure 2: MTOP adsorption capacity at different pH: (**a**) 0.91, (**b**) 1.78 and (**c**) 2.72. Several authors mention the effect of pH on the adsorption capacity and the percentage of removal of Cr (III) using OP [16; 17; 18; 19], which agree with the results obtained, despite the differences in the operating conditions and treatments applied to the OP, among which were water washes, treatments with HCl, NaOH (11.67 mg/g) and with formaldehyde (7.9 mg/g), obtaining adsorption capacity values from 23.76 mg/g with 180 h at pH of 5, up to 233.98 mg/g with 40 h of contact time. Among the conditions that differ in these treatments is the use of other adsorbents for the removal of Cr (III), different values of maximum adsorption capacity from 0.98 mg/g to 52 mg/g using hydrolcalcites [28], gelatin [36], jackfruit peel [10], ground foxtail [7] and lichens [32]. Microorganisms such as _Chlorella sorokiniana_, _Hylocomium splendens_ and algae such as _Nammochloris_, _Spirogyn_ spp. and _Sargassum filipendula_ have also been used, obtaining Cr (III) adsorption values of 23 mg/g [33], 4.14 mg/g [15], 37.7 mg/g [29], 30.21 mg/g [34] and 20.28 mg/g [35], respectively. In the literature, it is also mentioned that the adsorption kinetics of Cr (III) conform to several models, using jackfruit treated with EDTA, NaOH, and HNO\({}_{3}\) at pH = 4 [10]; _Citrus sinensis_ treated with water at pH = 4 [18]; rice hus at pH in the range of 5-6 [16]; carrot residues treated with HCl at pH in the range 2-5 [30]; _Sargassum_ treated with formaldehyde at pH = 2 [35]; _S. cerevisiae_ at pH = 4.3 [37], among the more reported are those of pseudo second order [15; 17; 18; 29; 30], pseudo first order [35], and Elovich [19], which is consistent with what was obtained in the present work. Similarly, it has been reported that the percentage of removal of Cr varies from 40% to 95% [10; 18; 19; 30; 35; 38; 39]; therefore, it is possible that WTOP is a biomaterial with optimal Cr (III) adsorption and removal capacity that allows considering this bioadsorbent as a candidate for use in the elimination of Cr (III) ions in aqueous solution.