## 4 Discussion
Sugarcane bagasse was analyzed as an adsorbent, and the maximum removal rate was observed at pH 6.5 [10]. Once the pH basic is near, nickel precipitation occurs [36], which also depends on the concentration of metal in the solution. This might be the reason for the decreased percentage of adsorption after pH 5.0. A similar increase in adsorption until pH 5 was reported previously [2,44]. These authors stated that metal adsorption is mostly based on an ion exchange mechanism between metal ions and existing counter-ions on negatively charged ionic groups and that metal ions must compete with protons for the binding sites. Therefore, higher pH will mean lower competition and higher adsorption. The mechanisms that occur during the retention of ions in a biosorbent are directly related to the functional groups present in the material. These groups are normally present in structures in the cell wall, which include cellulose organized in microfibers, surrounded by hemicelluloses, lignin, pectin and small quantities of proteins. This way, the retention of metal ions by the material surface also depends on its chemical composition, which explains the variability found for each material. Nevertheless, since only the initial pH of the solution is measured, the final pH of the water with the adsorbent might also contribute to the differences between materials. Figure 7 presents the adsorption isotherms of nickel for walnut shell, chestnut shell, pine wood and burnt pine wood. Results show that equilibrium is reached for around 100 mg/L for most of the materials. Walnut shell adsorption was not very consistent for concentrations higher than 100 mg/L but this is probably due to the major errors obtained when determining higher concentrations in atomic adsorption. The data suggest that equilibrium was reached earlier in chestnut shell which can be due to the combination of higher pore volume and the second highest pore size, since this is in accordance with [PERSON] and [PERSON] [40], where higher pore size and pore volume adsorption equilibrium is achieved earlier. The Langmuir model (Figure 8) assumes that when adsorption occurs in a monolayer surface, with a finite number of active sites and homogeneous energy of adsorption, no additional adsorption can occur at the same site [11]. The theory is based on the fact that adsorption occurs at uniform sites [45], and takes place without interaction between the adsorbed molecules [37]. Each active site is equal, with the capability to attach particles independent of other active sites being filled or not [11]. On the other hand, Freundlich isotherm (Table 2) is derived by assuming a heterogeneous surface with a non-uniform distribution of heat of adsorption over the surface [24]. The heterogeneity of the adsorbent surface affects adsorption equilibrium and kinetics [46]. Heterogeneous surfaces include more than one type of adsorption site, and each type has a distinct heat of sorption. It is a model that is suitable for multilayer adsorption, for adsorbents with a heterogeneous surface and a uniform distribution of adsorption heat [11]. The model considers that metal ions are infinitely accumulated on the surface of the adsorbent [45]. Thus, the adsorptive capacity is related to the concentration of metal ions in equilibrium in the solution. The experimental results showed that the Langmuir model fitted the sorption processes best because it presented higher R\({}^{2}\) for all biosorbents tested, with values ranging between 0.994 for pine wood and 0.998 for chestnut shell. As found in this study, the sorption promoted by activated carbon prepared from almond husk was better described by the Langmuir isotherm with R\({}^{2}\) = 0.983 and 0.995 [43]. Similarly, the nickel sorption by protonated rice bran fitted well with the Langmuir isotherm model with R\({}^{2}\) about 0.941 [5]. Even though the Langmuir model presented the best results, the Freundlich model also predicted the adsorption process well.