## 3 Results and Discussion  
### 3.1. Sorption of Zn(II), Cu(II), and Pb(II) by the Bean Peel  
Figure 1 shows the variation of the normalized concentration (\(C_{t}\)/\(C_{0}\)) of Zn(II), Cu(II) and Pb(II) in the mixed aqueous solutions with time (h), either for the experiments with the experimental design used or for the controls. Figure 1: **(a)** Variation of the normalized concentration (\(C_{t}\)/\(C_{0}\)) of Zn(II) (upper line graphics), Cu(II) (middle line graphics), and Pb(II) (bottom line graphics) as function of time (0–6 h), with the respective standard deviation, for initial concentrations of 100 \(\upmu\)g/L (left column), 250 \(\upmu\)g/L (middle column), and 500 \(\upmu\)g/L (right column) and for different pH values (4.0, 5.6, 7.0). **(b)** Variation of the corrected, normalized concentration (\(C_{t}\)/\(C_{0}\)) of Cu(II) as function of time (h), with the respective standard deviation, for initial concentrations of 100 \(\upmu\)g/L, 250 \(\upmu\)g/L and 500 \(\upmu\)g/L and for different pH values (4.0, 5.6, 7.0). As shown in Figure 1a, the concentration of controls for Zn(II) remained stable over time (variations lower than 7.5%, except for 100 \(\upmu\)g/L at pH 7.0 with variations up to 14%) for the three studied pH values. Regarding the controls of Cu(II), they showed high stability at pH 4.0 and 5.6 (variation lower than 8.4%), while at pH 7.0, the Cu(II) concentration decreased with time, having been more notorious for the concentration of 100 \(\upmu\)g/L (up to 40%). This decrease may be explained by the formation of copper carbonate (CuCO\({}_{3}\)) but also of copper hydroxide (CuOH\({}^{+}\)), whose concentration increases from about a pH value of 5.0 up to 8.0 [38]. Concerning the controls of Pb(II), only those at pH 4.0 remained at stable concentrations over time (variation lower than 4.7%), while at higher pH values, their concentrations decreased a lot (up to 87% at pH 5.6 and up to 98% at pH 7.0). These findings may be explained by the formation of species of lead carbonate (PbCO\({}_{3}\)) and lead hydroxide (PbOH\({}^{+}\)) from pH values of 4.5 and 4.0, respectively, and up to about a pH value of 8.0 [38], which may have contributed to their precipitation, decreasing the Pb(II) ion concentration in the aqueous solution. In addition, Figure 1a also shows that the normalized concentration of Pb(II) for pH 5.6 and 7.0 was generally lower for the controls than for the experiments, evidencing that the removal was not due to the adsorption by the bean peel but due to the decreased number of Pb(II) ions in the solution. Therefore, in the case of copper and lead, since the controls at pH 7.0 for Cu(II) and at pH 5.6 and 7.0 for Pb(II) presented variations of concentration systematically above 15%, it was necessary to correct the concentration values obtained on the removal experiments to obtain the values of removal, effectively referring to the interaction between the metals and the bean peel. Table 2 shows the removal percentages of the biomaterial white bean peel upon removing the Zn(II), Cu(II), and Pb(II) elements by application of the experimental design. As highlighted in Figure 1, generally, the removal of elements increased with the increase of the contact time, thus indicating the uptake of the elements by the white bean peel. This can be clearly seen, for example, at the initial concentration of 100 \(\upmu\)g/L for Zn(II) and Cu(II) at pH 5.6, where an increase in time of contact from 2 h to 6 h leads to an increase in the adsorption from 54% to 91% and 37% to 68%, respectively, and at the initial concentration of 250 \(\upmu\)g/L for Pb(II) at pH 4.0, where with the increase of contact time from 2 h to 6 h, the removal increased from 55% to 94%. Concerning the increase of the initial concentration of the elements in the solution, it was verified that the higher the concentration, the greater the removal of the metals from the solution.