## 3 Results and Discussion
### Optimization of the Sorption Process of Zn(II), Cu(II) and Pb(II) by the Bean Peel
The relationship between the predicted and experimental values of removal percentages was shown to be linear with a strong correlation (\(R^{2}\) = 0.9437; see Supplementary Materials Figure S3), confirming that the chosen model is appropriate for estimating copper removal. After identification of the best models to predict the removal percentages of Zn(II) and Cu(II), and based on their equations, the optimal conditions under which the greatest removal of these elements by the biomaterial is attained were determined. Concerning Zn(II), it was determined that the optimal conditions are obtained when a contact time of 5 h 18 m is applied for all initial concentrations studied, achieving a removal percentage higher than 95%. Regarding the optimal conditions for Cu(II), with a contact time of 6 h, it was determined that it is possible to achieve a removal of between 52% and 87% for the initial concentration of 100 \(\mu\)g/L, between 55% and 90% for the initial concentration of 250 \(\mu\)g/L, and between 66% and 100% for the initial concentration of 500 \(\mu\)g/L, with the lowest removal percentages obtained for the pH value of 7.0. As shown in Figure 3, a decrease in pH value leads to an increase in the copper adsorption capacity of the bean peel, and at a neutral pH for rainwater (pH 5.6), it is possible to remove percentages up to 69%. Considering that the solutions used contain a mixture of Zn(II), Cu(II), and Pb(II) metals, it was necessary to find optimal conditions that could be applied simultaneously to them. In fact, when using mixed solutions, competition of elements for the available sites may occur at the surface of the biomaterial white bean peel. The greater affinity of the bean peel for adsorbing Zn(II) when compared with Cu(II) at a higher pH was highlighted by the higher values of removal. Therefore, to obtain the highest removal percentage for Cu(II) for all pH  
Figure 3: Response surfaces obtained from Equation (5) for Cu(II) removal (%), showing the interaction between the removal percentage, time (h), pH, and initial concentration (\(\mu\)g/L). values evaluated, the optimal conditions consist of a contact time of 6 h, estimating removal percentages higher than 52% in the range of concentrations evaluated. On the other hand, at 6 h of contact time, the removal of zinc is higher than 91% for all concentrations and pHs evaluated, slightly lower than estimated for 5 h 18 m. Concerning the Pb(II), it is expected that at 6 h for pH 4.0, the removal will be higher than 74% for 100 \(\mathrm{\SIUnitSymbolMicro g}\)/L and higher than 90%, on average, for the concentrations of 250 \(\mathrm{\SIUnitSymbolMicro g}\)/L and 500 \(\mathrm{\SIUnitSymbolMicro g}\)/L (Figure 1), while for the remaining pH values, it is not possible to estimate the removal due to the loss of Pb(II) by precipitation. For the optimal conditions defined, namely 6 h of contact time between the white been peel and the PTE solution, and at the neutral pH for atmospheric water (5.6), the amount of PTEs sworbed by gram of sorbent (\(q_{t}\)) was, for Zn(II), 0.10 mg/g for 100 \(\mathrm{\SIUnitSymbolMicro g}\)/L and 0.55 mg/g for 500 \(\mathrm{\SIUnitSymbolMicro g}\)/L; for Cu(II), 0.081 mg/g for 100 \(\mathrm{\SIUnitSymbolMicro g}\)/L and 0.48 mg/g for 500 \(\mathrm{\SIUnitSymbolMicro g}\)/L; and for Pb(II), 0.41 mg/g for 500 \(\mathrm{\SIUnitSymbolMicro g}\)/L. Comparing the amount of Zn(II) sworbed per gram of white bean peel for 6 h at pH 5.6 with that obtained for the whole white bean (1 bean) in the study of [PERSON] et al. [31], which was 0.0057 mg/g for 100 \(\mathrm{\SIUnitSymbolMicro g}\)/L and 0.033 mg/g for 500 \(\mathrm{\SIUnitSymbolMicro g}\)/L, the results show that in an ultrapure water solution, the amount sworbed by gram of biosorbent was higher with the bean peel than with the whole bean, which highlights the advantage of using only the peel instead of the whole bean.