## 3 Results and Discussion
### Sorption of the Mixture of Zn(II), Cu(II) and Pb(II) by Bean Peel in Rainwater  
With the purpose of understanding the effect of the rainwater matrix on the removal of Zn(II), Cu(II), and Pb(II) elements by the bean peel, experiments with six rainwater samples spiked with a mixture of elements with concentrations of 100 \(\upmu\)g/L and 500 \(\upmu\)g/L were performed, at their natural pHs. However, rainwater samples already presented PTEs with concentrations of between 5.3 and 21.9 \(\upmu\)g/L, 1.2 and 3.3 \(\upmu\)g/L, and 0.4 and 6.1 \(\upmu\)g/L for Zn(II), Cu(II), and Pb(II), respectively (see Supplementary Materials Table S3). Rainwater samples were numbered from one to six (1-6), and their natural pHs were 6.1, 5.8, 5.9, 6.0, 6.3, and 6.1, respectively, which are close to the neutral pH for atmospheric water. The optimal conditions for the removal of the elements, 6 h of the contact time between the bean peel and the aqueous solution of PTEs, were applied to the rainwater samples spiked with the mixture of the elements, and the results obtained are displayed in Table 4. This table also presents the data predicted by the models for Zn(II) and Cu(II). Considering the results achieved for the spike of 100 \(\upmu\)g/L, were observed removals for Zn(II) of 83% for sample 1 and between 38% and 53% for the remaining samples. When applying the spike of 500 \(\upmu\)g/L, the percentages were similar to those obtained with the spike of 100 \(\upmu\)g/L. With the exception of sample 1, the results obtained are substantially inferior to those predicted by the model for Zn(II) removal. Nevertheless, [PERSON] et al. [31] removed Zn(II) from rainwater samples spiked with 100 \(\upmu\)g/L and 500 \(\upmu\)g/L using the whole bean (5 beans in 2 h, to guarantee a low level of organic matter released by the sorbent) having obtained removal percentages higher than 58%, which were generally superior to those obtained with the bean peel in the present study. Therefore, since the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Spike/Sample**} & \multirow{2}{*}{**Zn(II)**} & \multicolumn{2}{c}{**Cu(II)**} & \multicolumn{2}{c}{**Pb(II)**} \\ \cline{2-6}  & & **Predicted by the** & **Model (\%)** & **Experimental (\%)** & **Predicted by the** & **Experimental (\%)** \\ \hline
**100 \(\upmu\)g/L** & & & & & \\ Sample 1 & 83 & 94 & 48 & 63 & 89 \\ Sample 2 & 53 & 94 & 25 & 66 & 48 \\ Sample 3 & 38 & 94 & 12 & 65 & 38 \\ Sample 4 & 45 & 94 & 16 & 64 & 38 \\ Sample 5 & 47 & 94 & 11 & 61 & 11 \\ Sample 6 & 53 & 94 & 27 & 63 & - \\ \hline
**500 \(\upmu\)g/L** & & & & & \\ Sample 1 & 89 & 91 & 78 & 77 & 97 \\ Sample 2 & 47 & 91 & 31 & 80 & 49 \\ Sample 3 & 32 & 91 & 23 & 79 & 35 \\ Sample 4 & 47 & 91 & 36 & 78 & 58 \\ Sample 5 & 52 & 91 & 35 & 74 & 60 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental and predicted results from response models for Zn(II), Cu(II) and Pb(II) removal from rainwater samples. Zn(II) sorbed per gram of biosorbent was higher for the bean peel than for the whole bean (presented above), such findings may be due to the usage of only one bean peel in the present study, while in the work of [PERSON] et al. [31] five whole beans were used, increasing the available surface area for the interaction with the metals. Regarding the biosorption of Cu(II) by the bean peel, for the spike of 100 \(\mathrm{\SIUnitSymbolMicro g}\)/L, the removal percentage was notably inferior to Zn(II) for all samples, with sample 1 removing 48% and the remaining five samples removing between 12% and 27%. For the spike of 500 \(\mathrm{\SIUnitSymbolMicro g}\)/L, an increase in removal percentages was observed (an increase of \(\sim\)15%), as expected, since according to the model chosen for Cu(II), an increase in concentration leads to an increase in removal. However, when comparing the experimental results obtained for the samples with the results predicted from the model, lower percentages were obtained experimentally for all the rainwater samples.