## 4 Conclusions  
The white bean peel sorbent was able to reduce the concentration of Zn(II), Cu(II) and Pb(II) in an aqueous solution, and the removal increased with time and with the increase of their initial concentration in water. The Zn(II) removal was not affected by the initial pH value of the solution, but the Cu(II) and Pb(II) removals were, since at pH 7.0 for Cu(II) and at pH 5.6 and 7.0 for Pb(II) occur the formation of carbonates of the elements that precipitate. It was possible to achieve models that describe the removal of Zn(II) and Cu(II) in mixed solutions of Zn(II), Cu(II) and Pb(II) by response surface methodology, and they are of second-order polynomial surface: for Zn(II) the variables are the initial concentration, time, the quadratic form of time, and the interaction between concentration and time; for Cu(II) the variables are the time, pH, and the quadratic form of the initial concentration. The optimal conditions obtained through the models, being 6 h of contact time between the sorbent and the solution, predict removals higher than 91% for Zn(II) and 52% for Cu(II). When applying the optimal conditions to rainwater samples, the results showed that the rainwater matrix affected the removal of the PTEs since generally were obtained lower removals than predicted by the models. SEM and FTIR-ATR studies showed that no clear changes occurred in the morphology and in chemical structure of the peel during the sorption process. FTIR-ATR spectra highlighted a decrease in the intensity of some peaks attributed to the stretching of O-H bonds and C=O bonds of carboxylic groups, suggesting that such groups in the surface of the peel were involved in the removal of the PTEs. This research showed, for the first time, that the peel of the white bean, isolated from the bean core, may be used as a sorbent for the removal of a mixture of PTEs from rainwater. The white bean peel is a natural, low-cost, and sustainable sorbent that can be easily used for rainwater treatment, but more research is needed in order to combine this sorbent with other processes of removal of contaminants/interferents, namely organics that should be previously removed, to guarantee an effective rainwater treatment. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app15020627/s1](https://www.mdpi.com/article/10.3390/app15020627/s1)]([https://www.mdpi.com/article/10.3390/app15020627/s1](https://www.mdpi.com/article/10.3390/app15020627/s1)), Figure S1: UV-Vis spectra of aqueous solutions from organic matter released by 1, 2, and 5 bean peels to ultrapure water (50 mL) after 2 h, 4 h, and 6 h; Figure S2: Percentage of Zn(II) removal predicted by model in function of the experimental percentage of removal; Figure S3: Percentage of Cu(II) removal predicted by model in function of the experimental percentage of removal; Figure S4: EDX spectra of interior (A) and exterior (B) surface of the white bean peel after 6 h of contact with PTE solution 500 \(\mu\)g/L at pH 4.0; Table S1: Models obtained by application of best subsets regression for Zn(II) removal; Table S2: Models obtained by application of best subsets regression for Cu(II) removal; Table S3: Characterization of rainwater samples regarding pH and concentrations of Zn(II), Cu(II), and Pb(II). Conceptualization, P.S.M.S. ; methodology, G.H.C., A.C.E. and P.S.M.S. ; validation, G.H.C., A.C.E. and P.S.M.S. ; formal analysis, G.H.C. ; investigation, G.H.C., A.C.E. and P.S.M.S. ; resources, A.C.E. and P.S.M.S. ; writing--original draft preparation, G.H.C. and P.S.M.S. ; writing--review and editing, G.H.C., A.C.E. and P.S.M.S. ; visualization, P.S.M.S. ; supervision, A.C.E. and P.S.M.S. ; project administration, P.S.M.S. ; funding acquisition, A.C.E. and P.S.M.S. All authors have read and agreed to the published version of the manuscript.