White bean (Phaseolus vulgaris L.) as a sorbent for the removal of zinc from rainwater

Abstract

The present work aimed to assess the sorption capacity of the common white bean (Phaseolus vulgaris L.) to remove Zn(II) from rainwater, rendering it suitable for use in buildings, and the efficiency of the process was evaluated for two initial Zn(II) concentrations, representing high (100 μg L^-1) and very high (500 μg L^-1) levels of Zn(II) in rainwater. The effects of the amount of beans (1, 5 and 10 beans per 50 mL), as well as the initial pH values of the zinc solution [acid (4), neutral (5.6) and basic (7) for atmospheric waters] were also assessed. The removal of Zn from water was affected by the change in pH values. When 5 and 10 beans were used, after 4 h and 2 h of contact time, respectively, the accumulated Zn(II) on the beans was released back into the solution, and this release occurred first for the highest tested pH value. The sorption rate of Zn(II) from the solution increased with the increasing amount of beans, but for 5 and 10 beans this only took place up to 4 h and 2 h, respectively. Furthermore, the removal percentages of Zn(II) increased with the increase of the initial concentrations of the metal in water. Kinetic studies revealed that a pseudo-first-order model provided the best fitting for the experimentally obtained values. Fourier transform infrared spectroscopy coupled with attenuated total reflectance (FTIR-ATR) analyses of the bean shells and cores indicated that contact with a Zn(II) solution did not cause notable alterations to the chemical structures of these bean components. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses suggested that the process of sorption occurred at the surface of the beans (shells). The results obtained in this study also suggest that the matrix of rainwater samples did not interfere with the removal of metal, and that the process of the removal of Zn(II) by the white beans can be applied to real samples. On the whole, results indicate that for the removal of Zn(II) from rainwater, 1 bean up to 6 h, or 5 and 10 beans up to 2 h can be used per 50 mL for the removal of up to 60% of Zn(II) present in water, thus constituting a viable solution for the effective reduction of this metal in rainwater.

Keywords: Biosorbent; Rainwater; Sorption; White bean; Zinc.