Two-step, low-cost and biocompatible removal of hexavalent chromium using yerba mate (Ilex paraguariensis) and montmorillonite-rich bentonite

Abstract

Due to its widespread occurrence, potent ecotoxicity, environmental persistence, and bioaccumulation potential, aqueous Cr contamination remains a major global concern. Herein a novel two-step, low-cost and biocompatible process for removing Cr from polluted water was proposed which comprises an initial Cr(VI) to Cr(III) reduction step using an aqueous yerba mate (Ilex paraguariensis) extract (YM), which contains high polyphenol concentrations, followed by a second Cr(III) removal step using montmorillonite-rich bentonite (MMT), a phyllosilicate mineral belonging to smectite group that has a high adsorption capacity. The solution obtained after the first step was analysed by UV–visible spectrophotometry and the solid obtained after the second step was analysed by X-ray diffraction (XRD) using a Small-Angle X-ray Scattering (SAXS) instrument and by infrared Fourier transform (FTIR) spectroscopy. Batch experiments using a starting Cr(VI) concentration of 10 mg L−1, and optimised via response surface methodology, showed that optimal conditions were pH 3.0 and a YM/Cr(VI) = 0.600 for the first step, achieving 98% Cr(VI) reduction within 150 min, with 1.91 g L−1 MMT for the second step, resulting in 80% Cr(III) removal within 120 min. Analysis of residues using UV–vis spectrophotometry,XRD and FTIR suggested the formation of aqueous complexes between Cr(III) and the oxidation products of YM by Cr(VI) and the adsorption of those complexes and Cr(III) into the interlaminar region and external surface of MMT bythe formation of surface complexes. Overall, the results demonstrate YM-MMT as highly effective, and low-cost biogenic sorbent for chromium removal from acidic waters.