Enhanced removal of Cr(VI) in the Fe(III)/natural polyphenols system: role of the in situ generated Fe(II)

This study developed a cost-effective and eco-friendly method by coupling plant extracts (take green tea for example) and Fe(III) to reduce Cr(VI) and precipitate Cr(Ill). At acidic pH, 1.43 mM Fe(III) combined with 1.33 g/L green tea extracts could reduce 93% of Cr(VI) in 180 min, which was much larger than (similar to)50% by green tea extracts alone. Moreover, 52% of Cr(III) could automatically precipitate out as mixed Fe(III)-Cr(III) (oxy)-hydroxide solids. In the viewpoint of mechanism, polyphenols in green tea extracts were the reactive constituents and transformed Fe(III) to Fe(II), by which step the aqueous Fe(II) level was maintained to continuously reduce Cr(VI) to Cr(III), and thus accelerating Cr(VI) reduction. The generated Fe(III) partially participated in the reaction with polyphenols again and some Fe(III) formed precipitates with Cr(III). Overall, the electron transfers in the polyphenol-Fe-Cr cyclic reactions made Fe(III) used for multiple times, thus accelerated Cr(VI) reduction. The applicability of the combined process was further verified by removing 100% and 70% of Cr(VI) from electroplating wastewater and contaminated soil, respectively. As polyphenols can be derived from plant wastes and Fe(III) is naturally abundant, this study provides a promising method for in situ remediation of Cr(VI)-contaminated sites.