The interactions of polyphenols with Fe and their application in Fenton/Fenton-like reactions

Polyphenols are aromatic substances with one or more hydroxyl groups together with other substituents attached to aromatic rings. Polyphenols can be found in regular foods and drinks, including teas, coffee, vegetables, fruits, etc. Polyphenols have anti-oxidant, pro-oxidant, anti-histamine, anti-bacterial, anti-inflammatory, and anti-viral properties. They present chelation and reduction activities to iron ion and the anti-oxidant properties are mainly induced by the iron chelation effect. The metal chelation activity of polyphenols is primarily related to the presence of ortho-dihydroxy groups. Polyphenols can reduce Fe(III) to Fe(II) via a one-electron transfer pathway and they are oxidized to semiquinones. Due to the chelation and reduction properties, polyphenols are applied by researchers to promote the efficiency of Fenton/Fenton-like reactions for contaminant removal. The application of polyphenols in Fenton/Fenton-like reactions is reviewed in this article for the first time. The polyphenols reviewed include dihydroxybenzenes (such as catechol and hydroquinone), phenolic acids (such as gallic acid, protocatechuic acid, and tannic acid), flavonoids (such as catechin and epigallocatechin-3-gallate), and polyphenol extracts from tea, woods, coffee, etc. As a substance that pervades almost every aspect of human life, polyphenols present promising effect on the waste treatment in the environmental field, which deserves more studies.