Properties of polyphenols and polyphenol-containing wastewaters and their treatment by Fenton/Fenton-like reactions

Polyphenols commonly exist in agro-industrial wastewaters, such as soft drink wastewater, cork wastewater, oil mill wastewater, landfill leachate, pulp mill wastewater, winery wastewater, and tannery wastewater. Poly -phenols at high concentration tend to suppress or even eliminate the efficiency of biological wastewater treat-ment processes. Fenton/Fenton-like reactions as complementary treatments to biological processes have been reviewed in this article for the removal of polyphenols in the above wastewaters. Only a few researchers have taken advantage of the chelating and reducing properties of polyphenols and used polyphenol-containing wastewaters, such as cork wastewater and oil mill wastewater, to enhance the efficiency of Fenton/Fenton-like processes for the treatment of other contaminants. Some other researchers use individual polyphenols such as gallic acid, protocatechuic acid, and catechol as models of polyphenol-containing wastewaters. The treatment of these compounds by Fenton/Fenton-like reactions is also reviewed in this article. In the Fenton/ Fenton-like processes, hydroxyl radical (center dot OH) is considered as the main species for decomposing polyphenols. The possible reaction mechanisms of center dot OH with polyphenols is discussed, as well as the reaction impact factors, including the number and position of hydroxyl groups in phenolic rings and the properties of other functional groups (electron-donating groups or electron-withdrawing groups). The center dot OH attack on phenolic compounds (e.g. phenol) in Fenton reaction can generate polyphenols, which, in turn, enhances the efficiency of Fenton process.