Product formation from phenolic compounds removal by laccases: A review

• Laccases are used for the removal of recalcitrant micropollutants in water.
• Insoluble reaction products are generated by reaction between laccases and substrates.
• Insoluble reaction products are most of the time not completely identified.
• Highlighting that some insoluble products do not present any estrogenic or toxic effects.

Lignolytic enzymes are very promising tools for biotechnological applications in water treatment. Among them, laccases (EC 1.10.3.2: p-diphenol:dioxygen oxidoreductase) are versatile enzymes with the ability to oxidize a wide range of aromatic and non-aromatic compounds (polyphenols, aryl diamines, anilines, methoxy substituted phenols, organic/inorganic metal compounds, etc.). Laccases have confirmed their potential for the degradation of recalcitrant residual phenolic compounds in water. Typically, oxidation of a laccase substrate leads to the formation of free radical with the concomitant loss of one electron and reduction of molecular oxygen into water. The formed radicals can react via non-enzymatic reactions i.e. oxidative condensation, and oxidative phenolic coupling or oxidative coupling, leading to the formation of soluble or non-soluble reaction products, several of them of polymeric nature. As many studies have highlighted the generation of products potentially more toxic than the parent molecules with the use of tertiary treatments such as advanced oxidative processes (AOPs) applied in water treatment plans, the identification of reaction products generated by the degradation of phenolic contaminants in water by laccases, and their potential toxicity, have to be investigated. This review summarizes and discuss progress made this last decades in identification of products and their risk assessment regarding their potential toxicity for a list of the most studied recalcitrant phenolic pollutants in water, such as dyes, bisphenol A, triclosan, nonylphenol, natural and synthetic hormones, etc.