Detoxification of substituted phenols by oxidoreductive enzymes through polymerization reactions

Abstract

Laccases from the fungiRhizoctonia praticola andTrametes versicolor as well as horseradish peroxidase and tyrosinase were evaluated for their ability to polymerize phenolic contaminants. The removal of phenols through polymerization depended on the chemical structure and concentration of the substrate, pH of the reaction mixture, activity of the enzyme, length of incubation, and temperature. The enzymes retained their activity throughout a broad range of pH (pH 3.0 to 10) and temperature (5 to 55°C). The removal of halogenated phenols decreased with increasing number of chlorines and increasing molecular weight of the substituent. Laccases fromR. praticola andT. versicolor removed 2,4-dichlorophenol at initial concentrations of up to 1,600 mg/L. The amount of the substrate removed increased with increasing enzyme activity. The precipitates formed during polymerization of 2,4-dichlorophenol constituted a mixture of oligomers with average molecular weights of up to 800 for the fraction soluble in dioxane. Mass spectra revealed the loss of chlorine atoms during enzymatic polymerization. The release of chloride ions into solution during polymerization amounted to up to 20% of the chlorine initially associated with the 2,4-dichlorophenol molecule. Dechlorination contributes to the overall detoxification effect which results from enzymatic polymerization.