Phenol removal by enzymes: benefits and shortcomings

Comparisons have been performed of different phenol-oxidizing enzymes (phenol-oxidases, tyrosinases, laccases) from different sources. Specific reference has been made to substrate specificity, operational stability and contaminant immobilization efficiency, in order to assess suitability for environmental applications. Phenol-oxidase and tyrosinase enzymes, from plant sources, have been tested in batch experiments by using minced tissues, acetone powders and raw extracts. Substrate specificity is usually broad and absorption also takes place when tissues are used. Operational stability is low, due to product inactivation. Final reaction-products are insoluble polymers that can be removed by physical methods or immobilized into the humic matrix of the soil. As regards suitability for environmental applications, stability problems generally discourage the use of purified, immobilized enzymes. Minced tissues or raw extracts from plant tissues may be used successfully in wastewater at low phenol concentration without enzyme recovery, in view of the reasonable cost of the sources. As regards phenol-contaminated soil, the widespread presence of endogenous, mainly abiotic, activity makes the addition of plant enzymes unproductive for the most reactive contaminants such as di-phenols. Since batch experiments with soil slurries in the presence of variously substituted mono-phenols show exiguous natural activity, the use of plant extract or tissues with high phenol-oxidase activity is not completely ruled out. Comparison with literature data on wastewater and contaminated soil aerobic treatment shows that the rate of phenol removal by enzymes and natural abiotic catalysts can proceed faster than biodegradation, due to toxicity effects on microbial metabolism.