Influence of treatment conditions on the oxidation of micropollutants by Trametes versicolor laccase
Many organic compounds present at low concentrations in municipal wastewater, such as various pharmaceuticals and biocides, are recalcitrant in conventional wastewater treatment plants (WWTPs). To improve their biodegradation, oxidoreductase enzymes such as laccases were tested. The goal was to find optimal conditions for the transformation of two anti-inflammatory pharmaceuticals (diclofenac (DFC) and mefenamic acid (MFA)), one biocide (triclosan (TCN)) and one plastic additive (bisphenol A (BPA)) by Trametes versicolor laccase. Experiments were conducted in spiked solutions at different pH values (from 3 to 9), enzyme concentrations (70–1400 U l1), reaction times (0–26 hours) and temperatures (10, 25 and 408C) following a Doehlert experimental design. A semi-empirical model was developed to understand better the combined effects of the four factors and to determine optimal values. This model was able to fit well the experimental data and showed good predictive ability. All four factors had a significant effect on the micropollutant oxidation with the greatest influence shown by pH. Results for single compounds were different from those obtained for mixtures of micropollutants. For instance, DFC transformation occurred at much higher rates in mixtures under alkaline conditions. Optimal conditions were compound-dependent, but were found to be between pH 4.5 to 6.5 and between 258C to more than 408C. With a suitable laccase concentration, we obtained a high removal rate (>90%) of the four individual compounds (range of times: 40 min to 5 hours), showing the potential of laccases to improve biodegradation of environmentally persistent compounds.