Laccases: Potential treatment for removal of micropollutants in municipal wastewater?
Relatively high concentrations of micropollutants such as pharmaceuticals and biocides in municipal wastewater treatment plants (WWTP) effluents underscore the necessity to develop new treatments. Organisms that produce oxidative enzymes such as laccases are a potential means to improve biodegradation of these compounds. Laccases are polyphenol oxidases widespread among plants, fungi and bacteria. They are able to catalyse the oxidation of various aromatic compounds, including several pharmaceuticals and biocides. The goals of this study were to evaluate the potential of four strains of Streptomyces bacteria (S. cyaneus, S. ipomoea, S. griseus and S. psammoticus) and the white-rot fungus Trametes versicolor for i) their ability to produce laccase in municipal wastewater with diverse growth substrates, ii) their laccase activity at different pH, temperature and salinity values, and iii) their ability to degrade selected micropollutants, especially diclofenac (DFC), mefenamic acid (MFA) and bisphenol A (BPA), in buffer solutions and in municipal wastewater. Among the Streptomyces strains evaluated, only S. cyaneus produced extracellular laccase in quantities sufficient to envisage its potential use in wastewater treatment. Laccase activity produced by T. versicolor was however more than 20 times higher, the highest activity being observed in wastewater with ash branches as the sole carbon source, a cheap and easily available substrate. S. cyaneus (LSc) and T. versicolor (LTv) laccases were mainly active for acidic pH conditions (optimal pH in the range < 4 to 6). LTv showed significant activity for wider pH and temperature ranges, especially for near-neutral pH and between 10-25°C, conditions found in municipal wastewater. Both laccases could oxidize the selected micropollutants in buffer solutions, with faster degradation observed with LTv, especially at pH 7. However, in municipal wastewater (pH 7.6-7.8), no significant removal of DFC and MFA was observed with LTv alone (in 12 h, 23°C, 1000 U l-1), unless the pH was decreased to 6.7-7 and mediator (ABTS) added. High removal (> 80%) of these two compounds was reached in less than 24 h when T. versicolor was present in the wastewater, even at an initial pH of 7.9. This suggests that, despite the potentially efficacious oxidative capacity of laccase, treatment of micropollutants in municipal wastewater without pH adjustment is only feasible with the presence of the whole organism. The ability of T. versicolor to degrade recalcitrant pollutants such as DFC in municipal wastewater is therefore very promising. New challenges have now to be faced to develop a treatment suitable for municipal WWTPs.
Record created on 2013-11-05, modified on 2016-08-09