Understanding the role of mediators in the efficiency of advanced oxidation processes using white-rot fungi

The coupling of biological with advanced oxidation processes for the removal of 13 pharmaceuticals is reported in this study. The efficiency of this system was examined by using two ligninolytic fungi, Trametes versicolor and Ganoderma lucidum. Pharmaceuticals' removal was examined by using four different quinone substances (1,4-benzoquinone, 2,6-dimethoxy-1,4-benzoquinone, 2-methyl-1,4-naphthoquinone and gallic acid). The quinone redox cycling mechanism of the fungal species promotes the production of hydrogen peroxide for subsequent decomposition to non-selective and highly oxidizing hydroxyl radicals in presence of Fe(III)-oxalate. Results evidenced that the most effective mediators for T. versicolor and G. lucidum were 2,6-dimethoxy-1,4-benzoquinone (DMBQ) and gallic acid (GA), respectively. Time-course experiments showed that higher concentrations of Fe(II) were produced using DMBQ and GA, suggesting that the efficiency of the mediator in reducing Fe(III) is linked to the removal of pharmaceuticals. The application of DMBQ in the redox cycling process of T. versicolor resulted to 60-100% removal for all the pharmaceuticals, due to the high affinity of laccase enzyme for DMBQH(2). The advanced bio-oxidation of T. versicolor was tested for hospital wastewater treatment. Results showed a decrease of the elimination degree for pharmaceuticals as compared to the previous bioassays conducted with a synthetic medium. Nevertheless, significant removals (30-100%) were achieved by using DMBQ and GA as mediators of the redox cycle for T. versicolor under the non-sterilized conditions of the hospital wastewater. The use of GA is of great importance since its phenolic form, equivalent to the hydroquinone form, is considered as a critical parameter in Fe(III) reduction.