Enhancing the Production of Hydroxyl Radicals by Pleurotus eryngii via Quinone Redox Cycling for Pollutant Removal

The induction of hydroxyl radical ((OH)-O-center dot) production via quinone redox cycling in white-rot fungi was investigated to improve pollutant degradation. In particular, we examined the influence of 4-methoxybenzaldehyde (anisaldehyde), Mn2+, and oxalate on Pleurotus eryngii (OH)-O-center dot generation. Our standard quinone redox cycling conditions combined mycelium from laccase-producing cultures with 2,6-dimethoxy-1,4-benzoquinone (DBQ) and Fe3+-EDTA. The main reactions involved in (OH)-O-center dot production under these conditions have been shown to be (i) DBQ reduction to hydroquinone (DBQH(2)) by cell-bound dehydrogenase activities; (ii) DBQH(2) oxidation to semiquinone (DBQ(center dot-)) by laccase; (iii) DBQ(center dot-) autoxidation, catalyzed by Fe3+-EDTA, producing superoxide (O-2(center dot-)) and Fe2+-EDTA; (iv) O-2(center dot-) dismutation, generating H2O2; and (v) the Fenton reaction. Compared to standard quinone redox cycling conditions, (OH)-O-center dot production was increased 1.2- and 3.0-fold by the presence of anisaldehyde and Mn2+, respectively, and 3.1-fold by substituting Fe3+-EDTA with Fe3+-oxalate. A 6.3-fold increase was obtained by combining Mn2+ and Fe3+-oxalate. These increases were due to enhanced production of H2O2 via anisaldehyde redox cycling and O-2(center dot-) reduction by Mn2+. They were also caused by the acceleration of the DBQ redox cycle as a consequence of DBQH(2) oxidation by both Fe3+-oxalate and the Mn3+ generated during O-2(center dot-) reduction. Finally, induction of (OH)-O-center dot production through quinone redox cycling enabled P. eryngii to oxidize phenol and the dye reactive black 5, obtaining a high correlation between the rates of (OH)-O-center dot production and pollutant oxidation.