Enhancing the production of hydroxyl radicals by Pleurotus eryngii via quinone redox cycling for pollutant removal

Abstract

The induction of hydroxyl radical (.OH) 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 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 production under these conditions have been shown to be (i) DBQ reduction to hydroquinone (DBQH2) by cell-bound dehydrogenase activities; (ii) DBQH2 oxidation to semiquinone (DBQ.-) by laccase; (iii) DBQ.- autoxidation, catalyzed by Fe3+-EDTA, producing superoxide (O 2 .-) and Fe2+-EDTA; (iv) O2 .- dismutation, generating H2O2; and (v) the Fenton reaction. Compared to standard quinone redox cycling conditions, .OH 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 O2 .- reduction by Mn 2+. They were also caused by the acceleration of the DBQ redox cycle as a consequence of DBQH2 oxidation by both Fe3+-oxalate and the Mn3+ generated during O2 .- reduction. Finally, induction of .OH 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 production and pollutant oxidation.