Hydroxyl radicals can be generated by reactions between hydrogen peroxide and iron oxides, such reactions have been suggested to cause biotoxicity of iron oxide nanoparticles. Similar reactions are also involved in degradation of wood and soil organic matter, where microbial metabolites mediate the hydroxyl radical production. In wood degradation 2,5-dimethoxy hydroquinone (2,5-DMHQ) has been identified as a key component. This metabolite has been isolated from several distantly related species of brown-rot fungi suggesting that the biosynthetic pathways for 2,5-DMHQ were present in a common ancestor; in turn implying that 2,5-DMHQ is a widespread fungal metabolite. 2,5-DMHQ has a dual function in radical-induced decomposition of organic matter where reactions with soluble Fe(III) generate both Fe(II) and hydrogen peroxide, which are the necessary ingredients for hydroxyl radical production. This study focused on 2,6-DMHQ, an isomer of the common fungal metabolite, and its reaction with ferrihydrite and goethite under variable pH and oxygen concentrations. The overall aim was to investigate under what conditions and with what mechanisms hydroxyl radicals were formed during 2,6-DMHQ oxidation at water-iron oxide interfaces. Combined wet-chemical and spectroscopic analyses showed that both minerals effectively oxidized 2,6-DMHQ under aerobic conditions. Moreover, comparison between aerobic and anaerobic results revealed that ferrihydrite promoted 2,6-DMHQ oxidation via a combination of reductive dissolution and heterogeneous catalysis while goethite mainly caused catalytic oxidation. These results were in agreement with changes in the reduction potential (EH) of the iron oxides as a function of dissolved Fe(II) where EH of goethite dropped below that of 2,6-DMHQ at low [Fe(II)] whereas EH of ferrihydrite was sufficiently high to cause oxidation at substantially higher [Fe(II)]. Under favorable conditions the oxidation of 2,6-DMHQ by the iron oxides generated substantial amounts of hydroxyl radicals. The extent of radical production was a function of the relative contribution from the reductive dissolution and catalytic oxidation pathways of 2,6-DMHQ, and controlled by pH, oxygen concentrations, and mineral properties. Overall, the results showed that reactions between hydroquinones and iron oxides rapidly and locally generate appreciable amounts of reactive hydroxyl radicals that can decompose organic matter-mineral associations.

Generation of hydroxyl radicals by hydroquinones and iron oxide nanoparticles

254th National Meeting (2017)

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