Pathway for the Production of Hydroxyl Radicals during the Microbially Mediated Redox Transformation of Iron (Oxyhydr)oxides

The reduction of ferric iron (Fe(III)) to ferrous iron (Fe(II)) by dissimilatory iron-reducing bacteria is widespread in anaerobic environments. The oxidation of Fe(II) in aerobic environments has been found to produce hydroxyl radicals (•OH); however, the role of iron-reducing bacteria in the process has not been well understood. Here, Shewanella oneidensis MR-1-mediated redox transformation of four typical iron (oxyhydr)oxides and the production of reactive oxygen species were investigated. The results showed that the production of •OH was mainly determined by the insoluble Fe(II) formed during microbially mediated reduction and also mediated by the mineralogical phase. Moreover, this study for the first time observed the exogenetic iron-independent production of •OH by S. oneidensis MR-1, and the integrated pathway of •OH generation during the iron redox process was revealed. Superoxide (O2•-) was indicated as a key intermediate species that was produced by both abiotic and biotic pathways, and •OH was generated by both the exogenetic iron-dependent Fenton-like reaction and exogenetic iron-independent pathways. S. oneidensis MR-1 played a pivotal role in both the reduction of Fe(III) and the production of O2•-. These findings contribute substantially to our understanding of the generation mechanism of reactive oxygen species at oxidation-reduction boundaries in the environment.