The Impact of γ Radiation on the Bioavailability of Fe(III) Minerals for Microbial Respiration

Conservation of energy by Fe(III)-reducing species such as Shewanella oneidensis could potentially control the redox potential of environments relevant to the geological disposal of radioactive waste and radionuclide contaminated land. Such environments will be exposed to ionizing radiation so characterization of radiation alteration to the mineralogy and the resultant impact upon microbial respiration of iron is essential. Radiation induced changes to the iron mineralogy may impact upon microbial respiration and, subsequently, influence the oxidation state of redox-sensitive radionuclides. In the present work, Mossbauer spectroscopy and electron microscopy indicate that irradiation (1 MGy gamma) of 2-line ferrihydrite can lead to conversion to a more crystalline phase, one similar to akaganeite. The room temperature Mossbauer spectrum of irradiated hematite shows the emergence of a paramagnetic Fe(III) phase. Spectrophotometric determination of Fe(II) reveals a radiation-induced increase in the rate and extent of ferrihydrite and hematite reduction by S. oneidensis in the presence of an electron shuttle (riboflavin). Characterization of bioreduced solids via XRD indicate that this additional Fe(II) is incorporated into siderite and ferrous hydroxy carbonate, along with magnetite, in ferrihydrite systems, and siderite in hematite systems. This study suggests that mineralogical changes to ferrihydrite and hematite induced by radiation may lead to an increase in bioavailability of Fe(III) for respiration by Fe(III)-reducing bacteria.