Iron (hydr)oxides are commonly found in soils and sediments in a wide variety of forms ranging from poorly- (e.g., ferrihydrite) to highly-ordered (e.g., goethite, magnetite) oxides. The structure, composition and degree of crystallinity of these minerals vary greatly depending on the physical and chemical conditions of their surroundings. The transformation of Fe (hydr)oxides under microbial Fe(III)- reducing or abiotic Fe(II)-oxidizing conditions is well characterized. However, Fe(II) oxidation in anaerobic environments by bacteria that couple Fe(II) oxidation with nitrate (NO3-) reduction remains poorly understood. Reactors containing synthetic Fe (hydr)oxides (goethite, ferrihydrite or lepidocrocite) and a natural inoculum, were subjected to four complete redox cycles by addition of either nitrate or oxygen to induce oxidation and glucose to induce reduction half-cycles. Here we demonstrate that aqueous Fe(II) concentrations appear independent of the initial Fe (hydr)oxide used when reduction is followed by biological oxidation (linked to nitrate reduction). However, when biological reduction is followed by oxidation via exposure to O2, dissolved Fe(II) concentrations vary depending on initial Fe (hydr)oxide mineralogy. Additionally, characterization with XRD and XAS reveals that in addition to forming more thermodynamically stable Fe (hydr)oxides during redox cycling (e.g. magnetite and goethite), formation of less stable oxides (e.g. ferrihydrite) may occur under certain conditions. Our results reveal that variations in initial mineralogy and oxidation mechanism may control the mineralogical evolution of Fe (hydr)oxides in natural environments. Examination of Fe (hydr)oxide development as a result of both biotic and abiotic redox processes is crucial for linking biological and geochemical processes occurring in soils and sediments.

Impact of oxidation mechanism on the evolution of Fe (hydr)oxide mineralogy during biogeochemical redox cycling

248th National Meeting (2014)

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