Heterogene Oxidation von Fe(II) in wässrigen Systemen: Abhängigkeit der Produktbildung von Umweltfaktoren
Final Report Abstract
Fe(III) solid phases are the products of Fe(II) oxidation either by abiotic reactions or by Fe(II)-oxidizing bacteria. The scope of this study was to investigate the influences of a) geochemical solution conditions (pH, carbonate, phosphate, humic acids) and b) mineral templates on the Fe(II) oxidation rate and Fe(III) mineralogy. Fe(III) mineral characterization was performed using 57Fe-Mössbauer spectroscopy, My-X-ray diffraction, and electron microscopy after oxidation of dissolved Fe(II) within Mops-buffered suspensions of cells or iron minerals. Acidovorax sp. BoFeN1, a nitrate-reducing, Fe(II)-oxidizing bacterium was used for bio-mineralization experiments. Lepidocrocite (Gamma-FeOOH) (90%), which also forms after chemical oxidation of Fe(II) by dissolved O2, and goethite (Alpha-FeOOH) (10%) formed at pH 7.0 in the absence of any strongly complexing ligands. Higher solution pH, increasing concentrations of carbonate species, and increasing concentrations of humic acids promoted goethite formation and caused little or no changes in Fe(II) oxidation rates. Phosphate resulted in Fe(III) solids unidentifiable to our methods and significantly slowed Fe(II) oxidation rates. Our results demonstrate that Fe(III) mineralogy formed by bacterial Fe(II) oxidation can be explained primarily by solution chemistry rather than by biotic factors. Fe(II) oxidation in heterogeneous systems results in the formation of secondary phases modifying the surfaces and thus the reactivity of primarily minerals. Our scope was to identify secondary Fe(III) mineral phases formed after abiotic Fe(II) oxidation on common iron oxides using nitrobenzene as a model oxidant in pH-buffered suspensions. In addition to oxide sorbent growth, Fe(II) sorption and oxidation by nitrobenzene resulted in the formation of secondary Fe(III) minerals. Goethite formed on three hematite morphologies (rhombohedra, needles, and hexagonal platelets), and acicular needle shapes typical of goethite appeared on the micron-sized hexagonal platelets. The proportion of goethite formation on the three hematites was linked to a number of surface sites. Only goethite was observed to form on a goethite sorbent. In contrast, lepidocrocite formed on magnetite and maghemite sorbents (consistent with homogeneous Fe(II) oxidation by O2) and assumed spherulite morphologies. On hematite, the directed formation of goethite as opposed to lepidocrocite suggests hematite may possess a templating ability for the Alpha-FeOOH atom arrangement as opposed to Gamma-FeOOH. The initiation of all secondary Fe-oxide formations occurred after four to six equivalents of monolayer coverage on the supporting mineral sorbent. Overall, Fe(III) product identity formed during heterogeneous Fe(II) oxidation appears to be governed mainly by the identity of the underlying sorbent and partly by the amount of available surface sites.
Publications
- (2010). Biomineralization of lepidocrocite and goethite by nitrate-reducing Fe(II)-oxidizing bacteria: Effect of pH, bicarbonate, phosphate, and humic acids. Geochimica et Cosmochimica Acta, 74(13), 3721-3734
Larese-Casanova, P., Haderlein, S. B., & Kappler, A.
- (2012). Heterogeneous oxidation of Fe(II) on iron oxides in aqueous systems: Identification and controls of Fe(III) product formation. Geochimica et Cosmochimica Acta, 91, 171-186
Larese-Casanova, P., Kappler, A., & Haderlein, S. B.
(See online at https://doi.org/10.1016/j.gca.2012.05.031) - (2012). Potential function of added minerals as nucleation sites and effect of humic substances on mineral formation by the nitrate-reducing Fe(II)-oxidizer acidovorax sp. BoFeN1. Environmental Science and Technology, 46(12), 6556-6565
Dippon, U., Pantke, C., Porsch, K., Larese-Casanova, P., & Kappler, A.
(See online at https://doi.org/10.1021/es2046266)
