Characterization of Iron-Oxides Formed by Oxidation of Ferrous Ions in the Presence of Various Bacterial Species and Inorganic Ligands

The oxidation of ferrous ions in the presence of an excess of dissolved oxygen at neutral pH generally leads to the formation of lepidocrocite. The effect of inorganic ligands (PO4, SO4, or Si(OH)4) in concentrations typical of those in sediment pore waters, and of microorganisms (Escherichia coli K12, Pseudomonas aeruginosa PA01, Bacillus subtilis or Bacillus licheniformis) on the mineralogy, chemical composition, morphology and spatial distribution of the iron-oxides were examined using various complementary techniques, including TEM, XRD, and EXAFS. The presence of inorganic ligands during the oxidation can affect the mineralogy as well as the size and structure of the Fe-oxide particles. While the presence of sulfate (SO4/Fe = 0.5) had little effect on the outcome of the Fe-oxide synthesis, low quantities of phosphate (PO4/Fe = 0.05) inhibited lepidocrocite and large quantities of aqueous silica (Si/Fe = 5) favored the formation of 2-line ferrihydrite. The presence of any of the four representative species of bacterial cells in the various systems did not modify the mineralogy of the Fe-oxides. However, the size of the Fe-oxide particles tended to be reduced, and the presence of the cells also affected the spatial organization and the morphology of the particles. In addition, in some systems, some of the iron remains adsorbed on the cells and does not contribute to the formation of mineral phases.