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Biogeochemical Properties of Bacteriogenic Iron Oxides

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Bacteriogenic iron oxides (BIOS) are composite materials that consist of intact and partly degraded remains of bacterial cells intermixed with variable amounts of poorly ordered hydrous ferric oxide (HFO) minerals. They form in response to chemical or bacterial oxidation of Fe 2+ , which gives rise to Fe 3+ . Once formed, Fe 3+ tends to undergo hydrolysis to precipitate in association with bacterial cells. In acidic systems where the chemical oxidation of Fe 2+ is slow, bacteria are capable of accelerating the reaction by several orders of magnitude. At circumneutral pH, the chemical oxidation of Fe 2+ is fast. This requires Fe 2+ oxidizing bacteria to exploit steep redox gradients where low p O 2 slows the abiotic reaction enough to allow the bacteria to compete kinetically. Because of their reactive surface properties, BIOS behave as potent sorbents of dissolved metal ions. Strong enrichments of Al, Cu, Cr, Mn, Sr, and Zn in the solid versus aqueous phase (log 10 K d values range from 1.9 to 4.2) are common; however, the metal sorption properties of BIOS are not additive owing to surface chemical interactions between the constituent HFO and bacteria. These interactions have been investigated using acid-base tritrations, which show that the concentration of high pK a sites is reduced in BIOS compared to HFO. At the same time, hydroxylamine insoluble material (i.e., residual bacterial fraction) is enriched in low pK a sites relative to both BIOS and HFO. These differences indicate that low pK a or acidic sites associated with bacteria in BIOS interact specifically with high pK a or basic sites on intermixed HFO.

Keywords: bacteria; bacteriogenic; ferric; ferrous; geochemistry; iron; oxidation; oxide

Document Type: Research Article


Affiliations: Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada

Publication date: 2005-04-01

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