GEOC 96 |
| The slow dissolution and low solubility of iron oxides in oxygen rich environments limit the turnover of iron and its availability to organisms. Several non-enzymatic processes are known to increase iron solubility and to accelerate iron oxide dissolution, all of which are used by microorganisms and plants to enhance iron bioavailability, namely, proton-promoted, ligand-promoted, and (photo-)reductive dissolution. We have studied iron isotope fractionation by these dissolution processes to trace their relative importance in complex natural environments and to study the molecular mechanisms of the dissolution processes. The dissolution of goethite (α-FeOOH)by proton-promoted dissolution (0.5 M HCl), ligand promoted dissolution (DFO-B, oxalate, malonate, EDTA, pH 3 – 8.5), and photoreductive dissolution (oxalate) was investigated in batch reactor studies. Samples were taken over time, filtered, and iron isotope ratios were measured by MC-ICPMS. Iron isotope fractionation was not observed during proton-promoted dissolution, indicating that the goethite was not isotopically zoned. A strong enrichment of lighter iron isotopes (54Fe) relative to the bulk mineral was observed in the early stages of ligand controlled and reductive dissolution in the presence of oxalate. We have modeled these observations using a modified Rayleigh process. Iron isotope fractionation was also observed in the presence of DFO-B and malonate. A significant effect of pH on isotope fractionation was observed in the presence of EDTA. In this presentation we will discuss if and how observations of isotope fractionation during iron oxide dissolution are useful for the elucidation of reaction mechanisms on the molecular scale. |
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Physical Chemistry of Soil and Aquifer Systems: A Symposium in Honor of Garrison Sposito
1:30 PM-4:45 PM, Tuesday, 12 September 2006 Moscone Center -- Room 256, Oral
Division of Geochemistry |