Evidence of Iron Isotope Exchange at the Ferrihydrite Surface
Abstract
Ferrihydrite is the primary iron phase to precipitate from aqueous Fe(III) solutions, and is a precursor to diagenetically produced minerals such as hematite and goethite. Isotopic exchange between aqueous Fe and the ferrihydrite surface could impact measured iron isotope compositions in a variety of settings where ferrihydrite might react with ambient aqueous iron as it aggregates, accumulates, or is diagenetically altered. We chose a manufactured 3-nm sized ferrihydrite for these experiments with a large surface area available for exchange (~500m2/g), so this study likely represents an end member case in terms of isotopic exchange. Natural ferrihydrites are generally larger aggregate particles with substantially less surface area (<150m2/g), which may reduce the bulk effects of isotopic exchange. However, even small amounts of isotopic exchange could dramatically affect isotopic compositions of aqueous iron. We have successfully demonstrated that iron isotope exchange occurs between iron oxyhydroxide (ferrihydrite) and aqueous ferric iron using a 57Fe tracer approach. Our results show that ~25% iron isotope exchange occurs after ten days, with no further exchange evident over the duration of the study (25pm5% exchange from 10-80 days). The nanoparticulate (3nm) ferrihydrite used in this study has ~30% of its Fe atoms at the surface. We suggest that our 57Fe isotope exchange data reflect the complete exchange of available surface Fe atoms after the first ten days and that further diffusional exchange is not detected over the duration of this experiment. This study highlights the usefulness of using enriched isotope tracers to investigate the rate of exchange between aqueous iron atoms and those at the ferrihydrite surface under equilibrium conditions, and is a critical first step toward better interpreting iron isotope variations measured in natural settings. The demonstration of iron isotope exchange in this study emphasizes that ferric precipitates should not be regarded as isotopically unreactive and their potential impact on the iron isotope compositions of ambient fluids cannot necessarily be ignored. It seems unlikely, however, that the isotopic compositions of the solid ferrihydrite will be strongly affected during diagenesis. Further investigation into the extent of surface exchange on ferrihydrites of various particle sizes is necessary to better characterize the iron isotope variations measured in natural systems. Changes in environmental conditions such as temperature and pH should also affect the reactivity of iron at the ferrihydrite surface, and require further experimental work. It remains unknown if the isotopic exchange properties of the nanoparticles used in this study may be scaled to larger crystals.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.B43B..05P
- Keywords:
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- 0400 Biogeosciences