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Title: Fe(II)-catalyzed Recrystallization of Goethite Revisited

Results from enriched 57Fe isotope tracer experiments have shown that atom exchange can occur between structural Fe in Fe(III) oxides and aqueous Fe(II) with no formation of secondary minerals or change in particle size or shape. Here we derive a mass balance model to quantify the extent of Fe atom exchange between goethite and aqueous Fe(II) that accounts for different Fe pool sizes. We use this model to reinterpret our previous work and to quantify the influence of particle size and pH on extent of goethite exchange with aqueous Fe(II). Consistent with our previous interpretation, substantial exchange of goethite occurred at pH 7.5 (≈ 90%) and we observed little effect of particle size between nanogoethite (81 x 11 nm) and microgoethite (590 x 42 nm). Despite ≈ 90% of the bulk goethite exchanging at pH 7.5, we found no change in mineral phase, particle size, crystallinity, or reactivity after reaction with aqueous Fe(II). At a lower pH of 5.0, no net sorption of Fe(II) was observed and significantly less exchange occurred accounting for less than the estimated proportion of surface Fe atoms in the particles. Particle size appears to influence the amount of exchange at pH 5.0 and we suggestmore » that aggregation and surface area may play a role. Results from sequential chemical extractions indicate that 57Fe accumulates in extracted Fe(III) goethite components. Isotopic compositions of the extracts indicate that a gradient of 57Fe develops within the goethite with more accumulation of 57Fe occurring in the more easily extracted Fe(III) that may be nearer to the surface. We interpret our particle size, pH, and sequential extraction findings as consistent with the mechanism of interfacial electron transfer and bulk conduction previously proposed to explain the substantial Fe atom exchange observed in goethite in contact with aqueous Fe(II).« less
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Publication Date:
OSTI Identifier:
Report Number(s):
47824; KC0302060
DOE Contract Number:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Environmental Science & Technology, 48(19):11302-11311
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org:
Country of Publication:
United States
Environmental Molecular Sciences Laboratory