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 observable 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 Fe(III) oxides 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 exchange between aqueous Fe(II) and goethite and hematite. 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 (≈ 110 m2/g) and microgoethite (≈ 40 m2/g). Despite about 90% of the bulk goethite exchanging at pH 7.5, we found no change in mineral phase, average 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. A similar trend with pH was observed for hematite with less exchange occurring at lower pH values. In contrast to goethite, however, we observed some influence of particle size on extent of exchange with hematite.

Fe(II)-catalyzed recrystallization of hematite and goethite revisited

249th National Meeting (2015)

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