Fe(II)-Catalyzed Recrystallization of Goethite Revisited

Results from enriched Fe-57 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 (approximate to 90%) and we observed little effect of particle size between nanogoethite (average size of 81 x 11 nm; approximate to 110 m(2)/g) and microgoethite (average size of 590 x 42 nm; approximate to 40 m(2)/g). Despite 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. Particle size appears to influence the amount of exchange at pH 5.0 and we suggest that aggregation and surface area may play a role. Results from sequential chemical extractions indicate that Fe-57 accumulates in extracted Fe(III) goethite components. Isotopic compositions of the extracts indicate that a gradient of Fe-57 develops within the goethite with more accumulation of Fe-57 occurring in the more easily extracted Fe(III) that may be nearer to the surface.