Fe(II)-Catalyzed Recrystallization of Goethite Revisited

Abstract

Results from enriched (57)Fe 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 (average size of 81 × 11 nm; ≈ 110 m(2)/g) and microgoethite (average size of 590 × 42 nm; ≈ 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 (57)Fe accumulates in extracted Fe(III) goethite components. Isotopic compositions of the extracts indicate that a gradient of (57)Fe develops within the goethite with more accumulation of (57)Fe occurring in the more easily extracted Fe(III) that may be nearer to the surface.

<strong>Cr release from Cr-substituted goethite during the aqueous Fe(II)-induced recrystallization</strong>

“…Fe isotope fractions ( f ) were calculated by dividing the counts in each isotope channel by the sum of the total counts across all four channels (masses of 54, 56, 57, and 58) [32]. Fe atom exchange rates were calculated with the following equation [33,34]:…”

“…Simultaneously, the compositions of 56 Fe(II) (f 56 Fe) in all five treatments increased quickly during the first day, and then continued increasing slowly on subsequent days. These results suggest that Fe atom exchange occurred between Fe(II) aq and Fe(III) oxide of the goethites, in which some 57 Fe atoms of Fe(II) aq entered into the structure of pure goethite and Cr-goethite, while some Fe atoms from the natural isotopic Fe (i.e., 56 Fe(II) from iron oxides) of the pure goethite and Cr-goethites were released into the solution as Fe(II) aq [33,34]. For the pure goethite and Cr-goethite, the composition changes in 57 Fe(II) and 56 Fe(II) were substantially different.…”

“…The increase of f 56 Fe in Fe(II) aq in pure goethite was much higher than that for the Cr-goethite. To further quantify the Fe atom exchange, the change rates in all treatments were calculated using the Equation (1) [32,33] (Figure 5). The Fe atom exchange rate in the pure goethite reached 19.17% following reaction with Fe(II) aq for 30 days, which was much higher than that for all other Cr-goethites.…”

“…Environment factors (e.g., pH [33], initial Fe(II) aq concentration [23], and particle size [32]) have substantial effects on the interplay between the Fe(II) aq and Fe(III) oxide of iron (hydr)oxide, which further affects the release behavior of Cr(III) in Cr-goethites. To obtain deep insight into the effect of Fe(II) aq -induced recrystallization of goethites on the release behavior of Cr(III), we investigated the effects of pH and initial Fe(II) aq concentration on Cr(III) release during the Fe(II) aq -induced recrystallization of Cr-goethites.…”

“…At a pH of 7.5, Cr-goethites achieved the largest amount of Fe atom exchange among the three pH value conditions. At a pH of 5.5, a smaller proportion of Fe(II) aq was sorbed onto the surface of goethites due to the high point of zero charge (7.0 to 8.5) of goethite [42], which reduced the energetics of electron transfer between Fe(II) aq and Fe(III) oxide in Cr-goethites [33,43,44]. Therefore, substantially less Fe atom exchange occurred over 30 days at a pH of 5.5 when compared with that at a pH of 7.5.…”