No abstract

The reaction of aqueous Fe(II) with Fe(III) oxides is a complex process, comprising sorption, electron transfer, and in some cases, reductive dissolution and transformation to secondary minerals. To better understand the dynamics of these reactions, we measured the extent and rate of Fe isotope exchange between aqueous Fe(II) and goethite using a 57Fe isotope tracer approach. We observed near-complete exchange of Fe atoms between the aqueous phase and goethite nanorods over a 30-day time period. Despite direct isotopic evidence for extensive mixing between the aqueous and goethite Fe, no phase transformation was observed, nor did the size or shape of the goethite rods change appreciably. High-resolution transmission electron microscopy images, however, appear to indicate that some recrystallization of the goethite particles may have occurred. Near-complete exchange of Fe between aqueous Fe(II) and goethite, coupled with negligible change in the goethite mineralogy and morphology, suggests a mechanism of coupled growth (via sorption and electron transfer) and dissolution at separate crystallographic goethite sites. We propose that sorption and dissolution sites are linked via conduction through the bulk crystal, as was recently demonstrated for hematite. Extensive mixing between aqueous Fe(II) and goethite, a relatively stable iron oxide, has significant implications for heavy metal sequestration and release (e.g., arsenic and uranium), as well as reduction of soil and groundwater contaminants.

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.

The largest Fe isotope fractionations occur during redox changes, as well as differences in bonding, but these are expressed only in natural environments in which significant quantities of Fe may be mobilized and separated. At the circumneutral pH of most low-temperature aqueous systems, Fe 2+ aq is the most common species for mobilizing Fe, and Fe 2+ aq has low 56 Fe/ 54 Fe ratios relative to Fe 3+-bearing minerals. Of the variety of abiologic and biologic processes that involve redox or bonding changes, microbial Fe 3+ reduction produces the largest quantities of isotopically distinct Fe by several orders of magnitude relative to abiologic processes and hence plays a major role in producing Fe isotope variations on Earth. In modern Earth, the mass of Fe cycled through redox boundaries is small, but in the Archean it was much larger, reflecting juxtaposition of large inventories of Fe 2+ and Fe 3+. Development of photosynthesis produced large quantities of Fe 3+ and organic carbon that fueled a major expansion in microbial Fe 3+ reduction in the late Archean, perhaps starting as early as ∼3 Ga. The Fe isotope fingerprint of microbial Fe 3+ reduction decreases in the sedimentary rock record between ∼2.4 and 2.2 Ga, reflecting increased bacterial sulfate reduction and a concomitant decrease in the availability of reactive iron to support microbial Fe 3+ reduction. The temporal C, S, and Fe isotope record therefore reflects the interplay of changing microbial metabolisms over Earth's history.