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The reduction potential of Fe(II)-Fe(III) (oxyhydr)oxide systems provides an important control on the biogeochemical cycling of redox-sensitive elements such as carbon and nitrogen as well as trace metals and organic contaminants in natural systems. As such, an in-depth understanding of the factors controlling the reduction potential of such systems is critical to predicting the likely transformation, transport and fate of these species in natural and perturbed environments. In this study the mineralogy and reduction potential of ferrihydrite suspensions at pH 6.50 and pH 7.00 were determined over the course of their Fe(II)-catalyzed transformation to lepidocrocite and goethite using X-ray absorption spectroscopy and mediated electrochemical approaches. The measured reduction potentials were compared to those of analogous Fe(II)-Fe(III) (oxyhydr)oxide suspensions reacted for 5 min containing pure ferrihydrite (Fh), lepidocrocite (L) and goethite (Gt). The reduction potentials of the pure Fe(II)-Fe(III) (oxyhydr)oxide suspensions were, respectively, +47.5, −13.5 and −122.3 mV vs. SHE at pH 6.5, and −22.9, −84.1 and −189.7 mV vs. SHE at pH 7. These values are in good agreement with reduction potentials calculated using the Nernst equation and reported thermodynamic solubility products indicating that these suspensions had reached equilibrium within 5 min. The reduction potential of the pH 6.50 Fe(II)-ferrihydrite suspension decreased from +47.4 mV to –126.4 mV over a week, and from −20.1 mV to −188.4 mV (all vs. SHE) after 24 h at pH 7. The changes in reduction potential over time matched well to those calculated from the relative proportion of each pure Fe(III) (oxyhydr)oxide present suggesting that Fe 3+ activity was influenced by the mix of iron oxides present rather than the most insoluble solid species. Finally, evidence is provided that adsorbed Fe(II) has the capacity to reduce a significantly larger fraction of a reducible species than the aqueous Fe(II) species with which it is in equilibrium. As an Fe(III) (oxyhydr)oxide suspension in equilibrium with aqueous and adsorbed Fe(II) species possesses a single, unique reduction potential, this suggests that adsorbed Fe(II) is a more facile reductant than aqueous Fe(II). [ABSTRACT FROM AUTHOR]

Using a novel combination of in situ potentiometric experiments and quick-scanning XAS we present Fe K-edge XAS spectra (to k = 12 Å −1 ) during Fe III hydrolysis and precipitation in 0.33 M Fe(ClO 4 ) 3 , Fe(NO 3 ) 3 , FeCl 3 and Fe 2 (SO 4 ) 3 solutions up to pH 4.8. Edge-sharing Fe III polymers appeared almost immediately upon hydrolysis with strong evidence for a μ-oxo dimer species forming in the Fe(ClO 4 ) 3 , Fe(NO 3 ) 3 and FeCl 3 solutions. The effects of SO 4 on hydrolysis and polymerization pathways included inhibition of both the formation of the μ-oxo dimer and double corner Fe III bonding, ultimately resulting in the precipitation of schwertmannite. As such, under these experimental conditions, double corner Fe III bonding appears to be critical to the formation of ferrihydrite. The spectral trends indicated that the decomposition/transformation of the dimer was sudden and broadly coincident with shortening average Fe–O bond distances, increased Fe neighbors at ∼3.43 Å and a pre-edge energy transformation suggestive of decreased ligand field strength as well as increasing proportions of tetrahedral Fe III . This result suggests that the incorporation of tetrahedral Fe III into ferrihydrite occurs only at the latter stages of extended polymerization. [ABSTRACT FROM AUTHOR]

In this study, the kinetics and stable isotopic fractionation of nitrate (NO 3 −) reduction in Fe(II)/Fe(III) homogeneous/heterogeneous systems were investigated at circumneutral pH (i.e., pH 6.5 to 7.5) and ambient temperature (24 °C). Of the suite of iron (oxy)(hydr)oxides examined, NO 3 − reduction by Fe(II) was only significant in the presence of layered double hydroxide green rust (GR) minerals. Comparison of type 1 and type 2 GRs demonstrated that type 1 (GR(Cl−)) had the fastest NO 3 − reducing ability, with an Fe(II)-normalized pseudo first-order kinetic rate constant of k obs (NO 3 −) = 16.2 × 101 M−1 d−1. Anion concentrations (SO 4 2− or Cl−) and the ratio of [Fe(II)] to [Fe(III)] also influenced reduction kinetics. Ammonium (NH 4 +) was the predominant reaction end-product (>50%) with the remaining reduced N species concluded to be comprised of N 2(g) as other intermediates (nitrite (NO 2 −), nitric oxide (NO) and nitrous oxide (N 2 O) were not detected during NO 3 − reduction. Apparent nitrogen (δ15N-NO 3 −) isotopic fractionation (15ε) for GR(Cl−) and GR(SO 4 2−) were were calculated to be 12.9 (CI: 8.2, 16)‰ and 37.9 (CI: 34, 41)‰, respectively. Similarly, oxygen (δ18O-NO 3 −) isotopic fractionation (18ε) differed between GR(Cl−) and GR(SO 4 2−): 4.53 (CI: 2.5, 5.8)‰ and 14.4 (CI: 12, 16)‰. However, the ratio of 18ε:15ε for both minerals could be fitted to a linear regression of slope 0.369 (CI: 0.361, 0.377). These findings reaffirm the importance of GR minerals to NO 3 − reduction, especially in iron-rich systems hosting dynamic redox oscillations, including hyporheic zones, estuarine sediments and groundwater aquifers. These results also demonstrate that N and O stable isotope kinetic fractionation analyses as well as the ratio of 18ε:15ε can provide a means to distinguish between this abiotic anoxic reaction from anaerobic bacterial NO 3 − reduction processes. [ABSTRACT FROM AUTHOR]

In this study, the kinetics of Fe(II) oxidation in the presence of the iron oxyhydroxides ferrihydrite, Si-ferrihydrite, schwertmannite, lepidocrocite and goethite are investigated over the pH range 4–5.5. Despite limited sorption of Fe(II), the rate of Fe(II) oxidation is up to 70-fold faster than in the absence of any Fe oxyhydroxide phase over pH 4.5–5.5. Enhanced Fe(II) oxidation was minor or negligible at pH 4 with undetectable amounts of Fe(II) adsorbed to the iron oxyhydroxides at this pH. Heterogeneous rate constants derived from kinetic modeling were normalized to the concentration of adsorbed Fe(II) and deviated by no more than 13.8% at pH 4.5, 5 and 5.5, indicating that oxidation is proportional to the concentration of adsorbed Fe(II). Average rate constants were found to be: 2.12 ± 0.20, 1.30 ± 0.09, 1.69 ± 0.22, 1.20 ± 0.08 and 0.68 ± 0.09 M −1 s −1 for ferrihydrite, goethite, lepidocrocite, schwertmannite and Si-ferrihydrite, respectively. The role of reactive oxygen species, such as hydrogen peroxide, the hydroxyl radical and superoxide, towards the overall oxidation of Fe(II) was examined but found to have only a minor impact on Fe(II) oxidation when compared to the effect of heterogeneous oxidation. [ABSTRACT FROM AUTHOR]

Ferrihydrite (Fh) is a major Fe(III)-(oxyhydr)oxide nanomineral distinguished by its poor crystallinity and thermodynamic metastability. While it is well known that in suboxic conditions aqueous Fe(II) rapidly catalyzes Fh transformation to more stable crystalline Fe(III) phases such as lepidocrocite (Lp) and goethite (Gt), because of the low solubility of Fe(III) the mass transfer pathways enabling these rapid transformations have remained unclear for decades. Here, using a selective extractant, we isolated and quantified a critical labile Fe(III) species, one that is more reactive than Fe(III) in Fh, formed by the oxidation of aqueous Fe(II) on the Fh surface. Experiments that compared time-dependent concentrations of solid-associated Fe(II) and this labile Fe(III) against the kinetics of phase transformation showed that its accumulation is directly related to Lp/Gt formation in a manner consistent with the classical nucleation theory. 57Fe isotope tracer experiments confirm the oxidized Fe(II) origin of labile Fe(III). The transformation pathway as well as the accelerating effect of Fe(II) can now all be explained on a unified basis of the kinetics of Fe(III) olation and oxolation reactions necessary to nucleate and sustain growth of Lp/Gt products, rates of which are greatly accelerated by labile Fe(III). [ABSTRACT FROM AUTHOR]