Christopher A. Gorski

Redox Behavior of Magnetite: Implications for Contaminant Reduction

The factors controlling rates of contaminant reduction by magnetite (Fe3O4) are poorly understood. Here, we measured the reduction rates of three ArNO2 compounds by magnetite particles ranging from highly oxidized (x = Fe2+/Fe3+ = 0.31) to fully stoichiometric (x = 0.50). Rates of ArNO2 reduction became almost 5 orders of magnitude faster as the particle stoichiometry increased from x = 0.31 to 0.50. To evaluate what was controlling the rate of ArNO2 reduction, we measured apparent 15N kinetic isotope effects ((15)N-AKIE) values for nitrobenzene and magnetite open-circuit potentials (E(OCP)). 15N-AKIE values were greater than unity for all magnetite stoichiometries investigated, indicating that mass transfer processes are not controlling the rate of ArNO2 reduction by magnetite. E(OCP) measurements showed that the E(OCP) for magnetite was linearly related to the stoichiometry, with more stoichiometric magnetite having a lower potential. Based on these results, we propose that conceptual models that incorporate both redox and Fe2+ diffusion processes, rather than those that rely solely on diffusion of Fe2+, are more appropriate for understanding contaminant reduction by magnetite. Our work indicates that particle stoichiometry should be considered when evaluating rates of contaminant reduction by magnetite.

Determination of nanoparticulate magnetite stoichiometry by Mössbauer spectroscopy, acidic dissolution, and powder X-ray diffraction: A critical review

Abstract A solid solution can exist of magnetite (Fe3O4) and maghemite (γ-Fe2O3), which is commonly referred to as nonstoichiometric or partially oxidized magnetite. The degree of stoichiometry in magnetite is quantitatively measured by determining the ratio of Fe2+ to Fe3+. Magnetite stoichiometry (x = Fe2+/Fe3+) strongly influences several physical properties, including the coercitivity, sorption capacity, reduction potential, and crystalline structure. Magnetite stoichiometry has been extensively studied, although very little work exists examining the stoichiometry of nanoparticulate samples (<<100 nm); when the stoichiometry was measured for nanoparticulate samples, it was not validated with a secondary technique. Here, we review the three most common techniques to determine magnetite stoichiometry: (1) acidic dissolution; (2) Mössbauer spectroscopy; and (3) powder X-ray diffraction (pXRD), specifically with nanoparticulate samples in mind. Eight samples of nonstoichiometric magnetite were synthesized with x ranging from 0 to 0.50 and with the particle size kept as similar as possible (BET specific surface area = 63 ± 7 m2/g; particle size ≈ 20 nm). Our measurements indicate excellent agreement between stoichiometries determined from Mössbauer spectra and by acidic dissolution, suggesting that Mössbauer spectroscopy may be a useful means for estimating magnetite stoichiometry in nanoparticulate, multi-phases samples, such as those found in the environment. A significant linear correlation was also observed between the unit-cell length (a) of magnetite measured by pXRD and magnetite stoichiometry, indicating that pXRD may also be useful for determining particle stoichiometry, especially for mixed phased samples

Spectroscopic Evidence for Interfacial Fe(II)−Fe(III) Electron Transfer in a Clay Mineral

Interfacial electron transfer has been shown to occur between sorbed Fe(II) and structural Fe(III) in Fe oxides, but it is unknown whether a similar reaction occurs between sorbed Fe(II) and Fe(III)-bearing clay minerals. Here, we used the isotopic specificity of (57)Fe Mössbauer spectroscopy to demonstrate electron transfer between sorbed Fe(II) and structural Fe(III) in an Fe-bearing smectite clay mineral (NAu-2, nontronite). Mössbauer spectra of NAu-2 reacted with aqueous (56)Fe(II) (which is invisible to (57)Fe Mössbauer spectroscopy) showed direct evidence for reduction of NAu-2 by sorbed Fe(II). Mössbauer spectra using aqueous (57)Fe(II) showed that sorbed Fe(II) is oxidized upon sorption to the clay and pXRD patterns indicate that the oxidation product is lepidocrocite. Spectra collected at different temperatures indicate that reduction of structural Fe(III) by sorbed Fe(II) induces electron delocalization in the clay structure. Our results also imply that interpretation of room temperature and 77 K Mössbauer spectra may significantly underestimate the amount of Fe(II) in Fe-bearing clays. These findings provide compelling evidence for abiotic reduction of Fe-bearing clay minerals by sorbed Fe(II), and require us to reframe our conceptual model for interpreting biological reduction of clay minerals, as well as contaminant reduction by reduced clays.

Influence of Magnetite Stoichiometry on U VI Reduction

Hexavalent uranium (U(VI)) can be reduced enzymatically by various microbes and abiotically by Fe(2+)-bearing minerals, including magnetite, of interest because of its formation from Fe(3+) (oxy)hydroxides via dissimilatory iron reduction. Magnetite is also a corrosion product of iron metal in suboxic and anoxic conditions and is likely to form during corrosion of steel waste containers holding uranium-containing spent nuclear fuel. Previous work indicated discrepancies in the extent of U(VI) reduction by magnetite. Here, we demonstrate that the stoichiometry (the bulk Fe(2+)/Fe(3+) ratio, x) of magnetite can, in part, explain the observed discrepancies. In our studies, magnetite stoichiometry significantly influenced the extent of U(VI) reduction by magnetite. Stoichiometric and partially oxidized magnetites with x ≥ 0.38 reduced U(VI) to U(IV) in UO(2) (uraninite) nanoparticles, whereas with more oxidized magnetites (x < 0.38) and maghemite (x = 0), sorbed U(VI) was the dominant phase observed. Furthermore, as with our chemically synthesized magnetites (x ≥ 0.38), nanoparticulate UO(2) was formed from reduction of U(VI) in a heat-killed suspension of biogenic magnetite (x = 0.43). X-ray absorption and Mössbauer spectroscopy results indicate that reduction of U(VI) to U(IV) is coupled to oxidation of Fe(2+) in magnetite. The addition of aqueous Fe(2+) to suspensions of oxidized magnetite resulted in reduction of U(VI) to UO(2), consistent with our previous finding that Fe(2+) taken up from solution increased the magnetite stoichiometry. Our results suggest that magnetite stoichiometry and the ability of aqueous Fe(2+) to recharge magnetite are important factors in reduction of U(VI) in the subsurface.

Coal Fly Ash as a Source of Iron in Atmospheric Dust

Anthropogenic coal fly ash (FA) aerosol may represent a significant source of bioavailable iron in the open ocean. Few measurements have been made that compare the solubility of atmospheric iron from anthropogenic aerosols and other sources. We report here an investigation of iron dissolution for three FA samples in acidic aqueous solutions and compare the solubilities with that of Arizona test dust (AZTD), a reference material for mineral dust. The effects of pH, simulated cloud processing, and solar radiation on iron solubility have been explored. Similar to previously reported results on mineral dust, iron in aluminosilicate phases provides the predominant component of dissolved iron. Iron solubility of FA is substantially higher than of the crystalline minerals comprising AZTD. Simulated atmospheric processing elevates iron solubility due to significant changes in the morphology of aluminosilicate glass, a dominant material in FA particles. Iron is continuously released into the aqueous solution as FA particles break up into smaller fragments. These results suggest that the assessment of dissolved atmospheric iron deposition fluxes and their effect on the biogeochemistry at the ocean surface should be constrained by the source, environmental pH, iron speciation, and solar radiation.

Redox properties of structural Fe in clay minerals. 1. Electrochemical quantification of electron-donating and -accepting capacities of smectites

Clay minerals often contain redox-active structural iron that participates in electron transfer reactions with environmental pollutants, bacteria, and biological nutrients. Measuring the redox properties of structural Fe in clay minerals using electrochemical approaches, however, has proven to be difficult due to a lack of reactivity between clay minerals and electrodes. Here, we overcome this limitation by using one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in clay minerals and a vitreous carbon working electrode in an electrochemical cell. Using this approach, the electron-accepting and -donating capacities (Q(EAC) and Q(EDC)) were quantified at applied potentials (E(H)) of -0.60 V and +0.61 V (vs SHE), respectively, for four natural Fe-bearing smectites (i.e., SWa-1, SWy-2, NAu-1, and NAu-2) having different total Fe contents (Fe(total) = 2.3 to 21.2 wt % Fe) and varied initial Fe(2+)/Fe(total) states. For every SWa-1 and SWy-2 sample, all the structural Fe was redox-active over the tested E(H) range, demonstrating reliable quantification of Fe content and redox state. Yet for NAu-1 and NAu-2, a significant fraction of the structural Fe was redox-inactive, which was attributed to Fe-rich smectites requiring more extreme E(H)-values to achieve complete Fe reduction and/or oxidation. The Q(EAC) and Q(EDC) values provided here can be used as benchmarks in future studies examining the extent of reduction and oxidation of Fe-bearing smectites.

Redox properties of structural Fe in clay minerals. 2. Electrochemical and spectroscopic characterization of electron transfer irreversibility in ferruginous smectite, SWa-1.

Structural Fe in clay minerals is an important, albeit poorly characterized, redox-active phase found in many natural and engineered environments. This work develops an experimental approach to directly assess the redox properties of a natural Fe-bearing smectite (ferruginous smectite, SWa-1, 12.6 wt % Fe) with mediated electrochemical reduction (MER) and oxidation (MEO). By utilizing a suite of one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in SWa-1 and a working electrode, we show that the Fe2+/Fe3+ couple in SWa-1 is redox-active over a large range of potentials (from E(H) = -0.63 V to +0.61 V vs SHE). Electrochemical and spectroscopic analyses of SWa-1 samples that were subject to reduction and re-oxidation cycling revealed both reversible and irreversible structural Fe rearrangements that altered the observed apparent standard reduction potential (E(H)(ø)) of structural Fe. E(H)(ø)-values vary by as much as 0.56 V between SWa-1 samples with different redox histories. The wide range of E(H)-values over which SWa-1 is redox-active and redox history-dependent E(H)(ø)-values underscore the importance of Fe-bearing clay minerals as redox-active phases in a wide range of redox regimes.

Redox properties of structural Fe in clay minerals: 3. Relationships between smectite redox and structural properties.

Structural Fe in clay minerals is an important redox-active species in many pristine and contaminated environments as well as in engineered systems. Understanding the extent and kinetics of redox reactions involving Fe-bearing clay minerals has been challenging due to the inability to relate structural Fe(2+)/Fe(total) fractions to fundamental redox properties, such as reduction potentials (EH). Here, we overcame this challenge by using mediated electrochemical reduction (MER) and oxidation (MEO) to characterize the fraction of redox-active structural Fe (Fe(2+)/Fe(total)) in smectites over a wide range of applied EH-values (-0.6 V to +0.6 V). We examined Fe(2+)/Fe(total )- EH relationships of four natural Fe-bearing smectites (SWy-2, SWa-1, NAu-1, NAu-2) in their native, reduced, and reoxidized states and compared our measurements with spectroscopic observations and a suite of mineralogical properties. All smectites exhibited unique Fe(2+)/Fe(total) - EH relationships, were redox active over wide EH ranges, and underwent irreversible electron transfer induced structural changes that were observable with X-ray absorption spectroscopy. Variations among the smectite Fe(2+)/Fe(total) - EH relationships correlated well with both bulk and molecular-scale properties, including Fe(total) content, layer charge, and quadrupole splitting values, suggesting that multiple structural parameters determined the redox properties of smectites. The Fe(2+)/Fe(total) - EH relationships developed for these four commonly studied clay minerals may be applied to future studies interested in relating the extent of structural Fe reduction or oxidation to EH-values.

Influence of Fe(2+)-catalysed iron oxide recrystallization on metal cycling.

Recent work has indicated that iron (oxyhydr-)oxides are capable of structurally incorporating and releasing metals and nutrients as a result of Fe2+-induced iron oxide recrystallization. In the present paper, we briefly review the current literature examining the mechanisms by which iron oxides recrystallize and summarize how recrystallization affects metal incorporation and release. We also provide new experimental evidence for the Fe2+-induced release of structural manganese from manganese-doped goethite. Currently, the exact mechanism(s) for Fe2+-induced recrystallization remain elusive, although they are likely to be both oxide-and metal-dependent. We conclude by discussing some future research directions for Fe2+-catalysed iron oxide recrystallization.

Synthesis and properties of titanomagnetite (Fe3-xTixO4) nanoparticles: A tunable solid-state Fe(II/III) redox system

Titanomagnetite (Fe(3-x)Ti(x)O(4)) nanoparticles were synthesized by room temperature aqueous precipitation, in which Ti(IV) replaces Fe(III) and is charge compensated by conversion of Fe(III) to Fe(II) in the unit cell. A comprehensive suite of tools was used to probe composition, structure, and magnetic properties down to site-occupancy level, emphasizing distribution and accessibility of Fe(II) as a function of x. Synthesis of nanoparticles in the range 0≤x≤0.6 was attempted; Ti, total Fe and Fe(II) content were verified by chemical analysis. TEM indicated homogeneous spherical 9-12 nm particles. μ-XRD and Mössbauer spectroscopy on anoxic aqueous suspensions verified the inverse spinel structure and Ti(IV) incorporation in the unit cell up to x≤0.38, based on Fe(II)/Fe(III) ratio deduced from the unit cell edge and Mössbauer spectra. Nanoparticles with a higher value of x possessed a minor amorphous secondary Fe(II)/Ti(IV) phase. XANES/EXAFS indicated Ti(IV) incorporation in the octahedral sublattice (B-site) and proportional increases in Fe(II)/Fe(III) ratio. XA/XMCD indicated that increases arise from increasing B-site Fe(II), and that these charge-balancing equivalents segregate to those B-sites near particle surfaces. Dissolution studies showed that this segregation persists after release of Fe(II) into solution, in amounts systematically proportional to x and thus the Fe(II)/Fe(III) ratio. A mechanistic reaction model was developed entailing mobile B-site Fe(II) supplying a highly interactive surface phase that undergoes interfacial electron transfer with oxidants in solution, sustained by outward Fe(II) migration from particle interiors and concurrent inward migration of charge-balancing cationic vacancies in a ratio of 3:1.