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The high-pressure behavior of magnetite has been widely debated in the literature. Experimental measurements have found conflicting high-pressure transitions: a charge reordering in magnetite from inverse-spinel to normal-spinel [ Pasternak et al. J. Phys. Chem. Solids 65 1531 (2004); Rozenberg et al. Phys. Rev. B 75 020102 (2007)], iron high-spin to intermediate-spin transition in magnetite [ Ding et al. Phys. Rev. Lett. 100 045508 (2008)], electron delocalization in magnetite [ Baudelet et al. Phys. Rev. B 82 140412 (2010); Glazyrin et al. Am. Mineral. 97 128 (2012)], and a structural phase transition from magnetite to h-Fe3O4 [ Dubrovinsky et al. J. Phys.: Condens. Matter 15 7697 (2003); Fei et al. Am. Mineral. 84 203 (1999); Haavik et al. Am. Mineral. 85 514 (2000)]. We present ab initio calculations of iron's spin state in magnetite and h-Fe3O4, which help resolve the high-pressure debate. The results of the calculations find that iron remains high spin in both magnetite and h-Fe3O4; intermediate-spin iron is not stable. In addition, magnetite remains inverse-spinel but undergoes a phase transition to h-Fe3O4 near 10 GPa. Magnetite has a complex magnetic ordering, multiple valence states (Fe2+ and Fe3+), charge ordering, and different local Fe site environments, all of which were accounted for in the calculations. The lack of intermediate-spin iron in magnetite helps resolve the spin state of iron in perovskite, the major mineral in the lower mantle. In both magnetite and perovskite, x-ray emission spectroscopy (XES) measurements in the literature show a drop in satellite peak intensity by approximately half, which is interpreted as intermediate-spin iron. In both minerals, calculations give no indication of intermediate-spin iron and predict high-spin iron to be stable for defect-free crystals. The results question the interpretation of a nonzero drop in XES satellite peak intensities as intermediate-spin iron. [ABSTRACT FROM AUTHOR]

Reduction of mobile aqueous uranyl complexes can be hindered by the strength of the uranyl-ligand complex and the resistivity of the complexes to electron transfer. Semiconducting solids, such as magnetite (Fe 3 O 4) and ilmenite (FeTiO 3), can facilitate otherwise-slow electron transfer between the reductant and oxidant, especially if these species are adsorbed on their surfaces as inner-sphere complexes. Electrons can then be shuttled through the surface or the structure. This process can be facilitated by either finding a new electron transfer path or by weakening or destroying the ligand-uranyl complex. Using in situ electrochemical atomic force microscopy (EC-AFM), we imaged reduction products of uranyl as mediated by magnetite and ilmenite in the absence or presence of the organic ligands, ethylenediaminetetraacetate (EDTA) and oxalate. In situ images of uranyl reduction show gradual nucleation and growth of reduced uranium on the mineral surfaces with decreasing reduction potentials. X-ray photoelectron spectroscopy (XPS) measurements indicate pentavalent uranium as the dominant uranium species resulting from uranyl reduction mediated by magnetite and ilmenite. The reduction potentials of uranyl-organic species relevant to our reaction system, (UVIO 2 ∙oxalate)0 and (UVIO 2 ∙HEDTA)−, are derived using quantum–mechanical modeling and compared with the midpoint potentials obtained from cyclic voltammetry measurements. The spectroscopic, electrochemical, and computational data suggest that the catalysis of Fe-bearing minerals facilitates the reduction of uranyl-organic complexes. U(VI)-oxalate complexes are redox-active and participate directly in the reduction mediated by magnetite. In the presence of EDTA, reduction of uranyl may proceed in concert with ligand exchange between metals (i.e., U and Fe) on the mineral surface. The proposed mechanism is the breaking of uranyl-organic bonds upon interaction with the mineral surface followed by the formation of Fe-organic bonds, thereby allowing the reduction of hexavalent uranyl to pentavalent one. [ABSTRACT FROM AUTHOR]

Although previous studies have demonstrated redox transformations of selenium (Se) in the presence of Fe-bearing minerals, the specific mechanism of magnetite-mediated Se electron transfer reactions are poorly understood. In this study, the redox chemistry of Se on magnetite is investigated over an environmentally relevant range of Eh and pH conditions (+0.85 to −1.0 V vs . Ag/AgCl; pH 4.0–9.5). Se redox peaks are found via cyclic voltammetry (CV) experiments at pH conditions of 4.0–8.0. A broad reduction peak centered at −0.5 V represents a multi-electron transfer process involving the transformation of selenite to Se(0) and Se(−II) and the comproportionation reaction between Se(−II) and Se(IV). Upon anodic scans, the oxidation peak centered at −0.25 V is observed and is attributed to the oxidation of Se(−II) to higher oxidation states. Deposited Se(0) may be oxidized at +0.2 V when pH is below 7.0. Over a pH range of 4.0–8.0, the pH dependence of peak potentials is less pronounced than predicted from equilibrium redox potentials. This is attributed to pH gradients in the microporous media of the cavity where the rate of proton consumption by the selenite reduction is faster relative to mass transfer from the solution. In chronoamperometry measurements at potentials ⩾−0.6 V, the current–time transients show good linearity between the current and time in a log–log scale. In contrast, deviation from the linear trend is observed at more negative potentials. Such a trend is indicative of Se(0) nucleation and growth on the magnetite surface, which can be theoretically explained by the progressive nucleation model. XPS analysis reveals the dominance of elemental selenium at potentials ⩽−0.5 V, in good agreement with the peak assignment on the cyclic voltammograms and the nucleation kinetic results. [ABSTRACT FROM AUTHOR]

Hexavalent chromium is a highly toxic and readily mobile metal contaminant introduced to the environment through a variety of industrial operations. In the presence of reductants, such as Fe(II), and catalytic mineral surfaces, such as iron oxides surfaces, Cr(VI) may be reduced to a less toxic and relatively insoluble form Cr(III). In this study, we investigate the interaction between Cr(VI) and the surface of the Fe(II)-bearing mineral magnetite, Fe(II)Fe(III) 2 O 4 , as an example catalyst, using electrochemical atomic force microscopy (EC-AFM). With this method, the redox potential is controlled by an electrode, and Cr deposition on the magnetite surface is imaged over time as a function of redox potential and pH of the solution. Quantitative analyses of volumetric growth and surface coverage reveal that more precipitation occurs over time at very negative (− 500 mV at pH 3, − 750 mV at pH 7, and − 1000 mV at pH 11) and very positive (+ 1000 mV at pH 3 and + 500 mV at pH 11) electrochemical potentials. Up to 70% of the surface is covered with precipitates at pH 7, while less coverage is observed at pH 11 (< 8%) and pH 3 (< 2%). Particle growth at pH 3 is predominantly lateral in nature with a tendency to form a higher number of smaller adsorbate particles. At pH 11, growth is primarily vertical (perpendicular to the surface), and smaller particles tend to aggregate into larger clusters on the surface with increasingly negative redox polarization. These larger clusters (most likely subcrystalline Cr-(oxy)hydroxides) co-exist with rhombic, crystalline particles that likely have preferred crystallographic growth relationships with the magnetite substrate; the composition of this latter type of particle is most likely chromite, FeCr 2 O 4 , based on the matching crystallography and stability under the Eh/pH conditions of this experiment. Results from growth analyses were confirmed by batch experiments performed on the same samples used in the AFM, where changes in the Cr concentration in solution were monitored using inductively coupled plasma mass spectrometry (ICP-MS). Virtually all of the Cr was removed after 30 min at pH 7 (starting at 2 μM Cr(VI)) at all potentials, ~ 60% at pH 3, and very little removal at pH 11. X-ray photoelectron spectroscopy (XPS) analyses of the magnetite surface at pH 3 suggest that only Cr(III) phases are present and that Cr and/or Fe–Cr oxide phases are more stable at very reducing conditions (− 750 mV), while Cr and/or Fe–Cr (oxy)hydroxide phases are present under more moderately-reducing conditions (− 250 mV). At pH 11, any Cr deposited is below the detection limit for XPS analysis (< 1% surface coverage); however, Fe(II)/Fe(III) ratios (~ 0.3) are lower than the expected ratio of magnetite, which indicates that some oxidation of the surface has occurred. This oxidized layer, possibly maghemite (γ-Fe 2 O 3 ), hematite (α-Fe 2 O 3 ), or FeOOH, may inhibit the reduction of Cr(VI) on the surface. [ABSTRACT FROM AUTHOR]

Reduction of U(VI) to U(IV) on mineral surfaces is often considered a one-step two-electron process. However, stabilized U(V), with no evidence of U(IV), found in recent studies indicates U(VI) can undergo a one-electron reduction to U(V) without further progression to U(IV). We investigated reduction pathways of uranium by reducing U(VI) electrochemically on a magnetite electrode at pH 3.4. Cyclic voltammetry confirms the one-electron reduction of U(VI) to U(V). Formation of nanosize uranium precipitates on the magnetite surface at reducing potentials and dissolution of the solids at oxidizing potentials are observed by in situ electrochemical atomic force microscopy. XPS analysis of the magnetite electrodes polarized in uranium solutions at voltages from -0.1 to -0.9 V (E0U(VI)/U(V)= -0.135 V vs Ag/AgCl) show the presence of only U(V) and U(VI). The sample with the highest U(V)/U(VI) ratio was prepared at -0.7 V, where the longest average U-Oaxial distance of 2.05 ± 0.01 Å was evident in the same sample revealed by extended X-ray absorption fine structure analysis. The results demonstrate that the electrochemical reduction of U(VI) on magnetite only yields U(V), even at a potential of -0.9 V, which favors the one-electron reduction mechanism. U(V) does not disproportionate but stabilizes on magnetite through precipitation of mixed-valence state U(V)/U(VI) solids. [ABSTRACT FROM AUTHOR]

Pentavalent uranium is generally treated as an unstable intermediate when uranyl, U(VI)O 2 2+ , is reduced to U 4+ . However, mineral surfaces have been shown to stabilize pentavalent uranium, thus hindering further reduction (Ilton et al., 2005, 2010). The subject of this study is to identify the kinetic pathways that lead to U(V)O 2 + being a metastable species. Electrochemical methods provide an in situ approach for the investigation of the intermediate reaction of U(V)O 2 + on the surfaces of magnetite. Redox reactions of uranyl ions on particulate (∼3 μm) and bulk magnetite surfaces were investigated using cyclic voltammetry and potential step chronoamperometry using cavity microelectrodes and bulk (planar) mineral electrodes. The estimated redox potentials are consistent with the standard redox potential of UO 2 2+ /UO 2 + , indicating UO 2 2+ is first reduced to UO 2 + on the surfaces of both powder and bulk magnetite. The one-electron reduction of UO 2 2+ to UO 2 + was further confirmed by directly measuring the number of electrons transferred during the reduction process on the bulk magnetite electrode. Based on the charge conservation analysis and the positive correlation between the pH and the peak current for the UO 2 + transformation to UO 2 2+ , the peak corresponding to the oxidation of U 4+ to UO 2 2+ was assigned in the voltammograms of particulate magnetite. The presence of U 4+ indicates that the disproportionation of UO 2 + (2U(V) ↔ U(IV) + U(VI)) is occurring on the surface of particulate magnetite within the timeframe of the experiment. The lack of a peak for U 4+ in voltammograms for bulk magnetite suggests that the rate of the UO 2 + disproportionation reaction is slower on bulk magnetite than that on particulate magnetite. The catalytic property of particulate magnetite surfaces on the disproportionation reaction is explained by its ability to adsorb and desorb protons, which could facilitate the proton-coupled disproportionation reaction of UO 2 + . This increased catalytic activity and related adsorption and desorption kinetics of protons may be related to the increased number of under-coordinated surface sites near step edges on the magnetite powder. [ABSTRACT FROM AUTHOR]

Abstract: There are many studies describing the influence of parameters such as pH, pCO2, and complexing ligands on the sorption of the aqueous uranyl species onto mineral surfaces. However, few of these studies describe the reduction reaction mechanisms and the factors that influence the rate of reduction, despite the fact that the oxidation state of uranium is the most important factor controlling the mobility of uranium. In this study, the energetics and kinetics of the U(VI) reduction half-reaction on pyrite, hematite, and magnetite were investigated by electrochemical methods using a powder microelectrode (PME) as the working electrode. Anodic and cathodic peaks corresponding to the 1e- redox couple, U(VI)/U(V), were identified in cyclic voltammograms of pyrite, hematite, and magnetite at pH 4.5. A second oxidation peak, corresponding to the oxidation of U(IV), was identified and provides evidence for the formation of reduced uranium phase(s) on the mineral surfaces. In addition, uranium-containing precipitates were identified on pyrite surfaces after polarization in a PME. This study identifies the disproportionation of U(V) species on the surface as a possible rate-limiting step in the two-step U(VI) reduction mechanism: (1) charge transfer to form U(V) followed by, (2) a disproportionation reaction that forms U(IV) and U(VI). The Tafel slope (i.e., the derivative of the electrode potential with respect to log[current]) was used to evaluate electrochemical mechanisms. High Tafel slopes (>220mV/(logunit of current) on all minerals evaluated) suggest that uranyl reduction is mediated by insulating (hydr)oxide layers that are present on the semiconducting mineral surfaces. The onset potential for uranyl reduction was determined for pyrite (>+0.1V vs. Ag/AgCl), and hematite and magnetite (between−0.02 and−0.1V vs. Ag/AgCl). The onset potential values establish a baseline kinetic parameter that can be used to evaluate how solution conditions (e.g., dissolved reductants, complexing ligands, and polarizing ions) affect the kinetics of uranyl reduction. The results of this study demonstrate the potential for using PMEs to evaluate redox potentials and mechanisms for U(VI) reduction by Fe-oxides and sulfides under more complex solution conditions as well as other environmentally-relevant mineral-analyte systems. However, it should be noted that the determination of redox kinetics using Butler–Volmer theory has limitations when applied to semiconductor mineral electrodes. Charge depletion in semiconductor surface states can affect the kinetic values obtained for redox reactions on the surface. These limitations and a discussion of the flat band potential are considered in the interpretation of U redox kinetics in this study. [Copyright &y& Elsevier]