Article pubs.acs.org/est
Iron Atom Exchange between Hematite and Aqueous Fe(II) Andrew J. Frierdich,†,‡,⊥,∇ Maria Helgeson,†,⊥ Chengshuai Liu,§,# Chongmin Wang,∥ Kevin M. Rosso,∥ and Michelle M. Scherer*,† †
Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242, United States Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, United States § Guangdong Institute of Eco-Environmental and Soil Sciences, Guangzhou, 510650, P. R. China ∥ Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡
S Supporting Information *
ABSTRACT: Aqueous Fe(II) has been shown to exchange with structural Fe(III) in goethite without any significant phase transformation. It remains unclear, however, whether aqueous Fe(II) undergoes similar exchange reactions with structural Fe(III) in hematite, a ubiquitous iron oxide mineral. Here, we use an enriched 57Fe tracer to show that aqueous Fe(II) exchanges with structural Fe(III) in hematite at room temperature, and that the amount of exchange is influenced by particle size, pH, and Fe(II) concentration. Reaction of 80 nm-hematite (27 m2 g−1) with aqueous Fe(II) at pH 7.0 for 30 days results in ∼5% of its structural Fe(III) atoms exchanging with Fe(II) in solution, which equates to about one surface iron layer. Smaller, 50 nm-hematite particles (54 m2 g−1) undergo about 25% exchange (∼3× surface iron) with aqueous Fe(II), demonstrating that structural Fe(III) in hematite is accessible to the fluid in the presence of Fe(II). The extent of exchange in hematite increases with pH up to 7.5 and then begins to decrease as the pH progresses to 8.0, likely due to surface site saturation by sorbed Fe(II). Similarly, when we vary the initial amount of added Fe(II), we observe decreasing amounts of exchange when aqueous Fe(II) is increased beyond surface saturation. This work shows that Fe(II) can catalyze iron atom exchange between bulk hematite and aqueous Fe(II), despite hematite being the most thermodynamically stable iron oxide.
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INTRODUCTION Iron is the fourth most abundant element on Earth, the most common redox active transition metal, and a critical nutrient for all life, from microorganisms to humans. Oxic conditions on the Earth’s surface result in iron existing primarily as Fe(III) (oxyhydr)oxide minerals. Subsurface environments, however, commonly become anoxic due to microbial respiration which may lead to the formation of reduced iron species, such as aqueous Fe(II) (Fe(II)aq). The coexistence of Fe(II)aq and Fe(III) (oxyhydr)oxides is common at redox interfaces during the weathering of Fe(II)bearing primary rocks, the upwelling of anoxic spring water, acid mine drainage, and during microbial iron reduction. Reaction of Fe(II)aq with crystalline iron oxides macroscopically (based on wet-chemistry observations) behaves as cation sorption at predominantly passive oxide surfaces.1−6 Results from 57Fe Mössbauer spectroscopy, however, have shown that Fe(II) undergoes interfacial electron transfer with the underlying Fe(III) in the solid,7−12 and enriched iron isotope tracer experiments have shown that Fe(II)aq exchanges with Fe(III) in goethite and ferrihydrite, and with iron in magnetite.13−21 While electron transfer between Fe(II) and Fe(III) in hematite has been demonstrated,10,12,22−24 it remains unclear, however, whether Fe(II)aq exchanges with Fe(III) in hematite. Although there is no direct evidence demonstrating iron exchange in hematite with Fe(II)aq, there is indirect evidence that suggests exchange occurs. Reactions between Fe(II)aq and © XXXX American Chemical Society
trace element substituted hematite, for example, resulted in the release of isomorphic-substituted metals (e.g., Ni, Zn) to solution, suggesting that Fe(II)aq was exchanging with the mineral.25−27 Similarly, Ni incorporation into hematite was observed spectroscopically when Fe(II)aq was present.27 Other researchers, however, have not found evidence for Fe(II)promoted metal fixation into hematite,5 although metal fixation has been reported in several studies involving Fe(II)aq and goethite.6,27−29 Additional indirect evidence for exchange comes from observations of growth of hematite on one surface and pitting on another surface where bulk electron conduction was shown to link the crystallographic faces.22,30 Surface structural changes on hematite single crystals have also been observed from X-ray reflectivity measurements following reactions with Fe(II)aq.31 Exchange between Fe(II)aq and a reactive Fe(III) hematite surface layer has been inferred from iron isotope fractionations during microbial iron reduction;32−34 similar fractionations occur in abiotic experiments where Fe(II)aq is added to hematite suspensions, with the authors concluding that roughly one surface iron layer participated in exchange.35 Received: March 12, 2015 Revised: June 11, 2015 Accepted: June 12, 2015
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DOI: 10.1021/acs.est.5b01276 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Reactions were conducted in duplicate and were initiated by combining 0.1 mL of the 57Fe-enriched Fe(II)aq solution to 9.9 mL of a hematite suspension (2 g L−1). Solution pH was buffered by 25 mM HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) for pH 7.0, 7.5, and 8.0) or 25 mM MES (2(N-morpholino)ethanesulfonic acid) for pH 5.5 and 6.5; 25 mM KBr was present as a background electrolyte for reactions at all pH values. For experiments that explored the effect of Fe(II) concentration, the initial volume of the hematite suspension was adjusted such that after addition of 57Fe(II)aq, the final reactor volume was 10 mL. Reactors were wrapped in Al-foil and placed on an end-overend rotator to keep the suspension mixed. Individual reactors were sacrificed at specific time intervals and three operationally defined components were sampled. First, a 1 mL aliquot was taken from each reactor suspension and placed into a 2 mL microcentrifuge tube and spun inside the glovebox at ∼6200g for 1.5 min. The supernatant was discarded and the pellet was resuspended in 1 mL of 0.4 M HCl for 10 min. This suspension was centrifuged a second time and the supernatant was filtered (0.22 μm, nylon) into a clean 2 mL microcentrifuge tube and saved for analysis. The iron contained in this 0.4 M HCl extract is referred to as Fe(II)extr. The remaining hematite pellet was then dissolved in 1 mL of 4 M HCl at 70 °C. The remaining 9 mL of the original reactor suspension was filtered and acidified to 0.4 M HCl. This filtrate contains the Fe(II)aq sample fraction. Iron concentrations were determined by the Fe(II)selective reagent ferrozine;38 total Fe was determined after reduction of Fe(III) by hydroxylamine and Fe(III) was calculated by difference. Iron isotope analyses were carried out using a Thermo Scientific X Series II Quadrupole inductively coupled plasma mass spectrometer (ICPMS). Argide polyatomic interferences (e.g., 56[ArO]+, 54[ArN]+) were removed with a reaction cell containing a He:H2 mixture of 93%:7%. All iron samples were diluted to 0.5 μM Fe in 0.1 M HCl (trace metal grade) prior to analysis. Iron isotope mole fractions ( f) were calculated by dividing the counts per second (cps) of isotope n by the sum of the total iron isotope cps as given below:
The only direct measurement of exchange of Fe(II)aq with hematite comes from the work of Pedersen et al. (2005), who detected no iron atom exchange. These authors reacted 55Feenriched iron oxides in natural abundance Fe(II)aq solutions and measured the 55Fe radioisotope tracer released as aqueous 55 Fe(II).20 Significant amounts of 55Fe were detected in the Fe(II)aq fraction for reactions with goethite and ferrihydrite. However, no 55Fe release was detected during reaction of Fe(II) with 55Fe-enriched hematite. Pedersen et al. (2005) conducted their experiments at a relatively low pH (i.e., 6.5). Given the importance of solution pH on the amount of iron exchange between Fe(II)aq and goethite,17,36 it is plausible that exchange did not occur under the conditions of their study but may be more prominent in higher pH solutions. The wide variety of experimental conditions used in these studies make it difficult to reconcile the discrepancies regarding the extent to which Fe(II)aq exchanges with hematite. Here, we use a 57Fe-enriched stable isotope tracer to directly measure if Fe(II)aq undergoes atom exchange with Fe(III) in hematite under a variety of experimental conditions. We explore the parameters that we suspect are most likely to affect exchange including particle size, Fe(II) concentration relative to hematite solids, and solution pH. Hematite samples reacted with Fe(II)aq are compared through ex situ characterization with unreacted materials to determine whether changes in particle size, crystallinity, or reactivity occur following atom exchange. Our overall objective is to determine whether Fe(II) catalyzes iron atom exchange with hematite and how important environmental parameters, such as pH, particle size, and Fe(II) concentration influence the extent of exchange.
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MATERIALS AND METHODS Hematite Synthesis and Characterization. Two synthetic hematite materials were prepared in order to test the effect of particle size, or specific surface area, on the extent of iron isotope exchange. The first synthesis was carried out according to the method described by Pedersen et al. (2005). Briefly, 10.81 g of FeCl3·6H2O was added to 2 L of a 3.75 mM HCl solution preheated to 98 °C. This solution was held at 98 °C for 10 days. The second hematite material was prepared by heating 500 mL of a 0.2 M solution of Fe(ClO4)3·9H2O at 98 °C for 7 days.37 Both materials were washed by centrifugation, dried in an oven at 70 °C, and passed through a 325 mesh sieve. BET specific surface areas were determined by N2 adsorption at 77K and found to be 27 and 54 m2 g−1 corresponding to the first and second synthesized material. Hereon, these will be referred to as 27 and 54 m2 g−1 hematite. Scanning and transmission electron microscopies (SEM and TEM) revealed the average particle size to be 82 ± 17 nm (2σ, n = 265) and 48 ± 15 nm (2σ, n = 108) for 27 and 54 m2 g−1 hematite, respectively. Phase purity was confirmed by X-ray diffraction (XRD) and no crystalline phases other than hematite were identified (see Supporting Information (SI)). Iron Isotope Exchange Experiments. Our study is based on reacting hematite that has a natural abundance iron isotope composition with 57Fe-enriched solutions of Fe(II)aq. All exchange experiments were carried out in an anoxic glovebox (96% N2, 4% H2) with O2 concentrations kept below 1 ppm by continual atmospheric circulation over a Pd catalyst. A 57Feenriched solution of 0.1 M Fe(II)aq was prepared by dissolving 57 Fe metal (Isoflex, San Francisco, CA) in warm ( 0. No correction for mass-dependent isotope fractionation is applied to f tFe(II) since equilibrium and kinetic isotope fractionations are negligible (i.e., a few per mil which would change f nFe by
