Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite

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Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite Derek Peak†,* and Tom Regier‡ †

Department of Soil Science, College of Agriculture and Bioresources, University of Saskatchewan, 51 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A8 ‡ Canadian Light Source, Inc., 101 Perimeter Road, Saskatoon, Saskatchewan, Canada, S7N 0X4 S Supporting Information *

ABSTRACT: Ferrihydrite is a common iron hydroxide nanomineral commonly found in soils, sediments, and surface waters. Reactivity with this important environmental surface often controls the fate and mobility of both essential nutrients and inorganic contaminants. Despite the critical role of ferrihydrite in environmental geochemistry, its structure is still debated. In this work, we apply bulk sensitive Fe L edge Xray absorption spectroscopy to study the crystal field environment of the Fe in ferrihydrite and other Fe oxides of known structure. This direct probe of the local electronic structure provides verification of the presence of tetrahedrally coordinated Fe(III) in the structure of ferrihydrite and puts to rest the controversy on this issue.

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workers21 reported a single-domain model for ferrihydrite containing both tetrahedral and octahedral iron. This model was developed by calculating a pair distribution function (PDF) from the reduced structure function obtained via synchrotronbased X-ray total scattering data. They later revised their structural model slightly22 but the overall structure proposed is that of a 13 Fe unit where 12 Fe are octahedral and a central Fe is tetrahedral. This building block is iso-structural with the extensively studied Al13 soluble complex.23 Recent high quality XANES18 and EXAFS18,20 experiments are generally supportive of the model proposed by Michel and colleagues, with some disagreement on the amount of tetrahedral Fe that is present in the structure. This model stands in contrast to a competing multiple domain octahedral Fe structural model of ferrihydrite first proposed by Drits and co-workers in 1993.24 In this model, ferrihydrite is a mixture of two major components: a d-phase that is feroxyhite-like and an f-phase that is akaganeite/goethite like in structure. Recently, Manceau has evaluated the quality of this model and reanalyzed PDF and XAS measurements of other researchers25−27 and concluded it is the best model to describe all of the existing XANES, EXAFS, and scattering results. One reason for the contradictory structural models is that (a) many of the chosen spectroscopic techniques produce

INTRODUCTION Ferrihydrite is the initial solid product of Fe(III) hydrolysis in solution, and is the precursor to more crystalline forms of iron oxides.1 However, ferrihydrite is metastable in natural systems and persists both as discrete particles and as surface coatings in soils and sediments. Its large surface area and functional group density2 make it an important sorbent for nutrients3,4 and contaminants5−7 and it is also a common iron(III) source for iron reducing bacteria.8 Contaminant reactivity is often controlled by sorption reactions at the mineral/water interface. Accordingly, geochemical transport modeling relies upon surface complexation theory to describe the interfacial chemistry with functional groups and sorption reactions at the solid/water interface. To refine these models and improve their predictive ability, it is important to understand the structure of nanominerals such as ferrihydrite. For over two decades, the structure of synthetic ferrihydrite has been extensively studied by many scientists using a range of spectroscopic techniques including Mössbauer,9−11 EELS,12 XANES,13−18 EXAFS,19,20 and PDF/X-ray scattering.21,22 Rather than conclusively providing a model of the shortrange ordered material, the intense investigation of ferrihydrite has instead produced two competing structural models: one in which tetrahedral Fe is an important structural component, and a second in which only octahedral ferric iron is present. Although a ferrihydrite structural model which contained tetrahedral Fe was first proposed in 1988, the strongest evidence for tetrahedral Fe in ferrihydrite comes from recent studies using PDF and XAS. In a 2007 study, Michel and co© 2012 American Chemical Society

Received: Revised: Accepted: Published: 3163

July 23, 2011 February 8, 2012 February 27, 2012 February 27, 2012 dx.doi.org/10.1021/es203816x | Environ. Sci. Technol. 2012, 46, 3163−3168

Environmental Science & Technology

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significant amount of octahedral Fe3+ exist in the structure but a detailed comparison with all relevant reference compounds is not used in these studies.31,32 In particular, Pan and collaborators interpret their six line ferrihydrite STXM data to be representative of Fe3+ in octahedral co-ordination but do not compare their measurements to that of maghemite which is known to contain tetrahedral Fe3+.33 In this study, we apply a recently developed fluorescence measurement method called inverse partial fluorescence yield (IPFY), which is bulk sensitive and free of self-absorption effects34 to measure the Fe L-edge XANES of several iron (hydr)oxides. The results of these bulk sensitive measurements, when compared to the surface sensitive TEY measurements, confirm the presence of tetrahedral Fe(III) in the structure of synthetic 2 line ferrihydrite.

ambiguous results when used to probe the ferrihydrite structure and (b) the concentration of tetrahedral Fe may be close to the limits of detection for the method. Pre-edge features consistent with tetrahedral Fe are seen in Fe K-edge XANES spectra of ferrihydrite,13,18 but a plausible alternative explanation for the features that only involves octahedral Fe(III) has also been made.14,16 Mössbauer measurements have been utilized as proof that ferrihydrite only contains octahedral Fe(III) by the Drits model but were also used to set an upper-limit of tetrahedral Fe(III) in the Michel model. EXAFS is commonly employed for structural studies of short-range order materials, but EXAFS provides an average local environment, and ferrihydrite EXAFS studies have similarly been contradictory. In the case of ferrihydrite, overlap in the interatomic distances of tetrahedral iron sites and some octahedral linkages are a challenge for EXAFS to resolve; nonetheless recent studies using high-k, high-resolution data developed a model including tetrahedral iron to fit their ferrihydrite.20 However, it has been argued by Manceau27 that these results are over fit, and that the Drits model can acceptably explain the EXAFS results of ref 20 without including tetrahedral Fe. Fe L-edge XANES of ferrihydrite probes the local electronic structure at the Fe sites in bulk and nanocrystalline samples with great sensitivity, and presents a possible alternative to Kedge experiments. The L-edge XANES provides different information on the Fe crystal environment than does Fe K-edge XANES because of the allowed dipole transitions for 1s vs 2p electrons. K edge spectra reflect the density of unoccupied 2p states, commonly associated with the unoccupied O 2p orbitals in the iron oxides, while L edge spectra involve excitation into the unoccupied 3d states of Fe. The effect of the crystal field on these orbitals is to remove their degeneracy, introducing a fine structure in the absorption spectrum that is highly specific to the site symmetry. EELS at the Fe L-edge has been successfully used to probe the electronic structure of ferrihydrite12 but low energy resolution and increased beam damage to samples in EELS measurements make the approach less than ideal. Synchrotron-based L-edge XANES provides information similar to EELS, but with a much lower radiation dose and increased spectral resolution (0.1 eV vs 1 eV). Despite the utility of Fe L-edge XANES for studies of the structure of the iron oxides, the technique has, until now, not been specifically applied for this purpose. The primary reason for this is due to the lack of a suitable detection technique. Total electron yield (TEY) measurements are highly surface sensitive and will be heavily influenced by the surface oxidation or surface contamination of the samples. Fluorescence yield techniques on concentrated samples are bulk sensitive but may not be useful because of saturation effects.28 Total fluorescence measurements at the Fe L-edge are also susceptible to nonlinear backgrounds arising from the energy dependence in the O K emission.29 Scanning transmission X-ray microscopy (STXM) is one suitable method for the study of nanostructured compounds, where a suitable optical contrast exists. But many of the relevant reference materials for ferrihydrite may not readily exist at the size scale necessary for STXM analysis, making comparison with standards problematic. The above technical problems related to XANES measurement have resulted in only a limited application of Fe L-edge XANES for studies of bulk materials. In a recent study the TEY method was used to probe the surface structure Fe L-edge of ferrihydrite-silicate solids subjected to calcining.30 STXM studies on two and six line ferrihydrite have shown that

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MATERIALS AND METHODS Synthetic commercial hematite, magnetite, wü stite, and maghemite were purchased from Alfa Aesar and Sigma Aldrich, and a synthetic goethite, akaganeite, and ferrihydrite (2-line) were synthesized using the methods of ref 1 with the following modifications. For goethite synthesis, the initial hydrolysis product was aged at 298K for 14 days and prior to freeze-drying was acid washed in 0.4 M HCl to remove residual amorphous Fe(OH)3. The resulting product consisted of 100 nm rods that had BET surface area of 65 m2g−1. For ferrihydrite a solution of 1 M Fe(Cl)3 was neutralized with 1 M KOH under N2 sparging and continuous stirring: the resulting suspension was held at pH 7.5 for one hour using a computer-controlled titrator (Metrohm 718) and then washed via centrifugation 3 times with 0.1 M NaCl to remove entrained and adsorbed Fe3+ and Cl− and then 5 times with 18 MΩ H2O (Barnstead Diamond NanoPure) to remove soluble salts prior to flash freezing in N2(l) and freeze-drying. XANES measurements were performed at the Spherical Grating Monochromator (SGM) beamline of the Canadian Light Source in Saskatoon, SK. Dry samples were prepared by placing a small amount of homogenized (via agate mortar and pestle) powder on graphite tape. All samples were scanned from 700 to 735 eV in 0.1 eV steps, which encompasses both the Fe L2 and L3 absorption edges. Surface sensitive absorption spectra were recorded using TEY and bulk sensitive absorption spectra were measured using IPFY. TEY employs the intensity of emitted secondary electrons as a probe of the attenuation coefficient. At normal incidence, TEY is not susceptible to saturation effects because the attenuation length of the electrons is much shorter than the attenuation length of the incident photons. However, this also implies that TEY is only a probe of the surface (∼5 nm) of the solid rather than the bulk structure. To probe the bulk structure, IPFY measurements were made using an energy resolving fluorescence yield detector to monitor the intensity of the O Kα emission as the incident photon energy was scanned across the Fe L2 and L3 absorption threshold. For these experiments, a commercial silicon drift detector (SDD) with an energy resolution of ∼150 eV was used. IPFY was then calculated by taking the inverse of the O PFY after normalization to the incident photon flux as recorded by a gold mesh. To illustrate the IPFY technique and its advantages, Figure 1 shows the results from analysis of the synthetic ferrihydrite sample at the Fe L-Edge. In the upper portion of the figure, the excitation−emission matrix is presented. Two regions show significant fluorescence when the sample is scanned through 3164

dx.doi.org/10.1021/es203816x | Environ. Sci. Technol. 2012, 46, 3163−3168


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Figure 1. Fe L-Edge XANES spectrum of ferrihydrite. At top is the excitation−emission matrix for the sample that has been normalized to the incident flux. The middle portion contains the total fluorescence (obtained by averaging the total emission at each energy) as well as the partial fluorescence arising from either Fe or O emission. At bottom is the IPFY spectrum, which represents the bulk Fe L-Edge free from self-absorption effects.

Figure 2. Fe L-Edge XANES of a variety of mineral samples compared with ferrihydrite. The surface spectra are denoted by TEY (dashed line), whereas the bulk spectra are IPFY (solid lines).

octahedral Fe(III) in a spinel ferrite structure. In magnetite, one-third of the Fe sites are octahedral Fe(II), one-third are octahedral Fe(III) and the remaining third are octahedral Fe(III). Ferrous oxide contains only octahedrally coordinated Fe(II). It has been established via both experiment and theoretical calculations35 that Fe L-edge XAS spectra are dominated by the dipole-allowed 2p-3d transitions. Two major spectral features are observed in Fe XANES of these minerals. First of all, the Fe L-edge of all samples are well resolved into separate L3 (706− 713 eV) and L2 (718−725 eV) regions as a result of spin−orbit coupling. Second, there is additional splitting of these main peaks that occurs due to the electronic interaction between the Fe 3d and the O 2p orbitals. Specifically, the degeneracy between the 5 different Fe d orbitals is lifted, giving rise to the eg and t2 g orbitals with a splitting that is highly sensitive to the coordination environment. In goethite, akaganeite, and hematite, there is pronounced splitting of the L3 edge into two well-resolved peaks at approximately 708.1 and 709.5 eV. This is consistent with octahedral Fe(III) being the only coordination environment. The close agreement between the surface sensitive TEY and bulk sensitive IPFY suggests that there is little or no difference between the Fe coordination at the surface and the bulk of these materials. A slight shift (∼0.1 eV) to higher energy position for goethite relative to hematite occurs for the main L3 peak. This has not been reported in the literature due to the lack of high resolution XANES, but it can be reasonably explained from crystal field theory. Hematite is red, whereas goethite is ochrous, which is evidence of a slightly decreased

the Fe L2,3 edges: these correspond to Fe Lα and Lβ emission (625−725 eV) and O Kα emission (450−550 eV). Total fluorescence (TFY) is produced simply as the sum of all fluorescence at that excitation energy (Fe + O for ferrihydrite), and is analogous to the signal from commonly used total fluorescence detectors such as micro channel plates or diodes. The ability to separate the individual elemental contributions to the total fluorescence reveals that the energy dependence in the total signal arises not only from increasing Fe fluorescence, but instead from a strong decrease in the O Kα emission. As a consequence of this, significant distortion in the TFY occurs, including a sub-background absorbance in the Fe edge due to the decrease in the O emission being larger than the increase in Fe emission.34 The lower portion of Figure 1 shows the inverse of the normalized O fluorescence, which is proportional to the total attenuation coefficient of the material.34 The inverse of the partial fluorescence yield of the O in the sample therefore provides access to a high quality fluorescence spectrum of Fe that is free from self-absorption effects present in Fe PFY.

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RESULTS AND DISCUSSION In Figure 2, we combine the TEY (surface) and IPFY (bulk) spectra of goethite (α-FeOOH), akaganeite (β-FeOOH) hematite (α-Fe 2 O 3 ), maghemite (γ-Fe 2 O 3 ), magnetite (Fe3O4), and wüstite (FeO) standards with the TEY and IPFY of ferrihydrite. These standards were chosen to provide a spectral reference for both octahedrally and tetrahedrally coordinated Fe(III) as well as octahedral Fe(II). In goethite, hematite, and akaganeite all of the iron is octahedrally coordinated Fe(III). Maghemite contains both tetrahedral and 3165



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important information about the iron coordination in this nanomineral. First of all, the splitting of the L3 peak into two peaks with the same energies as hematite, goethite, and maghemite provides unsurprising validation of Fe(III) in this iron hydroxide. Also of note is that the surface and bulk XANES of ferrihydrite are quite similar, suggesting that the same coordination of Fe exists in the surface and in the bulk of this material. More importantly, the relative intensities and the relatively poor resolution of the splitting between the 2 bands of the L3 peak is clear evidence of the presence of Fe(III) in more than one configuration. This point is emphasized when ferrihydrite is plotted on top of maghemite, as shown in Figure 3. Indeed, the similarity between ferrihydrite and maghemite

crystal field splitting (distance between the 2 observed L3 peaks). An important point to make about these crystalline octahedral minerals is that, while the Fe−O bond distances and angles vary tremendously in these two crystal structures, the Ledge XANES splitting remains sharp and well resolved in both spectra. At the K-edge, it has been reported that ferrihydrite and akaganeite are indistinguishable; their L-edge spectra are quite different. This highlights the fact that L-edge absorption spectroscopy is much more sensitive to the average crystal field environment in materials than it is to specific bond lengths and angles. The latter are more aptly probed by measuring the EXAFS above the K-edge, which is sensitive to photoelectron scattering pathways. In contrast to ferric oxides and hydroxides, the fluorescence yield spectra of wüstite and magnetite differ significantly from their electron yield spectra, which is the result of strong surface modification in the Fe(II) containing samples. The bulk spectra for both minerals are dominated by a large broad peak at 709 eV that has a low energy shoulder at 706.5 eV. An additional shoulder is observed in the magnetite spectra at 708 eV. The spectral shapes are consistent with previous theoretical models of wüstite as an octahedral Fe(II) compound36 and magnetite as a mixed valence compound with Fe(II) and Fe(III) in octahedral sites and Fe(III) in a tetrahedral site.37 The low energy shoulders can be used as clear indicators of the presence of Fe(II) in the bulk of the samples.38 The electron yield spectra of the Fe(II)-containing compounds show some splitting in the L3, but it is not as well resolved as in hematite and goethite. For wüstite, there is a shift of the position of the L3 peaks to higher energy when compared to the bulk suggesting that the surface iron has been largely converted to octahedral Fe(III). But the persistence of the shoulder at 706.5 eV in the electron yield data demonstrates that some octahedral Fe(II) is present at or near the surface. The differences in the electron yield and fluorescence yield can readily be explained by the surface oxidation of wüstite from octahedral Fe(II) to tetrahedral Fe(III). Indeed, wüstite is well-known to partially oxidize and form tetrahedral Fe(III) and tetrahedral Fe(III) is required to refine the crystal structure of wüstite.36 This oxidation would be expected to occur preferentially on the surface of the material. Our results suggest that a similar surface modification is occurring for magnetite. Maghemite is a polymorph of the purely octahedral hematite that contains 20−33% tetrahedral Fe(III) depending upon site vacancies.33 The spectrum of maghemite differs from that of hematite in that the L3 peak is noticeably broader, resulting in a less well resolved splitting. The broadening has previously been modeled35 and assigned to a combination of the distortion of the symmetry of the Fe(III) octahedra and the presence of a second Fe(III) tetrahedral component with a reduced crystal field splitting. This tetrahedral Fe(III) is predicted to have a main peak position slightly lower than octahedral Fe(III), which causes the maghemite L3 to broaden. Since the ligand field splitting remained well resolved in the highly disordered akaganeite standard, it is unlikely that disorder alone can account for the observed broadening in ferrihydrite. There is little difference between the surface and bulk spectra of this mineral, which shows that, in contrast to wüstite, tetrahedral iron is present throughout the structure of the mineral rather than only on the surface. Given the above discussion about the surface and the bulk of the known Fe oxide standards, the spectrum of synthetic ferrihydrite powder, shown in Figure 2, reveals some new and

Figure 3. Fe L3-Edge XANES bulk spectra of minerals compared to ferrihydrite.

can only be readily explained by the presence of tetrahedral Fe(III) in ferrihydrite. It is also possible to estimate the relative percentages of octahedral and tetrahedral iron via fitting the IPFY data for maghemite and ferrihydrite with theoretical tetrahedral and octahedral sites obtained from the literature.35 The fitting produced results (Supporting Information, SI) suggest that 30−40% tetrahedral Fe is present in both mineral phases. This is somewhat higher than the 10−12% tetrahedral Fe in ferrihydrite proposed based upon PDF calculations22 and Fe K-edge XANES,18 but agrees reasonably well with the values proposed by Maillot et al.20 obtained via EXAFS. It must be noted that the molar absorptivity for octahedral and tetrahedral Fe3+ was assumed to be identical in the above relative % calculations; this may overestimate the true tetrahedral amount if this assumption is false. However, our LC modeling was extremely sensitive to changing the E0 of the calculated sites; if allowed to independently vary, it can produce ∼10% variation 3166



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differences between the powder ferrihydrite used for structural analysis and the initial hydrolysis product in solution often referred to as hydrous ferric oxide, or HFO. In conclusion, this Fe L-Edge XANES study provides direct proof for tetrahedral Fe(III) in the structure of ferrihydrite, and supports the PDF-based model of Michel that proposed ferrihydrite is iso-structural with aluminum polymer systems.21,22 Recent soft X-ray spectroscopic studies at the Al Kand L-edge have also directly observed that amorphous aluminum oxides contain tetrahedral Al3+ that converts to octahedral Al as the minerals crystallize into octahedral minerals.42,43 This may imply that similar polymerization/ condensation/precipitation mechanisms may be active for both iron and aluminum. Additionally, the use of the bulk-sensitive IPFY approach detailed in this work can provide new structural information on many other amorphous transition metal oxide and hydroxide phases of environmental importance that are difficult to differentiate and identify with K-edge XAS or EXAFS.

in the tetrahedral component. However, in no case could using only octahedral sites reproduce the features of the ferrihydrite L-edge XANES; tetrahedral Fe in 15% or more was always required to obtain a reasonable fit. Because researchers had previously shown that electron beam damage could reduce Fe(III) to Fe(II) in EELS studies of ferrihydrite,12 we carefully evaluated our XANES results for photoreduction. This was tested by either collecting multiple scans on a single spot or by running scans with a reduced beam flux. We observed no change in the Fe L-Edge XANES of any of our iron oxide samples. Furthermore, no systematic difference in TEY vs IPFY is observed in our ferrihydrite samples that would be consistent with reduction occurring over the course of the measurements. Finally, if photoreduction were occurring to a large extent on ferrihydrite, then it should also be detected for goethite and hematite. The hematite and maghemite spectra match theoretical calculations for Fe2O3 in the literature very closely,35 which would not be expected if photoreduction were important. On the basis of the above evidence, we are confident that tetrahedral Fe(III) is the source of the observed spectral features of ferrihydrite. The presence of tetrahedral Fe in ferrihydrite has several potential effects for contaminant stability and reactivity in soils, sediments, and engineered systems. It is expected that any surface functional groups formed at tetrahedral sites would have stronger bonding with H2O or OH ligands based upon Pauling’s electrostatic bond strength. This could have effects such as slowing the rate or extent of ligand exchange between Fe−OH2+ groups and oxyanions at low pH. This is consistent with bonding mechanisms observed for ligands such as sulfate39 and selenate40 adsorption on ferrihydrite (predominantly outersphere except below pH 4) and goethite/hematite (innersphere complexes form over a much wider pH range). This same chemical property (stronger Fe−O bonding in surface groups) would enhance metal cation bonding on ferrihydrite tetrahedral sites, since protons of Fe−OH would be more readily exchanged to form Fe−O−Me inner-sphere complexes. Of more general importance is the role that tetrahedral iron might play in the stability of ferrihydrite in natural systems. It has been shown in synthetic laboratory experiments41 the transformation of high surface area amorphous Fe(OH)3 into crystalline goethite and/or hematite has been shown to either incorporate trace metals into the resulting iron crystal structure or else release adsorbed metals as surface area decreases. The active mechanism depends upon solution conditions as well as the metal present. The presence of tetrahedral iron in ferrihydrite may play a role in the persistence of this high surface area mineral as a metastable phase in natural systems. The consistency between the electron yield and fluorescence yield spectra of ferrihydrite allows us to additionally conclude that the tetrahedral Fe(III) is structural. However, in a nanoparticle such as ferrihydrite, the distinction between surface and bulk spectra may lose some of its meaning. Many researchers report less than 10 nm average particle diameter for ferrihydrite so the electron yield is expected to be sensitive to the majority of the ferrihydrite particles. It is therefore possible that the source of tetrahedral Fe(III) in ferrihydrite is the production of under-coordinated Fe(III) at the surface of dried samples as a result of the freeze-drying process commonly used in mineral synthesis. Future L-edge studies of ferrihydrite in aqueous suspensions should be conducted to verify whether tetrahedral Fe forms in the synthesis of this material in solution or instead upon drying. This could elucidate whether there are

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ASSOCIATED CONTENT

* Supporting Information S

Additional Fe L-edge XANES of octahedral ferric oxoanion salts, simulations of L-edge XANES from mixing (a) multiple octahedral sites or (b) octahedral and tetrahedral sites. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Robert Green (University of Saskatchewan) for his helpful comments and suggestions about L-edge modeling. D.P. acknowledges funding for this research from the National Sciences and Engineering Research Council of Canada Discovery grant program. Research described in this work was performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan.

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REFERENCES

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Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite

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