Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite”
W
maghemite and ferrihydrite, and the calculated tetrahedral Fe(III) spectrum confirmed that the spectral shape of the ferrihydrite could not, in any reasonable way, be attributed to anything but some component of tetrahedral Fe(III). The primary concern in the comment by Hocking et al. seems to be the effect of changing the effective nuclear charge and/or the covalency of the system and the potential for this to allow for an arbitrary result in our linear combination fit. We believe these are valid concerns only if the ligands field strength experienced by the Fe sites are allowed to vary. This variation in ligand field strength could arise either from changing the ligands involved or by allowing the energy position of the spectral components to vary in calculations. As shown by Hocking, by introducing the energy positions of the t2 g and eg peaks as free parameters into the fit, agreement between the measurement and an incorrect model can easily be achieved. We recognized this, and therefore our fits did not involve any free parameters other than the spectral intensities. All energy positions in our fits in Figures S3 and S4 were obtained directly from measurement. The Fe(III) octahedral energy was taken from the hematite measurement and the Fe(III) tetrahedral energy was extracted from the subtraction of hematite from maghemite. Finally, we wish to address the data presented by Hocking et al. for various Fe(III) octahedral compounds containing a wide variety of ligands, all with different ligand field strengths and symmetries. We again agree that changing the strength and symmetry of the ligand can have a measurable effect on the position and separation of the t2 g and eg orbitals for octahedral Fe(III), but this is hardly a good model for ferrihydrite. In our system, the Fe in ferrihydrite is expected to be coordinated directly to O or OH, neither of which are expected to change the ligand field splitting enough in a purely octahedral system to produce the observed ferrihydrite spectrum (see FeOOH and Fe2O3). We did also consider the possibility that some of the Fe in our samples could be coordinated to chloride ions from the ferric chloride used in the precipitation. Since Cl is a weak field ligand, this is a legitimate concern. However, no difference was observed in the absorption spectra for ferrihydrite samples precipitated with ferric chloride versus nitrate (a strong field ligand).9 Thus, with O as the only possible ligand in the system, the magnitude of the variation in t2 g and eg energy positions illustrated by Hocking are simply not plausible for Fe octahedral when energy is constrained via calibration. In conclusion, it is our opinion that our observations of tetrahedral Fe(III) in ferrihydrite are not affected by the issues raised by Hocking et al. L-edge absorption spectroscopy is a direct probe of the coordination environment in transition metals and the recently developed IPFY method is ideally suited for these types of measurements. The limited linear
e appreciate the concern that has been expressed by Hocking et al. regarding the use of linear combination fitting to extract information from the L-edge spectra of ferrihydrite. We share these concerns, which was the reason that we only make a rough estimate on the amount of tetrahedrally coordinated Fe(III) in ferrihydrite.1 The role of effective nuclear charge, bond covalency, and potential errors in linear combination fitting put forth by Hocking et al. are correct, but do not affect our basic conclusion that the Fe Ledge of Ferrihydrite shows evidence of tetrahedrally coordinated Fe. We would like to take this opportunity to present some additional discussion regarding our methodology that we hope will clarify some of the concerns felt by Hocking. Due to the dipole allowed p to d transitions, L-edge absorption spectroscopy is a very useful method for directly probing the local electronic structure of transition metals in nanomaterials.2,3 A direct comparison of L-edge absorption spectra with either calculation or reference standard spectra will, in many cases, immediately yield both oxidation state and coordination. The lack of long-range order in nanomaterials does not affect the L-edge spectra because the core excitations are highly localized to the first coordination sphere of the excited transition element. These attributes of L-edge absorption spectroscopy make the method a valuable complement to EXAFS, electron microscopy, and X-ray or neutron scattering when studying nanoparticle structures. Until recently there have been major difficulties in the reliable application of L-edge measurements at soft X-ray energies due to saturation problems with fluorescence yields4 and surface contamination or oxidation issues with electron yield measurement. Previous L-edge measurements on ferrihydrite using EELs5,6 and STXM7 reported only octahedral Fe, but these studies were limited by a lack of both spectral resolution and a full set of reference standard measurements. Our use of the inverse partial fluorescence yield (IPFY) technique8 for the measurement of ferrihydrite and other Fe oxide minerals allowed us to make reliable, bulk sensitive, Fe Ledge measurements of these materials. These novel measurements serve as the basis for our conclusions. Upon initial measurement, our attention was first drawn to the similarity between the absorption spectrum of ferrihydrite and maghemite, known to contain ∼35% tetrahedrally coordinated Fe(III). The close agreement between these two spectra was the initial motivation for our statement of “direct observation of tetrahedral Fe(III) in ferrihydrite” but this similarity alone was not sufficient for such claims. Measurement of a variety of samples with Fe(III) in octahedral coordination supported the tetrahedral hypothesis as all of these systems exhibited well separated t2 g and eg orbitals, in contrast to the spectrum of ferrihydrite and maghemite where the separation is much less distinct. It was also clear from comparison with FeO and Fe3O4 that Fe(II) was not present in the ferrihydrite in any significant quantity. Finally, the similarity between the residuals generated by subtracting the hematite spectrum from that of © 2012 American Chemical Society
Published: September 11, 2012 11473
dx.doi.org/10.1021/es303399u | Environ. Sci. Technol. 2012, 46, 11473−11474
Environmental Science & Technology
Correspondence/Rebuttal
combination fitting in our paper did not employ any free parameters except the spectral weights, and since all Fe in our samples was coordinated to O or OH, the ligand effects demonstrated by Hocking et al. are not applicable to our study.
Derek Peak*,† Tom Z. Regier‡ †
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Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon SK S7N 5A8, Canada ‡ Canadian Light Source, Inc., Saskatoon SK, Canada
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; phone: 1 (306) 966-6806. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Peak, D. P.; Regier, T. Z. Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite. Environ. Sci. Technol. 2012, 46 (6), 3163−3168. (2) de Smit, E.; Swart, I.; Creemer, J. F.; Hoveling, G. H.; Gilles, M. K.; Tyliszczak, T.; Kooyman, P. J.; Zandbergen, H. W.; Morin, C.; Weckhuysen, B. M.; de Groot, F. M. F. Nanoscale chemical imaging of a working catalyst by scanning transmission X-ray microscopy. Nature 2008, 456, 222−225. (3) de Groot, F. M. F.; Kotani, A. Core Level Spectroscopy of Solids; CRC Press, 2008. (4) Eisebitt, S.; Böske, T.; Rubensson, J. T.; Eberhardt, W. Determination of absorption coefficients for concentrated samples by fluorescence detection. Phys. Rev. B 1993, 47, 14103−14109. (5) Gloter, M.; Zbinden, F.; Guyot, F.; Gaill, C.; Colliex, C. TEMEELS study of natural ferrihydrite from geological-biological interactions in hydrothermal systems. Earth Planet. Sci. Lett. 2010, 222 (3−4), 947−957. (6) Pan, Y. H.; Vaughan, G.; Brydson, R.; Bleloch, A.; Gass, M.; Sader, K.; Brown, A. Electron-beam-induced reduction of Fe3+ in iron phosphate dihydrate, ferrihydrite, haemosiderin and ferritin as revealed by electron energy-loss spectroscopy. Ultramicroscopy 2010, 110, 1020−1032. (7) Chan, C. S.; Fakra, S. C.; Emerson, D.; Fleming, E. J.; Edwards, K. J. Lithotrophic iron-oxidizing bacteria produce organic stalks to control mineral growth: implications for biosignature formation. ISME J. 2011, 5, 717−727. (8) Achkar, A. J.; Regier, T. Z.; Wadati, H.; Kim, Y-J; Zhang, H.; Hawthorn, D. G. Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects. Phys. Rev. B 2011, 83, 081106. (9) Peak, D. P.; Regier, T. Z. Response to Comment on “Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite”. Environ. Sci. Technol. 2012, 46 (12), 6885−6887.
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