Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite”
T
he comments of Manceau on our recent publication 1 correctly focus on some of the challenges involved in applications of spectroscopy to determine the structure of ferrihydrite. His two major criticisms are that perhaps our synthesis methods influenced the structure of the 2-line ferrihydrite we measured, and that an alternative explanation based on theoretical calculations should also be considered. We are happy to reply to both of these criticisms. First of all, we agree that the study of ferrihydrite and iron oxide minerals in general is complicated by the complexity and diversity of phases that are produced in synthesis from forced hydrolysis. Our decision to use ferric chloride rather than ferric nitrate was based primarily upon the requirements of applying the IPFY technique 2 rather than mineralogical considerations. Since IPFY for iron oxide samples relies upon the intensity of the O Kα emission from the water to obtain self-absorption-free bulk Fe L-edge spectra, we were concerned that the presence of O in residual or adsorbed nitrate might complicate the measurement. However, we have also collected Fe L-edge spectra of ferrihydrite produced from ferric nitrate (Figure 1), and there were essentially no differences from the sample produced from ferric chloride published in our manuscript. For this reason, we conclude that our spectra observations at the Fe L-edge are not specific to the formation of ferrihydrite in the presence of Cl ions. One of Dr. Manceau’s primary arguments is that the observation of the XRD of 2-line ferrihydrite does not preclude the presence of nanocrystalline β-FeOOH or other nanoscale products. Indeed, the presence of nanophases in hydrolysis products is not limited only to ferric chloride systems; researchers have also shown that 6-line ferrihydrite readily crystallizes to nanometer-sized goethite particles which then condense to goethite needles.3 To minimize the possibility of the formation of other nanominerals in our samples, we have collected ferrihydrite spectra immediately after hydrolysis by holding the titrated precipitate for only 30 min at pH 7−7.5. The samples were then drop coated onto a substrate, and measured immediately after drying. The results of this hydrolysis, using both Fe(NO3)3 and FeCl3, are shown in Figure 1. For comparison, the spectra of both crystalline αFeOOH and β-FeOOH are also shown in Figure 1. We concluded that since there is no significant difference between our drop-coated ferrihydrite precipitates and the powder sample previously measured 1 that the phase present in all cases is a 2-line ferrihydrite. Another of Manceau’s main points is that the difference between the absorption spectrum of ferrihydrite and purely octahedral Fe oxides (e.g., goethite and akaganeite in Figure 1) can be readily explained by an increase in bond disorder. Bond distance variation produces a variety of different distorted FeO6 octahedra within the β-FeOOH structure compared to αFeOOH. His hypothesis is that this disorder would induce broadening of the t2 g and eg* peaks at the L3 edge, thus blurring the splitting. As Figure 1 demonstrates, this broad© 2012 American Chemical Society
ening does not, for FeOOH minerals, shift the energy position of the peaks, and it would also not produce a spectral shape that is observed in our measurements of ferrihydrite. Instead, the peak positions of ferrihydrite are similar to those of goethite, with the exception of a broad asymmetric increase at 708.5− 709.5 eV. As shown in the Supporting Information for the article in question, this increased spectral intensity occurs at the position expected for tetrahedral Fe3+ based both upon literature5,6 and our measurements of maghemite,1 thus leading to our claim that tetrahedral Fe is present in the structure. Whether or not our observations could also be explained with octahedral Fe becomes the basis of Manceau’s modeling results. In the next portion of his comment, Dr. Manceau employs multiplet calculations in an attempt to demonstrate that our observations could arise from purely octahedral Fe. With respect to the calculations performed by Manceau, we certainly agree with the general trend that when ligand field splitting of an FeO6 octahedron is decreased below 1.0, it will produce absorption features consistent with tetrahedral Fe. This is because the calculation will in fact force an electronic structure that may be more appropriate for a tetragonal (VFe) or tetrahedral Fe as ΔE is decreased. The main questions that then arise are what values of ΔE are chemically reasonable, how much broadening to the calculations should be added, and how to assign energy positions to the calculations. Comparison of the calculation from Dr. Manceau’s Comment at ΔE = 1.4 to those made by 6 for hematite using the same code, and the experimental measurement of hematite,1 reveal considerably more broadening and different t2 g/eg* intensities in Dr. Manceau’s comment than in other studies. We do not know the source of this discrepancy but it may complicate interpretations built on the modeling results . It should also be noted that his calculated 20% tetrahedral +80% octahedral spectrum does not match very well with our observed ferrihydrite data in Figure 1 or in 1 and does not produce the experimental asymmetric broadening. We recognize that in both Manceau’s calculations and our simulations, there are assumptions that have to be made that can influence the outcome. These include which parameters are allowed to vary in the calculations, and the need to assign an energy scale to both our simulations and his calculation output. We approached this problem in our model by subtracting a hematite spectrum from maghemite and fixing the energy of the tetrahedral component to the peak maximum in that residual spectrum. The quantity of tetrahedral Fe predicted in our simulation is thus strongly correlated to this assigned energy and will vary if the tetrahedral peak position in ferrihydrite is different than in maghemite. We felt this was a better approach than allowing it to be a floating variable, but we concede that the uncertainty in our estimation of the amount of tetrahedral Fe present may be somewhat large. Nonetheless, we maintain Published: June 5, 2012 6885
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Correspondence/Rebuttal
Figure 1. (left) Comparison of Fe-L3 edge of ferrihydrite prepared from either FeCl3 or Fe(NO3)3 to goethite and akaganeite FeOOH samples. (right) Calculated hematite Fe L3-edge from Manceau’s comment compared to another calculation using the same code,6 calculations from another code 5 and an experimentally measured hematite TEY spectrum.
and other physical processes in addition to the diversity of chemical variables which affect its structure. As a final comment, we respect the passion and expertise that not only Dr. Manceau but also many other active researchers who are proponents of either the Drits7 or Michel8 ferrihydrite structural models have brought to this important scientific debate. We attempted to report our observations using a complementary technique without overinterpretation, and we believe that soft X-ray measurements using energy resolved fluorescence yield techniques at the Fe L-edge may continue to provide unique insight to this discussion.
that in both our drop-coated and powder ferrihydrite samples the presence of tetrahedral Fe is the simplest and most consistent explanation for the observed spectra. The fact that we see little difference in our ferrihydrite prepared from Cl− or NO3− salts (Figure 1) and that we observe more tetrahedral Fe with L-Edge XANES measurements 1 than has typically been proposed based upon hard Xray analysis8,9 may provide some additional insight into this system. Is it possible that sample dehydration prior to analysis, whether by freeze-drying, drop-coating, or cryogenic preparation, might remove important structural water and produce the observed tetrahedral iron? For nanoparticles, this effect would be much more pronounced than for traditional minerals due to much of the total Fe being present on the surface. Presumably, the vacuum requirements of the soft X-ray beamline would enhance H2O loss on samples prior to analysis, which would be consistent with more tetrahedral Fe in our results. Since most researchers prepare their ferrihydrite via forced hydrolysis, particle size may be quite variable which would affect the amount of under-coordinated Fe sites. This could also produce not only octahedral and tetrahedral, but also potentially 5 coordinate Fe and would further complicate data analysis. Dr. Manceau discusses a related phenomenon when he mentions that Fe−O bond distances do not respond to thermal transformation as one would expect when ferrihydrite converts into hematite. However, it has also been shown that goethite and lepidocrocite (purely octahedral systems) can produce maghemite upon dehydration and heating and acicular hematite can readily form via heating goethite or other needle-shaped ferric oxyhydroxides.10 It is perhaps possible that the mineralogy of ferrihydrite might depend upon hydration state
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
AUTHOR INFORMATION
Corresponding Author
*Phone: 1 (306) 966-6806; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Peak, D.; Regier, T. Z. Direct observation of tetrahedrally coordinated Fe(III) in ferrihydrite. Environ. Sci. Technol. 2012, 46, 3163−3168. (2) 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.
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(3) Yuwono, V. M.; Burrows, N. D.; Soltis, J. A.; Penn, R. L. Oriented aggregation: Formation and transformation of mesocrystal intermediates revealed. J. Am. Chem. Soc. 2010, 132, 2163−5. (4) Regier, T. Z.; Krochak, J.; Sham, T. K.; Hu, Y. F.; Thompson, J.; Blyth, R. I. R. Performance and capabilities of the Canadian Dragon: The SGM beamline at the Canadian Light Source. Nucl. Instrum. Methods A 2007, 582, 93−95. (5) Crocombette, J. P.; Pollak, M.; Jollet, F.; Thromat, N.; Gautiersoyer, M. X-ray-absorption spectroscopy at the Fe L(2,3) threshold in iron-oxides. Phys. Rev. B 1995, 52, 3143−3150. (6) de Groot, F. M. F.; Glatzel, P.; Bergmann, U.; van Aken, P. A.; Barrea, R. A.; Klemme, S.; Havecker, M.; Knop-Gericke, A.; Heijboer, W. M.; Weckhuysen, B. M. 1s2p resonant inelastic X-ray scattering of iron oxides. J. Phys. Chem. B 2005, 109, 20751−20762. (7) Drits, V. A.; Sakharov, B. A.; Salyn, A. L.; Manceau, A. Structural model for ferrihydrite. Clay Miner 1993, 28, 185−207. (8) Michel, F. M.; Barron, V.; Torrent, J.; Morales, M. P.; Serna, C. J.; Boily, J.-F.; Liu, Q.; Ambrosini, A.; Cismasu, A. C.; Brown, G. E., Jr. Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 2787−2792. (9) Maillot, F.; Morin, G.; Wang, Y.; Bonnin, D.; Ildefonse, P.; Chaneac, C.; Calas, G. New insight into the structure of nanocrystalline ferrihydrite: EXAFS evidence for tetrahedrally coordinated iron(III). Geochim. Cosmochim. Acta 2011, 75, 2708−2720. (10) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrences, And Uses, 2nd ed.; Wiley-VCH, 2003.
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