Correspondence/Rebuttal pubs.acs.org/est
Comment on “Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite”
W
e have read with interest the discussion1,2 on recent work by Peak and Regier3 on the interpretation of the Fe L-edge spectrum of ferrihydrite. Having extensively studied Fe L-edge XAS spectra,4−8 we felt it important to point out some of the limitations in applying linear combinations to these types of spectra. In agreement with Manceau2 we do not believe that assignment of the presence or absence of tetrahedral sites in ferrihydrite is possible using linear combinations of L-edge spectra alone. However, our reasons for this conclusion are more general. While the TT-Multiplet code is very useful for interpreting Ledge XAS spectra, it suffers limitations, particularly when analyzing real “unknown data”, such as the sites occupied by Fe in ferrihydrite. Fundamentally, L-edge spectral shape can be considered as a combination of the six factors shown in Figure 1 and described in references 5, 8, and 9. The crystal field effect
data sets, rather than relying on simulations. Figure 2A depicts a series of high spin Fe(III) data of four compounds having a
Figure 2. (A) Experimental data for a series of octahedral Fe compounds (Adapted from ref 5. Copyright 2010 American Chemical Society); inset shows the L3 edge scaled and superposed to an overlay. (B) Spectra of K3[Fe(ox)3] and K3[Fe(cat)3], and the average of the two spectra. (C) Octahedral coordination of Fe(III) in the compounds considered. (D) Comparison of the spectra of ferrihydrite and hematite showing the region where the tetrahedral coordination contribution appears (Adapted from ref 3. Copyright 2012 American Chemical Society).
single octahedral Fe site of similar but slightly different bonding environment (Figure 2C), published elsewhere.5,11 The energies of the spectra are referenced to each other by use of an internal standard.5 While these are molecular compounds and not minerals, they are useful as they enable us to identify spectral effects associated with individual octahedral Fe sites. Several things that have significant implications on linear combination analysis are clear from Figure 2. First, the spectra of K3[Fe(cat)3] and [Fe(pha)3] are offset to lower energy relative to Na[Fe(ida)2] and K3[Fe(ox)3], due to slight differences in effective nuclear charge (point 1, Figure 1). Second, the spectral intensity of K3[Fe(cat)3] and [Fe(pha)3] are lower than those of K3[Fe(ox)3] and [Fe(ida)2]− (point 5, Figure 1). Neither of these differences is “automatically” accounted for by the TT-muliplet code. To examine the combined effect of these factors on linear combination spectral simulations, we have generated a spectrum that is the sum of a 1:1 mix of the K3[Fe(cat)3] and K3[Fe(ox)3] experimental spectra (Figure 2B). The bonding in these compounds is not substantially different from that found in a range of octahedral
Figure 1. Contributions to L-edge spectral shape of a high spin Fe(III) compound.5,8
pointed out by Manceau2 (Figure 1, point 2) is only one potential contribution to the observed shape and intensity and energy shift of an L-edge spectrum. The other effects shown in Figure 1 can also contribute substantially, the most pertinent of these to the spectrum of ferrihydrite is the Zeff of the metal, which can shift the average energy of a spectrum.10,11 The TT-Multiplet program, in general, simulates well the atomic and ligand field effects of an L-edge. However, the effects of Zeff (energy shift) and covalency (total intensity) must be adjusted with consideration of the experimental observations. While these factors can certainly be accounted for,5,8,9 a spectral observation that is considered to be the sum of a number of unknowns presents complications. To demonstrate how these limitations may affect the analysis of data by linear combination, it is useful to examine some real © 2012 American Chemical Society
Published: September 11, 2012 11471
dx.doi.org/10.1021/es303084e | Environ. Sci. Technol. 2012, 46, 11471−11472
Environmental Science & Technology
Correspondence/Rebuttal
(8) Hocking, R. K.; Solomon, E. I., Ligand Field and Molecular Orbital Theories of Transition Metal X-ray Absorption Edge Transitions. In Structure and Bonding Molecular Electronic Structures of Transition Metal Complexes; Mingos, D. M. P., Day, P., Dahl, J. P., Eds.; Springer, 2012; Vol. 142, pp 1−30. (9) deGroot, F. M. F. Multiplet effects in X-ray spectroscopy. Coord. Chem. Rev. 2005, 249, 31−63. (10) George, S. J.; Lowery, M. D.; Solomon, E. I.; Cramer, S. P. Copper L-edge spectral studies: A direct experimental probe of the ground state covalency in the blue copper site in plastocyanin. J. Am. Chem. Soc. 1993, 115 (7), 2968−2969. (11) Wasinger, E. C.; deGroot, F. M. F.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. L-edge X-ray Absorption Spectroscopy of NonHeme Iron Sites: Experimental Determination of Differential Orbital Covalency. J. Am. Chem. Soc. 2003, 125, 12894−12906. (12) Malliot, F.; Morin, G.; Wang, Y.; Bonnin, D.; IIdefonse, P.; Chaneac, C.; Calas, G. New insight into the structure of nanocrystalline ferrihydrite: EXAFS evidence for tetrahedrally coordinated iron (III). Geochem. Cosmochim. Acta 2011, 75, 2708−2720. (13) Hiemstra, T.; van Riemsdijk, W. H. Geochim. Acta 2009, 73, 4423−4436. (14) Manceau, A. Critical evaluation of the revised akdalaite model for ferrihydrite. Am. Mineral. 2011, 96, 521−533. (15) Michel, F. M.; Ehm, L.; Antao, S. M.; Lee, P. L.; Chupas, P. J.; Lui, G.; Strongin, D. R.; Schoonen, M. A.A.; Phillips, B. L.; Parise, J. B. The structure of ferrhydrite a nano-crystalline material. Science 2007, 316, 1726−1729. (16) Michel, F. M.; Barronc, V.; Torrent, J.; Moralesd, M. P.; Sernad, C. J.; Boilye, J. F.; Liuf, Q.; Ambrosing, A.; Cismasua, A. C.; Brown, G. E., Jr. Ordered ferrmagnetic form of ferrihydrite reveals links among structure, composition and mangetism. Proc. Natl. Acad. Sci. 2010, 107 (107), 2787−2792. (17) Manceau, A.; Gates, W. P. Surface Structural Model for Ferrihydrite. Clay Miner. 1997, 45 (3), 448. (18) Ford, R. G.; Bertsch, P. M. Distinguishing between surface and bulk dehydration-dehydroxylation reactions in synthetic geothites by high-resolutions thermogravimetric analysis. Clays Clay Miner. 1999, 47, 329−337. (19) Xu, W.; Hausner, D. B.; Harrington, R.; Lee, P. L.; Strongin, D. R.; Parise, J. B. Structural water in ferrhydrite and constraints this provides on possible structure models. Am. Mineral. 2011, 96, 513− 520. (20) Ghose, S. K.; Waychunas, G. A.; Trainor, T. P.; Eng, P. J. Hydrated goethite (α-FeOOH) (1 0 0) interface structure: Ordered water and surface functional groups. Geochim. Cosmochim. Acta 2010, 74, 1943−1953.
iron (FeO6) centers in minerals; the 50% mix was chosen to simplify visualization of spectral effects. The energy offset of the original spectra with respect to one another has a substantial effect in the spectral region used to interpret tetrahedral iron (Figure 2B, circle) as is also illustrated in the comparison spectra of ferrihydrite and hematite (Figure 2D), the latter having no tetrahedral iron. In an unknown environment such as ferrihydrite,12−20 these effects will combine to produce a variety of spectral modifications, a simple example of which we have seen with our summation of only two spectra. Thus we demonstrate a serious limitation to interpreting L-edge spectra obtained by applying linear combination analysis to the Fe L-edge spectra when the components which are being used are themselves poorly understood.
Rosalie K. Hocking*,† Will P. Gates‡ John D. Cashion§ †
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School of Pharmacy and Molecular Science, James Cook University, Townsville, QLD, 4811, Australia, and School of Chemistry, Monash University, Melbourne, Vic 3800, Australia ‡ Department of Civil Engineering, Monash University, Melbourne, Vic 3800, Australia, and SmecTech Research Consulting § School of Physics, Monash University, Melbourne, Vic 3800, Australia
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; phone: +61 3 9905 4593; fax: +61 3 9905 4597. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Peak, D.; Regier, T. Z. Response to Comment on “Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite”. Environ. Sci. Technol. 2012, 46, 6885−6887. (2) Manceau, A. Comment on “Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite”. Environ. Sci. Technol. 2012, 46, 6882−6884. (3) Peak, D.; Regier, T. Direct Observation of Tetrahedrally Coordinated Fe(III) in Ferrihydrite. Environ. Sci. Technol. 2012, 46, 3163−3168. (4) Hocking, R. K.; Debeer George, S.; Gross, Z.; Walker, F. A.; Hodgson, K. O.; Hedman, B.; Solomon, E. I. Fe L- and K-edge XAS of Low-Spin Ferric Corrole: Bonding and Reactivity Relative to Low-Spin Ferric Porphyrin. Inorg. Chem. 2009, 48 (4), 1678−1688. (5) Hocking, R. K.; Debeer George, S.; Raymond, K. N.; Hodgson, K. O.; Hedman, B.; Solomon, E. I. Fe L-edge XAS determination of the Differential Orbital Covalency of Siderophore Model Compounds: Electronic Structure Contributions to High Stability Constants. J. Am. Chem. Soc. 2010, 132 (11), 4006−4015. (6) Hocking, R. K.; Wasinger, E. C.; Yan, Y.; DeGroot, F. M. F.; Walker, F. A.; Hodgson, K. O.; Hedman, B.; Solomon, E. I. Fe L-edge XAS of Low Spin Heme Relative to Non-heme Fe Complexes: Delocalization of Fe d electrons into the Porphyrin Ligand. J. Am. Chem. Soc. 2007, 129 (1), 113−125. (7) Hocking, R. K.; Wasinger, E. C.; deGroot, F. M. F.; Hodgson, K. O.; Hedman, B.; Solomon, E. I. Fe L-edge XAS studies of K4[Fe(CN)6] and K3[Fe(CN)6]: A direct probe of back-bonding. J. Am. Chem. Soc. 2006, 128 (32), 10442−10451. 11472
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