6938 2H
J. Phys. Chem. B 2004, 108, 6938-6940
MAS NMR Studies of Deuterated Goethite (r-FeOOD) Kathryn E. Cole,†,§ Younkee Paik,†,§ Richard J. Reeder,‡,§ Martin Schoonen,‡,§ and Clare P. Grey*,†,§ Department of Chemistry, Department of Geosciences, and Center for EnVironmental Molecular Science, SUNY at Stony Brook, Stony Brook, New York 11794-2275 ReceiVed: March 30, 2004; In Final Form: April 19, 2004
2 H MAS NMR spectroscopy was applied to study the deuterated form of iron oxyhydroxide goethite (RFeOOD), typically one of the major inorganic components of soil. High-resolution spectra can be obtained above the Ne´el temperature, either by raising the temperature or by cation doping. The results indicate that NMR spectroscopy is a feasible method for characterizing the local internal and surface structures of these classes of materials and challenge the often-held assumption that iron-containing materials are impossible to investigate by using high-resolution MAS NMR methods.
Introduction
2H
Goethite, an iron oxyhydroxide (R-FeOOH), is a major inorganic component in soils and often gives soil its characteristic red-brown color. It also possesses a strong uptake capacity for toxic metals1,2 and (hazardous) oxyanions such as AsO43- and represents a major sorbent for radioactive ions in nuclear waste sludges.3-6 Predicting the fate of pollutants in the environment (e.g., following release from nuclear waste repositories) and the design of effective remediation strategies, to clean up natural environments and industrial and nuclear wastes, requires an understanding of the mechanisms for ion sequestration on goethite and related materials.7 Experimental studies on both synthetic and natural materials have shown that the surface properties of goethite, especially the reactivity of individual O-H groups, are important in determining mobility, reversibility, and fate of sorbed species.1-8 Semiempirical models have also been proposed to understand and predict surface properties and uptake behavior of goethite and other minerals, by utilizing molecular level information such as charge of the bulk metal ions, bond connectivity and distances between metal and oxygen atoms, and the strength of hydrogen bonding in the structure.9,10 In principle, solid-state NMR spectroscopy represents an ideal method for studying these sorption processes, either by analyzing the spectra of the sorbed ions directly or by investigating the role that the -OH groups on the surface and in the channels of goethite and other crystalline and amorphous iron oxides and hydroxides play in controlling ion uptake. However, these systems are generally considered unsuitable for NMR experiments due to their paramagnetic and/or magnetic properties. Nonetheless, we have used 2H MAS NMR spectroscopy to study the deuterium local environments in deuterated, structurally similar, highly defective manganese oxides and hydroxides,11 and we have shown that the approach yields detailed information concerning the types of surface and bulk OH groups, metal oxidation state, and the nature of the hydrogen bonding. Here we show that solid-state
Figure 1. Structure of goethite.
* Corresponding author. Fax: 1-631-632-5731. E-mail: cgrey@notes. cc.sunysb.edu. † Department of Chemistry, SUNY at Stony Brook. ‡ Department of Geosciences, SUNY at Stony Brook. § Center for Environmental Molecular Science, SUNY at Stony Brook.
MAS NMR spectroscopy can similarly be applied to study deuterium local environments in deuterated goethite. We demonstrate that by controlling the magnetic properties of the goethite, either by doping or synthesizing nanoparticles, NMR spectra may be obtained at temperatures relevant to the environmental processes.
Goethite (Figure 1) has orthorhombic symmetry (space group Pbnm) and is isostructural with groutite (R-MnOOH) and diaspore (R-AlOOH).12 The Fe3+ ions are antiferromagnetically ordered at room temperature. The magnetic structure has been studied by various techniques including Mo¨ssbauer spectroscopy,12,13 neutron diffraction,14 susceptibility measurements,15,16 and positron annihilation lifetime spectroscopy.17 The Ne´el temperature (TN) for the antiferro- to paramagnetic transition is as high as 130 °C for pure materials12,16 but drops below room temperature as the level of cation doping (by cations such as Al3+)18 or iron vacancies increases and/or crystallite size decreases.13,19,17 A decrease in TN is also observed for materials with higher concentration of iron vacancies. Experimental Section The chemical composition, structure, and physical properties of the material observed in nature can be readily reproduced in the laboratory by controlling the synthesis conditions.20-23 Deuterated goethite samples were synthesized in this study according to the method of Schwertmann et al.20 First, 10 mL of 1.0 M iron(III) nitrate was mixed with 90 mL of 0.7 M KOH
10.1021/jp0486090 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/12/2004
Letters
Figure 2. 2H variable temperature (VT) MAS NMR spectra of (a-e) pure goethite, (f) nanoparticle goethite, and (g) Al-doped goethite (Al/ Fe ≈ 0.1). The spectra were acquired at the indicated temperature with a CMX-200 spectrometer at a Larmor frequency of 30.72 MHz with a 4 mm Chemagnetics probe and MAS frequencies of 17 kHz.
in D2O. The samples were stored in plastic bottles at either 25 °C for 18 days or 60 or 70 °C for 10 days. Protonated goethite was also synthesized at 70 °C, in H2O for comparison. The solutions were filtered and washed with D2O (or H2O) while monitoring the pH of the effluent until it decreased below 8. The filtrate was then freeze-dried. Goethite nanoparticles and the Al-doped samples were prepared by oxidizing the Al3+doped iron(II) oxyhydroxide (“green rust” ) formed by hydrolysis of FeCl2 and AlCl3, as outlined in ref 24. Results and Discussion The powder X-ray diffraction data of the samples synthesized at 60 and 70 °C were consistent with those of goethite, and no noticeable differences were observed between the protonated and deuterated forms. The sample synthesized at 25 °C remained amorphous, possibly due to the sluggish reaction in D2O. The FTIR spectrum of the protonated goethite sample (70 °C) shows broad OH stretching vibrations at 3000-3400 cm-1 which are replaced by two sharp peaks at 2360 and 2380 cm-1 on deuteration, consistent with the literature.25 Thermogravimetric analysis (TGA) showed that the surface water desorbs by 200 °C in both samples, and accounts for ∼1.3 and ∼1.8% of the protonated and deuterated sample masses, respectively. The major weight loss commences above 225 °C and an ∼11% mass loss is seen for the deuterated (∼12% for the protonated) sample by 500 °C. This is associated with the structural transformation from goethite to hematite (R-Fe2O3).26 The XRD, FTIR, and TGA plots of the deuterated and protonated samples can be found in the Supporting Information. The samples of deuterated goethite synthesized at 60 and 70 °C were vacuum-dried at 100 °C for the 2H NMR experiments. Both samples were analyzed at room temperature using 2H MAS NMR, and no signal was observed. However, a dramatic improvement in the resolution was observed on heating the sample to 125 °C (Figure 2), which is ascribed to the antiferroto paramagnetic phase transition of the sample that occurs at this temperature (TN approximately 120-130 °C).12,17 Below
J. Phys. Chem. B, Vol. 108, No. 22, 2004 6939 TN, the hyperfine magnetic field at the iron nuclei of pure goethite is as high as 38 T,12 which is greater than the strength of the static field of the magnet used for the NMR experiments (4.7 T). The 2H spins are clearly broadened by these large local magnetic fields below TN. The line width decreases from 125 to 150 °C, as residual antiferromagnetic correlations decrease and the electronic spin relaxation time, T1e decreases, due to increased thermal motion. A large, isotropic shift of approximately 90 ppm is observed above TN. This is ascribed to the Fermi contact (hyperfine) shift mechanism arising from the transfer of unpaired electron density from unpaired electrons on the Fe3+ ions (d5 S ) 5/2) to the 1s orbitals on the 2H atoms.11,27 The isotropic shift remains constant with temperature (within experimental error); this is ascribed to the strong residual antiferromagnetic (AF) couplings between the iron spins.12,17 The shift is also smaller than that observed for the isostructural compound groutite (MnOOD; δiso ) 410 ppm), although this compound contains d3 ions, again consistent with residual AF couplings in goethite. The sideband pattern resembles a Pake doublet, indicating that the deuterium ions are rigidly bound to the oxygen sites. As the temperature is increased above 200 °C, the signal intensity decreases due to the decomposition reaction of goethite (R-FeOOD) to form hematite (R-Fe2O3) and water (D2O),26 as evidenced by a sharp single resonance at 5 ppm from the released water molecules. In startling contrast to the more standard compounds, high-resolution 2H MAS NMR spectra could be obtained at room temperature for the goethite nanoparticles and the Al3+-doped samples (Figure 2f-g). This is consistent with a decrease in TN to below room-temperature either on doping or on decreasing the particle size.13 Extensive 2H NMR studies on organic and inorganic solids have shown that the size of 2H nuclear quadrupolar coupling constant (QCC) correlates with the (O-D- - - -O) hydrogen bonding distance.28-30 This correlation can be expressed via the following equation, where “R” represents the O-O distance in Å: QCC (kHz) ) 442.7-4882/R3.29 A 2H QCC of 208 ((5) kHz was obtained by simulation of the 2H MAS NMR spectra by using the approach outlined in refs 31 and 32 and by assuming that the electric field gradient has a cylindrical symmetry. (Much smaller dipolar coupling constants were estimated from the simulations and ranged from 18 to 37 kHz depending on the angle between quadrupolar and dipolar (paramagnetic) tensors. An O-D- - - -O hydrogen bonding distance of 2.75 ((0.03) Å is calculated from the estimated QCC, which is consistent with the O-O distances (R ) 2.7482.752 Å) reported for natural goethites,20,33 and samples synthesized at high temperatures (>50 °C). The results suggest an asymmetric double-well potential for deuterium in the goethite structure. Similar 2H MAS NMR spectra are obtained for the protonated material, after soaking the sample in D2O for 72 h and drying, indicating that the local environments are similar in the samples prepared in H2O and D2O. In summary, we have obtained high-resolution 2H MAS NMR spectra from deuterated goethite, above the Ne´el transition. The distance between the hydrogen-bonded oxygen atoms can be extracted from the 2H QCC and is in good agreement with that obtained from the crystal structure. The results demonstrate the feasibility of applying NMR methods to characterize the local internal and surface structures of these classes of materials. The application of similar approaches to the study of related materials can be readily envisaged. Acknowledgment. This worked was funded by the NSFfunded Center for Environmental Molecular Science (CHE0221934). We thank X. Z. Guo (Materials Science Department)
6940 J. Phys. Chem. B, Vol. 108, No. 22, 2004 for assistance with the TGA. Helpful discussions with Dr. E. Elzinga (Geosciences Department) are gratefully acknowledged. Supporting Information Available: XRD, FTIR, and TGA and DTG plots of both the proton and deuterium forms of goethite. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bruemmer, G. W.; Gerth, J.; Tiller, K. G. J. Soil Sci. 1988, 39, 37. (2) Coughlin, B. R.; Stone, A. T. EnViron. Sci. Technol. 1995, 29, 2445. (3) Sparks, D. L.; Scheidegger, A. M.; Strawn, D. G.; Scheckel, K. G. In Mineral-Water Interfacial Reactions; Sparks, D. L., Grundi, T. J., Eds.; American Chemical Society: Washington, DC, 1998; p 108. (4) Peak, D.; Ford, R. G.; Sparks, D. L. J. Colloid Interface Sci. 1999, 218, 289. (5) Peak, D.; Sparks, D. L. EnViron. Sci. Technol. 2002, 36, 1460. (6) Dixit, S.; Hering, J. G. EnViron. Sci. Technol. 2003, 37, 4182. (7) Sparks, D. L.; Grundi, T. J. Mineral-Water Interfacial Reactions: Kinetics and Mechanisms; American Chemical Society: Washington, DC, 1998. (8) Elzinga, E. J.; Peak, D.; Sparks, D. L. Geochim. Cosmochim. Acta 2001, 65, 2219. (9) Venema, P.; Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 183, 515. (10) Ostergren, J. D.; Brown, G. E., Jr.; Parks, G. A.; Persson, P. J. Colloid Interface Sci. 2000, 225, 483. (11) Paik, Y.; Osegovic, J. P.; Wang, F.; Bowden, W.; Grey, C. P. J. Am. Chem. Soc. 2001, 123, 9367.
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