Environ. Sci. Technol. 2008, 42, 7891–7897
Characterization of Ferrihydrite-Soil Organic Matter Coprecipitates by X-ray Diffraction and Mo1ssbauer Spectroscopy K A R I N E U S T E R H U E S , * ,† FRIEDRICH E. WAGNER,‡ ¨ USLER,§ WERNER HA MARIANNE HANZLIK,| HEIKE KNICKER,§ KAI U. TOTSCHE,† ¨ GEL-KNABNER,§ AND INGRID KO UDO SCHWERTMANN§ Institut fu ¨ r Geowissenschaften, Friedrich-Schiller-Universita¨t, 07749 Jena, Germany, Physik Department, Technische Universita¨t Mu ¨ nchen, 85747 Garching, Germany, Lehrstuhl fu ¨ r Bodenkunde, Technische Universita¨t Mu ¨ nchen, 85350 Freising, Germany, and Department Chemie, Technische Universita¨t Mu ¨ nchen, 85748 Garching, Germany
Received March 28, 2008. Revised manuscript received August 15, 2008. Accepted August 18, 2008.
In soils and sediments ferrihydrite often precipitates from solutions containing dissolved organic matter, which affects its crystallinity. To simulate this process we prepared a series of 2-line ferrihydrite-organic matter coprecipitates using water extractable organic matter (OM) from a forest topsoil. The products were characterized by X-ray diffraction, Mo¨ssbauer spectroscopy, N2-gas adsorption and transmission electron microscopy. With increasing C/Fe ratios of the initial solution the d-spacings of the two major XRD peaks increased, while peak shoulders at 0.22 and 0.16 nm weakened. The asymmetry of the 0.26 nm peak decreased and disappeared at a C/Fe ratio of 0.78. The quadrupole splitting of the Mo¨ssbauer spectra at 300 K increased from 0.78 to 0.90 mm s-1, the mean magnetic hyperfine field at 4.2 K dropped from 49.5 to 46.0 T, and the superparamagnetic collapse of the magnetic hyperfine splitting was shifted toward lower temperatures. These data reflect a strong interference of OM with crystal growth leading to smaller ferrihydrite crystals, increased lattice spacings, and more distorted Fe(O,OH)6 octahedra. Even small amounts of OM significantlychangeparticlesizeandstructuralorderofferrihydrite. Crystallinity and reactivity of natural ferrihydrites will therefore often differ from their synthetic counterparts, formed in the absence of OM.
Introduction Ferrihydrite (Fh) is a poorly crystalline Fe oxyhydroxide ubiquitously occurring in soils, sediments, mine wastes, and other aqueous environments (1-3). The combination of a large specific surface area (up to 700 m2 g-1) and a high amount of pH-dependent functional hydroxyl groups at the * Corresponding author phone: +49-3641-948642; fax: +49-3641948622; e-mail:
[email protected]. † Friedrich-Schiller-Universita¨t. ‡ Physik Department, Technische Universita¨t Mu ¨ nchen. § Lehrstuhl fu ¨ r Bodenkunde, Technische Universita¨t Mu ¨ nchen. | Department Chemie, Technische Universita¨t Mu ¨ nchen. 10.1021/es800881w CCC: $40.75
Published on Web 10/08/2008
2008 American Chemical Society
surface makes ferrihydrite an efficient sorbent for ions and organic molecules (4). In consequence, ferrihydriteseven if present at a low concentrationsmay determine the mobility and availability of many nutrients and pollutants. In soils, ferrihydrite adsorbs high amounts of soil organic matter. So called “sorptive protection” by soil minerals like ferrihydrite is assumed to stabilize OM from biological degradation and to represent a major mechanism of carbon sequestration (5, 6). Conversely, association of ferrihydrite with dissolved OM may retard or inhibit the transformation of ferrihydrite into more crystalline Fe oxides. The structure of ferrihydrite is still under discussion (1, 7, 8). Identification and characterization is hampered by its small particle size and poor crystallinity. As in all Fe oxides and hydroxides, the characteristic structural unit is the Fe (O, OH)6 octahedron, which forms (short) edge-linked di- or trioctahedral chains. These are cross-linked at octahedral corners to neighboring chains. The published structural models differ in the proposed stacking order of the anionic layers, the distribution of Fe on the octahedral sites, the presence of Fe in tetrahedral coordination, or the existence of face sharing octahedra. Several models describe ferrihydrite as consisting of microdomains of different crystal structures and varying crystal order (9, 7, 10), whereas others emphasize that its surface structure is different from that of the bulk (11, 12). A specific surface structure is a typical property of nanoparticles, which may become manifested in contracted or expanded surface layers, a different surface composition, or an enhanced disorder at the particle surface (13). This may strongly influence structural analyses because of the large surface area/volume ratio of small particles. For example, Zhao et al. (11) assigned as much as 35% of the Fe atoms in ferrihydrite to the surface. Likewise, chemisorption of Si and borate by ferrihydrite was found to significantly alter its XRD pattern and Mo¨ssbauer spectrum (14, 15). Especially in soils, ferrihydrite often forms in the presence of dissolved OM. This involves processes such as adsorption, coprecipitation, flocculation/coagulation, and poisoning of crystal growth (16, 17). Schwertmann et al. (18) found that coprecipitation of ferrihydrite with water extractable soil organic matter lowered its crystallinity and decreased the magnetic ordering temperature as well as the magnetic hyperfine field. In a similar approach, Mikutta et al. (19) precipitated ferrihydrite in the presence of synthetic acid polysaccharides to investigate the influence of microbial exudates on the mineral properties. They observed a stronger aggregation of ferrihydrite particles due to the polymers, a slightly decreasing magnetic hyperfine field, but no significant change of the crystal structure (XRD) or the local coordination of Fe (EXAFS). This study aims at a better understanding of the formation of Fe(III) oxides under natural conditions, specifically in naturally occurring solutions with dissolved humic substances. We therefore synthesized a series of ferrihydrites using solutions with increasing amounts of OM extracted from soil. Assuming that the OM is normally soluble, but carried down by the precipitating ferrihydrite due to sorption or occlusion, we address the products obtained at various C/Fe ratios as “coprecipitates”. These coprecipitates were characterized by X-ray diffraction, Mo¨ssbauer spectroscopy, and transmission electron microscopy. Our main objectives were to quantify how coprecipitation affects the loading capacity of ferrihydrite for OM and how the occupation of growth sites changes the ferrihydrite structure. Such investigations are a first step to relate the properties of natural VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Synthesis Conditions: C/Fe ratio, C Concentration, Fe Concentration, and pH of the Initial Solution; And Properties of the Coprecipitates: Calculated C/Fe ratio, C Concentration, Specific Surface Area (SSA), Average Quadrupole Splitting at 300 K, Average Hyperfine Field at 4.2 K, and the Superparamagnetic Blocking Temperaturea
C/Fe initial solution
C initial solution (mg l-1)
Fe initial solution (mg l-1)
0.00 0.09 0.28 0.57 0.78 1.17 1.48 2.28
0 188 120 317 353 307 296 568
436 2038 430 554 450 261 200 249
a
nd is not detected.
b
pH initial solution
C content coprecipitate (mg g-1)
C/Fe coprecipitate (calculated)
SSA (m2 g-1)
2.3 1.9 2.3 2.2 2.1 2.3 2.3 2.2
5 39 79 122 135 159 142 170
0.01 0.08 0.17 0.30 0.35 0.46 0.40 0.54
386 250 13 16 23 17 5
Experimental Section Recovery of Water Extractable Soil Organic Matter. Waterextractable soil organic matter (OM) was extracted from the litter layer of a podzol close to Freising, Germany. Samples were air-dried and passed through a 2 mm sieve to remove coarse plant remains. Aliquots of 150 g of soil and 700 mL of deionized H2O were shaken end-over-end for 16 h at room temperature and then centrifuged for 1 h at 4000 rpm. The supernatant was pressure-filtered through polyvinylidene fluoride (Durapore; 0.45 µm pore width) membranes, concentrated in low temperature rotary evaporators and freezedried. This material is supposed to represent the polysaccharide-rich composition of a typical soil solution. A solid state 13C NMR spectrum (18) suggests a total aromatic C content of less that 6%. The signal for carbonyl-amide C accounts for 7% of the total C and is centered at 175 ppm (typical for esters). A small peak at 17 ppm was assigned to terminal methyl C groups. With 65-70% of the total C the O-alkyl C groups represent the most important functional group of the extracted OM. The freeze-dried OM contains 371 mg kg-1 C and 14 mg kg-1 N. Synthesis of Ferrihydrite. Two-line ferrihydrite was produced by titrating a 0.01 M Fe(NO3)3 solution with 0.1 M KOH to a pH of 7 (20). Organic matter-ferrihydrite coprecipitates were prepared by dissolving the Fe(NO3)3 in a solution of extracted OM and deionized H2O and raising the pH afterward. Products with different OM loadings were produced by varying the C/Fe ratio of the initial solution between 0.1 and 2.3 (Table 1). The precipitates were separated by centrifugation, washed twice with deionized H2O, and freeze-dried. Analytical Methods. Total C content was measured with a CN analyzer (Vario EL, Elementar GmbH) of solids and dried supernatants. Specific surface area (SSA) was measured by N2 gas adsorption with an Autosorb1 instrument (Quantachrome) and calculated according to the BET- equation from 11 data points in the relative pressure range of 0.05-0.3. Prior to the measurements, samples were outgassed for at least 16 h at 343 K in a flow of He to remove adsorbed water from the sample surfaces. For transmission electron microscopy (TEM), ∼2 mg of Fh were ultrasonically dispersed in ∼20 mL of deionized H2O. A drop of this suspension was airdried on a TEM grid and examined with a JEOL JEM 2011, working at an accelerating voltage of 120 kV. Powder X-ray diffraction (XRD) patterns were obtained with a Phillips vertical goniometer (PW 1050) using Co KR radiation at 40 kV and 30 mA, step-scanning from 5 to 90 °2Θ with increments of 0.02 °2Θ and a counting time of 5 s per step. 9
Hyperfine field at 4.2 K (T)
Blocking temperature (K)
0.78 nd 0.83 0.86 0.87 0.87 0.89 0.90
49.5 47.9 48.3 47.7 46.9 46.5 46.5 46.0
84.0 56.0 53.0 47.0 nd nd nd 36.0
b
Smaller than detection limit with 0.1 g sample material, i.e. < 10 m2 g-1.
ferrihydrites to the conditions of their formation and to assess differences between natural and synthetic ferrihydrites.
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Data were profile-fitted with XFIT (21) using a linear background and pseudo-Voigt functions. The asymmetric ferrihydrite peak at ∼0.25 nm (9) was fitted by two pseudoVoigt peaks. The noise of each diffractogram was quantified by the difference of the observed diffractogram and a filtered curve. For filtering we combined a running median with a Savitzky-Golay filter. If the root-mean-square error (rmse) of the fitted minus the observed curve approached the level of noise, no further peak was included. The inclusion of all peaks clearly increased the goodness of fit, as compared to a linear combination fit of any combination of fewer peaks: inclusion of the last accepted peak decreased the rmse by at least a factor of 10 in all cases. Mo¨ssbauer spectra were recorded in transmission geometry using a sinusoidal velocity waveform and a source of 57Co in Rh. For measurements below ambient temperature, a liquid He bath cryostat was used. Temperatures above 4.2 K were reached during the slow warming of the cryostat after the liquid He had run out. These spectra average over regions of typically 2-3 K. The magnetically split spectra were fitted with broadened patterns corresponding to static Gaussian distributions of hyperfine fields. Spectra exhibiting electric quadrupole interactions were fitted with patterns corresponding to Gaussian distributions of quadrupole splittings. In the transition region, where magnetically split patterns and pure quadrupole patterns coexist, superpositions of both distributions were used. Further details of the fitting procedure have been described in ref19.
Results and Discussion Particle Size, Surface Area, and C Concentration. Transmission electron micrographs show that the OM-free reference ferrihydrite forms aggregates composed of particles ∼3-5 nm in diameter (Figure 1a). This is consistent with the specific surface area (SSA) measurement: assuming a density of ∼4 g cm-3 (3), a spherical geometry and no surface roughness, a SSA of 386 m2 g-1 (Table 1) corresponds to a particle diameter of ∼4 nm. In the presence of OM, ferrihydrites form considerably smaller particles (Figure 1b and c). Their SSA, however, does not reflect their small particle size (250 m2 g-1 for Fh at a C/Fe ) 0.09 and 5-23 m2 g-1 for all other samples; Table 1). These small SSAs can be explained by (i) a reduced accessibility for N2 due to the formation of denser aggregates by the associated OM or (ii) by a masking of the mineral surface by OM. According to Chiou et al. (22) natural OM exhibits only very low N2-BET specific surface areas of ca. 1 m2 g-1. An OM coverage of approximately onethird of the oxide surface for the Fh at C/Fe ) 0.09, and an almost complete coverage for the other coprecipitates can therefore account for their low N2-BET specific surface areas.
FIGURE 1. TEM images show aggregates of pure ferrihydrite (a), and of ferrihydrite-OM coprecipitates, which formed at different C/ Fe ratios of the initial solution (b, c). Ratios in parentheses refer to the C/Fec of the coprecipitates.
FIGURE 2. Carbon concentration of coprecipitates vs C/Fe of the initial solution (a), percentage of C, which is not precipitated, but remaining in the supernatant vs C/Fe of the initial solution (b), and correlation between the nonprecipitated amounts of C and Fe (c). Carbon concentrations of coprecipitates increase with increasing C/Fe ratio of the initial solution from 39 to 170 mg g-1 C in the products (Table 1, Figure 2a). The highest achieved loading of 170 mg C per g ferrihydrite corresponds to ca. 460 mg OM per g ferrihydrite. This loading is roughly twice as large as observed in sorption experiments by Tipping (23), but smaller than the maximum loading of 318 mg C per g ferrihydrite reached in sorption experiments with water extractable soil OM by Kaiser et al. (24). One has to bear in mind, however, that different OM sorbtives were used in these experiments and that loadings expressed on a mass basis strongly depend on the SSA of the sorbent. If we assume that ferrihydrites in the coprecipitates have the same surface area as the OM-free control sample (386 m2 g-1), the highest achieved loading corresponds to 0.77 mg C per m2 ferrihydrite (this is an overestimation because of the fact that TEM and XRD results point to smaller particles and therefore higher mineral surface area for the coprecipitates). Compared to maximum loadings from sorption experiments with water extractable soil OM on ferrihydrite (1.1 mg C m-2 ferrihydrite (24);) and estimated sorption maxima on natural pedogenic Fe oxides from acid forest soils (1.2 mg C m-2 Fe oxide (25);) the maximum loading obtained in our coprecipitation experiment is rather low. We conclude that coprecipitation produces OM loadings of the same magnitude as pure sorption processes and do not encapsulate large amounts of OM in aggregates. A mass balance calculation comparing the amount of C and Fe in the initial solution and in the products shows that increasing proportions of both elements are not precipitated, but remain in solution as the C/Fe ratio of the initial solution increases (Figure 2b and c). Amounts of nonprecipitated Fe and C are correlated (Figure 2c) and correspond to an atomic C/Fe ratio of 13.3 in the supernatant. The linear correlation suggests that the formation of a specific soluble Fe-OMcomplex is responsible for the mobilization of the nonprecipitated Fe. However, this result may also be interpreted as an experimental artifact, i.e., an increasing formation of very small Fe oxides with increasing C/Fe, associated with a higher loss of solid material during centrifugation.
Structural Characteristics. The XRD of the OM free reference sample (C/Fe ) 0) shows the usual peaks of 2-line ferrihydrite at ∼0.26 nm and ∼0.15 nm (Figure 3). Their peak shoulders can be explained by smaller peaks at 0.22, 0.20, 0.17, and 0.16 nm, which correspond to d-spacings for ferrihydrites with higher than two-line crystallinity. The asymmetry of the 0.26 nm-peak (9) was quantified by an additional peak at 0.29 nm. With an increasing C/Fe ratio of the initial solution the XRD pattern changes systematically: The small peaks at 0.22, 0.20, 0.17, and 0.16 nm, as well as the one at 0.29 nm weaken until they finally disappear, while the two main peaks broaden and shift toward higher d-spacings (Figure 3 and Figure 4a-c). The decreasing number of peaks and the observed peak broadening indicate a decreasing size of coherent scattering domains or an increase of stacking disorder in the anionic layers. Based on EXAFS, Waychunas et al. (26) suggested that the precipitation process of ferrihydrite starts with the formation of small chains of Fe octahedra, continues by linking these chains via their edges to plate-like dioctahedral and trioctahedral chains, and only then proceeds by cross-linking the chains at their octahedral corners. This scenario is in line with our observations: At high initial C/Fe ratios, frequent reactions between Fe (O, OH)6 octahedra and organic molecules may prevent cross-linking and result in particles with only two XRD peaks, ascribed to (110) and (300). This indicates that only perpendicular to these two hk0 planes enough lattice planes are stacked to produce an XRD peak, suggesting a plate-like shape of the coherent scattering domains. Less interference with organic molecules allows for the development of the additional hkl planes (112) and (115), i.e, the low intensity peaks at 0.22 and 0.16 nm, which might be connected to the crosslinking process. The observed peak shift points to increased spacings for the 110 and 300 planes. Such peak shifts are also found for ferrihydrites doped with borate (15) and Si (14). In both studies, the shifts were considered to reflect changes in the lattice spacings, and the non-Fe metals were assumed to chemisorb on the surface of the ferrihydrite nanoparticles. Increasing d-spacings are also in accordance with Michel et VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. XRD data in relation to the C/Fe ratio of the initial solution: d-spacings (a, b) and full width at half-maximum (fwhm) of the 110 and 300 X-ray peaks (c).
FIGURE 3. X-ray diffractograms of pure ferrihydrite, some coprecipitates with different initial C/Fe ratios (ratios in parentheses refer to the C/Fec of the coprecipitates) and the extracted organic matter. The heavy black line denotes the measured data points. Also shown are fit envelope, background, fitted peaks, and the residuum. al. (8), who observed systematically changing unit cell dimensions with decreasing crystal size. The asymmetry of the 0.26 peak is known for ferrihydrites regardless their degree of crystallinity. It was interpreted as being caused by stacking disorder in the close packed anionic planes by Janney et al. (7) or to represent ultra dispersed hematite in ferrihydrite by Drits et al. (9). However, the most intense hematite peak appears at ∼0.27 nm, whereas we had to include a peak at 0.29 nm to quantify the peak asymmetry. 7894
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The diffractogram of the OM was fitted by a broad peak at ∼0.49 nm, which also appears in coprecipitates with higher C contents (i.e., at C/Fe of 0.78, 1.18, 1.48, and 2.28). Three sharp peaks at 0.24, 0.20, and 0.14 nm in the diffractograms of OM and ferrihydrite at C/Fe ) 2.28 can be assigned to Al and are caused by the sample holder. Magnetic Characteristics by Mo ¨ ssbauer Spectroscopy. All spectra recorded at room temperature show broadened electric quadrupole doublets with isomer shifts (IS) of 0.35-0.36 mm s-1 against R-Fe. This is consistent with values reported in other studies (0.31-0.38 mm s-1; compiled by ref12) and is in the range for trivalent Fe of 0.24-0.54 mm s-1 (27). The isomer shifts fall into the overlap range of tetrahedrally (0.24-0.44 mm s-1) and octahedrally (0.34-0.54 mm s-1) coordinated Fe and do not help to decide whether 4-fold coordinated Fe is present (27, 28). The broadening of the doublets can be attributed to a distribution of electric field gradients. Good fits were obtained with an asymmetrical Gaussian distribution of quadrupole splittings (QS), i.e., with a distribution having a larger variance on the high QS side than on the low QS side. The mean QS increases with increasing C/Fe ratios of the initial solution from 0.77 mm
FIGURE 5. Dependence of the Mo¨ssbauer results for the mean electric quadrupole splitting (QS) at ambient temperature (a) and of the mean magnetic hyperfine field (Bhf) at 4.2 K (b) on the C/Fe ratio of the initial solution. s-1 for the organic free ferrihydrite to 0.90 mm s-1 for the ferrihydrite with a C/Fe ) 2.28 (Figure 5a). Increasing QS values indicate an increasing distortion of the octahedral geometry around the Fe atom (29). All 4.2 K Mo¨ssbauer spectra show a broadened sextet, indicating complete magnetic ordering at this temperature. None of the spectra contain a quadrupole doublet component representing iron that does not exhibit a magnetic splitting, as has been reported by Schwertmann et al. (18). The mean magnetic hyperfine field (Bhf) decreases with increasing C/Fe of the initial solution from 49.5 T of the pure ferrihydrite to 46.0 T for the Fh at C/Fe ) 2.28 (Figure 5b). Such a decrease can be explained by decreasing particle size and crystallinity (30) or by decreasing interparticle interactions (31). Both effects may apply here: decreasing particle size with increasing initial C/Fe ratios is evident from the TEM images (Figure 1), decreasing crystallinity is shown by XRD, and decreasing interparticle interactions are plausible because of the increasing amounts of diamagnetic organic materials between individual ferrihydrite particles. Interestingly, by using polysaccharide-rich natural OM we found much larger changes in QS and Bhf than that described in ref19, where ferrihydrite was synthesized in the presence of various pure polysaccharides. For some samples, the temperature dependence of the Mo¨ssbauer spectra was recorded between 4.2 and 116 K (Figure 6). In this temperature range, the magnetic hyperfine splitting collapses, which is shown exemplary for the coprecipitate obtained at a C/Fe ) 0.57 (Figure 7). The vanishing of the magnetic splitting at temperatures well below the Ne´el temperature TN (TN ∼ 500 K for 6-line Fh (32)) is explained by superparamagnetic relaxation. When the relaxation becomes sufficiently fast, the magnetic effects in the Mo¨ssbauer spectra disappear. The superparamagnetic relaxation rate ω increases with increasing temperature and decreasing particle size according to ω ) ωo exp[-KV ⁄ kT]
(1)
FIGURE 6. Mean hyperfine field (Bhf) and magnetic fraction (Amag) as obtained from Mo¨ssbauer spectra taken at different temperatures. where ωo is a constant depending on the material and perhaps its particle shape, as is the anisotropy constant K, V is the particle volume, k is Boltzmann’s constant, and T is the temperature. In a ferrihydrite powder, particle size and particle shape would have distributions, so that a distribution of relaxation rates has to be expected at any given temperature. The narrower the distribution of properties of the particles, the more abrupt is the transition from the magnetically split sextet to the quadrupole doublet (29). Therefore, the width of the transition is an indicator of the homogeneity of particle sizes, shapes, and perhaps other properties. Schwertmann et al. (18) reported that for 6-line ferrihydrite, the same spectral shapes (and hence relaxation rates) occur at a temperature twice as high as with 2-line ferrihydrite. Neglecting particle shape, this means that the mean particle volumes are about two times larger in 6-line than in 2-line ferrihydrite. The particle diameter would differ by much less, namely by roughly the cubic root of 2, i.e., by a factor of 1.26. Consequently, rather small differences in the mean particle size result in rather drastic differences in the temperature dependence of the Mo¨ssbauer spectra. The magnetically split fraction (Amag) of the Mo¨ssbauer spectra and the mean hyperfine field (Bhf) in this fraction are shown in Figure 6 for all samples, for which the temperature dependence was measured. Whereas according to Schwertmann et al. (18) 6-line ferrihydrite still has a magnetic fraction of 100% at 100 K, the organic-free 2-line ferrihydrite from our coprecipitation series starts to lose its magnetic order above 72 K (Figure 6b), where the Bhf has decreased to ∼20 T. For ferrihydrite precipitated in the presence of OM, both the proportion of the sextet as well as the Bhf decrease even more rapidly. A surprisingly large change occurs between the organic free ferrihydrite and the one with the lowest OM content (Fh at C/Fe ) 0.09), indicating that even small amounts of OM may have a significant effect VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Supporting Information Available Extension of Figure 7 shows five Mo¨ssbauer spectra taken at different temperatures of one coprecipitate (C/Fe ) 0.57 or C/Fec ) 0.30). This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited
FIGURE 7. Mo¨ssbauer spectra of coprecipitates obtained from the solution with an initial C/Fe ratio of 0.57 (C/Fec ) 0.30) taken at different temperatures.
on the crystal size and/or structure. If the observed decrease in the superparamagnetic blocking temperature (Table 1; defined as the temperature at which the magnetic fraction has dropped to 0.5) is only attributed to particle volume (eq 1) the particle size for the coprecipitate obtained at C/Fe ) 2.28 is smaller by a factor of 0.75 than the pure 2-line ferrihydrite. The superparamagnetic behavior, and hence the blocking temperature may, however, also be influenced by interparticle interactions, which will be reduced when OM separates the ferrihydrite particles. Mo¨ssbauer and XRD results show that even small concentrations of water soluble soil OM have a clear impact on structure, size, and magnetic properties of the precipitated ferrihydrite. With increasing C/Fe of the initial solution, that is with an increasing probability of reactions between OM and ferrihydrite or the early polymers of Fe(O,OH)6 octahedra, structural features of ferrihydrite change systematically: organically poisoned ferrihydrite shows larger lattice spacings, fewer crystal planes, smaller particle diameters, and a magnetic hyperfine splitting that disappears at lower temperatures. Natural ferrihydrites must, therefore, be expected to differ in many properties from their synthetic analogues.
Acknowledgments Many thanks to Petra Mu ¨ller and Maria Greiner for assistance in the laboratory, and to Roman Ska´la for helpful XRDdiscussions. The thorough reviews by three anonymous referees and the editorial work by D. Dzombak are gratefully acknowledged. This study was supported by the FriedrichSchiller-Universita¨t Jena and the Technische Universita¨t Mu ¨ nchen. 7896
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