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Properties of Goethites Prepared under Acidic and Basic Conditions in the Presence of Silicate
Journal of Colloid and Interface Science 216, 106 –115 (1999) Article ID jcis.1999.6285, available online at http://www.idealibrary.com on
Properties of Goethites Prepared under Acidic and Basic Conditions in the Presence of Silicate Susan Glasauer,* ,1 Josef Friedl,† and Udo Schwertmann† *Department of Microbiology, College of Biological Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada; and †Institute for Soil Science, Technical University, Munich, D-85350 Freising, Germany Received December 31, 1998; accepted April 14, 1999
Most studies on Si–Fe interaction during crystallization have considered alkaline systems, although acid conditions are most common in natural systems. Furthermore, the results of these studies are conflicting, especially regarding the effect of Si on the crystallinity of the products. Si has been found to increase or decrease product crystallinity (e.g., 6 –10), depending on the pH during synthesis, the iron salt used, the synthesis temperature, and even the method of analysis. It has been shown that increasing concentrations of Si in an Fe(III) salt solution inhibit goethite formation and favor ferrihydrite, the metastable precursor. Cornell and Giovanoli (8) reported better crystallinity for goethite formed in M KOH at higher Si concentrations; they postulated that Si adsorbs preferentially on specific crystal planes, slowing the growth and hence leading to better development of crystal faces. Under similar initial conditions, however, Quin et al. (10) concluded that increasing Si concentrations resulted in poorly crystalline goethite. There have been no studies on goethite grown in an Fe(III) system over a wide range of Si concentrations at acid and alkaline pH and ambient temperatures to enable a comparison of the rates and products of crystallization. Because the chemical behavior, e.g., speciation and polymerization, of Si and Fe are pH dependent, differences in the properties of Si–Fe oxides formed at different H 1 concentrations can be expected. Furthermore, the location of Si in the Fe oxide precipitates is still ambiguous. Previous studies have shown that a close association exists between Fe oxides and Si when they are coprecipitated. Si associated with natural goethites that resisted extraction with alkali was attributed to Si possibly linking individual crystals into aggregates (11). Laboratory experiments on synthetic goethites coprecipitated with Si showed that only about 40% of total Si and 20% of total Fe were dissolved from a synthetic Si– goethite in 0.7 M HNO 3 after 14 days at RT (12). Smith and Eggleton (13) determined Si in natural goethite crystals by electron microprobe analysis and found that the amounts of Si were consistent with the estimated amount of Si that can be located between individual domains. These studies provide evidence that not all associated Si is easily accessible at the surface without necessarily being structurally incorporated.
Goethite in natural environments usually grows in the presence of dissolved silicate. To study silicate-associated goethite with specific properties, goethite was synthesized in an Fe(III) system at RT under acidic (OH/Fe 5 2; pH 1.6 –1.8) and basic (OH/Fe 5 4; pH 12–13) conditions at Si concentrations between 10 25 and 1 M. The goethites were characterized by transmission (TEM) and scanning (SEM) microscopy, X-ray diffraction (XRD), Mo¨ssbauer spectroscopy (MS), and chemical analyses. Despite large differences in size and morphology, all goethite crystals were dominantly bound by 110 and 021 faces. Nanosized crystals (ca. 20 nm) were formed at low pH, where the influence of Si was weak. In the basic system, where Si retarded crystallization, much larger (tens to hundreds of nanometers) crystals were formed whose shape varied from acicular and multidomainic at low Si (10 25 M) to monodomainic, blocky crystals at high (10 22 M) Si concentration that had reduced growth along [001] in favor of [100] and [010]. The sizes and shapes of the crystals are discussed in terms of nuclei and growth unit concentrations in the system. Part of the retained Si could be released by phosphate and NaOH (surface-Si), and part was only liberated into HCl congruently with Fe (up to 46 and 17 g kg 21 for the acid and basic goethites, respectively). Neither XRD nor MS were able to prove structural Si-for-Fe substitution, probably because no tetrahedral positions are available to accommodate Si in the goethite structure. It is assumed that this nonsurface Si fraction is located between the crystal domains. © 1999 Academic Press Key Words: goethite; ferrihydrite; silica; synthesis; Mo¨ssbauer.
INTRODUCTION
The interactions of Si with Fe oxides (used as a collective term) influence a variety of natural processes. For example, Si protects Fe phases against corrosion (1), and the association with Si can increase the solubility of Fe 31 in acid mine drainage waters (2). Si has been postulated to encourage the formation of soil aggregates by linking Fe oxide particles (3, 4). The crystallization of Fe phases, especially with respect to the mineralogy and particle morphology, is affected by Si (for a review see (5)). 1
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PROPERTIES OF PREPARED GOETHITES
The mechanism by which Si associates with Fe(III) oxides at an atomic scale is still obscure. To help elucidate this, dissolution experiments can be used to give information on the location of trace elements associated with Fe oxides (14 –17). Previous studies have postulated that Si might substitute for Fe (e.g., 18, 19). In the present study, synthetic Fe oxide–silicate systems were used to gain better insight into the structural and chemical properties of Si associated with goethite, which was selected as a model Fe(hydr)oxide due to its high natural abundance and surface reactivity. MATERIALS AND METHODS
Syntheses The acid syntheses followed the procedure of Mørup et al. (20). Iron nitrate (0.7 mol) was dissolved in 350 mL 2 M HNO3 and diluted with water to 1.4 L. Various amounts of sodium silicate solutions (see below) were added with 1 M NaOH, with continuous stirring, to yield an OH/Fe molar ratio of 2. The resulting ferrihydrite suspension was aged 60 days at 23°C; during this time, the pH decreased from 1.8 to 1.6 for all treatments. The same procedure was followed for the basic series (21), in which NaOH was added to the Fe(III) solution to give a final OH/Fe ratio of 4. The pH decreased from 12.9 to 12.5 over 60 days. Periods of longer aging up to 600 days were also tested, as was the effect of seeding by addition of 9.3% goethite. The products were centrifuged, washed several times and then dialyzed in distilled, deionized water until the conductivity of the dialysis water equaled that of deionized water. The resulting yellow-brown product was freeze-dried. Plasticware was used for the syntheses and for aging the products. In the following, the two series and their goethite products are referred to as acidic and basic series and goethites, respectively. Two ranges of silicate concentrations for the acidic and basic syntheses were used. In the first, the initial concentrations ([Si] i), added as Na 2SiO 3, were 0, 10 25, 10 24, 10 23, 10 22, and 10 21 M (yielding Si i/(Fe i 1 Si i) molar ratios from 0 to 0.286). The range between 10 23 and 10 22 M Si was further detailed into treatments with 2, 4, 6, and 8 3 10 23 M Si. The basic series included an additional member at 1 M Si.
107
able Si by shaking it with 0.1 M Na 2HPO 4 solution in 0.01 M NaNO 3 for 24 h at RT. Fe was determined using either AAS (Unicam 939) or a colorimetric technique for the oxalate extracts, because oxalate clogged the AAS burner head. For colorimetry, a 2.5 mL aliquot of 25% sulfosalicylic acid was added to 5 mL of filtered sample. The solution was titrated with concentrated NH 3 (25%) to a yellow color, filled to 50 mL volume, and measured at 436 nm (Spectronic 501, Bausch & Lomb). To separate ferrihydrite (soluble) from goethite (insoluble) (23), an amount of suspension containing 20 –100 mg solid was diluted to 50 mL with a pH 3, 0.2 M NH 4 oxalate solution, shaken for 2 h in the dark, filtered using 0.05 mm membrane filters, and Fe was measured on the filtrates. Using filter papers with larger pore sizes resulted in higher values for oxalate-extractable Fe; small sol particles probably escaped into the filtrate and dissolved. Successive extractions with oxalate were used to purify goethite from ferrihydrite in the product precipitated at high pH with [Si] i 5 8 3 10 23 M. To study dissolution behavior, the following experiments were conducted: (1) The dissolution in oxalate of the Si-free acidic and basic goethite was determined by shaking a 1 g/L suspension in 0.2 M oxalate solution at pH 3 in the dark, periodically removing aliquots with stirring, and analyzing for Fe. (2) The dissolution behavior over time and the effect of Si were measured on acidic goethites prepared at 0, 10 22, and 10 23 M [Si] i and on basic goethites prepared at 10 23 and 8 3 10 23 M [Si] i, after removal of the associated ferrihydrite. Samples of 20 mg were shaken in 10 mL 6 M HCl for different lengths of time. (3) The relationship between Fe- and Sirelease in an acidic and a basic goethite grown at [Si] i 5 10 23 M was obtained from shaking a 10 g/L suspension in 3 and 6 M HCl, respectively, with the concentrations chosen so that the different types of goethite would dissolve at comparable rates. Five milliliter aliquots were removed at intervals with stirring and filtering (0.1 mm), followed by analysis for Fe and Si using ICPS (4). Changes in dissolution rate and surface area of the basic goethite precipitated at [Si] i 5 10 23 M were studied on individual 20 mg samples shaken with 10 mL 6 M HCl for different lengths of time and filtered (0.1 mm membrane filters). Fe was measured on the filtrates, and surface area was measured on the material retained by the filter paper after it was rinsed with deionized water and dried at 60°C.
Chemical Analysis To determine Si, a colorimetric method after Koester (22) was used, in which the yellow complex of Si(OH) 4 and molybdic acid is reduced to molybdenum blue. ICPS (Liberty 2000, Varian Australia) was used when high concentrations of Fe, which interferes with the colorimetric determination, were present. Total Si was determined in a dissolution of freezedried samples using small (1–2 mL) amounts of 6 M HCl, followed by immediate dilution to 25 mL. Hydroxyl-extractable Si was determined by treating the sample with 0.2 M NaOH for 3 h at 60°C and phosphate-extract-
Mineral Identification and Surface Area For XRD, samples were gently ground with 10 wt% elemental Si as an internal line position standard, and lightly pressed into an aluminum top-fill holder with unglazed glass to give a smooth surface. Diffractograms were recorded on a Philips PW 1070/1820 vertical goniometer with CoK a radiation using a 1° divergence slit, steps of 0.02°2u, and count times of 10 s/step. The mean coherence length (MCL) was calculated by the Scherrer equation, using the program MCD (24), after correcting for instrumental broadening. MCLs along
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the a, b, and c axes were calculated by multiplying MCL hkl with cos w, w being the angle between the vector perpendicular to hkl and the vector along the axis (25). MCL a was taken from the 110, 120, and 130 lines and MCL b from the 020, 040, and (for the basic goethites) 060 lines. The unit cell edge lengths were calculated using the ten strongest lines between 0.418 and 0.142 nm. Where broader than 0.6°2u, the line position was corrected for apparent line shift (26) using the equation of Schulze (27). For TEM, about 1 mg solid was suspended in 20 ml ethanol and treated for 1 min with ultrasound. The suspensions were dropped onto carbon-coated copper grids, dried, and observed (Zeiss EM 10 A and Phillips CM 10). Thin sections were made by embedding suspended material after the method of Spurr. For SEM, dried suspensions were fixed onto tape and sputtered with gold before they were examined (Cambridge Instruments 360 S). Mo¨ssbauer spectra were recorded at various temperatures using sinusoidal or constant acceleration Mo¨ssbauer spectrometers equipped with sources of 57Co in Rh kept at room temperature. The spectrometers were calibrated using foils of a-Fe at room temperature. A liquid helium bath cryostat was used to cool both source and absorber to 4.2 K. The mean hyperfine fields B hf were determined by fitting appropriate numbers of Lorentzian-line-shaped electrical quadrupole doublets and split-Gaussian distributions of magnetic hyperfine sextets, as described by Friedl and Schwertmann (28). Surface areas were determined by applying the BET (29) equation to N 2 adsorption isotherms using freeze-dried samples, thoroughly homogenized and outgassed at 60°C under N 2 for 24 h (Quantasorb). Surface areas were calculated from three-point BET. Microporosity was determined from an N 2 adsorption isotherm with 35 pressures after outgassing the samples at 75°C for 24 h (Micromeretics Gemini 2360) and calculated by t-plot analysis (30). Briefly, the volume of adsorbed N 2 is plotted vs t, the thickness of a statistical monolayer, which is a function of the relative gas pressure. RESULTS
Goethite Formation Goethite was the only crystalline oxide formed with time at the expense of dispersed (acidic system) and coagulated (basic system) ferrihydrite. In numerous samples, the goethite was associated with residual ferrihydrite, the proportion of which increased with increasing [Si] i (initial [Si]) in the system. The ferrihydrite/goethite ratio is reflected in the ratio of oxalate soluble Fe after 2 h oxalate extraction (Fe o) to total Fe (Fe t). For both systems, Fe o/Fe t was ,0.1 at [Si] i , 4 3 10 23 M. At higher [Si] i, the ratio increased abruptly up to 1 in the basic series and more gradually up to 0.5 in the acid series. In the acid system at [Si] i of 10 23 and 10 22 M, goethite continued to form, and Fe and Si were continuously removed
from solution, over 660 days. At [Si] i of 10 21 M, goethite formation also continued over 660 days, although the concentration of Fe in the sol remained the same. Seeding with 9.3% goethite–Fe did not accelerate goethite formation in this treatment. In the basic system at [Si] i of 10 22 M, Fe o/Fe t decreased from 0.97 at 60 days to 0.66 at 300 days and to 0.46 at 660 days, indicating continuous growth of goethite. For this sample, seeding enhanced goethite formation by 21%. This agrees with earlier results of Schwertmann and Taylor (31), in which the addition of 6 – 8% goethite to a Si-containing (0.266 M) suspension of lepidocrocite enabled the transformation to the more stable goethite at RT in M KOH. Surface Area and Microporosity The SAs of the acid goethites increased nonlinearly from ca. 140 to 250 m 2/g at high [Si] i, which can be attributed to a decrease in crystal size. In contrast, for the basic goethites purified from ferrihydrite, SA decreased with increasing Si i/ (Fe i 1 Si i) down to only 13 m 2/g for the highest [Si] at which goethite formed (Fig. 1, upper). The SA of the unpurified samples had values up to 300 m 2/g, a common value for ferrihydrite showing 2 XRD lines, which this sample exhibited. With increasing [Si] the micropore surface increased from 0.8 to 7.2% of the BET surface in the acidic series and increased with [Si] i, whereas in the basic series it ranged from 0 to 10% without a clear trend with [Si] i. The large, block-like goethite crystals that grew at [Si] i of 8 3 10 23 M under basic conditions had no micropores. Chemical Composition and Dissolution Behavior Total Si retained by the goethites (Si t/(Si t 1 Fe t)) shows a curvilinear relationship with log Si i/(Fe i 1 Si i) (Fig. 1, lower). No saturation with Si was observed. The higher relative Si retention of the acid goethites is paralleled by their poorer crystal development. The OH- and phosphate-extractable Si fractions (Table 1) ranged between 0.24 and 1.0, and OH extracted somewhat more Si than did phosphate most likely because OH has a depolymerizing effect, in addition to one of anion exchange, whereas phosphate only exchanges for silicate. A 2- to 4-h treatment in 0.2 M oxalate solution at pH 3– 4 in the dark is widely used to separate ferrihydrite from wellcrystallized Fe oxides, in both synthetic and natural (soil) samples (23, 32). This method has so far not been applied to goethites produced under strongly acidic conditions as in this study. After 2 h, the acidic Si-free goethite dissolved to 7%, and the Si-free basic goethite only to 0.2% partly because of the higher surface area of the acidic goethite (136 vs 51 m 2/g) as shown earlier (33). When the oxalate extraction was continued beyond 2 h, the Si-free acidic goethite dissolved completely after 36 h, whereas after 48 h only 6% of its basic analog had dissolved. The ratio of the dissolution rates (acidic basic) is 19, much higher than that predicted by the ratio of the surface areas (2.7). Thus, in addition to the smaller crystal size,
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TABLE 1 Total, P-, and OH-Extractable Si for Acidic and Basic Goethites Formed in the Presence of Si Initial Si (M)
Total Si (mmol/g)
P-extractable Si (mmol/g)
OH-extractable Si (mmol/g)
Acidic goethites 25
1 3 10 1 3 10 24 1 3 10 23 2 3 10 23 4 3 10 23 6 3 10 23
a
0.5 (0.1) 2.6 (0.6) 36.1 (3.1) 105.7 (8.0) 207.5 (16.8) 292.5 (19.9)
b
b
1.0 (0.1) 14.7 (0.5) 47.5 (1.8) 107.4 (6.4) 117.3 (4.9)
1.4 (0.5) 15.8 (4.3) 51.1 (8.9) 112.3 (18.6) 115.9 (13.8)
Basic goethites 13 13 13 23 a b
FIG. 1. Relationship between Si in the system [log Si i/Si i 1 Fe i)] and Si retention [Si t/(Si t 1 Fe t)] (lower) and surface area (upper) for acid and basic goethites.
it is likely that the higher structural disorder of the acidic goethite contributes significantly to its higher dissolution rate. This indicates that the 2-h extraction time recommended for the conventional oxalate method should not be modified without mineralogical control (34). The dissolution-vs-time relationships for selected acidic goethites could best be fitted (r 2 5 .0.99) between Fe diss/Fe t of 0.03 and 0.90 with a zero-order model, whereas for the basic goethites the cubic rate law was more appropriate; both of these models imply steady-state dissolution conditions at the surface or a shape-preserving dissolution mechanism (35). In both cases, the dissolution rate was directly related to surface area (Table 2) and, thus, Si t/(Si t 1 Fe t) had an opposite effect on rates for the acidic and basic goethites because increasing Si i/(Fe i 1 Si i) led to smaller crystals in an acidic medium and to larger crystals in a basic one. The dissolution features investigated by TEM with a polydomainic (basic) goethite (not shown) started with tipping of
25
10 10 24 10 23 10 22
b
0.8 (0.3) 2.7 (0.5) 25.5 (2.0) 46.4 (3.9)
1.3 (0.1) 14.4 (0.5) 34.9 (1.4)
b
1.7 (0.3) 14.6 (3.4) 32.1 (4.9)
( ) indicates one standard deviation. [Si] below detection limits.
the domain terminals followed by extensive hole formation in the interior, as described earlier (36 –38). Thus, the dissolution cannot be considered shape-preserving. For the same goethite, SA increased during dissolution, as expected, whereas the dissolution rate, expressed as per unit SA, decreased drastically during the initial 1 to 2 h (Fig. 2). From this and the dissolution morphology we speculate that various parts of the crystal differ in dissolvability due to an inhomogeneous distribution of structural defects. Dissolution–time curves also gave information about the Fe and Si distribution in the goethites (Fig. 3). For an acidic goethite with Si t/(Si t 1 Fe t) of 0.0034 mol z mol 21 dissolving in 3 M HCl, the Si-fraction not extracted by OH or phosphate appeared in solution congruently with Fe after the first 30 min, until dissolution was complete after 10.25 h. The regression is Si diss/Si t 5 0.68 z Fe diss/Fe t 1 0.35 (r 2 5 0.99). The basic TABLE 2 Dissolution Rates of Si Goethites in 6 M HCl at RT Si t/Si t 1 Fe t (mol/mol)
Surface area (m 2/g)
Dissolution rate (min 21 z 10 3)
Acidic goethites 0 0.0034 0.046
136 148 254
19.2 20.3 57.5
Basic goethites 0.0024 0.0170
36 13
1.60 0.35
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GLASAUER, FRIEDL, AND SCHWERTMANN
FIG. 2. Change in surface area and dissolution rate per unit SA in 6 M HCl for a basic goethite formed at [Si] i 5 10 23 M.
goethite (Si t/(Si t 1 Fe t) 5 0.0024) dissolved completely in 6 M HCl in 14 h and lost about 40% of its Si in the first 30 min. The regression is Si diss/Si t 5 0.62 z Fe diss/Fe t 1 0.39 (r 2 5 0.99). The value of the intercept may be interpreted as surface-bound Si, supported by the fact that about the same fraction was extracted by phosphate (Table 1). This amount results in a calculated Si adsorption of ca. 0.1 mmol/m 2, which is much below possible surface saturation of ca. 2–3 mmol/m 2 assuming a mechanism identical to that of phosphate adsorption, i.e., bidentate complexes with two singly coordinated Fe–OH groups at the (110) crystal faces (39). The remaining Si can only be dissolved by continued dissolution of the goethite. Dissolution studies on the other acidic and basic goethites also showed an initial rapid release of Si followed by a congruent dissolution with Fe.
of 10 23 M tend to be slightly larger than those grown in the absence of Si, but the difference is not statistically significant. Most of the crystals lie with their c axis (needle length) parallel to the viewing plane. Since the crystals are short, however, some of them stand upright, i.e., with their c axis perpendicular to the viewing plane, so that their cross section becomes visible (Fig. 4a) (9, 20, 40, 41). This diamond-shaped cross section provides further evidence that the crystals are mainly bounded by (110) faces, as first shown by Schwertmann (37). Measurements on cross sections of 30 such particles standing on end averaged to 7 and 15 nm for needle thickness (a axis) and width (b axis), respectively. With the basic goethites, the increase in MCL a and MCL b and in their respective EM sizes with increasing [Si] i is more pronounced than for the acidic goethites. At no or low [Si] i, the crystals are multidomainic, as seen from SEM (Fig. 4b), resulting in relatively high EM/MCL ratios. With increasing [Si] i, the number of domains per cohesive “bundle” decreased, until blocky crystals consisting of a single domain were formed at [Si] i of 8 3 10 23 M; these were stunted along the c axis but still bounded by 110 faces, as seen from cross sections (Fig. 4c), and 021 faces. This goethite also had the lowest ratios of EM/MCL. A similar blocky morphology was observed for goethites that grew in the presence of Si either from lepidocrocite (31) or from ferrihydrite (10, 42), both at high pH. Hyperfine Properties The hyperfine fields, B hf, of the basic goethites at 4.2 K with Si t/(Si t 1 Fe t) molar ratios of 0, 0.004, 0.010, and 0.017 were uniformly 50.5 T. This value is close to that of 50.6 T for an
Unit Cell Size, Crystal Size, and Crystal Morphology For the basic goethites, no difference in the unit cell size relative to pure goethite could be detected. The same applies to the acidic goethites after correction for apparent line shift due to line broadening. There was only a slight increase in the unit cell b edge length at the highest [Si] i of the acidic goethites but no change in the cell volume (not shown). Both XRD line broadening and electron microscopy supply information on the crystal size. A comparison for both goethites is presented in Table 3. The acidic goethites are very small in all dimensions and the reasonable agreement between MCL a and MCL b on the one hand and the respective TEM figures on the other indicates that the crystals are essentially monodomainic; i.e., the EM/MCL ratios are close to one. A value of this ratio above one implies that the crystals are composed of more than one domain. The crystals grown at [Si] i
FIG. 3. Fraction of dissolved Si and Fe of an acidic goethite in 3 M HCl and a basic goethite in 6 M HCl, both formed at [Si] i 5 10 23 M.
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TABLE 3 Dimensions of Acidic and Basic Goethites, Calculated from MCLs and Measured on Crystals Observed in TEM and SEM MCL (nm) Initial Si (M)
Si t/Si t 1 Fe t b
a
EM (nm) b
a
b
c
EM a /MCL a
EM b /MCL b
13 (2) 17 (4)
19 (4) 27 (5)
1.2 0.9
1.9 1.1
154 (61) 235 (53) 280 (77) 236 (110)
761 (228) 715 (131) 740 (178) 642 (217)
3.4 3.1 2.9 1.7
5.4 5.9 5.5 2.4
Acid goethites 0 1 3 10 23
0 0.003
6 (1) 8 (1)
7 (2) 16 (3)
7 (1) 7 (3) Basic goethites
0 2 3 10 23 4 3 10 23 8 3 10 23
0 0.004 0.010 0.017
9 (1) 17 (9) 26 (2) 59 (6)
29 (4) 40 (1) 51 (3) 99 (11)
30 (7) c 53 (7) c 75 (17) e 103 (32)
a
( ) indicates one standard deviation. mol/mol in goethite. c #5 particles measured (otherwise $15). b
“ideal” goethite (43). All goethites showed completely magnetically split patterns at 298 K, and the splitting as well as the magnetic sextet fraction decreased as the temperature increased from 298 to 373 K; this is demonstrated in Fig. 5 by spectra for the samples with no Si and with Si t/(Si t 1 Fe t) of 0.017. At a given temperature, magnetic hyperfine splitting increased with Si t/(Si t 1 Fe t) from 0 to 0.010 mol/mol, followed by a slight decrease at a ratio of 0.017 (Fig. 6). At 363 K, a temperature about 40 K below the Neel temperature of an ideal goethite, the Si-free goethite showed only a doublet, whereas at Si t/(Si t 1 Fe t) of 0.010 and 0.017 a weak sextet was still detectable, demonstrating the slightly better crystallinity of these goethites. The increase in B hf with increasing Si t/(Si t 1 Fe t) may be explained as the effect of larger crystal size, which is increasingly counteracted by the effects of diamagnetic Si at higher Si content. The average B hfs of the acid goethites at 4.2 K were 50.4, 50.5, 49.9, 49.9, and 49.8 T for Si t/(Si t 1 Fe t) of 0, 0.003, 0.020, 0.038, and 0.046 mol/mol, showing little change with increasing Si. Small crystal size can explain the lower B hfs compared to the basic goethites (44). The corresponding B hfs at RT decreased with increasing Si: 23.3, 22.4, 15.9, 0, and 9.8 T. The exception to the trend is the sample with 0.038 Si, which also had the smallest B hf at 4.2 K. Only the goethite with no Si showed complete magnetic splitting, whereas the other samples had additional doublets with broadened lines due to superparamagnetic relaxation of part of the sample. DISCUSSION
In the framework of earlier results, two aspects merit discussion: the nature of the Si retained by goethite and its effect on the crystal properties.
Incorporation of Si Considerable amounts of the added Si remained non-OHextractable, leading to Si t/(Si t 1 Fe t) molar ratios up to 0.017 for the basic goethites and up to 0.046 for the acidic goethites. This Si fraction dissolved congruently with Fe, and although such behavior may indicate isomorphous substitution of Fe (as for Al, Mn, and numerous other metals; see (5)), it does not prove such a substitution. Four different positions can be suggested schematically for the association of ions with goethite: external surfaces, internal surfaces (between domains), clusters, and structural substitution. External surfaces are those at the solid/solution interface, and it can be assumed that NaOH dissolves most of this Si fraction by anion exchange and depolymerization. Internal surfaces are the interfaces between closely aggregated crystals or between domains of crystals. Ions can also form clusters/inclusions within the structure, as demonstrated for Fe 31 in corundum (45) and in diaspore (46). A final possibility is that ions substitute within the crystal structure. Substitution by a smaller cation will contract the unit cell, as determined by XRD. Measured unit cell volumes did not differ significantly from those of pure goethites and, hence, gave no indication that Si substituted for Fe in the structure. If substitution took place, the amount was too small to be detected using XRD methods. Furthermore, Si in sixfold coordination with O has an ionic radius that is significantly smaller (0.040 nm) than that of high spin Fe 31 (0.064 nm), which is six-coordinated with O and OH in goethite. In nature, however, six-coordinated Si is seldom found and fourfold coordination is rather the rule; the mineral stishovite, a SiO 2 polymorph, is a rare exception. A difference in crystal radius of 615% is usually taken as the upper limit for the replacement of Fe 31 to avoid strain, and this is much less than the difference in radii between Fe 31 and Si 41.
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FIG. 4. EM photos of goethites: (a) Diamond-shaped cross sections of crystals of acid goethite bounded by (110) faces (TEM), scale 5 50 nm; (b) multidomainic basic goethite grown at [Si] i 5 2 z 10 23 (SEM); (c) Cross section of a basic goethite crystal cut perpendicular to [001], showing (110) faces (TEM), scale indicates 100 nm.
Mo¨ssbauer spectroscopy has been used to determine the effect of the substitution of Fe(III) by diamagnetic cations on the magnetic characteristics of goethite (e.g., 47). Magnetic ordering is created by the presence of paramagnetic structural elements and can be disturbed by diamagnetic elements, such as Al and Si. In analogy to Al, Si within the goethite crystal should, therefore, similarly affect the magnetic properties (44).
A reduction of magnetic ordering was suggested by Quin et al. (10) for goethites synthesized at pH 10 –11 in the presence of Si; ordering could not, however, be attributed unequivocally to goethite because ferrihydrite was a major component of their samples. Decreased magnetic ordering due to Si could not be observed for our basic Si goethites. On the contrary, the improved magnetic splitting at RT and higher temperatures (Fig.
PROPERTIES OF PREPARED GOETHITES
113
FIG. 5. Mo¨ssbauer spectra of a basic goethite with zero Si and with Si t/(Si t 1 Fe t) of 0.017 mol/mol recorded at 298, 343, 353, and 363 K.
5) indicates that the Neel temperature was always greater for the Si goethites than for the pure one. The slight reduction at the highest Si may be due to reduced superferromagnetic interaction (20, 48) caused by surface Si, which may have led to a slightly reduced B hf and increased superparamagnetic fraction of this sample. On the other hand, the earlier magnetic breakdown observed at Si t/(Si t 1 Fe t) of 0 and 0.004 mol/mol can be explained by smaller crystal size, in agreement with TEM and XRD. For the acidic goethites, lower magnetic ordering and broader XRD peaks proved that under acidic conditions goethite of lower crystal order and smaller crystal size was formed as the Si i/(Si i 1 Fe i) increased, in agreement with TEM observations. As with the basic goethites, a decrease in the fraction that was magnetically ordered parallels reduced crystal size and superferromagnetic interaction due to Si at the crystal surfaces or between the domains.
In conclusion, excluding a statistical distribution of Si in the structure of goethite, we assume that the Si fraction inaccessible to OH and phosphate, but which dissolves congruently with Fe, may either aggregate the nanosized crystals (acidic goethite) or be located between the domains of the multidomainic crystals (basic goethites). Both may cause a spatial distribution of Si that is sufficiently random to result in congruent dissolution with Fe. Si Effect on Crystal Formation As shown by Schwertmann (37), synthetic goethite crystals are usually bounded by 110 and 021 faces, which are the slowest growing surfaces and therefore determine the morphology. This seems to apply in the case of the acidic and basic Si goethites as well. In other words, although the Si-affected crystals vary considerably in size and shape, their habit ap-
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system even at RT. In other words, the supply of the soluble species from the dissolution of two-line ferrihydrite does not seem to limit the rate of nucleation and crystallization. Silicate, which occurs as an anion at alkaline pH, can, however, interfere with goethite formation either by hindering nucleation and crystal growth or by reacting with ferrihydrite and thereby retarding the supply of growth units. This would explain why fewer and larger crystals form. CONCLUSIONS
FIG. 6. Hyperfine fields of basic goethites as a function of Si t/(Si t 1 Fe t) molar ratios, measured at various temperatures.
peared to be rather uniform. This agrees with recent kinetic studies of Weidler (49). When the crystals grew more slowly, e.g., in the presence of Si, they were not only larger, but the proportion of the 021 surface increased at the expense of the 110 surface, i.e., the crystal growth rate in the a and b directions increased at the expense of that in the c direction. Cornell et al. (42) concluded that this may be due to the preferential binding of silicate to the terminal 021 faces, thereby retarding their growth rate. The reasons for large variations in size of the same habit as a function of pH are not clear. It seems to reflect the ratio between rate of nucleation and supply of mono- and dimeric growth units (50). The adsorption of Si on goethite is minimal at low pH because silicate forms protonated or neutral polymers and, in addition, stable complexes with Fe 31, i.e., FeOSi(OH) 321 (51), which reach the maximum formation near pH 2 (2). Both forms of Si lower the activity of silicate anion in solution. Nucleation of goethite will not, therefore, be substantially hindered, and many small crystals will develop. The rate of crystallization will, however, be slow because at low pH the unhydrolyzed Fe species dominate. At high pH (11–12), the fully hydrolyzed species Fe(OH) 42, from which nuclei and crystals form, predominates. This may explain why goethite grows so rapidly in a purely alkaline
There was no evidence for Si substitution in the goethite crystal; instead, the data support a random distribution of Si between strongly aggregated nanocrystals (acidic-precipitated system) or domains of large multidomainic crystals (basicprecipitated system). The different behaviors in acidic and basic solution reflect the influence of pH on the rates of crystal nucleation and growth via the effect on chemical speciation of Si and Fe. These results open up several avenues for further investigations. One question is how solution chemistry determines the size and morphology of goethite particles, i.e., how the mechanism by which Fe units are supplied to the growing crystal reacts to changes in pH or to the presence of growth inhibitors like Si. Also of interest is determining the influence of surface functional groups and surface defects on crystal reactivity, with particular attention to the 021 face. A more applied challenge is how to disperse ferrihydrite particles and the strongly aggregating goethite nanocrystals, which resisted separation by dispersing agents or by ultrasound. ACKNOWLEDGMENTS We thank C. B. Koch for assistance with the Mo¨ssbauer analyses and P. G. Weidler for the analyses of microporosity. This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Schw 90/48-2).
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Ohmori, M., and Matijevic, E., J. Colloid Interface Sci. 160, 288 (1993). Reardon, E. J., Chemical Geology 25, 339 (1979). Marsan, F. A., and Torrent, J., Soil Sci. Soc. Am. J. 53, 1140 (1989). Pinheiro-Dick, D., and Schwertmann, U., Geoderma 74, 49 (1996). Cornell, R. M., and Schwertmann, U., “The Iron Oxides,” VCH Verlag, Weinheim, 1996. Schwertmann, U., and Thalmann, H., Clay Miner. 11, 189 (1976). Karim, Z., Clays Clay Miner. 32, 181 (1984). Cornell, R. M., and Giovanoli, R., J. Chem. Soc., Chem. Commun. 413– 414 (1987). Koch, C. J. W., Madsen, M. B., Mørup, S., Christiansen, G., Gerward, L., and Villadsen, J., Clays Clay Miner. 34, 17 (1986). Quin, T. G., Long, G. J., Benson, C. G., Mann, S., and Williams, R. J. P., Clays Clay Miner. 36, 165 (1988). Torrent, J., Schwertmann, U., and Barron, V., Clays Clay Miner. 40, 14 (1992). Gerth, J., Bru¨mmer, G. W., and Tiller, K. G., Z. Pflanzenerna¨hr. Bodenk. 156, 123 (1993). Smith, K. L., and Eggleton, R. A., Clays Clay Miner. 31, 392 (1983).
PROPERTIES OF PREPARED GOETHITES 14. Lim-Nunez, R., and Gilkes, R. J., in “Proc. 1985 Int. Clay Conference, Denver” (L. G. Schultz, H. van Olphen, and F. A. Mumpton, Eds.) 1987, p. 197. 15. Hochella, M. F., in “Mineral-Water Interface Geochemistry” (M. F. Hochella and A. F. White, Eds.), Reviews in Mineralogy 23, p. 87. Mineralogical Society of America, Washington, DC, 1990. 16. Schwertmann, U., in “Iron Nutrition and Interactions in Plants,” Y. Chen and Y. Hadar, eds., pp. 1–25. Kluwer Academic, Netherlands, 1991. 17. Schwertmann, U., and Pfab, G., Geochim. Cosmochim. Acta 58, 4349 (1994). 18. Murad, E., Mineralogical Mag. 43, 355 (1979). 19. Vempati, R. K., Loeppert, R. H., Dufner, D. C., and Cocke, D. L., Soil Sci. Soc. Am. J. 54, 695 (1990). 20. Mørup, S., Madsen, M. B., Franck, J., Villadsen, J., and Koch, C. J. W., J. Magnetism Magn. Mater. 40, 163 (1983). 21. Koch, C. J. W., Madsen, M. B., and Mørup, S., Surf. Sci. 156, 249 (1985). 22. Koester, H. M., “Die chemische Silikatanalyse,” Springer-Verlag, Berlin and Heidelberg, 1979. 23. Schwertmann, U., Z. Pflanzenerna¨hr., Du¨ng., Bodenkunde 105, 194 (1964). 24. Stanjek, H., Dissertation, Technische Universita¨t Mu¨nchen, 1991. 25. Schulze, D., and Schwertmann, U., Clay Miner. 19, 521 (1984). 26. Klug, H. P., and Alexander, L. E., “X-Ray Diffraction Procedures,” 2nd ed., Wiley, New York, 1974. 27. Schulze, D., Clays Clay Miner. 32, 36 (1984). 28. Friedl, J., and Schwertmann, U., Clay Miner. 31, 455 (1996). 29. Brunauer, S., Emmett, P. H., and Teller, E., J. Am. Chem. Soc. 60, 309 (1938). 30. DeBoer, J. H., Lippens, B. C., Linsen, B. G., Broekhoff, J. C. P., van den Heuvel, A., and Osinga, T. J., J. Colloid Interface Sci. 21, 405 (1966). 31. Schwertmann, U., and Taylor, R. M., Clays Clay Miner. 20, 159 (1972).
32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.
44. 45. 46. 47. 48. 49. 50. 51.
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McKeague, J. A., and Day, J. H., Can. J. Soil Sci. 46, 13 (1966). Schwertmann, U., Can. J. Soil Sci. 53, 244 (1973). Campbell, A., and Schwertmann, U., Clay Miner. 20, 515 (1985). Blesa, M. A., and Maroto, A. J. G., J. Chim. Phys. 83, 757 (1986). Cornell, R. M., Posner, A. M., and Quirk, J. P., Soil Sci. Soc. Am. Proc. 38, 377 (1974). Schwertmann, U., Clay Miner. 19, 9 (1984). Weidler, P. G., Dissertation, Technische Universitaet Muenchen, 1995. Torrent, J., Barron, V., and Schwertmann, U., Soil Sci. Soc. Am. J. 54, 1007 (1990). Atkinson, R. J., Parfitt, L., and Smart, R. C., J. Chem. Soc. Faraday Trans. 70, 1472 (1974). Murphy, P. J., Posner, A. M., and Quirk, J. P., J. Colloid Interface Sci. 56, 270 (1976). Cornell, R. M., Giovanoli, R., and Schindler, R. W., Clays and Clay Miner. 35, 21 (1987). Goodman, B. A., in “Clay Mineralogy: Spectroscopic and Chemical Determinative Methods” (M. J. Wilson, Ed.), p. 68. Chapman & Hall, London, 1994. Murad, E., Am. Mineralogist 67, 1007 (1982). Viktorov, V. V., Phys. Stat. Sol. 174, 529 (1992). Hazeman, J. L., Manceau, W., Sainctavit, Ph., and Melgrange, C., Phys. Chem. Min. 19, 25 (1992). Murad, E., and Schwertmann, U., Clay Miner. 18, 301 (1983). Mørup, S., Hyperfine Interactions 60, 959 (1990). Weidler, P. G., Hug, S. J., Wetche, T. P., and Hiemstra, T., Geochim. Cosmochim. Acta 62, 3407 (1998). Schwertmann, U., Friedl, J., and Stanjek, H., J. Colloid Interface Sci., in press. Weber, W. J., and Stumm, W., J. Inorg. Nucl. Chem. 27, 237 (1965).
Properties of Goethites Prepared under Acidic and Basic Conditions in the Presence of Silicate Susan Glasauer,* ,1 Josef Friedl,† and Udo Schwertmann† *Department of Microbiology, College of Biological Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada; and †Institute for Soil Science, Technical University, Munich, D-85350 Freising, Germany Received December 31, 1998; accepted April 14, 1999
Most studies on Si–Fe interaction during crystallization have considered alkaline systems, although acid conditions are most common in natural systems. Furthermore, the results of these studies are conflicting, especially regarding the effect of Si on the crystallinity of the products. Si has been found to increase or decrease product crystallinity (e.g., 6 –10), depending on the pH during synthesis, the iron salt used, the synthesis temperature, and even the method of analysis. It has been shown that increasing concentrations of Si in an Fe(III) salt solution inhibit goethite formation and favor ferrihydrite, the metastable precursor. Cornell and Giovanoli (8) reported better crystallinity for goethite formed in M KOH at higher Si concentrations; they postulated that Si adsorbs preferentially on specific crystal planes, slowing the growth and hence leading to better development of crystal faces. Under similar initial conditions, however, Quin et al. (10) concluded that increasing Si concentrations resulted in poorly crystalline goethite. There have been no studies on goethite grown in an Fe(III) system over a wide range of Si concentrations at acid and alkaline pH and ambient temperatures to enable a comparison of the rates and products of crystallization. Because the chemical behavior, e.g., speciation and polymerization, of Si and Fe are pH dependent, differences in the properties of Si–Fe oxides formed at different H 1 concentrations can be expected. Furthermore, the location of Si in the Fe oxide precipitates is still ambiguous. Previous studies have shown that a close association exists between Fe oxides and Si when they are coprecipitated. Si associated with natural goethites that resisted extraction with alkali was attributed to Si possibly linking individual crystals into aggregates (11). Laboratory experiments on synthetic goethites coprecipitated with Si showed that only about 40% of total Si and 20% of total Fe were dissolved from a synthetic Si– goethite in 0.7 M HNO 3 after 14 days at RT (12). Smith and Eggleton (13) determined Si in natural goethite crystals by electron microprobe analysis and found that the amounts of Si were consistent with the estimated amount of Si that can be located between individual domains. These studies provide evidence that not all associated Si is easily accessible at the surface without necessarily being structurally incorporated.
Goethite in natural environments usually grows in the presence of dissolved silicate. To study silicate-associated goethite with specific properties, goethite was synthesized in an Fe(III) system at RT under acidic (OH/Fe 5 2; pH 1.6 –1.8) and basic (OH/Fe 5 4; pH 12–13) conditions at Si concentrations between 10 25 and 1 M. The goethites were characterized by transmission (TEM) and scanning (SEM) microscopy, X-ray diffraction (XRD), Mo¨ssbauer spectroscopy (MS), and chemical analyses. Despite large differences in size and morphology, all goethite crystals were dominantly bound by 110 and 021 faces. Nanosized crystals (ca. 20 nm) were formed at low pH, where the influence of Si was weak. In the basic system, where Si retarded crystallization, much larger (tens to hundreds of nanometers) crystals were formed whose shape varied from acicular and multidomainic at low Si (10 25 M) to monodomainic, blocky crystals at high (10 22 M) Si concentration that had reduced growth along [001] in favor of [100] and [010]. The sizes and shapes of the crystals are discussed in terms of nuclei and growth unit concentrations in the system. Part of the retained Si could be released by phosphate and NaOH (surface-Si), and part was only liberated into HCl congruently with Fe (up to 46 and 17 g kg 21 for the acid and basic goethites, respectively). Neither XRD nor MS were able to prove structural Si-for-Fe substitution, probably because no tetrahedral positions are available to accommodate Si in the goethite structure. It is assumed that this nonsurface Si fraction is located between the crystal domains. © 1999 Academic Press Key Words: goethite; ferrihydrite; silica; synthesis; Mo¨ssbauer.
INTRODUCTION
The interactions of Si with Fe oxides (used as a collective term) influence a variety of natural processes. For example, Si protects Fe phases against corrosion (1), and the association with Si can increase the solubility of Fe 31 in acid mine drainage waters (2). Si has been postulated to encourage the formation of soil aggregates by linking Fe oxide particles (3, 4). The crystallization of Fe phases, especially with respect to the mineralogy and particle morphology, is affected by Si (for a review see (5)). 1
To whom correspondence should be addressed.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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PROPERTIES OF PREPARED GOETHITES
The mechanism by which Si associates with Fe(III) oxides at an atomic scale is still obscure. To help elucidate this, dissolution experiments can be used to give information on the location of trace elements associated with Fe oxides (14 –17). Previous studies have postulated that Si might substitute for Fe (e.g., 18, 19). In the present study, synthetic Fe oxide–silicate systems were used to gain better insight into the structural and chemical properties of Si associated with goethite, which was selected as a model Fe(hydr)oxide due to its high natural abundance and surface reactivity. MATERIALS AND METHODS
Syntheses The acid syntheses followed the procedure of Mørup et al. (20). Iron nitrate (0.7 mol) was dissolved in 350 mL 2 M HNO3 and diluted with water to 1.4 L. Various amounts of sodium silicate solutions (see below) were added with 1 M NaOH, with continuous stirring, to yield an OH/Fe molar ratio of 2. The resulting ferrihydrite suspension was aged 60 days at 23°C; during this time, the pH decreased from 1.8 to 1.6 for all treatments. The same procedure was followed for the basic series (21), in which NaOH was added to the Fe(III) solution to give a final OH/Fe ratio of 4. The pH decreased from 12.9 to 12.5 over 60 days. Periods of longer aging up to 600 days were also tested, as was the effect of seeding by addition of 9.3% goethite. The products were centrifuged, washed several times and then dialyzed in distilled, deionized water until the conductivity of the dialysis water equaled that of deionized water. The resulting yellow-brown product was freeze-dried. Plasticware was used for the syntheses and for aging the products. In the following, the two series and their goethite products are referred to as acidic and basic series and goethites, respectively. Two ranges of silicate concentrations for the acidic and basic syntheses were used. In the first, the initial concentrations ([Si] i), added as Na 2SiO 3, were 0, 10 25, 10 24, 10 23, 10 22, and 10 21 M (yielding Si i/(Fe i 1 Si i) molar ratios from 0 to 0.286). The range between 10 23 and 10 22 M Si was further detailed into treatments with 2, 4, 6, and 8 3 10 23 M Si. The basic series included an additional member at 1 M Si.
107
able Si by shaking it with 0.1 M Na 2HPO 4 solution in 0.01 M NaNO 3 for 24 h at RT. Fe was determined using either AAS (Unicam 939) or a colorimetric technique for the oxalate extracts, because oxalate clogged the AAS burner head. For colorimetry, a 2.5 mL aliquot of 25% sulfosalicylic acid was added to 5 mL of filtered sample. The solution was titrated with concentrated NH 3 (25%) to a yellow color, filled to 50 mL volume, and measured at 436 nm (Spectronic 501, Bausch & Lomb). To separate ferrihydrite (soluble) from goethite (insoluble) (23), an amount of suspension containing 20 –100 mg solid was diluted to 50 mL with a pH 3, 0.2 M NH 4 oxalate solution, shaken for 2 h in the dark, filtered using 0.05 mm membrane filters, and Fe was measured on the filtrates. Using filter papers with larger pore sizes resulted in higher values for oxalate-extractable Fe; small sol particles probably escaped into the filtrate and dissolved. Successive extractions with oxalate were used to purify goethite from ferrihydrite in the product precipitated at high pH with [Si] i 5 8 3 10 23 M. To study dissolution behavior, the following experiments were conducted: (1) The dissolution in oxalate of the Si-free acidic and basic goethite was determined by shaking a 1 g/L suspension in 0.2 M oxalate solution at pH 3 in the dark, periodically removing aliquots with stirring, and analyzing for Fe. (2) The dissolution behavior over time and the effect of Si were measured on acidic goethites prepared at 0, 10 22, and 10 23 M [Si] i and on basic goethites prepared at 10 23 and 8 3 10 23 M [Si] i, after removal of the associated ferrihydrite. Samples of 20 mg were shaken in 10 mL 6 M HCl for different lengths of time. (3) The relationship between Fe- and Sirelease in an acidic and a basic goethite grown at [Si] i 5 10 23 M was obtained from shaking a 10 g/L suspension in 3 and 6 M HCl, respectively, with the concentrations chosen so that the different types of goethite would dissolve at comparable rates. Five milliliter aliquots were removed at intervals with stirring and filtering (0.1 mm), followed by analysis for Fe and Si using ICPS (4). Changes in dissolution rate and surface area of the basic goethite precipitated at [Si] i 5 10 23 M were studied on individual 20 mg samples shaken with 10 mL 6 M HCl for different lengths of time and filtered (0.1 mm membrane filters). Fe was measured on the filtrates, and surface area was measured on the material retained by the filter paper after it was rinsed with deionized water and dried at 60°C.
Chemical Analysis To determine Si, a colorimetric method after Koester (22) was used, in which the yellow complex of Si(OH) 4 and molybdic acid is reduced to molybdenum blue. ICPS (Liberty 2000, Varian Australia) was used when high concentrations of Fe, which interferes with the colorimetric determination, were present. Total Si was determined in a dissolution of freezedried samples using small (1–2 mL) amounts of 6 M HCl, followed by immediate dilution to 25 mL. Hydroxyl-extractable Si was determined by treating the sample with 0.2 M NaOH for 3 h at 60°C and phosphate-extract-
Mineral Identification and Surface Area For XRD, samples were gently ground with 10 wt% elemental Si as an internal line position standard, and lightly pressed into an aluminum top-fill holder with unglazed glass to give a smooth surface. Diffractograms were recorded on a Philips PW 1070/1820 vertical goniometer with CoK a radiation using a 1° divergence slit, steps of 0.02°2u, and count times of 10 s/step. The mean coherence length (MCL) was calculated by the Scherrer equation, using the program MCD (24), after correcting for instrumental broadening. MCLs along
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the a, b, and c axes were calculated by multiplying MCL hkl with cos w, w being the angle between the vector perpendicular to hkl and the vector along the axis (25). MCL a was taken from the 110, 120, and 130 lines and MCL b from the 020, 040, and (for the basic goethites) 060 lines. The unit cell edge lengths were calculated using the ten strongest lines between 0.418 and 0.142 nm. Where broader than 0.6°2u, the line position was corrected for apparent line shift (26) using the equation of Schulze (27). For TEM, about 1 mg solid was suspended in 20 ml ethanol and treated for 1 min with ultrasound. The suspensions were dropped onto carbon-coated copper grids, dried, and observed (Zeiss EM 10 A and Phillips CM 10). Thin sections were made by embedding suspended material after the method of Spurr. For SEM, dried suspensions were fixed onto tape and sputtered with gold before they were examined (Cambridge Instruments 360 S). Mo¨ssbauer spectra were recorded at various temperatures using sinusoidal or constant acceleration Mo¨ssbauer spectrometers equipped with sources of 57Co in Rh kept at room temperature. The spectrometers were calibrated using foils of a-Fe at room temperature. A liquid helium bath cryostat was used to cool both source and absorber to 4.2 K. The mean hyperfine fields B hf were determined by fitting appropriate numbers of Lorentzian-line-shaped electrical quadrupole doublets and split-Gaussian distributions of magnetic hyperfine sextets, as described by Friedl and Schwertmann (28). Surface areas were determined by applying the BET (29) equation to N 2 adsorption isotherms using freeze-dried samples, thoroughly homogenized and outgassed at 60°C under N 2 for 24 h (Quantasorb). Surface areas were calculated from three-point BET. Microporosity was determined from an N 2 adsorption isotherm with 35 pressures after outgassing the samples at 75°C for 24 h (Micromeretics Gemini 2360) and calculated by t-plot analysis (30). Briefly, the volume of adsorbed N 2 is plotted vs t, the thickness of a statistical monolayer, which is a function of the relative gas pressure. RESULTS
Goethite Formation Goethite was the only crystalline oxide formed with time at the expense of dispersed (acidic system) and coagulated (basic system) ferrihydrite. In numerous samples, the goethite was associated with residual ferrihydrite, the proportion of which increased with increasing [Si] i (initial [Si]) in the system. The ferrihydrite/goethite ratio is reflected in the ratio of oxalate soluble Fe after 2 h oxalate extraction (Fe o) to total Fe (Fe t). For both systems, Fe o/Fe t was ,0.1 at [Si] i , 4 3 10 23 M. At higher [Si] i, the ratio increased abruptly up to 1 in the basic series and more gradually up to 0.5 in the acid series. In the acid system at [Si] i of 10 23 and 10 22 M, goethite continued to form, and Fe and Si were continuously removed
from solution, over 660 days. At [Si] i of 10 21 M, goethite formation also continued over 660 days, although the concentration of Fe in the sol remained the same. Seeding with 9.3% goethite–Fe did not accelerate goethite formation in this treatment. In the basic system at [Si] i of 10 22 M, Fe o/Fe t decreased from 0.97 at 60 days to 0.66 at 300 days and to 0.46 at 660 days, indicating continuous growth of goethite. For this sample, seeding enhanced goethite formation by 21%. This agrees with earlier results of Schwertmann and Taylor (31), in which the addition of 6 – 8% goethite to a Si-containing (0.266 M) suspension of lepidocrocite enabled the transformation to the more stable goethite at RT in M KOH. Surface Area and Microporosity The SAs of the acid goethites increased nonlinearly from ca. 140 to 250 m 2/g at high [Si] i, which can be attributed to a decrease in crystal size. In contrast, for the basic goethites purified from ferrihydrite, SA decreased with increasing Si i/ (Fe i 1 Si i) down to only 13 m 2/g for the highest [Si] at which goethite formed (Fig. 1, upper). The SA of the unpurified samples had values up to 300 m 2/g, a common value for ferrihydrite showing 2 XRD lines, which this sample exhibited. With increasing [Si] the micropore surface increased from 0.8 to 7.2% of the BET surface in the acidic series and increased with [Si] i, whereas in the basic series it ranged from 0 to 10% without a clear trend with [Si] i. The large, block-like goethite crystals that grew at [Si] i of 8 3 10 23 M under basic conditions had no micropores. Chemical Composition and Dissolution Behavior Total Si retained by the goethites (Si t/(Si t 1 Fe t)) shows a curvilinear relationship with log Si i/(Fe i 1 Si i) (Fig. 1, lower). No saturation with Si was observed. The higher relative Si retention of the acid goethites is paralleled by their poorer crystal development. The OH- and phosphate-extractable Si fractions (Table 1) ranged between 0.24 and 1.0, and OH extracted somewhat more Si than did phosphate most likely because OH has a depolymerizing effect, in addition to one of anion exchange, whereas phosphate only exchanges for silicate. A 2- to 4-h treatment in 0.2 M oxalate solution at pH 3– 4 in the dark is widely used to separate ferrihydrite from wellcrystallized Fe oxides, in both synthetic and natural (soil) samples (23, 32). This method has so far not been applied to goethites produced under strongly acidic conditions as in this study. After 2 h, the acidic Si-free goethite dissolved to 7%, and the Si-free basic goethite only to 0.2% partly because of the higher surface area of the acidic goethite (136 vs 51 m 2/g) as shown earlier (33). When the oxalate extraction was continued beyond 2 h, the Si-free acidic goethite dissolved completely after 36 h, whereas after 48 h only 6% of its basic analog had dissolved. The ratio of the dissolution rates (acidic basic) is 19, much higher than that predicted by the ratio of the surface areas (2.7). Thus, in addition to the smaller crystal size,
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TABLE 1 Total, P-, and OH-Extractable Si for Acidic and Basic Goethites Formed in the Presence of Si Initial Si (M)
Total Si (mmol/g)
P-extractable Si (mmol/g)
OH-extractable Si (mmol/g)
Acidic goethites 25
1 3 10 1 3 10 24 1 3 10 23 2 3 10 23 4 3 10 23 6 3 10 23
a
0.5 (0.1) 2.6 (0.6) 36.1 (3.1) 105.7 (8.0) 207.5 (16.8) 292.5 (19.9)
b
b
1.0 (0.1) 14.7 (0.5) 47.5 (1.8) 107.4 (6.4) 117.3 (4.9)
1.4 (0.5) 15.8 (4.3) 51.1 (8.9) 112.3 (18.6) 115.9 (13.8)
Basic goethites 13 13 13 23 a b
FIG. 1. Relationship between Si in the system [log Si i/Si i 1 Fe i)] and Si retention [Si t/(Si t 1 Fe t)] (lower) and surface area (upper) for acid and basic goethites.
it is likely that the higher structural disorder of the acidic goethite contributes significantly to its higher dissolution rate. This indicates that the 2-h extraction time recommended for the conventional oxalate method should not be modified without mineralogical control (34). The dissolution-vs-time relationships for selected acidic goethites could best be fitted (r 2 5 .0.99) between Fe diss/Fe t of 0.03 and 0.90 with a zero-order model, whereas for the basic goethites the cubic rate law was more appropriate; both of these models imply steady-state dissolution conditions at the surface or a shape-preserving dissolution mechanism (35). In both cases, the dissolution rate was directly related to surface area (Table 2) and, thus, Si t/(Si t 1 Fe t) had an opposite effect on rates for the acidic and basic goethites because increasing Si i/(Fe i 1 Si i) led to smaller crystals in an acidic medium and to larger crystals in a basic one. The dissolution features investigated by TEM with a polydomainic (basic) goethite (not shown) started with tipping of
25
10 10 24 10 23 10 22
b
0.8 (0.3) 2.7 (0.5) 25.5 (2.0) 46.4 (3.9)
1.3 (0.1) 14.4 (0.5) 34.9 (1.4)
b
1.7 (0.3) 14.6 (3.4) 32.1 (4.9)
( ) indicates one standard deviation. [Si] below detection limits.
the domain terminals followed by extensive hole formation in the interior, as described earlier (36 –38). Thus, the dissolution cannot be considered shape-preserving. For the same goethite, SA increased during dissolution, as expected, whereas the dissolution rate, expressed as per unit SA, decreased drastically during the initial 1 to 2 h (Fig. 2). From this and the dissolution morphology we speculate that various parts of the crystal differ in dissolvability due to an inhomogeneous distribution of structural defects. Dissolution–time curves also gave information about the Fe and Si distribution in the goethites (Fig. 3). For an acidic goethite with Si t/(Si t 1 Fe t) of 0.0034 mol z mol 21 dissolving in 3 M HCl, the Si-fraction not extracted by OH or phosphate appeared in solution congruently with Fe after the first 30 min, until dissolution was complete after 10.25 h. The regression is Si diss/Si t 5 0.68 z Fe diss/Fe t 1 0.35 (r 2 5 0.99). The basic TABLE 2 Dissolution Rates of Si Goethites in 6 M HCl at RT Si t/Si t 1 Fe t (mol/mol)
Surface area (m 2/g)
Dissolution rate (min 21 z 10 3)
Acidic goethites 0 0.0034 0.046
136 148 254
19.2 20.3 57.5
Basic goethites 0.0024 0.0170
36 13
1.60 0.35
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GLASAUER, FRIEDL, AND SCHWERTMANN
FIG. 2. Change in surface area and dissolution rate per unit SA in 6 M HCl for a basic goethite formed at [Si] i 5 10 23 M.
goethite (Si t/(Si t 1 Fe t) 5 0.0024) dissolved completely in 6 M HCl in 14 h and lost about 40% of its Si in the first 30 min. The regression is Si diss/Si t 5 0.62 z Fe diss/Fe t 1 0.39 (r 2 5 0.99). The value of the intercept may be interpreted as surface-bound Si, supported by the fact that about the same fraction was extracted by phosphate (Table 1). This amount results in a calculated Si adsorption of ca. 0.1 mmol/m 2, which is much below possible surface saturation of ca. 2–3 mmol/m 2 assuming a mechanism identical to that of phosphate adsorption, i.e., bidentate complexes with two singly coordinated Fe–OH groups at the (110) crystal faces (39). The remaining Si can only be dissolved by continued dissolution of the goethite. Dissolution studies on the other acidic and basic goethites also showed an initial rapid release of Si followed by a congruent dissolution with Fe.
of 10 23 M tend to be slightly larger than those grown in the absence of Si, but the difference is not statistically significant. Most of the crystals lie with their c axis (needle length) parallel to the viewing plane. Since the crystals are short, however, some of them stand upright, i.e., with their c axis perpendicular to the viewing plane, so that their cross section becomes visible (Fig. 4a) (9, 20, 40, 41). This diamond-shaped cross section provides further evidence that the crystals are mainly bounded by (110) faces, as first shown by Schwertmann (37). Measurements on cross sections of 30 such particles standing on end averaged to 7 and 15 nm for needle thickness (a axis) and width (b axis), respectively. With the basic goethites, the increase in MCL a and MCL b and in their respective EM sizes with increasing [Si] i is more pronounced than for the acidic goethites. At no or low [Si] i, the crystals are multidomainic, as seen from SEM (Fig. 4b), resulting in relatively high EM/MCL ratios. With increasing [Si] i, the number of domains per cohesive “bundle” decreased, until blocky crystals consisting of a single domain were formed at [Si] i of 8 3 10 23 M; these were stunted along the c axis but still bounded by 110 faces, as seen from cross sections (Fig. 4c), and 021 faces. This goethite also had the lowest ratios of EM/MCL. A similar blocky morphology was observed for goethites that grew in the presence of Si either from lepidocrocite (31) or from ferrihydrite (10, 42), both at high pH. Hyperfine Properties The hyperfine fields, B hf, of the basic goethites at 4.2 K with Si t/(Si t 1 Fe t) molar ratios of 0, 0.004, 0.010, and 0.017 were uniformly 50.5 T. This value is close to that of 50.6 T for an
Unit Cell Size, Crystal Size, and Crystal Morphology For the basic goethites, no difference in the unit cell size relative to pure goethite could be detected. The same applies to the acidic goethites after correction for apparent line shift due to line broadening. There was only a slight increase in the unit cell b edge length at the highest [Si] i of the acidic goethites but no change in the cell volume (not shown). Both XRD line broadening and electron microscopy supply information on the crystal size. A comparison for both goethites is presented in Table 3. The acidic goethites are very small in all dimensions and the reasonable agreement between MCL a and MCL b on the one hand and the respective TEM figures on the other indicates that the crystals are essentially monodomainic; i.e., the EM/MCL ratios are close to one. A value of this ratio above one implies that the crystals are composed of more than one domain. The crystals grown at [Si] i
FIG. 3. Fraction of dissolved Si and Fe of an acidic goethite in 3 M HCl and a basic goethite in 6 M HCl, both formed at [Si] i 5 10 23 M.
111
PROPERTIES OF PREPARED GOETHITES
TABLE 3 Dimensions of Acidic and Basic Goethites, Calculated from MCLs and Measured on Crystals Observed in TEM and SEM MCL (nm) Initial Si (M)
Si t/Si t 1 Fe t b
a
EM (nm) b
a
b
c
EM a /MCL a
EM b /MCL b
13 (2) 17 (4)
19 (4) 27 (5)
1.2 0.9
1.9 1.1
154 (61) 235 (53) 280 (77) 236 (110)
761 (228) 715 (131) 740 (178) 642 (217)
3.4 3.1 2.9 1.7
5.4 5.9 5.5 2.4
Acid goethites 0 1 3 10 23
0 0.003
6 (1) 8 (1)
7 (2) 16 (3)
7 (1) 7 (3) Basic goethites
0 2 3 10 23 4 3 10 23 8 3 10 23
0 0.004 0.010 0.017
9 (1) 17 (9) 26 (2) 59 (6)
29 (4) 40 (1) 51 (3) 99 (11)
30 (7) c 53 (7) c 75 (17) e 103 (32)
a
( ) indicates one standard deviation. mol/mol in goethite. c #5 particles measured (otherwise $15). b
“ideal” goethite (43). All goethites showed completely magnetically split patterns at 298 K, and the splitting as well as the magnetic sextet fraction decreased as the temperature increased from 298 to 373 K; this is demonstrated in Fig. 5 by spectra for the samples with no Si and with Si t/(Si t 1 Fe t) of 0.017. At a given temperature, magnetic hyperfine splitting increased with Si t/(Si t 1 Fe t) from 0 to 0.010 mol/mol, followed by a slight decrease at a ratio of 0.017 (Fig. 6). At 363 K, a temperature about 40 K below the Neel temperature of an ideal goethite, the Si-free goethite showed only a doublet, whereas at Si t/(Si t 1 Fe t) of 0.010 and 0.017 a weak sextet was still detectable, demonstrating the slightly better crystallinity of these goethites. The increase in B hf with increasing Si t/(Si t 1 Fe t) may be explained as the effect of larger crystal size, which is increasingly counteracted by the effects of diamagnetic Si at higher Si content. The average B hfs of the acid goethites at 4.2 K were 50.4, 50.5, 49.9, 49.9, and 49.8 T for Si t/(Si t 1 Fe t) of 0, 0.003, 0.020, 0.038, and 0.046 mol/mol, showing little change with increasing Si. Small crystal size can explain the lower B hfs compared to the basic goethites (44). The corresponding B hfs at RT decreased with increasing Si: 23.3, 22.4, 15.9, 0, and 9.8 T. The exception to the trend is the sample with 0.038 Si, which also had the smallest B hf at 4.2 K. Only the goethite with no Si showed complete magnetic splitting, whereas the other samples had additional doublets with broadened lines due to superparamagnetic relaxation of part of the sample. DISCUSSION
In the framework of earlier results, two aspects merit discussion: the nature of the Si retained by goethite and its effect on the crystal properties.
Incorporation of Si Considerable amounts of the added Si remained non-OHextractable, leading to Si t/(Si t 1 Fe t) molar ratios up to 0.017 for the basic goethites and up to 0.046 for the acidic goethites. This Si fraction dissolved congruently with Fe, and although such behavior may indicate isomorphous substitution of Fe (as for Al, Mn, and numerous other metals; see (5)), it does not prove such a substitution. Four different positions can be suggested schematically for the association of ions with goethite: external surfaces, internal surfaces (between domains), clusters, and structural substitution. External surfaces are those at the solid/solution interface, and it can be assumed that NaOH dissolves most of this Si fraction by anion exchange and depolymerization. Internal surfaces are the interfaces between closely aggregated crystals or between domains of crystals. Ions can also form clusters/inclusions within the structure, as demonstrated for Fe 31 in corundum (45) and in diaspore (46). A final possibility is that ions substitute within the crystal structure. Substitution by a smaller cation will contract the unit cell, as determined by XRD. Measured unit cell volumes did not differ significantly from those of pure goethites and, hence, gave no indication that Si substituted for Fe in the structure. If substitution took place, the amount was too small to be detected using XRD methods. Furthermore, Si in sixfold coordination with O has an ionic radius that is significantly smaller (0.040 nm) than that of high spin Fe 31 (0.064 nm), which is six-coordinated with O and OH in goethite. In nature, however, six-coordinated Si is seldom found and fourfold coordination is rather the rule; the mineral stishovite, a SiO 2 polymorph, is a rare exception. A difference in crystal radius of 615% is usually taken as the upper limit for the replacement of Fe 31 to avoid strain, and this is much less than the difference in radii between Fe 31 and Si 41.
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GLASAUER, FRIEDL, AND SCHWERTMANN
FIG. 4. EM photos of goethites: (a) Diamond-shaped cross sections of crystals of acid goethite bounded by (110) faces (TEM), scale 5 50 nm; (b) multidomainic basic goethite grown at [Si] i 5 2 z 10 23 (SEM); (c) Cross section of a basic goethite crystal cut perpendicular to [001], showing (110) faces (TEM), scale indicates 100 nm.
Mo¨ssbauer spectroscopy has been used to determine the effect of the substitution of Fe(III) by diamagnetic cations on the magnetic characteristics of goethite (e.g., 47). Magnetic ordering is created by the presence of paramagnetic structural elements and can be disturbed by diamagnetic elements, such as Al and Si. In analogy to Al, Si within the goethite crystal should, therefore, similarly affect the magnetic properties (44).
A reduction of magnetic ordering was suggested by Quin et al. (10) for goethites synthesized at pH 10 –11 in the presence of Si; ordering could not, however, be attributed unequivocally to goethite because ferrihydrite was a major component of their samples. Decreased magnetic ordering due to Si could not be observed for our basic Si goethites. On the contrary, the improved magnetic splitting at RT and higher temperatures (Fig.
PROPERTIES OF PREPARED GOETHITES
113
FIG. 5. Mo¨ssbauer spectra of a basic goethite with zero Si and with Si t/(Si t 1 Fe t) of 0.017 mol/mol recorded at 298, 343, 353, and 363 K.
5) indicates that the Neel temperature was always greater for the Si goethites than for the pure one. The slight reduction at the highest Si may be due to reduced superferromagnetic interaction (20, 48) caused by surface Si, which may have led to a slightly reduced B hf and increased superparamagnetic fraction of this sample. On the other hand, the earlier magnetic breakdown observed at Si t/(Si t 1 Fe t) of 0 and 0.004 mol/mol can be explained by smaller crystal size, in agreement with TEM and XRD. For the acidic goethites, lower magnetic ordering and broader XRD peaks proved that under acidic conditions goethite of lower crystal order and smaller crystal size was formed as the Si i/(Si i 1 Fe i) increased, in agreement with TEM observations. As with the basic goethites, a decrease in the fraction that was magnetically ordered parallels reduced crystal size and superferromagnetic interaction due to Si at the crystal surfaces or between the domains.
In conclusion, excluding a statistical distribution of Si in the structure of goethite, we assume that the Si fraction inaccessible to OH and phosphate, but which dissolves congruently with Fe, may either aggregate the nanosized crystals (acidic goethite) or be located between the domains of the multidomainic crystals (basic goethites). Both may cause a spatial distribution of Si that is sufficiently random to result in congruent dissolution with Fe. Si Effect on Crystal Formation As shown by Schwertmann (37), synthetic goethite crystals are usually bounded by 110 and 021 faces, which are the slowest growing surfaces and therefore determine the morphology. This seems to apply in the case of the acidic and basic Si goethites as well. In other words, although the Si-affected crystals vary considerably in size and shape, their habit ap-
114
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system even at RT. In other words, the supply of the soluble species from the dissolution of two-line ferrihydrite does not seem to limit the rate of nucleation and crystallization. Silicate, which occurs as an anion at alkaline pH, can, however, interfere with goethite formation either by hindering nucleation and crystal growth or by reacting with ferrihydrite and thereby retarding the supply of growth units. This would explain why fewer and larger crystals form. CONCLUSIONS
FIG. 6. Hyperfine fields of basic goethites as a function of Si t/(Si t 1 Fe t) molar ratios, measured at various temperatures.
peared to be rather uniform. This agrees with recent kinetic studies of Weidler (49). When the crystals grew more slowly, e.g., in the presence of Si, they were not only larger, but the proportion of the 021 surface increased at the expense of the 110 surface, i.e., the crystal growth rate in the a and b directions increased at the expense of that in the c direction. Cornell et al. (42) concluded that this may be due to the preferential binding of silicate to the terminal 021 faces, thereby retarding their growth rate. The reasons for large variations in size of the same habit as a function of pH are not clear. It seems to reflect the ratio between rate of nucleation and supply of mono- and dimeric growth units (50). The adsorption of Si on goethite is minimal at low pH because silicate forms protonated or neutral polymers and, in addition, stable complexes with Fe 31, i.e., FeOSi(OH) 321 (51), which reach the maximum formation near pH 2 (2). Both forms of Si lower the activity of silicate anion in solution. Nucleation of goethite will not, therefore, be substantially hindered, and many small crystals will develop. The rate of crystallization will, however, be slow because at low pH the unhydrolyzed Fe species dominate. At high pH (11–12), the fully hydrolyzed species Fe(OH) 42, from which nuclei and crystals form, predominates. This may explain why goethite grows so rapidly in a purely alkaline
There was no evidence for Si substitution in the goethite crystal; instead, the data support a random distribution of Si between strongly aggregated nanocrystals (acidic-precipitated system) or domains of large multidomainic crystals (basicprecipitated system). The different behaviors in acidic and basic solution reflect the influence of pH on the rates of crystal nucleation and growth via the effect on chemical speciation of Si and Fe. These results open up several avenues for further investigations. One question is how solution chemistry determines the size and morphology of goethite particles, i.e., how the mechanism by which Fe units are supplied to the growing crystal reacts to changes in pH or to the presence of growth inhibitors like Si. Also of interest is determining the influence of surface functional groups and surface defects on crystal reactivity, with particular attention to the 021 face. A more applied challenge is how to disperse ferrihydrite particles and the strongly aggregating goethite nanocrystals, which resisted separation by dispersing agents or by ultrasound. ACKNOWLEDGMENTS We thank C. B. Koch for assistance with the Mo¨ssbauer analyses and P. G. Weidler for the analyses of microporosity. This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Schw 90/48-2).
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