Speciation and Crystal Chemistry of Iron(III) Chloride Hydrolyzed in

Apr 21, 2000 - ... (UMR 6635, CNRS-Aix-Marseille III), Europole Méditerranéen de l'Arbois, BP 80, .... S. Thoral, J. Rose, J. M. Garnier, A. van Gee...
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Langmuir 2000, 16, 4726-4731

Speciation and Crystal Chemistry of Iron(III) Chloride Hydrolyzed in the Presence of SiO4 Ligands. 1. An Fe K-Edge EXAFS Study Emmanuel Doelsch,*,† Je´rome Rose,† Armand Masion,† Jean Yves Bottero,† Daniel Nahon,† and Paul M. Bertsch‡ CEREGE (UMR 6635, CNRS-Aix-Marseille III), Europole Me´ diterrane´ en de l’Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France, and Savannah River Ecology Laboratory, AACES, University of Georgia, PO Drawer E, Aiken, South Carolina 29802 Received October 19, 1999. In Final Form: February 1, 2000 The hydrolysis of Fe-Si systems with Si/Fe ratios between 0 and 4 leads to the formation of poorly crystalline or, more frequently, of long-range disorganized precipitates. The increase of Si/Fe molar ratios results in an dramatic change of Fe polymerization. The formation of double and single corner-sharing Fe linkages is reduced compared to pure Fe hydrolysis products. The growth regime depends on the Si concentration in the system. Three-dimensional and two-dimensional growth of Fe colloids occurs at low and high Si/Fe ratios, respectively, systems with Si/Fe ratios around 1 representing a crossover between these two regimes. Though Si neighbors cannot be detected unequivocally by Fe K-edge EXAFS, their presence in the close environment of Fe atoms is evident from the change in Fe speciation.

Introduction The hydrolysis and condensation mechanisms of Fe(III) in aqueous solutions have received considerable attention in the past 10 years because of their importance in a variety of fields such as catalysis,1,2 coagulation in water treatment,3,4 and soil sciences.5-7 The amorphous colloidal phases are of special interest due to their enhanced capability of adsorption and transport of organic and inorganic pollutants resulting from their high specific surface area and potentially high number of sorption sites.8-10 The nature and the structure of the solid phase strongly depend on the nature and concentration of the ligands present during the hydrolysis. For instance the hydrolysis of iron(III) chloride and iron(III) nitrate leads rapidly to akaganeite (β-FeOOH) and slowly to ferrihydrite and goethite (R-FeOOH), respectively. For these two iron salts, the nucleation and growth mechanisms have been thor* Corresponding author. Phone: (+33) 442 97 15 43. Fax: (+33) 442 97 15 59. E-mail: [email protected]. † CEREGE. ‡ University of Georgia. (1) Monte, F. d.; Morales, M. P.; Levy, D.; Fernandez, A.; Ocana, M.; Roig, A.; Molins, E.; O’Grady, K.; Serna, C. J. Langmuir 1997, 13, 36273634. (2) Ennas, G.; Musinu, A.; Piccaluga, G.; Zedda, D.; Gatteshi, D.; Sangregorio, C.; Stanger, J. L.; Concas, G.; Spano, G. Chem. Mater. 1998, 10, 495-502. (3) Vilge-Ritter, A.; Rose, J.; Masion, A.; Bottero, J.-Y.; Laine, J.-M. Colloids Surf., A 1999, 147, 297-308. (4) Lefebvre, E.; Legube, B. Water Res. 1993, 27, 433-447. (5) Iron in soils and clays minerals; Stucki, J. W., Goodman, B. A., Schwertmann, U., Eds.; NATO ASI Series; D. Reitel Publishing Co.: Dordrecht, The Netherlands, 1985; p 893. (6) Schwertmann, U.; Cornell, R. M. Iron oxides in the laboratory. Preparation and characterization; VCH: New York, 1991. (7) Cornell, R. M.; Schwertmann, U. The iron oxides-structure, properties, reactions, occurrence and uses; VCH: New York, 1996. (8) Bottero, J. Y.; Arnaud, M.; Villieras, F.; Michot, L. J.; Donato, P. D.; Franc¸ ois, M. J. Colloid Interface Sci. 1993, 159, 45-52. (9) Buffle, J.; Leeuwen, H. V. Environmental particles. In Environmental analytical and physical chemistry series; Lewis Publishers: Boca Raton, FL, 1991; Vol 1, p 554. (10) Buffle, J.; Leeuwen, H. V. Environmental particles. In Environmental analytical and physical chemistry series; Lewis Publishers: Boca Raton, FL, 1993; Vol 2, p 426.

oughly studied mainly by SAXS and EXAFS.11-16 In the case of chloride, the polymerization of Fe follows a set pathway including the formation of an intermediate polycation “Fe24” whose local structure is that of akaganeite,12 whereas the growth of iron nitrate phases involves a variety of different subunits.15-18 Complexing ligands such as PO4 ions or natural organic matter strongly limit the polymerization of iron(III) by occupying its growth sites, dimers and trimers typically being the largest Fe species.3,19-21 The potentially strongly complexing silicate ions are of great relevance to environmental issues22 and material sciences such as membrane fouling23 and steels, videotape, corrosion, etc.24 Numerous studies concerning natural or synthetic samples25-35 have examined the influence of Si on the (11) Combes, J. M.; Manceau, A.; Calas, G.; Bottero, J. Y. Geochim. Cosmochim. Acta 1988, 53, 583-594. (12) Bottero, J. Y.; Manceau, A.; Villieras, F.; Tchoubar, D. Langmuir 1994, 10, 316-319. (13) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398-402. (14) Tchoubar, D.; Bottero, J. Y. C. R. Acad. Sci. Paris, Ser. 2a 1996, 322, 523-534. (15) Bottero, J. Y.; Tchoubar, D.; Arnaud, M.; Quienne, P. Langmuir 1991, 7, 1365-1369. (16) Rose, J.; Manceau, A.; Masion, A.; Bottero, J. Y. Langmuir 1997, 13, 3240-3246. (17) Schwertmann, U.; Friedl, J.; Pfab, G. J. Solid State Chem. 1996, 126, 336. (18) Schwertmann, U.; Friedl, J.; Stanjek, H. J. Colloid Interface Sci. 1999, 209, 215-223. (19) Rose, J.; Flank, A. M.; Masion, A.; Bottero, J. Y.; Garcia, F. Langmuir 1997, 12, 6701-6707. (20) Rose, J.; Flank, A. M.; Masion, A.; Bottero, J. Y.; Elmerich, P. Langmuir 1997, 13, 1827-1834. (21) Rose, J.; Vilge, A.; Olivie-Lauquet, G.; Masion, A.; Frechou, C.; Bottero, J.-Y. Colloids Surf., A 1998, 136, 11-19. (22) Iler, R. K. The chemistry of silica. Solubility, Polymerisation, Colloid and Surface Properties and Biochemistry; Wiley-Interscience: New York, 1979. (23) Khatib, K.; Rose, J.; Barres, O.; Stone, W.; Bottreo, J. Y.; Anselme, C. J. Membr. Sci. 1997, 130, 53-62. (24) Manceau, A.; Ildefonse, P.; Hazemann, J. L.; Flank, A. M.; Gallup, D. Clays Clay Miner. 1995, 43, 304-317. (25) Schwertmann, U.; Thalmann, H. Clay Miner. 1976, 11, 189199. (26) Zahurul, K. Clays Clay Miner. 1984, 32, 181-184. (27) Deng, Y. Water Res. 1997, 31, 1347-1354.

10.1021/la991378h CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000

Fe(III) Hydrolyzed in the Presence of SiO4

crystallization of Fe oxides or oxide hydroxides, i.e., its influence on phases which underwent heating and/or aging. It has been shown that the presence of Si in solution plays an important role in this process by affecting the nature, the shape, and the size of the Fe precipitates even at low levels of silica. For example, the presence of Si inhibits the formation of goethite and favors the formation of ferrihydrite.25,29 Moreover, the presence of Si delays the transformation of the metastable ferrihydrite into more crystalline products.26,28-31,35 Despite all these studies, the major question about the precise localization of Si in the FeOOH precipitates remains ambiguous.32 Surprisingly little is known about the nucleation and growth mechanisms of Fe-Si systems during the first steps of hydrolysis which precede any crystallization process. The present work aims at a better understanding of these phenomena by studying the nanostructure of freshly precipitated Fe(III)/Si phases at various Si/Fe molar ratios and pH values.

Langmuir, Vol. 16, No. 10, 2000 4727 Table 1. Crystallographic and EXAFS Parameters of Reference Minerals Ra (Å)

Nb

σc

Ld (Å-2)

∆Ee

Qf

1.0

-2.5

0.02

EXAFS

Fe-Fe Shell 3.08 6 0.08

andradite

EXAFS XRD

Fe-O Shell 2.02 6 0.07 2.02 6

1.0

5.22

andradite

EXAFS XRD

Fe-Si Shell 3.37 6 0.05 3.36 6

0.27

4.15

andradite

EXAFS XRD

Fe-Ca Shell 3.40 6 0.05 3.36 6

0.1

0.3

γ(FeOOH)

0.01

0.01

a R: distance between the two atoms of each atomic pair. b N: number of atoms in shell of iron. c σ: Debye-Waller factor (disorder parameter; Å2). d L corresponds to the first term in the λ(k) expression (λ(k) ) 2k/L). e ∆E: separation between absorption edges for mineral and Fe foil. f Q ) ∑[(k3χtheo) - (k3χexp)]2/(k3χexp)2.

Materials and Methods Materials. Samples with Si/Fe molar ratios ranging from 0 to 4 were prepared by simultaneously adding 5.406 g of FeCl3‚ 6H2O and the appropriate amount of tetraethyl orthosilicate (TEOS) to 50 mL of bidistilled water and 10 mL of 1 M HCl in a 250 mL polyethylene flask. The volume was adjusted to 100 mL with bidistilled water to obtain a final Fe concentration of 0.2 M. The pH was set at 3, 5, 7, and 10 by addition of 10 M sodium hydroxide solution under vigorous stirring and achieved within 30 min. This short time is needed because the first step of the hydrolysis of TEOS leads to the formation of Si(OH)4 and is followed by the polymerization of these monomeric species.36,37 Nevertheless, it has been shown that a high H2O/Si ratio resulted in an almost complete conversion of Si(OC2H5)4 into monomeric Si(OH)4 before significant condensation occurred.36 Therefore it can be assumed that, in the present case, monomeric Si(OH)4 is allowed to react with iron during hydrolysis. Methods. Precipitates were centrifuged at 40 000 rpm during 2 h. The supernatants obtained after centrifugation were analyzed for Fe and Si by ICP-AES with an Jobin-Yvon JY38+. The centrifugates were freeze-dried and outgassed at 10-3 Torr. The solids were ground to a fine powder and analyzed by X-ray diffraction with a Philips PW 3710 X-ray diffractometer using a Co KR radiation at 40 kV and 40 mA (a counting time of 12 s per 0.02° step was used for the 2θ range 3.5-78°). Selected X-ray diffraction peaks were then fitted with the PROFIT 1.0 program (Philips Analytical). Fe K-edge EXAFS measurements were performed in the transmission mode at the following synchrotron sources: (i) beam line D42, DCI, ring at 1.85 GeV and 200 mA (LURE, Orsay, France); (ii) beam line X23A2, NSLS, ring at 2.5 GeV and 300 mA (BNL, Upton, NY). EXAFS data reduction38 was accomplished according to a procedure previously described.39 k3χFe(k) spectra were Fourier transformed from k to R space by using a Kaiser apodization (28) Cornell, R. M.; Giovanolli, R.; Schindler, P. W. Clays Clay Miner. 1987, 35, 21-28. (29) Cornell, R. M.; Giovanoli, R. J. Chem. Soc. 1987, 413-414. (30) Parfitt, R. L.; Gaast, S. J. V. d.; Childs, C. W. Clays Clay Miner. 1992, 40, 675-684. (31) Mayer, T. D.; Jarrel, W. M. Water Res. 1996, 30, 1208-1214. (32) Glasauer, S.; Friedl, J.; Schwertmann, U. J. Colloid Interface Sci. 1999, 216, 106-115. (33) Anderson, P. R.; Benjamin, M. M. Environ. Sci. Technol. 1985, 19, 1048-1053. (34) Kandori, K.; Uchida, S.; Kataoka, S.; Ishikawa, T. J. Mater. Sci. 1992, 27, 719-728. (35) Vempati, R. K.; Loeppert, R. H. Clays Clay Miner. 1989, 37, 273-279. (36) Pouxviel, J. C.; Boilot, J. P.; Beloeil, J. C.; Lallemand, J. Y. J. Non-Cryst. Solids 1987, 89, 345-360. (37) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990. (38) Michalowicz, A. J. Phys. 4 1997, C2, 235. (39) Manceau, A.; Calas, G. Clay Miner. 1986, 21, 341-360.

Figure 1. X-ray diagrams of Si/Fe pH ) 3 samples. window (t ) 2.5).40 This procedure results in radial distribution functions (RDF) uncorrected from phase shift functions; i.e., RDF peaks are displaced from crystallographic distances by about 0.3-0.4 Å. The contributions of the various shell were signaled out by a back Fourier transform (including a removal of the Kaiser window contribution), from real to k space. These partial EXAFS functions were then least-squares fitted by a theoretical function in order to determine the structural and chemical parameters: Rj (distances between neighbors), Nj (number of neighbors), and the nature of atomic neighbors in the jth shell around Fe. To fit partial EXAFS spectra by theoretical functions, amplitude (Fi-j) and phase shift functions (φi-j) for the different i-j pairs are required. FFe-Fe(k,Rj) and (φFe-Fe) were determined experimentally from γ-FeOOH. FFe-O(k,Rj) and (φFe-O) were experimentally determined from andradite, an iron silicate, Ca3Fe2Si3O12. For FFe-Si(k,Rj) and (φFe-Si) theoretical functions were used41 due to the lack of convenient references. Their validity was ascertained with pure and well-crystallized iron silicate reference (andradite42). These references were used to determine the electron mean free path length (λ) which was then kept fixed for our unknown samples (Table 1). The uncertainties on R and N are (0.01 Å and 10%, respectively.43

Results Chemical Analysis. For all the samples, the introduced Fe and Si was almost completely precipitated during hydrolysis; Fe and Si contents in the supernatant were typically lower than 0.5% and 8% of the initial amount, respectively. This translates to Si/Fe ratios in the precipitated phases close to the initial ones. (40) Manceau, A. M.; C. J. Phys. Chem. Miner. 1988, 15, 283-295. (41) Ankudinov, A. L.; Rehr, J. J. Phys. Rev. B 1997, 56, R1712. (42) Hazen, R. M.; Finger, L. W. Amer. Mineral. 1989, 74, 352-359. (43) Teo, B. K. EXAFS: basic principle and data analysis; SpringerVerlag: Berlin, 1986.

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Figure 2. Relationship between Si/Fe molar ratios and the product of the intensity and width at half-maximum of the (110) X-ray diffraction peak for pH ) 3 series samples.

X-ray Diffraction. At pH ) 3, the spectra of the samples display peaks resulting from the presence of halite, which is a residue of the synthesis and broad lines resulting from the presence of akaganeite (Figure 1).

Doelsch et al.

Figure 3. X-ray diagrams of Si/Fe ) 1 samples: /, halite; A, akaganeite.

These broad lines are indicative of small crystals6 and/or of poor order. We monitored the evolution of the (110) reflection at 0.75 nm which is one of the most intense line diffraction for akaganeite. When I × (fwhm) (product of intensity and full width at half-maximum, with correction

Figure 4. Fe radial distribution function (uncorrected for phase shift functions).

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Table 2. Structural Parameters for Fe (Backscatterer in the First Coordination Sphere) Contributions Deduced from EXAFS Analysis for Si/Fe ) 0 and Si/Fe ) 4 Samplesa Fe-O1 shell

Fe-O2 shell

R window (Å)

R (Å)

N

pH ) 3 pH ) 5 pH ) 7 pH ) 10 pH ) 10

1.04-2.05 0.98-1.98 1.25-1.96 1.25-1.96 1.25-1.96

1.99 1.99 2.02 2.03 2.01

5.26 5.32 3.25 3.05 5.07

Sample Si/Fe ) 0 0.11 0.11 0.06 1.89 0.81 0.06 1.89 0.78 0.11

pH ) 3 pH ) 5 pH ) 7 pH ) 10

0.96-2.00 0.98-2.00 0.98-2.00 0.98-2.00

2.02 2.00 2.00 2.01

4.68 5.28 5.00 3.63

Sample Si/Fe ) 4 0.09 1.88 1.09 0.09 1.85 0.72 0.09 1.85 1.00 0.07 1.87 1.25

a

R (Å)

σ

N

Fe-O3 shell R (Å)

σ

0.02 0.02

2.18 2.21 2.25

σ

10-3Q

0.03 0.02 0.11

1.22 4.80 1.03 0.47 15.17

N

0.72 0.41 0.92

0.06 0.07 0.08 0.07

1.63 1.61 1.41 1.12

Cf. Table 1 for explanations of EXAFS parameters.

Table 3. Structural Parameters for Fe (Backscatterer in the Second Coordination Sphere) Contributions Deduced from EXAFS Analysis for Si/Fe e 0.5a Fe-Fe1 shell R window (Å)

R (Å)

N

Fe-Fe2 shell σ

R (Å)

N

Fe-Fe3 shell σ

Fe-Fe4 shell

R (Å)

N

σ

R (Å)

N

σ

Q

3.43 3.44 3.43 3.45

2.41 1.67 0.90 1.52

0.10 0.10 0.10 0.10

3.77 3.89 3.83 3.88

0.61 0.89 0.65 0.50

0.11 0.12 0.11 0.11

0.08 0.01 0.02 0.02

Sample Si/Fe ) 0

pH ) 3 pH ) 5 pH ) 7 pH ) 10

2.26-3.54 2.21-3.57 2.25-3.48 2.25-3.53

3.02 2.97 2.99 2.98

2.31 1.26 1.68 1.55

0.11 0.06 0.09 0.08

3.11 3.11 3.11

pH ) 3 pH ) 5 pH ) 7 pH ) 10

2.20-3.58 2.20-3.56 2.20-3.58 2.15-3.58

2.95 2.95 2.96 2.97

0.91 0.77 1.17 1.59

0.05 0.04 0.08 0.09

Sample Si/Fe ) 0.25 3.08 0.76 0.03 3.08 0.84 0.02 3.10 1.01 0.06 3.11 0.84 0.06

3.45 3.48 3.44 3.43

0.91 0.40 1.05 1.26

0.10 0.05 0.09 0.11

3.80 3.87 3.90 3.90

0.81 0.50 0.41 0.50

0.10 0.11 0.11 0.11

0.04 0.05 0.05 0.02

pH ) 3 pH ) 5 pH ) 7 pH ) 10

2.20-3.54 2.20-3.54 2.20-3.54 2.20-3.52

2.96 2.96 2.96 2.95

1.02 1.06 0.82 0.95

0.06 0.08 0.05 0.06

Sample Si/Fe ) 0.5 3.09 0.82 0.04 3.09 0.82 0.06 3.10 0.79 0.03 3.09 0.75 0.03

3.43 3.43 3.48 3.45

1.09 0.40 0.49 0.80

0.08 0.09 0.08 0.09

3.83 3.87 3.84

0.56 1.00 0.34

0.11 0.11 0.11

0.07 0.02 0.05 0.08

a

0.61 0.71 0.46

0.03 0.05 0.02

Cf. Table 1 for explanations of EXAFS parameters.

for background) of the (110) peak is plotted as a function of Si/Fe molar ratios, a clear trend is observed for the pH ) 3 samples (Figure 2). First, we observed a strong decrease of the value of the product I × (fwhm) from Si/Fe ) 0 to Si/Fe ) 0.25, followed by a less steep but steady decrease from Si/Fe ) 0.25 to Si/Fe ) 4. The samples at pH ) 5 display a broad peak with very low in intensity (Figure 3) which result from the presence of akaganeite, which is a very minor phase in these samples. Whatever the Si/Fe molar ratio, all the samples at pH ) 7 and 10 are totally amorphous to X-ray diffraction as shown for Si/Fe ) 1 samples (Figure 3). EXAFS Spectroscopy. Radial Distribution Functions (RDF). The radial distribution functions (RDF) obtained from Fourier transforming the reduced EXAFS data consist of two peaks resulting from Fe-backscatterer interactions (Figure 4). The first peak at ≈1.5 Å (RDF distances uncorrected for phase shift function) corresponds to the contribution of O, OH, H2O, or Cl in the first ligand sphere surrounding Fe atoms.12 The second peak between 2.2 and 3.6 Å indicates the presence of atoms in the nextnearest shells around iron. For each pH, the shape of this second peak evolves from very broad at low Si/Fe (e0.5) to a sharper line for Si/Fe g 1, thus indicating a modification of the local environment of Fe as the Si content increases. Analysis of the Ligand Coordination Shell. The contribution of the first coordination shell was isolated and back-transformed to k space to determine structural parameters. EXAFS structural parameters used for each calculated spectrum are listed in Table 2. The structural

parameters for Fe-O atomic pairs were calculated for only two series Si/Fe ) 0 and Si/Fe ) 4. When more than one beat node was present, experimental EXAFS spectra were fitted using two or three atomic shells. Moreover, the calculation of EXAFS spectra with three atomic shells significantly improves the quality of the least-squares fit (Table 2). In our samples, the first coordination shell only consists of O backscatterers, contrarily to less hydrolyzed Fe systems where it includes up to 2 Cl atoms.12,19 Analysis of the Next-Nearest Coordination Shells. A previous study24 illustrated the difficulty to detect neighboring Si atoms by Fe K-edge EXAFS: including a Si shell in the calculation was not necessary to obtain a good spectral fit for various mineral reference phases (nontronite, hisingerite). Furthermore, the authors demonstrated that adding or omitting the Si shell did not affect the Fe-Fe distances derived from the spectral fitting.24 In the present case also, the quality of the fit was not affected whether a Si shell was used in the calculations. Therefore, only Fe backscatterers were considered in the analysis of the second and third coordination shell. As suggested by the shoulders on the second peak of the RDF, especially at low Si/Fe (see arrows on Figure 4), several Fe-Fe contributions had to be taken into account in the modeling of the partial EXAFS spectra corresponding to this peak. Depending on the sample, the calculated EXAFS spectra included 3 or 4 Fe-Fe contributions in order to have an accurate fit to the experimental signal (Table 3). Attempts to model with less than 4 Fe-Fe contributions failed because of beat node, high σ value, and amplitude problems (Figure 5). The 4 Fe-Fe con-

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Figure 5. Comparison of partial EXAFS spectra corresponding to the second and third coordination spheres around iron.

tributions correspond to edge-sharing iron octahedra at approximately 3.00 and 3.10 Å (Fe-Fe1 and Fe-Fe2, respectively), double corner-sharing iron octahedra at approximately 3.45 Å (Fe-Fe3), and single corner-sharing iron octahedra at approximately 3.85 Å (Fe-Fe4) (Table 3). For all the samples (except Si/Fe ) 0, pH ) 3), the Fe-Fe1, Fe-Fe2, and Fe-Fe3 contributions were detected (Table 3).

Doelsch et al.

Figure 6. Comparison between the partial EXAFS curve of Si/Fe ) 0 pH ) 3 (corrresponding to the 2.26-3.54 Å region of the RDF) and that of akaganeite.

Discussion The absence of akaganeite lines on the XRD patterns for pH > 5 is consistent with previous reports on the synthesis of this iron oxide from ferric chloride; at pH g 5, the chloride ions, which are thought to stabilize the structure,12 are displaced by OH- ions.6 Modifications of nucleation and growth mechanisms of the solids formed at pH 3 in the presence of Si are evident from the evolution of the product I × (fwhm) (Figure 2). This product allows one to quantify the presence and the crystallinity of the phase studied. The decrease of I × (fwhm) versus the Si/ Fe molar ratios (Figure 2) clearly demonstrates that the Si content in the samples affects the crystallinity of the final products. Moreover, we observe a progressive loss of crystallinity with increasing Si concentration. Contrary to previous studies,25,29 no ferrihydrite was detected by X-ray diffraction. This is probably due to the preparation of samples with a short aging and without heating. A closer insight into the nature of the precipitates and their modifications with pH and Si concentration is provided by the EXAFS structural data. In the absence of Si, all four Fe-Fe linkage types (edge and single and double corner) were detected (Table 3). These results contrast with previous studies of the hydrolysis of iron(III) chloride reporting no single corner linkages between Fe octahedra.11,12 Furthermore, the structure of akaganeite does not include single corner linkage. However, to ascertain the relevance of this Fe-Fe4 contribution, the Si/Fe ) 0 pH ) 3 sample, which contains akaganeite, was compared to the EXAFS spectrum of the well-crystallized akaganeite phase (Figure 6): the presence of beat nodes at high k (11 < k < 14 Å-1) indicates the presence of an additional contribution and thus justifies the use of the Fe-Fe4 shell. Attribution to purely multiple scattering

Figure 7. Evolution of total number of iron neighbors.

effects can also be excluded since it has been shown recently that the multiple scattering path Fe-O-Fe (with an effective distance close to the Fe-Fe4 contribution) only exists in a single corner-sharing geometry.16 Thus, even if multiple scattering may contribute to the EXAFS signal, it supports the presence of an single corner linkage. The unexpected presence of single corner linkages in our Si/Fe ) 0 samples is probably due to the sample preparation procedure. In previous studies,11,12 the samples were obtained by partial and slow hydrolysis of the Fe salt. In the present case, the addition of base occurred at a higher rate to avoid Si polymerization before the desired pH value is reached.36 This rapid hydrolysis results in less well organized solid phases, particularly since they were not allowed to age. The presence of Si largely affects the local structure of the precipitates. Although Fe K-edge EXAFS does not allow univocal detection of neighboring Si atoms because of the large difference in atomic number,24 their influence on the Fe speciation in the resulting solids is evident. The polymerization of Fe is very sensitive to the Si concentration but does not follow a linear trend. The total number of Fe neighbors decreases sharply at a low Si/Fe ratio content (Si/Fe ) 0.25), remains low for intermediate Si/ Fe ratios (0.5 and 1.0), and increases again for higher Si/Fe ratios (Si/Fe > 1) (see trend arrows on Figure 7). However, even at the highest Si concentration, the number

Fe(III) Hydrolyzed in the Presence of SiO4

Langmuir, Vol. 16, No. 10, 2000 4731

Table 4. Structural Parameters for Fe (Backscatterer in the Second Coordination Sphere) Contributions Deduced from EXAFS Analysis for Si/Fe g 1a Fe-Fe1 shell R (Å)

R (Å)

N

Fe-Fe2 shell σ

R (Å)

N

Fe-Fe3 shell σ

Fe-Fe4 shell

R (Å)

N

σ

R (Å)

N

3.47 3.48 3.43 3.41

0.45 0.72 0.24 0.92

0.06 0.11 0.04 0.10

0.08 0.05 0.04 0.08

σ

Q

pH ) 3 pH ) 5 pH ) 7 pH ) 10

2.20-3.50 2.20-3.47 2.20-3.58 2.20-3.47

2.94 2.96 2.95 3.00

0.62 0.73 0.84 1.83

0.02 0.05 0.10 0.10

Sample Si/Fe ) 1 3.08 0.93 0.02 3.10 1.09 0.05 3.07 1.41 0.09 3.13 0.47 0.05

pH ) 3 pH ) 5 pH ) 7 pH ) 10

2.20-3.50 2.20-3.45 2.20-3.47 2.20-3.54

2.92 2.93 2.93 2.94

0.39 0.41 0.50 0.47

0.02 0.05 0.03 0.02

3.07 3.08 3.09 3.08

Sample Si/Fe ) 2 1.85 0.07 1.88 0.09 1.42 0.02 1.24 0.05

3.45 3.44 3.47 3.50

0.73 1.01 0.59 0.54

0.10 0.10 0.10 0.11

0.04 0.07 0.05 0.07

pH ) 3 pH ) 5 pH ) 7 pH ) 10

2.20-3.54 2.20-3.54 2.20-3.54 2.20-3.34

3.00 3.02 2.97 2.96

1.86 2.00 1.93 1.33

0.09 0.10 0.11 0.11

3.12 3.14 3.11 3.10

Sample Si/Fe ) 4 0.95 0.06 0.60 0.04 1.73 0.08 1.87 0.09

3.44 3.49 3.44 3.45

0.76 0.63 1.03 0 .13

0.08 0.11 0.11 0.02

0.04 0.04 0.02 0 .01

a

Cf. Table 1 for explanations of EXAFS parameters.

Figure 8. Evolution of number of iron neighbors.

of Fe neighbors (NSi/Fe ) 4,pH ) 3 ) 3.5) does not reach the number determined for the Si/Fe ) 0 samples (NSi/Fe ) 0, pH ) 3 ) 5.3) or comparable systems in the literature,11,16 thus showing the inhibiting effect of Si on the polymerization of Fe. Nevertheless, this inhibition is significantly weaker than for PO4 ligands, which, from the lowest P/Fe ratios, blocked the polymerization at the Fe dimer stage at low pH < 3.19,20 The evolution of the number of neighbors by type of linkage follows the same trend at each experimental pH (Figure 8): (i) The number of edge sharing linkages (at ≈3 Å) follows the same trend as the total number of neighbors, i.e., decrease from Si/Fe ) 0 to Si/Fe ) 0.5 followed by an increase for Si/Fe > 1. (ii) The number of double corner linkages (at ≈3.4 Å) decreases from Si/Fe ) 0 to Si/Fe ) 1 and varies little for higher Si concentrations. (iii) The number of single corner linkages (at ≈3.8 Å) decreases with increasing Si content (number of single corner linkages ) 0 for Si/Fe > 0.5). This suggests the presence of two growth regimes for Fe species in Fe-Si systems depending on the Si concentration. At low Si/Fe, because of the limited number of available SiO4 ligands, the growth of iron species occurs by edge-but also by corner-sharing linkages (number of single corner + double corner linkages > 1 in most samples) (Table 3). These corner-sharing linkages are a prerequisite to the formation of three-dimensional structures such as goethite and akaganeite (double corner) or lepidocrocite

(single corner). Increasing the Si/Fe ratio to 1 results in decreased complexation of Fe growth sites by SiO4 ligands and thus in less polymerized Fe species, as shown by the decrease of the number of neighbors for all types of linkages (Figures 7 and 8; Table 4). For Si/Fe > 1, the inhibition of corner-sharing linkages by Si ligands remains strong and the increased Fe polymerization mainly occurs through edge-sharing linkages (Figure 8; Table 4), which strongly suggests a two-dimensional growth of the Fe species. This enhanced polymerization seems contradictory to the high Si concentration. However, since Si is in excess of Fe, the formation of polymeric Si species despite the fast hydrolysis conditions may be occurring, thus resulting in a reduced number of complexing SiOH4 groups and consequently allowing the formation of larger Fe species. A remarkable point is that, at these high Si/Fe ratios, the proportion of edge linkages exceeds that of the Si/Fe ) 0 systems which supports the hypothesis of a twodimensional growth. Conclusion The presence of Si ligands modifies the hydrolysis pathways of Fe(III) chloride. The precipitated phases are poorly crystalline or amorphous. Level of Fe polymerization (number of Fe-Fe contributions) and growth regime are strongly depending on the Si/Fe ratio of the system. Three-dimensional and two-dimensional growth of Fe species occurs for low and high Si/Fe ratios, respectively. A crossover between these two growth regimes is observed for Si/Fe ratios around 1, where the polymerization of Fe reaches a minimum. Acknowledgment. This work was partially supported by CNRS-NSF collaboration agreement No. 7383 and award number DE-FC09-96SR-18546 from the US Department of Energy to the University of Georgia Research Foundation. The authors wish to thank A. Traverse and F. Bouamrane who are in charge of the beam line D42 of DCI (LURE, Orsay, France) and J. Woicik and Z. Fu who are in charge of the beam line X23A2 of NSLS (Brookhaven National Lab., supported by US DOE) for their helpful and kind assistance. LA991378H