Nucleation and Growth Mechanisms of Iron Oxyhydroxides in the

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Langmuir 1997, 13, 3886-3889

Nucleation and Growth Mechanisms of Iron Oxyhydroxides in the Presence of PO4 Ions. 4. Structure of the Aggregates Armand Masion,*,† Je´roˆme Rose,† Jean-Yves Bottero,† Denise Tchoubar,‡ and Franc¸ ois Garcia§ Laboratoire des Ge´ osciences de l’Environnement URA 132 CNRS, CEREGE, Europole Me´ diterrane´ en de l’Arbois, BP 80, 13545 Aix en Provence Cedex 04, France, Centre de Recherche sur la Matie` re Divise´ e, Laboratoire de Cristallographie UMR 812, Universite´ d’Orle´ ans, BP 6706, 45067 Orle´ ans Cedex 2, France, and Elf-Atochem, Centre de Recherche Rhoˆ ne-Alpes, rue Henri Moissan, BP 69, 69310 Pierre-Be´ nite, France Received January 13, 1997. In Final Form: April 23, 1997X The structure of the aggregates formed by partial hydrolysis of Fe-PO4 solutions was investigated at the semilocal scale by small angle X-ray scattering. Aging has no effect on the structure of Fe-PO4 colloids, except for a P/Fe molar ratio of 0.2 and an hydrolysis ratio R ) 1.0 where aggregation of the subunits occurred between 18 and 24 months of aging. At P/Fe ) 0.2, the fractal dimension Df of the aggregates (approximately 100 Å) increases from 2.30 to 2.85 with increasing R. For P/Fe ) 0.5, larger and more open aggregates are formed and the hydrolysis ratio has little influence on the semilocal structure. The P-Fe distance and the initial stoichoimetry control the structure of the resulting aggregates.

Introduction The hydrolysis products of metal ions such as Al or Fe have received considerable attention since they act as support phases for the transport of organic and inorganic substances. The ability of such phases for the transport depends on their structure. Many factors such as hydrolysis ratio R ([OH]/[metal]), aging, temperature, nature of the counterion of the metal salt, presence, nature, and concentration of ligands have a strong influence on the structure of formed phases. In the case of Al(III), the hydrolysis in the absence of ligands leads to the formation of an amorphous phase, the subunit of which being the Al13 polycation.1-3 The structure, described in terms of fractal dimension Df, was shown to become more dense with increasing hydrolysis ratio.2-5 Upon aging, the aggregates formed at R > 2.8 undergo a solid state transformation to yield crystalline Al(OH)3,6 whereas the phases formed at lower R remain amorphous.7 The presence of ligands strongly influences structure and fate of the hydrolysis products. Inorganic ligands such as phosphate or silicate hinder or inhibit the crystallization.8,9 The same effect is also observed with organic ligands,9-11 * To whom correspondence should be addressed. Phone: (33) 442 97 15 34. Fax: (33) 442 97 15 40. E-mail: [email protected]. † Laboratoire des Ge ´ osciences de l’Environnement URA 132 CNRS. ‡ Centre de Recherche sur la Matie ` re Divise´e, Laboratoire de Cristallographie UMR 812, Universite´ d'Orle´ans. § Elf-Atochem, Centre de Recherche Rho ˆ ne-Alpes. X Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Bottero, J. Y.; Tchoubar, D.; Cases, J. M.; Fiessinger, F. J. Phys. Chem. 1982, 86, 3667-3673. (2) Kumru, S. S.; Bale, H. D. J. Appl. Crystallogr. 1994, 27, 682692. (3) Bertsch, P. M.; Parker, D. R. In The environmental chemistry of aluminum; Sposito, G., Ed.; CRC Press: Boca Ratan, FL, 1996; pp 117168. (4) Axelos, M.; Tchoubar, D.; Bottero, J. Y.; Fiessinger, F. J. Phys. (Paris) 1985, 46, 1587-1593. (5) Bottero, J. Y.; Axelos, M.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117, 47-57. (6) Bradley, S. M.; Kydd, R. A.; Howe, R. F. J. Colloid Interface Sci. 1993, 159, 405-412. (7) Hsu, P. H.; Bates, T. F. Miner. Mag. 1964, 33, 749-768. (8) Hsu, P. H. Soil Sci. 1979, 127, 219-226. (9) Violante, A.; Huang, P. M. Clays Clay Miner. 1985, 33, 181-192. (10) Violante, A.; Violante, P. Clays Clay Miner. 1980, 28, 425-434.

S0743-7463(97)00041-3 CCC: $14.00

even after several years of aging.12 This hindrance of the crystallization is foreseeable from the structure of the fresh phases. The aggregates formed in the presence of organic acids are very dense (Df > 2.3) and consist essentially of uncondensed monomers,13 since the ligands have the double effect of hindering the polymerization of Al14-16 and depolymerizing polynuclear Al species.13,17 The case of the Fe(III) hydrolysis is similar in some respect to the one of Al(III). The fresh hydrolysis products are amorphous to X-ray diffraction. The nature of the counterion is known to influence the structure of the crystalline phases obtained upon aging. For example, the presence of Cl- ions induces the formation of β-FeOOH whereas R-FeOOH is formed with NO3-.18-20 Electron microscopy has been used to determine size and shape of the particles formed during this process. They consist of spherical clusters (15-40 Å) which aggregate to form rodlike structures.18-22 More recent small angle X-ray scattering (SAXS) studies performed on fresh partially hydrolyzed ferric nitrate and chloride solutions confirm these results.23,24 For hydrolysis ratios R < 2.0, the aggregates are linear. This arrangement of the subunits (11) Wang, M. K.; White, J. L.; Hem, S. L. Clays Clay Miner. 1983, 31, 65-68. (12) Violante, A.; Gianfreda, L.; Violante, P. Clays Clay Miner. 1993, 41, 353-359. (13) Masion, A.; Bottero, J. Y.; Thomas, F.; Tchoubar, D. Langmuir 1994, 10, 4349-4352. (14) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Genevrier, F.; Boudot, D. Environ. Sci. Technol. 1991, 25, 1553-1559. (15) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Montigny, F.; Genevrier, F. Environ. Sci. Technol. 1993, 27, 2511-2516. (16) Masion, A.; Thomas, F.; Bottero, J. Y.; Tchoubar, D.; Tekely, P. J. Non-Cryst. Solids 1994, 171, 191-200. (17) Masion, A.; Thomas, F.; Tchoubar, D.; Bottero, J. Y.; Tekely, P. Langmuir 1994, 10, 4353-4356. (18) Murphy, P. J.; Mosner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 270-283. (19) Murphy, P. J.; Mosner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 284-297. (20) Murphy, P. J.; Mosner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 312-319. (21) Dousma, J.; DeBruyn, P. L. J. Colloid Interface Sci. 1976, 56, 527-539. (22) Dousma, J.; DeBruyn, P. L. J. Colloid Interface Sci. 1978, 64, 154-170. (23) Bottero, J. Y.; Tchoubar, D.; Arnaud, M.; Quienne, P. Langmuir 1991, 7, 1365-1369.

© 1997 American Chemical Society

Structure of Fe-PO4 Colloids

has been attributed to magnetic dipolar interactions which tend to linearize the structure.24 At higher hydrolysis ratios however, the structure of the aggregates is more branched when R increases from 2.0 to 3.0, with Df values raging from 1.7 to 2.0,23,24 which correspond to a clustercluster aggregation mechanism.25 In these SAXS studies, the size of the subunits was found to depend on the nature of the counterion and on R: in ferric chloride solutions, the subunits have a constant size of 16 Å over the whole R range,24 whereas their size increases with R from 7 to 13.5 Å in nitrate solutions.23 This variation in subunit size is very well correlated to the Fe speciation determined by extended X-ray absorption fine structure (EXAFS) spectroscopy during the first steps of hydrolysis. The hydrolysis of ferric chloride follows a well-defined pathway resulting in the formation of a Fe24 polycation,26 contrary to the nitrate salt where the nucleation process appears as rather erratic.27 Similar to the Al(III) hydrolysis, the presence of organic or inorganic complexing ligands has a strong influence on the formed Fe structure, mainly translating to the formation poorly crystalline phases, e.g., refs 28-30. In the case of PO4 ligands, the loss of crystallinity is explained by the speciation of Fe in partially hydrolyzed Fe-PO4 solutions:31-33 the polymerization of Fe in such systems is limited to the edge sharing dimer stage, these dimers being the predominant Fe species for hydrolysis ratios larger than 1.0. The aim of the present paper is to investigate the structure at the semilocal scale of the aggregates formed in these systems by means of SAXS. Materials and Methods Sample Preparation. The partially hydrolyzed Fe-PO4 samples were those used in previous EXAFS and SAXS studies.31-33 Briefly, FeCl3-H3PO4 mixtures at P/Fe molar ratios of 0.2 and 0.5 were base (NaOH) hydrolyzed under vigorous stirring. The samples were analyzed at hydrolysis ratios R of 1.0, 1.5, and 2.0. All samples were aged 18 and 24 months before analysis. Small Angle X-ray Scattering Experiments. The SAXS measurements were performed using the synchrotron facility at the Laboratoire pour l’Utilisation du Rayonnement Electromagnetique (Universite´ de Paris Sud, Orsay, France), beam line D24 of the DCI storage ring (E ) 1.85 GeV, I ) 320 mA). The curves were recorded at λ ) 1.89 Å. Data acquisition time was 5000 s for each sample and the covered Q range was 0.009-0.180 Å-1 for the samples aged 18 months and 0.009-0.750 Å-1 for the samples aged 24 months (connection of the curves obtained at sample-detector distances corresponding to Q ranges of 0.0090.180 and 0.032-0.750 Å-1). Q ) 4π(sin Θ)/λ is the wave vector modulus. Background correction, smoothing, and normalization of the curves were carried out as described previously.33,34 Determination of the Fractal Dimension Df. The fractal dimension Df is the most convenient tool to describe the structure (24) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398-402. (25) Jullien, R.; Botet, R. Aggregation and fractal aggregates; World Scientific: London, 1987. (26) Bottero, J. Y.; Manceau, A.; Villie´ras, F.; Tchoubar, D. Langmuir 1994, 10, 316-319. (27) Rose, J.; Manceau, A.; Masion, A.; Bottero, J. Y. Langmuir, in press. (28) Cornell, R. M.; Schwertmann, U. Clays Clay Miner. 1979, 27, 402-410. (29) Kandori, K.; Uchida, S.; Katoaka, S.; Ishikawa, T. J. Mater. Sci. 1992, 27, 719-728. (30) He, Q. H.; Leppard, G. G.; Paige, C. R.; Snodgrass, W. J. Water Res. 1996, 30, 1345-1352. (31) Rose, J.; Manceau, A.; Bottero, J. Y.; Masion, A.; Garcia, F. Langmuir 1996, 12, 6701-6707. (32) Rose, J.; Flanck, A. M.; Masion, A.; Bottero, J. Y.; Elmerich, P. Langmuir 1997, 13, 1827-1834. (33) Masion, A.; Rose, J.; Bottero, J. Y.; Tchoubar, D.; Elmerich, P. Langmuir, in press. (34) Masion, A.; Tchoubar, D.; Bottero, J. Y.; Thomas, F.; Villie´ras, F. Langmuir 1994, 10, 4344-4348.

Langmuir, Vol. 13, No. 14, 1997 3887

Figure 1. Scattering curves (log-log) for P/Fe ) 0.2, R ) 1.0 at 18 and 24 months of aging. The curves are shifted in intensity for better clarity. of amorphous phases. In our case, it was derived from SAXS measurements. For an aggregate, the scattered intensity is35

I(Q) ) I0(Q) S(Q)

(1)

where I0 is the scattering by the subunits of the aggregate and S(Q) is the interference function describing the arrangement of the subunits within the aggregate. For a fractal aggregate S(Q) scales as Q-Df.35 For our samples, the function I0 corresponds to the computed scattering of the subunits whose nature and proportion have been determined by modeling of the outermost part of the scattering curves.33 The fractal dimension is easily obtained by calculating the slope of log(I(Q)/I0(Q)) vs log Q plots.

Results and Discussion Attempts to fit the central part of the scattering curves of the Fe-PO4 systems by the scattering of homogeneous particles (spheres, platelets, needles ...) failed. The curves are characteristic of aggregates. Effect of Aging. The scattering curves of the samples taken at 18 months and 24 months of aging were identical for all studied systems except for P/Fe ) 0.2, R ) 1.0 (Figure 1). For this sample, the curve at t ) 18 months is nearly flat for Q between 0.025 and 0.180 Å-1. This indicates that the solution consists of isolated species with a size smaller than 18 Å. Thus the species determined from EXAFS data at the local scale,31,32 viz, isolated monomers and dimers, possibly linked to a PO4 tetrahedron, may represent the actual composition of the solution. The increase of the scattered intensity for Q < 0.025 Å-1 (Figure 1) does not correspond to a real structure within the solution since an aggregate of this size (≈125 Å) built with small subunits (Fe monomers and dimers) would have a significant contribution to the scattering at larger Q. This increase may correspond to a concentration effect. This aspect will be discussed below. The curve recorded at 24 months of aging (Figure 1) clearly shows that aggregation of the subunits occurred in this time span. The nature of the subunits does not seem to have changed with time, since EXAFS analyses at different aging were perfectly reproducible.31 Thus the local range order is unaffected by the aging. Nevertheless, the factors leading to the aggregation of the subunits after 18 months of stability remain unclear. Systems at P/Fe ) 0.2. Only the samples at 24 months of aging are considered in this part of the discussion. The subunits of the aggregates formed at this P/Fe ratio have been determined previously:31-33 for R ) 1.0, the subunits are Fe monomers (≈60%) and edge sharing Fe dimers (35) Vicsek, T. Fractal growth phenomena; World Scientific: London, 1989; p 355.

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Masion et al.

Figure 2. log(S(Q)) vs log(Q) plots for Fe-PO4 mixtures at various hydrolysis ratios and 24 months of aging: (a) P/Fe ) 0.2; (b) P/Fe ) 0.5; inset, enlarged curve for R ) 1.0 and 0.20 Å-1 < Q < 0.75 Å-1 (y axis × 3; x axis unchanged). The curves are shifted in intensity for better clarity. Table 1. Local and Semilocal Structure of Aggregates Formed by Partial Hydrolysis of Fe-PO4 Mixtures, Samples Aged 24 Months P-Fe distance (Å)a

no. of Fe around Pa

Df ((0.05)

Q range (Å-1)

R ) 1.0 R ) 1.5 R ) 2.0

3.35 3.19 3.15

P/Fe 7) 0.2 1.7 2.4 3.2

2.30 2.39 2.85b

0.043-0.250 0.038-0.295 0.063-0.173

R ) 1.0 R ) 1.5 R ) 2.0

3.24 3.30 3.25

P/Fe ) 0.5 0.8 2.0 1.8

1.98 1.84

0.070-0.430 0.030-0.325

a EXAFS data after Rose et al., 1996 and 1997.31,32 fractal dimension.

b

Apparent

(≈40%), and at higher hydrolysis ratios the dimers represent almost 100%. The scattering curves shown on Figure 2a clearly show that these subunits form small aggregates. For R ) 1.0, the size of the aggregate is approximately 120 Å, as shown by the asymptotical behavior of the curve for Q < 0.025 Å-1 (Figure 2a). The fractal dimension Df ) 2.30 calculated for Q between 0.043 and 0.250 Å-1 (Table 1) is relatively high and does not correspond to a classical aggregation mechanism. The Df value (2.40) of the sample at R ) 1.5 is comparable to the fractal dimension obtained for R ) 1.0 (Table 1). However, its slightly higher value shows a trend to a densification of the system. The size of the aggregates at R ) 1.5 is again around 100 Å. In this case, however, the asymptotical behavior of S(Q) is less pronounced than for R ) 1.0. This may originate from long-range correlation between the dense 100 Å particles. For the R ) 2.0 sample, the I0 function could not be determined,33 and thus only an apparent fractal dimension (2.85) was derived from the slope of the log(I) vs log(Q) plot. However, a comparison of the true Df and the apparent Df values for all other samples shows that the differences are around 0.15. Thus, since EXAFS studies showed that the same type of subunits are present at R ) 1.5 and R ) 2.0,31,32 it is reasonable to assume that the fractal dimension of the aggregates formed at R ) 2.0 is Df ) 2.85 ( 0.15 (Table 1). This Df value is significantly higher than for R ) 1.5 and confirms the trend observed by comparing the systems at R ) 1.0 and R ) 1.5, viz., the structure of the aggreagtes becomes more dense with increasing R. Also, this high Df value indicates the formation of very dense particles forming the rapidly settling precipitate observed macroscopically.31 The size of the aggregates (≈100 Å) lies in the same range as for the lower hydrolysis ratios. The evolution of Df at the semilocal scale with increasing R is very well correlated to the evolution of the structure at the local scale. EXAFS analyses of the same samples31,32

Figure 3. Electron density profiles from the center of (a) a monomer-PO4 cluster and (b) a dimer-PO4 cluster, up to 10 Å assuming a 2.8 Å thick hydration shell. The broken lines indicate the electron density F0 of the surrounding solution. The arrows point to the number of H2O molecules for which the electron density of the hydration shell matches F0.

allowed the determination of P-Fe distances and the number of iron neighbors around one phosphorus atom (Table 1). The P-Fe distance progressively decreases from 3.35 to 3.15 Å when R increases from 1.0 to 2.0. This shortening by 0.20 Å of the P-Fe distances indicates a densification of the cluster structure which impacts the structure at higher scale as shown by the increase of Df from 2.30 at R ) 1.0 to 2.85 at R ) 2.0 (Table 1). Furthermore, the number of iron atoms bound to one PO4 tetrahedron also increases with R (Table 1) without modifying the overall size of the aggregates. This induces a supplementary densification of the system. Thus, at R ) 2.0 where three iron dimers are linked to one PO4,31 the high Df value of 2.85 and the short P-Fe distance indicate a compact arrangement of the atoms within the cluster which probably has a ball-like shape rather than a starshaped structure as assumed previously from EXAFS data.31,32 Systems at P/Fe ) 0.5. The scattering curve for R ) 1.0 shows some similarity with the curve corresponding to P/Fe ) 0.2, R ) 1.0 at 18 months of aging (Figures 1 and 2b). Thus it can be assumed from the general shape of the curve that the composition of the P/Fe ) 0.5, R ) 1.0 sample corresponds to the subunits determined previously (i.e., ≈60% monomers, ≈40% dimers)31-33 as isolated species within the solution. The curve displays two poorly defined maxima at Q ) 0.32 Å-1 and Q ) 0.50 Å-1 (Figure 2b). This behavior can be compared to the scattering of particles with several electron densities such as micelles or lipoproteins,36,37 where the peaks originate from the contrast variation between the hydrophobic and hydrophilic pole and the surrounding medium. In our case, the solvent has a density F0 ) 0.367 electron Å-3. For a Fe monomer-PO4 species with a radius of 3.47 Å, its density F1 is 0.748 electron Å-3. If we consider a hydration shell (thickness 2.8 Å corresponding to the diameter of H2O) constituted by 10 water molecules around this species, the density of this shell is F2 ) 0.112 electron Å-3. Of course, the actual number of water molecules in the hydration shell may be different, but there is an electron density difference with the surrounding solution for up to 30 water molecules in the shell, which is an unreasonably high value. The same reasoning is also valid with a dimer-PO4 cluster, for which the hydration shell must include 41 water molecules to match the electron density of the bulk (Figure 3). Thus, the presence of a low-density solvaent layer around higher density subunits can explain the maxima at high Q on the scattering pattern.2 The increase of the intensity at low Q (