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Langmuir 1997, 13, 3882-3885
Nucleation and Growth Mechanisms of Iron Oxyhydroxides in the Presence of PO4 Ions. 3. Speciation of Fe by Small Angle X-ray Scattering Armand Masion,*,† Je´roˆme Rose,† Jean-Yves Bottero,† Denise Tchoubar,‡ and Pierre Elmerich§ 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 d’Application de Levallois, 95 rue Danton, 92300 Levallois-Perret, France Received November 18, 1996. In Final Form: April 14, 1997X The speciation of Fe(III) within the aggregates formed in partially hydrolyzed Fe-PO4 solutions was determined by small angle X-ray scattering. The simulation of the scattering curves by models consisting of hard spheres representing the various Fe species shows that the presence of PO4 hinders the polymerization of Fe(III). At a hydrolysis ratio R ([OH]/[Fe]) of 1.0, 60% of Fe are Fe monomers and 40% are edge sharing dimers. At higher R, the aggregates consist essentially of edge-sharing Fe dimers (>84%). The Fe speciation derived from the modeling of the scattering curves confirmed and refined the results obtained on the same samples by extended X-ray absorption at the Fe k-edge spectroscopy in a previous study.
Introduction The speciation of metal ions, and especially Al(III) and Fe(III), in nonhomogeneous solid phases (aggregates) is of great interest in natural systems as well as in industrial processes. The speciation of Al during its hydrolysis in the absence of ligands different from OH has been extensively studied over the past 2 decades mainly by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies.1-5 The existence of the tridecameric Al13 polycation in solution and its proportion under diverse synthesis conditions have been clearly determined.1,3,6-8 The presence of organic ligands during Al hydrolysis resulted in strong hindrance of the formation of soluble Al13 due to the complexation of the precursors of the tridecamer.9,10 The modeling of small angle X-ray scattering (SAXS) curves yielded a quantitative speciation of Al contained in the aggregates formed at higher pH.11-13 * 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 d’Aplication de Levallois. X Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. J. Phys. Chem. 1980, 84, 2933-2939. (2) Akitt, J. W.; Farthing, A. J. Chem. Soc., Dalton Trans. 1981, 1606-1608. (3) Bertsch, P. M.; Thomas, G. W.; Barnhisel R. I. Soil Sci. Soc. Am. J. 1986, 50, 825-830. (4) Bottero, J. Y.; Axelos, M.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117, 47-57. (5) Bradley, S. M.; Kydd, R. A.; Howe R. F. J. Colloid Interface Sci. 1993, 159, 405-412. (6) Furrer, G.; Trusch, B.; Mu¨ller C. Geochim. Cosmochim. Acta 1992, 56, 3831-3838. (7) Kloprogge, J. T.; Seykens, D.; Jansen, J. B. H., Geus, J. W. J. Non-Cryst. Solids 1992, 142, 94-102. (8) Parker, D. R.; Bertsch, P. M. Environ. Sci. Technol. 1992, 26, 914-921. (9) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Genevrier, F.; Boudot, D. Environ. Sci. Technol. 1991, 25, 1553-1559. (10) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Montigny, F.; Genevrier, F. Environ. Sci. Technol. 1993, 27, 2511-2516. (11) Masion, A.; Tchoubar, D.; Bottero, J. Y.; Thomas, F.; Villie´ras, F. Langmuir 1994, 10, 4344-4348.
S0743-7463(96)02015-X CCC: $14.00
The solid phase consisted essentially of uncondensed Al monomers,12 the initially formed Al13 being depolymerized by the organic ligands.13 The speciation of Fe(III) in the early stages of the hydrolysis is less understood than that for Al. The use of extended X-ray absorption fine structure (EXAFS) spectroscopy was necessary to provide precise data on the nucleation and polymerization steps:14,15 for FeCl3 solutions and with increasing pH, Fe octahedra form edgesharing dimers, trimers by adding a double corner sharing monomer onto the dimer, and finally the Fe24 polycation which has the local structure of β-FeOOH. An unstable Fe13 polymer having a structure analogous to Al13 was described on the basis of IR results,16 but no evidence of this species was found in the EXAFS studies. SAXS studies of partially hydrolyzed ferric salts showed that, whatever the salt used, the size of the aggregates was approximately 10 nm and the structure evolved from linear to branched with increasing hydrolysis ratio R ) [OH]/ [Fe].17,18 Differences were found in the subunit size: with ferric nitrate it ranged from 7 to 13.5 Å depending on R,17 whereas it remained constant at 16 Å with the chloride salt.18 Recently, the hydrolysis of FeCl3 in the presence of PO4 ligands has been investigated by Fe k-edge and P k-edge EXAFS spectroscopy.19,20 The PO4 ions displayed a strong affinity toward Fe. They were bound to Fe octahedra from the lowest pH values,20 thus occupying growth sites and limiting the polymerization of Fe to edge sharing dimers or possibly trimers.19,20 Double corner (12) Masion, A.; Bottero, J. Y.; Thomas, F.; Tchoubar, D. Langmuir 1994, 10, 4349-4352. (13) Masion, A.; Thomas, F.; Tchoubar, D.; Bottero, J. Y.; Tekely, P. Langmuir 1994, 10, 4353-4356. (14) Combes, J. M.; Manceau, A.; Calas, G.; Bottero, J. Y. Geochim. Cosmochim. Acta 1989, 53, 583-594. (15) Bottero, J. Y.; Manceau, A.; Villie´ras, F.; Tchoubar, D. Langmuir 1994, 10, 316-319. (16) Bradley, S. M.; Kydd, R. A. J. Chem. Soc., Dalton Trans. 1993, 2407-2413. (17) Bottero, J. Y.; Tchoubar, D.; Arnaud, M.; Quienne, P. Langmuir 1991, 7, 1365-1369. (18) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398-402. (19) Rose, J.; Manceau, A.; Bottero, J. Y.; Masion, A.; Garcia, F. Langmuir 1996, 12, 6701-6707. (20) Rose, J.; Flanck, A. M.; Masion, A.; Bottero, J. Y.; Elmerich, P. Langmuir 1997, 13, 1827-1834.
© 1997 American Chemical Society
Fe Speciation by SAXS
Langmuir, Vol. 13, No. 14, 1997 3883
Figure 1. Normalized SAXS curves (log-log plots) of Fe-PO4 systems for P/Fe ratios of 0.2 and 0.5 and hydrolysis ratios R ) 1.0, 1.5, and 2.0. [Fe] ) 1.5 M. Normalized scattered intensities In are shifted to improve the clarity of the figure.
sharing trimers were not formed in the presence of PO4.19 In these studies, the speciation of Fe was determined on the basis of EXAFS spectra modeling parameters. The uncertainty on the proportions was typically in a 15-20% range. The present work aims at comparing the Fe speciation determined from EXAFS data for Fe-PO4 systems to the one obtained by modeling of SAXS curves. Materials and Methods Materials. The Fe-PO4 samples were identical to those used for the EXAFS studies.19,20 Stock solutions of 1.5 M FeCl3‚6H2O were mixed with powder phosphoric acid to obtain P/Fe molar ratios of 0.2 and 0.5. These mixtures were base hydrolyzed (10 M NaOH) until hydrolysis ratios R of 1, 1.5, and 2 were reached. Methods. (a) Small Angle X-ray Scattering. SAXS curves were recorded on the synchrotron D24 beam line, DCI storage ring (E ) 1.85 GeV, I ) 320 mA), Laboratoire pour l’Utilisation du Rayonnement Electromagnetique (Universite´ de Paris Sud, Orsay, France). The wavelength was set to λ ) 1.89 Å to avoid the fluorescence of Fe. The recording time was 5000 s for each sample. Two sample-detector distances were used and the corresponding Q ranges were 0.009-0.180 Å-1 and 0.032-0.750 Å-1. Q is the wave vector modulus and is equal to 4π(sin Θ)/λ, where 2Θ is the scattering angle. (b) Data Treatment. Background correction and smoothing of the scattering curves were performed as described in a previous study.11 Normalized curves were obtained by calculating In following eq 1:
In(Q) )
I(Q) PO
(1)
where PO is the invariant and is equal to
PO )
∫ Q I(Q) dQ
1 2π2
∞
0
2
(2)
After normalization the curves for each sample were connected to cover the whole Q range available except for P/Fe ) 0.2, R ) 2.0 where the outermost part of the curve (Q > 0.180 Å-1) could not be recorded properly because the formed precipitate settled too fast at the bottom the sample holder to allow long enough recording times at high Q to obtain an usable scattering curve.
Qualitative Analysis of the Experimental Curves The normalized experimental log(In) vs log(Q) are presented in Figure 1. Attempts to fit the experimental curves by the scattering of homogeneous particles (spheres, platelets, needles) were unsuccessful. Furthermore, none of the curves displayed correlation peaks which would indicate a characteristic interparticle distance within the samples. This is in agreement with the amorphous and disordered nature of the samples pointed out previously.19 The subunits of the aggregates scatter at the largest angles. In the range from Q ) 0.350 Å-1 to Q ) 0.750 Å-1
Figure 2. Fe speciation and cluster structure for partially hydrolyzed Fe-PO4 systems determined from EXAFS data after Rose et al., 1996.19,20
(-0.45 < log(Q) < -0.13) the slopes of the scattering curves ranged from 0.5 to 1.1, meaning that the subunits are smaller than the experimental detection limit (i.e., 4.3 Å). This indicates also that the subunits were either isolated or formed linear structure at this scale. Modeling of the Outermost Part of the Scattering Curves Preliminary Hypotheses. Only the part of the curves where log(Q) > -0.45 was considered for the modeling. As stated above, no branched structures are to be expected in this Q range. The nature of the possible Fe species has been determined previously by EXAFS.19,20 Figure 2 summarizes the tentative clusters structures obtained from these data. It shows clearly that in the presence of PO4, the hydrolysis of Fe(III) was limited to the dimer stage. Thus, the major Fe species within the aggregates were monomers and dimers formed by edge sharing. A third species consisting of an edge-sharing Fe dimer linked to a PO4 tetrahedron was also considered to take into account the Fe-P associations identified by EXAFS. Simulation Procedure. The calculation of the theoretical scattering curves can be achieved following several procedures. The most rigorous way takes into account all interatomic distances and the atomic scattering factors. The atomic scattering factors can be easily determined from tables computed by numeric calculation of HartreeFock wave function. However, though atomic coordinates can be obtained from EXAFS data for small subunits, it is not possible to determine the interatomic distances for associations of subunits within amorphous systems such as the Fe-PO4 systems in the present study. One way to overcome this difficulty is to assimilate the Fe species to simple geometric shapes. Indeed, it can be hypothesized that the subunits of the aggregates (Fe monomers and dimers, Fe dimer-PO4) have a spherical shape. The validity of this approximation has been established in a previous study, where Al oligomers were considered as hard spheres.11 The scattering curves corresponding to the spheres were equivalent to those calculated using the
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Masion et al.
Table 1. Speciation of Fe(III) in Partially Hydrolyzed Fe-PO4 Systems Derived from the Modeling of SAXS Curves and Compared to the EXAFS Estimations after Rose et al., 1996 and 1997,19,20 Proportions (%) of Fe Involved in the Different Subunits P/Fe ) 0.2 R ) 1.0 SAXS Fe monomer Fe dimer (edge sharing) PO4-Fe dimer cluster
56.9 42.8 0.0
P/Fe ) 0.5 R ) 1.5
EXAFS ? ≈30
SAXS 0.0 88.9 10.4
≈0 ≈100
Figure 3. Spheres used for the modeling of the SAXS curves and representing the subunits of the aggregates. r value is radius of the sphere. Left to right: Fe monomer, edge-sharing Fe dimer, PO4-Fe dimer cluster.
atomic scattering factors. The intensity scattered by a hard sphere is:21
(
I(Q) ) ∆F2V2 3
)
sin Qr - Qr cos Qr Q3r3
2
R ) 1.0
EXAFS
(3)
where r and V are the radius and the volume of the sphere, respectively. The radii of the spheres modeling the Fe species were determined from the interatomic distances obtained from the EXAFS data.19,20 Thus, we considered three different sizes: r ) 2.10 Å for the Fe monomer, r ) 3.28 Å for the edge-sharing Fe dimer, and r ) 4.17 Å for the PO4-dimer cluster (Figure 3). The modeling itself was performed following the procedure described in a previous study.11 Briefly, the theoretical scattering curves corresponding to the spheres were computed one by one. The fit of linear combinations of theoretical curves computed for each Fe species to the experimental signal was carried out by a least-squares method. The uncertainty on the derived proportions is less than 10%. Results and Discussion P/Fe ) 0.2 Systems. For P/Fe ) 0.2, only the samples at R ) 1.0 and R ) 1.5 could be modeled. The proportions yielded by the SAXS curve modeling are volume proportions. They were converted into percentage of Fe involved in the different subunits and are listed in Table 1. For R ) 1.0 (Figure 4a), the simulation shows that approximately 60% of Fe is monomer, the remaining 40% is involved in edge-sharing dimers (Table 1). These proportions are in agreement with the EXAFS results indicating that 30% of Fe forms dimers (Figure 2, Table 1). Furthermore, the proportion of monomers which was difficult to determine by EXAFS could be easily derived from the SAXS curve modeling. However, the SAXS simulation could not distinguish between free and complexed Fe monomers. For R ) 1.5, the modeling of the SAXS curve shows that almost 100% of Fe forms edge sharing dimers (Figure 4a, Table 1). Again, this corre(21) Porod, G. In Small angle X-ray scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982; pp 17-51.
R ) 1.5
SAXS
EXAFS
SAXS
61.0 38.9 0.0
≈60
1.0 98.6 0.0
≈30
R ) 2.0
EXAFS ≈0 ≈100
SAXS 16.1 83.6 0.0
EXAFS ≈0 ≈100
Figure 4. Modeling of the SAXS curves of Fe-PO4 systems: (a) P/Fe ) 0.2; (b) P/Fe ) 0.5; black lines, experimental; gray lines, calculated. R values are the hydrolysis ratio. Experimental and calculated intensities were shifted to improve the clarity of the figure.
sponds to the proportions determined by EXAFS (Figure 2, Table 1). However, in the SAXS modeling, Fe is mainly detected as isolated dimer (90% vs only 10% for the PO4dimer cluster) whereas the presence of PO4 is clearly detected by EXAFS.19,20 This is probably due to the same problem encountered during the Fe k-edge EXAFS study: the difference in atomic weight between Fe and P hindered the detection of the lighter element. For R ) 1.0, no Fe-P contribution was detected at the Fe k-edge19 whereas it was clearly visible at the P k-edge.20 Since SAXS is sensitive to differences in electron density, the contribution of Fe atoms to the scattering signal is much larger than for the P atoms. Therefore, in Fe-PO4 clusters, mainly the Fe dimers are “seen” and thus can appear as isolated species (r ) 3.28 Å) even if they are linked to PO4 tetrahedra (r ) 4.17 Å) as hypothesized from EXAFS analysis (Figure 2). Thus, in the present conditions, the modeling of the SAXS curves cannot confirm the cluster models derived from EXAFS data. P/Fe ) 0.5 Systems. The outermost part of the scattering curves could be modeled for all three hydrolysis ratios. At R ) 1.0 (Figure 4b, Table 1), the model consists of Fe monomers as major species (61%) and of a strong proportion of Fe dimers (39%). These proportions match those determined by EXAFS, viz. 60% monomers and 30% edge sharing dimers (Figure 2, Table 1). Similar to the P/Fe ) 0.2 R ) 1.0 sample, the modeling of the SAXS curve allowed attributing the “missing” iron of the EXAFS estimations to a given species. Again, the comparison of the proportions derived from EXAFS and SAXS (Table 1) tends to indicate that the proportions of dimers are slightly underestimated by EXAFS (or slightly overestimated by SAXS), the agreement between the two being nevertheless very good. At R ) 1.5 (Figure 4b, Table 1), the best fit to the experimental signal is obtained with a model consisting almost exclusively of dimers (r ) 3.28 Å). This corresponds to the proportion determined by EXAFS (Figure 2, Table 1). Similarly to the samples of the P/Fe ) 0.2 series, the Fe dimers appear again as isolated species (r ) 3.28 Å) in the SAXS curve modeling whereas EXAFS results demonstrate that they are involved in the formation of chainlike clusters where the dimers are linked by PO4 tetrahedra (Figure 2). For the gel phase at R ) 2.0 (Figure 4b, Table 1), the proportions yielded by the SAXS curve
Fe Speciation by SAXS
Langmuir, Vol. 13, No. 14, 1997 3885
detected as part of a larger pentameric species. Thus, to appear “isolated”, the monomers are probably not bound to other Fe species. Since the P/Fe ) 0.5, R ) 2.0 sample is a gel, these monomers cannot be really isolated but are part of the structure. Their role in the structuring of the gel can be thought of as bridges not between Fe dimers but between PO4 tetrahedra (Figure 5). In that case, the single corner Fe-Fe distances detected by EXAFS could correspond to linkages between Fe dimers.
Figure 5. Comparison between the structure of the gel phase (P/Fe ) 0.5, R ) 2.0) determined from EXAFS data and determined from SAXS results.
modeling are again in very good agreement with the EXAFS results. Indeed, SAXS indicates that a large majority of Fe (approximately 85%) is involved in dimers. However, the model giving the best fit to the experimental SAXS curve includes 15% monomeric Fe (Table 1). This allows a refinement of the model for the gel described from EXAFS data (Figure 2). In this model, the gel consist of small chains formed by linking a PO4-dimer units together. Since Fe-Fe corner sharing contributions were detected by EXAFS,19 the link between two chains was thought to be Fe monomer bridges binding two Fe dimers by single corner sharing, thus forming a sort of Fe pentamer (Figure 5). However, the modeling of the SAXS curve shows the presence of “isolated” Fe monomers. If all the monomers present were building bridges between two dimers, these monomers would not appear as isolated species in the SAXS modeling since there is no electronic contrast with the surrounding species. They would be
Conclusion The modeling of the SAXS curves allowed the confirmation of EXAFS results showing that the hydrolysis of Fe(III) is limited to the dimer stage in Fe-PO4 systems. The speciation of Fe is rather monotonous: the edge sharing dimer is the major subunit of the aggregates (>84%) as soon as the hydrolysis ratio exceeds 1. The low electron density of P compared to Fe makes the Fe species appear as isolated and thus hinders the determination of the stoichiometry of Fe-P associations which was possible from EXAFS data. It must be noticed that there is an excellent agreement between EXAFS and SAXS results as far as Fe species are concerned: the ranking (major vs minor species) is identical in all studied samples and the proportions yielded by the two techniques are quite similar. Thus there is a cross-validation of the results obtained by EXAFS and SAXS. On one hand, the EXAFS results were confirmed. On the other hand, the agreement between the results shows that the hypotheses made for the SAXS curve modeling were justified. Combining the two techniques yields a powerful tool to propose reliable structural models. Acknowledgment. This work forms part of the EEC exchange research program within the COST action #D5 “Chemistry at Surfaces and Interfaces” and benefited from the financial support of Elf-Atochem SA. The authors also wish to thank the staff at the Laboratoire pour l’Utilisation du Rayonnement Electromagne´tique, Orsay, France, and especially C. Bourgaux for her help during the scattering experiments. LA962015+
