Langmuir 1994,10, 316-319
316
Structure and Mechanisms of Formation of FeOOH(C1) Polymers Jean-Yves Bottero,*ftAlain Manceau,t Fr6d6ric Villieras,? and Denise Tchoubars Laboratoire Environnement et Minbralurgie, U A 235 CNRS, ENSG and INPL, Rue du Doyen M Roubault, 54501 Vandoeuvre Cedex, France, Groupe de Gbochimie de I'Environnement, LGIT-IRIGM, Universitb J Fourier et CNRS, BP 53X,38000 Grenoble Cedex, France, and Centre de Recherche sur les SystBmes Microdivisbs (Laboratoire de Cristallographie), UMR 813 CNRS, Universitb d'Orlbans, BP 6703, 45000 Orlkans la Source, France Received February 19, 1993. In Final Form: September 10,199P Iron oxyhydroxide, especially in its so-called "amorphous" form, plays a key role in the retention and migration of organic and inorganic compounds in soils and aquatic media. The local structure of these "amorphous" species can be directly investigated using synchrotron-basedX-ray absorption spectroscopy. In order to study the nucleation mechanisms,FeC13 solutions were hydrolyzed by NaOH and the precursors obtained at different molar ratios (0 Ir = (NaOH)/Fe I2.7) were studied by EXAFS. For r 1 1.5,Fe polymers formed at equilibrium are hexacoordinated and their local structure is the same as 8-FeOOH. For r = 1.5, the spectra obtained at different aging times show that the starting nuclei are dimers with edge-sharing octahedra. From t 50 min, trimers with edge and corner-sharing octahedra are detected in solution. After 1h, 8-Fe00H-like polycations,formed by the coalescenceof the trimers, can be observed. These polymers are extremely stable because C1- ions are still incorporated in the structure and are easily displaced by OH-.
-
Introduction The polymerization of metals (Al, Si, Fe, Cr, etc.) via hydrolysis has received considerable attention in the past 10 years in domains as different as catalyst syntheses or environmental sciencewhere noncrystalline particles play a key role in the adsorption and transportation of organic or inorganic pollutants.' In general, hydrolysis leads to small clusters (Le. "sol") which aggregate to form larger ones and then are transformed to an infinite phase (Le. "gel"). The structure of the amorphous precipitates depends on the structure of the species formed in the undersaturation region. Because of their natural abundance and industrial use, a large amount of work has been recently devoted to sol-gel systems containing Si, Al, or Cr.28 In the case of Al(III), the hydrolysis process occurs in two steps: First, the All3 polycation is formed from monomers, dimers,4s5and trimers.6 In the second step the A113 polycations aggregate to form crystals Al(OH)3 via a solid-state tran~formation.~ The early stages of hydrolysis of Cr(II1) involve the nucleation of dimers, trimers, and tetramers. These small polymers then condense into transitory Cr polycations where Cr atoms share a common hydroxo bridge. Further evolution of these transition t Laboratoire Environnement e t Minbralurgie, UA 235 CNRS.
Universite J Fourier e t CNRS. 8 Universite d'Orl6ans.
@Abstractpublished in Advance ACS Abstracts, December 15,
phases occurs through intramolecular condensation associated with deprotonation. In this final state, Cr atoms share a common tetracoordinated oxo ligand.8 In spite of numerous studies by using potentiometry?JO UV-visible spectroscopy,ll centrifugation and electron microscopy,12and more recently small angle X-ray scatthe structering and photon correlation ~pectroscopy,~3J~ ture of Fe(II1) polycations is not well known due to the intrinsic higher lability of Fe3+. Based on potentiometric titration, several mechanisms for the formation of these polycations have been proposed. They consider a condensation of dimers or trimers via deprotonation15 which leads to Fe octahedra sharing hydroxo and oxo bridges. Very recently the study of Fe(II1) polymers formed in highly concentrated gels (1 M) through extended X-ray absorption f i e structure or EXAFSspectroscopyl6showed that they were well organized at the local scale consisting in edge and corner sharing octahedra continuously increased with the hydrolysis ratio r. The same kind of evolution was observed on the same gels by Magini et al.17 using wide angle X-ray scattering. These results seem to be in contradiction with small angle X-ray scattering or SAXS13data which indicate that in more dilute Fe chloride solutions (Fe = 0.1 M) the hydrolysis leads to the formation of a unique polycation of 1.5 nm. The previous results obtained in 1M solution could be interpretated in terms of crystal growth and aggregation of polycations which is faster in concentrated than in dilute sols and corresponds
1993.
(1)Buffle, J.; Van Leeuwen, H. P. In Sampling and Characterization of Enuironmental Particles; Lewis Publishers: Chelsea, in press. (2) Brinkner, C. J.; Scherer, G.W. Sol-gel science: the physics and chemistry of sol-gel processing; Academic Press: San Diego, Ca, 1990; n 908.
(3) Van Beek, J. J.; Seykens, D.; Jansen, J. B. H.; Schuiling, R. D. J. Non-Cryst. Solids 1991,134,14-22. (4) Akitt, J. W.; Lester, L.; Kandelwal, F. H. J. Chem. SOC. 1972,26, 609-612. (5) Bottero, J. Y.; Tchoubar, D.; Cases, J. M.; Poirier, J. E.; Fiessinger, F. J. Phys. Chem. 1980, 2933-2939. (6) Henry, M.; Jolivet, J. P.; Livage, J. In Aqueous Chemistry of Metal
Cations: Hydrolysis, Condensation and Complexation. Struct. Bonding
1992, 77. (7) Bottero, J. Y.; Axelos, M. A. V.; Tchoubar, D.; Cases, J. M.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117,47-57. (8) Stiinzi, H.; Marty, W. Inorg. Chem. 1983,20,2145-2150.
0743-7463194l2410-0316$04.50/0
(9) Dousma, J.; De Bruyn, P. L. J. Colloid Interface Sci. 1976, 56, 527-539. (10) J Dousma, J.; De Bruyn, P. L. J. Colloid Interface Sci. 1978,64, 154-170. (11) Knight, R. J.; Sylva, R. N. J. Inorg. Nucl. Chem. 1975,37,779873. (12) Murphy, P. J.; Posner, A. M.; Quirk, J. P. J. Colloid andlnterface Sei. 1976, 56, 284-297. (13) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Amaud, M.Langmuir 1991, 7, 1365-1369. (14) Bottero, J. Y.; Tchoubar, D.; Amaud, M.; Quienne, P. Langmuir 1991, 7,398-402. (15) Schneider, W. Hydrolysis of Iron (111). Chaotic olation versus nucleation. Comments Inorg; Chem. 1984,3, 205-223. (16) Combes, J. M.; Manceau, A,; Calas, G.; Bottero, J. Y. Geochim. Cosmochzm. Acta 1989,53,583-594. (17) Magini, M.; Radnai, T. J. Chem Phys. 1979, 71, 4255-4262.
0 1994 American Chemical Society
Formation of FeOOH(C1) Polymers
Langmuir, Vol. 10,No. 1, 1994 317
to an increase of the number of Fe nearest neighbors. Specifically,this paper aims at answering the two following questions relative to the hydrolysis of dilute FeIW1 solutions at 0.1 M: do j3-FeOOH precursors possess the same structure as the well-crystallized solids; what is the mechanism of their formation? For this reason and based on previous SAXS results,13partially FeC1~6Hz00 . 1 M solutions were studied by using extended X-ray absorption fine structure (EXAFS), which is particularly well-suited to investigate disordered systems in solutions and gas adsorption to analyze both the structure and the mechanisms of formation of the polycations that form at the early stage of hydrolysis.
Materials and Experimental Methods Materials. The samples were prepared by dissolving FeCls.GH20 (Merckref 3904) 0.2 M inO.1 M HC1. The hydrolysis was carried out by NaOH addition under vigorous stirring to prevent important local supersaturation versus FeOOH. At the end of the partial neutralization the Fe concentration is 0.1 M. All the samples at r = (NaOH)/(Fe) = 0 , 0 . 7 , 1.0, 1.5, 1 . 8 , 2 , 2.2, 2.5,2.7 are aged 15 days and no precipitates occurred except for the r = 2.7 sample which correspondsto the flocculation threshold. The sample r = 1.5 was used to determine the nucleation mechanism and was studied at t < 10 min, t = 60 min, and t = 15 days after the end of NaOH addition. Well-crystallized akaganeite 8-FeOOH and goethite a-FeOOH were used as reference. In order to investigate the texture of the colloids present in the sols, the partially hydrolyzed species present in r = 2.5 were centrifuged at 50000g during 1 h. The centrifugate was freezedried and outgassed at 10-9 Torr vacuum at room temperature during 48 h before experiment. 8-FeOOHreference samples were also outgassed in the same condition. Experimental Methods. X-ray Absorption Spectroscopy. X-ray absorption experiments at the FeK-edge were performed with the LURE synchrotron source (Orsay, France), the positron storage ring running at 1.85 GeV and 280-330 mA. Detailed discussions of the principles of X-ray absorption spectroscopy have been previously presented.18 EXAFS data reduction was accomplished according to a procedure previously described.lQA Kaiser window was used for Fourier transforms20 and the spectral fitting procedure was carried out using theoretical backscattering amplitudes and phase shifts.21 The first atomic shell around Fe absorber was fitted using the mean free-path parameters derived from the analysis of FeK-edge EXAFS in FeClr6H20 and y-FeOOH for chlorine and oxygen atoms, respectively, and a,@FeOOH were used as a reference for Fe neighboring shells. The local structure and EXAFS structural parameters of Fe oxides have been previously described.22 Low-Pressure Argon Adsorption. Low-pressure argon isotherms were obtained through a quasi-equilibrium continuous gas adsorption pr0cedure.~3 This continuous method allows calculation of an experimental derivative curve dVadd In (Pi PO), where V, is the adsorbed gas volume and P/Po the relative pressure. The experimental curve is fitted using derivative isotherm summation (DIS).24Local isotherms are described by three parameters: (i) the normal interaction between the adsorbate and the adsorbent; (ii) the lateralinteractions between molecules;(iii) the quantity of gas adsorbed on eachsite. Normal interactions depend on the position of the maxima of the derivative isotherm on the ln(P/Po) axis and on the intensity of lateral interactions. Lateral interactions are easily detected through the shape of the derived isotherms. The height of the (18) Eisenberger, P.; Kincaid, B. M. Science 1978,200, 1441-1447. (19) Manceau, A.; Calas, G.Clay Miner. 1986, 21, 341-360. (20) Bonnin, D.;Calas, G.; Suquet, H.;Pezerat, H.Phys.Chem.Miner. 1985, 12, 55-64. (21) Teo, B. K.; Lee, P. A. J. Am. Chem.SOC.1980,101, 2815-2830. (22) Manceau, A.; Combes, J. M. Phys.Chem.Miner. 1988,15,283295. (23) Michot, L.; Franqois, M.; Cases, J. M. Langmuir 1990,6,677-681. (24) Villieras, F.; Cases, J. M.; Franqois, M.; Michot, L.; Thomas, F. Langmuir 1992,8, 1789-1795.
Figure 1. Radial distribution function, uncorrected for phase shift, for unhydrolyzed r = 0 and partially hydrolyzed FeCl&HzO solution from r = (NaOH)/(Fe) = 0.7 to r = 2.7, and 6-FeOOH reference sample. Table 1. Ultraviolet Absorption Spectroscopy Data for Different Fe(II1) Species species wavelength, nm references FeC12+ 220,335 26 Fe3+,FeOH2+ 240 27 Fe(OH)2+ 300 27 Fe2(0H)2'+ 335 27 FeCL317,365 26 ~
maxima gives the monolayer capacity of the local isotherm. The relationship between these parameters and the position of maxima is obtained by establishing second-order derivative equations of the fundamental equation of gas adsorption on a homogeneous surface. Multilayer adsorption isothermswith lateralinteractions can be fitted if a BET-like correction is introduced. For each domain, experimental parameters are determined and adjusted by the operator. The isotherm and derivative isotherm are computed by summation of each local model in order to compare them with the experimental ones.
Results
XAS Spectroscopy. The radial distribution function (RDF) obtained from Fourier transforming the reduced EXAFS data consists of three peaks at maximum (Figure 1) resulting from first, second, and third Fe/backscatterer interactions. For unhydrolyzed solution r = 0, RDF obtained by Fourier transforming FeK EXAFS spectrum shows only one peak characteristic of Fe monomers (Figure 1). The analysis of this peak leads toa coordination shell consisting of two C1- in trans-position at 2.31 A and four oxygen atoms (0,OH, HzO) at 2.01 A, which is very similar to the one known in FeClr6HzO crystals.26 This is in partial agreement with UV data which were interpretated in terms of unhydrolyzed, partially hydrolyzed monomers, monochloride complexes, and dimer^^^^^^ (Table 1). The unhydrolyzed solution does not contain Fe polymers. In sols aged for 2 weeks and for r ranging from 0.7 t o 1.5, other RDF peaks characteristic of Fe-Fe pairing are detected (Figure 1). Fe-Fe distances were found to be equal to 3.02 and 3.45 A and correspond t o referenced well-crystallized (cr,@,r)-FeOOHstructures, to hydroxo (25) Lind, M. D. J. Chem. Phys. 1967,47,990-993. (26) Asakura, K.;Nomura, M.; Kuroda, H.Bull Chem. SOC.Jpn. 1986, 53,1543-1546. (27) Ciavatta, L.; Grimaldi, M. J. Inorg. Nucl. Chem. 1975,37,163170.
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318 Langmuir, Vol. 10, No.1, 1994
specific surface area of 300 m2/g. The polycations formed in r = 2.5 possess ultramicropores of -0.6 nm associated with larger micropores (Figure 2). Well-crystallized B-FeOOH (36 m2/g) also exhibits micropores of 0.5-0.6 nm (Figure 2).
90
p 60
Discussion Local Structure versus rat t = 15 Days. The ratio of the amplitude of the two Fe peaks in the RDF (Ahydrorol A,) increases first from 1 to 1.7 as r increases from 0.7 to 1.5 and remains constant above r = 1.5. The number of hydroxo bridges thus increases faster than the number of oxo bridges in the earliest stages of the hydrolysis. Above r = 1.5, this ratio is equal to that known for 8-FeOOH and very different from that of a-FeOOH (Figure 3). The local structure of polycations both containing hydroxo edge (3.01 A) and oxo corner (3.45A) bridges thus evolves up to r = 1.5 and remains constant thereafter. The value of r = 1.5 appears to be a threshold for the formation of polycations and was used to study in deeper details the mechanism of the polycation formation. The coordination number (CN) values corresponding to the first cationic shell at 3.01 A are found to vary from 0.8 f 0.2 (r = 0.7) to 1.5 f 0.2 (r = 2.7) and 2 f 0.2 (8FeOOH). CN values around 1 are consistent with the presence of dimers and trimers. The value of 1.5 f 0.3 is consistent with the formation of a larger polymer. The pore size distribution of freeze-dried r = 2.5 sample and 8-FeOOH, which has intracrystal channels, corresponds to micropores of 0.5-0.6 nm, which seem to be of the same nature. They correspondto the intramicropores. The larger micropores, important for the dry r = 2.5 sample, correspond to interaggregate pores formed during drying. So the larger polymers possess a 8-FeOOH-like local structure. Mechanism of Nucleation. For t S 10 min of aging time, only two structural peaks are present (Figure 4). The first one at low distance corresponds to Fe-0 and
> 30 u-20 10
0 0.6 0.5 0.6 0.7 0.8 0.9 1
1.1 1.2 1.3 1.6
Effective Pore Diameter (nm) Figure 2. Pore size distribution (dVddEPD = adsorbed volume/effective pore diameter) vs EPD for freeze-dried sample r = 2.6 and well crystallized 8-FeOOH. -a
Fe OOH ---0Fe OOH
C
.-0
A
Y
3 1 5 Distance (A) Figure 3. Comparison of the radial distribution function for r = 2.7 (t 15 days), a- and 8-FeOOH. Fe polymers have a p-FeOO%k e local structure. 1
2
-
E
edge (Fe[OH]rFe) and oxo corner ( F A - F e ) linkages between Fe octahedra.22 Ar Continuous Gas Adsorption Technique. The freeze-driedsols (r= 2.3 containing polycations have been studied by using Ar continuous gas adsorption following a previously published procedure. Solid samples have a
3.01 A
1
*3.45 A
Fe,
Polycation
A
I
3.01A
Dimer
Trimer
Distance (A) Figure 4. Radial distributionfunction for r = 1.5vs aging time t: (a) t = 10 min; (b)60min; (c) 16 days. Arrows indicate Fe-(OH)z-Fe pairing at 3.01 A (edge-linkages) and Fe-O-Fe ones at 3.45 A (corner-linkages). Structural models of dimers, trimers, and Feu polycations are shown on the right.
Formation of FeOOH(C1) Polymers Fe-C1 pairs and the second one to Fe neighbors at 3.01 A. From t = 30 min, a second Fe contribution appears at 3.45 8, which corresponds to corner linkages through oxo bridges. After 2 weeks of aging, larger polycations or more numerous polycations are present with the same local structure as 0-FeOOH. At the light of these results, the nucleation mechanism of the first Fe polymers that form at the beginning of the hydrolysis can be depicted as follows. The data recorded at various aging times show that Fe polymers are first formed by hydroxylation. These polymers could be dimers, trimers, etc., where only edge sharing Fe octahedra are present. The next step consists in the formation of oxo bridges. Schneider15 postulated the formation of trimers or tetramers through oxolation processes from dimers and monomers. Large polymers are then thought to result from the coalescence of small polymers through oxo bridges (Figure 4). Combining the different constraints from EXAFS (CN = 1.5 f 0.31, Ar absorption on freeze-dried polymers both proving the 0-Fe00H-like structure and knowing that the size is constant whatever r > 1.5 at 1.5 nm as calculated from SAXS,13a model of polymer is proposed (Figure 4). The polymer should contain 24 Fe atoms. This process involved here has strong similarities with that described for the formation of A113 from trimers in partially hydrolyzed A1C13 solutions? The great difference is that the Keggin structure does not exist with Fe(II1). C1- for OH- Exchange. Nucleation and coalescence are only promoted by bridging ligands as OH or 0. The analysis of r = 0 solution has shown that C1- ions are complexed by Fe. It is necessary to displace C1- ions from the first coordination shell for the building of small and large polymers. The amount of C1- in the inner Fe coordination sphere can be calculated from EXAFS results. The higher chloride for oxygen scattering amplitude accounts for the large amplitude of the RDF peak (Figure 5). Its amplitude drops with r increasing but its position does not change until r 2,2.2. Beyond these values the position corresponds to that observed for 0-FeOOH. These results indicate that C1- ions are progressively displaced out of the first coordination sphere of Fe along with the hydrolysis. SAXS experiments14 have shown that large polycations form aggregatesfrom r 2 1.5. These aggregates are highly linear for r < 2.2 and branched for r > 2.2. For r < 2.2 there is, in average, one C1- ion in the coordination of Fe. It could play a key role in aggregating polycations as it was observed in the case of AlI3gels.' For r > 2.2 only
-
Langmuir, Vol. 10, No. 1, 1994 319
C
.-c0, 0 C
.-c0, 3 .-nL c,
.-VI
0 A
.-Um
m
E
1
3 0 Distance ( A ) Figure 5. Radial distribution function for r = 0, r = 1.5, r = 2.0, r = 2.5, and r = 2.7 ( t h = 15 days) and 8-FeOOH showing the position of the peak corresponding to the first coordination shell Fe-0,Cl.
0
2
a very small fraction of C1- ions are bound to Fe atoms and polycations should be aggregated essentially through OH or 0 bonds.
Conclusion This study provides the first direct experimental evidence for the existence of dimers and trimers in inorganic Fe(II1) solution. In addition the local structure of large Fe(II1) polycations has been elucidated by using both EXAFS spectroscopy and gas adsorption. These two methods have shown that Fe(II1) polycations probably result from the coalescence of hydroxo and oxo bridged trimers, a 0-FeOOH-likelocal organization process. Based on these results and on the fact that their size is 1.5 nm as derived from SAXS experiments, a structural model is proposed which consists of 24 Fe octahedra linked as in 0-FeOOH. Acknowledgment. This work was financially supported by the programme Dynamique et Bilan de laTerre; "Fleuves et Erosion" (INSU, CNRS), No. 603.
