Langmuir 1996, 12, 6701-6707
6701
Nucleation and Growth Mechanisms of Fe Oxyhydroxide in the Presence of PO4 Ions. 1. Fe K-Edge EXAFS Study Je´roˆme Rose,† Alain Manceau,‡ Jean-Yves Bottero,*,† Armand Masion,† and Francois Garcia§ Laboratoire des Ge´ osciences de l’Environnement, URA 132 CNRS et Universite Aix-Marseille III, CEREGE FU 017, BP 80, Europole Me´ diterrane´ en de l’Arbois, 13545 Aix-en-Provence Cedex 4, France, Groupe de Ge´ ochimie de l’Environnement, LGIT-IRIGM, Universite´ J Fourier et CNRS, BP 53, 38041 Grenoble Cedex 9, France, and Elf-Atochem, Centre de Recherche Rhoˆ ne-Alpes, rue Henri Moissan, BP69, 69310 Pierre-Be´ nite, France Received June 25, 1996. In Final Form: September 9, 1996X Fe K-edge EXAFS spectroscopy has been used to determine the nucleation mechanisms and to propose a model of colloid and gel formation during the hydrolysis of FeCl3 in the presence of PO4 in acidic solutions. The displacement of OH or O ligands by PO4 anions drastically changes the normal path of Fe(III) hydrolysis in pure water: (i) The Fe(III) nuclei are mainly dimers and linear trimers formed through equatorial edge sharing. (ii) One Cl remains in the first coordination shell of Fe even when gels or precipitates are formed. (iii) Precipitates or gels are obtained through the aggregation of different subunits. In the precipitates, the subunits correspond to three iron dimers associated through one PO4 tetrahedron. The precipitates are formed by aggregating the subunits through a Fe-Fe corner-sharing link (Fe-Fe ) 4 Å). In gels, the subunits are two dimers associated through one PO4 tetrahedron. The aggregation of subunits through PO4 leads to the formation of chains. Fe monomer bridges link these chains together by corner sharing, thus forming the gel. The growth mechanisms of Fe-PO4 colloids are dependent on the initial P/Fe ratio.
Introduction The hydrolysis of Fe3+ or Fe2+ was mainly studied through the identification of the formed crystalline phases.1-3 Relatively few investigations on nucleation processes of Fe2+ or Fe3+ were undertaken.4,5 The structure of the nuclei of Fe(III) at a low hydrolysis ratio, far from supersaturation against R- and β-FeOOH, has been characterized only very recently.6 It has been shown that during the hydrolysis of FeCl3 solutions a particular polycation can be formed at room temperature.6 The structural model proposed for this polycation consists in the arrangement of 24 Fe atoms in a β-FeOOH local structure. The basic unit is a trimer formed by two edgesharing octahedra linked to one double-corner octahedron. The nucleation kinetics and the structure of polycations depend on the exchange rate of Cl for O or OH in the first coordination sphere of Fe.5,6 Controlling the hydrolysis allows us to control the structure and the size of the nuclei of the colloids formed.7,8 However the major difficulty in controlling the hydrolysis of Fe(III) is due to its electronic configuration, 3d5. The filling of the d electronic layer * To whom correspondence should be addressed. † URA 132 CNRS et Universite ´ Aix-Marseille III. ‡ Universite ´ J Fourier et CNRS. § Centre de Recherche Rho ˆ ne-Alpes. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) Manceau, A.; Drits, V. A. Clay Miner. 1993, 28, 165-184. (2) Combes, J. M.; Manceau, A.; Calas, G. Geochem. Cosmochim. Acta 1990, 54, 1083-1091. (3) Shwertmann, U.; Cornell, R. M. Iron hydroxyde in the laboratory; VCH: New York, 1991; p 137. (4) Schneider, W. Hydrolysis of iron(III)schaotic olation versus nucleation. Comments Inorg. Chem. 1984, 3, 205-223. (5) Combes, J. M.; Manceau, A.; Calas, G.; Bottero, J. Y. Geochem. Cosmochim. Acta 1989, 53, 583-594. (6) Bottero, J. Y.; Manceau, A.; Villie´ras, F.; Tchoubar, D. Langmuir 1994, 10, 316-319. (7) Brinker, C. J.; Sherer, G. W. Sol-gel science: the physics and chemistry of sol-gel processing; Academic Press: San Diego, CA, 1990; p 908. (8) Bottero, J. Y.; Bersillon, J. L. Aluminium and Iron(III) chemistry. Aquatic humic substances; American Chemical Society: Washington, DC, 1989; p 425.
S0743-7463(96)00629-4 CCC: $12.00
during the hydrolysis corresponds to a very low energy. Thus the nucleation and growth of Fe colloids are spontaneous (∆G < 0). Such a constraint can be avoided by using strong complexing anions such as PO4, SO4, and SiO4 known to built stable bonds with Fe atoms. The complexation of Fe(III) hinders the formation of wellcrystallized minerals9-12 even at ligand/Fe molar ratios as low as 0.02.9 Nevertheless the first steps of the nucleation and oligomerization of Fe atoms in the presence of phosphate anions are poorly understood. This is essentially due to the great difficulty in determining the speciation of Fe in such systems. The recent developments of element specific structural methods such as EXAFS (extended X-ray absorption fine structure) spectroscopy allowed a breakthrough in the knowledge of the structure of colloids. The present work aims at studying the local structure of Fe colloids in the first stages of the hydrolysis of Fe(III)/PO4 systems at various [P]/[Fe] molar ratios. In this first of two companion papers, Fe K-edge EXAFS spectroscopy was used to characterize the local structure around the Fe atoms and to study the formation mechanisms of the colloids. Materials and Methods Materials. Stock solutions of 1.5 M iron(III) chloride were prepared from reagent grade FeCl3‚6H2O dissolved into distilled water. Powder phosphoric acid was added into the solution, and the volume was adjusted to 100 mL with distilled water. The weight of phosphoric acid was calculated to obtain P/Fe molar ratios of 0.2 and 0.5. A 10 M sodium hydroxide solution was injected into the sample with a rate of 0.55 mL/h in order to carry out the hydrolysis in a 200 mL vessel fit out with four baffles. The injection was made under vigorous stirring to prevent (9) Kandori, K.; Uchida, S.; Kataoka, S.; Ishikawa, T. J. Mater. Sci. 1992, 27, 719-728. (10) Music, S.; Popovic, S.; Orehovec, Z. J. Colloid Interface Sci. 1993, 160, 479-482. (11) Andreeva, D.; Mitov, I.; Tabakova, T.; Andreev, A. J. Mater. Sci.: Mater. Electron. 1994, 5, 168-172. (12) Norton, G. A.; Richardson, R. G.; Markuszewski, R. Environ. Sci. Technol. 1991, 25, 449-455.
© 1996 American Chemical Society
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Table 1. Samples Studied and Their Physical States P/Fe
n ) [OH]/[Fe]
time injection (h)
0.2
0 1 1.5 2
0 32 46 60
0.5
0 1 1.5 2
0 40 54 68
physical state clear solution sol sol two phases: sol and precipitate clear solution clear solution sol gel
important local oversaturation with respect to FeOOH(s). Hydrolysis ratios n ) [OH]/[Fe] of 1, 1.5, and 2 were obtained with the two series P/Fe ) 0.2 and 0.5. Table 1 summarizes the fresh samples studied (t e 6 days) and their physical states. Methods. X-ray Absorption Spectroscopy. Fe K-edge EXAFS measurements were performed in transmission mode at the DCI synchrotron source (LURE, Orsay, France) on the EXAFS 1 beam line. The positron storage ring was running at 1.85 GeV and 280-330 mA. Theoretical Aspects. EXAFS consists in recording the absorption coefficient µ of a given sample as a function of the wavelength in the X-ray range. The spectral scan is performed in the vicinity of an X-ray absorption edge (K, L, or M) of the chosen target element. A photoelectron is ejected from the central atom to the continuum. The electronic wave associated can be backscattered by surrounding atoms (up to three or four nearest atomic shells), resulting in an interference between outgoing and incoming waves. This interference gives rise to a sinusoidal variation of µ vs energy (E), known as EXAFS. In the single-scattering approximation, the interference function χ(k) can be linked to the structural parameters at the local scale around the X-ray absorber via the following equation:
χi(k) )
∑χ ) ∑A (k) sin[2kR + φ (k)] ij
j
j
j
ij
(1)
j
where χij(k) corresponds to the contribution of the jth atomic shell at the distance Rj from the central atom i. We consider an atomic shell to be a set of atoms of the same type at the same distance from the central atom i. k stands for the modulus of the wave vector (expressed in Å-1) associated with the electronic wave. φij(k) corresponds to the phase shift function of the i-j atomic pair. It contains contributions from both the absorbing element (δi) and the backscattering atom (φj). Aj(k) is the amplitude factor, which is equal to
NjFj(k,Rj) -2σj2k2 2Rj/λ(k) e e kRj2
(2)
where Nj corresponds to the number of atoms in the jth shell. σj is the Debye-Waller factor, which accounts for thermal vibrations and static disorder. Fj(k,Rj) is the amplitude function, which depends on the atoms in the jth shell. λ(k) is the electron mean free path length that accounts for inelastic scattering processes. Its k dependence is expressed as λ(k) ) 2k/L. Data Reduction. EXAFS data reduction was accomplished according to a procedure described previously.13 χFe(k) spectra were Fourier transformed from k to R space by using a Kaiser apodization window (τ ) 2.5).14 This procedure results in radial distribution functions uncorrected from phase shift functions; i.e., RDF peaks are displaced from crystallographic distances by ≈0.3-0.4 Å. The contributions of the various shells 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 (eq 1) in order to determine the structural and chemical parameters: Rj, Nj, and the nature of the atomic neighbors in the jth shell around Fe. (13) Manceau, A.; Calas, G. Clays Miner. 1986, 21, 341-360. (14) Manceau, A.; Combes, J. M. Phys. Chem. Miner. 1988, 15, 283295.
Determination of Empirical Amplitude and Phase Function. In order to fit partial EXAFS spectra by theoretical functions (eq 1), amplitude and phase shift functions for the different i-j pairs are required. FFe(k,Rj) was determined experimentally from γ(FeOOH) (6 Fe atoms at 3.08 Å; Table 2). For FP(k,Rj), φFe-Fe and φFe-P theoretical functions were used15 due to the lack of convenient references. Their validity was ascertained with pure and well crystallized iron phosphate references (strengite,16 barbosalite,17 vivianite,18 rockbridgeite19). These references were used to determine L (Å-2), which was then kept fixed for our unknown samples. These parameters are listed in Table 2. The Nj and Rj determined using EXAFS spectroscopy for all reference minerals are in a 15% and 3% range, respectively, around the crystallographic data. Thus the uncertainties in the experimental N and R are in the same percentage range.
Results EXAFS Spectra. EXAFS spectra of samples at various n ratios (0, 1, 1.5, 2) for P/Fe ) 0.2 and 0.5 are shown in Figure 1a and b, respectively. For each P/Fe series, modifications of the phase of EXAFS curves can be noticed at k ) 5.6 Å-1. The zero crossing moves from 5.6 to 5.8 Å-1 when n increases from 0 to 1. This phase shift points out a modification of the local environment of iron as the hydrolysis ratio increases. Radial Distribution Functions (RDFs). RDFs of the eight samples are shown in Figure 2. The first peak at 1.7 Å (RDFs distance uncorrected for phase shift function) corresponds to the contribution of the nearest O, OH, H2O, or Cl belonging to the first coordination sphere of Fe atoms.6,14 The second and third peaks at ≈2.7 and ≈3.5 Å, respectively, indicate the presence of atoms in the second and third coordination spheres around iron. The second peak can be assigned to the nearest cations. For both P/Fe series, at n ) 0, the RDFs do not exhibit more than one peak, suggesting that iron octahedra are probably isolated in solution. For a higher hydrolysis ratio and for both P/Fe series the amplitudes of the second and third peaks (see arrows Figure 2) increase as n increases, particularly in P/Fe ) 0.5 samples. This is the consequence of a larger coordination number in the second and third atomic spheres around iron. Thus one can assume that a polymerization occurs when n increases. Analysis of the Ligand Coordination Sphere (First Coordination Sphere). Partial EXAFS spectra obtained by back Fourier transforming the first RDF peak are shown in Figure 3. EXAFS structural parameters used for each calculated spectra are listed in Table 3. For each spectrum one or two beat nodes can be noticed (see arrows in Figure 3). The presence of one beat node indicates that the partial EXAFS spectrum is composed of two sinusoidal curves with different frequencies. Thus theoretical EXAFS spectra were computed using two atomic shells. In some cases three atomic shells were needed to take into account both beat nodes, especially the one at high k values. At high k, this amplitude beating is possibly due to the interference of the structural sinusoidal signal with background noise and might not correspond to structural information. Nevertheless, the high signal/noise ratio of EXAFS spectra at high k-range (Figure 1) suggests that structural contribution to the beat nodes exists. This is supported by the fact that the recalculation of partial EXAFS spectra with three atomic shells significantly improves the quality of the least-square fit (Figure 3a) (Table 3). (15) McKale, A. G.; Veal, B. W.; Paulikas, A. P.; Chan, S. K.; Knapp, G. S. J. Am. Chem. Soc. 1988, 110, 3736. (16) Moore, P. B. Am. Mineral. 1966, 51, 168-176. (17) Lindberg, M. L.; Christ, C. L. Acta Crystallogr. 1959, 12, 695697. (18) Fedji, P.; Poullen, J. F. Bull. Mineral. 1980, 103, 135-138. (19) Moore, P. B. Am. Mineral. 1970, 55, jan-feb.
Fe Oxyhydroxide in the Presence of PO4 Ions
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Table 2. Crystallographic and EXAFS Parameters of Reference Mineralsa Re strengite barbosalite vivianite rockbridgeite
EXAFS XRD EXAFS XRD EXAFS XRD EXAFS XRD
Fe-Fe1 shellb (Å) Nf typeg
2.85 2.85 2.95 2.96 2.70 2.70
1.5 1.5 0.5 0.5 0.3 0.3
F
Re
Fe-Fe2 shellb (Å) Nf typeg
3.73 3.67
2.5 2.5
C
E F
Ld (Fe-Fe)
1.7 1.7
2.88 2.88
1.1 1.3
F+E
1.7
Re
Fe-P1 shellc (Å) Nf typeg
3.28 3.29 3.22 3.21 3.21 3.22 3.19 3.22
3.0 3.0 2.5 2.5 2.0 2.0 1.3 1.3
C C C C
Re
Fe-P2 shellc (Å) Nf typeg
3.43 3.38 3.36 3.40 3.40 3.41 3.32 3.34
1.0 1.0 1.5 1.5 1.0 1.0 2.3 2.3
Ld (Fe-P)
Qh
C
2
0.05
C
2
0.17
C
2
0.15
C
2
0.17
a γ(FeOOH) was used as a reference for F -2 b Fe-Fe. For this sample σ ) 0.08 (Å, L ) 1.7 (Å ), N ) 6, R ) 3.08 Å, and Q ) 0.098. Fe-Fe1 and Fe-Fe2 correspond to the first and second Fe-Fe shells, respectively. c Fe-P1 and Fe-P2 correspond to the first and second Fe-P shells, respectively. d L corresponds to the first term in the λ(k) expression (λ(k) ) 2k/L). e R: distance between the two atoms of each atomic pair. f N: number of atoms in the second and third spheres of iron. g F ) face sharing between two iron octahedra. E ) edge sharing between two iron octahedra. C ) corner sharing between two iron octahedra or one octahedron and one phosphate tetrahedron. h Q ) ∑[(k3χtheo) - (k3χexp)]2/(k3χexp)2.
Figure 1. Fe k3χ(k) spectra for liquid samples: (a) P/Fe ) 0.2; (b) P/Fe ) 0.5. Arrows indicate phase shifts between n ) 0 and n g 1 for both series.
For unhydrolyzed solutions (n ) 0), the first coordination sphere of iron consists in 2 Cl at 2.3 Å and 4 (O, OH, H2O) at 2 Å. This structure of the iron octahedron is close to the one present in crystalline FeCl3‚6H2O (4 O at 2.06 Å and 2 Cl at 2.3 Å)20 and previously determined in solution.6 For both P/Fe ratios, when n reaches 1, one Cl atom of the iron octahedra is exchanged by one O, OH ligand. When n increases from 1 to 2, the chemical nature of the iron octahedron is not affected: one chlorine atom still remains in the first coordination sphere of iron. Analysis of the Second and Third Coordination Spheres. The curves resulting from back Fourier transforms of the second and third RDF peaks are shown in Figure 4. The corresponding structural parameters are reported in Table 4. Fe, P, and Cl were used as backscatterer atoms in the second and third coordination spheres around iron. The calculation of the curves was performed using three or four atomic shells. The 2.4-3.8 (20) Asakura, K.; Nomura, M.; Kuroda, H. Bull. Chem. Soc. Jpn. 1985, 58, 1543-1550.
Figure 2. Fe radial distribution functions (uncorrected for phase shift functions) for (a) P/Fe ) 0.2 and (b) P/Fe ) 0.5. Arrows indicate two peaks and one shoulder for each sample with n g 1. This corresponds to the presence of three different atomic pairs.
Å R-range of the RDF curves, on which back Fourier transforms are applied, corresponds to two peaks. In addition, each spectrum exhibits a weak shoulder which accounts for a third contribution. Thus the recalculation of partial EXAFS curves with three shells appears reasonable. In some cases, the use of a fourth shell greatly improves the accuracy of the modeled curve (Figure 4; Table 4). For each sample a Fe-Fe contribution (Fe-Fe1) at a distance of 2.97-3.10 Å is detected. This distance is characteristic of hydroxo edge linkages between Fe octahedra.1,5 The number of Fe1 neighbors (NFe1) increases from 0.3 at n ) 1 to 0.9-1 at higher n values regardless of the P/Fe ratio. For the P/Fe ) 0.2 series, no Fe-P contribution was detected at n ) 1. A Fe-P distance is detected at a distance
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Figure 3. Comparison of partial EXAFS spectra corresponding to the first coordination sphere: (a) P/Fe ) 0.2, n ) 2, (i) calculation using three atomic shells, (ii) calculation using two atomic shells; (b) P/Fe ) 0.5; (solid line) experimental spectrum; (dotted line) calculated spectrum. Arrows show that the last beat node is not restitute when only two atomic shells are used in the recalculation of the partial EXAFS curve. Table 3. Structural Parameters for Fe (Backscatterer in the First Coordination Sphere) Contributions Deduced from EXAFS Analysis for P/Fe ) 0.2 and P/Fe ) 0.5
sample
Ra window (Å)
Fe-O1 shellc physical state
n)0 n ) 1.0 n ) 1.5 n)2
1.1-2.2 clear solution 1.2-2.2 clear solution 1.3-2.2 sol 1.0-2.1 gel
Fe-Cl1 shellc
Ra Ra Ra (Å) NO1b (Å) NO2b (Å) NCl1b
P/Fe ) 1.3-2.3 clear 2.02 solution n ) 1.0 1.0-2.2 sol 2.00 n ) 1.5 1.3-2.3 sol 1.98 n ) 2 (i) 1.0-2.3 precipitate 1.97 n ) 2 (ii) 1.0-2.3 precipitate 2 n)0
Fe-O2 shellc
Q
0.2d 4.0
2.28
2.0
0.02
4.9 2.30 4.0 2.1 1.1 2.27 4.0 2.1 1.6 2.30 5.0 2.28
1.0 1.0 0.8 1.0
0.10 0.06 0.05 0.09
2.27
2.0
0.13
1.97 4.3 2.1 1.1 2.30
0.9
0.04
1.98 5.0 1.98 5.0
1.1 0.9
0.06 0.02
P/Fe ) 0.5 2.02 4.0
2.27 2.27
a R: distance between the two atoms of each atomic pair. b N: number of atoms in the first coordination sphere of Fe. c Fe-Oi: ith oxygen shell around Fe. Fe-Cl1: first chlorine shell around Fe. d For the P/Fe ) 0.2 series at n ) 2, two conditions for the recalculation of partial EXAFS curve are shown. The first one using three shells (n ) 2 (i)) corresponds to the best Q value. The second (n ) 2 (ii)) exhibits EXAFS parameters for the recalculation of the partial EXAFS curve with only two atomic shells.
RP which decreases from 3.39 to 3.2 Å, for n ) 1.5 and 2, respectively. Correlatively the number of phosphorus atoms NP increases from 0.4 to 0.6 (Table 4). For the P/Fe ) 0.5 series, phosphorus atoms are already present in the second coordination sphere at n ) 1. Similar to the case
Figure 4. Comparison of partial EXAFS spectra corresponding to the second and third coordination spheres around iron: (a) P/Fe ) 0.2; (b) P/Fe ) 0.5; (solid line) experimental spectrum; (dotted line) calculated spectrum. Arrows indicate differences between experimental and recalculated curves when only three shells are used in the recalculation of partial EXAFS curves for both P/Fe series at n ) 2.
for the P/Fe ) 0.2 series, the decrease of RP (3.55-3.27 Å) is correlated to an increase of NP (0.8-1). At n ) 2, the number of P neighbors reaches 1. In the third coordination sphere a new Fe-Cl contribution (Fe-Cl2) is detected at a distance of ≈3.7 Å. In each sample, the number of Cl2 neighbors NCl2 is equal to NFe1 (Table 4). Another Fe-Fe contribution (Fe-Fe2) is detected in the third coordination sphere for some samples (Table 4). The distance of 4.08 Å between the two irons of this atomic pair is close to the single-corner linkage in γ(FeOOH) mineral (3.8-3.9 Å).1 This difference in the Fe-Fe distance for the same linkage type could account for the slight dissymmetry in the first coordination shell around iron in the samples (Table 3), resulting in a greater distance distribution of oxygen atoms in the range 1.952.1 Å. Discussion Role of P in the Hydrolysis Hindrance. During the hydrolysis of pure FeCl3, the formation of polynuclear Fe species translates to an increasing value of NFe1, which reaches 1.5 for n ) 2.0.6 In our Fe-PO4 systems, this number of nearest Fe neighbors remains low even at high hydrolysis ratios and never exceeds 1 (Table 4). This low NFe1 strongly suggests that the presence of PO4 ligands hinders the polymerization of Fe. Figure 5 illustrates this difference in the number of neighbors by a great difference in the intensity of the second RDF peak. Departure of Cl from the First Coordination Sphere of Fe. Previous studies5,6 have shown that the
Fe Oxyhydroxide in the Presence of PO4 Ions
Langmuir, Vol. 12, No. 26, 1996 6705
Table 4. Structural Parameters for Fe (Backscatterer in the Second and Third Coordination Spheres) Contributions Deduced from EXAFS Analysis for P/Fe ) 0.2 and P/Fe ) 0.5 sample
Ra window (Å)
physical state
Fe-Fe1 shell RFe1a (Å) NFe1b
Fe-P shell RPa (Å) NPb
Fe-Cl2 shell RCl2a (Å) NCl2b
Fe-Fe2 shell RFe2a (Å) NFe2b
Q
P/Fe ) 0.2 n ) 1.0 n ) 1.5 n ) 2 (i)c n ) 2 (ii)
2.4-3.8 2.3-3.7 2.4-3.8 2.4-3.8
sol sol precipitate precipitate
2.97 3.02 3.00 3.02
0.3 0.9 0.7 0.7
3.39 3.20 3.21
0.4 0.6 0.5
3.72 3.70 3.72 3.72
0.4 0.9 0.7 0.7
n ) 1.0
2.3-3.7
3.12
0.3
P/Fe ) 0.5 3.55
0.8
3.75
0.4
n ) 1.5 n ) 2 (i) n ) 2 (ii)c
2.3-4.0 2.3-3.8 2.3-3.8
clear solution sol gel gel
3.10 3.02 3.02
1.0 1.0 1.0
3.38 3.27 3.27
0.7 1.0 1.0
3.73 3.73 3.73
1.0 0.6 0.76
4.02
0.5
4.04
0.3
0.29 0.17 0.08 0.15 0.16
4.10 4.10
0.2 0.2
0.22 0.14 0.20
a R: distance between the two atoms of each atomic pair. b N: number of atoms in the second and third coordination spheres of iron. At n ) 2 and for both P/Fe series, the first and the second rows (n ) 2 (i) and n ) 2 (ii)) correspond to the recalculation of partial EXAFS curves using four and three atomic shells, respectively. c
Figure 5. Comparison of radial distribution functions (RDFs) for samples hydrolyzed in the presence and absence of PO4. Arrows show the decrease of the amplitude of the second and third RDF peaks when n increases.
formation of the intermediate dimeric and trimeric Fe species during the hydrolysis of pure FeCl3 implies the departure of both Cl atoms from the first coordination sphere of Fe (initially constituted by 4 O at 2 Å and 2 Cl at 2.3 Å). The gradual decrease of NCl1 from 2 at n ) 0 to 1 at n ) 1.5 and finally to 0.3 at n ) 2,5,6 i.e. when the trimer is formed, is in perfect agreement with the proposed hydrolysis mechanism. In the presence of complexing PO4 ligands, the structure of the first coordination sphere in the very first stages of the hydrolysis (n e 1.5) is similar to that of iron in samples without phosphate. For both systems and at n ) 0, the iron atoms are surrounded by 4 O at 2 Å and 2 Cl at 2.3 Å. When n increases, one Cl is exchanged by an O, leading to an additional Fe-O bond. But significant differences with respect to the FeCl3 system appear at higher hydrolysis ratios. At n ) 2 one Cl atom still remains in the first coordination shell of iron. This means that a potential condensation site is still occupied by Cl even for high n values. Different Types of Fe-Fe Linkages. Two Fe-Fe distances are detected in the present samples. The first Fe-Fe contribution at 2.97-3.1 Å corresponds to octahedra associations through one edge.1,5 This type of linkage is detected in all hydrolyzed samples. The second Fe-Fe distance at ≈4 Å is attributed to single-corner sharing and is present in four samples only (Table 4). No Fe-Fe distance characteristic of double-corner sharing (at ≈3.45 Å1,5,6) was detected in our Fe/PO4 systems. Tentatively it is possible to identify the speciation of Fe to models of oligomers for which a clear solution or sol is
Figure 6. Psosible structures for Fe edge-sharing oligomers. exp formed (Figure 6). NFe corresponds to NFe1 in Table 4, i.e. 1 the average number of Fe neighbors at ≈3 Å around each theo iron atom. Figure 6 details NFe for several isolated 1 small iron oligomers with only edge sharing.21 At n ) 1 exp for both P/Fe ratios, NFe ) 0.3 (Table 4). If we assume 1 that only dimers formed by edge sharing or isolated iron monomers are present, the proportion of monomers exp theo )/(NFe (for iron involved in this dimer formation is (NFe 1 1 dimers)) × 100 ) 0.3/1 × 100 ) 30%. In fact one can assume that not only dimers but also larger edge-sharing iron oligomers (e.g. trimers A and B and tetramers A, B, C, and D in Figure 6) could exist in solution. But because theo (for iron oligomers) > 1, trimers and of their NFe 1 tetramers are probably only minor species. Thus, at n )
(21) Wells, A. F. Structural inorganic chemistry; Oxford University Press: Oxford, U.K., 1984; p 1382.
6706 Langmuir, Vol. 12, No. 26, 1996
Figure 7. Possible mechanisms for the formation of the Fe dimer.
1, the majority of the iron is not linked to other irons through edge sharing. exp ≈ 1. In a first At n > 1 and for both P/Fe ratios, NFe 1 approximation, this value corresponds to a proportion of 100% dimers, which represent the predominant species. However, the presence in solution of small oligomers formed by edge sharing (see trimers A and B and tetramers A, B, C, and D on Figure 6) and even Fe monomers cannot exp is close to 1, these oligomers be excluded. But, since NFe 1 are probably only minor species. The presence of a second Cl contribution at ≈3.7 Å implies, as shown in Figure 6, that Cl2 is in the axial position on the Fe octahedra. In addition, our data indicate that NCl2 ≈ NFe1. Only dimer A, trimer B, and tetramer D (Figure 6) fulfill both conditions. Nevertheless this axial configuration of Cl atoms does not agree with the well accepted equatorial position described in the literature on hydrolysis processes.4,22 Figure 7 shows two possible mechanisms for the formation of the Fe dimer. If the formation of the Fe-O-Fe bond involves a coordinance initially occupied by a chloride, the Cl atoms on the resulting structure are in the equatorial plane formed by the two Fe-O-Fe bridges. In this case, the Fe-Cl2 distance is 4.8 Å, which is inconsistent with our data. Moreover, this configuration of the dimer enables the formation of a trimer through double-corner sharing. Again, the data (Fe-Fe distances, number of Fe neighbors) show that this species is not present in our samples. The second mechanism shown in Figure 7 does not involve sites originally occupied by chlorine atoms. The Cl atoms are in the axial position on the resulting structure, and the Fe-Cl2 distance of this configuration (3.7 Å) is in good agreement with our EXAFS data (Table 4). Thus, the most likely species present is dimer A of Figure 6. Possible minor species are trimer B and tetramer D (Figure 6). It is noticeable that the axial configuration of Cl automatically implies the formation of linear clusters through equatorial edge sharing. The second Fe-Fe2 contribution at a distance of 4 Å corresponding to the association of two iron octahedra through one corner is not detected in all samples (Table 4). At n ) 1 and for P/Fe ) 0.2, the value of NFe2 is 0.5. This contribution can originate from the presence of (22) Henry, M.; Jolivet, J. P.; Livage, J. Aqueous chemistry of metal cations: hydrolysis, condensation and complexation. Struct. Bond. 1992, 153-206.
Rose et al.
isolated iron dimers sharing one corner. It can also correspond to a third Fe octahedron linked through a single corner to one dimer. Both previous assumptions are coherent with NFe1 and NFe2. For other samples for which the Fe-Fe2 contribution exists, since NFe1 is close to 1, the majority of the iron is presumably involved in the formation of edge-sharing dimers. In these cases, the FeFe2 contribution corresponds probably to the linkage of one Fe to a dimer through one single corner. Finally this polymerization of iron during the hydrolysis of FeCl3 in the presence of PO4 is not trivial and very different from the hydrolysis of pure FeCl3 in solution. In this latter case, the dimer stage is followed by the formation of a trimer through a Fe atom sharing a double corner and corresponding to a Fe-Fe distance of 3.45 Å. This distance, predominant in FeOOH structures, was not detected in the present study. Type of Fe-P Linkage. For both P/Fe series, Fe-P distances vary from 3.2 to 3.55 Å. The nature of the linkage with such Fe-P distances can be inferred from the comparison with well crystallized iron phosphate references. In the majority of these mineral structures, PO4 tetrahedra are corner sharing with Fe octahedra (rockbridgeite,19 strengite,16 vivianite,18 barbosalite,17 whitmoreite,23 lipscombite,24 Fe2(PO4)Cl,25 Fe2+Fe3+2(PO3OH)4(H2O)426). In all these minerals Fe-P distances are in a 3.2-3.58 Å range. Bridging through edge sharing between PO4 and Fe is only present in Fe3(H2O)(PO4)227 where Fe(II) is 5-fold coordinated and the Fe-P distance is very short (2.75 Å). Thus, it can be assumed that, in the present Fe-PO4 systems, phosphate tetrahedra are bound to Fe octahedra through corner sharing all along the hydrolysis process. Probable Structures of Clusters during Growth of Particles. Three types of atoms are detected in the second and third coordination spheres around iron: Fe, P, and Cl. Concerning iron associations, the most likely structure is the Fe dimer with, in some cases, a singlecorner sharing Fe monomer attached to it. The Fe-PO4 interaction results in a PO4 tetrahedron-Fe octahedron linkage through single-corner sharing. By taking into account this structural information and combining it with the initial stoichiometry, it is possible to propose structural models of the formed Fe-PO4 species. For P/Fe ) 0.2 at n ) 1.5, each iron atom is surrounded on average by NP ) 0.4 phosphorus atom (Table 4). On the basis of the initial P/Fe stoichiometry, and since the majority of iron in solution is in a dimeric form, this NP value indicates that almost 100% of the dimers are involved in small clusters consisting of two Fe dimers bridged by one PO4 tetrahedron (Figure 8). At a higher hydrolysis ratio (n ) 2), the NP value of 0.6 (Table 4) is consistent with one PO4 tetrahedron bridging three iron dimers. If we consider this latter cluster as the subunit of the precipitate, the Fe-Fe2 distance (at 4.04 Å) detected for this sample probably accounts for the linkage between two subunits through Fe-Fe single-corner sharing (Figure 8). Correlatively the increase of NP corresponds to a shortening of the Fe-P distance down to 3.3-3.2 Å. This could lead locally to the formation of a denser basic unit. Thus, these two factors (the formation of dense subunits (23) Moore, P. B.; Anthony, R.; Kampf, A.; Irving, J. Am. Mineral. 1974, 59, 900-905. (24) Vencato, I.; Mattievich, E.; Mascarenhas, Y. P. Am. Mineral. 1989, 74, 456-460. (25) Anderson, J. B.; Rea, R. J.; Kostiner, E. Acta Crystallogr. 1976, B32, 2427-2431. (26) Vencato, I.; Mascarenhas, Y. P.; Mattievich, E. Am. Mineral. 1986, 71, 222-226. (27) Moore, P. B.; Araki, T. Am. Mineral. 1975, 60, 454-459.
Fe Oxyhydroxide in the Presence of PO4 Ions
Langmuir, Vol. 12, No. 26, 1996 6707
Figure 8. Tentative models of nucleation and growth of hydrolyzed Fe species for the P/Fe ) 0.2 series.
and the linkages between them) could explain the macroscopic precipitation observed at n ) 2 for P/Fe ) 0.2. At P/Fe ) 0.5, following the same reasoning as above, combining NP with the initial P/Fe stoichiometry suggests that one PO4 tetrahedron bridges no more than two dimers (Figure 9). Thus, at this higher P/Fe ratio, one PO4 bridging two edge-sharing iron dimers can be considered as a subunit. At n ) 1.5 and 2, Fe-Fe2 distances probably represent linkages between subunits through Fe-OHFe bonds. The increase of NP from 0.8 to 1 for n ) 1.5 and 2, respectively, may correspond to the association of the small subunits through PO4 bridges (Figure 9), thus explaining the formation of an infinite phase, i.e. the gel observed macroscopically. Conclusion This study describes the first steps of nucleation and the growth mechanisms, at the atomic scale, during the hydrolysis of iron chloride in the presence of phosphate ions. The local structure of the polymers around iron atoms was studied by EXAFS spectroscopy at the Fe
Figure 9. Tentative models of nucleation and growth of hydrolyzed Fe species for the P/Fe ) 0.5 series.
K-edge. This study shows that the iron(III) chloride hydrolysis is limited to the dimer stage. The presence of chloride ions in the first coordination sphere and of phosphorus atoms in the second one inhibits the linkage of other iron octahedra on the dimer through double-corner sharing, corresponding to the normal evolution of the iron hydrolysis. For low hydrolysis ratios the presence of phosphorus atoms around iron octahedra is not detectable, but the polymerization of iron is already hindered. For higher n ratios (1.5 e n e 2) phosphate ions bond Fe dimers together, possibly forming dense clusters. Further growth at n ) 2 occurs by bridging through Fe single-corner sharing or PO4-Fe linkages of pre-existing units, depending on the initial P/Fe ratio. Acknowledgment. This work forms part of the EEC exchange research program within COST action no. D5 ‘Chemistry at Surfaces and Interfaces’. LA9606299
