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Hydrolysis of Iron(II) Chloride under Anoxic Conditions and Influence of SiO4 Ligands Emmanuel Doelsch,† Je´roˆme Rose,*,† Armand Masion,† Jean Yves Bottero,† Daniel Nahon,† and Paul M. Bertsch‡ CEREGE (UMR 6635) CNRS-Aix-Marseille III, Interfaces Physicochemical Group, Europoˆ le Me´ diterrane´ en de l’Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France; and Savannah River Ecology Laboratory, AACES, University of Georgia, PO Drawer E, Aiken, South Carolina 29802 Received October 26, 2001. In Final Form: February 27, 2002 This study describes the evolution of the nanoscale aqueous ferrous species as a function of the hydrolysis ratio and the influence of SiO4 ligands. Samples were synthesized under anoxic conditions and studied by EXAFS at the Fe K-edge. At low hydrolysis ratio (R ) [OH-]added/[Fe(II)]initial ) 0.1), the data are consistent with the formation of a planar Fe cluster with five Fe(II) octahedra. Although Si neighbors cannot be detected unequivocally by Fe K-edge EXAFS, they affect the condensation of ferrous species by hindering the linkages between Fe(II) octahedra. Indeed, at low hydrolysis ratio (R ) 0.1) and Si/Fe ) 1, only small sized Fe clusters such as planar Fe(II) trimers are formed. Moreover, this study clearly indicates that the polymerization of Fe(II) is more affected by the presence of SiO4 than in the case of Fe(III).
Introduction The oxidation-reduction steps of iron are of a particular importance in the global geochemical cycling and industrial applications. Indeed, 17 × 1020 mol of Fe are stored in the sedimentary rocks and 3.5 × 1012 mol/year are transformed from oxidized to reduced reservoirs and vice versa.1 Moreover, these oxidation-reduction processes are of particular interest for the hydrometallurgical industry since pure and high quality iron oxides and oxyhydroxides can be synthesized from Fe(II) solutions.2,3 Most of the studies dealing with the Fe(II)-Fe(III) system concern the composition and the structure of final products from the oxidation of an Fe(II) solution. Under acidic conditions (pH < 5), the Fe oxides precipitate directly from Fe(III) aqueous species (Fe(II) ions oxidizes before precipitation), and the final products can be ferrihydrite, goethite, or hematite depending on temperature and the nature of the ferrous salt used.3 In alkaline solutions (pH > 8), the end product is magnetite.3 For slightly acidic to slightly alkaline conditions, intermediate Fe(II)-Fe(III) hydroxy-salt phases are formed.3 These so-called green rusts (GRs) have been intensively studied4-6 because they govern the mechanisms and kinetics of corrosion of ironbased alloys in aqueous media. They have a layered structure made of brucitelike sheets, positively charged due to trivalent Fe. The neutrality is ensured by anions such as Cl-, CO32-, or SO42-. The GRs are stable only at * Corresponding author. E-mail:
[email protected]. † Europo ˆ le Me´diterrane´en de l’Arbois. ‡ University of Georgia. (1) Stumm, W.; Sulzberger, B. Geochim. Cosmochim. Acta 1992, 56, 3233-3257. (2) Chen, T. T.; Cabri, L. J. In Iron control in hydrometalurgy; Monhemius, J. E. D. a. A. J., Ed.; Ellis Horwood: Chichester, England, 1986; pp 19-55. (3) Cornell, R. M.; Schwertmann, U. The iron oxides-structure, properties, reactions, occurrence and uses; VCH: New York. 1996; 573 pages. (4) Refait, P.; Genin, J. M. R. Corros. Sci. 1993, 34, 797-819. (5) Refait, P.; Genin, J. M. R. Clay Mineral. 1997, 32, 597-613. (6) Refait, P.; Abdelmoula, M.; Genin, J.-M. R. Corros. Sci. 1998, 40, 1547-1560.
low redox potential, and upon oxidation they transform to lepidocrocite or goethite.7 The effects of a number of ions such as carbonate,8 aluminum,9 and silicate10,11 have been extensively studied during the oxidation of FeCl2 solutions, Fe(OH)2, or GRs. The presence of chloride in the initial solution promotes lepidocrocite,12 whereas the presence of Si hinders the formation of lepidocrocite and ferrihydrite forms instead.10,11 Most of the published work on ferrous iron deals with the kinetics of oxidation1,13,14 and the effect of anions on the oxidation.15-17 Surprisingly little is known about the nucleation and growth mechanisms of Fe(II) polymers under anoxic conditions during the first steps of hydrolysis which precede crystallization. To our knowledge, no study has been dedicated to the description of mechanisms of Fe(II) condensation. In a review of the aqueous chemistry of metal cations, Henry et al. (1992)18 detail the condensation mechanisms for some divalent cations such as Pb(II) or Ni(II). In summary, two processes have been described: • The condensation begins at R ) [OH-]/[M] ) 1 (with M for metal) with the formation of a dimer. For divalent cations, the negative partial charge of the oxygen in the (7) Schwertmann, U.; Fechter, H. Clay Mineral. 1994, 29, 87-92. (8) Carlson, L.; Schwertmann, U. Clay Mineral. 1990, 25, 65-71. (9) Taylor, R. M.; Raupach, M.; Chartres, C. J. Clay Mineral. 1990, 25, 375-389. (10) Schwertmann, U.; Thalmann, H. Clay Mineral. 1976, 11, 189199. (11) Zahurul, K. Clays Clay Mineral. 1984, 32, 181-184. (12) Taylor, R. M. Clays Clay Mineral. 1984, 17, 369-377. (13) Davison, W.; Seed, G. Geochim. Cosmochim. Acta 1983, 47, 6779. (14) Barry, R. C.; Schnoor, J. L.; Sulzberger, B.; Sigg, L.; Stumm, W. Water Res. 1994, 28, 323-333. (15) Tamura, H.; Goto, K.; Nagayama, M. J. Inorg. Nucl. Chem. 1976, 38, 113-117. (16) Tamura, H.; Kawamura, S.; Hagayama, M. Corros. Sci. 1980, 20, 963-971. (17) Gunten, U. V.; Schneider, W. J. Colloid Interface Sci. 1991, 145, 127-139. (18) Henry, M.; Jolivet, J. P.; Livage, J. In Structure and bonding; Springer-Verlag: Berlin, Heidelberg, Germany, New York, and Tokyo, 1992, pp 153-206.
10.1021/la011605r CCC: $22.00 © 2002 American Chemical Society Published on Web 04/26/2002
Hydrolysis of Iron(II) Chloride
µ2-OH bridges (i.e.: M-O(H)-M) induces the condensation which occurs by the transformation of all the µ2-OH bridges into µ3-OH bridges (i.e.: M-O(-M)(H)-M). A compact symmetrical tetramer [M4(OH)4(OH2)12]4+ is thus obtained with four metal atoms at the vertexes of a tetrahedron. • At R ) 2, the neutralization of the divalent metal solution leads to hydroxide nucleation and growth. The resulting hydroxide has a typical lamellar CdI2 structure based on metal sheets having µ3-OH bridges on each side. The formation of this hydroxide could be preceded by the formation of planar tetramers which grow along perpendicular directions. As there is no structural relation between a compact tetramer observed at R ) 1 and a planar tetramer, Jolivet et al.19 suggest that [M(OH)2(OH2)4]0 will form a dimer which will link with another one to form the planar tetramer. Contrary to what is seen for Fe(II), many studies20-25 have been devoted to the description of the structure of the intermediate polycations, which are formed during the reaction of ferric salt titration. In the case of chloride, the hydrolysis, condensation, and polymerization of Fe follows a set pathway including the formation of a dimer, a trimer and an intermediate polycation “Fe24” whose local structure is that of akaganeite,21 whereas the growth of iron nitrate phases involves a variety of different subunits.24-27 In a previous study,28 Fe K-edge EXAFS was used to study the local structure of freshly prepared Si/Fe(III) precipitates at various Si/Fe(III) molar ratios from 0 to 4 and pH values at 3, 5, 7, and 10. The level of Fe(III) polymerization (number of Fe-Fe contributions) and the growth regime are strongly dependent on the Si/Fe(III) molar ratio of the system. At Si/Fe < 1, the growth of Fe species is shown to occur through edge and corner sharing bonds, whereas for Si/Fe > 1 growth occurred mainly by edge sharing. Products with a Si/Fe ) 1 ratios are characterized by nanosized clusters. Thus, at low (0-0.5) and high (2-4) Si/Fe ratios, the growth of Fe colloids is respectively three- and two-dimensional. Si/Fe ratios around 1 represent a crossover between these two growth regimes. The objectives of the present study are (i) to determine and to compare the nucleation and polymerization processes for Fe(II) and Fe(II)-Si systems as a function of the hydrolysis ratio (R ) [OH-]/[Fe2+]) and (ii) to compare the local structure of the formed species in Fe(III)-Si and Fe(II)-Si systems. The present investigation is achieved through EXAFS at the Fe K-edge, which allows one to characterize the speciation at the nanostructural scale. (19) Jolivet, J.-P. De la solution a` l’oxyde; Intere´ditions/CNRS e´ditions: Paris, 1994; 387 pages. (20) Combes, J. M.; Manceau, A.; Calas, G.; Bottero, J. Y. Geochim. Cosmochim. Acta 1988, 53, 583-594. (21) Bottero, J. Y.; Manceau, A.; Villieras, F.; Tchoubar, D. Langmuir 1994, 10, 316-319. (22) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398-402. (23) Tchoubar, D.; Bottero, J. Y. C. R. Acad. Sci. Paris, Ser. IIa 1996, 322, 523-534. (24) Bottero, J. Y.; Tchoubar, D.; Arnaud, M.; Quienne, P. Langmuir 1991, 7, 1365-1369. (25) Rose, J.; Manceau, A.; Masion, A.; Bottero, J. Y. Langmuir 1997, 13, 3240-3246. (26) Schwertmann, U.; Friedl, J.; Pfab, G. J. Solid State Chem. 1996, 126, 336. (27) Schwertmann, U.; Friedl, J.; Stanjek, H. J. Colloid Interface Sci. 1999, 209, 215-223. (28) Doelsch, E.; Rose, J.; Masion, A.; Bottero, J.-Y.; Nahon, D.; Bertsch, P. M. Langmuir 2000, 16, 4726-4731.
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Materials and Methods Materials. Si/Fe(II) samples (with molar ratios equal to 0, 0.5 and 1) were synthesized following the method of Peterson et al.29 with some modifications. To exclude oxygen, sample synthesis was performed in glovebox containing 90% N2 and 10% H2. The H2 was used to activate a Pd catalyst to remove oxygen from the glovebox atmosphere. Oxygen was removed from all solutions by purging with N2-H2. Oxygen content was determined using dissolved oxygen ampules (Chemetrics, Inc., 0-40 ppb range). Solutions were used only after the dissolved oxygen concentration was 0.1, only one FeFe contribution (at 3.26 Å) is needed to have good agreement between the experimental and calculated spectra as shown for Si/Fe(II) ) 1, R ) 1 in Figure 4. For less hydrolyzed samples (R ) 0.1), the presence of a beat node at 12 Å-1 requires the use of two Fe-Fe contributions at 3.27 and 3.15 Å to obtain a satisfactory fit for all studied Si/Fe(II) ratios(Figure 4b). Analysis of Data by Using FEFF and FEFFIT. Figure 5 compares the RDF of Fe(OH)2 calculated with FEFF in two cases: single and multiple scattering. No difference is observed on the first and the second peak of the RDF, which means that multiple scattering does not affect the first and the second coordination sphere. The signal results only from the local environment of the Fe central atom: 6 O at 2.14 Å for the first coordination sphere and 6 Fe at 3.26 Å for the second coordination sphere. The peak at 5.25 Å is present on the two RDF of Fe(OH)2 also. Thus, it can be attributed to the contribution of a single scattering path such as Fe-O with Fe and O on two different octahedra. The main difference occurs at 6 Å with the appearance of a peak for the RDF of Fe(OH)2 spectrum calculated with the multiple scattering paths. Therefore, this peak can only be explained by the contribution of multiple scattering path such as Fe-O-Fe-O, Fe-Fe-O or Fe-Fe-Fe (Figure 5). On the basis of these preliminary remarks, we used only the single scattering path of the Fe(OH)2 reference calculated by FEFF in order to model the first and the second coordination sphere of the samples Si/Fe(II) ) 0 and Si/Fe(II) ) 1 at R ) 0.1 with FEFFIT. The results are presented in Table 3 and in Figure 6. For the first coordination sphere, we found six oxygens at a unique distance from the central Fe atom (r ) 2.13 Å for Si/Fe(II) ) 0 and r ) 2.12 Å for Si/Fe(II) ) 1). This is different from what we found for Si/Fe(II) ) 0, R ) 0.1 (Table 1) where two Fe-O shells at 2.14 and 2.02 Å were needed to improve the quality of the fit. For the second coordination sphere, two Fe-Fe shells were found at 3.16 and 3.26 Å. The number of neighbors are drastically different for Si/Fe(II) ) 0 and 1 at R ) 0.1. We find 2.71 neighbors at 3.26 Å and 0.40 neighbors at 3.16 Å for Si/Fe(II) ) 0, while for
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Figure 4. Comparison of partial EXAFS spectra: (a) samples Si/Fe(II) ) 0 at R ) 0.1 and 1 corresponding to the first peak of the RDF; (b) samples Si/Fe(II) ) 1 at R ) 0.1 and 1 corresponding to the second peak of the RDF. Table 2. Structural Parameters for Fe (Backscatterer in the Second Coordination Sphere) Contributions Deduced from EXAFS Analysis for Si/Fe(II) ) 0, 0.5, and 1 Series Fe-Fe1 shell Ra
r window (Å)
0 0.09 0.1 1 2.5
2.3-3.3 2.3-3.4 2.3-3.3
0.1 1 2.5
2.3-3.4 2.2-3.4 2.3-3.4
0.1 1 2.5
2.4-3.3 2.3-3.3 2.3-3.3
rb
Nc
σd
Fe-Fe2 shell r
N
σ
Qe
3.27 3.27 3.28
0.00 0.00 2.41 5.25 6.00
0.08 0.09 0.09
0.04 0.02 0.03
Si/Fe(II) ) 0.5 3.15 0.21 0.08 3.27 3.25 3.27
1.32 4.36 6.00
0.08 0.09 0.09
0.03 0.02 0.01
0.49 3.90 6.00
0.02 0.09 0.09
0.02 0.04 0.02
Si/Fe(II) ) 0 0.00 0.00 3.15 0.40 0.08
3.16
Si/Fe(II) ) 1 0.45 0.02
3.28 3.26 3.27
a R: hydrolysis ratio. b r: distance between the two atoms of each atomic pair (Å). c N: number of atoms in shell of iron. d σ: DebyeWaller factor (disorder parameter; Å). e Q ) ∑[(k3χtheo)- (k3χexp)]2/ (k3χexp)2.
Si/Fe(II) ) 1 we find only 0.75 neighbors at 3.26 Å and 0.58 at 3.16 Å. These results are similar to those reported in Table 2. Discussion Speciation of Fe(II) Aqueous Species. As explained in the Materials section, sections a and b of the Fe(II) titration (Figure 1) correspond to the titration of HCl since the quantity of NaOH added corresponds to the initial HCl concentration. Thus, the Fe(II) titration concerns only sections c, d, and e. This titration curve is quite similar to previous results40 showing a buffer region corresponding to hydrolytic consumption of hydroxyls (plateau section c, R ) 0.1-2) and pH increase at R > 2 (d). This pH increase corresponds to the formation of the Fe(OH)2 hydroxide since it occurs at R ) 2.18 To have a more accurate view of our results we plotted the samples in a Eh vs pH diagram also known as Pourbaix diagram (Figure 7). This diagram was built by taking into account chloride green rust (GR1(Cl-) with a chemical (40) Bertsch, P. M.; Miller, W. P.; Anderson, M. A.; Zelazny, L. W. Clays Clay Mineral. 1989, 37, 12-18.
formula set at FeII3FeIII(OH)8Cl‚nH2O4 for which data are reported elsewhere.41 The distribution of samples in the Pourbaix diagram is as follows •From R ) 0 to R ) 0.09, samples are in the domain of soluble Fe(II). •The R ) 0.1 sample is in the domain GR1(Cl-). •The R ) 1 sample is located on the boundary between GR1(Cl-) and Fe(OH)2 areas. •The R ) 2.5 sample is in the Fe(OH)2 domain. The location of the R ) 0.1 sample on the Eh vs pH diagram could suggest the presence of GR1(Cl-)-like material. This remark poses the problem of the initial oxidation state of aqueous iron species. Indeed, the formation of green rust is described during the steps of the oxidation of Fe(OH)2 and thus implies the presence of Fe(III) cations.4-6,41 For our experiments, all the precautions to avoid oxidation have been taken. Nevertheless, the initial FeCl2‚4H2O salt has a greenish color, which suggests the presence of some Fe(III). Therefore, it seems reasonable to argue that samples having the lowest hydrolysis ratios (R < 1) are relatively rich in Fe(III) because of the low solubility of Fe(III) species in comparison with Fe(II) aqueous species for these pH of synthesis (pH < 7.6, Figure 1). A closer insight into the nature of the precipitates and their modifications with hydrolysis ratio is provided by the EXAFS structural data (Tables 1 and 2). Six O neighbors characterize the first coordination sphere of our samples, which means that Fe atoms are in octahedral coordination. The distance between the Fe central atom and the O neighbors increases with the hydrolysis ratio (i.e.: from 2.11 Å for Si/Fe(II) ) 0, R ) 0 to 2.14 Å for Si/Fe(II) ) 0, R ) 2.5). For Si/Fe(II) ) 0, R ) 2.5, we find six oxygen atoms at a distance of 2.14 Å from the central Fe atom which is in good agreement with the Fe(OH)2 reference. For other Fe-O distances, an explanation can be proposed by the analysis of the Si/Fe(II) ) 0, R ) 0.1 sample. Indeed, we found two atomic shells of O backscatterers at two distances 2.14and 2.02 Å. This last distance is not characteristic of divalent iron hydroxide but can be attributed to the Fe(III)-O distance.28 Therefore, it (41) Genin, J. M. R.; Bourrie, G.; Trolard, F.; Abdelmoula, M.; Jaffrezic, A.; Refait, P.; Maitre, V.; Humbert, B.; Herbillon, A. Environ. Sci. Technol. 1998, 32, 1058-1068.
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Figure 5. Comparison of Fe(OH)2 RDF calculated by FEFF for single and multiple scattering and illustration of multiple scattering paths on Fe(OH)2 structure. Table 3. Structural Parameters for Fe (Backscatterer in the First and the Second Coordination Sphere) Contributions Deduced from FEFFIT Analysis of Si/Fe(II) ) 0, R ) 0.1 and Si/Fe(II) ) 1, R ) 0.1 Experimental Data Fe-O shell Si/Fe(II) ) 0, R ) 0.1 Si/Fe(II) ) 1, R ) 0.1
Fe-Fe1 shell
ra
Nb
σ2 c
r
2.13 2.12
6.00 6.00
0.006 0.005
3.16 3.16
Fe-Fe2 shell
N
σ2
r
0.4 0.58
0.006 0.005
3.26 3.26
quality of fit
N
σ2
r-factord
χν2 e
2.71 0.75
0.006 0.005
0.016 0.013
14.5 6.2
a R: distance between the two atoms of each atomic pair (Å). b N: number of atoms in shell of iron. c σ2: Debye-Waller factor (disorder parameter; Å). d N
∑{[Re(f )]
2
i
+ [Im(fi)]2}
i)1
r-factor )
N
∑{[Re(χ
2 datai)]
+ [Im(χdatai)]2}
i)1
with fi being the function to minimize, N the number of function evaluations, and χ the data. χν2 )
[
Nidp N
2
N
∑{[Re(f )]
2
i
i)1
]
e
+ [Im(fi)]2} /ν
with fi being the function to minimize, N the number of function evaluations, Nidp the number of indepedent measurements in a spectrum, N the uncertainties in the functions to minimize, and ν the degree of freedom in the fit. For the latter two parameters, good fits are characterized by r-factor ) 0.02 and χν2 ≈ 10.
indicates that some ferric iron ions are still present in the solution. The distance at 2.11 Å for the samples at R < 0.1 can be interpreted as a mean distance between 2.14 (for Fe(II)-O) and 2.00 Å (for Fe(III)-O). From the analysis of the second coordination sphere of the Si/Fe(II) ) 0 samples, we can observe the following points. • Up to R ) 0.09, no Fe-O-Fe linkage is formed. No dimers and trimers are formed at this low R value, contrary to what was found for ferric chloride.21 • For R ) 0.1, two Fe-Fe distances are necessary to obtain a good fit: 3.27 and 3.15 Å. The distance at 3.27 Å is very close to the Fe-Fe distance in the Fe(OH)2 reference mineral (i.e.: 3.26 Å), but 3.15 Å is not a common distance between two iron neighbors in any iron(II) or iron(III) oxyhydroxide or oxide. By considering that both Fe(II) and Fe(III) cations coexist in the same structure, this uncommon distance can be understood. Indeed, the edge-sharing linkage between two Fe(II) octahedra is
characterized by a distance of 3.26 Å and for edge-sharing linkage between two Fe(III) octaedra it is characterized by a distance of 2.95-3.11 Å.28 Therefore, an edge-sharing linkage between an Fe(II) and an Fe(III) octahedron should have a mean distance ranging from 3.10 to 3.18 Å, which is very close to what we found (i.e., 3.15 Å). The low number of neighbors (Ntotal ) 2.8) is inconsistent with the local structure of Fe(OH)2 which is characterized by 6 Fe neighbors at 3.26 Å. This remark can be validated by the RDF of Si/Fe(II) ) 0, R ) 0.1 sample (Figure 2). Indeed, the absence of the peak at 5.25 Å (Fe-O single scattering between two different octahedra, Figure 5) and of the peak at 6 Å (multiple scattering, Figure 5) allow us to assert that only small clusters with a few Fe atoms are formed. Moreover, the data do not correspond to a compact or a planar tetramer as observed for Pb(II) or Ni(II).18 • For R ) 1 and R ) 2.5, there are 5.25 and 6.00 neighbors respectively at 3.27 Å, which corresponds to a local
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Figure 8. Evolution of total number of iron neighbors.
Figure 6. Results of FEFFIT analysis: (a) Si/Fe(II) ) 0, R ) 0.1; (b) Si/Fe(II) ) 1, R ) 0.1.
Figure 7. Eh-pH equilibrium diagram of the system Fe/ chloride containing solution at 25 °C for [Cl-] ) 0.4 mol/L. Gr(Cl-) was taken at Fe4(OH)8Cl.nH2O. f indicates samples.
structure very close to Fe(OH)2. This is confirmed by the presence of the two peaks at 5.25 and 6 Å on the RDF (Figure 2). The hydrolysis, condensation, and polymerization of Fe(II) under anoxic conditions follow a set pathway including first the formation of small cluster with a planar structure
and then intermediate polycations whose local structure is that of Fe(OH)2. The presence of Fe(III) ions in the vicinity of Fe(II) ions is only detected for R ) 0.1 samples. Thus, their presence is not a result from a progressive oxidation of the ferrous solutions, which would have led to the detection of Fe(III) whatever the hydrolysis ratio. It is probably linked to the initial state of the iron(II) chloride salt used in the experiments which can contain Fe(III) atoms in a minor quantity. Influence of Si Content on the Fe(II) Condensation and Comparison with the Fe(III)-Si System. By EXAFS, the presence of a “light” element such as Si in the vicinity of a “heavy” element such as Fe is not easily detected. As previously shown,35 including a Si shell in the calculation was not necessary to obtain a good spectral fit. Furthermore, the authors demonstrated that adding or omitting the Si shell did not affect the Fe-Fe distances derived from the spectral fitting. Using only Fe-Fe shells to model our experimental data is sufficient to describe evolution of the local Fe environment in our samples. The results of the second coordination shell analysis show that the SiO4 ligands affect the local environment of iron (Figure 8), especially at R ) 0.1 (Figure 3a, Tables 2 and 3) and R ) 1 (Figure 3b, Tables 2 and 3). Indeed, there is a strong decrease of the number of neighbors from Si/Fe(II) ) 0 with Ntotal ) 2.8 to Si/Fe(II) ) 1 with Ntotal ) 0.94 at R ) 0.1. Under the same pH conditions, the decrease of Ntotal in Si/Fe(III) systems was less drastic, from Ntotal ) 3.94 at Si/Fe(III) ) 0 to Ntotal ) 2.49 at Si/Fe(III) ) 1,28 which is a reduction by a factor 1.6 vs a factor 3 for the ferrous systems. Thus, it appears that Si atoms hinder more strongly the condensation of Fe in Si/Fe(II) than in Si/ Fe(III) systems. In the studied Fe(II) samples, the only type of linkage observed is the edge-sharing linkage between two Fe octahedra. Thus, it is difficult to consider different growth regimes of the Fe(II) species depending on the Si content as opposed to what was demonstrated for Si/Fe(III) systems.28 To have a more accurate view of the local structure of the samples Si/Fe(II) ) 0 and 1 synthesized at R ) 0.1, we used FEFFIT results to build model clusters (Figure 9). For each cluster, we introduced an Fe(III) octahedron (Fe1 in both cases) with a distance rFe(II)-Fe(III) ) 3.15 Å. Five octahedra were needed to obtain a total number of iron atoms equal to 2.7 for Si/Fe(II) ) 0 at R ) 0.1 (Figure 9a). For Si/Fe(II) ) 1, only three octahedra are needed to obtain NFe total ) 1.33 (Figure 9b). These values are close
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increases. Indeed, whatever the Si/Fe(II) ratio, we found NFe-Fe ) 6 (Table 2, Figure 8) which means that the local structure of the samples is identical to the Fe(OH)2 structure. For this hydrolysis ratio, the pH of synthesis is very high (pH ) 12.8) and corresponds to higher solubility of Si.31 At pH ) 10 and for oxidizing conditions (Fe(III)-Si), Si-O-Fe bonds were still detected.42 Under the present anoxic conditions, the Si atoms do not seem hinder the growth of Fe(II) species at high pH. Conclusion
Figure 9. 3D structural clusters: (a) for Si/Fe(II) ) 0, R ) 0.1 with NFe total ) 2.7, NFe (3.15 Å) ) 0.33, and NFe (3.26 Å) ) 2.33; (b) for Si/Fe(II) ) 1, R ) 0.1 with NFe total ) 1.33, NFe (3.15 Å) ) 0.66, and NFe (3.26 Å) ) 0.66; (c) possible localization of Si atoms in the Si/Fe(II) ) 1, R ) 0.1 3D cluster.
to those determined from the experimental EXAFS data. Nevertheless, it is obvious that the clusters presented in Figure 9 are only tentative 3D structures and that the actual clusters may have slightly different structures. Even if the presence of Si is not detected by Fe K-edge EXAFS, these clusters provide some interesting information. Indeed, the absence of Fe4 to Fe6 octahedra for Si/ Fe(II) ) 1, R ) 0.1 cluster suggest the possibility of FeO-Si linkages which hinder further Fe(II) condensation. Possible localizations of the Si tetrahedra are illustrated in Figure 9c. At R ) 2.5, no modification of the local structure of the samples is detected by EXAFS when the Si content
To our knowledge, this study allows one to describe for the first time the nucleation and the growth of the ferrous iron species under anoxic conditions. The formation of the first nuclei occurs at R ) 0.1 and corresponds to the formation of small clusters (NFe-Fe < 3) having a planar structure, different from the compact or the planar tetramer observed other divalent cations such as Pb(II) or Ni(II).18 The growth of these nuclei takes place for 0.1 < R < 2 during the hydrolytic consumption of hydroxyls. The formation of linkages between several nuclei or one nucleus and several Fe(II) monomers (the present data do not allow us to distinguish between these two mechanisms) leads to a local structure very close to Fe(OH)2 (NFe-Fe ) 5.2 for Si/Fe(II) ) 0, R ) 1). Finally, for R > 2, the local structure of samples is identical to the ferrous iron hydroxide. The presence of Si ligands hinders strongly the Fe(II) condensation at low hydrolysis ratios (R ) 1) which has not been observed for Si/Fe(III) samples. For R ) 2.5, the high solubility of Si does not imply the existence of numerous linkages with Fe(II) ions. Therefore, the samples have the same local structure whatever the Si/Fe ratios. Acknowledgment. The authors wish to thank A. Traverse and F. Bouamrane who are in charge of the beam line D42 of DCI (LURE, Orsay, France) for their helpful and kind assistance and P. Refait who provided us with the Fe(OH)2 EXAFS spectrum and numerous pieces of advice for the synthesis of samples. This work was partially supported by CNRS-NSF collaboration agreement No. 7383. P.M.B. was supported by financial assistance award No. DE-FC09-96SR18546 from the U.S. Department of Energy to the University of Georgia Research Foundation. LA011605R (42) Doelsch, E.; Stone, W. E. E.; Petit, S.; Masion, A.; Rose, J.; Bottero, J.-Y.; Nahon, D. Langmuir 2001, 17, 1399-1405.
