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Crystallization of ferric hydroxide into spinel by adsorption on colloidal magnetite
Crystallization of Ferric Hydroxide into Spinel by Adsorption on Colloidal Magnetite P H I L I P P E BELLEVILLE, J E A N - P I E R R E JOLIVET, ELISABETH T R O N C , 1 AND JACQUES LIVAGE Chimie de la MatiOre Condens~e (CNRS URA 1466), Universit8 Pierre et Marie Curie, Paris 05, France
Received April 5, 1991; accepted September 25, 1991 Interaction between "ferric hydroxide" and colloidal magnetite in alkaline medium has been characterized by transmission electron microscopy,X ray powder diffraction, Mrssbauer spectroscopy,and kinetics of dissolution in acidic medium. An adsorption reaction takes place immediately. It involves an interfacial electron transfer which starts magnetite oxidation and makes ferric hydroxide transform into a spinel layer growing epitacticallyon the particle surface. © 1992AcademicPress,Inc. INTRODUCTION "Ferric hydroxide" is a very poorly crystallized hydrated oxyhydroxide, made up of a short-range ordered arrangement of Fe(O, OH, OH2) octahedra ( 1, 2). The local structure is close to that of goethite ( ~ - F e O O H ) . With time it generally transforms into goethite a n d / o r hematite (a-Fe20 3 ) depending on the conditions (pH, temperature, concentration, foreign ions . . . . ) ( 3 - 6 ) . Goethite forms by the dissolution of ferric hydroxide and reprecipitation ofgoethite in solution, whereas hematite arises from a solid state reaction within ferric hydroxide particles. Divalent transition metal cations (M H) at sufficiently high levels ( M " / F e I" above ca. 0.2, roughly) cause ferric hydroxide to convert into spinel ( 7 - 9 ) . This generally proceeds by dissolution and reprecipitation. In the case of FeII, however, the conversion to spinel can also be effected topotactically (10). Fe H adsorption on ferric hydroxide leads to interfacial electron transfer. Delocalization of excess electrons within ferric hydroxide induces local spinel ordering. The degree of ordering increases with the Fe n level J To whom correspondence should be sent at Chimie de la Mati~re Condensre, T54 E5, U.P.M.C., 4 Place Jussieu, 75252 Paris Cedex 05, France.
and governs the stability of the material in solution. F e " / F e "I compositions above ca. 0.07 are sufficient to ensure stability in alkaline media. Colloidal magnetite (Fe304), on the other hand, exhibits a remarkable redox behavior (l 1-13). Electron transfer between Fe n and Fe I" ions in the octahedral sublattice makes the solid respond to surface phenomena by ionic and electronic transfers, eventually reversible, through the interface. The transformation Fe304 --~ T-Fe203 can thus take place under a great variety of conditions in aqueous media. In each case the conversion is driven by an adsorption reaction which traps mobile electrons at surface sites. Surface Fe ~ ions are released into solution or oxidized in s i t u depending on the conditions. Electron hopping and small structural changes ensure the renewal of ferrous sites at the surface and keep the reaction going up to completion. The oxidation process in the particle is the same as for aerial oxidation (14), but p h e n o m e n a inducing the interfacial transfer and external conditions ruling the behavior of surface Fe II are very different. In view of the properties of each system, ferric hydroxide and magnetite, we attempted to characterize their interaction. Preliminary
453 0021-9797/92 $3.00 Journal q[('olloid and lnte(/bce Sciemz,, Vol. 150, No. 2, May 1992
Copyright © 1992 by Academic Press~ Inc. All rights of reproduction in any form reserved.
454
BELLEVILLE
results have been previously reported ( 15 ); a detailed investigation is reported here. EXPERIMENTAL
Materials Magnetite was prepared by alkalinizing a stoichiometric mixture of FeCI3 and FeCI2 with NH3 at room temperature. In order to minimize dissolution-reprecipitation processes, suspended magnetite was stabilized by aging for a week. Great care was taken to exclude oxygen at all stages of the preparation. A varying proportion of FeCI3 ( 1 M) was added to an aliquot of suspended magnetite (pH 8, [Fe] - 0.3 M ) under vigorous stirring and bubbling with argon. NH3 was simultaneously added to keep the pH above 8 and avoid release o f F e n from the magnetite ( 11 ). Fem immediately precipitated and base consumption corresponded to the stoichiometry OH /Fead = 3. The suspension was then centrifuged, rinsed with degassed distilled water, and stored under argon. The pH was near 8; it was not measured during the aging. A reference sample of magnetite was stored under the same condition. The various systems will be characterized by the ratio of added Fe Ill (Fela~) relative to total initial iron (Fe~). -
111
Methods Chemical analysis. Fe n and total Fe concentrations in the suspensions were determined after dissolving the solid in concentrated HC1. Fe n was first titrated potentiometrically with K2Cr207. After titration, all Fe was reduced with a SnCI2 solution and the mixture was again titrated with K2Cr207. Chemical reactivity. The various solid species were characterized by their behavior against dissolution in acidic medium (2 M HC1) according to the procedure described elsewhere (16). Under such conditions, the dissolution frees both Fe n and Fe m ions. The kinetics was determined by reducing Fem ions as they were released into solution. KI (0.2 M) was used as a reducing reagent and formed Journal of CoUoid and Interface Science. Vol. 150, No. 2, May 1992
ET AL.
I2 was titrated with Na2S203. Acid type and concentration are such that ferric hydroxide dissolves quasi-instantaneously, whereas crystalline phases dissolve more slowly. Their dissolution rate is mainly determined by the degree of structural ordering, the particle size, and the FeU/Fe nl composition, Fe n ions promoting dissolution ( 17, 18). Data analysis using pseudo-first-order rate laws yielded dissolution rate constants and proportions of the various Fe nl species. X ray diffraction. Diffractograms were recorded on a Siemens D500 powder diffractometer equipped with a graphite monochromator, using the CuKa radiation. Powders were obtained from suspensions by centrifuging, washing, and drying under argon. Transmission electron microscopy. Micrographs and diffraction patterns were obtained on a Jeol 100 CXII apparatus. Samples were prepared by evaporating very dilute ultrasonicated solutions onto carbon-coated grids, dspacings were calibrated using an Au pattern. Particle size distributions were estimated by measuring the size of about 400 particles. Mrssbauer spectrometry. M6ssbauer spectra were recorded using a conventional spectrometer (Elscint-Inel) with a 57Co/Rh source. Samples were made up of suspensions frozen in a polymeric film; this was carried out by dispersing the suspension in an aqueous solution of polyvinylic alcohol and drying the mixture in an argon flow. Velocities were calibrated using an iron foil. Isomer shifts are given relative to metallic iron at room temperature. RESULTS
AND
DISCUSSION
The composition of aged suspensions always corresponded to the Fen/Fe In stoichiometry of the initial mixture of magnetite and ferric hydroxide. Operating conditions effectively avoided oxidation.
Characterization of the Reaction A detailed investigation of the interaction between precipitated Fe nl and stoichiometric
ADSORPTION OF FERRIC HYDROXIDE
455
FIG. 1. Electronmicrographsof a mixture Fe304 + 2.25 Fe(OH)3 immediatelyafter the precipitation of ferric hydroxide(a), and after 6 days aging (b). magnetite was carried out at the composition llI Fead/Fei = 0.75, which corresponds to the stoichiometry Fe304 + 2.25 Fe(OH)3. Electron microscopy. TEM observations of the system immediately after precipitation (Fig. 1a) show that it is two-phase. It consists of spinel particles of ca. 100 A mixed up with much smaller particles of ferric hydroxide. Under aging, ferric hydroxide decreases in proportion, but its characteristics remain unaltered. It is present only in a small amount in the 6-day-old mixture (Fig. I b). Spinel particles are then characterized by size distribution (Fig. 2a) with a mean (Dm) of 122 A (standard deviation tr = 27 A). Comparison with particle size distribution of starting magnetite ( D m = 106 A, a = 24 A) (Fig. 2b) indicates significant growth of the particles. X ray diffraction. The X ray diffraction pattern of the 6-day-old mixture (Fig. 3a) is analogous to that of starting magnetite (Fig. 3b). The unit-cell parameter is of ca. 8.36 A in each case; this is typical of nonstoichiometric magnetite and suggests that the starting magnetite partly oxidized during the process of drying. The variation of the line width at half-maximum as a function of the Bragg angle shows that there is no particular structural disorder due to the adsorption reaction. The apparent size deduced from the Scherrer formula is
equal to 120 A compared to 109 A for starting magnetite. Such data are consistent with TEM results and show that particle growth is coherent with respect to diffraction. Dissolution kinetics. Dissolution kinetics of starting magnetite and mixture at different aging times are shown in Fig. 4. Magnetite dissolution exhibits only one kinetic stage. After Fe nl precipitation, three stages are observed: a quasi-instantaneous stage followed by a rather slow stage and then a faster one. The quasi-instantaneous stage corresponds to the dissolution of ferric hydroxide. The other two kinetic stages are characteristic of the disso-
p% 10
|
i
10
10
D nm
20
FIG. 2. Particlesizedistribution for a 6-day-oldmixture Fe304 + 2.25 Fe(OH)3(a), and starting Fe304 (b). Journal ~![ColloM and lnterJace Science, Vol. 150, No. 2, May 1992
456
BELLEVILLE ET AL.
106 A, in diameter, and iron is homogeneously distributed all through the particle, Fem uptake at 6-day-aging corresponds to an adsorbed layer 9 ~, thick, which is consistent with TEM results ( 8 A). Mrssbauer spectroscopy. Spectra of starting magnetite and mixture at different aging times are shown in Fig. 5. The magnetite spectrum I I essentially consists of two sextets; one is due to Fe 3+ ions in tetrahedral sites and the other 311 to Fe 3+ and Fe 2÷ ions in octahedral sites. As the electron exchange is fast at the Mrssbauer time scale, these ions appear in the average state Fe 25+ and the relative areas of the two 440 220 sextets are theoretically in the ratio Fe3+/Fe 25+ 400 511 A = 1/2. In addition to the spinel pattern, the 22~ , 422 mixture immediately after Fe "x precipitation yields a quadrupole doublet typical of para' 3'o 50 70 o2e magnetic ferric hydroxide (19). Comparing FIG. 3 X. ray diffractionpattern of a 6-day-oldmixture the spinel pattern with that of starting magFe304 + 2.25 Fe(OH)3(a), and starting Fe304 (b). netite, particularly at negative velocities, one notices an increase in the intensity of the Fe 3÷ component relative to the Fe 2s÷ one. This is lution of partly oxidized magnetite (16). The typical of nonstoichiometric magnetite; unslow stage and the faster one correspond to paired octahedral Fe 3÷ ions give the same conthe attack on the particle surface and core, tribution as tetrahedral Fe 3÷ ions (20, 21 ). The respectively; the difference in reactivity comes spectrum's evolution with aging time indicates from a difference in composition, the surface progressive disappearance of ferric hydroxide being poorer in Fe". and increasing nonstoichiometry of spinel. Data analysis (Table I) shows that, immeHyperfine field distributions related to pardiately after Fem precipitation, ferric hydrox- ticle size distribution (22) and features related ide represents only 22% of total Fem. Since to fast electron exchange between ions in octhe amount of added Fem corresponds to 53% of total Fe m, we deduce that nearly 60% of added Fem has been taken up immediately by magnetite particles. Fem uptake goes on under aging; it is nearly complete (---90%) after 23 days aging. The characteristics of the dissolution of the spinel particles are progressively .c]--- " d altered: the rate constant relative to surface dissolution (Table I) decreases progressively and the kinetic stage relative to dissolution of 0 5 10 t rain 15 the core deviates more and more from a FIG. 4. Kinetics of dissolution in acidic medium of pseudo-first-order rate law (Fig. 4). This instarting Fe304(a) and a mixture Fe304 + 2.25 Fe(OH)3 dicates increasing oxidation of the surface and immediately after Fe(OH)3 precipitation(b), after aging existence of a composition gradient, increas- for 1 day (c), 6 days (d), and 23 days (e). Corepresents ingly smooth, through the particle. Assuming the amount of total Fem, and c that of FenI undissolved adsorption takes place onto a spherical particle at time t. Journal of Colloid and Interlace Science, Vol. 150, No. 2, M a y 1992
457
A D S O R P T I O N OF FERRIC H Y D R O X I D E TABLE I Characteristics of the Kinetics of Acidic Dissolution of Magnetite and a Mixture ii~ Fe304 q- 2.25 Fe(OH)3 (Fe~d/Fei = 0.75) Fe304
Aging time (day) Rate constant k (10-2 mn -~) Fraction immediatelydissolved relative to total Fem (%) Uptake Fe~JFei (%)
Fe304 + 2.25 F e ( O H ) 3
-12.7
0 3.4
0 --
22 44
tahedral sites (23) make the analysis of the spectra particularly complex. In order to estimate the proportion of ferric hydroxide, we used a simplified model involving one sextet with line widths constrained to be all equal for the Fe 3÷ component, and equal by pairs for the Fe :'5÷ component. Corresponding parameters are given in Table II. The proportion of feric hydroxide deduced from the relative area of the corresponding doublet, assuming identical recoilless fractions for all iron atoms, is consistent with dissolution kinetics data. The increase, in similar proportions, of Fe 3÷ and Fe 25+ hyperfine fields compared to starting magnetite indicates an increase in the size of spinel particles. Too-strong overlapping of the Fe 3+ and Fe 25+ components does not, however, allow reliable estimation of the variation of the composition of these particles.
i.'."
t=0
"
:" ," ;"
: ;-
.: :
• :
23d
ld
~'"
":
. .
~; "
;'~ "to
'
:, :. :. .
: ."
:
1,"
~'" " i.
--"~
~,~,..'~
. ; . f • . :
. ".,::" : j , ". .g,.: :. :
60
initial
Fe304
.;
-6
0
6
v (rnrn/s)
-6
0
6
F I G . 5, M 6 s s b a u e r spectra ( 3 0 0 K ) o f a m i x t u r e
1 2.3
Fe304
+ 2.25 Fe(OH)3at differentaging times (t), and starting Fe304.
6 2.1
14 55
10 61
23 1.6 7 65
These results show that ferric hydroxide precipitated in the presence of colloidal magnetite is transformed into a spinel layer which grows on the particle surface in epitaxy with the lattice. The reaction proceeds between solid phases by an adsorption reaction which is assisted by electron transfer through the interface magnetite/ferric hydroxide. The process is similar to that involved in the conversion of ferric hydroxide to spinel by Fe n adsorption (10). In this case, electron transfer between Fe n and Fem ions, evidenced by M6ssbauer spectroscopy, locally induces cubic close-packed ordering, and delocalization of excess electrons within the whole particle results in spinel crystallization. In pure ferric hydroxide iron atoms are only coupled via anions. As FeII-Fe m electron transfer is a direct process, metal-metal bonding is essential (24). In order for it to take place, electron transfer implies that suitable structural rearrangements have occurred. The loose structure and high level of hydration of ferric hydroxide make such rearrangements proceed easily by olation-oxolation processes with water elimination. The presence of extra electrons in this structurally versatile material is thus the driving force for spinel crystallization. In the present case, the extra electrons are supplied by magnetite. Correlatively magnetite oxidizes. Oxidation of surface iron ions starts oxidation in depth ( 11, 13, 14). Excess positive charges left by the transfer of electrons are compensated locally by the creation of vacancies in the octahedral sublattice; excess cations diffuse toward the surface where they coordiJournal
o/ Colloid
and
Inter~ace
Science,
Vol. 150, No. 2, M a y 1992
458
BELLEVILLE ET AL. TABLE II Mtissbauer Parameters of a Mixture Fe304 + 2.25 Fe(OH)3at Different AgingTimes (t) Spinel Fe 3+
Starting Fe304 mixture t=0 1 day 6 days 23 days
Fe T M
Ferric hydroxide
IS
H
IS
H
IS
EQ
A (%)
0.29 0.29 0.29 0.30 0.30
46.7 47.3 47.4 47.4 47.6
0.55 0.56 0.55 0.54 0.54
42.6 43.5 43.7 43.6 43.6
0.35 0.35 0.34 0.34
0.77 0.73 0.75 0.70
13 9 7 2
Note. Isomer shifts (IS) and quadrupole splittings (QS) are in mm/s; hyperfinefields (H) are in T. A is the relative surface area of the quadrupole doublet. nate to adsorbed species. Electrons hopping among octahedral sites from the core ensure the renewal of surface Fe" ions and keep the adsorption reaction going. The electron transfer proceeding preferentially between crystallographically equivalent sites drives the epitactic crystallization of the adsorbed layer. Further support for the fundamental role of Fe n is given by the effect of oxidation of starting magnetite. Various experiments showed that partial oxidation of the initial colloid slowed down the reaction and considerably limited its extent. In the case of quasi-complete oxidation (Fen/Fe "I = 0.03), all added Fe I" remained in the form of free ferric hydroxide, which immediately dissolved during dissolution kinetics experiments. Under aging, spinel particles underwent no change and ferric hydroxide transformed into goethite (Fig. 6b). Hence, precipitated Fe IIxdoes not interact with particles of oxidized spinel.
Fig. 7. In the experimental conditions the reaction seems to limit itself to an uptake equivalent to the initial iron amount ( E e a d s / E e i "~ 1 ). In this way the particle doubles its iron content, doubling faster with larger amounts of added Fem (10 days at R = 1, 1 day at R = 5). As the reaction takes place between solid phases, the amount of ferric hydroxide at the surface of a magnetite particle is a determining factor; the larger this amount, the larger the contact area between the two phases, and hence the faster the reaction proceeds.
S÷H
a
Journal t~/Colloid and lntetJace Science, Vol. 150, No. 2, May 1992
H
S H
HS
H
H S
H
S
Extent of the Reaction The extent of the reaction between precipitated Fe nl and stoichiometric magnetite was investigated by varying the relative amount of added Fe Ill . The ratio R = ~tr~"Itad/Feiwas varied from 0.75 up to 5 and the phenomena were characterized by X ray diffraction and dissolution kinetics. The variation of F e l l l uptake as a function of added Fe "I and aging time is represented in
s S
oo
GG ~
i
i
10
i
i
i
!
S+G
GG
i
20
30
og
40
FIG. 6 X. ray diffractionpatterns of aged mixtures Fe304 + 6 Fe(OH)3 (a) and Fe2.69Oaq- 2.02 Fe(OH)3 (b). S, H, and G stand for spinel, hematite, and goethite, respectively.
ADSORPTION OF FERRIC HYDROXIDE
459
suming the mobile electrons are transferred from an immediately reactive superficial layer £ half a unit cell thick, the newly formed spinel layer has a Fen/Fe m composition of 0.15; this is consistent with the limit for spinel topo.5 tactically produced from ferric hydroxide to be stable (10). In the second stage, the reaction is essentially kept going by electrons supplied from i i ! | I 1 2 3 4 5 the interior, which implies correlative vacancy R = Fe~l/Fe i creation and outward diffusion of cations through the adsorbed layer. The onset of spinel FIG. 7. Adsorption of Fe(OH)3 onto colloidal Fe304. ordering in ferric hydroxide is a fast process Fe "~ uptake (Fe~a~) as a function of the amount of added even at low Fe Hlevels (10). Hence, cation miFem (Fe~a) relative to initial iron (Fei) immediately after Fe HI addition (IS]) and after aging for 1 day (m), 10 days gration in the spinel is likely to be the rate( e ) , and 25 days (A). determining step; it slows down the reaction, which practically stops when the thickness of The value R = 0.75 led to a stable system the adsorbed layer reaches 2 spinel unit cells at room temperature ( <25 °C). Spinel was the (Feads/Fei -~ 1). Beyond, the reaction becomes only product detected even after aging for sev- too slow, surface Fe" ions are too much diluted eral months. Larger R values resulted in he- to be reactive enough; the electrons do not matite formation (Fig. 6a); the larger the diffuse in ferric hydroxide in excess which amount of added Fe H~, the more quickly he- evolves independently. At the end of the rematite appeared (one month at R = 1, a few action, the overall F e " / F e m composition in days only at R ~< 2). In such cases the uptake the particle is equal to 0.2; because of its forcould not be determined quantitatively by mation process the adsorbed layer may be nodissolution kinetics since hematite dissolution ticeably hydroxylated. is slow and interferes with spinel dissolution. In similar experimental conditions (pH, The 3-month-old system at R = 5 also con- temperature, concentration), ferric hydroxide tained some goethite in addition to spinel and alone or in the presence of 3~-Fe203 is transhematite. The presence of hematite (and goe- formed into goethite. The transformation of thite) in the systems indicates that when Fe "~ excess ferric hydroxide into hematite in the is added in too-large amounts, the reaction is presence of magnetite may be caused by slight not fast enough to prevent independent evo- pH variations (not measured) during the system evolution (6) or by the presence of Fe H. lution of a fraction of ferric hydroxide. Data presented in Fig. 7 show that the re- This transformation probably starts up at the action exhibits two kinetic stages: a quasi-in- contact of spinel particles. Surface concentrastantaneous stage followed by a much slower tion in Fe ~I is too low to keep crystallization one. Such two-stage behavior is a systematic into spinel going, but Fe" ions may neverthefeature of interracial reactions involving col- less induce local dehydration of ferric hydroxloidal magnetite (11-13). The first stage in- ide, creating hematite nuclei which orient the volves Fe ~1ions that are at the surface of the transformation of all unadsorbed ferric hyF e 3 0 4 particle. They lead to an uptake of Fe ~H droxide. Such a process is analogous to that which is approximately equal to half of the involved in the thermal transformation 3'initial iron amount (Feads/Fei - 0.5) whatever Fe203 ~ a-Fe203 (25): dehydration between the amount of added Fem (insofar as it is not surfaces of neighboring particles in aggregates limiting). This results in the buildup of a spinel create local strains which make the whole aglayer one unit cell thick, roughly (=8 A). As- gregate transform. 1
Journal ¢~[('olh~id and Interf~lce Science, Vol. 150, No. 2, M a y 1992
460
BELLEVILLE ET AL.
CONCLUSION F e r r i c h y d r o x i d e i n c o n t a c t w i t h colloidal m a g n e t i t e t r a n s f o r m s i n t o a spinel layer w h i c h e x t e n d s t h e i n i t i a l lattice. T h e t r a n s f o r m a t i o n proceeds by an adsorption reaction which inv o l v e s c o u p l e d e l e c t r o n i c a n d i o n i c transfers t h r o u g h t h e interface. E l e c t r o n d e l o c a l i z a t i o n i n t h e w h o l e s y s t e m is the d r i v i n g force for the reaction. ACKNOWLEDGMENT We are grateful to M. Lavergne (CRMP, Universit6 Pierre et Marie Curie) for electron microscopy experiments. REFERENCES 1. Feitknecht, W., Giovanoli, R., Michaelis, W., and Miiller, M., Helv. Chim. Acta 56, 2847 (1973). 2. Combes, J. M., Manceau, A., Calas, G., and Bottero, Y., Geochim. Cosmochim. Acta 53, 583 (1989). 3. Feitknecht, W., and Michaelis, W., Helv. Chim. Acta 50, 212 (1962). 4. Fischer, W. R., and Schwertmann, U., Clays Clay Miner 23, 33 (1975). 5. Schwertmann, U., and Murad, E., Clays Clay Miner. 31, 277 (1983). 6. Cornell, R. M., and Giovanoli, R., Clays Clay Miner. 33, 424 (1985). 7. Cornell, R. M., and Giovanoli, R., Clays Clay Miner. 35, 11 (t987). 8. Cornell, R. M., and Giovanoli, R., Polyhedron 7, 385 (1988). 9. Cornell, R. M., Clay Minerals 23, 329 (1988).
hmrnal of Colloid and h11er/ace Science. Vol. 150. No. 2, May 1992
10. Tronc, E., Belleville, P., Jolivet, J. P., and Livage, J., Langmuir, in press. 11. Jolivet, J. P., and Tronc, E., ,L Colloid Interface Sci. 125, 688 (1988). 12. Tronc, E., Jolivet, J. P., Massart, R., and Lefebvre, J., J. Chem. Soc. Faraday Trans. 1 80, 2619 (1984). 13. Jolivet, J. P., Tronc, E., Barbr, C., and Livage, J., J. Colloid Interface Sci. 138, 465 (1990). 14. Sidhu, P. S., Gilkes, R. J., and Posner, A. M., J. Inorg. Nucl. Chem. 39, 1953 (1977). 15. Tronc, E., Jolivet, J. P., Belleville, P., and Livage, J., Hyperfine Interact. 46, 637 (1989). 16. Jolivet, J. P., Belleville, P., Tronc, E., and Livage, J., submitted for publication. 17. Sidhu, P. S., Gilkes, R. J., Cornell, R. M., Posner, A. M., and Quirk, J. P., Clays Clay Miner. 29, 269 (1981). 18. Blesa, M. A., and Maroto, A. J. G., J. Chim. Phys. 83, 757 (1986). 19. Murad, E., and Schwertmann, U., Am. Mineral. 65, 1044 (1980). 20. Topsoe, H., Dumesic, J. A., and Boudart, M., J. Phys. (Les Ulis. Ft.) 35, C6, 411 (1974). 21. Haneda, K., and Morrish, A. H., J. Phys. (Les Ulis, Ft.) 38, C1,321 (1977). 22. Morup, S., Dumesic, J. A., and Topsoe, H., in "Applications of M/Sssbauer Spectroscopy" (R. L. Cohen, Ed.), Vol. 2, p. I. Academic Press, New York, 1981. 23. Vandenberghe, R. E., and De Grave, E. in "Mrssbauer Spectroscopy Applied to Inorganic Chemistry" (G. J. Long and F. Grandjean, Eds.), Vol. 3, p. 59. Plenum, New York, 1989. 24. Shermann, D. M., Phys. Chem. Miner. 14, 355 (1987). 25. Tronc, E., and Jolivet, J. P., Hyperfine Interact. 54, 737 (1990).
Received April 5, 1991; accepted September 25, 1991 Interaction between "ferric hydroxide" and colloidal magnetite in alkaline medium has been characterized by transmission electron microscopy,X ray powder diffraction, Mrssbauer spectroscopy,and kinetics of dissolution in acidic medium. An adsorption reaction takes place immediately. It involves an interfacial electron transfer which starts magnetite oxidation and makes ferric hydroxide transform into a spinel layer growing epitacticallyon the particle surface. © 1992AcademicPress,Inc. INTRODUCTION "Ferric hydroxide" is a very poorly crystallized hydrated oxyhydroxide, made up of a short-range ordered arrangement of Fe(O, OH, OH2) octahedra ( 1, 2). The local structure is close to that of goethite ( ~ - F e O O H ) . With time it generally transforms into goethite a n d / o r hematite (a-Fe20 3 ) depending on the conditions (pH, temperature, concentration, foreign ions . . . . ) ( 3 - 6 ) . Goethite forms by the dissolution of ferric hydroxide and reprecipitation ofgoethite in solution, whereas hematite arises from a solid state reaction within ferric hydroxide particles. Divalent transition metal cations (M H) at sufficiently high levels ( M " / F e I" above ca. 0.2, roughly) cause ferric hydroxide to convert into spinel ( 7 - 9 ) . This generally proceeds by dissolution and reprecipitation. In the case of FeII, however, the conversion to spinel can also be effected topotactically (10). Fe H adsorption on ferric hydroxide leads to interfacial electron transfer. Delocalization of excess electrons within ferric hydroxide induces local spinel ordering. The degree of ordering increases with the Fe n level J To whom correspondence should be sent at Chimie de la Mati~re Condensre, T54 E5, U.P.M.C., 4 Place Jussieu, 75252 Paris Cedex 05, France.
and governs the stability of the material in solution. F e " / F e "I compositions above ca. 0.07 are sufficient to ensure stability in alkaline media. Colloidal magnetite (Fe304), on the other hand, exhibits a remarkable redox behavior (l 1-13). Electron transfer between Fe n and Fe I" ions in the octahedral sublattice makes the solid respond to surface phenomena by ionic and electronic transfers, eventually reversible, through the interface. The transformation Fe304 --~ T-Fe203 can thus take place under a great variety of conditions in aqueous media. In each case the conversion is driven by an adsorption reaction which traps mobile electrons at surface sites. Surface Fe ~ ions are released into solution or oxidized in s i t u depending on the conditions. Electron hopping and small structural changes ensure the renewal of ferrous sites at the surface and keep the reaction going up to completion. The oxidation process in the particle is the same as for aerial oxidation (14), but p h e n o m e n a inducing the interfacial transfer and external conditions ruling the behavior of surface Fe II are very different. In view of the properties of each system, ferric hydroxide and magnetite, we attempted to characterize their interaction. Preliminary
453 0021-9797/92 $3.00 Journal q[('olloid and lnte(/bce Sciemz,, Vol. 150, No. 2, May 1992
Copyright © 1992 by Academic Press~ Inc. All rights of reproduction in any form reserved.
454
BELLEVILLE
results have been previously reported ( 15 ); a detailed investigation is reported here. EXPERIMENTAL
Materials Magnetite was prepared by alkalinizing a stoichiometric mixture of FeCI3 and FeCI2 with NH3 at room temperature. In order to minimize dissolution-reprecipitation processes, suspended magnetite was stabilized by aging for a week. Great care was taken to exclude oxygen at all stages of the preparation. A varying proportion of FeCI3 ( 1 M) was added to an aliquot of suspended magnetite (pH 8, [Fe] - 0.3 M ) under vigorous stirring and bubbling with argon. NH3 was simultaneously added to keep the pH above 8 and avoid release o f F e n from the magnetite ( 11 ). Fem immediately precipitated and base consumption corresponded to the stoichiometry OH /Fead = 3. The suspension was then centrifuged, rinsed with degassed distilled water, and stored under argon. The pH was near 8; it was not measured during the aging. A reference sample of magnetite was stored under the same condition. The various systems will be characterized by the ratio of added Fe Ill (Fela~) relative to total initial iron (Fe~). -
111
Methods Chemical analysis. Fe n and total Fe concentrations in the suspensions were determined after dissolving the solid in concentrated HC1. Fe n was first titrated potentiometrically with K2Cr207. After titration, all Fe was reduced with a SnCI2 solution and the mixture was again titrated with K2Cr207. Chemical reactivity. The various solid species were characterized by their behavior against dissolution in acidic medium (2 M HC1) according to the procedure described elsewhere (16). Under such conditions, the dissolution frees both Fe n and Fe m ions. The kinetics was determined by reducing Fem ions as they were released into solution. KI (0.2 M) was used as a reducing reagent and formed Journal of CoUoid and Interface Science. Vol. 150, No. 2, May 1992
ET AL.
I2 was titrated with Na2S203. Acid type and concentration are such that ferric hydroxide dissolves quasi-instantaneously, whereas crystalline phases dissolve more slowly. Their dissolution rate is mainly determined by the degree of structural ordering, the particle size, and the FeU/Fe nl composition, Fe n ions promoting dissolution ( 17, 18). Data analysis using pseudo-first-order rate laws yielded dissolution rate constants and proportions of the various Fe nl species. X ray diffraction. Diffractograms were recorded on a Siemens D500 powder diffractometer equipped with a graphite monochromator, using the CuKa radiation. Powders were obtained from suspensions by centrifuging, washing, and drying under argon. Transmission electron microscopy. Micrographs and diffraction patterns were obtained on a Jeol 100 CXII apparatus. Samples were prepared by evaporating very dilute ultrasonicated solutions onto carbon-coated grids, dspacings were calibrated using an Au pattern. Particle size distributions were estimated by measuring the size of about 400 particles. Mrssbauer spectrometry. M6ssbauer spectra were recorded using a conventional spectrometer (Elscint-Inel) with a 57Co/Rh source. Samples were made up of suspensions frozen in a polymeric film; this was carried out by dispersing the suspension in an aqueous solution of polyvinylic alcohol and drying the mixture in an argon flow. Velocities were calibrated using an iron foil. Isomer shifts are given relative to metallic iron at room temperature. RESULTS
AND
DISCUSSION
The composition of aged suspensions always corresponded to the Fen/Fe In stoichiometry of the initial mixture of magnetite and ferric hydroxide. Operating conditions effectively avoided oxidation.
Characterization of the Reaction A detailed investigation of the interaction between precipitated Fe nl and stoichiometric
ADSORPTION OF FERRIC HYDROXIDE
455
FIG. 1. Electronmicrographsof a mixture Fe304 + 2.25 Fe(OH)3 immediatelyafter the precipitation of ferric hydroxide(a), and after 6 days aging (b). magnetite was carried out at the composition llI Fead/Fei = 0.75, which corresponds to the stoichiometry Fe304 + 2.25 Fe(OH)3. Electron microscopy. TEM observations of the system immediately after precipitation (Fig. 1a) show that it is two-phase. It consists of spinel particles of ca. 100 A mixed up with much smaller particles of ferric hydroxide. Under aging, ferric hydroxide decreases in proportion, but its characteristics remain unaltered. It is present only in a small amount in the 6-day-old mixture (Fig. I b). Spinel particles are then characterized by size distribution (Fig. 2a) with a mean (Dm) of 122 A (standard deviation tr = 27 A). Comparison with particle size distribution of starting magnetite ( D m = 106 A, a = 24 A) (Fig. 2b) indicates significant growth of the particles. X ray diffraction. The X ray diffraction pattern of the 6-day-old mixture (Fig. 3a) is analogous to that of starting magnetite (Fig. 3b). The unit-cell parameter is of ca. 8.36 A in each case; this is typical of nonstoichiometric magnetite and suggests that the starting magnetite partly oxidized during the process of drying. The variation of the line width at half-maximum as a function of the Bragg angle shows that there is no particular structural disorder due to the adsorption reaction. The apparent size deduced from the Scherrer formula is
equal to 120 A compared to 109 A for starting magnetite. Such data are consistent with TEM results and show that particle growth is coherent with respect to diffraction. Dissolution kinetics. Dissolution kinetics of starting magnetite and mixture at different aging times are shown in Fig. 4. Magnetite dissolution exhibits only one kinetic stage. After Fe nl precipitation, three stages are observed: a quasi-instantaneous stage followed by a rather slow stage and then a faster one. The quasi-instantaneous stage corresponds to the dissolution of ferric hydroxide. The other two kinetic stages are characteristic of the disso-
p% 10
|
i
10
10
D nm
20
FIG. 2. Particlesizedistribution for a 6-day-oldmixture Fe304 + 2.25 Fe(OH)3(a), and starting Fe304 (b). Journal ~![ColloM and lnterJace Science, Vol. 150, No. 2, May 1992
456
BELLEVILLE ET AL.
106 A, in diameter, and iron is homogeneously distributed all through the particle, Fem uptake at 6-day-aging corresponds to an adsorbed layer 9 ~, thick, which is consistent with TEM results ( 8 A). Mrssbauer spectroscopy. Spectra of starting magnetite and mixture at different aging times are shown in Fig. 5. The magnetite spectrum I I essentially consists of two sextets; one is due to Fe 3+ ions in tetrahedral sites and the other 311 to Fe 3+ and Fe 2÷ ions in octahedral sites. As the electron exchange is fast at the Mrssbauer time scale, these ions appear in the average state Fe 25+ and the relative areas of the two 440 220 sextets are theoretically in the ratio Fe3+/Fe 25+ 400 511 A = 1/2. In addition to the spinel pattern, the 22~ , 422 mixture immediately after Fe "x precipitation yields a quadrupole doublet typical of para' 3'o 50 70 o2e magnetic ferric hydroxide (19). Comparing FIG. 3 X. ray diffractionpattern of a 6-day-oldmixture the spinel pattern with that of starting magFe304 + 2.25 Fe(OH)3(a), and starting Fe304 (b). netite, particularly at negative velocities, one notices an increase in the intensity of the Fe 3÷ component relative to the Fe 2s÷ one. This is lution of partly oxidized magnetite (16). The typical of nonstoichiometric magnetite; unslow stage and the faster one correspond to paired octahedral Fe 3÷ ions give the same conthe attack on the particle surface and core, tribution as tetrahedral Fe 3÷ ions (20, 21 ). The respectively; the difference in reactivity comes spectrum's evolution with aging time indicates from a difference in composition, the surface progressive disappearance of ferric hydroxide being poorer in Fe". and increasing nonstoichiometry of spinel. Data analysis (Table I) shows that, immeHyperfine field distributions related to pardiately after Fem precipitation, ferric hydrox- ticle size distribution (22) and features related ide represents only 22% of total Fem. Since to fast electron exchange between ions in octhe amount of added Fem corresponds to 53% of total Fe m, we deduce that nearly 60% of added Fem has been taken up immediately by magnetite particles. Fem uptake goes on under aging; it is nearly complete (---90%) after 23 days aging. The characteristics of the dissolution of the spinel particles are progressively .c]--- " d altered: the rate constant relative to surface dissolution (Table I) decreases progressively and the kinetic stage relative to dissolution of 0 5 10 t rain 15 the core deviates more and more from a FIG. 4. Kinetics of dissolution in acidic medium of pseudo-first-order rate law (Fig. 4). This instarting Fe304(a) and a mixture Fe304 + 2.25 Fe(OH)3 dicates increasing oxidation of the surface and immediately after Fe(OH)3 precipitation(b), after aging existence of a composition gradient, increas- for 1 day (c), 6 days (d), and 23 days (e). Corepresents ingly smooth, through the particle. Assuming the amount of total Fem, and c that of FenI undissolved adsorption takes place onto a spherical particle at time t. Journal of Colloid and Interlace Science, Vol. 150, No. 2, M a y 1992
457
A D S O R P T I O N OF FERRIC H Y D R O X I D E TABLE I Characteristics of the Kinetics of Acidic Dissolution of Magnetite and a Mixture ii~ Fe304 q- 2.25 Fe(OH)3 (Fe~d/Fei = 0.75) Fe304
Aging time (day) Rate constant k (10-2 mn -~) Fraction immediatelydissolved relative to total Fem (%) Uptake Fe~JFei (%)
Fe304 + 2.25 F e ( O H ) 3
-12.7
0 3.4
0 --
22 44
tahedral sites (23) make the analysis of the spectra particularly complex. In order to estimate the proportion of ferric hydroxide, we used a simplified model involving one sextet with line widths constrained to be all equal for the Fe 3÷ component, and equal by pairs for the Fe :'5÷ component. Corresponding parameters are given in Table II. The proportion of feric hydroxide deduced from the relative area of the corresponding doublet, assuming identical recoilless fractions for all iron atoms, is consistent with dissolution kinetics data. The increase, in similar proportions, of Fe 3÷ and Fe 25+ hyperfine fields compared to starting magnetite indicates an increase in the size of spinel particles. Too-strong overlapping of the Fe 3+ and Fe 25+ components does not, however, allow reliable estimation of the variation of the composition of these particles.
i.'."
t=0
"
:" ," ;"
: ;-
.: :
• :
23d
ld
~'"
":
. .
~; "
;'~ "to
'
:, :. :. .
: ."
:
1,"
~'" " i.
--"~
~,~,..'~
. ; . f • . :
. ".,::" : j , ". .g,.: :. :
60
initial
Fe304
.;
-6
0
6
v (rnrn/s)
-6
0
6
F I G . 5, M 6 s s b a u e r spectra ( 3 0 0 K ) o f a m i x t u r e
1 2.3
Fe304
+ 2.25 Fe(OH)3at differentaging times (t), and starting Fe304.
6 2.1
14 55
10 61
23 1.6 7 65
These results show that ferric hydroxide precipitated in the presence of colloidal magnetite is transformed into a spinel layer which grows on the particle surface in epitaxy with the lattice. The reaction proceeds between solid phases by an adsorption reaction which is assisted by electron transfer through the interface magnetite/ferric hydroxide. The process is similar to that involved in the conversion of ferric hydroxide to spinel by Fe n adsorption (10). In this case, electron transfer between Fe n and Fem ions, evidenced by M6ssbauer spectroscopy, locally induces cubic close-packed ordering, and delocalization of excess electrons within the whole particle results in spinel crystallization. In pure ferric hydroxide iron atoms are only coupled via anions. As FeII-Fe m electron transfer is a direct process, metal-metal bonding is essential (24). In order for it to take place, electron transfer implies that suitable structural rearrangements have occurred. The loose structure and high level of hydration of ferric hydroxide make such rearrangements proceed easily by olation-oxolation processes with water elimination. The presence of extra electrons in this structurally versatile material is thus the driving force for spinel crystallization. In the present case, the extra electrons are supplied by magnetite. Correlatively magnetite oxidizes. Oxidation of surface iron ions starts oxidation in depth ( 11, 13, 14). Excess positive charges left by the transfer of electrons are compensated locally by the creation of vacancies in the octahedral sublattice; excess cations diffuse toward the surface where they coordiJournal
o/ Colloid
and
Inter~ace
Science,
Vol. 150, No. 2, M a y 1992
458
BELLEVILLE ET AL. TABLE II Mtissbauer Parameters of a Mixture Fe304 + 2.25 Fe(OH)3at Different AgingTimes (t) Spinel Fe 3+
Starting Fe304 mixture t=0 1 day 6 days 23 days
Fe T M
Ferric hydroxide
IS
H
IS
H
IS
EQ
A (%)
0.29 0.29 0.29 0.30 0.30
46.7 47.3 47.4 47.4 47.6
0.55 0.56 0.55 0.54 0.54
42.6 43.5 43.7 43.6 43.6
0.35 0.35 0.34 0.34
0.77 0.73 0.75 0.70
13 9 7 2
Note. Isomer shifts (IS) and quadrupole splittings (QS) are in mm/s; hyperfinefields (H) are in T. A is the relative surface area of the quadrupole doublet. nate to adsorbed species. Electrons hopping among octahedral sites from the core ensure the renewal of surface Fe" ions and keep the adsorption reaction going. The electron transfer proceeding preferentially between crystallographically equivalent sites drives the epitactic crystallization of the adsorbed layer. Further support for the fundamental role of Fe n is given by the effect of oxidation of starting magnetite. Various experiments showed that partial oxidation of the initial colloid slowed down the reaction and considerably limited its extent. In the case of quasi-complete oxidation (Fen/Fe "I = 0.03), all added Fe I" remained in the form of free ferric hydroxide, which immediately dissolved during dissolution kinetics experiments. Under aging, spinel particles underwent no change and ferric hydroxide transformed into goethite (Fig. 6b). Hence, precipitated Fe IIxdoes not interact with particles of oxidized spinel.
Fig. 7. In the experimental conditions the reaction seems to limit itself to an uptake equivalent to the initial iron amount ( E e a d s / E e i "~ 1 ). In this way the particle doubles its iron content, doubling faster with larger amounts of added Fem (10 days at R = 1, 1 day at R = 5). As the reaction takes place between solid phases, the amount of ferric hydroxide at the surface of a magnetite particle is a determining factor; the larger this amount, the larger the contact area between the two phases, and hence the faster the reaction proceeds.
S÷H
a
Journal t~/Colloid and lntetJace Science, Vol. 150, No. 2, May 1992
H
S H
HS
H
H S
H
S
Extent of the Reaction The extent of the reaction between precipitated Fe nl and stoichiometric magnetite was investigated by varying the relative amount of added Fe Ill . The ratio R = ~tr~"Itad/Feiwas varied from 0.75 up to 5 and the phenomena were characterized by X ray diffraction and dissolution kinetics. The variation of F e l l l uptake as a function of added Fe "I and aging time is represented in
s S
oo
GG ~
i
i
10
i
i
i
!
S+G
GG
i
20
30
og
40
FIG. 6 X. ray diffractionpatterns of aged mixtures Fe304 + 6 Fe(OH)3 (a) and Fe2.69Oaq- 2.02 Fe(OH)3 (b). S, H, and G stand for spinel, hematite, and goethite, respectively.
ADSORPTION OF FERRIC HYDROXIDE
459
suming the mobile electrons are transferred from an immediately reactive superficial layer £ half a unit cell thick, the newly formed spinel layer has a Fen/Fe m composition of 0.15; this is consistent with the limit for spinel topo.5 tactically produced from ferric hydroxide to be stable (10). In the second stage, the reaction is essentially kept going by electrons supplied from i i ! | I 1 2 3 4 5 the interior, which implies correlative vacancy R = Fe~l/Fe i creation and outward diffusion of cations through the adsorbed layer. The onset of spinel FIG. 7. Adsorption of Fe(OH)3 onto colloidal Fe304. ordering in ferric hydroxide is a fast process Fe "~ uptake (Fe~a~) as a function of the amount of added even at low Fe Hlevels (10). Hence, cation miFem (Fe~a) relative to initial iron (Fei) immediately after Fe HI addition (IS]) and after aging for 1 day (m), 10 days gration in the spinel is likely to be the rate( e ) , and 25 days (A). determining step; it slows down the reaction, which practically stops when the thickness of The value R = 0.75 led to a stable system the adsorbed layer reaches 2 spinel unit cells at room temperature ( <25 °C). Spinel was the (Feads/Fei -~ 1). Beyond, the reaction becomes only product detected even after aging for sev- too slow, surface Fe" ions are too much diluted eral months. Larger R values resulted in he- to be reactive enough; the electrons do not matite formation (Fig. 6a); the larger the diffuse in ferric hydroxide in excess which amount of added Fe H~, the more quickly he- evolves independently. At the end of the rematite appeared (one month at R = 1, a few action, the overall F e " / F e m composition in days only at R ~< 2). In such cases the uptake the particle is equal to 0.2; because of its forcould not be determined quantitatively by mation process the adsorbed layer may be nodissolution kinetics since hematite dissolution ticeably hydroxylated. is slow and interferes with spinel dissolution. In similar experimental conditions (pH, The 3-month-old system at R = 5 also con- temperature, concentration), ferric hydroxide tained some goethite in addition to spinel and alone or in the presence of 3~-Fe203 is transhematite. The presence of hematite (and goe- formed into goethite. The transformation of thite) in the systems indicates that when Fe "~ excess ferric hydroxide into hematite in the is added in too-large amounts, the reaction is presence of magnetite may be caused by slight not fast enough to prevent independent evo- pH variations (not measured) during the system evolution (6) or by the presence of Fe H. lution of a fraction of ferric hydroxide. Data presented in Fig. 7 show that the re- This transformation probably starts up at the action exhibits two kinetic stages: a quasi-in- contact of spinel particles. Surface concentrastantaneous stage followed by a much slower tion in Fe ~I is too low to keep crystallization one. Such two-stage behavior is a systematic into spinel going, but Fe" ions may neverthefeature of interracial reactions involving col- less induce local dehydration of ferric hydroxloidal magnetite (11-13). The first stage in- ide, creating hematite nuclei which orient the volves Fe ~1ions that are at the surface of the transformation of all unadsorbed ferric hyF e 3 0 4 particle. They lead to an uptake of Fe ~H droxide. Such a process is analogous to that which is approximately equal to half of the involved in the thermal transformation 3'initial iron amount (Feads/Fei - 0.5) whatever Fe203 ~ a-Fe203 (25): dehydration between the amount of added Fem (insofar as it is not surfaces of neighboring particles in aggregates limiting). This results in the buildup of a spinel create local strains which make the whole aglayer one unit cell thick, roughly (=8 A). As- gregate transform. 1
Journal ¢~[('olh~id and Interf~lce Science, Vol. 150, No. 2, M a y 1992
460
BELLEVILLE ET AL.
CONCLUSION F e r r i c h y d r o x i d e i n c o n t a c t w i t h colloidal m a g n e t i t e t r a n s f o r m s i n t o a spinel layer w h i c h e x t e n d s t h e i n i t i a l lattice. T h e t r a n s f o r m a t i o n proceeds by an adsorption reaction which inv o l v e s c o u p l e d e l e c t r o n i c a n d i o n i c transfers t h r o u g h t h e interface. E l e c t r o n d e l o c a l i z a t i o n i n t h e w h o l e s y s t e m is the d r i v i n g force for the reaction. ACKNOWLEDGMENT We are grateful to M. Lavergne (CRMP, Universit6 Pierre et Marie Curie) for electron microscopy experiments. REFERENCES 1. Feitknecht, W., Giovanoli, R., Michaelis, W., and Miiller, M., Helv. Chim. Acta 56, 2847 (1973). 2. Combes, J. M., Manceau, A., Calas, G., and Bottero, Y., Geochim. Cosmochim. Acta 53, 583 (1989). 3. Feitknecht, W., and Michaelis, W., Helv. Chim. Acta 50, 212 (1962). 4. Fischer, W. R., and Schwertmann, U., Clays Clay Miner 23, 33 (1975). 5. Schwertmann, U., and Murad, E., Clays Clay Miner. 31, 277 (1983). 6. Cornell, R. M., and Giovanoli, R., Clays Clay Miner. 33, 424 (1985). 7. Cornell, R. M., and Giovanoli, R., Clays Clay Miner. 35, 11 (t987). 8. Cornell, R. M., and Giovanoli, R., Polyhedron 7, 385 (1988). 9. Cornell, R. M., Clay Minerals 23, 329 (1988).
hmrnal of Colloid and h11er/ace Science. Vol. 150. No. 2, May 1992
10. Tronc, E., Belleville, P., Jolivet, J. P., and Livage, J., Langmuir, in press. 11. Jolivet, J. P., and Tronc, E., ,L Colloid Interface Sci. 125, 688 (1988). 12. Tronc, E., Jolivet, J. P., Massart, R., and Lefebvre, J., J. Chem. Soc. Faraday Trans. 1 80, 2619 (1984). 13. Jolivet, J. P., Tronc, E., Barbr, C., and Livage, J., J. Colloid Interface Sci. 138, 465 (1990). 14. Sidhu, P. S., Gilkes, R. J., and Posner, A. M., J. Inorg. Nucl. Chem. 39, 1953 (1977). 15. Tronc, E., Jolivet, J. P., Belleville, P., and Livage, J., Hyperfine Interact. 46, 637 (1989). 16. Jolivet, J. P., Belleville, P., Tronc, E., and Livage, J., submitted for publication. 17. Sidhu, P. S., Gilkes, R. J., Cornell, R. M., Posner, A. M., and Quirk, J. P., Clays Clay Miner. 29, 269 (1981). 18. Blesa, M. A., and Maroto, A. J. G., J. Chim. Phys. 83, 757 (1986). 19. Murad, E., and Schwertmann, U., Am. Mineral. 65, 1044 (1980). 20. Topsoe, H., Dumesic, J. A., and Boudart, M., J. Phys. (Les Ulis. Ft.) 35, C6, 411 (1974). 21. Haneda, K., and Morrish, A. H., J. Phys. (Les Ulis, Ft.) 38, C1,321 (1977). 22. Morup, S., Dumesic, J. A., and Topsoe, H., in "Applications of M/Sssbauer Spectroscopy" (R. L. Cohen, Ed.), Vol. 2, p. I. Academic Press, New York, 1981. 23. Vandenberghe, R. E., and De Grave, E. in "Mrssbauer Spectroscopy Applied to Inorganic Chemistry" (G. J. Long and F. Grandjean, Eds.), Vol. 3, p. 59. Plenum, New York, 1989. 24. Shermann, D. M., Phys. Chem. Miner. 14, 355 (1987). 25. Tronc, E., and Jolivet, J. P., Hyperfine Interact. 54, 737 (1990).