Synthesis of Fe(II-III) hydroxysulphate green rust by coprecipitation

Solid State Sciences 4 (2002) 61–66 www.elsevier.com/locate/ssscie

Synthesis of Fe(II-III) hydroxysulphate green rust by coprecipitation Antoine Géhin a , Christian Ruby a , Mustapha Abdelmoula a , Omar Benali b,c , Jaafar Ghanbaja d , Philippe Refait e , Jean-Marie R. Génin a,∗ a Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, UMR 7564 CNRS-Université Henri Poincaré, Equipe Microbiologie et Physique

and Département Matériaux et Structures, ESSTIN, 405 rue de Vandoeuvre, F-54600 Villers-lès-Nancy, France b Laboratoire de Chimie du Solide Minéral, UMR 7555 CNRS-Université Henri Poincaré, Paris, France c Faculté des Sciences (Chimie), Université Ibn Tofail, BP 133 Kénitra, Morocco d Faculté des Sciences, Université Henri Poincaré, F-54500 Vandoeuvre-Lès-Nancy, France e Laboratoire d’Etudes des Matériaux en Milieux Agressifs, EA 3167 Université de La Rochelle, Avenue Michel Crépeau, F-17042 La Rochelle cedex 1, France Received 7 May 2001; received in revised form 26 September 2001; accepted 2 October 2001

Abstract Iron(II-III) hydroxysulphate precipitates were prepared in absence of other compounds by coprecipitation that consists of mixing solutions of iron(II) and iron(III) salts with NaOH solution in adequate proportions. Precipitates were characterised by transmission Mössbauer spectroscopy (TMS), XRD, TEM and AFM. Mössbauer spectrum measured at 15 K was composed of Fe(II) and Fe(III) doublets with δ = 1.33 and 0.51 mm s−1 , and ∆ = 2.88 and 0.43 mm s−1 , respectively. The Fe(II)/Fe(III) ratio of 2 is independent of the solution ratio 2− III 2+ 2− set at 3 that confirms the composition, [FeII 4 Fe2 (OH)12 ] ·[SO4 ·m H2 O] . XRD analysis led to hexagonal unit cell parameters of aH = bH = 0.318 ± 0.004 nm and cH = 1.090 ± 0.004 nm. The value aH = 0.314 ± 0.002 nm was obtained by TEM diffraction pattern corresponding to (h k 0) planes and displayed a distribution of hexagonal plates (10 < diameter < 500 nm). Sample aged 7 days at 70 ◦ C showed larger plates (100 < diameter < 1000 nm). AFM on aged sample allowed to measure the thickness of isolated hexagonal crystallite.  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Iron(II-III) hydroxysulphate; Green rust; Coprecipitation; XRD; TEM; AFM; Mössbauer spectroscopy

1. Introduction Green rust (GR) compounds belong to the family of layered double hydroxides (LDH) and have the normalised III x+ n− chemical formula [FeII (1–x) Fex (OH)2 ] ·[(x/n)A · (mx/n)H2O]x− , where An− are intercalated anions such as 2− − CO2− 3 [1,2], SO4 [3], or Cl [4]. In this structure, positively III x+ alternate charged hydroxide layers [FeII (1–x) Fex (OH)2 ] with negatively charged interlayers of anions An− and of m water molecules per anion. They are chemically reactive due to the presence of iron(II). For instance, the synthetic 2− II III 2+ GR that incorporates SO2− 4 , [Fe4 Fe2 (OH)12 ] ·[SO4 · m H2 O]2− , was demonstrated to be able to reduce in abiotic conditions polluting species such as nitrate, Se(IV) and Cr(VI) ions [5–7]. * Correspondence and reprints.

E-mail address: [email protected] (J.-M.R. Génin).

Moreover, a mineral with structural properties similar to that of a GR was recently evidenced in hydromorphic soils by means of Mössbauer and Raman spectroscopies [8, 9]. This GR mineral plays a major role in the solubility and transport of iron in soil solutions and aquifers since it masters the dissolved Fe(II) concentration [9,10]. Once oxidized into the ferric form, the iron precipitates. This hinders its mobility [10]. Because the mineral is dilute and scattered, no direct diffraction or morphological studies are presently available. In contrast, transmission electron microscopy (TEM) was used to look at synthetic GR particles in some few studies [11–13], where it was clearly shown that they consist of relatively large hexagonal crystallites (≈ 100–1000 nm). This study is aimed at a better comprehension of the formation of GRs by precipitation from Fe(II)–Fe(III) containing solutions in natural environments, as it occurs in soils or by corrosion of steel surfaces. GRs are commonly prepared in the laboratory by aerial oxidation of Fe(OH)2 pre-

1293-2558/02/$ – see front matter  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 1 ) 0 1 2 1 9 - 5

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cipitates [14–18] and very seldom prepared by coprecipitation of Fe(II)–Fe(III) containing solutions as often done for other LDHs. In our survey, the iron(II-III) hydroxysulphate GR(SO2− 4 ) was thus prepared by coprecipitation, its physico-chemical characteristics were studied by transmission Mössbauer spectroscopy (TMS) and X-ray diffraction (XRD), and compared with those of hydroxysulphates prepared by other methods. Moreover, the morphological properties of GR(SO2− 4 ) particles were studied with TEM and for the first time with atomic force microscopy (AFM).

2. Experimental method 2.1. Preparation of GR(SO2− 4 ) GR(SO2− 4 ) was synthesised by precipitation from a mixture of iron(II) and iron(III) salts with sodium hydroxide. Ferrous sulphate heptahydrate FeSO4 ·7 H2 O and dehydrated ferric sulphate Fe2 (SO4 )3 were dissolved in 100 mL of demineralized water in a 200 mL flask. A [Fe(II)]/[Fe(III)] ratio of 3 was chosen with {[Fe(II)] + [Fe(III)]} = 0.2 M. A magnetic stirring ensured a fast and complete dissolution at a rotation velocity of 500 rpm. Then, 100 mL of NaOH, corresponding to a [OH− ]/{[Fe(II)] + [Fe(III)]} ratio of 3/2, i.e., [OH− ] = 0.3 M, was added to the solution under the same conditions of magnetic stirring. After few seconds, the flask was sheltered from the air to avoid any oxidation. The samples were aged 24 hours before analysis. At this stage the pH of the solution was 6.9. The influence of the experimental conditions, i.e., [Fe(II)]/[Fe(III)] ratio and NaOH concentration, were fully investigated and will be the subject of another paper [19]. The coprecipitations led to mixtures containing either 2− GR(SO2− 4 ) and goethite α-FeOOH, or GR(SO4 ), magnetite Fe3 O4 and ferrous hydroxide Fe(OH)2 . In particular, if the stoichiometric ratio found in the hydroxysulphate green rust is used, i.e., [Fe(II)]/[Fe(III)] = 2 and [OH− ]/{[Fe(II)] + [Fe(III)]} = 2, the resulting precipitate was found to be a mixture comprising green rust, magnetite and ferrous hydroxide. The case related in this paper is that allowing to obtain pure Fe(II-III) hydroxysulphate. Finally, some samples were aged at a higher temperature of 70 ◦ C for a week in order to increase the crystal size.

of a closed Mössbauer cryogenic workstation with vibrations isolation stand manufactured by Cryo Industries of America® . Helium exchange gas was used to thermally couple the sample to the refrigerator, allowing variable temperature operation from 15 to 300 K. The samples were filtered on a paper under inert atmosphere, set in the sample holder and introduced in the cryostat for Mössbauer measurements. Computer fittings were done using Lorentzian-shape lines with widths constrained to be equal for each line. The parameters, which result from computer-fitting must be both mathematically (minimisation of χ 2 ) and physically significant; in particular the width of all lines must be small enough (FWHM ∼ = 0.3 mm s−1 ). Products were also analysed by XRD, using the Co Kα1 wavelength (λ = 0.1788965 nm). The final product to be analysed were merely filtered and rapidly coated with glycerol to avoid any oxidation [20]. The samples were observed by TEM (CM20/STEM Philips), using a voltage of 200 kV, i.e., a wavelength λ = 2.5 × 10−3 nm. One drop of the suspension was laid on a copper grid, which was introduced in the microscope under a 10−8 Torr vacuum. The AFM images were obtained with an Explorer Ecu+ scanning probe thermomicroscope. The experiments were performed in the air with contact mode using silicon nitride tips (arm length of 200 µm and tip radius < 50 nm). One drop of the suspension was deposited on a glass substrate mounted on a metallic plate with a double-sided sticky tape. The glass substrate was allowed to dry during few minutes before each AFM experiment. The particles were not moved during scanning of the sample surface.

3. Results 3.1. XRD and TMS analyses All main lines found in the XRD pattern (Fig. 1) are typical of GR(SO2− 4 ). Angular positions of the diffraction peaks lead to a series of dhkl spacings close to that proposed by Bernal et al. [3]. Various small lines (t) are attributed to Na2 SO4 thenardite mineral. Its presence comes from the operating mode where Na+ and SO2− 4 ions are involved. Na2 SO4 precipitated after filtration when remnants of water

2.2. Characterisation of products Four methods were used in order to characterise the products: transmission Mössbauer spectroscopy (TMS), Xray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Mössbauer spectra were measured by means of a constant-acceleration spectrometer with a 50 mCi source of 57 Co in Rh and calibrated with a 25 µm foil of α-Fe at room temperature. Spectra of green rust compounds, which are sensitive to aerial oxidation, were measured at 15 K. The cryostat consisted

Fig. 1. XRD pattern of Fe(II-III) hydroxysulphate green rust.

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Fig. 2. 57 Fe Mössbauer spectrum of Fe(II-III) hydroxysulphate green rust measured at 15 K.

Table 1 GR(SO2− 4 ) hyperfine parameters measured at 15 K D1 D2

δ (mm s−1 )

∆ (mm s−1 )

RA (%)

FWHM (mm s−1 )

1.33 0.51

2.88 0.43

66.8 33.2

0.29 0.29

δ: isomer shift with respect to metallic α-iron at room temperature; ∆: quadrupole splitting; RA: relative abundance; FWHM: full width at half maximum is constrained to be equal for each Lorentzian-shaped line.

evaporated. Additional lines denoted by an asterisk would indicate that some ordering of sulphate ions occurs inside the interlayers of the structure, which would lead to a larger √ lattice parameter a = a 3 as observed previously [21]. The two peaks would then correspond to d100 and d101 using a as lattice parameter. Since unusual XRD diffraction lines noted t and ∗ in Fig. 1 were observed, Mössbauer spectroscopy was used to confirm that the Fe(II-III) hydroxysulphate was the only compound that contained Fe. The Mössbauer spectrum of the precipitate (Fig. 2) displays only two doublets D1 and D2 typical of GR(SO2− 4 ) [15,18,21,22]: D1 with a large quadrupole splitting ∆ = 2.9 mm s−1 corresponds to high spin Fe(II) ions in octahedral sites, and D2 with a small quadrupole splitting ∆ = 0.44 mm s−1 corresponds to high spin Fe(III) ions in octahedral sites. The hyperfine parameters of D1 and D2 are reported in Table 1. Moreover, the intensity ratio D1 /D2 is close to 2, as it is the case when the GR is obtained by aerial oxidation of Fe(OH)2 , III implying that the chemical formula is kept at [FeII 4 Fe2 − 2+ 2− (OH )12 ] [SO4 ·m H2 O] [15,18]. In conclusion, the TMS data indicate that the hydroxysulphate GR was the only solid formed during the coprecipitation process. Moreover, it appears that its chemical composition is not affected by the preparation method and always characterised by a Fe(II)/Fe(III) ratio of 2, independently of the initial ratio found in the solution, in this case fixed at 3.

Fig. 3. TEM picture of Fe(II-III) hydroxysulphate green rust. (a) Hexagonal particles of GR(SO2− 4 ). (b) Poorly crystallised phase.

3.2. TEM and AFM analyses The TEM pictures of GR(SO2− 4 ) aged 24 hours at ambient temperature or one week at 70 ◦ C are displayed in Figs. 3 and 4 with the corresponding diffraction patterns, respectively. In Fig. 3, two different morphologies are clearly identified in the sample aged at ambient: one is well ordered, pointed by (a) in Fig. 3, and consists of a distribution of small and thin hexagonal plates and a second one is poorly crystallised, pointed by (b). The hexagonal particles, which constitute the first morphology (a), exhibit a weak contrast since their thickness is small; the particles are more or less regular and of various sizes going from 10 to about 500 nm. The hexagonal plates overlap each other so that the diffraction pattern is of polycrystalline type and constituted of rings (Fig. 3). The same sample was aged at 70 ◦ C during one week. The TEM picture is displayed in Fig. 4a where there exists only one morphology. The poorly crystallised part observed in Fig. 3 has completely disappeared. There is a distribution of large hexagonal plates, the diameter of which varies from 100 to 1000 nm. Moreover the strong contrast in the TEM image shows that the thickness of the hexagonal particles is larger than that observed in Fig. 3. The hexagonal plates are generally agglomerated but there exist zones where the particles are rather well separated. Thus, a diffraction pattern of an isolated GR(SO2− 4 ) particle was obtained. The electron diffraction pattern is composed of spots distributed on a hexagonal lattice. In some places of the picture, some particles whose hexagonal plane seems to be perpendicular to the surface of the grid are seen as

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Fig. 4. (a) TEM picture of Fe(II-III) hydroxysulphate green rust aged 1 week at 70 ◦ C. The arrows indicate GR(SO2− 4 ) particles whose hexagonal plane are perpendicular to the surface of the grid. (b) Indexed diagram corresponding to the electron diffraction pattern.

pointed by arrows in Fig. 4a. The diameter and the thickness of three grains were estimated (Fig. 5). The same sample was also analysed by AFM (Fig. 6). Agglomerated hexagonal plates are observed. However, their edges are clearly visible. In the rare sufficiently isolated hexagons, the diameter and thickness were measured with precision: a typical roughness

profile is displayed in Fig. 6. The evolution of the thickness of various hexagonal plates as a function of the diameter is presented in Fig. 5: there are three points of TEM data, the others are obtained by AFM measurements. The diameter and thickness of hexagonal particles of GR(SO2− 4 ) vary from 115 to 1020 nm and from 17 to 80 nm, respectively. These

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Fig. 5. Evolution of the GR(SO2− 4 ) particle thickness as a function of diameter.

Fig. 7. (a) Crystal lattice of GR(SO2− 4 ). (b) Macroscopic crystal of

GR(SO2− 4 ).

Fig. 6. (a) AFM image of agglomerated hexagonal plates of GR(SO2− 4 ). (b) Roughness profile along the line indicated in (a).

values show that the diameter of the particles grow more quickly than their thickness. In other words, the growth of the hexagonal crystals of GR(SO2− 4 ) is anisotropic. 4. Discussion The existence of GR(SO2− 4 ) hexagonal particles was already shown by other authors [12,13] using TEM. However, electron diffraction was not used in these previous studies to obtain any crystallographic information about GR(SO2− 4 ). According to the early publication of Bernal et al. [3], the structure of GR(SO2− 4 ) can be described in a hexagonal lattice with parameters a = 0.32 nm and c = 1.09 nm (Fig. 7). From the dhkl distances observed in the XRD pattern, the values a = 0.316 ± 0.004 nm (XRD) and c = 1.09 ± 0.004 nm (XRD) of GR(SO2− 4 ) are calculated.

From the TEM diffraction pattern of Fig. 4a, the parameter a can also be computed. The zone axis is [0 0 1] indicating that the habit plane of the crystal is merely the hexagonal basal plane (0 0 0 1) and the (h k 0) indexing of the different spots are given in Fig. 4b. This result differs slightly from the conclusions obtained by McGill et al. [11], who studied the TEM electron diffraction picture of a green rust crystal formed by corrosion of cast iron; the observed habit plane was thus most closely related to (5, 6, 6)R (R for the rhombohedral lattice of a green rust 1 structure), a plane situated at 5◦ 48 from the hexagonal basal plane (0 0 0 1). The dhk0 distances were measured and value a = 0.317 ± 0.002 nm (TEM) was deduced in good agreement with the previous value obtained by XRD. TEM did not allow us to confirm the existence of SO2− 4 ordering since not a single spot could be assigned to the corresponding peaks designated by an asterisk and detected by XRD. Because of the [0 0 1] zone axis, the parameter c cannot be determined from the TEM diffraction pattern. The values of a and c determined by TEM and XRD are in good agreement with those given by Bernal [3]. The graph of Fig. 5 shows that the growth of the GR(SO2− 4 ) particles is anisotropic: the growth kinetics of the face (0 0 1) is significantly slower than that of the lateral faces {1 0 0}. The faces that are visible are generally those having the lowest growth rate. The mean growth rate Vhkl of a (h, k, l) face is practically inversely proportional −1 to the corresponding dhkl distance [23]: Vhkl ∝ dhkl . The V100 and V001 growth rates can be compared √ and the ratio V100/V001 estimated: V100 /V001 = (2c)/( 3a) ∼ = 4. As ) hexagonal plates shown in Fig. 7, the shape of the GR(SO2− 4

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is understood: the anisotropy of the hexagonal lattice is in direct relation with the shape of the macroscopic GR(SO2− 4 ) particles.

5. Conclusion In this study, the Fe(II-III) green rust hydroxysulphate GR(SO2− 4 ) was prepared by coprecipitation as done for other LDHs. Mössbauer spectroscopy and XRD analyses demonstrated that GR(SO2− 4 ) was the only solid that formed. Its composition was found to be the same as that observed for GR(SO2− 4 ) samples prepared by other methods. Its hexagonal particles were observed with transmission electron microscopy. Using aged samples, a single crystal TEM diffraction pattern could be obtained and confirmed the value of the lattice parameter a obtained by XRD. The height of the hexagonal plates was measured for the first time with atomic force microscopy. It varies between 30 and 80 nm whereas the diameter varies between 300 and 1000 nm. The origin of the anisotropic growth of the GR(SO2− 4 ) crystal is linked to the difference between the growth rates of the {1 0 0} and (0 0 1) faces.

Acknowledgements Suggestions by the reviewers for improving this manuscript are fully acknowledged.

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