Mustapha Abdelmoula

Mechanisms of formation and structure of green rust one in aqueous corrosion of iron in the presence of chloride ions

The crystal structure of the Fe(II)-Fe(III) hydroxychloride known as Green Rust one (GR) was investigated by X-ray diffraction and confirmed to be analogous with that of iowaite. It is a rhombohedral crystal [R3m, a=0.3190(1)nm, c=2.385(6)nm] consisting of Fe(OH)2 like-hydroxide sheets which alternate regularly with interlayers composed of Cl ions and H2O molecules and follow the stacking sequence AcBiBaCjCbAk..., where A, B, C are OH layers, a, b, c Fe layers and i, j, k interlayers. By means of transmission Mossbauer spectroscopy analyses at 20K of the solid phases formed during the oxidation of Fe(OH)2 into GR(Cl), it was demonstrated that the composition of the GR compound varied continuously from FeII3FeIII(OH)8Cl\nH2O, with n probably equal to 2, to approximately FeII2.2FeIII(OH)6.4Cl\nH2O. The formation of GR(Cl) involves an in situ incorporation of the Cl ions from the solution into the interlayers of GR(Cl) and a corresponding oxidation of Fe(II) to Fe(III) without any structural changes. This mechanism, which allows the oxidation of iron at the only cost of chloride intercalation, explained why GR formation would be favoured vs other possibilities such as direct ferric oxyhydroxide or magnetite formation. To confirm this assumption, in situ transmission Mossbauerspectroscopy analyses were performed on iron coupons polarised potentiostatically in KCl solutions of pH about 9. At a potential of 0.55VSHE a ferrous hydroxide layer is formed on the metal while at 0.35VSHE the corrosion product is mainly composed of GR.

The preparation and thermodynamic properties of Fe(II)-Fe(III) hydroxide-carbonate (green rust 1) ; Pourbaix diagram of iron in carbonate-containing aqueous media

Abstract Carbonate-containing green rust 1, GR1(CO32−), is prepared by oxidation of Fe(OH)2 in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO4·7H2O solutions and carbonate ions are added as a Na2CO3 solution. For sufficiently large concentrations of sodium carbonate, SO42− ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)2 oxidizes into GR1(CO32−). In the second stage of reaction, GR1(CO32−) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO32−), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 A and c = 22.45 ± 0.05 A . From the Mossbauer analysis of the stoichiometric GR1(CO32−), which leads to a Fe2+:Fe3+ ratio of 2:1, the chemical formula is established to be: [Fe4(II)Fe2(III)(OH)12][CO3·2H2O]. The 78 K Mossbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe2+ doublets d1 and D2 corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s−1 and quadrupole splittings (QS) of 2.93 and 2.67 mm s−1, respectively, and one Fe3+ doublet D3 with an IS of 0.47 mm s−1 and QS of 0.43 mm s−1. These three doublets were already used to fit the Mossbauer spectrum of chloride-containing GR1(Cl−) [see J.M.R. Genin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Genin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO32−) is estimated to be ΔG °f = − 966.250 cal mol−1. Knowing the chemical formula and ΔG °f of GR1(CO32−) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.

Surface chemistry and structural properties of mackinawite prepared by reaction of sulfide ions with metallic iron

Tetragonal FeS1−x mackinawite, has been synthesized by reacting metallic iron with a sodium sulfide solution and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), transmission Mossbauer spectroscopy (TMS) and X-ray photoelectron spectroscopy (XPS). Based on XRD and TEM analyses, synthetic mackinawite exhibits crystallization and is identical to the natural mineral. Unit cell parameters derived from XRD data are a = b = 0.3670 nm and c = 0.5049 nm. The bulk Fe:S ratio derived from the quantitative dispersive energy analysis is practically 1. XPS analyses, however, showed that mackinawite surface is composed of both Fe(II) and Fe(III) species bound to monosulfide. Accordingly, monosulfide is the dominant S species observed at the surface with lesser amount of polysulfides and elemental sulfur. TMS analysis revealed the presence of both Fe(II) and Fe(III) in the mackinawite structure, thus supporting the XPS analysis. We propose that the iron monosulfide phase synthesized by reacting metallic iron and dissolved sulfide is composed of Fe(II) and S(-II) atoms with the presence of a weathered thin layer covering the bulk material that consists of both Fe(II) and Fe(III) bound to S(-II) atoms and in a less extent of polysulfide and elemental sulfur.

Iron control by equilibria between hydroxy-Green Rusts and solutions in hydromorphic soils

In order to verify Fe control by solution - mineral equilibria, soil solutions were sampled in hydromorphic soils on granites and shales, where the occurrence of Green Rusts had been demonstrated by Mossbauer and Raman spectroscopies. Eh and pH were measured in situ, and Fe(II) analyzed by colorimetry. Ionic Activity Products were computed from aqueous Fe(II) rather than total Fe in an attempt to avoid overestimation by including colloidal particles. Solid phases considered are Fe(II) and Fe(III) hydroxides and oxides, and the Green Rusts whose general formula is [FeII1−xFeIIIx(OH)2]+x· [x/z A−z]−x, where compensating interlayer anions, A−, can be Cl−, SO42−, CO32− or OH−, and where x ranges a priori from 0 to 1. In large ranges of variation of pH, pe and Fe(II) concentration, soil solutions are (i) oversaturated with respect to Fe(III) oxides; (ii) undersaturated with respect to Fe(II) oxides, chloride-, sulphate- and carbonate-Green Rusts; (iii) in equilibrium with hydroxy-Green Rusts, i.e., Fe(II)-Fe(III) mixed hydroxides. The ratios, x = Fe(III)/Fet, derived from the best fits for equilibrium between minerals and soil solutions are 1/3, 1/2 and 2/3, depending on the sampling site, and are in every case identical to the same ratios directly measured by Mossbauer spectroscopy. This implies reversible equilibrium between Green Rust and solution. Solubility products are proposed for the various hydroxy-Green Rusts as follows: log Ksp = 28.2 ± 0.8 for the reaction Fe3(OH)7 + e− + 7 H+ = 3 Fe2+ + 7 H2O; log Ksp = 25.4 ± 0.7 for the reaction Fe2(OH)5 + e− + 5 H+ = 2 Fe2+ + 5 H2O; log Ksp = 45.8 ± 0.9 for the reaction Fe3(OH)8 + 2e− + 8 H+ = 3 Fe2+ + 8 H2O at an average temperature of 9 ± 1°C, and 1 atm. pressure. Tentative values for the Gibbs free energies of formation of hydroxy-Green Rusts obtained are: ΔfG° (Fe3(OH)7, cr, 282.15 K) = −1799.7 ± 6 kJ mol−1, ΔfG° (Fe2(OH)5, cr, 282.15 K) = −1244.1 ± 6 kJ mol−1 and ΔfG° (Fe3(OH)8, cr, 282.15 K) = −1944.3 ± 6 kJ mol−1.

Effect of orthophosphate on the oxidation products of Fe(II)-Fe(III) hydroxycarbonate: the transformation of green rust to ferrihydrite

Abstract Hydroxycarbonate green rust GR(CO32−) has been synthesized by oxidation of aqueous suspensions of Fe(OH)2 by aeration at the air–liquid interface, in the presence of HCO3− ions at pH 7.5 to 9. During the oxidation of GR(CO32−), ferrihydrite formed first and then turned into goethite by dissolution and precipitation. The oxidation of GR(CO32−) in the presence of orthophosphate ions, which were added as Na2HPO4 · 7H2O salt, also involved the formation of ferrihydrite but not that of goethite, because the dissolution of ferrihydrite is inhibited by the adsorption of phosphate ions on its surface. The oxidation was slowed down because of the suppression of the catalytic effect of iron(III) hydroxide on the oxidation of Fe(II). In anoxic conditions without phosphate, a mixture of GR(CO32−), goethite, and ferrihydrite was observed to transform spontaneously into a mixture of siderite and magnetite. It is thermodynamically consistent, which shows that GR(CO32−) is metastable with respect to the two-phase system FeCO3–Fe3O4. In the presence of phosphate, this transformation was inhibited and GR(CO32−) did not transform in anoxic conditions. Anionic phosphate species dissolved in solution did not give rise to a corresponding GR, i.e., phosphate species did not substitute for carbonate inside the interlayers of the GR. Moreover, iron phosphates did not appear, neither during the oxidation of GR(CO32−) in the presence of oxygen nor in anoxic conditions.

Conversion electron Mössbauer spectroscopy and X-ray diffraction studies of the formation of carbonate-containing green rust one by corrosion of metallic iron in NaHCO3 and (NaHCO3 + NaCl) solutions

Abstract The corrosion of α-iron in 0.1 mol 1−1 NaHCO3 solutions, with or without additional NaCl, leads to the formation of a deep-green homogeneous layer which covers the metallic surface. It is analysed by X-ray diffraction (XRD) and conversion electron Mossbauer spectroscopy (CEMS) and proves to be made of carbonate-containing green rust one, GR1(CO32−), an Fe(II)-Fe(III) hydroxide-carbonate with chemical formula [Fe4 staggered(II)Fe2 staggered(III)(OH)12][CO3·2H2O]. If left in solution, the green rust layer oxidizes into α-FeOOH goethite. The corrosion process is: Fe → GR1(CO32−) → α-FeOOH, without previous formation of ferrous hydroxide, as expected from the Pourbaix diagram of iron in carbonate-containing aqueous media. If removed from the solution and oxidized in the air, the green rust layer oxidizes into a mixture of ferrihydrite or δ-FeOOH, i.e. poorly crystallized hydrated ferric oxide, and of a compound that could be called ferric green rust which keeps, in spite of the oxidation of the Fe(II) ions, the original stacking sequence. Iron samples corroded in (0.1 mol1−1 NaHCO3 + 4 mol1−1 NaCl) solutions are also covered with carbonate-containing green rust one layers and the chloride-containing green rust one, [Fe3 staggered(II)Fe(III)(OH)8][Cl·nH2O]is not observed even though the ClHCO3-ratio of the solution is as large as 40:1.

Structure and stability of the Fe(II)–Fe(III) green rust “fougerite” mineral and its potential for reducing pollutants in soil solutions

Abstract Fe(II)–Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [FeII(1−x) FeIIIx (OH)2]x+·[(x/n) An−·(m/n) H2O]x−, where x is the ratio FeIII/Fetot, and reflects the structure in which brucite-like layers alternate with interlayers of anions An− and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl−) and the hydroxysulphate GR2(SO42−) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl−) the structure of the fougerite GR mineral, [FeII(1−x) FeIIIx (OH)2]x+·[x OH−·(1−x) H2O]x-  Fe(OH)(2+x)·(1−x) H2O, is proposed displaying interlayers made of OH− ions and water molecules (in situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH−) would be R3m, with lattice parameters a≅0.32 and c≅2.25 nm. Stability conditions and the Eh-pH diagram of Fe(OH)(2+x) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: μ°[Fe(OH)(2+x)]=(1−x) μ°[Fe(OH)2]+x μ°[Fe(OH)3]+RT [(1−x) ln (1−x)+x ln x]+A0 x (1−x), where μ°[Fe(OH)2]=−492.5 kJ mol−1, μ°[Fe(OH)3]=−641 kJ mol−1 and A0=−243.9 kJ mol−1 at the average temperature of 9±1°C. The upper limit of occurrence of GR mineral at x=2/3, i.e. Fe3(OH)8, is explained by its unstability vs. α-FeOOH and/or magnetite; Fe(OH)3 is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and Eh-pH diagrams of Fe(OH)(2+x) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO3−, Se(VI) or Cr(VI) are fully illustrated.

Experimental synthesis of chlorite from smectite at 300°C in the presence of metallic Fe

Abstract The alteration and transformation behaviour of montmorillonite (bentonite from Wyoming, MX-80) in low-salinity solutions (NaCl, CaCl2) in the presence of metallic Fe (powder and 8 × 4 × 1 mm plate) and magnetite powder was studied in batch experiments at 300°C to simulate the mineralogical and chemical reactions of clays in contact with steel in a nuclear waste repository. The evolutions of pH and solution concentrations were measured over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, Transmission Mossbauer Spectroscopy and TEM (EDS, HR imaging and EELS). Dissolution of the di-octahedral smectite of the starting bentonite was observed, in favour of newly formed clays (chlorite and saponite), quartz, feldspars and zeolite. The formation of Fe-chlorite was triggered by contact with the metallic Fe plate and Fe-Mg-chlorite at distance from the Fe plate (>2 mm).

Experimental study of the transformation of smectite at 80 and 300°C in the presence of Fe oxides

Abstract The alteration and transformation behaviour of montmorillonite (Wyoming bentonite) was studied experimentally to simulate the mineralogical and chemical reaction of clays in contact with steel in a nuclear waste repository. Batch experiments were conducted at 80 and 300°C, in low-salinity solutions (NaCl, CaCl2) and in the presence or otherwise of magnetite and hematite, over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, transmission Mössbauer spectroscopy and EDS-TEM. Experimental solutions were characterized by ICP-AES and ICP-MS. The main results are that no significant change in the crystal chemistry of the montmorillonite occurred at 80°C, while at 300°C, the presence of Fe oxides leads to a partial replacement of montmorillonite by high-charge trioctahedral Fe2+-rich smectite (saponite-like) together with the formation of feldspars, quartz and zeolites.

Electrochemical formation of a new Fe(II)Fe(III) hydroxy-carbonate green rust: characterisation and morphology

Abstract Electrochemical behaviour of iron in deaerated 0.2 M carbonate/bicarbonate solution pH 9.6 and T =25°C was investigated. Oxidation of iron leads to the formation of green rust as intermediate product and ferrihydrite as ultimate product. Characterisation of green rust was done through FTIR, TEM, XRD and Mossbauer spectroscopy. Results point out differences in the structure and morphology, such as size of hexagonal crystal lattice, parameter c and Fe(II)/Fe(III) ratio, between our electrochemically formed GR and GR1(CO 3 2− ) obtained by oxidation of Fe(OH) 2 . Co-precipitation of Fe(II) and Fe(III) species during the electrochemical procedure leads to a new GR. We propose the following chemical formula for this GR, [Fe (II) 2 Fe (III) 2 (OH) 8 ] 2+ ·[CO 3 ] 2− . However, the possibility that incorporation of both CO 3 2− and Cl − ions could occur is not ruled out.