Structure Of Green Rust Research Articles

Effect of Sb on precipitation of biogenic minerals during the reduction of Sb-bearing ferrihydrites

Antimony (Sb) is a naturally occurring element; it is enriched in the environment by anthropogenic activities. Like other metalloid species, Sb partitions to mineral phases such as oxyhydroxides. In reducing environments, Fe(III) may serve as a terminal electron acceptor during dissimilatory iron reduction leading to its transformation. Relatively little is known concerning the effect of Sb(V) on the precipitation of biogenic minerals in relation to microbiologically mediated redox reactions. To further our understanding, Sb-bearing ferrihydrites (0.5 g) with variable Sb/(Fe + Sb) molar ratios of 0.04, 0.06 and 0.1, were incubated in the presence of Shewanella oneindensis MR-1 (1 × 108 cell mL−1) under N2 atmosphere. Additionally, we synthesized abiotic GR1(CO32−) in the presence of Sb(V) to examine the effect of Sb(V) on this mineral formation and stabilization. A combination of wet chemistry and solid analysis techniques (XRD, Mössbauer and Raman spectroscopies) was used to characterize the reactions.The Sb loading affected the rate and the extent of bio-reduction compared with pure ferrihydrite. Only a minor fraction of the total Sb, less than 0.5%, was released into the solution by the end of the incubation period, suggesting that the metalloid partitioned mainly in a newly formed phase. Furthermore, XPS analyses showed the presence of Sb(V) and Sb(III) species on the biogenic minerals. Magnetite was the main biogenic precipitate (91%) in the absence of Sb(V). Increasing of the molar ratios [Sb/(Fe + Sb)] to 0.1 resulted mainly in the precipitation of carbonated green (47%) rust and goethite (37%). Abiotic green rust synthesis carried out in the presence of Sb(V) indicated the latter’s stabilizing effect on the green rust structure, as for phosphate species. Thus, it is likely that Sb(V) preserve biogenic green rust, hindering its transformation to more thermodynamically stable phases.

Immobilization of selenium(VI) in artificially contaminated kaolinite using ferrous ion salt and magnesium oxide

We used kaolinite artificially contaminated with Se(VI) as well as Pb(II) as a representative of common metal contaminant in soils and immobilized these heavy metals using immobilization agents prepared by mixing Fe(II) salt and MgO. The expected role of MgO was to increase the pH of the system to immobilize Pb(II). The increase in pH was also expected to lead to the formation of Fe(II)-bearing minerals including green rusts, which in turn reduce and immobilize Se(VI). It was observed that Pb(II) was easily immobilized below the permissible leaching limit in Japan (10 μg/L), regardless of the type and dosage of the immobilization agent. However, Se(VI) was not effectively immobilized without the addition of Fe(II) salt. This observation indicated that the reduction of Se(VI) by Fe(II) is the dominant immobilization mechanism of Se(VI). The adverse influence of dissolved oxygen on the immobilization efficiency of Se(VI) also supported this indication. It was also observed that the addition of Fe(II) as FeSO4 resulted in lower immobilization efficiencies than the addition of Fe(II) as FeCl2, thereby indicating that SO42− negatively influenced the reduction of Se(VI) by Fe(II) possibly by interfering with the intake of Se(VI) into the green rust structure. The analysis of Se-loaded Fe precipitates obtained from batch experiments that mimicked the conditions of the immobilization treatment showed that Se(-II)/Se(0) and magnetite (Fe3O4) are the final products.

Fougerite, a new mineral of the pyroaurite-iowaite group: Description and crystal structure

AbstractFougerite (IMA 2003-057) is a mixed M(II)-M(III) hydroxysalt of the green rust group, where M(II) can be Fe or Mg, and M(III) is Fe. The general structural formula is: [Fe1−2+xFex3+Mgy(OH)2+2y]+x[x/nA−n.mH2O]−x where A is the interlayer anion and n its valency, with 1/4 ≼ x/(1+y) ≼ 1/3 and m ≼ (1−x+y). The structure of green rusts and parent minerals can accommodate a variety of anions, such as OH−, Cl−, CO32−,SO42−${\rm{CO}}_3^{2 - },\;{\rm{SO}}_4^{2 - }$. The structure of the mineral was studied by Mössbauer, Raman and X-ray absorption spectroscopies (XAS) at the FeK edge. Mössbauer spectra of the mineral obtained at 78 K are best fitted with four doublets: D1 and D2 due to Fe2+ (isomer shift δ ≈ 1.27 and 1.25 mm s−1, quadrupole splitting ΔEQ ≈ 2.86 and 2.48 mm s−1, respectively) and D3 and D4 due to Fe3+ (δ ≈ 0.46 mm s−1, ΔEQ ≈ 0.48 and 0.97 mm s−1, respectively). Microprobe Raman spectra obtained with a laser at 514.53 nm show the characteristic bands of synthetic green rusts at 427 and 518 cm−1. X-ray absorption spectroscopy shows that Mg is present in the mineral in addition to Fe, that the space group is and the lattice parameter a ≈ 0.30–0.32 nm. The mineral forms by partial oxidation and hydrolysis of aqueous Fe2+, to give small crystals (400–500 nm) in the form of hexagonal plates. The mineral is unstable in air and transforms to lepidocrocite or goethite. The name is for the locality of the occurrence, a forested Gleysol near Fougères, Brittany, France. Its characteristic blue-green color (5BG6/1 in the Munsell system) has long been used as a universal criterion in soil classification to identify Gleysols. From a thermodynamic model of soil-solution equilibria, it was proposed that for the eponymous mineral, Fougères-fougerite, OH− may be the interlayer anion. In other environments, the interlayer anion may be different, and other varieties of fougerite may exist. Fougerite plays a key role in the pathways of formation of Fe oxides.

Speciation of iron; characterisation and structure of green rusts and Fe II–III oxyhydroxycarbonate fougerite

Fe II–Fe III hydroxysalt green rusts (GR) are very reactive layered double hydroxides that can be characterised mainly by XRD and Mössbauer spectroscopy. Mössbauer spectra of the main compounds presented in this article are reviewed with special emphasis on those of GRs constituted of one or two ferrous doublets and one ferric doublet. The structure of GR has a stacking sequence of alternating brucite-like layers of Fe II–III(OH) 6 octahedrons and anion interlayers as confirmed from XRD patterns. There exist two kinds of structure named green rust one (GR1) and green rust two (GR2) with R 3 ¯ m and P 3 ¯ m 1 space groups, respectively. Cation and anion distributions within Fe layers and interlayers match abundances of Mössbauer spectrum components. In the view of that, it can be suggested that the fougerite mineral is partially deprotonated Fe II–III hydroxycarbonate with a general formula Fe II 6 ( 1 − x ) Fe III 6 x O 12 H 2 ( 7 − 3 x ) CO 3 , where x is within the range [ ( 1 3 ) , ( 2 / 3 ) ]. Cation substitution by Mg II and Al III cannot be excluded relating fougerite with other minerals like pyroaurite and hydrotalcite. To cite this article: J.-M.R. Génin et al., C. R. Geoscience 338 (2006).

Competitive Formation of Hydroxycarbonate Green Rust 1 versus Hydroxysulphate Green Rust 2 in Shewanella putrefaciens Cultures

The formation of hydroxysulphate green rust 2, a Fe(II-III) compound commonly found during corrosion processes of iron-based materials in seawater, has not yet been reported in bacterial cultures. Here we used Shewanella putrefaciens, a dissimilatory iron-reducing bacterium to anaerobically catalyze the transformation of a ferric oxyhydroxide, lepidocrocite (γ-FeOOH), into Fe(II) in the presence of various sulphate concentrations. Biotransformation assays of γ-FeOOH were performed with formate as the electron donor under a variety of concentrations. The results showed that the competitive formation of hydroxycarbonate green rust 1 (GR1(CO3 2−)) and hydroxysulphate green rust 2 (GR2(SO4 2 −)) depended upon the relative ratio (R) of bicarbonate and sulphate concentrations. When R ≥ 0.17, GR1(CO3 2 −) only was formed whereas when R < 0.17, a mixture of GR2(SO4 2 −) and GR1(CO3 2 −) was obtained. These results demonstrated that the hydroxysulphate GR2 can originate from the microbial reduction of γ-FeOOH and confirmed the preference for carbonate over sulphate during green rust precipitation. The solid phases were characterized by X-ray diffraction, transmission Mössbauer spectroscopy and scanning electron microscopy. Diffuse reflectance infrared Fourier transform spectroscopy confirmed the presence of intercalated carbonate and sulphate in green rust's structure. This study sheds light on the influence of dissimilatory iron-reducing bacteria on microbiologically influenced corrosion.

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 H 2O 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 Mo ssbauer 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 Fe II3Fe III(OH) 8Cl\\ nH 2O, with n probably equal to 2, to approximately Fe II2.2Fe III(OH) 6.4Cl\\ nH 2O. 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 Mo ssbauerspectroscopy analyses were performed on iron coupons polarised potentiostatically in KCl solutions of pH about 9. At a potential of 0.55V SHE a ferrous hydroxide layer is formed on the metal while at 0.35V SHE the corrosion product is mainly composed of GR.

Mössbauer effect evidence of a ferrous sulphate layer in the structure of green rust 2 and its atmospheric oxidation

Mossbauer spectra of green rust 2 run under vacuum from 78 K to 460 K, display a gradual change in the abundances of two out of three ferrous sites, with the ferric sites due to electronic exchange. Further oxidation in atmospheric conditions gives spectra displaying that the two first Fe2+ ions become oxidized to the ferric state, but that the last Fe2+ site remain unoxidized: thus, a ferrous sulphate layer in the structure of green rust 2 is left unchanged.

On the Structure of Green Rust of Iron

On the Structure of Green Rust of Iron