GREEN RUSTS: Quick view.
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 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).
Standard Gibbs free energies of formation or chemical potentials of various GRs (synthetics and mineral) have been computed, Eh-pH Pourbaix diagrams drawn and stability conditions determined. These thermodynamic data can be used most importantly to predict the possibility that GRs minerals reduce some anions in contaminated soils, e.g. NO3−, Se(VI) or Cr(VI).
Left. Gleysol of the site from Fougères (Brittany, France). The blue-green colour indicates the occurrence of Fougerite. [Trolard, 2006].
Middle. TEM picture of Fe(II-III) hydroxysulphate green rust. [Géhin et al. 2002].
Right. General view of an ordered representation of the crystal structure of sulphate green rust, GR2(SO42−). [Simon et al. 1998].
Thesis
Green rusts (GRs) were long ago recognised as transitory intermediate products during the wet corrosion of iron-base materials (Girard, Thesis work, 1935). The preparation of GRs by oxidation of Fe(OH)2 was the predilection of corrosion scientists that essentially tried to simulate the process of oxidation of Fe species. In the 80's and 90's, a lot of studies showed the reactivity of Fe(II)aq with the various ferric oxyhydroxides plays a key role in the availability of iron in soils and aquifers. Thus, reactions such as the dissolution of ferric oxyhydroxides in the presence of Fe(II)aq and the co-precipitation of GRs are of major importance in environment.
My thesis work aimed at determining systematically the influence of the [Fe(II)]/[Fe(III)] and [OH−]/[Fetotal] ratios upon the co-precipitation of GRs. A Fe-compound mass-balance diagram was shown to be a powerful tool to study the various steps of the reactions of co-precipitation. Solid phases were fully characterised with transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and transmission Mössbauer spectroscopy (TMS).
The results from this research have important implications for :
(1) synthesis of GRs, and layered double hydroxides;
(2) mechanisms of GRs formation in environement,and widely iron biogeochemical cycle;
(3) attenuation of pollution (trace elements, heavy metals, radionuclides) in natural systems, due to chemical properties of GRs (sorption, redox, etc.)
Related articles.
Ruby C., Géhin A., Aissa R. and Génin J.-M. R., Mass-balance and E-pH diagrams of Fe(II-III) green rust in aqueous sulphated solution. Corrosion Science, 48, 3824-3837 (2006).
Ruby C., Géhin A., Aissa R., Ghanbaja J., Abdelmoula M. and Génin J.-M. R., Chemical stability of hydroxysulphate green rust synthetised in the presence of foreign anions: carbonate, phosphate and silicate. Hyperfine Interactions, 167, 803-807 (2006).
Ruby C., Aissa R., Géhin A., Cortot J., Abdelmoula M. and Génin J.-M. R., Green rusts synthesis by coprecipitation of FeII-FeIII ions and mass-balance diagram. C.R. Géosciences, 338, 420-432 (2006).
Ruby C., Upadhyay C., Géhin A., Ona-Nguema G. and Génin J-M R, In Situ Redox Flexibility of FeII-III Oxyhydroxycarbonate Green Rust and Fougerite. Environmental Sciences and Technology, 40, 4696-4702 (2006).
J.-M. R. Génin, R. Aïssa, A. Géhin, M. Abdelmoula, O. Benali, V. Ernstsen, G. Ona-Nguema, C. Upadhyay, C. Ruby. Fougerite and FeII-III hydroxycarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)(2+x). Solid State Sciences, 7, 545-572 (2005).
R. Aissa, C. Ruby, A. Géhin, M. Abdelmoula and J.-M. R. Génin. Synthesis by Coprecipitation of Al-Substituted Hydroxysulphate Green Rust FeII4 FeIII(2−y)AlIIIy (OH)12SO4, nH2O. Hyperfine Interactions, 156/157, 445-451 (2004).
F. Bochet, A. Géhin, C. Ruby, J. Ghanbaja, M. Abdelmoula and J.-M. R. Génin. Coprecipitation of Fe(II-III) hydroxycarbonate green rust stabilised by phosphate adsorption. Solid State Sciences, 6, 117-124 (2004).
Ph. Refait, A. Géhin, M. Abdelmoula and J.-M. R. Génin. Coprecipitation thermodynamics of iron(II-III) hydroxysulphate green rust from Fe(II) and Fe(III) salts. Corrosion Science, 45, 659-676 (2003).
C. Ruby, A. Géhin, M. Abdelmoula, J.-M. R. Génin and J.-P. Jolivet. Coprecipitation of Fe(II) and Fe(III) cations in sulphated aqueous medium and formation of hydroxysulphate green rust. Solid State Sciences, 5, 1055-1062 (2003).
A. Géhin, C. Ruby, M. Abdelmoula, O. Benali, J. Ghanbaja, Ph. Refait and J.-M. R. Génin. Synthesis of Fe(II-III) hydroxysulphate green rust by coprecipitation. Solid State Sciences, 4, 61-66 (2002).
G. Ona-Nguema, M. Abdelmoula, F. Jorand, O. Benali, A. Géhin, J. C. Block and J.-M. R. Génin. Iron(II,III) hydroxy-carbonate green rust formation and stabilisation from lepidocrocite bioreduction. Environmental Science and Technology, 36, 16-20 (2002).
G. Ona-Nguema, M. Abdelmoula, F. Jorand, O. Benali, A. Géhin, J. C. Block and J.-M. R. Génin. Microbial reduction of lepidocrocite g-FeOOH by Shewanella putrefaciens; the formation of a green rust. Hyperfine Interactions, 139/140, 231-237 (2002).
L. Legrand, M. Abdelmoula, A. Géhin, A. Chaussé and J.-M. R. Génin. Electrochemical formation of a new Fe(II)-Fe(III) hydroxy-carbonate green rust: characterisation and morphology. Electrochimica Acta, 46, 1815-1822 (2001).