Oxidation modes and thermodynamics of FeII-III oxyhydroxycarbonate green rust: dissolution-precipitation versus in-situ deprotonation; about the fougerite mineral

Abstract : FeII-III hydroxycarbonate green rust GR(CO32-), FeII4 FeIII2 (OH)12 CO3 • 3H2O, is oxidized in aqueous solutions with varying reaction kinetics. Rapid oxidation with either H2O2 or dissolved oxygen under neutral and alkaline conditions leads to the formation of ferric oxyhydroxycarbonate GR(CO32-)*, FeIII6 O12 H8 CO3 • 3H2O, via a solid-state reaction. By decreasing the flow of oxygen bubbled in the solution, goethite -FeOOH forms by dissolution-precipitation mechanism whereas a mixture of non-stoichiometric magnetite Fe(3-x)O4 and goethite is observed for lower oxidation rates. The intermediate FeII-III oxyhydroxycarbonate of formula FeII6(1-x) FeIII6x O12 H2(7-3x) CO3 • 3H2O, i.e. GR(x)* for which x  [1/3, 1], is the synthetic compound that is homologous to the fougerite mineral present in hydromorphic gleysol; in situ oxidation accounts for the variation of ferric molar fraction x = [FeIII]/{[FeII]+[FeIII]} observed in the field as a function of depth and season but limited to the range [1/3, 2/3]. The domain of stability for partially oxidized green rust is observed in the Eh-pH Pourbaix diagrams if thermodynamic properties of GR(x)* is compared with those of lepidocrocite, -FeOOH, and goethite, -FeOOH. Electrochemical equilibrium between GR(x)* and FeII in solution corresponds to Eh-pH conditions close to those measured in the field. Therefore, the reductive dissolution of GR(x)* can explain the relatively large concentration of FeII measured in aqueous medium of hydromorphic soils containing fougerite.
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Christian Ruby, Mustapha Abdelmoula, Sebastien Naille, Aurélien Renard, Varsha Khare, et al.. Oxidation modes and thermodynamics of FeII-III oxyhydroxycarbonate green rust: dissolution-precipitation versus in-situ deprotonation; about the fougerite mineral. Geochimica et Cosmochimica Acta, Elsevier, 2009, 74, pp.953-966. ⟨10.1016/j.gca.2009.10.030⟩. ⟨hal-00522806⟩

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