Oxidation modes and thermodynamics of Fe(II-III) oxyhydroxycarbonate green rust: Dissolution-precipitation versus in situ deprotonation

• Published in: Geochimica et cosmochimica acta 2010-02, Vol.74 (3), p.953-966
• Main Authors: Ruby, Christian, Abdelmoula, Mustapha, Naille, Sebastien, Renard, Aurelien, Khare, Varsha, Ona-Nguema, Georges, Morin, Guillaume, Genin, Jean-Marie R
• Format: Article
• Language: English
• ISSN: 0016-7037
• DOI: 10.1016/j.gca.2009.10.030

Fe(II-III) hydroxycarbonate green rust GR(CO(3)(2-)), Fe(II)(4) Fe(III)(2) (OH)(12) CO(3)[sup].3H(2)O, is oxidized in aqueous solutions with varying reaction kinetics. Rapid oxidation with either H(2)O(2) or dissolved oxygen under neutral and alkaline conditions leads to the formation of ferric oxyhydroxycarbonate GR(CO(3)(2-))*, Fe(III)(6) O(12) H(8) CO(3)[sup].3H(2)O, via a solid-state reaction. By decreasing the flow of oxygen bubbled in the solution, goethite a-FeOOH forms by dissolution-precipitation mechanism whereas a mixture of non-stoichiometric magnetite Fe((3-)(x))())O(4) and goethite is observed for lower oxidation rates. The intermediate Fe(II-III) oxyhydroxycarbonate of formula Fe(II)(6(1-)(x))()) Fe(III)(6)(x)) O(12) H(2(7-3)(x))()) CO(3)[sup].3H(2)O, i.e. GR(x)* for which x [epsilon (Porson)] [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 = [Fe(III)]/{[Fe(II)]+[Fe(III)]} 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 E(h)-pH Pourbaix diagrams if thermodynamic properties of GR(x)* is compared with those of lepidocrocite, g-FeOOH, and goethite, a-FeOOH. Electrochemical equilibrium between GR(x)* and Fe(II) in solution corresponds to E(h)-pH conditions close to those measured in the field. Therefore, the reductive dissolution of GR(x)* can explain the relatively large concentration of Fe(II) measured in aqueous medium of hydromorphic soils containing fougerite.