Oxidation modes and thermodynamics of FeII-III oxyhydroxycarbonate green rust: Dissolution-precipitation versus in situ deprotonation

FeII-III hydroxycarbonate green rust GR(CO32-), Fe-4(II) Fe-2(III) (OH)(12) CO3 center dot 3H(2)O, 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-)*, Fe-6(III) O-12 H-8 CO3 center dot 3H(2)O, via a solid-state reaction. By decreasing the flow of oxygen bubbled in the solution, goethite alpha-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 oxyhydroxyearbonate of formula Fe(II)6((1-x)) Fe-6x(III) O-12 H2(7-3x) CO3 center dot 3H(2)O, i.e. GR(x)* for which x is an element of [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, gamma-FeOOH, and goethite, alpha-FeOOH. Electrochemical equilibrium between GR(x)* and Fell 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. (C) 2009 Elsevier Ltd. All rights reserved.