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

摘要

FeII–III hydroxycarbonate green rust GR(CO32), FeII 4 FeIII 2 (OH)12 CO33H2O, 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(CO3 2)*, FeIII 6 O12 H8 CO33H2O, 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(3x)O4 and goethite is observed for lower oxidation rates. The intermediate FeII–III oxyhydroxycarbonate of formula FeII 6(1x) FeIII 6x O12 H2(73x) CO33H2O, i.e. GR(x)* for which x e [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, c-FeOOH, and goethite, a-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.