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

Abstract

Fe II–III hydroxycarbonate green rust GR(CO 3 2−), Fe II 4 Fe III 2 (OH) 12 CO 3·3H 2O, is oxidized in aqueous solutions with varying reaction kinetics. Rapid oxidation with either H 2O 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 3·3H 2O, via a solid-state reaction. By decreasing the flow of oxygen bubbled in the solution, goethite α-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 3·3H 2O, i.e. GR( x)∗ for which x ϵ [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, γ-FeOOH, and goethite, α-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.