Synthesis of green rusts by oxidation of Fe(OH)2, their products of oxidation and reduction of ferric oxyhydroxides; –pH Pourbaix diagrams

FeII–FeIII hydroxysalt green rusts (GR), { Fe II 2 / 3 Fe III 1 / 3 ( OH ) 2 } ( 1 / 3 ) + ⋅ { [ ( 1 / 3 ) / n ] A n − ⋅ ∼ ( m / n ) H 2 O } ( 1 / 3 ) − , are prepared by oxidising a Fe(OH)2 precipitate in presence of anions A n − ( Cl − , SO 4 2 − , CO 3 2 − ) . Electrode potential E h and pH recorded during aerial oxidation depend on the initial ratio R = { [ OH − ] / [ Fe II ] } . For R = R h = 2 , i.e. Fe(OH)2 stoichiometry, two stages are observed and magnetite Fe3O4 forms. In contrast, an excess of Fe2+ ions, i.e. R < R h , gives rise to 3 stages described in a mass balance diagram: A around pH 7.2 ends at inflection point t g , B around pH 6.5 at t f and C around pH 3.5 for strong acids and 9 for buffered solutions. GR forms alone at t g if initial ratio R = R g = 1.67 . Fast oxidation by H2O2, dry oxidation or voltammetry lead to deprotonated GR whereas bioreduction yields deprotonated { Fe II 6 ( 1 − x ) Fe III 6 x O 12 H 2 ( 7 − 3 x ) } 2 + ⋅ [ CO 3 2 − ⋅ ∼ 3 H 2 O ] 2 − fougerite with x ∈ [ 1 / 3 , 2 / 3 ] . E h –pH diagrams for hydroxycarbonate, chloride, or sulphate are presented. To cite this article: J.-M.R. Génin et al., C. R. Geoscience 338 (2006).