The Transformation of γ-FeO(OH) to Fe3O4 and Green Rust II in an Aqueous Solution

A mechanism of the transformation of γ-FeO(OH) to Fe3O4 and/or green rust II in an aqueous solution containing FeII ion in the presence or absence of the SO42− ion at 298 K is described. γ-FeO(OH) is dissolved by two mols of FeII ions adsorbed on the surface of γ-FeO(OH), and a dissolved species (DS) is formed, which is then transformed to Fe3O4 or green rust II. The equilibrium of the formation of green rust II from γ-FeO(OH) is represented by: K=[H+]2⁄(Kh2[Fe2+]2[SO42−]) K=2.14×108, where Kh is the hydrolysis constant of Fe2+. This equation shows that, for a given proton concentration(pH), higher concentrations of Fe2+ and SO42− ions facilitate the formation of green rust II from γ-FeO(OH). When little green rust II is formed, the DS is transformed to Fe3O4.