Transformation of γ-FeO(OH) to Fe3O4 by adsorption of iron(II) ion on γ-FeO(OH)

Abstract

When the pH of a reaction mixture of γ-FeO(OH) and the iron(II) ion is raised from 5 to 9, the γ-FeO(OH) is transformed to stoicheiometric Fe₃O₄ at 25 °C. The transformation reaction is triggered by the adsorption of the iron(II) ion on γ-FeO(OH) at a pH above 7.3, and the adsorbed Fe^II–γ-FeO(OH) is subsequently transformed to Fe₃O₄. In each step, one proton is released, a total of two protons being released in the reaction. The reaction of the transformation step is written as follows: [γ-FeO(OH)]₂FeOH⁺→ Fe₃O₄+ H₂O + H⁺ and includes a dissolution–precipitation process. The adsorption of iron(II) ion on Fe₃O₄ seems to take place along with the formation of the intermediate by the adsorption of iron(II) ion on γ-FeO(OH).