Transformation of γ-FeO(OH) to Fe₃O₄by adsorption of iron(<scp>II</scp>) 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 FeII–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).