Cadmium(II)-, Magnesium(II)-, and Zinc(II)-bearing Ferrites Formed from γ-FeOOH at Various Reaction pH’s

The reaction of γ-FeOOH with the metal (Cd(II), Mg(II), or Zn(II)) and Fe(II) ions in solution gave the metal ion-bearing ferrites without oxidation. The ferrites obtained were the stoichiometric Mg(II)- and Zn(II)-bearing ferrites [Mg0.27Fe2.73O4.00 (pH 10.6), Zn0.38Fe2.61O4.00 (pH 10.5)] at pH 7.5–11.4, and the nonstoichiometric Cd(II)-bearing ferrites [Cd0.19Fe2.68O4.00 (pH 10.2)] at pH 7.5–10.5 (25 °C). The Cd(II)- and Mg(II)-ion content in the ferrites increases with an increase in the reaction pH, but the Zn(II)-ion content is almost constant at pH 7.5–10.5. This pH dependence of the metal-ion content in the ferrites agrees with that of the fractional adsorption of these metals on γ-FeOOH. This suggests that only the adsorbed metal ions are incorporated into the ferrites. The pH dependence of these metal-ion contents in the air-oxidation reaction is also similar to that in the transformation reaction from γ-FeOOH. This transformation reaction seems to be one process in the air-oxidation reaction.