Synthesis of magnetite particles by oxidation of hydroxyl-chloride green rust suspension under controlled conditions

Magnetite (Fe₃O₄) particles were synthesized by oxidation of a hydroxyl chloride green rust (GR(Cl^-)) suspension at room temperature. The formation process of Fe₃O₄ particles was characterized by X-ray diffraction, magnetization and electrochemical measurements. The results showed that a small amount of fine Fe ₃O₄ particles were nucleated when the supernatant solution of the as-synthesized GR(Cl^-) suspension was replaced by deaerated water. By controlling the injection of oxygen gas at room temperature, Fe ₃O₄ particles of about 70 nm in diameter formed from such GR(Cl^-) suspension, while goethite (α-FeOOH) particles were mainly obtained from the as-synthesized GR(Cl^-) suspension under the same oxidation conditions. Hence, the saturation magnetization of final oxidation products obtained from the GR(Cl^-) suspension in which the supernatant solution was replaced was about 60 emu/g, which was six times larger than that obtained from the as-synthesized GR(Cl^-) suspension. In the early stage of the oxidation process, the oxidation-reduction potential (ORP) in the GR(Cl^-) suspension in which supernatant solution was replaced was lower than that in the as-synthesized GR(Cl^-) suspension. In addition, the value of pH of the former suspension was higher than that of the latter suspension. It is concluded that the formation of Fe₃O₄ particles is enhanced in solution with relatively low ORP and high pH.