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 Fe3O4 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, Fe3O4 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.

In situ measurements of X-ray absorption spectra at an Fe K absorption edge were carried out during the transformation of chloride-containing green rust (GR(Cl -)) to oxyhydroxide (that is, lepidocrocite (γ -FeOOH)), by oxidation via aqueous solution. Results showed that the Fe K absorption edge in X-ray absorption near-edge structure (XANES) spectrum shifted toward the higher energy side with increasing oxidation time. The factor analysis of the XANES spectra, using reference data, implied that dissolution of GR(Cl -) and precipitation of γ -FeOOH occur simultaneously. The fraction of γ -FeOOH in the suspension increased continuously with increasing oxidation time, in response to the continuous decrease of the fraction of GR(Cl -). The results lead us to conclude that GR(Cl -) transforms to γ -FeOOH during oxidation under the present experimental conditions.

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