Characterization of Oxidation of Green Rust (Cl<SUP>−</SUP>) Containing Copper in Aqueous Solution

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To clarify the role of Cu in the oxidation of green rust (GR(Cl−)), which contains Fe(II) and Fe(III) ions, Cu-containing GR(Cl−) suspensions were synthesized and oxidized under various reaction conditions using nitrogen gas containing oxygen as the oxidant. X-ray diffraction (XRD) measurements were performed for analyzing the solid particles formed in the synthesis and oxidation steps. Upon the addition of Cu ions, a part of GR(Cl−) was oxidized to Fe3O4, while the Cu ions were reduced to metallic Cu (Cu(0)) in the GR(Cl−) suspensions. The pH and oxidation-reduction potentials (ORPs) of the aqueous solutions were measured during oxidation. The results showed that after oxidation, the pH of the solutions decreased, while the ORP increased. After the introduction of oxygen, GR(Cl−) was converted to iron oxides such as γ-FeOOH, α-FeOOH, and Fe3O4. The rate of conversion of GR(Cl−) to Fe3O4 was enhanced after the addition of Cu ions. The oxidation processes of the GR(Cl−) suspension containing copper revealed a reaction process in the ORP curve, which may be attributed to the reaction of Cu(0)/Cu(II) in the aqueous solution.