Formation of Iron Oxide and Oxyhydroxides in Aqueous Solutions and Their Physical Properties

Abstract

The oxidation processes of Fe(II) hydroxo-complexes to α-, β-, γ-, and δ-FeOOH and Fe3O4 in aqueous solutions were investigated by processing measurements with infrared, far infrared, and Mossbauer spectra and X-ray diffraction of oxidation products. The effect of oxidation rate and addition of Cu2+ or PO43- ion on the oxidation of Fe (II) hydroxo-complexes were also studied.It has been found that in the present experimental condition the process of oxidation depends on the pH-value of solution as follows: in basic solution (pH 12.5∼13.5) Fe(OH)2→α-FeOOH, in the slightly acidic solution (pH 6∼6.5) β-Fe2(OH)3Cl→Green rust I→γ-FeOOH, and in neutral or slightly basic solution (pH 7∼8) Fe(OH)2→Fe3O4 and β-Fe2(OH)3Cl→Green rust I→Fe3O4. The infrared spectra of Fe(OH)2 and Green rust I, an oxidation intermediate, have been obtained.The rapid oxidation of Fe(II) hydroxo-complexes in solutions with the addition of H2O2 results in the formation of δ-FeOOH. The oxyhydroxide has also been obtained by removing water from the Fe (II)-complexes, and leaving them in air to be oxidized. The broad absorption band at 450cm-1 has been shifted for 480cm-1 with the increase in size of δ-FeOOH crystallite formed by the oxidation of Fe(OH)2 with H2O2.In the presence of Cu2+ or PO43- ion, freshly precipitated Fe(II) hydroxo-complexes are oxidized into amorphous δ-FeOOH with air in solutions. This transformation is considered to have been caused by the catalytic effect of Cu2+ or PO43- ion in the oxidation of Fe(II)-complexes.