Reduction and Oxidation Effect on the Solubility and Transformation of Iron Oxides

Abstract

AbstractReports on the solubility of Fe during fluctuating redox conditions in soils and sediments are often difficult to interpret. This study was undertaken to examine the solubility relationships of Fe in suspensions of amorphous Fe(OH)3, Fe2O3(hematite), and soils subjected to changing redox conditions. A decrease in pe + pH was accomplished by the addition of organic matter and H2(g) in soils, and by H2(g) with a Pt catalyst in Fe(III) oxide suspensions. Addition of H2(g) and a Pt catalyst to Fe(III) oxide suspensions resulted in their dissolution and subsequent precipitation of two distinct Fe3O4 products. With small additions of O2 (0.2 mL of air), equilibrium was maintained with Fe3O4(magnetite). X‐ray diffraction confirmed its presence. When large amounts of atmospheric O2(g) were added (15 mL of air), the precipitate designated Fe3O4(amorp) was formed with a mean log of the equilibrium constant based on activities (Ko) of 40.65 ± 0.11. X‐ray diffraction showed this precipitate to be amorphous. This precipitate controlled Fe solubility within the pe + pH range of 4.0 to 12.7. In soil suspensions subject to changing redox, the Fe3O4(amorp) precipitate and Fe(OH)3(amorp) appear to control Fe solubility. Crystalline Fe(III) oxides including Fe2O3(hematite) and FeOOH(goethite) can only control Fe solubility under more highly stable, oxidizing conditions. Only with prolonged and intensive reduction is Fe3O4(magnetite) expected to form and control Fe solubility.