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

Reports 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 , Fe 2 O 3 (hematite), and soils subjected to changing redox conditions. A decrease in pe + pH was accomplished by the addition of organic matter and H 2 (g) in soils, and by H 2 (g) with a Pt catalyst in Fe(III) oxide suspensions. Addition of H 2 (g) and a Pt catalyst to Fe(III) oxide suspensions resulted in their dissolution and subsequent precipitation of two distinct Fe 3 O 4 products. With small additions of O 2 (0.2 mL of air), equilibrium was maintained with Fe 3 O 4 (magnetite). X-ray diffraction confirmed its presence. When large amounts of atmospheric O 2 (g) were added (15 mL of air), the precipitate designated Fe 3 O 4 (amorp) was formed with a mean log of the equilibrium constant based on activities (K°) of 40.65 ± 0.11. X-ray diffraction showed this predpitate 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 Fe 3 O 4 (amorp) precipitate and Fe(OH) 3 (amorp) appear to control Fe solubility. Crystalline Fe(III) oxides including Fe 2 O 3 (hematite) and FeOOH(goethite) can only control Fe solubility under more highly stable, oxidizing conditions. Only with prolonged and intensive reduction is Fe 3 O 4 (magnetite) expected to form and control Fe solubility.