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 suspension s of amorphous Fe(OH)(3), Fe2O3(hematite), and soils subjected to chan ging redox conditions. A decrease in pe + pH was accomplished by the a ddition 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 ca talyst to Fe(III) oxide suspensions resulted in their dissolution and subsequent precipitation of two distinct Fe3O4 products. With small ad ditions of O-2 (0.2 mt of air), equilibrium was maintained with Fe3O4( magnetite). X-ray diffraction confirmed its presence. When large amoun ts of atmospheric O-2(g) were added (15 mt of air), the precipitate de signated Fe3O4(amorp) was formed with a mean log of the equilibrium co nstant based on activities (K degrees) of 40.65 +/- 0.11. X-ray diffra ction showed this precipitate to be amorphous. This precipitate contro lled Fe solubility within the pe + pH range of 4.0 to 12.7, In soil su spensions 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 wit h prolonged and intensive reduction is Fe3O4(magnetite) expected to fo rm and control Fe solubility.