Graduate Thesis Or Dissertation
 

Genesis of iron oxides in unsaturated porous media

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  • The kinetics of physical and chemical processes controlling the precipitation of iron oxide minerals in unsaturated media were investigated in this study. The characteristic morphology, mineralogy, and chemistry of iron oxides were also explored. Laboratory column experiments using fine and coarse grades of silica sand were conducted to compare the abiotic and biotic mechanisms governing the formation of Fe(III) oxides in unsaturated porous media. The samples of iron precipitates were analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM) and optical microscopy to identify the mineral and elemental composition of iron oxides. Two sets of iron bands appeared during the course of the experiments. Red oxidized bands (ferrihydrite) were formed at the upper extent of the capillary fringe. Whereas, black bands (magnetite) were formed below the oxidized iron bands. Additional visual observations were made to distinguish between abiotic and biotic origins of iron oxide minerals. A conceptual mechanistic framework for abiotic iron oxide formation was also proposed leading to the development of the kinetic equations for an unsaturated system that was exposed to a source of reduced iron at one end and oxygen at the other end. Finally, a numerical model was developed to investigate abiotic iron oxide formation with time at a particular location within an unsaturated porous media under different scenarios. These scenarios could include the use of different media, different unsaturation conditions and different time scales. This study reveals that abiotic iron oxide formation in unsaturated media is mainly a diffusion controlled mechanism. The location of iron oxide precipitation is regulated by the diffusion rates of Fe(II) (aq) and O₂ (g) and thus elucidates the importance of air-water and water-mineral interface properties and diffusive gradients on iron transformations in natural systems. These results can be implied to explain the formation of Fe (III) oxide cemented bands under hydrological processes and time scale in systems representative of natural environments.
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