Tuesday, November 6, 2007 - 10:00 AM
185-3
Mass Transfer Impacts on Iron Transformations within Soil Aggregates.
Celine Pallud1, Yoko Masue-Slowey2, Christof Meile3, and Scott Fendorf2. (1) Environmental Science, Policy and Management, UC Berkeley, 137 Mulford Hall, Berkeley, CA 94720, (2) Geological and Environmental Sciences, Stanford University, Bldg. 320, Rm. 118, Stanford, CA 94305, (3) Department of Marine Sciences, University of Georgia, Athens, GA 30602
Iron (hydr)oxides are ubiquitous in soils and sediments and exert a pronounced effect on the fate and transport of nutrients and contaminants. In such natural environments, iron is subject to biotic and abiotic redox transformations and iron cycling depends on a tight interplay between hydrodynamic transport and (bio)geochemical reactions. In structured soils, solutes move preferentially (by advection) through macropores and slowly (by diffusion) into intra-aggregate micropores, leading to the establishment of redox gradients at the aggregate scale. In this study, artificial soil aggregates, representing systems of intermediate complexity, were used to study the coupling of physical, chemical, and biological processes affecting iron oxides transformations, under environmentally relevant geometries. We used novel aggregate-based reaction flow cell experiments and reactive transport modeling to determine mass transfer and biogeochemical redox controls on the cycling of iron ranging from micropore- to aggregate-scales. Aggregates were made of ferrihydrite coated-sand and inoculated with Shewanella putrefaciens. Lactate was added in the effluent solution along a simulated macropore. Chemical gradients, spatial distribution of bacteria, and solid phase constituents were determined, to quantify magnitude, as well as temporal and spatial heterogeneity in biotransformation rates of iron. After 9 days of reaction, a slight and uniform transformation of ferrihydrite results in approximately 10% (mol Fe) of goethite and 10% (mol Fe) magnetite. While this distribution remains steady within the outer portion of the aggregate, toward the aggregate center, goethite becomes the dominant product (60% (mol Fe)) after 36 days of reaction. Due to the localized buildup of both Fe(II) and bicarbonate, up to 15% (mol Fe) siderite also results within aggregate centers while no magnetite was detected.