Environ. Sci. Technol. 2010, 44, 74–79
Spatial Patterns and Modeling of Reductive Ferrihydrite Transformation Observed in Artificial Soil Aggregates ´ L I N E P A L L U D , * ,† M A T T E O K A U S C H , † CE SCOTT FENDORF,‡ AND CHRISTOF MEILE§ Department of Environmental Science, Policy and Management, University of California, Berkeley, California 94720, Environmental Earth System Science, Stanford University, Stanford, California 94305, and Department of Marine Sciences, University of Georgia, Athens, Georgia 30602
Received June 12, 2009. Revised manuscript received August 31, 2009. Accepted September 11, 2009.
Within soils, biogeochemical processes controlling elemental cycling are heterogeneously distributed owing, in large part, to the physical complexity of the media. Here we quantify how diffusive mass-transfer limitation at the soil aggregate scale controls the biogeochemical processes governing ferrihydrite reductive dissolution and secondary iron mineral formation. Artificial cm-scale aggregates made of ferrihydritecoated sand inoculated with iron-reducing bacteria were placed in flow-through reactors, mimicking macro- and microporous soil environments. A reactive transport model was developed to delineate diffusively and advectively controlled regions, identify reaction zones and estimate kinetic parameters. Simulated iron (Fe) breakthrough-curves show good agreement with experimental results for a wide-range of flow rates and input lactate concentrations, with only a limited amount (e12%) of Fe lost in the reactor outflow over a 31 day period. Model simulations show substantial intra-aggregate, mm-scale radial variations in the secondary iron phase distributions, reproducing the trends observed experimentally where only limited transformation of ferrihydrite was found near the aggregate surface, whereas extensive formation of goethite/lepidocrocite and minor amounts of magnetite and/or siderite were observed toward the aggregate center. Our study highlights the important control of variations in transport intensities on microbially induced iron transformation at the soil aggregate scale.
1. Introduction Redox transformations of Fe (hydr)oxides play a central role in the biogeochemistry of soils and sediments. Under nonsulfidic reducing conditions, most Fe(III) reduction is mediated by dissimilatory iron-reducing bacteria (DIRB) that couple the oxidation of organic matter or H2 with the reduction of Fe(III) (hydr)oxides (1-3). Ferrihydrite (Fe(OH)3 · nH2O) is commonly found in environments with fluctuating redox conditions (4) and is characterized by its * Corresponding author phone: (510)-642-6359; fax: (510)-6435098; e-mail:
[email protected]. † University of California. ‡ Stanford University. § University of Georgia. 74
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high bioavailability, surface area, and intrinsic reactivity, thus helping to control the persistence and mobility of various trace metals, organic contaminants, and radionuclides (5-7). Upon ferrihydrite reduction, Fe(II) is produced, which can lead to the formation of secondary iron mineral phases (6, 8-10). Low Fe(II) concentrations (
