Mineralogical and Microbial Controls on Iron Reduction in a Contaminated Aquifer-Wetland System

Iron reduction is an important redox reaction in anaerobic environments for both biological and chemical cycling of elements such as carbon. However, the controls on the rate and extent of iron reduction are poorly understood and unlike other major terminal electron accepting processes, iron reduction has the added complexity that its oxidized form (ferric iron) exists primarily as one of several solid phases in environments with pH greater than 3. Thus, the distribution and form of ferric iron minerals are important controls on iron reduction in natural systems. For the first phase of this research a series of sequential chemical extractions was performed on a core taken from a landfill-leachate-contaminated wetland-aquifer system at the Norman Landfill, Norman, OK. The phases targeted by the sequential extractions consist of easily water soluble salts and ions present in the soil solution; weakly acid soluble iron (such as siderite and ankerite); easily reducible iron (such as ferrihydrite and lepidocrocite); moderately reducible iron (such as goethite, akageneite, and hematite); organically bound iron; magnetite; and pyrite. The second phase of this research involved creating in situ microcosm experiments that exposed native microbial communities to a test solution amended with 2-line ferrihydrite (Fe5HO8∙4H2O), electron donor (lactate and acetate), and a conservative tracer for a period of eleven days. The kinetics of iron reduction were then evaluated over time and the resulting changes in microbial community structure documented through DNA and RNA analysis. Results document the spatial distribution of iron phases at the contaminated wetland-aquifer interface. Results of the sequential extractions indicate that ferrihydrite was present throughout the core. Accordingly, ferrihydrite was used in subsequent experiments on in situ microcosms to evaluate the kinetic controls on the microbial reduction of ferrihydrite. The results of these experiments show that microbial communities actively responded to the introduction of the amended ferrihydrite solution by increasing their community size and reducing ferrihydrite to an iron (II) phase in increasing amounts over an eleven day period