DISTRIBUTION OF IRON OXIDES IN A CONTAMINATED WETLAND-AQUIFER SYSTEM AND KINETICS OF MICROBIAL IRON REDUCTION

Iron reduction is an important redox reaction in anaerobic environments in terms of 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. A series of sequential chemical extractions were 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. Results describe the spatial distribution of iron phases at the contaminated wetland-aquifer interface. Sequential extractions suggest ferrihydrite was an important phase and thus was used in subsequent experiments to evaluate the kinetic controls on the microbial reduction of ferrihydrite. Additional in situ microcosm experiments consisted of exposing native microbial communities to test solution amended with 2-line ferrihydrite (Fe5HO8 4H2O), electron donor (lactate and acetate), and a conservative tracer. The kinetics of iron reduction were then evaluated over time and the resulting changes in microbial community structure documented.