DISTRIBUTIONS OF MICROBIALLY MEDIATED IRON METABOLISM IN ACID MINE DRAINAGE-DERIVED SEDIMENTS

Throughout the Appalachian basin historical and modern coal mining activity has exposed associated iron sulfide deposits to infiltration of oxygenated water. Resultant oxidation of iron sulfides yields iron rich, low pH fluids referred to as acid mine drainage (AMD). When AMD emerges at the terrestrial surface, and is exposed to atmospheric oxygen, dissolved Fe(II) is oxidized, yielding insoluble iron(III) hydroxide and oxide phases. In many cases, this process is mediated by iron oxidizing bacteria (FeOB), and continuous precipitation of iron(III) phases leads to the formation of massive iron(III) deposits that are referred to as “iron mounds.” In iron mound sediments iron may undergo redox cycling due to the activities of FeOB and iron reducing bacterial (FeRB). The depth-dependent distributions of these activities was monitored over 1500 hours in columns that contained iron mound sediment and were incubated in the laboratory. Iron(II) concentrations and electrical potential were monitored throughout the experiments at various depths within the sediment columns. Rapid iron(II) oxidation was observed in the upper ~1 cm of the sediments, and over the course of the incubations, iron(III) reduction (as indicated by iron(II) accumulation) was observed in the deeper portions of the sediments. However, iron(II) accumulation patterns and electric potential signals(EP) indicative of iron(II) oxidation were observed in oxygen-depleted portions of the sediments. Electrical potential was inversely related to iron(II) concentration and peaks within the first few centimeters of the interface in biologically active incubations. Little iron transformations or depth dependent EP variation were observed in biologically inactive incubations, indicating that the iron transformations observed in non-sterile sediments were biologically mediated. Columns incubated with initial concentrations of soluble iron(II) displayed different EP patterns than those incubated without iron(II). Results of these experiments indicated that FeOB are most active near the fluid-sediment interface, and FeRB are most active in deeper portions of the sediments.