MOBILIZATION OF HEAVY METALS AND URANIUM LINKED TO BIOSTIMULATION OF MICROBIAL FE(III) REDUCTION IN CONTAMINATED CREEK SOILS

Understanding the fate and transport of metals and radionuclides in soil environments is necessary for evaluating risks to pristine sites. Microbial activity, through natural attenuation or bioremediation, may affect contaminant dynamics by different mechanisms, e.g., sorption and precipitation processes or redox state transformations. In the former uranium mining area Ronneburg, Germany iron-rich surficial creek soils are contaminated with heavy metals and uranium. We assessed the potential for enhancing bioremediation of these contaminants via the addition of carbon substrates and identified active microbial populations affecting contaminant mobility. Addition of ethanol or lactate stimulated microbial Fe reduction which was associated with an increase in concentrations of soluble Co, Ni, Zn, As, and U. Mobilization of U was highly unexpected as U(VI) reduction often overlaps with Fe reduction. This suggests that there was a release of sorbed metals during reductive dissolution of Fe(III) oxides. Subsequent sulfate reduction was concurrent with a decrease in U, Co, Ni, As, and Zn concentrations. The relative contribution of U(IV) in the solid phase changed from 19 to 89% after anoxic incubation. The active Fe(III)-reducing community was dominated by δ-Proteobacteria (Geobacter) in ethanol-amended microcosms while, lactate-amended microcosms had more diverse microbial communities (e.g., Acidobacteria, Firmicutes, δ-Proteobacteria, and β-Proteobacteria). Although stimulated communities were related to organisms known to enhance reductive immobilization in U contaminated environments (e.g., Geobacter) their activity at our site was associated with increased contaminant mobility. Sites in the U.S. where Geobacter are major players in reductive immobilization of U are often carbon poor and associated with less heavy metal co-contaminants. In a surficial stream with high organic carbon inputs and a complex mixture of metals and radionuclides stimulating microbial activity may in fact enhance metal mobility, potentially causing metal-enriched soil horizons to be a source of metal contaminants to groundwater. Therefore, the unique geochemistry and microbial communities at different contaminated sites can greatly affect the success of bioremediation strategies.