Biogeochemical Importance of the Bacterial Community in Uranium Waste Deposited at Key Lake, Northern Saskatchewan

ABSTRACT

The long-term stability of immobilized elements of concern in uranium tailings deposited in the Deilmann Tailings Management Facility (DTMF), northern Saskatchewan, is dependent upon maintenance of highly oxic conditions within the tailings mass. The main objective of this study was to investigate the effect of stimulating microbial activity on the redox potential and state of ferrihydrite, which are considered to be the primary controlling condition and mineral phase, respectively, within the tailings. To determine the potential for biologically mediated decreases in redox potential and ferrihydrite reduction, a series of microcosm assays were performed. Non-sterile material from the tailings–water interface of the DTMF site was inoculated with indigenous flora previously isolated from the tailings material and enriched with a carbon source (50 ppm trypticase soy broth) and incubated under continuous-flow or intermittent-flow conditions, and compared with an uninoculated, no-carbon control that received continuous flow. Highly reducing conditions with redox potentials of less than −300 mV were detected after 2 days of incubation within the carbon-enriched tailings of microcosms receiving continuous flow, and less than −280 mV after 11 days of incubation within carbon-enriched tailings in microcosms receiving intermittent flow. The lowest recorded Eh value (−545 mV) was recorded after 14 days in a carbon-enriched microcosm receiving intermittent flow. In contrast, the redox conditions in the control microcosm never dropped below −93 mV; thus, it was clear that microbial activity and available carbon drove the Eh conditions to become highly reducing. The occurrence of low redox conditions was concomitant with the bulk chemical detection of Fe (II) in the effluent of treated microcosms. Sites of microbial ferrihydrite reduction were also detected using scanning transmission X-ray microscopy where Fe (II) species were observed in close proximity with bacterial cells. Analysis of the microbial diversity present within the microcosms confirmed that microbes indigenous to the DTMF system have the potential to generate conditions suitable for the proliferation of sulfate and iron reducing bacteria, such as Desulfosporosinus, which was detected by high-throughput 16S rRNA gene sequencing.

KEYWORDS:

• Biofilm
• iron reduction
• community structure
• sulfate reducing bacteria
• sediments

Acknowledgments

Nurul Huda Khan is thanked for on-site sample collection. STXM data were acquired at beamline 10ID1 at the CLS. The CLS is supported by NSERC, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, the Western Economic Diversification Canada, and the University of Saskatchewan. We thank the staff scientists at the CLS (Chithra Karunakaran and Jian Wang) for their support.

Funding

Cameco Corporation, Natural Science and Engineering Research Council of Canada-Collaborative Research and Development (NSERC-CRD) and NSERC are acknowledged for their financial support.