Geochemical and Microbial Controls of the Decomposition of Depleted Uranium in the Environment: Experimental Studies using Soil Microorganisms

Abstract

The decomposition of depleted uranium (DU) alloy in the presence of microorganisms characteristic of clay-rich soils has been studied using microcosm experiments. To assess the possible roles of specific groups of soil microorganisms, enrichment culture experiments were undertaken where the indigenous microbes were isolated from the soil and selectively cultured by adding growth medium supplemented with a specific terminal electron acceptor and electron donor, producing an inoculum to which a DU “coupon” was added. Experiments were conducted using microcosms with enriched consortia of either aerobic or anaerobic bacteria, including fermentative organisms and those that respire Fe(III), sulfate or nitrate. In a series of experiments, the rate and extent of DU breakdown was determined for each of the consortia. Changes in solution chemistry (pH, Eh, total Fe, Fe(II), nitrate, sulfate, glucose) were monitored and the proportions of dissolved uranium and solid breakdown products determined. The latter were characterized using environmental scanning electron microscopy (ESEM), X-ray powder diffractometry (XRD), and X-ray photoelectron spectroscopy (XPS). The microbial communities in the microcosms exposed to the DU were studied using PCR-based 16SrRNA profiling techniques. In the aerobic microcosms, significant DU corrosion as determined by mass loss (of ∼1.8%) occurred over 40 days; however, within experimental error, as much or more DU was consumed (∼3% weight loss) in an abiotic control experiment. The slower rate in the biotic experiment may reflect formation of a passivating film at the surface. Under anaerobic conditions, DU corrosion was less extensive than under aerobic conditions, with abiotic control experiments again showing comparable or greater mass loss. The order of decreasing corrosion rates under abiotic anaerobic conditions was: Fe (III)-reducing conditions (∼3% mass loss) > fermenting conditions (∼1% mass loss) > nitrate-reducing conditions (∼0.2% mass loss) > sulfate-reducing conditions (∼0.1% mass loss). In the equivalent biotic systems, corrosion was only significantly different to the abiotic treatments under Fe(III)-reducing conditions (∼2% mass loss), fermenting conditions (∼0.1% mass loss) and under nitrate-reducing conditions where biotic corrosion was so slow that mass loss was only measurable after extending the experiment to 300 days (after which there was 0.2% mass loss). The microbial communities in the different enrichment cultures were distinct with no evidence of differences between the microbial communities sampled from the DU coupon surface and those from the ‘planktonic’ population. After 40 days of corrosion in the aerobic systems, pale yellow, black and bright yellow alteration products had formed. These products were identified as chernikovite, uraninite and schoepite, respectively. The limited extent of alteration in the anaerobic systems did not yield identifiable mineral alteration products. Overall, these experiments show that microbes are surprisingly ineffective at promoting the breakdown of DU, which is dominated by chemical corrosion and leads to end-products that reflect the geochemical environment in which the corrosion has occurred, and that may include relatively stable mineral phases. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.

Keywords:

• depleted uranium
• soil microorganisms

ACKNOWLEDGMENTS

The Natural Environment Research Council (grant NE/C506799/1) is gratefully acknowledged for funding this research. Dstl (Porton Down, UK) and AWE (Aldermaston, UK) are thanked for supplying and cutting the DU, respectively. A. Bewsher, S. Caldwell, P. Lythgoe and J. Waters are thanked for technical support.