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

• Published in: Geomicrobiology journal 2011-06, Vol.28 (5-6), p.457-470
• Main Authors: Alvarez, Rebeca, Livens, Francis R., Lloyd, Jonathan R., Holt, James P., Boothman, Christopher, Wincott, Paul, Handley-Sidhu, Stephanie, Keith-Roach, Miranda, Vaughan, David J.
• Format: Article
• Language: English
• Subjects: depleted uranium; soil microorganisms
• ISSN: 0149-0451; 1521-0529
• DOI: 10.1080/01490451.2010.508018

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 cultu