Actinide Biocolloid Formation in Brine by Halophilic Bacteria
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ABSTRACT We examined the ability of a halophilic bacterium (WIPP I A) isolated from the Waste Isolation Pilot Plant (WIPP) site to accumulate uranium in order to determine the potential for biocolloid facilitated actinide transport. The bacterial cell surface functional groups involved in the complexation of the actinide were determined by titration. Uranium, added as uranyl nitrate, was removed from solution at pH 5 by cells but at pH 7 and 9 very little uranium was removed due to its limited solubility. Although present as soluble species, uranyl citrate at pH 5, 7, and 9, and uranyl carbonate at pH 9 were not removed by the bacterium because they were not bioavailable due to their neutral or negative charge. Addition of uranyl EDTA to brine at pH 5, 7, and 9 resulted in the immediate precipitation of U. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) analysis revealed that uranium was not only associated with the cell surface but also accumulated intracellularly as uranium-enriched granules. Extended X-ray absorption fine structure (EXAFS) analysis of the bacterial cells indicated the bulk sample contained more than one uranium phase. Nevertheless these results show the potential for the formation of actinide bearing bacterial biocolloids that are strictly regulated by the speciation and bioavailability of the actinide.
INTRODUCTION The Waste Isolation Pilot Plant (WIPP) repository located in Southeastern New Mexico is specifically designed for the long-term isolation of transuranic (TRU) waste from the accessible environment. The effectiveness of WIPP relies on its ability to limit the migration of actinides, predominantly Pu, Am, Np, and U, in the subsurface bedded salt environment. Potential migration mechanisms include transport as soluble species or as colloids, which are generally defined as particles < 1 pm suspended in a liquid. Actinide-bearing colloids that may be generated in the repository include mineral fragments, products from corrosion of the steel waste containers, intrinsic colloids (hydrolysis and condensation of actinide ions), and organic colloids (products from the biodegradation of cellulosic material, humic and fulvic acids, and microbial cells). The concentration of naturally occurring microbes in the hypersaline environments at WIPP is about l04 to 107 cells ml1 and have exceeded 108 cells ml1 in a simulated cellulose-rich repository environment [1, 2]. Because bacterial cells fall within the colloidal size range, typically on the order of 0.5 to 1.0 pm in length [2], and can be mobile in the subsurface, the potential for biocolloid facilitated transport of actinides under WIPP repository conditions is being Investigated. Bacterial cell walls possess a variety of negatively charged functional groups (COO, PO4, OH-) capable of complexation with cations [3]. Surface charge is dependent upon the proton condition of the external milieu. The thermodynamic controls on charge buildup and metal binding at specific microbe-solution interfaces has been studi
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