Speciation of Uranium in Biologically Reduced Sediments in the Old Rifle Aquifer

Wednesday, May 16, 2012 - 1:30pm

Juan S. Lezama Pacheco

The speciation and dynamics of Uranium(IV) in naturally and artificially bioreduced sediments, as well as its local nanometer-to-millimeter scale physical and chemical environment, controls its stability, susceptibility to oxidation, and subsequent transport behavior in aquifers. Uraninite (UO2) is widely believed to be the most stable and therefore desirable form of uranium in bioreduced sediments. However, non-polymerized forms of U(IV), generally referred to as monomeric U(IV), also have been observed in laboratory axenic culture, column, and microcosm studies.

Similar types of studies generally have not been performed in reduced aquifers but are required to develop a better understanding of U(IV) reaction products produced in these complex subsurface systems. In this talk I will present results from an in-situ technique for studying U(IV) products of biological U(VI) reduction and their dynamics in aquifers over the scale of days to years. This technique uses in-well columns to obtain direct access to sediment U(IV) species, evolving microbial communities, and trace and major ion groundwater constituents.

Whole sediments from these in-situ columns have been examined using X-ray and electron microscopy, and X-ray absorption spectroscopy. Our results indicate that the speciation of Uranium is closely related to that found in pure cultures, suggesting that direct enzymatic reduction by bacteria is the primarily pathway for reduction of U(VI) in stimulated aquifers. Predictions based on these results are key to assess the effectiveness of remediation approaches like bioremediation, currently proposed as a possible strategy for cleanup and remediation of contaminated sites.

Moreover, in order to make accurate predictions for the stability of these U forms, more detailed knowledge is required about the different intermediate species, kinetics and formation processes of these products. I will present some concepts and ideas of an in-situ flow-through experiments that could enable us to understand the formation of U intermediates, giving a more detailed picture of the formation of these products and the rate-limiting steps that could control it.

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