Speciation of uranium(IV) in a bioreduced aquifer
Résumé
Uranium contamination of groundwater is a concern at several US Department of Energy sites, such Old Rifle, CO. Uranium transport in the environment is mainly controlled by its oxidation state, since oxidized U(VI) is soluble, and then mobile. Bio-remediation techniques of contaminated aquifers aim at immobilizing uranium in a reduced form. Previous laboratory and field studies have shown that adding an electron donor (lactate, acetate, ethanol) to the groundwater stimulate the activity of metal-reducing bacteria, which promotes U(VI) reduction in contaminated aquifers. However, obtaining information on chemical and physical forms of U(IV) species for sediments biostimulated in the field is challenging due to the low concentration of uranium in the aquifers (typically < 10 ppm). An in-situ technique has been developed for studying uranium reduction dynamics during such bioremediation episodes. This technique uses in-well columns to obtain direct access to chemical and physical forms of U(IV) produced, evolving microbial communities, and trace and major ion groundwater constituents.
The aim of this work was to assess the formation of U(IV) during the early stages of a bio-remediation experiment, under Fe-reducing conditions, at the Old Rifle site, CO. Several in-well chromatographic columns packed with sediment were deployed and were sampled at different days. X-ray absorption spectroscopy and X-ray microscopy were used to obtain information on Fe and U speciation and distribution. Chemical extractions of the reduced sediments have also been performed, to study the rate of Fe(II) and U(IV) accumulation.