Uranium in Framboidal Pyrite from a Naturally Bioreduced Alluvial Sediment

Authors

• Nikolla P. Qafoku, Pacific Northwest National LaboratoryFollow
• Ravi K. Kukkadapu, Pacific Northwest National LaboratoryFollow
• James P. McKinley, Pacific Northwest National Laboratory
• Bruce W, Arey, Pacific Northwest National Laboratory
• Shelly D. Kelly, Argonne National Laboratory
• Chongmin Wang, Pacific Northwest National Laboratory
• Charles T. Resch, Pacific Northwest National Laboratory
• Philip E. Long, Pacific Northwest National Laboratory

Date of this Version

2009

Citation

Environ. Sci. Technol. 2009, 43, 8528–8534

Abstract

Samples of a naturally bioreduced, U-contaminated alluvial sediment were characterized with various microscopic and spectroscopic techniques and wet chemical extraction methods. The objective was to investigate U association and interaction with minerals of the sediment. Bioreduced sediment comprises ∼10% of an alluvial aquifer adjacent to the Colorado River, in Rifle, CO, that was the site of a former U milling operation. Past and ongoing research has demonstrated that bioreduced sediment is elevated in solid-associated U, total organic carbon, and acid-volatile sulfide, and depleted in bioavailable Fe(III) confirming that sulfate and Fe(III) reduction have occurred naturally in the sediment. SEM/EDS analyses demonstrated that framboidal pyrites (FeS₂) of different sizes (∼10-20 μm in diameter), and of various microcrystal morphology, degree of surface weathering, and internal porosity were abundant in the <53 μm fraction (silt + clay) of the sediment and absent in adjacent sediments that were not bioreduced. SEM-EMPA, XRF, EXAFS, and XANES measurements showed elevated U was present in framboidal pyrite as both U(VI) and U(IV). This result indicates that U may be sequestered in situ under conditions of microbially driven sulfate reduction and pyrite formation. Conversely, such pyrites in alluvial sediments provide a long-term source of U under conditions of slow oxidation, contributing to the persistence of U of some U plumes. These results may also help in developing remedial measures for U-contaminated aquifers.