Uranium in Framboidal Pyrite from a Naturally Bioreduced Alluvial Sediment

• Published in: Environmental science & technology Vol. 43; no. 22; pp. 8528 - 8534
• Main Authors: Qafoku, Nikolla P, Kukkadapu, Ravi K, McKinley, James P, Arey, Bruce W, Kelly, Shelly D, Wang, Chongmin, Resch, Charles T, Long, Philip E
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15-11-2009
• Subjects: Applied sciences; Aquifers; Biodegradation, Environmental; Characterization of Natural and Affected Environments; Exact sciences and technology; Extraction processes; Geologic Sediments - chemistry; Iron - chemistry; Microscopy, Electron, Scanning; Pollution; Rivers; Sediments; Spectrum analysis; Sulfides - chemistry; Uranium; Uranium - chemistry; Colorado River; USA, Colorado R; Scanning electron microscopy; Radioactive pollution; XANES spectrometry; Pollutant behavior; Pollution control; Plume; Radioisotope; Sulfates; Sediments; Material weathering; Long term; EXAFS spectrometry; Aquifers; Persistence; Iron III; Uranium; Decontamination; Morphology; Weathering; Oxidation; Pollution abatement; Total organic carbon; Porosity; Ground water
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es9017333

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.