title: Uranium in framboidal pyrite from a naturally bioreduced alluvial sediment.
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 approximately 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 (FeS(2)) of different sizes ( approximately 10-20 microm in diameter), and of various microcrystal morphology, degree of surface weathering, and internal porosity were abundant in the <53 microm 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.
authors.affiliation: Pacific Northwest National Laboratory, Richland, Washington 99352, USA. nik.qafoku@pnl.gov
authors.forename: Nikolla P
authors.initials: NP
authors.lastname: Qafoku
authors.forename: Ravi K
authors.initials: RK
authors.lastname: Kukkadapu
authors.forename: James P
authors.initials: JP
authors.lastname: McKinley
authors.forename: Bruce W
authors.initials: BW
authors.lastname: Arey
authors.forename: Shelly D
authors.initials: SD
authors.lastname: Kelly
authors.forename: Chongmin
authors.initials: C
authors.lastname: Wang
authors.forename: Charles T
authors.initials: CT
authors.lastname: Resch
authors.forename: Philip E
authors.initials: PE
authors.lastname: Long
mesh_terms: D001673:Biodegradation, Environmental; D019015:Geologic Sediments / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008855:Microscopy, Electron, Scanning; D045483:Rivers; D013440:Sulfides / Q000737:chemistry*; D014501:Uranium / Q000737:chemistry*
chemical_list: D013440:Sulfides; C011342:pyrite; D014501:Uranium; D007501:Iron