Uranium Deposition in a Weathered Fractured Saprolite/Shale DEBRA H. PHILLIPS, † DAVID B. WATSON,* ,‡ AND YUL ROH § Environmental Engineering Research Centre, School of Planning, Architecture, and Civil Engineering, Queen’s University of Belfast, Belfast BT9 5AG, Northern Ireland, U.K., Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6038, and Faculty of Earth Systems and Environmental Sciences, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Korea Chemical analysis and scanning electron microscopy (SEM) microanalysis were carried out on cores of contaminated geological material collected around four closed waste disposal ponds to examine the extent of nitric acid extractable U (U NA ) association with P, S, and extractable Fe, Al, and Mn oxides within deeply weathered fractured shale. The solid phase in many regimes on the site has been exposed to highly buffered acidic (<3.5) groundwater and has been aggressively weathered. Higher correlations occur between U NA and total P and S (r 2 ) 0.76, 0.69, respectively), citrate bicarbonate dithionite extractable Fe (Fed) and Al (Ald) (r 2 )0.87, 0.80, respectively), and acid oxalate extractable or amorphous/poorly crystalline Fe (Feo) (r 2 )0.63) in core material from a field plot known as Area 1 compared to core material from another field plot known as Area 3. In Area 3 core material, linear regression analysis of U NA and total P and S, and Fed, Ald and Feo gave r 2 values of 0.67, 0.4, 0.06, 0.24, and 0.45, respectively. These results showed similar relationships with SEM-wavelength dispersive spectroscopy (WDS) mapping of this material. It is noteworthy that Area 1 geological material has not been as aggressively weathered as Area 3 material due to its physical location from the waste source. In all of the cores, most of the Fe and Al oxides were crystalline, while most of the Mn oxides were amorphous. The greater adsorption and/or fixation of anion complexes of P-U (uranium phosphate) and S onto Fe and Al oxides from Area 1 cores compared to Area 3 core material is probably due to a higher amount of crystalline Fe and Al oxides compared to amorphous Fe and Al oxides and higher Al substitution in Fe oxides in Area 1. This unique study illustrates the relationships between U NA , total P and S, and Al, Fe and Mn oxides in fractured shale under field conditions which can be used in planning remediation of this site and other similar sites. Introduction The U.S. Department of Energy’s Environmental Remediation Sciences Program operates a Field Research Center (FRC) at an Oak Ridge Tennessee Y-12 site in the area encompassing a group of four closed and capped radioactive liquid waste disposal ponds to test and understand bioremediation methods for metals and radionuclides. Research is needed on core material from the Environmental Remediation Sciences Division (ERSD) FRC site to determine the extent of U contamination across the site (1) and the relationship that U phosphate has with secondary Fe, Al, and Mn oxides, particularly Fe oxides (2), to help elucidate the fate and transport of U so that experiments can be better planned. Mobility of U in groundwater can be retarded by precipitation of U-bearing materials and adsorption of U to various geomaterials, especially at low U concentrations (3), particularly when complexed with P (4, 5) and S (6). U-P associations are more insoluble than U-O associations (7). Complexes of uranyl phosphate associated with iron oxides are reported in samples from acidic environments on the Oak Ridge Reservation (ORR) (4). SEM microanalysis of highly weathered Nolichucky shale saprolite from the FRC shows the material as having a ‘sponge-like’ internal porosity with large mineral crystallites and Fe-bearing micas, while less weathered shale from the site is dense, compact and contained fine-textured illite and nanosized goethite (8). Stucki et al. (2007) (9) report that, after biostiumulation of material from the FRC site which was dominated by goethite, there were equal amounts of goethite and hematite; however, total Fe oxide content decreased to 2 /3 of the amount of the original sample. Goethite has been reported to strongly adsorb anions and cations in previous studies (5). Ferrihydrite (1) has been identified at the FRC site. Both of these Fe oxides are known to strongly adsorb U (5, 10). Uranium uptake by Fe oxides can initially be by adsorption, and over time U can be fixed in Fe-rich material. This precipitation process leads to the long-term retardation of U in the system (3). Al hydroxides can retain many types of anions (11). Sulfate adsorption by Al hydroxides increases with decreasing pH; however, sulfate is held less tightly than phosphate over a range of pH values (12, 13). Sulfide minerals are also efficient scavengers of soluble uranyl. Uranium mobility is significantly affected by interaction with sulfide surfaces (6), and it can be adsorbed by sulfate-bearing minerals (14, 15). Secondary deposits of clays and Fe and Mn oxides from the weathering of the shale coat many of the surfaces on the highly fractured and jointed saprolite on the Y-12 plant (2, 16), indicating they are hydrologically active (17). Reactive contaminant transport through the fractured, weathered shale on the ORR is affected by sorption, redox, and dissociation reactions of these surface coatings of Fe, Mn, and Al oxides (18). Phillips et al. (2006) (2) report that differential natural and anthropogenic weathering of frac- tured shale beneath the FRC site influences the distribution of U contamination in the geological material and ground- water. Selective dissolution is often used to quantify oxides in soils. The dithionite citrate bicarbonate (DCB) method extracts crystalline and poorly crystalline/amorphous oxides (free oxides) (19-24), while acid ammonium oxalate (AAO) extracts the poorly crystalline/amorphous oxides (22, 23, 25- 27). At the FRC site, the major crystalline Fe oxides identified are goethite (2) and hematite (1), while no specific crystalline Al oxides have been detectedspossibly due to being below detection by XRD (2). AAO extracts 90-100% of the ferri- hydrite (poorly crystalline/amorphous Fe oxide) and other Fe oxides that are recent byproducts of weathering (24). Gibbsite (aluminous hydroxide [Al(OH 3)] is the major crys- talline Al hydroxide in soil material and is extracted with the * Corresponding author phone: (865)241-4749; fax: (865)576-8646; e-mail: watsondb@ornl.gov. † Queen’s University of Belfast. ‡ Oak Ridge National Laboratory. § Chonnam National University. Environ. Sci. Technol. 2007, 41, 7653-7660 10.1021/es070819d CCC: $37.00  2007 American Chemical Society VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7653 Published on Web 10/19/2007