2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 4
Presentation Time: 8:45 AM

X-Ray Absorption Spectroscopic Analysis of U(VI) Uptake by Mackinawite in the Presence of Carbonate

GALLEGOS, Tanya J.1, WEBB, Samuel M.2, BETTERTON, William J.1 and BULLOCK, John1, (1)U.S. Geological Survey, Box 25046, DFC MS 973, Denver, CO 80225-0046, (2)Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Ctr, Building 137, MS 69, 2575 Sand Hill Road, Menlo Park, CA 94025, tgallegos@usgs.gov

Reaction products formed upon interaction of uranyl (UO22+) with mackinawite, a reduced iron sulfide, were studied using x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) to determine the effects of pH, carbonate, and reoxidation on uptake of uranium by mackinawite. Samples were synthesized with uranium loadings of 2,379 and 23,790 ppm U at pH 5, 7 and 9. Some samples were also equilibrated with 1.67×10-2 M bicarbonate, a common ground-water constituent and lixiviant (leachate) used in in-situ leach (ISL) uranium mining. XAS data demonstrated that solid-phase uranium reaction products exhibit a shift in absorption edge position from the original U(VI) location to a lower energy, indicative of a reduced oxidation state (U(IV)). XAS data confirmed that near-neighbor coordination consisted of U-O and U-U bonds consistent with the formation of a discrete uraninite (UO2) precipitate at pH 5, 7 and 9 in the presence and absence of carbonate. When reduced uranium/mackinawite samples were exposed to air, XRD demonstrated that mackinawite oxidized to form iron oxides. Furthermore, XAS data showed that UO2(s) was oxidized to U(VI) as evidenced by near-neighbor coordination to Fe, typical of sorption of UO22+ onto Fe surface groups, and an absorption edge consistent with the original uranyl indicating a U(VI) valence state. Thus, upon oxidation, the system still provides uptake capacity for U(VI). Solution-phase analysis demonstrated that under anoxic conditions, mackinawite completely removed 12 ppm and 143 ppm of initial U(VI) added to 5-g/L and 60-g/L mackinawite suspensions, respectively, at pH 5,7 and 9. This research provides a basis for further examination of the feasibility of using mackinawite as a remediation medium for ground-water restoration following ISL mining and in-situ remediation of U(VI)-contaminated ground waters.