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

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.