Effect of bicarbonate and oxidizing conditions on U(IV) and U(VI) reactivity in mineralized deposits of New Mexico

We investigated the effect of bicarbonate and oxidizing agents on uranium (U) reactivity and subsequent dissolution of U(IV) and U(VI) mineral phases in the mineralized deposits from Jackpile mine, Laguna Pueblo, New Mexico, by integrating laboratory experiments with spectroscopy, microscopy, and diffraction techniques. Uranium concentration in solid samples from the mineralized deposit obtained for this study exceeded 7000 mg kg−1, as determined by X-ray fluorescence (XRF). Results from X-ray photoelectron spectroscopy (XPS) suggest the co-existence of U(VI) and U(IV) at a ratio of 19:1 at the near surface region of unreacted solid samples. Analyses made using X-ray diffraction (XRD) and electron microprobe detected the presence of coffinite (USiO4) and uranium-phosphorous-potassium (U-P-K) mineral phases. Imaging, mapping, and spectroscopy results from scanning transmission electron microscopy (STEM) indicate that the U-P-K phases were encapsulated by carbon. Despite exposing the solid samples to strong oxidizing conditions, the highest aqueous U concentrations were measured from samples reacted with 100% air saturated 10 mM NaHCO3 solution, at pH 7.5. Analyses using X-ray absorption spectroscopy (XAS) indicate that all the U(IV) in these solid samples were oxidized to U(VI) after reaction with dissolved oxygen and hypochlorite (OCl−) in the presence of bicarbonate (HCO3−). The reaction between these organic rich deposits and 100% air saturated bicarbonate solution led to significant alterations in uranium speciation.