In situ AFM and XPS Investigation of U 6+ Reduction by Fe 2+ on Hematite and Pyrite

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In situ AFM and XPS Investigation of U6+ Reduction by Fe2+ on Hematite and Pyrite Jingjie Niu1, Udo Becker2 and Rodney Ewing3 1, 2, 3 Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, U.S.A. ABSTRACT Uranyl adsorption/reduction by Fe2+ on hematite and pyrite has been studied at neutral pH under anoxic and CO2-free conditions. XPS results confirm that more U3O8 precipitates on hematite than on pyrite reacted for 24 h in 160 ȝM uranyl nitrate and 160 ȝM Fe2+ solution at initial pH 7.3. These results are explained in terms of co-adsorption energy and U atom Mulliken charge transfer by quantum mechanical calculations. Moreover, in situ fluid tapping-mode AFM experiments on hematite indicate a deceleration of the U reduction rate within 24 h due to the passivation of the surface caused by the formation of orthorhombic U3O8 crystals. In addition, crystals observed using AFM show morphologies of orthorhombic schoepite appearing on hematite after 5 h. INTRODUCTION Uranyl ions, from uranium ore mining and radioactive waste disposal, often exist as strong aqueous complexes with inorganic ligands in surface and subsurface environments[1][2]. The mobility of uranyl ions in low-temperature geochemical systems may be inhibited by sorption and/or reduction on iron sulfide and iron oxide mineral surfaces in the presence of Fe2+ [3][4][5]. XPS experiments confirm that uranyl adsorbs onto the mineral surface and that formation of UO2+x(s) immobilizes uranyl from solution[5][6][7]. However, the detailed role of mineralcatalyzed uranyl reduction by Fe2+ has not been well studied. This study, at the nano-scale, has investigated the kinetics of precipitate formation during uranyl reduction, and the role of hematite and pyrite in catalyzing these reactions. These results contribute to the understanding of the mechanisms that control the mobility of uranium in the geosphere and provide the basis for the development of effective remediation strategies for uranium-rich waste streams[8]. EXPERIMENTAL METHODS All AFM (Bruker EnviroScope) experiments were performed in a N2 atmosphere. Fresh mineral surfaces were first cut in a glove bag (Coy glove bag, < 0.5% O2) and mounted in 35 mm culture dishes. Then, FeSO4, UO2(NO3)2, and 2.45 ml 0.1 M NaNO3 were added to obtain concentrations in Table I, and 0.1 M NaOH was used to adjust pH. The reaction time t=0 of in situ experiments was defined as the time when U6+ and Fe2+ were injected into the culture dish. Next, in situ fluid tapping-mode AFM images and time were recorded accordingly with a scan rate of 0.702 Hz and scan sizes of 10 ȝm, 5 ȝm, or 3 ȝm. After 5 h, the culture dish was sealed, transferred into the glove bag and kept there until t=24 h. Then, the solution was removed and the sample surface was imaged using AFM in tapping mode. Finally, half of each sample was coated with gold and measured using a Kratos Axis Ultra XPS with a Mg Kα source (1200 eV),


a working current of 7 mA and a working voltage of 13 kV. All binding energies were