The Influence of Reaction Pathways on the Reduction of U(VI) to U(IV): The Role of the Intermediate U(V) Species
EMSL Project ID
39953
Abstract
This proposal is focused on the interfacial oxidation/reduction mechanisms of polyvalent metal ions important in subsurface systems. The current research is targeted at unraveling the reaction pathways involved in the reduction of U(VI) to U(IV). This emphasis was sparked by our previous unexpected discovery that an intermediate oxidation state of uranium (i.e., U(V)) was stabilized on the surfaces of ferrous micas after heterogeneous reduction of U(VI)aq by structural FeII. In this regard, U presents a case study for trying to elucidate the role of an intermediate oxidation state during multi-electron transfer reactions. These observations were based on short term experiments and did not address fundamental issues concerning reduction mechanisms, such as whether U(IV) is formed by two sequential one electron transfers from the mineral to sorbed uranyl, or by disproportionation of sorbed U(V). Subsequently, the solid phase reductant was changed from FeII containing micas to magnetite in order to reduce the structural complexity of the substrate. A major discovery was that U(V) formed at the near-surface of magnetite but U(IV) did not, despite long reaction times and sufficiently reducing conditions to stabilize UO2(c). Consequently, not only did U(V) not disproportionate, but it was not reduced by a second electron transfer. Further, thermodynamic modeling and EXAFS indicated that U was likely incorporated in secondary products, which appear to have stabilized U(V). We propose to evaluate potential coordination sites for the stabilization of U(V) and explore conditions under which U(IV) will or will not form from U(V) in those coordination sites. This will allow us to relate the U(V) to U(IV) transition to both the redox state of the system and the bonding environment of U(VI), U(V) and U(IV). These studies will primarily focus on additional experimental measurements using magnetite, where the formation of secondary reaction products will be controlled, as well as experiments using U(VI) containing phases specially designed to directly test the influence of the coordination environment on the reduction of U(VI) to U(V) to U(IV). The bonding environment and valence states of U and Fe will be interrogated by XPS, XAS, HRTEM, and Mossbauer all housed in the EMSL, except for XAS. Molecular dynamics and ab initio calculations will be employed to help constrain and generalize the interpretation of the experimental and spectroscopic results. The EMSL experimental and computational facilities are state of the art and have been integral to the success of the work thus far.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-01
End Date
2013-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Ilton ES, and PS Bagus. 2011. "XPS Determination of Uranium Oxidations States." Surface and Interface Analysis 43(13):1549-1560. doi:10.1002/sia.3836
Ilton ES, JS Lazama Pacheco, JR Bargar, Z Shi, J Liu, L Kovarik, MH Engelhard, and AR Felmy. 2012. "Reduction of U(VI) Incorporated in the Structure of Hematite." PNNL-SA-87404, Pacific Northwest National Laboratory, Richland, WA.
Massey MS, JS Lezama-Pacheco, ME Jones, ES Ilton, JM Cerrato, JR Bargar, and S Fendorf. "Competing retention pathways of uranium upon reaction with Fe(II)." Geochimica et Cosmochimica Acta. doi:10.1016/j.gca.2014.07.016
Sebastien Kerisit, Andrew R. Felmy, and Eugene S. Ilton (2011)
Atomistic Simulations of Uranium Incorporation into Iron (Hydr)Oxides. Environ. Sci. Tech., 45, 2770-2776
Singer DM, SME Chatman, ES Ilton, KM Rosso, JF Banfield, and G Waychunas. 2012. "Identification of simultaneous U(VI) sorption complexes and U(IV) nanoprecipitates on the magnetite (111) surface." Environmental Science & Technology 46(7):3811-3820. doi:10.1021/es203877x
Singer DM, SME Chatman, ES Ilton, KM Rosso, JF Banfield, and G Waychunas. 2012. "U(VI) sorption and reduction kinetics on the magnetite (111) surface." Environmental Science & Technology 46(7):3821-3830. doi:10.1021/es203878c
Skomurski FN, ES Ilton, MH Engelhard, BW Arey, and KM Rosso. 2011. "Heterogeneous Reduction of U6+ by Structural Fe2+ From Theory and Experiment." Geochimica et Cosmochimica Acta 75(22):7277-7290. doi:10.1016/j.gca.2011.08.006