Changing Np Redox Speciation in the Synchrotron Beam
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Changing Np Redox Speciation in the Synchrotron Beam S. Skanthakumar1, 2, D. Gorman-Lewis4, A. Locock4, M.-H. Chiang1, M.P. Jensen1, P.C. Burns1, 4 , J. Fein4, C.D. Jonah1, K. Attenkofer3, and L. Soderholm1, 2, 4 1 Chemistry Division, 2Actinide Facility, 3Advanced Photon Source, Argonne National Laboratory, Argonne, IL, and 4Civil Engineering and Geological Sciences, University of Notre Dame, South Bend, IN. ABSTRACT X-ray absorption data, obtained from Np(V) in two environmentally-related samples, show evidence of significant metal-ion reduction in the synchrotron beam. The data are presented and possible sources of reduction are discussed. INTRODUCTION Neptunium (Np; atomic number 93) is a radiotoxic, long-lived manmade element that is produced as a biproduct in nuclear reactors and therefore it is abundant in their waste. Np is considered to be the most problematic of the 5f elements for long-term waste storage because of its potentially high solubility in groundwater. Subsurface environmental transport of groundwater contaminants is greatly influenced by their chemical speciation, complexation, precipitation, sorption and mineralization processes. Under environmentally relevant conditions, Np can be found in the III, IV, Np(V)O2+ and Np(VI)O22+ oxidation states. In the laboratory, acidic Np solutions contain primarily the neptunyl(V) [O=Np=O]+ ion. Whereas this, and the corresponding neptunyl(VI) ions are soluble in aqueous solutions at near-neutral pH, Np(IV) forms hydrous oxides or the dioxide, both of which are very insoluble. Therefore, in order to effectively model the fate and transport of Np in geologic systems, it is important to understand its redox speciation, in solution, as adsorbed species, and in the solid state. Prediction of speciation requires both knowledge of the metal-solution and -crystal chemistries, with an appreciation for the limitations of empirical models to adequately describe the complex interactions found in a natural environment. As part of an ongoing program, we have become interested in mechanisms by which Np can be concentrated or sequestered in the environment. Studies underway include the influence of bacteria on Np redox properties, solubility, and aggregation [1-3] and the incorporation of Np into uranyl mineral phases [4-6]. Characterization of Np speciation has relied on x-ray absorption spectroscopy (XAS), which is a proven tool for the study actinide of speciation, both in solution and in the solid state [7]. Np oxidation states can be discerned from the x-ray absorption nearedge structure (XANES) [8, 9] and confirmed by the coordination environment, determined from the extended x-ray absorption fine structure (EXAFS) [10]. During the studies of Np(V) in both the bacterial and mineral samples, significant Np redox activity was noted while samples were exposed to the synchrotron beam. Metal reduction in a synchrotron beam has previously been observed for aqueous CuCl2 solutions, where it was attributed to hydrated electrons produced by the radiolysis of water by
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