Hydrothermal Synthesis and Structure of Neptunium(V) Oxide
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0985-NN12-02
Hydrothermal Synthesis and Structure of Neptunium(V) Oxide Tori Z. Forbes1, Peter C. Burns1,2, L. Soderholm1,2, and S. Skanthakumar2 1 Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN, 46556 2 Chemistry Division, Argonne National Laboratory, Argonne, IL, 60439
ABSTRACT Single crystals of Np2O5 have been synthesized by low-temperature hydrothermal reaction of a (NpO2)+ stock solution with natural calcite crystals. The structure of Np2O5 was solved by direct methods and refined on the basis of F2 for all unique data collected on a Bruker X-ray diffractometer equipped with an APEX II CCD detector. Np2O5 is monoclinic, space group P2/c, with a = 8.168(2) Å, b = 6.584(1) Å, c = 9.3130(2) Å, β = 116.01(1)˚, V = 449.8(2) Å3, and Z = 1. The structure contains chains of edge-sharing neptunyl pentagonal bipyramids linked into sheets through cation-cation interactions with distorted neptunyl square bipyramids. Additional cation-cation interactions connect the sheets into a three-dimensional framework. The formation of Np2O5 on the surface of calcite crystals has important implications for the precipitation of isolated neptunyl phases in natural aqueous systems. INTRODUCTION 237
Np is an important constituent of commercial spent nuclear fuel and is considered problematic for the long-term storage of nuclear waste in a geologic repository [1]. Initial concentrations of 237Np in nuclear waste are relatively small, but due to its long half-life (2.14 x 106 years), it becomes a major contributor to the total potential radioactive dose of the system after several thousand years [1]. Limiting the transport of neptunium within the environment is vital for the success of the repository; therefore it is important to understand its geochemical behavior. Neptunium is a redox-sensitive element with the pentavalent (NpO2)+ moiety dominant under a wide range of environmental conditions [2]. The solid phases of Np(V) are considered relatively soluble and Np(V) aqueous species do not easily adsorb to mineral surfaces, therefore the mobility neptunium in the environment is of considerable concern [3]. Thermodynamic modeling of the repository systems suggests the pentavalent neptunium oxide, Np2O5, could be a stable solid in groundwater aquifers with low ionic strength [3]. Solubility studies conducted by Nitsche et al. [4] and Wes Efurd et al. [5] indicated precipitation of Np2O5 in simulated groundwater containing (NpO2)+ at 90 °C. Current thermodynamic modeling sometimes uses Np2O5 as the solubility limiting phase in geochemical systems and predicts a neptunium solubility of 3 x 10-5 mol kg-1 based upon this phase [3]. However, the precipitation of Np2O5 under these conditions is perhaps unexpected because previous synthesis methods of Np2O5 require high temperature ozonation or molten salt techniques [6-8]. Using mild hydrothermal conditions we have synthesized high quality single crystals of Np2O5 on the surface of natural calcite crystals and have determ
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