Presence and Persistence of Uranyl Peroxide Nanoclusters in Contact with Geological Media
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0893-JJ09-06.1
Presence and Persistence of Uranyl Peroxide Nanoclusters in Contact with Geological Media Karrie-Ann Kubatko1, Katie M. Gunderson1, Mark Antonio2, Peter C. Burns1,2 and Lynne Soderholm1,2 1
Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46545 2 Chemistry Division, Argonne National Laboratory, Building 200, M163, Argonne, IL 60439 ABSTRACT Recently uranyl peroxide nanoclusters containing 24, 28, and 32 uranyl polyhedra were chemically and structurally characterized under alkaline conditions. Such nanoclusters could conceivable form from oxidative alteration of nuclear waste in a geological repository by incorporating peroxide formed by alpha-radiolysis of water or in tanks where high-level waste is stored. The stability and persistence of uranyl peroxides in the vadose zone will be impacted by their interaction with geological media. Here we report batch experiments of solutions containing monodisperse nanoclusters in contact with crushed welded tuff. Within the first 72 hours, U concentrations in solution remained unchanged; however concentrations of Si, Al, Ca, Mg, Na, and Fe increased due to the dissolution of calcium-bearing aluminum silicate minerals in the welded tuff. Despite the presence of excess Li+ in solution, within two weeks crystals precipitated in which Ca2+ replaced Li+ in the nanocluster cage.
INTRODUCTION Nano-scale particles are of considerable importance in many environmental systems as they often form at low temperatures, can impact the transport of heavy metals and radionuclides in geologic media, and are small enough that their properties are often particle-size dependent [1]. Recently the synthesis, characterization, and stability of self-assembling nanospheres containing 24, 28, and 32 uranyl polyhedra have been reported from alkaline solutions containing hydrogen peroxide [2] (Figure 1). Uranyl peroxide clusters could form in alkaline solutions in contact with nuclear waste due to formation of hydrogen peroxide by the alpha radiolysis of water. Crystals containing uranyl peroxide nanospheres precipitated from alkaline peroxide solutions under ambient conditions. Small-angle X-ray scattering data revealed an evolving structural organization in solution with well-defined clusters present as early as two days after preparation [2]. The overall change in small-angle X-ray scattering data with time indicates a ripening of the solution and is consistent with a self-assembly process that yields the initial nanospheres in solution, which subsequently precipitated. This new class of uranyl nanoclusters is a rare example of self-assembling nanospheres. The potential for their likely presence in natural and contaminated environmental samples provides impetus for further study of this unusual family of nanospheres.
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Figure 1. Uranyl nanoclusters containing (a) 24, (b) 28, and (c) 32 uranyl polyhedra.
EXPERIMENTAL DETAILS Solutions Previous studies of solutions containing 24-polyhedral nanocluste
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