Carbonate Minerals Could Immobilize Neptunium in Groundwater


The radioactive metallic element neptunium (Np) is created when uranium (U)-based nuclear fuel is burned in electricity-producing commercial reactors and in plutonium-producing reactors operated for military purposes. Np(V) has been accidentally released to the environment at former Department of Energy (DOE) weapons production sites as well as other locations through a variety of circumstances. Because Np(V) has a high aqueous solubility, it is readily transported in groundwater. Predictions for the transport of Np(V) in groundwater are based on studies of U(VI), in part because U(VI) is easier and cheaper to study. However, there are major differences in the crystal chemistry of Np(V) and U(VI), suggesting they might be incorporated into mineral structures differently, and thereby immobilized in groundwater differently. In a recent study, researchers from the University of Notre Dame and Pacific Northwest National Laboratory examined factors that impact the structural incorporation of Np(V) and U(VI) ions into carbonate and sulfate minerals. Using spectroscopic and imaging instruments in RadEMSL, a radiochemistry facility at DOE’s Environmental Molecular Sciences Laboraty, the team found that carbonate minerals incorporated both ions at far higher levels than sulfate minerals. In addition, they found that Np(V) and U(VI) are incorporated into carbonate minerals at dramatically different levels, and that Np(V) can be readily incorporated into carbonate minerals, thereby reducing its mobility in groundwater.

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Balboni, E., J. M. Morrison, Z. Wang, M. H. Engelhard, and P. C. Burns. 2015. “Incorporation of Np(V) and U(VI) in Carbonate and Sulfate Minerals Crystallized from Aqueous Solution,” Geochimica et Cosmochimica Acta 151,133-49. DOI: 10.1016/j.gca.2014.10.027.