Scientists found the incorporation of neptunium (V) (NpO2+, neptunyl) and uranium (VI) (UO22+, uranyl) in a variety of mineral structures were quite different despite similar molecular geometries. Specifically, dramatically different incorporation levels were found for U(VI) and Np(V) in carbonate minerals grown from similar aqueous solutions in ambient conditions.
Scientific results clearly demonstrate crystal-chemical behaviors of Np(V) and U(VI) actinyl ions are sufficiently different than predicted. The new experimental results indicate that low-temperature mineral structures in the subsurface are highly selective for the incorporation of Np(V) or U(VI) actinyl ions, a mechanism that may limit their mobility and utilized as part of a groundwater remediation strategy.
In a study published in Geochimica et Cosmochimica Acta, researchers from the University of Notre Dame and Pacific Northwest National Laboratory (PNNL) examined factors that impact structural incorporation of Np(V) neptunyl and U(VI) uranyl ions into carbonate and sulfate minerals. The visiting scientists worked with resident experts and used spectroscopic and imaging instruments in RadEMSL. RadEMSLis a radiochemistry facility, which is part of EMSL, Environmental Molecular Sciences Laboratory, a Department of Energy Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at PNNL.
The radioactive metallic element Np is created when uranium-based nuclear fuel is burned up in electricity-producing commercial reactors and in plutonium-producing reactors operated for military purposes. One potential remediation strategy for environmental contamination is to reduce the transport of highly soluble pentavalent Np [Np(V)] by inducing incorporation of Np(V) actinyl ions into low-temperature minerals, such as carbonate and sulfate, that form naturally in the subsurface.
Because Np does not occur naturally in any appreciable quantity and there is no natural analogue for its environmental chemistry, predictions for environmental transport of Np(V) are based on studies of hexavalent uranium [U(VI)], in part because it is easier and cheaper to conduct studies with uranium. However, there are major differences in crystal chemistry of Np(V) and U(VI), suggesting they would not be incorporated in mineral structures the same way.
Researchers examined factors that impact structural incorporation of Np(V) neptunyl and U(VI) uranyl ions into carbonate and sulfate minerals. Overall, the growing carbonate minerals incorporated both actinyl ions at far higher levels than sulfate minerals. The experimental data also revealed remarkable differences with incorporation of Np(V) and U(VI) into carbonate minerals.
Funding and Program: Office of Biological and Environmental Research, Subsurface Geochemical Research Program
Publication: Balboni E, JM Morrison, Z Wang, MH Engelhard, and PC Burns. 2015. "Incorporation of Np(V) and U(VI) in Carbonate and Sulfate Minerals Crystallized from Aqueous Solution." Geochimica et Cosmochimica Acta 151:133-149. DOI:10.1016/j.gca.2014.10.027