Surface-Mediated Formation of Plutonium Nanoparticles

Researchers observe metal-trapping on mineral surfaces.

Image courtesy of Argonne National Laboratory
AFM micrograph of the muscovite (001) basal plane showing PuO2 nanoparticles after overnight exposure to 2 mM Pu(III) in 100 mM NaCl at pH 2.6 (image range: 200 nmx200 nm).

The Science

Researchers for the first time directly observed nanometer-sized plutonium particles spontaneously occurring within hours of exposure of an acidic plutonium solution to the naturally occurring mineral muscovite using a combination of X-ray scattering and spectroscopy, as well as atomic force microscopy. The results support previous observations, revealing that this process occurs more rapidly than expected, and does not necessarily require an unusually reactive mineral surface.

The Impact

Understanding the tendency of plutonium (Pu) to form nanoparticles under environmentally relevant conditions provides insight into the complexity of its geochemical transport in natural systems. Specifically revealing how Pu interaction with mineral interfaces may slow its transport, the described process revealed a 30-fold increase in the effective capacity of the muscovite mineral surface to bond Pu(III). This adsorption route represents a new heterogeneous pathway for immobilizing metal nanoparticles at mineral-water interfaces that can be significant in separating and isolating this type of element in the environment.

Summary

The release of the radioactive element plutonium (Pu) into the environment is a major concern due to the high radiotoxicity and long half-life of some Pu isotopes. However with the absence of any geological record for Pu it is difficult to understand its chemistry or predict its fate and transport in the environment. Previous research with other heavy metals and actinide elements showed that transport in nature is often controlled by the metal atoms bonding to the surface of minerals. In these experiments, by researchers at Argonne National Laboratory, Pu(IV) nanoparticles are observed to form spontaneously from an initially Pu(III) solution when exposed for a few hours to muscovite mineral basal surfaces. This results in a broad Pu agglomeration extending >7 nanometers above the muscovite surface associated with the formation of Pu(IV) oxide nanoparticles. The present result demonstrates that the interaction of this chemically-active charged metal atom with a relatively non-reactive mineral surface can effectively ‘catalyze’ a change in the oxidation state of the adsorbed ion leading to the formation of nanoparticles attached to the mineral surface. This result provides new insight into understanding the environmental transport of plutonium and other contaminants capable of similar redox/polymerization reactions.

Contact

Paul Fenter
Chemical Sciences and Engineering Division,
CSE-200 Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
Voice: 630-252-7053
http://www.cse.anl.gov/Go/IPG/
fenter@anl.gov

Funding

This work was conducted at Argonne National Laboratory supported by the Department of Energy. The X-ray data were collected at the GeoSoilEnviroCARS beamline 13-ID-C (supported by an NSF-BES collaborative grant) and the X-ray Operations and Research beamline 6-ID-B at the Advanced Photon Source (supported by BES), Argonne National Laboratory. S.S.Lee, F. Bellucci and P. Fenter are supported by the BES Geosciences program and K.E. Knope. R.E. Wilson and L. Soderholm are supported by the BES Chemical Sciences research program. M. Schmidt is jointly supported by the United States Department of Energy Office of Science/BER, NSF, and the EPA.

Publications

M. Schmidt, S. S. Lee, R. E. Wilson, K. E. Knope, F. Bellucci, P. J. Eng, J. E. Stubbs, L. Soderholm, and P. Fenter, “Surface-Mediated Formation of Pu(IV) nanoparticles at the Muscovite-Electrolyte Interface”, Environmental Science & Technology, 47, 14178-14184 (2013).  [DOI: 10.1021/es4037258]

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