Environmental Radiochemistry
Fate and Transport of Radionuclides
A. Intrinsic Pu colloids on large goethite grain. B. High resolution transmission electron microscope (TEM) image of an individual Pu4O7 nanoparticle (darker image) on goethite (lighter background). The Pu colloids shows a distorted structure from the commonly observed fcc PuO2 structure to bcc Pu4O7 as a result of epitaxial growth.
(Powell et al., 2011 ES&T 45:72698-270)
Seaborg scientists, Annie Kersting and Mavrik Zavarin currently lead field, experimental and modeling efforts to understand the fate and transport of actinides in the environment. Current research is focused on determining the dominant biogeochemical processes that control plutonium (Pu) transport in the soil and groundwater. (See here for more research funded through the Office of Science Biological & Environmental Research program.).
Recently their team has used the TEM to characterize the attachment of Pu colloids on minerals at the nanoscale. They showed that intrinsic Pu-oxide colloids undergo a structural distortion from PuO2 to Pu4O7 as it grows on the surface of the Fe-oxide, goethite.
Nano Secondary-ion Mass Spectrometry (NanoSIMS) elemental map of Pu-contaminated Hanford sediments. A 20 x 20 um ion images of 57Fe, 27Al, and 239Pu show the Pu associated with the Fe rim of a quartz grain.
(Kips et al., 2012 in prep)
We are investigating the biogeochemical processes controlling actinide migration in the environment. Recent work examines Np and Pu sorption to mineral colloids over environmental concentrations that span >10 orders of magnitude (10-6~10-16 mol/L). Recent results indicate that the sorption affinity and desorption kinetics are influenced by the existence of a high affinity, sites at low concentrations. See S&TR; Oct./Nov. 2011, Plutonium Hitches a Ride on Subsurface Particles for further reading.
Actinide contaminated sediments from 20-30m below ground of the Hanford Site are being studied to try and isolate the Pu from the sediments. The NanoSIMS was used to determine that the Pu is co-located with the Fe surface coatings on quartz grains.
Molecular dynamics simulation of Pu(OH)4 in liquid water. Left: Optimized Pu(OH)4 structure in the gas phase (i.e., no coordinated waters) is tetrahedral. Right: Snapshot from ab initio molecular dynamics simulation of Pu(OH)4 in a periodic simulation cell of 96 water molecules at ambient temperature and pressure. Bulk water stabilizes a distorted planar geometry for Pu(OH)4.
(Huang et al., 2012 submitted)
At extremely low environmental concentrations, no spectroscopic techniques exist to interrogate electronic structure of elements on mineral surfaces. We are applying state-of-the-art ab initio molecular dynamics simulations of Pu(OH)4 in a bulk water environment. Results show that the lowest energy structure for gas phase Pu(OH)4 is tetrahedral, but in the presence of bulk liquid, Pu stabilizes to a distorted planar structure.
Schematic representation of the sorption process identified from solid state NMR spectroscopy. The sorption of paramagnetic cations to deprotonated surface sites blocks diffusion of magnetization energy from protonated sites that reduces the observed NMR signal intensity for these sites.
(Mason et al., 2012)
We have been developing nuclear magnetic resonance (NMR) spectroscopic methods to investigate the structure of Pu sorbed on mineral surfaces at environmental concentrations. Pu and other actinides exhibit paramagnetic magnetic ordering which has a profound effect on the NMR spectral response. This response translates to a reduction in signal intensity of surface fuctional sites which are associated with paramagnetic species. Chemometrics analyses of these data sets can identify the specific surface binding sites for paramagnetic species.