Biogeochemical Processes Responsible for the Enhanced Transport of Plutonium under Transient Unsaturated Condtions.
EMSL Project ID
30449
Abstract
Plutonium is an element of concern because of its radiotoxicity. At Savannah River Site (SRS), as a result of processing Pu for nuclear weapons, some ~60 Ci of 238Pu and 239Pu was released into environment through direct disposal into seepage basins and air emission between 1954 and 1989. In aqueous solutions, Pu exists in various oxidation states (III, IV, V and VI). Under typical groundwater conditions, Pu(V/VI) is 500 times more mobile than Pu(III/IV). To understand the fate and mobility of Pu in vadose zone, solid plutonium (III), (IV) and VI) were placed in SRS lysimeters that were exposed to natural weather conditions with the intent of measuring the influence of Pu oxidation state on Pu mobility through the SRS subsurface sediments. Sediment cores from these lysimeters were collected 11 years later and total Pu concentration as a function of depth were determined. Laboratory studies showed that Pu is quickly reduced from Pu(VI) to Pu(IV) by these sediments. These tests also showed that Pu(IV) oxidation may also be occurring. Reactive transport modeling of the system also supported the hypothesis that both oxidation and reduction of Pu are occurring in the lysimeter sediments.The general hypothesis is that Pu transport through vadose zone is controlled by its oxidation state, which in turn is controlled by biogeochemical conditions that are closely related to Fe-mineralogy of the SRS sediments and fluctuations in pore water level and composition. Thus, the objectives of the EMSL proposal is to analyze sediments primarily by variable temperature Mössbauer spectroscopy to characterize a) the complex Fe-mineralogy of the pristine sediment [Fe-oxides and clays; type of Fe-oxide, and nature of Fe-oxide (Al-substituted?), etc], b) sediments that were treated with selective chemical reagents, e.g., acid ammonium oxalate (AAO at pH 3) to selectively remove poorly-crystalline Fe-oxide, etc, since the extent of bioreduction and biotransformation, and Pu sorption/reduction/oxidation onto these minerals is dictated by type of Fe-mineral, c) type of biogenic Fe(II) in bioreduced sediments.
Sediments samples, abiotic from EMSL and bioreduced sediments from Savannah River National Laboratory (SRNL) will be used in parallel Pu oxidation-reduction/sorption/microbe studies at SRNL and Lawrence Livermore National Laboratory (LBNL)/UC-Berkeley, partner institutes of the project funded by DOE-OBERs ERSP project. Also, at EMSL, some SEM and TEM analyses will be carried out to provide complimentary information to Mössbauer. A graduate student or postdoctoral fellow from SRNL or LBNL/UC-Berkeley will travel to EMSL to assist Dr. Kukkadapu with the analysis on site. Finally, we anticipate using this data in writing manuscript(s) and then in the renewal of our ERSP proposal.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2008-09-22
End Date
2010-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Hixon, A. D., Y. J. Hu, D. I. Kaplan, R. D. Kukkadapu, H. Nitsche, O. Qafoku, and B.A. Powell. 2010. Influence of Iron Redox Transformations on Plutonium Sorption to Sediments. Radiochemica Acta 98:685-694.
Hu, Y. J., L. K. Schwaiger, C. H. Booth, R. K. Kukkadapu, E. Cristiano, D. I. Kaplan, H. Nitsche. 2010. Molecular Interactions of Plutonium(VI) with Synthetic Manganese-Substituted Goethite. Radiochimica Acta 98:655-667.