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March 26, 2008
Plutonium Oxidation State Transformations after 11 Years in Vadose Zone Sediments
D.I. Kaplan1, M. Duff1, B.A. Powell2, and M.E. Denham1
1Savannah River National Laboratory, Aiken, SC;
2Clemson University, Environmental Engineering and Earth Sciences, Clemson, SC
Pu mobility in the subsurface environment is largely controlled by its oxidation state; Pu(V) and Pu(VI) are
typically two orders-of-magnitude more mobile than Pu(III) or Pu(IV). Micro- XRF studies reveled that Pu was very unevenly distributed
on the sediment and Mn concentrations were too low (630 mg kg-1) and perhaps of the wrong mineralogy to influence Pu
distribution. Pu-XANES of the Pu(III) and Pu(IV)-amended lysimeters contained essentially identical Pu distributions, suggesting
steady state had been achieved: 37% Pu(III), 67% Pu(IV), 0% Pu(V), and 0% Pu(VI). To our knowledge, this is the first observation
of steady state Pu(III) in association with natural sediments.
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Daniel Kaplan |
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Martine Duff |
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Brian Powell |
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Miles Denham |
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Plutonium is an element of concern in disposal and remediation scenarios because of its radiotoxicity and very long
half-lives for several isotopes. Once released into the environment, the movement of Pu is strongly related to its oxidation
state. In aqueous solutions, Pu can exist in oxidation states III, IV, V, and VI, with two or three of these oxidation
states commonly present at equilibrium. Pu(V) and Pu(VI) are typically two orders of magnitude more mobile than Pu(III) or Pu(IV).
A series of lysimeters were established on the Savannah River Site located in Aiken, South Carolina, to evaluate the
long-term transport of Pu through vadose zone sediments. The vadose zone is the region below the root zone and above the
underlying water table and is where radioactive waste in the U.S. is disposed. Five lysimeters were evaluated for this study:
an unamended control and four lysimeters amended with PuVIO2(NO3)2,
PuIV(NO3)4, PuIV(C2O4)2, or PuIIICl3.
The lysimeters consisted of inverted 52L bottomless carboys that were connected to separate leachate collection reservoirs and
filled with nearby vadose zone sediment. Pu-amended filter discs were buried and after being exposed to natural weather conditions
for 11 years, the cores were collected containing the sediment and filter.
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Figure 1. Micro-SXRF elemental map images of sediment next to the: PuIV(NO3)4-amended
filter disc. Regions "a" and "c" identify the locations where the Pu-XANES spectrum presented in Figure 2 were collected. Relative
concentrations on these maps increase from black (least concentrated), blue, red, and yellow (most concentrated). Data collected at
X-ray beamline X26A. |
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For the Pu(II)- and Pu(IV)-amended lysimeters, more than 95% of the Pu remained within 1.25 cm of the source after 11 years;
the other 5% of Pu moved at an overall rate of 0.9 cm yr-1. As expected, Pu moved more rapidly through the Pu(VI)
lysimeter, at an overall rate of 12.5 cm yr-1. The micro-x-ray fluorescence (micro-XRF) imaging of the sediment samples
led to two key findings (Figure 1). First, Pu was not evenly dispersed throughout the samples. Instead, sediment
Pu was rarely detected and when it was detected, it existed at high concentrations. Such a distribution is typical of (co)precipitated
or colloidal phases. If the Pu were in fact individual Pu particles, these colloids would not be mobile because mobile colloids
tend to be less than 1 µm, whereas those present in the µ-XRF were greater than 50 µm. The second general finding was that Pu
concentrations did not correlate with Mn or Fe. Pu has been shown repeatedly to sorb strongly to Mn- and Fe-oxyhydroxides.
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Figure 2. The Pu-XANES spectra for the standards [Pu(IV) and (VI)] and regions "c" and "a" on the
sediment taken next to the Pu(IV)-spiked filter in the Pu(NO3)4- amended lysimeter (Lysimeter #9; see Figure 1).
The spectra demonstrate the edge energy shift with oxidation state at the half height of the edge step (i.e., when Normalized
Fluorescence Counts = 0.5). The average edge energy for sediment from Pu(NO3)4-amended lysimeter (region "c")
is roughly midway between Pu(III) and Pu(IV) which, based on relative edge energy, would be consistent with a mixed Pu(III/IV) species.
Data collected at X-ray beamline X26A. |
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Figure 2 presents examples of sediment and standard microprobe Pu-X-ray Absorption Near Edge Spectroscopy
(XANES) spectra at the Pu L3 absorption edge. Calibration error estimates on these values are about 25%. The average
oxidation state of Pu in the PuIIICl3 and PuIV(NO3)4 filter discs indicate
that the Pu oxidation states had changed since being placed in the ground (Table 1). In both cases, no Pu(V) or
Pu(VI) was detected and varying fractions of Pu(III) and Pu(IV) are present. The proportion of Pu(III) and Pu(IV) measured in the
samples appear to have been influenced by the original oxidation state of the material. This suggests that the two source materials
may not have yet reached steady state. Pu(III) is rarely, if ever, reported in natural sediments exposed to air. Based on thermodynamic
considerations, Pu(III) may have a wide natural occurrence especially in acidic environments and therefore its existence may warrant
further investigation.
| Lysimeter Sample |
Pu(III) ± Stdev(a) (%) |
Pu(IV) ± Stdev(a) (%) |
Number of XANES Measurements |
| Pu(NO3)4 filter disc |
21 ± 17 |
77 ± 17 |
7 |
| PuCl3 filter disc |
52 ± 3 |
48 ± 3 |
3 |
| Sediment in contact with Pu(NO3)4 filter disc(b) |
38 ± 18 |
63 ± 18 |
10 |
| Sediment in contact with PuCl3 filter disc(b) |
35 ± 17 |
65 ± 17 |
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(a)Only Pu(III) and Pu(IV) species were assumed to be present for these calculations.
(b)Sediment was separated from the filter disc prior to XANES measurement. |
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Table 1. The calculated average distribution for Pu(III) and Pu(IV) in the lysimeter samples
based on edge energy in comparison to spectra for Pu(IV) in PuO2(s) and Pu(III) in PuBr3(s). |
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BEAMLINE
X26
FUNDING
U.S. Department of Energy
PUBLICATION
D.I. Kaplan, B.A. Powell, M.C. Duff, D.I. Demirkanli, M.E. Denham, R.A. Fjeld, and F.J. Molz, "Plutonium Source Solubilities and Oxidation
State Transformations in Vadose Zone Sediments," Environ. Sci. Technol., 41(21): 7417–7423 (2007).
FOR MORE INFORMATION
Daniel Kaplan
Savannah River National Laboratory
Aiken, SC
Email: daniel.kaplan@srnl.doe.gov
