Sources, Speciation and Mobility of Plutonium and Other Transuranics in the Groundwater at the Savannah River Site (Sept. 2003-Sept. 2006) Page: 3 of 5
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stripping Szilard-Chalmers process (Dai et al. 2002). The best evidence of this preferential
formation and stabilization of the 240Pu in the oxidized forms is seen in isotope data from
separated oxidized and reduced Pu fractions. In 2004 the reduced form of Pu has a 240Pu/239Pu
ratio of 0.05-0.06 (wells 1 & 2 in 2004) to 0.15 (wells 3 & 4 in 2004). Weapons grade Pu is
characterized by a low 240Pu content, with 240Pu/239Pu ratios usually <0.07 (Oughton et al., 2000).
Average SRS materials have ratios <0.05 to 0.15, related to weapons production and reactor
operations, while global fallout from weapons testing has a very constant ratio near 0.18 in soils
(Krey et al., 1976; Kelley et al., 1999). Unusually high 240Pu/239Pu ratios >1 and as high as 10 in
Wells 3 & 4 are specific to the oxidized fraction. Clearly this indicates a source that is specific
to the enrichment of 240Pu in more oxidized form.
Plutonium sorption is controlled largely by solubility, as opposed to surface complexation,
adsorption or absorption (Kaplan et al. 2006b). Shown in Figure 1 is the amount of Pu desorbed
from two sets of samples of varying pH. The first sample was a well characterized
Pu(V)/PuO2(am) sample in contact with 0.4 M NaClO4. The second sample was a subsurface
SRS sediment from the lysimeter study that had been in contact with Pu for 24 years. Aqueous
Pu concentrations were on average 3.4 orders-of-magnitude less than the solubility values
reported by Rai et al. (2001). The lower Pu concentrations in the sediment data could be
attributed to the sediment surfaces adsorbing/complexing dissolved Pu, in addition to Pu
dissolution occurring, or to the Pu solid phase being more crystalline in nature (and thus less
soluble) than that studied by Rai et al. (2001).
y = -0 718x- 1606 ORaiet al. (2001)
- This Study
-6 Figure 1. Comparison of sediment desorption
-8 data (this study) and a Pu(V) PuO2(am) 0.4 M
y =-0 653x- 5 033 NaClO4 system (no sediment present, 0.0018-
-12 ,um filtrates); Rai et al. (2001), Kaplan et al.
2 3 4 5 6 7 8 9 (2006).
pH orp H
The other important process governing Pu solubilization and desolubilization was redox. This is
well known to influence Pu chemistry; however, there are no reported rates of Pu oxidation in
sediment in the literature. An established solvent extraction, ultrafiltration technique (Kenney-
Kennicutt and Morse 1985) was modified (Powell et al. 2004) so that we could determine Pu
oxidation states in both the solid and aqueous phase at environmental Pu concentrations. In this
experiment, Pu(V) was added to a SRS sediment and within 24 hours, 92% of it had converted to
Pu(IV) (Figure 2). Oxidation experiments revealed that Pu oxidation was five orders-of-
magnitude slower than the reduction rate. In fact, transport modeling of the 11-year old
lysimeter sediments (Figure 10) indicated that Pu remained in the oxidized state for only ~0.01%
of the time. The remainder of the time it existed primarily as Pu(IV). Pu oxidation state
distribution in the sediment of the lysimeter after 24 years of equilibrating was Pu(III) =
0.0 .0%, Pu(IV) = 98.3%, Pu(V) = 0.02 + 0.03%, and Pu(VI) = 0.09 + 0.05%.
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Buesseler, K. O.; Kaplan, D.; Peterson, S. & Dai, M. Sources, Speciation and Mobility of Plutonium and Other Transuranics in the Groundwater at the Savannah River Site (Sept. 2003-Sept. 2006), report, November 7, 2006; Woods Hole, Massachusetts. (https://digital.library.unt.edu/ark:/67531/metadc878554/m1/3/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.