Research Eleven-Year Field Study of Pu Migration from Pu III, IV, and VI Sources D A N I E L I . K A P L A N , * ,† DENIZ I. DEMIRKANLI,‡ LEO GUMAPAS,‡ BRIAN A. POWELL,‡ ROBERT A. FJELD,‡ FRED J. MOLZ,‡ AND STEVEN M. SERKIZ† Savannah River National Laboratory, Aiken, South Carolina, 29808, and Department of Environmental Engineering & Science, Clemson University, Clemson, South Carolina, 29634
Understanding the processes controlling Pu mobility in the subsurface environment is important for estimating the amount of Pu waste that can be safely disposed in vadose zone burial sites. To study long-term Pu mobility, four 52-L lysimeters filled with sediment collected from the Savannah River Site near Aiken, South Carolina were amended with well-characterized solid Pu sources (PuIIICl3, PuIV(NO3)4, PuIV(C2O4)2, and PuVIO2(NO3)2) and left exposed to natural precipitation for 2-11 years. Pu oxidation state distribution in the Pu(III) and Pu(IV) lysimeters sediments (a red clayey sediment, pH ) 6.3) were similar, consisting of 0% Pu(III), >92% Pu(IV), 1% Pu(V), 1% Pu(VI), and the remainder was a Pu polymer. These three lysimeters also had near identical sediment Pu concentration profiles, where >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. Solute transport modeling of the sediment Pu concentration profile data in the Pu(VI) lysimeter indicated that some transformation of Pu into a much less mobile form, presumably Pu(IV), had occurred during the course of the two-year study. This modeling also supported previous laboratory measurements showing that Pu(V) or Pu(VI) reduction was 5 orders of magnitude faster than corresponding Pu(III) or Pu(IV) oxidation. The slow oxidation rate (1 × 10-8 hr-1; t1/2 ) 8000 yr) was not discernible from the Pu(VI) lysimeter data that reflected only two years of transport but was readily discernible from the Pu(III) and Pu(IV) lysimeter data that reflected 11 years of transport.
Introduction The United States disposes of some radioactive waste in the vadose zone, the subsurface region between the surface and the underlying aquifer. This zone, along with engineered barriers, is expected to reduce human risk to exposure by (1) separating the waste from the surface biosphere, (2) protecting the surfaces of the encapsulated waste from natural * Corresponding author phone: 803-725-2363; fax: 803-725-4704; e-mail:
[email protected]. † Savannah River National Laboratory. ‡ Clemson University. 10.1021/es050073o CCC: $33.50 Published on Web 12/06/2005
2006 American Chemical Society
weathering processes, and (3) minimizing contact with surface (rainfall) and subsurface (ground) water that may act as a vector to transport waste contaminants. Thus, understanding the long term and transitory geochemical processes occurring in the vadose zone is important to ensure safe disposal of nuclear waste. Plutonium is among the key radionuclides under consideration for disposal in the vadose zone as low level waste. Its tendency to migrate through sediments is largely controlled by its oxidation state. Plutonium is capable of simultaneously existing in groundwater in four oxidation states: Pu(III), Pu(IV), Pu(V), and Pu(VI) (1). This is the result of the four oxidation states, especially the higher three states, having quite similar reduction potentials in neutral and basic solutions (2, 3). These states exist as Pu3+, Pu4+, PuVO2+, and PuVIO2+2. The tendency to form complexes, hydrolyze, polymerize, and sorb to solids tends to follow the effective charge of the ion: Pu4+ > PuVIO2+2 > Pu3+ > PuVO2+. The fact that the PuVIO2+2 ion has an effective charge greater than the Pu3+ ion may be explained by the linear structure of the (O-Pu-O)2+ ion, in which the charge of the equatorial side becomes intensively exposed, creating an effective charge of 3.3 (4). Partitioning of Pu(IV) species to sediment is generally 2 or 3 orders-of-magnitude greater than that of Pu(V) and Pu(VI) (5, 6). Additionally, Pu(IV) is appreciably less soluble than Pu(V) and Pu(VI) (7, 8). Pu has been shown to undergo oxidation state transformations via interactions with several minerals (9-16). Powell et al. (15) reported that Pu(V) reduction by magnetite (Fe3O4) followed first-order kinetics between pH 3 and 8. They also reported that Pu(V) adsorption from the aqueous phase, not surface reduction, was the ratelimiting step. Ferric iron containing minerals, such as hematite (R-Fe2O3) and goethite (R-FeOOH), have also been shown to promote surface-facilitated reduction of Pu(V) (1012, 16). Pu(V) adsorption and subsequent reduction occurred faster on goethite than hematite (16). With goethite, KeeneyKennicutt and Morse (10) observed not only Pu(V) reduction but also oxidation. Conversely, oxidation of Pu(IV) to Pu(V) or Pu(VI) has been shown to occur through a water-catalyzed reaction on PuIVO2(s) (17) and also through oxidation of Pu(IV) on manganese oxide (14, 18-20). Pu mobility through a lysimeter amended with a PuIV(NO3)4 solid was monitored for 11 years (21). Effluent Pu concentrations from the lysimeter did not exceed those of a control (no Pu) lysimeter (data not shown; ∼1 pCi/L 239Pu). Analysis of sediment core samples revealed that >95% of the Pu added to the lysimeter remained within 1.25 cm of the source (21). Laboratory studies showed that the lysimeter sediment quickly reduced Pu(V) to Pu(IV) (the pseudo-firstorder reduction rate constant, k′obs, was 0.11 hr-1; t1/2 ) 6.3 h). Of particular interest was that this same sediment could be induced to release very low concentrations of sorbed Pu under aerobic, but not anoxic (Ar atmosphere) conditions, presumably by oxidation of sorbed Pu(IV) to the more mobile Pu(V) species. Transport modeling supported the postulation that Pu oxidation occurred in the lysimeter sediment. The objective of the present study was to compare the mobility of Pu in the PuIV(NO3)4 amended lysimeter to that in similar lysimeters amended with PuIIICl, PuIV(C2O4)2 (Puoxalate), and PuVIO2(NO3)2. Particular attention was directed at comparing the Pu oxidation state distributions in the sediments and using solute transport modeling to make inferences about the controlling transport mechanisms. VOL. 40, NO. 2, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic of the lysimeter used in study. Pu source consisted of Pu added as a liquid to filter paper and then sandwiched between two clean filter papers. Lysimeters were left exposed to natural conditions for 2-11 years.
Materials and Methods Lysimeter. A series of lysimeters was established on the Savannah River Site located in Aiken, South Carolina to evaluate the long-term transport of Pu through vadose zone sediments. Five lysimeters were evaluated for this study: an unamended control and four lysimeters amended with PuVIO2(NO3)2, PuIV(NO3)4 (previously reported by Kaplan et al. (21)), PuIV(C2O4)2, or PuIIICl3. The lysimeters consisted of inverted 52-L bottomless carboys that were connected to separate leachate collection reservoirs (Figure 1). Lysimeters were filled with well-mixed subsurface sediment collected from near the field location. The sediment was collected from a 4-m-deep pit from which the surface soil had been removed. The sediment used in this study was primarily collected from the vadose zone and contained no observable biological material. For the amended lysimeters, the Pu source material was added as a liquid to a filter paper, permitted to dry, and then buried 21.6 cm below the lysimeter sediment surface on the centerline of the carboy. The lysimeters were left exposed to natural weather conditions for 11 years, except for the PuVIO2(NO3)2 lysimeter, which was left for only two years. During operation, leachate from the lysimeter was periodically sampled and analyzed for total aqueous Pu concentrations by alpha spectrometry. At the end of these periods, the lysimeters were cored and total Pu concentrations were measured by alpha spectrometry in 1.25- or 2.5cm, depth-discrete sediment samples. After collection, the sediment cores were stored moist and in the dark at room temperature, except for the PuVIO2(NO3)2 lysimeter core which was processed immediately after coring and first reported in Hawkins (22). Analyses. All sediment characterization was conducted with the sediment in the moist state. Moist samples were used in an effort to minimize experimental artifacts introduced by drying sediments, such as changing the reactivity 444
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and mineralogy of Fe-, Al-, and Mn-oxides or oxidizing organic matter. Sediments were characterized for the following: pH in a 1:1 water-sediment slurry; particle-size distribution by the sieve and hydrometer method; zero-pointof-charge by the point-of-zero-salt effect batch method; Fe(II) and Fe(III) concentration by the 1,10-phenanthroline method, followed by ICP-ES analysis; total Fe and Mn concentrations by aqua regia digestion (1 part HNO3 to 3 parts HCl) followed by ICP-ES analysis; and organic matter determined by the weight-loss-on-ignition method conducted at 360 °C for 2 h (23-25). Mineralogy was determined by X-ray diffraction analyses of the 98% of the Pu measured in sedimentfree simulated Savannah River Site groundwater (26). Finally, it should be noted that the Kd construct describes a rapidly reversible process. Although this is an important Pu geochemical process, it often leads to additional surface reactions that may not be readily reversible (15, 18, 28). The laboratory measurements used to parametrize the model’s Kd value likely measured truly reversible (adsorption) and nonreversible (precipitation) Pu sorption (26). Water was assumed to move by dispersion and advection through the sediment at a constant unsaturated water content estimated as 0.3 cm3 cm-3. While the sediment cores were in storage, water was assumed to move by diffusion only. The system of equations is
RV
∂CV ∂CV ∂2 C V - kr (RV - 1)CV + ) -v +D ∂t ∂z ∂z2 ko(RIV - 1)CIV (1)
and
RIV
∂CIV ∂CIV ∂2CIV + kr (RV - 1)CV ) -v +D ∂t ∂z ∂z2 ko(RIV - 1)CIV (2)
where RV, RIV ) retardation factors for Pu(V) and Pu(IV), CV ) Pu(V) aqueous concentration, CIV ) Pu(IV) aqueous concentration, D ) dispersion coefficient, v ) mean seepage velocity, kr ) first-order reduction rate constant for Pu(V), and ko ) first-order oxidation rate constant for Pu(IV). The mean seepage velocity, v, was calculated to be 0.04 cm h-1, VOL. 40, NO. 2, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Selected Physical and Chemical Properties of the Lysimeter Sediment parameter
value
pH organic matter, wt % sand/silt/clay, wt % surface area, m2 g-1 zero-point-charge (pH) clay mineralogy
6.3 goethite>hematite> gibbsite>HIVa 15,900 1.9%/98.1% 630
total Fe, mg kg-1 Fe(II)/Fe(III), wt % total Mn, mg kg-1
a Relative concentrations of minerals in clay-size (
