Environ. Sci. Technol. 2007, 41, 7417-7423
Influence of Sources on Plutonium Mobility and Oxidation State Transformations in Vadose Zone Sediments D A N I E L I . K A P L A N , * ,† B R I A N A . POWELL,‡ MARTINE C. DUFF,† DENIZ I. DEMIRKANLI,§ MILES DENHAM,† ROBERT A. FJELD,§ AND FRED J. MOLZ§ Savannah River National Laboratory, Aiken, South Carolina 29808, Lawrence Livermore National Laboratory, Lawrence, California 94551, and Department of Environmental Engineering and Science, Clemson University, Anderson, South Carolina 29634.
Well-defined solid sources of Pu(III) (PuCl3), Pu(IV) (Pu (NO3)4 and Pu (C2O4)2), and Pu(VI) (PuO2(NO3)2) were placed in lysimeters containing vadose zone sediments and exposed to natural weather conditions for 2 or 11 years. The objective of this study was to measure the release rate of Pu and the changes in the Pu oxidation states from these Pu sources with the intent to develop a reactive transport model source-term. Pu(III) and Pu(IV) sources had identical Pu concentration depth profiles and similar Pu release rates. Source release data indicate that PuIV(C2O4)2 was the least mobile, whereas PuVIO2(NO3)2 was the most mobile. Synchrotron X-ray fluorescence (SXRF) revealed 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. The high stability of sorbed Pu(IV) is proposed to be due to the formation of a stable hydrolyzed Pu(IV) surface species. Plutonium X-ray absorption near-edge spectroscopy (XANES) analysis conducted on sediment recovered at the end of the study from the PuIV(NO3)4- and PuIIICl3-amended lysimeters contained essentially identical Pu distributions: approximately 37% Pu(III), 67% Pu(IV), 0% Pu(V), and 0% Pu(VI). These results were similar to those using a wet chemistry Pu oxidation state assay, except the latter method did not detect any Pu(III) present on the sediment but instead indicated that 93-98% of the Pu existed as Pu(IV). This discrepancy was likely attributable to incomplete extraction of sediment Pu(III) by the wet chemistry method. Although Pu has been known to exist in the +3 oxidation state under microbially induced reducing conditions for decades, to our knowledge, this is the first observation of steady-state Pu(III) in association with natural sediments. On the basis of thermodynamic considerations, Pu(III) has a wide potential distribution, especially in acidic environments, and as such may warrant further investigation.
* Corresponding author phone: (803)725-2363; fax: (803)725-4704; e-mail:
[email protected]. † Savannah River National Laboratory. ‡ Lawrence Livermore National Laboratory. § Clemson University. 10.1021/es0706302 CCC: $37.00 Published on Web 10/03/2007
2007 American Chemical Society
Introduction The Savannah River Site (SRS) produced about 40 metric tons of plutonium, one-third of the nation’s inventory, for nuclear weapons between 1954 and 1989. Most of this Pu left the site in the form of hockey puck-sized disks for the Rocky Flats site in Colorado. Today, 34 metric tons of Pu is planned to be shipped back to SRS from the Pantex site and converted into commercial reactor fuel. The SRS is also expected to receive 13 metric tons of a less-pure form of Pu from other Department of Energy facilities, for which there is no set disposal plan. Existing Pu waste stored in the site’s 49 waste tanks is also presently being processed into glass logs for eventual disposal in a yet-unnamed federal repository. The SRS also contains several low-level waste disposal facilities, of which some contain Pu in the vadose zone. As a result of processing Pu, some 60 Ci of 238Pu and 239Pu over the years have been released into the environment through direct disposal into seepage basins and air emission. As these Purelated activities continue at the SRS, a better understanding of the key geochemical processes responsible for Pu transport are essential to minimize the uncertainty, and therefore the cost, associated with managing the risk of dealing with this highly toxic and radioactive substance. 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 (1). Pu(III, IV) are more stable in acidic and reduced media, whereas oxidized species (V, VI) are more stable in higher pH environments (2, 3). It is generally observed that Pu(III) and Pu(IV) sorb much stronger than Pu(V) and Pu(VI) (1). Reactive mineral surfaces, such as iron and manganese oxides, have been observed to facilitate oxidation or reduction of Pu under field and laboratory systems. Powell et al. (4, 5) studied Pu(V) reduction following adsorption by synthetic magnetite, hematite, and goethite, and they were able to develop overall reaction rates from experimental data. They also observed no reduction of Pu(V) when there was no adsorption on either goethite or hematite, further suggesting that reduction is a surface-mediated process. Several other studies also reported Pu oxidation state transformations by interactions with solid surfaces of iron and manganese oxides (6-10). A series of lysimeters were established in the early 1980s to evaluate the long-term fate of Pu in the SRS vadose zone. The 52 L lysimeters were amended with carefully prepared solid sources of PuVIO2(NO3)2, PuIV(NO3)4, PuIV(C2O4)2, or PuIIICl3, backfilled with homogenized sandy loam sediment, and then left exposed to natural weather conditions for either 2 or 11 years (11-13). The PuIV(NO3)4-amended lysimeter had >95% of the Pu remaining within 1.25 cm of the source after 11 years; however, there were detectable levels of Pu as far as 30 cm from the source. Laboratory studies showed that the sediment quickly reduced Pu(V) to Pu(IV) (11). Transport modeling supported the postulation that Pu oxidation occurred in the lysimeter sediments as the inclusion of an oxidation term in the model produced simulations that captured the Pu depth profile data. By not including the oxidation process in the model, Pu mobility was grossly underestimated by a factor of 3.5. The aqueous Pu concentration in these sediments is limited by solubility of the solidphase Pu and subsequent sorption of the solubilized Pu (13). Pu oxidation state distribution in the Pu(III) and Pu(IV) lysimeter sediments were similar, consisting of 0% Pu(III), VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7417
>92% Pu(IV), 1% Pu(V), and 1% Pu(VI), and the remainder was operationally defined by the solvent extraction scheme as Pu polymer (12). These three lysimeters also had nearly identical sediment Pu concentration profiles. Reactive transport modeling supported laboratory measurements showing that Pu(V) or Pu(VI) reduction was 5 orders of magnitude faster than corresponding Pu(III) or Pu(IV) oxidation. Similarly, model-calibrated (curve-fitted) values of oxidation rate constants were 5 orders of magnitude smaller than reduction rate constants (13). The calibrated parameter values were robust and well-defined through all lysimeter simulations. The present study focuses on the Pu sources in the SRS lysimeters, with particular attention directed at Pu release rates and oxidation state changes as they approach steady state with the vadose zone sediment. Specifically, the objectives of this study were to (1) measure the amount of Pu released from the Pu sources after 11 years of weathering in the vadose zone, (2) measure changes in the Pu source material oxidation state after 11 years of weathering, and (3) measure Pu oxidation states on sediments and using microprobe synchrotron X-ray fluorescence (micro-SXRF) imaging create elemental maps of Pu and other elements on sediments. Regarding this last objective, the Pu oxidation state was measured by X-ray absorption near-edge spectroscopy (XANES), and the results were compared to those from an indirect, wet chemistry method (13). The motivation of this research was to provide a conceptual model for a source-term for a reactive transport model that would improve prediction of long-term Pu release into the environment.
Materials and Methods Lysimeter. A series of lysimeters were established on the SRS located in Aiken, South Carolina to evaluate the longterm 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, PuIV(C2O4)2, or PuIIICl3. These lysimeters have been described previously (11-13). The lysimeters consisted of inverted 52 L bottomless carboys that were connected to separate leachate collection reservoirs. Lysimeters were filled with well-mixed subsurface sediment collected from a nearby vadose zone at a 4 m depth; no biological material was observable in the sediment. It had the following properties: pH 6.2, goethite > hematite > gibbsite > hydroxyl-interlayered vermiculite. The procedure used to prepare the PuVIO2(NO3)2, PuIV(NO3)4, PuIV(C2O4)2, or PuIIICl3 source materials has been presented previously (12). The materials were prepared from weapons grade plutonium and then pipetted in the aqueous phase onto paper filter disks, where they were permitted to air-dry. The filter disks were then sandwiched between two filter disks to ease handling and minimize accidental Pu loss. The final mass of Pu added to each disk was approximately 2 × 107 Bq 239Pu (∼0.5 mCi 239Pu). For the amended lysimeters, the Pu filter disks were 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 2 years. During operation, leachate from the lysimeter was periodically sampled and analyzed for total aqueous Pu concentrations by R spectrometry. At the end of the study, the lysimeters were cored vertically down the middle and total Pu concentrations were measured by R spectrometry in 1.25 or 2.5 cm, 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. The 7418
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 21, 2007
PuVIO2(NO3)2 lysimeter core was processed immediately after coring and reported in ref 15. Analytical. Total Pu concentrations in each lysimeter sediment depth-discreet section was determined from a 5.0 g sample that had been leached overnight with aqua regia. 236Pu spikes were added to each acid digest as a chemical yield tracer. 236Pu was not detected in unspiked samples. Pu in the acid-leached samples was combined with ferric nitrate and then reduced using hydroxylamine and ferrous sulfamate. An anion complexing reagent (aluminum nitrate) was then added, and the solutions were oxidized with sodium nitrite. Pu was then extracted from the matrix using thenolytrifluoroacetone (TTA). The TTA extract was mounted by evaporation on a counting dish and analyzed by R spectroscopy. X-ray-Based Studies. Microprobe synchrotron X-ray fluorescence and Pu X-ray XANES data for Pu on filter disks and sediments recovered from the Pu(III)- and Pu(IV)amended lysimeters were collected at the National Synchrotron Light Source (NSLS), at the Brookhaven National Laboratory located in Upton, NY. The synchrotron hard X-ray fluorescence microprobe on the bending magnet at X26A of the NSLS was used with a channel-cut Si(110) monochromator. Microfocusing optics were used to produce the X-ray beam (16). A double elliptical Au- or Rh-coated KirkpatrickBaez mirror system angled at 2 mrad was used to focus a 350 µm × 350 µm monochromatic X-ray beam at the Pu L3 absorption edge (18 054 eV) to a 7 µm vertical × 13 µm horizontal beam (16). Milligram quantities of sediment and filter paper samples were contained within fitted plastic inserts with polypropylene and kapton windows, placed in a metal frame and mounted on an automated, digital x-y-z stage at 45° to the beam. Fluorescent X-rays were detected with a nine-element Ge array detector (30 mm2 area; Canberra) mounted at 90° to the incident beam and about 1 cm from the sample. Elemental mappings on the samples were conducted using micro-SXRF imaging at several areas, ranging from 100 µm × 100 µm to 300 µm × 300 µm in size. For elements with absorption energies below 18.5 keV, SXRF was performed by collecting 20 s live counts in the elemental regions of interest and rastering the sample in 4-5 µm steps in the x-y plane. Elemental imaging was conducted with the Pu-containing filters and for the sediment samples that were directly associated with the Pu-containing filters. The Pu-XANES data for oxidation state calibration of Pu were also collected at the NSLS using Pu standards that were provided by Drs. Don T. Reed and Lester Morse (both formerly of Argonne National Laboratory). The standards were embedded in polystyrene resin and placed in aluminum metal frames. The frames were mounted on an automated, digital x-y-z stage oriented at 45° to the beam. Fluorescent X-rays were detected with a Si(Li) energy dispersive detector (30 mm2 area, Canberra) or a nine-element Ge array detector mounted at 90° to the incident beam and 2 cm from the standard mounts. Micro-XANES spectra were collected on the Pu LR emission line from 50 eV below the Pu absorption edge to 180 eV above the Pu edge in varying step increments from 0.4 to 2.5 eV. Energy calibration was made with wellcharacterized Pu solid standards: PuBr3(s), Pu(IV)O2(s), Ba3Pu(VI)O6(s), and a Zr foil (K edge at 17 998 eV).
Results and Discussion The total Pu concentration in the lysimeter sediments is presented in Figure 1. Plutonium moved below and above the source material, driven downward primarily by advective forces from vadose zone water movement and upward from plant uptake and not evaporation (14). Approximately onethird of the infiltrating groundwater underwent evapotranspiration, promoting upward Pu mobility (14). The Pu(III)
FIGURE 1. Total sediment Pu concentrations in (top) lysimeter sediment amended with Pu(III) or Pu(IV) and (bottom) Pu(VI) and Pu(IV). and both Pu(IV) profiles were essentially identical, especially below the source materials (Figure 1, top). Greater than 95% of the Pu remained within 1.25 cm of the source material in these studies that were exposed to rainfall for 11 years. Plutonium from PuVIO2(NO3)2 traveled appreciably deeper in a shorter period of time, 2 years compared to 11 years for the three other lysimeters. Each lysimeter received approximately 0.5 mCi (2 × 107 Bq 239Pu; Table 1). The amount of Pu leached with the aqueous phase into the collection reservoir during the 11 year study was less than or equal to the activity recovered in the control (no Pu) lysimeter leachate (which included fallout Pu activity; 0.32 × 10-3 µCi/2 yrs). Of the Pu(III) or Pu(IV) introduced into the lysimeters, 17-29% leached into the 10 cm diameter sediment cores recovered at the end of the study (i.e., leached into the core, not including Pu in the filter disk and Pu leached into sediment outside the core) (Table 1). The PuVIO2(NO3)2 lysimeter was exposed to rainfall for only 2 years, and the Pu traveled deeper into the profile than in the other lysimeters. As with the other lysimeters, Pu concentrations in the leachate during the 2 years in question were not greater than those released from the control lysimeter. The remarkable similarity of the amount of Pu released into the sediment from the Pu(III) and Pu(IV) sources suggests the mobility of Pu was controlled by a phase different from the initial phases. PuIV(C2O4)2 has a solubility in the order of 1 × 10-4 M, whereas PuIIICl3 and Pu(NO3)4 are deliquescent, making their solubility values very high. Sixty-eight paired Eh and pH measurements were taken during a series of twice-weekly wet/dry cycles, using lysimeter sediment. These cycles were designed to simulate cyclic
oxidizing conditions. During the wet cycle, ∼1 cm of uncontaminated groundwater (25 mL) covered 15 g of sediment for 1 day. The water was then vacuum filtered through the sediment, and the sediment was permitted to air-dry for 2-3 days, being completely air-dried for at least 1 day. The water was then reintroduced for another wet cycle and brought up to a 25 mL volume. pH and Eh measurements were taken while the water was in contact with the sediment. As such, these measurements represent the oxidized limits expected for this system because of the continuous introduction of oxygenated water and the frequent drying. These values were plotted on a Pu Pourbaix diagram (Figure 2; Geochemist Workbench, version 6; Rockware, Inc., Golden, CO). These data points exist within an inscribed area identified by ref 17 as the range of conditions found naturally in undisturbed environmental systems (Figure 2). Being a simulated vadose zone system, it is not surprising that the measured pH/Eh data points cluster toward the more oxidized range of the inscribed area. They fall within the Pu(OH)40(aq) zone, suggesting that Pu(IV) would be the most dominant oxidation state. Using the mean pH (6.5) and Eh (0.38 V) of these oxidized samples, it was calculated that the speciation would be Pu(III) ) 1.3 × 10-4%, Pu(IV) ) 99.8%, and Pu(V) ) 0.2%. Natural conditions would be expected to remain moist longer, thereby increasing natural microbial activity and creating more reducing conditions than were experimentally simulated. X-ray-Based Studies. Extensive micro-SXRF imaging of the sediment samples was conducted. Three sets of elemental maps are presented as examples to demonstrate two key findings (Figure 3). First, Pu was not evenly dispersed throughout the samples. Instead, sediment Pu was rarely found in a given 100 µm × 100 µm field of investigation and was highly concentrated. Such a distribution is typical of (co)precipitated or colloidal phases. If these are individual colloid Pu particles, it is highly unlikely that the ones imaged are mobile because mobile colloids tend to be 50 µm. The second general finding was that Pu concentrations did not correlate with Mn. This can be attributed to the very low Mn concentrations in the sediment, 630 mg kg-1 of total sediment. Furthermore, there were few (or no) high-concentration areas of Mn detected by SXRF imaging, suggesting the absence of manganese oxides. Pu has been shown to concentrate in edge-sharing manganese oxides in the presence of iron oxides or other minerals in Yucca Mountain tuff (19). Microprobe Pu-XANES measurements, which provide information on the average oxidation state of Pu over small regions, were conducted on the filter disks and the sediments immediately in contact with the Pu(NO3)4 and PuCl3 filter disks. Plutonium concentrations on the filters and in the sediments in contact with the filters were suitable for PuXANES measurements. Pu concentrations on other sediment samples or from the clay fractions of other sediment samples were too low for suitable XANES data acquisition. The differences in total Pu counts between the sediments in contact with the filters and the clay fractions from the sediments that were sampled at greater depths were g3 orders of magnitude. The Pu-XANES spectra were acquired for 17 sediment samples, 10 filter disks, and 3 standards. The XANES data analyses are based on the half-height of the normalized absorption edge step for Pu at the Pu L3 edge (19). Assignment of Pu oxidation states was based on the following assumptions: (1) only Pu(III) and Pu(IV) were present in the samples and (2) the Pu(III) and Pu(IV) oxidation state standards provide an accurate estimation of Pu-XANES edge energy calibration for our samples. We feel comfortable with the first assumption because no evidence of Pu(V) or Pu(VI) was VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7419
TABLE 1. Plutonium Leached from Various Sources PuVIO2(NO3)2a µCi initial Pu activity on filter disk sediment Pu activity Pu in leachateb Pu on filterc
477.0 29.6
