Environ. Sci. Technol. 2004, 38, 5053-5058
Influence of Oxidation States on Plutonium Mobility during Long-Term Transport through an Unsaturated Subsurface Environment D A N I E L I . K A P L A N , * ,† BRIAN A. POWELL,‡ DENIZ I. DEMIRKANLI,‡ ROBERT A. FJELD,‡ FRED J. MOLZ,‡ STEVEN M. SERKIZ,† AND JOHN T. COATES‡ Department of Environmental Engineering & Science, Clemson University, Clemson, South Carolina, 29634, and Westinghouse Savannah River Company, Aiken, South Carolina, 29808
Lysimeter and laboratory studies were conducted to identify the controlling chemical processes influencing Pu(IV) mobility through the vadose zone. A 52-L lysimeter containing sediment from the Savannah River Site, South Carolina and solid PuIV(NO3)4 was left exposed to natural wetting and drying cycles for 11 years before the lysimeter sediment was sampled. Pu had traveled 10 cm, with >95% of the Pu remaining within 1.25 cm of the source. Laboratory studies showed that the sediment quickly reduced Pu(V) to Pu(IV) (the pseudo-first-order reduction rate constant, k′obs, was 0.11 h-1). Of particular interest was that this same sediment could be induced to release very low concentrations of sorbed Pu under oxidizing 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 inclusion of an oxidation term in the model produced simulations that capture 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. It is concluded that both oxidation and reduction mechanisms can play an important role in Pu transport through the vadose zone and should be considered when evaluating disposal of Pu-bearing wastes.
Introduction The vadose zone, the subsurface region between the surface and the underlying aquifer, is expected to provide an important buffer between disposed nuclear waste and the biosphere. For example, the vadose zones at the Savannah River Site in South Carolina and the Hanford Site in Washington are expected to account for >95% of the Pu residence time as it travels between disposal sites and hypothetical receptors (1, 2). Thus, understanding the long term and transitory geochemical processes occurring in this zone is especially important to ensure safe disposal of nuclear waste. * Corresponding author phone: (803)725-2363; fax: (803)725-4704; e-mail:
[email protected]. † Clemson University. ‡ Westinghouse Savannah River Company. 10.1021/es049406s CCC: $27.50 Published on Web 09/02/2004
2004 American Chemical Society
The environmental mobility of Pu is profoundly influenced by its oxidation state. Plutonium(IV) is 2 to 3 orders-ofmagnitude slower moving than Pu(V) or (VI) (3). The pH and Eh of natural environments is such that Pu often exists simultaneously as Pu4+ and PuVO2+ and, to a much smaller extent, as Pu3+ and PuVIO22+ (4, 5). Many iron (oxy)hydroxide minerals, humic acid, and natural sediments (5-9) can reduce Pu from the mobile oxidized forms to the less mobile reduced forms. These findings have prompted the implicate assumption in some risk calculations involving existing and proposed repositories that once Pu is leached from the waste form (which typically exists as PuIVO2(s)), it will remain as a reduced species. Recent work has shown that PuIVO2(s) is not the thermodynamically stable form as had previously been understood (10). Rather, some of the PuIVO2 surface is oxidized in the presence of water, forming as much as 27% Pu(VI), the more mobile form of Pu. Additionally, some naturally occurring manganese oxides are capable of oxidizing Pu(IV) (11-13). The fact that any Pu would remain in the +5 or +6 oxidation states has profound implications for subsurface waste disposal. For example, one may expect 2 orders of magnitude less Pu(VI) than Pu(IV) could be disposed at the E-Area Low-Level Waste Facility at the Savannah River Site (14). Furthermore, not knowing what percentage of Pu exists as Pu(VI) introduces additional uncertainty into waste disposal and remediation programs, including the Yucca Mountain and Waste Isolation Pilot Plant (WIPP) programs. This uncertainty typically translates into the need to make costly conservative estimates during facility design and operations. The objective of this study was to evaluate Pu mobility through a lysimeter containing Savannah River Site sediment. The lysimeter contained a well-characterized PuIV(NO3)4(s) source and was exposed to natural rainfall and drying cycles for 11 years prior to being sampled and analyzed for total Pu concentrations as a function of sediment depth. Supplemental laboratory studies were conducted to identify and quantify key Pu geochemical processes, with particular attention directed at the role of Pu oxidation state. Transport modeling was used to further evaluate and corroborate laboratory results with lysimeter results.
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 various radionuclides through vadose zone sediments. Two of these lysimeters were examined for this study, one was unamended, the Control, and the other contained PuIV(NO3)4(s). The lysimeters consisted of inverted 52-L bottomless carboys that were connected to separate leachate collection reservoirs (Figure 1). The lysimeters were backfilled with well-mixed nearby subsurface sediment. For the amended lysimeter, 1.7 × 107 Bq (3.4 × 10-5 mol) of aqueous weapons-grade Pu was added as PuIV(NO3)4 to a filter paper that was buried 21.6 cm below the lysimeter sediment surface (described in more detail below). The lysimeter was exposed to natural weather conditions for 11 years. At the end of this period, total Pu concentrations were measured by alpha spectrometry in 1.25or 2.5-cm, depth-discrete sediment samples. During operation, leachate from the lysimeter was periodically sampled and analyzed for total aqueous Pu concentrations by alpha spectrometry. Laboratory Study of Plutonium Reduction by Sediment. The objective of this study was to monitor the oxidation VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic of the lysimeter used in this study. Source consisted of Pu(IV) added as a liquid to filter paper and then sandwiched between two clean filter papers. Lysimeters were left exposed to natural conditions for 11 years. state of sorbed Pu after adding a 239Pu(V) spike solution to the control (unamended) lysimeter sediment. A 0.3-mL aliquot of a 5.0 × 10-7 M 239Pu(V) spike solution was added to centrifuge tubes containing 24 300 ( 400 mg L-1 sediment suspension in 0.02 M NaCl. The spike solution contained 98 ( 3% Pu(V) at time zero and 99 ( 6% Pu(V) at the conclusion of the experiment. The samples were mixed in the dark for up to 3000 min. Control samples containing no solid phase were prepared to monitor the stability of Pu(V) over the course of the experiment (no change in Pu oxidation state were observed in these controls). Pu oxidation state distribution was determined by the method used by Kenney-Kennicutt and Morse (6) and revised by Morgenstern and Choppin (13) and Powell et al. (15). The procedure permits monitoring the oxidation state distribution in both the solid and aqueous phases by measuring the Pu oxidation state before and after acidifying the sample to desorb Pu from the solid phase. Briefly, a 2.5 mL suspension aliquot was passed through a 12-nm filter (Microsep 30K MWCO Omega Membrane Centrifugal Device; Pall Corporation, East Hills, NY), and the filtrate was then analyzed for Pu oxidation state distribution by the thenolytrifluoroacetone (TTA; Alfa Asear, Ward Hill, MA) and bis-(ethyhexyl)phosphoric acid (HDEHP; Alfa Asear, Ward Hill, MA) method described below. The remaining 7.5 mL of the suspension sample was acidified to pH 1.5 using HClO4. It is assumed that this step quantitatively desorbs all the Pu(V) and Pu(VI) (and some portion of the Pu(IV)); this assumption was tested using Th(IV), Np(V), and U(VI) analogues (also discussed below). The mass balance equation for the acidified sample is
[Pu]total ) [Pu(IV)]solid + [Pu(IV)]aq + [Pu(V)]aq + [Pu(VI)]aq (1) Importantly, this is an indirect measurement of oxidation state and as such is limited by its underlying assumptions. Analogue conformational studies were conducted in which 230Th(IV), 237Np(V), or 233U(VI) were added to a 2.5 g L-1 control sediment suspension in 0.02 M NaCl. The samples were mixed in the dark for a 1-h adsorption step. The samples were then 5054
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acidified to pH 1.5, leached for various times, and passed through a ∼12-nm filter. The acidified and filtered samples were subjected to Pu oxidation state analyses, as described above. Additional verification of this technique is presented by Powell et al. (15). Sediment-free controls containing Pu(IV), Pu(V), and Pu(VI) solutions were also included in the measurements to confirm that oxidation state of the Pu in the aqueous state were not altered during the analysis. Laboratory Study of Sediment Pu Desorption under Oxidizing Conditions. This experiment was conducted to determine whether sorbed Pu from the Pu(IV) lysimeter could be desorbed into the aqueous phase under natural oxidizing conditions. The lysimeter sediment sample containing the highest Pu concentration, 1.89 × 108 Bq g-1, was used in this study. Approximately 15 g of sediment and 25 mL of boiled (degassed) Savannah River Site groundwater was placed on the top section of two disposable 0.22-µm filtration assembly units. The units were placed in an Ar(g) (reducing) environment for 14 days to ensure that all of the Pu was Pu(IV). After the 14-day equilibration period, one of the filtration units was exposed to air, i.e., oxidizing conditions. Both sediments were then cycled through two wet/dry cycles per week, for a total of 21 cycles. During the wet cycle, ∼1 cm uncontaminated groundwater (25-mL) covered the sediment for 1 day. The water was then vacuum filtered through the sediment, and the sediment was permitted to air-dry for 2 to 3 days, being completely air-dried for at least 1 day. The air-dried sediment sample contained ∼5.9% moisture as determined gravimetrically by drying at 105 °C. In the reducing environment, the groundwater was sparged with Ar. In the oxidizing environment, the groundwater was sparged with air. The filtrate from the two samples was periodically sampled and analyzed for total Pu concentrations. Analytical Methods. Aqueous Pu oxidation state distribution analysis was performed by parallel extraction of the Pu solution into 0.5 M TTA in cyclohexane at pH 0.5 and 0.5 M HDEHP in heptane at pH 0.5 (15-18). Total Pu concentrations were measured with an alpha-beta discriminating liquid scintillation counter (Wallac Inc., Model 1415). To determine the total Pu concentration in the lysimeter sediment, an aliquot of each sample was aggressively leached with aqua regia (1 part HNO3 to 3 parts HCl). 246Pu spikes were added to each acid digest as a chemical yield tracer; 246Pu was not detected in unspiked samples. All of the Pu in the samples was reduced using ferric nitrate followed by hydroxylamine as well as ferrous sulfamate. An anion complexing reagent (aluminum nitrate) was then added, and the solutions were oxidized with sodium nitrite. The Pu was then extracted from the matrix using a TTA solution. The TTA layer was mounted on a counting dish and analyzed by alpha spectroscopy. The control sediment was stored moist and in the dark at 4 °C. All characterization and subsequent equilibration tests were conducted with the sediment in the moist state. Moist samples were used in an effort to minimize experimental artifacts introduced by over drying sediments, such as changing the reactivity of Fe-, Al-, and Mn-oxides or oxidizing organic matter. The sediments were characterized for the following: pH in a 1:1 water-sediment slurry; particlesize distribution by the sieve and hydrometer method; zeropoint-of-charge by the point-of-zero-salt effect batch method; and free Fe by the dithionite-citrate buffer method (19). Organic matter was determined by the weight-loss-onignition method conducted at 360 °C for 2 h (20). Mineralogy was determined by X-ray diffraction analyses of the hematite > gibbsite > HIVa 15 888 5346 2925 630 9 4
a
Relative concentrations of minerals in clay-size (
