Chapter 17 2-
Biogeochemistry of Chelating Agents Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/04/18. For personal use only.
Rates and Mechanisms ofCo(II)EDTA Interactions with Sediments from the Hanford Site Melanie A. Mayes, X. L. Yin, M. N. Pace, and Philip M. Jardine Environmental Sciences Division, O a k Ridge National Laboratory, P . O . B o x 2008, MS-6038, O a k Ridge, TN 37831
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The migration of Co from buried radioactive waste in the Hanford subsurface may be facilitated by chelation with organics such as EDTA. The goal of this research was to quantify the rates and mechanisms of Co(II)EDTA interactions with twenty Hanford subsurface sediments using kinetic batch experiments. Our results suggested that oxidation to Co(III)EDTA occurred in all sediments, oxidizing up to 75% of the initial Co(II)EDTA concentrations. Oxidation was sustained over 30 days, though rates typically decreased as a function of time. Dissociation of the Co(II)EDTA complex and adsorption/precipitation of free Co tended to increase with contact time. Correlations were derived between sediment Mn content and 1) the rate of Co(II)EDTA disappearance, and 2) the production of Co(III)EDTA . These correlations will provide an improved mechanistic understanding and predictive capability of the interactions of chelated metals in the Hanford subsurface. 2-
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© 2005 American Chemical Society
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Introduction At the U.S. Department of Energy (DOE) Hanford Reservation in southeastern Washington, prolonged production of plutonium and other nuclear weapon components generated large quantities of radioactive waste. High-level wastes were co-disposed with complex, caustic mixtures of sodium hydroxide and nitrate in large underground storage tanks. Approximately 200 million liters of high-level waste in 177 storage tanks contained 400 million C i of radioactivity (1). Sixty-seven of the 149 single-shelled tanks have leaked to varying degrees into a deep vadose zone (-100 m) where recharge is minimal since the region experiences low annual precipitation (-170 mm). Past releases of radionuclides have altered the distribution of moisture in the vadose zone, causing groundwater levels to rise -30 m below the tanks (2). Further, the areal extent of contamination is great, consisting of 1900 waste sites and 500 contaminated facilities over the large area (1700 km ) of the Reservation. Predictions of radionuclide migration have generally been unsuccessful in matching the observed subsurface distribution, thus transport of radionuclides has been described as "accelerated" (3). Both hydrological and geochemical factors contribute to such "enhanced" mobility of radionuclides at Hanford. The subsurface media consists of unconsolidated sediments of Miocene-Pleistocene age, which are diverse in terms of grain size and mineralogy. Hydrologie processes potentially contributing to accelerated transport include preferential finger and funnel flow (4- 6), thin "film flow" along borehole walls (7), and lateral transport within fine-grained sedimentary units (3, 8-11). Geochemical factors contributing to enhanced migration include extreme temperatures within the tanks due to radioactive decay (12), the concentrated, caustic nature of the tank waste (pH > 10,1= 1-10 M ) which may dissolve and re-precipitate subsurface minerals (13), and co-disposal with organic complexants (14). During the R E D O X method of fuel reprocessing, the procedure for separation of U and Pu involved the addition of chelating agents to ensure all constituents remained in solution (2). Further, millions of kilograms of organic complexants including citrate, E D T A , H E D T A , and glycolate, were used to enhance the recovery of U and ^Sr, and these solutions were subsequently discharged into the underground storage tanks. Analyses of tank wastes indicate that some carbon compounds have survived the high temperatures of the tanks, though high concentrations of carbonate suggest that considerable degradation has also occurred (2). However, many tank leaks occurred early in the Hanford's history, so it is likely that chelators and metalchelate complexes leaked into the vadose zone before and during high temperature events in the waste tanks. In the T X and T Y Waste Management Areas ( W M A ) , in particular, E D T A and C o were both significant components of the waste stream, and some of these tanks have discharged waste into the 2
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280 subsurface. In addition, a large (-435,000 L ) leak from the T-106 tank resulted in the discharge of ^ C o into the subsurface (2). Observed C o migration through the vadose zone is substantial, as observed from drywell gamma logs, which is probably linked to the formation of stable complexes with E D T A (2). In the B - B X - B Y W M A , the migration of C o is occurring at a rate of -0.5 m y" (9), which is several orders of magnitude greater than the infiltration rate. These observations suggest that radionuclides are continuing to migrate through the vadose zone, and the relatively rapid rate of C o migration suggests that the metal may be present in a chelated form. The purpose of this study is to quantify the rates and mechanisms of Co(II)EDTA interactions using batch kinetic studies with twenty different Hanford subsurface sediments. Multiple sediment types were used because of the area, depth, and sedimentary diversity of the waste disposal area. Mass balance was used to quantify dissociation, adsorption, and oxidation reactions at natural sediment p H . Selective extractions were utilized to characterize soil chemical properties. The relationship between observed Co(II)EDTA " reaction and soil chemical properties was quantified using statistical correlations. These results will allow the reactivity of Co(II)EDTA " to be predicted using known sediment geochemistry in the Hanford subsurface. 60
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Materials and Methods The Hanford Reservation is located within a large bend in the Columbia River, consequently all subsurface deposits reflect past depositional environments of the river system. The vadose and saturated zones are comprised of three sedimentary formations with different depositional environments: fluvial (Middle Ringold Formation), alternating fluvial and lacustrine sedimentation (Upper Ringold Formation), periodic desiccation and soil development (Plio-Pleistocene Unit), and successive glacial outwash deposits (Hanford flood deposits) (15). Subsequent weathering may have also altered the oxidation state and the distribution of some reactive minerals (e.g., M n , A l , and Fe oxides), particularly in the older Ringold (Pliocene age) and Plio-Pleistocene units. The Hanford Hff and Hfc sediments were obtained from approximately 20m below ground surface in the Environmental Restoration Disposal Facility (ERDF) in the 200W area (10, 15), while the five IDF sediments were composited from an exploratory borehole in the proposed Interim Disposal Facility (IDF) in the 200E area (16). The sample number corresponds to the depth below ground surface (in feet) of the IDF composites (Table I). Samples of the Plio-Pleistocene Unit (75, 17) and the Upper Ringold Fm. (11, 18) were collected from various beds in the White Bluffs, which is located at the northern boundary of the Reservation. Samples of the Middle Ringold Fm. were
{Wm
Hanford Hanford Hanford Hanford Hanford Hanford Hanford Plio-Pleistocene U . Ringold U. Ringold U. Ringold U. Ringold U. Ringold U. Ringold U. Ringold U . Ringold U. Ringold M . Ringold M . Ringold M . Ringold M . Ringold
Formation
1
0.13 0.11 0.12 0.12 0.14 0.15 0.09 0.04 0.18 0.15 1.67 0.15 0.15 1.88 0.40 0.23 4.00 0.14 0.04 0.96 0.91
Mnfgkg )
Extraction data takenfromBaraett et al., (2000).
§
Hfc Hff EDF45 IDF110 IDF150 IDF200 IDF215 PP US LS SS4 SS4a SS4b S5 PS2 PS1 WB TF1 TF2 TF3 TF4
Sediment 0.95 0.23 0.85 0.85 0.97 0.92 1.15 0.31 0.36 0.25 1.44 2.48 0.69 2.20 1.14 0.67 n/a 0.29 0.01 1.45 0.01
1
AKgkg ) 6.62 4.90 5.13 6.00 5.68 6.80 7.05 1.61 7.49 6.62 18.4 46.3 16.4 23.1 5.25 1.68 25.3 19.3 6.54 55.7 19.4
Fe(gkg')
no
2
Co
d
3
l
2
1
2
Rate (h ) loss Co(II)EDTA ' 2.01 e-4 4.65 e-4 2.63 e-4 1.17 e-3 3.56 e-4 1.85 e-4 1.84 e-4 2.36 e-4 1.17 e-3 8.67 e-4 5.24 e-4 5.54 e-4 5.79 e-4 5.45 e-4 4.40 e-3 3.22 e-3 3.64 e-3 3.91 e-4 1.17 e-4 3.35 e-3 2.68 e-3
st
0.602 0.954 0.935 0.909 0.949 0.808 0.899 0.796 0.992 0.967 0.976 0.768 0.911 0.909 0.975 0.988 0.930 0.870 0.817 0.864 0.967
Rater
2
and CoilDEDTA *, and 1 order rate coefficients.
K (cm g ) wm Co(II)EDTA 2.41 0.73 1.79 4.93 2.94 0 3.22 0 3.90 0 1.85 0 1.31 0 1.06 3.00 1.49 18.9 0.39 8.61 4.14 345 2.87 0 0 5.23 1.60 373 0.87 52.0 2.35 2.24 147 2.64 0 0.89 0.63 41.0 0 41.0 0.92
Table I. Hanford area sediments, extractable oxides, distribution coefficients of Co
282 collected from the Taylor Flats area, which is located to the east of the Reservation. The sediments, which are mostly sands with some silts and clays, were sieved to
