Oxidation of Molecular Tritium by Intact Soils Clyde W. Sweet" and Charles E. Murphy, Jr. E. I. du Pont de Nemours & Co., Savannah River Laboratory, Aiken, South Carolina 29808
The effects of environmental factors on the rate of oxidation of molecular tritium (T2)to tritiated water (HTO) were determined for intact soils during field exposures. Maximum deposition velocities of -0.03 cm/s were measured for T2 at low wind speeds for a variety of soils over a wide range of conditions. At higher wind speeds, deposition velocities as high as 0.13 cm/s were measured. Deposition velocities were slightly inhibited in wet soils and at 0 "C. In dry soils, oxidation of Tz to HTO occurred deeper in the soil profile, but deposition velocities were unaffected.
Introduction Molecular tritium (T2) is a radioactive pollutant released from chemical-separation facilities for nuclear fuel. The release of T2 to the environment may increase in the future because of increased fuel reprocessing activities and potential releases from fusion power reactors. Since permissible concentrations of tritium in the molecular form are roughly 10 000 times higher than those for tritiated water (HTO), environmental processes influencing the oxidation of T2 to HTO need to be understood to assess the significance of T2 pollution. McFarlane et al. (1-4) showed that T2 is oxidized by soil microbes under laboratory Conditions and that HTD is rapidly accumulated in the water and organic components of plants via soil oxidation and transpiration. Tritium oxidizing activity was found in a wide variety of soils in the vicinity of nuclear fuel reprocessing facilities (3).Liebl and Seiler (5) found that soils are a sink for hydrogen gas under both laboratory and field conditions; however, the rates which they determined are higher than those reported by McFarlane et al. (1-4). Recently, Garland and Cox (6) reported measurements of T2 uptake by grassland soils. They found that deposition velocity varied seasonally and suggested that this might be due to differences in soil moisture. In this work, we report T2 deposition rates determined for intact, undisturbed soils under field conditions. The effects of temperature, moisture, soil type, and wind speed on deposition are discussed as well as the distribution of deposited HTO in the soil under different environmental conditions. Methods Soil exposures were conducted in a Plexiglas (Rohm and Haas Co.) chamber (57 X 103 X 31) cm3 mounted on a rectangular stainless-steel skirt as shown in Figure 1. The skirt was driven 10-20 cm into the soil, and the chamber was clamped to a lip and gasket around the skirt. This isolated a column of intact surface soil and a 300-L airspace above the soil. A small pump was used to circulate the air inside the chamber. Air was pulled through a column containing silica gel and Ascarite (Arthur H. Thomas Co., Philadelphia, PA) at a rate of 5 L/min. This rate was sufficient to prevent water condensation on the sides of the chamber and to keep the C02 concentration below 500 ppm as measured by an infrared gas analyzer. A steady-state C02 concentration was reached after This paper was presented a t the 181st National Meeting of the American Chemical Society in Atlanta, GA, March 30-April 3, 1981. * Address correspondence to this author a t Clinch Valley College, Wise, VA 24293. 0013-936X/81/0915-1485$01.25/0 @ 1981 American Chemical Society
-2 h. Since Ascarite removes all of the C02 from the air flowing through the column, the steady-state concentration times the flow rate equals the C02 input from the soil. A 5-L flexible Teflon (E. I. du Pont de Nemours & Co.) bag provided a reservoir of air to compensate for sample withdrawal and changes in atmospheric pressure. Three sites on the Savannah River Plant, a government facility near Aiken, SC, were used to determine uptake rates of T2 under field conditions. One site had a sandy soil (Orangeburg, pH 4.2) under a loblolly pine stand covered with a 2-cm layer of pine litter. A second site had a similar soil but no vegetation cover. A third was under a mature bottomland hardwood stand and consisted of a sandy loam (Okenee, pH 3.8) with a 5-cm leaf litter layer. In a typical experiment, 20 pCi of T2 was introduced into the chamber. After allowing 5 min for mixing, we collected 1-L air samples at regular intervals for the next 2 or 3 h. Concentrations of Tz in the air were measured with an ionization chamber. After the exposure period, the chamber was removed, and soil cores were taken to determine the distribution of HTO in the soil. The amount of HTO in the atmospheric water vapor was determined by freeze-drying the silica gel and measuring the tritium in the resulting water by liquid scintillation counting. Soil samples were analyzed by complete freeze-drying to recover free-water tritium. T o recover all of the organically bound tritium as HTO, we combusted the soil a t 700 "C. Tritium determinations were by liquid scintillation counting. Control experiments were done by introducing 1%helium gas into the airspace and measuring its disappearance by gas chromatography. The chamber was also set up over a plastic sheet to determine T2 loss characteristics in the absence of soil. Laboratory experiments were carried out by passing air containing T2 over the surface of a 50-g soil sample in a flowthrough system. The air was exhausted past a dry-ice trap to remove any water vapor. The T2 converted to HTO could be determined by analysis of the soil water and atmospheric water by liquid scintillation counting.
Results and Discussion The results of a typical experiment are shown in Figure 2. When soil is present, T2 disappears rapidly from the air. Approximately 80-90% is gone after 1h. In control experiments, helium levels remained constant in the chamber over a 2-h period. Similarly, in the absence of soil, T2 is not lost from the chamber, indicating that Tz losses are not due to leakage. The small decrease in T2 during the initial part of the control experiment is probably due to absorption by plastic and metal components of the chamber. This effect was greatly reduced when metallic chamber components were taped or painted. After 2 h, ionization-chamber values usually began to increase because of the buildup of radon gas escaping from the soil (7). For this reason, experiments were usually ended after 2 h. A deposition velocity (Vd) for Tz can be calculated from these data by using the following formula (8): vd
= V In ( C o / C ) / ( AV t )
(1)
where C = final T2 concentration, CO= initial Tz concentration, V = airspace volume in cm3, A = soil surface area in cm2, and V t = time of uptake in seconds. Volume 15, Number 12, December 1981 1485
I
I
4
cl, I
I
PSCARITE SILICA GEL
Flgure 1. Soil exposure chamber: (P) circulating pump; (G.B.) flexible
gas bag.
He t Soil
80
70t1
-
60 50 -
-
-
40 30 -
20 -
-
T2
10I
OO
60
30
Figure 2.
I
1
90
120
Time, min
-
Effect of wind velocity on T2 uptake in soil. I
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Soil e
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-
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Environmental Science & Technology
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Table I shows T2 deposition velocities for the three soils under different environmental conditions. Maximum values for v d were -0.03 cm/s. Deposition velocities were slightly lower in wet hardwood soils and at cold soil temperatures. There was no correlation between COz evolution and v d . Deposition was strongly inhibited by surface treatment with ethanol. Values for v d calculated from the data of McFarlane et al. (I,4 ) range from less than 0.001 to 0.03 cm/s. The lower values may be due to soils being tested at 140% water-holding capacity in some of their work. In this work, intact soils with high moisture contents show reduced T2 uptake. Laboratory studies with similar soils (9) show even greater reductions in v d for saturated soils. Presumably, this is due to an inhibition of gaseous diffusion into the soil (6). Liebl and Seiler (5) found higher v d values for molecular hydrogen (H2) than those reported here for Ta. Part of this difference may be due to an isotope effect since T2 has a much higher molecular weight than Hz. However, experimental conditions can also alter v d values. In laboratory experiments where air containing a constant concentration of T p was passed over soil, v d can be calculated from the following formula: Vd
= T/(CAt)
(2)
where T = total amount of T2 absorbed by soil, C = c o n c h tration of Tz in the air, A = surface area of soil (cm2),and t = time of exposure (seconds). Figure 3 shows the effect on v d of changing flow velocity in this system. At low velocities, v d values were similar to those measured in the field chamber; however, at higher flow rates, v d values increased to levels comparable to those reported by Liebl and Seiler (5).Higher wind velocities disrupt the boundary layer at the soil surface and lead to higher deposition velocities for T2. The field exposures in this work were all done at flow velocities of less than 0.5 m/s, conditions representative of those near the soil surface in a pine stand. Deposited tritium can be recovered from soil by freezedrying and combustion. McFarlane (2) has pointed out possible errors that may arise because of isotopic fractionation during freeze-drying. In this work, these problems are minimized because the samples are completely dried so that differential rates of evaporation for water and HTO will not affect the final result. Table I1 shows the results from a typical experiment. Normally, -75% of the HTO can be recovered in the top 2.5 cm of mineral soil. The rest is in the litter layer and deeper in the soil profile. Most of the tritium is in the fraction removed by freeze-drying. A small amount of tritium remaining in the dried residue can be recovered by combustion. Tritium is as-
Table II. Distribution of Tritium in Soil After Tritium Depositiona pine stand, pCi
PINE
I
8%
,
3%
I
1%
1
air litter-FC litter-CC soil-FD 0-2.5 cm 2.5-5 cm 5-10 cm soil-C total
0.02 2.5 0.2 15.9 1.o 0.1 0.3 20.0
treated soil,
wci
0.02 0.4 0.1 2.0 4.6 11.6 0.3 19.0
a 20 pCi added as T?; tritium reported in pCi HTO. Surface soil treated with 95% ethanol, 1 L/(lOOO c d ) . FD = tritium recovered in water from freezedrying; C = tritrium recovered in water from combustion.
4BOTTOMLAND
Relative Soil HTO Concentration,
Yo of
toto1
‘Soil moisture content, 0-5 cm depth
Figure 4. Effect of moisture levels on tritium deposition profiles.
Since ethanol was found to be an effective inhibitor of Tz-oxidizing soil microbes, an experiment was also done with ethanol-inhibited soil. From the deposition results in Table 11, it is apparent that Tz diffuses deeper into the soil under these conditions. In a field situation, Tz diffuses into the soil until it reaches a level where there is enough microbial activity present to oxidize it to HTO. A similar process takes place in very dry soils. Laboratory experiments (9) indicate that soils with low moisture contents (less than 1%moisture) have reduced rates of T2 absorption. In contrast, our field studies show that Tz uptake is unimpaired when the soil surface dries out to the same extent. Examination of the soil core profiles from the two sandy soils in Figure 4 shows that tritium is deposited at greater depths under dry conditions. The T2 appears to diffuse rapidly to a deeper site where moisture levels allow microbial activity and oxidation to HTO. The highly organic bottomland soil shows a similar change in the tritium deposition profile with increasing dryness. In this soil, high moisture levels (above 30%) inhibit tritium uptake but do not result in a change in the tritium deposition profile. Other workers ( 1 0 , l l )have described the fate of HTO in the surface soil of forested and grassland ecosystems. Unless rainfall occurs, the tritium will gradually diffuse downward or evaporate. Relatively high peak concentrations will be maintained near the surface for several days (10). If HTO is deposited in the upper layers of mineral soil, it will be taken up by plants. Although the human dose due to this process
would be low ( 2 ) ,it needs to be considered to evaluate the impact of Tz pollution. McFarlane et al. ( 3 )reported that T2 uptake by soil is independent of soil type and chemical properties under laboratory conditions. The results from field experiments reported here are in general agreement with this conclusion. However, we have found that T2 uptake is considerably faster in intact soils. Deposition is only inhibited under conditions of high soil moisture or low temperature. In the Southeast, T2 can be expected to enter the hydrosphere rapidly by oxidation to HTO in the soil.
Acknowledgment We are indebted to Dr. Robert Fallon for helpful discussions and experimental work which contributed toward accomplishing this research. Literature Cited (1) McFarlane, J. C.; Rogers, R. D.; Bradley, D. V., Jr. Enuiron. Sci. Technol. 1978,12,590-3. (2) McFarlane, J. C.; Beckert, W. F.; Brown, K. W. J. Enuiron. Qual. 1979.8. 269-76. .,., -(3) McFarlane, j . C.; Rogers, R. D.; Bradley, D. V., Jr. Enuiron. Sci. Technol. 1978,12,607-8. (4) McFarlane, J. C. Enuiron. Exp. Bot. 1978,18,131-8. (5) Liebl, K. H.; Seiler, W. Microb. Prod. Util. Gases, [Proc.Symp.], 1975 1976,215-29. (6) Garland, J. A.; Cox, L. C. Water, Air, Soil Pollut. 1980, 14, 103-14. (7) Tanner, A. B. In “The Natural Radiation Environment”; Adams, J. A. S., Lowder, W. N., Eds.; University of Chicago Press: Chicago, 1964; pp 161-90. (8) Murphy, C. E., Jr.; Corey, J. C. In “Radioecology and Energy Resources”; Cushing, C. E., Jr., Ed.; Dowden, Hutchinson and Ross: Stroudsburg, PA, 1976; pp 108-12. (9) Fallon, R. D. Savannah River Laboratory, unpublished data, 1980. (10) Garland, J. A. Water, Air, Soil Pollut. 1980,13,317-33. (11) Jordan, C. F.; Koranda, J. J.; Kline, J. R.; Martin, J. R. Bioscience 1970,20,807-12.
Received for review April 6 , 1981. Accepted August 7, 1981. This paper was prepared in connection with work under Contract No. DE-ACO9-76SROOO01 with the U.S. Department of Energy.
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