Environ. Sci. Technol. 1994, 28, 1417-1421
Detritiation of Water by Isotopic Exchange: Experimental Results Mlchael W. Slattery' and Neil L. Ingraham
Desert Research Institute, Water Resources Center, University of Nevada System, P.O. Box 19040, Las Vegas, Nevada 89109
Isotopic exchange is a major process controlling the light isotopic composition of meteoric water in many hydrologic systems. The application of isotope exchange to the removal of tritium from water is a natural extension of understanding the behavior of the light isotopes of water. This paper presents new experimental data on a simple, low-cost vapor-sparging technique using the process of isotopic exchange, which capitalizes on the isotopic disequilibrium between a tritiated wastewater and a tritium free, water-saturated vapor. Isotopic equilibrium was observed between the tritiated water and exhaust vapors for tritium as well as for deuterium and oxygen-18 within 1-2 s (over a 20-cm bubble rise). This steady-state equilibrium provides maximum efficiency in reducing the tritium concentrations by isotopic exchange. About 50 % of the initial tritium activity (either 5000 or 630 000 TU) of a tritated water was transferred to the vapor phase over an average of 45 days. Exchange rates analysis for all isotopes studied indicates that for a given experimental geometry and isotopic equilibrium the factors controlling the magnitude of the exchange are the absolute humidity and the flow rate of the tritium-free airstream.
Introduction Significant quantities of technogenic tritium as HTO are found at many nuclear processing and testing facilities throughout the United States and the world. Ever stricter water pollution regulations require a better understanding of the movement of tritium in the hydrosphere as well as the development of remediation methods. Detritiation of water has thus been the subject of major research efforts as waste from processing, reprocessing,and testing facilities begins to reach off-site areas in excessof regulatory limits. The principle objective of research in this area is to study new methods to remove tritium from water and so reduce or eliminate off-site aqueous tritium releases. Current HTO treatment or detritiation technologiessuch as liquidand vapor-phase HdHTO catalytic exchange, electrolysis, water distillation, solid sequestering, and solar evaporation are generally both energy and capital intensive (I). Most such processes result in a solid residue which must then be packaged as a mixed hazardous waste and sent to a landfill. An economical, low-technical process amenable to field applications is thus of great interest. Bubble-mediated exchange is often used for mass transfer purposes, and many studies have been conducted on the mass transfer characteristics of oxygen and carbon dioxide gases (2-4). Two of the most common experimental methods for measuring gaseous transfer are the photographic method in which the change in bubble size is related to the mass transfer, and the pressure method
based on volume change measurements recorded in a micronanometer (5). These methods correctly evaluate the mass transfer as a liquid-side resistance process in which the main transfer is limited by the liquid-film boundary layer. Gas-phase controlled exchange coefficients for water vapor are about five times greater than their liquid-phase counterparts and, thus, control the transfer of water isotopes (6). The bubble-mediated exchange of deuterium, oxygen-18, and tritium however has received little attention in the literature. Isotope exchange proceeds between water and the saturated bubbles at a rate controlled by the equilibrium isotope fractionation factor, allowing the rate of exchange to be quantified by simple mass balance. Recently, experimental data on deuterium, oxygen-18, and tritium exchangebetweenwater and water vapor under quiescent conditions revealed relationships between surface area, volume, and instantaneous and steady-state isotopic compositions yielding exchange rates for theses isotopes (7, 8). These researchers showed that isotopic exchange between vapor and liquid phases can produce isotopic shifts in water toward being either more or less enriched in the heavier isotopes, depending upon the composition of the juxtaposed vapor. This result suggests that a disequilibrium between phases can be utilized to extract (exchange) one isotope for another on a molecular basis rather than a volumetric exchange (evaporation). This paper presents new experimental data on a simple, low-cost air-sparging technique which capitalizes on the isotopic disequilibrium between a tritiated wastewater and a tritium-free water-vapor feed stream. In addition to the measurements of tritium exchange, both deuterium and oxygen-la are monitored in these experiments to better understand the behavior of these isotopes during the sparging process.
Theory
* Address correspondence to this author at his present address: Byrd Polar Research Center, Ohio State University,108 Scott Hall, 1090 Carmack Road, Columbus, OH 43210; e-mail address: mslatter@I magnus.acs.ohio-state.edu.
The transfer of water vapor to and from a body of water is a dynamic process rendering evaporation, when a net loss of liquid water occurs, and condensation under conditions of a net gain. The transfer of water molecules back and forth allows isotope exchange without a net loss or gain of water mass. This process of a one-for-one transfer of moleculesproduces stark changes in the isotopic composition of the liquid without net additions or subtractions to the bulk water mass (8). The rate of evaporation into the atmosphere is limited by the diffusion of water vapor through a quiescent layer of air near the surface (9, IO). In the model development described in ref 9, the stagnant layer immediately adjacent to the water surface is saturated with water vapor; molecular diffusion is the dominant transport mechanism within this layer. The region above this quiescent layer has the same water content as the bulk atmosphere where transport is turbulent. In this composite two-layer boundary system at the air-water interface, the flux of gas through each boundary layer is given by an equation
0 1994 American Chemical Society
Environ. Sci. Technol., Vol. 28, No. 8, 1994 1417
0013-936X/94/092S-1417$04.50/0
SATU RATION SYSTEM
EXCHANGE SYSTEM
A .-
A .-
15 iiters/minute Compressed Air
Vapor globe port
Ac ry lic C o n t a in ment Box
Saturation Columns (7 liter volume each)
Figure 1. Schematic of the bubble-mediated tritium exchange experiment. The saturation system produces high (>95% ) relative humidity' water vapor of known isotopic composition Rhrough evaporation and exchange with water A. The tritium transfer (liquid to vapor) is induced in the exchange vessel by a constant flux of water-saturated, zero-tritium air through the bulk liquid (water B).
of the form
Q = kAC
(1)
where Q is the flux of gas through a boundary, AC is the concentration across a particular layer, and k is the corresponding exchange constant. A mass balance on the volume of water and the airstream yields dCL V= FCin- FC,,, dt where VL and CLare the volume and concentration of the tritiated water, F is the air flow rate, Cin and Coutare the concentration of the input and output airstreams, and t is time. At isotopic equilibrium, the equality Cout
= kCL
(3)
exists and yields
dC,+ k-F dt
= FCin
VL
(4)
Equation 4 is a linear first-order differential equation applicable if the change in isotopic composition of the liquid phase over time is known. The solution to eq 4 is (5) Experimental Section The derived equations were validated, and the rate of la0/160,D/H, and T/H exchange by the air-sparging method was measured in three separate experiments by bubbling air with tritium-free vapor through 30 L of tritiated water. A schematic of the experimental design is shown in Figure 1. Laboratory air was bubbled through three gas-washingcolumns each filled with 7 L of tritiumfree water to saturate it with a water vapor of known isotopic composition. 1418
Environ. Sci. Technol.. Vol. 28. No. 8, 1994
The stable isotopic composition of the water in the gaswashing columns was chosen such that the resultant vapor was tritium free and enriched in D and l80relative to the tritiated water in the reaction vessel. This stable isotopic disequilibrium facilitated the study of the tritiumexchange process by monitoring the stable isotopic exchange and allowing considerably more sampling without reducing the volume of water in the vessel because of the small sample size required for stable isotope analysis. The temperature, relative humidity, flow rate, and stable isotopic composition of the saturated air were monitored before injection into the reaction vessel containing the tritated water. The isotopic composition of the vessel water and exchange vapors were also monitored. The exchange vessel was weighed daily to record changes in water content. The saturated air was then bubbled through the reaction vessel filled with tritiated water. Sampling. The input and output vapors and tritiated exchange water in the reaction vessel were sampled at regular intervals for stable isotope analysis. The exchange water in the reaction vessel was sampled for stable isotope analysis by inserting 10-pL capillary pipets through the vent on the containment box. Water vapor for these isotopes was sampled by releasing the vacuum on a 1-L glass globe previously evacuated to Torr within the atmosphere of interest. The water vapor was condensed, and the volume of water vapor captured was recorded and provided a secondary check on the absolute humidity of the airstream. Aliquots of 1 mL of the exchange water for tritium analysis were collected at each sampling point. Samples of the exchange (exit) vapors for tritium analysis in runs 2 and 3 were collected by using an in-line extraction technique specially designed for this purpose. This extraction system consisted of multiple low-flow cold traps (COz-acetone slush) that condensed the tritiated water vapor into 1-mL samples. Gas Analysis. Hydrogen gas was extracted from 5-pL water aliquots using zinc as the reducing agent (11). The Hz gas is then analyzed on a Nuclide 3-60HD mass
OOT-----,
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\ '
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0,ooO TU Run 3, Co- 630,000TU
1-
.-. '. \ \
-04.
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r-squared = -099
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100
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'
200
300 Time in days
400
500
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0.2
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._ I 600
Figure 2. First-order rate plot of tritium depletion from all three trials. The solid line is a least squares regresslon of the data.
spectrometer. Reproducibility for the hydrogen analysis is 1.0%. Oxygen isotope ratio determination was based on the conversion of the water sample to COO by the guanidine hydrochloride method (12). The resulting carbon dioxide gas was then analyzed on a Finnegan Model Delta-E isotope-ratio mass spectrometer. Laboratory reproducibility for oxygen isotopes is 0.2%. The stable isotopic compositions are reported in the standard 6 notation as %O variation from V-SMOW. Analyses for tritium were performed by liquid scintillation counting on either a Beckman LS 1801 liquid scintillation counter or a Packard Minaxi 4300 liquid scintillation counter. Lowactivity waters were enriched by an electrolytic method, while concentrated waters were counted immediately.Each sample was typically counted five times (about 20-50 min each). Results are reported in the standard tritium unit (TU), where
---.
0.0
0
20
10
30
40 50 Time in Days
60
70
80
Figure 3. Reduction In tritium concentrationover time In 30 L of water at 24.5 'C. Broken lines are eq 5 drawn for the indicated values of air flow rates. 650000 1
'
,
.' 1
! 1.127
/ confidence intervals i
13Hatom 1TU= 1 X 10l8'H atoms
(6)
Results The depletion trends of tritium in the exchange vessel for all three trials are shown in Figure 2 and confirm that the exchange is a first-order kinetic process as shown by eq 5. The rate constant k,derived for all three runs from a regression of the data in Figure 2, is 1.85 X This value agrees well with the theoretical vapodliquid density ratio (7) as expected for the bulk-phase transfer of water at the experimental temperature. The difference in the rates of tritium removal for all three runs is shown in Figure 3; the broken lines are eq 5, drawn for the indicated values of air flow rate. The data for run 2 deviate from the trend predicted by eq 5 at 15 L/min, while the data from run 1 are in good agreement with this model. The deviation of run 2 from the first and third runs (and that predicted by eq 5 for 15 L/min, Figure 3) appears constant and suggests a systematic error such as flow rate for this run. The late-time
Figure 4. Liquid-vapor tritium concentrations showing calculated fractionation factors for tritium at 24.5 O C obtained in run 3. The upper line is the concentration of the liquid phase, and the lower line is the concentration measured In the vapor phase. The calculated fractionation factors for each liquid-vapor pair are also shown.
data for run 2 trend are along an apparent flow rate curve of about 12.5 L/min in Figure 3. About 45% of the initial 5000 TU in the vessel water of run 1was transferred to the vapor phase over a 43-day period. For runs 2 and 3, (initial tritium concentration -630 000 TU) reductions of 42 % (57 days) and 24 % (26 days) were recorded. The results obtained from this study show that for the experimental design employed here, approximately 45% of the tritium can be transferred to the vapor phase over a period of 50 days, regardless of the initial activity of the exchange water. The tritium was exchanged at a rate of about 47 TU/day and about 5000 TU/day for initial concentrations of 5000 and 630 000 TU, respectively. A maximum shift of about 260 000 TU over 57 days was observed in run 2. The time course of liquid-vapor tritium concentrations for run 3 is shown in Figure 4 and is representative of all three trials. The results of tritium analyses of the exit vapors confirm that the effluent was in constant equilibrium with the tritium in the liquid phase (Figure 4). Environ. Sci. Technol., Vol. 28, No. 8, 1994 1419
m0
t
10.0
i
/
AnnlyticalError
1
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' 1
I
flux out of the vessel was in stable isotopic equilibrium with the tritiated water at all times and that for a onestage exchange column of this geometry, the efficiency of the system is at a maximum. Water loss from the vessel of about 5 % by evaporation and slight undersaturation of the tritium-free airstream showed minimal effects on the isotopic evolution of the vessel water. The effect of 5 % evaporation on the vessel water under open (Rayleigh) conditions was calculated as being only 2%0in 6l80 and 10%0in D as shown in Figure 5.
Discussion -130 0
-15 0
-10 0
.5.0
00
50
Delta 18-0
Flgure 5. 6i80-6Devolutlon of the exchange water for all experiments. In all three runs the stable isotopic compositions of the tritiated water shifttoward those of the column exchange waters. The observed 5 % evaporatlon in the trltiated water would produce a small stable isotopic shift as shown.
Equilibrium was observed at all levels of tritium activity studied. The observedfractionation factors for tritium range from 1.106 to 1.133 and agree well with the generally accepted value of 1.11at 25 "C (13,14). The calculated fractionation factors confirm the precision achieved with the vapor extraction unit and suggests that, regardless of initial activity studied, the system so designed is in constant equilibrium with respect to tritium. The exchange of tritium between the vapor stream and the vessel water is also reflected in the concurrent exchange of the stable isotopes of hydrogen and oxygen. The tritiated water becomes enriched in D and l80during the exchange process relative to their initial values (Figure 5). The deuterium composition of the exchange water showed a steady enrichment in all three trials, with a maximum of 6l%0,while the P O composition increased a maximum of ~ . O % O . The isotopic composition of the input vapors from the gas-washing columns was variable, with a maximum range of 3%0of 6l80 and 2 5 L of 6 D. This stable isotopic enrichment progressessmoothlyover time, toward a steady-state composition, even though the isotopic composition of the input vapor varied slightly due to evaporation of the water in the gas-washing columns. The stable isotopic trends for runs 1 and 3 begin parallel to the Meteoric Water Line (MWL, 6 D = P O + 10; (15)) and slope away after about 23 days, while the run 2 data deviate from the MWL at a constant slope of about 7.5. The lower slope could be the result of evaporation caused by the lack of complete saturation of the tritium-freevapor. Evaporation would also reduce the effectiveness of the exchange process, as shown for run 2 in Figure 3 by enrichment of tritium in the liquid phase. However, the observed deviation for run 2 in Figure 3 is also consistent with a lower flow rate. Measurement of the vessel water and exit vapors for stable isotopes indicates that the liquid phase and the exit vapors were in stable isotopic equilibrium at all times. The mean observed D/H and l80/l6Ofractionation factors between the exit vapors and exchange waters deviate less than 2Yw from the accepted values for deuterium and oxygen at 24.5 "C(16,17). These results confirm that the 1420
Envlron. Scl. Technol., Vol. 28, No. 8, 1994
Our results show that the exit vapors are in isotopic equilibrium with the vessel water at all times and that the isotopic shifts observed are the result of isotope exchange rather than evaporation. The remediation of tritiated water using the process of isotope exchange with tritiumfree vapor is based on the large isotopic disequilibrium between the two fluid phases of water. At ambient temperatures, tritium prefers the liquid phase over the vapor phase as described by the equilibrium fractionation factor (l.ll),but only by about 11 %. If the liquid phase contains more than the 11% excess predicted by equilibrium distribution, the transfer direction will reverse, and tritium will move from the liquid phase to the vapor phase. Thus, it is the large isotopic imbalance that allows the process of isotopic exchange to be effective in tritium remediation. The near-instantaneous (in terms of seconds) isotopic equilibrium reached between the vapor and liquid phases allows higher rates of tritium extraction to be realized by simply increasing the flow of saturated air. Increasing the flux of water vapor may be served by simply increasing the rate of saturated air flow or by increasing the absolute humidity by raising the temperature. Lowering the temperature of the exchange process increases the equilibrium fractionation factor, which would facilitate the exchange;however,a reduction in temperature also serves to decreasethe absolute humidity, thus effectively reducing the flow rate. The effects of these two parameters (fractionation factor and absolute humidity) thus affect tritium remediation in competing ways, the balance of which favors increasing the temperature. An inverse log-log relationship is observed between the tritium-free air flow rate and the time required to reduce the tritium concentration in water to a desired level for a given set of experimental conditions. The volume of tritium-free water required to exchange a given amount of tritium is fixed by the fractionation factor and cannot be optimized. However, the time required for the exchange process can be optimized with reasonable choices of air flow rate and temperature. The isotope exchange process is a low-cost method of tritium remediation, which overcomes several major disadvantages inherent to current methodologies. The isotope exchange method produces no hazardous waste byproducts that result from many other detritiation schemes, The processed water is essentially reusable if tritium is the only contaminant, which is often the case. The process is generally unaffected by the presence of contaminants other than tritium, such as deuterium (heavy water) or other radionuclides which eventually concentrate in evaporative processes. These considerations allow large
volumes of variously contaminated water to be processed using this technique. Using the process of isotope exchangetoreclaim tritiated water releases the tritium to the atmosphere as water vapor in the same manner as solar evaporation. The process of evaporation, however, concentrates tritium in the liquid phase exacerbating the problem, requiring the evaporation process to go to completion. Tritium vapor release from an exchange facility, however, is also a controllable pointsource release as opposed to the flux from a solar evaporation pond. This allows the exchange process to be curtailed if necessary, as with inclement weather or a localized inversion. Bubble-mediated exchange also has possible in situ applications. In this case, a groundwater interception scheme rather than outright remediation may be envisioned, such as at the boundary of a nuclear facility underlain by a tritiated groundwater plume. Dual horizontal wells, stacked vertically, would serve as injection and extraction tubes to cycle a water-saturated air flow. The air flow, as a curtain of vapor bubbles, intercepts the slow-moving groundwater ( 1-5 ft/day) in a hydrostratigraphic unit of known depth at right angles, creating a single zone of isotope exchange capable of reducing tritium activities below those allowed outside of the facility boundaries. The slow-movinggroundwater ensures that only small quantities of water are processed per unit time. Another advantage of this method is the fact that the water need not be brought to the surface for treatment, possibly inducing uncontaminated water to move or mix with a tritated plume. Such a scheme could be used as a barrier to eventual off-site contamination rather than an actual remediation scheme because of inherent natural complexities of the soil zone. N
Analysis of exchange rates for all isotopes studied indicates that for a given geometry, and once isotopic equilibrium is attained, the flow rate of air controls the isotope mass transfer as predicted by eq 5. This observation suggests that to optimize the extraction rates the number of independent bubbler stages should be incremed in addition to maximizing the air flow rate. The increase in air flow should be limited to the level where isotopic equilibrium between phases is always maintained. Acknowledgments
This research was funded by DOE Grant DE-ACOB90NV10845. Stable isotope analyses were performed by C. Shadel, and technical assistance was provided by G. Lucas, both at the Desert Research Institute, Las Vegas. Electrolytic enrichment and subsequent liquid scintillation counting of several samples was performed by P. McQueen at the Desert Research Institute, Reno, NV. Literature Cited
King,C. H.; Brunt, V. V.;King,R. B.; Garber,A. R. Presented at the Waste Management Symposium on Savannah River Site Environmental Restoration and Waste Management for the New Decade, Tuscon, AZ, Feb 1991. Calderbank, P. H.; Lochiel, A. C. Chem. Eng. Sci. 1964,19, 87-93.
Deindoerfer, F. H.; Humphrey, A. E. Ind. Eng. Chem. 1961, 53,755.
Guy, C.; Carreau, P. J.; Paris, J. Can. J. Chem. Eng. 1992, 70, 55-60.
Bischof,F.; Sommerfeld,M.; Durst,F. Chem.Eng. Sci. 1991, 46 (12),3115-3121. Liss, P.S.Deep-sea Res. 1972,20,221-238. Ingraham, N. L.; Criss, R. E. J. Geophys. Res. 1994,D11, . . 20547-20553.
Conclusions
The bubble-mediated isotope exchange process appears to be a promising alternative remediation technique with unique advantages over current methods. These include the low capital and maintenance costs, retrieval of the process water, and the fact that no residual contaminated byproducts are generated. Isotopic equilibrium for the isotopes of hydrogen and oxygen was found between the exchange vessel liquid and the exiting vapor at all times during the three experiments. It is clear that the exchange of stable isotopes is being forced by the control of the column waters with respect to the initial composition of the vessel water rather than byevaporation. Measurements of tritium, deuterium, and oxygen-18concentrations in both liquid and vapor phases revealed that isotopic equilibrium was attained within the length of the bubble path, indicating that the exchange efficiency for these isotopes was at or near 100%. The near-instantaneous isotopic equilibrium attained over a short bubble rise in conjunction with a high degree of isotopic disequilibrium between phases result in a steady and known decrease in tritium concentration of the liquid phase.
Slattery, M. W. M.S. Thesis, University of Nevada, Las Vegas, 1993. Craig, H.; Gordon, L. Spoleto 1965,165. Dorsey, N. E. Properties of Ordinary Water-Substance in all its Phases: Water-Vapor,Water,and all the Ices; ACS Monograph 81;American Chemical Society: Washington, DC, 1968. Kendall, C.; Coplen, T. Anal. Chem. 1985,57,1437-1440. Dugan,J. P.; Borthwick,J.; Harmon, R. S.; Gagnier,M. A,; Gahn, J. E.; Kinsel, E. P.; Macleod,S.; Viglino, J. A.; Hess, J. W. Anal. Chem. 1985,57,1734-1736. Jacobs, D. G. Sources of Tritium and Its Behavior upon Release to the Atmosphere; Atomic Energy Commision Critical Review Series TID-24635;1968. Sepall, 0.; Mason, S. G. Can. J . Chem. 1960,38,2024-2025. Craig, H. Science 1961,133,1702-1703. Baertschi, P.; Thuerkauf, M. Helv. Chim. Acta 1960,43, 80-89.
Craig, H.; Gordon,L.; Horibe, Y. J.Geophys. Res. 1963,68, 5079-5087. Received for review September 9, 1993.Revised manuscript received May 9,1994.Accepted May 11, 1994." 0
Abstract published in Advance ACS Abstracts, June 1, 1994.
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