10342
J. Phys. Chem. B 1998, 102, 10342-10346
Isotopic Exchange and Vapor Pressure Isotope Effect in Tritium Oxide Adsorption on Silica Gel Robert Rosson, Richard Jakiel, and Bernd Kahn* EnVironmental Resources Center, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: August 4, 1998; In Final Form: October 1, 1998
Consistently lower tritium concentrations found in water distilled from silica gel than in the water vapor that had been adsorbed from air stimulated this study of isotopic exchange and the vapor pressure isotope effect for tritiated relative to nontritiated water. Tritium concentrations were compared for various amounts of water vapor adsorbed on and desorbed from a fixed amount of silica gel dried at 100 °C. Tests of the hypothesis that a constant percent water and hydroxyl groups bound in the silica gel undergo isotopic exchange with the adsorbed tritiated water were performed with a type of silica gel generally used in ambient air monitoring. Separate tests were performed to evaluate the additional impact of the vapor pressure isotope effect in reducing the tritium concentration in water vapor relative to the condensed phase. This occurs (1) in the tests when air is swept through tritiated water to generate airborne HTO for adsorption on the silica gel and (2) in distilling water adsorbed on silica gel for tritium analysis. A value for exchangeable water on the silica gel of 5.9 ( 0.2% was found when adsorbing between 7 and 22% water. The isotope effect reduced the tritium concentration in air relative to water by 11% at 20 to 22 °C and by 4% in azeotropic distillation at 80 °C.
Introduction Tritium oxide (HTO) is adsorbed on silica gel to monitor airborne tritium in the environment.1-4 Adsorption of HTO also has been recommended for structural studies of silica gel by measuring surface hydroxyl group concentrations.5,6 Tracer studies, however, indicate that tritium concentrations measured in water desorbed from the silica gel are consistently lower than in the original tracer solution.3,6,7 Presented here is a study to examine the contributions of isotopic exchange and the vapor pressure isotope effect in reducing the tritium concentration of HTO adsorbed on and then desorbed from silica gel. Isotopic exchange between HTO and H2O can occur when tritiated water vapor is adsorbed on silica gel that contains nontritiated water and hydroxyl groups. Water is considered to be retained by silica gel in at least two forms:6,8,9
(tSi)2O + H2O ) (tSi)2O‚H2O ) 2 tSiOH
(1a) (1b)
Tritium exchange with these forms would be represented, respectively, by
(tSi)2O‚H2O + HTO ) (tSi)2O‚HTO + H2O (2a) tSiOH + HTO ) tSiOT + H2O
(2b)
The silica gel used for environmental monitoring of airborne HTO is usually dried at 100 °C before use, but some water, and particularly some hydroxyl groups, remain at much higher temperatures. The percent exchangeable water in the silica gel used for this study10 could equal the 6.5% of the maximum weight loss between 100 and 950 °C reported by the manufacturer. However, some water associated with weight loss may not be
exchangeable. However, some drying the silica gel may yield either one or one-half molecule water per exchangeable hydrogen site, as shown in eq 1a and b; some exchangeable water or hydroxyl groups may be retained above the temperature to which the silica gel was heated. In tritium adsorption and desorption tracer experiments, the vapor pressure isotope effect for HTO relative to H2O results in successively greater enrichment of HTO in the condensed phase until vaporization is taken to completion. The isotope separation factor R is defined as the mole ratio of HTO to H2O in the condensed phase relative to the same mole ratio in the gas phase;11 for tritiated water that is not extremely concentrated, this is simply the tritium concentration in water divided by the tritium concentration in the vapor. Values of R of 1.10 at 20 °C, 1.04 at 80 °C, and 1.03 at 100 °C have been reported by Baumgartner and Kim;11 they compiled published values, confirmed the values with their own measurements, and provided a curve of R vs temperature that shows a decrease of R with increasing temperature. To measure the extent of isotopic exchange, air was pumped through a tritium tracer solution to pick up tritiated water vapor and then through a column to deposit the vapor on silica gel. The vapor-loaded silica gel was transferred to a flask for desorbing the water by distillation to measure its tritium concentration. The same system was used to measure the isotope effect by condensing samples of the vapor in a cold trap to obtain R, the ratio of the tritium concentration in the tritium tracer solution to the concentration in the frozen water vapor. The corrections for the isotope effect in adsorbing water on silica gel, and in desorbing water for tritium measurements, were used in a material balance to calculate the percent exchangeable water in the silica gel. Tests with segmented columns were performed to indicate whether the isotopic exchange on silica gel occurred in the column or in the distilling flask. The silica gel was heated
10.1021/jp9832746 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/24/1998
Tritium Oxide Adsorption
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10343
Figure 1. Isotope effect and silica gel column testing apparatus.
TABLE 1: Measurements of Tritiated Water Tracer Adsorbed on Silica Gel Columns Run tritium net count rate, c/min, 10 mL silica gel wt, g initial final 1 2 3 4 5 6 7 8 9 10
325.0 300.0 301.7 346.4 307.4 328.2 295.7 316.9 300.0 274.4
349.1 340.5 344.5 390.0 352.1 375.1 349.4 373.2 360.8 335.6
wt adsorbed water, g/100 g (W)
tracer solution (Cm)
recovered water (C′ m)
tritium fraction (C′m/Cm)
relative tritium concentration in vapor(f1)
7.4 13.5 14.2 12.6 14.5 14.3 18.2 17.8 20.3 22.3
15.0 1510.0 14.1 14.3 62.7 62.7 14.4 15.6 2400.0 13.9
7.2 930.0 8.5 8.4 42.7 42.0 10.0 10.7 1680.0 10.2
0.480 0.614 0.603 0.587 0.681 0.670 0.694 0.686 0.698 0.734
0.91 0.92 0.92 0.92 0.92 0.92 0.93 0.93 0.93 0.93
average
incrementally and weighed to determine whether the percent exchangeable water was identical with the weight loss of silica gel at 1050 °C. Procedure Air was swept through 140 mL tritiated water solution at 2022 °C in the closed loop shown in Figure 1. To measure the tritium concentration in vapor, 3 mL samples of vapor were collected periodically in the cold trap. Aliquots of the vapor and the tritiated water solution were measured for tritium content with a liquid scintillation counter. The concentration ratio was calculated to obtain R and the activity in adsorbed vapor relative to the measured initial solution activity. By closing the valves to the cold trap, air carrying tritiated water vapor was pumped through weighed silica gel (approximately 300 g) in a plastic column, 30 cm long with an inside diameter of 5 cm. An air flow rate of 2 to 3 L/min was maintained until between 21 and 66 g water were collected. This range had been selected to match water vapor collection during routine environmental surveillance throughout the year. The moist column was weighed to determine the amount of adsorbed water, and the silica gel was transferred to a distillation flask with reflux condenser. The silica gel was mixed, and 320 mL cyclohexane was added for azeotropic distillation at 80 °C. Two portions of distilled watersthe first 12 mL and either the next 12 mL or, for smaller amounts of adsorbed water, as much
exchangeable water, % (X) 0.062 0.061 0.068 0.065 0.050 0.052 0.054 0.052 0.059 0.066 0.059 ( 0.007
water as could be obtainedswere collected, and aliquots were analyzed for tritium concentration. The tritium concentrations in the desorbed water were compared to the tritium concentrations measured initially in the tritiated water solution to calculate the extent of isotopic exchange with nontritiated water and hydroxyl groups on the silica gel. Columns were divided with glass wool separators into four segments, each of 75 g silica gel. The silica gel used here was Tel-tale, grade 42, from the same manufacturer. It is an indicator silica gel that gradually changes from blue when dry to pink when moist. This silica gel had 6.3% exchangeable water.7 Each column was exposed to tritiated water in the closed loop until the first two segments were moist and the third segment was partially moist. The segments were separated for individual weighing and distilling. Various amounts of tritiated water were distilled from the flasks, either directly at 100 °C or by azeotropic distillation at 80 °C. Tritium concentrations in sequential amounts of distilled water were measured to compare them with the tritium concentration in the initial solution for determining the vapor pressure isotope effect. Weight loss, generally attributed to water loss, was measured thermogravimetrically from 100 to 1050 °C for six replicate samples of the grade 41 silica gel. The samples were heated in crucibles to a selected temperature, cooled to room temperature, weighed, and then reheated to a higher temperature.
10344 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Rosson et al.
Figure 2. Tritium concentration in HTO gas and liquid phases at 20-22 °C.
The curve in Figure 2 of tritium concentration in the residual solution relative to the amount of evaporated water for the 140 mL solutions used in this study is based on the equation
TABLE 2: Tritium Liquid-Vapor Ratio Due to Isotopic Separationa volume vaporized, mL 0 0-2.7 2.7 7.1-9.8 9.8 20.2-22.9 22.9 33.2-35.7 35.7 42.0-44.5 44.5 63.8-66.5 66.5 86.5-88.9 88.9 average a
tritium count rate, c/min,mL vapor liquid
relative tritium concentration vapor liquid
744 673
liquid -vapor ratio
1.000 0.905
747 675
1.004
1.105
1.003
1.106
1.019
1.109
1.017
1.092
1.031
1.099
1.079
1.117
1.098
1.115 1.106 ( 0.010
0.907 746
684
0.919 758
693
0.931 757
698
0.938 767
719
0.966 803
733
C1i ) C1i-1 (W0 - Wi - R-1∆W)/(W0 - Wi - ∆W) (4)
0.985 817
Note: initial water volume was 140 mL.
Results and Discussion The tritium concentration measured in the water distilled from the silica gel, C′m, was consistently lower than measured in the tritium tracer solution before air was bubbled through it to adsorb vapor on the silica gel Cm as shown by the tritium fraction in Table 1. The lowest relative tritium concentrations occurred for the lowest amounts of adsorbed water. One cause of the low measured tritium fraction is the vapor pressure isotope effect. The tritium concentration in the vapor swept from the tracer solution C is lower than the concentration measured in the solution Cm by a correction factor f1 ) C/Cm. Moreover, the tritium concentration in the water vapor distilled from the silica gel to collect the adsorbed water for tritium analysis C′m is lower than the concentration on the silica gel C′ by a correction factor f2 ) C′m/C′. Hence, the actual tritium fraction is related to the measured tritium fraction by
C′/C ) C′m/Cmf1f2
(3)
Tritium concentrations measured simultaneously in the tracer solution and the vapor (see Table 2 and Figure 2) yield the isotope separation factor R of 1.11 at 20-22 °C, consistent with the value of 1.10 reported at 20 °C by Baumgartner and Kim.11
In eq 4, R is 1.11, C1i is the concentration of the ith gram of water Wi being evaporated, W0 is the total mass of water being evaporated, and ∆W is the differential amount of water evaporated, e.g., 1 g, for which the concentration is being calculated. Measured and calculated values agree for the range of water evaporated in this study. The lower curve in Figure 2 shows the tritium concentration in vapor, 1.11 below the calculated curve for tritium in water. Measured tritium concentrations in water that represent C′ and in the distillate from direct and azeotropic distillation C′m are given in Table 3 and shown as horizontal bars in Figure 3, for comparison with the calculated curves in Figure 3. The curves are based on R values of 1.04 at 80 °C and 1.03 at 100 °C given by Baumgartner and Kim,11 applied in eq 4. The last two columns in Table 3 indicate that the values of f2 based on R averaged over point values agree with measurements within 1%. The measured ratio includes any effect of tritium oxide exchange with glass surfaces in the distillation apparatus. This test does not address the possibility of a difference in the isotope effect between distillation from water and from silica gel. The correction factor f1 listed in Table 1 is the average of the point values in Figure 2 for the amount of adsorbed water per run. On the basis of the values in Table 3, f2 ) 0.97 was used for all water samples recovered from silica gel by azeotropic distillation. According to the material balance for tritiated water on silica gel, the percent exchangeable water in the silica gel X is related to the percent adsorbed water W by
X ) W [(C/C′) - 1]
(5)
where C/C′ is the inverse of the tritium fraction in eq 3 and column 7 in Table 1. The values of X calculated according to eq 5 and 3 are listed in the last column in Table 1. The average value of inferred exchangeable water Xa is 5.9%, with a standard deviation for 10 measurements of ( 0.7%, and a standard error of ( 0.2%. All C′/C values are within 6% of the curve of W/(W + Xa) in Figure 4. The applicability of the
Tritium Oxide Adsorption
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10345
TABLE 3: Tritium Concentration in Distilled Water ratioa
tritium count rate, c/min, 10 mL total water, g azeotropic (800 °C) 45 60 direct (100 °C) 60
a
water distilled g
water (C′)
condensed vapor (C′m)
measured
calculatedb
0-13 14-26 0-15 16-30
1510
1450 1490 1440 1490
0.96 0.98 0.95 0.98
0.97 0.98 0.96 0.98
328 336 342 355
0.97 1.00 1.01 1.05
0.98 0.99 1.00
0-15 16-30 31-45 45-60
1510 337
Ratio of tritium condensed vapor/water. b From Figure 3.
Figure 3. Tritium cncentration in distillate relative to initial concentration in liquid.
curve to tritium fractions from 7 to 27% adsorbed water supports the hypothesis of a constant fraction of exchangeable water in silica gel. The thermogravimetrically determined weight loss shown in Table 4 was 6.5% between 100 and 1050 °C. This value also was given by the manufacturer as the maximum water content to 950 °C. Most weight was lost between 100 and 850 °C. The small excess of weight loss over exchangeable water suggests that most but not all of the weight loss represents water and hydroxyl groups that promptly exchange with tritiated water. On the other hand, the same comparison with grade 42 silica gel by the same manufacturer found 6.3% exchangeable water compared to 5.3% weight loss.7 Two segmented silica gel columns were operated to determine the importance of mixing the water-loaded silica gel before distillation. The tritiated water balance per segment can be written as
C′i/C ) (C′i-1/C)Wi/(Wi + Xa)
(6)
where Wi refers to the percent water entering the ith segment and C′i-1 is the concentration of tritium in water entering that segment, i.e., the calculated concentration adsorbed on the preceding segment. In alternative a, exchange is complete in the segment so that Wi is all of the water entering the segment; for alternative b, exchange is completed in the distillation flask so that Wi is the water retained in the segment.
The measured values given in Table 5 suggest that, to ensure complete exchange, the silica gel must be mixed before distilling so that any dry material at the end of the column is in contact with moist silica gel from the front of the column. The measured tritium fractions are closer to the values for calculation a for the first two, moist, segments, and closer to calculation b in the third, relatively dry, segment. The measured results are approximate in that they were corrected for average isotope effects based on the total amount of water adsorbed on all segments, without considering the effect on the separate segments of an initially lower value of f1. Conclusions Consistently lower tritium concentrations measured in water distilled from silica gel relative to tritium concentrations of airborne HTO can be explained by isotopic exchange of adsorbed HTO with nontritiated water and hydroxyl groups in the silica gel. For the silica gel being studied, its inferred exchangeable water was 5.9 ( 0.2%, compared to its weight loss between 100 and 1050 °C of 6.5 ( 0.1%. The implication of this observation is that adsorbed tritiated water is diluted by nontritiated water and hydroxyl groups in dried silica gel, but that not all such groups in the studied material exchange promptly. This dilution by isotopic exchange can be expected for other types of silica gel to an extent that depends on the structural binding of water and hydroxyl groups remaining in silica gel heated to 100 °C.
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Figure 4. Concentration correction factor for airborne tritium oxide collected on silica gel.
TABLE 4: Silica Gel Weight Loss
1 2 3 4 5 6 avg. % wt loss a
300 °Ca
600 °Ca
temperature 850 °Ca
950 °C
1050 °C
0.1078 0.1088 0.1170 0.1149 0.1177 0.1129 2.2 ( 0.2
0.2631 0.2605 0.2558 0.2517 0.2575 0.2480 4.9 ( 0.1
0.3251 0.3431 0.3368 0.3194 0.3353 0.3180 6.4 ( 0.2
0.3381 0.3526 0.3403 0.3231 0.3397 0.3232 6.5 ( 0.2
0.3388 0.3534 0.3426 0.3250 0.3414 0.3253 6.5 ( 0.2
net weight at 100 °C, g
sample
5.5061 5.3870 5.1248 5.0647 5.1058 5.0023
Weight loss relative to weight at 100 °C, grams.
TABLE 5: Test of Tritium Exchange in 4-Segment Columnc adsorbed water no. of segment run 1 In 1 2 3 4 run 2 In 1 2 3 4
weight, g
count rate, c/min
36.0 19.3 11.7 4.2 0.8
343 ( 3 286 ( 3 205 ( 2 79 ( 1
51.0 21.3 16.4 10.0 3.3
347 ( 3 273 ( 3 244 ( 3 120 ( 2
calcda
tritium fraction calcdb measured
0.88 0.69 0.35 0.05
0.80 0.57 0.27 0.04
0.92 ( 0.02 0.66 ( 0.02 0.26 ( 0.02
0.92 0.79 0.58 0.24
0.82 0.64 0.43 0.18
0.87 ( 0.02 0.78 ( 0.02 0.38 ( 0.02
a Assume exchange in entire segment during flow. b Assume exchange in distillation flask only. c Notes: 1. Each segment contained 75 g silica gel. 2. Measured tritium fraction is count rate ratio divided by 0.93 × 0.97 (f1, and f2, respectively) for each segment.
Tritium concentrations measured in determining the amount of exchangeable water on silica gel and in desorbing water from silica gel by distillation are affected by the vapor pressure isotope effect. This effect reduces the concentration of tritium in vapor relative to the condensed phase by only 3% near the boiling point of water but by 11% at room temperature. The correction factor for this effect depends on the amount of water
transferred between phases relative to the amount present and disappears only if all of the water is transferred, i.e., the condensed phase is taken to dryness or all vapor is collected. Acknowledgment. This work was supported by the Georgia Department of Natural Resources. We thank Mr. Jeffrey Lahr for thermogravimetric measurements. References and Notes (1) Arnett, M. W.; Mamatey, A. R. Savannah River site environmental report for 1996. Report no. WSRC- TR-97-0171 (Aiken SC 29808); Westinghouse Savannah River Co., 1997; p 76. (2) Bishop, C. T.; Carfagno, D. G.; Farmer, B. M.; Lacy, V. C.; Yanko, W. H. Fusion Technol. 1985, 8, 2427. (3) Patton, G. W.; Cooper, A. T., Jr.; Tinker, M. R. Health Phys. 1997, 72, 397. (4) Thompson, J. L.; Duce, S. W.; Keller, J. H. An atmospheric tritium and carbon-14 monitoring system. Report no. NUREG/CR-0386, U.S. Government Printing Office: Washington, DC, 1978. (5) Unger, K.; Gallei, E. Kolloid Z. Z. Polym. 1970, 237, 358. (6) Unger, K. K. Porous Silica; Elsevier Scientific Publishing Co.: New York, 1979; pp 6-9, 72-76. (7) Rosson, R.; Klima, S.; Jakiel, R.; Kahn, B.; Fledderman, P. Health Phys. 1998. Submitted for publication. (8) Iler, R. K. The chemistry of Silica; John Wiley & Sons: New York, 1979; pp 624-645. (9) Bergna, H. E. The Colloid Chemistry of Silica. Advances in Chemistry Series 234; American Chemical Society: New York, 1994; pp 21-38. (10) Silica gel grade #41. W. R. Grace & Co., Davison Chemical Division, Baltimore, MD 21213 (11) Baumgartner, F.; Kim, M.-A. Appl. Radiat. Isot. 1990, 41, 395.
