Environ. Sci. Technol. 1993, 27,2826-2830
Adsorption of Organic Vapors on I c e and Quartz Sand at Temperatures Below 0 O C Kai-Uwe Goss
Ecological Chemistry and Geochemistry, University of Bayreuth, D-95440 Bayreuth, Germany The adsorption of volatile organic compounds at low temperatures was studied on ice with different salt concentrations and on quartz sand. A chromatographic method was applied which allowed measurements at very low concentrations. The observed adsorption of the nonpolar compounds on ice was comparable to the adsorption on the surface of a water film as determined in a previous study at higher temperatures. For all tested substances, a decrease of adsorption was observed with increasing salt concentration in the ice. The surface of fresh ice was subject to an aging process which led to a decrease in the adsorption of the polar compounds. Measurements that were carried out after completion of this aging process showed an adsorption of the polar substances that was different from the adsorption on the surface of adsorbed water films. The adsorption behavior of volatile substances on the surface of a water film covering quartz sand did not change when experimental temperatures dropped below the freezing point. Introduction Vapor adsorption on ice and snow is thought to be an important process for the environmental fate of organic compounds in polar regions. Also, the global distribution of nondegradable substances may be governed by this process. Wania and Mackay (1) give a good review of the so called 'global distillation'. They suggest that certain Compounds preferentially accumulate in polar regions. This may be due to an enhanced adsorption on ice and snow as well as on mineral surfaces. One investigation studying the adsorption of organic vapors on ice has been published (2),but the experiments were confined to two substances (n-pentane and n-hexane) at high concentrations in the nonlinear range of the adsorption isotherm, and it is therefore not possible to extrapolate the results to environmental conditions. The goal of this work was to measure adsorption coefficients for different organic compounds on ice at temperatures between -4 and -20 "C at low concentrations. The influence of the salt content on the adsorption behavior was also investigated. Snow was not studied because it undergoes a sintering process that leads to an exponential decrease of the specific surface area (31, and therefore a reproducible adsorption cannot be expected. The temperature dependence of vapor adsorption below 0 "C was also studied on quartz at 70% relative humidity (RH). In a previous paper ( 4 ) ,the adsorption of organic vapors on quartz sand at temperatures well above 0 "C has been described. From the results it was concluded that, at relative humidities between 30 and 10076, adsorption takes place at the surface of a thin water film covering the quartz surface. The properties of this water film and thus the vapor adsorption might change at temperatures below 0 "C. In this case, adsorption coefficients measured at higher temperatures could not be 2626
Environ. Sci. Technoi., Vol. 27, No. 13, 1993
extrapolated to temperatures below 0 "C. The work of Hoff et al. (5) suggest that there may be a change in the temperature dependence of vapor adsorption at temperatures below 0 "C. The experiments were carried out using a chromatographic method which had been employed previously ( 4 , 6). This method uses the retention of the test substances observed during the chromatographic process in a column filled with the test sorbent to obtain information about the adsorption behavior. One important advantage of this approach is that adsorption can be studied at very low concentrations where the adsorption isotherm is linear, In this case, the adsorption equilibrium can be described by an adsorption coefficient [K= {mgof substance/surface area of sorbent (cm2))/{mgof substance/volume of gas phase (cm3))1. Experimental Section For the production of the pure ice, double-distilled water was used. For the salt-containing ice, 4.4 mg/L CaS04 and 15mg/L NaCl were added. Ion concentrations found in the meltwater of snow and glacier ice were reported to be less than 5 mg/L for single ions (7,8). To avoid a freezing out of the ions, the salt solutions were frozen step by step in thin layers with liquid nitrogen. Then the ice was pounded and sieved to obtain the fraction between 0.63 and 1mm. Smaller particles were not used because there is an increase of the sintering effect with decreasing size of the ice pieces (9). A steel column filled with ice was installed in a gas chromatograph and kept at the desired temperature with an external thermostat. To avoid sublimation of the ice, the air stream used as carrier gas was saturated with water vapor in a water saturator at a temperature above the experimental temperature. The excesswater vapor was removed by passing the gas stream over ice at the experimental temperature before it entered the gas chromatograph. A description of the other experimental conditions as well a5 a discussion of how true adsorption equilibrium could be provided is given in ref 4. Vapor adsorption strongly depends on the specific surface area of the sorbent (6,10,1 2 ) . For this reason, the measured adsorption coefficients should be normalized using the specific surface area of the sorbent to make them comparable. To determine the surface area of the ice samples, the BET nitrogen adsorption technique of Nelsen and Eggertsen (12) was used, Unfortunately, the specific surface area of the ice samples was below the detection limit of 0.1 m2/g. Assuming the ice particles to be of spherical shape with a diameter of 0.8 mm, the surface area would have been 0.0082 m2/g. The actual surface area must have been larger than this value but still smaller than 0.1 m2/g. For the calculation of the adsorption coefficients, an estimated value of 0.05 m2/gwas used for all three ice species, which should not deviate by more than a factor 2 from the real value. It is expected that the 00 13-938X/93/0927-2828$04.00/0
0 1993 Amerlcan Chemical Society
In K I c m l
In K , c r 1
1
Ar1s0
P
Chlorooenzene
GO37
30375
3038
03385
0039
00395
0C43
:/T I:
:
0.995) for all substances on the three ice species
Table I. Coefficients of the Regression Lines In K = D + B/T on Ice ice
iCecaBo, B
substance
-D
B
-D
chlorobenzene p-xylene m-xylene n-nonane 1,3-dichlorobenzene 1,2-dichlorobenzene 2,3-benzofuran anisole
22.15 22.82 22.97 24.39 22.71 22.27 23.73 24.34
4233 4533 4579 5021 4901 4876 5175 5204
21.24 22.15 22.03 23.58 23.68 22.22 23.87
iceN.cl
3899 4225 4200 4644 5030 4731 5122 ~~
-D
B
19.58 20.70 20.54 22.90 22.94 23.27 23.27 24.22
3353 3793 3754 4443 4686 4885 4842 5052
~~
(Figure 1). Multiplying the slopes of the regression lines with the negative value of the gas constant (R)yields the heats of adsorption A", of the substances. The good linearity of the lines in Figure 1indicate that the heats of adsorption are independent of temperature in the tested temperature range. Table I gives the slopes and intercepts of the regression lines. The confidence intervals of the slopes lay within f5-25 % at the 95 % probability level. Experiments using ice with 4.4 mg/L CaSOr and 15mg/L NaCl exhibited adsorption coefficients that were about a factor 1.6 and 2.0 smaller, respectively, than those measured for pure ice (Figure l). It was not clarified whether only the total ion concentration or also the kind of ions affects the adsorption behavior. Since natural ice or snow also contains salt in this concentration range, this effect needs more detailed studies in the future. The slopes B of the regression lines in Table I also exhibit a decrease with increasing salt concentration in the ice. However, this decrease is barely more than the confidence interval Environ. Sci. Technol., Vol. 27, No. 13, 1993 2827
Table 11. Comparison of Heats of Adsorption AHs on Ice and Water Film Adsorbed on Ca-Kaolinite and Comparison of Adsorption Coefficients K a t -12 OC and 100% RH
substance
AH,(ice) AH&waterfilm)
K(ice) K(water film)
chlorobenzene p-xylene m-xylene 2,3-benzofuran 1,3-dichlorobenzene 1,2-dichlorobenzene
1.07 0.95 0.96 1.05 1.01 0.99
2.90 1.40 1.39 2.37 4.17 3.94
333
of the slopes, and in two cases there is no decrease at all. Thus, it is not clear whether this effect is really significant. Some work has been done supporting the view that a liquid-like transition layer exists on the surface of ice a t temperatures above -30 "C (9). Furthermore, Orem and Adamson (2) concluded from their results that the adsorption behavior of ice above -35 "C closely resembles that of liquid water. Therefore, it is interesting to compare the ice results with those for kaolinite from a previous study (6). The adsorption on kaolinite had been determined for the same substances but at higher temperatures and for different relative humidities. It was found that extrapolation of these data to 100% RH yields the adsorption on a bulk water surface. A comparison of the heats of adsorption AHE for ice and Ca-kaolinite (6) is given in the first column of Table 11. The data for kaolinite were measured at 70% RH. I t is reasonable to assume that these values are close to those at 100% RH because no significant dependence of AI& on the relative humidity was determined in the previous work ( 4 , 6 ) . The results show a very good agreement between heats of adsorption on both sorbents. The next column in Table I1 gives the ratio of the adsorption coefficients on salt-free ice and the corresponding values for Ca-kaolinite extrapolated to 100% RH and -12 OC. Unfortunately, the significance of this comparison is restricted because there are some uncertainties concerning the data used. The specific surface area of the ice, and thus also the corresponding adsorption coefficients, may vary within a factor of about 2 (see above). Also the extrapolation of the values for kaolinite from temperatures between 50 and 80 OC to -12 "C and from 70% RH to 100% leads to uncertainties. Because of these uncertainties, the observed differences (Table 11) are not significant, but on the other hand the existence of deviations in the real data cannot be excluded. It is interesting to note that the extrapolation of the normalized adsorption coefficients measured at 70 % RH between 50 and 80 "C on kaolinite with a specific surface area 300 times as large as that of ice are at least in the same order of magnitude as the normalized adsorption coefficients on ice. This finding as well as the similarity of the heats of adsorption support the hypothesis that adsorption on ice is comparable to adsorption at the surface of subcooled water. The adsorption experiments with ice as a sorbent were also carried out with highly polar organic compounds. In this case, successive measurements at a constant temperature revealed a decrease in adsorption on all three ice species. Figure 2 shows this time-dependent decrease of adsorption on pure ice. The first interpretation, namely, that this finding was due to a decrease of the surface area caused by sintering of the ice particles, had to be discarded because the nonpolar substances exhibited a constant 2828
Envlron. Scl. Technol., Vol. 27, No. 13, 1993
""\
:
\ "
J-
:o4
c
-
L.-
t
l
&
L
+
_
Y
_
c
, ,
,,! LLT
,
L-__ LL
,
Figure 2. Adsorption coefficlentson pure ice durlngtheaging process.
adsorption behavior. The next question was whether the carbon dioxide in the carrier gas might play a role, for example, by competitive adsorption. However, the same effect occurred when nitrogen was used instead of air as the carrier gas. The phenomenon also could not be explained by an alteration of the ice surface by the substances themselves (i.e., irreversible adsorption). The effect on any of the polar substances was the same no matter whether other adsorption experiments with polar and nonpolar compounds were carried out in between or not. It has been suggested (4,6)that the adsorption of polar, oxygen-containing molecules on the surface of adsorbed water films is governed by the formation of hydrogen bonds. The adsorption of the nonpolar substances is not influenced by such polar forces. Thus, the time-dependent effect observed may have been caused by an aging process of the ice surface that altered its ability to form hydrogen bonds whereas the surface area itself remained constant. This would explain why nonpolar substances were not affected. The surface of the ice particles was the result of fractures. The surface structure of fresh ice particles thus may have been similar t o the bulk structure of ice. Since this is certainly not the most favorable thermodynamic state at the air-ice interface, it may give rise to a migration process in the course of which water molecules leave their position in the surface layer and reenter it with another orientation or at another position. Weyl ( 1 4 ) suggested that the nascent surface of a salt crystal also undergoes rearrangements of this kind, lowering the free energy of the surface. The aging process was not observed when the ice sample was stored for 2 weeks in a deepfreezer at a temperature of -29 "C. In this case diffusion is the only possible transport process, and at low temperatures diffusion velocities are slow. However, there is a much more intensive exchange of water molecules between the ice and the gas phase when a water-saturated gas is streaming through the column during the experiments. The time-dependent decrease of adsorption of anisole was slow enough to allow the determination of the heat of adsorption on fresh ice (Table I). This was not possible for the more polar compounds. Figure 3 shows the adsorption coefficients of the polar compounds as a function of temperature after 4 weeks, when the aging process approached an end. In Table I11 the slopes and intercepts of the regression lines as well as the corresponding heats of adsorption AHE(ice)which were calcu-
In K icnl
-4 5
1
-5 0
{
-6 5
-1
c
-
-7
7
5
t
cc375
" cc3a
' 03385
" '
" :c39
' GC3%
"
" " '
'
c040
' ' ' I ~0405
334
!/-
e 3
Figure 3. In Kvsthe reclprocalabsolute temperature for the adsorption of polar compounds on 4-week-old Ice.
substance
-D
B
acetone ethylacetate epichlorhydrine anisole
18.36 21.92 22.43 23.00
2891 3879 4263 4729
-24.0 -32.2 -35.4 -39.3
, ,
2-36
, C ~ E S 1 O'C
' 0 3 1
3~375
cLda
.a>ai
.E;.? I
Table 111. Coefficients of Regression Lines In K = D + B / T for Adsorption of Polar Compounds on 4-Week-Old Ice and Comparison of Heats of Adsorption and Condensation Mdice) (kJ/mol)
Ek , /
Mn(wf)
(kJ/mol)
(kJ/mol)
-32.1 -34.9 -41.2 -43.9
-49.4 -50.2 -46.0
lated from the slopes B are shown. For comparison, the heats of condensation AHc and the heats of adsorption on a water film AH.Jwf) (4,6)are given in the last two columns of Table 111. The heats of adsorption on the 4-week-old ice are smaller than those for the adsorption on the surface of a water film adsorbed on polar sorbents (quartz sand or kaolinite) at higher temperatures and smaller than the corresponding heats of condensation. Furthermore, the adsorption coefficients for acetone and ethyl acetate on ice are 2.5 orders of magnitude smaller than those for the adsorption on quartz sand extrapolated to 100% RH and to the same temperatures below 0 "C. These facts point to weak interactions between the polar compounds and the ice, similar to those between the nonpolar compounds and the polar sorbents studied earlier (4,6). Obviously, the adsorption of the polar compounds on old ice is not based on the formation of hydrogen bonds as has been suggested for their adsorption on the surface of a water film. This may be due to a different orientation of the surface HzO molecules of the ice and supports the assumptions concerning the postulated aging process. The presented results may also be valid at lower temperatures than those tested here. However, an extrapolation should not exceed the limit of -35 "C because Orem and Adamson (2)observed a change in the adsorption behavior of ice at temperatures below -35 "C. Taking into account differences in the specific surface areas, it may be possible to extrapolate the adsorption behavior on ice to that on snow. The surface of snowflakes which grow by the condensation of water should already be in a thermodynamic equilibrium. Thus, adsorption on the surface on snowflakesmay be comparable to the adsorption on the 4-week-old ice studied in this work. A comparison of the adsorption coefficients for ice presented here with the results of field measurements with snow or ice would be of interest, but for this purpose the specific surface area of the natural sorbents has to be known. Considering
OJS'
/T
Flgure 4. In Kvs the reciprocalabsolute temperature for the adsorptlon on quartz sand at temperatures around 0 "C.
the sintering of snow, this represents a problem. Up to now, no literature data have been found which appeared suitable for comparison with the data from this work. Nevertheless, the presented results may contribute to a better understanding of the transport and distribution processes or organic chemicals in areas of higher latitude. Adsorption of Quartz Sand at Low Temperatures. Vapor adsorption on quartz sand at relative humidities 130% takes place at the surface of an adsorbed water film (4). To investigate whether this water film, and thus the adsorption behavior at its surface, undergoes significant changes at temperatures below the freezing point, the adsorption coefficients were measured at 70 % RH and at temperatures between +4 and -12 "C. Below 0 "C two saturation vapor pressures exist: one over ice and a higher one over subcooled water. The problem is that it is not clear which one has to be used to define the relative humidity for the adsorption experiments. Kast (15) showed that the amount of water adsorbed on a surface is only a function of relative humidity and not of temperature. Using this criterion, the abovementioned problem can be solved. If the sand-filled coiumn, after having been equilibrated with 70% RH at a temperature 10 "C, is cooled to a temperature below 0 "C, the amount of adsorbed water will keep constant when the new conditions also correspond to 70% RH. If the relative humidity at the new temperature is not 7 0 % , the amount of adsorbed water will change until a new equilibrium is reached. At low temperatures this would take some time because the water transport capacity of the carrier gas is low. The adsorption behavior of test substances depends on the thickness of the water film covering the mineral surface. An increase in the thickness of the water film goes ahead with a decrease of the adsorption forces between the mineral surface and the substances ( 4 , 6 ) . This provides us with a tool to decide whether there is a change in the amount of adsorbed water with time. When the relative humidity in the column was 70% of thevapor pressure over subcooledwater, adsorption experiments directly after a temperature decrease and some days later at the same conditions gave the same results, indicating that the adsorbed water film remained unchanged. This was not true when the relative humidity ' of the lower vapor pressure over ice. In this case, was 70% aslow increase in the adsorption of all test substances was Envlron. Scl. Technol.. Vol. 27,No. 13, 1993 2828
surface of a water film covering a mineral sorbent is principally the same below and above 0 "C. This result is in good agreement with the work of others who studied water adsorbed to mineral surfaces at temperatures well below 0 "C using differential thermal analysis or analysis jqzc of the dielectric relaxation (16,17). They found adsorbed &~?~o~~ib water to be unfrozen even at very low temperatures. The exact temperature limit above which adsorbed water will 1.09 be unfrozen is not yet clear.
Table IV. Coefficients of Regression Lines In K = D + B / T at 70% RH for Quartz Sand at Temperatures around 0 OC and Comparison with Results on Ca-Kaolinite from a Previous Work (6)
substance
-D
E
AH&quartz) AH,(Ca - kaolinite)
l,l,l-trichlorethane benzene tetrachlorethylene n-octane to1uene chlorobenzene m-xylene
20.98
3338
1.18
21.79 21.19
3656 3551
1.07 1.09
24.91 22.74 22.51 24.51
4886 4261 4277 5102
1.16 1.01 1.08 1.07
0.88 1.13
1.00 0.75 0.90 0.88
observed between the measurements directly after the temperature decrease and some days later. This points to a desorption of water. The lower the new temperature was, the larger the difference between the old and the new equilibria. This finding suggests that the vapor pressure over subcooled water should be used for the definition of the relative humidity over the mineral surface below 0 "C. This is how it was done in the experiments presented here. In Figure 4,the logarithms of the measured adsorption coefficients are plotted vs the reciprocal of temperature. The adsorption coefficientsare related to a specificsurface area of 0.26 m2/g for the quartz sand, which is a more reliable value than the 0.34 m2/gthat had been determined in a previous work for the same adsorbent (4). The plots do not show any change in the adsorption behavior at or below 0 "C. The data cannot be directly compared to those measured on quartz sand at higher temperatures ( 4 ) because the substances are not the same. However, they can be compared to extrapolated data from Ca-kaolinite measured earlier at higher temperatures and the same relative humidity (6). This earlier work had shown that the water film which covers the mineral surfaces at relative humidities above 30% levels out differences in the surface properties so that adsorption coefficients on quartz sand and kaolinite were the same when normalized to the specific surface area (6). The comparison at temperatures below 0 " C also reveals that neither the heats of adsorption nor the adsorption coefficients K exemplarily given for -12 "C show any significant difference (Table IV). I t can be concluded that the vapor adsorption behavior on the
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Environ. Sci. Technol., Vol. 27, No. 13, 1993
Acknowledgments
I thank M. Hinkel and M. S. McLachlan for critical revision of the manuscript. This work was supported by the Deutsche Forschungsgemeinschaft. Literature Cited (1) Wania, F.; Mackay, D. Ambio 1993, 22, 10-18. (2) Orem, M. W.; Adamson, A. W. J. Colloid Interface Sci. 1969,31, 278-286. (3) Jellinek, H. H. G.; Ibrahim, S. H. J. Colloid Interface Sci. 1967,25, 245-254. Goss, K.-U. Environ. Sci. Technol. 1992, 26, 2287-2294. Hoff, R. M.; Muir, D. C. G.; Grift, N. P. Environ. Sci. Technol. 1992, 26, 266-275. Goss, K.-U. Environ. Sci. Technol., in press. Collins, D. N. Arct. Alp. Res. 1979, 11, 307-333. Johannessen, M.; Henriksen, A. Water Resour. Res. 1978, 14, 615-619. Hobbs, P. V. InIcePhysics; Clarendon Press: Oxford, 1974; pp 392-460. Ong, S. K.; Lion, L. W. Water Res. 1991,25, 29-36. Pennell, K. D.; Rhue, R. D.; Rao, P. S. C.; Johnston, C. T. Environ. Sci. Technol. 1992,26, 756-763. Nelsen,F. M.;Eggertsen,F. T. Anal. Chem. 1958,30,13871390. Vidal-Madjar, C.; Gonnord, M.-F.;Goedert, M.; Guiochon, G. J. Phys. Chem. 1975, 79,732-741. Weyl, W. A. J. Colloid Interface Sci. 1951, 6, 389-405. Kast, W. In Adsorption from the gas phase; VCH: Weinheim, 1988; pp 34-35. Anderson,D. M.; Tice, A. R. Solid Sci. SOC.Am. Proc. 1971, 35,47-54. Hoekstra, P.; Doyle, W. T. J. Colloid Interface Sci. 1971, 36, 513-521.
Received for review March 10, 1993. Revised manuscript received August 16,1993. Accepted August 25, 1993.' @
Abstractpublishedin Advance ACSAbstracts, October 15,1993.
