Environ. Sci. Techno/. 1995, 29, 1982-1989
Sorption of N m r Orgrnic Vapors by Ice 8M( Snow J O H N T . H O F F , * , +F R A N K W A N I A , + DONALD MACKAY,' AND ROBERT GILLHAM+ Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, and Department of Chemical Engineering a n d Applied Chemistry, University of Toronto, Toronto, Ontario, Canada MSS 1A4
Two complementary methods of measuring the sorption of nonpolar organic vapors to snow and ice surfaces are described. Sorption measurements are presented for n-hexa ne, n-hepta ne, n-octa ne, benzene, chlorobenzene, 1,4-d ic hlo ro benzene, di c h loromethane, trichloromethane, tetrachloromethane, l,l,l-trichloroethane, trichloroethene, and tetrachloroethene over the temperature range from -10 to 0 "C. A gas chromatographic retention time method using a column containing ice-coated Chromosorb P has the advantage of giving precise determinations of the air-ice surface partition coefficient for volatile chemicals. A snow pack method involves measuring the sorption of chemical from a gas stream into a bed of snow, the quantity sorbed being determined by the change in gas concentration and by directly measuring the snow concentration. This method more closely simulates environmental conditions, but the snow surface area is uncertain. It is concluded that to a first approximation partitioning a t the air-ice interface can be estimated by extrapolating adsorption constants for the air-water interface. Implications concerning the fate of organic contaminants in environmental snow samples are discussed.
in snow. Finally, snow and ice particles have been implicated as promoting reactions of organic contaminants in the upper atmosphere. Several field investigations of the presence of nonpolar organic chemicals in snow and ice have sought to establish baseline contamination levels in remote regions such as the Antarctic (1-3) and the Arctic (4-91, to study geographical distribution patterns of organic contaminants (10-13), and to quantify wet depositional fluxes (14-18). The behavior of organic contaminants during snow melt has been investigated in the laboratory (19)and under field conditions (20).Recently, depth profiles in glaciers have been used to deduce temporal trends of the deposition of persistent organic contaminants (21-23). The current understanding of the interaction of organic vapors with the surface of snow and ice is inadequate for a quantitative treatment of these processes. Orem and Adamson (24) measured the adsorption of alkanes on ice. Goss (25) measured sorption of organic vapors on the surface of ice by gas chromatography (GC) and performed sorption experiments with quartz sand at temperatures below 0 "C and relative humidities of 70%,concluding that the sorption of nonpolar compounds by ice surfaces was similar to that observed for water films on mineral surfaces at higher temperatures (26). This paper describes investigations of the sorption of nonpolar organic vapors on ice and snow surfaces using two different, but complementary, approaches. The first is an extension of a previous study measuring the adsorption of volatile organic compounds at the air-water interface ofwater-coated Chromosorb P by gas chromatography (27) to temperatures below 0 "C. The second method has been developed to measure partition coefficients from air to bulk snow by equilibrating artificial snow with a gas stream of constant vapor concentration. We present results and conclusions from these measurements, propose a conceptual model of the partitioning of organic chemicals to snow, and suggest research priorities to improve our understanding of the interaction between nonpolar organic contaminants and snow and ice.
Experimental Section Introduction Snowand ice are important components of cold ecosystems at high latitudes and altitudes as well as in temperate ecosystems in winter and undoubtedly influence the fate and behavior of persistent organic contaminants. Because of the high surface area and low temperature, falling snowflakes may efficiently scavenge organic constituents from the atmosphere. During permanent or seasonal snow and ice cover, there is continuous diffusive and advective exchange of organic chemicals with the atmosphere. Snow undergoes metamorphic changes, both during fall and after deposition, including compaction, sintering, freeze-thaw cycles, and melting. These changes are likely to have a significant effect on the behavior of chemicals contained * Correspondingauthor e-mail address:
[email protected]. +
University of Waterloo.
* University of Toronto.
1982 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 8, 1995
Measurement of Sorption by Inverse Gas Chromatography. In a previous study (27),the adsorption of organic compounds at the air-water interface was studied by inverse gas Chromatography (28). The method, which is described in more detail in the earlier paper was slightly modified to allow measurements at lower temperatures. Briefly, the retention times for organic vapors and methane in a column ofwater-coated Chromosorb P were measured as a function of temperature. The retention times were used to calculate the dimensionless column capacityfactor, K,, according to
I(, = tr/tm- I where tr and tm are the retention times for the sorbed compound and the unsorbed methane, respectively. Kc is equal to the net retention volume, Vn,divided by the volume of gas in the column, V,, both per gram of sorbent (m3/g). V,, is given by
0013-936)(/95/0929-1982$09.00/0
0 1995 American Chemical Society
V, = ki,,,,pi
+ f&Vw
at temperatures above 0 "C (2)
and
V, = ka,ic&i
at temperatures below 0 "C (3)
where Ai (m*/P, and V, (m3/g) are the surface area and volume of water per gram of sorbent, kia,,,,., and ki,,i,. are the water surface-air and ice surface-air partition coefficients (m),andK,is thedimensionlesswater-airpartition coefficient. V,,,is calculated from the retention time for methane and the flow rate of the carrier gas in the column. A 1 m x 6 mm (i.d.1 glass column was packed with approximately 10 g of water-coated Chromosorb P (0.31g of organic-free water@ of dry carrier). The column and a conditioner column packed with the same material were immersed in alow temperature bath, ensuringthat the He carrier gas was equilibrated with water as it entered the analytical column. The temperature was monitored as it drifted upward at the rate of about 0.06 " C h i n from -20 to+lO°C. Theretentiontimesweremeasuredof 13organic chemicals (methane, n-hexane, n-heptane, n-octane, benzene, chlorobenzene, 1.4-dichlorobenzene. dichloromethane, trichloromethane, tetrachloromethane. 1.1.1trichloroethane, trichloroethene. tetrachloroethene). Very small quantities (-1 nmol) of the organic vapors were injected, ensuring that the retention times were measured in the linear Henry's law portions of the isotherms. These experiments were repeated several times, and the retention times were found to be reproducible within a few percent. Meamuement ofAirto BulkSnowPardtion~flicients In an Equilibration Vessel. A method was developed in which a gas stream with constant, adjustable concentration of a nonpolar organic compound, 1,4-dichlorobenzene (DCB), passed through a vessel containing snow. The concentration of DCB in the exit gas from the snow vessel was monitored while the snow came to equilibrium with the incoming carrier gas. The final amount of DCB sorbed to the snow was also determined. The experiments were performed with artifcially produced snow made from frozen chunks of double-distilled watermechanicallypulverizedinablenderfilledwithliquid nitrogen. The ice powder was sieved (mesh size 1 mm), allowed to age at around -12 "C, sieved again, and placed into the experimental glass vessels. Two batches of snow were used in the experiments: batch 1was aged for several months, while batch 2 was allowed to age for only 2 weeks. The experimental apparatus shown in Figure 1 was set up in a cold room at - 12 "C. Nitrogen gas was purified by passing it through a sorbent trap and a liquid nitrogen trap. The gas stream was split into two; one passed through a saturator column (12 cm x 2 cmi.d.) filledwithglass beads (20/30mesh) coated with DCB to generate a saturated gas flow. Theother stream passed through adummy saturator columnofequalflowresistance. TheconcentrationofDCB in the combined streams was controlled by adjusting the flow rates through the two pathways. To avoid subliming the snow in the sorption vessel, both gas streams were prehumidified by passing them through snow at the same temperature. After a stabilizing period, the mixed gas stream was passed through the third snow-filled vessel (20 cm x 5 cm i.d.1. The concentration of DCB entering and leaving this vessel was monitored by trapping the vapor on a sorbent trap (Tenax TA. 35/60 mesh) and thermally desorbing it
A gas cylinder B in-line regulator C liquid Nitrap D sorbent trap
.
E flowmeter F snow vessel
0 saturator column H Sway switching valve J sampling tube
U
FIGURE 1. Schematicdiagram ofme experimntal appamasforthe determination ofthe equilibrium psrtnioningof 1,Cdichlorobenmne between air and bulk snow.
into a gas chromatograph. Initially. sorption to the snow resulted in a low exit gas concentration. With time, the concentration increased until it reached the inlet value. The amount sorbed was estimated by integrating this concentration over time. The gas flow was then stopped, the snow vessel was sealed, the snow was allowed to melt, and the meltwater was transferred to a flask. An aliquot of the meltwater was diluted with distilled water and sparged for 2 h with a pure gas stream onto a sorbent trap, which wasanalyzedasfortheairsamples.Theemptysnowvessel was also sparged to measure the residue of DCB left in the snow vessel. These quantities were combined to give a second estimate ofthe amount of DCB sorbed to the snow. ComparisonofMethods. Theconsistencyofthe results obtained with the two methods was tested by measuring thesorption of 1.4-dichlorobenzenewithexchangedsorbing material; i.e., the artificial snow (batch 2) was examined using the retention time method, and the ice-coated ChromosorbPwasused in theequilibrationvesselmethod. The specific surface areas inferred from the sorption measurements served as a basis for comparing the two methods. The specific surface area of the ice-coated ChromosorbPwasalsomeasuredbyadsorptionofnitrogen gas at 77 K between relative presssures of 0.03 and 0.30 using a Gemini 2375 surface area analyzer (Micromentics Instrument Corp., Norcross, GAL
Results Measurements of Sorption on Waterllce-Coated Chmmosorb P by Imrerse Gas Chromatography. Representative data showing the change of retention time as a function of temperature are given in Figure 2. Three kinds of behavior were exhibited by (i) nonsorbing methane (ii) the alkanes, and (iii) the more water-soluble compounds. The retention times for methane decrease with increasing temperature due to the effect of temperature on carrier gas volume and hence flow rate. The increase in the retention time for methane seen at 0 OC is caused by the change in gas volume that accompanies melting of the ice. The retention times for the sorbed compounds are consistent with theassumptionthat adsorptionat theai-iceinterface is responsible for sorption at temperatures below 0 "C. VOL. 29. NO. 8.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY.
1-
In (Wm) -l2I
n-octane
methane 0.97
I
In (Wm)
.
L
I ,b
0.92
.Io
.5
0
5
lt
410
'
5.
0
'
5'
temperature in ' C
temperature in "C
'
-12.5
-
-13
-
.10.5
benzene
.
.
'
chlorobemzene,
'
'
10 I
retention time In min
retention time in min
trichloromethane
tetrachloroethene
0.0036 0.0037 0,0038 0 OW9 0,001 WT
-
in ( W m ) .11.5
-12
-
.12.5
-
trichloromethane
WT In (kia'm) -11
.11.5
-12
.12.5
temperature in ' C
temperature in "C
FIGURE 2. Retention times of four selected chemicals (methane, woctene, trichloromethene, and tetrachloroethene) in a gas chromatographic column filled with waterhce-coated Chromosorb P es a function of temperature.
In Kc 2.5 1
I
4
0'
.13 -
..
,.m. '. '
.13.5
-
,w'
.13
'
-13.5
d
0.0036 0,0037 0.0038 0.0039 0,004 WT
O.CU35 0.0058 0.0037 0.0038 0.0019 0.004 WT
FIGURE 4. Temperature dependence of the air-interface partition coefficient for benzene, chlorobenzene, woctane, and trichloromethane on waterAce-coated Chromosorb P. The solid and dashed lines represent actual end extrapolateddata from Hartkopf and Karger (28).
2 1.5 1
0.5
0.00355 0.0036 0.00365 0.0037 0.00375 0.0038 Kfr FIGURE 3. Temperature dependence of the column capacity factor for 1,l.l-trichloroethane (m),trichloroethene (01,and tetrachloroethene (+) above and below the freezing point of water.
whereas adsorption and dissolution in water contribute to sorption at temperatures above 0 "C. The retention times for the alkanes decrease monotonically with increasing temperature, with a small change in slope noticeable at 0 "C. The retention times for the more water-soluble compounds increase sharply at 0 "C due to solution in water. The effect of solution in water can be estimated from a plot of In & versus reciprocal absolute temperature. Some examples are shown in Figure 3. The plots are linear except in the immediate vicinity of the discontinuity. The magnitude of the change in & at 0 "C was calculated from the regression lines shown in the figure. Kwa was then calculated by eqs 2 and 3. The Kwavalues obtained for the C1and Cz chlorinated hydrocarbons compare favorablywith those of Gossett (29). The average ratio of the experimental 1984 * ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, N O . 8, 1995
Kwa values to those of Gossett is 1.02 with a standard deviation of 0.08. Adsorption of organicvapors on ice can be characterized by plots of In kia versus reciprocal absolute temperature, some examples of which are shown in Figure 4. Each plot contains data for two experiments to show the reproducibility. The kia values were calculated from the measured net retention volumes by eq 3, with Ai estimated by fitting V,, for n-heptane at 0 "C with kia calculated from Hartkopf and Karger's (28)enthalpy and entropy values. The actual (above 0 "C) and extrapolated (below 0 "C) kia values for water (28)are also shown for comparison. The experimental ki, values for ice are somewhat larger than the extrapolated ki, values for water. According to the van't Hoff equation, the slope of In ki, versus 1IT is approximately -AHa/R, where AHais the enthalpy of adsorption (30). The regression coefficientsand the adsorption enthalpies calculated from data for between two and five experiments are given in Table 1. The adsorption enthalpies reported by Hartkopf and Karger and the enthalpies of condensation, AH,, are also given for comparison. The adsorption enthalpies for ice-coated Chromosorb P are larger than those for adsorption on water and for condensation. Measurementof Bulk Snowt o mPartitionCOeBBdents with the EquilibrationVeasel. Nine measurements of DCB partitioningwere obtained at several vapor concentrations. Figure 5 illustrates the change in DCB concentration in the gas leaving the snow vessel during an experiment. The amount of DCB sorbed on the snow at equilibrium was estimated by integrating these concentration profiles and by direct analysis of the snow. The average ratio of directly measured amount to amount estimated by integration is
snow concentration in uglL meltwater
TABLE 1
Coefficiowts of R o m i o a Ewer In (&&I) = A B(774, M a for Ico-Coated Cbnuosorb P, M a for Wmr, and MC
+
compound
-A
B
n-hexane n-heptane n-octane benzene chlorobenzene 1,4-dichlorobenzene dichloromethane trichloromethane tetrachloromethane l,l,l-trichloroethane trichloroethene tetrachloroethene
30.28 31.53 33.59 30.06 31.23 31.33 27.87 30.63 29.62 30.91 30.85 31.67
4225 4821 5632 4606 5226 5539 3736 4671 4143 4667 4658 4978
-Anab - A k C
-AH,' 37.3 (1.3) 42.3 (1.7) 49.0 (1.7) 40.5 (1.3) 45.6 (1.7) 48.2 (1.7) 33.3 (1.3) 41.0 (1.7) 36.6 (1.3) 41.0 (1.3) 40.9 (1.3) 43.6 (1.3)
28 31 36 31 35
31.8 36.4 41.4 33.9 41.0 46.4 28.9 31.4 32.6 32.6 34.7 39.7
23 27 23
a This work; 95% confidence intervals are given in parentheses. From ref 28. From ref 60.
.
' ,'
300
200
I00
0 0
artificial snow
Cummrmilu@t
400
20
40
60
80
100 120 140
air concentration in ugR T
w
* *
aa
FIGURE6. Measured 1A-dichlorobenzeneequilibrium concentration in bulk snow as a function of gas concentration for two artificial snow samples: batch 1 (0)and batch 2 (B).
lime in mlmdrr
io0
.- b
4b
A
**
80-
**
*.,,I 20
?20
0
** ,
80
120
180
***
, . ,
240 300 360 hrm in m m u h
, 420
,
v
, . , . , 480
540
800
,I
BBO
FIGURE 5. Concentration of l,4-dichlorobenzene in gas leaving the equilibration vessel filled with artificial snow (a) and ice-coated Chromosorb P (b) as a function of time. The concentration values before time 0 reflect the incoming gas concentration.
1.02 with a standard deviation of 0.20. Figure 6 shows the measured bulk snow concentration plotted against incoming gas concentration, a linear relationship being apparent. The measurement error is evident from the considerable scatter. A dimensionless bulk snow-to-air equilibrium partition coefficient KsAcan be defined as the ratio between the bulk snow concentration (mol/m3of meltwater), which includes the chemical that is contained in the interstitialair,and the gas concentration (mol/m3 of air). Ks.4 was found to be 2.8 (6 = 0.77) for batch 1 and 3.8 for batch 2 (9 = 0.92). The smaller Ks.4 value for batch 1is believed due to the longer aging period, resulting in a reduced specific surface area (31). Batch 1 also had a slightly higher specific gravity (0.39)compared to batch 2 (0.37). Comparison of Methods. The change of gas concentration of DCB after passage through the snow vessel containing Chromosorb Pis shown in Figure 5. The larger specific surface area of the Chromosorb P compared to the snow results in a longer time period for equilibration. Because it proved difficult to measure the concentration of
DCB in the ice-coated Chromosorb P, the amount sorbed was estimated by integrating the gas concentration over time. The specific surface area of the Chromosorb P was calculated by assuming that DCB is either adsorbed or present in the interstitial air and using kia (4.0 x m) as determined by the GC method. The two determinations gave an average of 0.8 m2/g of dry Chromosorb P, which is comparable to 1.0 m2/g obtained by the retention time method and 1.0 m2/gobtained by the nitrogen adsorption (BET) method. It proved to be difficult to measure sorption on artificial snow using the retention time method. The glass column was packed in a chest freezer at -20 "C and transferred to the gas chromatograph quickly to minimize exposure to higher temperatures. The small specific surface area of the artificial snow required correction for adsorption on the glass column. The corrected net retention volume was then converted to specific surface area using eq 3 and the kia value for ice-coated Chromosorb P. The two determinations resulted in an average specific area of 0.03 m2/gfor snow 2. The bulksnow-to-air partition coefficientsconvert to specific surface areas of 0.03 m2/gforbatch 1and of 0.05 m2/g for batch 2. This agreement is considered to be satisfactory considering the magnitude of experimental error. It is thus concluded that the two methods give similar results.
Discussion At temperatures close to that of melting, the surface of ice is covered with a transitional liquid-like layer (32). According to Jellinek (331,this layer is thickest at the melting point (approximately 10 nm) and decreases exponentially with decreasing temperature until it vanishes at about -30 "C. Adsorption of nonpolar organic vapors on ice at temperatures near melting is thus expected to be similar to that on water. Orem and Adamson (24) observed that the reduced adsorption isotherms for n-hexane on water VOL. 29, NO. E, 1995 / ENVIRONMENTALSCIENCE &TECHNOLOGY
isas
and ice are very similar. Vidal-Madjar et al. (34) obtained good agreement between theoretically calculated adsorption potentials and measured adsorption enthalpies for saturated and aromatic hydrocarbons when the surface of water was assumed to be that of hexagonal ice. Our results indicate that the experimental kia values for ice at temperatures near melting are close to those expected for water at the same temperatures. The root mean square difference between the observed and calculated In kia values at 0 "C is 0.17, a factor of 1.19in ki,. The largest difference, that for benzene, is 0.26, a factor of 1.30. The experimental and calculated ki, values for alkanes are nearly coincident at temperatures above 0 "C (e.g., see the plot for n-octane in Figure 4). The absence of a discontinuity in In ki, for alkanes at 0 "C suggests that the surface area of the interface does not change appreciably on freezing and that Chromosorb P does not influence sorption of alkanes at temperatures above 0 "C other than by providing support for the air-water interface. As the temperatures decrease below 0 "C, the experimental In kia values diverge from the extrapolated values for water. Although the kia values for ice-coated Chromosorb P are systematically higher than those for water, the differences are always less than a factor of 1.8 at -10 "C. The reported ki, values of Goss (25)for chlorobenzene on pounded ice exceed ours by a factor of 2.0 over the temperature range studied. His specific surface area, 0.05 m2/g,was estimated "to within a factor of 2" by sieving the ice particles to a certain size range. Alowestimated surface area would explain his higher kiavalues for chlorobenzene, and his kiavalues for other compounds would then deviate to the same degree. The only other compound for which comparison is possible is n-nonane. Goss' kia values for n-nonane are larger than the extrapolated values for water by a factor of 5.5. The finding that the adsorption enthalpies for ice are larger than those for condensation contrasts with previous studies, which have generally concluded that water and ice are low energy surfaces. Older studies (24,35-38) usually found differential enthalpies of adsorption greater than those for condensation, but the more recent studies (25, 28, 39, 40) have found the opposite. For the nonpolar compounds investigated by Hartkopf and Karger, the average ratio of AHa to AH, is 0.88,and Goss' data are consistent with this. Our measurements give a significantly higher ratio, 1.17, although part of the difference may be in the calculation of AH, (30). The larger values of AH, obtained in this study suggest that the surface of ice-coated Chromosorb Pis more energetic than that of the ice powder used by Goss. Alternatively, adsorption enthalpies for water may increase when water freezes. The surface free energy of water at 0 "C is 76 mJ/m2,whereas that of ice is 109 mJ/m2(41).The London dispersion force, which accounts for 29% of the surface free energy of water, is primarily responsible for adsorption of nonpolar organic compounds (41). It is not known whether the dispersion energy increases when water freezes. More careful measurements of adsorption enthalpies may shed some light on this issue. The observation that sorption of chlorinated hydrocarbons decreases with increasing temperature contrasts with recent findings for COz (42)and for SO2 (43). The close agreement of our kia and Kwa values for 0 "C with those calculated from the data of Hartkopf and Karger (28) and of Gossett (29)definitely rules out a very thick quasi1986 ENVIRONMENTAL SCIENCE & T E C H N O L O G Y / VOL. 29, N O . 8 , 1 9 9 5
liquid layer (500 nm) as implied by the SO2 sorption experiments of Conklin and Bales (431. The average thickness of ice coating the Chromosorb P was 350 nm. Our results are thus consistent with, but do not prove, the existence of a thin quasi-liquid layer. The measurement error obtained by regression analysis of the In Kc vs 1/T data suggests a layer thickness less than 20 nm, which is consistent with the recent ellipsometry measurements by Furukawa et al. (44). The two experimental methods give similar results, suggesting that they both measure adsorption at the airice interface. The GC retention time method is fast, is relatively simple, and can produce precise data. The sorption vessel method allows more time for equilibration, it could employ natural snow samples, and it is not limited to volatile chemicals. It is also more laborious and requires a cold room. It is recommended that, whenever possible, the surface area determined by adsorption of a compound with a known ki, be checked by an independent method such as the nitrogen adsorption method. It is concluded, as Goss (25) has previously concluded, that in the temperature range 0 to -10 "C the sorption of nonpolar organic vapors to ice surfaces is similar in magnitude to that expected by extrapolation for a water surface, but the few available data suggest that ki, for ice may be up to a factor of 2 larger than that for water. Existing data and correlations for air-water interfacial partitioning are thus tentatively suggested as a means of estimating air-ice partitioning. Conceptual Model of EquilibriumPartitioningin Snow. We suggest that for a quantitative treatment of the partitioning of nonpolar organic chemicals to snow and ice phases six simultaneous partitioning mechanisms must be considered: (i) incorporation into the solid ice crystal (subscript s); (ii) dissolution into the quasi-liquid surface layer (subscript1); (iii)adsorption at the &-liquid interface (subscript i); (iv) adsorption onto mineral particles (subscript m); (v) partitioning into organic and biotic phases (subscript 0);and (vi) partitioning into the interstitial air phase (subscript a). The relative importance of each of these processes depends on the properties of the chemical (e.g., vapor pressure and water solubility) and the snow (e.g., mineral and organic content). In fugacity terms, the equilibrium partitioning of an amount M (mol) of a nonpolar organic chemical in a snow pack of volume V (m3)at a prevailing fugacityf(Pa) can be expressed as
+
+
M = fvcvszs UIZ, + aizi a,z,
+ vozo+ vaZa)
(4)
where each sub-compartment x in the snow pack has been assigned a volume fraction v, (m3/m3),or specific surface area a, (m2/m3)and a & value (mol m-3 Pa-') or interfacial z, value (mol m-z Pa-'), each term representing a process listed above. Well-established methods exist to estimate capacity terms z or Z based on physical-chemical properties and partition coefficients of the chemical (45): air Z, = l / R T
(5)
where R is the gas constant (8.314J mol K-I) and Tis absolute temperature (K)
liquid-like layer
for ki, can be deduced and the constant changed to express kia (in units of m) and to express CS, (in m0l/m3), giving
if assumed to be pure water, where Kwa may be extrapolated from values above 0 "C organic lipid phase zo
= ZIKOW
(7)
where KO, is the octanol-water partition coefficient
log kia = -0.769 log C - 5.97
+ log Ga
(12)
Values of CS, (subcooled liquid) deduced from vapor pressure and Henry's law constants are given in Table 2. Values of Kwa were calculated also from these Henry's law constants. Extrapolation to -10 "C was done assuming log ki,(--l0 "C) = log kia(20"C)
mineral surfaces
1 + -2.303R ( AHa 263 1 - 293) (13)
Z, = ZIK,
where A H , was estimated from the relationship of 22 nonpolar compounds (28)
where Ks is a mineral-water sorption coefficient
AHa = 0.878 f 0.057(SD)
Ks(m)= cs(mol/m2)/C,(mol/m3)
(14)
M C
We assume that z, is negligible in the absence of information to the contrary. A capacity term for the solid ice phase Z, is difficult to estimate. Organic chemicals may initially become part of snow crystals during riming (capture of supercooled cloud droplets by snow crystals), but they may later freeze out and accumulate at the ice-air interface. The exclusion of organic molecules from ice crystals is so complete that freeze concentration has been used as a means to concentrate organic chemicals in aqueous solution (46). For the present purposes, we assume Zs to be negligible. The capacity of the ice-air interface can be estimated to a first approximation from
zi= kia/RT= k i g a
(10)
using extrapolated values of kia for adsorption of organic chemicals on the air-water interface. The specific surface area of snow and ice ai is uncertain, but it is likely to range over orders of magnitude. Jellinek and Ibrahim (31)obtained a nitrogen BET surface area of 7.77 m2/gfor one sample of fresh snow, and Adamson et al. (47) reported values ranging from 0.2 to 1.3 m2/g. The size and geometry of snow crystals ( 4 4 4 9 ) suggest a range of 0.1-1.0 m2/g for fresh snow. IllustrativeCalculation. A calculation of the equilibrium partitioning of DCB, hexachlorobenzene (HCB), and y-hexachlorocyclohexane (y-HCH)in a hypothetical snow pack at -10 "C illustrates the role of adsorption at the airice interface. The following properties were assumed to apply to the hypothetical snow pack specific gravity specific surface area liquid layer thickness organic content density of ice
0.4 (newfirn type snow, ref 49) 1 m2/g (based on data from refs 31 and 47) 10 nm (based on ref 32) 1 g/m3 of snow (assumed) 9 17 kg/m3
Table 2 lists the selected physical-chemical properties.The correlation of kiw (cm) as a function of liquid solubility C (mol/cm3)for nonpolar organic compounds of Hoff et al. (27)at 20 "C was used, namely log ki, = -0.769 log C - 8.58
(1 1)
Since kia is by definition kiwKwa,the corresponding equation
with AHc values from refs 50 and 5 1 as given in Table 2. The Kwa values at - 10 "C were calculated using data from refs 52 to 54, and the log KO, values were obtained from refs 55 and 56 assuming no change with temperature. The ki, value for DCB obtained from this correlation was 7.3 x lop5m, which is about twice the measured value of 3.4 x m. This approach is thus regarded as presently giving no better than a factor of 2 accuracy. The estimated values for y-HCH and HCB are speculative since measured kia values are unavailable. Volume fractions, specific surface areas, and capacity values of four snow phases derived from the snow and chemical properties are given in Table 3 as well as the equilibrium distribution between these phases. This calculation, even with its possible error, suggests that adsorption at the interface is much more important than dissolution in the very thin quasi-liquid layer for the partitioning of nonpolar organic chemicals in snow. It is likely that in all three cases at least 96% is sorbed to the surface with no more than 1%in the organic phase and 1% in solution. Two percent of the most volatile chemical (DCB)is present in the interstitial air, but higher values are expected for more volatile chemicals. It has been pointed out previously that interfacial effects will be most pronounced when the chemical exhibits a strong tendency to partition to the interface and when the interfacial area is large compared to the volume of the air or water phase (27). This should be the case for snow, which combines a large surface area and aliquid layer ofverylimited thickness. Environmental Implications. These findings have several implications for the behavior of organic chemicals in snow and ice under environmental conditions. Falling snowflakes of high specific surface area are potentially efficient scavengers of gaseous organic compounds from the atmosphere, but the degree of approach to equilibriumis uncertain. Czuczwaet al. (15)found higher scavenging efficiencies for gaseous nonpolar organic compounds such as alkylbenzenes in snow than in rain. Concurrent measurements of snow and air concentrations are required to determine the scavenging coefficients. When measuring the concentrations of organic pollutants in freshly fallen snow, there is potential loss due to volatilization and contamination by dry particle fallout. Snow samples should be immediately sequestered in airtight containers. Conventional precipitation samplers are VOL. 29, NO. 8,1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
1987
TABLE 2
Physical-Chemical Propeities of Three Chemicals Used as Input Parameters for Illustrative Calculations vapor pressure of subcooled liquid (Pa) Henry's law constant (Pa.m3/mol) solubility of subooled liquid (mol/m3) octanol-water partitioning coefficient entropy of fusion (J/(mol.K)) enthalpy of condensation of subcooled liquid (kJ/mol) a
1,4-DCB
HCB
y-HCH
log 17 = 10.71 - 2444/8 log H = 6.47 - 1181/F log CG, = 4.24 - 1263/f log KOW= 3.48 Atu$ = 58.2' AHc = 46.8a
log I7 = 8.50 - 2873/Tb log H = 10.05 - 2493/P log CG, = -1.55 - 3 8 0 / f log Kow = 5.5h AfusS = 44.8' AH, = 55.0b
log = 13.79 - 4334/P log H = 7.54 - 2382/8 log cb, = 6.25 - 1952/f log Kow = 3.8h AfusS= 61.1k AH, = 83.0b
*
Ref 50. Ref 51. Ref 52. Ref 53. e Ref 54. 'Calculated pH. g Ref 55. Ref 56. Ref 57. Ref 58.
TABLE 3
Illustrative Calculation of Equilibrium Partitioning of Three Chemicals in Hypothetical Snow Pack at -10 "C interstitial air volume fraction or specific surface area partition coefficients 1,CDCB HCB 1/-HCH capacity value 1,4-DCB HCB y-HCH equilibrium distribution 1,4-DCB HCB 1/-HCH
va in m3/m3
organic lipid
quasi-liquid layer LI in m3/m3 4 x 10-3
vo in m3/m3 10-6
ice surface
4.57 10-4 4.57 10-4 4.57 x 10-4
22.8 580 71 103 4 in (mol/(m3-Pa)) 1.04 x 0.265 32.5
2510 316 000 6300 Z, in (mol/(m3*Pa)) 26.2 8.39 104 2.05 x 105
ai in m3/m3 4 105 kia in m 7.29 10-5 0.110 0.626 zi in (mol/(m2-Pa)) 3.33 x 10-8 5.02 x 10-5 2.86 x 10-4
1.89% 0.001% 0.0002%
0.31% 0.01% 0.11%
0.19% 0.42% 0.18%
97.62% 99.58% 99.7 1Yo
0.56
Kwa
2, in (mol/(m"Pa))
thus generally not suitable for sampling snow. Melting should be performed in a sealed vessel (possibly in the presence of a liquid solvent such as hexane), and the headspace concentration should be determined. Temperature and surface area are important auxiliaryvariables. Techniques for determining the association of chemical with the interface and with aerosol particles scavenged by snow are also needed. Two recent studies have concluded that the behavior of higher molecular weight PAHs in a melting snow pack can be explained by their association with solid particles (19) and that particle scavenging is responsible for the scavenging of PAHs by snow (59). However, the fact that the chemicals were associated with particles in melted snow does not necessarily imply that they were originally associated with particles in the unmelted snow. Because the capacity of snow for sorbing chemicals is controlled by the available surface area, concentrations in the snow pack may decrease as the snow ages and the area drops. Jellinek and Ibrahim (31)observed an exponential decrease of surface area with time. The rate increased with increasing temperature. The fugacity of the contaminant will therefore increase during metamorphosis, resulting in a non-equilibrium situation in which the chemical may diffuse from the snow packinto the atmosphere. The snow will probably attain a relatively low surface area, perhaps in the range of 0.01-0.1 m2/g,before melting. Chemicals that are sorbed at the interface will presumably be redistributed among the remaining phases during melting. Understanding the complex set of partitioning and transport processes during aging and melting is required for understanding and predicting the risks associated with organic chemical pulses in initial snowmelt and runoff. The assertion that snow pack concentrations decline during metamorphosis is substantiated by field investiga1988 a ENVIRONMENTAL SCIENCE & T E C H N O L O G Y / VOL. 29, N O . 8 , 1 9 9 5
Kow
tions. Hargrave et al. (7) observed that snow from the Canadian High Arctic had lower concentrations of HCB and hexachlorocyclohexanes late in the summer than in May. Patton et al. (9) found that freshly fallen snow had up to six times higher concentrations than old subsurface snow. Gregor (21)found elevated concentrations of many hydrophobic chemicals in the uppermost layers compared with the deeper strata of the glacier. Sampling the same layer again after 1 year, he found substantially lower concentrations and concluded that large quantities disappeared from the snow, probably during the summer season. Therefore, caution is advised when using snow pack concentrations to infer spatial or temporal patterns of deposition of organic chemicals.
Acknowledgments The authors are grateful for financial support from the Natural Sciences and Engineering Research Council of Canada, the Atmospheric Environment Service of Environment Canada, the Waterloo Centre for Groundwater Research and the Gottlieb Daimler-und-KarlBenz-Siftung.
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Received for review October 31, 1994. Revised manuscript received April 5, 1995. Accepted April 14, 1995.@ ES940677K @
Abstract published in Advance ACS Abstracts, June 1, 1995.
VOL. 29. N O . 8. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1989
