Environ. Sci. Technol. 1982, 16, 283-286
Air-Sea Exchange of High Molecular Weight Organic Pollutants: Laboratory Studies Elliot Atlas, Ronald Foster, and C. S. Glam”
Department of Chemistry, Texas A&M University, College Statlon, Texas 77843 Laboratory studies have determined air-water partition coefficientsand mass-transfer rates of predominant organic pollutants. Results demonstrate the rapid volatilization of certain high molecular weight organic compounds from distilled water and seawater solutions. Behavior of these compounds can be interpreted in terms of the Whitman resistance model of mass transfer across the air-water interface.
W
Introduction The exchange of organic chemicals across the air-water interface is a significant process affecting the transport and distribution of organic compounds in the environment (1, 2 ) . Organic pollutants may readily volatilize from water contaminated by runoff or direct discharge (3-6). Conversely, pristine waters, such as open-ocean regions, may be a sink for chemicals transported long distances via the atmosphere ( 7 , 8 ) . The fact that many organic pollutants, even of high molecular weight, are found as gases in the atmosphere suggests that vapor-phase exchange with water bodies may be a major factor in their environmental behavior. Although the mechanism(s) of gas transfer across an air-liquid surface are not exactly known, different theoretical models of the process have been used with some success (9). These models have been recently discussed by Smith et al. (10) and thus will be discussed only briefly here. The two-resistance model of Whitman (11)describes the diffusive transfer of a chemical across thin gas and liquid films adjacent to the interface. According to the model, the total “resistance” to transfer across the interface is the s u m of “resistance” in the gas phase plus “resistance” in the liquid phase. The general mathematical expression for this process has been shown to be R L = 1/KL = 1/k1+ l/k,(H/RT) (1) where R L = total resistance to mass transfer, KL = total mass-transfer coefficient, kl = liquid-phase mass-transfer coefficient, k, = gas-phase mass-transfer coefficient, H = Henry’s law constant, R = gas law constant, T = temperature, and (HIRT) = dimensionless partition coefficient. This model has been used to predict the interfacial transfer of carbon dioxide (12),oxygen (12),sulfur dioxide (13),and various organic compounds (5,13-16). Laboratory validation of this model has been mixed. For example, Liss (12) and Tsivoglou et al. (17) found reasonable agreement between their experimental data and theoretical predictions. Other experiments have determined that the air-liquid transfer of gases under certain conditions do not follow the two-resistance model (e.g., ref 18). . Alternate models have been devised to explain experimental results. The “penetration” model of Higbie (19) and the surface-renewal model of Danckwerts (20) agree well with a body of experimental data (see ref 18). These models predict that the mass transfer of a gas across an air-water interface is proportional to the square root of the molecular-diffusion coefficient of the gas. The tworesistance model, however, predicts a linear relationship between diffusion coefficient and mass flux. Dobbins (21) 0013-936X/82/0916-0283$01.25/0
presents data which show that
k1 a Dln (2) where D1= molecular diffusion coefficient and n = 0.5 5 n d 1. Under very turbulent conditions n approached 0.5 (surface-renewal model), while under less turbulent conditions n approached 1 (two-resistance model). Thus, choice of a particular model to predict mass-transfer rates may depend on the degree of turbulence in the system. Typically, experimental techniques are used that measure mass-transferrates of two compounds simultaneously. The ratio of mass-transfer rates measured in the laboratory can then be used to predict interfacial transfer rates in the environment. If the exchange rate, Ken-, of a tracer gas such as oxygen (17) or ethylene (16) is known for natural conditions, then the mass-transfer rate of the unknown, Ken,’, is
Ken”’= (K”/Ktracer)~ad(,n,t“ (3) Prior studies have concentrated on the exchange of gases (12,13,17,18),aromatic hydrocarbons (5),and the,more volatile chlorinated methanes, ethanes, and ethylenes (10, 15,16). Studies of air-water transfer of dieldrin have been reported recently (14). Calculations by Mackay and Leinonen (4) have suggested that high molecular weight chlorinated hydrocarbons such as the PCBs and DDTs may readily volatilize from aqueous solution, although only few data are available to support this hypothesis (22,23). This paper reports mass-transfer rates and Henry’s law constants of high molecular weight organic pollutantsincluding hexachlorobenzene, PCB (Aroclor 1242), chlordane, dieldrin, p,p’-DDE, and dibutyl phthalate (DBP). Experiments of volatilization of compounds from aqueous solutions allow a test of the two-resistance diffusion model for high molecular weight organic vapors. Experimental Section Partition coefficients were measured at room temperature (-23 “C)by using a procedure similar to that described by Mackay et al. (24). Small variatipns in room temperature should not significantly affect the test results. Distilled water and seawater used in the experiments were free of interfering organic contaminants. Total organic carbon content of the solutions was not measured. Solutions were spiked with test compounds by first spiking a Teflon-coated stir bar with solvent containing the test compound. After the solvent had evaporated, the bar was placed in the solution and the solution was equilibrated by stirring for 24-48 h. In all cases except one, the initial concentration of test compound was near or below its reported solubility. The spiked solution was purged with water-saturated helium, and an exponential decline in concentration was measured. The purging vessel was a glass cylinder 38 cm X 6 cm. According to equations shown by Mackay (24),a 38-cm purging cylinder will produce equilibrium conditions for the compounds being tested. The effect of using different purging gases was not tested in this study. The concentration decrease in solution was related to the partition coefficient by In (CJCO) = - ( H / R T ) ( G / V ) t (4)
0 1982 American Chemical Society
Envlron. Scl. Technol., Vol. 16, No. 5, 1982 283
where Co = initial concentration of compound (ng/l), C, = concentration (ng/l) of compound at time t (min), G = gas flow rate (L/min), V = volume of solution (L), and H/RT = partition coefficient (see eq 1) (dimensionless). The concentrations were determined by extracting 5-mL aliquots of solution with isooctane and using gas chromatography with an electron-capture detector for quantitation. Use of glass capillary columns allowed concentration changes of individual PCB isomers to be monitored. Reproducibility of the partition-coefficient measurements was 10-20%. Mass-transfer constants were determined by monitoring the volatilization of a compound from a stirred solution. The overall mass transfer coefficient, KL, is obtained from the relationship (5, 10) In (C,/Co) = - ( K L / L ) ~ (5) where L is the height of the liquid in the vessel and the other symbols are the same as in eq 4. Prior to spiking, the solution was purged to remove oxygen. The deoxygenated, spiked solution was placed in a 19-cm diameter by 10-cm deep glass container and stirred with a 7.5-cm magnetic stir bar at 100-200 rpm. Changing the stirring rate altered the magnitude of the exchange coefficients, but the exchange rate of test compounds relative to oxygen remained constant over the range of stirring speeds used. Water depth was typically 4-5 cm. The mass-transfer coefficient of oxygen KLo2 (=kloz) was determined from eq 5, where C now represents the concentration difference between the measured O2 concentration and the equilibrium concentration with the atmosphere. Oxygen concentration was measured with a Yellow Springs Instrument YS154A dissolved oxygen probe. Experiments with natural seawater allowed the direct determination of KGHzo(=kgHz0) by using the procedure described by Liss (12). The flux of water vapor was determined by monitoring the change in salinity over the course of the experiment. F = [pW(S- S o ) / S W ) l L (6) where F = water vapor flux [g/(cm2rnin)], pw = density of seawater (g/cm3),S = salinity at end of experiment (%o), So = initial salinity (%o), At = length of experiment (rnin), and L = height of water in the vessel (cm). Salinity was measured with an inductive salinometer. The seawater used had a salinity of 36%0and was passed through an Amberlite XAD-2/ charcoal filter to remove interfering organic compounds. From the flux, the mass-transfer coefficient KGHzowas calculated from KGHzo= kgHzo = F/(C,, - C,) (7) where C,, = saturation water concentration at the water surface and C, = concentration of water vapor in the laboratory air.
Results and Discussion A. Partition Coefficients. Hexachlorobenzene (HCB), PCB (Aroclor 1242), p,p'-DDE, and a- and y-chlordane had similar partition coefficients: 0.03-0.05 in distilled water and 0.1-0.2 in seawater (Table I). The larger partition coefficient in seawater is expected because of smaller solubility of these organic compounds in salt solution, although only a few solubility data are available for compounds in both distilled water and seawater. The magnitude of the salting-out of PCBs and HCB is in excellent agreement with that predicted from the data of Dexter and Pavlou (25)and our own laboratory data (26). The range of partition coefficients found for these compounds indicates that they should readily volatilize from 284
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Table I. Partition Coefficients ( H I R T )of Organic Compounds in Distilled Water and Seawater (360h0) partition coeff distilled compound water seawater lit valuesa hexachloro0.054 0.07 0.028 benzene PCB (Aroclor 0.032 0.12 0.017-0.044 1242)
p,p'-DDE 0.050 0.15 0.0008-0.099 0.055 0.23 0.001 2-0.0039 7-chlordane a-chlordane 0.036 0.17 0.0012-0.0039 a-hexachloro0.00096 0.00044 0.000 19-0.000 32 hexane dieldrin (0.0044) (0.086) 0.000 02-0.000 3 1 di-n-butyl (0.011) (0.145) 0.000 064 phthalate These values do not a From EPA compilation (3). represent equilibrium partition coefficients; the relatively high values may be related to other processes occurring that will remove dieldrin and DBP from solution. Table 11. Partition Coefficients of PCB Isomers in Distilled Distilled Water and Seawater (3601ao) partition coeff distilled PCB isomer water seawater ratio 2,4' 2,5,2' 2,3,2' 2,5,4' 2,3,3' 2,4,2',6' 2,5,2',5' 2,3,2',5' 3,4,4' 2,3,4,2' 2,5,3',4' 2,3,4,4'
+ 2,3,6,4' + 2,3,6,2'5'
0.039 0.041 0.033 0.038 0.033 0.031 0.038 0.032 0.034 0.035 0.036
0.079 0.11 0.13 0.15 0.16 0.20 0.20 0.22 0.19 0.17 0.16
2.0 2.8 3.8 3.9 4.9 6.2 5.2 6.9 5.5 4.8 4.4
0.034
0.19
5.4
-
water. In terms of the two-resistance diffusion model, these compounds are primarily (>80 %) liquid-phase controlled. Coefficients for the individual PCB isomers tested in this study showed only a slight variation (Table 11). Hexachlorocyclohexane(HCH), dieldrin, and di-n-butyl phthalate (DBP) should not be rapidly purged from solution because of their relatively high solubilities (low partition coefficient). HCH followed its expected behavior. However, dieldrin and DBP were removed from solution more rapidly than expected. Their removal rate in these experiments approached, or exceeded, the volatilization rate of the more volatile HCB, PCBs, DDE, and chlordane. Experiments showed that wall losses could not explain the disappearance of these compounds. Seawater enhanced the removal of these compounds by 10-20 times. This interesting result suggests a process related to interaction at the bubble-water interface. (When purged with a gas, seawater produces smaller bubbles (higher interfacial area) compared to those of distilled water.) However, we are unable to offer a reasonable explanation of this phenomenon here. B. Mass-TransferCoefficients. Experiments with HCB, PCB, DDE, and chlordane yielded rates of volatilization that were 20-30% of the oxygen reaeration rate (Table 111). Dieldrin, a-HCH, and DBP volatilized much more slowly than the compounds listed above. Volatilization rates were on the order of 1-5% of the oxygen reaeration rate. Furthermore, relative volatilization/ reaeration rates were not significantly different between distilled water and seawater. Our studies are in good
Table 111. Average Ratios of Mass-Transfer Coefficients of of Organic ComDounds. KT x . to Mass-Transfer Coefficient of Oxygen, KLdz ( K L % =?1-37 cm/h) KLX/KLoa
compound HCB PCB (1242) p,p'-DDE r-chlordane a-chlordane a-HCH dieldrin DBP
distilled water seawater 0.33 0.21 0.20 0.20 0.14 0.029 0.047 0.0083
0.30 0.28 0.20 0.23 0.14 0.036 0.054 0.0080
overall av 0.30 5 0.05 0.24 0.06 0.20 0.02 0.20 t 0.05 0.14 0.01 0.032 f 0.008 0.050 f 0.013 0.0082 ?: 0.0012
*
* *
agreement with other reported volatilization rates. Paris et al. (22)measured PCB volatilization rates which are in close agreement with our study and the recent work of Slater and Spedding (14) measured KL of dieldrin, which was in the range of values we report. Also, additional experiments performed with carbon tetrachloride compared well with other published results (10). Our results can be interpreted in terms of the Whitman two-resistance mass-transfer model with appropriate corrections for differences in diffusion between oxygen and the organic molecules tested. The basis of the two-resistance model is that the mass-transfer coefficient, kl, can be related to the rate of molecular diffusion across a stagnant film adjacent to the interface. Recalling that the liquid-phase mass-transfer coefficient of oxygen, k1°2, is equal to the total (Le., experimental) mass-transfer coefficient,KLo2,then the transfer coefficient, Kt, for an organic compound is
k? = KL02[D(x)/D(02)]"= KLo2fl"
(8)
where n = 1 according to the resistance model and n approaches 0.5 for the surface-renewalmodel. Similarly, the gas-phase transfer coefficient is
k,' = KG~ZO[D(X)/D(H~O)]" = K ~ ~ ~ ' f 2 " (9) Equations 8 and 9 can be substituted into eq 1 to yield the final result:
(10) ~ / K L "=. l/[KLozffi"]+ ~ / [ K G ~ " ~ ~ " ( H / R T)] where KLx = the overall (experimental) mass-transfer coefficient for organic x. Thus, eq 10 allows a test of the two-resistance model by comparing an experimentally determined KLx to that predicted from the theoretically related quantities-KLO2, K G ~ Z HIRT, O, f 1 , and f 2 . KLo2 is monitored in all experiments; KG~zO is directly measured in experiments with seawater; HIRT, the partition coefficient, was previously determined by purging experiments; fl and f 2 can be estimated from empirical relationships relating molecular diffusivity to molecular structure and composition. Smith et al. (10) have discussed the problem of estimating molecular diffusion coefficients from the molecular weight, molecular diameter, or empirical relationships derived from experimental data. We agree that empirical relationships such as the Hayduk-Laudie correlation or the Othmer-Thakar equation (27) should yield the most consistent and accurate estimates of molecular diffusion. Figure 1 compares the calculated diffusion ratios, D(x)/D(02),to experimentally determined ratios of the total mass-transfer coefficients, KLx/KLoz. For "liquid-phase" controlled compounds, the two-resistance diffusion model predicts that D(x)/D(02)= KLx/KLoz.HCB, PCB, DDE, and chlordane closely follow this relationship. Carbon
"0
2
4
.6
8
10
D ( x 110 (02I
Flgure 1. Comparison of dlffuslon ratios of organic compounds to ratlos of mass-transfer coefficients. Theoretical relationship according to two models of gas exchange is shown by solid line; (0)this work, others are literature data [see Smith et al. ( l o ) ] .
Table IV. Comparison of Measured Mass-Transfer Coefficients in Seawater to Coefficients Predicted from the Two-Layer Diffusion Model (Eq 10 in Text) (KLoz = 13-20 cm/h, kgHZo = 1700-9300 cm/h) diffusion ratioa l / K L , h/cm compound
f,
f,
measd
predicted
0.27 0.23 0.27 0.29 HCB 0.22 0.26 0.25 0.22 PC B 0.21 0.19 0.34 0.34 DDE 0.34 r-chlordane 0.20 0.18 0.40 a-chlordane 0.20 0.18 0.48 0.36 0.25 0.22 2.15 2.48 a-HCH 0.21 0.18 1.44 0.37 (1.51)b dieldrin 0.20 0.19 9.61 0.41 (11.47)' DBP Calculated by using HIRT = 0.001 (ref a See eq 10. Calculated by using H/RT = 0.0001 (ref 3 and un14). published data).
tetrachloride, however, is anomalous; although this result is in agreement with data presented by Smith et al. (10). The compounds a-HCH, DBP, and dieldrin are gas-phase controlled, and their mass-transfer ratios cannot be equated with D(x)/D(0,). A more complete test of the two-resistance model which incorporates both gas and liquid-phase resistances can be obtained by using eq 10. Measured oxygen aeration rates, water-vapor fluxes, and partition coefficients are used to predict the total mass-transfer coefficient of organic compounds. Excellent agreement is obtained for all compounds except dieldrin and DBP (Table IV). These two compounds showed unusual behavior in the purging experiments, which produced anomalously high partition coefficients. From an estimate of HIRT based on literature data for these compounds, better agreement is obtained (see Table IV). The results presented here demonstrate the applicability of the two-resistance model to predicting the air-sea exchange rates of certain high molecular weight organic pollutants. The data suggest that the equation presented by Smith et al. (eq 11) may be valid for compounds of a
Kx/Koz= [ D ( X ) / D ( O ~ ) ] O . ~ ~
(11)
limited molecular weight range or may be applicable to a limited class of compounds. As presently formulated, no single model can accurately rationalize all experimentally Environ. Scl. Technol., Vol. 16, No. 5, 1982 285
determined volatilization rates. Thus, prediction of the volatilization of an organic compound in the environment should be more accurate if based on laboratory determinations of its mass-transferrate rather than on any model of gas exchange. Field validation is even more desirable (16).In addition, other processes such as interaction with bursting bubbles may provide additional mechanisms for interfacial transfer of high-molecularweight organic pollutants.
(11) (12) (13) (14)
Literature Cited
(18) (19) (20) (21)
(1) Giam, C. S.; Atlas, E.; Chan, H. S.; Neff, G. Rev. Int. Oceanogr. Med. 1977,47, 79. (2) Hunter, K. A,; Liss, P. S. Mar. Chem. 1977, 5, 361. (3) Environmental Protection Agency, Water-Related Environmental Fate of 129 Priority Pollutants, 1 & 2, EPA44014-79-029 a & b, 1979. (4) Mackay, D.; Leinonen, P. J. Environ. Sci. Technol. 1975, 9, 1178. (5) Southworth, G. R. Bull. Enuiron. Contam. Toxicol. 1979, 21, 507. (6) Mackay, D.; Yuen, T. K. Water Pollut. Res. J. Can. 1980, 15, 83. (7) Bidleman, T. F.; Rice, C. P.; Olney, C. F. In “Marine Pollutant Transfer”; Windom, H. L., Duce, R., Eds.; Lexington Books: Lexington, MA, 1976; p 323. (8) Atlas, E.; Giam, C. S. Science (Washington,D.C.) 1981,211, 163. (9) Sherwood, T. K.; Pigford, R. L.; Wilke, C. R. “Mass Transfer”; McGraw Hill: New York, 1975. (10) Smith, J. H.; Bomberger, D. C.; Haynes, D. L. Environ. Sci. Technol. 1980,14, 1332.
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(15) (16) (17)
(22) (23) (24) (25) (26) (27)
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Received for review May 18, 1981. Revised manuscript received December 10,1981. Accepted January 25,1982. This work was supported by the National Science Foundation (Grant No. OCE77-12482) and the Robert A. WelchFoundation (GrantNo. A240).
