Literature Cited (1) Stender, J. H., Fed. Regist., 39,3756 (1974). (2) Preliminary Report on US.Production of Selected Synthetic Organic Chemicals, U.S. International Trade Commission, Washington, D.C., March 16,1978. (3) Bowen, B., Anal. Chem., 48,1584-7 (1976). (4) Savitsky, A,, Siggle, S., Anal. Chem., 46, 153-5 (1974). (5) Bowman, M. C., King, J . R., Holder, C. L., Int. J . Enuiron. Anal. Chem., 4,205-23 (1976). (6) El-Dib, M. A., J . Assoc. Off. Anal. Chem., 54,1383-7 (1971). (7) Yasuda, S. K., J . Chromatogr., 104,283-90 (1975). ( 8 ) Ivan. G.. Aiutacu. R.. J . Chromatoer.. 88.391-7 (1974). (9) Jakovijevic, I. M.’, Zynger, J., Bishara, R: H., Anal. Chern., 47, 2045-6 (1975). (10) . , Holder. C. L.. Kine. J. R.. Bowman. M. C.. J . Toxicol. Enuiron. Health, 2,’lll-29 (1576). ’ (11) Mieure, J. P., Dietrich, M. W., J . Chrornatogr, Sci., 11,559-70 (1973). (12) Masuda, Y., Hoffmann, D., Anal. Chem., 41,650-2 (1969).
(13) Fazio, T., Damico, J. N., Howard, J . W., White, R. H., Watts, J. O., J . Agric. Food Chem., 19,250-3 (1971). (14) Fine, D. H., Rounbehler, D. P., Oettinger, P. E., Anal. Chim. Acta, 78,383-9 (1975). (15) Fine, D. H., Ross, R., Rounbehler, D. P., Silvergleid, A., Son, L., J . Agric. Food Chern., 24,1069-71 (1976). (16) Singer, G. M., Lijinsky, W., J. Agric. Food Chem., 24, 550-5 (1975). (17) “Pesticide Analytical Manual”, Vol. 1, Sections 211 and 212, Food and Drug Administration, Washington, D.C., 1973. (18) Stalline. D. L.. Tindle. R. C.. Johnson, J. L., J. Assoc. Off. Anal. Chem , 5g, 28-32 (1972). (19) Diachenko, G. W., Laboratory Information Bulletin No. 1745, Field Sciences Branch. Food and Drug Administration. Rockville, Md., Dec 11, 1974. (20) “1974 Directory of Chemical Producers, United States of America”, Stanford Research Institute, Menlo Park, Calif., 1974. I
Received f o r review June 26, 1978. Accepted October 7, 1978.
Determination of Air-Water Henry’s Law Constants for Hydrophobic Pollutants Donald Mackay’, Wan Ying Shiu, and Russell P. Sutherland Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A4
rn A novel system is described for the determination of Henry’s law constants ( H )for hydrophobic compounds between air and water with an accuracy of about 5%. The method involves measurement of the compound concentration in only the water phase while being stripped isothermally from solution a t a known gas flow rate. Determinations of H were made for benzene, toluene, ethylbenzene, chlorobenzene, naphthalene, biphenyl, and phenanthrene, and agreement with available literature data was satisfactory. Since, if any two of the three quantities H , vapor pressure, and aqueous solubility are known, the third may be calculated, it is suggested that the method may be useful for obtaining accurate solubility and vapor pressure data or for verifying existing data. T h e method may be suitable for elucidating the extent of sorption of volatilizing compounds in aqueous environments and quantifying the role of sorption in reducing volatilization rates. Volatilization from water bodies to the atmosphere is recognized as a significant environmental pathway for solutes such as gases and some hydrophobic organic pollutants such as hydrocarbons and chlorinated hydrocarbons. A knowledge of the Henry’s law constant ( H )is essential in calculating the direction and rate of transfer. If the solute concentrations in the air and water are determined, it immediately gives the direction of the transfer, and in cases where the solute is close to equilibrium an accurate value of H is essential. In most cases, although the direction of transfer is obvious, the distribution of resistance to mass transfer between the air and water phases and hence the overall rate depends on H . The volatilization process is generally accepted as consisting of diffusion of the solute from the bulk of the water to the interface, followed by transfer across the interface, and finally diffusion from the interface to the bulk of the air phase. Measurements of concentration profiles show that most of the diffusive resistance lies a few millimeters above and below the interface and it is believed that the interface itself offers little or no resistance. The diffusive flux in each phase is conventionally expressed as the product of the solute concentration 0013-936X/79/0913-0333$01.00/0 @ 1979 American Chemical Society
difference and a mass transfer coefficient which is essentially a mass conductivity and can be regarded as a diffusivity divided by a diffusion pathlength. Calculating the volatilization rate requires summation of the two phase resistances (which are essentially the reciprocals of the conductivity) and often one phase resistance dominates. This summation requires a knowledge of H , since it is usually assumed that the solute concentrations immediately on either side of the interface are in equilibrium and the magnitude of the concentration driving force which can be achieved in each phase thus depends on H. The relevant equations which have been developed elsewhere are summarized as follows ( 1 ):
N = KoL(C - P/H)
(1)
l / K o L = 1 l K ~-I-R T I H K G
(2)
In these equations N is the mass flux (gmol m-* h-l), K L and K G are the liquid- and gas-phase mass transfer coefficients (m h-l), K O Lis the overall liquid phase mass transfer coefficient (m h-l), H is the Henry’s law constant (atm m3 gmol-l), C is the solute concentration in the liquid phase (g-molm-3), P is the solute partial pressure in the air (atm), T is the absolute temperature (K), and R is the gas constant (m3 atm g-mol-l K-l). The ratio of the resistances in the gas and liquid phases (rGL)can be shown to be: rGL = R T K L I H K G
(3)
Liss and Slater ( 2 ) have suggested that typical environmental values of K L and K G are, respectively, 0.2 m h-’ for 0 2 transfer and 30 m h-l for H20 transfer. Thus the ratio ( K G I K L )is of the order of 150 and should usually lie in the range of 50 to 300. I t is recognized that increasing solute molecular weight decreases K G and K L , probably as a result of a decrease in diffusivity, but the effect on both coefficients is expected to be similar in magnitude; thus the ratio may be relatively constant. The distribution of resistances rGL between the two phases is illustrated in Figure l a t 25 “C as a function of H for various values of this transfer coefficient atm m3 gmol-I ratio. If the value of H is greater than 5 X (which implies a compound of relatively high vapor pressure Volume 13, Number 3, March 1979
333
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Figure 1. Effect of Henry's law constant on ratio of gas to liquid phase resistances rGLfor three KGIKLratios
andlor low solubility), then Equations 1 and 2 reduce to Equation 4 below and the resistance lies almost totally in the liquid phase. If the Henry's law constant is below 5 X atm m3 g-mol-l (which implies a compound of low vapor pressure and/or high solubility) then the equations reduce to Equation 5 below and the resistance lies in the vapor. In the intermediate range the resistance of each phase is significant and calculation of the overall transfer rate depends on the value of H and thus an accurate value is essential. When H > 5 X 10-3 atm m3 g-mol-1
N = KL(C - PIH) When H
99% approach to equilibrium. The mass transfer equation indicates that systems of lower H will approach equilibrium faster. It is relatively simple to test the approach to equilibrium for any system by determining the 336 Environmental Science & Technology
Io
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20
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Figure 5. Effect of humic acid sorbent addition (22.3 mg L-') on naphthalene concentration (as indicated by fluorescence intensity) during volatilization
sensitivity of the experimentally determined H to the liquid depth. The system was first tested using benzene, toluene, and ethylbenzene, substances for which H can be accurately calculated from solubility and vapor pressure data. The experimental and calculated H values given in Table I demonstrate the accuracy of the apparatus. The calculated H values were obtained by dividing the vapor pressure (atm) by the solubility (g-mol m-3). Further runs were performed on systems for which H was unknown and the results of these experiments are also given. For acenaphthene and phenanthrene, the UV absorbance was close to the lower limit a t which accurate measurement is possible, and these results are thus suspect. A more accurate analytical method is needed for compounds with solubilities below 5 g m-3. In all cases, the logarithm of the absorbance-time curve was linear, except during the first few minutes of a run, in which there may have been surface effects or heterogeneous water concentrations. Once steady evaporation was established, no curvature was observed. This linearity indicates that H is constant over the entire concentration range. It should be noted that the theoretical analysis presented earlier assumed that the volume of the solute vapor formed is negligible compared to that of the stripping gas. This is generally valid
except possibly a t the beginning of evaporation of high concentrations of volatile solutes in which the solute vapor volume can be appreciable. This effect results in an initial curvature to the absorbance-time line, and if desired, the equation for the curve can be derived. T o investigate if the system is capable of elucidating if sorption reduces volatilization rate (as is frequently claimed may occur in natural waters), a series of tests was undertaken in which, during the volatilization of naphthalene, a small quantity of concentrated aqueous suspension of fulvic or humic acid was added. A typical result is given in Figure 5. The concentrations were measured in this case by fluorescence, since UV determination proved to be impossible due to interference. Tests showed that interference with fluorescence was negligible. The fluorescence intensity (which is proportional to concentration) of the aqueous solution, measured directly to give the dissolved naphthalene concentration, is shown on the upper lines. The dissolved concentration fell by a factor of 1.44 on addition of 22 mg L-l of humic acid, implying 30% sorption. Equilibrium was apparently reached in 10 min, but it is possible that continuous concentration measurement could yield data on the sorption kinetics. The lower lines illustrate the change in total concentration as measured by liquid extraction of the sample and fluorescence intensity measurement of the extract. The log concentration vs. time curve changes slope to a new value corresponding to an apparent reduction in H by a factor of ( 1 F ) , where F is the ratio of sorbed to dissolved concentration. By measuring the displacement or slope change of these curves, either for dissolved or total concentration, information can be obtained on the extent to which sorption reduces volatilization rate.
+
Discussion The results demonstrate that the gas stripping procedure is an effective nieans of measuring H for aromatic hydrocarbon-water systems. The H values were within 3% of values reported in the literature. In the other systems studied, an acceptable level of precision corresponding to a standard deviation of less than 6% was achieved. The method is believed to be the best available for measuring H of low vapor pressure hydrophobic organic compounds. It has several advantages in that it is fast (five determinations can be made in a 10-h period), it is inherently simple, and it requires measuring only the relative change in concentration (not the absolute concentration) in only one phase. In other systems, the absolute concentrations in both liquid and vapor phases must be determined. The first part of this study was limited to compounds which could be analyzed by UV spectroscopy and only those components with absorbances greater than about 0.2 could be studied. As a result, H values for higher molecular weight hydrocarbons and chlorinated hydrocarbons could not be determined accurately. In the case of acenaphthene, for example, low absorbance readings gave poorly reproducible results. I t is suggested that for such compounds, more appropriate analytical procedures would be either fluorescence analysis or sampling of radiolabeled compounds with measurement of concentration by liquid Scintillation counting. The apparatus can be used to measure solubilities if vapor pressure data are available. This may be the most accurate method of measuring solubility, since there are no problems arising from the presence of dispersed or particulate organic solutes, nor is there any need to prepare a completely saturated solution. The method also permits the determination of vapor pressure data from solubility and H data. For ex-
ample, it is possible to calculate the vapor pressure of the solid hydrocarbons from the data in Table I. Only for naphthalene is solid vapor pressure data available. Although the system does not achieve a complete approach to equilibrium, in most cases the approach is satisfactory and the degree of approach can be measured easily and, if desired, taken into account in calculating H. The humic acid results suggest that the technique may be useful in elucidating the extent to which sorbents reduce volatilization rates. By injecting a known quantity of organic or mineral sorbent, or an electrolyte, or another organic solute, it is possible to measure the change in volatilization rate. Several experimental techniques could be used, including measurement of dissolved, total, or sorbed concentration, or the gas-phase composition of one or more solutes, using absorption, spectroscopy, fluorescence, liquid scintillation counting of radiolabeled solutes, or gas chromatography. Multiple concentration measurement has the advantage of providing mass balance verification. In principle, any technique which yields concentration data can be used to determine the effect of sorption on H, and even elucidate the sorption kinetics. In the case of electrolytes, the “salting out” effect will cause a steepening of the total concentration vs. time curve, corresponding to an increase in H and decrease in solubility. When suspensions of organic or mineral matter are used, it is essential that all parts of the liquid are turbulent to eliminate the possibility of settling in quiescent regions. Loss of a small quantity of the liquid as spray is of little consequence since it does not alter the solute concentration. If samples of the liquid are removed, the liquid volume ( V ) is reduced and the log concentration-time curve may become nonlinear. Allowing for this effect in the calculation of H is straightforward.
Conclusions A technique has been devised, tested, and verified for the measurement of air-water Henry’s law constants for hydrophobic solutes. The method is particularly suitable for low solubility, low vapor pressure solutes such as higher molecular weight hydrocarbons or halogenated hydrocarbons, for which accurate solubility and vapor pressure data may not be available or obtainable. It has been shown that the method has the potential to quantify the effect of sorbents and electrolytes on air-water equilibrium and possibly give some kinetic information on sorption processes. Literature Cited (1) Mackay, D., Leinonen, P. J., Enciron. Sei Technol., 9, 1178
(1975). ( 2 ) Liss, P. S., Slater, P. G., lVature (London),247,181 (1974). ( 3 ) Dilline. U’. L.. Tefertiller. N. B.. Kallos. G. J.. Enuiron. Sei. Techno’i, 9,833 (1975) (4) Mackav. D.. Shiu. W. Y.. J Chem En2 Data. 22.399 (1977). , . (5) McAuliffe, C., J . Phys. Chem., 70, 1267 (1966). (6) Aquan-Yuen, M., Mackay, D., Shiu, W.Y., J . Chem. Eng. Data,
in Dress. (7) Lk’east, R. C., “Handbook of Chemistry and Physics”, 53rd ed, CRC Press, 1972-73. ( 8 ) Zwolinqki. B J . Wilhoit. R. C . “Handhook of V a m r Pressures and Heats ;)f Vaporization of Hydrocarbons and kelated Compounds”. API, 44-TRC Publications in Science and Engineering, 1971.
Receiced ,for reuieu May 16, 1978. Accepted October 12, 1978. The authors are grateful t o Environment Canada, Atmospheric Enuironnient Seruice Inland Waters Directorate, Imperial Oil Limited, and the U S . Entironmental Protection Agency for financial support of this u3ork.
Volume 13, Number 3, March 1979
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