Environ. Sci. Technol. 1990,2 4 , 1332-1338
Atkinson, R.; Aschmann, S. M.; Fitz, D. R.; Winer, A. M.; Pitts, J. N., Jr. Znt. J . Chem. Kinet. 1982, 14, 13-18. (37) Taylor, W. D.; Allston, T. D.; Moscato, M. J.; Fazekas, G. B.; Kozlowski, R.; Takacs, G. A. Znt. J. Chem. Kinet. 1980, (36)
12, 231-240. (38) Cvetanovic, R. J. J . Phys. Chem. Ref. Data 1987, 16, 261-326. (39) Singleton,D. L.; Cvetanovic,R. J. J. Phys. Chem. Ref. Data 1988,17, 1377-1437. (40) Hynes, A. J.; Wine, P. H.; Semmes, D. H. J. Phys. Chem. 1986,90,4148-4156. (41) Barnes, I.; Bastian, V.; Becker, K. H. Znt. J . Chem. Kinet. 1988, 20, 415-431. (42) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J . Phys. Chem. 1983,87, 4978-4981. (43) Atkinson, R.; Pitts, J. N., Jr.; Aschmann, S.M. J . Phys. Chem. 1984,88, 1584-1587. (44) Wallington, T. J.; Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. Znt. J . Chem. Kinet. 1986,18, 837-846. (45) Atkinson, R.; Tuazon, E. C.; Arey, J. Znt. J. Chem. Kinet.,
in press. (46)
(51)
Logan, J. A. J. Geophys. Res. [Atmos.] 1985, 90,
(52)
Mosher, B. W.; Duce, R. A. J . Geophys. Res., C: Oceans
10463-10482.
Atmos. 1983, 88, 6761-6768. (53) Uematsu, M.; Duce, R. A.; Prospero,J. M.; Chen, L.; Menill,
J. T.; McDonald,R. L. J. Geophys. Res., C: Oceans Atmos. 1983,88, 5343-5532. (54) (55) (56) (57) (58) (59) (60) (61)
Martinez, R. I.; Herron, J. T. Znt. J . Chem. Kinet. 1978,
Mayland, H. F.; James, L. F.; Panter, K. E.; Sonderegger, J. L. Selenium in Agriculture and the Environment. SSSA Spec. Publ. 1989, No. 23, 15-50. Mackenzie,F. T.; Lantzy, R. J.; Paterson, V. J. Znt. Assoc. Math. Geol. 1979, I I , 99-142. Prospero, J. M.; Charlson, R. J.; Mohnen, V.; Jaenicke, R.; Delany, A. C.; Moyers, J.; Zoller, W.; Rahn, K. Rev. Geophys. Space Phys. 1983,21, 1607-1629. Arimoto, R.; Duce, R. A.; Ray, B. J.;Unni, C. K. J. Geophys. Res., [Atmos.] 1985, 90, 2391-2408. Ross, H. B. Tellus 1985, 37, 78-90. Lag, J.; Steinnes, E. Geoderma 1978,20, 3-14. Cutter, G. A. Anal. Chim. Acta 1978, 98, 59-66. Suzuki, Y.; Sugimura,Y.; Miyake, Y. J.Meteorol. SOC.Jpn. 1981,59,405-409.
10, 433-452. (47)
Prinn, R.; Cunnold, D.; Rasmussen, R.; Simmonds, P.;
Alyea, F.; Crawford,A.; Fraser, P.; Rosen, R. Science 1987,
238, 945-950. (48) Arey, J.; Atkinson, R.; Zielinska,B.; McElroy, P. A. Environ. Sci. Technol. 1989, 23, 321-327. (49) Platt, U. F.; Winer, A. M.; Biermann, H. W.; Atkinson, R.; Pitts, J. N., Jr. Environ. Sci. Technol. 1984, 18, 365-369. (50) Atkinson, R.; Winer, A. M.; Pitts, J. N., Jr. Atmos. Environ. 1986,20, 331-339.
Received for review December 21, 1989. Revised manuscript received April 4, 1990. Accepted April 16, 1990. We gratefully acknowledge the financial support of this research by the US. Department of the Interior, Bureau of Reclamation, through Cooperative Agreement 0-FC-20-09220. While this research has been funded by the Department of the Interior, it has not been subjected to agency review and therefore does not necessarily reflect the views of the agency and no official endorsement should be inferred.
Effect of Cosolvents on Hydrocarbon Partition Coefficients for Hydrocarbon Mixtures and Water MaJdA. El-Zoobi, Gerard E. Ruch, and Frank R. Groves, Jr.'
Chemical Engineering Department, Louisiana State University, Baton Rouge, Louisiana 70803 This paper presents a method of predicting partition coefficients for components of hydrocarbon mixtures with cosolvents in contact with water. It is shown theoretically that the hydrocarbon partition coefficients correlate with the water-phase cosolvent concentration. The cosolvent concentration in the water phase can in turn be estimated from experimental data or predicted with the aid of the UNIQUAC activity coefficient correlation. The method is confirmed by comparison with literature data for mixtures containing benzene, n-hexane, and cyclohexane with cosolvents ethanol and methanol. The results are useful for transport and fate studies for hydrocarbon spills.
Introduction Leakage of hydrocarbons from underground storage tanks is a matter of current environmental concern. If hydrocarbons contact groundwater, they may contaminate drinking water aquifiers. The interaction between organic materials and groundwater is a complex process involving mass transfer as well as equilibrium considerations. However, hydrocarbon solubility in water is an important physical property involved in this process. The solubility of most hydrocarbons is small but can be increased considerably if a cosolvent, completely miscible with water, is present in the hydrocarbon mixture. Alcohols, including methanol and ethanol, may be added to gasoline as octane enhancers. These cosolvents are therefore particularly important in determining the transport and fate of hy1332
Environ. Sci. Technol., Vol. 24, No. 9, 1990
drocarbon mixtures in the environment. Arbuckle (I, 2) discussed aqueous solubility of hydrophobic materials in a general way and related solubility to thermodynamic activity coefficients. Relatively little work on the effect of hydrophilic cosolvents on the aqueous solubility of hydrophobic solutes has been reported in the literature. Yalkowsky and Roseman (3) suggested that cosolvent effects could be approximated by treating the mixed solvent as a linear combination of its components. Bannerjee (4) measured the effect of benzyl alcohol on the solubility of chlorobenzene, 1,2-dichlorobenzene, and toluene in water at 298 K. The data were correlated with the aid of activity coefficients predicted by the UNIFAC group contribution method. Munz and Roberts (5) studied the effect of cosolvents methanol and 2-propanol on the aqueous-phase activity coefficient for chloroform, carbon tetrachloride, and hexachloroethane. The activity coefficients were derived from measurements of the Henry's law constant but could be used to compute solubility. The data were compared to UNIFAC predictions in the dilute concentration range. It was found that UNIFAC overpredicted the cosolvent effects. Because of the approximate nature of the UNIFAC method, deviations of its predictions from the experimental data were to be expected. The objective of this paper is to present a quick approximate method for determining hydrocarbon concentrations in water in equilibrium with hydrocarbon mixtures containing cosolvents. This hydrocarbon-water interaction
0013-936X/90/0924-1332$02.50/0
0 1990 American Chemical Society
coefficients. For the special case of a pure h drocarbon in equilibrium with water, xiHC 1.0 and yiHB= 1.0, and
COSOLVENT
=
KHC
= xHCW = l / y H C w
(5)
Hydrocarbon solubilities are small even in the presence of moderate amounts of cosolvents. Hence, as an approximation we now treat the aqueous phase as a watercosolvent mixture, infinitely dilute in hydrocarbons. With this approximation, eq 4 can be rearranged as follows:
HYDROCARBON
WATER
Flgure 1. Schematic diagram of ternary equilibrium data.
is conveniently described by means of equilibriumpartition coefficients for the components of the mixture:
Ki = XiW/XiHC
(1)
where xi is the mole fraction of component i and the superscripts W and HC refer to water and hydrocarbon phases, respectively. We first show that hydrocarbon partition coefficients for components of mixtures can be obtained easily from experimental data on single hydrocarbon-cosolvent mixtures. The method is confirmed by using data on solutions of benzene, n-hexane, and cyclohexane with methanol and ethanol as cosolvents. The key variable in determining the hydrocarbon partition coefficients is the cosolvent mole fraction in the aqueous phase. This variable can be conveniently obtained from the cosolvent mole fraction in the hydrocarbon phase provided that the partition coefficient for the cosolvent is known. The next section of the paper presents experimental data on the methanol and ethanol partition coefficients. Finally, we describe a simple method for predicting the cosolvent partition coefficient based on the UNIQUAC activity coefficient correlation. The overall result is a method of predicting the partition coefficient of a component of a hydrocarbon mixture of known composition. Theoretical Treatment of the Hydrocarbon Partition Coefficient Equilibrium relations for hydrocarbon-cosolvent-water mixtures are conveniently represented on a triangular diagram as in Figure 1. A system with overall composition in the two-phase region, point A, separates into a hydrocarbon phase, point B, and an equilibrium water phase, point C, connected by a tie line, BAC. In this paper we assume that the hydrocarbon-phase composition is known and want to predict the water-phase composition, point C, more particularly the hydrocarbon mole fraction in the water phase. The basic condition for equilibrium is the equality of the fugacities of the components in the hydrocarbon and water phases. fiHC
= fiW
(2)
In terms of mole fractions, xi, and activity coefficients, yi, this can be written yiHCxiHC
= yiwxiW
(3)
It follows then that the partition coefficient for a component is given by K i = xiW/xiHC = yiHC/yiW (4) Thus, the partition coefficients are governed by the activity
KHC/YHC~‘ = ~/(YHC~)(6) where (yHCw)-is the activity coefficient of hydrocarbon at infinite dilution in the water phase. This equation has interesting implications. The ratio, KHC/yHCHC depends only on the activity coefficient of the hydrocarbon in the water phase and therefore only on the mole fraction of cosolvent in the water phase. A plot of this ratio for a particular hydrocarbon, e.g., benzene, for a variety of hydrocarbon mixtures vs water-phase cosolvent mole fraction should give a single line. The ratio K H C / yHCHC for benzene for a given water-phase cosolvent concentration should be the same whether the hydrocarbon phase contains only benzene or a mixture of benzene with other hydrocarbons. The form of the graph of KHC/yHCHC vs xcsw can be investigated by using an activity coefficient correlation equation to predict (yHCw)”. The Margules equation is a convenient correlation for this purpose since it expresses the activity coefficient as a power series in component mole fractions. Approximate results are easily obtained by neglecting higher powers of small mole fractions. The Margules equations for a ternary mixture of hydrocarbon (component l ) , water (component 2), and cosolvent (component 3) can be written as follows (6): In y1 = ~ 2 ~ [ A 1+2 2x1 6 4 2 1 - A d 1 + ~ ? [ A i 3+ 2x1 (A31 - AiJ1 + x2~3[(A21+ A12 + A31+ A13 - A23 - A32)/2 + xl(A21 A12 + A31 - A131 + (x2 - ~ g ) ( A 2 3- A32) - (1 - 2~1)C*] (7)
For infinite dilution of hydrocarbon (xl = 0) this reduces to In -ylm = A12x22+ A12~3’+ C
+
1 ~ 2 ~ 3C 2 ~ 2 ~ 3 ’
(8)
where we have also neglected terms in x33. In this expression the constants are C1 = (A21 A12 + A31 + A13 - A23 - A32)/2 - C*
+
+
(A23
- A32)
and
c, = -2(A23 - A321
(9)
After setting x2 = (1 - x3) we obtain In ylm= A12 + (Cl - 2ApJx3
+ (A13 + A12 - C1+ C2)x32
(10) where we have again neglected the terms in x:. Now the coefficients of x3 and x: in eq 10 are of the same order of magnitude; so for x3 small (say
