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Evaporation of phenols from synthetic liquid fuel spills
Environment International, Vol. 7, pp. 203-206, 1982 Printed in the USA.
0160-4120/82/030203-04503.00/0 1982 Pergamon Press Ltd.
EVAPORATION OF PHENOLS FROM SYNTHETIC LIQUID FUEL SPILLS G. R. Southworth Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee37830, USA
(Received 30 July 1981;Accepted 6 November 1981) The environmental impact of a synthetic-fuel spill on a waterway will be determined in part by the rate of dissolution of toxic components into the underlying water and by the rate of removal of those components from the oil by evaporation. In this study, the two film mass transfer model of volatilization was applied to the study of the evaporation of phenols from two synthetic liquid fuels. The rates of evaporation of phenol and alkylated phenols were measured under laboratory conditions. Liquid and gaseous mass transfer coefficients were measured experimentally, and used to calculate Henry's Law coefficients for various phenol-oil pairs from the observed evaporation rates. It was concluded that evaporation rates of phenols from synthetic oils would be significantly less than rates of dissolution in most spills.
Introduction
role of volatilization in reducing the toxic hazard of spills.
Transportation-related spills into inland waters are likely to occur as a commercial synthetic liquid fuels industry is developed and products are shipped via barge, rail, and pipeline. Water soluble fractions of synthetic liquid fuels have been demonstrated to have much higher acute toxicity to aquatic organisms than water soluble fractions of petroleum and its distillate and residual oils (Giddings et al., 1980; Giddings, 1979; Giddings and Washington, 1981). Phenols are the quantitatively dominant class o f c o m p o u n d s in the water soluble fractions of synthetic oils (Giddings et al., 1980) and undoubtedly account for much of the observed acute toxicity. Phenols are m a j o r constituents of most coalderived liquid fuels (Guerin et al., 1981). The environmental impact o f a synthetic-fuel spill on a waterbody will be determined in part by the rate of dissolution of toxic components into the underlying water, and by the rate o f removal of those components from the oil by evaporation (Jordan and Payne, 1980). The light aromatics (naphthalenes, benzenes) associated with much of the acute toxicity of petroleum products (Anderson et al., 1974) evaporate much more rapidly than they dissolve (Harrison et al., 1975; McAuliffe, 1977), and thus volatilization acts to ameliorate toxic effects of spills. The objective of this study was to investigate the rates of evaporation of various phenols f r o m two synthetic liquid fuels in an effort to ascertain the
Methods
The volatilization process was described using the mass transfer expression used previously by Liss and others (Liss, 1973, Mackay and Leinonen, 1975; Southworth, 1979). The mass flux o f a contaminant ( A ) across the oil-air interface is thus characterized by the expression: mass flux A (mol/cm 2 h)
Hkgkl Hkg + k l
( [ A ] t - [AIg/H),
(1)
where k, and k, are gas- and liquid-phase mass transfer coefficients (cm h-'), respectively, H is the Henry's law coefficient (equilibrium M concentration ratio of A in gas/liquid), and [A], and [A], are M concentrations of A in each phase. Two key assumptions made in this study were that Henry's Law was obeyed and that the v a p o r pressure of individual phenols above a solution of phenol in synthetic oil varied as a linear function of M concentration in the oil. (Henry's Law is obeyed when vapor pressure 203
204 of compound A varies as a linear function of mole-fraction A in the liquid phase. At low solute concentrations, mole fraction and molarity approximate a linear relationship, and the second assumption is valid). The concentration of phenols in the atmosphere ([A]g) can be assumed to be zero under the experimental conditions employed, and the mass flux then becomes a direct proportion of the phenol concentration in the oil. The evaporation of phenol, methyl phenols (C1phenols), dimethyl and ethyl phenols (C2-phenols), and trimethyl, methylethyl, and propyl phenols (C3-phenols) from dodecane (an alipathic hydrocarbon of low volatility similar in viscosity to the synthetic fuels) and two coal-derived synthetic liquid fuels was investigated by placing 30 mL of oil in a pyrex culture dish (13.5-cm diameter × 2 cm deep) in a laboratory hood at a fixed sash setting (wind velocity - 1 m/sec at 5 cm above the oil surface) and periodically determining the concentration of phenols remaining. Samples were taken at 0, 2, 4, and 23 h for synfuel #1 and 0, 2, 4, 6, 24, and 48 h for synfuel #2. Evaporation of phenols from dodecane was carried out by adding approximately 5-10 mg/L each of phenol, 2-methylphenol, 3,5-dimethylphenol, and 2,4,6trimethylphenol separately to dodecane and monitoring the disappearance of each by UV spectrophotometry. Phenols in the synthetic fuels were analyzed by gas chromatography on an 1.8 m × 6 mm packed glass column (3.6o7o Dexsil 400 mesh on Supelcopore 100-200), using a Perkin Elmer model 3920 B gas chromatograph with flame ionization detector, 110 to 250 °C at 4°/min programmed temperature. Phenols were isolated from the oil by placing 100 #L synthetic oil in l0 mL hexane and extracting twice with 5 mL 0.2 N NaOH. The NaOH solutions were combined, acidified to pH 1-2 with HC1, and extracted twice with 5 mL ethyl ether. The ether extracts were combined and adjusted to 10 mL volume. A 5 /~L sample of ether was then injected into the gas chromatograph. Individual isomers were not resolved on this column, but isomers were separated into groups having similar boiling points. For example, the methyl phenols were separated into two peaks, one containing 2-methylphenol and another containing 3-methylphenol and 4-methylphenol. When the studies using known compounds (dodecane experiments) are compared with data from the synthetic fuels, the known compounds are compared with gas chromatographic peaks that contain those isomers. The mass transfer coefficients characterizing the system were estimated under conditions of liquid or gas phase control of the volatilization process (Southworth, 1979). The gas phase mass transfer coefficient for the system (ke) was determined from the rate of evaporation of dodecane at 23 °C, using the literature value of 2.0 #g/L for the equilibrium vapor concentration of dodecane in air at 23 °C, 1.0313 x 105 Pa (1 atm) (Weast, 1965). The liquid-phase mass transfer coefficients were estimated by measuring the rate of volatilization of
G.R. Southworth ~'CO2 from the three oils under the test conditions. x4CO2 was added to the oils by placing 50 mL oil in a closed bottle with 100 mL of acidified NaHCO3 solution for 24 h. '4CO2 remaining in the oil as a function of time was measured by periodically withdrawing 50 #L samples of oil and counting 14C by liquid scintillation spectrometry. Volatilization of '4CO2 was assumed to be completely liquid-phase controlled and thus to provide a direct measure of k, The evaporation rates of phenols were analyzed as a first-order exponential decay, [Air = [A], e-kt,
(2)
where [A]i and [A], are the initial and final concentrations of individual phenols in the oil over time t, and k is a first-order rate constant. The concentration in the oil at time t is multiplied by the ratio of oil volume at time t to initial oil volume to adjust for evaporationof the volatile oil components. Oil volumes were estimated from the measured mass of oil remaining as a function of time and the measured change in density of the oil over a 24-h period. The calculated density of the oil remaining at any time was used to convert the oil mass measures to volume, assuming a linear change in density with time. Estimates of k obtained from successive time intervals were in good agreement. The first-order decay constant (k) is related to the overall mass transfer coefficient [HK~k~/(Hk~ + kt)], kd = Hk, k,/(Hk, + k~),
(3)
where d is the depth of the oil layer (cm). Estimates of k and the mean oil depth over the period of evaporation were inserted into Eq. (3) to obtain an estimate of the overall mass transfer coefficient. In many cases, phenols did not evaporate faster than the oil, and little or no decrease in concentration was noted. In such instances the overall mass transfer coefficient was also estimated directly from Eq. (1), using the measured mass flux and the mean observed concentration of individual phenols over the duration of the experiment. This procedure produced virtually identical estimates to those obtained using the exponential decay procedure. The estimates of the Henry's Law coefficient were calculated from the overall mass transfer coefficients, using estimates of ks obtained from the rate of dodecane evaporation (adjusted for molecular weight) and of k, obtained from 1'CO2 evaporation from the three oils tested.
Results Phenols constituted a significant fraction of each of the synthetic fuels examined. Synfuel #2 contained 25 g/L phenol, 60 g/L Ct-phenols, 34 g/L C2-phenols, and
E v a p o r a t i o n of phenols from synthetic-fuel spills
205
17 g/L C3-phenols. Synfuel #1 contained considerably less; 9 g/L phenol, 19 g/L Cl-phenols, 9 g/L C2phenols, and 4 g/L Crphenols. The evaporation of volatile oil constituents removed about 25%o of the initial volume of synfuel #1 in the first 2 h, increasing the concentrations of all phenols. Thereafter, phenols and Cl-phenols decreased in concentration, while C2-phenols and Crphenols changed little in concentration. After 24 h, about 50%o of the oil volume had evaporated. Synfuel #2 contained less highly volatile constituents, and thus no rapid increase in phenol concentrations was noted. Phenol and Cl-phenols decreased in concentration over 24 h of evaporation, while C2-phenols and C3-phenols decreased very little or increased in concentration. Approximately 20% of the oil evaporated in 24 h. Phenols evaporated 10 to 40 times faster from dodecane than from the synthetic fuels (Table 1). Their evaporation rate from synfuel #1 was about twice that from synfuel #2, As would be expected, the volatility of the various phenols in any given oil was ordered according to boiling points, with the higher boiling, multimethylated phenols evaporating much more slowly from the oils than C,-phenols and phenol. Nevertheless, evaporation rates for the phenols were all relatively low. The calculated half-life for evaporation of phenol from a 2-mm thick film of synfuel #1 under the experimental conditions is 7.7 h, assuming no change in volume of the oil. C3-phenols would have a half-life of 173 h under the same conditions. The gas-phase mass transfer coefficient, k,, estimated from dodecane evaporation, was 834 cm h -1 (normalized to mol. wt. 100, since k , ~ 1 mol. wt. (Liss and Slater, 1975). The liquid-phase mass transfer coefficient, kt, varied with individual oils, with values of 4.6, 3.0, and 1.2 cm h -~ measured for dodecane, synfuel #1, and synfuel #2, respectively. Such variation was anticipated as a result of the variation in viscosities of the three oils. The individual mass transfer coefficients were combined with the estimates of the overall mass transfer coefficients (Table 1) to calculate the Henry's Law coefficients for phenols in the various oils (Table 2). The observed evaporation rates of phenols among the three Table 1. Overall mass transfer coefficients [Hk,/(Hk, + k~)] for the evaporation of phenols from dodecane and synthetic liquid fuels at 23 °C, 0.2-cm oil depth, and - 1 m / s e c wind velocity. Mass flux phenols ( m o l / c m 2 h) = (phenol concentration) x [Hk, k/(Hk~ + k,)]. Units of the overall mass transfer coefficient are cm h-'. Dodecane
Synfuel # 1
Synfuel #2
Phenol Ci-phenol*
3.6 x 10-' 1.1 x 10-1
C2-phenols*
3.2 x 10-2
C3-phenols
8.8 x 10-3
!.8 1.2 5.2 3.3 2.0 9
8.4 5.4 2.8 1.9 1.1 6
x x X x x x
l0 -2 l0 "2 10-3 10-3 10-3 10 "4
x x x x x x
10-3 10 -3 10-3 10 -3 10-3 10-4
*Double entries indicate individual isomers or groups of isomers having similar gas chro mato graphic retention times.
Table 2. Henry's Law ( H ) coefficients for phenols in dodecane and synthetic oils at 23 °C, estimated from evaporation rates and mass transfer coefficients. Units of H are molar concentration ratio (dimensionless). Dodecane
Synfuel #1
Synfuel #2
Phenol C~-phenol*
4.6 x 10-4 1.4 x 10-4
C2-phenols*
4.3 x 10 -5
C~-phenols
1.2 x l0 -s
2.1 1.5 6.7 4.4 2.7 1.1
1.0 6.8 3.5 2.5 1.4 8
x x x x x x
10 -s 10"s 10-* 10-6 10-6 10-6
x × x × x ×
10-s 10-6 l0 -6 l0 -6 l0 -~ l0 -~
*Double entries indicate separate isomers or groups of isomers having similar gas chromatographic retention times.
oils was found to be almost entirely due to variation in H, with the variability in k, (liquid-phase transport) having little impact on volatilization rate at such low rates of evaporation. The variation of H with degree of methylation of the phenols and oil type is apparent in Table 2. Phenols apparently have much higher activity coefficients when dissolved in an alkane than in a more aromatic material such as the synthetic fuels, as evidenced by the 10- to 20-fold higher value of H in dodecane. The twofold variation in H between synfuels #1 and #2 may reflect a higher activity coefficient for phenols in synfuel #1, which is more highly hydrogenated and probably more aliphatic in character than synfuel #2. The effect of increasing alkylation on H is dramatic, with Ca-phenols having Henry's Law coefficients about 20 times less than phenol. The observed variation in H i s slightly greater than the variation in equilibrium vapor pressures of the compounds at 25 °C.
Discussion Evaporation of toxic compounds from spills is a process that competes with dissolution to reduce the hazard of spills to aquatic biota (MacAuliffe, 1977). The evaporation rates observed in this study must be extrapolated to realistic field conditions in order to predict the likely rate of evaporation of phenols from a spill. This can be done by using Eq. (1) to describe the rate of evaporation and substituting values of k8 and k, that typify field conditions and values of H obtained in this study. Prior studies have assumed typical values of k, and k~ to be 3000 cm h -1 and 20 cm h -1, respectively, over open water (Liss and Slater, 1974; Dilling, 1977; Mackay and Leinonen, 1975). Since k, varies primarily as a function of wind velocity and turbulence, values over water should differ little from values over oil. Values of kt estimated from 14CO2 evaporation from oils were similar to values observed in water under similar conditions (Southworth, 1979). Literature based estimates of mass transfer coefficients for aqueous systems
206
thus seemed to be reasonable estimates for field conditions. If k, is assumed to be 20 cm h -1 (a value observed in water at 5-10 m s -1 wind, Cohen et al., 1978), and k, is assumed to be 8000 cm h -~ (a value ten times that observed in this study at - 1 m s -~ wind velocity, and somewhat above that predicted at 10 m s -1 wind for a compound of mol. wt. 100 from Liss, 1973), an overall mass transfer coefficient for phenol, in synfuel #1 of 0.17 cm h -1 is calculated. This corresponds to a half-life of 0.8 h in a film 2 mm in depth, under vigorous conditions of wind mixing. Observations in our laboratory of the dissolution of phenol from synfuel #1 (S. E. Herbes, personal communication) under gentle mixing conditions yielded a dissolution rate about twice this evaporation rate. Vigorous wind mixing would undoubtedly increase dissolution as well as evaporation, making it safe to assume that evaporation of phenol is unlikely to exceed dissolution under any reasonable set of environmental conditions, and would probably be much less in most cases. Efforts are underway in our laboratory to apply a mass transfer equation similar to the evaporation expression to the process of dissolution, which will permit a more direct comparison of dissolution and evaporation rates for the phenol series. The evaporation of volatile components of the synthetic oil itself acted to increase the concentrations of phenols despite their simultaneous loss to the atmosphere. In an accidental spill, evaporation of the oil would act to increase the rate of dissolution of the phenols, despite reducing the total amount eventually dissolving into the water. The large variation in evaporation rates of the same compound among the three oils illustrates the wide variation in activity coefficients possible as a function of oil composition. An assumption often made in evaporation or dissolution studies is that the activity coefficient is unity, and the solution thus is ideal with respect to Raoult's Law. While such an assumption is approximately valid for phenols in the two crude synfuels studied, it may be inappropriate to apply to more highly hydrotreated synthetics. In conclusion, it appears that although evaporation of phenols is a significant process with respect to the fate of those compounds, it is not rapid enough to pre-
G.R. Southworth
vent substantial dissolution from occurring in aqueous spills of synthetic liquid fuels. Acknowledgments--I would like to thank S. E. Herbes and M. B. Browman for helpful suggestions on the manuscript. Research sponsored by the Office of Health and Environmental Research, U.S. Department of Energy, under contract W-7405-eng-26 with Union Carbide Corporation. Publication No. 1874, Environmental Sciences Division, ORNL.
References Anderson, J. W., Neff, J. M., Cox, B. A., Tatum, H. E., and Hightower, G. M. (1974) Characteristics of dispersions and watersoluble extracts of crude and refined oils and their toxicity to estuarine crustaceans and fish, Mar. Biol. 27, 75-78. Cohen, Y., Cocchio, W., and Mackay, D. (1978) Laboratory study of liquid-phase controlled volatilization rates in presence of wind waves, Environ. Sci. Technol. 12, 553-558. Dilling, W. L. (1977) Interphase transfer processes. II. Evaporation rates of chloro methanes, ethanes, ethylenes, propanes, and propylenes from dilute aqueous solutions. Comparisons with theoretical predictions, Environ. Sci. Technol. 11,405-409. Giddings, J. M. (1980) Acute toxicity to Selenastrum capricornutum of aromatic compounds from coal conversion, Bull. Environ. Contam. Toxicol. 23, 360-364. Giddings, J. M., Parkhurst, B. R., Gehrs, C. W., and Millemann, R. E. (1980) Toxicity of a coal liquefaction product to aquatic organisms, Bull. Environ. Contam. Toxicol. 25, 1-6. Giddings, J. M. and Washington, J. N. (1981) coal liquefaction products, shale oil, and petroleum: Acute toxicity to freshwater algae, Environ. Sci. TechnoL 15, 106-108. Guerin, M. R., Rubin, I. B., Rao, T. K., Clark, B. R., and Epler, J. L. (1981) Distribution of mutagenic activity in petroleum and petroleum substitutes, Fuel 60, 282-288. Harrison, W., Winnik, M. A., Kwong, P. T. Y. and Mackay, D. (1975) Environ. Sci. TechnoL 9, 231-234. Jordan, R. E. and Payne, J. R. (1980) Fate and Weathering o f Petroleum Spills in the Marine Environment Ann Arbor Science Publishers, Inc., Ann Arbor, MI. Liss, P. S. (1973) Processes of gas exchange across an air-water interface, Deep Sea Res. 20, 221-238. Liss, P. S. and P. G. Slater (1974) Fluxes of gases across the air-sea interface, Nature 247, 181-184. Mackay, D., and Leinonen, P. J. (1975) Rate of evaporation of low solubility contaminanta from water bodies to atmosphere, Environ. Sci. Technol. 9, 1178-1180. McAuliffe, C. D. (1977) Evaporation and solution of C~ to C1o hydrocarbons from crude oils on the sea surface, Fate and Effects o f Petroleum Hydrocarbons in Marine Organisms and Ecosystems, D. A. Wolfe, ed., pp. 363-373. Pergamon Press, New York, NY. Southworth, G. R. (1979) The role of volatilization in removing polycyclic aromatic hydrocarbons from aquatic environments, Bull. Environ. Contam. Toxicol. 21, 507-515. Weast, R. C. (1965) Handbook o f Chemistry and Physics, 46th ed., p. D-118. Chemical Rubber Co., Cleveland, OH.
0160-4120/82/030203-04503.00/0 1982 Pergamon Press Ltd.
EVAPORATION OF PHENOLS FROM SYNTHETIC LIQUID FUEL SPILLS G. R. Southworth Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee37830, USA
(Received 30 July 1981;Accepted 6 November 1981) The environmental impact of a synthetic-fuel spill on a waterway will be determined in part by the rate of dissolution of toxic components into the underlying water and by the rate of removal of those components from the oil by evaporation. In this study, the two film mass transfer model of volatilization was applied to the study of the evaporation of phenols from two synthetic liquid fuels. The rates of evaporation of phenol and alkylated phenols were measured under laboratory conditions. Liquid and gaseous mass transfer coefficients were measured experimentally, and used to calculate Henry's Law coefficients for various phenol-oil pairs from the observed evaporation rates. It was concluded that evaporation rates of phenols from synthetic oils would be significantly less than rates of dissolution in most spills.
Introduction
role of volatilization in reducing the toxic hazard of spills.
Transportation-related spills into inland waters are likely to occur as a commercial synthetic liquid fuels industry is developed and products are shipped via barge, rail, and pipeline. Water soluble fractions of synthetic liquid fuels have been demonstrated to have much higher acute toxicity to aquatic organisms than water soluble fractions of petroleum and its distillate and residual oils (Giddings et al., 1980; Giddings, 1979; Giddings and Washington, 1981). Phenols are the quantitatively dominant class o f c o m p o u n d s in the water soluble fractions of synthetic oils (Giddings et al., 1980) and undoubtedly account for much of the observed acute toxicity. Phenols are m a j o r constituents of most coalderived liquid fuels (Guerin et al., 1981). The environmental impact o f a synthetic-fuel spill on a waterbody will be determined in part by the rate of dissolution of toxic components into the underlying water, and by the rate o f removal of those components from the oil by evaporation (Jordan and Payne, 1980). The light aromatics (naphthalenes, benzenes) associated with much of the acute toxicity of petroleum products (Anderson et al., 1974) evaporate much more rapidly than they dissolve (Harrison et al., 1975; McAuliffe, 1977), and thus volatilization acts to ameliorate toxic effects of spills. The objective of this study was to investigate the rates of evaporation of various phenols f r o m two synthetic liquid fuels in an effort to ascertain the
Methods
The volatilization process was described using the mass transfer expression used previously by Liss and others (Liss, 1973, Mackay and Leinonen, 1975; Southworth, 1979). The mass flux o f a contaminant ( A ) across the oil-air interface is thus characterized by the expression: mass flux A (mol/cm 2 h)
Hkgkl Hkg + k l
( [ A ] t - [AIg/H),
(1)
where k, and k, are gas- and liquid-phase mass transfer coefficients (cm h-'), respectively, H is the Henry's law coefficient (equilibrium M concentration ratio of A in gas/liquid), and [A], and [A], are M concentrations of A in each phase. Two key assumptions made in this study were that Henry's Law was obeyed and that the v a p o r pressure of individual phenols above a solution of phenol in synthetic oil varied as a linear function of M concentration in the oil. (Henry's Law is obeyed when vapor pressure 203
204 of compound A varies as a linear function of mole-fraction A in the liquid phase. At low solute concentrations, mole fraction and molarity approximate a linear relationship, and the second assumption is valid). The concentration of phenols in the atmosphere ([A]g) can be assumed to be zero under the experimental conditions employed, and the mass flux then becomes a direct proportion of the phenol concentration in the oil. The evaporation of phenol, methyl phenols (C1phenols), dimethyl and ethyl phenols (C2-phenols), and trimethyl, methylethyl, and propyl phenols (C3-phenols) from dodecane (an alipathic hydrocarbon of low volatility similar in viscosity to the synthetic fuels) and two coal-derived synthetic liquid fuels was investigated by placing 30 mL of oil in a pyrex culture dish (13.5-cm diameter × 2 cm deep) in a laboratory hood at a fixed sash setting (wind velocity - 1 m/sec at 5 cm above the oil surface) and periodically determining the concentration of phenols remaining. Samples were taken at 0, 2, 4, and 23 h for synfuel #1 and 0, 2, 4, 6, 24, and 48 h for synfuel #2. Evaporation of phenols from dodecane was carried out by adding approximately 5-10 mg/L each of phenol, 2-methylphenol, 3,5-dimethylphenol, and 2,4,6trimethylphenol separately to dodecane and monitoring the disappearance of each by UV spectrophotometry. Phenols in the synthetic fuels were analyzed by gas chromatography on an 1.8 m × 6 mm packed glass column (3.6o7o Dexsil 400 mesh on Supelcopore 100-200), using a Perkin Elmer model 3920 B gas chromatograph with flame ionization detector, 110 to 250 °C at 4°/min programmed temperature. Phenols were isolated from the oil by placing 100 #L synthetic oil in l0 mL hexane and extracting twice with 5 mL 0.2 N NaOH. The NaOH solutions were combined, acidified to pH 1-2 with HC1, and extracted twice with 5 mL ethyl ether. The ether extracts were combined and adjusted to 10 mL volume. A 5 /~L sample of ether was then injected into the gas chromatograph. Individual isomers were not resolved on this column, but isomers were separated into groups having similar boiling points. For example, the methyl phenols were separated into two peaks, one containing 2-methylphenol and another containing 3-methylphenol and 4-methylphenol. When the studies using known compounds (dodecane experiments) are compared with data from the synthetic fuels, the known compounds are compared with gas chromatographic peaks that contain those isomers. The mass transfer coefficients characterizing the system were estimated under conditions of liquid or gas phase control of the volatilization process (Southworth, 1979). The gas phase mass transfer coefficient for the system (ke) was determined from the rate of evaporation of dodecane at 23 °C, using the literature value of 2.0 #g/L for the equilibrium vapor concentration of dodecane in air at 23 °C, 1.0313 x 105 Pa (1 atm) (Weast, 1965). The liquid-phase mass transfer coefficients were estimated by measuring the rate of volatilization of
G.R. Southworth ~'CO2 from the three oils under the test conditions. x4CO2 was added to the oils by placing 50 mL oil in a closed bottle with 100 mL of acidified NaHCO3 solution for 24 h. '4CO2 remaining in the oil as a function of time was measured by periodically withdrawing 50 #L samples of oil and counting 14C by liquid scintillation spectrometry. Volatilization of '4CO2 was assumed to be completely liquid-phase controlled and thus to provide a direct measure of k, The evaporation rates of phenols were analyzed as a first-order exponential decay, [Air = [A], e-kt,
(2)
where [A]i and [A], are the initial and final concentrations of individual phenols in the oil over time t, and k is a first-order rate constant. The concentration in the oil at time t is multiplied by the ratio of oil volume at time t to initial oil volume to adjust for evaporationof the volatile oil components. Oil volumes were estimated from the measured mass of oil remaining as a function of time and the measured change in density of the oil over a 24-h period. The calculated density of the oil remaining at any time was used to convert the oil mass measures to volume, assuming a linear change in density with time. Estimates of k obtained from successive time intervals were in good agreement. The first-order decay constant (k) is related to the overall mass transfer coefficient [HK~k~/(Hk~ + kt)], kd = Hk, k,/(Hk, + k~),
(3)
where d is the depth of the oil layer (cm). Estimates of k and the mean oil depth over the period of evaporation were inserted into Eq. (3) to obtain an estimate of the overall mass transfer coefficient. In many cases, phenols did not evaporate faster than the oil, and little or no decrease in concentration was noted. In such instances the overall mass transfer coefficient was also estimated directly from Eq. (1), using the measured mass flux and the mean observed concentration of individual phenols over the duration of the experiment. This procedure produced virtually identical estimates to those obtained using the exponential decay procedure. The estimates of the Henry's Law coefficient were calculated from the overall mass transfer coefficients, using estimates of ks obtained from the rate of dodecane evaporation (adjusted for molecular weight) and of k, obtained from 1'CO2 evaporation from the three oils tested.
Results Phenols constituted a significant fraction of each of the synthetic fuels examined. Synfuel #2 contained 25 g/L phenol, 60 g/L Ct-phenols, 34 g/L C2-phenols, and
E v a p o r a t i o n of phenols from synthetic-fuel spills
205
17 g/L C3-phenols. Synfuel #1 contained considerably less; 9 g/L phenol, 19 g/L Cl-phenols, 9 g/L C2phenols, and 4 g/L Crphenols. The evaporation of volatile oil constituents removed about 25%o of the initial volume of synfuel #1 in the first 2 h, increasing the concentrations of all phenols. Thereafter, phenols and Cl-phenols decreased in concentration, while C2-phenols and Crphenols changed little in concentration. After 24 h, about 50%o of the oil volume had evaporated. Synfuel #2 contained less highly volatile constituents, and thus no rapid increase in phenol concentrations was noted. Phenol and Cl-phenols decreased in concentration over 24 h of evaporation, while C2-phenols and C3-phenols decreased very little or increased in concentration. Approximately 20% of the oil evaporated in 24 h. Phenols evaporated 10 to 40 times faster from dodecane than from the synthetic fuels (Table 1). Their evaporation rate from synfuel #1 was about twice that from synfuel #2, As would be expected, the volatility of the various phenols in any given oil was ordered according to boiling points, with the higher boiling, multimethylated phenols evaporating much more slowly from the oils than C,-phenols and phenol. Nevertheless, evaporation rates for the phenols were all relatively low. The calculated half-life for evaporation of phenol from a 2-mm thick film of synfuel #1 under the experimental conditions is 7.7 h, assuming no change in volume of the oil. C3-phenols would have a half-life of 173 h under the same conditions. The gas-phase mass transfer coefficient, k,, estimated from dodecane evaporation, was 834 cm h -1 (normalized to mol. wt. 100, since k , ~ 1 mol. wt. (Liss and Slater, 1975). The liquid-phase mass transfer coefficient, kt, varied with individual oils, with values of 4.6, 3.0, and 1.2 cm h -~ measured for dodecane, synfuel #1, and synfuel #2, respectively. Such variation was anticipated as a result of the variation in viscosities of the three oils. The individual mass transfer coefficients were combined with the estimates of the overall mass transfer coefficients (Table 1) to calculate the Henry's Law coefficients for phenols in the various oils (Table 2). The observed evaporation rates of phenols among the three Table 1. Overall mass transfer coefficients [Hk,/(Hk, + k~)] for the evaporation of phenols from dodecane and synthetic liquid fuels at 23 °C, 0.2-cm oil depth, and - 1 m / s e c wind velocity. Mass flux phenols ( m o l / c m 2 h) = (phenol concentration) x [Hk, k/(Hk~ + k,)]. Units of the overall mass transfer coefficient are cm h-'. Dodecane
Synfuel # 1
Synfuel #2
Phenol Ci-phenol*
3.6 x 10-' 1.1 x 10-1
C2-phenols*
3.2 x 10-2
C3-phenols
8.8 x 10-3
!.8 1.2 5.2 3.3 2.0 9
8.4 5.4 2.8 1.9 1.1 6
x x X x x x
l0 -2 l0 "2 10-3 10-3 10-3 10 "4
x x x x x x
10-3 10 -3 10-3 10 -3 10-3 10-4
*Double entries indicate individual isomers or groups of isomers having similar gas chro mato graphic retention times.
Table 2. Henry's Law ( H ) coefficients for phenols in dodecane and synthetic oils at 23 °C, estimated from evaporation rates and mass transfer coefficients. Units of H are molar concentration ratio (dimensionless). Dodecane
Synfuel #1
Synfuel #2
Phenol C~-phenol*
4.6 x 10-4 1.4 x 10-4
C2-phenols*
4.3 x 10 -5
C~-phenols
1.2 x l0 -s
2.1 1.5 6.7 4.4 2.7 1.1
1.0 6.8 3.5 2.5 1.4 8
x x x x x x
10 -s 10"s 10-* 10-6 10-6 10-6
x × x × x ×
10-s 10-6 l0 -6 l0 -6 l0 -~ l0 -~
*Double entries indicate separate isomers or groups of isomers having similar gas chromatographic retention times.
oils was found to be almost entirely due to variation in H, with the variability in k, (liquid-phase transport) having little impact on volatilization rate at such low rates of evaporation. The variation of H with degree of methylation of the phenols and oil type is apparent in Table 2. Phenols apparently have much higher activity coefficients when dissolved in an alkane than in a more aromatic material such as the synthetic fuels, as evidenced by the 10- to 20-fold higher value of H in dodecane. The twofold variation in H between synfuels #1 and #2 may reflect a higher activity coefficient for phenols in synfuel #1, which is more highly hydrogenated and probably more aliphatic in character than synfuel #2. The effect of increasing alkylation on H is dramatic, with Ca-phenols having Henry's Law coefficients about 20 times less than phenol. The observed variation in H i s slightly greater than the variation in equilibrium vapor pressures of the compounds at 25 °C.
Discussion Evaporation of toxic compounds from spills is a process that competes with dissolution to reduce the hazard of spills to aquatic biota (MacAuliffe, 1977). The evaporation rates observed in this study must be extrapolated to realistic field conditions in order to predict the likely rate of evaporation of phenols from a spill. This can be done by using Eq. (1) to describe the rate of evaporation and substituting values of k8 and k, that typify field conditions and values of H obtained in this study. Prior studies have assumed typical values of k, and k~ to be 3000 cm h -1 and 20 cm h -1, respectively, over open water (Liss and Slater, 1974; Dilling, 1977; Mackay and Leinonen, 1975). Since k, varies primarily as a function of wind velocity and turbulence, values over water should differ little from values over oil. Values of kt estimated from 14CO2 evaporation from oils were similar to values observed in water under similar conditions (Southworth, 1979). Literature based estimates of mass transfer coefficients for aqueous systems
206
thus seemed to be reasonable estimates for field conditions. If k, is assumed to be 20 cm h -1 (a value observed in water at 5-10 m s -1 wind, Cohen et al., 1978), and k, is assumed to be 8000 cm h -~ (a value ten times that observed in this study at - 1 m s -~ wind velocity, and somewhat above that predicted at 10 m s -1 wind for a compound of mol. wt. 100 from Liss, 1973), an overall mass transfer coefficient for phenol, in synfuel #1 of 0.17 cm h -1 is calculated. This corresponds to a half-life of 0.8 h in a film 2 mm in depth, under vigorous conditions of wind mixing. Observations in our laboratory of the dissolution of phenol from synfuel #1 (S. E. Herbes, personal communication) under gentle mixing conditions yielded a dissolution rate about twice this evaporation rate. Vigorous wind mixing would undoubtedly increase dissolution as well as evaporation, making it safe to assume that evaporation of phenol is unlikely to exceed dissolution under any reasonable set of environmental conditions, and would probably be much less in most cases. Efforts are underway in our laboratory to apply a mass transfer equation similar to the evaporation expression to the process of dissolution, which will permit a more direct comparison of dissolution and evaporation rates for the phenol series. The evaporation of volatile components of the synthetic oil itself acted to increase the concentrations of phenols despite their simultaneous loss to the atmosphere. In an accidental spill, evaporation of the oil would act to increase the rate of dissolution of the phenols, despite reducing the total amount eventually dissolving into the water. The large variation in evaporation rates of the same compound among the three oils illustrates the wide variation in activity coefficients possible as a function of oil composition. An assumption often made in evaporation or dissolution studies is that the activity coefficient is unity, and the solution thus is ideal with respect to Raoult's Law. While such an assumption is approximately valid for phenols in the two crude synfuels studied, it may be inappropriate to apply to more highly hydrotreated synthetics. In conclusion, it appears that although evaporation of phenols is a significant process with respect to the fate of those compounds, it is not rapid enough to pre-
G.R. Southworth
vent substantial dissolution from occurring in aqueous spills of synthetic liquid fuels. Acknowledgments--I would like to thank S. E. Herbes and M. B. Browman for helpful suggestions on the manuscript. Research sponsored by the Office of Health and Environmental Research, U.S. Department of Energy, under contract W-7405-eng-26 with Union Carbide Corporation. Publication No. 1874, Environmental Sciences Division, ORNL.
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