Environ. Sci. Technol. 1986, 20, 296-299
Solution of Hydrocgrbons in a Hydrocarbon-Water System with Changing Phase Composition due to Evaporation David R. Burrls and Wllllam G. MacIntyre"
Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062 Pure water was brought into contact with a four-component liquid hydrocarbon phase comprised of methylcyclohexane, ethylbenzene, tetralin, and l-methylnaphthalene. The headspace above the hydrocarbon phase was continually purged with Nz to provide a controlled evaporative loss. Hydrocarbon concentrations in the water and hydrocarbon phases changed due to dissolution and evaporation and were measured as a function of time. A surface renewal mass transfer model is congruent with the observed hydrocarbon concentrations in the aqueous phase. Hydrocarbon-phase composition and component interactions are jmportant in determining the time dependence of the aqueous-phase composition. The results have implications concqrning the fate of components of petroleum products discharged in the aquatic environment. W
Introduction Several oil spill weathering studies have investigated the fate of spills in the aquatic environment (e.g., 1-5). Weathering i s a complex process that includes evaporation, dissolution, dispersiw, advection, photochemical oxidation, and microbial degradation. Examination of the overall weathering process is further complicated by the complex composition of petroleum qnd difficulties in environmental sampling. Petroleum is a liquid mixture of many components, primarily aromatic and aliphatip hydrocarbons. Solubility experiments using relatively sivple mixtures of hydrocarbons, either binary (6-9) or 12-component (9), have contributed to the understanding of the aqueous dissolution of more complex hydrocarbon mixtures. Solute concentrations in water-liquid hydrocarbon mixture systems at equilibrium have been determined by a number of investigators (6-10) to follow the equation ci* = cioxiyi
(1)
where Ci* and Cy are $he equilibrium solute concentrations for component i resulting from the mixture and pure compound, respectively. xi is the mole fraction of component i in the hydrocarbon phase. yi is the activity coefficient of component i in the hydrocarbon phase. If component i is a solid, its calculated supercooled liquid solubility must be used for Cio. Experiments done at this laboratory have shown that solute-solute interactions in the aqueous phase are not measurable within the precision of water solubility determinations. Equilibrium or near-equilibrium conditions are rarely encountered in the environment. Burris and MacIntyre (8, 9) fqund that in nonequilibrium conditions, binary hydrocarbon mixtures of constant composition in contact with water yielded hydrocarbon concentrations in water that were in nearly the same ratios as found at equilibrium (i.e., hydrocarbon concentrations relative to each other remained nearly constant), It was suggested (9) that a surface renewal mass transfer model may explain this result. In this model, small parcels of water in a tbin layer at the water-hydrocarbon interface reach near-equilibrium with the hydrocarbon phase prior to their transport by turbulent eddies into the more dilute underlying water. 296
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The resulting nonequilibrium bulk water solute concentrations are thus in nearly the same ratio as that occurring in the equilibrium situation. In an environmental spill, the hydrocarbon-phase composition changes with time, primarily due to evaporation. The changing composition of the hydrocarbon phase should then be used in combination with the above model to explain processes involved in the dissolution of a spill. Simultaneous dissolution and evaporation were observed here in isolation from the other weathering processes under controlled laboratory conditions using a four-component hydrocarbon mixture. Water solubjlities of the individual components and equilibrium and nonequilibrium solute concentrations of the complete mixture of constant composition were determined. Detailed examination of the solution of hydrocarbons into water from a hydrocarbon phase of changing composition has not been reported in the literature. Knowledge of the aqueous solution of hydrocarbons from a hydrocarbon phase of changing composition is needed to determine the fate of petroleum discharges in the aquatic environment. Experimental Section The water solubility vessel design used for both the single-component and the four-component hydrocarbon mixture (0.7474 mole fraction methylcyclohexane; 0.0507 mole fraction ethylbenzene; 0.1001 mole fraction tetralin; 0.1017 mole fraction 1-methylnaphthalene) of constant composition is given in Burris and MacIntyre (8). Minimal evaporation takes place in this vessel, allowing the hydrocarbon-phase composition to remain constant. A magnetic stirrer mixed the water phase without disturbing the water-phase-hydrocarbon-phase interface. Laser light scattering experiments were used to show that droplets q e not formed when this procedure is used. The four-component hydrocarbon mixture copposition was choseq as being a simple representation of a petroleuq fuel. Water samples (approximately 2Q mL each) were obtained at specified times after hydrocarbon-phase addition in the approach-to-equilibrium experiment and after a minimum of 48 h for the equilibrium solubility experiments. Six samples were obtained for each of the equilibrium solute concentration determinations. Two samples were obtained for each of the approach-to-equilibrium solute concentration determinations. The vessels were maintained at 28 f 1 O C . Water samples were solvent extracted with pentane and analyzed by glass sapillary gas chromatography with a flame ionization detector (GC-FID) using internal standard quantification as described in Burris and MacIntyre (8). The water solubility vessel design used for the fourcomponent hydrocarbon mixture (initial composition given above) of changing composition due to evaporation is shown in Figure 1. The vessel is a 45 cm X 15 cm i.d. Pyrex cylinder with an aluminum plate cover sealed with silicon rubber. The aluminum plate was drilled and tapped for two O-ring seals holding Pyrex tubes for sample removal, inlet and outlet for N2 gas, and a septum fitting. Sample tubes were located to collect water at 6 cm below the hydrocarbon-phase-water-phase interface and 2 cm
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0 1986 American Chemical Society
Table I. Aqueous-Phase Cohcentrations (in mg/L) in a Closed Vessel Approach-to-EquilibriumSolubility Experiment Using Four-Component Hydrocarbon Mixture time, h
-b
component
1
MCH EB THN MN
1.16 (0.32)" 1.18 (0.32) 0.63 (0.17) 0.70 (0.19)
48 3 5.40 5.50 2.95 3.25
(equilibrium)
6
(0.32) (0.32) (0.17) (0.19)
8.53 8.71 4.77 5.27
(0.31) (0.32) (0.17) (0.19)
10.5 (0.31) 10.4 (0.31) 6.00 (0.18) 6.77 (0.20)
" Numbers in parentheses are normalized fractions of total hydrocarbon concentration at specified time.
,
methylcyclohexane
06
TIME lhourri
Figure 1. Water soiubiih vessel used in experimeht in which the hydrocarbon-phase composition changed due to evaporation: (a) aqueous phase, (b) hydrocarbon phase, (c) magnbtic stirbar, (d) water sampling tubes, (e) removable septum fitting.
from the vessel bottom to provide representative sampling of the water phase. The initial volumes of water and hydrocarbon mixture were 6.5 L and 200 mL, respectively. Mixing was done with a 2.5 cm X 0.5 em magnetic stirbar rotating at 150 rpm. The hydrocarbon-phase-water-phase interface was not disturbed by the stirring actiorl. The cylinder was maintained at 28 f 1OC. A 500 mL/min Nz headspace purge was maintained, except during sampling. At each sampling time a water sample (approximately 20 mL) was obtained from each sampling tube by applying air pressure through the septum. The Nz values were closed during sampling. Water samples wefe analyzed as described above. Also at each sampling time, a hydrocarbon-phase $ample (approximately0.1 mL) was obtained with a pipet through the septum fitting with the septum temporarily removed. The hydrocarbon-phase sample was diluted in pentane and analyzed by GC-FID. Suppliers and stated purities of chemicals used were as follows: Aldrieh Chemical Co.-1-methylnaphthalene, 99% ; 1,2,3,4-tetrahydronaphthalene(tetralin), 99% ; ethylbenzene, 99%; Lancaster Synthesis Ltd.-methylcyclohexane, 99.9 % ; Burdick and Jackson-pentane, HPLC grade. All chemicals were used as received except 1-methylnaphthalene, which was further purified by elution through an activated silica gel column.
Redults and Discussion Single compound solubilities in water (2 f SD) at 28 OC were metliylcyclohexane (MCH) 13.5 f 0,l mg/L; ethylbenzene (EB)178 & 2 mg/L; tetralin (THN) 46.7 & 0.4 mg/L; and 1-methylnaphthalene (MN) 34.7 f 0.2 mg/L. These single compound solubilities ih water are in reasonable agreement with literature values. Solute csncentrations for the four-component hydrocarbon mixture-water closed system are given in Table I for the equilibrium and approach-to-equilibrium experiments. Normalized ratios for each compound at each sampling time to the total hydrocarbon solute concentration are also tabulated,
Figure 2. Hydrocarbon-phase composition as a function of time.
0
IO
20
30
40
50
60
70
80
90
I20
TlME Ihourrl
Figure 3. Hydrocarbon concentrations in the aqueous phase as a function of time.
'"1 140
IM
I
/-
ethylbenzene
GIii 60 -
TIME (hour4
Flgure 4. Aqueous-phase concentrationsas a percent of saturation concentration with respect to hydrocarbon-phasecomposition as a function of time.
Results of the experiment using the four-coinponent hydrocarbon mixture and water system that was open to evaporative loss are given in Figure 2, which shows the hydrocarbon-phase composition vs. time, and Figure 3, which shows the hydrocarbon solute concentration, Ci, vs. time. The solute concentrations in Figure 3 are the avEnviron. Sci. Technol., Vol. 20, No. 3, 1986 297
erages of results from the upper and lower sampling tubes. Values between the two tubes generally agreed within 1070, indicating a well-mixed water phase. Figure 4 shows hydrocarbon solute concentrations as a percent of their saturation concentrations (Ci*) for the hydrocarbon-phase composition existing at each sampling time. Measured xi and Cio data along with yi values determined as described below were used in eq l to calculate Ci* values at each sampling time. yi values for the initial hydrocarbon-phase composition were determined by using eq 1 with Ci* data in Table I along with xi and Cio values. Within the precision of water solubility determinations, these are expected to be good yi estimates. All other yi values were calculated by modifying values predicted by the UNIFAC method (11). UNIFAC determined activity coefficients (yiu)can have significant errors. These errors (fi) were determined by using yy values and solubility-determined yi values for the initial hydrocarbon-phase composition: Yiu -
(2) initial composition
yy values were then corrected by using the following equation: (3) The results of these experiments can be interpreted on the basis of a surface renewal mass transfer model (12)in which all water parcels have approximately the same exposure time before being replaced by turbulent exchange with the bulk water. Assumptions in the application of this model are as follows: the interfacial boundary must be unbroken; there must be complete coverage of the surface water by the hydrocarbon phase; and water parcel exposure time at the interface must be sufficient to attain a near-saturation water-phase concentration, with respect to the hydrocarbon phase, in the interfacial water parcel. Approach-to-equilibrium solubility experiments using different stirring speeds show that the last assumption holds if the mixing energy is less than that required to break the hydrocarbon-water interface and form droplets. Experiments reported here cannot coilfirm this mass transfer model because the interfacial boundary was not observed directly and turbulence was not measured. The model can be used to qualitatively explain the experimental observations. The approach-to-equilibrium results in Table I are in agreement with earlier work done with binary hydrocarbon mixtures (8,9). Nonequilibrium solute concentration ratios are essentially the same as those found at equilibrium and are congruent with the surface renewal mass transfer model. In an environmental petroleum discharge, evaporation of the spill generally is concurrent with dissolution. The experiment using the vessel in Figure 1was designed to examine the two processes simultaneously. Aqueous SOlution of hydrocarbons in a water-phase-hydrocarbonphase system is dependent on the composition of the hydrocarbon phase. Hydrocarbon concentrations in water resulting from contact with a hydrocarbon phase of changing composition can be interpreted in terms of the hydrocarbon-phase composition. The hydrocarbon-phase composition (see Figure 2) changed as expected from the vapor pressures of the components. The MCH mole fraction decreased rapidly and approached zero by 50 h. The EB mole fraction increased due to the decreasing MCH contribution up to 30 h then decreased to near zero by 121 h. The MN mole 298
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fraction increased more rapidly than THN until 72 h, at which point the THN mole fraction started to decrease. The hydrocarbon solute concentration ratios (see Figure 3) were essentially the same as in the equilibrium situation for the initial hydrocarbon-phase composition during the first 11.5 h. This was expected since the hydrocarbonphase composition did not change much until after 11.5 h. The solute proportions changed substantially after this time. The MCH solute concentration reached a maximum around 24 h. This was expected since Figure 4 shows that MCH became saturated with respect to the hydrocarbon phase at about 26 h. Once a component is oversaturated, with respect to the hydrocarbon phase, in the aqueous phase it will tend to return to the hydrocarbon phase and be available for removal by evaporation. Similarly, the EB solute concentration reached a maximum at about 43 h. This maximum occurred because EB was saturated with respect to the hydrocarbon phase at about 43 h as shown in Figure 4. The initial hydrocarbon mixture activity coefficients for THN and MN were 1.28 and 1.91, respectively, but by 48 h they were both essentially unity. The effect of this change in activity coefficient values was seen in the ratio of MN and THN solute concentrations, which was fairly constant up to about 30 h. After 30 h the MN concentration decreased relative to THN since both activity coefficients were approaching unity. This observation is in agreement with the surface renewal mass transfer model. It would be difficult to scale and apply these experimental results to a particular petroleum discharge situation because of differences in nonequilibrium initial conditions, turbulent air and bulk water flows, the complexity of petroleum, and the loss of hydrocarbon solutes through an air-water interface when petroleum does not cover the entire water surface. Some information, however, can be gained from these experiments that is helpful in elucidating the fate of spilled oil in the aquatic environment. A mixture of liquid hydrocarbon components will dissolve and evaporate in a fashion similar to that observed in these experiments, assuming an unbroken hydrocarbon-water interface, The hydrocarbon transport from the hydrocarbon phase to the aqueous phase appears to follow a surface renewal model. Hydrocarbon-phase composition and interactions and their variation with time are important factors in controlling the aqueous-phase concentrations. Aqueous-phase hydrocarbon concentrations are of importance in determining the fate of petroleum discharges in the aquatic environment, particularly with respect to metabolism by and toxicity to biota.
Acknowledgments We thank C. L. Smith, M. A. Mittiga, and J. Greaves for their assistance. Registry No. Methylcyclohexane, 108-87-2; ethylbenzene, 100-41-4;tetralin, 119-64-2; 1-methylnaphthalene, 90-12-0; water, 7732-18-5.
Literature Cited (1) Smith, C. L.; MacIntyre, W. G. In “Proceedings of the Joint Conference on Prevention and Control of Oil Spills”; API, Washington, DC, 1971; pp 457-461. (2) Harrison, W.; Winnik, M. A,; Kwong, P. T.; Mackay, D. Environ. Sci. Technol. 1975, 9, 231-234. (3) McAuliffe, C. D. In “Fate and Effects of Petroleum Hydrocarbons in Marine Organisms and Ecosystems”; Wolfe, D. A., Ed.; Pergamon: New York, 1977; pp 363-372. (4) Frankenfeld, J. W. In “Proceedings of the Joint Conference on Prevention and Control of Oil Spills”; API, Washington, DC, 1973; pp 485495.
Environ. Sci. Technol. 1986, 20, 299-302 Banerjee, S. Environ. Sci. Technol. 1984, 18, 587-591. Fredenslund, A.; Gmehling, J.; Rasmussen, P. “VaporLiquid Equilibria Using UNIFAC, A Group-Contribution Method”; Elsevier Scientific: Amsterdam, 1977. Welty, J. R.; Wicks, C. E.; Wilson, R. E. “Fundamentals of Momentum, Heat and Mass Transfer”;Wiley: New York,
(5) Payne, J. R.; Kirstein, B. E.; McNabb, G. D.; Lambach, J. L.; de Oliveira, C.; Jordan, R. E.; Hom, W. In “Proceedings of the 1983 Oil Spill Conference”; API, Washington, DC, 1983; pp 423-434. (6) Green, W. J.; Frank, H. S. J . Solution Chem. 1979, 8, 187-196. (7) Leinonen, P. J.; Mackay, D. Can. J . Chem. Eng. 1973,51, 230-233. (8) Burris, D. R.; MacIntyre, W. G. Environ. Toricol. Chem. 1985,4, 371-377. (9) Burris, D. R.; MacIntyre, W. G. In ”Oil and Freshwater”; Vandermeulen, J. H., Ed.; Pergamon: Toronto, in press.
1984.
Received for reveiw July 17,1985. Accepted October 24,1985. This work was supported by a grant (AFOSR-83-0036)from the U.S. Air Force Office of Scientific Research.
NOTES Diagenetic Trace-Metal Profiles in Arctic Lake Sediments Jeffrey C. Cornwell” Institute of Marine Science, University of Alaska, Fairbanks, Alaska 9970 1
rn Heterogeneous trace-metal profiles have been found in the slowly accumulating sediments of Toolik Lake, AK. Elevated Ba, Co, Cu, Mo, and Ni concentrations near the sediment-water interface are inconsistent with inputs from recent watershed disturbances. The apparent cause of trace-metal enrichment is comigration with Mn and Fe in pore water resulting in narrow bands of trace-metal enrichment associated with high (>15%) concentrations of Mn and Fe. The misinterpretation of diagenetic enrichments as anthropogenic features is a potential problem in mildly reducing, Mn- and Fe-rich lake sediments.
Introduction The surfaces of hydrous Mn and Fe oxides are important sites for the removal of trace metals from natural waters (1-3). Selective extraction techniques show that trace metals on suspended particulates and in sediments are often associated with metal oxide phases (4-6). In Toolik Lake, AK, postdepositional migration of Mn and Fe within sediment results in Mn- and Fe-rich horizons (7). These arise from the reduction of Mn and Fe oxides, upward diffusion of Mn2+ and Fe2+,and precipitation of oxides closer to the sediment-water interface (8-10). The purpose of this study is to examine the effect of Mn and Fe migration on the distribution of trace metals (Ba, Co, Cu, Ni, and Zn) commonly found associated with lacustrine ferromanganese deposits ( 4 , 5). I will demonstrate that postdepositionalchemical reaction can result in trace-metal distributions that are similar to those influenced by anthropogenic inputs. Toolik Lake is a 1.5 km2,multibasin kettle lake formed 12 000 years ago in the northern foothills of the Brooks Range, AK. Disturbances to the 65 km2 watershed, including the construction of the Dalton Highway and the trans-Alaska oil pipeline, commenced in the early 1970s. The fine-grained sediment is dominated by the presence *To whom correspondence should be addressed at the Department of Oceanography,Texas A&M University, College Station, TX 77843.
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of high concentrations (>15%) of Mn and Fe in narrow horizons. The diagenetic formation of the Mn- and Fe-rich layers occurs in Toolik Lake because of (1)large inputs of Mn and Fe relative to other sediment components (7), (2) low sediment accumulation rates (11), and (3) the moderately reducing sediment conditions arising from low rates of organic matter input to the sediment (12).
Methods The cores presented in this study, B and C, were obtained in 1980 from 5.5- and 8.0-m water depths respectively, using a KB corer modified to hold a 6.6 cm (inner diameter) plastic core liner. Undisturbed cores resulted from slowly lowering, not dropping, the corer into the sediment. Additional cores for pore water and solid-phase analysis of major sediment components were collected at each site (7). The cores were extruded, and 1.0-cm sections were frozen in plastic bags. The water content was determined by drying at 65 “C, and a ceramic mortar and pestle was used for homogenization. Dried, ground sediment samples (0.25 g) were digested at 70 O C with 2.0 mL of concentrated HN03 and 4.0 mL of concentrated HCl until dryness (13). After addition of 10.0 mL of 1.5 N HC1, the mixture was briefly heated and the solution removed by pipetting. Cu, Ni, Zn, Co, and Ba were ’analyzed by atomic absorption spectrometry (AAS) with blanks carried throughout the entire procedure. Lead was not analyzed because of very low (
