Environ. Sci. Technol. 1992,26,983-990
depends upon the aqueous concentration, not the volume, provided the volume is large enough to prevent significant depletion of the sample. A methyl silicone film decreases the limit of detection to 1ppb (w/v) for compounds with moderate distribution constants, while reducing the realtive standard deviation for volatile compounds from 20% to 6%. The further development of unique coatings will enhance the application of the method to complex matrices such as heavily contaminated water from landfill sites where the distribution constant may be affected by the high organic content of the water; the effect of other contaminants on K can be minimized by choosing a stationary phase specific to certain classes of compounds so that the distribution constant is maximized. The range of possible stationary phases can also be extended to physically fragile coatings by incorporating stainless steel tubing into the fiber design. Increasing the stationary-phase volume by either increasing the film thickness or increasing the exposed length of fiber will reduce the limits of detection even further (13). The method is readily portable and has enormous potential for field monitoring, by desorbing the fiber either into a conventional GC in a mobile laboratory or for use with portable instruments. The method is inexpensive, as the fiber/steel tubing combination costs approximately $2.00/ft.
Acknowledgments Many thanks to Professor Jim Barker of the groundwater center for his encouragement and advice. Registry No. Water, 7732-18-5; benzene, 71-43-2; toluene, 108-88-3; ethylbenzene, 100-41-4; xylene, 1330-20-7.
Literature Cited (1) Liska, I.; Kurpick, J.; LeClerc, P. A. J. High Resolut. Chromatog. 1989, 12, 517. (2) Glaze, W. H.; Lin, C. C.; Burleson, J. L.; Henderson, J. E.; Mapel, D.; Rawley, R.; Scott, D. R. Optimization of L i p uid-Liquid Extraction Methods for the Analysis of Organics in Water; U.S. Dept. of Commerce, National Technical Information Service, Springfield, VA, 1983. (3) Junk, G. A.; Richard, J. J. Anal. Chem. 1988, 60, 451. (4) USEPA Method 624. Fed. Regist. 1984, 49, 141. (5) Ho, J. S.; Hodaklevic, P.; Bellar, T. A. Am. Lab. 1989, July, 41. (6) Doherty, L. Am. Environ. Lab. 1991, 6 , 11. (7) Zlatkis, A.; Ranatunga, R. P. J.; Middleditch, B. S. Anal. Chem. 1990,62, 2471. (8) Pankow, J. F.; Ligocki, M. P.; Rosen, M. E.; Isabelle, L. M.; Hart, K. M. Anal. Chem. 1986,58,429. (9) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145. (10) Mackay, D. M.; Roberts, P. V.; Cherry, J. A. Environ. Sei. Technol. 1985, 19, 384. (11) Chiou, C. T. Environ. Sei. Technol. 1985, 19, 57. (12) Louch, D.; Motlagh, S.; Pawliszyn, J. Extraction Dynamics of Organic Compounds from Water Using Liquid-Coated Fused Silica Fibers. Anal. Chem., in press. (13) Potter, D. W.; Pawliszyn, P. Parts per Trillion Detection of Substituted Benzenes in Water Using Solid Phase Microextraction and Gas-Chromatography-Ion Trap Mass Spectrometry. Submitted to Anal. Chem.
Received for review August 30,1991. Revised manuscript received December 16, 1991. Accepted December 28, 1991. Financial support from the Natural Sciences and Engineering Research Council of Canada, Varian Canada, Varian Associates, Imperial Oil Canada, and Supelco Canada is gratefully acknowledged.
Estimating the Equilibrium Aqueous Concentrations of Polynuclear Aromatic Hydrocarbons in Complex Mixtures Wllllam F. Lane” Remediation Technologies, Inc., 127 Kingston Drive, Suite 105, Chapel Hill, North Carolina 27514
Raymond C. Loehr Department of Civil Engineering, The University of Texas at Austin, Austin, Texas 78712
By use of a two-phase liquid-liquid equilibrium model, the distribution of nonpolar solutes between water (polar phase) and soil organic matter (nonpolar phase) was related to principles of equilibrium chemistry. Batch equilibrium experiments were conducted with field-contaminated soils. Aqueous concentrations were measured directly, predicted through the use of organic cosolvents, and calculated from Raoult’s law, thereby providing a three-way comparison of solute behavior in water. Results showed that composition of the nonpolar phase strongly influences the solute concentrations in the polar phase, suggesting that Raoult’s law is applicable to complex mixtures. Tar-water partitioning experiments demonstrated that the distribution of solutes in complex mixtures is analogous to partitioning among multiple solvents.
Introduction The deposition of polynuclear aromatic hydrocarbons (PAHs) in soils has occurred at many locations, including manufactured gas plant (MGP) sites and wood-treating facilities. A t such sites, these constituents have the potential to impact human health and the environment. 0013-936X/92/0926-0983$03.00/0
Many research efforts have been undertaken to better understand the risks associated with the presence of PAH compounds in soils. Selection of technical options and implementation of management practices must include an understanding of the fundamental relationships between the components of the complex mixtures in the environment (soil, water, natural organic matter, and contaminant phase) which contain the PAH compounds. The behavior of the compounds in the aqueous phase is of critical importance because solute transport and transformation processes are known to occur predominantly in water. The presence of 16 PAH compounds in the aqueous phase was the focus of this research. The equilibrium aqueous concentrations were estimated in three ways: (a) by use of batch equilibrium experiments, they were measured directly; (b) they were predicted by using mixtures containing varying fractions of water and miscible organic cosolvents; and (c) they were calculated from Raoult’s law, which describes the equilibrium behavior of a solute between two phases. The use of cosolvents to enhance solubilization of sparingly soluble compounds has been demonstrated and described in the pharmaceutical literature (1). The ap-
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 5, 1992 983
plicability of cosolvent theory to sorption and solubilization of nonpolar contaminants in soils also has been investigated by several researchers (2-4). A protocol for batch equilibrium experiments with cosolvents served as the basis for the experimental work. The objectives of this research were as follows: to investigate the partitioning behavior of PAH compounds associated with mixtures of contaminated soils and water; to relate the behavior of PAH compounds in complex mixtures to fundamental principles of equilibrium chemistry; and to develop methods of estimating equilibrium concentrations of PAH compounds in complex mixtures. The research examined the behavior of PAH compounds only. However, the conclusions drawn from the work may reflect the behavior of other groups of nonpolar organic compounds which also are frequently found as contaminants in soils and groundwater. Theory Chemical Equilibrium. Raoult’s law describes the behavior of solutes in an ideal two-phase mixture at equilibrium (5). At equilibrium, the chemical potential of each solute is uniform among all phases. Using Raoult’s law convention, whereby the pure liquid solute is considered the standard state, the activity (ai)of each solute in a liquid-liquid mixture at equilibrium is uniform in both phases and can be expressed as ai = q y r = ,b1 YLb
(1)
where x: and x: are the mole fractions of the solute in each phase and yP and yp are activity coefficients. The exact solution of eq 1requires the use of the mole fractions and activity coefficients at equilibrium, i.e., when solvent a and solvent b are mutually saturated (6). In soils and sediments, sorption of nonpolar organic compounds has been shown to be well-correlated with the organic fraction of the sorbent material (7). Partitioning of nonpolar contaminants into organic matter has been widely regarded as the principal mechanism of sorption of hydrophobic compounds in soils and sediments (&IO), particularly those with greater than 0.1% organic carbon (I1,12). (Some researchers (13,14)have disputed the role of organic carbon in partitioning processes.) The partitioning of nonpolar solutes into organic matter is analogous to dissolution in a nonpolar solvent phase (15). Partition coefficients (Kp,K,, etc.) are commonly used to designate the solute’s relative affinity for the sorbent phase (soil and organic matter) over the aqueous phase. This research investigated the applicability of equilibrium solution theory (Raoult’s law) to the partitioning behavior of PAH compounds in mixtures of contaminated soils and water. The reader is referred to refs 5 and 16 for a discussion of Raoult’s law. Raoult’s law predicts that the concentration of a solute in either phase of a two-phase mixture can be found from the solute concentration in the other phase (eq 1). The activity coefficient of a solute is equal to the inverse of the mole fraction solubility, Xf (6), and eq 1 is written as x ; = xb1 YLbX?
(2)
The description of a solute partitioning in soil and water provides the basis for a mixture consisting of two slightly miscible phases: (a) soil organic matter and contaminants (nonpolar phase) and (b) water (polar phase). The buried materials at MGP and wood-treating sites have physical and chemical characteristics similar to those of coal tar. Therefore, for convenience, the combined contaminant/ organic matter phase will herein be denoted as the “tar” 084
Environ. Sci. Technol., Vol. 26, No. 5, 1992
phase. The soil organic matter/contaminant phase is designated as the tar phase to indicate that the nonpolar phase includes a large mass of contaminants that are not native to the soils but may contribute significantly to the sorptive capacity of the soil. Equation 2 can be written for nonpolar solutes, such as PAHs in contaminated soils: XT
= xF$XT
(3)
x)
where variables denoted by w ( x $ and signify waterphase conditions and symbols designated with t ( x ! and 7;)represent tar-phase variables. The mole fraction terms for the aqueous phase, x $ and X$,can be converted to molar terms by assuming constant proportionality between molar concentrations and mole fractions in dilute aqueous solutions (17). Equation 3 is rewritten as CTmo1.w
= xFYFs&mlar
(4)
where c & is~ the~ molar ~ ~ concentration of the solute in the aqueous phase and Sp is the molar solubility. The activity coefficient in the nonpolar phase, $, is generally assumed to equal 1 (10, 15,18). Multiplying both sides of eq 4 by the molecular weight of component i, MWi, produces cYmg/L
= cF(MWt)S&m1ar
(5)
where c; equals the tar-phase concentration (mg/mg) of the solute i and MW, is the average molecular weight of the tar phase. The supercooled liquid solubility is used for S$ in eq 5 because equilibrium between two liquid phases (water and tar)implies that partitioning solutes are in the liquid state. Cosolvents. The use of organic cosolvents to enhance the solubilization of nonpolar solutes in water has been reported to follow a log-linear model ( I , 19, 20) log s y = log S$ + afc
(6)
where SF is the solubility of the solute in the mixed solvents (cosolvent and water), Sy is the aqueous solubility, u is the cosolvency power, and f, is the volume fraction (0 If , I1)of cosolvent in the solvent mixture. The effect of cosolvents on sorption/partitioning also has been discussed in the literature (2-4,21,22). Cosolvents provide a useful tool for estimating concentrations of nonpolar solutes mixed with pure water. Measurement of the mixed-solvent solubility (Sy) at various cosolvent fractions (f,) provides a set of data which can be regressed on a log-linear scale to determine a slope (g) and y-intercept (ST). The y-intercept is equal to the predicted solute concentration in a pure aqueous solution (no cosolvent). In this research, the prediction of aqueous concentrations using cosolvent mixtures has been extended to the measurement of poorly soluble compounds found in the aqueous phase of complex mixtures. Hence, there are three methods by which equilibrium aqueous concentrations of hydrophobic solutes such as polynuclear aromatic hydrocarbons can be estimated. First, aqueous concentrations can be measured directly in batch equilibrium partitioning experiments. Second, equilibrium experiments with organic cosolvents can produce sufficient data for prediction of the aqueous concentrations from log-linear regressions. Finally, the concentrations can be calculated from the characteristics of the nonpolar (tar) phase. Tar-Water Partition Coefficients. As discussed above, the sorption/partitioning of hydrophobic solutes in nonpolar soil organic matter is comparable to dissolution in a nonpolar solvent. The introduction of nonpolar con-
Table I. Description of Soils Investigated
site identification
facility location in US.
24 loWn 24 medium” 24 highu 97 lowb 97 highb 118 lowb 118 highb 4Ogb 470b
northeast northeast northeast midwest midwest midwest midwest northeast northeast
total PAH concn: mg/kg 52 354 730 1907 1108 38 41 7 184
total organic C, g/kg (wet wt) 2.8 5.4 15 13 13 28 6.8 400 440
OGas holder residue disposal. bMGP, manufactured gas plant. ‘Soil concentration of 16 PAH compounds (wet weight basis).
taminants into a soil may affect the degree of partitioning (Le., in a contaminated soil, a higher mass of sorbate will be bound to the soil matrix) but the mechanism for sorption will likely remain the same as in uncontaminated soil (23). Partition coefficients typically are used to compare the distribution of a solute in a mixture of two distinct phases (22). The use of partition coefficients has been extended to partitioning in soil organic matter (10) and the partitioning of solutes in organic pollutant phases (1424). By use of terminology reported by Chiou et al. (6),the partitioning of solutes between a free tar phase (pollutants plus organic matter) and water can be related to the solute’s aqueous solubility by log K,, = - log S - log V,.- log yr* + log (yw*/y,) (7) where K,, is a tar-water partition coefficient, S is the supercooled liquid molar aqueous solubility (equivalent to Sy in eq 5 ) , is the average molar volume of watersaturated tar, -yt* is the activity coefficient of the solute in water-saturated tar, yw* is the compound’s activity coefficient in tar-saturated water, and y wis the activity coefficient in pure water. The activity coefficient in the tar phase, yt*, is nearly constant for nonpolar compounds and is assumed to equal approximately 1. The activity coefficients in pure water and tar-saturated water are assumed equivalent (yw* = yw). Thus, under ideal conditions, eq 7 predicts a linear relationship (slope = -1) between log K,, and log S , with a y-intercept of - log V. This relationship was evaluated in this research.
v,.
Materials and Methods Batch Equilibrium Experiments. Samples of aged contaminated soils were obtained from five locations around the United States (Table I). Nine soil samples were received by the Environmental and Water Resources Engineering laboratory at the University of Texas at Austin (EWRE-UT). The soil samples were obtained at sites which had been associated with production of manufactured gas from coal. At sites 24,97, and 118, samples were taken at more than one location. The soil samples at these three sites were further classified by the location of sampling relative to the contaminant source. Samples taken near the source were classified as “high,”while those taken farther from the source were subsequently classified as “medium” or ”low.” Each soil sample received in the laboratory was analyzed for the presence of the following 16 PAH compounds: naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene, fluoranthene, pyrene, benz[alanthracene, chrysene, benzo[b]fluoranthene, benzo[klfluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene,
benzo[ghi]perylene, and indeno[ 1,2,3-cd]pyrene, Approximately 10 g of each soil was extracted for a minimum of 18 h with 100% methylene chloride in a Soxhlet extraction apparatus. Samples from the methylene chloride extracts were analyzed with a Hewlett-Packard HP 5890A gas chromatograph with an H P 5970 Series mass selective detector (GC/MS). The total concentrations of these 16 PAH compounds in the soils are shown in Table I. Batch experiments were conducted to determine the equilibrium aqueous concentrations of the PAH compounds. A 24-h equilibration time was selected based on a study of soil-solvent desorption kinetics conducted in the EWRE-UT laboratory using similar contaminated soils. The batch desorption protocol was as follows. Three of the nine soil samples (site 24 medium, site 24 high, and site 470) were placed under a fume hood for 24 h to air-dry the soil prior to the batch equilibrium experiments. The remaining soils were not air-dried. All soil samples were passed through a 2-mm (No. 10) sieve. Approximately 9 g of soil was placed in separate 50-mL Nalgene Teflon (Nalge 3114) centrifuge tubes. To each tube, 36 mL of pure water or a water-cosolvent mixture was added, making the soiksolution ratio 1:4. The soil to solution ratio was selected on the basis of a separate study conducted in the EWRE-UT laboratory. The amount of headspace in each tube was kept to a minimum (typically -1 mL). Triplicate equilibration tubes were prepared for each soil sample at each cosolvent fraction. Cosolvent mixtures were prepared using water (ASTM Type 2), methanol (Mallinckrodt Specialty Chemicals, HPLC grade), and 2-propanol (Fisher Scientific, HPLC grade). For each mixture, water and one of the cosolvents were mixed to various volumetric fractions. For example, a mixture with 30% methanol cf, = 0.3) was prepared by mixing 30 mL of methanol and 70 mL of water. Calcium chloride was added to the cosolvent-water mixtures to achieve a matrix of 0.01 N CaC1, to provide a constant ionic strength and minimize nonsettling particles. The centrifuge tubes containing the soil-solvent slurries were tumbled in a rotating shaker at 30 revolutions per minute (rpm) for 24 h in a 20 OC room. The tubes were removed and centrifuged at 10 000 rpm for 15 min. The supernatant was removed from each tube and stored in a glass vial with a Teflon cap. The vials were stored at 4 OC prior to analysis of the samples. The samples were analyzed by high-performance liquid chromatography (HPLC). A Waters Model 700 Satellite WISP autosampler was used in conjunction with a Waters Model 484 tunable absorbance detector. The wavelength was set at 254 nm. Initial conditions of the HPLC solvent flow (65% water and 35% acetonitrile) were maintained for the first 2 min of each sample analysis. A constant (linear) gradient was established in order to bring the solvent flow from initial conditions to 100% acetonitrile in 14 min. Next, 100% acetonitrile was pumped through the column for 4 min, and finally, the solvent flow was returned to initial conditions over a 5-min period using a nonlinear gradient. Each analysis lasted 25 min. The solvents and sample passed through a 15-cm-long chromatography column (Supelco 5-8318) with Supelcosil LCPAH (5 pm) packing. The solvent flow rate was 2 mL/ min. With some soils, the actual aqueous concentrations of some of the PAH compounds could be measured directly and were included with the cosolvent mixture data. However, for other soils, the PAH concentrations were below quantitation limits. To confront this problem, an Environ. Sci. Technol., Vol. 26, No. 5, 1992
985
100
1
i -
............ .
. Maurnurn ............ Extractable ....... Antbra
100
[
-
. . ..
Sigma= 775
/ 1 0.01
I
0
0.2
AnthracelW Z-Pmopamlpater I
1
Anthracane Methandpeter
a
I
0.4 0.6 Cosolvent Fraction
~
0 01
I
0.8
1
Flgure 1. Solubilization of anthracene: site 97 hlgh soil. Solid symbols are extracts prepared wlth Sep-Pak cartridges.
additional laboratory method for measuring PAH concentrations in pure water, i.e., containing no cosolvents, was developed for use with the equilibrium experiments. This method involved equilibrating the soil with pure water using the procedure described above. After 24 h of mixing and then centrifuging, 20 mL of the supernatant was injected through a Sep-Pak CISsample preparation cartridge (Waters Associates, No. 51910). The PAH compounds were sorbed in the hydrophobic contents of the Sep-Pak cartridge. The PAHs were removed from the cartridge by injecting 2 mL of acetonitrile, thus producing a 10-fold concentration of the components in the supernatant. The 2-mL samples were analyzed by HPLC. Tar-Water Partitioning Experiments. Two samples of coal tar isolated from soil collected at site 24 were received from the Soil Science Department at the University of Florida in mid-1990. A portion of each tar sample was equilibrated with water (ASTM Type 2, no cosolvent present) to determine a value for the equilibrium distribution coefficient (K,,) for each PAH compound in the tar and water. The K,, value for a single PAH compound is equal to the PAH concentration in tar (C,,mass/mass) divided by the PAH concentration in water (C,, mass/volume). Both concentrations are measured following equilibration of the tar and water. Typical units for Kt, are liters per kilogram. The K,, values were determined by combining approximately 0.1 g of tar with 40 mL of water in a Nalgene Teflon extraction tube. The mixtures were tumbled for 24 h at 30 rpm. The mixtures were then centrifuged at 10OOO rpm for 15 min. Samples of the two distinct phases (tar and water) were removed from the tubes. The aqueous fraction was concentrated with a Sep-Pak cartridge and then analyzed directly by HPLC to determine the PAH concentrations in the water phase (C,). The tar fraction was diluted by a factor of 100 in 100% methylene chloride. The final diluent was prepared with acetonitrile and methylene chloride in a ratio of 5:l. The diluted tar samples were analyzed by HPLC to determine the concentration of PAHs in the resultant tar phase (CJ.
Results and Discussion Batch Equilibrium Experiments. The soil samples were used in conjunction with mixtures consisting of water and either of two cosolvents, methanol or 2-propanol in the batch experiments. These liquid samples were collected and analyzed by HPLC for the concentrations of 16 PAH compounds. Figure 1 shows illustrative results of equilibration tests with the site 97 high soil. The data indicate the concenB86 Envlron. Sci. Technol., Vol. 26, No. 5, 1992
i.;, 0
j
-/ A
Anthracene 2-PropanolWater
1 -
Anthracene MathanolWater A
J
I
02
04
06
08
1
Cosolvent Fraction
Figure 2. Solubilization of anthracene: site 97 high soil; linear portion regression. Solid symbols are extracts prepared wlth Sep-Pak cartridges.
trations of anthracene in the supernatant after 24 h of mixing and are shown on a log-linear scale as suggested by eq 6. Also shown in Figure 1 are the quantitation limit for anthracene analysis by HPLC (0.08 mg/L) and the maximum extractable anthracene, which equals the anthracene concentration that would be obtained if all the compound from the 9-g soil sample were extracted and solubilized in the 36-mL liquid sample. Figure 1 shows curvature as the data approach the maximum available compound. The linear portion of the curve (f, I0.5) is the relevant part. Hence, data in the upper curved portion are not used in the calculations and the remaining data are linearly regressed, as shown in Figure 2. The regression lines shown were determined from all data in the linear portions of the plots. That is, data from triplicate samples at each cosolvent fraction were included in the linear regressions. (The data were not averaged.) The regression lines were not forced through the data on the y-axis. This experimental approach was used for all the soils and cosolvent mixtures to determine a values and aqueous PAH concentrations. The slopes of the two plots in Figure 2 equal the u values from eq 6. The a values shown in Figure 2 represent the cosolvency power of the solvent-water mixtures. The y-intercepts (to which the two lines converge) represent predicted values for the equilibrium anthracene concentration in pure aqueous solution (no cosolvent). In Figures 1 and 2, the data on the y-axis (f, = 0) are below the quantitation limit for anthracene. The data shown were found by 10-fold concentration of samples with Sep-Pak cartridges. Aqueous concentrations of other PAH compounds were typically below 0.1 mg/L in the batch equilibrium experiments and hence, in many instances, the development and use of a method of concentrating samples was required to directly quantify the aqueous (f, = 0) PAH concentrations. Regression lines for the equilibrium experiments had squared coefficients of correlations (r2) greater than 0.85 with most r2 values exceeding 0.95. a values for PAH compounds in the two cosolvent-water mixtures are shown in Tables I1 and 111. Values were not consistent for specific compounds in different soils. Also, the a values did not increase with hydrophobicity as expected. It is not clear whether the soil type alone directly impacts the a values or if the composition and state of the organic phase (including the tar) in each soil is also significant. u values from the equilibrium experiments were compared with values from solubilization experiments from Morris et al. (25). The ratio of the desorption a to soh-
Table 11. u Values from 2-Propanol-Water Equilibrium Experiments"
24 low naphthalene acenaphthylene fluorene anthracene phenanthrene fluoranthene pyrene benz [a]anthracene chrysene benzo[ blfluoranthene benzo[k]fluoranthene benzo[a]pyrene dibenz[a,h]anthracene benzo[ghi] perylene indeno[1,2,3-cd]pyrene
24 medium
24 high
site 97 high
2.47
2.81
4.96 6.63 6.12 7.43 7.61 8.06 5.48 4.00 4.38 6.30
6.10 7.75 7.25 8.75 8.66 6.91 8.88 5.87 3.58 8.94
3.85 4.79 5.93 5.59
5.37 6.15 5.91 6.23 6.12 5.13 5.19 4.70 6.01 4.79
6.01 6.05 5.42
97 low
6.65 6.49
118 low
118 high
2.73 3.17 5.29 5.84 6.18 6.97 6.39 4.74 7.88
2.09
409
470 4.69 5.46
4.83 5.70 5.06 4.37
4.08
" Blank space indicates insufficient chemical concentration in extracts or too few data points to determine pound.
u
3.39 5.98 6.36 7.94 3.01 3.77 3.29 2.92 2.92 3.41 2.31 2.21
7.60
3.51
values for the specific com-
Table 111. u Values from Methanol-Water Equilibrium Experiments"
24 low naphthalene acenaphthylene fluorene anthracene phenanthrene fluoranthene pyrene benz [ a ]anthracene chrysene benzo[ blfluoranthene benzo [ a ]pyrene
24 medium
24 high
4.32 3.39 5.46 4.23
2.75 3.43 4.07 3.97
5.45 5.00 5.58
4.28 4.68 2.80
4.17 4.25
site 97 low 1.63 5.08 3.58 4.63 4.33 5.00 5.06 4.61 4.00 3.45
97 high
118 low
1.85
2.02 2.20 3.37 5.29 4.24 4.37 4.68
4.32 5.35 5.05 5.76 5.72 9.09 5.23 6.53 4.65
118 high
409
3.60 3.94 3.37 2.70
Blank space indicates insufficient chemical concentration in extracts or too few data points to determine pound. Soil 470 was not used in the methanol-water experiments.
u
values for the specific com-
Table IV. a Values From Equilibrium Experimentsn
24 low naphthalene anthracene naphthalene anthracene phenanthrene pyrene chrysene
24 medium
1.49
24 high
97 low
2-Propanol-Water 0.74 0.85 1.43 1.61 1.88 Methanol-Water 0.48
0.99
1.30 1.28 1.27 1.12
site 97 high
0.97 1.20 1.00 0.56
1.10
1.31 1.18 0.81
0.55 1.27 1.53 1.33 1.05
118 low
118 high
0.82 1.41
0.63
0.60 1.26 1.28 1.09
409
470 1.41
0.82
1.09 0.78
a Blank space indicates insufficient chemical concentration in extracts or too few data points to determine alpha values for the specific compound.
bilization u is defined as a. Table IV lists the a values for five PAH compounds. The values vary from 0.48 to 1.88, which is consistent with the results of Fu and Luthy (2) and Rao et al. (4). The variation in the solubilization effects of the cosolvents, as reflected in the range of a values, has been attributed to solvent-sorbent interactions (4, 26). The u values are expected to correspond to the hydrophobicity of the solutes (25). Figure 3 illustrates the scatter of u values of site 97 high soil when plotted against log K,, values. The data from the equilibrium experiments with soil and water appear larger (by a factor of -2) than the solubilization u from Morris et al. (25), which also are shown on the graph.
As mentioned previously, three of the soil samples were air-dried prior to the equilibrium experiments. The airdrying procedure was removed from the protocol for the remaining soils because it was presumed that sorption of pollutants on air-dried soil did not reflect sorption in field soils. A comparative experiment was conducted to evaluate the effect of air-drying soils. Small variations (&lo%)in u values and y-intercepts were observed, implying that air-drying had a minimal effect on the degree of solubilization. Thus, the data from both the air-dried and wet soil batch experiments were used for the evaluations presented. Attainment of chemical equilibrium within 24 h was verified by conducting an extended equilibrium experiEnviron. Scl. Technol., Vol. 26, No. 5, 1992 987
1
~
I
'
-
2-PropanolWater MathanolWater A I
8 b '
10
e n
r
Table V. Summary of Estimated Equilibrium Aqueous Concentrations
-
!
-
compound
predicted aq conc, measd mg/L calcd conc aq conc, 2-propanol methanol from Raoult's mg/L data data law! mg/L Site 24 Low
4
A
-
LJ
2L
0.009 0.012 0.015
fluorene phenanthrene pyrene
0.36n 0.14"
naphthalene fluorene anthracene
2.29 0.31 0.03
naphthalene fluoranthene benzo[a]pyrene
11.07 0.06 0.005
n
i
1
0 0
anthracene phenanthrene pyrene
1
2
0.009 0.012 0.016
0.076 0.012 0.012
0.312 0.117 0.004
0.269 0.293 0.043
3.48 0.536 0.056
3.52 0.169 0.042
11.48 0.047 0.006
13.1 0.034 0.001
12.88 0.013
b
5.78 0.054 0.002
0.063 0.001 0.012
0.44 0.0003 0.0004
0.009 0.009
0.019 0.004 0.0003
b b
0.00001 0.00001
c c
0.046 0.0008 0.0008
Site 24 Medium
.
1
3
,
4
1
,
5
1
8
1
1
7
1
8
b
Flgure 3. D values for PAH compounds: site 97 high soil. Solid symbols show data from ref 25.
ment. A batch equilibrium test was conducted for site 24 high medium soil with 2-propanol-water. Soil samples were equilibrated with cosolvent mixtures for 1,2,4, and 6 days. The results indicated small differences (&lo%) in predicted and measured concentrations of the detected PAH compounds, indicating that 24 h was sufficient to approximate equilibrium in the batch experiments. Comparison of Measured, Predicted, and Calculated Concentrations. To relate the results of the equilibrium experiments with principles of equilibrium chemistry (Raoult's law), the experimental data were compared with calculations performed using eq 5. The PAH concentrations in the tar phase, ci, were determined by an indirect method. Other data (27) showed that eight coal tars had an average organic carbon content of 71%. (The standard deviation was 18.3%.) The concentrations of tar in the soil samples used in this study were estimated by using a mean total organic carbon content of 71 % and by assuming that the total organic carbon detected in each soil was equivalent to the amount of organic carbon in the tar contained in the same soil. A total organic carbon (TOC) analysis was conducted on each soil using a modified Mebius procedure (28). The organic carbon content of each soil was converted to a tar concentration by dividing the TOC by 0.71. The PAH concentrations in the tar phase were found by dividing the PAH concentrations in each soil by the tar concentration in the same soil. The assumption that the total organic carbon in each soil was equal to the organic carbon in the tar could not be validated from the work done in this research because information on the natural organic matter in the soils investigated was not available. However, typical organic carbon contents are known for various soil types. For example, the organic carbon contents of well-drained mineral soils typically vary between 2% and 6% (29). Seven of the nine soils used in this research contained less than 6% organic carbon, making it difficult to determine the degree to which the organic carbon in the soils was associated with the organic carbon in the tar. The average molecular weight of the nonpolar phase was found from available data for eight coal tars (27). An average value of 537 (with a standard deviation of 457) was reported and used for all soils. Supercooled liquid solubilities (P were ) found by converting crystalline solubilities (Sc)from the literature (30, 31) using the following equation (32): log Sscl= log Sc O.Ol(MP - 25) (8) where MP is the melting point ("C) of the solute.
+
Envlron. Scl. Technol., Vol. 26, No. 5, 1992
0.298 0.132 0.009
Site 24 High
Log Kow
988
0.01 0.013 0.015
2.52 0.330 0.029
Site 97 Low 10.47 0.046
0.004
Site 97 High naphthalene pyrene indeno[l,2,3cdlpyrene
14.79 0.017 0.006
naphthalene anthracene phenanthrene
0.07
14.12 0.013 0.005
Site 118 Low b 0.01
0.065 0.005
b
Site 118 High phenanthrene fluoranthene chrysene
0.01 0.01 0.02
fluoranthene pyrene
0.12 0.04
naphthalene fluorene phenanthrene
b b b
0.009 0.009 0.008
0.010
Site 409 0.001 0.0001
Site 470 0.02 0.001 0.0003
c
By direct measurement; all others in column by Sep-Pak method. bThe sample was analyzed but the compound was not detected. Insufficient data were available for regression analysis. Concentrations were calculated using ea 5.
Table V compares measured, predicted, and calculated aqueous concentrations for PAH compounds for which there were comparative data. Aqueous concentrations which were found by direct measurement were similar ( & E % ) to those predicted by regressing cosolvent data. For most compound-soil pairs, agreement between the three methods of estimating values is within 1 order of magnitude. Clearly, there is some correlation between Raoult's law and much of the equilibrium data. We conclude that this correlation is not simply fortuitous but rather a reflection of the partitioning mechanisms behaving according to solution theory. Nonetheless, the accuracy of the calculations from Raoult's law (eq 5) is limited by the assumptions used in the derivations and by the data which were combined in the calculations. Further investigation, such as characterization of the tar and soil organic matter, as well as improvement of the experimental method for direct measurement of aqueous concentrations, may provide insight into the applicability of equilibrium principles such as Raoult's law to other mixtures of contaminated soil and water. The need for well-characterized
7
Table VI. Logarithms o f Tar-Water P a r t i t i o n Coefficients f r o m Site 24 T a r
compound
1% Ktw (sample 1)
naphthalene fluorene anthracene phenanthrene fluoranthene pyrene benz[a]anthracene chrysene
3.7 4.9 a 5.2 a 6.1 a a
!a
log Kt, (sample 2) 1 2 3 4
3.89 5.14 6.11 5.50 a
6.34 a
6.43
3.83 5.09 5.72 5.40 a 6.29 a 6.43
3.86 5.10 5.76 5.42 6.17 6.30 a 6.44
1
41 3
7
-., 8,
OM
0
61-
2
Y
A
3
2
;
Sample #1 Sample #2
I
3
4
Statistical Line of Best Fit logKtw=-l.OologS+1.05 d
/ Site 24 Tar Site 24 Tar Sample #1 Sample #2 ~
c
0 4
Picel Data
A
1
;
A
,
I
-5
-4
I
,
-3
-2
1
Figure 5. Inverse relatlonship between tar-water partitlon coefflcients and supercooled liquid solubilities. Additional data from ref 18.
-
E
I
-7
B
Log S(sc1) (molesll)
2
+
.E
t
3.86 5.11 5.78 5.45 6.21 6.34 6.21 6.43
Compound not detected or quantified in the experiments.
Statistical Line of Best Fit log Ktw = 1 1310g KOW 033
Bo
6
I
I
I
5
6
7
Log Kow
Flgure 4. Linear free-energy relationshlp between tar-water partitioning and octanol-water partltionlng. Addltional data from ref 18.
soils in sorption experiments has been demonstrated by reports in the literature which emphasize the relationships between the physical-chemical characteristics of soil-organic material and sorption of nonionic sorbates (33,34). Tar-Water Partitioning. Tar-water partition coefficients (K*) were determined for the two coal tar samples (denoted as sample 1and sample 2). Sample 1was mixed with water once, while two separate mixtures with sample 2 were used. Duplicate extracts were taken from each mixture involving sample 2. Results are shown in Table
VI. The calculation of Kh values parallels the determination of octanol-water partition coefficients (KO,). The correlation between partitioning of solutes in different solvent-water systems is the basis for the linear free-energy relationship (22). Figure 4 shows the linear free-energy relationship between the Ktwvalues (Table VI) and KO, values for the PAH compounds that were measured. (KO, values were taken from ref 35.) Included in the figure are data from Picel et al. (I@, who examined partitioning of PAHs in mixtures of coal-oil and water. A linear trend is evident on the log-log scale, suggesting that the linear free-energy relationship holds for tar-water partitioning. Figure 4 also shows that the data from this research correlate well with the data from ref 18. An inverse log-log relationship between partition coefficients and aqueous solubilities is predicted by eq 7. Figure 5 illustrates the relevance of the relationship to the K,, values determined in this research. Additional data from ref 18 are shown. As discussed with eq 7 , the y-intercept is equal to the negative logarithm of the molar volume of the water-saturated tar. Assuming a density of 0.99 g/mL ( I @ , the molecular mass of the coal tar is estimated to be 88 g/mol, which is approximately one-sixth
of the mean value assumed for the tars used in this study, 537 g/mol (27). The large difference in predicted and assumed molecular weight values suggests some inaccuracies in the K,, values. The questionable reliability of aqueous concentrations used to determine the Ktwvalues may limit the acceptability of the tar-water partition coefficients. In addition, the values of Tt* in eq 7 may not have equaled 1 for the PAH compounds, and hence the y-intercept would not correspond directly to the molar volume. Summary A two-phase liquid-liquid equilibrium model (eq 5) based on Raoult’s law (eq 1)was investigated for its relevance to the partitioning of 16 PAH compounds in mixtures of contaminated soil and water. Mixtures of water and organic cosolvents were used to estimate the true equilibrium aqueous concentrations since the aqueous concentrations of several PAH compounds were below the quantitation limits and direct analysis of the samples was not practicable. A log-linear relationship (log chemical concentration versus cosolvent fraction) satisfactorily fit solubilization data for PAH compounds from nine contaminated soils. The square of the coefficient of correlation (r2)was 0.95 or greater for most of the relationships. Equilibrium aqueous concentrations of PAH compounds predicted by data from 2-propanol-water mixtures were similar (&15%) to predicted concentrations from methanol-water data. cr values (the slopes of the log-linear cosolvent solubilization relationships) were not consistent for individual compounds in different soil samples. Solute-sorbent and solvent-sorbent interactions varied among the different soils. Similarly, cr values did not show any correlation with compound hydrophobicity as reported in the literature. The lack of correlation between Q values and hydrophobicity of the PAH compounds was attributed to solventsorbent interactions. Extended equilibration experiments demonstrated that aqueous concentrations of PAH compounds did not change significantly after 24 h of mixing, indicating that chemical equilibrium was established in the initial 24 h. A derivation of Raoult’s law leading to eq 5 was combined with literature data for properties of coal tars to calculate equilibrium aqueous concentrations of PAH compounds. These results strongly suggest the relevance of equilibrium solution theory to partitioning of PAH compounds in complex mixtures. The calculated concentrations were within 1order of magnitude of several of the Environ. Sci. Technol., Vol. 26, No. 5, 1992 989
measured and predicted values, but some estimated concentrations for specific solutesoil pairs varied over 2 orders of magnitude. Tar-water partition coefficients (&J were well-correlated with octanol-water partition coefficients and supercooled liquid solubilities, suggesting that partitioning behavior of PAH compounds in different solvent-water systems is similar. However, the accuracy of the Kt, values is uncertain based on the molecular weight of tar predicted by the data. The experimental determination of aqueous concentrations of PAHs limited the accuracy of the tarwater partition coefficients. In summary, the measurement and estimation of equilibrium aqueous concentrations provided information related to the partitioning behavior of PAH compounds in complex mixtures. Moreover, the fundamental principles of equilibrium chemistry, such as Raoult’s law, were able to predict the behavior of PAH compounds in complex mixtures.
Acknowledgments We gratefully acknowledge the assistance of Dr. M. Elrashidi and Dr. I. Murarka, the EPRI project managers, as well as that of Nadine Gordon, David Erickson, and Karen Spaniel at the University of Texas at Austin, and the cooperation of Dr. P. S. C. Rao and Linda S. Lee at the University of Florida. Registry No. Naphthalene, 91-20-3; acenaphthylene, 208-96-8 fluorene, 86-73-7; anthracene, 120-12-7; phenanthrene, 85-01-8; fluoranthene, 206-44-0; pyrene, 129-00-0; benz[a]anthracene, 56-55-3; chrysene, 218-01-9; benzo[b]fluoranthene, 205-99-2; benzo[k]fluoranthene, 207-08-9; benzo[a]pyrene, 50-32-8; dibenz[a,h]anthracene, 53-70-3; benzo[ghi]perylene, 191-24-2; indeno[l,2,3-cd]pyrene, 193-39-5.
Literature Cited Yalkowsky, S. H. In Techniques of Solubilization of Drugs; Yalkowsky, S. H., Ed.; Marcel Dekker: New York, 1981. Fu, J.; Luthy, R. G. J. Enuiron. Eng. 1986, 112, 328. Rao,P. S.C.; Hornsby, A. G.; Kilcrease, D. P.; Nkedi-Kizza, P. J. Environ. Qual. 1985, 14, 376. Rao, P. S. C.; Lee, L. S.; Pinal, R. Environ. Sci. Technol. 1990, 24, 647. Prausnitz, J. M.; Lichtenthaler, R. N.; Azevedo, E. G. Molecular Thermodynamics of Fluid-Phase Equilibria; Prentice-Hall: Englewood Cliffs, NJ, 1986. Schmedding, D. W.; Manes, M. Environ. Sci. Chiou, C. T.; Technol. 1982, 16, 4. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241. Chiou, C. T.;Peters, L. J.; Freed, V. H. Science 1979,206, 831.
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(9) Freeman, D.H.;Cheung, L. S. Science 1981, 214, 790. (10) Chiou, C. T.In Reactions and Movements of Organic Chemicals in Soils, Sawhney, B. L., Brown, K., Eds.; Special Publication No. 22; Soil Science Society of America: Madison, WI, 1989; pp 1-29. (11) McCarty, P. L.; Reinhard, M.; Rittman, B. E. Environ. Sci. Technol. 1981, 15, 40. (12) Schwarzenbach, R. P.; Westall, J. Enuiron. Sci. Technol. 1981, 15, 1360. (13) Kyle, B. G. Science 1981, 213, 683. (14) Mingelgrin, U.; Gerstl, Z. J. Environ. Qual. 1983, 12, 1. (15) Chiou, C. T.;Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983, 17, 227. (16) Atkins, P. W. Physical Chemistry; W. H. Freeman: New York, 1986. (17) Mackay, D.Environ. Sci. Technol. 1977, 11, 1219. (18) Picel, K.C.;Stamoudis, V. C.; Simmons, M. S. Water Res. 1988,22, 1189. (19) Yalkowsky, S. H.; Valvani, S. C.; Amidon, G. L. J. Pharm. Sci. 1976, 65, 1488. (20) Fu,J.; Luthy, R. G. J. Environ. Eng. 1986, 112, 346. (21) Brusseau, M. L.;Wood, A. L.; Rao, P. S. C. Environ. Sci. Technol. 1991, 25, 903. (22) Leo, A,; Hansch, C. J. Org. Chem. 1971, 36, 1539. (23) Boyd, S. A,; Sun, S. Environ. Sci. Technol. 1990,24, 142. (24) Cline, P. V.; Delfino, J. J.; Rao, P. S. C. Environ. Sci. Technol. 1991, 25, 914. (25) Morris, K. R.; Abramowitz, R.; Pinal, R.; Davis, P.; Yalkowsky, S. H. Chemosphere 1988, 17, 285. (26) Nkedi-Kizza, P.; Rao, P. S. C.; Hornsby, A. G. Environ. Sci. Technol. 1985, 19, 975. (27) Meta Environmental, Inc. Draft Report Prepared for Electric Power Research Institute, RP2879-01, 1990. (28) Page, A. L., Miller, R. H., Keeney, D. R., Eds. Methods of Soil Analysis, P a r t 2. Chemical and Microbiological Properties; American Society of Agronomy and Soil Science Society of America; Madison, WI, 1982. (29) Brady, N. C. The Nature and Properties of Soils; Macmillan: New York, 1984. (30) Mackay, D.;Shiu, W. Y. J. Chem. Eng. Data 1977,22,399. (31) Sims, R. C.; Overcash, M. R. Residue Rev. 1983, 88, 1. (32) Yalkowsky, S. H.; Valvani, S. C. J. Pharm. Sci. 1980, 69, 912. (33) Garbarini, D. R.; Lion, L. W. Environ. Sci. Technol. 1986, 20, 1263. (34) Murphy, E. M.; Zachara, J. M.; Smith, S. C. Enuiron. Sci. Technol. 1990,24, 1507. (35) U.S. EPA. Water Engineering Research Laboratory (WERL) Treatability Database, Cincinnati, OH, 1989. Received for review July 22,1991. Revised manuscript received J a n u a r y 9, 1992. Accepted J a n u a r y 10, 1992. The research reported here was conducted with support from the Electric Power Research Institute through Project E P R I RP-2879-7 (Environmental Partitioning a n d Release of Organics). Dr. M. Elrashidi and Dr. I. Murarka were the E P R I project managers.
