Environ. Sci. Technol. 1997, 31, 1692-1696
A Distributed Reactivity Model for Sorption by Soils and Sediments. 7. Enthalpy and Polarity Effects on Desorption under Supercritical Fluid Conditions THOMAS M. YOUNG* AND WALTER J. WEBER, JR. Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125
Mechanisms of phenanthrene desorption from five subsurface materials in supercritical carbon dioxide (SC CO2) were investigated by measuring isotherms in the presence of fixed quantities of a polar cosolvent (methanol) and by calculating desorption enthalpies from the temperature dependence of the isotherms. The addition of 7.4 mol % methanol to the SC CO2 phase resulted in 2-11-fold reductions in Freundlich capacity factors at 120 atm and 50 °C. The capacity reduction greatly exceeded that expected from the 21% increase in phenanthrene solubility accompanying cosolvent addition. Isotherms became more linear at 120 atm and 50 °C upon methanol addition for all sorbents except a shale sample. Solubility and organic carbon-normalized phenanthrene sorption capacities declined with increasing solvent polarity, in the order dry SC CO2 > methanol-amended SC CO2 > aqueous solution, and declined with sorbent organic carbon content. Sorption enthalpies from an ideal gas reference state ranged from -106 to -70 kJ/mol, similar to phenanthrene’s heat of condensation of -70.7 kJ/mol. The results indicate that the primary mechanism for polar cosolvent desorption enhancement is competitive displacement of phenanthrene from polar sites within soil organic matter or on mineral surfaces.
Introduction Releases of volatile and semivolatile organic chemicals to the subsurface occur under water saturation levels that range from very low in arid environments with deep groundwater to near 100% above most shallow aquifers. The thermodynamics and mechanisms of organic chemical sorption on natural materials differ significantly as a function of subsurface moisture content. At high relative humidities, sorption isotherms have been reported to become more linear and have far lower capacity than in oven-dried soils (1-5). Displacement of organic chemicals from high-energy mineral sites by water at higher moisture levels has been suggested as the major mechanism underlying these observations (13). The preceding paper in this series proposed the use of supercritical carbon dioxide (SC CO2) as a tool for probing * Corresponding author present address: Department of Civil and Environmental Engineering, University of California, Davis, California 95616-5294; e-mail:
[email protected]; telephone: 916-754-9399; fax: 916-752-7872.
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soil-solute interactions and showed that, for dry soils, sorption capacity depended on the quantity and the oxygen content of the soil organic matter (SOM) and also on swelling properties related to SOM rigidity (6). More general application of SC CO2 for studying desorption requires an understanding of how sorption behavior changes in the presence of polar competitors such as water. In this study, the mechanism of the desorption process in SC CO2 systems was investigated by calculating enthalpies from the temperature dependence of the isotherms and by examining desorption in bisolute systems containing both phenanthrene and methanol. Methanol was used as a competitive solute rather than water because its greater solubility in SC CO2 ensures a single solvent phase and because similar impacts of methanol and water on PAH sorption from SC CO2 have been reported (7). Overall, the isotherms measured in dry and methanol-amended SC CO2 and the calculated desorption enthalpies for several sorbents and conditions provide a clearer picture of (i) the mechanisms of desorption from SOM and (ii) the potential of SC CO2 as a tool for probing desorption resistance.
Background A substantial portion of current knowledge regarding mechanisms of organic chemical sorption on natural solids has been inferred from isotherms measured for the same soil and solute in different solvents. Sorption experiments in vapor phases, nonpolar solvents, and aqueous solutions have suggested that sorption behavior depends strongly on surface hydration, as the soil’s water content varies from ovendried to water-saturated. Oven-dried soils and minerals have been found to exhibit nonlinear, high-capacity sorption isotherms in vapor or nonpolar organic systems relative to the same soil-solute combinations in aqueous solution (e.g., ref 1). Sorption isotherms have been reported to be welldescribed by the BET model, with capacities directly related to specific surface areas (2, 8). Adsorption enthalpies for volatile organic vapors on air or oven-dried sorbents have been found to vary between -40 and -30 kJ/mol (e.g., ref 8), values similar to the sorbate’s heat of condensation from the vapor phase. Sorption isotherms from hexane on dry soils have been found to be nonlinear, with high solubilitynormalized sorption capacities (9, 10). Isosteric heats of sorption from hexane similar to those from water have been reported in one study (11) but were observed to be significantly more exothermic in another (10). A conceptual model that views sorption on oven-dried sorbents as adsorption to dehydrated mineral surfaces has been advanced to explain these results. As vapor-phase relative humidities increase, sorption has been shown to be strongly suppressed, with isotherms approaching linearity (2-4, 8). Below a sorbent’s monolayer water capacity, increasing humidity reduces sorption as water displaces sorbed organic molecules from polar sites. Above the monolayer capacity, the sorption mechanism shifts primarily to accumulation at the air-water interface, and the reduced sorption appears to result from the decreased capacity of bulk water surfaces for adsorbate (2-4). Adsorption enthalpies in moist vapor systems similar to the solute’s heat of condensation (-∆Hv) have been reported, with little variation as relative humidity levels increased from 30 to 70% (3, 4). In aqueous systems, sorption isotherms have been shown to be nearly linear for many materials, although large deviations exist (12, 13). Sorption capacity in hydrated systems has been correlated with organic matter content and structure but has shown little relationship to specific surface
S0013-936X(96)00569-X CCC: $14.00
1997 American Chemical Society
area (13-17). Sorption enthalpies in aqueous systems near zero, significantly less exothermic than the solute’s heat of condensation, have been reported (18). Sorption in aqueous systems can consequently be viewed as uptake within a heterogeneous soil organic matter phase rather than primarily as a surface reaction similar to that prevailing in vapor-phase systems. Sorption isotherms on dry soils in SC CO2 typically have been reported to display the nonlinearity and high-capacity characteristic of low humidity vapor-phase sorption isotherms (6, 7). Corresponding isotherms for organic chemical sorption in the presence of a competitively sorbing polar solute were not identified in the literature. However, a recent study has shown that a reduction in isotherm capacity and linearity upon methanol or water addition to SC CO2 was consistent with dynamic extraction data (19). Addition of water or methanol to SC CO2 at levels as low as 1 mol % have been shown to reduce anthracene’s distribution coefficient by more than a factor of 3 (7). The addition of cosolvents to SC CO2 often results in appreciable solubility increases, making both solubility and isotherm measurements essential in inferring mechanisms.
FIGURE 1. Effect of methanol modifier addition on phenanthrene desorption isotherms in SC CO2 at 120 atm and 50 °C for Ohio shale and Houghton soil.
Experimental Section The sorbents, spiking procedure, supercritical fluid extraction apparatus, and analytical methods employed were described in the preceding paper (6). Desorption isotherms were measured by charging spiked soil to the extraction cell, pressurizing the system, and introducing 250 µL of methanol into the recirculating SC CO2 using the sample loop. Recirculation was continued until a stable reading was obtained on the UV detector (90-120 min). Isotherms were measured at a pressure of 120 atm and a temperature of 50 °C for the same five sorbents studied previously. Additional isotherms at 40 and 60 °C were determined for Ohio shale, Chelsea soil, and Webster soil. Assuming that all of the added methanol was present in the supercritical fluid phase, the amount of added methanol was approximately 7.4 mol % at 50 °C. The actual concentration in the fluid phase was reduced by the amount of methanol that sorbed to the solid phase, which was not determined in this study. However, assuming that methanol did not sorb beyond a monolayer coverage, less than 1.5% of the injected methanol was sorbed. This assumption is supported by data on methanol sorption by silica from SC CO2 under conditions similar to those employed in this work (20). The solubility of phenanthrene in methanol-amended SC CO2 was determined by packing an extraction cell with excess phenanthrene crystals and glass beads and recirculating the fluid for 60-90 min and taking a sample of the fluid phase for off-line analysis via HPLC. Enthalpy Calculations. To calculate sorption energies in supercritical fluids, significant deviations of the fluid phase from ideality must be considered. The large isothermal compressibility of SC CO2 allows numerous solvent molecules to cluster around a solute molecule to take advantage of the energy available from favorable solute-solvent interactions (21). Up to 100 solvent molecules may participate in a cluster, resulting in large negative partial molar volumes and enthalpies for solutes at infinite dilution (22, 23). The method employed here to calculate desorption enthalpies (∆Hads 2 ) includes corrections for both solute partial molar enthalpy in the fluid phase and fluid compressibility according to the following (23-25):
∆Hads 2 )
[( ) ] ∂ ln KD ∂T
- β RT2 + (h h F2 - hIG 2 )
FIGURE 2. Effect of methanol modifier addition on phenanthrene desorption isotherms in SC CO2 at 120 atm and 50 °C for Chelsea and Webster soils. concentration; fOM ≡ mass fraction of organic matter in the sorbent; FOM ≡ density of organic matter; β ≡ volume expansivity of the supercritical phase ) -(1/F)(∂F/∂T); F ≡ fluid-phase density; R ≡ ideal gas constant; T ≡ absolute temperature; h h F2 ≡ partial molar enthalpy of solute in the supercritical fluid phase; and hIG 2 ≡ molar enthalpy of solute in the ideal gas reference state. KD values were calculated directly from isotherm data presented in ref 6 for Ohio shale, Chelsea soil, and Webster soil at three temperatures (40, 50, and 60 °C), four pressures (120, 150, 210, and 310 atm), and at least six solid-phase loadings. Linear regressions of ln KD on T were performed independently for each solid-phase loading and pressure. The slope values were combined to calculate a single sorption enthalpy for each sorbent because no statistically significant effect of either pressure or solid-phase loading was observed. Volume expansivities of the supercritical phase were obtained by numerical differentiation of a highly accurate, empirical equation of state (26). The value of h h ∞2 - hIG 2 required for the calculations was obtained by interpolation and extrapolation of literature data for phenanthrene in SC CO2 at 50 °C (23). Values obtained by this approach ranged from -98 to -45 kJ/mol as pressure increased from 120 to 310 atm at 50 °C.
Results and Discussion (1)
P,qe
where KD ≡ solute distribution coefficient ) (qefOMFOM)/Ce; qe ≡ solid-phase solute concentration; Ce ≡ fluid-phase solute
Isotherms at 120 atm and 50 °C with and without added methanol are compared for four of the sorbents in Figures 1 and 2. Isotherms with methanol were also obtained for Wurtsmith subsurface material under these conditions and for Ohio shale, Webster soil, and Chelsea soil at two additional
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TABLE 1. Freundlich Desorption Isotherm Parameters in Supercritical CO2 with Methanol Modifier at 120 atm soil Chelsea
temp (°C)
KF (µg/g)/(mg/L)n
n
R2
obs
40
10.4 (13.0)a 11.2 (9.0) 21.7 (13.7) 7.12 (5.7) 18.6 (13.0) 26.8 (13.3) 10.7 (13.3) 9.12 (10.1) 11.9 (18.5) 28.0 (4.3) 1.39 (1.13)
0.96 (0.49) 0.89 (0.31) 0.79 (0.27) 1.13 (0.20) 0.83 (0.17) 0.91 (0.13) 1.06 (0.57) 1.07 (0.49) 0.96 (0.69) 1.04 (0.05) 1.16 (0.95)
0.99
4
1.00
4
1.00
4
1.00
5
1.00
5
1.00
4
0.99
5
0.99
5
0.98
5
1.00
6
1.00
3
50 60 Ohio shale
40 50 60
Webster
40 50 60
Houghton
50
Wurtsmith
50
a
Values in parentheses represent 95% confidence intervals.
temperatures (40 and 60 °C). All 11 isotherms were welldescribed (R 2 > 0.98) by the Freundlich model:
qe ) KFCne
(2)
where KF is the Freundlich unit-capacity coefficient and n is a joint measure of the relative magnitude and diversity of energies associated with a particular sorption process. Parameters were estimated using a nonlinear least-squares algorithm (quasi-Newton, SYSTAT v. 5.03 for Windows, SYSTAT, Inc., Evanston, IL), and the model fits are shown on the plots. Isotherm parameters, associated 95% confidence intervals, numbers of observations, and regression R 2 values are summarized in Table 1. Each of the sorbents displayed a large decrease in sorption capacity and, with the exceptions of Ohio shale at 120 atm and 50 °C and Chelsea soil at 120 atm and 60 °C, an increase in isotherm linearity following methanol addition. The smallest change in sorption capacity upon adding methanol, a reduction by a factor of 2.5 in KF value, was observed for the Ohio shale. The largest change, more than 1 order of magnitude, was found for the Webster soil. The desorption enhancement caused by the methanol cannot be solely explained by increased solubility. Phenanthrene solubility in the presence of 7.4 mol % methanol at 120 atm and 50 °C was 1278 mg/L, an increase of 21% over the value measured without added methanol. This result is similar to, but smaller than, solubility increases reported for anthracene and perylene in SC CO2 with methanol cosolvent (7). The change in solubility upon addition of methanol is therefore insufficient to completely explain the observed isotherm shifts. Competitive displacement of phenanthrene molecules from polar sites within soil organic matter or on mineral surfaces is probably responsible for methanol’s ability to stimulate desorption beyond the solubility enhancement. Free water had been removed from the soils by freeze-drying following aqueous spiking. Phenanthrene can sorb to dehydrated mineral or SOM functional groups via several mechanisms. Dipole-induced dipole interactions (Debye energy) and dispersive interactions (London dispersion energy) operate between polarizable phenanthrene molecules and polar functional groups in SOM (carbonyl, hydroxyl,
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FIGURE 3. Effect of solvent polarity and soil organic matter content on sorption capacity (normalized by solubility and organic carbon content). phenolic, or carboxyl groups) or at mineral surfaces (oxide or hydroxide groups). Weak hydrogen bonds can also be formed between phenanthrene’s π-electrons and Lewis acid sites such as hydroxyl groups or bound cations. This suggestion is supported by the positive correlation between oxygen content and sorption capacity observed in the dry supercritical fluid systems (6). Solvent polarity determines what fraction of the polar sites are available for phenanthrene sorption. Nonpolar CO2 molecules are incapable of strong dispersive interactions because of their low polarizability and linear configuration and are unable to participate in permanent dipole-dipole interactions. Consequently, CO2 does not exclude phenanthrene from polar SOM or mineral sites. Polar molecules such as water or methanol, however, can participate in both dipole-dipole and stronger acid-base interactions and are highly effective in displacing nonpolar molecules from polar sorption sites. This explanation has been advanced to explain the strong suppression of vapor-phase sorption of organic chemicals resulting from increased relative humidity (1, 2). By analogy to the mechanisms that have been suggested for vapor-phase sorption, the addition of the methanol displaced the phenanthrene from the high-energy polar sites, restricting it to lower energy, less polar sorption sites, partitioning into free methanol on the soil surface, or adsorption at the methanol-CO2 interface. The result is a transition from nonlinear isotherms for all of the freeze-dried sorbents to isotherms that are all nearly linear after methanol addition, having Freundlich exponents that do not differ from unity at the 95% confidence level, and with dramatically reduced isotherm capacities. To examine the effect of solvent polarity on sorption capacities across varied sorbent types, distribution coefficients were normalized by solubility and organic carbon content as follows:
K*OC )
qe fOC(Ce/S)
(3)
where qe is calculated from the Freundlich isotherm parameters for a particular Ce value and S is the phenanthrene solubility in the solvent. If sorption is simply a partitioning process, solubility-normalized capacity of a sorbent for a particular solute will vary little between solvents. Figure 3 summarizes the calculated K*OC values (mL/g) for aqueous, methanol-modified SC CO2, and SC CO2 solvent systems as a function of the sorbents’ organic carbon fraction at roughly equivalent Ce/S values of 10-3. The plot shows an approximately linear decrease in log K*OC with increasing log fOC and a substantial decrease in sorption capacity (SC CO2 >
methanol-modified SC CO2 > aqueous solution) as solvent polarity increases. Declining organic carbon-normalized sorption capacities with increasing organic matter content have been noted previously (18, 27) and may result from more adsorption-like behavior in less flexible mineral-bound SOM or the contribution of mineral sites to sorption in low organic matter soils. In Figure 3, the Ohio shale points are highlighted to illustrate their deviation from the trend established by the other sorbents in water and methanol-amended SC CO2. The less polar, more rigid macromolecular structure of the shale’s organic matter fraction has been identified as the cause of the unusually large KOC values and isotherm nonlinearity in aqueous systems (13, 18). The elimination of the capacity discrepancy between shale and the other soils in dry SC CO2 must result from a convergence of sorption mechanisms, either because hydrophobically-driven sorption is suppressed or because of a change in the glassy character of the shale’s kerogen structure. Changes in water ordering near surfaces have been suggested as a cause of sorption capacity reductions at humidities above those required for monolayer surface coverage, and this effect would be expected to differ between rigid, less polar kerogen and more flexible, polar SOM (3-5). Alternatively, glassy SOM domains may be responsible for shale’s anomalous sorption behavior in aqueous solution. The existence of glassy domains has been experimentally confirmed, and the applicability of two-site sorption models from the polymer literature has been demonstrated (28, 29). When glassy polymers sorb significant volumes of solvent molecules, expansion of the network eventually allows polymer chains free rotation, causing a transition from a glassy structure to an amorphous one, just as an increase in thermal energy can convert the polymer from glassy to amorphous upon crossing the glass transition temperature. This phenomenon, known as a glass transition pressure, has been observed for glassy polymers in supercritical carbon dioxide. Glass transition pressures from 38 to 43 atm have been reported for poly(methyl methacrylate) with a glass transition temperature of 105 °C (30). The sorption behavior of the Ohio shale can thus be explained by assuming that the kerogen structure resembles amorphous synthetic polymers under supercritical conditions, while it resembles glassy polymers under normal environmental conditions. Correlations between sorption capacity and sorbent characteristics in methanol-modified SC CO2 and aqueous systems provide another means of examining mechanistic hypotheses. For both systems, the best correlation between sorption capacity and soil properties was obtained for a model that included the fraction of organic carbon (fOC) and the molar hydrogen to oxygen ratio (H/O), similar to a model developed by Grathwohl (17):
KD,SFE/MeOH ) H -2.4 + 38.6fOC + 5.0 O
(R 2 ) 1.00) (4)
H O
(R 2 ) 1.00) (5)
KD,Aqueous ) -18229 + 12780fOC + 8723
where KD represents the distribution coefficient for phenanthrene between the solid and fluid phases (in mL/g) at an SC CO2 concentration of 1 mg/L or an aqueous concentration of 10 µg/L. The change in sign of the fOC term from negative in dry soils to positive in methanol-amended SC CO2 is consistent with a change in sorption mechanism upon methanol addition. In dry soils sorption occurs primarily at oxygen-bearing functional groups within the organic matter fraction. After the addition of methanol, it is the reduced, more hydrophobic organic matter sites that are positively correlated with sorption capacity, presumably because the methanol has occupied the more hydrophilic sites.
TABLE 2. Comparison of Phenanthrene Desorption Enthalpy in Supercritical CO2 and Water
sorbent Ohio shale Chelsea soil Webster soil a
∆Hads (kJ/mol) SC CO2, ideal gas ref state
∆Hads (kJ/mol) SC CO2, supercooled liquid solute ref state
∆Hads (kJ/mol) aq, supercooled liquid solute ref state
-77.2 (28.3)a -106.5 (42.2) -70.4 (31.6)
-1.3 (28.3) -30.6 (42.2) 5.5 (31.6)
18.4 (11.4) 1.0 (24.0) -8.2 (12.2)
Values in parentheses represent 95% confidence intervals.
The effect of methanol addition may explain the discrepancy in sorption isotherm linearity in SC CO2-soil systems reported by previous investigators (7, 31). Nonlinear isotherms have been obtained for phenanthrene sorption from supercritical CO2 for five different conditions on a sandy loam soil that had been oven-dried at 100 °C (32). Freundlich parameters for phenanthrene sorption at 45.7 °C and 131.6 atm were KF ) 20.1 (µg/g)/(mg/L)n and n ) 0.73. In contrast, linear isotherms were reported for phenanthrene sorption from SC CO2 at 100 atm and temperatures between 35 and 55 °C for another sandy soil (31). Linear distribution coefficients were found to be 1.24, 3.06, and 16.45 (µg/g)/ (mg/L) at 35, 45, and 55 °C, respectively. Soils that were used in the two studies (7, 31) had similar characteristics with respect to organic matter contents (0.8% vs 1.07%) and specific surface areas (11.4 vs 13.1 m2/g). The presence of residual moisture in an air-dried soil sample at typical ambient humidities or in the CO2 employed might result in monolayer water coverage on polar sorption sites. This effect would be sufficient to account for the considerable difference in isotherm capacity and linearity observed in the two studies. The change in isotherm linearity and capacity upon methanol addition observed in this study is also qualitatively consistent with isotherms inferred from dynamic extraction data (19). Enthalpies of desorption from Ohio shale, Webster soil, and Chelsea soil relative to an ideal gas reference state ranged from -70.4 to -106.5 kJ/mol, but relatively large 95% confidence intervals precluded the identification of statistically significant differences between the sorbents (Table 2). The most striking feature of the enthalpy data is the strongly exothermic nature of the phenanthrene sorption process. The values obtained for each of the dry sorbents correspond closely to and are not statistically different from phenanthrene’s heat of condensation (-∆Hv) of -70.7 kJ/mol (33). Investigators studying the sorption of nonpolar organic chemicals on oven-dried soils and minerals in vapor-phase systems also report sorption enthalpies closely related to the solute’s heat of condensation (3, 8). Naphthalene sorption enthalpy on quartz sand at relative humidities above calculated monolayer water coverages were -46.4 kJ/mol (3). The enthalpy of naphthalene desorption into the vapor phase from activated carbon, a material derived from natural organic matter or its precursors, was measured as 64 kJ/mol using differential scanning calorimetry (34). These values are similar to naphthalene’s -∆Hv of -51.7 kJ/mol. Sorption to ovendry soils from nonpolar organic solvents also proceeds with high enthalpies. For example, isosteric heats of parathion sorption on dry Woodburn soil from hexane approached -100 kJ/mol (10). To compare the enthalpy values obtained in this study with phenanthrene sorption enthalpies on the same materials in aqueous systems (18), it is necessary to convert the present values to a pure supercooled liquid solute reference state. At
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50 °C the enthalpy difference between phenanthrene’s supercooled liquid and ideal gas states was calculated to be -75.9 kJ/mol (35, 36). The resulting enthalpy values for sorption from the supercooled liquid state in SC CO2 and aqueous solution are compared in Table 2. No statistically significant difference was observed between the two enthalpy measurements, suggesting that relatively weak, nonspecific forces govern sorption in both systems and that the higher sorption capacities in SC CO2 result from greater availability of favorable sorption sites. Sorption enthalpies of nonpolar organic compounds on mineral surfaces were also found to vary little as humidity was varied from 30 to 70% (3, 4) even though sorption capacity declined logarithmically with increasing moisture levels. The enthalpies measured in this study are in agreement with those calculated from the data of Andrews (32) but differ from those of Erkey et al. (31). The original sorption enthalpy that was reported by Andrews et al. (7) did not include corrections for solvent compressibility or the large negative partial molar enthalpy of phenanthrene in the supercritical phase. Correcting for these factors following the approach of Shim and Johnston (23) yields a sorption enthalpy from an ideal gas reference state of -68 kJ/mol, similar to phenanthrene’s -∆Hv and the values obtained in this study. In contrast, Erkey et al. (31) calculated phenanthrene sorption enthalpies of -20.7 kJ/mol on a low organic matter content, sandy soil using the same calculation approach and the same database of partial molar enthalpies in the supercritical phase. The differences in reported sorption enthalpies may arise from experimental uncertainties or from differing soil characteristics. In many respects, sorption of nonpolar organic chemicals by soils in SC CO2 resembles vapor-phase sorption processes. The sorption capacity is suppressed as methanol competitively displaces phenanthrene from polar sites within SOM and on mineral surfaces, similar to the effect of increased relative humidity. The energetic heterogeneity of the sorption process is also reduced upon methanol addition, as reflected by the increase in isotherm linearity, as higher energy polar functional groups are removed from the pool of available sorption sites. The reduction in capacity and linearity is not sufficient to measurably affect sorption energetics, however, as enthalpies from the supercooled liquid solute state are near zero in solvents of varied polarity. The results suggest that, through judicious choice of temperature, pressure, and polar cosolvent content, SC CO2 may be a promising desorption test medium for investigating a wide range of environmentally relevant SOM-pollutant interactions.
Acknowledgments This research was funded by the U.S. Environmental Protection Agency, Office of Research and Development, Risk Reduction Engineering Laboratory, Cincinnati, OH, under Cooperative Agreement CR 818213-01-0. The project officer was Dr. James Ryan. The authors thank Dr. Kurt Pennell, a research scientist in our program at the time of this work and currently an Assistant Professor in the School of Civil Engineering at Georgia Tech University, for his help in formulating the link between organic sorption in supercritical fluid and vapor-phase systems and Patrick Carpentier, an undergraduate research assistant, for his assistance in performing the experimental work. We also thank two anonymous reviewers for thorough and constructive comments on this work.
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Received for review June 28, 1996. Revised manuscript received January 2, 1997. Accepted January 16, 1997.X ES960569T X
Abstract published in Advance ACS Abstracts, April 1, 1997.
