Environ. Sci. Technoi. IQQ3,27, 2789-2794
Sorption of Organic Vapors at the Air-Water Interface in a Sandy Aquifer Material John T. Hoff,'lt Robert Glliham,t Donald Mackay,* and Wan Ying Shld
Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1,Canada, and Institute for Environmental Studies, University of Toronto, Toronto, Ontario M5S 1A4, Canada
It was hypothesized that the mineral surfaces in a sandy aquifer material would be sufficiently hydrated in a GC column fed with humidified carrier gas to provide an airwater interface for the sorption of nonpolar organic vapors. To test this, the properties of three sorbents were investigated: an aquifer material, an oxidized version of the aquifer material, and a soil taken from the vadose zone overlying the aquifer. Isotherms, vapor-phase sorption constants, partial molar enthalpies of sorption, and the incremental free energy of sorption for a methylene group were measured using a variety of volatile organic compounds. The sorbents were also characterized with respect to specific surface area, organic carbon content, and water content. The sorptive properties of the oxidized aquifer material closely resembled those of water-coated silica, supporting the hypothesis. It was estimated that the air-water interface was responsible for up to 50% of the observed sorption of alkanes in the aquifer material under the experimental conditions. Introduction
Sorption is an important factor affecting the fate and transport of organic chemicals in the vadose zone. The amount of water present in the soil strongly influences sorption, and a major challenge for modeling sorption in the vadose zone is to define soil moisture regimes and the chemical mechanisms which operate in those regimes. When a soil is sufficiently moist, the existence of bulk-like water can be assumed, and partitioning between soil and soil gas can be described by (1) KZ = ( K t + Vw)/KH Kdv(cm3/g),the vapor-phase sorption constant, represents the partitioning of a chemical between soil (solids plus water) and soil gas in the unsaturated zone. Kdv is equal to the quantity of sorbed chemical per gram of dry soil divided by the concentration of nonsorbed chemical in the gas phase. Kda (cm3/g),the aqueous-phase sorption constant, represents the partitioning between soil solids and bulk water in the saturated zone. KH is the dimensionless Henry's law constant for partitioning between air and water, and Vw (cm3/g)is the water content of the soil. Models for predicting transport of organic chemicals in the vadose zone are usually based on eq 1(1,2),and several studies have shown that it provides reasonably accurate estimates when the water content of the soil is sufficiently high (3-6). When the water content is lower, sorption exceeds that predicted by eq 1. Chiou and Shoup (6) postulated that soil consists of two sorptive phases, mineral and organic, and that partially hydrated mineral surfaces
* Corresponding author.
+ University
of Waterloo. University of Toronto.
0013-936X/93/0927-2789$04.00/0
0 1993 American Chemical Society
are mainlyresponsible for sorption when the water content is sufficiently low. More recent studies (7-10)have shown that water and organic vapors compete for adsorption sites when the relative humidity (RH) is less than about 30 % Several authors (3, 5, 11, 12) have suggested that the air-water interface contributes to sorption of organic vapors in the vadose zone. An air-water interface is formed when soil particles are coated by a continuous film of water thick enough to reproduce the surface chemical properties of bulk water. Recently, Pennell et al. (13)estimated that accumulation at the air-water interface accounted for approximately half of the amount of p-xylene sorbed by Woodburn soil at 90% RH. However, the conditions necessary for sorption at the air-water interface to be important are poorly understood. Studies of retention mechanisms in gas-liquid chromatography (14, 15) have shown that when silica is coated with a layer of water at least 5-10 molecules in thickness, organic vapors are sorbed at the air-water interface and in bulk water. Then, Kdv is given by
.
where KIA(cm) is the air-water interface sorption constant and Aw (cm2/g)is the area of the air-water interface. The area of the air-water interface decreases as water content increases due to surface tension forces. Accumulation at the interface will be appreciable when the interface-water partition coefficient, KIW(= K ~ K H 1, is comparable to Vw/Aw. The magnitude of KIWcan be estimated from water solubility and surface tension data using correlations given by Hoff et al. (16). The main objective of this study was to test the hypothesis that the mineral surfaces in a sandy aquifer material are sufficiently hydrated in a GC column at 9092 % RH to provide an air-water interface for the sorption of nonpolar organic vapors. Appreciable air-water interface sorption was expected, because the organic carbon content of the aquifer material is very low, and the minerals are thought to have affinities for water similar to silica. The sorptive properties of an aquifer material, an oxidized version of the same, and a soil were studied using gas chromatographic methods. The aquifer material was oxidized to reduce or eliminate sorption to organic carbon as a possible mechanism, and the soil was included to show the predominant influence of organic carbon. The experimental data were compared with literature data for water-coated silica and other relevant materials. Finally, the possible role that air-water interface sorption plays in vadose zone soils was considered. Materials and Methods
Sorbents. The aquifer material, which was collected from the saturated zone of the Borden aquifer (C.F.B. Borden, Alliston, Ontario) at a depth of 5 m, is a fine- to medium-grained sand consisting of quartz, feldspars, carEnviron. Sci. Technol., Vol. 27. No. 13, 1993
2788
bonates, and amphibole with less than 2 % clays. The physical properties of the aquifer material have been previously described (17). The aquifer material was oxidized by heating it in a muffle furnace at 375 "C for 7 h. The soil was collected from a depth of 15-20 cm in an area overlying the Borden aquifer. The soil had a finer texture and contained recognizable plant remains (roots and stems). The specific surface areas of the sorbents were determined by the nitrogen BET technique (18)and by heptane adsorption (see below). The organic carbon contents were determined by a combustion technique on whole samples after HC1 digestion to remove carbonates (19). The amounts of water taken up by the sorbents were determined by weighing the packed columns before and after drying them in the GC oven. Sorption Measurements. Gas chromatography is a useful tool for studying sorption, because the measurements are rapid and precise, very low gas-phase concentrations can be used, and there is little restriction on the kinds of sorbent materials that can be studied. The methods of gas chromatography are described in refs 20 and 21. In this study, approximately 30 g of sorbent was packed in a 1-mglass column,and humidified carrier gas (nitrogen) was passed through the column for at least 24 h before sorption was measured. The carrier gas was saturated with water vapor before it entered the column by passing it through a series of gas dispersion tubes held at the temperature of the column by means of a 10-L water bath placed in the oven of the gas chromatograph, a HewlettPackard 5890A. The columns were operated at a flow rate of approximately 15 cm3/min, which resulted in pressure drops between 19 and 27 kPa and a carrier gas residence time of approximately 45 s. The average relative humidity in the column was between 90 and 92%, while the actual relative humidity decreased from 100% at the column inlet to between 79 and 84% at the outlet (22). Dorris and Gray (15) observed that the chemical properties of the air-water interface were obtained in a column of silica operated under similar conditions. Sorption was measured when the gas-phase solute concentrations were vanishingly small and at higher concentrations. The infinite dilution measurements involved injecting the diluted vapors of various saturated, aromatic, and chlorinated hydrocarbons together with methane gas using a 10-pL syringe. The retention times were measured by a Hewlett-Packard 33926 integrator and used to calculate the sorption constant values. In gas chromatography, the term that is used for the sorption constant, KdV,is the net retention volume, VN (cm3/g), which is calculated from the retention times by (3) VN = [ ( T , - T M ) / T M VM ] where TR(min) is the retention time for the retained solute, and T M(min) is that for the unretained tracer, methane. The volume of gas phase per gram of sorbent in the column, VM (cm3/g),was determined by (a) calculation from the density of the soil solids (measured by water displacement) and the internal volume of the column (measured with water) and (b) calculation by the formula VM = FJ(TcoL/TFLOW)TM
(4)
where F (cm3/g) is the soap bubble-measured flow rate, J is the dimensionless pressure correction factor (20), and 2780
Environ. Sci. Technol., Vol. 27, No. 13, 1993
~~
-
~
Table I. Specific Surface Area, Organic Carbon Content, and Water Content Data for Sorbents
sorbent oxidized aquifer material aquifer material soil
specific surface organic water content area (m2/g) carbon at 90-92% nitrogen heptane content (%) RH ( % ) 0.40
0.41
0.003
0.24
0.40 0.88
0.39
0.016 1.06
0.24 1.40
1.00
TCOLand TFLOW are the absolute temperatures of the column and flowmeter, respectively. The KdVvalues were measured at four to five temperatures ranging from 5 to 25 OC. For greater accuracy and precision,the values for 20 "C were interpolated from linear plots of ln(VNl2') vs 1/T, where T is the absolute temperature. According to the van't Hoff equation, the slope is equal to -AHSIR, where iws is the enthalpy of sorption and R is the universal gas constant (20, 23). Isotherms were determined for the humidified sorbents by injecting small volumes (0.02-1O.OpL) of liquid heptane into a heated (100 "C) injector. The relative pressure, p/po, of heptane ranged from 0 to 0.6. The peak profiles were sent by cable from the integrator to a small computer for data storage and calculation. Isotherms were calculated from peak profiles by the peak maxima modification of the elution by characteristic point method (20,24). Briefly, the sorbedconcentrations, Cs,were obtained by integrating the envelope of peak maxima according to (5) wherep is the patial pressure of the solute in the gas phase and VN is given by eq 3. Heptane isotherms were also determined for the ovendried (110"C) sorbents to obtain the specific surface areas. To place the retention times into a convenient range, the sorbents were diluted 1:9 (wt/wt) with nonporous glass beads (-100 to +200 mesh). Surface areas were deduced from the resulting isotherms by the BET equation (18). The area occupied by a heptane molecule, 0.66 nm2, was calculated by assuming that the molecules are close packed spheres. The surface area of the sorbent mixture was corrected for that of the glass beads, which was measured separately.
Results Surface Areas, Organic Carbon, and Water Contents. The data in Table I show that the surface areas obtained by heptane sorption are similar to those obtained by nitrogen sorption, Such correspondences have been observed in other studies (3, 7). The surface area of the aquifer material is close to the value reported by Ball et al. (17), 0.42m2/g7which was obtained by krypton sorption. The surface area of the soil is about twice that of the aquifer material. The surface areas of the oxidized and unoxidized aquifer materials are essentially equal, indicating that the heat treatment did not alter the surface area. Heating at 375 OC reduced the organic carbon content by about 80%, however, and it also caused the color of the sand to redden, presumably due to the oxidation of ferrous iron. The organic carbon content of the aquifer material is similar to the value reported by Ball, 0.021 %
.
A
0 8 r
Table 11. Experimental Sorption Enthalpies, AH, (exper), Sorption Enthalpies Calculated for Oxidized Aquifer Material, AH. (calc), and Enthalpies of Condensation, AHc
07-
06N
'E z
05-
x
04-
compound
a
03-
pentane hexane heptane octane dichloromethane chloroform 1,2-dichloroethane benzene toluene ethyl benzene chlorobenzene
ALL-
01
JI
-
vN
I
~
~
~
~
-
~
)
Flgure 1. Superimposed chromatographic peaks for heptane in the humidifiedoxidized aquifer material at 1.5 O C . The circles indicate the peak maxima: the dashed line indicates the quantity, VnXp), in eq 5.
-AH, (exper) (kJ/mol) oxidized -A",(ca1c)a -MC* aquifer aquifer material material soil (kJ/mol) (kJ/mol) 32
29 34 40
29 33
34 33
31 43 38
34 39 44
38 44
24 29 32 37 31 31 33 32 31 43 36
27 32 37 42 29 31 33 34 38 42 40
a Calculated by eq 6 using enthalpies of adsorption and solution for water from refs 14 and 15. Obtained from ref 14.
I - S O I L 1174PC)
2-AQUIFER MATERIAL (22 8'Cl 3-OXIDIZED
01
02
03
RELATIVE PRESSURE,
04
AQUIFER MATERlAL116.9DCI
05
PIP0
Flgure 2. Sorption isotherms for heptane in the humidified sorbents at various temperatures.
The observed water content of the oxidized and unoxidized aquifer materials, 0.24 f 0.03 % , translates to an average water layer thickness of 6 nm (20molecular layers), which is greater than the thickness acquired by nonporous silica at 90-92% RH, 2.9-3.6 nm (25). The excess water was probably condensed in small pores. The existence of such pores is evident from the surface area vs pore size distribution determined by Ball. The water content is consistent with the hypothesis that the aquifer material surfaces were sufficiently hydrated to produce an airwater interface. Sorption Isotherms. Peak profiles for heptane in the humidified oxidized aquifer material are shown in Figure 1. The peaks are superimposed to show the relationship between the net retention volume, VN, and the partial pressure, p , of heptane in the column. Peak maxima are indicated by small circles, and the dashed line represents VN@)in eq 5. The coincidence of the diffuse edges of the peaks at high partial pressure and the fact that the peaks become Gaussian as the partial pressure approaches zero indicate that peak shape at high partial pressure is primarily due to isotherm nonlinearity, although nonequilibrium effects, such as tailing, may also be present (20, 24). The monotonic increase in VNwith increasing partial pressure produces an isotherm which curves upward. Representative isotherms for heptane in the three sorbents are shown in Figure 2. Replicate isotherm determinations indicated that sorbed concentrations are reproducible to within 10%. The isotherms exhibit BET type I11 or anti-Langmuir shape (upward curvature),
indicating a low energy of interaction with the sorbent surfaces. Type I11isotherms result when the solute-solute interaction is greater than the solute-sorbent interaction, i.e., the enthalpy of sorption is less than or equal to the enthalpy of condensation. Pennell et al. (13) obtained type I11 isotherms for p-xylene on Webster soil at 90% RH, and Chiou and Shoup (6) obtained type I11 isotherms for benzene and m-dichlorobenzene on Woodburn soil at 90 % RH. Various studies have shown that type I11 isotherms result when nonpolar organic vapors are sorbed at the air-water interface (25,26). It was observed that the net retention volume for heptane in the soil increased slightly at very low partial pressure. This behavior is consistent with the larger enthalpy of sorption observed for the soil (see below). Sorption Enthalpies, Sorption Constants, a n d AGCHZ. The sorption constants, the enthalpies of sorption and the free energy of interaction for a methylene group, A G ~ Hwere ~ , measured for gas-phase solute concentrations on the order of mol/cm3. Correspondence with Henry's law was indicated by the fact that the Kd' values were independent of concentration. To examine whether equilibrium was attained in the GC column, the dependence of Kdv for heptane on gas residence time was measured using columns of oxidized aquifer material and soil. The heptane peaks were generally narrow and symmetrical for the oxidized sand but were broader and slightly asymmetrical for the soil. To eliminate possible bias from peak asymmetry, the retention times were calculated by the method of moments rather than using the peak maxima (20). For the oxidized sand, Kd' remained constant when Z'M increased from 0.5 to 4 min, but for the soil Kdv increased slightly suggestingthat equilibrium was obtained for the oxidized aquifer material but not for the soil, The experimental enthalpies of sorption are given in Table 11. Each value is an average of two to five determinations; the average relative standard deviation was 4%. The enthalpies of sorption for water and the enthalpies of condensation are also given in Table 11.The sorption enthalpies for water were calculated by (27) = [(Kdv- vw/K,)/KdvI mInblface + Vw/ (K&,j")l m B d k (6) where mInterface and m B u l k are the sorption enthalpies Environ. Scl. Technol., Vol. 27, No. 13, l9g3 2791
Table 111. Experimental and Calculated Sorption Constants, Kdv, for Alkanes in Humidified Sorbents at 20 OC Kd" (cm3/g)
compound
oxidized aquifer material expt calca
aquifer material expt calca
soil
expt
calca
pentane 0.026 0.026 0.125 0.061 hexane 0.028 0.062 0.050 0.061 0.359 0.145 heptane 0.070 0.118 0.134 0.115 1.07 0.273 octane 0.174 0.282 0.360 0.275 3.05 0.655 a Calculated by eq 2 assuming that Aw is equal to the dry surface area (Table I) and using K I Avalues from ref 14.
for the air-water interface and for bulk water, respectively (14). The experimental sorption enthalpies for alkanes and for several chlorinated and aromatic compounds in the oxidized aquifer material compare well with those calculated by eq 6, suggesting that water is responsible for sorption of these compounds in the oxidized aquifer material. The experimental sorption enthalpies for the soil are significantly larger, suggesting that some other mechanism is responsible for that sorbent. The experimental sorption constant values for pentane to octane at 20 "C are given in Table 111. Because they were determined at infinite dilution, the Kdv values are proportional to the initial slopes of the isotherms. The KdVvalues increase from pentane to octane. The rate of increase reflects the free energy of interaction for a methylene group, which is calculated by (15) AGcHz = -RT ln[K~(C,+,)/K~(C,)l
(7)
where Kdv(Cn+l)/Kdv(Cn) is the sorption constant ratio for two alkanes differing by a methylene group. The experz -2.2, -2.4, and -2.6 kJ/mol for imentalvalues of h G c ~are the oxidized aquifer material, the aquifer material, and the soil, respectively. These values can be compared with literature values for water-coated silica and for water. Literature values for water-coated silica, determined by gas chromatography, are -2.0 kJ/mol at 12.5 "C (14),-2.1 kJ/mol at 25 "C (12), and -2.0 kJ/mol at 20 "C (15). Literature values for bulk water, determined by surface tension measurements, are -2.1 kJ/mol at 20-25 "C (2831) and-1.8 kJ/molat 15-20 "C (32,33). The experimental value of AGCHZfor the oxidized aquifer material is close to the literature values for water-coated silica and the airwater interface. The incremental free energy for a methylene group can be used to calculate the London (dispersion) force component of the surface free energy, y~ (mN/m), which is usually determined by measuring contact angles for a series of nonpolar organic liquids on a solid surface (34). The value of y~ for the oxidized aquifer material, 26 mN/m, is close to that for quartz, sapphire, and Pyrex glass at 95% RH, 25 mN/m (35). The value of y~ for the soil, 36 mN/m, falls into the range of values for synthetic organic polymers, 20-46 mN/m (36),and is similar to values for some natural polymers [e.g., lignin, 36-43 mN/m (37); cellulose, 34-52 mN/m (22,37);and keratin, 25-34 mN/m (38)l. The y~value for the soil thus suggests the influence of organic carbon. The fact that the sorption enthalpy and AGCHZvalues increase in the sorbent series from the oxidized aquifer material to the soil implies that more than one mechanism 2792
Envlron. Sci. Technol., VoI. 27, No. 13, 1993
is involved, because enthalpy and AGCHZare intensive thermodynamic variables, independent of the surface area, mass, or volume of a sorptive phase. The increasing AHs and AGCHZvalues appear to reflect the increasing importance of organic carbon in the sorbent series.
Discussion Sorption Mechanisms. According to the information given in the Introduction, the following phases can contribute to sorption of organic vapors in water unsaturated soil: (a) organic carbon, (b) bulk water, (c) airwater interface, and (d) partially hydrated mineral surfaces. Sorption by partially hydrated mineral surfaces is unlikely given the water contents of the sorbents. This conclusion is also supported by the type I11 isotherm, sorption enthalpy, and A G ~ values H ~ that were obtained for the oxidized aquifer material. The water contents together with the Henry's law constants for alkanes (16) imply that partitioning into bulk water is not significant. Therefore, the air-water interface and organic carbon are probably responsible for the sorption of alkanes in the three sorbents. The relative importance of organic carbon would decrease as the surface area to organic carbon ratio increases among the sorbents. This ratio increases by 2 orders of magnitude from the soil to the oxidized aquifer material, suggestingthat sorption by organic carbon would be negligible for the oxidized aquifer material. To examine this hypothesis, the experimentalKdVvalues were compared with air-water interface contributions calculated by assuming that Aw is equal to the dry surface area (Table I) and the KIAvalues were taken from ref 14. As seen in Table 111,the calculated Kdvvalues exceed the measured values for the oxidized aquifer material, implying that Aw is smaller than the specific surface area. As a further test for consistency, Kdv values for chlorinated and aromatic compounds in the oxidized aquifer material were compared with those predicted by eq 2 using the following procedure. The measured KdV values were adjusted for partitioning in water using K H values from ref 16 and the observed water content (Table I). The adjusted Kdv values were then regressed on K I Avalues taken from ref 16. The correlation seen in Figure 3 supports the hypothesis. The area of the air-water interface implied by the regression is 0.17 m2/g. The fact that the implied Aw is smaller than the specific surface area is consistent with the surface area vs pore size distribution for the -20 to +40 mesh fraction (17), which indicates that most of the surface area is contained in pores smaller than 12 nm. Assuming a contact angle of 0" and an adsorbed layer thickness of 9.5 molecules (25), pores smaller than 12 nm would be filled with condensed water at 90% RH (28). The surface area VI pore size distribution should be similar in the oxidized and unoxidized aquifer materials, because the specific surface areas and the water contents were observed to be similar. Importance of Air-Water Interface Sorption. If the air-water interface is responsible for sorption of alkanes in the oxidized aquifer material and if organic carbon predominates in the soil, both phases would be expected to contribute to sorption in the unoxidized aquifer material. Assuming that the phases sorb independently and that heating at 375 "C did not change the surface area vs pore size distribution, the air-water interface contribution to sorption in the unoxidized and oxidized aquifer materials
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Acknowledgments
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chloroethene, dichlorobenzene, and tetrachlorobenzene were taken from refs 39 and 40; KIWvalues were taken from from ref 16; and Aw value was taken as 0.17 m2/g. With these assumptions, the amounts sorbed at the airwater interface ranged from 7 to 14%. The calculations suggest that equilibrium was not obtained with respect to soil solids and that the air-water interface may account for significantly less than 50% of sorption for longer time scale experiments. Sorption at the air-water interface is probably not significant in the vadose zone overlying the Borden aquifer, because the water and organic contents are too high (41). The mechanism may be important in low organic content desert soils and in temperate soils when vapor extraction depletes the water content of the vadose zone. It may also be important in the top centimeter of soil exposed to the atmosphere, which may become dry. Progress in future studies may be assisted by varying the air-water interfacial area and by using the nitrogen BET technique to measure it.
1
2
3
4
5
K I A ( PLm
Flgure 3. Plot of experimental Kdvvalues adjusted for dissolution in water vs literature KIAvalues from ref 16 for various compounds in
the oxidized aquifer material at 25 OC. Symbols: (1) heptane, (2) benzene, (3) toluene, (4) ethyl benzene, (5) Isopropyl benzene, (6) chlorobenzene, (7) mdichlorobenzene, (8) dichloromethane, (9) trichloromethane, (10)tetrachloromethane, (1 1) trichloroethene, (12) 1-bromobutane, (13) 1,2-dIchloroethane.
should be equal. The data in Table I11 then imply that air-water interface sorption accounts for 48-56 % of K d v for alkanes in the aquifer material. If the value of Aw implied by the regression is used to calculate the airwater interface contributions, the percentages are 33-52 %. Air-water interface sorption would be less important for the chlorinated and aromatic compounds due to the greater importance of partitioning into bulk water. The chromatography studies mentioned in the Introduction imply that equilibrium would be obtained with respect to air-water interface sorption under the experimental conditions, and this conclusion is supported by theoretical considerations (20),but recent studies have shown that only a fraction of the sorption sites are rapidly accessible in conventional batch Kde measurements (39, 40). The measurements of Ball and Roberts (40)indicated that time periods of several hundred days are required to reach equilibrium. Equilibrium was achieved more rapidly when the aquifer material was pulverized, suggesting that diffusion in large particles is rate limiting. To determine whether the estimated importance of airwater interface sorption is biased by the short time scale of the experiments, we calculated hypothetical distributions for several solutes among the sorptive phases of the aquifer material using the followingprocedure. The terms on the right-hand side of eqs 1 and 2 were multiplied by KH, yielding quantities proportional to the amounts of solute partitioned to soil solids (Kda), to the air-water interface (KrwAw), and to bulk water (VW). To evaluate these quantities, Kda values for tetrachlomethane, tetra-
We thank the anonymous reviewers for constructively critical comments. Reviews by Gilles Dorris, Loren Hepler, and Peter Tremaine and suggestions from Suresh Rao and George Parks are also acknowledged. The nitrogen surface areas were provided by Holly Johnston, and the sorbent samples were provided by Stephanie O'Hannesin and Katherine O'Leary. We are grateful to the Natural Sciences and Engineering Research Council of Canada and to the Ontario government for financial support. Literature Cited (1) Jury, W. A.; Spencer, W. F.;Farmer,W. J. J.Enuiron. Qual. 1983,12,558-564. (2) Baehr, A. L. Water Resour. Res. 1987,23, 1926-1938. (3) Call, F. J. Sci. Food Agric. 1957, 8, 630-639. (4) Spencer, W. F.; Cliath, M. M. Soil Sci. SOC.Am. Proc. 1970, 34, 574-578. (5) Ong,S. K.; Lion, L. W. J. Enuiron. Qual. 1991,20,180-188. (6) Chiou, C. T.;Shoup, T. D. Environ. Sci. Technol. 1985,19, 1196-1200. (7) Rhue, R. D.; Rao, P. S. C.; Smith, R. E. Chemosphere 1988, 17, 727-741. ( 8 ) Rhue, R. D.; Rao, P. S. C.; Pennel, K. D.; Reve, W. H. Chemosphere 1989,18, 1971-1989. (9) Rao, P. S. C.; Ogwada, R. A.; Rhue, R. D. Chemosphere 1989, 18, 2177-2191. (10) Pennell, K. D.; Rhue, R. D.; Hornsby, A. G. J. Enuiron. Qual. 1992, 21, 419-426. (11) Shearer, R. C.; Letey, J.; Farmer, W. J.; Klute, A. Soil Sci. SOC.Am. Proc. 1973, 37, 189-193. (12) Okamura, J. P.; Sawyer, D. T. Anal. Chem. 1973,45,80-84. (13) Pennell, K. D.; Rhue, R. D.; Rao, P. S. C.; Johnston C. T. Environ. Sci. Technol. 1992, 26, 756-763. (14) Hartkopf, A.; Karger, B. L. Acc. Chem. Res. 1973,6, 209216. (15) Dorris, G. M.; Gray, D. G. J. Phys. Chem. 1981,85,36283635. (16) Hoff, J. T.; Mackay, D.; Gillham, R.; Shiu, W. Y. Environ. Sci. Technol. 1993,27, 2174-2180. (17) Ball, W. P.; Buehler, Ch.; Harmon, T. C.; Mackay, D. M.; Roberts, P. V. J. Contam. Hydrol. 1990,5, 253-295. (18) Lowell, S.; Shields,J. E. Powder SurfaceArea and Porosity, Chapman and Hall: New York, 1991. (19) Churcher,P. L.; Dickhout, R. D:J. Geochem.Explor. 1987, 29,235-246. Environ. Sci. Technol., Vol. 27, No. 13, 1993 2783
Conder, J. R.; Young, C. L. Physicochemical Measurement by Gas Chromatography; John Wiley and Sons: New York, 1979. Laub, R. J.; Pecsok R. L. Physicochemical Applications of Gas Chromatography; John Wiley and Sons: New York, 1978. Dorris, G. M.; Gray, D. G. J. Chem. SOC.Faraday Trans. 1 1981, 77,713-724. Atkinson, D.; Curthoys, G. J. Chem. Educ. 1978, 9, 564566. Huber, J. F. K.; Gerritse, J. J. Chromatogr. 1971,58, 137. Pashley, R. M. J.Colloid Interface Sci. 1980, 78,246-248. King, J. W.; Chatterjee, A,; Karger, B. L.; J.Phys. Chem. 1972, 76, 2769-2777. Castells, R. C.; Arancibia, E. L.; Nardillo, A. M. J.Phys. Chem. 1982,86,4456-4460. Massoudi,R.; King, A. D., Jr. J.Phys. Chem. 1974,22,22622266. Jho, C.; Nealon, D.; Shogbola, S.;King, A. D., Jr. J. Colloid Interface Sci. 1978, 65, 141-154. Clint, J. H.; Corkill, J. M.; Goodman, J. F.; Tate, J. R. J. Colloid Interface Sci. 1968, 28, 522-530. Posner, A. M.; Anderson, J. R.; Alexander, A. E. J. Colloid Interface Sci. 1952, 7, 623.
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Aveyard, R.; Haydon, D. A. Trans. Faraday SOC.1965,61, 2255. Jones, D. C.; Ottewill, R. H. J. Chem. SOC.1955, 4076. Dorris, G. M.; Gray, D. G. J.Colloid Interface Sci. 1980,77, 353-362. Bernett, M. K.; Zisman, W. A. J.Colloid Interface Sci. 1969, 29, 411-423. Shafrin, E. G.; Zisman, W. A. J.Phys. Chem. 1960,64,519524. Dorris, G. M.; Gray, D. G. J.Colloid Interface Sci. 1979,71, 93-106. Alter, H.; Cook, H. J.Colloid Interface Sci. 1969,29,439443. Curtis, G. P.; Roberts, P. V.; Reinhard, M. Water Resour. Res. 1986, 22, 2059-2067. Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991,25, 1223-1236. Hughes, B. M. M.Sc. Thesis, University of Waterloo, 1991.
Received for review February 22, 1993. Revised manuscript received August 23, 1993. Accepted August 25, 1993." 0
Abstract published in Advance ACSAbstracts, October 15,1993.
