Environ. Sci. Technol. 1003, 27,928-937
Application of Headspace Analysis to the Study of Sorption of Hydrophobic Organic Chemicals to a-AI203 Judlth A. Perllnger,tl* Steven J. Elsenrelch,'g* and Paul D. Capell Environmental Engineering Sciences, Department of Civil and Mineral Engineering, University of Minnesota, Minneapolis, Minnesota 55455, and Water Resources Division, United States Geological Survey, St. Paul, Minnesota 55 101
The sorption of hydrophobic organic chemicals (HOCs) to tr-AlzOs was investigated with a headspace analysis method. The semiautomated headspace analyzer gave rapid, precise, and accurate results for a homologous series alkylbenzenes even at low percentages of solute mass sorbed (3-50% ). Sorption experiments carried out with benzene alone indicated weak interactions with wellcharacterized aluminum oxide, and a solids concentration effect was observed. When the sorption coefficients for benzene alone obtained by headspace analysis were extrapolated up to the solids concentrationstypically used in batch sorption experiments, the measured sorption coefficients agreed with reported sorption coefficients for HOCs and sediments of low fractional organic carbon content. Sorbed concentrations increased exponentially with aqueous concentration in isotherms with mixtures of alkylbenzenes,indicating solute-solute interactions at the mineral surface. Sorption was, however, greater than predicted for partitioning of a solute between its pure liquid phase and water, indicating additional influences of the surface and/or the structured liquid near the mineral surface.
Introduction
Volatile, hydrophobic organic chemicals (HOCs) have become common groundwater contaminants ( I , 2). Relative to sorption of HOCs in groundwater, the influence of sorption on the fate and transport of HOCs in surface waters is relatively well understood. Sorption of HOCs to solids rich in natural organic matter such as surface sediments and soils has been extensively studied and can be described using property- and structural-activity relationships (3-6). The organic carbon content of the subsurface decreases with depth ( 7 ) and can be less than 0.001 g of OC/g of solid. Thus, partitioning of HOCs to the natural organic matter of the solids is reduced, and sorption to mineral surfaces may become an important mechanism of retardation of HOCs in aquifers. Relationships describing sorption to soil and sediment organic matter fail to predict sorption at low organic carbon content. Sorption of HOCs to mineral surfaces has been the subject of few studies (8,9).A comprehensive theoretical (or empirical) model for sorption of HOCs to mineral surfaces does not exist. Before such a model can be developed,a fundamental understanding of the properties of the solute, solvent, and sorbent that influence the sorption processmust be gained. Until recently, analytical methods were inadequate to measure the small extent of
* Corresponding author. + Present address: Swiss Federal Institute for Water Resources and Water Pollution Control, CH-6047,Kastanienbaum, Switzerland. 3 University of Minnesota. § United States Geological Survey. 028
Environ. Sci. Technol., Vol. 27, NO. 5, 1993
HOC sorption to mineral surfaces. In addition, researchers studying sorption to mineral surfaces frequently used sorbents that were not thoroughly characterized and/or consisted of a composite of minerals with varying characteristics. This made the determination of the influence of particular characteristics of the sorbent on sorption of HOCs impossible. Headspace analysis was employed in this study to avoid analytical artifacts that can result from phase separation by filtration or centrifugation (e.g., incomplete separation of colloidal-size particles from the aqueous phase, solute volatilization). Although headspace analysis has not been used to date to study sorption of HOCs to mineral surfaces, it has been used in a wide variety of applications including the measurement ofvapor pressure (IO,I I ) , solubility (121, activity coefficients ( I O ) , Henry's law constants (13, 14), dimerizationof solutes ( I I , I 5 ) ,sorption tonaturalsorbents (16),and humic acid/HOC association (17). The objectives of this study were 2-fold: (1)to develop and evaluate the use of a headspace analysis method for studying sorption of HOCs to mineral surfaces, and (2) to study the sorption of HOCs to well-characterized minerals. This paper presents the materials and methods used in the headspace analysis, a summary of the methods and results that verified its use to study sorption, and the results of sorption experiments using cu-AlzOs. Materials and Methods
Headspace Analysis System Design. The headspace analysis system was a modification of the design of Hussam and Carr (IO). The system consisted of a thermostated experimental cell and a gas standard bulb connected in parallel to an automated sampling valve system and attached to a gas chromatograph (Figure 1). The thermostated cells used to hold the water or waterisolid slurries were modified 4- or 6-L Pyrex filter flasks. Large volume cells were used to allow the addition of solutes without predilution in cosolvents. Two ports on top of the cell allowed the sampling of the cell and the addition of solute via an autoburet. A ground glass stopper near the top of the cell provided access for cleaning and filling the cell. A Teflon stopcock (2-mm borehole) located on top of the cell was used for the addition of solutes by a syringe. The contents of the cell were stirred with an immersible-base magnetic stirrer. Teflon-covered magnetic stir bars were used. The cell was immersed in a thermostated water bath up to the fittings on top of the cell to maintain uniform temperature of the water and the headspace. The temperature of the water bath was maintained at a selected temperature f O . l "C. A 12.5-L Pyrex bulb containing known masses of the solutes at partial pressures comparable to those in the headspace of the cell provided the calibration standard for analyses. The bulb was equipped with aTeflon needle valve for evacuation and a Mininert valve for solute injection. The temperature of the gas bulb was monitored 0013-936X/93/0927-0928$04.00/0
0 1993 American Chemical Society
CARRIER GAS GLASS BULB
GAS CHROMATOGRAPH
Figure 1. Headspace analysis system.
by a thermocouple inside a 10-cm-long 2-mm i.d. “well” on the bulb surface. Sample analyses were performed with a HewlettPackard 5840A gas chromatograph (GC) equipped with a 15-mDB-1 Mbore capillary column (0.53-mm i.d., 1.5-pm film thickness, Chromtech) and a flame-ionization detector (FID). The Nz carrier gas flow rate through the column was 11mL/min, and the N2 makeup gas flow rate was 20 mL/min. The GC injection port temperature was 250 “C, and the FID temperature was 300 “C. The oven temperature was ramped from 50 to 55 “C. Total analysis time for the alkylbenzene series (benzene,Cl-C4-benzene) was 13.1 min. The gas sampling system (Figure 1) consisted of a vacuum pump and nickel tubing (0.01-in. id.) and sampling valves which were heated to 200 “C. The tubing ran from the base of the injection port on the GC to a 6-port switchingvalve equipped with a 100-pLsample loop. From the 6-port valve, the tubing ran to a three-way tee and then to the stainless steel sampling valves on the gas bulb and the experimental cell. Stainless steel tubing (500 pL) between the toggle valve and the pump provided a ballast to ensure that the sample tubing was thoroughly flushed between samples. Sampling. Three steps were involved in sampling the headspace of the cell. First, the transfer lines were evacuated. The 6-port valve was switched to “load“ position to allow carrier gas to flow through the valve and directly into the GC. The vacuum pump was turned on, and the toggle valve was opened for 30 s to create a vacuum between the vacuum pump and the sampling valves on the cell and the glass bulb. In the second step, the cell was sampled. The toggle valve was closed to isolate the vacuum pump from the system, and the sampling valve on the cell was opened for 15 s. This allowed 677 pL of gas to flow from the cell through the lines, sample loop, and ballast until the pressure was equal in the cell and the sampling system. In the final step, the sampling valve was closed and the 6-port valve was switched to the “inject” position to allow carrier gas to flow through the sample loop and carry the sample into the GC. A similar procedure was followed for sampling the bulb. Solutes. The compounds selected for this study were the homologous series of alkylbenzenes including benzene, toluene, ethylbenzene, propylbenzene, and butylbenzene. The high solubility and Henry’s law constants of these compounds were required by the relatively high detection limits of the flame-ionization detector. All neat compounds except propylbenzenewere purchased from Aldrich Chemical Co. (Milwaukee, WI) and were used without further purification. The stated purities were as follows:
benzene 99.9 % , toluene 99.9 % , ethylbenzene 99 % , and butylbenzene 99 % . The propylbenzene was purchased from Crescent Chemical Co. (Hauppauge, NY; 99% purity). Solids Characterization. Aluminum oxide was obtained from Alcan Chemicals (Cleveland, OH). Before characterization and use in sorption experiments, it was combusted for 24 h at 500 “C to eliminate organic matter. To thoroughly characterize the solids, particle size distribution, specific surface area, surface charge, organic carbon content (foe), and surface morphology were measured. Size distributions of particle suspensions were measured with an ELZONE 80XY particle-size analyzer before and after sorption experiments were performed. A 2-mL aliquot of the suspensions was added to 110 mL of 0.024 M NaCl to dilute the particles and increase the ionic strength to the minimum needed for particle counting. The specific surface area of the aluminum oxide was determined by multipoint BET analysis (Quantasorb instrument) using nitrogen gas as the sorbent. Surface charge was measured according to the methods described by Stumm and Morgan (18). The f O c remaining after combustion of the aluminum oxide was measured with a Coulometrics instrument. Sorption Experiments. All sorption experiments were carried out at a pH of 7.0, an ionic strength of 0.01 M, and a temperature of 20 “C as follows. The cell was filled with known masses of double-distilled water and solids. The pH and ionic strength of the solution were adjusted with a NaCl/NaHC03 buffering system such that the solution was at equilibrium with C02cg,in the headspace of the cell at pH 7.0 at an ionic strength of 0.01 M. The cell was immersed in the water bath, attached to the headspace analysis system, and covered to prevent penetration of light. Liquid solutes were injected into the cell with a syringe. The mass of mixture added to either the cell or the bulb was determined as the difference in weight of the syringe before and after adding the mixture of compounds. The individual alkylbenzenes were found to diffuse into the Teflon parts of the autoburet at different rates, thus changing their molar ratios. Therefore, the autoburet was not used for solute addition in the experiments reported here. Following the addition of solutes, the contents of the cell were continuously stirred until equilibrium was reached. The headspace of the cell was sampled in triplicate. The gas bulb, which contained the same alkylbenzene mixture as the cell, was sampled in triplicate immediately followingthe sampling of the cell. To produce a sorption isotherm, solute was repeatedly added to the cell by a syringe. Following equilibration, the headspace concentration of the solutes was measured after each solute addition. The maximum concentration of solute in the aqueous phase was never allowed to exceed a value of onehalf the solute’s aqueous solubility. Theory and Computations. The Henry’s law convention was used to compute aqueous activity from the measured gaseous concentration. Thus, it was necessary to know the Henry’s law constants, KH,of the alkylbenzenes very precisely. Experimentally,KHwas determined by adding a known total mass of solute, MT (mol), to the cell 97% filled with distilled water, allowing the system to come to equilibrium, and measuring the concentration of alkylbenzene in the air, CA (mol L-l). The partial pressure p (atm) of solute in the headspace was then Environ. Sci. Technol., Vol. 27, No. 5, 1993 929
computed by assuming ideal gas behavior (p = CART,where atm L mol-1 K-I and T is the absolute R = 8.206 X temperature in degrees Kelvin). The concentration of solute in the water, cw (mol L-I), was determined by mass balance: MT = MA + Mw
(1) where M Aand Mw are the moles of solute in the air and water, respectively. In terms of measured parameters, this is (2) MT = CAVA+ C ~ V W where VA and Vw are the volumes of air and water, respectively. The aqueous concentration (CW) is then:
cw = (MT - CAVA)/Vw (3) In computing the Henry’s law constant (atm m3 mol-l) from the experimental data, KH = 10-3p/c~, the gaseous and aqueous activity coefficients were assumed to be equal to unity. The assumption of unit gaseous activity is supported by Mackay and Shiu (19). The assumption of unit aqueous activity is discussed below. The concentration of solute in the sorbed phase, cs, was determined by mass balance, knowing the Henry’s law constant. If the cell is partially filled with a sediment/ water slurry, an alkylbenzene added to the cell partitions between the gas, liquid, and solid phases: MT =MA
+ Mw + Ms
(4)
or (5) where Ms is the moles of solute sorbed, MSOLID is the mass of solids added to the cell (g), and the other terms are as previously defined. Substituting 1 0 - 3 ~ ~ R Tfor / Kcw, ~ the only unknown in the mass balance is cs (mol g l ) : MT = CAVA+ (10-3C~RT/K~)Vw + C S ~ ~ S O L I D (6) Solving for CS:
The concentration in the sorbed phase may be expressed in two other ways: as fractional organic carbon content (foe*) of the particles and as surface coverage of the alkylbenzenes on the particle surface. The fractional organic carbon (g of C/g of solid) that results from sorbing an alkylbenzene to the mineral surface may be computed as: foc*
= cs.C.MW
(8)
where C is the moles of carbon atoms per mole of alkylbenzene, and MW is the molecular weight of carbon. When a mixture of alkylbenzene is sorbed, the total foc* is simply the sum of the contributions of each alkylbenzene. The surface coverage of alkylbenzenes sorbed to the mineral surface can be computed as: surface coverage = cs.TSA/(2.AREA) (9) where TSA is the total surface area of the alkylbenzenes (m2/mol), and AREA is the specific surface area of the solids (m2/g). It is assumed in this Computation that the alkylbenzenes sorb parallel to the mineral surface. Because only one side of the alkylbenzene would sorb to the surface under this assumption, the total surface area of the g30
Environ. Sci. Technol.. Vol. 27, NO. 5, 1993
molecule is divided by two. The total surface coverage resultingfrom sorbing a mixture of alkylbenzenes is equal to the sum of the individual contributions to surface coverage. A surface coverage of one indicates monolayer coverage. Total surface areas of the alkylbenzenes were computed by R. Dickhut (Ph.D. Dissertation, University of Wisconsin-Madison) according to the method of Pearlman (20). It should be noted that the computed TSA values are larger than appropriate for use in eq 9 because Pearlman considers the surface morphology of the molecule, whereas eq 9 requires only the “parking arean of the molecule. This area should be less than the TSA value by a constant for each solute, but no effect was made to correct the TSA values. Whereas the TSA/2 value for benzene molecules computed according to the method of Pearlman is 50 nm2,No11 (21) cites values for the parking area of benzene sorbing onto silica of 40 nm2 for parallel sorption and 25 nm2 for upright sorption. Results
Verification of the Headspace Analysis Method. The methods described above to determine the concentration of solute in the sorbed phase are dependent upon the absence of losses from the cell by leakage, the establishment of equilibrium, and the absence of processes other than air-water exchange and sorption to the oxide minerals. Processes that could influence the computed aqueous and sorbed concentrations include sorption to the glass container above or below the air-water interface, sorption at the air-water interface, or association of the alkylbenzenes in solution, causing nonideal behavior. The time to reach equilibrium and the absence of leakage from the cell were determined by adding a known amount of an alkylbenzene mixture to the cell partially filled with water or a suspension of aluminum oxide and measuring the concentration of alkylbenzenes in the gas phase over time. In equilibration experiments carried out over 5 days with air and water in the cell, the gaseous concentration of the alkylbenzenes varied by 2 % or less after an initial drop in concentration in the first 160 min of the experiment. The initial high concentrations were caused by the volatilization of the alkylbenzenes from a separate organic (alkylbenzene) phase that floated on top of the water before dissolving. Subsequently, there was no systematic decrease in concentration; hence, no leakage occurred. The routine equilibration time when alkylbenzene mixtures were added to only air and water was 180 min. Equilibration experiments with alkylbenzene mixtures and suspensions required 350 min to reach equilibrium. The coefficient of variation of the gaseous Concentration of the alkylbenzenes in an experiment with 1g/L aluminum oxide was I1% over 9 days after the first 6 h. No slow sorption step occurred for this system. Benzene dissolved into the water most rapidly of the alkylbenzenes because of its highsolubility. When benzene alone was the sorbate, at least 240 min of equilibration time was allowed. The absence of processes other than air-water exchange and sorption to the aluminum oxide was tested by titration of a mixture of alkylbenzenes into distilled water at 20 “C to give mole fractions in the range of 10-8-10-6 and measurement of the equilibrium gaseous concentration at each titration increment. The aqueous mole fractions used in the sorption experiments also ranged from lo-* to
E' +
I
41
U
d 2
0 TOLUENE ÐYLBENZENE 7 PROPYLBENZENE BUTYLBENZENE
n 0.00
0.02
0.04
C o n c e n t r a t i o n in W a t e r
0.06
(mole m-3)
Flgure 2. Lack of nonlinear sorption to the cell wall or the air-water interface demonstrated by plotting measured partial pressure versus aqueous concentration computed according to eq 3. Linear regressions through the data for each alkylbenzene had zero intercepts, and the slopes of the regressions correspond to the Henry's law constants of each alkylbenzene.
The aqueous concentration was computed at each increment by mass balance (eq 3). According to eq 3, sorption to the cell wall or the air-water interface, both of which can be described by a sorption coefficient K = cs/cw or K = C ~ / C Awould cause an error in the calculation of KH.If the sorption to the cell wall or the air-water interface were not linear, plots of CA versus cw would not be linear and regressions would yield nonzero intercepts. Linear regressions of plots of partial pressure versus aqueous concentration had zero intercepts (Figure 21, and there was no curvature as aqueous concentration increased (r2 2 0.964). The absence of sorption to the glass cell was further tested by measuring the KH value of benzene at two water volumes. In each case, the same K Hvalue was determined, indicating negligible sorption to the glass container or to the air-water interface. According to calculations based on the air-water partitioning model of Valsaraj (22),less than 2 X mol of total alkylbenzenes would be bound to the air-water interface at the highest aqueous concentrations used in the sorption experiments with alkylbenzenemixtures, 2 order of magnitude less than the lowest sorbed mass reported here. This indicates that partitioning at the air-water interface is negligible relative to sorption to the suspended solids. The lack of curvature of the partial pressure versus aqueous concentration data also verifies that the activity coefficients of the alkylbenzenes do not differ from one (ideal behavior) in this concentration range. The measured Henry's law constants compare favorably with values reported in the literature (Table I). By inference, the mass balance technique for computing the concentration in the aqueous phase must be accurate, and sorption to the cell wall or to the air-water interface must be negligible. Linear regressions of the natural logarithm of the measured K H values given in Table I versus the inverse of absolute temperature in the range 10-30 "C have correlation coefficients 20.9945, substantiating the internal consistency and precision of the data in this temperature range. The measured Henry's law constant of benzene at 20 "C was not significantly different than the value given in Table I when benzene alone was titrated into the cell at mole fractions of 10-8-10-6. This indicates that the presence of the other alkylbenzenes had no influence on
the KHvalue of benzene in the sorption experiments with alkylbenzene mixtures. Aluminum Oxide Characterization. The mineral used in these sorption experiments was aluminum oxide. The aluminum oxide was identified by X-ray diffraction analysis as a-Al203 or corundum. The corundum was virtually free of organic matter, with a fractional organic carbon content of 0.000012 f 0.000003g of C/g. The mean diameter of the corundum was 6.0 f 0.5 pm. The specific surface area of the corundum was 0.55 m2/g,which is two times greater than the value computed by assuming that the particles are nonporous spheres with a diameter of 6.0 pm and a density of 3.97 g/cm3(Alcan Co.). The particles have a pH,,, equal to 9.0 f 0.1. Sorption Experiments with Benzene Alone. The four isotherms in which benzene was the only solute do not level off at surface coverages of one (Figure 3). The Freundlich model, C, = KC,", was fit to the data by leastsquares regression with n = 1 (linear regression) and by using n as a fitting parameter (Table 11). The mean square of the regressions, where e and v errors, MSE = (Sei2/u)2, are the residuals and the degrees of freedom, respectively, were 1-13 % smaller when n was used as a fitting parameter. All n values were slightly less than 1when n was used as a fitting parameter, indicating that the sorption of the benzene molecules was limited by the available mineral surface at each solids concentration. Because the difference between the two models is small (see Figure 3), the linear model was used in the discussion of the solids concentration effect (below). It is important to determine whether or not the sorption coefficients (calculated by linear regression as the slope of the isotherm) were statistically significant and different at the low masses of benzene sorbed in these experiments (3-10% of the total mass). Sensitivity analysis of the computed sorption coefficient revealed that it was most sensitive to variations in the mass of solute added to the cell (MT), with variations in the Henry's law constant ( K H )response , factor (RF), and area counts from the cell sample (Ac) also being important. The error was propagated through the linear regression according to the method of Woodward (23). The relative standard deviations for the parameters used in the computation of the aqueous and sorbed concentrations are given in Table 11. The propagated errors (95% confidence limits; Table 11) indicate that the sorption coefficients were significantly greater than zero and significantly different from one another even at the low masses of solute sorbed. The sorbed concentrations (Figure 3) and the sorption coefficients (Table 11) decrease as solids concentration increases. This so-called "solids concentration effect" has been observed in laboratory studies (24-26) and in field studies (27,281. These data are surprising in that anumber of the possible causes that have been suggested for the decrease in the sorption coefficient with increasing solids concentration cannot be invoked for these experiments, as discussed further below. Sorption Experiments with Alkylbenzene Mixtures. Sorption experiments with alkylbenzene mixtures were carried out at three solids concentrations: 0.1 g/L, 1.0 g/L, and low aqueous concentrations [1(LC) g/Ll, 1.0 g/L and high aqueous concentrations [l(HC) g/Ll, and 2.75 g/L. In these experiments, an alkylbenzene molar mixture of 5:5:5:4:1 benzene:toluene:ethylbenzene:propy1benzene:butylbenzene was titrated into the cell. The Environ. Scl. Technol., Vol. 27, No. 5, 1993 931
Table I. Comparison of Measured Henry's law Constants and Standard Deviations (lo3 atm m3 mol-') with Literature Values 10 O c a benzene toluene ethylbenzene propylbenzene butylbenzene 15 "C" benzene toluene ethylbenzene propylbenzene butylbenzene 20 O C C benzena toluene ethylbenzene propylbenzene butylbenzene 25 O c a benzene toluene ethylbenzene propylbenzene butylbenzene 30 O c a benzene toluene ethylbenzene propylbenzene butylbenzene
this study 2.86 f 0.010 2.89 f 0.010 3.02 f 0.010 4.35 f 0.001 5.35 f 0.008
ref 14 3.30 f 0.050 3.81 f 0.200 3.26 f 0.0205 5.68 f 0.196 -
3.75 f 0.020 3.85 f 0.025 4.22 f 0.025 6.21 f 0.040 8.17 f 0.140
3.88 f 0.246 4.92 f 0.363 4.51 f 0.254 7.31 f 0.352 -
ref 12
ref 19 -
-b -
-
I
I
3.82 f 0.05 3.91 i 0.07 4.53 f 0.21 5.86 f 0.21 -
-
-
-
-
4.54 f 0.100 4.92 f 0.121 5.75 f 0.156 8.37 f 0.134 11.0 f 0.312
4.52 f 0.218 5.55 f 0.145 8.01 f 0.217 8.81 f 0.109
-
-
-
-
5.96 f 0.008 6.51 f 0.034 7.84 f 0.038 11.6 f 0.085 16.7 f 0.112
5.28 f 0.249 6.42 f 0.039 7.88 f 0.23 10.8 f 0.283 -
6.00 f 0.07 6.56 f 0.2 7.87 f 0.25 10.5 f 0.30 -
5.43 f 0.25 6.61 f 0.35 8.0 f 0.7 7.0 f 3 13.0 f 2.5
7.31 f 0.051 8.27 f 0.112 10.3 f 0.029 15.3 f 0.164 21.4 f 0.642
7.20 f 0.377 8.08 f 0.108 10.5 f 0.239 13.7 f 0.189 -
-
-
-
-
a Two replicates. The symbol - indicates that the Henry's constant values were not measured a t the given temperature. Thirty-six replicates.
T -
: h
; 3.
A 1.0 g/L
Table 11. Parameters Determined for Benzene Sorption to Corundum According to the Freundlich Modela n=l
150
n used as fitting parameter
v
solids concn (g L-I)
z 0 E-
K f 95% CL (mL 9')
K (mL E')
n
18 15 6.7 2.1
0.89 0.98 0.92 0.90
4
a P
3 U
1.0 2.75 6.0 10.0
10 3
z 0
U
B
w
m
a With n set equal to 1and when n was used as a fitting parameter (isotherms shown in Figure 3). The relative standard deviations (in percent, based on replicate measurements) used in propagating the error in the sorption coefficients were as follows: M T , 3; K H ,2; RF, 5. 2; Ac, 1; VA, 5; Vw, 0.1; MSOLID,
5 3
a 0 rn 30 33
47 f 6.0 19 f 3.1 12 f 1.0 5.0 f 0.57
20
43
6C
A Q U E O U S CONCENTRATION (pmole/L)
Flgure 3. Sorbed concentrationsand surface coverages of benzene sorbed to corundum versus equilibrium aqueous concentration at four solids concentrations. The error bars indlcate the 95% confidence limits of the sorbed concentrationcomputed by error propagation.The y-intercepts of llnear regressions through the data of each isotherm (solid lines) were not significantly different from zero. The dotted lines show the Freundiich model when n was used as a fitting pafameter. The slope of the linear regressions (equal to the sorption coefficient, Freundlich model, n = 1) decreases with increasingsolids concentration.
fraction of the total molar mass of the alkylbenzenessorbed (15-50% ) was higher than when benzene alone was sorbed at the same solids concentrations. The sorbed concentrations of benzene in the alkylbenzene mixture increased to values that were up to five times greater than sorbed concentrations when benzene alone was sorbed (Figure 4), indicating that the presence of the other alkylbenzenes enhanced the sorption of benzene. 932 Environ. Sci. Technol., Vol. 27, No. 5, 1993
Concentrations in the sorbed phase of the 1.0 g/L, high aqueous concentration (Figure 5a), 0.1 g/L, and 2.75 g/L isotherms (not shown) increase exponentially with increasing aqueous concentration. When the low and high aqueous concentration isotherms at 1.0 g/L solids concentration are combined, resulting in a wider aqueous concentration range (Figure 5b, only benzene results shown), the exponential increase is even more apparent. The exponential shape of the isotherms indicates interaction of the sorbed solutes. When the force of interaction between sorbed solute molecules is significant relative to that between solute and sorbent, cooperative sorption occurs corresponding to this type of isotherm (29). The increase in the computed sorption coefficient for each datu point [K = sorbed concentration (mol/g)/ aqueous concentration (mol/mL)l with foe*, computed according to eq 8 (Figure 6; only benzene results shown),
5-
h
OEENZENE
60
A
0 1 g/L,
0
A
5
v
A
A
Benzene alone 1 (HC) g/L, Benzene in mixture
A
A PROPYLEENZENE
3
0 BUTYLBENZENE
z
A
AA
40-
A
z
A
E z
8
20-
A A A
t;l E 0"
m
w
100
A
0
0
A
b
A
0
W
Q
~
@
0
0
0 D
-
200 AQUEOUS CONCE NTRATlON
300
AQUEOUS CONCENTRATION (pmole/L)
400
(prnole/L)
e
Flgure 4. Comparlson of the sorption Isotherm of benzene (alone) to the sorption isotherm of benzene in the aikyibenzene mixture at a corundum concentration of 1.0 g/L at high [l(HC) g/L] aqueous concentrations. The aqueous and sorbed concentration of each of the alkylbenzenes at each titration polnt is shown in Figure 5a. The increase in the sorbed concentration of benzene wlth the alkyibenzene mixture at surface coverages greater than 7 (foe* > 0.001) indicates that the presence of the other alkylbenzenes enhances the sorption of benzene.
may result from surface interactions of the alkylbenzene molecules. The linear increase in the log(sorption coefficient) with log(f,,*) for foc* > 0.001 may indicate partitioning of the alkylbenzenes from solution to alkylbenzenes sorbed to the mineral surface (3). Because there are many layers sorbed (10-104, eq 9), an immiscible layer of alkylbenzenes may exist at the mineral surface. The sorbed concentration, and thus the sorption coefficient, decreased upon addition of solids because the sorbed molecules were then spread over a greater surface area of the mineral. The foc* and surface coverage of the solids decreased as solids concentration increased. Solute interactions would be expected to diminish as surface coverage decreases and hence as solids concentration increases. The 1.0 g/L, low aqueous concentration, and 2.75 g/L sorption isotherms extend to low surface coverages where a sorption mechanism other than partitioning of solute molecules to sorbed solute molecules may become important. For these isotherms, total surface coverages varied from 0.1 to 14 layers. These isotherms correspond to the data points at foc* < 0.001 [log(foc*)= -31 in Figure 6. At low foe*, the sorption coefficients do not increase as markedly with foc* as for foc* > 0.001, although they are positively correlated withfo,*. Although surface interactions of sorbed solute molecules may occur at foc* < 0.001,. these interactions appear to have less influence on the sorption coefficientthan at foc* > 0.001.
0
0
W
2ol
w
0
0 0
10
U
0
rn
8
0
are * 50
0
1 00
150
2 0
AQUEOUS C 0NC ENTRATIO N (p-no Ie/L) Flgure 5. (a, Top) Sorbed concentrations and surface coverages (m2/ m2) of the aikylbenzenes versus aqueous concentration at a solid concentration of 1.O g/L and high aqueous concentrations. Aqueous concentrations are < 10% of aqueous solubilities. Sorbed concentrations increase exponentially with aqueous concentrations. At the same aqueous concentration, sorptlon increased wlth increasing compound hydrophobicity. (b, Bottom) Isotherms of benzene in the alkylbenzene mixture sorbed to corundum at solids concentrations of 1.O g/L and low aqueous concentration [ 1(LC) g/L] and high aqueous concentration [ 1(HC) g/L]. The initial aqueous concentrations of the alkylbenzene cosoiutes in the 1(LC) g/L experlment were all approximately 5 X M and increased to approximately 6 X M. The aqueous and sorbed concentrations of the alkyibenzene cosolutes in the 1(HC) g/L experiments are presented in panel a. I
. -
0
G
I
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Discussion Sorption Experiments with Benzene Alone. The lack of leveling off of the benzene isotherms beyond surface coverages of one indicates that the benzene molecules do not interact with one another on the mineral surface at these calculated surface coverages (0.1-7 layers), in contrast to the alkylbenzene mixture isotherms. The lack of leveling off at surface coverages of one also indicates that the surface coverage computation, which assumes parallel adsorption, may not accurately describe the sorption process. There are three possible explanations for this. First, the surface area available for sorption of benzene may be different (higher) than the surface area
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109(foe*) Flgure 6. Computed sorption coefficients increased ilnearly wlth foco (6,resulting from sorbed aikylbenzenes) at foe* > 0.001. Below an foe* of 0.001 (surface coverage of approximately lo), surface interactions of the aikylbenzenes are much reduced. Included are data of Capel (39; open squares) in which sorptlon experiments wlth bentonite as sorbent and an alkylbenzene mixture were carried out using headspace analysis (see text for details).
available for Nz sorption (used to measure the specific surface area of corundum by BET analysis). This explaEnviron. Scl. Technol., Vol. 27, No. 5, 1993
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Loa! 0 2.0-2.5 X 2.5-3.0 0 3.0-3.5 A 3.5-5.0
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Log(S0LIDS CONCENTRATION) ( g i L )
Flgure7. Sorptioncoefficient(log) is plottedversus solids concentration (log) for studies from the literature in which HOCs were sorbed to minerals with f,, 0.001 (data from refs 5 and 44-49). The line is the regression of the sorption coefficients of benzene to corundum versus solids concentration from this study [log(KJ= -0.94 log (solids concentration)4- 1.71. When extrapolated up to the solids concentratlon used in typical Sorptionexperiments,the sorption coefficientsmeasured by headspace analysis are in the range of those reportedin the literature.
nation is intuitively contradictory, however, because N2 has approximately three times less surface area, and so would be expected to reach regions on the mineral surface where benzene could not. Thus, if anything, the surface area available for benzene sorption would be expected to be lower than that determined by N2 sorption. Rhue et al. (30)found that the surface areas available forp-xylene, toluene, and ethylbenzene sorption on bentonite, kaolin, Webster soil, and Lula aquifer material were similar to those measured by Nz sorption in the absence of water. Second, the benzene molecule may sorb at an orientation other than parallel to the mineral surface. This, however, is contradictory to the expectation that the x-system of benzene would interact with the mineral surface, causing parallel sorption. Using the surface area for upright sorption of benzene to silica of 25 nm2 cited by No11 (21), the final surface coverage in the 1.0g/L solids concentration experiment is 1.75, still somewhat greater than monolayer coverage. If the first layer of benzene molecules sorbed parallel to the mineral surface,the second layer of benzene molecules could interact with the first layer either by London forces or perhaps by hydrogen bonding using water molecules as “bridges”. Recently, Suziki et al. (31) demonstrated that water molecules formed hydrogenbonded clusters positioned above the benzene mystem using vibrational rotational tunneling far-IR spectroscopy. Alternatively, the sorption process is not adsorption of benzene to the mineral surface, but rather partitioning of benzene into the water layer adjacent to the mineral surface (vicinal water), as posited by Schwarzenbach et al. (9). The range of measured sorption coefficients (5-50 mL/ g, Table 11) is higher than that of reported sorption coefficients (approximately 0.01-10 mL/g) from studies using HOCs and low focsorbents (Figure 7). The solids concentration effect that causes a decrease of approximately 1 order of magnitude in the sorption coefficients per order of magnitude increase in solids concentration may explain the difference between the sorption coeffi934
Environ. Sci. Technol., Vol. 27, No. 5, 1993
cients here and in the literature, which were carried out at much higher solids Concentrations (50-5000 g/L). In order to compare the sorption coefficients of benzene sorbed to corundum with those reported in the literature, a linear regression of the log(K) versus log(so1ids concentration) data was performed. The linear regression allows extrapolation of the sorption coefficients up to the solids concentrations typically used in batch sorption experiments with low-f,, materials (50-5000 g/L) and HOCs. The equation of the regression was log(K) = -0.94 log(so1ids concentration)+ 1.76 (10) n = 4 , r = 0.976 The regression is plotted in Figure 7 together with sorption coefficients reported for batch experiments with low foc materials cfoc 0.001) and HOCs of varying hydrophobicities [log(K,,) = 2.1-5.01. In Figure 7, the literature data are separated according to halflog units of KO,[i.e., log(K,,) = 2.0-2.5, 2.5-3.0, etc.] to facilitate comparison of the sorption coefficient of benzene [log(Ko,) = 2.11 with the literature values. From Figure 7 it is clear that when extrapolated up to the solids concentrations reported in the literature, the benzene-corundum sorption coefficients fall within the range of reported values. However, the data in the literature cover many types of solids with differing surface characteristics. Experimental data on one or more solids over a range of solids concentrations are needed. Various explanations for the solids concentration effect have been put forth in the literature. They have been reviewed in detail and discussed with respect to these and, therefore, only the results in another publication (8), main points will be discussed here. Of the explanations put forth in the literature, a number cannot be invoked to explain the present results. In headspace measurements, physical separation of the solid from the aqueous phase is not necessary for the determination of the sorbed and aqueous concentrations. Therefore, incomplete separation of colloid-associated HOC from the aqueous phase (32) cannot be invoked. If HOC is sorbed to colloidal-sized mineral particles, it will not bias the sorption results. The only way that the presence of colloids could affect the sorption coefficient in these experiments is if the mass ratio of colloidal-size particles to larger particles changed as a function of solids concentration. To decrease the sorption coefficient with increasing solids concentration, the ratio would have had to decrease, thus decreasing the surface area available for sorption. Such an aggregation of particles was not apparent from particle-size distributions of the particles taken before and after the sorption experiments (33). Even though these particle-size distributions do not give information about particles 2 pm if it were occurring. I t is concluded that neither the presence of colloids nor particle aggregation explains the observed decrease in the sorption coefficient. As discussed earlier, no slow sorption step was found for this system. Thus, lack of attainment of sorption equilibrium, reviewed by Elzerman and Coates (34),cannot explain the solids concentration effect. Slow sorption kinetics would not be expected for this system because the corundum was shown by scanning electron microscopy and comparison of geometric versus BET surface area to be nonporous, and, therefore, intramineral diffusion is
negligible. The combusted corundum has low natural organic carbon content (foc = 0.000012), and thus intraorganic matter diffusion is probably negligible. As previously discussed, particle-size distributions taken before and after the sorption experiments do not change. Thus, there is no evidence of particle aggregation, and intraaggregate diffusion is, therefore, unlikely to explain the solids concentration effect. Of the explanations that have been put forth for the solids concentration effect, only a particle interaction model (35,36)can be invoked to explain these data. This model assumes that the “loosely sorbed” or “reversibly sorbed” component of the HOC can be desorbed upon particle collision. As solids concentration increases, the frequency of particle collision increases, and the resulting HOC desorption decreases the sorption coefficient. For the benzene-corundum results presented here, all of the sorbed alkylbenzene would be expected to be at the surface rather than in particle aggregates, pores, or natural organic matter. The data of van Hoof (37) also support the view that only the surface associated HOC is affected by the solids concentration effect. In batch sorption experiments of 4-monochlorobiphenylto well-characterized polystyrene particles, the measured solids concentration effect was greatest initially and decreased with exposure time (Le., the sorption coefficient of batch experiments carried out at different solids concentrations approached the same value over time). If it is assumed that the initial mechanism by which monochlorobiphenyl bound to the surface was surface adsorption followed by diffusion of the PCB molecule into the bulk solid, then there may have been a solids concentration effect exerted on surface adsorption. It remains an open question whether the energy of particle collision is great enough to cause solute desorption. We suggest the possibility that the energy for desorption originates from structuring forces that arise in the solvent when particles approach one another. Israelachvili and McGuiggan (38) reviewed recent measurements of forces between liquids and solids at a resolution of 1 A. In particular, they reviewed studies that have found new, “structuring” forces that result when surfaces approach one another within distances of 5-10 molecular diameters of the solvent (approximately 10-30 A when water is the solvent). At separation distances greater than 5-10 molecular diameters, the force laws are as expected from continuum theories; that is, an attractive van der Waals force and a repulsive double-layer force if the surfaces are charged. At separation distances less than 5-10 molecular diameters, however, the liquid can no longer be treated as a structureless continuum, and an additional solvation force, or “hydration”force if the solvent is water, is present that oscillates with the distance between the surfaces, varying between attraction and repulsion, with a periodicity equal to the mean diameter of the solvent molecules. In aqueous solution the hydration forces between hydrophilic surfaces can also be monotonically repulsive, with characteristic decay lengths of 6-15 A, making them relatively long range. According to the authors, these effects are closely related to the phenomenon of the spontaneous condensation of a liquid phase of solute from a solvent. These surface-induced nucleations can occur even when two surfaces are very far apart, as much as 100
A.
Perhaps it is these hydration forces that cause the solids concentration effect observed for the benzene-corundum sorption coefficients. As solids concentration increases, particles approach each other closely more frequently, which imposes a new structure on the water between the particles. This could cause either a decrease in the activity coefficient of sorbed water or an increase in the activity coefficient of sorbed solute. In either case, a decrease in the measured sorption coefficient would result because both sorbed solute and sorbed water are assumed to behave ideally in the sorption coefficient computation. This postulation seems more plausible still if HOC sorption is viewed as a partitioning of HOCs into vicinal water. Then factors that affect vicinal water would affect the sorption coefficient. The vicinal water influenced by hydration forces cannot accommodate the HOC molecules, and the net effect is to decrease the sorption coefficient when the sorbed solute and water are assumed to behave ideally. Sorption Experiments with Alkylbenzene Mixtures. The sorption coefficients of bentonite have the same dependence on foc* as the sorption coefficients of corundum (Figure 6). These sorption experiments were performed by Capel (39) using the headspace analyzer of Hussam and Carr (10) with bentonite as sorbent, an equimolar mixture of alkylbenzenes as solutes, and a pH, ionic strength, and temperature of 6 4 0 . 1 M, and 25 “C, respectively. The bentonite had a specific surface area of 31 m2/g (Micromeretics Consultants, Norcosse, GA), a diameter of 6 pm, and an foc of 0.00014. In this experiment the alkylbenzene mixture was first added to the cell containing air and water only and allowed to equilibrate. A bentonite slurry was then titrated incrementally into the cell to give four solids concentrations between 0.01 and 10mg/L, and the equilibrium headspace concentration was measured following each addition of bentonite. Thus, the influence of the type of mineral surface on the sorption reaction appears to be negligible at high surface coverages. The correlation between the results of Capel (39) and the results reported here indicates that the high sorbed concentrations were not the result of adding the alkylbenzene mixtures to the cell after the solids were added; in Capel’s experiments the alkylbenzenes were preequilibrated in the cell before adding the solids to the cell. The linear increase of the sorption coefficient with foc* when foc* > 0.001 (demonstrated in Figure 6 for benzene) suggests that when alkylbenzene mixtures were sorbed to corundum, the sorption process was partitioning of alkylbenzenes between solution and (previously sorbed) alkylbenzenes. In column experiments using low foc (
