Environ. Sci. Technol. 2004, 38, 4439-4444
Hindered Gas-Phase Partitioning of Trichloroethylene from Aqueous Cyclodextrin Systems: Implications for Treatment and Analysis N. KASHIYAMA AND T. B. BOVING* Department of Geosciences, University of Rhode Island, Kingston, Rhode Island 02881
Chemically enhanced flushing has shown great promise for attenuating subsurface nonaqueous phase liquid (NAPL) contamination. One particular chemically enhanced remediation technology is cyclodextrin enhanced flushing (CDEF). CDEF has been demonstrated as a viable alternative to conventional and innovative remediation methods. However, the presence of cyclodextrin (CD) in solution complicates the treatment and analysis of volatile organic compounds, such as trichloroethylene (TCE). The principal reason for the complications is the presence of TCE in three compartments instead of two, i.e., the aqueous solution, the vapor phase, and complexed inside the soluble CD molecule. Aqueous TCE-CD systems were examined at various concentration and temperature conditions and their respective Henry’s law constants were measured. The presence of CD significantly decreased Henry’s law constant of TCE. On the basis of these results, a quantitative model was developed to predict the additional effort that becomes necessary when air-stripping TCE from CDEF flushing solution. The modeling results demonstrate that the presence of CD requires significantly higher gas flow rates or longer residence times of the flushing solution inside an air stripper. Similarly, current gas chromatographic purge-and-trap methods for TCE analysis in CD solution appear to underestimate the aqueous phase TCE concentration if the CD concentration of the sample is not accounted for. Although this model was developed specifically for CD-TCE systems, it is likely that these results have implications for other VOCs and other solubilization enhancing agents, such as surfactants or cosolvents.
three phases requires different approaches and treatment technologies. For example, the removal of residual VOCNAPL from the solid phase can be enhanced by flushing aqueous solutions of solubility enhancing chemicals (e.g., surfactants or cyclodextrin) through the contaminated pore space. When NAPL comes in contact with the flushing solution, the chemical agent facilitates NAPL partitioning into the aqueous phase. The dissolved contaminant can then be pumped to the surface for further treatment, e.g., by air stripping. Alternatively, a VOC can be removed from the subsurface by sparging air through the contaminated aquifer or by applying a vacuum to the unsaturated zone. In both cases, the VOC partitions into the vapor phase and is brought to the surface for further treatment. Because the vaporization of VOC is such an effective phase transfer process (3), and, compared to water, much lower gradients are required to initiate soil air flow (4), many remediation and wastewater treatment approaches are based on vaporization (5-8). Also, many standard chemical analysis methods relay on vaporization of VOC from the aqueous into the gas phase (e.g., purge-and-trap after EPA method 5030). The factors controlling vaporization include the contaminant phase, the contaminant’s vapor pressure, soil permeability, and environmental factors such as temperature (9). Typically, Henry’s law is used to describe the distribution of a volatile contaminant between the aqueous and gas phase. There are two forms of the Henry’s law constant. One form is the dimensionless Henry’s law constant (Hc), which is described as the ratio of a compound’s concentration in gas phase (Cg; mol/m3) and that in aqueous phase (Caq; mol/m3) at equilibrium
Hc )
Cg Caq
(1)
The second commonly used form of Henry’s law is expressed as
kH )
Pv Caq
(2)
where kH is the Henry’s constant (atm‚m3/mol) and Pv is the vapor pressure (atm). The vapor pressure as a function of temperature can be expressed as Pv ) CgRgT. Thus, both forms of the Henry’s law can be related by
kH ) HcRgT
1. Introduction The presence of nonaqueous phase liquids (NAPLs) is considered the single most important factor limiting site remediation (1). NAPL-type contaminants, such as trichloroethylene (TCE), are common groundwater contaminants found at numerous sites (2). If a large amount of spilled dense nonaqueous phase liquids (DNAPL) is present, the contaminants may migrate into the saturated zone. Trapped in the pore space by capillary forces, residual NAPL then becomes the source of long-term groundwater pollution. Especially volatile organic contaminants (VOC), such as most chlorinated solvents, tend to partition between the liquid, solid, and vapor phase. The remediation of each of these * Corresponding author phone: 401-874-7053; fax: 401-874-2190; e-mail:
[email protected]. 10.1021/es049956q CCC: $27.50 Published on Web 07/20/2004
2004 American Chemical Society
(3)
where T is absolute temperature (K), and Rg is the universal gas constant (atm‚L/mol‚K) According to ref 10, Henry’s law is valid as long as the vapor phase does not react with the aqueous phase. However, the presence of chemical flushing agents in solution is expected to impact HC. This is because the addition of chemical flushing agents enhances the solubility of hydrophobic compounds, for example, by forming chemical inclusion complexes or aggregates (micelles), or facilitating the transport by contaminant absorption. Although these VOC-flushing agent interactions are mostly reversible, breaking them up, e.g., by air stripping, may require more effort compared to stripping VOC from just water. Important design parameters that determine the size and cost of an air stripping system are residence time and the air flow rates necessary to achieve the desired treatment goal. Both VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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parameters are controlled by a range of reactor-specific and environmental variables (such as type of reactor, bubble size and density, temperature; e.g., ref 11). The addition of solubility-enhancing agents and the possible water-air mass transfer restrictions associated with their presence add a further variable to the air stripper design process. Among the chemical flushing agents that have shown potential for contaminant solubility enhancement are complexing agents, surfactants, and dissolved organic matter (12). This study focuses on a particular type of complexing agents: cyclodextrins. These cyclic sugars have received significant attention as possible remedial agents for sites contaminated with VOC-NAPL (13). The toriodial interior of the cyclodextrin oligomer is formed by six to eight glucopyranose units and creates a hydrophobic cavity that is attractive for hydrophobic contaminants to partition into. The exterior of the cyclodextrin molecule is polar, making cyclodextrins very water soluble (14). The cyclodextrin molecule forms weak complexes with many types of hydrophobic contaminants, such as pesticides, polycyclic aromatic hydrocarbons (PAHs), and explosives (15-17). The formation of cyclodextrin-contaminant complexes significantly increases the apparent solubility of many low-solubility organic contaminants and is the basis for cyclodextrin use in groundwater remediation. During a typical CD-enhanced flushing treatment (CDEF) application, CD solution is injected via injection wells located upstream of the source zone. After passing through the contaminated zone, the contaminant-cyclodextrin complex is then pumped to the surface from one or more extraction wells located downstream. CDEF technology has been successfully used in the field several times for both pilot- and full-scale operations (18-21). The separation of VOC by air stripping from the CD flushing solution and subsequent reuse of the CD is a principal requirement for cost-efficient use of this remedial approach (21). However, it is currently not known whether or how much the presence of CD in the aqueous solution impacts the aqueous-gas-phase exchange of VOCs. The primary objective of this study is thus to determine the Henry’s law constant for TCE in cyclodextrin solutions at various concentrations and different temperature conditions and use these data to develop a quantitative model for TCE partitioning from cyclodextrin solution into the gas phase. TCE was selected because it is one of the most common VOCs in contaminated groundwater. By quantifying the distribution of TCE between the gas and aqueous phase at various temperatures and CD concentrations, crucial information is gained for designing effective air stripping systems for treating recovered CD flushing solution. In addition, quantification of a CD concentration and temperature dependency of TCE vaporization will improve the interpretation of chemical analysis data of aqueous TCE samples.
2. Materials and Methods Equilibrium partitioning in closed systems (EPICS) procedures described in refs 22 and 23 were employed to determine the Henry’s law constant. In principle, properly sealed glass vials containing TCE dissolved in aqueous hydroxypropylβ-cyclodextrin (HPCD) solution were allowed to equilibrate with the gas phase in the vial’s headspace. The incubation was carried out under strictly controlled temperature conditions. The gaseous concentration, or headspace concentration, of TCE in the sealed vials at equilibrium was measured by gas chromatography. The HPCD (CAVASOL W7 MTL) was donated by Wacker Inc. In the following, CD refers to HPCD. The 40.1 wt % 4440
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aqueous CD solution (technical grade) was used as received. The TCE was purchased from Aldrich (ACS reagent, 99.5%). Henry’s law constant for TCE in cyclodextrin solution was determined at six different temperatures (10, 20, 30, 40, 50, and 60 °C). The following set of procedures was repeated at each temperature. Aqueous solutions of cyclodextrin at 1, 2, 5, and 10% (by weight) were prepared. For each concentration (including a blank of deionized water), three vials (22-mL standard headspace glass vials) were filled with 2 mL of cyclodextrin solution. A second set of vials was filled with 5 mL of cyclodextrin solution. All vials were sealed with Teflon rubber septa and aluminum clamp caps. Triplicates were used to ensure quality control. The vials were incubated in an oven at the precise temperature for which Henry’s law constant was to be obtained. After 24 h equilibration time, 2 µL of aqueous TCE solution (saturated; 1084 mg/L at 22 °C) was injected into each vial using a gastight syringe. Because the ratio of the injected volume to the total vial volume was very small, the resulting pressure increase inside the vial was considered negligible. All vials were stored in inverted position to minimize TCE losses and to prevent the gas phase from contacting the septum. After the TCE was injected, the vials were again incubated for 24 h. Finally, a headspace vapor sample from each vial was withdrawn using a gastight syringe. The syringe was heated to the same temperature as the sample to prevent condensation. The vapor sample was immediately injected into a Shimadzu GC-17A gas chromatograph equipped with a FID and a DB-5MS column (30 m long, i.d. 0.32 mm, film 0.25 µm). The flow rates of ultrahigh purity H2, air, and N2 gases were 45 , 450, and 30 mL/min, respectively. The column flow rate was 1.7 mL/min (velocity 27 cm/sec). The following two-step temperature program was applied: initial temperature was set at 35 °C with a 1 min hold, then rising at the rate of 2 °C/min to 50 °C with no hold; rising at the rate of 8 °C/min to 140 °C with a 5 min hold (program duration: 24.75 min). Dimensionless Henry’s law constants were calculated from the ratios of the 2- and 5-mL vials for each temperature and each cyclodextrin concentration. The distribution of TCE between the aqueous and the gas phase at equilibrium in each vial can be given by the following expression (22):
Vial 1: maq1 ) M - mg1 )
Cg1Vaq1 Hc
(4)
Vial 2: maq2 ) M - mg2 )
Cg2Vaq2 Hc
(5)
where M is the total amount of TCE injected to the vials; maq1 and maq2 are the mass of TCE partitioned into the aqueous phases of vials 1 and 2, respectively; mg1 and mg2 are the mass of TCE partitioned into the gas phases of vials 1 and 2, respectively; Vaq1 and Vaq2 are the volumes of aqueous phases in the vials; Cg1 and Cg2 are the concentrations of TCE in the gas phase. Combining eqs 4 and 5 yields
maq1 - maq2 ) mg2 - mg1 )
Cg1Vaq1 Cg2Vaq2 Hc Hc
(6)
Thus, the dimensionless Henry’s law constant (Hc) can be expressed as
Hc )
(Cg1Vaq1 - Cg2Vaq2) (Cg2Vg2 - Cg1Vg1)
(7)
FIGURE 1. Henry’s law constants for aqueous TCE solutions (atm‚ m3/mol). The experimental data and regression line modeled by van’t Hoff equation are compared to data from refs 23-25.
FIGURE 2. Effect of temperature and CD concentration on the Henry’s law constants (atm‚m3/mol).
TABLE 1. Henry’s Law Constants at Different Temperatures and Various Cyclodextrin (CD) Concentrations average kH of TCE (L‚atm/mol) temp. (°C)
water
1% CD
2% CD
5% CD
10% CD
10 20 30 40 50 60
0.0043 0.0071 0.0131 0.0199 0.0357 0.0543
0.0026 0.0046 0.0089 0.0128 0.0196
0.0025 0.0024 0.0061 0.0084 0.0132
0.0016 0.0026 0.0029 0.0039 0.0045
0.0006 0.0007 0.0011 0.0016 0.0017
Further simplification results in
( )
Cg1 V - Vaq2 Cg2 aq1 Hc ) Cg1 Vg2 V Cg2 g1
( )
(8)
The integrated peaks areas from the GC-FID outputs are directly proportional to the sample concentration. Therefore, the ratio of concentrations(Cg1/Cg2) in eq 8 can be replaced by the ratio of integrated areas (Ag1/Ag2)
Hc )
rVaq1 - Vaq2 Vg2 - rVg1
(9)
where r is the ratio of integrated GC-FID areas (Ag1/Ag2). Using eq 9, the dimensionless Henry’s law constant is calculated by measuring the relative area of Ag1 and Ag2. This approach has the advantage that no actual concentrations have to be determined. In the following discussion, the average of three kH measurements is reported.
3. Results and Discussion Henry’s Law Constant for TCE in Water. Measured kH values for TCE in water in absence of cyclodextrin are plotted in Figure 1. It shows that kH increases exponentially with increasing temperature, indicating that TCE is less likely to stay in the solution at elevated temperatures. These experimental data are well predicted by the van’t Hoff equation. On the basis of the best-fit regression curve for TCE in water, van’t Hoff equation takes the following form:
ln kH ) 10.864 - 4602.60/T
(10)
where T is temperature (K). Our results for the TCE-water
FIGURE 3. Relationship between kH (atm‚m3/mol) and CD concentration at various temperatures. system at various temperatures are concordant with those of previous studies (23-25). Henry’s Law Constant for TCE in CDaq. The Henry’s law constants for TCE in solutions of various cyclodextrin concentrations are shown in Table 1. The results indicate that at a given temperature, kH decreases with increasing cyclodextrin concentration (Figures 2 and 3). To systematically understand this result, we first deduced a hypothetical relationship between TCE solubility and cyclodextrin concentration. Next, a model was developed to quantify this relationship. The model was then tested and calibrated using the data obtained in this study. Relationship Between kH for TCE and Cyclodextrin Concentration. As suggested by ref 15, cyclodextrin forms 1:1 inclusion complexes with many nonpolar organic compounds (eqs 11 and 12).
TCE(g) T TCE(aq)
(11)
TCE(aq) + cyclodextrin T cyclodextrin - TCE (12) The equilibrium constants for these reactions are
[TCE(aq)] 1 ) PTCE kH
(13)
[cyclodextrin - TCE] [TCE(aq)]‚[cyclodextrin]
(14)
K1 ) K2 )
where kH is the (true) Henry’s constant of TCE. The measured Henry’s constant in the presence of cyclodextrin-TCE VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Relationship between kH/kH-app values and CD concentration (%) at various temperatures. complex (apparent Henry’s constant; kH-app) can thus be defined as
kH-app )
PTCE [TCE(aq)] + [cyclodextrin - TCE]
(15)
kH-app is, by definition, identical to kH in the absence of cyclodextrin. As the concentration of cyclodextrin in the solution increases, kH-app decreases due to formation of the cyclodextrin-TCE complex. From eqs 14 and 15
kH-app ) ) kH kH-app
PTCE [TCE(aq)] + K2‚[TCE(aq)]‚[cyclodextrin] kH 1 + K2‚[cyclodextrin]
) 1 + K2‚[cyclodextrin]
FIGURE 5. Linear relationship between the equilibrium constant K2 and temperature of the CD solution.
(16)
Eq 16 predicts that the ratio of kH-app (the measured value for a cyclodextrin solution) to kH (the measured value for water) is proportional to the concentration of cyclodextrin in the solution. Values of kH/kH-app at each temperature are plotted against the aqueous cyclodextrin concentration in Figure 4a-e. It shows that data follow the approximately linear relationship as predicted in eq. 16. A similar linear relationship between CD and n-alcohols has been described by ref 26. Relationship Between kH for TCE-CD Systems and Temperature. With regard to Figure 4a-e, the data also indicate that the equilibrium constant K2 is a function of temperature, i.e., the slope of the linear regression is increasing with increasing temperature. By plotting the slopes obtained fromFigure 4a-e versus temperature, it can be 4442
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shown that K2 is approximately linearly related (R ) 0.94, n ) 5) to the temperature of the CD solution in the range between 20 and 60 °C (Figure 5). Thus, K2 can be approximated as
K2 ) RT + β
(17)
where R ) 0.0229, β ) -5.3776, and T is temperature (K). By combining eqs 10, 15, and 16, the apparent Henry’s law constant can be expressed as the function of cyclodextrin concentration and temperature
kH-app )
eA-B/T 1 + (RT + β)‚[cyclodextrin]
(18)
The values for constants A, B, R, and β were obtained by fitting to the results of this study. Thus, kH-app in the
TABLE 2. Values for kH/kH-app at Different Temperatures and Various CD Concentrationsa kH/kH-app temp (°C)
1% CD
2% CD
5% CD
10% CD
20 30 40 50 60
3.21 3.08 2.36 2.64 2.56
3.29 5.95 3.43 4.01 3.82
5.14 5.50 7.22 8.73 11.18
12.68 19.93 19.27 21.27 29.13
aApparent Henry’s constant, k H-app, is the measured Henry’s constant, kH, in the presence of the cyclodextrin-TCE complex (see eq 19)
FIGURE 6. Predicted removal rates of TCE from the water/TCE/CD system at 60 °C and a gas flow rate of 6 m3/min. temperature range between 20 and 60 °C can be expressed as
kH-app )
e10.864-4602.60/T (19) 1 + (0.0229T - 5.3776)‚[cyclodextrin]
Table 2 shows the relative Henry’s constant (kH/kH-app) at various temperatures and CD concentrations. In combination with a mass balance equation (27) (see eq 20), eq 19 can be used to predict the efficiency of air stripping TCE from cyclodextrin solution.
( )
k HG C ln ) t Co VRT
(20)
In eq 20, C and C0 are the actual solute concentration in liquid phase and the initial contaminant concentration (mol/ L), respectively, G is the gas flow rate (L/hr), V is the volume of the liquid (L), R is the gas constant (L‚atm/mol‚K), T is the system temperature (K), and t is time. Values for kH for an arbitrary cyclodextrin concentration in the range from 1% to 10% and at temperature T can be calculated from eq 19. Figure 6 compares the predicted removal rate of TCE from the water/TCE/cyclodextrin system at various cyclodextrin concentrations at 60 °C and a gas flow rate of 6 m3/min. It shows that the efficiency of air stripping is significantly lowered as the concentration of cyclodextrin increases. The time required for removing TCE is proportional to the concentration of cyclodextrin in the solution. For example, if C0 TCE ) 100 mg/L, the time required to remove 99% TCE (CTCE ) 1 mg/L) from a 10% cyclodextrin solution is about 23.6 times longer (944 min; note: data point is off the chart in Figure 6) compared to stripping from water (40 min). In summary, the presence of CD in the aqueous phase hinders the treatment of dissolved TCE by air stripping and is expected to influence the chemical analysis of TCE. It was shown that Henry’s law constant for TCE decreases exponentially with increasing CD concentration. For aqueous
TCE-water systems without cyclodextrin, the van’t Hoff equation well predicts Henry’s constant of TCE at temperatures between 10 and 60 °C. When CD is present in the solution, kH may decrease by more than an order of magnitudesdepending on temperature and CD concentration. Further, increasing the temperature of the CD solution cannot fully compensate for lower kH; particularly if the CD concentration is equal to or greater than 5%. The results of these studies were used to develop and test a model for air stripping TCE from CD solution at any given temperature between 20 and 60 °C and CD concentration ranging from 1% to 10%. The model indicates that significantly more effort is necessary to air-strip TCE from CD solution compared to stripping from water. These findings have important implications for the design of air stripper treatment systems during a CDEF application. This is because much longer residence times and/or higher gas flow rates are necessary to attenuate TCE in the CD flushing solution. Similarly, if in situ air sparging of TCE or soil vapor extraction technology is considered, proportionally more effort is needed if the groundwater contains CD. Although this model was developed specifically for CD-TCE systems, it is likely that it can be appliedsafter appropriate modificationssto other VOCs and other solubilization enhancing agents, such as surfactants or cosolvents. Finally, CD concentration and temperature-dependent vaporization are expected to impact the chemical analysis of TCE by purge-and-trap. For example, compared to water, less TCE is purged from the aqueous phase at a given temperature and purge duration if CD is present in solution. The resulting TCEaq concentration, therefore, must appear lower than it actually is. This problem can be partially circumvented by preparing TCE standards in aqueous CD solutions. However, as this research shows, TCE vaporization is a nonlinear function of the CD concentration. Field samples that contain an unknown or fluctuating amount of CD are therefore difficult to analyze precisely. This is because their analysis would require preparation of several standard curvesseach at a specific CD concentration. More work is necessary to modify existing CD-TCE analytical methods and examine practical solutions that permit accurate TCE analysis without the need for multiple standard curves.
Acknowledgments This study was made possible by a grant from the Environmental Security and Technology Certification Program, ESTCP (Grant CU-0113).
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Received for review January 8, 2004. Revised manuscript received May 7, 2004. Accepted May 10, 2004. ES049956Q
