Environ. Sci. Technol. 2002, 36, 104-111
Implications of Alcohol Partitioning Behavior for In Situ Density Modification of Entrapped Dense Nonaqueous Phase Liquids T O H R E N C . G . K I B B E Y , * ,† C. ANDREW RAMSBURG,‡ KURT D. PENNELL,‡ AND KIM F. HAYES§ School of Civil Engineering and Environmental Science, University of Oklahoma, Norman, Oklahoma 73019-1024, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0512, and Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 40809-2125
Surfactant-based remediation techniques have the potential to be very effective for removing dense nonaqueousphase liquids (DNAPLs) from contaminated sites. However, a risk associated with surfactant-based remediation of DNAPLs is the potential for unwanted downward mobilization of the DNAPL contaminants, making them more difficult to remove from the subsurface. The work described here examines the use of hydrophobic alcohol solutions to reduce the densities of entrapped DNAPLs, converting them to light nonaqueous-phase liquids (LNAPLs). Results of partitioning studies are presented for alcohol-DNAPL systems, in the absence and presence of surfactants. Results indicate that alcohol concentrations near saturation are necessary for conversion of DNAPLs to LNAPLss particularly for high-density DNAPLs such as trichloroethylene (TCE) and tetrachloroethylene (PCE). Although surfactants can increase the mass of alcohol that can be delivered to a contaminated zone, they appear to change the partitioning equilibrium such that higher alcohol concentrations are required to achieve the same result. Results of this work indicate the importance of minimizing dilution during density modification applications and suggest the concept of using an alcohol macroemulsion flood for density conversion. Implications of the results of this work for remediation system design are discussed.
Introduction Chlorinated organic liquids have seen widespread use as a result of their typically low flammability and excellent performance as solvents. However, because they are more dense than water and are often resistant to biodegradation, their use has resulted in extensive and persistent environmental contamination problems. For example, the chlorinated compounds trichloroethylene (TCE), tetrachloroethylene (PCE), vinyl chloride, methylene chloride, 1,1dichloroethene, 1,2-dichloroethane, carbon tetrachloride, and chloroform have been individually detected at between * Corresponding author phone: (405 325-0580; fax: (405)325-4217; email:
[email protected]. † University of Oklahoma. ‡ Georgia Institute of Technology. § The University of Michigan. 104
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22 and 59% of National Priorities List (NPL) sites (1). When chlorinated organic liquids (and other high density organic liquids) are present in the environment in separate-phase form, they are often referred to as dense nonaqueous phase liquids (DNAPLs) because of their high densities relative to that of water. A 1993 U.S. EPA study examining site data for 712 NPL sites concluded that there was a medium-to-high likelihood of DNAPL contamination at approximately 60% of all NPL sites (2). Surfactant-based remediation technologies are gaining acceptance for remediation of aquifers contaminated by organic liquids. Conventional pump-and-treat technologies (where clean water is pumped through a contaminated zone to remove the contamination) have proven to be ineffective for removal of organic liquids, potentially requiring decades of pumping at many sites before contaminants can be fully removed (3). Surfactant solutions can be used to improve the performance of pump-and-treat technologies by dramatically accelerating the dissolution of organic liquids and/ or reducing the water/organic liquid interfacial tension, allowing organic liquids to be mobilized and pumped out of the aquifer as a separate phase. However, with surfactant-based remediation field tests showing increasing promise, some risks remain difficult to quantify and control in the field. For example, surfactantinduced reduction of the interfacial tension of entrapped DNAPLs carries with it the risk that the DNAPLs will migrate downward in the subsurface due to their high density, making them more difficult to remove. The a priori prediction of the likelihood of unwanted downward mobilization at any particular site is severely complicated by the potential for unexpected effects from impurities in DNAPLs. Impurities with the potential to lead to problems include surfactants that may have been used with the organic liquid to improve its performance or high polarity, low interfacial tension compounds such as alcohols or ketones. Measurements by the authors with a sample of waste tetrachloroethylene (PCE) from a dry cleaners in Ann Arbor, MI, found that while many properties of the PCE were identical to those of pure PCE, the interfacial tension of the waste PCE in the presence of water was very low (less than 1 dyn/cm, as compared with 47 dyn/cm for pure PCE). The waste PCE contained approximately 2 g/L of nonvolatile residuesslikely surfactants and starches used in the drycleaning process. Similarly, the presence of alcohols or other hydrogen-bonding compounds in an organic liquid contaminant mixture can lead to unexpectedly low interfacial tensions when surfactants are used for remediation (4). Although surfactants can be selected to minimize interfacial tension reduction, reducing the risks of unwanted mobilization, there is no guarantee at any particular site that interactions between the remediation surfactants and impurities in the DNAPL will not lead to unwanted mobilization. As such, the emphasis of the work described here was on providing a quantitative evaluation of the feasibility of using hydrophobic alcohols to reduce the densities of entrapped DNAPLs, prior to using surfactants for remediation. Previous work by the authors (4-9) used two-dimensional aquifer cells and batch studies to examine the use of hydrophobic alcohols, with and without surfactants, for in situ density modification. Although the work has produced promising results for compounds with densities slightly greater than water (e.g., chlorobenzene), compounds with higher densities, such as PCE or trichloroethylene (TCE), have presented difficulties. Without surfactants, multiple pore volumes of solution were necessary to reduce the density sufficiently for 10.1021/es010966q CCC: $22.00
2002 American Chemical Society Published on Web 12/01/2001
TABLE 1. Properties of Chemicals Used in This Study mol wt (g/mol)
density (g/mL)
74.12
0.8098
interfacial tension (mN/m)
aqueous solubility (g/L)
final X1-butanol required to convert to LNAPL
Alcohols 1-butanol
63 DNAPLs
tetrachloroethylene (PCE) trichloroethylene (TCE) chlorobenzene (CB)
a
165.83 131.39 112.56
1.623 1.464 1.107
47.48 34.5 37.41
149 1099.8 307.4
0.77 0.71 0.36
surfactants
mol wt (g/mol)
cmc (mg/L)
description
SN120 Tween 80
569 1310
54 13
ethoxylated alcohol POE 20 sorbitan monooleate
X1-butanol values are reported on a water-free basis. Interfacial tension and aqueous solubility data are from ref 10.
TCE to be safely mobilized without unwanted downward migration (7). With surfactantssincluding surfactants that by themselves would not produce low enough interfacial tensions for mobilizationsunwanted downward mobilization was found to be a problem (7). Because alcohol partitioning into an entrapped organic liquid involves diffusion of the alcohol into the entrapped liquid, partitioning of alcohol can modify the properties of the interface of an entrapped organic liquid before sufficient alcohol can partition into the core of the liquid to reduce its density. This produces a situation where the interface of the organic liquid has the properties of a separate-phase alcohol, while the core retains the high density of the chlorinated liquid. In the presence of water, immiscible alcohols have interfacial tensions that are 1 order of magnitude lower than most chlorinated organic liquids (10). When surfactants are added, the interfacial tension will be reduced further, and unwanted downward mobilization may occur. Even if surfactants are not used directly, any surfactants already present as an impurity in the entrapped organic liquid could potentially lead to unwanted mobilization. Related work was reported by Lunn and Kueper (11), who evaluated the use of aqueous 2-butanol solutions for the density modification of PCE. Lunn and Kueper conducted column experiments and evaluated the phase behavior of the 2-butanol/water/PCE system. In the column experiments, a pool of PCE was perched on a fine sand lens in a packed column, and an aqueous solution of 2-butanol was passed through the PCE in an up-flow column arrangement. The aqueous 2-butanol solution was found to reduce the density of the PCE sufficiently to prevent downward mobilization when pure 1-propanol was used to displace the PCE. One concern identified in the paper was the necessity of up-flow gradients for the approach to work. Because PCE densities in the system were not reduced below that of water in the experiments, up-flow gradients were still needed during displacement to prevent downward motion. In contrast, the objective of the work reported here is to reduce the density of DNAPLs below that of water, eliminating the need for up-flow gradients. Relationships between the reported 2-butanol phase behavior results presented by Lunn and Kueper and the results of this work will be discussed in the Discussion section. An alternate method for preventing unwanted downward mobilization of DNAPLs was presented by Miller et al. (12). The approach relies on initially flooding the contaminated area with a high-density brine. Because the brine has a density higher than most DNAPLs (1.8 g/cm3), buoyancy forces will prevent mobilized DNAPL from moving downward. (Essentially, rather than reduce the density of the DNAPL, the method increases the density of the aqueous phase.) One concern with the method is that it may be difficult to contain the brine solution in the contaminated area; all one- and
two-dimensional experiments demonstrated in the paper use carefully enclosed regions to retain the brine in the area surrounding the DNAPL. Nevertheless, it may be feasible to contain the dense brine in some environmental systems using carefully designed hydraulic controls and extensive brine injection. The focus of the work described here was on the use of hydrophobic alcohols to reduce the densities of DNAPLs below that of water, converting them to light nonaqueous phase liquids (LNAPLs), eliminating the risks of unwanted downward mobilization. This paper presents a detailed evaluation of the partitioning behavior of 1-butanol in the presence of changing NAPL composition and in the presence and absence of surfactants. Because density modification by alcohol partitioning requires a substantial NAPL composition change, a quantitative understanding of the influence of NAPL composition on partitioning behavior is essential for design of density modification applications. Surfactants have been suggested as a means of increasing the amount of dissolved alcohol that can be introduced to the subsurface (7, 11, 12), so the evaluation of alcohol partitioning behavior in the presence of surfactants is an important aspect of this work.
Materials and Methods Materials. Properties of chemicals used in this study are shown in Table 1. HPLC-grade 1-butanol (Mallinckrodt, Paris, KY), and Milli-Q water (Millipore Corp., Bedford, MA) were used for all experiments. 1-Butanol was selected because its moderately high solubility (6.3%) allowed a substantial mass of alcohol to be added to solution, while still being sufficiently hydrophobic to provide a driving force for partitioning. DNAPLs examined include chlorobenzene (CB), trichloroethylene (TCE), and tetrachloroethylene (PCE). These DNAPLs were selected because of their relevance as environmental contaminants and because they cover a range of polarities and densities. All DNAPLs were purchased from Sigma (St. Louis, MO) and were used as received. The commercial nonionic surfactants used in this study included an alcohol ethoxylate, Witconol SN120 (Witco Corp., New York NY); an ethoxylated sorbitan fatty acid ester, Tween 80 (ICI, Wilmington, DE); and a nonethoxylated sorbitan fatty acid ester, Span 80 (ICI). SN120, Tween 80, and Span 80 were provided by their respective manufacturers. All surfactants were used as received. Alcohol Partitioning Measurements. Experiments presented in this paper were conducted in two different laboratories using similar methods. Detailed PCE and TCE data collected at Georgia Institute of Technology (Figure 1; GT data) were determined using batch experiments conducted in 10-mL graduated (0.1-mL) conical centrifuge tubes. To each centrifuge tube, 1-butanol, DNAPL, and water were added in various volume ratios such that the total volume VOL. 36, NO. 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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was 10 mL. The tubes were mixed for 72 h on LabQuake shaker tables. After being equilibrated, the samples were centrifuged at 1100 rpm for 5 min to separate the aqueous and nonaqueous phases. The final volume of each phase was recorded. For each sample, approximately 0.4 g of the aqueous phase was diluted with approximately 1.1 g of 2-propanol in a chromatography vial. Similarly, approximately 0.1 g of the nonaqueous phase was diluted with approximately 1.4 g of 2-propanol. All samples were analyzed using a Hewlett-Packard (Palo Alto, CA) 6890 gas chromatograph equipped with a flame ionization detector and an HP-5 (Hewlett-Packard) column. Remaining data in Figure 1 (UM data) were determined in 4-mL vials using a 1:1 water to NAPL ratio. Relative amounts of 1-butanol and PCE or CB in the NAPL were varied. Following equilibration, concentrations in NAPL and water phases were measured and analyzed using a Hewlett-Packard 5890 gas chromatograph equipped with a flame ionization detector and a J&W Scientific (Folsom, CA) DB-624 column. Data corresponding to surfactant solutions were determined by adding varying volumes of PCE or CB (between 10 and 1000 µL) to 3 mL of surfactant/1-butanol solution. Surfactant solutions were prepared to have similar equilibrium 1-butanol solubilities. The SN120 solution was prepared at 86 g/L, while the Tween 80 solution was prepared at 115 g/L. Both solutions initially contained 113 g/L 1-butanol. (Solubilities of 1-butanol for SN120 and Tween 80 were determined to be approximately 137 and 125 g/L, respectively.) Samples were allowed to equilibrate for 96 h, and then one or both of the phases were sampled (due to low NAPL volumes in some samples, NAPL sampling was not possible in some cases), and concentrations of all components (1-butanol, PCE or CB, surfactant) were analyzed. Surfactants were analyzed using a Hewlett-Packard 1050 HPLC with a Sedere SEDEX 55 evaporative lightscattering detector using methods described elsewhere (13). Unless otherwise noted (e.g., Figure 6), alcohol mole fractions in the NAPL reported in the paper are based on the quantities of alcohol and the other organic liquid (e.g., PCE, TCE, CB) in the NAPL and do not include any water that may be partitioned into the NAPL. Reasons for this approach, in addition to the implications of including or excluding partitioned water from density calculations, are discussed in detail below in the Water Partitioning into the NAPL section. In short, a careful evaluation of the impact of partitioned water on density calculations shows that partitioning of water can be neglected for most practical density conversion modeling purposes. Density calculations conducted for this paper are based on an assumption of constant partial molar volumes during mixing; as such, densities were calculated by calculating a molar weighted average of the densities of all components in the mixture. The assumption of constant partial molar volumes is supported by results presented by Lunn and Kueper (11), who show a near-linear dependence of density on mole fraction for alcohol-DNAPL mixtures. UNIFAC Calculations. UNIFAC is a group contribution method for estimation of activity coefficients in nonideal liquid mixtures. It was developed by Fredenslund et al. (14), and has been widely used and tested (e.g., refs 15-17). UNIFAC uses a matrix of empirically determined parameters to calculate the effects of individual component functional groups on mixture behavior and has been found to provide very good quantitative predictions for many liquid mixtures. UNIFAC calculations for this paper were conducted using a program based on the computational core of Choy and Reible’s excellent UNIFAC activity coefficient calculator (18). For this work, the core of the program was rewritten in C++ and was used to search for compositions where both 1-butanol and water had equal activities in both aqueous and mixed NAPL phases (i.e., 1-butanol activity the same in 106
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both phases and water activity the same in both phases). This calculation procedure simultaneously solves for the solubility of the water-saturated mixed NAPL components (1-butanol and PCE, CB, or TCE) in water and for the solubility of water in the water-saturated mixed NAPL. Note that the implication of this solution procedure is that even though mole fractions reported in this paper do not include water, UNIFAC predictions do consider the influence of water partitioned into the NAPL because of the need to simultaneously calculate the complete compositions (including water) of the two phases.
Results and Discussion To reduce the density of a DNAPL by alcohol partitioning, a substantial volume of alcohol must partition into the DNAPL. For example, Table 1 shows calculated final mole fractions of partitioned 1-butanol necessary for PCE, TCE, and CB to be converted to LNAPLs. For a DNAPL as dense as PCE, the composition of the final NAPL must be more than 77% 1-butanol by moles (not including the moles of partitioned water, which may also be significant). This dramatic composition change would be expected to be accompanied by a change in the affinity of the alcohol for the NAPL, complicating the modeling and design of density modification applications. Because of the extensive composition changes necessary for density modification, the problem of determining alcohol partitioning coefficients as a function of composition is essentially equivalent to the problem of predicting the aqueous concentration of alcohols in equilibrium with watersaturated organic liquid mixtures. For chemically similar compounds, ideal mixing is expected, and the concentration of each compound in equilibrium with the mixture is found to be proportional to its mole fraction in the mixture (e.g., ref 16). It has long been recognized that mixtures containing compounds with significant chemical differences do not form ideal mixtures (e.g., see the extensive discussion in ref 19). For mixtures of alcohols and other polar compounds with nonpolar compounds, it is often found that the polar compounds will tend to have equilibrium concentrations greater than the concentrations that would be predicted by mole fraction. For example, Banerjee (16) examined binary mixtures containing benzyl alcohol and chlorobenzene, 1,2dichlorobenzene, toluene, or ethyl acetate. It was observed that the aqueous concentrations of benzyl alcohol were always disproportionally higher than would be predicted by mole fraction alone. Figure 1 shows aqueous concentration of 1-butanol as a function of NAPL composition for binary mixtures of 1-butanol with PCE, TCE, and CB. Note that the PCE data were taken from two different experiments conducted in different laboratories with different methods; the high degree of agreement between PCE experiments reflects the repeatability of experimental measurements. As expected, Figure 1 shows that concentrations of 1-butanol in water are higher than would be predicted from ideal mixing over the range of mole fractions examined. Of the three DNAPLs shown, PCE is the least polar, having the highest interfacial tension and lowest aqueous solubility. As might be expected, deviation from ideal mixture solubility behavior is greater for the PCE than for TCE or CB, although the 1-butanol solubility curves for all three compounds are quite similar. In particular, the 1-butanol solubility curves for TCE and CB are nearly identical, despite the molecular differences between the two. An important implication of this solubility behavior for density modification is immediately apparent: the aqueous alcohol solution must be very close to saturation for there to be a driving force for additional alcohol to partition into
FIGURE 1. Aqueous concentration of 1-butanol in equilibrium with PCE/1-butanol, TCE/1-butanol, and CB/1-butanol mixtures. X1-butanol values are reported on a water-free basis in this and all other figures.
FIGURE 3. 1-Butanol partition coefficient as a function of mole fraction of 1-butanol in NAPL. Parameters for quadratic fits are given in Table 2.
TABLE 2. Equation Constants for Quadratic Fit of 1-Butanol Partition Coefficient as a Function of Mole Fraction of 1-Butanol (for X1-butanol Values Calculated on a Water-Free Basis) compd PCE TCE CB a
FIGURE 2. Influence of equilibrium concentration on maximum achievable 1-butanol mole fraction. the NAPL. This point is illustrated in Figure 2, which shows the solubility data for PCE from Figure 1. Figure 2 shows that if a PCE-contaminated zone is flooded with an infinite volume of a 40 g/L solution of 1-butanol, the highest mole fraction of 1-butanol that could result in the NAPL would be approximately 0.12snowhere near the approximately 0.77 needed to achieve a calculated density of 1.0. Flooding with a 50 g/L 1-butanol solution could produce a maximum partitioned mole fraction of approximately 0.53. It is apparent from Figure 2 that a 1-butanol solution concentration greater than approximately 56 g/L would be needed to convert PCE to an LNAPL. Given that the aqueous solubility of 1-butanol is approximately 63 g/L, 1-butanol concentrations very close to the aqueous solubility would be needed to produce the desired density reduction for PCE. Results presented by Lunn and Kueper (11) for 2-butanol partitioning into PCE showed reduced density-reduction effectiveness for lower concentration alcohol solutions and decreasing benefits with added volumes of alcohol solution; both of those trends result from the solubility behavior of the alcohol/NAPL mixture illustrated in Figure 2. Flushing with a particular alcohol concentration will cause the NAPL composition to asymptotically approach the equilibrium mole fraction corresponding to the solution concentration. An alcohol solution with a concentration below that required for density conversion (approximately 56 g/L for the 1-butanol/PCE system) will not cause density conversion, no matter how many pore volumes are used. An important implication of this solubility behavior for field
a
b 10-3
-6.81 × -7.02 × 10-3 -9.00 × 10-3
c 10-2
2.22 × 2.12 × 10-2 2.39 × 10-2
4.51 × 10-4 1.68 × 10-3 9.52 × 10-4
Kx ) a(X1-butanol)2 + b(X1-butanol) + c. Kx in units of (mol fraction/(g/L)).
remediation system design is that the pumping system would need to be carefully designed to minimize dilution of the alcohol solution in the area being treated, since any dilution could reduce the concentration of alcohol below that necessary for density conversion. Modeling the transport and partitioning of alcohol into DNAPLs and the subsequent density reduction of the DNAPL requires an understanding of the effect of changing composition on alcohol partitioning. Figure 3 shows the data from Figure 1 represented in the form of a partition coefficient aq ). (A partition coefficient based on (Kx ) X1-butanol/C1-butanol mole fraction in the NAPL was used to eliminate the need for applications to track volume changes in the NAPL.) At X1-butanol ) 0, the partition coefficient corresponds to the coefficient that would be measured for partitioning low concentration 1-butanol between water and an excess amount of NAPL. At X1-butanol ) 1, the partition coefficient is fixed at a value of (1-butanol solubility)-1. Between, the curves are well-described by quadratic fits (see Table 2 for fit constants). Using this information, it would be possible to quantitatively incorporate alcohol partitioning into a transport model for modeling the transport and partitioning of alcohols in DNAPL-contaminated zones, and associated density reduction was observed. Figure 4 shows equilibrium aqueous concentrations corresponding to the partition coefficient fits in Figure 3, illustrating that the quadratic partition coefficient fits provide a reasonable description of mixed dissolution behavior. Water Partitioning into the NAPL. The solubility of water in polar NAPLs, such as alcohols, can be very high. The solubility of water in 1-butanol has been reported to be 51.2% by moles (10). This value is supported by UNIFAC calculations and represents a significant quantity of water in the NAPL. Figure 5 shows UNIFAC calculated quantities of water in the mixed NAPL as a function of the mole fraction of 1-butanol in the mixture (calculated without water) for PCE, TCE, and CB. For the most part, substantial quantities of water are VOL. 36, NO. 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Partition coefficient fits from Table 2 converted to concentration.
FIGURE 5. Calculated mole fraction of water in NAPL mixtures. expected to be present in the NAPL, particularly when alcohol makes up a significant fraction of the mixed NAPL. Why, then, are mole fractions in this paper calculated without considering water in the NAPL? The primary reason is that water has very little influence on the density of the mixed NAPL within the range of interest. For example, Figure 6 shows the density of the mixed PCE/1-butanol with and without water (calculated assuming constant partial molar volumes) as a function of NAPL composition. The results in Figure 6 show that the effects of ignoring the presence of partitioned water has only a minimal influence on calculated density until the density has dropped below that of water. In addition, the error is conservative, in that if water is ignored, the density is predicted to be slightly higher than the density calculated considering water. The reason for the negligible influence of partitioned water on the density is that as the fraction of 1-butanol in the mixture increases, the density of the mixture is decreasing, approaching that of water. Although increasing quantities of water are present, the extent to which it can influence the density decreases as the density approaches that of water. Perhaps the most important point is that water partitioning cannot reduce the density of a DNAPL below the density of water, and ignoring water partitioning will have no influence whatsoever on the composition that corresponds to density conversion from DNAPL to LNAPL. Given the subtle effect of water partitioning on density (Figure 6) and the fact that partition coefficients can be identified on the basis of NAPL components and alcohol alone (Figures 3 and 4), it seems reasonable that practical modeling of the density conversion process could safely ignore partitioned water. Neglecting water in calculations allows modeling of 108
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FIGURE 6. Calculated densities of PCE/1-butanol mixtures, with and without partitioned water. the partitioning process without requiring explicit tracking of water partitioning, significantly simplifying the modeling process. Errors introduced by this simplification will be conservative and are likely to be small relative to errors introduced by unknowns in subsurface heterogeneity. Application of Surfactants. The use of surfactants to increase the amount of alcohol that can be dissolved and delivered to the DNAPL has been previously suggested (4-7, 11). Previous work by the authors explored the use of surfactants for density modification applications in experimental systems (4-7). Lunn and Kueper (11) also suggested using surfactants to increase aqueous concentrations of alcohol delivered to the DNAPL for partitioning. In the work reported here, we present experiments quantitatively evaluating the impact of surfactants on alcohol partitioning behavior. Figure 7 shows aqueous concentrations of 1-butanol in equilibrium with NAPL/1-butanol mixtures for two different NAPLs. Plots reflect the effects of two different surfactants, SN120 and Tween 80, on the solubility of 1-butanol. Note that concentrations of the surfactants have been selected so that the solubility of pure 1-butanol will be approximately the same in each surfactant solution (137 g/L for SN120; 125 g/L for Tween 80 at the concentrations used). The most significant result from this figure is that although the surfactant has in each case approximately doubled the amount of 1-butanol in solution relative to the case without surfactant, the trends with mole fraction are unchanged. That is, in the presence of surfactant, the equilibrium alcohol concentration is well above that that would be predicted by ideal mixing. As a result, the same interpretation used in Figure 2 still applies when surfactants are added. Although surfactant increases the amount of 1-butanol that can be added to solution, it also increases the amount that must be added to get a desired equilibrium NAPL mole fraction. Figure 8 shows the results from Figure 7 presented in the form of a partition coefficient. The curves shown are based on the quadratic partition coefficient fits from Figure 3 (parameters listed in Table 2), scaled by the ratio of the aqueous solubility of pure 1-butanol to the solubility of 1-butanol in surfactant solution. This figure illustrates that this scaling procedure does a very reasonable job of fitting the measured partition coefficients in the presence of surfactant solution, suggesting that surfactant decreases the partition coefficient primarily through increasing the aqueous concentration of 1-butanol. Scaling the partition coefficient by the ratio of the solubilities is equivalent to the scaling procedure for correcting Henry’s law constants for micellar solubilization (20). As is the case with Henry’s law constants, increased solubilization capacity increases the affinity of the alcohol for the surfactant-containing aqueous phase, ef-
FIGURE 7. Effect of surfactants on aqueous concentration of 1-butanol in equilibrium with PCE/1-butanol and CB/1-butanol mixtures. Note that mole fraction does not consider surfactant partitioned into NAPL.
FIGURE 8. Partition coefficient fits from Table 2, scaled by solubility enhancement. fectively reducing its affinity for the NAPL. The use of this scaling approach for water/NAPL systems assumes that the surfactant primarily influences the interactions of the alcohol in the aqueous phase and not the NAPLsa reasonable assumption for a surfactant system designed to increase the concentration of alcohol in the aqueous phase. Figure 9 shows 1-butanol solution concentration predictions in the presence of surfactants, based on the scaled partition coefficient curves from Figure 8. The curves do not match the data as well as in the surfactant-free case (Figure 4), but that may reflect the greater scatter in the surfactant data when compared with the surfactant-free data. Nevertheless, it is apparent that the curves provide a very reasonable quantitative description of the solubility trends when surfactants are present. Determination of Partition Coefficients for Modeling Density Modification. Because alcohol partition coefficients
FIGURE 9. Scaled partition coefficients (Figure 8), used to determine equilibrium concentration. vary significantly with changing NAPL composition, it is necessary to determine coefficients over the entire range of interest in order to design or model a density modification application. Several approaches can be used. First, if phase diagram data are available for the DNAPL and alcohol of interest, the data can be used directly to determine and fit partition coefficients (as was done in this paper). Alternately, UNIFAC (or another chemical property estimation method) could be used to calculate the alcohol partition coefficients for mixed NAPLs. For example, Figure 10 shows UNIFACpredicted 1-butanol partition coefficients for the NAPL mixtures. Although the predictions are not as good as the VOL. 36, NO. 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 10. UNIFAC-calculated alcohol partition coefficients for NAPL mixtures. Curves represent UNIFAC-predicted partition coefficients as a function of NAPL composition. empirical fits in Figure 3, they are still quite good considering that they are predicted on the basis of molecular structure alone. Note that UNIFAC could be directly incorporated into a transport model for prediction of partition coefficients with changing composition. However, a more easily implemented approach for using UNIFAC would be to fit quadratic polynomials through selected calculated points (e.g., X1-butanol ) 0, 0.5 and 1.0). Although the resulting curve would depend on the points selected (the UNIFAC predictions have more inflections than can be fit with a quadratic polynomial), the resulting curve should be sufficiently accurate for modeling density modification. An empirically fit partition coefficient, as described in this paper, could be implemented with only minor changes to existing transport models. A third approach that could be used to determine partition coefficients as a function of composition would be to use published correlations for determining alcohol partition coefficients to fix the curve at the X1-butanol ) 0 end. For example, Dwarakanath and Pope (21) presented an equivalent alkane carbon number-based method for predicting alcohol partition coefficients. In that paper, they present measured and predicted partition coefficients for 24 alcohols in the presence of 9 NAPLs (including 6 DNAPLs). Although the focus of that work was on the use of alcohols as partitioning tracers for subsurface characterization, the methods used would be equally applicable to density modification applications. Note that the value of the partition coefficient at X1-butanol ) 0 is not itself particularly important for determining alcohol partitioning behavior during density modification. A system with a low partition coefficient at X1-butanol ) 0 may still be a good selection for density modification, depending on the partitioning behavior at higher X1-butanol values. As previously described, the partition coefficient at X1-butanol ) 1 is given by (1-butanol solubility)-1. With both ends of the partition coefficient curve fixed, only a single experimental partition coefficient measurement would be necessary (perhaps at X1-butanol ≈ 0.5, for example) to determine a quadratic expression for partition coefficient as a function of mole fraction. Note that it might be possible in some situations to simply use a linear function for partition coefficient (fixed at values determined at X1-butanol ) 0 and 1) (this would be functionally equivalent to the selfassociation model of alcohol partitioning; 19, 22, 23), although the inaccuracy of this approach would be greatest at intermediate alcohol mole fractions, where partitioning behavior has the greatest influence on density conversions potentially resulting in large errors in predictions. If surfactants or cosolvents were to be used in solution, partition coefficients could be scaled by measured alcohol solubility 110
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enhancement of the surfactant solution, as described in the previous section. Implications for Density Modified Displacement Applications. Perhaps the most important result of the work presented here is that alcohol solutions near saturation must be used in order to achieve density modification for highdensity DNAPLs, such as PCE or TCE. Infinite volumes of low-concentration alcohol solutions will not achieve density modification if the required final alcohol mole fraction in the NAPL is higher than the equilibrium mole fraction corresponding to the alcohol concentration added. This means that careful attention must be paid during remediation system design to prevent dilution of the alcohol solution in the contaminated zone. The results of this work also demonstrate that there is no apparent benefit to using surfactants for density modification. Although surfactants increase the mass of alcohol that can be passed through the DNAPL contaminated zone, they also change the equilibrium to require more alcohol for density conversion. In addition, the combination of surfactants with alcohols may in some cases lead to unwanted interfacial tension reduction and downward mobilization (4). One approach that is suggested by these results is the use of an alcohol macroemulsion to deliver alcohol to contaminated zones. Because separate-phase alcohol droplets would be present in the contaminated zone, the water surrounding the droplets would be at or near saturation at all times, creating a constant alcohol source for partitioning into the DNAPL. We have created and tested a 1-butanol macroemulsion stabilized by a mixture of Tween 80 and Span 80 surfactants. Two-dimensional aquifer cell experiments with the alcohol macroemulsion showed substantial promise and will be described elsewhere. The results of this work should provide a useful starting point for the design of future density modification applications.
Acknowledgments Funding for the research was provided by the Great Lakes and Mid-Atlantic Hazardous Substance Research Center under Grant R819605-01 from the Office of Research and Development, U.S. Environmental Protection Agency. Additional matching funds were provided by the State of Michigan Department of Environmental Quality. The content of this publication does not necessarily represent the views of either agency.
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Received for review May 11, 2001. Revised manuscript received October 12, 2001. Accepted October 16, 2001. ES010966Q
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