Surfactant Phase Behavior with Field Degreasing Solvent

This paper focuses on optimizing the phase behavior of anionic surfactants for a complex DNAPL consisting of many components typical of degreasing sol...
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Environ. Sci. Technol. 2000, 34, 4842-4848

Surfactant Phase Behavior with Field Degreasing Solvent VARADARAJAN DWARAKANATH† AND G A R Y A . P O P E * ,‡ Duke Engineering and Services, 9111 Research Boulevard, Austin, Texas 78758, and Department of Petroleum and Geosystems Engineering, University of TexassAustin, CPE 2.502, Austin, Texas 78712

Surfactant phase behavior data are presented for dense nonaqueous phase liquids (DNAPLs). The experimental procedures and results published in this paper led to the successful selection of a surfactant formulation that was used in a field demonstration at a site contaminated with DNAPL rich in trichloroethene (TCE). Experiments were conducted to investigate the effects of temperature, electrolyte, cosolvent, xanthan gum polymer, and surfactant hydrophobe length on surfactant phase behavior. Experimental results are presented for a field degreasing solvent that is a DNAPL from Operable Unit 2 (OU2) of Hill Air Force Base, Layton, UT. These experiments show that procedures previously developed and reported only for pure component DNAPLs such as TCE can be useful for selecting suitable surfactants for complex field DNAPLs. In addition to the usual criteria of large increases in contaminant solubilization and lowering of interfacial tensions, we identify rapid formation of microemulsions with acceptably low viscosities as an additional requirement for surfactant screening. Our results indicate that the best surfactant formulations equilibrate to low viscosity microemulsions within a few hours. Extensive soil column studies have shown that such behavior leads to DNAPL recoveries exceeding 99% without significant problems such as pore plugging and high surfactant retention.

Introduction This paper focuses on optimizing the phase behavior of anionic surfactants for a complex DNAPL consisting of many components typical of degreasing solvents. The principal objective in this research was to identify critical experimental measurements that are required to select surfactant formulations for use in field surfactant floods. The selected surfactant recovered more than 99% of the DNAPL in laboratory soil column experiments (1) and 98.5% of the DNAPL in a surfactant-enhanced aquifer remediation (SEAR) demonstration at Hill AFB OU2 (2, 3). Surfactant-enhanced aquifer remediation (SEAR) is one of the few highly effective technologies for the removal of nonaqueous phase liquids (NAPLs) from soils and is particularly advantageous for remediating DNAPLs from groundwater with high hydraulic conductivity. Surfactants greatly increase remediation efficiency by increasing the solubility of the NAPL constituents and by lowering the interfacial * Corresponding author phone: (512)471-3235; fax: (512)471-9678; e-mail: [email protected]. † Duke Engineering and Services. ‡ University of TexassAustin. 4842

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tension between the NAPL and water (4-6). The applicability of surfactants for remediating soils contaminated by a variety of commonly encountered NAPLs has been demonstrated by many laboratory studies (1, 7-13). Dwarakanath et al. (1) have shown that the residual NAPL saturation of both pure TCE and a TCE-rich field DNAPL can be reduced to less than 0.0002 in laboratory soil column experiments, which corresponds to NAPL recoveries of as high as 99.9%. Recent field demonstrations of SEAR have shown that NAPL saturations can be reduced to as low as 0.0003 (2, 3, 14, 15). These very favorable results are due in part to the application of careful phase behavior studies to surfactant selection such as illustrated in this paper for a field degreasing solvent. One of the first studies of surfactant phase behavior was done by Winsor (16), who defined the various phase environments observed once surfactant, water, and oil were mixed (Winsor type I-III systems). Enhanced oil recovery (EOR) research in the 1970s and 1980s resulted in a much improved understanding of microemulsion phase behavior and surfactant characteristics desirable for EOR (17-19). The low temperature, the low electrolyte concentration, and the unconfined nature typical of contaminated groundwater aquifers are in sharp contrast to the high temperatures, high electrolyte concentrations, and confined nature of oil reservoirs. Despite these differences, researchers have used the same principles of surfactant phase behavior developed for EOR to design and perform phase behavior experiments. For example, Martel et al. (20, 21), conducted phase behavior experiments at varying NAPL concentrations to develop ternary phase diagrams with surfactant, NAPL, and water. Earlier work by Kile and Chou (22) characterized the solubility of chlorinated hydrocarbons by surfactants. Similar work by Edwards et al. (23, 24) characterized the solubility enhancement as well as interfacial tension reduction of polycyclic aromatic hydrocarbons by nonionic surfactants both below and above the critical micelle concentration. Baran et al. (25-27) identified surfactant mixtures that produced Winsor type III microemulsions with common NAPLs such as TCE and jet fuel. Surfactant phase behavior is sensitive to several variables, and adequate characterization is required before application in field operations. For example, a decrease in temperature is usually accompanied by an increase in equilibration times and microemulsion viscosity. Slow equilibration and high microemulsion viscosity are manifested in the form of mass transfer limited dissolution and are undesirable (28, 29). A higher cosolvent concentration can often be used to reduce the microemulsion viscosity to acceptable values, but this also brings about a reduction in contaminant solubilization. An increase in the surfactant hydrophobe length increases the contaminant solubilization but also increases equilibration times and the propensity for mixtures of surfactant, water, and NAPL to form liquid crystals, gels, and macroemulsions (18). As the concentration of electrolytes such as sodium and calcium increases, the phase behavior shifts from a water external microemulsion (Winsor type I) to a middle phase microemulsion (Winsor type III) to a NAPL external microemulsion (Winsor type II). The preferred behavior for SEAR is either type I or type III (6). The usual protocol for surfactant selection is to measure contaminant solubilization and interfacial tension between the NAPL and the surfactant solution (4). Viscosity is an equally important property, but until recently it has only rarely been reported in the surfactant remediation literature. Even though several papers report viscosities of aqueous surfactant solutions (1, 30-33), only Dwarakanath et al. (1) 10.1021/es0009121 CCC: $19.00

 2000 American Chemical Society Published on Web 10/14/2000

and Weerasooriya et al. (32) report viscosity data for microemulsions. These viscosities need to be known to predict the increase in the hydraulic gradient when the surfactant is injected and to adjust the injection and extraction rates. In the approach for surfactant selection presented herein, the measurement of microemulsion viscosities is presented as a surfactant screening tool in addition to the measurement of contaminant solubilization, interfacial tension reduction, and aqueous surfactant and NAPL viscosities as reported by previous researchers (4). Since a range of contaminant concentrations may be expected in the subsurface during surfactant flooding, microemulsion viscosities are measured over a range of contaminant concentrations. Low surfactant and microemulsion viscosities are required for reasonable flow rates under maximum available hydraulic gradients in most aquifers. Because of its potential benefit to SEAR, we include in this paper some phase behavior data for surfactant and polymer mixtures. The use of water-soluble polymer with surfactants increases the viscosity of aqueous surfactant solutions from 2-3 to 4-7 cp, mitigates aquifer heterogeneities, and thus would be expected to improve SEAR performance in most cases (1, 6, 11, 34, 35). Polymer has been used for 35 years to improve the efficiency of both water-flooding and surfactant-enhanced oil recovery (35). The resistance to flow in porous media is due to the internal friction of the fluid, i.e., its viscosity and the resistance due to the pore geometry. A controlled increase in the viscosity of a fluid in a porous medium increases its resistance to flow in the more conductive pores resulting in relatively low velocity in these pores as compared to a less viscous fluid like water. In fact, it is well-known that a sufficient increase in viscosity will completely mitigate the channeling of fluids in heterogeneous porous media (34, 35). Unlike viscous polymer solutions, viscous surfactant solutions do not always result in more favorable displacement behavior. The high viscosities observed in some aqueous surfactant solutions and microemulsions are caused by stacking of surfactant micelles, formation of liquid crystal structures, and interaction between the surfactant hydrophobes (18), which often result in high surfactant retention and pore plugging (1). Viscous surfactant solutions consisting of such highly structured micelles typically exhibit thixotropic viscosities. Such behavior is more complex and difficult to control and predict than Newtonian behavior that is characteristic of the surfactant formulations described in this paper. Anionic surfactants were used in this study because they generally exhibit low sorption on aquifer material due to its negative surface charge. We report the effect of temperature, cosolvent concentration, surfactant hydrophobe length, electrolyte, and polymer on microemulsion phase behavior and viscosity. The approach presented in this paper was used to select surfactants for the SEAR field demonstrations at Hill Air Force Base (AFB) Operable Unit 2 (OU2) (2, 3) to remove degreasing solvent from the soil at this site. High efficiency SEAR rests on the ability of the surfactant to rapidly equilibrate into low viscosity microemulsions insitu when it mixes with the DNAPL in the soil. Microemulsions are composed of submicroscopic particles that are suspended by Brownian motion and hence are thermodynamically stable (18). Macroemulsions are unstable and often very viscous when first formed, difficult to control and predict, and otherwise complicate SEAR greatly. Cosolvents tend to break macroemulsions and promote the formation of microemulsions (6, 18). When NAPL constituents are solubilized at the center of a micelle, it is termed Winsor type I; conversely, when water is solubilized at the center of a micelle, it is termed Winsor type II. An intermediate bicontinuous region is termed Winsor type III. Winsor type III behavior is associated with ultralow

TABLE 1. Physical Properties of the Hill OU2 DNAPL density (g/cm3) viscosity (cp) equivalent alkane carbon no. interfacial tension (dyn/cm)

1.38 0.8 -5.41 8.6

interfacial tensions and was used for EOR (6). When anionic surfactants are used, the transition from Winsor type I to type III to type II can be caused by the addition of cations such as sodium or calcium as well as by a number of other variables. Winsor type III microemulsions have both NAPL and water solubilized in a micelle. The electrolyte concentration at which equal volumes of NAPL and water are solubilized in the microemulsion is termed optimum salinity. The NAPL solubilization parameter is the volume of NAPL solubilized per unit volume of pure surfactant, and the water solubilization parameter is the volume of water solubilized per unit volume of pure surfactant. The NAPL and water solubilization parameters are equal at optimum salinity. Surfactant phase behavior is commonly represented using both volume fraction diagrams and ternary diagrams. Volume fraction diagrams provide an understanding of the sensitivity of the surfactant to additional electrolyte. The surfactant, cosolvent, contaminant, and cosurfactant concentrations are fixed, while the concentration of the electrolyte is varied. Volume fraction diagrams provide information on the electrolyte concentration at which a transition from Winsor type I to type III to type II is observed. In addition, these diagrams provide information on the solubilization of the NAPL components in the microemulsion and the optimum salinity. Ternary phase diagrams represent surfactant phase behavior as a function of varying concentrations of surfactant, NAPL, and water. In these experiments, the electrolyte concentration in the water is fixed, and the volume fraction of surfactant, NAPL, and water is varied.

Experimental Section Materials and Methods. The pure phase TCE was obtained from Aldrich Chemicals, Milwaukee, WI. The field DNAPL was obtained from OU2 at Hill AFB, Layton, UT. The OU2 DNAPL consists of about 80% chlorinated solvents and about 20% grease. The solvent mixture is about 73% TCE, 14% 1,1,1trichloroethane (TCA), 8% tetrachloroethene (PCE), 3% Freon 113, and less than 1% each of carbon tetrachloride, toluene, and dichloromethane. The physical properties of the Hill OU2 DNAPL are summarized in Table 1. The pipets used for phase behavior were obtained from Fisher Scientific, Pittsburgh, PA. The surfactants, sodium dihexyl sulfosuccinate (commercial name Aerosol MA-80I) and sodium diamyl sulfosuccinate (commercial name Aerosol AY), were obtained from CYTEC industries, Morristown, NJ. 2-Propanol (IPA) was obtained from Fisher Scientific, Pittsburgh, PA. Phase behavior procedures were similar to those used in earlier work (1, 25-27). The procedure involved mixing 2 mL of NAPL with 2 mL of aqueous surfactant solution (with cosolvent wherever applicable) in a 5-mL pipet. The ends were flame-sealed to prevent loss due to volatilization. After pouring the contaminant and surfactant, the initial NAPLsurfactant interface was carefully noted to obtain an accurate measurement of the actual volume of NAPL and aqueous surfactant solution. The pipet was mixed gently and allowed to equilibrate for several hours. For the experiments conducted at temperatures other than 23 °C, the samples were placed in a water bath. The samples were allowed to equilibrate for 12 h, then mixed vigorously, and returned to the water bath. The phase volumes were observed every 24 h until equilibrium was reached. Once equilibrium was reached, the visual differences in the initial VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Comparison of GC measured and volumetric estimates of contaminant solubilization.

FIGURE 2. Ternary diagram for sodium dihexyl sulfosuccinate, TCE, and 0.01 wt % NaCl at 23 °C. levels and the microemulsion-DNAPL interface were recorded. The volumes of the NAPL, microemulsion, and water (wherever applicable) were calculated. The visual volume differences were used to estimate contaminant solubilization. In addition to the phase behavior pipets, separate samples were prepared in 8-mL borosilicate glass vials. In these vials, 4 mL of DNAPL and 4 mL of surfactant were mixed and allowed to equilibrate. The microemulsions collected from these samples were analyzed to determine the concentration of solubilized contaminant in a SRI model 8610C gas chromatograph (GC) using a flame ionization detector to verify the validity of volumetric estimates. A 1.83 m long Carbopak, 0.31 cm diameter packed column with 1% SP 1000 was used to verify the concentration of the solubilized contaminant in microemulsion. Helium was used as the carrier gas, and the flow rate was set at 40 mL/min. The GC was calibrated between 50 and 10 000 mg/L TCE. For the OU2 DNAPL, a 20 000 mg/L stock was prepared by dissolving a fixed mass of DNAPL into methanol. This stock was subsequently diluted to obtain calibration standards between 50 and 10 000 mg/L. The viscosities were measured using a couette rheometer made by Contraves (Model Contraves Low 4844

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Shear-30). The DNAPL-water interfacial tensions were measured using a spinning drop tensiometer (model 300) manufactured at The University of Texas at Austin.

Results Comparison of Volumetric and GC Data. The advantage of using graduated borosilicate glass pipets is that the changes in the NAPL-microemulsion or microemulsion-water interfaces can be used to estimate contaminant solubilization from the volume changes of each phase. Such volumetric data are easy to measure and inexpensive and thus lend themselves to rapid screening and observation of very large numbers of samples. A comparison of GC measurements and volumetric estimates of contaminant solubilization is shown in Figure 1. The volumetric estimates of contaminant solubilization when compared to the GC measurements are within the experimental error of (10% of the GC measurements and thus justify the use of the volumetric data for surfactant screening purposes when the volume changes are high as in this example. When volume changes are less than about 0.05 mL, the volumetric data become increasingly less accurate, and GC data would be preferred.

FIGURE 3. Effect of temperature on optimum salinity.

TABLE 2. Effect of IPA on Winsor Type I Microemulsion Viscosity

TABLE 3. Effect of IPA on Optimum Salinity and Optimum Solubilization Parameter with TCE and OU2 DNAPL

OU2 DNAPL 8% sodium dihexyl sulfosuccinate, NaCl, vol solubilization, microemulsion % IPA wt % fraction mg/L viscosity, cp

8% sodium dihexyl sulfosuccinate, % IPA

optimum salinity, % NaCl

TCE optimum solubilization parameter

0 2 4 8 20

1.25 nm 1.06 0.935 0.5

6.5 nm 5.3 5 4.5

4 4 4 4 4 8 8 8 8 8

0.67 0.5 0.65 0.67 0.69 0.58 0.4 0.5 0.55 0.57

0 0.10 0.12 0.19 0.24 0 0.11 0.13 0.14 0.17

0 140 000 163 000 257 000 325 000 0 152 000 178 000 193 000 229 000

2.6 5.5 8.1 7.9 7.5 2.8 5.2 5.3 7.2 4.6

Ternary Phase Diagram. A ternary phase diagram for sodium dihexyl sulfosuccinate at 23 °C is shown in Figure 2. The overall composition of each mixture is represented by the squares, and the circles represent the equilibrium microemulsion composition. For example, in Figure 2, point A represents a mixture with an overall composition of 50 vol % TCE, 11 vol % surfactant, and 39 vol % water. This mixture separates into two phases once it reaches equilibrium. The upper phase is a microemulsion with 39% volume fraction TCE, represented by point B. The lower phase is almost pure DNAPL, represented by point O. The line connecting point A and point O is called a tie line and connects the equilibrium compositions of the two coexisting phases. The low salinity of the water (0.01 wt % sodium chloride [NaCl]) resulted in a Winsor type I system as indicated by the tie lines. The binodal curve represents the line below which two phases exist. The maximum height of the binodal curve is 0.14 and is another common measure of the effectiveness of a surfactant. Effect of Cosolvent Concentration. IPA was used as a cosolvent for minimizing the occurrence of gels/liquid crystals/emulsions, lowering equilibration times, and reducing the viscosity of the contaminant-rich microemulsion (18). The microemulsion samples without IPA generally exhibited persistent milky macroemulsions with TCE and chocolate brown macroemulsions with the Hill DNAPL on mixing and took approximately 24 h to equilibrate. The addition of IPA reduced the equilibration times to less than 12 h for

a

OU2 DNAPL optimum optimum salinity, solubilization % NaCl parameter nma 1.16 1.05 0.83 nm

nm 6.2 5.5 4.4 nm

nm, not measured.

microemulsions prepared with the pure TCE as well as the field DNAPL, and persistent macroemulsions were not observed. The effect of the addition of IPA on the viscosity of Winsor type I microemulsions is summarized in Table 2. In these experiments, the surfactant concentration was 8 wt % sodium dihexyl sulfosuccinate on an aqueous basis. All these measurements were conducted at 12 °C since this is the groundwater temperature at Hill AFB OU2. When 12 vol % DNAPL is added to the aqueous surfactant solution (163 000 mg/L) containing 4 wt % IPA, the viscosity increases from 2.6 cp to a peak value of 8.1 cp. Increasing the IPA concentration to 8% causes only a moderate decrease in the microemulsion viscosities. Table 3 shows that both the optimum salinity and the optimum solubilization parameter decrease as the concentration of IPA is increased, the surfactant is more effective without IPA except for the need to reduce the viscosity and equilibration times. Effect of Electrolyte Type. The effect of the type of electrolyte used for mixing the surfactant formulation is very important for anionic surfactants. For a surfactant solution containing 8 wt % sodium dihexyl sulfosuccinate on an aqueous basis and 50 vol % TCE, the optimum salinity decreased from 1.1 wt % NaCl to 0.5 wt % CaCl2. For 8 wt % sodium dihexyl sulfosuccinate and 8 wt % IPA and 50 vol % OU2 DNAPL, the optimum salinity decreased from 0.83 wt % NaCl to 0.46 wt % CaCl2. Effect of Temperature. A change in the temperature affects the solubility of the surfactant in water and thus the surfactant phase behavior. If the surfactant solubility in water decreases with a decrease in temperature, which is typical VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Volume fraction diagrams for 8 wt % sodium dihexyl sulfosuccinate, 4 wt % IPA, and OU2 DNAPL at different temperatures.

FIGURE 5. Contaminant solubilization by 8 wt % sodium dihexyl sulfosuccinate, 4 wt % IPA, and OU2 DNAPL at different temperatures. of sulfonates such as used in this study, then less electrolyte is needed to achieve equal affinity of the surfactant for the water and the NAPL, thereby reducing the optimum salinity (18). Lower temperatures also tend to result in slower equilibration and more problems with viscous phases, thus making the selection of surfactants suitable for SEAR much more difficult. Extensive experimental data summarized in Bourrel and Schechter (18) show that the optimum salinity increases linearly with increases in temperature for most anionic surfactants. Consistent with this well-known trend, the phase behavior experiments with OU2 DNAPL and sodium dihexyl sulfosuccinate show a decrease in optimum salinity as the temperature is decreased to groundwater values as shown in Figure 3. Less well-known is that even though the optimum salinity decreases with a decrease in temperature, the solubilization parameters and the volume fraction diagrams remain unchanged when the electrolyte concentration is normalized by the optimum salinity as shown in Figure 4. The normalized electrolyte concentration is the concentration of the electrolyte divided by the optimum salinity. The results in Figure 4 indicate that the relative width of the Winsor type 4846

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III region with respect to the optimum salinity remains constant. Contaminant solubilization is plotted against normalized electrolyte concentration in Figure 5. Within the experimental error, the behavior of the surfactant is the same at different temperatures. Effect of Average Surfactant Hydrophobe Length. An understanding of the effect of average surfactant hydrophobe length on phase behavior is necessary for surfactant selection for field applications. A longer surfactant hydrophobe gives higher oil solubility, higher NAPL solubilization parameters, and lower optimum salinities (18). Higher NAPL solubilization translates into a reduced mass of surfactant required for remediation and therefore lower remediation costs. However, a longer surfactant hydrophobe also makes the surfactant more susceptible to liquid crystal formation as longer surfactant hydrophobes have a greater tendency for stacking up to form liquid crystals (18). Liquid crystal formation often gives viscosities that are too high and equilibration times that are too high. The general effect of the hydrophobe length on solubilization parameter and optimum salinity for both TCE and OU2 DNAPL is illustrated in Figure 6. The fraction of sodium

FIGURE 6. Effect of surfactant hydrophobe length on optimum salinity and optimum solubilization parameter.

FIGURE 7. Effect of polymer on the phase behavior of 4 wt % sodium dihexyl sulfosuccinate, 8 wt % IPA, and TCE. dihexyl sulfosuccinate was increased from 0.5 to 1 while simultaneously decreasing the fraction of the shorter sodium diamyl sulfosuccinate from 0.5 to 0. A decrease in optimum salinity and an increase in solubilization parameter can be observed when increasing the fraction of sodium dihexyl sulfosuccinate. This can be directly attributed to the increased surfactant hydrophobe length of the sodium dihexyl sulfosuccinate as compared to the sodium diamyl sulfosuccinate. A higher solubilization parameter can be obtained by blending sodium dioctyl sulfosuccinate, which is a longer surfactant. However, it should be noted that in two soil column experiments as reported by Dwarakanath et al. (1), the use of sodium dioctyl sulfosuccinate led to a reduction in permeability making this surfactant undesirable. The longer hydrophobe results in a higher contaminant solubilization, but it is also more prone to forming liquid crystals that cause permeability reduction. Therefore, such behavior must be investigated in soil column experiments as described in Dwarakanath et al. (1).

Effect of Polymer. Aquifer remediation by in-situ flooding processes is limited by heterogeneities. Water-soluble polymers can be added to aqueous surfactant solutions to increase the viscosity of the solution, which mitigates the effects of heterogeneity and improves the sweep efficiency of the surfactant flood (6, 35). The first step in screening polymers for this use is to measure the effect of the polymer on the phase behavior. Figure 7 shows a volume fraction diagram without polymer and with 500 mg/L of xanthan gum polymer. The phase behavior did not change in this example, which suggests that xanthan gum can be used to improve the performance of surfactant flooding.

Discussion We have presented an approach for surfactant selection that includes the measurement of aqueous surfactant and microemulsion viscosities in addition to the traditional techniques presented in the literature (4). The principal goal of our research was to develop experimental techniques that VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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would identify surfactants that form acceptably low-viscosity microemulsions with a field degreasing solvent in addition to the high contaminant solubilization and acceptable interfacial tension reduction. We have presented results that show the effect of cosolvent, temperature, electrolyte, surfactant hydrophobe length, and polymer on surfactant phase behavior. Specifically we illustrate the effect of the addition of IPA on microemulsion viscosity. The addition of a cosolvent such as IPA has the favorable effects of decreasing both microemulsion viscosity and equilibration time. Since fast equilibration corresponds to fast mass transfer and thus the most efficient use of the surfactant close to its equilibrium solubilization potential (28, 29), cosolvent use with surfactants is highly recommended for most SEAR applications. However, the cosolvent also has the unfavorable effect of decreasing the optimum solubilization parameter, which may necessitate the use of a larger mass of surfactant. Therefore, a compromise should be reached such that the microemulsion produced as a result of surfactant flooding has an acceptably low viscosity for propagation in the subsurface. We illustrate such a compromise by presenting results with a surfactant formulation consisting of 4% IPA with 8% sodium dihexyl sulfosuccinate and 0.7% sodium chloride that was used at a field surfactant flood demonstration at Hill AFB OU2 (2, 3). This field surfactant flood recovered 98.5% of the DNAPL and reduced the final DNAPL saturation to 0.035%.

Acknowledgments Funding for this research was provided by the State of Texas (Grant ATP-D-404), the Advanced Applied Technology Demonstration Facility (AATDF), Hill Air Force Base, and Radian International. We would also like to acknowledge Taimur Malik, Meng Lim, Hans Meinardus, Bruce Rouse, Shekhar Jayanti, and Vinitha Weerasooriya for assistance and support in various phases of this work and Jacqui Avvakoumides for her work in editing this manuscript. We also acknowledge the constructive comments of the reviewers of this manuscript. Finally, we would like to acknowledge the late Professor Bill Wade for insightful discussions on surfactant phase behavior.

Literature Cited (1) Dwarakanath, V.; Kostarelos, K.; Shotts, D.; Pope, G. A.; Wade, W. H. J. Cont. Hydrol. 1999, 38 (4), 465-488. (2) Brown, C. L.; Delshad, M.; Dwarakanath, V.; McKinney, D. M.; Pope, G. A.; Wade, W. H.; Jackson, R. E.; Londergan, J. T.; Meinardus, H. W. In Innovative Subsurface Remediation: Field Testing of Physical, Chemical, and Characterization Technologies; Brusseau, M. L., Sabatini, D. A., Gierke, J. S., Annable, M. D, Eds.; ACS Symposium Series 725; American Chemical Society: Washington, DC, 1999; p 64. (3) DE&S (Formerly INTERA). Final Report on the Demonstration of Surfactant Enhanced Aquifer Remediation of Chlorinated Solvent DNAPL at Operable Unit 2, Hill AFB, Utah. Prepared for AFCEE Technology Transfer Division, Brooks AFB, San Antonio, TX, 1998. (4) Fountain, J. C.; Klimek, A.; Beikirch, M. G.; Middleton, T. M. J. Hazard. Mater. 1991, 28 (3), 295. (5) Fountain, J. C.; Starr, R. C.; Middleton, T.; Beikirch, M.; Taylor, C.; Hodge, D. Ground Water 1996, 34 (5), 910. (6) Pope, G. A.; Wade, W. H. In Surfactant-Enhanced Subsurface Remediation Emerging Technologies; Sabatini, D. A., Knox, R. C., Harwell, J. H., Eds.; ACS Symposium Series 594; American Chemical Society: Washington, DC, 1995; p 142. (7) Pennell, K. D.; Jin, M.; Abriola, L.; Pope, G. A. J. Contam. Hydrol. 1994, 16 (1), 35. (8) Shiau, B.; Sabatini, D. A.; Harwell, J. H. Ground Water 1994, 32 (4), 561. (9) Shiau, B.; Rouse, J. D.; Sabatini, D. A.; Harwell, J. H. In SurfactantEnhanced Subsurface Remediation Emerging Technologies; Sabatini, D. A., Knox, R. C., Harwell, J. H., Eds.; ACS Symposium

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(10) (11) (12) (13) (14)

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(33)

(34) (35)

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Received for review January 21, 2000. Revised manuscript received July 20, 2000. Accepted July 25, 2000. ES0009121