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A new concept: the use of neutrally-buoyant microemulsions for DNAPL remediation
Journal of Contaminant Hydrology 34 Ž1998. 383–397
A new concept: the use of neutrally-buoyant microemulsions for DNAPL remediation K. Kostarelos
a,1
, G.A. Pope
b,)
, B.A. Rouse
b,2
, G.M. Shook
c,3
a
b
Department of CiÕil Engineering, The UniÕersity of Texas at Austin, Austin, Texas 78712, USA Department of Petroleum and Geosystems Engineering, The UniÕersity of Texas at Austin, Austin, Texas 78712, USA c Idaho National Engineering and EnÕironmental Laboratory, Idaho Falls, ID 83415-2107, USA Received 23 October 1997; accepted 11 May 1998
Abstract Even in the absence of mobilization of dense nonaqueous phase liquid ŽDNAPL., the microemulsion that forms when the surfactant solubilizes a dense contaminant such as trichloroethylene will be more dense than water and tends to migrate downward. This paper addresses the issue of migration with a new concept: surfactant enhanced aquifer remediation at neutral buoyancy. Laboratory results of surfactant remediation in two-dimensional model aquifers show that downward migration of microemulsion containing solubilized dense contaminants can be reduced to an acceptable level, even in the absence of capillary barriers in the aquifer. One model experiment was designed to exhibit a small degree of vertical migration and full capture of the microemulsion at the extraction well. The second experiment was designed to demonstrate the effect of large buoyancy forces that lead to excessive downward migration of the microemulsion. Density measurements of aqueous solutions containing sodium dihexyl sulfosuccinate surfactant, isopropanol, trichloroethylene, and sodium chloride are presented. A companion paper presents the results of the flow and transport calculations needed for this approach to surfactant flooding. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Groundwater; Microemulsion; Remediation; Surfactant; DNAPL; Contaminants
)
Corresponding author. Fax: q1-512-471-9678; E-mail: [email protected] Fax: q1-512-471-6548; E-mail: [email protected]. 2 E-mail: [email protected]. 3 E-mail: [email protected]. 1
0169-7722r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 7 7 2 2 Ž 9 8 . 0 0 0 9 1 - 6
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K. Kostarelos et al.r Journal of Contaminant Hydrology 34 (1998) 383–397
1. Introduction The use of surfactants for remediation of subsurface sites contaminated by dense, non-aqueous phase liquids ŽDNAPLs. has been investigated by several research groups ŽEllis et al., 1986; Abdul et al., 1990; Gannon et al., 1992; Pennell et al., 1994; Pope and Wade, 1995; Shiau et al., 1995; Fountain et al., 1996; Martel and Gelinas, 1996; Hirasaki et al., 1997; Jawitz et al., 1998.. Mobilization may cause the DNAPL to move deeper into former uncontaminated zones ŽPankow and Cherry, 1996.. For this reason, some researchers recommend using solubilization rather than mobilization as the mechanism to recover the contaminants in order to avoid downward movement of the DNAPL as a separate phase. However, solubilization of contaminant by the surfactant creates a liquid phase Žpreferably a classical microemulsion as described by Bourrel and Schechter, 1988. that is denser than the surrounding groundwater and can travel downward before being captured by an extraction well. This paper discusses a method of solubilizing a contaminant while maintaining a relatively low microemulsion density, and introduces the concept of neutral buoyancy in order to control vertical migration and ensure recovery of the contaminant. Surfactant solutions can be designed to solubilize large amounts of contaminant, which translates into fast recovery rates in the field. However, the higher solubilization also results in increases in microemulsion density and a consequently higher risk of downward migration in the aquifer. Using a surfactant solution of intermediate contaminant solubilization keeps the microemulsion density low. This minimizes the risk of downward migration while still offering relatively rapid clean-up. A theoretical and numerical analysis in a companion paper ŽShook et al., 1998. shows that the vertical migration of a microemulsion that is denser than the surrounding groundwater is described in terms of the following dimensionless scaling groups: a gravity number Ž NG ., an effective aspect ratio Ž R L . and a mobility ratio Ž M8.. Numerical studies presented in Shook et al. Ž1998. further demonstrate that vertical migration is reduced with a reduction in NG or R L for all values of M8. Reductions in NG may be achieved in any of three ways: reducing microemulsion phase density, increasing microemulsion viscosity, or increasing the injection rate. In the experiments discussed in this paper, all three means of reducing NG were used to control vertical migration. Adding a light alcohol such as isopropanol reduces the microemulsion density. In fact, adding sufficient alcohol reduces the density to that of groundwater Židentically neutral buoyancy., though from a design perspective this much reduction is typically not required. In general, adding alcohol to a surfactant solution affects the amount of contaminant that is solubilized and consequently the interfacial tension between the microemulsion and contaminant phase ŽHuh, 1979.. The reduction in trichloroethylene solubilization due to adding isopropanol was very small. Microemulsion viscosity can be increased by adding a mobility control agent such as foam ŽHirasaki et al., 1997. or polymer ŽLake, 1989; Pope and Wade, 1995.. In this research work, polymer was used to increase viscosity. The two tank experiments presented in this paper employ surfactant solutions specifically formulated to solubilize DNAPL without mobilizing it. The injected surfac-
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tant solutions and corresponding flow rates yield a trapping number ŽJin, 1995; Pennell et al., 1996. low enough to prevent mobilizing the DNAPL, but still solubilize a significant amount of trichloroethylene. More information on surfactant solutions and their behavior can be found in Bourrel and Schechter Ž1988. and their application to EOR is discussed in Pope and Baviere ´ Ž1991.. The first experiment showed that vertical migration of the microemulsion could be reduced to an acceptable level when the density was close to that of water. By an acceptable level it is meant that hydraulic control over the microemulsion was maintained and full recovery of the DNAPL was achieved. The second experiment shows that loss of control over the microemulsion can occur if buoyancy effects are not minimized.
2. Description of chemicals, equipment and methods 2.1. Chemicals The surfactant used, sodium dihexyl sulfocuccinate, is commercially available from Cytec in an 80% active form called Aerosol MA-80 and is a food grade surfactant. Sodium dihexyl sulfosuccinate is a twin tailed ester sulfonate shown below.
Laboratory studies at the University of Texas ŽShotts, 1996; Dwarakanath, 1997; Dwarakanath et al., 1998; Kostarelos, 1998. have shown that it has very favorable properties for surfactant remediation, and this has been confirmed by a recent successful field demonstration ŽBrown et al., 1998.. Several alcohols were screened, but the density data presented in this paper are limited to ethanol and isopropanol ŽIPA. because these gave the most favorable results. Isopropanol was used in the tank experiments. Polymer was used in the surfactant solutions for both experiments. Adding a small amount of water-soluble polymer to an injected solution to increase its viscosity has been done for improved oil recovery for more than 30 yr and found to be very beneficial. The increased viscosity lowers the mobility of the displacing fluid compared to the mobility of the displaced fluid, resulting in better sweep efficiency and hydrodynamic stability ŽLake, 1989; Sorbie, 1991; Pope and Wade, 1995.. Phase mobility is
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defined as the phase permeability divided by the phase viscosity. The ratio formed by dividing the mobility of the displacing fluid by the mobility of the displaced fluid is called the mobility ratio. For these tank experiments, the displacing fluid is the surfactant solution and the displaced fluid is the resident water. As shown in this paper and the companion paper ŽShook et al., 1998., altering the viscosity can also be used to obtain the desired gravity and mobility ratios for the neutral buoyancy approach to surfactant flooding. Xanthan gum, a biopolymer used as a common food additive, was chosen for these experiments. For additional discussion of the chemicals used, see Kostarelos Ž1998.. 2.2. Tank description and experimental methods The tank was made of two parallel plates of 3r8-in. glass with aluminum end and bottom pieces, as shown in Fig. 1. The inside dimensions of the tank are 103 cm long, 46.7 cm high, and 5.1 cm wide. The sand was packed to a height of 40 cm and the remaining space was left open at the top to model an unconfined aquifer. The tank was packed by raining dry sand through a series of screens that served to distribute the rain evenly across the tank width, while moving the rainer along the tank length. Before water saturating, carbon dioxide gas was passed through the tank to displace any air and prevent trapping any air within the sand. The two end pieces functioned as injectionrextraction wells and this arrangement provided a minimum amount of dead volume. A 1r16-in. o.d. stainless steel tube was inserted from above and used to release trichloroethylene ŽTCE. into the sandpack. The TCE was held in a small reservoir that was elevated to impose a gradient into the tank and overcome the entry pressure of the porous medium. The intrinsic permeability was measured with a steady state water flood and a conservative tracer test was performed to measure the saturated pore volume of the model aquifer contacted by the water flood. Trichloroethylene Ždyed with Oil Red ‘O’
Fig. 1. Initial experimental set-up.
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from Fisher Scientific. was spilled from a point source within the sand creating a contamination zone near the injection well and close to the top of the pack. The TCE spill was followed by a partitioning tracer test to estimate the residual DNAPL saturation. Next, the surfactantrpolymer flood and a final partitioning tracer test were conducted. With the exception of the TCE spill, all fluids were injected from a port that was screened from the top to the bottom of the tank, and produced in a similar manner. The surfactant injection was conducted at constant flow rate using a dual-action piston pump while recording any variations in pressure drop across the tank. Other fluids were injected under constant head using a Mariotte tube while measuring the flow rate. A partitioning tracer test involves injecting and extracting several tracers and measuring their retention times. These tracers are selected so that they partition between the DNAPL and groundwater with different partition coefficients. The tracers used to measure the TCE average saturation were isopropanol, 2,3-dimethyl-2-butanol, 2methyl-2-hexanol and 1-heptanol. More information on the tracer tests can be found in Kostarelos Ž1998.. When transported through the contaminated sandpack, the tracers exhibit chromatographic separation. The retardation of each tracer depends on the volume of the DNAPL and its partition coefficient and can be used to calculate the DNAPL volume andror average saturation using the method of first temporal moments. Additional information on partitioning tracer tests can be found in Jin et al. Ž1995. and Dwarakanath Ž1997.. This approach served both as a check on the initial DNAPL saturation and a measure of the final DNAPL saturation as well as one of several experiments that have been done to validate its use in the field. More details regarding the tank design, experimental procedures, and the partitioning tracer data can be found in Kostarelos Ž1998..
3. Experimental results 3.1. Physical property data Microemulsion density as a function of alcohol and TCE concentration was measured for two alcohols: ethanol and isopropanol. Densities of single-phase microemulsions were measured at 258C using a pycnometer. Density measurements were made on solutions of 5 wt.% MA-80 Ž4 wt.% active sodium dihexyl sulfosuccinate., 0.6 wt.% sodium chloride, alcohol concentrations ranging from 0 to 8 wt.%, TCE concentrations ranging from 0 to 6 wt.%, and water. Polymer was omitted in these experiments because it has negligible effect on density at the concentrations used for the tank experiments. Densities ranged from 0.9944 to 1.029 grcm3, which is considered to be within the range of interest for this study. Fig. 2 shows the microemulsion density as a function of alcohol and TCE concentrations. All of these trends appear to be linear over the range studied within the precision of the data, which is "0.001 grcm3. Both alcohols reduce the density of the microemulsion, with the reduction slightly more for IPA than for ethanol. A least-squares fit was made to the measured density data using a linear equation with microemulsion density Žgrcm3 . as the dependent variable, the concentration of
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Fig. 2. Microemulsion density for increasing concentration of alcohol Žeither ethanol or isopropanol., 4 wt.% sodium dihexyl sulfosuccinate, 0.6 wt.% sodium chloride, and trichloroethylene.
each component Žmass fraction. as independent variables and apparent density Ž r ). of each component as constant coefficients as follows: rm .e.s r brine )C brine q r MA )C MA q ralcohol )Calcohol q r TCE )C TCE . The values of apparent density given in Table 1 can be used to calculate the microemulsion density for other solutions consisting of these components. Although the dependence of the microemulsion density on the concentration of each component is linear for the range studied, this does not mean that the solution is ideal in the thermodynamic sense. In fact it is highly non-ideal as indicated by the apparent density of the TCE differing considerably from the value for pure TCE of 1.46 grcm3. 3.2. Model aquifer results The same tank was used for both tank experiments, and the same type of sand and packing methods were also used. The primary difference between the experiments is in the solutions used and the injection rates, each being designed for the intended purpose of each experiment.
Table 1 Apparent density for solutions of brine, sodium dihexyl sulfosuccinate, alcohol, and trichloroethylene Components
brine, 0.6 wt.% NaCl sodium dihexyl sulfosuccinate alcohol trichloroethylene
Apparent density, grcm3 Ethanol
Isopropanol
1.0014 1.1781 0.8358 1.3213
1.0014 1.1838 0.8046 1.3187
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Fig. 3. Front side of tank used in experiments after TCE Žred. spill for first tank experiment. 389
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Fig. 4. TCE location on back side of tank before surfactant flood.
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Fig. 5. Close-up of TCE location on front side of tank before surfactant.
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3.2.1. Surfactant flood results— first tank experiment (DK-5) The intrinsic permeability of the sand pack was measured using fresh water. The permeability of the sand pack was 1 = 10y1 1 m2 Ž10 Darcy. based upon measurements of pressure drop and flow rate. The pore volume estimated from a conservative tracer test was 7670 cm3. About 170 cm3 of TCE, or 2.2% average saturation, was released for this experiment. A photograph of the front of the tank ŽFig. 3. shows the location of the TCE Žred. after the release. Photographs of the sand pack after the TCE spill are shown in Fig. 4 Žback side. and Fig. 5 Žfront side.. The first experiment was designed to demonstrate that, with appropriate attention to buoyancy effects, vertical migration of the microemulsion is manageable. The surfactant flood was started at a constant injection rate of 4.4 cm3rmin Ž0.33 mrD Darcy velocity.. In this experiment, the aqueous surfactant solution contained 4 wt.% sodium dihexyl sulfosuccinate, 0.6 wt.% sodium chloride, 8 wt.% isopropanol, and 0.05 wt.% xanthan gum. This chemical composition results in an equilibrium TCE solubility of approximately 3.3 wt.%. The resulting microemulsion phase density under these conditions is 1.003 grcm3. At the Darcy velocity of 0.33 mrD, the microemulsion had a viscosity of approximately 5 mPa s Ž5 cp.. Injection of such a viscous fluid caused the head to increase steadily. After 1.2 pore volumes, the rate was lowered to 2.2 cm3rmin to reduce the head to an acceptable level Table 2. The ability of the surfactant solution to solubilize the contaminant without mobilizing DNAPL can be observed qualitatively using the photographs of the surfactantrpolymer flood. The red-dyed trichloroethylene was solubilized without any mobilization of the trichloroethylene DNAPL from its location before the surfactant flood based on visual observation through the glass walls of the tank. Vertical migration of the TCE-rich microemulsion, h, was smaller than the depth of the recovery wells, H, as the experimental design intended. Fig. 6 shows the location of the TCE Žfront side view. after about 13 h of surfactant injection. The TCE visible in the upper left corner of Fig. 5 has migrated to the right without downward migration. The TCE visible in the center of Fig. 5 has started to show some migration. In Fig. 7, after about 20 h of surfactant injection, migration continues to be horizontal without any vertical migration.
Table 2 Experimental conditions Property
First experiment ŽDK-5.
Second experiment ŽDK-6.
salinity Žwt.%. surfactant concentration Žwt.%, active. isopropanol concentration Žwt.%. xanthan gum concentration Žwt.%. solubilization ratio solubilized trichloroethylene Žwt.%. microemulsion density Žgrcm3 . surfactant density Žgrcm3 . flow rate Žcm3 rmin. viscosity of injected solution Žcp.
0.4 4.0 8.0 0.05 0.57 3.3 1.003 0.993 4.4 5
0.7 4.0 0.0 0.03 1.0 5.9 1.028 1.009 0.80 3
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Fig. 6. TCE location after 13 h of surfactant flooding.
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Fig. 7. TCE location after 20 h of surfactant flooding.
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Fig. 8. TCE concentration in effluent.
Effluent samples were collected at frequent intervals and measured by gas chromatography ŽGC. for TCE concentration. As shown in Fig. 8, the TCE concentration increased from about 0.05 wt.% to about 0.5 wt.% before declining. The TCE concentration at 3.1 pore volumes was about 0.004 wt.%. At this time, a short length of nylon tubing was removed from the outlet and replaced with stainless steel tubing and the flood continued. The TCE concentration was not detectable on a gas chromatograph calibrated down to 0.0001 wt.% between 3.1 and 3.4 pore volumes when the experiment was stopped. 3.2.2. Surfactant flood results— second tank experiment (DK-6) A second experiment was designed to demonstrate the potential for unacceptable vertical migration in cases where the buoyancy forces are unacceptably large. To achieve this, we increased the gravity number by three means: increasing the microemulsion density by omitting IPA, decreasing the Darcy velocity, and reducing viscosity. These changes result in an unacceptably large increase in buoyancy forces, leading to excessive downward migration of the microemulsion. The measured permeability of the sand pack used was 5 Darcy with a pore volume, measured by tracers, of 7020 cm3. A 45 cm3 TCE spill was used, producing a 0.6% average saturation. The surfactant flood was conducted at a flow rate of 0.80 cm3rmin using an aqueous surfactant solution of 4 wt.% sodium dihexyl sulfosuccinate, 0.7 wt.% sodium chloride, and 0.03 wt.% xanthan gum. This solution resulted in a TCE solubility in the microemulsion of 5.9 wt.%, and a microemulsion phase density of 1.028 grcm3. At the Darcy velocity of this experiment Ž0.17 mrD., the microemulsion viscosity was approximately 3 mPa s Ž3 cp.. Visual observation indicated that the solubilized TCE underwent dramatic downward migration, reaching the bottom of the sand pack after injection of only 0.25 pore volumes of surfactant. This experiment demonstrated that the TCE-rich microemulsion
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will migrate downward despite horizontal bulk fluid movement. This implies that the TCE can migrate outside of the capture zone of an extraction well. From a comparison of the two experiments, it is concluded that proper design is essential to recover a DNAPL such as TCE, using the surfactant-enhanced remediation process. It is further apparent from these experiments that density differences can be overcome, and DNAPL can be recovered without uncontrolled vertical migration.
4. Conclusions This paper presents results of laboratory experiments that illustrate a new means of using surfactants for DNAPL removal from groundwater. An aqueous solution of surfactant and alcohol is injected at a specified rate and forms a microemulsion in situ when it solubilizes the DNAPL. Sufficient amounts of light alcohol such as isopropanol can be added to reduce the density of the microemulsion to make it neutrally buoyant with respect to the groundwater or as close as needed to control the migration of the microemulsion plume to ensure its complete capture by the extraction wells. Polymer can be added to increase its viscosity and aid in the control of the gravity and viscous forces needed to achieve this goal. We call this approach Surfactant Enhanced Aquifer Remediation at Neutral Buoyancy, or SEAR-NB. The flow and transport calculations needed to design such a SEAR-NB flood are presented in a companion paper. Using SEAR-NB, DNAPL can be removed from aquifers with no capillary barrier or an unknown or insufficient capillary barrier. This greatly increases the potential to use surfactants to remediate aquifers contaminated by DNAPLs without undue risk of spreading the contaminants downward into uncontaminated groundwater.
Acknowledgements The Idaho National Engineering and Environmental Laboratory provided funding for this research. Several students and technical staff at the University of Texas including T. Bermudez, G. Baum, V. Dwarakanath, Doug Shotts, Meng Lim, Neil Deeds, Regina Eco, Filipe Ip, Kiam Ooi and B. Savicki provided assistance with the tank experiments. We also wish to thank David Daniel for his advice and assistance in the experiments and Robert Knox for fabricating the sand tank.
References Abdul, A.S., Gibson, T.L., Rai, D.N., 1990. Selection of surfactants for the removal of petroleum products from shallow sandy aquifers. Ground Water 28 Ž6., 920. Bourrel, M., Schechter, R.S., 1988. Microemulsions and Related Systems. Marcel Dekker, New York. Brown, C.L., Delshad, M., Dwarakanath, V., McKinney, D.C., Pope, G.A., Wade, W.H., Jackson, R.E., Londergan, J.T., Meinardous, H.W., 1998. A successful field demonstration of surfactant flushing of an unconfined aquifer contaminated by chlorinated solvents. ACS Symposium Series, American Chemical Society, Washington, DC., In press.
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Dwarakanath, V., 1997. Characterization and remediation of aquifers contaminated by non-aqueous phase liquids using partitioning tracer and surfactants. Ph.D. Dissertation, University of Texas at Austin, Austin Texas. Dwarakanath, V., Kostarelos, K., Pope, G.A., Shotts, D., Wade, W.H., 1998. Surfactant remediation of soil columns contaminated by nonaqueous phase liquids. J. Contam. Hydrol., in press. Ellis, W.D., Morgan, D.R., Ranjithan, S.R., 1986. Treatment of contaminated soils with aqueous surfactants. EPAr600r2-85r129, Hazardous Waste Engineering Research Laboratory. Fountain, J.C., Starr, R.C., Middleton, T., Beikirch, M., Taylor, C., Hodge, D., 1996. A controlled field test of surfactant-enhanced aquifer remediation. Ground Water 35 Ž5., 910–916. Gannon, O.K., Bribing, P., Raney, K., Ward, A., Wilson, J., Underwood, J.L., Deblak, K.A., 1992. Soil clean up by in situ surfactant flushing: III. Laboratory results. Journal of Science Technology, September, pp. 1073–1094. Hirasaki, G.J., Miller, C.A., Szafranski, R., Lawson, J.B., Akiya, N., 1997. Surfactantrfoam process for aquifer remediation. SPE paper 37257. 1997 SPE International Symposium on Oil Field Chemistry, Feb 18–21, Houston, TX. Huh, C., 1979. Interfacial tensions and solubilizing ability of a microemulsion phase that coexists with oil and bring. J. Colloidal and Interface Science 71, 408–426. Jawitz, J.W., Annable, M.D., Rao P.S.C., Rhue, R.D., 1998. Field implementation of a Winsor type I surfactantralcohol mixture for in situ solubilization of a complex LNAPL as a single-phase microemulsion. Jin, M., 1995. Surfactant enhanced remediation and interwell partitioning tracer test for characterization of NAPL contaminated aquifers. Ph.D. Dissertation, University of Texas, Austin, TX. Jin, M., Delshad, M., Dwarakanath, V., McKinney, D.C., Pope, G.A., Sepehrnoori, K., Tilburg, C., Jackson, R.E., 1995. Partitioning tracer test for detection, estimation and remediation performance assessment of subsurface nonaqueous-phase liquids. Water Resour. Res. 31 Ž5., 1201–1211. Kostarelos, K., 1998. Surfactant enhanced aquifer remediation at neutral buoyancy. Ph.D. Dissertation, University of Texas at Austin, Austin, TX. Lake, L.W., 1989. Enhanced Oil Recovery. Prentice Hall, Englewood Cliffs, NJ. Martel, R., Gelinas, P.J., 1996. Surfactant solutions developed for NAPL recovery in contaminated aquifers. Ground Water 34 Ž1., 143–154. Pankow, J.F., Cherry, J.A., 1996. Dense Chlorinated Solvents and other NAPLs in Groundwater. Waterloo Press, Portland, OR., 500 pp. Pennell, K.D., Jin, M., Abriola, L.M., Pope, G.A., 1994. Surfactant enhanced remediation of soil columns contaminated by residual tetrachloroethylene. J. Contaminant Hydrology 16 Ž1., 35. Pennell, K.D., Pope, G.A., Abriola, L.M., 1996. Influence of viscous and buoyancy forces on the mobilization of residual tetrachloroethylene during surfactant flushing. Environ. Sci. and Technol. 30 Ž4., 1328–1335. Pope, G.A., Bavire, M., 1991. Basic concepts in enhanced oil recovery processes. In: Bavire, M. ŽEd.., Crictical Reports on Applied Chemistry. Elsevier Science Publishing, London, UK. Pope, G.A., Wade, W.H., 1995. Lessons from enhanced oil recovery research for surfactant enhanced aquifer remediation. In: Sabatini, D.A., Knox, R.C., Harwell, J.H. ŽEds.., Surfacant-enhanced subsurface remediation: emerging technologies, ACS symposium series No. 594. American Chemical Society, Washington, DC, pp. 142–160. Shiau, B., Rouse, J.D., Sabatini, D.A., Harwell, J.H., 1995. Surfactant selection for optimizing surfactant-enhanced subsurface remediation. In: Sabatini, D.A., Knox, R.C., Harwell, J.H. ŽEds.., Surfacant-enhanced subsurface remediation: emerging technologies, ACS symposium series No. 594. American Chemical Society, Washington, DC, pp. 65–79. Shook, G.M., Kostarelos, K., Pope, G.A., 1998. Minimization of vertical migration of DNAPLs using surfactant enhanced aquifer remediation at neutral buoyancy. J. Contaminant Hydrology, this edition. Shotts, D.R., 1996. Surfactant remediation of soils contaminated with chlorinated solvents. M.S. Thesis, The University of Texas at Austin. Sorbie, K.S., 1991. Polymer Improved Oil Recovery. Blackie and Son, Bishopbriggs, Glasgow, G64 2NZ.
A new concept: the use of neutrally-buoyant microemulsions for DNAPL remediation K. Kostarelos
a,1
, G.A. Pope
b,)
, B.A. Rouse
b,2
, G.M. Shook
c,3
a
b
Department of CiÕil Engineering, The UniÕersity of Texas at Austin, Austin, Texas 78712, USA Department of Petroleum and Geosystems Engineering, The UniÕersity of Texas at Austin, Austin, Texas 78712, USA c Idaho National Engineering and EnÕironmental Laboratory, Idaho Falls, ID 83415-2107, USA Received 23 October 1997; accepted 11 May 1998
Abstract Even in the absence of mobilization of dense nonaqueous phase liquid ŽDNAPL., the microemulsion that forms when the surfactant solubilizes a dense contaminant such as trichloroethylene will be more dense than water and tends to migrate downward. This paper addresses the issue of migration with a new concept: surfactant enhanced aquifer remediation at neutral buoyancy. Laboratory results of surfactant remediation in two-dimensional model aquifers show that downward migration of microemulsion containing solubilized dense contaminants can be reduced to an acceptable level, even in the absence of capillary barriers in the aquifer. One model experiment was designed to exhibit a small degree of vertical migration and full capture of the microemulsion at the extraction well. The second experiment was designed to demonstrate the effect of large buoyancy forces that lead to excessive downward migration of the microemulsion. Density measurements of aqueous solutions containing sodium dihexyl sulfosuccinate surfactant, isopropanol, trichloroethylene, and sodium chloride are presented. A companion paper presents the results of the flow and transport calculations needed for this approach to surfactant flooding. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Groundwater; Microemulsion; Remediation; Surfactant; DNAPL; Contaminants
)
Corresponding author. Fax: q1-512-471-9678; E-mail: [email protected] Fax: q1-512-471-6548; E-mail: [email protected]. 2 E-mail: [email protected]. 3 E-mail: [email protected]. 1
0169-7722r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 7 7 2 2 Ž 9 8 . 0 0 0 9 1 - 6
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1. Introduction The use of surfactants for remediation of subsurface sites contaminated by dense, non-aqueous phase liquids ŽDNAPLs. has been investigated by several research groups ŽEllis et al., 1986; Abdul et al., 1990; Gannon et al., 1992; Pennell et al., 1994; Pope and Wade, 1995; Shiau et al., 1995; Fountain et al., 1996; Martel and Gelinas, 1996; Hirasaki et al., 1997; Jawitz et al., 1998.. Mobilization may cause the DNAPL to move deeper into former uncontaminated zones ŽPankow and Cherry, 1996.. For this reason, some researchers recommend using solubilization rather than mobilization as the mechanism to recover the contaminants in order to avoid downward movement of the DNAPL as a separate phase. However, solubilization of contaminant by the surfactant creates a liquid phase Žpreferably a classical microemulsion as described by Bourrel and Schechter, 1988. that is denser than the surrounding groundwater and can travel downward before being captured by an extraction well. This paper discusses a method of solubilizing a contaminant while maintaining a relatively low microemulsion density, and introduces the concept of neutral buoyancy in order to control vertical migration and ensure recovery of the contaminant. Surfactant solutions can be designed to solubilize large amounts of contaminant, which translates into fast recovery rates in the field. However, the higher solubilization also results in increases in microemulsion density and a consequently higher risk of downward migration in the aquifer. Using a surfactant solution of intermediate contaminant solubilization keeps the microemulsion density low. This minimizes the risk of downward migration while still offering relatively rapid clean-up. A theoretical and numerical analysis in a companion paper ŽShook et al., 1998. shows that the vertical migration of a microemulsion that is denser than the surrounding groundwater is described in terms of the following dimensionless scaling groups: a gravity number Ž NG ., an effective aspect ratio Ž R L . and a mobility ratio Ž M8.. Numerical studies presented in Shook et al. Ž1998. further demonstrate that vertical migration is reduced with a reduction in NG or R L for all values of M8. Reductions in NG may be achieved in any of three ways: reducing microemulsion phase density, increasing microemulsion viscosity, or increasing the injection rate. In the experiments discussed in this paper, all three means of reducing NG were used to control vertical migration. Adding a light alcohol such as isopropanol reduces the microemulsion density. In fact, adding sufficient alcohol reduces the density to that of groundwater Židentically neutral buoyancy., though from a design perspective this much reduction is typically not required. In general, adding alcohol to a surfactant solution affects the amount of contaminant that is solubilized and consequently the interfacial tension between the microemulsion and contaminant phase ŽHuh, 1979.. The reduction in trichloroethylene solubilization due to adding isopropanol was very small. Microemulsion viscosity can be increased by adding a mobility control agent such as foam ŽHirasaki et al., 1997. or polymer ŽLake, 1989; Pope and Wade, 1995.. In this research work, polymer was used to increase viscosity. The two tank experiments presented in this paper employ surfactant solutions specifically formulated to solubilize DNAPL without mobilizing it. The injected surfac-
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tant solutions and corresponding flow rates yield a trapping number ŽJin, 1995; Pennell et al., 1996. low enough to prevent mobilizing the DNAPL, but still solubilize a significant amount of trichloroethylene. More information on surfactant solutions and their behavior can be found in Bourrel and Schechter Ž1988. and their application to EOR is discussed in Pope and Baviere ´ Ž1991.. The first experiment showed that vertical migration of the microemulsion could be reduced to an acceptable level when the density was close to that of water. By an acceptable level it is meant that hydraulic control over the microemulsion was maintained and full recovery of the DNAPL was achieved. The second experiment shows that loss of control over the microemulsion can occur if buoyancy effects are not minimized.
2. Description of chemicals, equipment and methods 2.1. Chemicals The surfactant used, sodium dihexyl sulfocuccinate, is commercially available from Cytec in an 80% active form called Aerosol MA-80 and is a food grade surfactant. Sodium dihexyl sulfosuccinate is a twin tailed ester sulfonate shown below.
Laboratory studies at the University of Texas ŽShotts, 1996; Dwarakanath, 1997; Dwarakanath et al., 1998; Kostarelos, 1998. have shown that it has very favorable properties for surfactant remediation, and this has been confirmed by a recent successful field demonstration ŽBrown et al., 1998.. Several alcohols were screened, but the density data presented in this paper are limited to ethanol and isopropanol ŽIPA. because these gave the most favorable results. Isopropanol was used in the tank experiments. Polymer was used in the surfactant solutions for both experiments. Adding a small amount of water-soluble polymer to an injected solution to increase its viscosity has been done for improved oil recovery for more than 30 yr and found to be very beneficial. The increased viscosity lowers the mobility of the displacing fluid compared to the mobility of the displaced fluid, resulting in better sweep efficiency and hydrodynamic stability ŽLake, 1989; Sorbie, 1991; Pope and Wade, 1995.. Phase mobility is
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defined as the phase permeability divided by the phase viscosity. The ratio formed by dividing the mobility of the displacing fluid by the mobility of the displaced fluid is called the mobility ratio. For these tank experiments, the displacing fluid is the surfactant solution and the displaced fluid is the resident water. As shown in this paper and the companion paper ŽShook et al., 1998., altering the viscosity can also be used to obtain the desired gravity and mobility ratios for the neutral buoyancy approach to surfactant flooding. Xanthan gum, a biopolymer used as a common food additive, was chosen for these experiments. For additional discussion of the chemicals used, see Kostarelos Ž1998.. 2.2. Tank description and experimental methods The tank was made of two parallel plates of 3r8-in. glass with aluminum end and bottom pieces, as shown in Fig. 1. The inside dimensions of the tank are 103 cm long, 46.7 cm high, and 5.1 cm wide. The sand was packed to a height of 40 cm and the remaining space was left open at the top to model an unconfined aquifer. The tank was packed by raining dry sand through a series of screens that served to distribute the rain evenly across the tank width, while moving the rainer along the tank length. Before water saturating, carbon dioxide gas was passed through the tank to displace any air and prevent trapping any air within the sand. The two end pieces functioned as injectionrextraction wells and this arrangement provided a minimum amount of dead volume. A 1r16-in. o.d. stainless steel tube was inserted from above and used to release trichloroethylene ŽTCE. into the sandpack. The TCE was held in a small reservoir that was elevated to impose a gradient into the tank and overcome the entry pressure of the porous medium. The intrinsic permeability was measured with a steady state water flood and a conservative tracer test was performed to measure the saturated pore volume of the model aquifer contacted by the water flood. Trichloroethylene Ždyed with Oil Red ‘O’
Fig. 1. Initial experimental set-up.
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from Fisher Scientific. was spilled from a point source within the sand creating a contamination zone near the injection well and close to the top of the pack. The TCE spill was followed by a partitioning tracer test to estimate the residual DNAPL saturation. Next, the surfactantrpolymer flood and a final partitioning tracer test were conducted. With the exception of the TCE spill, all fluids were injected from a port that was screened from the top to the bottom of the tank, and produced in a similar manner. The surfactant injection was conducted at constant flow rate using a dual-action piston pump while recording any variations in pressure drop across the tank. Other fluids were injected under constant head using a Mariotte tube while measuring the flow rate. A partitioning tracer test involves injecting and extracting several tracers and measuring their retention times. These tracers are selected so that they partition between the DNAPL and groundwater with different partition coefficients. The tracers used to measure the TCE average saturation were isopropanol, 2,3-dimethyl-2-butanol, 2methyl-2-hexanol and 1-heptanol. More information on the tracer tests can be found in Kostarelos Ž1998.. When transported through the contaminated sandpack, the tracers exhibit chromatographic separation. The retardation of each tracer depends on the volume of the DNAPL and its partition coefficient and can be used to calculate the DNAPL volume andror average saturation using the method of first temporal moments. Additional information on partitioning tracer tests can be found in Jin et al. Ž1995. and Dwarakanath Ž1997.. This approach served both as a check on the initial DNAPL saturation and a measure of the final DNAPL saturation as well as one of several experiments that have been done to validate its use in the field. More details regarding the tank design, experimental procedures, and the partitioning tracer data can be found in Kostarelos Ž1998..
3. Experimental results 3.1. Physical property data Microemulsion density as a function of alcohol and TCE concentration was measured for two alcohols: ethanol and isopropanol. Densities of single-phase microemulsions were measured at 258C using a pycnometer. Density measurements were made on solutions of 5 wt.% MA-80 Ž4 wt.% active sodium dihexyl sulfosuccinate., 0.6 wt.% sodium chloride, alcohol concentrations ranging from 0 to 8 wt.%, TCE concentrations ranging from 0 to 6 wt.%, and water. Polymer was omitted in these experiments because it has negligible effect on density at the concentrations used for the tank experiments. Densities ranged from 0.9944 to 1.029 grcm3, which is considered to be within the range of interest for this study. Fig. 2 shows the microemulsion density as a function of alcohol and TCE concentrations. All of these trends appear to be linear over the range studied within the precision of the data, which is "0.001 grcm3. Both alcohols reduce the density of the microemulsion, with the reduction slightly more for IPA than for ethanol. A least-squares fit was made to the measured density data using a linear equation with microemulsion density Žgrcm3 . as the dependent variable, the concentration of
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Fig. 2. Microemulsion density for increasing concentration of alcohol Žeither ethanol or isopropanol., 4 wt.% sodium dihexyl sulfosuccinate, 0.6 wt.% sodium chloride, and trichloroethylene.
each component Žmass fraction. as independent variables and apparent density Ž r ). of each component as constant coefficients as follows: rm .e.s r brine )C brine q r MA )C MA q ralcohol )Calcohol q r TCE )C TCE . The values of apparent density given in Table 1 can be used to calculate the microemulsion density for other solutions consisting of these components. Although the dependence of the microemulsion density on the concentration of each component is linear for the range studied, this does not mean that the solution is ideal in the thermodynamic sense. In fact it is highly non-ideal as indicated by the apparent density of the TCE differing considerably from the value for pure TCE of 1.46 grcm3. 3.2. Model aquifer results The same tank was used for both tank experiments, and the same type of sand and packing methods were also used. The primary difference between the experiments is in the solutions used and the injection rates, each being designed for the intended purpose of each experiment.
Table 1 Apparent density for solutions of brine, sodium dihexyl sulfosuccinate, alcohol, and trichloroethylene Components
brine, 0.6 wt.% NaCl sodium dihexyl sulfosuccinate alcohol trichloroethylene
Apparent density, grcm3 Ethanol
Isopropanol
1.0014 1.1781 0.8358 1.3213
1.0014 1.1838 0.8046 1.3187
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Fig. 3. Front side of tank used in experiments after TCE Žred. spill for first tank experiment. 389
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Fig. 4. TCE location on back side of tank before surfactant flood.
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Fig. 5. Close-up of TCE location on front side of tank before surfactant.
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3.2.1. Surfactant flood results— first tank experiment (DK-5) The intrinsic permeability of the sand pack was measured using fresh water. The permeability of the sand pack was 1 = 10y1 1 m2 Ž10 Darcy. based upon measurements of pressure drop and flow rate. The pore volume estimated from a conservative tracer test was 7670 cm3. About 170 cm3 of TCE, or 2.2% average saturation, was released for this experiment. A photograph of the front of the tank ŽFig. 3. shows the location of the TCE Žred. after the release. Photographs of the sand pack after the TCE spill are shown in Fig. 4 Žback side. and Fig. 5 Žfront side.. The first experiment was designed to demonstrate that, with appropriate attention to buoyancy effects, vertical migration of the microemulsion is manageable. The surfactant flood was started at a constant injection rate of 4.4 cm3rmin Ž0.33 mrD Darcy velocity.. In this experiment, the aqueous surfactant solution contained 4 wt.% sodium dihexyl sulfosuccinate, 0.6 wt.% sodium chloride, 8 wt.% isopropanol, and 0.05 wt.% xanthan gum. This chemical composition results in an equilibrium TCE solubility of approximately 3.3 wt.%. The resulting microemulsion phase density under these conditions is 1.003 grcm3. At the Darcy velocity of 0.33 mrD, the microemulsion had a viscosity of approximately 5 mPa s Ž5 cp.. Injection of such a viscous fluid caused the head to increase steadily. After 1.2 pore volumes, the rate was lowered to 2.2 cm3rmin to reduce the head to an acceptable level Table 2. The ability of the surfactant solution to solubilize the contaminant without mobilizing DNAPL can be observed qualitatively using the photographs of the surfactantrpolymer flood. The red-dyed trichloroethylene was solubilized without any mobilization of the trichloroethylene DNAPL from its location before the surfactant flood based on visual observation through the glass walls of the tank. Vertical migration of the TCE-rich microemulsion, h, was smaller than the depth of the recovery wells, H, as the experimental design intended. Fig. 6 shows the location of the TCE Žfront side view. after about 13 h of surfactant injection. The TCE visible in the upper left corner of Fig. 5 has migrated to the right without downward migration. The TCE visible in the center of Fig. 5 has started to show some migration. In Fig. 7, after about 20 h of surfactant injection, migration continues to be horizontal without any vertical migration.
Table 2 Experimental conditions Property
First experiment ŽDK-5.
Second experiment ŽDK-6.
salinity Žwt.%. surfactant concentration Žwt.%, active. isopropanol concentration Žwt.%. xanthan gum concentration Žwt.%. solubilization ratio solubilized trichloroethylene Žwt.%. microemulsion density Žgrcm3 . surfactant density Žgrcm3 . flow rate Žcm3 rmin. viscosity of injected solution Žcp.
0.4 4.0 8.0 0.05 0.57 3.3 1.003 0.993 4.4 5
0.7 4.0 0.0 0.03 1.0 5.9 1.028 1.009 0.80 3
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Fig. 6. TCE location after 13 h of surfactant flooding.
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Fig. 7. TCE location after 20 h of surfactant flooding.
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Fig. 8. TCE concentration in effluent.
Effluent samples were collected at frequent intervals and measured by gas chromatography ŽGC. for TCE concentration. As shown in Fig. 8, the TCE concentration increased from about 0.05 wt.% to about 0.5 wt.% before declining. The TCE concentration at 3.1 pore volumes was about 0.004 wt.%. At this time, a short length of nylon tubing was removed from the outlet and replaced with stainless steel tubing and the flood continued. The TCE concentration was not detectable on a gas chromatograph calibrated down to 0.0001 wt.% between 3.1 and 3.4 pore volumes when the experiment was stopped. 3.2.2. Surfactant flood results— second tank experiment (DK-6) A second experiment was designed to demonstrate the potential for unacceptable vertical migration in cases where the buoyancy forces are unacceptably large. To achieve this, we increased the gravity number by three means: increasing the microemulsion density by omitting IPA, decreasing the Darcy velocity, and reducing viscosity. These changes result in an unacceptably large increase in buoyancy forces, leading to excessive downward migration of the microemulsion. The measured permeability of the sand pack used was 5 Darcy with a pore volume, measured by tracers, of 7020 cm3. A 45 cm3 TCE spill was used, producing a 0.6% average saturation. The surfactant flood was conducted at a flow rate of 0.80 cm3rmin using an aqueous surfactant solution of 4 wt.% sodium dihexyl sulfosuccinate, 0.7 wt.% sodium chloride, and 0.03 wt.% xanthan gum. This solution resulted in a TCE solubility in the microemulsion of 5.9 wt.%, and a microemulsion phase density of 1.028 grcm3. At the Darcy velocity of this experiment Ž0.17 mrD., the microemulsion viscosity was approximately 3 mPa s Ž3 cp.. Visual observation indicated that the solubilized TCE underwent dramatic downward migration, reaching the bottom of the sand pack after injection of only 0.25 pore volumes of surfactant. This experiment demonstrated that the TCE-rich microemulsion
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will migrate downward despite horizontal bulk fluid movement. This implies that the TCE can migrate outside of the capture zone of an extraction well. From a comparison of the two experiments, it is concluded that proper design is essential to recover a DNAPL such as TCE, using the surfactant-enhanced remediation process. It is further apparent from these experiments that density differences can be overcome, and DNAPL can be recovered without uncontrolled vertical migration.
4. Conclusions This paper presents results of laboratory experiments that illustrate a new means of using surfactants for DNAPL removal from groundwater. An aqueous solution of surfactant and alcohol is injected at a specified rate and forms a microemulsion in situ when it solubilizes the DNAPL. Sufficient amounts of light alcohol such as isopropanol can be added to reduce the density of the microemulsion to make it neutrally buoyant with respect to the groundwater or as close as needed to control the migration of the microemulsion plume to ensure its complete capture by the extraction wells. Polymer can be added to increase its viscosity and aid in the control of the gravity and viscous forces needed to achieve this goal. We call this approach Surfactant Enhanced Aquifer Remediation at Neutral Buoyancy, or SEAR-NB. The flow and transport calculations needed to design such a SEAR-NB flood are presented in a companion paper. Using SEAR-NB, DNAPL can be removed from aquifers with no capillary barrier or an unknown or insufficient capillary barrier. This greatly increases the potential to use surfactants to remediate aquifers contaminated by DNAPLs without undue risk of spreading the contaminants downward into uncontaminated groundwater.
Acknowledgements The Idaho National Engineering and Environmental Laboratory provided funding for this research. Several students and technical staff at the University of Texas including T. Bermudez, G. Baum, V. Dwarakanath, Doug Shotts, Meng Lim, Neil Deeds, Regina Eco, Filipe Ip, Kiam Ooi and B. Savicki provided assistance with the tank experiments. We also wish to thank David Daniel for his advice and assistance in the experiments and Robert Knox for fabricating the sand tank.
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