Environ. Sci. Technol. 2003, 37, 4246-4253
Use of a Surfactant-Stabilized Emulsion To Deliver 1-Butanol for Density-Modified Displacement of Trichloroethene C . A N D R E W R A M S B U R G , * ,†,‡ KURT D. PENNELL,‡ TOHREN C. G. KIBBEY,§ AND KIM F. HAYES† Department of Civil and Environmental Engineering, University of Michigan, 181 EWRE, 1351 Beal Avenue, Ann Arbor, Michigan 48109-2125, School of Civil and Environmental Engineering, 311 Ferst Drive, Georgia Institute of Technology, Atlanta, Georgia 30332-0512, and School of Civil Engineering and Environmental Science, University of Oklahoma, Norman, Oklahoma 73019-1024
A novel surfactant-enhanced aquifer remediation technology, density-modified displacement (DMD), has been developed to minimize risk of dense non-aqueous-phase liquid (DNAPL) downward migration during displacement floods. The DMD method is designed to be implemented using horizontal flushing schemes, with in situ DNAPL density conversion accomplished by the introduction of a partitioning alcohol (e.g., 1-butanol) in a predisplacement flood (preflood). Subsequent NAPL displacement and recovery is achieved by flushing with a low-interfacial-tension (lowIFT) surfactant solution. The efficiency of the DMD method may be enhanced for heavier DNAPLs, such as trichloroethene (TCE), by increasing alcohol delivery and the extent of partitioning during the preflood. The objective of this study was to evaluate the use of a macroemulsion, consisting of 4.7% (vol) Tween 80 + 1.3% (vol) Span 80 + 15% (vol) 1-butanol to achieve efficient in situ density conversion of TCE (relative to that obtained with use of an aqueous preflood solution) prior to low-IFT displacement and recovery from a two-dimensional aquifer cell. The cell was configured to represent a heterogeneous unconfined aquifer system with an overall NAPL saturation between 2% and 3%. After flooding with approximately 1.2 pore volumes of the macroemulsion, a low-IFT solution consisting of 10% (vol) Aerosol MA + 6% (vol) 1-butanol + 15 g/L NaCl + 1 g/L CaCl2 was introduced to displace and recover NAPL. Visual observations and quantitative measurements of effluent fluids demonstrated that in situ density conversion and displacement of TCE-NAPL was successful, with effluent NAPL densities ranging from 0.97 to 0.99 g/mL. For the experimental system employed herein, 93% recovery of the introduced TCE mass was realized after flushing with a combined 2.4 pore volumes of the density conversion and low-IFT solutions. These results demonstrate the increased efficiency of the DMD method * Corresponding author phone: (734)936-3175; fax: (734)763-2275; e-mail:
[email protected]. † University of Michigan. ‡ Georgia Institute of Technology. § University of Oklahoma. 4246
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when surfactant-based emulsions are used to enhance 1-butanol delivery and partitioning behavior.
Introduction Laboratory-scale experiments have demonstrated that nonaqueous-phase liquids (NAPLs) can be efficiently recovered from porous media using two-phase (immiscible) displacement technologies (1-3). In unconfined aquifer systems, however, implementation of this approach for dense nonaqueous-phase liquid (DNAPL) source zone remediation is likely to result in uncontrolled downward migration of the displaced free product. Hydrodynamic control approaches, including vertical flushing (4, 5) and high-density brine solutions (6, 7), have been used to mitigate such downward migration; however, both of these methods require confinement of the DNAPL source zone. Alternatively, several studies have investigated the use of partitioning alcohols to lower DNAPL density, thereby reducing the risk of downward migration (8-19). Much of this work has been conducted at the batch or column scale, and typically has been used in concert with vertical flushing (8-10, 12-15). More recent experiments demonstrated the utility of coupling in situ density conversion of DNAPLs to light non-aqueous-phase liquids (LNAPLs) using a partitioning alcohol (1-butanol) with low-interfacial-tension (low-IFT) displacement and recovery of the NAPL (11, 16, 18, 19). This process, referred to as density-modified displacement (DMD), was implemented in 2-D aquifer cell studies, under horizontal flow, using a 6% (wt) aqueous solution of 1-butanol for in situ density conversion of chlorobenzene (CB) and trichloroethene (TCE) (19). Although the subsequent low-IFT displacement effectively recovered both CB-NAPL and TCE-NAPL, a comparatively greater amount of the aqueous 1-butanol solution (∼5 pore volumes) was required for density conversion of TCE. While expected, this result suggests that the DMD process may be limited by alcohol delivery for relatively heavier DNAPLs, such as TCE and tetrachloroethene (PCE). To further optimize the DMD method, preflood approaches must be developed that can support high concentrations of partitioning alcohol (1-butanol) without phase separation, while maintaining favorable alcohol partitioning into the NAPL. Surfactants have been widely used as additives to improve delivery of sparingly soluble compounds in several industries including pharmaceuticals, food manufacturing, and agriculture. Kibbey et al. (17) reported that while micellar surfactant solutions (Winsor type I microemulsion) may increase the mass of 1-butanol delivered during preflood applications, their use may also reduce alcohol partitioning into NAPLs. For example, micellar solubilization of 1-butanol by two surfactant solutions (86 g/L Witconol SN-120 and 115 g/L Tween 80) increased the total aqueous-phase concentration of 1-butanol required to achieve a given 1-butanol concentration in CB-, TCE-, PCE-NAPLs (17). Such behavior limits the effectiveness of Winsor type I microemulsions for the delivery of partitioning alcohols during DMD density conversion prefloods. Consequently, it was hypothesized that 1-butanol could be delivered to the subsurface using a properly designed macroemulsion (17). It is well established that the shear forces present as a fluid mixture passes through a porous medium may cause emulsification, or enhance the stability of existing macroemulsions in low-IFT systems (e.g., refs 20-25). While there remains concern over potential for macroemulsion-induced 10.1021/es0210291 CCC: $25.00
2003 American Chemical Society Published on Web 08/15/2003
TABLE 1. Relevant Properties of Chemicals Used (25 °C)a
a
chemical
molecular weight
solubility in water (g/L)
water solubility in organic (wt %)
liquid density (g/mL)
dynamic viscosity (cP)
1-butanol trichloroethene
74.12 131.39
74.5 1.100
20.5 0.320
0.81 1.47
2.27b 0.53
All data from Riddick and Bunger (39).
b
30 °C.
pore clogging resulting from droplet deposition, flocculation, and coalescence (26-30), a recent study suggests deposition is a complex function of soil and emulsion surface chemistry (31). The size of the macroemulsion droplets relative to pore sizes also plays an integral role in hydraulic conductivity reductions. Smaller emulsified droplets, relative to pore sizes, produce less head loss (26, 32). In fact, several macroemulsions (formed either in situ or injected) have been reported to be transported with only limited reductions in hydraulic conductivity (22, 25, 31, 32). With respect to surfactant-enhanced aquifer remediation (SEAR) applications, a number of surfactant solutions possessing high solubilization capacity for chlorinated solvents have been observed to form emulsions (23, 24, 33, 34). Emulsions are defined as dispersions of one liquid in another incompletely miscible liquid (35). Typically, emulsions are subdivided into microemulsions and macroemulsions, with the chief distinction being that the former are thermodynamically stable (35, 36). Macroemulsion systems are thermodynamically unstable due to their larger surface area per drop, resulting in an excess free energy that cannot be balanced by entropic contributions (37). Many macroemulsions are, however, metastable (i.e., stable for a given period ranging from hours to months to years), lending themselves to industrial and commercial applications. Fountain et al. (23) and Fountain (24) observed that many emulsions formed upon mixing nonionic surfactant solutions and PCE- or TCE-DNAPL were dense and persistent. Fountain et al. (23) recognized that emulsion transport may increase the rate of contaminant mass removal from the subsurface, but were unable to recover the emulsified contaminant under flow conditions typically found in aquifers (23, 24). Okuda et al. (25) reported that a 1% solution of Triton X-100 (octylphenylpolyoxyethylene) formed emulsions when contacted with residual PCE-DNAPL in column studies. More importantly, emulsification of PCE accounted for up to 30% of the contaminant mass recovered from the column. More recently, Gupta and Mohanty (32) observed piston-like displacement of PCE, emulsified by a nonioinic surfactant (Glucopon-425N) in column sand packs. Increases in pressure drop were noted (not quantified) in the presence of the emulsion, but this reportedly aided in stabilizing flow behavior (32). In previous DMD studies, the volume of preflood solution required to achieve in situ density conversion of NAPL was significantly greater in the case of TCE (∼5 pore volumes) than of CB (∼1 pore volume). This result indicates a potential limitation in the applicability of the DMD method for relatively heavier DNAPLs, such as TCE and PCE. The efficiency of the DMD method may be improved if alcohol can be delivered in greater quantities during the preflood. However, attempts to increase 1-butanol delivery in a micellar surfactant solution indicate that the presence of surfactants alters (unfavorably) the equilibrium partitioning behavior of 1-butanol (17). Consequently, the objective of this study was to develop and evaluate a macroemulsion for the delivery of 1-butanol for in situ density conversion of TCE-NAPL. Initial qualitative screening of Tween 80 + Span 80 + 1-butanol mixtures indicated favorable macroemulsion stability (i.e., stability on the order of hours) with a combination of 4.7% (vol) Tween 80 + 1.3% (vol) Span 80. Macroemulsion viscosity
and IFT in the presence of TCE are also important factors to be considered in emulsion development as an increase in viscosity, or decrease in IFT, may lead to premature NAPL mobilization during the density conversion preflood. Thus, density, viscosity, and IFT measurements were used to design a macroemulsion that was metastable and compatible with a low-IFT surfactant formulation used to displace the densityconverted NAPL. The developed emulsion was subsequently tested in two-dimensional aquifer cells to assess the ability of an emulsion delivery (preflood) approach to enhance the overall efficiency of the DMD method.
Materials and Methods Materials. Tween 80 (polyoxyethylene (20) sorbitan monooleate) and Span 80 (sorbitan monooleate) were obtained from ICI Surfactants and used without further purification. Both Tween 80 and Span 80 are food-grade nonionic surfactants, and possess a synergistic emulsifying relationship. An anionic surfactant, Aerosol MA-80I (sodium bis(1,3-dimethylbutyl)sulfosuccinate) was obtained from Cytec Industries with 80% active ingredient and with inactive ingredients comprised mostly of water and 2-propanol. HPLC-grade 1-butanol was obtained from Fisher Scientific. Oil-Red-O, an organic soluble dye, was obtained from Fisher Scientific and used at a concentration of 4 × 10-4 M to color the TCE (HPLC-grade, Aldrich Chemical) red for visualization purposes. Previous work has shown minimal effects of the dye on the physical properties of TCE at low concentration (38). Ergioglaucin A (Fluka Chemicals) was used at a concentration of 3 × 10-5 M to color the aqueous phase blue-green. All aqueous solutions were prepared with water that was purified using a Nanopure analytical deionization (Barnstead/Thermolyne Corp.) system. Relevant properties of the chemicals used in this study are given in Table 1. Ottawa Federal Fine (30-140 mesh) and F-70 (40-270 mesh) sands were obtained from U.S. Silica Co. Federal Fine sand was selected as the background medium because the particle size distribution, intrinsic permeability (∼4 × 10-11 m2), and organic carbon content (50%) was also observed during the macroemulsion preflood in Box 2 (Supporting Information, Figure E). Recent studies have found that in situ macroemulsification of NAPLs is an important removal mechanism, accounting for 3068% of DNAPL removal if conditions are favorable for emulsion transport (25, 31, 49). The large increase in effluent 4250
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FIGURE 5. Representative early (a, ∼700 mL of displacement solution or ∼2110 mL total) and late (b, ∼1080 mL of displacement solution or ∼2490 mL total) time photographs of TCE-NAPL displacement in Box 1. The screened interval of the injection well is shown here by the black line located in the lower left corner of the aquifer cell, generating flow that is predominantly left to right. TCE concentrations after ∼1100 mL was attributed to breakthrough of the milky-white TCE-surfactant macroemulsion. The second increase in TCE effluent concentration at ∼2400 mL corresponded to breakthrough of the low-IFT
TABLE 4. Mass Recoveries of Major Components from Boxes 1 and 2 parameter
Box 1
Box 2
amount of TCE available to flooding (g) amount of TCE recovered as NAPL (g) amount of TCE solubilized/emulsified (g) total amount of TCE recovered (g) recovery of TCE (%) amount of 1-butanol introduced (g) amount of 1-butanol recovered in NAPL (g) amount of 1-butanol recovered in aqueous phase (g) total amount of 1-butanol recovered (g) recovery of 1-butanol (%) amount of Aerosol MA introduced (g) amount of Aerosol MA recovered in NAPL (g) amount of Aerosol MA recovered in aqueous phase (g) recovery of Aerosol MA (%)
37.7 2.3 32.7 35 92.7 240 11.6 185 197 82.0 140 4.2 117 86.6
52.2 4.9 44.0 48.9 93.6 220 6.3 178 184 83.8 not measured not measured not measured not measured
FIGURE 6. Effluent aqueous-phase concentrations from Box 1. Vertical lines represent introduction of flushing solutions into the influent. displacement flood, as indicated by the increase in Aerosol MA concentrations. Concentrations of TCE varied during the displacement process, consistent with nonuniform NAPL migration into the end chamber. The tailing in effluent concentrations observed after flushing with ∼4000 mL was largely the result of viscous instabilities, which led to partial breakthrough of the postdisplacement water flood. Effluent aqueous-phase concentrations of 1-butanol and TCE obtained from Box 2 are consistent with those from Box 1, although surfactant analysis was not conducted for Box 2 effluent samples (Supporting Information, Figure E). Mass recoveries of TCE and 1-butanol from both boxes 1 and 2 are shown in Table 4. Most notably, the 93% recovery of TCE was achieved after flushing with a total of 2.4 pore volumes (macroemulsion and low-IFT solution), indicating the efficiency of the macroemulsion delivery process. Comparable SEAR solubilization aquifer cell studies required up to 8 pore volumes to recover 70% of released PCE-DNAPL (e.g., ref 38). On the basis of Box 1 aqueous and NAPL effluent samples, the Aerosol MA recovery was determined to be 87%. Analysis of effluent NAPL samples revealed significant partitioning of the Aerosol MA into the NAPL, with concentrations ranging from 24 to 125 g/L. The high concentrations of surfactant in the NAPL are indicative of Winsor type II behavior, and likely increased partitioning of water into the NAPL. The total Aerosol MA mass partitioned into the NAPL, however, amounted to only 3% of the Aerosol MA introduced into the aquifer cell, demonstrating the relatively minimal loss of surfactant to the NAPL under these experimental conditions. Effluent densities of both the aqueous and organic (NAPL) phases were measured over the course of the experiment (Figure 7). Horizontal lines appearing in Figure 7 represent
the influent solution densities and indicate the minimum value required for density conversion. Vertical lines represent the time of introduction of each solution into the partially screened, influent well. Breakthrough of the macroemulsion corresponded to the increase in aqueous-phase effluent density at ∼1100 mL. Increased aqueous-phase effluent densities resulted from the large mass (∼60%) of TCE incorporated into the macroemulsion and partitioning of 1-butanol into the TCE-NAPL. In the absence of TCE-NAPL emulsification and 1-butanol partitioning, the aqueous-phase effluent density would have decreased to a value near that of the influent macroemulsion (0.98 g/mL). These results are consistent with previous studies which reported that surfactant flushing solutions may become more dense due to DNAPL solubilization (or emulsification), and consequently migrate downward (e.g., refs 38 and 50). Upon introduction of the low-IFT solution, effluent densities increased, but did not reach the influent value of 1.015 g/mL. This effect was attributed to emulsion breaking and mixing along the displacement front, and solubilization of densitymodified TCE-NAPL. Effluent TCE-NAPL densities ranged from 0.97 to 0.99 g/mL, and all were less than their corresponding aqueous-phase samples. The relatively low TCE-NAPL densities measured in the effluent demonstrate successful in situ density conversion of TCE-NAPL, and support the observed upward NAPL displacement during the low-IFT displacement flood (Figure 5). The subsequent decrease and scatter of the aqueous-phase densities resulted from mixing during the final water flood. Effluent aqueousphase samples from Box 2 substantiate the trends observed in Box 1 (Supporting Information, Figure F). The potential for hydraulic conductivity reduction is an important consideration in flooding with macroemulsions. Consequently, the drop in head across the aquifer cell, as well as aqueous- and non-aqueous-phase viscosities, is plotted as a function of effluent volume in Figure 8. The head loss increased throughout the macroemulsion flood, reaching a maximum of 7.4 cm, which corresponds to a gradient in head of ∼12%. Over the course of Box 1, the flow rate was maintained at ∼3.5 mL/min until the remaining mobilized TCE-NAPL was recovered during water flooding, at which time (∼3500 mL) the flow rate was increased to ∼12 mL/min to expedite recovery of the resident surfactant solution. Given the relatively constant flow rate during the preflood and displacement flood, the increased head loss during these floods (i.e., until ∼2800 mL) was attributed in part to increased aqueous-phase viscosity (Figure 8). The viscosity of the organic liquid phase is also an important parameter in low-IFT displacement, as both the aqueous and organic phases must flow through an unconfined domain. Results indicate these NAPL samples had higher viscosities than those reported in previous DMD studies (∼8VOL. 37, NO. 18, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Effluent densities and corresponding aqueous-phase concentrations of TCE and 1-butanol from Box 1. Horizontal lines represent influent solution densities. Vertical lines represent introduction of flushing solutions into the influent. duction of high-solubilization-capacity flushing solutions, a macroemulsion density-conditioning preflood would reduce the risk of downward displacement, and more readily achieve neutral buoyancy(50). The large amount of TCE-NAPL mass recovered during the macroemulsion preflood (∼60%) coupled with efficient alcohol delivery and mass transfer indicates that use of macroemulsion flooding strategies with the DMD method holds great promise for the remediation of DNAPL source zones, allowing for horizontal flushing schemes without the need for physical confinement.
Acknowledgments
FIGURE 8. Effluent viscosities and corresponding head loss across the aquifer cell from Box 1. Vertical lines represent introduction of flushing solutions into the influent. 11 cP versus ∼2-4 cP) (18, 19). While the NAPL viscosities measured in Box 1 were comparatively high, NAPL viscosities as high as 15.6 cP have been reported during the displacement of complex, multicomponent DNAPLs from a medium of similar intrinsic permeability (2 × 10-7 cm2) (13). Finally, it is important to note that the viscous instabilities observed during the NAPL displacement flood could be reduced with further optimization of the low-IFT solution (13, 14, 19). In summary, use of the 1-butanol macroemulsion reduced the volume of preflood solution required for in situ TCENAPL density modification in aquifer cell experiments from ∼5 pore volumes with a 6% (wt) 1-butanol aqueous solution (19) to ∼1.2 pore volumes. Although the primary focus of this study was to evaluate the efficiency of a macroemulsion for delivery of a partitioning alcohol to the entrapped and pooled NAPL, the displacement solution employed herein represents one of several possible approaches. For example, displacement following a macroemulsion preflood could be realized using alcohol flooding or other low-IFT surfactant formulations. Furthermore, when used prior to the intro4252
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We thank ICI Surfactants (Tween 80 and Span 80) and Cytec Industries (Aerosol MA-80I) for supplying the surfactants used in this work. Additional appreciation is expressed to Dr. Gary Pope (University of Texas at Austin) and Dr. Varadarajan Dwarakanath (Intera Engineering) for assistance with the initial selection of low-IFT surfactant formulations. We also thank Kacy Cullen and Jonafel Crowe (Georgia Institute of Technology) for their assistance with the image analysis and Coulter counter systems, respectively. Funding for the research was provided by the Great Lakes and Mid-Atlantic Hazardous Substance Research Center (GLMA-HSRC) under Grant No. R-819605-01 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 paper does not necessarily represent the views of either agency and has not been subject to agency review.
Supporting Information Available Figures show droplet size distributions of the influent emulsion, surfactant partitioning as a function of salt content, and representative photographs, effluent concentrations, and effluent densities from Box 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review November 18, 2002. Revised manuscript received April 28, 2003. Accepted June 13, 2003. ES0210291
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