Physicochemical Transport Processes Affecting the Removal of

Environ. Sci. Technol. 1996, 30, 1852-1860

Physicochemical Transport Processes Affecting the Removal of Residual DNAPL by Nonionic Surfactant Solutions ITARU OKUDA, JOHN F. MCBRIDE, SIMON N. GLEYZER, AND CASS T. MILLER* Department of Environmental Sciences and Engineering, The University of North Carolina, CB 7400, 106 Rosenau Hall, Chapel Hill, North Carolina 27599-7400

Aquifers contaminated with dense nonaqueous phase liquids (DNAPLs) are extremely difficult to remediate with standard pump-and-treat methods. Enhanced remediation methods, such as flushing with cosolvent or surfactant solutions, promise to reduce remediation times but result in complex physicochemical systems for which we still lack both fundamental understanding and reliable process-based models. The overall objective of this work was to observe and quantify various physicochemical transport processes acting during the removal of a typical residual DNAPL (tetrachloroethylene, PCE) by solutions containing a nonionic surfactant (Triton X-100). To achieve this goal, we measured the phase behavior of the water/PCE/Triton X-100 system in batch systems and performed a set of glass bead column experiments to investigate residual PCE removal mechanisms, nonreactive tracer transport, and Triton X-100 transport. We observed and quantified removal of residual PCE by a number of processes (dissolution, micellar emulsions/ microemulsion transport, macroemulsion transport, and DNAPL mobilization) as a function of the surfactant concentration used to elute the PCE from the porous medium. We concluded that macroemulsion transport was an important process, accounting for up to 30% of total PCE removalsa process which, to date, has not been accounted for in mathematical models of surfactant-enhanced remediation. We also observed that viscous fingering developed during elution of surfactant in the idealized one-dimensional column system, suggesting that this phenomenon will also affect the efficiency of surfactant recovery in field-scale applications.

Introduction The difficulty in remediating contaminated aquifers with standard pump-and-treat methods is due, in part, to the * Corresponding author telephone: (919) 966-2643; fax: (919) 9662583; e-mail address: casey [email protected].

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 6, 1996

slow dissolution rates of residual and pooled dense nonaqueous phase liquids (DNAPLs) into water (1). To overcome this problem, technologies such as cosolvent and surfactant flushing have been proposed (2-9); these methods, applied in enhanced oil recovery, use chemicallyactive additives. The success of these enhanced-remediation methods generally relies upon two DNAPL removal mechanisms: solubilization and mobilization. In water/NAPL/surfactant systems, solubilization is defined operationally as the mass transfer of a dissolved or colloidal (micellar emulsions and/or microemulsions) NAPL component from the immobile, residual NAPL to the mobile, aqueous phase (9-12). The processes of dissolution, micellar emulsification, and/or microemulsification are thermodynamically stable (13, 14). Mobilization, which has received only limited attention in the environmental literature (6, 8), occurs when viscous and/or gravitational forces acting on the NAPL exceed the capillary forces responsible for immobilizing it. Mobilization of residual NAPL occurs until a new mechanical equilibrium, governed by the Young-Laplace equation, is reached (e.g., ref 15). To produce significant mobilization, the interfacial tension has to be reduced to near 10-3 mN/m (6, 15). Such ultra-low interfacial tension can be achieved when the water/NAPL/surfactant system forms a Winsor Type III system (6, 16-18)sa special condition in which hydrophobic NAPL and the aqueous phase are delicately balanced by the surfactant and form what is termed a middle phase. Although this approach has been studied in tertiary oil recovery (15, 19, 20), environmental investigations are few and many questions remain (8, 17, 18). NAPL removal by solubilization and mobilization have been the main focus of past investigations of surfactant flushing, and these two NAPL removal processes are often modeled as the only NAPL removal processes occurring during surfactant flushing (e.g., refs 21 22). However, because water/NAPL/surfactant systems often exhibit complex phase behavior, mechanisms of NAPL removal cannot simply be attributed to solubilization and mobilization. A third mechanism pertinent to surfactant-enhanced removal of NAPL is the formation and transport of NAPL in water macroemulsionsssmall droplets of NAPL, suspended in the aqueous phase, that do not coalesce quickly. Table 1 summarizes both the operational definitions used in this paper and the characteristics of various forms of NAPL in a water/NAPL/surfactant system. The formation and transport of macroemulsions are important because (i) surfactants that are good solubilizers tend to form macroemulsions (5); (ii) macroemulsions may affect the aqueous-phase permeability of porous media (23-25); (iii) macroemulsions may be mobile, so controlling their movement during surfactant flushing in the subsurface is essential; and (iv) solubilization kinetics may be affected by macroemulsification. The importance of macroemulsions, however, has not been considered explicitly in the past (4, 7, 8), and their transport in porous media has received little attention (25). In response to this lack of quantitative research on the importance of the multiple physicochemical processes affecting surfactant-enhanced removal of residual DNAPL, the objectives of this work were (i) to develop a methodology

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TABLE 1

Definitions Used and Characteristics of Various Forms of NAPL and NAPL Component in a Water NAPL Surfactant Systema experimental fractionation

characteristic thermodynamic dimension stability

form

solubilized NAPL

dissolved microemulsion

macroemulsified NAPL macroemulsion

mobilized NAPL

a

molecular 1 µm

coalescable droplets

characteristics

stable stable

dissolved (monomeric) NAPL in aqueous phase any form of emulsion that is transparent and easily transported with bulk aqueous phase due to its small size (1 µm) whose mesostable transport behavior can be significantly different from bulk fluid separate-phase NAPL that is released when capillary force acting on residual NAPL is overcome by viscous and/or gravitational forces

The distinction among various forms of NAPL was made in relation to their transport characteristics.

TABLE 2

Experimental Parameters Chemicals chemical formula mol wt (g/mol)

chemical name PCE TX-100

tetrachloroethylne octylphenyl polyoxyethylene (9.5)

C2Cl4 C34H62O12

density (g/cm3 at 20 °C)

viscosity (mPa s-1 at 20 °C)

1.62a 1.08 at 10%

0.89a 1.06 at 1%b 2.19 at 10%b

166 660 (av)

Glass Bead Columnc porous medium exp no.

exp

1 and 2 residual PCE removal with 1% and 10% TX-100 3-8

surfactant transport

length (cm)

particle size (mm)

porosity

capillary barrier length (cm)

particle size (mm)

dispersion coeff (cm2/h at Darcy velocity)

0.5 0.125 < dp < 0.210 1.8 at 6.1 cm/h 0.177 < dp < 0.425 0.367 dp10/dp50/dp90 ) (CV ) 2%) (without PCE) 0.193/0.257/0.292 not used not used 1.7 at 9.2 cm/h 23.5 0.177 < dp < 0.425 0.356 dp10/dp50/dp90 ) (without PCE) 0.193/0.257/0.292

20.1

a From Table 3.4 of Cohen and Mercer (33). b Measured with falling ball viscometer (Haake Instruments, Paramus, NJ). c Glass column: 2.50 cm internal diameter, adjustable length (max 30 cm).

to investigate the multiple operative transport processes affecting DNAPL removal in well-characterized experimental systems by surfactant solutions known to yield stable macroemulsions; (ii) to evaluate the relative importance of these transport processes as a function of surfactant solution concentration; and (iii) to examine the results of these studies in light of current surfactant-enhanced remediation models.

Materials and Methods Materials. After preliminary phase behavior studies of 10 surfactants, we selected Triton X-100 (TX-100, ICN Biomedical, Costa Mesa, CA) for the following reasons: (i) TX100 showed promising solubilization and phase-behavior characteristics; (ii) it is one of the most well-studied nonionic surfactants (13, 26, 27); (iii) the concentration of TX-100 can be determined by conventional analytical methods; and (iv) sorption of TX-100 was found to be minimal in the model systems employed in this study. TX-100 was used as received (100% pure as a mixture of isomers) without further purification. Reagent-grade tetrachloroethylene (PCE, Aldrich Chemical, Milwaukee, WI) was used as a representative DNAPL without further purification. A sample of 0.01 N NaCl (Fisher Scientific, Fair Lawn, NJ) was used as a nonreactive chloride tracer, and deionized and

distilled water was used as the aqueous phase. The porous media were built from glass beads (McMaster-Carr Supply, Atlanta, GA) to provide a well-characterized, inert solid phase. The glass beads were packed into a 2.5 cm diameter glass column. Selected material properties are presented in Table 2. Analytical Methods. TX-100 and PCE concentrations were quantified by using a high-pressure liquid chromatography instrument (HPLC, Model 510, Waters, Milford, MA), equipped with a reverse-phase column (C-18, Beckman, Schaumburg, IL) and an ultraviolet (UV) detector (220 nm, Model 57, Applied Biosys, Foster City, CA) with isocratic operation (0.5 mL/min) of mobile phase (75/25 vol % 2-propanol/aqueous solution). For the independent surfactant transport experiments, the experimental system did not contain PCE, and an evaluation confirmed that chromotographic separation of TX-100 isomers did not occur in the glass bead column. Consequently, a UV-visible spectrophotometer (Model U-2000, Hitachi, Danbury, CT) was used to determine the surfactant concentration. The nonreactive tracer (0.01 N NaCl) concentration was analyzed with an in-line electrical conductivity meter (Cole-Parmer, Niles, IL). All quantitative analyses using the HPLC, the spectrophotometer, and the electrical conductivity meter were performed within the linear response range of each

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instrument with r2 > 0.999, and the coefficient of variation (CV) of each measurement was approximately 2%. An X-ray attenuation instrument was used to perform the nondestructive measurement of the porous medium’s solid volume fraction, the PCE saturation in the porous medium filled with PCE and water, and the total PCE concentration in the porous medium filled with PCE and aqueous surfactant solution (28). The X-ray instrument consisted of a tungsten-target X-ray tube and a germanium detector, aligned and mounted on a computer-controlled scanning system. The attenuation of a narrow energy band of X-rays (30.0-36.0 keV) was used to calculate the various phase fractions. The collimated radiation beam traversing the porous-medium was 5 mm by 10 mm (width × height) in cross section. At each vertical sampling location, the beam covered 50% of the horizontal column cross section. The random errors associated with these measurements were as follows: estimated standard error was 0.00027 for porosity, 0.0022 for PCE saturation, and 3.4 g/L for total PCE concentration; thus, the resolution of the total PCE concentration measurement was 10.2 g/L. An estimated upper bound for bias in these measurements was 0.0017 for porosity, 0.0013 for PCE saturation, and 2.0 g/L for total PCE concentration. Because 10% TX-100 and 1% TX-100 solutions had a linear attenuation coefficient only 3% less than that of water, they were not detectable by the X-ray instrument (PCE had a linear attenuation coefficient 617% greater than that of water). Phase Behavior in Batch Experiments. To determine the phase behavior of the ternary system, a series of batchcontacting experiments was conducted by mixing selected amounts of water, PCE , and TX-100 in 25-mL glass graduated cylinders to obtain a total volume of 10-15 mL. Each batch system was shaken by hand for about 10 min and then allowed to equilibrate at 24 ( 1 °C. After a 3-day equilibration period, a visual observation of the phase behavior in each cylinder was recorded, and duplicate HPLC analyses were conducted for PCE and TX-100 in several aqueous-phase samples. A 3-day equilibration period was selected because the longest duration column experiment required at least 3 days to collect and process effluent samples for subsequent chemical analysis. The water/PCE/TX-100 (WPT) system formed PCE in water macroemulsions, which are small droplets of PCE suspended in water. The direct analysis of macroemulsionphase composition was attempted, but a consistent sampling of this phase proved difficult. For this reason, the composition of the macroemulsion phase was estimated from (i) the apparent volume of settled macroemulsions, (ii) HPLC analysis of aqueous-phase composition, and (iii) total composition of the WPT system in each cylinder, assuming 26 vol % occupancy of the aqueous phase within the settled macroemulsions; 26% is a lower limit for the porosity of packed uniform spheres (14). Although dictated by the sampling logistics of the column experiments, the 3-day equilibration period ensured that the separate macroemulsion phase was relatively stable and thus composed of macroemulsions likely to be prevalent in a field-scale surfactant flushing operation. As pointed out by Sharma and Shaw (20), the clear separation of emulsions associated with the bulk aqueous phase is somewhat subjective because emulsions are often polydispersed in size. At the end of the 3-day equilibration period, the relatively stable macroemulsions consisted of droplets of a size class prone to sedimentation and filtering

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FIGURE 1. Schematic of experiment setup used for surfactant flushing experiments.

in porous media; these processes could prevent the macroemulsions from being transported with the bulk aqueous phase. Surfactant Transport Experiments. To study dispersion and sorption/desorption of TX-100, a series of independent TX-100 and chloride tracer (0.01 N NaCl) transport experiments was conducted at 0.028, 0.1, 1, 5, and 10% surfactant concentrations in the absence of residual PCE. The column was a 2.5 cm diameter, 30 cm long, adjustable packedlength glass column (ACE Glass, Vineland, NJ), packed with 23.5 cm of the 0.257 mm median diameter glass beads (Table 2; also see Figure 1 for a similar column system). The column was packed as uniformly as possible by depositing the air-dried glass beads at a steady rate of flow through a delivery tube, equipped with a stainless mesh on the bottom end, without stopping the flow or agitating the glass beads during the packing. The porous medium was supported on the bottom by a nylon filter and stainless screen and confined at the top by a glass-frit plunger (ACE Glass, Vineland, NJ). Residual PCE Removal Experiments. A series of column experiments was performed to investigate the dynamics of surfactant-enhanced removal of residual PCE in a wellcharacterized porous media system by using the setup depicted in Figure 1. The column used for residual PCE removal experiments was essentially the same as the one used for the TX-100 and nonreactive tracer transport studies, except that 0.5 cm of the 0.168 mm median diameter glass beads were placed first in the empty inverted column, followed by 20.1 cm of the 0.257 mm median diameter glass beads (Table 2). The small layer of fine glass beads served as a capillary barrier to create a sharp residual PCE saturation boundary near the top of the column and to prevent the formation of a continuous PCE ring at the boundary between the glass beads and the top plunger. After packing the column, degassed, deionized, and distilled water was pumped from the bottom until the column was saturated with water. Next, the porosity of the porous medium was measured along the entire length of the column at 5-mm increments by using the X-ray instrument. Then PCE was introduced with the syringe pump from the bottom of the column at a high flow rate (Darcy velocity ) 122 cm/h) until the X-ray instrument detected PCE just below the 0.5 cm long capillary barrier. The column was then flushed with approximately 8 pore vol of water at a Darcy velocity of approximately 60 cm/h to create the residual PCE saturation. Finally, a sharp residual saturation boundary at the top of the column and a uniform distribution of residual PCE throughout the

TABLE 3

Results of Column Experiments exp

TX-100 concn (%)

1

1

0.155 (CV ) 4%)

6.1

2

10

0.165 (CV ) 6%)

5.7

none

8.6-9.2

3-8

0.028-10

residual saturation

Darcy velocity (cm/h)

PCE recovery in effluent

TX-100 recovery (%)

total 103% aq 80%*; 59%** macro 20%*; 41%** mobil nd total 103% aq 58%*; 85%** macro 35%*; 9%** mobil 6% not present

X-ray measurement

flow interruption

108

∆z ) 1 cm ∆t ) 0.16 h

59 h at 15 pore vol

100

15 s per location ∆z ) 4 cm ∆t ) 0.04 h

20 h at 4 pore vol

99-106

15 s per location none

none

Abbreviations: CV, coefficient of variation (%) ) (standard deviation/mean) × 100; *, estimate from HPLC analysis of effluent supernatant; **, estimate from batch solubility data; aq, removal by PCE solubilization; macro, removal by macroemulsion transport; mobil, removal by mobilization; nd, not detected. a

column were verified by using the X-ray instrument. These procedures and the 2.5 cm diameter column were used to minimize potential development of two- and threedimensional phenomena in the idealized one-dimensional column system, such as dissolution fingering (29, 30) and viscous fingering. Dispersion characteristics of the liquid-saturated glass bead columns were quantified by performing 0.01 N NaCl tracer experiments with and without residual PCE present. The effluent concentration of NaCl was monitored in real time with the electrical conductivity meter, and the results were used to determine the dispersion coefficient in the porous medium. Residual PCE removal was then investigated by pumping a degassed aqueous solution, containing either 10 or 100 g/L (1 or 10%) of TX-100 in deionized and distilled water, in a downflow direction with a peristaltic pump. Total PCE concentration measurements during this flushing experiment were made by using the X-ray instrument at multiple locations along the column and as a function of time, noted in Table 3. Effluent samples were collected in 2- 40-mL increments, and effluent concentrations of TX-100 and PCE were measured after a 3-day equilibration period. Last, surfactant flushing was halted, the column system was allowed to equilibrate, and flushing continued to determine whether or not complete PCE removal had been achieved. Degassed, deionized, and distilled water was used to elute TX-100 after the flow interruption (immediately after the flow interruption during the 10% TX-100 experiment, and 0.4 pore vol after the flow interruption during the 1% TX100 experiment). Flow rates, velocities, sampling locations, and flow interruption details are summarized for each experiment in Table 3.

Results and Discussions Phase Behavior. Figure 2 shows a ternary phase diagram of the WPT system. At low surfactant concentration, the system consisted of a white PCE in water macroemulsion phase and a transparent aqueous phase. Within 2 h after mixing the components, the two phases separated into two distinct volumes in the graduated cylinder, and these volumes changed little during the 3-day equilibration period. Macroemulsions are formed by kinetic energy exerted by the surrounding fluid. Their stability against flocculation/coalescence processes is affected by van der Waals attraction, by repulsive forces arising from interaction of electrical double layers, by steric repulsions, and by other forces acting on droplets (13, 14). In general, macroemul-

FIGURE 2. Aqueous corner of triangular, ternary phase diagram. Open markers represent the number of separate phases visually observed after 3-day equilibration. HPLC data points were obtained from the HPLC analysis of aqueous phase. Complex refers to phase behavior of a highly viscous system that was difficult to interpret.

sions are considered thermodynamically unstable, because they eventually separate into two phases (13, 14). However, unlike a suspension of PCE droplets in water created by mechanical agitation in the absence of surfactant, the observed macroemulsions of the WPT system formed nearly spontaneously when the mixture was made and remained stable during 1 month of equilibration. Therefore, the macroemulsion phase of the WPT system was considered to exist in pseudo-equilibrium with the aqueous phase. At higher surfactant concentrations, the macroemulsion phase disappeared as the system became miscible. The phase composition at this immiscible-miscible phase transition, which is shown as part of a binodal line intersecting solid triangles in Figure 2, was determined by titration with TX-100 to miscibility and by HPLC analysis of the aqueous phase. A mass balance calculation indicated that the macroemulsion phase consisted predominantly of PCE, ensuring the nature of the macroemulsion phase as suspended droplets (>1 µm) of PCE. A large one-phase (miscible) region of the WPT system is shown in Figure 2. In this region, the solution was transparent to bluish in color, which suggests that the emulsified droplets were 20 wt % PCE and surfactant concentrations, the system became very viscous, and interpretation of the observed phase behavior was difficult. The molar solubility ratio (MSR)sthe moles of PCE per mole of TX100 in the linear portion of the apparent solubility curveswas 6.7 ( 0.2 within the TX-100 concentration range


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FIGURE 3. (a) TX-100 breakthrough and (b) elution curves in the absence of PCE.

of 0.5-12%. This MSR value is greater than those of other water/PCE/surfactant systems (20): 0.39 for sodium dodecyl sulfate; 0.45 for T-MAZ 28; 2.27 for T-MAZ 20; and 3.15 for T-MAZ 60. TX-100 may yield small MSR values in other water/NAPL/TX-100 systems [e.g., 0.04 for pyrene, 0.11 for phenanthrene, and 0.34 for naphthalene (12)]. Surfactant Transport in the Absence of Residual PCE. Figure 3a,b shows the breakthrough and elution curves, respectively, of the surfactant and tracer in the absence of residual PCE. Comparison of surfactant and tracer breakthrough curves shown in Figure 3a demonstrated that no strong deviation of TX-100 transport characteristics from that of the tracer was evident at g1% TX-100. In addition, a UV wavelength scan of effluent samples at different stages of TX-100 breakthrough did not reveal a shift in the UV absorbance spectrum, and thus chromatographic separation of TX-100 isomers was unlikely. These results indicated that the sorption of surfactant by glass beads was not significant at high surfactant concentrations (g1%). Independent batch sorption experiments also confirmed this result. For these reasons, the polydispersed characteristics of TX-100 were not considered in this work. In natural systems and at low surfactant concentrations, the potential for sorption and chromatographic separation should not be neglected (12, 26, 31). The elution of TX-100 (Figure 3b) was generally symmetrical to the breakthrough of TX-100. At 10% TX-100, however, the elution curve of the surfactant exhibited early elution and subsequent tailing; the 5% TX-100 elution curve (not shown in Figure 3b) was similar. We hypothesized

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that this result was caused by viscous fingering, because the aqueous phase was displacing a more viscous surfactant solution during elution (see Table 2 for viscosity data). To test this hypothesis, the elution of 10% TX-100/nonreactive tracer mixtures was studied. When mixed with surfactant, the elution of the tracer (plus sign within open circle in Figure 3b) was identical to the elution of the surfactant (open circle in Figure 3b). These results indicated the presence of a viscosity effect in the idealized onedimensional column. The effect of viscosity is expected to be more pronounced in greater spatial dimensions, and it is currently being investigated by using a two-dimensional flow cell. Because the complete recovery of injected surfactant from porous media is desired after the removal of contaminant, this issue is also important in the design of an enhanced-remediation strategy using surfactants. Residual PCE Removal Experiments. Surfactant flushing experiments were conducted with 1% and 10% TX-100 solutions. These concentrations were selected, as suggested by Martel et al. (10), based on the phase behavior shown in Figure 2. At 1% TX-100, the phase composition in the pore space during PCE removal was expected to be in the two-phase zone during most of the displacement process. On the other hand, the phase composition in the pore space was expected to be near-miscibility at 10% TX-100. Therefore, residual PCE removal at 10% TX-100 should have been substantially different than that at 1% TX-100. In addition to facilitating a rapid cleanup (i.e., within a few pore vol), we expected that flushing with 10% TX-100 would partially suppress the formation of macroemulsions (9, 10). Because of the small number of macroemulsion transport studies in porous media to date, the extent of possible macroemulsion transport was difficult to assess a priori; based on the earlier study by Fountain et al. (5), the transport of any WPT macroemulsions should be limited. If the removal process is restricted to equilibrium dissolution in the aqueous phase, then the estimated number of pore vol required to dissolve the PCE was 1.5 pore vol at 10% (solubility: 150 g/L) and 23 pore vol at 1% (solubility: 10 g/L). An additional pore vol of liquid must be added to these estimates because the PCE-containing solution also has to be displaced from the column. Other factors such as dissolution kinetics, mobilization, transport of macroemulsions, dispersion, dissolution fingering, and viscous fingering will influence these estimates of PCE removal efficiency. The X-ray data showed that the column, used for both experiments, was uniformly packed with an average porosity of 0.367 and a CV of 2%. The average residual PCE saturation was 0.155 (CV ) 4%) at the start of the 1% TX100 flushing experiment and 0.165 (CV ) 6%) at the start of the 10% TX-100 flushing experiment. The tracer breakthrough curve in the clean column (in the absence of residual PCE) had a dispersion coefficient of 1.80 cm2/h; this value was obtained by fitting the data to the advectivedispersive equation (Program CXTFIT; 32). The tracer breakthrough curve in the same column at residual PCE saturation had a statistically significant (5% level), smaller dispersion coefficient of 1.25 cm2/h. This smaller dispersion coefficient represented a 67% increase in the Peclet number [(pore water velocity)(column length)/(dispersion coefficient)]; a 56.7% decrease in Peclet number was observed in a 4.8 cm inside diameter, 6.5 cm long column of 0.5585-mm sand at 15.8% residual dodecane (7). A larger Peclet number indicates that the residual PCE created a narrower

FIGURE 5. Change in total PCE concentration at several locations within column during 1% TX-100 flushing experiment as determined with X-ray instrument.

FIGURE 4. Effluent concentration during 1% TX-100 flushing, (a) PCE and (b) TX-100.

water-filled pore-size distribution by occupying the larger pores in the glass-bead medium. PCE Removal by Using 1% TX-100. The PCE effluent breakthrough and elution curves during 1% surfactant flushing are given in Figure 4a. A noticeable change in PCE concentration occurred at about 0.8 pore vol, which was in good agreement with the breakthrough of TX-100. The amount of mobilized PCE (separate-phase PCE) was negligible. The formation of macroemulsions was visually observed as a white band in the column, and the effluent was turbid from the breakthrough of PCE at 0.8 pore vol, up to 10 pore vol. The total PCE concentration in the effluent increased steadily and reached approximately 23 g/L, which was above the batch aqueous-phase solubility (10 g/L), and thus reflected the existence of macroemulsions. The aqueous-phase PCE concentration also exceeded the batch solubility value. The total PCE concentration started to decrease at 6 pore vol, and >95% of the PCE initially present in the column was removed within 15 pore vol. To examine the relative contribution of macroemulsion transport to total PCE removal, the separation of macroemulsions in the effluent samples was attempted by sedimentation during the 3-day equilibration period. The supernatant, however, was somewhat cloudysnot only after the equilibration period but even after a 30-min centrifugation at 200 G, suggesting the existence of suspended

macroemulsions in the aqueous phase. This cloudy supernatant was not observed in any batch systems over a wide range of surfactant concentrations. Figure 4b shows the effluent breakthrough and elution curves of TX-100 during 1% TX-100 flushing. The breakthrough of surfactant occurred at about 0.8 pore vol. The concentration increased rapidly, reaching that of the injected solution. Essentially all TX-100 existed in the aqueous phase, and the breakthrough curve was consistent with the result of the independent 1% TX-100 transport experiment. This similar transport behavior suggested that both the partitioning of TX-100 into or onto the PCE phase and the sorption of TX-100 onto the glass beads were small. Figure 5 shows the change in total PCE concentration at three locations in the column during 1% TX-100 flushing. Because the X-ray instrument measures the total PCE concentration in the pore space, the concentration includes residual PCE (trapped separate-phase PCE), mobilized PCE (mobile separate-phase PCE), solubilized PCE in the aqueous phase, and PCE macroemulsions. The x-axis is normalized to the pore vol above the measurement location. The data show evidence of mass-transfer limitation and reflect the influence of column length on observed data when mass-transfer limitation exists. In addition, the data indicate that the mass-transfer processes responsible for the elution of PCE approached equilibrium by the time the elution front reached the end of the 20.6 cm long column, because the elution fronts converged at locations furthest from the inlet. A gradual removal of PCE from the column was observed, which was consistent with the effluent PCE elution curve. PCE Removal by Using 10% TX-100. Figure 6a shows the PCE breakthrough and elution curves during 10% TX100 flushing. At 0.81 pore vol, coincident with TX-100 breakthrough, a pure phase of PCE was observed as drops in the effluent sample. This spike of mobilized pure-phase PCE accounted for approximately 6% of the initial residual PCE. After 0.86 pore vol, the effluent became turbid, indicating the existence of macroemulsions. The total concentration leveled off at 180 g/L, which was greater than the batch aqueous-phase solubility (150 g/L). The aqueousphase PCE concentration, however, never attained the batch solubility value. The presence of macroemulsions continued until 2.2 pore vol. The removal of residual PCE was essentially complete at 2.5 pore vol, although the flow


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FIGURE 7. Change in total PCE concentration at several locations within column during 10% TX-100 flushing experiment as determined with X-ray instrument.

FIGURE 6. Effluent concentration during 10% TX-100 flushing, (a) PCE and (b) TX-100.

interruption at 4.0 pore vol indicated that a small amount of PCE still remained in the column. Approximately half of the PCE concentration during elution was associated with the aqueous-phase; the rest was associated with the macroemulsion phase and mobilized separate-phase PCE. Because the total mass recovery of residual PCE (cumulative mass of PCE in effluent) was satisfactory (103%), the systematic discrepancy between the effluent aqueous phase PCE concentration and the batch solubility was attributed to a sampling error rather than a systematic analytical error. The surfactant breakthrough and elution curves during 10% TX-100 flushing are shown in Figure 6b. The breakthrough of surfactant occurred at 0.86 pore vol, after which the concentration of surfactant steadily increased to the concentration of the injected solution (10%) and leveled off. There was a decrease in total surfactant concentration when the effluent became a single phase (at 2.0 pore vol); this decrease was presumably an experimental artifact. Almost all surfactant in the system was in the aqueous phase and not in the macroemulsion phase. The temporal distribution of PCE in the column, assessed with the X-ray instrument, changed in three distinguishable stages (Figure 7). In the first stage, the PCE concentration at a given location remained at its residual saturation value. As the surfactant front passed an X-ray measurement location, the concentration increased (second stage). This increase indicated the formation and transport of a PCE

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bank originating upgradient, whose total concentration continued to increase as a function of distance from the inlet and whose maximum total concentration was significantly greater than the concentration corresponding to the residual saturation. The bank was visually observable as a white band of PCE macroemulsions. As the bank passed and the elution front reached the measurement location, the PCE concentration dropped sharply, leaving almost no PCE behind (third stage). When the data were plotted as in Figure 5, convergence of the elution fronts occurred for locations furthest from the inlet. There was a small decrease in PCE concentration at the front of the PCE bank; the decrease was most pronounced at 17.75 cm. This small inflection was attributed to the mobilization of PCE at the measurement location. The other interesting feature of the temporal distribution of PCE was a sharp PCE concentration peak just before the concentration rapidly dropped (e.g., at 9.75 cm). Because the TX-100 concentration was approaching 10% near the end of the PCE bank, this sharp peak can be attributed to a phase transition from two-phase (macroemulsion) behavior to one-phase (miscible) behavior. Comparison of 1% and 10% TX-100 Flushing Experiments. Based on the effluent PCE concentration data, 15 pore vol of 1% TX-100 removed >95% of the initial residual PCE. Similarly, approximately 4 pore vol of 10% TX-100 solution removed >95% of the initial residual PCE, although >85% of the initial residual PCE was removed within 2.5 pore vol. These results demonstrate the efficient removal of residual PCE by TX-100. To achieve the same level of cleanup by other methods, approximately 103 pore vol of water, 30 pore vol of 60% methanol solution (30), 20 pore vol of 4% polyoxyethylene (20) sorbitan monooleate (8), or 1.5 pore vol of a 4% sodium dioctyl and diamyl sulfosuccinates mixture (8) are required; these values are reported for one-dimensional column systems similar to those used in our experiments. Both 1% and 10% TX-100 flushing experiments demonstrated a significant removal of residual PCE as macroemulsions (Table 3). Due to the presence of macroemulsions in the effluent, the total PCE concentrations exceeded the solubility limit (10 g/L for 1% TX-100 and 150 g/L at 10% TX-100) in both experiments. PCE removal during 1% TX-100 flushing was thus faster than predicted from batch equilibrium solubility alone. From the batch solubility limit of 10 g/L, one would predict the removal of

the initial residual PCE mass in approximately 24 pore vol rather than the actual experimental result of 15 pore vol. The relative contributions of solubilization, macroemulsion transport, and mobilization to the total PCE removal were estimated with two different methods, because the transport behavior of macroemulsions appeared to be affected by their characteristics (Table 3). In the first method, the PCE breakthrough and elution curves (Figures 4a and 6a) were integrated for total PCE, mobilized PCE, and PCE in supernatant after 3-day equilibration. In addition to the solubilized PCE, the supernatant samples contained macroemulsified PCE that remained suspended in the bulk aqueous phase after the 3-day equilibration period; this characteristic of the supernatant was evident during 1% TX-100 flushing. The contribution from macroemulsion transport was calculated from the difference between total PCE and the sum of mobilized PCE and PCE in the supernatant. The estimate of supernatant PCE is believed to differentiate the fraction of macroemulsified PCE that can be readily transported within the bulk aqueous phase from the fraction of macroemulsified PCE whose transport can be strongly affected by gravity, i.e., downward migration of macroemulsions and mobilized PCE. In the second method, the relative contribution of solubilized PCE to total PCE removal was calculated from the batch solubility as a function of the measured TX-100 concentrations (Table 3). The reason for the discrepancy between the observed solubilized PCE concentrations during the PCE removal experiments and the equilibrium PCE solubility from the batch experiments is not clear. But because the droplet-size distribution of macroemulsions is affected by kinetic energy during formation, by surfactant concentration, and by filtering in porous media (23-25), it is possible that this and other characteristics of macroemulsions formed and transported in porous media are also a function of porous media properties. Based on the results of the two methods used to assess the relative contribution of the different processes to the removal of residual PCE, we concluded that the transport of macroemulsions was significant in our experiments; as much as 30% of total PCE removal was accounted for by macroemulsion transport, while solubilization was still the dominant mechanism of PCE removal. Because macroemulsion transport was significant, the development of reliable constitutive models that predict the behavior of macroemulsions in porous media is strongly recommended. Development of such models may be difficult because the behavior of macroemulsions in porous media is very complexsmainly because macroemulsions are subject to filtration and thus would have transport characteristics similar to that of solid particles in porous media (23). Some of the important parameters, as reviewed by Kokal et al. (25), include (i) emulsion stability, (ii) concentration of emulsion and rheological properties, (iii) emulsion dropletsize distribution, (iv) pore size distribution of porous media, (v) wettability of the surfaces, and (vi) flow velocity. Existing models have been reviewed by Kokal et al. (25). Martel et al. (10) proposed a miscible-displacement approach to NAPL removal, in which a high-concentration surfactant solution is used to completely dissolve residual NAPL, thereby avoiding the reduction of permeability caused by macroemulsions. Based on the batch behavior of the WPT system, there was reason to believe that 10% TX-100 flushing could result in more efficient removal of PCE than 1% TX-100 flushing (estimated 2.5 versus 24 pore

vol, respectively). In the column experiments, 10% TX-100 flushing was more efficient than 1% flushing with respect to the time for residual PCE removal; 4.0 pore vol versus 15 pore vol, respectively, was required to remove >95% of the initial residual PCE. Overall, however, 1% TX-100 flushing showed greater PCE removal efficiency (mass of PCE removed per unit mass of surfactant) than 10% TX100 flushing. This result is consistent with the earlier empirical finding by Fountain et al. (5). The greater efficiency of 1% TX-100 seemed to be related to the processes by which macroemulsions were formed and transported in the porous medium. First, recall that flushing with 1% TX-100 yielded some macroemulsions suspended in the aqueous phase that could not be separated from the aqueous phase even after centrifugation. It is possible that the macroemulsion droplets formed during 1% TX-100 flushing were smaller, and thus more easily transported, than the macroemulsion droplets formed during 10% TX-100 flushing or in the 1% TX-100 batch system; this behavior was not observed during 10% TX-100 flushing or in any batch systems. Second, significant macroemulsification occurred during the 10% TX-100 flushing experiment; this was not observed in the batch experiment at 10% TX-100. Even if g10% TX-100 is used to achieve total miscibility, macroemulsification is still expected to occur spontaneously at the surfactant front where (i) the concentration of TX-100 is lower because of dilution caused by hydrodynamic dispersion and (ii) the surfactant solution is nearly saturated with PCE because of the solubilization occurring upgradient. The formation of a concentrated bank of macroemulsified PCE at the surfactant front, as observed during 10% TX-100 flushing (Figure 7), is likely to increase the bulk viscosity within the bank, because the bulk viscosity of emulsions generally follows an empirical extension of the Einstein limiting law for a dilute, rigid suspension (14). This condition will reduce the aqueous-phase permeability of the porous medium, causing bypassing of surfactant solution around the macroemulsion bank and the DNAPL-contaminated region and resulting in less effective removal of NAPL, especially in greater spatial dimensions (see also ref 5). Implication of Macroemulsion Transport. Mobilization and solubilization have been the primary focus of past research on surfactant-enhanced remediation studies; this focus is evident from the mechanisms incorporated in existing mathematical models (21, 22). However, the effluent PCE concentration and total PCE concentration observed within the column in our experiments quantitatively demonstrated both that macroemulsions can be formed in porous media, as was reported by Fountain et al. (5), and that they can be transported significantly in one-dimensional columns, which was not reported previously. The direct consequence of macroemulsion transport was the enhanced removal/mobility of NAPL in the idealized one-dimensional experiments. Some of the enhanced PCE removal efficiency can be explained by the likelihood that NAPL solubilization kinetics may be increased by macroemulsification because important mass transfer parameters, such as effective interfacial area between NAPL and bulk aqueous phase, are significantly affected by macroemulsification. Yet, in an aquifer contaminated with DNAPL, DNAPL removal by macroemulsion transport may not be desirable because the macroemulsions might be transported downward by gravity and further aggravate the extent


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of contamination. Other concerns include the increase in the bulk viscosity of an aqueous phase by suspended macroemulsions and the reduction of aquifer permeability by filtration of macroemulsions. Macroemulsions are ubiquitous in water/NAPL/surfactant systems (5, 13, 14), and the processes mentioned above are extremely important in the implementation of surfactant-enhanced NAPL remediation. However, recent surfactant flushing models (21, 22) do not account for the formation and transport of macroemulsions and consider only solubilization and mobilization. Therefore, further experimental and theoretical research and model development are necessary, especially for multiple spatial dimensions.

Acknowledgments The authors would like to thank Dr. Paul Imhoff for his valuable suggestions and comments. This research was supported by Grant 5 P42 ES05948-02 from The National Institute of Environmental Health Sciences, Research Triangle Park, NC; Grants DAAL03-91-G-0155 and DAAL0392-G-0111 from The Army Research Office, Research Triangle Park, NC; and the Hoechst Celanese Corporation/ University of North Carolina at Chapel Hill Research Partnership, supported by the Hoechst Celanese Corporation, Sommerville, NJ.

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(13) Attwood, D.; Florence, A. T. Surfactant Systems: Their chemistry, pharmacy and biology; Chapman and Hall: New York, 1983. (14) Adamson, A. W. Emulsions, foams, and aerosols. In Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: Inc.: New York, 1990; pp 525-559. (15) Reed, R. L.; Healy, R. N. Some physicochemical aspects of microemulsion flooding: A review. In Improved Oil Recovery by Surfactant and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; Academic Press: New York, 1977; pp 383-437. (16) Winsor, P. A. Trans. Faraday Soc. 1948, 44, 376-398. (17) Shiau, B. J.; Sabatini, D. A.; Harwell, J. H. Ground Water 1994, 32, 561-569. (18) Baran, J. R.; Pope, G. A.; Wade, W. H.; Weerasooriya,V.; Yapa, A. Environ. Sci. Technol. 1994, 28, 1361-1366. (19) Nelson, R. C.; Pope, G. A. Soc. Pet. Eng. J. 1978, 18, 325-338. (20) Sharma, M. K.; Shah, D. O. Macro- and microemulsions in enhanced oil recovery. In Macro- and Microemulsion: Theory and Applications; Shah, D. O., Ed.; Academic Press: New York, 1985; pp 149-172. (21) Abriola, L. M.; Dekker, T. J.; Pennell, K. D. Environ. Sci. Technol. 1993, 27, 2341-2351. (22) Brown, C. L.; Pope, G. A.; Abriola, L. M.; Sepehrnoori, K. Water Resour. Res. 1994, 30, 2959-2977. (23) Soo, H.; Radke, C. J. Ind. Eng. Chem. Fundam. 1984, 23, 342347. (24) Hofman, J. A. M. H.; Stein, H. N. Colloids Surf. 1991, 61, 317329. (25) Kokal, S. L.; Maini, B. B.; Woo, R. Flow of emulsions in porous media. In Emulsions: Fundamentals and Applications in the Petroleum Industry; Schramm, L. L., Ed.; American Chemical Society: Washington, DC, 1992; pp 219-262. (26) Ramirez, W. F.; Shuler, P. J.; Friedman, F. Soc. Pet. Eng. J. 1980, 20, 430-438. (27) Chhatre, A. S.; Kulkarni, B. D. J. Colloid Interface Sci. 1992, 150, 528-534. (28) McBride, J. F.; Miller, C. T. Research Report No. ER/1.94; Center for Multiphase Research, Department of Environmental Sciences & Engineering, University of North Carolina: Chapel Hill, NC, 1994. (29) Imhoff, P. T.; Gleyzer, S. N.; McBride, J. F.; Vancho, L. A.; Okuda, I.; Miller, C. T. Environ. Sci. Technol. 1995, 29, 1966-1976. (30) Imhoff, P. T.; Thyrum, G. P.; Miller, C. T. Water Resour. Res., in press. (31) Adeel, Z.; Luthy, R. G. Environ. Sci. Technol. 1995, 29, 10321042. (32) Parker, J. C.; van Genuchten, M. Th. Determining transport parameters from laboratory and field tracer experiments. Va. Agric. Exp. Stn. 1984, No. 84-3. (33) Cohen, R. M.; Mercer, J. W. DNAPL Site Evaluation; C. K. Smoley/ CRC Press: Boca Raton, FL, 1993.

Received for review June 22, 1995. Revised manuscript received February 5, 1996. Accepted February 9, 1996.X ES9504395 X

Abstract published in Advance ACS Abstracts, April 15, 1996.

Physicochemical Transport Processes Affecting the Removal of

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