Environ. Sci. Techno/. 1993,27,2332-2340
Surfactant-Enhanced Solubilization of Residual Dodecane in Soil Columns. 1. Experimental Investigation Kurt D. Pennell, Linda M. Abrlola,' and Walter J. Weber, Jr.
Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2 125
The widespread detection of organic solvents and other petroleum-based products in groundwater has prompted intensive study of nonaqueous-phase liquid (NAPL) transport and dissipation in subsurface environments. Although such research has significantly advanced our understanding of multiphase flow and transport in porous media, the development of viable remediation technologies remains a formidable task. Nonaqueous-phase liquids frequently enter the unsaturated zone as discrete liquid phases which are transported downward as a result of gravitational and capillary forces. If the spill is of sufficient size to reach the water table and overcome capillary entry barriers, a NAPL that is more dense than water (DNAPL) will tend to migrate vertically through the saturated zone. In contrast, a light nonaqueous-phase liquid (LNAPL) will tend to spread laterally along the water table, forming a lens of free product. However, fluctuations in the water table can result in vertical displacement of the LNAPL free product and its subsequent redistribution within the saturated zone. As the NAPL is transported through the subsurface, a portion of the organic phase will be retained within soil pores as immobile ganglia or globules due to interfacial forces. The entrapped NAPL may occupy between 5 and 40% of the pore volume (1-3). Due to the low solubility of many NAPLs, the residual organic phase frequently represents a long-term source of groundwater contamination. In addition, concentrations of NAPLs in groundwater rarely exceed 10%of their aqueous solubility. This phenomenon has been attributed to irregular NAPL distributions, nonuniform flow patterns, and dilution effects (4) as well as rate-limited mass transfer between the organic and aqueous phases ( 1 , 5 , 6 ) . Thus, conventional pump-and-treat technologies, which are based on NAPL dissolution, have proven to be an ineffective and extremely costly means of aquifer restoration (7).
Surfactant-enhanced aquifer remediation (SEAR) has been proposed as an alternative method for recovering residual NAPLs from contaminated aquifers. This technique is based primarily on two processes: (a) micellar solubilization of NAPLs and (b) mobilization of entrapped NAPLs due to interfacial tension reductions. The first approach capitalizes on the ability of micellar solutions to increase the aqueous solubility of hydrophobic organic compounds. Surfactants are amphiphilic compounds, possessing both hydrophilic and lipophilic moieties. Below the critical micelle concentration (CMC),surfactants exist solely as monomers and have a minimal effect on the solubility of most hydrocarbons (8,9). As the surfactant concentration approaches the CMC, the lipophilic moieties of the surfactant monomers associate with one another to form micelles consisting of a hydrophobic core surrounded by a hydrophilic mantle. A dramatic enhancement in NAPL solubility is commonly observed above the CMC, which is attributed to the incorporation of hydrophobic compounds within surfactant micelles (8-14). Thus, aqueous micellar solutions could be employed in pumpand-treat systems to enhance the recovery of residual NAPLs. The second approach to SEAR is based on the formation of middle-phase microemulsions which lead to ultralow interfacial tensions dyn/cm) between the organic and aqueous phases (15). This allows for the displacement or mobilization of residual NAPLs from porous media under normal flow regimes. This type of system has been investigated by the petroleum industry to enhance oil recovery. In such applications, surfactant solutions often contain cosolvents and brine to promote the formation of middle-phase microemulsions ( 16-18). In theory, this approach could offer an efficient means of recovering NAPLs from contaminated aquifers. However, several issues must be resolved prior to employing such systems for aquifer remediation, including the potential migration of mobilized NAPLs into fine-textured or uncontaminated aquifer materials. Aqueous surfactant solutions have been successfully used to remove sorbed or deposited polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from soil materials (8, 19-22). However, the recovery of hydrocarbons existing as free product or a t residual saturations has proven to be far more difficult. Initial success was achieved by the Texas Research Institute (23),which reported approximately 80 % recovery of the gasoline present in Ottawa sand using a mixture of 2% Richonate YLA, an alkyl benzene sulfonate, and 2% Hyonic PE-90, an ethoxylated nonylphenol. However,the application of similar surfactant formulations at a field site contaminated by JP-4 jet fuel, fire retardant, and chlorinated solvents resulted in the complete plugging of two wells and had no significant effect on the concentration of contaminants beneath the remaining wells (24). Ziegenfus (25) also reported the immediate plugging of soil columns following the application of aqueous solutions of Hyonic PE-90. The failure of these experiments may have
2332 Environ. Scl. Teohnoi., Vol. 27, No. 12, 1993
0013-938X/93/0927-2332$04.00/0
The solubilization of dodecane by polyoxyethylene (20) sorbitan monooleate, a nonionic surfactant, was investigated as a potential means of recoveringnonaqueous-phase liquids from contaminated aquifers. Residual saturations of dodecane were established by injecting 14C-labeled dodecane into water-saturated soil columns and displacing the free product with water. Flushing with a 43 g/L surfactant solution increased the concentration of dodecane in the column effluent by 5 orders of magnitude. However, effluent dodecane concentrations were considerably less than the equilibrium value of 3500 mg/L measured in batch studies. Subsequent column experiments conducted a t several flow velocities and with periods of flow interruption confirmed the existenceof rate-limited, rather than instantaneous, solubilization of residual dodecane. The results of this study demonstrate the sizable capacity of surfactant solutions to enhance the recovery of residual dodecane, even under conditions of rate-limited solubilization.
Introduction
0 1993 American Chemical Soclety
been due to reductions in permeability brought about by the dispersion of fine materials and subsequent blockage of pore throats or by the formation of highly viscous emulsions. In contrast, Fountain et al. (26) identified a number of surfactant formulations capable of removing tetrachloroethylene (PCE) from soil columns after injecting 7-14 pore volumes of surfactant solution. The mixed results of surfactant remediation studies conducted to date emphasize the need for fundamental research on the processes governing surfactant-enhanced aquifer remediation. The overall purpose of this study was to assess the potential utility of surfactant flushing as an aquifer remediation strategy. The data reported herein are for a single nonionic surfactant, polyoxyethylene (POE) (20) sorbitan monooleate (trade name: Witconol2722; Tween 80). This food-grade surfactant is commonly used in the manufacture of shortenings, whipped toppings, and dietary supplements. Due to its low toxicity and potential for biodegradation (27),POE (20)sorbitan monooleate is wellsuited for aquifer remediation. Dodecane, a saturated hydrocarbon, was selected as the model NAPL. The specific objectives of this research were to measure the effect of POE (20) sorbitan monooleate on the aqueous solubility of dodecane and to quantify the removal of residual dodecane from soil columnsby surfactant flushing. The soil column experiments were designed to test the validity of the local equilibrium assumption for describing the solubilization of residual dodecane. In addition, surfactant sorption and the effect of entrapped dodecane on column dispersivity were assessed. The results of these studies are utilized in a companion paper (28)to evaluate the capabilities of a one-dimensional numerical simulator to model surfactant-enhanced NAPL solubilization in porous media.
Materials and Methods Materials. Polyoxyethylene (20) sorbitan monooleate (Lot B2601, provided by the Witco Corp., was used as received. This surfactant is formed by the esterification of 1,4-sorbitan with oleic acid, followed by condensation with 20 mol of ethylene oxide to increase the aqueous solubility of the product. The average molecular weight and density of POE (20) sorbitan monooleate are 1310 g/mol and 1.08 g/mL, respectively, while the literature values for the CMC and hydrophilic-lipophilic balance (HLB)are 13 mg/L and 15, respectively (27,28). It should be noted that POE (20) sorbitan monooleate is commercially manufactured, and thus, the reported molecular structure represents an "average" that varies in both the degree of ethoxylation and the nature of the hydrophobic group. Dodecane was selected as a representative of the saturated hydrocarbons found in gasoline and jet fuel. Purified-grade dodecane was obtained from Fisher Scientific, while I4C-labeled dodecane was purchased from Sigma Chemical. Dodecane has a density of 0.75 g/cm3 and an aqueous solubility of 3.7 Fg/L (31). The surface tension of dodecane is 24.9 dynlcm (32), while the interfacial tension between dodecane and water is 52.8 used in the miscible dyn/cm (33). Tritiated water (~HzO), displacement experiments, was obtained from American Radiolabeled Chemicals. All solutions were prepared with deionized, distilled water that was passed through a
Nanopure (Barnstead) purification system to remove organic contaminants. Ottawa sand was used as the solid phase for the surfactant sorption and soil column experiments. Ottawa sand is a nonporous silica sand containing little if any organic material and has been found to exhibit no detectable sorption of TCE (34). The 20-30 mesh (0.850.55-mm)size fraction was obtained from Fisher Scientific and rinsed with Nanopure water to remove fine materials prior to use. The specific surface area of this sand has been reported to be 0.1 m2/g (35). Dodecane Solubilization. The solubility of dodecane in aqueous solutions of POE (20) sorbitan monooleate was measured in a temperature-controlled room using 25-mL borosilicate glass vials equipped with open-top screw caps and Teflon-backed septa. A total of 1mL of 14C-labeled dodecane was contacted with 20 mL of POE (20) sorbitan monooleate solution, ranging in concentration from 20 to 160g/L. The specific activity of the 14C-labeleddodecane was approximately 0.5 FCi/mL. Duplicate samples of each surfactant concentration were equilibrated on an oscillating shaker (Labquake Industries) for 48 h at 22 f 0.1 OC. The sample vials were then inverted and allowed to settle for 24 h to achieve complete phase separation. Duplicate 1-mL samples were taken from the aqueous phase of the inverted vials using a gas-tight syringe and transferred to scintillation vials containing 10 mL of Ecolume cocktail (ICN Biomedicals). The activity of dodecane in the aqueous phase was assayed by standard liquid scintillation counting (LSC)procedures using a LKB Wallac Model 1219 Rackbeta scintillation counter. Surfactant Sorption. Surfactant sorption experiments were conducted using 35-mL centrifuge tubes capped with Teflon-backed septa. Approximately 30 g of Ottawa sand was weighed into each centrifuge tube, to which an aqueous solution of POE (20) sorbitan monooleate was added until no headspace existed in the tube (-30 mL). The initial concentration of surfactant ranged from 0.5 to 23 g/L. The contents of each centrifuge tube were mixed on a rotary tumbler for 72 h at 22 f 1"C. The solid and aqueous phases were separated by centrifugation at 2500 rpm for 1 h using a DuPont Sorvall RC5B centrifuge. The concentration of surfactant in the aqueous phase was determined by ultraviolet (UV) analysis using a Varian DMS 200 UV-visible spectrophotometer at a wavelength of 230 nm (36). Due to the low organic carbon content of the Ottawa sand, no detectable interference from natural organic matter was observed at this wavelength. Soil Column Procedures. Three soil column experiments were conducted to study the transport of a nonreactive tracer (3HzO)and the solubilization of residual dodecane using procedures adapted from Lee et al. (37) and Powers et al. (61,respectively. The column apparatus consisted of a Kontes preparative chromatography column made of borosilicate glass (4.8-cm i.d.1 equipped with an adjustable end plate that allowed the bed length to be varied from 1 to 13 cm (Figure 1). Both end plates were fitted with a 40-mesh PTFE screen to enhance radial distribution at the column inlet and to reduce dispersion at the outlet. The top end plate wa8 lined with 20 pm of PTFE filter paper to yield organicwetting conditions, while the bottom end plate was lined with Whatman No. 42 filter paper to yield water-wetting conditions. The column was packed under vibration with air-dried Ottawa sand in Environ. Sci. Technol., Voi. 27, No. 12, 1993
2333
14m
8."
COlleROI
... : :. DodecaMl
HPLC Pump
DisplacedWater or suriaefant
Schematic diagram of lhs experimental apparatus used lo measure lhe removal of resMual dodecane from soil columns packed wkh Onawa sand. Flgura 1.
Table 1. Physical Parameters of Soil Columns before (i) and after (0)Entrapment of Residual Dodecane.
L
Pb
column (ern) (glcrns) mw.i Pi 1
2 3
6.50 6.35 6.43
1.80 1.79 1.80
ai
8.
(cm) (9%) mw.,
a.
Po (em)
0.324 107.5 0.061 20.36 0.258 42.2 0.154 0.326 106.0 0.060 15.84 0.274 45.9 0.139 0.323 19.70 0.259 56.5 0.114
*Parameter definitions are niven in the text.
2-cm increments. To minimize layering, the bed surface was mixed with a small spatula between increment additions. Approximately 40 pore vol of de-aired water was pumped into the bottom of the soil column using a Rainin HPLC pump. The pump was equipped with a 25-mL pump head and back-pressure regulator t o reduce variations in the flow rate. The weight of the column was monitored during this process to determine soil bulk density ( p b ) and porosity (n)and t o ensure complete water saturation (s,) (Table I). A pulse of tritiated water was then displaced through thecolumn a t apore-water velocity ofapproximately 20cm/h. The specificactivity oftritiated water used in the miscible displacement experiments was 15 nCi/mL. Column effluent was collected in glass vials and immediately capped. Samples of 1 mL from each effluent fraction were transferred to scintillation vials containing 10 mL of Ecolume cocktail, and the vials were assayed by LSC procedures as described previously. I4C-Labeleddodecane was then introduced through the top of the soil column using a syringe pump (Harvard Apparatus) a t a rate of 0.3 mL/min. The specific activity of dodecane used in the soil column experiments was approximately 1.0 pCi/mL. Dodecane, which is less dense than water, was pumped in a downflow mode to achieve stable displacement of water from the column (38). The Whatman No. 42 filter paper on the bottom end plate allowed water to be displaced from the column but prevented theexitofdodecane. When approximately70% of the pore volume was occupied by dodecane, the flow wasreversedand waterwasintroducedthroughthebottom of the column. Dodecane was displaced from the column by pumping 5 pore vol of water through the column a t a superficial or Darcy velocity of 1 cmih, followed by an additional 15 pore vol of water at a Darcy velocity of 25 cmih. The residual saturation (so) of dodecane was 2334
Envirm. Sci. Technol.. Vol. 27. NO. 12, 1993
where C, is the molar concentration of organic in a surfactant solution; C.,,, is the molar concentration of organic a t the CMC; C. is the molar concentration of is the molar concentration of surfactant; and C,, surfactant a t the CMC. Given the extremely waterinsoluble nature of dodecane, the apparent solubility of
dodecane may increase slightly at surfactant concentrations approaching the CMC, as noted by Kile and Chiou (IO) for DDT. In addition, the presence of an organic solute may reduce the CMC of a surfactant by altering the activity of surfactant monomers (43). Thus, the value of Ccm,oand Ccm,,cannot be explicitly determined from the data presented herein. However, the MSR can be calculated from the slope of the solubility curve above the CMC, when the concentrations are expressed on a molar basis. Using the average molecular weight of POE (20) sorbitan monooleate, an MSR value of 0.62 was obtained from the data shown in Figure 2. This value is about two times greater than MSR values reported for the solubilization of PAHs by several nonionic surfactants (8). A second approach frequently used to characterize the solubilization of hydrophobic organic compounds is based on the partitioning of organic compounds between surfactant micelles and the aqueous phase. The micelle aqueous-phase partition coefficient (K,) can be defined as
K, = X,/Xa where X, is the mole fraction of organic in the micellar phase, and Xais the mole fraction of organic in the aqueous phase. The mole fraction of organic in the micellar phase can be calculated from the MSR (8): X, = MSR/ (1+ MSR)
(3) For dilute solutions, the mole fraction of organic in the aqueous phase can be estimated as follows (8): (4)
where V, is the molar volume of water (e.g., 0.01805 L/mol at 25 "C). Assuming that the value of Cc,c,ois equal to the solubility of dodecane in water, the log K, was 8.99. The K, values derived from surfactant-organic systems can be correlated to the partitioning of organic compounds between octanol and water, represented by K,. Edwards et al. ( 8 ) obtained a linear relationship between log K, and log KO, for the solubilization of several organic compounds by Triton X-100. Using a log KO, of 6.51, estimated from the aqueous solubility of dodecane (44), the log K, calculated for dodecane-POE (20) sorbitan monooleate falls above this line. These data suggest that POE (20) sorbitan monooleate has a greater capacity to solubilize hydrophobic organic compounds than Triton x-100. Surfactant Sorption. The sorption of POE (20) sorbitan monooleate by Ottawa sand over a solution concentration range of approximately0.02-21 g/L is shown in Figure 3. Sorption data were expressed as the amount of surfactant sorbed per unit weight of sorbent (Q,)versus the concentration of surfactant in the solution phase at equilibrium (C,).These data conformed to the Langmuir equation which can be written in the form: (5) where Q,, represents the maximum sorption capacity, and b is a constant equal to the rate of adsorption divided by the rate of desorption. The sorption data were fit to the linear form of eq 5 using a least-squares, linear regression procedure (SYSTAT,Inc.) to obtain the values of Qm, and b shown in Figure 3.
Langmuir Parameters Oms = 1 33 mgig
-P -F
I. .
1.5
7)
1.0 0) I
C
a
E 0.5
" 0
I
I
I
I
5
10
15
20
25
Surfactant Concentration (giL)
Flgure 3. Sorption of polyoxyethylene sorbitan monooleate by Ottawa sand at 22 O C .
Polyoxyethylene (20) sorbitan monooleate consists of a lipophilic group, which may be retained by hydrophobic surfaces due to van der Waals attractions or partitioning phenomena, and a polyoxyethylene head group that may be adsorbed on hydrophilic surfaces via hydrogen bonding (45-47). At low surface coverages, surfactant monomers are likely to lie parallel to the sorbent surface. As the surfactant concentration increases, the hydrophobic portion of the surfactant may be displaced from mineral surfaces, allowing for lateral interactions between adjacent hydrophobic groups of sorbed monomers (48). Above the CMC, the sorbed surfactant phase can exist as either a monolayer (45)or a bidimensional aggregate (bilayer) (48). Such behavior typically results in simple (L2) or stepped (L4) Langmuir isotherms which reach a limiting value at or near the CMC (43,49,50). Although the sorption data are consistent with a simple Langmuir isotherm, the limiting sorption capacity of 1.33 mg/g was approached at surfactant concentrations well above the reported CMC of POE (20) sorbitan monooleate (13 mg/L). This type of behavior, however, has been observed for the sorption of several nonionic surfactants on polar materials (19, 49). When expressed on amolar basis, the maximum sorption of POE (20) sorbitan monooleate on Ottawa sand was 1.02 rmollg or 10.2 pmo1/m2,which is within the range of values typically reported for nonionicsurfactants (49). In general, the sorption of ethoxylated nonionicsurfactants on mineral surfaces and activated carbon decreases as the number of ethylene oxide (EO) groups increases (48, 51, 52). This effect can be attributed to the increase in the aqueous solubility of surfactant monomers and the corresponding reduction in affinity for hydrophobic surfaces. In addition, the area occupied by surfactant monomers in the adsorbed state becomes larger as the length of the POE chain increases. Thus, the maximum sorption capacity of sorbitan monooleates would be expected to decrease as the degree of ethoxylation is increased. Miscible Displacement of 3Hz0.Effluent breakthrough curves (BTCs) for 3H20 were measured before and after dodecane entrapment to determine the effect of residual dodecane on dispersivity (Figure 4). Note that both BTCs are highly symmetrical and do not exhibit tailing, which suggest that immobile water had no measurable effect on the transport of 3Hz0. Therefore, the measured BTCs were fit to an analytical solution of the one-dimensional advection-dispersion reaction (ADR) solute transport equation. Assuming conditions of hoEnviron. Sci. Technol., Vol. 27, No. 12, 1993 2335
0.8
-
-? d
(a)
d
? b
0.6
v = 15.2 cmlhr So = 0%
b
I
0.4 0.2
0
5
Pore Volumes (p) Figure 4. Measured and simulated breakthrough curves for 3H20 displacement through Ottawa sand (a) before and (b) after dodecane entrapment (column 2).
mogeneity, local equilibrium, isotherm linearity, and isotherm singularity, the one-dimensional ADR equation may be written in dimensionless form as:
R = 1+ p,,Kp/ns,;
C* = C/Ci; p = vt/L; P = vL/D; X = x / L (7) where R is the retardation factor, Pb is the soil bulk density, Kp is the partition coefficient, n is the porosity, sw is the water saturation, C is the solution-phase solute concentration, Ci is the influent solute concentration, p is the pore volume, u is the pore-water velocity, t is the time, L is the column length, P is the Peclet number, D is the hydrodynamic dispersion coefficient, and x is distance. As anticipated, the values of R obtained for all 3H20 BTCs were equal to 1, which indicates that no retardation of tritiated water occurred in the presence or absence of residual dodecane. Prior to the entrapment of dodecane, the Peclet numbers for columns 1 and 2 were 107.5 and 106,respectively (Table I). The dispersion coefficient (D), derived from the Peclet number, is frequently observed to be proportional to the pore-water velocity,D = au,where a is the dispersivity. The relatively small values of a are typical of those reported for columns packed with uniform quartz sands (37) (Table I). Following the entrapment of residual dodecane, the values of a for columns 1 and 2 increased by a factor of 2.5 and 2.3, respectively. The increased dispersivity was attributed to the presence of immobile globules or ganglia of dodecane which may result in greater local velocity variations due to the blockage of pore spaces and increased tortuosity of mobile water flow paths. 2338
Envlron. Scl. Technol., Vol. 27, No. 12, 1993
Soil Column Experiments. Results of a preliminary column experiment indicated that the effluent concentration of dodecane was approximately 500 mg/L after flushing with a 43.2 g/L solution of POE (20) sorbitan monooleate a t a pore-water velocity of 6 cm/h. Although this represents a 5 order of magnitude increase in dodecane solubility, the effluent concentration was still 7 times less than the equilibrium value of 3500 mg/L measured in batch experiments. Therefore, the column experiments reported herein were specificallydesigned to investigate rate-limited solubilization of residual dodecane. Two experimental procedures were employed for this purpose: (1)the flow interruption method and (2) the measurement of effluent dodecane concentrations a t several flow rates. The results of three column experiments conducted a t pore-water velocities ranging from 6 to 25 cm/h and flow interruption periods of 3.5-100 h are shown in Figure 5. The porewater velocities used correspond to Darcy velocities of 0.4-1.7 miday, which are within the range of velocities expected in aquifers under natural and remediation conditions (6,521. At each flow rate the solubilization of residual dodecane was measured under pseudo-steadystate conditions. This condition is established during the initial stages of surfactant flushing, prior to significant changes in the interfacial area and mass of the residual NAPL (6). Over the course of each experiment less than 10 % of the dodecane was removed from the soil column. The flow interruption technique has been utilized to investigate several rate-limited processes including intraparticle and film diffusion (53,54)and the sorption of hydrophobic organic solutes by soils (55). The procedure involves the displacement of a solution at a constant flow rate and the interruption of flow for a specified period of time, followed by the recommencement of flow. In Figure 5a, points A, B, and C correspond to the steady-state effluent concentration of dodecane prior to flow interruption, the volume at which flow was stopped, and the post-interruption concentration of dodecane, respectively. The eluted surfactant solution containing elevated concentrations of dodecane represents the pore volume residing within the column during the flow interruption period. The sizable increase in the effluent concentration of dodecane following flow interruption is indicative of rate-limited, rather than instantaneous, solubilization of residual dodecane. As the duration of flow interruption or residence time was increased, the extent of dodecane solubilization within the soil column approached the equilibrium value measured in batch experiments (Figures 5 and 6a). A residence time of approximately 100 h was required to achieve equilibrium solubilization of the residual dodecane. Following elution of the interruptionresident solution, the effluent concentration returned to a steady-state value corresponding to the applied porewater velocity. In effect, the stoppage of flow allows for greater contact time between the micellar solution and residual dodecane, which results in a corresponding increase in the aqueous-phase concentration of dodecane. As the pore-water velocity of the surfactant solution was increased, the steady-state concentration of dodecane in the column effluent decreased (Figures 5 and 6b). These findings also indicate that the solubilization of residual dodecane was reduced when the residence time of the micellar solution within the soil column decreased. A similar trend has been reported for the dissolution of residual styrene in soil columns (6). However,the observed
4000 Column 1
3500
3000
s a
-g
-,
_.
Equilibrium ._ _. _,Solubility._
._ ._
_,
_
_. ._
..
._
1
I
Duration of flow Interruption (hrs)
2500
2500
E
2000
2000
0 Column 2
m
B
$
v
E
m
U
A
T
-0
1500
0"
1000
1000
500 61
1 I
O d
I
0
200
100
B
300
400
20
40
60
80
100
Duration of Flow Interruption (hrs)
500
600 4000 Column 2 Equilibrium Solubility
3500
..
._._ 49 2
3000
2
?
2500
g
2000
m
_ -
22.2
Duration of flow interruption (hrs;
I
T F
400
v
300
,2,3
200
m
B
R
500
71
1500
pore-water velocity (cmlhr;
100
1000
I
5.8
500
5 0
100
200
300
400
500
600
700
4000
E 2
a
Column 3
Duratlon 01 flow interruption (hrs; 2500
c
125
i
1500 too0 500
0
25
Figure 6. Effect of (a) duratlon of flow Interruption and (b) pore-water veloclty on the effluent concentration of dodecane.
3000
2000
20
800
Volume (ml)
E
15
Pore-Water Velocity (cm/hr)
0
-=& -
10
I 400
Volume (ml)
Flgure 5. Measured effluent concentrationsof dodecane after flushing (a) colum 1, (b) column 2, and (c) column 3 wlth an aqueous solutlon (43.2 g/L) of polyoxyethylene sorbitan monooleate.
deviation from equilibrium for dodecane solubilization was 6-7 times greater than that reported for styrene dissolution at comparable flow velocities. These data, taken in concert with the flow interruption results, demonstrate the importance of rate-limited solubilization to the recovery of residual dodecane from soil columns. Although nonequilibrium solubilization occurred within the soil columns, flushing with a 43.2 g/L solution of POE (20) sorbitan monooleate greatly enhanced the recovery of residual dodecane. Based on mass balance calculations, 196,479, and 579 mg of dodecane were solubilized after flushing soil columns 1, 2, and 3 with 0.5, 0.8, and 0.7 L of surfactant solution,respectively. The recoveryof equivalent amounts of dodecane by water flushing would require from 5.3 X lo4 to 5.6 X lo5L of water, assuming that the equilibrium solubility of dodecane (3.7 pg/L) was reached.
These data demonstrate the dramatic improvement in residual NAPL recovery that can be realized by the use of aqueous surfactant solutions in a conventional pumpand-treat remediation framework. Based on the results of the flow interruption and steady-state experiments, the flow schedule could be optimized to further improve the efficiency of dodecane solubilization, thereby minimizing the amount of surfactant required for dodecane recovery. Thus, the identification and characterization of ratelimited NAPL solubilization could be essential to the development of efficient and cost-effective SEAR technologies on a larger scale. The majority of solubilization studies conducted to date have focused on equilibrium rather than rate-limited conditions. However, two mechanistic models have been proposed to describe the solubilization of organic liquids. The first model incorporates the dissolution of an organic liquid into the aqueous phase, adsorption of the organic at the micelle/water interface, and subsequent incorporation within the micelle (56-59). In the second model, the micelles are thought to diffuse to the organidwater interface, dissociate into monomers which are adsorbed at the interface, and then re-form into micelles containing the associated organic liquid (60-63). The rate of solubilization has been shown to increase with the surfactant concentration above the CMC and the polarity of the organic solute (60, 62). In addition, the rate of n-hexadecane solubilization in mixtures of linear alkylarene sulfates and an ethoxylated alcohol was found to be strongly correlated to the molecular structure of the surfactant formulation (60). The flow interruption studies presented herein demonstrate that the rate of dodecane solubilization decreased as the solubilization limit was Environ. Sci. Technol., Vol. 27. No. 12, 1993 2337
approached (Figure 6a). Similar kinetic relationships have been reported for the solubilization of naphthalene, anthracene, pyrene, and dibenzanthracene (57,58). These results could be attributed to a reduction in the rate of organic incorporation within the micelle (model 1) or a reduction in the rate of micelle diffusion to or from the organidwater interface (model 2) as the micelles beome saturated with dodecane. The tritiated water BTCs show that, under flowing conditions, immobile water zones had a negligible impact on transport in the aqueous phase. These results suggestthat diffusion of the dissolved organic species to surfactant micelles would not be limited by physical nonequilibrium between regions of mobile and immobile water. However, the inherent complexity of micellar solubilization precludes a definitive mechanistic interpretation of the flow interruption results. In this study, the surfactant formulation and experimental conditions were selected to promote the solubilization, rather than the mobilization, of residual dodecane. However, these processes are not mutually exclusive and, thus, may occur simultaneously during surfactant-enhanced aquifer remediation. It is imperative, therefore, to evaluate the potential for NAPL mobilization prior to conducting surfactant flushing tests. Two dimensionless groups, the capillary number (Nca)and Bond number ( N B ) can be used to assess the relative importance of viscous and buoyancy forces, respectively, which act to mobilize the organic liquid versus the capillary forces, which act to retain the organic liquid within a porous medium (64,651. These dimensionless numbers can be defined as follows:
where k is the intrinsic permeability, k,, is the relative permeability to water, fl is the pressure change, uow is the interfacial tension between the organic and aqueous phases, L is the length of the column, p, is the viscosity of water, q is the Darcy velocity, Ap is the difference in fluid densities, andg is the acceleration due to gravity. As defined above, the capillary and Bond numbers may be superposed for vertical displacement of NAPLs in the direction of the buoyancy force. Using an interfacial tension of 52.8 dyn/cm for dodecane and water (32), an intrinsic permeability of 1.86 X lo4 cm2 (66),and a relative permeability of 0.5, the values of N B and Nca during the conservative tracer experiments are estimated as 4.3 x 10-6 and 2.5 X 10-7, respectively. Since dodecane was not displaced as a free product under these conditions, the sum of the capillary and Bond numbers (4.55 X lo4) is assumed to be less than the critical value required for dodecane mobilization. During surfactant flushing, however, a few small drops of dodecane were occasionally observed on the surface of the column effluent after the flow rate was increased. The apparent mobilization of a small amount of entrapped dodecane could be explained by an increase in the sum of the capillary and Bond numbers above a critical value for this system. The interfacial tension between a 4 ?6 solution of POE (20) sorbitan monooleate and dodecane was found to be approximately 5.2 dyn/cm using an axisymetricdrop-shape analysis (ADSA) apparatus (67). Based on the flow rates employed during the surfactant flushing experiments, the 2338 Envlron. Scl. Technol., Vol. 27, No. 12, 1993
sum of the Bond and capillary numbers would vary from to 4.7 X lo5. This range of values approximately 4.5 X provides an estimate of the critical sum of the capillary and Bond numbers required to induce mobilization of entrapped dodecane in the column apparatus. These data are consistent with the findings of Morrow and Songkran (64),who reported that the residual saturation of Soltrol130 in columns packed with glass beads was relatively constant when the sum of the capillary and Bond numbers It should be noted, was less than approximately 2 X however, that the critical value derived from the capillary and Bond numbers is system specific and can vary over an order of magnitude depending on properties of the organic liquid, matrix, and experimental design. Conclusions The addition of POE (20) sorbitan monooleate to water resulted in a sizable and linear enhancement in the apparent solubility of dodecane at surfactant concentrations above the CMC. These results were attributed to the incorporation or partitioning of dodecane within the hydrophobic core of surfactant micelles. The ability of this surfactant to recover residual dodecane from Ottawa sand was evaluated in one-dimensional soil column experiments. Based on tritiated water breakthrough curves, the presence of entrapped dodecane was shown to increase dispersivity by a factor of 2.4 but had no discernible effect on physical nonequilibrium. Following the introduction of a 43.2 g/L surfactant solution, the concentration of dodecane in the column effluent increased by approximately 5 orders of magnitude but was considerably less than the equilibrium value measured in batch experiments. This discrepancy was attributed to rate-limited solubilization of dodecane based on (a) the increase in effluent dodecane concentrations following flow interruption and (b) the reduction in steady-state effluent concentrations as the pore-water velocity was increased. The rate of dodecane solubilization during periods of flow interruption was found to decrease as the apparent solubility limit was approached. Surfactantsorption conformed to a Langmuir isotherm and had a minimal impact on the solubilization of residual dodecane. These data are utilized in a companion paper (28) to evaluate a numerical model capable of simulating coupled surfactant transport and solubilization of residual NAPLs and to investigate the implications of rate-limited solubilization on the recovery of dodecane from soil columns. Future research efforts will focus on the characterization of surfactant-enhanced solubilization of entrapped organic liquids for a range of surfactant-NAPL systems relevant to aquifer remediation. Acknowledgments We thank Mr. Mamadou Diallo and Dr. Fred Desai for assistance with the screening of nonionic surfactants and interfacial tension measurements, respectively, and Ms. Joanne Geils of the Witco Corp. for supplying polyoxyethylene (20) sorbitan monooleate. Funding for this research was provided by the Office of Research and Development, US. Environmental Protection Agency under Grant R815750 to the Great Lakes and Mid-Atlantic Hazardous Substance Research Center. Partial funding of the research activities of the Center was also provided by the State of Michigan, Department of Natural Resources and the Department of Energy. Additional
funding was provided by the Ford Motor Co. and the U S . Environmental Protection Agency through Cooperative Agreement CR-818647with the R. S. Kerr Environmental Research Laboratory. The research described in this article has not been subjected to Agency review and, therefore, does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
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Received for review November 16, 1992. Revised manuscript received July 7, 1993. Accepted July 16, 1993.' @Abstractpublished in Advance ACS Abstracts, September 1, 1993.
