Surfactant-Enhanced Subsurface Remediation - American Chemical

Chapter 2

Impact of Surfactant Flushing on the Solubilization and Mobilization of Dense Nonaqueous-Phase Liquids 1

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L. M. Abriola , K. D. Pennell , G. A. Pope , T. J. Dekker , and D. J. Luning-Prak Downloaded by UNIV OF ROCHESTER on June 13, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0594.ch002

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1Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109 2Department of Petroleum Engineering, University of Texas, Austin, TX 78712 This paper provides an overview of on-going research related to surfactant-enhanced recovery of entrapped dense nonaqueous phase liquids (DNAPLs) in porous media. Three issues which may prove of particular importance to the design of successful field remediation schemes are highlighted: (1) rate-limited micellar solubilization; (2) the control of organic liquid mobilization; and (3) the influence of physical heterogeneities on organic distribution and recovery. Experimental and modeling investigations which explore each of these issues are presented and discussed. These studies reveal that (1) micellar solubilization is substantially rate-limited under flow conditions anticipated in engineered recovery schemes; (2) buoyancy forces may play an important role in DNAPL mobilization; and (3) entrapment of organic liquids in low permeability zones will strongly influence the performance of surfactant-enhanced solubilization operations at the field-scale. Organic solvents and other petroleum-based products are frequently released to the environment as a separate organic phase or nonaqueous phase liquid (NAPL). When a NAPL migrates through the subsurface, capillary forces act to retain a portion of the organic liquid as discrete ganglia within the pones. These immobile ganglia may occupy between 5 and 40% of the pore volume at residual saturation (i, 2) and frequently represent a long-term source of aquifer contamination due to the low aqueous solubility of most NAPLs. Of particular concern are sites contaminated with dense NAPLs or DNAPLs. Such compounds, because of their large densities and low viscosities, are not typically confined to the unsaturated or capillary fringe zones. These dense liquids tend to migrate vertically under gravitational forces, and given sufficient spill volume, will displace water within the saturated zone and may spread deep within an aquifer formation. It is now generally recognized that conventional pump-and-treat remediation methods are an ineffective and costiy means for aquifer restoration when NAPLs are present (3,4). The failure of this technique can be attributed, in large part, to the low aqueous solubilities of NAPLs and their relatively slow rates of dissolution. Over the past few years considerable interest has focused on surfactant flushing as an alternative method for recovering residual NAPLs from 0097-6156/95/0594-0010$12.00/0 © 1995 American Chemical Society

In Surfactant-Enhanced Subsurface Remediation; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF ROCHESTER on June 13, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0594.ch002

2. ABRIOLA ET AL.

Impact of Surfactant Flushing on DNAPLs

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contaminated aquifers (e.g., 5, 6). This technique is based on the ability of surfactants to: (a) increase die aqueous solubility of NAPLs via micellar solubilization and (b) mobilize or displace the entrapped NAPL through reductions in the interfacial tension between the organic and aqueous phases. Surfactant washing has been successfully employed to remove sorbed or deposited polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from soil materials (7, 8). Initial applications of surfactant solutions to recover entrapped organic liquid contaminants, however, achieved mixed results (e.g., 9, 10) . Recently, surfactant flushing has been successfully employed to remove automatic transmission fluid and residual dodecane from soil columns (II, 12). To date, relatively few studies have addressed the use of surfactants to recover DNAPLs from aquifer materials. Pennell et al. (13) reported that mixtures of sodium sulfosuccinate surfactants were capable of removing more than 99% of the residual tetrachloroethylene (PCE) from soil columns packed with F-95 Ottawa sand. This paper presents an overview of our recent experimental and modeling research on surfactant enhanced remediation of DNAPLs. Some important issues which may have a significant impact on the field application of surfactant technologies to DNAPLs are highlighted. Specifically, these issues include: (a) rate-limited micellar solubilization; (b) the onset and extent of DNAPL mobilization during surfactant flushing; and (c) the influence of formation heterogeneity on DNAPL distribution and recovery. The presentation focuses on a single DNAPL, PCE, which was selected as representative of the chlorinated solvents typically encountered at contaminated sites. Illustrative data are presentedrelatingto the application of several commercially-available surfactant formulations, including polyoxyethylene (POE) (20) sorbitan monooleate (Witconol 2722), sodium diamyl sulfosuccinate (Aerosol AY 100); sodium dioctyl sulfosuccinate (Aerosol OT 100), and sodium dihexyl sulfosuccinate (Aerosol MA 100). These surfactants were selected to produce a range of desired interfacial tensions between the surfactant solution and PCE, and for their phase behavior and capacity to solubilize PCE. Micellar Solubilization Although equilibrium batch solubilization measurements are useful for screening surfactants, it is important torecognizethat such batch measurements may not be adequate for the prediction of surfactant performance in natural porous media. Previous soil column experiments in our laboratories have revealed that the micellar solubilization process is often rate-limited. We have observed substantial mass transfer rate-limitations for solubilization ofresidualdodecane and PCE in silica sands whenflushingwith aqueous solutions of Witconol 2722 (12,13). The CMC for Witconol 2722 isreportedto be 13 mg/L (12). To further investigate solubilization rates, a series of batch and column experiments was conducted following the general procedures described by Pennell et al. (13). In the batch studies, an excess amount of PCE was contacted with a 4% (wt) solution of Witconol 2722 in 25-mL centrifuge tubes, which were gently mixed on a reciprocating shaker. The aqueous phase was destructively sampled over time and analyzed for PCE using a direct injection gas chromatography method developed for aqueous samples containing surfactant (14). The solubilized concentration of PCE was found to increase with time, approaching an equilibrium value of 38,500 mg/L after approximately 24 hours of mixing. The log K for this system was computed as 4.73 and the MSR as 7.18. The sharp rise and asymptotic plateau of the solubilization rate curve was similar to that reported by Arytyunyan and Beileryan for the solubilization of hydrocarbons in solutions of soc&um pentadecylsulfonate (75). m



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SURFACTANT-ENHANCED SUBSURFACE REMEDIATION

Soil column experiments were then performed to explore the rate of PCE solubilization in a natural sandy porous medium. Borosilicate glass columns (5 cm i.d.) were packed with Oil Creek sand, supplied by the R.S. Kerr Environmental Research Laboratory, and saturated with de-aired water. Residual PCE saturations were established by injecting PCE liquid into water-saturated soil columns in an upflow mode, and men displacing the free product with water, in a downflow mode. Following the entrapment of PCE, a 4% solution of Witconol 2722 was pumped through the column and effluent samples were analyzed by the GC method described above. The results of a representative surfactant flushing experiment are shown in Figure 1. At a Darcy velocity of -3.4 cm/hr, the effluent concentration of PCE was found to approach a steady-state value of -30,000 mg/L, which is -8,500 mg/L less than the batch measured equilibrium value. To further investigate ratelimited solubilization of PCE, a flow interruption procedure was employe! Flow interruption data in Figure 1 suggest that equilibrium was attained within 18.5 hours. Note that subsequent interruptions of flow for 94,20, and 69 hours yielded effluent concentrations in excess of the equilibrium value. This behavior was attributed to the formation of unstable macroemulsions which led to elevated concentrations of PCE in the aqueous phase. The existence of macroemulsions was indicated by visual examination of the effluent samples. Centrifugation of these samples at 7,500 rpm (23 cm dia. SORVALL SS34 rotor) was sufficient to break the macroemulsion, and the resulting aqueous phase concentrations were consistent with the equilibrium value (triangles shown in Figure 1). Figure 2 shows the results of an experiment conducted to assess the influence of flow velocity on the steady-state concentration of PCE exiting the column. These data indicate that the rate of residual PCE solubilization was insensitive to velocity over the range of Darcy velocities employed (1.6 to 8.3 cm/hr). Results of these experiments may be compared with a similar set of experiments conducted for solubilization of dodecane in a quartz sand (13). For this system, the log K was computed as 9.02 and the MSR as 0.69. Effluent concentrations of dodecane were shown to approach equilibrium only after flow was interrupted for 100 hours. Concentrations exhibited some sensitivity to flow rate as the Darcy velocity was increased from 2 to 8 cm/hr (Figure 3). The greater sensitivity of dodecane concentrations to flow interruption and flow velocity indicate that dodecane was solubilized at a slower rate than PCE. These findings are consistent with data presented by Carroll and Ward, who found that the rate of solubilization increases with the polarity of the organic solute (16,17). Two mechanistic models, incorporating several coupled processes, have been proposed to describe rate-limited solubilization of organic liquids. The first model involves dissolution of the organic liquid into the aqueous phase and adsorption at the micelle-water interface, followed by diffusion into surfactant micelles (e.g., 15). In the second model, surfactant micelles are thought to diffuse to the organicwater interface, dissociate into monomers which are then adsorbed at the interface, and reform into micelles containing the organic liquids (e.g., 16,18). Further investigations are underway to characterize the solubilization process in more detail in order to ascertain the appropriateness of each modeling approach, to identify rate-limiting steps, to determine optimal surfactant concentration, and to develop predictive tools for flushing performance. m

NAPL Mobilization Although the discussion above focuses on micellar solubilization, aqueous surfactant solutions also have the capacity to displace or mobilize residual NAPLs from porous media. This process has been shown to be an efficient means for recovering residual PCE from soil columns (5,13). For example, Pennell et al.


2. ABRIOLA ET AL.

Impact of Surfactant Flushing on DNAPLs

70000' 94hrs Duration of flow interruption

60000

6 9 h r s

20hrs


18.5 hrs

-O" samples Δ centriruged samples batch equilbrium —ι • r4 6 Total Pore Volumes

Figure 1. Effect of flow interruption on effluent concentrations of PCE during flushing of Oil Creek sand with a 4% solution of POE (20) sorbitan monooleate. 40000-

3 30000-

•

s Ο

• • • • • • ° ^

Α

Δ Δ Δ Ζ ^ +

+

+ + + *

χχχ

•

20000-

3 Ο CO

Flow rate Ο 0.5 mL/min • 1 mL/min Δ 1.5mL/min + 2 mL/min χ 2.5 mL/min batch equilibrium

10000"

o-pooooTotal Pore Volumes

Figure 2. Effect of variations in flow rate on the effluent concentration of PCE during flushing of Oil Creek sand with a 4% solution of POE (20) sorbitan monooleate.


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(13) reported that after injecting less than 2 pore volumes of a 4% Aerosol AY/OT solution, more than 99% of the residual PCE was removed from the column, with 80% being displaced as a separate organic phase. Despite the obvious potential of this recovery approach, the implications of this process on surfactant-based remediation scenarios must be carefully evaluated. Of particular concern is the possible downward migration of DNAPLs through an aquifer formation. Thus, it is essential to develop a means for evaluating the onset and extent of NAPL mobilization during surfactant flushing. To induce NAPL mobilization, the reduction in interfacial tension (IFT) between the aqueous and organic phases must be sufficient to overcome the capillary forces acting to retain organic liquids within a porous medium. The capillary number (Ncâ) and Bond number (NB) are dimensionless groups that can be employed to assess the impact of viscous and buoyancy forces on the mobilization of NAPLs in porous media (13,19, 20). These expressions can be defined as follows: N

c

a

=

j^rwPwI^* = uwSL σ Δχ a ηΛΙ/

N

s

=

(1)

n u /

Apgkk

rw

(2)

Darcy 1500 I- velocity (cm/hr)

300

400

500

600

Volume Injected (mL) Figure 3. Impact of variations in flow velocity and duration of flow interruption on the recovery of dodecane from soil columns containing 20-30 mesh Ottawa sand (adapted from (12)).

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1E-06

1E-05

1E-04 N

N

Ca. B,

N

ca

1E-03 + N

1E-02

B

Figure 4. Tetrachloroethylene desaturation curve for 20-30 mesh Ottawa sand.


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0.09 dyne/cm with the Aerosol AY/OT solution. During this phase of the experiment the Bond number was slightly greater than the capillary number, revealing the major importance of buoyancy forces in the displacement of PCE from the column. The sum of the capillary and Bond numbers required to induce PCE mobilization was ~2 Χ 10" , while almost complete removal was achieved at -1 X 10~3. Predictions based upon the capillary number alone would have been inadequate to characterize the mobilization process. It should be recognized that the critical sum required for mobilization is system specific and may vary by an order of magnitude depending upon the experimental design and the properties of the organic liquid and porous medium. 4


Numerical Modeling Mathematical models can be developed and used to explore the potential impact of various physical and chemical processes on the performance of surfactant enhanced aquifer remediation (SEAR) at the field scale. Such mathematical approaches are necessarily limited by our understanding of processes and their interactions, and our ability to estimate appropriate model parameters. Although numerical simulators have been extensively employed in the petroleum literature to predict the performance of tertiary oil recovery schemes, mathematical models have only recently been applied to SEAR. Taking their lead from the petroleum literature, most of these modeling studies (22-25) have assumed local thermodynamic equilibrium among system constituents. As discussed above, however, there is a growing body of laboratory evidence to suggest that, at least for the micellar solubilization process, mass transfer rate limitations may be important Abriola et al. (26) developed a conceptual model for surfactant enhanced solubilization, which incorporated mass transfer rate limitations. The model was implemented in a one-dimensional numerical simulator and was used to reproduce a series of surfactant column flushing experiments (12). Extrapolation of laboratory scale observations to the field, however, requires a multi-dimensional modeling approach. Multi-dimensional simulators can serve as important tools, permitting exploration of the potential influence of surfactant/organic properties, solubilization kinetics, aquifer formation heterogeneities, and flushing strategies on SEAR performance. The example model simulations presented below are based upon the sequential modeling approach presented by Dekker and Abriola (27). In this approach, the initial introduction and subsequent migration and entrapment of the organic pollutant in the subsurface is treated as an immiscible flow problem and modeled using VALOR, a two-dimensional multiphase flow simulator (28). Following entrapment and redistribution, dissolution and surfactant enhanced solubilization of the organic are then simulated using a twodimensional extension of the model presented in Abriola et al. (26) under the assumption that no further free phase migration of the NAPL occurs. The contrasting time scales of the initial flow and entrapment process and the ensuing solubilization process make it reasonable to treat the two processes independendy. A linear driving force expression is used to model mass transfer between the aqueous and organic phases. Estimates of NAPL-aqueous interfacial area are obtained by assuming spherical NAPL blob geometry, and interfacial area is decreased with time as the NAPL is solubilized. (See also 29-31 for further discussion of the interphase mass transfer model.) As described, the focus of this modeling approach is, thus, enhanced, rate-limited solubilization; simulation of surfactant-enhanced mobilization of the NAPL is precluded. The example model simulations presented below serve to illustrate model capabilities and highlight the influence of heterogeneity on SEAR effectiveness.



2. ABRIOLA ET AL.

Impact ofSurfactant Flushing on DNAPLs

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Consider the DNAPL spill event illustrated in Figure 5. Here a spill of PCE is simulated in a perfectly stratified saturated formation of fine sands. The simulation domain is composed of four layers of aquifer material with two contrasting permeability values kj and fo, where ki

Surfactant-Enhanced Subsurface Remediation - American Chemical

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