‘s ecause of the limitations c pump-and-treat technologj , attention is now focused on the feasibility of surfactant use to increase its efficiency. Surfactants have been studied for use in soil washing and enhanced oil recovery. Although similarities exist between the applications, there are significant differences in the objectives of the technologies and the limitations placed on surfactant use. In this article we review environmental studies concerned with the fate and transport of surface-active compounds in the suhsurface environment and discuss key issues related to their successful use for in situ aquifer remediation, particularly with respect to nonaqueous-phase liquids.
B
Definition The word surfactant is a conbaction of the descriptive phrase surface-active agent. Surfactants are surface active because they concentrate at interfacial regions: airwater, oil-water, and solid-liquid interfaces, for example. The surface activity of surfactants derives from their amphiphilic structure, meaning that their molecules contain one soluble and one insoluble moiety. In aqueous systems, a surfactant has a polar or ionic hydrophilic moiety and a nonpolar hydrophobic moiety, referred to as the head and tail
CANDIDA
C. W E S T
U S . Environmental Protection Agency Ada, OK 74820
JEFFREY H. HARWELL The University of Oklahoma Norman, OK 73029 groups, respectively. One of the most common surfactants, sodium dodecyl sulfate, is a good example of this structure: The dodecyl chain has a very low water solubility whereas the sulfate group has a very high water solubility. Surfactants are classified according to the nature of the hydrophilic portion of the molecule (see Table 1).The head group may carry a negative charge (anionic], a positive charge (cationic], both negative and positive charges (zwitterionic], or no charge (nonionic]. Differences in the chemistry of surfactants due to the nature of the hydrophobic tails (degree of branching, carbon number, aromaticity] are usually less pronounced than those due to the hydrophilic head group. A phenomenon unique to surfactants is the self-assembly of molecules into dynamic clusters called
2324 Envircn. Sci. Techncl., Vol. 26, No. 12, 1992
micelles [see Figure 11. It is important to distinguish that although all amphiphilic molecules are surface active and can be expected to be in excess at interfaces, not all amphiphilic compounds are commonly referred to as surfactants. (One can, of course, insist that hecause every amphiphile is surface active to some extent, all amphiphiles can be called surfactants. This usage, however, is never found in industry, turns the word “surfactant” into a synonym for “amphiphile,” and ignores the differences in the magnitude of the surface activity of true surfactants compared to that of amphiphiles such as alcohols. Therefore, we will follow the industrial convention of restricting the term surfactant to highly surface-active compounds such as those capable of forming micelles.] Though perhaps an oversimplification, a good deal of insight into the properties of micelles can be gained by thinking of them in terms of the “oil drop model.” The micelle is pictured as a 3-4-nm diameter droplet of oil with an ionic or polar coating. Micelle formation occurs above a critical concentration of surfactant monomers, referred to as the critical micelle concentration (CMC],which is different for every surfactant ( 2 ) . CMCs typically range between 0.1 and 10 mM. In a micelle, the individual mono-
0013-936W92/0926-2324$03.0010@ 1992 American Chemical Society
fulvic solutions have been examined (3-5)and observed to be inversely correlated to the acidity of the humate as well as dependent on the solution pH and the source of the humic or fulvic material (6).PicNatural surface-active chemicals colo and Mbagwu (7)likened humic Many studies of naturally occur- acids in their dissociated form to ring surface-active compounds and anionic surfactants and explored their interactions with contami- how their surface activity contribnants in aqueous solution have uted to soil aggregation stability. been published in the environmen- Pramauro and Pellizzetti (8)include tal literature. The amphiphilic re- humic substances in a general disquirements are often fulfilled by hy- cussion of surface-active comd r o p h i l i c , oxygen-containing pounds. Kan and Tomson (9) comfunctional groups joined to hydro- pared Triton X-100 and bovine phobic aliphatic and aromatic moi- serum albumin as model facilitators eties. Humic substances, for exam- of transport by dissolved organic ple, may exist associated with solid carbon ( I O )of naphthalene and matrices or as dissolved aggregates phenanthrene through soil colthat have micelle-like structures (2). umns. Bacteria, yeasts, and fungi have The surface a ’‘ “ ’ hum’- --d
mers are oriented with their hydrophilic moieties in contact with the aqueous phase and their hydrophobic moieties tucked into the interior of the aggregate.
the capability to synthesize surfactants, commonly referred to as biosurfactants. The primary classes of microbial surface-active compounds are glycolipids, amino acidcontaining lipids, phospholipids, fatty acids, peptides, and polymeric biosurfactants. These can be associated with the cell wall or excreted as extracellular material. Excreted surfactants cause emulsification of hydrocarbons in solution, whereas cell-associated surfactants facilitate hydmcarbon uptake (11). General discussions on bioemulsi6cation have been written by Zajic and Panchal (12)and Gutnick and Minas (13).
‘ABLE 1
ixamples of the four surfactant types
r
1
nrimelhyl nonium bromide
3 - ~ 4 k yamino1
on= acid
I of
Zwtttenonic
RI;H,CH,CH~CW-
surfactant rnlcellization Spherical micelle
’I onomers and micelles)
Remediation applications Because pump-and-treat systems are often ineffective for aquifer restoration (14, 15) there is interest in developing new methodologies to improve their efficiency (16). Surfactant-soil interactions are being studied to observe immobilization of contaminants for subsequent in situ treatment, release of contaminants from mineral surfaces as a result of competition, or redistribut i o n of s o r b e d c o n t a m i n a n t s between mobile and immobile organic phases (17-20;Puls, R. W., EPA Robert S. Kerr Environmental Research Laboratory, Ada, OK, personal communication, 1992). The remainder of this discussion will focus on the use of surfactant solutions to remediate nonaqueousphase liquids (NAPLs) in the saturated zone of the subsurface. At many sites, NAPLs exist in the vadose zone and the saturated zone as organic “ganglia”; this is known as residual saturation. If sufficient volume has been released, excess NAPL may also accumulate as free liquid in the subsurface, depending on the density of NAPL and the permeability of boundaries encountered by the NAPL flow. Both the residual saturation and the bulk free phase represent long-term sources of dissolved groundwater contamination. New remediation technologies must be directed at removing these sources (21,22).The use of surfactants to remediate such sites is being examined in research laboratories (23-26)and in the field (27-29). There are two general mechanisms by which surfactants can enhance the removal of NAPL sources from the saturated zone. From an engineering standpoint, the easiest to apply is solubilization, which is a result of micelle formation. The second mechanism, mobilization of NAPLs, depends on the tendency of
Environ. Sci. Technoi.. VoI. 26. No. 12. 1992 2325
surfactants to lower interfacial tension. Solubilization. In pump-andtreat remediation, the amount of contaminant removed from the subsurface with each volume of groundwater pumped to the surface depends (in part) on the aqueous solubility of the contaminant. When surfactant is added to the aqueous phase, the organic interior of micelles acts as an organic pseudophase into which organic contaminants can be partitioned. This phenomenon is called solubilization (30) and can be likened to the capacity of humic and fulvic acid solutions to solubilize hydrophobic organic solutes (32-34). In an aqueous system, the extent to which a solute will concentrate in a micelle can be related to the octanol-water partition coefficient (KO,)of the solute (35-37). In general, the larger the KO, of a solute the greater will be its tendency to concentrate inside the micelle. Thus, micelles in a solution represent an increased capacity of the mobile aqueous phase for the organic solute over that of pure water (38). Enhanced solubility of contaminants has been described using a two-phase separation model for solute behavior ( 3 9 , expressed as S,*/S, (the ratio of the apparent aqueous solubility to the true aqueous solubility). This is an adaptation of a pseudophase micelle model by Shinoda and Hutchison (40), expressing the solubility of an organic compound as the sum of its concentration in the monomer solution and the micelles. The increase in the mobile-phase solubility of a contaminant in a surfactant solution can be dramatic. For example, the water solubility of trichloroethylene (TCE) at 15 "C is approximately 7.5 mM (980 mg/L) (41). In a 0.5 M solution of sodium dodecyl sulfate at 15 "C, TCE solubility is 150 mM (19,600 mglL) (Sabatini, D. A.; Harwell, J. H., University of Oklahoma, unpublished data), which represents a 20-fold increase in the mass per volume of water produced from an extraction well (assuming equilibrium conditions prevail). Mobilization. Surfactant-enhanced remediation can also be based on mobilization of the residual NAPL. This is the phenomenon on which surfactant-enhanced oil recovery work was based (42). Mobilization has greater potential than solubilization to increase the rate of remediation, but can be riskier because
of the movement of free-phase liquid. In the saturated zone, the interface between the water-wet soil surface and NAPL is characterized by NAPL-water interfacial tension (IFT). The forces that trap organic liquids are dominated by capillarity (adhesive-cohesive forces), which is proportional to the IFT at the liquid (NAPL)-liquid (water) interface. An excellent discussion of capillary trapping is included in a report by Wilson et al. (43). When the NAPL-groundwater IFT is high, a large pressure drop per unit of distance (hydraulic gradient) between the injection and extraction wells, APIL, is required to push a residual droplet of NAPL out of its pore space. Typical values of IFT for NAPLs a n d w a t e r a r e 3 0 - 5 0 dynesIcm (43). The product of the IFT multiplied by the increase in interfacial area that would result from the mobilization of the NAPL droplet represents an expenditure of energy. If the IFT were lower, less energy would be required to form the new interfacial area and to push the residual saturation from the pore, and mobilization would occur at lower values of APIL. The ratio [(APIL)IyNw](where yNW is the NAPL-water IFT) is proportional to the capillary number, Nc. In experiments to determine the value of AP/L required to displace a residual saturation of a NAPL from a pore space, it has been found that when SN/SNR (the ratio of the fractional NAPL saturation after flushing with water to the residual NAPL saturation before flushing with water) is plotted as a function of Nc, there is a critical value, N*,, at which SN approaches zero; N*, is found to be constant whether it is reached by increasing APIL or by decreasing yNw (44, Figure 2). Design of a surfactant system for mobilization of a trapped phase thus begins with designing a system to create an ultralow IFT, less than dynelcm ( 2 ) . It is critical that the surfactant structure achieve a proper balance of hydrophilic and hydrophobic interactions at the interface to achieve large reductions in the IFT (45). This is distinct from solubilization, in which it is only necessary that surfactant micelles be present and remain stable in the aqueous phase. In order to attain an ultralow IFT, the surfactant (or surfactant mixture) must be matched to the nature of the trapped phase ( 2 , 45-47). Often a surfactant system (a surfactant
2326 Environ. Sci. Technol., Vol. 26,No. 12, 1992
plus a low concentration of a cosurfactant) is developed to optimize the efficiency and stability of the system. Two methods can be used to examine the appropriateness of a surfactant for a specific contaminant: the hydrophilicllipophilic balance (HLB)method and the Winsor method. HLB method, The required surfactant to remediate a particular organic contaminant can be obtained from an empirical scale called the HLB scale. The HLB scale was designed for matching surfactant structure to an organic chemical to be emulsified in water. Each surfactant has an HLB number indicative of the types of oils it can emulsify. The more water soluble the surfactant, the higher the HLB number. Most surfactant manufacturers supply their surfactants' HLB numbers. The HLB requirement of an organic chemical is directly related to its hydrophobicity; as hydrophobicity decreases (more water soluble) the HLB requirement increases. For example, the water solubility of dodecane is less than that of dodecanol, which is less than that of dodecanoic acid, and their HLB requirements are 10, 14, and 16, respectively. Generally, as the KO, of the organic increases (water solubility decreases), the HLB requirement decreases. The HLB method is not adequate for selecting surfactants because the impact of temperature and electrolytes on surfactant performance have not been quantitatively incorporated into the HLB method. Winsor systems and parameter diagrams. During the last 20 years, a systematic approach for designing surfactant systems to produce ultralow IFTs has been developed (45, 48-50) and is readily adaptable to aquifer conditions. The basis of this approach is the observed relationship between IFT and the formation of a middle-phase microemulsion (also called a Winsor Type I11 microemulsion) (Figure 3) (adapted from Reference 1; 52-52). A microemulsion.is a type of emulsion that is a true thermodynamic phase. In a Winsor Type I system, the surfactant is too water soluble and is in the form of oil-swollen micelles in the aqueous phase. In the Winsor Type I1 system, the surfactant is too oil soluble and virtually all the surfactant is found in the form of water-swollen reverse micelles in the oil phase. The Winsor Type I11 system is said to be balanced, the surfactant
Correlation of retidual NAPL saturation with capillary number and mobilization
r
having nearly equal affinity for both phases. This forms a new phasethe middle-phase microemulsionthat contains almost all of the surfactant and large quantities of the organic chemical and water. The volume of organic chemical per unit weight of surfactant in the middle phase is called the solubilization parameter. The IFT is reduced when the middle phase is formed, is minimized when approximately equal volumes of the organic chemical and water are used in forming the middle phase, and decreases as the solubilization parameter increases. A strategy for designing a balanced system was developed for use in enhanced oil recovery. The major element of this systematic approach is the three-parameter diagram (45, 46,50)such as that in Figure 4. This particular type of three-parameter diagram is called a salinity scan. The abscissa of the salinity scan is the weight of NaCl per liter of the system (oil plus water). The ordinate is the weight (by percent) of surfactant and cosurfactant in the system. The solid curves that run from the upper left to the lower right are boundaries between Winsor Type I or I1 systems and Winsor Type 111 systems (the region between the curves] and are identified experimentally. The dashed line through the center of the threephase region is the point at which
1
equal volumes of oil and water are incorporated into the middle-phase microemulsion. The intersection of the curves is the point at which only one phase is present: at this point, all of the water and oil in the system has been solubilized into a microemulsion phase, the solubility parameter is at a maximum, the interfacial tension between the oil and the water phases goes to zero, the capillary number goes to a maximum, and the oil is mobilized. There are many parameters that can be varied in a three-parameter diagram. In Figure 4 the ratios of the surfactant to the cosurfactant and of the brine (NaCl and water) to oil are kept constant, and the NaCl concentration is varied. A modification of this diagram could be used to obtain an ultralow NAPL-groundwater IFT.For instance, the NaCl concentration could be kept constant and the ratio of surfactant to cosurfactant varied. It is critical, however, that the phase behavior of the system be studied at the aquifer temperature because surfactant phase behavior can be highly temperature dependent. Surfactant system stability Successful surfactant use to enhance remediation goes beyond selection of a surfactant (or surfactant system) that will efficiently solubilize or mobilize NAF'Ls. The surfac-
tant must also be matched to the subsurface conditions so that it remains active. We believe that apparent failures of early attempts to use surfactants to enhance pump-andtreat remediation may have been the result of choosing surfactants incompatible with the contaminated medium. Several aspects of surfactant behavior should be taken into account. Surfactant sorption and precipitation. Surfactants that go to organic liquid-water interfaces can also be expected to go to solid-liquid interfaces in the system (I, 53-61). Surfactant adsorption is a threat to the success of surfactant-enhanced remediation because it reduces the active concentration of surfactant. Sorption of surfactants on inorganic surfaces may be significant because surfactant aggregation may occur at the solid-liquid interface. At very low concentrations, adsorbed surfactant monomers begin to aggregate and form micelle-like structures called admicelles or hemimicelles, depending on whether the aggregates have one or two surfactant layers (Figure 1). Once these structures form on the solid surface, sorption of additional surfactant may rapidly increase until a complete bilayer of surfactant covers the solid surface. The tendency of the surfactant to form admicelles or hemimicelles is dependent on interactions between the hydrophilic moiety of the surfactant and the solid surface. When there is hydrogen bonding, as between ethoxylated nonionics and silicon oxide surfaces, sorption may be either high or low, depending on the water solubility of the surfactant. The higher the water solubility of the surfactant, the lower the tendency for adsorption. When there is an attractive coulombic interaction between the head group and the surface, as is the case with cationic surfactants and aquifer solids (which are generally negatively charged), a complete bilayer of surfactant may cover the surface. However, when the coulombic interaction is repulsive, as between an anionic surfactant and silicon oxide surfaces, sorption may be very low, especially at low electrolyte concentrations. Surfactant selection for remediation should attempt to minimize surfactant adsorption to aquifer sediments. Although anionic surfactants CM be expected to sorb less than nonionics to most mineral surfaces, they are subject to losses by precipitation (62-65).Because of their abil-
Envimn. Sci. Technol.. Vol. 26, No. 12. 1992 2327
/
ity to form micelles, the precipitation behavior of ionic surfactants is dramatically different from that of simple inorganic salts. Any factor that lowers the CMC of a surfactant system will decrease the susceptibility of the surfactant to precipitation. For example, mixtures of ionic and nonionic surfactants often have a lower CMC and consequently a higher tolerance for ions that would precipitate the ionic surfactant by itself. Surfactant solutions are also sensitive to temperature. The temperature at which the solubility of an ionic surfactant becomes equal to the CMC of the surfactant is known as the Krafft point. Below the Krafft point, there are very low concentrations of monomers and no micelles are formed. At temperatures higher than the Krafft point, the surfactant’s solubility increases dramatically because the monomer concentration is high enough for micelle formation. The micelles act like a separate phase into which the surfactant can partition with little increase of surfactant activity in solution. Groundwater systems cause concern because the temperature can drop below the Krafft point of the surfactant components and that can cause the surfactant concentration
to drop below the CMC, thereby rendering the surfactant useless. It is possible, however, to lower the Krafft point of a surfactant structure to make it more tolerant of groundwater conditions. This is done by branching the hydrophobic moiety, increasing the bulkiness of the hydrophilic moiety, or by using a cosurfactant. Surfactant losses into trapped residual phases. Nonionic surfactants can partition into trapped residual phases if their solubilities in the NAPL are high. Additionally, nonionic surfactants are subject to coacervation, the formation of a separate surfactant-rich aqueous phase and an aqueous phase at a high critical temperature called the cloud point. The cloud point can be noted by the development of solution turbidity. Some nonionic surfactants also have a low critical temperature that is very much like the Krafft point temperature of ionic surfactants. Because nonionic surfactants are susceptible to phase trapping when either critical temperature is approached, it is imperative to understand the phase behavior of the surfactant system under the aquifer conditions. Ionic surfactants are generally insoluble in nonaqueous liquids, although their divalent salts may have signif-
2328 Environ. Sci. Technol., Vol. 26, No. 12, 1992
icant solubility in the nonaqueous phase. Chromatographic separation of surfactant mixtures. When a surfactant solution is injected into a reservoir, it tends to separate chromatographically into individual components. If the system has been formulated to produce an ultralow IFT at conditions existing in the aquifer, then as the formulation is changed by chromatographic separation of the system components, it will no longer be an optimum formulation and may not mobilize the residual organics. Mixed surfactant behavior. The tendency of surfactants to precipitate, sorb, and form coacervate phases can be reduced by using mixtures of surfactants. This effect is so important that most commercial and industrial surfactant applications involve several surfactant types or several isomers of a particular surfactant type. Mixed surfactant behavior can be expected to play an important role in the development of commercially viable surfactant systems for enhanced remediation processes (66). Surfactant acceptability. A key issue in the use of surfactants in remediation of aquifers is the recalcitrance and toxicity of surfactants in the subsurface environment and
possible interference of active bic degradation of contaminants. In th, past, studies of surfactant transport and fate in the environment concentrated on the impact of anthropogenic detergents because they are ubiquitous (67).Many of these studies were confined to surface receiving waters (68).Extensive toxicological data on aquatic animals exist primarily for the anionic surfactants (69). Cationic surfactants are known to be potent germicides (70)and are toxic in the mglL range to a wide variety of aquatic organisms, although their toxicity may be mitigated by their high reactivity to solids (71, 72).It has been proposed that the use of naturally occurring surfaceactive compounds for mobilization of contaminants would allay these concerns (34).Biosurfactants may have advantages over synthetic chemical surfactants in that they are readily biodegradable; many are tolerant to wide variations in temperature, pH, and salt concentrations (73);they may be produced in situ (74);and, in some cases, they are cheaper to produce than synthetic surfactants. It has been reported that the presence of some surfactants decreases the rate of biodegradation of a contaminant or stops it altogether (75, 76);in other cases, the presence of a surfactant enhances the rate of biodegradation (77,78).It is not clear which result is desirable without knowing the role that contaminant biodegradation plays in a given remediation plan. Concluding remarks Surface-active compounds are ubiquitous in the subsurface. They are present as a result of indigenous biological activity and disposal of waste products and effluents, and are now being considered for use in remediation of aquifers. Terms to describe the environmental chemistry,hydrology, and transport mechanisms of surface-active compounds are being adapted for use in the field of subsurface remediation. It is important to understand these processes for successful aquifer remediation by subsurface injection of surfactants. To date, much of the environmental research on surfactants has been concerned with the efficiency of surfactant solubilization (soil washing], and little has been done to predict the behavior and ultimate fate of these compounds in aquifer environments.
Winsor Type II systems
Winsor Type 111 systems
Boundary between
The viability of a surfactant-based remediation process depends on selecting surfactants for optimum efficiency (minimizing losses to sorption, precipitation, a n d phase changes), environmental acceptability (yet to be defined), and balanced biological degradation. The cost of using surfactants versus alternative remediation technologies must be balanced against the differences i n contaminant recovery. Most importantly, it must be ascertained that the use of surfactants in subsurface remediation will not add to the deterioration of groundwater.
Candido C. West is an environmental research scientist with EPA at the Robert S. Kerr Environmental Research Labomtory in Ada, OK. She holds a Ph.D. from the Department of Environmentol Science and Engineeringat Rice University, Houston, TX. Her research interests focus on the influence of natuml organic matter and synthetic surfactants on the tmnsport and fate of organic contaminants in subsurface environments.
Acknowledgments We would like to express our appreciation for the views and suggestions of John Westall of Oregon State University and David Sabatini of the University of Oklahoma. The information in this document has been subjected to administrative review by the United States Environmental Protection Agency. It does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Jeffrey H. Harwell is a professor of chemical engineeringand director of the School of Chemical Engineering and Materials Science at the University of OWahoma. He directs research involving fundamental properties and industriol applications of surfactants. He received a B.S. degree in chemistryand an M . S . degree in chemical engineering from Texas A.9M University and a Ph.D. in chemical engineering from the UN'versity of Texas at Austin. He received the Victor K. M e r Award of the American Chemical Society in 1983 and was co-chairman of the 65th Colloid and Surface Science Symposium, held in 1991.
Envimn. Sci. Technol.. Vol. 26, NO.12. 1992 23ZS
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