&EPA
United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Ada, OK 74820
Research and Development EPA/600/S-96/002
August 1996
ENVIRONMENTAL
RESEARCH BRIEF
Surfactant-Enhanced DNAPL Remediation: Surfactant Selection,
Hydraulic Efficiency, and Economic Factors
David A. Sabatini1-3, Robert C. Knoxu, and Jeffrey H. Harwell2-3
INTRODUCTION
Chlorinated hydrocarbons are ubiquitous ground water
contaminants due to their widespread use as organic
solvents and cleaners/degreasers. The immiscibility of
chlorinated organics with ground water causes them to
exist as nonaqueous phase liquids (NAPLs); this results in
their occurrence in the subsurface as residual and free
phases. Having a density greater than water, they are often
referred to as dense nonaqueous phase liquids (DNAPLs).
Water solubilities of these chlorinated hydrocarbons are
frequently several orders of magnitude above their drinking
water standards, yet low enough to limit dissolution during
pump-and-treat remediation. Remediation of residual DNAPL
contamination can require hundreds to thousands of
flushings of the ground water (pore volumes) using
conventional pump-and-treat methods. Strongly sorbing
(hydrophobic) compounds will experience a similar fate
(e.g., PAHs, PCBs). The inefficiency of conventional pump-
and-treat methods for these contaminants has recently
been addressed, with surfactants being mentioned as a
promising technology for enhancing conventional
approaches (Haley, et al. 1991; Palmer and Fish, 1992).
Two obstacles to widespread implementation of surfactant-
enhanced subsurface remediation are (1) gaining regulatory
1School ofCivil Engineering and Environmental Science
2School of Chemical Engineering and Materials Science
3The Institute for Applied Surfactant Research
The University of Oklahoma, Norman, OK 73019
approval for the injection of surfactants, and (2) the
economics as impacted by surfactant costs (suggesting
minimization of hydraulic and physicochemical surfactant
losses). The objective of this brief is to present research
results addressing these two issues, as further delineated
below. Of course, proper surfactant selection is imperative
to successful enhancement of DNAPL extraction; factors
affecting surfactant selection arediscussed inthisdocument.
Gaining regulatory approval is an obstacle common to all
chemical amendments being considered for subsurface
remediation. Surfactants with U.S. Food and Drug
Administration direct food additive status and which are
commonly used in consumer products are the focus of this
research. The economics of surfactant-enhanced
remediation processes will potentially be limited by
surfactant losses in the subsurface, including hydraulic
losses to uncontaminated portions of the aquifer and
physicochemical losses (e.g., sorption, precipitation).
Hydraulic approaches for minimizing surfactant losses are
presented, including discussion of vertical circulation wells.
Finally, results of a preliminary economic analysis for
surfactant-enhanced DNAPL remediation are presented,
along with suggestions for optimizing the economics of this
technology.
BACKGROUND
Surfactants (surface-active-agents) accumulate at surfaces or
interfaces, a result of the dual nature of surfactant molecules.
Surfactant molecules have hydrophobic (water disliking) and
hydrophilic (water liking) moieties. Surfactants are commonly
utilized in detergents and food products to alter the surface
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chemistry of the system in a desirable manner. Above a certain
concentration surfactant molecules self-assemble into aqueous-
phase spherical aggregates with the hydrophobic portions of the
molecule in the interior of the aggregate and the hydrophilic
portions at the exterior. This aggregate is referred to as a micelle
and the surfactant concentration above which micelles form is
referred to as the critical micelle concentration (CMC).
Surfactants can improve subsurface remediation by solubilization
and mobilization. Solubilization significantly increasesthe aqueous
concentration of the contaminant via micellar partitioning, thereby
reducing the number of pore volumes which must be pumped to
extract the DNAPL. Optimal mobilization utilizes ultra-low
interfacial tensions (evidenced concomitant with formation of
middle-phase microemulsions) to significantly reducethe capillary
forces trapping the DNAPL, thereby allowing the oil to be readily
extracted with the water.
Solubilization enhancementfor neutral organic compounds results
from the partitioning of the contaminant into the hydrophobic
core (oil-like center) of the micelle. Two parameters that
describe this process are the molar solubilization ratio (MSR) and
the micellar-water partition coefficient (KJ. Given a graph of
aqueous contaminant concentration versus surfactant
concentration, the molar solubilization ratio (moles of contaminant
per mole of surfactant) is the slope of the straight line portion of
the plot above the CMC. The micellar-water partition coefficient
(K ) is the molar ratio of the contaminant in the micellar phase
divided by the molar ratio of the contaminant in the aqueous
phase, andean be determined given values of MSR, contaminant
water solubility, and the molar concentration of water (Shiau et
al., 1994; Edwards et al., 1991).
Optimal mobilization requires alteration of the surfactant system
to produce a middle-phase microemulsion; these systems have
the potential to elute the residual saturation in several pore
volumes due to significant reductions in interfacial tensions.
Micellar systems transition from normal to swollen micelles
(WinsorType I), to middle-phase systems (WinsorType III), and
finally to reverse micelles (Winsor Type II system, surfactants
reside in the oil phase) as properties of the surfactant system are
varied. As the surfactant goes from the water phase (hydrophilic)
to the oil phase (lipophilic), the hydrophilic-lipophilic balance
(HLB) decreases. Thus, surfactants with high values of HLB are
highly water soluble. As shown in Figure 1, fora very hydrophilic
surfactant system (right side of figure), the surfactant resides in
the water phase as micelles, and a portion of the oil phase
partitions into the micellar phase. For a very lipophilic surfactant
system (the left side of the figure), the surfactant resides in the
oil phase as reverse micelles. Intermediate between these
extremes, a third phase appears which consists of water, oil and
surfactant; this is the middle-phase system (so designated by
virtue of its intermediate density). As shown in Figure 1, these
phase transitions can be achieved for ionic surfactants by
adjusting the salinity or hardness of the aqueous phase (commonly
utilized in surfactant-enhanced oil recovery — Bourrel and
NfT
Hep
law
tnll ''lU-Ha
-'T'p M.siP
HLO M
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Schechter, 1988). However, introduction of high salt
concentrations is not desirable in aquifer restoration as remediation
of brine contamination is also a difficult problem. In this research,
middle-phase systems are achieved by altering the HLB of a
binary surfactant system.
Surfactant-enhanced environmental remediation research to
date has evaluated a wide spectrum of issues (e.g., Valsaraj and
Thibodeaux, 1989; Vignon and Rubin, 1989; Edwards et al.,
1991; Smith and Jaffe, 1991; Jafvert and Heath, 1991; Peters et
al., 1992; West, 1992; West and Harwell, 1992; Abdul et al.,
1992; Fountain, 1992; Pennel et al., 1993; Baran et al., 1994;
Sabatini et al., 1995). The objective of this document is to
discuss the technical feasibility and limitations of using food
grade surfactants for solubilization and mobilization of chlorinated
solvents, describe physicochemical and hydraulic approaches
that minimize surfactant losses while not compromising
remediation enhancement, and present a preliminary evaluation
of the economics of implementing surfactant-enhanced
subsurface remediation. This document emphasizes the
importance of proper design of the surfactant system and
hydraulic regime for the technical and economical feasibility of
surfactant-enhanced subsurface remediation.
MATERIALS AND METHODS
The chlorinated organics evaluated in this research were
tetrachloroethylene (PCE), trichloroethylene (TCE), and trans-
1,2-dichloroethylene (1,2-DCE). Table I summarizes
characteristic parameters for these contaminants. These
compounds were selected based on their ubiquitous occurrence
as subsurface contaminants and their range in hydrophobicity
(as noted by their relative K^ values). The surfactants evaluated
in this research were selected based on their status as FDA direct
food additive compounds and their relative HLB values. The food
grade surfactants are largely combinations of fatty acids and
sugars. The S-MAZ surfactants and T-MAZ surfactants are
sorbitan esters and ethyloxylated sorbitan esters, respectively
(with ethylene oxide groups ranging from 0 to 80). Table II
summarizes properties of select surfactants evaluated in this
research. Surfactant solubilization, mobilization, precipitation
and sorption studies were conducted according to standard
procedures (as documented in Shiau et al., 1994, 1995a,b;
1996a,b; Rouse etal., 1993; 1995; 1996). Sorption assays were
conducted utilizing the Canadian River alluvium (CRA) material
which consists of 72% sand, and 27% silt and clay, and has an
organic carbon content of 0.07%.
SURFACTANT SELECTION FOR OPTIMIZING
SUBSURFACE REMEDIATION
Solubilization of chlorocarbons was evaluated using single
surfactant systems (SDS and three of the T-MAZ surfactants).
Solubilization curves for the three chlorinated organics (PCE,
TCE, 1,2-DCE) with T-MAZ 60 are presented in Figure 2. The
aqueous phase concentration of the chlorinated organics
increases linearly above the CMC. However, at higher surfactant
concentrations (exceeding the surfactant solubility limit) negative
deviations from this linear trend are observed (Shiau et al.,
1994). These results illustrate several points: (1) below the
CMC, surfactant addition has little to no effect on the solubility of
these contaminants, and (2) the higher the surfactant
concentration is above the CMC the greater is the chlorocarbon
solubility enhancement due to the increased number of micelles
(i.e., operate well above the CMC to achieve maximum
enhancement, but below the surfactant's solubility limit). From
Figure 2 the 1,2-DCE solubility enhancement is approximately
two-fold at 10 mM T-MAZ 60 and approximately eight-fold at
50 mM T-MAZ 60; the enhancement approaches two orders of
magnitude for PCE. The salinity of these systems was less than
30 mg/L and the hardness was 16 mg/L (this is true throughout
this section unless noted otherwise).
Solubilization parameters (MSR and Km, on a molar basis) are
summarized in Table III for SDS, T-MAZ 28, T-MAZ 20, and T-
MAZ 60. The data in Table III demonstrate that more hydrophobic
compounds (e.g., PCE) will realize the greatest enhancement
(higher Km indicates greater contaminant partitioning into micelles).
Km is observed to vary more between contaminants for any
surfactant than between surfactants for a given contaminant.
Thus, for the solubilization mechanism, surfactant selection is
relatively independent of the contaminant(s) and will most likely
be made based on factors such as cost, susceptibility to losses,
toxicity. At the same time, when estimating (extrapolating) Km
values or when modeling the solubilization process, it should be
recognized that micellar solubilization (and thus Km) varies as a
function of contaminant type (nonpolar versus polar/ionic) and
aqueous contaminant concentrations belowtheir water solubility
(Nayyaret al., 1994; Rouse etal., 1995). Aqueous contaminant
concentrations below water solubility may be experienced due to
nonequilibrium solubilization, mixed NAPL phases, etc. (Pennell
et al., 1993; Rouse et al., 1995; Pennel et al., 1994; Soerens,
1995).
•"• 350
0 300
O 250
K 200 -
s
1
2 100 .
i
•
^ ',
•' « *
...:: - "
10
15 20 25 30
T-MAZ 60 CONC. (mM)
35
40
50
1.2DCE
* TCE
PCE
Figure 2: Solubilization of DNAPLs (PCE, TCE, 1,2-DCE) in T-MAZ 60 @
15°C, After Shiau etal., 1994a. T MAZ 60 CMC = 0.02 mM.
Molecular Weight: PCE, 166mg/mM; TCE, 131 mg/mM;DCE,
97 mg/mM; and T-MAZ 60, 1310 mg/mM. Data Shown is
Average of Replicates, Errors within Size of Symbols.
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Table I: Contaminant Properties
Chemical Molecular
Formula
Tetrachloroethylene(PCE) C2CI4
Trichloroethylene(TCE) C2HCI3
Trans-1 ,2-Dichloroethylene (DCE) C2H2CI2
Molecular
Weight
166
131
97
Aqueous
Solubility (mg/L)
200
1100
6300
log K«a
2.6
2.38
0.48
aK = octanol-water partition coefficient
After Shiauetal. (1994)
Table II: Surfactant Properties
Surfactant
MWa
Type"
HLBC
SMDNSd
Sodium Dodecyl Sulfate (SDS)
POE (80)e sorbitan monolaurate (T-MAZ 28)
POE (20)e sorbitan monolaurate (T-MAZ 20)
POE (20)e sorbitan monostearate (T-MAZ 60)
Aersol OT^AOT)
POE (20)e sorbitan tristearate
Sodium stearoyl-2-lactylate
Sorbitan monooleate
Sorbitan trioleate (S-MAZ85K)
260
288
3866
1266
1310
445
1842
450
428
956
A
A
N
N
N
A
N
A
N
N
>40
40
19.2
16.7
14.9
NA9
10.5
10
4.6
2.1
*MW= molecular weight
bA = anionic, N = nonionic
cHLB=hydrophilic-lipophilic balance
dSMDNS = Sodium mono and dimethyl naphthalene sulfonate
e() = number ofethylene oxides
'Bis-2-ethylhexyl sodium sulfosuccinate
gNA = not available, but included in table in order of HLB (i.e., between 14.9 and 10.5)
After Shiauetal., 1994
Table III: Solubilization Parameters for Chlorinated Organics
Chlorinated Organic
Surfactant
MSR Log Km
PCE
TCE
1,2-DCE
After Shiauetal., 1994
SDS
T-MAZ 28
T-MAZ 20
T-MAZ 60
SDS
T-MAZ 28
T-MAZ 20
T-MAZ 60
SDS
T-MAZ 28
T-MAZ 20
T-MAZ 60
0.39
0.45
2.27
3.15
0.34
1.68
3.29
3.95
1.37
2.46
7.49
6.91
4.5
4.55
4.9
4.94
3.27
3.66
3.75
3.77
2.76
2.85
2.95
2.94
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Initial efforts to achieve middle-phase systems, without
consideration of surfactant structure, were unsuccessful. The
HLB of the surfactant systems was varied from 2.1 to 40.
Although phase inversion was observed in this HLB range (Type
II to Type I), a clear middle-phase was not achieved in the
transition (instead a mesophase was realized). Using the
branched surfactant Aerosol OT (AOT) and sodium mono- and
di-methyl naphthalene sulfonate (SMDNS), a middle-phase
microemulsion was realized in the transition region. Thus, it is
observed that surfactant selection (structure) is critical to achieving
middle-phase systems.
Mobilization with AOT and SMDNS was achieved by varying the
SMDNS concentration while holding the AOT concentration
constant. Figure 1 shows a phase diagram for 1,2-DCE using
AOT and SMDNS as the surfactant system. At low SMDNS
concentrations a Type II system is realized (surfactant has
partitioned into the oil phase). Increasing the SMDNS
concentration diminishes the potential for AOT to partition into
the oil, thereby allowing it to accumulate at the interface and
produce a middle-phase system. At yet higher SMDNS
concentrations, the system is over-optimized and the surfactants
reside in the water phase (Type I system). Middle-phase
systems were achieved via this approach for PCE, TCE, and 1,2-
DCE individually (Shiau et al., 1994) and in binary and ternary
mixtures of these chlorocarbons (Shiau et al., 1996a).
Salinity and hardness can affect the formation of middle-phase
systems. Potential impacts of naturally occurring hardness on
middle-phase systems are illustrated in Figure 3 (TCE middle-
phase systems at two levels of calcium (hardness)). Higher
hardness levels required more SMDNS to maintain the surfactant
balance and achieve a middle-phase system (the optimal SMDNS
concentration is higher). The increased calcium concentration
will tend to drive the ionic AOT into the oil phase, thus requiring
additional SMDNS to achieve the middle-phase microemulsion.
Similarly, lower temperatures increased the SMDNS
concentrations necessary to obtain middle-phase systems (Shiau
et al., 1994). As expected, surfactant systems capable of
achieving middle-phase systems are a function of the DNAPL
(forsingle component) or DNAPLcomposition (for multicomponent
oils). For example, the optimal SMDNS concentration for 0.5%
AOT with PCE, TCE, and 1,2-DCE is 1.40, 2.43 and 2.19 wt%,
respectively (Shiau et al., 1994).
For multicomponent residual phases, the optimal surfactant
concentration is a function of the mole fraction of each phase in
the mixed waste and the optimal SMDNS concentration for each
individual phase. Figure 4 illustrates this by considering a ternary
DNAPL system with a constant mole fraction of 1,2-DCE (0.5050)
and a variable mole fraction of PCE and TCE (the mole fraction
of PCE is shown on the ordinate and the mole fraction of TCE is
equal to 0.4950 minus the PCE mole fraction). Thus, an ordinate
value of 0 corresponds to a binary system of TCE and 1,2-DCE
while an ordinate value of 0.4950 corresponds to a binary system
of PCE and 1,2-DCE. The abscissa corresponds to the average
equivalent weight of the surfactant system ([SMDNS]* plus AOT,
where AOT is constant at 0.5 wt%; these units are utilized to
facilitate the regular solution analysis). The ideal solution
analysis is based on ideal mixing while the regular solution plot
accounts for nonidealities (parameters for the ternary regular
solution predictions were determined independently in three
binary systems). As can be seen in Figure 4, regular solution
theory is better able to predict phase behavior in the ternary
system than ideal solution theory (Shiau et al., 1996a). It is
especially encouraging that, given analyses from binary systems,
one can better predict results for ternary systems of variable
DNAPL composition.
2.3 2.4 2.5
WT% of SMDNS ADDED
0.5
0.45 -
0.4 -
0.35 -
0.3 -
0-26 -
0.2 -
0.16 -
0-1 -
0.05 -
0
AVE. EQUIL WT. FOR PCE/TCE/DCE MIXTURE
AOT = 0.5 WT%, W/0 = 1, OIL = PCE/TCE/DCE
MOLE FRACTION
OF DCE(Xd) = 05050
282 284 286 268
AVE. SURFACT. EQUIL. WT. FOR MIXTURE
n [Ca] = 16 mg/l (gw)
[Ca] = 478mg/l
• REGULAR SOLUTION
- IDEAL SOLUTION # EXPERIMENTAL
Figure 3: Phase Diagram for TCE at 15°C for AOT and SMDNS for Two
Calcium ValuesfSMDNS* Optimal Concentration at Each Calcium
Level), Adapted from Shiau etal., 1994.
Figure 4: Phase Diagram for Ternary DNAPL System for Constant Mole
Fraction of 1,2-DCE and Varying Mole Ratio of PCE and TCE.
-------
These results illustrate the sensitivity of middle-phase systems
to aquifer conditions (temperature, hardness, etc.), contaminant
composition, and surfactant structure, and should alert users to
potential problems if proper surfactant selection and design
efforts are not utilized when implementing mobilization via
middle-phase microemulsions. Also, vertical migration of released
residual may be realized depending on aquifer conditions and the
hydraulics of the extraction system. This again illustrates the
care that must be taken in utilizing mobilization. Despite these
limitations, mobilization has the potential to be significantly more
efficient than solubilization and should not be prematurely
dismissed as a viable technology. In fact, vertical circulation
wells can potentially mitigate and even take advantage of any
tendencies for vertical migration. Both of these issues are
discussed below in more detail.
In comparing the efficiency of the solubilization and mobilization
mechanisms, Table IV documents the enhancements resulting
from these two mechanisms for a similar weight percent of
surfactant (via solubilization with T-MAZ 60 and mobilization with
Aerosol OT and SMDNS). As observed in Table IV, the
enhancement is two orders of magnitude for PCE via solubilization
and three and one-half orders of magnitude via mobilization
(relative to water alone). For 1,2-DCE the enhancement by
solubilization is approximately one order of magnitude, while
being two orders of magnitude for mobilization. The increase in
efficiency via mobilization versus solubilization is dramatic. The
data in Table IV also demonstrate that surfactant-enhanced
subsurface remediation will be of the greatest benefit for more
hydrophobic compounds.
In Figure 5 solubilization and mobilization results from one-
dimensional column studies are presented (Shiau et al., 1995b).
Mobilization (AOT/SMDNS) achieved higher contaminant
concentrations and eluted the PCE more quickly than solubilization
(T-MAZ-60) (> 97% extracted in ca. 3 pore volumes). The tail on
the solubilization curve indicates the reduced extraction rate and
thus slow approach to complete PCE elution for solubilization
(likely due to interfacial area constraints); while ca. 85% of the
PCE is eluted within 10 pore volumes, less than 90% has been
eluted by 30 pore volumes. Again, the potential advantages of
mobilization over solubilization are apparent. From Figure 5, the
aqueous PCE concentration prior to surfactant introduction is
observed to be 80 mg/L. Based on this initial concentration,
Table IV: Comparison of Solubilization and Mobilization
1000000 r
Chlorinated GW>
Organic Solubility
(mg/L)
Solubilization Mobilization11
(6.5wt%T-MAZ60) (5.0 wt% AOTand
(mg/L) SMDNS)(mg/L)
PCE 80.6
TCE 990
1,2-DCE 5,340
16,900
14,700
37,800
619,000
594,000
557,000
*GW= Ground Water
''Surfactant concentration is based on initial aqueous volume;
contaminant concentration is in middle phase system.
After Shiau etal., 1994
100000
i
o
o
o
UJ
o
a.
10000
1000
*
*
10 15 20 25
RELATIVE PORE VOLUME
30
35
AOT/SMDNS
* T-MAZ-60
Figure 5: Column Results for PCE Elution via Solubilization (T-MAZ 60)
and Mobilization (AOT/SMDNS) Systems with CRA Medium.
water dissolution would require in excess of 400 pore volumes to
dissolve the residual PCE. Considering reduced mass transfer
as the interfacial area decreases, the actual number of pore
volumes for water dissolution is expected to be much greater
than 400 pore volumes. The relative advantages of surfactant-
enhanced remediation over water dissolution are apparent. It
should be noted that the concentrations in Figure 5 are still
several orders of magnitude above the drinking water standard.
While surfactanttechniques significantly expedite mass removal,
protracted amounts of time may be required to achieve drinking
water standards (although much less than with water alone).
It is observed that the food grade surfactants are technically
viable for use in subsurface remediation activities. This may be
advantageous for obtaining regulatory approval for utilizing
surfactants in subsurface remediation. However, edible
surfactants may be more susceptible to losses (e.g., precipitation,
sorption; Shiau et al., 1995a), which can cause system failure
(e.g., pore clogging due to precipitation) and will result in
increased costs of the surfactant (thereby hindering the economic
viability of the process). For this reason, research in our
laboratories has evaluated methods for minimizing precipitation
and sorption of food grade surfactants (e.g., use of hydrotropes
or cosolvents to decrease the sorption and precipitation of these
surfactants — Krebbs-Yuill, 1995). Also, in other research
surfactants with indirect food additive status have been evaluated
that are less susceptible to precipitation and sorption (e.g., alkyl
diphenyloxide disulfonates - Rouse et al., 1993; ethoxylated
alkylsulfates - Rouse et al., 1996). Space limitations prevent a
detailed discussion of techniques for limiting surfactant losses;
the interested reader is directed to the above references and
Shiau et al. (1995b) for more detailed discussions. Thus,
surfactant selection is not only critical to the technical
-------
implementation of surfactant-enhanced subsurface remediation,
but is also imperative to minimizing subsurface losses and
improving the economics of implementing the technology. This
will be further discussed in a later section focusing on economic
considerations.
IMPROVING THE HYDRAULIC EFFICIENCY OF
SURFACTANT INJECTION/EXTRACTION
The technical and economic feasibility of surfactant-based
remediation processes will depend on the ability to achieve
hydraulic control in the subsurface while also attempting to
maximize hydraulic efficiency. Hydraulic efficiency can be
increased by: (1) minimizing the volume of injected surfactant
solution; (2) minimizing the volume of fluid to be pumped to the
surface (reducing treatment costs); (3) targeting injected
chemicals to the contaminated zones of the aquifer (thereby
preventing the movement of injected fluids towards clean portions
of the aquifer); and (4) maximizing capture of resulting water-
surfactant-contaminant mixtures.
One method of improving the hydraulic efficiency of surfactant-
enhanced subsurface remediation is the strategic placement of
impermeable and/or hydraulic barriers. Impermeable physical
barriers (e.g., grout curtains, slurry walls, sheet piling) can be
used to deflect flows into or away from contaminated zones by
creating zones of low permeability. Hydraulic barriers (e.g.,
injection wells, infiltration galleries) can be used to deflect flows
into or away from contaminated zones by creating zones of
increased hydraulic potential (head). A recent numerical modeling
study assessed the relative effectiveness of hydraulic and
impermeable barriers for improving the efficiency of DNAPL
remediation processes, both with and without surfactants (Gupta,
1993). The overall conclusion drawn from these results was that
mass transfer of the contaminant from the residual phase to the
fluidmov'mg through the contaminated zone should be maximized,
regardless of the fluid used (e.g., air, water, chemical solution).
Simple upgradient injection of surfactants followed by
downgradient extraction is tremendously inefficient due to loss of
surfactant to uncontaminated zones. Injection of surfactant
solutions inside partially encircling impermeable barriers with
downgradient deflector wells was found to be the most efficient
of the systems evaluated for the surfactant-based processes.
The impermeable barrier cuts off upgradient water (eliminates
dilution of surfactant solution) and prevents migration of surfactant
solutions into uncontaminated areas. Hydraulic barriers (deflector
wells) provide increased gradient in addition to directional control
while also having the advantage of being a temporary measure.
The volumes (mass) of surfactant solution required to exceed the
CMC in the contaminated zone decreased significantly (up to
65%) with barriers over simple injection/extraction (Gupta, 1993).
The use of operational measures has also been suggested for
improving pump-and-treat efficiency; such measures include
cyclic(pulsed) pumping, push-pull pumping, and variable injection/
extraction ratios (Keely, 1989). Pulsed pumping was first
proposed by petroleum engineers to improve recovery from
hydrocarbon reservoirs (Aguilera, 1980). Disadvantages
associated with pulsed pumping identified in laboratory and field
studies include increased remediation times (due to decreased
concentration gradients during resting phase), operation and
maintenance issues, and lack of necessary hydraulic control
(Stallard and Anderson, 1992; Armstrong et al., 1994; and
Voudrias and Yeh, 1994).
Simultaneous injection to and extraction from a common vertical
borehole creates a circulating flow pattern (Figure 6) within a
sphere or ellipsoid around the borehole (referred to as vertical
circulation wells — VCWs), a concept previously evaluated in
petroleum production. A myriad of potential benefits of VCWs
can be delineated, including: (1) reduced costs over systems
involving multiple wells; (2) effective hydraulic control achieved
Injection Interval
vertical Migration
of Mobilized DNAPL
Diffusion Limited Zone
Limiting Streamline
•XXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXX1
Figure 6: Vertical Circulation Well (VCW) Flow Pattern.
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over limited volumes of the formation; (3) ability to capture
DNAPLs that might sink when mobilized; (4) applicable to both
light NAPLs (LNAPLs or floaters) and DNAPLs (sinkers); (5)
minimizes surfactant losses; (6) minimizes volumes of fluids
produced at the surface requiring treatment; and (7) induced
mounding can remediate portions of the contaminated vadose
zone around the well. Obviously, these and alternate systems
must be evaluated on a case-by-case basis to determine optimal
configurations.
Two-dimensional steady state flow induced by the VCW system
in an aquifer with a regional gradient can be described using the
complex potential, Q,
= O
(1)
where, Q is the complex potential, is the hydraulic potential,
and *¥ is the stream function. Lines of constant O are called
equipotentials and they describe the head distribution within the
aquifer. Lines of constant *¥ are called streamlines and they
describe the flow paths of ground water within the aquifer.
The two screened intervals behave as a line source and a line
sink, respectively. By superposition, the complex potential for
the line source and line sink can be combined, along with the
complex potential for a regional gradient (lateral flow), to produce
the overall complex potential forthe aquifer. Using the equations
for line sources/sinks and regional gradient developed by Strack
(1989), the complex potential becomes:
Figure 7 shows equipotentials and streamlines for a VCW in an
aquifer having a local regional gradient, where the bottom
screened interval of the VCW is extracting at a rate higher than
the upper screened interval is injecting (Qout >Qin). By having Qout
> Qin, it is possible to approach complete capture of the injected
solution; however, the extracted solution is significantly diluted
by fresh ground water (this effect increases as the Qou/Qin ratio
increases). This will result in diluted extraction concentrations for
both the injected surfactant solution and the solubilized (or
mobilized) contaminant. It is important to recognize that the
performance of the surfactant solution relative to extraction of
the contaminant is masked in the VCW system effluent by
dilution due to the fresh ground water that is extracted. It is also
important to note that the dilution occurs in the borehole and not
in the formation; thus, surfactant concentrations in the aquiferwill
not drop below the CMC due to this phenomenon.
The relative performance oftheVCWsystem versus thetraditional
injection/extraction (two-well) system was assessed using a two-
dimensional (sand tank) model packed with glass beads. The
tank, constructed of aluminum, is 36 inches wide by 18 inches
high, with a 2 inch depth. The tank has adjustable constant-head
end reservoirs, a glass front plate for visual observation, and
piezometers for sampling and head measurement distributed
horizontally and vertically throughout the tank. A known mass of
DNAPL was gravity fed from the surface in each test prior to
removal via surfactant solutions. Aqueous DNAPL concentrations
were quantified throughout each test. Enhanced solubilization
and mobilization (microemulsification) were evaluated for each
hydraulic configuration (in both cases the injection and extraction
pumping rates were 100 ml/min with no regional gradient).
a .
-£|ln
Z-l) ln| Z-l] - Z + l] ln(Z+l|]
-Q
where,
and
D- z. -z.
(2)
D = total length of vertical circulation well (both screens
and spacer)
a = separation distance between screens (spacer)
z = x + iy
Z = X + iY
Qo = lateral Darcy velocity
o = strength of injection/extraction interval
z1 = bottom of lower screen
z4 = top of upper screen
Z =
H
(3)
The variable transformation from z to Z is simply a change from
the global (x,y) coordinate system to a local (X,Y) system based
on the geometry of the vertical circulation well.
Horizontal Direction
(dimensionlcss)
Figure 7: Cross Section Showing Streamlines for a VCW System in an
Aquifer with a Regional Gradient. Injection in Upper Zone,
Extraction in Lower Zone, Regional Gradient from Right to Left.
-------
Preliminary studies of the VCW system and two-well systems
have also been conducted in three-dimensional sand tanks
(Roberts et al., 1993; Chen, 1995).
Plotted in Figure 8 are the mass recovery curves for the two-well
and VCW systems using the surfactant-enhanced solubilization
mechanism for residual saturation of PCE. A4.56 weight percent
(wt%) solution of T-MAZ 20 was used during the solubilization
studies. The interfacial tension of this solution is 5 dynes/cm.
The curves show that the VCW system achieves higher effluent
contaminant concentrations (increased mass recovery at early
extraction volumes) due primarily to reduced dilution by
uncontaminatedgroundwater(i.e., improved hydraulicefficiency).
The surfactant recovery is likewise expected to be greater in the
VCW as surfactant lost to the uncontaminated portions of the
aquifer is minimized. Decreasing efficiency in the VCW system
is observed at ca. 5 liters. If the initial efficiency of the VCW had
been maintained it appears that the remediation would have
been complete within 10 liters; however, in reality it required
greater than 30 liters to approach 100% recovery of the PCE.
Thus, the tailing of the VCW mass removal curve negates the
advantage of the initially higher concentrations
Plotted in Figure 9 are the mass recovery curves for the two-well
and VCW systems via surfactant-enhanced mobilization (again
for residual saturation of PCE). The surfactant system used was
1.3 wt%AOT and 3.6 wt%SMDNS, which produced an interfacial
tension of 5 x 10"3 dynes/cm. It can be seen in Figure 9 that the
two-well system approaches a plateau at ca. 70% recovery of
the PCE; visual observations indicated that some ofthe mobilized
residual was lost from the flowlines due to vertical migration. It
is very encouraging to note that this did not occur in the VCW
system and that removal efficiencies approached 100% for PCE
in the VCW system. However, a significant decline in the
extraction efficiency ofthe VCW system occurred at ca. 5 liters.
It is interesting to note that, if not for this decline in efficiency, the
VCW system could have remediated the PCE before significant
PCE extraction in the two-well system was observed.
The change in efficiency for mobilization using the VCW system
(ca. 5 liters in Figure 9) was due to the accumulation of a
diffusion-limited mass of contaminant atthe fresh water-surfactant
solution interface (i.e., the outermost streamline) of the VCW
(see Figure 6). Because the fluid outside the streamline (i.e., the
ground water) contains no surfactant, the surfactant system at
this interface is diluted and no longer exists as a middle-phase
system. At this interface, slow diffusion-limited dissolution of this
contaminant mass reduces the efficiency of the extraction
process, as reflected in the long tailing ofthe mobilization mass
recovery above 5 liters. It is suggested that this phenomenon
can be addressed through relatively simple operational variations
in the VCW system; i.e., reversing injection-extraction direction,
operating with three screened intervals with alternating injection
and extraction points, etc.
In summary, the VCW system has potential advantages overthe
two-well system relative to hydraulic capture of the mobilized
residual. The efficiency ofthe VCW system will be maximized
when operational variations prevent the limiting effects of dilution
at the outermost streamline. The VCW system also has the
distinct advantage of higher surfactant recovery compared to the
two-well system and potentially lower volumes of water to be
treated at the surface.
ECONOMIC CONSIDERATIONS FOR TECHNOLOGY
IMPLEMENTATION
In conventional pump-and-treat remediation of a site containing
residual saturation of a DNAPL, it is not uncommon for contaminant
concentrations in the ground waterto be well belowtheirsolubility
20 30 40
Fluid Extracted (liters)
20
3D 40 50
Fluid Extracted (liters)
60
70
30
- Two-Well System
VCW System
- Two-Well System
VCW System
Figured: Effluent Mass Recovery Curves for Surfactant enhanced
Solubilization.
Figure 9: Effluent Mass Recovery Curves for Surfactant-enhanced
Mobilization.
-------
values. For a given mass of residual DNAPL and a given ground-
water extraction rate, the time required to remediate an individual
DNAPL should be proportional to its aqueous solubility. This is
plotted in Figure 10, where R is the ratio of the total mass of the
particular DNAPL in the soil to the recovery water flow rate
(pounds per gallon per minute) and the aqueous concentrations
of the chlorocarbons are assumed to be 10% of their solubility
limit(e.g.,duetononequilibriumeffects, dilution). Thus, increasing
values of R generally indicate either more DNAPL mass for a
given volume of ground water extracted, or lower ground-water
extraction rates for a given DNAPL mass; both of these will result
in greater remediation times. Also shown in Figure 10 is the
remediation time required for surfactant-enhanced pump-and-
treat remediation; surfactant introduction significantly reduces
the remediation time for the more hydrophobic DNAPLs (i.e.,
aqueous solubilities less than about 3000 mg/L).
The curves for surfactant-amended pump-and-treat remediation
in Figure 10 were generated assuming that the recovered water
has a sodium lauryl sulfate surfactant concentration of 1.7 wt. %
or 10 times its CMC (CMC = 0.006 M) (Rosen, 1989), and the
concentration of DNAPL in the surfactant micelles corresponds
to an aqueous DNAPL concentration of 10% of its aqueous
solubility. Molar solubilization ratio (MSR) values utilized were
0.275 for tetrachloroethylene (PCE, aq. solubility = 200 mg/L),
0.20 for trichloroethylene (TCE, aq. solubility =1100 mg/L)
(Sabatini and Knox, 1992), 0.15 for 1,1,2,2-tetrachloroethane
(aq. solubility = 2900 mg/L), and 0.10 for 1,2-dichloroethane
(1,2-DCA, aq. solubility = 8690 mg/L). The relationships shown
in Figure 10 provide reasonable estimates of relative costs for
sodium lauryl sulfate as evaluated in this preliminary economic
analysis. It is recognized that incorporating changes in mass
recovery over the course of remediation (as documented in
Figure 5) will more accurately capture the absolute times (and
costs) of remediation. However, the baseline case of conventional
pump-and-treat will be similarly impacted, minimizing the impact
on the comparative observations presented below.
Another major concern with surfactant-enhanced pump-and-
treat technologies is the initial cost of surfactant required to fill the
aquifer. This cost can be greaterthan $20 million fora very large
aquifer or less than $1 million when targeted to the residual
DNAPL zone. Figure 11 shows surfactant capital cost for PCE
remediation with varying aquifer volumes and surfactant prices.
Two different R values (ratio of PCE in the aquifer to water
removal rate) are shown. The surfactant concentration is set to
theoretically allow complete remediation of PCE in seven years.
An aquifer volume of 50 million gallons corresponds roughly to 25
acres cross-sectional area by 20 feet thickness at 30 percent
porosity; 10 million gallons corresponds to 5 acres with the same
thickness and porosity. This range is not unusual for dissolved
contaminant plumes; however, residual saturation zones are
10000 =
50
1000 =
£
2
R=25,000; No Surf.
R=1 5,000, No Surf.
R=5,000; No Surf.
R=25,000; w/Surf
*
R=1 5,000; w/Surf
100 1000 10000
Aqueous Solubilty, mg/L
40-
30-
20-
10-
R=15,000; 18xCMC
""; 6.25xCMC
$1.00 lib.
0 10 20 30 40 50 60
Aquifer Pore Volume, Million Gallons
Figure 10: The Remediation Time Required for Very Insoluble Contaminants
with 1.7wt percent Surfactant and Water (R = ratio of mass of
contaminant in aquifer to water removal rate — Ib/gpm). Figure 11: Initial Surfactant Cost as a Function of Aquifer Pore Volume.
10
-------
typically only a fraction of this size. These results show that
surfactant remediation should be targeted to the residual
saturation zone (where it provides the greatest benefit) as well
as the importance of minimizing surfactant losses (i.e., sorption
and precipitation).
Based on the capital costs of the surfactant and given that
multiple pore volumes of injection will be utilized, economics
dictate surfactant recovery and reuse. In a typical hydraulic
regime, recovery wells will likely produce more water than is
injected to maintain hydraulic control, which can result in a
significant bleed stream. Figure 12 demonstrates the cost of
surfactant losses without a recovery step from the bleed stream.
The cost of surfactant recovery from the bleed stream is typically
only a small fraction of the cost of lost surfactant.
In order to identify the most economic surfactant recovery
process, a base case was studied. In the base case, DNAPL
contaminated water is extracted at the rate of 500 gpm, the bleed
stream is 150 gpm, and 350 gpm of treated water is reinjected
along with surfactant. It is assumed that DNAPL concentrations
in the reinjected water should be at or below the 0.5 ppb level.
The treatment process will be based on chemical/physical
differences between the DNAPL, surfactant and ground water.
The initial separation can be based on differences in micellar/
aqueous properties, volatility, surfactant/solution properties, or
organic/aqueous properties. Potential unit operations to achieve
the recommended separation are listed in Table V. Two process
Table V: Unit Operations Considered
40
30-
03
Q.
20-
10-
Surfactant @ $2.25/lb
50/lb.
ry Cost
0 50 100 150 200 250 300
Bleed, Gallons per Minute
Figure 12: The Costof Surfactant Losseswithouta Recovery Step fromthe
Bleed Stream.
SEPARATE DNAPL FROM FEED
Airstrip
Steam Strip
Vacuum Steam Strip
Pervapo ration
Organic Extraction
Biotreatment
SEPARATE SURFACTANT FROM BLEED
Waste
Foam Fractionation
Al, Ca Precipitation; Cation Exchange
Micellar Enhanced Ultrafi It ration (MEUF)
MEUF combined with Foam Fractionation
DESTROY/SELL DNAPL
Incineration
Carbon Adsorption
Recycle/Sell/Store
Biotreatment
schemes are recommended based on economics and proven
ability to remove DNAPL and surfactant to the desired levels. If
the recovered DNAPL has no value, standard air stripping of the
process feed will remove the DNAPL and leave the surfactant for
reinjection. Catalytic incineration of the DNAPL can be utilized,
followed by caustic scrubbing to remove HCI. The surfactant is
recovered from the bleed stream using a combination of
ultrafiltration and foam fractionation. Ultrafiltration is most
efficient down to the CMC level and foam fractionation is very
efficient below the CMC level. If the DNAPL has sufficient value
to a reclaimer or can be recycled to an adjacent operation,
vacuum-steam distillation becomes attractive. Surfactant is
recovered from the bleed stream as in the previous case.
The initial design and cost estimates in this preliminary analysis
assume that surfactants have only minor impacts on stripping
efficiency. Recent research has demonstrated that micellar
solubilization decreases the overall efficiency of the stripping
process by reducing the aqueous contaminant activity and thus
the driving force for volatilization (Lipe et al., 1996). Foaming
may also result, which can create significant operating problems
(Lipe et al., 1996); vacuum stripping has been shown to minimize
this effect (Choori, 1994). Also, for nonvolatile compounds
liquid-liquid extraction has demonstrated potential given that the
reduced activity due to micellar solubilization is considered
(Hasegawa et al., 1996). Incorporation of such effects was
beyond the scope of this analysis.
The hypothetical case studied represents an extremely large
volume of residual DNAPL contamination; such a case was
selected to highlightthe important factors in designing surfactant-
enhanced subsurface remediation. It was assumed that 5 acres
of contaminated soil, 20 feet in depth, had an average porosity
of 0.30, and the void fraction contained 13 volume percent
DNAPL. The DNAPLs present were assumed to consist of equal
volumes of PCE, TCE and 1,2-DCA. This equates to 5.74 million
pounds of PCE, 5.16 million pounds of TCE and 4.46 million
pounds of DCA. The surfactant was assumed to cost $1.00 per
pound and was added in sufficient quantities to achieve 15 times
its CMC in the recovered water. In this analysis, 20 percent of
11
-------
the surfactant was assumed to be lost due to adsorption in the
first two pore volumes. This surfactant loss is consistent with
surfactant adsorption values of 10 mg/g or greater (Rouse et al.,
1993), and illustrates the economic importance of minimizing
surfactant losses. The recovery wells were assumed to operate
at 500 gpm and the bleed stream necessary was assumed to be
150gpm. The recovery stream was assumed to contain 330 mg/
L TCE, 400 mg/L PCE and 950 mg/L 1,2-DCA (based on MSR
values above and assuming aqueous concentrations at 10 percent
of solubility limits and no competitive effects of solubilizates),
giving a theoretical remediation time of seven years.
A total of 68 injection and recovery wells were assumed along
with piping to the treatment facility. The DUALL Division of MET-
PRO Corporation recommends three air strippers in series to
treat 500 gpm process water containing PCE, TCE and 1,2-DCA.
The columns are 9 feet in diameter with 28 feet of packing.
Global Environmental estimates that a catalytic incinerator
operating at 1000° F with 30 ft3 of Englehard catalyst will achieve
99 percent destruction efficiency. (Allied also manufactures a
catalyst that can be used with high chloride streams.) The hot
gas stream must be quenched to about 180°F. A 5-foot diameter
caustic scrubber is recommended for removing HCI from the
cooled gas stream with a design efficiency of 99.5 percent. The
cost of ultrafiltration and foam fractionation will depend on the
specific surfactant used. Conservative costs were utilized based
on previous work at the University of Oklahoma (Dunn et al.,
1985). Based on this analysis, the approximate capital cost is
$3,570,000 and the annual O&M cost is $940,000 (see Krebbs-
Yuill et al., 1995 for more details).
As an alternative, costs for the vacuum-steam stripping case
were estimated. AWD, a division of DOW, estimates installed
costs of $3 million for a 5-foot diameter column, package boiler
(1500 to 2000 Ib/hr steam), and instrumentation. The overall
capital cost forthis approach is $5,070,000, with the annual O&M
costs at $765,000 (see KrebbsYuill et al., 1995 for more details).
Costs will increase rapidly if the water requires significant
pretreatment for inorganics and if operating problems are
experienced due to the presence of the surfactant. Again, the
actual tower dimensions may increase due to micellar effects
(Lipeetal., 1995).
For a given contamination site, there is a cost tradeoff between
the number of wells (assuming the optimum pumping rate per
well is known and fixed), the time required for remediation, the
size of the treatment process, and the initial surfactant cost. This
is shown for our base case in Figure 13 where the above-ground
process is air stripping-incineration with surfactant recovery of
the bleed. The negative net present value, discounted at
10 percent, is plotted against the gpm of well water recovered.
Again, it is emphasized that these numbers should be viewed in
a relative sense (remembering the simplifying assumptions
made in the analysis). The surfactant concentration is varied to
give a theoretical remediation time of seven years; therefore, the
initial surfactant cost decreases with increasing gpm. The cost
of wells and the above ground processing costs increase with
increasing flow rate. For the base case, the optimum flow rate,
corresponding to the minimum negative net present value, is
about 500 gpm. Initial surfactant costs due to adsorption losses
were not included since they were assumed not to vary between
cases.
20
15-
Q. 10
10
n>
0)
0)
Total Cost
V^ Optimum
•Well/Process
Cost
Initial Surfactant
/ Cost
200 400 600 800
Water Recovery Rate, gpm
1000
Figure 13: Cost Optimization fora Five Acre Surfactant Flood.
The cost benefits of a small, well defined area of contamination
can be shown in the following example using pump-and-treat
technology to remediate residual DNAPL from only 1/4 acre
covering a depth of 20 feet and treating 35 gpm using air stripping
and incineration. The contaminated soil contains approximately
125,000 pounds of residual PCE and estimates indicate it will
take another 41 years to remediate. The operating cost is
$125,000 per year and the net negative present value of this
operating cost, discounted at 10 percent, is $1.225 Million.
Adding $350,000 of surfactant (@ $1.00/lb) and $120,000
capital for surfactant recovery, the remediation time is decreased
to about 3 years. If the additional operating cost is about $30,000
per year, the negative net present value is decreased to $545,000.
Not only is the cost to the company reduced, but also a very
undesirable environmental liability will be eliminated sooner.
Part of the initial surfactant cost may be recovered at the end of
the remediation project. This would be accomplished by continued
water flooding at a reduced rate without surfactant addition. Only
the surfactant recovery portion of the above-ground process
would be operated. The surfactant should retain a fair fraction
of its original value, especially to another surfactant-enhanced
remediation project (assuming regulatory approval of used
surfactant). As an alternate, a site may be compartmentalized
with a smaller initial volume of surfactant shifted from compartment
to compartment (resulting in decreased capital costs of surfactant
12
-------
but increased remediation time). Maximizing recovery of
surfactant from the subsurface will also be necessary if there is
environmental concern regarding residual surfactant in the aquifer.
In summary, surfactant-enhanced pump-and-treat remediation
is effective for DNAPL chemicals with relatively low aqueous
solubilities, but requires surfactant reuse to be economical.
From this preliminary analysis, the best surfactant recovery
process is probably a combination of microfiltration and foam
fractionation. Total project costs can be minimized by (1)
defining the area of residual saturation, thus minimizing the
volume of soil requiring surfactant flooding, and (2) balancing the
number of wells and volume of water to be treated with the cost
of surfactant and surfactant concentration required to achieve
remediation. For relatively small areas of residual saturation
(i.e., acres or less which is common even for dissolved plumes
having dimensions in miles), surfactant-enhanced pump-and-
treat via solubilization can be less expensive, on a present value
basis, than pump-and-treat alone. The economics for mobilization
will be even more favorable than solubilization, especially if
surfactant slugs of less than a pore volume can be utilized
followed by polymer drives, etc.; future research should evaluate
such methods for environmental applications. When external
costs are incorporated into the analysis (e.g., regulatory or public
mandates, liability, property transactions, etc.), it is obvious that
surfactant-enhanced subsurface remediation has the potential
for widespread implementation.
NOTICE
This work was funded by the U.S. Environmental Protection
Agency under Cooperative Agreement CR818553-01-0 with the
Robert S. Kerr Environmental Research Center, Ada, OK. It has
been subjected to Agency review and approved for publication.
Mention of trade names or commercial products does not
constitute endorsement or recommendation for use. The authors
acknowledge input and guidance from Dr. Candida West of
RSKERC. The many students who worked faithfully and diligently
on this project (including Bor Jier Shiau, Yili Chen, and Barbara
Krebbs-Yuill) are gratefully acknowledged by the authors. The
authors also acknowledge the helpful input of the anonymous
reviewers.
QUALITY ASSURANCE STATEMENT
All research projects making conclusions or recommendations
based on environmentally related measurements and funded by
the Environmental Protection Agency are required to participate
in the Agency Quality Assurance Program. This project was
conducted under an approved Quality Assurance Program Plan.
The procedures specified in the plan were used without exception.
Information on the plan and documentation of the quality
assurance activities and results are available from the Principal
Investigator.
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