xvEPA
United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/S-97/005
September 1997
ENVIRONMENTAL
RESEARCH BRIEF
Innovative Measures for Subsurface Chromium Remediation:
Source Zone, Concentrated Plume, and Dilute Plume
David A. Sabatini1, Robert C. Knox1, Edwin E. Tucker2 and Robert W. Puls3
Introduction
Many sites in the United States are contaminated with
toxic metals such as lead, cadmium, and chromium. Under
normal conditions, subsurface chromium contamination
exists in two stable oxidation states: hexavalent, [Cr(VI)]
and trivalent [Cr(lll)]. Cr(VI) is both toxic and mutagenic;
Cr(lll) is of less health concern and, because of its lower
water solubility, its aqueous concentrations are generally
below water quality standards. In the subsurface, Cr(VI)
generally exists in theanionicchromate(CrO42-)or bichromate
(Cr2O72-) forms which are relatively soluble and mobile. Thus,
the risk associated with ground-water contamination is high
and remediation of ground water contaminated with Cr(VI) is
of critical importance. The conventional approach for
remediating contaminated ground water sites has been
water-based pump-and-treat. In recent years it has been
recognized that this approach can require protracted periods
of time to approach treatment goals (Keely, 1989; Palmer and
Wittbrodt, 1991; Palmer and Fish, 1992).
The behavior of chromium in soils depends upon various
factors, including: the form of chromium present, soil pH and
mineralogical properties, and presence of organic matter.
'Civil Engineering and Environmental Science and Institute for Applied
Surfactant Research, University of Oklahoma, Norman, OK 73019
2Chemistry and Biochemistry, University of Oklahoma, Norman, OK
73019
3 Subsurface Protection and Remediation Division, National Risk
Management Research Laboratory, ORD, U.S. EPA, Ada, OK
74820
Several authors have studied the behavior of chromium in
soils as a function of these factors (Bartlett and Kimble,
1976; Zachara et al., 1989; Ainsworth et al., 1989; Eary and
Rai, 1991; Anderson et al., 1994). The presence of organic
matter and Fe(ll) is responsible for reduction of Cr(VI) to
Cr(lll) (Eary and Rai, 1991; Anderson et al., 1994). Addition
of multivalent anions (e.g., phosphate and sulfate) was
found to decrease chromate adsorption due to competition
for the same adsorption sites (i.e., ion exchange — Bartlett
and Kimble, 1976; Zachara et al., 1989; Puls et al., 1994a).
Tucker et al. (1992) showed the ability of a cationic
polyelectrolyte to partially solubilize a solid BaCrO4 phase.
Puls etal.(1994a) showed the ability of an anionic surfactant,
sodium dodecyl sulfate (SDS), to significantly enhance the
elution of chromate in column studies.
Subsurface chromium contamination at the U.S. Coast
Guard Support Center, near Elizabeth City, North Carolina, is
the focus of this research. A brief description of the site is
given here to provide a context for this research. For further
details on the site, the interested reader should consult Puls
et al. (1994a, b). A chrome plating shop was operated at the
Coast Guard facility for more than thirty years. Activities in
this shop resulted in the release of chromic acid into the
soils below the shop. While some Cr(VI) was reduced to
Cr(lll) in the vadose zone soils, the reducing capacity of
these soils was eventually overwhelmed and ground-water
contamination resulted. Although the most concentrated
portion of the dissolved plume is down gradient from the
source zone, the plume has not yet separated from the
source zone. This indicates that the remaining source zone
soils continue to leach chromium into the ground water. The
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dilute portion of the plume has migrated down gradient
towards an adjacent river.
As such, the contamination at this site can be divided into
three regions: (1) the source zone soils, (2) the concentrated
portion of the ground-water plume, and (3) the dilute portion
of the ground-water plume. Applying conventional pump-
and-treat methods to all three regions will be highly inefficient.
Pump-and-treat remediation of the concentrated plume
without regard forthe other regions will also be inefficient, as
the dilute portion of the plume will continue to migrate to the
river and the source zone materials will continue to leach
into the plume. Therefore, using an integrated approach to
simultaneously address these three regions will be most
effective. The technologies chosen for each of the three
regions must be tailored to the unique characteristics of that
region.
This environmental research brief reports on innovative
measures for addressing each of these three regions. For
the source zone, surfactant-enhanced chromium extraction
is evaluated for expediting the removal of chromium from
thesourcezonesoils, thereby mitigating thecontinual feeding
of the ground-water plume. For the concentrated plume,
polyelectrolyte-enhanced ultrafiltration (PEUF) is evaluated
as an innovative treatment process with desirable operating
characteristics (less sludge production, higher quality final
water, etc.). Relative to the dilute plume, the hydrogeological
effectiveness of hydraulically passive, chemically reactive
barrier systems is evaluated (i.e., in situ reduction of Cr(VI)
to Cr(lll)). It is proposed that the collective use of these three
innovative technologies will significantly improve the
remediation of subsurface chromium contamination.
Research evaluating each of these technologies is the focus
of subsequent sections.
Surfactant-Enhanced Chromium Extraction: Source
Zone Soils
Background:
This section focuses on an innovative technology forexpediting
elution of chromium from the source zone soils. If the source
zone soils are not addressed, the plume will continually be
replenished and the efficacy of the plume treatment methods will
be compromised. As mentioned above, introduction of multivalent
anions and surfactants has been observed to enhance chromium
elution from contaminated soils. This research further evaluates
the use of surfactant systems to expedite chromium extraction
from the source zone soils.
Surfactants (surface-active-agents) are amphiphilic molecules
consisting of lipophilic and hydrophilic groups. This amphiphilic
structure results in the surface active nature of surfactants and
causes them to concentrate in interfacial regions. Above a
specific concentration, surfactant molecules form dynamic
aggregates or micelles. The concentration above which micelles
form is known as the critical micelle concentration (CMC).
Aqueous-phase micelles have a hydrophobic interior and a
hydrophilic exterior, causing them to behave like dispersed oil
drops. Solubilization is the phenomenon by which non-polar
species partition into the organic interior of the micelles. Micelles
and polar/ionic species interact mainly through hydrogen bonding
and electrostatic forces (Shimamoto and Mima, 1979). Recently,
surfactant-enhanced pump-and-treat remediation has been of
great interest (Nash, 1987; Abdul and Gibson, 1991; Harwell,
1992; West, 1992; Edwards et al., 1992; Palmer et al., 1992;
Fountain, 1992; West and Harwell, 1992; Rouse et al., 1993;
Shiau et al., 1994; and Sabatini et al., 1995).
The main factor considered in evaluating surfactant systems
for chromium remediation is the efficiency of chromate extraction
(ratio of chromate removal by surfactant to chromate removal by
water). It is hypothesized that surfactants can displace the
adsorbed chromate by either ion exchange, precipitation-
dissolution and/or counterion binding mechanisms, and that
further enhancement in extraction may be achieved if surfactants
with solubilized complexing agents are used. Laboratory batch
and column studies were conducted to evaluate these hypotheses
using contaminated soil from the U.S. Coast Guard site.
The surfactants used in this research along with some of their
relevant properties are summarized in Table I. These surfactants
were selected based on their type (anionic, zwitterionic, etc.),
their susceptibility to losses (e.g., precipitation and sorption),
their ease of regulatory acceptance (having USFDA direct food
additive status), and experience with them in prior research
(Rouse etal., 1993; Shiau etal., 1994). Chromium contaminated
soil samples were obtained from the U.S. Coast Guard Support
Center, Elizabeth City, North Carolina, at depths from 1.5 ft to
6.5 ft. The fraction organic carbon content in Elizabeth City soils
varied from 0.0006 to 0.0027 (Puls et al., 1994a). Batch and
column studies, as well as chemical analyses, were conducted
according to standard procedures, as documented elsewhere
(Nivasetal., 1996).
Batch Extraction Studies:
Batch studies were conducted using 3.0g of soil and 15 ml of
solution. Figure I compares the Cr(VI) extraction from Elizabeth
City soil using deionized (D.I.) water and surfactants. The
amount of Cr(VI) removed is observed to increase with anionic
surfactant concentration. At concentrations greater than the
CMC, Cr(VI) removal was relatively constant (see Figure I and
Table II). Upon equilibration D.I. water solubilized 2.64 ppm of
Cr(VI). The ratio of maximum Cr(VI) removal by surfactants to
that of D.I. water ranged from 2.1 for Dowfax 8390 to 2.8 for
Deriphat-160 (see Table II).
Anionic surfactants could enhance Cr(VI) extraction by ion
exchange, precipitation-dissolution, and counterion binding. The
experimental data are not consistent with counterion binding as
the enhancement would begin and increase above the CMC.
Surfactant precipitation-Cr(VI) dissolution also does not appear
to be the mechanism since surfactants with high precipitation
resistance are expected to be much less efficient (e.g., Dowfax
8390). However, the data are consistent with ion exchange as
the primary extraction mechanism because the enhancement
occurs below the CMC and is independent of the precipitation
potential of the surfactant. Additional analysis of the sorption
data further corroborates this conclusion (as presented in Nivas
etal., 1996).
It was hypothesized that the extraction of chromate from the
contaminated soil could be enhanced if a chromium complexing
agent is solubilized into the surfactant micelles. Solubilization
assays were conducted for sodium dodecyl sulfate (SDS) and
aerosol OT (AOT) using diphenyl carbazide (DPC) as the
solubilizate. In both casesthe aqueous solubility of DPC increased
with concentration beyond the CMC (maximum surfactant
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Table 1 Relevant Properties of Surfactants and Complexing Agent Used in this Study
Surfactant
SDS*
AOTd
Dowfax-8390
Deriphat-160
T-Maz 20
DPCo
Type
Anionicf
Anionicf
Anionic
Zwitter ionic
A/on ionic?
—
Chemical
Formula
C12H25OS03Na
(C9H1702)2CH2CHS03Na
C16H33C12H70(S03Na)2
R-NH(CH2COONa)2
C12H2402H3(CH2CHO)2002CH2CH03
C6H5(NH)2CO(NH)2C6H5
Average
Molecular
Weight
288.4
444.6
642
373
1228
242
CMC
(mM)
8.20F
1. 124s
3. Off
-
0.039"
—
Obtained from
Fisher Scientific
American Cyanamid Co.
Dow Chemical Co.
Henkel Corp.
PPG/Mazer
Fisher Scientific
* SDS - sodium dodecyl sulfate
b USFDA Direct Food Additive
0 Mukerjee and Mysels, 1971
d AOT - aerosol OT
eShiau, etal., 1994
'Obtained from Dow Chemical Company
9 Complexing agent - Diphenyl Carbazide
10
I
o
o
9
8-
6-
2-
A. _|_ A A A
• T JJk A A
A
CMC Dowfax CMC SDS
CMC AOT
Water
AOT
SDS
A
Dowfax 8390
X
Deriphat-160
I T
5 10 15 20
Initial Surfactant Concentration, mM
Figure I Extraction of Cr(VI) by surfactants from Elizabeth City soil in
batch systems.
concentration evaluated was 20 mM). While the aqueous DPC
concentration is 1.56 mM, at 20 mM surfactant concentrations
SDS and AOT solubilized 8.67 mM and 5.33 mM of DPC,
respectively.
Figure II shows the extraction of Cr(VI) using surfactant-
solubilized DPC. Use of DPC in the batch studies required
shifting to a solids based concentration (mg Cr / Kg soil). The
results of these studies are summarized in Table III. Upon
equilibration, water solubilized 11.3 mg of Cr/Kg of soil. The
chromium removal increased with surfactant concentration, with
maximum removal being evidenced above the CMC. The
maximum ratio of chromium removal, with and without surfactants,
ranges from 9.3 for SDS to 12.0 for Dowfax 8390 (see Table III);
thus, the addition of a chromium complexing agent enhanced the
extraction of Cr(VI) by an order of magnitude greater than that
obtained with D.I. water. The ratio of surfactant concentration at
which these removals were achieved to the CMC ranged from 2.5
for SDS to 8.3 for AOT. In all cases the surfactant-solubilized
DPC outperformed the surfactant only results (see Table II). This
demonstratesthatthe micellar-DPC extraction can further improve
chromium removal relative to surfactant only systems. The
results of these studies show an enhancement of 3.7 to 5.7 times
over those of surfactants alone.
Table II Results of Cr(VI) Extraction Studies by Surfactants and Hydrotropes from Elizabeth City Soil
Extracting
Agent
D.I. water
AOT
SDS
Dowfax 8390
Deriphat-160
Extracting agent
cone, at max.
Cr(VI) removal,
mM
3
10
1
5
Ratio
to
CMC
2.7
1.2
0.3
Max.
Cr(VI)
Cone.,
ppm
2.6
5.2
6.4
5.6
7.4
Ratio ofCrfVI)
removal by extracting
agent to that by water
1
2.0
2.5
2.1
2.8
-------
Total Cr removed, mg/kg of soil
8 Ł g g 8 i i
A A
+
+
Z
CMC Dowfax
CMC AOT
A
A
X
CMC SDS
x 2
Water
AOT
X
SDS
A
Dowfax 8390
5 10 15
Initial Surfactant Concentration, mM
20
Figure II Extraction ofCr(VI) by surfactants with DPC from Elizabeth
City soil in batch systems.
Column Extraction Studies:
Column studies were conducted in 2.5 cm diameter by 15 cm
long glass chromatography columns; the average porosity and
pore water velocities were 0.39 and 9.1 cm/h, respectively.
Figure III shows the results of column studies evaluating Cr(VI)
removal using water, 10 mM AOT and 10 mM Dowfax 8390.
Deriphat-160 was not included in these experiments due to its
extensive sorption in the batch studies. The results are presented
as the Cr(VI) concentration versus the number of pore volumes
of solution injected. Table IV summarizes the results from these
column studies (for further details on column studies see Nivas
et al., 1996). Dowfax 8390 showed maximum Cr(VI) removal
followed by AOT and D.I. water. In all three cases the maximum
Cr(VI) effluent concentration was obtained between 1.34 and
1.76 pore volumes.
The trends in the column results are similar to those observed
for batch tests. For the column studies, when the Cr(VI)
concentrations in the effluent went below 0.5 ppm, injection was
stopped. After a week of no flow, the pumping of 10 mM Dowfax
8390 was continued in order to determine if the Cr(VI) removal
mechanism was rate limited, as shown in Figure III. The effluent
Cr(VI) concentration increased from 0.34 ppm to 3.34 ppm after
the flow interruption. This demonstrates that the Cr(VI) removal
mechanism may have been rate limited. Injection of 10 mM
Dowfax 8390 to the originally waterflushed column also increased
the effluent Cr(VI) concentrations from 0.2 ppm to 2.38 ppm (see
Figure III).
Figures IV and V show Cr(VI) removal using surfactant-
solubilizedDPC. The results ofthese studies are also summarized
in Table IV. The maximum Cr(VI) removal in the effluent is higher
than that observed with surfactants alone (10.3 ppm and 19.7 ppm
for AOT-DPC and Dowfax 8390-DPC columns, respectively).
This is about 1.7 to 1.9 times greater than that obtained in
columns flushed by AOT and Dowfax 8390 alone. In batch
systems, addition of DPC enhanced the extraction by 3.7 to
5.7 times over that of surfactants alone. Thus, the removal
efficiency enhancement in column studies is less than that
observed in the batch systems. The number of pore volumes
required to reach maximum Cr(VI) removal concentrations was
longer than anticipated as evidenced in Figure V (11.22 and
12.06 pore volumes for AOT and Dowfax 8390 with DPC,
respectively, versus 1.54 and 1.76, respectively, for surfactants
alone). The slope of the percentage removed relative to water
alone curves is very low until about 10 pore volumes. The lack
of peaks at 1.54 to 1.76 pore volumes, which were observed in
surfactant only - no DPC columns, indicates that monomer/Cr(VI)
ion exchange is not occurring.
It is postulated that two competing mechanisms are taking
place in the surfactant-DPC systems. First, DPC exits the
micelles and forms a Cr(VI)-DPC complex with the solids-
associated chromate (this process is apparently rapid enough to
prevent monomer-Cr(VI) ion exchange). Second, the extraction
of the Cr(VI)-DPC complex into the micellar core occurs, due to
complexation with the micellar-phase DPC. It appears that prior
to 10 pore volumes the former mechanism dominates; hence,
there is no rapid increase in the removal of Cr(VI) in the early
stages of injection. During this phase the formation of Cr(VI)-
DPC complex was clearly seen by the pink coloration of the
medium (the pink color being indicative of the Cr-DPC complex).
Once the solids-associated chromium has been complexed, the
extraction of chromate anions and solubilization of Cr-DPC
complex takes place. This is indicated by the increase in the
slope of both curves after 10 pore volumes in Figure V. However,
at about 12 pore volumes the removal efficiency of both the
systems again starts to decrease, possibly due to the diminishing
solubilization potential of micelles already saturated with DPC.
At this point injection of pure micellar solutions of surfactants
would be expected to enhance the solubilization of the solid
associated Cr(VI)-DPC complex.
At about 14 pore volumes the AOT-DPC column was switched
to flushing with 20 mM AOT only (no DPC). This initially
increased the Cr(VI) removal concentration in the effluent from
Table III Results ofCr(VI) Batch Extraction Studies by Surfactants with Solubilized DPC from Elizabeth City Soil
Extracting
Agent
D.I. water
SDS
AOT
Dowfax 8390
Extracting
agent cone, at
max. Cr
removal, mM
20
10
10
Ratio to
CMC
2.5
8.3
3.3
Max. Aqueous
Cr Cone.,
mg/Kg of soil
11.3
106
119
135
Ratio of Cr removal
by extracting agent to
that by water
1
9.3
10.4
12.0
-------
120
Flow interruptions followed
by 10mM Dowfax 8390 injection
O
10
20 30 40
Pore Volumes Injected
50
Water
% Removal, Water "
10mM AOT -*- 10 mM Dowfax 8390
— % Removal, 10mM AOT % Removal Dowfax
Figure III Removal ofCr(VI) by D.I. water and surfactants in columns.
0.86ppmto4.22ppm(seeFigure IV); however, the enhancement
was only temporary. Subsequent introduction of Dowfax 8390
alone, at a concentration of 10 mM, again increased the Cr(VI)
concentrations in the effluent (from 1.83 ppm to 7.11 ppm, as
shown in Figure IV). The injection of 10 mM Dowfax 8390 was
continued until 25 pore volumes, by which time around 175% of
Cr(VI) was removed relative to D.I. water.
Similarly, the column flushed with solubilized DPC in 10 mM
Dowfax 8390 was switched to surfactant only at about 16 pore
volumes. This switch increased the Cr(VI) concentration in the
effluent from 10.11 ppm to 15 ppm. The injection of 10 mM
Dowfax 8390 was continued until Cr(VI) removal in the effluent
was negligible. The total Cr(VI) removed was 213% relative to
D.I. water in less than 20 pore volumes, as opposed to the
surfactant only case which achieved only 125% after 25 pore
volumes. An interesting point to note is that in this column run,
the tailing of the concentration with time was not observed (see
Figure IV). The lack of an elution tail is obviously advantageous
for chemically-enhanced pump-and-treat processes.
It is suggested that the removal of Cr(VI) from the soil can be
further enhanced by optimizing the time of switching from injection
of surfactant with DPC to surfactant alone and/or by increasing
the surfactant concentration. The time of switching should be
immediately after the complete formation of Cr-DPC complex of
the solids-associated chromate. For example, at about 10 pore
volumes the formation of Cr-DPC complex appeared to be
Table IV Cr(VI) Extraction from Columns by Water, Surfactants Alone, and Surfactant- Solubilized DPC
Extracting Agent
D.I. Water
AOT
Dowfax 8390
AOT with DPC
Dowfax with DPC
Max. Cr(VI)
removed in
effluent, ppm
5.3
7.0
11.8
10.3
19.7
Ratio of Cr(VI)
removed by
extracting agent to
that by D.I. water
1
1.3
2.2
1.9
3.7
Total Pore
volumes
flushed
35.9
26.5
24.4
24.3
18.9
-------
i
10 15
Pore Volumes Injected
20
25
Figure IV Removal ofCr(VI) by surfactants with DPC in columns.
250
I 200
2
Ł
« 150-
20mMAOTw/DPC
10mM Dowfaxw/DPC
10mMDowfax8390.
(D
EC
o
100-
I
10 15 20
Pore Volumes Injected
25
Figure V
Percentage removal of Cr(VI) by surfactants with DPC in
columns.
complete. Switching at this time to pure micellar solutions would
have resulted in extraction of Cr(VI) in fewer pore volumes. By
increasing the concentration of the surfactant-DPC mixture, the
surface capacity for DPC could be satisfied in fewer pore volumes;
higher surfactant concentrations for complex extraction would
likewise expedite this process. It should be noted that the
concentrations for the column runs were selected based on
results from batch studies which indicated that extraction efficiency
was leveling off at surfactant concentrations of 10-20 mM; this
illustrates the importance of conducting column studies and not
relying solely on batch results.
In summary, batch resultsdemonstratethatsurfactant systems
have the potential to enhance chromium elution by a factor of 2
to 3 (surfactants alone) to an order of magnitude (surfactant-
complexing agent systems versus water alone). While column
studies corroborated results of batch tests, they also illuminated
operational considerations that require additional research. Thus,
whilethistechnology shows great promise forenhancing chromium
extraction from soils, further laboratory and field scale studies are
necessary prior to full scale implementation.
Polyelectrolyte-EnhancedUltrafiltration:
Concentrated Plume
Background:
The research presented in this section focuses on an innovative
treatment process for ground water extracted from the
concentrated portion of a contaminant plume. This treatment
process has improved operational characteristics relative to
existing technologies and will thus further optimize the pump-
and-treat methodology for the concentrated plume. The next
section will discuss a passive approach for dealing with the dilute
portion of the plume (where pump-and-treat would become less
efficient).
For several years, a particulararea of research atthe University
of Oklahoma has involved the combination of colloids (e.g.,
surfactant micelles and polyelectrolytes) with ultrafiltration
membranes to remove contaminant materials from aqueous
streams (Christian et al., 1988; Sasaki et al., 1989; Christian
et al., 1990; Christian et al., 1992; Tucker et al., 1992; Krehbiel
etal., 1992). These colloid-enhanced ultrafiltration (UF)
techniques have the potential to be used in low-energy, efficient,
and selective processes for removal of target ions or molecules.
One focal point has been the development of an efficient process
for removal of toxic metals and metalloids in anionic form (e.g.,
chromate, arsenate and selenate) from aqueous streams or
ground water. In addition to removal of these ions to potable water
levels, the ultrafiltration method does not, unlike many current
techniques, further degrade the overall water quality in the
process of removing one or more toxic components. A specific
goal has been to selectively remove the toxic ion in the form of a
compact solid waste and produce effluent water with essentially
the same ionic strength as that which enters the process.
In contrast with reverse osmosis, ultrafiltration is not generally
a direct method for removal of ions from aqueous streams. The
pore size of UF membranes is characterized by a molecular
weight cutoff (MWCO) value which is a rough estimate of the size
of molecules which are retained by the membrane. For instance,
a MWCO of 5000 Daltons indicates that a molecule with a
molecular weight larger than 5000 will be largely retained by the
membrane. In a hypothetical example, inorganic ions such as
chloride, sodium, and chromate in aqueous solution are much
smaller than this and will freely pass through the UF membrane.
However, if a soluble polymer of molecular weight greater than
the MWCO is added to the stream to be filtered and, if the polymer
binds chromate, then both the polymer and the chromate will be
retained bythemembrane. FiguresVI andVII presentqualitatively
a polyelectrolyte/chromate mixture and subsequent binding of
-------
cr
r
Na+
CrOf
Na+
Ci;
cr
Cl- CrOf
Na+
Cl- Na+
Figure VI
Homogeneous aqueous mixture of polyelectolyte and
chromate.
Figure VII Chromate ion replaces chloride ion on the cationic polymer.
the chromate ion to the polymer. The solution passing through the
membrane (permeate) will be depleted in chromate while the
solution retained by the membrane (retentate) will be enriched in
chromate (and polymer). The concentration of chloride and
sodium ions will be essentially the same on both sides of the
membrane since these ions are not bound to the polymer.
Conventional removal of chromate is commonly done through
lowering aqueous pH by addition of acid, adding a reducing agent
such as sodium sulfite, and finally, adding lime or other base to
raise the solution pH and precipitate the reduced chromium as
the Cr(lll) hydroxide. The chromate reduction process uses a
substantial quantity of acid, base and reducing agent with the
result that the effluent water from this conventional process has
increased ionic strength due to salt loading (Kosarek, 1981). For
example, the product water from one recently implemented
chromate reduction process has a sulfate ion concentration of
1700 ppm. This may be compared with an input level of only
34 ppm sulfate in the feed ground water (Buehler, 1993). Further
use or disposition of the product water containing such high
sulfate ion concentrations might be restricted. Additionally, a
large sludge volume is produced from the reduction process
because other metal hydroxide compounds precipitate along
with the chromium hydroxide. Chromium is a minor component in
this precipitate due to both indiscriminate hydroxide precipitation
and the fact that the hydrous oxides thus produced have a very
low solids content, typically on the order of 10%-20%.
The polyelectrolyte-enhanced ultrafiltration (PEUF) process
for chromate (CrO42~) removal involves addition of a cationic
polyelectrolyte[poly(dimethyldiallylammonium chloride)] to a feed
stream containing Cr(VI); the solution is then filtered with a UF
membrane cartridge, so that the chromate-polymer mixture will
be retained in a small volume (the retentate), while a large
fraction of the feed solution is produced as highly purified water.
Either chromate or bichromate {HCrCy} will bind to the
polyelectrolyte. Model calculations show that at pH 6.5 there is
essentially a 50:50 mixture of the chromate and bichromate
anions in aqueous solution (with a trace amount of dichromate
anion). At pH 7.0, ca. 76% of the total chromium exists as the
chromate anion. As long as the solution pH is above 6.0, a
significant fraction of the Cr(VI) in solution will exist as a divalent
anion and will replace monovalent chloride counterions on the
polymer. Other anions in ground water such as sulfate and
phosphate will also bind to the polymer but sulfate is expected to
be the main competing ion in the pH range 6-8 due to mineral
solubility considerations.
The binding of chromate with polymer creates membrane
selectivity since the polymer-chromate complex is too large to
pass through pores in the membrane. Ions which are not bound
to the polymer pass freely through the membrane. Due to the
membrane transparency for unbound ions there is no salt or brine
retention by the membrane and the process can be carried out at
low applied pressures (10-100 psi). Figure VIII depicts the
membrane separation.
Permeate water from the membrane is of lower ionic strength
than the feed water and is substantially reduced in chromate
concentration. In laboratory experiments PEUF has been used to
remove 99.9% of chromate from feed water containing as much
as 5 ppm Cr(VI). The retentate solution from the membrane is a
concentrated solution of polymer and chromate (and sulfate, if
present). This mixture is treated to separate the polyelectrolyte
for reuse in the process. Although several mechanisms might be
used for polymer-chromate separation in the retentate liquid, one
method is theaddition of barium chloride at less than stoichiometric
ratios to precipitate sulfate and chromate as a compact solid
waste.
Field Test:
A small scale field test of the PEUF process forthe removal of
chromate from ground water at the U. S. Coast Guard Support
Base in Elizabeth City, North Carolina, was conducted in March
1993. Figure IX gives a schematic of the overall process which
was employed. The field test consisted of a complete process
implementation to remove Cr(VI). The ground water from three
different monitoring wells contained Cr(VI) levels of 2.1 to 3.8 ppm,
as well as several hundred ppm of dissolved solids including ca.
Feed Solution
Permeate
Na+ Cl-
Retentate t
poly + chromate
Figure VIII Ultrafiltration of the aqueous mixture.
-------
POLYELECTROLYTE-ENHANCEDULTRAFILTRATION
for Chromate Removal
Membrane System
Ground
Water _
Feed
Polymer
Feed
Polymer
Makeup
Permeate Disposal
H2O,Na+,CI_- °r. . ..
Remjection
Retentate
Polymer 1[
Feed
Figure IX Qualitative schematic of Elizabeth City UF test apparatus.
90 ppm sulfate, as shown below. The feed water pH was above
6.0 and no pH modification was used in this test. The Cr(VI) and
sulfate concentrations detected in several wells were as follows:
Well #2, 3.1 ppm, 85 ppm; Well #12, 1.5 ppm, 82 ppm; Well #13,
3.8 ppm, 86 ppm.
The single stage pilot UF apparatus was capable of producing
150 gal/day of purified water. The pilot test was conducted over
a five day period. The maximum operating time per day was
12 hours and the longest continuous period of operation was
8 hours. An initial charge of 7 gal. of polymer concentrate was
used and this was not augmented during the test period. The UF
system was operated at an applied pressure of 50 psig with a feed
water recovery (as permeate) fraction of 0.75 to 0.80. Ground
water was pumped from a monitoring well at a nominal rate of 450
mL/min, mixed with polyelectrolyte solution (150 mL/min), and
then ultrafiltered to produce a permeate (purified water) and a
retentate solution containing polymer, chromate, and sulfate
ions.
The retentate solution was treated by addition of a small
quantity (less than stoichiometric) of barium chloride to precipitate
barium sulfate and barium chromate and to partially regenerate
the polymer. The renewed polymerwasthen filtered and recycled
to mix with fresh ground water. A very limited quantity of solid
precipitate was produced in this test. It was not possible to
accurately measure the total barium chromate and barium sulfate
solids because these collected in both the polymer retentate tank
and on the cartridge filters used in the test. An estimate of the
maximum quantity of precipitate which could be produced can
easily be made. If it is assumed that the well water contains
sulfate at 90 ppm and chromate at 3 ppm (as hexavalent chromium)
then complete removal of both these ions as the barium salt
would produce 0.83 grams of BaSO4 and 0.055 grams of BaCrO4
for each gallon of ground water treated.
The polyelectrolyte-enhanced ultrafiltration process performed
flawlessly during the several day test period. Some permeate
analyses for Cr(VI) were done on site by ion chromatography with
UV detection of the diphenylcarbazide complex. Ion
chromatography analyses were also conducted for sulfate.
Chromate levels in the permeate over a several day period were
at virtually the detection limit ofthe ion chromatography apparatus
[<50 ppb as Cr(VI)]. Sulfate levels were in the range of 1 to 5 ppm
in the permeate. Additional on-site colorimetric analysis with
diphenylcarbazide, and analysis of permeate samples brought
back to the laboratory, indicated a possible range of Cr(VI)
concentration of 30 to 70 ppb, as measured in various UF
permeate samples over the course ofthe test.
The PEUF process has several advantages in ground-water
treatment for chromate removal. The purified water produced by
the PEUF process has an ionic strength similarto or less than the
original ground water. In the Elizabeth City test a few ppm of
Cr(VI) and ca. 80 ppm of sulfate were removed from the ground
water and replaced with chloride ion. The product water was
substantially reduced in ionic strength relative to the feed ground
water. This may be contrasted with the great increase in ionic
strength of product water (relative to feed water) from the
conventional reduction-precipitationtreatmentforCr(VI) removal.
-------
Thus, the PEUF product water is suitable for reuse, or reinjection
to aid in the remediation process, or direct disposal. The
chromate waste is concentrated into a small volume which makes
possible either further treatment or easy recovery of Cr(VI). The
PEUF process is continuous and does not use large volumes of
acid or base for pH modification in contrast with conventional
reduction treatment. The polyelectrolyte can be reused many
times in the process. Additional direct applications of the PEUF
process are found in the removal of arsenic and selenium from
aqueous streams. Toxic anion removal from aqueous streams by
the PEUF process is the subject of a patent issued to the
University of Oklahoma (U.S. #5,302,290).
Hydraulically-Passive Redox Barriers: Dilute Plume
Background:
A variety of field, laboratory and modeling studies were
conducted to evaluate the viability of using hydraulically-passive,
chemically-reactive barriers for remediation of the dilute
subsurface chromium contaminant plume at Elizabeth City. Ideally,
these barrier systems would operate by installing the reactive
barrier in the path of migration of the contaminant plume. The
plume would move through the reactive barrier due to the existing
regional gradient; hence, the barrier would be hydraulically
passive. Material in the reactive barrier would cause an in situ
reaction that results in immobilization of the dissolved chromium.
Various in situ reactions or contaminant removal mechanisms
have been proposed for reactive barriers. Starr and Cherry
(1994) discuss five groups of ;ns/fu reactors based on contaminant
removal mechanisms (e.g., sorption, precipitation), chemical
delivery (e.g., dissolution of reactants, nutrients), or physical
process (e.g., air sparging) proposed for the reactive material or
reactive portion of the barrier. The focus of their study was on the
hydraulic efficiency of a dual barrier (funnel-and-gate) system
that uses low permeability sections (funnels) to direct contaminated
water through higher permeability materials (gates) in which
reactions would effect contaminant removal.
A redox barrier has been proposed for the dilute chromium
plume at Elizabeth City, North Carolina (Powell etal., 1995; Puls
et al., 1995). The reactive material in the redox barrier would be
native aquifer material amended with iron filings. The proposed
redox reaction would involve reduction of Cr(VI) to Cr(lll) and
oxidation of ferrous (Fe+2) iron to ferric (Fe+3) iron. The reacted
species would then precipitate as insoluble hydroxides (Powell et
al., 1995). This research focuses on the hydrogeological
considerations of implementing such barriers.
Field Studies:
A series of site characterization activities were completed at
the Elizabeth City site. These activities focused on quantifying
hydraulic characteristics of the shallow ground-water formation.
Hydraulic conductivity values for the various monitoring wells
were developed from slug tests (Bouwer, 1978). Variations in the
regional gradient were assessed from synoptic water level
observations of the monitoring well network. Finally, the potential
impacts on the regional gradient of the river (e.g., wind waves,
tidal fluctuations) immediately adjacent to the field site were
assessed through continuous water level recorders installed in
selected monitoring wells. Ongoing drilling operations provided
aquifer material for subsequent laboratory studies. Analyses
from periodic sampling episodes were used to generate estimates
of total mass and migration of the center of mass of the dilute
chromium plume.
In general, the monitoring wells at the Elizabeth City site
(Figure X) are completed at two different depths (15 ft or 20 ft)
within the shallow aquifer. The shallow wells are screened over
the bottom 5 feet; the deeperwells are screened over the bottom
10 feet. Hydraulic conductivity values derived from the slug tests
(Table V)showtheaquiferto be a fairly uniform, highly conductive,
sandy material. However, there does appear to be a slightly
lower permeability lens running east-west (parallel to the river)
about 60 to 80 feet out from the source zone. The regional
gradient (Figure XI) is slightly southeast to northwest and does
not deviate seasonally more than a few degrees.
Hangar Bldg. 79
MW3
MW21 •
MW26
MW17 -
MW18
MWs 30, 29, 28
MW15
MWs 24,23,22
MW16
Nprth
MW27
MW33
MW19 RX5 MW20
20ft
MW34_ +
MW32 sparge wells
Pasquotank River
Figure X Monitoring well locations at Elizabeth City.
-------
Table V Hydraulic Conductivity Values from Slug Tests on Selected Monitoring Wells (Calculated Based on Both Partially and Fully
Penetrating Assumptions)
Well
Ml/1/3
MW11
MW16
MW16a
MW17
MW18
MW19
MW20
MW20b
MW22
MW27
MW27b
MW28
MW28b
MW29
MW29b
MW32
MW32b
Casing
Diameter
(in)
2
4
2
2
2
2
2
2
2
2
2
2
Total
Depth
(ft)
15
25
15
15
15
15
15
50
20
20
20
15
Elevation
Top of
Casing
(ft)
107. 16
107. 15
106.74
106. 74
106.7
105.86
105.88
106.8
106.29
106.45
106. 73
106.4
Screened
Interval below
T.O.C.
(ft)
92. 16 ®
82. 15 ®
91.74®
91.74®
91.7®
90.86®
90.88®
56.8®
86.29 ®
86.45 ®
86. 73 ®
91.4®
97.16
87.15
96.74
96.74
96.7
95.86
95.88
66.8
96.29
96.45
96.73
96.4
Hydraulic Conductivity
(ft/day)
partially fully
penetrating penetrating
5.1
15.9
2.4
2.6
2.9
1.3
2.6
8.2
5.3
4.7
0.9
0.3
8.9
8.8
5.3
5.4
6.1
2.5
4.9
15.2
2.3
2.5
2.8
1.2
2.5
8
5.2
4.3
0.8
0.3
8.6
8.6
5.1
5.3
5.9
2.4
The observed movement of the contaminant plume is slower
than would be predicted based on the measured hydraulic
gradient and conductivity values and projected adsorption effects.
It was hypothesized that weather-related or tidal effects in the
adjacent river could have reversed the gradient of the shallow
ground-water formation for extended periods in the past. The
supposed gradient reversal was proposed as one factor
contributing to the slow migration of the contaminant plume. A
three-month period of continuous water level observations showed
infrequent disturbances in the regional gradient. In addition, the
magnitudes of these disturbances were small and readily
attenuated by the highly conductive aquifer.
Laboratory Studies:
Laboratory studies were conducted to assess the hydraulic
characteristics of aquifer materials amended with iron filings.
Aquifer materials from four different depth ranges in the
unsaturated zone were retrieved from RX5 (Table VI). Constant
head column tests, using three different flow rates, were completed
on the materials from each depth range. Hydraulic conductivity
values reported are the geometric mean of the three measured
values. The same tests were conducted on samples of aquifer
materials amended with iron filings. The results of the hydraulic
conductivity determinations showed no definite trend with respect
to the addition of iron filings. Although more decreases than
increases in hydraulic conductivity were observed for the iron
amended samples, the magnitude of the changes between the
different depth ranges far exceeds the variations due to the
addition of iron filings.
Changes in hydraulic conductivity associated with redox
barriers will result in conflicting effects. The hydraulic conductivity
of the reactive barrier must be higherthan the surrounding aquifer
in order to induce flow of the contaminant plume through the
reactive materials. However, increased hydraulic conductivity
values will result in decreased residence times within the reactive
Figure XI Regional gradient at Elizabeth City.
10
-------
Table VI Laboratory Hydraulic Conductivity Values and Ratios of
Aquifer Materials With and Without Iron Filings
Source (depth)
%Fe K (ft/day) K Ratio
RX5(10'-11')
RX5(13'-14')
RX5(15'-16')
RX5(17'-18')
0
5
20
0
20
0
5
0
20
0.56
0.15
0.28
0.71
0.19
73.6
34.8
6.36
7.41
1.00
0.27
0.49
1.00
0.27
1.00
0.47
1.00
1.16
barrier, which could reduce the effectiveness of the redox reaction.
Moreover, to achieve the higher hydraulic conductivity values, a
medium with large grain sizes would have to be added to the
native materials. The larger grain sizes will have reduced specific
surface areas which could also reduce the effectiveness of the
redox reaction. However, Starr and Cherry (1994) note that
changes in hydraulic conductivity of more than 1 orderof magnitude
above the native material result in relatively little increase in the
amount of flow through a reactive barrier.
Breakthrough curves for chloride were developed for the
aquifer materials with and without iron filings (Figure XI I) at the 13
to 14 foot depth range. The iron amended materials appear to
show less dispersion than the native materials.
Modeling Studies:
Estimates of total chromium mass in the subsurface and
location of the center of mass were developed for four sampling
episodes. The mass estimates (Table VII) and center of mass
locations were developed using spatial moments and triangular
elements (Freyberg, 1986; Knox, 1993). Refinements in the
monitoring well network over time resulted in an increased
number of wells. However, not all chromium sampling episodes
included all available wells. If the spatial network includes the
monitoring wells with low (background) levels of chromium that
are located outside the actual dissolved chromium plume, the
areal extent of the chromium plume appears to increase. Hence,
total mass estimates of dissolved chromium are artificially inflated.
The latter sampling episodes focused only on those monitoring
wells that had previously shown elevated concentrations of
dissolved chromium.
Spatial moment calculations show that the center of mass of
the chromium plume has migrated out approximately 80 feet from
the source area toward the river (Figure XIII). However, samples
from monitoring wells near the river taken during the most recent
sampling episode indicate that the chromium plume could be
discharging into the river. Total mass of dissolved chromium in
the plume is probably less than 15 kg (Table VII).
Numerical simulations of alternate barrier configurations were
completed using the USGS Method of Characteristics (MOC)
model and input data derived from the field and laboratory
studies. All simulations utilized the configuration of the plume
fromthe most recent sampling episode as initial conditions. Initial
simulations showed that the permeable barrier had to be specified
as having a transmissivity value 1000 times higher than the
surrounding native materials in order to cause discernable
changes in the flowlines of the formation. For each configuration,
the solid contour lines indicate the impact of the barrier on the flow
path and shape of the plume after eight years of transport. The
solid contour lines indicate what portions of the plume would
actually pass through the barrier. Those portions of the plume
passing through the barrier are assumed to be totally reduced
(other research evaluates Cr(VI) removal; Powell et al., 1995).
Observation wells were placed on the periphery of the barriers to
assess how much of the plume was captured.
Four basic barrier configurations (Figure XIV) were considered;
the transverse barrier, the longitudinal barrier, the Y-shaped
barrier, and the funnel-and-gate system. The funnel-and-gate
system was similar to the Y-shaped barrier, but utilized
impermeable funnels to direct flow through the permeable gate
similarto the systems analyzed by Starr and Cherry (1994). Each
barrier configuration occupied the same number of cells in the
transport grid.
Figure XV depicts simulated chromium concentrations in
peripheral observation wells overtime for each of the four barrier
configurations; hence, the figure depicts how much chromium is
not removed by each configuration. In general, the transverse
barrier performed best in terms of capturing the plume. Starr and
Cherry (1994) also found that barriers were most effective when
they were oriented predominantly orthogonal to the regional
gradient. The Y-shaped barrier performed slightly betterthan the
funnel-and-gate system. Given the increased complexity of
construction of the funnel-and-gate systems, questions arise as
to the viability of these systems. However, these simulations also
showed that the hydraulic conductivity of the barrier must be
much higher (two orders of magnitude) than the surrounding
media in order to significantly alter the ground-water flow,
regardless of the configuration. Forthe highly conductive formation
at Elizabeth City, these results indicate the most viable alternative
to be a transverse redox barrier wider than the contaminant
plume be placed down gradient of the plume.
BTCRX5-513-141
Figure XII Breakthrough curves for aquifer materials with and without
iron filings.
11
-------
Table VII Mass Estimates of Dissolved Chromium
Sampling Episode
June 1991
April 1992
July 1992
March 1993
June 1993
September 1993
Total Mass
(kg)
9.4
8.0
4.12
4.63
5.04
14.3
Monitoring
Samples in
12
10
12
7
11
8
Well
Grid
Summary / Recommendations
Advanced remediation efforts should consider innovative
processes for each of three regions: (1) the source zone soils,
(2) the concentrated portion of the contaminant plume, and (3)
the dilute portion of the plume. This environmental research brief
reports on innovative measures for addressing each of these
three regions. For the source zone, surfactant-enhanced
chromium extraction was evaluated for expediting the removal of
chromium from the source zone soils, thereby mitigating the
continual feeding ofthe ground-water plume. Forthe concentrated
plume, polyelectrolyte-enhanced ultrafiltration (PEUF) was
evaluated as an innovative treatment process with desirable
operating characteristics (less sludge production, higher quality
final water, etc.). Relative to the dilute plume, the hydrogeological
effectiveness of hydraulically-passive, chemically-reactive barrier
systems were evaluated (in situ reduction of Cr(VI) to Cr(lll)).
Batch studies demonstrated that surfactant systems can
enhance chromium elution by a factor of 2 to 3 (surfactants alone)
to an order of magnitude (surfactant-complexing agent systems)
versus water alone. While column studies corroborated results
of batch tests, they also illuminated operational considerations
that require additional research. Thus, while this technology
shows great promise for enhancing chromium extraction from
soils, further laboratory and field-scale studies are necessary
prior to full-scale implementation.
The polyelectrolyte-enhanced ultrafiltration (PEUF) system
performed very well during the pilot-scale field demonstration,
reducing permeate chromium concentrations to near or below
the detection limit (<50 ppb as Cr(VI)). Permeate sulfate levels
were reduced to the 1 to 5 ppm range as well. These results were
achieved without significant increases in the ionic strength ofthe
treated water, as experienced using conventional reduction-
precipitation processes. This along with the reduced chemical
demands ofthe PEUF process, make it a very attractive process.
The successful field demonstration reported herein will help to
expedite its utilization in full scale systems.
The hydraulic characterization activities demonstrated that
the aquifer is relatively homogeneous with respect to hydraulic
conductivity and that the natural gradient, which is from the
source area toward the Posquotank River, did not deviate
significantly with time. Numerical modeling studies focused on
identifying optimal orientation and configuration for the
hydraulically-passive, chemically-reactive barrier. The difference
in hydraulic conductivity between the barrier and the aquifer
necessary to alter the flow lines of the contaminant plume was
found to be three orders of magnitude. These differences are not
practical given the fairly high conductivity ofthe natural aquifer.
The optimal orientation forthe barrierwas found to be orthogonal
to the flow path of the plume. Hence, a simple linear, down
gradient barrier, wider than the plume and orthogonal to the flow
path, is recommended. As constructed, the barrier is a continuous,
hanging (i.e., not keyed into an underlying impermeable formation)
wall 150 feet long. The barrier is only slightly more permeable
than the natural aquifer, but is longer than the plume is wide;
hence, the entire contaminant plume should eventually traverse
the barrier. Headless across the barrier will be monitored to
determine residence times ofthe flowing fluids.
While each of these processes is at different stages of
development, it is proposed that the collective use of these three
innovative technologies will significantly improve the remediation
of subsurface chromium contamination. Future research should
continue to evaluate the synergism of these and other innovative
technologies as we strive to optimize remediation of subsurface
contamination.
June 93
June 91
April 92
MW17
July 92
Sept 93
North
March 93
5ft
RX5
+
Pasquotank River
Figure XIII Center of mass locations based on spatial moment
calculations.
Disclaimer
The U.S. Environmental Protection Agency through its Office
of Research and Development partially funded and collaborated
in the research described here under Cooperative Agreement
No. CR-820736 to Rice University. It has been subjected to the
Agency's peer and administrative review and has been approved
for publication as an EPA document. Mention of trade names or
commercial products does not constitute endorsement or
recommendation for use.
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 ofthe quality assurance
activities and results are available from the Principal Investigator.
12
-------
Transverse Barrier
Cr Plume
ouu
700
600 •
500 •
400
300
200
100 •
,*. Observation
*** Well
§r « years
^T^
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i i i i i i i i i
100
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400 500
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Heads and Velocities
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X(ft)
400 500
100
200 300
X(ft)
400 500
Figure XIV Redox barrier configurations used in numerical simulations (a) transverse barrier; (b) longitudinal barrier; (c) Y-shaped barrier; (d) funnel
and gate.
13
-------
Cr Plume
800
700 -
600 -
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§400 -
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300
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800
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t t t t t t 11 t 11 t t t t t t t t t ttt_U. 1.404-1 I I I I If
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1111111111111111 111 tLU-H-,1.6bt ttttttt
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1111111111 \ \ \\\ \\\ \ \ UA
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100
H h
200 300
X(ft)
400 500
Figure XIV Redox barrier configurations used in numerical simulations (a) transverse barrier; (b) longitudinal barrier; (c) Y-shaped barrier; (d) funnel
and gate.
14
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Cr Concentrations at Observation Wells
0.5
0.4 --
to.3
o
o
O
6 °-2
'V ,*w*
''
/ Longitudinal Barrier
/••v
Time (years)
Figure XV Chromium concentrations for monitoring wells peripheral to redox barriers.
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