SEPA
United States
Environmental Protection
Agency
EPA/540/SR-00/503
October 2000
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology
Summary
In Situ Bioremediation by
Electrokinetic Injection
Randy A. Parker, Elif Chiasson, Robert J. Gale and John Pardue
Electrokinetics, Inc. through a coop-
erative agreement with the U.S. Envi-
ronmental Protection Agency's National
Risk Management Research Laboratory
(NRMRL), conducted a laboratory evalu-
ation of electrokinetic transport as a
means to enhance in-situ bioremediation
of trichloroethene (TCE).
Four critical aspects of enhancing
bioremediation by electrokinetic injec-
tion were investigated:
1) Determination of the efficiency of
injection of representative nutrient
ions into both homogeneous and
heterogeneous soil masses with
high and low hydraulic
permeabilities;
2) Shake flask studies to confirm that
the injectable ions are capable of
enhancing TCE degradation and to
determine the kinetic parameters;
3) Bench-scale studies to optimize in-
jection and TCE degradation mecha-
nisms; and
4) A prototype study to inject ions into
a one-meter long soil mass and to
compare TCE degradation rates
versus a control.
Transport rates for ions by ion migra-
tion exceeded the electroosmotic rate,
thereby permitting a reasonably uniform
distribution of the process additives
across soil mass boundaries. Soil me-
dia with widely varying hydraulic
permeabilities are shown to have simi-
lar ion injection rates when stratified.
This tends to ensure a homogeneous
distribution of ionic additives and nutri-
ents in heterogeneous zones requiring
bioremediation.
Shake flask tests of benzoic acid-en-
hanced TCE degradation gave first or-
der rates of 0.040 ± 0.005/day and 0.033
± 0.005/day for concentrations at 6 and
50 ppm, respectively. Sulfate was found
to be as effective as benzoic acid in
enhancing TCE degradation at the 6 ppm
level, but not as effective at the 50 ppm
level.
Prototype cell experiments of TCE
degradation were demonstrated follow-
ing the electrokinetic injection of ben-
zoic acid. Although volatile losses of TCE
from the columns were significant and
made precise analyses difficult, a deg-
radation rate of 0.039 ± 0.007/day for
TCE, measured at the peripheral of the
column, was in good agreement with the
shake flask degradation rate value. Col-
umn studies showed similar rates to
flask studies at its peripheries but it was
not possible to inject benzoic acid ho-
mogeneously throughout the one-meter
column. It is concluded that the rate of
injection of a carbon source must be
compatible with its microbial degrada-
tion kinetics in order to ensure homoge-
neous distribution. A theoretical model
has been developed for carbon source
injection and for the control of injection
by modification of the electrode well com-
positions.
This Emerging Technology Summary
was developed by NRMRL, Cincinnati,
OH, to announce key findings of the re-
search project that is fully documented in
a separate report of the same title (see
Project Report ordering information at
back).
80% Recycled/Recyclable
Printed with vegetable-based ink on
§aper that contains a minimum of
0% post-consumer fiber content
processed chlorine free
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Introduction
Electrokinetic remediation employs elec-
trode^ placed across a soil mass with a
low-level DC electrical current density or a
low electrical potential difference to trans-
port species under coupled and/or un-
coupled conduction phenomena. This also
results in physiochemical and hydrologi-
cal changes in the soil matrix. Generally,
externally applied fluid, or groundwater,
acts as the conductive medium. The mi-
gration of species under electric fields is
influenced by the prevailing electrolysis
reaction at the electrodes. The electrical
conduction phenomena, with other physi-
cal and chemical transformations in the
soils, will comprise the basic mechanisms
for the electrokinetic remediation tech-
nique. This technology has mainly been
studied for the remediation of inorganic
species; however, it is possible to employ
electrokinetics in bioremediation to engen-
der an effective level of uniformly injected
nutrients, electron acceptors/donors, mi-
crobes, and other process additives in a
heterogeneous soil matrix. The process
additives can be injected into the system
at the electrodes by the electrolysis reac-
tions that occur when the soil is charged,
or by cycling the process fluid. Develop-
ments in understanding multi-species
transport under electric fields suggests that
electrokinetic injection can be efficiently
used to overcome the difficulties associ-
ated with the hydraulic injection of spe-
cies. Electrolyte neutralization techniques
could be used to effectively inject posi-
tively charged species at the anode and
negatively charged species at the cath-
ode, while co-ions in the species intro-
duced could facilitate depolarization of the
electrode reactions to maintain a desir-
able pH environment and a low electrical
conductivity.
The eJectrokinetic technique is currently
an emerging technology for extracting in-
organic or selected organic species. Addi-
tionally, as depicted in Figure 1, it is
envisioned to use the technique for the
introduction of nutrients and process addi-
tives in soils.
Ion Injection Heterogeneity and
Optimization Studies
Two types of ion injection studies were
conducted. The first study investigated the
feasibility of injection of ammonium and
sulfate ions into a heterogeneous soil bed
(EK-01). These distinct strata comprised a
fine sand layer placed on top of a clay
layer. The second study investigated in-
jection schemes for ammonium and .sul-
fate ions, and nitrate or phosphate ions to
optimize the ion injection methodology
(EK-02 to EK-04). Generally, electroos-
Figure 1. Schematic diagram of possible field processing configuration.
motic flow ensues from anode to cathode.
Therefore, an asymmetric ion injection oc-
curs for anions and cations. Although the
migration rates for injected ions may far
exceed the electroosmotic flow rate, cat-
ions present in the soil may accumulate
close to the anode and inhibit uniform in-
jection of'the cationic species. As a conse-
quence, three experiments were designed
using natural silty-clay and the same elec-
trical field to assess the improvements pos-
sible from flushing the anolyte. In EK-02,
ammonium and nitrate ions were injected
from the anolyte and catholyte, respec-
tively, without flushing the anode compart-
ment. In EK-03, the same ions were
injected under the same conditions, but
with continuous flushing of the anolyte. In
EK-04, phosphate was used in the
catholyte instead of nitrate to assess the
effects of chemical species under identical
conditions. The bulk electrical conductivity
of the silty-clay was 46 ± 3.2 uS/pm. A
constant current density of 47uA/cm2 was
selected for these tests, which rendered a
voltage gradient of 1.05 V/cm.
Shake Flask Studies of TCE
Degradation
Shake flask studies were conducted in
order to examine the ability of benzoic
acid to stimulate TCE degradation for a
representative soil. These studies were
conducted aerobically in 120 ml serum
bottles with Teflon lined stoppers. A slurry
was produced by the addition of sterile
water (autoclaved) and homogenized by
vigorous shaking. Bottles received 80
grams of slurry (40% water by mass).
Three treatments were examined: a killed
control 0.2% (HgCI2), a live control, and
an amended (40mg/kg benzoic acid). Each
treatment was incubated at temperatures
of 20, 25, 30, and 45°C. All bottles re-
ceived TCE added in water to achieve
their final concentrations. Treatments in-
cubated at 25°C contained TCE concen-
tration at 6 ppm, all others contained TCE
concentrations of 50 ppm. In addition to
TCE, all treatments received a mineral salt
whose composition is listed in Table 1. An
innoculum, which was a 10g soil sample
of known active TCE degradation capabil-
ity, was added to the soil slurry. The
innoculum was added to reduce the in-
duction time before degradation com-
menced. This would not be necessary for
on-site treatment since the bacterial popu-
lation would have time to adapt to the
presence of TCE or other contaminants.
In addition, microcosm studies were used
to investigate two physical parameters
(concentration and temperature) which
might be important to this process. Experi-
ments were conducted for 64 days and
Table 1. Composition of Mineral Salt Media
Compound Amount (mMxIQ-3)
KH2PO4
K2HPO4
NaHCO3
MnCI2.4H2O
HBO2
ZnCI2
CoCI2.6H2O
NiCI2
CuCI2.2H2O
Na, MoO4.2H,O
1980
2010
1428
2.5 -
8
0.4
0.2
0.2
0.2
0.4
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samples were collected every 8 days.
Samples were taken by homogenizing the
complete contents of 3 bottles from each
treatment by shaking. After shaking, 5
grams of slurry was removed and placed
in 10 ml of methanol for analysis.
Bench Scale Studies of
Degradation Mechanisms.
Bench-scale tests were conducted to in-
vestigate the ability of electrokinetic injec-
tion to transport benzoic acid into the soil
column and to promote TCE degradation.
Four treatments were examined: control
(C), a benzoate-amended control (AC),
electrokinetic control with no benzoate
(EKC), and electrokinetic amended (EKA).
All four experiments were conducted for
two months. Samples were taken at 1, 30,
and 60 days. The quantitative results from
these experiments were inconclusive due
to volatilization of the low (5 ppm) initial
concentrations that were used to mimic
concentration at a natural field site.
Prototype Scale Study of TCE
Contaminated Loess Soil
Prototype studies using electrokinetic in-
jection and a control experiment without
electrolysis were compared. Glass columns
were packed with Loess soil (a surrogate
soil) that contained 100mg/kg TCE. Each
column had five sampling ports along its
length. The injection period for benzoic
acid was 25 days at constant voltage of 1
V/cm gradient across the soil. This period
was sufficient to allow at least one pore
volume of electrolyte to pass from anode
to cathode. After 25 days the soil samples
were analyzed to establish a baseline for
TCE, then samples were taken at 30-day
intervals during the bioremediation stage.
However, the TCE concentrations were
well below the initial value of 100 ppm at
the start of bioremediation. This loss is
due to both volatilization and removal of
excess water upon loading the'column!!
Electric Potential and Flow
Results for injection into a
Heterogeneous Bed
The electrical gradients in the sand and
clay layers were 1.6 and 2.4 V/m respec-
tively for an average current density of
151 uA/cm2. The back calculated conduc-
tivity of the clay layer (based on actual
cross sectional area) was 8090 uS/cm,
while that of sand was 10-12 uS/cm. Thus
the clay layer, more conductive and about
three times the depth of the sand layer,
would carry most of the current initially.
With time the conductivity increased more
rapidly in the sand layer than in the clay
layer. The" final voltage gradient dropped
to 0.5V/cm in both layers after 1028 hours
of processing. The coefficient of electroos"
motic permeability is not a constant, but
increased in time from an initial value of
1.5x10-= to a final value of 9.5x10* cmWs.
The transport rate for ammonium and
sulfate ions was faster across the sand
layer in the first 200 hours due to its higher
electrical gradient. The effective transport
rates were 18 and 13 cm/day for the sand
and clay layers, respectively as found for
these soils independently. The transport
rates decreased and after 1028 hours of
processing the final ammonium concen-
trations were 300-350 mg/L across the
sand layer and 250-300 mg/L across the
clay layer.
, There was no sulfate ion transport in the
clay layer until the boundary condition and
the anode increased above the initial sul-
fate concentration. The sulfate transport in
the clay occurred from cathode to anode
against the electroosmotic flux of about 3
cm/day. After 596 hours, the sulfate ions
saturated both the sand and clay layers at
700-800 mg/L, and reached 900-1000 mg/
L in the sand and 1000-1200mg/L in the
clay layer after 1028 hours of processing.
It was demonstrated that the technique
of neutralizing the electrochemically gen-
erated acid and base and injecting de-
sired counter-ions can be effectively used
in heterogeneous layers with widely vary-
ing properties. The transport rates, on the
order of 10-20 cm/day, are practical for
field processing conditions with electrical
gradients of 1V/cm or less. The current
densities for this injection technology are
quite small, 15-150 uA/cm2, in this study.
Notwithstanding chemical complications,
the uniform electrokinetic injection of pro-
cess additives and nutrients for in-situ
bioremediation is confirmed to be viable
in these soils.
Effect of TCE Concentration on
Degradation Rates „ ,__'.„.._
Experiments to determine whether there
is a correlation between TCE concentra-
tion and degradation rates were conducted
using the same soil and techniques, but
with concentrations of TCE varied by an
order of magnitude. Concentration data
from experiments conducted at the 6 ppm
and 50 ppm levels were normalized to
allow for direct comparison of degradation
rates. The results indicate that under en-
hanced conditions the degradation of TCE
is independent of the initial concentration
of TCE. However, it is apparent that sul-
fate is as effective as benzoic acid for en-
hancing degradation at the 6 ppm level,
but the effects are less significant at the
higher TCE concentrations.
The degradation curves for the controls
are similar in shape, but the overall degra-
dation is less in the experiments with
higher concentrations of TCE than with
lower levels of contamination. This is due
to the fact that higher concentration of TCE
would require higher concentrations of
substrate, which was the limiting factor in
the Loess soil.
Effect of Temperature on
Degradation Rates
Experiments to assess the correlation
between temperature and degradation
rates were carried out at 20, 30, and 45°C
with and without benzoate amendment.
Tests showed that little degradation was
indicated in control treatments regardless
of temperature, while benzoate amend-
ments clearly stimulated degradation at all
temperatures examined. 30°C appears to
be the optimal temperature, but degrada-
tion was observed over the complete range
of 20°-45°C, indicating that the increased
temperature caused by the electrokinetic
process might actually stimulate degrada-
tion. Temperatures beyond 45°C may in-
hibit degradation and these temperatures
might be reached in the subsurface envi-
ronment if excessive current densities are
used in electrokinetic processing. However,
in the methodology developed by Electro-
kinetics, Inc. the injection period using elec-
trokinetics preceded the bioremediation
period and any loss of microbes may only
result in an extended induction period.
Prototype Study of TCE
Contaminated Loess Soil
Prototype scale experiments were con-
ducted to assess electrokinetic assisted
TCE degradation versus a control. Follow-
ing injection into a 1m column the TCE
first order degradation rate close to the
periphery was determined to be 0.039 ±
0.007/day for TCE, in good agreement with
degradation rate value from the anaerobic
slurry flask tests,(p.P47^±,p.009/day). The
benzoate amended columns contained
lower chlorinated metabolites of TCE deg-
radation, primarily cis-dichloroethylene
(cis-DGE) and trans dichloroethylene
(trans-DCE). These two breakdown prod-
ucts have been shown to be biological
degradation products of TCE. TCE ap-
peared to be minimal in the unamended
columns and only trace levels of metabo-
lites were detected.
Conclusions
These experiments demonstrate that
electrokinetic injection to engineer degra-
dation of recalcitrant hydrocarbons, or
other difficult to degrade contaminants, is
feasible in principle. Levels of ionic miner-
als suitable for bioremediation enhance-
ment are readily injected at adequate
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United States
Environmental Protection Agency
Centerfor Environmental Research Information
Cincinnati, OH 45268
PRESORTED STANDARD
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use
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EPA/540/SR-00/503
concentrations in a practical time frame. It
is cautioned that knowledge of the degra-
dation rates of the carbon source enhanc-
ers is critical to ensuring their
homogeneous distribution. Fundamental
studies are needed of the consumption
rates of nutrients and carbon sources by
microbes in order to engineer and opti-
mize injection protocols. To achieve homo-
geneous injection, the penetration rate must
exceed the local degradation rate of the
carbon source.
Publications detailing this work are avail-
able on request from:
Electrokinetics, Inc
11552 Cedar Park Ave
Baton Rouge, LA 70809
Telephone: (225) 753-8004
Robert J. Gale, ElifChiasson and John Pardue are with Electrokinetics, Inc., Baton Rouge,
LA 70809.
Randy A. Parker is the EPA Project Officer and author and'can be contacted at:
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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