United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-97/054 August 1997
&EPA Project Summary
Theoretical and Experimental
Modeling of Multi-Species
Transport in Soils Under Electric
Fields
Yalcin B. Acar, Akram N. Alshawabkeh, and Randy A. Parker
This project investigated an innova-
tive approach for transport of inorganic
species under the influence of electric
fields. This process, commonly known
as electrokinetics uses low-level direct
current (dc) electrical potential differ-
ence across a soil mass applied
through inert electrodes placed in an
open flow arrangement. The applica-
tion of low-level dc current across elec-
trodes placed in the soil mass causes
physiochemical and hydrological
changes in the soil-water-electrolyte
medium leading to contaminant trans-
port and removal.
The feasibility and efficiency of trans-
porting lead under electric fields was
investigated in this study at pilotscale
in three one-ton Georgia kaolinite speci-
mens spiked with lead nitrate solution.
Electrode spacing was set at 72.4 cen-
timeters (cm). Tests were conducted
on specimens with a lead nitrate con-
centration of 856 milligrams per kilo-
gram (mg/kg) to 5,322 mg/kg. Pore pres-
sures and temperatures developed
across the soil mass, electric potential
distributions, pH distributions, and lead
transport were investigated.
The results demonstrate that heavy
metals and species that are solubilized
in the anodic acid front can be effi-
ciently transported by electromigration
under an electric field applied across
electrodes placed in soils. After 2,950
hours of processing and an energy ex-
penditure of 700 kWh/m3, 55% of the
lead removed across the soil was found
precipitated within the last two cm close
to the cathode, 15% was left in the soil
before the 2 cm zone, 20% was found
precipitated on the fabric separating
the soil from the cathode compartment,
and 10% was unaccounted for. Overall,
The project adequately demonstrated
the potential applicability of the pro-
cess, and it appears that the process
is appropriate for testing on a larger
scale.
A theoretical model was also devel-
oped for multi-component species
transport under coupled hydraulic, elec-
tric, and chemical potential differences.
A mass balance of species and pore
fluid coupled with a charge balance
across the medium resulted in a set of
differential equations. Sorption, aque-
ous phase and precipitation reactions
were modeled by a set of algebraic
equations. Instantaneous chemical
equilibrium conditions were assumed.
Transport of H+, OH", and Pb2+ ions, the
associated chemical reactions, electric
potential, and pore pressure distribu-
tion across the electrodes in electroki-
netic remediation were modeled. Model
predictions of acid transport, lead trans-
port, and pore pressure distribution dis-
played agreement with the pilotscale
results validating the formalisms of-
fered for multi-component transport of
reactive species under an electric field.
The model also bridges the gap be-
tween the electrochemistry and me-
chanics in electroosmotic consolidation
of soils.
This project summary was developed
by the National Risk Management
Laboratory's Land Remediation and
Pollution Control Division to announce
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key findings of the research project
that is fully documented in a separate
report of the same title (see project
report ordering information at back).
Introduction
Electrokinetic remediation technology
using low level dc electrical potential dif-
ference is shown in Figure 1. The applied
electrical current or electric potential dif-
ference leads to electrolysis reactions at
the electrodes generating an acidic me-
dium at the anodes and an alkaline me-
dium at the cathodes. The acids gener-
ated at the anode advances through the
soil toward the cathode by transport
mechanisms including ion migration due
to electrical gradients, pore fluid advec-
tion due to prevailing electroosmotic flow,
pore fluid flow due to any externally ap-
plied or internally generated hydraulic po-
tential difference, and diffusion due to gen-
erated chemical gradients. The alkaline
medium developed at the cathode will first
advance toward the anode by ionic migra-
tion and diffusion. However, the mass
transport of the H+ ions will neutralize this
base front preventing its transport toward
the anode. Free chemical species present
in the pore fluid and/or desorbed from the
soil surface will be transported toward the
electrodes depending on their charge. The
primary driving mechanisms of species
transport are the same as the acid or
base transport mechanisms. As a result
of the transport of chemical species in the
soil pore fluid, cations will collect at the
cathode and anions at the anode. Heavy
metals and other cationic species will be
removed from the soil with the effluent, or
they will be deposited at the cathode.
Species transport mechanisms under
electric fields are envisioned to be em-
ployed in remediating soils from organic
and inorganic species (electrokinetic re-
mediation), injections of microorganisms
and nutrients in bioremediation, injections
of grouts in soil stabilization and waste
containment, soil and pore fluid character-
ization and species extraction using pen-
etrating probes, diversion systems for con-
taminant plumes, and leak detection sys-
tems in containment barriers.
Methodology
The primary objective of the pilotscale
testing was to demonstrate removal of
lead in the soil samples. Three pilotscale
tests (PST) were conducted using lead-
spiked Georgia kaolinite. This mineral was
selected for this study because of its low
activity and high electroosmotic water
transport efficiency relative to other clay
minerals. Lead nitrate [Pb(No3)2] salt was
used as the source of lead because of its
high solubility in water and its ability to
provide the necessary ionic forms of lead
and nitrate.
Two pilotscale tests were conducted on
kaolinite spiked with lead at concentra-
tions of about 856 micrograms per gram
(|ig/g) and 1533 |ig/g. A third pilotscale
test was conducted on a kaolinite/sand
mixture spiked with lead at a concentra-
tion of 5322 |ig/g. Three rows of elec-
Extraction
exchange
II
F
(
3rocessing
\
\
Process control system
-**
t
t
•+
Extraction
exchange
II
Processing
— r
Figure 1. Schematic of electrokinetic soil processing showing migration of ionic species, transport of acid front and/or processing fluid across the process
medium (Acar and Alshawabkeh)
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trades were placed in compartments in
the soil. A polyacrylite frame was placed
in each compartment to hold the elec-
trodes. Each cathode and anode series
consisted of a row of five equally spaced
electrodes with a center-to-center spacing
of 18.3 cm. The central row of electrodes
was designated as anode while the two
outer rows were designated as cathodes.
Electrode row spacing was set at 72.4
cm. The schematic diagram of the con-
tainer with electrode configuration is shown
in Figure 2.
Since the electroosmotic flow was ex-
pected to occur from the anode toward
both cathodes, a water tank was con-
nected to the anode reservoir to supply
the required amount of fluid flow. Water
was collected from both cathode compart-
ments in separate containers. In order to
avoid introduction of advection due to ex-
ternal hydraulic potential differences, the
hydraulic head was kept constant at zero
head difference at the cathode and anode
compartments during experiment. This pro-
vision permitted evaluation of the singular
effect of the electrical potential gradients
on water flow within the system. The
pilotscale sample in each test was com-
posed of two identical halves or cells. An
electrical current of 1.7 amperes (A) was
supplied to the sample at the anode. This
current was divided to supply the two cells
with .85A each. The cross sectional area
of the soil treated was 6,398 cm2 (91.4 cm
width x 70.0 cm height) and the applied
current density was 0.14 milliamperes per
centimeter (mA/cm). Voltage probes, ten-
siometers, and thermocouples were used
to monitor changes in voltage distribution,
suction, and temperature across one cell,
while the second cell was used to assess
the concentration changes with time. Ex-
periment related parameters are summa-
rized in Table 1.
Pilotscale test specimens were sampled
before, during and after the test runs. For
the first pilotscale test (PST1), the soil
was divided into three horizontal layers,
top layer (layerl), middle layer (Iayer2)
and bottom layer (layerS). Each layer was
divided into 10 longitudinal sections of 7.0
cm length and six lateral sections of equal
size. A total of 180 soil samples were
taken from the first pilot scale test. Each
sample represented a soil volume of 2703
cm3. The same sample collection proce-
dure was used for the second (PST2) and
third (PST3) pilotscale tests, but with dif-
ferent soil volumes per sample. A total of
400 soil samples were collected from the
second pilot scale test, and 80 samples
were collected from the third pilotscale
test.
Results and Discussion
Lead Transport
Three layers at different elevations were
analyzed for lead distribution in PST1. Fig-
ure 3 shows a three-dimensional contour
diagram of the mean and standard devia-
tion of the final lead distribution across
the middle layer in this test. Most of the
lead was found precipitated in the last
section close to the cathode, which con-
tained about 54% of the initial lead. The
soil across the electrodes (excluding the
last section ) contained about 40% of the
initial lead concentration, most of which
was found in permitivity of the cracks that
Wooden
container
wall
Gundseal
0.32 cm thick.
HDPE/Bentonite
composite
6.35 cm x 76.2 cm
electrodes
Sampling tubes
(7 time intervals)
Cathode
Anode
O O O O
o o o o
o o o o
o o o o
J3"O O O
o o o o
o o o o
o o o
o o o
o o o
o o o
o o o
o o o
o o o
Cathode
Plexiglass
frame
Fabri-Net
(geotextile-Gundnet
composite)
12.7cm
72.4cm
12.7cm
9.1 cm
4 at 18.3cm 9.1 cm
Figure 2. Schematic diagram of pilotscale test container and electrode configuration
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Table 1. Initial Conditions for Pilotscale Tests
Parameter Test 1
Test 2
Tests
Current (mA)
Container Dimensions (cm)
850.0
1700.0
1700.0
Width
Depth
Length
Test Duration (hr)
Current Density (|iA/cm2)
Initial Soil pH
Initial Pb Concentration (|ig/g)
Initial Water Content (%)
Initial Dry Density (g/cm3)
Initial Saturation (%)
91.4
91.4
91.4
1,300
132.8
4.7
856
44.1
1.222
91
70.0
70.0
70.0
2,950
132.8
4.5
1,533
44.3
1.22
91
70.0
70.0
70.0
2,500
132.8
4.2
5,322
24.6
1.80
90
C/Cr
100
Distance across
the anode (cm) 30
20
10
100
C/Cr
50
60
30
20
10
40
Distance from
anode (cm)
(a)
100
10
o
y 1
o
0.1
0.01
10 20 30 40 50
Distance from anode (cm)
(b)
60 70
Figure 3. Final lead concentration/initial lead concentration (C/C0) across the middle layer of PSTI (a) 3-D contour diagram and (b) mean and standard
deviation.
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formed in the soil due to consolidation.
Approximately 1% of the initial lead con-
centration was electrodeposited and/or pre-
cipitated at the electrodes.
In PST2 most of the soil across the top
layer (up to the last 7 cm of the cathode
zone) displayed more than 90% removal
with a final concentration of less than 150
|ig/g. Most sections of the soil contained a
final lead concentration of less than 50
|ig/g, with removal efficiencies of up to
98% of the initial lead. The bottom and
middle layers exhibited similar lead con-
centration profiles. More than 90% removal
(up to 98% in most parts of the layers)
was achieved across the specimen. All
layers demonstrated that most of the lead
was transported to the cathode zone. Fig-
ure 4 shows final lead concentrations and
the mean and standard deviation of final
lead concentrations across the middle layer
of PST2. Soil type, pore fluid chemistry,
and pH were the major factors that af-
fected lead sorption and retardation.
Energy Expenditure
Energy expenditure is evaluated per unit
volume of soil treated in units of kilowatt-
hours per cubic meter (kWh/m3). Energy
expenditures in pilotscale tests increased
slightly with time within the first 500 hours
of processing to about 50 kWh/m3, then it
increased linearly with time to about 325
kWh/m3 in PST1 after 1300 hours, 700
kWh/m3 in PST2 after 2950 hours, and
700 kWh/m3 in PST3 after 2500 hours of
processing. Energy costs ranged from
$16.3 to $35 per cubic meter of soil with
an electric power cost of $.05/kWh. The
results indicate that steady state condi-
tions were realized within the first 500
hours for PST1 and PST2, and within 700
hours for PST3. Energy expenditure was
directly related to the corresponding elec-
tric potentials. Nonlinear changes in en-
ergy expenditure early in the tests were
associated with the increase in the total
voltage applied within the first 1000 hours.
SoilpH
Final pH distributions in PST2 across
the middle layer of Cell A are shown in
figure 5. One-dimensional pH profiles
across the soil specimen were similar and
did not display significant changes with
depth. The results also indicated that the
pH within the anode region decreased to
about 2.5 in PST1 and to 1.5 in PST2. In
the cathode region the pH remained within
the initial value in both tests (4.7 in PST1
and 4.5 in PST2). Slight differences in pH
between the two tests at the anode region
could be related to the differences in pro-
cessing time (2950 hours for PST2 and
1300 hours for PST1). After 2950 hours of
conducting PST2, the acid front moved to
C/C0
Distance across
the anode (cm)
C/CC
Distance from
anode (cm)
O
O
1. 4
1.2
0.2
nn
:
• ,
*1
)
|
i
i
j
i
TlJt.
<
t -
)
---
10 20 30 40
Distance from anode (cm)
(b)
50
60
70
Figure 4. Final lead concentration/initial lead concentration (C/C0) across cell B in the middle layer of PST2 (a) 3-D contour diagram and (b) mean and
standard deviation.
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pH
70
60
50
Distance across 49
the anode (cm)
30
PH
20
10
(a)
B.U
7.0
I 5.0
CL
'5
1 n
-^ ^
__^r____,
Initial pH
* *
"""•"""•
r~*
X
-f--
5A
*
10 20 30 40 50
Distance from anode (cm)
(b)
60
70
Figure 5. Final pH distributions across cell A in the middle layer of PST2 (a) a 3-D contour diagram and (b) mean and standard deviation.
30-35 cm from the anode. For PST1, the
acid front was at a distance of 16 to 18
cm from the anode after 1300 hours of
processing.
Temperature Changes
Thermocouples were used to monitor
temperature changes across the soil. In
PST2, the initial temperature of the sample
before processing was 23°C. The tem-
perature increased at different rates across
the soil and at the electrode compart-
ments. Soil sections near the cathode ex-
perienced the highest increase in tem-
perature (23°C to 42°C) while the anolyte
experienced the lowest increase in tem-
perature (23°C to 35°C). The heat flux
due to electrical gradients was a function
of the electric potential gradient and the
heat transfer conductivity of the medium.
The majority of the temperature increase
occurred during the first 1000 hours of
processing. This temperature increase co-
incided with an increase in the voltage
across the soil. Temperature changes be-
tween the soil and the electrode compart-
ments were related to the voltage distribu-
tion. Most of the voltage drop occurred at
the cathode region resulting in a higher
increase in temperature in that zone.
Transport Modeling
Test results from PST3 were used for
comparison with a mathematical model of
coupled-reactive multi-component species
transport under an electric field in a satu-
rated soil. The modeling of transport pa-
rameters result in a system of differential/
algebraic equations. The partial differen-
tial equations describe fluid flow, charge,
and species transport, while algebraic
equations describe the chemical reactions
in the soil pore fluid.
Figure 6 compares the predicted lead
profile after 50 days to the measured pro-
file after 53 days. Both profiles display a
significant decrease in lead concentration
across the specimen. The predicted pro-
file is lower than the measured profile in
the first 40-45 cm from the anode, but
display similar qualitative agreement in the
cathode region. The model results and
their comparisons with pilotscale test re-
sults demonstrate that the principles of
multi-species transport under an electric
field can be formalized.
Conclusions
Ionic migration is the dominant trans-
port mechanism for heavy metals under
an electric field, particularly when the co-
efficient of electroosmotic permeability is
less than 10~5 square centimeters per volt-
seconds (cmWs). The contribution of ad-
vective transport under electric gradients
(electroosmosis) depends on the soil type.
However, even when the coefficient of
electroosmotic permeability is in the order
of 10~4 cmWs (e.g. lower activity clays at
high water contents), mass flux of H+ and
OH- ions are at least 10 times greater by
ion migration than electroosmosis. At these
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1.4104
1.2 10
1 104
D)
,§
T3
ra
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Yalcin B. Acar and Akram N. Alshawabkeh are with Electrokinetics, Inc., Baton
Rouge, LA 70809, and Randy A. Parker is with the National Risk Management
Research Laboratory
Randy A. Parker is the EPA Project Officer (see below).
The complete report, entitled "Theoretical and Experimental Modeling of Multi-
Species Transport in Soils under Electric Fields, "(Order No. PB97-193056; Cost:
$49.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Land Remediation and Pollution Control Division
National Risk Management Research Laboratory
U. S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-97/054
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