United States National Risk Management

Environmental Protection Research Laboratory

Agency Cincinnati, OH 45268

Project Summary

Research and Development EPA/600/SR-97/054 August 1997

This project investigated an innovative 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 difference across a soil mass applied

through inert electrodes placed in an

open flow arrangement. The application of low-level dc current across electrodes placed in the soil mass causes

physiochemical and hydrological

changes in the soil-water-electrolyte

medium leading to contaminant transport and removal.

The feasibility and efficiency of transporting lead under electric fields was

investigated in this study at pilotscale

in three one-ton Georgia kaolinite specimens spiked with lead nitrate solution.

Electrode spacing was set at 72.4 centimeters (cm). Tests were conducted

on specimens with a lead nitrate concentration of 856 milligrams per kilogram (mg/kg) to 5,322 mg/kg. Pore pressures 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 efficiently transported by electromigration

under an electric field applied across

electrodes placed in soils. After 2,950

hours of processing and an energy expenditure 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

Theoretical and Experimental

Modeling of Multi-Species

Transport in Soils Under Electric

Fields

Yalcin B. Acar, Akram N. Alshawabkeh, and Randy A. Parker

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 process, and it appears that the process

is appropriate for testing on a larger

scale.

A theoretical model was also developed for multi-component species

transport under coupled hydraulic, electric, 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, aqueous 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 distribution across the electrodes in electrokinetic remediation were modeled. Model

predictions of acid transport, lead transport, and pore pressure distribution displayed agreement with the pilotscale

results validating the formalisms offered for multi-component transport of

reactive species under an electric field.

The model also bridges the gap between the electrochemistry and mechanics 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
2

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 difference is shown in Figure 1. The applied

electrical current or electric potential difference leads to electrolysis reactions at

the electrodes generating an acidic medium at the anodes and an alkaline medium at the cathodes. The acids generated at the anode advances through the

soil toward the cathode by transport

mechanisms including ion migration due

to electrical gradients, pore fluid advection due to prevailing electroosmotic flow,

pore fluid flow due to any externally applied or internally generated hydraulic potential difference, and diffusion due to generated chemical gradients. The alkaline

medium developed at the cathode will first

advance toward the anode by ionic migration 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 employed in remediating soils from organic

and inorganic species (electrokinetic remediation), injections of microorganisms

and nutrients in bioremediation, injections

of grouts in soil stabilization and waste

containment, soil and pore fluid characterization and species extraction using penetrating probes, diversion systems for contaminant plumes, and leak detection systems 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 leadspiked 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 concentrations of about 856 micrograms per gram

(μg/g) and 1533 μg/g. A third pilotscale

test was conducted on a kaolinite/sand

mixture spiked with lead at a concentration of 5322 μg/g. Three rows of elecFigure 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)

Processing

Extraction

exchange

Processing

Extraction

exchange

Process control system

AC/DC

converter

Pb2+

Cu2+

Cu2+

Pb2+

Ca2+

Ca2+

H O+

3

Cl -

PO3 -

4

CNNO -

3

F -

OHProcessed media

Acid front

and/or anodic

process fluid

Anode +

Cathodic

process

fluid

Cathode -
3

trodes were placed in compartments in

the soil. A polyacrylite frame was placed

in each compartment to hold the electrodes. 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 container with electrode configuration is shown

in Figure 2.

Since the electroosmotic flow was expected to occur from the anode toward

both cathodes, a water tank was connected to the anode reservoir to supply

the required amount of fluid flow. Water

was collected from both cathode compartments in separate containers. In order to

avoid introduction of advection due to external hydraulic potential differences, the

hydraulic head was kept constant at zero

head difference at the cathode and anode

compartments during experiment. This provision permitted evaluation of the singular

effect of the electrical potential gradients

on water flow within the system. The

pilotscale sample in each test was composed 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, tensiometers, 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. Experiment related parameters are summarized 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 (layer1), middle layer (layer2)

and bottom layer (layer3). 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 procedure was used for the second (PST2) and

third (PST3) pilotscale tests, but with different 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. Figure 3 shows a three-dimensional contour

diagram of the mean and standard deviation 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 contained 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

Figure 2. Schematic diagram of pilotscale test container and electrode configuration

Contaminated Soil

Gundseal

0.32 cm thick.

HDPE/Bentonite

composite

6.35 cm x 76.2 cm

electrodes

Sampling tubes

(7 time intervals)

Anode

Cathode

Cathode Plexiglass

frame

Fabri-Net

(geotextile-Gundnet

composite)

12.7 cm

72.4 cm

12.7 cm

72.4 cm

12.7 cm

Wooden

container

wall

9.1 cm 4 at 18.3 cm 9.1 cm
4

Table 1. Initial Conditions for Pilotscale Tests

Parameter Test 1 Test 2 Test 3

Current (mA) 850.0 1700.0 1700.0

Container Dimensions (cm)

Width 91.4 70.0 70.0

Depth 91.4 70.0 70.0

Length 91.4 70.0 70.0

Test Duration (hr) 1,300 2,950 2,500

Current Density (μA/cm2) 132.8 132.8 132.8

Initial Soil pH 4.7 4.5 4.2

Initial Pb Concentration (μg/g) 856 1,533 5,322

Initial Water Content (%) 44.1 44.3 24.6

Initial Dry Density (g/cm3) 1.222 1.22 1.80

Initial Saturation (%) 91 91 90

Figure 3. Final lead concentration/initial lead concentration (C/Co) across the middle layer of PSTI (a) 3-D contour diagram and (b) mean and standard

deviation.

100

10

1

0.1

0.01

10

20

30

40

50

60

70

80

90

0

10

20

30

40

50

60

100

10

1

0.1

0.01

Distance from

anode (cm)

Distance across

the anode (cm)

C/Co

C/Co

(a)

100

10

1

0.1

0.01

0 10 20 30 40 50 60 70

Distance from anode (cm)

(b)

C/Co
5

formed in the soil due to consolidation.

Approximately 1% of the initial lead concentration was electrodeposited and/or precipitated 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

μg/g. Most sections of the soil contained a

final lead concentration of less than 50

μg/g, with removal efficiencies of up to

98% of the initial lead. The bottom and

middle layers exhibited similar lead concentration 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. Figure 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 affected lead sorption and retardation.

Energy Expenditure

Energy expenditure is evaluated per unit

volume of soil treated in units of kilowatthours 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 conditions 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 electric potentials. Nonlinear changes in energy expenditure early in the tests were

associated with the increase in the total

voltage applied within the first 1000 hours.

Soil pH

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 processing time (2950 hours for PST2 and

1300 hours for PST1). After 2950 hours of

conducting PST2, the acid front moved to

Figure 4. Final lead concentration/initial lead concentration (C/Co) across cell B in the middle layer of PST2 (a) 3-D contour diagram and (b) mean and

standard deviation.

2.0

1.5

1.0

0.5

0.0

10

20

30

40

50

60

70

80

90

0

10

20

30

40

50

60

2.0

1.5

1.0

0.5

0.0

Distance from

anode (cm)

Distance across

the anode (cm)

C/Co

C/Co

(a)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 10 20 30 40 50 60 70

Distance from anode (cm)

C/Co

(b)
6

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 temperature increased at different rates across

the soil and at the electrode compartments. Soil sections near the cathode experienced the highest increase in temperature (23°C to 42°C) while the anolyte

experienced the lowest increase in temperature (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 coincided with an increase in the voltage

across the soil. Temperature changes between the soil and the electrode compartments were related to the voltage distribution. 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 saturated soil. The modeling of transport parameters result in a system of differential/

algebraic equations. The partial differential 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 profile after 53 days. Both profiles display a

significant decrease in lead concentration

across the specimen. The predicted profile 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 results demonstrate that the principles of

multi-species transport under an electric

field can be formalized.

Conclusions

Ionic migration is the dominant transport mechanism for heavy metals under

an electric field, particularly when the coefficient of electroosmotic permeability is

less than 10-5 square centimeters per voltseconds (cm2/Vs). The contribution of advective 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 cm2/Vs (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

(a)

0

10

20

30

40

50

60

Distance from

anode (cm)

Distance across

the anode (cm)

10

20

30

40

50

60

70

80

90

6

5

1

0

2

3

4

6

5

1

0

2

3

4

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 10 20 30 40 50 60 70

Distance from anode (cm)

Soil pH

(b)

Initial pH

pH

pH
7

Figure 6. A comaprison of the third pilotscale test (PST3) results and modeled total lead concentration after 50 days.

flow rates however, mass flux of other

species is of the same order of magnitude. Advection under a hydraulic head

difference is significant for soils with hydraulic conductivities greater than 10-5 cm/

s. Diffusion is significant only at concentration fronts and pH fronts because of

the high concentration gradients.

Pilotscale tests conducted at initial lead

concentrations of 1533 μg/g and 5322 μg/

g resulted in higher removal efficiencies

than tests conducted at lower initial concentrations. Tests with higher initial lead

concentrations displayed significant lead

removal from the soil. More than 90% (up

to 98% in most soil regions) removal was

achieved across the second pilot scale

test except the last 7 cm near the cathode. High concentrations of lead are encountered near the cathode in all tests

due to lead hydroxide precipitation in the

high pH environment. Almost no lead is

found in the effluent or on the electrodes.

The results indicate that the placement of

the cathode, whether in direct contact with

the soil or at a distance from the soil,

greatly influences the amount of lead precipitated or electrodeposited at the cathode.

 The results of the pilotscale tests conducted in this study demonstrate the feasibility of electrokinetic soil remediation in

full field applications. Precipitation of heavy

metals within the cathode region due to

the alkaline environment is expected to

decrease the efficiency of the process and

increase energy expenditure. Enhancement techniques such as controlling the

chemistry at the cathode may be necessary to increase the efficiency of removal.

The full report was submitted in fulfillment of Cooperative Agreement No. CR816828-01-1 by Electrokinetics, Inc., under the sponsorship of the U.S. Environmental Protection Agency.

Model t=50 day

PST top layer

PST middle layer

PST bottom layer

0

2000

4000

6000

8000

1 104

1.2 104

1.4 104

Initial concentration

0 10 20 30 40 50 60 70

Distance from anode (cm)

Total lead (mg/kg)
8

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 MultiSpecies 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

EPA/600/SR-97/054

Official Business

Penalty for Private Use $300

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35