www.elsevier.com/locate/enggeo

Engineering Geology 77

Ultrasonically enhanced electrokinetic remediation for removal of

Pb and phenanthrene in contaminated soils

Ha Ik Chunga,*, Masashi Kamonb,1

aGeotechnical Engineering Research Department, Korea Institute of Construction Technology, 2311 Daewhadong, Ilsangu,

Goyangsi, Gyeonggido, Republic of KoreabGraduate School of Global Environmental Studies, Kyoto University, Yosida Honmarchi, Sakyoku, Kyoto 606-8501, Japan

Received 1 June 2003; accepted 1 July 2004

Available online 13 September 2004

Abstract

Electrokinetic and ultrasonic remediation technologies were studied for the removal of heavy metal and polycyclic aromatic

hydrocarbon (PAH) in contaminated soils. The study emphasized the coupled effects of electrokinetic and ultrasonic techniques

on migration as well as clean-up of contaminants in soils. The laboratory soil flushing tests combined electrokinetic and

ultrasonic technique were conducted using specially designed and fabricated devices to determine the effect of both techniques.

The electrokinetic technique was applied to remove mainly the heavy metal and the ultrasonic technique was applied to remove

mainly organic substance in contaminated soil. A series of laboratory experiments involving electrokinetic and electrokinetic

and ultrasonic flushing tests were carried out. Natural clay was used as a test specimen and Pb and phenanthrene were used as

contaminants. An increase in out flow, permeability and contaminant removal rate was observed in electrokinetic and ultrasonic

tests. Some practical implications of these results are discussed in terms of technical feasibility of in situ implementation of

electrokinetic ultrasonic remediation technique.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Polycyclic aromatic hydrocarbon; Electrokinetic and ultrasonic remediation technologies; Phenanthrene

1. Introduction

Several clean-up techniques have been developed

for contaminated soils; examples include pump-and-

0013-7952/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.enggeo.2004.07.014

* Corresponding author. Tel.: +82 31 910 0216; fax: +82 31

910 0211.

E-mail addresses: hichung@kict.re.kr (H.I. Chung)8

kamon@geotech.gee.kyoto-u.ac.jp (M. Kamon).1 Tel.: +81 75 753 5114; fax: +81 75 753 5116.

treat, soil vapor extraction, soil washing, water

flushing, steam extraction, bioremediation, etc. The

pump-and-treat method is ineffective due to the

requirement of large equipment with high energy

and slow removal of contaminants. The soil vapor

extraction method is not applicable to remove con-

taminants from saturated soil deposits and ground

water. The effectiveness of bioremediation depends

greatly on suitable microorganism and nutrients in the

subsurface. Therefore, much still remain to be done in

(2005) 233–242

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242234

order that a generally accepted methodology can be

developed for a broad range of applications.

Electrokinetic soil processing is a new, innovative

and cost-effective remediation technology that

employs conduction phenomena under electric cur-

rents for transport, extraction and separation in

cohesive soils. A low level direct electric current is

applied across contaminated soil deposits through

inert electrode placed in holes or trenches in the soil

filled with processing fluid. The driving mechanisms

for species transport are ion migration by electrical

gradients, pore fluid advection by prevailing electro-

osmotic flow, pore fluid flow due to any externally

applied or internally generated hydraulic potential

difference, and diffusion due to generated chemical

gradients as shown in Fig. 1. As a result, cations are

accumulated at the cathode and anions at the anode,

while there is a continuous transfer of hydrogen and

hydroxyl ions across the medium. Various laboratory

and field studies on the feasibility of the electro-

kinetic process have shown that heavy metals and

other cationic species can be removed from the

contaminated soil. The feasibility and cost effective-

ness of electrokinetics for the extraction of heavy

metals such as lead, copper, zinc and cadmium from

Fig. 1. The effects of electrokinetic phenomena on

soils have been demonstrated by many researchers

(Mitchell, 1986; Runnels and Larson, 1986; Hamed,

1990; Pamukcu et al., 1990; Pamukcu, 1994; Acar et

al., 1992, 1994, 1989; Acar and Alshawabkeh, 1993;

Acar and Hamed, 1991; Yeung, 1993; Yeung et al.,

1994; Lee, 2000; Han, 2000; Reddy and Saichek,

2002).

Numerous researchers (Iovenitti et al., 1995; Reddi

et al., 1993; Simikin and Verbitskaya, 1989) have

proposed possible mechanisms for the effect of

acoustic waves on fluid flow through porous media.

The cavitation and capillary forces are principally

responsible for the movement of fluid in porous

media. Capillary forces play an important role in

liquid percolation through fine pore channels. The

liquid films adsorbed onto pore walls during the

percolation process can be destroyed by mechanical

vibration. The seismoacoustic fields lower the capil-

lary pressure and seismoacoustic wave affects on

increasing of water saturation and flow in soil

stratum. The mechanisms responsible for the

observed increase in transport rates and unit-operation

processes due to ultrasonic energy can be divided into

two categories: (1) effects on fluid particles involving

displacement, velocity and acceleration, and (2)

porous soil media (Chung and Kang, 1999).

Fig. 2. The effects of ultrasonic phenomena on porous soil media (Kim, 2000).

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242 235

phenomena including radiation pressure, cavitation,

acoustic streaming and interfacial instabilities. An

increase in hydraulic conductivity and contaminant

removal was observed from the effect of acoustic

excitation on cohesionless soils. Various researchers

have shown the enhancement of the contaminant

migration and recovery by acoustic waves through

laboratory and field tests (Simikin and Verbitskaya,

1989; Suslick, 1988; Frederick, 1965; Murdoch et al.,

1998; Iovenitti et al., 1995; Kim, 2000). They

summarized the potential effects of acoustic wave

Fig. 3. The effects of ultrasonic phenom

on the porous grain framework of the soil and pore

fluid on Figs. 2 and 3.

Natural concentrations of heavy metals and poly-

cyclic aromatic hydrocarbons (PAHs) on soil deposits

are not high; however, studies have indicated that many

areas near urban complexes, gas station, metalliferous

mines or major roads display abnormally high concen-

trations of these elements. So it is important to prevent

the transport of these contaminants into the environ-

ments by remediating the source zones where high

concentrations of heavy metals and PAHs exist. The

ena on pore fluids (Kim, 2000).

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242236

objective of this paper is to present a laboratory

investigation that was performed to evaluate the

coupled effects of electrokinetic and ultrasonic reme-

diation technologies. Bench-scale electrokinetic and

electrokinetic and ultrasonic experiments were con-

ducted using natural clay that was spiked with Pb as a

representative heavy metal and phenanthrene as a

representative PAH. Pb is a positive charged ionic

contaminant, on the other hand, phenanthrene is a

neutrally charged nonionic contaminant. The measured

parameters during the tests included the hydrogen ion

concentration (pH) values, electrical potential, electro-

osmotic flow and at the conclusion of the experiments,

the soil was extruded and pH values and residual Pb

and phenanthrene concentrations were measured along

the length of each soil specimen. The results were

analyzed to assess the electrokinetic and ultrasonic

remedial efficiency.

2. Experimental methodology

Two types of experiments including electrokinetic

remediation experiment (EK) and electrokinetic and

ultrasonic remediation experiment (EK+US) were

carried out. The electrokinetic remediation processor

consisted of five parts: anode and cathode electrode,

test chamber, inlet and outlet, supply water reservoir

and electric power supplier. The graphite electrode is

Fig. 4. Test setup for electrokineti

used and situated on the one side (cathode part) and

the other side (anode part) of the test chamber. A

constant current of 50 mA was applied to the anode

and cathode electrodes. The test chamber is made of a

plexiglass and it has a 10 cm wide, a 10 cm height and

a 20 cm length. The chamber was filled with

contaminated soil. The inlet and outlet tubes were

installed at both parts of the chamber. The inlet tube is

connected to a reservoir and outlet tube is connected

to a burette for measuring the outflow quantity. A

reservoir was filled de-aired and de-ionized water. The

solutions in test chamber were circulated constantly

by pump to check the anolyte and catholyte pH. The

electrokinetic and ultrasonic remediation processor

consisted of eight parts: anode and cathode electrode,

test chamber, inlet and outlet, supply water reservoir,

electric power supplier, generator, converter and

acoustic horn. The transmitting acoustic horn, which

is mounted on top of the soil sample, is used for

generation ultrasound. The ultrasonic processor has a

maximum power output of 200 W with a frequency of

excitation equal to 30 kHz. The detailed schematic

view of the experimental setup is shown in Fig. 4.

Natural clay sampled at Inchon City area in Korea

was used as a soil specimen. Table 1 contains the

physical and chemical properties of this clay used in

this experiment. The index properties of this soil were

specific gravity 2.58, liquid limit 32.8%, plastic limit

22.5%, percent finer than #200 sieve 90%, specific

c and ultrasonic experiment.

Fig. 5. Accumulated flow volume with time.

Table 1

Physical and chemical properties of test clay

Items Values

Specific gravity 2.58

Percent finer than no. 200 sieve 90.0

Specific surface area (cm2 g�1) 5249

Coefficient of uniformity 4.5

Hydraulic conductivity (cm s�1) 2�10�7

Electrical conductivity (As cm�1) 1500

pH 6.5–7.13

Chemical constituents (%)

SiO2 69.20

Al2O3 13.97

Fe2O3 4.30

The others 12.53

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242 237

surface area 5,249 cm2/g, coefficient of uniformity

4.5, activity 0.114 and coefficient of permeability

2d 10�7 cm/s. The chemical properties were organic

matter content 6.74%, initial pH 6.5–7.13 and initial

electrical conductivity 1500 s/cm. Natural clay is

classified into CL in USCS soil classification system.

Natural clay was dried, crushed, pulverized and

mixed with Pb and phenanthrene solution. Pb and

phenanthrene were used as a surrogate contaminant to

demonstrate the soil contaminated by heavy metal and

PAH. For the preparation of contaminated soil, the

admixed soil specimens were thoroughly mixed with

lead of 500 mg/kg concentration and phenanthrene of

500 mg/kg concentration. The mixtures were stirred

with stainless steel spoons and all mixing operations

were performed within glass beakers. The motivation

for this mixing technique was to ensure that the Pb

and phenanthrene would be distributed evenly

throughout the soil. For each clay sample that was

prepared, the water content was optimum moisture

content, the dry density was 95% of maximum dry

density, and the degree of saturation was 100%. The

mixture was cured in a bowl more than 24 h. The

mixtures were compacted in a test cell.

The test specimen was then subjected to ultrasonic

waves at 30 kHz frequency from ultrasonic test setup

and to electric current at 50 mA from electrokinetic test

setup. Tests were continued to maximum 15 days (360

h). The hydraulic head was not applied to the cells, the

water level of inlet and outlet is same, so hydraulic

gradient was zero for all tests. A constant current of 50

mA was applied to the cells and the voltage was

measured on a daily basis during experiment. The pH

measurement was made in the anode and cathode

reservoir. The time-dependent water movement

through the soil due to electroosmosis was measured

on the outflow by graduated cylinder. After the

electrokinetic and electrokinetic and ultrasonic tests,

the cell was disassembled from the electrokinetic

system and the soil specimen was extruded and sliced

in 10 sections. The value of pH and the concentration of

Pb and phenanthrene were measured in each slice to

assess the change of chemical property in the soil

specimen and the efficiency of the process in lead and

phenanthrene removal.

3. Results and discussion

3.1. Cumulative outflow

Under the actions of electroosmotic flow and

electromigration by electric power and acoustic flow

by ultrasonic waves in fine soil, the pore water and

contaminant in fine soil is allowed to flow and migrate

from the inlet (anode side) to outlet (cathode side).

The effluent was collected at cathode side in a 500-ml

polypropylene cylinder. The accumulated flow vol-

ume with time is presented in Fig. 5. The basic pattern

of flow was relatively consistent for each experiment.

The figure shows the accumulative water flow is

varied and increased with time. The outflow began a

little later in the beginning stage of experiment on the

application of electrical and ultrasonic energy due to

time elapse on the development of their phenomena in

soil porous media.

The outflow for electrokinetic remediation test was

steadily increased in the beginning stage with an

Fig. 7. Final pH distribution across the soil specimen.

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242238

increase in the operating duration up to 250 h during

experiment, and continuously constant to the end of

experiment. On the other hand, the outflow for

electrokinetic and ultrasonic remediation test was

steadily increased in the beginning stage with an

increase in the operating duration to the end of

experiment of 360 h. In the case of electrokinetic

remediation process, a lead hydroxide is formed in

soil pore and reduced ionic strength and soil pore

space with time, so the outflow is reduced to the end

of experiment. But in the case of electrokinetic and

ultrasonic remediation process, the movement of

molecule and the porosity and permeability of soil is

occurred, so the outflow is not reduced and steadily

increased to the end of experiment as shown in Fig. 5.

The accumulated flow volume with time is higher

for electrokinetic and ultrasonic remediation test than

for electrokinetic remediation test. It means that the

ultrasonic process has a role to increase the liquid

outflow due to sonication effects. The accumulated

quantity of outflow was 120 ml/h for electrokinetic

remediation test and 143 ml/h for electrokinetic and

ultrasonic remediation test after 360 h. The final

accumulated quantity of outflow increased with 19%

due to the coupled effects for electrokinetic and

ultrasonic test by comparing with for electrokinetic

test.

3.2. pH of anolyte and catholyte

The pH of anolyte and catholyte was measured by

pH meter from the influent and effluent. Fig. 6 shows a

plot of the anolyte and catholyte pH with respect to

time. Anolyte pH was measured at the inflow reservoir

Fig. 6. Anolyte and catholyte pH with time.

and catholyte pHwasmeasured at the outflow reservoir

during an electrokinetic remediation test.

The anolyte pH dropped to values of 2–3 and the

catholyte pH rose to value of 11–13 upon the start of

the electrokinetic test regardless of different two types

of tests. Subsequently, the pH remained relatively

constant with further processing. This results from the

electrolysis of water. The oxygen gas and hydrogen

ion is generated and decreased the pH at the anode, on

the other hand hydrogen gas and hydroxide ion is

generated and increased pH at the cathode.

3.3. pH of soil

The profiles of pH across the soil specimen by

different remediation tests are shown in Fig. 7. This

demonstrates that the acid front generated at the anode

advances steadily towards the cathode, while base

front generated at the cathode remains in the cathode.

The acid front progressed to the cathode by advection,

migration and diffusion and neutralized the base at the

cathode.

The fronts have met at a normalized distance of

approximately 0.8 from the anode in the case of

electrokinetic test. On the other hand, the whole area

of soil specimen was acidified in the case of electro-

kinetic and ultrasonic test. The soil pH remained

between 4 and 8 for the entire length of the soil

specimens for electrokinetic test, but the soil pH

remained between 3 and 6 for electrokinetic and

ultrasonic test. The final pH value across the soil

specimen slightly decreased in the case of enhance-

ment test by introducing ultrasonic wave to electric

field due to increase of outflow by applied ultrasonic

energy.

Fig. 9. Normalized concentration of Pb in soil specimen.

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242 239

3.4. Electrical potential gradient

The variation of electrical potential across the soil

specimen for electrokinetic test and electrokinetic and

ultrasonic test during experiments is presented typi-

cally in Fig. 8. The electrical potential was steadily

increased during all experiment. The increase of

electrical potential in the tests is probably associated

with formation of a lead hydroxide, which will reduce

ionic strength and soil pore space. Thus, the resistance

in the soil specimen is increased and electrical

potential is increased with elapsed time.

The final electrical potentials by electrical cur-

rents from this figure were about 11 V/cm in the

electrokinetic test and about 10 V/cm in the

electrokinetic and ultrasonic test. This result shows

that the electrical potentials slightly decreased about

10% in the case of electrokinetic and ultrasonic

experiment due to removal of lead hydroxide and

decrease of electrical resistance in soil due to the

movement of molecule and the increasing of

porosity and permeability by vibration, cavitation

and sonication effects.

3.5. Pb concentration in soil specimen

The soil specimen was extruded and cut into 10

small sliced specimens with same length after

conducting the experiment to measure pH and

concentration throughout total soil specimen length.

Pb concentrations of each sliced soil specimen were

measured by laboratory chemical analysis facilities.

The Pb contaminant was allowed to migrate from the

inlet (anode side) to outlet (cathode side) under the

Fig. 8. Electrical potential with time.

actions of electroosmosis and electromigration by

electric fields and migration by ultrasonic waves.

From the results of these phenomena, finally the

contaminant is accumulated to the cathode zone or

wholly passed the cathode zone and gone outside

from the soil specimen.

Fig. 9 demonstrates the normalized Pb concen-

tration (final concentration C/initial concentration Co)

profile across the soil specimen with different

experimental conditions. The results show that the

Pb ion is migrated and transported toward the cathode

zone, and removed from the soil specimen. Thus the

soil specimen is cleaned due to extraction of

contaminant compared with initial condition. The Pb

concentration is relatively higher at the anode zone

than at the cathode zone. Overall, considering the

initial concentration of Pb in the soil, two types of

remediation techniques removed significant amounts

of Pb.

The normalized Pb concentration is varied with

experimental conditions. The normalized Pb concen-

tration in soil specimen is low in the case of

electrokinetic and ultrasonic remediation process by

comparing with in the case of electrokinetic reme-

diation process. The heavy metal contaminant such

as Pb is easily migrated and removed by electro-

osmosis and electromigration phenomena induced

from electrokinetic process. The removal efficiency

of heavy metal by electrokinetic and ultrasonic

technique is higher than that by electrokinetic

technique. The residual concentration of Pb in soil

specimen is average 0.12 C/Co for electrokinetic test

and average 0.09 C/Co for electrokinetic and ultra-

sonic test.

Fig. 11. Removal efficiency of Pb in soil specimen.

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242240

3.6. Phenanthrene concentration in soil specimen

Phenanthrene concentration in soil specimen was

measured from 10 small sliced specimens in each

experiment. The phenanthrene contaminant was

allowed to migrate from the inlet to outlet under the

actions of electroosmosis and ultrasonic wave. Since

phenanthrene is a neutrally charged contaminant,

electromigration is not act for migration of phenan-

threne toward outlet compartment. Fig. 10 demon-

strates the normalized phenanthrene concentration

profile across the soil specimens determined at the

conclusion of experiments. The results show that the

phenanthrene is migrated and transported toward the

cathode zone, and removed from the soil specimen.

The phenanthrene concentration is relatively higher at

the anode zone than at the cathode zone. Considering

the initial concentration of phenanthrene in the soil,

significant amounts of phenanthrene removed for all

techniques. The PAHs such as phenanthrene is

migrated and removed by electric and sonic phenom-

ena induced by electrokinetic and ultrasonic process.

The normalized phenanthrene concentration in soil

specimen is low in the case of electrokinetic and

ultrasonic process by comparing with in the case of

electrokinetic process. The removal efficiency of

phenanthrene by electrokinetic and ultrasonic techni-

que is higher than that by electrokinetic technique.

Reddi et al. (1993) also investigated the effect of

ultrasonic energy on enhancement of the transporta-

tion of water in porous soils. They observed an

increase in the permeability of soil specimen. The

increased permeability due to sonication can be

Fig. 10. Normalized concentration of phenanthrene in soil

specimen. Fig. 12. Removal efficiency of phenanthrene in soil specimen.

attributed to water and contaminant migration. The

residual concentration of phenanthrene in soil speci-

men is average 0.15 C/Co for electrokinetic remedia-

tion test and average 0.1 C/Co for electrokinetic and

ultrasonic remediation test.

3.7. Contaminants removal rate

The contaminant removal efficiency is calculated

from the inverse of residual concentration of contam-

inant in soil specimen. The removal efficiency of Pb is

average 88% for electrokinetic test and average 91%

for electrokinetic and ultrasonic test as shown in Fig.

11. This results show that the removal efficiency of Pb

in electrokinetic and ultrasonic process is increased

about 3.4% comparing with electrokinetic process due

to ultrasonic effect. Fig. 12 shows the removal

efficiency of phenanthrene is average 85% for

electrokinetic test and average 90% for electrokinetic

and ultrasonic test. This results show that the removal

H.I. Chung, M. Kamon / Engineering Geology 77 (2005) 233–242 241

efficiency of phenanthrene in electrokinetic and

ultrasonic process is increased about 5.9% comparing

with electrokinetic process.

The mass balance was calculated from the amount

of contaminants in soil, influent, and effluent. The

mass balances in electrokinetic test are 92% (final

amount of Pb in soil and effluent: 12% and 80%,

respectively, error 8%) for Pb contaminant and 90%

(final amount of phenanthrene in soil and effluent: 9%

and 81%, respectively, error 10%) for phenanthrene

contaminant. And the mass balances in electrokinetic

and ultrasonic test are 95% (final amount of Pb in soil

and effluent: 15% and 80%, respectively, error 5%)

for Pb contaminant and 96% (final amount of

phenanthrene in soil and effluent: 10% and 86%,

respectively, error 4%) for phenanthrene contaminant.

The high mass balances over 90% were obtained in all

experiments. The 5–10% error is acceptable for the

accuracy needed in this study.

When ultrasonic energy is applied in contaminated

soil, the viscosity of fluid phase decreased and flow

rate increased, the molecular movement increased and

sorbed contaminants mobilized, and the cavitation

developed and porosity and permeability increased.

Thus, the removal efficiency of contaminant is higher

for electrokinetic and ultrasonic test than for simple

electrokinetic test. From these results, it can be

suggested that the electroosmosis and ultrasonic

migration phenomena are efficient for the removal

of nonionic matter, and the electromigration phenom-

enon is efficient for the removal of ionic matter. Thus,

the contaminant removal rate is highest in electro-

kinetic and ultrasonic soil flushing system due to the

coupled effect of electrokinetic and ultrasonic techni-

que. This could suggest that introduction of enhance-

ment techniques, addition of ultrasonic process onto

electrokinetic process, could be effective for increas-

ing of contaminant removal rate from the contami-

nated soil.

4. Conclusions

The objective of this laboratory investigation was

to evaluate the coupled effect of electrokinetic and

ultrasonic technique for extraction of ionic and

nonionic matter from contaminated soils. A series of

tests were conducted for electrokinetic remediation

and electrokinetic and ultrasonic remediation. The

main conclusions drawn from the results of this

investigation were as follows:

1. The water and contaminant in porous soil media

is allowed to flow and migrate under the actions

of electroosmotic flow and electromigration by

electric power and acoustic flow by ultrasonic

waves for electrokinetic and ultrasonic remedia-

tion process.

2. The accumulated outflow and contaminant

removal rate are higher by addition of vibration,

cavitation and sonication effects in the case of

electrokinetic and ultrasonic remediation process

than in the case of electrokinetic remediation

process.

3. The accumulated quantity of outflow was 120 ml/

h for electrokinetic remediation test and 143 ml/h

for electrokinetic and ultrasonic remediation test

after 360 h, so the quantity of outflow increased

with 19% due to the coupled effects of electro-

kinetic and ultrasonic phenomena.

4. The final pH value across the soil specimen

slightly decreased due to increase of outflow and

further advance of acid front in the case of

enhancement test by introducing ultrasonic wave

to electric field.

5. The removal rates of Pb and phenanthrene are

average 88% and 85% for electrokinetic test and

average 91% and 90% for electrokinetic and

ultrasonic test, thus the removal efficiencies in

electrokinetic and ultrasonic process are increased

about 3.4% for Pb and 5.9% for phenanthrene by

the coupled effects of electrokinetic and ultra-

sonic phenomena comparing with simple electro-

kinetic process.

6. The new remedial technique combined with

electrokinetic and ultrasonic process can be

effectively applied for the removal of ionic and

nonionic contaminants from the ground contami-

nated with various contaminants.

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