inetic Removal of Phenanthrene from Clay Soil by Periodic Electric Potential Application
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH
Part AToxic/Hazardous Substances & Environmental Engineering
Vol. A39, No. 5, pp. 11891212, 2004
Enhanced Electrokinetic Removal of
Phenanthrene from Clay Soil by Periodic Electric
Potential Application
Krishna R. Reddy* and Richard E. Saichek
Department of Civil and Materials Engineering,
University of Illinois at Chicago, Chicago, Illinois, USA
ABSTRACT
Electrokinetically enhanced in-situ flushing using surfactants has the potential to
remove polycyclic aromatic hydrocarbons (PAHs) from low permeability clay
soils; however, previous research has shown that the applied electric potential
produces complex physical, chemical, and electrochemical changes within clay
soils that affect mass transfer and overall efficiency. This article presents the
results of a laboratory investigation conducted to determine the contaminant
mass removal by using a periodic voltage application. The periodic voltage effects
were evaluated by performing four different bench-scale electrokinetic tests with
the voltage gradient applied continuously or periodically, under relatively low
voltage (1.0 VDC/cm) and high anode buffering (0.1 M NaOH) as well as high
voltage (2.0 VDC/cm) and low anode buffering (0.01 M NaOH) conditions. For
all the tests, kaolin soil was used as a representative clay soil and it was spiked
with phenanthrene, a representative PAH, with a target concentration of 500
mg/kg. A nonionic polyoxyethylene surfactant, Igepal CA 720, was used as the
flushing solution in all the tests. The voltage was applied according to a cycle of
five days of continuous application followed by two days of down time, when
*Correspondence: Krishna R. Reddy, Graduate Research Assistant, University of Illinois at
Chicago, Department of Civil and Materials Engineering, 842 West Taylor St., Chicago, IL
60607, USA; Fax: 312-996-2426; E-mail: kreddy@uic.edu.
1189
DOI: 10.1081/ESE-120030326
1093-4529 (Print); 1532-4117 (Online)
Copyright & 2004 by Marcel Dekker, Inc.
www.dekker.com ORDER REPRINTS
the voltage was not applied. The results of these experiments show that
considerable contaminant removal can be achieved by employing a high,
2.0 VDC/cm, voltage gradient along with a periodic mode of voltage application.
The increased removal was attributed to increased phenanthrene solubilization
and mass transfer due to the reduced flow of the bulk solution during the down
time as well as to the pulsed electroosmotic flow that improved flushing action.
Key Words:
Electrokinetics;
Electroosmosis;
Electrokinetic
remediation;
Phenanthrene; PAHs; Soils; Clays; Surfactants; Sorption; Solubilization;
Remediation.
INTRODUCTION
Polycyclic aromatic hydrocarbon (PAH) contamination exists at numerous sites
throughout the United States and worldwide. In particular, former manufactured
gas plant (MGP) sites have become a high priority for remediation because these
sites are often highly contaminated with PAHs and pose a substantial risk to human
health and the environment.
[1]
Since PAH contaminants are persistent and
hydrophobic, they are exceptionally challenging for cleanup efforts, and conven-
tional remediation techniques, such as thermal, stabilization and solidification,
and bioremediation methods, are often inefficient or costly, especially when the
contaminants are located within clayey and/or organic soils. An innovative
remediation technique, electrokinetically enhanced in-situ flushing, has great
potential to remediate PAH-contaminants in clayey soils. In essence, electrokine-
tically enhanced in-situ flushing is a combination of the electrokinetic and the in-situ
flushing remediation techniques, and these methods are integrated because the
electrokinetic transport mechanism of electroosmosis greatly improves flow and
soilsolution-contaminant interaction in low permeability clayey soils.
[2]
Moreover,
since PAHs are hydrophobic, surfactants are used as flushing solutions to
accomplish PAH removal by micellar solubilization.
Compared to most conventional remediation technologies, the electrokinetic
method is relatively safe, easy to implement, and economical, but the contaminant
mass transport mechanisms and the physical, chemical, and electrochemical
processes that are involved are complex and are not well understood. In around
1914, Helmholz and Smoluchowski presented one of the first theories concerning
electroosmosis,
[3]
and, although their theory is more applicable for soils with fairly
large pores, it is still widely accepted. HelmholzSmoluchowski (HS) theory
basically states that the electroosmotic flow velocity (
eo
) is directly proportional to
the applied voltage gradient (E
z
), zeta potential ( ), and dielectric constant (D) of the
fluid, and it is inversely proportional to the fluid viscosity ( )
[3,4]
:
v
eo
¼
D E
z
ð
1Þ
The zeta potential depends on the interfacial chemistry between the liquid and
solid phases,
[5]
and West and Stewart
[6]
define it as the electric potential at the
junction between the fixed and mobile parts in the double layer. The zeta potential is
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Reddy and Saichek ORDER REPRINTS
thought to be a function of many parameters including the types of clay minerals and
ionic species that are present as well as the pH, ionic strength, and temperature.
[7]
The pH at which the net total clay particle surface charge is zero is known as the
point of zero charge (PZC or pH
pzc
), and various methods, such as titration,
electrophoretic mobility measurement, or flocculation measurement, are used to
determine this value.
[8]
Generally, the mineral surface of a low acid buffering clayey
soil may become protonated under low pH conditions, and a solution that has a pH
below the pH
pzc
would cause the mineral surface and the zeta potential to become
more positively charged than a solution with a pH above the pH
pzc
. Consequently,
by HS theory, the electroosmotic flow towards the cathode could reduce, cease
or even reverse when pH is lowered from values above the pH
pzc
to values below
the pH
pzc
.
[5]
During electrokinetics, electrolysis reactions occur at the electrodes generating
H
þ
ions at the anode and OH ions at the cathode. The electromigration of H
þ
and
OH
and other ions into the soil towards the oppositely charged electrode could
result in the adsorption of ions to the mineral surface, which affects the charge on the
mineral surface and the zeta potential. In addition, chemical reactions between
migrating ions could produce changes in the conductivity of the pore solution, and
this may affect the voltage gradient at local regions along the soil profile.
[9]
Moreover, changes in pH may increase mineral dissolution,
[10]
which could increase
the amount of ions in solution, ion migration, and electroosmotic flow. Low pH
solutions may cause clay flocculation, which may produce a more open clay
structure
[11]
with a greater hydraulic conductivity. From HS theory, it is evident
that the variations in zeta potential and voltage gradient occurring at local regions
will generate a nonuniform electroosmotic flow rate along the soil profile, and this
could result in differences in pore pressure and/or cause consolidation in some
soils.
[12]
To address the problem of pH changes caused by the electrolysis reaction at
the anode, several investigators have employed various buffering solutions, and these
solutions appear to sustain or increase electroosmotic flow.
[1315]
Due to the complex transport and physicochemical processes, the electrokinetic
process can be difficult to analyze, and the situation is exacerbated when surfactant
solutions are used to remove PAHs. As the surfactant concentration increases in a
dilute aqueous solution, a concentration known as the critical micelle concentration
(CMC) is reached where the surfactant molecules begin to aggregate into tiny (1
10 nm diameter) structures called micelles.
[16]
The interior of the micelle provides a
hydrophobic region where PAH molecules may reside, and, as the surfactant
concentration increases above the CMC, due to micellar solubilization, PAH
solubility will increase considerably. One problem with surfactant solutions is that
they may possess a low dielectric constant,
[17]
which, by H-S theory, will reduce the
electroosmotic flow rate compared to water.
Of the three main surfactant types, nonionic surfactants may be the best for the
electrokinetically enhanced in-situ flushing process. This is because they are good
solubilizers, they are relatively nontoxic,
[18]
they do not strongly adsorb to clay soils
like cationic surfactants, and they should not oppose the electroosmotic flow
towards the cathode like anionic surfactants. However, as with all surfactants,
nonionic surfactant molecules are amphiphilic (partly hydrophobic and partly
hydrophilic) and attracted to interface locations, and several investigators have
Enhanced Electrokinetic Removal of Phenanthrene
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found that surfactant sorption to interfaces may continue at concentrations that are
substantially above the CMC.
[1921]
Since PAHs typically adsorb to organic
substances, and surfactants are organic, solubilized PAH molecules may partition
to sorbed surfactant molecules. In addition, some nonionic surfactants, such as the
polyoxyethylene surfactants, have molecules that may acquire charges. These
charged surfactant molecules may occur due to hydrogen bonding with the
oxyethylene group
[2224]
or by the formation of complexes