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This is to certify that the
thesis entitled

PHENANTHRENE DEGRADATION IN LOW PLASTICITY
CLAY USING AN ALTERNATING CURRENT

presented by

Disha C. Shah

has been accepted towards fulfillment
of the requirements for the

MS. degree in Environmental Engin_eering

 

 

 

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24 Tune 7,0724

 

Date

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PHENANTHRENE DEGRADATION IN LOW PLASTICITY CLAY
USING AN ALTERNATING CURRENT

By

Disha C. Shah

A THESIS

Submitted to:
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Civil and Environmental Engineering

2006

ABSTRACT

PHENANTHRENE DEGRADATION IN LOW PLASTICITY CLAY
USING AN ALTERNATING CURRENT

By

Disha C. Shah

Polycyclic aromatic hydrocarbons (PAHs) are relatively common and persistent
contaminants present in soils at contaminated sites. In this study, the effect of sinusoidal
alternating current (AC) on the concentration of phenanthrene in spiked low plasticity
clay was evaluated in a lab-scale setup. The key hypothesis evaluated was that an AC
signal can generate oxidation reactions in clay soil due to the capacitive properties of
charged clay mineral surfaces saturated with dielectric water to cause degradation of
benzene rings in PAHs. The effect of an AC application in phenanthrene spiked clay was
evaluated based on these two key parameters: (1) initial concentration; and (2) magnitude
of applied AC. Four cells were subjected to an AC having fi’equency equal to 60 Hz for
time periods ranging from 60 to 160 days. The clay was spiked for initial phenanthrene
concentration of 500 and 50,000 mg/kg and the peak to peak voltage applied ranged from
20 to 70 V (root mean square average ~ 14.1 to 49.4 V). The key finding of this study
was that in the lab-scale setup, sinusoidal AC did not decrease the concentration of
phenanthrene in the spiked low plasticity clay over the test period when compared to

control samples subjected to similar temperature conditions but not subjected to an AC.

Acknowledgements

This project has been partially funded by the National Science Foundation (NSF)
grant No. BBS-0402772. This report has not been reviewed by the NSF.

I would like to thank all individuals who helped me in making this project possible.
I wish to express my sincere gratitude to my advisor Dr. Milind V. Khire who helped me
whenever I was in need. I am also thankful to my committee members Dr. Phanikumar
Mantha and Dr. Hui Li for their valuable time and support. I am thankful to my
colleagues Emmanuel Pepprah and Gizelle Torrizo for their co-operation during the
experiments.

I am grateful to Mr. Saradhi Balla and Mr. Kirk Riley for giving me an opportunity
to work as Research assistant for two semesters for TOSC. I am also thankful to John
Wu and Karen Goryl of Detroit Diesel Corporation for giving me an opportunity to work
with them as Summer Intern and gain some experience in the “real world.”

I would also like to thank my parents and my elder brother Rutvik whom I see as
my role model. I would like to thank God to give me such a wonderful and supporting
family. Apart from them, I would also like to thank Nandagopal Methil Sudhakaran for
his healthy criticism and for the constant effort to make me independent in each way.
Finally, I would like to thank my friends back in India and here in the US. for their

constant encouragement and support.

TABLE OF CONTENTS

 

 

 

 

 

 

 

LIST OF TABLES vi
LIST OF FIGURES vii
LIST OF ABRREVIATIONS -- ix
LIST OF SYMBOLS -- - x
CHAPTER 1: INTRODUCTION- - - 1
1.1 BACKGROUND ........................................................................................................... 1
1.2 PAH CONTAMINATION IN SOILS ........................................................................... 2
1.2.1 Physical and Chemical Properties of PAH’S ................................................. 2
1.2.2 History and Sources of PAH Contamination .................................................. 4
1.2.3 Conventional PAH Cleanup Technologies ..................................................... 5
1.2.4 Key Challenges ............................................................................................. 12
CHAPTER 2: ELECTROCHEMICAL REMEDIATION 14
2.1 ECRT PRINCIPLES .................................................................................................... 16
2.1.1 Point of zero charge ...................................................................................... 16
2.1.2 Contaminant Transport Mechanisms ............................................................ 18
2.1.3 Physics of ECRT .......................................................................................... 21
2.1.4 ECRT Chemistry .......................................................................................... 28
2.2 FACTORS AFFECTING ELECTROKINETIC REMEDIATION ............................ 28
2.2.1 Electrolyte Composition and pH .................................................................. 28
2.2.2 Zeta Potential ................................................................................................ 29
2.2.3 Voltage and Current Levels. ......................................................................... 30
2.2.4 Soil Structure ................................................................................................ 30
2.2.5 Water Content ............................................................................................... 31
2.2.6 Soil Chemistry .............................................................................................. 32
2.2.7 Electrode Material, Configuration, and Spacing .......................................... 32
2.3 EFFICIENCY AND APPLICABILITY OF THE TREATMENT .............................. 34
2.4 COST OF THE TREATMENT ................................................................................... 36
2.4.1 Cost of Fabrication and Installation of Electrodes ....................................... 36
2.4.2 Electric Energy Costs ................................................................................... 36
2.4.3 Cost of Enhancement Agent ......................................................................... 37
2.4.4 Cost of Post Treatment ................................................................................. 37
2.4.5 Fixed Costs ................................................................................................... 38
CHAPTER 3: EXPERIMENTAL METHODOLOGY _ - 19
3.1 EXPERIMENTAL PROCEDURE .............................................................................. 39
3.1.1 Soil Properties ............................................................................................... 39
3.1.2 Sample Preparation ....................................................................................... 39
3.1.3 Electrolytic Cell Setup .................................................................................. 45

iv

3.2 SAMPLING AND ANALYTICAL METHODS ........................................................ 45

 

 

 

3.2.1 Sampling Frequency ..................................................................................... 46
3.2.2 Sampling Method ......................................................................................... 46
3.2.3 Extracting the Sample ................................................................................... 53
3.2.4 Preparing the Stock Solution ........................................................................ 53
3.2.5 Gas Chromatography Analysis ..................................................................... 55
3.3 QUALITY CONTROL OF THE EXPERIMENT ...................................................... 60
CHAPTER 4: RESULTS AND DISCUSSION 61
4.1 AVERAGE PHENANTHRENE CONCENTRATION .............................................. 61
4.1.1 mekr-ZOV .................................................................................................. 64
4.1.2 mek=70Vand 35V .................................................................................... 64
4.2 EFFECT on pH ............................................................................................................ 68
4.3 EFFECT ON TEMPERATURE .................................................................................. 75
4.4 ELECTRICAL PARAMETERS ................................................................................. 86
4.5 CORROSION OF ELECTRODES ............................................................................. 91
CHAPTER 5: SUMMARY AND CONCLUSIONS -- 94
REFERENCES 96

LIST OF TABLES

Table 1-1: Physical and Chemical Properties of Phenanthrene ........................................... 3
Table 3-1: Testing Program. .............................................................................................. 41
Table 3-2: Index Properties of MSU Blue Clay ................................................................ 44
Table 3-3: Preparation of Standard Solutions for 5,000 ppm Phenanthrene Solution ...... 56
Table 3-4: Preparation of Standard Solution of 50,000 ppm Phenanthrene Solution ....... 57
Table 4-1: Summary of Tests ............................................................................................ 62
Table 4-2: Average Phenanthrene Concentration in Test and Control Cells .................... 63
Table 4-3: pH and Temperature in Test Cells ................................................................... 72
Table 4-4: Thickness of Electrodes at the End of Test ...................................................... 93

vi

LIST OF FIGURES

Figure 2-1: Typical Electrochemical Cell Setup ............................................................... 15
Figure 2-2: Resistance in an AC Circuit ............................................................................ 23
Figure 2-3: Clay a “Leaky” Capacitor ............................................................................... 24
Figure 2-4: Capacitance in an AC Circuit ......................................................................... 26
Figure 2-5: Different Configuration of Electrodes ............................................................ 35
Figure 3-1: Overall Experimental Procedure ..................................................................... 40
Figure 3-2: Pictures of Experimental Test cells ................................................................ 42
Figure 3-3: Hot Air Oven .................................................................................................. 47
Figure 3-4: Samples kept for Drying in Hot Air Oven ...................................................... 48
Figure 3-5: Steel Extraction Cells ..................................................................................... 49
Figure 3-6: Glass Vials Used for Extraction ..................................................................... 50
Figure 3-7: Interior of the Temperature Chamber ............................................................. 51
Figure 3-8: Variacs Used for the Test Cells ...................................................................... 52
Figure 3-9: Dionex Accelerated Solvent Extractor Model No. 200 .................................. 54
Figure 3-10: Gas Chromatograph Instrument .................................................................... 59
Figure 4-1: Average Phenanthrene Concentration vs. Time for T-l and C-1 ................... 65
Figure 4-2: Average Phenanthrene Concentration vs. Time for T-2 and C-2 ................... 66
Figure 4-3: Average Phenanthrene Concentration vs. Time for T-3 and C-3 ................... 67
Figure 4-4: Average Phenanthrene Concentration vs. Time for T-4 and C-4 ................... 69
Figure 4-5: Effect on pH when DC is applied. .................................................................. 70
Figure 4-6: pH vs. Time for Test Cell T-l ........................................................................ 73
Figure 4-7: pH vs. Time for Test Cell T-2 ........................................................................ 74

vii

Figure 4-8: pH vs. Time for Test Cell T-3 ........................................................................ 76

Figure 4-9: pH vs. Time for Control Cell C-3 ................................................................... 77
Figure 4-10: pH vs. Time for Test Cell T-4 ...................................................................... 78
Figure 4-11: pH vs. Time for Control Cell C-4 ................................................................. 79
Figure 4-12: Temperature vs. Time for Test Cell T - 1 .................................................... 80
Figure 4-13: Temperature vs. Time for Test Cell T - 2 .................................................... 81
Figure 4-14: Temperature vs. Time for Test cell T — 3 ..................................................... 82
Figure 4-15: Temperature vs. Time for Control Cell C — 3 ............................................... 83
Figure 4-16: Temperature vs. Time for Test Cell T — 4 .................................................... 84
Figure 4-17: Temperature vs. Time for Control Cell C — 4 ............................................... 85
Figure 4-18: Cp (Parallel Capacitance) vs. Time for Test Cell T-l .................................. 87
Figure 4-19: Cs (Series Capacitance) vs. Time for Test Cell T-l ..................................... 88
Figure 4-20: Z (Impedance) vs. Time for Test Cell T-l .................................................... 89
Figure 4—21: Z (Impedance) vs. Time for Test Cell T-2 .................................................... 90
Figure 4-22 Z (Impedance) vs. Time for Test Cell T-3 .................................................... 92

viii

LIST OF ABBREVIATIONS

AC = Alternating Current

DC = Direct Current

GC = Gas Chromatography

ASE = Accelerated Solvent Extractor

GF = Granular Filter

DI WATER = Distilled water

DRE = Destruction and Removal Efficiency
ECRT = Electrochemical Remediation Technology
E0 = Electro osmosis

H-S = Helmholtz - Smoluchowski

L'ITD = Low Temperature Thermal Desorption
PAH = Polyaromatic hydrocarbons

PCB = Polychlorinated Biphenyls.

PL = Plastic Limit

LL = Liquid Limit

P1 = Plasticity Index

USCS = Unified Soil Classification System

CL = Clay

FID = Flame Ionisation Detector

VOC = Volatile organic compounds

SVOC = Semi-volatile organic compounds

ix

LIST OF SYMBOLS

I = Current

V= Voltage

Z = Impedance

SP = surface potential

szpc = pH at zero point of charge
0'0 = permanent charge

a” = variable charge

0,3 = inner sphere charge

005 = outer sphere charge

cp = total particle charge

qA = Electro osmotic flow rate

kg = coefficient of electro osmotic permeability
E = Electric field strength

A = cross-sectional area normal to flow direction
VA = Electro osmotic flow velocity
D = Dielectric constant

4' = Zeta Potential

)7 = viscosity of fluid

n = porosity of medium

V0 = voltage amplitude

a) = angular frequency
f = frequency

10 = maximum current

Vrms = Root mean square voltage
Xc = Capacitive Reactance

C = Capacitance

Canary = Energy costs

C2 = cost in $/kWh

,8 = lumped property of contaminant

d) = electrode spacing or phase difference

u = ionic mobility of species

r= tortuosity factor

R4 = delay factor

0: electrical conductivity of soil

Co = Target initial concentration of phenanthrene
Cp = Capacitance in parallel

Cs = Capacitance in series

xi

CHAPTER 1

INTRODUCTION

1.1 BACKGROUND

Contaminated soil or sediment consists of hazardous substances in solid, liquid
and/or gaseous form mixed within the soil. Usually, contaminants in the soil are
physically or chemically attached to soil particles, or, if they are not attached, are trapped
in the void space of soil matrix or are in aqueous phase of the pore water. Leaching of
contaminants from soil into infiltrated water poses major groundwater and surface water
contamination concerns. Because of food and drinking water contamination concerns, it
is necessary to remediate contaminated soil.

Key contaminants in soils or sediments can be grouped into: (1) inorganic; or (2)
organic. Among inorganic contaminants, arsenic and lead are common contaminants
(ATSDR 1990). Among organic contaminants, volatile organic compounds (V OCs) such
as benzene and semi-volatile organic compounds (SVOCs) such as polychlorinated
biphenyls, polycyclic aromatic hydrocarbons (PAHs) and pesticides are the common
contaminants (ATSDR 1990). Among many PAHs that exist in the environment,
phenanthrene, naphthalene, benzo(a)pyrene and fluoranthene are most common in soils
or sediments. Phenanthrene is the contaminant that was used in this study as the model
PAH. Due to widespread occurrence of PAHs, novel treatment technologies for

contaminated soils is necessary.

1.2 PAH CONTAMINATION IN SOILS
1.2.1 Physical and Chemical Properties of PAH’S

PAHs represent a large class of organic compounds containing two or more fused
aromatic rings made up of carbon and hydrogen atoms. At ambient temperature, PAHs
are in solid state. The general characteristics common to the class are: relatively high
melting and boiling points, low vapor pressure, and very low water solubility which tends
to decrease with increasing molecular weight. PAHs are soluble in many organic solvents
and are highly lipophilic. They are chemically rather inert. Reactions that are of interest
with respect to their environmental fate and possible sources of loss during atmospheric
sampling are photodecomposition and reactions with nitrogen oxides, nitric acid, sulfur
oxides, sulfuric acid, ozone, and hydroxyl radicals. As PAHs are hydrophobic and have
low solubility in water, their affinity for the aquatic phase is relatively low. However, in
spite of the fact that most PAHs are released into the environment via the atmosphere,
considerable concentrations are also found in the hydrosphere because of their low
Henry's law constant. As the affinity of PAHs for organic phases is greater than that for
water, their partitioning coefficients between organic solvents, such as octanol, and water
are relatively high. Their affinity for organic fractions in sediment, soil, and biota is also
high. Hence, PAHs accumulate in organisms in water and sediments and in the food
chain. Table 1-1 presents a summary of physical and chemical properties of

phenanthrene.

Table 1-1: Physical and Chemical Properties of Phenanthrene

 

Description

Phenanthrene

 

CAS No.

85-01-8

 

Chemical Structure

990

 

 

 

 

 

 

 

 

Chemical formula C14H10
Molecular Weight (g/mol) 178.22
Appearance colorless crystalline solid ‘c
Melting point (°C) 100 ‘5
Boiling point (°C) 340 b
Vapor Pressure 6.8x104 mm Hg at 25°C b
Density 0.980 g/cm3 at 4°C“
Insoluble in Water (1.2 mg/L) b
Solubility Soluble in glacial acetic acid, and other organic

solvents like ethanol, benzene, carbon disulfide,

carbon tetrachloride, diethyl ether and toluene ’

 

 

Octanol / water Partition coefficient

 

4.15 - 4.45 ‘r

 

Note :a Budavari et al., 1989; b ATSDR ,1990; c US. EPA, 1987; d Mabey et al., 1982.

 

1.2.2 History and Sources of PAH Contamination

PAHs occur abundantly in nature and are made by natural processes as well as
man-made processes. Pure PAHs usually exist as colorless, white, or pale yellow-green
solids in nature. PAHs can be derived in nature by burning gas, coal or oil, forest fires
and volcanic eruptions. PAHs can also be formed by cigarette smoke, burning jet engine,
oils, fi'om internal combustion engines, heating of any type of carbon-based material like
coal, tar, rubber, wood, oil, etc. Other sources where PAHs are produced include refining
of petroleum oil, coal ash used as fill, and incomplete combustion of crude oil.
Commercial processing of coal leads first to coal-tars, which are firrther processed to
yield pitch, asphalt, impregnating oils (creosotes for the preservation of wood), and
residue oils such as anthracene oil (IARC 1985). The concentration of PAH in coal-tars is
generally 0.75 to 1 %. Naphthalene and phenanthrene are the most abundant compounds,
occurring at concentrations of 5-10%. Long term health effects of exposure of PAHs
include kidney and liver damage, jaundice and short-term health effects include eye
irritation, nausea, vomiting, diarrhea and confusion (Illinois Department of Public
Health).

Phenanthrene is a PAH derived from fractional distillation of high-boiling coal tar
oil. It is ubiquitous in air, land and water following incomplete combustion of fossil fuels
and woods. Based on inadequate data from animal bioassays and no human data
available, US. EPA (1993a, 1987) has placed phenanthrene in weight-of-evidence group
D, not classifiable as to human carcinogenicity.

Currently there is no commercial production of phenanthrene in United States (US.

EPA 1987) Phenanthrene can be used in the manufacture of dyestuffs, explosives, drugs,

in the synthesis of phenanthrenequinone, and in biochemical research (Sax and Lewis
1987). A derivative, cyclopentenophenanthrene, has been used as a starting material for
synthesizing bile acids, cholesterol, and other steroids (IARC 1983). Some of the known
sources of phenanthrene in the atmosphere are vehicular emissions, coal and oil burning,
wood combustion, coke plants, aluminum plants, iron and steel works, foundries,
municipal incinerators, synfuel plants, and oil shale plants (U .S. EPA 1987),
manufacturing natural gas plants (MGP), mining, herbicides, pesticides and oil spills. It is
widely distributed in the aquatic environment and has been identified in surface water,
tap water, wastewater, and dried lake sediments. It has also been identified in seafood
collected from contaminated waters and in smoked and charcoal-broiled foods (IARC

1983). Phenanthrene is one of the many PAHs on EPA's priority pollutant list (ATSDR 1990).

1.2.3 Conventional PAH Cleanup Technologies

Generally PAHs are neutral and non-polar organic compounds comprising of two
or more benzene rings, which are fused together through two or more carbon atoms
(National Research Council 1997). Various conventional remediation technologies have
been used for PAH contaminated soils or sediments. Presented below is a brief

description of each of the conventional technologies used.

1.2.3.1 Thermal Desorption

For thermal desorption, contaminated soils are heated such that organic
contaminants and water volatilize. Thermal Desorption can be categorized into two
processes based on the temperature ranges: (1) low temperature thermal desorption (90 to
320 0C) and high temperature thermal desorption (320 to 560 °C). Low temperature

thermal desorption (LTTD) is an ex-situ means of physically separating Volatile and

 

semi-volatile organic contaminants from soils. Contaminated soil is placed in a chamber,
and heated to volatilize the associated hydrocarbons. The exhaust gases, containing the
hydrocarbons are consumed in an oxidizing unit, and are subsequently cooled and passed
through a bag house to remove particulate matter. The treated soil can then be returned to
the original excavation site as clean backfill. LTTD technology can be utilized as a
mobile soil remediation solution to satisfy on-site clean up. The cost of operating the
system can range from $50/tonne to $80/tonne, depending upon the soil type, moisture
content and the level of contamination (U SEPA 1990). Clay soil is difficult to treat using
this method because of the tendency to stick to process equipment and to aggregate into
large clumps which inhibits heat transfer in the thermal desorption process (U SEPA

1998)

1.2.3.2 Soil Washing
Soil washing is a water-based or co-solvent based process for scrubbing soils ex-
situ or in-situ to remove contaminants. The process removes contaminants from soils in
one of the following two ways:
. by dissolving or suspending them in the wash solution
0 by concentrating them into a smaller volume of soil through particle size
separation, gravity separation, and attrition scrubbing (similar to those
techniques used in sand and gravel operations).
Contaminants sorbed onto fine soil particles are separated from bulk soil in a water-
based system on the basis of particle size. The wash water is augmented with a basic
leaching agent, surfactant, or chelating agent or by adjustment of pH to help remove

organics and heavy metals. Soils and wash water are mixed ex-situ in a tank or other

 

treatment unit. The wash water and various soil fractions are usually separated using
gravity settling or by using hydrocyclones. Semi volatiles, pesticides, heavy metals and a
wide range of organic and inorganic contaminants can be cleaned from coarse grained
soils. By using this method, however it is difficult and expensive to remove organics

adsorbed onto clay-size particles.

1.2.3.3 Soil Flushing

In-situ soil flushing is the extraction of contaminants from the soil with water or
other suitable aqueous solutions. Soil flushing is accomplished by passing the extraction
fluid through in-place soils using an injection or infiltration process. Extraction fluids
with the desorbed contaminants or reaction by—products can be of disposal concern. They
must be recovered from the underlying aquifer and possibly recycled or reused in the
process. Low permeability clay soils or heterogeneous types of soils are difficult and

expensive to treat using this treatment method (U SEPA 1997).

1.2.3.4 Incineration

High temperatures ranging from 870 to 1,200 °C are used to volatilize and combust
(in the presence of oxygen) organics in hazardous wastes. Often auxiliary fuels are
employed to initiate and sustain combustion. The destruction and removal efficiency
(DRE) for properly operated incinerators exceeds the 99.99% requirement for hazardous
waste and can be operated to meet the 99.99% requirement for PCBs and dioxins. Heavy
metals can produce a bottom ash that requires stabilization before disposal.

Volatile heavy metals, including lead, cadmium, mercury, and arsenic, leave the

combustion unit with the flue gases and require the installation of gas scrubbing systems

 

for removal. Thus, this treatment technology may not be effective for soil containing
metals and soil having a low organic content may be costly to incinerate. This technology

is not applicable to soil requiring in-situ treatment (US EPA 1997).

1.2.3.5 Chemical Extraction

Chemical extraction does not destroy wastes but is a means of separating hazardous
contaminants from soils, sludges, and sediments. Thus, reducing the volume of the
hazardous waste that must be treated. The technology uses an extracting chemical. Waste
contaminated soil and extractant are mixed in an extractor, thereby dissolving the
contaminants. The extracted solution is then placed in a separator, where the
contaminants and extractant are separated for treatment and further use. Hydrochloric
acid and organic solvent can be used as extractant and are respectively called Acid
Extraction and Solvent Extraction. Solvent extraction is generally least effective on very
high molecular weight, organic, and very hydrophilic substances. Higher clay content

also reduces extraction efficiency and requires longer contact times.

1.2.3.6 Solidification/Stabilization

Solidification or stabilization reduces the mobility of hazardous substances and
contaminants in the environment through both physical and chemical means.
Contaminants are physically bound or enclosed within a stabilized mass (solidification),
or chemical reactions are induced between the stabilizing agent and contaminants to
reduce their mobility (stabilization). Generally inorganics including radionuclides can be
“locked” using Solidification/Stabilization. It is not common and less effective for using

this technology for organics such as PAHs.

 

1.2.3.7 Composting

Contaminated soil is excavated and mixed with bulking agents and organic
amendments such as wood chips, hay, manure, and vegetative (e. g., potato) wastes.
Proper amendment selection ensures adequate porosity and provides a balance of carbon
and nitrogen to promote thermophilic, microbial activity. Composting is a controlled
biological process by which organic contaminants (e.g., PAHs) are converted by
microorganisms under aerobic and anaerobic conditions to innocuous and stabilized
byproducts. Typically, thermophilic conditions (54 to 65 °C) must be maintained to
properly compost soil contaminated with hazardous organic contaminants. The increased
‘ temperatures result from heat produced by microorganisms during the degradation of the
organic material in the waste. In most cases, this is achieved by the use of indigenous
microorganisms. The composting process can be applied to soils and lagoon sediments

contaminated with biodegradable organic compounds.

1.2.3.8 Enhanced Bioremediation

Enhanced bioremediation is a process in which indigenous or inoculated micro-
organisms (e.g., fungi, bacteria, and other microbes) degrade or metabolize organic
contaminants found in soil and/or ground water, converting them to innocuous end-
products. Nutrients, oxygen, or other amendments may be used to enhance
bioremediation and contaminant desorption from subsurface materials. Enhanced
bioremediation techniques have been successfully used to remediate soils and
groundwater contaminated with fuel, volatile organic compounds (VOCs), semi-volatile
organic compounds (SVOCs including PAHs), and pesticides. Pilot-scale studies have

demonstrated microbial degradation of soils contaminated with munitions waste. While

 

bioremediation cannot degrade inorganic contaminants such as metals, it can be used to
immobilize these contaminants. Bioremediation is not well suited for soils with low
permeability (e.g., fine clays). Higher permeability is required to allow the nutrients to
reach the indigenous microorganisms. The bioremediation of soil contaminated with
polycyclic aromatic hydrocarbons (PAH) often is limited by a low bioavailability of the
contaminants. Tiehm et a1. (1997) showed that a surfactant-enhanced PAH mobilization
resulted in an increased PAH degradation as compared to the treatment without

surfactant.

1.2.3.9 Phytoremediation

It is a typical in-situ treatment system. Phytoremediation is a process that uses
plants to remove, transfer, stabilize, and destroy contaminants in soil and sediment.
Contaminants can be organic or inorganic. Phytoremediation is applicable for the
remediation of select metals, pesticides, solvents, explosives, crude oil and PAHs. It is
not effective for strongly sorbed (e.g., PCBs) and weakly sorbed contaminants (FRTR).
Phytoremediation of organic pollutants depends on plant-microbe interactions in the
rhizosphere, but the extent and intensity of such rhizosphere effects are likely to decrease
with increasing distance from the root surface. Also results from natural and engineered
phytoremediation systems provide strong evidence that vegetated soils mitigate
polycyclic aromatic hydrocarbon (PAH) contamination. However, the mechanisms by
which PAH mitigation occurs and the impact of plant organic matter on PAH attenuation

remains unclear. (Gregory et a1. 2005)

10

 

1 .2.3.10 Photodegradation

Photodegradation, also known as photolysis, is a method used where sunlight or
some other source of ultraviolet rays, in the presence of hydrogen peroxide (H202), are
used to lyse the ring structure of the PARS. This leaves the PAHs more susceptible to
microbial attack. Normally, the ring structure is broken down by wavelengths of 295 to
300 nm, (website, university of waterloo, CA). It has been found that the heavier the
molecule is, the more susceptible it is to photodegradation. Also, as light penetration
decreases with increased depth, photodegradation also decreases (CRC press online,

1999).

1.2.3.11 Electrokinetic Remediation Technology

Electrokinetics is a process that separates and extracts heavy metals, radionuclides,
and organic contaminants from saturated or unsaturated soils, sludges, and sediments.
Electrokinetic remediation is a process in which a low-voltage direct-current electric field
is applied across a section of contaminated soil to move and remove contaminants. The
electrical current causes electro osmosis and ion migration, which move the aqueous
phase contaminants in the subsurface from one electrode to the other. Contaminants in
the aqueous phase or contaminants desorbed from the soil surface are transported towards
respective electrodes depending on their charge. The contaminants may then be extracted
to a recovery system or deposited at the electrode. Surfactants and complexing agents can
be used to increase solubility and assist in the movement of the contaminant. In addition,
reagents may be introduced at the electrodes to enhance contaminant removal rates (EPA
1995). Saichek and Reddy (2005) surfactants were added to increase solubility of

phenanthrene.

ll

 

1.2.4 Key Challenges

The conventional technologies discussed above have specific disadvantages as

listed below:

1. The conventional treatment technologies are ineffective in low permeability soil
like natural clays. Most of the problem of PAH contamination persists in clayey
soils since they have a high tendency to sorb PAHs. Clay soils adsorb strongly
and have high a cation exchange capacity. They are also good conductor of
electricity when saturated. It is comparatively easier to remediate sandy soils
because of high hydraulic conductivity (or 10'5 cm/s). It is easier to pump
treatment liquids and extract effluents into a sandy soil and treat it because of
large pore spaces. Whereas, the same technology for clayey soil having
hydraulic conductivity less than 1045 cm/s would require tremendous amount of
pressure to treat the same area of land because of comparatively tighter
arrangement of clay particles.

2. When ex-situ treatment is carried out, the treatment is too expensive due to the
cost of soil excavation.

3. Certain conventional treatment technologies do not allow reuse of the treated
soil.

4. Not all the treatments are efficient and cost effective.

5. Naphthalene and phenanthrene are by far the most abundant PAH compounds,
occurring at concentrations of 5-10%, Hence remediation of soil contaminated
with Phenanthrene is considered herewith in this research. In addition Reddy et

al. (1997) have studied the effect of direct current applied in phenanthrene

12

contaminated soil. In this research project, an alternating current is used to

evaluate the degradation of phenanthrene in spiked low plasticity clay.

 

l3

CHAPTER 2

ELECTROCHEMICAL REMEDIATION

Most conventional technologies are not well suited for low permeability soils or
soils having high organic matter content. Electrochemical remediation has been a topic of
research for the remediation of contaminated clayey soil. (Acar et a1. 1995; Saichek and
Reddy 2005). Acar et al. (1995) have shown that Electrokinetic remediation is
appropriate for organics, inorganics and radionuclides.

Electrochemical remediation involves passage of direct or alternating current
through soil. The technique can be performed in-situ or ex situ. Its advantages include
close control over the direction of movement of water and dissolved contaminants, even
through heterogeneous soils, possible retention of contaminants within a confined zone
(Page & Page 2002). The ECRT is very different from the classical Electrokinetic
remediation technology. A typical electrochemical setup is shown in Figure 2-1.
Typically electrodes are inserted initially into the area surrounding the contaminated soil.
A relatively low voltage alternating current is applied across the electrodes. Since the
process requires a conducting pore fluid, it is generally applicable for clayey soils that are
saturated (Acar et a1. 1995).

A power generation system, which includes a power supply, a transformer kit was
used to generate the required alternating current and apply it to the test cells. The
transformer is used to adjust the voltage. A sinusoidal voltage was generated and current

I, was calculated using Ohm’s law as shown in Eq. 2-1:

14

 

AC Power

Supply
120 v

 

 

 

 

 

 

 

 

Variac

 

 

 

Neutral

 

 

 

 

 

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
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I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
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i re 2-1:Typical ec oc emical ell Se

 

I = V/Z (2-1)
where V: voltage applied across the cell and Z = impedance of the system

2.1 ECRT PRINCIPLES
2.1.1 Point of zero charge

Schwarzenbach et a1. (1993) calculated that most of the counter ions are located
within 1 to 30 nm of the clay mineral surface. Clay mineral surfaces can be covered with
hydroxyl ions, electron rich atoms or ligands, and a diffuse double layer consisting of
counter ions at 1 to 30 nm of the surface after reaching equilibrium. Moreover, the
surface hydroxyls may undergo proton exchange reactions with the aqueous solution.
These reactions mainly depend on the solution composition and the pH. Apart from H+
and OH- , other species can also bind with the surface and contribute to the surface
charge, which as a group, they are called potential determining species (Stumm and
Morgan 1996).

Consider an oxide molecule at the surface where the metal (M) atom is bound to
the surface and the hydroxyls exhibit the following reactions Eq. 2-2 and 2-3 with H+
(Schwarzenbach et al. 1993).

M—OH+2 -) M—OH + H+ (2-2)
M—OH 9 M—0' + 11" (2-3)

As pH increases, both the reactions move in forward direction and the surface
potential decreases as it becomes more difficult to move H+ away from the oxide surfaces

(Schwarzenbach et a1. 1993) . Here M—OH‘L2 is called protonated surface sites and MO‘ is

16

 

called deprotonated surface sites. When only H+ and OH" species are considered, the
abundance of protonated and deprotonated sites determines the surface charge. In case of
equal concentration of these species, the surface attains a zero net charge, and the pH at
which the surface potential (w) exhibits a zero net Charge is called the zero point of
charge (szpc).

Thus, at low pH, the protonated (M—OH”) specie has greater concentration and
determines the surface charge. The surface potential is positive in this case. Whereas at
higher pH, MO' is higher in concentration and surface potential is negative and
deprotonated specie determines the surface charge.

The charges on clay minerals are basically divided into two main categories:
permanent or variable charge. Sposito (1989) describes permanent charge (00) of the
mineral surface as the negative charge due to the isomorphic substitution, while the
variable charge due to protonation of the surface. The variable charge (on) is a function
of pH and is equal to the number of moles of H+ minus the number of moles of OH' that
forms complexes with the surface functional groups. In addition to the permanent and
variable surface charge densities, other permanent species besides H+ and OH- may bind
to the mineral surface. Sposito (1989) describes these species as inner sphere or outer
sphere charges. Hence total particle charge (op) is presented in Eq. 2-4

0p = 00 + 0H + 015 + cos (2-4)

Where 00 is always negative whereas other charges depends on the composition of
the solution. The pH at which net particle charge up = O is szpc since the ionic species in

solution do not contribute to the charge of mineral surface.

17

 

2.1.2 Contaminant Transport Mechanisms
The electrochemical remediation is accomplished through various contaminant
transport mechanisms such as electromigration, electro osmosis, electrophoresis and

diffusion (Probstein and Hicks 1993).

2.1.2.1 Electromigration

Electromigration refers to migration of ionic species in an applied electric field. In
case of clay soil, electromigration is one of the dominant modes of contaminant transport.
The ionic mobility is related to electrical conductivity of the solution and is therefore
affected by concentration, ionic charge, and temperature. The major influences on
electromigration during electrochemical remediation are field strength (as for electro
osmosis), ionic concentration, and charge of ionic species present. Typical ionic
mobilities in soil are 3 x 10'8 mz/Vs, except for H+ and OH’, which have mobilities one

order of magnitude greater (Page & Page 2002).

2.1.2.2 Electrophoresis

Electrophoresis is the movement of electrically charged particles suspended in a
liquid by the action of an electric field (Glenn et a1. 1996). Mainly, electrophoresis is due
to migration of colloidal or larger sized particles. Electrophoresis of clay colloids may
play an important role in decontamination if the colloids migrate with adsorbed
chemicals. Electrophoresis movement can contribute to the transport of contaminants in

the form of colloidal electrolytes or ionic species.

18

 

2.1.2.3 Electro osmosis

Electro osmosis (E0) is the transport of pore fluid under an electrical gradient
(USEPA 1997). Electro osmotic flow may provide the key for increasing soil—solution—
contaminant interaction though low permeability regions because it is capable of
producing a substantial flow through these regions that is far superior to the flow
achieved by an ordinary hydraulic gradient. In addition, once the contaminants are
desorbed from the soil and are solubilized into solution, electro osmosis facilitates
advective transport of the contaminant-laden solution towards the electrode compartment
for removal.

Electro osmosis provides increase in soil solution and contaminant interaction
through low permeability regions because it is capable of producing a substantial flow
through these regions that is far superior to the flow achieved by an ordinary hydraulic
gradient. Moreover, advective transport is facilitated due to electro osmosis for
transporting the contaminated solution towards the electrodes.

Clay surfaces are negatively charged, hence when they are surrounded by aqueous
solution, counter ions concentrate adjacent to the particle surface of the clay. This region
is known as diffuse double layer. Under an electrical potential, electro osmotic flow is
produced because the locally excess ions migrate in a plane parallel to the surface of the
particle towards the oppositely charged electrode. After application of electric gradient,
the counter ions in the double layer may be tightly held in place due to electrostatic
forces, but the excess counter ions that are far away from the double layer are mobile
(Eykholt 1992). Therefore, a shear layer exists that divides the fixed and mobile region.

This shear plane is located at a specific distance from the charged particle clay surface.

19

 

The electric potential at this distance is called zeta potential (Q). Zeta potential can be a
function of many parameters including type of clay mineral, ionic species that are
present, pH, temperature and ionic strength (Vane and Zang 1977). The zeta potential of
the soil varies during the process and can affect the rate and direction of electro osmotic
flow. (Miller 1955; Shapiro and Probstein 1993; Eykholt and Daniel 1994; West and
Stewart 1995).

Electro osmotic flow rate can be represented by Eq. 2-5 which is analogous to
Darcy’s law (Page & Page 2002).

qA = - keEA (25)

where qA = Electro osmotic flow rate; kc = coefficient of electro osmotic
permeability; E = electric field strength or negative potential gradient; and A = cross
sectional area normal to flow direction.

The Helmholtz — Smoluchowski (H-S) theory is a widely accepted theory for
electro osmotic flow. It applies to the system with larger pore size relative to electric
diffuse double layer. The H-S equation is represented in Eq. 2-6:

VA: - D 2; 13/11 (2-6)

where VA: electro osmotic flow velocity; D = Dielectric constant; C = zeta
potential; E = applied voltage gradient; and n = viscosity of fluid.

According to Eykholt (1992), for negatively charged mineral surfaces, the zeta
potential 6; is negative, the counter ions are cations and the electrical gradient E is positive
in the direction of the cathode. So by H-S equation (Eq. 2-3), the velocity and flow

should be directed towards the cathode. Conversely, for positively charged particle

20

surfaces, the zeta potential is positive and hence counter ions are anions, but the electrical
gradient remains positive in the direction of the cathode. Which means, due to positive
zeta potential, the velocity and direction of electro osmotic flow is negative and towards
anode.

Further, the electro osmotic flow can be given by Eq. 2-7 as follows.

qA=DCEnA/n (2-7)

where n = porosity of the medium.
Combining Eq. 2-2 and 2-4, coefficient of electro osmotic permeability can be

found out.

kc=D§n/n (2'8)

2.1.2.4 Diffusion

Diffusion of ions becomes significant near the electrodes as concentration gradients
develop (Probstein and Hicks 1993). However, diffusion is a slow process and
Electromigration and Electro osmosis are two dominant processes occurring in clayey

soils.

2.1.3 Physics of ECRT

Direct current (DC) involves current flowing in one direction only, whereas for
alternating current (AC), the voltage oscillates in a sine wave (Eq. 2-9) or any other
shaped wave pattern varying with time. Equation 2-9 shows the AC voltage firnction.

V = Vosinwt (2-9)

21

 

where V = alternating voltage; Vo = voltage amplitude; to = frequency; and t = variable
time. Resistance in an AC circuit is represented in the Figure 2-2. Because Ohm’s law
(Eq. 2-1) is applicable to an AC circuit, combining Eq. 2-1 and Eq. 2-9, we get the
following relation.

I = (VD/Z) sinmt = losinmt (2-10)

where I0 = maximum current.
In a household circuit in North America, the AC frequency is 60 Hz. The angular
frequency is related to the frequency, f, by Eq. 2-11.

(x) = 21tf (2-11)

Vo represents the maximum voltage, which in a household circuit in North America
is about 170 volts. The average value of the voltage in the household is 120 volts. The
averaging method used to obtain such value is called root mean square (rrns). Voltages
and currents for AC circuits are generally expressed as rms values. For a sine wave, the
relationship between the peak and the rrns average is as shown in Eq. 2-12.

vm, = 0.707 x vo (2-12)

A capacitor consists of two parallel plates separated by dielectric fluid or material.
An ideal capacitor, once fully charged, blocks the flow of current across the dielectric
material between the plates. Whereas, a “leaky” capacitor allows a current to flow once
charged. Clay can be considered as a leaky capacitor as shown in Figure 2-3 where the
charged flaky particles act as the parallel plates and the aqueous solution in the pores

(e.g., water) acts as a dielectric fluid. In an ideal capacitive unit, current shall lead the

22

VT

 

 

Voltage

Current

time

 

 

Figure 2-2: Resistance in an AC Circuit

23

 

 

®O©®®G@
Q 3 G G
G) G) G)
C9 0 9 G) G G tA_dsorbectll‘I
ca Ions on e
@G 9690 0C9 clay particle
Q 3 Q 0 surface
(+3 _ G) 69
(+30 0 690 G G
0 3 I G 0
Dielectric pore Clay Mineral
water Surface having
—ve charge

Figure 2-3: Clay a “Leaky” Capacitor

24

voltage by 90° which also means the same as the phase difference between current and
voltage is 90° or that current reaches the peak before voltage. The AC power supply
produces an oscillating voltage. The description of full one cycle of voltage (Elementary
Physics, 2000, Boston University) is as follows

Step 1: At point a (see Figure 2-4) the voltage is zero and the capacitor is
uncharged. Initially, the voltage increases quickly. The voltage across the capacitor
matches the power supply voltage, so the current is large to build up charge on the
capacitor plates. The closer the voltage gets to its peak, the slower it changes, meaning
less current has to flow. When the voltage reaches a peak at point b, the capacitor is fully
charged and the current is momentarily zero.

Step 2 - After reaching a peak, the voltage starts dropping. The capacitor must
discharge now, so the current reverses direction. When the voltage passes through zero at
point c, it's changing quite rapidly. To match this voltage the current must be large and
negative.

Step 3 - Between point’s c and d, the voltage is negative. Charge builds up again on
the capacitor plates, but the polarity is opposite to what it was in step one. Again the
current is negative, and as the voltage reaches its negative peak at point (1 the current
drops to zero.

Step 4 - After point (1, the voltage heads toward zero and the capacitor must
discharge. When the voltage reaches zero it's gone through a full cycle so it's back to

point ‘a’ again to repeat the cycle.

25

 

 

 

 

 

Voltage

/

a
+ ++

 

Figure 2-4: Capacitance in an AC Circuit

26

 

The larger the capacitance of the capacitor, the more charge has to flow to build up
a particular voltage on the plates, and the higher the current will be. The higher the
frequency of the voltage, shorter the time available to change the voltage, so the larger
the current has to be. The current, then, increases as the capacitance increases and as the
frequency increases. This is known as capacitive reactance and is measured in ohms.
There is an inverse relationship between current and resistance, so the capacitive
reactance is inversely proportional to the capacitance and the frequency: Hence a
capacitor in an AC circuit exhibits a kind of resistance called capacitive reactance (Xe),
measured in ohms. This depends on the frequency of the AC voltage, and is given by Eq.
2-13:

x, = l/mC = 1/21th (2-13)

where Xc = capacitive reactance; (o = angular frequency; f = frequency and C =
capacitance.

In an ideal capacitor, no energy is lost because the capacitor stores charge and
releases it. In this case, energy is stored in the electric field between the capacitor plates.
The amount of energy stored in a capacitor is given by Eq. 2-14:

Energy in a capacitor: E = 1/2 CV2 (2-14)

Electrons have negative (-) electrical charges. Because opposite charges attract,
they will move toward an area consisting of positive (+) charges. This movement is made
easier in an electrical conductor, such as a metal wire. The electricity in the wire with AC
supply moves in one direction for a while and then reverses its direction. The charge at

the ends of the wire alternates between negative (-) and positive (+). If the charge is

27

negative (-), that pushes the negatively charged electrons away from that terminal. If the

charge is positive (+), the electrons are attracted in that direction.

2.1.4 ECRT Chemistry
The predominant process occurring in the electrochemical cell is the electrolysis of
water in the moist soil, which results in formation of H+ ions and oxygen at the anode and
OH' and Hydrogen at the cathode as represented in Eq. 2-15 and 2-16.
H20 -)1/2 02(g) + 2H+(aq) + 2e_ Water is oxidized at the anode (2-15)
2H20 + 2e‘ -) H2(g) + 20H_(aq) Water is reduced at cathode (2-16)
Due to the electrolysis reactions, there is a decrease in pH near the anode and
increase in pH at the cathode as the generated protons and hydroxyl ions migrate towards
oppositely charged electrode. An acid front moves across the soil sample from the anode
toward the cathode and a base fiont moves from the cathode to the anode (Acar et a1.
1991). The acid front moves faster than the base front because the mobility of H+ exceeds
that of OH' and the direction of electro-osmotic flow is generally toward the cathode. The
soil usually becomes acidic throughout most of its volume, except close to the cathode.
There are exceptions to this, e. g., when the soil itself is highly buffered or alkaline

(Reddy et a1. 1997).

2.2 FACTORS AFFECTING ELECTROKINETIC REMEDIATION
2.2.1 Electrolyte Composition and pH

Changes to the electrolyte composition occur when dissolved substances are
removed from the soil sample, by entering the electrolyte, by depositing on cathode as is

the case for the metal ions like copper (Page & Page 2002). The dominant process is

28

 

electrolysis of water in moist soils which results in an immediate local change in pH due
to the formation of H+ and oxygen at anode and OH" ions and hydrogen at cathode. The
acidity at the anode and alkalinity at the cathode increase with time

Due to the change in pH, some soil minerals dissolve because of the changing
composition of the solution. Due to increase in acidity, concentration of certain ions like
Mg, Fe and Al increases and increase in alkalinity increases the concentration of Si and
Al ions in the solution (Grim 1968).

For metals, various ions may be stable at different pH values. Metal cation like
Zn+2 is stable under acidic conditions, and oxyanion like ZnOz' under alkaline conditions.
At the point where pH changes suddenly, the solubility of these metal ions is at a
minimum and they precipitate as metal hydroxides (Gray and Schlocker 1969).
Successfiil removal of the contaminants depends on maintaining metal ion solubility at
either a low pH or a high pH throughout the soil and avoiding the conditions resulting in

precipitation. Thus, control of pH is of major importance.

2.2.2 Zeta Potential

The clay particle surfaces usually carry negative charge, and hence the zeta
potential of clay soils is usually negative resulting into net electro osmotic flow towards
cathode. Values of Zeta potential varies due to change in pH and ionic strength of
solution. Increased acidity causes zeta potential to become less negative and even to
attain positive values of zeta potential at low pH (Lorenz 1969). Flow rate decreases if
the pH is depressed below neutral and increases at alkaline pH values (Shapiro and

Probstein 1993). Zeta potential becomes more positive with increase in ionic strength of

29

the solution. Increasing concentration of metals like Cu, Co and Cd causes the zeta

potential to become more positive (Hunter and James 1992).

2.2.3 Voltage and Current Levels.

The electric current intensities used in most Electrokinetic studies (Alshawabkeh et
a1. 1999) are in the order of a few tens of milliamperes per square centimeter. Although a
high current intensity can generate more acid and increase the rate of transport to
facilitate the contaminant removal process, it increases power consumption tremendously
as power consumption is proportional to the square of electric current. An electric current
density in the range of 1 to 10 A/m2 has been demonstrated to be the most efficient for
the process (Alshawabkeh et a1. 1999). However, appropriate selection of electric current
density and electric field strength depends on the electrochemical properties of the soil to
be treated, in particular the electrical conductivity. The higher the electric conductivity of
the soil is, the higher the required electric current density will need to be to maintain the
electric field strength required. An optimum electrical current density or electric field
strength should be selected based on the soil properties, electrode spacing, and time

requirements of the process.

2.2.4 Soil Structure

Clogging of pores and eventual cessation of flow can result from deposition of
metal hydroxide compounds in the metal contaminated soil. These metal hydroxides are
formed due to reaction between hydroxyl ions near the cathode and heavy metals present
in the soil. Some clay types like montrnorillonite show dramatic changes in the soil

structure resulting in water loss and shrinkage (Grundl and Reese 1997). Therefore, it is

30

 

necessary to prevent precipitation in the soil while remediating heavy metal contaminated

soil.

2.2.5 Water Content

The extent of saturation of the soil is another factor that alters the electro-osmotic
flow rate (Gray and Mitchell 1967; Mitchell and Yeung 1991; Lindgren et a1. 1994;
Laursen 1997) and hence decontamination of the soil. Uneven moisture distributions and
regions of consolidation may arise during electroremediation associated with the
development of negative pore pressures (Hamed et a1. 1991; Acar and Alshawabkeh
1996; Eykholt 1997). The negative pore pressures result from uneven flow rates caused
by variations in pH and hence electric field strength and zeta potential (Miller 1955; Misc
1961; Esrig 1968; Eykholt 1997).

Another cause of drying of soils during electroremediation is the heating effect of
the electric current (Femandez-Gonzalez 1966; Shapiro and Probstein 1993). Although
increasing temperature increases the rate of any chemical reaction and therefore might be
advantageous under carefully controlled conditions (Krause and Tarrnan 1995; Penn and
Savvidou 1997), desiccation can cause problems, particularly on large-scale samples and
in field trials (Lageman et a1. 1989; Ho et a1. 1997; Lindgren et a1. 1998). The effect is
likely to be more severe in unsaturated soils but occurs even in soils which are initially
saturated. Some of the consequences of drying out are shrinkage and cracking of the soil,

development of uneven flow paths, and eventual cessation of fluid flow.

31

2.2.6 Soil Chemistry

Characteristics of the soil, which influence kinetics of contaminant removal,
include adsorption, ion exchange, and buffering capacity (Grim 1968; Sposito 1984). Soil
contaminant interaction is especially high in fine-grained soils. Adsorption isotherms and
ion exchange capacities have been determined directly in electroremediation experiments
(Hamed et a1. 1991; Yeung et al. 1996; Puppala et a1. 1997; Reddy et al.1997; Sah and
Chen 1998; Chen et al. 1999). Ionic contaminants must be desorbed before they can be
removed and decontamination is most rapid at concentrations above the ion exchange
capacity of the soil. Soils which adsorb strongly and have high cation exchange
capacities, e. g., illitic and bentonitic clays, are thus more difficult to decontaminate than
clays, like kaolinite, with low cation exchange capacities. This correlates with electro
osmotic flow rates, which are high in kaolin and low in illitic and bentonitic clays (Gray
and Mitchell 1967).

Changes in pH affect the adsorption characteristics of the soil. H+ ions, generated
by electrolysis of water, are effective at desorbing metal cations as the acid front passes
through the soil. Similarly, OH' ions produced at the cathode aid desorption of anions,
such as CrO4'2 (Weng et a1. 1994) and there is evidence that they may also help certain
organic molecules to desorb (Segall et a1. 1980). The availability of H+ and OH’ ions in

the pore solution is determined by the buffering capacity of soils.

2.2.7 Electrode Material, Configuration, and Spacing
Chemically inert and electrically conducting materials such as graphite, coated
titanium, or platinum are used as anode to prevent dissolution of the electrode and

generation of undesirable corrosion products in an acidic environment. If necessary,

32

sacrificial electrodes can also be used as anode. Any conductive materials that do not
corrode in a basic environment can be used as cathode (Alshawabkeh et a1. 1999).
Important considerations for the choice of electrode material are: (1) electrical
conduction properties of the material; (2) availability of the material; (3) ease of
fabrication to the form required for the process; (4) ease of installation in the field; and
(5) material, fabrication, and installation costs. Regardless of the material selected for the
electrode, the electrode has to be installed properly in the field so that it can make good
electrical contact with the subsurface. Moreover, the design must make provisions to
facilitate exchange of solution with the subsurface through the electrode. The size, shape,
and arrangement of electrodes and the distance between them will also affect rates of
decontamination. Most bench-scale and large-scale laboratory and pilot-scale field
studies on electrokinetic remediation performed to date have been one-dimensional (1-
D). Effective and efficient firll-scale field application will require an optimal electrode
configuration. l-D, two-dimensional (2-D), or axisymmetrical electrode configurations
may be adopted.

For a l-D electrode configuration, sheet electrodes can be installed in the field by a
procedure similar to the installation of wick drains. The electrodes are practically driven
into the soil. Using electrode trenches is another 1-D application. However, an
approximately l-D electric field can also be obtained by lines of rod electrodes placed in
boreholes. It is probably the easiest method to install and the most cost-effective
approach for in-situ remediation. However, this configuration may develop spots of
inactive (dead) electric field between electrodes of the same polarity. Hexagonal, square,

or triangular electrode configurations can be used for 2-D field implementation.

33

 

In a hexagonal electrode configuration, electrodes form honeycomb cells
containing a cathode surrounded by six anodes as shown in Fig. 2-5(a). In a square
configuration, electrodes form square cells containing a cathode surrounded by four,
eight, or may be even more anodes as shown in Fig. 2-5(b). Similarly, in a triangular
configuration, electrodes form triangles containing one cathode surrounded by three
anodes as shown in Fig. 2-5(c). In these configurations, the cathode is placed at the center
and the anodes are placed on the perimeter to maximize the spread of the acidic
environment generated by the anodes and to minimize the extent of the basic
environment generated by the cathode. These configurations of electrodes generate 2D
nonlinear electric fields. Spots of inactive electric field can still develop in these
configurations during electrokinetic processing of contaminated soils. However, the areas
of these inactive spots are smaller than those developed in the approximate l-D
configuration containing parallel lines of anodes and cathodes. In a 1-D configuration, the
electric current density, i.e., current per unit area, is independent of location. In 2-D
configurations, however, the electric current density increases linearly with distance
toward the cathode. Therefore, the electric field strength also increases linearly with

distance toward the cathode (Alshawabkeh et al. 1999).

2.3 EFFICIENCY AND APPLICABILITY OF THE TREATMENT
New unproven technologies may offer greater effectiveness at a lower cost but with
greater risks. Traditional clean up technologies as discussed before can be expensive and

time consuming and sometimes not effective. Nevertheless, given the substantial

34

 

(a) Hexagonal

l

awe

:
.-..n.»?..m?—_n-u€>. ..........

ewe-mo

 

 

d
0)) Square 0 3110 C
.........i...,,,,,§) O cathode
\ \3 I"?

(c) Triangular

Figure 2-5: Different Configuration of Electrodes

35

 

limitations and high costs of conventional treatment technologies, the need to find

cheaper and more effective technologies will continue to increase.

2.4 COST OF THE TREATMENT

In general, the total costs for full-scale in-situ implementation of the technology
may have five major components: (1) Costs for fabrication and installation of electrodes;
(2) cost of electric energy; (3) cost of enhancement agent if necessary; (4) cost of any
post-treatment if necessary; and (5) fixed costs (Alshwabekh et al. 1999). These costs are

discussed in detail as follows.

2.4.1 Cost of Fabrication and Installation of Electrodes

Cost of electrodes depends on the material type, installation type and dimensions of
the electrodes. The number of electrodes per unit volume of soil to be treated depends on
electrode configuration and spacing. The installation costs depend on the method of
installation, depth of the electrodes to be installed, and number of electrodes to be
installed. The total costs of electrodes per unit volume of soil to be treated include the

material and fabrication costs of the electrodes and their installation costs in the field.

2.4.2 Electric Energy Costs

The total energy costs for the remediation depends mainly on the cost per KWh of
electricity. Alshwabekh et a1. (1999) gives the formula for energy expenditure for
remediation cost per unit volume of the soil. The formula is presented in Eq. 2-17.

Cenergy = C2 x (D/ 3,600,000 x [3 (2-17)

36

 

where Cmrgy = Energy cost; C2 = cost in $/kwh and B is a lumped property of the
contaminant and the soil as presented in Eq. 2-18. It represents the rate of transport of
specific specie per unit electric current density; (D is electrode spacing or phase

difference;

[3 = nut + K, / Rd 0' in (m3/c or (m/s)/ (A/m2)) (2-18)

where o = electric conductivity of soil (S/m); n = porosity; u = ionic mobility of species;
I = tortuosity factor; Kc: coefficient of electro osmotic conductivity of soil and Rd is the
delay factor similar to the retardation factor of the advection dispersion contaminant
transport and can be introduced to account for the extra time required for the acid

transport, metal desorption, dissolution, etc.

2.4.3 Cost of Enhancement Agent
If the use of enhancement agent is necessary, the cost of chemical should be
considered. Cost of the enhancement agent can be based on the desired remedial

efficiency.

2.4.4 Cost of Post Treatment

If the effluent from the process requires post treatment or if a portion of the treated
soil needs to be removed due to accumulation of high concentration of contaminant that
cannot be extracted, there will be post treatment costs. These costs are highly site and
contaminant specific. They are also dependent on the enhancement agent used in the

process. Therefore, they have to be quantified on a case-by-case basis.

37

 

2.4.5 Fixed Costs

Fixed costs include mobilization and demobilization costs of various equipment,
site preparation, security, progress monitoring, insurance, labor, contingency, and
miscellaneous expenses. The equipment will not be consumed in a particular project.

However, there is capital, depreciation, or rental cost involved.

Thus the total costs per unit volume of the soil to be treated can be given by

summation of all the types of costs discussed above.

38

i.

CHAPTER 3

EXPERIMENTAL METHODOLOGY

3.1 EXPERIMENTAL PROCEDURE

The overall experimental procedure followed is outlined below in Figure 3-1. Each
step is discussed in detail in the following sections. Four sets of experiments were
designed as presented in Table 3-1 to gain a better understanding of the degradation
phenomena of phenanthrene in the soil. Also controls were designed for each test to study
the effect of natural biodegradation and heat. Figure 3-2 displays some of the pictures of

the experimental setup in the lab. Images in this thesis are presented in color.

3.1.1 Soil Properties

The clay used in the study was obtained from Michigan State University campus in
East Lansing. Here the clay has been referred to as “MSU blue clay” because of its
grayish-blue appearance. The index properties of the MSU blue clay are presented in
Table 3-2. According to Unified Soil Classification System (U SCS), MSU blue clay is

classified as low plasticity clay (CL).

3.1.2 Sample Preparation
3.1.2.1 Grinding the Sample

MSU blue clay consisted of chunks. Hence, first step was to process the soil sample
to make it finer and uniform. It was crushed using a mallet. The crushed soil was passed
through Sieve # 20 (mesh size = 0.85 mm).The soil passing through the sieve was oven

dried and spiked.

39

 

 

Sample
Preparation

 

 

 

r- ----------------------------------- '

 

 

Spiking the clay _.

for a target
concentration

 

 

 

 

Electrical
cell setup

 

 

 

 

 

Sampling
and Analysis

 

 

 

a. Grinding/Crushing 5

b. Sieving using sieve No. 20 i

a. Add Acetone and phenanthrene
and mix for 24 hrs in mechanical
shaker

b. Dry the sample for 24 hrs again
and crush into fine

c. Add DI water (70% water content)
to the spiked clay

a. Transfer the ingredients into :
plastic container. 5
b. Cut two copper electrodes of i
proper size and insert in the ;
container with proper wiring. :
c. Start the power as per the .
designed experiment :

I

a. Sample representative samples at
regular intervals or fiequency and
dry it in oven for 48 hrs..

b. Extract 10 g of sample after drying
in the oven.

c. Prepare the extracts for analysis
using Gas Chromatograph

Figure 3-1: Overall Experimental Procedure

40

_ ... - ,1 “mm

 

Table 3-1: Testing Program.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Target
AC Voltage
Test Phenanthrene Cell size Container
Name applied
1]) Concentration (m x m x m) type
(V peak)
(mg/kg)
Test 1 T-l 500 20 0.136 x 0.136 x 0.078 Plastic
Control 1 01 500 Amhism IOOOL Glass Bottle
conditions
Test 2 T-Z 50,000 20 0.136 x 0.136 x 0.078 Plastic
Control 2 C-2 50,000 Ambient 0 136 x 0 136 x 0 078 Plastic
conditions ' ' '
Test 3 T-3 500 70 4.63 x 2.28 x 1.71 Plastic
At 70 °C in
Control 3 C-3 500 temperature 2000 L Glass Bottle
chamber
Test 4 T-4 50,000 35 0.136 x 0.136 x 0.078 Plastic
At 70 °C in .
Control 4 04 50,000 temperature 0.136 x 0.136 x 0.078 Plastic
chamber
Notes: 1) The frequency of the input AC voltage was 60 Hz.

41

2) Two cell sizes were selected as per the second last column.

 

Large sample cell
46.3 x 22.8 x 17.1 cm

 

Small sample cell
13.6 x13.6 x 7.8 cm

 

PHENANTHRENE
CO=5000O mO/kg

DISHA , CZ
LOW VOLTAGE

 

Figure 3-2: Pictures of Experimental Test cells

42

3.1.2.2 Spiking the sample
The sample was spiked for two different concentrations: 500 mg/kg and 50,000
mg/kg of Phenanthrene in dry clay. Following procedure was followed to spike the
sample.
1) Weighed 0.5 g and 50 g of Phenanthrene in a glass vial to achieve 500 and
50,000 mg/kg of spiked target concentrations respectively.
2) Phenanthrene was dissolved in just enough amount (approximately 100 ml) of
acetone and mixed thoroughly.
3) Dissolved solution was added into 1,000 g of oven-dried processed soil in a

glass bottle.

 

4) Acetone was added into the soil until the whole sample was in a slurry form and
then mixed properly using a mechanical shaker for 24 hrs.

5) The mixture was left to air dry in an aluminum foil tray under an exhaust hood
for approximately 24 hrs until acetone volatilized.

6) After drying, the sample was crushed into finer and uniform soil using a mallet.

7) A target gravimetric water content equal to 70% was achieved by adding 700 ml
of DI water into 1,000 g of spiked sample into a glass bottle.

8) The spiked clay with added water is kept in a mechanical shaker for 24 hrs and

then the contents were transferred into the electrical cell.

43

 

Table 3-2: Index Properties of MSU Blue Clay

 

 

 

 

Plastic Limit (PL) 24 %
Liquid Limit (LL) 49.5 %
Plasticity Index (P.I.) 25.5 %

 

 

 

44

3.1.3 Electrolytic Cell Setup

After transfening the contents into the plastic container, the cell was connected to

the power source using the following procedure.

1) Copper electrodes of appropriate size (depending on the size of the electrolytic
cell) were cut. 190 mm x 120 mm was the size of electrodes for the larger cell
and 110 mm x 65 mm size electrodes were cut for the smaller cell. The initial
thickness of the electrodes was 2 mm. The electrodes were inserted at both ends
of the container with a space of about 2 cm from the walls and the cell. The

spacing between the electrodes was 37 cm in the larger cell and 10.5 cm in the

 

smaller cell.

2) The electrodes were connected to two cables (red & black) and were connected
to a variac.

3) The variac was set to a desired voltage output as per Table 3-1.

4) Plugged in the variac into the wall power outlet (120 VAC) to get the
experiment started.

5) Initial samples were collected at time t=0. Details for analysis and sampling are
discussed in the next section. As the experiment progressed, samples were
collected at a predetermined frequency to evaluate potential changes in the
phenanthrene concentration.

3.2 SAMPLING AND ANALYTICAL METHODS
Presented below is a list of materials and equipment used during the sampling and

analysis.

45

Instruments/Equipments: Balance, Hot Air oven @ 40 °C, Transformers, couple of
wires, clamps, switches, and Accelerated Solvent Extractor and Gas Chromatograph for
extraction and analysis

Chemicals and Materials: Phenanthrene (98% pure), Acetone, Dichloromethane,
crucibles, steel extraction cell kit, a pestle, filter paper, Diatomaceous earth, Ottawa Sand,
beaker, 2 syringes, small GC vials, large glass extraction vials, etc. Figure 3-3 to 3-8

displays pictures of some of the instruments and equipments used during the experiment.

3.2.1 Sampling Frequency

Sampling frequency varied for each experiment. However, typically test cells were
sampled at a frequency of 0, 2, 4, 8, and 15 and 30 and then after every 2 weeks for
experiment for Test T-3, T-4, C-3. The second and fourth day sampling period was
skipped because previous experiments indicated no appreciable change in concentration
during this period. The sampling was done at t = 0 before starting the experiment to

establish the average initial concentration.

3.2.2 Sampling Method

Crucibles were cleaned with Alconox detergent and DI water before any sampling
event. Also, before any type of monitoring or sampling event, the power was turned off.
Samples were collected from anode, cathode and middle of the test cell. This was
followed by thoroughly mixing the sample in the test cell. All the samples were kept in
the oven for drying purpose for 48 hrs at 40 °C and collecting three more samples at

random locations. Phenanthrene extraction was followed after oven drying.

46

 

Figure 3-3: Hot Air Oven

47

 

Figure 3-4: Samples kept for Drying in Hot Air Oven

48

 

Figure 3-5: Steel Extraction Cells

49

   
 

m .’§_;"", — "27'2" “— ‘7“7-131-7 ~71-éztgr —--‘

41/ \‘L/ 4..., - ~ .- 31" -.-.-

Figure 3-6: Glass Vials Used for Extraction

50

E.
l
l

 

Figure 3-7: Interior of the Temperature Chamber

51

 

 

 

Figure 3-8: Variacs Used for the Test Cells

52

3.2.3 Extracting the Sample

In order to analyze the concentration of phenanthrene using gas chromatography,
phenanthrene had to be extracted in a solution phase. Extraction of phenanthrene from the
clay was achieved using the Dionex Accelerated solvent extractor (ASE) Model No. 200
as shown in the Figure 3-9.

The oven-dried sample was crushed lightly with a pestle. Ten grams of the sample
was weighed in a crucible. After that, 2.5 g of diatomaceous earth was added to the
sample to remove any moisture, if present. The sample was mixed properly using a
spatula. The mixture was poured into the steel extraction cell of the ASE. A filter paper
(GF/B filter size 19.8 mm, Whatrnan Glass fiber fiter) was kept at the bottom of the cell
before adding the sample. The extraction cell was filled with Ottawa sand until the cell
was full. The lid of the cell was tightly closed. When the extraction cells got ready, glass
vials (same in number as the extraction cells) were pre-weighed and kept in the lower
compartment of ASE. The extraction cells that were prepared were kept in the upper
compartment of ASE. Acetone and Dichloromethane was used as solvents to extract
phenanthrene from the clay in an aqueous phase. Once the extraction was done, the glass
vials that are filled with sample extract are weighed. These samples were stored in a

refrigerator if they were not analyzed right away.

3.2.4 Preparing the Stock Solution
The stock solution was prepared before analyzing the sample extract. The stock
solution was used as standard for the GC. The method to prepare stock solution for both

500 and 50,000 mg/kg of phenanthrene concentration is described below.

53

L2"-

 

 

Figure 3-9: Dionex Accelerated Solvent Extractor Model No. 200

54

o Added 0.125 g of Phenanthrene to 25 ml solvent (acetone + dichloromethane at
1:1 ratio by volume) to produce 5,000 ppm phenanthrene solution.

0 Added 1.25 g of Phenanthrene to 25 ml solvent (acetone + dichloromethane at
1 :1 ratio by volume) to produce 50,000 ppm phenanthrene solution.

0 Prepared 5 vials for standards for different concentration ranging from 9.98

mg/L to 50,000 mg/L as shown in Table: 3-3 & Table 3-4.

3.2.5 Gas Chromatography Analysis

After the PAHs have been extracted and separated from the soil into an aqueous
phase, they are analyzed using Gas Chromatography. The main advantage of using gas
chromatography is that the contaminant of interest is separated fi'om the other chemical
species by passing the sample through a specially designed column. Instrumental analysis
using spectrophotometers is difficult since extreme care must be used to avoid the
interference of other chemical species, and the standards and calibration curves must be
prepared in the same manner as the regular tests are to be run (Sawyer and McCarty
1978)

Gas chromatography involves a sample being vaporized and injected onto the head
of the chromatographic column. The sample is transported through the column by the
flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase
which is adsorbed onto the surface of an inert solid. Nitrogen is used as the carrier gas
which is chemically inert. The carrier gas system also includes a molecular sieve to

remove water and other impurities.

55

Table 3-3: Preparation of Standard Solutions for 5,000 ppm Phenanthrene Solution

 

 

 

 

 

 

 

Concentration (mg/L) Volume of phenanthrene Volume of acetone (ml)
solution (stock in uL)
9.98 6 3
19.92 12 3
49.5 30 3
99.8 60 3
192.31 80 2

 

 

 

56

 

Table 3-4: Preparation of Standard Solution of 50,000 ppm Phenanthrene Solution

 

 

 

 

 

 

 

 

Concentration (mg/L) Volume of phenanthrene Volume of acetone (ml)
solution (stock in mL)

1,000 0.5 (5,000 mg/L stock) 2

5,000 Stock 5,000 ppm Stock
10,000 0.5 (50,000 mg/L stock) 2
16,666.67 1 (50,000 mg/L stock) 2
21,428.57 1.5 (50,000 mg/L stock) 2

50,000 Stock 50,000 ppm Stock

 

 

 

57

 

The injector contains a heated chamber containing a glass liner into which the
sample is injected through the septum. The carrier gas enters the chamber and can leave
by three routes (when the injector is in split mode). The sample vaporizes to form a
mixture of carrier gas, vaporized solvent and vaporized solutes. A proportion of this
mixture passes onto the column, but most exits through the split outlet. The septum purge
outlet prevents septum bleed components from entering the column.

The effluent from the column is mixed with hydrogen and air, and ignited. Organic
compounds burning in the flame produce ions and electrons which can conduct electricity
through the flame. A large electrical potential is applied at the burner tip, a collector
electrode is located above the flame. The current resulting from the pyrolysis of any
organic compounds is measured. Flame Ionization Detectors (FIDs) are mass sensitive
rather than concentration sensitive; this gives the advantage that any changes in mobile
phase flow rate do not affect the detector's response. The F ID is a useful general detector
for the analysis of organic compounds. It has high sensitivity, a large linear response
range, and low noise. It is also robust and easy to use, however, it destroys the sample.
Figure 3-10 shows the Gas Chromatograph used during the experiments for this project.
The make and model no. of Gas Chromatograph used during the project was Perkin
Elmer and Autosystem 610N0081901 respectively. The oven temperature of Gas
Chromatograph was maintained to 150 degree Celsius at 0 min and increase of 40
degrees per minute to 250 degree Celsius. The maximum temperature of oven was
maintained at 350 °C. The carrier gas, i.e. nitrogen gas flow rate was maintained to 10

ml/min during the analysis of the phenanthrene.

58

 

Figure 3-10: Gas Chromatograph 1nstrument

59

3.3 QUALITY CONTROL OF THE EXPERIMENT

During the course of the experiments, appropriate quality control procedure
including replicate samples and instrument calibration were followed. The calibration
standards for the phenanthrene analyses by GC were prepared for two different
concentration ranges, and each analysis was calibrated with at least 5 standards ranging
from 9 mg/L to 192 mg/L for 5,000 ppm phenanthrene solution and at least 6 standards
ranging from 1,000 to 50,000 mg/L for 50,000 ppm phenanthrene solution. The linearity
of the calibration graph was always checked to make sure that the regression coefficient

was close to one.

60

CHAPTER 4

RESULTS AND DISCUSSION

The key objective was to evaluate if phenanthrene, a PAH, can be broken down
(decomposed) using an application of alternation current. This objective was
accomplished by carrying out experiments listed in Table 4-1.

The key hypothesis tested in this project was alternating current when passed
through a clay soil generates oxidation reactions due to capacitance of clay-water double
layer system. Such reactions are caused by electrons supplied and released due to the
charging and discharging of the clay-water double layer system during the positive and
negative cycles of an AC. These reactions can breakdown PAHs. Table 4-1 below

presents a summary of the tests conducted.

4.1 AVERAGE PHENANTHRENE CONCENTRATION

Extraction efficiency was measured by spiking 20 g of clay with 0.001 g of
phenanthrene to achieve a target concentration of 500 mg/kg. Acetone was used to
dissolve phenanthrene and then mixed with the soil thoroughly in a crucible. The sample
was analyzed following the same method as described in Section 3.2.3 to 3.2.5 for
extraction and analysis of phenanthrene. The extraction efficiency ranged from 83 % to
99 %.

The concentration of phenanthrene measured in the clay for the tests presented in
Table 4-1 is presented in this section. Average phenanthrene concentration of the test
samples and their controls was measured is shown in Table 4-2. Average Phenanthrene
concentration was represented by analysis of 6 samples collected from each test cells and

3 samples fi'om the control cells. The error bars shown in the plots represent the

61

Table 4-1: Summary of Tests

 

 

 

 

 

 

 

Initial Test Z 1"” = Sam le size
Test Concentration duration V...“ V"... (a; V,....,/Zavg m xlin x m
(Co. mgfig) LmA)
0.136 x 0.136
T-l 500 60 20 14.1 140 100 x 0.078
0.136 x 0.136
T—2 50,000 60 20 14.1 215 65 x 0.078
T-3 500 160 70 49.4 245 200 4‘631x721'28 x
T-4 50,000 60 35 24.7 4,120 6 (“36 x (“36
x 0.078

 

 

 

 

 

 

 

 

62

 

Table 4-2: Average Phenanthrene Concentration in Test and Control Cells

 

 

 

 

 

 

 

 

 

 

 

Test / Target Average Phenanthrene Concentration (mg/kg)
Test . Test
Sample Concentration .
ID (1 . . C duration
“mph“ (9mm) t=0 t=5 t=10 t=15 t=30 t=60
day day day day day day
T-l L°w 500 60 322 263 345 334 327 312
voltage
C-l C°nltf_°llf°’ 500 100 413 418 386 -- 351 329
T-2 Low 50,000 60 47,972 41,644 51,687 51,936 41,879 47,512
Voltage
C-2 “Diff; f“ 50,000 60 49,671 49,601 58,358 55,381 57,274 54,142
r-3 H‘gh 500 160 412 400 408 372 374 345
Voltage
Control for
O3 T8 500 120 427 381 447 393 364 323
T-4 H‘gh 50,000 60 38,748 40,440 53,045 41,745 47,931 47,141
Voltage
C-4 “”0”“ 50,000 60 36,572 49,496 46,604 54,781 51,586 50,461

 

T-4

 

 

 

 

 

 

 

 

 

Note: Frequency f equal to 60 Hz used for all tests listed above

63

 

maximum and minimum range of the phenanthrene concentration in the experimental

cells.

4.1.1 mG= 20 V
4.1.1.1 Target concentration equal to 500 mg/kg

Test T-l was carried out for a total duration of 60 days. Figure 4-1 shows the graph
of average phenanthrene concentration versus Time for Test T-l. Control 01 for the test
was kept in the ambient environment without applying any electricity in a 1,000 L glass
bottle. There is no significant difference between the control and the test cell in the

concentration of phenanthrene.

4.1.1.2 Target concentration equal to 50,000 mg/kg

Test T-2 was carried out for a total duration of 60 days. Figure 4-2 shows the
graph of average phenanthrene concentration versus Time for Test T-2. A control C-2 for
test was kept in the ambient environment without applying any electricity in a similar
type of plastic container. There is no significant difference between the control and the

test cell in the concentration of phenanthrene.

4.1.2 mG= 70 V and 35V
4.1.2.1 Target concentration equal to 500 mg/kg

Test T-3 was carried out at a peak to peak voltage of 70 V for duration of 160 days.
Figure 4-3 shows the graph of average phenanthrene concentration versus time for Test
T-3. Control C-3 for the test was kept in the temperature chamber at 70 °C to simulate the
elevated temperature conditions without applying any electricity in a 2,000 L glass bottle.

There is some decrease in the phenanthrene concentration due to the

64

 

500

 

 

 

 

 

 

 

 

 

 

 

a
x
\
U)
5
C
Q 400
g
C
8
e 300
C
0
0)
cc)
5 200 . : : ;
g Sample size: ‘36 cm x 13.6‘ cm x 7.8 cm‘
5 co = 500 mg/kg
(D
V =20V,f=60H ,
i 100 p-p Z ................................................ .4
V =20/1.414=14.1 V , -
0’ "m ‘ e r 1
8 Zavg = 140 ohms g '
5 I =v IZ =100mA i I 0-1
< 0 I
0 20 40 60 80 100 120
Elapsed Time (d)

Figure 4-1: Average Phenanthrene Concentration vs. Time for T-l and C-1

65

Average Phenanthrene Concentration (mg/kg)

7104

6104

5104

4104

3104

2104

110‘

010°

 

 

C0=50,000 mg/kg

V =20V,f=60 Hz,

V = 20/1.414 =14.1 V
Z =2150hms

I =V IZ =65mA

3

a

3

 

5

may

Sarrple size: 13.6 cm x 13.6 cm x 7.8 cm

 

.............................

 

................................................

O T-2
I 02

 

 

 

0 10

2o 30 4o
Elapsed Time (d)

50

60

Figure 4-2: Average Phenanthrene Concentration vs. Time for T-2 and C-2

66

70

Average Phenanthrene Concentration (mg/kg)

500

400

300

200

100

 

 

 

 

 

 

 

 

 

 

l l . y
'3 'l l
I
~ ________________________ g
sample size: 46.36 1122.86 x 17.14 cm
Co=500mglkg ;
v =70/1.414=49.4v ;
"m 5
Z” = 245 ohms E O T-3
Inns: vans/2mg = 200 m J: I C-3
0 50 100 150 200

Elapsed Time (d)

Figure 4-3: Average Phenanthrene Concentration vs. Time for T-3 and C-3

67

application of an alternating current in the test cell. However, the control also shows a
similar decrease. The decrease in phenanthrene concentration in both the test and control

cells may be due to biological activity resulting in biodegradation.

4.1.2.2 Target concentration equal to 50,000 mg/kg

Test T-4 was carried out for a peak to peak voltage of 35 V for duration of 60 days.
Figure 4-4 shows the graph of average phenanthrene concentration versus time for Test
T-4. Control 04 for the test was kept in a temperature chamber at 70 °C to simulate the
high temperature conditions without applying any electricity in a similar plastic
container. There is no significant difference between the control and the test cell in the

concentration of phenanthrene.

4.2 EFFECT on pH

To minimize extreme pH changes, which occur when a DC is applied, an AC is
used in these tests. Copper electrode plates were used in the experiments. Figure 4-5
presents a plot of pH versus normalized distance from Anode presented by Reddy and
Saichek (2004) to show the effect on pH due to continuous application of DC electrical
signal on the sample. The pH in the soil was measured along the length of the soil sample
in this study. The pH was observed in the range of 2.5 to 5.5 at the end of the test.
According to Reddy and Saichek (2004) low pH values caused a minor increase in electro
osmotic flow due to higher dissolution rate. In this study, the average pH values ranged

from 7.5 to 6.0 which show a relatively small variation in pH when an AC is applied.

68

 

 

 

 

 

 

 

 

 

 

 

2104 Sample size: 13.6 cm x13.6 cm x 7.8 cm " ,.. """"""""""""""" """""""
c0 = 50.000 mglkg

vw=35 V,f=60 Hz,
1 104 vrms = 35/1.414 = 24.7 v ............................ _

 

 

 

 

 

 

 

 

 

Average Phenanthrene Concentration (mg/kg)

 

Zavg = 4,120 ohms . 114
| = V /Z = 6 mA :
rrns rrns avg : I C-4
0 l
0 10 20 30 4o 50 60 70
Elapsed Time (d)

Figure 4-4: Average Phenanthrene Concentration vs. Time for T-4 and C-4

69

 

 

pl-l

 

 

 

 

 

 

 

14 “
(b) High Voltage Gradient (2.0 VDClcm)

12 " 8. Low Bufferlng (0.01 M NaOH)
10 '1

8 " +Test C - Continuous J

' -°-Test D - Periodic
6 ..
4 ' . °— 7 '
.—
2 ..
o I I I l I l I I 1

 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Normalized Distance from Anode

Figure 4-5: Effect on pH when DC is applied. (Adapted from Reddy and Saichek 2004)

70

Table 4-3 shows a summary of the pH values at anode, center and cathode region
for the tests conducted. Due to the electrolysis reactions occurring at anode and cathode,
H+ ion is formed at anode which should result in a decrease in the pH at anode and
increase in pH at cathode due to formation of OH' ions. The results show that pH at
anode decreases faster with time, indicating formation of H+ ion at anode and faster
movement of acid front because of high mobility of H+ as compared to OH' ions (Acar et
al. 1991). However, the pH does not increase near the cathode significantly due to less
mobility of OH‘ ion and because the direction of electro-osmotic flow is generally
towards the cathode. The pH values in the middle of the cell show that soil is gradually
becoming acidic throughout most of its volumes, except near to the cathode. (Reddy et al.
1997)

Figures 4-6, 4-7, 4-8 and 4-10 show pH versus time for Test cell T-l, T-2, T-3 and
T-4 respectively. The pulsating AC is connected to anode and neutral is connected to
cathode. Figure 4-6 shows that pH at cathode decreased initially but then gradually
started increasing and the pH at anode started decreasing gradually. However, it was
observed that pH at cathode was always higher at all the measurement points as
compared to pH at anode.

Figure 4-7 shows pH versus time for Test cell T-2. Similar high values of pH at
cathode can be observed at the end of the test with a decrease in pH fi'om 10.6 to 7.1 at
anode. pH values in middle of the soil shows decrease in pH gradually after many

fluctuations indicating acidic characteristics throughout the volume of soil.

71

Table 4-3: pH and Temperature in Test Cells

 

t=0

t=5

t=10

t=15

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test pH/Temp Location day day day day day (Ely
Anode 7.0 7.4 7.2 6.3 6.1 6.6

pH Mid 7.6 7.7 7.1 7.0 5.8 6.1

T __ 1 Cathode 7.4 7.1 6.4 5.9 6.3 6.3
Temp Anode 22.3 26.5 28.4 27.1 26.8 26.6

(0C) Mid 22.4 27.2 28.3 27.6 26.8 25

Cathode 22.2 26.9 28.1 27.4 26.6 24

Anode 7.4 6.7 8.4 6.4 6.0 5.5

pH Mid 7.8 7.5 7.8 6.3 6.2 5.4

T _ 2 Cathode 7.7 7.3 7.7 5.6 5.9 5.6
Temp Anode 23.1 27 .4 26.4 25.5 25.5 23.7
(0C) Mid 23.2 27.9 26 26 25.4 24.4
Cathode 22.5 27.5 25.7 25.8 25.3 23.4

Anode 10.6 9.1 8.5 9.6 8.4 7.8

pH Mid 8.7 9.3 9.2 8.8 8.3 8.3

T _ 3 Cathode 7.8 9.4 9.2 9.4 8.6 8.2
Temp Anode 22.9 57.8 52.6 47.9 53.1 47.4

(0C) Mid 23.0 59.2 61.1 58.6 54.7 59

Cathode 22.7 57.7 52.4 52.2 49.2 48

Anode 8.9 6.8 7.5 7. 0 6.0 6.4

pH Mid 9.0 8.0 8.0 7.8 7.0 6.9

T _ 4 Cathode 9 6.6 7.0 7.2 6.9 6.5
Temp Anode 24.5 42.5 38.6 37.0 37.9 30.4
(0C) Mid 24.4 43 41.6 40.0 38.2 33.4
Cathode 24.1 41.5 38.1 37.6 36.0 31.2

 

72

 

1O

 

 

 

 

San'plesizez13.6cmx13.60mx7.8cm
Co=500nelkg

VW=20V,f=60Hz,

V =20/1.414=14.1V

ms

2 =140ohrrs

avg

| =V IZ =100mA

rrns rms avg

 

 

4 i

 

 

l l L

 

0 10 i 20 30 40 50 60 70
Elapsed Time (d)

Figure 4-6: pH vs. Time for Test Cell T-l

73

 

Sample size: 13.6 cm x 13.6 cm x 7.8 cm
C 0 = 50,000 mglkg

Vp_p=20V,f=60Hz,
V = 20/1.414 =14.1 V
rrns

Z =2150hms

avg

I =V /Z =65mA
rrns rrns avg

   
 
 
   

 

 

 

 

 

 

—O—Cathode
+ Center 5
—0- Anode E
l l l l l
O 1 0 20 30 4O 50 60 7O

Elapsed Time (C!)

Figure 4-7: pH vs. Time for Test Cell T-2

74

Figure 4-8 shows pH versus time for test cell T-4. The pH values at anode show a
sudden decrease in 10-day sampling event with a higher pH at cathode. Figure 4-10
shows pH versus time for test cell T-3. However, it does not conform to the expected
trend. This may be due to calibration error during pH measurement. The 30-day reading
shows a decrease in pH at cathode which may be due to slower base front as compared to
movement of acid front towards anode.

Figure 4-9 and 4-11 show the variation of pH in the control cells C-3 and C-4 kept
for higher concentration (50,000 mglkg) in ambient enviromnent conditions and in
temperature chamber @ 70 0C, respectively. As shown in Figure 4-10, under ambient
conditions the pH was observed to decrease gradually alter a sudden increase in pH from

7.8 to 5.8.

4.3 EFFECT ON TEMPERATURE

Temperature can be important factor affecting the effectiveness of the remediation
process. Temperature was monitored throughout the experiments. Figures 4-12, 4-13, 4-
14, and 4-16 show the temperatures at anode, center and cathode of the test cells T-l, T-
2, T-3 and T-4 during the term of the experiment. Figure 4-15 and 4-17 show the
temperatures in control cells C-3 and C-4. All tests where an AC was applied show
heating and temperature rise of the soil sample due to the Joule heating phenomenon.

The graph for cell T-l and T-3 shows constant temperatures ranging from 22 to 28
°C at most times which is because of ambient conditions. For test cell T-2 the plot of
temperature versus time displays a higher temperature range from 50 to 60 °C due to
higher amount of electric current passing through the cell. The plot for test cell T-4

displays that the temperature increases up to 42 °C at a point.

75

1O

 

 

 

 

 

Sample size: 46.36 x2286 x 17.14 cm
00 = 500 mglkg

V =70V,f=60Hz

 

 

 

 

 

_-, ....................... l p-p

. v“: 70/1.414 = 49.4 v

+Center | =v [z =200mA

—O—Anode ""3 ""3 ”9

0 i 50 100 150 200
Elapsed Time (d)

Figure 4-8: pH vs. Time for Test Cell T-3

76

10

 

 

r I I Sample size: 13.6 cm x13.6 cm x 7.8 cm A
g g g CO = 50,000 mglkg Control cell C-3

 

 

 

 

 

10 20 30 40 50 60 70
Elapsed Time ((1)

Figure 49: pH vs. Time for Control Cell C-3

77

 

1O

 

 

Sample size: 13.6 cm x 13.6 cm x 7.8 cm
C o = 50,000 mglkg

: ; : VW=35V,f=60Hz,
' ............. _____________ v =35/1.414=24.7v
: 2 "“3
‘ ' Z =4,1200hms
3V9
I =v /z =6mA
was me avg

  
   

 

 

.......................................................................................

 

 

 

 

 

 

 

+ Cathode
—I— Center
—O—- Anode E E E E E
3 L L 1 l 1
O 1 O 20 30 40 5O 60 7O
Elapsed Time (d)

Figure 4-10: pH vs. Time for Test Cell T-4

78

 

10 i l 1 Sample size: 13.6 cmx13.6 omx7.86m

Co = 50,000 mglkg Control cell C-4

 

 

9_ ..................................................................................................... _a
8_ ......................................................................................................... _

 

 

 

 

 

6
5 l l l l l l l
0 10 20 30 40 50 60 70
Elapsed Time ((1)

Figure 4-11: pH vs. Time for Control Cell 04

79

Temperature (°C)

30

22

20

 

 

 

 

 

 

 

Sample size: 13.6 cm x13.6 cm x 7.8 cm
C0 = 500 mglkg

 

 

 

. . . Vp_p=20V,f=60Hz,
: """"""" 2 """"""""""" vrms = 20/1.414 =14.1 v
+ Cathode Z“ = 140 ohms
-I— Center = =
+ Anode i i 1m vm/zavg 100 mA
0 10 20 30 40 50 60 70 8O
Elapsed Time (d)

Figure 4-12: Temperature vs. Time for Test Cell T — 1

8O

 

Temperature (°C)

30

28

N
O)

N
A

22

20

 

 

 

Sample size: 13.6 cm x 13.6 cm x 7.8 cm
C0 = 50,000 mglkg

Vp_p=20V,f=60 HZ,
V = 20/1.414 =14.1 V
rrns

................

 

 

 

 

 

 

 

 

—O— Cathode z = 215 ohms
1&2: 1:: vm/zmg = 65 mA
0 10 i 20 30 40 50 60
Elapsed Time (d)

Figure 4-13: Temperature vs. Time for Test Cell T — 2

81

70

 

 

Temperature (°C)

70

60

50

40

30

20

 

................................................

 

 

L —O— Cathode

 

 

 

 

 

 

 

 

 

 

+Cente' l =v IZ =200mA
j —-O—Anode . "“5 ""5 M
0 50* 100 150 200
Elapsed Time ((1)

Sample size: 46.36 x 22.86 x 17.14 cm
Co = 500 mglkg

V” = 70V, f= 60 Hz

V = 70/1.414 = 49.4 V
rms

Z = 245 ohms
avg

 

 

Figure 4-14: Temperature vs. Time for Test cell T — 3

82

If Iii-5..

7O

60

01
O

Temperature(°C)
A
O

30

20

 

 

 

 

 

 

 

l l l

g g Sample size: 13.6 cm x 13.6 cm x 7.8 cm

; ; Co = 50,000 mglkg Control cell C -3 5'“
o—v—ki I + a

l l l l l l l

 

 

0 10 20 30 40 50 60 7o
Elapsed Time (d)

Figure 4-15: Temperature vs. Time for Control Cell C — 3

83

Temperature (°C)

70

60

50

40

30

20

 

 

 

 

 

 

 

    

 

 

 

I l 1 ‘Sample size: 13.6 cm x 13.6 cmjx 7.8 cm
C0 = 50,000 mglkg
VW=35V,f=60 Hz,
V = 35/1.414 = 24.7 V
_ ................................................ m
2 = 4,120 ohms
avg
l = V /Z = 6 mA
rrns rms avg
+ Cathode
. + Center 5
l I Anode L l l l
0 10 20 30 4O 50 60 7O
Elapsed Time (d)

Figure 4-16: Temperature vs. Time for Test Cell T — 4

84

 

Temperature °C

50

4O

30

20

 

 

 

 

 

 

 

Sample size: 13.6 cm x 13.6 cm x 7.8 cm
g g C0 = 50,000 mglkg Control cell C-4
1 l

10 20 30 40 50 60 70
Elapsed Time (d)

Figure 4-17: Temperature vs. Time for Control Cell C — 4

85

 

 

This is due to reinitialization of the cell T-4 when it was started. Initially the cell was set
up at 70 V in parallel which was too high for the sample size of the test cell, which made
it heat fast enough and the sample started boiling at a point. The test was re-started again
but the temperatures of soil did not reach ambient values as compared to the T-l cell. It
always had higher temperature ranging form 28 to 40 °C. The control cells C-1, C-2 and
C-3 displayed a constant temperature of approximately 22 °C. While O4 is control cell

kept in Temperature chamber at 70 °C.

4.4 ELECTRICAL PARAMETERS

Electrical parameters including capacitance in series and parallel and Impedance of
the cell were measured periodically during the tests. All measurements were done at a
100 Hz fiequency using an LCR meter. The meter is designed to measure the parameters
at different range of frequency from 100 Hz to 10000 Hz. The LCR meter did not have an
option to measure these parameters at 60 Hz which was the frequency of the applied AC.
Other electrical parameters like capacitance can be measured in series and parallel both
and denoted as Cs and Cp with units in F arads. The Cp and Cs were observed to decrease
as time passed for all the tests. Figure 4-18 and 4-19 show plots of Cp and Cs versus time
for test cell T-l respectively.

Soil impedance or resistance is sensitive to moisture content of the soil and thus
can be used to calculate the volumetric water content of the soil. Change in temperature
can result in change in impedance of soil. Figure 4-20 displays variation of resistance
with time for test cell T-l. With time, the total resistance of the cell increased from 102 to
196 ohms in 60 days with an average Z value of 140 ohms. As time passed, pH of the soil

decreased and resistance of the soil increased. Figure 4-21 show fluctuations in

86

Parallel Capacitance at 100Hz, Cp (Farads)

 

 

 

 

 

 

 

 

l l l Sample size: 13.6 cm x13.6 cm x 7.8 em
= 1 CO = 500 mglkg
V =20V,f=60HZ.
V =20/1.414=14.1 V
m
2 = 140 ohms
mg
‘ l = V /Z = 100 mA
1 ms rms avg
l l i l l L l
0 10 20 30 40 50 60 70

Elapsed Time (d)

Figure 4-18: Cp (Parallel Capacitance) vs. Time for Test Cell T-l

87

 

Series Capacitance at 100Hz, Cs (Farads)

 

 

 

 

Sample size: 13.6 cm x 13.6 cm x 7.8 cm
Co = 500 mglkg

V” = 20 V, f= 60 Hz,

V = 20/1.414 =14.1 V

 

 

 

 

 

Z“ = 140 ohms
2 s 5 'm: vans/Z = 100 "‘A
10-5 l l 1 m
0 10 20 30 40 50 60
Elapsed Time (d)

Figure 4-19: Cs (Series Capacitance) vs. Time for Test Cell T-l

88

70

 

 

Impedance, 2 (ohms)

250

200

150

100

50

 

 

 

 

 

. Sample size: 13.6 cm x 13.6 cm x 7.8 cm
C o = 500 mglkg

VW=20V,f=60 Hz,
V = 20/1.414 =14.1 V
rrns

Z =1400hms

3‘9

l =V IZ =100mA
rms rms avg

 

 

 

0 10 20

3O 40 50 60

Elapsed Time (d)

Figure 4-20: Z (Impedance) vs. Time for Test Cell T-l

89

70

 

”a: u
_A

 

Impedance, Z (ohms)

500

400

300

200

100

 

Sample size: 13.6 cm x 13.6 cm x 7.8 cm
C o = 50,000 mglkg

Vp_p=20V,f=60Hz,

V = 20/1.414 =14.1 V
rms

Z =2150hms

avg

l =V IZ =65mA

rrns rms avg

 

 

 

 

 

 

 

 

 

 

 

l l l l l l l
0 10 20 30 4o 50 60
Elapsed Time (d)

Figure 4-21: Z (Impedance) vs. Time for Test Cell T-2

90

70

impedance with an average value of 250 ohms in test cell T-2. Figure 4-22 shows a plot
of impedance of Test cell T-3 versus time displaying sudden changes in impedance with

an average value of 245 ohms.

4.5 CORROSION OF ELECTRODES

It is known that DC stray currents can accelerate the corrosion process in buried
metallic structures. This is also evident from Figure 4-5 in which relatively low pH
values were observed with higher dissolution rate for a DC application. At the positions
where the AC current leaves the metal to enter the surrounding soil, electrochemical
corrosion may appear. The amount of corrosion (weight of metal lost) is related to the
electrical charge that has passed the metal-soil interface. In terms of net electrical charge
AC stray currents would not cause corrosion as the current-direction is reversed during
each half cycle, Hence the net direction of the current is zero. However, the actual AC
current flowing due to an AC voltage present may not be symmetrical for the positive and
the negative half-cycle, due to the non-linear electrical behavior of the interface between
metal and soil. Low frequency alternating current induces less corrosion as compared to
high frequency of alternating current (Chin 1984). Table 4-4 shows thickness of
electrodes at the end of the test period.

Test cell T-2 shows some corrosion of the electrode but the other three test cells do
not show significant corrosion. Electrodes of test cell T-4 has corroded more compared to

Test cell T-3 due to higher intensity of current passing through it.

91

 

 

Impedance, Z (ohms)

600

500

400

300

200

100

 

 

 

 

 

 

‘ 1 Sample size: 46.36 x 22.86 x 17.14 cm
3 C0 = 500 mglkg
v” = 70v, f= 60 Hz
7‘ """"""""""""""""""""""""""""""""" v = 70/1.414 = 49.4 v
rrns
Z = 245 ohms
ave
I = V /Z = 200 mA
_____________________________________________ "“3 "“5 M
i l l 1
0 50 100 150 200
Elapsed Time (d)

Figure 4-22: Z (Impedance) vs. Time for Test Cell T-3

92

 

 

 

 

Table 4-4: Thickness of Electrodes at the End of Test

 

 

 

 

 

 

 

 

Test Thickness of electrode (mm)
Anode Cathode
T-l 1.94 (3%) 1.96 (2%)
T-2 1.93 (3.5%) 1.72 (14%)
T-3 1.74 (13%) 1.79 (10.5%)
T-4 1.89 (5.5%) 1.62 (19%)

 

 

 

Notes: 1) The initial thickness of electrodes was 2 mm.
2) The values in the parenthesis show the percent
decrease in the thickness.

93

 

1.?

CHAPTER 5

SUMMARY AND CONCLUSIONS

Polycyclic aromatic hydrocarbons (PAHs) are contaminants that are present in soils
at contaminated sites ubiquitously. While conventional technologies such as “soil
washing” exist to cleanup PAHs in coarse-grained soils, these technologies are
ineffective in clayey soils due to the strong affinity of PAHs to clay mineral surfaces and
low hydraulic conductivity of clay soils. In this project, the effect of sinusoidal
alternating current (AC) on the concentration of phenanthrene in spiked low plasticity
clay was evaluated in a lab-scale setup. The key hypothesis evaluated was, an AC
electrical signal can generate oxidation reactions in clay soils due to the capacitive
properties of charged clay mineral surfaces saturated with dielectric water to cause
degradation of benzene rings in PAHs. No enhancement agents were added to the spiked
clay during the application of the AC.

The effect of application of an AC on the concentration of phenanthrene in spiked
clay was evaluated on the basis of these two key parameters: (1) initial concentration of
phenanthrene; and (2) the magnitude of applied AC. Total four sample cells were
subjected to an AC for a time period ranging from 60 to 160 days. The initial
phenanthrene concentration in the spiked clay was 500 and 50,000 mg/kg and the peak to
peak voltage applied ranged from 20 to 70 volts (root mean square average = 14.1 to 49.4
volts). This resulted in a root mean square current that ranged from 0.006 to 0.2 arnperes.
Copper plate electrodes having an initial thickness equal to 2 mm were used to apply the

AC. The key parameters monitored throughout the testing period included: (1)

94

 

 

 

phenanthrene concentration in the clay; (2) pH of the clay; (3) temperature of the clay;
(4) electrode thickness change; and (5) capacitance and impedance of the clay.

The phenanthrene concentration in the cells monitored over the duration of the tests
when compared with control samples (samples not subjected to an AC) indicated
insignificant decrease or change in the phenanthrene concentration in the clay.
Adsorption of phenanthrene on the plastic cell walls was not measured in this study.
However, results from glass and plastic control cells indicated that the adsorption is
relatively small compared to the initial spiked concentrations used in this study.

Over the duration of the tests, the average pH of the spiked clay dropped from
about 7.5 to 6.0. It was also observed that the thickness of the copper electrodes
decreased from 2 mm to about 1.7 mm during the 60-day duration of the tests. Formation
of an acid front is primarily responsible for the dissolution of the electrodes. However,
compared to a DC application, the electrode decay rate is relatively slow when an
equivalent AC is used. Due to Joule heating, the temperature of the cells subjected to AC
increased to up to 60° C within 160 days after an AC was applied. Control sample
subjected to a similar temperature increase (without an AC application) indicated no
appreciable decrease in phenanthrene concentration. The impedance of the clay increased
during the testing period primarily due to the increase in the temperature of the clay.

Based on the results, the key conclusion of this study is that in a lab-scale setup,
sinusoidal AC at a frequency equal to 60 Hz does not cause appreciable decrease in the
concentration of phenanthrene in spiked low plasticity clay. Further studies including
degradation of phenanthrene in other media and chemicals subjected to an AC needs to

be carried out.

95

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