Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
Acid front migration during the electrolysis of clay
Final Year Honours Dissertation 2008
Zoe Thiele
10220183
Supervisors: Dr David Reynolds and Professor Andy Fourie
School of Environmental Systems Engineering
(i)
ABSTRACT
Past and present land use practices and chemical handling practices have resulted in
significant and widespread groundwater contamination issues in Australia and around the
world. Remediation can be complex and expensive, and there is currently no practical option
for the remediation of contaminated clay soils. Electrokinetic treatment is an innovative new
method for subsurface remediation. Electrokinetic treatment involves the application of an
electric field across a contaminated soil profile to remove contaminants in the soil. Direct
current electric fields are used to mobilise contaminants or treatments through the soil profile.
Mechanisms of electrokinetic transport include electromigration, electroosmosis and
electrophoresis. During electrolysis, oxidation at the anode generates acid which migrates
through the soil profile towards the cathode, whilst reduction at the cathode produces a base
front which migrates through the soil towards the anode. The pH gradient generated by this
process can affect electrokinetic processes and properties of the clay such as the zeta
potential. The zeta potential is the difference in potential between the slipping plane in the
double layer and the bulk medium, and governs some properties of electrokinetic transport
such as electroosmosis.
The primary aim of this study was to investigate the migration of the acid front through clay
cores during electrokinetic treatment. A series of one dimensional column electrokinetic
experiments were conducted on commercially available clay to investigate the development of
a pH gradient during electrolysis. In addition, properties of the three clays including
hydraulic conductivity, particle size distribution, buffering capacity and the zeta potential-pH-
ionic strength relationships were investigated. The migration of acid through the clay core
was measured after different amounts of time by direct measurement of pore water pH.
Migration of the acid front was measured in the three clays at a rate of 1.9 to greater than 2.5
cm day-1. Migration of the acid front was fastest through the clay which showed the most
negative zeta potential, which may indicate electroosmosis, in addition to electrokinetic
migration, was a significant transport mechanism under these conditions.
(ii)
ACKNOWLEDGEMENTS
The author would like to acknowledge Dr David Reynolds and Professor Andy Fourie for
guidance and feedback throughout the year. The author would also like to acknowledge
Golder Associates for funding the project.
Thanks also to Tim, Ming and Dan for sharing ideas, references and the laboratory space. A
big thankyou to A. Prof. Yee-Kwong Leong for the zeta potential results and Stafford
McKnight from the University of Ballarat for the XRD analysis. And finally, thanks to
Simon for assistance with the final editing and moral support throughout the year.
(iii)
TABLE OF CONTENTS
Abstract ........................................................................................................................................... i Acknowledgements........................................................................................................................ ii Table of Contents ......................................................................................................................... iii List of Figures ................................................................................................................................ v List of Tables................................................................................................................................. vi List of Abbreviations................................................................................................................... vii Chapter 1. Introduction............................................................................................................. 1.1
1.1 General .......................................................................................................................... 1.1 1.2 Aims and objectives of this study ................................................................................. 1.3
Chapter 2. Literature Review ................................................................................................... 2.1 2.1 Properties of clays ......................................................................................................... 2.1
2.1.1 Soil structure ................................................................................................ 2.1 2.1.2 The Electrical Double Layer ........................................................................ 2.3 2.1.3 Zeta Potential................................................................................................ 2.5 2.1.4 Hydraulic conductivity................................................................................. 2.7
2.2 Principles of Electrolysis............................................................................................... 2.8 2.2.1 Transport Mechanisms ................................................................................. 2.8 2.2.2 Acid and base production and migration.................................................... 2.13
2.3 Summary ..................................................................................................................... 2.15 Chapter 3. Research Methods ................................................................................................... 3.1
3.1 Experimental Design ..................................................................................................... 3.1 3.2 Clay Characteristics....................................................................................................... 3.1
3.2.1 X-ray diffraction (XRD)............................................................................... 3.1 3.2.2 Grain size analysis........................................................................................ 3.2 3.2.3 Moisture content........................................................................................... 3.2 3.2.4 Unit weight................................................................................................... 3.3 3.2.5 Acid neutralising capacity............................................................................ 3.3 3.2.6 Hydraulic conductivity testing ..................................................................... 3.3 3.2.7 Zeta potential determination......................................................................... 3.5
3.3 1D core experiments...................................................................................................... 3.5 3.3.1 Intermediate Junction pH sensor .................................................................. 3.6 3.3.2 Preparation of clay cores .............................................................................. 3.6 3.3.3 Electrolytic cell setup ................................................................................... 3.7 3.3.4 Measurement of the pH profile .................................................................... 3.9
Chapter 4. Results and Discussion............................................................................................ 4.1 4.1 Clay Characteristics....................................................................................................... 4.1
4.1.1 X-ray diffraction........................................................................................... 4.1 4.1.2 Grain size analysis........................................................................................ 4.2 4.1.3 Moisture content........................................................................................... 4.3
(iv)
4.1.4 Unit weight................................................................................................... 4.4 4.1.5 Acid neutralising capacity............................................................................ 4.5 4.1.6 Hydraulic conductivity testing ..................................................................... 4.6 4.1.7 Baseline clay pH........................................................................................... 4.7 4.1.8 Zeta potential determination......................................................................... 4.7
4.2 1D core experiments.................................................................................................... 4.11 4.2.1 Results from core experiments................................................................... 4.11 4.2.2 General observations in pH trends and clay characteristics....................... 4.19 4.2.3 Quality control and instrument performance ............................................. 4.20
Chapter 5. Conclusions and Recommendations ...................................................................... 5.1 References ................................................................................................................................... 5.1 Appendices .................................................................................................................................. 5.1 Appendix A – Raw Data ............................................................................................................ 5.2 Appendix B – Particle Size Analysis......................................................................................... 5.8
(v)
LIST OF FIGURES
Figure 1: Structure of clay minerals........................................................................................ 2.2
Figure 2: The electrical double layer....................................................................................... 2.4
Figure 3: Mechanism of Electroosmosis............................................................................... 2.11
Figure 4: Experimental setup for determination of hydraulic conductivity using the falling
head test................................................................................................................................... 3.4
Figure 5: PVC Core dimensions.............................................................................................. 3.6
Figure 6: Electrolyte containers .............................................................................................. 3.9
Figure 7: pH profile measurements....................................................................................... 3.10
Figure 8: Zeta potential for Studio Porcelain .......................................................................... 4.8
Figure 9: Zeta potential for Kagero clay ................................................................................. 4.8
Figure 10: Zeta potential for SWE clay .................................................................................. 4.9
Figure 11: Zeta potential of different clay types at 4.0 g/L NaCl ......................................... 4.10
Figure 12 Anolyte and catholyte pH change over time......................................................... 4.12
Figure 13: pH profiles for Studio Porcelain clay .................................................................. 4.13
Figure 14: pH profiles for Kagero clay ................................................................................. 4.14
Figure 15: pH profiles for SWE clay .................................................................................... 4.15
Figure 16: Migration of pH 4 region..................................................................................... 4.17
Figure 17: Migration of pH 3 region..................................................................................... 4.17
Figure 18:Transverse variability in pH measured in a three day SP core ............................. 4.23
(vi)
LIST OF TABLES
Table 1: Effective ionic mobilities of selected common anions and cations ........................ 2.10
Table 2: Percentage mineral composition of clay types based on XRD ................................. 4.1
Table 3: Particle size distribution of clays .............................................................................. 4.2
Table 4: Moisture content of clay types .................................................................................. 4.3
Table 5: Moisture content of SP cores after EK treatment ..................................................... 4.3
Table 6: Unit weight of clay packed in PVC cores ................................................................. 4.5
Table 7: Acid neutralising capacity of clay types ................................................................... 4.6
Table 8: Hydraulic conductivity of the three clay types by the falling head test .................... 4.6
Table 9: Baseline clay pH ....................................................................................................... 4.7
Table 10: Migration rate of acid front by line of best fit....................................................... 4.18
Table 11: General observations in pH trends ........................................................................ 4.19
Table 12: Average and standard deviation of primary and duplicate pH measurements...... 4.22
(vii)
LIST OF ABBREVIATIONS
1D One dimensional
ANC Acid neutralising capacity
CEC Cation exchange capacity
DC Direct current
DI De-ionised
EDL Electrical double layer
EK Electrokinetic
H+ Hydrogen ion
ISCO In situ chemical oxidation
NaCl Sodium chloride
OH- Hydroxide ion
SP Studio porcelain (clay type)
XRD X-ray diffraction
Chapter 1. Introduction School of Environmental Systems Engineering
1.1
CHAPTER 1. INTRODUCTION
1.1 GENERAL
Past and present land use practices and chemical handling practices have resulted in
significant and widespread groundwater contamination issues in Australia and around the
world. Groundwater contamination can be detrimental to the environment, people and the
economy. Groundwater remediation can be complex, expensive and often ineffective. Project
timescales for groundwater remediation are often in the order of decades. While advances in
remediation technology have in many instances resulted in partial contaminant removal, there
is often a cost barrier and a time barrier to complete remediation of a contaminated site, with
compete remediation impractical.
Various remediation methods exist, such as pump and treat, in situ chemical oxidation (ISCO)
and soil vapour extraction, however these methods have limitations and are only suited to
particular types of soils. Remediation of low permeability media such as clays pose a
significant challenge due to difficulties in mobilising the contaminant, or transporting the
treatment to the contaminant. Remediation of clays using methods such as ISCO is often not
feasible due to the cost and timescale of treatment. Combining electrokinetic (EK) treatment
with ISCO has been shown to significantly enhance transport of treatment to contaminant,
resulting in reduced timescales and cost of remediation.
Electrokinetic treatment is an innovative new method for subsurface remediation. EK
treatment involves the application of an electric field across a contaminated soil profile to
remove or reduce contaminants in the soil. Direct current (DC) electric fields are utilised to
mobilise contaminants or treatments through the soil profile. Mechanisms of electrokinetic
transport include electromigration, electroosmosis and electrophoresis. While overall
mechanisms governing electrokinetic transport are generally well described (1993;
Alshawabkeh 2001; Isosaari et al. 2007), there are many complex and interacting processes
governing the system due to heterogeneous geochemical behaviour (Eykholt and Daniel 1994;
Denisov et al. 1996; Alshawabkeh et al. 2004; Isosaari et al. 2007). Further work is required
before EK treatment can be utilised as a viable remediation option.
Chapter 1. Introduction School of Environmental Systems Engineering
1.2
In order to improve the feasibility of EK treatment as a remediation option, research at the
University of Western Australia (UWA) is currently looking into developing a three-
dimensional, coupled hydraulic and electrokinetic flow and reactive solute transport model.
The focus of this research group is in developing the technology which will enable the
remediation of low permeability contaminated soils, for which currently there is no practical
solution. This project was aimed at investigating the geochemistry of EK treatment in three
clays whilst also providing baseline data, which will be suitable for the validation of models
to simulate electroosmosis, electromigration, electrolysis, and pH changes.
Chapter 1. Introduction School of Environmental Systems Engineering
1.3
1.2 AIMS AND OBJECTIVES OF THIS STUDY
The aim of this study was to investigate the geochemistry of EK treatment, with application to
the field of electrokinetic remediation of low permeability media, and to provide baseline data
which can be used for the validation of EK treatment modelling. Three main objectives were
defined to address this aim. These objectives are outlined below.
Objective 1: To obtain three different commercially available clays and investigate
physical/chemical parameters of these clays.
Commercially available clay types were chosen based on initial zeta potential testing to
ensure the three clays showed different zeta potential/pH relationships. Other parameters
which were measured to classify the clay included grain size distribution, hydraulic
conductivity, acid neutralising capacity (ANC), density, moisture content and x-ray
diffraction (XRD).
Objective 2: To quantify the relationship between pH and zeta potential in three different
clay types.
Baseline data was collected to quantify the relationship between pH and zeta potential in three
different clay types and at different salt concentrations. This was used to give an indication of
differences between the three clays and the indicative zeta potential for the in situ clay cores.
Objective 3. To quantify 1D acid/base migration during electrolysis of three clay types.
One-dimensional column tests were conducted, measuring the changes in pH along the core
profile over time. Three different clay types were investigated at a constant voltage. Time
treatment ranged from 1 to 5 days.
Chapter 2. Literature Review School of Environmental Systems Engineering
2.1
CHAPTER 2. LITERATURE REVIEW
From a soil remediation perspective, the potential for EK enhanced remediation was initially
observed as a method for transporting contaminants (such as heavy metals) through the soil
profile for treatment or removal (Acar and Alshawabkeh 1996; Denisov et al. 1996). EK
applications have also included electroosmotic dewatering and consolidation in geotechnical
applications (Eykholt and Daniel 1994; Alshawabkeh et al. 2004). More recent studies have
shown EK treatment as being effective in transporting treatment chemicals such as oxidants
through the contaminated soil profile, for in situ remediation (Roach and Reddy 2006;
Isosaari et al. 2007; Reynolds et al. 2008). EK remediation has even been utilised to transport
polycyclic aromatic hydrocarbon degrading bacteria through soil (Wick et al. 2004).
Various studies have already shown the success of combining EK with ISCO (Roach and
Reddy 2006; Reynolds et al. 2008), however further research is required to develop the
method to be a commercially viable remediation option. The following sections outline the
properties of clay necessary for understanding some of the electrochemical phenomena that
occur during EK treatment, and then goes on to describe some of the physical processes
behind electrochemical remediation of low permeability media. The literature review
concludes with a discussion of aspects relating to the production and migration of acid and
base through the clay profile, which is the focus of the present study.
2.1 PROPERTIES OF CLAYS
2.1.1 Soil structure
Whilst clays are defined as fine grained soils (typically grain size less than 0.002 mm), it’s
their unique structural characteristics that differentiate them from other soils. It is important
to note that clay minerals are different to clay size minerals, as any mineral can be of clay
particle size (Gerrard 2000). Non clay minerals of clay particle size are commonly referred to
as silts. The building blocks of clay minerals include a basic structural unit of a silica
tetrahedron and an alumina octahedron (Craig 1987). The silica tetrahedron consists of a
central silica cation, surrounded by four oxygen anions. This forms a silica layer by linking
together and sharing oxygen atoms to balance charge. The alumina octahedron structural unit
consists of a central aluminium atom with six oxygen or hydroxyl atoms in an octahedron
Chapter 2. Literature Review School of Environmental Systems Engineering
2.2
(Budhu 2007), which also joins to other octahedral units to form a alumina layer. The
layering of these silica and alumina sheets form the basic units of the different clay types. The
three main groups that make up common clays include kaolinite, illite and montmorillonite
(Budhu 2007).
Kaolinite, one of the more common clay types, is classified as a two layer clay, with a sheet of
silica tetrahedra joined through octagonal hydroxyls with an alumina octahedral sheet
(Murray 1999). The surfaces of the kaolinite sheets are theoretically electrically neutral,
however at the edges of the 1:1 layer, the surfaces are disrupted and hydroxyl groups occupy
sites of broken bonds (Alkan et al. 2005).
Figure 1: Structure of clay minerals
The silica tetrahedron and the alumina octahedron structural units shown in the left hand panels combine in layers to form
Kaolinite, Illite and Montmorillonite clays shown in the right hand panel. Figure adapted from Budhu (2007).
Chapter 2. Literature Review School of Environmental Systems Engineering
2.3
Illite consists of one alumina sheet between two silica sheets, with a layer of potassium ions
binding successive layers together (see Figure 1). Montmorillonite has a similar structure,
however the silica-alumina-silica layers are joined by weak van der Waals forces and
exchangeable ions (Budhu 2007). Montmorillonite is commonly referred to as swelling or
expansive clay, as the weak bonding between the silica-alumina-silica layers allows for
substantial ingress of water. Some of the important properties that characterise clay minerals
are particle size and shape, surface chemistry, surface area, surface charge and plasticity
(Murray 1999).
Determination of soil structure
A common semi-quantitative method of analysis for fine grained sediments such as clays is
XRD (Poppe et al. 2001), which utilises x-rays (electromagnetic radiation with wavelength in
the order of 10-8 to 10-12 m). X-rays can be produced from bombarding a metal target with
high energy electrons and are useful in the study of crystal structure as they have wavelengths
which are comparable to the atomic separation distance in solids (Serway and Beichner 2000).
Each different clay type will have a different crystalline lattice structure, hence will diffract x-
rays in a characteristic pattern (Poppe et al. 2001). This can be used to identify clay samples
against a reference library and can also be used to asses mineral composition of mixed
materials. The result is semi quantitative as the intensity of a minerals diffraction peaks does
not represent an accurate measure of abundance. This is because different minerals and
different atomic planes within a mineral will vary in their ability to diffract x-rays (Poppe et
al. 2001). It also depends on the sample mount and x-ray machine conditions, which vary
between laboratories.
2.1.2 The Electrical Double Layer
There are a number of increasingly complicated models describing the distribution of charge
around a solid layer. One way to describe and visualise surface characteristics is known as
the diffuse double layer or electrical double layer (EDL), although it is often described as the
electrical interfacial layer, as a two layer model can be simplistic for many applications
(Delgado et al. 2007). In the majority of cases the surface of a clay particle is negatively
charged due to the presence of broken bonds and isomorphous substitution (Alshawabkeh et
al. 1999; Aydin et al. 2004; Laxton and Berg 2006). Electrical neutrality is maintained in the
Chapter 2. Literature Review School of Environmental Systems Engineering
2.4
solid state through a layer of adsorbed cations, which in solution diffuse away from the
particle surface to form EDL (Alshawabkeh 1999). The EDL region contains a higher
proportion of cations than the bulk solution, which are pulled towards the solid particle
surface by electrostatic attraction and away by diffusion into the bulk solution (see Figure 2).
Close to the solid surface is a region of bound ions, which may be bound by purely Columbic
interactions or by a special chemical affinity to the particle surface (Delgado et al. 2007). The
bound layer is also known as the Stern layer or hydrodynamically stagnant layer. Beyond the
bound layer is a diffuse layer with a higher concentration of cations than the bulk solution,
which merges into the neutral bulk solution. The ions in the EDL are exchangeable, in that
they can be replaced by different ions having the same net charge.
Figure 2: The electrical double layer
The diffuse double layer or EDL is formed by the electrostatic attraction of cations to the net negative charge on the clay
particle surface. This results in a region surrounding the solid particle with a higher proportion of cations than the bulk
solution. Figure from Gillen (2006).
The thickness of the EDL is variable and is a function of pH, electrolyte concentration and
electrolyte type (Alkan et al. 2005). The net charge of the particles in the EDL is
approximately equal to and opposite to the net charge on the particle (Vane and Zang 1997).
If the pore water contains a higher concentration of ions, this will mean that to achieve
electrical neutrality, a smaller volume of pore fluid is required in the EDL, hence the
thickness of the EDL is smaller. The individual ions in the EDL can carry different charge
Chapter 2. Literature Review School of Environmental Systems Engineering
2.5
(for example calcium ions Ca2+ and sodium ions Na+), which will change the thickness of the
EDL, as only half as many calcium ions as sodium ions would be required for electrical
neutrality. Hence electrolyte type will also determine the thickness of the EDL. Pore water
pH also plays a major role in the EDL, however this can be complicated and non-linear. The
interaction and exchange of H+ and OH- ions with bound ions can increase or decrease the
thickness of the double layer.
A term for describing the number of exchangeable cation sites on a clay mineral is known as
the cation exchange capacity (CEC). An exchange of bound ions with ions in solution can
occur if the bonding is not too strong and the sites are accessible to the soil solution (Troeh
and Thompson 2005). The CEC of kaolinite (typically 3-15 meq/100g) is much lower then
many other clay minerals, such as illite (20-40 meq/100g) and smectite (60-120 meq/100g),
due to its 1:1 silicate structure which does not allow for cation exchange between layers
(Troeh and Thompson 2005). While in most cases cation exchange dominates clay minerals,
anion exchange is also possible due to broken edge bonds. Broken edge bonds can be either
positive or negative and result from the termination of the continuous ionic sequence which
makes up the clay lattice structure. Anions or cations are required at these edges for electrical
neutrality and in some cases these are exchangeable, contributing to the cation or anion
exchange capacity (Troeh and Thompson 2005).
2.1.3 Zeta Potential
The zeta potential is the difference in potential between the slipping plane in the EDL and the
bulk medium. The surface charge on a discrete clay particle is generally negative, resulting in
a negative zeta potential. As the zeta potential is related to the properties of the EDL as
described above, factors that effect the EDL will also change the zeta potential. These factors
include pore fluid ionic strength (determined by concentration and composition) and pH.
The relationship between pH and zeta potential is complex and depends on the clay properties
and the properties of the pore fluid. In some cases the zeta potential is reported to decrease
linearly with the logarithm of the pH of the soil medium (Acar and Alshawabkeh 1993). The
isoelectric point is defined as the pH value at which the zeta potential is zero (Greenwood et
Chapter 2. Literature Review School of Environmental Systems Engineering
2.6
al. 2007). At low pH, the magnitude of the zeta potential may become positive below the
isoelectric point, depending on the surface chemistry and characteristics of the clay
(Thepsithar and Roberts 2006). The relationship between pH and zeta potential is also highly
dependent on the concentration and type of ions present in the pore fluid. There is no general
relationship between the zeta potential-pH curve and pore water ionic concentration, as
different ions can increase or decrease the magnitude of zeta potential with regard to pH.
Studies by Kaya and Yukselen (2005) demonstrated that in the presence of alkali earth ions
(LiCl and NaCl), the zeta potential of quartz powder and kaolinite become more negative over
a range of pH values. However, the opposite was observed for CaCl2 and the hydrolysable
heavy metals such as Zn2+ and Cu2+, and results for montmorillonite were often opposite to
those obtained for kaolinite (Kaya and Yukselen 2005; Kosmulski and Dahlsten 2006).
Measurement of zeta potential
The zeta potential is a property of charged interfaces and should be independent of the
technique used for its measurement (Delgado et al. 2007). There is currently no method to
measure zeta potential directly, however there are a number of ways it can be quantified
through measurement of its influence on particle behaviour. In most cases to measure zeta
potential of clay requires a colloidal solution of discrete clay particles and properties of the
movement of the clay particles are measured. One such method involves using optical
methods to measure particle velocity in a applied magnetic field. Optical methods measure
light scattering off moving particles. For this method samples need to be a very dilute colloid
solution (Vane and Zang 1997).
Zeta potential can also be measured by electroacoustics, which allows for pastes or slurries to
be measured. An alternating voltage across the colloidal solution causes the charged particles
to oscillate at the same frequency, generating soundwaves that are measured over a range of
frequencies (Shilov et al. 2004). The larger the zeta potential the faster the particle moves,
generating stronger sound waves (Greenwood et al. 2007). The method of electroacoustics
also allows for simultaneous measurement of the zeta potential and particle size, and allows
for measurements of much more concentrated solutions than other techniques (Greenwood
2003). Electroacoustics can also refer to measurement of the reciprocal mechanism, whereby
a sound wave passes though the medium, generating a macroscopic potential difference called
the ultrasonic vibration potential effect (Greenwood 2003).
Chapter 2. Literature Review School of Environmental Systems Engineering
2.7
2.1.4 Hydraulic conductivity
It is well established that the flow of water through saturated soil can be represented by
Darcy’s law (Equation 1).
q=Aki Equation 1
vx=q/A=ki Equation 2
Where q is the volume of water flowing per unit time, A is the cross sectional area of soil
corresponding to the flow and k is the hydraulic conductivity (also known as coefficient of
permeability). This can also be written in the form of Equation 2, where vx is the average
flow velocity and i is the hydraulic gradient, also written as dH/dx, which is the change in
hydraulic head H over a distance dx (Craig 1987). Darcy’s law is valid for all soils where the
flow is laminar (Budhu 2007). The average flow velocity calculated by the expression above
is the flow divided by the entire cross sectional area. As flow in saturated soil medium only
occurs through the interconnected pore space, another term to describe the flow velocity
accounting for porosity is the seepage velocity (Equation 3)
vs=vx/n Equation 3
Where n is the porosity of the soil, which is the ratio of the volume of voids to the total
volume (Budhu 2007). The hydraulic conductivity ranges in the order of 1 cm/s in clean
gravels to 1x10-7 cm/s in some clays (Budhu 2007). Hydraulic conductivity can vary
depending on soil type, particle size, pore fluid properties, void ratio, pore size, homogeneity
(layering and fissuring) and entrapped gasses (Budhu 2007).
Chapter 2. Literature Review School of Environmental Systems Engineering
2.8
2.2 PRINCIPLES OF ELECTROLYSIS
An electrolytic cell is an electrochemical cell in which a non-spontaneous reaction is driven
by an external source of current. Oxidation is defined as the removal of electrons from a
species and in a electrolytic cell oxidation reactions occur at the anode. Reduction is the
addition of electrons to a species and occurs at the cathode. In a basic electrochemical cell the
predominant redox reaction is often the dissociation of water into hydrogen ions and
hydroxide ions producing hydrogen and oxygen gas (Alshawabkeh et al. 2004).
Anode half reaction: 2H2O O2 + 4H + 4e- (Eo=-1.229)
Cathode half reaction: 2H2O + 2e- H2 + 2OH- (Eo=-0.828)
The standard reduction potential, E0, is a measure of the tendency of the reactants in their
standard states to proceed to products in their standard states (Acar and Alshawabkeh 1993).
Secondary reactions can occur at the anode and cathode due to the reduction and oxidation of
other constituents in the anolyte and catholyte. The type of reactions occurring at the
electrodes is dependent on the concentration of species and electrochemical potential of redox
reactions (Acar and Alshawabkeh 1993) and the standard reduction potential for the redox
reaction. For example other reactions that may occur at the cathode include the production of
hydrogen gas from hydrogen ions and the precipitation of metals from metal ions and metal
hydroxides (Acar and Alshawabkeh 1993).
2.2.1 Transport Mechanisms
Mechanisms of electrokinetically induced transport include electromigration, electroosmosis
and electrophoresis (Acar and Alshawabkeh 1993). The relative contribution of each transport
mechanism to the total mass transport depends on soil properties such as pore water
composition, soil type, water content and processing conditions (Acar and Alshawabkeh
1993). Diffusion and advection can also contribute to transport during EK treatment.
Chapter 2. Literature Review School of Environmental Systems Engineering
2.9
Electromigration
Electromigration is the movement of ionic species due to an applied electric field. Negative
ions (anions) move towards the positively charged anode and positive ions (cations) move
towards the negatively charged cathode. This movement is dependent on the applied electric
field and the effective ionic mobility. The effective ionic mobility of an ion is a function of
its molecular diffusion coefficient, soil porosity, tortuosity factor and charge (Acar and
Alshawabkeh 1993). The effective ionic mobility can be estimated by assuming the Nerst-
Townsend equation (Equation 4) is valid for ions in the pore fluid of a saturated soil medium.
RTFzD
nuu jjjj*
* == τ Equation 4
Where uj* is the effective ionic mobility, uj is the ionic mobility in free solution, τ is the
tortuosity factor, n is the porosity factor, Dj* is the effective diffusion coefficient, zj is the
valence of the ion, F is faradays constant, R is the universal gas constant and T is the absolute
temperature (Acar and Alshawabkeh 1993). The velocity and migrational flux of a species
under electrical potential gradients only can then be defined as Equation 5 and Equation 6.
)(v *m Edxdu j −= Equation 5
)(J *mj Edxdcu jj −= Equation 6
Where E is the electric potential, cj is the molar concentration vm, is the velocity of an ion and
Jjm is the migrational flux. The effective ionic mobility for the hydrogen ion and the
hydroxide ion are reported at much higher magnitude than other common ions such as sodium
or nitrate (Acar and Alshawabkeh 1993). This is due to their interaction with water molecules
in solution and is discussed in more detail later in this chapter. The effective ionic mobilities
for some common ions (assuming a clay with porosity 0.6 and average tortuosity of 0.35) are
included in Table 1.
Chapter 2. Literature Review School of Environmental Systems Engineering
2.10
Table 1: Effective ionic mobilities of selected common anions and cations
Common cations
Effective ionic mobility
(x106 cm2/Vs) Common
anions Effective ionic mobility
(x106 cm2/Vs)
H+ 760 OH- 432
Na+ 109 NO32- 155
Ca2+ 130 CO32- 156
Table values from Acar and Alshawabkeh (1993)
Electrophoresis
Electrophoresis is the transport of charged solids suspended in liquid (colloids) and
macromolecules under an electric field (Lee 2005). Electrophoresis does not play a major role
in mass transport through saturated clay cores but has been observed where micelles are
formed due to the addition of surfactants or in remediating slurries (Acar and Alshawabkeh
1993). The phenomenon of electrophoresis can be used to measure zeta potential of clay
particles suspended in solution.
Electroosmosis
Electroosmotic flow refers to the bulk pore fluid flow through a soil medium under an applied
electric field. Under the influence of an electric field, the mobile layer of cations in the EDL
move towards the cathode. This net migration of cations in the EDL drags the pore water
with it, resulting in a hydraulic flow (Alshawabkeh et al. 1999). This mechanism is most
apparent in fine grained soils. If the soil pore is idealised as a capillary, fine grained soils
have a larger number of capillaries of smaller diameter than coarse grained soils. Thus the
influence of the EDL on the net pore water in fine gained soils is proportionately greater,
resulting in greater movement of pore water. In addition, electroosmosis is most significant
when the pore water ion concentration is low (Ottosen et al. 2008).
Chapter 2. Literature Review School of Environmental Systems Engineering
2.11
Electroosmotic flow can be related empirically to the applied electric field by Equation 7.
Qe= ke ie A Equation 7
Where Q is the fluid volume flow rate (m3/s), ke is the coefficient of electroosmotic
conductivity (m2/V-s), ie is the electric field strength (V/m) and A is the cross sectional area.
By this model electroosmotic flow is dependent on the coefficient of electroosmotic
permeability (ke) which is a empirical measure of the fluid flux per unit area of the soil per
unit electric gradient (Alshawabkeh 2001). The ke value is dependent on zeta potential,
viscosity of the pore fluid, porosity and electrical permittivity of the medium.
Figure 3: Mechanism of Electroosmosis
When a DC current is applied across a saturated soil profile, the movement of cations in the electrical double layer drags the
bulk solution with it, resulting in movement of the bulk solution towards the cathode in most clays (Vane and Zang 1997).
Zeta potential controls the direction and magnitude of the electroosmotic flow (Thepsithar and
Roberts 2006). The magnitude of the zeta potential controls the magnitude of osmotic flow,
whilst the sign controls the direction (Kaya and Yukselen 2005). Electroosmotic flow is
usually in the direction of the cathode due to soil surface having a net negative charge,
resulting in a negative zeta-potential, however the direction can be reversed under conditions
Chapter 2. Literature Review School of Environmental Systems Engineering
2.12
of positive zeta potential. As discussed in Section 2.1.3, some clays with certain pore water
compositions can exhibit a positive zeta potential for low pH ranges. Whilst the pH of the
pore fluid is above the pH value at the iso-electric point, electroosmotic flow will be towards
the cathode, however if it drops below the iso-electric point, flow may reverse and be in the
direction of the anode (Thepsithar and Roberts 2006).
Electroosmotic flow depends mainly on the porosity, applied voltage gradient and the zeta
potential of the soil matrix and is independent of the pore size distribution and the presence of
macro pores (Acar and Alshawabkeh 1993; Isosaari et al. 2007). This gives electroosmotic
flow an advantage at generating uniform fluid and mass transport in fine grained soils over
hydraulic gradients, which are highly dependent on the soil fabric and macro-structure (Acar
and Alshawabkeh 1993). While the hydraulic conductivity of soils can vary by many orders
of magnitude, the coefficient of electroosmotic conductivity is generally in the range of 1x10-9
to 1x10-8 m2/V-s (Alshawabkeh et al. 1999).
Optimising electroosmotic flow is complex and highly dependent on clay and pore water
chemistry (Isosaari et al. 2007). Studies have been conducted in enhancing the zeta potential
in order to optimise electroosmotic flow (Popov et al. 2004). While the pH of the pore water
is a dominant variable controlling processes such as zeta potential and electroosmosis, the
extent to which this occurs is highly dependent on clay type (Eykholt and Daniel 1994).
Diffusion and advection in a saturated soil medium
Whilst not an electrokinetically driven process, diffusion and advection can play a significant
role during EK treatment. Diffusion can be significant depending on time and length scales
and due to the enhanced chemical gradients which are produced in the soil during EK
treatment. Diffusion in a saturated soil matrix differs from diffusion in aqueous solution as
the flow paths for diffusion are much longer and more tortuous (Yeung and Datla 1995). The
effective diffusion coefficient may be empirically defined by Equation 8.
Di* = τ x Di Equation 8
Where Di* is the effective diffusion coefficient of species i, Di is the diffusion coefficient of
species i in free solution and τ is the tortuosity of soil ranging from 0 to 1 (Yeung and Datla
Chapter 2. Literature Review School of Environmental Systems Engineering
2.13
1995). Di can be obtained from literature, and with the measurement of Di*, a value for τ can
be obtained experimentally (Yeung and Datla 1995).
Advection of species during EK treatment can result from hydraulic gradients, which may
either be present from external processes (for example if the technique was applied to a soil
profile with groundwater flow, or to enhance pump and treat systems) or from internal
processes relating to EK treatment. Non-uniform pore pressure distributions can develop
across the soil profile during EK treatment and these can result in hydraulic gradients driving
advection processes (Yeung and Datla 1995).
2.2.2 Acid and base production and migration
Oxidation at the anode generates acid that migrates through the soil profile towards the
cathode. Reduction at the cathode produces a base front that migrates through the soil towards
the anode (Acar and Alshawabkeh 1993). The rate at which the acid front and base front will
migrate through the soil profile depends on many complex and interacting factors relating to
the soil properties and electrokinetic treatment. The effective ionic mobility’s of the
hydrogen ion and the hydroxide ion are reported as significantly higher than other common
ions. This is due to transfer mechanisms within and between water molecules. The hydrogen
and hydroxide ions are able to be transferred through a series of hydrogen bonded water
molecules by rearrangement of hydrogen bonds in a string of aligned water molecules.
While it is common to refer to the hydrogen ion as a free ion in solution, this is a simplified
model. A lone proton in solution has a very high charge density and has a strong attraction to
surrounding water molecules and will bond covalently to one of the lone electron pairs to
form a hydronium ion, H3O+, which is also attracted by hydrogen bonds to several other water
molecules, forming a mixture of species with the general formula of H(H2O)n+ (Silberberg
2000). Whist the detail of the mechanism is still not fully understood, the suggestion that the
effective movement of a proton involves the rearrangement of bonds in a group of water
molecules is known as the Grotthuss mechanism (Atkins and de Paula 2002). Agmon (1995)
suggested the mechanism for this involves the H9O4+ unit, which is a trigonal planer H3O+ ion
linked to three H2O molecules. A rapid re-orientation of this arrangement results in a new
Chapter 2. Literature Review School of Environmental Systems Engineering
2.14
H9O4+ cluster in which the positive charge is located one molecule across from its original
location (Atkins and de Paula 2002). Regardless of the detail of the mechanism, the measured
ionic mobility of H+, and to a lesser extent OH-, is significantly higher then other ions, as
shown in Table 1.
Acar and Alshawbkeh (1993) reported the base front which developed at the cathode in
kaolinite will first advance towards the anode by ionic migration and diffusion, but its
migration will be impeded due to the neutralisation from hydrogen ions migrating in the
opposing direction. In addition, the acid front advancing through the soil from the anode to
the cathode has been observed as resulting in a decrease in the electroosmotic permeability
associated with an increase in conductivity in the anode compartment and a corresponding
drop in zeta potential (Acar and Alshawabkeh 1993). The establishment of a pH profile
across the clay also results in a zeta potential gradient, with the increase in pH at the cathode
resulting in the zeta potential becoming more negative, and the decrease in pH near the anode
resulting in the zeta potential becoming less negative (Kaya and Yukselen 2005).
Acid production and migration can be beneficial in cases where the objective is to achieve the
mobilisation of metals, however it can be detrimental in some applications. Changes in pH
can have a complicating effect during EK enhanced ISCO, and in some cases pH gradients
across the soil profile are undesirable. For example the mechanism of oxidation of phenol by
permanganate is complex and highly dependent on pH (Roach and Reddy 2006; Thepsithar
and Roberts 2006) and pH profiles can significantly complicate the process. In cases where
the pH gradient needs to be minimised, the production of acid and base at the electrodes can
be controlled through the use of buffers in the anolyte and catholyte. In addition the extent to
which the acid front moves through the soil depends on the buffering capacity of the clay,
which is also highly variable between different clay types. The buffering capacity describes
the process whereby ions adhered to the clay particle surface can exchange with the H+ and
OH- ions, removing them from the pore water and preventing migration of ions adsorbed to
the immobile surface (Acar et al. 1993). The lower cation exchange capacity of kaolinite
clays is reported to result in a much lower buffering capacity compared with other clay
minerals such as illite or montmorillonite (Acar and Alshawabkeh 1993).
Various methods have been developed to control the electrolyte pH to suppress or enhance the
effect of acid or base migration through the soil profile (Lee and Yang 2000). One method is
Chapter 2. Literature Review School of Environmental Systems Engineering
2.15
to introduce weak acids or bases into the respective electrolyte compartments to neutralise any
acid or base produced, for example the addition of acetic acid to the cathode to neutralise
hydroxyl ions (Alshawabkeh et al. 1999). For cases where an acidic medium is preferred for
the removal of metals, selective membranes have also been investigated for preventing
migration of hydroxyl ions (Alshawabkeh et al. 1999).
2.3 SUMMARY
Whilst it is well established that acid and base production and migration through the clay core
will occur during EK treatment, it has been shown that there are many complex and
interacting mechanisms which cause migration. Properties of the clay will effect the timing
and extent to which migration by electrokinetic or electroosmotic mechanisms will occur.
More research is required, looking into how variations in clay composition will affect the EK
migration mechanisms during treatment.
Chapter 3. Methodology School of Environmental Systems Engineering
3.1
CHAPTER 3. RESEARCH METHODS
3.1 EXPERIMENTAL DESIGN
The objectives of this study were to investigate some of the physical/chemical properties of
three different clays types, to quantify the relationship between pH and zeta potential in these
clays and to perform a series of 1D electrolysis tests to observe acid/base migration. The
following sections outline the experimental tests used to address each of these objectives.
3.2 CLAY CHARACTERISTICS
Three commercially available clay types were chosen for this study based on advice from the
manufacturer on compositional differences and some preliminary testing to ensure the zeta
potential-pH relationship differed between the three clay types. Commercial clay was chosen
in preference to finding a natural clay source as it allowed for large quantities of relatively
homogeneous clay to be obtained for this and future studies within reasonable cost and time
constraints. The three clays are mixtures consisting of a kaolin component. The clay types
selected for analysis were Clayworks Studio Porcelain (SP), Kagero (K), and SWE.
3.2.1 X-ray diffraction (XRD)
To examine the composition of the three clay types, samples were sent for XRD analysis to
the University of Ballarat. Prior to sample submission, a 10 g subsample of each clay was
dried in an oven at approximately 105ºC for two days. These were then crushed using a
mortar and pestle and air-freighted to the University of Ballarat for analysis.
X-ray diffraction traces were obtained from the powdered samples with Siemens D500 and
D501 diffractometers using Fe-filtered CoKα radiation. The operating conditions were
40kV/30mA, step scan 0.02θ/2θ at 1o/2θ/min, with a fixed 1o divergence and receiving slits
and a 0.15o scatter slit (pers com McKnight 2008).
Additional traces were obtained from air-dried and glycolated mounts prepared by
sedimentation in 1% sodium hexaphosphate aqueous suspensions. The mineral phases
present were identified by computer-aided searches of the 2007 ICDD PDF4/Minerals subfile
Chapter 3. Methodology School of Environmental Systems Engineering
3.2
and the USGS “Clay Flowsheet” (Poppe et al. 2001). Quantitative XRD results were obtained
using SiroQuant™ ver 3.0, which is a “whole pattern” Rietveld analysis technique and
refinement of the most suitable mineral structures available in the current software package
databank (pers com McKnight 2008). Results from this method are regarded as semi-
quantitative.
3.2.2 Grain size analysis
Clay samples were sent to Golder Associates Perth Laboratory for grain size analysis and
Atterberg limits. Tests were carried out in accordance with Australian Standard AS1726-Soil
Classification. Sieve analysis determined the percentage composition of coarse grained soils
(75 to 0.075 mm) through analysis of the mass of soils passing through successively finer
sieve mesh. Fine grained particles (less then 0.075) require hydrometer analysis, which
involved mixing a small amount of soil into a suspension and allowing to settle over time
(Budhu 2007). The larger particles settle fastest, resulting in a layered profile according to
grain size. This is measured using hydrometer which is lowered into suspension and will sink
to a position where the buoyancy force balances the weight of the hydrometer, giving a
reading of density of the suspension at different times (Budhu 2007).
3.2.3 Moisture content
Moisture content is defined as the weight ratio of water to solids. Moisture content was
measured for the three clay types to determine variation in moisture content between clay
types and consistency of moisture content within each clay type. For each moisture content
determination, a small amount (roughly 20 g) of wet clay was weighed accurately in a small
aluminium foil tray which was pre-weighed. This was then placed in an oven at 105ºC and
left for a minimum of two days. The dry sample and aluminium tray were weighed
immediately upon removal from the oven. Moisture content was calculated as:
w = Ww/Ws x 100% Equation 9
Where w is the moisture content expressed as a percentage, Ww is the weight of water (given
by the weight of the wet sample minus the weight of the dry sample) and Ws is the weight of
the dried sample.
Chapter 3. Methodology School of Environmental Systems Engineering
3.3
3.2.4 Unit weight
The unit weight or density is the weight of soil per unit volume and is dependent on moisture
content. The unit weight was measured for the three clays to quantify differences between
clay types and as a quality control procedure to check the consistency of packing into PVC
cores for the column experiments. The unit weight was measured by weighing a PVC core of
known dimensions, packing clay into the PVC core, then weighing the PVC and clay,
ensuring the open ends of the PVC core were packed flat (no protruding clay or void space).
Unit weight was then calculated by Equation 10.
γ = W/V Equation 10
Where γ is the unit weight, W is the weight of packed clay (given by the total weight of PVC
and packed clay minus the weight of the PVC core) and V is the internal volume of the PVC
core.
3.2.5 Acid neutralising capacity
Samples of the three clays were submitted to the analytical laboratory ALS for the
determination of acid neutralising capacity (ANC). Samples were dried at 85ºC prior to
testing. The standard method ALS uses in the determination of ANC is referenced as USEPA
600/2-78-054, and involves a fizz test to semi-quantitatively estimate the likely reactivity,
prior to acid titration. The sample is then reacted with an excess quantity of acid. Acid
titration is used to determine the amount of acid that has not reacted with the clay and this is
used in comparison to a blank titration to calculate the ANC, which is reported as kg H2SO4
equivalent or converted into %CaCO3.
3.2.6 Hydraulic conductivity testing
The hydraulic conductivity of the clays was determined using the falling head test (Craig
1987), which is a test suited to fine grained soils such as clays. Figure 4 shows the setup for
the measurement of k includes a cylindrical core with the specimen, which has coarse filter
paper at each end to prevent loss of sample, a standpipe to measure change in hydraulic head
with internal area a, and a constant head reservoir at the outlet.
Chapter 3. Methodology School of Environmental Systems Engineering
3.4
Figure 4: Experimental setup for determination of hydraulic conductivity using the falling head test.
The change in hydraulic head is measured over time to determine flow through the sample.
The time, t1, for the water to fall from h0 to h1 is measured. Given a cylindrical core specimen
of length l and internal cross sectional area A, the hydraulic conductivity is obtained by
applying Darcy’s law:
lhAk
dtdha =− Equation 11
∫∫ =−11
0 0
th
h
dtl
Akhdha Equation 12
1
0
1
lnhh
Atalk = Equation 13
Hydraulic conductivity is a property of the clay and the pore fluid. The fluid used to
determine hydraulic conductivity was de-ionised (DI) water with a small amount of sodium
chloride (NaCl) salt added to make a solution of 0.4 g/L NaCl, which is consistent with the
electrolyte solution used in the EK tests. The top of the standpipe was covered to minimise
loss of solution by evaporation.
Chapter 3. Methodology School of Environmental Systems Engineering
3.5
3.2.7 Zeta potential determination
Clay samples for zeta potential testing were prepared by first drying the sample in an oven at
approximately 105ºC for two days. Dried samples were then ground into fine powder using a
mortar and pestle. Approximately 21g of clay powder was added to 119 g of laboratory de-
ionised water or a salt solution. Salt solutions were made up with the addition of sodium
chloride salt (Ajax fine chem. analytical reagent A465-5KG) to laboratory DI water to the
desired concentration. All weight measurements were accurate to 0.005 g. The clay powder
and water mixture was then shaken vigorously by hand to disperse the clay powder into the
solution.
Zeta potential determination was carried out in the Surface Forces and Dispersion Rheology
Laboratory in the School of Mechanical Engineering at UWA by A. Prof. Yee-Kwong Leong.
Samples were diluted with the appropriate salt solution to 260ml and then sonicated (Sonics
VibracellTM) for 30 seconds prior to zeta potential measurement to homogenise the solution
and break any agglomerates. Zeta potential measurements were measured using a ZetaProbe,
which utilises the principle of electroacoustics to determine zeta potential. This principle is
discussed in Section 2.2.3. The ZetaProbe is capable of taking single point measurements, or
titrating pH to get a relationship between pH and zeta potential for the sample. The particle
density and concentration were determined for the calculation of the solids volume fraction
required in the calculation of zeta potential.
The Kagero clay contained a relatively large amount of coarse grained material in the samples
that needed to be removed by sedimentation prior to dilution with the appropriate salt solution
and sonification. After sonification a sample of these slurries was collected for solids content
determination. Zeta potential-pH curves were determined for each of the three clays using DI
water, 0.4 mg/L NaCl, and 4 mg/L NaCl solution.
3.3 1D CORE EXPERIMENTS
Column experiments utilised a similar physical setup as used by Lee (2005), Gillen (2006)
and Reynolds et al. (2008). An electric current was passed through clay packed into PVC
Chapter 3. Methodology School of Environmental Systems Engineering
3.6
sleeves and sealed between two plastic vessels to hold the electrolyte solution and the
electrodes.
3.3.1 Intermediate Junction pH sensor
An intermediate Junction pH sensor (IonodeTM IJ44) was used to determine pH. The pH
sensor was chosen specifically for its ability to be inserted directly into fine sediment. In the
standard electrode design, a porous membrane (typically ceramic) will separate the reference
fluid within the sensor from the sample electrolyte. If this becomes clogged, the movement of
ions across the membrane is restricted and the probe returns erroneous readings, making this
type of probe unsuitable for direct measurement of soft solids such as clay. The intermediate
junction pH sensor contains an electrolyte reference gel contained within a removable plastic
sleeve, allowing a small amount of unrestricted movement of the electrolytes where the sleeve
contacts the main stem. With regular removal, cleaning and replacement of the electrolyte
gel, this design allows for restricted solution contact without a porous membrane that would
be prone to clogging.
3.3.2 Preparation of clay cores
Clay was packed into PVC sleeves for electrokinetic treatment. The PVC sleeves were 99.1
mm long (x direction) with an internal diameter of 43.9 mm (Figure 5). This gave an internal
volume of 150 cm3. The clay was packed into the sleeve by inserting the sleeve into a solid
block of clay, and trimming the clay at the open ends. Cores were then weighed to determine
unit weight using the method described in 3.2.4. This was performed to identify any
inconsistency in packing.
Figure 5: PVC Core dimensions
Chapter 3. Methodology School of Environmental Systems Engineering
3.7
To prevent loss of clay sample from the PVC core during treatment, a filter material (Jif®
cloths) was pre-cut to the dimensions of the open ends and glued (Tarzan’s Grip®) to the
PVC edge, covering the open end. Cores were left for approximately 10 minutes for the glue
to dry, then left to soak in a bucket of the electrolyte solution for a minimum of three days.
This was intended to equilibrate the cores with the electrolyte solution prior to EK treatment.
The experimental method was modified slightly in the case of the SP clay, as it was observed
that this clay had a particularly fine component that was not constrained by the filter cloth.
An additional layer of filter paper was used in conjunction with the filter cloth in this case.
3.3.3 Electrolytic cell setup
The electrolytic cell consisted of two identical electrolytic half-cells, which contained the
electrodes and the electrolyte fluid. The two half cells were connected by the clay core.
Electrolyte fluid
The electrolyte solution consisted of laboratory DI water with a small amount of NaCl salt
(Ajax fine chem. analytical reagent A465-5KG) added to make a solution of 0.4 g/L NaCl.
Some electrolyte is required to enable the electrolyte solution to conduct charge, as
theoretically, pure DI water would not conduct charge. A weak NaCl solution was chosen for
the anolyte and catholyte as the purpose of the experiment was to observe the production and
migration of acid and base at the electrodes and minimise any secondary reactions which may
occur at the electrodes. In addition a concentrated NaCl solution could have resulted in error
in the measurement of pH due to sodium error from the pH probe. An example of an
undesirable secondary reaction would be the production of chlorine gas, Cl2, from the
oxidation of the chloride ion, Cl-, at the anode. This effect is able to be reduced by keeping a
low chloride concentration. The pH and conductivity of the anolyte and catholyte was
recorded (TPS WR81 pH-cond-salinity) at the beginning and end of each test. The power to
the electrodes was turned off before conductivity and pH measurements of the anolyte or
catholyte were made.
Chapter 3. Methodology School of Environmental Systems Engineering
3.8
Electrodes
The electrodes were mixed copper/titanium/platinum supplied by McCoy Engineering. The
electrodes was supplied as a single length of wire which was cut into lengths of
approximately 150 mm long.
Power supply
The electric power was supplied by a Powertech (MP-3092) dual tracking DC laboratory
power supply or a Manson (NP-9613) DC regulated power supply, both capable of supplying
between 0 and 40V, and a maximum current of three amps. Voltage was set at a constant 20V
for all tests, with maximum current enabled. Insulated copper wire was used to connect the
power supply to the electrodes. The copper wire was wrapped around the top of the electrode
and sealed securely by electrical tape. The connection and seal between the copper wire and
the electrodes needed to be water tight to prevent corrosion, as although the connection was
kept above the level of the electrolyte solution, condensation would settle on the electrodes
and wire leading to corrosion of the copper wire if a sufficient seal was not achieved. The
voltage was checked and adjusted using a hand held digital voltameter (DSE Q1459).
Voltage measurements were taken using the handheld voltameter in contact directly with the
electrodes at the point where the electrode contacted the top of the electrolyte solution.
Measurements were taken at the beginning and end of each test.
Electrolyte containers
The electrolyte fluid and electrodes were contained within two discrete nine litre plastic
buckets with a purpose built plastic flange for the connection of the PVC core (Figure 6).
Within the plastic flange, two rubber o-rings were used to create a watertight seal once the
PVC core was put in place. To assist with obtaining a water tight seal, and inserting and
removing the PVC cores, a valve and o-ring lubricant (Molykote® 111) was applied liberally
to the inside of the plastic flange over the o-rings and outside of the PVC core. The PVC core
was directly inserted into the first bucket then the second bucket by hand. Care was required
not to dislodge the o-rings during insertion, which could result in leakage. Once the water
tight seal was confirmed the buckets were filled with 5L of the electrolyte fluid. The
electrodes were then secured in position using adhesive tape and the buckets were covered
Chapter 3. Methodology School of Environmental Systems Engineering
3.9
with cling wrap to reduce electrolyte losses through evaporation. The electrodes were placed
to hang vertically immediately adjacent to the PVC core.
Figure 6: Electrolyte containers
3.3.4 Measurement of the pH profile
Removal of the clay core
Following the removal of the electrodes and electrolyte fluid from the containment vessels,
the PVC core was carefully extracted from the plastic flanges on each vessel without
disturbing the clay material within the core. To remove the intact clay core from the PVC
sleeve, a flat plastic disc with a radius slightly smaller than the internal radius of the PVC
sleeve was placed at the mouth of one end of the PVC sleeve and pushed through the core
using a ceramic rod. As the clay cores were saturated and often very soft, particular care was
required not to stress the integrity of the clay during removal and potentially result in
fractured and collapsed cores. The orientation of the anode and cathode ends were marked
with a permanent marker on the PVC sleeve. Upon removal of the clay core from the sleeve
it was important to note the orientation during removal to keep track of the anode and cathode
ends of the core.
Chapter 3. Methodology School of Environmental Systems Engineering
3.10
pH profile measurements
Immediately upon removal from the PVC sleeve, the clay core was measured for pH profile
using an intermediate junction pH sensor (Ionode IJ44) described earlier. The pH sensor was
calibrated using a two point calibration with pH 4.00 and pH 6.88 standard solutions (Rowe
Scientific) prior to taking any measurements. The electrolyte gel (TPS saturated KCl gel for
I.J. electrodes) was replaced at least weekly. The core was sectioned such that seven
equidistant points were measured in a straight line along the length of the clay, being careful
to record which end was associated with the anode and which end was associated with the
cathode (Figure 7). The pH probe was inserted directly into the clay to a depth of
approximately 20 mm. The pH value was recorded after the pH values stabilised. After one
set of pH measurements were recorded along the length of the clay core, the core was rotated
about its longitudinal axis such that another line of seven points could be measured along its
length. This was done to obtain an average value based on two replicate measurements.
Figure 7: pH profile measurements
Quality control methods
The pH probe was calibrated with standard calibration solutions each day prior to
measurement. The electrolyte gel was changed as a minimum once a week as recommended
in the manufactures instruction manual. Towards the end of the experimental program the
electrolyte gel was changed daily when measurements were taken. This was due to ingress of
Chapter 3. Methodology School of Environmental Systems Engineering
3.11
fine sediment into the plastic cover, which could potentially lead to erroneous readings and
contribute to drift.
The experimental procedure was designed to test 1D flow. As such, it was assumed that the
clay packed within the core was relatively homogeneous in all directions and the electric field
applied over the clay core was homogeneous in y and z (x, y and z directions shown in to
Figure 5), thus any heterogeneity in the y and z directions in either the clay structure and
density or the electric field could result in non-uniform migration. To investigate the
variability in the y and z direction within a core, five linear profiles along the x-axis of one
clay core were measured.
Replicate pH profiles along every core were recorded to give two profiles for each clay core
for each test. In addition, two independent electrolytic cells were set up and run
simultaneously for each time treatment, to give a replicate core for each clay and time
treatment.
For at least two time treatments for each clay type, a control core was set up and measured
using the same methods and over the same timeframe as the treatment cores. The control core
was left to soak in the electrolyte solution for the same period as the treatment cores were
under the influence of the electric field. Thus any processes owing to diffusion into or out of
the core affecting pH were accounted for.
Chapter 4. Results and Discussion School of Environmental Systems Engineering
4.1
CHAPTER 4. RESULTS AND DISCUSSION
4.1 CLAY CHARACTERISTICS
4.1.1 X-ray diffraction
The XRD results yielded valuable information on the composition of the commercial clays, as
it was observed that the dominant clay component was kaolin and two of the clays (the SWE
and the SP clay) had a large component of quartz mineral. Quartz is a relatively inert material
which can often occur as particles across the entire particle size range, including clay size
particles (Gerrard 2000).
Table 2: Percentage mineral composition of clay types based on XRD
SWE Kagero SP
Phase Weight% Phase Weight% Phase Weight%
Quartz 44 Kaolin 61.7 Quartz 40.2
Kaolin 33.2 Quartz 20.6 Kaolin 35.3
Illite 17.4 Albite 9.7 Albite 20.3
Albite 3 Illite 6.9 Illite 3.2
Rutile 1.4 Orthoclase 0.7 Rutile 0.3
Mixed layer illite 1
Mixed layer illite 0.2
Mixed layer illite 0.6
Orthoclase 0 Rutile 0.2 Orthoclase 0.1
The major components of all three clays included quartz, kaolin clay, illite clay and albite,
which is a sodium rich feldspar not uncommon in clay. Trace components included rutile
(naturally occurring titanium dioxide), orthoclase (feldspar mineral) and mixed layer illite
clay.
Chapter 4. Results and Discussion School of Environmental Systems Engineering
4.2
The mineral composition of the SWE and SP clays appeared to be very similar, with
approximately the same fractional amounts of quartz and kaolin clay, with the main difference
between the two clays being the relative amounts of illite clay and albite mineral. The Kagero
clay appeared to have a significantly larger component of kaolin clay than the other two clay
types. These different mineral compositions may be reflected in the differences in the zeta
potential-pH relationship and other pH properties such as buffering capacity and baseline pH.
4.1.2 Grain size analysis
The grain size analysis results were reported according to Australian Standard AS1726 (Soil
classification), which separates silts and clays according to grain size, however this is a
simplifying convention as silts and clays can have overlapping gain size distributions and
ideally should be classified according to mineral structure. The Kagero clay had a
significantly higher proportion of larger particle sizes then the SP and the SWE, with 31% of
the mass in the sand sized particle range. This result is consistent with the grainy appearances
of the Kagero clay. The SWE and SP clay did not have any significant amount of visible
grains, and this is confirmed by the sieve analysis showing only 2% of fine sand was present
in these clays. The SWE and SP clays were very similar in grain size distribution. The SWE
clay had the largest component of grains below 0.002 mm.
Table 3: Particle size distribution of clays
SP Kagero SWE
Clay description CLAY (With Silt) Sandy CLAY (with Silt) CLAY (With Silt)
Sand size particles (0.075-2.36mm)
2% 31% 2%
Silt size particles (0.002-0.075mm)
50% 31% 42%
Clay size particles (less than 0.002mm)
48% 38% 56%
Full sieve and hydrometer analysis results are included in Appendix B.
Chapter 4. Results and Discussion School of Environmental Systems Engineering
4.3
4.1.3 Moisture content
The variation in moisture content between clays was determined by the average moisture
content from three sub samples within a 10kg bag of each clay type. The moisture content
ranged from 33% for the Kagero to 23.6 % for the SP.
Table 4: Moisture content of clay types
Average moisture content (% w/w, n=3) Standard Deviation
(% w/w, n=3)
SP 23.6 0.4
Kagero 33.0 0.3
SWE 28.1 0.3
Measurement of moisture content was also performed to give an indication of variation within
each clay type, as variability in the clay mixture could result in heterogeneity in the packed
clay cores for the 1D core experiments. Three sub-samples from various parts of the clay
pack showed a small standard deviation (Table 4) indicating the clay within each 10kg pack
was relatively homogenous. The clay used in these 1D core experiments was from a single 10
kg pack of each clay type, hence variability between clay packs was not required for the
purposes of this investigation.
Table 5: Moisture content of SP cores after EK treatment
Average moisture content (% w/w) Standard Deviation
(% w/w)
SP 3 day treatment 28.0 (n=4) 0.013
SP 5 day treatment 28.4 (n=2) 0.017
SP 5 day control 23.4 (n=2) 0.006
The moisture content was measured in the centre of the SP cores after three and five day EK
treatment and in a five day control core for comparison (Table 5). This showed a significant
difference in water content between treatment cores and control cores (two tailed t-test,
p=0.0025), with an increase in moisture content noted after EK treatment. In addition, the
Chapter 4. Results and Discussion School of Environmental Systems Engineering
4.4
average water content was higher in the five day treatment then the three day treatment,
however this was not statistically significant (two tailed t-test, p=0.79). As these
measurements were made on only a few cores, and only for the SP clay, this may not be
representative of all treatments. It does, however, correlate with the observation of swelling
in the EK treated cores, which was observed as lateral expansion of the clay cores resulting in
bulging of the filter cloth at either end of the clay core and in some cases leading to rupture of
the clay cores.
As clay is a three phase medium consisting of solid (clay particles), gas (air) and liquid (pore
water), for a increase in moisture content to occur water must displace air, clay particles, or
both. In this case as lateral expansion of the clay out of the PVC core was observed the
ingress of water has resulted in swelling of the clay matrix, with possibly some displacement
of air. If an increase in water content was measured with no observation of swelling this
would indicate ingress of water was displacing only the gas phase. The development of
positive pore pressures and soil swelling has been noted in similar experiments (Alshawabkeh
et al. 2004; Zhou et al. 2006) and is thought to be caused by non-uniform electro-osmotic
flow created by non-uniform electric gradients and non-uniform changes in zeta potential
(Alshawabkeh et al. 2004).
It is also significant to note that the moisture content of the control core was not significantly
different to the moisture content of the SP clay prior to soaking, indicating that allowing the
clay cores to stand in the electrolyte solution prior to treatment may not have resulted in
significant ingress of electrolyte solution into the clay. This result was based on only one
core and more measurements would be required to confirm if this observation was
representative of the treatment. Ingress of electrolyte fluid would require displacement of soil
particles or displacement of gas held in the soil pores, which requires overcoming the
displacement pressure of air in successively smaller capillaries. A hydraulic pressure head
may be required to promote ingress of electrolyte solution into the clay prior to EK treatment.
4.1.4 Unit weight
The unit weight for the clay types varied from 1.99 g/cm2 for the Studio Porcelain, to 1.85
g/cm2 for the Kagero (see Table 6).
Chapter 4. Results and Discussion School of Environmental Systems Engineering
4.5
Table 6: Unit weight of clay packed in PVC cores
Average unit weight (g/cm3) Standard Deviation
(g/cm3)
Range in unit weight (g/cm3)
SP (n=10) 1.99 0.01 0.04
Kagero (n=9) 1.85 0.01 0.04
SWE (n=3) 1.92 0.01 0.03
The unit weight was also determined for the packed clay cores as a quality control procedure
to check the consistency of packing into PVC cores for the column experiments. In general,
the variation in packed density showed little variability for all clay types, with a standard
deviation of 0.01 g/cm3 and the largest range of 0.04 g/cm3. In terms of the variation in the
total weight of the packed core, the largest range was 6.6 g, which was 2.2 % of the average
weight of a packed core. This indicated a small variability in packing consistency. Packing
consistency in future experiments may be improved by a method of standard compaction at
standard moisture content, however this may result in minimal improvement in quality control
for the increased procedural complexity. In general, the method used for packing the cores
was considered suitable for the purposes of this investigation. Packing efficiency was also
confirmed by the general appearance of the control cores after removal from the PVC sleeve,
as there were no observations of voids within these cores.
4.1.5 Acid neutralising capacity
The ANC of each clay type is presented in Table 7 in the units of percentage calcium
carbonate equivalent. The SP clay gave the highest ANC, followed by the Kagero and the
SWE. The greatest difference in this case was between the SP and the SWE, due to
compositional differences.
Chapter 4. Results and Discussion School of Environmental Systems Engineering
4.6
Table 7: Acid neutralising capacity of clay types
ANC (% CaCO3)
SP 0.7
Kagero 0.5
SWE 0.3
SWE (duplicate) 0.4
The ANC could affect the rate of migration by neutralising/consuming a proportion of the
acid front, reducing the drop in pH observed in the profile, where the higher ANC would have
the greatest effect. A duplicate sample for the SWE was submitted to the laboratory to check
the accuracy of the lab method and the result is presented in Table 7. Whilst there was a
difference of 0.1 between the primary and the duplicate sample, this was small and not
considered to effect the quality of the results for the purposes of this investigation.
4.1.6 Hydraulic conductivity testing
The hydraulic conductivity was determined for the three clays using the falling head test and
the results are presented in Table 8. The hydraulic conductivity measurements were repeated a
number of times for each clay to confirm the apparatus set up was giving consistent results.
The consistency of the results is represented by the standard deviation shown in Table 8.
Table 8: Hydraulic conductivity of the three clay types by the falling head test
Hydraulic conductivity
x10-7 cm/s
Standard deviation
x10-7 cm/s
SP 1.6 0.09
Kagero 5.0 0.13
SWE 3.0 0.09
The Kagero clay had a significantly higher hydraulic conductivity than the SP and SWE clay,
consistent with the grain size analysis which showed the SP and SWE clays have a larger
component of fine grained particles.
Chapter 4. Results and Discussion School of Environmental Systems Engineering
4.7
4.1.7 Baseline clay pH
Measurements of baseline clay pH were taken to determine the differences between clay types
and the variation within each 10 kg bag of clay, the results of which are presented in Table 9.
The baseline pH is controlled by composition of the clay and the pore fluid. The pH
measurements were taken directly from the clay as supplied from the manufacturer, which
was at a moisture content of approximately 20 to 30%. The equilibrium between the clay
particles and the pore water changes depending on moisture content and pore water
composition, and this can also influence pH.
Table 9: Baseline clay pH
Average (n=4) Standard deviation
SP 7.45 0.14
Kagero 7.17 0.18
SWE 7.63 0.18
The baseline pH of the three clay types ranged from7.63 in the SWE to 7.17 in the Kagero.
There was some variation across the bag, represented by a standard deviation in pH
measurements of 0.18, which was greater than the accuracy of the pH probe and hence
reflects small scale spatial variability in clay pH.
4.1.8 Zeta potential determination
Zeta potential-pH curves were determined for each of the three clays using DI water, 0.4
mg/L NaCl and 4 mg/L NaCl solution and these results are presented in Figure 8, Figure 9
and Figure 10 below. For the SP clay the relationship between zeta potential and pH in the DI
water and clay slurry was negative and linear in the pH region from 2 to 9 (Figure 8). The
addition of NaCl resulted in a more negative zeta potential and a less linear relationship
between zeta potential and pH. The difference between a NaCl concentration of 0.4 and 4 g/L
was minimal in the pH