FY thesis ZT - University of Western Australia · Acid front migration during the electrolysis of...

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

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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.

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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.

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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

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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

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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

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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

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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.


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.


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


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).


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


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


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


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).


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).


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.


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.


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).


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


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


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


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


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


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.


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.


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.


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


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


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.


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


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.


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


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.


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.


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


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).


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.


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.


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

FY thesis ZT - University of Western Australia · Acid front migration during the electrolysis of...

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