Electrokinetic applications in the remediation of NAPL ... · Figure 1-1: Application of...

Electrokinetic applications in the remediation of NAPL contaminated sites

University of Western Australia Centre for Water Research

Abstract Subsurface NAPL contaminated sites act as a long-term environmental problem, and may

pose as a threat to human health. Contaminant removal by excavation, however, is often

unfeasible for economic reasons. The demand for an innovative and cost-effective in situ

remediation technology has led to the employment of electrokinetic phenomena in

contaminant mobilisation and recovery.

The electrokinetic technique employs a low-level direct current or electrical potential

difference to induce mass transport by coupled and uncoupled conduction phenomena.

Traditional electrokinetic technologies have predominantly focussed on the migration of

heavy metals toward treatment or recovery zones, and have been seen to have limited

applicability to NAPL contaminated sites due to the nonpolar nature of NAPLs. This study

investigated the viability of remediating NAPL contaminated sites by the use of

electrokinetically driven treatment compounds. Treatment compounds suitable for such use

include potassium permanganate and sodium persulphate, which are commonly used to

target contamination through in situ chemical oxidation.

A series of electrokinetic tests were conducted to assess the viability of such a remediation

technique. The laboratory setup comprised an electrolytic cell, in which anodic and cathodic

compartments were separated by a sediment core sample, thereby simulating the movement

of a treatment compound through a saturated porous medium. Breakthrough curves were

obtained for various applied electrical potential differences across the sediment core sample.

Mass transport was found to be linearly proportional to the applied electrical potential

difference. However, mass transport without the application of an electrical potential

difference was measured to exceed mass transport employing electrical potential differences.

Additionally, it was found that the choice of materials used in electrokinetic operations may

have a significant impact on the rates of mass transport achieved.



Acknowledgements Dr David Reynolds for your help and inspiration throughout the year;

Dianne Krikke for aiding my efforts with great patience;

Matthew Stovold for all your help in the laboratory;

David Thomas for your ideas and the two white buckets you gave me;

Family and friends for bearing with me throughout the project.



Table of Contents 1. INTRODUCTION ..................................................................................................... 1

2. LITERATURE REVIEW.......................................................................................... 4

2.1. MECHANISMS OF MASS TRANSPORT IN AN ELECTRIC FIELD..................................... 4 2.1.1 Molecular diffusion ............................................................................................. 4 2.1.2 Electromigration ................................................................................................. 7 2.1.3 Electroosmosis................................................................................................... 9 2.1.4 Electrophoresis ................................................................................................ 11 2.1.5 Acid front formation .......................................................................................... 12

2.2. RELATIVE CONTRIBUTION OF MASS FLUXES.......................................................... 13

2.3. SITE APPLICABILITY .............................................................................................. 13

2.3.1 Formation of hazardous by-products ................................................................ 14 2.3.2 Site impacts ..................................................................................................... 15

2.4. TIMESCALES AND EFFICIENCY .............................................................................. 15

2.5. COST ................................................................................................................... 17

3. METHODOLOGY.................................................................................................. 19

3.1. LABORATORY CONFIGURATION............................................................................. 19

3.1.1 Electrolyte fluid................................................................................................. 20 3.1.2 Sediment core sample...................................................................................... 21 3.1.3 Electrolyte containers ....................................................................................... 22 3.1.4 Power supply ................................................................................................... 23 3.1.5 Electrodes ........................................................................................................ 23 3.1.6 Additional equipment and instrumentation........................................................ 24

3.2. ELECTROKINETIC TESTING ................................................................................... 25 3.2.1 Testing program ............................................................................................... 25 3.2.2 Electrokinetic performance and diffusion test procedure .................................. 25 3.2.3 Free solution control test procedure ................................................................. 26

3.3. SEDIMENT CORE ANALYSES.................................................................................. 26

4. RESULTS AND DISCUSSION ............................................................................ 27

4.1. DIFFUSION CONTROL TESTS ................................................................................. 27 4.1.1 Expected and observed results ........................................................................ 27 4.1.2 Discrepancies .................................................................................................. 29

4.2. ELECTROKINETIC PERFORMANCE TESTS .............................................................. 30

4.2.1 Sustenance of mass transport.......................................................................... 32 4.2.2 Apparatus decomposition................................................................................. 35 4.2.3 pH .................................................................................................................... 38



4.2.4 Voltage and current.......................................................................................... 39

4.3. EFFECTIVENESS OF ELECTROKINETIC MASS TRANSPORT ..................................... 41

4.4. FREE SOLUTION CONTROL TEST........................................................................... 43

4.5. SEDIMENT CORE ANALYSES.................................................................................. 44

5. CONCLUSIONS .................................................................................................... 46

5.1. SCIENTIFIC SIGNIFICANCE..................................................................................... 46

5.2. FUTURE RESEARCH.............................................................................................. 46

6. GLOSSARY........................................................................................................... 48

7. REFERENCES ...................................................................................................... 49



List of Figures Figure 1-1: Application of electrokinetics in the removal of heavy metals....................................... 2

Figure 1-2: NAPL remediation via electrokinetically enhanced in situ chemical oxidation.............. 2

Figure 2-1: Profile of a diffusing chemical front................................................................................ 6

Figure 2-2: Position of a diffusing contaminant front at 100 and 10,000 years ............................... 7

Figure 2-3: Cumulative volume of water transported by electroosmosis....................................... 10

Figure 2-4: Electroosmotic permeability and hydraulic conductivity for a range of soils ............... 11

Figure 2-5: Damaged circuitry due to electromigration voiding ..................................................... 12

Figure 2-6: Field configuration employed in the electrokinetic remediation of the Chemical Waste

Landfill in Albuquerque, New Mexico....................................................................................... 16

Figure 2-7: Post-treatment mass balance for uranyl ion removal.................................................. 17

Figure 3-1: Schematic diagram of laboratory configuration ........................................................... 19

Figure 3-2: Photo of laboratory configuration................................................................................. 20

Figure 3-3: Estimated anolyte concentration due to diffusive mass flux........................................ 21

Figure 3-4: Artificial and gneiss core samples ............................................................................... 22

Figure 3-5: Electrolyte container .................................................................................................... 23

Figure 3-6: Copper electrode ......................................................................................................... 24

Figure 4-1: Expected relative concentration of the chloride ion 5cm from the source .................. 27

Figure 4-2: Anolyte TDS for the diffusion control tests .................................................................. 28

Figure 4-3: Linear regression of diffusion control test data............................................................ 28

Figure 4-4: Anolyte chloride concentration for the diffusion control tests ...................................... 29

Figure 4-5: Anolyte TDS for the electrokinetic performance tests ................................................. 31

Figure 4-6: Linear regression of electrokinetic performance test data .......................................... 33

Figure 4-7: Exponential regression of Test 40V1 data ................................................................... 33

Figure 4-8: Residual plot of Test 40V2 data under linear regression............................................. 34

Figure 4-9: Anolyte TDS after 10 days as a function of voltage .................................................... 35

Figure 4-10: Anolyte and catholyte colours at the completion of various electrokinetic tests. ...... 36

Figure 4-11: Electrode mass remaining after the 20V and 40V2 electrokinetic tests. ................... 36

Figure 4-12: Pore space clogging .................................................................................................. 37

Figure 4-13: Decomposition of the flange attachment ................................................................... 38

Figure 4-14: Electrical potential difference across the core sample for the electrokinetic tests.... 39

Figure 4-15: Electrical current through the core sample for the electrokinetic tests. .................... 39

Figure 4-16: Electrical resistance of the core sample for the electrokinetic tests.......................... 40

Figure 4-17: Anolyte TDS for all electrokinetic and successful diffusion control tests. ................. 41

Figure 4-18: Anolyte TDS for the free solution control test............................................................ 43

Figure 4-19: Photos of the anolyte and catholyte during the free solution control test ................. 44



List of Tables Table 2-1: Diffusion coefficient, ionic mobility, and effective ionic mobility at 25ºC ........................ 8

Table 4-1: r2 values of electrokinetic performance test data under regression. ............................ 32

Table 4-2: Saturated density, dry bulk density, and porosity of sediment core samples. ............. 44

Table 4-3: Hydraulic conductivity and dry bulk density of sediment core samples ....................... 45

List of Appendices Appendix A: Apparatus designs

Appendix B: Diffusive breakthrough concentrations

Chapter 1: Introduction

University of Western Australia 1 Centre for Water Research

1. Introduction Subsurface NAPL (non-aqueous phase liquid) contaminated sites act as a long term

environmental problem, and may pose as a threat to human health (Saichek 2005). The

remediation of such sites, however, is often extremely difficult and costly. Contaminant

removal by excavation is one of the more effective methods of remediation, but it is highly

expensive and becomes unfeasible at many locations (Reynolds 2005). As a result, the

successful remediation of NAPL contaminated sites is rarely achieved, despite the significant

annual expenditures (Reynolds 2005).

The demand for an innovative and cost effective in situ remediation technology has led to the

employment of conduction phenomena in soils under an electric field to remove subsurface

chemical species (Acar 1993). This technique, variably known as electrokinetic remediation,

electroreclamation, electrokinetic soil processing, electrochemical decontamination,

electrorestoration or electrochemical soil processing (ITRC 1997), uses low-level direct

current in the order of mA per cm² of cross-sectional area, or an electric potential difference

in the order of a few volts per centimetre, between electrodes placed in the ground in an

open flow arrangement (Acar 1993).

The application of a direct current through porous media is known to induce mass transport

via several mechanisms. Of these, the two most significant transport phenomena are

electromigration and electroosmosis (Acar 1993, ITRC 1997, Kim 2002). Electromigration

refers to the migration of ionic species towards the oppositely charged electrode, whilst

electroosmosis refers to the bulk flow of pore fluid towards an electrode dependent on

medium properties. Electromigration has been demonstrated to be the more important of the

two, and is either enhanced or retarded by electroosmosis depending on operating

conditions (Kim 2002).

Electrokinetic remediation is a developing technology with a relatively long research history

dating back to the 1930�s (ITRC 1997). The technology has traditionally focussed on the

removal of heavy metals by migration towards treatment or recovery zones (Figure 1-1),

showing some success in laboratory and pilot-scale studies (Acar 1997, USEPA 1995).

However, electrokinetic methods have been seen to have limited applicability to NAPL

contaminated sites (Reynolds 2005, USEPA 1995) due to the nonpolar nature of NAPL

contaminants.



Figure 1-1: Application of electrokinetics in the remediation of a heavy metal contaminated site (Acar 1993).

The purpose of this study was to investigate the viability of remediating NAPL contaminated

sites by the use of an electrokinetically driven treatment compound. In particular, only the

treatability of saturated porous media was considered. Because of the nonpolar nature of

NAPLs, significant contaminant transport cannot be achieved under an electric field (Acar

1997), particularly when contaminants are present in residual form. Hence, rather than

removing the contaminant from the porous matrix, remediation of the site by the

electrokinetic delivery of a charged treatment compound was considered (Figure 1-2).

Treatment compounds suitable for such use include oxidants that are commonly used to

target contamination through in situ chemical oxidation, such as potassium permanganate

and sodium persulphate (Reynolds 2005). Delivery of the treatment is proposed to be

achievable by deploying a surface flood of the treatment compound, and establishing an

electrical gradient as shown in Figure 1-2. Residual permanganate from the process would

then oxidise naturally occurring carbon and become innocuous (Reynolds, D. A. 2005, pers.

comm., 11 September).

Figure 1-2: Proposed method of NAPL remediation via electrokinetically enhanced in situ chemical oxidation.



To simulate such a technique, an electrolytic cell was established in which the anodic and

cathodic compartments were separated by a saturated sediment core sample. The catholyte

simulated the surface flood of the treatment compound, whilst the anolyte simulated the

NAPL reservoir. Various electrical potential differences were then applied across the

sediment core sample, in order to observe the effect of voltage gradient on the rate at which

the treatment compound could be delivered through the porous matrix and into the NAPL

reservoir.

Chapter 2: Literature Review


2. Literature Review

2.1. Mechanisms of mass transport in an electric field

The application of a low-level direct current in a soil mass induces physicochemical and

hydrological changes to the medium to which it is applied, leading to species transport by

coupled and uncoupled conduction phenomena (Acar 1993, Kim 2002). The driving

mechanisms for species transport under an electric field are diffusion by chemical gradient,

ionic migration by electrical gradient, pore fluid advection by prevailing electroosmotic flow,

and electrophoretic migration of colloidal particles (Acar 1997). Several compositional and

environmental variables affect the contribution of each flux to the total mass flux. These

include soil mineralogy, pore fluid composition and conductivity, electrochemical properties of

the species in the pore fluid, and the porosity and tortuosity of the porous medium (Acar

1993).

2.1.1 Molecular diffusion

A solute in water will move from a region of greater concentration to a region of lower

concentration in a process known as Fickian or molecular diffusion. Diffusion occurs

irrespective of bulk fluid motion or applied electrical potential difference, and the quantity of

mass diffused obeys Fick�s first law (Equation 2-1):

dxdCDF −= 2-1

where F is the diffusional mass flux, D is the diffusion coefficient at infinite dilution, and dxdC

is the concentration gradient. The negative sign indicates that solute movement occurs from

regions of higher concentration to regions of lower concentration. Values of D for some

common ions are presented in Table 2-1.

Diffusion through porous media is limited to the flowpaths of the soil matrix. As a result

diffusion through porous media cannot occur as fast as it can through water. To account for

the longer flowpaths ions must traverse due to the presence of mineral grains, Bear (1972)

uses the concept of tortuosity. Tortuosity is a measure of the effect of flowpath geometry on

the movement of fluids through porous media (Fetter 1993). Bear (1972) derived a coefficient

of molecular diffusion in porous media, or effective diffusion coefficient, to account for the

effect of flowpath geometry on fluid dynamics (Equation 2-2):

DTD ** = 2-2



where *D is the effective diffusion coefficient, and *T is the tortuosity of the isotropic

medium. For isotropic media, the tortuosity tensor *ijT reduces to the single scalar *T , the

value of which can be determined empirically. *ijT is a nonrandom porous medium operator

that transforms the average components of an external force acting at a physical point of a

porous medium into the average components of its projections along the streamlines (Bear

1972). The operator takes into account the effect of divergence of streamlines in a porous

medium that cannot be visualised as made up of capillary tubes of constant cross-section.

The value of *T is always less than 1 for porous media, and has been found by Freeze and

Cherry (1979) to typically range from 0.5 to 0.01 for laboratory studies using porous geologic

materials. Perkins and Johnson (1963) (as appears in Fetter (1993)) determined that *T

was equal to 0.7 for sand column studies using uniform sand. For media where the angle

between the channel axis and streamlines vary between 0º to 90º such that 45º can be

chosen as a representative value, Bear (1972) mathematically derived that a value of 3

2 can

be used to estimate the medium�s tortuosity.

Acar (1993) and Mattson (2002) express the coefficient of molecular diffusion in saturated

porous media as the product of the species� diffusion coefficient at infinite dilution D, the

medium�s porosity n, and a tortuosity factor τ which includes all other factors of tortuosity

(Equation 2-3). Bear (1972) acknowledges this representation of the effective diffusion

coefficient, stating that it may be better to use this form in particular circumstances.

nDD * τ= 2-3

For systems where concentrations change with time, mass flux via molecular diffusion

follows Fick�s second law (Equation 2-4):

2

2

xCDC

∂∂=

∂∂

t 2-4

where dtdC is the change in concentration with time. Crank (1956) found that for chemical

diffusion occurring from a source region to a surrounding region that is infinitely diluted,

chemical concentration in the surrounding region can be determined by Equation 2-5:

Dtx erfc C(x,t) Ci

20= 2-5



where Ci(x,t) is the concentration at distance x from the source at time t since diffusion began,

C0 is the constant concentration of the source, and erfc is the complementary error function

(Equation 2-6).

∫ −−=z

)d(erfc z0

221 ηηπ

exp 2-6

Equation 2-5 is a solution to Equation 2-4 for the initial condition where the region

surrounding the source is at infinite dilution, and the boundary condition where the source

maintains its chemical concentration over time. The complementary error function is related

to the normal distribution, and hence the solution given by Equation 2-5 is normally

distributed as expected for diffusional processes (Fetter 1993). Figure 2-1 shows the relative

concentration profile of a chemical species diffusing under such initial and boundary

conditions. The rate of solute transport achievable by molecular diffusion is represented

graphically by Figure 2-2.

Figure 2-1: Profile of a diffusing chemical front as predicted by the complementary error function. The profile follows a normal distribution, hence 84% of the values will be less than the value that is one standard deviation more than the mean, and 16% of the values will be less than the value that is one standard deviation less than the mean (Fetter 1993).

The equations presented here are limited to transport phenomena taking place in a single-

phase fluid saturating a porous medium, where no transport takes place through the solid

phase and where there is no interaction between the constituents of the fluid and the solid

surfaces of the porous matrix (Bear 1972). Clearly, diffusion rates for solutes adsorbed onto

the surfaces of the porous matrix will be less than that for non-adsorbed species. In addition,

electrical neutrality must be maintained by migrating ions as they diffuse (Fetter 1993). For

instance, in the case of NaCl solution, the chloride ion cannot diffuse faster than the sodium

ion unless there is another positive ion in the region into which the chloride ion is diffusing.



Figure 2-2: Position of a contaminant front diffusing away from a source region at 100 and 10,000 years. The shaded areas illustrate the diffusive distances achievable for effective diffusion coefficients ranging from 1x10-11m2s-1 to 1x10-10m2s-1. Diffusion distances were obtained using Equation 2-5, and parameters typical of nonreactive chemical species in clayey geologic deposits (Freeze 1979).

2.1.2 Electromigration

Electromigration or migration refers to the transportation of ionic species under the influence

of an applied electric field (Kim 2002). The electric field will cause ions to move towards the

electrode of opposite charge. Migrational mass flux can be determined by a simple extension

of Fick�s first law (Boudreau 2004), as given by Equation 2-7:

EcuJ m *−= 2-7

where Jm is the migrational mass flux, *u is the effective ionic mobility, c is the concentration,

and E is the applied electrical potential difference.

Acar (1993), Jacobs (1996), Yeung (1990), and Boudreau (2004) theoretically estimate

effective ionic mobility by extending the Nernst-Townsend-Einstein relationship for ionic

species in free solution (Mattson 2002), assuming that the relationship between effective

ionic mobility and the molecular diffusion coefficient extends to ionic species in soil pore

fluids (Equation 2-8):

TRFzDnuu

** == τ 2-8

Effective ionic mobility then becomes a function of the ion�s effective diffusion coefficient *D ,

charge z, Faraday�s constant F = 96,485, the universal gas constant R = 8.314, and absolute

temperature T. Acar (1993) found that for charged species under a unit electrical gradient,

the ratio of effective ionic mobility to effective diffusion coefficient is approximately 40 times



the charge of the species. Table 2-1 shows the diffusion coefficient, ionic mobility at infinite

dilution, and effective ionic mobility in soil for selected ionic species.

Table 2-1: Diffusion coefficient, ionic mobility at infinite dilution, and effective ionic mobility in soil for selected ionic species. Typical porosity and tortuosity values of 0.6 and 0.35 were used to calculate ionic mobility (Acar 1993). Values of D are valid for ions in water at 25ºC. Diffusion coefficients are approximately 50% lower at 5ºC.

Species )scm(10 D -12-6 )sVcm(10 u -1-12-6 )sVcm(10u -1-12-6*

+H 93 3625 760+Na 13 519 109−OH 53 2058 432−Cl 20 790 166

When a current is generated purely by virtue of electromigration in the free pore fluid, the

total current I relates to the migrational mass flux of each species Jj through Faraday's law

for equivalence of mass flux and charge flux (Equation 2-9):

∑∑

∑=

==j

n

iiii

jjj

jj

cuz

cuzt III

1

*

*

2-9

where tj is the transference number of the jth species, identifying the contribution of the jth ion

to the total effective electrical conductivity (Acar 1993). Transference number of an ionic

species is then dependent on the species� ionic mobility relative to that of other species in the

pore fluid, and its concentration relative to the total electrolyte concentration of the pore fluid.

Consequently, transference number, and therefore migrational mass flux of an ionic species,

is dynamic and will increase in response to an increase in the relative concentration of that

species during electrokinetics operations (Acar 1993). In the same manner, transference

number and migrational mass flux of an ionic species will be reduced if the relative

concentration of that species decreases during the remediation process. Electrolysis

reactions at the electrodes may cause either of these two to occur, thereby increasing or

decreasing the efficiency of electromigration depending on the availability of the chemical

species present, and the electrochemical potential of the associated electrolysis reactions

(Acar 1993).

As stated, Equation 2-9 is valid only when electromigration is the sole mechanism of current

transmission. This neglects migration within the diffuse double layer (layer of ions in a

primary ionic shell together with the structure of counterions from the electrolyte (Kisza in

press)), surface conductance, and assuming the porous medium�s constituents to be electric

isolators (Acar 1993).



2.1.3 Electroosmosis

Electroosmosis is the bulk flow of pore fluid towards an electrode under the influence of an

applied electric field (ITRC 1997). Electroosmotic flow is produced when counterions in the

diffuse double layer adjacent to the mineral surface migrate toward the oppositely charged

electrode, transferring momentum to the surrounding fluid molecules via viscous forces as

they migrate (Saichek 2005). Consequently, non-ionic species in the pore fluid will also be

transported via this mechanism. Due to the charge of most mineral surfaces, electroosmosis

typically occurs towards the direction of the cathode (ITRC 1997).

The Helmholtz-Smoluchowski theory for electroosmosis is a theoretical description of pore

fluid flow under electrical gradients (Acar 1993), and is still widely accepted despite the

degree of assumptions involved (Saichek 2005). The theory introduces the coefficient of

electroosmotic permeability ke, which describes the volumetric flow rate of pore fluid through

a unit cross-sectional area of the medium due to a unit electrical potential difference (Acar

1993). Electroosmotic permeability has been expressed by Saichek (2005) in the form of

Equation 2-10, where D is the dielectric constant of the fluid, η is the fluid viscosity, n is the

medium porosity, and ζ is the zeta potential:

nDke ηζ= 2-10

Mitchell (1993) reports ek values to range from -1-12-5 Vscm1.5x10 for clayey silts to

-1-12-4 Vscm2x10 for quick clays. Kim (2002), however, states that ek values in the order of

-1110 to -810 are more commonly observed, and verifies this experimentally. Electroosmotic

permeability relates to the electroosmotic mass flux of a chemical species Je by the

relationship shown in Equation 2-11 (Acar 1993):

EkccJ ew

e −= 2-11

where wcc is the ratio of chemical species to water in the pore fluid and E is the applied

electrical potential difference.

The zeta potential term introduced in Equation 2-10 can be defined as the electric potential at

the junction between the fixed and mobile parts of the diffuse double layer West (1995), the

value of which can be determined by Equation 2-12:

tcBA log−=ζ 2-12



where A and B are empirically determined constants, and ct is the total concentration of the

electrolyte. Eykholt (1994) describes the value of the zeta potential to be a complex function

of the interfacial chemistry between both solid and liquid phases. Acar (1993) reports zeta

potential to decrease linearly with the logarithm of the pore fluid�s pH, thereby reducing

electroosmotic permeability and the rate of species transport by electroosmosis. Reverse

electroosmotic flow may even trigger at low pH values, if the isoelectric point of the porous

medium is reached and the zeta potential changes sign (Acar 1993). Acar (1993) found this

phenomenon to be replicable, with reverse electroosmotic advection occurring from the

cathodic compartment to the anodic compartment when the cathode reaction was acid-

depolarised. Figure 2-3 (Kim 2002) shows the cumulative volume of water transported via

electroosmosis, exemplifying the effects of pH and electrical potential on the efficiency of

electroosmotic advection.

Figure 2-3: Cumulative volume of water transported by electroosmosis under various conditions. Negative values indicate reverse electroosmotic advection. In experiment A, electroosmotic flow decreased over time due to lowered pH and increased conductivity of the pore fluid. In experiment B, electroosmotic flow was significantly enhanced by an increased electrical potential difference. In experiment C, pH was maintained at approximately 2, resulting in reverse electroosmotic advection (Kim 2002).

In contrast to fluid flow under hydraulic gradients, electroosmotic flow under an applied

electric potential difference is primarily dependent on medium porosity and zeta potential

rather than pore size distribution or the presence of macropores (Acar 1993). Electroosmosis

is then an effective means of generating uniform fluid advection, and hence solute mass flux,

through fine-grained media (Mitchell 1993). Mitchell (1993) illustrates this by comparing

water flow through clay under hydraulic gradients with electroosmotic flow under electrical

gradients. For a clay with a hydraulic conductivity of -1-10 ms1x10 , it was calculated that a

hydraulic gradient of 1000 would be required to produce a flow equivalent to that achievable

by electroosmosis in a 20V electric field. Figure 2-4 further illustrates the independence of



electroosmotic permeability from pore sizes, maintaining a relatively constant value across

various soil types despite differences in hydraulic conductivities ( hk ).

Figure 2-4: Electroosmotic permeability and hydraulic conductivity for a range of soils. Variations in electroosmotic permeability are small relative to those of hydraulic conductivity (Electrokinetic Limited 2004).

The efficiency and economics of mass transport via electroosmosis depends on the quantity

of water advected per unit electrical charge passed (Mitchell 1993). Mitchell (1993) states

that the amount of water advected may vary over several orders of magnitude, depending on

factors such as soil type, water content, and electrolyte concentration. (Acar 1993) reports

maximum electroosmotic flux to be achievable in silts and in low activity clays constituting a

high water content. According to Grundl (1996), electroosmosis in a constant voltage system

at hydraulic steady state should continue as long as a supply of active redox ions can be

maintained, and the permeability of the medium is unaltered. Heterogeneous reactions

between the pore fluid and soil may affect ion availability and/or soil permeability, thereby

resulting in the cessation of electroosmosis (Grundl 1996). Ion starving by metal hydroxide

precipitation, and acid degeneration of clay minerals are examples of such reactions.

2.1.4 Electrophoresis

Electrophoresis refers to the transportation of charged particles and macromolecules under

an electric field. Electrophoretic movement is induced by the electrostatic attraction and

repulsion between charged particles and the electrodes when a DC field is established

across a colloidal suspension (Mitchell 1993). The phenomenon becomes significant only

when surfactants are introduced to form charged micelles with other species, or when the

technique is employed in remediating slurries (Acar 1993). As a result, electrophoresis is not

significant in this study, and the mechanisms of the transport phenomenon will not be

discussed.



2.1.5 Acid front formation

The mechanisms of mass transport described in Sections 2.1.1 to 2.1.4 assume no

interaction between the solid and liquid phases of the soil matrix. However, this is not a valid

assumption as charged species are highly attracted to and sorbed onto oppositely charged

mineral surfaces (Acar 1993). Desorption of target ionic species from charged mineral

surfaces is essential in effective electrokinetic transport of these species. For instance, in the

traditional application of electrokinetics to remediate heavy metal contaminated soil,

mobilisation of the metal contaminant via acidification has proven to be a critical precursor to

successful contaminant removal. This is achieved by the electrolytic generation of hydrogen

ions at the anode and its subsequent transport across the medium.

Acar (1993) states that soil conditions near the electrodes will be dominated by electrolysis

reactions (Equations 2-13) at the early stages of the remediation process. An acidic medium

will be generated at the anode whilst an alkaline medium will be generated at the cathode.

High ionic mobilities, together with high relative concentrations will then result in the

preferential transport of the hydrogen and hydroxide ions. Ionic mobility of the hydrogen ion

is approximately twice that of the hydroxide ion, hence the chemistry of the system will be

dominated by the hydrogen ion unless the progression of the acid front is retarded by the

buffering capacity of the soil (Acar 1993).

−+ ++→ eHOOH 442 22

−− +→+ OHHeOH 222 22 2-13

Species desorption via acidification may have secondary impacts on mass transportation

rates. As cationic species are desorbed from mineral surfaces and transported towards the

cathode, voids may develop in the porous medium. In microelectronic circuitry, a similar

phenomenon is encountered due to momentum transfer between electrons and metal ions

(Decuzzi 2003), in a process known as electromigration voiding (Figure 2-5).

Figure 2-5: Damaged circuitry due to electromigration voiding. Such voiding is analogous to medium decomposition during the electrokinetic remediation process (Carchia 1999).



As in microelectronics, voiding of a porous medium will induce a feedback process that

increases mass transport (via increased porosity and tortuosity), which in turn generates

further voiding. The extent of voiding will be dependent on the concentration and mobility of

the cationic species present.

2.2. Relative contribution of mass fluxes

Electromigration and electroosmosis are the dominant mechanisms of mass transport in the

electrokinetic remediation of soils (Kim 2002). The relative contribution of each mechanism to

the total mass flux is dependent on soil type, water content, chemical species present, pore

fluid concentration, and processing conditions (Acar 1993). Acar (1993) introduces a

dimensionless mass transport number eλ to describe the ratio of migrational mass flux to

electroosmotic mass flux under equal electrical gradients. eλ reduces to the ratio of effective

ionic mobility to electroosmotic permeability as shown in Equation 2-14:

ee

m

e ku

JJ *

==λ 2-14

Acar (1993) found that mass transport by electromigration in highly electroosmotic-

permeable Georgia kaolinite ( -5 x101=ek ) was at least 10 times greater than mass transport

by electroosmosis. The value of eλ reached as high as 300 in the latter stages of the

remediation process, fluctuating due to the time dependent dynamic chemistry of the system

under the influence of an electric field (Acar 1993). The high values of eλ were attributed to a

reduction in electroosmotic permeability arising from acid front development and the coupled

increase in pore fluid conductivity. Acar (1993) hence states that electroosmotic flow towards

the cathode decreases in time both by a decrease in electroosmotic permeability, and by a

reduction in electrical potential gradient. This statement is supported by Kim (2002).

2.3. Site applicability

Saichek (2005) states electrokinetic remediation as a flexible technology, capable of use for

a variety of different soil types and contaminants. However, the soil�s physical, chemical, and

biological characteristics that may limit the application of the technology have yet to be

adequately quantified (USAEC 2000). The USAEC (2000) found that laboratory treatability

tests of site specimens may be inaccurate, and may give a false indication of technology

applicability to the site. On the other hand, Acar�s (1997) bench-scale and pilot-scale studies

showed that the effects of soil type posed as no major restriction to the technology.

Nevertheless, quantification of the effect of site characteristics on technology performance is

necessary, as an incorrect application of current or voltage density to the site may lead to the



formation of new contaminant species, and adversely impact on the soil�s physical, chemical,

and biological properties (USAEC 2000).

In previous applications of electrokinetics to remove heavy metal species from soil samples,

soil pH buffering capacity was significant in determining the success of the technology.

Contaminant removal was not easily achieved when pH buffering capacity was high, due to

insufficient acid desorption and dissolution of adsorbed and/or complexed metal species

(Kim 2002). In soil specimens rich in calcium carbonate, Acar (1993) encountered buffering

capacities 20 to 60 times that of Georgia kaolinite, implying that the production and

introduction of 20 to 60 times more acid was necessary in these specimens than in the

kaolinite specimen.

Zeta potential impacts the extent of electromigration in the diffuse double layer, and

consequently the level of mass transport via electroosmotic advection. For soils exhibiting a

low zeta potential such as the Sandia soil, electromigration is negligible in the diffuse double

layer (Mattson 2002), thereby rendering the effect of electroosmosis insignificant. For soils

where zeta potential becomes negative as a consequence of high levels of acidification, the

direction of electroosmotic flow changes towards the anode (Kim 2002). In such a case,

electroosmotic flow will enhance the effect of electromigration on anionic species, but will

oppose the direction of cationic migration.

Kim (2002) found that the movement of ionic species through soil is somewhat dependent on

the nature of the species itself, particularly its adsorption affinity and mobility in the soil. It

was found that species with high mobilities in soils and weak affinities for particulate surfaces

were more easily transported. Kim (2002) also suggests that weakly bound fractions of

contaminants in soils are more easily removed by electrokinetic remediation, whereas

residual fractions are significantly more difficult to remove.

Acar (1997) and the ITRC (1997) state that soils with high water content and low activity will

result in the most efficient conditions for the electrokinetic remediation process. It was

elaborated that high activity soils will significantly retard the transportation of ionic species

and will also result in high buffering capacities for pH changes.

2.3.1 Formation of hazardous by-products

The presence of naturally occurring or anthropogenic organic and inorganic soil species may

result in the electrolytic generation of potentially hazardous by-products during remediation

processes (USAEC 2000). The use of amendment additions, such as acetic acid in the

depolarisation of the cathode reaction, may also lead to the formation of hazardous by-



products. The USAEC (2000) state that fugitive emissions of species such as chlorine,

trihalomethanes, and acetone could be generated during remediation processes and may

pose as potential health hazards to site workers and the public. In a field demonstration

conducted by the USAEC (2000) at Naval Air Weapons Station (NAWS) Point Mugu in

California, the application of an electric field even lead to an increase in organic

contaminants at the site. This was primarily attributed to the production of trihalomethane as

a result of chlorine build up in the anodic well. Vinyl chloride concentrations were also

increased due to the acceleration of natural dehalogenation processes. According to the

USAEC (2000), current laboratory treatability tests of site specimens cannot adequately

predict the formation of potentially hazardous by-products that may result from the

application of an electric field to site-specific constituents.

2.3.2 Site impacts

Electrokinetic remediation is an in situ technology requiring relatively little site disturbance

and limited use of heavy machinery (ITRC 1997). However, the establishment of a DC

voltage field between electrodes placed in a saturated porous medium is known to produce

several electrochemical side effects. In addition to the conduction phenomena discussed in

Section 2.1, the following may occur: ion exchange, soil desiccation by heat generation,

mineral decomposition, precipitation of salts or secondary minerals, electrolysis, hydrolysis,

physical and chemical adsorption, oxidation, reduction, and soil fabric changes (Mitchell

1993). Mitchell (1993) describes the interaction of these effects as complex, and that

continuous changes in soil properties that cannot be readily accounted for must be expected

due to the simplifying assumptions made in electrokinetic theory. Changes may be beneficial

to, or may impair the efficiency of electrokinetic transport (Mitchell 1993).

Whilst the effects of a DC voltage field through a saturated porous medium are known, the

application of such a field has not produced any observable impacts in past remediation

efforts (USAEC 2000). Regulators have expressed concerns regarding the capacity of soils

to sustain growth due to the physical, chemical, and biological changes that may occur as a

result of electrokinetic applications (ITRC 1997). The technology may therefore be more

applicable at industrial sites where such concerns are not an issue. The full impacts of

electrokinetic remediation must be established before large-scale implementation of the

technology can take place (USAEC 2000).

2.4. Timescales and efficiency

The ITRC (1997) state that the use of electrokinetic technologies to remove contaminants

from soils may take longer than conventional technologies. According to the ITRC (1997),

this occurs since the concentration of the target species becomes low with the progression of



time, corresponding to a reduced operational efficiency of the technology. On the other hand,

Kim (2002) describes the electrokinetic technique as one of the most promising remediation

technologies, offering high efficiency and time effectiveness in the decontamination of soils.

In either case, typical migration rates achievable by electrokinetics are approximately 2.5cm

per day (ITRC 1997). Hence, the electrokinetic remediation of soils using electrodes spaced

at 2m to 3m would require 100 days. Increased spacing between the electrodes, whilst

requiring longer processing periods, is expected to result in less electric power expenditure

per unit volume of soil processed (Acar 1997). Figure 2-6 illustrates the configuration

employed at a field demonstration in Albuquerque during 1996. In this configuration,

cathodes and anodes were spaced at less than 2m apart.

Figure 2-6: Field configuration employed in the electrokinetic remediation of the Chemical Waste Landfill in Albuquerque, New Mexico (ITRC 1997).

In the field demonstration conducted by the USAEC (2000) at NAWS Point Mugu, electrodes

were spaced at 4.3m. However, no contaminant movement or acid front development was

observed after 3 months. In contrast, Acar (1993) was able to ultimately remove uranyl

species from spiked kaolinite specimens as shown in Figure 2-7. Kim (2002) also employed

electrokinetics to remove metals from various tailing soils. Electric fields were applied to soil

cells ranging from 15cm to 20cm in length for 120 hours. As much as 90.3% and 95.4% of

the lead and zinc in the samples was removed. However, when electrokinetics was applied

to low pH tailing soils, lead and zinc removal dropped to 17.2% and 38.9% due to the effect

of reverse electroosmosis (Kim 2002). Hence, electrokinetic mass transport rates are highly

site specific, and extended application of the technology may yield no results if site

conditions are unfavourable.



Figure 2-7: Post-treatment mass balance in electrokinetic remediation experiments for uranyl ion removal from spiked kaolinite specimens (Acar 1993).

2.5. Cost

Electrokinetic remediation is an economic and cost effective remediation technology (Acar

1993, ITRC 1997, Kim 2002, Reddy 2004). However, the cost of the technology is highly

dependent on the chemical and hydrological properties of the site of remediation. Factors

which have been found to significantly influence the cost of the technology include: soil

characteristics, concentration of background ionic species, degree of contamination, depth of

contamination, site preparation requirements, the use of cathode-depolarisation techniques,

and electricity and labour rates (ITRC 1997). Pilot-scale field studies using electrodes spaced

at 1.0m to 1.5m indicate that the energy costs associated with heavy metal extraction are in

the order of US$25 per cubic metre treated (ITRC 1997). In commercial applications of

electrokinetic remediation, the ITRC (1997) encountered technology costs ranging from

approximately US$20 per cubic yard (Electrokinetics, Inc.) to approximately US$225 per

cubic yard (Geokinetics International).

Electrokinetics may also be used in conjunction with other technologies. For instance,

Electro-Petroleum, Inc conducted the electrokinetically enhanced bioventing remediation of

an underground storage tank spill (USEPA 1995). Gasoline levels of up to 2,200ppm were

reduced to well below the target level of 100ppm after approximately 90 days of operation,

incurring an estimated cost of only US$50 per tonne.

The USAEC (2000), however, state that those marketing the technology have not accurately

represented its cost. Price estimates have not always included the indirect costs associated

with excavation, permits, and the treatment of residues (ITRC 1997), nor capital and



operational costs (USAEC 2000). The USAEC (2000) extrapolated full-scale costs of the

technology from those incurred at the field demonstration at NAWS Point Mugu. Extrapolated

capital costs amounted to US$890,988, which included the costs of pre-deployment

treatability testing, installation of utilities, acquisition of processing equipment, construction

work, and technology mobilisation, setup, and demobilisation. Operational and maintenance

costs amounted to US$302,062, which included labour, materials, utilities, fuel, and

performance testing and analysis. The total cost for the 1000 cubic yards treated therefore

amounted to US$1,193,050, yielding a unit cost of US$1,193 per cubic yard. This figure far

exceeds those presented above, and renders electrokinetic remediation more expensive

than conventional excavation and incineration, which typically costs between $400 and $500

per cubic yard (Dev 1988). However, the figures obtained by the USAEC (2000) have been

acknowledged to contain possible inaccuracies due to the information available.

Chapter 3: Methodology


3. Methodology

3.1. Laboratory configuration

Although the mechanics of electrokinetic remediation are complex and not thoroughly

understood, the technology is relatively easy to implement compared to most conventional

technologies (Reddy 2004). Due to the simplicity of the electrokinetic technique, laboratory

configurations employed in the testing of the technology are often similar. The methods used

in this study are comparable to those of Reddy (2004), but with no incorporation of the effect

of hydraulic gradient on electrokinetic mass transport rates.

Figure 3-1 and Figure 3-2 illustrate the laboratory configuration employed in this study. The

setup constituted an electrochemical cell divided by a saturated sediment core sample. The

catholyte simulated the surface flood of the treatment compound that was shown in Figure

1-2, whilst the anolyte simulated the subsurface NAPL reservoir. In this manner, one-

dimensional subsurface transportation of a charged treatment compound under chemical and

electrical gradients could be simulated. The two electrochemical compartments were placed

horizontally and fluid levels were set to produce no hydraulic gradient. Field applications of

the proposed remediation technique will hence have the potential to be accelerated by the

employment of some form of hydraulic head. The purpose of this study was to focus only on

the electrokinetic component of the remediation technique, and not the technique as a whole.

Several designs for the laboratory setup were considered. Appendix A details construction

and assembly notes for the various designs considered, including the design implemented.

Figure 3-1: Schematic diagram of laboratory configuration.



Figure 3-2: Photo of laboratory configuration.

3.1.1 Electrolyte fluid

The catholyte comprised a solution of sodium chloride to simulate the surface flood of

potassium permanganate. Clearly, the chloride ion was used to simulate the movement of

the permanganate ion, and was the target species of electrokinetic transport. Chloride, whilst

being of equal charge to permanganate, is smaller, conservative (Lipson 2005), and hence

more mobile than permanganate. The results achieved may therefore be optimistic with

respect to observed chloride and expected permanganate transport rates. Chloride was used

for safety reasons and for its ready availability. The catholyte solution was formed by

dissolving 500g of sodium chloride (minimum assay 99.9%) in 9L of deionised water to

produce a solution with a TDS value of 55.6ppk. The use of deionised water removed the

presence of background ionic species, enabling solute concentration to be accurately

measured by solution TDS.

The anolyte comprised only deionised water to simulate the NAPL reservoir. Once again, this

removed the presence of background ionic species, enabling the breakthrough concentration

of the chloride ion to be accurately measured by solution TDS. The anolyte, being housed in

a container identical to that of the catholyte, required a volume of 9L to prevent the

establishment of a hydraulic gradient.

Using smaller volumes of water would have resulted in higher concentration gradients,

thereby accelerating the process. However, variations in water volume result in an inversely

proportional variation in solute concentration. Concentration gradients, and therefore

electrokinetic and diffusive mass fluxes, are then dependent on changes in water volume.

The use of larger volumes of water renders the effects of water loss on mass transport rates

insignificant, thereby minimising the distortion of results. Water losses, whilst unlikely, may

occur due to evaporation, sealing failure, apparatus failure, or any water sampling required.

In addition, the use of large volumes of water allows the maintenance of relatively constant



solute concentrations. This in turn establishes boundary conditions as described in Section

2.1.1, allowing diffusive mass flux to be quantified using Crank�s (1956) solution to Fick�s

second law. As a result, diffusive mass flux achievable through a sediment core sample was

estimated (Figure 3-3) and the maximum volume of water still allowing the detection of

diffusion was selected, with allowances for overestimation of tortuosity. Electrokinetic mass

transport rates were expected to be significantly faster than diffusion, hence detection limits

were set by diffusion rates.

0 2 4 6 8 10 120

50

100

150

200

250

300

Time (days)

Con

cent

ratio

n (p

pm)

Maximum diffusion rate

Minimum diffusion rate

Figure 3-3: Estimated anolyte concentration due to diffusive mass flux. Diffusion through porous media with tortuosity values of 1 and 0.1 are shown. Hence, rates of diffusion achievable in the laboratory are expected to lie within the region formed by the two curves.

The lower curve in Figure 3-3 was obtained using Crank�s (1956) solution to Fick�s second

law (Equation 2-5) for tabulated values of the complementary error function, and cubic spline

interpolations for finer resolution. Fick�s first law (Equation 2-1) was then applied to obtain

mass flux for each timestep. A conservative and simplifying assumption made was that

diffusive flux occurred only in response to the concentration gradient between the catholyte

and core sample. The upper curve was obtained directly via Fick�s first law under the

assumption that the concentration gradient across the core did not diminish with time.

Consequently, both curves provide conservative estimates of diffusion for the tortuosities

shown. Appendix B details the derivation of these values.

3.1.2 Sediment core sample

The saturated porous matrix between the surface flood and NAPL reservoir was simulated by

the sediment core sample. Initially, the sediment cores used were composed of gneiss

(metamorphically altered arkosic sedimentary rock). However, due to the inability to saturate

the cores as a result of their impermeability, artificial high permeability cores were

manufactured. It is important to note that only saturated media were used in this study, and

that electrokinetic transport through unsaturated media may be slower for reasons such as

reduced tortuosity and increased electrical resistance (Mattson 2002). Wieczorek (2005)



states that soil desiccation during the remediation process may even lead to the standstill of

electrokinetic transport. Whilst electrokinetic remediation of the unsaturated zone is of

importance (Mattson 2002), it is not the purpose of this study.

All artificial sediment cores were manufactured simultaneously and in an identical manner to

ensure uniformity between the samples. The cores were composed of a mix of Melcann

Rapid Set Concrete, builder�s sand, and water, placed in PVC pipe sections. Many ratios of

rapid set concrete, sand, and water were trialled, and it was found that a ratio of 2:6:1

produced a sufficiently permeable core of reasonably high structural integrity. PVC pipe

sections were 5cm in length, and 38mm in internal diameter. All cores were saturated in

deionised water for a minimum of seven days prior to installation in the apparatus. A different

core sample was used for each electrokinetic test such that the core was free of chloride ions

at test commencement. Figure 3-4 shows the two types of cores used, saturating in

deionised water.

Figure 3-4: Artificial and gneiss core samples saturating in deionised water.

Structural integrity of the sediment cores is of significance, as the inability of the core to

withstand minor physical stresses may result in partial core decomposition during apparatus

assembly, thereby impacting on uniformity between the core samples. Core dimensions,

however, are nominal, and electrokinetic testing time can be adjusted by electrolyte volumes

and concentrations. Pipe diameters of 38mm were selected such that the electrolyte

containers initially used for the gneiss core samples did not require remanufacturing.

3.1.3 Electrolyte containers

Both catholyte and anolyte were housed in plastic containers (Figure 3-5A), as metallic

casings would exhibit electrolytic degeneration and subsequent leakage during electrokinetic

testing. Where possible, the use of metals was avoided in the manufacture of the containers,

however, metals were required in the construction of the flanges (Figure 3-5C). Plastic lids

were used to seal the containers to prevent evaporative losses and maintain constant

electrolyte volumes.



Figure 3-5: Electrolyte container. Containers were constructed with a single plastic flange, attached by metal nuts, bolts, and a washer. Figure 3-5B shows a close-up of the flange, and Figure 3-5C shows the flange attachment from inside the container.

It is not required of the anodic container be of the same size as the cathodic container. It is

only necessary to maintain equal water levels in both catholyte and anolyte, such that fluid

advection by hydraulic gradient will not occur. Hence, the anodic container could be made

slimmer, allowing a greater detection capacity of the chloride ion. However, all containers

were constructed in the same manner for ease of manufacture.

Two electrolyte container designs were developed for use in this study. Design and assembly

notes for each can be found in Appendix A. The design implemented was selected for its

simplicity and cost effectiveness.

3.1.4 Power supply

The Powertech MP-3092 Laboratory Power Supply was used to provide the electric field

through all sediment core samples. The power supply was dual tracking and capable of

supplying 0-40VDC of electrical potential at up to 3A, with current control capabilities. The

power supply constituted digital voltage and current meters for each output, capable of

resolutions of dV and cA. Maximum current was enabled for all electrokinetic tests, and was

not achieved through any of the cores. Voltages were varied for the different tests, ranging

from 0-40V.

3.1.5 Electrodes

The electrodes used in both cathodic and anodic compartments were composed of copper

wire. 168/0.12mm OFC wires were entwined about enamelled copper wires to produce disc-

shaped electrodes as shown in Figure 3-6. The electrodes spanned the cross-sectional area

of the sediment cores, such that the voltage field between the electrodes was uniformly

distributed over the cross-sectional area of the cores. This produced one-dimensional

species transport under chemical and electrical gradient.



Figure 3-6: Copper electrode spanning the PVC pipe of a sediment core sample.

One such copper electrode was placed at each end of the sediment core sample, and was

connected to the power supply by an insulated copper wire. All connections were tested

using the inbuilt digital voltage meter of the power supply. Shorting the circuit by connection

of the electrodes was found to consistently produce zero resistance, implying that the entire

voltage drop in all electrokinetic tests occurred across the sediment core sample.

3.1.6 Additional equipment and instrumentation

IEC Magnetic stirrers (cat. CH2080-001) were used to promote the uniformity of both

catholyte and anolyte, such that concentration gradients did not exist within each electrolyte.

This maintained the horizontal concentration gradient across the sediment core sample and

enabled accurate TDS and pH measurements of the electrolytes.

Selleys All Clear Hydrophobic Silicone Sealant was applied at every join of the apparatus

assembly to ensure the sealing integrity of the system. PVC sealants were avoided due to

the need to dismantle and clean the apparatus between electrokinetic tests. Successful

system sealing was accomplished in all tests.

TDS and pH measurements were obtained using the TPS WP-81 Conductivity-TDS-pH

Meter. TDS readings were taken using the TPS k=1/ATC/Temp Sensor (cat. 122201), and

pH readings were taken using the TPS Combination pH Sensor (cat. 121207). Readings

were taken from the centre of each electrolyte container, although the location of the

readings was insignificant due to the uniformity of the electrolytes as a result of the magnetic

stirrers.



3.2. Electrokinetic testing

3.2.1 Testing program

The electrokinetic testing program consisted of five electrokinetic performance tests varying

in voltage gradient, three diffusion control tests in which no electrical potential difference was

applied, and one free solution control test in which ions were allowed to migrate through free

solution unimpeded by a sediment core sample. Voltage gradients applied across the

sediment cores in the electrokinetic performance tests ranged from 10-40V, and no other

variables were tested concurrently with voltage. Electrolyte volumes and concentrations, core

type (except in the free solution control test), and laboratory configuration remained constant

for all tests, and all testing was conducted in a constant temperature laboratory (25ºC). Tests

were operated until substantial mass transport had taken place, or until the electrical

connection was terminated by decomposition of the anode or connecting wire. Electrokinetic

performance and diffusion control tests were typically operated for 9-10 days, whilst the free

solution control test was operated for 200 minutes.

3.2.2 Electrokinetic performance and diffusion control test procedure

In all electrokinetic performance and diffusion control tests, the apparatus was first

assembled without the electrolytes. Appropriate levels of deionised water were then placed in

both cathodic and anodic compartments, and magnetic stirrers were activated. The power

supply was then set to produce the required voltage gradient, and any subsequent reduction

of voltage was then the result of electrolyte conductivity. In all eight electrokinetic

performance tests, current was not limited. Recording commenced once sodium chloride was

added to the cathodic compartment. The dissolution of sodium chloride at the concentrations

used was rapid due to the high solubility of the salt, hence a stable concentration gradient

across the sediment core sample was quickly established. Therefore, at the time of

commencement, the anolyte was devoid of ionic species, and the catholyte concentration of

sodium chloride was 500g/9L.

Concentration and pH of both catholyte and anolyte were then measured at daily intervals

from test commencement to completion. During electrokinetic performance tests, electrical

potential difference and electrical current between the electrodes was also measured. Only

measurements of net electrokinetic performance were obtained, and isolation of the various

electrokinetic transport phenomena was not intended.



3.2.3 Free solution control test procedure

The free solution control test was operated in a similar manner to the electrokinetic

performance and diffusion control tests. However, no sediment cores or magnetic stirrers

were used in the free solution control test, and recording frequencies were higher. The

purpose of this control test was to quantify the effects of a voltage gradient on the movement

of the chloride ion in free solution, that is, a medium with a tortuosity value of one. Maximum

electrical potential (40V) was used in the free solution control test, and maximum electrical

current (3A) was also enabled. Magnetic stirrers could not be used as they would induce

significant mass transport by fluid advection without the presence of a sediment core.

Catholyte mixing prior to testing commencement was therefore done manually while the pipe

connecting the cathodic and anodic compartments was sealed.

3.3. Sediment core analyses

Several hydrological properties of the manufactured sediment cores were analysed, such

that the electrokinetic tests performed could be made comparable to applications of the

technology on other porous media. Randomly selected core samples were analysed in terms

of their porosity, dry bulk density, and hydraulic conductivity. The dry bulk density bρ of each

core sample was obtained by dividing the weight of the dry sample by its volume. Porosity n

was then calculated by subtracting the sample�s dry bulk density from its saturated density

satρ , and dividing the result by the density of water wρ as shown in Equation 3-1:

w

bsatnρ

ρρ −= 3-1

Hydraulic conductivity was derived using the constant head hydraulic conductivity method of

Clothier (1981). This was achieved by placing a known quantity of the core sample in a

permeater and saturating it. A constant head was then applied and the flow rate through the

sample was measured. The height of the sample L and the height of the applied constant

head h were then used to calculate the applied hydraulic gradient φ grad by Equation 3-2:

LLhgrad +=φ 3-2

Finally, flow rate q and hydraulic gradient were used to calculate the core sample�s hydraulic

conductivity hK via Darcy�s Law (Equation 3-3):

φ gradKq h−= 3-3

Chapter 4: Results and Discussion


4. Results and Discussion

4.1. Diffusion control tests

Diffusion control tests were conducted in which no electrical potential difference was applied

across the sediment core sample. Due to their importance as control tests, three were

required, such that anomalies in any one test could be identified and taken into consideration

in subsequent analyses. The application of no electrical potential difference in these tests

implied that mass transport was achieved purely by virtue of molecular diffusion.

4.1.1 Expected and observed results

Under the conditions employed in the diffusion control tests, Crank�s (1956) solution to Fick�s

second law (Equation 2-5) can be used to determine the expected breakthrough profile.

Hence, diffusion of the chloride ion through 5cm of porous media can be expected to follow

the trends shown in Figure 4-1. The time scale shown will be dilated according to the

tortuosity of the medium. At the early stages of the diffusion process, chloride concentrations

can therefore be expected to follow an exponential or linear pattern. The vertical scale shown

will be reduced according to the dilution of the chloride ion in the anolyte.

0 2 4 6 80

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Time (days)

Rel

ativ

e C

once

ntra

tion

Figure 4-1: Expected relative concentration of the chloride ion at a distance of 5cm from the source. This is the concentration of the sediment core sample at the junction between core and anolyte.

Observed anolyte TDS in the three diffusion control tests is illustrated by Figure 4-2. It can be

seen that the results vary widely from test to test, despite the fact that each test was

conducted in an identical manner. As the tests differed only in the sediment core samples

used, the apparent variations can only be attributed to differences between the samples. It is

possible that heterogeneities between the various core samples produced differences in core

porosity and tortuosity, thereby affecting diffusion rates as observed.



0 1 2 3 4 5 6 7

500

1000

1500

2000

2500

3000

3500

Time (days)

Con

cent

ratio

n (p

pm)

Test 1

Test 3

Test 2

Figure 4-2: Anolyte TDS for the three diffusion control tests.

It is of note that the behaviour of Test 3 was vastly different to that of the other two tests.

Whilst Test 1 and Test 2 exhibited highly linear trends with high r2 values as illustrated by

Figure 4-3, Test 3 followed a trend that was more logarithmic in nature. Under linear

regression, Test 3 data produced an r2 value of only 0.3941. However, when the outlier was

removed, that is, the first point in the time series data was ignored, an r2 value of 0.93003

was achieved. According to Figure 4-1, a logarithmic trend should be apparent only after an

extended period of time, and the plateau observed in Test 3 should only be existent when the

source has almost completely diffused into the anolyte. It is hence more probable that the

first data point is incorrect than that molecular diffusion had proceeded to a state of near-

equilibrium.

0 1 2 3 4 5 6 7

0

500

1000

1500

2000

Time (days)

Con

cent

ratio

n (p

pm)

Test 1: R2=0.9604

Test 2: R2=0.9176

Linear Regression

Figure 4-3: Linear regression of Test 1 and Test 2 data. Exponential regression yielded r2 values of only 0.63757 and 0.44177.

The observed TDS spike at the beginning of Test 3 may have been caused by the presence

of some residual chemical species in the anodic compartment prior to test commencement.

Dissolution of the species into solution over the day would have then caused the observed

spike in anolyte TDS. However, the sudden increase in anolyte TDS was accompanied by a



corresponding reduction in catholyte TDS, implying that actual mass transport had taken

place from the catholyte to the anolyte. The observation is then more likely to be sourced to a

fluid advection event driven by some form of hydraulic gradient. Such a gradient may have

been induced by a difference in height between the cathodic and anodic compartments.

Whilst unlikely due to the strict quality controls adhered to, this may have occurred as a

result of incorrect apparatus assembly. As it is likely that Test 3 was affected by some form

of experimental error, the results of the test have not been included in further analyses.

The maintenance of electroneutrality as described in Section 2.1.1 dictates that the sodium

ion in the catholyte migrate in accordance with the chloride ion. As a result, the observed

TDS values in the diffusion control tests were produced by the presence of both sodium and

chloride ions in solution. To isolate the concentration of the chloride ion, the observed TDS

values were multiplied by the proportional mass of chloride. Actual chloride concentrations

are shown in Figure 4-4, together with diffusion rates possible through a range of porous

media and free solution. It can be seen that the diffusion rates achieved through the core

samples were significantly higher than rates achievable through free solution.

0 1 2 3 4 5 6 7 80

500

1000

1500

2000

2500

Time (days)

Con

cent

ratio

n (p

pm)

Test 2Test 2

Test 1

Figure 4-4: Anolyte chloride concentration for the diffusion control tests. Test 3 was excluded due to inaccuracies in the data set. The dotted lines represent the range of diffusion rates achievable through media with tortuosity values ranging from 0.1 to 1.

4.1.2 Discrepancies

Though the dissimilarities between the tests can be soundly justified by core heterogeneity,

the reason accompanying the excessively high concentrations in all diffusion control tests is

less apparent. The linear form of the breakthrough curves was anticipated. The vertical scale

of the curves, however, clearly shows that mass transport occurred at rates faster than

physically possible via molecular diffusion alone. This implies the presence of mass transport

via mechanisms other than molecular diffusion, and/or the introduction of external chemical

species into solution.



The possibility of chemical species being introduced into solution is unlikely. Rigorous

controls on electrolyte containment and isolation were adopted, and no contact between

electrolyte and external bodies was established except during TDS and pH readings.

Moreover, during such readings, all sensors were cleaned in deionised water prior to

immersion within the electrolyte. Supporting the unlikelihood that new chemical species were

introduced into solution is the combined TDS of both catholyte and anolyte, which remained

a constant value throughout all diffusion control tests.

It is possible that molecular diffusion was enhanced by advective transport, mechanical

dispersion and/or turbulent diffusion. Some fluid advection was likely to have occurred in

response to the horizontal density gradient between the catholyte and anolyte. Simmons

(2001) found that density dependent flow can be significant for density differences as small

as 0.35-2.8%, rendering considerable advection possible with the 1.06% density difference

established in all the tests. Such flow, however, was assumed negligible in the design of the

laboratory setup. Advective transport driven by elevation head was unlikely, as such mass

transport could not have been sustained due to the high permeability of the cores. A

difference in height would initially induce advection but would quickly cease, in a manner

similar to the behaviour of Test 3. Constant hydraulic head, however, may be generated

when water is continually lost such as via evaporation, and though the rate of evaporation of

deionised water is greater than that of saline solutions (Al-Shammiri 2002), the electrolyte

containers were sealed and evaporative losses were contained. Magnetic stirring was also

unlikely to have produced significant fluid advection due to the core barrier and the minimal

fluid velocities generated. In addition, any advection caused by one stirrer would have been

opposed by counter advection from the other stirrer. Mechanical dispersion and turbulent

diffusion, however, may have been promoted by the magnetic stirrers, thereby enhancing

chloride mass transport.

Whilst a component of the observed rates of chloride transport is then likely to be attributable

to laboratory induced fluxes as described, these fluxes also affect mass transport under

electrical gradients. Hence, the mass transport rates achieved by the diffusion control tests

are nevertheless comparable to those achieved under the influence of an electric field.

Though the scales of the data sets obtained in this study may then be larger than attainable

in the field, the results are still directly applicable to field application of the technology.

4.2. Electrokinetic performance tests

Electrokinetic performance tests were implemented using electrical potential differences of

10V, 20V, 30V, and 40V. The results of the performance tests are illustrated in Figure 4-5.



Due to the highly atypical results of the first 40V test (40V1) relative to the other electrokinetic

performance tests, a second 40V test (40V2) was conducted. The 40V2 test produced results

considerably more comparable with those achieved prior. With the exception of the 40V1 test,

it can be seen that mass transport rates were influenced by the magnitude of the applied

electrical potential difference. Higher electrical potential differences across the sediment core

induced higher rates of mass transport.

0 2 4 6 8 10 12

500

1000

1500

2000

2500

3000

10V

20V

30V

40V1

40V2

Time (days)

Con

cent

ratio

n (p

pm)

Figure 4-5: Anolyte TDS for the five electrokinetic performance tests.

The distinctly poor initial performance of the 40V1 test was likely produced by the core

sample in use. Uncharacteristic core tortuosity, porosity, activity, or pH may have generated

the unusually high resistance observed. High core activity was unlikely to have been

responsible for the low transport rates due to the conservative nature of the chloride ion

(Lipson 2005). High pH, and consequently high electroosmotic retardation of the chloride ion,

was also unlikely as all core samples were composed from the same material and

manufactured simultaneously. The core properties most likely to have influenced mass

transport rates as observed were therefore tortuosity and porosity. The core sample in use

may have received an uneven distribution of large mineral grains, been subject to

compaction during manufacture, and/or constitute dense cement patches, all of which reduce

total pore volume and generate more tortuous pathways. These in turn reduce mass

transport by both electrokinetic phenomena and molecular diffusion.

Alternatively, the observation may have been the result of experimental error such as

laboratory induced fluid advection opposing the chloride ion flow. However, such advection

could not have been sustained for the period of time that low TDS was observed, as



discussed in Section 4.1.2. Hence, the most probable cause of low anolyte TDS was the

heavy impedance of the chloride ion by the core sample in use.

The exponential increase in anolyte TDS at the latter stages of the 40V1 test is also unusual

relative to the other electrokinetic performance tests. Since the retardation of the chloride ion

has already been discounted as a result of the ion�s conservative nature, the most viable

explanation of this observation is the decomposition of the sediment core during test

execution. As discussed in Section 2.1.5, the generation of an acidic medium at the anode

solubilises cationic species in the porous medium, enabling their subsequent transport under

electrical gradients. The desorption and transportation of these species from the porous

medium then leads to the development of voids, thereby increasing mass transport rates.

The large growth in anolyte TDS was then likely to be the product of both increased chloride

flux by medium voiding, and the presence of dissolved ionic species from the sediment core

sample.

It is understood that the behaviour of the 40V1 test was most likely caused by heterogeneity

between the core samples used, and this is supported by the inability of the 40V2 test to

replicate such behaviour despite being conducted under identical conditions. Heterogeneity

as such is a form of experimental error; therefore, the results of the 40V1 test are not directly

comparable with those achieved in the other electrokinetic performance tests. Consequently,

40V1 test data has not been included in some of the subsequent analyses.

4.2.1 Sustenance of mass transport

Anolyte TDS for most electrokinetic performance tests increased linearly with time. Both

linear and exponential regression was performed on each of the data sets obtained from the

performance tests (Table 4-1), and a linear model was found to be significantly more

appropriate for all tests, with the exception of the 40V1 test. In addition, the r2 values of the

data sets under linear regression indicate that the linear model is highly accurate. In the case

of the 40V1 test, it was found that anolyte TDS followed an exponential pattern. The fitting of

performance test data to linear and exponential models is illustrated in Figure 4-6 and Figure

4-7.

Table 4-1: r2 values of electrokinetic performance test data under linear and exponential regression.

Test (r2) Linear Regression (r2) Exponential Regression10V 0.97324 0.8052820V 0.98875 0.7987630V 0.99397 0.60222

40V1 0.73009 0.9622740V2 0.97512 0.42753



0 2 4 6 8 10 120

500

1000

1500

2000

2500

3000

3500

Time (days)

Con

cent

ratio

n (p

pm)

30V: R2=0.99397

40V2: R2=0.97512

20V: R2=0.98875

10V: R2=0.97324

Linear Regression

Figure 4-6: Linear regression of electrokinetic performance test data. Test 40V1 was excluded due to its nonlinear nature.

The linear fit of performance test data indicates that electrokinetics is an effective means of

generating a consistent mass flux through porous media. The data suggests that the

migration of a flooded ionic species through porous media will occur at a rate independent of

time. This implies that the proposed method of contaminant remediation may be significantly

faster than traditional applications of the electrokinetic technology, which become inefficient

over time due to reduced concentrations of the target species (ITRC 1997). It was anticipated

that the flooding of the target species would maintain the species� relative concentration in

the electrolyte, and hence its transference number, thereby enabling a reasonably constant

mass flux to take place via electromigration (Equation 2-9). Such linear transport is

supported by the performance test data.

0 1 2 3 4 5 6 7 8

500

1000

1500

2000

Time (days)

Con

cent

ratio

n (p

pm)

40V1: R2=0.9604

Exponential Regression

Figure 4-7: Exponential regression of Test 40V1 data.



The exponential breakthrough curve is of greater significance. If it is possible to generate

increasing mass transport rates with time as shown, then the electrokinetically enhanced

remediation of NAPL contaminated sites could be achieved in timeframes considerably less

than previously anticipated. Whilst exponential breakthrough has been demonstrated to be

possible under particular circumstances, such a pattern of mass transport was not replicable.

Hence, the 40V1 test only illustrates an optimistic remediation speed and exponential

breakthrough should not be expected under normal circumstances.

Though the curves shown in Figure 4-6 are all sufficiently linear, as indicated by high r2

values, closer analysis of the 40V2 curve suggests that constant mass transport cannot be

sustained indefinitely. It can be seen that the 40V2 curve exhibits a considerable decline in

mass transport rate directly after test commencement, and the nonrandom nature of its

residuals under linear regression (Figure 4-8) indicates that a linear model may be

inappropriate. Further illustrating the curve�s nonlinearity is the location at which its linear

model intersects the vertical axis. Whilst the linear models of the other curves intersect the

vertical axis near the origin, as expected for physical processes, the 40V2 one does not.

0 2 4 6 8 10 12

-300

-200

-100

0

100

Time (days)

Res

idua

l (pp

m)

Figure 4-8: Residual plot of Test 40V2 data under linear regression.

The reduced performance of the 40V2 test promptly after its commencement may have been

caused by the time dependent dynamics of the system due to the applied electric field.

Reduced electrical potential difference over time as a result of electrolyte conductivity may

have reduced electromigration, as dictated by Equation 2-7. Decreased relative

concentration of the chloride ion due to the introduction of other species via core

decomposition may have also reduced electromigration. However, such reductions in mass

flux cannot have exclusively affected the 40V2 test, and are therefore unlikely causes of the

test�s unique performance. Furthermore, the reduced performance of the 40V2 test was

unlikely to have been the result of the opposing electroosmotic flow, as electroosmosis is

known to diminish with time (Acar 1993, Kim 2002, Reddy 2004, Wieczorek 2005). Finally,



pH in both electrochemical compartments stabilised after the first day, rendering it an unlikely

cause of the observed reduction in mass flux. The electrokinetic phenomena underlying the

distinct performance of the 40V2 test are therefore unclear.

It can also be seen that the 40V2 curve convergences with the 30V curve in the latter stages

of the electrokinetic process. This indicates the possibility that the application of any

electrical potential difference larger than 30V will ultimately produce no greater mass

transport than achievable at 30V. The existence of such an upper limit of mass transport is

purely conjectural, but it is likely that an upper limit does exist. Medium desiccation by heat

generation (Mitchell 1993) and electroosmosis will increase the porous medium�s resistivity,

eventually leading to the possibility of electrokinetic standstill Wieczorek (2005).

Analysis of the mass transported after 10 days as a function of the applied electrical potential

difference across the sediment core sample yields Figure 4-9. Linear regression of the data

points indicates that a linear model is appropriate. Removal of the 30V point produces an r2

value of 0.98921, whilst removal of the 40V2 point produces an r2 value of only 0.93469. This

indicates that it is more likely mass transport was uncharacteristically fast in the 30V

electrokinetic performance test, than that mass transport in the 40V2 test was

uncharacteristically slow. As such, an upper limit of mass transport achievable in 10 days is

not apparent with the available data. It is conjectured, however, that the curve will acquire a

logarithmic character at higher voltages, as expected for natural processes.

0 5 10 15 20 25 30 35 400

500

1000

1500

2000

2500

Electrical Potential Difference (V)

Con

cent

ratio

n (p

pm)

R2=0.93597

Regression

Data Values

Figure 4-9: Anolyte TDS after 10 days as a function of voltage. The 40V1 data set was not included. Linear regression of the four points yielded an r2 value of 0.93597.

4.2.2 Apparatus decomposition

The decomposition of the copper anode via electrolysis was likely to have reduced mass flux

in all electrokinetic performance tests, and may account for the reduced performance of the

40V2 test. Whilst the decomposition of copper metal into copper ions does not itself impede



mass transport, the formation of insoluble copper compounds does. Acar (1993) found that

the precipitation of insoluble chemical species during electrokinetic operations resulted in the

clogging of soil pores, hindering subsequent mass transport through the porous medium.

The presence of various copper species was significant in both the catholyte and anolyte of

each electrokinetic performance test, as illustrated by Figure 4-10. By the completion of each

performance test, the anolyte was blue-green in colour, which is characteristic of the cupric

ion Cu2+ (de Sales 2005) and its compounds. The intensity of the colour was based on the

applied electrical potential difference across the sediment core sample. Higher electrical

potential differences produced anolytes more intense in colour, indicating the production of

higher concentrations of the cupric ion and its compounds.

Figure 4-10: Anolyte and catholyte colours at the completion of various electrokinetic performance tests.

The catholyte was dominated by a black precipitate, the concentration of which was again

dependent on the applied electrical potential difference. Once again, higher electrical

potential differences produced higher concentrations of the precipitate. It was deduced that

the precipitate was cupric oxide CuO, which is black in colour (Mostafa 1981) and insoluble

(Keast 1985). Hence, it was observed that the use of higher voltage gradients resulted in the

production of more copper ions, as one would expect. This is supported by Figure 4-11,

which illustrates the effect of voltage gradient on the extent of anode decomposition.

Figure 4-11: Electrode mass remaining after the 20V (Figure 4-11A) and 40V2 (Figure 4-11B) electrokinetic performance tests.

Cupric oxide was not only observed in the catholyte. Its formation was found to occur

throughout the entire sediment core sample in some electrokinetic performance tests (Figure

4-12). Such precipitation of insoluble copper species was likely to have significantly retarded



chloride migration, and occurred to the greatest extent in the higher voltage tests. The anodic

end of the core samples (Figure 4-12C) were most affected by cupric oxide precipitation, with

the effects of pore obstruction diminishing towards the cathodic end (Figure 4-12B). In

addition to the production of cupric oxide, it is well documented that cupric ions react to form

the insoluble copper oxychloride CuCl2·3Cu(OH)2 in the presence of chloride ions (Beltran-

Garcia 2000). This was most significant in the 40V tests, as can be seen in Figure 4-12A,

rendering it a possible cause of the reduced performance of the 40V2 test.

Figure 4-12: Clogging of the medium�s pore spaces by the precipitation of insoluble copper species. Figure 4-12A shows the anodic face of the sediment core sample used in the 40V2 test, and Figure 4-12B and Figure 4-12C show the cathodic and anodic faces of the core sample used in the 30V test.

It is clear, then, that the use of copper in electrokinetic remediation is highly inappropriate.

Inert conductors such as graphite, platinum, or electrokinetic geosynthetics should be used in

order to avoid the introduction of secondary products that complicate the electrochemistry of

the system. Though it is unrealistic to remove the possibility of soil pore clogging as metals

occur naturally in soils, the effects of pore clogging by introduced species should be

minimised. Electrokinetic enhancement by acid-depolarisation of the cathode reaction may

even be required under less favourable circumstances. Such acid-depolarisation has proven

to be successful in the remediation of heavy metal contaminated soils (ITRC 1997), and may

prove critical in the application of the proposed remediation technique at highly metallic sites.

Electrolyte containers also experienced deterioration during electrokinetic performance

testing. During such tests, the metallic flange attachment of the anodic containers became

heavily corroded (Figure 4-13A), eventually leading to the complete loss of the washer

(Figure 4-13B). This indicates that the use of metals in electrokinetic applications should be

completely avoided, or protective measures should be taken to ensure that any metals used

are not corroded in the remediation process.



Figure 4-13: Decomposition of the flange attachment due to the electrochemistry of the anodic compartment.

4.2.3 pH

Acar (1997) found that during electrokinetic remediation, catholyte pH typically increased to a

value of 11, whilst anolyte pH typically decreased to less than 2. Most pH changes were also

found to occur within the first 100 hours of processing. In the electrokinetic performance tests

conducted, however, an anolyte pH of 2 was never reached. Whilst the catholyte pH of 11

was consistently achieved, the lowest anolyte pH recorded was only 3.6. Moreover, an acid

front was never generated and the two electrochemical compartments maintained a steady

pH over time.

The large factor of dilution in the tests conducted cannot account for the inability of the anode

to generate a sufficiently acidic medium, as the cathode produced the expected pH. However,

the standard reduction potential for the oxidation of chloride ions to chlorine gas at 25ºC is

1.36V, whereas that for the electrolysis of water to hydrogen ions is only 1.23V. Hence, it is

possible that chloride was preferentially oxidised in place of water, thereby prohibiting the

development of a highly acidic medium. Such findings are consistent with Wieczorek (2005),

who also found that the presence of chloride reduced the electrolysis of water, producing a

more basic anolyte than otherwise.

In any case, electrokinetic remediation by the use of a surface flood is potentially a versatile

means of contaminant removal. Such a technique of remediation has been shown to be

effective regardless of the development of an acid front, contrasting with traditional

applications of the technology in which the development of a pH front is considered a

precursor to successful contaminant removal (USAEC 2000). However, whilst the pH range

of the remediation technique may be large, the effect of pH on its success must be

understood before field application of the technology. It is speculated that the technique is

most effective at low pH ranges, allowing electroosmosis to proceed in the direction of the

anode, and solubilising naturally occurring metals, thereby leading to improved mass

transport by medium voiding.



4.2.4 Voltage and current

The electrical potential difference across the sediment core sample in all electrokinetic

performance tests remained relatively constant (Figure 4-14). Such maintenance of the

voltage gradient suggests that, in each test, electrolyte conductivity did not significantly

change with time. Additionally, the observation that voltage gradients did not appreciably

change from test commencement to test completion suggests that electrolyte conductivity

was insignificant throughout the tests. This contradicts with the mass transport rates

observed, and also with Figure 4-15, which illustrates high electrolyte conductivities

throughout all the electrokinetic performance tests.

0 2 4 6 8 10 120

5

10

15

20

25

30

35

40

Time (days)

Ele

ctric

al P

oten

tial D

iffer

ence

(V)

10V

20V

30V

40V240V1

Figure 4-14: Electrical potential difference across the sediment core sample for the five electrokinetic performance tests.

0 2 4 6 8 10 120

20

40

60

80

100

120

140

Time (days)

Cur

rent

(mA)

10V

20V

30V

40V1

40V2

Figure 4-15: Electrical current through the sediment core sample for the five electrokinetic performance tests.



The apparent discrepancy can be explained in terms of core decomposition during the

electrokinetic process. Medium decomposition by voiding and other electrochemical

processes reduces the electrical resistance of the porous medium over time as discussed in

Section 2.1.5. Heterogeneities in the medium may also reduce its resistivity during

electrokinetic operations. The result is increased current flow despite stagnancy in electrical

potential difference. In terms of the 40V2 electrokinetic test, current flow was likely to have

decreased in response to severe soil pore clogging and the subsequent increase in the core

sample�s electrical resistivity. In a pilot test of electrokinetic remediation, Acar (1997)

encountered voltage drops of up to 20V/cm due to pore clogging by the precipitation of

insoluble metal species. Hence, in the 40V2 electrokinetic performance test, the effective

voltage gradient across the sediment core sample may have been significantly reduced by

the presence of insoluble copper species. The 30V test also exhibited a reduction in current

flow similar to that of the 40V2 test. The reduction, however, was not sustained throughout

the test, most likely due to limited pore clogging.

Figure 4-16 demonstrates the change in electrical resistivity of the sediment core samples

during the electrokinetic performance tests. The large spikes in current flow and electrical

resistivity were most likely due to the low resolution of the current meter. In addition, current

flow is a measure of the instantaneous conductivity of the electrical circuit; hence local

variations of electrolyte conductivity in time may have also caused the observed spikes. It is

of note that the trends of electrical resistance and current flow observed are consistent with

the previously given justifications for the performances of the electrokinetic tests.

2 4 6 8 10 12300

400

500

600

700

800

900

1000

1100

1200

1300

Time (days)

Res

ista

nce

(ohm

s)

10V

20V

30V

40V2

Figure 4-16: Electrical resistance of the sediment core sample for the electrokinetic performance tests. Test 40V1 was excluded due to the uncharacteristically high electrical resistance of the sediment core sample used in the test.



4.3. Effectiveness of electrokinetic mass transport

It has been shown that electrokinetic mass transport increases in accordance with the

applied electrical potential difference across the porous medium. However, comparison of

the electrokinetic performance tests with the diffusion control tests (Figure 4-17) shows that

mass flux may be greater when no voltage gradient is applied at all. This fundamentally

undermines a well proven and established theory, and implies one of two things - that the

data derived from this study is inconclusive, or electrokinetic theory is incorrect. Clearly, it is

more likely to be the former.

0 2 4 6 8 10 12

500

1000

1500

2000

2500

3000

10V

20V

30V

40V2

Time (days)

Con

cent

ratio

n (p

pm)

Control Test 2

Control Test 1

40V1

Figure 4-17: Anolyte TDS for all electrokinetic performance and successful diffusion control tests.

It has been consistently found that diffusive flux is an insignificant form of mass transport

relative to electrokinetic phenomena under an electric field (Acar 1993, USEPA 1995, ITRC

1997, USAEC 2000, Kim 2002, Mattson 2002, Saichek 2005). The results achieved in the

diffusion control tests, then, must be in some way incomparable to those achieved in the

electrokinetic performance tests. That is, either the TDS values achieved for unenhanced

mass transport were too high, or those achieved for electrokinetically enhanced mass

transport were too low.

It may be possible that the opposing electroosmotic flow in each of the electrokinetic

performance tests dominated the dynamics of the test system. The pH conditions employed

in this study were slightly basic relative to past studies, resulting in increased electroosmotic

flow in the direction of the cathode (Acar 1993). However, electroosmotic mass flux is known

to be considerably less significant than mass flux by electromigration, even in highly

electroosmotic-permeable media as discussed in Section 2.2. Kim (2002) validates this,

describing electroosmosis as a secondary transport mechanism that either enhances or



retards electromigration. Hence, electroosmosis was unlikely to have countered the migration

of the chloride ion to the extent that was observed in the electrokinetic performance tests.

As stated in Section 4.2.3, chloride ions undergo oxidation to chlorine gas at the anode,

which either remains dissolved in solution or escapes from the anolyte. In both cases, the

chlorine gas will not contribute to anolyte conductivity, and hence will not be detected in TDS

readings. The inability of the anolyte to attain the expected pH of 2 indicates that chloride

oxidation took place to a large extent in place of water electrolysis. In such a case, the TDS

values achieved in the electrokinetic performance tests could be significantly

misrepresentative of the actual mass flux that took place. Such losses of the chloride ion,

however, are unavoidable unless the electron transfer process required for chloride oxidation

is prevented. This may be possible with the use of a charge-permeable membrane to encase

the anode, thereby disallowing contact between the anode and the surrounding anolyte

whilst enabling a voltage gradient. Alternatively, an unreactive ion may be used in place of

the chloride ion. However, no such preventative measures were taken, and it is possible that

all TDS measurements obtained from the electrokinetic performance tests were

underestimates of the actual mass transported.

Severe soil pore clogging may have retarded the electrokinetic performance tests, producing

the low anolyte TDS values observed. However, it was shown in Section 4.2.4 that core

decomposition accompanies such pore clogging, and it was found that core decomposition

proceeded to a greater extent than pore clogging for most tests. Therefore, whilst soil pore

clogging was likely to have reduced the performance of the electrokinetic tests, it could not

have been responsible for the extent of reduced performance observed.

Analysis of the laboratory setup may provide further explanations for the discrepancies

observed. Since mass transport in the diffusion control tests occurred significantly faster than

predicted, it is almost certain that laboratory induced mass fluxes were generated. Though it

was assumed that such fluxes would act uniformly on both diffusion control and electrokinetic

performance tests alike, it is possible that some fluxes acted exclusively on the diffusion

control tests. It is unclear whether such mass fluxes are significant, or if they even exist.

Therefore, this is a matter that requires further investigation before any future testing.

The results may be accounted for in terms of sediment core heterogeneity. Whilst it is true

that the core samples used in the diffusion control tests may have been extremely porous

and negligibly tortuous relative to those used in the electrokinetic performance tests, such a

possibility is highly unlikely. Such heterogeneity implies immense differences between the



various core samples used. However, all practical measures were taken to prevent any

heterogeneity during the manufacture of the cores.

It has been established, then, that whilst the diffusion control tests are incomparable with the

electrokinetic performance tests, the reasons why are unclear. Many unlikely factors such as

core sample heterogeneity and unprecedented magnitudes of electroosmosis may be

responsible, but are extremely improbable. The only plausible explanation is that of chloride

loss by electrolysis, which was believed to be negligible in the design of the experiment.

Future research in this field must therefore take into consideration the possibility of all these

errors, and corresponding preventative measures must be applied in future tests.

4.4. Free solution control test

The purpose of the free solution control test was to quantify the effects of a voltage gradient

on the movement of the chloride ion without impedance due to medium tortuosity.

Consequently, the results provide an indication of phenomena that may occur at the latter

stages of the electrokinetic remediation process. The rates of mass transport achieved in the

free solution control test are illustrated in Figure 4-18. It can be seen that mass flux

decreased in a logarithmic manner over time. However, this may not be indicative of the end

stages of the remediation process, as the chloride ion in the catholyte was not replenished

during the test. It is intended that field applications of the technology will maintain the

concentration of the chemical flood over time. Without chemical replenishment, it can be

seen that mass transport occurs rapidly until a plateau is reached, after which mass transport

is relatively negligible.

0 50 100 150 2000

5

10

15

20

Time (days)

Con

cent

ratio

n (p

pk) R2=0.98667

Logarithmic Regression

Figure 4-18: Anolyte TDS for the free solution control test. Logarithmic regression of the data produced an r2 value of 0.98667.

The high rates of mass transport in the direction of the anode, however, were coupled with

high rates of mass transport in the direction of the cathode. The speed of cathodic migration



is illustrated by Figure 4-19B, in which a distinct plume of copper compounds can be seen to

evolve in the cathodic compartment. By the completion of the test, the anolyte had evolved

into an opaque solution (Figure 4-19A), most likely due to the backward flux of insoluble

copper species from the cathodic compartment. The catholyte, as illustrated by Figure 4-19C,

eventually evolved into a green slurry. The bubbles that can be seen in Figure 4-19C were

most likely the result of hydrogen gas production by the electrolysis of water.

Figure 4-19: Photos of the anolyte and catholyte during the free solution control test. Figure 4-19A shows the anolyte near test completion, Figure 4-19B shows the development of a plume in the cathodic compartment, and Figure 4-19C shows the catholyte near test completion.

The extent of copper precipitation in the free solution control test again suggests that the

presence of naturally occurring metals in soils may eventually pose as a bottleneck to the

remediation process. As previously discussed, the use of acid-depolarisation techniques may

be required to counter such precipitation, and may be critical in the remediation of some soils.

Acar (1993) states acetic acid as the preferred depolarising acid since it is environmentally

safe, does not fully dissociate, and is soluble with most cations.

4.5. Sediment core analyses

The USEPA (1995) state that the success of the electrokinetic technology is generally more

dependent on contaminant characteristics than on medium characteristics. The effects of

compositional variations between the core samples were then unlikely to have produced vast

differences between the electrokinetic tests performed. Some of the hydrological properties

of the manufactured sediment core samples are shown in Table 4-2 and Table 4-3.

Table 4-2: Saturated density, dry bulk density, and porosity of randomly selected sediment core samples.

Core A Core BSaturated Density (g/cm3) 1.99 2.07Dry Bulk Density (g/cm3) 1.76 1.88

Porosity 0.233 0.183



The dry bulk densities rendered were reasonably consistent with a mean value of 1.52g/cm3,

which is typical of sand. The porosity of the core samples was found to be high, with values

of approximately 0.2. Hydraulic conductivities ranged from 0.0280cm/s to 0.09425cm/s, thus

remaining in the same order of magnitude. The sediment core samples analysed were

therefore relatively uniform, suggesting that the tests performed in this study were subject to

reasonably similar conditions. However, the results show otherwise, varying widely from test

to test. It is thus recommended that any future research conducted employ the use of the

same porous medium for all electrokinetic performance and diffusion control tests. Only such

an approach will ensure uniformity between the tests, but will require that the constituents of

the medium to be inert such that they do not react to the applied electric field. A porous

medium composed of glass beads may then be appropriate.

Table 4-3: Hydraulic conductivity and dry bulk density of randomly selected sediment core samples. The hydraulic gradients employed and the flow rates achieved are also shown.

Core D Core E Core F Core G Hydraulic Gradient 2.32 2.52 2.43 2.25

Flow Rate (cm/s) 0.078 0.071 0.209 0.212 Hydraulic Conductivity (cm/s) 0.0337 0.0280 0.0861 0.0943

Dry Bulk Density (g/cm3) 1.31 1.52 1.44 1.24

The hydraulic conductivities and porosities rendered in the sediment core samples can be

considered high. Hence, the mass transport rates observed in this study may be optimistic

relative to what may be commonly encountered in the field. Further testing using porous

media of varying characteristics is required before field application of the remediation

technique.

Chapter 5: Conclusions


5. Conclusions

5.1. Scientific significance

Subsurface NAPL contamination is a serious environmental problem, and the remediation of

contaminated sites is rarely achieved despite significant annual expenditures. This study

considered the viability of employing electrokinetic phenomena in conjunction with in situ

chemical oxidation to remediate such sites. Laboratory investigations were conducted,

simulating the movement of a charged treatment compound into a subsurface NAPL

reservoir via a porous medium. Five electrokinetic performance tests were implemented, as

well as three diffusion control tests and one free solution control test.

Mass transport through the porous medium was found to increase linearly with the applied

electrical potential difference. However, mass transport in the diffusion control tests was

measured to exceed mass transport in the electrokinetic performance tests, suggesting

electrokinetics to be ineffective in the remediation of NAPL contaminated sites. It was

established that such an observation was most likely due to inaccuracies in the methods

employed. Hence, further investigation is suggested before any conclusions are derived,

giving particular focus to the sources of error in this study. Importantly, this study identified

the significance of various system components on the effectiveness of the electrokinetic

technique, such as the composition of the electrodes and the porous medium.

5.2. Future research

It is recommended that future tests consider the use of electrokinetic geosynthetics to

encase the electrodes. The use of geosynthetics prevents the introduction of secondary

chemical species into the electrolyte that may complicate the electrochemistry of the system,

and may also prevent the electron transfer process that oxidises the flooded species. As an

additional measure, the flooded species should be a charged compound that does not

undergo electrolysis.

To ensure uniformity between the various tests, a single porous medium should be used. As

such, it must be composed of a material that is relatively inert and easily cleaned between

tests, such as glass beads. The extent of electroosmotic advection must also be quantified,

or at least qualified. This may be achievable with the use of a nonpolar dye in the anolyte.

Furthermore, since mass flux was found to increase linearly with the applied voltage gradient,

larger voltage gradients should be employed such that an upper limit for the electrokinetic

process can be qualified. Finally, enhancement techniques such as acid-depolarisation

should be explored. The introduction of acid into the system will mobilise naturally occurring

Chapter 5: Conclusions


metal species in the soil and lead to the development of voids, thereby improving mass

transport via a feedback process. Additionally, the use of acids may induce reverse

electroosmosis, thereby enhancing the ionic migration of the negatively charged treatment

compound.

Chapter 6: Glossary


6. Glossary advection transport by bulk fluid motion

counterion accompanying ionic species that maintains electric neutrality

diffuse double layer layer of ions in the primary ionic shell, together with the structure of counterions from the electrolyte

electrolysis electrically induced chemical reaction

electromigration transportation of ionic species by electrical gradient

electroosmosis bulk flow of pore fluid due to viscous drag exerted by ions migrating in the diffuse double layer

electroosmotic permeability

flow rate of pore fluid through a unit cross-sectional area of the porous medium in response to a unit electrical potential difference

electrophoresis transportation of charged particles by electrical gradient

heterogeneous of non-uniform composition

hydraulic conductivity

flow rate achievable per unit hydraulic gradient applied

hydraulic gradient change in hydraulic head per unit distance in a given direction

in situ �in place�

in situ chemical oxidation

remediation technology employing the use of oxidants to degrade organic contaminants in situ

micelle aggregate of amphipathic molecules

molecular diffusion mass transport by chemical gradient

NAPL non-aqueous phase liquid, such as TCE and automotive fuels

porosity ratio of pore volume to total volume

relative concentration

ratio of concentration to source concentration

sediment core cylindrical section of a sediment sample

sorption attachment of an aqueous species to the surface of a solid

TDS total dissolved solids, which is related to conductivity

tortuosity measure of the effect of flowpath geometry on fluid dynamics

transference number fraction of total current carried by the ionic species

zeta potential electrical potential at the junction between the fixed and mobile parts of the diffuse double layer

Chapter 7: References


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Team, Emerging Technologies Project.

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from Tailing Soils�, Journal of Environmental Engineering, vol. 128, no. 8, pp. 705-715.

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electrodes in molten salts�, Electrochimica Acta.

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Attenuation in Fractured Bedrock�, Ground Water, vol. 43, no. 1, pp. 30-39.

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unsaturated soil: 1. Theory, model development, and testing�, Journal of Contaminant

Hydrology, vol. 54, no. 1-2, pp. 99-120.

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Appendix A


Appendix A. Apparatus designs

A1. Initial design 1

A1.1. Electrolyte container (isometric view)

Electrolyte containers house both

catholyte and anolyte, and are connected

via a central pipe (Appendix A1.2)

encasing the sediment core sample. The

upper flange allows the insertion of the

magnetic stir bar and electrolyte fluid, and

can be sealed via parafilm to prevent

evaporative losses. The side flange

connects to the central pipe.

A1.2. Central pipe (isometric view)

The central pipe houses the sediment core

sample in the core station (see right).

Rubber sealant on the wall of the core

station encases the sediment core,

ensuring ion flow does not occur around

the core. Grooves are placed on either

side of the core station to hold the rubber

sealant in place when inserting or

removing the core sample. Ridges at each

end of the central pipe connect to an

electrolyte container as shown in Appendix

A1.3.

Appendix A


A1.3. Central pipe assembly (side view)

The various components of the

electrochemical cell are assembled as

shown. Dotted lines represent internal

walls, and shaded regions represent

rubber sealants and o-rings. As can be

seen, all joins are rubber-sealed and a

constant internal diameter equivalent to

that of the core sample is maintained.

A2. Initial design 2

A2.1. Electrolyte container (isometric view)

Once again, electrolyte containers house

both catholyte and anolyte, but are now

directly connected by a sealed sediment

core sample as shown in Appendix A2.3.

As before, the upper flange allows the

insertion of the magnetic stir bar and

electrolyte fluid, and can be sealed via

parafilm to prevent evaporative losses.

The side flange connects to the sealed

sediment core sample.

Appendix A


A2.2. Sealed sediment core sample (photo)

To prevent water losses through the

sediment core sample during electrokinetic

testing, Bondall TerraTite elastomeric

waterproofing membrane system is

applied to seal the core. The application of

such a waterproofing membrane is

analogous to the use of a pipe as

discussed in Appendix A1.2.

A2.3. Sealed sediment core assembly (side view)

The various components of the

electrochemical cell are assembled as

shown. Once again, dotted lines represent

internal walls. The major advantages of

this design relative to the design

discussed in Appendix A1 are simplicity

and cost effectiveness. Visual verification

of system sealing is also possible as the

core is exposed for view. The major

disadvantage of the design is the

requirement of each core sample to be

individually treated well in advance of use

in electrokinetic testing.

The setup shown was initially employed in

electrokinetic testing, but was

unsuccessful due to the impermeability of

the gneiss core samples (Section 3.1.2).

Appendix A


A3. Final design

A3.1. Electrolyte container

The design implemented in electrokinetic

testing was highly similar to that discussed

in Appendix A2. The final electrolyte

container design was the same as

discussed in Appendix A2.1, with

orthogonal views as shown to the left.

However, due to the difficulty of flange

construction during manufacturing, the

upper wall of the container was left open in

place of a flange. This change was

considered insignificant.

A3.2. Artificial sediment core sample The sediment cores used in electrokinetic

testing were artificially derived as

described in Section 3.1.2. They are

analogous to the sealed core samples

discussed in Appendix A2.2, and are

assembled in a manner identical to that

shown in Appendix A2.3. Figure 3-1

illustrates the full laboratory setup.

Appendix B


Appendix B. Diffusive breakthrough concentrations

B1. Diffusion with medium tortuosity of 0.1 (lower-estimate)

Time Elapsed

(days)

Relative Core Breakthrough Concentration

Concentration Gradient(g/cm 4)

Total Mass Transported

(g)

Anolyte Concentration

(ppm) 0 0.00000 0.01111 0.00000 0.000 1 0.00002 0.01111 0.02431 2.701 2 0.00009 0.01111 0.04641 5.157 3 0.00029 0.01111 0.06851 7.612 4 0.00065 0.01110 0.09060 10.067 5 0.00122 0.01110 0.11268 12.520 6 0.00206 0.01109 0.13475 14.972 7 0.00321 0.01108 0.15679 17.421 8 0.00473 0.01106 0.17880 19.867 9 0.00666 0.01104 0.20078 22.309

10 0.00906 0.01101 0.22270 24.745 11 0.01197 0.01098 0.24457 27.174 12 0.01545 0.01094 0.26637 29.596 13 0.01954 0.01089 0.28808 32.009

B2. Diffusion with medium tortuosity of 1.0 (upper-estimate)

Time Elapsed

(days)

Concentration Gradient (g/cm 4)

Total Mass Transported

(g)

Anolyte Concentration

(ppm)0 0.01111 0.000 0.0001 0.01111 0.221 24.5572 0.01111 0.442 49.1153 0.01111 0.663 73.6724 0.01111 0.884 98.2295 0.01111 1.105 122.7876 0.01111 1.326 147.3447 0.01111 1.547 171.9028 0.01111 1.768 196.4599 0.01111 1.989 221.016

10 0.01111 2.210 245.57411 0.01111 2.409 267.67512 0.01111 2.608 289.77713 0.01111 2.807 311.879

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