The Use of Electrokinetics for Remediation of Contaminated ... · PDF fileRemediation of...

The Use of Electrokinetics for Remediation of Contaminated

Groundwater in Low Permeability Sediments

Honours Dissertation

Michael Gillen 10219736

October 2006

Supervisor: Dr David A. Reynolds

School of

Environmental Systems

Engineering

Abstract

i

Abstract

Chlorinated solvents, in particular trichloroethene and tetrachloroethene, form a major

worldwide source of groundwater contamination. One of the many problems that impede

the successful remediation of contaminated sites occurs when DNAPL sources diffuse

into low permeability zones. Conventional remediation techniques aimed at removing

these contaminants have proven expensive and ineffective, and as a result contamination

will often persist for many decades. Electrokinetics is an emerging technology that has

found many applications in environmental contaminant removal. It has in the past been

demonstrated to successfully remove and/or destroy contaminants such as heavy metals

and radionuclides from groundwater. It is particularly effective in removing these

compounds from heterogeneous and low-permeability soils, such as clay. More recently,

the use of electrokinetic phenomena to mediate and enhance chemical

oxidation/reduction has been considered. Specifically, electrokinetics may be utilised to

transport oxidising or reducing agents into these low permeability zones, facilitating the

remediation of these contaminants.

This study investigates the feasibility of utilising electrokinetics to transport potassium

permanganate (KMnO4) and nanoscale zero-valent iron (ZVI) through glass and clay

porous media. A series of two-dimensional bucket and tank experiments were conducted

to ascertain whether potassium permanganate (40 gL-1) and ZVI (9 gL-1) could be

effectively transported through low permeability glass and clay media utilising

electrokinetic behaviour. Results indicated that electrokinetic effects can significantly

increase the rate of mass transport of these reagents through the low permeability glass

media and clay. They can therefore be successfully transported into zones of low

permeability that would otherwise be bypassed by traditional flow and transport

techniques (e.g. oxidant flushing). Further studies should focus on the application of these

techniques to larger-scale two and three-dimensional models incorporating natural

sediments, and eventually their applicability in the field.

.

Acknowledgements

ii

Acknowledgements

The author would like to acknowledge the support and assistance of many people who

have contributed to the completion of this project.

Dr David Reynolds for your consistent support and guidance throughout the year;

Dr Edward Jones for your invaluable time and advice in the laboratory;

Hon San Leong, for providing a great deal of time assisting in the laboratory;

Dianne Krikke for aiding my laboratory efforts;

The gang at SESE;

Thanks must also be extended to family and friends for their ongoing support and

understanding.

Table of Contents

iii

Table of Contents LIST OF FIGURES......................................................................................................................................V LIST OF TABLES................................................................................................................................... VIII GLOSSARY................................................................................................................................................ IX 1 INTRODUCTION................................................................................................................................1 2 LITERATURE REVIEW....................................................................................................................4

2.1 ELECTROKINETICS: BACKGROUND AND HISTORY .........................................................................4 2.2 PRINCIPLES OF ELECTROKINETICS: TRANSPORT PHENOMENA.......................................................5

2.2.1 Electromigration......................................................................................................................7 2.2.2 Electroosmosis.........................................................................................................................9 2.2.3 Electrophoresis ......................................................................................................................16 2.2.4 Relative Contributions ...........................................................................................................18

2.3 PRINCIPLES OF ELECTROKINETICS: AQUEOUS CHEMISTRY .........................................................19 2.4 CHLORINATED SOLVENTS IN THE ENVIRONMENT........................................................................21

2.4.1 DNAPL Transport and Distribution ......................................................................................22 2.4.2 Phase Transfer Processes......................................................................................................24 2.4.3 Diffusion ................................................................................................................................25 2.4.4 Sorption and Transformation.................................................................................................27

2.5 CHLORINATED SOLVENT REMEDIATION......................................................................................28 2.5.1 Potassium Permanganate ......................................................................................................31 2.5.2 Zero-Valent Iron ....................................................................................................................34

3 EXPERIMENTAL METHODS........................................................................................................41 3.1 COLUMN METHODOLOGY ...........................................................................................................42

3.1.1 Materials and Configuration .................................................................................................43 3.1.2 Experimental Procedure ........................................................................................................45

3.2 BUCKET METHODOLOGY ............................................................................................................46 3.2.1 Materials and Configuration .................................................................................................46 3.2.2 Trial 1: Inward Transport .....................................................................................................49 3.2.3 Trial 2: Outward Transport...................................................................................................50

3.3 TANK METHODOLOGY ................................................................................................................51 3.3.1 Heterogeneous Configuration A ............................................................................................52 3.3.2 Heterogeneous Configuration B ............................................................................................56

4 RESULTS ...........................................................................................................................................61 4.1 COLUMN EXPERIMENTS ..............................................................................................................61 4.2 BUCKET EXPERIMENTS ...............................................................................................................65

4.2.1 Trial 1: Inward Transport .....................................................................................................65 4.2.2 Trial 2: Outward Transport...................................................................................................65

4.3 TANK EXPERIMENTS ...................................................................................................................68 4.3.1 Heterogeneous Tank Configuration A ...................................................................................68 4.3.2 Heterogeneous Tank Configuration B ...................................................................................74

5 DISCUSSION .....................................................................................................................................80 5.1 ELECTROOSMOTIC PERMEABILITY OF THE CLAY ........................................................................85 5.2 OXIDATION EFFICIENCY AND POTENTIAL BARRIERS...................................................................86 5.3 ACID/BASE CHEMISTRY ..............................................................................................................88 5.4 CLAY FRACTURING .....................................................................................................................89 5.5 FIELD CONFIGURATION ...............................................................................................................90

Table of Contents

iv

6 CONCLUSIONS ................................................................................................................................93 7 REFERENCES...................................................................................................................................95 APPENDIX A ............................................................................................................................................101

List of Figures

v

List of Figures FIGURE 2.1 ELECTROOSMOTIC PERMEABILITY AND HYDRAULIC CONDUCTIVITY FOR SELECTED SEDIMENTS.

THE RELATIVE VARIATION OF KE ACROOS THE RANGE OF SOILS IS SMALL COMPARED TO THE

VARIATION IN HYDRAULIC CONDUCTIVITIES (ELECTROKINETIC LIMITIED, 2004).................................11 FIGURE 2.2 THE SOLID-LIQUID INTERFACE, SHOWING THE DIVISION BETWEEN THE COMPACT AND MOBILE

PARTS OF THE ELECTRICAL DOUBLE LAYER (PALLIAT, 2000)................................................................13 FIGURE 2.3 PRINCIPLES OF ELECTROKINETIC TRANSPORT IN SATURATED POROUS MEDIA (YEUNG, 2006). ....19 FIGURE 2.4 DIFFUSION OF POOLED DNAPL SOURCE INTO CLAY DEPOSIT (PANKOW AND CHERRY, 1996).....23 FIGURE 2.5 THE DISSOLUTION AND DIFFUSION OF A DNAPL SOURCE TRAPPED IN A FRACTURE INTO

SURROUNDING MATERIAL (PANKOW AND CHERRY, 1996)....................................................................26 FIGURE 2.6 LASAGNE CONFIGURATION, CONSISTING OF THREE TREATMENT ZONES WITHIN TWO ELECTRODES

(HO ET AL., 1999). ................................................................................................................................39 FIGURE 3.1 SCHEMATIC OF COLUMN APPARATUS, SHOWING THE CATHODIC RESERVOIR (KMNO4) AND

ANODIC RESERVOIR (H2O) CONNECTED BY A POROUS MEDIA CORE (JONES ET AL., IN PREP.)................43 FIGURE 3.2 POWERTECH MP - 3092 LABORATORY POWER SUPPLY...............................................................44 FIGURE 3.3 PLAN VIEW SCHEMATIC OF THE BUCKET APPARATUS. THE POLARITY OF THE ELECTRODES WAS

INTERCHANGED BETWEEN THE TWO CONDUCTED TRIALS. ....................................................................47 FIGURE 3.4 BUCKET APPARATUS SHOWING: A) STAINLESS STEEL CYLINDER AND CLAY BLOCK IN THE

CENTRE; AND, B) CLAY BLOCK WITH HOLLOWED-OUT WELL IN THE CENTRE, 600-850ΜM GLASS BEADS,

AND ELECTRODES. ................................................................................................................................48 FIGURE 3.5 COMPLETED BUCKET APPARATUS FILLED WITH POTASSIUM PERMANGANATE SOLUTION.............49 FIGURE 3.6 EXOTECH MASTERFLEX L/S PERISTALTIC PUMP. .........................................................................50 FIGURE 3.7 BUCKET APPARATUS SHOWING INNER WELL WITH ELECTRODE INSERTED INSIDE AND PUMP INLET

TO WITHDRAW EXCESS SOLUTION. ........................................................................................................51 FIGURE 3.8 2-D TANK APPARATUS SHOWING THE HETEROGENEOUS OF MEDIA AND ELECTRODE

CONFIGURATION ...................................................................................................................................53 FIGURE 3.9 SCHEMATIC OF 2D TANK APPARATUS SHOWING CLAY LENSES IN A HIGH-PERMEABILITY

BACKGROUND. ......................................................................................................................................57 FIGURE 3.10 2D HETEROGENEOUS TANK APPARATUS WITH GLASS AND CLAY MEDIA, SHOWING THE

FORMATION OF PREFERENTIAL PATHWAYS AROUND THE CLAY BLOCKS. ..............................................59 FIGURE 3.11 HETEROGENEOUS TANK CONFIGURATION UTILISING COARSE AND FINE GLASS BEADS TO CREATE

A HETEROGENEOUS MEDIUM BETWEEN THE TWO ELECTRODES. ............................................................60 FIGURE 4.1 COMPARISON OF NACL TRANSPORT THROUGH GLASS MEDIA BY DIFFUSION AND 20V DC

CURRENT (JONES ET AL., IN PREP.). .......................................................................................................61 FIGURE 4.2 TRANSPORT RATES OF KMNO4 THROUGH GLASS MEDIA BY DIFFUSION, 10 AND 20V DC

ELECTRIC CURRENT (JONES ET AL., IN PREP.). .......................................................................................63

List of Figures

vii

FIGURE 4.3 COMPARISON OF KMNO4 TRANSPORT THROUGH CLAY MEDIA BY DIFFUSION, 10 AND 20V DC

ELECTRIC CURRENT. THE DIFFUSION AND 10V TRIALS WERE REPLICATED (JONES ET AL., IN PREP.). ....64 FIGURE 4.4 BUCKET APPARATUS AT THE CONCLUSION OF THE SECOND TRIAL. THE CLAY SLURRY ON TOP OF

THE GLASS BEADS APPEARS TO HAVE RISEN TO THE SURFACE AT THE EDGE OF CLAY, AND SPREAD OVER

THE TOP. ...............................................................................................................................................66 FIGURE 4.5 DISSECTED CLAY CYLINDER SHOWING MANGANESE AND FERROUS IRON PRECIPITATION. ...........67 FIGURE 4.6 TANK TRIAL 1 AT COMMENCEMENT (A) AND TERMINATION (B) OF THE EXPERIMENT; DURATION =

140 MINUTES; NO ELECTROKINETIC EFFECTS PRESENT..........................................................................69 FIGURE 4.7 TANK TRIAL 2 AT COMMENCEMENT OF EXPERIMENT (A), START OF ELECTRIC CURRENT (B), AND

TERMINATION OF EXPERIMENT (C); DURATION = 140 MINUTES; ELECTROKINETIC EFFECTS PRESENT

FROM T=50 TO T=140 MINUTES............................................................................................................70 FIGURE 4.8 TANK TRIAL 3 AT COMMENCEMENT (A) AND TERMINATION (B) OF EXPERIMENT; DURATION = 140

MINUTES; ELECTROKINETICS EFFECTS PRESENT FOR COMPLETE DURATION. .........................................70 FIGURE 4.9 TANK TRIAL 4 AT COMMENCEMENT OF EXPERIMENT (A), START OF ELECTRIC CURRENT (B), AND

TERMINATION OF EXPERIMENT (C); DURATION = 230 MINUTES; ELECTROKINETIC EFFECTS PRESENT

FROM T=110 TO T=230 MINUTES..........................................................................................................71 FIGURE 4.10 SNAPSHOTS OF THE LOW-PERMEABILITY ZONE TAKEN AT 10 MINUTE INTERVALS DURING TRIAL

TWO. TOP IMAGES ARE ORIGINAL COLOUR AND THE BOTTOM LINE HAS BEEN ENHANCED FOR THE

CALCULATION OF INTRUSION AREA.......................................................................................................72 FIGURE 4.11 CROSS-SECTIONAL AREA INTRUSION OF MNO4 IONS INTO THE LOW-PERMEABILITY ZONE

PRESENTED AS THE PERCENTAGE OF FACE AREA. ..................................................................................73 FIGURE 4.12 ZERO-VALENT IRON TANK TRIAL 2 AT COMMENCEMENT (A), START OF ELECTRIC CURRENT (B),

AND CONCLUSION OF EXPERIMENT (C); DURATION = 190 MINUTES; ELECTROKINETIC EFFECTS PRESENT

FROM T=90 TO T=190 MINUTES. NOTE THAT DUE TO THE LOW COLOUR CONTRAST, THE IRON FRONT

HAS BEEN HIGHLIGHTED WITH A GREY LINE AT T=190. ........................................................................74 FIGURE 4.13 PHOTOGRAPHS FROM FINE-GLASS LENS TRIALS WITHOUT ELECTROKINETICS (A,B) AND TRIAL 2

WITH ELECTROKINETICS (C,D)...............................................................................................................75 FIGURE 4.14 DISSECTION OF THE CLAY LENSES ALONG THE X-, Y- AND Z-AXIS..............................................76 FIGURE 4.15 COMPARISON OF DISSECTED CLAY BLOCK FROM THE NORMAL AND ELECTROKINETIC TESTS.

BLOCKS ON THE RIGHT OF THE CALLIPERS ARE TAKEN FROM THE ELECTROKINETIC TEST. ...................77 FIGURE 4.16 ELECTRIC FIELD LINES BETWEEN TWO PARALLEL RODS IN A SINGLE PLANE...............................77 FIGURE 4.17 TEMPORAL EVOLUTION OF CURRENT THROUGH THE POWER SUPPLY DURING THE 2D TANK

TRIALS WITH GLASS AND CLAY MEDIA. .................................................................................................78 FIGURE 5.1 POTASSIUM PERMANGANATE TRIAL 3; ELECTRIC CURRENT PRESENT FROM START OF

EXPERIMENTAL RUN. THE DELAYED MNO4- FRONT TO THE LEFT OF THE FINE-GLASS LENS IS LIKELY A

RESULT OF IONIC MIGRATION TO THE LEFT OF THE TANK, DUE TO THE INFLUENCE OF THE CATHODE....82 FIGURE 5.2 CONCEPTUAL INVASION OF CLAY BLOCKS BY MNO4

- ..................................................................83

List of Figures

vii

FIGURE 5.3 CONCEPTUAL APPLICATION OF COUPLED ELECTROKINETICS AND ISCO TO LOW PERMEABILITY

SEDIMENTS............................................................................................................................................91

List of Tables

viii

List of Tables

TABLE 2.1 DIFFUSION COEFFICIENTS AND IONIC MOBILITIES OF SELECTED ANIONS AND CATIONS. ..................9 TABLE 2.2 SUMMARY OF THE EFFECTIVENESS OF VARIOUS TREATMENT TECHNOLOGIES UNDER

HETEROGENEOUS SEDIMENTS (ATHMER AND HUNTSMAN, 2005). ........................................................30 TABLE 2.3 VARIOUS CHEMICAL PROPERTIES OF TRICHLOROETHENE (REITSMA AND DAI, 2000;

SCHWARZENBACH, 2003). ....................................................................................................................34

Glossary

ix

Glossary Anode Electrode at which oxidation takes place; the positive terminal of a

power supply.

Anolyte Solution containing the anode of an electrochemical cell.

Cathode Electrode at which reduction takes place; the negative terminal of a

power supply.

Catholyte Solution containing the cathode of an electrochemical cell.

Chelate Process by which an ion (often a metal) is bound into a stable,

largely unreactive ring-like structure.

DNAPL Dense Non-Aqueous Phase Liquid. Organic liquid that is

immiscible with, and denser than water.

Ionic strength Quantity proportional to the amount of electrostatic interaction

between ions in a solution. Equal to 0.5 ΣmiZi where m is the

molar concentration of species ‘i’, and Z the charge of the species.

Fracture Open joints, fissures, and/or faults that present through a porous or

non-porous media.

Nanoscale Physical quantity with dimensions of the order 10-9 – 10-8 m.

Oxometal Compound consisting of a metal bound to one or more oxygen

atoms.

PCE Tetrachloroethene (also perchloroethene/perchloroethylene) –

C2Cl4. Solvent used in the production of textiles and metals.

Glossary

x

Peristalsis Synchronised and coordinated series of contractions and

expansions which are used to move the contents of a flexible tube.

Streaming potential Potential difference created across a soil core created by hydraulic

flow under a unit hydraulic gradient.

TCE Trichloroethene (also trichloroethylene) – C2HCl3. Solvent used

widely in the dry-cleaning industry.

Zero-valent Compound having zero-valency (i.e. uncharged).

Introduction

1

1 Introduction

The industrial use of chlorinated solvents has contributed significantly to groundwater

contamination around the world. Of the many solvents that threaten ground water

resources, trichloroethene (TCE – C2HCl3), tetrachloroethene (PCE – C2Cl4)

dichloroethene (DCE), vinyl chloride (VC) are some of the more prevalent (Zhang,

1998). Many years of improper disposal, coupled with their persistence in the

environment means they form a major and continuing source of groundwater

contamination. Indeed, chlorinated solvents form the largest group of pollutants on the

U.S Environmental Protection Agency’s pollutant list (Kim and Gurol, 2005)

Chlorinated solvents are referred to as dense non-aqueous phase liquids (DNAPLs), and

often exist in the subsurface as a pure-phase liquid. The migration and distribution of a

DNAPL source through the subsurface is dictated to a large extent by the presence of

heterogeneities and preferential flow pathways (Pankow and Cherry, 1996). DNAPL

sources often pool on top of clay lenses or horizons, and where fractured clay is present,

can become distributed through fractured clay networks. Over an extended time frame of

decades dissolution and diffusion of the source zone into the surrounding low-

permeability layer is observed.

There are many established techniques available for remediation of sites contaminated by

chlorinated solvents; however, most technologies utilise pore fluid advection to facilitate

migration of contaminants out of the contaminated zone (e.g. pump-and-treat) or the

migration of treatment compounds into the contamination zone (e.g. oxidant flushing).

Indeed, traditional technologies such as pump-and-treat, oxidant flushing, and biological

treatment are invariably ineffective at removing contaminants from low permeability

zones, due to the long time scales associated with advection and dispersion through clay

sediments.

Introduction

2

Electrokinetics (EK) is a developing technology that has been utilised to remove or

destroy heavy metals, organic compounds, and radionuclides in groundwater (Acar and

Alshawabkeh, 1993). The application of a direct current (DC) through a saturated porous

medium is known to induce a number of coupled and un-coupled solute transport

phenomena through the solid matrix and pore liquid. These are the result of the various

methods of charge conduction through the saturated media. Electroosmosis is the bulk

flow of the pore fluid to one electrode due to the complex electrical interactions at the

solid-liquid interface; ion migration is the transport of an ionic species to the oppositely

charged electrode; and electrophoresis is the term used the migration of charged colloidal

particles towards the opposite electrode (Paillat, 2000).

Electrokinetic phenomena have been utilised in a number of applications to either

mediate or enhance a variety of other remediation technologies. Electrokinetics has, for

example, been coupled with adsorption/degradation or reductive dechlorination in a

process called Lasagne (Ho et al., 1995; 1999), whereby heavy metals are transported

through a number of treatment zones.

The fundamental advantage driving the interest behind EK soil remediation is the

relatively homogeneous solute transport that can be achieved through a heterogeneous

subsurface. Whereas hydraulic flow is characterised by highly preferential flow pathways

due to large variations in hydraulic conductivity, the transport of ions under the

application of an electric current is relatively uniform through a heterogeneous medium.

This study is part of a larger body of work with the aim of coupling EK remediation

technology with in-situ chemical oxidation (ISCO) techniques. Specifically, the

technology utilises electrokinetic phenomena to transport charged chemical

oxidising/reducing agents into low-permeability sediments for the destruction of various

contaminants.

This study builds on the promising results of one-dimensional column experiments (Jones

Introduction

3

et al., in prep.) which examined the efficacy of utilising electrokinetics to transport

potassium permanganate (KMnO4) ions and nanoscale zero-valent iron (VZI) through

porous media cores. The purpose of this study is to ascertain the success of the

electrokinetic technology at transporting the permanganate ion (MnO4-) and nanoscale

ZVI particles through a two-dimensional heterogeneous medium. Towards this end a

number of one-dimensional and two-dimensional laboratory experiments were conducted

using MnO4- and nanoscale ZVI as the target transport species. The permanganate ion is a

powerful reducing agent (Kim and Gurol, 2005) whilst ZVI particles are powerful

reducing agents (Gillham and O’Hannesin, 1994).

This paper describes the one- and two-dimensional experimental activities that were

undertaken by the author, and those completed in the preceding study by Jones et al. (in

prep.).It also provides considerations for further work towards the field application of a

coupled electrokinetics and ISCO technique.

Literature Review

4

2 Literature Review

2.1 Electrokinetics: Background and History

Electrokinetic (EK) remediation, also electroreclamation, electrokinetic soil processing,

and electrochemical decontamination, utilises a low-level direct current (DC) between

electrodes embedded in a saturated porous medium to facilitate the removal of

contaminants from the soil. Electrokinetics is a broadly exercised term, with application

in microelectronics and medicine, and in general can be described as the study of the

motion of liquids or particles that result from an electric field. The use of electrokinetics

in environmental site remediation has arisen as an innovative and cost-effective

alternative to traditional remediation methods.

Considerable interest has specifically arisen in the application of electrokinetics to the

remediation of low permeability sediments. The application of a DC voltage in the order

of 1-4 V/cm across a soil mass results in the mass transport of aqueous species by a

number of conduction phenomena. The transport processes are accompanied by sorption,

precipitation, hydrolysis, dissolution, and other chemical processes (Acar and

Alshawabkeh, 1993).

The potential for the electrokinetic remediation of soils was realised in the 1980s when

scientists noted high levels of heavy metals carried in pore water transported by

electroosmosis (Azzam and Oey, 2000). Since then a number of different applications of

the electrokinetic remediation technology have emerged which have examined the

removal of heavy metals and radionuclides from low permeability sediments utilising

electrokinetic phenomena. (e.g. Lageman, 1993; Ho et al., 1999; 1995; Kim et al., 2002;

Yang, 2001; Azzam and Oey, 2000). The general principle however does not directly

apply to many organic contaminated which often have low aqueous solubilities and have

Literature Review

5

zero valency.

The influence of an electric current applied through a porous media was first noted at the

beginning of the 19th century when Reuss (1808) noted the transport of water towards the

cathode. Much of the early theoretical work on electrokinetic theory was done by

Smoluchowski. Interested readers are referred to a historical perspective of

Smoluchowski’s work by Lyklema (2003).

Several variations on the basic electrokinetic concept exist: electrokinetically enhanced

bioremediation; electrokinetically deployed oxidation/reduction; periodic reversal of

current to repeatedly pass contaminants through a treatment zone; and electroheated

extraction (Cauwenberghe, 1997). Electrokinetically enhanced bioremediation, for

example, utilises electrokinetic effects to deliver biologically active slurries

(Cauwenberghe, 1997) and nutrients (e.g. Rabbi et al., 2000) to low permeability media.

Electrokinetic technology is relatively easy to implement in laboratory and field

applications, but the fundamental reactions which accompany it are complex (Saichek

and Reddy, 2005a); a thorough knowledge of the physical, chemical and electrochemical

processes that occur in a saturated soil is essential to successfully implement field-scale

site remediation. A description of the electrochemical processes that occur during the

implementation of electrokinetic technology is presented in section 2.2 and 2.3.

2.2 Principles of Electrokinetics: Transport Phenomena

This section provides a brief review of the physical and chemical mechanisms by which

mass transfer may be achieved. Under the influence of an electric field there are five

mechanisms which act to transport aqueous contaminants through a porous media:

advection, dispersion, electromigration, electroosmosis, and electrophoresis (Acar and

Alshawabkeh, 1993). Advection and dispersion are results of hydraulic flow processes,

Literature Review

6

created by differences in hydraulic head across a porous media. Dispersion is a

combination of molecular diffusion – the spontaneous motion of particles from areas of

high concentration to low concentration – and mechanical dispersion, which is a direct

consequence result of the tortuosity of the medium. Electromigration, electrophoresis,

and electroosmosis are all direct results of the electric field and induced charge

conduction through the saturated porous media.

The electrical conductivity is a measure of the ability of a medium to conduct an electric

current, and is define for a saturated porous medium to be:

σs = σsur + σfl n τ, (Musso, 2003) (2-1)

where σs is the overall conductivity of the soil mass, σsur is the surface conductivity of the

porous material, σfl is the electrical conductivity of the pore fluid, n the porosity, and τ

the tortuosity factor (Musso, 2003; Szymczyk et al., 2002). Simply stated, the

conductivity of the saturated porous material is a function of both the aqueous solution

and soil matrix properties. As it turns out, surface conductivity and zeta-potential are the

two major factors affecting electroosmotic flow (e.g. Lu et al., 2006; Grundl and

Michalski, 1996).

Surface conductance is associated with the movement of ions within the electrical double

layer. Early theoretical treatment linked the surface conduction phenomenon to the

mobile ions in the diffuse region of the double layer. Recently however, the importance

of the compact layer contribution to the surface conductivity has been recognised.

Tangential migration of ions in the compact layer has been shown to contribute

substantially to surface conductivity (Szymczyk et al., 2002).

Another two modes of electrical conduction prevail in a saturated porous medium: redox

reactions (e.g. electrolysis of water), and conduction along pore surfaces (Grundl and

Michalski, 1995). A number of other processes occur in soils due to the application of an

Literature Review

7

electric current, and include: ion exchange, hydrolysis, pH change, soil desiccation, and

reaction/precipitation (Azzam and Oey, 2000). A brief description of the three major

mechanisms of solute transport – electroosmosis, electromigration, and electrophoresis –

is presented below.

2.2.1 Electromigration

Electromigration refers to the mass flux of aqueous ions under the influence of an electric

field. Negative species move towards the anode and positive species move towards the

cathode. The velocity of an ion in an electric field of 1 V/m is the ionic mobility. The

ionic mobility of many inorganic ions falls in the range of 1×10-8 to 10×10-8 m2V-1s-1

(Yeung, 2006). The effective ionic mobility uj* of chemical species in porous media may

be estimated from the effective diffusion coefficient and charge on the ion by the Nernst-

Einstein equation:

RTD

u*

j*j

Fz j= (2-2)

where zj is the valence of the ion, F is Faraday’s constant (F = 96 485 Coulomb/mol), R

the ideal gas constant (8.315 J K-1 mol-1), and T temperature. The effective diffusion

coefficient Dj* of the chemical species xj in a saturated porous medium is related to the

diffusion coefficient in an aqueous solution Dj, a tortuosity factor, and a porosity factor.

Typical values for porosity and tortuosity fall in the range of 0.2 and 0.5 and 0.1 to 0.7

respectively (Acar and Alshawabkeh, 1993). It has been noted that the ratio between the

effective diffusion coefficient and effective ionic mobility of a chemical species is

approximately 40 times the charge on the species (Acar and Alshawabkeh, 1993). The

range of effective ion mobilities in fine-grained soils has been suggested to be 3×10-9 to

1×10-8 m2V-1s-1 (Yeung, 2006).

Literature Review

8

The migrational mass flux Jj ( gm-2s-1) is related to uj* by:

Jj = - uj* Cj E (2-3)

where Cj is the molar concentration and E is the electric potential (Acar and

Alshawabkeh, 1993). It is important to note that that as such the Nernst-Einstein equation

has not been validated experimentally (Yeung, 2006).

The acid-base chemistry of a soil under the influence of electrokinetic phenomena is

controlled by the migration of hydrogen ions away from the anode, and migration of the

hydroxyl ions away from the cathode. H+ migrates towards the cathode at a rate defined

by the effective ionic mobility, uj* = 760 cm2/Vs. The OH- ions migrate towards the

anode, with a value of uj* of 432 cm2/Vs (Acar and Alshawabkeh, 1993). Given that the

effective ionic mobility of H+ is ~1.8 times greater than that of OH- the system is likely to

be dominated by the movement of the hydrogen, often referred to in the literature as the

acid front.

Ion migration has been shown to be transport mechanism for many aqueous ions; the

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

times the charge on ion. (Acar and Alshawabkeh, 1993). Moreover, changes in the

tortuosity and pore size distribution has been noted to exhibit little influence on the

effective diffusion coefficients of inorganic chemicals (Shackelford and Daniel, 1991).

This is an important implication for mass flux rates – to a limited extent the ionic

mobility is largely independent of the hydraulic conductivity. Values of diffusion and

ionic mobility for common anions and cations are given in Table 2.1.

Literature Review

9

Table 2.1 Diffusion coefficients and ionic mobilities of selected anions and cations.

Species Dj (×10-6 cm2/s) uj (×10-6 cm2/Vs) uj* (×10-6 cm2/Vs)

H+ 93 3625 760

Na+ 13 519 109

Ca+ 8 617 130

OH- 53 2058 432

MnO4- 40 - -

NO3- 19 740 155

The transference number describes the distribution of electrical conduction by ion

migration between various ionic species. The transference number of an ionic species is

both a function of relative mobility and concentration of the species. The transference

number of a selected species increases with increasing aqueous concentration; that is, if

the concentration of a species decreases with respect to the total electrolyte concentration

the mass transport of the target species will decrease. This can potentially become a

problem if ion migration is the dominant mechanism of mass transport for the species in

question.

2.2.2 Electroosmosis

Electroosmosis is the term given to the movement of the bulk aqueous solution through a

porous medium due to applied direct current (DC) electric field. Electroosmosis was first

observed in the 1800’s and was first used in geotechnical engineering to dewater clayey

soils in the 1930’s (Azzam and Oey, 2000). The electroosmotic effect is a result of the

complex electrical interactions that exist at the interface between a solid and liquid.

When a direct current (DC) electric field is applied across a section of saturated porous

media the ions in the pore fluid act as conducting medium, an electric current is

Literature Review

10

generated. The diffuse part of the electrical double layer (EDL) migrates along the

potential gradient, transferring momentum to the bulk pore fluid. For most minerals

surfaces under normal environmental pH regimes (5<pH<9) the diffuse part of the EDL

is dominated by positive counterions, hence net flow is towards the cathode.

In general, soil minerals are negatively charges due to asymmetry in the lattice structure.

Surface charge density changes with mineral type, increasing in the following order:

sand, silt, and clay (Acar and Alshawabkeh, 1993). Under normally occurring pH regimes

in the environment (5<pH<9), clay minerals exhibit a negative surface charge (Grundl

and Michalski, 1995). Indeed, most solid surfaces may bear electrostatic charges when in

contact with a liquid. Glass plates, for example, have been shown have a negative surface

charge when in contact with neutral-pH water (Gu and Li, 2000).

The most popular theoretical rationalization for electroosmosis was introduced by

Helmholtz in 1879 and modified in 1914 by Smoluchowski (Saicheck and Reddy, 2005).

The Helmholtz-Smoluchowski (H S) equation is only rigorously valid where κα >>1; that

is, the thickness of the double layer (κ-1) is small compared to the pore radius (r). The

Smoluchowski equation does not include in its formulation the phenomena of surface

conduction (Saichek and Reddy, 2005). An alternative model proposed by Schmidt

(1950) implies that Ke varies with the square of the average pore radius. Despite the

simplicity of the Smoluchowski model it is still widely regarded and used (Saichek,

2005). Indeed, Paillat et al (2000) has validated the theoretical description of the coupled

electroosmosis and streaming potential phenomena for a simple homogeneous glass

porous media. Furthermore, theoretically predicting electroosmotic flow becomes more

difficult when the medium is a natural one, and the solid mass chemically reacts with the

pore water. The equation is described as:

Ve = Ke ie (2-4)

where Ve is the electroosmotic flow velocity [L/T], Ke is the electroosmotic permeability

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11

[L2/VT] and ie is the electric potential gradient ΔU/l [V/L]. Ke, the electroosmotic

permeability of the medium (an analogue to hydraulic conductivity) is a function of the

dielectric constant of the medium, viscosity of water, zeta potential, and porosity:

μξε nKe = (Azzam and Oey, 2000) (2-5)

Where ε is the permittivity of the medium, ξ the zeta-potential, n is the porosity, and μ the

liquid viscosity. Values of Ke have been suggested from field and experimental work to

generally fall in the range of 1× 10-9 to 10× 10-9 m2/Vs for the bulk of porous media

systems (Yeung, 2006; Musso, 2003). One of the most important implications of this

model is that unlike hydraulic flow, electroosmotic flow is not affected by the pore size.

More importantly however is the result that electroosmotic flow is easier to achieve than

hydraulic (pressure-driven) flow in low-permeability zones such as clay. This is

summarized effectively in Figure 2.1. The relative insignificance of the electroosmotic

permeability to hydraulic conductivity of the selected sediments is highlighted by the

contrast between the blue and green curves.

Figure 2.1 Electroosmotic permeability and hydraulic conductivity for selected sediments. The relative variation of Ke acroos the range of soils is small compared to the variation in hydraulic conductivities

(Electrokinetic Limitied, 2004).

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While electroosmosis is relatively simple to observe in practice, the theoretical aspect is

very complex. Yeung and Hsu (2005), for example, noted the reversal of electroosmotic

flow direction through a clay column midway through their experiments. Grundl and

Michalski (1995) investigated the characteristics of electroosmotic flow through glacial

till, consisting primarily of illite and smectite. They reported flow rates of up to 2.4 mL/h,

using electric fields up to 100V/m. A decrease of electroosmotic flow with time was

noted, thought to be due to a decrease in electroosmotic permeability. Kim et al. (2002)

also noted that the volume of transported pore water decreased gradually through the

electrokinetic experiments, and electroosmosis ceased completely by the end of the

experiments. The successful implementation of electrokinetic remediation based on

electroosmosis flow to transport contaminants requires a working knowledge of factors

which affect the electroosmosis flow.

Electroosmosis has been viewed to be suitable for remediation of clayey soils

contaminated with non-ionic contaminants such as TCE (Popov et al., 2004).

Electroosmosis in clay soils can be enhanced by the addition of certain complexing

agents, which have the function of increasing (more negative) the surface zeta-potential.

Popov et al (2004) demonstrated the enhancement of zeta-potential by the addition of

various chelating agents.

The Electrical Double Layer

At the interface between a solid and liquid the adsorption of some ionic species or electric

charging of the liquid by the solid induces an electrical double layer (EDL) (Hunter,

1987). The EDL consists of a region where oppositely-charged ions – called counterions

– are electrostatically adsorbed to the solid surface. The accepted theory describes the

presence of two different regions of the EDL: the compact region where the counterions

in the liquid are assumed to be hydrodynamically immobile; and the diffuse layer where

the counterions are hydrodynamically mobile (see Figure 2.2). The EDL forms the

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foundation for the study and understanding of many colloidal systems.

The resulting distribution of ions in the solution is therefore a balance of advective,

electrostatic and diffusive processes. The thickness of the compact is in the order of the

ionic radii of the counterions; the thickness of the diffuse layer is a function of the fluid

properties and can be expressed as:

σεδ e

eD

= (2-6)

where ε and σ are the electric conductivity and permittivity of the fluid respectively, Do is

the ionic diffusion coefficient (Palliat, 2000). The arbitrary separation of the double layer

into a mobile and immobile layer has been criticized as unrealistic (e.g. Hunter, 1987),

but while not strictly correct in a statistical mechanical sense, it is a practical method of

modelling and understanding the system. The electrical potential at the plane between the

compact and diffuse parts of the EDL is a particularly important feature of the solid-

liquid interface.

Figure 2.2 The solid-liquid interface, showing the division between the compact and mobile parts of the

electrical double layer (Paillat, 2000).

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The Zeta Potential

The zeta potential can be defined as the electrical potential at the plane of shear between

the compact and diffuse regions of the EDL, and is considered the intrinsic electrical

property of a solid liquid interface. Indeed, the zeta-potential forms a large basis for the

study of colloids and surfaces (Hunter, 1987). The sign and magnitude of the zeta

potential directly relate to the strength and direction of the electroosmotic flow.

Most sediments exhibit a negative surface charge, hence the zeta potential for most

minerals is negative. For example, values of the zeta-potential in most clay types are

generally in the range of 0 to -50 mV (Yeung, 2006). The polarity and magnitude of the

zeta-potential depends on a number of factors including: mineral surface, pore electrolyte

concentration, and pore pH. The point of zero charge (PZC) is defined as the pH at which

the electrical potential at the edge of the compact part of the EDL is 0 mV (Hunter,

1987). Increasing acidity and ionic strength cause the zeta-potential to become less

negative. The relationship between zeta-potential and pH has been noted to vary

significantly for different varieties of clay (Vane and Zang, 1997).

Gu and Li (2000) experimentally determined variability of the zeta-potential of glass

plates in aqueous solution under a variety of pH, electrolyte and surfactant conditions.

The zeta-potential was found to be -62.2 mV for neutral water, and decrease with

increasing pH. At low pH, the selective adsorption of H+ onto the glass surface slightly

counteracts the negative zeta-potential. Similarly, at high pH, adsorption of OH- onto the

glass surface causes a negatively increasing zeta-potential. The voltages often applied

during electrokinetic remediation are sufficient to sustain hydrolysis reactions at the

electrodes. The acidity increase and selective adsorption of H+ onto the solid media at the

anode causes compaction of the EDL, and a decrease in the soil zeta potential and Ke

(Azzam and Oey, 2000). In a similar fashion, the addition of electrolytes weakens the

zeta-potential because of the compressed EDL. The increase in electrolyte concentration

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effectively causes diffusion to ‘push’ the diffuse double layer closer to the glass surface.

The relationship between the zeta-potential and pH for kaolinite has been empirically

determined:

ξ = -38.6 + 281e-0.48 pH (Kim et al., 2002) (2-7)

Streaming Potential

The streaming potential method is the most widely used technique for measuring

properties intrinsic to electroosmosis (Szymczyk et al., 2002). In the same manner that an

electric field may induce bulk pore fluid flow, the advection of the fluid due to a pressure

gradient may create an electric field due to the migration of the diffuse double layer.

When a pressure gradient is applied to a porous medium the ions in the diffuse part of the

double layer are carried towards the low-pressure side. This movement of charges is

called the streaming current, Is. The accumulation of charge at one end of the media

induces an electric field, which causes charge movement in the opposite direction. At

some point a steady state is reached where the current due to the pressure gradient is

equal and opposite in direction to the current due to the electric field.

LUKV ee

Δ= (2-8)

LhKV e

hh = (2-9)

where Vh is the hydraulic flow velocity [L/T], Ve the electroosmosis flow velocity, Kh is

the hydraulic conductivity [L2 T-1], Ke the coefficient of electroosmotic permeability, he is

the hydraulic head difference [L]. The resulting potential difference across the media that

is measured is called the streaming potential, and can be linked back to the zeta-potential

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by the following:

flsUσμξε

= (2-10)

Where Us is the streaming potential, ε is the permittivity of the pore fluid, ξ the zeta-

potential, μ the pore fluid viscosity, and σfl the conductivity of the pore fluid. Note that

streaming potential in this equation is defined per unit pressure difference. Paillat (2000)

observed that the streaming current for glass porous media is proportional to the

hydraulic flow rate D, and inversely proportional to r0 – the hydraulic radius of the

porous media. That is, at a given flow rate the streaming current is higher where the pore

radius is small and the charge density is larger.

An obvious limitation of equation 2-8 is the absence the surface conductivity contribution

to the current conduction. Indeed, this assumption has been shown to result in significant

underestimation of the zeta-potential (Szymczyk et al., 2002).

Another method for measurement of the zeta-potential is the electrophoresis technique in

which the porous material is crushed into fine colloidal-sized particles and dispersed in an

aqueous solution. The electrophoretic movement of the particles is then measured in an

electric field. However, a question which results from using this procedure is whether the

process of crushing the material fundamentally changes the electrical properties of the

medium (Gu and Li, 2000).

2.2.3 Electrophoresis Electrophoresis is the movement of charged colloids and particulates in an applied

electric field. The particle mobility is a function of the zeta-potential on the particle

surface. Indeed, the quantity and variety of electrical charge on the particle surface

controls a great deal of the fundamental particle behaviour in aqueous solutions. The

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electrophoretic velocity of a particle was first described by Helmholtz, and later refined

by Smoluchowski:

μξε EV = (2-11)

Where V is the particle velocity, E is the electric field vector, ε the dielectric permittivity

of the medium, μ the viscosity of the fluid, and ξ is the zeta potential of the particle

surface (Lyklema, 2003). In a similar manner to the solid-liquid boundary of the porous

media (discussed in section 2.2.2) electroosmosis, aqueous ions accumulate at the surface

of suspended particles to satisfy electroneutrality. The zeta potential is defined as in

section 2.2.2, and again varies with the ionic strength and pH of the bulk solution. The

electrophoretic mobility (μe) of a particle can also be defined as:

μξεμ =e (Hunter, 1987) (2-12)

The functional relationship between each of the variables is similar to that used to

describe electroosmosis flow; the relationship between zeta-potential and mobility is

clearly visible. In addition to electrokinetic transport, the mobility of colloids and

particulates in a porous medium also depends upon flocculation, sedimentation,

deposition and sorption/desorption of particles onto the soil matrix. Particles with high

surface charge density often exhibit sufficient electrostatic repulsion to remain suspended

in solution as discrete particles; conversely, reduced surface charge can often cause

flocculation and sedimentation. The mobility and stability of particles is of critical

importance to the field application of nanoscale zero-valent iron (ZVI).

Electrophoresis can apply to a number of different types of particles, including clay. For a

clay-water system, negatively charged clay particles migrate towards the anode (Paillat,

2000). Paillat (2000) also suggested that in a three-phase system containing clay, water

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and oil, the clay particles aligned themselves at the interface between oil and water, and

carried some oil particles towards the anode by electrophoresis.

2.2.4 Relative Contributions

The relative contributions of electroosmosis, electrophoresis and ion migration to

transport of a desired species varies with the nature of the species, soil type, water

content, and electrolyte composition and concentration. A dimensionless number may be

introduced to quantify the relative contributions electroosmosis and migration λe:

e

je K

u *

=λ (2-13)

Where uj* is the effective ion mobility and Ke the coefficient of electroosmotic

permeability (Acar and Alshawabkeh, 1993). λe may change with the soil-water

chemistry, some reasons for which include. Mass transport by ion migration has been

shown to be between 10 and 300 times higher than transport by electroosmosis in

Georgia kaolinite (Acar and Alshawabkeh, 1993).

An important consideration for the use of electrokinetics as a remediation tool is the

direction of each transport. As previously discussed the direction of electroosmosis flow

depends on the point of zero-charge and the pH of the pore solution. Under normal

environmental conditions electroosmosis flow is towards the cathode. The direction of

ion migration depends on the sign of the particle charge. For negatively charged ions such

as MnO4- ion migration is towards the anode – in this instance ion migration and

electroosmosis and ion migration are acting against each other.

The direction of electrophoresis again depends on the point of zero-charge and the pore

solution pH. Under a pH range of ~4-10, the zeta-potential of the polymer-coated iron

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19

particles is negative, and hence transport is directed towards the anode. Again, in this

instance, electrophoresis and electroosmosis are acting in opposite direction to transport

the iron particles.

Figure 2.3 Principles of electrokinetic transport in saturated porous media (Yeung, 2006).

2.3 Principles of Electrokinetics: Aqueous Chemistry

Hydrolysis reactions also influence the electrochemical properties of The aqueous

chemistry of un-enhanced electrokinetics operations are dominated by acidic conditions

produced by the electrolysis of water at the anode. The voltages commonly applied in

electrokinetics are sufficient to cause the ionisation of water. The following equations

dominate at the electrodes:

Anode: 2H20 O2 + 4H+ + 4e- , E0 = -1.229 V (2-14)

Cathode: 2H20 +2 e- H2 + 2OH- , E0 = -0.828 V (2-15)

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where E0 is the standard reduction potential. These equations result in the formation of a

steep pH gradient between the electrodes. The hydrogen ions in solution start advancing

towards the cathode, whilst hydroxyl ions advance towards the anode, both driven by ion

migration. However, the difference in effective ionic mobility (recall mobility is affected

by ion size), the hydrogen ions advance far more quickly than the hydroxyl ions. The

result is that the pore fluid chemistry is dominantly acidic. This is often referred to as an

acid front (Acar and Alshawabkeh, 1993). The acid/base conditions have important

implications for contaminant removal: the acidic condition facilitate the solubilization of

contaminants from the clay mineral surface; the base front causes precipitation of metal

hydroxides which can potentially clog pore space; the presence of hydrogen ions can

affect the electroosmotic flow; and the high concentration of protons may decrease mass

transport efficiency by ion migration.

Several methods of overcoming the soil acidification problem have been suggested in the

literature. One of these involves addition of a weak acid such as acetic acid to the cathode

to buffer the basic front formation (Acar and Alshawabkeh, 1993). The acetic acid

minimizes precipitation of metal hydroxide compounds. Depolarization of the anode

compartment with a weak base compound is also an option available to control the acid

front formation. As with much engineering design a compromise is needed: the reduced

ion migration (from a decrease in transference number) due to the high H+ concentration

must be weighed against the desorption capacity of the pore fluid. The buffering capacity

of a clay is also an important factor in considering the chemistry of a system, and varies

between different clays. Milwhite kaolinite, for example, has a far higher acid buffering

capacity than Georgia kaolinite (Yeung and Hsu, 2005).

The installation of cation-selective membranes between the cathode and soil to prevent

migration of the hydroxyl ions has also been suggested and experimentally verified in

bench scale tests (Li et al., 1988). Specialized electrodes may also be utilized to prevent

the acid and base front formation, but operation and maintenance costs (Athmer and

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21

Huntsman, 2005). More recently, bench trials have shown that external circulation of the

cathode compartment electrolyte to the anode compartment lead to an increase in the

effective remediation period and contaminant removal efficiency (Lee and Yang, 2000).

2.4 Chlorinated Solvents in the Environment

Chlorinated solvents are part of a larger group of common and persistent contaminants,

known collectively as hydrophobic organic compounds (HOCs). Other compounds

included in this group are a range of petroleum products. The many years of wide

industrial use and improper disposal of chlorinated solvents in many cleaning

applications has led to a number of contaminated groundwater sites around the world,

including residential areas in Perth, WA (Benker et al., 1996). Compounds such as TCE

can be harmful to the respiratory, circulatory, and central-nervous systems (Yang, 2001),

while PCE is a suspected carcinogen.

Like many immiscible organic liquids, chlorinated solvents are often introduced to

groundwater as a dense non-aqueous phase liquid (DNAPL). These dense liquids are so

named because they have a higher density than water (the density of TCE is 1.46 g/cm3).

Once in the environment, DNAPL source areas are often difficult to find. In contrast,

light non-aqueous phase liquids (LNAPL), tend to ‘float’ on the groundwater surface

when spilled and are easily detectable (Pankow and Cherry, 1996).

The slow dissolution of a DNAPL in the groundwater can contribute to, and sustain,

contaminant plumes for decades to centuries (Pankow and Cherry, 1996). A number of

physical, chemical, and biological processes act on DNAPLs in groundwater that are of

interest to contaminant hydrogeologists: advection, dissolution, dispersion, volatilization,

vaporization, and sorption to aquifer materials. This section briefly discusses some of

these important processes, and how they can contribute to problems with contaminated

site remediation.

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2.4.1 DNAPL Transport and Distribution

DNAPLs exist in the groundwater in residual or pooled form, where pooled sources

suggest the presence of numerous interconnected DNAPL-filled pores, conversely a

residual DNAPL source is generally confined to single pores spaces (Brewster). Once

introduced to the vadose zone, DNAPLs will migrate downwards, driven by the DNAPL-

water density difference. A DNAPL source will continue migrating downwards until it

reaches a low permeability horizon (E.g. aquitard or clay lens), at which point it will start

to spread out laterally (Pankow and Cherry, 1996). The DNAPL will remain in a pooled

form above the low permeability material until the entry pressure is reached. Often,

DNAPLs can also preferentially migrate into deep, dead-end fractures. Diffusion from a

DNAPL in a fracture into the surrounding material can account for significant

contaminant mass transfer into the surrounding aquifer material.

Indeed, subsurface heterogeneity and preferential flow pathways (Eg. fractures and

macropores) have been shown in field (E.g. Lenczewski 2006; Brewster et al., 1996) and

laboratory (E.g. Jorgensen et al., 1998; Reitsma, 2000) studies to strongly affect DNAPL

transport. The transport and distribution of DNAPLs in the subsurface is affected

significantly by heterogeneity of the porous media. Low permeability layers, fractures

and macropores all act to distribute a DNAPL non-uniformly in groundwater. Moreover,

heterogeneity of the porous media – such as found in dead and pores promotes the

formation of discrete DNAPL zones (Li and Schwartz, 2003; Jorgensen et al., 1998).

Results of field experiments conducted by Brewster et al. (1996) further suggested the

long term mass distribution of a DNAPL source introduced from the surface is focused in

high permeability zones overlying lower permeability zones.

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Figure 2.4 Diffusion of pooled DNAPL source into clay deposit (Pankow and Cherry, 1996).

Fractured porous media, particularly in clay deposits, also act to distribute contaminant

mass heterogeneously though the subsurface; a small volume of DNAPL released in a

fractured media will often distribute over a very large volume of the bulk medium.

Fractures can trigger significant contaminant mass disappearance over small timescales,

particularly where the fracture apertures are small, fracture spacings are small, and the

porous matrix has an appreciable porosity (Pankow and Cherry, 1996). Significant

contaminant mass has dissolved and diffused into the sediment matrix, and hence

appreciable mass can remain in the original source zone location after the source has

disappeared. The mass diffused into the porous matrix will begin to diffuse back out

again when the DNAPL source has disappeared and the fractures are flushed with clean

water.

For a DNAPL source to enter into a fracture network the capillary pressure at the base of

the DNAPL pool must overcome the entry pressure of the fracture, where:

Pcapillary = (ρnw – ρw) g h (2-16)

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In most environmental scenarios, the DNAPL is the non-wetting liquid and water is the

wetting fluid. The entry pressure to t rough walled fracture can be estimated by :

ePentry

θσ cos2= (2-17)

Where e is the aperture of the largest fracture, σ the interfacial tension between the

DNAPL and water, and θ the angle of contact angle with the fracture walls (Pankow and

Cherry, 1996).

2.4.2 Phase Transfer Processes

The phase transfer process occurs at the interface between the DNAPL and the aqueous

solution. It follows that larger DNAPL volumes and surface areas will result in higher

dissolution rates. This was confirmed by Kim and Gurol (2005) during experimental trials

of DNAPL TCE oxidation by potassium permanganate (KMnO4). In addition, dissolution

rates also increase when sited in a well-mixed aqueous solution.

The DNAPL dissolution process has investigated in a number of laboratory and field

studies. Among others, the three dimensional DNAPL distribution – also known as

DNAPL architecture – has a significant impact on dissolution rates. For example, recent

work by Lenczewski et al. (2006) showed the presence of fractures in fine-grained

sediments (Eg. Saprolite) facilitate the rapid dissolution of the DNAPL TCE. The

DNAPL was noted in this case to preferentially distribute through the length of fractures

and macropores in the clay till – glacial sediment – dissolution to occur over of each

fractures. The first experimental evidence of rates of transport and dissolution of a

DNAPL in fractured clayey till Jorgensen et al. (1998) provided first (Glacial deposits).

Clay Till deposits exist in many drinking water aquifers in the northern hemisphere.

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Recent work by Rivett and Feenstra (2005) investigated the field-scale dissolution of

multi-component DNAPL source inserted beneath the water table and exposed to a

natural groundwater flow regime. Most soil cores extracted close downstream of the

source exhibited low (<50% saturation) dissolved organic concentrations. This was

attributed to substantial flow bypassing of the source zone; the source was identified as

the main heterogeneity in the study site, with a higher permeability than the surrounding

aquifer material. The exception was a central plume at saturation concentration, ascribed

to a series of preferential stream tubes carrying undiluted source material to a focal point

(Rivett and Feenstra 2005). This highlights the problems inherent to remediation

techniques such as surfactant flushing and pump-and-treat – the DNAPL source can often

be bypassed by the bulk fluid flow.

Due to the dissolution and diffusion processes, contaminant mass can persist long after

the DNAPL source zone has disappeared. When the source zone has been depleted the

mass in the surrounding low permeability material will start diffusing back into the

fractures or high permeability material.

2.4.3 Diffusion

Over long periods of time a pooled DNAPL will slowly diffuse into zones of low

permeability such as clay layers. The contribution of fractures in clay also exerts a

dominant influence on the diffusive loss of chemical species from fracture networks of

fracture aperture, soil matrix porosity, and the effective diffusion coefficient (Pankow and

Cherry, 1996). Molecular diffusion in an aqueous solution is due to the random,

Brownian molecular motion. Aqueous species migrate from areas of high concentration

to low concentration as described by Fick’s first law:

xCDJ∂∂

−= φ (2-18)

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Where J is the diffusive mass flux [ML-2 T-1] ,φ the soil porosity (dimensionless) , D the

effective diffusion coefficient [L2T-1], C the aqueous-phase concentration [mol V-1], and

x is the distance. The apparent, or effective tortuosity factor relates the free-solution

diffusion coefficient to the effective diffusion coefficient. Diffusion of contaminant mass

from fracture networks in porous media is a significant mechanism of mass loss from a

DNAPL source. Figure 2.5 shows the conceptual diffusion of a chlorinated compound

from a fracture into the surrounding aquifer material. Perhaps most importantly is that

once the DNAPL has been depleted (stage 3 - Figure 2.5), the diffused mass will begin to

diffuse back into the fracture network. In this manner, clay lenses and aquitards can act

as long-term sources of contaminant mass into to the groundwater, after the DNAPL

source has disappeared (Pankow and Cherry, 1996)..

Figure 2.5 The dissolution and diffusion of a DNAPL source trapped in a fracture into surrounding

material (Pankow and Cherry, 1996).

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Pankow and Cherry (1996) presented an analytical solution for the mass transfer from a

single fracture surface into the surrounding soil matrix. The governing equation for

diffusion from a single fracture is:

tC

xC

RD

∂∂

=∂∂

2

2

(2-19)

Where C is the local concentration and R is the retardation factor (which accounts for

sorption onto the soil matrix), t the edge of the aperture, is assumed to be the saturation

solubility. This can easily be solved analytically for the diffusive flux J at time t. This

result can be integrated to determine the total mass diffused per unit area of fracture face:

1/21/2 (RDt) 4 S M

πφ= (2-20)

Where R is the retardation factor, S is the aqueous solubility of the compound in

question, D the effective diffusion coefficient, and φ the porosity. The simplified solution

suggests that within 50 years, DNAPL in fractures with an aperture of 9μm

(trichlorobenzene) to 10mm (Dichloromethane) would disappear completely. Note that

fracture aperture is defined as the distance between the fracture walls.

2.4.4 Sorption and Transformation

Various chlorinated ethenes (e.g. TCE, PCE, VE, DCE) can also be naturally attenuated

in groundwater due to a number of biological, dispersion, dilution, volatilization, and

transformation/destruction (Clement 2000). A field study in 1995 displayed the

prevalence of natural attenuation through anaerobic dehalogenation. The dechlorination

of TCE, DCE and VC were shown to be associated with biological sulphate reduction and

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methanogenesis

Sorption of chlorinated solvents can reduce the mobility and rate of biotic and abiotic

transformations (Pankow and Cherry, 1996). The degree of sorption depends on the

chemical properties of the compounds as well as properties and composition of aquifer

materials. The sorption of hydrophobic compounds largely to organic material present in

the aquifer, and degree of sorption is inferred from the Kow value of the organic liquid

(Schwarzenbach, 2003). Kow is the octanol-water partitioning coefficient and describes

the partitioning of a given compound between a polar solvent (water) and a non-polar

solvent (octanol). It therefore provides an indication of the polarity of a given compound

– a non-polar compound will partition itself with the bulk of the mass in the octanol

solvent.

The degree of sorption for organic liquids will therefore generally depend on the organic

matter content of a given soils and the hydrophobicity of the contaminant. A result of this

is that weakly hydrophobic compounds such as TCE are sorbed onto aquifer material to a

lesser extent than highly hydrophobic compounds such as poly-chlorinated biphenyls

(PCBs).

2.5 Chlorinated Solvent Remediation

Generally, a site contaminated with chlorinated solvents consists of two components, a

source zone and a groundwater plume (Pankow and Cherry, 1996). The primary goal of

groundwater remediation is to return the aquifer water to drinking water quality, and

ideally to return the aquifer to natural background levels, prior to any contamination. It is

important when planning a remediation strategy to consider the location and form of and

source zones, the extent and composition of contaminant plume, as well as relevant

hydrogeological information.

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A number of in-situ treatment methods are available for soil and groundwater

remediation, including: vacuum treatment (also soil vapour extraction), flushing,

surfactants/cosolvent flushing, biodegradation, chemical oxidation/reduction, and thermal

treatment (also steam stripping). Thermal treatment of volatile organic compounds makes

use of the volatilization of NAPLs at high temperatures, which can be collected using

vacuum extraction techniques.

Chemical oxidation and reduction is another emerging technology for treatment organic

contaminants and chlorinated solvents. In-situ application of various chemical reagents,

such as Fenton’s Reagent (Yeh et al., 2003), hydrogen peroxide (H2O2), ozone (Masten

and Davies, 1997), potassium permanganate (Schnarr and Truax, 1998), persulfate

(S2O82-) (Liang and Bruell, 2004), and zero-valent iron (Orth and Gillham, 1996), have

had varying degrees of success. The biologically-mediated reductive dehalogenation of

trichloroethane and TCE has also been shown to occur in microcosms (Barrio-Lage et al.,

1986). The following two sections summarize the use of potassium permanganate and

nanoscale ZVI in the dehalogenation of chlorinated solvents.

There are however many difficulties in the remediation of sites contaminated by organic

liquids such as chlorinated solvents. Examples include: residual NAPL sources decrease

the relative hydraulic conductivity of the medium (hydraulic flow is likely to circumvent

these zones); complex unstructured fracture networks mean that the initial DNAPL

distribution is often difficult to locate; dead-end fractures not connected to active

groundwater flow impede flushing remediation technologies; presence of significant

contaminant mass in low-permeability sediments mean timescales of treatment are often

very large (Pankow and Cherry, 1996). Traditional pump-and-treat technologies, for

example, fail due to the presence of preferential flow pathways. Indeed, any technology

which utilises flushing is likely to be ineffective. Table 2.2 describes the fundamental

remediation patterns of each technique in the subsurface.

Surfactants are another class of compound similar to cosolvents that both lower the

Literature Review

30

interfacial surface tension and increase the solubility of HOCs through a process called

micellar stabilization Surfactant-enhanced electroosmosis remediation techniques

reviewed by Saichek (2005) Surfactant flushing suggested as remediation technology to

increase solubility and removal rates through fracture networks. These methods, however,

are not effective in cases where the DNAPL source has dissolved into surrounding porous

media.

Table 2.2 Summary of the effectiveness of various treatment technologies under heterogeneous sediments (Athmer and Huntsman, 2005).

In general, the remediation approach is constrained by the form and extent of the

contamination at the site in question, and an appropriate method should be chosen based

on site-specific characteristics. For example, McGuire et al. (2006) reviewed the success

of ISCO, enhanced bioremediation, thermal treatment, and surfactant flushing at

removing DNAPL source zones at 59 contaminated sites. They reported median

concentration reduction of 88% of the parent chlorinated compound. The latter two

treatment technologies remediate contaminants by enhancing mobility via micellar

stabilization or phase transfer processes. Unfortunately, while these techniques appear to

be useful on removing DNAPL source zones, electrokinetics is the only one which is

efficient in generating transport though heterogeneous media.

Electrokinetic remediation has also been tested in conjunction with in-situ surfactant

Treatment Process Transport characteristics Heating Pattern

Soil vapour extraction Preferential/non-uniform None

Pump and treat Preferential/non-uniform None

Steam stripping Preferential/non-uniform Non-uniform

Surfactant flushing Preferential/non-uniform None

In-Situ Chemical Oxidation Preferential/non-uniform Non-uniform

Soil Heating None Uniform

Electrokinetics (electroosmosis) uniform Uniform

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31

flushing, with emphasis on application to heterogeneous soils. Saichek and Reddy (2005)

noted considerably enhanced phenanthrene removal efficiency in a heterogeneous soil

system containing kaolinite using electrokinetics. This technique utilises the

electroosmosis phenomena to mobilize contaminant trapped in low-permeability

sediments. Cosolvents lower the interfacial tension of the solvent (water); large

concentrations effectively increase the ‘organic’ character of the mixture. Cosolvent

molecules partially enclose HOC compounds; in effect, the HOC is partially dissolved in

water, and partially dissolved in the cosolvent.

Electrokinetically enhanced bioremediation has also been considered as a possible

technique to enhance bioremediation of organic compounds such as TCE (e.g. Rabbi et

al., 2000). Electrokinetic phenomena are utilised to stimulate and enhance microbial

activity in the subsurface by the delivery of nutrients, electron donors/receptors, and

microbes. Complete biodegradation is however difficult to realise due to the complex

geochemical, biological and physical processes that occur in the subsurface environment

(Rabbi, 2000). Biodegradation technologies are often limited by factors such as

temperature, moisture content, pH, oxygen concentrations, and nutrient supply (Saichek

and Reddy, 2005a)

2.5.1 Potassium Permanganate

Permanganante is an oxometal reagent, and efficiently oxidizes a wide range of organic

and inorganic compounds in an aquifer. Reagents of this sort have been known for many

decades to directly attack carbon double-bonds through direct oxygen transfer (Stewart,

1964; Wiberg and Saegebarth, 1957 from Yan and Schwartz, 1999), and exhibits a high

effectiveness over a wide range of pH 3-12 (Struse et al., 2002).

Potassium permanganate has been used for many years in wastewater treatment

processes, and has proven to be a readily available and effective oxidizing agent in a

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32

number of other applications, including the oxidation of various chlorinated solvents and

other organic contaminants (Li and Schwartz, 2000). The oxidative dechlorination of

various chlorinated compounds has been displayed in many batch scale (e.g. Uryncowicz,

Yan and Schwartz, 1999; Kim and Gurol, 2005) and laboratory (e.g. Thomson, 2000;

Reitsma and Marshall, 2000) experiments. The failure of traditional pump-and-treat

technologies to remove DNAPL source and groundwater plumes from contaminated sites

has prompted many investigations into the feasibility of in-situ application of a variety of

oxidising agents (e.g. Thomson, 2000). In-situ chemical oxidation (ISCO) is an emerging

remediation technology that utilises chemical oxidation as a mass destruction technology

to remove contaminants from soils ‘in place’.

The majority of work utilising oxidising/reducing agents to destroy chlorinated

compounds rely on flushing of the treatment compound through the contaminated area.

However, both porous media heterogeneity and heterogeneous DNAPL distribution can

vastly reduce the efficiency of remediation techniques based on flushing of the source site

with an oxidising agent (Rivett 2005; Hayden 2006).

Advantages of in-situ chemical oxidation include the relatively small cost involved, and

excavation is not required. There are however several disadvantages to oxidant flushing:

plugging of the porous media by reaction by-products (CO2 and MnO2); and modification

of the hydraulic properties of the medium due to the presence of DNAPL (Reitsma and

Marshall, 2000). Reitsma and Marshall (2000) performed various laboratory experiments

to delineate these factors. A DNAPL source was injected into a coarse sand lens

surrounded by fine sand in a three-dimensional tank. They noted that initially most flow

was diverted around the coarse sand lens due to the reduced hydraulic conductivity

caused by the presence of the DNAPL.

The oxidation of chlorinated solvents by aqueous ions such as permanganate occurs only

in aqueous solution or at the DNAPL surface. Thus, the dissolution rate of DNAPLs often

controls the overall rate of oxidation. The fundamental mechanism of TCE oxidation by

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33

KMnO4 was elucidated in the laboratory by Kim and Gurol (2005).

The impact of oxidation products – namely MnO2 and CO2 – on the remediation process

has been studied by Li and Schwartz (2000) in a series of column and tank experiments.

Column experiment realised a 96.9% removal of TCE within 365 hours. The tank

experiments however recorded only a 35% TCE removal after 313 hours – a result of the

formation of stagnation zones and preferential flow pathways, often around source zones.

These are thought to persist due the relatively small interfacial area between the source

and the surrounding aqueous solution, and the porous media at the interface becomes

clogged. The generation of CO2 bubbles has also been observed to transport TCE vapour

(Reitsma and Marshall, 2000).

The overall reaction of potassium permanganate with TCE is given as:

C2HCl3 + 2MnO4- 2MnO2 + 2CO2 + 3Cl- + H+ (2-21)

A description of the intermediate reaction pathways has been described by Yan and

Schwartz (2000). :

C2HCl3 + MnO4- ª I γCA + MnO2 +3Cl- (2-22)

Where I is a cyclic compound, CA is a carboxylic acid and γ is their stoichiometric

coefficient. The reaction rate of the first step of 2-22 has been observed to second-order,

pH-independent over 4 < pH < 8, and the limiting reaction step (Yan and Schwartz,

2000). Equation 2-22 demonstrates the formation of an intermediate cyclic compound

during the reaction process. This paper does not provide a comprehensive description of

the dechlorination reaction pathway; interested readers are referred to works by Yan and

Schwartz (2000) and Kim and Gurol (2005) for further reading.

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34

Table 2.3 Various chemical properties of trichloroethene (Reitsma and Dai, 2000; Schwarzenbach, 2003).

Parameter Value

Diffusion coefficient 10.1×10-6 cm/s

Aqueous solubility 1100 mgL-1

Melting point -73°C

Boiling point 87°C

Density 1.46 mgL-1

2.5.2 Zero-Valent Iron

Despite their diminutive size, nanoscale metal particles potentially represent a cost-

effective solution to a great number of environmental contamination problems (Zhang,

2003). The effectiveness of these particles in the dechlorination of organic contaminants

has been demonstrated in many laboratory (Zhang et al., 1998; Zhang, 2003; Nutt et al.,

2005) and field (Zhang, 2003; Elliot and Zhang, 2001; Quinn et al., 2005) trials. The use

of granular iron in the degradation of pure-phase DNAPL chlorinated solvents has also

been effectively demonstrated in laboratory and field tests (Wadley et al., 2005). The use

of granular zero-valent iron particles has been demonstrated to be both a robust and cost-

effective process to remediate TCE-contaminated sites (Ho et al., 1999).

Zero-valent metals include iron, zinc, nickel and other transition metals; iron is generally

used over other zero-valent metals due to its high availability and low cost. The

mechanisms and kinetics of chemical transformations by granular zero-valent iron (ZVI)

have been described experimentally as early as 1994 (Matheson and Tratnyek, 1994).

Two factors contribute to the effectiveness of using nanoscale metal particles for

remediation of contaminated groundwater: the small particle size (1-100nm) means that

particles can effectively be transported in the groundwater; and they offer a great deal of

flexibility in field applications, whether deployed in a fixed reactive matrix or in mobile

Literature Review

35

slurry mixtures. Research has suggested that nanoscale iron particles may be effectively

used in the dechlorination of organic solvents (TCE, PCE), immobilization of heavy

metals (As, Cd, Cr), and transformation of fertilizers (e.g. NO3-) (Zhang, 2003).

Dehalogenation of chlorinated compounds is accepted to occur at the metal/solution

interface and involves a number of steps: adsorption of contaminant onto metal surface,

chemical reaction on surface, desorption of reaction product, and transport into bulk

solution. Evidence suggests that the dehalogenation of chlorinated compounds by zero-

valent iron occurs in several steps (Matheson and Tratnyek, 1994), where a single

chlorine atom is removed in each step:

Fe + XCly + H+ Fe(II) +XHCly-1 + Cl- (2-22)

where X is a hydrocarbon chain. The reactivity of ZVI depends largely on the reactive

surface area of the iron particles, the metal surface may often form oxide or organic

coatings, which act as reactive barriers. Early experiments have shown that preceding

reductive dechlorination experiments with rinsing of the ZVI with dilute acid solution

increased the rate of reaction, due to the dissolution of organic and unreactive oxides

coating the surface (Matheson and Tratnyek, 1994). Zhang et al (1998) displayed that the

effectiveness of bimetallic particles to be enhanced by a combination of the following:

increased surface area; increasing surface reactivity; and reducing production of by-

products. Liu et al (2006) found that the rate of TCE degradation by ZVI decreased as the

concentration of ferrous iron (Fe(II)) increased, using acid-washed ZVI. Alternately, an

increase in ferrous ions promoted TCE degradation when un-washed ZVI was used. The

reasons behind this are not clear.

Permeable reactive barriers are passive in-situ treatment systems and require no energy to

move the contaminants; they do however rely of the natural dissolution and advection of

the contaminant through the treatment zone. Of a number of chemical agents suggested

for use in permeable reactive barriers, zero-valent iron (ZVI) has shown the greatest

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36

promise for the reduction of a number of environmental contaminants (Liu et al., 2006).

The use of granular iron as a passive treatment method for the in-situ remediation of

contaminant plumes carrying dissolved chlorinated solvents has been studied over the

past decade (Gillham and O’Hannesin, 1994; Orth and Gillham, 1996). Orth and Gillham

(1996) reported very low (3-3.5% of initial TCE) quantities of chlorinated reaction

products, providing evidence of the benefits of granular iron for TCE destruction. The

first field trial of the technology was conducted by O’Hannesin and Gillham (1998) for

trichloroethene (TCE) and tetrachloroethene (PCE).

The technique adopted by Gillham and O’Hannesin (1994) consisted of an excavated

‘treatment zone’, filled with granular iron and sand. Contaminated groundwater then

flows through the treatment zone, resulting in a plume containing dechlorination products

which in the case of chlorinated ethenes such as TCE and PCE are principally ethene and

ethane (Orth and Gillham, 1996).

While the reductive capabilities of nanoscale ZVI have been recognized, there are still

disadvantages to practical remediation applications; namely, the colloid chemistry of

these particles is such that they tend to agglomerate and adhere to surfaces (Zhang, 2003).

Factors affecting nanoparticle movement through a porous media have been described in

depth by many colloid science texts (e.g. Hunter, 1987), and are summarized in section

2.2.3 of this paper. Adams (2006) suggested agglomeration and sedimentation of Fe0

particles as a mechanism that decreased the mobility of the iron particles through the

porous media matrix.

Vinyl alcohol-co-vinyl acetate-co-itaconic acid (commonly ‘poly’) is a favourable agent

for minimizing agglomeration of metallic particles. The polymer is coated on to the

surface of iron particles. It also means that where the zeta-potential of uncoated nanoscale

VZI changes sign at pH~8.1, the zeta-potential of the coated ZVI stays negative over 4.5

< pH <10 (Durant and Zhang, 2006).

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37

Chemistry

Two simultaneous reactions control the fate of zero-valent iron in groundwater –

corrosion by oxygen of water, and reductive dechlorination. Oxygen is the preferred

oxidising agent, and under aerobic conditions the reaction proceeds as:

2Fe0 +4H+ + O2 Fe(II) +2H2O (2-23)

In the absence of oxygen water can also serve as an oxidising agent, and thus iron

corrosion may still occur in anaerobic conditions:

Fe0 +2H2O Fe(II) + H2 +2OH- (2-24)

Equations 21 and 22 suggest dissolved oxygen and water may compete with chlorinated

solvents for electrons from the VZI.

For common chlorinated organic solvents such as TCE, the overall reactions can be

written as:

3Fe0 + C2HCl3 +3H+ 3Fe(II) +C2H4 + 3Cl- (2-25)

Note that in the above equation it has been assumed that the primary reaction product is

ethene (Orth and Gillham, 1996). Recall from earlier in this section that dechlorination is

thought to occur in stages, whereby one chlorine atom is removed at each stage of the

dechlorination process. The daughter products from the above equation would then

undergo the same chemical transformation until all chlorine atoms were removed. Note

from equation 22 that acidity is consumed during the dechlorination process.

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38

Another dechlorination pathway involves the ferrous ion, a daughter product of Fe0:

2Fe(II) + RX + H+ 2Fe(III) + RH + X- (2-26)

where R is an arbitrary hydrocarbon chain.

The effect of pH on the reaction rate of ZVI with TCE was investigated by Chen et al.

(2001), for pH values between 1.7 and 10. The rate of degradation was found to decrease

with increasing pH; degradation was not observed at pH values of 9 and 10, and rates

decreased linearly between 8 and 3.8. Although decreasing the pH increases the rate of

TCE degradation, it also facilitated the disappearance of ZVI to corrosion. The difference

in reaction rates was attributed to precipitation of Fe(OH)3 on the particle surface – at

high pH values Fe(OH)3 has a lower solubility and may precipitate of the ZVI surface,

decreasing the number of reactive sites on the iron particles. This phenomenon was also

observed by Chen et al (2001).

Zero-Valent Iron and Electrokinetics

A hybrid technology called ‘Lasagne’ that combines in-situ treatment technologies with

electrokinetic transport mechanisms has emerged over the last decade. The general

concept of which is to use electrokinetics to transport contaminants into ‘treatment

zones’, where they are removed by adsorption, immobilization, or degradation (Ho et al.,

1995). A field-scale experiment by Ho et al (1999) utilized three treatment zones,

consisting of a slurry mixture of iron filings and kaolin clay between the anode and

cathode plates. The pH gradient problem was overcome in the Lasagne process by the

periodic flow reversal (Ho et al., 1995).

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39

Figure 2.6 Lasagne configuration, consisting of three treatment zones within two electrodes (Ho et al., 1999).

The use of electrokinetics combined with a zero-valent metal reducing agent has been

investigated more recently by Chang and Cheng (2006). Remediation of PCE from

contaminated soils in laboratory batch experiments showed promising results with respect

to maintaining near-neutral pH, removal efficiency, and maintaining original soil

properties.

More recently an approach similar to that used by Ho et al (1999) and Chang and Cheng

(2006) has been investigated. These have focused on the application of electrokinetic

phenomena to transport the treatment agent to the contaminant site, rather than transport

the contaminants to the treatment site. Adams (2006) investigated the application of

electrokinetic phenomena to deliver ZVI through low permeability zones. Attempts to

induce transport of ZVI through one-dimensional columns by electrokinetic and

hydraulic mechanisms were not effective. The iron was also found to form cationic

complexes at the cathode. The electrokinetic transport of VZI was later recognised to

require coating of the nanoscale particles with an amphiphilic synthetic polymer.

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40

Subsequent studies have successfully demonstrated the transport of coated ZVI through

glass porous media using electrokinetics (Jones, in prep.) The aim of this investigation

was to investigate the transport of permanganate ions and nanoscale ZVI particles could

be transported into low permeability sediments in a two-dimensional physical apparatus.

Experimental Methods

41

3 Experimental Methods

The aim of this study was to develop and assess an electrokinetic technique to enhance

transport of permanganate (MnO4-) ions and nanoscale zero-valent iron (ZVI) through

porous media. The experimental methods focused on extending the one-dimensional

microscale work by Jones et al. (in prep.) into a two-dimensional mesoscale model.

Towards this end a number of two-dimensional physical modelling experiments were

conducted to visualise transport of MnO4- and nanoscale ZVI through heterogeneous

porous media under the application of an electric current. This section describes the

experimental materials and procedures used by the author. It also includes a description

of the 1D column experiments that were conducted as a precursor to this study.

One-dimensional column tests were previously conducted by Jones et al. (in prep.) to

quantitatively assess and compare mass transport rates of MnO4- and ZVI through

sediment cores due to diffusion and electrokinetics. The aim of the 2D experiments was

to confirm, in a qualitative sense, that electrokinetics is effective in increasing the rate of

mass transport through low-permeability porous media. The first series of two-

dimensional trials included two bucket experiments, and was designed to investigate the

electrokinetic transport of permanganate ions in a radial direction. The second series of

2D trials took place in a transparent Perspex tank, which simulated the cross section of a

heterogeneous subsurface environment. The simulation of a heterogeneous medium was

achieved by choosing two different media of contrasting saturated hydraulic

conductivities. The porous media used in this study consisted of both glass beads and

artificial pottery clay. The experiments were intended to provide a more realistic

analogue to field-scale remediation conditions than the 1D column experiments.

The proposed electrokinetic remediation technique utilises two well-established

technologies: in-situ chemical oxidation (ISCO) and electrokinetic (EK) techniques. The


42

oxidative capability of MnO4- have been proved in many batch-scale tests (e.g. Kim and

Gurol, 2005; Yang and Schwartz, 2003). Similarly, batch-scale experiments have

demonstrated the use of ZVI as a powerful reducing agent (e.g. Orth and Gillham, 1996;

Golpagar et al., 1997). The effectiveness of oxidant flushing has been demonstrated for

MnO4- for one- and two-dimensional bench-scale tests (e.g. Li and Schwartz, 2004) as

well as field-scale applications (E.g. Schnarr et al., 1998). The oxidation technology has

been well-established over the past decade.

This study focuses primarily on a two-dimensional concept validation for the use of

electrokinetics to transport MnO4- and VZI through low-permeability sediments. This

section describes the experimental materials and methods used by the author, as well as

those used by Jones et al. (in prep.) in their one-dimensional column experiments.

3.1 Column Methodology

Column experiments were conducted by Jones et al. (in prep.) using the apparatus

described in Figure 3.1. These methods have also been developed and utilised in recent

studies by Lee (2005) and Adams (2006). The configuration consists of an

electrochemical cell separated by a saturated sediment core.

The purpose of these trials was to observe and quantify rates of mass transport through a

one-dimensional porous media core under a range of different electrical conditions. Ion

migration and electrophoresis were expected to be the dominant transport modes for both

MnO4- and ZVI respectively, and therefore the anodic reservoir was filled with deionized

water and the cathodic reservoir contained a solution of either KMnO4 or VZI. Transport

of the target species was expected to occur towards the anode for both diffusion and

electrokinetics trials (Figure 3.1).


43

Figure 3.1 Schematic of column apparatus, showing the cathodic reservoir (KMnO4) and anodic reservoir (H2O) connected by a porous media core (Jones et al., in prep.).

3.1.1 Materials and Configuration

A 40gL-1 potassium permanganate solution was prepared by dissolving 40g of KMnO4

crystals (Sigma Aldrich) for every litre of deionised water. The reservoirs had a capacity

of approximately 9L; therefore a total mass of 360g of solid potassium permanganate was

used in the manufacture of the anodic electrolyte.

Two porous media types were chosen for the column experiments. GB7(A) sandblasting

glass beads (Temco Distributors) with a nominal diameter of 150-250μm and specific

gravity of 2.46-2.49 gcm-3 were chosen because of the low cost, availability, relative

consistency of size and shape, and negligible reactivity. Walkers’ Ultra-white Modelling

clay is an artificial kaolinite-based clay and was purchased for its softness, availability

+ -

H2O

Electroosmosis

Electromigration

Diffusion Porous Media Core

Anodic Reservoir Cathodic Reservoir

DC Power Supply

KMnO4

40g/l H20

+’ve electrode -‘ve electrode

Filters


44

and cost.

A Powertech dual tracking DC power supply (model MP – 3092) was used to supply

power to the electrodes throughout this project (Figure 3.2). It was capable of supplying a

maximum voltage of 40 volts and a maximum current of 3 amps. The power supply has

two outputs, which can be used independently or in a master/slave configuration.

Figure 3.2 Powertech MP - 3092 Laboratory Power Supply

The approach for the column experiments involved measuring the electrical conductivity

of the anodic reservoir at regular intervals. The electrical conductivity of a solution is

both a function of the ionic strength and mobility of the constituent ions; electrical

conductivity and ion concentration has been shown to follow a log-log relationship for

simple monovalent and divalent compounds (Zimmt and Odegaard, 1993). Conductivity

can, then, be used in these trials as an indirect measure of MnO4- concentration in the

anolyte bucket, and therefore also a measure of mass transport rate through the core.

Conductivity measurements (μS) were converted to a concentration (ppm) using the

following relationship:

2 µS = 1ppm = 1mg/L (3-1)


45

Rates of mass transport were measured by diffusion and electrokinetics under various DC

voltages. Conductivity measurements were taken using a TPS Conductivity-Salinity-pH-

Temp Meter (Model WP-81). Total dissolved solids (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).

The reservoirs utilised in the previous study by Jones et al. (in prep.) had been used in

previous studies (E.g. Lee, 2005; Adams, 2006), and were used in an unmodified

condition. It is important to ensure that there is no liquid mass loss from the anolyte

solution, as this affects the ion concentration and electrical conductivity of the electrolyte.

As a precaution a silicone sealant was applied to the joins at either end of the core to

minimize leaking from joints, and covers were placed on top of the buckets to minimize

evaporation losses.

Porous media cores were created for the experiments by packing either glass media or

clay into a Perspex cylinder (70mm in length, 50mm in diameter). Cores were left to

saturate fully in deionized water for at least seven days prior to installation between the

reservoirs. The ends of each core were covered with a filter to keep the core in a

compacted condition, while allowing for a full degree of flow across the core.

Mixed metal (Pt/Ti/Cu) electrodes (McCoy Engineering) were employed in both the

anode and cathode reservoirs. Electrodes were approximately 150mm in length and 3mm

in diameter. The electrodes were secured in the reservoirs adjacent to the core for the

duration of the experiments.

3.1.2 Experimental Procedure

A series of experiments was conducted for MnO4- using the configuration described.


46

Each core is connected to the electrolyte reservoirs by inserting the core into the side

port, applying pressure and twisting. The cathodic reservoir was filled with the 40gL-1

MnO4- solution and the anodic reservoir was filled with deionized water. Each

experiment was conducted for a predetermined period of time, with measurements of

conductivity (μS) and TDS (ppt) taken periodically. For the glass core trials, voltages of

20V and 40V DC were utilised, corresponding to a voltage gradient of 2.85 V/cm and 5.7

V/cm respectively. For the clay core trials voltages of 10V and 20V were utilised,

resulting in a voltage gradient of 1.425 V/cm and 2.85 V/cm respectively.

3.2 Bucket Methodology

The bucket experiments were designed to observe the electrokinetic transport of MnO4-

through a clay cylinder in a radial direction. Two bucket experiments were conducted

using the apparatus described in Figure 3.3. A 400 mm diameter plastic bucket with an

inlet/outlet valve at the bottom was chosen for these experiments. The bucket casing

housed a hollowed-out clay cylinder surrounded by glass beads. A stainless steel cylinder

was emplaced in the glass media to act as the anode in the first trial, and cathode in the

second. The configuration was designed so that electrokinetic transport of MnO4- could

be observed in a radial motion through the clay cylinder.

3.2.1 Materials and Configuration

Figure 3.3 illustrates the experimental configuration employed in this section of the

experimental methods. The centre of the bucket was filled with a block of clay (Walker’s

Handbuilding Earthenware) which was rolled into a rough cylinder and hollowed out

with a small well in the centre (Figure 3.4). It was secured to the bottom by applying

downward pressure and twisting. The clay was wrapped in several layers of plastic

flyscreen to assist with handling and rolling. The clay was chosen over the variety used in


47

the 1D column experiments for its rigidity.

Figure 3.3 Plan view schematic of the bucket apparatus. The polarity of the electrodes was interchanged between the two conducted trials.

Surrounding the clay, the bucket was packed with 600-850μm sandblasting glass beads.

The glass beads used in this study have been used in earlier groundwater modelling

studies, and the hydraulic conductivity had been determined to be approximately 225

m/day (Harman, 2002). Figure 3.4 shows the bucket apparatus at two stages of

construction.

-/+

+/-

K2 Clay cylinder

Inner well

Stainless steel cylinder

Plastic bucket casing

Inlet/outlet value

DC

Power Supply

600-850 μm glass beads


48

Figure 3.4 Bucket apparatus showing: a) stainless steel cylinder and clay block in the centre; and, b) clay block with hollowed-out well in the centre, 600-850μm glass beads, and electrodes.

A300mm diameter stainless steel cylinder was used as the outer electrode, and a multi-

component Pt/Ti/Cu electrode similar to that used in the column experiments was used in

the inner well. The Pt/Ti/Cu electrode is one of several that had been fabricated for use in

previous electrokinetic experiments (Adams, 2006), and were chosen based on their

negligible reactivity.

The approach for the bucket methodology was similar to that used for the column trials;

measurements of the electrical conductivity of the cathodic and anodic electrolytes were

used to determine mass transport of MnO4- ions. The conductivity and TDS were

measured with the TPS-81 conductivity meter used in the column experiments. Note that

clay particles have been observed to be transported by electrophoresis towards the anode

(REF), and the mineral lattice may affect the ionic strength of the water. The Powertech

power pack described in section 3.1.1 was utilised in these experiments.

a) b)


49

Figure 3.5 Completed bucket apparatus filled with potassium permanganate solution.

3.2.2 Trial 1: Inward Transport

The first bucket trial was conducted using the stainless steel cylinder as the cathode, and

the mixed metal electrode (McCoy Engineering) as the anode. Transport was anticipated

to occur via electromigration towards the anode, moving radially inwards. The bucket

was saturated with a 40gL-1 potassium permanganate solution (Sigma Aldrich) from the

bottom to upwardly displace trapped air in the pore space. This was achieved by filling

via the inlet/outlet valve using a peristaltic pump (Figure 3.6). A peristaltic pump was

utilised as the pumping fluid is fully contained within a small rubber tube, and does not

contact or damage the pump at any time. The centre well was filled with deionised water.


50

Figure 3.6 Exotech Masterflex L/S peristaltic pump.

When the bucket became fully saturated the power pack was switched on, delivering 10V

DC at approximately 10-20mA. The test ran for approximately 48 hours; conductivity

measurements were taken from the inner well (TPS Australia, Model WP-81

Conductivity / TDS-pH-mV) every 6-8 hours. Following the termination of the test the

contents of the bucket were removed and discarded, and the bucket refilled with new

materials.

3.2.3 Trial 2: Outward Transport

The second trial is identical in concept to the first, except that the direction of ion

transport is reversed; that is, the steel cylinder acted as the anode, and the mixed metal

electrode acted as the cathode. The glass media in the bucket was filled with deionized

water from the bottom valve, using the same filling method as for trial 1. A 40gL-1

potassium permanganate solution was placed in the inner well. A screened PVC well was

placed in the glass media (Figure 3.7) so the conductivity of the glass media pore solution

could be measured. It was constructed by drilling 20 holes through a section of PVC and

wrapping with the same plastic flyscreen used to wrap the clay.

It was important for this experiment to note that the electroosmosis flow is directed


51

towards the cathode (inner reservoir), and so the peristaltic pump was deployed to

remove the excess liquid from the cathodic solution. As a result of the continual inflow of

deionised water, the permanganate solution was slowly diluted, and was intermittently

topped up with 40gL-1 solution (Figure 3.7).

Figure 3.7 Bucket apparatus showing inner well with electrode inserted inside and pump inlet to withdraw excess solution.

The trial was conducted for approximately 144 hours, with conductivity measurements

taken every 6-8 hours. The Powertech power supply delivered 10V at approximately 10-

20mA for the duration of the experiment.

3.3 Tank Methodology

The final series of experiments were designed as qualitative visualisations of

electrokinetic transport through a 2D heterogeneous medium. They took place in a two-

dimensional Perspex tank (constructed by the Department of Civil and Structural

Engineering, University of Western Australia). The purpose of these trials was to

visualize the effects of electrokinetics on the transport of MnO4- and nanoscale ZVI


52

through low-permeability media. The two-dimensional tank apparatus is intended to

provide a more physically realistic model of field conditions than the 1D experiments.

Two different heterogeneous configurations were constructed: Heterogeneous

Configuration A, and Heterogeneous Configuration B. The first tank configuration

(configuration A) was constructed using a single rectangular low-permeability lens

composed of fine 45-90μm glass beads (Burwell Technologies). The background media

was composed of coarse 600-850μm glass beads (Figure 3.8). The second tank

configuration (configuration B) was constructed with ten rectangular low-permeability

clay blocks, set in a background of 600-850μm glass beads (Figure 3.9). A constant

voltage gradient was supplied to the system using the Powertech DC power supply.

3.3.1 Heterogeneous Configuration A

These experiments were designed to simulate a simple two-dimensional heterogeneous

aquifer. The purpose of these was to visually qualify how the presence of the

heterogeneity affected ion transport patterns within the two-dimensional domain and to

ascertain whether EK could enhance the ion transport through the low-permeability zone.

Porous Media

The configuration utilised the placement of the anode into the low-permeability lens, and

the cathode in the background porous media (Figure 3.8). A single rectangular low-

permeability lens (dimensions 100mm x 200 mm x 52 mm), which was composed of 45-

90μm fine glass beads, was emplaced in a background porous medium composed of 600-

850μm glass beads. The 600-850 and 45-90μm glass beads were chosen because they

represented the greatest hydraulic conductivity difference between any of the available

grades of glass beads. Fibreglass filter paper was used to separate the glass media from

the side reservoirs.


53

Figure 3.8 2-D tank apparatus showing the heterogeneous of media and electrode configuration

Tank Packing

Every effort was made to pack the tank homogeneously using the procedure outlined by

Oliviera et al. (1996). The small size of the tank, presence of the metal electrodes, and the

difficulty in handling the small glass beads made this task difficult. This study is not

primarily a quantitative investigation, and as such small inconsistencies in the packing

media were considered unimportant. Two G-clamps was attached to the back plate of the

Perspex tank to prevent bowing of the face plates and significant movement of the tank.

Flow direction

- + DC Power Supply

Coarse 600-850μm glass beads

Fine 45-90μm glass beads

Electroosmosis

Electromigration Electrophoresis

449mm

249mm

Inflow

Outflow

Cathode Anode

Peristaltic pump

MnO4- / ZVI

solution

100mm


54

Tank Configuration

When the tank was fully packed, the constant head reservoirs were set to a height

difference of approximately 12mm The resulting hydraulic gradient was approximately

0.0267, and the resulting flow was from left to right (Figure 3.8). The tank was allowed

to circulate for 24 hours with clean water to displace as much air as possible before trials

were conducted. The input reservoir was filled with the reagent (either MnO4- or ZVI)

which was recirculated through the tank with the Exotech peristaltic pump.

Visualization of 2D tank experiments

The transparent Perspex tank is ideal for process visualization. For each experiment the

transport of MnO4- and VZI was monitored photographically using a JVC 3CCD (5

Megapixel) digital video recorder. An Inaebeam tungsten filament lamp was used to

illuminate the tank. Following each experiment the digital video was downloaded to a

desktop PC and processed to a MPEG format. Screen-shots were retrieved from the

digital video media at 10 minute time intervals to provide qualitative information

regarding the temporal intrusion of the low-K zone by MnO4-.

To provide a quantitative view of the transport of MnO4- through the fine glass media, the

intrusion of MnO4- through the low-permeability zone was computed as a percentage of

the total cross-sectional area. This was achieved by cropping snapshot images to include

the fine glass zone, and running the sequence of images through a MATLAB script. A

shading tool was applied to the MnO4- pink discolouration in each snapshot to provide a

high contrast between uninvaded and invaded regions. The MATLAB script was written

explicitly for this purpose, and computed the invaded cross-sectional area by calculating

the number of pixels with shading above a threshold value (see Appendix A).


55

Power supply

The Powertech DC power supply described in previous sections was utilised to deliver a

voltage of 40V, corresponding to a voltage gradient of approximately 4 V/cm. This was

chosen to obtain maximal transport rates.

Experimental Activities

A total of four trials were conducted with MnO4

- as the target species; a further two tests

were conducted using nanoscale VZI as the target species.

Potassium Permanganate: A 40gL-1 solution was created using the same procedure as

for the column experiments. The first trial with potassium permanganate was run for 2:30

hours with no electrokinetic effects present, investigating the transport of MnO4- through

the low-permeability lens due to advection and diffusion alone. The second trial was run

for 2:30 hours with an electric current established after the first 50 minutes, investigating

the electrokinetic transport of MnO4- through the low-permeability zone. The third trial

was run for a duration of 3:00 hours with electrokinetic effects present for the entire

duration. The fourth and final test was run for a duration of 3:30 hours, and no

electrokinetic effects were present in the first 90 minutes.

Between each test the permanganate solution was removed from the left constant-head

tank and the tank was flushed with clean water for 24 hours or until all permanganate was

removed from the tank. Note that it was not necessary to remove traces of permanganate

from the low-permeability layer in the bottom of half of the tank because it is not of

particular interest in this study.

Zero-valent Iron: Two further trials took place in the 2D tank apparatus focusing on ZVI

as the target transport species. The nanoscale ZVI used in this study was 50-250 nm in


56

diameter and was obtained from Lehigh University. The ZVI was treated with Poly

(acrylic acid) sodium salt (MW ~ 3,000), 40% solution in water (Mw / Mn 1.5), which is

a tri-block copolymer (Polysciences Inc).

A 3gL-1 ZVI solution was created using the following procedure: 1) 30g of iron was

measured; 2) the iron slurry was poured into a bucket containing 10L of deionised water;

3) 1-2mL of PVA synthetic polymer was added to the bucket, and; 4) the mixture was

stirred using a power drill with fluted attachment for 30 minutes. The resulting solution

was approximately 3gL-1. It was recognised throughout the first trial that the 3gL-1 ZVI

solution was not concentrated enough to provide a sufficient visual contrast in the tank

apparatus. A second test was later conducted with a 9gL-1 solution in an identical manner

to the first. Each trial was run for 4:00 hours with an electric current established after the

first 120 minutes.

3.3.2 Heterogeneous Configuration B

The second two-dimensional tank configuration was developed to simulate a more

complex heterogeneous two-dimensional aquifer, and to visually qualify the transport

patterns of MnO4- through the given domain. A heterogeneous medium was created with

coarse 600-850μm glass beads forming the background media and clay blocks / fine 45-

90μm glass beads to form low-permeability lenses. Clay blocks (dimensions of 90mm x

15mm x 52mm) were created using Walkers Ultra White Modelling Clay. A total of four

tests were conducted with the two different low-permeability materials.

These experiments examine a variation on the electrokinetic technique investigated in the

first series of 2D tank trials. That is, both electrodes were placed in the background glass

media, with the small low-permeability lenses located between them (Figure 3.9).

Hydraulic flow through a heterogeneous medium will be dominated by preferential

pathways, where flow bypasses low-permeability zones. Accepted theory suggests that


57

the presence of electrokinetic transport will enhance mass transport rates through

sediments with a wide range of hydraulic conductivities. The purpose of these trials was

to observe the transport of permanganate ions through the clay blocks with and without a

DC current.

Figure 3.9 Schematic of 2D tank apparatus showing clay lenses in a high-permeability background.

The DC current was supplied by the Powertech power supply. To attain a sufficiently

high voltage gradient between the electrodes from the two independent outputs of the

supply were connected in series to produce a DC voltage of 80V. The electrodes were

Flow direction

- + DC Power Supply

Clay/glasss

3

4

5

6

7 8

9

10 11

Advection Eelectromigration

Electroosmosis

1 2

449mm

249mm

Inflow Outflow

Inflow

1 10≈KPeristaltic

pump

MnO4- / ZVI

solution


58

placed approximately 430mm apart; the resulting voltage gradient was approximately

1.86 Vcm-1.

Clay Media

The tank was packed using a method similar to that employed in the first 2D tank

experiments. The clay blocks were lowered into the tank and placed on the background

beads. For a successful visualization technique, the clay blocks were required to be well-

sealed to the front plate of the tank, excluding flow between the clay blocks and the front

plate by hydraulic means. The clay blocks were secured to the front Perspex plate by

applying pressure to the back of the clay blocks with a craft knife. The location of the

clay blocks was marked with a permanent marker on the front face of the sand tank.

The constant head tanks were set to a height difference of 14mm. The resulting hydraulic

gradient was approximately 0.0312, and flow was from left-to-right (Figure 3.10). Figure

3.10 shows the 2D tank apparatus at the commencement and 20 minutes into the first

simulation. The formation of preferential flow pathways around the clay blocks is clearly

visible.

These tests were originally conceived with the intention of using visualization to

ascertain MnO4- transport through the clay blocks. This approach was abandoned after

initial observation of the tank apparatus, showing intrusion of MnO4- over the front face

of many of the clay blocks (Figure 3.10b). The alternative approach adopted involved

removing the clay blocks from the tank after termination of the experiment and dissecting

them to display the final MnO4- distribution through the interior.

The first trial was run for 90 minutes without an electric current, while the second test

was run for 90 with an electric current present.


59

a) b)

Figure 3.10 2D heterogeneous tank apparatus with glass and clay media, showing the formation of preferential pathways around the clay blocks.

Following termination of each test the permanganate solution was flushed from the

reservoirs via valves on the bottom of the tank. The tank was then flushed with clean

water for 1-2 hours to remove the permanganate solution from the glass media, and then

drained by opening both valves on the bottom of each head tank. The clay blocks were

then removed and left to dry under normal room conditions for 12-24 hours to partially

harden. Following the drying period the clay blocks were dissected both laterally and

longitudinally and photographed.

Fine Glass media

The following experiments were designed in response to the low quality of the

visualization of MnO4- transport though the clay blocks described in the previous pages.

The tank configuration is similar to that utilised in pervious experiments, except the clay

blocks were replaced with fine glass beads to act as the low-permeability material. The

experimental configuration is displayed in Figure 3.11. Two trials were conducted: the

first without an electric current present; the second with an electric current present for the


60

full duration of the experiment.

Figure 3.11 Heterogeneous tank configuration utilising coarse and fine glass beads to create a heterogeneous medium between the two electrodes.

Results

61

4 Results

4.1 Column Experiments

Experiments were conducted in a preceding study by Jones et al. (in prep.) to test the

performance of electrokinetics for transporting MnO4- through one-dimensional porous

media cores composed on glass beads and artificial modelling clay. Prior to the

electrokinetic tests with permanganate solution, two initial trials were run using NaCl as

the target species. The NaCl solution was contained in the cathode reservoir, and hence

only Cl- ions migrated towards the anode. Figure 4.1 demonstrates the temporal increase

in concentration of the anodic reservoir, due to the migration of Cl- ions. The zeta-

potential of the glass media is negative under normal pH conditions (Gu and Li, 2000),

electroosmosis is assumed to be towards the cathode. This confirms the assumption that

ion migration is the dominant mode of transport for Cl- through glass media.

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

0.1200

0.1400

0.1600

0 20 40 60 80 100 120 140

Time (Hours)

Rel

ativ

e C

once

ntra

tion

(C/C

0)

Diffusion

20 Volts

Figure 4.1 Comparison of NaCl transport through glass media by diffusion and 20V DC current (Jones et al., in prep.).

Results

62

Following the preliminary investigation of NaCl transport a number of further

experiments were conducted with MnO4- as the target species. Column experiments were

run for a period of approximately 5-6 days, during which varying degrees of

permanganate transport were observed. Discolouration of the cathodic reservoir solution

and the core were visually noted throughout the experiment. The extent of mass transport

was determined by the conductivity measurements taken in each of the reservoirs. The

results of the column tests are all presented as the relative concentration measured in the

cathodic reservoir. That is, the concentration of the cathodic reservoir was scaled against

the concentration of the anodic reservoir.

The rate of mass transport through glass media was highest for the 20V electrokinetics

experiments, and smallest for the diffusion experiments (Figure 4.2). By 170 hours the

relative concentration of the anolyte for the 20V EK test had increased almost fourfold

from the diffusion test. Similarly, the 10V EK test displayed approximately a two-fold

increase in the relative concentration of the anolyte reservoir. The 10V electrokinetic

experiment was reproduced to increase confidence in results; both trials produced

consistent results.

Results

63

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.00 50.00 100.00 150.00 200.00 250.00

Time (Hrs)

Rel

ativ

e C

once

ntra

tion

(C/C

0)

Diffusion

10 Volts

10 Volts Repeated

20 Volts

Figure 4.2 Transport rates of KMnO4 through glass media by diffusion, 10 and 20V DC electric current

(Jones et al., in prep.).

Interestingly, mass transport rates through clay cores do not show the same trends as the

glass media cores. The mass transport rates achieved through the clay cores were highest

for the 10V electrokinetics experiment; when it was expected form the glass media trials

that the 20V trial would be the greatest (Figure 4.3). The diffusion and 10V EK

experiments were repeated for confidence in results. The two diffusion and two 10V EK

tests show consistent results. The 10V results do however demonstrate considerable data

scatter, supporting the suggestion by Paillat (2000) that the electrokinetic characteristics

of a medium are increasingly difficult to predict in more complex natural sediments such

as clays. Moreover, it suggests suggesting that the destabilization and fracturing of the

clay under larger electric currents is both complex and not well characterized.

Results

64

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 50 100 150 200 250

Time (Hours)

Rel

ativ

e C

once

ntra

tion

(C/C

0)

Diffusion Test 1

Diffusion Test 2

10 Volts Test 1

10 Volts Test 2

20 Volts

Figure 4.3 Comparison of KMnO4 transport through clay media by diffusion, 10 and 20V DC electric

current. The diffusion and 10V trials were replicated (Jones et al., in prep.).

Fracturing of the clay cores was observed to varying extents in each of the electrokinetic

tests, and was most prominent for the 20V trial; the fractures were aligned both

perpendicular and parallel to the axis of the column. The observed decrease in mass

transport rates in the 20V EK experiment as compared to the 10V EK experiment is due

to a number of chemical and physical processes that occur in a porous media during

electrokinetic processes, and include: changes to the mineral structure by ion

sorption/desorption; changes to the acid/base chemistry and buffering capacity of the soil;

insoluble precipitate formation; hydrolysis; chemical oxidation/reduction, and; changes to

the pore solution composition (Yeung, 2006).

A combination of these factors may have contributed to the unexpected results observed

in the lower-than-expected results obtained in the 20V EK experiments through the clay

cores. The fact that this observation was not replicated in the glass media suggests the

results are a function of the complex mineralogy of the clay particles. Furthermore, it

Results

65

highlights the complex nature of many types of sediment, and the need to thoroughly

investigate the process efficiency.

4.2 Bucket Experiments

Two bucket experiments were conducted with the aim of quantifying MnO4- transport in

a radial direction through a clay medium. A number of practical issues arose early in each

experiment which proved difficult to overcome. The electroosmosis through the clay and

resulting drying/overflowing of the centre well made any quantitative measurements

difficult. The following section briefly describes each of the bucket trials.

4.2.1 Trial 1: Inward Transport

Several hours after the commencement of the first trial, the inner well of the clay cylinder

was observed to be completely dry. The electroosmosis flow had, in this instance, caused

the liquid in the well to be transported towards the cathode. This caused near-complete

drying and some fracturing of the clay around the inner well. Fractures were primarily

aligned along the axis of the clay cylinder.

These observations dictated that frequent filling of the well with extra deionised water

would be necessary, making ion detection by electrical conductivity impractical. As a

result, the inward transport experiments were abandoned.

4.2.2 Trial 2: Outward Transport

The conductivity of the measuring well was taken at three separate instances prior to the

commencement of the electrokinetic test. Each measurement differed marked from the

Results

66

previous, and is thought to be due to dissolution of clay particles and ion desorption from

the mineral lattice. The significant effect of the clay chemistry made it difficult to use

conductivity as a measurement of permanganate transport. Conductivity measurements

taken throughout the trial were also erratic and differed significantly with the depth of

measurement.

The pump failed at one point during the experiment due to a blockage at the inlet tube

with clay slurry. The centre well over-flowed with permanganate solution (note the

orange-brown discolouration of the clay in Figure 4.4), making any further measurements

of conductivity redundant.

Figure 4.4 Bucket apparatus at the conclusion of the second trial. The clay slurry on top of the glass beads

appears to have risen to the surface at the edge of clay, and spread over the top.

Despite the limitations behind conductivity measurements, the trial was continued for a

full six days in lieu of gaining some visual confirmation of MnO4- transport through the

clay. Upon completion the contents of the bucket were emptied, and the clay cylinder was

dissected to observe the final distribution of MnO4- through the clay.

Results

67

Figure 4.5 Dissected clay cylinder showing manganese and ferrous iron precipitation.

Some precipitation of ferrous iron and manganese is evident in the well, but there was no

obvious presence of MnO4- ions through the clay cylinder (Figure 4.5). The clay slurry on

top of the glass media is another interesting point; the slurry appears to have developed at

the outer perimeter of the clay cylinder, moved parallel to the clay surface, and spread

across the glass surface (Figure 4.4).

These observations contrast those of the 1D column experiments discussed in section 4.1.

The only conceptual difference between the two systems is the different clay. The clay

utilised in the bucket trials was more rigid than that used in the column trials, and

exhibited a more granular texture. A number of properties of the clay may have affected

mass transport rates through it, including: water content; and clay mineral structure

(Paillat, 2000). The electric current applied to the sediment mass may also have been

insufficient to induce significant mass transport in the timescale of the experiment.

Results

68

4.3 Tank Experiments

4.3.1 Heterogeneous Tank Configuration A

Experiments in the 2D tank apparatus were first configured with a single rectangular low-

permeability lens (fine glass beads) located in a high permeability background media

composed of coarse glass beads (Figure 3.8). They were conducted into the use of

electrokinetics to enhance transport of both MnO4- and nanoscale zero-valent iron (ZVI)

through the 2D cross-section of a simple analogue heterogeneous aquifer.

The contrast in hydraulic conductivities was anticipated to create sufficient heterogeneity

in the system. The hydraulic conductivity of the 600-850μm glass has been determined to

be 225m/day (Harman, 2002), and the conductivity of the 45-90μm glass beads was

determined experimentally by the author to be approximately 9m/day. The resulting

conductivity contrast was in the order of ×25.

The conductivity of the 45-90μm beads was determined using the falling head method;

The flow rate through a porous media core is measured under the action of a diminishing

head from some initial height h0 to another height ht, over a time interval Δt. The

hydraulic conductivity is calculated as:

s

c

AAmLK = (3-1)

where L is the length of porous media core , As the cross sectional area of sample, Ac

cross sectional area of water tube attached to top of sample, and m is the gradient of line

ln(ho/ht) against t. h0 the initial height of water from surface of media, ht – Final water

height from media surface (Wilson et al., 2000). The conductivity was determined to be

Results

69

~9m/day (see appendix A).

The results of the four trials with permanganate and two trials with ZVI are presented

below. The distribution of MnO4- ions through the low-permeability zone is easily

visually distinguishable between trials one and two (Figures 4.6, 4.7, 4.8, 4.9). The

intrusion of the permanganate ions is more pronounced ion in the trials with

electrokinetic effects present.

Potassium permanganate: trial 1

a) T=0 min b) T=140 min

Figure 4.6 Tank trial 1 at commencement (a) and termination (b) of the experiment; duration = 140 minutes; no electrokinetic effects present.

Results

70


a) T = 0 min b) T=50 min c) T= 140 min

Figure 4.7 Tank trial 2 at commencement of experiment (a), start of electric current (b), and termination of experiment (c); duration = 140 minutes; electrokinetic effects present from T=50 to T=140 minutes.


a) T=0 min b) T=140 min

Figure 4.8 Tank trial 3 at commencement (a) and termination (b) of experiment; duration = 140 minutes; electrokinetics effects present for complete duration.

Results

71


a) T=0 min b) T=120 min c) T=230 min

Figure 4.9 Tank trial 4 at commencement of experiment (a), start of electric current (b), and termination of experiment (c); duration = 230 minutes; electrokinetic effects present from T=110 to T=230 minutes.

The electrical current applied through the tank increased with time for each trial,

indicating a decrease in electrical resistance of the tank contents. Current conduction

through a pore solution occurs by the migration of ions and charged particles; the

increasing quantity of MnO4- ions increased the conduction capacity of the soil and hence

the power used during the process.

Trial three (Figure 4.8) demonstrated significantly lower MnO4- intrusion of the fine-

glass lens than trial two (Figure 4.7) over the trial period. This may be attributed to the

electrostatic repulsion of the negatively-charged permanganate ions from the cathode

(negative supply terminal). That is, for MnO4- ions on the left of the cathode, ion

migration is towards the left of the tank, away from the fine-glass lens.

The results from each of the four trials are summarised together using the process

outlined in section 3.3.1. To quantify the extent of transport through the low-

permeability zone the cross-sectional area invaded by MnO4- was calculated as a

function of time.

Results

72

Figure 4.10 provides an example of the process used to compute the area at each

time interval. The first row of images was taken directly from the snapshots taken

from the digital video media. The second row of images was edited by manually

applying a shading tool to areas with purple discolouration with a black fill,

providing disambiguation for MATLAB analysis. The process was followed for

each of the remaining three trials. The MATLAB script was then executed for

each collection of photographs. The MATLAB script is located in appendix A.

Figure 4.10 Snapshots of the low-permeability zone taken at 10 minute intervals during trial two. Top images are original colour and the bottom line has been enhanced for the calculation of intrusion area.

The output from the MATLAB script is displayed in Figure 4.11. The contrast between

the first three trials is clearly visible by T=140 minutes. Trial four was run for a different

length of time to the previous three because the advection was significantly lower than

for the other three trials. The reason for this discrepancy was not clear. The height of the

constant head tanks may have been misread; the permeability of he media may have

decreased during previous tests due to formation of MnO2 precipitates or compaction of

the glass beads.

Results

73

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

0 50 100 150 200 250

Time (minutes)

Face

are

a in

vade

d (%

) Normal

Electrokinetic Test 1



Figure 4.11 Cross-sectional area intrusion of MnO4 ions into the low-permeability zone presented as the percentage of face area.

Zero-valent iron: trial 1

Two trials in the 2D tank apparatus were conducted using nanoscale zero-valent iron

(ZVI) as the target transport species. The first was conducted using a prepared 3gL-1 iron

solution; however, this solution was too dilute to achieve a discernable visual contrast in

the porous medium, and results from this trial are emitted from this paper. Following this

discovery, the second trial was conducted with a concentrated solution of approximately

9gL-1. Results from this trial are presented in Figure 4.12. It clearly demonstrates the

transport of nanoscale ZVI through the heterogeneous porous medium. The iron front in

Figure 4.12c has been manually highlighted for clarity.

Results

74

a) T=0 b) T=90 c) T=190

Figure 4.12 Zero-valent iron tank trial 2 at commencement (a), start of electric current (b), and conclusion of experiment (c); duration = 190 minutes; electrokinetic effects present from T=90 to T=190 minutes. Note that due to the low colour contrast, the iron front has been highlighted with a grey line at T=190.

Although no comparative tests were run without electrokinetic effects, these trials

importantly demonstrate that transport of ZVI through a 2D heterogeneous porous

medium is possible, with enhancement of transport occurring due to electrokinetic

effects. 1D column experiments by Adams (2006) using untreated ZVI did not yield any

transport through cores composed of glass beads, thought to be due the particle

flocculation and sedimentation. It appears that the treatment of ZVI with Polyacrylic-acid

(PAA) minimizes flocculation, allowing particles to be transported through the porous

medium.

4.3.2 Heterogeneous Tank Configuration B

A total of four trials were conducted using the configuration described in section 3.3.2;

two conducted with fine-glass lenses, and a further two conducted with clay lenses. The

results form the trials with fine-glass lenses are presented in Figure 4.13. Figure 4.13b

and Figure 4.13d show the tank apparatus at the conclusion of each experiment (T=90

minutes) for the trial without and with electrokinetics respectively. It is apparent from

these photographs that the electrokinetics has enhanced transport of MnO4- ions through

the fine-glass lenses, as demonstrated by the discolouration of the lenses. Theory suggests

that ion migration will be the dominant form of transport through the fine-glass; a

Results

75

uniform electric field between two parallel rods should induce uniform lateral transport,

as evidenced in Figure 4.13d; blocks 9, 10 and 11. This is again a sound indication that

ion migration is the dominant transport mechanism, as suggested by theory. The non-

uniformity of transport seen in the other lenses is likely due to the irregular geometry of

created when packing the tank.

a) T=0 mins b) T=90 mins

c) T=0 mins d) T=90 mins

Figure 4.13 Photographs from fine-glass lens trials without electrokinetics (a,b) and trial 2 with electrokinetics (c,d).

The image of the tank in Figure 4.13b demonstrates that the distinct variation in hydraulic

conductivities between the 600-850 micron glass media and clay blocks was sufficient to

prevent any observable invasion of the clay blocks by MnO4-.

Results

76

The trials utilising clay blocks as a low-permeability medium were initially conceived

with the intention of examining MnO4- transport through the clay blocks by visualization.

It was however noted after the first 20 minutes of the trial that MnO4- ions were observed

to be discolouring the clay blocks. It was difficult however to unambiguously confirm the

transport of MnO4- through the clay lenses by visual means. An alternative approach was

considered, whereby the clay blocks were removed and dissected after the conclusion of

the test to gain visual confirmation of MnO4- transport through the clay blocks. The

dissections were performed by cutting along the length of the block (parallel to the flow

direction) and across the width of block perpendicular to flow direction (Figure 4.14)

Figure 4.14 Dissection of the clay lenses along the x-, y- and z-axis

A comparison of two blocks taken from the trials with and without electrokinetic effects

is presented in Figure 4.15. Block 1 (Figure 4.15b) shows significant colouration

differences to the clay taken from the normal test (left of callipers) and the electrokinetic

test (right of callipers). The block from the electrokinetic test (Figure 4.15a) does not

display the same extent of colouration as block 1, which is likely to be a consequence of

the placement of the clay blocks in the electrokinetic domain. It is also interesting to note

that the MnO4- front extended through the clay block from each of the four edges visible

in the photograph (front, back, left hand, and right hand faces).

Flow

Direction

y x

z

Results

77

a) Block 5 b) Block 1

Figure 4.15 Comparison of dissected clay block from the normal and electrokinetic tests. Blocks on the right of the callipers are taken from the electrokinetic test.

Theory suggests that ion migration should occur in the cathode-anode direction, crossing

equipotential lines at a 90° angle. The electric field between two long parallel rods should

be approximately uniform between the two electrodes (Figure 4.16). Charged species

transport under the scenario described in Figure 4.16 would then be expected to roughly

follow the electric field lines (negatively charged species will migrate in the opposite

direction to the arrows). The intrusion of MnO4- on the right-hand side of the block

(closest to anode) under the application of an electric current (Figure 3.2) was not

expected.

Figure 4.16 Electric field lines between two parallel rods in a single plane.

DC power supply

+ -

Electric field lines

Results

78

The current through the power supply varied throughout the electrokinetic test, from

~60mA at commencement of the experiment to ~470mA at the conclusion of the

experiment, indicating a decrease in conductivity of the conducting medium. The

increasing aqueous concentration of MnO4- provided a greater capacity for charge

conduction through the porous medium. Figure 4.17 displays the trend in electric current

supplied to the tank apparatus throughout the experiment.

0

50

100

150

200

250

300

350

400

450

500

0 20 40 60 80 100

Time (minutes)

Cur

rent

(mA

)

Figure 4.17 Temporal evolution of current through the power supply during the 2D tank trials with glass and clay media.

Block 10 also displayed a relatively low extent of MnO4- transport. This was unexpected

due to its close proximity to the inflow reservoir. The Pt/Ti/Cu metal electrodes,

however, did not extend the full depth of the background glass media and therefore may

not have been exposed to the same electrical forces as those in the upper reaches of the

tank.

The final distribution of MnO4- presents an interesting interpretation on the migration

through the block. Once again, if ion migration is indeed the dominant transport

mechanism through the low permeability clay material, then it may be expected that the

Results

79

transport through the clay would be directed from left to right, towards the anode. It is

interesting then to note that under the electrokinetic conditions block 5 (Figure 4.15a)

exhibits MnO4- transport from the horizontal edges (top and bottom) of the clay block,

reminiscent of mechanical dispersion. It appears the electrokinetic effect has enhanced a

pseudo-dispersion effect through the lateral clay boundaries. An important result of this is

that clay lenses which are long and relatively flat will be invaded from the top and bottom

edges as well as the end boundaries.

Discussion

80

5 Discussion

The results from experimental activities in this study confirmed the results of the

preceding column experiments (Jones et al. in prep.) that ion migration is indeed the

dominant transport mechanism for MnO4- through simple glass and clay porous media.

Results of two-dimensional experiments indicate a significant of MnO4- transport through

the low-permeability materials under the application of an electric current.

The 2D tank experiments further verified that the application of an electric current

through a low-permeability lens enhances transport due to the ion migration. Moreover,

the experiments demonstrated that whilst the presence of heterogeneities clearly affects

the hydraulic transport of MnO4-, electrokinetic transport is not as significantly affected.

Furthermore, the 2D tank experiment also revealed that electrokinetics may be utilised to

transport nanoscale zero-valent iron (ZVI) particles through fine-glass media and that

electrophoresis is the dominant transport mode for the given experimental configuration.

While the experiments conducted in this study made no attempt to investigate the relative

extent of mass transport under various voltage gradients, results of the column

experiments indicated that in general mass transport of MnO4- transport increased with

the applied voltage.

Results of the first bucket experiment yielded no useful results, due to limitations behind

the physical modelling approach. Conversely, the second trial was run satisfactorily for a

pre-determined period of time, following which the clay cylinder was dissected to gain

visual confirmation of MnO4- transport. There was no obvious visual presence of MnO4

-

ions through the clay cylinder. Importantly, however, there was no control bucket-type

experiment conducted with which to provide comparative results. It is therefore difficult

to unambiguously ascertain any transport patterns without a control test. A control test

should consist of an identical physical apparatus without any electrokinetic effects

Discussion

81

present, to observe the diffusion of MnO4- through the clay cylinder.

The reasons for the lack of observable transport through the clay cylinder are not clear.

Several options appear to reasonably support the observations recorded, and can be

summarised as follows: the transport of MnO4- did occur, but the concentration of the

inner well solution was too dilute to gain a strong visual representation of transport

through the cylinder; the electric potential gradient across the clay cylinder was

insufficient to significant mass transport by ion migration, or; the electroosmosis flow

was sufficient through the chosen clay media to overcome the ion migration towards the

anode.

The electroosmosis flow continually diluted the KMnO4- concentration of the inner well,

which would decrease transport via ion migration, via a lowering of the transference

number. It does not seem as reasonable that moving to a different variety of clay would

alter the value of λe (recall section 2.2.4) to the point where a reversal of dominant

transport modes is observed. It therefore seems most likely that while mass transport by

ion migration was not visually observed, it was indeed likely to have occurred.

The two-dimensional tank experiments further supported the electrokinetically-enhanced

transport of MnO4- through low-permeability lenses. After 90 minutes the intrusion of

MnO4- into a rectangular fine-glass lens was significantly greater when an electric current

was applied through the media. The second electrokinetic trial (permanganate trial 3)

highlighted possible deficiencies in the transport process when ion migration is directed

away from the anode and cathode. Figure 5.1 conceptually displays the effect of the

cathode on the transport of MnO4- to the right end of the tank.

Discussion

82

Figure 5.1 Potassium permanganate trial 3; electric current present from start of experimental run. The delayed MnO4

- front to the left of the fine-glass lens is likely a result of ionic migration to the left of the tank, due to the influence of the cathode.

The results from heterogeneous tank configuration B with clay blocks as the low-

permeability media provide an interesting insight into the transport of MnO4- through the

clay. Whereas the theory suggests the formation of a transport front perpendicular to the

current flow, significant vertical transport of MnO4- was observed through the clay blocks

(Figure 4.15a). While the transport front was more pronounced along the left vertical

edge (up-gradient), the transverse invasion of MnO4- on the front and back faces of the

clay blocks was greater than expected. Furthermore, transport of MnO4- through the clay

blocks was observed into the right-hand face of the clay blocks, albeit to a lesser extent

than noted for the left-hand face (Figure 4.15a). This indicates a small but significant

movement of MnO4 in the opposite direction to the ion migration.

As no MnO4- was observed through the clay blocks in the trial without electrokinetics

diffusion is not a likely candidate to describe the distribution of MnO4- through the clay

blocks. Figure 5.1 displays the relative contributing mechanisms to the final MnO4-

distribution. It is hypothesised that electroosmosis may be responsible for the pink

discolouration on the down-gradient side of the clay block (Figure 4.15b). The reasoning

behind the pink discolouration at the top and bottom of the clay is not as easily explained.

Direction of ion migration

Discussion

83

Figure 5.2 Conceptual invasion of clay blocks by MnO4- .

Several possibilities have been considered by the author. One practicable explanation

involves a resistivity-mediated flow-path bifurcation into the low-permeability media.

This is a result of Ohms law, which tells us that an electrical current will take the path of

least resistance, and thus may preferentially migrate through a lower resistance material

to minimize the travel time between electrodes. For this to occur however, the electrical

resistivity of the clay media is required to be lower than that of the surrounding glass

media. This is a complex function of soil matrix properties, soil water ionic strength, and

the mobility of the constituent ions, and is difficult to ascertain at this preliminary stage

of investigation. A second explanation is the instigation of a dispersive-like effect,

produced due to the pore structure and tortuosity of the two different porous media.,

Conceptually, a dispersive effect created by the migration of aqueous species through a

tortuous medium may occur due to the

It is sufficient at this stage of investigation, and indeed for the scope of this study, to note

that electrokinetics enhanced the rate of mass transport through both the vertical and

horizontal faces of t he clay blocks. A benefit of this is that a reactive front would fully

traverse a thin clay layer in a shorter time-scale than if the reactive front advanced solely

from the end face of the block. Further work would prove useful for elucidating the

mechanisms of MnO4- entry into clay blocks when electrokinetic effects are applied. It

Clay block Lateral ion migration

MnO4-

MnO4-

MnO4-

MnO4-

Advection

MnO4-

+-

MnO4-

Discussion

84

would be interesting to observe the temporal evolution of the MnO4- distribution through

the clay blocks, and also to note the timescale required for complete invasion. The spatial

dimensions of trials can be scaled to potential field scenarios to provide estimates of the

timescale required for complete remediation of a low-permeability material.

It should be cautioned that the experiments conducted in this study were for well-

controlled and characterised systems, composed of geometrically simple and chemically

inert glass media (Burwell Abrasive Blasting Equipment, 2004), as well as modelling

clay. In natural sediments there are many electrochemical phenomena that may affect the

composition of the soil matrix and pore solution chemistry, which can either enhance or

inhibit the removal process (Yeung, 2006). To extent this concept to these materials, a

working understanding of the complex chemical and physical properties of the medium is

required, coupled with knowledge of the MnO4- reduction under electrokinetic

conditions.

A number of practical considerations for EK systems in natural sediments that have been

noted to affect the performance of EK techniques, and include: soils with high water

content, high saturation, and low activity are favourable for mass transport rates via

electromigration and electroosmosis (soils of high activity exhibit a strong buffering

capacity and maintain a unidirectional electroosmotic flow, but may require enhancement

to desorb sorbed contaminants); soil type ; soil pH needs to be sufficiently low to

mobilize metal contaminants and sufficiently high to maintain a positive (towards

cathode) electroosmotic flow; acid/base buffering capacity of the soil, and; the zeta-

potential (Yeung, 2006).

Other factors which warrant consideration include: the migration of ions is not selective,

and pore solution with a high ionic strength may decrease the transport efficiency of ions;

acidification of soils generated at the anode may not be environmentally desirable, and

inhibit the remediation process, and; the process is not very cost-effective when the target

species concentration is low and background ionic concentrations are high; the risk of

Discussion

85

secondary contamination from over-utilisation of MnO4-; and field configuration and

implementation (Yeung, 2006).

The proposed technology has several advantages over the more well-established

variations on the electrokinetic remediation process. Established practices utilise

electrokinetic effects to transport contaminants out of contaminated soils, where they can

be removed and treated (E.g Ho et al., 1999; Lageman, 1993; Yeung and Hsu, 2005).

Recently, Rabbi et al. (2000) investigated a method which utilised electroosmosis to

inject nutrients into a clay lens to enhance biodegradation. In a similar manner, the

proposed technology utilises ion migration to transport an oxidising agent (MnO4-) into

clay lenses.

The sorption of dissolved solvent compounds to aquifer materials is another potential

barrier which has impeded complete remediation of soils contaminated with heavy metals

(Acar and Alshawabkeh, 1993; Li et al., 1998). The acidic conditions generated by the

electrolysis of water at the anode is utilised to desorb heavy metal ions sorbed to the

mineral lattice of clay particles. A further advantage of the proposed technology is that

the contaminants are not required to migrate anywhere, and hence the degree of sorption

does not effect the process efficiency.

5.1 Electroosmotic Permeability of the Clay

Jones et al. (in prep.) estimated the electroosmotic permeability of the kaolinite-based

modelling clay to be approximately 5.12 ×10-5 cm2/Vs, using the volume change recorded

in the two electrolyte reservoirs. This value is in agreement with values suggested by

Mitchell (1993, cited by Acar and Alshawabkeh, 1993). Recall however that the

coefficient of electroosmotic permeability depends on the soil pore water ionic strength

and pH, and hence varies considerably throughout the duration of an electrokinetic test.

Discussion

86

Given the estimated values for the electroosmotic permeability and measured transport

rates for MnO4- a dimensionless transport number can be computed as described in

section 2.2.4. The effective diffusion coefficient can be estimated from the diffusion

experiments conducted using the reservoir apparatus. Figure 4.3 gives an approximate

mass transport rate of 0.0568ghr-1 over the 190 hour recording period, corresponding to a

flux rate of 2.01× 10-7 gcm-2s-1. Using the Fickian description of molecular diffusion the

effective diffusion coefficient can then be calculated according to the following:

xC

JD

∂∂

=*

* (5-1)

where the concentration gradient is approximated by a 0.04gcm-3 concentration difference

over the 7 cm column. The value of D* calculated is 3.52× 10-5 cm-2s-1. Using the Nernst-

Einstein equation (2-2) the effective ionic mobility of MnO4- through the clay media can

be estimated. The value of uj* was determined to be 1.5× 10-3cm2s-1, one order of

magnitude outside the characteristic range for common inorganic molecules suggested by

Yeung (2006) (recall section 2.2.1). It should be reiterated that this calculation is very

rough, and should only be considered as an order-of-magnitude estimate.

For the clay media used in this study it is also possible to define the dimensionless

transport ration λe. Equation 2-13 yields a value of approximately 29.3, expressive of the

dominance of ion migration to the mass flux of MnO4- through the clay. Once again,

readers are cautioned that this value is only a rough estimate and does not give any

indication of how the relative mass transport varies during electrokinetic testing.

5.2 Oxidation Efficiency and Potential Barriers

The proposed technology is a coupled electrokinetics and in-situ chemical oxidation

Discussion

87

(ISCO), and as such, the electrokinetic component has been considered in isolation.

While there have been many studies of the transport (E.g. Struse and Seigrist, 2000) and

oxidation efficiency (E.g. Scroth et al., 2001; Schnarr et al., 1998; Yang and Schwartz,

1999) of permanganate ions in clayey sediments, the oxidation process has not, to the

author’s knowledge, been explicitly considered in conjunction with electrokinetics

through a low-permeability medium.

The efficiency of ISCO techniques with MnO4- as the oxidising agent has been

demonstrated to decrease due to pore clogging and reduction of permeability due to the

formation of reaction by-products; principally MnO2 and CO2 (recall equation 2-20). The

formation of low-permeability ‘crusts’, particularly around DNAPL sources has been

noted by Conrad et al. (2003) in their bench-scale experiments to remediate DNAPL TCE

from a heterogeneous analogue aquifer by permanganate solution flushing.

Lee et al. (2003) also observed that the efficiency of the dechlorination reaction of TCE

with MnO4- decreased with time due to the clogging of pores with reaction by-products.,

most noticeably on the interface of the DNAPL source and the outer edge of the TCE

plume In a similar study, Li and Schwartz (2004) demonstrated the solubilisation of

MnO2 precipitates using citric and oxalic acids.

It is reasonably expected that DNAPL sources will not be present in low permeability

lenses such as clays, given the magnitude of the entry pressure. For example, a pool

thickness of 106cm is required for the entry of a DNAPL TCE source into a medium for

which K= 0.001cm/s and porosity is 0.3., and evidence suggests that the density of

chlorinated solvents is sufficient to observe thick pools only rarely (Pankow and Cherry,

1996). Furthermore, clays often possess a hydraulic conductivity of 10-7 – 10-5 cm/s , and

require a higher entry pressure and hence DNAPL pool depth. Moreover, it suggests that

the maximum concentration that might be expected in subsurface clay is established by

the saturation concentration of a given chlorinated organic compound. Clogging of low

permeability sediments such as clays appears to be a minor limitation to the application

Discussion

88

of the proposed technology.

In addition, the reduction of the average pore volume by the formation of insoluble MnO2

precipitates and CO2 gas has been noted to significantly affect the species transport by

advection and dispersion, triggering the formation of preferential flow pathways around

zones of chlorinated organic compounds. The equivalent reduction in pore space,

however, has significantly less effect on the transport of species by electrokinetics (Acar

and Alshawabkeh, 1993; Paillat, 2000). Factors associated with the reaction of MnO4- can

be viewed as minor limitations to the proposed electrokinetic remediation technique.

5.3 Acid/Base Chemistry

The hydrolysis of water at the anode and cathode creates hydrogen and hydroxyl ions

respectively (Acar and Alshawabkeh, 1993). The movement of H+ to the cathode by ion

migration dominates the chemistry of the system, because the effective ionic mobility is

two orders of magnitude higher for H+. The column experiments described in sections 3.1

and were characterised by the transport of H+ towards the cathode by ion migration and

electroosmosis, and the migration of OH- is directed towards the anode by ion migration.

Similarly, in the 2D tank experiments the transport of H+ was directed towards the

cathode by ion migration and electroosmosis, and OH- was directed towards the anode by

ion migration and advection.

The transport of these ions through a soil core has important implications for soil

chemistry and transport regimes. The effect of soil acidification on electroosmosis and

removal of inorganic contaminants has been studied by many researchers (e.g. Grundl

and Michalski, 1996; Kim et al., 2002; Lee and Yang, 2000) over the past decade. The

migration of the acid front towards the cathode causes the compression of the electrical

double layer (EDL) and resultant variation in the zeta-potential of the medium. The sign

of the zeta-potential changes as the hydrogen concentration passes the point of zero

Discussion

89

charge, and as a result the direction of electroosmosis is reversed.

The extent of acidification depends on the buffering capacity of the soil. Buffering

capacity of soils depends on ionic strength of pore fluid, mineral type, and measurement

can not be generalized based on mineral type. A number of methods have been proposed

to mitigate this problem, including circulation of anodic and cathodic electrolytes (Chang

and Liao, 2006), and depolarization of the anodic and cathodic electrolytes (Acar and

Alshawabkeh, 1993). Yeung and Hsu (2005) observed a continued forward

electroosmotic flow direction by the addition of 0.05M NaCO3 added to the anodic

reservoir.

Yang and Schwartz (1999) investigated the oxidative destruction of chlorinated ethenes

by potassium permanganate. They examined the effect, among others, of pH on reaction

rates between MnO4- and TCE. High concentration TCE (0.76 mM) was .reacted with 3.8

mM MnO4- without the pH buffer. A drop in pH from 7 to 2–3 was observed within

several minutes. This was proposed to be a function of the formation of carboxylic acids.

Furthermore, results showed that the reaction rate of TCE with MnO4- was not dependent

on pH for 4 < pH < 8.

The experiments conducted in this study utilise ion migration, rather than electroosmosis

as the dominant transport mechanism; indeed, electroosmosis acts in the opposite

direction to ion migration. Moreover, the possible decrease or reversal of electroosmotic

flow can be considered to be an advantage to the proposed technology, as it will increase

the overall mass transport rate.

5.4 Clay Fracturing

Clay fracturing was clearly observed during the 10 and 20V 1D column experiments, the

two-dimensional bucket experiments, but not during the sand tank experiments. Clay

Discussion

90

fracturing has been noted in many previous studies. Grundl and Michalski (1996)

observed visible fractured of clay under a voltage gradient of 1 V/cm. They did not

investigate the causes of fracturing, but suggested desiccation due to the heating that

resulted from charge conduction. Figure 5.3 demonstrates one possible application of the

proposed technology. The reason the clay blocks were not visually observed to fracture

during the 2D tank experiments may be due to the small duration of the tank experiments

compared to that for the column experiments.

While fracturing was observed in both the 1D column and bucket experiments, it will not

necessarily occur in field situation, due to significant overburden pressure applied by the

overlying sediment material. The fracturing of clay can be considered as an advantage in

the electrokinetic remediation process, serving to increase the oxidation efficiency

5.5 Field Configuration

The field application of the proposed technology must be considered with appropriate

foresight when considering the design of further laboratory and pilot-scale field work.

Yeung (2006) described several operational parameters which affect the success of an

electrokinetics remediation technique, and include: electrode material and shape,

electrode configuration, enhancement techniques. Several observations made during

experimental activities also present for in-depth consideration of implementation of field-

scale remediation schemes.

Figure 5.3 demonstrates one possible field application of the proposed technology. A

source of permanganate ions should be introduced to the subsurface by a simple and cost-

effective method. An approach should be conceived whereby the time and power costs

associated with the remediation process are minimised. The placement of the electrodes

will have an impact on the direction and extent of ion migration through the clay lens.

The two-dimensional experiments conducted in this study were conducted utilising two

Discussion

91

simple cylindrical electrodes in a planar arrangement; however, a number of

configurations are available in a realistic three-dimensional scenario. The optimal spatial

arrangement of electrodes should be an avenue of further investigation.

One potential limitation to the field scale application of the proposed technology is the

requirement to introduce a sufficiently concentrated KMnO4 solution to the subsurface.

Schnarr et al. (1998) injected 10gL-1 KMnO4 solution into the subsurface by six injection

wells, with screened openings located at varying depths.

Figure 5.3 Conceptual application of coupled electrokinetics and ISCO to low permeability sediments.

The choice of electrode material is also critical to the technology success. A recent study

by Lee (2005) highlighted the virtue of the use of non-reactive electrodes in a laboratory

application of any electrokinetic technology. Indeed, Kim et al. (2002) employed

platinum mesh for the anode, whilst titanium mesh was employed as the cathode. Larger

scale field applications of electrokinetic technology have, on the other hand, utilised steel

rods inserted into a ‘curtain’ of granular iron filing and Loresco coke (Ho et al., 1999).

The iron filings were used to minimise production of H+ at the anode (Ho et al., 1999),

while the Loresco coke provides cathodic protection to the steel. This study, as well as its

precursors, utilised mixed metal Pt/Ti/Cu electrodes, and depending on the scale of field

Clay lens saturated with dissolved organic liquids

DC power supply - +

Ion migration

MnO4-

source

Discussion

92

application may prove to be too costly for field use.

Chang and Liao (2006) investigated the effect of electrode composition on various

electrochemical properties of the porous medium. They found that under identical

experimental conditions, graphite electrodes consumed less power than both mixed metal

Pt/Ti electrodes and stainless steel electrodes. Furthermore they noted that after 24 hours

operation of the electrokinetic cell that the pH of the pore solution was approx. 7.0 for the

graphite electrodes, and 4.0 for both the Pt/Ti and stainless steel electrodes.

Conclusions

93

6 Conclusions

The feasibility of a new technology coupling electrokinetic (EK) phenomena with in-situ

chemical oxidation (ISCO) was examined. The proposed technology utilises ion

migration under the influence of an electric current to transport charged reactive species

(permanganate ions – MnO4- and; zero-valent iron – ZVI) into low-permeability lenses

for the chemical destruction of contaminants. This study presents a number of EK

experiments with the specific aim of examining the feasibility of utilising electrokinetics

to transport MnO4- and nanoscale ZVI through porous media. Results indicate that

electrokinetics can be utilised to enhance rates of mass transport rates through low-

permeability porous media (glass beads and kaolinite-based clay) under the detailed

experimental conditions. Furthermore, electromigration and electrophoresis were

confirmed to be the dominant transport mechanisms for MnO4- and ZVI respectively.

This study was primarily a qualitative one, and no attempt was made to rigorously

quantify the effects of EK transport regimes.

There are many benefits of the proposed remediation technique: cessation or reversal of

electroosmotic flow due to pH change is not critical to system performance; clogging of

pore space by MnO2 is not likely to significantly affect system performance; the rate of

chlorinated solvents oxidation by MnO4- is not dependent on pH; the reagent species can

be delivered to the subsurface in concentrations sufficient to generate significant mass

transport by ion migration; sorption of dissolved organic liquids does not effect the

efficiency of the remediation process, and; excess reagent delivered to the subsurface

would oxidise natural organic matter, minimising secondary contamination.

Further work is required to elucidate the nature of electrokinetic transport of MnO4-

through a range of natural sediments with various zeta-potentials (e.g. bentonite).It

should also entail examining the transport and reduction of MnO4- through soils

Conclusions

94

contaminated with chlorinated solvents. Field application of the proposed technology

requires consideration of a number of chemical and physical properties of the system,

including: the effect of acidity on transport rates and oxidation efficiencies;

enhancements methods as necessary and; field materials and configuration.

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Appendix A

101

Appendix A MATLAB script used to calculate the area invaded by MnO4

- as detailed in section 3.3.1 %Program for determining the percentage of a given RGB image that is %coloured black %Author: Michael Gillen %10219736 num_images=24; v_final=zeros(1,num_images); for i=14:num_images %create a string of the path and filename for each picture in the series temp_string=strcat('E:\Uni\matlabpics\EK_1_edit\','0',int2str(i),'.bmp'); new_img=imread(temp_string); [a,b,c]=size(new_img); counter=0; for m=1:a for n=1:b if new_img(m,n,2)==[0,0,0] %test if pixel is black counter=counter+1; end end end v_final(i)=100*counter/(a*b);%percentage of area that is black in colour end v_final (Acar & Alshawabkeh 1993; Athmer & Huntsman 2005; Azzam & Oey 2001; Barrio-Lage et al. 1986; Benker et al. 1996; Brewster et al. 1995; Burwell Abrasive Blasting Equipment 2004; Cauwenberghe 1997; Chang & Cheng 2006; Chang & Liao 2006; Chang et al. In Press; Chen et al. 2001; Cho & Park In Press; Clement et al. 2000; Conrad et al. 2002; Crimi 2005; Crimi & Siegrist 2003; Durant & Zhang 2006; Electrokinetic Limited 2004; Elimelech et al. 2000; Elliot & Zhang 2001; Fu et al. 2003; Fure et al. 2006; Gillham & O'Hannesin 1994; Golpagar et al. 1997; Grundl & Michalski 1996; Gu & Li 2000; Hayden & J 2006; Ho et al. 1999; Ho et al. 1995; Hrapovic et al. 2005; Hunter 1987; Johnson et al. 2004; Johnson et al. 1996; Jones et al. in prep.; Jorgensen et al. 1998; Kang et al. 2004; Kim & Gurol 2005; Kim et al. 2002; Kington 2005; Lageman 1993){Lee, 2003 #18; Lee, 2000 #81; Lee, 2005 #29; Lenczewski, 2003 #34; Lenczewski, 2006 #33; Li, 2004 #19; Li, 2000 #57; Li, 1998 #78; Liang, 2004 #20; Liu, 2006 #69; Lu, 2006 #43; Lyklema, 2003 #71; Mala, 1997 #40; {McGuire, 2006 #45; McNab, 2000 #75; Musso, 2003 #1; Nutt, 2005 #66; Oliviera, 1996 #41; Orth, 1996 #62; Paillat, 2000 #38; Parker, 2003 #51; Rabbi, 2000 #23; Reitsma, 2000 #56; Rivett, 2005 #47; Saichek, 2005 #4}Matheson, 1994 #72; McDowall, 2005 #76}(Saichek & Reddy 2005; Schnarr et al. 1998; Schroth et al. 2001; Schwarzenbach 2003; Seol 2000; Shackelford & Daniel 1990; Song & Carraway 2005; Struse & Siegrist 2000; Struse et al. 2002; Szymczyk et al. 2002; Thomson 2000; Tunnicliffe & Thomson 2004; Urynowicz & Siegrist 2005; Wada & Umegaki 2001; Wadley et al. 2005; Yan & Schwartz 2000; Yang 2001; Yang & Schwartz 1999; Yeung 2006; Yeung & Hsu 2005; Zhang 2003; Zhang et al. 1998)

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