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Transport of Nanoscale Zero
Valent Iron Using
Electrokinetic Phenomena
2006
Angus Adams
10126688
Supervisor: Dr. David Reynolds
Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena
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University of Western Australia ii
Abstract
DNAPL (Dense Phase NonAqueous Liquid) contamination is a large problem facing
today’s groundwater sources. One of the most prevalent classes of DNAPLs is the
halogenated aliphatics class of chemical compounds. Many techniques currently exist
for the treatment of groundwater zones contaminated with halogenated aliphatics, but
are not always feasible due to certain factors, such as cost, aquifer or hydraulic
conductivity restraints. Using electrokinetics to deliver nanoscale zero valent iron to
remediate the contamination zone is one possibility of overcoming such problem sites.
Electrokinetic phenomena are induced by applying a direct current voltage across the
target zone to induce movement of the desired species. Species can be moved via
electroosmosis, electromigration, electrophoresis or a combination of the three.
Traditional electrokinetic studies have utilised electrokinetics to induce movement of
the contaminant to the electrodes. This study however, examines the ability of
electrokinetic phenomena to deliver the treatment chemical (nanoscale zero valent
iron) to the desired zone.
Interaction of nanoscale zero valent iron slurry with varying classes of electrodes was
investigated and found to form iron cation complexes at the cathode. The ability to
transmit nanoscale zero valent iron through a porous media matrix was also
investigated, and found that transmission rates were extremely small. The attempts to
induce electrokinetically driven movement using both the cathode and the anode
indicated that both electrodes were not capable of significant movement of the
nanoscale zero valent iron through the porous media matrix, although the nanoscale
zero valent iron did exhibit a much stronger affinity for the cathode than the anode. It
was also found that the nanoscale zero valent iron was ineffective at penetrating the
porous media matrix under a hydraulic gradient, probably due to the nanoscale zero
valent iron agglomerating to form particles that were too large to effectively migrate
through the porous media matrix. It was thus determined that electrokinetic induced
movement of nanoscale zero valent iron is not feasible in cases where the nanosale
zero valent iron can not be moved due to a hydraulic gradient.
Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena
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Acknowledgements
Dr. David Reynolds’ mentoring and guidance throughout the study;
Cara Moreland for her support and hours of devoted editing;
Diane and Robert Adams for enabling me to get this far;
Matthew Chatley for the brainstorming and workshop skills;
Dr. Ismail Yusoff for his laboratory help.
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Table of Contents
Table of Contents ..................................................................................................... iv
1 Introduction......................................................................................................11
2 Literature Review..............................................................................................13
2.1 Remediation in the Saturated Zone............................................................14
2.2 Methods of Remediation in the Saturated Zone .........................................15
2.2.1 Excavation ............................................................................................15
2.2.2 Pump-and-treat......................................................................................15
2.2.3 Soil Vapour Extraction..........................................................................15
2.2.4 Thermal Treatment................................................................................16
2.2.5 In-situ Flushing.....................................................................................16
2.2.6 Passive Reactive Barriers ......................................................................16
2.2.7 Mass destruction ...................................................................................17
2.2.8 Biological Remediation.........................................................................18
2.2.9 Containment..........................................................................................18
2.3 Zero valent iron.........................................................................................18
2.3.1 Zero valent iron history .........................................................................18
2.3.2 Zero Valent Iron Chemistry...................................................................19
2.3.3 Zero Valent Iron Advantages.................................................................24
2.3.4 Nano-scale Zero Valent Iron .................................................................24
2.3.5 Zero valent iron delivery .......................................................................24
2.4 Diffusion...................................................................................................25
2.5 Electrokinetics ..........................................................................................26
2.5.1 Electroosmosis ......................................................................................27
2.5.2 Electrophoresis......................................................................................28
2.5.3 Electromigration ...................................................................................29
2.5.4 Electrolysis ...........................................................................................30
2.6 DNAPLS and chlorinated solvent contamination of groundwater ..............31
2.7 Agglomeration chemistry..........................................................................32
2.8 Site applicability .......................................................................................33
2.9 Costings....................................................................................................34
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3 Methodology Development ...............................................................................36
3.1 Instrument Calibration ..............................................................................36
3.2 Power supply ............................................................................................37
3.3 Iron Concentration in Slurry Determination ..............................................37
3.4 Single containment vessel experiments .....................................................38
3.5 Dual containment vessel experiment with unhindered flow .......................38
3.5.1 Electrodes .............................................................................................38
3.5.2 Containment Vessel ..............................................................................40
3.5.3 Mixing method......................................................................................41
3.5.4 NaCl experiment ...................................................................................42
3.5.5 Nanoscale Zero Valent Iron Supply.......................................................43
3.5.6 Nanoscale zero valent iron experiment with no porous media................43
3.6 Dual containment vessel experiment with porous media flow and orbital
mixing method......................................................................................................45
3.6.1 Mixing method......................................................................................45
3.6.2 Orbital mixer board construction...........................................................45
3.6.3 Manufacturing of additional side ports in the connecting tube ...............46
3.6.4 Needle selection ....................................................................................47
3.6.5 Silica filling of tube/screen installation..................................................47
3.6.6 Initial containment vessel experiment with porous media flow and orbital
mixing...............................................................................................................50
3.6.7 Sampling technique development ..........................................................50
3.6.8 Second dual containment vessel experiment with porous media flow and
orbital mixing....................................................................................................50
3.6.9 Side port construction in the connecting tube ........................................51
3.6.10 Filling of connecting tube with porous media. ...................................51
3.6.11 Third dual containment vessel experiment with porous media and
orbital mixing....................................................................................................52
3.7 Dual containment vessel experiment with porous media flow and
mechanical mixing method ...................................................................................52
3.7.1 Mechanical mixing................................................................................52
3.7.2 Non-metallic mixing paddle construction ..............................................52
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3.8 Experiment using mechanical mixing........................................................53
3.9 Initial direct injection of nanoscale zero valent iron into porous media
experiment............................................................................................................54
3.10 Direct injection of nanoscale zero valent iron into porous media experiment
with enhanced conductivity...................................................................................55
3.11 Initial hydraulic advection experiment ......................................................56
3.12 Iron concentration sampling......................................................................56
4 Results ..............................................................................................................58
4.1 Iron concentration determination...............................................................58
4.2 Single containment vessel experiment.......................................................58
4.3 Dual containment vessel experiment with no porous media.......................60
4.3.1 Sodium Chloride (NaCl) experiments....................................................60
4.3.2 Initial zero valent iron experiment.........................................................62
4.3.3 Second zero valent iron experiment.......................................................63
4.4 Dual containment vessel experiment with porous media and orbital
mixing… ..............................................................................................................64
4.4.1 Initial experiment ..................................................................................64
4.4.2 Second experiment................................................................................65
4.4.3 Third experiment...................................................................................66
4.5 Dual containment vessel experiment with porous media and mechanical
mixing.. ................................................................................................................68
4.6 Dual containment vessel experiment with porous media and direct
injection................................................................................................................70
4.7 Dual containment vessel experiment with porous media and direct injection
with enhanced conductivity...................................................................................72
4.8 Hydraulic advection experiment................................................................74
5 Discussion.........................................................................................................76
5.1 Iron concentration determination...............................................................76
5.2 Single containment vessel .........................................................................77
5.3 Dual containment vessels with unhindered flow........................................78
5.3.1 NaCl experiment at 20 volts ..................................................................78
5.3.2 NaCl experiment at 10 volts ..................................................................79
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5.3.3 Initial experiment with zero valent iron and porous media.....................79
5.3.4 Second experiment with zero valent iron and no porous media..............79
5.4 Dual containment vessel experiment with porous media and orbital
mixing… ..............................................................................................................80
5.5 Experiment with mechanical mixing .........................................................81
5.6 Experiment with direct injection ...............................................................81
5.6.1 Initial direct injection experiment..........................................................81
5.6.2 Direct injection experiment with enhanced conductivity........................82
5.7 Hydraulic advection experiment................................................................83
6 Conclusion ........................................................................................................84
6.1 Electrokinetics and nanoscale zero valent iron...........................................84
6.2 Recommendations.....................................................................................84
7 Glossary ............................................................................................................86
8 References.........................................................................................................88
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List of Tables Table 2.1 - Suitability of zero valent metals for treatment of various compounds......19
Table 2.2 – Electroosmotic Flux Factors...................................................................28
Table 3.1 – Summary of experimentation stages.......................................................36
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Figure 2.1 – Diagram demonstrating three mechanisms of halogenated aliphatic
degeneration by zero valent iron...............................................................................22
Figure 2.2 – Zero valent iron reactions .....................................................................23
Figure 3.1 – Capped Pt/Ti/Cu electrodes ..................................................................40
Figure 3.2 – Aperture with silicone sealant applied to prevent leaking......................41
Figure 3.3 – Containment vessels with connecting tube ............................................41
Figure 3.4 – Top view of connecting tube with three sampling ports fitted with
flexible tubing ..........................................................................................................46
Figure 3.5 – Side view of connecting tube with three sampling ports fitted with
flexible tubing ..........................................................................................................47
Figure 3.6 – Connecting tube filled with porous media .............................................48
Figure 3.7 – Connecting tube ready for insertion between two containment vessels. .49
Figure 3.8 – Semi filled connecting tube capped with pink screens. ..........................49
Figure 3.9 – Wooden paddle used for mechanical mixing. ........................................53
Figure 3.10 – Connecting tube featuring injection of nanoscale zero valent iron
through the flexible tubing .......................................................................................55
Figure 3.11 – Containment vessel with slit in side for constant hydraulic head .........56
Figure 4.1 – Steel electrodes after operation in nanoscale zero valent iron slurry. .....59
Figure 4.2 – Slurry reaction at cathode. ....................................................................60
Figure 4.3 – NaCl experiment conducted at 20 volts. ................................................61
Figure 4.4 – NaCl experiment conducted at 10 volts. ................................................62
Figure 4.5 – Aged Zero Valent Iron Experiment Iron Concentration.........................63
Figure 4.6 – Aged Zero Valent Iron pH ....................................................................63
Figure 4.7 – Total iron concentration versus time for second experiment without
porous media............................................................................................................64
Figure 4.8 – Voiding along the top of the connecting tube ........................................65
Figure 4.9 – Connecting tube featuring voiding ........................................................66
Figure 4.10 – Voiding due to orbital motion of mixer ...............................................67
Figure 4.11 – Cathode and Anode after experimentation...........................................67
Figure 4.12 – Orbital experiment mixing experiment nanoscale zero valent iron
concentrations ..........................................................................................................68
Figure 4.13 – Nanoscale zero valent iron penetration of porous media......................69
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Figure 4.14 – Cathode and anode after experimentation............................................69
Figure 4.15 – Mechanical mixing experiment nanoscale zero valent iron
concentrations ..........................................................................................................70
Figure 4.16 – Core sample of connecting tube featuring no visible nanoscale zero
valent iron penetration..............................................................................................71
Figure 4.17 – Direct injection experiment nanoscale zero valent iron concentration of
both anodic and cathodic containment vessels ..........................................................71
Figure 4.18 – Iron concentrations for the NaCl dosed direct injection experiment.....73
Figure 4.19 – pH and conductivity record of the NaCl dosed direct injection
experiment ...............................................................................................................73
Figure 4.20 – Amperage drawn during the NaCl dosed direct injection experiment ..74
Figure 4.21 – Core sample of connecting tube after hydraulic advection experiment.75
Figure 4.22 – Hydraulic Advection Experiment Iron Concentrations ........................75
Figure 5.1 – Powered electrodes immersed in a nanoscale zero valent iron slurry .....78
Chapter 1: Introduction
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1 Introduction
Dense NonAqueous Phase Liquid (DNAPL) contamination of groundwater suitable
for human consumption is prevalent throughout the world (Pankow and Cherry 1996).
DNAPLs can reside in the groundwater for years, providing a source of pollution for
decades (Pankow and Cherry 1996). One of the most prevalent DNAPLs to pollute
current groundwater supplies is trichloroethylene (TCE) (Westrick 1983).
Numerous treatment strategies exist for remediation of polluted groundwater, such as
excavation, pump and treat, passive reactive barriers and containment. These
strategies often are not feasible for many contaminated sites due to the characteristics
of the site. Many treatment strategies are not effective for sites posessing a low
hydraulic conductivity as they rely on hydraulic soil flushing.
Electrokinetics, also known as electroreclamation, electrokinetic soil processing,
electrokinetic extraction, electrodialytic remediation and electrochemical
decontamination is the application of a DC current to induce the movement of
chemical species. Electrokinetic phenomena comprise of (i) electromigration – the
movement of charged ions due to an electric potential difference, (ii) electrophoresis –
the movement of colloids or macromolecules due to an electric potential difference
and (iii) electroosmosis – the bulk movement of water due to an electric potential
difference. Electrokinetics is not affected by the hydraulic conductivity of the soil
matrix, and thus has the potential to be a treatment technique for soils possessing low
hydraulic conductivities (Van Cauwenberghe 1997). Traditional electrokinetic
remediation techniques often rely on the elecktrokinetic movement of the contaminant
to the electrode. This study focuses on the ability to move a treatment compound –
nanoscale zero valent iron – to the source of the contamination.
Zero valent iron posseses the ability to degrade halogenated aliphatics, such as TCE.
The zero valent iron oxidises halogenated aliphatics, yielding a dehalogenated
aliphatic and an iron cation (Matheson 1994). Nanoscale zero valent iron has been
shown to be even more effective than granular zero valent iron at reducing
halogenated aliphatics (Gavaskar et al. 2005b).
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Due to the inability of electrokinetics to induce movement of DNAPLs, this study’s
purpose was to investigate the ability of electrokinetic phenomena to transmit
nanoscale zero valent iron through a saturated porous media matrix. Two
containment vessels were hydraulically connected via a connecting tube filled with
porous media. The anode was positioned in a containment vessel and the cathode in
the other. The nanoscale zero valent iron was then placed in one containment vessel
and the other was monitored for an increase in iron concentration. Multiple trials
were conducted to test both the anode and the cathode for nanoscale zero valent iron
transmission. Various methods of suspending the nanoscale zero valent iron were
also tested. Samples were then analysed using an atomic absorption spectroscopy
(AAS) for total iron content. The interaction between a nanoscale zero valent iron
slurry and powered anodes and cathodes was also investigated.
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2 Literature Review
Freeze and Cherry (1979) define groundwater contaminants as “all solutes introduced
into the hydrologic environment as a result of man’s activities” and groundwater
pollution as when “contaminant concentrations attain levels that are considered to be
objectionable”. Another definition of groundwater contamination provided by Miller
(1980) is “the degradation of the natural quality of groundwater as a result of man’s
activities”. Matthess (1982) believes polluted groundwater occurs when the
concentration of the contaminant exceeds the maximum permissible concentration for
potable water.
To put the expectations of groundwater remediation into perspective, billions of
dollars have been spent by the petroleum industry to increase yields of hydrocarbons
from identified reserves. Whilst this great sum of money has been invested in
hydrocarbon extraction, the industry considers an exceptional yield to be between 30-
40% of the total mass of petroleum products. Contrastingly, it is a normal occurrence
to expect a 99.9% removal of contaminants from a polluted groundwater source to
consider it remediated (Pankow, 1996).
Groundwater contamination and pollution has been recognized as early as the mid
nineteenth century, as evidenced by Dr. John Snow’s work connecting seepage from
privy vaults to the cholera contamination of wells in 1854 (Malman and Mac 1961).
The problem of groundwater contamination is a vast one. It was estimated that it
would take 4 to 5 years to conduct one series of test on the public water supply wells
in Illinois, which represent just below 7% of total wells in the state (Illinois EPA
1986).
In 1982, it was estimated that one percent of economically producible groundwaters
were contaminated. This contamination may be more significant than the figure
implies, due to many of the contaminated sites being in close proximity to heavily
populated areas (Gass 1982).
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Groundwater contamination can occur in four distinct ways (Barcelona et al 1990):
i) Infiltration. This is the most common mechanism for contamination of
groundwater to occur. It involves the contaminant moving from the
surface to the groundwater below it through pore spaces in the soil.
ii) Direct Migration. This occurs when a source already within the saturated
zone leaks into the surrounding groundwater, such as a pipeline.
iii) Interaquifer Exchange. The mixing of uncontaminated groundwater with
contaminated groundwater when the bodies of water are hydraulically
connected.
iv) Recharge from Surface Water. When contaminated surface water bodies
come into contact with nearby groundwater.
Sources of groundwater contamination fall into six different categories (OTA 1984):
1) Sources designed to discharge substances
2) Sources designed to store, treat, and/or dispose of substances;
discharge through unplanned release
3) Sources designed to retain substances during transport or
transmission
4) Sources discharging substances as consequence of other planned
activities
5) Sources providing conduit or inducing discharge through altered
flow patterns
6) Naturally occurring sources whose discharge is created and/or
exacerbated by human activity.
2.1 Remediation in the Saturated Zone
Various techniques exist for groundwater remediation in the saturated zone.
Irrespective of the technique utilised to clean up a contaminated site, factors such as;
i) Soil characteristics, heterogeneity and complexity
ii) Groundwater characteristics, heterogeneity and complexity
iii) Geochemical characteristics, heterogeneity and complexity
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must be analysed as they all influence remediation strategies (Henry et al 2002).
There are three classes of DNAPL remediation techniques used today. They are;
i) Containment
ii) Dissolved-Phase Destruction
iii) Saturated Zone Removal.
2.2 Methods of Remediation in the Saturated Zone
2.2.1 Excavation
The simplest method of remediating contaminated groundwater in the saturated zone
is by excavation, where the contaminated zone is excavated and removed. Suitable
excavation sites can be limited by cost, size and accessibility of contamination.
2.2.2 Pump-and-treat
Another technique is the ‘pump and treat’ method, in which a series of wells are
constructed to withdraw the contamination via pumping. Pump-and-treat is the most
used technique for remediation of chlorinated solvent sites (Henry et al 2002). The
application of this technology consists of extracting groundwater from one or more
strategically constructed wells. The contaminated material is then collected and can
then be treated externally. Pump and treat methods can be prohibitively expensive
and are also influenced by hydraulic conductivity, and in some cases, have operated
for long periods of time (sometimes over a decade) without appreciably reducing the
contamination concentration (Pankow 1996). Pump-and-treat techniques are ‘best
thought of as a management tool to prevent, by hydraulic manipulation of the aquifer,
continuation of contaminant migration’ (Mackay et al 1989), which highlights the
limited abilities of such technology.
2.2.3 Soil Vapour Extraction
Soil vapour extraction is the most accepted technique for in-situ contaminant
remediation in the vadose zone (Henry et al 2002); however it can be applied to the
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saturated zone in some cases, which is known as multiphase extraction. It involves
applying a strong vacuum (up to 660 mm of mercury) to subsurface soils and
groundwater (Henry et al 2002). Air sparging can be conducted simultaneously with
soil vapour extraction, which agitates the targeted zone with air bubbles to volatilize
the contaminants, which are subsequently extracted. Soil vapour extraction is
however limited by its inapplicability to many sites, such as deeply penetrating and
hard to get to DNAPL contamination zones, as well as sites of low hydraulic
conductivity. (Henry et al 2002).
2.2.4 Thermal Treatment
To enhance contaminant removal, certain sites can be treated thermally. Thermal
treatment involves increasing the temperature of a contaminated zone to increase
volatility and vapour pressure of the contaminant/s, which can then be removed via
soil vapour extraction. Thermal treatment is limited to sites that can use soil vapour
extraction, and can also prove to be relatively expensive in the generation of heat
(Henry et al 2002).
2.2.5 In-situ Flushing
Injection of a chemical agent into the contaminated zone to increase solubility and/or
mobility is referred to as in-situ flushing. Typical additives for flushing involve co-
solvents (often in the form of alcohol), and surfactants (Henry et al 2002). In-situ
flushing is limited in that it is not applicable to soils with low hydraulic conductivity
(Thal 2006), and is only suitable for treating the most permeable sections of the
contamination site (Henry et al 2002).
2.2.6 Passive Reactive Barriers
A widely used technique to treat contaminated groundwater in-situ is by utilising
Passive Reactive Barriers (PRB). The location of the PRB must first be ascertained,
then the existing soil must be excavated and the void space filled with the reactive
medium with relatively high hydraulic conductivity.
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Some of the initial field testing of PRBs was done using zero valent elemental iron
filings (Gillham 1993) (Gillham 1994). More exotic remediation chemicals emerged
following zero valent iron’s success, such as dissolved chemicals and genetically
engineered bacteria (Pankow, 1996), however, the material of choice for use in PRBs
is zero valent iron (Henry et al 2002). Incorrect understanding of the frequently
complex hydrogeology of various contamination sites can lead to incorrect barrier
wall placement, which can leave contaminated zones outside of the barriers untreated,
such an example is the Hill Air Force Base in Utah, which left 3000 gallons of
DNAPL untreated outside of the installed barrier wall (Henry et al 2002).
Passive Reactive Barriers have proven to be effective at treating a great number of
contaminated groundwater plumes; however, they have certain limitations (Pankow
1996):
i) They only target contaminant plume, and not the source of contamination.
They therefore have to wait for the contaminant to be leached into or
advected with the groundwater before treatment can be initiated.
ii) They are unfeasible solutions in certain situations of complex
hydrogeologic conditions, such as fractured rock.
iii) Most Passive Reactive Barriers have been installed to a depth of
approximately 15 metres, although there have been instances of depths up
to 35 metres (Henry et al 2002). They cannot penetrate deep into the soil,
rendering them wholly ineffective with deep plumes, as they cannot reach
the target zone.
iv) A comprehensive understanding of the hydrogeologic conditions at the
contamination site is required for this technology to work, as the
positioning of PRB is of utmost importance.
2.2.7 Mass destruction
Mass destruction techniques are sometimes also employed, in which a reactive
chemical is pumped to the contaminant source zone, and is flushed throughout the
zone. Chemicals such as permanganate (MnO4-), hydrogen peroxide (H2O2), sodium
(Na), potassium (K), perchlorate (ClO4), ozone (O3) and certain enzymes have been
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used in the past to oxidise organic contaminants. Reducing chemicals, such as
sodium salts of dithionite (Na2S2O4) have also been used in the past (Henry et al
2002). However, these techniques require tight controlling of chemical conditions,
such as pH and eH values, and are often costly due to the relatively expensive nature
of the chemicals involved. Hydrogeoligic structure and flow paths can also limit the
effectiveness of such techniques (Pankow 1996), thus decreasing the viability of this
technique for low hydraulic conductivity zones. Henry et al (2002) states that the
chemical additives remain largely in the most permeable zones, and thus rarely reach
lesser permeable zones.
2.2.8 Biological Remediation
The majority of bioremediation approaches rely on stimulation of biodegradation by
the addition of organic carbon. The current effectiveness of biological remediation is
limited, as demonstrated by field observations which reveal a persistence of
hydrocarbons at treated sites (Henry et al 2002).
2.2.9 Containment
Various containment strategies exist for trapping a contaminant source zone or plume
inside an impermeable barrier and preventing it from spreading further without
treatment of the contaminant. As this is not a remediation technique, and merely a
prevention of additional contamination, it shall not be considered further.
2.3 Zero valent iron
2.3.1 Zero valent iron history
Nano-scale zero valent iron is an exciting technology for treating contaminated
groundwater. Iron was first recognised and patented in 1972 as a chlorinated
pesticide degrader (Sweeny 1972). In 1981, Sweeny (1981a 1981b) utilised iron
powders to degrade various hydrocarbons, such as trichloroethylene. Additional
suggestions for using zero valent iron to degrade trichloroethylene and
trichlorotethane were made in the late 1980’s by Senzaki (1988). However, it was not
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until after this point in time that focused work was conducted on using zero valent
iron to remediate polluted groundwater. Work published in the 1990’s revealed the
power of iron at remediating contaminated groundwater (Reynolds et al 1990),
(Gillham et al 1992). In 1993 a patent was lodged by the University of Waterloo for
using zero valent iron for treating contaminated groundwater in-situ, demonstrating
the identification of zero valent iron as a remediation constituent.
2.3.2 Zero Valent Iron Chemistry
Zero valent iron has been shown to react and degrade many types of chemicals
(Gavaskar 2005a), including halogenated aliphatics, polyhalogenated aromatics and
nitrates (Zawaideh 1997) and trichloroethene (Henry et al 2002). A table listing the
various compounds zero valent iron has proven to reduce is present in Table 1.1
below.
Table 2.1 - Suitability of zero valent metals for treatment of various compounds
(Henry et al 2002).
Treatment Material Contaminants Treated Untreatable Contaminants
methanes dichloromethane
ethanes 1,2 dichloroethane
ethenes aromatic hydrocarbons
Zero valent metals propanes polychlorinated biphenyls
chlorinated pesticides chlorobenzenes
freons chlorophenols
nitrobenzenes
Cr, U, As, Tc, Pb, Cd
The standard half reaction for zero valent iron reacting to yield a ferrous cation and 2
electrons is:
Fe0 Fe2+ + 2e- This reaction has a standard reduction potential of +0.44 V (Atkins 1998).
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Alkyl halides have a typical half reaction as such, where RX indicates a halogenated
hydrocarbon, and X- represents a halogen anion:
RX + 2e- + H+ RH + X- These types of half reactions have reduction potentials ranging from +0.5 V to +1.5 V
at pH 7 (Matheson 1994), the variation is attributed to the wide range of alkyl halides
that this reaction applies to.
When combined, these two half reactions yield a thermodynamically spontaneous
reaction:
Fe0 + RX + H+ Fe2+ + RH + X-
This constitutes the most basic mechanism for halogenated hydrocarbon degradation
by zero valent iron, yielding a ferrous cation, an aliphatic hydrocarbon and a halogen
anion (Matheson 1994).
A second mechanism for degeneration of halogenated hydrocarbon by zero valent
iron is the oxidation of zero valent iron to a ferrous cation by water (Matheson 1994).
The ferrous ion then further oxidises to a ferric cation by the following half equation:
Fe2+ Fe3+ + e- This oxidation reaction can be coupled with the reduction half equation to reduce the
alkyl halide shown above to yield:
Fe2+ + RX + H+ Fe3+ + RH + X-
This is a second mechanism for the degradation of an alkyl halide (Matheson 1994)
by zero valent iron and is portrayed in Figure 2.1 as reaction (B).
Matheson (1994) describes a third mechanism, portrayed by reaction (C) in Figure
2.1, which involves the zero valent iron reacting with water to yield the ferrous cation,
the hydroxyl anion and hydrogen gas (H2). It is a combination of the two following
half reactions:
Fe0 Fe2+ + 2e-
H2O + e- H2 + OH-
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To produce:
Fe0 + 2H2O Fe2+ + 2H2 + 2OH-
The H2 gas generated can then continue on to react with an alkyl halide in a reaction
known as an addition reaction to yield a dehalogenated aliphatic, a halogen anion and
a proton in the following manner:
RX + H2 RH + X- + H+ It is important to note that the H2 can only react with the alkyl halide if a suitable
catalyst is present. Matheson (1994) states that the iron surface, defects or additional
solid constituents may provide such catalysis.
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Figure 2.1 – Diagram demonstrating three mechanisms of halogenated aliphatic
degeneration by zero valent iron (Matheson 1994).
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It is important to note that the protons generated from the above reactions are capable
of further reducing chlorinated hydrocarbons. Subsequently, it has been shown that
hydrogenation is insignificant, and an oxide layer encapsulates most iron particles.
Alkyl halides are now thought to react with zero valent iron in corrosion pits in which
Fe0 is exposed, as shown in the top most diagram in Figure 2.2, for the oxide layer
acting as a semi-conductor to facilitate the reduction-oxidation reaction, as shown in
the middle diagram in Figure 2.2, or for the oxide layer to coordinate Fe2+ to reduce
the alkyl halide (Center for Groundwater Research 2002).
Figure 2.2 – Zero valent iron reactions
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(Center for Groundwater Research, 2002)
2.3.3 Zero Valent Iron Advantages
Using zero valent iron has the following advantages as a remediation technology
(Zawaideh 1997):
i) It is relatively inexpensive
ii) It is non-toxic
iii) It degrades certain chemical faster than other techniques of remediation,
such as biotic remediation
iv) It has a high energy effectiveness.
2.3.4 Nano-scale Zero Valent Iron
Nano-scale zero valent iron is more effective at reaching deep zones of
contamination, and is more effective at contaminant degradation than iron of larger
sizes (Geiger et al 2003). Nano-scale zero valent iron can induce greater rates of
reaction because of its greater specific surface area, which allows a greater exposure
of the iron particle to the contaminant per unit weight of iron than other larger
particles (Tratnyek 2003). Additionally, as particle size decreases and tends towards
10nm, thermodynamic properties, such as work-function and free energy begin to
alter and can increase reactivity (Campbell et al 2002). Gavaskar et al (2005b) has
found that nanoscale zero valent iron is significantly more reactive than granular iron,
and states that it can remediate a plume in a much shorter time scale. Additionally,
injection of nanoscale zero valent iron has proved to be less arduous (Gavaskar
2005b). Henry et al (2002) states that nanoscale zero valent iron has a superior pore
penetration ability when compared to larger particulate zero valent iron.
2.3.5 Zero valent iron delivery
Delivery mechanisms for nanoscale zero valent iron to DNAPL source zones include
pneumatic fracturing and injection, direct push injection and closed-loop recirculation
wells (Gavaskar 2005a), (Quinn et al 2005). Difficulty in administering the zero
valent iron to the target area has been expressed using these methods. Electrokinetics
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has the possibility of providing a solution to the problem of administering nanoscale
zero valent iron to the intended zone.
2.4 Diffusion
Diffusion in a liquid medium is the net flux of a certain constituent from a zone of
higher concentration to a zone of lower concentration (Quickenden, 2003). Diffusion
can also be explained by the Second Law of thermodynamics, which states that ‘The
entropy of an isolated system increases in the course of a spontaneous change’
(Atkins 1998). Diffusion occurs irrespective of bulk fluid motion or electrical
potential gradients.
Diffusion can be described by Fick’s laws, the first being:
dx
dCDF !=
where
F = mass flux of species per unit area per unit time.
D = diffusion coefficient.
C = solute concentration.
dx
dC = the rate of change of concentration with respect to distance.
The negative term is used to specify that bulk motion is from higher concentration to
a lower concentration, and no vice-versa (Fetter 1994).
Fick’s second law stats that the rate of change of concentration with respect to time is
equal to the product of the diffusion coefficient and the rate of change of the rate of
change of the concentration with respect to distance (Fetter 1994).
2
2
dx
CdD
dt
dC=
Where dC/dt = rate of concentration change with respect to time.
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The diffusion process when flowing through a porous media is slightly different, due
to two reasons. One being that the distance a particle must travel is increased due to
the fact that the particle must flow around the media. The second reason is that a
large percentage of the cross sectional area of the length the particle is flowing
through is blocked by the presence of the porous media. To account for this, the
effective diffusion (D*) is calculated in the following manner;
D* = wD where w is a determined empirical coefficient, typically ranging between 0.5 to 0.01.
A nonempirical relationship was determined in 1971, such that:
D* = D (√τ) Where τ = tortuosity (the actual length of flow path divided by the straight-line
distance between the start and end point of the flow) (Fetter 1994).
2.5 Electrokinetics
Electrochemical remediation has a number of terms, such as electrokinetic
remediation, electroreclamation, electrokinetic soil processing, electrokinetic
extraction, electrodialytic remediation and electrochemical decontamination. All these
terms refer to the application of a low-intensity direct electrical current (DC) between
an anode and cathode situated at the site of contamination to induce or increase one or
more transport processes (Lageman et al 2003). In a soil matrix, electric current tends
to be conveyed through micropores, which is the location of many contaminants, such
as DNAPLs (Lageman et al 2003). Several phenomena arise from the application of
such an electric field, such as:
1) Electroosmosis
2) Electrophoresis
3) Electromigration
4) Electrolysis
The first pioneering effort of using an electric field to improve the chemical quality of
soils was in the 1930s. Puri and Anand (1936) used an electric potential difference to
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determine if it could influence the extraction of sodium ions in soil. Utilisation of
electroosmosis was used by Casagrande (1948) to stabilise soil formations by
dewatering. Electrokinetic research boomed in the late 1980s and early 1990s
(Lageman et al 2003), with more than 400 papers written in this time. It is an
indication of the significant level of interest in this field.
2.5.1 Electroosmosis
Electroosmosis is the bulk movement of fluid due to the imposition of an electric
field. Most soil surfaces possess, to some degree, a charge, predominantly a negative
polarity. The negative charge present on the surface of the particles attract positive
ions to them, forming what is know as a ‘double-layer’, or ‘zeta-potential’ (Zeng
2001). When an electric field is applied, the positive ions accumulated at the surface
of the particles begin to move towards the cathode. This movement also draws the
surrounding fluid with it via friction (Electroosmosis 2006), thus initiating water flux.
The water flow rate is determined by the forcing due to a potential difference, and the
frictional forces experienced at the solid-liquid interface. Total flow rate (qA) is
determined by
AL
Vkq eA
=
Where
ke = electroosmotic permeability
L
V = electrical potential gradient
A = cross sectional area.
Electroosmotic flow can be determined by:
Aikq ee=
where q is electroosmotic flow rate, ke is electroosmotic permeability, ie is electrical
potential gradient and A is the area normal to the flow. Electroosmotic flow can be
affected by a number of factors, and are summarised in the table below:
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Table 2.2 – Electroosmotic Flux Factors
Electroosmotic Flux Affecting Factor Effect
Electric field Changes in proportion to potential applied Buffer pH Increases as pH increases Ionic Strength Decreases as ionic strength increases Temperature Decreases as temperature increases
Organic Modifier Generally decreases as concentration increases
Negative Surfactant Increases as concentration increases Positive Surfactant Decreases as concentration increases Neutral hydrophilic polymer Decreases as concentration increases
Electroosmosis induced flows are not affected by pore size (Zeng 2001). Therefore
electroosmosis has the potential to be an effective mechanism for treatment of soils
sites which feature poor hydraulic conductivity due to pore sizing and therefore
difficult to treat using methods reliant on hydraulic conductivity.
2.5.2 Electrophoresis
Electrophoresis is the movement of colloids or macromolecules induced by an electric
field. Due to the varied nature of colloidal particles and macromolecules, it is
extremely difficult to characterise electrophoresis. Probably the largest sector that
deals with electrophoresis is the biotechnology sector. Gel phoresis is used
extensively in this field to spearate nucleic acids and proteins base on their ability to
move through a gel under an electric (DC) potential.
The force (Fe) experienced by a charged particle under an electric gradient is
EqFe !=
where
q = charge
E = electric field
This force is countered by the frictional force (Ff), which acts against the movement
of the particle.
fvFf !=
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where
v = velocity of the particle
f = friction coefficient
These two equations can be used to determine the effective electrophoretic mobility
factor (µ), where;
E
v
f
q==µ
This derived effective electrophoretic mobility is not necessarily a good
approximation for nanoscale zero valent iron because of its physical and chemical
properties. Factors such as particle size, surface charge density, pH and solution ionic
strength all have an influence on the effective electrophoretic mobility (Taylor et al
2004).
The Smoluchowski equation derives a relationship between the zeta potential and
effective electrophortic mobility as such (Taylor et al 2004):
!
"#µ =
where
ξ = zeta-potential
ε = electric permitivity
η = viscosity
Electrophoretic induced movement is difficult to characterise for nanoscale zero
valent iron, due to the varying nature of the nanoscale zero valent iron particle size
distribution and effective surface charge.
2.5.3 Electromigration
Electromigration is the movement of charged species, such as Fe2+ or OH- to the
electrode of opposite charge. Migrational flux (Jjm) is dependant on effective ionic
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mobility, electrical potential, valence and temperature (Acar, 1993). The relationship
of migrational flux and its variable is as follows:
)(*EcuJ jj
m
j !"##=
where *
ju = effective ionic mobility
cj = molar concentration
E = electrical potential
Although no completely correct method to determine effective ionic mobility has
been devised, extending the Nerst-Townsend-Einstein relation yields:
RT
FzDu
jj
j
*
*=
where *
jD = effective diffusion coefficient
zj = valence
F = Faraday’s constant
R = universal gas constant
T = absolute temperature
The effective ionic mobility of a species is typically an order of magnitude larger than
the effective diffusion coefficient and, assuming a unit electrical gradient, is
approximately 40 times the valence (Acar 1993). This highlights the importance of
electromigration, and it’s much larger influence on mobility of charged species than
the diffusion mechanism.
2.5.4 Electrolysis
Electrolysis is the application of an electric current to induce a non-spontaneous
chemical reaction, such as the splitting of H2O to H+ and OH- (Atkins 1996). When
electrodes are inserted into an aqueous medium, two important reactions may take
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place, generation of oxygen gas and protons at the anode, and generation of hydrogen
gas and hydroxyl ions at the cathode, as shown below:
2H2O O2 + 4H+ + 4e- anode
4H2O + 4e- 2H2 + 4OH- cathode
It is important to note that the pH of the bulk solution remains constant as the number
of protons produced equals the number of hydroxyl ions produced.
2.6 DNAPLS and chlorinated solvent contamination of groundwater
DNAPLs are defined as Dense NonAqueous Phase Liquids. The term is used in
hydrogeological circles to describe a liquid that is immiscible with water and has a
specific gravity greater than water. When situated with water, DNAPLs form a
separate phase and do not mix to any significant degree. A great number of DNAPLs
are chlorinated hydrocarbons, such as trichloroethylene (U.S. Geological Society
2006).
Dense nonaqueous phase liquids accumulate in groundwater as pools that can slowly
release contaminants into the surrounding groundwater over multiple decades
(Pankow and Cherry 1996). DNAPLS are a real threat to groundwater quality
because of their ability to migrate below the water line in aquifers and their persistent
presence once there (Groundwater Protection and Restoration Group 2006), that they
remain the largest cleanup problems (Anonymous 1995) and are amongst the most
prolific groundwater contaminant (Pankow and Cherry 1996).
Remediation of DNAPLs subsurface pools have been shown to rapidly collapse the
pollutant plume originating at the DNAPL pool. It is therefore important to remediate
the pool of DNAPL, and not concentrate solely on the emanating plume (U.S.
Geological Society 2006).
The Ground Water Supply Survey (Westrick 1983) found that one of the two most
prevalent volatile organic chemicals in groundwater was trichloroethylene. This
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highlights the significance of trichloroethylene as a contaminant of groundwater
supplies, and thus the importance of developing strategies to reduce such
contamination.
Chlorinated solvents are problematic for a number of reasons:
1) Their high volatilities lead people to believe that it is safe to
disposal of solvents by pouring them on the ground, and that it
would all volatialise into the atmosphere. Although a large amount
does, a significant component of the solvent can penetrate the soil
and enter the groundwater.
2) The high densities enable the solvent to easily penetrate through
the vadose zone and the groundwater zone.
3) The low absolute solubilities result in the contamination having a
long life span, because it can not be effectively dissolved away by
the groundwater
4) The high relative solubilities result in the saturated level of
chlorianted solvents to be much higher than the safe concentrations
for human consumption.
5) The low interfacial tension between chlorinated solvent and water
allow the solvent to penetrate into small pore spaces.
6) The low degree of retardation by soil material results in the
solvent not being significantly retarded and thus allowing the
chlorinated solvent to move with the groundwater
7) The low degradability of chlorinated solvent result in the
substance remaining in the groundwater for a long period of time.
2.7 Agglomeration chemistry
Zero valent iron is not a polar substance, and carries no overall charge (hence the
zero-valent term). However, zero valent iron has been known to agglomerate and
form colloidal particles (Thomas, D., 2005, pers. comm., 16 September), which
requires an attractive driving force to draw the particles together. Since they do not
have an overall charge, an explanation for the formation and maintenance of these
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colloids is that they are held together by Van Der Waals forces. It is the effect of
these Van Der Waals forces that result in the polarity of zero-valent iron, which thus
leads to agglomeration. It was attempted to exploit this polarity by using
electrokinetics to induce the zero valent iron to move from the position of application
to the desired position.
2.8 Site applicability
Electrokinetic phenomena have been used with success in many distinctly different
soil types (Acar 1997). Electromigration rates are not particularly dependant on fluid
permeability, rather pore water electrical conductivity and tortuosity. Electrokinetic
remediation is a viable technique in both saturated and unsaturated zones (Van
Cauwenberghe 1997).
Electrokinetics is suitable for zones of low hydraulic conductivity (Van
Cauwenberghe 1997). In such soils the low hydraulic conductivity makes traditional
soil flushing techniques such as pump-and-treat unfeasible. This fact is immensely
important, as electrokinetic inducement of nanoscale zero valent iron may prove to be
a solution to halogenated aliphatic groundwater contamination in zones which are not
suitable for techniques amenable for sites with high hydraulic conductivity.
Before a site can be deemed suitable to be electrokinetically remediated, certain
parameters need to be ascertained (Van Cauwenberghe 1997). Spatial electrical
conductivity variability must be examined at the site to determine if it will interfere
with the voltage gradient. Pore water pH must be determined to gain an
understanding of how it may affect the nanoscale zero valent iron. Pore water
electrical conductivity must also be taken into account, to establish the anticipated
efficiency of the technique. The chemical make-up of the soil and pore water must
also be considered as it has the potential to interact and react with the nanoscale zero
valent iron (Van Cauwenberghe 1997).
Electrokinetics does have limitations however, and is not suitable for use irrespective
of the site. Electrokinetic remediation is not suitable when the pH conditions are such
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that the anode is susceptible to unacceptable levels of corrosion. Sites which contain
chemical species that may influence the pH when exposed to an electrical gradient
must be examined for suitability. Foreign anomalies such as rubble or metallic
building bodies may affect the effectiveness of the electrokinetic phenomena (Van
Cauwenberghe 1997).
2.9 Costings
Nanoscale zero valent iron varies in price depending on supplier and current market
prices. Gavaskar (2005a) quotes prices varing from US$ 20/lb to US$ 70/lb. Factors
such as raw material cost, licencing fees and manufacturing costs all impact on the
price of nanosccale zero valent iron.
PRBs using zero valent iron have lower costs than pump-and-treat and have higher
initial outlay, but maintenance and long-term operation costs are lower (Henry 2002).
Commercially, the longest running PRB costs US$ 50 000 per year, as opposed to the
US$ 300 000 spent before on the same site using pump-and-treat.
Factors which influence costings include (Van Cauwenberghe 1997):
1) Electricity price
2) Labour cost
3) Initial contaminant concentrations
4) Target contaminant concentrations
5) Conductivity of pore water
6) Concentration of other ions.
7) Soil characteristics
8) Moisture content
9) Extent of contaminantion
10) Zone preparation
Acar (1997) found the energy expenditure to electrokinetically remediate a site to be
between 325 kWhm-3 to 700kWhm-3. Assuming an energy cost of 10 cents/kWh, this
translates to $33 m-3 to $70 m-3. Van Cauwenberghe (1997) quotes energy
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consumption rates of 500 kWhm-3, which corresponds to a cost of $50 m-3. Further
prices quoted from various vendors range from $25 m-3 to $300 m-3 (Van
Cauwenberghe 1997).
Gavaskar (2005a) conducted field trials on three separate sites using zero valent iron
and found that it cost US$ 289 000 to treat a contamination site in Hunters Point
(USA) of 1287 m3 in size containing 6.4 kg of TCE. Another site in Jacksonville
(USA) of size 1265 m3 containing an estimated 27.7 kg of TCE cost US$ 412 000 to
remediate. A third 12426 m3 site in Lakehurst (USA) incurred a cost of US$ 255 500
to remediate. The three sites give costs of $US 224 m-3, US$ 326 m-3 and US$ 21 m-3
respectively. The breakdown of the costs of remediating the three sites is not
consistent, as is the type of zero valent iron used, and therefore the cost per cubic
metre is not entirely consistent. This discrepancy in cost between the sites may also
be due to differences in TCE source zone location, TCE contamination extent, extent
of remediation and aquifer variability.
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3 Methodology Development
The experimental procedure was developed throughout the experimental period. The
experimentation began with investigation of effects of both the cathode and anode in
contact with a nanoscale zero valent iron slurry. The mass transport of chemical
species known to be susceptible to electrokinetic effects (sodium and chloride ions)
between two hydraulically connected containment vessels was then conducted.
Following this, electrokinetic mass transport of nanoscale zero valent iron was
attempted between a congruent pair of hydraulically connected containment vessels.
The experimental set-up was then transmogrified to simulate a groundwater
environment more closely by forcing the nanoscale zero valent iron to flow through
porous media. Mixing methods to maintain the nanoscale zero valent iron suspended
in solution was also experimented with. Induced movement of nanoscale zero valent
iron that had been directly injected into the porous media were also attempted.
Finally, movement of the nanoscale zero valent iron by application of a hydraulic
gradient was trialled. These major steps are summarised in the table below, and
documented in more detail later in this chapter.
Table 3.1 – Summary of experimentation stages Stage Experiment 1 Nanoscale zero valent iron interaction with electrodes in a single containment vessel
2 Electrokinetic movement of nanoscale zero valent iron between two hydraulically connected containment vessels
3 Electrokinetic movement of nanoscale zero valent iron through porous media featuring orbital mixing
4 Electrokinetic movement of nanoscale zero valent iron through porous media featuring mechanical mixing
5 Electrokinetic movement of nanoscale zero valent iron directly injected into porous media
6 Electrokinetic movement of nanoscale zero valent iron directly injected into porous media with enhanced conductivity
7 Hydraulic advection of nanoscale zero valent iron through porous media
3.1 Instrument Calibration
Various parameters were measured in the experiments to characterise the effect of
electrokinetic phenomena on nanoscale zero valent iron. All pH, temperature and
conductivity measurements were conducted using a TPS Conductivity-Salinity-pH-
Temp. Meter, Model WP-81. The pH probe was calibrated using a 2-point calibration
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technique. It was first rinsed with de-ionised water, dried, then immersed in a pH
7.00 calibration standard solution Biolab pH 7 potassium dihydrogen orthophosphate
buffer solution, batch number AF405379 and allowed to equilibrate. After
equilibrium was reached, the probe was removed and rinsed with de-ionised water
again, dried, and placed in Rowe Scientific pH 4.00 calibration standard solution –
potassium hydrogen phthalate, code CB 2660, batch AK051017 and allowed to reach
equilibrium again. The slopes given from the calibrations ranged between 98.9% and
98.2%.
The conductivity probe was calibrated using a 1-point calibration technique. The
probe was rinsed with de-ionised water, and then immersed in 58Scm-1 calibration
solution, allowed to equilibrate and then calibrated, the probe was then considered fit
for use.
3.2 Power supply
A Powertech dual tracking DC power supply, model MP – 3092 was used to supply
power to the electrodes throughout the research. It was capable of supplying a
maximum voltage of 40 volts, and a maximum current of 3 amps. The power supply
had two outputs, capable of being used independently or in a master/slave
configuration. Voltage used in the experiments never exceeded 20 volts, and typical
currents were approximately 0.01 amps.
3.3 Iron Concentration in Slurry Determination
The container of iron was thoroughly shaken for 1 minute before sampling. A 120
mL sample was poured into a measuring cylinder on a tared electronic scale. The
sample was then weighed, and it was then attempted to calculate the percentage iron
content.
Another 100g sample was poured into a drying container. After 16 hours of heating at
80 degrees Celsius, the drying container was reweighed. A percentage iron
calculation was then conducted.
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3.4 Single containment vessel experiments
A test was devised to first initiate experimentation with zero valent iron and
electrokinetics to observe any forthcoming effects. A 2L single containment vessel
was initially filled with water. 20 mL of zero valent iron was then added to the water
and mixed metal oxide electrodes were inserted into the slurry. The electrodes were
positioned at opposite corners of the containment vessel, and the voltage set to 20
volts. The voltage was applied for 8.5 hours. The containment vessel was left
uncovered for the duration of the experiment to prevent the possible build-up of
noxious gases.
Following this test, two stainless steel nails 100mm in length and 38mm in diameter
were used as electrodes. These were placed in opposite corners of a 2L containment
vessel. The vessel was filled with tap water and 10 mL of zero valent iron slurry was
added. A voltage of 20 volts was applied between the electrodes in the uncovered
bucket for a period of 41 hours.
3.5 Dual containment vessel experiment with unhindered flow
The dual containment vessel experiment with unhindered flow was designed to begin
experimentation of movement of species from one vessel to another using
electrokinetics. The nanoscale zero valent iron was to be placed in one containment
vessel with the aim to enhance its movement into another containment vessel through
a connecting tube using electrokinetics.
3.5.1 Electrodes
Care was taken during electrode selection as not all materials were deemed suitable
for usage. Iron electrodes were avoided due to the inability of the analytical
technique used for analysis of iron content to distinguish between zero valent iron and
iron released from the electrode. Copper electrodes are susceptible to corrosion (Lee,
2005) and were therefore not suitable. Similarly, any other common metal that could
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corrode was not accepted because of the possibility of influencing the iron in solution.
Mixed metal oxide electrodes were also considered due to their notable performance
and relatively low cost. However, they were also eventually rejected as suitable
electrodes due to the inability of to be completely certain that there were no iron
compounds present that could influence results.
The electrodes that were eventually selected consisted of three layers. The inner core
comprised of copper for its ability to carry charge as copper has the second highest
electrical conductivity of all known elements (5.88 x 107/ S m-1) (Kittel, 1986). The
outer layer of the electrode was a fine plating of platinum due to platinum’s ability to
resist chemical attack. This was the most vital section of the electrode, as it would be
in contact with the aqueous solution, and therefore must not contaminate it with
foreign iron atoms. Between the platinum and copper layers was a layer of titanium,
which provided a buffer between the solution it was to be placed in and the copper
core, in case the fine platinum plating was penetrated due to chemical corrosion or,
the more likely event of mechanical scratching.
The electrodes were purchased from McCoy Engineering in lengths averaging 23 cm.
They were all cut from a single strand of electrode and therefore had an exposed
copper core at each end. This was undesirable, as the copper at the tip of the end of
the electrode that was immersed in the aqueous solution would be exposed to
chemical attack and could rapidly corrode, thus contaminating the containment
vessels. To prevent this from occurring, it was necessary to cap the end with a non-
permeable material. Both ends of the electrodes were ground on a bench grinder.
This achieved two goals. Firstly, it coarsened one end and removed any unwanted
compounds so that a good connection could be made to the power source. Secondly,
on the other end, it resulted in a stronger bonding of the capping substance. An epoxy
resin was mixed, comprising of 2 parts by weigh Araldite BY 157 TS LC from
Vantico (>60% Bisphenol A, >10% Butandiol diglycidyl ether, 10-30% Bisphenol F )
and 1 part by weight hardener Aradur 2764 – CH from Vantico. PVC electrical
cowling of 20 mm outside diameter was then cut in lengths of approximately 30 mm,
into which the mixed epoxy was injected. The electrodes were then inserted along
their longest axis into the cowling to a depth of approximately 2 cm. They were then
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fixed in place and the epoxy allowed to harden over 24 hours. As mentioned above,
both ends had been ground with a bench grinder, which had removed the platinum
layer, leaving the titanium middle layer exposed. Although one end was encased in
epoxy resin, an approximately 8mm section immediately above the epoxy was not
sealed (due to it being outside the encapsulating epoxy), and had had the platinum
ground off, thus exposing the titanium beneath it. This was not considered a
significant problem, because the titanium would oxidise, forming a TiO2 layer that
electrically insulates and stops the copper from corroding. As this was not a large
section of the electrode, the reduction in capacity to deliver current to the bulk
solution was not considered significant. Following the hardening period, the
electrodes were then deemed suitable for use, and are shown in Figure 3.1.
Figure 3.1 – Capped Pt/Ti/Cu electrodes
3.5.2 Containment Vessel
The testing apparatus consisted of two 15 litre plastic vessels, each featuring an
aperture in one side. These apertures were fitted with a circular plastic fitting, with an
o-ring on the inside diameter. This left an aperture in the side of each vessel of 50
mm radius. The two vessels were joined by a 100 mm length of clear Perspex tube.
After evidence of severe leaking around the aperture and screws fixing the circular
plastic fitting, a silicon-based sealant was applied liberally to any outside surfaces
suspected of leakage, as shown in Figure 3.2. After 24 hours of setting, the
containment vessels were again connected via the connecting tube and filled with
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water and it was tested for leaks. This procedure was repeated until there was no
leakage and the finished set-up is shown in Figure 3.3.
Figure 3.2 – Aperture with silicone sealant applied to prevent leaking
Figure 3.3 – Containment vessels with connecting tube
3.5.3 Mixing method
Mixing of the water in the containment vessels would advect the nanoscale zero
valent iron, thus masking any movement of nanoscale zero valent iron due to
electrokinetic effects. It was therefore decided to not stir the containment vessel
solutions in this experiment.
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3.5.4 NaCl experiment
Tests were conducted regarding using electrokinetics to induce movement of charged
ionic species. NaCl was chosen as the ionic species to conduct the experiment due to
their non-hazardous nature, ease of acquisition and low cost. These tests were
designed to observe electrokinetic phenomena in action, and to ensure that the
methodology and equipment were correct. As such, the containment vessels were the
ones used in the zero valent iron experiments, the electrodes used were fabricated
platinum/titanium/copper electrodes (the same as the zero valent iron experiment
electrodes) and the connecting tube between the containment vessels had the same
dimensions as the tubes used in the zero valent iron experiments.
3.5.4.1 NaCl test at 20 volts
24 litres of 23.4°C tap water, with a maximum total iron concentration of 0.16 mgL-1
(Water Corporation, 2006) was added to two containment vessels connected by a 100
mm connecting tube between the apertures. The aperture in the containment vessel
containing NaCl was blocked so that there was no advection from one containment
vessel to the other. 100 g of sodium chloride was added to one of the two
containment vessels. The fabricated Pt/Ti/Cu electrodes were positioned such that
they were suspended above the aperture fitted in each containment vessel, and
projected downwards across the diameter of the aperture, the cathode (negative
electrode) being positioned in the containment vessel dosed with NaCl. The pH and
conductivity probes were positioned in the corner of the containment vessel that held
the anode (positive electrode), closest to the aperture and electrode. The containment
vessel dosed with NaCl was then stirred for two minutes to ensure thorough mixing.
The blockage between the two containment vessels was then removed after motion in
the stirred containment vessel had ceased, and conductivity was monitored every
minute until it stabilised. Although the water in the containment vessel that was not
dosed with additional NaCl was saline to a small degree, the experiment was designed
to measure and characterise the change in salinity and therefore, this small amount of
additional salt was not a problem.
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3.5.4.2 NaCl test at 10 volts
A second test was run with slightly different operating parameters to determine the
effect of voltage on electromigration. First the containment vessels were filled with
24 litres of 22.5°C tap water with maximum Fe concentration 0.16 mgL-1, (Water
Corporation, 2006). The connecting tube was blocked at the aperture of the
containment vessel containing the negative electrode, to ensure no advection occurred
between buckets. 101 g of NaCl was added to the containment vessel holding the
negative electrodes, and stirred for two minutes to ensure dissolution. The cathode
was then placed such that it was suspended into the NaCl doped containment vessel,
projecting downwards across the aperture, and the anode positioned similarly in the
other containment vessel. The blockage was then removed and conductivity was then
measured every minute until the conductivity levels stabilised. The conductivity
probe was agitated before readings were taken to give a more representative sample.
3.5.5 Nanoscale Zero Valent Iron Supply
Each sample of nanoscale zero valent iron used in the various experiments in this
document was obtained in the following manner. The container of the nanoscale zero
valent iron was shaken vigorously for 1 minute to ensure homogeneity. The required
volume of nanoscale zero valent iron was poured into a measuring cylinder. It was
then transferred into the required containment vessel.
3.5.6 Nanoscale zero valent iron experiment with no porous media
Following the NaCl experiments, it was decided to continue with the no porous media
experiments and conduct a similar experiment, but this time using zero valent iron in
place of NaCl.
3.5.6.1 Initial nanoscale zero valent iron experiment with no porous
media
Two containment vessels were connected via 100 mm length of 49.8 mm diameter
clear Perspex tube and filled with normal tap water. The buckets were placed on a
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purpose made wooden platform sitting on top of a Ratek EOM5 Orbital Mixer. A
mixed metal oxide electrode was connected via dual strand copper wiring to both the
positive and negative port of a Powertech dual tracking DC power supply model MP –
3092, and a potential difference of 20 volts was applied. This voltage was used
because a large voltage was desired to induce the electrokinetic effects, without
overloading the power supply. A mixed metal oxide electrode was positioned at the
aperture of each containment vessel. A water sample was taken from the containment
vessel containing the positive electrode, and the solution from the same vessel was
monitored for pH and conductivity. 72 grams of zero valent iron that was received in
August 2005 was introduced to the containment vessel containing the negative
electrode. The zero valent iron varied in size from a powdery substance to one
roughly spherical piece with a radius of approximately 2 cm. The orbital mixer was
not operated, due to the possibility of the mixing conveying some of the iron to the
positively charged containment vessel because the connecting Perspex tube did not
hold any inhibiting material, i.e. filled solely with water. After 300 minutes, the
orbital mixer was powered and mixing of the water contained in the containment
vessels began. The experiment continued to run for a further 90 minutes with the
orbital mixer running. Water was sampled from the surface, just outside the aperture
of the positively charged containment vessel for further analysis.
3.5.6.2 Second nanoscale zero valent iron experiment with no porous
media
The two containment vessels were connected by a 100mm, 49.8mm inside diameter
piece of tubing. 24 litres of water were added to the containment vessels, which were
then isolated from each other by application of a plug, thus blocking flow from one
containment vessel to the other. Electrodes were connected 20 volts via the power
source and positioned at both ends of the connecting tube in the containment vessels.
31 mL of the freshly prepared iron slurry was then introduced into the negative
containment vessel. The pH probe was placed in the corner of the negative
containment vessel, closest to the side aperture. The pH in the anodic containment
vessel was monitored, and water samples were taken by submergence of a 21 mL
sample vial at the aperture of the connecting tube, on the anodic side.
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3.6 Dual containment vessel experiment with porous media flow and
orbital mixing method
After the experiments with a connecting tube free of porous media, an experimental
set-up that simulated a groundwater environment was required. As such, it was
decided to fill the connecting tube with inert 250 micron silica beads to imitate porous
media in the subsurface.
3.6.1 Mixing method
The zero valent iron was significantly denser than water, and was observed to descend
to the bottom of the containment vessel upon introduction to the water body. In order
to have the iron more heterogeneously distributed throughout the containment vessel
it was introduced into, it was deemed necessary to agitate the water body. Various
methods of mixing were posed as suitable means to suspend the iron in the aqueous
solution. Magnetic stirrer plates and stirrer bars were quickly disregarded, due to the
interactions the imposed magnetic fields would have on the zero valent iron. Electric
mixers were considered, however, concern was raised over the ability of the electric
motor to endure constant operation for hours at a time. Two electric mixers were
purchased from a department store for usage, and the accompanying documents did
not recommend stirring for more than one and five minutes respectively, so they were
deemed unsuitable and not used. All stirrer fittings available for both Sunbeam™ and
Breville™ electric mixers also contained high levels of iron, which could influence
results. It was then decided upon to use a Ratek EOM5 orbital mixer, which uses
elliptical motion of a base-plate to induce agitation in the containment vessels.
3.6.2 Orbital mixer board construction
A board was constructed for the containment vessels to be placed upon. A 750 mm x
450 mm wooden board was measured to fit the lip of the Ratek EOM5 orbital mixer
plate. Four rectangular wooden stoppers were attached to the underside of the
wooden board, positioned such that they were situated along the outside of each side
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of the Ratek EOM5 orbital mixer plate; this was to prevent any slippage whilst the
mixer plate was in motion.
3.6.3 Manufacturing of additional side ports in the connecting tube
Samples from within the porous media in the connecting tube were to be taken, using
a needle and syringe. The connecting tube was 100mm long, had an inside diameter
of 49.8 mm and an outside diameter of 51.8 mm. Once fitting inside the containment
vessels was complete, the tube length between the two containment vessels was
69mm. Manufacturing of three sampling ports was conducted using three 7 mm
diameter tubes. These were fitted along the longitudinal axis of the tubing at
equidistant intervals. These tubes were inserted into the connecting tube, and
PARFiX silicone sealant was applied to completely seal the join. The completed
connecting tube apparatus can be seen in Figure 3.4 and Figure 3.5.
Figure 3.4 – Top view of connecting tube with three sampling ports fitted with flexible tubing
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Figure 3.5 – Side view of connecting tube with three sampling ports fitted with flexible tubing
3.6.4 Needle selection
A needle was required to sample in the connecting tube porous media. An envisaged
problem would be the aperture of the needle becoming clogged by the porous media.
To combat this possible problem, a 10 mL needle with side port injection capabilities,
part # 008962SGE was acquired from Alltech Associates Australia. This needle
featured a side-port aperture, rather than the more conventional location at the tip of
the needle, which would decrease the chance of blockage from porous media
particles.
3.6.5 Silica filling of tube/screen installation
In order for the 250 micron silica porous media to remain inside the connecting tube
after installing it into the apertures of the containment vessels, PARFiX silicone
sealant was applied to the circular edge of the connecting tube. A single sheet of
Chux® Regular Superwipes was then placed on the edge and pressure was applied to
fix the sheet to the connecting tube edge. After 24 hours of drying, an additional
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sheet was affixed on top of the first sheet, rotated by 90 degrees, so that the small
apertures in the sheet were perpendicular to the first sheet. After the PARFiX silicone
sealant had set, 250 micron silica bead porous media were poured into the connecting
tube and packed by application of pressure. Another sheet of Chux® Regular
Superwipes was then fixed in place with silicone sealant on the top end of the
connecting tube, thus sealing the silica inside. A fourth sheet of Chux® Regular
Superwipes was rotated 90 degrees, then affixed on top of the first sealing sheet on
the top end of the tube as seen in Figure 3.6. The connecting tube was then fitted into
the containment vessel apertures ready for usage.
Figure 3.6 – Connecting tube filled with porous media
The filling of the connecting tube with porous media method was improved following
the initial effort, by standing the connecting tube upright in a container holding water
before addition of porous media. This was done to compact the porous media to
reduce the chance of a large void forming in the silica after insertion into the
containment vessel apertures. After filling the tube with silica, the end was capped
with Chux® Regular Superwipes in the same manner as outlined previously, and is
shown in Figure 3.7 and Figure 3.8.
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Figure 3.7 – Connecting tube ready for insertion between two containment vessels.
Figure 3.8 – Semi filled connecting tube capped with pink screens.
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3.6.6 Initial containment vessel experiment with porous media flow and
orbital mixing
Two containment vessels were connected by insertion of a 49.8 mm inside diameter
connecting tube. The tube was filled with porous media beads and an electrode was
positioned over the aperture of each containment vessel. The containment vessels had
24 L of tap water added to them; 30 mL of zero valent iron slurry was then added to
the cathodic containment vessel. Preliminary testing of pH, temperature and
conductivity was conducted. The entire set-up was positioned on the orbital mixer on
a setting 2.5, and an initial sample of the anodic containment vessel water was done.
Sampling was to be conducted periodically after.
3.6.7 Sampling technique development
The proposed sampling technique consisted of inserting a needle connected to a
syringe through the sample port on the side of the connecting tube, into the porous
media inside of it. The plunger on the syringe was then pulled, allowing water into
the syringe. However, after extracting approximately 1.5 mL, the needle ceased to
function. The suspected problem was that the hole was plugged by a silica bead, thus
preventing flow into or out of the needle. It was thus deemed an unsuitable technique
for sampling. It was decided upon to sample in the same manner as in previous
experiments, which consisted of manually stirring the solution in the containment
vessel to ensure homogeneity, and immersing a 21 mL sampling vial in the solution in
the containment vessel at the aperture to obtain the water sample.
3.6.8 Second dual containment vessel experiment with porous media flow
and orbital mixing
The experiment was set-up in the same manner as the previous experiment, with two
containment vessels connected by a connecting tube filled with porous media. 24 L
of water was added, with a cathode positioned at the aperture of one containment
vessel and an anode positioned at the aperture of the other containment vessel. The
containment vessels were placed on a board on top of the orbital mixer. The power
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source delivered 20 volts to the electrodes, and the orbital mixer was set on setting of
2.5. 30 mL of nanoscale zero valent iron was added to the cathodic containment
vessel. Water was sampled in the anodic containment vessel just outside the aperture
of the connecting tube in 21 Ml sample vials.
3.6.9 Side port construction in the connecting tube
To improve the filling of the connecting tube with porous media, it was required to
drill an aperture into the side of the connecting tube to allow the porous media to be
poured into it. A 7mm aperture was drilled into the side of the tube, equidistant from
both ends. The aperture was sealed after filling with porous media by plugging with 7
mm diameter flexible tubing, followed by application of silicone PARFiX sealant
around the join.
Following filling of the connecting tube, it was thought that easier filling could be
achieved by positioning the filing aperture closer to one end. This allowed the
connecting tube to be tipped on one side when the tube was semi-filled, which moved
all the porous media to the end furthest from the filling aperture, thus allowing easier
addition of further porous media.
3.6.10 Filling of connecting tube with porous media.
A third method was then employed to further the efforts to mitigate voiding occurring
in the connecting tube porous media. Both ends were sealed using PARFiX silicone
sealant and Chux® Regular Superwipes in the same manner as outlined previously.
Once the silica gel had set after 24 hours, the porous media was inserted into the
connecting tube through the previously manufactured 7 mm sample port. The porous
media was dry to allow easy insertion. Following dry packing, the tube was
immersed in water, which resulted in the porous media compacting further.
Additional dry porous media were then poured through the small aperture and
compacted using a thin pine skewer. This process was repeated until no more porous
media could be compressed into the connecting tube. At this stage, the screening
cloths on both sides were convex, bulging outwards due to the pressure exerted by the
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porous media. A flexible polymer tube piece with outside diameter 7 mm was
inserted into the small aperture to seal it, and PARFiX silicone sealant was applied to
the joint and allowed to set to complete the join.
3.6.11 Third dual containment vessel experiment with porous media and
orbital mixing
The identical experimental set-up was used for this experiment as used in the previous
experiment, and sampling was conducted periodically in the same manner. The
hydraulically connected tube packed with silica beads was properly packed with no
voiding across the top. pH was periodically sampled in the anodic containment
vessel, and water samples were also taken to be analysed for iron content by
submergence of a 21 mL sample vial at the aperture of the connecting tube in the
anodic containment vessel. The experiment was run until the porous media in the
connecting tube eroded to form a void space across the length of the connecting tube
containing the porous media.
3.7 Dual containment vessel experiment with porous media flow and
mechanical mixing method
3.7.1 Mechanical mixing
The orbital mixers were used on a number of experiment runs. However, it was
suspected that they could cause erosion of the porous media in the connecting tube.
To alleviate this problem, a XUI 13 mm Hammer drill, Model XHD-200 variable
speed drill was fitted with a manufactured wooden paddle to agitate the water
contained in the containment vessel.
3.7.2 Non-metallic mixing paddle construction
A 9.6 mm diameter, 390mm long piece of wooden dowel was fitted and glued into the
side of an 80 mm x 39mm x 13 mm rectangular piece of wood. A 10 mm hole was
drilled approximately 20 mm into a rectangular piece of wood. A piece of dowel was
inserted and glued into the hole to form a non-ferrous mixing paddle. This dowel
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could be inserted into the chuck of an electric power drill to rotate and mix the fluid in
the containment vessel. The apparatus is shown in Figure 3.9
Figure 3.9 – Wooden paddle used for mechanical mixing.
3.8 Experiment using mechanical mixing
This experiment was conducted to examine the effects of placing the anode in the
containment vessel that the nanoscale zero valent iron was in and placing the cathode
in the other containment vessel that had no zero valent iron in it. 24 L of tap water
was added to two containment vessels connected by a tube packed with porous media.
The electrodes were suspended over a wooden pole placed across the containment
vessels, and were placed at the ends of the connecting tube. A voltage of 20 volts was
applied to the electrodes. The fabricated wooden paddle was fitted into the XUI 13
Hammer drill and then suspended into the anodic containment vessel. The drill was
operated at regular intervals to re-suspend any nanoscale zero valent iron that had
settled out of the suspension. 30 mL of nanoscale zero valent iron slurry was added to
the anodic containment vessel. The experiment was run for one week. Sampling was
conducted periodically and consisted of agitating the fluid in the containment vessel
to ensure the sample was homogenous and taking the sample from just below the
surface near the electrode. pH was also monitored in the cathodic containment vessel.
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The water that was removed lowered the depth of the water in the containment
vessels. To combat the drop in water level, tap water was added each day to maintain
the volume of water used in the experiment. This added water was not considered
enough to alter any physical or chemical process due to slight changes in
concentration. The experiment was run for 7.5 days, as per the previous experiment,
because for the electrokinetic effect on moving the nanoscale zero valent iron
particles to be considered useful, it must be able to move the particles the 100mm in
this time interval.
3.9 Initial direct injection of nanoscale zero valent iron into porous
media experiment
Two containment vessels connected by a porous media filled connecting tube were
filled with 24 L of tap water. Electrodes were suspended just outside the aperture of
each containment vessel and a voltage of 20 volts was applied. The connecting tube
had a 7 mm diameter flexible tube (termed injection ports) inserted into each of the
three 7 mm apertures along the top of the connecting tube, and PARFiX silicone
sealant sealed the joints. Approximately 1.5 mL of nanoscale zero valent iron was
injected into the middle flexible tube as seen in Figure 3.10. It was the intention that
electrokinetic processes induce movement of the nanoscale zero valent iron to an
electrode. Water in both containment vessels was agitated with a non-metallic stirring
pole and sampled periodically just underneath the surface near the electrodes. The
experiment was run for 7.8 days as it was deemed that if the nanoscale zero valent
iron could not be moved in this time period, it would not be effective as a remediation
technique.
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Figure 3.10 – Connecting tube featuring injection of nanoscale zero valent iron through the flexible tubing
3.10 Direct injection of nanoscale zero valent iron into porous media
experiment with enhanced conductivity
A subsequent experiment was conducted again featuring direct injection of nanoscale
zero valent iron into the connecting tube. In this experiment, the injection site was
220 mm from the aperture in the anodic containment vessel and 670 mm from the
aperture in the cathodic containment vessel. This was done as it was suspected that
the nanoscale zero valent iron would have a greater affinity for the cathode than the
anode. In the event of the nanoscale zero valent iron moving to the cathodic
containment vessel, the longer pathway through the porous media would impart more
confidence in the validity of the result. 50 g of sodium chloride (NaCl) was added to
each containment vessel to determine if the increased conductivity would enhance the
electrokinetic effects on the nanoscale zero valent iron. pH and conductivity were
monitored in the cathodic containment vessel, as was the amperage drawn by the
power supply. Cathodic containment vessel water samples were taken by
submergence of a 21 mL sample vial at the connecting tube aperture. The experiment
was run for 10.2 days, because for the electrokinetic transport of nanoscale zero
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valent iron to be considered in the field, it would have to be able to move 100mm in
this time period.
3.11 Initial hydraulic advection experiment
A connecting tube filled with porous media was inserted between two containment
vessels. A slit was cut in the side of a containment vessel, to ensure a constant
hydraulic head. The containment vessels were filled so that the water level was at the
slit that was cut into the side of the containment vessel and 30 mL of nanoscale zero
valent iron was added to the other containment vessel. 10 pore space volumes (780
mL) of water were added to the containment vessel containing the iron, to provide a
hydraulic gradient. The zero valent iron containment vessel was stirred frequently to
mitigate settling. The final set-up is shown in Figure 3.11
Figure 3.11 – Containment vessel with slit in side for constant hydraulic head
3.12 Iron concentration sampling
The samples were analysed for total iron content using a SpectraAA-100 atomic
Absorption Spectroscopy (AAS) Machine. The machine was calibrated using a two
point calibration technique, using standards of 10 ppm total iron and 3.9 ppm total
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iron. The readings were reliable to a minimum concentration of 2 ppm. The machine
did exhibit a slight amount of creep in the measurements, and to mitigate this, the
machine was re-calibrated every 20 samples. Samples were prepared by addition of a
70% nitric acid (HNO3) solution, to ensure the dissolution of all the iron.
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4 Results
The results for the numerous experiments are presented below. The implications of
the results are discussed in the discussion chapter.
4.1 Iron concentration determination
A 120 mL sample of nanoscale zero valent iron slurry weighed 99.9g. This gave a
specific gravity of less than unity. 8325.0120
9.99= .
A sample of nanoscale zero valent iron was dried to determine the slurry’s water
content. The results showed that the drying vessel that had been dried over 16 hours
contained solids weighing 68.2 g. This resulted in a water content to be 100 - 68.2 =
31.8 %.
4.2 Single containment vessel experiment
After 15 minutes, the test involving mixed metal oxide electrodes was stirred, which
produced a fizzing noise, probably due to gas generation at the electrodes. An hour
later, a significant amount of effervescence was observed at the cathode. The surface
around the cathode also had a brittle film form. Two hours after the electrodes were
supplied power, a brown sludge had formed around the cathode (shown in Figure 4.2),
and the bubbling continued, which was accompanied by an audible fizzing noise.
There was little change to the anode, however approximately 75 % of the mixed metal
oxide coating had been removed from the cathode.
The test using steel electrodes began in a similar fashion to the mixed metal oxide
electrode test, with bubbling occurring at the cathode. A brown sludge formed at the
cathode approximately 3 hours after the electrodes were powered. This brown sludge
continued to grow and propagate over the surface of the fluid until the cessation of the
experiment. The electrodes were examined after the conclusion of the experiment.
The cathode appeared unchanged, whereas the anode was coated in a thick brown
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coating. This brown coating was easily wiped away by a cloth, exposing a black
coloured surface. While the thickness of the cathode did not appear to be changed,
the anode was significantly thinner. The thickness of both electrodes before the
experiment was 3.9 mm, and after the experiment the cathode was still 3.9 mm in
diameter, but the anode was 3.6 mm in diameter. Both electrodes are shown in Figure
4.1
Figure 4.1 – Steel electrodes after operation in nanoscale zero valent iron slurry.
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Figure 4.2 – Slurry reaction at cathode.
4.3 Dual containment vessel experiment with no porous media
4.3.1 Sodium Chloride (NaCl) experiments
The results for both the NaCl experiment run at 20 volts, and the experiment operated
at 10 volts revealed the ions in solution did indeed migrate from one containment
vessel to the other. The increase in concentration of ions was indicated by the
increase in conductivity, as they are approximately proportional (Zimmt, 1993).
4.3.1.1 NaCl experiment at 20 volts
The conductivity results from the NaCl experiment conducted at 20 volts are shown
below in Figure 4.3.
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NaCl expt
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0.00 200.00 400.00 600.00 800.00 1000.00 1200.00
Time (mins)
Co
nd
ucti
vit
y (
mS
)
Figure 4.3 – NaCl experiment conducted at 20 volts.
It can be seen from Figure 4.3 that during the first 22 minutes of the experiment, the
conductivity fluctuated noticeably. The conductivity of the solution in the anodic
vessel then increased over time, thus indicating the migration of ions from the dosed
cathodic containment vessel to the anodic containment vessel. The conductivity
plateaued after approximately 4 hours.
4.3.1.2 NaCl experiment at 10 volts
The conductivity results from the NaCl experiment conducted at 10 volts are shown
below in Figure 4.4.
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NaCl 10V Run
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0 200 400 600 800 1000 1200
Time (mins)
Co
nd
ucti
vit
y (
mS
)
Figure 4.4 – NaCl experiment conducted at 10 volts. It can be seen from the above graph that the conductivity rapidly increased from
below 1 mS/cm to approximately 5.3 mS/cm within 12 minutes. The conductivity
then fluctuated for 5 hours and then exhibited an upward trend.
4.3.2 Initial zero valent iron experiment
Upon powering the electrodes in the initial zero valent iron experiment, effervescence
was observed at both electrodes, being more pronounced at the negative electrode.
As time progressed, the pH was observed to drop from 7.45 to 7.05 in 190 minutes.
Accurate measurement of the pH was not possible following initiation of the orbital
mixer, due to the pH probe fluctuations. After 390 minutes, no migration of the iron
was visually observed.
Figure 4.5 and Figure 4.6 below show the iron concentration and pH level
respectively for the first zero valent iron experiment.
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Aged ZVI Experiment
-0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0 50 100 150 200 250 300 350 400
Time (mins)
Fe
co
nc
ne
tra
tio
n (
mg
/L)
`
Figure 4.5 – Aged Zero Valent Iron Experiment Iron Concentration
Aged ZVI Experiment
6.2
6.4
6.6
6.8
7
7.2
7.4
7.6
0 50 100 150 200 250 300 350 400
Time (mins)
pH
Figure 4.6 – Aged Zero Valent Iron pH
4.3.3 Second zero valent iron experiment
Although efforts were made to mitigate nanoscale zero valent iron advection from the
cathodic containment vessel to the anodic containment vessel, the removal of the plug
created a great deal of water movement that advected the iron from one containment
vessel to the other. Due to the absence of porous media between the two containment
vessels, eddies were induced that advected a significant amount of the zero valent iron
slurry from the negative containment vessel to the positive containment vessel when
the plug was removed. There was also a slight hydraulic head difference between the
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containment vessels due to the addition of the nanoscale zero valent iron slurry, which
also conveyed nanoscale zero valent iron through the connecting tube. This
compromised the experiment, as nanoscale zero valent iron had been moved by non-
electrokinetic phenomena. Figure 4.7 shows the results of the analysis of the samples
taken during this experiment.
No Porous Media Experiment
0
1
2
3
4
5
6
7
8
-200 0 200 400 600 800 1000 1200 1400
Time (mins)
Fe
Co
nc
en
tra
tio
n (
mg
/L)
Figure 4.7 – Total iron concentration versus time for second experiment without porous media.
4.4 Dual containment vessel experiment with porous media and
orbital mixing
4.4.1 Initial experiment
The first attempt at filling the connecting tube with porous media resulted in the
formation of a significant cavity space located along the top of the long axis of the
connecting tube. This resulted in the rapid advection of the aqueous solution
containing nanoscale zero valent iron through the connecting tube, along the top of
the tube through the space with no porous media, into the other containment vessel.
The cavity was suspected to be caused by compaction of the porous media once wet,
thus reducing the volume occupied by porous media and leaving a void space above,
seen in Figure 4.8. The advection of the nanoscale zero valent iron through the cavity
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from one containment vessel to the other would mask any electrokinetic transport, and
hence the experiment was stopped with inconclusive results.
Figure 4.8 – Voiding along the top of the connecting tube
4.4.2 Second experiment
When the connecting tube filled by the second method was installed into the
containment vessel apertures, a cavity formed in the same position as before, i.e.
along the long axis above the porous media. The reason for this cavity space
formation could not be explained by the compaction of silica after it was wet, since it
was installed into the connecting tube in an aqueous matrix. The Chux® Regular
Superwipes screening cloth affixed over the ends of the connecting tube were flush
with the tube ends during filling with porous media because the tube was standing
upright. However, when turning the connecting tube on its side to fit it into the
containment vessel apertures, the screening cloth bulged outwards, increasing the
available volume for containment of the porous media, and thus resulting in the cavity
space across the long axis of the tube. Upon addition of the nanoscale zero valent
iron, it was visibly seen to immediately flow into the connecting tube through the
cavity along the top of the tube, as seen in Figure 4.9. The experiment was then
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stopped, again because of the advection of the nanoscale zero valent iron masked any
electrokinetic induced movement.
Figure 4.9 – Connecting tube featuring voiding
4.4.3 Third experiment
The third experiment yielded much better results than the previous two experiments
due to the porous media in the connecting tube not containing a large void. The
experiment proceeded to run in a satisfactory manner until 92 hours after
commencement, when the waves induced by the elliptical motion of the orbital mixer
caused the media in the connecting tube to erode away. The erosion of the porous
media resulted in a void forming across the top of the connecting tube, similar to the
previous two experiments, seen in Figure 4.10. This allowed water to be advected
from one containment vessel to the other solely without passing through the porous
media. When removed from the containment vessel, the cathode was coated in a
black coating that could not be easily removed, seen in Figure 4.11.
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Figure 4.10 – Voiding due to orbital motion of mixer
Figure 4.11 – Cathode and Anode after experimentation
Figure 4.12 shows the total iron concentration for samples taken over the duration of
the experiment.
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Orbital mixing Expermient with no Voiding
-0.2
0
0.2
0.4
0.6
0.8
1
0 1000 2000 3000 4000 5000 6000 7000
Time (mins)
Fe
Co
nc
en
tra
tio
n (
mg
/L)
Figure 4.12 – Orbital experiment mixing experiment nanoscale zero valent iron concentrations
A slight increase in total iron concentration was initially observed, followed by a
decrease after 50 hours. The slightly increasing trend is then observed once more up
until 92 hours, when a large spike in iron concentration is observed, coinciding with
the time the cavity was formed from the eroded porous media. The concentration of
iron in all the samples was very small (less than 0.2 mg/L), with the exception of the
last sample.
4.5 Dual containment vessel experiment with porous media and
mechanical mixing
For the duration of the experiment, water slowly leaked from the join between the
connecting tube and containment vessel. The leakage was very slow, less than a drop
every 10 minutes. However, when this water loss was combined with additional
water loss from evaporation and removal for sampling, it had the potential to induce
iron migration by the formation of a hydraulic head. To prevent the hydraulic head
forming, both containment vessels were periodically topped up to exactly 24 L with
additional tap water. Nanoscale zero valent iron penetration can be seen in Figure
4.13 and the electrodes after the experiment are shown in Figure 4.14.
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University of Western Australia 69
Figure 4.13 – Nanoscale zero valent iron penetration of porous media
Figure 4.14 – Cathode and anode after experimentation
Figure 4.15 shows the total iron concentration results for the duration of the
experiment.
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Mechanical Mixing Experiment
0
0.2
0.4
0.6
0.8
1
0 2000 4000 6000 8000 10000 12000
Time (mins)
Fe
Co
nc
en
rati
on
(m
g/L
)
Figure 4.15 – Mechanical mixing experiment nanoscale zero valent iron concentrations
It can be seen that the iron concentrations for this experiment remained fairly constant
over the entire period. The concentration reached a peak level of 0.118 mg/L, fell to a
low of 0.017 mg/L, and had a range of 0.101 mg/L.
A visual inspection of the core of the porous media in the connecting tube revealed
the nanoscale zero valent iron to have penetrated into the porous media on the anodic
side. The porous media was starkly white from the cathodic end to 18 mm from the
anodic end, when it was contrastingly a dark black. The iron penetrated 18 mm in
over 7.5 days, resulting in a transmission rate of 2.39mm/day.
4.6 Dual containment vessel experiment with porous media and direct
injection
After a visual inspection of the porous media core following completion of this
experiment, the nanoscale zero valent iron directly injected into the side injection port
did not seem to move significantly. In fact, it had not even entered the porous media
in the main connecting tube. The nanoscale zero valent iron could be easily observed
visually, as it was a black colour, and the porous media was starkly white. After the
187 hours (7.8 days) had passed, the porous media 1 mm below the injection port had
not changed colour, and was still a very clear white colour. After inspection, the
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University of Western Australia 71
entire core sample did not appear to have any trace of black nanoscale zero valent
iron, as seen in Figure 4.16. Total iron concentrations can be seen in Figure 4.17.
Figure 4.16 – Core sample of connecting tube featuring no visible nanoscale zero valent iron penetration
Direct Injection Experiment
0
0.2
0.4
0.6
0.8
1
0 5000 10000 15000
Time (mins)
Fe
Co
nc
en
tra
tio
n (
mg
/L)
Anodic Fe
Concentration
(mg/L)
Cathodic Fe
Concentration
(mg/L)
Figure 4.17 – Direct injection experiment nanoscale zero valent iron concentration of both anodic and cathodic containment vessels
Chapter 4: Results
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The iron levels in the anodic containment vessel were significantly higher than the
iron levels monitored in the cathodic containment vessel. This was assumed to be due
to residual iron from a previous experiment being present in the anodic containment
vessel. This was not deemed to be problematic because the concentrations were
analysed for a change in iron concentration, and not absolute concentration. Thus, the
general discrepancy between the anodic and cathodic containment vessels iron
concentrations is not an indication of electrokinetic transport phenomena, and merely
a difference in baseline iron concentrations. The maximum and minimum iron
concentrations in the anodic containment vessel were 0.323 mg/L and 0.064 mg/L
respectively, and had a range of 0.259 mg/L. The maximum and minimum iron
concentrations in the anodic containment vessel were 0.113 mg/L and 0.014 mg/L
respectively, and had a range of 0.099 mg/L.
4.7 Dual containment vessel experiment with porous media and direct
injection with enhanced conductivity.
The nanoscale zero valent iron that was injected into the injection port did not visibly
move after 9 days in this experiment. Visual inspection of the core revealed the
porous media to be completely white with no black sections, thus indicating the
nanoscale zero valent iron had not moved through the connecting tube. Figure 4.18
shows the total iron concentration levels for the duration of the experiment.
NaCl Dosed Direct Injection Experiment
0
0.2
0.4
0.6
0.8
1
0 2000 4000 6000 8000 10000 12000 14000 16000
Time (mins)
Fe
Co
nc
en
tra
tio
n (
mg
/L)
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Figure 4.18 – Iron concentrations for the NaCl dosed direct injection experiment
It can be seen from Figure 4.18 that the total iron concentration remained relatively
constant over the entire duration of the experiment. Peak concentration was 0.25
mgL-1 and the lowest concentration was 0.16 mgL-1. The concentration range was
0.09 mgL-1. Figure 4.19 shows the conductivity to immediately increase from 4.31 mS/cm to 9.70
mS/cm in a time span of 8 hours. The conductivity then remains fairly constant,
fluctuating by only 0.58 mS/cm for the rest of the experiment’s duration. The pH also
climbed from an initial value of 8.27, and exceeded a pH of 10 after 33 hours. It then
further increased to a peak value of 11.42, and then fluctuated between 10.7 and 11.25
for the remainder of the experiment.
NaCl Dosed Direct Injection Experiment
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0 5000 10000 15000 20000
Time (mins)
pH
Conductivity (mS/cm)
Figure 4.19 – pH and conductivity record of the NaCl dosed direct injection experiment
As seen in Figure 4.20, the current drawn at the beginning of the experiment was
similar to other experiments, at 0.02 amps. The enhanced salinity did have a marked
effect on the amperage drawn, peaking at double the original reading.
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NaCl Dosed Direct Injection Experiment Current
Levels
0
10
20
30
40
50
0 2000 4000 6000 8000 10000 12000 14000 16000
Time (mins)
Am
pe
rag
e (
mA
)
Figure 4.20 – Amperage drawn during the NaCl dosed direct injection experiment
4.8 Hydraulic advection experiment
The volume required to provide 10 pore space volumes was calculated in the
following manner.
Volume of connecting tube = 105.22!!"
= 196.35 mL
Void volume = 35.1964.0 !
= 78.5 mL
A core sample was taken following the completion of the experiment, seen in Figure
4.21, to ascertain the degree of nanoscale zero valent iron penetration. After a period
of 380 minutes, the iron had penetrated a length of 15 mm. This correlated to a
transmission rate of 2.37 mm/hr.
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Figure 4.21 – Core sample of connecting tube after hydraulic advection experiment
Figure 4.22 shows the total iron concentration for the duration of the experiment
Hydraulic Advection Experiment
0.000
0.200
0.400
0.600
0.800
1.000
0 100 200 300 400
Time (mins)
Fe
co
nc
en
tra
tio
n (
mg
/L)
Figure 4.22 – Hydraulic Advection Experiment Iron Concentrations
The iron concentration fluctuated from a peak value of 0.23 mgL-1 to a minimum
value of 0.11 mgL-1. There did not seem to be any clear upward trend in the total iron
concentrations for this experiment.
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5 Discussion
5.1 Iron concentration determination
Since iron is more than seven times the density of water, a specific gravity of less than
unity was not expected. Upon visual inspection of the slurry, the same gas that was
suspected for the pressure build up in the packaging was present in the slurry as an
emulsified froth. It was thought that this gas had a specific gravity less than unity,
and thus was the explanation for the very low density of the slurry. As the details of
the gas were not divulged to the author, the iron concentration was therefore not able
to be ascertained by weighing a known volume. This is because when as the density
of a constituent was not known, an additional variable exists, making the system of
equations used an unsolvable system.
The sample that was dried in the drying oven may have gained weight depending on
the degree of enhanced oxidation of the nanoscale zero valent iron. If the iron
corroded very rapidly due to the elevated temperatures, each iron atom is capable of
bonding with 3 oxygen atoms. Although not every iron atom would react in this way,
a large degree of oxidation in the elevated temperatures in the drying oven would
result in the oxygen atoms contributing significantly to the weight of the sample. The
molecular mass of oxygen and iron is 16.0 g/mol, and 55.8 g/mol respectively. Three
additional oxygen molecules would contribute 48 g, or 2.461008.5548
48=!
+% of
the weight.
When first opened, there was a significant spillage of the iron slurry. This was due to
the encapsulating plastic withholding a build up of pressure from the sample. When
the packaging was opened, an expulsion of the build up of gas was combined with a
large leakage of the nanoscale zero valent iron slurry container. This leakage may
have removed a significant amount of water from the slurry, and would hence
increase the slurry’s iron concentration.
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The slurry had been made at an unknown time in the past and so it was known that the
slurry had been existent long enough for an amount of evaporation to occur. This
would also have increased the iron concentration.
This method for determining the iron concentration in the slurry was deemed to be the
optimum method, and gave an iron concentration of 682 g/L.
5.2 Single containment vessel
The major change witnessed with the steel anode was explained by corrosion. The
observed change agreed with the concept of the zero valent iron undergoing an
oxidation reaction at the anode, converting from Fe0 to Fe2+ and/or Fe3+. Once
formed, the ferrous and/or ferric ions could then be solvated by the surrounding water
molecules. This would result in the reduction in mass and diameter observed with the
steel anode. The anode surface was black, suggesting the formation of either FeO or
Fe3O4. This did not occur with the mixed metal oxide anode because its external
surface did not contain significant amounts of Fe0 to be oxidised.
The brown sludge that formed in both experiments (Figure 5.1), could be explained
by formation of ferric oxide (Fe2O3). Commonly known as rust, it has a characteristic
brown appearance that can be seen in Figure 5.1. Both experiments featured the
brown sludge forming at the cathode. The theory that the iron was reacting with the
oxygen generated by the electrodes was quickly discounted because the generation of
oxygen occurs at the anode, and not at the cathode, which was where the brown
sludge appeared. The appearance of the brown sludge can be explained by
combination of hydroxyl radicals and positively charged solvated iron particles. Once
solvated, the positively charged iron particles migrate by the process of
electromigration to the cathode. (OH)- radicals are generated at the cathode due to the
electrolysis of water. The (OH)- ions combine with the positive Fe ions to form a
brown iron hydroxide solid. It was this iron-hydroxide solid that was observed at the
cathode.
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Figure 5.1 – Powered electrodes immersed in a nanoscale zero valent iron slurry
5.3 Dual containment vessels with unhindered flow
5.3.1 NaCl experiment at 20 volts
The fluctuation at the beginning of the experiment (up until approximately 22
minutes) can be explained by the anode’s influence on the dissolved salts in solution.
As the water contained a small amount of charged ions, the electrode induced these
ions to movement close to the conductivity sensor. As the ions passed the probe, the
probe would record the increase in conductivity. These reading would give a false
reading of the actual conductivity, as it measured the higher conductivity of the
immediate surroundings, and not the overall conductivity of the anodic containment
vessel. It can be seen that following the initial fluctuations, the readings did indeed
stabilise, and give more credible results. The steady increase in conductivity
following the initial 22 minutes show that the electrodes did indeed function in the
desired manner, and induced electrokinetic phenomena to move the charged ions in
solution.
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5.3.2 NaCl experiment at 10 volts
The 10 volt experiment did not exhibit the same initial fluctuations as the 20 volt
experiment, indicating that the lower voltage did not induce such a large force on the
existing ions in solution as the 20 volt experiment, thereby the number of ions that
were moved into close proximity of the conductivity probe was not as great. The
probe was moved at certain times to refresh the water surrounding it. This had a
pronounced effect on the reading, which accounts for the fluctuations observed for the
first 5 hours. Again, the results demonstrate the electrokinetic phenomena to be
powerful enough to induce a significant ion flux at this lower voltage.
5.3.3 Initial experiment with zero valent iron and porous media
The initial test used nanoscale zero valent iron that had been procured the previous
calendar year. The slurry had almost completely no water content and had
agglomerated to a significant degree. As a result of this, the iron did not disperse into
the containment vessel, rather it settled quickly to the bottom. The observed pH
fluctuations that made it difficult to measure the exact pH was explained by the
movement of the water body continually exposing the probe to a different section of
water, however, the pH did drop approximately more than half a pH point after
mixing. This pH change was thought to be due to hydrolysis of water occurring at the
electrodes.
5.3.4 Second experiment with zero valent iron and no porous media
The total iron concentration results verify that the sampling technique used is capable
of detecting iron influxes. The concentration of iron peaked at a level of 6.96 mg/L,
which was more than any other experiment. The fluctuating nature of the total iron
concentration is due to agitation variance. The higher occurrences of iron
concentration coincide with times of agitation. The induced eddies that conveyed
nanoscale zero valent iron between the containment vessels upon removal of the plug
was the reason why the levels of total iron concentration are so high. Therefore, the
merits of this experiment are that it demonstrated the analysis technique was capable
Chapter 5: Discussion
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of detecting nanoscale zero valent iron when it was known to be definitely present,
and that agitation does re-suspend previously settled iron particles in the solution.
5.4 Dual containment vessel experiment with porous media and
orbital mixing
The packing of the connecting tube with porous media proved to be quite difficult. In
an effort to mitigate the effects of voiding across the top of the connecting tube, the
porous media was packed very compactly. This compaction may have made it more
difficult for transmission of particles than if it had been packed more loosely. As the
third experiment did not immediately feature voiding across the top of the connecting
tube, the total iron concentration analysis of this experiment yielded interesting
results. Due to the erosion caused by the elliptical motion of the orbital mixers in this
experiment, a void formed across the connecting tube.
This void allowed the advection of iron particles into the anodic containment vessel
before the final sample was taken (taken at t = 5872 minutes, total iron concentration
= 0.919 mg/L. This influx of iron due to non-electrokinetic effects explains the
extremely high iron level recorded at this time. Once this sample is disregarded, the
iron concentration is seen to be quite stable, and very low. The apparent slight
increase in iron concentrations is not due to electrokinetic phenomena, but rather to
instrumentation creep. This is evidenced by the drop in apparent total iron
concentration at t = 3000 minutes, which coincided with the re-calibration of the AAS
machine. All remaining total iron concentration levels were very low, indicating the
nanoscale zero valent iron had not successfully been moved through the connecting
tube to the anodic containment vessel in the time period.
When removed from the containment vessel, the cathodic electrode was coated in a
black layer. This was presumed to be a coating of nanoscale zero valent iron. This
layer had significantly adhered to the cathode, and did not easily rub-off by hand or
scaping and rubbing with paper. This validated the notion that the nanoscale zero
valent iron is attracted to cathodic sources.
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5.5 Experiment with mechanical mixing
The mechanical mixing experiment results show no significant influx of nanoscale
zero valent iron into the cathodic containment vessel. The slight variances in the
concentrations are attributed to the random fluctuations of the AAS machine, and also
the re-calibrations that were conducted at t = 1090 minutes and t = 6300 minutes.
Overall, the total iron concentrations are insignificant and show no influx of iron
particles over the duration of the experiment.
It is thought that the mechanical agitation of the nanoscale zero valent iron slurry in
the anodic containment vessel resulted in a forcing of the nanoscale zero valent iron
through the porous media in the connecting tube. The penetration of the dark
coloured iron into the porous media reveals the ability of the nanoscale zero valent
iron to move through media, thus demonstrating the iron particle transmission ability
of the porous media.
After more than a week of being submerged in a nanoscale zero valent iron slurry, the
anode did not accumulate a solid black coating, rather a thin film of water and
nanoscale zero valent iron that could easily be removed by wiping with either a finger
or a piece of paper. It is this fact, coupled with the observed coating on the cathode,
which suggests the nanoscale zero valent iron has an affinity for the cathode and not
the anode.
5.6 Experiment with direct injection
5.6.1 Initial direct injection experiment
The total iron concentrations varied considerably between the anodic and cathodic
containment vessels. This is due to the presence of residual iron from a previous
experiment in the anodic containment vessel. This was not considered to be
problematic, as the water samples from each containment vessel were analysed for
changes in total iron concentration, and not absolute iron concentrations. The
relatively large decrease observed after 5.5 days in both anodic and cathodic samples
is explained by the re-calibration of the AAS machine. The total iron concentration
fluctuations for both containment vessels were very small, indicating no significant
transmission of nanoscale zero valent iron through the porous media.
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Mixing prior to each sampling run may have moved iron through connecting tube,
however, since mixing was done equally on both sides, and only just before sampling,
it was not thought to have a significant effect. The flexible injection port tubing was
removed and examined after the experiment. The tubing was sliced longitudinally
and inspected. White porous media was observed at the base of the tubing where it
had been inserted into the connecting tube, to a thickness of 3 mm. This white porous
media had been forced into the injection port tubing when the tubing had been
installed. On top of the white porous media was the nanoscale zero valent iron slurry
that was a distinct black fluid. The iron slurry did not appear to have moved into the
connecting tube and had remained in the injection port for the duration of the
experiment. This suggests that the electrokinetic effects had no significant influence
on the nanoscale zero valent iron, because it did not seem to have been moved.
Ferric (Fe3+) and ferrous (Fe2+) ions could move towards the negatively charged
cathode due to electromigration, but not to the anode due to electro-repulsion. As
both containment vessels experienced similarly insignificant total iron concentration
increases, the transmission of Fe2+ and Fe3+ ions was determined to be insignificant.
5.6.2 Direct injection experiment with enhanced conductivity
The total iron concentration did not significantly increase over time. The fluctuations
observed were due to the small fluctuations of the AAS machine, and not due to an
influx of nanoscale zero valent iron. This coupled with the evidence that the
nanoscale zero valent iron did not leave the injection port flexible tubing suggests that
electrokinetic phenomena were unsuccessful in mobilising the nanoscale zero valent
iron.
The solutions in the two containment vessels were dosed with additional NaCl,
however, the porous media in the connecting tube was wet with non-dosed tap water.
This resulted in the connecting tube acting as a resistive barrier between the two
containment vessels because of its lower relative salinity. The amperage increased to
30 mA after 3.5 days, which indicated that the ions had penetrated through the porous
Chapter 5: Discussion
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University of Western Australia 83
media. The amperage further increased to 40 mA after 10 days, further enforcing the
notion that the added ions had gone through the connecting tube. The enhanced ion
concentrations and thus higher currents involved in this experiment show that
elevated conductivity does not have an appreciable effect on the ability to move
nanoscale zero valent iron using electrokinetic phenomena.
5.7 Hydraulic advection experiment
Despite 10 pore space volumes of fluid passing through the containment vessel, the
nanoscale zero valent iron had only moved a small fraction the length of the
connecting tube, (15 mm (or 15 %) of the length of the tube). This demonstrated that
the nanoscale particles had difficulty travelling through the porous media network of
voids, as they had moved such a small distance after such a comparatively large
discharge.
This experiment was conducted after all previous experiments, resulting in using the
most aged nanoscale zero valent iron slurry. Since the slurry had had the most time
agglomerate, it would have had the largest mean particle size. It is therefore
inconclusive as to if the nanoscale zero valent iron had better penetration capabilities
at a younger age.
However, this experiment does add credence to the theory that the nanoscale zero
valent iron is difficult to move due to its difficulty in fitting through the void
networks. It has therefore not been established if electrokinetic phenomena can be
used to induce movement of nanoscale zero valent iron. It has however been shown
that electrokinetic phenomena are not able to induce a meaningful transmission of
nanoscale zero valent iron in cases where the nanoscale zero valent iron cannot be
moved effectively by a hydraulic gradient.
Chapter 6: Conclusion
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6 Conclusion
6.1 Electrokinetics and nanoscale zero valent iron
Electrokinetic effects were shown to possess the ability to move charged aqueous
ions, such as sodium and chloride ions. The experiments conducted showed that the
nanoscale zero valent iron was not able to be moved through the porous media
effectively by electrokinetic effects. The nanoscale zero valent iron particles did not
move significantly towards either the anode or the cathode in a number of
experiments. It was also shown that the nanoscale zero valent iron was also not able
to be moved through the porous media under a hydraulic gradient. It was therefore
concluded that electrokinetic phenomena could not move nanoscale zero valent iron
in situations where the hydraulic inducement of nanoscale zero valent iron is not
possible.
The nanoscale zero valent iron did have an affinity for the cathode and was not
attracted to the anode significantly. This was evidenced in the altered appearances of
the cathode in the orbital mixing experiment and the anode in the mechanical mixing
experiment.
6.2 Recommendations
An aspect of the experiment that had a large degree of variability was the packing of
the connecting tube with porous media. The packing process used required a great
deal of time to be spent packing the porous media. It was not determined whether the
packing process impeded nanoscale zero valent iron flow because it was so closely
packed. It is therefore recommended that it be ascertained whether the packing of the
porous media has an impact on the ability of the nanoscale zero valent iron to pass
through it.
The difference in the interaction of the nanoscale zero valent iron slurry with the
cathode and with the anode provides an interesting area for further research. The
Chapter 6: Conclusion
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University of Western Australia 85
apparent affinity of the nanoscale zero valent iron particles to the cathode and the lack
of reaction between slurry and anode require more investigation.
It is also recommended to investigate the interaction between the nanoscale zero
valent iron and silica porous media particles at various pH levels. This will give a
greater understanding of the processes involved in the various experiments, and
possible retardation mechanisms.
A further recommendation is to investigate the usage of additional chemical species
such as surfactants or polymers containing both hydrophobic and hydrophilic
constituents to interact with the nanoscale zero valent iron. This has the possibility to
prevent or mitigate agglomeration. Preventing agglomeration has the potential to
increase the ability of the nanoscale zero valent iron to penetrate the porous matrix
and also decrease the force required to move the average sized particle.
Chapter 7: Glossary
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7 Glossary
AAS – Atomic Absorption Spectroscopy.
Agglomeration – adherence of particles into single particle.
Aliphatic – An organic carbon compound in which the atoms are joined in open
chains.
Alkane – A hydrocarbon that contains only single bonds.
Alkyl Halide – An alkane that has had at least one hydrogen replaced with a halogen.
Anode – The positive electrode, where oxidation occurs.
Cathode – The negative electrode, where reduction occurs.
DNAPL – Dense Non-Aqueous Phase Liquid.
Electrode – An electrically conductive structure that transfers electrons.
Ferric – An iron cation that has a charge of 3+.
Ferrous – An iron anion that has a charge of 2+.
Halogenated – A compound containing at least one element that is a halogen.
Halogen – An element coming from group 17 of the periodic table.
Hydrocarbon – An organic chemical consisting of hydrogen and carbon.
Halogenated Hydrocarbon – A hydrocarbon that has had at least one hydrogen
replaced with a halogen atom.
Chapter 7: Glossary
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Pore Space Volume – The volume of a designated zone that is comprised of voids.
Saturated Zone – The zone in which all pore spaces are completely filled with water.
Vadose zone – The zone between land surface and water table that contains water
content less than saturation.
ZVI – Zero Valent Iron.
Chapter 8: References
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8 References
Anonymous, 1995, ‘Spotlight on innovations in remediation’
Civil Engineering; Oct 1995, 65, 10, pp. 26. Available from: ABI/INFORM Global
Atkins, P.W., 1998, ‘Physical Chemistry’, Oxford University Press
Barcelona, M., Keely, J.F., Pettyjohn, W.A., 1990, ‘Contamination of Ground Water:
Prevention, Assessment, Restoration’, Noyes Data Corporation, Park Ridge, New
Jersey.
Casagrande, L. (1948) ‘Electro-osmosis in soils’. Geotechnique, 1948(A), vol. 1, 159-
177
Center for Groundwater Research. (2002), Zero Valent Iron, [online], available from:
< http://cgr.ebs.ogi.edu/iron/> [4 April 2006].
Chew, C.F. and Zhang, T.C., 1997, ‘Nitrate Removal Using Electrokinetic/Iron Wall
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19-22, Kansas State University.
Electroosmosis, 2006, Electroosmotic Flow, [online], available from: <
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Fetter, C.W., 1994, ‘Applied hydrogeology’, Prentice-Hall Inc
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Freeze, R.A., and Cherry J.A.. 1979, ‘Groundwater’. Prentice-Hall, Inc., Englewood
Cliffs, NJ.
Chapter 8: References
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Gass, T.E., 1980, ‘To What Extent Is Ground Water Contaminated’, Water Well
Journal 34 (11):26-27.
Gavaskar, A., Tatar, L., Condit, W., 2005a, ‘Cost and Performance Report Nanoscale
Zero-Valent Iron Technologies for Source Remediation’, Contract Report CR-05-007-
ENV, Naval Facilities Engineering Service Center, Port Hueneme, California 93043-
4301
Gavaskar, A., Tatar, L., Condit, W., 2005b, ‘Nanoscale Zero Valent Iron
Technologies For Source Remediation’, Naval Facilities Engineering Service Center,
Port Hueneme, California 93043-4301.
Gillham, R.W., O’Hannesin, S.F., 1994, ‘Enhanced degredation of halogenated
aliphatics by zero-valent iron’, Ground Water, 32, 958-967.
Gillham, R.W., O’Hannesin, S.F., 1993, ‘Metal enhanced abiotic degradation of
halogenated aliphatics: Laboratory tests and field trials’, Proceedings of the HazMat
Central Conference, Chicago, Illinois, March 9-11
Gillham, R. W., O’Hannesin, S. F., 1992. “Metal-Catalyzed Abiotic Degradation of
Halogenated Organic Compounds.” IAH Conference: Modern Trends in
Hydrogeology. Hamilton, Ontario, May 10-13, pp. 94-103.
Groundwater Protection and Restoration Group, 2006, available from: <
http://www.dnapl.group.shef.ac.uk/starter.htm> [22 July 2006].
Henry, S. M., Warner, S.D., 2002, ‘Chlorinated solvent and DNAPL remediation:
innovative strategies for subsurface cleanup’, Oxford University Press.
Illinois Environmental Protection Agency. 1986. A Plan for Protecting Illinois
Groundwater. Illinois Environmental Protection Agency, Springfield, IL.
Chapter 8: References
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University of Western Australia 90
Kittel, C., 1986, ‘Introduction to Solid State Physics’, John Wiley and Sons, 6 Edition
Lageman, R., Lisbeth M., Ottosen, Alexandra B. Ribeiro
Electrochemical remediation of CCA polluted soil. Proc., Special Seminar on Soil
Remediation. COST Action E22, 6th Workshop Optimisation and Remediation of
Preservatives, Zagreb, Croatia, 21-22 Sep. 2003
Lee, J., 2005, ‘Electrokinetic applications in the remediation NAPL contaminated
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