Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena

Transcription

1 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena 2006 Angus Adams Supervisor: Dr. David Reynolds

2 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena 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. University of Western Australia ii

3 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena 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. University of Western Australia iii

4 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena Table of Contents Table of Contents... iv 1 Introduction Literature Review Remediation in the Saturated Zone Methods of Remediation in the Saturated Zone Excavation Pump-and-treat Soil Vapour Extraction Thermal Treatment In-situ Flushing Passive Reactive Barriers Mass destruction Biological Remediation Containment Zero valent iron Zero valent iron history Zero Valent Iron Chemistry Zero Valent Iron Advantages Nano-scale Zero Valent Iron Zero valent iron delivery Diffusion Electrokinetics Electroosmosis Electrophoresis Electromigration Electrolysis DNAPLS and chlorinated solvent contamination of groundwater Agglomeration chemistry Site applicability Costings...34 University of Western Australia iv

5 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena 3 Methodology Development Instrument Calibration Power supply Iron Concentration in Slurry Determination Single containment vessel experiments Dual containment vessel experiment with unhindered flow Electrodes Containment Vessel Mixing method NaCl experiment Nanoscale Zero Valent Iron Supply Nanoscale zero valent iron experiment with no porous media Dual containment vessel experiment with porous media flow and orbital mixing method Mixing method Orbital mixer board construction Manufacturing of additional side ports in the connecting tube Needle selection Silica filling of tube/screen installation Initial containment vessel experiment with porous media flow and orbital mixing Sampling technique development Second dual containment vessel experiment with porous media flow and orbital mixing Side port construction in the connecting tube Filling of connecting tube with porous media Third dual containment vessel experiment with porous media and orbital mixing Dual containment vessel experiment with porous media flow and mechanical mixing method Mechanical mixing Non-metallic mixing paddle construction...52 University of Western Australia v

6 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena 3.8 Experiment using mechanical mixing Initial direct injection of nanoscale zero valent iron into porous media experiment Direct injection of nanoscale zero valent iron into porous media experiment with enhanced conductivity Initial hydraulic advection experiment Iron concentration sampling Results Iron concentration determination Single containment vessel experiment Dual containment vessel experiment with no porous media Sodium Chloride (NaCl) experiments Initial zero valent iron experiment Second zero valent iron experiment Dual containment vessel experiment with porous media and orbital mixing Initial experiment Second experiment Third experiment Dual containment vessel experiment with porous media and mechanical mixing Dual containment vessel experiment with porous media and direct injection Dual containment vessel experiment with porous media and direct injection with enhanced conductivity Hydraulic advection experiment Discussion Iron concentration determination Single containment vessel Dual containment vessels with unhindered flow NaCl experiment at 20 volts NaCl experiment at 10 volts...79 University of Western Australia vi

7 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena Initial experiment with zero valent iron and porous media Second experiment with zero valent iron and no porous media Dual containment vessel experiment with porous media and orbital mixing Experiment with mechanical mixing Experiment with direct injection Initial direct injection experiment Direct injection experiment with enhanced conductivity Hydraulic advection experiment Conclusion Electrokinetics and nanoscale zero valent iron Recommendations Glossary References...88 University of Western Australia vii

8 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena List of Tables Table 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 University of Western Australia viii

9 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena 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 Figure 4.2 Slurry reaction at cathode 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 University of Western Australia ix

10 Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena 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 University of Western Australia x

11 Chapter 1: Introduction 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). University of Western Australia 11

12 Chapter 1: Introduction 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. University of Western Australia 12

13 Chapter 2: Literature Review 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). University of Western Australia 13

14 Chapter 2: Literature Review 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 University of Western Australia 14

15 Chapter 2: Literature Review 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 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 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 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 University of Western Australia 15

16 Chapter 2: Literature Review 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) 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) 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 cosolvents (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) 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. University of Western Australia 16

17 Chapter 2: Literature Review 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 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 (MnO - 4 ), hydrogen peroxide (H 2 O 2 ), sodium (Na), potassium (K), perchlorate (ClO 4 ), ozone (O 3 ) and certain enzymes have been University of Western Australia 17

18 Chapter 2: Literature Review used in the past to oxidise organic contaminants. Reducing chemicals, such as sodium salts of dithionite (Na 2 S 2 O 4 ) 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 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) 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 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 University of Western Australia 18

19 Chapter 2: Literature Review 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 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 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: Fe 0 Fe e - This reaction has a standard reduction potential of V (Atkins 1998). University of Western Australia 19

20 Chapter 2: Literature Review 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: Fe 0 + RX + H + Fe 2+ + 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: Fe 2+ Fe 3+ + e - This oxidation reaction can be coupled with the reduction half equation to reduce the alkyl halide shown above to yield: Fe 2+ + RX + H + Fe 3+ + 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 (H 2 ). It is a combination of the two following half reactions: Fe 0 Fe e - H 2 O + e - H 2 + OH - University of Western Australia 20

21 Chapter 2: Literature Review To produce: Fe 0 + 2H 2 O Fe H 2 + 2OH - The H 2 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 + H 2 RH + X - + H + It is important to note that the H 2 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. University of Western Australia 21

22 Chapter 2: Literature Review Figure 2.1 Diagram demonstrating three mechanisms of halogenated aliphatic degeneration by zero valent iron (Matheson 1994). University of Western Australia 22

23 Chapter 2: Literature Review 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 Fe 0 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 Fe 2+ to reduce the alkyl halide (Center for Groundwater Research 2002). Figure 2.2 Zero valent iron reactions University of Western Australia 23

24 Chapter 2: Literature Review (Center for Groundwater Research, 2002) 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 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 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 University of Western Australia 24

25 Chapter 2: Literature Review 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: F =!D dc dx where F = mass flux of species per unit area per unit time. D = diffusion coefficient. C = solute concentration. dc = the rate of change of concentration with respect to distance. dx 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). dc = dt D 2 d C 2 dx Where dc/dt = rate of concentration change with respect to time. University of Western Australia 25

26 Chapter 2: Literature Review 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 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 University of Western Australia 26

27 Chapter 2: Literature Review 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 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 (q A ) is determined by q A = Where k e V L A k e = electroosmotic permeability V = electrical potential gradient L A = cross sectional area. Electroosmotic flow can be determined by: q = k e i e A where q is electroosmotic flow rate, k e is electroosmotic permeability, i e 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: University of Western Australia 27

28 Chapter 2: Literature Review Table 2.2 Electroosmotic Flux Factors Affecting Factor Electric field Buffer ph Ionic Strength Temperature Organic Modifier Negative Surfactant Positive Surfactant Neutral hydrophilic polymer Electroosmotic Flux Effect Changes in proportion to potential applied Increases as ph increases Decreases as ionic strength increases Decreases as temperature increases Generally decreases as concentration increases Increases as concentration increases Decreases as concentration increases 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 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 (F e ) experienced by a charged particle under an electric gradient is F e = q! E where q = charge E = electric field This force is countered by the frictional force (F f ), which acts against the movement of the particle. F f = v! f University of Western Australia 28

29 Chapter 2: Literature Review where v = velocity of the particle f = friction coefficient These two equations can be used to determine the effective electrophoretic mobility factor (µ), where; µ = q = f v E 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 Electromigration Electromigration is the movement of charged species, such as Fe 2+ or OH - to the electrode of opposite charge. Migrational flux (J j m ) is dependant on effective ionic University of Western Australia 29

30 Chapter 2: Literature Review mobility, electrical potential, valence and temperature (Acar, 1993). The relationship of migrational flux and its variable is as follows: m * J j = u j # c j # "(! E) where * u j = effective ionic mobility c j = 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: u * j = where D * j z j RT F * D j = effective diffusion coefficient z j = 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 Electrolysis Electrolysis is the application of an electric current to induce a non-spontaneous chemical reaction, such as the splitting of H 2 O to H + and OH - (Atkins 1996). When electrodes are inserted into an aqueous medium, two important reactions may take University of Western Australia 30

31 Chapter 2: Literature Review place, generation of oxygen gas and protons at the anode, and generation of hydrogen gas and hydroxyl ions at the cathode, as shown below: 2H 2 O O 2 + 4H + + 4e - anode 4H 2 O + 4e - 2H 2 + 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 University of Western Australia 31

32 Chapter 2: Literature Review 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 University of Western Australia 32

33 Chapter 2: Literature Review 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 University of Western Australia 33

34 Chapter 2: Literature Review 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$ per year, as opposed to the US$ 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 University of Western Australia 34

35 Chapter 2: Literature Review 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 $300 m -3 (Van Cauwenberghe 1997). Gavaskar (2005a) conducted field trials on three separate sites using zero valent iron and found that it cost US$ to treat a contamination site in Hunters Point (USA) of 1287 m 3 in size containing 6.4 kg of TCE. Another site in Jacksonville (USA) of size 1265 m 3 containing an estimated 27.7 kg of TCE cost US$ to remediate. A third m 3 site in Lakehurst (USA) incurred a cost of US$ 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. University of Western Australia 35

36 Chapter 3: Methodology Development 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 Electrokinetic movement of nanoscale zero valent iron between two hydraulically 2 connected containment vessels Electrokinetic movement of nanoscale zero valent iron through porous media featuring 3 orbital mixing Electrokinetic movement of nanoscale zero valent iron through porous media featuring 4 mechanical mixing Electrokinetic movement of nanoscale zero valent iron directly injected into porous 5 media Electrokinetic movement of nanoscale zero valent iron directly injected into porous 6 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 University of Western Australia 36

37 Chapter 3: Methodology Development 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 AF 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 AK 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. University of Western Australia 37

38 Chapter 3: Methodology Development 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 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 University of Western Australia 38

39 Chapter 3: Methodology Development 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 10 7 / 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 nonpermeable 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 University of Western Australia 39

40 Chapter 3: Methodology Development 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 TiO 2 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 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 University of Western Australia 40

41 Chapter 3: Methodology Development 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 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. University of Western Australia 41

42 Chapter 3: Methodology Development 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 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. University of Western Australia 42

43 Chapter 3: Methodology Development 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 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 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 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 University of Western Australia 43

44 Chapter 3: Methodology Development 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 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. University of Western Australia 44

45 Chapter 3: Methodology Development 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 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 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 University of Western Australia 45

46 Chapter 3: Methodology Development of the Ratek EOM5 orbital mixer plate; this was to prevent any slippage whilst the mixer plate was in motion 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 University of Western Australia 46

47 Chapter 3: Methodology Development Figure 3.5 Side view of connecting tube with three sampling ports fitted with flexible tubing 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 # SGE 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 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 University of Western Australia 47

48 Chapter 3: Methodology Development 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. University of Western Australia 48

49 Chapter 3: Methodology Development Figure 3.7 Connecting tube ready for insertion between two containment vessels. Figure 3.8 Semi filled connecting tube capped with pink screens. University of Western Australia 49

50 Chapter 3: Methodology Development 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 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 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 University of Western Australia 50

51 Chapter 3: Methodology Development source delivered 20 volts to the electrodes, and the orbital mixer was set on setting of 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 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 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 University of Western Australia 51

52 Chapter 3: Methodology Development 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 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 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 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 University of Western Australia 52

53 Chapter 3: Methodology Development 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. University of Western Australia 53

54 Chapter 3: Methodology Development 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 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. University of Western Australia 54

55 Chapter 3: Methodology Development 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 University of Western Australia 55

56 Chapter 3: Methodology Development valent iron to be considered in the field, it would have to be able to move 100mm in this time period 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 University of Western Australia 56

57 Chapter 3: Methodology Development 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 (HNO 3 ) solution, to ensure the dissolution of all the iron. University of Western Australia 57

58 Chapter 4: Results 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 99.9 specific gravity of less than unity. = 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 = 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 University of Western Australia 58

59 Chapter 4: Results 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. University of Western Australia 59

60 Chapter 4: Results Figure 4.2 Slurry reaction at cathode. 4.3 Dual containment vessel experiment with no porous media 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) NaCl experiment at 20 volts The conductivity results from the NaCl experiment conducted at 20 volts are shown below in Figure 4.3. University of Western Australia 60

61 Chapter 4: Results NaCl expt Conductivity (ms) Time (mins) 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 NaCl experiment at 10 volts The conductivity results from the NaCl experiment conducted at 10 volts are shown below in Figure 4.4. University of Western Australia 61

62 Chapter 4: Results NaCl 10V Run Conductivity (ms) Time (mins) 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 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. University of Western Australia 62

63 Chapter 4: Results Aged ZVI Experiment Fe concnetration (mg/l) ` Time (mins) Figure 4.5 Aged Zero Valent Iron Experiment Iron Concentration Aged ZVI Experiment ph Time (mins) Figure 4.6 Aged Zero Valent Iron ph 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 University of Western Australia 63

64 Chapter 4: Results 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 nonelectrokinetic phenomena. Figure 4.7 shows the results of the analysis of the samples taken during this experiment. No Porous Media Experiment Fe Concentration (mg/l) Time (mins) 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 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 University of Western Australia 64

65 Chapter 4: Results 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 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 University of Western Australia 65

66 Chapter 4: Results stopped, again because of the advection of the nanoscale zero valent iron masked any electrokinetic induced movement. Figure 4.9 Connecting tube featuring voiding 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 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 University of Western Australia 66

67 Chapter 4: Results 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. University of Western Australia 67

68 Chapter 4: Results Orbital mixing Expermient with no Voiding 1 Fe Concentration (mg/l) Time (mins) 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 University of Western Australia 68

69 Chapter 4: Results 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. University of Western Australia 69

70 Chapter 4: Results Mechanical Mixing Experiment Fe Concenration (mg/l) Time (mins) 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 mg/l, fell to a low of mg/l, and had a range of 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 University of Western Australia 70

71 Chapter 4: Results entire core sample did not appear to have any trace of black nanoscale zero valent iron, as seen in Figure Total iron concentrations can be seen in Figure Figure 4.16 Core sample of connecting tube featuring no visible nanoscale zero valent iron penetration Direct Injection Experiment Fe Concentration (mg/l) Time (mins) 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 University of Western Australia 71

72 Chapter 4: Results 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 mg/l and mg/l respectively, and had a range of mg/l. The maximum and minimum iron concentrations in the anodic containment vessel were mg/l and mg/l respectively, and had a range of 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 Fe Concentration (mg/l) Time (mins) University of Western Australia 72

73 Chapter 4: Results 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 for the remainder of the experiment. NaCl Dosed Direct Injection Experiment ph Conductivity (ms/cm) Time (mins) 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. University of Western Australia 73

74 Chapter 4: Results NaCl Dosed Direct Injection Experiment Current Levels 50 Amperage (ma) Time (mins) 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 = "! 2.5 2! 10 = ml Void volume = 0.4! = 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. University of Western Australia 74

75 Chapter 4: Results 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 Fe concentration (mg/l) Time (mins) 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. University of Western Australia 75

76 Chapter 5: Discussion 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 48 additional oxygen molecules would contribute 48 g, or! 100 = % 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. University of Western Australia 76

77 Chapter 5: Discussion 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 Fe 0 to Fe 2+ and/or Fe 3+. 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 Fe 3 O 4. This did not occur with the mixed metal oxide anode because its external surface did not contain significant amounts of Fe 0 to be oxidised. The brown sludge that formed in both experiments (Figure 5.1), could be explained by formation of ferric oxide (Fe 2 O 3 ). 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. University of Western Australia 77

78 Chapter 5: Discussion Figure 5.1 Powered electrodes immersed in a nanoscale zero valent iron slurry 5.3 Dual containment vessels with unhindered flow 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. University of Western Australia 78