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DNAPL Behavior in Fractured Rock – Transport Facilitated by Pickering Emulsions

B.H. Kueper, A. Roy-Perreault, and M.R. West Department of Civil Engineering

Queen’s University Kingston, Ontario, Canada K7L 3N6

Abstract Laboratory experiments demonstrate that finely divided solid materials (bentonite, iron oxide, magnesium oxide and manganese oxide) can form stable o/w emulsions for NAPLs such as PCBs, TCE, and diesel fuel. These emulsions, referred to as Pickering emulsions, are stabilized by finely divided solid materials that coat the NAPL-water interface; they do not require the presence of a surfactant to form or to be stabilized. In addition to the presence of finely divided solids, Pickering emulsions require energy to initially emulsify the NAPL. It is proposed that drilling through NAPL source zones in fractured bedrock may provide both the energy and the finely divided solids to form and stabilize Pickering emulsions. Pickering emulsions can represent a form of facilitated NAPL transport in fractured bedrock in that the emulsion droplets have the potential to migrate through large aperture fractures. Evidence of such behavior would include (i) measured aqueous phase concentrations greater than the solubility of the NAPL, (ii) the presence of aqueous phase contamination beyond where advective-dispersive transport subject to sorption can account for it, and (iii) short-term concentration spikes immediately following drilling activities. Pickering emulsions have been visually observed exiting bedrock fractures at a site in the north-eastern U.S. in the vicinity of a fault plane within a DNAPL source zone where extensive drilling had taken place. 1.0 - Introduction Groundwater contamination associated with dense, non-aqueous phase liquids (DNAPLs) such as chlorinated solvents, PCB oils, coal tar, and creosote has been frequently observed throughout industrialized areas of the world. Upon release to the subsurface, DNAPL will distribute itself in the form of disconnected blobs and ganglia of organic liquid referred to as residual DNAPL, as well as in continuous distributions referred to as pools (Figure 1). Residual and pooled DNAPL will form in both unconsolidated deposits, as well as in fractures present in rock and clay (Kueper and McWhorter, 1991; Longino and Kueper, 1999). As groundwater flows past residual and pooled DNAPL, mass transfer from the DNAPL to the water results in the formation of aqueous phase plumes. With respect to transport mechanisms, it is well established that the DNAPL phase itself will migrate subject to capillary, viscous, and gravity forces. For moderate to high viscosity DNAPLs, and/or large release volumes, DNAPL migration can also be influenced by mass transfer processes such as dissolution and vapourization. Solute transport in the aqueous phase is subject to advection, dispersion, volatilization, sorption, NAPL-water partitioning, and potential degradation mechanisms. In dual porosity media, such as fractured sedimentary rock and fractured clay, solute transport in the aqueous phase can also be significantly influenced by both forward and back diffusion processes. With respect to DNAPL migration, it is commonly believed that the primary means of movement are bulk flow of the contiguous DNAPL phase following its initial release to the subsurface, and pool remobilization in response to actions such as increases in the hydraulic gradient, lowering of interfacial tension, short-circuiting along borings and monitoring wells, or increases in DNAPL saturation. An additional form of potential DNAPL migration, however, is movement in the form of an emulsion. To date, contaminant hydrogeologists have generally associated this form of facilitated transport with chemical remediation methods such as surfactant and alcohol flushing. In these cases, it is the presence of a chemical agent (e.g., surfactant) that can

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both form and stabilize an emulsion. An additional DNAPL emulsion transport process that has not yet received widespread attention in the hydrogeology community, however, is migration as a Pickering emulsion.

Figure 1 – Schematic illustrating presence of residual and pooled DNAPL in both porous and fractured media. Pickering emulsions are stabilized by finely divided solid materials and do not require the presence of a surfactant to form or to be stabilized. Pickering emulsions have been formally studied in the petroleum, pharmaceutical, and chemical engineering disciplines, but have not yet been systematically examined in the context of contaminant hydrogeology. In particular, we are not aware of any work in any discipline examining the potential for PCB DNAPLs to form Pickering emulsions. It is hypothesized here that Pickering emulsions can form in fractured bedrock in the presence of finely divided solid material that is either naturally present (e.g., oxides, clays), formed as a result of drilling activities, or introduced as a result of grouting and borehole sealing activities. The existence of Pickering emulsions in a contaminant hydrogeology context is consistent with observations at an industrial site in the north-eastern U.S. where the authors have visually identified a solid-stabilized PCB DNAPL emulsion exiting from rock fractures. The emulsion was subjected to X-Ray Fluorescence analysis which revealed the presence of inorganic elements such as chlorine, calcium, silicon and sulfur in the white coating (Rawson, 2002). It is believed that the finely divided solid material was derived from either faults known to exist in the immediate area of the observation, and/or from extensive drilling activities that are known to have taken place within the DNAPL source zone immediately upgradient of the observed emulsions. Other evidence of the existence of Pickering emulsions includes the observation of elevated PCB concentration spikes (greater than solubility in some cases) in monitoring wells at the Smithville, Ontario PCB site that are correlated with drilling activities. The purpose of this paper is to describe the formation and stabilization of Pickering emulsions, with an emphasis on polychlorinated biphenyls (PCBs). Data is also presented showing the formation of fuel oil and chlorinated solvent Pickering emulsions. This paper presents the results of laboratory batch experiments examining the use of bentonite, iron oxide, magnesium oxide and manganese oxide as stabilizing agents, and

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discusses the implications of Pickering emulsions on field characterization activities such as drilling through DNAPL source zones in fractured bedrock. 2.0 – Overview of Pickering Emulsions Emulsions stabilized by finely divided solids are referred to as Pickering emulsions, first recognized by Ramsden and Oxon (1903) and later studied in detail by Pickering (1907). A wide variety of solid particles have been used in the past as stabilizers for this type of emulsion, including iron oxides, hydroxides, metal sulphates, silica, clay and carbon (Binks, 2002). The main stabilization mechanism involved in the formation of Pickering emulsions is the creation of a mechanical barrier at the droplet interface that inhibits coalescence. Along with this mechanical barrier, the change in rheological properties at the NAPL/water interface and the reduced mobility of the particles associated with it contribute to the stabilization of the system (e.g., Tambe and Sharma, 1993; Abend et al, 1998). The simple presence of finely divided solids is not adequate to form a Pickering emulsion; energy must be imparted to the NAPL-water system to first disperse the NAPL phase into droplets, which are then subsequently stabilized by the solid particles. The type of emulsion formed, oil (i.e., NAPL) droplets dispersed in water (o/w) versus water droplets dispersed in oil (w/o), is dependant upon the wettability of the solid particles. This is typically quantified by the contact angle between the solid particles and the NAPL-water interface. When NAPL and water are in the presence of particles preferentially wetted by the water phase, an o/w emulsion is observed. For particles preferentially wetted by oil, w/o emulsions are expected (Binks, 2002). The contact angle also affects the stability of the emulsion through the energy of attachment of particles at the interfaces (Binks and Lumsdon, 2000a). The energy of attachment, and therefore the stability of the emulsion, increases as the contact angle approaches 90°, where it reaches a maximum. Very hydrophilic or hydrophobic emulsifiers, for example, are known to form large droplets which are susceptible to coalescence (Binks and Lumdson, 2000b). In general, factors that affect surface chemistry and contact angle will impact the stability and characteristics of the emulsion. Inversion of the emulsion can occur when changing the water volume fraction of the system (Binks and Lumsdon, 2000b). In addition, inversions have been observed by altering the ionic balance of the solution by increasing the pH. There is a minimum concentration of solid particles required for an emulsion to be formed (Tambe and Sharma, 1993). Beyond this, the droplet size is expected to decrease with an increase in the solid particle concentration (Gelot et al., 1984; Yan and Masliyah, 1995). At some point, however, the diameter decreases to where it becomes independent of the solid concentration and remains relatively constant (Ashby and Binks, 2000). Changes in the droplet size over time have been observed and are typically attributed to either the coalescence of droplets, or Ostwald ripening. Ostwald ripening is induced by an equilibration process which takes place between droplets of different sizes and results in an increase in the mean droplet diameter. This is caused by the increased solubility of the liquid contained in the smaller droplets, as predicted by the Kelvin equation. However, Ashby and Binks (2000) proposed that this concept needs to be adapted as it does not take into account the changes in interfacial tension which occur in Pickering emulsions when the droplet diameter is varied. 3.0 – PCB Laboratory Batch Experiments 3.1 - Solid particles This study employed bentonite, iron oxide, magnesium oxide and manganese oxide as emulsion stabilizers. Bentonite is often used as a component in well-drilling muds and in grout slurries for well completion activities. Bentonite is a montmorillonite-rich clay, a subcategory of the smectite group. Iron oxide (also known as ferric oxide, or iron oxide III) is a red brown powder, essentially insoluble in water. The mineral form of iron oxide is hematite or, more rarely, maghemite. Magnesium oxide is a white powder, slightly soluble (0.86 g/l) in water (Alfa Aesar, 1999) and appears in mineral form as periclase. Manganese oxide, also known as manganese oxide (IV), is a black powder essentially insoluble in water and appears in mineral form as pyrolusite. The size distribution of each solid material was measured with a Coulter Laser Scattering Analyzer employing

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Fraunhofer diffraction (LS 200, VSM+). The median particle diameter (d50) was calculated to be 3.6 µm for bentonite, 0.5 µm for iron oxide, 3.8 µm for magnesium oxide, and 3.1 µm for manganese oxide. In order to act as efficient emulsion stabilizers, it is necessary that the size of the stabilizing particles be smaller than the size of the emulsion droplets to be stabilized. Further information regarding the solid particles can be found in Roy-Perreault et al. (2004). 3.2 - NAPL Phase The primary NAPL examined in this study is a polychlorinated biphenyl (PCB). In liquid form, PCBs are denser than water, colourless, viscous oils made up of up to 209 individual congeners. The different PCB congeners have varying levels of chlorination with most of the formerly available commercial PCB oils being mixtures of these compounds. The PCB mixture used in this study was Aroclor 1242, which accounted alone for 52% of the United States’ PCB production (EPA, 1998). Aroclor 1242 is composed primarily of the C12H7Cl3 congener and consequently contains approximately 42% chlorine by weight (Monsanto, 1988). The Aroclor 1242 utilized in this study is characterized by a density of 1.383 g/cc, a viscosity of 63 cP (22 oC), and a DNAPL-water interfacial tension of 39 dynes/cm (measured using ASTM D971). The Aroclor 1242 was obtained from Trans-Cycle Industries (Kirkland Lake, Ontario, Canada), a firm specializing in the disposal and recycling of PCBs. The aqueous solubility of Aroclor 1242 is reported to range from 0.10 mg/l (24°C) to to 0.703 mg/l (23°) as summarized by Mackay et al. (1992). In this study, a PCB solubility value of 0.24 mg/l (EPA, 1979b) is adopted because of its good correlation with our experimental results. In addition to PCB, this study examined the formation and stability of Pickering emulsions using trichloroethylene (TCE) and diesel fuel (Jet-A-1) as NAPLs (see Section 4). 3.3 - Methodology The solid concentration, χ, is expressed as χ = ms/mw where ms is the mass of the solid and mw is the mass of the water. The solid dispersions were prepared by adding a known mass of solid powder to a known volume of deionised water, followed by mixing and further dilution to arrive at solutions with concentrations of 0.5, 1, 2, 3 and 4 wt%. Different volumes of the solid dispersion were placed in 40 ml glass vials equipped with Teflon lined plastic caps. A 3 ml aliquot of PCB was then added to each dispersion. The resulting oil volume fraction, φoil, is the ratio of the volume of oil to the total volume of the liquid phase. The standard mixing technique consisted of shaking by hand, vigorously and continuously, for one minute. Three parameters were varied amongst a total of 24 tests including the agitation technique and intensity, the concentration of the solid dispersions and the oil volume fraction. The employed agitation techniques included hand shaking, mechanical agitation, and use of a homogenizer for durations of between 1 and 5 minutes. The concentration of the solid dispersion was varied from 0 to 5 wt%, while the oil volume fraction was varied from 0 to 90 vol%. Sample stability was assessed 6 hours after preparation for each test. Each sample was observed under the microscope and droplet size measurements were performed. Selected samples were also evaluated as a function of time with measurements taken at 24 hours, 1 week, and every month for 3 months. Quality control samples were prepared and analyzed including blanks, replicates (16 samples) and duplicates (23 samples). The size distribution of the emulsions was evaluated using a stereo microscope (Olympus SZ11) providing magnification ranging from 10x to 220x. A CCD black and white camera, mounted on the microscope, was connected to a computer containing an image analysis system (LECO 2001, software version 2.02). Several photographs of each emulsion were taken and analyzed by measuring the diameter of droplets in an aliquot and processing the information to obtain an average droplet size and the size distribution. A minimum of 100 droplets were used to compile the average droplet size (dave). Further details are provided by Roy-Perreault et al. (2004).

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3.4 – Results Blank samples composed solely of PCB DNAPL and water were tested in order to assess the stability of emulsions in the absence of the solid dispersions. PCB droplets were formed when the PCB in water solution was agitated, but these droplets immediately sedimented to the bottom of the container and the droplets quickly coalesced to form two separate, continuous phases. This indicates that while the energy provided during the test is sufficient to break the continuous oil phase into small droplets, the droplets are unstable, resulting in coalescence. Preliminary tests were performed with all four solid dispersions to assess whether the emulsified droplets would be stable when diluted with pure water. It was observed that after dilution (1:100) with pure water, the droplets were stable against dissolution and coalescence, even after rigorous agitation. The tests were performed using an initial solid concentration of 2 wt% and an oil volume fraction of 30 vol%. All samples were observed for a period of 30 days. In general, bentonite, iron oxide and magnesium oxide were effective at stabilizing the oil droplets throughout the testing program. For the base case conditions (φoil = 30 vol%; χ = 2 wt%) the formed emulsions were stable with average droplet diameters 6 hours after preparation of 80 µm for magnesium oxide, 102 µm for bentonite, 134 µm for the iron oxide, and 258 µm for the manganese oxide dispersion. The droplet size distributions for the base case conditions 6 hours and 3 months after preparation are presented in Figures 2(a) and 2(b), respectively. Photographs of the emulsions formed under base case conditions are presented in Figure 3. Unlike the other three solid dispersions, the manganese oxide dispersions did not form stable emulsions over the range of employed solid concentrations and oil-water volume fractions. It was observed that the manganese particles would preferentially reside within the PCB phase, indicating a non-optimal contact angle with respect to Pickering emulsion stabilization.

0%

10%

20%

30%

40%

50%

0 100 200 300 400 500Droplet diameter (µm)

Freq

uenc

y

Bentonite

Iron oxide

Magnesium oxide

Manganese oxide

0%

10%

20%

30%

40%

50%

0 100 200 300 400 500

Droplet diameter (µm)

Freq

uenc

y

Bentonite

Iron oxide

Magnesium oxide

(a) (b) Figure 2 - Size distribution of Pickering emulsion of droplets (χ = 2 wt%; φoil = 30 vol%). (a) 6 hours after preparation, (b) 3 months after preparation. Manganese oxide data is not shown at 3 months due to the relatively unstable nature of the emulsion. In general, the average droplet size varied little over the 3 month testing program for most of the samples. Some samples exhibited varying droplet size during the first week, following which little change was observed. The relatively little change in average droplet size is attributable to a lack of droplet coalescence and a very weak Ostwald ripening effect. Ostwald ripening is induced by an equilibration phenomena taking place between the continuous phase and the droplets of different sizes. The solubility of the material concealed in the droplets varies as a function of the size of the drop as expressed by the Kelvin equation (Kabalnov et al., 1987):

6

)2exp()()(

rRTVcrc mσ

∞= (1)

where c(r) is the aqueous phase solubility of oil contained within a drop of radius r, c(∞) is the solubility in a system with only a planar interface, σ is the interfacial tension between the two phases, Vm is the molar volume of the oil, T is the temperature and R the universal gas constant. Bentonite

Iron oxide

Magnesium oxide

Figure 3 - Optical microscopy photographs of PCB Pickering emulsions (φoil = 30 vol%; χ = 2 wt%) for the different solid dispersions, 6 hours after preparation. Equation (1) predicts that material enclosed in smaller droplets has a greater solubility than material in larger droplets, implying that smaller droplets dissolve more easily into the continuous phase. The dissolved material diffuses through the continuous phase and can re-condense into larger droplets, leading to an overall increase in the average droplet diameter (Ashby and Binks, 2000). The rate of ripening, ω, as formulated by Lifshitz, Slezov and Wagner (1987) is:

RT

DVcdtrd mc

9)(8)( 3 σω ∞

== (2)

500µm

500µm

500µm

7

where D is the free solution diffusion coefficient and rc, is the mean droplet radius at any given time. This equation has been experimentally verified for surfactant stabilized emulsions and predicts a linear increase in the mean droplet radius with time. A further implication is that the system should approach zero variability in droplet size at late time. The change in solubility was calculated using (1) for PCB droplets within the range of diameters observed. It was concluded that Ostwald ripening had little if any effect on the solubility of the oil contained in the droplets. While Ostwald ripening has a significant impact on the solubility of droplets with diameters smaller than 0.01 µm, droplets with a diameter from 1-1000 µm are predicted to display solubility changes of less than 1%. The rate of ripening of the system was calculated to be 2.7 x10-25 m3/s, which corresponds to an increase of 0.7 µm/month on the average radius. The variation of the volumetric oil fraction had a significant impact on the stability, average droplet size, and size distribution of the emulsions formed. It was observed that emulsions could not be stabilized by any solid dispersion for oil fractions greater than 70 vol%. The maximum PCB fraction that allows stable emulsions to be formed is 70 vol% when stabilized with bentonite, iron oxide or magnesium oxide dispersions, and 60 vol% for manganese oxide. The average droplet size of the emulsion goes through a minimum at intermediate oil fractions and reaches a maximum at the upper and lower limits. This behaviour has also been reported in the emulsion literature by Ashby and Binks (2000) who worked with toluene stabilized by Laponite clays, a synthetic smectite. In addition to the change in the average droplet diameter, the size distribution of the droplets at low oil fraction is spread out and the standard deviation is high. As the oil fraction increases, the distribution sharpens and exhibits a wide distribution at higher oil contents. The solid concentration had little if any impact on the potential for formation of emulsions as long as the concentration was at least 0.5 wt% for all three solids used. In the case of bentonite, however, for lower concentrations (0.5 wt%), some non-spherical droplets were observed. With the exception of the samples stabilized with manganese oxides, the stability to coalescence as a function of time was not affected by the solid concentration in the colloidal dispersion for any of the solids tested. The average droplet size is maximised at low solid concentrations, and decreases at intermediate solid concentrations. This can be explained by the fact that once a minimum droplet coverage is attained, the emulsion’s droplet size is expected to decrease with an increase in the solid particle concentration (Gelot et al, 1984; Yan and Masliyah, 1995). Figure 4 illustrates PCB emulsions for solid concentrations of 5 vol%, 7 vol% and 10 vol%.

Figure 4 – PCB Pickering emulsions (φoil = 30 vol%) stabilized with bentonite solid dispersion. The concentration of solid in the dispersion equals 5 vol% (left), 7 vol% (middle) and 10 vol% (right). The opaque residue on top of the settled emulsion droplets is bentonite powder.

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All tests performed with agitation techniques that provided less intensity than the standard method were unsuccessful at forming emulsions, and immediately after agitation, the PCB DNAPL and water would separate into two distinct phases. When using an agitator (Mistral multi-mix 4500) to shake the samples no emulsions were formed after either 1 or 5 minutes of agitation. The homogenizer (Hamilton Beach, Scovill) was partially successful. Because the spindle of this instrument is situated close to the bottom of the container it would emulsify only the upper part of the PCB phase in which it was immersed. The emulsion formed was then in contact with a coalesced PCB phase, which contributed to the instability of the system. These results suggest that a minimum amount of energy needs to be imparted to the system in order to form NAPL droplets susceptible to stabilization by finely divided solid particles. 4. 0 - Experiments with TCE and Diesel Fuel A series of batch experiments were performed using TCE and diesel fuel (Jet-A-1) as the NAPLs of interest. The solid dispersions (2%) included bentonite, iron oxide, magnesium oxide and manganese oxide as emulsion stabilizers. Emulsions formed in the absence of any solid dispersion immediately coalesced into bulk phases. Emulsions of diesel/magnesium oxide, diesel/iron oxide, diesel/bentonite, TCE/magnesium oxide, and TCE/bentonite were stable and coalescence was not observed. Emulsions of diesel/manganese oxide, TCE/iron oxide, and TCE/manganese oxide were unstable and coalesced to varying degrees. To investigate the effect of diluting the formed Pickering emulsions, each of the stable emulsions was placed into a beaker of de-ionized water. The diesel/iron oxide and diesel/magnesium oxide emulsions coalesced immediately into separate bulk phases. The diesel/bentonite emulsions yielded stable droplets with reduced diameters. The TCE/magnesium droplets became semi-stable with dilution in that a fraction of the original droplets coalesced, releasing magnesium oxide into the water column. The TCE/bentonite droplets did not visually coalesce with dilution, and appeared to be the most stable of the various tested combinations. Figure 5 illustrates microscope photographs of the TCE/bentonite emulsion initially (a), after 6 days (b), and after 1 ml of the emulsion was diluted with 100 ml of water (c). Figure 6 presents the corresponding droplet size distribution graphs, illustrating that there is little discernable change in the overall range of droplet sizes.

(a) (b) (c) Figure 5 – TCE/bentonite Pickering emulsion (a) initially, (b) after 6 days, and (c) after 1 ml of the emulsion was diluted with 100 ml of water.

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0

2

4

6

8

10

12

14

16

0 50 100 150 200 250 300 350 400Droplet Diameter (um)

Freq

uenc

y (%

)

Bentonite Initial

Bentonite - 6 days

Bentonite - Diluted

Figure 6 – TCE/bentonite Pickering emulsion droplet size distributions 5.0 – PCB DNAPL Column Experiments 5.1 – General Description The completed PCB batch experiments (Section 3) indicate that two necessary conditions for the formation of a stable Pickering emulsion are (i) sufficient energy to break the NAPL phase into droplets, and (ii) the presence of finely divided solids to stabilize the emulsions. To assess whether or not groundwater flowing through residual and pooled PCB DNAPL could provide the necessary energy, a series of 15 porous media column experiments were performed. The employed glass column had an internal diameter of 5.0 cm and a maximum packed height of 49.0 cm. The experimental set-up schematic is presented in Figure 7. Water and the aqueous solid dispersions were introduced into the sand column through constant head reservoirs. The effluent exiting of the column was collected in 200 ml Teflon capped glass jars. All experiments where DNAPL was at residual saturation were conducted with downward flow. Upward flow conditions were used for the DNAPL pool experiments. For the upward flow experiment, the experimental set-up was the same as that presented in Figure 7, but with the inlet reservoirs feeding the bottom of the column and the effluent sampled from the top. Both a medium sand (12/20 mesh) and a fine sand (40/60 mesh) were used in these experiments. The medium and fine sand hydraulic conductivities were 0.67 cm/s and 9.1 x 10-3 cm/s, respectively. The medium sand presented non-laminar flow conditions for gradients higher than 0.25. The flow experiments with iron oxide and magnesium oxide dispersions were carried out using 3 wt% solid dispersions. For the bentonite dispersions, both 1 wt% and 3 wt% were used. For the residual DNAPL experiments, wet sand was mixed with a known volume of PCB in order to obtain an initial non-wetting phase saturation (SNW) of 20%. The fist step of the experiment consisted of flushing deionised water at increasing gradients (up to 1.1) through the column until no more of the DNAPL phase was observed in the effluent. This was done in order to ensure that the DNAPL existed at residual saturation levels. For the pooled DNAPL experiments, a base layer of medium sand was emplaced followed by a 3 cm layer of fine sand, which as in turn overlain by a 5 cm layer of the medium sand. DNAPL was placed above the fine sand layer, which provided the capillary resistance (sufficiently high entry pressure) to support the DNAPL as a pool.

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Figure 7 - Schematic of the downward flow laboratory-scale injection system. A total of 15 experiments were performed as outlined in Table 1. The emulsification potential of each solid was verified under 5 different hydraulic gradients (0.01, 0.05, 0.25, 0.5 and 1.0). In the case of the bentonite dispersion, two extra experiments were conducted: one at a higher solid concentration in the dispersion (3 wt% rather than 1 wt%) and one with the PCB under pooled conditions as opposed to the residual saturation used in the other tests. These latter tests were conducted using a hydraulic gradient of 1.0. The low gradient experiments always preceded the higher gradient experiments, and the column was repacked every time a new solid dispersion was used. This was completed a total of 4 times; once for each tested solid and once for the pooled DNAPL experiment. The presence/absence of emulsification in the system was assessed by three different methods including (i) microscopic and visual observations of aliquots from the effluent, (ii) evaluation of the PCB concentration in the effluent by GC analysis, and (iii) by monitoring of the changes in the hydraulic conductivity of the system. The microscopic and visual observations were performed systematically on every effluent sample taken. The evaluation of the PCB concentration was carried out using gas chromatography (EPA, 1999).

H2O Solid

dispersion

drain Sample drained by gravity

Data acquisition system

5.04

adjustableheight

2-way valves

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Table 1 - Summary of column experiments involving residual and pooled DNAPL

Test # Solid Dispersion (χ) Hydraulic gradient (∇h) PCB saturation

1 0.01

2 0.05

3 0.25

4 0.50

5

Bentonite (1 wt%)

1.00

6 Bentonite (3 wt%) 1.00

residual

7 Bentonite (1 wt%) 1.00 pooled

8 0.01

9 0.05

10 0.25

11 0.50

12

Iron oxide (3 wt%)

1.00

13 0.25

14 0.50

15

Magnesium oxide (3 wt%)

1.00

residual

5.2 – Results and Discussion No emulsions were observed in any of the effluent samples with either visual inspection or with the microscope in any of the 15 performed experiments. In addition, none of the GC analyses revealed PCB concentrations that could have been conclusively related to the presence of emulsions. This implies that no PCBs were mobilized under the form of an emulsion, and that the only form of PCBs in the effluent samples was dissolved (aqueous phase) PCB. As a final check, visual inspection of the residual DNAPL left in the column once the experiments were completed did not reveal the presence of any emulsions trapped in the medium. For the pooled DNAPL experiment, water samples taken (upon disassembly of the column) from different intervals above the perched PCB pool were observed using an optical microscope and no emulsions were seen in the samples taken from within the pool, or in any of the samples taken hydraulically downgradient of the pool. The reductions of the hydraulic conductivity observed were only related due to the change in flow properties of the solid dispersions. It was observed that once the solid dispersion was replaced by clear water, the initial hydraulic conductivity of the sand was recovered, also implying that emulsions were not present in the sand pack. The overall conclusion from this series of column experiments is that the employed hydraulic conditions were not capable of shearing DNAPL droplets from either the residual or the pooled DNAPL. It appears that a higher amount of energy would need to be added to the system to allow such droplets to form. 6.0 – Conclusions The performed laboratory experiments demonstrate that bentonite, iron oxide, and magnesium oxide are efficient stabilizers leading to the formation of PCB DNAPL Pickering emulsions. Pickering emulsions were also formed using diesel and TCE as the NAPLs of interest. Two necessary conditions to form a Pickering

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emulsion are (i) sufficient energy to initially emulsify the NAPL, and (ii) the presence of finely divided solids of appropriate wettability and sufficient concentration to then stabilize the emulsion against coalescence. It is suggested here that drilling through NAPL source zones in fractured bedrock can provide both the energy and solid material required to form and stabilize Pickering emulsions. Evidence of this occurring at a site would include observations such as (i) measured aqueous phase concentrations greater than the solubility of the NAPL, (ii) the presence of aqueous phase contamination beyond where advective-dispersive transport subject to sorption can account for it, and (iii) short-term concentration spikes immediately following drilling activities. Pickering emulsions have been observed exiting bedrock fractures at a site located in the north-eastern U.S. The presence of Pickering emulsions could also explain the concentration spikes associated with drilling activities at the Smithville PCB site located in south-western Ontario. With respect to travel distance, this study did not examine the filtering and straining aspects of bedrock fractures. It should be expected, however, that Pickering emulsion transport will be restricted to the larger aperture fractures, and the larger aperture regions of individual fractures. Pickering emulsion transport is therefore to be most likely observed at sites that exhibit high fracture permeability. Other factors such as pH changes, electrical charge, and Ostwald ripening can also limit or completely prevent the transport of a Pickering emulsion through bedrock fractures. These factors are currently poorly understood, and further research is required to better define those sites and site conditions that are conducive to Pickering emulsion transport through fractured bedrock. Acknowledgements The authors would like to acknowledge the financial assistance of the General Electric Company, the Natural Sciences and Engineering Research Council (NSERC) of Canada, Queen's University at Kingston, and the Senator Frank Carrel Fellowship. Special thanks is given to Dr. J. Rawson for providing the initial motivation for this project. References Abend, S., Bonnke, N., Gutschner, U., Lagaly, G., 1998. Stabilization of Emulsions by Heterocoagulation of Clay Minerals and Layered Double Hydroxides. Colloid Polym. Sci., 276, 730-737. Alfa Aesar, 1999. Material Safety Data Sheet (MSDS) for magnesium oxide (cas # 1309-48-4), section 9: physical and chemical properties. Reviewed on 04/15/1999. Ashby, N.P., Binks, B.P., 2000. Pickering Emulsions Stabilized by Laponite Clay Particles. Phys.Chem.Chem.Phys., 2 (24), 5640-5646. Binks, B.P., 2002. Particles as surfactants-Similarities and Differences. Curr. Opin. Colloid. In., 7, 21-41. Binks, B.P., Lumsdon, S.O., 2000a. Effect of Oil Type and Aqueous Phase Composition on Oil-Water Mixtures Containing Particles of Intermediate Hydrophobicity. Phys. Chem. Chem. Phys., 2 (13), 2959-2967. Binks, B.P., Lumsdon, S.O., 2000b. Influence of Particles Wettability on the Type and Stability of Surfactant-Free Emulsions. Langmuir, 16, 8622-8631. EPA, 1979b. Water-Related Environmental Fate of 129 Priority Pollutants. Volume II, EPA-440/4-79-029a. US Environmental Protection Agency, USA, Washington, DC, 40-2 to 43-10. EPA, 1998. Water Quality Standards; Establishment of Numeric Criteria for Priority Toxic Pollutants; States' Compliance-Revision of Polychlorinated Biphenyls (PCBs) Criteria. Federal Register Environmental Document, 63 (63), 40 CFR 131, 16182-16188. Gelot, A., Friesen, W., Hamza, H.A., 1984. Emulsification of Oil and Water in the Presence of Finely Divided Solids and Surface-Active Agents. Colloid Surface, 12 (3-4), 271-303. Kabalnov, A.S., Pertzov, A.V., Shchukin, E.D., 1987. Ostwald Ripening in Emulsions. 1. Direct Observations of Ostwald Ripening in Emulsions. J. Colloid. Interf. Sci., 118 (2), 590-597. Kueper, B.H. and McWhorter, D.B., 1991. The behavior of dense, non-aqueous phase liquids in fractured clay and rock. Journal of Ground Water, Vol. 29, No. 5, pp. 716-728.

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Longino, B.L. and Kueper, B.H., 1999. Non-wetting phase retention and mobilization in rock fractures. Water Resources Research, Vol. 35, No. 7, pp. 2085-2093. Mackay, D., Shiu, W.Y., Ma, K.C., 1992. Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals. Volume 1: Monoaromatic Hydrocarbons, Chlorobenzenes and PCBs. Lewis Publishers, USA, 576-579. Monsanto, 1988. Material Safety Data Sheet (MSDS) for Polychlorinated Biphenyls (PCBs). USA, MO. Reviewed on 10/01/1988. Pickering, S.U., 1907. Emulsions. J. Chem. Soc. (London), 91, 2001-2021. Ramsden, W.M.A., Oxon, M.D., 1903. Separation of Solids in the Surface-Layers of Solutions and Suspensions. Observations on Surface-Membranes, Bubbles, Emulsions and Mechanical Coagulation). Preliminary Account. Proc. Roy. Soc. London, 72, 127-64. Rawson, J., 2002. Personal communication. Roy-Perreault, A., Kueper, B.H. and Rawson, J.R., 2004. Formation and stability of PCB Pickering emulsions. Submitted for publication. Tambe, D.E., Sharma, M.M., 1993. Factors Controlling the Stability of Colloid-Stabilized Emulsions, I. An Experimental Investigation. J. Colloid. Interf. Sci., 157 (1), 244-253. Yan, N., Masliyah, J.H., 1995. Characterisation and demulsification of solids-stabilized oil-in-water emulsions. Part 1: Partitioning of clay particles and preparation of emulsions. Colloid Surface A, 96, 229-242. Biographical Sketches Dr. B.H. Kueper is a professor in the Department of Civil Engineering at Queen’s University located in Kingston, Ontario. Dr. Kueper’s research program is focused on the behavior and remediation of dense, non-aqueous phase liquids (DNAPLs) in both porous and fractured media. He is an Associate Editor with both the Journal of Ground Water, and the Journal of Contaminant Hydrology, and holds B.A.Sc. (Civil Engineering) and Ph.D. (Hydrogeology) degrees from the University of Waterloo. Andreanne Roy-Perreault received here B.Sc. (Biochemistry) degree from Laval University in 2000, her B.Sc. Eng. (Geological Engineering) degree from Laval University in 2001, and her M.Sc. Eng. (Civil Engineering) degree from Queen’s University in 2003. Her interests include the fate and transport of PCBs and other organic compounds in groundwater, as well as methods of site remediation. Mike West is a Ph.D. student in the Department of Civil Engineering at Queen’s University located in Kingston, Ontario. He holds an undergraduate degree (Civil Engineering) from Queen’s. His research interests include the migration of dense, non-aqueous phase liquids (DNAPLs) and aqueous phase contaminants in fractured media, matrix diffusion processes, and methods of site remediation. Contact Information Dr. B.H. Kueper, Department of Civil Engineering, Queen’s University, Kingston, Ontario, Canada, K7L 3N6. Phone: 613-533-6834, Fax: 613-533-2128, email: kueper@civil.queensu.ca.