Remediation of Barium Contaminated Groundwater. A Study of Barium Sulphate Mobility

Transcription

1 Final Year Project School of Water Research Remediation of Barium Contaminated Groundwater A Study of Barium Sulphate Mobility Mala H. Batu Supervisor: Dr Christoph Hinz Co-Supervisor: Dr Chris Barber 3 rd November

2 Abstract Concentrations of barium exceeding Australian Drinking Water Guidelines have been found in some aquifers near Perth, Western Australia. Barium has implications for human health, because of the link between high barium in drinking water and cardiovascular disease. In general high barium concentrations are found in deep confined aquifers, which are depleted in sulphate. Therefore the possibility of injection of sulphate-amended water to precipitate barium sulphate (BaSO 4 ), in situ during aquifer storage and recovery (ASR) is a solution to reduce concentrations of barium. This paper will investigate the mobility of precipitated BaSO 4, as a colloid through three types of sand with differing grain sizes. The stable colloid kaolinite will be used as a reference for comparison with the BaSO 4. The mobility of the colloids was studied experimentally using a flow-through small diameter sand column through which solutions containing colloidal suspensions BaSO 4 and kaolinite were eluted. The absorbance of column eluant was measured with a flow-through UV-VIS spectrophotometer set at 420nm. The mixing interface between barium chloride and sodium sulphate solutions was also investigated in the column for a study of in situ precipitation of BaSO 4. The effects of variable sand grain size and flow rate were studied to assess potential colloid behaviour during ASR. The results proved that kaolinite is a stable colloid while BaSO 4 is unstable. The BaSO 4 suspensions produced erratic results, indicating that the BaSO 4 colloid was readily flocculating and attaching to sand grains as well as column tubing. The positively charged BaSO 4 particles would attach and collect on charged mineral surfaces, but occasionally these become displaced. Kaolinite (which has an overall negative charge) in contrast showed a smooth elution pattern. The in situ precipitation test showed that BaSO 4 was produced when sodium sulphate was introduced into the barium chloride environment. Measured absorbance of the eluant showed a characteristic pattern indicating that BaSO 4 was accumulating in the column with successive dispersive mixing events. This may be a reversible reaction as the BaSO 4 may be bleeding out from the column. Filtration coefficients were measured and deduced that columns of smaller sand size filter out more kaolinite and barite colloids. 2

3 Acknowledgements The author expresses great appreciation for the excellent supervision of Dr Christoph Hinz and Dr Chris Barber, for their advice, guidance and support throughout the year. The author would also like to acknowledge Henning Prommer, Edgardo Alarcon Leon and Michael Smirk. Finally, the author especially wishes to thank family and friends for their love and constant support throughout the year. 3

4 Table of Contents 1 INTRODUCTION 1 2 LITERATURE REVIEW DEFINITION OF A COLLOID COLLOID FORMATION AGGREGATION MECHANISMS IN COLLOID TRANSPORT RESTRICTIONS IN COLLOID TRANSPORT COLLOID MOBILIZATION IONIC STRENGTH EFFECT OF PH COLLOID COLUMN EXPERIMENTS BARIUM SULPHATE BARIUM AS A COLLOID ROLE OF DISPERSANTS TURBIDITY SURFACE CHARGE BARITE AND PERMEABILITY BARIUM IN GROUNDWATER 14 3 METHODOLOGY SAND COLUMN SANDS COLUMN PACKING AND SATURATION TRACER KAOLINITE CALIBRATION CURVE BREAKTHROUGH CURVES FOR CONSTANT FLOW RATE BREAKTHROUGH CURVES FOR VARIABLE FLOW RATE 22 4

5 3.3 BARIUM SULPHATE CALIBRATION CURVE BREAKTHROUGH CURVES FOR BARIUM SULPHATE SUSPENSION CREATING A MIXING INTERFACE BETWEEN BARIUM CHLORIDE AND SODIUM SULPHATE CALCULATIONS COLLOID FILTRATION RATE (K F ) DISPERSION COEFFICIENT (D) 26 4 RESULTS SODIUM CHLORIDE TRACER KAOLINITE CALIBRATION CURVES BREAKTHROUGH CURVES FOR DIFFERENT SAND SIZES VARIABLE FLOW RATE BARIUM SULPHATE SUSPENSION MIXING INTERFACE 33 5 DISCUSSION SODIUM CHLORIDE TRACER KAOLINITE BREAKTHROUGH CURVES FOR DIFFERENT SAND SIZES COLLOID FILTRATION RATE DEPENDENCE ON PORE WATER VELOCITY BARIUM SULPHATE BEHAVIOUR OF BASO MIXING INTERFACE 41 6 CONCLUSION 43 7 REFERENCES 45 5

6 List of Figures FIGURE 2-1 EXPERIMENTAL DETERMINATION OF THE COLLOID FILTRATION RATE (K F ) USING A LATEX-GLASS MODEL WITH A NACL TRACER. (A) STEP-INPUT BREAKTHROUGH CURVES, (B) PULSE-INPUT BREAKTHROUGH CURVES, AND (C) COLLOID FILTRATION RATE CALCULATED FROM BOTH METHODS....7 FIGURE 2-2 CONCEPTUAL VIEW OF (A) INITIAL DEPOSITION, (B) BLOCKING, (C) AND RIPENING (KRETZSCHMAR ET AL 1999)...7 FIGURE 2-3 SEM MICROGRAPHY OF A SYNTHETIC BASO 4 CRYSTAL IN ITS RHOMBOHEDRAL STRUCTURE (DUNN ET AL 1999)...10 FIGURE 2-4 PARTICLE SIZE DISTRIBUTION FOR BASO4 (SUN AND SKOLD 2001)...11 FIGURE 2-5 THE INFLUENCE OF SOLUTION PH ON THE SURFACE CHARGE OF BASO 4 (COLLINS 1998)...13 FIGURE 2-6 EQUILIBRIUM BETWEEN CONCENTRATIONS OF BARIUM AND SULPHATE (BARBER AND PROMMER 2002)...16 FIGURE 3-1 SCHEMATIC OF APPARATUS FOR NACL TRACER EXPERIMENTS...19 FIGURE 3-2 DISPLACEMENT OF BACL 2 BY NA 2 SO 4 IN THE COLUMN TO INVESTIGATE THE MIXING INTERFACE...25 FIGURE 4-1 BREAKTHROUGH CURVE CONSTRUCTED WITH EC MEASUREMENTS FROM EXPERIMENTS INVOLVING COLUMN OF MM AT TWO DIFFERENT FLOW RATES FIGURE 4-2 RELATIONSHIP BETWEEN THE ABSORBANCE (NM) AND CONCENTRATION (PPT) FOR KAOLINITE FIGURE 4-3 BREAKTHROUGH CURVES FOR 1PPT KAOLINITE IN DIFFERENT SAND GRAINS AT FLOW RATE 1.25ML/MIN FIGURE 4-4 ABSORBANCE MEASURED WITH PORE VOLUME FOR 1PPT KAOLINITE IN COLUMN WITH 1.4-2MM SAND SIZE WITH CHANGING FLOW RATE FROM 0.8ML/MIN TO 1.25ML/MIN TO 1.8ML/MIN...30 FIGURE 4-5 BREAKTHROUGH CURVE FOR SAND SIZE 1.4-2MM WITH 1PPT KAOLINITE FLOWING THROUGH WHERE THE FLOW RATE CHANGES FROM 0.8ML/MIN- 1.25ML/MIN- 1.5ML/MIN FIGURE 4-6 BREAKTHROUGH CURVE FOR 30PPM BASO 4 SUSPENSION IN 1.4-2MM SAND COLUMN AT FLOW RATE 0.52ML/MIN FIGURE 4-7 ALTERNATING SEQUENCE OF PUMPING IN NA 2 SO 4 INTO A BACL 2 COLUMN ENVIRONMENT CONTAINING A SAND SIZE OF 1.4-2MM (A) MM (B) AT FLOW RATE 1ML/MIN

7 FIGURE 4-8 ALTERNATING SEQUENCE OF PUMPING IN NA 2 SO 4 INTO A BACL 2 COLUMN ENVIRONMENT CONTAINING A SAND SIZE OF MM AT FLOW RATE 1ML/MIN FIGURE 5-1 RELATIONSHIP BETWEEN COLLOID FILTRATION RATE AND PORE WATER VELOCITY FOR 1.4-2MM SAND

8 List of Tables TABLE 2-1 DRINKING WATER STANDARDS (NH&MRC/ARMCANZ 1994, ROBERTSON 1991, LANCIOTTI ET AL 1989)...15 TABLE 2-2 AQUIFERS IN PERTH THAT MAY BE DEPLETED IN SULPHATE...16 TABLE 3-1 COLUMN DIMENSIONS...17 TABLE 4-1 PEAK RELATIVE CONCENTRATIONS AND PORE VOLUMES TO REACH AVERAGE RELATIVE CONCENTRATION FOR DIFFERENT SAND TYPES WITH 1PPT KAOLINITE FLOWING AT 1.25ML/MIN TABLE 4-2 ABSORBANCE OF BASELINE LEVELS AFTER EACH INTERCHANGE FOR EACH SAND...35 TABLE 5-1 THE PORE WATER VELOCITY (CM/MIN), DISPERSIVITY (CM) AND DISPERSION COEFFICIENTS (CM 2 /MIN), FOR EACH SAND SIZE...37 TABLE 5-2 COLLOID FILTRATION RATES CALCULATED FOR EACH SAND AT A FLOW RATE OF 1.25ML/MIN...38 TABLE 5-3 COLLOID FILTRATION RATES MEASURED IN 1.4-2MM SAND FOR DIFFERENT PORE WATER VELOCITIES

9 1 Introduction Elevated barium concentrations have been found in aquifers near Perth, Western Australia exceeding Australian Drinking Water Guidelines. The Australian non-exceedance guideline for barium is 0.7mg/L (NH&MARC/ARMCANZ 1994), while concentrations have been found in the Myalup region of the Yarragadee aquifer in Perth to be 0.8-4mg/L (Barber and Prommer 2002). High barium concentrations have serious health implications for humans, as it can increase the risk of constriction of blood vessels and contractions of ailmentary canal, convulsions and paralysis. Exposure in low doses from drinking water can also increase the risk of heart attack (NH&MARC/ARMCANZ 1994). In general high concentrations of barium have been found in deeper, confined aquifers that are sulphate depleted. This phenomenon occurs in Illinois (Gilkeson et al 1981) and Arizona (Robertson 1991) in the US; Florence, Italy (Baldi et al 1996); and in China (Zhou and Li 1992). This is caused by the natural dissolution of barium from the aquifer minerals where sulphate has been bacterial reduced. It can be explained by the solubility of barium sulphate (BaSO 4 ). BaSO 4 precipitate is created by the interaction of Ba 2+ ions and SO 2-4 ions (Equation 1-1). The formation of the precipitate is dictated by the solubility of the precipitate, where the solubility product K sp is 1.1 x (Shaw et al 1998). This value indicates that BaSO 4 is very insoluble. K sp is derived from multiplying the activities (M) of dissolved Ba 2+ and SO 2-4 ions together (Equation 1-2). So, if an aquifer is depleted in sulphate- the activity of sulphate will decrease. Therefore the barium activity has to increase- leading to an increase in concentration of barium. This is also true for the opposing case. If the barium activity decreases, the activity of the sulphate has to increase. Therefore to decrease the barium concentrations, the sulphate concentration has to increase. Equation 1-1 Ba 2+ (aq) + SO 4 2- (aq) BaSO 4(s) Equation 1-2 K sp = activity Ba2+ x activity SO42- A possible solution to lower the barium concentrations is to inject sulphate-amended water to precipitate BaSO 4, in situ during aquifer storage and recharge. In theory, injecting sulphate 1

10 into the sulphate-depleted aquifer should precipitate out BaSO 4, reducing the barium concentrations to allowable levels. This technique has been employed in one simulation study by Scrivner et al (1996), where sodium sulphate (Na 2 SO 4 ) and sodium sulphide was added to soils in a waste landfill to reduce metal concentrations of barium. However this technique has not been used in the field. Therefore the research needs to be conducted to judge if this is a feasible option. This leads to the reason behind this study. The aim of this study is to investigate the mobility of precipitated BaSO 4 as a colloid through three types of sand with differing grain size, and use a well studied colloid- kaolinite, as a reference for comparison with BaSO 4. BaSO 4 will be studied as a colloid to understand what happens when BaSO 4 is formed in an aquifer, and investigate how it is transported with groundwater flow. Three types of sand were selected to understand the transport in different environments, but not to represent the environments in the Perth aquifers, which is quite heterogeneous. A well-studied colloid kaolinite will be used for comparison purposes, as little work has been done on the mobility of BaSO 4 as a colloid. Colloid mobility is important in the aquifer especially if the colloid is a contaminant. The transport of the colloid will indicate how far it may be advected and dispersed throughout the porous media, showing the extent of contamination in an aquifer. This is especially important in terms of the pumping of groundwater from the aquifer for drinking water consumption. However in this case the formation of the colloid will reduce the amount of contamination. The formation of the BaSO 4 colloid during ASR is a relatively new concept to help reduce contamination. Little is known about the behaviour of this colloid in terms of its formation and transport through the aquifer. Many processes including adsorption and filtration will impact heavily on the movement of this colloid. Therefore this study will provide preliminary information about the behaviour of the colloid, to see if this option is feasible. 2

11 2 Literature Review This review will cover the general definition of a colloid and the mechanisms involved in the transport of colloids through a porous media. The geochemistry of barium sulphate will also be discussed and the previous investigations into the transport of this colloid. 2.1 Definition of a Colloid Colloidal particles exist within the size range of 1nm to 1µm (Buddemeier & Hunt 1988). These particles move according to Brownian motion, which is caused by the motion from the bombardment by fluid molecules that move with a random thermal nature. This Brownian motion helps keep the colloids in suspension. Manahan (2000) classifies colloidal particles in terms of: Hydrophilic colloids are large molecules or ions that strongly interact with water, which results in the spontaneous formation of more colloids. Examples are proteins and synthetic polymers. Hydrophobic colloids interact less with water and are more stable, as they have a charge. This surface charge and the opposing charge ions in the surrounding solution form an electrical double layer. This enables the colloids to repel each other. Clay particles are a good example of these colloids. Association colloids are composed of miscelles. These are aggregates of molecules and ions. A hydrocarbon chain may surround a spherical colloidal particle where the ionic head is attracted to the positive ions in solution, and the tails entrain the colloid. The colloid BaSO 4 investigated in this study is classified as a hydrophobic colloid. Colloids have large specific areas usually greater than 10m 2 /g (Kretzschmar et al 1999). This enables particles or other colloids to adsorb onto the surface of these colloids, or for the colloids to adsorb onto grains in the aquifer. In general colloids are relatively stable and will remain in suspension over large time periods. This will be the case unless the colloids coagulate or deposit onto the soil. This study will test this theory, as BaSO 4 is known to be unstable in suspension. This will be discussed later in the review. The behaviour of the 3

12 colloids will also depend on the high specific area, high interfacial energy and high surface charge/ density ratio (Manahan 2000). 2.2 Colloid Formation Colloids are mostly created by (Kretzschmar et al 1999)- in situ by changes in solution chemistry, which causes particles to be released. precipitation from saturated solutions introduction of biocolloids (viruses and bacteria) from anthropogenic inputs. This study will focus on colloid formation by precipitation from ASR. ASR can produce drastic changes in water chemistry by injecting sulphate-amended water into the aquifer to induce the precipitation of BaSO 4 colloids. A similar study was performed by Liang et al (1993) who injected oxygenenated water into an anoxic aquifer to induce the precipitation of iron (III) hydroxide colloids to reduce the iron (II) concentrations in the aquifer Aggregation Aggregation occurs when colloids attract and form larger particles. This process is prevented by the electrostatic repulsion of the electrical double layer. Aggregation can be classified into two sections- coagulation and flocculation. Coagulation occurs when the repulsion is reduced so much that the colloidal particles can join and form larger particles. The repulsion is overcome by van der Waals forces. When the particles come close to other particles through Brownian motion, van der Waals do the rest and pull the particles together. The particles will then stick together and squeeze out the solution between the particles. Hydrophyllic colloids tend to be more stable so they will not to coagulate, while hydrophobic colloids are less stable and will want to coagulate. Flocculation uses bridging compounds that connect the colloids through chemical bonds. Polyelectrolytes cause flocculation to occur. 2.3 Mechanisms in Colloid Transport Restrictions in colloid transport The following processes restrict colloid movement (McGechan and Lewis 2002): 4

13 Straining/Filtration Adsorption Electrostatic charge mechanisms: which include the double diffuse layer (DDL). Straining occurs where the colloid is greater than the size of the pore, and therefore cannot pass through. According to Hunt et al (1987) and Ibaraki and Sudickey (1995), filtration is achieved by particles that are captured within pores that are larger than the colloid. However other authors refer to this as straining. Adsorption occurs when reactive substances attach to surfaces of solids e.g. colloids attach themselves to the soil matrix. The adsorption capacity of a solid depends on the surface area to volume ratio. If it has high adsorption capacity, it means that the solid is small but has a large specific area. Therefore there are more adsorption sites for the colloid to attach itself to. Another important factor is the surface charge of the colloid. Colloids usually behave as charged particles, which can interact strongly with the soil (McGechan and Lewis 2002). The diffuse double layer is also known as the electrical double layer. This layer is created by the arrangement of ions dissociated from a charged surface. These ions are subject to two forces: adsorption generated by the electric field of the charged surface and diffusive force formed from the concentration gradients trying to equilibrate the ion concentration throughout the whole solution (Brady and Weil 1996). It will be formed when the colloid and the particle it comes into contact with. The thickness of the layer is usually large (10µm) for monovalent cations that have low ionic strength; and thin (0.1µm) for multivalent ions that have a high ionic strength (McGechan and Lewis 2001). Ionic strength will be discussed later. This study does not investigate ionic strength, but it is recommended that it been done in the future. To simplify analysis, this study will combine all of these terms and define the process as filtration. Where filtration is the mechanism of removing colloid out of solution by trapping it within the pores. This process is composed of transport of the colloid through the porous media and attachment of colloid to the surface of the soil matrix. 5

14 2.4 Colloid Mobilization Colloid transport through granular porous media is modelled by the advection-dispersion equation (Kretzschmar et al 1999): Equation 2-1 C t = D 2 z C 2 v C z k f C Where C(t) is the colloidal concentration over time; D the dispersion coefficient that describes how the colloids spread through the media; v the pore water velocity; k f the colloid filtration rate; and z the distance travelled through the porous media. Based on colloid filtration theory, this study assumes that colloid filtration is an irreversible process. That indicates that once the colloids have been filtered out by the porous media they will remain there indefinitely. Colloid filtration occurs in two steps- first the colloids are transported through the porous media by Brownian diffusion (transport step); and then the colloids may attach to the soil matrix in the aquifer (attachment step). The transport step is affected by the size and density of the colloids; accessibility to the surface area of the soil matrix; pore structure; and the pore water velocity. Surface and solution chemistry between the colloid and the soil surface will affect the attachment step. These include the van der Waals forces, electric double layer, steric repulsion and hydration forces (Kretzschmar et al 1999). The k f can be calculated from two methods. These are the step-input or short-pulse column experiments (Figure 2-1). This study will employ the step-input method to determine the k f, as both methods produce very similar results. Akbour et al (2002) determined the colloid filtration rates (k f ) for kaolinite using the pulse-input method. The rates varied between 1-12 hr -1. Even though this study will use the step-input method, vales will be able to be compared. 6

15 Figure 2-1 Experimental determination of the colloid filtration rate (k f ) using a latexglass model with a NaCl tracer. (a) Step-input breakthrough curves, (b) pulse-input breakthrough curves, and (c) colloid filtration rate calculated from both methods. After the initial attachment to the soil matrix, blocking and ripening processes can occur (Figure 2-2). Blocking occurs when the colloid-colloid attraction is unfavourable due to repulsion. Therefore the filtration rate decreases because the mobile colloids will repel against the colloids that are already attached to the soil matrix. Conversely when colloid-colloid attraction is favourable, the mobile colloids will attach to the colloids already attached to the soil matrix. These phenomenon will not be studied exclusively, only the determination of if the colloids are filtered or not. Figure 2-2 Conceptual view of (a) initial deposition, (b) blocking, (c) and ripening (Kretzschmar et al 1999). 7

16 Colloid mobilization is caused by- Decrease in ionic strength Increasing in ph Increase in flow velocity High pumping rates Rapid infiltration Fractured flow This review will focus on the first three factors, as they are important experimental parameters Ionic Strength Ionic strength can be reduced by infiltration of precipitation, irrigation or aquifer recharge. Dilute solutions containing monovalent ions are the most effective at mobilizing colloids. The dilute solutions will also reduce the hydraulic conductivity. And solutions with high ionic strength and contain bivalent ions will not cause colloid mobilization. Low ionic strength solutions will produce large ( thick ) double layers, because the surface charge of the colloid needs to be balanced by a large layer because the ion concentration is lower. Decreasing the ionic strength will increase the repulsion, as when two colloids approach each other, more of the double layers will overlap increasing repulsion and therefore promote colloid mobilization (Ryan and Elimelech 1996). The ionic strength is important in terms of coagulation occurring, because high ionic strength enhances coagulation because the attractive van der Waals forces dominate and the double layers compressed. At low ionic strength a larger repulsive barrier is formed because there is a large electrostatic repulsion. Therefore there will be less attachment of colloids to the soil matrix (Kretzschmar et al 1999) Effect of ph Kolakowski and Matijevic (1979) showed that chromium hydroxide colloids that were attached to glass beads during low ph. However as the ph increased from 9.6 to 11.5 the colloids were released. The increase in ph increases the level of repulsion between the colloids and the medium. But this is with glass beads. The scope of this project will use natural sediments. 8

17 An increase in ph promotes colloid mobilization. Laboratory experiments were conducted with columns containing disturbed and undisturbed sediments. Bunn et al (2002- as cited in Ryan and Elimelech 1996) experiments were carried out on two columns- one with disturbed sediments and the other undisturbed sediments. Disturbed meaning the sediment was not changed; and ph was increased sequentially to the same sediment. Disturbed sediments will be used in the present study. Undisturbed meaning fresh sediment was used each time the ph was changed. Twice as many colloids were released from the disturbed sediment as the undisturbed sediments. This was attributed to the colloids being released during sediment drying and coatings on the sand grains were disturbed, releasing colloids attached to it. Roy and Dzombak (2001) and Grolimund et al (1996) have also produced work that has indicated that the disturbed sediment releases more colloids that undisturbed. Increasing the ph increases the electrostatic repulsion between the colloids and the sediment. Therefore increasing colloid release. However a limit does occur when the ph is 12.5 in disturbed and 13.1 in undisturbed (Bunn et. al. 2002). At these levels the increasing ionic strength reduces the electrostatic repulsion and colloids release is diminished. Bunn et al (1996) hypothesised that for colloid release to occur, the ph of the influent would have to exceed the ph of the point of zero charge (ph pzc ) on the ferric oxyhydroxides. If the influent ph was below the ph pzc, then the positive ferric hydroxides would bind to the negative clay colloids. However if the influent ph is above then this bond would become a repulsion, as the ferric oxyhydoxides become negatively charged. BaSO 4 point of zero charge will be discussed later in the study. 2.5 Colloid Column Experiments Most experiments conducted with colloids have suspensions containing colloids passed through soil samples. Material from the bottom of the column as well as the soil column is then analysed. Lahav and Tropp (1980) used this method with suspensions containing synthetic latex microspheres of diameter 0.12 and 0.21µm. The column sample contained a 9

18 high concentration of trapped spheres in the surface layers after analysis. Water entering and leaving the column has also been analysed. From this high macroporosity aided colloid transport; and the ph in the colloid suspension affected the ionic charge (Seta & Karathanasis 1997). Artificially created fractures have also been constructed in the soil samples. Toran and Palumbo (1992) compared transport with and without fractures, however they found that colloids smaller than the pores were being captured. This was attributed to chemical effects. Experiments with metals involved the migration of copper and zinc in soil columns containing clay and silty clay (Karanthanasis 1999). The transport of the metals was enhanced with suspensions containing colloids by 5-50 fold, compared to suspensions with no colloids. 2.6 Barium Sulphate Barium sulphate (BaSO 4 ) is formed according to the following reaction- Equation 2-2 BaSO 4 Ba SO 4 where the equilibrium constant and solubility product, K sp = 1.23 x The solubility, S = 1.1 x 10-5 mol/l (Manahan 2000). BaSO 4 is typically an orthorhombic crystal, but it can also form several other different shapes (Figure 2-3). Collins (1998) measured an average size of 16µm. Figure 2-3 SEM micrography of a synthetic BaSO 4 crystal in its rhombohedral structure (Dunn et al 1999) 10

19 Figure 2-4 Particle size distribution for BaSO4 (Sun and Skold 2001) A particle size distribution was formed from a 5mM BaSO4 suspension in Sun and Skold (2001) work. The average effective diameter of the particles was 156nm with a range of nm (Figure 2-4). SEM micrographs and scintillation counters were used to measure the barium particle sizes (Aliaga et. al. 1989). However they produced significantly different results. The SEM micrographs measured particles with size 20µm, while the counter measured particles in the range of 0.5-5µm. This inconsistency was attributed to the irregular sizes of the particles. 2.7 Barium as a Colloid Synthetic BaSO 4 is the one of the most insoluble sulphate minerals, as it has a pk sp of 9.96 (Bishop 1988). It is commonly used in powder coatings, paints, inks, rubber, pigments, storage batteries, plastics, paper etc. BaSO 4 is formed by three steps- nucleation, crystal growth and aggregation. The size of the BaSO 4 particles formed is dependent on the conditions during these phases. 2.8 Role of Dispersants Sun and Sköld (2001) created barium sulphate (BaSO 4 ) particles by precipitation and used light scattering measurements to determine its turbidity. Turbidity was measured for a wide range of temperatures and concentrations. Titrating BaCl 2 and NaSO 4 created the solution of BaSO4, while light scattering, ph and conductivity were measured throughout the process. 11

20 However BaSO 4 is a very unstable mineral, so two different dispersants were used in separate experiments. The dispersants were a- 5% aqueous solution of non-ionic surfactant, and a 2% aqueous solution of polyacryl amide. Polyacryl amide (PAM) is used as a flocculent and a stabilizer. This depends on the concentration- at low concentrations it acts as a flocculent, and at high concentrations a stabilizer. The BaSO 4 suspension had a ph of approximately 7.4 throughout all the experiments. Results show that BaSO 4 precipitate is stable throughout the ph range of The main findings include- 1. BaSO 4 particles aggregated forming larger particles in the absence of a dispersant. Therefore the BaSO 4 suspension becomes unstable. 2. In the presence of the non-ionic surfactant, aggregation does not occur beyond 50 C, therefore forming a stable suspension. The surfactant disperses the particles and stops them from aggregating. 3. For BaSO 4 to form a concentration of 1.5mM is needed in the presence of a PAM. Therefore the critical nucleation is 1.5mM i.e. turbidity at this concentration leads to nucleation and crystal growth. It is inferred that this is due to the Ba 2+ sequestering action of the PAM. From this work it can be deduced that BaSO 4 is unstable, but can be stabilised with use of a non-ionic surfactant or PAM. However these will not be used in this study, as this does not represent what is happening in the field. In is not practical to add a one these agents to the aquifer to stabilise the BaSO 4 colloid. 2.9 Turbidity Turbidity is associated with light scattering, because when a beam of light passes through a solution, the light will scatter when it hits the BaSO 4 particles. The scattering reduces the intensity of the light. Therefore the turbidity depends on the concentration and size of the BaSO 4 particles. In this present study will use turbidity measurements to show firstly that BaSO 4 colloid particles are forming in the sand columns. The analytical method to form BaSO 4 will also be used. 12

21 2.10 Surface Charge The surface charge in soils is important in colloid transport (Brady & Weil 1996). The colloids can be seen as charged particles that interact with the soils. The stability of a colloid depends on hydration and the surface charge. Hydration prevents contact with the colloidal particles, which results in the formation of larger particles. The surface charge determines whether aggregation occurs. If the particles have the same charge they will repel, and if they have opposite charges they will attract. In terms of surface charge, when there are excess sulphate ions, they adsorb onto the BaSO 4 particles, causing an overall negative charge. If there are excess barium ions, then a positive charge will result. The ph will affect the surface charge of a colloid. The point of zero charge occurs when at a given ph, when the overall net charge of a colloid will be zero (Manahan 2000). This will favour the onset of aggregation and precipitation. DLVO theory says that highly charged particles will form stable suspensions, while low surface charge will allow particles to come together and form larger particles. Collins (1998) measured the surface electrical properties of barite, and investigated the effect polyasparate has on the surface charge. This polyasparate has functions to help inhibit barium sulphate precipitate. Collins (1998) shows that barite is positively charged below ph of 5 (and negatively charged above). That is the point of zero charge for BaSO 4 is 5 (Figure 2-5). Figure 2-5 The influence of solution ph on the surface charge of BaSO 4 (Collins 1998). 13

22 In theory below a ph of 5 the colloid will be positively charged and will attract to a negatively charged soil matrix. While above a ph of 5 it will repel from the soil matrix. Affect of changing the ph around the point of zero charge will not be analysed in this study Barite and Permeability Aliaga et al (1989) investigated how permeability was reduced in sandpacks with the aid of barium and calcium sulphate precipitation. Usually one solution of barium ions and another containing sulphate ions was pumped at 0.2cm 3 /min simultaneously into the sandpacks. This resulted in a precipitate forming inside the sandpack, when the two solutions encountered each other. Some of this precipitate would then collect within the sand, causing the permeability to decrease. A 60% reduction was recorded in the permeability trend. However this trend did oscillate due to the instability of the precipitate. Once again another study has found that BaSO4 colloid was unstable. This shows that particle bridges were forming and breaking throughout the experiment. However these oscillations decrease as the pore size is decreased. The group also experimented with calcium sulphate or potassium sulphate already mixed into the sandpacks. Then barium chloride was injected into the sandpacks, causing precipitation of barium sulphate. A permeability decrease of approximately 60% was also found in this experiment. Experiments performed using a CT scan enabled the detection of the wave-like progress of barium sulphate through the sandpack. The leading edge of the barium sulphate was spread out- more than what would be accounted for by dispersion or the linear function deduced by the group. The CT scan was also used to display the path that the barium sulphate takes in the sandpack. It does not progress uniformly, but a tortuous path Barium in Groundwater High barium concentrations in aquifers is not only localised to Perth, as occurrences of elevated concentrations of barium have been found all over the world. In the US, (where the guideline level is 1mg/L (Table 2-1), high concentrations have been found in New Jersey aquifers (Czarnik and Kozinski 1994); public supply wells in Illinois (Voelker 1989); and 14

23 alluvial basins in Arizona, Nevada, New Mexico and California (Robertson 1991). Barium was found to exceed the European Union standards (Table 2-1) in Birmingham, UK (Ford and Telham 1994) and the Netherlands (Frapporti et al 1996); 30% of St Petersberg aquifers exceeded 2mg/L (Barvish and Shvarts undated); and Tuscany, Italy contained barium concentrations ranging from 7µg/L-1160µg/L. Table 2-1 Drinking Water Standards (NH&MRC/ARMCANZ 1994, Robertson 1991, Lanciotti et al 1989) Country/Organisation Drinking Water Guidelines (mg/l) Australia 0.7 USA 1 European Union (EU) 0.1 World Health Organisation (WHO) 0.7 Work conducted by Barber and Prommer (2002) established the equilibrium between barium and sulphate concentrations (Figure 2-6). Therefore concentrations below Australian guidelines have an equilibrium of 20mg/L of sulphate. So aquifers below 20mg/L of sulphate were considered depleted of sulphate. Therefore concentration needs to be raised to 100mg/L to reduce concentrations of barium to below the Australian guidelines if equilibrium established. Therefore suggest in situ treatment of barium by ASR where sulphate containing up to mg/L would induce precipitate of barium sulphate in a short storage period. 15

24 log Barium ug/l mg/l 1 mg/l 0.1 mg/l 0.01 mg/l Sulphate mg/l Figure 2-6 Equilibrium between concentrations of barium and sulphate (Barber and Prommer 2002) Barber and Prommer (2002) also compiled some of the aquifers in Perth that may be depleted in sulphate (Table 2-1). There are three formations in Perth that have recorded depleted sulphate levels with depleted sulphate, which would indicate elevated concentration of barium. Superficial aquifers have less than 20mg/L but due to an unconfined aquifer and the existence of silicate minerals such as feldspars, this is not likely. The only barium data for the formations was for the Myalup region. This region recorded elevated concentrations of barium 0.8-4mg/L, well above 0.7mg/L. Table 2-2 Aquifers in Perth that may be depleted in sulphate Formation Region Range of SO 2-4 (mg/l) Mirrabooka Leederville Gwelup 5-26 Mirrabooka 3-90 Wanneroo 0-19 Yarragadee Wanneroo 0-39 Myalup 5-19* 16

25 3 Methodology 3.1 Sand Column Sands Three sand size grains were investigated in this study mm mm mm These three were chosen to understand colloid mobility in a large sand grain, a medium sand and a small sand grain Column Three glass columns with the same dimensions were constructed to house the three sizes of sands. The column properties are listed below (Table 3-1). Each column has tubing and valves attached that will control flow to and from the column. A valve is placed before and after the column, so that flow can bypass the column if necessary. Table 3-1 Column dimensions Inner Diameter (cm) 1 Length (cm) 7.5 Volume (cm 3 ) Packing and Saturation Each column was packed with a different sand size. Care was taken when packing the column with sand, as the sand needs to be packed tightly and uniformly so that the flow travels uniformly throughout the column. The column was then saturated with deionised water (DI) ready for experiments Materials The following materials are needed to pack and saturate the columns- 3 x columns 17

26 3 x sand sizes (1.4-2mm, mm, mm) Retort Stand Plunger CO 2 DI water Procedure 1. Wash and dry column thoroughly. 2. Weigh column and tubing associated with it. 3. Place column upright in a retort stand. 4. Start to fill column with some sand. Use a plunger to pack sand and level it out in the column. 5. Add some more sand and use the plunger to pack it. 6. Repeat step 4 until the entire column is packed. 7. Seal column and weigh. 8. Now fill the column with CO 2. This is used to push out all of the air from the column. 9. Saturate the column with DI water by either distillation or pumping water through at a very slow flow. The DI water will absorb the CO 2 producing a saturated column. Care must be taken not to let air back into the column Tracer A sodium chloride (NaCl) tracer was used to determine the dispersivity (cm) of each sand size. Dispersivity is a material property, and from this the Dispersion Coefficient (cm 2 /min) can be derived. This coefficient is used to describe the extent that colloids will spread throughout the porous media. 0.2mM NaCl was used as a background solution, and then a pulse of 0.5mM NaCl entered the column. This tracer was used to ensure that it would be unreactive in the column, and that the background and pulse had similar ionic strength. Two flow rates were chosen (one high, and one low), so that the appropriate range of dispersion coefficients values could be calculated Materials 3 columns with differing sand size 0.2mM NaCl 18

27 0.5mM NaCl Flow-through electrical conductivity detector Peristaltic pump Timer Procedure Valve Column UV-VIS PUMP Beaker 1 Beaker 2 Figure 3-1 Schematic of apparatus for NaCl tracer experiments The following should be conducted for each of the columns. 1. Set up apparatus as shown (Figure 3-1). 2. Flush column with 0.2mM NaCl solution. Ensure that no air enters the column. 3. Set flow rate to 1.4mL/min. 4. Simultaneously input pulse into column and start EC measurements at an interval of 8secs. Again ensure that no air enters the column. 5. Input pulse for exactly 5mins. Therefore 7mL of 0.5mM flows through the column. 6. After 5mins replace the pulse with the background solution. 7. Stop recording EC measurements once a stable reading has been reached. 8. Repeat steps 2-8 for flow rate 0.6mL/min. 19

28 3.2 Kaolinite Calibration Curve Calibration curve was devised to convert absorbance measured by the UV-VIS spectrophotometer to concentration. Producing kaolinite solutions of differing concentrations and measuring their corresponding absorbance with the UV-VIS spectrophotometer achieved this. This relationship was then plotted to find a linear relationship, which should have a correlation coefficient greater than 0.98 to ensure that the relationship is strong. Concentrations of kaolinite were chosen so that there was a broad range, but also that a relationship could be formed from the values. Therefore the concentrations were chosen at regular intervals between 0.1-5ppt. The solutions were not too turbid, so that the absorbance detected would be too high for the UV-VIS spectrophotometer to measure. Also solutions with too high a concentration deviate from a linear relationship with absorbance Materials 10mL of each Kaolinite concentration- 5, 2.5, 1, 0.75, 0.5, 0.1ppt. 5 x 4.5 ml cubettes Magnetic Stirrer Stirrer Piece UV-VIS Spectrophotometer Procedure 1. Fill a cubette with the 5ppt kaolinite solution and measure the absorbance in the UV- VIS spectrophotometer. 2. Place the solution back into the 5ppt kaolinite container and place the container on the magnetic stirrer with stirrer piece inside. 3. Turn on stirrer and mix for 1min. 4. Take container off stirrer and fill the same cubette with the solution. 5. Place cubette in the UV-VIS spectrophotometer and measure the absorbance. 6. Repeat steps 1-9 for the following concentrations of kaolinite- 2.5ppt, 1ppt, 0.75ppt, 0.5 ppt and 0.1ppt. 20

29 3.2.2 Breakthrough Curves for Constant Flow Rate Breakthrough curves were constructed for- each sand size different concentrations The following method and procedure will outline how to produce a breakthrough curve for each column at one flow rate (1.25mL/min) and at one concentration (1ppt kaolinite). Breakthrough curves are used to determine the colloid filtration rate (k f ). k f is the rate at which colloids are filtered out of solution by the porous media. This is very important in understanding colloid mobility in an aquifer. Having different concentrations, as this is to represent what is happening in the aquifer. It also to see the affect different concentrations and flow rates on colloid transport. Concentrations were chosen so that they were within the sensitivity range of the UV-VIS. Not enough of a turbid solution would produce no absorbance readings and a too turbid solution would be beyond the limits of the UV-VIS detectable range. Also the concentration would have to have a linear relationship with absorbance (Section 3.2.1). Flow rates were chosen depending on how many pore volumes would flow through the column in a minute. If too many pore volumes flowed through in a minute, the UV-VIS would not be able to represent what truly was happening the in column. The interval that the UV-VIS would measure absorbance may miss what is coming out of the column. Therefore a flow rate 1.25mL/min ensured that a pore volume would exit the column every minutes for the different sand sizes Materials 1ppt kaolinite solution DI water Sand column Peristaltic pump Flow-through UV-VIS spectrophotometer Timer 21

30 Now to see the affects with the column, change the experimental set-up so that solution now flows through the column. Repeat steps 1-5 to record the absorbance from the column. These are the A(t) readings which can then be converted to C(t). This procedure can be conducted for each column. To construct the breakthrough curve pore volume is plotted against C(t)/C O Breakthrough Curves for Variable Flow Rate This experiment was devised to understand if k f was affected by a changing flow rate. A slow flow rate (0.8mL/min) was used in the beginning of the experiment and then it was gradually increased to 1.25mL/min and then 1.8mL/min during a pulse of kaolinite to the column Procedure First the initial concentration of kaolinite (C O ) must be measured before it enters the column. Then the following is conducted- 1. Set-up the experimental apparatus as shown (Figure 2-1). 2. Flush column thoroughly with DI water in beaker Set flow rate to 1.25mL/min. 4. Simultaneously start UV-VIS measurements and input the kaolinite solution by switching to beaker Once a stable reading has been reached, switch back to beaker Again once a stable reading has been met, stop recording absorbance. The maximum absorbance is the initial absorbance of the kaolinite (A O ), which can then be converted to C O using the linear relationship in the calibration curve (Section 3.2.1). The same experimental apparatus was used in Section The initial concentration was again measured before it enters the column, but this time the following procedure was implemented- 1. Set flow rate to 0.8mL/min and set path to beaker 1 to flush column. 2. Simultaneously start measuring absorbance and switch path to beaker Once a stable reading has been reached, change flow rate to 1.25mL/min. 4. Again once a stable reading has been reached, increase the flow rate to 1.8mL/min. 5. Once a stable reading is met, switch to beaker Stop measuring absorbance once the reading is stable. 22

31 3.3 Barium Sulphate Calibration Curve Calibration curve was devised to convert absorbance measured by the UV-VIS spectrophotometer to concentration. The materials and procedure used to create the calibration curve was adapted from the analytical Turbidimetric method (Clesceri et al 1998). Producing BaSO4 solutions of differing concentrations and measuring their corresponding absorbance with the UV-VIS spectrophotometer achieved this. This relationship was then plotted to find a linear relationship, which should have a correlation coefficient greater than Materials The following materials were used in the creation of the calibration curve- 10mL of each Na 2 SO 4 concentrations- 10, 7.5, 5, 2.5, 1ppm. 10mL of Buffer B BaCl 2 (Dihydrate) Analytical Reagent 5 x 30mL containers with cap 5 x 4.5 ml cubettes Weighing Balance Magnetic Stirrer Stirrer Piece UV-VIS Spectrophotometer Procedure 7. Take 10mL of 10ppm Na 2 SO 4 and place in a small 30mL container. 8. Add 2mL of Buffer B solution. Screw cap on container and shake. 9. Fill a cubette with the solution and measure the absorbance in the UV-VIS spectrophotometer. 10. Place the solution back into the 10ppm Na 2 SO 4 container and place the container on the magnetic stirrer with stirrer piece inside. 11. Turn on stirrer. 23

32 12. Measure 1.25g of BaCl 2 and add to 10ppm Na 2 SO 4 container and mix for 1 minute exactly. This is to ensure all the BaCl 2 is dissolved inside the solution 13. Take container off stirrer and fill the same cubette with the solution. 14. Leave for exactly 5mins. 15. Place cubette in the UV-VIS spectrophotometer and measure the absorbance. 16. Repeat steps 1-9 for the following concentrations of Na 2 SO 4-7.5ppm, 5ppm, 2.5ppm and 1ppm. Timing is important in this experiment, as the BaSO 4 is forming and precipitating out of solution. Mixing allows for the Na 2 SO 4 and BaCl 2 to fully mix, and allowing it to settle for 5 mins ensures that the same amount of precipitate is formed for each concentration of solution Breakthrough Curves for Barium Sulphate Suspension The same materials and procedure were used as in Section except that- the background solution was 10ppm Na 2 SO 4 (beaker 1) the pulse was 10ppm BaSO4 suspension (beaker 2), which was constantly mixed by a magnetic stirrer. However results from this experiment proved erratic, so the concentration of BaSO 4 was increased to 30ppm to see if the sensitivity range was the problem. Other concentrations were experimented, which also produced the same results. Explanations of the erratic results are discussed in Section Therefore another method of experimenting with BaSO 4 colloids was found Creating a Mixing Interface between Barium Chloride and Sodium Sulphate Since creating BaSO 4 outside the column and then letting it infiltrating the column produced erratic results, another method was chosen. The BaSO 4 would be produced within the column by flushing the column with BaCl 2, and then displacing it with a Na 2 SO 4 solution ( Figure 3-2). BaSO 4 should be formed at the interface of the two solutions. This is representative of the situation of ASR. Sulphate-amended water is used to displace the water in aquifer, which contains barium. It will also give an idea of how much colloids are produced in the interface and their mobility with the interface. The affects of dispersion should be easily investigated too. Once Na2SO4 has displaced the BaCl2, the reverse was done. BaCl 2 then displaced Na 2 SO 4 to see what affect this process had. 24

33 BaCl 2 Mixing Zone Flow Na 2 SO 4 Figure 3-2 Displacement of BaCl 2 by Na 2 SO 4 in the column to investigate the mixing interface Materials The same experimental apparatus is used as in Section except- beaker 1 contained 9mM BaCl 2 constantly mixed by a magnetic stirrer. beaker 2 consisted of 10mM Na 2 SO 4. The concentrations were chosen so that SO 2-4 was in excess of Ba 2+. This is to maintain that all the Ba 2+ would react and form BaSO Procedure To create a mixing interface the following was conducted in each column- 1. Flush the column with BaCl2. 2. Set the flow rate to 1.25mL/min. 3. Simultaneously start recording absorbance and switch flow to beaker Once a stable reading has been established, switch back to beaker Again when the reading is stable switch back to beaker Repeat steps 4-5 twice. 7. Once the reading is stable stop measurements. Throughout this experiment the ph and EC was measured every 3mins. 25

34 3.4 Calculations Colloid Filtration Rate (k f ) Colloid deposition rates are determined from step-input or short-pulse column experiments (Kretzschmar et al 1999). This study has assumed that this is a filtration process not a deposition process, so it will refer to the colloid deposition rate as the colloid filtration rate. Again this study has focused on step-input experiments to derive the breakthrough curves. The following method to determine the k f is adapted from Kretzschmar et al (1999) work. Columns that have a high Peclet Number can calculate the colloid filtration rate from: Equation 3-1 k f v p = L ln F where v p is the pore water velocity, L the length of the column and F the ratio between C/C O once the plateau has reached a stable value Dispersion Coefficient (D) The Dispersion Coefficient was derived using a spreadsheet created by Bromly (2003) based on Yu et al (1999) moment method. It can be calculated by measuring the EC during the NaCl tracer experiments (Section 3.1.4). 26