Remediation of Barium Contaminated Groundwater
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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
3rd November 2003
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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 (BaSO4), in situ during aquifer storage and recovery (ASR) is a solution to
reduce concentrations of barium. This paper will investigate the mobility of precipitated
BaSO4, 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 BaSO4.
The mobility of the colloids was studied experimentally using a flow-through small diameter
sand column through which solutions containing colloidal suspensions BaSO4 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
BaSO4. 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 BaSO4 is unstable. The BaSO4
suspensions produced erratic results, indicating that the BaSO4 colloid was readily
flocculating and attaching to sand grains as well as column tubing. The positively charged
BaSO4 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 BaSO4 was produced when
sodium sulphate was introduced into the barium chloride environment. Measured absorbance
of the eluant showed a characteristic pattern indicating that BaSO4 was accumulating in the
column with successive dispersive mixing events. This may be a reversible reaction as the
BaSO4 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.
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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.
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Table of Contents
1 INTRODUCTION 1
2 LITERATURE REVIEW 3
2.1 DEFINITION OF A COLLOID 3
2.2 COLLOID FORMATION 4
2.2.1 AGGREGATION 4
2.3 MECHANISMS IN COLLOID TRANSPORT 4
2.3.1 RESTRICTIONS IN COLLOID TRANSPORT 4
2.4 COLLOID MOBILIZATION 6
2.4.1 IONIC STRENGTH 8
2.4.2 EFFECT OF PH 8
2.5 COLLOID COLUMN EXPERIMENTS 9
2.6 BARIUM SULPHATE 10
2.7 BARIUM AS A COLLOID 11
2.8 ROLE OF DISPERSANTS 11
2.9 TURBIDITY 12
2.10 SURFACE CHARGE 13
2.11 BARITE AND PERMEABILITY 14
2.12 BARIUM IN GROUNDWATER 14
3 METHODOLOGY 17
3.1 SAND COLUMN 17
3.1.1 SANDS 17
3.1.2 COLUMN 17
3.1.3 PACKING AND SATURATION 17
3.1.4 TRACER 18
3.2 KAOLINITE 20
3.2.1 CALIBRATION CURVE 20
3.2.2 BREAKTHROUGH CURVES FOR CONSTANT FLOW RATE 21
3.2.3 BREAKTHROUGH CURVES FOR VARIABLE FLOW RATE 22
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3.3 BARIUM SULPHATE 23
3.3.1 CALIBRATION CURVE 23
3.3.2 BREAKTHROUGH CURVES FOR BARIUM SULPHATE SUSPENSION 24
3.3.3 CREATING A MIXING INTERFACE BETWEEN BARIUM CHLORIDE AND SODIUM SULPHATE 24
3.4 CALCULATIONS 26
3.4.1 COLLOID FILTRATION RATE (KF) 26
3.4.2 DISPERSION COEFFICIENT (D) 26
4 RESULTS 27
4.1 SODIUM CHLORIDE TRACER 27
4.2 KAOLINITE 28
4.2.1 CALIBRATION CURVES 28
4.2.2 BREAKTHROUGH CURVES FOR DIFFERENT SAND SIZES 29
4.2.3 VARIABLE FLOW RATE 30
4.3 BARIUM SULPHATE 32
4.3.1 SUSPENSION 32
4.3.2 MIXING INTERFACE 33
5 DISCUSSION 37
5.1 SODIUM CHLORIDE TRACER 37
5.2 KAOLINITE 38
5.2.1 BREAKTHROUGH CURVES FOR DIFFERENT SAND SIZES 38
5.2.2 COLLOID FILTRATION RATE DEPENDENCE ON PORE WATER VELOCITY 39
5.3 BARIUM SULPHATE 40
5.3.1 BEHAVIOUR OF BASO4 40
5.3.2 MIXING INTERFACE 41
6 CONCLUSION 43
7 REFERENCES 45
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List of Figures
FIGURE 2-1 EXPERIMENTAL DETERMINATION OF THE COLLOID FILTRATION RATE (KF) 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 BASO4 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 BASO4 (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 BACL2 BY NA2SO4 IN THE COLUMN TO INVESTIGATE THE MIXING
INTERFACE. .........................................................................................................................25
FIGURE 4-1 BREAKTHROUGH CURVE CONSTRUCTED WITH EC MEASUREMENTS FROM
EXPERIMENTS INVOLVING COLUMN OF 0.5-0.71MM AT TWO DIFFERENT FLOW RATES. .............27
FIGURE 4-2 RELATIONSHIP BETWEEN THE ABSORBANCE (NM) AND CONCENTRATION (PPT) FOR
KAOLINITE. .........................................................................................................................28
FIGURE 4-3 BREAKTHROUGH CURVES FOR 1PPT KAOLINITE IN DIFFERENT SAND GRAINS AT FLOW
RATE 1.25ML/MIN. ..............................................................................................................29
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. .....31
FIGURE 4-6 BREAKTHROUGH CURVE FOR 30PPM BASO4 SUSPENSION IN 1.4-2MM SAND COLUMN
AT FLOW RATE 0.52ML/MIN. ................................................................................................32
FIGURE 4-7 ALTERNATING SEQUENCE OF PUMPING IN NA2SO4 INTO A BACL2 COLUMN
ENVIRONMENT CONTAINING A SAND SIZE OF 1.4-2MM (A) 0.5-0.71MM (B) AT FLOW RATE
1ML/MIN.............................................................................................................................34
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FIGURE 4-8 ALTERNATING SEQUENCE OF PUMPING IN NA2SO4 INTO A BACL2 COLUMN
ENVIRONMENT CONTAINING A SAND SIZE OF 0.125-0.25MM AT FLOW RATE 1ML/MIN. ............35
FIGURE 5-1 RELATIONSHIP BETWEEN COLLOID FILTRATION RATE AND PORE WATER VELOCITY
FOR 1.4-2MM SAND..............................................................................................................40
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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. ..................................................................................................................30
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 (CM2/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..............................................................................................................39
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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
(BaSO4). BaSO4 precipitate is created by the interaction of Ba2+ ions and SO42- ions (Equation
1-1). The formation of the precipitate is dictated by the solubility of the precipitate, where the
solubility product Ksp is 1.1 x 10-10 (Shaw et al 1998). This value indicates that BaSO4 is very
insoluble. Ksp is derived from multiplying the activities (M) of dissolved Ba2+ and SO42- 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 Ba2+(aq) + SO4
2-(aq) →→→→ BaSO4(s)
Equation 1-2 Ksp = activityBa2+ x activitySO42-
A possible solution to lower the barium concentrations is to inject sulphate-amended water to
precipitate BaSO4, in situ during aquifer storage and recharge. In theory, injecting sulphate
Remediation of Barium Contaminated Groundwater
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into the sulphate-depleted aquifer should precipitate out BaSO4, reducing the barium
concentrations to allowable levels. This technique has been employed in one simulation study
by Scrivner et al (1996), where sodium sulphate (Na2SO4) 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 BaSO4 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
BaSO4.
BaSO4 will be studied as a colloid to understand what happens when BaSO4 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 BaSO4 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 BaSO4 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.
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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 BaSO4 investigated in this study is classified as a hydrophobic colloid.
Colloids have large specific areas usually greater than 10m2/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 BaSO4 is known to be
unstable in suspension. This will be discussed later in the review. The behaviour of the
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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 BaSO4 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.
2.2.1 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
2.3.1 Restrictions in colloid transport
The following processes restrict colloid movement (McGechan and Lewis 2002):
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• 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.
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2.4 Colloid Mobilization
Colloid transport through granular porous media is modelled by the advection-dispersion
equation (Kretzschmar et al 1999):
Equation 2-1Ck
z
Cv
z
CD
t
Cf−
∂
∂−
∂
∂=
∂
∂2
2
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; kf 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 kf 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 kf, as
both methods produce very similar results. Akbour et al (2002) determined the colloid
filtration rates (kf) 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.
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Figure 2-1 Experimental determination of the colloid filtration rate (kf) 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.
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).
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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.
2.4.1 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).
2.4.2 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.
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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 (pHpzc) on the ferric oxyhydroxides. If the
influent pH was below the pHpzc, 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. BaSO4 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
Remediation of Barium Contaminated Groundwater
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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 (BaSO4) is formed according to the following reaction-
Equation 2-2 BaSO4 ↔↔↔↔ Ba2+ + SO42-
where the equilibrium constant and solubility product, Ksp = 1.23 x 10-10. The solubility, S =
1.1 x 10-5 mol/L (Manahan 2000).
BaSO4 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 BaSO4 crystal in its rhombohedral structure
(Dunn et al 1999)
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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 125-
225nm (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 BaSO4 is the one of the most insoluble sulphate minerals, as it has a pKsp of 9.96
(Bishop 1988). It is commonly used in powder coatings, paints, inks, rubber, pigments,
storage batteries, plastics, paper etc. BaSO4 is formed by three steps- nucleation, crystal
growth and aggregation. The size of the BaSO4 particles formed is dependent on the
conditions during these phases.
2.8 Role of Dispersants
Sun and Sköld (2001) created barium sulphate (BaSO4) particles by precipitation and used
light scattering measurements to determine its turbidity. Turbidity was measured for a wide
range of temperatures and concentrations. Titrating BaCl2 and NaSO4 created the solution of
BaSO4, while light scattering, pH and conductivity were measured throughout the process.
Remediation of Barium Contaminated Groundwater
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However BaSO4 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 BaSO4 suspension had a pH of approximately 7.4 throughout all the experiments. Results
show that BaSO4 precipitate is stable throughout the pH range of 0-14. The main findings
include-
1 . BaSO4 particles aggregated forming larger particles in the absence of a dispersant.
Therefore the BaSO4 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 BaSO4 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 Ba2+ sequestering action
of the PAM.
From this work it can be deduced that BaSO4 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 BaSO4 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 BaSO4 particles. The scattering reduces the
intensity of the light. Therefore the turbidity depends on the concentration and size of the
BaSO4 particles.
In this present study will use turbidity measurements to show firstly that BaSO4 colloid
particles are forming in the sand columns. The analytical method to form BaSO4 will also be
used.
Remediation of Barium Contaminated Groundwater
13
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 BaSO4 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 BaSO4 is 5 (Figure 2-5).
Figure 2-5 The influence of solution pH on the surface charge of BaSO4 (Collins 1998).
Remediation of Barium Contaminated Groundwater
14
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.
2.11 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 0.2cm3/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.
2.12 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
Remediation of Barium Contaminated Groundwater
15
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 50-100mg/L would induce precipitate of barium sulphate in a short storage period.
Remediation of Barium Contaminated Groundwater
16
0
1
2
3
4
5
6
7
0 200 400 600
Sulphate mg/L
log
Bar
ium
ug
/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 SO4 2-(mg/L)
Mirrabooka - 1-18
Gwelup 5-26
Mirrabooka 3-90
Leederville
Wanneroo 0-19
Wanneroo 0-39Yarragadee
Myalup 5-19*
10 mg/L
1 mg/L
0.1 mg/L
0.01 mg/L
Remediation of Barium Contaminated Groundwater
17
3 Methodology
3.1 Sand Column
3.1.1 Sands
Three sand size grains were investigated in this study-
• 1.4-2mm
• 0.5-0.71mm
• 0.125-0.25mm
These three were chosen to understand colloid mobility in a large sand grain, a medium sand
and a small sand grain.
3.1.2 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 (cm3) 5.89
3.1.3 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.
3.1.3.1 Materials
The following materials are needed to pack and saturate the columns-
• 3 x columns
Remediation of Barium Contaminated Groundwater
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• 3 x sand sizes (1.4-2mm, 0.5-0.71mm, 0.125-0.25mm)
• Retort Stand
• Plunger
• CO2
• DI water
3.1.3.2 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 CO2. 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 CO2 producing a saturated column. Care
must be taken not to let air back into the column.
3.1.4 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 (cm2/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.
3.1.4.1 Materials
• 3 columns with differing sand size
• 0.2mM NaCl
Remediation of Barium Contaminated Groundwater
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• 0.5mM NaCl
• Flow-through electrical conductivity detector
• Peristaltic pump
• Timer
3.1.4.2 Procedure
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.
UV-VIS PUMP
Column
Beaker 1 Beaker 2
Valve
Remediation of Barium Contaminated Groundwater
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3.2 Kaolinite
3.2.1 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.
3.2.1.1 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
3.2.1.2 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.
Remediation of Barium Contaminated Groundwater
21
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 (kf). kf 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 1.1-2.2
minutes for the different sand sizes.
3.2.2.1 Materials
• 1ppt kaolinite solution
• DI water
• Sand column
• Peristaltic pump
• Flow-through UV-VIS spectrophotometer
• Timer
Remediation of Barium Contaminated Groundwater
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3.2.2.2 Procedure
First the initial concentration of kaolinite (CO) 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 1.
3. Set flow rate to 1.25mL/min.
4 . Simultaneously start UV-VIS measurements and input the kaolinite solution by
switching to beaker 2.
5. Once a stable reading has been reached, switch back to beaker 1.
6. Again once a stable reading has been met, stop recording absorbance.
The maximum absorbance is the initial absorbance of the kaolinite (AO), which can then be
converted to CO using the linear relationship in the calibration curve (Section 3.2.1).
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)/CO.
3.2.3 Breakthrough Curves for Variable Flow Rate
This experiment was devised to understand if kf 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.
The same experimental apparatus was used in Section 3.2.3. 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 2.
3. 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 1.
6. Stop measuring absorbance once the reading is stable.
Remediation of Barium Contaminated Groundwater
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3.3 Barium Sulphate
3.3.1 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
0.98.
3.3.1.1 Materials
The following materials were used in the creation of the calibration curve-
• 10mL of each Na2SO4 concentrations- 10, 7.5, 5, 2.5, 1ppm.
• 10mL of Buffer B
• BaCl2 (Dihydrate) Analytical Reagent
• 5 x 30mL containers with cap
• 5 x 4.5 mL cubettes
• Weighing Balance
• Magnetic Stirrer
• Stirrer Piece
• UV-VIS Spectrophotometer
3.3.1.2 Procedure
7. Take 10mL of 10ppm Na2SO4 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 Na2SO4 container and place the container on
the magnetic stirrer with stirrer piece inside.
11. Turn on stirrer.
Remediation of Barium Contaminated Groundwater
24
12. Measure 1.25g of BaCl2 and add to 10ppm Na2SO4 container and mix for 1 minute
exactly. This is to ensure all the BaCl2 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 Na2SO4- 7.5ppm, 5ppm, 2.5ppm
and 1ppm.
Timing is important in this experiment, as the BaSO4 is forming and precipitating out of
solution. Mixing allows for the Na2SO4 and BaCl2 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.
3.3.2 Breakthrough Curves for Barium Sulphate Suspension
The same materials and procedure were used as in Section 3.2.2 except that-
• the background solution was 10ppm Na2SO4 (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 BaSO4 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 5.3.1. Therefore another method of experimenting with BaSO4 colloids
was found.
3.3.3 Creating a Mixing Interface between Barium Chloride and Sodium Sulphate
Since creating BaSO4 outside the column and then letting it infiltrating the column produced
erratic results, another method was chosen. The BaSO4 would be produced within the column
by flushing the column with BaCl2, and then displacing it with a Na2SO4 solution (
Figure 3-2). BaSO4 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. BaCl2
then displaced Na2SO4 to see what affect this process had.
Remediation of Barium Contaminated Groundwater
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Figure 3-2 Displacement of BaCl2 by Na2SO4 in the column to investigate the mixing
interface.
3.3.3.1 Materials
The same experimental apparatus is used as in Section 3.2.2 except-
• beaker 1 contained 9mM BaCl2 constantly mixed by a magnetic stirrer.
• beaker 2 consisted of 10mM Na2SO4.
The concentrations were chosen so that SO42- was in excess of Ba2+. This is to maintain that
all the Ba2+ would react and form BaSO4.
3.3.3.2 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 2.
4. Once a stable reading has been established, switch back to beaker 1.
5. Again when the reading is stable switch back to beaker 2.
6. Repeat steps 4-5 twice.
7. Once the reading is stable stop measurements.
Throughout this experiment the pH and EC was measured every 3mins.
BaCl2
Na2SO4
Flow
Mixing Zone
Remediation of Barium Contaminated Groundwater
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3.4 Calculations
3.4.1 Colloid Filtration Rate (kf)
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 kf 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 FL
vk p
f ln−=
where vp is the pore water velocity, L the length of the column and F the ratio between C/CO
once the plateau has reached a stable value.
3.4.2 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).
Remediation of Barium Contaminated Groundwater
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4 Results
4.1 Sodium Chloride Tracer
Figure 4-1 Breakthrough curve constructed with EC measurements from experiments
involving column of 0.5-0.71mm at two different flow rates.
Electrical conductivity (EC) measurements were made by adding a pulse of 0.5mM NaCl
solution to a background solution of 0.2mM NaCl in each of the columns (Figure 4-1). These
experiments were performed at two different flow rates- 0.6 and 1.4mL/min. Results show
that the EC measurements are consistent with a breakthrough pattern. Plotting relative EC, the
maximum critical value is 1 in all cases.
Changing the flow rate in each sand size did not make an impact on the results, as the same
pulse volume was added to each column. However the gradient at which the EC increases is
steeper for the larger sand size columns.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7 8
Pore Volume
Re
lati
ve
EC
1.4mL/min 0.6mL/min
Remediation of Barium Contaminated Groundwater
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4.2 Kaolinite
4.2.1 Calibration Curves
y = 1.6916x - 0.0897
R2 = 0.9964
-0.5
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2
Absorbance (nm)
Co
nce
ntr
atio
n (
pp
t)
Figure 4-2 Relationship between the absorbance (nm) and concentration (ppt) for
kaolinite.
A relationship was established between the concentration of the differing kaolinite solutions
and the absorbance, which was measured with a UV-VIS spectrophotometer (Figure 4-2).
Fitting a linear relationship to this data produced a correlation coefficient of 0.9964 indicating
a strong linear relationship. Therefore the relationship exists such that-
Equation 4-1 Concentration = 1.6916 x Absorbance – 0.0897
This relationship is strong but has a negative intercept of 0.0897, which is unusual. This was
ignored, as this value is small in comparison to the concentration used.
Remediation of Barium Contaminated Groundwater
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4.2.2 Breakthrough Curves for Different Sand Sizes
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10 12 14 16 18 20
Pore Volumes
C/C
o
1.4-2mm 0.5-0.71mm 0.125-0.25mm
Figure 4-3 Breakthrough curves for 1ppt kaolinite in different sand grains at flow rate
1.25mL/min.
The breakthrough curves for 1ppt kaolinite solution in different sand with a constant flow rate
of 1.25mL/min are shown above (Figure 4-3). The largest sand size (1.4-2.0mm) produced the
highest peak of relative concentration with a value of 0.9695. The middle sand grain size (0.5-
0.71mm) produced a peak of 0.948, and the smallest grain size (0.125-0.250mm) had the
smallest relative concentration peak- 0.942 (Table 4.1).
Remediation of Barium Contaminated Groundwater
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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.
Sand Size (mm) Peak Relative Concentration
1.4-2 0.9695
0.5-0.71 0.9480
0.125-0.25 0.9420
4.2.3 Variable Flow Rate
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.
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0 5 10 15 20 25 30 35 40 45
Pore Volumes
Ab
so
rban
ce (
nm
)
Flow rate change
(0.8-1.25mL/min)
Flow rate change
(1.25-1.8mL/min)
Remediation of Barium Contaminated Groundwater
31
Increasing the flow rate in a column will increase the concentration of kaolinite that is
produced in the eluant (Figure 4-4). As soon as the flow rate is increased from 0.8mL/min to
1.25mL/min or 1.25mL/min to 1.5mL/min, a rise in concentration is recorded by the UV-VIS
spectrophotometer. The change in flow rate from 0.8mL/min to 1.25mL/min occurs at 18.75
pore volumes, and the increase from 1.25mL/min to 1.5mL/min takes place at 31.875 pore
volumes.
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.
From the measured concentration a breakthrough curve can be created for changing the flow
rate within the 1.4-2mm sand column (Figure 4-5). When the flow rate changes during the
experiment, the peak in relative concentration increases. The stable plateau reaches 0.867 at
flow rate 0.8mL/min; a reading of 0.966 for 1.25mL/min; and 0.989 at 1.8mL/min.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35 40 45
Pore Volume
C/C
o
Remediation of Barium Contaminated Groundwater
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4.3 Barium Sulphate
4.3.1 Suspension
This graph represents one of many results that were obtained from pumping a BaSO 4
suspension through a sand column. Results were not reproducible, however this was one of
the “better” data that shows the erratic behaviour of BaSO4. This breakthrough curve displays
a 30ppm BaSO4 suspension flowing through the 1.4-2mm sand column at a flow rate of
0.52mL/min (Figure 4-6). There is no constant peak or plateau produced in the data, only
“jagged” peaks. Initially there is a marked increase in concentration near 2 pore volumes, but
then the concentration “zig zags” until reaching a constant relative concentration of around
0.365. However this does not remain constant, as the relative concentration fluctuates. If any
quantitative analysis could be made it would be the fact that the relative concentration hovers
between 0.3 and 0.5.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7 8 9 10 11
Pore Volume
C/C
o
Figure 4-6 Breakthrough Curve for 30ppm BaSO4 suspension in 1.4-2mm sand column
at flow rate 0.52mL/min.
Remediation of Barium Contaminated Groundwater
33
4.3.2 Mixing Interface
The mixing interface between BaCl2 and Na2SO4 was observed by pumping Na2SO4 into a
BaCl2 environment, and then once a Na2SO4 environment had been established, BaCl2 was
then pumped in. This was repeated a number of times. These experiments were conducted in
each sand for a constant flow rate of 1mL/min. Experiments performed with 1.4-2mm and
0.5-0.71mm produced marked peaks each time the environment changed from either BaCl2 to
Na2SO4, or Na2SO4 to BaCl2 (Figure 4-7). The red lines indicate when the environment was
changed. When comparing these peak heights in the two sand sizes, no relationship could be
established. This is because all the peaks in the larger sand reached absorbance values near
0.23-0.27 nm, yet peak heights in the 0.5-0.71mm sand did not reach a common range.
For the medium column the first peak from Na2SO4 entering BaCl2 column was 0.25nm but
the next peak (when BaCl2 entered Na2SO4 environment) reached 0.43nm. The peak then
dropped for then next interchange to 0.32nm but then increased to 0.40nm. An alternating
peak sequence was reproduced in the 1.4-2mm column, as the peaks increased when Na2SO4
entered the BaCl2 environment not the other way around as seen in the 0.5-0.71mm column.
In both columns there is am increasing trend of the baseline levels of absorbance recorded
after each environment change (Table 4-2). Both baselines levels of absorbance begin at 0nm
but then they increase once the first environment has been changed. Column with 1.4-2mm
has it’s baseline level increased to 0.02nm, and column 0.5-0.71mm to around 0.04nm after
the first interchange. The baseline levels seem to increase by approximately 0.25-0.3nm in the
1.4-2mm sand, and increases by about 0.45nm in the 0.5-0.71mm sand after each interchange.
Remediation of Barium Contaminated Groundwater
34
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 5 10 15 20 25 30 35 40
Pore Volume
Ab
sorb
ance
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 5 10 15 20 25 30 35 40
Pore Volume
Ab
sorb
ance
Figure 4-7 Alternating sequence of pumping in Na2SO4 into a BaCl2 column
environment containing a sand size of 1.4-2mm (a) 0.5-0.71mm (b) at flow rate
1mL/min.
Remediation of Barium Contaminated Groundwater
35
Table 4-2 Absorbance of baseline levels after each interchange for each sand
Absorbance (nm) Width of peaks (pore volumes)
1.4-2mm sand 0.5-0.71mm sand 1.4-2mm sand 0.5-0.71mm
sand
1st interchange 0.02 0.04 2.16 1.35
2nd interchange 0.05 0.08 2.5 2.4
3rd interchange 0.07 0.12 2 1.8
4th interchange 0.10 0.17 2.4 1.5
5th interchange 0.12 - 2.1 -
The width of the peaks can also be compared with these sands. The larger sand produced
peaks over a larger pore volume than the medium sand. The 1.4-2mm sand recorded an
average of 2.2 pore volumes while the 0.5-0.71mm sand produced peaks over 1.7 pore
volumes.
The results from the smallest column (0.125-0.25mm) produced no peaks in absorbance, but
did share this increasing trend in baseline values with the other sands (Figure 4-8).
0
0.025
0.05
0.075
0.1
0.125
0.15
0 2 4 6 8 10 12 14 16 18 20
Pore Volume
Ab
sorb
ance
(n
m)
Figure 4-8 Alternating sequence of pumping in Na2SO4 into a BaCl2 column
environment containing a sand size of 0.125-0.25mm at flow rate 1mL/min.
Remediation of Barium Contaminated Groundwater
36
Baseline levels increased initially by approximately 0.03nm but then the rate started to
decrease. The difference at the last interchange was 0.02nm.
Remediation of Barium Contaminated Groundwater
37
5 Discussion
5.1 Sodium Chloride Tracer
EC measurements were used to calculate the dispersivity (λ) and the dispersion coefficient
(D) for each size of sand. For the results to make sense the relative EC should have a critical
value of 1. This indicates that none of the pulse of 0.5mM NaCl is being filtered by the
column- i.e. whatever enters the column leaves the column.
A spreadsheet devised by Bromly (2003) uses the Yu et al (1999) moment method to
calculate the λ and D from the EC measurements. This spreadsheet was used in this study for
this same purpose by entering values for EC, properties of the sand and the flow rate. The λ
for each sand is listed below (Table 5-1).
Table 5-1 The pore water velocity (cm/min), dispersivity (cm) and dispersion coefficients
(cm2/min), for each sand size.
Sand Size
(mm)
Pore Water
Velocity
(cm/min)
Dispersivity
(cm)
Comparison of
dispersivity
(cm)
Dispersion
Coefficient
(cm2/min)
1.4-2 1.778 0.148 0.278 0.268
0.5-0.71 2.225 0.136 0.249 0.302
0.125-0.25 2.281 0.134 0.221 0.306
The dispersivity values for comparison were obtained from Bromly (2003). The dispersivity
values from this study were lower in all cases, but still in the same magnitude. They also
decreased with decreasing sand size indicating that colloids will spread or disperse more in a
column that has larger sand grains. Therefore it is easier for the colloids to disperse in a pore
volume that is larger and has fewer obstructions in the way to restrict colloid movement.
The dispersion coefficient was calculated by multiplying the pore water velocity with the
dispersivity. Obviously it too will increase for larger sand grains. The pore water velocity
increases with sand size, as the pore volume decreases. The flow rate remains the same, but
Remediation of Barium Contaminated Groundwater
38
since the volume the solution has to travel is smaller, the velocity has to increase to keep the
flow rate constant.
5.2 Kaolinite
5.2.1 Breakthrough Curves for Different Sand Sizes
When 1ppt kaolinite was placed through the differing sand sizes at a constant flow rate of
1.25mL/min, one important observations was made (Section 4.2.2)- the larger the sand grain
size, the larger relative concentration peaks. The largest sand grain size produced the largest
relative concentrations peak indicating that there is less colloid filtration occurring inside the
column. If a value of 1 was reached for the relative concentration, this means that there is no
colloid filtered inside the column. Therefore the closer the relative concentration peak is to 1,
the less colloid mass is being retained in the column.
From the relative concentration peak the colloid filtration rate (kf) can be calculated (Section
3.4.1). The kf for each sand is listed below (Table 5-2). These values were consistent with kf
calculated by Akbour et al (2002) for kaolinite colloids. The kf decreases with decreasing sand
size. Therefore there are more colloids being filtered in the columns with a smaller sand size.
This makes sense, as there is a smaller pore volume in a column with finer sand grains. The
colloids are more likely to become trapped within this finer pore space, and will then fall out
of solution and collect with the column. Conversely for a larger sand grain size there are
larger pore spaces, so the likelihood of colloids becoming trapped decreases and there are less
colloid filtered.
Table 5-2 Colloid filtration rates calculated for each sand at a flow rate of 1.25mL/min.
Sand Size (mm) kf (min-1)
1.4-2 0.0148
0.5-0.71 0.0168
0.125-0.25 0.0175
Remediation of Barium Contaminated Groundwater
39
5.2.2 Colloid Filtration Rate Dependence on Pore Water Velocity
The flow rate affects the concentration in the eluant from the column. As the flow rate is
increased, a higher concentration of kaolinite is recorded indirectly by the UV-VIS
spectrophotometer. This is because at higher flows more colloidal mass can be transported
through the column.
From the kf’s calculated from the experiment with variable flow rate (Section 4.2.3), it
indicates that kf is dependent on the pore water velocity. When the flow rate is increased (and
hence velocity), the relative concentration peak increases, and also the kf. The kf’s for each
velocity are listed below (Table 5-3). The data was then plotted to see what the relationship
was (Figure 5-1).
Table 5-3 Colloid filtration rates measured in 1.4-2mm sand for different pore water
velocities.
Pore Water Velocity (cm/min) Colloid Filtration Rate (min–1)
2.29 0.0435
3.57 0.0165
5.14 0.0076
This relationship needs to be studied more, as three points of data is not enough to establish a
proper relationship. But from equation 3.1 it shows that there is a relationship between the
two parameters. The negative sign shows that there is a negative relationship, which agrees
with the results.
Remediation of Barium Contaminated Groundwater
40
0
0.01
0.02
0.03
0.04
0.05
0 2 4 6
Pore Water Velocity (cm/min)
Co
llo
id F
iltr
ati
on
Rate
(min
-1)
Figure 5-1 Relationship between colloid filtration rate and pore water velocity for 1.4-
2mm sand.
5.3 Barium Sulphate
5.3.1 Behaviour of BaSO4
Creating a BaSO4 suspension and pumping it into the column produced very erratic results
that were not reproducible (Section 4.3.1). Trends that did occur were that the peak in relative
concentration fluctuated in a range but did not reach some critical value. Initially the relative
concentration rose to some value then diminished to some constant range. This erratic
behaviour may be attributed to the BaSO4 colloids readily flocculating inside the column and
attaching to the sand grains. Colloids may be forming particle bridges between sand grains,
and these bridges may be broken by a mobile colloid. As these bridged colloids become
dislodged, large spikes in absorbance (and therefore concentration) will be recorded. Even
experiments conducted with no column produced these erratic results indicating that colloids
were attaching to the tubing in the apparatus as well. It can be deduced that colloids are being
deposited in the column from most results. The breakthrough curve for the 30ppm BaSO4
suspension in the 1.4-2mm sand column showed a peak range between 0.3-0.5 (Figure 4-6).
Since this peak does not reach anywhere near 1, this shows that colloids are being filtered out
Remediation of Barium Contaminated Groundwater
41
of suspension and are being deposited inside the column. However due to the quality of the
data, it can only be analysed qualitatively. Therefore a kf can not be calculated.
This behaviour is consistent with past studies done by Sun and Skold (2001) and Aliaga et al
(1992). Both studies encountered the precipitate to be unstable in their experiments. Aliaga et
al (1992) even tried to stabilise the precipitate by adding a non-ionic surfactant and PAM.
5.3.2 Mixing Interface
The peaks in the experiments investigating the mixing interface between BaCl2 and Na2SO4
represent the formation of BaSO4 precipitate (Section 4.3.2). The peaks in absorbance show
that the UV-VIS spectrophotometer is detecting a more turbid solution than BaCl2 or Na2SO4
alone. This can only be explained by the fact that when the two solution meet inside the
column, they interact and form BaSO4, but only at the interface. This is called the zone of
mixing. This explains why the peak only occurs over a few pore volumes not over many pore
volumes. The BaSO4 is being formed at the interface of the two solutions and is then
transported out of the column by the flow. The sand grains promote some mixing of the two
solutions also, to help this reaction occur.
The baseline levels of absorbance are on the increase, which indicates that some proportion of
BaSO4 colloids that have accumulated inside the column. This is in agreement with the results
from creating BaSO4 outside the column and pumping it in, as it proves filtration is occurring
(Section 5.3.1). However the increasing levels suggest that the BaSO4 colloids may be
“bleeding” out of the column at some rate. Therefore this filtration process may be a
reversible reaction. Again this can only be analysed qualitatively. Therefore from the results a
kf can not be calculated.
The larger sand produced peaks over the largest pore volume in comparison with the medium
sand. This means that the BaSO4 colloids are being produced and dispersing throughout the
column more in the larger sand. The colloids are spreading out over a larger pore volume, and
are thus more mobile. This would indicate that its dispersion coefficient would be greater in
the larger sand. However this can not be computed due to the type the data. This observation
would be attributed to the greater pore volume within the 1.4-2mm sand column. A greater
Remediation of Barium Contaminated Groundwater
42
pore volume allows for the colloids to disperse more easily than in a smaller pore volume.
The medium sand size column has more sand grains than the large column, so this allows for
more obstructions for a mobile colloid. The more sand grains there are, the harder the path is
to traverse through the column for the colloid. This is agreement with the results from the
NaCl tracer experiments (Section 4.1). It was found that the dispersion coefficient was larger
for sand grains of larger size.
No peaks in absorbance are observed in the finest sand column. This is because BaSO4 is
being produced in the column but a major proportion is not leaving the column. It can be
deduced that BaSO4 is being produced in the column by the increasing trend of baseline
absorbance after each interchange. Again we can see these “bleeding” phenomena, which is
evident by the increasing baseline levels.
Remediation of Barium Contaminated Groundwater
43
6 Conclusion
From the experiments performed in this study the following was found-
• the colloid filtration rate (kf) is higher for smaller sand sizes. This means that more
colloids will be filtered out in the smaller sand.
• kf is dependent on pore water velocity (vp). The larger the vp, the lower the kf will be,
as a higher velocity will flush out more colloids.
• the dispersivity (λ) and the dispersion coefficient (D) are greater for larger sands. This
is because in larger sand there is a larger pore volume and less grain surface area for
the colloids to attach to. Therefore the colloids will disperse more throughout the sand.
• when a BaSO4 suspension is produced outside the column, the colloids readily
flocculate and create particle bridging between the sand grains. These bridges may be
broken by the displacement by mobile colloids. From these erratic results it is difficult
to determine a kf, but qualitatively it can be inferred that the sand filters out a
proportion of the BaSO4 colloids.
• injecting Na2SO4 into BaCl2 column environment was used to simulate ASR, where
BaSO4 was produced in the zone of mixing. The zone of mixing is greatest for larger
sands, as there is greater dispersion. However in the finest sand it appeared that no
BaSO4 was produced in the eluant, as it was accumulating inside the column. In all the
sands there appeared to be some “bleeding” of BaSO4, which indicates that colloids
were being released at some rate from the sands. Therefore the filtration of BaSO4
may be a reversible reaction.
This study gave introductory knowledge of the mobility of BaSO4 as a colloid. This is a
complex issue though as the behaviour of BaSO4 was very unstable. Most of the
conclusions about BaSO4 could only be done qualitatively not quantitatively. Therefore it
is recommended that further work be conducted to describe the behaviour quantitatively,
which was done for kaolinite. Work should be done to understand why BaSO4 is so
unstable. Possible factors responsible could be the effect of ionic strength or point of zero
charge. Experiments involving BaSO4 suspensions with differing ionic strength and pH
above and below the point of zero charge should be conducted. This should give an idea
why the precipitate is unstable.
Remediation of Barium Contaminated Groundwater
44
Creating the mixing interface is the best experiment to represent what’s happening in the
aquifer. Na2SO4 represents sulphate-amended water being pumped into the aquifer.
BaSO4 is formed in the zone of mixing and is then filtered out by the porous media. It
then continues to accumulate in the column. Therefore with the formation of BaSO4 this
reduces the barium concentration in the water. Results show that the porous media
“bleeds” out BaSO4. This indicates that the filtration process is reversible as BaSO4 is
adsorbing to the sand grains but is then released after some time. More research should be
conducted to understand if this is indeed a reversible reaction. The mixing interface could
be investigated more by injecting Na2SO4 into a BaCl2 column environment and reversing
the flow to promote mixing. This would be representative of what occurs in the aquifer,
because more dispersion and mixing is expected in the field.
Kaolinite colloids proved a stable colloid for comparison. Therefore further research to
determines the kf for BaSO4 can be compared with kaolinite.
kf is dependant on pore water velocity, so this has implications for ASR. During ASR
velocities are greater near the well and decrease proportionately to the radius squared
from the well. Therefore near the well the filtration of colloids will be smaller, so the
colloids should form and disperse away from the well.
Another recommendation is to conduct experiments in soils characteristic to Perth’s
geology because the sands that were chosen were based on size only.
Remediation of Barium Contaminated Groundwater
45
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