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Flocculation of silica by High Molecular Weight Polysaccharides.

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Dev. Chem. Eng. Mineral Process. 12(3/4), pp, 341-354, 2004.
Flocculation of Silica by High Molecular
Weight Polysaccharides
K.K. Foong and R. Amal*
Centrefor Particle and Catalyst Technologies, School of Chemical
Engineering and Industrial Chemistry, The University of New South
Wales, Sydney, New South Wales 2052, Australia
W.O.S. Doherty and L.A. Edye
Sugar Research Institute, Mackay, Queensland 4740, Australia
Flocculation of colloidal silica particles by naturally occurring polysaccharides is of
interest to soft drink and beer manufacturers. Compounds found in jloc include
polysaccharides, proteins, lipids, waxes and silica. These compounds may originate
@om the water, sugar, yeast, other beverage ingredients or additives used in the
beverage manufacture. In this paper, the flocculation of silica particles by
polysaccharide extracts of cane sugar was investigated. The change in the particle
size with time was monitored by small angle light scattering. Studies were performed
under both static and agitated conditions. Under static conditions, it was observed
that faster flocculation kinetics were obtained at higher polysaccharide loadings as
the time taken to form thefloc network was reduced. Higher silica concentrations
resulted in larger and denserflocs. The time lapse prior to the appearance of visible
cottonball floc was reduced substantially when the solutions were agitated.
Introduction
In soft drink and beer manufacturing, floc formation can arise due to the production
process. These flocs form as a result of aggregation of the compounds used in the
preparation of these beverages. The conditions under which the floc occurs are not
fully understood and removal of the floc from the process would be of great financial
benefit to the manufacturers.
High molecular weight polysaccharides found in soft drinks include starch,
dextran, syrup hemicellulose, indigenous sugar cane polysaccharide (only in cane
sugars), and sakaran. Polysaccharides were found to be a component in flocs isolated
* Authorfor correspondence (r.amal@unsw.edu.au).
341
K.K. Foong,R. Amal, W 0 . S . Doherty and L.A. Edye
from cane and beet sugars. Miki et al. [ 11 found that the polysaccharide content in floc
isolated from refined cane sugars was 66.9%. Cohen et al. [2] isolated 23 ppm of
insoluble matter from a refined cane sugar. They found that the high molecular weight
polysaccharide content of the insoluble matter was 12.7% and the silica content was
12.8%. From the literature surveyed, it is still unclear how much of the floc formed is
due to impurities from the sugar itself or from water used to make up the beverage.
In soft drink manufacture, studies 'have been performed on the presence of floc in
beet and cane sugars since 1959 [2-91. There is a lesser concern with the formation of
floc in beet sugars since the floc causing substance (saponin) can be readily
eliminated. T h s is not the case with cane sugars. Most of the studies on flocs in cane
sugars are on floc composition rather than the mechanisms of floc formation. A
proposed mechanism on the floc formation process has been given but has not been
proven experimentally [6]. It is widely believed that the formation of floc is caused by
polysaccharides. The use of silica to remove floc-causing polysaccharides has not
been explored at all. However, in the beer industry studies have been carried out on
the removal of haze in beers using silica hydrogel. Even though the floc formed is
caused by different substances present in the beer, the use of silica hydrogel to
remove floc-causing compounds has been widely documented [ 10-171.
The role of silica in the floc formation process in cane sugars is poorly understood
at present. It is believed that both high molecular weight polysaccharides and silica
play a pivotal role in the floc formation process. Polysaccharides are a major organic
component in floc whereas silica is the largest inorganic component that makes up the
floc. This paper presents studies on flocculation of colloidal silica particles by
polysaccharide extracts from cane sugar in synthetic beverages.
Experimental Procedure
The sugar used was food grade refined white sugar from sugarcane. Citric acid,
sodium benzoate, sodium azide and the silica particles were purchased from Aldrich
Chemical Company. MilliQ water purified by a Millipore unit (Milli-Q PLUS) was
used to prepare all solutions. Analyses were performed by the Brookhaven three-inone system (which incorporates the 90+ (particle sizing), ZetaPlus and ZetaPals (zeta
potential measurements)) and the Malvern Mastersizer S.
The Brookhaven 90+ (Brookhaven Instruments Corporation, USA) employs the
technique of Photon Correlation Spectroscopy (PCS) to determine the particle size.
PCS, also known as dynamic light scattering, involves measuring the total light
scattered by particles undergoing Brownian motion or diffusion with time.
Information such as diffusion coefficients, size and size distribution of the particles,
and kinetic phenomena can be derived from this instrument. For the measurement of
larger particles, static light scattering can be used to obtain the size of particles. Using
ths method, the angular dependence of the scattered light from the total number of
aggregates and single particles are measured. The main difference between static light
scattering and dynamic light scattering is that with static light scattering, intensity
data is displayed as a function of angle of scattered light, whereas with dynamic light
scattering, intensity fluctuations with time are obtained. The Malvern Mastersizer S is
an instrument that uses static light scattering. The Brookhaven ZetaPlus and ZetaPals
direct a laser light source and apply an electric field through the sample. The
342
Flocculation of Silica by High Molecular Weight Polysaccharides
frequency of light scattered by the particles into a detector would be Doppler shifted
by the applied electric field an amount proportional to the velocity of the particles.
The velocity at which the particles move is proportional to the charge of the particles.
In these studies, the Brookhaven ZetaPals was used.
a. Preparation of the silica stock solution
One gram (1 g) of silica was added to 1 litre of MilliQ water and mixed in a
volumetric flask. The mixture was then transferred to a measuring cylinder and left
overnight to allow the large particles to settle. The solution was then centrifuged at
3000 rpm for 10 minutes and the supernatant collected. The supernatant solution was
stored as a stock solution at 4°C. The silica concentration in the stock solution was
found to be 12 ppm silica from Inductively Coupled Plasma-Atomic Emission
Spectroscopy (ICP-AES) after digestion of the particles with hydrofluoric acid. The
size of the particles was measured by the Brookhaven 90+, which gave an average
particle size of approximately 400 nm.
A stock solution of 24 ppm silica solution was prepared using the above method
but using 2 g of silica per litre of water. For this stock silica solution, particles were
centrifuged at 8000 rpm for 10 minutes. Ths stock solution had a similar particle size
to that of the 12 ppm stock solution.
b. Preparation of synthetic beverage
Refined sugar (100 g) was added to approximately 800 g MilliQ water. The solution
was mixed for 1 hour before being passed through a 0.22 micron filter. The following
solutions were added in this particular order: 7.14 mL of 26.6 w/v% citric acid, 7.14
mL of 15 w/v% sodium benzoate, and 3.57 mL of 1 w/v% sodium azide. The
beverage was then made up to 1000 g with MilliQ water to give a sucrose
concentration of 10 w/w%. The pH of the resulting solution was between pH 2.8-3.2.
A beverage with a 20 w/w % sucrose concentration was prepared using a similar
method but with 200 g of refined sugar.
c. Preparation of a 10 w/w% sugar/l pprn silica acidic beverage solution
A sample of the 12 ppm stock solution of silica was placed in an ultrasonic bath for 5
minutes and then diluted to 2 ppm. Approximately 80 g of the 2 ppm solution was
measured out. Added to this solution was 714 micro-litres (pL) of 26.6 w/v% citric
acid, 714 pL of 15 w/v% sodium benzoate, and 357 pL of 1 w/v% sodium azide. The
solution was then made up to 100 g using the 2 pprn silica suspension. Twenty-five
grams of this solution was added to 25 g of the 20 w/w % sugar stock solution,
resulting in a 10 w/w% sugar / 1 ppm silica mixture for analysis by the Malvern
Mastersizer S. Similarly, acidic beverages containing 4 ppm, 6 ppm, and 12 ppm
silica were prepared.
d. Preparation of a 10 w/w% sugar /polysaccharide solution
In order to characterize the size of the micro-flocs due to self-aggregation of the
polysaccharide, initial polysaccharide flocs formed in beverages were characterized
by the Brookhaven 90+ system.
Five grams ( 5 g) of the 10 w/w% sugar solution was placed in a McCartney bottle
and heated to 85°C for 10 minutes while being magnetically stirred. The
343
K.K. Foong, R. Amal, W.O.S.
Doherty and L.A. Edye
polysaccharide extracts, which had been preheated at 85°C for 30 minutes, were then
added. The resulting mixture was stirred for 1 minute before being placed in a 5 mL
polypropylene cuvette, whereby it was measured by the Brookhaven 90+ for 15
minutes.
e. Measurement of zeta potential
Five grams of a sugar/silica mixture was placed in a 25 rnL McCartney bottle and
heated to 85°C for 10 minutes, while being magnetically stirred. The polysaccharide
extracts, which had been preheated at 85°C for 30 minutes, were then added. The
resulting sugar/silica mixture was then stirred for 1 minute before being placed in a 5
mL polypropylene cuvette and stored at room temperature for 15 minutes prior to
measurement of the zeta potential using the Brookhaven ZetaPals. Zeta potential
measurements were performed on both 10 w/w % sugar11 ppm silica and 10 w/w%
sugar16 ppm silica suspensions.
f. Preparation of beverages for analysis by the Malvern Mastersizer S
The sugadsilica suspension was preheated to 85°C for 20 minutes, while being
magnetically stirred. Polysaccharide extracts, preheated to 85°C for 30 minutes, were
then added to the sugar/silica mixture and stirred for 1 minute. The
sugar/silica/polysaccharidesuspension was then stored at room temperature for 15
minutes before being placed in the instrument sample cell for analysis by the Malvern
Mastersizer S (Malvern Instruments, USA).
For runs under static conditions, the solution was left in the Malvern Mastersizer
for 24 hours with measurements taken every 30 minutes. If no significant aggregation
was observed withm 24 hours, the suspension was monitored regularly for 24 days.
Suspensions were stored in a 250 mL plastic container.
For runs under agitated conditions, a GFL (Model 3005) orbital shaker from Lomb
Scientific (Australia) was used. The shaker was set at 40 orbits per minute (0.p.m).
After measurements in the Malvern Mastersizer, the suspension was placed on the
orbital shaker.
Runs were performed with 35 ppb, 50 ppb, 75 ppb, 100 ppb, 200 ppb and 500 ppb
polysaccharide using 1 ppm, 4 ppm, 6 pprn and 12 ppm silica concentrations under
both static and shaking conditions. A run was also performed on a 10% sugar/6 ppm
silica mixture (without the addition of polysaccharides) for 24 days to determine any
growth of silica particles.
g. The visualfloc test
This test was used for suspensions that did not show significant aggregation within 24
hours. The sugar/silica/polysaccharidesuspensions were prepared using the above
method. After the suspensions were heated, they were moved to a 250 mL container
and stored at room temperature and examined daily for flocs. Floc observations were
made using a projection black box.
Observed flocs were defined as follows: “cottonball” or “cotton-like’’ flocs include
flocs greater than or equal to 5 mm in length, either floating in the suspension or
sedimented at the bottom of the container. Sandy silica aggregates, which had settled
at the base of the container and have not formed cottonball floc, are denoted “SA”.
The term ‘no floc’ was defined as no observable floc formation in the beverage.
344
Flocculation of Silica by High Molecular Weight Polysaccharides
Results and Discussion
i Silica and polysaccharide characterisation
The zeta potential of the silica solution (with the addition of citric acid, sodium
benzoate and sodium azide) was around -30 mV (pH 3). The zeta potential of the
polysaccharide extracts in sugar was determined to be -7 mV (pH 3). A study was
then conducted varying the polysaccharide extract concentration with two beverages
containing either 1 ppm or 6 pprn silica. The polysaccharide concentrations used
ranged from 50 ppb to 10,000 ppb. Figure 1 shows the results obtained from this
study.
0
l&!OO
2000
3000
4000
+
5000
6000
7000
8000
9000
1C
0
+ 1ppm silica
Figure 1. The effect of varying polysaccharide concentration on silica Concentration.
Figure 1 shows that with an increase in polysaccharide concentration, the zeta
potential value of the particle shifts to a less negative value, indicating that the
polysaccharide is adsorbed on the silica surface and that there are interactions
between the silica particles and polysaccharide.
For low silica concentrations (1 ppm), around 1000 ppb polysaccharide was
required to cover the surface of silica (shown by the leveling out in the graph in
Figure 1). For a higher silica particle concentration ( 6 pprn), around 6000 ppb
polysaccharide was required to saturate the silica surface with polysaccharide. The
polysaccharide does not completely neutralise the silica charge as both the silica and
the polysaccharides are negatively charged.
The size distribution of silica particles was first determined on a 10 w/w YOsugar/
6 ppm silica system with no polysaccharide addition. The solution was measured over
24 days with no observed changes in the size distribution indicating that any silica
flocculation occurring during this time is negligible. The silica particles were found to
have a mean size of 0.4 microns (see Figure 2).
345
K.K. Foong, R. Amal, W.O.S.Doherty and L.A. Edye
At high polysaccharide concentrations, flocs were formed in the absence of silica
particles. Th~sis due to the self-aggregation of polysaccharide particles in the extracts
forming micro-flocs, which interlink with one another to form large flocs over time.
In order to understand the aggregation mechanism of the silicdpolysaccharide system,
the size of these micro-flocs was characterised using the Brookhaven 90+. The
average size of the micro-flocs was 0.3 micron (see Figure 3). It was found that at
polysaccharide concentrations greater than 500 ppb, these micro-flocs formed larger
flocs.
ii. Aggregation of silica by polysaccharides under static conditions
Aggregation of silica particles by polysaccharides was studied by monitoring the
change in vol% of particles less than 1 micron and greater than 100 micron with time.
Results for the aggregation of silica of various loadings (1-12 ppm) with 500 ppb
polysaccharide are presented in Figures 4 and 5. Monitoring the vol% of sub micron
particles accounts for the unflocculated silica particles (as already illustrated in Figure
2), while the particles greater than 100 micron represent the aggregates that will
eventually emerge as cottonball flocs. As the silica loading increases, the aggregation
rate is faster. The amount of sub I-micron particles drops significantly within 24
hours at the high silica loadings (12 ppm), which indicates that most of the silica
particles have been flocculated. This is in accordance with aggregation theory which
says that the more particles there are in solution, the greater the chance of collisions
occurring between particles, thus resulting in an increase in aggregation rate and
larger flocs being formed.
Figure 6 shows the change in vol% (~01%of particles after 24 hours - vol% of
particles after 0 hrs) for particles greater than 100 microns for polysaccharide
concentrations of 35 ppb, 50 ppb, 75 ppb, 200 ppb and 500 ppb with different silica
loadings. As the polysaccharide concentration increases so the rate of aggregation
increases, resulting in the formation of larger flocs after 24 hours. Figure 6 also
illustrates that for suspensions containing a polysaccharide concentration less than 75
ppb, there is no significant change in the volume percent of particles greater than 100
microns within 24 hours. This indicates that at these conditions, aggregation is slow
as only limited polysaccharide micro-flocs are available to flocculate the silica. In
fact, many days were required for the detection of particles greater than 100 microns
by the Malvem Mastersizer. For example, for 35 ppb polysaccharide and 12 ppm
silica, it took 9 days to observe the presence of flocs greater than 100 microns.
Even though the aggregation process is slow, the Malvern Mastersizer can detect
the growth of aggregates with polysaccharide concentrations less than 75 ppb. At low
polysaccharide concentrations, aggregates are formed by adsorption destabilisation.
These aggregates are denser when hgher silica loadings are applied, and they settle at
the bottom of the container and are seen as sandy aggregates.
In addition to the light scattering measurements, a visual floc test was used to
determine the time required to observe the cottonball floc formation. The suspensions
were visually monitored for 24 days. Table 1 lists the time taken to observe the
formation of cotton-like flocs larger than 5 mm. For low concentrations (less than 100
ppb) of polysaccharides, although no cottonball floc was observed, some sedimentary
silica aggregates (SA) were formed at the bottom of the plastic container. These
aggregates are also indicated in Table 1.
346
Flocculation of Silica by High Molecular Weight Polysaccharides
0.01
0.1
1
10
Particle size (microns)
100
1000
Figure 2. The size distribution of silica particles from the Malvern Mastersizer S.
100
a
**
**
80
.
.-3
u)
f 60
CI
40
20
. .
.
I
0
0.01
0.1
1
Particle size (microns)
Figure 3. Size distribution of polysaccharide micro-floes fLom the Brookhaven 90+
( I 0% sugar / 500 ppb polysaccharide system).
347
K.K. Foong, R. Amal, W.O.S. Doherty and L A . Edye
The main problems with the visual floc test are its subjective nature and the
variability between duplicate samples. Furthermore, the various types of flocs formed
are not easily differentiated and smaller aggregates formed cannot be detected by this
method. For the runs with polysaccharide concentrations less than 100 ppb, no
cottonball floc was observed within 24 days. The visual observation indicates that at
low polysaccharide concentrations (< 100 ppb) and high silica loadings (6 ppm and
12 ppm), the polysaccharide micro-floc formation is limited, resulting in no cottonball
floc but sandy silica aggregates at the bottom of the container.
iii. Aggregation of silica by polysaccharides under agitated conditions
The results for 500 ppb polysaccharide and various silica concentrations are shown in
Figure 7. Only the vol% change up to 5 hours was shown, because after that time
significant settling of floc particles has occurred with the mn containing 12 ppm
silica. Results from subsequent measurements were non-representative, as the
Malvern Mastersizer could not detect settled flocs. Settling of aggregates begins
when the aggregates reach a certain size; this settling will occur more quickly with
higher silica loadings as more silica is incorporated in the floc network. Due to the
lower number of silica particles at the 1 ppm silica loading, the density of the floc
formed is less than those formed at the higher silica loadings (6 ppm and 12 ppm),
resulting in less flocs settling. The settling of these flocs causes the non-representative
measurements at the high silica loadings, since the undetectable presence of the large
flocs by the light scattering instrument would invariably result in higher proportions
of smaller particles measured in the suspension. Therefore, only results monitored up
to 5 hours are presented.
It can be clearly seen in Figure 7 that only the run with 12 ppm silica shows an
increase in the volume O h of particles greater than 100 micron after 2 hours, whereas
the suspensions with 1 ppm and 6 ppm silica show a vol% change after 3 hours. This
indicates that flocculation is faster at hgher silica concentrations. With shaking, there
is no initial settling of the agitated silica particles. This means that at a fixed
concentration of silica, more particles are available for flocculation than under static
conditions. Therefore with shalung, the collision frequency between the aggregates
will be higher than with no shaking, due to the larger amount of particles present for
flocculation. More particles present means that larger aggregates are formed at a
faster time.
The visual floc test indicated that the time taken to observe cotton-like flocs was
reduced with the application of shaking compared to static conditions. The influence
of shaking on the time required to observe cotton-like flocs is shown in Table 2. It
was also observed that a higher concentration of silica resulted in larger amounts of
cotton-like flocs at the bottom of the container.
Runs were also performed using lower concentrations of polysaccharide (< 100
ppb) to determine whether the trend occurred for concentrations at which
polysaccharides are present in authentic beverages. From these runs, a similar trend to
that of the run with 500 ppb polysaccharide was observed.
A comparison of Tables 1 and 2 shows that the time taken to visually observe the
flocs is reduced substantially when shaking is applied to the system, which is in
accordance with aggregation theory. For polysaccharide concentrations less than 75
ppb, observation of cotton-like floc was made only under the influence of shaking.
348
Flocculation of Silica by High Molecular Weight Polysaccharides
.p..-....
....f............._
f .......... ...
0
8
4
12
time (hr)
24
20
16
Figure 4. Volume percent of particles less than I micron for a 500 ppb
polysaccharide system with various concentrations of silica under static conditions.
100
0
,
I
0
I
4
8
12
16
20
24
time (hr)
Figure 5. Volume percent of particles greater than 100 micron for a 500 ppb
polysaccharide run with various concentrations of silica under static conditions.
349
K.K. Foong, R. Amal, N O S . Doherty and L.A. Edye
100
!ct
s-
e
20 -
0
100
200
300
Polyaaccharlde conc (ppb)
400
500
Figure 6. The volume percent change of particles greater than 100 microns between
the beginning of the experiment (t = 0 hours) and ending of experiment (t = 24 hours)
for different polysaccharide concentrations under static conditions.
Table 1. The number of days required to form cotton-likeflocs and silica aggregates
under static conditions.
Without shaking, the mechanism of floc formation occurs at a much slower rate
than under the effect of agitation, since the main driving force for the collision of the
particles in the absence of agitation is Brownian motion. With shaking, the collision
rate between the polysaccharide micro-flocs is faster and results in formation of the
floc network in a shorter time. From the light scattering measurements, it is found that
for a polysaccharide concentration of 500 ppb undergoing agitation, aggregates
greater than 100 micron started to form within 2 to 3 hours for silica loadings ranging
from 1 to 12 ppm. Without agitation, aggregates greater than 100 micron were
observed only after 6 hours for silica loadings ranging from 4 to 12 ppm. No large
aggregates were observed for 1 ppm of silica loading. This also explains why
cottonball flocs were seen at much lower polysaccharide concentrations with shaking.
350
Flocculation of Silica by High Molecular Weight Polysaccharides
100
90
80
A
c 70
:
'5
60
0
50
A
' 30
# 40
0
A
#
4
0
0
20
0
0
10
/
z
0
0
0
c
I
c
1
2
7
3
4
5
time (hr)
Figure 7. Volume % ofparticles greater than 100 micron for a system containing
500 ppb polysaccharide and various concentration of silica.
Table 2. Time taken to form cotton-likeflocs under agitated conditions.
For the formation of cottonball floc, the polysaccharide micro-flocs have to
aggregate to form a network large enough to trap the silica particles in order for the
aggregates to be observed. With polysaccharide loadings greater than 100 ppb, the
micro-flocs formed due to the self-aggregation of polysaccharides combine with other
micro-flocs to form a floc network. The floc network entraps silica particles forming
large aggregates, which are ultimately observed as the cottonball flocs. A schematic
of the mechanism of the cottonball floc formation is presented in Figure 8.
At low polysaccharide concentrations (less than 100 ppb), the floc network takes
longer to form or even does not form. At these concentrations, the polysaccharide floc
network is more difficult to form. Thus, the micro-flocs adsorb on to the silica
particles and assist the aggregation of the silica particles through adsorption
destabilization. A schematic of this mechanism is presented in Figure 9.
351
K.K. Foong, R. Amal, K0.S. Doherty and L.A. Edye
The presence of silica particles enhances the s u e and density of the cottonball
flocs formed. As the silica loading increases, the collision rate between silica and
polysaccharide micro-flocs increases thus enhancing the aggregation process.
However, the presence of greater number of silica particles in the network results in
larger and denser aggregates resulting in larger amount of flocs settling at the bottom
of the container.
Figure 8. Mechanism of cottonball floc formation at high polysaccharide
concentrations (> 100 ppb).
352
Flocculation of Silica by High Molecular Weight Polysaccharides
Figure 9. Mechanism of cottonball floc formation at low polysaccharide
concentrations (< I00 ppb).
Conclusions
These studies show that sub-micron silica particles can be flocculated by
polysaccharide extracts from sugar cane. The flocculation mechanism is highly
dependent on the polysaccharide concentration. At h g h polysaccharide
concentrations, a larger floc network is formed yielding cottonball flocs in which
silica particles are trapped. At low polysaccharide concentrations, the silica particles
are flocculated by adsorption destabilisation, forming dense aggregates.
Without agitation, cotton ball flocs are observed after 13 days of aggregation for
100 ppb of polysaccharide and 12 ppm silica. However, under the influence of
shalung, the time required to observe the flocs is 1 day. In addition, with agitation, a
polysaccharide concentration as low as 35 ppb is sufficient to yeld cottonball flocs,
whereas under static conditions cottonball flocs are observed only with
polysaccharide concentrations of 100 ppb or greater. Hence, it can be concluded that
agitation plays an important role in flocculation of silica particles by the
polysaccharide extracts of sugar.
Acknowledgements
K. Foong wishes to thank the Australian Government for their financial assistance
through the ARC SPIRT grant to this project.
353
K.K. Foong, R. Arnal, W.O.S.
Doherty and L.A. Edye
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354
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