Desalination 281 (2011) 100–104
Contents lists available at ScienceDirect
Desalination
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Potential use of a novel modified seaweed polysaccharide as flocculating agent
Héctor J. Prado a, c, María C. Matulewicz a,⁎, Pablo R. Bonelli b, Ana L. Cukierman b, c
a
b
c
Departamento de Química Orgánica, CIHIDECAR (CONICET-UBA), Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, (C1428EGA), Buenos Aires, Argentina
PINMATE, Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, (C1428EGA), Buenos Aires, Argentina
Cátedra de Farmacotecnia II, Departamento de Tecnología Farmacéutica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Junín 956, (C1113AAD), Buenos Aires, Argentina
a r t i c l e
i n f o
Article history:
Received 24 May 2011
Received in revised form 20 July 2011
Accepted 21 July 2011
Available online 24 August 2011
Keywords:
Cationic polysaccharides
Cationized agarose
3-chloro-2hydroxypropyltrimethylammonium chloride
Flocculation
Kaolin
Water treatment
a b s t r a c t
Flocculation performances of cationized agaroses of different degrees of substitution in the range 0.04–0.77
are reported for the first time. The cationized agaroses were successfully synthesized by the reaction of
agarose with 3-chloro-2-hydroxypropyltrimethylammonium chloride in alkaline medium. Two of the
cationized agaroses with degrees of substitution of 0.19 and 0.58, presented colloid flocculation performances
comparable to commercial cationic polyacrylamides, as determined from assays using kaolin suspensions as
model systems. Zeta potential measurements for the cationized agarose that presented the best performance
suggested that bridge formation, rather than charge neutralization, should be the main mechanism
responsible for flocculation of kaolin particles. Cationized agaroses may constitute a new flocculating agent
with promising properties for water treatment, especially if the source of the agarose is a seaweed species of
easy availability.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Impurities present in water sources can be in the form of dissolved/
colloidal organic matter, dissolved salts and as suspended materials such
as clay, silica, microbial cells and algae. Many of these impurities represent
varied health risks and must be removed [1].
Organic polymers have been employed in coagulation/flocculation for
at least four decades. In comparison with alum (KAl(SO4)2·12H2O), some
of the advantages of polymers in water treatment are lower requirements
of coagulant dose, smaller volume of sludge, smaller increase in the ionic
load of the treated water, reduced level of aluminum in treated water and
cost savings of up to 25–30% [1,2].
As most colloids present negative charges, cationic polyelectrolytes are
of great interest for their applications as flocculants. However, there is
scarce information regarding the relation between polymer structure and
flocculation performance in the production of potable water [1,3].
The modification of natural polysaccharides has been explored as a
way of combining their best attributes with those of synthetic polymers
currently in use [4]. Polysaccharides are fairly shear stable, in contrast with
long-chain cationic polyacrylamides and are biodegradable. Concerns
about cationic polyacrylamides toxicity are increasingly limiting their
application in some countries through legal restrictions [5–8]. A variety of
⁎ Corresponding author at: Intendente Güiraldes 2160, Ciudad Universitaria, C1428EGA,
Buenos Aires, Argentina. Tel./fax: +54 11 45763346.
E-mail address: [email protected] (M.C. Matulewicz).
0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2011.07.061
polysaccharide backbones has been used as substrate for cationization and
their flocculation performance has been tested; these include starch [5,9–
16], glycogen [4,14], dextran [17], cellulose [18], carboxymethylcellulose
[19], and guar gum [14,20]. Etherification with 2-hydroxy-3-(N,N,Ntrimethylammonium)propyl groups has been most commonly applied in
order to introduce cationic groups into polysaccharides [2,4–6,9–20].
In the present work, cationic agaroses with different degrees of
substitution (DS) were prepared by reaction of commercial agarose with
3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC),
and their flocculation performance using kaolin suspensions as model
systems was evaluated for the first time. Kaolin suspensions have
received increasing attention in recent years because kaolin is a key
component in industrial effluents [21]. Besides, in order to gain insight
into the mechanisms by which flocculation of kaolin particles occurs for
this kind of cationic polyelectrolyte, zeta potential measurements for
the best performing cationized agarose were carried out.
2. Experimental
2.1. Materials
The agarose employed was Agar Bacteriological (Agar No. 1), (Oxoid
Ltd., Basingstoke, Hampshire, UK). 3-chloro-2-hydroxypropyltrimethylammonium chloride, 60% w/w aqueous solution was provided by
Sigma-Aldrich, Inc. (St Louis, MO, USA).
For the flocculation tests, USP type kaolin from Sigma-Aldrich
Inc. was employed. The mean size of the kaolin particles in tris/HCl
H.J. Prado et al. / Desalination 281 (2011) 100–104
buffer of pH 8 was 600 nm, as determined by dynamic light scattering
with the Zetasizer Nano-Zs intrument (Malvern Instruments Ltd.,
Worcestershire, UK). Cationic polyacrylamide-based Zetag 7557 and
Zetag 8185 were kindly provided by BASF Argentina S.A., through its
distributor PPE Argentina S.A. According to the information supplied
by the manufacturer, Zetag 7557 and 8185 have a molecular weight
between 1.5 × 10 6 − 2.0 × 10 6 and higher than 2.0 × 10 6, respectively;
both products have high charge density (DS N 0.60). Zetag 7557
presents an apparent viscosity of around 1800 cP for 1% w/v solutions,
while Zetag 8185 is more viscous with an apparent viscosity of around
2400 cP, for the same concentration. All the other reagents were of
analytical grade.
101
Fig. 1. Chemical structure of cationized agaroses.
2.2. Methods
2.2.1. Synthesis of cationized agaroses
1 g of agarose (6.54 mmol of the mean unit, 153 Da) was employed
in all the experiments and the final volume of reaction was 100 mL
(final concentration 1% w/v). The concentrations of the reagents NaOH
and CHPTAC are expressed as molar ratios with regard to the mean
unit of the polysaccharide. Agarose was dispersed in water in a round
bottom flask, and heated at 90 °C for 15 min. The flask was immersed
in an oil bath over a hotplate provided with magnetic stirring, at 50 °C.
The solution of CHPTAC and the solution of NaOH of the required
molarity were added [22]. The other reaction conditions are reported
in Table 1, whereas the structure of the product is shown in Fig. 1.
2.2.2. Determination of the DS by elemental analysis
The DS was obtained from determinations of elemental nitrogen
(DSEA) in a Carlo Erba EA 1108 CHNS elemental analyzer (Carlo
Erba, Milan, Italy). Native agarose does not present nitrogen in its
composition.
The equation applied for DSEA calculation [11], adapted to agarose
was:
DSEA =
ð 153 × %N Þ
ð 153 × %NÞ
=
ð100 × 14Þ−ð 152 × %NÞ
1400−ð 152 × %NÞ
where 153 is the molecular weight of the mean unit of agarose (162 +
144)/2 = 153; %N is the percentage of elemental nitrogen on dry
basis; 14 is the atomic weight of nitrogen and 152 is the molecular
weight of the cationic substituting group.
2.2.3. Determination of the molecular weight
Number average molecular weight (Mn), was determined by the
Park and Johnson's method [23]. For each DSEA, the mean anhydrous
monosaccharide unit (MWanh) was calculated as follows:
Four different concentrations for each sample (2, 4, 6 and
8 mg mL − 1) were prepared and filtered through 0.02 μm membranes. All the assays were performed at 25 °C, except for the native
agarose that was measured at 35 °C in order to avoid gelation. Glass
couvettes were employed.
2.2.4. Flocculation performance
The performance of the cationized agaroses with different degrees
of substitution in the flocculation of kaolin suspensions was evaluated
and compared with that determined for commercial polyacrylamidebased flocculants Zetag 7557 and Zetag 8185.
Flocculation efficiency was obtained from absorbance measurements. For this purpose,10 flasks each with 50 mg of kaolin in 50 mL
of tris/HCl buffer (0.05 M tris(hydroxymethyl)aminomethane, 0.03 M
HCl) of pH 8.0 were prepared (solids content 1 g L − 1) [5]. The test was
performed with one flask at a time. The flask was agitated for 5 min in
a vortex stirrer (stirring speed 300 rpm, orbital diameter 4.5 mm) in
order to suspend the kaolin. Different volumes of cationized agarose
solution (0.5 mg mL − 1) were added to each flask to obtain different
weight ratios of cationized agarose to kaolin in the range 0.5–
10.0 mg g − 1. Then, stirring was repeated for another 5 min. A portion
of the sample was immediately poured in a rectangular spectrophotometer cell (1 cm optical path) and its turbidity at 500 nm was
registered at time 0 and once a minute, for 15 min [2]. Additionally,
control flasks were prepared and treated in the same way that the
other samples except for the addition of the flocculating agent. The
residual turbidity (RT) [5,9] was calculated as follows:
RT =
At
× 100
A0
MWanh = 153 + ðDSEA × 152Þ−ðDSEA × 1Þ:
where At is the absorbance of the sample at 500 nm at different times,
and A0, the absorbance of the sample at 500 nm at the beginning of the
test. This test was performed at 25 °C, by triplicate for each cationized
agarose of different DS and for the commercial cationic polyacrylamides.
The mass average molecular weight (Mw) was determined employing the Zetasizer Nano-Zs (Malvern Instruments Ltd., Worcestershire,
UK) provided with a 4 mW He–Ne (633 nm) laser and a ZEN3600 digital
correlator. This instrument performs measurements employing static
light scattering techniques.
2.2.5. Zeta potential determinations
The zeta potential (ξ potential) for different ratios of cationized
agarose (CAG19 of DSEA = 0.19) to kaolin in tris/HCl buffer of pH 8.0 was
determined. With that purpose, the Zetasizer Nano-Zs was employed.
Capillary cells were used in these determinations.
3. Results and discussion
Table 1
Reaction conditions of the experiments.
Experiment CHPTAC: agarose NaOH: CHPTAC Time DSEA Molecular weight
molar ratio
molar ratio
(h)
Mn (kDa) Mw (kDa)
CAG04
CAG19
CAG58
CAG77
1.00
2.00
4.00
8.00
1.15
2.30
2.30
1.73
18
2
2
2
0.04
0.19
0.58
0.77
113.2
134.5
180.4
196.6
192.4
242.0
306.7
354.0
The main characteristics of the cationized agaroses synthesized
and employed in the flocculation assays are presented in Table 1. In
contrast to native agarose, the cationized agaroses were soluble in water
at room temperature, at the concentration employed (0.5 mg mL− 1);
CAG04 (DSEA = 0.04) required an initial mild heating to achieve solubilization. A study of the parameters involved in the reaction and full
characterization of this and other cationized agaroses has been earlier
reported [22]. It should be mentioned that the nomenclature used in
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H.J. Prado et al. / Desalination 281 (2011) 100–104
the present study for cationized agaroses differs from that employed
previously; herein, the acronym CAG is followed by the degree of
substitution of the product (i.e. CAG19 denotes cationized agarose with
DSEA of 0.19).
3.1. Flocculation performance
Fig. 2 (a–d) shows the performance of the cationized agaroses
in the flocculation of kaolin particles for variable ratios of cationized
agaroses to kaolin. For these tests four cationized agaroses of different
substitution degree were employed: CAG04 (DSEA = 0.04), CAG19
(DSEA = 0.19), CAG58 (DSEA = 0.58) and CAG77 (DSEA = 0.77). Besides,
for comparative purposes, in Fig. 3 (a–b) the results corresponding
to the commercial cationic polyacrylamides (Zetag 7557 and Zetag
8185) evaluated in analogous conditions are presented.
From the flocculation results, a plot of residual turbidity after
15 min (end of the test) as a function of the ratio of flocculating agent
to kaolin was built (Fig. 4). This figure allows an easier comparison of
the behavior of the different flocculating agents and determination of
optimal concentrations.
As can be appreciated in the aforementioned figures, among
the cationized agaroses, the one that presents the best flocculation
performance (minimum residual turbidity after 15 min) is CAG19,
for ratios of 2–4 mg of flocculating agent per gram of kaolin. For
the agaroses CAG58 and CAG77, the best results were obtained for
a specific concentration of flocculating agent (2 mg per gram of
kaolin for CAG58 and 1 mg per gram of kaolin for CAG77), being
this concentration lower, the more substituted the polysaccharide
was. Therefore, CAG19 also presented the advantage that it was less
sensitive to variations in the concentration of flocculating agent,
maintaining the optimal concentrations in a broader range than the
other products.
Comparing the flocculation performance of the cationized agaroses
with that of commercial flocculating agents based on cationic
polyacrylamides, employed in their optimum concentrations, it can
be inferred that the cationized agaroses CAG19 and CAG58 presented
an intermediate behavior between those determined for Zetag 7557
and 8185. This is noteworthy, in the case of a natural product
derivative with a molecular weight of around two orders of magnitude lower than the commercial products. The lower performance
of Zetag 8185 with regard to Zetag 7557 could be related, among
other factors, to the higher viscosity of the former, as informed by
the manufacturer, and/or to differences of charge density. In turn,
the lower viscosity of CAG19 [22] could also contribute to its best
performance among all the cationized agaroses tested.
At concentrations higher than optimal values, the excess of
polymer seems to reduce the flocculation performance in all cases.
This is probably due to an increase in system viscosity and in the zeta
potential of particles, indicating that the negative charge of kaolin
particles could be reversed and, in some cases, the suspensions could
be restabilized electrostatically. At high polymer concentrations,
CAG19 behavior was similar to Zetag 7557 (Fig. 4).
Besides, CAG04 delayed flocculation of kaolin suspensions instead
of promoting it. This behavior could be attributed to its particular
characteristics. Cationic agarose CAG04 is a highly viscous material,
although it does not macroscopically gelled as native agarose. CAG04
Fig. 2. Evolution of the flocculation for cationized agarose CAG04 (DSEA = 0.04) (a), CAG19 (DSEA = 0.19), CAG58 (DSEA = 0.58) and CAG77 (DSEA = 0.77) (c).
H.J. Prado et al. / Desalination 281 (2011) 100–104
103
Fig. 3. Evolution of the flocculation for Zetag 7557 (a) and Zetag 8185 (b).
required an initial mild heating in order to achieve solubilization.
Perhaps, as a result of its very low DS, a certain degree of levogirous
double helixes [24] as in native agarose may be still formed, hindering
flocculation promotion. Although this result is undesirable for treatment of effluents, it could be useful for other applications, i.e. in
the stabilization of suspensions used in liquid pharmaceutical dosage
forms.
Fig. 5 shows the evolution of the flocculation for the different
flocculating agents, at their optimal concentrations. It is observed that
polyacrylamides induced flocculation faster than cationized agaroses
and that in general, the flocculating agents that presented better
performance in the early minutes of the test were the same as those
with lower residual turbidity after 15 min. However, even though the
final residual turbidity was lower for the optimal concentration of
CAG19 than for the respective concentrations of CAG58 and Zetag
8185, their flocculation performance order fluctuated in the time
interval studied, being almost identical, for instance, at 8 min.
3.2. Zeta potential determinations
Zeta potential serves as an important parameter in characterizing
the electrostatic interaction between particles in dispersed systems
and the properties of the dispersion as affected by this electrical
phenomenon [25]. Because of its relevance in flocculation, the zeta
potential of kaolin in tris/HCl buffer of pH = 8.0 was determined, by
the addition of different quantities of the cationized agarose that
Fig. 4. Residual turbidity (RT) after 15 min for the evaluated products.
presented the best performance in the flocculation tests (CAG19). The
results are illustrated in Fig. 6; measurements were carried out under
the same conditions as for the flocculation performance tests.
From Fig. 6, it can be observed that kaolin particles in tris/HCl
buffer, without the addition of cationized agaroses, presented a zeta
potential of −17.4 mV. This value is consistent with others reported
for kaolin [26,27].For the optimum ratios of flocculating agent to
kaolin (2–4 mg of CAG19 per gram of kaolin) zeta potential is different
from zero. Mechanisms of flocculation can be divided in bridge
formation, charge neutralization (including “charge patch” effects),
and depletion flocculation. This last mechanism is not significant
in water treatment. The other mechanisms depend crucially on the
adsorption of the polymer onto particles surfaces [1,21]. Hence,
according to present zeta potential measurements, main flocculation
mechanism for the best performing cationized agarose would be
bridge formation, rather than charge neutralization. This finding is
also supported by the lower sensitivity to the variation in the concentration of the flocculating agent. In addition, it should be hypothesized that although the overall charge of the particles is positive
at optimal concentrations, isolated patches of negative charges could
be formed.
As cationized agaroses are novel products, there are no previous
studies regarding their application in flocculation. However, the closer
systems studied involve kaolin particles flocculated with other
polysaccharides cationized with the same group. In the case of
Fig. 5. Evolution of the flocculation for the different flocculating agents, at their optimal
concentrations.
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H.J. Prado et al. / Desalination 281 (2011) 100–104
References
Fig. 6. Zeta potential of the kaolin particles with the addition of different amounts of
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4. Conclusions
Cationized agaroses of different degrees of substitution, in the
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would be mainly due to bridge formation, rather than to charge
neutralization. Cationized agarose may constitute a new flocculating
agent with promising properties for water treatment, especially if the
source of the agarose is a seaweed species of easy availability.
Acknowledgements
This work was supported by grants of the National Research
Council of Argentina (CONICET, PIP 112-200801-00234) and the
University of Buenos Aires (UBA, X137). M.C.M, P.R.B. and A.L.C. are
Research Members of CONICET. H.J.P. received a Doctoral Fellowship
from CONICET. Color versions of Figs. 2 to 5 are available in the
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