Journal of Environmental Protec tion, 2013, 4, 51-55
doi:10.4236/jep.2013.41b010 Published Online January 2013 (http://www.SciRP.org/journal/jep)
51
Sorption of Pb(II), Cd(II), and Ni(II) Toxic Metal Ions by
Alginate-Bentonite
Teoh Wah Tzu, Takuma Tsuritani, Kazunori Sato
Department of En vi ronment al Engineeri ng, Nagaoka Univers ity of Technology, Nagaoka, Japan.
Received 2013
ABSTRACT
The sorption of Pb(II), Cd(II), and Ni(II) toxic metal ions from aqueous solution by composite alginate-bentonite and
alginate wa s investigated. The affinity and sorption cap acity of the toxic metal ions for both type of samples were eva-
luated. The Langmuir maximum sorption capacity for each toxic metal ion increased for alginate-bentonite as compared
to alginate. However, affinity for toxic metal ion remained unchanged for both alginate-bentonite and alginate in the
order of Pb(II) > Cd(II) > Ni(II). Alginate-bentonite also shortens the d uration required f or complete sorptio n. Elemen-
tary mapping anal ysis depicts the gradient di ffusion of toxic metal ions i nto the centre of alginate -bentonite beads indi-
cated that sorption was contributed by surface adsorption and diffusion.
Keywords: Toxic Metals; Al ginate; Bentonite; Wastewater
1. Introduction
The presence o f toxic metal ions s uch as Pb (II), Cd(II), and
Ni(II) in the aqua tic enviro nment is posi ng seriou s health
threats to all livings. These toxic metals have long
half-life and the toxicity is cumulated in human bodies
for long duration. Industrialization and the growth of
population is the main contributor in the release of these
toxic pollutants. Since toxic metal ions in aquatic envi-
ronment are of high mobility and able to spread to large
area in short time, the removal of these toxic metal ions
from aquatic environment is one of the challenges in
science. Efforts to develop effective and efficient sorbent
that fulfill both the cost as well as the performance are in
grea t demand .
Mineral clay such as bentonite, has been found to be
capable of removing toxic metal ions in wastewater through
sorption process [1]. The abundance of this naturally
occurred low cost bentonite makes it a strong candidate
for toxic metal ions removal application. However, the
use of powder sorbent such as bentonite introduces prac-
ticality problems since separation of solid and wastewa-
ter is difficult and costly [2]. This problem can be over-
come by powder immobilization technique which re-
tained the bentonite in suitable polymer support. Utiliz-
ing renewable bioresources such as alginate natural po-
lysaccharide polymer to immobilize bentonite powder
contribute to feasibility of developing low cost sorbent.
Apart from the cost and availability, alginate is also
known to have good affinity to adsorb divalent toxic
metal ions [3 -4] si nce it i s ri c h i n ne ga ti ve l y c ha r ge f unc-
tional group such as carboxylate group.
Studi es of the alginate-bento ni t e were more focused on
the application for drug delivery system and only very
limited research focusing on toxic metal ions removal. In
this study, the Pb(II), Cd(II), and Ni(II) sorption capacity
as well as sorption rate of alginate bead and alginate-
bentonite beads was evaluated using data which best-
fitted with Langmuir isotherm model. The sorbents were
characterized to understand the sorption mechanism.
2. Materials and Method
2.1. Materials
Sodium alginate (EP grade) and bentonite powder (EP
grade) were used without any further purification process.
0.2 M calcium chloride (CaCl2) solution was prepared
from calcium chloride salt (GR grade). Pb(II), Cd(II) and
Ni(II) solutions for sorption experiments were prepared
from lead nitrate (purity over 99%), cadmium nitrate
tetrahydrate (purity over 99%), and nickel nitrate hex-
ahydrate (purity over 99%). The toxic metal ions con-
centration was determined by Inductively Coupled Plas-
ma Atomic Emissio n Spectroscopy/ICP -AES (Shi madz u,
ICP-7510), standard calibration was carried out using
100 mg/L Pb(II), 100 mg/L Cd(II), and 100 mg/L Ni(II)
standard solutions that diluted to produce 1 mg/L, 10
mg/L and 100 mg/L solution. All the above mentioned
chemicals were supplied by Nacalai Tesque.

Sorption of Pb(II), Cd(II), and Ni(II) Toxic Metal Ions by Alginate-Bentonite
52
2.2. Alginate-Bentonite Beads Pre paration and
Characterization
Alginate-bentonite beads were prepared by adding 4.0 g
of bentonite into 100 ml of distilled water, the mixture
was stirred for 0.5 hour and 1.5 g of sodium alginate was
slowly added. The solution was stirred until a uniform
creamy color alginate-bentonite solution was formed. To
form alginate-bentonite beads with consistent diameter,
the solution was dispensed into 0.2 M CaCl2 solution
from a syringe (1.2 mm in diameter) using an automated
dispensing pump (As One, SPE-1) at fixed height. The
beads were immersed in the CaCl2 solution for 3 hours to
enhance the polymerization process and subsequently
rinsed with distilled water. The gel beads were dried at
ambient atmosphere until constant weight was obtained.
To prepare of alginate beads, the procedure is similar
except bentonite was excluded in the preparation.
The morphology of the dried alginate and alginate-
bentonite beads before and after sorption experiments
were analyze using scanning electron microscope/SEM
(Shi madzu, EPMA-2000). To investigate the diffusion of
toxic metal ions into the alginate-bentonite beads, ele-
mental mapping analysis was carried out on a cross-sec-
tioned of post-sorption alginate-bentonite bead. All the
samples were sputtered with gold prior to the SEM ob-
serva tion and elemental mapping analysis.
2.3. Sorption Experiment
Batch isotherm sorption experiments were conducted by
placing 0.02 g of sorbent, i.e. alginate beads or alginate-
bentonite bead in respe cti ve 50 ml gla ss bottle c ontaining
30 ml of various concentration of Pb(II), Cd(II), and
Ni(II) solution. The toxic metal ions concentration used
in this experiment was 0.2 mM, 0.4 mM, 0.6 mM, 0.8
mM, 1.0 mM, 1.5 mM. The bottles were agitated at 100
rpm usi ng a water bath s haker (BT 100, Ya ma to Scientific)
for 24 hours; the temperature of the waterbath was set at
303 K. Final concentration of toxic metals ion after the
sorption process was determined by ICP-AES.
To study the sorption against time, the samples were
placed in 100 ml of 1 mM Pb(II), Cd(II), and Ni(II) solu-
tion respectively; 3 ml of the toxic metal ions solution
was sampled at the specified interval, namely 0.5 hour
(h), 1 h, 2 h, 3 h, 6 h, 12 h, and 24 h. T he concentratio n
of Pb(II), Cd(II), and Ni(II) at interval time was deter-
mined by ICP-AES.
3. Discussions
3.1. Characterization of the Beads
SEM was employed to determine the surface properties
of the samples before and after the toxic metal ions sorp-
tion. The surface morphology of alginate and algi-
nate-bentonite bead was given in Figure 1. SEM analysis
a
b
c
d
Figure 1. Surface morphology for: (a) alginate bef ore sorp-
tion, (b) alg inate beads af ter s orptio n, (c) alginate-bentonite
befo re s o rption, and (d) a lginate-bentonite after sorption.
a
b
c
d

Sorption of Pb(II), Cd(II), and Ni(II) Toxic Metal Ions by Alginate-Bentonite
53
revealed that compact ridge-like structure was observed
at the surface of alginate beads showed in Figure 1a
which related to the high degree of dehydration during
the drying process. After the toxic metal ions sorption
process; the morphology of the alginate bead was not
significantly change as showed in Figure 1b. The
ridge-like folded structure may attributed by the repeat-
ing of swelling followed by the shrinking during the
drying process. Figure 1c revealed that inclusion of
bentonite in alginate changed the surface morphology of
alginate. The ridge-like structure of alginate was not de-
tected in alginate-bentonite. This morphology changes
after bentonite inclusion can be explained by the cross-
linking process of alginate. Alginate consists of linear
mono mers fro m ma nnuroni c aci d (M) and guluro nic ac id
(G). These monomers formed different sequence of mo-
nomer blocks such as M blocks, G blocks and MG blocks.
G monomers are responsible for the reaction with diva-
lent metal cation cross-linker such as Ca(II) and subse-
quentl y bind the G blocks from M and GM blocks close-
ly [5]. Bentonite which is uniformly suspended in algi-
nate gel solution disrupted the sectional alginate chain
formation during the cross-linking process and thus al-
tered surface properties of alginate beads.
An e xaminat ion o f the S EM a nalysi s on al ginate - ben-
tonite bead after sorption process showed in Figure 1d,
indicated the presence of pores on the surface of alginate-
bentonite bead. Bentonite is an expandable clay mineral
consist mainly montmorillonite in a 3 layers structures.
The basic structural unit is composed of 2 tetrahedrally
coordinated sheets of silicon ion layer sandwiched the
octahedrally coordinated sheet of aluminum ion layer as
illustrated in Figure 2. The inter-layer spacing expands
when it adsorbed water or metal ions [6]. During the ab-
sorption process, the alginate-bentonite beads swell and
allowed the toxic metals ions to penetrate into the inner
part of the beads. The Pb(II) and Cd(II) which had larger
ionic radii compared to Ca(II) were bonded within
Octahedral
Al2O3/MgO
La y er
Metal ions
Tetrahedral
SiO2 Layer
Figure 2 Basic struct ure of montmorillonit e.
the inter layer spacing causing irreversible expansion of
the inter-layers spacing. The pores observed can be
attribute to the irr eversible e xpansion of inter la yer spac-
ing of bentonite due to toxic metal ions sorption as well
as structural changes due to drying process.
3.2. Sorption Isotherm
Isotherm data were fitted with the Langmuir and Freun-
dlich model, the Langmuir model gives the best repre-
sentation. The linear mathematical expression of Lang-
muir model is given as Equation (1):
Ce/qe = (Ce/qmax) + [(1/b)*(1/qmax)] (1)
where qe (mmol/m2) is the toxic metal sorption at the
equilibrium state, Ce (mM) is the final concentration of
toxic metal at equilibrium, b is the Langmuir constant
that measure the affinity of the sorbent-sorbate, and qmax
(mmol/m2) is the theoretical maximal sorption capacity
of the monolayer.
The data of isotherm sorption by alginate beads and
alginate-bentonite beads were fitted with Langmuir mod-
el and tabulated in Table 1 and Table 2 respectively.
Accor din g to t he La ng muir a f finit y co nsta nt, b, the affin-
ity of the toxic metals for alginate beads is as followed:
Pb(II ) > Cd(II) > Ni(II). Inclusion of bentonite in algi-
nate beads did not change the preferential state. This
outcome is consistent with the findings from other re-
searchers, the alginate affinity for toxic metal ions was
found to follow the order of: Pb (II)) > Cd(II) > Cu(II) [7];
while affinity of bentonite for toxic metals was follo wed
the order of: Pb(II) > Cd(II) > Cu(II) [2], both of the
samples were found to have similar affinity and therefore
inclusion of be nto nite did not a lte r the affinity order.
Table 1. Coefficients of Langmuir Isotherm for Alginate.
Metal ion Alginate
qmax b R2
Pb(II) 2.5 38.4 0.9933
Cd(II) 1.6 10.0 0.9981
Ni(II) 1.2 3.3 0.9979
Table 2. Coefficients of Langmuir Isotherm for Alginate-
bentonite.
Metal ion
Alginate-bentonite
qmax b R2
Pb(II) 5.8 46.4 0.9913
Cd(II) 4.2 14.2 0.9970
Ni(II) 3.4 5.5 0.9995

Sorption of Pb(II), Cd(II), and Ni(II) Toxic Metal Ions by Alginate-Bentonite
54
Inclusion of bentonite in alginate beads significantly
improved the sorption capacity (qmax) for each toxic metal.
The calculated maximum sorption capacity of alginate-
bentonite beads for Pb(II), Cd(II), and Ni(II) is 5.8
mmol/m2, 4.2 mmol/m2, and 3.4 mmol/m2 respectively;
this values are consistently higher compared to alginate
beads, namely 2.5 mmol/m2, 1.6 mmol/m2, and 1.2
mmo l / m2. Alginate-bentonite beads are consistently show-
ing better sorption performance regardless of the type of
toxic metals.
Both alginate and bentonite are hydrophilic materials
with negatively charge functional groups that attract the
cationic metals sp ecies lead i ng to so rptio n. For b entonite,
other than having the cation exchange capacity at the
outer surface and edges of its octahedral and tetrahedral
sheet, it also possessed excellent sorption properties
which is available within its interlayer space. This con-
tributed to higher sorption capacity for the alginate-ben-
tonite beads.
3.3. Toxic Metal Sorption Against Time
Figure 3 showe d the sorption amount (qt) of Ni(II), Cd(II)
and Pb(II) up to 24 hours. The sorption process was rapid
for the first 3 hours for both alginate and alginate-
bentonite. However, alginate-bentonite consistently showing
higher sorption amount for Ni(II), Cd(II), and Pb(II).
Furthermore, alginate-bentonite is able to reach sorption
equilibrium in shorter duration particularly for Pb(II). It
is obvious that alginate-bentonite is both effective and
efficient in toxic metals sorption when compared to
alginate.
Figure 3c showed that Pb(II) sorption by alginate-
bentonite reached saturation at 12 hours whereas Pb(II)
sorption by alginate sho wing no sign of saturation as yet
at 24 hours. For alginate-bentonite, the first sharpest in-
creased in Pb(II) sorption (0.5 hour to 3 hours) was at-
tributed to the sorption of Pb(II) on the surface of algi-
nate-bentonite beads. The second portion of the plot (3
hours to 12 hours) indicating gradual sorption stage,
where diffusion is rate limiting. The last portion (12 hour
onward) attributed to the final equilibrium stage. In the
case of alginate beads, the second stage which attributed
by diffusion process was much longer (3 hours onward
without showing saturation at 24 hours) compared to
alginate-bentonite .
The shorter duration time required by alginate-ben-
tonite to achieve sorption saturation is attributed by the
inclusion of be ntonite that is easily s welled when contact
with liquid, this expansion of inter-layer spacing within
the cross-linked alginate polymer chain makes the beads
swell faster and to a larger extent. Hence the average
opening between the alginate chains will be greater. The
Pb(II) will exhibit a greater intra-gel diffusivity if the
openings between the chains are greater [5].
To elucidate the occurrence of Pb(II) diffusion into the
alginate-bentonite beads, element mapping analysis was
performed on a cross-sectioned alginate-bentonite bead
shown in Figure 4. The mapping analysis showed that
Pb(II) was detected as far as 200 μm from the edge of the
bead, the intensity of Pb(II) distribution was gradually
decreased toward the centre of the beads indicating dif-
fusion of Pb(II) into the alginate-bentonite bead.
The shorter duration time required by alginate-ben-
tonite to achieve sorption saturation is attributed by the
inclusion of be ntonite that is easily s welled when contact
with liquid, this expansion of inter-layer spacing within
the cross-linked alginate polymer chain makes the beads
swell faster and to a larger extent. Hence the average
opening between the alginate chains will be greater. The
Pb(II) will exhibit a greater intra-gel diffusivity if the
openings between the chains are greater [5].
Figure 3. Sorption of alginate-bentonite beads and alginate
beads: (a) Ni(II), (b) Cd(II) , and (c) Pb(II).

Sorption of Pb(II), Cd(II), and Ni(II) Toxic Metal Ions by Alginate-Bentonite
55
100 μm
200 μm
Figure 4. Ele menta l map pi ng a naly sis at t he cr oss -sectioned
of post-sorption alginate-bentonit e bead.
To elucidate the occurrence of Pb(II) diffusion into the
alginate-bentonite beads, element mapping analysis was
performed on a cross-sectioned alginate-bentonite bead
shown in Figure 4. The mapping analysis showed that
Pb(II) was detected as far as 200 μm from the edge of the
bead, the intensity of Pb(II) distribution was gradually
decreased toward the centre of the beads indicating dif-
fusion of Pb(II) into the alginate-bentonite bead.
4. Conclusion
Alginate-bentonite beads had evidently shown effective-
ness and efficiency for the sorption of Pb(II), Cd(II) and
Ni(II) toxic metals compared to alginate. The toxic met-
als affinity is in the order of Pb(II) > Cd(II) > Ni(II).
Langmuir sorption capacity coefficient shows that inclu-
sion of bentonite in alginate increased the surface active
sites for toxic metals sorption. Mapping analysis con-
firmed the diffusion of Pb(II) into the beads, attributed
by the expandable inter-layer spacing of bentonite that
enlarged the openings between the chains of alginate
polymer. Combination of this low cost bentonite and
alginate sorbents had proven to show synergy effects in
the sorption of toxic metals from aquatic environment.
5. Acknowledgements
The authors gratefully acknowledge the support of re-
search grant from Nagaoka University of T echno log y.
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