Natural Resources, 2011, 2, 107-113
doi:10.4236/nr.2011.22015 Published Online June 2011 (http://www.scirp.org/journal/nr)
107
The Factors on Removal of Zinc Cation from
Aqueous Solution by Bentonite
Shuli Ding1, Juanjuan Shen1, Bohui Xu1, Qinfu Liu2, Yuzhuang Sun1
1Key Laboratory of Resources Reconnaissance of Hebei Province, Hebei University of Engineering, Handan, China; 2 School of
Resources and Safety Engineering, China University of Mining & Technology, Beijing, China.
Received January 29th, 2011; revised March 7th, 2011; accepted March 15th, 2011.
ABSTRACT
The removing zinc cation from aqueous solution by Ca-bentonite and Na-exchanged bentonite was studied. The factors
such as the initial concentration of Zn2+, the liquid-to-solid ratio, pH, adsorption time, stirring speed, coexisting ions,
temperature and bentonite particle size were investigated. The results show that the adsorption process of bentonite
accorded with the Freundlich isotherm model, the removal of Zn2+ by Ca-bentonite and Na-exchanged bentonite
reached equilibrium in 2 h, and adsorption of Na- bentonite was superior to Ca-bentonite. The adsorption rate of zinc
increased with increasing pH, temperature, stirring speed, time span and with decreasing bentonite particle, the initial
concentration of Zn2+ and the liquid-to-solid ratio. In mixed solution which contains Pb2+and Cr6+, Pb2+ has no influ-
ence on the removal of Zn2+ by both the bentonites while Cr6+can decease it.
Keywords: Bentonite, Zn2+, Adsorption
1. Introduction
Heavy metals are typical and persistent environmental
pollutants. Because of its toxicity and nonbiodegradable
nature, metals are of special significance. The presence
of heavy metals in wastewater and surface water is be-
coming a severe environmental and public health prob-
lem. Recently, natural materials, which are good sorbents
and inexpensive have received much attention in heavy
metal wastewater treatment [1-3]. Bentonite is the most
abundant phyllosilicate mineral and its main mineral
component is montmorillonite. Bentonite which was
used in wastewater treatment has the advantage of simple
device, low cost, no repollution etc.
The adsorption capacity of montmorillonite for heavy
metal ions has been concerned since 1960s [4-8]. In lit-
erature various types of bentonite adsorbents have been
cited. H. Omar et al. [9] investigated the adsorption po-
tential of raw and formaldehyde modified-bentonite for
the removal of 60Co radionuclide from radioactive waste
solutions. S. T. Akar et al. [10] combined white rot fungi
and montmorillonite, this proposed was successfully
used to sequester of copper ions from real wastewater in
continuous mode. E. Alvarez-Ayuso et al. [11], con-
cluded that both the Ca-montmorillonite and na-mont-
morillonite can remove heavy metals from the industrial
effluent efficiently. W. Matthes et al [12]. studied sorp-
tion of Al and Zr-hydroxy intercalated and pillared ben-
tonite and found that the adsorption of heavy metal by
montmorillonite was majorly due to the coordination of
surface hydroxyl groups as well as the interchange with
cation among crystal layers.
The major objective of this work is to study the ad-
sorption of Zn2+ in aqueous solution as well as its influ-
encing factors. Handan Ca-bentonite and Na-exchanged
bentonite were used in the experiments with the view to
discovering their metal ion recovery potential from ef-
fluents. The adsorption effect of Cr6+ and Pb2+ compete
with Zn2+ in aqueous solution was analyzed.
2. Materials and Methods
2.1. Equipment
The X-ray diffraction (D/max-2200 diffractometer at a
scanning speed 4˚ (2θ)/min, using CuK α radition at
40kV, 100mA) was used to identify the Mineral compo-
sition. 721 model spectro-photometer was used to meas-
ure the content of Zn2+, Cr6+ and Pb2+ in solutions. Also,
the AW120 electron balance, HJ-5 constant temperature
magnetic force beater, pHS-3C acidimeter, anion and
cation exchange column, GL-20G-Ⅱmodel high-speed
refrigerated centrifuge and some other instruments were

108 The Factors on Removal of Zinc Cation from Aqueous Solution by Bentonite
used in experiments.
2.2. Materials
The Ca-bentonite M-1 was obtained from Handan district
(Hebei Province, China). It was purified in the laboratory
to remove carbonates, iron, hydroxide and organic matter.
The bentonite was then crushed, ground, sieved through
200 mesh and dried at 105˚C in an oven. Its mineralogi-
cal composition, chemical components and physico-
chemical properties are listed in Table 1 to 3 respec-
tively.
All the other reagents used were of analytical grade
and were obtained from Merck (Darmstadt, Germany).
Metal salts of ZnCl2·2H2O, Pb(NO3)2, potassium di-
chromate(K2Cr2O7) were used to prepare metal ion solu-
tions. The solutions (1000 g/L) were prepared by dis-
solving appropriate amounts of metal salts in doubly
distilled water. The working solutions were prepared by
diluting the stock solutions to appropriate volumes. pH
adjustments of these solutions were made by 0.01, 0.1
and 1 M HNO3 and NaOH solutions.
2.3. Preparation of Na-Bentonite
The Na-exchanged bentonite M-2 was prepared as fol-
lows: the raw bentonite M-1 was mixed up with Na2CO3
according to 100:3, the slurry was made with deionized
Table 1. Mineral composition of Ca-bentonite
Mineral Percentage
calcium montmorillonite 61.7
kaolinite 0 ~ 6
illite 2 ~ 8
feldspar 2 ~ 11
quartz 2 ~ 9
gypsum 2 ~ 20
cristobalite 1 ~ 8
Table 2. Chemical component of Ca-bentonite
Components Percentage
SiO2 61.10
Al2O3 18.19
Fe2O3 5.48
CaO 1.59
MgO 1.90
Na2O 1.30
K2O 1.28
TiO2 0.54
LOI (loss on ignition) 8.62
Table 3. Physicochemical properties of Ca-bentonite
Properties Value
Cation exchange capacity (mmol/g) 0.61
Specific surface area (m2/g) 20
Colloid index (mL/g) 4.13
Epansion multiple (mL/g) 9
water in liquid-to-solid ratio of 10:1 afterwards. This
suspension was stirred using the magnetic stirrer for 1 h,
then standing, removal of the underlying sandy, centri-
fuging, drying, grinding. The sample was subsequently
referred to as Na-exchanged bentonite adsorbent.
2.4. Method
Adsorption on the samples was determined in batch
sorption experiments in the single species system. Ad-
sorption experiments were carried out by shaking a cer-
tain particle size of the bentonite with 100 mL of metal
ions solution at a certain concentration in covered poly-
ethylene containers. The pH of the solution was kept
constant by the addition of HNO3 or NaOH solutions as
needed. Subsequently, the suspension was stirred on the
magnetic stirrer at controlled temperature and stirring
rate. Standing for 2 h after equilibrium and then centri-
fuged for 10 min at 9000 r/min. The supernatants col-
lected were analyzed for the metal ion. All of these un-
certain influence conditions were predetermined before
respective adsorption experiments based on previous
literature [13,14]. The amounts of the metal ion adsorbed
by the adsorbents were calculated by dithizone spectro-
photometry [15].
The sorption rate () and amount of metal ion ad-
sorbed by the bentonite adsorbents () were calculated
as:
%P
Q


o
%100 oe PCCC
(1)


oe
CCV
Qm

 (2)
where and Ce are the concentration of metal ion
in the initial and equilibrium concentration of metal ion
in solution (mg/L), is the volume of metal ion solu-
tion used (mL) and m is the weight of the adsorbent used
(g).
Co
V
3. Results and Discussion
3.1. Effect of Contact Time
A fixed particles size of the adsorbents reagent (through
200 mesh) was added to 100 mL of zinc ions solution
which adjusted pH to 5 at concentration 0.005 mol/L,
with liquid-to-solid ratio of 100 mL/g at 20˚C. At differ-
ent time an interval of 0-120 min, the adsorption rate of
Zn2+ on the bentonite adsorbents has shown in Figure 1.
As seen from the figure, there is a rapid uptake within
the first 15 min. Increasing contact time allows greater
amounts of Zn2+ to be removed from the aqueous solu-
tion. 120 min later, the adsorption reached equilibrium.
Ünlü N. et al. [16] took kinetic considerations into
account and found that the presence of functional groups
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The Factors on Removal of Zinc Cation from Aqueous Solution by Bentonite 109
Figure 1. Effect of contact time on the adsorption of Zn2+
ions onto bentonite and Na-bentonite.
is greatly determined by porous structure of polymeric
matrics. As was shown by H Woehlecke et al. [17], in
our study the observed rapid uptake can be attributed to
the porous structure of the bentonite adsorbents.
3.2. Effect of Temperature
In order to evaluate the effect of temperature on adsorp-
tion characteristics of the bentonites, the experiment was
studied at a constant initial concentration of 5 × 10−3
mol/L, bentonite particles size through 200 mesh, liq-
uid-to-solid ratio of 100 mL/g and pH 5 at stirring speed
of 1200 r/min, adsorption time 120 min. The results of
the studies on the influence of temperature on cation ad-
sorption are presented in Figure 2 in terms of amount of
metal removed versus temperature. It can be seen that the
percentage zinc uptake on both M-1 and M-2 increases
with increasing temperature. The reason for this is that as
the temperature increases, the ionic velocity accelerates.
The ion exchange reaction was hastened.
In earlier publication, the effect of temperature on ad-
sorption is important not only because it affects the rate
and extent of adsorption but also due to the fact that
temperature dependence of adsorption provides informa
tion about possible adsorbate-adsorbent interaction [18-
20].
Figure 2. Effect of solution temperature on the adsorption
of Zn2+ ions onto bentonite and Na-bentonite.
Figure 3. Relationship between initial metal ion concentra-
tion and Zn2+ adsorption.
3.3. Effect of Initial Concentration
The effect of initial concentration was investigated under
the following conditions: the bentonite particles size
(through 200 mesh), liquid-to-solid ratio (100 mL/g), pH
(through 200 mesh), liquid-to-solid ratio (100 mL/g), pH
(5), temperature (20˚C), stirring speed (1200 r/min), ad-
sorption time (120 min). Figure 3 has shown the adsorp-
tion of zinc ions onto M-1 and M-2 samples. The amount
of metal ion adsorbed increased gradually with different
initial concentration varying from 0 to 0.02 mol/L.
However, the adsorption rate decreased with increasing
initial metal ion concentration.
The adsorption mechanism of Zn2+ by bentonite in
zinc chloride solution is:
2
ZnM-MontZn-Mont 2M
  (3)
where M is the cation in montmorillonite interlayer, M-
Mont and Zn-Mont are M-montmorillonite and zinc-
montmorillonite.
With other conditions unchanged, the increase of Zn2+
concentration controls the direction of adsorption reac-
tions for positive reaction. The amount of zinc ion ad-
sorbed increased. On the other hand, the higher concen-
tration of Zn2+ is, the more Zn2+ remained after equilib-
rium. For this reason, the percentage of zinc ion adsorbed
decreased with increasing initial concentration.
3.4. Effect of Liquid-to-Solid Ratio
In condition of initial concentration is 5×10−3 mol/L,
bentonite particles size through 200 mesh, pH
5,temperature of 20℃, stirring speed of 1 200 r/min,
contact time 120 min. The amount of zinc adsorbed re-
duced with increasing liquid-to-solid ratio as shown in
Figure 4. It indicates that the adsorption capacity of M-1
and M-2 improved with the increase of bentonite dosage.
In terms of Equation (3), the increase of bentonite
dosage (that is liquid-to-solid ratio reduced) is propitious
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110 The Factors on Removal of Zinc Cation from Aqueous Solution by Bentonite
Figure 4. Effect of liquid-to-solid ratio on the adsorption of
Zn2+ ions onto bentonite and Na-bentonite.
Figure 5. Effect of pH on the adsorption of Zn2+ ions onto
bentonite and Na-bentonite.
to positive reaction with other conditions unchanged. As
a result, the concentration of Zn2+ remained after equilib-
rium declined, register as a raise in percentage of metal
ion adsorbed.
3.5. Effect of PH
To determine the pH necessary for adsorption, liquid-
to-solid ratio is 100 mL/g of solution containing 0.005
mol/L metal ion and particles size through 200 mesh
were stirred at 1200 r/min at varying time intervals (0-
120 min) at 20˚C. Figure 5 indicates that the adsorption
capacity of M-1 and M-2 was dependent on pH.
In general, the adsorption of ion on clay mineral in-
cludes exchange adsorption and specific adsorption. The
exchange adsorption is of pH-independent constant,
which has relation to permanent charge of clay mineral
[21]. Therefore, it can be seen that the pH-dependent
percentage of zinc ion adsorbed belongs to reinforcement
of specific adsorption.
3.6 Effect of Stirring Speed
The impact studies of stirring speed on zinc ion uptake
capacities under the following conditions: initial concen-
tration is 5 × 10−3 mol/L, bentonite particles size through
200 mesh, pH is 5, temperature of 20˚C and adsorption
Figure 6. Effect of stirring speed on the adsorption of Zn2+
ions onto bentonite and Na-bentonite.
time is 120 min. As shown in Figure 6, we can get the
effect of stirring speed.
Experimental data is processed to obtain regression
equation of stirring speed and adsorption rate respec-
tively:
11.78 31.25 M
W



, (4)
10.9957 M
R
22.2331 39.003 M
W



, (5)
20.999 4 M
R
where 1M W

and 2M
W

are adsorption rate of M-1and
M-2,

is stirring speed.
From above equations and Figure 6, we can draw the
conclusion that adsorption rate of bentonite are increased
by stirring speed and they are linear correlation. The
reason is that with other conditions unchanged, acceler-
ate stirring speed makes the velocity of Zn2+ increased
and then facilitates adsorption. Furthermore, a higher
stirring speed would be propitious to bentonite dispersion,
increase BET of bentonite and enhance bentonite adsorp-
tion.
3.7. Effect of Particle Size
Figure 7 shows the removal efficiencies for different
particle sizes under the conditions of the initial concen-
tration 5 × 10−3 mol/L, liquid-to-solid ratio 100 mL/g,
20˚C, pH 5, stirring speed of 1200 r/min, adsorption time
120 min. As can be seen from the graph, the smaller par-
ticle size is, the higher adsorption rate is. This may be in
connection with crystal morphology and surface proper-
ties of montmorillonite. The crystal of montmorillonite
overlapped in lamellar-type. Binding force between the
crystal face is weaker, this led to peeling along the layer
easily. When particle size becomes smaller, radius-
thickness ratio would largen and the BET double in-
crease. The electroactive of grain cross-section simulta-
neously enhances, resulting in the removal rate of Zn2+
by bentonite improved obviously [19].
3.8 Effect of the coexisting ions
In order to investigate the respective effect of the Pb2+
and Cr6+ presence in the aqueous solution on the sorption
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The Factors on Removal of Zinc Cation from Aqueous Solution by Bentonite 111
Figure 7. Effect of particle size on the adsorption of Zn2+
ions onto bentonite and Na-bentonite.
Figure 8. Effect of competing ionic concentration on the
adsorption of Zn2+ ions onto bentonite and Na-bentonite.
of Zn2+ onto the bentonite adsorbents, we prepared vari-
ous amounts of Pb2+ and Cr6+ solutions with maintaining
all other conditions as above. The results, shown in Fig-
ure 8, clearly indicate that the simultaneous presence of
Cr6+ and Zn2+ in aqueous solutions reduced the equilib-
rium adsorption capacity of bentonite adsorbents, while
there was hardly effect in the simultaneous aqueous solu-
tion of Pb2+ and Zn2+.
This may be attributed to the ionic radius, hydrated ra-
dii and valence of the ions [22]. The higher the electro
valence is, the smaller is the ion radius with easier inter-
changes reaction. Cr6+ is of the greatest electro valence,
the least hydrated radii and the strongest competitiveness
among Cr6+, Pb2+ and Zn2+.
Accordingly, Cr6+ displaces zinc cation, resulting in a
decline of percentage of zinc ion. Pb2+ is of larger hy-
drated radii than Zn2+. Similarly, Pb2+ could not displace
zinc cation, resulting in no effect on adsorption of Zn2+.
3.9 Adsorption isotherm
The adsorption behavior of M-1 and M-2 samples was
Figure 9. Adsorption isotherm of Zn2+ions onto bentonite
and Na-bentonite.
Figure 10. Relationship between lgQ and lgCe.
studied and the results obtained depicted in Figure 9. As
shown in Figure 10, the plot of (lg ) versus () for
adsorption of Zn2+ onto bentonites can be seen.
Qe lgC
In this study, linear coefficients of determination test
were used. Linearised forms of the two curves were:
M-1 e,M-1
lg1.8671lgC 1.849Q

, (6)
M-1
R 0.9278
M-2 e,M-2
lg0.7829lgC 0.8811Q

, (7)
M-2
R0.99
e
As is seen from above, both (6) and (7) are found to fit
the linearised form of Freundlich equation [23]:
e
lgqlg1/ lgC
F
Kn

 (8)
where e is the amount of solute adsorbed per unit
weight of adsorbent (mg/g), e the equilibrium concen-
tration of solute in the bulk solution (mg/l),
q
C
F
K
the con-
stant indicative of the relative adsorption capacity of the
adsorbent (mg/g) and the constant indicative of the
intensity of the adsorption. We can conclude that adsorp-
tion isotherms of bentonites for zinc ion could be ex-
pressed mathematically in terms of Freundlich models.
Rearranged these equations, we obtain:
1/n
0.5356
1
0.014 2C
M
Qe
,M-1
e ,M-2
(9)
1.2773
2
0.131 5C
M
Q (10)
4. Conclusions
Local bentonite and its Na-modified form were tested as
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112 The Factors on Removal of Zinc Cation from Aqueous Solution by Bentonite
adsorbent material for the removal of zinc ions from
waste solutions. The results indicate that removal of Zn2+
by Na-exchanged bentonite is more effective than by
Ca-bentonite. Compared to the published data in the
same field, it is found to be in agreement with most of
them.
The adsorption experiments were conducted under
different conditions. The extent of zinc adsorption in-
creased with increase in pH, temperature, stirring speed,
contact time and decrease in particle size, initial concen-
tration, liquid-to-solid ratio.
In a complex system, the simultaneous presence of
Cr6+ and Zn2+ in aqueous solutions reduced the equilib-
rium adsorption capacity of bentonite adsorbents, while
there was no effect in the simultaneous aqueous solution
of Pb2+ and Zn2+.
The adsorption of Zn2+ ions onto bentonite and Na-
exchanged bentonite can be expressed with Freundlich
type sorption isotherms.
5. Acknowledgements
This research was financially supported by National
Natural Science Foundation of China (Grant No.41072
031, 41072119) and Natural Science Foundation of He-
bei Province (Grant No. D2009000833).
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