The Factors on Removal of Zinc Cation from Aqueous Solution by Bentonite

doi:10.4236/nr.2011.22015

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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.

E-mail: [email protected]

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:





%100 oe

PCCC

(1)





CCV



 (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).

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

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

(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:

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

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







, (4)

10.9957

R

22.2331 39.003







, (5)

20.999 4

R

where 1M



and 2M



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

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.

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

As is seen from above, both (6) and (7) are found to fit

the linearised form of Freundlich equation [23]:

lgqlg1/ lgC



 (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),

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

0.014 2C

Qe

,M-1

,M-2

(9)

1.2773

0.131 5C

Q (10)

4. Conclusions

Local bentonite and its Na-modified form were tested as

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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