Jordan Journal of Chemistry Vol. 3 No.4, 2008, pp. 409-423

JJC

Thermodynamics and Kinetics of Co (II) Adsorption onto Natural and Treated Bentonite

W. K. Mekhemera, J. A. Hefneb, N. M. Alandisa*, O. A. Aldayel, S. Al-Raddadi

a King Saud University, P. O. Box 11495, Riyadh 11452 b King Abdulaziz City For Science and Technology P. O. Box 6086 Riyadh 11442 Received on April 20, 2008 Accepted on Sept. 22, 2008

Abstract

The present study examined the application of a commercial natural bentonite (untreated,

NB) for the removal of Co (II) ions from aqueous solutions. Specific surface area of the sample

was determined by means of N2 adsorption-desorption at 77 K applying the BET method. X-ray

powder diffraction (XRD) was also used to characterize the (NB). The effects of pH, adsorption time, cobalt ion concentration, bentonite doses, temperature and the NB treatment (calcination

of NB at 700OC and washing by water) on the adsorption process of Co (II) were examined. The

optimum pH for adsorption was found to be 6.0. The data demonstrated that the amount of Co

(II) adsorbed on bentonite increases rapidly during the first hour, thereafter adsorption becomes

linear with time. Pseudo-second-order model described best the kinetics of the reaction. The

thermodynamic parameters of the adsorption (the Gibbs free energy, entropy, and enthalpy)

were determined and discussed. The adsorption of Co(II) on NB appeared to follow Langmuir

adsorption. The process was found to be spontaneous and endothermic under normal

conditions. The adsorbed amount of Co (II) on washed bentonite (WB) was increased by 100%

compared to NB and bentonite calcined at 700ºC (CB). Our results show that bentonite could

especially WB be considered as a potential adsorbent for Co (II) removal from aqueous

solutions.

Keywords: Adsorption; Clay minerals; Bentonite; Cobalt; Wastewater treatment.

Introduction

The presence of metals in aquatic environment is of great concern due to their

toxicity to many life forms. Co (II) is a very toxic element[1]. The increased use of Co (II)

in nuclear power plants and in many industries such as mining, metallurgical,

electroplating, paints, pigments and electronic industries has resulted in Co (II) finding

its way to natural bodies of water. The tolerance limit for Co (II) in potable water has

been fixed as 0.05 mg L-1 [2]. It is, therefore, desirable that wastewater from industries

is treated to remove Co (II) ions before being discharged into the bodies of water.

Various treating methods such as chemical precipitation[3], reverse osmosis, ion

exchange, solvent extraction[1], coagulation[4] and adsorption are utilized to remove the

metal ions from aqueous solutions[5]. Among all these methods, adsorption appears to

be the most effective, especially for effluents with moderate and low concentrations[2,3].

* Corresponding author: e-mail: nandis@ksu.edu.sa

410

Activated carbon is widely used as an adsorbent with excellent adsorption

characteristics. However, due to the high cost of activated carbon, its use is restricted

sometimes for commercial considerations. In addition to cost, adsorptive properties

and availability are also criteria for choosing an adsorbent to remove pollutants. This

has encouraged research into discovering materials that are both efficient and cheap.

Low cost adsorbent materials used by various investigators include saw dust, slurry,

biomass and cellulose, peat, chitin, orange waste, rice husk and wheat bran [4, 5] , and

the technical feasibility of these various low-cost adsorbents for heavy metal removal

from contaminated water has been reviewed by Babel and Kurniawan[6]. Most of these

adsorbents contain functional groups associated within the major constituents of the

substrate. Other low-cost adsorbents showing capability to adsorb heavy metals are

zeolites. The use of zeolites for Co (II) removal was intensively investigated in recent

years [7]. Various types of clays have been reported[8] as low-cost adsorbents for heavy

metal removal. Sepiolite clay has been studied for the removal of Co (II) ions for the

purpose of remediation of polluted water[9]. Natural clay minerals also gained

significant attention among scientists mainly due to their valuable ion exchange

capacity. Smectite clays, such as bentonite and montmorillonite, are fundamental soil

components and are abundant in nature. Bentonite has a 2: 1 layer structure and it

consists of alumina octahedral layer sandwiched between two silica tetrahedral layers.

Large deposits of natural bentonite in many countries provided local industries

promising benefits such as cost efficiency since they are able to treat wastewater

contaminated with heavy metals at low cost[10].

The aim of the present investigation is to study the adsorption of Co (II) ions

onto natural bentonite and to understand the way cobalt ions interact with bentonite.

Towards this aim, the effect of various parameters such as the effects of pH, bentonite

doses, temperature, bentonite treatment (calcination and washing) and contact time on

the adsorption process has been investigated.

Materials and methods Instrumentation

Elemental concentration was analyzed using a Perkin- Elmer Optima 5300 DV

ICP optical emission spectrometer coupled with peristaltic pump and AS-93 plus auto

sampler unit. The optimized condition for the ICP operation is given as follows:

Typical operation conditions for ICP measurement

Instrument PE- Optima 5300 DV Spray Chamber Baffled Cyclonic Nebulizer Low Flow Gem Cone Nebulizer Flow 0.6 L/ min Auxiliary Flow 0.2 L/ min Rf power 1500 watts Plasma Flow 15 L/min Sample Flow Rate 1.5 to 2.5 mL/ min Equilibration Time 15 sec Torch Position -3

411

Chemicals

Cobalt(II) nitrate [Co (NO3)2.6H2O] was used in this study. pH adjustments were

carried out using 0.1N hydrochloric acid (HCl) and 0.1N sodium hydroxide (NaOH). All

Co(II) solutions were prepared with ultra-pure water (specific resistivity of 18 M Ω. cm)

obtained from an E-pure (Barnstead, USA) purifier system.

Adsorbent

The commercial untreated bentonite (NB) was obtained from a local supplier

(Bariod Saudi Arabia Limited). It was characterized by X-ray diffraction (XRD) and

chemical analysis. diffraction (XRD) and chemical analysis. The chemical composition

of the tested samples was as follows: (Wt%) SiO2 50.77, Al2O3 19.89, CaO 3.98 , Na2O

1.96, Fe2O3 19.18, MgO 3.03 and Ti 1.18.

The XRD pattern of the commercial untreated bentonite is shown in Figure 1. It

indicates that the dominant component is montmorillonite (64.9 %), kaolinite (10.6 %),

Geothite (9.5 %), Hematite (9.4 %) and Boehmite (5.6 %).

Figure 1. The XRD patterns of the NB

A 1.0 kg commercial bentonite (untreated) (NB) was sieved through different

mesh sizes: 0.250, 0.180, 0.075, 0.045 and 0.0375 mm. The percentage weight of

each size was calculated and presented in the following table according to Udden-

Wentworth Grain-Size Scale, mm and PHI (Ø). All bentonite samples used in this study

with a range of the particle sizes as shown in Table 1:

412

Table 1. Particle size distribution of commercial bentonite (untreated) (NB).

% Weight Scale* Mm PHI (Ø)

0.1 MEDIUM SAND 0.250 2.0

88.2 FINE SAND 0.180 2.5

6.5 VERY FINE SAND 0.075 3.75

3.8 COARSE SILT 0.045 4.5

1.4 COARSE SILT 0.0375 4.75

* According to Udden-Wentworth Grain-Size Scale

To enhance the adsorption capacity of commercial bentonite material, a 50 g

sample was washed several times with deionized water to remove any particles

adhering to the surface, salt and any other water-soluble contamination, then was

oven-dried over night at 60 °C under vacuum. The dried bentonite sample was ground

and sieved. The washed sample was labeled as WB and stored in sealed

polypropylene bottles.

To study also the effect of thermal treatment on the adsorption capacity of NB,

about 200 gm of bentonite was taken from NB sample and was heated in a furnace at

700οC for 24 h.The sample was cooled down to room temperature over silica gel then

ground and passed through sieves. The calcined bentonite was labeled as CB and

stored in sealed polypropylene bottles.

Surface area determination

The specific surface area of commercial bentonite (untreated ) (NB) was

determined by applying the BET method (Brunauer, Emmet, Teller). In this method the

external surface area can be determined from the quantity of gas adsorbed to form a

monolayer over the surface of the solid. The bentonite sample was weighed in different

portions (0.2 – 0.3 g) and placed in a glass cell, using Analysis Adsorptive Nitrogen

(N2) Instrument. Before gas sorption experiment performed, solid surfaces freed from

contaminants (degassed) such as water by using heat under vacuum. The sample

after cleaned brought to a constant temperature by means of an external bath. Then,

small amount of nitrogen gas (the absorbate) were admitted in steps into the

evacuated sample chamber.

The monolayer volume , Vm is then determined and used to calculate the

specific surface area. The determined surface area of NB was S NB = 45.9 m2 g−1.

These values are lower than those expected for bentonite [11], but it may be explained

by the lack of treatment. In fact, Bourg et al. [12], and Goldberg et al[13] have measured

for a bentonite without pretreatment, a specific surface areas of 23.9 and 18.6 m2 g−1:

it is obvious that pores are blocked and the N2 molecules can not penetrate easily into

the interlayer regions, causing an underestimation of the specific surface area[14- 15 ].

413

General procedure

Adsorption of Cobalt with NB was carried out in a batch reactor. 2000 mg/L of

Co (II) stock solution was prepared by dissolving 9.877 g of Co (NO3)2 6H2O in 1 L

deionized water. Standard Co (II) solutions ranging between 40 and 2000 mg/L were

prepared by diluting the stock solutions. 0.5 gm of bentonite mixed with 50mL of Co(II)

solution with different concentrations (40-2000 mg /L) were applied in the shaker.

162 rpm stirring rate and 298 K temperature in all experiments were chosen. The

concentration of Co (II) remained in the solution, after being centrifuged was analyzed

by a Perkin-Elmer Optima 5300 DV ICP optical emission spectrometer. In this study,

the effects of several factors such as pH, concentration of solution, bentonite doses,

heat treatment, washing of NB, temperature (293,313 and 333K) and contact time on

Co (II) removal efficiency were examined.

Adsorption isotherms

From the above batch adsorption experiments, the adsorbed amount (qe) of Co

(II) per unit of sorbent mass was calculated as follows:

.................................................................................. (1)

Where Co is the initial cobalt concentration, Ce is the concentration of cobalt at

equilibrium (mg/L), m is the clay mass (mg) and V is the solution volume (L).

Kinetic studies

The kinetic experiments were conducted in batch mode. The experimental

details were as follows: 0.5 g NB was added to 50 mL of 200 mg/L Co (II) solution. The

suspension was shaken for a period between 5 and 1440 min with a rotary shaker at a

speed of 162 rpm. After being centrifuged, Co (II) was analyzed by a Perkin-Elmer

Optima 5300 DV ICP optical emission spectrometer. All experiments were carried out

in duplicate.

Effect of pH

The influence of pH in the range of 2-12 was studied keeping all other

parameters constant (Cobalt concentration = 500 and 1000 mg/l; stirring

speed = 162 rpm; contact time = 1 h, adsorbent dose = 0.5 g, temp. = 25°C). The pH

of Co (II) solution was adjusted after adding the adsorbent by using a dilute NaOH and

HCl solutions. Also the pH of bentonite suspension was measured before and after Co

(II) adsorption on NB surface as following: 50 ml of Co (II) solution was added to

bentonite. The pH is registered at the first moments of addition and wait until

equilibrium is established, then measure the pH ( after 24 hours).

Effect of adsorbent dose

The adsorption efficiency of Co(II) on NB was studied at different adsorbent

doses [0.1- 5gm /50 ml cobalt solution] at cobalt concentrations (500 and 1000 mg/l),

keeping stirring speed (162 rpm), temperature (25 °C) and contact time (1 h) constant.

414

Desorption experiment

Desorption experiments were performed in order to estimate the Co(II) recovery

from NB surface. NB saturated with different adsorbed amount Co(II) ions were dried

at 100°C and then 0.5 g of these samples were added in a glass reactor containing

50mL deionized water under constant temperature of 25°C. The resulting bentonite

suspension was mechanically agitated for 1 h with a stirring rate of 162 rpm. After

being centrifuged, desorbed Co (II) was analyzed by a Perkin-Elmer Optima 5300 DV

ICP optical emission spectrometer.

Results and discussion Kinetic study

The adsorbed amount of Co (II) onto natural bentonite is presented in Figure 2

as a function of contact time. More than 80% of Co (II) adsorbs in the first 30 min

reaching soon after equilibrium. Although the equilibrium is achieved in a short time

(about 30 min), a contact time of 1 h was selected for further testing.

Lagergren first-order reaction rate model (Eq.2) and Ho et al.’s pseudo-second-

order reaction model[16-17] (Eq.3) were used to describe the kinetics of cobalt

adsorption on bentonite. The first-order Lagergren rate equation used by researchers [18- 22] to study the kinetics of heavy metal adsorption is in the following form:

ln (qe−qt) =ln(qe) −k1 t ................................................................................ (2)

where k1 is the Lagergren rate constant for adsorption (min−1), qe the amount of metal

ion adsorbed at equilibrium (mg/g), and qt is the amount of metal ion adsorbed at any

given time t (mg/g).

Ho et al. [23] used a pseudo-second-order reaction rate equation to study the

kinetics of adsorption of heavy metals on peat. The Ho et al. pseudo-second-order

equation is given by:

.................................................................................... (3)

h = k2q2 ..................................................................................................... (4)

where k2 (g/mg min) the rate constant of pseudo-second-order adsorption, h the initial

adsorption rate (mg/g. min) and qe and qt are the amount of adsorbed Co(II) on

adsorbent (mg/g) at equilibrium and at time t, respectively.

The plot of ln(qe−qt) versus t (not presented here ) shows a straight line with a

very low correlation factor, R (0.19), indicating that the cobalt removal with natural

bentonite is not a first-order reaction . Ho’s pseudo-second-order model described

best the kinetic data (Figure 3) ( a correlation coefficient R close to 1). The rate

constant k2, the correlation coefficients, the initial adsorption rate (h) and the removal

capacities at saturation (qe) were calculated from the values of the slopes and

intercepts according Ho’s pseudo-second-order model and are compared in the table 2

with the Lagergren first-order model.

415

The half-adsorption time of the metal, t1/2, i.e. the time required for the bentonite

to uptake half of the amount adsorbed at equilibrium, is often considered as a measure

of the rate of adsorption and for the second-order process is given by the

relationship[24]:

.............................................................................................. (5)

The determined value of t1/2 for the bentonite to uptake half of the amount

adsorbed of Co(II) at equilibrium was 10.7 min for 200 mg/L initial concentration

Table 2. Parameters for adsorption of Co(II) onto NB derived from the pseudo-first-

and second-order kinetic models.

0

5

10

15

20

0 500 1000 1500 2000Time (min)

q t

Figure 2. Effect of contact time on Co (II) adsorption onto NB (Co (II) concentration

200 mg/L, 25οC and NB dose = 10 g /L

y = 0.0633x + 0.1211R2 = 0.9997

05

10152025

0 100 200 300 400Time (min)

t/qt

Figure.3. Pseudo-second-order adsorption kinetics of Co (II) on NB surface

Pseudo-first-order Pseudo-second-order

qe (mg/g) k1 (min-1) R2 qe

(mg/g)

k2 (g/mg

.min)

h (mg/g

min)

R2

0.72 -0.0005 0.019 15.8 5.9 x10-3 1.5 0.9998

416

Batch pH studies

The pH of the aqueous solution is an important variable that controls cationic

adsorption onto clay surface. This is due to the change of clay surface properties and

the metal species with pH change. The plots of adsorbed amount versus pH of cobalt

(Figure 4) have inflection points at pH 6 where significant adsorption of cobalt actually

begins . With an increase of pH of the solution from 2.0 to 6.0, the removal capacity

increased from 30% to 40% and 23% to 26% at an initial Co (II) concentration of 500

and 1000 mg L− 1, respectively. It is known that the increase of pH decreases the

competition between the protons and the metal ions for surface sites and results in

increased uptake of metal ions by the bentonite.

The effect of pH on the adsorption of cobalt on bentonite may be explained on

the basis of aqua complex formation of the oxides present in the bentonite (NB). A

positive charge develops on the surface of the oxides of bentonite in an acidic medium

as follows:

-------SiOH + H+ Si -----OH+2 ............................................... (6)

A lowering of cobalt adsorption at low pH is due to the fact that surface charge,

thus developed is not suitable for cobalt adsorption. At low pH values, the high

hydrogen ion concentration at the interface (the hydrogen ions are more specifically

adsorbed than Co ions) repels the positively charged metal ions electrostatically and

prevents their approach to the bentonite surface.

In an alkaline medium , above the point of zero charge (The point of zero charge

for NB suspension at pH = 4) , the oxide surface of the NB becomes negatively

charged as shown in equation (7) and (8) , favoring the adsorption of cobalt.

---SiOH+OH− ---SiO−+H2O ...................................................... (7) ---SiO−+M ---Si---O---M .......................................................... (8)

Brigatti et al. [25] has not considered the precipitation tendency of metal ions in

the presence of sepiolite. Increasing pH was reported to increase the adsorption of

metal ions from kaolinite suspensions [26]. Gutierrez and Fuentes [27] studied the

adsorption behavior of Sr, Cs and Co by Ca-montmorillonite and showed that Co

adsorption increases above the pH of precipitation of Co (OH)2. In the present study,

The precipitation of Co(OH)2 has been observed during the adsorption experiment,

therefore the drastic increase in cobalt removal above pH = 6 was due to the

precipitation of cobalt ions as insoluble Co(OH)2(S) precipitate ( as the solubility product

of CoO , pKsp = -13.547) rather than the adsorption on the negatively surface charges

of bentonite. This type of behavior had been also observed for other hydrolysable

metals [28-29].

Similar comments were made by Bangash et al.1992 [30]. The variation of pH

values at which cobalt precipitates may be due to the difference in the aqueous

medium employed in their experiments (e.g. Co++ concentrations). In the present

study, an optimum pH of 6.0 was selected for cobalt – bentonite system.

417

Measuring the pHs of the NB suspension before and after Co (II) adsorption can

give good information for revealing the above comments. Figure 5. shows the pH

values of NB suspension before and after Co (II) adsorption. It was observed that the

pH of bentonite suspension decreases with increasing Co(II) concentration before and

after Co(II) adsorption. This result is probably due to the formation of the acidic Co(II)-

aqua complex. Therefore, the larger the Co(II) concentration the higher is the solution

acidity. Also, as can be seen from this figure, an increasing in the suspension pH after

Co(II) adsorption is observed. This may be attributed to the reaction of acidic Co(II)-

aqua complex with the insoluble carbonate in the bentonite sample (such as calcite).

Another reason is that increasing the ionic strength leads in general to increased

solubilities at this low concentrations..

0

50

100

150

0 2 4 6 8 10 12 14

pH of bentonite suspension

Ads

orbe

d am

ount

(mg/

g)

500 mg/L1000 mg/L

Figure 4. Effect of pH on Co(II) removal by NB at initial Co(II) concentrations 500

and 1000 mg/L, bentonite dose = 10 g/L

6.0

7.0

8.0

9.0

10.0

0 500 1000 1500 2000 2500

initial Co(II) concentrations (mg/L)

PH o

f ben

toni

te (N

B)

susp

ensio

n after adsorptionbefor adsorption

Figure 5. pHs of bentonite suspension before and after Co(II) adsorption onto NB

surface at 25οC

Effect of bentonite (NB) dosage:

Adsorption % of Co (II) on NB was studied at different bentonite (NB) doses [0.1,

0.3, 0.5, 0.7, 1, 2, 3, and 5 g/50 ml, respectively] keeping initial Cobalt concentration

(500 and 1000 mg/l), temperature (25 °C) and contact time (1 h) constant. The results

showed that with increasing the adsorbent dose, the adsorption% of Co (II) was

418

increased (Figure 6). The increase in the adsorption percentage with NB doses can be

explained by the increase in the adsorbent surface area and the availability of more

adsorption sites [31].

020406080

100120

0 2 4 6bentonite doses(g)

Ads

orpt

ion

%

500 mg/L1000 mg/L

Figure 6. Effect of bentonite (NB) doses on adsorption % of Co (II) at different Co (II)

concentration (500 and 1000mg/L) and at 25 °C

Adsorption isotherm

Figure 7 plots the adsorption amount of Co (II) by NB at various temperatures

(293, 313 and 333 K). It was observed that with increasing the temperature the

adsorbed amount of Co (II) on NB increased, indicating that the heat of adsorption is

positive (endothermic process). A value of 25.8, 28, and 34.7 mg/g had been obtained

as the adsorption capacity of bentonite for cobalt from batch experiments at 298, 313

and 333 K respectively. Some reported values of the adsorption capacity for cobalt on

sepiolit [ 9] are 0.79 mg and 38.6 mg on modified bentonite [3]. A comparison of these

values with the one obtained in this study showed that bentonite used in this research

exhibited high capacity for Co (II) adsorption from aqueous solutions.

0

10

20

30

40

0 500 1000 1500 2000 2500

Initial Co(III) concentrations (mg/L)

Ads

orbe

d am

ount

(m

g/g)

298K313 K333 K

Figure7. Adsorption isotherms of Co (II) onto NB adsorbent at different temperatures

All batch experimental data were fitted to the isotherm models of Langmuir

(eq.9) [33] and Freundlich (eq.)[34-35].

………………………………………………………….(9).

419

Where KL = kads/kd is the Langmuir constant; qm is the maximum adsorption capacity

(mg .g−1) and qe is the adsorbed amount of Co(II) at equilibrium (mg .g−1). Rearranging

Eq. (9) yields:

.............…………….……………………………..(10)

.............…………….……………………………..(11)

Where KF and n are Freundlich constants that are related to the adsorption capacity

and adsorption intensity, respectively.

Figure 8 displays linear plots of Ce/qe versus Ce at 293, 313 and 333 K. For

Langmuir isotherm in Figure 8, the values of qm and KL were determined from

experimental data by linear regression. According to Freundlich isotherm (not present

here), the values of KF and n were obtained similarly. The data in Table 3 presents the

results, along with associated correlation coefficients (R2). Also the data in Table 3

reveals that according to the correlation coefficients, the Langmuir model yields a

better fit than the Freunlicuh model.

0

20

40

60

80

0 500 1000 1500 2000Equilibrium concentration, Ce ( mg/L)

Ce/

qe

293 K313 K333 K

Figure 8. Linearized Langmuir isotherm models for Co(II) adsorption by the NB

adsorbent at (a)293 K, (b) 313 K and 333 K (adsorbent dosage, 10 g/L and shaking

time, 2 h Table 3. Parameters of Langmuir and Freundlich adsorption isotherm models for Co(II)

on NB adsorbent at different temperature.

T(K) Langmuir Frendlich

qm (

mg/g

KL(L/mg) KL (L/mol) R2 Kf n R2

293

313 333

25.8

28

34.7

0.019

0.017

0.0132

1119

1001

777.5

0.988

0.992

0.989

4.95

5

5.75

4.22

4.14

4

0.89

0.88

0.88

420

Adsorption thermodynamics:

The thermodynamic parameters of the adsorption, i.e. the standard enthalpy

∆H°, Gibbs free energy ∆G° and entropy ∆S° were calculated using the following

equations[36]:

∆G = -RT ln KL .............…………….……………………………..(10)

ln KL = ∆S/ R – ∆H /RT .............…………….………………... (13)

where R is the general gas constant (kJ .mol−1. K−1) , KL = kads/kd is the Langmuir

adsorption constant and T is the temperature (K). ∆H° and ∆S° values can be obtained

from the slope and intercept of the Van’t Hoff plots of ln KL (from the Langmuir

isotherm) versus 1/T [37-38]. The results of these thermodynamic calculations are

shown in Figure 9 and Table 4. The negative value for the Gibbs free energy for Co(II)

adsorption shows that the adsorption process is spontaneous and that the degree of

spontaneity of the reaction increases with increasing temperature. The overall

adsorption process is endothermic (∆H = 8.37 kJ mol−1). This result explains why the

Co (II) adsorption capacity of NB for increases with increasing temperature. Table 4

also shows that the ∆S value was positive, indicating that the heavy metal ions near

the surface of the adsorbent is more ordered than in the subsequent adsorbed state

(the Co(II) ions exist in the aqueous phase in a very well-ordered state, namely as an

aqua complex). In the other words, it is the dehydration of the cations that leads to the

observed increase in entropy [39]. Adsorption is thus likely to occur spontaneously at

relatively normal and high temperatures because ∆H > 0 and ∆S > 0.

y = -1007.5x + 10.054R2 = 0.9377

6.56.66.76.86.9

77.1

0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.0035

1/ T

ln K

L

Figure 9. Plot of the Langmuir adsorption constant (ln KL) vs. temperature (1/T). The

thermodynamic parameters in Table 4 are determined from this graph

421

Table 4. Thermodynamic constants for the adsorption of Co(II) on bentonite (NB) at

various temperatures:

T (K) Ln KL ∆Gº

(k/mol-1)

∆Hº

(kJmol-1)

∆Sº

(kJmol-1K-1)

293

313 333

6.6 6.9 7

-16 -17.9

-19.3

8.37

0.083

Effect of bentonite treatment

To study the effect of bentonite treatment , equal amount of each sample of NB,

WB and CB (0.5 g) mixed with 50 mL of Co(II) solution (500 and 1000 mg /L) were

applied in the shaker. 162 rpm stirring rate and 298 K temperature were chosen. The

concentration of Co (II) remained in the solution after being centrifuged was analyzed.

Figure10. shows the maximum adsorbed amount of Co (II) ions on natural bentonite

NB , WB and CB. The adsorbed amount of Co (II) on WB was higher than on NB and

on CB. This result may be due to removal of dissolved and excess salts presented in

untreated commercial bentonite (NB) upon washing. Therefore, the fraction of

exchange sites on bentonite surfaces increased and consequently become available

for more adsorption of Co (II) ions from solution. Also in this figure, it was observed

that the adsorption capacity of CB was very lowered compared to NB. It is beleved that

at 700ºC not only dehydration and dehydroxylation occur but also atotal/partial

structural collapse of the montmorillonite eventually occurs. This would explain the

reported low adsorption capacity [40 -42].

010203040506070

NB CB WBAdso

rbed

am

ount

(mg/

g)

500 mg/L

1000 mg/L

Figure.10. The adsorbed amount of Co(II) on NB, calcined bentonite at 700oC (CB)

and washed bentonite (WB) at initial concentration, 500 and 1000 mg/L.

Desorption

In the desorption studies, deionized water was used as desorption agent. The NB

samples loaded with different adsorbed amount of Co (II) ions (initial cobalt

concentration = 500, 1000 and 2000 mg/L) were placed in 50 ml deionized water at

25ºC and the amount of cobalt ions desorbed within 1h measured. Figure11 shows the

422

data of the adsorbed and desorbed amount of Co (II) ions. The data show that there is

about 4, 7 and 8 mg of Co (II) ions desorbed from NB surface loaded by 22.3, 24 and

30 mg Co(II) /g NB.

05

101520253035

500 mg/L 1000 mg/L 2000 mg/L

initial Co(II) concentrations (mg/L)

adso

rbed

and

des

orbe

d am

ount

(mg/

g) adsorbed amount

desorbed amount

Figure.11. Adsorbed and desorbed amount of Co (II) from NB surfaces

Conclusions The following conclusions were drawn from this study.

1. The optimum pH for Co (II) adsorption was 6.0 and the maximum cobalt removal

at this pH was 55%.

2. The kinetic studies indicated that equilibrium for cobalt adsorption on bentonite is

established in less than 1h.

3. The Ho et al.’s pseudo-second-order reaction rate model was found to describe

best the kinetic data.

4. Isotherm analysis of the data showed that the adsorption pattern for cobalt(II) on

bentonite followed the Langmuir isotherm.

5- The adsorption process is endothermic and increases with increasing

temperature.

6- Washing of natural bentonite provides a simple possibility to modify the

adsorption capacity of bentonite.

7- As the adsorbent dose was increased the adsorption % was increased.

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