Comparison of adsorption performances of vermiculiteand clinoptilolite for the removal of pyronine Y dyestuff

Mahmut Toprak • Abdullah Salci •

Ali Riza Demirkiran

Received: 1 August 2013 / Accepted: 15 November 2013

� Akademiai Kiado, Budapest, Hungary 2013

Abstract In this study, the adsorption of pyronine Y (PyY) from aqueous solution

on two adsorbents, vermiculite and clinoptilolite, was investigated with respect to

contact time, initial dye concentration, pH, adsorbate concentration and solution

temperature. Moreover, the dye removal performance of vermiculite was compared

with that of clinoptilolite under the same experimental conditions. The adsorption of

dye on adsorbents reached equilibrium in 15–25 min. The dye removal performance

of vermiculite was comparable with that of clinoptilolite at high adsorbent con-

centrations above 5.0 g/L resulting in nearly 91 % dye removal with 10 mg/L initial

dye concentration. The equilibrium experiments were analyzed by the Langmuir

and Freundlich isotherms. As a result, the adsorption of PyY by clinoptilolite fitted

the Freundlich isotherm well, while that by vermiculite fitted the Langmuir isotherm

well. The first order kinetic, pseudo-second order kinetic and intra-particle diffusion

models were used to investigate the kinetic data. The adsorption kinetics of PyY on

adsorbents was described by the pseudo-second order kinetic equation. Moreover,

the activation parameters were also calculated. It was found that the reaction for dye

uptake by vermiculite and clinoptilolite is the presence of an energy barrier.

Keywords Pyronine Y � Vermiculite � Clinoptilolite � Dye

Introduction

Synthetic dyes are widely used in many fields of industry, for example the textile,

cosmetic and pharmaceutical industries. Today, the worldwide productions of

M. Toprak (&) � A. Salci

Department of Chemistry, Bingol University, Bingol 12000, Turkey

e-mail: mahtoprak@gmail.com

A. R. Demirkiran

Department of Agriculture, Bingol University, Bingol 12000, Turkey

123

Reac Kinet Mech Cat

DOI 10.1007/s11144-013-0651-5

synthetic dyes are approximately 8 9 105 tons and most of them are discharged

directly into wastewater. The discharge of dye wastewater is a serious environmental

problem [1–5]. Colored water is harmful to aquatic animals because it contains a

variety of organic compounds and toxic substances [6–8]. Besides, the presence of

dyes in various sources of water diminishes light penetration, preventing photosyn-

thesis of the aquatic plants. In addition, colored water may affect human health

because of the mutagenic and carcinogenic effects of dyes. Furthermore, many of these

dyestuffs are resistant to biological degradation because of their synthetic origins [9–

16]. Therefore, these dyes need to be removed before the wastewater can be released

into the environment. The removal of dyes from wastewater has been conventionally

carried out by physical and chemical methods. Among these methods, adsorption is

one of the effective techniques because it is rapid and relatively easy to use. This

process is based on the principle of transferring the dyes from the wastewater to solid

phase. Activated carbon is an ideal adsorbent for wastewater treatment. However, its

use is restricted owing to the high price and regeneration problems [17, 18]. For this

reason, it is necessary to develop low-cost and easily available alternative adsorbents

for the treatment of effluent. In recent years the use of clinoptilolite, a natural zeolite,

and vermiculite, a mica-type lamellar mineral, has been investigated in terms of the

cost and potential for wastewater treatment [19–21]. Qiu et al. [22] investigated the

adsorption of safranine T and Amido Black 10B from aqueous solution with clin-

optilolite. The maximum adsorption capacity and adsorption affinity of the clinoptilolite

to the two dyes were calculated and predicted using the Langmuir model. Tang et al. [23]

studied the adsorption of methyl orange on vermiculite modified by cetyltrimethylam-

monium bromide (CTMAB) and suggested that the methyl orange removal rate of

CTMAB-vermiculite was better than that of vermiculite. Zhao et al. [24] investigated

the adsorption of methylene blue onto silica nano-sheets derived from vermiculite using

acid leaching. Results showed that the adsorption of methylene blue by silica nano-sheet

fitted the Langmuir equilibrium isotherm very well. Sismanoglu et al. [25] studied the

adsorption of reactive dyes onto clinoptilolite. It was found that the adsorption rate

decreased with increasing dosage of the reactive dyes. As a result, vermiculite and

clinoptilolite have been investigated for a wide variety of effluent applications.

However, the adsorption process of pyronine Y (PyY) dye which is a xanthine derivative

from aqueous solution on these adsorbents has not been reported so far.

In the current study, we report the ability of natural clinoptilolite and vermiculite

to remove PyY by adsorption from aqueous solution. In addition, this study provides

fundamental information on PyY adsorption capacities and adsorption constants for

clinoptilolite and vermiculite. The PyY removal performance of clinoptilolite was

compared with that of vermiculite under the same conditions. Therefore, this study

chooses to investigate the kinetic, equilibrium and activation parameters of

clinoptilolite and vermiculite to remove basic dye from aqueous solution.

Materials and methods

PyY was purchased from Sigma and used without further purification (chemical

formula: C17H19ClN2O, MW: 302.80 g/mol). The clinoptilolite sample was

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123

obtained from Goztepe (Istanbul, Turkey). The vermiculite sample used was

obtained from Karamursel (Kocaeli, Turkey). The clinoptilolite and vermiculite

powders of 20–50 mesh were used for adsorption experiments. Some physical

properties and the chemical composition of the adsorbents are shown in Tables 1

and 2. A stock solution was prepared by dissolving precisely 302 mg of PyY in

250 mL distilled water. All working solutions of PyY were prepared by diluting the

stock solution to required concentrations. Isotherm studies were carried out using

different amounts of adsorbents with 10 mL dye solutions of known initial

concentration (10 mg/L) at the desired pH and temperature. To adjust the pH of the

solution, a strong acid (0.1 mol/L HCI) or strong base (0.1 mol/L NaOH) was used.

The pH of the solutions was recorded with a Thermo Scientific pH meter. The

concentration of PyY in aqueous solution was determined by a spectrophotometer

(UV-1600, Shimadzu) at a wavelength of 548 nm. All adsorption experiments were

conducted with 50 mL flasks containing 10 mL of solution at constant temperatures

of 22, 30, 40 and 50 �C and the experiments were performed three times. The

amounts of dye adsorbed on clinoptilolite and vermiculite were calculated from the

concentrations in aqueous solutions before and after adsorption. The solid phase

loading was calculated by Eq. 1:

qe ¼ðC0 � CeÞ:V

1000:mð1Þ

where qe is the amount of dye adsorbed per gram of adsorbent in mg/g, C0 is the

initial dye concentration in mg/L, Ce is the equilibrium (residual) dye concentration

Table 1 Typical physical

properties of adsorbentsClinoptilolite Vermiculite

Bulk density (kg/m3) 650–850 64–160

Cation exchange (meg/g) 1.5–1.9 0.5–1

Water adsorption (%) 42–50 20–45

pH 7–8 6–9

Surface area (m2/g) 39 3.14

Porosity (%) 45–50 25–50

Table 2 Typical chemical

analysis of adsorbentsComposition (%) Clinoptilolite Vermiculite

SiO2 65–72 38–46

AI2O3 10–12 10–16

MgO 0.9–1.2 16–35

CaO 2.5–3.7 0.5–3

K2O 2.3–3.5 2–6

Fe2O3 0.8–1.9 4–12

TiO2 0–0.1 0.7–3

MnO 0–0.08 0.01–2

Na2O 0.3–0.65 0.1–1

Other 4.87–18.2 8.2–17.2

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in mg/L, V is the volume of the solution in mL and m is the mass of the adsorbent in

g.

The removal efficiency was calculated by Eq. 2:

%Q ¼ A0 � Ae

Ao

:100 ð2Þ

where, Q is the removal efficiency, A0 is the absorbency of initial dye, and Ae is the

absorbency of equilibrium dye.

Results and discussion

Effect of experimental conditions on the adsorption process

It is important to be able to estimate the rate at which dyestuff is removed from

wastewater in order to design an adsorption treatment facility. To determine the

equilibration time, the adsorption of PyY dye onto adsorbents was studied as a

function of contact time. The initial PyY concentration was 10 mg/L. An adsorption

experiment was carried out to find the effect of adsorption time on the adsorption of

PyY dye into vermiculite at 22 �C for solid concentration of 5.0 g/L and the result is

displayed in Fig. 1. The result indicated that the percent of adsorption increased

with increasing time. In about 25 min, the adsorbent can reach the adsorption

equilibrium. The amount of adsorbed dye did not show important changes after

15 min. As shown in Fig. 1, when the adsorption time increased, the amount of PyY

dye bound to clinoptilolite at 22 �C for a solid concentration of 3.0 g/L increased

dramatically. In about 10 min, the adsorbent can reach the adsorption equilibrium.

The amount of adsorbed dye did not exhibit important changes after 15 min

compared with the adsorption of PyY on the vermiculite, the clinoptilolite had a

faster adsorption rate. To determine the influence of initial PyY concentration on the

amount of adsorbed dye, the initial PyY concentration varied from 3 to 20 mg/L at

22 �C. It is seen that percent removal efficiency decreased with increasing initial

dye concentrations for both vermiculite and clinoptilolite. The experiments were

carried out against amounts of adsorbent concentrations in the range of 0.5–6.0 g/L

for 30 min at 10 mg/L of initial PyY concentration. The results of the experiments

are shown in Fig. 2. The results given in Figs. 1 and 2 show that the adsorption

capacity of clinoptilolite was higher than vermiculite. For adsorbent concentrations

lower than 5.0 g/L, PyY removal performance of clinoptilolite was better than that

of vermiculite because of the larger specific surface area (m2/g) of clinoptilolite.

However, the PyY removal performance of adsorbents was comparable at high

adsorbent concentrations above 3.0 g/L probably due to the large adsorption area

provided at high vermiculite concentrations. The results indicated that vermiculite

may be as effective an adsorbent as clinoptilolite at high adsorbent concentrations

for the removal of dyestuff. pH is an important factor in dyestuff adsorption. The

removal efficiency as a function of time for PyY on adsorbents at five different pH

values is illustrated in Fig. 3. As shown in Fig. 3, the uptake of dye increased by

increasing the initial pH and the dye adsorption by clinoptilolite was significantly

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affected over the pH range of 2.0–7.0. There was a sharp increase in the removal

when the solution pH increased from 2.0 to 7.0. Vermiculite had the maximum dye

removal (91 %) over a pH of 7, which decreased to 71 % at a pH of 2.0. The

increase in the adsorption with the rise in solution pH may be explained as the

increase in electrostatic force of attraction between the adsorbate and the adsorbent.

Similar studies have also shown that clinoptilolite and vermiculite will have higher

adsorption at higher pH values [22, 24].

Adsorption isotherm models

The applicability of adsorption on an adsorbent for the removal of dyes can be

explained by adsorption isotherms. In this study, the Langmuir model and the

0 10 20 30 40 50 60 70 80 90

1.50

1.65

1.80

1.95

2.10

2.25

2.40

2.55

Time (minute)

qe

(mg

/g)

Clinoptilolite Vermiculite

Fig. 1 Effect of adsorption time on the adsorption of dye on adsorbents

0 1 2 3 4 5 60

20

40

60

80

100

Rem

oval

Eff

icie

ncy

(%)

Adsorbent Concentration (g/L)

ClinoptiloliteVermiculite

Fig. 2 Effect of sorbent concentration on the adsorption of dye on adsorbents

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Freundlich model were used to define the adsorption of PyY on clinoptilolite and

vermiculite. The Langmuir model is based on the assumption that the adsorption

takes place at particular homogenous sites in the adsorbent and supposes a uniform

surface, a monolayer adsorbing adsorbate at constant temperature. The linear form

of the Langmuir model can be given by Eq. 3:

Ce

qe

¼ 1

q0KL

þ Ce

q0

ð3Þ

where Ce is the equilibrium (residual) adsorbate concentration in mg/L, q0 (mg/g) is

the maximum amount of adsorbate per unit weight of adsorbent to form a complete

monolayer on the surface bound at high Ce, KL (L/mg) is a constant related to the

energy of adsorption, q0 and KL are the Langmuir constants. The adsorption capacity

q0 and adsorption constant KL can be determined from the slope and intercept of a

linearized plot of Ce/qe against qe. The essential characteristics of the Langmuir

isotherm can be described in terms of a dimensionless constant separation factor

(RL), which is described by Eq. 4:

RL ¼1

1þ KLC0

ð4Þ

Here, KL is the Langmuir constant and C0 is the highest initial PyY concentration.

The value of RL demonstrates the type of isotherm to be either favorable

(0 \ RL \ 1), unfavorable (RL [ 1), linear (RL = 1) or irreversible (RL = 0). A

basic assumption of the Freundlich theory is that the adsorption takes place on a

heterogeneous surface. The Freundlich isotherm is valid for multilayer adsorption

on adsorbent surfaces as well as non-ideal adsorption. The linear form of the

Freundlich model can be given by the following Eq. 5:

2 4 6 8 10 1220

30

40

50

60

70

80

90

100

Rem

oval

Eff

icie

ncy

(%)

pH

Vermiculite Clinoptilolite

Fig. 3 Effect of initial solution pH on the removal of PyY on adsorbents

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logqe ¼ logK þ 1

nlogCe ð5Þ

Here, K and n are the mono-component Freundlich constants related to the

adsorption capacity and adsorption intensity of the adsorbent, respectively. For the

adsorption isotherm experiments done with each of the adsorbents, the isotherm

constants were obtained after fitting the data to the respective equations through

linear regression analysis. Each isotherm consisted of eight adsorbate concentrations

which varied from 3 to 20 mg/L. The Langmuir and Freundlich adsorption

isotherms of PyY on adsorbents are shown in Figs. 4 and 5, respectively. The results

of fitting experimental data with the Langmuir and Freundlich isotherms for the

adsorption of PyY on adsorbents are represented in Table 3. The suitability of

isotherms for the system was compared by utilizing the correlation coefficients, R2

values. For PyY adsorption on vermiculite, as shown in Table 3, the R2 obtained

from the Langmuir isotherm model (R2 = 0.9976) was higher than that obtained

from the Freundlich isotherm model. The low values of RL for the adsorbent confirm

the favorable uptake of PyY process. Therefore, the Langmuir equation better

exhibits the adsorption process. For PyY adsorption on clinoptillite, the R2 obtained

from the Freundlich isotherm model (R2 = 0.9825) was higher than that obtained

from the Langmuir isotherm model. Therefore, the Freundlich adsorption model is

suitable for modeling the adsorption of PyY on clinopitlolite.

Adsorption kinetic models

In order to predict the kinetic mechanism that governed the adsorption process,

pseudo-first order, pseudo-second order, and intra-particle diffusion models were

applied to analyze the experimental data at different temperatures. The pseudo-first

order equation has often been used to define the adsorption of an adsorbate from an

aqueous solution. This equation is based on the supposition that the change of solute

0 1 2 3 4 5 6 7 8 90.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

y = 0.114x + 0.5018

R² = 0.9514

y = 0.466x + 0.2649

R² = 0.9976

Ce

Ce/

q e

Vermiculite Clinoptilolite

Fig. 4 The Langmuir isotherm plots for clinoptilolite and vermiculite

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uptake with time is dependent on the difference in satiety concentration and the

amount of solid uptake with time. The linear form of the pseudo-first order model is

given by Eq. 6 [26]:

logðqeq � qtÞ ¼ logqeq �kpf t

2:303ð6Þ

Here, qeq (mg/g) and qt (mg/g) are the amounts of dye adsorbed on the adsorbent

at equilibrium and at time t, respectively, and kpf (/min) is the first order adsorption

rate constant. The values of log(qeq-qt) were calculated from the kinetic data. The

plot of log(qeq-qt) against t should give a straight line with slope -kpf and intercept

logqeq. The results of fitting the experimental data with the pseudo-first order

(Fig. 6) for the adsorption of the dye on clinoptilolite and vermiculite are presented

in Table 4. As can be seen, the linear regression R2 values for PyY adsorption on

clinoptilolite and vermiculite changed in the range of 0.8894–0.9268 and

0.9195–0.9533, respectively. These results show that the experimental data are

not described by the pseudo-first order model. The pseudo-second order equation is

based on the assumption that the change of solute uptake with time is directly

proportional to the amount of solute adsorbed on the surface of adsorbent and the

amount of dye adsorbed at equilibrium. The linear form of pseudo-second order

model is given by Eq. 7 [27]:

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

y = 1.3728x - 0.2564

R² = 0.9825

y = 0.2871x + 0.0714

R² = 0.9344

log Ce

logq

e

Vermiculite Clinoptilolite

Fig. 5 The Freundlich isotherm plots for clinoptilolite and vermiculite

Table 3 Values of the constans in Langmuir and Freundlich models

Adsorbent Langmuir Freundlich

qmax

(mg/g)

KL

(L/mg)

RL r2 n K (mg/g)

(L/mg)1/nr2

Clinoptilolite 8.772 0.227 0.595–0.1805 0.9514 1.389 1.564 0.9825

Vermiculite 2.145 1.75 0.159–0.0277 0.9976 3.483 1.787 0.9344

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t

qt

¼ 1

kpsq2eq

þ t

qeq

ð7Þ

Here, kps (g/mg/min) is the pseudo-second order rate constant and qeq is as

defined above. The pseudo-second order rate constant (kps) and the equilibrium

adsorption capacity (qeq) can be calculated experimentally from the slope and

intercept of the plot of t/qt versus t. The results of fitting experimental data with the

pseudo-second order model (Figs. 7, 8) for the adsorption of PyY on clinoptilolite

and vermiculite at different temperatures are given in Table 4. As can be seen, the

linear regression R2 values for PyY adsorption on clinoptilolite and vermiculite

changed in the range of 0.9993–0.9999 and 0.9912–0.9999, respectively. The above

0 5 10 15 20 25 30 35 40 45 50 55 60 65-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

y = -0.0524x + 0.2697R² = 0.9195

y = -0.0351x - 0.0306R² = 0.8864

Clinoptilolite Vermiculite

log

(qe-

q t)

t(minute)

Fig. 6 Pseudo-first order kinetic plots for the adsorption of PyY on clinoptilolite and vermiculite

Table 4 Adsorption kinetic parameters of PyY on adsorbents

T Pseudo-first order Pseudo-second order Intraparticle diffusion

kpf qae r2

1 qbe

kps qae r2

2kid c r2

3

PyY-clinoptilolite

22 0.0808 2.325 0.8864 2.3963 0.195245 2.325 0.9993 0.1706 1.327 0.528

30 0.0598 2.540 0.8642 2.5974 0.200093 2.540 0.9998 0.1092 1.7543 0.5654

40 0.0594 2.594 0.8216 2.6511 0.221626 2.594 0.9996 0.1142 1.7798 0.4990

50 0.0580 2.458 0.9268 2.4801 0.307654 2.458 0.9999 0.0382 2.1652 0.5046

PyY-vermiculite

22 0.1674 1.559 0.9195 1.7540 0.159291 1.559 0.9912 0.1602 0.4505 0.7129

30 0.1552 1.570 0.9522 1.6257 0.287040 1.570 0.9995 0.0931 0.9395 0.63

40 0.1545 1.570 0.9809 1.6069 0.436986 1.570 0.9998 0.0714 1.0894 0.613

50 0.1398 1.579 0.9533 1.6020 0.668231 1.579 0.9999 0.0488 1.248 0.6009

a Experimentalb Calculated

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results show that the pseudo-second order model fitted the equilibrium data better

than pseudo-first order model. Therefore, the adsorption kinetics of PyY by

clinoptilolite and vermiculite could be described by the pseudo-second order model.

The best fit of the pseudo-second order expression suggest that the chemisorption

mechanism is involved in the adsorption. The adsorbate species are most likely

transported from the solution phase to the solid surface of adsorbent particle through

an intra-particle diffusion process. The transport of adsorbate species onto the

surface of the adsorbent is often the rate limiting step in the adsorption. In the intra-

particle diffusion model, it is assumed that the adsorption capacity varies almost

proportionally with t1/2 and the model is commonly given by Eq. 8 [28]:

0 10 20 30 40 50 60 70 80 900

3

6

9

12

15

18

21

24

27

30

33

36

y = 0.4032x + 0.3081R² = 0.9999

y = 0.3772x + 0.6413R² = 0.9996

y = 0.385x + 0.6674R² = 0.9998

y = 0.413x + 1.1163R² = 0.9993

t/q t

t (minute)

22°C

30°C

40°C

50°C

Fig. 7 Pseudo-second order kinetic plots for the adsorption of PyY on clinoptilolite

0 10 20 30 40 50 60 70 800

10

20

30

40

50

y = 0.6242x + 0.583R² = 0.9999

y = 0.6223x + 0.8862R² = 0.9998

y = 0.6151x + 1.3181R² = 0.9995

y = 0.5701x + 4.099R² = 0.9912

t/q

t

t (minute)

22°C30°C40°C50°C

Fig. 8 Pseudo-second order kinetic plots for the adsorption of PyY on vermiculite

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qt ¼ kidt1=2 þ C ð8Þ

where t (min) is the contact time, kid (mg/g min1/2) is the intra-particle diffusion

constant, and C is the constant. Plots between qt versus t1/2 are shown in Fig. 9. The

values of the parameters and the correlation coefficients obtained by using exper-

imental data are listed in Table 4. As can be seen, the low R2 values determined for

the intra-particle diffusion model show that adsorption of PyY on clinoptillite and

vermiculite does not occur in the pores of a solid in accordance with surface

adsorption.

Activation parameters

The activation parameters of the adsorption process will help us to understand the

adsorption mechanisms and to improve the practical application of adsorbents to

wastewater treatment. The k2 constants of the second order kinetic equation for

adsorption of PyY on adsorbents at different temperatures listed in Table 4

(C0 = 10 mg/L, pH = 7.0) have been used to determine the activation energy of

PyY adsorption on adsorbents using the Arrhenius equation (Eq. 9):

lnk2 ¼ �Ea

RgTþ lnA ð9Þ

Here, A is the Arrhenius factor, Rg is the gas constant (8.3145 J/mol K) and Ea is

activation energy (J/mol). The plot of lnk2 against 1/T should give a straight line

with slope -Ea/Rg and intercept lnA. The results are shown in Fig. 10. According to

the results calculated in Fig. 10, the Ea values were found to be 12.3 kJ/mol for

clinoptilolite, and 40.2 kJ/mol for vermiculite. The positive values of activation

energy show the presence of an energy barrier in the adsorption process. Parameters

including free energy (DG*), enthalpy (DH*) and entropy (DS*) of activation can be

obtained using the Eyring equation (Eq. 10):

0 2 4 6 8 100.0

0.5

1.0

1.5

2.0

2.5

qt

t1/2

Clinoptilolite Vermiculite

Fig. 9 Intrapaticle diffusion plots for the adsorption of PyY on clinoptilolite and vermiculite

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lnk2

T¼ ln

kb

hþ ln

DS�

Rg

� DH�

RgTð10Þ

Here, kb and h. e Boltzmann’s constant (1.38 9 10-23 J/K) and Planck’s constant

(6.626 9 10-34 J s), respectively, and T is the absolute temperature. The values of

activation parameters including enthalpy (DH*) and entropy (DS*) for PyY-

clinoptilolite and PyY-vermiculite systems have been obtained from the slope and

the intercept of the Eyring plots in Fig. 11. The Gibbs free energies of activation

have been calculated using Eq. 11:

DG� ¼ DH� � TDS� ð11Þ

0.0031 0.0032 0.0033 0.0034

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

y = -4842x + 14.617R² = 0.9821

y = -1489x + 3.3501R² = 0.8103

lnk

2

1/T

Clinoptilolite Vermiculite

Fig. 10 Arrhenius plots for the adsorption of PyY on clinoptilolite and vermiculite

0.00306 0.00315 0.00324 0.00333 0.00342

-7.6

-7.2

-6.8

-6.4

-6.0y = -4531.6x + 7.8781R² = 0.9793

y = -1178.5x - 3.3885R² = 0.7346

1/T

ln(k

2/T

)

Clinoptilolite Vermiculite

Fig. 11 Plots of ln k2

Tversus 1

Tfor the adsorption of PyY on clinoptilolite and vermiculite

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As listed in Table 5, the values of DG* and DH* are positive, confirming again

the presence of an energy barrier in all the systems. The negative value of DS*

points out the diminishing randomness at the solid/liquid interface during the

adsorption of dye on vermiculite and clinoptilolite. In addition, the second order rate

constants increased with the rise in temperature. Therefore, the adsorption of PyY

on adsorbents was more favorable at a high temperature in the investigated range.

Similar results have been recorded on the adsorption of methylene blue onto silica

nanosheets derived from vermiculite.

Conclusions

Dyestuff removal performances of vermiculite and clinoptilolite were compared at

different initial adsorbent concentrations (3.0–5.0 g/L) and a constant initial PyY

concentration of (10 mg/L). The adsorption performance of clinoptilolite was better

than that of vermiculite at low adsorbent concentrations below 4.0 g/L. However,

the performances of vermiculite and clinoptilolite were comparable in terms of the

rate and the extent of PyY removal at high adsorbent concentrations above 4.0 g/L.

More than 91 % dyestuff removal efficiencies were obtained for both vermiculite

and clinoptilolite after 30 min. with adsorbent concentrations above 3.0 g/L. Two

adsorption isotherms were investigated to correlate the equilibrium adsorption data

and the isotherm constants were calculated for both adsorbents. As a result, the

adsorption of PyY by vermiculite was a good fit for the Langmuir isotherm, while

that by clinoptilolite was a good fit for the Freundlich isotherm. The adsorption

kinetics of PyY by clinoptilolie and vermiculite could be better described in the

pseudo-second order model. The positive values of activation energy show the

presence of an energy barrier. This study shows that vermiculite and clinoptilolite

are effective adsorbents for the removal of PyY from aqueous solutions. In this

sense, it could be suggested that could be utilized as simple and low-cost alternative

adsorbents for the removal of PyY from wastewater.

Acknowledgments We are grateful to the Research Fund of Bingol University (Project Number:

BUBAP199-121-2013) for their financial support.

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Table 5 The activation parameters for the adsorption process of dye on clinoptilolite and vermiculite

Adsorbent DH* (kJ/mol) DS* (J/molK) DG* (kJ/mol)

22 �C 30 �C 40 �C 50 �C

Clinoptilolite 9.8 ± 0.1 -225 ± 12 75.7 78.2 80.4 104.3

Vermiculite 37.7 ± 0.3 -132 ± 8 76.6 77.7 79.0 80.3

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