Comparison of adsorption performances of vermiculite and ...

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In this study, the adsorption of pyronine Y (PyY) from aqueous solution on two adsorbents, vermiculite and clinoptilolite, was investigated with respect to contact ...
Reac Kinet Mech Cat (2014) 111:791–804 DOI 10.1007/s11144-013-0651-5

Comparison of adsorption performances of vermiculite and clinoptilolite for the removal of pyronine Y dyestuff Mahmut Toprak • Abdullah Salci Ali Riza Demirkiran



Received: 1 August 2013 / Accepted: 15 November 2013 / Published online: 3 December 2013 Ó Akade´miai 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 concentrations 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, Bingo¨l 12000, Turkey e-mail: [email protected] A. R. Demirkiran Department of Agriculture, Bingol University, Bingo¨l 12000, Turkey

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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 photosynthesis 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 clinoptilolite. 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 cetyltrimethylammonium 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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793

obtained from Go¨ztepe (Istanbul, Turkey). The vermiculite sample used was obtained from Karamu¨rsel (Kocaeli, Tu¨rkey). 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 ¼

ðC0  Ce Þ:V 1000:m

ð1Þ

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 adsorbents

Table 2 Typical chemical analysis of adsorbents

Clinoptilolite

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

Composition (%)

Clinoptilolite

Vermiculite

SiO2

65–72

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

TiO2

38–46

4–12

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 :100 Ao

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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795

2.55 2.40

qe (mg/g)

2.25

Clinoptilolite Vermiculite

2.10 1.95 1.80 1.65 1.50 0

10

20

30

40

50

60

70

80

90

Time (minute) Fig. 1 Effect of adsorption time on the adsorption of dye on adsorbents

Removal Efficiency (%)

100

80

60

40

20 Clinoptilolite Vermiculite 0 0

1

2

3

4

5

6

Adsorbent Concentration (g/L) Fig. 2 Effect of sorbent concentration on the adsorption of dye on adsorbents

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

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Reac Kinet Mech Cat (2014) 111:791–804 Vermiculite

Clinoptilolite

Removal Efficiency (%)

100 90 80 70 60 50 40 30 20 2

4

6

8

10

12

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

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 1 Ce ¼ þ qe q0 KL 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 ¼

1 1 þ KL C0

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:

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797

1 logqe ¼ logK þ logCe ð5Þ n 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 5.0

Vermiculite Clinoptilolite

4.5

y = 0.466x + 0.2649

4.0

R² = 0.9976

3.5

Ce /qe

3.0 2.5 2.0 1.5 1.0 y = 0.114x + 0.5018

0.5

R² = 0.9514

0.0 0

1

2

3

4

5

6

7

8

9

Ce Fig. 4 The Langmuir isotherm plots for clinoptilolite and vermiculite

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0.8 0.7 0.6 0.5

logqe

0.4 0.3 0.2 0.1

y = 0.2871x + 0.0714 R² = 0.9344

0.0 -0.1

y = 1.3728x - 0.2564

-0.2

R² = 0.9825

-0.3 -0.4

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

log Ce 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/n

r2

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

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]: kpf t ð6Þ 2:303 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]: logðqeq  qt Þ ¼ logqeq 

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799

1.0 Clinoptilolite Vermiculite

0.5 0.0

log (qe-qt)

-0.5 -1.0 y = -0.0351x - 0.0306 R² = 0.8864

-1.5 -2.0

y = -0.0524x + 0.2697 R² = 0.9195

-2.5 -3.0 -3.5 0

5

10

15

20

25

30

35

40

45

50

55

60

65

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 kpf

qae

Pseudo-second order

Intraparticle diffusion

r12

qbe

kps

qae

r22

kid

c

r32

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

Experimental

b

Calculated

t 1 t ¼ þ 2 qt kps qeq 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

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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 intraparticle 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]:

36

y = 0.413x + 1.1163 R² = 0.9993

33 30

y = 0.385x + 0.6674 R² = 0.9998

27

t/qt

24

y = 0.3772x + 0.6413 R² = 0.9996

21 18

y = 0.4032x + 0.3081 R² = 0.9999

15

22°C

12

30°C

9

40°C

6

50°C

3 0 0

10

20

30

40

50

60

70

80

90

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

y = 0.5701x + 4.099 R² = 0.9912

50

y = 0.6151x + 1.3181 R² = 0.9995

t/q t

40

y = 0.6223x + 0.8862 R² = 0.9998

30

y = 0.6242x + 0.583 R² = 0.9999

20

22°C 30°C 40°C 50°C

10

0 0

10

20

30

40

50

60

70

80

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

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801

qt ¼ kid t1=2 þ C

ð8Þ 1/2

where t (min) is the contact time, kid (mg/g min ) 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 experimental 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 þ lnA Rg T

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): 2.5

2.0

qt

1.5

1.0

0.5

Clinoptilolite Vermiculite 0.0 0

2

4

6

t

8

10

1/2

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

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-0.5

lnk 2

-1.0 -1.5

y = -1489x + 3.3501 R² = 0.8103

-2.0 -2.5

Clinoptilolite Vermiculite

-3.0 0.0031

0.0032

0.0033

0.0034

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

-6.0 y = -4531.6x + 7.8781 R² = 0.9793

Clinoptilolite Vermiculite

ln(k2 /T)

-6.4

-6.8

-7.2 y = -1178.5x - 3.3885 R² = 0.7346

-7.6 0.00306

0.00315

0.00324

0.00333

0.00342

1/T Fig. 11 Plots of ln kT2 versus

1 T

for the adsorption of PyY on clinoptilolite and vermiculite

ln

k2 kb DS DH  ¼ ln þ ln  T h Rg Rg T

ð10Þ

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 PyYclinoptilolite 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

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803

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

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 Bingo¨l University (Project Number: ¨ BAP199-121-2013) for their financial support. BU

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