Adsorption of textile dyes on Pine Cone from colored wastewater: Kinetic, equilibrium and thermodynamic studies

Adsorption of textile dyes on Pine Cone from colored wastewater: Kinetic, equilibrium and thermodynamic studies

Desalination 268 (2011) 117–125 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m...

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Desalination 268 (2011) 117–125

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l

Adsorption of textile dyes on Pine Cone from colored wastewater: Kinetic, equilibrium and thermodynamic studies Niyaz Mohammad Mahmoodi a, Bagher Hayati b, Mokhtar Arami b,⁎, Christopher Lan c a b c

Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iran Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran Department of Chemical & Biological Engineering, University of Ottawa, Ottawa, Canada

a r t i c l e

i n f o

Article history: Received 22 August 2010 Received in revised form 28 September 2010 Accepted 2 October 2010 Available online 1 November 2010 Keywords: Adsorption Textile dyes Pine Cone Kinetics Equilibrium Thermodynamic

a b s t r a c t In this paper, the adsorption of Acid Black 26 (AB26), Acid Green 25 (AG25) and Acid Blue 7 (AB7) onto Pine Cone (PC) was investigated in aqueous solution. Surface study of PC was investigated using Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). The effect of operational parameters such as adsorbent dosage, dye concentration, inorganic anion (salt), pH and temperature onto dye removal was studied. The intraparticle diffusion model, the pseudo-first order and the pseudo-second order were used to describe the kinetics data. Equilibrium isotherms were analyzed using Langmuir, Freundlich and Tempkin adsorption models. Thermodynamic parameters of dye adsorption were obtained. The experimental data fitted well to the pseudo-second order kinetics model for dyes. The results indicated that isotherm data of AB26 and AG25 followed Langmuir isotherm and isotherm data of AB7 followed Freundlich isotherm models. The thermodynamic data indicated that the adsorption was endothermic process. Dye desorption studies in aqueous solution at pH 12 showed that maximum desorption of 93%, 97% and 94.5% were achieved for AB26, AG25 and AB7, respectively. It can be concluded that PC could be effectively employed as an effective biosorbent for the removal of dyes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Dyes have been used as colorants at different industries such as textile, food, paper, cosmetic, etc. [1]. More than 0.7 million tons of synthetic dyes are produced annually worldwide. In addition, over 10,000 different dyes and pigments have been applied in those industries. Researches indicate that approximately 15% of produced synthetic dyes per year have been lost during processing operations that involve the production and handling with many organic compounds hazardous to human health [2]. Wastewaters of dye production and application industries present an environmental problem because of the aesthetic nature due to the fact that the color is visible even in a low dye concentration. The textile industry consumes large quantities of water at its different steps of dyeing and finishing, among other processes. The non-biodegradable nature of dyes and their stability toward light and oxidizing agents complicate the selection of a suitable method for their removal [3,4]. In addition, toxicity bioassays have demonstrated that most of them are toxic.

Several methods such as membrane [5], electrochemical [6], coagulation/flocculation [7], biological [8–10], etc. have been used for dye removal from wastewater. Among the treatment methods, adsorption is considered to be relatively superior to other techniques because of low cost, simplicity of design, availability and ability to treat dyes in more concentrated form [11,12]. The research of the recent years mainly focuses on utilizing agricultural natural materials as low cost and available adsorbents [13–21] (Table 1). A literature review showed that Pine Cone was not used to remove dyes from colored wastewater. In this study, Pine Cone was used as an agricultural natural adsorbent to remove Acid Black 26 (AB26), Acid Green 25 (AG25) and Acid Blue 7 (AB7) from aqueous solution. Effective parameters such as adsorbent dosage, dye concentration, inorganic anion (salt), pH and temperature were investigated on dye removal. Kinetic, isotherm and thermodynamic studies were conducted to evaluate the adsorption capacity of Pine Cone. 2. Experimental 2.1. Chemicals and materials

⁎ Corresponding author. Tel.: +98 21 64542614; fax: +98 21 66400245. E-mail addresses: [email protected] (N.M. Mahmoodi), [email protected] (M. Arami). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.10.007

Pine Cone (PC) was obtained from a local fruit field in the Iran. The PC was first washed to remove the adhering dirt and then were dried, crushed, and sieved. After drying, they were sieved through a

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2.2. Adsorption tests

Table 1 Adsorption capacities of natural adsorbents to remove of cationic dyes. Adsorbent

Adsorbate

Adsorption capacity (mg/g)

Ref.

Canola hull Canola hull Canola hull Tamarind hull Tamarind hull Soy meal hull Soy meal hull Soy meal hull Orange peel Orange peel Pine cone Pine cone Pine cone

Basic Blue 41 Basic Red 46 Basic Violet 16 Basic red 18 Basic violet 16 Direct Red 81 Acid Blue 92 Acid Red 14 Direct Red 23 Direct Red 80 Acid Black 26 Acid Green 25 Acid Blue 7

67.6 49.0 25.0 65.5 46.1 120.5 114.9 109. 9 10.7 21.0 62.9 43.3 37.4

[15] [15] [15] [17] [17] [19] [19] [19] [21] [21] Present study Present study Present study

3.36 mm mesh. Acid dyes (Acid Black 26 (AB26), Acid Green 25 (AG25) and Acid Blue 7 (AB7)) were used. Dyes were obtained from Ciba Ltd. The chemical structure of dyes was are shown in Fig. 1. Other chemicals were Analar grade from Merck. The pH of the solutions was adjusted using H2SO4 or NaOH.

NaO3S

OH

N H

N

N

N

N

The adsorption measurements were conducted by mixing various amounts of PC (0.1–0.3 g) for AB26 and (0.2–0.6 g) for AG25 and AB7 in jars containing 200 mL of dye solutions (50 mg/L) at various pH values (2–12). The pH of the solution was adjusted using H2SO4 or NaOH. Adsorption experiments were carried out at different dye concentrations using optimum amount of PC (1 g/L adsorbent for AB26 and 2 g/L adsorbent for AG25 and AB7) at pH 2 and 25° C for 30 min. The dye removal was monitored and determined at certain time intervals (2.5, 5, 10, 15, 20, 25 and 30 min) during the adsorption processes. A CECIL 2021 UV–vis spectrophotometer was used to determine the dye concentration. To investigate the inorganic anion (salt) effect on dye removal efficiency, different salts such as NaCl, Na2SO4, NaHCO3, and K2CO3 were added to dye solution. At the end of the adsorption experiments, the samples were centrifuged and the dye concentration was determined. For thermodynamic studies, dyes adsorptions at different temperatures (25–65° C) were performed. 2.3. Surface studies The FTIR spectrum of PC was achieved by Perkin-Elmer Spectrophotometer Spectrum One in the range of 450–4000 cm− 1. Scanning Electron Microscope (SEM) LEO 1455VP was used to obtain the SEM images of original PC and dye adsorbed PC. The experiments were conducted with 200 mL dye solutions (50 mg/L dye concentration, pH 2, 1 g/L PC for AB26 and 2 g/L for AG25 and AB7). After the mixing, PC was separated from the solutions by centrifugation and dried for 120 min at 50° C. 2.4. Desorption studies

SO3Na AB26 CH2CH3 +

N

C H2

The adsorbent that was used for the adsorption of dye solution was separated from solution by centrifugation and then dried. It was agitated with 200 mL of distilled water at different pH values (2–12) for the predetermined equilibrium time of the adsorption process. The desorbed dye was determined. 3. Results and discussion 3.1. Surface characteristics

SO3Na

H2C SO3Na

N CH2CH3 AB7

In order to investigate the surface characteristic of PC, FTIR of PC was studied (Fig. 2). The FTIR of PC shows that the peak positions are at 3383.78, 2923.4, 2845.41, 1690.81, 1444.32 and 1048.65 cm− 1. The band at 3383.78 cm− 1 is due to O–H and N–H stretching. While the bands at 1690.81 and 1523.51 cm− 1 reflect the carbonyl group stretching (amide) and N H bending, respectively. Bands at 1305.59 and 1168.67 cm− 1 correspond to C–H bending and C–O stretching, respectively [22,23].

NaO3S O

HN

CH3

O

N H

CH3

NaO3S AG25 Fig. 1. The chemical structure of dyes.

Fig. 2. The FTIR spectrum of PC.

N.M. Mahmoodi et al. / Desalination 268 (2011) 117–125

3.2. Effect of adsorbent dosage The effect of PC dosage on the amount of dye adsorbed was investigated by contacting 200 mL of dye solution with initial dye concentration of 50 mg/L using jar test at room temperature (25° C) at pH 2 for 30 min. Different amounts of PC (0.1, 0.15, 0.2 and 0.3 g) for AB26 and (0.2, 0.3, 0.4 and 0.6 g) AG25 and AB7 in 200 mL of dye solutions were applied. After equilibrium, the samples were centrifuged and the concentration in the supernatant dye solution was analyzed. The plots of dye removal (%) versus time at different adsorbent dosage are shown in Fig. 3. The increase in adsorption of

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dyes with PC dosage was due to the availability of more active surface sites of PC for adsorption. A given mass of PC can adsorb only a fixed amount of adsorbate. So the initial dosage of adsorbate solution is very important.

3.3. Effect of dye concentration The effect of initial dye concentration on dye removal was studied. Experiments were done at different dye concentrations (25, 50, 75 and 100 mg/L) (PC: (1 g/L for AB26 and 2 g/L for AG25 and AB7), pH 2 and 200 mL dye solution). The dye removal decreases by increasing the initial dye concentration as shown in Fig. 4. It can be attributed

(a) (a)

100

100

90

90

Adsorbent (g/L)

70 60

0.5 0.75 1 1.5

50 40 30 20

80

Dye removal (%)

Dye removal (%)

80

60

25

50

50

40

75

30

100

20

10

10

0 0

5

10

15

20

25

30

0 0

time (min)

5

10

15

20

25

30

time (min)

(b)

(b)

100

60

Adsorbent (g/L)

100

1 1.5 2 3

80

40

20

Dye removal (%)

80

Dye removal (%)

Dye (mg/L)

70

Dye (mg/L) 25 50 75 100

60 40 20

0 0

5

10

15

20

25

0

30

0

time (min)

10

15

20

25

30

time (min)

(c)

(c)

100

100

1 1.5 2 3

60

40

20

Dye removal (%)

Adsorbent (g/L)

80

Dye removal (%)

5

Dye (mg/L)

80

25 50 75 100

60 40 20

0 0

5

10

15

20

25

30

time (min) Fig. 3. The effect of adsorbent dosage on dye removal by PC (a) AB26, (b) AG25 and (c) AB7 (pH 2, T 25 °C and C0: 50 mg/L).

0

0

5

10

15

20

25

30

time (min) Fig. 4. The effect of dye concentration on dye removal by PC (a) AB26, (b) AG25 and (c) AB7 (pH 2, T 25 °C and PC: 1 g/L for AB26 and 2 g/L for AG25 and AB7).

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that the active sites on adsorbent for dye removal decreases when dye concentration increases.

decreases in the presence of inorganic anions because these anions are small anions and compete with dyes on the adsorption of PC surface. 3.5. Effect of pH

3.4. Effect of inorganic anion (salt) To investigate the effect of inorganic anion on dye removal efficiency, 1 mmol of NaCl, Na2SO4, NaHCO3, and K2CO3 were added to 200 mL dye solution. Fig. 5 illustrates that dye removal capacity by PC

(a) 100 Salt

Dye removal (%)

80

non NaCl Na2SO4

60

NaHCO3 K2CO3

40

20

0 0

5

10

15

20

25

30

time (min)

(b) 100

The results of blank dye solution studies indicate that change of the initial pH (pH 2–12) of dye solution has negligible effect on the λmax of AB26, AG25 and AB7 dyes. This observation proves that any chemical structural change of dye molecules is not occurred at pH 2–12. The maximum absorbance wavelength (λmax (nm)) of AB26, AG25 and AB7 at different pH values are shown in Table 2. The effect of initial pH on the adsorption of AB26, AG25 and AB7 onto PC is shown in Fig. 6. The adsorption capacity increases when the pH decreases. Maximum adsorption of dyes occurs at acidic pH (pH 2). PC comprises of various functional groups, such as amino, hydroxyl and carbonyl groups which are affected by the pH of solutions. In other words, the predominant charges on the PC at acidic pH are positive and because of having SO− 3 group in the structure of dyes, it seems the dominant mechanism of the adsorption is electrostatic attraction. Therefore, at various pH values, the electrostatic attraction as well as the structure of dye molecules and PC could play very important roles in dye adsorption on PC. At pH 2, a considerable high electrostatic attraction exists between the positively charged surface of the adsorbent and dye anions, due to the ionization of functional groups of adsorbent and negatively charged anionic dye molecules. As the pH of the system increases, the number of negatively charged sites increases. A negatively charged site on the adsorbent does not favor the adsorption of anionic dyes due to the electrostatic repulsion [21,24]. Also, lower adsorption of AB26, AG25 and AB7 dyes at alkaline pH is due to the presence of excess OH− ions destabilizing anionic dyes and competing with the dye anions for the adsorption sites. Thus, the effective pH was 2 and it was used for further studies.

Dye removal (%)

Salt 80

non NaCl Na2SO4

60

NaHCO3 K2CO3

40

20

0 0

5

10

15 20 time (min)

25

30

3.6. Effect of temperature The adsorption studies were carried out at different temperatures 25, 35, 45 and 65° C (Fig. 7). The adsorption capacity increases with the increasing temperature, indicating that the adsorption is an endothermic process. This may be a result of increase in the mobility of the dye with increasing temperature [25]. An increasing number of molecules may also acquire sufficient energy to undergo an interaction with active sites at the surface. Furthermore, increasing temperature may produce a swelling effect within the internal structure of the PC enabling large dye molecule to penetrate further [26]. 3.7. Adsorption kinetics

(c) 100

Dye removal (%)

Salt 80

non NaCl Na2SO4

60

NaHCO3 K2CO3

40

Several models can be used to express the mechanism of solute adsorption onto an adsorbent. In order to design a fast and effective model, investigations were made on adsorption rate. For the examination of the controlling mechanisms of adsorption process, such as chemical reaction, diffusion control and mass transfer, several kinetics models are used to test the experimental data [27–30]. Table 2 The effect of initial pH of dye solutions on the maximum absorbance wavelength (λmax (nm)) of AB26, AG25 and AB7. pH

20

0 0

5

10

15

20

25

30

time (min) Fig. 5. The effect of salt on dye removal by PC (a) AB26, (b) AG25 and (c) AB7 (pH 2, T 25 °C, C0: 50 mg/L and PC: 1 g/L for AB26 and 2 g/L for AG25 and AB7).

2 4 6 8 10 12

λmax AB26

AG25

AB7

550 552 551 550 549 552

605 603 602 604 605 603

640 638 641 639 641 641

N.M. Mahmoodi et al. / Desalination 268 (2011) 117–125

45

(a)

40

70

35

60

30

Temperature (0C)

50

25

AB26 AG25 AB7

20 15 10

qe (mg/g)

qe (mg/g)

121

25 35 45 55 65

40 30 20

5 0 2

0

4

6

8

10

10

12

pH

0 0

Fig. 6. The effect of pH on the adsorption of dyes on PC (C0: 50 mg/L, T 25 °C and PC: 1 g/ L for AB26 and 2 g/L for AG25 and AB7).

5

10

15

20

25

30

35

40

Ce (mg/L)

(b) 40

The possibility of intraparticle diffusion resistance affecting adsorption was explored by using the intraparticle diffusion model as [27]:

35 Temperature (0C)

ð1Þ

+I

where qt, kp and I are amount of dye adsorbed (mg/g) at time t (min), the intraparticle diffusion rate constant (mg/g min1/2) and the intercept, respectively. Values of I give an idea about the thickness of the boundary layer. According to this model, the plot of uptake should be linear if intraparticle diffusion is involved in the adsorption process and if these lines pass through the origin then intraparticle diffusion is the rate controlling step [27]. When the plots do not pass through the origin, this is indicative of some degree of boundary layer control. This further shows that the intraparticle diffusion is not the only rate limiting step, but also other kinetic models may control the rate of adsorption. Pseudo-first order equation is generally represented as follows [28]: dqt = dt = k1 ðqe −qt Þ

ð2Þ

where qe and k1 are the amount of dye adsorbed at equilibrium (mg/g) and the equilibrium rate constant of pseudo-first order kinetics (1/min), respectively. After integration by applying conditions, qt = 0 at t = 0 and qt = qt at t = t, then Eq. (2) becomes logðqe −qt Þ = log ðqe Þ−ðk1 = 2:303Þt

ð3Þ

Data were applied to the pseudo-second order kinetic which is expressed as [29,30]: dqt = dt = k2 ðqe −qt Þ

ð4Þ

where k2 is the equilibrium rate constant of pseudo-second order (g/mg min). On integrating the Eqs. (4) and (5) is obtained. t = qt =

2 1 = k2 qe

+ ð1 = qe Þt

ð5Þ

To understand the applicability of the model, linear plots of t/qt versus t at different dye concentration values (25, 50, 75 and 100 mg/L) for the adsorption of dyes onto PC are shown in Fig. 8. The values of kp, I, r21 (correlation coefficient for intraparticle diffusion model), k1, k2, r22 (correlation coefficient for pseudo-first order adsorption kinetics) and r23 (correlation coefficient for pseudosecond order adsorption kinetics) were calculated and shown in

25

25 35 45 55 65

20 15 10 5 0 0

5

10

15

20

25

30

35

Ce (mg/L)

(c) 40 35 Temperature (0C)

30

qe (mg/g)

qt = kp t

qe (mg/g)

30 1=2

25

25

20

35 45

15

55 65

10 5 0 0

10

20

30

40

Ce (mg/L) Fig. 7. Adsorption isotherms for dyes on PC at different temperatures (a) AB26, (b) AG25 and (c) AB7 (pH 2, C0: 50 mg/L and PC:1 g/L for AB26 and 2 g/L for AG25 and AB7).

Table 3. Adsorption kinetics data of dyes showed that the rates of adsorption followed pseudo-second order kinetics.

3.8. Adsorption isotherms To optimize the design of an adsorption system for the adsorption of adsorbates, it is important to establish the most appropriate correlation for the equilibrium curves. Various isotherm equations such as Langmuir, Freundlich and Tempkin adsorption isotherms were studied.

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(a)

Table 3 Kinetics constants for dye adsorption at 25, 50, 75 and 100 mg/L dye concentrations (200 mL, pH 2 and PC: 1 g/L for AB26 and 2 g/L for AG25 and AB7).

1.4

t/qt (min.g/mg)

1.2

(qe)Exp. Intraparticle Dye concentration diffusion model (mg/L) I r21 kp

Dye (mg/L)

1

25 50 75 100

0.8 0.6 0.4 0.2 0 0

5

10

15

20

25

30

35

time (min)

(b) 3

t/qt (min.g/mg)

2.5 Dye (mg/L)

2

25 50 75 100

1.5 1

5

10

15

20

25

30

(c) 3

t/qt (min.g/mg)

2.5 Dye (mg/L)

1

3.52 7.75 0.70 7.89 6.44 12.24 0.75 19.20 8.82 12.30 0.84 36.64 9.95 12.81 0.86 39.35

0.23 0.31 0.34 0.36

0.49 0.76 0.91 0.93

23.70 42.55 55.56 62.11

0.05 0.02 0.01 0.01

1 1 1 1

AG25 25 50 75 100

11.00 20.04 27.58 31.3

1.63 3.16 4.38 5.56

4.11 6.35 8.33 6.15

0.64 9.33 0.73 15.55 0.57 21.50 0.89 20.46

1.03 0.62 0.58 0.29

0.94 0.96 0.96 0.87

11.27 20.75 28.49 34.01

0.17 0.06 0.04 0.01

1 1 1 1

AB7 25 50 75 100

11.27 19.72 26.74 31.60

1.72 3.13 4.34 5.31

3.87 6.03 7.61 8.27

0.69 9.91 0.75 15.03 0.77 21.98 0.81 26.95

0.90 0.57 0.57 0.56

0.95 0.95 0.97 0.98

11.55 20.49 27.86 33.33

0.13 0.05 0.03 0.02

1 1 1 1

1=n

log qe = log KF + ð1 = nÞlog Ce

0 5

10

15

20

25

ð8Þ

ð9Þ

where KF (L/mg) and 1/n are adsorption capacity at unit concentration and adsorption intensity, respectively. 1/n values indicate the type of isotherm to be irreversible (1/ n = 0), favorable (0 b 1/n b 1), unfavorable (1/n N 1) [3]. Eq. (9) can be rearranged to a linear form:

0.5

0

r23

23.05 40.50 52.13 57.46

qe = KF Ce

25 50 75 100

1.5

(qe)Cal. k2

where C0 is the initial dye concentration (mg/L). RL values indicate the type of isotherm to be irreversible (RL = 0), favorable (0 b RL b 1), linear (RL = 1) or unfavorable (RL N 1) [3]. The Freundlich equation is derived by assuming a heterogeneous surface with a non-uniform distribution of heat of adsorption over the surface. Freundlich equation can be expressed by [34]:

time (min)

2

r22

AB26 25 50 75 100

RL = 1 = ð1 + KL C0 Þ 0

(qe)Cal. k1

pseudo-second order

The essential characteristic of the Langmuir isotherm can be expressed by the dimensionless constant called equilibrium parameter, RL, defined by

0.5 0

Pseudo-first order

30

time (min) Fig. 8. Pseudo-second order adsorption kinetics of dyes on PC (a) AB26, (b) AG25 and (c) AB7 (pH 2, T 25 °C and PC:1 g/L for AB26 and 2 g/L for AG25 and AB7).

ð10Þ

Tempkin isotherm contains a factor that explicitly takes into the account adsorbing species adsorbent interactions. The Tempkin equation is given as [35,36]: qe = RT = b lnðKT Ce Þ

ð11Þ

which can be linearized as: In the Langmuir theory, the basic assumption is that the adsorption takes place at specific homogeneous sites within the adsorbent. This equation can be written as follows [31–33]: qe = Q 0 KL Ce = ð1 + KL Ce Þ

ð6Þ

where Ce, KL and Q0 are the equilibrium concentration of dye solution (mg/L), the Langmuir constant (L/mg) and the maximum adsorption capacity (mg/g), respectively. The linear form of Langmuir equation is: Ce = qe = 1 = KL Q0 + Ce = Q0

ð7Þ

qe = B1 lnKT + B1 lnCe

ð12Þ

where B1 = RT = b

ð13Þ

Tempkin equation assumes that the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent — adsorbate interactions and the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy [35,36]. A plot of qe versus ln Ce enables the determination of the isotherm constants B1 and KT from the slope

N.M. Mahmoodi et al. / Desalination 268 (2011) 117–125 Table 4 Isotherm constants for dye adsorption at different temperatures (200 mL, pH 2, C0: 50 mg/L and PC: 1 g/L for AB26 and 2 g/L for AG25 and AB7). Temperature Langmuir isotherm (°C) model

AB26 25 35 45 55 65 AG25 25 35 45 55 65

KL

RL

r42

KF

n

r52

62.89 63.29 64.52 67.11 66.22

0.23 0.27 0.31 0.33 0.51

0.08 0.07 0.06 0.06 0.04

1 1 0.99 0.99 1

19.47 20.53 23.53 25.64 29.31

3.31 3.38 3.73 3.90 4.39

0.98 3.87 0.98 4.66 0.99 8.60 0.99 11.81 0.99 26.74

37.45 42.19 37.59 38.61 39.37

0.10 0.14 0.17 0.21 0.28

0.13 0.11 0.17 0.19 0.22

0.17 0.12 0.11 0.09 0.07

0.13 0.15 0.10 0.10 0.08

1 7.25 1 7.70 1 8.51 1 9.84 0.99 11.77

0.99 7.95 0.96 8.72 0.98 9.71 0.98 10.3 0.98 10.96

2.36 2.41 2.49 2.69 3.04

2.58 2.72 2.87 2.87 2.92

0.98 0.98 0.98 0.99 0.99

1 1 0.99 1 1

KT

1.26 1.44 1.78 2.61 1.54

1.65 2.06 2.76 3.15 3.74

B1

r26

11.38 11.22 10.35 10.22 9.26

0.99 0.99 0.99 0.99 0.99

8.24 8.15 8.05 7.61 6.92

7.48 7.25 7.02 7.08 7.11

Table 5 Thermodynamic parameters of dye adsorption on PC (200 mL, pH 2, C0: 50 mg/L and PC: 1 g/L for AB26 and 2 g/L for AG25 and AB7). Temperature (°C)

Tempkin isotherm model

Q0

43.29 38.02 38.31 38.17 38.31

AB7 25 35 45 55 65

Freundlich isotherm model

0.99 0.99 0.99 0.99 0.99

0.98 0.97 0.96 0.97 0.97

123

Thermodynamic parameters ΔG0 (kJ/mol)

ΔH0 (kJ/mol)

ΔS0 (J/mol K)

AB26 25 35 45 55 65

− 3.44 − 3.95 − 4.46 − 4.97 − 5.48

11.81

51.17

AG25 25 35 45 55 65

− 1.65 − 2.07 − 2.50 − 2.93 − 3.35

11.04

42.58

AB7 25 35 45 55 65

− 4.38 − 5.20 − 6.02 − 6.84 − 7.65

2.02

81.88

    0 0 lnKc = ΔS = R − ΔH = RT and the intercept, respectively. KT is the equilibrium binding constant (L/mol) corresponding to the maximum binding energy and constant B1 is related to the heat of adsorption. The Q0, KL, RL, r24 (correlation coefficient for Langmuir isotherm), KF, n, r25 (correlation coefficient for Freundlich isotherm), KT, B1 and, r26 (correlation coefficient for Tempkin isotherm) are given in Table 4. Fig. 8 shows the adsorption isotherms of dyes (qe versus Ce) using PC at different temperatures. The results indicated that isotherm data of AB26 and AG25 followed Langmuir isotherm and isotherm data of AB7 followed Freundlich isotherm models.

3.9. Adsorption thermodynamics The thermodynamic parameters such as change in Gibbs energy (ΔG0: kJ/mol), enthalpy (ΔH0: kJ/mol) and entropy (ΔS0: kJ/mol K) were determined using the following equations [37]: Kc = CA = CS

0

0

ð16Þ

where R (8.314 J/mol K), T (K), CA, CS and Kc (L/g) are the gas constant, the absolute temperature, the amount of dye adsorbed on the adsorbent of the solution at equilibrium (mol/L), the equilibrium concentration of the dye in the solution (mol/L) and the standard thermodynamic equilibrium constant, respectively. By plotting a graph of ln Kc versus 1/T, the values ΔH0 and ΔS0 can be estimated from the slopes and intercepts, respectively (Fig. 9). Table 4 shows the negative values of ΔG0 and positive ΔH0. Data indicate that the AB26, AG25 and AB7 adsorption processes are endothermic processes. The positive value of ΔS0 suggests increased randomness at the solid/solution interface occurs in the internal structure of the adsorption of AB26, AG25 and AB7 dye onto PC. The positive values of ΔH0 indicate the presence of an energy barrier in the adsorption process and endothermic process [38]. The change in free energy for physisorption and chemisorption are between −20 and 0 kJ/mol and −80 to −400 kJ/mol, respectively [39]. The values of ΔG

ð14Þ

100

3

2 AB26 AG25 AB7

1.5 1

Desorption (%)

80

2.5

lnkc (L/g)

0

ΔG = ΔH −T ΔS

ð15Þ

60

AB26 AG25 AB7

40 20

0.5 0 0 2.90E-03 3.00E-03 3.10E-03 3.20E-03 3.30E-03 3.40E-03

0

2

4

6

8

10

12

14

pH

1/T (1/K) Fig. 9. Plots ln kC versus 1/T for adsorption of dyes.

Fig. 10. Effect of pH on desorption of dyes from PC (a) AB26, (b) AG25 and (c) AB7 (T 25 °C and PC: 1 g/L for AB26 and 2 g/L for AG25 and AB7).

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Fig. 11. SEM images of original and dye adsorbed PC after 20 min dye adsorption process (250 mL solution, C0: 50 mg/L, PC: 1 g/L for AB26 and 2 g/L for AG25 and AB7, pH 2, T 25 °C and agitating speed of 200 rpm).

in Table 5 are within the ranges of −20 and 0 kJ/mol indicating that the physisorption is the dominating mechanism. 3.10. Desorption studies Desorption studies help to elucidate the mechanism and recovery of the adsorbate and adsorbent. Desorption tests showed that maximum dye releasing of 93.16% for AB26, 97% for AG25, and 98% for AB7 were achieved in aqueous solution at pH 12 (Fig. 10). As the pH of the system increases, the number of negatively charged sites increased. A negatively charged site on the adsorbent favors desorption of dye anions due to the electrostatic repulsion. At pH 12, a significantly high electrostatic repulsion exists between the negatively charged surface of the adsorbent and anionic dye. 3.11. SEM analysis Scanning electron microscopy (SEM) has been a primary tool for characterizing the surface morphology and fundamental physical properties of the adsorbent surface. It is useful for determining the particle shape, porosity and appropriate size distribution of the adsorbent. Scanning electron micrographs of raw PC and dye adsorbed PC are shown in Fig. 11. From Fig. 11, it is clear that PC has considerable numbers of pores where, there is a good possibility for dyes to be trapped and adsorbed into these pores. The SEM of PC samples show very distinguished dark spots which can be taken as a sign for effective adsorption of dye molecules in the cavities and pores of this adsorbent.

4. Conclusions Kinetic, equilibrium and thermodynamic studies were done for the adsorption of Acid Black 26 (AB26), Acid Green 25 (AG25) and Acid Blue 7 (AB7) from aqueous solutions onto PC. Results of adsorption showed that PC can be effectively used as a biosorbent for the removal of anionic dyes. PC exhibited high adsorption capacities toward AB26, AG25 and AB7. The kinetics studies of dyes on PC indicated that the adsorption kinetics of dyes on PC followed the pseudo-second order at different dye concentration values. The equilibrium data have been analyzed. The results showed that the AB26 and AG25 followed Langmuir isotherm and AB7 followed Freundlich isotherm. Thermodynamic studies indicated that the dye adsorption onto PC was a spontaneous, endothermic, and physical reaction. Desorption studies were conducted and the results showed that at alkaline pH values high electrostatic repulsion existed between the negatively charged surface of the adsorbent and anionic dye. Based on the data of present study, PC is an eco-friendly adsorbent for dye removal from colored textile wastewater.

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