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Microporous and Mesoporous Materials 101 (2007) 388–396

Adsorption kinetics and thermodynamics of an anionicdye onto sepiolite

Mahir Alkan, Ozkan Demirbas� *, Mehmet Dogan

Balikesir University, Faculty of Science and Literature, Department of Chemistry, 10145 Balikesir, Turkey

Received 18 July 2006; received in revised form 2 December 2006; accepted 4 December 2006Available online 23 January 2007

Abstract

The adsorption kinetics of CI reactive blue 221 (RB221), an anionic dye, onto sepiolite was investigated in aqueous solution in a batchsystem with respect to stirring speed, contact time, initial dye concentration, pH, and temperature. Experimental results have shown thatthe acidic pH, increasing initial dye concentration and temperature favours the adsorption. Experimental data related to the adsorptionof RB221 on sepiolite under different conditions were applied to the pseudo-first-order equation, the pseudo-second-order equation andthe intra-particle diffusion equation, and the rate constants of first-order adsorption (k1), the rate constants of second-order adsorption(k2) and intra-particle diffusion rate constants (kint) were calculated, respectively. The experimental data fitted very well the pseudo-sec-ond-order kinetic model and also followed the intra-particle diffusion model up to 20 min, whereas diffusion is not only the rate control-ling step. The activation energy of system (Ea) was calculated as 7.73 kJ mol�1. The thermodynamics parameters of activation such asGibbs free energy, enthalpy, entropy were also evaluated and found that DG*, DH*, and DS* are 47.9 (49.4,50.0,51.3) kJ mol�1, 5.30(5.21,5.13,5.05) kJ mol�1, and �145.3 (�148.8,�142.7,�143.1) J K�1 mol�1 at 20 (30,40,50) �C, respectively. The results indicate thatsepiolite could be employed as a low-cost material for the removal of textile dyes from effluents.� 2006 Elsevier Inc. All rights reserved.

Keywords: Adsorption; Sepiolite; CI reactive blue 221; Dye; Kinetic models

1. Introduction

The treatment of textile waste comprising of dyestuffsand other non-biodegradable organics and inorganic posesconsiderable problems in the wastewater treatment indus-try. Conventional primary and secondary processes areinsufficient in removing these contaminants, with themajority of the research in this field conducted using ter-tiary treatments [1]. Reactive dyes are the most commondyes used due to their advantages, such as bright colours,excellent colourfastness and ease of application [2,3]. Theyexhibit a wide range of different chemical structures, pri-marily based on substituted aromatic and heterocyclicgroups. A large number of reactive dyes are azo com-

1387-1811/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.12.007

* Corresponding author. Tel.: +90 266 612 1278; fax: +90 266 612 1215.E-mail addresses: malkan@balikesir.edu.tr (M. Alkan), ozkan@bali-

kesir.edu.tr (O. Demirbas�), mdogan@balikesir.edu.tr (M. Dogan).

pounds that are linked by an azo bridge [4]. Many reactivedyes are toxic to some organisms and may cause directdestruction of creatures in water [5]. In addition, since reac-tive dyes are highly soluble in water, their removal fromeffluent is difficult by conventional physicochemical andbiological treatment methods [6,7]. In general, there are fivemain methods of reducing colour in textile effluent streams:adsorption, oxidation–ozonation, biological treatment,coagulation–flocculation, and membrane processes [8,9].

Adsorption phenomenon in solution systems plays avital role in many areas of practical environmental technol-ogy, which are mainly in water and wastewater treatmentdue to several advantages such as high efficiency, simpleoperation and easy recovery/reuse of adsorbent [1].Amongst the numerous techniques of dye removal, adsorp-tion is a procedure of choice for the removal of dissolvedorganic compounds from wastewater. Adsorption has aspecific advantage of removing the complete dye molecule,

Nomenclature

qe equilibrium dye concentration on adsorbent,mol g�1

R2 linear regression coefficientt time, sT temperature, KW mass of adsorbent, gqt the amount of dye adsorbed per unit mass of the

adsorbent at time t, mol g�1

C0 initial dye concentration in aqueous solution,mol l�1

k1 adsorption rate constant for pseudo-first-orderkinetic equation, s�1

k2 adsorption rate constant for pseudo-second-order kinetic equation, g mol�1 min�1

t1/2 the half-adsorption time of dye, skint intra-particle diffusion rate constant,

mol s�1/2 g�1

Ct dye concentration in solution at any time t,mol l�1

Ea activation energy, kJ mol�1

Rg gas constant, J K�1 mol�1

k0 Arrhenius factor, g mol�1 s�1

kB Boltzmann’s constanth Planck’s constantDG* free energy of activation, kJ mol�1

DH* enthalpy of activation, kJ mol�1

DS* entropy of activation, J K�1 mol�1

SO2CH2CH2OSO3H

NHC2H5

Fig. 1. The structure of CI RB221.

M. Alkan et al. / Microporous and Mesoporous Materials 101 (2007) 388–396 389

unlike certain removal techniques, which destroy only thedye chromophore leaving the harmful residual moieties inthe effluent [10]. Activated carbon is widely used as anadsorbent due to its high surface area, high adsorptioncapacity, but it is relatively high price, which limits theirusage [11]. For this reason, many researchers have investi-gated low-cost, locally available, biodegradable substitutesmade from natural sources to remove dyes from wastewa-ter [12–15]. Clays, such as sepiolite [16–18], zeolite [19],montmorillonite [20], perlite [21–23] and bentonite [24,25]are being considered as alternative low-cost adsorbents.

Sepiolite, which forms an important group of clay min-erals, is a natural hydrated magnesium silicate, (Si12)(Mg8)O30(OH6) Æ (OH2)4 Æ 8H2O and currently used in anumber of different applications such as many industrial,catalytic and environmental applications. Structurally, itis formed by blocks and channels extending in the fibredirection. A significant number of silanol (Si–OH) groupsare present at the surface of these minerals [26]. The pres-ence and concentration of surface functional groups playsan important role in the adsorption capacity and theremoval mechanism of the adsorbates [27]. Because of theirstructural morphology, sepiolites have received consider-able attention with regard to the adsorption of organicson the clay surfaces and to their use as support for catalysts[28]. The abundance and availability of sepiolite mineralreserves as a raw material source and its relatively low costguarantee its continued utilization in the future, and mostof the world sepiolite reserves are found in Turkey [16].

Several studies were achieved using sepiolite as an adsor-bent including catalyst support, wastewater treatment,solid wastes, reducing the toxic effect of some heavy metalsand pesticides [29]. Most literature on dye removal isrelated to cationic dyes. To our knowledge, compared withcationic dyes, only a little information exists on the use ofsepiolite, as an adsorbent for the removal of anionic dyesand also needs to research. The water-soluble anionic dyesare commonly used to dye fabrics like wool, nylon and silk.

Due to the weak interactions between the negativelycharged surface in clays and anionic dyes, a few studieson the adsorption of acid dyes have been carried out usingsepiolite as an adsorbent [16–18,30], but none of them hasinvestigated the kinetics and thermodynamics of adsorp-tion of RB221 onto sepiolite. In this study, it has beeninvestigated the adsorption kinetics of CI reactive blue221 (RB221), a anionic dye, on sepiolite from aqueoussolutions as a function of stirring speed, contact time,initial dye concentration, pH and temperature. The exper-imental data were analyzed using pseudo-first and second-order kinetics models, and intra-particle diffusion models.Furthermore, activation thermodynamics parameters werecalculated. Results of this study will be useful for futurescale up using this material as a low-cost adsorbent forthe removal of anionic dyes.

2. Material and methods

2.1. Sepiolite

RB221 was selected as a model reactive dye andobtained from Setas and Eksoy Textile Co. (Bursa, Tur-key). The chemical structure of RB221 is shown in Fig. 1.Sepiolite was obtained from Eskis�ehir, Turkey. The chem-ical composition of the sepiolite found in Turkey is given inTable 1; the cation exchange capacity (CEC), density andspecific surface area in Table 2 [17].

Table 1Chemical composition of sepiolite

Component Weight %

SiO2 53.47MgO 23.55CaO 0.71Al2O3 0.19Fe2O3 0.16NiO 0.43LoI 21.49

LoI: Loss of ignition.

Table 2Some physicochemical properties of sepiolite

Colour Cation exchangecapacity(meq 100 g�1)

Density(g cm�3)

Specific surfacearea (m2 g�1)

Particlesize (lm)

White 25.00 2.55 342 75

390 M. Alkan et al. / Microporous and Mesoporous Materials 101 (2007) 388–396

2.2. Purification of sepiolite

Sepiolite was treated before using in the experiments asfollows [31]: the suspension containing 10 g l�1 sepiolitewas mechanically stirred for 24 h, after waiting for about2 min, the supernatant suspension was filtered through fil-ter paper. The solid sample was dried at 105 �C for 24 h,ground then sieved by 75 lm sieves. The particles under75 lm were used in further experiments.

2.3. Method

RB221 was dried at 110 �C for 2 h before use in experi-ments. All solutions of RB221 were prepared with distilledwater. The adsorption kinetics of RB221 onto sepiolite wascarried out from its solution. For the experiments ofadsorption kinetics, 2 g sepiolite sample was added into aliter of RB221 solution at desired concentration, pH andtemperature. The pH of the solution was adjusted withNaOH or HCl solution by using an Orion 920A pH-meterwith a combined pH electrode. pH-meter was standardizedwith NBS buffers before every measurement. A preliminaryexperiment revealed that about 120 min periods is requiredfor the adsorption process to reach the equilibrium concen-tration. A magnetic stirrer at 20 �C and 500 rpm for120 min continuously agitated the mixture. Preliminaryexperiments showed that stirring speed has no importanteffect on adsorption rate. Therefore, the experiments madeat 500 rpm. The effect of temperature on the adsorptionrate has been studied at 20, 30, 40 and 50 �C. A constanttemperature bath was used to keep the temperature con-stant. At the end of the adsorption period, the solutionwas centrifuged for 15 min at 5000 rpm. The samples atappropriate time intervals were pipetted from the reactorby the aid of the very thin point micropipette, which pre-vents the transition to solution of sepiolite sample. Preli-minary experiments had shown that the effect of the

separation time on the amount of adsorbed dye was negli-gible. The absorbency of dye was measured three times foreach sample with a Cary j1Ej UV–Vis spectrophotometer(Varian) at 602 nm wavelength, at which the maximumabsorbency occurred. The amounts of dye adsorbed onsepiolite at any time, t, were calculated from the concentra-tions in solutions before and after adsorption. At anytimes, the amount of RB221 adsorbed (mol g�1) (qt), ontosepiolite was calculated from the mass balance equation asfollows:

qt ¼ ðC0 � CeÞVW

ð1Þ

where qt is the amount of adsorbed dye on sepiolite at anytime (mol g�1); C0 and Ce are the initial and equilibriumliquid-phase concentrations of RB221 (mol l�1), respec-tively; V is the volume of RB221 solution (l), and W isthe mass of sepiolite sample used (g) [32–34].

3. Results and discussion

3.1. Adsorption rate

To design an effective and quick adsorption model it wasconsidered necessary to carry out adsorption with kineticsviewpoint and effects of contact time, stirring speed, initialdye concentration, pH and temperature on the uptake rateof the dye were monitored very carefully. The kineticadsorption data was processed to understand the dynamicsof adsorption process in terms of the order of rateconstant.

3.1.1. Effect of contact time and initial dye concentration

The initial concentration provides an important drivingforce to overcome all mass transfer resistances of the dyebetween the aqueous and solid phases. The effect of con-centration on contact time was also investigated as a func-tion of initial dye concentration. The effect of initial dyeconcentration and contact time on the removal rate ofRB221 by sepiolite is shown in Fig. 2. As shown, theadsorption increases with increasing initial RB221 concen-tration. The removal of dye depends on the concentrationof the dye. Again, adsorption increased with an increasein contact time. The equilibrium was attained at 120 min.Similar result was found for the adsorption kinetics ofmalachite green onto activated carbon prepared fromTuncbilek lignite [35]. The amount of RB221 adsorbedat equilibrium increases from 2.350 · 10�6 to 3.584 ·10�6 mol/g by increasing initial RB221 concentration from2.5 · 10�5 to 10 · 10�5 mol/l under conditions of initial pH3 and 20 �C. The results show that dye uptake is rapid forthe first 20 min and thereafter it proceeds at a slower rateand finally attains saturation. The initial rapid phase maybe due to increased the number of vacant sites availableat the initial stage, as a result there exist increased concen-tration gradient between adsorbate in solution and adsor-bate in the adsorbent [36]. Generally, when adsorption

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

3.75

0 20 40 60 80 100 120t (min)

q t(m

ol g

-1)x

106

C0 (mol L-1)x105

: 10.0 : 7.5

pH : 3.0 : 5.0 T : 20 ºC : 2.5

Fig. 2. The effect of contact time and initial concentration on the removalrate of RB221 onto sepiolite from aqueous solutions.

M. Alkan et al. / Microporous and Mesoporous Materials 101 (2007) 388–396 391

involves a surface reaction process, the initial adsorption israpid. Then, a slower adsorption would follow as the avail-able adsorption site gradually decreases.

3.1.2. Effect of pHEffect of pH on the removal rate of RB221 is shown in

Fig. 3. As the pH increased, the removal rate decreased.The pH value of the dye solution plays an important rolein the whole adsorption process and particularly on theadsorption capacity. Similar observations have beenreported by other workers for adsorption of reactive dyesindicating that the carbon has a net positive charge on its

1.50

1.75

2.00

2.25

2.50

0 20 40 60 80 100 120t (min)

q t (

mol

g-1

) x

106

pH : 3 : 5

T : 20 ºC : 7 C0 : 2.5x10-5 mol L-1 : 9

Fig. 3. The effect of contact time and initial pH on the removal rate ofRB221 onto sepiolite from aqueous solutions.

surface [37]. In clay-aqueous systems, the potential of thesurface is determined by the activity of ions (e.g., H+ orpH), which react with the mineral surface. For clay miner-als the potential determining ions are H+ and OH� andcomplex ions formed by bonding with H+ and OH� [38].It is remarkably noted from zeta potential measurementsthat sepiolite has an isoelectrical point at pH 6.6 [39]. Inthis case, sepiolite surface has positive zeta potential belowpHiep and negative zeta potential above pHiep. The brokenbonds along the surface of the clay crystals result in hydro-lysis. In acid medium, the positive charge develops on thesurface of adsorbent and may be written as [32–34]

@SO–OHþHþ�@SO–OHþ2 ð2Þ

As can also be seen in Eq. (2), as the pH of the dye solu-tions becomes lower than pH 6.6, the association of dyeanions with more positively charged sepiolite surfacebecause of increasing SO–OHþ2 groups could more easilytake place. Since the solution is acidified by hydrochloricacid, the outer surface of positively charged interface willbe associated with Cl� ions. The chloride ions areexchanged with dye ions.

@SO–OHþ2 þDye��@SO–OHþ2 Dye� ð3Þ

This can also be explained as follows: at low pH, sepio-lite surface is closely associated with the hydronium(H3O+) ions. The surface of sepiolite becomes positivelycharged, thereby increasing electrostatic attractionsbetween negatively charged dye anions and positivelycharged adsorption sites and causing an increase in thedye adsorption. With the gradual increase in the pH ofthe solution, a decrease in the positive charge on the oxideor solution interface has been observed and the adsorbentsurface appears negatively charged due to deprotonationof the adsorbent surface. The removal of dye decreases athigher pH values may be due to the abundance of OH�

ions and because of electrostatic repulsion between thenegatively charged surface of adsorbent and the anionicdye molecules [40].

3.1.3. Effect of temperature

A study of the temperature dependence of adsorptionreactions gives valuable knowledge about the enthalpyand entropy changes during adsorption. Fig. 4 exhibitscontact time versus adsorbed amount graph at differenttemperatures. The equilibrium adsorption capacity ofRB221 onto sepiolite was found to increase with increasingtemperature, increasing from 3.521 · 10�6 mol g�1 at 20 �Cto 4.360 · 10�6 mol g�1 at 40 �C indicating that the dyeadsorption on the adsorbent was favored at higher temper-atures. The fact that the adsorption rate of RB221 ontosepiolite at various temperatures increases may be due toan increase in the mobility of RB221 ions with an increasein the temperature in the solution [41,42]. This effect sug-gests that an explanation of the adsorption mechanismassociated with the removal of RB221 onto sepioliteinvolves a physical process. A further verification of the

2.40

2.70

3.00

3.30

3.60

3.90

4.20

4.50

0 20 40 60 80 100 120t (min)

q t(m

ol g

-1)

x 10

6

Temperature (ºC) : 50 : 40

pH : 3.0 : 30 C0 : 10x10-5 mol L-1 : 20

Fig. 4. The effect of contact time and temperature on the removal rate ofRB221 onto sepiolite from aqueous solutions.

Intr

a-p

arti

cle

diff

usi

on

(kd

if)

Mas

s

tran

sfer

t 1/2

(min

)

(fo

rse

con

d-

ord

er)

g�1)

R2

kd

if-2

·10

3

(mo

lm

in�

1/2

g�1)

R2

kd

if-3

·10

3

(mo

lm

in�

1/2

g�1)

R2

R2

0.99

956

.00.

999

8.1

0.99

90.

491

2.13

8

0.99

888

.81.

000

11.5

1.00

00.

503

2.28

40.

999

123.

10.

999

17.7

0.99

80.

518

2.29

3

1.00

012

6.5

0.99

818

.51.

000

0.67

02.

549

0.99

941

.00.

998

6.1

0.99

90.

550

1.49

70.

999

51.4

0.99

96.

61.

000

0.65

21.

778

0.99

948

.31.

000

3.1

0.99

90.

602

1.80

0

1.00

015

4.2

1.00

010

.50.

999

0.57

02.

970

0.99

919

0.0

0.99

820

.60.

996

0.68

73.

196

0.99

921

6.5

0.99

845

.00.

999

0.67

53.

419

392 M. Alkan et al. / Microporous and Mesoporous Materials 101 (2007) 388–396

endothermic nature of the process was done by calculatingthe half-life of process at each temperature, which wasfound to increase with increasing temperatures (Table 3).

lcu

late

d

·10

6

ol

g�1)

kd

if-1

·10

3

(mo

lm

in�

1/2

8410

3.0

1013

6.1

2026

2.2

8427

5.3

3584

.782

75.3

5268

.1

9225

1.1

2030

0.6

2239

2.4

3.2. Adsorption kinetics

Several kinetic models are used to examine the control-ling mechanism of adsorption process such as chemicalreaction and diffusion control.

ion

on

tose

pio

lite

Sec

on

d-o

rder

·10

2

in�

1)

R2

k2

·10�

5

(gm

ol�

1

min�

1)

Exp

erim

enta

l

qe

·10

6

(mo

lg�

1)

Ca

qe (m

10.

999

1.23

12.

350

2.3

51.

000

1.81

32.

774

2.8

11.

000

2.44

13.

167

3.2

50.

999

2.69

73.

521

3.5

31.

000

1.00

62.

120

2.1

90.

999

0.87

42.

060

2.0

80.

999

0.64

02.

020

2.0

10.

999

3.00

13.

700

3.7

00.

999

3.26

93.

910

4.0

90.

999

3.63

84.

360

4.5

3.2.1. The first-order kinetic model

The rate constant of adsorption is determined from thefirst-order rate expression given by Lagergren and Svenska[43].

lnðqe � qtÞ ¼ ln qe � k1t ð4Þ

where qe and qt are the amounts of dye adsorbed (mol/g) atequilibrium and at time t (min), respectively, and k1 is therate constant of adsorption (min�1). The values of k1 andcorrelation coefficients at different conditions are presentedin Table 3.

Tab

le3

Kin

etic

sva

lues

calc

ula

ted

for

RB

221

adso

rpt

Tem

per

atu

re

(�C

)

Co

nce

ntr

atio

n

·10

5(m

ol

l�1)

pH

Fir

st-o

rder

R2

k1

(m

202.

53

0.99

94.

9

205.

03

0.94

68.

920

7.5

30.

971

7.9

2010

.03

0.93

49.

0

202.

55

0.99

84.

920

2.5

70.

994

4.8

202.

59

0.97

74.

1

3010

.03

0.95

65.

440

10.0

30.

997

6.4

5010

.03

0.99

86.

2

3.2.2. The second-order kinetic model

The second-order kinetic model is expressed as [44]

tqt

¼ 1

k2q2e

þ 1

qe

t ð5Þ

where k2 is the second-order constant (g/mol min) and canbe determined experimentally from the slope and interceptof plot t/qt versus t.

3.2.3. Intra-particle diffusionAn empirically found functional relationship, common

to the most adsorption processes, is that the uptake varies

20

30

40

50

60

/qt (

min

g m

ol-1

)x10

-6

pH : 3 : 5 : 7 T : 20 ºC : 9 C0 : 2.5x10-5 mol L-1

M. Alkan et al. / Microporous and Mesoporous Materials 101 (2007) 388–396 393

almost proportionally with t1/2, the Weber–Morris plot,rather than with the contact time t [45].

qt ¼ kintt1=2 þ C ð6Þ

where kint is the intra-particle diffusion rate constant.According to Eq. (6), a plot of qt versus t1/2 should be astraight line with a slope kint and intercept C when adsorp-tion mechanism follows the intra-particle diffusion process[46].

0

10

0 20 40 60 80 100 120t (min)

t

Fig. 6. Second-order kinetic equation for adsorption of RB221 onsepiolite at different initial pHs.

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120t (min)

t/qt (

min

g m

ol-1

)x10

-6

Temperature (ºC) : 50 : 40 : 30 pH : 3.0 : 20 C0 : 10x10-5 mol L-1

Fig. 7. Second-order kinetic equation for adsorption of RB211 onsepiolite at different temperatures.

3.2.4. Kinetics analysis

The kinetic parameters for adsorption of RB221 underdifferent conditions were calculated from Eqs. (4)–(6) andare given in Table 3. The correlation coefficients (R2), forthe first-order kinetic model are between 0.934 and 0.999,and the correlation coefficients (R2), for the pseudo-sec-ond-order kinetic model are in the range of 0.999–1.000.Again, the experimental qe values agree with the calculatedones, obtained from the linear plots (Table 3). Calculatedcorrelation coefficients (R2) are closer to unity for sec-ond-order kinetics model; therefore the adsorption kineticscould well be approximated more favorably by second-order kinetic model for RB221 adsorption, based on theassumption that the step may have chemisorption, providesthe best correlation of the data. The k2 (g/mol min) and R2

values were calculated from Figs. 5–7 and listed in Table 3.Similar phenomena have been observed in the biosorptionof remazol black B on biomass [47], adsorption of congored [48] and 2-chlorophenol [49] on coir pith carbon andthe adsorption of methylene blue on perlite [34]. Moreover,the pseudo-second-order rate constants indicate a steadyincrease from 1.231 · 105 to 3.638 · 105 g mol�1 min�1

with an increase in the solution temperatures from 20 to50 �C (Table 3).

For intra-particle diffusion model, Ho [50] pointed outthat it is essential for the qt versus t1/2 plots to go thoughthe origin if the intra-particle diffusion is the sole rate lim-iting step. Generally, if the adsorption steps are indepen-

0

10

20

30

40

50

60

0 20 40 60 80 100 120t (min)

t/qt (

min

g m

ol-1

)x10

-6

C0 (mol L-1)x105

: 2.5 : 5.0 : 7.5 pH : 3.0 : 10.0 T : 20 ºC

Fig. 5. Second-order kinetic equation for adsorption of RB211 onsepiolite at different initial dye concentrations.

dent of one another, the plot of qt versus t1/2 usuallyshows two or more intercepting lines depending upon theexact mechanism [51]. The intra-particle diffusion plotsfor the effect of pH are given in Fig. 8 (other figures notshown). Table 3 shows the corresponding model fittingparameters. In the present study, any plot did not passedthrough the origin. The plots present multilinearity, indi-cating that three steps take place. The first, sharper portionis attributed to the diffusion of adsorbate through the solu-tion to the external surface of adsorbent or the boundarylayer diffusion of solute molecules. The second portiondescribes the gradual adsorption stage, where intra-particlediffusion is rate limiting. The third portion is attributed tothe final equilibrium stage [52]. Values of the intra-particlediffusion constants (kint1, kint2 and kint3) were obtainedfrom the slopes of the linear portions of the plots and arelisted in Table 3. The correlation coefficients for theintra-particle diffusion model (R2) were 0.996 and 1.000.These values indicate that the adsorption of RB221 ontosepiolite may be followed by an intra-particle diffusion up

1.70

1.80

1.90

2.00

2.10

2.20

2.30

2.40

2 4 6 8 10 12t1/2 (min)1/2

q t (

mol

g-1

)x10

6

pH : 3 : 5

T : 20 ºC : 7 C0 : 2.5x10-5 mol L-1 : 9

kint1

kint2

kint3

Fig. 8. Intra-particle diffusion plots for adsorption of RB221 ontosepiolite at different pHs.

12.4

12.5

12.6

12.7

12.8

12.9

3.00 3.13 3.25 3.38 3.50

1/T (K-1)x103

lnk 2

C0 : 1.0x10-4 M pH : 3

Fig. 9. Arrhenius plots for adsorption of RB211.

394 M. Alkan et al. / Microporous and Mesoporous Materials 101 (2007) 388–396

to 20 min. This indicates that although intra-particle diffu-sion was involved in the adsorption process, it was not therate-controlling step. The values of kint increased withincreasing initial RB221 concentration. The driving forceof diffusion is very important for adsorption processes.Generally, the driving force changes with the adsorbateconcentration in the bulk solution. The increase of adsor-bate concentration results in an increase of the drivingforce, which will increase the diffusion rate of RB221.

A comparison between the adsorption orders and mod-els of basic dyes on various adsorbents are present in Table4. As seen in Table 4, similar results were found for basicred 22 on pith, for methylene blue on perlite and for meth-ylene blue on fly ash.

3.3. Activation thermodynamic parameters

Because the k2 values have been determined, severalthermodynamic parameters including the Arrhenius activa-

Table 4Adsorption order and mechanism of some dyes on various adsorbents

Adsorbents Reaction order Adsorbat

Fly ash Pseudo-second-order MethylenBiosorbent Pseudo-first-order MethylenBottom ash Pseudo-first-order QuinolineCalcined alunite Pseudo-second-order ReactiveBagasse fly ash Pseudo-second-order Orange-GActivated carbon First order MethylenPith Pseudo-second-order Basic redWood – AstrazonPerlit Pseudo-second-order MethylenPerlit Pseudo-second-order Methyl vPerlit Pseudo-second-order Victoria bModified peat–resin particle – Basic dyeSepiolite Pseudo-second-order RB221

tion energy (Ea), activation free energy change (DG*), acti-vation enthalpy change (DH*), and activation entropychange (DS*) can be calculated by using the followingequations [34]:

ln k2 ¼ ln A� Ea

RTð7Þ

ln k2 ¼kBT

hK� ð8Þ

DG� ¼ �RT ln K� ð9ÞDH � ¼ Ea � RT ð10Þ

DS� ¼ DH � � DG�

Tð11Þ

where A is the Arrhenius factor, kB and h are Boltzmann’sand Planck’s constants, respectively, R is the gas constant,and K* is the equilibrium constant at temperature T. A lin-ear plot of lnk2 versus 1/T for the adsorption of RB221onto sepiolite is constructed to generate the Ea valuefrom the slope (Fig. 9). The result obtained is 7.73 kJ mol�1

with a linear regression coefficient 0.9973. Moreover, thevalues of DG*, DH*, and DS* are 47.9 (49.4,50.0,51.3)kJ mol�1, 5.30 (5.21, 5.13,5.05) kJ mol�1, and �145.3

es Adsorption mechanism Ref.

e blue Particle diffusion [53]e blue – [54]yellow – [55]

dyes Mass transfer [7], Methyl violet – [56]e blue Intra-particle diffusion [46]22 Intra-particle diffusion [57]blue Intra-particle diffusion [58]e blue Intra-particle diffusion [34]iolet Intra-particle diffusion [32]lue – [33]

s Intra-particle diffusion [59]– In this study

M. Alkan et al. / Microporous and Mesoporous Materials 101 (2007) 388–396 395

(�148.8,�142.7,�143.1) J K�1 mol�1 at 20 (30, 40,50) �C,respectively.

The low DH* values for RB221 give a clear evidence thatthe interactions between RB221 and the surface hydroxylgroups of sepiolite may be weak. On the other hand, thepositive values of Ea, DG*, and DH* indicate the presenceof an energy barrier in the adsorption process. The positivevalues for these parameters are quite common because theactivated complex in the transition state is in an excitedform. The negative values of DS* suggest decreased ran-domness at the solid/solution interface and no significantchanges occur in the internal structure of the adsorbentthrough the adsorption of RB221 onto sepiolite.

3.4. Conclusions

Sepiolite has proved to be an effective adsorbent for theremoval of anionic dye via adsorption from aqueous solu-tion. Equilibrium adsorption was achieved in about 2 h.The adsorption is highly dependent on concentration, pHand temperature of the solution. pH affects the surfacecharge of the adsorbent and the degree of ionization ofadsorbate. Adsorption kinetic follows pseudo-second-order kinetic model confirming the chemisorption ofRB221 onto sepiolite. By second-order model to studythe mechanism of adsorption, calculated qe values agreedwell with the qe experimental values. Kinetic results showedthat the adsorption is a three-stage process comprising afast initial phase, a second slow and then a slower thirdphase. In the first phase, RB221 is adsorbed at the outersites of the adsorbent particles in a fast process that dom-inates the initial kinetics of adsorption. In the second andthird phases, dye molecules slowly diffuse inward andadsorb to the inner sites of the adsorbents. The tempera-ture effect was also used to calculate the thermodynamicactivation enthalpy, entropy, and free energy of adsorp-tion. The analysis of these thermodynamic parameters sug-gested that the adsorption is mainly physical because of thelow adsorption activation enthalpy.

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