Desalination 267 (2011) 64–72

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Desalination

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Dye removal from colored textile wastewater using chitosan in binary systems

Niyaz Mohammad Mahmoodi a, Raziyeh Salehi b, Mokhtar Arami b,⁎, Hajir Bahrami b

a Department of Environmental Research, Institute for Color Science and Technology, Tehran, Iranb Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran

⁎ Corresponding author. Tel.: +98 21 64542614; fax:E-mail addresses: nm_mahmoodi@aut.ac.ir (N.M. M

(M. Arami).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.desal.2010.09.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 May 2010Received in revised form 7 September 2010Accepted 8 September 2010

Keywords:Dye removalChitosanBinary systemColored textile wastewaterAnionic dye

In this paper, the removal of two anionic dyes from textile effluent in single and binary systems wasinvestigated. Direct Red 23 and Acid Green 25 were used as anionic dyes. The surface characteristics ofchitosan were investigated using Fourier transform infrared. The effects of operational parameters such aschitosan dosage, initial dye concentration, salt and pH on dye removal were studied. The isotherms of dyeadsorption were investigated. It was found that the isotherm data of Direct Red 23 and Acid Green 25 in singleand binary systems followed Tempkin isotherm. In addition adsorption kinetics of dyes was studied in singleand binary systems and rate sorption was found to conform to pseudo-second order kinetics with a goodcorrelation. Results indicated that chitosan could be used as a biosorbent to remove the anionic dyes fromcontaminated watercourses in both single and binary systems of pollutants.

+98 21 66400245.ahmoodi), arami@aut.ac.ir

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Many industries such as textile, paper, plastics and dyestuffsconsume substantial volume of water and also use chemicals duringmanufacturing and dyes to color their products. As a result, theygenerate a considerable amount of polluted wastewater [1–8]. Theirtoxic effluents are a major source of aquatic pollution and will causeconsiderable damage to the receiving waters if discharged untreated[4]. The pollution is characterized by biochemical oxygen demand(BOD), chemical oxygen demand (COD), suspended solids, bad smell,toxicity (high concentration of nutrients, presence of chlorinatedphenolic compounds, sulfur and lignin derivatives, etc.) and especiallycolor [4,5]. Color is the first contaminant to be recognized inwastewater and the presence of very small amounts of dyes inwater is highly visible and undesirable [7,8].

It is now recognized that adsorption using low-cost adsorbents isan effective and economic method for water decontamination.Chitosan is derived by deacetylation of the naturally occurringbiopolymer (chitin) which is the second most abundant polysaccha-ride in the world after cellulose. This natural polymer possessesseveral intrinsic characteristics thatmake it an effective biosorbent forthe removal of color. The majority of commercial polymers and ion-exchange resins are derived from petroleum-based raw materialsusing processing chemistry that is not always safe or environmental

friendly. Today, there is a growing interest in developing natural low-cost alternatives to synthetic polymers [9]. Chitin, found in theexoskeleton of crustaceans, the cuticles of insects and the cells walls offungi, is the most abundant amino polysaccharide in nature [10–12].This low-cost material is a linear homopolymer composed of β (1-4)-linked N-acetyl glucosamine (Fig. 1). It is structurally similar tocellulose, but it is an amino polymer and has acetamide groups at theC-2 positions in place of the hydroxyl groups. The presence of thesegroups is highly advantageous in providing distinctive adsorptionfunctions and conducting modification reactions. The raw polymer isonly commercially extracted from marine crustaceans primarilybecause a large amount of waste is available as a by-product of foodprocessing [10].

Synthetic dyes are an important group of recalcitrant organics andare often found in the environment as a result of their wideapplication. These industrial pollutants are common contaminantsin wastewater and are difficult to degrade due to their complexaromatic structure and synthetic origin. They are produced on a largescale. Although the exact number (and also the amount) of the dyesproduced in the world is not known, there are estimated to be morethan 100,000 commercially available dyes. Many of them are knownto be toxic or carcinogenic [13].

A literature review showed that chitosan has been investigated byseveral researchers as a biosorbent to remove dissolved dyes fromaqueous solutions in single systems of dyes [14–28] but dye removalusing chitosan was not studied in binary systems. In this study,chitosan was used as an adsorbent to remove dyes (Acid Green 25(AG25) and Direct Red 23 (DR23)) from single (sin.) and binary (bin.)systems. Effects of operational parameters such as chitosan dosage,

H

H

H

H

O

H

O

HONH2

OH

n

(a)

H

H

H

H

O

H

O

HONH

OH

n

CH3

O

(b)

H

H

H

H

O

H

O

HOOH

OH

n

(c)

Fig. 1. The chemical structure of (a) chitosan, (b) chitin and (c) cellulose.

65N.M. Mahmoodi et al. / Desalination 267 (2011) 64–72

initial dye concentration, salt and pH were investigated to evaluatethe adsorption capacity of chitosan in single and binary systems. TheLangmuir, Freundlich and Tempkin isotherms were used to fit theequilibrium data. Pseudo-first order, pseudo-second order andintraparticle diffusion kinetics models were studied.

O

O

HN

NH

NaO

NaO

(a)

N

OH

NH

NaO3S

NNH

CH3

O

(b)

Fig. 2. The chemical structure of dyes (a)

2. Experimental

2.1. Chemicals and methods

Chitosan (degree of deacetylation: 85%; averagemolecular weight:1000 kD) was supplied byMerck Enterprises Co. and used as received.Anionic dyes used in this study were Acid Green 25 (AG25) and DirectRed 23 (DR23). Dyes were purchased from Ciba Ltd and used withoutfurther purification. The chemical structure of dyes was shown inFig. 2. All other chemicals were of Analar grade and purchased fromMerck (Germany). UV–vis spectrophotometer CECIL 2021 wasemployed for absorbance measurements of samples. In order toinvestigate the surface characteristics of chitosan, Fourier transforminfrared (FTIR, Perkin-Elmer Spectrophotometer Spectrum One) inthe range 450–4000 cm−1 was studied.

2.2. Adsorption procedure

The dye adsorption measurements were conducted by mixingvarious amounts of chitosan (0.0625–0.5 g) for DR23 and AG25 insingle (sin.) and binary (bin.) systems of these dyes in jars containing200 mL of a dye solution at various pHs (2–10). The solution pH wasadjusted by adding a small amount of H2SO4 or NaOH. Dye solutionswere prepared using distilled water to prevent and minimize possibleinterferences in this study. Experiments were carried out at differentdye concentrations using 0.25 g/L chitosan for DR23 and AG25 insingle and binary systems at pH 2 and 25 °C for 30 min to attainequilibrium conditions (Table 1).

Dye concentrations in binary systems were calculated usingEqs. (1) and (2). For a binary system of components A and Bmeasured at λ1 and λ2, respectively, to give optical densities of d1 andd2 [29]:

CA = kB2d1−kB1d2ð Þ = kA1kB2−kA2kB1ð Þ ð1Þ

CB = kA1d2−kA2d1ð Þ= kA1kB2−kA2kB1ð Þ ð2Þ

where kA1, kB1, kA2, and kB2 are the calibration constants forcomponents A and B at the twowavelengths of λ1 and λ2, respectively.

CH3

CH3

3S

3S

N

SO3Na

OH

NH

ON

Acid Green 25 and (b) Direct Red 23.

Table 1The used initial dye concentration in single and binary systems of dyes.

Single system Binary system

C0, AG25 (sin.) C0,DR23 (sin.) C0, AG25 (bin.) C0,DR23 (bin.)

25 25 12.5 12.550 50 25 2575 75 37.5 37.5100 100 50 50

66 N.M. Mahmoodi et al. / Desalination 267 (2011) 64–72

The changes of absorbance were determined at certain timeintervals (2.5, 5, 7.5, 10, 15, 20, 25 and 30 min) during the adsorptionprocess. After experiments, the samples were centrifuged by HettichEBA20 and then the dye concentrationwas determined. To investigatethe salt effect on dye removal efficiency, Na2SO4, NaHCO3 and NaClwere added to dye solution. The results were verified with theadsorption isotherms (Freundlich, Langmuir and Tempkin) andkinetics (pseudo-first order, pseudo-second order and intraparticlediffusion model).

3. Results and discussion

3.1. Surface characteristics

Fig. 3 shows the FTIR spectra of raw and dye adsorbed of chitosan(raw chitosan (Fig. 3a), AG25 adsorbed chitosan (sin.) (Fig. 3b), DR23adsorbed chitosan (sin.) (Fig. 3c) and AG25–DR23 adsorbed chitosan(bin.) (Fig. 3d)). The FTIR spectrum of raw chitosan (Fig. 3a) showedthat the peak positions were at 3422, 1658, 1593, 1424, 1314 and1081 cm−1. The band at 3422 cm−1 was due to O–H and N–Hstretching. While the bands at 1658 cm−1 and 1593 cm−1 reflectedthe carbonyl group stretching (amide) and N–H bending, respectively.Bands at 1314 and 1081 cm−1 corresponded to C–H bending and C–Ostretching, respectively [30,31]. Peak intensity decreased after theabsorbing of dye for both single and binary systems of dyes.

3.2. Effect of operational parameters on dye removal

3.2.1. Adsorbent dosage effectThe effect of chitosan dosages on the amount of dye adsorbed was

investigated by contacting 200 mL of dye solution with initial dyeconcentration of 50 mg/L using jar test at room temperature (25 °C)for 30 min. Different amounts of chitosan (0.0625, 0.125, 0.25 and0.5 g) for DR23 and AG25were applied. After equilibrium, the sampleswere centrifuged and the concentration in the supernatant dyesolution was analyzed. Fig. 4 shows the effect of adsorbent dosage on

0.0

0.1

0.2

0.3

0.4

0.5

0.6

5001000150020002500300035004000

Wavenumber (cm-1)

Abs

orba

nce (a)

(b)

(c)

(d)

Fig. 3. FTIR spectra (a) raw chitosan (b) dye adsorbed chitosan (AG25; sin.) (c) dyeadsorbed chitosan (DR23; sin.) and (d) dye adsorbed chitosan (AG25–DR23; bin.).

the dye removal from single and binary systems of dyes. Thepercentage removal increased with the adsorbent dosage up to acertain limit and then it reached a constant value. Optimum adsorbentdosage for both single and binary systems of dyes (AG25 and DR23)was 0.05 g of chitosan for 200 mL of 50 mg/L dye solution. Theincrease in adsorption with adsorbent dosage can be attributed toincreased adsorbent surface and availability of more adsorption sites.However, if the adsorption capacity was expressed in mg adsorbedper gram of material, the capacity decreased with the increasingamount of sorbent. This may be attributed to overlapping oraggregation of adsorption sites resulting in a decrease in totaladsorbent surface area available to the dye and an increase indiffusion path length [16].

3.2.2. Dye concentration effectThe effects of initial dye concentration of AG25 and DR23 in single

and binary systems of dyes on the percentage of dye removal werestudied. Chitosan (0.05 g for AG25 and DR23 in single and binarysystems of dyes) was added to 200 mL of AG25 and DR23 in single andbinary systems of dyes at different dye concentrations of 25, 50, 75and 100 mg/L. These experiments were performed at pH 2. For bothsingle and binary systems of dyes, the equilibrium capacity decreasedwith an increase in the initial concentrations as shown in Fig. 5.

The amount of the dye adsorbed onto chitosan in both single andbinary systems of dyes increased with an increase in the initial dyeconcentration of solution if the amount of adsorbent was keptunchanged. This is due to the increase in the driving force of theconcentration gradient with the higher initial dye concentration. Atlow initial concentration, the adsorption of dyes by chitosan is veryintense and reaches equilibrium very quickly. This indicates thepossibility of the formation of monolayer coverage of the molecules atthe outer interface of the chitosan. At a fixed adsorbent dosage, theamount adsorbed increased with increasing concentration of solution,but the percentage of adsorption decreased. In other words, theresidual concentration of dye molecules will be higher for higherinitial dye concentrations. In the case of lower concentrations, theratio of initial number of dye molecules to the available adsorptionsites is low and subsequently the fractional adsorption becomesindependent of initial concentration [13,27,28].

3.2.3. Salt effectTo investigate salt effect on dye removal efficiency, 0.001 mol of

Na2SO4, NaHCO3 and NaCl were added to the dye solution. Fig. 6illustrates that the dye removal capacity of dyes (single and binarysystems) by chitosan is decreased in the presence of salts becausethese salts have small molecules and compete with dyes in adsorptiononto chitosan.

3.2.4. pH effectAccording to Kumar [19] and Yoshida [32], at lower pH more

protons will be available to protonate amino groups of chitosan toform groups NH3

+, thereby increasing electrostatic attractions be-tween negatively charged dye anions and positively chargedadsorption sites and causing an increase in dye adsorption. Thisexplanation agrees with our data on pH effect. It can be seen that thepH of aqueous solution plays an important role in the adsorption ofanionic dyes onto chitosan. The chitosan contains amino group, –NH2,which is easily protonated to form –NH+

3, in acidic solutions. Thehigh adsorption capacity is due to the strong electrostatic interactionbetween the –NH+

3 of chitosan and dye anions [33].The effect of pH on the adsorption of AG25 and DR23 in single and

binary systems of dyes onto chitosan is shown in Fig. 7. The maximumdye adsorption occurred at pH 2.

DR23 and AG25 dyes are dissociated to polar groups (R-SO3−).Chitosan is comprised of various functional groups such as amino,hydroxyl and carbonyl which could also be affected by the pH of

Adsorbent (g/L)

Adsorbent (g/L)

Adsorbent (g/L) Adsorbent

(g/L)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)

0.5

0.25

0.125

0.0625

(a)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

) 0.5

0.25

0.125

0.0625

(b)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

) 0.5

0.25

0.125

0.0625

(c) (d)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)

0.5

0.25

0.125

0.0625

Fig. 4. Effect of adsorbent dosage on dye adsorption onto chitosan (a) AG25 (sin.) (b) DR23 (sin.) (c) AG25 (bin.) and (d) DR23 (bin.) (T: 25 °C, agitation speed: 200 rpm, pH: 2 andinitial dye concentration: 50 mg/L).

67N.M. Mahmoodi et al. / Desalination 267 (2011) 64–72

solutions. Therefore, at different pH values, the electrostatic attractionas well as the organic property and structure of dye molecules andchitosan could play very important roles in the dye adsorption onchitosan. At pH 2, a significantly high electrostatic attraction existsbetween the positively charged surfaces of the adsorbent, due to theionization of functional groups of adsorbent and negatively chargedanionic dye. As the pH of the system increases, the numbers ofnegatively charged sites are increased. A negatively charged site onthe adsorbent does not favor the adsorption of anionic dyes due to theelectrostatic repulsion. Also, lower adsorption of AG25 and DR23 insingle and binary systems of dyes at alkaline pH is due to the presenceof excess OH− ions destabilizing anionic dyes and competing with thedye anions for the adsorption sites. The effective pH was 2 and it wasused in further studies.

3.3. Adsorption isotherm models in single and binary systems of dyes

In order to optimize the design of an adsorption system to removedyes from solutions, it is important to establish the most appropriatecorrelation for the equilibrium curve. Several isotherm models havebeen used in the literatures to describe the experimental data ofadsorption isotherms. The Langmuir model is the most frequentlyemployed model and given by [34]

qe = Q0KLCe = 1 + KLCeð Þ ð3Þ

where qe, Ce, Q0 and KL are the amount of solute adsorbed atequilibrium (mg/g), the concentration of adsorbate at equilibrium(mg/L), maximum adsorption capacity (mg/g) and Langmuir constant(L/mg), respectively.

The essential characteristics of the Langmuir isotherm can beexpressed by a dimensionless constant called equilibrium parameter,RL, which is defined by the following equation [35]:

RL = 1= 1 + KLC0ð Þ ð4Þ

where C0 is the initial dye concentration. The nature of the adsorptionprocess to be either unfavorable (RLN1), linear (RL=1), favorable(0bRLb1) or irreversible (RL=0).

In this work, an extended Langmuir model (Eq. (5)) was employedto fit the experimental data [36].

qe;I = Q0KL;I Ce;I

� �= 1 + ∑KL;I Ce;I

� �ð5Þ

where KL, I is the adsorption equilibrium constant of dye I inmixed dyesystem.

In dye adsorption from binary systems, the amounts of dyeadsorbed were expressed as

qe;1 = KL;1Q0;1Ce;1

� �= 1 + KL;1Ce;1 + KL;2Ce;2

� �ð6Þ

qe;2 = KL;2Q0;2Ce;2

� �= 1 + KL;1Ce;1 + KL;2Ce;2

� �ð7Þ

According to Eqs. (6) and (7), we have

KL;2Ce;2

� �= KL;1Ce;1

� �= Q0;1qe;2

� �= qe;1Q0;2

� �ð8Þ

Dye (mg/L)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)

255075100

(a)

Dye (mg/L)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)

255075100

(b)

Dye (mg/L)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)

12.52537.550

(c)

Dye (mg/L)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)12.5

25

37.550

(d)

Fig. 5. Effect of initial dye concentration on dye adsorption onto chitosan (a) AG25 (sin.) (b) DR23 (sin.) (c) AG25 (bin.) and (d) DR23 (bin.) (T: 25 °C, agitation speed: 200 rpm, pH: 2and 0.25 g/L of chitosan).

68 N.M. Mahmoodi et al. / Desalination 267 (2011) 64–72

After rearrangement, a linear form of the expanded Langmuirmodel in binary dye system was obtained.

Ce;1 = qe;1� �

= 1 = KL;1Q0;1

� �+ Ce;1 =Q0;1

� �+ qe;2Ce;1 = qe;1Q0;2

� �ð9Þ

According to Eq. (9), the values of Ce,1/qe,1 had linear correlationwith Ce,1 and Ce,1 qe,2/qe,1Q0,2 if the adsorption obeyed the expandedLangmuir model. By using Eq. (9) as the fitting model, the isothermparameters of an individual dye in the binary dye solutions wereestimated.

The Freundlich isotherm is derived by assuming a heterogeneoussurface with a non-uniform distribution of heat of adsorption over thesurface. Freundlich isotherm can be expressed by [37–40]:

qe = KFC1=ne ð10Þ

where KF is adsorption capacity at unit concentration and 1/n isadsorption intensity. 1/n values indicate the type of isotherm to beirreversible (1/n=0), favorable (0b1/nb1), unfavorable (1/nN1)[37]. Eq. (10) can be rearranged to a linear form:

log qe = log KF + 1 = nð Þ log Ce ð11Þ

The Tempkin isotherm is given as:

qe = RT = b ln KTCeð Þ ð12Þ

which can be linearized as:

qe = B1 lnKT + B1 lnCe ð13Þ

where

B1 = RT = b ð14Þ

where T and R are the absolute temperature (K) and the universal gasconstant (8.314 J/mol K), respectively. The constant b is related to theheat of adsorption. Also, B1 and KT are the Tempkin constant and canbe determined by a plot of qe versus ln Ce [41,42].

Tempkin isotherm contains a factor that explicitly takes into theaccount adsorbing species adsorbent interactions. This isothermassumes that (i) the heat of adsorption of all the molecules in thelayer decreases linearly with coverage due to adsorbent–adsorbateinteractions, and that (ii) the adsorption is characterized by a uniformdistribution of binding energies, up to somemaximumbinding energy[41,42].

To study the applicability of the Langmuir, Freundlich andTempkin isotherms for the dye adsorption onto chitosan in singleand binary systems of dyes at different pH values, linear plots of Ce/qeagainst Ce, log qe versus log Ce and qe versus ln Ce are plottedrespectively and the values of Q0, KL, KF, 1/n, B1, KT and r2 (correlationcoefficient values of all isotherms models) are shown in Table 2.

The correlation coefficient values (r2) show that the dye removalisotherm using Chitosan does not follow the Langmuir and Freundlichisotherms (Table 2). The calculated correlation coefficients (r2) forTempkin isotherm model show that the dye removal isotherm can beapproximated as the Tempkin model (Table 2). This means that theheat of adsorption of all the molecules in the layer decreases linearlywith coverage due to adsorbent–adsorbate interactions.

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

) No salt

NaCl

Na2SO4

NaHCO3 Na2SO4

NaHCO3

Na2SO4

NaHCO3

Na2SO4

NaHCO3

(a)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)

No salt

NaCl

(b)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

)

No salt

NaCl

(c)

0

20

40

60

80

100

0 5 10 15 20 25 30

time (min)

Dye

rem

oval

(%

) No salt

NaCl

(d)

Fig. 6. Effect of salt on dye adsorption onto chitosan (a) AG25 (sin.) (b) DR23 (sin.) (c) AG25 (bin.) and (d) DR23 (bin.) (T: 25 °C, agitation speed: 200 rpm, pH: 2, 0.25 g/L of chitosanand initial dye concentration: 50 mg/L).

69N.M. Mahmoodi et al. / Desalination 267 (2011) 64–72

3.4. Adsorption kinetics models in single and binary systems of dyes

Several kinetics models can be used to express the mechanism ofsolute sorption onto a sorbent. In order to design a fast and effectivemodel, investigations were made on adsorption rate. For theexamination of the controlling mechanisms of adsorption process,such as chemical reaction, diffusion control and mass transfer, severalkinetics models are used to test the experimental data [43,44].

0

25

50

75

100

125

150

175

200

0 2 4 6 8 10

pH

q e (

mg/

g)

AG25 (bin.)

DR23 (bin.)

AG25 (sin.)

DR23 (sin.)

Fig. 7. Effect of pH on dye adsorption onto chitosan (T: 25 °C, agitation speed: 200 rpm,0.25 g/L of chitosan and initial dye concentration: 50 mg/L at 30 min).

Pseudo-first order equation is generally represented as follows[45,46].

dqt = dt = k1 qe−qtð Þ ð15Þ

where qt and k1 are the amount of dye adsorbed at time t (mg/g) andthe equilibrium rate constant of pseudo-first order kinetics (1/min),respectively. After integration by applying conditions, qt=0 at t=0and qt=qt at t= t, then Eq. (15) becomes

log qe−qtð Þ = log qeð Þ− k1 = 2:303ð Þt ð16Þ

Data were applied to the pseudo-second order kinetic rateequation which is expressed as [45,47]:

dqt = dt = k2 qe−qtð Þ2 ð17Þ

where k2 is the equilibrium rate constant of pseudo-second order(g/mg min).

On integrating the Eq. (17),

t = qt = 1= k2q2e + 1 = qeð Þt ð18Þ

The possibility of intraparticle diffusion resistance affectingadsorption was explored by using the intraparticle diffusion model as

qt = kpt1=2 + I ð19Þ

where kp is the intraparticle diffusion rate constant.

Table 2Isotherm constants for dye adsorption at different pH values (2, 4, 5, 8 and 10) onto chitosan in single and binary systems of dyes (200 mL solution, T: 25 °C, initial dye concentration:50 mg/L and 0.25 g/L of chitosan for AG25 and DR23 in sin. and bin. systems).

System Langmuir isotherm Freundlich isotherm Tempkin isotherm

Q0 KL RL r12 KF 1/n r2

2 KT B1 r32

Single AG25

43 −0.205 −0.108 0.863 281 −2.197 0.836 0.0087 −52 0.938

DR23

34 −0.104 −0.238 0.926 1573 −1.044 0.940 0.0147 −86 0.982

Binary AG25

23 −0.230 −0.210 0.944 306 −1.22 0.831 0.0298 −42 0.854

DR23

20 −0.226 −0.215 0.929 336 −1.144 0.950 0.0284 −41 0.988

70 N.M. Mahmoodi et al. / Desalination 267 (2011) 64–72

Values of I give an idea about the thickness of the boundary layer,i.e; the larger intercept the greater is the boundary layer effect.According to this model, the plot of uptake should be linear ifintraparticle diffusion is involved in the adsorption process and ifthese lines pass through the origin then intraparticle diffusion is therate controlling step [48–50].When the plots do not pass through theorigin, this is indicative of some degree of boundary layer control andshows that the intraparticle diffusion is not the only rate limitingstep, but also other kinetic modelsmay control the rate of adsorption,all of which may be operating simultaneously.

To understand the applicability of pseudo-first order, pseudo-secondorder and intraparticle diffusionkineticsmodels, linearplots of log(qe–qt)versus contact time (t), t/qt versus contact time (t) (Fig. 8) and qt against

pH

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30

time (min)

t/qt

245810

(a)

(c)

(

pH

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30

time (min)

t/qt

245810

(

Fig. 8. Pseudo-second order adsorption kinetics of dyes onto chitosan (a) AG25 (sin.) (b) DR2of chitosan and initial dye concentration: 50 mg/L).

t1/2 at different pH values (2, 4, 5, 8 and 10) for the adsorption of dyes insingle and binary systems onto chitosan are plotted and the values ofkp, I, k1, k2, r2 (correlation coefficient values of all kineticsmodels) and thecalculated qe ((qe)Cal) are shown in Table 3.

The linearity of the plots (r2) demonstrates that pseudo-first orderand intraparticle diffusion kinetic models do not play a significant rolein the uptake of the dye by chitosan (Table 3). The linear fit betweenthe t/qt versus contact time (t) and calculated correlation coefficients(r2) for pseudo-second order kinetics model shows that the dyesremoval kinetics can be approximated as pseudo-second orderkinetics (Table 3). In addition, the experimental qe ((qe)Exp) valuesagree with the calculated ones ((qe)Cal), obtained from the linear plotsof pseudo-second order kinetics (Table 3).

pH

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30

time (min)

t/qt

2

4

5

8

10

b)

pH

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25 30

time (min)

t/qt

245810

d)

3 (sin.) (c) AG25 (bin.) and (d) DR23 (bin.) (T: 25 °C, agitation speed: 200 rpm, 0.25 g/L

Table 3Kinetics constants for dye adsorption at different pH values (2, 4, 5, 8 and 10) onto chitosan in single and binary systems of dyes (200 mL solution, T: 25 °C, initial dye concentration:50 mg/L and 0.25 g/L of chitosan for AG25 and DR23 in sin. and bin. systems).

System pH (qe)Exp Pseudo-first order Pseudo-second order Intraparticle diffusion

(qe)Cal. k1 r42 (qe)Cal. k2 r5

2 kp I r62

Single AG 25 (sin.)

2 178 55 0.308 0.831 179 0.026 1 24 79 0.5244 192 99 0.305 0.938 196 0.010 1 27 77 0.5945 91 68 0.208 0.954 98 0.006 0.999 15 24 0.7968 54 18 0.107 0.408 56 0.017 0.996 8 21 0.60910 38 23 0.099 0.634 39 0.033 1 6 15 0.626

DR23 (sin.)

2 155 79 0.180 0.912 161 0.006 1 23 54 0.6834 163 71 0.151 0.850 169 0.005 0.999 24 58 0.6615 138 93 0.248 0.941 145 0.006 1 22 44 0.7228 59 48 0.153 0.926 65 0.005 0.996 10 12 0.87610 38 15 0.163 0.700 40 0.003 0.999 5 14 0.626

Binary AG25 (bin.)

2 81 44 0.164 0.840 85 0.008 0.999 12 26 0.7224 75 42 0.175 0.935 83 0.009 1 12 24 0.7255 58 57 0.236 0.974 64 0.006 0.998 10 13 0.8488 35 31 0.183 0.959 39 0.008 0.997 6 6 0.87610 28 31 0.233 0.957 33 0.008 0.991 5 4 0.887

DR23 (bin.)

2 82 48 0.176 0.840 86 0.008 0.999 13 26 0.7294 75 41 0.150 0.819 79 0.008 0.998 11 24 0.7295 36 32 0.195 0.986 40 0.008 0.999 6 7 0.8628 25 17 0.191 0.939 26 0.021 0.999 4 7 0.78510 22 13 0.144 0.800 23 0.021 0.995 3 6 0.790

71N.M. Mahmoodi et al. / Desalination 267 (2011) 64–72

4. Conclusion

Equilibrium and kinetic studies were conducted for the adsorptionof AG25 and DR23 in single and binary systems of dyes from aqueoussolutions onto chitosan. Results showed that chitosan can beeffectively used as a biosorbent for the removal of anionic dyes. Thisbiosorbent exhibited high adsorption capacities toward DR23 andAG25 in single and binary systems of dyes. The kinetics studies of dyeson chitosan were performed and the data indicated that theadsorption kinetics of dyes on chitosan followed the pseudo-secondorder at different pH values. The equilibrium data had been analyzedusing Langmuir, Freundlich and Tempkin isotherms and the charac-teristic parameters for each isotherm have been determined. Theresults showed that the experimental data were correlated reasonablywell by Tempkin adsorption isotherm. Based on the data of presentstudy, one could conclude that the chitosan being an eco-friendlyadsorbent for dye removal from low concentration of acidic coloredtextile wastewater.

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