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Journal of Colloid and Interface Science 288 (2005) 30–35www.elsevier.com/locate/jcis

Adsorption of a model anionic dye, eosin Y, from aqueous solutionby chitosan hydrobeads

Sudipta Chatterjeea, Sandipan Chatterjeea, Bishnu P. Chatterjeea, Akhil R. Dasb,Arun K. Guhaa,∗

a Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, Indiab Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

Received 13 December 2004; accepted 17 February 2005

Available online 8 April 2005

Abstract

The process of sorption is being increasingly used for ecofriendly and economic remediation of textile dye effluents. The presstudy deals with the adsorption of a model anionic dye, eosin Y, from wastewater using conditioned chitosan hydrobeads. Coreduced the pH sensitivity and maintained the maximum sorption capacity of the beads near pH 8. To understand the chemicharacteristics of the adsorption process we studied, the kinetics and isotherm behavior of the system. It was observed that tplayed a significant role in the process. The Langmuir model was found to be most appropriate for the description of the adsorptioThe kinetic results followed a second-order equation. It was observed that 1 g of chitosan adsorbed∼76 mg of eosin Y. The dye was desorbfrom the beads by changing the pH of the solution, and the conditioned chitosan beads were reused five times without any loss of mand chemical efficacy. 2005 Elsevier Inc. All rights reserved.

Keywords: Bioremediation; Anionic dye; Chitosan hydrobeads; Chemicophysical study

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1. Introduction

Worldwide annual production of dyes is around 7× 105

tons, 5–10% of which is discharged into waste streamsthe textile industries[1]. The majority of these dyes aresynthetic origin and toxic in nature with suspected carcigenic and genotoxic effects[2–4]. The dye-bearing effluenwhen discharged into water bodies, affects photosynthaquatic life, and also humans.

A wide range of conventional treatment techniques sas chemical coagulation, activated sludge, trickling filcarbon adsorption, and photodegradation have been intigated extensively for removing dye from aquatic bodThe adsorption process produces an effluent that is

* Corresponding author. Fax: +91 33 2473 2805.E-mail address: bcakg@mahendra.iacs.res.in(A.K. Guha).

0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2005.02.055

,

-

of harmful substances like carcinogenic aromatic amproduced by microbial degradation[1,5] and free radicalsby photodegradation using UV light. The activated cbon generally used for dye adsorption is very expensBiosorption—use of waste biomaterials as sorbents—newly developed technique[6–9] for the removal of harmful substances from water bodies, but suffers serious limtions in the case of anionic dyes as most of the biomatecontain negatively charged cellulosic moieties, which lowadsorption due to coulombic repulsion. Recently, chitosa biopolymer of glucosamine, showed a higher capacityadsorption of colorant than activated carbon[10]. No andMeyers[11] demonstrated that swollen beads of chitosanhibit superior sorption capacity compared with flakes. Mo

over, the beads can be easily separated from the treated bulkfor repeated use. The ability of the anionic dye to adsorbonto chitosan beads can be attributed to the surface charge

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S. Chatterjee et al. / Journal of Collo

which depends on the pH of the operating system. Chitohas a positively charged surface below pH 6.5 (point of zζ potential), and reducing the pH increases the positivitythe surface[12,13], thus making the sorption process psensitive. Chitosan hydrobeads lose their integrity as asult of partial dissolution in acidic solvent, making theunsuitable for reuse. Crosslinking of chitosan with diffent bifunctional reagents[14–16]has been tried to increasthe integrity of the beads but has resulted in poor adstion. Thus, it is of interest to increase the integrity of tchitosan hydrobeads, as well as sorption properties, at apH.

We studied the efficacy of adsorption of a water-solumodel anionic dye, eosin Y, by sulfate-conditioned chitohydrobeads and the chemicophysical nature of adsorptiogather more information on the interaction of conditionchitosan and anionic dyes. Eosin Y (CI acid red 87), a ctar xanthene dye, was chosen as the model anionic dyavoid environmental hazards during investigation, as thisis not specifically listed as toxic by different health agenc

2. Materials and methods

2.1. Chemicals

Chitosan was prepared from shrimp shell by a modiHackmann procedure as described earlier[17]. Eosin Y waspurchased from BDH, India, and all other chemicals wprocured from E. Merck, Germany.

2.2. Preparation of chitosan hydrobeads

Chitosan beads were prepared by dropwise additiodegassed chitosan solution (2% w/v) in 7% v/v AcOto an alkaline coagulating mixture [H2O:MeOH:NaOH=4:5:1 w/w] as described by Mitani and co-researchers[18].The beads were collected by filtration and washed withter until neutral. The beads were conditioned using 0.1(NH4)2SO4 [19].

2.3. Estimation of eosin Y

The concentration of eosin Y in the solution was demined from the calibration curve drawn by measuringabsorbance at 517 nm in a Varian UV/vis spectrophototer.

2.4. Effect of conditioning

Fifty-milliliter aqueous solutions of eosin Y (500 mg L−1)were taken in different 250-ml Erlenmeyer flasks. The pHthe dye solutions was adjusted to 4.0. In one set of exp

ments, 1.5 g of swollen chitosan beads, and in another seof experiments the same amount of conditioned beads, wasadded. The flasks were agitated for 48 h (50 rpm) at 30◦C,

Interface Science 288 (2005) 30–35 31

which is far longer than required to reach equilibrium. Tbeads were separated from the solution by filtration throglass wool. The initial and equilibrium concentrations ofdye were estimated spectrophotometrically. The experimwas repeated at different pH values (4.0–12.0) of thesolution.

2.5. Equilibrium sorption experiment

An equilibrium sorption study was carried out with contioned chitosan beads as described above. The concentof the dye in the solution was allowed to vary from 1500 mg L−1. The experiment was conducted at pH 6.0. Tsame experiment was also repeated at two different tematures (viz. 40 and 50◦C).

2.6. Kinetic experiment

One and one-half grams of conditioned chitosan bewas added to different 250-ml Erlenmeyer flasks, each ctaining 50 ml of aqueous solution of eosin Y. The pHthe solution was adjusted to 6.0, and initial dye concentions were 20, 50, 100, and 200 mg L−1. The experiment waconducted at pH 6.0 and 30◦C, and the flasks were agitate(50 rpm) for 48 h. At regular intervals, the concentrationunadsorbed dye was determined.

2.7. Desorption experiment

After conducting the equilibrium study with 200 mg L−1

eosin Y, dye-loaded conditioned chitosan beads werelected by filtration through glass wool and used for dorption experiments. The beads were transferred to diffe250-ml Erlenmeyer flasks, each containing 50 ml water,the pH was adjusted to 10.0, 11.0, and 12.0. The flasksagitated at 50 rpm up to 20 h at 30◦C. The concentration othe eluted dye was determined at different time intervals

3. Results and discussion

3.1. Effect of pH

The effect of the conditioning of chitosan on dye asorption at different pH is illustrated inFig. 1. It appearsthat conditioned and unconditioned chitosan beads adsoequal amounts of eosin Y (76 mg/g dry chitosan) at pH 4.0In the case of unconditioned beads, dye adsorption decresharply with increasing pH. The adsorption of eosin Y onconditioned beads, on the other hand, remained practiunchanged up to pH 8.0; beyond this range a sharp decwas noted.

Adsorption of the anionic dye eosin Y by chitosan can

tattributed to the interaction between the protonated aminegroup of chitosan (R–NH2) and the anionic group of the dye(D–CO2Na) molecule:

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32 S. Chatterjee et al. / Journal of Colloi

Fig. 1. Adsorption of eosin Y by conditioned and nonconditioned chitobeads at different pH. Results represent averages of five replicated ements.

R–NH2 + H3O+ → R–NH+3 + H2O,

D–CO2NaH2O−→D–CO−

2 + Na+,

R–NH+3 + D–CO−

2 → R–NH+3 D–CO−

2 .

Decreasing the pH of the solution makes more protons aable to protonate the amine group of chitosan, with themation of a larger number of cationic amines. This resultincreasing dye adsorption by chitosan due to increasedtrostatic interactions. Conditioning of chitosan with ammnium sulfate would probably form R–NH+3 SO−

4 NH4, whichundergoes interaction with dye anion following the mecnism proposed by Muzzarelli and Rocchetti[19] in the caseof oxyanion adsorption by sulfate conditioned chitosan:

R–NH20.1 M (NH4)2SO4−−−−−−−−−→R–NH+

3 NH4SO−4 .

Adsorption of the colored anion on the chitosan surfprobably proceeds through the ion exchange reaction

R–NH+3 NH4SO−

4 + D–CO−2

→ R–NH+3 D–CO−

2 + NH4SO−4 .

3.2. Effect of temperature

Conditioned chitosan beads adsorbed eosin Y mosficiently at 30◦C. With a rise in temperature, uptake fasignificantly, indicating the exothermic nature of the proceBelow 30◦C, the process is slow and requires a long timeachieve equilibrium.

A comprehensive understanding of the nature of the inaction of adsorbate with adsorbents is essential for the meffective use of the adsorbent. To formulate a pragmaticgram for design, operation, and optimization, a correlaof the equilibrium data with the different relevant modelsisotherm is a prerequisite.

In the present study the experimental data for the eo

chitosan equilibrium isotherm were compared with those forthe Freundlich and Langmuir isotherm models. The Fre-undlich isotherm explains adsorption on a heterogeneous

Interface Science 288 (2005) 30–35

-

-

t

Fig. 2. Langmuir plot of eosin Y–chitosan bead equilibrium experimedata.R represents the correlation coefficient.P represents the probabilit(thatR = 0), and the equation represents the regression equation of the

surface with uniform energy. The linear form of the modegiven by

(1)logqe = logKF + 1/n logCe,

whereqe andCe are the sorbate concentrations at equirium in the solid (mg g−1) and liquid (mg L−1), respectively;1/n is the heterogeneity factor; andKF is the Freundlichconstant. The sorption process under investigation doeobey the Freundlich isotherm model. No linear relationsis found between the sorbet concentrations at equilibriumthe solid and liquid phases respectively (figure not show

On the other hand, in the Langmuir model it is assumthat the thickness of the adsorbed layer is monomoleculnature. The linear form of the isotherm,

(2)Ce

qe= 1

Q0b+ Ce

Q0,

has been used to analyze the experimental data, wheCeandqe are the equilibrium dye concentrations in the aque(mg L−1) and solid (mg g−1) phases, respectively;Q0 andb

are Langmuir constants related to the capacity of adsorpand energy of adsorption, respectively.

Plots of Ce/qe versusCe for eosin Y at three differentemperatures are presented inFig. 2. The linear form of theisotherms over the whole concentration range is conspous and the corresponding correlation coefficients are sfactory (R ≈ 1.0). These strongly suggest that the pressorption process occurs at a single surface and followsLangmuir sorption model closely. Langmuir monolayer suration capacityQ0 and energy of adsorptionb are listed inTable 1. The general shape of the equilibrium curves alowith the sharp curvature reflects the characteristics of Lamuir equilibrium with a high degree of irreversibility.

The essential features of the Langmuir isotherm canexpressed in terms of the dimensionless equilibrium paraterRL, which is defined asRL = 1/(1+bC0), whereb is the

Langmuir constant as described above andC0 is the initialdye concentration.RL values within the range 0< RL < 1indicate favorable adsorption.RL values calculated usingC0

S. Chatterjee et al. / Journal of Colloid and Interface Science 288 (2005) 30–35 33

Table 1Langmuir adsorption isotherm and second-order rate constants

Constant in Langmuir model Constant in second-order model

T (◦C) Q0 (mg g−1) b (L g−1) RL (usingC0 = 200 mg L−1)

Initial concentrationC0 (mg L−1)

h (mg g−1 min−1) qe (calculated) k2

20 0.1603 12.50 10.3× 10−4

30 80.84 0.1345 0.0360 50 0.1779 33.33 1.6× 10−4

40 75.82 0.0977 0.0487 100 0.2030 50.00 0.8× 10−4

50 72.83 0.0960 0.0495 200 0.5000 100.00 0.5× 10−4

Table 2Thermodynamic properties of the conditioned chitosan–eosin Y adsorption process

Concentration (mg L−1) T (◦C) Kc Thermodynamic parameter

Initial, C0(mg L−1)

Final,Ce(mg L−1)

�G0

(kJ mol−1)�H0

(kJ mol−1)�S0

(J mol−1 K−1)

50 5.120 30 8.770 −5.4678.010 40 5.242 −4.311 −17.10 −51.88.220 50 5.083 −4.366

100 16.678 30 4.996 −4.05218.880 40 4.297 −3.672 −12.52 −43.722.460 50 3.450 −3.119

150 44.720 30 2.240 −2.30349.820 40 2.010 −1.758 −8.34 −28.753.620 50 1.795 −1.473

200 88.240 30 1.267 −0.596

en

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fer-

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ass

92.330 40 1.16696.430 50 1.070

as high as 200 mg L−1 are well within the defined range (Ta-ble 1) and indicate the acceptability of the process.

Thermodynamic parameters such as change in freeergy (�G0), enthalpy (�H 0), and entropy (�S0) were de-termined using the following equations[20,21]:

KC = CAe/Ce,

�G0 = −RT lnKC,

logKC = �S0

2.303R− �H 0

2.303RT,

whereKC is the equilibrium constant,CAe is the amount ofdye (mg) adsorbed by each gram of adsorbent from 1the dye solution at equilibrium,Ce is the equilibrium con-centration (mg L−1) of the dye in the solution,T is the solu-tion temperature (in K), andR is the universal gas constan�H 0 and�S0 were calculated from the slope and intercof van’t Hoff plots of logKC versus 1/T . The results aresummarized inTable 2. The�G0 values indicate that the adsorption process is spontaneous in nature and more favoat lower concentrations of dye compared with higher ccentrations. The appreciably low free energy values indi

saturation of the process. The enthalpy values suggest thathe reaction is exothermic and thus less favorable at highertemperatures.

−0.387 −3.34 −8.1−0.171

-

e

Fig. 3. Effect of contact time on eosin Y–chitosan bead equilibrium at difent initial dye concentrations: (a) 200 mg/L; (Q) 100 mg/L; (") 50 mg/L;(2) 20 mg/L. Results represent averages of five replicated experiment

3.3. Kinetic study

The kinetic behavior of the adsorption process wstudied at 30◦C using different initial dye concentration

t(Fig. 3). It appears from the figure that the adsorption ca-pacity of the beads increases with increasing initial dyeconcentration, but the time required to reach equilibrium in

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34 S. Chatterjee et al. / Journal of Colloi

the process is independent of initial dye concentration aported by previous researchers[22,23]. The process is verfast initially and then slowly reache equilibrium within 20To determine the rate-controlling and mass transfer manism, kinetic data were correlated to linear forms ofpseudo-first-order rate model,

(3)log(qe − qt ) = logqe − k1

2.303t,

and the second-order rate model,

(4)t

qt

= 1

h+ 1

qet, h = k2q

2e,

whereqe andqt are the amounts of dye adsorbed (mg g−1)at equilibrium and at different intervals, respectively;k1(min−1) and k2 (g mg−1 min−1) are the pseudo-first-ordeand second-order rate constants; andh represents the initiaadsorption rate (mg g−1 min−1).

By correlation of the kinetic data with the above two ramodels, it was found that the plot oft/qt against time using different initial dye concentrations gives straight lin(Fig. 4) with high correlation coefficients(R ≈ 1.0). Thisindicates that the present sorption system follows predonantly the second-order rate model and the overall proappears to be controlled by chemisorption[24,25], whereasthe high degree of nonlinearity (figures are not showand poor correlation coefficient suggest the inability ofpseudo-first-order model to describe the kinetic profile ofadsorption process.

The average values of the rate constants are determfrom the intercepts of the curves and are given inTable 1,which listsk2, h, andqe (calculated) as a function of initiadye concentration. The second-order rate constant decrwith an increase in initial dye concentration, while the itial adsorption rate increases with an increase in initialconcentration.

3.4. Desorption study

Fig. 5 illustrates the desorption of eosin Y from chitosbeads at different pH with time. Eosin has been found todesorbed from the loaded beads just by increasing the pthe eluant to the alkaline range. The reaction responsiblthe desorption is

RNH+3 D–CO−

2 + NaOH→ RNH2 + D–CO2Na+ H2O.

The desorption process is observed to be reasonably fastially and then slowly to attain equilibrium after about 16The rate of desorption is found to increase with an increin pH of the eluant. About 65% of the dye is desorbed frthe beads at pH 12.0 within 2 h, whereas 98% can be elat pH 11.0 or 12.0.

The chitosan beads used in this study were reused for fivecycles without any loss of their sorption capacity (data notshown).

Interface Science 288 (2005) 30–35

d

s

f

-

Fig. 4. Second-order plot of adsorption of eosin Y onto chitosan beadsdifferent initial dye concentrations. Symbols are described in the legenFig 3. R represents the correlation coefficient,P represents the probabilit(thatR = 0), and the equation represents the regression equation of the

Fig. 5. Desorption of eosin Y from dye-loaded chitosan beads at diffetimes: (Q) pH 12.0; (!) pH 11.0; (2) pH 10.0. Results represent averagof five replicated experiments.

4. Conclusions

1. Conditioning of the chitosan beads with ammonium sfate reduces the pH sensitivity of the process.

2. Adsorption of the eosin dye on chitosan beads follothe Langmuir model.

3. The process is exothermic in nature. The highestciency of the adsorption process is observed at 30◦C.

4. The sorption process is very fast initially, attains equirium within a few hours, and follows the second-ordkinetic rate model.

5. The treated chitosan bead adsorbate is recyclable.

Acknowledgment

Sandipan Chatterjee acknowledges the Council of Scien-tific and Industrial Research (CSIR), Government of India,for providing a Senior Research Fellowship.

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S. Chatterjee et al. / Journal of Collo

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