Bioresource Technology 100 (2009) 2803–2809

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Bioresource Technology

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Enhanced adsorption of congo red from aqueous solutions by chitosanhydrogel beads impregnated with cetyl trimethyl ammonium bromide

Sudipta Chatterjee a, Dae S. Lee b, Min W. Lee c, Seung H. Woo a,*

a Department of Chemical Engineering, Hanbat National University, San 16-1, Deokmyeong-Dong, Yuseong-Gu, Daejeon 305-719, Republic of Koreab Department of Environmental Engineering, Kyungpook National University, Sankyuk-dong, Buk-gu, Daegu 702-701, Republic of Koreac Department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu,Pohang, Gyeongbuk 790-784, Republic of Korea

a r t i c l e i n f o

Article history:Received 19 September 2008Received in revised form 16 December 2008Accepted 16 December 2008Available online 8 February 2009

Keywords:AdsorptionChitosan beadCongo redCTABSurfactant

0960-8524/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.biortech.2008.12.035

* Corresponding author. Tel.: +82 42 821 1537; faxE-mail address: shwoo@hanbat.ac.kr (S.H. Woo).

a b s t r a c t

The adsorption of congo red (CR) onto chitosan (CS) beads impregnated by a cationic surfactant (CTAB,cetyl trimethyl ammonium bromide) was investigated. Chitosan beads impregnated at a ratio of 1/20of CTAB to CS (0.05% of CTAB and 1% of CS) increased the CR adsorption capacity by 2.2 times from162.3 mg/g (0% CTAB) to 352.5 mg/g (0.05% CTAB). The CR adsorption decreased with an increase inpH of the CR solution from 4.0 to 9.0. The Sips isotherm model showed a good fit with the equilibriumexperimental data and the values of the heterogeneity factor (n) indicated heterogeneous adsorptionof CR onto CS/CTAB beads, as well as CS beads. The kinetic data showed better fit to the pseudo sec-ond-order rate model than to the pseudo first-order rate model. The impregnation of CS beads by cationicsurfactants showed the highest adsorption capacities of CR compared to any other adsorbents and wouldbe a good method to increase adsorption efficiency for the removal of anionic dyes in a wastewater treat-ment process.

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1. Introduction

Wastewater effluents from different industries such as textiles,rubber, paper and plastics, contain several kinds of synthetic dye-stuffs (Chiou et al., 2004). A very small amount of dye in water ishighly visible. Further, discharging even a small amount of dye intowater can affect aquatic life and food webs due to the carcinogenicand mutagenic effects of synthetic dyes (Crini, 2006). Congo red [1-naphthalene sulfonic acid, 3,30-(4,40-biphenylenebis (azo)) bis (4-amino-) disodium salt] is a benzidine-based dye. This dye has beenknown to cause an allergic reaction and to be metabolized to ben-zidine, a human carcinogen. Synthetic dyes such as congo red (CR)are difficult to biodegrade due to their complex aromatic struc-tures, which provide them physico-chemical, thermal and opticalstability (Han et al., 2008). Although contaminated waste watermay be treated with conventional physical–chemical methods likereverse osmosis, ion exchange, chemical precipitation or limecoagulation, and oxidation, the application of these techniqueshas been restricted due to high energy consumptions or expensivesynthetic resins and chemicals. Moreover, these methods generatelarge amount of toxic sludge and are ineffective at lower concen-trations of dye (Blackburn, 2004; Chakraborty et al., 2003). There-

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fore, there is an urgent requirement for development of innovative,but low cost processes, by which dye molecules can be removed.

Adsorption technique is quite popular due to simplicity andhigh efficiency, as well as the availability of a wide range of adsor-bents. It has proved to be an effective method for removal of dyefrom wastewater (Allen et al., 2004). Activated carbon is the mostpopular adsorbent for removal of dyestuffs from wastewater(Kadirvelu et al., 2003). However, adsorbent grade carbon is cost-prohibitive and both regeneration and disposal of the used carbonare often very difficult. Therefore, there is a growing need to findlocally available, low cost, and effective materials for the removalof dyes. A number of non-conventional, low cost adsorbents suchas montmorillonite (Yermiyahu et al., 2003), bentonite (Lianet al., 2009), rice hull ash (Chou et al., 2001), leaf (Bhattacharryaand Sharma, 2004), fly ash (Mall et al., 2005), activated red mud(Tor and Cengeloglu, 2006), rice husk (Han et al., 2008), and fungi(Fu and Viraraghavan, 2002; Binupriya et al., 2008) have been usedfor the removal of congo red from aqueous solutions. However,some of these adsorbents do not have good adsorption capacitiesfor anionic dyes because most have hydrophobic or anionic sur-faces. Hence, there is a need to search for more effectiveadsorbents.

Chitosan, a linear biopolymer of glucosamine, has exhibitedexcellent adsorption capacity for anionic dyes and heavy metalions because chitosan molecules contain a large number of activeamine (–NH2) groups. In a recent review paper, chitosan was noted

2804 S. Chatterjee et al. / Bioresource Technology 100 (2009) 2803–2809

as one of the adsorbents having the highest adsorption capacity to-wards many classes of dyes (Crini, 2006). Moreover, chitosan canbe obtained at an industrial scale by chemical deacetylation ofcrustacean chitin, which is the second most abundant biopolymerin nature. To improve adsorption capacity of chitosan beads, sev-eral chemical modification methods, such as cross-linking (Vieiraand Beppu, 2006) and insertion of new functional groups (Sunet al., 2006; Wang and Wang, 2008a) have been performed. Re-cently, surfactant impregnation has been applied to several adsor-bents, such as clay (Krishna et al., 2000; Wang and Wang, 2008b),biomass (Bingol et al., 2004), and waste pith (Sureshkumar andNamasivayam, 2008). However, surfactant impregnation on chito-san for the adsorption of hazardous compounds has not been re-ported up to now. The aim of this study is to investigate theadsorption behavior of CTAB-impregnated chitosan beads (CS/CTAB beads) towards congo red (CR) dye, as well as to compareits adsorption capacity with chitosan beads (CS beads).

2. Methods

2.1. Preparation of chitosan (CS) beads

Chitosan (CS, >85% deacetylation), cetyl trimethyl ammoniumbromide (CTAB), and congo red (CR) were purchased from SigmaChemical Co., USA. All other chemicals were of analytical gradeand were also purchased from Sigma Chemical Co., USA. Chito-san-acetic acid solution was prepared by dissolving 10.0 g of chito-san powder in 300 ml of 5% (v/v) acetic acid solution. After dilutingthe chitosan-acetic acid solution to 1 l with deionized water, it wasleft overnight in stirring condition. Then the solution (1% CS) wasdropped into a precipitation bath containing 1000 ml of alkalinecoagulating mixture (H2O:MeOH:NaOH: 4:5:1, w/w) to form chito-san beads. The wet chitosan beads were collected and preserved indeionized water, after rinsing with deionized water to remove anyNaOH. Prior to use, the chitosan beads were kept for 30 min in thepH adjusted aqueous solution required for different experimentalconditions.

2.2. Preparation of CTAB modified CS beads (CS/CTAB beads)

The CTAB modified CS beads (CS/CTAB beads) were prepared asfollows: chitosan-acetic acid solution was prepared by dissolvingthe required amount of chitosan powder into the correspondingaqueous acetic acid solution. The desired amount of CTAB solutionwas added to the chitosan-acetic acid solution from a stock solu-tion of CTAB (2%) in deionized water and the CTAB concentrationin chitosan-CTAB-acetic acid solution was varied from 0.01% to0.1%. The final concentration of chitosan in chitosan-CTAB-aceticacid solution was fixed at 1%. This solution was kept in stirring con-dition for overnight at 50� C. Then the solution was dropped into aprecipitation bath containing 1000 ml of alkaline coagulating mix-ture (H2O:MeOH:NaOH: 4:5:1, w/w) to form CS/CTAB beads. Thewet CS/CTAB beads were collected and extensively rinsed withdeionized water to remove any NaOH.

2.3. Diameter and porosity of chitosan beads

The diameter (D) and porosity (e) of the CS beads as wellas CTAB modified CS beads can be determined by the amount ofwater within the pores of the beads (Zhao et al., 2007). The diam-eter (D) and porosity (e) can be calculated using the followingequations:

D ¼ 6WD=qMat þ ðWW �WDÞ=qW

p

� �1=3

ð1Þ

e ¼ ðWW �WDÞ=qW

WD=qMat þ ðWW �WDÞ=qW� 100% ð2Þ

where WW (g) is the wet weight of the beads before drying; WD (g)is the wet weight of the beads after drying; qw is the density ofwater, 1.0 g/cm3; and qMat is the density of material. The meanwet weight and dry weight of 3 sets of 10 beads will give the WW

(g) and WD (g) of a single bead.

2.4. Preparation of congo red (CR) solution

A stock solution (1000 mg/l) of congo red (CR) was prepared indeionized water, and experimental solutions of desired CR concen-trations were obtained by successive dilutions. The concentrationof CR in the experimental solution was determined from the cali-bration curve prepared by measuring absorbance of different pre-determined concentrations of CR solutions at kmax 497 nm usinga DR5000 spectrophotometer (HACH, USA).

2.5. Batch adsorption studies

The batch adsorption was carried out on a thermostat shaker at150 rpm and 30� C using 20 ml glass vials containing 0.2 g of wetCS beads and 10 ml of CR solutions of desired concentration andpH. In this study, CTAB-impregnated CS beads (CS/CTAB beads)were also used as adsorbent and CTAB concentration in the beadswas fixed at 0.05 w/v%. All the adsorption experiments were con-ducted in triplicate. The effect of CTAB concentration on CR(C0 = 500 mg/l) adsorption was tested by varying the CTAB concen-tration in CS/CTAB beads from 0.01% to 0.1%. The influence of pH onCR removal was studied by varying the initial pH of CR solutionsfrom 4 to 9 using 0.1N HCl or NaOH solutions, with the initial CRconcentration fixed at 500 mg/l. The change in pH during adsorp-tion was studied at acidic pH (pH 5), neutral pH (pH 7) and alkalinepH (pH 9), with the initial CR concentration at 50 mg/l. At the endof the adsorption period of 24 h, the supernatant solution was col-lected by centrifugation at 3,000 rpm for 30 min. The amount of CRin the solutions before and after adsorption was analyzed at497 nm using a DR5000 spectrophotometer (HACH, USA). The ef-fect of contact time was studied to determine the time taken byCS beads (as well as CS/CTAB beads) to reach equilibrium at pH 5with the initial CR concentration fixed at 500 mg/l. The CR concen-tration was measured at different time intervals up to 960 min.Each data point was obtained from an individual flask, and there-fore no correction due to withdrawn sampling volume was neces-sary. Equilibrium isotherm studies were carried out with differentinitial concentrations of CR (10–1,000 mg/l) at 30� C and pH 5.Langmuir, Freundlich and Sips isotherms were used to analyzethe equilibrium adsorption data. The amount of CR adsorbed(mg/g) was calculated based on a mass balance equation as givenbelow:

q ¼ ðC0 � CeqÞ � VW

ð3Þ

where q is the adsorbent capacity, mg/g; C0 is the initial concentra-tion of CR, mg/l; Ceq is the final or equilibrium concentration of CR,mg/l; V is the volume of experimental solution, l; and W is the dryweight of CS as well as CS/CTAB beads, g.

3. Results and discussion

3.1. General properties of CTAB-impregnated beads

To prepare CTAB-impregnated CS beads from chitosan-CTAB-acetic acid solution, CTAB concentration was fixed at 60.1% in solu-tion, because the CTAB concentration in this range did not produce

S. Chatterjee et al. / Bioresource Technology 100 (2009) 2803–2809 2805

any foam in the solution. But the CTAB concentration higher than0.1% caused excessive foam formation in the solution. The param-eters like wet weight (WW), dry weight (WD), bulk density andmaterial density of the beads were measured experimentally.Diameter and porosity of the beads were calculated from Eqs. (1)and (2), respectively. Table 1 shows that CS/CTAB beads had lesswater content (95.86%) than CS beads (96.70%), indicating thatmodification of CS beads with CTAB reduced water content andthus made the beads materially denser and mechanically stronger.Table 1 also shows that bulk density and material density of CS/CTAB beads were higher than CS beads. Porosity in CS/CTAB beadsincreased from 84.99% to 90.94% after addition of CTAB in beads.The diameter of CS/CTAB beads (2.57 mm) was less than that ofCS beads (2.87 mm), indicating CTAB modification makes CS/CTABbeads smaller than CS beads.

3.2. Effect of CTAB concentration

The effect of CTAB concentration (w/v%) in CTAB-impregnatedCS beads (CS/CTAB beads) on the adsorption of CR (C0 = 500 mg/l)was investigated at pH 5. The adsorption capacities of CS/CTABbeads increased with increasing CTAB concentration from 0.01%to 0.1% in CS/CTAB beads (where CS concentration in beads wasfixed at 1%) (data not shown). The adsorption of CR was increasedproportionally with increasing CTAB concentration from 0%(162.32 mg/g) to 0.05% (352.45 mg/g). Further increase in CTABconcentration in beads up to 0.1% slightly enhanced the adsorptioncapacity to 369.12 mg/g. CTAB has a positively charged head groupand consequently increase in CTAB concentration in beads in-creased the adsorption of anionic CR molecules due to enhancedelectrostatic interaction. However, when the CTAB concentrationin beads exceeded 0.05%, the adsorption capacities hardly in-creased, indicating CTAB concentration in CS/CTAB beads may havereached the saturation limit.

3.3. Effect of pH

The effect of pH ranging from 4.0 to 9.0 on the removal of CR(500 mg/l) was investigated with CS beads and CS/CTAB beads.The pH change in the CR solution was measured during adsorption.The pH rose initially and leveled off to 6, from an initial pH of 5. Nosignificant pH change was observed during adsorption when thepH of the initial CR solution was neutral (pH 7) or basic (pH 9).The adsorption was found to decrease with an increase in pH ofthe CR solution from 4.0 to 9.0 (data not shown) and the adsorptioncapacities of CS/CTAB and CS beads decreased from 371.2 to241.2 mg/g and 169.8 to 98.4 mg/g, respectively. As the pH of theCR solution increased, a proportional decrease in adsorption tookplace due to the successive deprotonation of positive chargedgroups on the adsorbent and electrostatic repulsion between neg-atively charged sites on the adsorbent and dye anions. There was

Table 1General properties of CS beads as well as CS/CTAB beads.

General properties Adsorbent

CS beads CS/CTAB beads

Wet weight (WW, mg) 10.86 8.42Dry weight (WD, mg) 0.36 0.35Water content in beads (%) 96.70 95.86Bulk density a (g/ml) 0.68 0.84Material density b (g/ml) 0.193 0.434Porosity (e, %) 84.99 90.94Diameter (D, mm) 2.87 2.57

a Wet weight of beads/volume of wet beads.b Dry weight of bead materials/volume of the materials.

also competition between OH� (at high pH) and dye anions for pos-itively charged adsorption sites. CS/CTAB beads showed higher CRadsorption than that of CS beads at all the pH levels tested in thisstudy because CTAB molecule has a positively charged head group.CTAB most likely interacted with CS through their non-polar (alkyl)groups in CTAB modified CS beads; the polar (positively) chargedhead groups point towards the bulk of the solution, making thesurface potential positive. Another possible mechanism is the elec-trostatic attraction between surfactant cations and negativelycharged groups of CS molecules because -OH groups of CS mole-cules in chitosan-CTAB-acetic acid solution (CS:CTAB = 1:0.05, w/w) lose hydrogen ions in alkali coagulating solution. In both mech-anisms, surfaces of CS/CTAB beads receive a positive potential,which increases the removal of CR anions.

3.4. Equilibrium adsorption isotherm

The equilibrium adsorption isotherm model is fundamental indescribing the interactive behavior between adsorbate and adsor-bent. Analysis of isotherm data is important for predicting theadsorption capacity of the adsorbent, which is one of the mainparameters required for the design of an adsorption system. Sev-eral isotherm models are used for this purpose. The Langmuir iso-therm model assumes monolayer coverage of adsorbate on ahomogeneous adsorbent surface. This model does not consider sur-face heterogeneity of the sorbent. It assumes adsorption will takeplace only at specific site on the adsorbent. The linearized formof the Langmuir model is given as:

Ce

qe¼ 1

KLþ aL

KLCe ð4Þ

where Ce is the equilibrium concentration of CR (mg/l) in the solu-tion, qe is the equilibrium CR concentration (mg/g) on the adsor-bent, and aL (l/mg) and KL (l/g) are the Langmuir constants with aL

related to the adsorption energy. The values of aL and KL are calcu-lated from the slope and intercept of the plot of Ce/qe vs. Ce, and Q0

[�KL/aL] signifies the maximum adsorption capacity (mg/g). Table 2indicates that the maximum adsorption capacity of CS/CTAB beads(Q0 = 385.86 mg/g) for CR is very much higher than that of CS beads(Q0 = 182.68 mg/g).

The essential feature of the Langmuir isotherm can be ex-pressed in terms of a dimensionless constant separation factor(RL) given by the following equation:

RL ¼1

ð1þ aLC0Þð5Þ

where C0 is the initial CR concentration (mg/l). RL values within therange 0<RL<1 indicate favorable adsorption. In this study, RL valuesof CS beads (0.029) and CS/CTAB beads (0.022) for the initial CR con-centration of 1,000 mg/l indicate favorable adsorption of CR ontoboth CS and CS/CTAB beads.

The Freundlich isotherm model is an empirical equation thatdescribes the surface heterogeneity of the sorbent. It considersmultilayer adsorption with a heterogeneous energetic distributionof active sites, accompanied by interactions between adsorbedmolecules. A linear form of Freundlich equation is:

ln qe ¼ ln KF þ1n

ln Ce ð6Þ

where KF (l/g) is the Freundlich adsorption isotherm constant relat-ing to the extent of adsorption and 1/n is related to the adsorptionintensity, which varies with the heterogeneity of the material. Thevalues of KF and 1/n calculated from the intercept and slope of theplot of log qe vs. log Ce are listed in Table 2.

Sips isotherm is employed to analyze the equilibrium data ob-tained during batch adsorption studies. Sips model is a combina-

Table 2Constants for equilibrium isotherm models with error analysis values.

Adsorbent aL (l/mg) kL (l/g) kL/aL [=Q0] (mg/g) RLa Error analysis

R2 v2

Langmuir Isotherm modelCS beads 0.034 6.21 182.7 0.029 0.997 34.88CS/CTAB beads 0.044 16.98 385.9 0.022 0.998 67.66

Adsorbent KF (l/g) 1/n Error analysis

R2 v2

Freundlich Isotherm modelCS beads 19.00 0.368 0.944 21.86CS/CTAB beads 31.47 0.426 0.965 64.08

Adsorbent qmax (mg/g) Keq (l/mg) n Error analysis

R2 v2

Sips isotherm modelCS beads 223.25 ± 14.09 0.082 ± 0.008 0.5794 0.998 1.57CS/CTAB beads 433.12 ± 42.59 0.048 ± 0.018 0.7763 0.991 25.11

a C0 is 1000 mg/l.

2806 S. Chatterjee et al. / Bioresource Technology 100 (2009) 2803–2809

tion of Langmuir and Freundlich models, having features of bothLangmuir and Freundlich equations. It is expressed as (Danyet al., 2004):

qe ¼qmaxKeqCn

e

1þ KeqCne

ð7Þ

Sips isotherm equation is characterized by the heterogeneityfactor, n, and it can be employed to describe the heterogenous sys-tem. Keq (l/mg) represents the equilibrium constant of Sips equa-tion and qmax (mg/g) is the maximum adsorption capacity. Whenn = 1, Sips isotherm equation reduces to the Langmuir equationand it implies a homogeneous adsorption process.

Fig. 1 shows the equilibrium adsorption of CR using CS beadsand CS/CTAB beads (qe vs. Ce) and the isotherms are plotted to-gether with the experimental data points. Both Langmuir and Sipsisotherm models show good fit to experimental data. Freundlichmodel assumes an infinite amount of adsorption theoretically be-cause it is an exponential equation. However, the experimentalevidence indicates that an isotherm plateau is reached at a limitingvalue of the solid phase concentration. This indicates that the Fre-undlich isotherm itself does not have any real physical significance.The values of the heterogeneity factor, n for CR adsorption onto CS

Ce (mg/l)

0 200 400 600 800 1000

qe

(mg

/g)

0

100

200

300

400

500

600

700Experimental data (CS beads)Langmuir plot (CS beads)Freundlich plot (CS beads)Sips plot (CS beads)Experimental data (CS/CTAB beads)Langmuir plot (CS/CTAB beads)Freundlich plot (CS/CTAB beads)Sips plot (CS/CTAB beads)

Fig. 1. Plots of qe vs. Ce for the adsorption of CR onto CS beads and CS/CTAB beads.

beads (0.5794) and CS/CTAB beads (0.7763) indicate that theadsorption process is heterogeneous and the Sips isotherm exhibitsthe best fit for this adsorption process.

3.5. Error analysis

An error is required to evaluate the fit of an isotherm equationto the experimental equilibrium data obtained. In this study, thelinear (Langmuir and Freundlich isotherm models) and non-linear(Sips isotherm model) coefficients of determination (R2) and a non-linear Chi-square test were performed for all the isotherm models.The Chi-square test statistic is basically the sum of the squares ofthe differences between the experimental data and the data ob-tained by calculating from models, with each squared differencedivided by the corresponding data calculated using the models.This can be represented mathematically as:

v2 ¼X ðqe � qe;mÞ

2

qe;mð8Þ

where qe,m is the equilibrium capacity obtained by calculating fromthe model (mg/g) and qe is the experimental data of the equilibriumcapacity (mg/g). If data from a model are similar to the experimen-tal data, v2 will be a small number, and if they differ, v2 will be a bignumber. Therefore, it is necessary to analyze the data using thenon-linear Chi-square test to confirm the best fit isotherm for thisadsorption system.

The results of the application of correlation coefficients (R2) andnon-linear Chi-square test (v2) on experimental data of the equi-librium capacity (qe) for the three adsorption isotherms are shownin Table 2. The Sips isotherm model appears to be the best fittingmodel for CR adsorption onto CS beads, with the highest correla-tion coefficient (R2 = 0.998) and lowest Chi-square value(v2 = 1.57) for CS beads. The Sips model has the lowest Chi-squarevalue (v2 = 25.11) and moderately high correlation coefficient(R2 = 0.991) for adsorption of CR onto CS/CTAB beads. Even thoughthe Langmuir isotherm model has the highest correlation coeffi-cient (R2 = 0.998) for CR adsorption onto CS/CTAB beads, the highChi-square value (v2 = 67.66) indicates that adsorption of CR ontoCS/CTAB beads is not homogeneous. Table 2 indicates that the Fre-undlich isotherm model has the lowest correlation coefficients andvery high Chi-square (v2) values for all the adsorbents used in thisstudy.

S. Chatterjee et al. / Bioresource Technology 100 (2009) 2803–2809 2807

3.6. Molar adsorption capacities

Table 3 shows the calculated values of maximum adsorptioncapacity (qmax) of CS beads, as well as CS/CTAB beads, estimatedfrom Sips isotherm model (with various units). The molar aminecontent (mmol NH2/g dry weight) of CS beads and CS/CTAB beadsare 5.30 and 5.05, respectively. The CTAB modified CS beads wereformed from the solution containing CTAB and CS in 1:20 ratio andmolar CTAB content in the beads was 0.13 mmol/g dry weight. Thevalue of mol CR/mol NH2 in CS beads was calculated assuming theCR binding onto CS beads only to NH2 groups of chitosan. A maxi-mum of 6% of the amine groups of CS beads can be used for CRbinding. Since the amount of CTAB was 0.13 mmol/g dry weightof CS/CTAB beads, CTAB can bind a maximum of 20.9% of totalCR binding in CS/CTAB beads (0.622 mmol/g dry weight). However,the maximum adsorption capacity of CS/CTAB beads was 1.94times higher than CS beads. This indicates that CR adsorption inCS/CTAB beads was increased more than maximum adsorptionby ionic interaction between a cationic charge of CTAB and an an-ionic charge of CR. This indicates that CTAB impregnation hadsome effects on enhancing CR adsorption other than ionic interac-tions. The enhancement of maximum adsorption capacity of CS/CTAB beads for CR adsorption would be partly due to hydrophobicinteractions between a hydrophobic tail of CTAB and hydrophobicmoieties of CR. CTAB impregnation increased the porosity of CS/CTAB beads as tabulated in Table 1. Recently it has been reportedthat the amount adsorbed by the adsorbent depends on its porosity(Seredych et al., 2009). Therefore the porosity increase by CTABimpregnation would be another possible reason for higher CRadsorption by CS/CTAB beads through accumulation of more CRmolecules into its structure.

3.7. Kinetic study

Adsorption kinetics is an important parameter for designingadsorption systems and is required for selecting optimum operat-ing conditions for batch adsorption study. To investigate theadsorption kinetics of CR onto CS and CS/CTAB beads, three differ-ent kinetics models, pseudo first-order and pseudo second-orderrate models and intra-particle diffusion models, were used in thisstudy.

The linearized form of the pseudo first-order rate equation is gi-ven as:

Table 3Maximum adsorption capacities in various units estimated from Sips isotherm model.

Adsorption capacity Adsorbent

CS beads CS/CTAB beads

mol NH2/g dry weight (molar amine content) 0.00530 0.00505mol CTAB/g dry weight – 0.00013qmax (mg CR/g dry weight) 223.25 433.12qmax (mmol CR/g dry weight) 0.320 0.622qmax (mg CR/ml bead) 5.02 15.16mol CR/mol NH2 0.060 0.123mol CR/mol CTAB – 4.79

Table 4Constants of different rate models.

Adsorbent C0 (mg/l) qe (exp) (mg/g) Pseudo-first-order equation

qe (cal) (mg/g) k1 (l/min)

CS beads 500 160.19 148.25 6.22 � 10�3

CS/CTAB beads 356.03 378.44 8.29 � 10�3

log ðqe � qtÞ ¼ log qe �k1

2:303t ð9Þ

where qe and qt are the amounts of nitrate adsorbed (mg/g) at equi-librium and at time (t), respectively, and k1 (1/min) is the rate con-stant of this equation. The values of pseudo-first-order rateconstants, k1 and qe values obtained from this rate model were cal-culated from slopes and intercepts of the plots of log (qe�qt) vs. tand are listed in Table 4. The correlation coefficient (R2) values forthe pseudo-first-order equation are 0.996 (CS beads) and 0.974(CS/CTAB beads). The value of k1, calculated from a pseudo-first-or-der equation for CS/CTAB beads is higher compared to CS beads,indicating that the mass transfer rate was higher for CS/CTAB beadsas compared to CS beads. The qe values calculated from this ratemodel agree well with qe (exp) values.

The pseudo second-order rate equation is expressed as:

tqt¼ 1

hþ 1

qet where; h ¼ k2q2

e ð10Þ

where h represents the initial adsorption rate (mg/g min) and k2 (g/mg min) is the pseudo second-order rate constant. The values of qe,k2 and h can be obtained from the slope and intercepts of the t/qt vs.t plots and are given in Table 4. The correlation coefficient (R2) val-ues for the pseudo-second-order equation are 0.996 (CS beads) and0.990 (CS/CTAB beads). A comparison of experimental and calcu-lated kinetic results using different kinetic models for adsorptionof CR (500 mg/L) onto CS beads, as well as CS/CTAB beads, is shownin Fig. 2. Both pseudo-first-order and pseudo-second-order ratemodels show good fit to experimental kinetic data, but the pseu-do-second-order rate model is a better fit than the pseudo-first-or-der. The h value calculated from pseudo-second-order equation ishigher for CS/CTAB beads than for CS beads, indicating a highermass transfer rate for CS/CTAB beads.

The intra-particle diffusion equation is given as:

qt ¼ kpt0:5 ð11Þ

kp, the intra-particle diffusion rate constant (mg/g min0.5) can beobtained from the slope of the qt vs. t0.5 plot and is listed in Table4. The plot of qt vs. t0.5 using initial kinetic data up to 180 min givescorrelation coefficients (R2) values of 0.990 for CS beads and 0.980for CS/CTAB beads. The linearity of the plots indicates that intra-particle diffusion might play a significant role in the initial stageof adsorption of CR onto CS beads, as well as CS/CTAB beads (Annad-urai et al., 2002), because the intra-particle diffusion equation is va-lid only for initial kinetic data.

3.8. Performance evaluation

The maximum adsorption capacity (qmax) of CTAB modified CSbeads calculated from the Sips isotherm model is listed in Table5 with literature values of qmax of other adsorbents for CR adsorp-tion. Dry weight of adsorbent (g) has been used for comparison ofqmax values (mg CR/g dry weight) of all the adsorbents listed in Ta-ble 5. All of the adsorbents used for CR adsorption have consider-ably lower qmax values than the CTAB modified CS beads used inthis study, except N,O-carboxymethyl chitosan (Wang and Wang,

Pseudo-second-order equation Intra particle diffusion

qe (cal) (mg/g) k2 (g/mg min) h (mg/g min) kp (mg/g min0.5)

183.42 5.16 � 10�5 1.74 8.70421.41 1.99 � 10�5 3.54 19.85

t (min)

0 200 400 600 800 1000 1200

qt (

mg

/g)

0

200

400

600

800Experimental data (CS beads)Pseudo-first-order (CS beads)Pseudo-second-order (CS beads)Intra particle diffusion (CS beads)Experimental data (CS/CTAB beads)Pseudo-first-order (CS/CTAB beads)Pseudo-second-order (CS/CTAB beads)Intra particle diffusion (CS/CTAB beads)

Fig. 2. Plots of qt vs. t for CR adsorption onto CS beads and CS/CTAB beads; initial CRconcentration, 500 mg/l, at pH 5.

Table 5Summary of congo red adsorption capacities of various adsorbents.

Type of adsorbent qmax

(mg/gdw)

Reference

Activated carbon prepared from coir pith 6.70 Namasivayam andKavitha (2002)

NaHCO3 pretreated Aspergillus niger biomass 8.19 Fu and Viraraghavan(2002)

Neem leaf powder 41.20 Bhattacharrya andSharma (2004)

Bagasse fly ash 11.89 Mall et al. (2005)Activated carbon (Laboratory grade) 1.88 Mall et al. (2005)Acid activated red mud 7.08 Tor and Cengeloglu

(2006)Mesoporous activated carbons 189 Grabowska and

Gryglewicz (2007)Montmorillonite 12.70 Wang and Wang (2007)Chitosan beads 93.71 Chatterjee et al. (2007)Chitosan/montmorillonite nanocomposite 54.52 Wang and Wang (2007)N,O-carboxymethyl chitosan 330.62 Wang and Wang

(2008a)4-vinyl pyridine grafted poly (ethylene

terephthalate) fibers18.1 Arslan and Yigitoglu

(2008)Anilinepropylsilica xerogel 22.62 Pavan et al. (2008)Ca-bentonite 107.41 Lian et al. (2009)CTAB modified chitosan beads 433.12 This work

2808 S. Chatterjee et al. / Bioresource Technology 100 (2009) 2803–2809

2008a). However, the higher qmax value and the simplicity of thepreparation method of CTAB modified CS beads makes it betteradsorbent than N,O-carboxymethyl chitosan for CR adsorption,which requires chemical grafting reactions.

The qmax value of CS/CTAB beads (433.12 mg CR/g dry weight)for CR adsorption was 1.94 times higher than that of CS beads(223.25 mg CR/g dry weight), as shown in Table 3. The maximumadsorption capacity of beads in the volume basis (mg CR/ml vol-ume of wet beads) has been obtained by multiplying qmax (mg/g)of beads with dry weight (g) of beads/ml volume of beads. How-ever, the volumetric qmax value in mg/ml of beads is much higher(3.02 times) for CS/CTAB beads (15.16 mg CR/ml bead) as com-pared to CS beads (5.02 mg CR/ml bead) because dry weight con-tent of CS/CTAB beads increased from 0.0225 g/ml to 0.035 g/mlafter CTAB modification. Hence, CTAB modification is more advan-tageous for practical use of CS/CTAB beads because it requiressmaller adsorbent volume than CS beads and the size of adsorbentcolumn will be less.

4. Conclusion

Cetyl trimethyl ammonium bromide (CTAB) was used to modifychitosan beads (CS beads) and the CS/CTAB beads were applied forthe removal of congo red (CR) from aqueous solution. CTAB at aconcentration of 0.05% in CS/CTAB beads substantially increasedthe adsorption capacity from 162.32 mg/g (0% CTAB) to352.45 mg/g (0.05% CTAB) at pH 5. The adsorption process wasdependent on pH, and the adsorption capacity increased withdecreasing pH. Equilibrium adsorption isotherm data indicated agood fit to the Sips isotherm model and the adsorption processwas heterogeneous. The adsorption process followed a pseudo-sec-ond-order rate model better than a pseudo-first-order rate model.Intra-particle diffusion played a significant role at the initial stageof adsorption process. The CS/CTAB beads showed the highestadsorption of CR among previously reported results in the litera-ture and the adsorption capacity significantly increased in the vol-ume basis of the beads, as compared to the dry weight basis.

Acknowledgement

This work was supported by grants from the Korea Science andEngineering Foundation (KOSEF) through the Advanced Environ-mental Biotechnology Research Center (AEBRC, R11-2003-006).

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