Bioresource Technology 101 (2010) 1800–1806

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Bioresource Technology

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Adsorption of congo red by chitosan hydrogel beads impregnatedwith carbon nanotubes

Sudipta Chatterjee a, Min W. Lee b,*, 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 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 a b s t r a c t

Article history:Received 29 July 2009Received in revised form 21 September 2009Accepted 22 October 2009Available online 4 December 2009

Keywords:AdsorptionCarbon nanotubesChitosanCongo redHydrogel beads

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.10.051

* Corresponding authors. Tel.: +82 54 279 8650; faxtel.: +82 42 821 1537; fax: +82 42 821 1593 (S.H. Wo

E-mail addresses: minos@postech.ac.kr (M.W. LeWoo).

The adsorption performance of chitosan (CS) hydrogel beads was investigated after multiwalled carbonnanotubes (MWCNTs) impregnation for the removal of congo red (CR) as an anionic dye. The study ofthe adsorption capacity of CS/CNT beads as a function of the CNT concentration indicated that 0.01%CNT impregnation was the most useful for enhancing the adsorption capacity. The sulfur (%) in the CS/CNT beads measured by energy dispersive X-ray (EDX) was 2.5 times higher than that of normal CS beadsafter CR adsorption. Equilibrium adsorption isotherm data of the CS/CNT beads exhibited better fit to theLangmuir isotherm model than to the Freundlich isotherm model, and the heterogeneity factor (n) valueof the CS/CNT beads calculated from the Sips isotherm model was close to unity (0.98). The maximumadsorption capacity of CS/CNT beads obtained from the Langmuir model was 450.4 mg g�1.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Textile industries discharge large amounts of colored wastewa-ter containing various dyes, some of which are mutagenic and car-cinogenic to human beings (Crini, 2006). Dyeing effluents arecharacterized by their fluctuating pH with large loads of suspendedsolids, high oxygen demand, resistance to biodegradability and sta-bility to light, heat and oxidizing agents (Ozcan et al., 2005; Sivarajet al., 2001). Azo dyes represent about 50% of all dye varieties andthese dyes are of great environmental concern due to their hugeapplications and recalcitrance. The removal of such type of dye-stuffs from effluent before discharging into natural water bodiesis extremely important from an environmental point of view. Sucheffluents cause abnormal coloration on the surface of the waterand block photosynthetic bacteria and aquatic plants from sunlight(Cheung et al., 2009).

Congo red (CR) (sodium salt of benzidinediazobis-1-naphthyl-amine-4-sulfonic acid) is a benzidine-based azo dye and it was se-lected in this study as a model anionic dye because of its complexchemical structure, high solubility in aqueous solution and its per-sistence, once it is discharged into natural environment. CR ismetabolized to benzidine, a known human carcinogen and expo-

ll rights reserved.

: +82 42 279 8299 (M.W. Lee),o).

e), shwoo@hanbat.ac.kr (S.H.

sure to this dye can cause some allergic responses. CR mainly oc-curs in the effluents discharged from textile, paper, printing,leather industries, etc. (Han et al., 2008) and during dyeing opera-tion, about 15% of CR ends up in wastewaters. There are many pro-cesses to remove CR molecules from colored effluents and thetreatment methods can be divided into three categories: (1) phys-ical methods such as adsorption (Namasivayam and Kavitha, 2002;Mall et al., 2005; Chatterjee et al., 2009b); (2) chemical methodssuch as ozonation (Gharbani et al., 2008; Khadhraoui et al.,2009), photo degradation (Wahi et al., 2005) and electrochemicalprocess (Elahmadi et al., 2009); and (3) biodegradation (Gopinathet al., 2009).

In general, the treatment of dyeing effluent is done by adsorp-tion, coagulation–flocculation, oxidation–ozonation, reverse osmo-sis, membrane filtration, biological degradation and electro-chemical processes (Kim et al., 2004; Shen et al., 2001). Nowadays,adsorption has been recognized as the most popular treatmentprocess for the removal of dye from an aqueous solution due toits simplicity, high efficiency, easy recovery and the reusability ofthe adsorbent (Garg et al., 2003; Aksu, 2005). The removal of dye-stuffs through adsorption onto activated carbon shows high effi-ciency, but it is an expensive process and there is difficulty in theregeneration of the adsorbent (Kadirvelu et al., 2003). In order todecrease the cost of treatment, attempts have been made to findmany alternative low cost adsorbents, such as activated carbonprepared from coir pith (Namasivayam and Kavitha, 2002), clayminerals (Gürses et al., 2004), rice husk (Han et al., 2008), leaf

S. Chatterjee et al. / Bioresource Technology 101 (2010) 1800–1806 1801

powder (Bhattacharrya and Sharma, 2004), fly ash (Acemioglu,2004), bacterial biosorbents (Vijayaraghavan and Yun, 2008), andfungus (Fu and Viraraghavan, 2002) for the removal of dye fromwater. However, low adsorption capacities of these adsorbents to-ward dyes limit their applications in practical field.

Chitosan (CS), the deacetylated product of chitin, exhibits a highadsorption capacity towards many classes of dyes due to its multi-ple functional groups, biocompatibility and biodegradability(Majeti and Kumar, 2000). CS-based adsorbents are versatile mate-rials in view of their use in different forms; from flake or powder,to hydrogel bead types. Recent review papers reported that CS-based adsorbents that are usually used in the form of hydrogelbeads have shown the highest adsorption capacity for numerousdyes (Crini and Badot, 2008). However, low the mechanicalstrength of CS hydrogel beads limits their commercial applicationas an adsorbent. Several chemical modification steps, includingchemical cross-linking (Chiou et al., 2004), poly amination (Kimand Cho, 2005) and carboxy alkyl substitution (Mourya and Inam-dar, 2008), have been performed to increase the mechanicalstrength of CS hydrogel beads. These methods may not satisfacto-rily improve mechanical stability, and sometimes cause a signifi-cant decrease in adsorption capacity (Crini and Badot, 2008).

Different nanofillers, such as clay and silica nanoparticles, havebeen used to reinforce the base-material chitosan in polymer nano-composite (Wang and Wang, 2007; Liu et al., 2005). Recently, thecarbon nanotube (CNT) has been used as a promising nanofillerfor the preparation of chitosan–CNT nanocomposites because ofits excellent mechanical, electrical and thermal properties (Wanget al., 2005; Kandimalla and Ju, 2006; Hao et al., 2007). Several re-ports demonstrated that CNTs have good adsorption capacities fordifferent materials due to their hollow and layered nanosizedstructures that have a large specific surface area (Long and Yang,2001; Li et al., 2002; Peng et al., 2003). CNTs are reported to bemore adsorbent than activated carbon for dioxin removal (Longand Yang, 2001), and recently CNTs have been used for the absorp-tion of dye molecules (Wu, 2007).

We have recently reported that the impregnation of CS hydrogelbeads with CS hydrogel beads impregnated with CNTs (CS/CNTbeads) resulted in significantly improved mechanical strength(Chatterjee et al., 2009c). In this study, CS/CNT beads were usedto remove CR from aqueous solution by batch adsorption processand parameters affecting the adsorption capacity of the CS/CNTbeads, including CNT concentration variation and pH, were inves-tigated. Some characterization studies, such as scanning electronmicroscopy (SEM), infrared (IR) spectroscopy and energy disper-sive X-ray (EDX), were performed in order to determine the modeof interaction between the congo red (CR) molecules and the beadsduring adsorption. Models fit to equilibrium isotherm and kineticdata were presented here to validate the usefulness of these novelCS/CNT hydrogel beads in the field of wastewater treatment.

2. Methods

2.1. Materials

Chitosan (CS, >85% deacetylation), cetyltrimethylammoniumbromide (CTAB), and congo red (CR) used in this study were pur-chased from Sigma Chemical Co., USA. The multiwalled carbonnanotubes (MWCNTs), manufactured by the catalytic chemical va-por deposition (CCVD) of CH4 over Fe–Mo/MgO catalysts, werepurchased from NanoSolution Co., Korea. The MWCNTs are 5–10 nm in diameter and 10–20 lm in length. The metal catalystand support in the as-prepared samples were removed by dissolv-ing them in hydrochloric acid, and then the carbon nanotube pow-der was filtered and washed with de-ionized water to remove any

acidic content on the surface of the MWCNTs. The purity of theresulting MWCNTs was more than 95 wt.%, which was determinedby TGA (thermogravimetric analysis). The BET surface area ofMWCNTs was 237.76 m2 g�1 (Chatterjee et al., 2009c).

2.2. Preparation of CS/CNT beads

The CS/CNT beads were synthesized using the same method de-scribed in our earlier publication (Chatterjee et al., 2009c). TheMWCNTs were first dispersed in a solution of CTAB (2 wt.%) in aweight ratio of 1:5 using 750 Watt ultrasonic processor (VC 750,Sonics) with a high power sonic tip operated at 20 kHz frequencyand 30% amplitude for 30 min (30 s on, 5 s off). The resultant blackdispersion of the MWCNTs was added to a CS solution (1 wt.% CS in2 vol.% acetic acid) to get a stable dispersion of MWCNTs and CSwith a weight ratio of 1:100. CS solution was prepared by dissolv-ing 10.0 g of CS powder in 0.4 dm3 of 5% (v/v) acetic acid solution,followed by dilution of CS–acetic acid solution to 1 dm3 with de-ionized water. The drop-wise addition of CS–MWCNT dispersioninto a precipitation bath containing 1 dm3 of alkaline coagulatingmixture (H2O:MeOH:NaOH = 4:5:1, w/w) gave rise to the CS/CNTbeads. The beads were extensively washed with de-ionized waterand preserved in an aqueous environment for future use.

2.3. Characterization of adsorbent

A cold type field emission scanning electron microscope (FE-SEM) (HITACHI, S-4800) was used to record the surface morphol-ogy of freeze-dried CS and CS/CNT beads. The thermogravimetricanalysis (TGA) of the beads was performed with a TGA851 thermo-gravimetric analyzer (Mettler Toledo) under N2(g) flow with a heat-ing rate of 10 �C min�1. The infrared (IR) spectra of the CS/CNTbeads before and after adsorption were characterized using a FTIRSpectrophotometer (Nicolet 6700, Thermal) with a KBr pellet. Anelemental analysis of the samples before and after adsorptionwas done by energy dispersive X-ray (EDX) spectrometer(JSM6390, JEOL).

2.4. Batch adsorption studies

Batch adsorption experiments were conducted with the CS/CNTbeads for the removal of CR from aqueous solutions. Equilibriumadsorption experiments were conducted by adding 0.2 g wetweight of CS/CNT beads into 0.01 dm3 of the CR solution of the de-sired concentration at pH 5 and at a temperature of 30 �C. Experi-mental solutions of CR were obtained by dilution from a stocksolution (1000 mg dm�3) of CR in de-ionized water. All of theadsorption experiments were conducted in triplicate and per-formed for 24 h in shaking conditions (200 rpm). The adsorptionperformance of the CS/CNT beads was studied as the function ofthe CNT concentration variation from 0.001 to 0.05 wt.% in theCS/CNT beads with an initial CR concentration of 500 mg dm�3.The effect of a pH change was studied by changing the initial pHof the CR solutions from 4 to 9; the initial CR concentration wasfixed at 500 mg dm�3. Equilibrium isotherm studies were carriedout with different initial concentration of CR (10–1000 mg dm�3)at a fixed temperature (30 �C). The non-linear forms of the Lang-muir, Freundlich and Sips isotherm models were used to analyzethe equilibrium adsorption isotherm data and the isotherm modelswere evaluated by the non-linear coefficients of determination (R2)and a non-linear Chi-square test (v2). Kinetic adsorption experi-ments using CS/CNT beads were performed at pH 5 with an initialCR concentration of 500 mg dm�3. Different time intervals of up to960 min were used for this study. The residual CR concentration inthe experimental solution (mg dm�3) was analyzed using a spec-trophotometer (HACH DR-5000, USA) at a kmax of 497 nm. The

Table 2Elemental analysis of CS and CS/CNT beads by EDX spectra.

Element Weight percentage (wt.%) of element

CS beads CS/CNT beads

Beforeadsorption

Afteradsorption

Beforeadsorption

Afteradsorption

C 56.60 60.07 60.16 63.87O 43.40 38.10 39.84 31.51S 0 1.83 0 4.62

1802 S. Chatterjee et al. / Bioresource Technology 101 (2010) 1800–1806

amount of CR adsorbed (mg g�1) was calculated based on a massbalance equation as given below:

qe ¼ðC0 � CeqÞ � V

Wð1Þ

where qe is the equilibrium adsorption capacity per gram dryweight of the adsorbent, mg g�1; C0 is the initial concentration ofCR in the solution, mg dm�3; Ceq is the final or equilibrium concen-tration of CR in the solution, mg dm�3; V is the volume of the solu-tion, dm3; and W is the dry weight of the hydrogel beads, g.

3. Results and discussion

3.1. General properties

The CS/CNT beads are dark black in color due to the addition ofthe MWCNTs, and it appears that the MWCNTs are uniformly dis-tributed throughout the beads. The diameter (D) and porosity (e) ofthe beads (Table 1) can be determined using equations consideringthe water content within the pores of the beads (Chatterjee et al.,2009a):

D ¼ 6WD=qMat þ ðWW �WDÞ=qW

p

� �1=3

ð2Þ

e ¼ ðWW �WDÞ=qW

WD=qMat þ ðWW �WDÞ=qW� 100% ð3Þ

where WW (g) is the weight of the wet chitosan beads before drying;WD (g) is the weight of the chitosan beads after drying; qw is thedensity of water, 1.0 g cm�3; and qMat is the density of material,g cm�3.

As shown in Table 1, the CS/CNT beads are found to have lesswater content (96.1%) than the CS beads (96.7%), indicating thatCNT impregnation has made the beads materially denser. Thematerial density of the CS/CNT beads was significantly higher thanthat of the CS beads due to the presence of CNT. While porosity inthe CS/CNT beads increased from 84.99% to 95.15% after the addi-tion of CNT into the beads, the diameter of the CS/CNT beads(2.66 mm) was less than that of the CS beads (2.87 mm).

3.2. Characterization

FE-SEM images of both CS beads and CS/CNT beads showed theheterogeneous and porous structure of the beads (figure notshown). The polymeric network of both types of beads was gener-ated by the CS molecules contained within the beads and theMWCNT impregnation made the beads denser compared to normalCS beads. A uniform distribution of CNTs throughout the CS matrixwith some small aggregates of CNTs was observed in the CS/CNTbeads.

TGA curves of the solid-state samples of the CS beads and theCS/CNT beads (figure not shown) exhibited small weight losses

Table 1General properties of CS and CS/CNT beads.

General properties CS beads CS/CNT beads

Wet weight (WW, mg) 10.9 9.8Dry weight (WD, mg) 0.36 0.38Water content (%) 96.7 96.1Bulk densitya (g cm�3) 0.68 0.58Material densityb (g cm�3) 0.193 0.792Porosity (e, %) 84.99 95.15Diameter (D, mm) 2.87 2.66

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

(about 3%) at temperatures between 35 and 100 �C due to thevaporization of the residual water. Both of the samples showedsimilar weight losses at temperatures ranging from 200 to600 �C. This occurs because the thermal decomposition of the sam-ples involves depolymerization and the decomposition of the glu-cosamine units of chitosan at 200–400 �C, which is followed bythe oxidative decomposition of the residues at temperatures be-tween 400 and 600 �C (Zhang et al., 2004). There was a differencein the weight loss between the CS beads (90.6%) and the CS/CNTbeads (73.0%) at 1200 �C, indicating that impregnation of the CSbeads with CNTs increased thermal stability.

FTIR spectra of the CS/CNT beads (figure not shown) showedsome characteristic peaks: 3417 cm�1 (wide peak of O–H stretch-ing overlapped with N–H stretching), 2923 cm�1 (C–H stretching),1636 cm�1 (C@O stretching of N-acetyl groups), 1371 cm�1 (asym-metric C–H bending of CH2 groups) and 1029 cm�1 (bridge C–O–Cstretching). The FTIR spectra of the CS/CNT beads exhibit a shiftingof the band from 3417 cm�1 to 3421 cm�1 after the adsorption ofCR. The peak at 1029 cm�1 has shifted to 1036 cm�1 and a newpeak appears at 1156 cm�1 after adsorption. The peak at1636 cm�1 corresponding to the C = O stretching of the N-acetylgroups of CS shifts to 1650.18 cm�1 after adsorption. Therefore,the adsorption of CR onto CS/CNT beads occurs by involving the–OH and –NH2 groups of CS.

The results of the EDX analysis (Table 2) show that the CS andthe CS/CNT beads consist of mainly carbon (C) and oxygen (O).The increase of carbon content in the CS/CNT beads was consistentwith the added amount of CNT. CR adsorption onto the CS beadsand the CS/CNT beads gives rise to a sulfur (S) peak in the EDXspectra of the samples. The higher S (wt.%) in the CS/CNT beads(4.62%) compared to that of the CS beads (1.83%) after CR adsorp-tion indicates that CNT impregnation increases the adsorptioncapacity of CS/CNT beads.

3.3. Effect of CNT concentration in beads

The effect of CNT concentration variation from 0.001 to0.05 wt.% in the CS/CNT beads on the absorption of CR with an ini-tial concentration of 500 mg dm�3 was monitored. The adsorptioncapacity of the CS/CNT beads is given as a function of the CNT con-centration in Fig. 1. The maximum adsorption was observed at0.01 wt.% CNT in the CS/CNT beads and thereafter the adsorptioncapacity decreased with an increase in the CNT concentration upto 0.05 wt.%. The high adsorption capacity of the CS/CNT beads ismainly due to the addition of a dispersant CTAB (Chatterjeeet al., 2009b). The enhancing effect of CTAB on adsorption wouldbe partly due to the hydrophobic interactions between the hydro-phobic tail of CTAB and the hydrophobic moieties of CR, and partlydue to the ionic interaction between a cationic charge on the CTABand an anionic charge on CR. CNTs are already reported as goodadsorbents for various materials due to its large specific surfacearea, and hollow and layered nanosized structures (Li et al.,2002; Peng et al., 2003). The CS/CNT beads containing more CNTsshould have given better adsorption capacity because CNTs have

CNT concentration (wt %) in CS/CNT beads0.00 0.01 0.02 0.03 0.04 0.05 0.06

q e (m

g g-1

)

300

350

400

450

C0 = 500 mg dm-3

CS concentration in CS/CNT beads = 1 wt %CTAB concentration in CS/CNT beadsa = 0.05 wt %

Fig. 1. Effect of CNT concentration variation (wt.%) in the CS/CNT beads on CRadsorption: initial CR concentration, 500 mg dm�3 and pH 5. aControl without CNTimpregnation contains same amount of CTAB (0.05 wt.%).

400

500

600 Experimental data Langmuir isotherm model Freundlich isotherm model Sips isotherm model 30 0C, pH 5

S. Chatterjee et al. / Bioresource Technology 101 (2010) 1800–1806 1803

a large specific surface area (237.8 m2 g�1), and strong interactionsbetween the benzene rings of CR and hexagonal arrays of carbonatoms in the CNTs occur during adsorption (Long and Yang,2001). However, the adsorption capacity of the CS/CNT beads in-creased with increase in CNT concentration up to a certain level(0.01 wt.%). The higher CNT concentrations in the CS/CNT beads ap-pear to induce the formation of larger aggregates of CNTs, whichobstruct the access of the CR to the adsorption site of CS and CTAB.This result indicated that the adsorption performance of the CS/CNT beads depends on the homogeneous and stable dispersion ofthe CNTs in the CS matrix. The addition of CNT can block some ofthe adsorption sites of CS and CTAB and the blocking could be en-hanced at increased level of CNT impregnation. Thereby, our re-sults suggest that the addition of CNTs could enhance theadsorption capacity of the CS/CTAB beads only if proper amountsof CNTs are added. The optimal content of CNT at 0.01 wt.% sug-gests that the positive effects of CNTs during adsorption are mostlynullified by negative effects due to aggregate formation and block-ing of adsorption sites by CNTs.

The effect of CNT impregnation on the adsorption capacity ofCS/CNT beads has been statistically evaluated by unpaired t test.The t and P values obtained from t test using data for beads withoutCNT and 0.01 wt.% CNT are 21.46 and less than 0.0001, respec-tively, and increase is statistically significant with a 95% confidenceinterval. The t and P values obtained for other CNT concentrationssuggest that the increase is statistically significant up to 0.02 wt.%CNT in beads. Moreover, the values (t = 9.42 and P = 0.0007) ob-tained from the t test using data for beads with 0.05% CNT impreg-nation and without impregnation indicate that the decrease is alsostatistically significant with a 95% confidence interval.

Ce (mg dm-3)0 200 400 600 800

q e (m

g g-1

)

0

100

200

300

Fig. 2. Plots of qe vs. Ce for the adsorption of CR onto the CS/CNT beads; pH 5 and30 �C.

3.4. Effect of pH

The effect of the initial pH, ranging from 4 to 9, on the adsorp-tion capacity of the CS/CNT beads for the adsorption of CR(500 mg dm�3) was investigated. Measurements of the pH of thesolution during adsorption indicated that a pH change occurredat the initial stage (within 1 h) of the adsorption process and thenthe pH leveled off to 6 from an initial value of 5. Since beads wereoriginally formed in a sodium hydroxide precipitation bath, theamine groups of CS molecules were not protonated. During adsorp-tion, protonation of amine groups is necessary for its interactionwith negatively charged CR molecules. A high concentration ofhydrogen ion (H+) at acidic pH leads to protonation of amine

groups and an increase in pH from initial pH 4 to 5.6 and pH 5 to6 during adsorption indicates a decrease in H+ concentration inthe solution. This type of pH change was not found during adsorp-tion when the initial pH of the CR solution was neutral (pH 7) orbasic (pH 9). This observation could be explained by the fact thatpH change due to protonation of amine groups does not occur atpH 7 and pH 9 due to absence of free H+ in solution. No significantpH change during adsorption was also found while initial pH of CRsolution was 6, suggesting that protonation of amine groups is noteffective due to low concentration of free H+ in solution. Theadsorption of CR onto CS/CNT was highly pH dependant and max-imum adsorption by the CS/CNT beads (423.1 mg g�1) occurred atpH 4. The adsorption was found to decrease with an increase in pHfrom 4.0 to 9.0 and the adsorption capacities of the CS/CNT beadsproportionally decreased from 423.1 to 253.2 mg g�1 (data notshown). In this full range of pH, the adsorption capacity of theCS/CNT beads was shown to be higher than that of the CS beads.

3.5. Adsorption isotherm

Fig. 2 shows the equilibrium adsorption of CR onto the CS/CNTbeads (qe vs. Ce). The non-linear forms of the Langmuir, Freundlichand Sips isotherm models have been used to interpret the experi-mental isotherm data. The non-linearized form of the Langmuiradsorption isotherm equation is:

qe ¼qmKLCe

1þ KLCeð4Þ

where Ce is the concentration of CR in solution at equilibrium(mg dm�3), qe is the adsorption capacity at equilibrium (mg g�1).The Langmuir constant, qm (mg g�1) is related to the maximumadsorption capacity (mg g�1) of the adsorbent and KL is the constantterm related to the energy of adsorption (dm3 mg�1).

The maximum adsorption capacity (qm) of the CS/CNT beads forCR was 450.4 mg g�1 (Table 3). In our previous report, the maxi-mum adsorption capacity of the CS/CTAB beads for CR was433.12 mg g�1 (Chatterjee et al., 2009b), which was the highest va-lue among the other adsorbents reported earlier for CR adsorption(Namasivayam and Kavitha, 2002; Han et al., 2008; Bhattacharryaand Sharma, 2004; Fu and Viraraghavan, 2002). Impregnation of0.05 wt.% CTAB increased the maximum adsorption capacity ofCS beads from 223.25 to 433.12 mg g�1 (Chatterjee et al., 2009b).CTAB was also used in the preparation of CS/CNT beads at the sameamount of CTAB. The small difference in maximum adsorption

Table 3Constants for isotherm models with error analysis values for CS/CNT beads.

Isotherm model Parameter value

Langmuir isotherm kL (dm3 mg�1) qm (mg g�1) RLa R2 v2

0.031 450.4 0.031 0.998 4.22Freundlich isotherm KF (dm3 g�1) 1/n R2 v2

61.73 0.313 0.905 113.46Sips isotherm qmax (mg g�1) Keq (dm3 mg�1) n R2 v2

500 0.029 0.980 0.998 10.41

a C0 is 1000 mg dm�3.

Table 4Constants of different rate models for CS/CNT beads at the initial concentration of 500 mg dm�3.

Kinetic model Parameter value

Pseudo-first-order equation qe (cal) (mg g�1) k1 (min�1) R2

412.6 7.80 x 10�3 0.994

Pseudo-second-order equation qe (cal) (mg g�1) k2 (g mg�1 min�1) h (mg g�1 min�1) R2

490.8 1.78 � 10�5 4.30 0.977

Intra-particle diffusion kp (mg g�1 min�0.5) R2

20.81 0.947

1804 S. Chatterjee et al. / Bioresource Technology 101 (2010) 1800–1806

capacity of CS/CNT beads (450.4 mg g�1) and CS/CTAB beads(433.12 mg g�1) indicated that CTAB molecules play a significantrole in enhancing adsorption performance of both varieties ofbeads. However, higher maximum adsorption capacity of CS/CNTbeads than CS/CTAB beads suggested that CNTs itself in the beadsadsorb CR during adsorption as clearly indicated in Fig. 1.

The RL value of the CS/CNT beads (0.031) obtained from Lang-muir isotherm model for the initial CR concentration of1000 mg dm�3 indicated favorable adsorption of CR onto the CS/CNT beads.

A non-linear form of Freundlich equation is:

qe ¼ kFC1ne ð5Þ

where KF (dm3 g�1) is the Freundlich constant related to the adsorp-tion capacity and 1/n measures the surface heterogeneity. The val-ues of KF and 1/n from this isotherm model are listed in Table 3.

The Sips model is expressed as:

qe ¼qmaxKeqCn

e

1þ KeqCne

ð6Þ

Keq (dm3 mg�1) represents the equilibrium constant of the Sipsequation and qmax (mg g�1) is the maximum adsorption capacity.The Sips isotherm model is characterized by the heterogeneity fac-tor, n, and specifically when n = 1, the Sips isotherm equation re-duces to the Langmuir equation and it implies a homogeneousadsorption process. The value of the heterogeneity factor (n) ofthe CS/CNT beads (0.98) indicates a homogeneous adsorption pro-cess (Table 3). The maximum adsorption capacity (qmax) of the CS/CNT beads obtained from the Sips isotherm was 500.0 mg g�1.

As shown in Fig. 2, the Langmuir isotherm model showed betterfit to the experimental isotherm data than the Freundlich isothermmodel. The results of the non-linear R2 and v2 for the three adsorp-tion isotherms (Table 3) indicated that the Langmuir isothermmodel appeared to be the best fitting model for the adsorption iso-therm data of the CS/CNT beads because it displayed the highest R2

(0.998) and the lowest Chi-square, v2 (4.22) values. The maximumadsorption capacity of CS/CTAB beads obtained from Langmuirisotherm model in our previous report is 385.9 ± 3.92 mg g�1

(Chatterjee et al., 2009b) and its difference from the maximumadsorption capacity of CS/CNT beads (450.4 ± 7.86 mg�1) is statisti-

cally significant with a 95% confidence interval. Thus, the effect ofCNT itself on adsorption capacity excluding the effect of CTAB isnot much high but obviously has some positive effect, which is alsoclearly supported by the increasing trend in Fig. 1.

3.6. Kinetic study

Experimental kinetic data using the CS/CNT beads for adsorp-tion of CR from a 500 mg dm�3 solution have been illustrated inFig. 3. The maximum adsorption by the CS/CNT beads occurredwithin 300 min and no further change in the uptake value wasfound after 360 min.

In order to show the most suitable model for the experimentalkinetic data, different kinetic models were used in this study. Theyare listed as follows: (i) pseudo-first-order rate model; (ii) pseudo-second-order rate model; (iii) and intra-particle diffusion model.The non-linear form of the pseudo-first-order rate equation isgiven as:

qt ¼ qeð1� e�k1tÞ ð7Þ

where qe and qt are the amounts of CR adsorbed (mg g�1) at equilib-rium and at a predetermined time (t), respectively, and k1 (min�1) isthe rate constant of this equation. The values of pseudo-first-orderrate constants, k1 and qe values are listed in Table 4. The R2 value ofthe non-linear form of the pseudo-first-order rate equation for theCS/CNT beads was 0.994 and the calculated value of qe

(412.6 mg g�1) from this rate model was very close to the experi-mental qe value (401.7 mg g�1).

The non-linear form of the pseudo-second-order rate equationis expressed as:

qt ¼q2

e k2t1þ qek2t

and h ¼ k2q2e ð8Þ

where h represents the initial adsorption rate (mg g�1 min�1) and k2

(g mg�1 min�1) is the pseudo-second-order rate constant. The val-ues of qe, k2 and h obtained from this rate model are given in Table4. The qe value of the CS/CNT beads obtained from the pseudo-sec-ond-order equation was 490.8 mg g�1 and the correlation coeffi-cient (R2) value of the CS/CNT beads for the non-linear form ofthe pseudo-second-order equation was 0.977, indicating that

t (min)

0 200 400 600 800 1000 1200

qt (

mg

g-1

)

0

100

200

300

400

500

Experimental dataPseudo-first-order modelPseudo-second-order modelIntraparticle diffusion modelC0 = 500 mg dm-3, pH 5

Fig. 3. Plots of qt vs. t for CR adsorption onto the CS/CNT beads; initial CRconcentration, 500 mg dm�3, pH 5.

S. Chatterjee et al. / Bioresource Technology 101 (2010) 1800–1806 1805

experimental kinetic data for CR adsorption by the CS/CNT beadsfollowed the pseudo-first-order rate model better than the pseu-do-second-order model.

The intra-particle diffusion equation is given as:

qt ¼ kpt0:5 ð9Þ

kp, the intra-particle diffusion rate constant (mg g�1 min�0.5) islisted in Table 4. The plot of qt vs. t0.5 using the initial kinetic dataup to 180 min gave correlation coefficient (R2) values of 0.947 forthe CS/CNT beads. The high R2 value indicates that intra-particle dif-fusion might play a significant role in the initial stage of the adsorp-tion of CR onto the CS/CNT beads.

While the value of k1 (7.80 � 10�3 min�1), calculated from apseudo-first-order model, for the CS/CNT beads is a bit lower thanthat (8.29 � 10�3 min�1) of the CS/CTAB beads. The h value(4.30 mg g�1 min�1) from a pseudo-second-order model and kp va-lue (20.81 mg g�1 min�0.5) from an intra-particle diffusion modelare higher than those (3.54 mg g�1 min�1 for h, and19.85 mg g�1 min�0.5 for kp) of the CS/CTAB beads. These valuesindicate that the mass transfer rate was somewhat increased bythe addition of CNT.

3.7. Performance and cost evaluation

The comparison of maximum adsorption capacity of CS/CNTbeads (450.4 mg g�1) with that of other adsorbents (in dry weightbasis of adsorbent) such as 330.62 mg g�1 of N,O-carboxymethylchitosan (Wang and Wang, 2008), 66.23 mg g�1 of palm kernelseed coat (Oladoja and Akinlabi, 2009), 49.7 mg g�1 of magneticallymodified fodder yeast cells (Safarik et al., 2007), 41.20 mg g�1 ofneem leaf powder (Bhattacharrya and Sharma, 2004), 11.89 mg g�1

of bagasse fly ash (Mall et al., 2005), 7.08 mg g�1 of acid activatedred mud (Tor and Cengeloglu, 2006), etc., and 51.81 mg g�1 ofautoclaved mycelial biomass of Trametes versicolor (Binupriyaet al., 2008) and 14.16 mg g�1 of sodium bicarbonate pretreatedAspergillus niger biomass (Fu and Viraraghavan, 2002) (in wetweight basis of biosorbent) indicates that maximum adsorptioncapacity of CS/CNT hydrogel beads was higher than any other pre-viously reported adsorbent. This indicates that CNT addition with aproper surfactant is advantageous in terms of adsorption capacity,as well as mechanical strength.

The application of CNT as adsorbent is not conventional becauseof its high production cost. Recent advancements in carbon nano-technology markedly reduce the production cost of CNT to approx-

imately $70/kg (in personal communication with a commercialproducer). In spite of that, direct use of CNT as adsorbent is stillexpensive because many low cost sorbents are available with goodadsorption capacities. We improved mechanical strength andadsorption capacity of CS hydrogel beads by adding only0.01 wt.% CNT and the cost might not change significantly withsuch addition because added CNT amount is well below the CSamount (1 wt.%) in the beads.

4. Conclusions

CS/CNT beads were prepared by the alkaline gelation of a stableCS–CNT dispersion. The maximum enhancement of the adsorptioncapacity for CR was achieved by only 0.01 wt.% CNT impregnation.The fitting of the equilibrium isotherm data of the CS/CNT beadsinto different isotherm models showed the best fit to the Langmuirisotherm model. The maximum adsorption capacity of the CS/CNTbeads obtained from the Langmuir model was 450.4 mg g�1. Thus,CNT impregnation may be a good method to enhance adsorptioncapacity and mass transfer rate of chitosan hydrogel beads as wellas their mechanical strength.

Acknowledgement

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology (Grant No.2009-0079636).

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