Bioresource Technology 102 (2011) 4402–4409

Contents lists available at ScienceDirect

Bioresource Technology

journal homepage: www.elsevier .com/locate /bior tech

Effect of the addition mode of carbon nanotubes for the production of chitosanhydrogel core–shell beads on adsorption of Congo red from aqueous solution

Sudipta Chatterjee a, Tania Chatterjee a, Seong-Rin Lim 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 and Materials Science, University of California, 3010 Kemper Hall, Davis, CA 95616, USA

a r t i c l e i n f o

Article history:Received 23 October 2010Received in revised form 31 December 2010Accepted 31 December 2010Available online 8 January 2011

Keywords:ChitosanCarbon nanotubeSodium dodecyl sulfateSurfactantAdsorption

0960-8524/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.12.117

⇑ 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 performance of chitosan (CS) hydrogel beads (CSBs) generated by sodium dodecyl sulfate(SDS) gelation with multi-walled carbon nanotube (CNT) impregnation was investigated for Congo redremoval as a model anionic dye. CNT-impregnated CSBs were prepared by four different strategies fordispersing CNTs: (a) in CS solution (CSBN1), (b) in SDS solution (CSBN2), (c) in CS solution containingcetyltrimethylammonium bromide (CTAB) (CSBN3), and (d) in SDS solution for gelation with CTAB-con-taining CS solution (CSBN4). It was observed from FE-SEM study that depending on nature of CNT disper-sion, CNTs were found on the outer surface of CSBN2 and CSBN4 only. The adsorption capacity of the CSBsvaried with the strategy used for CNT impregnation, and CSBN4 exhibited the highest maximum adsorp-tion capacity (375.94 mg/g) from the Sips model. The lowest Sips maximum adsorption capacity byCSBN3 (121.07 mg/g) suggested significant blocking of binding sites of CS by CNT impregnation.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Dyeing effluents discharged from various industries such astextiles, leathers, cosmetics, paper, and printing contaminate theenvironment by imparting undesirable color into water sources.Moreover, dye molecules in the effluents are known to be toxic,mutagenic, and carcinogenic (Blackburn, 2004; Crini, 2006).Among these dyes, azo dyes are difficult to remove because theyare recalcitrant organic molecules and resistant to biological deg-radation (Sun and Yang, 2003). Much attention is being paid to de-velop a suitable treatment method for effluent containing dyemolecules, and among numerous treatment techniques, adsorptionhas been considered to be the most suitable method for treatmentof dye containing effluents (Crini, 2006; Gupta and Suhas, 2009).Activated carbon is mostly used as a commercial adsorbent be-cause of its excellent adsorption capacity for various dyes, but itshigh cost and difficult regeneration process have initiated manyresearchers to search for cheaper and more effective adsorbents(Demirbas, 2009; Kadirvelu et al., 2003). Various natural polymersand waste materials like activated carbon prepared from coir pith(Santhy and Selvapathy, 2006), fungal biomass (Fu and Viraragha-van, 2002), bacterial biomass (Vijayaraghavan et al., 2008), cotton(Chairat et al., 2008), rice husk (Han et al., 2008), and chitosan (CS)(Chatterjee et al., 2009a) have shown tremendous adsorptioncapacity for various dyes. Moreover, biopolymers are renewable

ll rights reserved.

: +82 42 821 1593.

resources and more environmentally friendly than commercialmaterials (Crini, 2005, 2006).

CS, the deacetylated product of chitin, shows excellent adsorp-tion capacities towards many varieties of dyes, metal ions, inorgan-ics, and organic molecules (Crini and Badot, 2008; Chiou et al.,2004; Guibal, 2004; Chatterjee et al., 2009b, 2010a). CS has adiverse range of applications including biomedical fields, tissueengineering, cosmetics, textiles, and food industries because ofits non-toxic, biocompatible, and biodegradable nature (Kumaret al., 2004; Chung and Chen, 2008). The CS hydrogel bead (CB) isreported as a more useful adsorbent than the flake or powder formof this polymer (Varma et al., 2004). Nevertheless, the limited prac-tical application of CBs is due to their low mechanical strength andacid instability (Crini and Badot, 2008).

Carbon nanotube (CNT) is one of the most promising nanofillersused to enhance the mechanical properties of polymer matrices(Wang et al., 2005). Moreover, CNTs are effective at adsorbing dif-ferent materials (Li et al., 2002; Peng et al., 2003). It was reportedin an earlier publication that mechanical properties of CBs werehighly enhanced by CNT impregnation (Chatterjee et al., 2009c),and the maximum adsorption capacity of CNT-impregnated CBswas higher for Congo red (CR) than that of CBs (Chatterjee et al.,2010b). However, the acid instability of CBs still could not besolved by CNT impregnation, and the CNT-impregnated CBs werenot strong enough for actual use. Recently, mechanically improvedand acid stable CS hydrogel beads (CSBs) were developed by a newapproach, sodium dodecyl sulfate (SDS) gelation. Furthermore,beads by 4 g/l SDS gelation showed slightly higher adsorption

S. Chatterjee et al. / Bioresource Technology 102 (2011) 4402–4409 4403

capacity for CR over normal CBs (Chatterjee et al., 2010c). In thisstudy, the adsorption performance of CSBs after CNT impregnationwas investigated using CR as a model dye. Also, four different strat-egies to disperse CNTs into CSBs were investigated: (a) in CS solu-tion (CSBN1), (b) in SDS solution (CSBN2), (c) in CS solutioncontaining cetyltrimethylammonium bromide (CTAB) (CSBN3),and (d) in SDS solution for gelation with CTAB-containing CS solu-tion (CSBN4).

2. Methods

2.1. Materials

CS (>85% deacetylation), SDS, CTAB, and CR were purchased fromSigma Chemical Co., USA. The multi-walled CNTs, manufactured bycatalytic chemical vapor deposition (CCVD) of CH4 over Fe–Mo/MgO catalysts, were purchased from NanoSolution Co., Korea. TheCNTs were 5–10 nm in diameter and 10–20 lm in length. All otheranalytical grade chemicals were purchased from Sigma ChemicalCo., USA.

2.2. Formation of CNT-impregnated CSB

CSBs were impregnated with CNTs and the preparation methodswere similar to the method used for CSB formation (Chatterjeeet al., 2010c). In brief, CSBs were formed by dropwise addition ofCS solution (1 wt.% CS in 2 vol.% acetic acid) into SDS solutions ofdesired concentrations. Here, the preparation methods of four vari-eties of CNT-impregnated CSBs were applied. CNTs were impreg-nated into CSB by adding CNTs to a CS solution (CSBN1) and aSDS solution (CSBN2). For CSBN1 formation, CNTs were dispersedin CS solution (1 wt.% CS in 2 vol.% acetic acid) with weight ratiosvarying from 1:100 to 1:20 for CNT:CS, and the dispersion wasmade using a 750 Watt ultrasonic processor (VC 750, Sonics) witha high power sonic tip operated at 20 kHz frequency and 25% ampli-tude for 15 min (30 s on, 5 s off). In the case of CSBN2, the disper-sion of CNTs in SDS solution (5 g/l) was made with weight ratiosvarying from 1:50 to 1:10 under 25% amplitude ultrasonicationfor 15 min. The third variety, CSBN3, included prior dispersion ofCNT in CTAB solution with a weight ratio of 1:5 at 25% amplitudeultrasonication for 15 min. The resulting CNT dispersion was addedto a CS solution, and weight ratios of CNT to CS in the final disper-sion were varied from 1:100 to 1:20. The CTAB concentration in thefinal dispersion was maintained at 0.05 wt.%. The SDS concentra-tion required for gelation of CSBN3 was 7 g/l, whereas 5 g/l SDSsolution was used for gelation of CSBN1. The fourth variety, CSBN4,involved dropwise addition of CTAB-containing CS solution(0.05 wt.% CTAB in 1 wt.% CS) into CNT-containing SDS solution(7 g/l). The CNT concentration in SDS solution was varied from1:70 to 1:7. A gelling solution (10 ml) of desired concentrationwas used to form 20 CNT-impregnated CSB, and as-prepared beadsformed in gelling solution were collected after 3 h, followed by re-peated washing with deionized water.

2.3. Characterization of CNT-impregnated CSB

The scanning electron microscopy (SEM) of freeze-dried CSBN1,CSBN2, CSBN3, and CSBN4 was performed using field-emissionscanning electron microscopy (FE-SEM) (HITACHI, S-4800). CSBN1and CSBN2 were formed by 5 g/l SDS gelation with 0.01 wt.% CNTdispersed in CS solution (1 wt.% CS in 2 vol.% of acetic acid), and5 g/l SDS solution, respectively. CSBN3 was formed by 7 g/l SDSgelation after dispersing 0.01 wt.% of CNT in the solution contain-ing 1 wt.% CS and 0.05 wt.% CTAB. CSBN4 was formed from

0.05 wt.% CTAB-containing CS solution (1:20, wt./wt.), and 7 g/lSDS solution having 0.05 wt.% CNT.

2.4. Batch adsorption studies

Batch experiments were performed using four varieties ofCNT-impregnated CSBs, namely CSBN1, CSBN2, CSBN3, and CSBN4,and each variety was used to adsorb CR from aqueous solutions.The adsorption performance of CSBN1 and CSBN3 as a functionof CNT concentration in CS solution and that of CSBN2 and CSBN4in SDS solution was studied in the range between 0.002 and0.05 wt.%. Each adsorption experiment was conducted by adding0.2 g of wet beads into 10 ml of CR solution at the desired concen-tration held at pH 5 and 30 �C under shaking conditions (150 rpm)for 24 h. A stock solution (1000 mg/l) of CR was prepared to per-form adsorption experiments, and each adsorption experimentwas performed with an initial CR concentration of 100 mg/l, exceptequilibrium adsorption isotherm experiments. The effect of the ini-tial pH was studied by varying the initial pH of the dye solutionfrom 4 to 11. Equilibrium adsorption isotherm experiments wereconducted at 30 �C for an adsorption period of 24 h with CR con-centrations varying from 10 to 1000 mg/l. All experiments wereconducted in triplicate. The residual CR concentration in the exper-imental solution (mg/l) was analyzed using a spectrophotometer(HACH DR-5000, USA) at a kmax of 497 nm. The amount of CR ad-sorbed (mg/g) was calculated based on the mass balance equationgiven below:

qe ¼ðC0 � CeqÞ � V

W; ð1Þ

where qe is the equilibrium adsorption capacity of the adsorbent inmg/g; C0 is the initial concentration of CR in the solution in mg/l; Ceq

is the final or equilibrium concentration of CR in the solution in mg/l; V is the volume of the solution in l; and W is the dry weight of thehydrogel beads in g.

2.5. Adsorption isotherm models

The equilibrium isotherm data were interpreted using non-lin-ear forms of Langmuir, Freundlich, and Sips isotherm models,which are represented by the following equations, respectively:

qe ¼qmKLCe

1þ KLCe; ð2Þ

qe ¼ KFC1ne; ð3Þ

qe ¼qmaxKeqCn

e

1þ KeqCne

; 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 qm (mg/g) and KL (l/mg) are Langmuir isothermcoefficients. The value of qm represents the maximum adsorptioncapacity of the adsorbent, and KF [(mg/g)/(mg/l)1/n], and n areFreundlich isotherm coefficients. KF is related to the adsorptioncapacity of the adsorbent and n is an empirical parameterrepresenting the heterogeneity of site energies. Keq [(l/mg)n], qmax

(mg/g), and n are Sips isotherm coefficients. Keq and qmax representthe equilibrium constant and maximum adsorption capacity,respectively, and n, the heterogeneity factor of Sips isotherm model,represents the heterogeneity of the adsorption process. The fits ofexperimental data to the above mentioned isotherm models wereevaluated by the non-linear coefficients of determination (R2).

4404 S. Chatterjee et al. / Bioresource Technology 102 (2011) 4402–4409

3. Results and discussion

3.1. General properties

CSBN1 and CSBN3 are dark, black colored beads, and CNTs wereuniformly distributed throughout. In the cases of CSBN2 andCSBN4, incorporation of CNTs into the beads occurred in SDS solu-tion during gelation and CNTs formed some discontinuous layersover the main bead structure. The diameter (D) and porosity (e)of the bead can be determined using these equations (Chatterjeeet al., 2009a):

D ¼ 6WD=qMat þ ðWW �WDÞ=qW

p

� �1=3

ð5Þ

e ¼ ðWW �WDÞ=qW

WD=qMat þ ðWW �WDÞ=qW� 100%; ð6Þ

where, WW (g) is the weight of a single wet bead; WD (g) is theweight of the bead after drying; qW is the density of water, 1.0 g/ml; and qMat is the density of the material in g/ml.

As shown in Table 1, CSBN3 (95.97%) and CSBN4 (96.0%) hadless water content than CSBN1 (96.34%) and CSBN2 (96.28%), indi-cating that CTAB addition can change the water content of thebead. In this study, the material density and porosity of all varietiesof CNT-impregnated CSBs were similar, with variations from 0.78and 0.82 g/ml (95.01% and 95.40%), respectively. The diameters ofCSBN3 (2.85 mm) and CSBN4 (2.85 mm) were slightly less thanCSBN1 (2.94 mm) and CSBN2 (2.92 mm).

3.2. Estimation of CNT incorporation into CSB from SDS solution

CNT incorporation into CSBN2 during gelation from a stable dis-persion of CNTs (0.05 wt.%) in SDS (5 g/l) was determined by mea-suring the dry weight difference between CSBN2 and CSBs. Thetotal dry weight of 100 CSBs formed by 5 g/l gelation was0.041 g, and that of CSBN2 was 0.045 g. As shown in Table 1, theestimated amount of CNTs (g) incorporated per g dry weight ofCSBN2 was 0.09. The SDS and CS content in CSBs were 0.553 and0.447 g/g dry weight of bead, respectively (Chatterjee et al.,2010c), thus it can be assumed that the mass ratio of CS part (CSand CNT in CSBN1; CS in CSBN2; CS, CTAB, and CNT in CSBN3;and CS and CTAB in CSBN4) to SDS part (SDS in CSBN1; SDS andCNT in CSBN2; SDS in CSBN3; and SDS and CNT in CSBN4) inCNT-impregnated CSBs is approximately 1:1. Based on theassumption, the estimated CS and SDS contents (g) per g dry

Table 1General properties of various types of CNT-impregnated CSBs.

General properties CSBN1a CSBN2b CSBN3c CSBN4d

Wet weight (WW, mg) 13.1 12.9 11.9 12.0Dry weight (WD, mg) 0.48 0.48 0.48 0.48Water content (%) 96.34 96.28 95.97 96.00Bulk density e (g/ml) 0.62 0.61 0.59 0.61Skeletal density f (g/ml) 0.78 0.80 0.80 0.82Porosity (e, %) 95.35 95.40 95.01 95.13Diameter (D, mm) 2.94 2.92 2.85 2.85CNT content in dry beads (g/g) 0.005 0.090 0.005 0.150CS content in dry beads (g/g) 0.495 0.500 0.470 0.475SDS content in dry beads (g/g) 0.500 0.410 0.500 0.350CTAB content in dry beads (g/g) – – 0.025 0.025

CNT-impregnated CSB formation involved:a (0.01 wt.% CNT in 1 wt.% CS solution).b (0.05 wt.% CNT in 5 g/l SDS solution).c (0.01 wt.% CNT in 1 wt.% CS solution), 0.05 wt.% CTAB in CS solution.d (0.05 wt.% CNT in 7 g/l SDS solution), 0.05 wt.% CTAB in CS solution.e Wet weight of beads/volume of wet beads.f Dry weight of bead materials/volume of the materials.

weight of CSBN2 were 0.500 and 0.410, respectively. To formCSBN1, CNTs were added at 1:100 with respect to 1 wt.% CS solu-tion, hence the estimated amount of CNTs (g) incorporated per gdry weight of CSBN1 was 0.005. The estimated CS and SDS contents(g) per g dry weight of CSBN1 were 0.495 and 0.500, respectively.Therefore, CNT incorporation into CSBN2 was 18 times higher thanin CSBN1. In the case of CSBN2, the CNTs were incorporated at0.05 wt.% CNTs to 0.5 wt.% SDS solution, whereas for CSBN1, theCNTs were incorporated at 0.01 wt.% CNTs to 1 wt.% SDS solution.The amount of CNTs in the solution during formation of CSBN2was 10 times higher than that in CSBN1. However, the 18 timeshigher incorporation of CNTs into CSBN2 than into CSBN1 indi-cated that the amount of CNTs incorporated into the beads washigher by adding CNTs in SDS solution rather than in CS solution.

CNT incorporation in CSBN4 was also estimated from the dryweight difference between CSBN4 and the same variety withoutCNT impregnation. The total dry weight of 100 CSBN4 formed fromCNTs (0.05 wt.%) dispersed in SDS solution (7 g/l) was 0.063 g, andthe total dry weight of beads without CNT impregnation was0.053 g. As shown in Table 1, the estimated amount of CNTs (g)incorporated per g dry weight of CSBN4 was 0.15 and the esti-mated SDS content (g) per g dry weight of CSBN4 was 0.350. TheCTAB concentration in CS solution (1 wt.%) was maintained at0.05 wt.% and therefore the estimated CS amount in CSBN4 was0.475 g/g dry beads. Assuming the mass ratio of CS part to SDS partis 1:1 in CSBN3, the estimated amount of CNTs incorporated intoCSBN3 was 0.005 g/g dry weight because CNTs were added at1:100 with respect to 1 wt.% CS solution. Thereby, CNT incorpora-tion into CSBN4 was 30 times higher than that into CSBN3. More-over, the higher CNT amount in CSBN4 than in CSBN2 suggestedthat addition of CTAB into CS solution increased CNT incorporationinto CSBs from the SDS solution during gelation.

3.3. Characterization of CNT-impregnated CSBs

The characterization results of CNT-impregnated CSBs (CSBN1,CSBN2, CSBN3, and CSBN4) by FE-SEM study are given as Supple-mentary material in order to know the morphology and CNT distri-bution in the beads. Fig. S1 shows the FE-SEM images of outersurface of CNT-impregnated CSBs. As shown in Figs. S1A and S1C,no CNT is found on the outer surface of CSBN1 and CSBN3, respec-tively, while Figs. S1B and S1D show that CNTs are distributed onthe outer surface of CSBN2 and CSBN4, respectively. The imagesof CSBN2 (Fig. S1E) and CSBN4 (Fig. S1F) at high magnification indi-cate the random and dense network of CNTs with diameter of 20–40 nm on the outer surface of gel materials.

Fig. S2 shows the FE-SEM images of lateral view of inner sur-faces of cut materials of CSBN1 (Figs. S2A, S2C, and S2E), andCSBN3 (Figs. S2B and S2D) at different magnification. The lateralview of cut materials of CSBN1 and CSBN3 shows no CNT evenon the inner surface of these beads. The images at higher magnifi-cation indicate that CNTs are obtained between inner and outersurfaces of CSBN1 (Fig. S2E), and CSBN3 (Fig. S2D). The CNTs arefound between the inner and outer surfaces of CSBN2 and CSBN4(image not shown).

3.4. Effect of CNT impregnation in CSB

The effects of varying the CNT concentration (0.002–0.05 wt.%)in CS solution and in SDS solution on the CR adsorption (initialconcentration of 100 mg/l) by CSBN1 and CSBN2 have been givenin Fig. 1a. The adsorption capacity of CSBN1 was highest with0.01 wt.% CNT impregnation (116.64 mg/g), and further increasesin the CNT concentration (up to 0.05 wt.%) caused a slight reduc-tion in the adsorption capacity, from 116.64 to 114.83 mg/g.It was already reported that the adsorption capacity of

(a)

CNT concentration (wt %) in CS (CSBN1) or SDS (CSBN2) solution

0.00 0.01 0.02 0.03 0.04 0.05 0.06

q e (m

g/g)

104

106

108

110

112

114

116

118

120

CSBN1CSBN2

SDS gelation = 5 g/lCR = 100 mg/l, pH 5

(b)

CNT concentration (wt %) in CS (CSBN3) or SDS (CSBN4) solution

0.00 0.01 0.02 0.03 0.04 0.05 0.06

q e (m

g/g)

60

70

80

90

100

110

120

130

CSBN3CSBN4

CTAB = 0.05 wt %SDS gelation = 7 g/lCR = 100 mg/l, pH 5

Fig. 1. Effect of CNT concentration variation (wt.%) in CS solution (a) or in SDSsolution (b) during gelation on CR adsorption; initial CR concentration, 100 mg/land pH 5.

Table 2Density and adsorption capacities of various type adsorbent materials.a

Type Density ofmembranematerials (g/ml)

Adsorptioncapacity (mgCR/g)

Volumetric adsorptioncapacity (mg CR/mlmembrane materials)

CSBN1 0.49 116.64 ± 1.112 57.15 ± 0.544CSBN2 0.48 115.02 ± 1.128 55.21 ± 0.540CSBN3 0.51 75.71 ± 1.020 38.61 ± 0.518CSBN4 0.51 123.05 ± 0.532 62.76 ± 0.270

Data represents average of three replicates with ±standard deviations.a Initial CR concentration 100 mg/l.

S. Chatterjee et al. / Bioresource Technology 102 (2011) 4402–4409 4405

CNT-impregnated CBs for CR was enhanced by increasing CNT con-centrations up to a certain level (0.01 wt.%), and further increasesin CNT concentration had a negative effect on its adsorption capac-ity (Chatterjee et al., 2010b). The CS was used as a dispersant forCNTs in this study for CSBN1 and the amount of CS at 1 wt.% wasfound suitable for dispersing CNTs up to 0.01 wt.% and this was re-flected in increasing adsorption capacity by increasing CNTs in thebeads up to this limit. The further increase in CNT concentration inthe dispersion after 0.01 wt.% caused poor dispersion of CNTs inthe dispersion made by CS solution. Also, the positive effect ofCNT impregnation on CR adsorption is possibly due to the largespecific surface area of CNTs (237.8 m2/g) and the strong interac-tions between the benzene rings of CR and the hexagonal arraysof carbon atoms in the CNTs during adsorption. The adsorptioncapacity of CSBN1 was decreased by increasing the CNT concentra-tion higher than 0.01% because aggregate formation of CNTs andblocking of the adsorption sites of CS by CNTs are increased. In or-der to form CSBN2, SDS was used as a dispersant for CNTs and theconcentration of SDS (5 g/l) used for dispersion of CNTs was morethan 2 times of CMC concentration (2.33 g/l). The increase in CNTconcentration from 0.002 to 0.05 wt.% in 5 g/l SDS solution duringgelation increased the adsorption capacity of CSBN2 for CR(100 mg/l) from 107.89 to 115.02 mg/g and this could be due togood dispersion of CNTs in SDS solution and also on the presenceof CNTs on the surface of CSBN2. Thus, such an increase in the

adsorption capacity of CSBN2 by increasing CNT concentration inSDS solution suggested that the positive effects of CNTs were high-er than the negative effects due to aggregate formation and block-ing of adsorption sites by CNTs with increasing CNT concentrationas found in CSBN1. The slightly higher adsorption capacity ofCSBN1 than CSBN2 for CR could be due to the nature of the disper-sion medium for CNTs because dispersion in SDS molecules resultsin negatively charged CNTs by adsorption of SDS molecules ontoCNTs.

As shown in Fig. 1b, the adsorption capacity of CSBN3 was sig-nificantly decreased by increasing the CNT concentration from0.002 to 0.05 wt.% in the CS solution, and 0.05% CNT impregnationdecreased the adsorption capacity from 101.12 mg/g (0% CNT) to68.23 mg/g. The negative effect of CNT impregnation on theadsorption capacity of CSBN3 was possibly due to significantblocking of adsorption sites on CS, CTAB, and SDS molecules inthe beads by CNTs. The effective blocking of positively chargedamine groups of CS and CTAB of CSBN3 (the adsorption sites ofbeads) by CNTs could occur due to formation of CNT aggregatesin the mixture of cationic surfactant (CTAB) and CS as dispersant,followed by the complexation step using another anionic surfac-tant (SDS). Thus, these CNT aggregates in CSBN3 hinder the bindingsites of the beads to come into contact with CR during adsorption.So, the adsorption capacity of CNT-impregnated CSBs was found tobe dependant on the nature of dispersant for CNTs and also, themixture of dispersants (CTAB and CS) was observed less effectivethan single dispersant for CNTs. Fig. 1b also shows an increase inCNT concentration from 0.002 to 0.05 wt.% in SDS solution duringgelation, increasing the adsorption capacity of CSBN4 for CR(100 mg/l) from 101.47 to 123.05 mg/g. The increase in adsorptioncapacity of CSBN4 by increasing CNT concentration in SDS solutionsuggested that the positive effects of CNTs mostly occur due to theuse of SDS as dispersant which dispersed CNTs more uniformly inthe dispersion with fewer amounts of CNT aggregates. Also, thepresence of CTAB in the CS solution increased the incorporationof CNTs in the beads. In addition, unlike CNTs in CS solution forCSBN1 and CSBN3, CNTs were incorporated into CSBN4 from theSDS solution possibly causing less blocking of the binding sites ofCS molecules in CSBN4.

3.5. Volumetric adsorption capacity

Table 2 exhibits the density and volumetric adsorption capaci-ties of membrane materials made of CSBN1 and CSBN2, formedby 5 g/l SDS gelation, and CSBN3 and CSBN4, formed by 7 g/l SDSgelation. Densities of the membrane materials of all varieties ofCNT-impregnated CSBs were obtained by removing the core waterfrom the beads. As shown in Table 2, the densities of membranematerials were enhanced by CTAB addition, and the densities ofmembrane materials made of CSBN1, CSBN2, CSBN3, and CSBN4were 0.49, 0.48, 0.51, and 0.51 g dry weight/ml of membrane mate-rials, respectively. The volumetric adsorption capacities of the

4406 S. Chatterjee et al. / Bioresource Technology 102 (2011) 4402–4409

membrane materials made of beads (mg CR/ml membrane mate-rial volume) were obtained by multiplying qe (mg/g) by the densityof the membrane materials made of beads. The CSBN4 membranematerials (62.76 mg/ml ± 0.27) exhibited the highest volumetricadsorption capacity, whereas the CSBN3 membrane materials(38.61 mg/ml ± 0.518) exhibited the lowest volumetric adsorptioncapacity. Thereby, the volumetric adsorption capacity of the mem-brane materials made of CNT-impregnated beads depends on thestrategy used for bead formation and the effect of CNT impregna-tion, including both direct CR binding by CNTs and blocking effectsof CNTs.

3.6. Effect of initial pH

The effects of the initial pH (4–11) on the adsorption capacitiesfor CR (100 mg/l) are shown in Fig. 2a (CSBN1 and CSBN2) and inFig. 2b (CSBN3 and CSBN4). The pH change during adsorptionwas measured after certain time intervals and a pH change duringadsorption was observed when initial pH of the dye solution wasacidic (pH 4 and 5). The increase from an initial pH of 4 or 5 to finalpH 6 at equilibrium indicates protonation of amine groups on CSmolecules during adsorption. All varieties of CNT-impregnatedCSBs exhibited maximum adsorption when adsorption experi-ments were started at an initial pH of 4 and adsorption capacitywas found to decrease with an increase in the initial pH of theCR solution from 4 to 11. The adsorption capacities of CSBN1 and

(a)

Initial pH

4 6 8 10 12

q e (m

g/g)

40

60

80

100

120

CSBN1 (0.01 wt % CNT in CS solution)CSBN2 (0.05 wt % CNT in SDS solution)SDS gelation = 5 g/l, initial CR = 100 mg/l

(b)

Initial pH

4 6 8 10 12

q e (m

g/g)

20

40

60

80

100

120

140

160

CSBN3 (0.01wt % CNT in CS solution)CSBN4 (0.05 wt % CNT in SDS solution)CTAB = 0.05 wt %, SDS gelation = 7 g/lC0 = 100 mg/l

Fig. 2. Effect of initial pH of CR solution on adsorption onto various types of CNT-impregnated CSBs; initial CR concentration, 100 mg/l.

CSBN2 decreased from 116.03 to 55.56 mg/g and from 115.99 to56.02 mg/g, respectively, with an increase in the initial pH ofthe CR solution from 4 to 11 and the value of adsorption capacitywas reported at final pH of a particular initial pH of a dyesolution. CSBN3 and CSBN4 also exhibited a decrease in theiradsorption capacities from 75.98 to 40.32 mg/g and from 123.62to 79.23 mg/g, respectively, with an initial pH change from 4 to11. The high adsorption capacity at a low initial pH is due to en-hanced electrostatic interactions between positively chargedgroups on the beads and negatively charged CR molecules. Thepositive zeta potential value at pH 5 of CSB (+3.76 mV) formed by5 g/l SDS gelation clearly indicated that surface of CSB was posi-tively charged, so adsorption of CR onto those beads was mostlycontrolled by electrostatic attraction between CS and CR molecules.

3.7. Adsorption isotherm

Fig. 3a shows the adsorption isotherm data of CSBN1 and CSBN2fit to the Langmuir, Freundlich, and Sips isotherm models. Thecoefficients and non-linear R2 values for the three isotherm modelsare given in Table 3. The R2 values for the Langmuir (0.979) andSips (0.979) models for CSBN1 and the R2 values for the Langmuir(0.980) and Sips models (0.981) for CSBN2 suggested that isothermdata of CSBN1 and CSBN2 fit well to both isotherm models. The val-ues of n for CR adsorption onto CSBN1 and CSBN2 were 0.991 and1.100, respectively. Values for the maximum adsorption capacity of

(a)

Ce (mg/l)

0 200 400 600 800 1000

q e (m

g/g)

0

50

100

150

200

250

CSBN1 experimental (0.01 wt % CNT in CS solution)CSBN2 experimental (0.05 wt % CNT in SDS solution)Langmuir fittingFreundlich fittingSips fitting

(b)

Ce (mg/l)

0 200 400 600 800 1000

q e (m

g/g)

-100

0

100

200

300

400

500

CSBN3 experimental (0.01 wt % CNT in CS solution)CSBN4 experimental (0.05 wt % CNT in SDS solution)Langmuir fitting Freundlich fitting Sips fitting CTAB = 0.05 wt %, SDS gelation = 7 g/l

Fig. 3. Plots of qe versus Ce for the adsorption of CR onto various types of CNT-impregnated CSBs; pH 5 and 30 �C.

Table 3Constants for equilibrium isotherm models with error analysis values.

Adsorbent KL (l/mg) qm (mg/g) R2

Langmuir isotherm modelCSBN1a 0.073 199.98 0.979CSBN2b 0.077 191.15 0.980CSBN3c 0.029 124.97 0.990CSBN4d 0.162 370.37 0.985

Freundlich isotherm modelAdsorbent KF [(mg/g)/(mg/l)1/n] 1/n R2

CSBN1 42.02 0.251 0.896CSBN2 41.74 0.244 0.888CSBN3 19.11 0.288 0.889CSBN4 87.88 0.240 0.861

Sips isotherm modelAdsorbent qmax (mg/g) Keq [(l/mg)n] n R2

CSBN1 200.00 0.074 0.991 0.979CSBN2 188.32 0.065 1.100 0.981CSBN3 121.07 0.020 1.136 0.992CSBN4 375.94 0.166 0.932 0.985

a (0.01 wt.% CNT in 1 wt.% CS solution).b (0.05 wt.% CNT in 5 g/l SDS solution).c (0.01 wt.% CNT in 1 wt.% CS solution), 0.05 wt.% CTAB in CS solution.d (0.05 wt.% CNT in 7 g/l SDS solution), 0.05 wt.% CTAB in CS solution.

(a)

t (min)0 200 400 600 800 1000 1200

q t (m

g/g)

0

20

40

60

80

100

120

140

CSBN1 experimental (0.01 wt % CNT in CS solution)CSBN2 experimental (0.05 wt % CNT in SDS solution)pseudo-first-orderpseudo-second-orderintra-particle diffusion

(b)

t (min)0 200 400 600 800 1000 1200

q t (m

g/g)

0

20

40

60

80

100

120

140

CSBN3 experimental (0.01wt % CNT in CS solution)CSBN4 experimental (0.05 wt % CNT in SDS solution)pseudo-first-orderpseudo-second-orderintra-particle difussion

Fig. 4. Plots of qt versus t for CR adsorption onto various types of CNT-impregnatedCSBs; initial CR concentration, 100 mg/l, pH 5 and 30 �C.

S. Chatterjee et al. / Bioresource Technology 102 (2011) 4402–4409 4407

CSBN1 obtained from Langmuir (199.98 mg/g) and Sips (200.00mg/g) models were similar. The maximum adsorption capacityof CSBN2 obtained from Langmuir (191.15 mg/g) and Sips(188.32 mg/g) models also exhibited similar values. Thereby, theLangmuir and Sips isotherm models could satisfactorily describethe adsorption isotherm data for both CSBN1 and CSBN2. More-over, isotherm results indicated similar maximum adsorptioncapacity values for CSBN1 and CSBN2. However, evaluation of bothvarieties of beads on a cost basis suggested that 0.01% CNT additionto CS solution is more advantageous than making a dispersion of0.05% CNTs in 5 g/l of SDS solution because the cost might notchange significantly with such addition of CNTs in a 1% CS solution.

As shown in Fig. 3b, the isotherm results of CSBN3 and CSBN4exhibited good fits to both the Langmuir and Sips isotherm models.Table 3 shows that R2 values of CSBN3 from Langmuir (0.990) andSips (0.992) models were similar. CSBN4 also exhibited the same R2

values (0.985) from the Langmuir and Sips isotherm models. Thevalues of n for CR adsorption onto CSBN3 and CSBN4 were1.136 and 0.932, respectively. The maximum adsorption capacitiesof CSBN3 obtained from Langmuir (124.97 mg/g) and Sips(121.07 mg/g) isotherms were similar. CSBN4 also exhibited simi-lar Langmuir (370.37 mg/g) and Sips (375.94 mg/g) maximumadsorption capacities. CSBN4 formed by dropwise addition of CTABmixed CS solution (0.05% CTAB in 1% CS solution) into CNTs dis-persed in SDS solution (0.05% CNT in 7 g/l) was the best adsorbentin this study. The maximum adsorption capacity of CSBN3 was thelowest among all varieties of CSBs impregnated with CNTs, andCSBN3 was formed by CNT dispersion in CTAB prior to its additionin CS solution. Thereby, the positive effect of CNT impregnation onadsorption depends on the strategy used for bead formation.

Comparison of the maximum adsorption capacity of CSBN4(370.37 mg/g) with that of other adsorbents, such as 14.16 mg/gof sodium bicarbonate pretreated Aspergillus niger biomass (Fuand Viraraghavan, 2002), 41.20 mg/g of neem leaf powder(Bhattacharrya and Sharma, 2004), 66.23 mg/g of palm kernel seedcoat (Oladoja and Akinlabi, 2009), 208.33 mg/g of maghemite nano-particles (Afkhami and Moosavi, 2010), 7.08 mg/g of acid activatedred mud (Tor and Cengeloglu, 2006), 6.70 mg/g of activated carbonprepared from coir pith (Namasivayam and Kavitha, 2002),11.89 mg/g of bagasse fly ash (Mall et al., 2005), 330.62 mg/g ofN,O-carboxymethyl CS (Wang and Wang, 2008), 223.25 mg/g of

CBs (Chatterjee et al., 2009a), 208.30 mg/g of CSBs (Chatterjeeet al., 2010c), 433.12 mg/g of CTAB-impregnated CBs (Chatterjeeet al., 2009a), and 450.40 mg/g of CNT-impregnated CBs(Chatterjee et al., 2010b), suggested that CSBN4 could be used asan effective adsorbent for adsorption of CR from aqueous solutions.

3.8. Kinetic study

Experimental kinetic data for adsorption of CR by CSBN1 andCSBN2 from a 100 mg/l solution have been illustrated in Fig. 4a.Fig. 4b represents the experimental kinetic data of CSBN3 andCSBN4. The time required to reach equilibrium for all varieties ofbeads was 420 min. The coefficient values of Fig. 4a and b are givenin Table 4, and the fitting of experimental kinetic data to rate mod-els has been evaluated by the non-linear coefficients of determina-tion (R2).

The kinetics of CR adsorption by four varieties of CNT-impreg-nated CSBs was analyzed using a pseudo-first-order rate model, apseudo-second-order rate model and an intra-particle diffusionmodel. The coefficient values from Fig. 4a and b are given in Table4. The fits of experimental kinetic data to the above mentioned ratemodels were evaluated by the non-linear coefficient of determina-tion method (R2).The pseudo-first-order model can be representedin the following non-linear form:

qt ¼ qeð1� e�k1tÞ; ð7Þ

Table 4Constants of different rate models for CR adsorption with C0 = 100 mg/l.

Adsorbent qe (exp) (mg/g) Pseudo-first-order equation Pseudo-second-order equation Intra-particle diffusion

qe (cal) (mg/g) k1 (1/min) qe (cal) (mg/g) k2 (g/mg min) h (mg/g min) kp (mg/g min0.5)

CSBN1a 116.09 118.15 6.70 � 10�3 142.89 5.11 � 10�5 1.041 5.56CSBN2b 115.37 117.03 7.50 � 10�2 139.55 6.05 � 10�5 1.178 5.90CSBN3c 75.23 74.91 8.00 � 10�3 88.00 1.08 � 10�4 0.836 3.99CSBN4d 122.37 124.54 7.60 � 10�3 147.34 5.98 � 10�5 1.298 6.27

a (0.01 wt.% CNT in 1 wt.% CS solution).b (0.05 wt.% CNT in 5 g/l SDS solution).c (0.01 wt.% CNT in 1 wt.% CS solution), 0.05 wt.% CTAB in CS solution.d (0.05 wt.% CNT in 7 g/l SDS solution), 0.05 wt.% CTAB in CS solution.

4408 S. Chatterjee et al. / Bioresource Technology 102 (2011) 4402–4409

where qt and qe are the amounts of CR adsorbed (mg/g) at time t andequilibrium, respectively and k1 (1/min) is the equilibrium rateconstant of this equation.The non-linear form of the pseudo-second-order rate equation is expressed as

qt ¼q2

e k2t1þ qek2t

and h ¼ k2q2e ; ð8Þ

where h represents the initial adsorption rate (mg/g min) and k2 (g/mg min) is the equilibrium rate constant of the pseudo-second-order rate model.The correlation coefficients (R2) for both pseudo-first-order and pseudo-second-order rate equations were 0.997 forCSBN1 and 0.990 for CSBN2, indicating good fit to both rate models.For CSBN3, the correlation coefficients (R2) for the pseudo-first-order and pseudo-second-order equations were 0.986 and 0.991,respectively, and those for CSBN4 were 0.986 and 0.980, respec-tively. The R2 values obtained for CSBN3 and CSBN4 indicated betterfit to pseudo-first-order rate model than pseudo-second-order ratemodel. The experimental qe value (116.90 mg/g) for CSBN1 showedbetter fit to the pseudo-first-order (118.15 mg/g) than to the pseu-do-second-order (142.89 mg/g) model. The experimental qe value(115.37 mg/g) for CSBN2 fit more closely to the pseudo-first-order(117.03 mg/g) than the pseudo-second-order (139.55 mg/g) model.The qe value for CSBN3 obtained from the pseudo-first-order rateequation (74.91 mg/g) showed better fit to the experimental qe va-lue (75.23 mg/g) than to the pseudo-second-order rate equation(88.00 mg/g). The experimental qe value (122.27 mg/g) for CSBN4also showed better fit to the pseudo-first-order (124.54 mg/g) thanto the pseudo-second-order (147.34 mg/g) model.The intra-particlediffusion equation is given as

qt ¼ kpt0:5; ð9Þ

where kp is the intra-particle diffusion rate constant (mg/g min 0.5).The plot of qt versus t 0.5 using the initial kinetic data up to 180 mingave correlation coefficient (R2) values of 0.943, 0.953, 0.981, and0.980 and kp values of 5.56, 5.90, 3.99 and 6.27 mg/g min 0.5 forCSBN1, CSBN2, CSBN3, and CSBN4, respectively. The values of thecorrelation coefficients (R2) and rate constants for intra-particle dif-fusion suggested that intra-particle diffusion played a significantrole at the initial stage of adsorption.

4. Conclusions

Four varieties of CNT-impregnated CSBs developed involvingvarious strategies such as dispersion of CNTs into CS solution(CSBN1), SDS solution (CSBN2), and CS solution with prior disper-sion in CTAB (CSBN3), and also addition of CTAB-containing CSsolution into CNTs dispersed in SDS solution (CSBN4) were usedfor adsorption of CR from aqueous solutions. The adsorption capac-ity of CNT-impregnated CSBs was found to vary with nature of CNTdispersants as well as solutions containing oppositely chargedcompounds to the CNT dispersants, and CSBN4 exhibited the

maximum adsorption capacity (375.94 mg/g) among the fourvarieties of CNT-impregnated CSBs.

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).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.biortech.2010.12.117.

References

Afkhami, A., Moosavi, R., 2010. Adsorptive removal of Congo red, a carcinogenictextile dye, from aqueous solutions by maghemite nanoparticles. J. Hazard.Mater. 174, 398–403.

Bhattacharrya, K.G., Sharma, A., 2004. Azadirachta indica leaf powder as an effectivebiosorbent for dyes: a case study with aqueous Congo red solutions. J. Environ.Manage. 71, 217–229.

Blackburn, R.S., 2004. Natural polysaccharides and their interactions with dyemolecules: applications in effluent treatment. Environ. Sci. Technol. 38, 4905–4909.

Chairat, M., Rattanaphani, S., Bremmer, J.B., Rattanaphani, V., 2008. Adsorptionkinetic study of lac dyeing on cotton. Dyes Pigments 76, 435–439.

Chatterjee, S., Lee, D.S., Lee, M.W., Woo, S.H., 2010a. Enhanced molar sorption ratiofor naphthalene through the impregnation of surfactant into chitosan hydrogelbeads. Bioresour. Technol. 101, 4315–4321.

Chatterjee, S., Lee, M.W., Woo, S.H., 2010b. Adsorption of Congo red by chitosanhydrogel beads impregnated with carbon nanotubes. Bioresour. Technol. 101,1800–1806.

Chatterjee, S., Chatterjee, T., Woo, S.H., 2010c. A new type of chitosan hydrogelsorbent generated by anionic surfactant gelation. Bioresour. Technol. 101,3853–3858.

Chatterjee, S., Lee, D.S., Lee, M.W., Woo, S.H., 2009a. Enhanced adsorption of Congored from aqueous solutions by chitosan hydrogel beads impregnated with cetyltrimethyl ammonium bromide. Bioresour. Technol. 100, 2803–2809.

Chatterjee, S., Lee, D.S., Lee, M.W., Woo, S.H., 2009b. Nitrate removal from aqueoussolutions by cross-linked chitosan beads conditioned with sodium bisulfatebromide. J. Hazard. Mater. 166, 508–513.

Chatterjee, S., Lee, M.W., Woo, S.H., 2009c. Enhanced mechanical strength ofchitosan hydrogel beads by impregnation with carbon nanotubes. Carbon 47,2933–2936.

Chiou, M.S., Ho, P.Y., Li, H.Y., 2004. Adsorption of anionic dyes in acid solutions usingchemically cross-linked chitosan beads. Dyes Pigments 60, 69–84.

Chung, Y.C., Chen, C.Y., 2008. Antibacterial characteristics and activity of acid-soluble chitosan. Bioresour. Technol. 99, 2806–2814.

Crini, G., Badot, P.M., 2008. Application of chitosan, a natural aminopolysaccharide,for dye removal from aqueous solutions by adsorption processes using batchstudies: a review of recent literature. Prog. Polym. Sci. 33, 399–447.

Crini, G., 2006. Non-conventional low-cost adsorbents for dye removal: a review.Bioresour. Technol. 97, 1061–1085.

Crini, G., 2005. Recent developments in polysaccharide-based materials used asadsorbents in wastewater treatment. Prog. Polym. Sci. 30, 38–70.

Demirbas, A., 2009. Agricultural based activated carbons for the removal of dyesfrom aqueous solutions: a review. J. Hazard. Mater. 167, 1–9.

Fu, Y., Viraraghavan, T., 2002. Removal of Congo red from an aqueous solution byfungus Aspergillus niger. Adv. Environ. Res. 7, 239–247.

Guibal, E., 2004. Interactions of metal ions with chitosan-based sorbents: a review.Sep. Purif. Technol. 38, 43–74.

S. Chatterjee et al. / Bioresource Technology 102 (2011) 4402–4409 4409

Gupta, V.K., Suhas, 2009. Application of low-cost adsorbents for dye removal – areview. J. Environ. Manage. 90, 2313–2342.

Han, R., Ding, D., Xu, Y., Zou, W., Wang, Y., Li, Y., Zou, L., 2008. Use of rice husk foradsorption of Congo red from aqueous solution in column mode. Bioresour.Technol. 99, 2938–2946.

Kadirvelu, K., Kavipriya, M., Karthika, C., Radhika, M., Vennilamani, N., Pattabhi, S.,2003. Utilization of various agricultural wastes for activated carbon preparationand application for the removal of dyes and metal ions from aqueous solutions.Bioresour. Technol. 87, 129–132.

Kumar, R.M.N.V., Muzzarelli, R.A.A., Muzzarelli, C., Sashiwa, H., Domb, A.J., 2004.Chitosan chemistry and pharmaceutical perspectives. Chem. Rev. 104, 6017–6084.

Li, Y.H., Wang, S., Wei, J., Zhang, X., Xu, C., Luan, Z., Wu, D., Wei, B., 2002. Leadadsorption on carbon nanotubes. Chem. Phys. Lett. 357, 263–266.

Mall, I.D., Srivastava, V.C., Agarwal, N.K., Mishra, I.M., 2005. Removal of Congo redfrom aqueous solution by bagasse fly ash and activated carbon: kinetic studyand equilibrium isotherm analyses. Chemosphere 61, 492–501.

Namasivayam, C., Kavitha, D., 2002. Removal of Congo red from water by adsorptiononto activated carbon prepared from coir pith, an agricultural solid waste. DyesPigments 54, 47–58.

Oladoja, N.A., Akinlabi, A.K., 2009. Congo red biosorption on palm kernel seed coat.Ind. Eng. Chem. Res. 48, 6188–6196.

Peng, X., Li, Y., Luan, Z., Di, Z., Wang, H., Tian, B., Jia, Z., 2003. Adsorption of 1,2-dichlorobenzene from water to carbon nanotubes. Chem. Phys. Lett. 376, 154–158.

Santhy, K., Selvapathy, P., 2006. Removal of reactive dyes from wastewater byadsorption on coir pith activated carbon. Bioresour. Technol. 97, 1329–1336.

Sun, Q.Y., Yang, L.Z., 2003. The adsorption of basic dyes from aqueous solution onmodified peat-resin particle. Water Res. 37, 1535–1544.

Tor, A., Cengeloglu, Y., 2006. Removal of Congo red from aqueous solution byadsorption onto acid activated red mud. J. Hazard. Mater. 138, 409–415.

Varma, A.J., Deshpande, S.V., Kennedy, J.F., 2004. Metal complexation by chitosanand its derivatives: a review. Carbohydr. Polym. 55, 77–93.

Vijayaraghavan, K., Lee, M.W., Yun, Y.S., 2008. A new approach to study thedecolorization of complex reactive dye bath effluent by biosorption technique.Bioresour. Technol. 99, 5778–5785.

Wang, L., Wang, A., 2008. Adsorption properties of Congo red from aqueous solutiononto N,O-carboxymethyl-chitosan. Bioresour. Technol. 99, 1403–1408.

Wang, S.F., Shen, L., Zhang, W.D., Tong, Y.J., 2005. Preparation and mechanicalproperties of chitosan/carbon nanotubes composites. Biomacromolecules 6,3067–3072.