Bioresource Technology 101 (2010) 3853–3858

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Bioresource Technology

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A new type of chitosan hydrogel sorbent generated by anionic surfactant gelation

Sudipta Chatterjee, Tania Chatterjee, Seung H. Woo *

Department of Chemical Engineering, Hanbat National University, San 16-1, Deokmyeong-Dong, Yuseong-Gu, Daejeon 305-719, Republic of Korea

a r t i c l e i n f o

Article history:Received 24 October 2009Received in revised form 19 December 2009Accepted 21 December 2009Available online 2 February 2010

Keywords:AdsorptionChitosanHydrogel beadsSodium dodecyl sulfateSurfactant

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.12.089

* 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

A new type of chitosan hydrogel beads (CSB) with a core–shell membrane structure was generated bysodium dodecyl sulfate (SDS) gelation process. CSB exhibited higher mechanical strength and acid stabil-ity than chitosan hydrogel beads (CB) formed by alkali gelation. The effect of SDS concentration variationduring gelation on the adsorption capacity of CSB for congo red (CR) as a model anionic dye showed thatCSB formed by 4 g l�1 SDS gelation had the highest adsorption capacity. The maximum adsorption capac-ity of CSB (208.3 mg g�1) obtained from the Sips model was found slightly higher than that of CB(200.0 mg g�1). Membrane materials of CSB obtained after squeezing core water from the beads showedapproximately 25 times higher volumetric adsorption capacity than CB.

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1. Introduction

The effluents discharged from textile industry contain a largevariety of dyes and it is estimated that around 10–15% of the dyesare lost in the effluent during the dyeing processes (Crini, 2006;dos Santos et al., 2007). The discharge of dye containing effluentsto the receiving water bodies retards photosynthesis by aquaticplants and phytoplanktons due to abnormal colouration of water(Robinson et al., 2001). Furthermore, some dyes in the effluentsare toxic, carcinogenic or mutagenic to human beings (Crini,2006). Azo dyes represent about 50% of all dyes and more than53% of these commonly azo dyes are known to be highly resistantto biological degradation process (Manu and Chaudhari, 2002).Congo red (CR) is one of important azo dyes and it mainly occursin the effluents discharged from textile, paper, printing, leatherindustries, etc. CR is metabolized to benzidine, a known humancarcinogen.

Various techniques such as coagulation/flocculation (Hasaniet al., 2009), activated carbon adsorption (Kadirvelu et al., 2003),oxidation (Malik and Saha, 2003), ozonation (Selcuk, 2005), elec-trochemical oxidation (Radha et al., 2009), membrane separation(Chiu et al., 2009), biological degradation (Gopinath et al., 2009),etc., have been applied for the treatment of dye containing efflu-ents and most of the techniques are reported as expensive andnot environment friendly (Crini, 2006). Activated carbon is themost widely used adsorbent for dye removal, but it is too expen-sive and there is difficulty in the regeneration process (Kadirvelu

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et al., 2003). Various low-cost adsorbents such as coir pith acti-vated carbon (Namasivayam and Kavitha, 2002), fungal and algalbiomasses (Khalaf, 2008), bacterial biomass (Vijayaraghavanet al., 2008), neem leaf powder (Bhattacharrya and Sharma,2004), activated desert plant (Bestani et al., 2008), palm kernelseed coat (Oladoja and Akinlabi, 2009), etc., have been used forthe removal of dye from wastewater. However, low-adsorptioncapacities of some of these adsorbents for dyes initiate furthersearch for new adsorbents.

Chitosan (CS) is a natural heteropolymer of glucosamine andN-acetyl glucosamine residues, and obtained by alkaline deacety-lation of chitin (a polymer made up of N-acetyl glucosamineresidues), the next most abundant natural polysaccharide aftercellulose (Chung and Chen, 2008; Crini and Badot, 2008).Recently, CS hydrogel beads have generated great interest forremoval of environmentally hazardous chemicals due to their bio-degradability, low-cost, and multiple functional groups (Crini andBadot, 2008; Chatterjee et al., 2009a,b). CS hydrogel beads aremore extensively used than flake and powder forms of this poly-mer because of their high surface area and porosity (Varma et al.,2004). CS hydrogel beads have shown surprisingly high adsorp-tion capacities for numerous dyes and heavy metals, as comparedto various adsorbents surveyed in recent review papers (Crini,2006; Guibal, 2004; Gupta and Suhas, 2009). Nevertheless, CShydrogel beads formed by conventional physical or alkali gelationface serious impediments to commercial application, such as low-mechanical strength, low-acid stability, and a large material vol-ume. Different methods like chemical cross-linking (Chiou et al.,2004), carboxy alkyl substitution (Cestari et al., 2004), carbonnanotubes impregnation (Chatterjee et al., 2009c), etc., can

3854 S. Chatterjee et al. / Bioresource Technology 101 (2010) 3853–3858

improve mechanical stability of CS hydrogel beads, but still donot yield enough strength for use in actual situations. Moreover,sometimes these methods cause a significant decrease in adsorp-tion capacity (Crini and Badot, 2008).

Recently, the study on interactions between polymeric materi-als and anionic surfactants has been a subject of fundamental re-search (Thongngam and McClements, 2005; Trabelsi et al., 2006).According to literature survey, fabrication of CS hydrogel beadsby anionic surfactant gelation and its application in the adsorptionresearch field have not been reported to date. In this study, a newtype of CS hydrogel beads (CSB) was generated by anionic surfac-tant gelation process using sodium dodecyl sulfate (SDS). A de-tailed study of the mechanical properties of these materialstogether with its adsorption performance for CR as a model anionicdye was investigated.

2. Methods

2.1. Materials

CS (>85% deacetylation), SDS, and CR were purchased from Sig-ma Chemical Co., USA. All other analytical grade chemicals werepurchased from Sigma Chemical Co., USA.

2.2. Surfactant and alkali gelation

Formation of chitosan hydrogel beads by SDS gelation (CSB) in-volved dropwise (drop volume �20 ll) addition of CS solution(1 wt.% CS in 2 vol.% acetic acid) into SDS solution of various con-centrations between 1 and 50 g l�1 through a T-1000-B pipet tip,Axygen scientific, USA. The subsequent beads formation in thesame SDS solution depletes the amount of SDS. In order to attainthe same amount of SDS in the beads with subsequent bead forma-tion in the solution, maximum twenty hydrogel beads were formedin 10 ml SDS solution of desired concentration and beads were ta-ken from the same SDS solution after 3 h. CS hydrogel beadsformed by alkali gelation (CB) involved dropwise addition of CS(1 wt.%) in acetic acid solution (2 vol.%) to alkali mixture(H2O:MeOH:NaOH = 4:5:1, w/w). As-prepared beads were exten-sively washed with deionized water and preserved in aqueousenvironment for future use.

2.3. Structure analysis

Optical micrographs of CB, and CSB by 4, 10, 20 and 50 g l�1 SDSgelation were taken using Nikon Microscope Eclipse 80i, Japan. Ele-mental analyses of CB, and CSB formed with various SDS concen-trations (5, 10, 20 and 50 g l�1) were performed using Elementalanalyzer (EA1108, Fisons). Infrared (IR) spectra of CB and CSBformed with various SDS concentrations (5, 10, 20 and 50 g l�1)were characterized using a FTIR Spectrophotometer (Nicolet6700, Thermal) with KBr pellet.

2.4. Mechanical stability

The response of the CB and CSB to ultrasonication gives an in-sight into the mechanical stability of the beads and it was deter-mined by measuring weight loss after ultrasonication of 1 g wetbeads in 100 ml deionized water using 750 W ultrasonic processor(VC 750, Sonics) with a high power sonic tip operated at 20 kHzfrequency and under various power amplitudes (a) 21%, (b) 25%,(c) 30%, and (d) 35% for 10 min. The beads obtained after ultrason-ication were collected using 1 mm sieve and percentage dry weightloss of beads was measured from the weight difference of thebeads before and after ultrasonication.

2.5. Acid stability

Acid stability of gel particles was characterized by determiningthe mass loss due to gel dissolution in acidic solution. The swellingexperiment of beads in acid solution was performed by adding 1 gwet beads of CB and CSB in 50 ml deionized water of various pHlevels (pH 2–7) and the beads were allowed to swell for 6 h at30 �C in shaking condition. The percentage dry weight loss of thebeads was determined from the difference of wet weight of thehydrogel beads before and after swelling.

2.6. Adsorption study

The adsorption capacity of CSB formed by 4 g l�1 SDS gelationwas compared with CB for adsorption of CR from 100 mg l�1 aque-ous solution. Here CR was selected as a model anionic dye. Mem-brane materials of the CSB (CSM) were obtained after squeezingcore water from the beads and the adsorption performance ofCSM was also evaluated in this study. The batch adsorption exper-iments were performed in glass vials (20 ml) containing 10 ml CRsolution of desired concentration and 0.2 g wet adsorbent materi-als at pH 5 and 30 �C under shaking condition (150 rpm) for 24 h.The performance of CSB formed in various SDS solutions from 2to 50 g l�1 was studied for adsorption of 100 mg l�1 CR solution.Equilibrium isotherm studies were carried out at a fixed tempera-ture (30 �C) using CB and CSB formed by 4 g l�1 SDS gelation asadsorbent materials, and different initial concentrations of CR(10–1000 mg l�1) were prepared from a stock solution(1000 mg l�1). The kinetics experiments were performed to deter-mine equilibrium time for adsorption of CR (100 mg l�1) onto CBand CSB, and different time intervals up to 960 min were usedfor this study. The amount of CR in solution before and afteradsorption was analyzed at its absorption maximum (kmax,497 nm) using a DR5000 spectrophotometer (HACH, USA). All theexperiments were conducted in triplicate. The amounts of CR ad-sorbed by the adsorbent materials were calculated using the fol-lowing equation:

q ¼ ðC0 � CeqÞ � VW

ð1Þ

where q (mg g�1) is the amount of CR adsorbed by the adsorbentmaterials, C0 and Ceq (mg l�1) are the initial and equilibrium li-quid-phase concentration of CR, respectively, V (l) is the initial vol-ume of dye solution, and W (g) is the dry weight of adsorbentmaterials.

2.7. Adsorption isotherm models

The non-linear forms of the Langmuir, Freundlich and Sips iso-therm models were used to analyze the equilibrium isotherm dataand these models were evaluated by the non-linear coefficients ofdetermination (R2) and a non-linear Chi-square test (v2). Theexpression of the Langmuir model is given as:

qe ¼qmKLCe

1þ KLCeð2Þ

where Ce is the equilibrium concentration of CR (mg l�1) in the solu-tion, qe is the equilibrium CR concentration (mg g�1) on the adsor-bent, and qm (mg g�1) and KL (l mg�1) are Langmuir constantsrelated to the adsorption capacity and energy of adsorption,respectively.

The expression of the Freundlich model is:

qe ¼ KFC1ne ð3Þ

Table 1Elemental analysis of CSBs with various concentrations of SDS.

Element (%) SDS concentration (g l�1)

5 10 20 50

N 3.36 ± 0.03 3.40 ± 0.02 3.41 ± 0.03 3.37 ± 0.01C 47.52 ± 0.31 47.50 ± 0.34 47.42 ± 0.08 48.14 ± 1.98H 8.46 ± 0.07 8.46 ± 0.11 8.38 ± 0.05 8.49 ± 0.30S 6.14 ± 0.32 6.52 ± 0.16 6.71 ± 0.04 6.77 ± 0.01

Data represents average of three replicates with ± standard deviations.

S. Chatterjee et al. / Bioresource Technology 101 (2010) 3853–3858 3855

KF is the Freundlich constant related to the sorbent capacity of sor-bent and n is an empirical parameter representing the heterogene-ity of site energies.

Sips model is a combination of Langmuir and Freundlich modelsand it is expressed as:

qe ¼qmaxKeqCn

e

1þ KeqCne

ð4Þ

Keq (l mg�1) represents the equilibrium constant of Sips equa-tion and qmax (mg g�1) is the maximum adsorption capacity. Sipsisotherm model is characterized by the heterogeneity factor, n.

2.8. Desorption studies

Desorption studies were carried out with CR loaded beads ob-tained from an adsorption system with 10 ml of CR solution(100 mg l�1), and 0.2 g wet weight of beads (CB and CSB). Afteradsorption for 24 h at pH 5, the beads were separated by filtrationand CR concentration in the filtrate was measured spectrophoto-metrically. The beads were washed gently with deionized waterto remove unadsorbed CR molecules. After washing, beads weretransferred into 10 ml of deionized water adjusted to pH 10–13and CR desorbed from the beads to the solution was measuredafter 24 h.

SDS (g l-1)0 5 10 15 20 25

Dry

wei

ght l

oss

(%)

0

20

40

60

80

100

120

CB (21%)CB (25%)CB (30%)CB (35%)CSB (21%)CSB (25%)CSB (30%)CSB (35%)

Fig. 1. Mechanical stability test of CB and CSB formed at various SDS concentrations(5, 10 and 20 g l�1). The value in the parenthesis of the legend represents poweramplitude during ultrasonic operation.

3. Results and discussion

3.1. Formation of CS hydrogel beads by alkali and SDS gelation

Formation of CB involves neutralisation of each droplet of CSsolution in alkali solution and this process generates a physicalhydrogel bead that contains only water and CS in the free amineform. The neutralisation of NHþ3 sites into NH2 leads to the disap-pearance of ionic repulsions between polymer chains, and thephysical hydrogel bead is formed by physical cross-linking of poly-mer chains involving hydrogen bonds and hydrophobic interac-tions (Ladet et al., 2008). Physical hydrogel formation of CSsimply involves the process of hydrophobic/hydrophilic balance(Montembault et al., 2005).

Formation of CSB begins when drops of CS solution are added toSDS solution (2–50 g l�1). The mixing between the two solutions isnot instantaneous because CS solution droplets are viscous. Theoppositely charged surfactant (SDS) and polyelectrolyte (CS) comeinto contact with one another by means of counter-diffusion acrossthe surfactant/polyelectrolyte solution interface. Unlike the deeppenetration of small OH� ion during alkali gelation into the CSdroplet, complex formation occurs only at the interface due tothe large size of the surfactant molecule, and a gel capsule isformed around the drop. This effect results in shrinkage of thepolyelectrolyte drop, and hydrogel beads are thus formed by asso-ciative phase separation (Lapitsky and Kaler, 2004). Such a networkis stabilised by a combination of electrostatic, ion–dipole, andhydrophobic interactions (Thongngam and McClements, 2004).After the first soft membrane layer is rapidly formed, surfactantmolecules are still able to penetrate through the layer, and a sec-ond membrane layer is formed by contact with chitosan moleculesthat are diffused to the inner surface. Ultimately, a shell membranecomposed of thin layers is formed.

3.2. Structure of hydrogel beads

Optical microscope images of hydrogel beads (figure notshown) illustrate that CB consist purely of CS molecules, whereasCSB formed by 4 g l�1 SDS gelation consist of a sparsely gelled coreencapsulated by a dense shell, indicating a core–shell structure.

The images of CSB at different stages of gelation (figure not shown)clearly indicate that SDS complexation continues in SDS solutioneven after bead formation, and structural organisation of the beadschanges with time after formation in the same solution. Cross-sec-tions of CSB exhibit a multi-membrane structure. As SDS concen-tration is increased in the gelling solution, stronger and denserhydrogel beads are formed. Additionally, shrinkage is observedfrom osmotic action of the shell membrane, by which more watermolecules in the shell are released at high concentrations of SDS inthe gelling solution.

The results of elemental analysis of CSB formed by various SDSconcentrations are listed in Table 1. The sulfur (S) content of CSB isclose to the stoichiometric molar ratio, but this value slightly in-creases with an increase in SDS concentration in the gelling solu-tion. The determination of SDS content of CSB is purely based onS (%) obtained by elemental analysis because the amount of S inCSB is solely contributed by SDS molecules. Thereby, the increasein SDS concentration during gelation increases SDS content inCSB (Table 2).

FTIR spectra of CB (figure not shown) show characteristic peaksassignment of CB: 3442 cm�1 (wide peak of O–H stretching over-lapped with N–H stretching), 2878 cm�1 (C–H stretching),1651 cm�1 (amide II band, N–H bending and C@O stretching ofacetyl groups), 1382 cm�1 (O–H bending and C–N stretching) and1071 cm�1 (bridge C–O–C stretching and C–O stretching). Charac-teristic peaks obtained for CSB (figure not shown) with variousSDS concentrations during gelation (5, 10, 20, 50 g l�1) are:3441 cm�1 (wide peak of O–H stretching overlapped with N–Hstretching), 2923 and 2853 cm�1 (C–H stretching), 1635 cm�1

(amide II band, N–H bending and C@O stretching of acetyl groups),1467 cm�1 (asymmetric C–H bending of CH2 group), 1379 cm�1

(O–H bending and C–N stretching) and 1061 cm�1 (bridge C–O–Cstretching and C–O stretching). The appearance of new spectral

Table 2CS and SDS contents of CSBs with various SDS concentrations.

Adsorption SDS concentration (g l�1)

5 10 20 50

Sulfur (S) content (g) g�1 dry weight of beadsa 0.061 ± 0.003 0.065 ± 0.002 0.067 ± 0.000 0.068 ± 0.000SDS content (g) g�1 dry weight of beads 0.553 ± 0.028 0.587 ± 0.014 0.605 ± 0.003 0.610 ± 0.001CS content (g) g�1 dry weight of beads 0.447 ± 0.028 0.413 ± 0.014 0.395 ± 0.003 0.390 ± 0.000mol NH2 of CS g�1 dry weight of beads 0.0024 ± 0.0002 0.0022 ± 0.0001 0.0021 ± 0.0000 0.0021 ± 0.0000mol SDS mol�1 NH2 of CS 0.818 ± 0.094 0.936 ± 0.054 1.005 ± 0.008 1.030 ± 0.002

Data represents average of three replicates with ± standard deviations.a S content of CSB obtained by elemental analysis.

Table 3Adsorption capacities of different type adsorbent materialsa.

Type qe (mg CR g�1) qe,v (mg CR ml�1)

CB 83.1 ± 1.14 1.87 ± 0.03CSBb 115.8 ± 0.96 2.66 ± 0.03CSMb 103.5 ± 1.01 47.0 ± 0.12

a Initial CR concentration was 100 mg l�1.b The materials were produced by the gelation with 4 g l�1 SDS solution

3856 S. Chatterjee et al. / Bioresource Technology 101 (2010) 3853–3858

peaks especially in the region of 1650–1550 cm�1, and at2923 cm�1 indicates binding of dodecyl sulfate with the aminegroup of CS molecules during SDS gelation. Thereby, FTIR resultsclearly suggest that the interactions between CS and oppositelycharged SDS molecules lead to complex formation, in which gluco-samine units of CS form salt bonds with SDS. The final bead struc-ture forms via hydrophobic interactions between non-polarsegments of SDS in aqueous solution.

3.3. Mechanical stability of hydrogel beads

Mechanical stability testing of the beads is given as mass lossunder ultrasonication. Fig. 1 shows that CSB showed much lessmass loss than CB after ultrasonication, indicating that beadsformed by SDS gelation are mechanically more stable than physicalhydrogel beads of CS. The percentage dry weight loss of CSB afterultrasonication decreased with an increase in SDS concentrationduring gelation. It has been reported in earlier publication (Chat-terjee et al., 2009c) that mechanical stability of CB was greatly en-hanced by carbon nanotube (CNT) impregnation. The weight loss ofCNT impregnated CB was 2.02% after ultrasonication under 25%power amplitude for 10 min, whereas, that for CB and CB cross-linked with epichlorohydrin (ECH) was 51.61% and 40.21%, respec-tively. In this study, CSB formed by 5, 10 and 20 g l�1 SDS gelationexhibited 1.14%, 0.42% and 0% weight loss, respectively, after ultra-sonication under 25% power amplitude for 10 min. The less massloss of CSB than CNT impregnated CB or cross-linked CB indicatesthe effectiveness of SDS gelation method for enhancing mechanicalstability of CB.

3.4. Acid stability of hydrogel beads

Acid stability test of hydrogel beads indicated that the mass lossof CB was found to increase with a decrease in pH of the solution(figure not shown). Complete solubilisation of CB at pH 2 indicatespoor acid stability because free positive charges develop in beadsdue to protonation of the amine groups of CS and the resulting mu-tual repulsion causes swelling. CSB formed by 5 g l�1 SDS gelationdid not show any mass loss or pH-sensitive swelling under acidicconditions, even in a pH 2 solution. In the previous study (Chatter-jee et al., 2009c), CB after CNT impregnation did not show anyadditional acid stability. The acid stability of CB was only enhanced

after ECH cross-linking, and interestingly, the weight loss of CSB(0.4%) was obtained much less than CB cross-linked with ECH(24.8%) in a pH 2 solution. Therefore, SDS gelation could providea good acid stability of CS hydrogel beads without additionalcross-linking treatment.

3.5. Effect of SDS concentration on CR adsorption

SDS concentration variation (2–50 g l�1) during gelation exhib-ited effects on the adsorption capacity of CSB (mg g�1) for CR(100 mg l�1). The maximum adsorption capacity of CSB was ob-served with 4 g l�1 SDS gelation and at concentrations of SDS great-er than 4 g l�1, CSB showed a decrease in adsorption capacities. Theincrease of adsorption capacity up to 4 g l�1 SDS gelation is attrib-uted to hydrophobic interactions between CR and SDS moleculesdominating over the charge repulsions by the same molecules dur-ing adsorption. The increase in SDS concentration during gelationincreases the SDS content in CSB (Table 2) and makes the beadsdenser; this may cause increased charge repulsion between CRand negatively charged groups of free SDS molecules and obstructmass transfer of CR into the interior adsorption site duringadsorption.

3.6. Volumetric adsorption capacity

Table 3 shows that CSB formed by 4 g l�1 SDS gelation exhibitedbetter adsorption capacity (115.8 mg CR g�1 dry weight of beads)than CB (83.1 mg CR g�1 dry weight of beads) for adsorption froma 100 mg l�1 CR solution, and that was most likely due to increasedhydrophobic interactions between SDS and CR. The volumetricadsorption capacity of beads (mg CR ml�1 material volume) wasobtained by multiplying qe (mg g�1) of adsorbent materials withthe g dry weight ml�1 of materials, which was increased from0.023 g ml�1 for CB to 0.45 g ml�1 for CSM. The volumetric adsorp-tion capacity (47.0 mg ml�1) for CSM was significantly enhancedcompared to that for CB (1.9 mg ml�1), which represents a 25� in-crease. Therefore, significant enhancement of the volumetricadsorption capacity of CSM could enhance applicability in realwastewater treatment.

3.7. Adsorption isotherm

Fig. 2 exhibits the fitting of equilibrium adsorption isothermdata for CB and CSB by 4 g l�1 SDS gelation to the non-linear formof Langmuir, Freundlich and Sips isotherm models. The results ofnon-linear R2 and v2 for three adsorption isotherms are shown inTable 4. The Sips isotherm model appeared to be the best fittingmodel for CR adsorption onto CB because of highest R2 (0.993)and lowest Chi-square, v2 (2.69) values. The results of non-linearR2 (0.997) and v2 (1.60) of Langmuir model, and R2 (0.997) andv2 (1.70) of Sips model for adsorption of CR onto CSB indicated thatadsorption isotherm data for CSB exhibited good fit with bothmodels. The value of n for CR adsorption onto CSB was close to

Ce (mg l-1)0 200 400 600 800 1000

q e (m

g g-1

)

-50

0

50

100

150

200

250

Experimental data (CB)Langmuir non-linear (CB)Freundlich non-linear (CB)Sips non-linear (CB)Experimental data (CSB) Langmuir non-linear (CSB)Freundlich non-linear fit (CSB)Sips non-linear fit (CSB)4 g l-1 SDS gelation, pH 5, 30 0C

Fig. 2. Plot of qe vs. Ce for adsorption of CR onto CB and CSB formed by 4 g l�1 SDSgelation; pH 5 and 30 �C.

S. Chatterjee et al. / Bioresource Technology 101 (2010) 3853–3858 3857

unity (n = 1.02), indicating that adsorption onto CSB is homoge-neous. The close similarity between maximum adsorption capacityvalues obtained from Langmuir (208.1 mg g�1) and Sips(208.3 mg g�1) isotherm models for CSB beads indicated homoge-neous adsorption process. The maximum adsorption capacity ofCSB (208.3 mg g�1) and CB (200.0 mg g�1) obtained from the Sipsisotherm model was similar, whereas maximum adsorption capac-ity of CSB (208.1 mg g�1) obtained from the Langmuir isothermmodel was higher than that of CB (179.2 mg g�1). Thereby,mechanically improved CSB exhibited similar (Sips isotherm) orhigher (Langmuir isotherm) maximum adsorption capacity thanCB.

The comparison of maximum adsorption capacity of CSB(208.3 mg g�1) with that of other adsorbents used for adsorptionof CR such as 6.70 mg g�1 of activated carbon prepared from coirpith (Namasivayam and Kavitha, 2002), 41.20 mg g�1 of neem leafpowder (Bhattacharrya and Sharma, 2004), 66.23 mg g�1 of palmkernel seed coat (Oladoja and Akinlabi, 2009), and 330.62 mg g�1

of N,O-carboxymethyl CS (Wang and Wang, 2008) indicated thatCSB could be used as an effective adsorbent for adsorption of CRfrom aqueous solutions. Moreover, adsorption performance of thissorbent could be further improved by surface modification orimpregnation with various chemicals and materials as CR adsorp-

Table 4Constants for equilibrium isotherm models with error analysis values.

Langmuir isotherm modelAdsorbent KL (l mg�1) qm (mg g�1) Error analysis

R2 v2

CB 0.026 179.2 0.980 12.86CSB 0.046 208.1 0.997 1.60

Freundlich isotherm modelAdsorbent KF (l g�1) 1/n Error analysis

R2 v2

CB 26.91 0.291 0.938 23.00CSB 39.17 0.262 0.893 51.09

Sips isotherm modelAdsorbent qmax (mg g�1) Keq (l mg�1) n Error analysis

R2 v2

CB 200.0 0.054 0.72 0.993 2.69CSB 208.3 0.044 1.02 0.997 1.70

tion capacity increased by cetyl trimethyl ammonium bromideimpregnation (Chatterjee et al., 2010) or carbon nanotubes impreg-nation (Chatterjee et al., 2009d).

3.8. Kinetic study

Experimental kinetic data for adsorption of CR onto CB and CSBfrom a 100 mg l�1 solution have been illustrated in Fig. 3 and equi-librium adsorption time for both adsorption systems was 420 min.Two simplified kinetic models including pseudo-first-order andpseudo-second-order equations, and intra-particle diffusion mod-el, were used in this study.

The non-linear form of the pseudo-first-order rate equation isgiven as:

qt ¼ qeð1� e�k1tÞ ð5Þ

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 constants, k1 and qe valuesobtained from this rate model are given in Table 5.

The non-linear form of pseudo-second-order rate equation isexpressed as:

qt ¼q2

e k2t1þ qek2t

and h ¼ k2q2e ð6Þ

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 rate model are presented in Table5.

The correlation coefficient (R2) values of the pseudo-first-orderequation were 0.998 and 0.994 for CB and CSB, respectively. For thepseudo-second-order model, the correlation coefficient (R2) valueswere 0.988 and 0.991 for CB and CSB, respectively. The R2 values ofboth rate models for each adsorption system indicated that CRadsorption onto CB and CSB could be better explained by pseu-do-first-order rate model than pseudo-second-order rate model.As shown in Table 5, the qe values of CB (83.34 mg g�1) and CSB(116.92 mg g�1) obtained from pseudo-first-order rate model werein better agreement with qe (exp) value of CB (83.19 mg g�1) andCSB (115.90 mg g�1), respectively, than qe values of CB(96.59 mg g�1) and CSB (140.15 mg g�1) obtained from pseudo-second-order rate model.

t (min)0 200 400 600 800 1000 1200

q t (m

g g-1

)

0

20

40

60

80

100

120

Experimental data (CB)Pseudo-first-order (CB)Pseudo-second-order (CB)Intra particle diffusion (CB)Experimental data (CSB)Pseudo-first-order (CSB)Pseudo-second-order (CSB)Intra particle diffusion (CSB)4 g l-1 SDS gelationC0 = 100 mg l-1, pH 5

Fig. 3. Plot of qt vs. t for adsorption of CR onto CB and CSB formed by 4 g l�1 SDSgelation; initial CR concentration, 100 mg l�1; pH 5.

Table 5Constants of different rate models for CB and CSBa.

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

(mg g�1) qe (cal) (mg g�1) k1 (min�1) qe (cal) (mg g�1) k2 (g mg�1 min�1) h (mg g�1 min�1) kp (mg g�1 min�0.5)

CB 83.19 83.34 0.010 96.59 1.24 � 10�4 1.16 4.96CSB 115.90 116.92 0.007 140.15 5.61 � 10�5 1.10 5.76

a Initial CR concentration was 100 mg l�1.

3858 S. Chatterjee et al. / Bioresource Technology 101 (2010) 3853–3858

The intra-particle diffusion equation is given as:

qt ¼ kpt0:5 ð7Þ

kp is the intra-particle diffusion rate constant (mg g�1 min�0.5) andthe values of kp for CB and CSB were 4.96 and 5.76 mg g�1 min�0.5,respectively (Table 5). The correlation coefficients (R2) values for CB(0.947) and CSB (0.953) indicated that intra-particle diffusion has asignificant role in initial stage of adsorption in the study.

3.9. Desorption study

Desorption studies help to understand the mechanism ofadsorption and also help in the recovery of CB and CSB. The resultsindicated that CSB exhibited less increase in desorption (from16.18% ± 0.89 to 25.06% ± 1.72) than CB (from 25.92% ± 1.01 to36.84% ± 2.060) with pH change from 10 to 13. CB showed verylow-desorption for CR with pH change because of strong bond for-mation between CR and CS molecules and involvement of someinteractions other than electrostatic interactions. Moreover, theless desorption for CSB than CB with pH change suggests stronghydrophobic interactions between CSB and CR molecules.

4. Conclusions

CS hydrogel bead formed by SDS gelation (CSB) produced densecore–shell membrane structure, and it showed higher mechanicalstrength and acid stability than conventional CS hydrogel beadformed by alkali gelation (CB). CSB formed by 4 g l�1 SDS gelationexhibited slightly higher adsorption capacity than CB for congo red(CR). Membrane materials of CSB (CSM) generated by removingcore water had approximately 25 times higher volumetric adsorp-tion capacity than CB for CR. Thus, generation of CSM from CSBcould enhance the technical and commercial importance of con-ventional CB in the field of wastewater treatment by solving itsmajor limitations.

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 num-ber 2009-0079636).

References

Bestani, B., Benderdouche, N., Benstaali, B., Belhakem, M., Addou, A., 2008.Methylene blue and iodine adsorption onto an activated desert plant.Bioresour. Technol. 99, 8441–8444.

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.

Cestari, A.R., Vieira, E.F.S., dos Santos, A.G.P., Mota, J.A., de Almeida, V.P., 2004.Adsorption of anionic dyes on chitosan beads. 1. The influence of the chemicalstructures of dyes and temperature on the adsorption kinetics. J. ColloidInterface Sci. 280, 380–386.

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

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

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

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

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

Chiu, H.C., Liu, C.H., Chen, S.C., Suen, S.Y., 2009. Adsorption removal of anionic dyeby inorganic–organic hybrid anion-exchange membranes. J. Membr. Sci. 337,282–290.

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

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

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.

dos Santos, A.B., Cervantes, F.J., van Lier, J.B., 2007. Review paper on currenttechnologies for decolourisation of textile wastewaters: perspectives foranaerobic biotechnology. Bioresour. Technol. 98, 2369–2385.

Gopinath, K.P., Murugesan, S., Abraham, J., Muthukumar, K., 2009. Bacillus sp.mutant for improved biodegradation of congo red: random mutagenesisapproach. Bioresour. Technol. 100, 6295–6300.

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

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

Hasani, Z.M., Alavi, M.M.R., Arami, M., 2009. Coagulation/flocculation of dye-containing solutions using polyaluminium chloride and alum. Water Sci.Technol. 59, 1343–1351.

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.

Khalaf, M.A., 2008. Biosorption of reactive dye from textile wastewater by non-viable biomass of Aspergillus niger and Spirogyra sp.. Bioresour. Technol. 99,6631–6634.

Ladet, S., David, L., Domard, A., 2008. Multi-membrane hydrogels. Nature 452, 76–79.

Lapitsky, Y., Kaler, E.W., 2004. Formation of surfactant and polyelectrolyte gelparticles in aqueous solutions. Colloid Surf. A 250, 179–187.

Malik, P.K., Saha, S.K., 2003. Oxidation of direct dyes with hydrogen peroxide usingferrous ion as catalyst. Sep. Purif. Technol. 31, 241–250.

Manu, B., Chaudhari, S., 2002. Anaerobic decolorisation of simulated textilewastewater containing azo dyes. Bioresour. Technol. 82, 225–231.

Montembault, A., Viton, C., Domard, A., 2005. Physicochemical studies of thegelation of chitosan in a hydroalcoholic medium. Biomaterials 26, 933–943.

Namasivayam, C., Kavitha, D., 2002. Removal of congo red from water by adsorptiononto activated carbon prepared from coir pith, an agricultural solid waste. DyesPigm. 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.

Radha, K.V., Sridevi, V., Kalaivani, K., 2009. Electrochemical oxidation for thetreatment of textile industry wastewater. Bioresour. Technol. 100, 987–990.

Robinson, T., McMullan, G., Marchant, R., Nigam, P., 2001. Remediation of dyes intextile effluent: a critical review on current treatment technologies with aproposed alternative. Bioresour. Technol. 77, 247–255.

Selcuk, H., 2005. Decolourization and detoxification of textile wastewater byozonation and coagulation processes. Dyes Pigm. 64, 217–222.

Thongngam, M., McClements, D.J., 2004. Characterization of interactions betweenchitosan and an anionic surfactant. J. Agric. Food Chem. 52, 987–991.

Thongngam, M., McClements, D.J., 2005. Influence of pH, ionic strength, andtemperature on self-association and interactions of sodium dodecyl sulfate inthe absence and presence of chitosan. Langmuir 21, 79–86.

Trabelsi, S., Guillot, S., Raspaud, E., Delsanti, M., Langevin, D., Boué, F., 2006. Newnano- and microparticles with a liquid-crystal-like interior. Adv. Mater. 18,2403–2406.

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-carboxymethly-chitosan. Bioresour. Technol. 99, 1403–1408.