Micro- and nano-sized bentonite filled composite superabsorbents of chitosan and acrylic copolymer for removal of synthetic dyes from water

Applied Clay Science xxx (2014) xxx–xxx

CLAY-03162; No of Pages 11

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Applied Clay Science

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Research paper

Micro- and nano-sized bentonite filled composite superabsorbents ofchitosan and acrylic copolymer for removal of synthetic dyes from water

Ruma Bhattacharyya, Samit Kumar Ray ⁎Department of Polymer Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India

⁎ Corresponding author. Fax: +91 33 23508386.E-mail address: [email protected] (S.K. Ray).

http://dx.doi.org/10.1016/j.clay.2014.09.0150169-1317/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Bhattacharyya, R.,copolymer for removal of synthetic dyes f...,

a b s t r a c t
a r t i c l e i n f o
Article history:Received 29 April 2014Received in revised form 12 September 2014Accepted 13 September 2014Available online xxxx

Keywords:ChitosanCopolymerBentoniteDyeIsothermsKinetics

Bentonite clay, chitosan and acrylic copolymer gels have been separately reported for the adsorption of dyes. Inthe present work these three kinds of adsorbents were combined to make composite hydrogels. The hydrogelswere characterized by FTIR, XRD, DTA–TGA and SEM. The swelling, diffusion and network parameters of thehydrogels were also evaluated. These composite hydrogels were used for the removal of malachite green andmethyl violet dyes from water. The composite hydrogels showed high adsorption and removal% of both ofthese dyes. Themass transfer coefficient, diffusion coefficient and thermodynamic parameters of the dye adsorp-tion were also determined.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Chitin is the second most abundant biopolymer in nature aftercellulose. It consists of unbranched chains of (1, 4)-2-acetamido-2-deoxy-D-glucose. Chitosan is obtained by deacetylation of chitin. Chito-san is widely used as an adsorbent for the removal of metal ions anddyes. The presence of amino, acetamide and hydroxyl functional groupsin its structure serves as the active sites for adsorption (Wu et al., 2001).However, like most of the biopolymer based superabsorbents, the sta-bility of chitosan based superabsorbents or hydrogels is poor becauseof its extensive hydrophilicity and pH sensitivity (Wang et al., 2011).To overcome this limitation, chitosan has been modified by chemicalcrosslinking (Wan Ngah et al., 2011), making derivative of chitosan(Wan Ngah et al., 2011) or grafting a synthetic polymer to chitosan(Konaganti et al., 2010; Wang et al., 2011; Yang et al., 2013). Chitosanhas also been modified by forming its semi and full IPN with syntheticpolymers (Zhao et al., 2012; Maity and Ray, 2014) or making the com-posite hydrogels of chitosan (Wan Ngah et al., 2011).

Composite hydrogels of chitosan may be obtained by incorporatinginorganic filler (Wang et al., 2011; Nešic' et al., 2013; Auta andHameed, 2014) in its matrix or incorporating chitosan in the matrix ofa synthetic polymer (Zhao et al., 2012; Maity and Ray, 2014). In fact, in-tegration of various adsorbents to a single composite adsorbent resultsin increased mechanical strength, surface functionality, selectivity, re-generation and surface area (Auta and Hameed, 2014). Composite

Ray, S.K., Micro- and nano-sizAppl. Clay Sci. (2014), http://

hydrogel may be prepared by impregnation, grafting, crosslinking, che-lation and in situ polymerization (Chang and Juang, 2004).

During in situ polymerization the reactive monomers are allowed topolymerize in aqueous dispersion of inorganic fillers (Bhattacharyyaand Ray, 2013). By this method the fillers are strongly impregnated inthe matrix of the resulting polymer. The objective of the present studywas to synthesize a strong adsorbent by integrating three different ad-sorbents, i.e., a natural polymer such as chitosan, a synthetic copolymerand inorganic clay such as bentonite. By this integration favorable ad-sorption properties of these different adsorbents will be synergisticallycombined in a single composite hydrogel. The novelty of the presentwork lies on the technique used for combining the synthetic copolymerwith chitosan and clay. Instead of direct blending or grafting chitosanand clay were incorporated in situ during polymerization of the acrylicmonomers. Thus, in the present work acrylic acid, acrylamide andMBA (crosslinker) were allowed to free radically polymerize in thepresence of chitosan and clay in water. The monomers acrylic acid,acrylamide and crosslinker MBA were chosen since the copolymer gelobtained from these synthetic monomers was reported to be a goodadsorbent for industrial dyes (Li et al., 2011). Accordingly, severalhydrogels were prepared by varying molar ratio of acrylic acid andacrylamide, concentration of initiator and crosslinker. The chitosanincorporated copolymer gel (F0) showing the best swelling characteris-tics was identified and micro- and nano-sized bentonite clay was fur-ther incorporated during polymerization of F0 to produce compositehydrogels.

Bentonite is an inorganic adsorbent. Its main component montmo-rillonite consists of two layers of tetrahedral silica sheets sandwiching

ed bentonite filled composite superabsorbents of chitosan and acrylicdx.doi.org/10.1016/j.clay.2014.09.015

http://dx.doi.org/10.1016/j.clay.2014.09.015

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2 R. Bhattacharyya, S.K. Ray / Applied Clay Science xxx (2014) xxx–xxx

one layer of octahedral alumina sheet. It carries a permanent negativecharge because of isomorphous substitution of its Si+4 in tetrahedrallayers by Al3+ and Al3+ in octahedral layers by Mg2+ (Ma et al.,2012). This rich clay material is widely used alone or as one of the con-stituent adsorbents of a composite (Crini, 2006; Tahir and Rauf, 2006;Ma et al., 2012; Liu et al., 2014) for removal of synthetic dye.

Cationic synthetic dyes such as methyl violet or malachite green arewidely used for coloring various products because of its excellent fast-ness to materials, ease of applicability and durability. However, thesedyes are carcinogenic and mutagenic. The discharge of low concentra-tion of these dyes as aqueouswaste causes severe environmental pollu-tions. Thus, several low cost polymer hydrogel based adsorbents havebeen tried (Tang et al., 2012; Zhao et al., 2012; Bhattacharyya and Ray,2013;Maity and Ray, 2014) for the removal of these dyes. In the presentwork the unfilled F0 hydrogel showing the best swelling characteristichas been filled with 2, 4 and 6 wt.% (of total polymer) micro-sizedbentonite clay in situ during polymerization to produce three compositegels designated as MF2, MF4 and MF6, respectively. Similarly, the F0hydrogel was also filled with 0.5, 1.5 and 2 wt.% nano sized bentoniteclay to produce three composite hydrogels designated as NF0.5, NF1.5and NF2, respectively. These two kinds of composite gels were usedfor the adsorption of methyl violet and malachite green dyes over theconcentration range of 2.5–20 mg/L (low concentration range) and200–1000 mg/L (high concentration range) dye in water.

2. Experimental

2.1. Materials

The acrylic monomers, i.e., acrylic acid, acrylamide and N′N′methy-lene bis-acrylamide (MBA) were of analytical grades and used as ob-tained. These monomers were procured from Merck. Chitosan (N85%deacetylation) was procured from Merck and used as obtained. Redoxpair of initiators, i.e., ammonium persulfate and sodium metabisulfide,was also obtained fromMerck and used as obtained for polymerizationreaction. The two synthetic dyes, viz. malachite green andmethyl violetused in sorption studieswere purchased from SRL Chemical, India. Sodi-ummontmorillonite richmicro- (API, grade, 50–75 μm) and nano-sized(30–90 nm, aspect ratio 300–500, mineral's thickness 1 nm, cation ex-change capacity 120 meq/100 g) bentonite claywas supplied byAmrfeoPte. Ltd., Kolkata. For polymerization reactions, swelling and sorptionstudy distilled water was used.

2.2. Methods

2.2.1. Synthesis of the hydrogelsThe chitosan and copolymer based unfilled F0 gels were synthesized

from the acrylicmonomers and chitosan in a three-neck glass reactor bythe free radical solution polymerization at 30 °C for 3 h. At first 1 wt.%chitosan solutionsweremade in deionized water in a 250mL glass bea-ker by gradual addition of the required amount of chitosan and 2wt.% ofacetic acid to obtain a viscous solution of chitosan. The requiredamounts of acrylic acid and acrylamide were then added to the three-neck glass reactor placed on a constant temperature water bath. Thetemperature was maintained at 30 °C and aqueous solution of the initi-ators was added to the reactor followed by the addition of MBA(crosslinker). The hydrogel obtained was cut into small blocks andthen immersed into double distilled water for 48 h to remove thewater soluble oligomer, uncrosslink polymer and the unreacted mono-mers from the gel. The gel obtainedwas dried in a vacuumoven at 60 °Cto a constant weight. The dried gel was then disintegrated in a blender.For synthesizing micro-sized bentonite filled MF2, MF4, and MF6 andnano-sized bentonite filled NF0.5, NF1.5 and NF2 hydrogels, the re-quired amount of micro- and nano-sized bentonite filler was taken inthe polymerization mixtures and it was mixed in a sonicator bath

Please cite this article as: Bhattacharyya, R., Ray, S.K., Micro- and nano-sizcopolymer for removal of synthetic dyes f..., Appl. Clay Sci. (2014), http://

followed by polymerization in situ in this aqueous dispersion in a simi-lar way as in the case of F0.

2.2.2. Characterization of the hydrogelsThe Fourier transform infrared (FTIR) spectra of the hydrogels were

recorded on a FTIR spectrometer (Perkin Elmer, model-Spectrum-2,Singapore) using KBr pellet made by mixing KBr with fine powder ofthe polymer gel samples (10:1mass ratio of KBr to polymer). Differentialthermal analysis (DTA) and thermogravimetric analysis (TGA) of the hy-drogel sampleswere carried out in a Perkin Elmer instrument in nitrogenatmosphere at the scanning rate of 10 °C per minute in the temperaturerange of 60–600 °C. The crystallinity of the hydrogels was characterizedbyX-ray diffraction (XRD).Wide angle X-ray diffraction profile of the hy-drogel samples were studied at 25 °C with a diffractometer (model:X'Pert PRO, made by PANalytical B.V. The Netherlands) using Ni-filtered Cu Kα radiation (λ = 1.5418 Å) and a scanning rate of 0.005°/s(2θ/s). The angle of diffraction varied from 2 to 72°. The morphology ofthe gelswas observed by using SEMat amagnification of 1000 (Scanningelectron Microscope, model no. S3400N, VP SEM, Type-II, made byHitachi, Japan) with the accelerating voltage set to 10 kV.

2.2.3. Study of swellingThe dynamic swelling properties of the hydrogels were studied by

the gravimetric method in terms of the swelling ratio (SR) and equilib-rium swelling ratio (ESR) using Eqs. (1) and (1a), respectively. The SR ofthe dry gel samples of a specific weight (Wd) was determined from itsincreased weight due to swelling in water at different time intervals(Wt) while ESR was determined from its equilibrium weight (We),i.e., when there is no further change of weight with time. The buffer so-lutions of varied pH were prepared by dissolving phosphoric acid, po-tassium phosphate (KH2PO4) and potassium hydrogen phosphate(K2HPO4) in distilled water.

SRt g=gð Þ ¼ Wt−Wd

Wdð1Þ

ESR g=gð Þ ¼ We−Wd

Wdð1aÞ

2.2.4. Study of the dye removal capacity of the hydrogelsSmall amount of hydrogel sample (m g) was taken in 100 mL of the

dye solutionwith continuous stirring on amagnetic stirrer until equilib-riumwas reached. After equilibriumwas reached, the dye solution wasseparated by decantation from the hydrogel. The concentration of thedyes in this solution before and after addition of the gelwas determinedfrom a precalibrated curve of absorbance versus concentrations usingUV–visible Spectrophotometer (Perkin Elmer lambda 25). The absor-bance of the dye solutions was measured at a wavelength of 425 nmfor the malachite green and 584 nm for the methyl violet dye. Theamount of dye uptake (inmg) by unit mass (in g) of the hydrogel at dif-ferent time intervals (qt, mg/g) and at equilibrium (qe, mg/g) and ad-sorption or removal% was calculated using Eqs. (2a)–(2c), respectively.

qt ¼ C0−Ctð ÞVm

ð2aÞ

qe ¼ C0−Ceð ÞVm

ð2bÞ

removal% ¼ C0−Ceð ÞC0

� 100 ð2cÞ

Here Co and Ce are initial and equilibrium concentration of the dye inwater while Ct is dye concentration at time t, V is volume (L) of the dye



3R. Bhattacharyya, S.K. Ray / Applied Clay Science xxx (2014) xxx–xxx

solution containing the hydrogel and m is mass (g) of the dry hydrogelpolymer used for the experiment.

2.2.5. Error estimationThe swelling and the dye sorption data were fitted to non-linear ki-

netic and isotherm model equations without linearization to minimizeerrors for these fittings (Foo and Hameed, 2010) based on theLevenberg–Marquardt (L–M) algorithm where parameter values of amodel are adjusted in an iterative process using the chi square (χ2).The validity of the models was evaluated in terms of the regression co-efficient (r2), non-linearχ2 and the F values obtained from Anova anal-ysis in Origin software. For a good fitting, r2 should be close to unity, χ2

will be low while F value should be high (Foo and Hameed, 2010).

3. Results and discussion

3.1. Synthesis of hydrogel

The hydrogels of the copolymer and the chitosan (F0) weremade byfree radical polymerization of acrylic acid, acrylamide and MBA in thepresence of chitosan. The extent of the swelling or adsorption capacityof the hydrogels depend on the ionization or hydrogen bonding ofthe various functional groups such as amide (\CONH2), carboxylic(\COOH), amine (\NH2) and hydroxyl (\OH) groups present in itsstructures. The relative amounts of the acrylamide and acrylic acidin the copolymer depend on their reactivity ratios. Accordingly, thehydrogels were preparedwith three different comonomer ratios name-ly, acrylic acid: and acrylamidemolar ratio of 5:1, 7.5:1 and 10:1 and thecopolymer gel made with 7.5:1 comonomer ratio of acrylic acid and ac-rylamidewere found to give the best result of swelling inwater. Similar-ly, 12 wt.% of chitosan (of total monomer weight) in polymethacrylicacid gel was reported to give the highest adsorption for themethyl vio-let and Congo red dye (Maity and Ray, 2014). Thus, the micro- andnano-sized bentonite adsorptive clay was incorporated in the F0 gelmade with 7.5:1 comonomer ratio of the acrylic acid and acrylamideand 12wt.% chitosan. In the F0 gel networks are formed by crosslink co-polymerization of themono-functional acrylic acid and acrylamidewiththe bifunctional MBA. The macro-radicals from the amino groups of thechitosan also take part in polymerization as well as in the network for-mation (Maity and Ray, 2014). The micro- and nano-sized bentoniteclays are incorporated in these double networks during the in situ poly-merization. Further, during the polymerization some of the aminogroups of the chitosan form polyelectrolyte complex by reacting withthe carboxylic functional groups of the acrylic acid (Lee et al., 1999).The chemical structure of the two dyes is shown in Fig. 1a and b whilethe formation of the composite hydrogels and its interaction withthese dyes are shown in Fig. 2.

Malachite green dye

a

Fig. 1. Structure of the dyes. a, Malachi


3.2. Characterization of the hydrogels

3.2.1. FTIRThe FTIR of the micro- and nano-sized clay filled composite poly-

mers are shown in Fig. 3a and b, respectively. In both of these figuresFTIR of the chitosan is also shown for comparison. The chitosan is ob-served to show absorption at i) 3447 cm−1 due to the overlapping ofthe\OH stretching vibration, symmetric N\H vibration and the inter-molecular hydrogen bonding of the polysaccharide moiety of chitosan,ii) 2920 cm−1 due to the C\H stretching vibration, iii) 1659 cm−1

due to the carbonyl stretching vibration (amide-I), iv) 1594 cm−1 dueto the N\H stretching vibration (amide-II), v) 1419 and 1379 cm−1

due to the symmetrical deformation of its methyl (CH3) group, vi)1321 cm−1 due to the C\N stretching vibration (amide-III) and vii)1088 cm−1 due to the C\O stretching (Maity and Ray, 2014). All ofthese characteristic peaks of the chitosan are observed to shift in thefilled composite gels indicating the interaction between the functionalgroups of the constituent polymers in the composites. Accordingly, theabsorption band at around 3300 cm−1 in the filled NF or MF hydrogelsis due to the hydrogen bonded O\H and N\H stretching of the acryl-amide, MBA and chitosan (Dai et al., 2010). The absorption peaks ataround 1660–1680 cm−1 in the filled hydrogels indicate the carbonyl(\C_O) of acrylamide/MBA/acrylic acid units of the composites (Leeet al., 1999). The principle Si\O vibration band at 1024 cm−1 of thebentonite (Zheng et al., 2007) shifts to 1077–1087 cm−1 in the micro-particle filled gel (Fig. 3a) and 1080–1090 cm−1 in the nano-particlefilled composite (Fig. 3b) hydrogels. All of these shifting signify strongelectrostatic interaction among the various functional groups of thehydrogels and the fillers (Maity and Ray, 2014).

3.2.2. XRDXRD results of the polymers are shown in Fig. 4. Chitosan is a semi-

crystalline natural polymer as also reported elsewhere (Qi et al.,2004). Accordingly, chitosan is observed to show a strong diffractionpeak at around 20.1° associated with the mixture of (001) and (100)planes and one weak diffraction peaks at 10.6° associated with mixtureof its (001) and (100) planes (Dash et al., 2012;Maity and Ray, 2014). Aschitosan is incorporated in the amorphous copolymer of acrylic acid andacrylamide, i.e., in F0 the crystallinity of the chitosan disappears and itshows a broadened amorphous hallow at 24°. In fact, the crystallinityof the chitosan arises from the strong hydrogen bonding among itshydroxyl groups. However, as it is incorporated in the copolymer, thehydroxyl groups of the chitosan also show the electrostatic interactionwith carboxylic functional groups of the copolymer in the hydrogelsand hence its crystallinity is reduced to a great extent. For comparisonthe XRD of the bentonite is also shown in Fig. 4. The diffraction linesof montmorillonite are observed with several peaks of its 001 plane at2θ of 7.1, 20.3, 29, 35.6, and 62.3° as also reported elsewhere (Yang

Methyl violet dye

b

te green, and b, methyl violet dye.



+

Fig. 2. Structure of monomers and formation of composite hydrogel and its interaction with dye molecules.


et al., 2010). It also shows a minor peak at 2θ of 54.5° corresponding toits quartz impurity (Yang et al., 2010). Similar to chitosan, XRD peaks ofbentonite also disappear in NF2 indicating well dispersion and exfolia-tion of the bentonite particles in the polymer matrix (Choudalakis andGotsis, 2014). In fact, NF2 shows only one broadened peak at 22.8°and a reduced minor peak at 2θ of 6.9° due to bentonite.

3.2.3. SEMThe SEM of the hydrogels is shown in Fig. 5. The different phases of

polymers, i.e., the copolymer and the chitosan phase are observed inthe surface morphology of unfilled F0. The even distribution of themicro- and nano-sized clay is also observed in MF4 and NF2 indicatinga good compatibility of clay and polymer in the composite gels. No ag-glomeration of the nano-sized clay is observed in NF2. All of these indi-cate balanced integration of the two polymers and clay in the gels.

3.2.4. DTA and TGAThe DTA of the pure chitosan and F0 containing 6 and 12 wt.% chito-

san is shown in Fig. 6a. The decomposition of chitosan occurs in the tem-perature range of 265–340 °C with exothermic DTA peaks at 270 and325 °C due to the thermal decomposition of its amino and N-acetyl


residue (Nam et al., 2010). The F0 composite containing 12 wt.% chito-san is observed to show exothermic peaks at 250 and 373 °C, respec-tively due to the formation of \NH\CO\ bond between theprotonated/non-protonated amine groups of the chitosan and the car-boxylic/carboxylate groups of the acid in the composite above 200 °C(Nam et al., 2010). The other F0 gel containing 6 wt.% chitosan alsoshows similar exothermic peaks. The TGA of the same polymers isshown in Fig. 6b. The pure chitosan is observed to show around 5–10%weight loss in the temperature region of 86–290 °C corresponding tothe loss of absorbed water and 12–43% weight loss in the temperatureregion of 292–580 °C which accounts for the degradation of its mainchain (Rajendran and Sivalingam, 2013). The F0 containing 6 and12 wt.% chitosan shows multiple degradation profiles as observed inFig. 6b. Thus, the F0 hydrogel containing 12 wt.% chitosan is observedto show an initial weight loss of 5–10% in the temperature region of80–90 °C followed by 15–30%, 35–50%, 55–75% and 75%–85% in thetemperature region of 140–250 °C, 255–350 °C, 365–415 °C and 417–577 °C, respectively. The other composite gel also shows the multipleweight loss regions. In these cases the first region (~5–10%) of weightloss corresponds to the loss of the bound water while the weight lossat other temperature regions may be ascribed to splitting of main



a

1169

1398

2952

3422

34622939

14211682

34561176

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 500 1000 1500 2000 2500 3000 3500 4000 4500

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Wave number (cm-1)

Wave number (cm-1)

Tran

smitt

ance

CS

F0

MF4

MF2

MF6

1669547

1124

b

34202933

16601421

1360

1675

331629261655

34521410 16330

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Tran

smitt

ance

NF0.5

NF2

NF1.5

F0

CS

Fig. 3. FTIR of a) CS and micro-particle filled CPCS gel, b) CS and nano-particle filled CPCSgel.


chain and pendant hydroxyl and carboxylic groups of the compositegels (Milosavljevi'c et al., 2011).

3.3. Swelling of the hydrogels

The effect of i)molar ratios of acrylic acid and acrylamide, concentra-tion of ii) initiator and iii) crosslinker (MBA), weight% of iv)micro-sizedclay and v) nano-sized clay and vi) solution pH on equilibrium swellingratio (ESR) and swelling time (teq) is shown in Fig. 7a. In all of thehydrogels 12 wt.% (of total monomers) of chitosan were used. From

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80

Two theta (degree)

Inte

nsity

(-)

CSF0NF2Clay

Fig. 4. XRD of the polymer.


this figure it is observed that ESR increases while teq decreases with anincrease inmolar ratio of the acrylic acid in the copolymer or an increasein initiator concentration. However, the crosslinker shows an oppositetrend, i.e., with an increase in crosslinker (MBA) concentration theESR decreases but equilibrium time for swelling (teq) increases. The hy-drophilicity of the hydrogel increases with an increase in molar ratio ofacrylic acid in the copolymer. Accordingly, the hydrogel with acrylicacid:acrylamide molar ratio of 5:1, 7.5:1 and 10:1 are observed toshow ESR of 8.77, 22.24 and 24.12, respectively in Fig. 7a. Further, thecarboxylic functional groups of the hydrogels swell at a high rate toreach the equilibrium rapidly. Hence, teq decreases with an increase inacrylic acid content in the hydrogel. However, with an increase in acrylicacid content mechanical stability of the hydrogels also decreases be-cause of the extensive swelling in the presence of a large amount ofacrylic acid in the copolymer gel. Thus, in other hydrogels the acrylicacid: acrylamidemolar ratio was kept constant at 7.5:1 since the hydro-gel based on thismolar ratio showed an optimumbalance ofmechanicalstability and swelling. With an increase in the initiator concentrationmolecular weight of the hydrogel decreases (Odian, 1991) resulting inthe formation of more chain ends and network imperfection (Changet al., 2010). Thus, water molecules penetrate easily in this loose andimperfect network leading to an increase in the swelling and decreasein teq. Similarly, an increase in crosslinker concentrations leads to adense network resulting in reduced ESR. Aswatermolecules take longertime to penetrate to a tight network, teq increases with an increasein crosslinker concentration. The hydrogel prepared with 1 wt.%crosslinker, 1 wt.% initiator, 7.5:1 molar ratio of acrylic acid: acrylamideand 12 wt.% chitosan was designated as F0. F0 was further filled with 2,4 and 6 wt.% micro-sized clay (resulting gels were designated as MF2,MF4 and MF6, respectively) and 0.5, 1.5 and 2 wt.% nano sized clay(resulting gels were designated as NF0.5, NF1.5 and NF2, respectively).Because of agglomeration nano-sized clay cannot be incorporated tothe same extent asmicro-sized clay. In the presentwork stable hydrogelcould not be made with more than 6 wt.% micro-sized clay and 2 wt.%nano-sized clay. It is observed that among the filled hydrogels, theMF4 and NF2 gels show the highest ESR. The hydrophilic adsorptiveclay further increases thehydrophilicity of the composite gels. However,it also fills the network and hence teq increases significantly for thesefilled hydrogels while above 4 wt.% microfiller or 2 wt.% nano-fillerthe swelling is also observed to decrease. To evaluate the pH responsivenature of the hydrogels, MF4 and NF2 hydrogels were subjected toswelling at varying pH values as also shown in the same figure. It is ob-served that the swelling of the gels strongly depends onpHofwater andswelling increases significantly with an increase in solution pH from2.73 to 9.6. The swelling at low pH is due to protonation of the aminogroup (NH3

+) of chitosan and acrylamide moiety present in thehydrogels. With an increase in solution pH ionization of carboxylicgroups (COO−) of the hydrogel increases resulting in increased electro-static repulsion among the COO− groups and the penetration of largeamount of water in the gel network. Thus ESR increases at higher pH.

3.3.1. Swelling kineticsThe swelling ratio (SRt) of the hydrogels at different time intervals

(t) was also fitted directly to the following non-linear second orderrate Eq. (3) (Mall et al., 2006)

SRt ¼SR2

ecalkS2t1þ ks2Wecal

¼ rot1þ ks2Wecalt

: ð3Þ

Here, ks2 is the rate constant, SRecal is the calculated equilibriumswelling ratio and ro is the initial rate of swelling.

The non-linear data fitting was carried out in Origin-8 softwarebased on the Levenberg–Marquardt (L–M) algorithm where the valuesof parameters, i.e., ks2 and ro are adjustedusing the chi square (χ2) and Fvalues. The trendlines of these data fittings for the unfilled F0 and filledMF4 and NF2 composite gels are shown in Fig. 7b. A similar kind of



Fig. 5. SEM of the polymer a, F0, b, MF4, and c, NF2.


trendline was observed for the other hydrogels (not shown). Thetrendlines are similar, i.e., initially water uptake% increases with timeand finally it levels off at an equilibrium value. The values of ks2 andro, experimental and calculated ESR based on Eq. (3) for these compos-ite gels are shown in Table 1. The values of the statistical parameters,i.e., r2, χ2 and F of these fittings are also shown in Table 1. It is observedthat calculated ESR of the hydrogels is close to its experimental values.The low values of χ2 (~0.3–0.5), high values of F (~3600–8700) and

a. DTA of the polymer

-12

-10

-8

-6

-4

-2

0

2

4

6

8

0 100 200 300 400 500 600 700

Temperature (oC)

Hea

t flo

w (e

ndo

up)

CS F0 with 6% CS F0 with 12% CS

b. TGA of the polymer

0

20

40

60

80

100

0 300 600 900

Temperature (°C)

Res

idua

l wei

ght o

f pol

ymer

(%)

CS

F0 with 6% CS

F0 with 12% CS

Fig. 6. a, DTA and b, TGA of the polymers.


the values of regression coefficients r2 (N0.99, i.e., close to unity) asshown in Table 1 confirm the good fitting of the swelling data to secondorder rate Eq. (3).

3.3.2. DiffusionFor the understanding of the diffusionmechanism, the swelling data

were also fitted to Eqs. (4) and (5) (Ritger and Peppas, 1987) to obtainthe diffusion characteristics, viz., diffusion constant (kD), diffusional ex-ponent (n) and diffusion coefficient (D) of the hydrogels.

F ¼ mt

me¼ kDt

n ð4Þ

0

5

10

15

20

25

30

35

40

45

50

5.0:1A

A:AM

7.5:1A

A:AM

10:1A

A:AM0.5

% I1%

I

1.5% I

0.5% M

BA

1% M

BA

1.5% M

BA

2% M

BA2%

MF4%

MF6%

MF

0.5%NF

1.5%NF

2%NF

pH2.7

3pH

3.6

pH5.1

4pH

7pH

9.6

ESR

(-)

0

1000

2000

3000

4000

5000

6000

7000

Tim

e fo

r ESR

(min

)

0 6 0 0 1 2 0 0 1 8 0 0 2 4 0 0 3 0 0 0 3 6 0 0

0

5

1 0

1 5

2 0

2 5

0 800 1600 2400 3200 40000 .00 .20 .40 .60 .81 .0 F 0

M F 2M F 4M F 6N F 0 .5N F 1 .5N F 2

SR (g

/g g

el)

S w e lling tim e (m in), t

t

F (- )

a

b

Fig. 7. a, Equilibrium swelling ratio (ESR) of the hydrogels. b, Swelling curves of hydrogelssynthesized with varied filler wt% in hydrogel with non-linear fitting of data to secondorder rate equation and Peppas equation (inset) for diffusion.



Table 1Swelling and diffusion characteristics of the hydrogels.

Polymer ks2×105 ro×102 ESRexpt

(g/g gel)ESRcal

(g/g gel)r2/χ 2/F×10−2 kD×102 n D×106 r2/χ 2/F

F0 5.99 4.6 22.2 24.8 0.9948/0.50/45 1.2 0.57 1.96 0.9836/0.003/11MF2 5.21 4.9 24.2 27.6 0.9943/0.57/36 1.3 0.56 1.88 0.9802/0.004/10MF4 4.74 4.81 25.2 31.8 0.9962/0.47/61 1.4 0.54 1.48 0.9812/0.003/12MF6 6.44 4.5 21.3 24.5 0.9951/0.43/53 1.9 0.51 1.45 0.9708/0.0058/8NF0.5 5.14 4.8 24.6 28 0.9970/0.34/88 1.7 0.52 1.44 0.9771/0.004/11NF1.5 5.26 5.1 25.4 29 0.9973/0.32/107 2.0 0.50 1.30 0.9725/0.005/10NF2.0 5.10 6.2 22.3 25 0.9961/0.36/82 2.5 0.47 1.07 0.9585/0.008/12

ks2 (g gel/g water/minute), ro (g water/g gel/minute), kD (s−1), n (−), D (cm2/s).

40

50

60

70

80

90

100

110

Dye

ads

orbe

d (m

g/g

gel)

Ce = 100 mg/L


D ¼ πr2kD4

� �1n ð5Þ

Here F is the fractional water uptake (SRt/SRe) and r is the radius ofthe cylindrical hydrogel sample. The data fitting and non-linear regres-sion were similar to swelling kinetics as shown in the inset of Fig. 7b forthe abovementioned gels. The values of kD, n and D of the hydrogels arealso shown in Table 1. From Table 1 it is observed that the hydrogelsshow n values close to 0.5 indicating Fickian Case-1 diffusion, i.e., inthese cases rate of diffusion is slightly lower than rate of chain relaxa-tion (Lj Tomic' et al., 2007). The values of the statistical parameters,i.e., r2,χ2 and F for these non-linearfittings as shown in Table 1 also con-firm the good fitting of swelling data to diffusion equations.

3.3.3. Network parametersThe structures of the gel network were characterized in terms of the

averagemolecular weight between the crosslink (Mc), crosslink density(ρc) and mesh size (ς). These network parameters, viz. Mc of thehydrogels were obtained from volume fraction of polymer in swelledgel (ϕp) at equilibrium while water–polymer interaction parameter(χ), density of the hydrogels (ρp) and molar volume of water (Vs)were obtained using the Flory and Rehner model as reported elsewhere(Bhattacharyya and Ray, 2013). The values of the interaction parameter,χ were obtained from the polymer volume fraction (ϕp) (Xue et al.,2001). The mesh size, ς was obtained from ϕp and molecular weightof the repeating unit of the hydrogel (Mandal et al., 2012). Similarly,the other network parameter, viz. crosslink density (ρc) of the hydrogelwas obtained from Avogadro's number (NA = 6.023 × 1023/mol) andpolymer density (ρp) (Sunil and Surinderpal, 2006). The density of thehydrogels was obtained from its weight in air and a non-solvent (tolu-ene). The values of polymer density (ρp), polymer volume fraction(ϕp), interaction parameter (χ), molecular weight between crosslink(Mc), crosslink density (ρc) and mesh size (ς) of the hydrogels areshown in Table 2. In general, the small Mc and ς value and high ρcvalue signify a tighter network. The relative values of these network pa-rameters depend on density and equilibrium swelling values of thehydrogels. From Table 2 it is observed that the values of network

Table 2Network parameters of the hydrogels.

Polymer Density(g/cm3)

Polymervolumefraction(−)

Interactionparameter(−)

Mc × 10−7 Crosslinkdensity(ρc) × 10−16

Meshsize ς(nm)

F0 1.12 0.038 0.5128 1.17 5.78 34.23MF2 1.17 0.034 0.5113 1.95 3.62 46.10MF4 1.19 0.032 0.5107 2.44 2.94 52.53MF6 1.37 0.033 0.5109 2.54 3.25 53.18NF0.5 1.15 0.034 0.5113 1.91 3.63 45.62NF1.5 1.17 0.032 0.5107 2.32 3.04 51.09NF2.0 1.30 0.033 0.5110 2.36 3.32 52.0


parameters are in good agreement with swelling values and density ofthe hydrogels.

3.4. Dye adsorption

3.4.1. Effect of the solution pH on dye adsorptionThe solution pH is observed to influence the swelling results (Fig. 7a)

indicating the pH sensitivity of the hydrogels. The effect of the solutionpH on the dye adsorption for 100 mg/L of feed dye concentration ofmethyl violet and malachite green dye is shown in Fig. 8. The pH ofthe solution varied from 3 to 10 by using appropriate buffer solution.It is observed that the hydrogels show low dye adsorption at the lowestpH, i.e., at pH of 3. At this solution pH, the substituted amine groups ofthe two dyes and the amine groups of the composite gels (present inits chitosan moiety) remain protonated while the carboxylic functionalgroups of the hydrogels remain as COOH. Thus, the hydrogels show lowadsorption at low pH because of the electrostatic repulsion among theprotonated amine groups of the dyes and hydrogels. Up to pH 4 thedye adsorption is caused by hydrogen bonding among functional car-boxylic, hydroxyl and amine groups of the hydrogels and the dye. Asthe solution pH increases, ionization of COOH functional groups to car-boxylate (COO−) anion increases and the strong electrostatic interac-tion between cationic dyes and anionic gels causes higher dyeadsorption. The sudden increase of dye adsorption at pH of 7 may be

10

20

30

2 3 4 5 6 7 8 9 10 11pH of Feed

F0MV MF4MV

NF2MV F0MG

MF4MG NF2MG

Fig. 8. Effect of solution pH on dye adsorption for feed dye conc. of 100 mg/L.




attributed to the complete ionization of the carboxylic function groups(pKa of polyacrylic acid is 6.8 (Mittal, 2000) at this pH). The adsorptionof the dyes decreases above pH 7 because of an increase in deproton-ation of its amine groups. Panic et al. (2013) also reported a decreasein the adsorption of methyl violet from 60% to 7.7% by polymethacrylicacid based hydrogel for an increase of the solution pH from 6.8 to 9.

3.4.2. Effect of contact time on dye adsorptionThe effect of the contact time (t) on adsorption (qt) of themalachite

green and methyl violet dye for a fixed high (500 mg/L) and low(5 mg/L) feed dye concentration is shown in Fig. 9a and b for F0, NF2and MF4 hydrogels. In Fig. 9a qt and t data were fitted to the 1st orderkinetics while in Fig. 9b the same qt and t data were fitted to the 2ndorder kinetics. From these figures it is observed that for both high andlow (low concentration shown in inset of Fig. 9a and b) feed dye con-centrations, there are two distinct stages of the adsorption, viz. an initialhigh adsorption rate is followed by a slower rate of the adsorption. Atthe beginning of the adsorption experiment all the reacting functionalgroups are available for interacting with the dye molecules resultingin high adsorption rate. As these functional groups of the hydrogelsreact with dye molecules, the rate of adsorption decreases and reachesan equilibrium (qe) value. It is also observed that the saturation of thedye adsorption occurs much earlier for high feed dye concentration. In

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 00

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

0 8 0 0 1 6 0 0 2 4 0 0 3 2 0 0012345 F eed 5 m g/L (L o w )

t

q t

Dye

ads

orbe

d, q

tmg/

g ge

l

T im e , t (m in u te )

F 0 M G M F 4 M G N F 2 M G F 0 M V M F 4 M V N F 2 M V

a) 1st order kineticsF eed 5 0 0 m g/L (H igh )

B

A

0 200 400 600 800 1000 1200 1400 16000

100

200

300

400

500

0 800 1600 2400 3200012345 Feed 5 mg/L(Low)

Feed 500 m g/L(High)b) 2nd order kinetics

F0M G M F4M G NF2M G F0M V M F4M V NF2M VD

ye a

dsor

bed,

qtm

g/g

gel

T ime, t (m inute)

t

qt B

A

0 1 0 2 0 3 0 4 0 5 0 6 0- 2 . 6- 2 . 4- 2 . 2- 2 . 0- 1 . 8- 1 . 6- 1 . 4- 1 . 2- 1 . 0- 0 . 8- 0 . 6- 0 . 4- 0 . 2

0 . 00 . 2

0 1 0 2 0 3 0 4 0 5 0 6 0- 0 . 0 1 0- 0 . 0 0 8- 0 . 0 0 6- 0 . 0 0 4- 0 . 0 0 2

0 . 0 0 0

l n ( C t/ C o )

F e e d 5 0 0 m g / L F 0 M G M F 4 M G N F 2 M G F 0 M V M F 4 M V N F 2 M V

T i m e ( m i n u t e )

F e e d 5 m g / L

l n ( C t/ C o )

c) Determination of Mass transfer coefficient

B

T i m e ( m i n )

Fig. 9. Effect of contact time on dye adsorptionwith fitting of dye adsorption data to a) 1storder and b) 2nd order kinetics, and c) determination of mass transfer coefficient.


fact, at high feed dye concentration mass transfer resistance for trans-port of the dye molecules is reduced (Bayramoglu et al., 2009) andthus equilibrium time for the adsorption is attained much earlier forhigh feed dye concentration. However, the dye adsorption experimentwas carried out for 48 h to ensure equilibrium for both low and highconcentration range of the dye solution. From Fig. 9a or b it is also ob-served that similar to swelling experiments, filled composite gels viz.NF2 and MF4 show longer equilibrium time than unfilled F0 gel.

3.4.2.1. Adsorption kinetics. The dye adsorption data (qt) at various timeintervals (t) was directly fitted to the following non-linear 1st order(Eq. (6a)) and 2nd order (Eq. (6b)) rate equations

qt ¼ qe 1−exp −k1tð Þ½ � ð6aÞ

qt ¼q2ek2t

1þ k2qet: ð6bÞ

From Fig. 9a and b it is also evident that the dye adsorption data atvarious time intervals fit well to the 1st order (Fig. 9a) and 2nd order(Fig. 9b) kinetics. The values of the rate constant (k1 and k2), experi-mental and calculated qe for these non-linear fittings along with thevalues of the statistical parameters, viz. the r2, chi square (χ2) and theF values are shown in Table 3. The calculated qe of the low feed dye con-centration is observed to show a good match with experimental qe forboth the 1st and the 2nd order kinetics while for a high feed concentra-tion 1st order kinetic is observed to show a better fit. The values of thestatistical parameters also suggest a good fitting to these kinetics.

3.4.2.2. Mass transfer coefficient. At the initial stage (first 1 h) of dye ad-sorption the external mass transfer coefficient (kmtc) for the sorptionof the dye from the boundary layer to the hydrogel surface of area ‘a’was determined from the initial concentration of dye in water (Co)and also its concentration at time t (Ct) using Eq. (7) (Lazaridis et al.,2007).

lnCt

C0¼ −kf

aVt ð7Þ

Here ‘a’ is the surface area (m2) of the hydrogel while V is the solu-tion volume (m3). The linear plot of ln Ct

C0versus contact time t for both

low and high feed dye concentrations is shown in Fig. 9c (low conc.shown in inset). The slope of these linear curves gives the value ofkmtc. The linear curves show good fittings (r2 N 0.99). The values ofthe kmtc for a low (5 mg/L) and high (500 mg/L) feed concentrationsof the methyl violet and malachite green dye are shown in Table 3 forthe four hydrogels. It is observed that kmtc increases from F0 to NF2which is in tune with an increase in dye adsorption in the same order.

3.4.2.3. Film or pore diffusion. The resistance tomass transfer of dyemol-ecules from the aqueous feed to the hydrogel surface may be due to aboundary layer surrounding hydrogel (film diffusion) or resistancedue to thediffusion in the interior of the gel (pore diffusion). To evaluatethe nature of the diffusion the following Boyd model (Hameed and El-Khaiary, 2008) (Eq. (8)) was used

f ¼ qt

qe¼ 6

π2

Xαn¼1

1n2 exp −n2Bt

� �: ð8Þ

By applying the Fourier transform and integration the following ap-proximation is obtained (Hameed and El-Khaiary, 2008)

Bt ¼ −0:497−ln 1− qt

qe

� �: ð8aÞ



Table 3Kinetics and adsorption isotherm models, diffusion coefficient (D), mass transfer coefficient (kmtc), and thermodynamic parameters of the hydrogels for low and high conc. of malachite green (MR) and methyl violet (MV) dye.

Model F0MGlow/high

MF4MGlow/high

NF2MGlow/high

F0MVlow/high

MF4MVlow/high

NF2MVlow/high

Pseudo 2ndQexpt (mg/g) 3.98/388 4.06/410 4.3/423 3.68/384 3.87/402 4.09/417Qet (mg/g) 4.30/541 4.29/537 4.47/528 5.59/472 5.69/473 5.58/477k2 × 104 6.7/0.08 8.1/0.09 9/0.11 1.1/0.10 1.10/0.12 1.3/0.12R2 0.9754/0.9584 0.9870/0.9672 0.9854/0.9699 0.9435/0.9697 0.9350/0.9709 0.9233/.9745χ2 0.05/413 0.038/368 0.048/264 0.032/592 0.05/469 0.0647/481F value 777/1295 1528/1812 1502/3100 1523/1169 1264/1782 1125/1827

Pseudo 1stQexpt (mg/g) 3.98/388 4.06/410 4.3/423 3.68/384 3.87/402 4.09/417Qet (mg/g) 3.6/412 3.7/423 3.9/430 4.5/383 4.6/395 4.7/404k2 × 103 3.4/5.4 3.8/5.7 4.35/5.76 0.57/5.03 0.56/5.39 0.59/5.51R2 0.9585/0.9751 0.974/0.9829 0.9707/0.9858 0.9396/0.9817 0.9300/0.9823 0.9171/0.9833χ2 0.093/1115 0.092/1157 0.034/990 0.050/1446 0.069/1446 0.074/1446F value 494/477 782/574 1452/824 1173/475 1041/635 1077/735Kmtc × 102 (cm/s) 0.026/5.34 0.029/6.3 0.035/7.84 0.024/3.76 0.028/5.19 0.034/6.14

Pore diffusionD × 109 (m2/s) 4.89/46.43 5.573/41.566 4.944/39.122 3.095/36.977 2.757/22.136 2.260/19.84KD (mg/g min) 0.073/21.93 0.0795/21.924 0.0793/21.944 0.0537/19.367 0.053/15.687 0.051/15.403R2 0.9928/0.9756 0.9944/0.9697 0.9952/0.9684 0.9892/0.9592 0.9821/0.990 0.9838/0.8991

Langmuir–FreundlichQmax (mg/g) 108.8/3874 319.4/2714 575.2/2280 82.06/1930 142.80/2601 199.7/3057KLF (L/mg) 10−4 0.999/0.999 702.6/314.8 505.7/333.3 147.94/392 95.89/304.3 82.66/287.9R2 0.006/4.22 0.999/0.999 0.999/0.999 0.999/0.99 0.999/0.999 0.999/0.999χ2 80428/52329 0.0012/3.32 0.003/0.93 0.051/0.58 0.0021/0.96 0.0012/1.82F value 149.6/2016 420591/62244 178101/202260 9521/363129 175881/18671 32533/10785−ΔG0 (kJ/mol) at 303 K 12/14.3 15.8/16.2 16.3/16.8 11/12.3 13.2/15.1 15.3/14.8ΔH0 (kJ/mol) 4/4.5 4.2/4.8 5.2/5.8 3.4/3.8 3.9/4.4 4.2/3.8ΔS0 (kJ/mol) 0.04/0.03 0.041/0.035 0.05/0.041 0.03/0.026 0.038/0.037 0.04/0.038

9R.Bhattacharyya,S.K.Ray

/Applied

ClayScience

xxx(2014)

xxx–xxx

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thisarticle

as:Bhattacharyya,R.,Ray,S.K.,M

icro-and

nano-sizedbentonite

filledcom

positesuperabsorbents

ofchitosanand

acryliccopolym

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removalofsynthetic

dyesf...,A

ppl.ClaySci.(2014),http://dx.doi.org/10.1016/j.clay.2014.09.015


0 800 1600 2400 32000.00.51.01.52.02.53.03.54.0

0 800 1600 2400 32000.0

0.2

0.4

0.6

0.8

1.0

B(-

)

Time, t (minute)

F0MG MF4MG NF2MG F0MV MF4MV NF2MV

q t/qe

Time, t (minute)

5 mg/L

a) Fitting data toBoyd model for low conc.

0 200 400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

0 400 800 1200 1600012345678

Time, t (minute)

q t/qe

(-)


500 mg/L

b) Fitting data to Boyd model for high conc.

Time, t (minute)

Bt(-

)

Fig. 10. Fitting of dye adsorption data to Boyd model for a) low concentration (5 mg/L)and b) high concentration (500 mg/L).

70

75

80

85

90

95

100

70

75

80

85

90

95

100

0 100 200 300 400 500 6000

100

200

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400

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0 10 20 30 4005

1015202530

High conc. range

Dye

Rem

oval

%


Dye

ads

orbe

d (m

g/g

of g

el),

q e

Feed conc. of dye (mg/L), Ce

qe

Low conc. range

Fig. 11.Effect of feed concentration on dye adsorption and removal%withfitting of dye ad-sorption data to combined Langmuir–Freundlich model.


The dye adsorption (qt) at various time intervals (t) was plotted tothe Boyd model (Eq. (8a)) to obtain Boyd parameter ‘B’ as shown inFig. 10a for a low (5 mg/L) and Fig. 10b for a high (500 mg/L) feeddye concentration. The diffusion coefficient (D) through the sphericalgels of radius r was obtained from Boyd parameter (B) as

B ¼ π2DI

r2: ð8bÞ

In the inset of Fig. 10a and b, Bt was plotted against t and it is ob-served that for both low and high feed dye concentrations Bt vs t plotsgive straight lines passing through the origin indicating film diffusionor external mass transfer as the governing mechanism (Tang et al.,2012) for the dye adsorption. Similarly, the diffusion coefficient of thetwo dyes through these four kinds of hydrogels based on Eq. (8b) isalso shown in Table 3. The values of the diffusion coefficient for thedye adsorption also follow the same trend of kmtc, viz. it increasesfrom F0 to NF2.

3.4.3. Effect of the feed concentration on dye adsorption and removal%The effect of the feed concentrations of dyes in water (Ce) on the

equilibrium dye adsorption (qe, mg/g gel) and dye removal % for bothlow (Ce = 2.5–40 mg/L) and high ranges (Ce = 100–600 mg/L) offeed concentration is shown in Fig. 11 at pH 7. It is observed that qe in-creases with an increase in Ce whichmay be ascribed to the decrease inmass transfer resistance of the dye molecules between the liquid(water) and the solid (gel) phases (Bayramoglu et al., 2009). The dye


removal% of the hydrogels is also observed to decrease with an increasein Ce. With an increase in Ce the number of the active sites available onhydrogel decreases and hence the removal% decreases at higher Ce. It isalso observed from Fig. 11 that the hydrogels, i.e., F0,MF4 andNF2 showvery high qe and removal% over the entire feed concentration range. Ac-cordingly, for Ce of 2.5–40 mg/L, F0,MF4 andNF2 gels shows adsorptionof around 2–29 mg/g of malachite green and around 1.9–26 mg/g ofmethyl violet dye while for Ce of 100–600 mg/L, the qe was93–492 mg/g for malachite green and 91.6–482/g for methyl violetdye for the same hydrogels. Similarly, the removal% of 75–96.5 for themalachite green and 73.5–93 for the methyl violet is observed for Ce

of 100–600 mg/L while removal% of 67–86 for the malachite greenand 58–83 for the methyl violet is observed for Ce of 2.5–40 mg/L. It isobserved that for the same feed concentration (Ce), qe increaseswith in-corporation of micro- and nano-sized filler in the order F0 b MF4 b NF2for both of these dyes which is in tune with swelling results (Fig. 7a).From the results themalachite green dye is observed to showmarginal-ly higher adsorption than themethyl violet dyewhichmaybe attributedto the structural difference of the two dyes (Fig. 1) and their relativeelectrostatic interaction with the hydrogel molecules (Fig. 2). Bothmal-achite green and methyl violet are cationic triarylmethane dyes. Thesecationic dyes show strong electrostatic interaction with the hydrogelscontaining the anionic carboxylate (COO−) and also hydroxyl (\OH)functional groups. In malachite green there are two dimethyl aminogroups while the methyl violet dye contains three dimethylaminosubstituted phenyl rings (Fig. 1) with stable carbocation. Unlike mala-chite green, methyl violet shows high solubility in water, i.e., it ismore hydrophilic than malachite green. However due to the steric hin-drance of three phenyl rings to the approaching hydrogels,(Bhattacharyya and Ray, 2013) methyl violet shows marginally lowerdye adsorption than malachite green.

3.4.3.1. Adsorption isotherm. In Fig. 11 the Ce and qe data were also fitteddirectly to the non-linear three-parameter combined Langmuir–Freundlich model Eq. (9) (Cheung et al., 2000)

qe ¼ qm KLFCeð Þ1=n1þ KLFCeð Þ1=n: ð9Þ




Here qm is the maximum monolayer adsorption while KLF is thecombined Langmuir–Freundlich model such that

KLF ¼ kakd

: ð9aÞ

The parameter values viz. maximum monolayer adsorption (qmax),Langmuir–Freundlich constant (Klf) and ‘n’ are shown in Table 3. Thevalues of the statistical parameters viz., the r2, χ and the F values forthis non-linear fitting are also shown in Table 3. The hydrogels are ob-served to show very high qmax, while the values of r2, χ and F also sug-gest good fitting of the adsorption data to this model.

3.5. Thermodynamics of dye adsorption

The distribution of the dye molecules between the aqueous phaseand the solid hydrogels is expressed in terms of the distribution coeffi-cient, Kd which is obtained from the dye adsorbed at equilibrium by thehydrogel (qe) and equilibrium feed dye concentration (Ce) as

Kd ¼ qe

Ce: ð10Þ

The thermodynamic parameters viz., the change of the standard en-thalpy and entropy (ΔH0 and ΔS0) are obtained from Kd at differenttemperatures using Eq. (11)

lnKd ¼ −ΔH0

RTþ ΔS0

R: ð11Þ

Here R is the universal gas constant. The change of the standard freeenergy (ΔG0) is also obtained from Kd as

ΔG0 ¼ −RT ln Kd: ð12Þ

The values of the thermodynamic parameters of the dye adsorptionviz., ΔH0, ΔS0 and ΔG0 as obtained using Eqs. (10)–(12) are shown inTable 3 for 100 mg/L feed concentration of malachite green andmethylviolet dye. Other hydrogels also showed similar values. For determiningthese thermodynamic parameters Kdwas obtained fromdye adsorptiondata at 25, 35 and 50 °C. The negative ΔG0 values signify spontaneityand feasibility of the dye adsorption while positive ΔH0 values indicateexothermic nature of the adsorption. The positive ΔS0 also indicates anincrease in randomness in the solid (gel)–dye solution interface duringthe adsorption process.

3.6. Reusability of hydrogels

The reusability of the hydrogels were evaluated by repeated desorp-tion and sorption of the dyes for the five consecutive cycles. Desorptionwas not significant at experimental pH (7) while it was around 96% atpH of 3.5. This result indicates strong electrostatic interaction betweenthe dye and the hydrogel (Banat et al., 1996). The change of adsorptioncapacity of the hydrogels after 5 cycles of sorption/desorptionwasmar-ginal signifying good reusability of the hydrogels.

4. Conclusion

Superabsorbent hydrogels were synthesized from chitosan and thecopolymer of acrylic acid and acrylamide by free radical polymerizationin water using MBA as crosslinker. The hydrogel prepared with 1 wt.%initiator, 1 wt.% crosslinker, 12 wt.% chitosan and 7.5:1 comonomerratio of acrylic acid and acrylamide showed the best swelling character-istic. This hydrogel (F0) was further filled with micro-sized and nano-sized bentonite clay. The hydrogel containing 4 wt.% micro-sized clay(MF4) and 2 wt.% nano-sized clay (NF2) showed the highest swelling


characteristic among the respective filled gels. The F0, MF4 and NF2hydrogels were subsequently used for the adsorption of low andhigh concentration ranges of malachite green and methyl violet dye.These hydrogels showed high adsorption of both of these dyes. Theadsorption and removal% were found to increase in the followingorder— NF2 NMF4 N F0. However, malachite green dye showed higheradsorption than the methyl violet dye. The dye adsorption data werefound to fit well to the 1st and 2nd order kinetics and the combinedLangmuir–Freundlich isotherm. Themass transfer coefficient and diffu-sion coefficient of dye adsorptionwere also determined and their valueswere in good agreement with the above order of adsorption. The valuesof the thermodynamic parameters signified the spontaneity, feasibilityand the exothermic nature of the dye adsorption. These hydrogelsmay also be used for the removal of other similar dyes from water.

Acknowledgment

Authors are grateful to Council of Scientific and Industrial Research(CSIR, Sanction No. 22 (0547)/11/EMR-II) and Department of Scienceand Technology (DST, SERB/F/3664/2012–2013), Govt. of India fortheir financial support to purchase UV–vis spectrophotometer andFTIR spectrophotometer used for the present work.

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