Chemical Engineering Journal 260 (2015) 269–283

Contents lists available at ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Removal of congo red and methyl violet from water using nano clayfilled composite hydrogels of poly acrylic acid and polyethylene glycol

http://dx.doi.org/10.1016/j.cej.2014.08.0301385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Fax: +91 33 23508386.E-mail address: samitcu2@yahoo.co.in (S.K. Ray).

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

h i g h l i g h t s

� Composite hydrogels were made from acrylic acid, PEG and nano clay.� Nano clay fillers were incorporated in situ during polymerization.� The gels were characterized by FTIR, SEM, DTA–TGA and XRD.� Swelling, diffusion and dye adsorption of the gels were evaluated.� Composite hydrogels showed high dye adsorption and removal%.

a r t i c l e i n f o

Article history:Received 22 June 2014Received in revised form 8 August 2014Accepted 10 August 2014Available online 28 August 2014

Keywords:HydrogelSynthesisSwellingDye adsorptionKineticsAdsorption isotherms

a b s t r a c t

Superabsorbent hydrogels were synthesized from polyethylene glycol (PEG) and crosslinked polyacrylicacid. Nano clay filled composite hydrogels were also synthesized by incorporating 0.5, 1.0 and 1.5 wt%nano size bentonite filler in the hydrogels. These hydrogels were characterized by FTIR, SEM, XRD,DTA–TGA, swelling and diffusion characteristics. The filled composite hydrogels showing the best resultin swelling was further used for adsorption of congo red (CR) and methyl violet (MV) dye from water. Thecomposite hydrogel showed high adsorption and removal% for these dyes for both low (2.5–50 mg/L) andhigh range (100–600 mg/L) of feed dye concentration. The dye adsorption data were fitted to differentkinetics and adsorption isotherm models. The external mass transfer coefficient, diffusion coefficientand thermodynamic parameters of dye adsorption were also determined.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Chemical industries such as textile, plastic, paper, printing,pharmaceutical and food industries use more than 10,000 differentdyestuffs and pigments for coloring its products. However, around15 wt% of dyestuffs remains as pollutants in waste water [1]. Mostof these dyes are synthetic aromatic compounds. Varioustreatment processes such as coagulation/flocculation, membranetreatment, ion exchange, oxidation, degradation by biological, pho-tochemical or electrochemical method and adsorption are used forremoval of these non-biodegradable dyes from water. Amongst thevarious processes adsorption is extensively used because of its easyoperation, low cost and high efficiency. The hydrogels based oncrosslinked functional polymers such as poly ethylene glycol

(PEG) are very effective for selective adsorption of synthetic dyesfrom water.

Polyethylene glycol (PEG) is non-toxic and non-immunogenic.PEG is also biocompatible and soluble in water. This low cost poly-mer is extensively used in pharmaceutical preparation, tissue cul-ture and also as an adsorbent [2]. PEG derivatives can be freeradically polymerized to introduce several functional groups inits structures [3]. In an earlier study Kesenci and Erhan producedhydrogel beads carrying the amide functional groups based onethylene glycol dimethacrylate and acryl amide via suspensionpolymerization [4]. PEG derivatives are most commonly function-alized with vinyl groups at the chain-ends such as poly ethyleneglycol diacrylate (PEGDA). These active vinyl groups may becrosslinked to form PEG based superabsorbent hydrogel [5]. Thehydrogels based on PEG have been used as an effective adsorbent.Accordingly, Kwak et al. [5] prepared sodium methallyl sulfonate-co-poly(ethylene glycol) diacrylate (SMS-co-PEGDA) microspheresby suspension polymerization. These microsphere beads showed

270 R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283

high sorption for Co(II). Karadag et al. reported synthesis of a novelternary semi IPN by copolymerization of acrylamide and sodiumacrylate in the presence of PEG. This PEG based semi IPN hydrogelshowed high swelling in water and also high adsorption of JanusGreen B dye from water [6]. In another work acrylic acid was par-tially neutralized with potassium hydroxide in water and theresulting acrylate and unreacted acrylic acid monomers were poly-merized free radically with crosslinker methylene bis acrylamide(MBA) in aqueous solution of PEG. The semi IPN hydrogelsprepared by this method were reported to show high adsorptionfor methyl orange dye [7]. Semi IPN hydrogels were also synthe-sized by crosslink copolymerization of acrylamide, methyl succinic(itaconic) acid and MBA in aqueous solution of PEG. The resultingsuperabsorbent hydrogels were observed to show high swellingcharacteristics and sorption of basic blue 12 dye from water [8].Composite hydrogels were synthesized by intercalation of PEGand silica in bentonite by ultrasonic sol–gel technique. Theseorganic–inorganic hybrid hydrogels were found to be an effectiveadsorbent for volatile organics like phenol and dyes like malachitegreen and methyl blue [9].

From the above discussion it is evident that apart from biolog-ical applications, PEG based hydrogels have also been extensivelyused as adsorbents for removal of metal ions and organicpollutants. Thus, in the present work composite superabsorbenthydrogels were prepared from acrylic polymers, PEG and also aninorganic adsorbent, viz. bentonite.

Bentonite is a low cost inorganic adsorbent. It carries a perma-nent negative charge because of isomorphous substitution of Si+4

in tetrahedral layers by Al+3 and in octahedral layers by Mg+2

[10]. This rich clay material has been widely used alone or as oneof the constituents of a composite adsorbent for removal of syn-thetic dyes and metal ions from water [10–14]. In the present workcomposite hydrogels were prepared by crosslink copolymerizationof acrylic acid and MBA in the presence of PEG and bentonite inwater. The wt ratios of acrylic polymer and PEG were varied to pro-duce several hydrogels. The hydrogel showing the highest swellingin water was identified (designated as F0) and it was further filledwith 0.5, 1 and 1.5 wt% (of total polymer) nano size bentonite fill-ers by in situ mixing during polymerization. These filled hydrogelswere designated as F0.5, F1 and F1.5, respectively. The effect of var-ious parameters, viz., wt% of PEG and filler, type and concentrationof various salts and solution pH on swelling and diffusion of thesecomposite superabsorbents were studied. The filled or compositeF1 gel showing the optimum swelling characteristic and theunfilled F0 gel was then used for removal of the varied concentra-tions of two industrial dyes, i.e. congo red (CR) and methyl violet(MV) from water.

2. Experimental

2.1. Materials

Poly ethylene glycol (PEG) of molecular weight 4000 wasprocured from Merck and used as obtained. The redox pair of ini-tiators, i.e., ammonium persulfate and sodium metabisulfide wereobtained from Merck and used as obtained. The monomers, viz.,acrylic acid and N0N0 methylene bis acrylamide (MBA) were of ana-lytical grades and used as obtained. These monomers were alsoprocured from Merck. Sodium montmorilonite rich nano size(30–90 nm, aspect ratio 300–500, mineral’s thickness 1 nm, cationexchange capacity 120 meq/100 g) bentonite filler was gifted asfree samples by Amrfeo Pte. Ltd., Kolkata. Congo red (CR) andmethyl violet (MV) dyes used in adsorption studies werepurchased from SRL Chemical, India. Distilled water was used forpolymerization, swelling and sorption experiments.

2.2. Methods

2.2.1. Synthesis and characterization of the hydrogelsSemi IPN type hydrogels of PEG and polyacrylic acid were syn-

thesized by solution polymerization in a three-necked reactor atambient temperature (30 �C) for 3 h. At first 5 wt% aqueous solu-tion of PEG was prepared by gradual addition and stirring of therequired amount of PEG in distilled water in a 250 mL glass beaker.Polymerization of acrylic acid monomer and MBA crosslinker wasallowed in this solution in the presence of the redox pair ofinitiator, i.e., ammonium persulfate and sodium meta bisulfite. Forsynthesizing filled composite hydrogel, bentonite nano filler wastaken in the polymerization mixtures and the acrylic acid monomerwas allowed to polymerize in situ in this aqueous dispersion.

2.2.2. Characterization of the hydrogelsThe various functional groups present in the hydrogels were

characterized by Fourier transform infrared (FTIR) spectra. FTIRof the polymer samples were recorded on a FTIR spectrometer(Perkin Elmer, model-Spectrum-2, Singapore) using KBr pellet.KBr pellet was prepared in a mold by mixing dry KBr powder withfine powder of the hydrogel samples (10:1 wt ratio of KBr to poly-mer). The morphology of the composite gels were characterized byscanning electron microscopy (SEM) at a magnification of 1 K(Scanning Electron Microscope, Model No. S3400N, VP SEM,Type-II, made by Hitachi, Japan) and accelerating voltage set to10 kV. The crystallinity of the hydrogel samples was characterizedby its wide angle X-ray diffraction (XRD) profile in a diffractometer(Model: X’Pert PRO, made by PANalytical B.V., The Netherlands)using Ni-filtered Cu Ka radiation (k = 1.5418 Å) and a scanning rateof 0.005 degree (2h)/s) over 2–72� angle of diffraction. Differentialthermal analysis (DTA) and thermo gravimetric analysis (TGA) ofthe hydrogel samples were carried out in nitrogen atmosphere inthe temperature range of 60–600 �C at the scanning rate of 10 �Cper minute in a Perkin Elmer instrument.

2.2.3. Study of swellingThe swelling of the hydrogels in water was studied by gravimet-

ric method. Small amount of accurately weighed hydrogel samples(Wi) was immersed in distilled water at ambient temperature. Thegel samples were withdrawn from water at different time intervals(t) and weighed (Wt) after removing the excess surface watercarefully by using a filter paper without pressing the sample. Eachsample was weighed three times to minimize error and theaverage values of these three measurements were taken. Swellingexperiments were continued till the hydrogels reach its equilib-rium swelling value (We).

The swelling ratio (St) and equilibrium swelling ratio (Se) of thehydrogel samples were determined from the following Eqs. (1) and(1a), respectively:

Stðg=gÞ ¼Wt �Wi

Wið1Þ

Seðg=gÞ ¼We �Wi

Wið1aÞ

The swelling experiments were also carried out at different pH(3.9–9.7) by immersing the hydrogels in buffer solutions. The buf-fer solutions of varied pH were prepared by dissolving phosphoricacid, potassium phosphate (KH2PO4), potassium hydrogen phos-phate (K2HPO4), sodium chloride and sodium hydroxide in distilledwater.

2.2.4. Study of dye removal capacity of the hydrogels100 mg (w) of the hydrogel sample was brought in contact with

dye by immersing the gel sample in 100 mL (V) distilled water

R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283 271

containing varied concentration of dyes and after equilibrium wasreached, the dye solution was separated by decantation from thehydrogel. The concentration of dyes in these solution before andafter addition of the gel sample was determined from a precalibrat-ed curve of absorbance versus concentrations using UV–visibleSpetrophotometer (Perkin Elmer lamda 2.5). The adsorption exper-iments were also carried out at different pH (3.5–9) by immersingthe hydrogels in buffer solutions. The buffer solutions of varied pHwas prepared by dissolving phosphoric acid, potassium phosphate(KH2PO4), potassium hydrogen phosphate (K2HPO4) and sodiumhydroxide in distilled water. The absorbance of the dye solutionswas measured at wavelength of 495 cm�1 for CR and 584 cm�1

for MV dye. The adsorption capacity of the gel sample (qt, mg ofdye/g of gel) at different time intervals (t) and at equilibrium (qe

mg of dye/g of gel) and dye removal% (R%) was calculated from ini-tial concentration (Ci), concentration at time t (Ct) and at equilib-rium (Ce) of the dye using the following Eqs. (2a–2c), respectively:

qt ¼ðCi � CtÞV

wð2aÞ

qe ¼ðCi � CeÞV

wð2bÞ

R% ¼ ðCi � CeÞCi

� 100 ð2cÞ

2.2.5. Data fittingsLevenberg–Marquardt (L–M) algorithm was used for non linear

fittings of swelling and dye adsorption data. In this algorithm theparameter values of a specific model is adjusted by iteration usingchi square (v2). The validity of the models was evaluated in termsof regression coefficient (R2), non linear v2 and F values obtainedfrom Anova analysis in the same Origin software. For a good fitting,r2 should be close to unity, v2 will be low while F value should behigh [15].

3. Results and discussion

3.1. Synthesis of the hydrogels

The hydrogel is formed by crosslink copolymerization of acrylicacid monomer and MBA crosslinker in aqueous solution of PEG byfree radical polymerization. A semi IPN type structure of thehydrogel is formed where PEG remains incorporated in the cross-link network of polyacrylic acid. In fact, a crosslink copolymer ofacrylic acid and MBA is formed. PEG also takes part during poly-merization by forming macro radicals [16]. Several hydrogels wereprepared by varying weight ratios of PEG and acrylic acid. Themolecular weight of the gel is influenced by the concentration ofinitiator while the crosslink density of the gel depends on the con-centration of crosslinker (MBA). Hence, the concentration of theinitiator and crosslinker were also varied to prepare several gels.The gel showing the best swelling results were identified and thenano size bentonite fillers of varied concentration were incorpo-rated in situ in this unfilled gel to prepare the filled composite gels.The interaction of the dye molecules with these hydrogels is shownin Scheme 1.

3.2. Characterization of the hydrogels

3.2.1. FTIRThe FTIR of the three composite gels, i.e., F0.5, F1 and F1.5 are

shown in Fig. 1. The wave numbers corresponding to some of theabsorption bands of the polymers are also shown in the figure.The FTIR of the pure PEG is shown in the same figure for

comparison. For PEG a strong but broad absorption band appearingat 3442 cm�1 indicates its O–H and C–OH stretching while theabsorption band at 2885 cm�1 is due to its CH2 stretching vibra-tion. The absorption band at 1342 cm�1 corresponds to its –CH2

wagging vibration. Another characteristic absorption band of PEGat 1957 cm�1 is ascribed to its crystalline state. Similarly, theabsorption band at 1098 cm�1 is due to C–O–C stretching andthe absorption peak at 840 cm�1 corresponds to the vibration ofits –CH2–CH2–O group [16]. The 2885 cm�1 peak of PEG due toits CH2 stretching is observed to shift to 2917 cm�1 in F0.5,2919 cm�1 in F1 and 2929 cm�1 in F1.5. Similarly, the 3383 cm�1

peak of PEG due to its O–H and C–OH stretching is observed at3284 cm�1 in F0.5, 3291 cm�1 in F1 and 3294 cm�1 in F1.5. Thecarbonyl stretching of polyacrylic acid/MBA unit is observed at1654 cm�1 in F0.5, 1643 cm�1 in F1 and 1635 cm�1 in F1.5 [7,17].The Si–O vibration band at 1024 cm�1 of bentonite [18] andC–O–C stretching of PEG at 1098 cm�1 is observed to overlap at1093 cm�1 in F0.5, 1099 cm�1 in F1 and 1100 cm�1 in F1.5. Simi-larly, the 519 cm�1 absorption peaks corresponding to stretchingvibration of Si–O–Al of bentonite [19] shifts to 601 cm�1 in F0.5,600 cm�1 in F1 and 602 cm�1 in F1.5. All of these shifting and over-lapping signify strong electrostatic interaction amongst the variousfunctional groups of the hydrogels and the fillers [7,20].

3.2.2. XRDThe XRD of pure PEG, bentonite clay, F0.5, F1 and F1.5 gel is

shown in Fig. 2. The PEG is observed to show major XRD peaksat two theta of 19.8�, 23.8� and some minor peaks at 26.9�,36.6�, 40.5� and 46.2� as also reported elsewhere [21,22]. Theintermolecular hydrogen bonding between –OH groups of glycolis responsible for its crystallinity and hence XRD peaks. Similarto PEG, the clay is also observed to show several diffraction peaksof its montmorillonite 001 planes at 2h of 7.1�, 20.3�, 29�, 35.6�and 62.3� as also reported elsewhere [23]. It also shows a minorpeak at 2h of 54.6� corresponding to its quartz impurity [23].From Fig. 2 it is also observed that these XRD peaks of PEG andclay are absent in F0.5, F1 or F1.5. Instead of any XRD peaks thesecomposite hydrogels show an amorphous hallow at two theta ofaround 20�. As PEG and clay is incorporated in the amorphousacrylic gel, the intermolecular hydrogen bonding of PEG isreduced to a great extent because of strong electrostatic interac-tion between the –COOH groups of the acrylic gel and the –OHgroups of the PEG [20,21]. Similar to PEG, the XRD peaks of clayare also not observed in the composite gels. The absence of XRDpeaks of clay in the filled hydrogels also confirms well dispersionand exfoliation of the clay particles in the polymer matrix inthese gels [24].

3.2.3. SEMThe SEM of the filled composite gels is shown in Fig. 3. There is

no phase separation of the two polymers, i.e., the acrylic polymerand the PEG in the composite gel indicating good interpenetrationof the two polymers in the gel [7]. No agglomeration of the filler orPEG is also observed in the gel. Further, the nano filler is observedto be uniformly distributed in the composite gels.

3.2.4. DTAThe DTA of the polymer samples are shown in Fig. 4a. From this

figure it is observed that the virgin polyethylene glycol (PEG)shows an endothermic sharp melting peak at around 67 �C and aweak exothermic inflection in base line at around 172 �C due tooxidative degradation as also reported elsewhere [25]. PEG alsoshows exothermic peaks at 337 and 372 �C due to its decomposi-tion and charcoal evolution [22]. As the PEG is incorporated inthe matrix of polyacrylic acid and it is further incorporated withinorganic nano filler, the DTA of the composite gel changes. The

Scheme 1. Schematic of formation of the filled composite gel and its interaction with dyes.

272 R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283

filled composite gels, i.e., F0.5, F1 and F1.5 shows endothermicbroad melting peaks in the temperature range of 240–278 �C. Sim-ilarly, the exothermic degradation peak of pure PEG at around 337and 372 �C is also shifted to a single exothermic peak at around395–400 �C in the filled gel samples.

3.2.5. TGAThe TGA of the polymer samples are shown in Fig. 4b. PEG is

observed to show a weight loss of around 10 wt% up to a temper-ature of 275 �C which is associated with loss of physicallyadsorbed water [9]. In the temperature range of 280–400 �C, PEGundergoes decomposition of its main chain with 100% weight loss.The filled composite, viz. F0.5, F1 and F1.5 shows a multipleweight loss regions which is due to different degradation profileof its constituent polymer and fillers, viz., PEG, acrylic polymerand the bentonite nano filler. Hence, the composite gels showmultiple degradation profiles, i.e., a weight loss of around 5–15 wt% due to the loss of physically adsorbed water, 15–40 wt%weight loss in the temperature region of 250–340 �C and a weightloss of 50–80 wt% in the temperature region of 350–440 �C [22].The multiple degradation profile at higher temperature is due tothe splitting of main polymer chain of PEG and the copolymer.The filled gels are also observed to show a residue of 12–16 wt%at 540–550 �C indicating the presence of inorganic filler in thecomposite gels [22].

3.3. Swelling study

In the present work hydrogels were prepared by varying theamount of PEG in the hydrogel with 1 wt% each of initiator andcrosslinker. 1 wt% initiator and 1 wt% crosslinker were chosensince similar hydrogels prepared with these concentrations of ini-tiators and crosslinkers were observed to give the best swellingresults [26]. The effect of the wt% of PEG and filler, type of saltand its concentration and also pH of water on equilibrium swellingratio (ESR) of the hydrogels is shown in Fig. 5a while the effect ofcontact time on swelling ratio of hydrogels prepared with variedwt% of PEG and filler is shown in Fig. 5b. From Fig. 5a it is observedthat as the wt% of PEG increases, equilibrium swelling ratios (ESR)also increases. In fact, PEG contains several hydroxyl groups in itsstructure (Scheme 1) and thus with increase in wt% of this hydro-philic polymer in the copolymer, swelling ratio increases. However,as the PEG wt% increases further from 6 to 8 wt%, ESR of the gel isobserved to decrease. Though PEG increases hydrophilicity, it alsofills up the network of the gel and thus above 6 wt% PEG this porefilling effect offset the increase in hydrophilicity of the gel resultingin decrease in ESR. It is also observed from Fig. 5a that withincrease in wt% of PEG equilibrium swelling time increases from8155 min (0% PEG) to 9140 min (8 wt% PEG). The network struc-ture becomes more complicated by the incorporation of PEG andhence it becomes difficult for water to penetrate in the network

2885 PEG

957 PEG

1638 PEG

3442 PEG

1143 PEG

1342 PEG

1957 PEG

840 PEG

2440 F0.5

3450 F0.5

2440 F1

1689 F1.5

1067 F1.5

2435 F1.5

1929 F1.5

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1000 2000 3000 4000 5000Wave number (cm-1)

trans

mitt

ance

(-)

0

0.02

0.04

0.06

0.08

0.1

0.12

Tran

smitt

ance

(-)

PEG

F1

F1.5

F0.5

Fig. 1. FTIR of the polymers.

19.8o PEG

23.8o PEG

26.9o PEG

36.6o PEG40.5o PEG

clay

20.3o Clay

29o Clay

62.3o clay

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 20 40 60 80 100

Two theta ( degree)

Inte

nsity

PEG F0.5

F1 F1.5

Clay

Fig. 2. XRD of the polymer.

R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283 273

causing increase in equilibrium swelling time. In subsequent workthe hydrogel prepared with 6 wt% PEG and showing the highestswelling was filled with 0.5, 1 and 1.5 wt% nano size bentonitefiller. From Fig. 5a and b it is observed that the hydrophilic nano

filler also increases swelling up to 1 wt% in the gel above whichESR or SR decreases. Similarly, with increase in wt% of the filler,equilibrium swelling time also increases for these composite gels.The effect of varied wt% of monovalent sodium chloride and also1 wt% of bivalent calcium chloride and 1 wt% trivalent aluminumchloride on ESR is shown in Fig. 5a. It is observed that with increasein ionic concentration of sodium chloride or from monovalent(Na+1) to trivalent ion (Al+3), ESR decreases. In fact, swelling ofthe hydrogels is caused by repulsion of ionic charges of the func-tional groups (e.g., COO�) of the hydrogels which expands the gelnetwork with easy penetration of water molecules. In the presenceof salt solution the metal ions of the salts shields the ionic chargesof the gels causing less repulsive forces and hence less expansion ofthe gels. Thus, swelling decreases in the presence of the salt solu-tion [27]. The size of the metal ions also increases in the followingorder mono valent Na+1 < Ca+2 < Al+3. The bigger ion imposes moreshielding. Further, crosslink density of the gel with bigger ionsbecomes denser with decrease in swelling from monovalent totri valent ions [27]. The pH sensitivity of the gels is also evidentfrom the results of ESR of the composite gel prepared with 6 wt%PEG and 1 wt% nano filler (designated as F1). It is observed thatas pH increases, ionization of the functional groups of the gelincreases with more repulsive force and expansion of the network.As a result ESR increases at higher pH.

3.3.1. Swelling kineticsIn Fig. 5b the swelling data of the hydrogels prepared with var-

ied wt% of PEG and nano filler were also fitted directly to the fol-lowing second order rate Eq. (3) [28]:

St ¼S2

ecalkS2t1þ kS2Secal

¼ rot1þ kS2Secal

ð3Þ

Fig. 3. SEM of the polymers, (a) F0.5, (b) F1, (c) F1.5 (Scale 20 micron, magnification 1 K).

274 R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283

Here, kS2 is second order rate constant, Wecal is calculated ESR and ro

is initial rate of swelling.It is observed that the swelling data show good fitting to this

second order kinetics. The equilibrium swelling ratio, both experi-mental (Swexpt) and calculated (Swcal), rate of swelling (ro), secondorder rate constant (kS2) of the swelling and the values of statisticalparameters such as r2, v2 and F are shown in Table 1. The values ofSwexpt and Swcal are also observed to be very close while these datafittings give the values of r2 close to unity, low values of v2 andhigh values of F as observed in Table 1. These results also indicategood fitting of swelling data to second order kinetics.

3.3.2. Diffusion characteristicsIn Fig. 5c the fractional water uptake (F, ratio of swelling ratio at

a specific time and equilibrium swelling ratio) were plotted againstswelling time (t). Here also the data were fitted directly to the fol-lowing non linear Eq. (4) of Ritger and Peppas [29] to obtain thediffusion constant (kD). The diffusion coefficient (D), and diffusionexponent (n) of the hydrogels were obtained from the followingEq. (4a) [29]:

F ¼ St

Se¼ kDtn ð4Þ

D ¼ pr2 kD

4

� �1n

ð4aÞ

The values of D, kD and n along with the values of statistical param-eters such as r2, v2 and F of the PEG filled and both PEG and nanofiller filled hydrogels are also shown in Table 1. The values of nappears to be close to 0.5 indicating non Fickian diffusion [26] whilethe values of r2, v2 and F also suggest good fitting to the non linearequation of Ritger and Peppas. In subsequent work the unfilled F0and the filled F1 hydrogel were used for the study of adsorptionfor low (2.5–50 mg/L) and high (100–600 mg/L) range of feed con-centration of congo red (CR) and methyl violet (MV) dye in water.

3.4. Dye adsorption study

3.4.1. Effect of solution pH on dye adsorptionThe ionization and polarization of the various functional groups

present in the hydrogels such as amide (from acrylamide moiety),carboxylic and other acid groups (from acrylic acid moiety and fil-ler) and hydroxyl groups (from PEG moiety) changes with solutionpH, i.e., the unfilled F0 and filled F1 hydrogel is expected to be pHresponsive. Thus, the effect of solution pH on adsorption of the twodyes, i.e., CR and MV dye for a feed concentration of 5 mg/L of CR

-30

-20

-10

0

10

20

30

40

0 100 200 300 400 500 600

Sample temperature oChe

at f

low

(m

W)

Pure PEG F0.5 F1.0 F1.5 F0

0

20

40

60

80

100

0 100 200 300 400 500 600Temperature oC

Res

idua

l wt%

PEG F0.5 F1 F1.5

a

b

Fig. 4. (a) DTA and (b) TGA of the polymers.

R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283 275

and MV dye was studied as shown in Fig. 6. It is observed that thereis a sudden increase in dye adsorption from pH 4 to 7. This may beascribed to the abrupt increase in ionization of carboxylate ions ofthe gels above pka values of the polyacrylic acid (�7) as reportedelsewhere [30]. The adsorption of the dye molecules by the hydro-gels decreases above the solution pH of 9 because of deprotonationof the amines groups of the dyes.

3.4.2. Effect of contact time on dye adsorptionThe effect of contact time on adsorption of both low (3.5 mg/L)

and high (400 mg/L) feed dye concentration is shown in Fig. 7a forfitting of adsorption data to 1st order and Fig. 7b for 2nd orderkinetics. The dye adsorption curves are similar to swelling curves(Fig. 5b), viz. initially adsorption increases at a rapid rate followedby a slower rate of adsorption till it reaches the equilibrium. Ini-tially, at the beginning of the adsorption experiment, the functionalgroups of the hydrogel are available for these interactions and thusthe rate of adsorption is very high. As adsorption continues, thefunctional groups of the fixed amount of hydrogel present in theaqueous medium exhaust and thus rate of adsorption decreasesand finally it reaches a dynamic equilibrium with a maximumadsorption value (qmax) when the rate of adsorption by the hydro-gel equals the rate of desorption from the hydrogel [26]. It is alsoobserved from Fig. 7a and b that equilibrium time for 3.5 mg/L feedconcentration is much longer than 400 mg/L feed concentration. Infact, at high feed dye concentration mass transfer resistance fortransport of dye molecules is reduced and thus equilibrium timeis attained much earlier for high feed dye concentration [31].

3.4.2.1. Adsorption kinetics.3.4.2.1.1. Pseudo 1st order versus pseudo 2nd order. For evaluatingkinetics of the dye adsorption the adsorption data (qt) at varioustime intervals (t) were fitted to the following pseudo 1st order

equation (Eq. (5)) of Lagergren and pseudo 2nd order equation(Eq. (6)) of Ho and Mckay as also shown with solid lines inFig. 7a and b, respectively:

qt ¼ qecal½1� expð�k1tÞ� ð5Þ

qt ¼q2

ecalk2t1þ k2qecalt

ð6Þ

From these figures and the respective data of Table 2 it is evi-dent that for both low and high feed dye concentration pseudo1st order kinetics gives a good fitting while the pseudo secondorder kinetics shows good fitting only for low feed dye concentra-tion. For high feed dye concentration the calculated qe based onpseudo 2nd order kinetics is observed to be widely different fromexperimental qe.3.4.2.1.2. Mass transfer coefficient and diffusion coefficient and filmdiffusion. The external mass transfer coefficient (kmtc) for adsorp-tion of the dye molecules from boundary layer to hydrogel surfaceof surface area ‘a’ was determined from initial concentration of dyein water (C0 = 3.5 and 400 mg/L) and also its concentration at time t(Ct) for the first 90 min of experiment using the following Eq. (7)[32]:

lnCt

C0¼ �kmtc

aV

t ð7Þ

Here V is the volume of the dye solution. The value of kmtc wasobtained from the slope of the linear plot of ln Ct

C0vs. t as shown in

Fig. 8a. To determine the nature of diffusion of dye molecules, i.e.,if it is ‘pore’ or ‘film’ diffusion, the values of Boyd parameter (B)was obtained from the dye adsorption at time t (qt) and at equilib-rium (qe) using the following Boyd model Eq. (8) [33]:

Bt ¼ �0:497� ln 1� qt

qe

� �ð8Þ

F1.5

F1

F0.5

PEG8

PEG6

PEG4

PEG2

PEG0 NaCl 2.5

NaCl 1.5

NaCl 0.5

NaCl 4

CaCl2 1.5pH 2.73

pH 3.6

pH 5.5

pH 7

pH 9.5

AlCl31.51

4

7

10

13

16

ES

R (

-)

0

5000

10000

15000

20000

25000

Tim

e fo

r E

SR (

min

)

0 2000 4000 6000 8000 100000

2

4

6

8

10

12

P E G 0 P E G 2 P E G 4 P E G 6 P E G 8 F 0 . 5 F 1 F 1 . 5

Swel

ling

ratio

Swelling time, t (minute)

0 2000 4000 6000 8000 10000

0.0

0.2

0.4

0.6

0.8

1.0

P E G 0 P E G 2 P E G 4 P E G 6 P E G 8 F 0 . 5 F 1 F 1 . 5

F (-

)

t (min)

a

b

c

Fig. 5. (a) Effect of PEG, filler, salt and pH on equilibrium swelling ratio (ESR) of the hydrogels. (b) Swelling curves of hydrogels synthesized with varied PEG wt% and filler wt%in hydrogel with non linear fitting of data to second order rate equation and (c) Fitting swelling data to Peppas model of diffusion.

Table 1Swelling and diffusion coefficients of the hydrogels.

Polymer kS2 � 104 ro � 102 ESRexpt (g/g gel) ESRcal (g/g gel) r2/v2/F � 10�2 kD � 102 n D � 107 r2/v2 � 103/F � 102

PEG0 1.23 1.34 9.76 10.42 0.9826/0.235/7.78 5.6 0.32 0.56 0.9703/4/4.54PEG2 2.09 2.25 10.13 10.37 0.9689/0.453/8.13 9.6 0.26 1.21 0.9748/3.5/10PEG4 4.89 5.25 10.30 10.36 0.9871/0.186/28.85 15.4 0.21 2.41 0.9274/9.9/5PEG6 4.06 5.18 11.33 11.29 0.9763/0.391/17.61 14.2 0.22 2.14 0.9583/5.3/9.9PEG8 1.81 2.46 11.28 11.64 0.9686/0.528/13.36 10.3 0.25 1.33 0.9789/2.7/20F0.5 2.04 2.77 11.36 11.63 0.9644/0.582/13.46 11.03 0.24 1.48 0.9781/2.78/22F1 1.92 2.86 11.87 12.20 0.9656/0.603/15.36 11.05 0.24 1.49 0.9786/2.6/24F1.5 1.97 2.71 11.38 11.71 0.9667/0.523/17.50 11.07 0.23 1.49 0.9789/2.6/27

kS2 (g gel/g water/min), r0 (g water/g gel/min), kD (s�1), n (�), D (m2/s),

276 R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283

To determine the nature of diffusion of dye molecules, i.e., if it is‘pore’ or ‘film’ diffusion, Bt was plotted against t in Fig. 8b. It isobserved that initially some of the plots are linear and passingthrough the origin indicating external mass transferred controlledfilm diffusion or chemical reaction as the governing mechanism fordye adsorption [33]. However, the Boyd plots are not linear aftersome 1 h of adsorption and this loss of linearity signifies intra par-ticle transport controlled ‘pores diffusion’ as the governing mech-anism for dye adsorption. In Fig. 9a and b the adsorption datawere fitted to the following Webber’s pore diffusion model Eq.(9) for 3.5 mg/L (Fig. 9a) and 400 mg/L feed dye concentration(Fig. 9b), respectively:

qt ¼ kpt0:5 þ c ð9Þ

Here kp (mg g�1 min�0.5) is intra particle diffusion rate constant andc signifies mass transfer resistance due to boundary layer. It isobserved that the plots show three linear portions which confirmthat the external mass transfer at the initial stage is followed byintra particle pore diffusion into macro, meso and micro pores[33]. The slope of the second linear portion is the rate constantfor intra particle diffusion while its intercept is proportional tothe boundary layer thickness. The diffusion coefficient for the twodyes based on pore diffusion model was obtained using the follow-ing Eq. (10) [33]:

0

1

2

3

4

5

0 3 6 9 12pH of Feed

Dye

ads

orbe

d (m

g/g

gel)

F1CR

F0CR

F1MV

F0MV

Feed 5 mg/L dye

Fig. 6. Effect of pH on dye adsorption.

R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283 277

kp ¼6qe

r

ffiffiffiffiffiffiDp

p

rð10Þ

Here r is radius of the spherical hydrogel particle. The values of thediffusion coefficients of the dyes are also given in Table 2. The dif-fusion coefficient of CR dye is observed to be higher than MV dyewhich is also in tune with experimental results.

3.4.3. Effect of feed concentration on dye adsorptionThe effect of low range of feed dye concentration, i.e., 2.5–

50 mg/L and high range of feed dye concentration, i.e., 100–600 mg/L on adsorption and removal% of CR and MV dye for theunfilled F0 and filled F1 hydrogel at solution pH of 7 is shown inFigs. 10 and 11, respectively. From these figures it is evident thatthe composite hydrogels show high adsorption and removal% ofthe dyes. The high adsorption or removal% is caused by strong elec-trostatic interaction among functional groups of the cationic dyeswith the anionic hydrogels. It is also observed that for the samefeed concentration, the hydrogel shows higher adsorption andremoval% for CR dye than the MV dye. The MV dye shows lessadsorption in comparison to CR dye since it contains only tertiaryamine groups Thus, MV-hydrogel electrostatic interaction will beless than CR-hydrogel interaction [34]. Further, the F1 gel showhigher adsorption than unfilled F0 gel. The bentonite nano fillerwith its ion exchange functional groups shows increased electro-static interaction with both CR and MV dye. Thus, F1 gel showshigher adsorption than F0 gel. It is also observed that with increasein feed concentration of dye adsorption increases while removal%decreases. This may be because of the fact that the adsorptioncapacity of the hydrogel is fixed and at higher feed dye concentra-tion the amount of dye left in the solution is also high resulting indecrease in removal% [28].

3.4.3.1. Adsorption isotherm. In Figs. 10 and 11, the dye adsorptiondata at various feed concentrations were also fitted directly to thefollowing non linear two-parameter Langmuir (Eq. (11)), and Fre-undlich model (Eq. (12)), three-parameter Sip model (Eq. (13))and four-parameter Fritz–Schlünder (FS) (Eq. (14)) model [26].

Langmuir model

qe ¼qmaxKLCe

1þ KLCeð11Þ

where Ce is equilibrium concentration of dye in water, qmax is themonolayer capacity of the adsorbent hydrogels (mg g�1) and KL isLangmuir equilibrium constant (cm3 g�1). The characteristic ofLangmuir isotherm is expressed in terms of dimensionless separa-tion factor RL defined as

RL ¼1

KL þ Cið11aÞ

where, Ci is the feed dye concentration. The value of RL indicates ifthe Langmuir process is unfavorable (RL > 1), favorable (0 < RL < 1),linear (RL = 1) or irreversible (RL = 0).

Freundlich model

qe ¼ KFC1=ne ð12Þ

where, KF is Freundlich constant and ‘1/n’ signifies the nature of theisotherm.

Sip model

qe ¼KSCb

e

1þ ASCbe

ð13Þ

where KS and AS are Sips constant. This model is applicable foradsorbent with heterogeneous surfaces. At low concentration of

0 200 400 600 800 1000 1200 1400 1600 18000

1

2

3

0 50 100 150 200 2500

100

200

300

400

FICR FIMV F0CR F0MV

q t (m

g dy

el/g

gel

)Contact time, t (min)

1st order kinetics

Feed 3.5 mg/L dye

FICR FIMV F0CR F0MV

q t (m

g dy

e/g

gel)

Contact time, t (min)

Feed 400 mg/L dye

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 0 1 8 0 00

1

2

3

0 5 0 1 0 0 1 5 0 2 0 0 2 5 00

1 0 0

2 0 0

3 0 0

4 0 0

Feed 400 mg/L dye

Feed 3.5 mg/L dye

FICR FIMV F0CR F0MV

2 n d o rd e r k in e tic s

Contact time, t (min)

Contact time, t (min)

q t (m

g dy

e/g

gel)

FICR FIMV F0CR F0MV

q t (m

g dy

e/g

gel)

a

b

Fig. 7. Effect of contact time on dye adsorption with fitting of adsorption data to (a) 1st order and (b) 2nd order kinetics.

Table 2Parameter values of adsorption kinetics models and mass transfer coefficient for low and high conc. of congo red (CR) and methyl violet (MV) dye for F0 and F1 dye.

Model CR low MV low CR high MV high

F1/F0 F1/F0 F1/F0 F1/F0Pseudo 1stQexpt (mg/g) 3.1/2.73 3.03/2.72 360/321 352/317Qet (mg/g) 3.04/2.80 2.98/2.63 370/318 359/311k2 � 102 (min�1) 0.3/0.3 0.29/0.29 2.3/2.2 2.4/2.3R2 0.9972/0.9970 0.9960/0.9958 0.9959/0.9955 0.9950/0.9959v2 0.005/0.004 0.0063/0.0049 57.42/42.47 66.8/40.5F value 6069/6061 4333/4335 9693/9690 7987/9693

Pseudo 2ndqeexpt (mg/g) 3.1/2.73 3.03/2.72 360/321 352/317qecal (mg/g) 3.78/3.48 3.72/3.28 454/390 439/381k2 � 103 (g/mg min) 0.68/0.74 0.69/0.78 5.5 � 10�2/6.4 � 10�2 5.9 � 10�2/6.5 � 10�2

R2 0.9985/0.9982 0.9967/0.9977 0.9898/0.9896 0.9856/0.9898v2 0.0023/0.0020 0.0037/0.0028 143/106 194/101F � 10�2 119.66/119 73.55/73.57 38.8/38.6 27.39/38.66

Intraparticlekp (mg/g min1/2) � 102 7.9/7.3 7.8/6.8 2489/2141 2430/2091Intercept – – 75/64 76/63R2 0.9969/0.9967 0.9964/0.9947 0.9537/0.9530 0.9536/0.9518v2 0.0041/0.0034 0.0046/0.0035 153/113 152/108F � 10�2 45.46/45.42 39/39.5 22/22.29 21/22Dp (m2/s) 7.89 � 10�6/6.38 � 10�6 5.9 � 10�6/4.6 � 10�6 10.5 � 103/7.4 � 103 7.9 � 103/5.7 � 103

kmtc (m/s) 5.08 � 10�4/4.62 � 10�4 5 � 10�4/4.4 � 10�4 3.7 � 10�3/2.78 � 10�3 3.6 � 10�3/2.75 � 10�3

278 R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283

3 0 4 0 5 0 6 0 7 0 8 0 9 0

-0.20

-0.16

-0.12

-0.08

0 2 0 4 0 6 0 8 0-1.6

-1.2

-0.8

-0.4

0.0

F IC R F IM V F 0 C R F 0 M V

Feed 3.5 mg/L dye

Time, t (min)0

t

C

Cln

F IC R F IM V F 0 C R F 0 M V

Time, t (min)

Feed 400 mg/L dye

Determination of Mass transfer coefficient

0 200 400 600 800 1000 1200 1400 16000123456

0 20 40 60 80 100 120 140 160 180 2000123456

Boyd model. Feed 400 mg/L

Bt

FICR FIMV F0CR F0MV

Bt Time, t (minute)

Boyd model. Feed 3.5 mg/L

Time, t (minute)

FICR FIMV F0CR F0MV

a

b

Fig. 8. (a) Determination of mass transfer coefficient. (b) Fitting of adsorption data to Boyd model.

R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283 279

dye, it becomes Freundlich isotherm while at higher concentrationit shows a mono layer adsorption similar to Langmuir isotherm.

Fritz–Schlünder isotherm (FS)Most of the above model equations are combined in the

following generalized FS model Eq. (14)

qe ¼AFSCa

e

c þ BFSCbe

ð14Þ

In most of the cases c = 1 and the above model is written as

qe ¼AFSCa

e

1þ BFSCbe

ð14aÞ

where, AFS and BFS are Fritz–Schlünder constants while a, and b areequation exponent. This model equation reduces to Sip model whena = b and c = 1, Langmuir model when a = c = b = 1 and Freundlichmodel when c = 0. The experimental adsorption data are shownwith solid lines of different colors (thick lines for F1 and thin linesfor F0 gel) while the data fitting to these models are shown withdiscrete unfilled (for F0 gel) and filled (for F1 gel) legends ofdifferent colors in these Figs. 10 and 11. The objective of data fittingto various models was to evaluate the adsorption mechanism. It isevident from these figures and also from the values of model

parameters and statistical parameters given in Table 3 that theadsorption data fitted well to these models. The maximum monolayer adsorption capacity (qmax) of the hydrogel for both low andhigh concentration range is obtained from Langmuir model. FromTable 3 it is observed that for both low and high concentration rangeF0 and F1 gel shows high values of qmax. The separation factor, RL ofLangmuir model (Eq. (11a)) was obtained from initial feed dye con-centration (C0) and Langmuir constant, KL. From Table 3 it isobserved that for both low and high feed concentration range thevalues of RL was less than 1 which signifies favorable adsorption[35]. The heterogeneity of the adsorption was evaluated in termsof n value of Freundlich model. From Table 3 it is observed thatthe values of n are greater than 1 for both low and high range of feedconcentration indicating non linear heterogeneous adsorption [35].

3.4.3.1.1. Validity of the isotherm models. From Figs. 10 and 11 andalso from the values of statistical parameters, i.e., R2, v2 and F givenin Table 3 it is evident that the three-parameter Sip model or four-parameter Fritz–Schlünder (FS) model gives better fitting thantwo-parameter Langmuir or Freundlich models. Apart from lowvalue of v2 and high values of F, the values of R2 is also observedto be close to unity (�0.999) for these models for both low andhigh feed concentration of dyes. These values of statistical param-eters confirms very good fitting of adsorption data to Sip and FS

0 2 4 6 8 10 12 14 160

50

100

150

200

250

300

350

400

FICR FIMV F0CR F0MV

q t (m

g dy

e/g

gel)

t0.5(min)0.5

0 10 20 30 400.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

F1CR F1MV F0CR F0MV

q t (

mg/

g ge

l )

t0 .5 (m in0.5)

a

b

Fig. 9. Fitting of adsorption data to pore flow model for (a) low concentration(3.5 mg/L). (b) High concentration (400 mg/L).

0

5

10

15

20

25

30

35

0 10 20 30

Feed conc. of d

Dye

ads

orbe

d (m

g/g

gel)

FICRexpt FIMVexptFICRsip FICRFSFIMVSip FIMVFSF0CRLANG F0MVLANGF0CRsip F0MVsipFICRremoval% FIMVremoval%

Fig. 10. Effect of feed concentration (low range, 2.5–50 mg/L) on dye adsorption with fi

280 R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283

models. This fitting also signifies multi layer adsorption at low con-centration and monolayer adsorption at higher feed concentration[36].

3.4.4. Thermodynamics of dye adsorptionThe thermodynamic parameters of dye adsorption were deter-

mined from distribution coefficient, kd. The kd of dye betweenwater (liquid phase) and hydrogel (solid phase) at equilibriumwas obtained from equilibrium dye concentration in feed (Ce)and dye adsorbed by the hydrogel (qe) at equilibrium at three dif-ferent temperatures, i.e., at 25, 35 and 45 �C from the following Eq.(15):

kd ¼qe

Ceð15Þ

The change of Gibbs free energy of dye adsorption (DG0) isobtained from kd as

DG0 ¼ �RT ln kd ð15aÞ

Similarly, the change of standard enthalpy (DH0) and entropy(DS0) is obtained from kd using the following van’t Hoff Eq. (16):

ln kd ¼ �DH0

RTþ DS0

Rð16Þ

Here R is universal gas constant.The values of these thermodynamic parameters, i.e., DG0, DH0

and DS0 are shown in Table 3 for 400 mg/L feed concentration ofCR and MV dye. The negative DG0 values confirm feasibility ofthe dye adsorption process while positive DH0 values indicate exo-thermic nature of the adsorption. From Table 3 the standardentropy change (DS0) are also observed to be positive which signi-fies increase in randomness at the solid (gel)–dye solution inter-face during adsorption.

40 50 60

ye (mg/L)

0

10

20

30

40

50

60

70

80

90

100

Rem

oval

%

FICRLANG FICRFreudlichFIMVLANG FIMVFreundlichF0CRexpt F0MVexptF0CRFreundlich F0MVFreundlichF0CRFS F0MVFSF0CRremoval% F0MVremoval%

tting of adsorption data to Langmuir, Freundlich, Sip and Fritz–Schlünder models.

0

100

200

300

400

500

600

0 100 200 300 400 500 600 700Feed conc. of dye (mg/L)

Dye

ads

orbe

d (m

g/g

gel)

50

60

70

80

90

100

Rem

oval

%

FICRexpt FIMVEXPT F0CREXPTF0MVEXPT FICRFS FICRSipFICRLANG FICRLANG FIMVFSFIMVSip FIMVLANG FIMVFreunndF0CRFS F0CRSip F0CRLANGF0CRFreunnd F0MVFS F0MVSipF0MVLANG F0MVFreundlich FICRremoval%FIMVremoval% F0CRremoval% F0MVremoval%

Fig. 11. Effect of feed concentration (High range, 100–600 mg/L) on dye adsorption with fitting of adsorption data to Langmuir, Freundlich, Sip and Fritz–Schlünder models.

Table 3Adsorption isotherm parameters and thermodynamic parameters for low and high conc. of congo red (CR) and methyl violet (MV) dye.

Model CR low MV low CR high MV high

F1/F0 F1/F0 F1/F0 F1/F0LangmuirKL (L/mg) 0.008/0.003 0.007/0.008 4.63 � 10�4/5.16 � 10�4 5.18 � 10�4/5.05 � 10�4

qmax (mg/L) 110/225 111/92.14 2277/1927 2022/1926RL (�) 0.98–0.71/0.93–0.40 0.98–0.74/0.83–0.20 0.95–0.76/0.95–0.77 0.95–0.76/0.95–0.77R2 0.9999/0.9878 0.9992/0.9958 0.9999/0.9985 0.9996/0.9981v2 0.0122/1.33 0.087/0.387 13.15/41.7 9.17/49.5F � 10�2 1000/7.13 126/21.53 259/70 351/56.47

Freundlichn 1.2/1.1 1.19/1.18 1.16/1.18 1.17/1.18KF (L/mg) 1.21/0.82 1.13/0.97 1.98/2.02 2.07/1.96R2 0.9991/0.9893 0.9997/0.9932 0.9987/0.9982 0.9984/0.9980v2 0.119/1.17 0.035/0.641 40.31/46.28 45.56/51.51F � 10�2 101.85/8.07 315/13 84.38/62.68 70.66/54.37

SipKS (L/mg) 0.933/1.08 1.03/0.553 0.8/1.23 0.685/1.33bS 0.968/0.760 0.886/1.12 1.06/0.95 1.087/0.93AS 0.007/�0.013 0.003/0.009 0.0005/0.0004 0.0004/0.0004R2 0.9999/0.9883 0.9997/0.9956 0.9995/0.9981 0.998/0.9977v2 0.008/1.27 0.025/0.407 13.36/49.76 4.62/58.63F � 10�2 1007/496 286/13.65 170/38.86 464/31.84

FS*

KFS 99.94/1.49 1.21/1.1 � 107 5.65/2.42 235/2.35bFS 0.049/0.91 0.82/�2.86 0.711/0.819 0.232/0.821a 107/0.81 1.27/1.7 � 107 75/3.13 � 109 447/8.78 � 1010

�b 0.93/0.0002 1.8/0 1.06/5.16 0.93/�5.87R2 0.9999/0.9939 0.9998/0.9982 0.9995/0.9986 0.9998/0.9983v2 0.011/1.76 0.025/0.175 13.99/37.52 5.97/42.84F � 10�2 580/2.69 218/24 121.54/38.66 269.48/32.7�DG0 (kj/mol) at 303 K 9.5/9.3 8.2/7.8 9.4/9.1 7.5/7.3DH0 (kj/mol) 5.3/4.8 4.4/3.9 4.8/4.4 3.8/3.5DS0 (kj/mol) 0.028/0.024 0.025/0.023 0.027/0.022 0.022/0.018

* FS, Fritz–Schlünder.

R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283 281

3.5. Reusability of hydrogels

The reusability of the hydrogels were evaluated by repeateddesorption and sorption of the two dyes for five consecutive cyclesat a feed dye concentration of 400 mg/L and solution pH of 7. Thechange of adsorption capacity after these five repeated cycles ofsorption and desorption was marginal which indicates goodreusability of the hydrogels.

3.6. Comparison with reported results

The adsorption capacity of the present hydrogel F1 was com-pared with other reported hydrogels for separation of CR and MVdye in Table 4. From the results given in Table 4 it is evident thatmost of the reported works show adsorption results for high rangeof feed concentration. However, the present F1 hydrogel is

Table 4Comparison of present work with reported data.

Name of hydrogel Dye used in water, Temp, conc., pH Adsorption performance mg/g resin Refs.

Poly(HEMA-g-GMA) 700 mg/L MV dye in water at pH 5 121.5, Qmax = 0.189 [31]IPN2 2.5 mg/L in water at pH 7 and 25 �C 2.249 for BF, Qmax = 5.96

1.723 for MV, Qm = 3.93[26]

IPN2 500 mg/L in water at pH 7 and 25 �C 368.70 for BF, Qmax = 920283.76 for MV, Qmax = 613.8

[26]

CS/CTAB beads 1000 mg/L MV dye Qe = 373.29 [37]CS beads 1000 mg/L CR dye pH 5 Qe = 178.32 [37]Soya ash 25.9 mg/L MV dye at pH 9 4.209, Qmax = 5.76 [38]Supramolecular and composite gel of agarose 1000 mg/L MV dye at pH 7 Removal% 95.1% and 95.7% [39]CS/CNT beads 500 mg dm�3 CR dye at pH 4 Qe = 423.1 [40]CS/CTAB beads 500 mg/L CR dye at pH 5 Qe = 352 [41]Poly(AA-co-AM)/attapulgite 200–1000 mg/L MV dye at pH 7 917 for 1000 mg/L MV dye [42]F1 5 mg/L at pH 7 For CR Qmax = 110, Qe = 4.43, for MV Qmax = 111, Qe = 4.12 Present workF1 500 mg/L at pH 7 For CR Qmax = 2227, Qe = 423, for MV Qmax = 1927, Qe = 414 Present work

282 R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283

observed to show high adsorption capacity and Qmax for both lowand high range of feed dye concentration.

4. Conclusion

Several hydrogels were synthesized from acrylic acid, variedwt% of PEG and nano size bentonite filler. These hydrogels werecharacterized by FTIR, DTA–TG, XRD and SEM. The unfilled hydro-gel containing 6 wt% PEG (F0) and the filled hydrogel containing6 wt% PEG and 1 wt% nano size filler (F1) showed the best swellingresults. These unfilled F0 and filled F1 hydrogel were further sub-jected to adsorption of low and high concentration range of CRand MV dye. The hydrogels showed high adsorption capacity andremoval% of these dyes for both low and high range of feed concen-tration at a solution pH of 7. Between the two dyes the CR dye wasadsorbed more than the MV dye because of their structural differ-ence. The dye adsorption was also found to be exothermic in nat-ure. The dye adsorption data at different time intervals and feedconcentrations were fitted to various kinetic models and adsorp-tion isotherms. The diffusion of the dye molecules to the hydrogelwas found to be mass transfer controlled film diffusion at thebeginning of the experiment but after some 1 h of adsorption intraparticle pore diffusion became the governing mechanism ofadsorption. The adsorption was found to be heterogeneous withhigh mono layer adsorption for both low and high range of feedconcentration. The hydrogel may also be used for removal of othercationic dyes or metal ions from water.

Acknowledgements

Authors are grateful to Council of Scientific and IndustrialResearch (CSIR), 22(0547)/11/EMR-II and Department of Scienceand Technology (DST), SR/S3/CE/056/2009, Govt. of India for theirfinancial support to purchase UV–Vis spectrophotometer and FTIRspectrophotometer used for the present work.

References

[1] B. Kayranli, Adsorption of textile dyes onto iron based waterworks sludge fromaqueous solution; isotherm, kinetic and thermodynamic study, Chem. Eng. J.173 (2011) 782–791.

[2] J.M. Harris, Poly(ethylene glycol) Chemistry: Biotechnical and BiomedicalApplications. Plenum Press, New York, 1992.

[3] S.A. Bencherif, A. Srinivasan, J.A. Sheehan, L.M. Walker, C. Gayathri, R. Gil, J.O.Hollinger, K. Matyjaszewski, N.R. Washburn, End-group effects on theproperties of PEG-co-PGA hydrogels, Acta. Biomater. 5 (2009) 1872–1883.

[4] K. Kesenci, P. Erhan, Production of poly[(ethylene glycol dimathacrylate)-coacrylamide] based hydrogel beads by suspension copolymerization,Macromol. Chem. Phys. 199 (1998) 385–391.

[5] N.S. Kwak, J.R. Yang, C.W. Hwang, Taek Sung Hwang, The effect of a molecularweight and an amount of PEGDA(poly(ethylene glycol)diacrylate) on a

preparation of sodium methallylsulfonate-co-PEGDA microspheres andsorption behavior of Co(II), Chem. Eng. J. 223 (2013) 216–223.

[6] E. Karadag, O. Barıs, U. Züm, A study on water and dye sorption capacities ofnovel ternary acrylamide/sodium acrylate/PEG semi IPN hydrogels, Polym.Bull. 68 (2012) 1357–1368.

[7] Q. Tang, J. Wu, H. Sun, S. Fan, D. Hu, J. Lin, Synthesis of polyacrylate/poly(ethylene glycol)hydrogel and its absorption properties for heavy metalions and dye, Polym. Compos. 30 (2009) 1183–1189.

[8] S. Kundakci, H. Güney Ögüt, Ömer Barıs� Üzüm, Erdener Karadag, Equilibriumswelling characterization and dye uptake studies of acrylamide-co-methylenesuccinic Acid Hydrogels and semi-IPNs with PEG, Polym. Plast.Technol. 50 (2011) 947–956.

[9] P.S. Suchithra, L. Vazhayal, A.P. Mohamed, S. Ananthakumar, Mesoporousorganic–inorganic hybrid aerogels through ultrasonic assisted sol–gelintercalation of silica PEG in bentonite for effective removal of dyes, volatileorganic pollutants and petroleum products from aqueous solution, Chem. Eng.J. 200–202 (2012) 589–600.

[10] J. Ma, J. Qi, C. Yao, B. Cui, T. Zhang, D. Li, A novel bentonite-based adsorbent foranionic pollutant removal from water, Chem. Eng. J. 200–202 (2012) 97–103.

[11] S. Tahir, N. Rauf, Removal of a cationic dye from aqueous solutions byadsorption onto bentonite clay, Chemosphere 63 (2006) 1842–1848.

[12] Yi. Liu, Y. Kang, B. Mu, A. Wang, Attapulgite/bentonite interactions formethylene blue adsorption characteristics from aqueous solution, Chem.Eng. J. 237 (2014) 403–410.

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

[14] J. Vassilis, A. Marinos, S. Despoina, D. Gkantzou, M. Loizidou, Removal of Pb(II)from aqueous solutions by using clinoptilolite and bentonite as adsorbents,Desalination 210 (2006) 248–256.

[15] K.Y. Foo, B.H. Hameed, Insights into the modeling of adsorption isothermsystems, Chem. Eng. J. 156 (2010) 2–10.

[16] S. Nagarajan, K. Sabdham, V. Srinivasan, Block copolymerization ofpoly(ethylene glycol) with methyl methacrylate using redox macroinitiators,Macromol. Mater. Eng. 245 (1997) 9–22.

[17] J.W. Lee, S.Y. Kim, S.S. Kim, Y.M. Lee, K.H.Y. Lee, S.J. Kim, Synthesis andcharacteristics of interpenetrating polymer network hydrogel composed ofchitosan and poly(acrylic acid), J. Appl. Polym. Sci. 73 (1999) 113–120.

[18] L. Zheng, S. Xu, Y. Peng, J. Wang, G. Peng, Preparation and swelling behavior ofamphoteric superabsorbent composite with semi-IPN composed ofpoly(acrylic acid)/Ca-bentonite/poly(dimethyldiallylammonium chloride),Polym. Adv. Technol. 18 (2007) 194–199.

[19] K. Xu, J. Wang, S. Xiang, Q. Chen, W. Zhang, P. Wang, Study on the synthesisand performance of hydrogels with ionic monomers and montmorillonite,Appl. Clay Sci. 38 (2007) 139–145.

[20] J. Maity, S.K. Ray, Enhanced adsorption of methyl violet and congo red by usingsemi and full IPN of polymethacrylic acid and chitosan, Carbohydr. Polym.104C (2014) 8–16.

[21] D. Shanmukaraj, R. Murugan, Characterization of PEG: LiClO4 + SrBi4Ti4O15

nanocomposite polymer electrolytes for lithium secondary batteries, J. PowerSour. 149 (2005) 90–95.

[22] Mahmoud A. Hussein, Baha M. Abu-Zied, Abdullah M. Asiri, Preparation,characterization, and electrical properties of ZSM-5/PEG composite particles,Polym. Compos. 35 (2014) 1160–1168, doi: 10.1002/pc.22763.

[23] S. Yang, D. Zhao, H. Zhang, S. Lu, L. Chen, X. Yu, Impact of environmentalconditions on the sorption behavior of Pb(II) in Na-bentonite suspensions, J.Hazard. Mater. 183 (2010) 632–640.

[24] G. Choudalakis, A.D. Gotsis, Morphology and gas transport properties of acrylicresin/bentonite nanocomposite coatings, Prog. Org. Coat. 77 (2014) 845–852.

[25] M.J. Arias, J.R. Moyano, J.M. Gineas, Study by DSC and HSM of the oxazepam-PEG 6000 and oxazepam-D-mannitol systems: application to the preparationof solid dispersions, Thermochim. Acta 321 (1998) 33–41.

[26] R. Bhattacharyya, S.K. Ray, Kinetic and equilibrium modeling for adsorption oftextile dyes in aqueous solutions by carboxymethyl cellulose/

R. Bhattacharyya, S.K. Ray / Chemical Engineering Journal 260 (2015) 269–283 283

poly(acrylamide-co-hydroxyethyl methacrylate) semi interpenetratingnetwork hydrogel, Polym. Eng. Sci. 53 (11) (2013) 2439–2453.

[27] P.S. Keshava Murthy, Y. Murali Mohan, J. Sreeramulu, K. Mohana Raju, Semi-IPNs of starch and poly(acrylamide-co-sodium methacrylate): preparation,swelling and diffusion characteristics evaluation, React. Funct. Polym. 66(2006) 1482–1493.

[28] I.D. Mall, V.C. Srivastava, G.V.A. Kumar, I.M. Mishra, Characterization andutilization of mesoporous fertilizer plant waste carbon for adsorptive removalof dyes from aqueous solution, Coll. Surf. A. 278 (2006) 175–187.

[29] P.L. Ritger, N.A. Peppas, A simple equation for description of solute release IFickian and non-Fickian release from non-swellable devices in the form ofslabs, spheres, cylinders or discs, J. Contr. Rel. 5 (1987) 23–36.

[30] K.L. Mittal, Relevance to Adhesion Science and Technology, VSP, Utrecht, 2000.[31] G. Bayramoglu, B. Altintas, A.M. Yakup, Adsorption kinetics and

thermodynamic parameters of cationic dyes from aqueous solutions byusing a new strong cation-exchange resin, Chem. Eng. J. 152 (2009) 339–346.

[32] N.K. Lazaridis, G.Z. Kyzas, A.A. Vassiliou, D.N. Bikiaris, Chitosan derivatives asbiosorbents for basic dyes, Langmuir 23 (2007) 7634–7643.

[33] B.H. Hameed, M.I. El-Khaiary, Malachite green adsorption by rattan sawdust:isotherm, kinetic and mechanism modeling, J. Hazard. Mater. 159 (2008) 574–579.

[34] M. Al-Ghouti, M. Khraisheh, S.A. Ahmad, Thermodynamic behavior and theeffect of temperature on the removal of dyes from aqueous solution usingmodified diatomite: a kinetic study, J. Coll. Interf. Sci. 287 (2005) 6–13.

[35] L. Abramian, H. El-Rassy, Adsorption kinetics and thermodynamics of azo-dye Orange II onto highly porous titania aerogel, Chem. Eng. J. 150 (2009)403–410.

[36] T.S. Anirudhan, A.R. Tharun, Preparation and adsorption properties of a novelinterpenetrating polymer network (IPN) containing carboxyl groups for basicdye from aqueous media, Chem. Eng. J. 181–182 (2012) 761–769.

[37] S. Chatterjee, M.W. Lee, S. H. Woo, Influence of impregnation of chitosan beadswith acetyl trimethyl ammonium bromide on their structure and adsorption ofcongo red from aqueous solutions, Chem. Eng. J. 155 (2009) 254–259.

[38] V.K. Gupta, A. Mittal, V. Gajbe, J. Mittal, Adsorption of basic fuchsin usingwaste materials – bottom ash and deoiled soya – as adsorbents, J. Coll. Interf.Sci. 319 (2008) 30–39.

[39] J. Wang, H. Wang, Z. Song, D. Kong, X. Chen, Z. Yang, A hybrid hydrogel forefficient removal of methyl violet from aqueous solutions, Coll. Surf. B 80(2010) 155–160.

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

[41] S. Chatterjee, D.S. Lee, M.W. Lee, S.H. Woo, Enhanced adsorption of congo redfrom aqueous solutions by chitosan hydrogel beads impregnated with cetyltrimethyl ammonium bromide, Bioresour. Technol. 100 (2009) 2803–2809.

[42] Y. Wang, L. Zeng, X. Ren, H. Song, A. Wang, Removal of methyl violet fromaqueous solutions using poly(acrylic acid-co-acrylamide) attapulgite, J.Environ. Sci. 221 (2010) 7–14.