a systematic method of synthesizing composite superabsorbent hydrogels from crosslink copolymer for...

Journal of Industrial and Engineering Chemistry 19 (2013) 1191–1203

A systematic method of synthesizing composite superabsorbent hydrogels fromcrosslink copolymer for removal of textile dyes from water

Ruma Bhattacharyya a, Samit Kumar Ray a,*, Bidyadhar Mandal b

a Department of Polymer Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, Indiab Department of Chemistry, Bijoy Krishna Girls’ College, Howrah, India

A R T I C L E I N F O

Article history:

Received 1 August 2012

Accepted 11 December 2012

Available online 20 December 2012

Keywords:

Composite hydrogel

Network parameter

Dye removal

Adsorption kinetics and isotherm

Thermodynamic parameters

A B S T R A C T

A systematic method was employed to synthesize several hydrogels by crosslink copolymerization of

acrylamide (AM), hydroxyl ethylmethacrylate (HEMA) and N,N’-methylenebisacrylamide (NMBA) at

varied operating conditions. Composite hydrogels were also prepared by in situ incorporation of varied

amounts of sodium aluminosilicate filler to the monomer mixtures at optimum operating conditions.

These hydrogels were used for removal of rhodamine B and methyl violet dye from water at low (0.5–

3 mg/L) and high concentration (50–500 mg/L) ranges. The composite hydrogels showed much higher

dye adsorption than the unfilled hydrogels. Kinetic, adsorption and thermodynamic parameters for dye

adsorption were also evaluated.

� 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Superabsorbent polymers (SAPS) are crosslink hydrogel whichcan absorb much more water than general absorbing materials andthe absorbed water can hardly be removed from the gel even underpressure [1]. The presence of crosslink network prevents thishighly hydrophilic polymer from dissolution in water. Thesepolymers generally contain functional groups, such as NH2, CONH2,COOH, OH etc., in its structures. In water the long polymer chain ofthe SAPs extends due to electrostatic repulsion of these ionizedgroups resulting in extensive swelling of the crosslink network ofthese polymers. SAPs are generally synthesized by free radicalpolymerization of various vinyl monomers and their salts bysolution polymerization [2–5], inverse suspension or emulsionpolymerization [6–10] or by radiation polymerization [11–13].These SAPs are extensively used as absorbent in agriculture,horticulture, personal care products, dye removal from aqueouswaste, drug delivery, etc. [14–17]. In recent years variouscomposite SAPS are also being used for the above applicationsbecause of its reduced cost, higher water absorbency and increasedgel strength. These organic–inorganic composite SAPs are synthe-sized by incorporation of various clays, such as montmorillonite,kaolin, mica, attapulgite, sericite, zeolyte, in acrylic polymers [18–20]. Various vinyl monomers, such as acrylamide, acrylic acid, arenot very expensive and may be easily converted to a superabsor-

* Corresponding author. Fax: +91 33 23508386.

E-mail address: [email protected] (S.K. Ray).

1226-086X/$ – see front matter � 2012 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2012.12.017

bent hydrogel by free radical polymerization in water [10]. Unfilledand filled composite SAPs based on various acrylic polymers andinorganic/organic fillers have been widely used for removal ofindustrial dye or heavy metal ions from water. The performance ofSAPs based on acrylic copolymer is influenced markedly by variousreaction parameters, such as comonomer ratios, total monomerconcentration in water, concentration of initiator and comonomercrosslinker (e.g. NMBA) in reaction medium. Similarly, synthesis ofa composite or filled hydrogel from a hydrogel based on acryliccopolymer depends on type, concentration and mode of addition offiller. A systematic approach of optimizing various operatingparameters for selecting the right membrane for nanofiltration[21] or pervaporation [22] has already been reported. However,there is no systematic study about synthesis of an unfilled or filledhydrogel showing optimum adsorption or removal% of dye ormetal ion from water depending on specific reaction parametersand selecting optimum concentration of filler. Thus, the objectiveof the present work was to synthesize three acrylic copolymerhydrogels from AM and HEMA at varied comonomer ratio andevaluating its swelling characteristic at varied initiator, crosslinkerand total monomer concentration in water. The hydrogel showingthe optimum swelling characteristic was then filled with threedifferent concentrations of hydrophilic fillers, i.e. sodium alumi-nosilicate by in situ mixing of the filler during polymerization toproduce three filled or composite hydrogels. This aluminosilicatefiller was reported to increase hydrophilicity of polyvinyl alcoholmembranes [23]. The filled or composite gel showing the optimumswelling characteristic was then used for removal of variedconcentrations of two industrial dyes, i.e. rhodamine B and methyl

ing Chemistry. Published by Elsevier B.V. All rights reserved.


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R. Bhattacharyya et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1191–12031192

violet from water. The application of SAP for removal of lowconcentration of various dyes found in textile waste water is notwell studied because of absence of efficient SAP to be used as dyeadsorbent. Among the various dyes rhodamine B and methyl violetare extensively used in textile dying industries. These dyes havealso high tinctorial values. Even a low concentration of 1 mg/L ofthese dyes in water gives very distinct color [24]. Thus, in thepresent study the unfilled and filled hydrogel showing optimumswelling characteristic was used for removal of these two dyes overlow concentration range of 0.25–3 mg/L and high concentrationrange of 50–500 mg/L in water. The systematic method used in thepresent study for synthesizing hydrogel showing optimum dyeadsorption and removal % is schematically shown below:

A

Synthesis of PAM HEMA10, PAMHEMA5 , PAM HEMA1 hydrogels with AM:HEMA

comonomer ratio of 10:1, 5:1, 1:1

B

Synthesis of four differe nt hydrogels from each of the above three hydrogels of A i.e.

total 12 hydrogels at four different to tal monomer concent ration at fixed initiator and

crosslink er conce ntration

C

Synthesis of 3 different hydrogels from eac h of the three hydrogels of A at three different

initiator conce ntrations keepi ng total monomer and crosslink er conce ntrations constant and 3

different hydrogels from eac h of A at three different crosslinker con centration keepi ng total

monomer and initiator conce ntration constant to produce total 18 hydrogels

D

Stu dying wate r s wellin g characte ris tic and network para meter of these 18 hydrogels to

identify one PAM HEM A1 hydrogel synt hesi zed at optimum total monomer, initiator and

crosslink er conce ntration showing the highest equilibriu m swellin g (ES%)

E

Synthesis of 3 composite hydrogels from PAM HEMA1 i.e. F2PAMEMA1,F4PAM HEMA1

& F6PAM HEMA1 with insitu mixin g of 2,4 & 6 mass% alu minosilica te fille r during

polyme rization at optimum total monomer, initiator and crosslink er con centration

G

Stu dying adsorption and removal% of rhodamine B and methyl violet dye with unfilled

PAM HEMA1 & composite F4PAM HEMA1 hydrogel

F

Sele cting 1 composite hydrogel i.e. F4PAM HEMA1 showin g the highest ES%

2. Experimental

2.1. Materials

Monomers, i.e. acrylamide (AM from Fluka), hydroxyl ethyl-methacrylate (HEMA from Fluka), N,N’-methylenebisacrylamide(NMBA, from Fluka), redox initiator pair, i.e. ammoniumpersulfate (APS, from Fluka) and sodium metabisulfite (SMBS,Merck),were of analytical grade and used without furtherpurification. Hydrophilic aluminosilicate filler was used afterdrying at 110 8C for 2 h in oven. The specification of this filler isreported elsewhere [23]. Cationic dyes, rhodamine B and methylviolet (Basic Violet 1) used in sorption studies were purchasedfrom SRL Chemical, India. The structures of these two dyes areshown in Fig. 1.

2.2. Synthesis of poly(acrylamide-co-hydroxyethylmethacrylate)

[PAMHEMA] superabsorbent hydrogels

PAMHEMA hydrogels were prepared by free radical cross-linkcopolymerization of AM, HEMA and multifunctional NMBA. Thepolymerization reaction was carried out in a three necked glassreactor equipped with a mechanical stirrer, reflux condenser andthermometer. The dissolved oxygen of the reaction mixtures wasremoved by purging nitrogen gas for half an hour in the reactionmixtures before addition of the reacting monomers and cross-linker. After addition of the monomers and crosslinker thetemperature of the reaction mixtures was raised to 70 8C withaddition of required amounts of redox pair of initiator, i.e.

ammonium persulfate and sodium metabisulfite. The reactionwas then continued at this temperature till the reaction mixturesgelled. The structure of the monomers and the formation of thecrosslink hydrogel are shown in Fig. 2.

2.3. Synthesis of composite hydrogel by in situ filler incorporation

For synthesizing the composite hydrogel, the hydrophilicaluminosilicate (zeolyte) filler was first dispersed in water wellfor half an hour followed by addition of monomers and stirring foranother half hour. Then, the polymerization with in situ mixing offiller with these monomers was carried out in a similar way as inthe case of polymerization for the unfilled (without any filler) gel.The gel obtained was disintegrated in a blender, washed withwater and then isopropyl alcohol, followed by filtration and finallydried to constant weight at 30 8C in a vacuum oven.

Fig. 1. Structure of dyes: (a) rhodamine B (RB) and (b) methyl violet (MV).

Fig. 2. Structure of monomers and formation of crosslink hydrogel: (a) acrylamide, (b) hydroxyethylmethacrylate, (c) N,N’-methylenebisacrylamide, (d) crosslink copolymer

(hydrogel).

R. Bhattacharyya et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1191–1203 1193

0

10

20

30

40

50

60

70

80

5 10 15 20 25 30 35 40

Monomer conc. (mass%)

Poly

mer

yie

ld (

%)

0

1

2

3

4

5

6

7

8

9

10

PAMHEMA10 YEILD

PAMHEMA5YEILD

PAMHEMA1EILD

PAMHEMA10 GELTIME

PAMHEMA5GELTIME

PAMHEMA1GELTIME

Gel

tim

e (m

in)

Fig. 3. Variation of yield and gel time with total monomer concentration.

Table 1Composition and mechanical properties of the superabsorbent copolymer hydrogels.

Polymer code Comonomer

composition

(AM:HEMA) in

feed (molar ratio)

Peak ratio (P1/P2)

of OH:CH

Comonomer

composition

(AM:HEMA) in

hydrogel (molar)

Filler % TS (MPa)/EAB%

PAMHEMA1 1:1 0.610 1:1.75 Unfilled 24.21/121.55



F2PAMHEMA1 1:1 – 1:1.75 2 32.56/98.3

F4PAMHEMA1 1:1 – 1:1.75 4 39.21/89.21

F6PAMHEMA1 1:1 – 1:1.75 6 44.17/77.21


2.4. Characterization of the hydrogels

The synthesized hydrogels were characterized by the followingmethods.

2.4.1. FTIR spectroscopy

Fourier transform infrared (FTIR) spectra of the copolymerhydrogels were recorded on a FTIR spectrometer (Perkin Elmermodel-Spectrum-2, Singapore), using KBr pellet. The pellet wasmade by mixing KBr with fine powder of the polymer gel samples.(10:1 mass ratio of KBr to polymer).

2.4.2. Thermal analysis

Differential scanning calorimetry (DSC) of the polymer sampleswas carried out in a Mettler instrument in nitrogen atmosphere atthe scanning rate of 10 8C/min in the temperature range of 60–600 8C.

2.4.3. Scanning electron microscopy (SEM)

The three composite gel and one unfilled gel samples werecoated with gold (Au). The morphology of the filled gels wasobserved by using SEM (model no. S3400N, VP SEM, Type-II, madeby Hitachi, Japan) with the accelerating voltage set to 15 kV.

2.4.4. Mechanical properties

The tensile strength (T.S.) and elongation at break (E.A.B.) of theunfilled and filled samples were determined by an Instron-Tensiletester (Lloyd instruments, England). The experimental procedurefor mechanical properties is reported elsewhere [25]. In this work,cubic sample of 2 mm � 2 mm � 80 mm size was used. Thecrosshead speed of 100 mm/min was maintained. The cubicsamples were elongated at a strain rate of 5%/min. The T.S. andE.A.B. were calculated on the basis of initial cross section area of thesample. The T.S. and E.A.B. of the hydrogel samples are given inTable 1.

2.5. Study of swelling properties of the hydrogels

The dynamic swelling properties of the superabsorbenthydrogels were carried out in distilled water at ambienttemperature. The mass of the swollen gel was taken at differenttime intervals until there was no change of mass with time. Thewater uptake of the hydrogels (WC) was determined by using thefollowing Eq. (1):

WC ¼ Wt�WdWd

(1)

where Wt is the mass of swollen hydrogel polymer at time ‘t’ andWd is the mass of dry polymer. The amount of water adsorbed bydifferent hydrogels under equilibrium conditions, also calledequilibrium swelling (ES) was obtained when Wt did not changeany more (Wa) with time.

2.6. Study of dye removal capacity of the hydrogels

Solutions of the two kinds of dye, i.e. rhodamine B and methylviolet with varied concentration range (0.25–3 mg/L and 50–500 mg/L) in distilled water were prepared. 50 mg of hydrogel wastaken in known volume of the dye solution with continuousstirring on a magnetic stirrer until equilibrium was reached. Afterequilibrium was reached, the dye solution was separated bydecantation from the hydrogel. The concentration of dye solutionsbefore and after addition of hydrogel was determined byspectrophotometric measurement from a precalibrated curve ofabsorbance versus concentrations using Perkin Elmer lamda 25UV-visible Spetrophotometer. The absorbance of the dye solutionwas measured at wavelength of 554 nm for rhodamine B and584 nm for methyl violet. The amount of dye uptake (in mg) by unitmass (in g) of the hydrogel (Qe, mg/g) was calculated using thefollowing Eq. (2):

Qe ¼ ðC0V0�CeVÞWd

(2)

Here C0 and Ce are initial and final equilibrium (after contacttime t) concentration of dye solution (mg dm3) while V0 and V arevolume (dm3) of the initial and final dye solution containing thehydrogel and Wd is mass (g) of the dry hydrogel polymer used forthe experiment. The removal% of dye by the hydrogel polymerswas determined using the following Eq. (2a):

Removal% ¼ C0�Ce

C0� 100 (2a)

The results for swelling and dye uptake experiments werereproducible and the errors inherent in the measurements wereless than �3%.

-1

8

0

2

4

6

4000 40 0100020003000

%T

Wavenumber [cm-1]a. PAMHEMA1

0

90

20

40

60

80

4000 400100020003000

%T

Wavenumber [cm-1]b. PAMHEMA5

0

80

20

40

60

4000 400100020003000

%T

Wavenumber [cm-1]c. PAMHEMA10

Fig. 4. FTIR of PAMHEMA hydrogel: (a) PAMHEMA1, (b) PAMHEMA5, (c)

PAMHEMA10.

Fig. 5. DSC curve of PAMHEMA1 hydrogel.


3. Results and discussion

3.1. Synthesis of PAMHEMA hydrogels

Copolymerization of AM and HEMA was carried out in watersince both monomers are soluble in water. The pH of the reactionmedium was maintained at around 4 by using aqueous sulfuricacid since at this pH most uniform copolymer is formed [26]. Athigher pH amide group of AM would hydrolyze while at pH below 4crosslinking through imidization would occur at polymerizationtemperature [27]. The polymerization reaction was conducted in anitrogen atmosphere, to prevent oxidative degradation or theformation of copolymers of AM with oxygen. The hydrogel isformed by crosslink copolymerization of AM, HEMA and NMBA asshown in Fig. 2. During polymerization the bifunctional monomeror crosslinker NMBA undergoes copolymerization with both of theother two monomers, i.e. AM and HEMA to form the crosslinkcopolymer hydrogel. For making filled composite hydrogelPAMHEMA1 hydrogel showing the highest water uptake% wasincorporated with 2, 4 and 6 mass% (of total monomer mass)aluminosilicate filler to produce three filled or compositehydrogels. The composition of these six hydrogel polymers alongwith comonomer ratio of AM and HEMA taken for polymerization(feed) and approximate copolymer composition with respect toAM and HEMA is shown in Table 1.

3.1.1. Effect of reaction variables on polymer yield and gel time

The effect of total monomer concentration in water on polymeryield and gel time at fixed initiator and crosslinker concentration(both 0.5 mass% of total monomer mass) is shown in Fig. 3 forPAMHEMA10, PAMHEMA5 and PAMHEMA1 hydrogels. From thisfigure it is observed that with increase in total monomerconcentration polymer yield increases significantly for all the threegels. This may be attributed to the availability of greater amount ofactive primary radicals at higher monomer concentration in water[28]. It is also observed from this figure that for the same monomerconcentration polymer yield increases in the following order:

PAMHEMA10 < PAMHEMA5 < PAMHEMA1This trend may be explained in terms of difference in monomer

reactivity ratios. As the % of HEMA increases from PAMHEMA10 toPAMHEMA1, the extent of reaction increases because of muchhigher reactivity ratio of HEMA (rHEMA = 0.98) in comparison to AM(rAM = 0.14) [28]. Hence, polymer yield increases in the same order.From Fig. 3 it is also observed that gel time decreases with increasingmonomer concentration and also for the same monomer concen-tration gel time decreases from PAMHEMA10 to PAMHEMA1. Theincreased reaction rate at higher monomer concentration or forcomonomer compositions containing greater amounts of HEMA isresponsible for this trend. Similar kind of trend lines was obtainedfor PAMHEMA gels synthesized with varied initiator and crosslinkerconcentration at constant comonomer ratio and total monomerconcentration. In this case gel time increases with increasinginitiator concentration. This may be ascribed to formation of shorterpolymer chain leading to imperfect network formation in crosslinkcopolymerization [29]. As network is not easily formed, gel time isdelayed. Similarly as expected, with increase in crosslinkerconcentration gel time decreases since network in the polymer isformed at a much faster rate in presence of increased amount ofcrosslinker and polymer yield decreases because of formation ofnetwork at an early stage of polymerization.

3.2. Characterization of the hydrogel

3.2.1. FTIR spectroscopy

The FTIR spectra of PAMHEMA1, PAMHEMA5, and PAM-HEMA10 are shown in Fig. 4a–c, respectively. The ester stretching

vibration band at 1703.8 cm�1, the C–O stretching at1216.86 cm�1, and the O–H bending vibrations at 1082.83 cm�1

correspond to the HEMA comonomer of the copolymer [30]. Theother strong bands at 1450.21 and 2945.73 cm�1 correspond to theC–H bending (alkane, –CH3, alkane, –CH2–) and C–H stretching ofthe alkane, respectively. The stretching vibration band at1658.48 cm�1 corresponds to carbonyl of the AM moiety of thecopolymers. From these figures it is also observed that withincreasing amount of HEMA from PAMHEMA10 to PAMHEMA1 theintensity of the –OH group of HEMA increases. The formation ofhydrogen bonding is exhibited on the FTIR spectrum as a negativeshift of the stretching vibration of the functional group involved inthe hydrogen bond, which is typically a carbonyl group. Thehydrogen bond was identified by the clear formation of a shoulder


on the CONH2 group absorption peak, which appears at about1723 cm�1.

3.2.1.1. FTIR study and copolymer compositions. It is observed fromFig. 4a–c that increment of band intensity due to C–O stretching(for OH group of HEMA) is not proportional from PAMHEMA10to PAMHEMA1. This is because of the dependence of absorbance(A) on sample thickness which was not possible to maintainexactly same for all the three copolymer samples. Thus, peakratios (C–OH/C–H) of this C–OH group to C–H stretching werecalculated for the three copolymers to eliminate the thicknesseffect. As given in Table 1, this peak ratio of C–OH to C–Hstretching increases proportionately from PAMHEMA10 toPAMHEMA1 signifying increasing amount of HEMA in thecopolymer. However, exact copolymer compositions were notobtained since standard sample of the copolymer of knowncomposition was not available.

3.2.2. Thermal study

DSC curve of the unfilled PAMHEMA1 copolymer hydrogel isshown in Fig. 5. Similar kind of DSC curves was also obtained with

Fig. 6. SEM of the hydrogel: (a) PAMHEMA1(unfilled) hydrogel, (b) F2PAMHEMA1, (c) F

the other hydrogels with varied comonomer compositions.Crosslinking raises the glass transition temperature (Tg) of apolymer [28] while copolymerization reduces Tg of a copolymer byintramolecular plasticization. With crosslinking, due to increasedrestriction in segmental motion Tg of the copolymers increase in itscrosslinked gel form as shown for gel PAMHEMA1. As expected,with increasing amount of low Tg POLYHEMA, Tg of the copolymergel decreases in the following order: PAMHEMA1(98 8C) < PAMHEMA5 (115 8C) < PAMHEMA10 (125 8C).

3.2.3. SEM study

The SEM study was carried out to find distribution ofaluminosilicate filler in the matrix of composite hydrogel. SEMof PAMHEMA1, F2PAMHEMA1, F4PAMHEMA1 and F6PAMHEMA1is shown in Fig. 6a–d, respectively. From Fig. 6b–d it is observedthat with increase in filler loading the morphology becomescoarser and some micro cracks also develops in the interphase offiller and polymer. The unfilled hydrogel shows almost featurelessdense morphology as seen in Fig. 6a. At filler loading >6%agglomeration occurs as observed for F6PAMHEMA1 in Fig. 6e athigher magnification.

4PAMHEMA1, (d) F6PAMHEMA1, (e) F6PAMHEMA1 at higher magnification (1.5k).

0

200

400

600

800

1000

1200

1400

1600

1800

0 200 0 400 0 600 0 800 0 1000 0 1200 0 1400 0

PAMHEMA1

F2PAM HEMA1

F4PAM HEMA1

F6PAM HEMA1

1300

1500

1700

0 2 4 6 8Filler % in gel

Eq

uil

ibriu

m s

well

ing

(%

)

Swell ing time (min)

Wa

ter

up

tak

e (%

)

Fig. 7. Variation of swelling of the PAMHEMA1 hydrogel and ES% of the filled

hydrogels with filler%.


3.2.4. Mechanical properties

The tensile strength (T.S.) and elongation at break (E.A.B.) of theunfilled and composite gels are shown in Table 1. It is observed thatwith increase in amount of HEMA in the copolymer T.S. decreaseswhile E.A.B.% increases from PAMHEMA10 to PAMHEMA1.PolyHEMA is a low Tg soft polymer. Thus, with increase in itsamount T.S. decreases while E.A.B. increases. As this PAMHEMA gelis filled with filler, TS increases from F2PAMHEMA1 to F6PAM-HEMA1. Due to increased stiffness, E.A.B. is observed to decreasewith increasing filler loading in the composite gels.

3.3. Study of swelling properties of the hydrogels

The water uptake of initially dry hydrogels was measuredgravimetrically for a period of time using Eq. (1). Equilibriumswelling% (ES%) at varied comonomer ratios, total monomer,initiator and crosslinker concentration and filler loading wasdetermined from the respective swelling curve (not shown). Interms of swelling characteristics the optimized reaction condi-tions, i.e. comonomer ratio, total monomer, crosslinker andinitiator concentration for synthesizing hydrogels, were found tobe 1:1, 20%, 0.5% and 0.25%, respectively. The filled or compositehydrogels were synthesized at these reaction conditions by in situ

incorporation of varied dosages of the hydrophilic filler in thereaction mixtures of monomers, initiator and crosslinker.

3.3.1. Effect of filler incorporation on swelling

Effect of filler incorporation on water uptake % and ES% is shownin Fig. 7. From Fig. 7 it is observed that the filled compositehydrogels show higher water absorption than the unfilledPAMHEMA1 gel. For these composite gels initially with increasing% of filler water uptake% or ES% increases due to increasedhydrophilicity of the gel. The hydrophilic filler contributes to thisincreased hydrophilicity. However, it also fills the network andabove 4% filler loading this network filling more than compensatesthe effect of increased hydrophilicity resulting in decrease inswelling or ES%. The inorganic aluminosilicate fillers present in thehydrogel network act as additional network to reduce waterabsorption [31]. From Fig. 7 it is also observed that time forequilibrium swelling of filled hydrogels is much shorter thanunfilled PAMHEMA1 hydrogel. In this case both functional groupsof polymer as well as hydrophilic filler take part in waterabsorption and the network structure of gel filled with hydrophilicfillers takes much shorter time to reach saturation.

3.3.2. Studies of networking in the hydrogels

The size of gel network is usually characterized in terms ofaverage molecular weight between crosslinks, Mc and mesh size, z,measured by neutron scattering or quasielastic light scattering[11]. Mc is obtained from the following equation based on networktheory of Flory and Rehner [32]

Mc ¼ � VSrp V1=3p �V p

2

� �lnð1�V pÞþV pþxV2

p(3)

Table 2Network parameter of the superabsorbent copolymer hydrogels.

Name of the

polymer

Density of

polymer, g/mL

Volume fraction

of polymer in

swollen gel

Polymer-water

interaction

parameter, x

PAMHEMA1 2.802 0.0235 0.508

PAMHEMA5 2.758 0.0262 0.509

PAMHEMA10 2.749 0.0287 0.510

F2PAMHEMA1 2.835 0.0227 0.506

F4PAMHEMA1 2.842 0.0209 0.507

F6PAMHEMA1 2.857 0.0233 0.508

Here Vs is molar volume of solvent (water), Vp is volume fractionof polymer in the swollen gel under equilibrium, rp is density of thepolymer and x is polymer–solvent (water) interaction parameter.The density of the polymer sample was measured by the methodreported elsewhere [33]. For equilibrium swelling of mw g water/g dry hydrogel sample, Vp, x and f were calculated using thefollowing Eqs. (4)–(6), respectively [11].

V p ¼1=rp

ðmw=0:99Þþð1=rpÞ(4)

x ¼ V p

3 þ 0:5 (5)

and

f ¼ 1V p

(6)

The crosslink density in the network, rc, was calculated as

rc ¼ M0Mc

(7)

where, M0, the molar mass per crosslink is defined as

M0 ¼ nAMMAMþnHEMAMHEMAnAMþnhHEMA

(8)

Here n1 and M1 are number of moles and molecular mass of 1.The mesh size (& in A0) of the swollen polymeric network wascalculated from the following Eq. (9) [28].

& ¼ 2CnrC

h i1=2lV�1=3

s (9)

The Flory’s characteristic ratio, Cn was taken from the literature[27] and C–C bond length, ‘l’ was assumed as 1.54 8A. The values ofall of these network parameters of the three unfilled and threefilled copolymer hydrogels synthesized with total monomerconcentration of 20 mass%, crosslinker concentration of

Swelling

ratio, wAverage molar

mass between

crosslink, Mc

Crosslink

density, rc� 109

Mesh size of

network, &, A0

42.577 1.8 � 108 6.03 8.56 � 104

38.219 1.2 � 108 7.94 7.46 � 104

34.904 0.86 � 108 10.41 6.51 � 104

43.983 2.1 � 108 5.17 9.25 � 104

47.849 2.9 � 108 3.29 11.59 � 104

42.903 1.9 � 108 4.72 9.68 � 104

Table 3Diffusion exponent and diffusion constants of the superabsorbent copolymer hydrogel.

Name of the polymer Diffusion exponent, ‘n’ Diffusion coefficient, k � 102 Regression coefficient (r2)

PAMHEMA1 0.506 2.13 0.9838

PAMHEMA5 0.504 2.46 0.9991

PAMHEMA10 0.467 2.35 0.966

F2PAMHEMA1 0.523 1.88 0.9964

F4PAMHEMA1 0.504 2.15 0.9967

F6PAMHEMA1 0.532 1.76 0.9947

0.14

0.16

0.18

70

80

90

100

g/g

gel

)


0.50 mass% and initiator concentration of (of total monomermass) 0.25 mass% were calculated using Eqs. (3)–(9) and thesevalues are given in Table 2. From Table 2 it is observed that withincreasing amount of HEMA from PAMHEMA10 to PAMHEMA1,Mc as well as & increases. This effect is accompanied with decreasein crosslink density, rc and increase in swelling ratio, w of thehydrogel from PAMHEMA10 to PAMHEMA1. In fact, the size ofHEMA monomer is bigger than AM. Thus, with increase in amountof HEMA from PAMHEMA10 to PAMHEMA1, Mc or mesh size & ofthe network increases. From the data given in Table 2 it is alsoobserved that the composite hydrogels show higher Mc and & andhence a little lower rc and marginally higher w in comparison tounfilled hydrogels.

3.3.3. Studies of diffusion of water through the hydrogels

The penetration of water or soluble dye in to the gel network isgoverned by diffusion due to concentration gradient followed byrelaxation of the gel network by swelling. Depending on relativediffusion and relaxation the initial (up to 60% of the swelling data)fractional water uptake (F) by the hydrogel network can be roughlyexpressed [2] by the following Eq. (10).

F ¼ mtma¼ ktn (10)

or

ln F ¼ ln k þ n ln t (11)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 200 0 400 0 600 0 800 0 1000 0 1200 0 1400 0

PAMHEMA1RB

PAMHEMA1MB

F4PAMHEMA1RB

F4PAMHEMA1MV

0

50

100

150

200

250

300

350

0 10 0 20 0 30 0 40 0Time (mi nute)

Dye

ad

sorb

ed (

mg/g

of

gel

)

Time (min)

Dy

e a

dso

rbed

(m

g/g

of

gel

)

Fig. 8. Effect of contact time for 1 mg/L and 200 mg/L (shown in inset) feed dye

concentration.

Here mt and ma are mass of water absorbed at time t and atequilibrium, respectively; ‘n’ is diffusional exponent and ‘k’ isdiffusion coefficient. Here ‘k’ is related to structure of the gel and ‘n’is related to kind of transport through the gel. When diffusion ismuch slower than relaxation during swelling, i.e. for Case Idiffusion, n is 0.5 and k would be proportional to diffusioncoefficient over the initial sorption experiment. The swellingexponents ‘n’ and ‘k’ were calculated by plotting ln F versus ln t ofEq. (11). The values of ‘n’ and ‘k’ as obtained from slope andintercept of these trendlines along with respective regressioncoefficients are given in Table 3. From Table 3 it is observed that forall of the hydrogels the values of ‘n’ are very close to 0.5, i.e. thesehydrogel shows case-I diffusion. This may be ascribed to extensiveswelling of these superabsorbent hydrgels for which relaxationdue to extensive swelling of the polymers dominates diffusion ofwater in the network resulting in case-I diffusion. The values ofdiffusion coefficients of the hydrogels are also given in Table 3.

3.4. Study of dye removal capacity of the hydrogels

The PAMHEMA1 gel showing the highest ES% among all thethree unfilled gels and the F4PAMHEMA1 composite gel (contain-ing 4% filler) showing the highest ES% among all the three filled gels

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.5 1 1.5 2 2.5 3 3.50

10

20

30

40

50

60

PAMHEMAMVPAMHEMARBFPAMHEMARBFPAMHEMAMV

PAMHEMAMVremvPAMHEMARBremvFPAMHEMARBremv

0

50

100

150

200

250

300

350

0 10 0 20 0 30 0 40 0 50 0 60 0

0

20

40

60

80

100

120

PAMHEMAMVPAMHEMARBFPAMHEMARBFPAMHEMAMVPAMHEMAMVremvPAMHEMARBremvFPAMHEMARBremv

AM

Feed concentration of dye, mg/L

Dye

ad

sorp

tion

(m

Feed concentration of dye , mg/L

Dye

ad

sorp

tion

(m

g/g

gel

)

Rem

oval%

Rem

ov

al%

Fig. 9. Variation of dye adsorption of the unfilled and composite hydrogels with

initial concentration of dye in water for RB and MV dye: (a) low concentration range

(0.5–3 mg/L) and (b) high concentration range (50–500 mg/L).


were used for removal of rhodamine B and methyl violet dyes forboth low (0.25–3 mg/L) and high (50–500 mg/L) concentrationrange in water.

3.4.1. Effect of contact time

The effect of contact time on adsorption capacity of unfilledPAMHEMA1 and filled F4PAMHEMA1 hydrogels for rhodamine Band methyl violet dye at feed concentration of 1 mg/L and200 mg/L dye is shown in Fig. 8a and b, respectively. From thisfigure it is observed that like swelling in pure water the filledhydrogel polymers take much shorter time than the unfilledhydrogel polymer to reach the equilibrium adsorption value fordye adsorption. It is also observed that for both unfilled andfilled hydrogel equilibrium dye adsorption occurs much faster athigher feed concentration (200 mg/L, Fig. 8b). Dye adsorption byhydrogel is governed by film diffusion of dye moleculesfrom solution to surface of the hydrogels followed by porediffusion into the interior of the hydrogel [22]. At higherconcentration range mass transfer resistance for transport of dyemolecules is reduced and thus equilibrium time is reached muchfaster.

3.4.2. Effect of feed concentration of dye in water

The variation of dye adsorption by these two gels with feedconcentration of rhodamine B and methyl violet dyes is shownin Fig. 9a for low (0.25–3 mg/L) and Fig. 9b for high range (50–500 mg/L) of feed concentration. The removal% of the dyemolecules by the hydrogels is also shown in the same figures. Asthe concentration of dye increases, mass transfer resistance ofthe dye molecules between liquid (water) and solid (hydrogel)phase decreases. Hence, adsorption of the dye molecules by thehydrogels increases with increase in initial dye concentration inwater as observed in Fig. 9a or b. However, an adsorbent like thehydrogels used in this study can adsorb a fix amount of dye andthen it becomes saturated. Thus, above a certain initialconcentration of dye (around 1.5 mg/L for low concentrationrange, Fig. 9a and around 300 mg/L for high concentration range,Fig. 9b) there is no further increase in dye adsorption. Removal%for the dye is also observed to increase initially with feedconcentration and reaches maximum at this concentration.Removal% of filled gel is observed to be higher than unfilled gelfor both rhodamine B and methyl violet dye. For lowconcentration range, the maximum removal% from unfilled tofilled hydrogel is increased from around 79% to 85% for methylviolet and 86% to 92% for rhodamine B dye. Similar trend is alsoobserved for high concentration range of rhodamine B andmethyl violet dye. In this case removal% also increasessignificantly from unfilled to filled hydrogel. The differencesin adsorption capacity of the hydrogels for two different dyesshould be caused by the chemical interaction of functionalgroups of the dye and gel molecules. The much higheradsorption of rhodamine B dye than methyl violet dye overthe entire feed concentration may be ascribed to the presence ofcarboxylic group in the structure of rhodamine B dye (Fig. 1).This functional polar group shows strong interaction withhydroxy and amide group of the gel. Between the two dyemolecules the polarity of methyl violet is less than that ofrhodamine B in that it contains only a substituted amine group(Fig. 1). It is also observed from the figure that the filledcomposite gel shows much higher adsorption than the unfilledgel signifying interaction of hydrophilic filler with functionalgroups of dye molecules.

3.4.3. Adsorption kinetics and isotherm

Kinetics: The kinetics of dye adsorption by the unfilled andcomposite hydrogel polymers were determined by (1) pseudo first

order equation of Lagergren (Eq. (12)), (2) pseudo second orderequation of Ho and McKay (Eq. (13)), (3) intra-particle diffusionequation of Weber and Morris [34] (Eq. (14)) as given below

(1) Pseudo first order equation

Qt ¼ Qe 1 � expð�k1tÞ½ � (12)

(2) Pseudo second order equation

Qt ¼Q2

e k2t

1 þ k2Qet(13)

(3) Intra-particle diffusion model

Qt ¼ kidt0:5 þ A (14)

where Qt and Qe are dye adsorbed at time t and at equilibrium,respectively, k1, k2 and kid are rate constant for pseudo first order(min�1), second order (g/mg min) and intra particle diffusion (mg/g min0.5), respectively. The value of intercept ‘A’ in Eq. (14)indicates thickness of boundary layer. The greater the value of A,the higher will be the contribution of surface sorption to ratelimiting step for adsorption [35]. The various kinetic parameters,i.e. Qe, k1, k2, kid, and A were determined by directly fitting Qt and t

in Eqs. (12)–(14).Isotherm: The experimental adsorption equilibrium concentra-

tion of dye in unfilled and filled hydrogels (Qe) for different initialfeed concentrations of dye (Ce) was directly fitted to two parameternonlinear Langmuir (Eq. (15)) three parameter non linear Sip(Eq. (16)) and four parameter non linear Fritz–Schlunder (Eq. (17))equations. The Langmuir adsorption model is given by

Qe ¼ QmaxKLCe

1þKLCe(15)

where Ce is equilibrium concentration of dye in solution, Qmax is themonolayer capacity of the adsorbent hydrogels (mg/g) and KL isLangmuir equilibrium constant (dm3/g). The Langmuir isothermcan also be expressed in terms of a dimensionless parameterseparation factor RL as

RL ¼1

1 þ KLCi(15a)

Here, Ci is initial concentration of dye in water. For favorableLangmuir isotherm the values of RL should be between 0 and 1 [14].

Similarly, the three parameter Sip model is given by

Qe ¼ KSCbSe

1þASCbSe

(16)

where KS, AS and bS are Sips constant. At low concentration of dye,it becomes Freundlich isotherm while at higher concentration itshows a mono layer adsorption similar to Langmuir isotherm. Mostof the adsorption models are combined in the following general-ized four parameter Fritz–Schlunder model (FS)

Qe ¼ AFSCae

1þBFSCbe

(17)

where AFS, BFS, a and b are FS constant. The experimentalequilibrium dye concentration, Qe was plotted against contact time(t) (for kinetic parameters) and feed dye concentration (Ce, forisotherm parameters) with non linear fitting of Eqs. (12)–(17)using Origin-8 software based on Levenberg–Marquardt (L–M)algorithm. In L–M algorithm parameter values of the model areadjusted in an iterative process using Chi square. The validity of thekinetic and adsorption isotherm models were evaluated in terms ofregression coefficient (R2), non Linear Chi square test (x2) and F

values (obtained from Anova analysis). For a good fitting, R2 shouldbe close to unity, x2 will be low while F value will be high [36]. Thevarious kinetic and isotherm model parameters along withregression coefficient, chi square and F values are shown in

Table 4Kinetic parameters of the unfilled and filled hydrogels for low and high concentration of MV and RB dye.

Model PAMHEMAMV low/high PAMHEMARB low/high FPAMHEMAMV low/high FPAMHEMARB low/high

First order

Qeth (mg/g) 0.124/275 0.133/268 0.146/256 0.135//221

Qeexpt (mg/g) 0.1356/195 0.1451/234 0.1591/284 0.1467/271

k1 (min�1) 0.009/0.0123 0.011/0.012 0.0209/0.005 0.0178/0.0159

R2 0.9275/0.9068 0.9161/0.9114 0.9007/0.9732 0.8893/0.9174

x2 0.0001/589 1.14E�04/530 1.88E�4/112.64 1.8E�04/349

F value 625/446 593/459 582/987 502/562

Pseudo second

Qeth (mg/g) 0.128/392 0.136/265 0.148/341 0.137//335

Qeexpt (mg/g) 0.1356/195 0.1451/234 0.1591/284 0.1467/271

k2 (g/mg min) 0.109/9.658E�06 0.138/7E�05 0.273/3.9E�5 0.244/3.7E�5

R2 0.9620/0.9720 0.9577/0.9378 0.9407/0.9378 0.9432/0.9387

x2 6.01E�05/117 7.14E�5/185 1.12E�4/393 9.3E�05/366

F value 1200/950 1180/1066 978/671 975/667

Intra particle

kp (mg/g min1/2) 1.1E�04/12.89 8.4E�05/12.98 8.6E�05/15.94 6.5E�05/15.50

A 0.081/�17.99 0.094/27.72 0.0996/21.20 0.086/18.47

R2 0.7694/0.9570 0.8040/0.9665 0.9107/0.9776 0.9464/0.9167

x2 1.3E�04/181 9.5E�05/141 5.05E�05/141 2.9E�5/139

F value 531/612 885/1396 2183/1881 3109/1767


Table 4 for kinetic parameters and Table 5 for isotherm parametersfor adsorption of low and high concentration of rhodamine B andmethyl violet dye by the unfilled PAMHEMA and compositeF2PAMHEMA hydrogel. From Table 4 it is observed that theadsorption of dye follows more closely a second order kineticssince in this case, the theoretical equilibrium values are close toexperimental values with regression coefficients >0.98. It is alsoobserved that low range of feed dye concentration gives betterfitting to kinetic equations than high range of concentration. Thismay be due to much longer equilibrium time of the low range offeed dye concentration. The non-linear data fitting to pseudosecond order kinetic equation for low range of concentration of dye(1 mg/L) is shown in Fig. 10a which also shows close fitting. The

Table 5Adsorption isotherm and thermodynamic parameters of the hydrogels for low and hig

Model PAMHEMAMV high/low PAMHEMARB hi

Langmuir

KL (L/mg) 0.0014/0.4254 0.0018/0.4192

Qmax (mg/g) 564.75/0.267 581.34/0.289

R2 0.9734/0.9259 0.9696/0.9262

x2 (�104 for low) 168.34/1.7 270.78/1.91

F value 540/280 480/278

Sip

KS (L/mg) 0.1528/0.1922 0.0561/0.2051

b 1.357/2.295 1.622/2.306

AS (�104 for high) 4.41/1.286 1.67/1.277

R2 0.9741/0.9785 0.9791/0.9794

x2 (�105 for low) 163.56/4.85 185.87/4.85

371.5/651 467/673

FS

AFS (�109 for high) 2.21/0.5608 2/0.5839

a �2.93/0.9979 �2.40/�0.9739

BFS 105/5.98 3.93/5.77

b �3.95/�2.44 �3.49/�2.43

R2 0.9837/0.9909 0.9902/0.9913

x2 (�105 for low) 103.13/2.04 87.57/2.29

F value 443/1164 746/1195

�DG0 (kJ/mol)

303 K 14.75/9.02 14.76/9.24

313 K 14.79/9.09 15.33/9.67

323 K 14.85/9.23 15.90/10.1

DS0 (kJ K�1 mol�1) 0.054/0.041 0.05/0.043

DH0 (kJ K�1 mol�1) 1.61/3.49 2.49/3.81

values of rate constants as given in Table 4 are also comparable tothose given in literature [32] for similar feed concentration of dye.From Table 5 it is observed that the experimental dye adsorptionvalues fit well to all of these three models since in all of this nonlinear regression the values of regression coefficients (R2) aregreater than 0.95 with low x2 and high F values. The values of RL

(0.90–0.99) for all the hydrogels also signify favorable adsorptionof these two dyes. It is also observed that the values of the threecoefficients, i.e. Langmuir, Sip and FS along with maximummonolayer adsorption (Qmax) increases from unfilled PAMHEMA1to composite F2PAMHEMA1. The non linear direct fitting of lowrange of dye concentration to four parameter FS model is shown inFig. 10b which also shows close fitting of the experimental data.

h concentration of MV and RB dye.

gh/low FPAMHEMARB high/low FPAMHEMAMV high/low

0.0015/0.4432 0.0018/0.4449

821.72/0.3050 683.30/0.278

0.9773/0.9347 0.9687/0.9362

270.79/1.93 378.83/1.57

621/331 463/342

0.0806/0.2237 0.1653/0.2034

1.574/2.157 1.663/2.125

1.80/1.269 1.38/1.257

0.9858/0.9791 0.9799/0.9787

200.88/6.18 242.63/6.18

667.21/695 484.13/686

8.07/0.6370 3.31/0.5934

�2.57/�0.9462 �2.47/�0.9521

133/5.72 6.10/5.84

�3.66/�2.33 �3.57/�2.31

0.9951/0.9896 0.9920/0.9889

69.16/3.07 96.92/2.72

1457/1052 911/993

15.44/9.62 14.95/8.99

16.01/10.05 15.00/9.138

16.60/10.47 15.10/9.22

0.05/0.042 0.05/0.041

2.03/3.24 2.5/3.67

0 2000 4000 6000 8000 10000 12000

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

tQk1

tkQQ

e2

2

2

et

k1

PAMHEMA1RB

PAMHEMA1MB

F4PAMHEMA1RB

F4PAMHEMA1MV

Dy

e a

dso

rp

tio

n Q

t (m

g/g

of

gel)

Time (minute)

a

b

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

e

e

CFS

B1

CFS

A

eQ

B1

PAMHEMAMV

PAMHEMARB

FPAMHEMARB

FPAMHEMAMV

Eq

uil

ibri

um

Dy

e a

dso

rpti

on

, Q

e (m

g/g

of

gel

)

Feed dye conc., Ce (mg/L)

α

β

Fig. 10. Fitting of experimental dye adsorption data to kinetic and isotherm model:

(a) second order kinetics with dye concentration of 1 mg/L and (b) Fritz–Schlunder

model with low concentration range.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 0.5 1 1.5 2 2.5 3 3.5

ad

sorp

tio

n,

Qe (

mg/g

gel

)

FPAMH EMAMVexptFPAMH EMAMVl angmuirFPAMH EMAMVSIPFPAMH EMAMVFSFPAMH EMARBexptFPAMH EMARBlangmuirFPAMH EMARBSIPFPAMH EMARBFS

0

50

100

150

200

250

300

350

400

450

500

0 50 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0

ad

sorp

tio

n,

Qe (m

g/g

gel

)

FPAMHEMAMVexptFPAMHEMAMVlang muirFPAMHEMAMVS IPFPAMHEMAMVFSFPAMHEMARBexptFPAMHEMARBlang muirFPAMHEMARBS IPFPAMHEMARBFS

Equilibrium conc. of dye in water , Ce (mg/L)

Equil ibri um conc. of dye in water , Ce (mg/L)

Exp

erim

enta

l an

d c

alc

ula

ted

Dye

Exp

erim

enta

l a

nd

calc

ula

ted

Dye

a

b

Fig. 11. Fitting of experimental data to adsorption isotherm for composite hydrogel:

(a) low concentration range and (b) high concentration range.


The calculated dye adsorption values based on these threeadsorption models are plotted against experimental adsorptionvalues of the composite hydrogel for low and high concentrationrange in Fig. 11a and b, respectively. From these figures it is alsoclear that experimental dye adsorption values are also close tocalculated values based on these models.

3.4.4. Effect of temperature on dye adsorption and thermodynamic

parameters

The change in standard free energy (DG0), standard enthalpy(DH0) and standard entropy (DS0) during dye adsorption may beobtained from the following Eqs. (18)–(20):

DG ¼ DG0 þ RT ln Kd (18)

where Kd is distribution coefficient which can be calculated fromequilibrium dye adsorption (Qe) and equilibrium feed dyeconcentration (Ce) as [37]

Kd ¼Qe

Ce(18a)

At equilibrium; DG ¼ 0 and hence; DG0 ¼ �RT ln KF (18b)

The effect of temperature on distribution or thermodynamicconstant may be expressed as [37]

d ln Kd

dT¼ DH0

RT2(19)

Integrating, rearranging and assuming DH0 is independent oftemperature

ln Kd ¼DH0

RTþDS0

R(20)

The values of DH0 and DS0 are obtained from slope andintercept, respectively, of the linear trendline of ln Kd againstinverse of absolute temperature (1/T). Kd at three differenttemperatures of 30, 40 and 50 8C were obtained from dyeadsorption at feed dye concentration of 300 mg/L (high) and3 mg/L (low), respectively, using Eq. (18a). At higher tempera-ture the hydrogels showed increase in dye adsorption signifyingexothermic nature of this adsorption. The values of DG0 at thethree temperatures for low and high feed concentration areshown in Table 5 for both unfilled and composite gels. Thenegative values of DG0 confirm feasibility and spontaneity of thedye adsorption process. The values of DG0 also lie in between�20 and 0 kJ/mol indicating physisorption of this dye adsorptionprocess [38]. The positive values of DH0 also confirms theexothermic nature of the adsorption. From Table 5 it is alsoobserved that DS0 is positive which implies increase inrandomness in the solid (gel)–dye solution interface duringadsorption. The values of various thermodynamic parameters asshown in Table 5 are also comparable to those reportedelsewhere [35,37,38].

Table 6Comparison of present work with reported data.

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

Poly(HEMA-g-GMA) Resin 1000 mg/L of crystal violet at pH 7 76.8 [24]

Poly(AA-co-AM) as adsorbent 200–1000 mg/L of methyl violet at pH 7 917 for 1000 mg/L feed [38]

Composite poly(AA-co-VP) 40 mg/L of crystal violet at pH 7 4.1 [40]

Supramolecular and

composite gel of agarose

1000 mg/L of methyl violate at pH 7 Removal efficiencies

were 95.1 and 95.7% for

supramolecular gel and

hybrid gel, respectively.

[41]

Poly(AM-co-AA) 50 mg/L of methyl violet at pH 7 6.38 [42]

P(VP/MA) hydrogels 500 mg/L of methyl violet at pH 7 4.22 [43]

Jute stick 50 mg/L of rhodamine B at pH 7 4.6 [44]

PAMHEMA1 Methyl violate (MV) & rhodium B

(RB) 0.25–3 mg/L at pH 7

0.135 & 0.146 for MV &

RB dye at 1.5 mg/L feed

conc. with 78.7 & 85.7%

removal for MV& RB

Present work

PAMHEMA1 MV & RB 50–500 mg/L at pH 7 220 & 257 for MV & RB

dye at 300 mg/L feed conc. with 73 &

85% removal for MV& RB

Present work

F4PAMHEMA1 MV & RB 0.25–3 mg/L at pH 7 0.146 & 0.159 for MV & RB

dye, 84.9 & 91.6% removal

for MV & RB at 1.5 mg/L

Present work

F4PAMHEMA1 MV & RB 50–500 mg/L at pH 7 278 & 288 for MV & RB dye at 300 mg/L

feed conc. with 93 & 96%

removal for MV & RB

Present work

AA, acrylic acid; VP, vinyl pyrolidone; HEMA, hydroxyethylmethacrylate; GMA, glycedylmethacrylate; AM, acrylamide; MA, methacrylic acid.

Fig. 12. Photograph of the dry and swollen hydrogel: (a) dry hydrogel, (b) hydrogel

swollen with solution of MV, (c) hydrogel swollen with solution of RB.


3.4.5. Reusability of the hydrogels

The economic feasibility and reusability of the unfilled andfilled hydrogel was evaluated by desorption experiments in a batchsystem similar to adsorption experiment with dye loadedhydrogels. Almost complete desorption (up to 97.3%) was observedin 0.1 molar nitric acid. This result indicate that rhodamine B andmethyl violet dye was bonded to the hydrogel by electrostaticinteraction [39]. Five consecutive adsorption–desorption cycleswere used with the regenerated hydrogels without any significantdecrease in adsorption capacity signifying efficient reusability ofthe hydrogels.

3.4.6. Comparison with reported results

The performance of various polymeric hydrogels used forremoval of different dyes including rhodamine B and methyl violetfrom water is compared with the performance of the presenthydrogels for removal of these dyes in Table 6. It is observed fromTable 6 that most of the reported work used much higherconcentration (�50–1000 mg/L) than which is usually found intextile waste water. However, the present composite gel showshigh removal% for both low and high range of feed concentration.The results are also comparable to those reported with otherhydrogels as shown in Table 6. The photograph of the dry hydrogel,methyl violet dye swollen gel and rhodamine B dye swollen gel areshown in Fig. 12a–c, respectively.

4. Conclusions

A systematic method was used to synthesize several unfilledand composite hydrogels at varied reaction parameters. Thehydrogels were characterized by FTIR, DSC, SEM, swellingcharacteristics, network parameters and mesh sizes. A systematicmethod was used to optimize the reaction parameters in terms ofswelling characteristics of the resulting gels. One unfilled and onecomposite gel showing the highest swelling was used for removalof low and high concentration of rhodamine B and methyl violetdye from water. The hydrogels showed very high removal% forboth low and high feed concentration of dye in water. Thecomposite gels showed higher removal% and dye adsorption thanthe unfilled hydrogels. The hydrogels were also found to follow apseudo second order kinetics for adsorption of dyes. The


experimental dye adsorption values were found to fit well to twoparameter Langmuir, three parameter Sip and four parameterFritz–Schlunder models. Instead of linearization, the modelequations were directly used for fitting of experimental adsorptiondata by an iterative method using Levenberg–Marquardt (L–M)algorithm. Various thermodynamic parameters for these dyeadsorption confirmed spontaneity, feasibility and exothermicnature of the adsorption. Regenerated hydrogels were also foundto show high adsorption indicating economic feasibility of thehydrogels. These hydrogels may also be suitably used forconcentration of aqueous solutions of different chemicals as wellas for removal of low concentration of water soluble dyes similar tothose used in this study.

Acknowledgments

The authors are grateful to Department of Science andTechnology (DST-SERC), Govt. of India (SR/S3/CE/056/2009) forsponsoring this work.

References

[1] L.F. Buchholz, T. Graham, Modern Superabsorbent Polymer Technology, Wiley,New York, 1998.

[2] Y.M.P.S. Mohan, K.K. Murthy, M. Raju, Reactive & Functional Polymers 63 (2005)11.

[3] K. Kabiri, H. Omidian, S.A. Hashemi, M.J. Zohuriaan-Mehr, European PolymerJournal 39 (2003) 134.

[4] P.S.K. Murthy, Y.M. Mohan, K.M. Rao, K.M. Raju, International Journal of PolymericMaterials 54 (2005) 899.

[5] J. Chen, Y.J. Zhao, Journal of Applied Polymer Science 75 (2000) 808.[6] W.F. Lee, R.J. Wu, Journal of Applied Polymer Science 62 (1996) 1099.[7] W.F. Lee, R.J. Wu, Journal of Applied Polymer Science 64 (1997) 1702.[8] E.S. Rufino, E.E.C. Polymer 41 (2000) 4213.[9] W.F. Lee, P.L. Yeh, Journal of Applied Polymer Science 64 (1997) 2371.

[10] D.W. Lim, K.G. Song, K.J. Yoon, S.W. Ko, European Polymer Journal 38 (2002) 579.[11] W. Xue, S. Champ, M.B. Huglin, Polymer 42 (2001) 3665.[12] D. Saraydin, E. Karadag, O. Guven, Journal of Applied Polymer Science 79 (2001)

1809.[13] N.K. Goel, V. Kumar, Y.K. Bhardwaj, S. Sabharwal, Journal of Macromolecular

Science – Pure and Applied Chemistry 43 (2006) 327.

[14] G. Xu, G.Y. Wu, Li, Special Petrochemistry 1 (2002) 42.[15] R.G. Marcos, V.R. Adriano, H.T. Suelen, F.R. Adley, P.A.F. Judith, C.M. Edvani,

Carbohydrate Polymers 61 (2005) 464.[16] O. Baris, U. Zum, E. Karadag, Polymer-Plastics Technology and Engineering 45

(2006) 1277.[17] A.T. Kikuchi, Advanced Drug Delivery Reviews 54 (2002) 53.[18] W.F. Lee, Y.C. Chen, Journal of Euro Polymers 41 (2005) 1605.[19] A. Li, R.F. Liu, A.Q. Wang, Journal of Applied Polymer Science 98 (2005) 1351.[20] J. Lin, J. Wu, Z. Yang, M. Pu, Macromolecular Rapid Communications 22 (2001)

422.[21] P. Vandezande, X. Li, L.E.M. Gevers, I.F.J. Vankelecom, Journal of Membrane

Science 330 (2009) 307.[22] P. Das, S.K. Ray, S.B. Kuila, H.S. Samanta, N.R. Singha, Separation and Purification

Technology 81 (2011) 159.[23] N.R. Singha, T.K. Parya, S.K. Ray, Journal of Membrane Science 340 (2009) 35–44.[24] G. Bayramoglu, B. Altintas, M.Y. Arica, Chemical Engineering Journal 152 (2009)

339.[25] T. Qunwei, X. Sun, Q. Li, J. Wu, J. Lin, Journal of Colloid and Interface Science 339

(2009) 45.[26] R.C. Schulz, in: H.F. Mark (Ed.), Encyclopedia of Polymer Science and Engineering,

vol. 1, Wiley, New York, 1985, p. 169.[27] S. Ray, S.K. Ray, Industrial and Engineering Chemistry Research 45 (2006) 7210.[28] G. Odian, Principle of Polymerization, 3rd ed., John Wiley & Sons Inc., Singapore,

1991, pp. 198–334.[29] R.S. Tomar, I. Gupta, R. Singhal, A.K. Nagpal, Polymer-Plastics Technology and

Engineering 46 (2007) 481.[30] S. Ray, S.K. Ray, Journal of Membrane Science 270 (2006) 73.[31] A. Li, A. Wang, European Polymer Journal 41 (2005) 1630.[32] Z.Y. Ding, J.J. Akloins, R.J. Salovey, Polymer Science Part B: Physics 29 (1991) 1035.[33] J. Chen, H. Park, K. Park, Journal of Biomolecular Materials Research 44 (1999) 53.[34] M.M. Perju, E.S. Dragan, Ion Exchange Letters 3 (2010) 7.[35] Y. Yi Liu, A. Zheng, Wang, Journal of Environmental Science 22 (4) (2010) 486.[36] K.Y. Foo, B.H. Hameed, Chemical Engineering Journal 156 (2010) 2.[37] B. Kayranli, Chemical Engineering Journal 173 (2011) 782.[38] Y. Wang, L. Zeng, X. Ren, H. Song, A. Wang, Journal of Environmental Science 22 (1)

(2010) 7.[39] I.D. Mall, V.C. Srivastava, G.V.A. Kumar, I.M. Mishra, Colloids and Surfaces A 278

(2006) 175.[40] L.M. Zhang, Y. Zhou, Y.E. Wang, Journal of Chemical Technology and Biotechnol-

ogy 81 (2006) 799.[41] J. Wang, H. Wang, Z. Song, D. Kong, X. Chen, Z. Yang, Colloids & Surfaces B:

Biointerfaces 80 (2010) 155.[42] D. Solpan, S. Duran, D. Saraydin, O. Guven, Radian. Physics & Chemistry 66 (2003)

117.[43] D.S. Olpan, Z.K. Lge, Radian. Physics & Chemistry 75 (2006) 120.[44] G.C. Panda, S.K. Das, A.K. Guha, Journal of Hazardous Materials 164 (2009) 374.

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