European Polymer Journal 49 (2013) 4265–4275

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European Polymer Journal

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Hydroxyethyl Starch-g-Poly-(N,N-dimethylacrylamide-co-acrylic acid): An efficient dye removing agent

0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.10.012

⇑ Corresponding author. Tel./fax: +91 3222 275847.E-mail address: tridib_tripathy@yahoo.co.in (T. Tripathy).

Haradhan Kolya, Tridib Tripathy ⇑Postgraduate Division of Chemistry, Midnapore College, Midnapore, Paschim Medinipur 721101, West Bengal, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 August 2013Received in revised form 10 October 2013Accepted 11 October 2013Available online 23 October 2013

Keywords:Graft copolymerizationDye removalN,N-dimethylacrylamideAcrylic acidHydroxyethyl starchMalachite green

Hydroxyethyl Starch-g-Poly-(N,N-dimethylacrylamide-co-acrylic acid) was synthesized bysolution polymerization technique using potassium peroxydisulfate (K2S2O8) as the initia-tor at 90 �C. The synthesized graft copolymer was characterized by FTIR, NMR (both 1H and13C) Spectroscopy, molecular weight determination by GPC, TGA/DTG and SEM analysis.Biodegradation study was carried out by enzymatic hydrolysis. The number of carboxylicacid groups incorporated into the polymer was calculated by measuring neutralizationequivalent (N.E) of the graft copolymer titrimetrically. The synthesized graft copolymerwas used as the adsorbent for the removal of Malachite green, a cationic dye from its aque-ous solution. The operating variables studied were the amount of adsorbent, solution pH,contact time, temperature and the initial dye concentration. The adsorption data were usedto fit in the pseudo-first order and pseudo-second order rate equation in order to investi-gate the sorption mechanism. Equilibrium isotherm was analyzed using the Langmuir andthe Freundlich isotherms. In the present investigation it was found that the adsorptionkinetics followed a pseudo second order kinetics for the studied dye concentration range.The negative value of free energy change indicates the spontaneous nature of the adsorp-tion and also suggesting a chemisorption process.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The effluents from all the textile industry contain dyesubstances which are toxic to aquatic life and are generallynon-biodegradable [1]. Therefore, there is a need to devel-op technologies that can remove the colour substancesfrom the wastewater stream of the dying industries. Thereare large numbers of commercial dyes available in themarket, which are very much stable and difficult to deco-lourize due to their complex structures [2]. Reactive andacidic dyes are mainly used in textile industries due totheir bright colours and colour fastness [3]. Most reactivedyes are azo compounds where the aromatic rings are con-nected by azo bridges. Such azo dyes are toxic to someorganisms and represent an increasing environmental dan-

ger [4]. Of the various dyes, malachite green (MG) is widelyused for dyeing cotton, wool, silk, paper and leather. Butthis dye is harmful due to its toxicity and carcinogenic ef-fects [5,6]. It is known to be a liver tumor promoter to themammals [7]. Despite of its various toxic effects MG is stillextensively used in aquaculture and other industries.

Various traditional and advanced technologies havebeen utilized for the removal of colourants from the col-oured effluents, such as adsorption, nanofiltration bymembranes, biological treatment, flocculation and oxida-tion [8–10]. Adsorption techniques are widely used forthe dye removal because it has a specific advantage ofremoving the dye molecules completely [11]. For the re-moval of adsorbent from the waste water use of activatedcarbon is widely used throughout the world because of itsgreater capacity to absorb pollutants. However, commer-cial activated carbon has several disadvantages. It is quiteexpensive, and higher the quality the greater the cost.The regeneration procedure of saturated carbon is also

4266 H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275

expensive and results in loss of adsorbent. For this reasonthe search for the cost effective dye removing agents andeconomic method of removing dye from the waste waterhas also become the focus of many studies [12–15].

Developments of the polysaccharide based dye remov-ing agents or flocculating agents are of great interestsnow a day [16–18]. Natural polymers have the advantageof low cost and biodegradability. The biodegradability ofnatural polymers as such also acts as a drawback in thatit reduces the storage life and the flocculation or adsorp-tion performance [19]. Moreover, because of the variationin the composition of natural polymers from source tosource, their performance varies widely [20]. On the otherhand, synthetic polymers are more efficient and can be tai-lored to the needs of a particular application. Howevertheir biggest disadvantages are their shear degradabilityand non-biodegradability [21]. Recently a new class of floc-culating agents based on graft copolymers of natural poly-saccharides and synthetic polymers has been reported[22–24]. They essentially combine the best properties ofboth components. The graft copolymers are biodegradableto some extent and shear stable due to the presence ofpolysaccharide back bone.

Poly-(N,N-dimethylacrylamide) (PDMA) is highly watersoluble, biocompatible and its copolymer was used in oilrecovery [25,26]. In our previous study [27] we have car-ried out the synthesis, characterization and flocculationperformances of polyacrylamide (PAM) and poly-N,N-dim-ethylacrylamide (PDMA) grafted hydroxyethyl starch(HES) in some clay suspensions. It was concluded thatPDMA grafted HES showed better performance than thatof the PAM grafted HES because of the presence of morepolar –CONMe2 groups in grafted PDMA chains. It was alsofound that the partially alkaline hydrolyzed PAM graftedpolysaccharides in which both –CONH2 and –COONagroups are present showed better flocculation perfor-mances over the virgin graft copolymers [28]. The presentinvestigation is undertaken to incorporate both –COOHand –CONMe2 groups into the hydroxyethyl starch backbone by grafting a mixture of acrylic acid (AA) and N,N-dimethylacrylamide (DMA) onto the HES backbone, since–COOH groups cannot be generated from –CONMe2 groupby neither alkaline nor acidic hydrolysis [29]. Similar typeof graft copolymers were also reported for the removal ofmetal ions from the aqueous solution and also used as floc-culating agents [30,31].

The present article deals with the synthesis of a poly-meric flocculant to be used as a potent dye remover basedon HES and a mixture of DMA and AA. The graft copolymerwas prepared by the solution polymerization techniquewhere potassium perdisulfate (K2S2O8) was used as a rad-ical initiator. Various characterization techniques like1H NMR, 13C NMR, FTIR spectroscopy, thermal (TGA/DTGA)analysis, molecular weight determination by Gel Perme-ation Chromatography (GPC) method and SEM analysis. Atechnique has been developed for calculating the numberof –COOH groups of the graft copolymer by measuringneutralizing equivalent (N.E) trimetrically. A biodegrada-tion study was carried out by enzymatic hydrolysis. Evalu-ation of the effectiveness of the synthetic graft copolymeras flocculants for the colour removal was carried out in

water soluble synthetic dye suspension of malachite green(MG). The effects of variation of adsorbent mass, solutionpH, contact time and initial dye concentration on the sorp-tion capacity were evaluated. The sorption mechanism wasalso investigated using Langmuir and Freundlichisotherms. The equilibrium isotherm was analyzed andvarious thermodynamic parameters were also calculated.

2. Experimental

2.1. Materials

Hydroxyethyl starch (HES) and dimethylacrylamide(DMA) were procured from Aldrich Chemical Company,USA. Acrylic acid, potassium persulfate (K2S2O8), acetone,sodium nitrate, sodium hydroxide, dichloromethane(DCM), methanol, pet ether were procured from E. MerckLtd. Mumbai, India. Malachite green was procured fromLoba Chemie, Bombay, India. Doubly distilled water wasused for the synthesis. DMA and AA are vacuum distilledand the middle portion were used for the synthesis.

2.2. Synthesis and purification of the graft copolymer

2 g of hydroxyethyl starch was dissolved in 100 ml dis-tilled water with constant stirring of a slow stream ofnitrogen for about 20 min 8 ml of DMA and 8 ml AA wasadded to the HES solution. Then oxygen-free nitrogenwas purged through the solution for 30 min. At this stage0.10 g of K2S2O8 was added to the reaction mixture fol-lowed by further purging with nitrogen. The solution washeated to 90 �C and maintained throughout the reaction.The reaction was allowed to continue for 6 h. Then thereaction vessel was sealed and kept at room temperature(27 �C) for 24 h, after which it was terminated by addinga saturated solution of hydroquinone. At the end of thereaction the polymer was precipitated by adding DCMand acetone mixture (2:1). Then it was washed with meth-anol water (6:1 by volume) mixture to remove the occulat-ed homopolymers. After that it was re-precipitated byDCM and Pet ether solution (1:1 by volume). The polymerwas then dried in a hot air oven at 60 �C for 24 h, pulver-ized and sieved through a 125 lm sieve. The synthetic de-tails are given in Table 1.

2.3. Characterization of the graft copolymers

2.3.1. Determination of molecular weight and molecularweight distribution using gel permeation chromatography(GPC)

The molecular weight and molecular weight distribu-tion of the graft copolymer and the virgin polysaccharide(HES) were determined using GPC (Model: 2414, suppliedby water (I) Pvt. Ltd., USA). The flow rate was fixed at0.6 ml min�1 and the column temperature was kept at30 �C during the analysis. The results are shown in Figs. 1and 2 respectively.

Table 1Synthetic details of the graft copolymers.

Polymer Polysaccharide(g)

DMA(mol)

AA(mol)

Amount ofK2S2O8 (g)

% ofConversion

Graftingratio (%G)

Yield(g)

Mw

(Daltons)Nos. of –COOHgroups

HES-g-Poly-(DMA-co-AA) 2.0 0.078 0.117 0.1 88.18 810 16.2 12 � 106 48,023

% Of conversion is calculated from the relation.% Conversion = [(wt of graft copolymer – wt of polysaccharide)/amount of DMA + AA] � 100.%G = (wt. of grafted polymer/wt. of polysaccharide) � 100.

Fig. 1. The molecular weight and molecular weight distribution by GPC of HES.

H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275 4267

2.3.2. FTIR spectroscopyThe graft copolymer was subjected to FTIR spectral

analysis. A Perkin Elmer (Spectrum 2) spectrophotometerand the KBr pellet method was followed for spectralanalysis.

The IR spectrum of the graft copolymer is shown inFig. 3.

2.3.3. Thermal analysis (TG/DTG)The thermal analysis (TG/DTG) of the graft copolymer

was carried out with Stanton Red Croft (STA625) thermalanalyzer. TG/DTG analysis was performed upto a tempera-ture of 600 �C from the room temperature in an atmo-sphere of nitrogen. The heating rate was uniform in allcases at 10 �C min�1. The TG/DTG curves are shown inFig. 4.

2.3.4. NMR spectroscopy (1H and 13C)Both 1H and 13C NMR spectral analysis of all the graft

copolymers were carried out with a 500 MHz NMR instru-ment (JEOL, Tokyo, Japan) in D2O solvent at 25 �C. Thespectra are shown in Figs. 5 and 6.

2.3.5. Scanning electron microscopy (SEM) analysisScanning electron micrographs for HES and the synthe-

sized graft copolymer were shown in Fig. 7 respectively. A

Cam Scan Series-2 (Cambridge Scanning Company, UK)was used for SEM study.

2.3.6. Determination of the number of –COOH group bymeasuring neutralization equivalent (N.E.)

0.05 g of the synthesized graft copolymer was accu-rately weighted into a 100 ml of an Erlenmeyer flask. Thepolymer was dissolved in about 25 ml of water. The solu-tion was then titrated by 0.01 (N) NaOH solution conducto-metrically. From the conductometric titration graph(Fig. 8) volume of the NaOH required for neutralizationwas determined. The neutralization equivalent (N.E) wascalculated from the following equation [32].

N:E ¼ mg of acidNormality of NaOH�mL of NaOH added

ð1Þ

2.3.7. Biodegradation studies of the graft copolymer usingenzyme hydrolysis

Biodegradation of the graft copolymer was studied bythe enzymatic cleavage of the graft copolymer and alsothe virgin polysaccharide (HES) by the enzyme a-amylase.a-amylase is selective [33] for the cleavage of the a-glyco-sidic linkage of the polysaccharide thereby releasing theglucose residue. The percentage of glucose residue wasestimated by measuring optical density (O.D) values usingphotoelectric colorimeter. A typical procedure was as

Fig. 2. The molecular weight and molecular weight distribution by GPC of HES-g-Poly- (DMA-co-AA).

4000 3500 3000 2500 2000 1500 1000 500

35

40

45

50

Tra

nsm

itan

ce [

%]

Wavenumber [cm-1]

3462

1640

1380

Fig. 3. FTIR of HES-g-Poly (DMA-co-AA).

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800

Mas

s [%

]

Temperature [0C]

(a)

(b)

0

-2

-4

-6

-8

Fig. 4. (a) TG and (b) DTG of HES-g-Poly (DMA-co-AA).

4268 H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275

follows; 100 mg of HES and HES-g-Poly (DMA-co-AA) weredissolved in 100 ml of double distilled water separately at60 �C with constant stirring. 2 mg of a-amylase was dis-solved in 50 ml of phosphate buffer solution (pH 6.7). Theaqueous solution of HES and HES-g-Poly (DMA-co-AA)were taken separately in stoppered test tubes with varyingconcentration (100–1000 ll). After that the phosphate buf-fer (pH 6.7) solution was used to make up the volume ofeach solution upto 3 ml to maintain the solution pH at6.7. And then 2 ml of a-amylase solution was added ineach test tube with shaking. The mixture was incubatedat 37 �C and 45 min. To stop the enzymatic degradation,HES and HES-g-Poly (DMA-co-AA) were heated to 99 �Cfor 15 min after the addition of 2 ml of 2,4-dintrosalicylic-acid (DNS) (which indicate the presence of glucose in thesolution by its intensity of colour change yellow to winered). Then the intensity of colour change was measuredby a photoelectric colorimeter (Model: 304, Systronics,Ahmedabad, Gujarat, India) at wavelength 570 nm. Blanksolution was used for instrument setup. The result isshown in Fig. 9.

3. Dye adsorption study

3.1. Preparation of Malachite green solution and adsorptionexperiments

Aqueous solution of Malachite green was prepared indouble distilled water. The exact concentration of dye inthe experimental solutions was determined by UV–VISspectrophotometer (Shimudzu-1800, Japan). A Calibrationcurve of MG was prepared by measuring absorbance of dif-ferent predetermined concentrations of MG at kmax

612 nm. Concentration of dye in the experimental solutionwas calculated from the calibration curve. Adsorption anal-ysis was carried out using the graft copolymer as an adsor-bent. A typical experimental procedure was as follows; astock dye solution was prepared by dissolving 0.1 g ofdye into the 1.0 l distilled water and a stock polymer solu-tion was prepared by dissolving 1.0 g polymer in 500 mldistilled water. 2.5 ml of the polymer (0.008 g) from thestock solution was added to the 30 ml of the dye solution

ppm

HES-O-[CH2 - CH]m ........[ CH2 - CH-]n

CONMe2 COOH

2.0-3 ppm

1.3-1.8 ppm

1.3-1.8 ppm

DOH

Fig. 5. 1H NMR of HES-g-Poly (DMA-co-AA).

ppm

HES-O-[CH2 - CH]m ........[ CH2 - CH-]n

CONMe2 COOH30-48 ppm

30-48 ppm 30-48 ppm

30-48 ppm

175-180 ppm175-180 ppm

Fig. 6. 13C NMR of HES-g-Poly (DMA-co-AA).

H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275 4269

(0.003 g) in a 100 ml stoppered conical flask at pH 6.5 byusing 0.01 N sodium hydroxide solution. The solution pHwas measured by digital pH meter (Model 335, Systronics,Ahmedabad, Gujarat, India). After that the flask was shakenby a mechanical shaker upto 60 min, then the solution wasallowed to settle. Precipitation comes due to the interac-tion of the dye and the polymer. After which the solutionis centrifuged and the centrifugate is used for the adsorp-tion analysis by UV–VIS spectrophotometer at kmax

612 nm. The operating variables like, amount of adsorbent,solution pH, contact time, temperature and the initial dyeconcentration were studied to determine the dye concen-tration upto the equilibrium was reached and themaximum removal of dye was attained. The percentageadsorption of dye was calculated using the followingformula [34].

Dye removal capacity or colour removal

Adsorptionð%Þ ¼ ðC0 � CeÞC0

� 100 ð2Þ

and the equilibrium uptake was calculated using the fol-lowing formula:

qeq ¼ ðC0 � CeqÞ �VW

ð3Þ

where qeq is the equilibrium capacity of dye on the adsor-bent (mg g�1), C0 is the initial concentration of dye solution(mg l�1), Ceq is the equilibrium concentration of dye solu-tion (mg l�1), V is the volume of dye solution used (l) andW is the weight of adsorbent (g) used.

Fig. 7. (a) SEM of HES and (b) SEM of HES-g-Poly (DMA-co-AA).

8 10 12 14 16 18 20 22 24 260.15

0.20

0.25

0.30

0.35

Con

duct

ance

(0.01N) NaOH [ml]

Fig. 8. Conductometric titration curve of HES-g-Poly (DMA-co-AA) vs(0.01 N) of NaOH solution at 25 �C.

0 200 400 600 800 10000.00

0.16

0.32

0.48

HES

HES-g-Poly-(DMA-co-AA)

O.D

Concentration [ppm]

Fig. 9. Biodegradation studies of the graft copolymer using enzymehydrolysis of HES and HES-g-Poly (DMA-co-AA).

2 3 4 5 6 7 8

86

88

90

92

94

Dye

Rem

oval

[%

]

Polymer dose [mg]

Fig. 10. Effect of the amount of adsorbent on adsorption of MG by HES-g-Poly (DMA-co-AA) (conditions: dye conc 100 mg l�1, pH 5.5, contact time60 min and temperature 45 �C).

4270 H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275

All the batch experiments were carried out in triplicate,and the values reported here are the averages of threereadings.

3.2. Effect of operating variables

3.2.1. Effect of the amount of adsorbentThe study of the adsorption of malachite green on the

graft copolymer based adsorbent was performed by chang-ing the quantity (2–8 mg) of adsorbent in the test solutionwhile the maintaining the initial dye concentration(0.1 g l�1), temperature 45 �C, contact time (60 min) andpH (5.5) were kept constant. The result is shown in Fig. 10.

3.2.2. Effect of pHThe effect of pH on the adsorption study was performed

by varying the initial pH (3.5–8.0) under constant parame-ters at equilibrium condition. The result is shown in Fig. 11.

3.2.3. Effect of contact timeThe adsorption of dye was carried out by the graft

copolymer at a fixed adsorbent dose at different time inter-vals (20–80 min) by keeping other parameters constant.The result is shown in Fig. 12.

3 4 5 6 7 8 9

20

40

60

80

100D

ye r

emov

al [

%]

pH

Fig. 11. Effect of pH on the adsorption of MG by HES-g-Poly (DMA-co-AA)(conditions: adsorbent mass 7.0 mg, dye conc 100 mg l�1, contact time60 min and temperature 45 �C).

20 30 40 50 60 70 80

86

88

90

92

94

Dye

Rem

oval

[%

]

Time [min]

Fig. 12. Effect of contact time on the adsorption of MG by HES-g-Poly(DMA-co-AA) (conditions: adsorbent mass 7.0 mg, dye conc 100 mg l�1,pH 5.5 and temperature 45 �C).

20 30 40 50 60 70 80

86

88

90

92

94

Dye

Rem

oval

[%

]

Temperature [0C]

Fig. 13. Effect of temperature on the adsorption of MG by HES-g-Poly(DMA-co-AA) (conditions: adsorbent mass 7.0 mg, dye conc 100 mg l�1,pH 5.5 and contact time 60 min).

0 20 40 60 80 100 120 140 1600

20

40

60

80

100

Dye

rem

oval

[%

]

Time [min]

25 mg l-1

50 mg l-1

75 mg l-1

100 mg l-1

200 mg l-1

250 mg l-1

Fig. 14. Effect of initial dye concentration on the adsorption of MG byHES-g-Poly (DMA-co-AA) (conditions: adsorbent mass 7.0 mg, pH 5.5,contact time 60 min and temperature 45 �C).

H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275 4271

3.2.4. Effect of temperatureThe effect of increasing temperature on the adsorption

study was performed in the range of 25–80 �C under con-stant parameters at equilibrium condition. The result isshown in Fig. 13.

3.2.5. Effect of initial dye concentrationThe initial concentration of dye was varied from 50 mg

to 250 mg l�1 at time 20–150 min, pH 5.5 and temperature45 �C. The results are shown in Fig. 14.

3.2.6. Adsorption kineticsIn order to find out the mechanism of adsorption, three

kinetic models have proposed in the literature for theadsorption process [23].

� First order Lagergren equation

log ðqe � qtÞqe

¼ log qe � Kt2:303

ð4Þ

20 30 40 50 60 70 80

1.8

2.0

2.2

2.4

2.6

25 mg l -1

50 mg l -1

75 mg l -1

100 mg l -1

200 mg l -1

250 mg l -1

log

(qe-

q t)

Time [min]

Fig. 15. Lagergren plots on the adsorption of MG by HES-g-Poly (DMA-co-AA) at different initial dye concentration (conditions: adsorbent mass7.0 mg, pH 5.5 and temperature 45 �C).

0 20 40 60 80 100 120 140 1600

4

8

12

16

20

24t

/ qt [

min

g m

g-1]

Time [min]

25 mg l-1

50 mg l-1

75 mg l-1

100 mg l-1

200 mg l-1

250 mg l-1

Fig. 16. Pseudo second order kinetic on the adsorption of MG by HES-g-Poly (DMA-co-AA) at different initial dye concentration (conditions:adsorbent mass 7.0 mg, pH 5.5 and temperature 45 �C).

0 50 100 150 200 250 300360

370

380

390

400

q e [ m

g g-1

]

Time [min]

250C

350C

450C

550C

Fig. 17. Variation of equilibrium uptake of MG with time at varioustemperatures (conditions: dye conc 100 mg l�1, pH 5.5 and adsorbent7 mg).

4272 H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275

� The pseudo second order rate equationtqt¼ 1

kaq2e

þ tqe

ð5Þ

� The second order rate equation1

ðqe � qtÞ¼ 1

qeþ k2t ð6Þ

where k is the Lagergren rate constant of adsorption(min�1), ka is the pseudo-second-order rate constant ofadsorption (g mg�1 min�1), k2 is the second order rate con-stant (g mg�1 min�1), qe is the amount of dye adsorbed atequilibrium (mg g�1), qt is the amount of dye adsorbed attime (mg g�1).

In this study, the experimental data were used on thesemodels to find out the best fitted model of the adsorption.The plots are shown in Figs. 15 and 16. The calculated val-ues of k and ka are given in Table 2.

3.2.7. Equilibrium studies30 ml aqueous solution of malachite green (0.003 g)

was taken into 100 ml conical flasks and the pH was ad-justed at 5.5. 7 mg of solid graft copolymer was added toeach flask. The flasks were agitated on digital magneticstirrer with a hot plate at 250 rpm and temperature 45 �Cfor 150 min, which is expected enough to reach equilib-rium. At the end of the time the solution was centrifugedand measured its concentration by UV–VIS spectropho-tometer at kmax 612 nm. The results are shown in Fig. 17.

Table 2Measurement of rate constants (conditions: dye conc 25–250 mg l�1, pH 5.5, time

Polymer Initial dye conc (mg l�1) Pse

R2

HES-g-Poly (DMA-co-AA) 25 0.8150 0.7375 0.87

100 0.74200 0.62250 0.74

3.2.8. Thermodynamic parameter studiesThe thermodynamic parameters including change in

free energy (DG0), enthalpy (DH0), and entropy (DS0) werecalculated by using Van’t Hoff equations [22] which are

� LogCac ¼ �DH0

þ DS0

ð7Þ

20–150

udo-firs

Ceq 2:303 RT 2:303 R

� DG0 ¼ �2:303 RT logCac ð8Þ

Ceq

� DG0 ¼ �RT ln K ð9Þ

� lnK2 ¼ DH0

ðT2 � T1Þ ð10Þ

K1 RT1T2

� DG0 ¼ DH0 � TDS0 ð11Þ

where Cac is the amount adsorbed on solid at equilibrium(mg l�1), Ceq is the equilibrium concentration (mg l�1), Ris the universal gas constant, T is the temperature in Kelvin,K1 and K2 are the rate constants at temperature T1 and T2.

The calculated values of the DG0, DH0 and DS0 are listedin Table 3.

min, adsorbent dose 7 mg and tempr 45 �C).

t-order Pseudo-second-order

k (min�1) R2 ka (g mg�1 min�1)

3.8 � 10�2 1.00 2.3 � 10�5

4.0 � 10�2 1.00 8.8 � 10�6

4.2 � 10�2 1.00 1.8 � 10�6

4.3 � 10�2 1.00 7.1 � 10�7

4.18 � 10�2 1.00 3.3 � 10�6

4.16 � 10�2 1.00 3.9 � 10�6

Table 3Calculation of thermodynamic parameters (conditions: dye conc 100 mg l�1, pH 5.5, time 150 min, adsorbent dose 7 mg and tempr 25–55 �C).

Polymer pH Thermodynamic parameters

DG0 (KJ mol�1) DH0 (KJ mol�1) DS0 (KJ mol�1)

HES-g-Poly (DMA-co-AA) 5.5 �18.070 +2.624 +0.0650

Scheme 1. Probable mechanism of the graft copolymerization.

H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275 4273

4. Desorption study

The polymer bound dye after centrifuge was taken in a100 ml beaker. A suspension was made by the addition ofwater. The solution pH was adjusted as pH = 2 by addingglacial acetic acid. The solution was then stirred for15 min in a magnetic stirrer at temperature 75 �C. Thesolution was cooled and then the polymer was precipitatedout by adding methanol followed by centrifugation. By thisway 65–70% polymer was recovered.

5. Results and discussion

The graft copolymerization was carried out in aqueoussolution using K2S2O8 as the radical initiator. The syntheticdetails are given in Table 1. A probable mechanism of thegraft copolymerization is shown in Scheme 1.

Initially S2O2�8 decomposes to SO��4 (radical anion) and

SO2�4 by capturing electron [35] from –OH groups (where

lone pair of electron of the –OH groups is the source ofelectron) of the Hydroxyethyl Starch molecule forming aradical cation O HHES . This radical cation reacts withSO2�

4 forming macroradical (HES-O�). The macro radicalalso form by the reaction with a fresh molecule ofHydroxyethyl Starch (HES–OH) with SO��4 . This macro rad-ical (HES-O�) now react with monomers DMA and AA form-ing graft copolymer.

From the Figs. 1 and 2 it is obvious that the graft copoly-mer have higher molecular weight than HES. This is

because of the presence of PAA and PDMA chains ontothe HES backbone.

Fig. 3 shows the FTIR spectrum of the graft copolymer.The grafting is supported by the IR-spectroscopy. The spec-trum shows a characteristic absorption peak of tertiaryamide ( C

ONR2/R ) at 1640 cm�1 due to C

O stretching vibra-tion, one strong peak at 1380 cm�1 is for the ‘C–O’ groups.The strong absorption comes (greater peak intensity) dueto the submerging of ‘C–O’ units present in the polysaccha-ride backbone and that of C

OOH units of the grafted chains.

The peak 3462 cm�1 is for the –OH groups present in thepolymer molecule.

The TGA/DTGA curves of the graft copolymer are shownin Fig. 4. It is obvious from the curve that it had three re-gions. First at 50–99 �C, Second at 200–300 �C and thirdat 325–460 �C. The corresponding peaks where the degra-dation occurred were observed at 90 �C, 255 �C and420 �C. The initial weight loss of the polymer may havebeen caused by the presence of moisture in the sample.In the second region, the percent weight loss was causedby the loss of CO2 (–CO2 comes out from –COOH groups,in the graft copolymer). In the third zone, the weight losswas caused by the elimination of Me2NH and ‘CO’ groupsfrom the PDMA Chains [30,36]. A residue of about 16%was retained upto 800 �C.

Fig. 5 shows the 1H NMR spectrum of the graft copoly-mer in D2O at 25 �C. The bands at 2.0–3.0 ppm are attrib-uted to the methyl proton and those observed in1.3–1.8 ppm are due to the –CH2– and protons. The signal

4274 H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275

at 4.7 ppm corresponds to DOH, obtained by the deuteriumexchange with the –OH groups. In the 1H NMR spectrumthe disappearance of olefinic protons (Literature value�5.8 ppm) as well as the appearance of the signals corre-sponding to the saturated C–H groups are an evidence ofgrafting.

13C NMR spectrum of the graft copolymer in D2O at25 �C is shown in Fig. 6. The grafting is confirmed by the13C NMR spectroscopy also. The signals in between30 ppm and 48 ppm are due to the methyl (–CH3), methy-lene (–CH2–) and methine (–CH–) carbons coming fromboth the monomers (DMA and AA). The signals in between175 ppm and 180 ppm are due to the carbonyl groups ofboth the two monomers. The signal at the lower field is as-signed to carbonyl moieties from the carboxylic acid (–COOH) group.

A careful examination of the micrographs (Fig. 7) re-veals a large difference in the morphological appearanceof the polymer. HES has the granular structure which chan-ged drastically when it was grafted with DMA and AA. Thisobservation also supports the grafting.

Fig. 8 shows the conductometric titration graph of thegraft copolymer with 0.01 (N) NaOH. From the inflectionpoint in the graph the volume of alkali (20 ml) requiredto neutralize the fixed amount of graft copolymer in aque-ous solution was measured. The N.E. of the graft copolymeris calculated by using the equation (Eq. (1)).

The N.E is identical to the equivalent weight of the acid.If the acid has only one carboxylic (–COOH) group the N.Eand the molecular weight of the acid are identical. If theacid has more than one carboxyl group, the N.E is equalto the molecular weight (Mw) of the acid divided by thenumbers of carboxylic (–COOH) groups. Thus if the molec-ular weight (weight average molecular weight Mw) wasknown the number of –COOH groups can be calculatedfrom the following relation.

M:w ¼ ðN � EÞ � n ð12Þ

By using N.E value and the Mw (obtained by GPC) of thegraft copolymer, number of –COOH groups were calculatedby applying equation (Eq. (12)). The result is shown inTable 1.

It is obvious from the Fig. 9 that the less amount of glu-cose is obtained from the grafted hydroxyethyl starch,which means that the grafting has taken place into themonomeric glucose (cyclic) unit of the polysaccharidechain. It is also concluded that since a-amylase cleavesthe virgin polysaccharide and the graft copolymer, bothHES and HES-g-Poly (DMA-co-AA) are biodegradable.

6. Dye adsorption study

From Fig. 10 it is evident that the dye removal percent-age increases with increasing polymer (adsorbent) doseand reached at equilibrium after 7 mg. This is because ofthe fact that increasing in the adsorbent dose increasesthe additional sites for the adsorption upto the equilibriumpoint. After that it remains almost constant.

The pH of the dye solution plays an important role inthe whole adsorption process as evident from Fig. 11. From

this Figure it was observed that adsorption of dye increasesupto the pH 5.5 and after that it slowly decreases. Themaximum adsorption occurs at pH 5.5. At lower pHthe reason for the low adsorption is as follows; at thepH < 5.5 the amide groups get protonated, which generatesa positive charge on the polymeric chains. Again in acidicpH (pH < 5) malachite green also gets protonated resultingpositive charges into the MG molecules. Thus at pH below5.5 electrostatic repulsion occurs between the positivelycharged surface of the adsorbent and the positivelycharged dye molecules. When the pH of the solutionincreases (pH > 5.5) the amide groups remain as such (noprotonation occurs) but the carboxylic acid groupsdissociate to generate carboxylate ions which arenegatively charged. As the surface of adsorbent getsnegatively charged this enhances the adsorption of thepositively charged dye molecules (as at high pH, deproto-nation of MG occurs) through electrostatic force of attrac-tion. Therefore the adsorption of dye by adsorbentincreases at higher pH values, confirming the presence ofstrong chemical interactions between the dye and theadsorbent [16].

From Fig. 12 it is clear that the maximum adsorptiontakes place in 60 min at the dye concentration 100 mg l�1.The kinetics of dye removal by HES-g-(PDMA-co-PAA)indicated rapid binding of the dye to the sorbent upto20–60 min followed by a slow increase until a state ofequilibrium is reached. No change in the uptake capacityis observed with further increase in equilibrium time upto 80 min. The initial rapid phase may be due to increasein number of vacant sites available at the initial stage; asa result, there was an increased concentration gradient be-tween adsorbate in solution and the absorbent [15]. Gener-ally when adsorption involves a surface reaction process,the initial adsorption is rapid. Then a slower adsorptionwould follow as the available adsorption site graduallydecreases.

From Fig. 13 it is observed that the maximum dye re-moval percentage occurs at 45 �C. After that the removalof dye percentage decreases slowly. The reason is ex-plained in the equilibrium study.

Fig. 14 represents that, with increase in initial concen-tration of dye (25–250 mg l�1) increases the dye removalpercentage up to 100 mg l�1 dye concentration, after thatadsorption decreases significantly. Because at lower con-centration, all adsorbate ions present in the adsorptionmedium could interact with the binding sites of the adsor-bent but at higher concentration adsorption sites becomesaturated which decreases the percentage of uptakecapacity.

The Lagergren’s equation was the first kinetic equationwhich was extensively used to describe the adsorptionkinetics. Lagergren’s first order rate equation has beencalled pseudo-first order rate equation [37] where log (qe -� qt) (Eq. (4)) is plotted against t. If a straight line plot isobtained then the adsorption follows a pseudo first orderkinetics.

Fig. 15 shows the plot of log (qe � qt) against t at differ-ent dye concentrations (25–250 mg l�1). From which it isclear that the adsorption kinetics does not follow a pseudofirst order kinetics (no straight line is obtained). The value

H. Kolya, T. Tripathy / European Polymer Journal 49 (2013) 4265–4275 4275

of the correlation coefficient (R2) was also calculated fromthe plot (given in Table 2) which also suggests the same.

The adsorption kinetics may be described by a pseudo-second order model (Eq. (5)) [38] where t/qt was plottedagainst t. If the pseudo-second order kinetics is applicable,the plot becomes linear.

Fig. 16 shows the plot of t/qt vs t at different dye con-centrations (25–250 mg l�1), which is linear, thus fit withEq. (5). Therefore the adsorption follows a pseudo secondorder kinetics. The value of correlation coefficients (R2)was calculated (shown in Table 2). Here R2 = 1.0 also sug-gests the same.

The results (Fig. 17) show that adsorption of the dyewas maximum at 45 �C. Adsorption of a solute from thesolution phase onto the solid liquid interface occurs byremoving the solvent molecule (water) from the interfacialregion [39]. The interaction between solvent and solid sur-face reduced exposing a number of adsorption withincreasing temperature from 25 to 55 �C. This improvedthe possibility of interaction between dye and graftedcopolymer. It was observed that the adsorption decreasesafter 45 �C. It indicates the exothermic nature of the pro-cess. At high temperature the physical interaction betweenmalachite green and HES-g-Poly-(DMA-co-AA) weekenddue to weakening of hydrogen bonds and Vander Waalsinteraction.

7. Thermodynamics studies

From Table 3, the negative value of DG0 indicates thespontaneous nature of adsorption process. The positiveDH0 indicates the endothermic nature of dye adsorptionon to HES-g-Poly-(DMA-co-AA). In this case DH0 > 0 whichmeans, higher temperature will assist the adsorption ofdye on to HES-g-Poly-(DMA-co-AA) and increasing ran-domness at the solid solution interface during the fixationof the dye molecule on the active site of the adsorbent.

8. Conclusion

From the present investigation it can be concluded thatgrafting of the PDMA and AA chains simultaneously ontothe HES, an important class of starch derivatives can bedone successfully by potassium perdisulfate as the initia-tor. The graft copolymer can be effectively used as anadsorbent for the removal of Malachite Green from itsaqueous solution. Adsorption capacity depends on variousoperating variables like adsorbent mass, solution pH, con-tact time and initial dye concentration. The straight linesobtained by plotting t/qt vs t showed a good agreementof experimental data with the pseudo second order kineticmodel for different initial sorbent concentration. The Lang-muir adsorption isotherm model yielded a much better fit

than the Freundich model. The negative value of free en-ergy change indicated the spontaneous nature of adsorp-tion. Ionic interactions are mainly responsible for bindingM.G with the graft copolymer.

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