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International Journal of Biological Macromolecules 62 (2013) 557– 564

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules

jo ur nal homep age: www.elsev ier .com/ locate / i jb iomac

reparation, investigation of metal ion removal and flocculationerformances of grafted hydroxyethyl starch

aradhan Kolya, Tridib Tripathy ∗

ostgraduate Division of Chemistry, Midnapore College, Midnapore, Paschim Medinipur 721101, West Bengal, India

r t i c l e i n f o

rticle history:eceived 7 June 2013eceived in revised form7 September 2013ccepted 18 September 2013vailable online 27 September 2013

eywords:

a b s t r a c t

Ceric ion induced graft copolymerization of N,N-dimethyl acrylamide (DMA) and acryl amide (AM) werecarried out onto the hydroxyethyl starch (HES). These grafted copolymers were used for the removalof metal ions from their aqueous solutions. Flocculation performances of the synthesized graft copoly-mers were evaluated in 1.0 wt% silica suspensions. A comparative study of the flocculation performancesof the synthetic graft copolymers was also made. The different factors affecting metal ion absorp-tion, namely pH, treatment time, temperature and polymer dose were studied. A comparative studyof the metal ion removal capacity of the two synthetic graft copolymers was also made in five metal

raft copolymerizationlocculationetal ion removal,N-dimethyl acrylamideydroxyethyl starch

ions namely Ni(II), Zn(II), Cu(II), Pb(II) and Hg(II). The metal ion removal capacity follows the orderHg(II) > Cu(II) > Zn(II) > Ni(II) > Pb(II) in both the two synthetic polymers. Between the two graft copoly-mers, graft copolymer based on AM shows better performance than that based on DMA in all the metalsolutions. But the flocculation performance of DMA based graft copolymer showed better performancesthan that AM based graft copolymer. The former also performed best when compared to the commercialflocculants in the same suspension.

. Introduction

Heavy metal ions like mercury (Hg), lead (Pb), cadmium (Cd),hromium (Cr), etc. are highly toxic and cause environmental pol-ution [1,2]. Many industrial effluents are the source of such heavy

etal ions. Such toxic metal ions from industrial effluents, mine’saste water must be removed to avoid environmental pollution.

here are various methods to remove the toxic metal ions fromater solution including chemical precipitation, solvent extrac-

ion, membrane filtration, reverse osmosis and ion exchange. Theseethods are relatively expensive. Therefore, cost effective alterna-

ive methods or sorbents are needed for the treatment of heavyoxic metals contaminated waste streams [3].

Water-soluble synthetic and natural macromolecules are theost efficient materials used as flocculants in the process of water

larification. An attempt has been made in the past decades to com-ine the best properties of synthetic and natural polymers by graftopolymerization technique. It was also concluded that by graft-ng flexible polyacrylamide chains onto the various polysaccharide

ackbones it is possible to develop efficient and shear stable floccu-

ating agents for waste water treatment in industrial effluents andineral processing [4–6]. Various water soluble synthetic polymers

∗ Corresponding author. Tel.: +91 3222 275847; fax: +91 3222 275847.E-mail address: tridib tripathy@yahoo.co.in (T. Tripathy).

141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ijbiomac.2013.09.018

© 2013 Elsevier B.V. All rights reserved.

like polyacrylamide, polyacrylic acid, polystyrene sulfonic acid aswell as various polysaccharides which are used as flocculatingagents are also capable to coordinate metal ions [7,8]. Metal ions areadsorbed at the polymeric backbone mainly by secondary bondinginteractions like hydrogen bonding; coordinate bonding involvingthe metal ions and the electron donating groups present at thepolymer. In polyacrylamide the –CONH2 groups in polyacrylic acidthe –COO− and –COOH groups and in polystyrene sulfonic acid the–SO3H and –SO3

− groups serve this purpose. However in polysac-charides like starch, guargum, xantangum, etc. large number of –OHgroups play the metal ion coordination sites. Other polysaccharideslike sodium carboxymethylated cellulose (CMC), sodium alginate,chitosan apart from –OH groups –COO− (CMC and sodium alginate),–NH2 groups (chitosan) are present, which are capable to bind themetal ions. Thus if the metal ion attraction capabilities of both nat-ural polysaccharides and the synthetic polymers are combined, it ispossible to develop efficient low cost sorbents for the heavy metalions and also the shear stable flocculating agents. These can be doneby grafting various synthetic polymeric chains of vinyl monomersonto the polysaccharide backbones.

It was reported that dextran-g-polyacrylamide is a goodremover of Fe(III), Al(III), Ni(II) and Co(II) [9]. Some starch deriva-

tives prepared by grafting of some water soluble syntheticpolymers are studied by Khalil and Farag for the absorp-tion of various metal ions [10]. Metal ion sorption studies ofgraft copolymer k-carrageenan-g-N,N-dimethyl acrylamide and

5 of Bio

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pmaaTemmdcs

2

2

wanptz(Hc3AupiLB(fw

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58 H. Kolya, T. Tripathy / International Journal

MC-g-N-vinylformamide were also studied by Behari and co-orkers [11,12]. But no attempt has been made to investigate theetal ion removal and flocculation performances of graft copoly-ers based on N,N-dimethyl acrylamide and polysaccharides in

etail.Keeping this view in mind the present work is undertaken to

repare some sorbents for the removal of heavy metals from heavyetal contaminated aqueous waste streams and some flocculating

gents, by grafting poly-acrylamide (PAM) and poly N,N-dimethylcrylamide (PDMA) chains onto the hydroxyethyl starch backbone.he present investigation is also undertaken to investigate theffect of methyl groups on the ‘N’ atom of the amide group in theetal removal and flocculation performances of the graft copoly-er based on polysaccharide and N,N-dimethyl acrylamide in

etail. HES is a derivative of amylopectin, containing only branchedhains of glycoside molecules, devoid of a linear component. Itstructure is given in our previous communication [13].

. Experimental

.1. Materials

Hydroxyethyl starch (HES) and dimethyl acrylamide (DMA)ere procured from Aldrich Chemical Company, USA. Acryl-

mide, ceric ammonium nitrate (CAN), acetone and sodiumitrate were procured from E. Merck Ltd., Mumbai, India. Cop-er(II) acetate monohydrate [Cu(OAc)2·H2O], nickel(II) acetateetra hydrate [Ni(OAc)2·4H2O], lead(II) trihydrate [Pb(OAc)2·3H2O],inc(II) acetate dihydrate [Zn(OAc)2·2H2O] and mercuric chlorideAR) [HgCl2] and polyacrylamide (M.W 5 × 106) were supplied byimedia Laboratories, Mumbai, India. Silicon dioxide was pro-ured from Loba Chemie, Bombay, India. Digital pH meter (Model35) was procured from Systronics, Ahmedabad, Gujarat, India.tomic Adsorption Spectrophotometer (AAS) model-AA55B, GTAsing graphite furnace was procured from Varian Inc. The averagearticle size of silica is 101.4 nm and the suspension Zeta potential

s −52.3 at pH 7.2. Magnafloc-1011 was obtained from BASF Indiaimited, Mumbai, India. Telfloc-2230 was obtained as a gift fromalaji Paper and Newsprint Pvt. Ltd., Manikpara, WB, India. PolyN,N-dimethyl acrylamide) (M.W. is the order of 106) was procuredrom Scientific Polymer Product, Inc., USA. Doubly distilled wateras used for the synthesis.

.2. Synthesis of the graft copolymers

The detail synthetic procedure was given in our previous com-unication [13]. The synthetic details of the two graft copolymers

re given in Table 1. The graft copolymers were synthesized bysing ceric ion induced method.

.3. Determination of molecular weight and molecular weight

istribution using gel permeation chromatography (GPC)

The molecular weight and molecular weight distribution of HES--PAM and HES-g-PDMA was determined using GPC (Model: 2414,

able 1ynthetic details of the graft copolymers.

Polymer Polysaccharide (g) DMA (mol) Amount of CAN (mol × 10−4)

HES-g-PAM 1.5 0.140 4.564

HES-g-PDMA 1.5 0.109 4.564

Conversion = [(wt of graft copolymer − wt of polysaccharide)/amount of AM/DMA] × 10G = (wt of grafted polymer/wt of polysaccharide) × 100.a Percentage conversion is calculated from the relation.b Grafting ratio (%G) is calculated from the relation.

logical Macromolecules 62 (2013) 557– 564

supplied by water (I) Pvt. Ltd., USA). The flow rate was fixed at0.6 mL/min. and the column temperature was kept at 30 ◦C usingthe analysis.

2.4. Characterization of the graft copolymers

Graft copolymers were characterized by intrinsic viscosity mea-surement, FTIR, NMR spectroscopy and thermal analysis. Theresults were given in our previous article [13].

2.5. Adsorption of metal ions

20 mL solution of graft copolymer was added to 20 mL of metalion solution of known concentration (0.01 M) and then the mixturewas stirred by a magnetic stirrer. The amount of residual metal ionin the solution was determined by Atomic Adsorption Spectropho-tometer (AAA). Metal ion removal capacity of the copolymer wascalculated by using following relation [14].

Metal ion removal capacity (m mol/g of graft copolymer)

= (Ci − Cf ) × V

M(1)

where

Ci, initial metal ion concentration in the solution (m mol/L)Cf, metal ion concentration in the solution after metal ion removal(m mol/L)V, volume of the solution (L)M, the weight of the graft copolymer.

The pH of the metal ion solution is kept constant at 5.5 sinceat higher pH the metal ions are precipitated as metal hydroxide.Stock solution of copper(II) ion, nickel(II) ion, zinc(II) ion, lead(II)ion and mercuric(II) ion were prepared by dissolving Cu(OAc)2·H2O,Ni(OAc)2·4H2O, Zn(OAc)2·2H2O, Pb(OAc)2·3H2O and HgCl2 in dou-ble distilled water respectively. The concentration of metal ion insolution was 0.01 (M) in all five cases.

2.6. Desorption study

Desorption of the metal ions was carried out by boiling withacetic acid [15]. The metal loaded graft copolymer (0.2 g) was placedin a 50 mL of round bottom flask fitted with a condenser. Aceticacid (25 mL) was added to the sample and the mixture is refluxedfor 30 min. The mixture is cooled and centrifuged and after that5 mL of the filtrate was used for metal ion in Atomic AdsorptionSpectrophotometer.

2.7. Flocculation study

Standard Jar test [16] was carried out in standard Jar apparatus

supplied by Scientific Engineering Corporations, New Delhi, India.Turbidity measurements were carried out with a Digital NepheloTurbidity meter procured from E1 products, Haryana, India. Thedetailed procedure was given in our previous article [13]. 1.0 wt%

Percentage of conversiona Grafting ratiob (%G) Intrinsic viscosity (dL/g)

84.11 660.73 7.884.21 706.66 7.4

0.

H. Kolya, T. Tripathy / International Journal of Biol

Table 2Molecular weight and molecular weight distribution of HES, HES-g-PAM and HES-g-PDMA.

Polymer Mn (Da) MW (Da) Mp (Da) Mz (Da Mz+1 (Da)

sd

3

3

HrfuPhoo

3

3

aTcoaTp

HES 94,408 104,476 73,030 118,516 136,567HES-g-PAM 1,732,711 1,977,003 2,180,817 2,196,236 2,386,540HES-g-PDMA 4,370,859 4,936,798 568,639 5,409,715 5,798,830

ilicon dioxide was used for the flocculation study. The flocculantose varied from 1 to 12 ppm.

. Results and discussion

.1. Molecular weight and molecular weight distribution

The Molecular weight and molecular weight distribution ofES, HES-g-PAM and HES-g-PDMA were measured in GPC and the

esults are shown in Table 2, Figs. 1–3 respectively. It is obviousrom the result that the two graft copolymers have higher molec-lar weight than HES. This is because of the presence of PAM andDMA chains onto the HES backbone. Again PDMA grafted HES haveigher molecular weight than the PAM grafted HES. This is becausef the presence of two extra methyl groups in amide functionalityf PDMA.

.2. Metal ion adsorption study

.2.1. Effect of pHThe adsorption of different metal ions on to the HES-g-PAM

nd HES-g-PDMA were studied at different pH (1–6) separately.he results are shown in Fig. 4(a) and (b) respectively. In both theases the adsorption of the metal ion increases with the increase

f the pH but to a certain extent. Maximum adsorption was foundt pH 5.5. And after that it was observed that adsorption decreases.his is due to the precipitation of the metal hydroxides at higherH. All the metal ions used were adsorbed with different values,

Fig. 1. The molecular weight and molecular w

ogical Macromolecules 62 (2013) 557– 564 559

of which the highest adsorption was found for Hg(II) (2.3 m mol/gfor HES-g-PAM and 1.5 m mol/g for HES-g-PDMA at pH 5.5) andthe poorest adsorption was found for Pb(II) (0.2 m mol/g for HES-g-PAM and 0.1 m mol/g for HES-g-PDMA at pH 5.5) in both thecases. Other metal ions adsorption occurs in between the two. Theefficiency of absorption of different metal ions follows the orderHg(II) > Cu(II) > Zn(II) > Ni(II) > Pb(II). This is due to the difference intheir stability constant of metal ions and the ligand. Hg(II) formsa strong covalent bond with the ‘N’ atom of the amide function-ality. This will increase the stability of the Hg(II) complex withamide group. However this type of strong covalent attachment isnot possible for other metal ions. This is the reason for Hg(II) togets adsorbed strongly than the other metal ions with the amidefunctionality.

3.2.2. Effect of polymer doseThe adsorption of metal ions was studied by increasing the

polymer dose (both HES-g-PAM and HES-g-PDMA) from 0.5 to4.0 g/litter. The results are shown in Fig. 5(a) and (b) respectively.The maximum absorption was found at 2.0 g/L in both the casesand after that it levels off. The maximum absorption was found2.3 m mol/g for Hg(II) and 0.5 m mol/g for Pb(II) for HES-g-PAM thatfor HES-g-PAM 1.5 m mol/g and 0.1 m mol/g respectively.

3.2.3. Effect of treatment time and temperatureThe adsorption of metal ions was carried out by HES-g-PAM and

HES-g-PDMA at different time duration (3–24 h) and temperature(25–85 ◦C). The results are shown in Fig. 6(a) and (b) for HES-g-PAM,in Fig. 7(a) and (b) for HES-g-PDMA. In both cases the adsorption ofmetal ions increases with increasing treatment time upto 6 h afterthat it levels off. The metal ion absorption capacity increases withincreasing temperature up to a certain limit (45 ◦C) (Figs. 6b and 7b)

and after that it decreases. This is because of the hydrolyzing of theamide functionality in acidic medium (pH 5.5) to some extent forwhich the stability of the complex also decreases. This is observedin both HES-g-PAM and HES-g-PDMA.

eight distribution of HES functionality.

560 H. Kolya, T. Tripathy / International Journal of Biological Macromolecules 62 (2013) 557– 564

Fig. 2. The molecular weight and molecular weight distribution of HES-g-PAM.

ecular

3

TFdtdi

Fig. 3. The molecular weight and mol

.3. Desorption study

Both the two graft copolymers were used in desorption study.he results (adsorption and desorption) are shown in Table 3.rom the data it is concluded that, the adsorbed metal ions were

esorbed by the treatment with hot acetic acid without affectinghe graft copolymer. The absorbed amount of different metal ionecreases when desorbed. It is known that amide group (–CONH2)

s hydrolyzed easily by the mineral acids. Therefore desorption

weight distribution of HES-g-PDMA.

experiment for the extraction of metal ion is not suitable. To avoidthe hydrolysis of amide group acetic acid is used under hot condi-tion.

3.4. Comparison between HES-g-PAM and HES-g-PDMA in metal

ion absorption

The metal ion removal capacity of HES-g-PAM and HES-g-PDMAin optimum condition for all the metal ions is shown in Fig. 8. From

H. Kolya, T. Tripathy / International Journal of Biological Macromolecules 62 (2013) 557– 564 561

Fig. 4. (a) Metal ion removal capacity of HES-g-PAM at different pH. (b) Metal ionremoval capacity of HES-g-PDMA at different pH.

Table 3Metal ions adsorption–desorption values of the grafted PAM and PDMA.

Metal ion HES-g-PAM HES-g-PDMA

Adsorption (%) Desorption (%) Adsorption (%) Desorption (%)

Hg(II) 42 40 28 25Pb(II) 14 13 10 8Zn(II) 38 35 25 22Cu(II) 35 32 23 21

tawtcadfiatavm

Fig. 5. (a) Metal ion removal capacity of HES-g-PAM at different polymer dose. (b)Metal ion removal capacity of HES-g-PDMA at different polymer dose.

N

O

N

O

N

O MM+2

The major mechanisms of flocculation of polyelectrolytes aresurface charged neutralization and bridging. Surface charged neu-

Ni(II) 17 14 14 12

he figure it is seen that HES-g-PDMA is inferior to HES-g-PAM inll the five metal ions. The explanation comes from the followingay. In all the metal complexes with HES-g-PAM and HES-g-PDMA

he ligands are coordinated with the metal ions exclusively via thearbonyl oxygen atom [17]. The electron delocalization from themide nitrogen atom to the carbonyl group increases the electronensity on the carbonyl oxygen which reinforces the oxygen atomor the metalation (Scheme 1). In N,N-dimethyl amide functional-ty (–CONMe2) due to the presence of large sized methyl groupst the ‘N’ the approaching of metal ions for the coordination withhe carbonyl groups becomes difficult (Scheme 2) than that withmide(–CONH2) functional group. Thus, because of the larger of

olume of –NMe2 group over –NH2 group the coordination of theetal ions with –NMe2 group is weaker than that of –NH2 group.

Scheme 1. Metal ions coordination site of the acryl amide.

Hence PDMA grafted copolymer becomes weaker in coordinationwith the metal ions than that of Fig. 8 the PAM grafted copolymer.

3.5. Flocculation study

The flocculation performances of HES-g-PAM and HES-g-PDMAwere compared in 1.0 wt% silica suspension. The result is shownin Fig. 9. The turbidity of the supernatant liquid (NTU) afterflocculation was plotted against polymer dose. The flocculation per-formance of a flocculant was expressed in terms of turbidity. Thelower the turbidity, the better the performance will be. From theresult of flocculation it is observed that the graft copolymer basedon DMA showed better performance than AM. The reason will beas follows.

tralization occurs, if the charge of flocculant is opposite in sign tothat of the suspended particles. For neutral flocculants, the major

562 H. Kolya, T. Tripathy / International Journal of Biological Macromolecules 62 (2013) 557– 564

Crowdness surrounding the oxygen atom is

less. So metal ion can easily be coordinated

with ‘O’

Oxygen atom is surrounded by the bulky –NMe2

hence approaching of metal ion to coordinate with

oxygen atom becomes difficult.

N

O

HHN

O

MeMe

Scheme 2. Metal ion coordination capability of the acryl amide and N,N-dimethyl acrylamide.

FM

mltsTotctptocacc

Fig. 7. (a) Metal ion removal capacity of HES-g-PAM at different temperature. (b)Metal ion removal capacity of HES-g-PDMA at different temperature.

ig. 6. (a) Metal ion removal capacity of HES-g-PAM at different time duration. (b)etal ion removal capacity of HES-g-PDMA at different time duration.

echanism of flocculation is the polymer bridging [18]. When veryong chain polymer molecules are absorbed on the surface of par-icles, they tend to form loops that extend some distance from theurface into the aqueous phase, and their ends may also dangle.hese loops and ends may come into contact with, and attach tother particles. This is the bridging mode of flocculation. Essen-ially, a polymer bridging occurs because segments of a polymerhain get absorbed in various particles, thus bringing the particlesogether. For effective bridging to occur, there must be a sufficientolymeric chain length, which extends far enough from the par-icle surface to attach to other particles and also the flexibilityf the polymeric chains. In water medium, in the polyacrylamidehains, strong intramolecular hydrogen bonding between the –CO

nd –NH2 groups occurs, which makes the chain stiffen and heli-al [19]. Thus the approachability of the grafted PAM chains to theontaminant particle becomes less. But in case of PDMA chains,

Fig. 8. Comparison of the metal ion removal capacity of HES-g-PAM and HES-g-PDMA under optimum conditions.

H. Kolya, T. Tripathy / International Journal of Biological Macromolecules 62 (2013) 557– 564 563

Fi

tocflotcp

dg

twao

timgtst

Fs

ig. 9. Comparison of flocculation characteristics of HES-g-PAM and HES-g-PDMAn 1.0 wt% silica suspension.

he intramolecular hydrogen bonding does not occur. The absencef hydrogen bonding increases the flexibility of the grafted PDMAhains. So, in the PDMA grafted polysaccharides due to the betterexibility of the dangling grafted PDMA chains, the approachabilityf the grafted chains to the colloidal particles becomes easier thanhat of the grafted PAM chains. Hence, for PDMA grafted polysac-harides bridging will be better and easier than that of PAM graftedolysaccharides.

Again, in the amide functionality ( ( ) C

O

N

) the electronelocalization of the lone pair of nitrogen into the adjacent carbonylroup places a partial (+)ve charge on the nitrogen atom and a par-

ial negative charge on the oxygen atom [20]N

O

N

O

hich makes the amide group polar. Hence, the colloidal particlesre attracted by the amide groups in a polymer chain irrespectivef surface charges (Zeta potential) of the suspended colloidal par-

icles. The polarity of C

O

NMe2 group (present in DMA molecule)s higher than that of the –CONH2 group (present in acrylamide

olecule) due to the presence of electron donating methyl (–Me)roups at the nitrogen atom of the former, which helps the absorp-

ion of the colloidal particles into the grafted PDMA chains resultingtrong bridging. For this effective intense bridging capabilities ofhe PDMA chains, PDMA grafted polysaccharides showed better

ig. 10. Comparison of flocculation characteristics of PAM and PDMA in 1.0 wt%ilica suspension.

[[[[[[

Fig. 11. Comparison of flocculation characteristics of HES-g-PDMA, HES-g-PAMwith Magnafloc 1011 and Telfloc 2230 in 1.0 wt% silica suspension.

flocculation performance over PAM grafted polysaccharides. Thisfinding is further supported by the fact that when the flocculationperformance of poly-N,N-dimethyl acrylamide (PDMA) and poly-acrylamide (PAM) were compared in the silica suspension, it isobserved that the performance of PDMA are shown in Fig. 10.

The flocculation performance of HES-g-PDMA was comparedwith two commercially available flocculants Telfloc 2230 andMagnafloc-1011 in silica suspension. The result is shown in Fig. 11.Here it is observed that HES-g-PDMA shows better flocculationperformance than that of the two commercial flocculants. Thecommercial flocculants are polyacrylamide based linear polymer.Hence their performance is inferior to the graft copolymers. Theresults were in accordance with previous studies [21,22].

4. Conclusion

PDMA and PAM were successfully grafted onto the hydroxyethylstarch back bone by the ceric ion induced redox polymerizationtechnique in aqueous solution and were also used for the floccula-tion studies in 1.0 wt% silica suspension. These grafted copolymerswere used to remove the heavy metal ions from their aque-ous solution. PAM grafted HES showed better performance overPDMA grafted HES due to the steric crowding in the later case.But the flocculation performance of PDMA grafted HES showedbetter performances than PAM grafted HES due to the effec-tive bridging capacity in the HES-g-PDMA than HES-g-PAM. TheHES-g-PDMA showed better performance than some commercialflocculants. The metal ion absorption efficiency follows the orderHg(II) > Cu(II) > Zn(II) > Ni(II) > Pb(II) in all the above two cases. pH5.5 was found to be optimal for the removal of the studied metalions.

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