Swelling and Dye-Adsorption Characteristics of an Amphoteric Superabsorbent Polymer

Swelling and Dye-Adsorption Characteristics of an AmphotericSuperabsorbent PolymerNeelesh Bharti Shukla, Shruti Rattan, and Giridhar Madras*

Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

*S Supporting Information

ABSTRACT: Amphoteric superabsorbent polymers (SAPs) based on the anionic monomer sodium acrylate (SA) and thecationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) were synthesized by solutionpolymerization using N,N′-methylenebisacrylamide as a cross-linking agent. The ratio of anionic to cationic repeat units wasvaried to obtain anionic, cationic, and amphoteric SAPs. The synthesized SAPs were characterized by Fourier transform infraredspectroscopy. The equilibrium swelling capacity of the SAPs was found to be dependent on the nature and extent of the netcharge on the SAPs but independent of pH. The equilibrium swelling capacity was lowest for the SAP whose ratio of anionic tocationic repeat units was unity. The equilibrium swelling capacity increased as this ratio deviated from unity. The adsorption ofan anionic dye (orange G) and a cationic dye (methylene blue) was carried out from the individual solution as well as from theirmixture. The adsorption of the dyes was found to be dependent on the nature and amount of net charge on the SAPs butindependent of pH. The amount of the dye adsorbed decreased as the net charge on the amphoteric SAPs decreased. Theamphoteric SAPs with net negative or positive charge selectively adsorbed oppositely charged dyes from the mixture, but theamounts adsorbed were lower than those adsorbed from the individual dye solutions.

1. INTRODUCTION

Amphoteric superabsorbent polymers (SAPs) are three-dimen-sional cross-linked networks containing anionic, cationic, andeven neutral repeat units.1 The ionic units present in thenetwork create an osmotic pressure difference between the SAPand the swelling medium.2,3 Large amounts of water flow intothe network to balance the osmotic pressure difference,resulting in the swelling of the SAP. Amphoteric SAPs cancontain a net negative, positive, or neutral charge depending onthe content of the constituent repeat units. The swellingcharacteristics and the ability to adsorb charged species, such asdyes and metal ions, from aqueous solutions depend on thenature and amount of net charge on the amphoteric SAP.Acrylic acid,4−7 sodium acrylate,7−9 sodium styrene sulfo-

nate,10,11 and 2-acrylamide-2-methyl-1-propane sulfonicacid1,12−14 have been used as anionic monomers for thesynthesis of amphoteric SAPs. Commonly used cationiccomonomers are [2-(metharyloyloxy)ethyl] trimethylammo-nium chloride, [3-(methacryloylamino)propyl] trimethylam-monium chloride,5,10,12 diallyl dimethylammonium chlor-ide,4,14,15 4-vinylpyridine,6 and N,N′-dimethyl-N-ethylmetha-cryloxylethyl ammoniumbromide.7 Acrylamide6,7,10,14,15 and N-isopropylacrylamide13,16 are most widely used neutral mono-mers. Xu et al. studied the effects of anionic and cationic groupsratio on the swelling behavior of poly(acrylic acid-co-diallyldimethylammonium chloride) SAPs.4 English et al.synthesized balanced and unbalanced amphoteric SAPs basedon acrylamido methyl propylsulfonic acid, methacrylamidopropyl trimethyl ammonium chloride, and dimethyl acryl-amide.1 They studied the effects on the swelling transitions ofthe concentration of salt in the swelling medium and the totalpolymer ion concentration.

The ionic nature of SAPs has been used extensively for theremoval of charged dyes and metal ions from aqueous solution.Anionic SAPs based on acrylic acid have been employed for theremoval of various cationic dyes17−22 and metal ions.20,23,24

Cationic SAPs based on [2-(methacryloyloxy)ethyl]-trimethylammonium chloride have also been used to adsorbanionic dyes25 and metal ions.26 Amphoteric hydrogels havebeen used for a wide variety of applications,27−31 but to the bestof our knowledge, no studies of the adsorption of dyes overamphoteric SAPs have been reported.The SAPs were synthesized by the copolymerization of an

anionic monomer sodium acrylate (SA) and a cationicmonomer [2-(methacryloyloxy)ethyl]trimethylammoniumchloride (METAC) in the presence of the cross-linking agentN,N′-methylenebisacrylamide (MBA). The preparation of anamphoteric polymer by varying the ratio of the anionic tocationic group is a unique feature of this study. The SA/METAC ratio was varied to obtain a series of SAPs havingdifferent types and amounts of charge. The ratio of the anionicto cationic groups was found to determine the swelling andadsorption characteristics of the SAPs. In the present study, forthe first time, we report the adsorption of dyes over amphotericSAPs. An anionic and a cationic dye were adsorbed from theirindividual solutions and from their mixture. The effects of pHon adsorption and the Langmuir adsorption isotherms for theamphoteric polymers were also determined.

Received: April 23, 2012Revised: October 11, 2012Accepted: October 30, 2012Published: October 30, 2012

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2. EXPERIMENTAL SECTION

2.1. Materials. Monomer acrylic acid (AA) was purchasedfrom Merck Limited, Mumbai, India. Cationic monomer [2-(metharyloxy)ethyl]trimethylammonium chloride (METAC)was procured from Sigma-Aldrich (St. Louis, MO). Theinitiator, ammonium persulfate (APS), and cross-linkingagent, N,N′-methylenebisacryalmide (MBA), were procuredfrom S.D. Fine Chemicals Ltd. (Mumbai, India). AcceleratorN,N,N′,N′-tetramethylethylenediamine (TEMED) was ob-tained from Fluka. Sodium hydroxide (NaOH) used toneutralize AA was purchased from S.D. Fine Chemicals Ltd.Dyes orange G (OG) and methylene blue (MB) were obtainedfrom S.D. Fine Chemicals Ltd. Milli-Q deionized (DI) waterwas used for all experiments. The molecular weights ofmonomers SA and METAC are 94.06 and 207.70 g/mol,respectively.2.2. Synthesis of the Amphoteric SAPs. The anionic

monomer sodium acrylate (SA) and the cationic monomerMETAC were polymerized in solution to obtain poly(SA-co-METAC) amphoteric SAPs (see Scheme 1). SA and METACwere also homopolymerized to obtain the anionic and cationicSAPs poly(sodium acrylate) (PSA) and poly([2-(metharyloxy)-ethyl]trimethylammonium chloride) (PMETAC), respectively.SA was obtained by complete neutralization of AA by NaOH.The NaOH solution of required concentration was addeddropwise to AA under constant stirring in an ice bath. Therequired amount of METAC to achieve the desired copolymercomposition was added to the SA solution under stirring. Thecross-linking agent N,N′-methylenebisacrylamide (MBA), 0.5mol % of total monomers, was added to the SA/METACaqueous solution and allowed to dissolve completely understirring. The SA/METAC/MBA monomer/cross-linker mix-

ture was purged with nitrogen for 20 min. The initiator APS(0.5 mol % of total monomer) was added to the monomer/cross-linker mixture and dissolved under stirring. Theaccelerator TEMED (0.5 mol % of total monomer) wasadded to the reaction mixture, which was allowed to polymerizeat room temperature for 24 h. The reaction mixture was keptcovered with aluminum foil during the polymerization. Theobtained polymers were swollen in excess DI water, and thewater was repeatedly changed to extract the water-solublefraction and the residual monomers. The swollen polymerswere kept in a vacuum oven at 60 °C for 24 h, and drypolymers were obtained. Table S1 (Supporting Information)lists the compositions of the SAPs and the required amounts ofvarious reaction components.

2.3. Fourier Transform Infrared Spectroscopy andScanning Electron Microscopy. Fourier transform infrared(FTIR) spectra of all of the SAPs were recorded on a Perkin-Elmer Spectrum 1000 FT-IR spectrometer in transmittancemode in the range of 4000−500 cm−1. Scanning electronmicroscopy (SEM) images of the samples were obtained on aZeiss scanning electron microscope at an acceleration voltage of5 kV.

2.4. Determination of Equilibrium Swelling Capacityof the SAPs. The swelling capacity of the amphoteric SAPswas determined by the gravimetric method, in which 0.10(±0.0050) g of the dry polymer was kept in a plastic basket andimmersed in beakers containing 500 mL of DI water forswelling. The baskets containing SAPs were removed atdifferent times, and excess water was drained and wiped. Theswollen samples were weighed on a Denver Instruments TP214weighing balance and then returned to the respective beakersfor further swelling. The swelling capacity of the SAPs, S (g/g),

Scheme 1. Schematic of the Synthesis of Amphoteric SAP Poly(SA-co-METAC)

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301839z | Ind. Eng. Chem. Res. 2012, 51, 14941−1494814942

is defined with respect to the weights of dry (Wd) and swollen(Ws) polymers as

=−

SW W

Ws d

d (1)

The cross-linking densities, qc, of the amphoteric polymers canbe determined based on the swelling capacity, polymer density,and volume fraction of polymer in the swollen gel. Based on themethod detailed in the literature,32 the cross-link densities ofthe various amphoteric polymers synthesized in this study weredetermined.2.5. Adsorption of Dyes on the SAPs. The adsorption of

the anionic dye OG and the cationic dye MB on amphotericSAPs of various compositions was investigated. The molecularstructures and sizes of these dyes are listed elsewhere.18,33 Thedyes were adsorbed on the SAPs in batch experiments, in which400 mL of the dye solution of required concentration was takenin a beaker, and 0.10 (±0.0050) g of SAP was added to it. Thebeakers were kept covered during the experiments, andapproximately 0.5 mL of the dye solution was removed foranalysis at various times. The samples were filtered andanalyzed on a UV−vis spectrophotometer (Shimadzu UV-1700PharmaSpec instrument equipped with UVProbe 2.31 soft-ware). The samples were scanned in absorbance mode in thewavelength range of 400−800 nm. Predetermined calibrationcurves were used to convert the absorbance values at thewavelength corresponding to maximum absorbance (λmax) intodye concentrations.The following relations were used to determine the amount

of dye adsorbed per unit mass of SAP (q), the percentageremoval efficiency of the hydrogel (RE), and the partitioncoefficient (PC)

=−t q

C C VW

amount of dye adsorbed per unit weight of SAP at

time ( , mg/g)( )0

−=

q

C C VW

amount of dye adsorbed per unit weight of SAP at

equilibrium ( , mg/g)

( )eq

0 e

=−

×⎛⎝⎜

⎞⎠⎟

C CC

removal efficiency (RE, %) 1000

0

=−C CC

partition coefficient (PC) 0

where C0, C, and Ceq denote the dye concentration (mg/L) attime t = 0, at time t, and at equilibrium, respectively. W is theweight of the dry SAP (g), and V is the volume of dye solution(L). The efficacy of an adsorbent for the removal of dissolvedmaterials from a solution is given by the amount of dyeadsorbed per unit mass of SAP (q), the removal efficiency(RE), and the partition coefficient (PC).The adsorption of MB and OG from their individual

solutions as well as from their mixtures was carried out. For theadsorption of the individual dyes, solutions with a concen-tration of 25 mg/L were used. The concentration of each dye

was maintained at 25 mg/L in the mixtures. Homopolymeric(anion and cationic) and copolymeric (amphoteric) SAPs wereused for the adsorption studies. All of these experiments wererepeated at least three times. The adsorptions of MB and OGwere also carried out at different pH values of 2, 4, 7, 10 and12.The pH was adjusted using NaOH and HCl solutions.

3. RESULTS AND DISCUSSION

3.1. Fourier Transform Infrared Spectroscopy. TheFTIR spectra of cationic SAP poly([2-(metharyloxy)ethyl]trimethyl ammonium chloride) (SA/METAC = 0:1), ampho-teric SAP poly(sodium acrylat-co-[2-(metharyloxy)ethyl] tri-methyl ammonium chloride) (SA/METAC = 1:1), and anionicSAP poly(sodium acrylate) (SA/METAC = 1:0) are shown inFigure S1 (Supporting Information). In the FTIR spectrum ofPMETAC, the characteristic peak at 1732.8 cm−1 is attributedto the stretching vibrations of the carbonyl group, and thepeaks at 1489.1 and 955.5 cm−1 are due to the bending andstretching vibrations of quaternary ammonium group, respec-tively.34 The FTIR spectrum of PSA exhibits characteristicpeaks at 1735.6 and 1560.8 cm−1 corresponding to CO ofacrylate and (CO)O stretching of acrylate group, respec-tively. In the FTIR spectrum of poly(SA-co-METAC), thepeaks corresponding to the bending and stretching vibrations ofquaternary ammonium group appeared at 1490 and 954 cm−1,respectively. The peaks corresponding to the CO group ofacrylate and (CO)O stretching of acrylate group were foundto appear at 1726.2 and 1580 cm−1, respectively. The presenceof the characteristic peaks corresponding to the quaternaryammonium groups and the acrylate groups confirmed thecopolymerization of SA and METAC to form the amphotericSAP.The surface morphologies of the amphoteric SAPs with two

different scales are shown in Figure S2 (SupportingInformation). The surfaces of the SAPs are irregular andmicroporous, indicating an ability to absorb high amounts ofwater.

3.2. Equilibrium Swelling of the SAPs. The swelling ofthe SAPs occurs because of the osmotic pressure differencecaused by the presence of the ionic repeat units in the three-dimensional cross-linked network.3 Figure 1 shows thevariation in the swelling capacity of the SAPs with time. Allthe SAPs absorbed water and swelled at a higher rate in thebeginning. After a certain period of time, the water uptakebecame constant, and the SAPs achieved their equilibrium

Figure 1. Variations in the swelling capacities of the SAPs with time.



swelling capacity. At equilibrium, the polymer chains attainedthe elongated configurations, and an elastic retractive forcedeveloped preventing, further expansion of the network.3

3.2.1. Kinetics of Swelling of SAPs. The swelling of the SAPsfollowed first-order kinetics. The rate of the swelling of the SAPis given by

= −St

k S Sdd

( )s eq (2)

where S and Seq are the swelling capacities at any time t and atequilibrium, respectively, and ks is the swelling rate constant. Asat time t = 0, S = 0, so

= − −S S k t[1 exp( )]eq s (3)

Similar first-order swelling kinetics has been observed forpoly(acrylic acid-co-sodium acrylate-co-acrylamide) SAPs.3 Theexperimental data was fitted using eq 3, and the equilibriumswelling capacity and swelling rate constants were determined.The Seq and ks values obtained from the model are listed inTable 1.

3.2.2. Swelling of Anionic and Cationic SAPs. The anionicPSA swelled more than the cationic PMETAC, but the swellingrate constant of PSA was lower than that of PMETAC (Table1). The repeat unit of PSA has a lower molecular weight (SA,94.06 g/mol) than that of PMETAC (METAC, 207.70 g/mol).Thus, for the same amount of PSA or PMETAC, theconcentration of anionic repeat units (ionic repeat units perunit weight of the SAP) in PSA is higher than that of thecationic repeat units in PMETAC. The higher content ofanionic repeat units results in a higher osmotic pressuredifference, causing PSA to swell more than PMETAC. Thehigher rate of swelling of PMETAC could be attributed to thelarger size of the cationic pendant group. The cationic pendant

group in PMETAC is longer than the anionic pendant group ofPSA. The longer cationic group favors the solubilization of theSAP, resulting in a higher rate of water absorption. In additionto the size of the repeat units, the actual cross-link density ofthe SAPs and the molecular weights between the cross-linkscould also cause the difference in the equilibrium swellingcapacity and the swelling rate constant.

3.2.3. Swelling of the Amphoteric SAPs. The amphotericSAPs contained both anionic (SA) and cationic (METAC)repeat units. The amphoteric SAPs exhibited a much lowerequilibrium swelling capacity than the anionic or cationic SAPs(Table 1). The equilibrium swelling capacity of the amphotericSAPs also varied with the ratio of anionic to cationic repeatunits. The amphoteric SAP with an anionic-to-cationic repeatunit ratio equal to unity exhibited the lowest equilibriumswelling capacity (9.6 g/g). The equilibrium swelling capacityof the amphoteric SAPs increased as the ratio of anionic tocationic repeat units deviated from unity (Table 1).The swelling characteristics of SAPs depend on the attractive

and repulsive forces between the various charged repeat units.1

On one hand, the anionic and cationic repeat units of SAPsattract each other and prevent the network from expanding. Onthe other hand, the repulsive forces between anionic−anionicand cationic−cationic repeat units cause the network to expand.The swelling characteristics of SAPs are determined by the netcharge on the polymer network.The amphoteric SAP with equal amounts of anionic and

cationic repeat units exhibited the lowest equilibrium swellingcapacity, as the net charge on the SAP, and thus the osmoticallyactive sites, became zero.8 The net charge and the number ofosmotically active sites increased and the SAPs achieved higherequilibrium swelling capacities as the ratio of anionic to cationicrepeat units deviated from unity. The SAPs with SA/METACratios of 1:4, 2:3, 3:2, and 4:1 swelled more than the SAP withthe SA/METAC ratio of unity. The SAPs with net negativecharge (SA/METAC ratios of 3:2 and 4:1) showed higherequilibrium swelling capacities than the SAPs with net positivecharge (SA/METAC ratios of 2:3 and 1:4). This behavior issimilar to that exhibited by the anionic and cationic SAPs andcould be related to the larger size of the cationic pendant groupof METAC.A physical mixture of PSA/PMETAC (1:1 weight ratio) was

also subjected to swelling in DI water, and the equilibriumswelling capacity, 139.4 g/g, was found to lie between those ofthe individual homopolymeric SAPs. The equilibrium swellingcapacity of the physical mixture was much higher than that of

Table 1. Kinetic Parameters for the Swelling of the SAPs

SA/METAC molarratio

Seq(g/g)

standard deviation(g/g)

ks(h−1)

qc(×103)

0:1 113.7 3.6 2.48 0.451:4 36.9 1.5 2.54 2.872:3 23.6 0.7 1.17 5.661:1 9.6 1.0 3.30 29.933:2 24.2 1.0 1.65 4.654:1 54.9 5.1 1.42 0.901:0 167.6 3.6 0.65 0.11

Figure 2. Variations in the swelling capacities of the SAPs with time at different pH values for SA/METAC ratios of (a) 4:1 and (b) 1:4.



the copolymer poly(SA-co-PMETAC) with an anionic-to-cationic repeat unit ratio of unity. In a physical mixture, theanionic and cationic repeat units do not interact and neutralizeeach other as they do in an amphoteric SAP, and therefore, itshows a higher equilibrium swelling capacity.The effect of solution pH on the swelling of the amphoteric

polymers is shown in Figure 2. Figure 2a,b shows the swellingof the amphoteric polymers at pH values of 4, 7, and 10. In thispH range, the COO−−COO− and ammonium−ammoniumrepulsion leads to a high swelling capacity. The attractionbetween the ammonium and carboxyl groups restricts theswelling, and thus, the swelling is independent of pH. Theseresults are consistent with those observed for the swelling of anamphoteric polymer, poly(acrylic acid-co-diallyldimethyl am-monium chloride),4 and the swelling of an ampholyticpolymer,35 poly(aspartic acid).3.3. Adsorption of Dye on the SAPs. 3.3.1. Absorption

Spectra of the Dyes. Figure S3 (Supporting Information)shows the absorption spectra of MB, OG, and a mixture of MBand OG at time t = 0. The initial concentration of each dye was25 mg/L, in individual solution as well as in the mixture of thetwo dyes. In the spectra of the individual dyes, the characteristicpeaks of MB and OG appeared at 664 and 480 nm, respectively.In the mixture of the dyes, the characteristic peakcorresponding to OG was shifted to 495 nm, whereas thepeak corresponding to MB was shifted to 666 nm. In themixture of the two dyes, the intensity at the characteristic peaksof OG (at 480 nm) increased by 16.1%, whereas the intensity atthe characteristic peak corresponding to MB (at 664 nm)decreased by 17.1%. The shift in the positions of thecharacteristic peaks and the variation in the absorbance weredue to the interactions between the anionic and cationic dyes.3.3.2. Adsorption of the Dyes on the Anionic and Cationic

SAPs. The reduction in the concentrations of the dyes OG andMB with time is shown in parts a and b, respectively, of Figure3. Parts a and b of Figure S4 (Supporting Information) showthe increase in the amounts of OG and MB adsorbed byPMETAC and PSA, respectively, with time. MB (or OG) wasadsorbed by PSA (or PMETAC) because of the electrostaticattraction36 between the dye and the ionic repeat units of theSAP. PSA and PMETAC did not adsorb OG and MB,respectively, owing to the repulsion of the dye with the repeatunits of the SAP, but an increase in concentration wasobserved. This can be attributed to the swelling of the SAP. Ananionic (or cationic) SAP in an anionic (or cationic) dyesolution prefers water more than the dye. Thus, water isselectively absorbed, whereas the dye is excluded, resulting in

an increase in the concentration of the dye. The amount of MBadsorbed by PSA was higher than the amount of OG adsorbedby PMETAC. The molecular weight of SA is much lower thanthat of METAC. Thus, a given amount of PSA contains a largernumber of anionic repeat units than the number of cationicrepeat units contained in the same amount of PMETAC,resulting in higher adsorption of MB as compared to OG.Another reason could be the larger molecular sizes of OG andMETAC as compared to MB and SA, respectively, making theinnermost sites inaccessible for OG.

3.3.3. Adsorption of the Dyes on the Amphoteric SAPs.The reduction in the concentration of OG with time is shownin Figure 3a, and the variation in the amount of the OGadsorbed on amphoteric SAPs with net positive charge isshown in Figure S4a (Supporting Information). Figures 3b andS4b (Supporting Information) show the same for theadsorption of MB on amphoteric SAPs with net negativecharge. The extent of dye removal was dependent on the natureand amount of net charge of the amphoteric SAPs. The SAPwith an anionic/cationic repeat unit ratio of unity adsorbed thelowest amount of the dyes, as the net charge on this SAP waszero. The SAPs with net positive (or negative) charge did notadsorb cationic (or anionic) dye, but adsorbed the anionic (orcationic) dye. Among the amphoteric SAPs, OG and MB wereadsorbed in the following orders of SA/METAC ratios: 0:1 >1:4 > 2:3 > 1:1 and 1:0 > 4:1 >3:2 > 1:1, respectively. Theremoval efficiency of the SAPs also followed the same trends,whereas the partition coefficients followed the reverse order(Table 2). The swelling of the polymers also followed the sameorder of 0:1 > 1:4 > 2:3 > 1:1 and 1:0 > 4:1 >3:2 > 1:1 for OGand MB, respectively.

Figure 3. Variations in the concentrations of (a) OG and (b) MB with time for adsorption on the SAPs.

Table 2. Parameters for the Adsorption of Dye fromIndividual Solutions

SA/METAC ratio qeq (mg/g) RE (%) PC qm(mg/g) KL (L/g)

Orange G (OG)0:1 93.4 92.7 12.6 1497 0.0661:4 91.5 91.5 10.7 1307 0.0382:3 89.7 89.7 8.7 1000 0.0311:1 10.4 10.3 0.1

Methylene Blue (MB)1:1 21.6 22.5 0.33:2 93.0 94.4 17.0 714 0.0184:1 93.9 96.1 24.4 2525 0.0101:0 96.2 97.6 40.8 2645 0.012



The effect of pH on the adsorption of the dyes on thepolymer was investigated. Figure 4 shows the concentrationprofiles of OG and MB on 1:4 and 4:1 polymer at different pHvalues varying from 2 to 12. There was no significant change inthe pH during the course of the experiments. No appreciabledifference in the adsorption capacity of the SAP was observedat various pH values between 4 and 10. This indicates that, asfor swelling, pH value in the range between 4 and 10 does not aplay a significant role in the adsorption of the dyes foramphoteric polymers. Low adsorption of the cationic dye,methylene blue, at a low pH of 2 can be attributed to thepresence of H+ ions competing with the cation groups on thecationic dye for adsorption sites. At low pH, the positivelycharged surface sites on the adsorbent do not favor theadsorption of dye due to electrostatic repulsion. On the otherhand, the adsorption of OG (anionic dye) was the highest atpH values below 7 and decreased with further increase in pH.As the pH of the system increased, the number of negativelycharged sites increased and the number of positively chargedsites decreased. At high pH of 12, a negatively charged surfacesite on the adsorbent did not favor the adsorption of theanionic dye due to electrostatic repulsion.An adsorption isotherm describes the relationship between

the amount of the solute adsorbed at equilibrium by theadsorbent and the equilibrium concentration of solute in thesolution. The equilibrium data of adsorption were modeledusing the Langmuir isotherm. The linearized form of theLangmuir adsorption isotherm is

= +C

q qC

q K1 1eq

eq meq

m L (4)

where Ceq is the equilibrium adsorbate concentration in mg/L,qeq is the amount of the adsorbate adsorbed at equilibrium perunit weight of the adsorbent (mg/g), qm is the maximumamount of the adsorbate adsorbed per unit weight of theadsorbent (mg/g), and KL is the Langmuir adsorption constant(L/mg).Figure 5 shows the variation of the amount of dye adsorbed

at equilibrium with equilibrium concentration for theadsorption of OG and MB on the anionic, cationic, andamphoteric polymers. The linearized Langmuir adsorptionisotherms for the corresponding adsorption curves are shown asinsets, and the parameters determined from the linear form ofLangmuir adsorption isotherm are listed in Table 2. TheLangmuir isotherm is able to correlate the experimental datawell, indicating that the adsorption is due to monolayercoverage.

3.3.4. Adsorption of the Dyes from a Mixture on theAmphoteric SAPs. The adsorption of a mixture of OG and MBwas also carried out on the amphoteric SAPs. Figures 6 and S5(Supporting Information) show the variation of the dyeconcentration and the amount of dye adsorbed, respectively,with time for the adsorption of dyes on amphoteric SAPs ofvarious compositions. As the dyes having like charges did notget adsorbed over the homopolymeric SAPs, the adsorption ofthe mixture of the dyes was not carried out on these SAPs. TheSAP with an equimolar ratio of anionic to cationic repeat units

Figure 4. Variations in the concentrations of (a) OG and (b) MB with time at different pH values for SAPs with SA/METAC ratios of 1:4 and 4:1.

Figure 5. Langmuir isotherms for the adsorption of (a) OG and (b) MB on amphoteric polymers with different SA/METAC ratios.



showed the least adsorption of the dyes, and thus, it was notemployed for the adsorption of the mixture of dyes.We expected the adsorption of both the cationic and the

anionic dyes from the mixture by the amphoteric SAPs, butonly one dye was adsorbed. The adsorption of dyes from themixture followed a trend similar to that for the adsorption fromthe individual dye solutions, that is, the amphoteric SAPshaving net negative and positive charges adsorbed cationic andanionic dyes, respectively. The parameters determined from thelinear form of Langmuir adsorption isotherm for the adsorptionof dyes from the mixture are listed in Table 3. The dyes

containing the same charge as the net charge of the amphotericSAPs were not adsorbed. Although the trends were similar, theamount of the dyes adsorbed from the mixture was less thanthe amount adsorbed from the individual dye solutions. Thetwo dyes having opposite charges interacted strongly in theirmixture, as reflected by the change in the absorption spectrumof the mixture of dyes (Figure S3, Supporting Information).The lower extent of the adsorption of dyes from the mixturecould be due to the anionic−cationic dye interaction. Theextent of adsorption of OG on the amphoteric SAPs wasconsiderably lower than the extent of MB adsorption from themixture of the dyes (Figure 6). This is similar to theobservations wherein the individual dyes were separatelyadsorbed (section 3.3.2).

4. CONCLUSIONSThe amphoteric SAPs based on the anionic monomer SA andthe cationic monomer METAC were synthesized by solutionpolymerization using N,N′-methylenebisacrylamide as a cross-linker. The ratio of anionic to cationic repeat units was varied toobtain amphoteric SAPs with different natures and net charges.The anionic PSA showed higher equilibrium swelling capacitythan the cationic PMETAC. The equilibrium swelling capacity

of the poly(SA-co-METAC) SAPs increased as the ratio ofanionic to cationic repeat units deviated from unity due to theincrement in the osmotically active sites. The adsorption of theanionic dye OG and the cationic dye MB on the SAPs wascarried out, and the homopolymeric SAPs adsorbed oppositelycharged dyes. The amphoteric SAPs, depending on the natureand net charge, adsorbed only oppositely charged dye. Theadsorption of the dyes from their mixture was also carried out,and trends similar to that observed for the individual dyesolutions were observed, but the extent of adsorption wasfound to be lower. Thus, the nature and the amount of netcharge on the SAPs determined the selective adsorption of thedyes.

■ ASSOCIATED CONTENT

*S Supporting InformationRecipe for the synthesis of the SAPs (Table S1). FTIR spectraof the SAPs, PMETAC, poly(SA-co-METAC) and PSA (FigureS1). SEM image of poly(SA-co-METAC) with a ratio of1:1(Figure S2). Absorption spectra of MB, OG, and a mixtureof MB and OG (25 mg/L concentration of each dye) (FigureS3). Variations in the amounts of adsorbed OG and MB for theadsorption on the SAPs. (Figure S4). Variations in the amountsof adsorbed MB and OG from the mixtures for adsorption onthe amphoteric SAPs (Figure S5). This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Tel.: 091-80-22932321. Fax: 091-80-23600683. E-mail:[email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors thank the Department of Science and Technology,Government of India, for financial research support.

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Figure 6. Variations in the concentrations of MB and OG foradsorption from the mixture on the amphoteric SAPs.

Table 3. Parameters for the Adsorption of Dye from theMixture

SA/METAC ratio qeq (mg/g) RE (%) PC

Orange G (OG)1:4 75.7 74.2 2.92:3 69.2 67.1 2.0

Methylene Blue (MB)3:2 84.3 87.2 6.84:1 86.5 92.2 11.8



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Swelling and Dye-Adsorption Characteristics of an Amphoteric Superabsorbent Polymer

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