Swelling and Dye-Adsorption Characteristics of an Amphoteric

Article pubs.acs.org/IECR

Swelling and Dye-Adsorption Characteristics of an Amphoteric Superabsorbent Polymer Neelesh 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 the cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) were synthesized by solution polymerization using N,N′-methylenebisacrylamide as a cross-linking agent. The ratio of anionic to cationic repeat units was varied to obtain anionic, cationic, and amphoteric SAPs. The synthesized SAPs were characterized by Fourier transform infrared spectroscopy. The equilibrium swelling capacity of the SAPs was found to be dependent on the nature and extent of the net charge on the SAPs but independent of pH. The equilibrium swelling capacity was lowest for the SAP whose ratio of anionic to cationic repeat units was unity. The equilibrium swelling capacity increased as this ratio deviated from unity. The adsorption of an anionic dye (orange G) and a cationic dye (methylene blue) was carried out from the individual solution as well as from their mixture. The adsorption of the dyes was found to be dependent on the nature and amount of net charge on the SAPs but independent of pH. The amount of the dye adsorbed decreased as the net charge on the amphoteric SAPs decreased. The amphoteric SAPs with net negative or positive charge selectively adsorbed oppositely charged dyes from the mixture, but the amounts adsorbed were lower than those adsorbed from the individual dye solutions.

1. INTRODUCTION

The ionic nature of SAPs has been used extensively for the removal of charged dyes and metal ions from aqueous solution. Anionic SAPs based on acrylic acid have been employed for the removal 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 adsorb anionic dyes25 and metal ions.26 Amphoteric hydrogels have been used for a wide variety of applications,27−31 but to the best of our knowledge, no studies of the adsorption of dyes over amphoteric SAPs have been reported. The SAPs were synthesized by the copolymerization of an anionic monomer sodium acrylate (SA) and a cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) in the presence of the cross-linking agent N,N′-methylenebisacrylamide (MBA). The preparation of an amphoteric polymer by varying the ratio of the anionic to cationic group is a unique feature of this study. The SA/ METAC ratio was varied to obtain a series of SAPs having different types and amounts of charge. The ratio of the anionic to cationic groups was found to determine the swelling and adsorption characteristics of the SAPs. In the present study, for the first time, we report the adsorption of dyes over amphoteric SAPs. An anionic and a cationic dye were adsorbed from their individual solutions and from their mixture. The effects of pH on adsorption and the Langmuir adsorption isotherms for the amphoteric polymers were also determined.

Amphoteric superabsorbent polymers (SAPs) are three-dimensional cross-linked networks containing anionic, cationic, and even neutral repeat units.1 The ionic units present in the network create an osmotic pressure difference between the SAP and the swelling medium.2,3 Large amounts of water flow into the network to balance the osmotic pressure difference, resulting in the swelling of the SAP. Amphoteric SAPs can contain a net negative, positive, or neutral charge depending on the content of the constituent repeat units. The swelling characteristics and the ability to adsorb charged species, such as dyes and metal ions, from aqueous solutions depend on the nature and amount of net charge on the amphoteric SAP. Acrylic acid,4−7 sodium acrylate,7−9 sodium styrene sulfonate,10,11 and 2-acrylamide-2-methyl-1-propane sulfonic acid1,12−14 have been used as anionic monomers for the synthesis of amphoteric SAPs. Commonly used cationic comonomers are [2-(metharyloyloxy)ethyl] trimethylammonium chloride, [3-(methacryloylamino)propyl] trimethylammonium chloride,5,10,12 diallyl dimethylammonium chloride,4,14,15 4-vinylpyridine,6 and N,N′-dimethyl-N-ethylmethacryloxylethyl ammoniumbromide.7 Acrylamide6,7,10,14,15 and Nisopropylacrylamide13,16 are most widely used neutral monomers. Xu et al. studied the effects of anionic and cationic groups ratio on the swelling behavior of poly(acrylic acid-codiallyldimethylammonium chloride) SAPs.4 English et al. synthesized balanced and unbalanced amphoteric SAPs based on acrylamido methyl propylsulfonic acid, methacrylamido propyl trimethyl ammonium chloride, and dimethyl acrylamide.1 They studied the effects on the swelling transitions of the concentration of salt in the swelling medium and the total polymer ion concentration. © 2012 American Chemical Society

Received: Revised: Accepted: Published: 14941

April 23, 2012 October 11, 2012 October 30, 2012 October 30, 2012 dx.doi.org/10.1021/ie301839z | Ind. Eng. Chem. Res. 2012, 51, 14941−14948

Industrial & Engineering Chemistry Research

Article

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

2. EXPERIMENTAL SECTION 2.1. Materials. Monomer acrylic acid (AA) was purchased from Merck Limited, Mumbai, India. Cationic monomer [2(metharyloxy)ethyl]trimethylammonium chloride (METAC) was procured from Sigma-Aldrich (St. Louis, MO). The initiator, ammonium persulfate (APS), and cross-linking agent, N,N′-methylenebisacryalmide (MBA), were procured from S.D. Fine Chemicals Ltd. (Mumbai, India). Accelerator N,N,N′,N′-tetramethylethylenediamine (TEMED) was obtained from Fluka. Sodium hydroxide (NaOH) used to neutralize AA was purchased from S.D. Fine Chemicals Ltd. Dyes orange G (OG) and methylene blue (MB) were obtained from S.D. Fine Chemicals Ltd. Milli-Q deionized (DI) water was used for all experiments. The molecular weights of monomers 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 monomer METAC were polymerized in solution to obtain poly(SA-coMETAC) amphoteric SAPs (see Scheme 1). SA and METAC were also homopolymerized to obtain the anionic and cationic SAPs 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 added dropwise to AA under constant stirring in an ice bath. The required amount of METAC to achieve the desired copolymer composition was added to the SA solution under stirring. The cross-linking agent N,N′-methylenebisacrylamide (MBA), 0.5 mol % of total monomers, was added to the SA/METAC aqueous solution and allowed to dissolve completely under stirring. 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. The accelerator TEMED (0.5 mol % of total monomer) was added to the reaction mixture, which was allowed to polymerize at room temperature for 24 h. The reaction mixture was kept covered with aluminum foil during the polymerization. The obtained polymers were swollen in excess DI water, and the water was repeatedly changed to extract the water-soluble fraction and the residual monomers. The swollen polymers were kept in a vacuum oven at 60 °C for 24 h, and dry polymers were obtained. Table S1 (Supporting Information) lists the compositions of the SAPs and the required amounts of various reaction components. 2.3. Fourier Transform Infrared Spectroscopy and Scanning Electron Microscopy. Fourier transform infrared (FTIR) spectra of all of the SAPs were recorded on a PerkinElmer Spectrum 1000 FT-IR spectrometer in transmittance mode in the range of 4000−500 cm−1. Scanning electron microscopy (SEM) images of the samples were obtained on a Zeiss scanning electron microscope at an acceleration voltage of 5 kV. 2.4. Determination of Equilibrium Swelling Capacity of the SAPs. The swelling capacity of the amphoteric SAPs was determined by the gravimetric method, in which 0.10 (±0.0050) g of the dry polymer was kept in a plastic basket and immersed in beakers containing 500 mL of DI water for swelling. The baskets containing SAPs were removed at different times, and excess water was drained and wiped. The swollen samples were weighed on a Denver Instruments TP214 weighing balance and then returned to the respective beakers for further swelling. The swelling capacity of the SAPs, S (g/g), 14942

dx.doi.org/10.1021/ie301839z | Ind. Eng. Chem. Res. 2012, 51, 14941−14948


Article

was maintained at 25 mg/L in the mixtures. Homopolymeric (anion and cationic) and copolymeric (amphoteric) SAPs were used for the adsorption studies. All of these experiments were repeated at least three times. The adsorptions of MB and OG were also carried out at different pH values of 2, 4, 7, 10 and 12.The pH was adjusted using NaOH and HCl solutions.

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

S=

Ws − Wd Wd

(1)

The cross-linking densities, qc, of the amphoteric polymers can be determined based on the swelling capacity, polymer density, and volume fraction of polymer in the swollen gel. Based on the method detailed in the literature,32 the cross-link densities of the various amphoteric polymers synthesized in this study were determined. 2.5. Adsorption of Dyes on the SAPs. The adsorption of the anionic dye OG and the cationic dye MB on amphoteric SAPs of various compositions was investigated. The molecular structures and sizes of these dyes are listed elsewhere.18,33 The dyes were adsorbed on the SAPs in batch experiments, in which 400 mL of the dye solution of required concentration was taken in a beaker, and 0.10 (±0.0050) g of SAP was added to it. The beakers were kept covered during the experiments, and approximately 0.5 mL of the dye solution was removed for analysis at various times. The samples were filtered and analyzed on a UV−vis spectrophotometer (Shimadzu UV-1700 PharmaSpec instrument equipped with UVProbe 2.31 software). The samples were scanned in absorbance mode in the wavelength range of 400−800 nm. Predetermined calibration curves were used to convert the absorbance values at the wavelength corresponding to maximum absorbance (λmax) into dye concentrations. The following relations were used to determine the amount of dye adsorbed per unit mass of SAP (q), the percentage removal efficiency of the hydrogel (RE), and the partition coefficient (PC)

3. RESULTS AND DISCUSSION 3.1. Fourier Transform Infrared Spectroscopy. The FTIR spectra of cationic SAP poly([2-(metharyloxy)ethyl] trimethyl ammonium chloride) (SA/METAC = 0:1), amphoteric SAP poly(sodium acrylat-co-[2-(metharyloxy)ethyl] trimethyl ammonium chloride) (SA/METAC = 1:1), and anionic SAP poly(sodium acrylate) (SA/METAC = 1:0) are shown in Figure S1 (Supporting Information). In the FTIR spectrum of PMETAC, the characteristic peak at 1732.8 cm−1 is attributed to the stretching vibrations of the carbonyl group, and the peaks at 1489.1 and 955.5 cm−1 are due to the bending and stretching vibrations of quaternary ammonium group, respectively.34 The FTIR spectrum of PSA exhibits characteristic peaks at 1735.6 and 1560.8 cm−1 corresponding to CO of acrylate and (CO)O stretching of acrylate group, respectively. In the FTIR spectrum of poly(SA-co-METAC), the peaks corresponding to the bending and stretching vibrations of quaternary ammonium group appeared at 1490 and 954 cm−1, respectively. The peaks corresponding to the CO group of acrylate and (CO)O stretching of acrylate group were found to appear at 1726.2 and 1580 cm−1, respectively. The presence of the characteristic peaks corresponding to the quaternary ammonium groups and the acrylate groups confirmed the copolymerization of SA and METAC to form the amphoteric SAP. The surface morphologies of the amphoteric SAPs with two different scales are shown in Figure S2 (Supporting Information). The surfaces of the SAPs are irregular and microporous, indicating an ability to absorb high amounts of water. 3.2. Equilibrium Swelling of the SAPs. The swelling of the SAPs occurs because of the osmotic pressure difference caused by the presence of the ionic repeat units in the threedimensional cross-linked network.3 Figure 1 shows the variation in the swelling capacity of the SAPs with time. All the SAPs absorbed water and swelled at a higher rate in the beginning. After a certain period of time, the water uptake became constant, and the SAPs achieved their equilibrium

amount of dye adsorbed per unit weight of SAP at time t (q , mg/g) =

(C 0 − C )V W

amount of dye adsorbed per unit weight of SAP at equilibrium (qeq , mg/g) =

(C0 − Ce)V W

⎛C − C ⎞ removal efficiency (RE, %) = ⎜ 0 ⎟ × 100 ⎝ C0 ⎠

partition coefficient (PC) =

C0 − C C

where C0, C, and Ceq denote the dye concentration (mg/L) at time t = 0, at time t, and at equilibrium, respectively. W is the weight of the dry SAP (g), and V is the volume of dye solution (L). The efficacy of an adsorbent for the removal of dissolved materials from a solution is given by the amount of dye adsorbed 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 the adsorption of the individual dyes, solutions with a concentration of 25 mg/L were used. The concentration of each dye

Figure 1. Variations in the swelling capacities of the SAPs with time. 14943



Article

group in PMETAC is longer than the anionic pendant group of PSA. The longer cationic group favors the solubilization of the SAP, resulting in a higher rate of water absorption. In addition to the size of the repeat units, the actual cross-link density of the SAPs and the molecular weights between the cross-links could also cause the difference in the equilibrium swelling capacity and the swelling rate constant. 3.2.3. Swelling of the Amphoteric SAPs. The amphoteric SAPs contained both anionic (SA) and cationic (METAC) repeat units. The amphoteric SAPs exhibited a much lower equilibrium swelling capacity than the anionic or cationic SAPs (Table 1). The equilibrium swelling capacity of the amphoteric SAPs also varied with the ratio of anionic to cationic repeat units. The amphoteric SAP with an anionic-to-cationic repeat unit ratio equal to unity exhibited the lowest equilibrium swelling capacity (9.6 g/g). The equilibrium swelling capacity of the amphoteric SAPs increased as the ratio of anionic to cationic 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 SAPs attract each other and prevent the network from expanding. On the other hand, the repulsive forces between anionic−anionic and cationic−cationic repeat units cause the network to expand. The swelling characteristics of SAPs are determined by the net charge on the polymer network. The amphoteric SAP with equal amounts of anionic and cationic repeat units exhibited the lowest equilibrium swelling capacity, as the net charge on the SAP, and thus the osmotically active sites, became zero.8 The net charge and the number of osmotically active sites increased and the SAPs achieved higher equilibrium swelling capacities as the ratio of anionic to cationic repeat units deviated from unity. The SAPs with SA/METAC ratios of 1:4, 2:3, 3:2, and 4:1 swelled more than the SAP with the SA/METAC ratio of unity. The SAPs with net negative charge (SA/METAC ratios of 3:2 and 4:1) showed higher equilibrium swelling capacities than the SAPs with net positive charge (SA/METAC ratios of 2:3 and 1:4). This behavior is similar to that exhibited by the anionic and cationic SAPs and could be related to the larger size of the cationic pendant group of METAC. A physical mixture of PSA/PMETAC (1:1 weight ratio) was also subjected to swelling in DI water, and the equilibrium swelling capacity, 139.4 g/g, was found to lie between those of the individual homopolymeric SAPs. The equilibrium swelling capacity of the physical mixture was much higher than that of

swelling capacity. At equilibrium, the polymer chains attained the elongated configurations, and an elastic retractive force developed preventing, further expansion of the network.3 3.2.1. Kinetics of Swelling of SAPs. The swelling of the SAPs followed first-order kinetics. The rate of the swelling of the SAP is given by

dS = ks(Seq − S) dt

(2)

where S and Seq are the swelling capacities at any time t and at equilibrium, respectively, and ks is the swelling rate constant. As at time t = 0, S = 0, so S = Seq[1 − exp( − kst )]

(3)

Similar first-order swelling kinetics has been observed for poly(acrylic acid-co-sodium acrylate-co-acrylamide) SAPs.3 The experimental data was fitted using eq 3, and the equilibrium swelling capacity and swelling rate constants were determined. The Seq and ks values obtained from the model are listed in Table 1. Table 1. Kinetic Parameters for the Swelling of the SAPs SA/METAC molar ratio

Seq (g/g)

standard deviation (g/g)

ks (h−1)

qc (×103)

0:1 1:4 2:3 1:1 3:2 4:1 1:0

113.7 36.9 23.6 9.6 24.2 54.9 167.6

3.6 1.5 0.7 1.0 1.0 5.1 3.6

2.48 2.54 1.17 3.30 1.65 1.42 0.65

0.45 2.87 5.66 29.93 4.65 0.90 0.11

3.2.2. Swelling of Anionic and Cationic SAPs. The anionic PSA swelled more than the cationic PMETAC, but the swelling rate constant of PSA was lower than that of PMETAC (Table 1). 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, the concentration of anionic repeat units (ionic repeat units per unit weight of the SAP) in PSA is higher than that of the cationic repeat units in PMETAC. The higher content of anionic repeat units results in a higher osmotic pressure difference, causing PSA to swell more than PMETAC. The higher rate of swelling of PMETAC could be attributed to the larger size of the cationic pendant group. The cationic pendant

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. 14944



Article

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

an increase in the concentration of the dye. The amount of MB adsorbed by PSA was higher than the amount of OG adsorbed by PMETAC. The molecular weight of SA is much lower than that of METAC. Thus, a given amount of PSA contains a larger number of anionic repeat units than the number of cationic repeat 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 and METAC as compared to MB and SA, respectively, making the innermost 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 shown in Figure 3a, and the variation in the amount of the OG adsorbed on amphoteric SAPs with net positive charge is shown in Figure S4a (Supporting Information). Figures 3b and S4b (Supporting Information) show the same for the adsorption of MB on amphoteric SAPs with net negative charge. The extent of dye removal was dependent on the nature and amount of net charge of the amphoteric SAPs. The SAP with an anionic/cationic repeat unit ratio of unity adsorbed the lowest amount of the dyes, as the net charge on this SAP was zero. The SAPs with net positive (or negative) charge did not adsorb cationic (or anionic) dye, but adsorbed the anionic (or cationic) dye. Among the amphoteric SAPs, OG and MB were adsorbed 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. The removal 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 same order of 0:1 > 1:4 > 2:3 > 1:1 and 1:0 > 4:1 >3:2 > 1:1 for OG and MB, respectively.

the copolymer poly(SA-co-PMETAC) with an anionic-tocationic repeat unit ratio of unity. In a physical mixture, the anionic and cationic repeat units do not interact and neutralize each other as they do in an amphoteric SAP, and therefore, it shows 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 swelling of the amphoteric polymers at pH values of 4, 7, and 10. In this pH range, the COO−−COO− and ammonium−ammonium repulsion leads to a high swelling capacity. The attraction between the ammonium and carboxyl groups restricts the swelling, and thus, the swelling is independent of pH. These results are consistent with those observed for the swelling of an amphoteric polymer, poly(acrylic acid-co-diallyldimethyl ammonium chloride),4 and the swelling of an ampholytic polymer,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 MB and OG at time t = 0. The initial concentration of each dye was 25 mg/L, in individual solution as well as in the mixture of the two dyes. In the spectra of the individual dyes, the characteristic peaks of MB and OG appeared at 664 and 480 nm, respectively. In the mixture of the dyes, the characteristic peak corresponding to OG was shifted to 495 nm, whereas the peak corresponding to MB was shifted to 666 nm. In the mixture of the two dyes, the intensity at the characteristic peaks of OG (at 480 nm) increased by 16.1%, whereas the intensity at the characteristic peak corresponding to MB (at 664 nm) decreased by 17.1%. The shift in the positions of the characteristic peaks and the variation in the absorbance were due 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 and MB with time is shown in parts a and b, respectively, of Figure 3. Parts a and b of Figure S4 (Supporting Information) show the increase in the amounts of OG and MB adsorbed by PMETAC and PSA, respectively, with time. MB (or OG) was adsorbed by PSA (or PMETAC) because of the electrostatic attraction36 between the dye and the ionic repeat units of the SAP. PSA and PMETAC did not adsorb OG and MB, respectively, owing to the repulsion of the dye with the repeat units of the SAP, but an increase in concentration was observed. This can be attributed to the swelling of the SAP. An anionic (or cationic) SAP in an anionic (or cationic) dye solution prefers water more than the dye. Thus, water is selectively absorbed, whereas the dye is excluded, resulting in

Table 2. Parameters for the Adsorption of Dye from Individual Solutions SA/METAC ratio 0:1 1:4 2:3 1:1 1:1 3:2 4:1 1:0 14945

qeq (mg/g)

RE (%)

PC

Orange G (OG) 93.4 92.7 12.6 91.5 91.5 10.7 89.7 89.7 8.7 10.4 10.3 0.1 Methylene Blue (MB) 21.6 22.5 0.3 93.0 94.4 17.0 93.9 96.1 24.4 96.2 97.6 40.8

qm(mg/g)

KL (L/g)

1497 1307 1000

0.066 0.038 0.031

714 2525 2645

0.018 0.010 0.012



Article

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.

Ceq

The effect of pH on the adsorption of the dyes on the polymer was investigated. Figure 4 shows the concentration profiles of OG and MB on 1:4 and 4:1 polymer at different pH values varying from 2 to 12. There was no significant change in the pH during the course of the experiments. No appreciable difference in the adsorption capacity of the SAP was observed at various pH values between 4 and 10. This indicates that, as for swelling, pH value in the range between 4 and 10 does not a play a significant role in the adsorption of the dyes for amphoteric polymers. Low adsorption of the cationic dye, methylene blue, at a low pH of 2 can be attributed to the presence of H+ ions competing with the cation groups on the cationic dye for adsorption sites. At low pH, the positively charged surface sites on the adsorbent do not favor the adsorption of dye due to electrostatic repulsion. On the other hand, the adsorption of OG (anionic dye) was the highest at pH values below 7 and decreased with further increase in pH. As the pH of the system increased, the number of negatively charged sites increased and the number of positively charged sites decreased. At high pH of 12, a negatively charged surface site on the adsorbent did not favor the adsorption of the anionic dye due to electrostatic repulsion. An adsorption isotherm describes the relationship between the amount of the solute adsorbed at equilibrium by the adsorbent and the equilibrium concentration of solute in the solution. The equilibrium data of adsorption were modeled using the Langmuir isotherm. The linearized form of the Langmuir adsorption isotherm is

qeq

=

1 1 Ceq + qm qmKL

(4)

where Ceq is the equilibrium adsorbate concentration in mg/L, qeq is the amount of the adsorbate adsorbed at equilibrium per unit weight of the adsorbent (mg/g), qm is the maximum amount of the adsorbate adsorbed per unit weight of the adsorbent (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 the adsorption of OG and MB on the anionic, cationic, and amphoteric polymers. The linearized Langmuir adsorption isotherms for the corresponding adsorption curves are shown as insets, and the parameters determined from the linear form of Langmuir adsorption isotherm are listed in Table 2. The Langmuir isotherm is able to correlate the experimental data well, indicating that the adsorption is due to monolayer coverage. 3.3.4. Adsorption of the Dyes from a Mixture on the Amphoteric SAPs. The adsorption of a mixture of OG and MB was also carried out on the amphoteric SAPs. Figures 6 and S5 (Supporting Information) show the variation of the dye concentration and the amount of dye adsorbed, respectively, with time for the adsorption of dyes on amphoteric SAPs of various compositions. As the dyes having like charges did not get adsorbed over the homopolymeric SAPs, the adsorption of the mixture of the dyes was not carried out on these SAPs. The SAP with an equimolar ratio of anionic to cationic repeat units 14946



Article

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

■

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

S Supporting Information *

Recipe for the synthesis of the SAPs (Table S1). FTIR spectra of the SAPs, PMETAC, poly(SA-co-METAC) and PSA (Figure S1). SEM image of poly(SA-co-METAC) with a ratio of 1:1(Figure S2). Absorption spectra of MB, OG, and a mixture of MB and OG (25 mg/L concentration of each dye) (Figure S3). Variations in the amounts of adsorbed OG and MB for the adsorption on the SAPs. (Figure S4). Variations in the amounts of adsorbed MB and OG from the mixtures for adsorption on the amphoteric SAPs (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

showed the least adsorption of the dyes, and thus, it was not employed 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, but only one dye was adsorbed. The adsorption of dyes from the mixture followed a trend similar to that for the adsorption from the individual dye solutions, that is, the amphoteric SAPs having net negative and positive charges adsorbed cationic and anionic dyes, respectively. The parameters determined from the linear form of Langmuir adsorption isotherm for the adsorption of dyes from the mixture are listed in Table 3. The dyes

■

1:4 2:3 3:2 4:1

qeq (mg/g) Orange G (OG) 75.7 69.2 Methylene Blue (MB) 84.3 86.5

RE (%)

PC

74.2 67.1

2.9 2.0

87.2 92.2

6.8 11.8

AUTHOR INFORMATION

Corresponding Author

Table 3. Parameters for the Adsorption of Dye from the Mixture SA/METAC ratio

ASSOCIATED CONTENT

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

The authors declare no competing financial interest.

■

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

■

containing the same charge as the net charge of the amphoteric SAPs were not adsorbed. Although the trends were similar, the amount of the dyes adsorbed from the mixture was less than the amount adsorbed from the individual dye solutions. The two dyes having opposite charges interacted strongly in their mixture, as reflected by the change in the absorption spectrum of the mixture of dyes (Figure S3, Supporting Information). The lower extent of the adsorption of dyes from the mixture could be due to the anionic−cationic dye interaction. The extent of adsorption of OG on the amphoteric SAPs was considerably lower than the extent of MB adsorption from the mixture of the dyes (Figure 6). This is similar to the observations wherein the individual dyes were separately adsorbed (section 3.3.2).

REFERENCES

(1) English, A. E.; Tanaka, T.; Edelman, E. R. Polymer and solution ion shielding in polyampholytic hydrogels. Polymer 1998, 39 (24), 5893−5897. (2) Gürdaǧ, G.; Kurtuluş, B. Synthesis and Characterization of Novel Poly(N-isopropylacrylamide-co-N,N′-dimethylaminoethyl methacrylate sulfate) Hydrogels. Ind. Eng. Chem. Res. 2010, 49 (24), 12675− 12684. (3) Shukla, N. B.; Madras, G. Reversible Swelling/Deswelling Characteristics of Ethylene Glycol Dimethacrylate Cross-Linked Poly(acrylic acid-co-sodium acrylate-co-acrylamide) Superabsorbents. Ind. Eng. Chem. Res. 2011, 50 (19), 10918−10927. (4) Xu, S.; Wu, R.; Huang, X.; Cao, L.; Wang, J. Effect of the anionicgroup/cationic-group ratio on the swelling behavior and controlled release of agrochemicals of the amphoteric, superabsorbent polymer poly(acrylic acid-co-diallyldimethylammonium chloride). J. Appl. Polym. Sci. 2006, 102 (2), 986−991. (5) Kajjari, P. B.; Manjeshwar, L. S.; Aminabhavi, T. M. Novel Interpenetrating Polymer Network Hydrogel Microspheres of Chitosan and Poly(acrylamide)-graf ted-Guar Gum for Controlled Release of Ciprofloxacin. Ind. Eng. Chem. Res. 2011, 50 (23), 13280− 13287. (6) Dogu, S.; Kilic, M.; Okay, O. Collapse of acrylamide-based polyampholyte hydrogels in water. J. Appl. Polym. Sci. 2009, 113 (3), 1375−1382.

4. CONCLUSIONS The amphoteric SAPs based on the anionic monomer SA and the cationic monomer METAC were synthesized by solution polymerization using N,N′-methylenebisacrylamide as a crosslinker. The ratio of anionic to cationic repeat units was varied to obtain amphoteric SAPs with different natures and net charges. The anionic PSA showed higher equilibrium swelling capacity than the cationic PMETAC. The equilibrium swelling capacity 14947



Article

(7) Li, X.; Dong, Q.; He, P. Synthesis and water absorbency of polyampholytic hydrogels with antibacterial activity. J. Appl. Polym. Sci. 2009, 112 (1), 439−446. (8) Kudaibergenov, S. E.; Sigitov, V. B. Swelling, Shrinking, Deformation, and Oscillation of Polyampholyte Gels Based on Vinyl 2-Aminoethyl Ether and Sodium Acrylate. Langmuir 1999, 15 (12), 4230−4235. (9) Xu, S.; Cao, L.; Wu, R.; Wang, J. Salt and pH responsive property of a starch-based amphoteric superabsorbent hydrogel with quaternary ammonium and carboxyl groups (II). J. Appl. Polym. Sci. 2006, 101 (3), 1995−1999. (10) Baker, J. P.; Blanch, H. W.; Prausnitz, J. M. Swelling properties of acrylamide-based ampholytic hydrogels: Comparison of experiment with theory. Polymer 1995, 36 (5), 1061−1069. (11) Baker, J. P.; Stephens, D. R.; Blanch, H. W.; Prausnitz, J. M. Swelling Equilibria for Acrylamide-Based Polyampholyte Hydrogels. Macromolecules 1992, 25 (7), 1955−1958. (12) English, A. E.; Tanaka, T.; Edelman, E. R. Polyampholytic Hydrogel Swelling Transitions: Limitations of the Debye−Hückel Law. Macromolecules 1998, 31 (6), 1989−1995. (13) Ermatchkov, V.; Ninni, L.; Maurer, G. Thermodynamics of phase equilibrium for systems containing N-isopropyl acrylamide hydrogels. Fluid Phase Equilib. 2010, 296 (2), 140−148. (14) Starodoubtsev, S. G.; Churochkina, N. A.; Khokhlov, A. R. Hydrogel Composites of Neutral and Slightly Charged Poly(acrylamide) Gels with Incorporated Bentonite. Interaction with Salt and Ionic Surfactants. Langmuir 2000, 16 (4), 1529−1534. (15) Peng, G.; Xu, S.; Peng, Y.; Wang, J.; Zheng, L. A new amphoteric superabsorbent hydrogel based on sodium starch sulfate. Bioresour. Technol. 2008, 99 (2), 444−447. (16) Wada, N.; Yagi, Y.; Inomata, H.; Saito, S. Swelling behavior of N-isopropylacrylamide-based amphoteric gels. J. Polym. Sci. A: Polym. Chem. 1993, 31 (10), 2647−2651. (17) Shukla, N. B.; Madras, G. Kinetics of adsorption of methylene blue and rhodamine 6G on acrylic acid-based superabsorbents. J. Appl. Polym. Sci. 2012, 126 (2), 463−472. (18) Shukla, N. B.; Madras, G. Adsorption of cationic dyes on poly(acrylic acid-co-sodium acrylate-co-acrylamide) superabsorbents. J. Appl. Polym. Sci. 2012, 124 (5), 3892−3899. (19) Ü züm, Ö . B.; Kundakci, S.; Durukan, H. B.; Karadağ, E. Swelling and dye sorption studies of AAm/SA hydrogels crosslinked by glutaraldehyde and divinylbenzene. J. Appl. Polym. Sci. 2007, 105 (5), 2646−2654. (20) Singh, T.; Singhal, R. Poly(acrylic acid/acrylamide/sodium humate) superabsorbent hydrogels for metal ion/dye adsorption: Effect of sodium humate concentration. J. Appl. Polym. Sci. 2012, 125 (2), 1267−1283. (21) Li, S.; Zhang, H.; Feng, J.; Xu, R.; Liu, X. Facile preparation of poly(acrylic acid−acrylamide) hydrogels by frontal polymerization and their use in removal of cationic dyes from aqueous solution. Desalination 2011, 280 (1−3), 95−102. (22) Li, S.; Liu, X.; Zou, T.; Xiao, W. Removal of Cationic Dye from Aqueous Solution by a Macroporous Hydrophobically Modified Poly(acrylic acid−acrylamide) Hydrogel with Enhanced Swelling and Adsorption Properties. Clean: Soil, Air, Water 2010, 38 (4), 378−386. (23) Rodriguez, E.; Katime, I. Behavior of acrylic acid−itaconic acid hydrogels in swelling, shrinking, and uptakes of some metal ions from aqueous solution. J. Appl. Polym. Sci. 2003, 90 (2), 530−536. (24) Dai, J.; Yan, H.; Yang, H.; Cheng, R. Simple method for preparation of chitosan/poly(acrylic acid) blending hydrogel beads and adsorption of copper(II) from aqueous solutions. Chem. Eng. J. 2010, 165 (1), 240−249. (25) Shen, J.; Ren, L.; Zhuang, Y. Interaction between anionic dyes and cationic flocculant P(AM-DMC) in synthetic solutions. J. Hazard. Mater. 2006, 136 (3), 809−815. (26) Rivas, B. L.; Maturana, H. A.; Villegas, S. Synthesis, characterization, and properties of an efficient and selective adsorbent to mercury(II). Polym. Bull. 1997, 39 (4), 445−452.

(27) Kharlampieva, E.; Erel-Unal, I.; Sukhishvili, S. A. Amphoteric Surface Hydrogels Derived from Hydrogen-Bonded Multilayers: Reversible Loading of Dyes and Macromolecules. Langmuir 2006, 23 (1), 175−181. (28) Kozlovskaya, V.; Sukhishvili, S. A. Amphoteric Hydrogel Capsules: Multiple Encapsulation and Release Routes. Macromolecules 2006, 39 (18), 6191−6199. (29) Li, X.; Xu, S.; Wang, J.; Chen, X.; Feng, S. Structure and characterization of amphoteric semi-IPN hydrogel based on cationic starch. Carbohydr. Polym. 2009, 75 (4), 688−693. (30) Luo, Y.; Peng, H.; Wu, J.; Sun, J.; Wang, Y. Novel amphoteric pH-sensitive hydrogels derived from ethylenediaminetetraacetic dianhydride, butanediamine and amino-terminated poly(ethylene glycol): Design, synthesis and swelling behavior. Eur. Polym. J. 2011, 47 (1), 40−47. (31) Wang, D.; Song, Z.-Q.; Shang, S.-B. Characterization and biodegradability of amphoteric superabsorbent polymers. J. Appl. Polym. Sci. 2008, 107 (6), 4116−4120. (32) Shukla, S.; Bajpai, A. K. Preparation and characterization of highly swelling smart grafted polymer networks of poly(vinyl alcohol) and poly(acrylic acid-co-acrylamide). J. Appl. Polym. Sci. 2006, 102 (1), 84−95. (33) Vinu, R.; Akki, S. U.; Madras, G. Investigation of dye functional groups on the photocatalytic degradation of dyes by nano-TiO2. J. Hazard. Mater. 2010, 176, 765−773. (34) Wu, Y.; Zhang, N. Aqueous photo-polymerization of cationic polyacrylamide with hybrid photo-initiators. J. Polym. Res. 2009, 16 (6), 647−653. (35) Zhao, Y.; Su, H.; Fang, L.; Tan, T. Superabsorbent hydrogels from poly(aspartic acid) with salt-, temperature- and pH-responsiveness properties. Polymer 2005, 46 (14), 5368−5376. (36) Chen, C. Y.; Wang, P.; Zhuang, Y. Y. Dye removal from wastewater using the adsorbent developed from sewage sludge. J. Environ. Sci. 2005, 17 (6), 1018−1021.

14948


Swelling and Dye-Adsorption Characteristics of an Amphoteric

Recommend Documents