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Removal of cationic dyes by poly(acrylamide-co-acrylic acid) hydrogels in aqueous solutions
ARTICLE IN PRESS
Radiation Physics and Chemistry 77 (2008) 447–452 www.elsevier.com/locate/radphyschem
Removal of cationic dyes by poly(acrylamide-co-acrylic acid) hydrogels in aqueous solutions Dilek S- olpan, Sibel Duran, Murat Torun Department of Chemistry, Hacettepe University, 06800, Beytepe, Ankara, Turkey Received 20 June 2007; accepted 21 August 2007
Abstract Poly(acrylamide-co-acrylic acid (poly(AAm-co-AAc)) hydrogels prepared by irradiating with g-radiation were used in experiments on swelling, diffusion, and uptake of some cationic dyes such as Safranine-O (SO) and Magenta (M). Poly(AAm-co-AAc) hydrogels irradiated at 8.0 kGy have been used for swelling and diffusion studies in water and cationic dye solutions. The maximum swellings in water, and SO, and M solutions observed are 2700%, 3500%, and 4000%, respectively. Diffusions of water and cationic dyes within hydrogels have been found to be non-Fickian in character. Adsorption of the cationic dyes onto poly(AAm-co-AAc) hydrogels is studied by the batch adsorption technique. The adsorption type was found Langmuir type in the Giles classification system. The moles of adsorbed dye for SO and M per repeating unit in hydrogel (binding ratio, r) have been calculated as 3834 106 and 1323 106, respectively. These results show that poly(AAm-co-AAc) hydrogels can be used as adsorbent for water pollutants such as cationic dyes. r 2007 Elsevier Ltd. All rights reserved. Keywords: Hydrogel; Gamma-irradiation; Poly(acrylamide-co-acrylic acid); Swelling; Adsorption; Diffusion; Cationic dyes.
1. Introduction Adsorption has attracted considerable interest as a feasible procedure for removing color from effluents. Recently, a number of studies have been reported with regard to the adsorption equilibria and kinetic of dye removal processes using various adsorbents (AlvarezLorenzo and Concheiro, 2002), (Ekici et al., 2006). Commercially available adsorbents contain activated carbon (McKay, 1982, 1983), chitin, silica gel, wood, peat, bauxite, and polymeric adsorbents (Karadag˘ et al., 2006). Of these, activated carbon is one of the most widely used porous adsorbents because of its capability for efficiently adsorbing a broad range of different types of adsorbates (U¨zu¨m and Karadag˘, 2006). Since the cationic dyes used mostly in textile industry have large molecules, their adsorption onto activated carbon and other adsorbents may not be optimal. Polymeric adsorbents and hydrogel systems may offer distinct advantages in specifically defined cases. Corresponding author. Tel.: +90 312 2977990; fax: +90 312 2992163.
E-mail address: [email protected] (D. S- olpan). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.08.006
Hydrogels are crosslinked hydrophilic polymers that are swollen in water usually to equilibrium. Hydrogels find considerable applications and have been extensively studied, because they combine glassy behavior (in their dry state) with elasticity (when sufficient water is adsorbed). The behavior of highly swollen hydrogels is, of course, a function of the network characteristic (such as degree of swelling, diffusion coefficient, crosslink density, mesh size, etc.), which in turn is connected with chemical structures. Hydrogels have been used with widespread applications in different fields. The removal of color from textile wastewaters is a major environmental problem because of the difficulty of treating such waters by conventional methods. Some groups have used various adsorbents for the removal of acidic and basic dyes from aqueous solutions (S- olpan and Ko¨lge, 2006). This study describes the preparation and color-removal ability of a novel series of polymeric adsorbents. These adsorbents were prepared from the reaction of arylamide and acrylic acid. The ability of poly(AAm-co-AAc) hydrogels to adsorb cationic dyes from aqueous solution under several experimental conditions has been studied. The adsorption
ARTICLE IN PRESS D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452
448
effect of poly(AAm-co-AAc) hydrogels on cationic dyes indicates the very good potential of this procedure in achieving the desired objective. 2. Experimental 2.1. Materials
from the absorbance of the solutions. Swollen hydrogels removed from the water bath at regular intervals were dried superficially with filter paper, weighed, and placed in the same bath again. The measurements were continued until a constant weight was reached for each sample. 2.4. Adsorption studies
Acrylamide (AAm) and acrylic acid (AAc) monomers used in this study were obtained from BDH. Safranine-O (SO) and Basic Magenta (M) dyes were supplied from Allied Chemical. Some properties of monomers and dyes are listed in Table 1. 2.2. Preparation of hydrogels The solutions of monomers of AAm and AAc were prepared in three different compositions (AAm/AAc mole ratios, 30/70, 20/80, and 15/85). These solutions were placed in polyvinylchloride straws of 3 mm diameter and irradiated in air at ambient temperature in a Gammacell220 type g-irradiator. The percent conversion was determined gravimetrically. Dose of 8.0 kGy was applied at a fixed dose rate of 0.16 kGy h1. Hydrogels obtained in long cylindirical shapes were cut, washed with distilled water for removal of unreacted monomers, and dried in air and in vacuum, and stored for later evaluations. Poly(AAm-coAAc)1, poly(AAm-co-AAc)2, and poly(AAm-co-AAc)3 were prepared using three different compositions of poly(AAm-co-AAc) hydrogels, which were obtained by using AAm/AAc monomer mixtures at three different mole percents, which are 30/70, 20/80, and 15/85, respectively. 2.3. Swelling and diffusion studies Dried poly(AAm-co-AAc)1–3 hydrogels irradiated at 8.0 kGy were left to swell in distilled water ionic strength I ¼ 0.1, 50 mg dm3 of SO and M solutions each at 25 1C to measure the parameters of diffusion and swelling. The concentrations of dyes in the solutions were determined
Adsorption of the dyes from aqueous solutions was investigated in continuous adsorption equilibrium experiments. Effects of initial concentration and content of AAc in AAm/AAc monomer mixtures to prepare hydrogels on adsorption rate and capacity were studied. The cationic dye solutions were prepared in the concentration ranges of 5–60 mg dm3 for SO and M. Approximately 0.05 mg of poly(AAm-co-AAc)1–3 hydrogels, which are of different compositions, were transferred into 50 mL of dye solutions at 0.1 ionic strength and agitated magnetically at moderate rpm and allowed to be in equilibrium for 48 h at 25 1C. After adsorption, the poly(AAm-co-AAc) hydrogels were removed from the adsorption medium. The concentration of the dyes in aqueous phases were measured by a UV–visible spectrophotometer after the desired treatment period. 3. Results and discussion 3.1. Swelling and diffusion The swelling of poly(AAm-co-AAc) hydrogels in water and in aqueous solutions of 50 mg dm3 SO and M was calculated from the following relation (Buckley and Berger, 1962; Gu¨ven and S- en, 1990): % Swelling ¼ ½ðmt m0 Þ=m0 100,
where mt is the mass of swollen gel at time t, and m0 is the initial mass of the dry gel. The swelling curves of poly(AAm-co-AAc)3 hydrogel in distilled water and solutions of SO and M are shown in Fig. 1. Fig. 1 shows
Table 1 Chemical formula of monomers and dyes +
H H C C H CONH2 Acrylamide
H H C C H COOH Acrylic Acid
H3C
N
H2N
N+
NH2 Cl-
CH3
Cl-
NH2 C
H2N
Safranine-O -1
Molar mass (gmol ): 350 max(nm): 517 Color Index (CI):50240
(1)
Basic Magenta -1
Molar mass (gmol ): 338 max(nm): 539 Color Index (CI): 42500
NH2
ARTICLE IN PRESS D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452
that maximum swellings occur with water (2700%), SO (3500%), M (4000%). The following equation was used to determine the nature of diffusion of water and aqueous solutions of dyes into poly(AAm-co-AAc) hydrogels (Buckley and Berger, 1962): F ¼ ktn .
(2)
% Swelling
4000
3000
2000
Water Magenta Safranine-O
1000
1000
2000
3000
4000
6000
5000
In this equation, F denotes the amount of solvent fraction at time t. k is a constant incorporating characteristics of the macromolecular network system and the penetrant, and n is the diffusional exponent, which is indicative of the transport mechanism. This equation was applied to the initial stages of swelling, and plots of ln(F) versus ln(t) are presented in Fig. 2. The exponents (n) and k were calculated from the slope and intercept of the lines and the results are given in Table 2. For hydrogel characterization, the diffusion coefficient can be calculated by various methods. The short time approximation method is used for calculation of diffusion coefficients of poly(AAm-co-AAc) hydrogels. The short time approximation is valid for the first 60% of the swelling. The diffusion coefficients of the cylindrical poly(AAm-co-AAc) hydrogels are calculated from the following relations (Buckley and Berger, 1962): F ¼ 4ðDt=pr2 Þ1=2 pðDt=pr2 Þ p=3ðDt=pr2 Þ3=2 ,
0 0
449
t (min) Fig. 1. Swelling curves of poly(AAm-co-AAc)3 hydrogels in water and in 50 mg dm3 Safranine-O and Magenta solutions at pH 7.0 and 25 1C.
(3)
where D is in cm2 min1, t is the time at which the swelling is one-half the equilibrium value (V/V0 ¼ 12), and r is the radius of the swollen cylindirical sample. A graphical comparisons of Eqs. 2 and 3 shows the semi-empirical Eq. 3 and k ¼ 4(D/pr2) 12 and D ¼ 0.049/(t/4r2) 12. For the hydrogels, F versus t 12 curves are given in Fig. 3. The slopes
0.15
-1.5 -2.0
0.10 F
lnF
-2.5 -3.0 -3.5
H2O Magenta Safranine-O
-4.0 -4.5 2.0
H 2O Magenta Safranine-O
0.05
0.00 2.5
3.5
3.0
4.5
4.0
2
5.5
5.0
4
6
8
10
12
t1/2
lnt Fig. 2. Plots of lnF versus lnt of poly(AAm-co-AAc)3 hydrogels in water and in 50 mg dm3 Safranine-O and Magenta solutions at pH 7.0 and 25 1C.
Fig. 3. Plots of F versus t 12 of poly(AAm-co-AAc)3 hydrogels in water and in 50 mg dm3 Safranine-O and Magenta solutions at pH 7.0 and 25 1C.
Table 2 Swelling and diffusion parameters of poly(AAm-co-AAc) hydrogels in water and Safranine-O and Magenta solutions at pH 7.0 and 25 1C Poly(AAm-co-AAc)1 2
Water SO M D: cm2 min1.
Poly(AAm-co-AAc)2 5
2
Poly(AAm-co-AAc)3 5
k 10
n
D 10
k 10
n
D 10
k 102
n
D 105
0.35 0.32 0.36
0.76 0.83 0.79
2.48 2.95 2.98
0.40 0.24 0.33
0.70 0.85 0.88
2.22 2.44 2.89
0.39 0.35 0.38
0.70 0.85 0.87
3.05 3.13 3.63
ARTICLE IN PRESS D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452
of the lines of F versus t 1/2 give the diffusion coefficients that are listed for three different hydrogels in Table 2. Diffusion coefficients are important parameters for the penetration of some chemical species into polymeric systems. The n values in this table range between 0.70 and 0.87. The diffusion of aqueous solutions of dyes into poly(AAm-co-AAc) hydrogels was taken as a non-Fickian character. Penetration of solvent into hydrogel is more easy in aqueous solutions of SO and M, since there is a hydrophilic interaction with these dyes and hydrogel. The equilibrium swelling of hydrogels is high (3500–4000%) in aqueous solutions of SO and M, which are cationic dyes, but low (2700%) in water. The dyes contain unpaired electrons of N. These atoms behave like a hydrophilic group and hydrogen bonds with water. Thus, the swelling of poly(AAm-co-AAc) hydrogels increases as the dyes bring water into the gels. At the end of these evaluations, the hydrogels in the aqueous solutions were swollen in the following order: M4SO4water. The number that determines the type of diffusion n was found to be over 0.70 in the aqueous dye solutions. Hence, the diffusion of water is not easier than the other molecules. Non-Fickian transport is observed as the pH of the surrounding fluid increases above pKa (Ende and Peppas, 1996). For cylindirical hydrogels, in Non-Fickian type diffusion, n is between 0.45 and 1.00. Hence, the diffusion into poly(AAm-co-AAc) hydrogels was taken to be as a non-Fickian. It can be said that for higher swelling values of the hydrogels, the transport of water into the hydrogels becomes more non-Fickian. This is generally explained as a consequence of the slow relaxation rate of the polymer matrix. The cationic dye molecules interact with the carboxyl groups of acrylic acid in poly(AAm-co-AAc) hydrogel, so the hydrophilic groups of the poly(AAm-coAAc) are not bonded with water. Thus, the swelling of the hydrogel increased in the aqueous solutions of cationic dyes.
3.2. Adsorption studies To observe the adsorption of SO and M, poly(AAm-coAAc)3 hydrogels were placed in aqueous solutions of cationic dyes such as SO and M. The amount of adsorption per unit mass of the poly(AAm/AAc) hydrogels was evaluated by using the following equation: qe ¼ ½ðC 0 C e ÞV =W ,
(4)
where, qe is the amount of dye adsorbed on unit dry mass of the poly(AAm/AAc) hydrogels (mg g1), C0 and Ce are the concentrations of the dyes in the initial solution and the aqueous phase after treatment for a certain period of time, respectively (mg dm3), V is the volume of the aqueous phase (L) and W is the amount of dry poly(AAm/AAc) hydrogels (g).
In an adsorption system at equilibrium, the total solute concentration (Ct, mol dm3) is Ct ¼ Cb þ Ce,
(5)
where Cb is the equilibrium mole number of the solute on the adsorbent per dm3 solution (bound solute concentration) and Ce is the equilibrium concentration of the solute in the solution in mol dm3 (free solute concentration). The value of the bound concentration may be obtained by using Eq. (5). For a fixed free solute concentration, Cb is proportional to the polymer concentration on the binding system; the amount bound can therefore be conveniently expressed as the binding ratio, r, defined by r ¼ C b =P.
(6) 3
Thus, with Cb in mol dm and P in base mol (moles of monomer units) per dm3 solution, r represents the average number of molecules of solute bound to each monomer unit at that free solute concentration (Karadag˘ and Saraydın, 1996). 3.3. Effect of concentration of dye solution on adsorption To determine the effect of concentration of dye solution on adsorption, plots of the binding ratio (r) against the free concentrations of the dyes in solutions (C, m mol dye dm3) are shown in Figs. 4 and 5 for SO and M at pH 7 and 25 1C and 0.1 ionic strength, respectively. These figures show that adsorptions of dyes within different compositions of poly(AAm-co-AAc) hydrogels correspond to type-S adsorption isotherms in the Giles classification system for adsorption of a solute from its solution (Giles et al., 1960). In the S curves, the initial direction of curvature shows that adsorption becomes stronger as concentration increases. In practice, the S curve usually appears when three conditions are fulfilled: the solute molecule (a) is
rx106 (mol dye/repeating unit in hydrogel)
450
4000 3500 3000 2500 2000 1500 1000 Safranine-O
500 0 0
5
10
15
20
25
30
35
40
Cex106 (mol dye dm-3) Fig. 4. The variations of binding ratio (r) of poly(AAm-co-AAc)3 hydrogel–dye systems with the equilibrium concentration in Safranine-O solutions at pH 7.0 and 25 1C.
ARTICLE IN PRESS rx106 (mol dye/repeating uniy in hydrogel)
D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452 1400 1200 1000 800 600 400 Magenta
200 0 0
2
4
8
6
10
Cex106 (mol dye dm-3)
rx106 (mol dye/repeating unit in hydrogel)
Fig. 5. The variations of binding ratio (r) of poly(AAm-co-AAc)3 hydrogel–dye systems with the equilibrium concentration in Magenta solutions at pH 7.0 and 25 1C.
energy is unusually small as compared to the polar contribution. Not surprisingly, water provided many examples of type S isotherms. The hydrogel is covered with a layer of adsorbed water; however, the adsorbent–adsorbate interaction would be virtually reduced to the weak dispersion energy of water with dyes, so that a type S isotherm should result. In a system that gives rise to a type S isotherm, however, multiple layers are built up on some parts of the surface, while a monolayer is still incomplete on other parts. The binding ratio of cationic dyes SO and M into hydrogel systems gradually increases with the increase in content of AAc in AAm/AAc monomer mixtures used to prepare poly(AAm/AAc) hydrogel. The changes between the binding ratio (r) and AAc% are given in Fig. 6. 4. Conclusion Poly(AAm-co-AAc) hydrogel prepared from AAm/AAc monomer mixtures containing more AAc, showed maximum % swelling in distilled water. Diffusion of water and dye solutions within hydrogels was found to be of a non-Fickian character. Diffusion coefficients were calculated for poly(AAm-coAAc) hydrogel in water, SO, and M solutions. Initial swelling rate increased with increased pH and AAc content in hydrogel. Adsorption capacity of poly(AAm-co-AAc) hydrogels was changed with amount of AAc, and adsorption isotherms of poly(AAm-co-AAc) hydrogels were S type.
4000 3500 Magenta Safranine-O
3000
451
2500 2000 1500 1000 500 0 0
20
40 % AAc
60
80
Fig. 6. The variations of binding ratio (r) of poly(AAm-co-AAc)3 hydrogel-dye systems with the AAc content in poly(AAm-co-AAc) hydrogels at pH 7.0 and 25 1C.
monofunctional, (b) has moderate intermolecular attraction, causing it to pack vertically in a regular array in the adsorbed layer, and (c) meets strong competition for substrate sites, from molecules of the solvent or of another adsorbed species. The weakness of the adsorbent–adsorbate forces will cause the uptake at low concentrations to be less, but once a molecule has become adsorbed the adsorbate–adsorbate forces will promote the adsorption of further molecules–a cooperative process–so that the isotherm will become convex to the concentration axis. Type S isotherms may originate through the adsorption of either nonpolar molecules or polar molecules, always provided that the adsorbent–adsorbate force is relatively weak. A polar adsorbate of particular interest in this context is water, because the dispersion contribution to its overall interaction
References Alvarez-Lorenzo, C., Concheiro, A., 2002. Reversible adsorption by a pHand temperature-sensitive acrylic hydrogel. J. Control Rel. 23–80 (1-3), 247–257. Buckley, J.D., Berger, M., 1962. The swelling of polymer system in solvents. II. Mathematics of diffusion. J. Polym. Sci. 56, 175–188. Ekici, S., Is-ıkver, Y., Saraydın, D., 2006. Poly(acrylamide-sepiolite) composite hydrogels:Preparation, swelling and dye adsorption properties. Polym. Bull. 57, 231–241. Ende, M., Peppas, N.A., 1996. Transport of ionizable drugs and proteins in crosslinked poly(acrylic acid) and poly(acrylic acid-co-2-hydroxyethyl methacrylate) hydrogels. I. Polymer characterization. J. Appl. Polym. Sci. 59, 673–685. Giles, C.H., MacEwan, T.H., Nakhwa, S.N., Smith, D., 1960. A system of classification of solution adsorption isotherms and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. J. Chem. Soc. 74, 3973–3993. Gu¨ven, O., S- en, M., 1990. Preparation and characterization of poly(N-vinyl2-pyrrolidone) hydrogels. Polymer 32 (13), 2491–2496. Karadag˘, E., Saraydın, D., 1996. Interaction of some cationic dyes with acrylamide/itaconic acid hydrogels. J. Appl. Polym. Sci. 61, 2372–2376. Karadag˘, E., U¨zu¨m, O¨.B., Saraydın, D., Gu¨ven, O., 2006. Swelling characterization of g-radiation induced CAMA hydrogels in urea solutions. Mater. Des. 27 (7), 576–584.
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McKay, G., 1982. Adsorption of Dyestuffs from aqueous solutions with activated carbon. I. Equilibrium and batch contact-time studies. J. Chem. Tech. Biotechnol. 32, 759–772. McKay, G., 1983. Adsorption of Dyestuffs from aqueous solutions with activated carbon. III. Intraparticle diffusion processes. J. Chem. Tech. Biotechnol. 33A, 196–204.
S- olpan, D., Ko¨lge, Z., 2006. Adsorption of Methyl Violet in aqueous solutions by poly(N-vinylpyrrolidone-co-methacrylic acid) hydrogels. Radiat. Phys. Chem. 75, 120–128. U¨zu¨m, O¨.B., Karadag˘, E.J., 2006. Synthetic polymeric adsorbent for dye based on chemically crosslinked acrylamide/mesaconic acid hydrogels. Appl. Polym. Sci. 101, 405–413.
Radiation Physics and Chemistry 77 (2008) 447–452 www.elsevier.com/locate/radphyschem
Removal of cationic dyes by poly(acrylamide-co-acrylic acid) hydrogels in aqueous solutions Dilek S- olpan, Sibel Duran, Murat Torun Department of Chemistry, Hacettepe University, 06800, Beytepe, Ankara, Turkey Received 20 June 2007; accepted 21 August 2007
Abstract Poly(acrylamide-co-acrylic acid (poly(AAm-co-AAc)) hydrogels prepared by irradiating with g-radiation were used in experiments on swelling, diffusion, and uptake of some cationic dyes such as Safranine-O (SO) and Magenta (M). Poly(AAm-co-AAc) hydrogels irradiated at 8.0 kGy have been used for swelling and diffusion studies in water and cationic dye solutions. The maximum swellings in water, and SO, and M solutions observed are 2700%, 3500%, and 4000%, respectively. Diffusions of water and cationic dyes within hydrogels have been found to be non-Fickian in character. Adsorption of the cationic dyes onto poly(AAm-co-AAc) hydrogels is studied by the batch adsorption technique. The adsorption type was found Langmuir type in the Giles classification system. The moles of adsorbed dye for SO and M per repeating unit in hydrogel (binding ratio, r) have been calculated as 3834 106 and 1323 106, respectively. These results show that poly(AAm-co-AAc) hydrogels can be used as adsorbent for water pollutants such as cationic dyes. r 2007 Elsevier Ltd. All rights reserved. Keywords: Hydrogel; Gamma-irradiation; Poly(acrylamide-co-acrylic acid); Swelling; Adsorption; Diffusion; Cationic dyes.
1. Introduction Adsorption has attracted considerable interest as a feasible procedure for removing color from effluents. Recently, a number of studies have been reported with regard to the adsorption equilibria and kinetic of dye removal processes using various adsorbents (AlvarezLorenzo and Concheiro, 2002), (Ekici et al., 2006). Commercially available adsorbents contain activated carbon (McKay, 1982, 1983), chitin, silica gel, wood, peat, bauxite, and polymeric adsorbents (Karadag˘ et al., 2006). Of these, activated carbon is one of the most widely used porous adsorbents because of its capability for efficiently adsorbing a broad range of different types of adsorbates (U¨zu¨m and Karadag˘, 2006). Since the cationic dyes used mostly in textile industry have large molecules, their adsorption onto activated carbon and other adsorbents may not be optimal. Polymeric adsorbents and hydrogel systems may offer distinct advantages in specifically defined cases. Corresponding author. Tel.: +90 312 2977990; fax: +90 312 2992163.
E-mail address: [email protected] (D. S- olpan). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.08.006
Hydrogels are crosslinked hydrophilic polymers that are swollen in water usually to equilibrium. Hydrogels find considerable applications and have been extensively studied, because they combine glassy behavior (in their dry state) with elasticity (when sufficient water is adsorbed). The behavior of highly swollen hydrogels is, of course, a function of the network characteristic (such as degree of swelling, diffusion coefficient, crosslink density, mesh size, etc.), which in turn is connected with chemical structures. Hydrogels have been used with widespread applications in different fields. The removal of color from textile wastewaters is a major environmental problem because of the difficulty of treating such waters by conventional methods. Some groups have used various adsorbents for the removal of acidic and basic dyes from aqueous solutions (S- olpan and Ko¨lge, 2006). This study describes the preparation and color-removal ability of a novel series of polymeric adsorbents. These adsorbents were prepared from the reaction of arylamide and acrylic acid. The ability of poly(AAm-co-AAc) hydrogels to adsorb cationic dyes from aqueous solution under several experimental conditions has been studied. The adsorption
ARTICLE IN PRESS D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452
448
effect of poly(AAm-co-AAc) hydrogels on cationic dyes indicates the very good potential of this procedure in achieving the desired objective. 2. Experimental 2.1. Materials
from the absorbance of the solutions. Swollen hydrogels removed from the water bath at regular intervals were dried superficially with filter paper, weighed, and placed in the same bath again. The measurements were continued until a constant weight was reached for each sample. 2.4. Adsorption studies
Acrylamide (AAm) and acrylic acid (AAc) monomers used in this study were obtained from BDH. Safranine-O (SO) and Basic Magenta (M) dyes were supplied from Allied Chemical. Some properties of monomers and dyes are listed in Table 1. 2.2. Preparation of hydrogels The solutions of monomers of AAm and AAc were prepared in three different compositions (AAm/AAc mole ratios, 30/70, 20/80, and 15/85). These solutions were placed in polyvinylchloride straws of 3 mm diameter and irradiated in air at ambient temperature in a Gammacell220 type g-irradiator. The percent conversion was determined gravimetrically. Dose of 8.0 kGy was applied at a fixed dose rate of 0.16 kGy h1. Hydrogels obtained in long cylindirical shapes were cut, washed with distilled water for removal of unreacted monomers, and dried in air and in vacuum, and stored for later evaluations. Poly(AAm-coAAc)1, poly(AAm-co-AAc)2, and poly(AAm-co-AAc)3 were prepared using three different compositions of poly(AAm-co-AAc) hydrogels, which were obtained by using AAm/AAc monomer mixtures at three different mole percents, which are 30/70, 20/80, and 15/85, respectively. 2.3. Swelling and diffusion studies Dried poly(AAm-co-AAc)1–3 hydrogels irradiated at 8.0 kGy were left to swell in distilled water ionic strength I ¼ 0.1, 50 mg dm3 of SO and M solutions each at 25 1C to measure the parameters of diffusion and swelling. The concentrations of dyes in the solutions were determined
Adsorption of the dyes from aqueous solutions was investigated in continuous adsorption equilibrium experiments. Effects of initial concentration and content of AAc in AAm/AAc monomer mixtures to prepare hydrogels on adsorption rate and capacity were studied. The cationic dye solutions were prepared in the concentration ranges of 5–60 mg dm3 for SO and M. Approximately 0.05 mg of poly(AAm-co-AAc)1–3 hydrogels, which are of different compositions, were transferred into 50 mL of dye solutions at 0.1 ionic strength and agitated magnetically at moderate rpm and allowed to be in equilibrium for 48 h at 25 1C. After adsorption, the poly(AAm-co-AAc) hydrogels were removed from the adsorption medium. The concentration of the dyes in aqueous phases were measured by a UV–visible spectrophotometer after the desired treatment period. 3. Results and discussion 3.1. Swelling and diffusion The swelling of poly(AAm-co-AAc) hydrogels in water and in aqueous solutions of 50 mg dm3 SO and M was calculated from the following relation (Buckley and Berger, 1962; Gu¨ven and S- en, 1990): % Swelling ¼ ½ðmt m0 Þ=m0 100,
where mt is the mass of swollen gel at time t, and m0 is the initial mass of the dry gel. The swelling curves of poly(AAm-co-AAc)3 hydrogel in distilled water and solutions of SO and M are shown in Fig. 1. Fig. 1 shows
Table 1 Chemical formula of monomers and dyes +
H H C C H CONH2 Acrylamide
H H C C H COOH Acrylic Acid
H3C
N
H2N
N+
NH2 Cl-
CH3
Cl-
NH2 C
H2N
Safranine-O -1
Molar mass (gmol ): 350 max(nm): 517 Color Index (CI):50240
(1)
Basic Magenta -1
Molar mass (gmol ): 338 max(nm): 539 Color Index (CI): 42500
NH2
ARTICLE IN PRESS D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452
that maximum swellings occur with water (2700%), SO (3500%), M (4000%). The following equation was used to determine the nature of diffusion of water and aqueous solutions of dyes into poly(AAm-co-AAc) hydrogels (Buckley and Berger, 1962): F ¼ ktn .
(2)
% Swelling
4000
3000
2000
Water Magenta Safranine-O
1000
1000
2000
3000
4000
6000
5000
In this equation, F denotes the amount of solvent fraction at time t. k is a constant incorporating characteristics of the macromolecular network system and the penetrant, and n is the diffusional exponent, which is indicative of the transport mechanism. This equation was applied to the initial stages of swelling, and plots of ln(F) versus ln(t) are presented in Fig. 2. The exponents (n) and k were calculated from the slope and intercept of the lines and the results are given in Table 2. For hydrogel characterization, the diffusion coefficient can be calculated by various methods. The short time approximation method is used for calculation of diffusion coefficients of poly(AAm-co-AAc) hydrogels. The short time approximation is valid for the first 60% of the swelling. The diffusion coefficients of the cylindrical poly(AAm-co-AAc) hydrogels are calculated from the following relations (Buckley and Berger, 1962): F ¼ 4ðDt=pr2 Þ1=2 pðDt=pr2 Þ p=3ðDt=pr2 Þ3=2 ,
0 0
449
t (min) Fig. 1. Swelling curves of poly(AAm-co-AAc)3 hydrogels in water and in 50 mg dm3 Safranine-O and Magenta solutions at pH 7.0 and 25 1C.
(3)
where D is in cm2 min1, t is the time at which the swelling is one-half the equilibrium value (V/V0 ¼ 12), and r is the radius of the swollen cylindirical sample. A graphical comparisons of Eqs. 2 and 3 shows the semi-empirical Eq. 3 and k ¼ 4(D/pr2) 12 and D ¼ 0.049/(t/4r2) 12. For the hydrogels, F versus t 12 curves are given in Fig. 3. The slopes
0.15
-1.5 -2.0
0.10 F
lnF
-2.5 -3.0 -3.5
H2O Magenta Safranine-O
-4.0 -4.5 2.0
H 2O Magenta Safranine-O
0.05
0.00 2.5
3.5
3.0
4.5
4.0
2
5.5
5.0
4
6
8
10
12
t1/2
lnt Fig. 2. Plots of lnF versus lnt of poly(AAm-co-AAc)3 hydrogels in water and in 50 mg dm3 Safranine-O and Magenta solutions at pH 7.0 and 25 1C.
Fig. 3. Plots of F versus t 12 of poly(AAm-co-AAc)3 hydrogels in water and in 50 mg dm3 Safranine-O and Magenta solutions at pH 7.0 and 25 1C.
Table 2 Swelling and diffusion parameters of poly(AAm-co-AAc) hydrogels in water and Safranine-O and Magenta solutions at pH 7.0 and 25 1C Poly(AAm-co-AAc)1 2
Water SO M D: cm2 min1.
Poly(AAm-co-AAc)2 5
2
Poly(AAm-co-AAc)3 5
k 10
n
D 10
k 10
n
D 10
k 102
n
D 105
0.35 0.32 0.36
0.76 0.83 0.79
2.48 2.95 2.98
0.40 0.24 0.33
0.70 0.85 0.88
2.22 2.44 2.89
0.39 0.35 0.38
0.70 0.85 0.87
3.05 3.13 3.63
ARTICLE IN PRESS D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452
of the lines of F versus t 1/2 give the diffusion coefficients that are listed for three different hydrogels in Table 2. Diffusion coefficients are important parameters for the penetration of some chemical species into polymeric systems. The n values in this table range between 0.70 and 0.87. The diffusion of aqueous solutions of dyes into poly(AAm-co-AAc) hydrogels was taken as a non-Fickian character. Penetration of solvent into hydrogel is more easy in aqueous solutions of SO and M, since there is a hydrophilic interaction with these dyes and hydrogel. The equilibrium swelling of hydrogels is high (3500–4000%) in aqueous solutions of SO and M, which are cationic dyes, but low (2700%) in water. The dyes contain unpaired electrons of N. These atoms behave like a hydrophilic group and hydrogen bonds with water. Thus, the swelling of poly(AAm-co-AAc) hydrogels increases as the dyes bring water into the gels. At the end of these evaluations, the hydrogels in the aqueous solutions were swollen in the following order: M4SO4water. The number that determines the type of diffusion n was found to be over 0.70 in the aqueous dye solutions. Hence, the diffusion of water is not easier than the other molecules. Non-Fickian transport is observed as the pH of the surrounding fluid increases above pKa (Ende and Peppas, 1996). For cylindirical hydrogels, in Non-Fickian type diffusion, n is between 0.45 and 1.00. Hence, the diffusion into poly(AAm-co-AAc) hydrogels was taken to be as a non-Fickian. It can be said that for higher swelling values of the hydrogels, the transport of water into the hydrogels becomes more non-Fickian. This is generally explained as a consequence of the slow relaxation rate of the polymer matrix. The cationic dye molecules interact with the carboxyl groups of acrylic acid in poly(AAm-co-AAc) hydrogel, so the hydrophilic groups of the poly(AAm-coAAc) are not bonded with water. Thus, the swelling of the hydrogel increased in the aqueous solutions of cationic dyes.
3.2. Adsorption studies To observe the adsorption of SO and M, poly(AAm-coAAc)3 hydrogels were placed in aqueous solutions of cationic dyes such as SO and M. The amount of adsorption per unit mass of the poly(AAm/AAc) hydrogels was evaluated by using the following equation: qe ¼ ½ðC 0 C e ÞV =W ,
(4)
where, qe is the amount of dye adsorbed on unit dry mass of the poly(AAm/AAc) hydrogels (mg g1), C0 and Ce are the concentrations of the dyes in the initial solution and the aqueous phase after treatment for a certain period of time, respectively (mg dm3), V is the volume of the aqueous phase (L) and W is the amount of dry poly(AAm/AAc) hydrogels (g).
In an adsorption system at equilibrium, the total solute concentration (Ct, mol dm3) is Ct ¼ Cb þ Ce,
(5)
where Cb is the equilibrium mole number of the solute on the adsorbent per dm3 solution (bound solute concentration) and Ce is the equilibrium concentration of the solute in the solution in mol dm3 (free solute concentration). The value of the bound concentration may be obtained by using Eq. (5). For a fixed free solute concentration, Cb is proportional to the polymer concentration on the binding system; the amount bound can therefore be conveniently expressed as the binding ratio, r, defined by r ¼ C b =P.
(6) 3
Thus, with Cb in mol dm and P in base mol (moles of monomer units) per dm3 solution, r represents the average number of molecules of solute bound to each monomer unit at that free solute concentration (Karadag˘ and Saraydın, 1996). 3.3. Effect of concentration of dye solution on adsorption To determine the effect of concentration of dye solution on adsorption, plots of the binding ratio (r) against the free concentrations of the dyes in solutions (C, m mol dye dm3) are shown in Figs. 4 and 5 for SO and M at pH 7 and 25 1C and 0.1 ionic strength, respectively. These figures show that adsorptions of dyes within different compositions of poly(AAm-co-AAc) hydrogels correspond to type-S adsorption isotherms in the Giles classification system for adsorption of a solute from its solution (Giles et al., 1960). In the S curves, the initial direction of curvature shows that adsorption becomes stronger as concentration increases. In practice, the S curve usually appears when three conditions are fulfilled: the solute molecule (a) is
rx106 (mol dye/repeating unit in hydrogel)
450
4000 3500 3000 2500 2000 1500 1000 Safranine-O
500 0 0
5
10
15
20
25
30
35
40
Cex106 (mol dye dm-3) Fig. 4. The variations of binding ratio (r) of poly(AAm-co-AAc)3 hydrogel–dye systems with the equilibrium concentration in Safranine-O solutions at pH 7.0 and 25 1C.
ARTICLE IN PRESS rx106 (mol dye/repeating uniy in hydrogel)
D. S- olpan et al. / Radiation Physics and Chemistry 77 (2008) 447–452 1400 1200 1000 800 600 400 Magenta
200 0 0
2
4
8
6
10
Cex106 (mol dye dm-3)
rx106 (mol dye/repeating unit in hydrogel)
Fig. 5. The variations of binding ratio (r) of poly(AAm-co-AAc)3 hydrogel–dye systems with the equilibrium concentration in Magenta solutions at pH 7.0 and 25 1C.
energy is unusually small as compared to the polar contribution. Not surprisingly, water provided many examples of type S isotherms. The hydrogel is covered with a layer of adsorbed water; however, the adsorbent–adsorbate interaction would be virtually reduced to the weak dispersion energy of water with dyes, so that a type S isotherm should result. In a system that gives rise to a type S isotherm, however, multiple layers are built up on some parts of the surface, while a monolayer is still incomplete on other parts. The binding ratio of cationic dyes SO and M into hydrogel systems gradually increases with the increase in content of AAc in AAm/AAc monomer mixtures used to prepare poly(AAm/AAc) hydrogel. The changes between the binding ratio (r) and AAc% are given in Fig. 6. 4. Conclusion Poly(AAm-co-AAc) hydrogel prepared from AAm/AAc monomer mixtures containing more AAc, showed maximum % swelling in distilled water. Diffusion of water and dye solutions within hydrogels was found to be of a non-Fickian character. Diffusion coefficients were calculated for poly(AAm-coAAc) hydrogel in water, SO, and M solutions. Initial swelling rate increased with increased pH and AAc content in hydrogel. Adsorption capacity of poly(AAm-co-AAc) hydrogels was changed with amount of AAc, and adsorption isotherms of poly(AAm-co-AAc) hydrogels were S type.
4000 3500 Magenta Safranine-O
3000
451
2500 2000 1500 1000 500 0 0
20
40 % AAc
60
80
Fig. 6. The variations of binding ratio (r) of poly(AAm-co-AAc)3 hydrogel-dye systems with the AAc content in poly(AAm-co-AAc) hydrogels at pH 7.0 and 25 1C.
monofunctional, (b) has moderate intermolecular attraction, causing it to pack vertically in a regular array in the adsorbed layer, and (c) meets strong competition for substrate sites, from molecules of the solvent or of another adsorbed species. The weakness of the adsorbent–adsorbate forces will cause the uptake at low concentrations to be less, but once a molecule has become adsorbed the adsorbate–adsorbate forces will promote the adsorption of further molecules–a cooperative process–so that the isotherm will become convex to the concentration axis. Type S isotherms may originate through the adsorption of either nonpolar molecules or polar molecules, always provided that the adsorbent–adsorbate force is relatively weak. A polar adsorbate of particular interest in this context is water, because the dispersion contribution to its overall interaction
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