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Removal of anionic dyes from aqueous solutions by adsorption


Composites: Part B 43 (2012) 1570–1578

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Composites: Part B
journal homepage: www.elsevier.com/locate/compositesb

Removal of anionic dyes from aqueous solutions by adsorption of chitosan-based semi-IPN hydrogel composites
Sanping Zhao ?, Feng Zhou, Liyan Li, Mengjie Cao, Danying Zuo, Hongtao Liu
Key Laboratory of Green Processing and Functional Textiles of New Textile Materials of Ministry of Education, Wuhan Textile University, Wuhan 430073, PR China

a r t i c l e

i n f o

a b s t r a c t
Semi-IPN hydrogel composites for dye adsorption studies were prepared via photopolymerization of poly(ethylene glycol) (PEG) macromer and acrylamide (AAm) monomer in the presence of chitosan (CS). Swelling properties and kinetics of the hydrogel composites were investigated in aqueous solution and Acid Red 18 (AR 18) solution. The adsorption studies showed that the adsorption capacity for AR 18 increased with the increase of initial dye concentration and chitosan content in the hydrogels, but decreased with the increase of pH and ionic strength of dye solutions. Absorption kinetics of AR 18 followed pseudo second-order kinetic model at pH 2.0. The adsorption capacities for Acid Orange 7 (AO 7), Methyl Orange (MO) and Basic Violet 14 (BV 14) were also examined at pH 2.0, and the equilibrium adsorption data of AR 18, AO 7 and MO well ?tted the Langmuir isotherm. The hydrogel composites could be potentially used as absorbents for anionic dye removal in wastewater treatment process. ? 2012 Elsevier Ltd. All rights reserved.

Article history: Received 15 April 2011 Received in revised form 5 December 2011 Accepted 2 January 2012 Available online 9 January 2012 Keywords: A. Polymer–matrix composites (PMCs) B. Physical properties Adsorption pro?les

1. Introduction During the past three decades, the pollution from dye wastewater has attracted much attention due to the growing use of a variety of dyes in the textile, leather, paper, rubber, plastic and food industries [1,2]. Among them, synthetic dyes derived from coal-tar-based hydrocarbons such as benzene, naphthalene, anthracene, toluene, and xylene are more stable and more dif?cult to biodegrade due to their complex aromatic molecular structures [3]. Even tiny amount of dyes in water may cause signi?cant color change, and many of these dyes are known to be toxic or carcinogenic. They not only affect aquatic life but also traverse through the entire food web, resulting in biomagni?cation [4]. Numerous techniques including physical–chemical and biological decolorization methods have been developed to treat wastewater ef?uents containing dyes [5]. Of them, adsorption using adsorbents is considered as an effective and economic method for water decontamination. Many low-cost adsorbents have been proposed and investigated for their ability to remove dyes [6–8]. Recently, special attention has been paid to naturally polysaccharide-based adsorbents such as chitosan and its derivatives. Chitosan is a kind of abundant and naturally occurring hydrophilic cationic polysaccharide derived from chitin. It has been widely investigated as a biosorbent for the capture of dyes from aqueous solutions due to its easy availability, ready chemical modi?cations, environmental friendly behavior, low cost and
? Corresponding author. Tel./fax: +86 27 87426559.
E-mail address: zhaosanping@163.com (S. Zhao). 1359-8368/$ - see front matter ? 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2012.01.015

outstanding adsorption capacities for a wide range of dyes [9]. Generally, as an effective adsorbent for decoloring applications, chemical crosslinking of chitosan is required to improve the mechanical resistance and to reinforce the chemical stability of the chitosan in acidic media. Different crosslinkers such as glutaraldehyde (GLU) or ethylene glycol diglycidyl ether (EGDE) were employed to produce covalently crosslinked materials from gels to beads or particles for the removal of the dyes [10,11]. However, the dye adsorption capacities and uptake ef?ciency signi?cantly decreased due to the decrease of availability of amine functions for the complexation with dyes as well as the decrease of the accessibility to internal sites of the materials which could be explained by the loss in ?exibility of chitosan chain resulting from the crosslinking [12,13]. Semi-interpenetrating polymer network (semi-IPN) technology is a simple and feasible route to fabricate polysaccharide-based hydrogels, in which hydrophilic polysaccharide chain penetrates into another crosslinked polymeric network without any chemical bonds between them. The polysaccharide-based semi-IPN hydrogels have been extensively investigated in biomedical application ?elds due to their nontoxicity, biodegradability and biocompatibility [14–16]. It is demonstrated that the formation of semi-IPN structure can conserve the characteristics of each polymeric network, and the interlocked structure in the crosslinked networks enhances the stability of the materials, thereby ensuring the mechanical strength [17,18]. These semi-IPN hydrogels often possess porous network structure and allow solute diffusion through the hydrogel structure, and the polyelectrolytic polysaccharides in the crosslinked network have ionic functional groups such as

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carboxylic acid, amine, sulfonic acid groups, which can absorb and trap ionic dyes and heavy metals [19,20]. In previous studies, we reported the synthesis of different polysaccharide-based semi-IPN hydrogel systems such as guar gum, sodium alginate, chitosan by UV irradiation technology, and their swelling behaviors were investigated in details, and the release pro?le of BSA from the resultant hydrogels as controlled-release vehicles was also evaluated [18,21,22]. In this work, semi-IPN hydrogel composites were prepared by incorporating chitosan into hydrophilic copolymerized network of PEG macromer and AAm monomer and investigated as absorbents for the removal of anionic dyes from aqueous solutions. 2. Experimental 2.1. Materials PEG (Mn = 6000) was imported from Japan and distributed domestically. Glycidyl methacrylate (GMA, 97%, Aldrich, USA), 1-vinyl-2-pyrrolidone (NVP, 97%, Fluka), 4-dimethylaminopyridine (DMAP, 99%, TCI, Japan), and the photoinitiator, 2,2-dimethoxy-2phenyl acetophenone (DMPA) (Fluka, Switzerland) were used as received. Chitosan (Mw = 4.5 ? 105, degree of deacetylation = 90%) was purchased from RuJi Biotech Development Co., Ltd., (Shanghai, China). The other chemicals used were of analytical grade and used without further puri?cation. The dyes used in adsorption studies were Acid Red 18 (AR 18), Acid Orange 7 (AO 7), Methyl Orange (MO) and Basic Violet 14 (BV 14), which purchased from Shanghai Reagent Corp., China, and their structure and characteristics were exhibited in Table 1. Spectrophotometric measurements in adsorption studies were performed on a UV-2550 (SHIMADZU, Japan) UV–Vis spectrophotometer with a spectral resolution of 1 nm. 2.2. Synthesis of PEG macromer and the semi-IPN hydrogel composites PEG macromer was synthesized via the ring-opening reaction of the terminal hydroxyl groups of PEG with epoxy group of GMA

using DMAP as a catalyst, according to previously reported method [21]. Brie?y, 12 g of PEG was dissolved in 100 mL of dichloromethane in a 250-mL round-bottomed ?ask, 1.14 g of GMA and 0.5 g of DMAP were added to the solution. The mixture was stirred at room temperature for 48 h. The crude product was precipitated in an excess of anhydrous ethyl ether. The product was further puri?ed by dissolving in dichloromethane and precipitating in anhydrous ethyl ether twice, and the precipitate was dried under vacuum at room temperature for 48 h. The semi-IPN hydrogel composites were prepared via UV-initiated free-radical polymerization [22]. The feed compositions of the hydrogels in this study are listed in Table 2. Brie?y, acrylamide (AAm), PEG macromer and chitosan at different ratios were dissolved in 2 wt% acetic acid (HAc) solution at room temperature. Then a speci?ed amount of photoinitiator solution of DMPA in NVP (100 mg mL?1) was added to the mixture (1 wt% DMPA to the total amount of the macromer and AAm monomer), the resulting solution was homogeneously mixed and added to a Te?on model with a diameter of 5 cm. Following this, the mixture was exposed to 365 nm LWUV lamp of 16 W (ZF-7A type, Shanghai Jihui Scienti?c Instrumental Co. Ltd.) for 20 min to ensure the enough copolymerization of the macromer with AAm monomer. The distance between the reaction mixture and the light source is kept at 2 cm. For comparison, the copolymerized hydrogel without adding chitosan was synthesized in the same conditions. The photocrosslinked hydrogels were punched into disks (10 mm in diameter and 3 mm in thickness). To remove the residual unreacted monomers and other impurities in the just obtained hydrogel samples, the samples were immersed in distilled water for 3 days at room temperature, and the water was refreshed every 12 h. Finally, the samples were dried at room temperature for 2 days and then dried at 60 °C under reduced pressure for another 2 days.

2.3. Swelling studies in aqueous solution and AR 18 solution at pH 2.0 Swelling behavior was studied by a gravimetric method. Dried semi-IPN hydrogel samples were left to swell in aqueous solution and AR 18 solution (200 mg L?1) with a ?xed ionic strength

Table 1 Chemical structure and some properties of different dyes in this study. Name Acid Red 18(AR 18) Chemical structure Molar mass (g mol?1) 604.47 Color index number 16,225 kmax (nm) 506

Acid Orange 7 (AO 7)

350.32

15,510

483

Methyl Orange (MO)

327.33

13,025

505

Basic Violet 14 (BV 14)

323.82

42,500

545

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Table 2 Feed compositions for the preparation of the semi-IPN hydrogel composites. Sample code Feed components PEG macromer (g) 0.5 0.5 0.5 0.5 AAm (g) 0.7 0.7 0.7 0.7 Chitosan (mg) 0 70 140 240 2 wt% HAc (g) DMAP (mg)

2.5. Equilibrium adsorption isotherm The isotherm models of linear Langmuir and Freundlich were often applied to describe the equilibrium adsorption isotherm data. In the experiments of equilibrium adsorption isotherm, a ?xed amount of 60 mg dried hydrogel composite was immersed into 100 mL of dye solutions with a ?xed ionic strength (I = 0.01 mol L?1) at pH 2.0, the mixture was shaken to equilibrate for 3 days at 25 °C. The initial dye concentrations were in the range of 50– 350 mg L?1. The amount of adsorption at equilibrium (qe) was calculated according to the following equation:

PA-0 PA-1.5 PA-3 PA-5

4.5 4.5 4.5 4.5

13 13 13 13

(I = 0.01 mol L?1) at pH 2.0 at 25 °C. The swollen weight was measured for each sample at a given time after excess surface water was carefully wiped off with moistened ?lter paper. Each sample was measured three times and the average value of three measurements was taken. The swelling ratio (SR) was calculated as follows:

qe ? ??C 0 ? C e ? ? V ?=m

?7?

SR?%? ? ??W s ? W d ?=W d ? ? 100

? 1?

where Ws and Wd denote the weight of the swollen and dried hydrogel sample, respectively. The water uptake data were analyzed with the ?rst-order kinetic (Eq. (2)) and second-order kinetic (Eq. (3)) equations [23,24]:

where C0 and Ce are the initial and equilibrium solution concentration of dyes (mg mL?1), respectively. V and m are the volume of the dye solution (mL) and the weight of the dried semi-IPN hydrogel sample (g). The equilibrium data were analyzed in accordance with the Langmuir adsorption isotherm as follows [26]:

C e =qe ? 1=?b ? qm ? ? C e =qm

?8?

ln?W 1 =?W 1 ? W ?? ? k1 t   t =W ? 1= k2 W 2 1 ? t =W 1

? 2? ? 3?

where qm (mg g?1) is the maximum monolayer adsorption capacity, and b (L mg?1) is the Langmuir constant. The dimensionless separation factor, RL, is essential characteristic of the Langmuir isotherm, which is de?ned as follows:

where W1 is the maximum water uptake, W is the water uptake at time t, (W1 ? W) is the unrealized water uptake, and k1 and k2 is the speci?c rate constant. 2.4. Adsorption studies For the adsorption kinetics, dried semi-IPN hydrogel samples (60 mg) were immersed in 100 mL 200 mg mL?1 of dye solutions at 25 °C. If no special instructions, the ionic strength of dye solutions investigated was maintained 0.01 mol L?1 adjusting by NaCl, pH was 2.0 and all adsorption experiments were carried out at a constant shaking rate of 100 rpm min?1. During the adsorption process, 2 mL of the dye solution was taken out from the system at regular intervals of time and the dye concentrations were measured using a Shimadzu 2550 model UV–Vis spectrophotometer. The amount of the dye adsorbed by the hydrogel at time t, qt (mg g?1 dried hydrogel), was calculated using the following expression:

RL ? 1=?1 ? bC 0 ?

?9?

where b is the Langmuir constant and C0 is the highest initial dye concentration. The value of RL indicates the type of the isotherm to be either favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL = 1) or irreversible (RL = 0) [27]. The Freundlich model was described the non-ideal adsorption on heterogeneous surfaces, suggesting multilayer adsorption on adsorbent surfaces. The Freundlich constants KF and exponent (n) were calculated from the following equation [26]:

log qe ? log K F ? l=n ? log C e

?10?

where KF ((mg g?1)(L mg?1)1/n) is the Freundlich adsorption constant and the slope of the Freundlich equation 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, which may become more heterogeneous when 1/n gets close to zero. The value of 1/n below one suggests a normal Langmuir isotherm while 1/n above one is indicative of cooperative adsorption. 3. Results and discussion 3.1. Synthesis of PEG macromer and the semi-IPN hydrogel composites PEG macromer as a crosslinker was synthesized via the ringopening reaction between the AOH groups of PEG and epoxy group of GMA. Fig. 1 presents the 1H NMR spectrum of the macromer. The signals at 5.5–6.2 ppm belong to the protons of AC(CH3)@CH2 attached to the both ends of PEG. The signal at 3.63 ppm results from the protons of ACH2CH2A units of PEG. The signals at 4.1–4.3 ppm belong to the protons of ACH2CH2OAGMA and ACH2CH(OH)CH2A of GMA. The signal at 1.95 ppm is attributed to the protons of ACH3 group of GMA. The degree of methacrylation in the macromer could be calculated from the signal intensity ratio of the ACH2CH2A protons in the PEG (3.63 ppm) and the carbon–carbon double bonds (5.5–6.2 ppm), and was found to be 97.8%. These results indicate that the photcrosslinkable macromer was successfully synthesized. Semi-IPN hydrogels were prepared via in situ copolymerization of AAm monomer with PEG macromer in the presence of chitosan by UV irradiation technology. Fig. 2 depicts the FT IR spectra of chitosan, PEG macromer, PA-0 and PA-5 samples. The much

qt ? ??C 0 ? C t ? ? V ?=m

? 4?

where V is the volume of the solution (mL), m is the weight of the dried semi-IPN hydrogel samples (g), C0 is the initial dye concentration (mg mL?1), and Ct is the bulk dye concentration at the indicated time t (mg mL?1). The pseudo ?rst-order and pseudo second-order kinetic models were used to analyze the dye adsorption behaviors of the semi-IPN hydrogel composites. The pseudo ?rst-order rate equation is given as [25]:

ln?qe ? qt ? ? lnqe ? ?k1 t =2:303?
?1 ?1

? 5?

where qe (mg g ) and qt (mg g ) are the amounts of adsorbed dyes by the hydrogel at equilibrium and at time t, respectively, and k1 is the pseudo ?rst-order adsorption rate constant (min?1). The pseudo second-order kinetic model is given below [25]:

? ? t =qt ? 1= k2 q2 e ? t =qe

? 6?

where k2 denotes the second-order adsorption rate constant (g mg?1 min?1), qe is the adsorption capacity calculated by the pseudo second-order kinetic model (mg g?1).

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f H

a c d e b b e d c f COOCH2 CHCH2 OCH2CH2 (OCH2CH2) CH2CH2O CH2CHCH2OOC H x-2 C=C OH OH C=C CH3 H H CH3 g g h h

Fig. 1. 1H NMR spectrum of PEG macromer in CDCl3.

broader adsorption peaks around 3200–3500 cm?1 are ANAH bands from chitosan and PAAm component. Two characteristic bands of the amide I and amide II of the amide groups of PAAm component and chitosan for the two semi-IPN hydrogel samples were observed around 1500–1600 cm?1. The peak at 1110 cm?1 correspond to the ACAOACA stretching from PEG segments. 3.2. Swelling studies Semi-IPN hydrogel composites in this study displayed pH-sensitive property due to the ionization behavior of ANH2 groups of chitosan in response to external pH changes. The swelling behaviors related with pH-sensitivity of hydrogels containing chitosan were reported somewhere [22,28]. Herein, to study the effect of dye molecules on swelling properties of semi-IPN hydrogels, their swelling behaviors in aqueous solution and AR 18 solution were investigated at pH 2.0. As shown in Fig. 3. It was observed that semi-IPN hydrogel samples exhibited similar swelling behavior, the semi-IPN hydrogels showed a rapid water uptake, and the swelling ratio increases with the increase of chitosan content in the hydrogels both in aqueous solution and dye solution at pH 2.0. As compared with the swelling ratio of semi-IPN hydrogels

in aqueous solution, the semi-IPN hydrogels displayed a relatively decreased swelling ratio in AR 18 solution. This could be explained that the ASO? 3 groups of AR 18 dye molecules interacted with the ANH? 3 groups of chitosan in the hydrogel composites at pH 2.0, and the electrostatic interaction between the tri-sulfonated AR 18 molecules (three ASO? 3 groups per dye molecule) and the ANH? 3 groups of chitosan acted as the crosslinking points [29], the crosslink density of the semi-IPN hydrogel composites increased, resulting in the decrease of the swelling ratio of the hydrogel in the aqueous solution of trivalent anionic AR 18. To analyze the swelling kinetics, the ?rst-order kinetic and second-order kinetic equations (Eqs. (2) and (3)) were employed. It was found that the experimental data for water content W and time t could be described well by the second-order kinetic equation both in aqueous solution and AR 18 solution at pH 2.0. As shown in Fig. 4a and b. The experimental equilibrium swelling ratios of PA-1.5, PA-3, and PA-5 samples in aqueous solution and in AR 18 solution are 930%, 985%, 1070%, and 847%, 910%, 986%, respectively. The theoretical equilibrium swelling ratios in aqueous solution and in AR-18 solution are 1028%, 1072%, 1121%, and 944%,

Fig. 2. FT IR spectra of CS, PEG macromer and different hydrogels.

Fig. 3. The swelling kinetic curves of the semi-IPN hydrogels in aqueous solution and AR 18 solution (pH, 2.0; temperature, 25 °C).

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1006%, 1108%, respectively. Thus the kinetics model is in agreement with swelling experiments. Obviously, the swelling properties of semi-IPN hydrogels are dependent on the CS content at low pH.

3.3. Adsorption of AR 18 3.3.1. Effect of chitosan content The effect of CS content on AR 18 adsorption kinetics for the hydrogels was investigated in a pH 2.0 solution of AR 18 (Fig. 5). As can be seen from Fig. 5, the hydrogel without CS displayed a very low adsorption capacity for AR 18 molecules. When CS was introduced into the hydrogel, with increasing CS content in the hydrogels, adsorption capacities and adsorption rates of AR 18 onto the semi-INP hydrogel composites were greatly increased, and the equilibrium time of adsorption was found to be about 24 h. This may be attributed to the ionization behavior of the amino groups of CS. In a solution of pH 2.0, most ANH2 groups of CS are positively charged and convert into NH? 3 groups, which could make electrostatic attraction with SO? 3 groups on the acid dye. With the increase of the CS content in the hydrogels, more positively charged amino groups lead to much more expanding hydrogel network [22]. At the same time, more cationic sites are produced to form ionic complexes with AR 18 molecules [30]. The combination of swelling

Fig. 5. Time pro?les for AR 18 adsorption by different hydrogels (pH, 2.0; temperature, 25 °C).

behaviors of the hydrogels and the electrostatic attractions between dye molecules and CS results in the higher dye adsorption capacities and higher adsorption rates for the semi-IPN hydrogel composites. While no ionic sites in PA-0 sample were provided for trapping AR 18 molecules at pH 2.0, and its adsorption capacity for AR 18 is very low in comparison with semi-IPN hydrogel systems. Therefore, CS moieties in the semi-IPN hydrogel composites play main role for adsorption of AR 18. In order to study the controlling mechanisms of adsorption process, pseudo ?rst-order kinetic (Eq. (5)) and pseudo second-order kinetic (Eq. (6)) rate models were applied to analyze experimental data of AR 18 adsorption of the semi-IPN hydrogel composites (Fig. 6a and b). The results are shown in Table 3. Upon correlation of the kinetic data with above two rate models, it was found that the lines for pseudo second-order rate model have higher correlation coef?cients compared to the correlation coef?cient obtained from the linear plot of the pseudo ?rst-order rate model. Table 3 shows that the calculated qe(cal) value was much closed to the experimental qe(exp) value, indicating that the pseudo second-order model was more applicable for the system. 3.3.2. Effect of the pH of dye solution As discussed above, semi-IPN hydrogel PA-5 sample exhibited a higher adsorption capacity due to a higher CS content at pH 2.0. Thus, hydrogel PA-5 sample as a typical example was used to investigate the effect of pH, initial dye concentration and ionic strength on dye adsorption capacity and adsorption isotherms. The adsorption capacity of hydrogel PA-5 sample for anionic AR 18 was investigated in response to external pH variations. As shown in Fig. 7. Sharp decrease in dye removal was observed when surrounding pH was increased from 2 to 7, while over the range from 7 to 10, a lower adsorption capacity was found. This may be attributed to the ionization behavior of amino groups of chitosan chain which interlocked into hydrophilic crosslinked network of PEG macromer with AAm monomer. Chitosan has primary amino groups with pKa value close to 6.5 [31]. At pH below pKa of chitosan, the amino groups are positively charged. Decreasing the pH of dye solution makes more protons available to protonate the amine group of chitosan with the formation of a large number of cationic amines, which results in increasing dye adsorption due to increased electrostatic interactions. At pH above the pKa, chitosan’ amino groups are kept in the form of ANH2, which makes it insoluble, resulting in the shrinkage of the hydrogel composites. As a result, it is dif?cult for AR 18 molecules to diffuse into the

Fig. 4. Experimental data of water content W and time t in aqueous solution and AR 18 solution: plotted according to Eq. (2) (?rst-order kinetics) (a) and Eq. (3) (second-order kinetics) (b) for the hydrogel PA-1.5, PA-3 and PA-5.

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Fig. 7. The effect of pH values on equilibrium adsorption of AR 18 by hydrogel PA-5 at 25 °C.

Fig. 6. Adsorption kinetics of AR 18 by different hydrogels according to pseudo ?rst-order model (a) and pseudo second-order model (b).

inner of the hydrogels and the interactions between dye molecules and chitosan greatly decreased, leading to lower dye adsorption capacities. A certain amount of dye adsorption at higher pH implied the presence of different interactions in the adsorption process simultaneously, such as dye–dye interaction, dye-adsorbent, physical adsorption, and aggregation [9]. Nevertheless, for semiIPN hydrogel composites produced, the adsorption capacity of anionic AR 18 was greatly affected by the pH of medium and was signi?cantly decreased by increasing the pH. 3.3.3. Effect of the initial dye concentration Fig. 8 shows that the effect of initial AR 18 concentration on the equilibrium adsorption capacity of hydrogel PA-5 sample at pH 2.0. It was observed that the adsorption capacity increased dramatically with an increase in the initial concentration of dye solution when the amount of PA-5 sample was kept unchanged, and reached a plateau when the dye concentration increased to about 300 mg L?1. This is attributed to the increase in the driving force

of the concentration gradient with higher initial dye concentration as well as the highly swollen hydrogel network providing the accessible diffusion path for dye molecules into the hydrogel to interact with the CS chains [32]. It was found that at low initial dye concentration the adsorption of AR 18 by semi-IPN hydrogel composite is very intense. To compare the adsorption capacities of PA-0 and PA-5 hydrogel samples, 60 mg of both dried samples was immersed into 100 mL 30 mg L?1 AR 18 solution at pH 2.0 for 2 days, respectively. For PA-0 sample, the color of dye solution had no obvious change, and the hydrogel PA-0 after adsorption displayed light red color. However, the dye solution is almost colorless compared to the original red solution when applying PA-5 sample, and the hydrogel PA-5 showed deep red color, and the percentage of dye adsorption reached 98%. As depicted in Fig. 9. Although the amount of the dye adsorbed increased with increasing concentration of solution, the percentage of adsorption decreased. For example, the initial concentration of AR 18 increased from 100 mg L?1 to 200 mg L?1, the amount of dye adsorbed was 194 and 316 mg g?1, respectively, while the percentage of adsorption decreased from 91.3% to 80.3%. This is due to the electrostatic repulsion between the dye molecules with increasing concentration which results in a competition between the dye molecules for the limited active sites in the adsorbent. 3.3.4. Effect of ionic strength It is well known that some additives such salts and surfactants were utilized to accelerate or retard dye adsorption process. Sodium chloride is often used as a stimulator in dyeing process, which may screen the electrostatic interaction of opposite charges or enhance the degree of dissociation of the dye molecules to decrease or increase the amount of dye adsorbed. Herein, the effect of additional sodium chloride on the adsorption capacity of AR 18 on hydrogel PA-5 sample was investigated at pH 2.0 with different ionic strength adjusted by NaCl. As depicted in Fig. 10. The obtained results gave an indication that the adsorption capacities and

Table 3 The pseudo ?rst-order and pseudo second-order rate constant for AR 18 by the semi-IPN hydrogel composites. Sample code qe(exp) (mg g?1) Pseudo ?rst-order kinetic model k1 (min?1) PA-1.5 PA-3 PA-5 160 250 320 0.0028 0.0035 0.0037 qe(cal) (mg g?1) 101.0 200.6 270.9 R2 1 0.9678 0.9956 0.9952 Pseudo second-order kinetic model k2 (g mg?1 min?1) 2.49E?05 9.84E?06 7.04E?06 qe(cal) (mg g?1) 167.7 265.9 345.7 R2 2 0.9991 0.9996 0.9996

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Fig. 8. The effect of AR 18 initial concentration on the equilibrium adsorption of hydrogel PA-5 (pH, 2.0; temperature, 25 °C).

Fig. 10. The effect of ionic strength of AR 18 solutions on adsorption of hydrogel PA5 (pH, 2.0; temperature, 25 °C).

PA-5 sample

(b)

(a)

PA-0 sample

charged groups of chitosan in the hydrogel network, resulting in decreasing the removal of cationic dye. A low adsorption of the semi-IPN hydrogel for cationic dye might be attributed to other interactions such as physical adsorption and hydrogen bonding between backbone network of the hydrogel composite and dye molecules. At the same experimental conditions, for three anionic dyes, the order of the adsorption capacities of hydrogel PA-5 sample was AR 18 > AO 7 > MO. It seems that the higher adsorption capacity of AR 18 is due to its tri-sulfonated structure. However, some attempts to determine the correlation between the structure of the dyes and the adsorption capacities of adsorbents are failed [35]. Nevertheless, these semi-IPN hydrogels are more preferable for anionic dye molecules due to ionic complex formation between the dye molecules and the hydrogel at investigated conditions. 3.4. Adsorption isotherm Langmuir and Freundlich models are applied to describe equilibrium adsorption isotherms for three anionic dyes onto the hydrogel PA-5 sample at pH 2.0 and 25 °C, and depicted in Fig. 12a and b. The related parameters calculated from Eqs. (8)– (10) are given in Table 4. The best-?t isotherm model for the system was judged by the correlation coef?cients, R2 values. As seem from Table 4, the Langmuir model displayed the best ?t with the higher R2 L values when compared to the Freundlich model for the

(c)
Fig. 9. Photographs of AR 18 solution and dried gel samples before adsorption (A) and after adsorption for hydrogel PA-5 (B) and for hydrogel PA-0 (C) (AR 18 concentration, 30 mg L?1; pH, 2.0; temperature, 25 °C).

the initial adsorption rate decreased with increasing ionic strength of dye solution. The ionic strength increased as NaCl concentration increased and more Cl? ions may screen the positive sites of chitosan in the hydrogel, leading to the reduce of electrostatic attractive force, and accordingly, the amount adsorbed and the initial adsorption rate for AR 18 decrease, which is in agreement with the reported results [10,33]. 3.3.5. Adsorption of different dyes The adsorption studies were extended to the AO 7 and MO (anionic dyes), and BV 14 (cationic dye) in order to investigate adsorption capacities of different dyes onto semi-IPN hydrogel composite. Fig. 11 shows the adsorption kinetics of three dyes on hydrogel PA5 sample at pH 2.0. It was observed that after immersed into dye solutions, AO 7 and MO, two anionic dyes, could rapidly be adsorbed into the hydrogel until higher adsorption capacities were achieved. This is due to strong electrostatic attraction between anionic dye molecules and the cationic groups of chitosan in the hydrogel at investigated conditions [34]. However, for cationic BV 14, it can be noticed that the adsorption of BV 14 is much lower than that of anionic dyes. This can be referred to repulsion interactions present between positive charges in the BV 14 and positively

Fig. 11. Time pro?les of adsorption for different dyes by hydrogel PA-5 (pH, 2.0; temperature, 25 °C).

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CS was embedded into the hydrophilic polymeric hydrogel network. The swelling measurement results showed that the swelling ratio was higher in aqueous solution than in tri-sulfonated AR 18 solution at pH2.0, and the swelling kinetics were in consistent with second-order kinetic model. The adsorption studies of AR 18 by the hydrogel composites showed that the adsorption capacities increased by increasing the CS content and initial dye concentration but deceased with the increase of pH and ionic strength of dye solutions. As compared with cationic BV 14, the hydrogel composite displayed high adsorption capacity for anionic dyes like AR 18, AO 7 and MO, and the order of the adsorption capacity was AR 18 > AO 7 > MO. The adsorption of three anionic dyes by the hydrogel composite ?tted well with the Langmuir equation. The electrostatic attraction between anionic dye molecules and ANH? 3 groups of the chitosan chain was attributed to the main adsorption process. Acknowledgments The authors are grateful for the ?nancial support provided by the Nature Science Foundation of Hubei Province (2010CDB 04903), Foundation of Hubei Educational Committee (Q2010 1603), and SRF for ROCS, SEM. References
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Fig. 12. The Langmuir isotherm model (a) and Freundlich isotherm model (b) for AR 18, AO 7 and MO by the hydrogel PA-5 (pH, 2.0; temperature, 25 °C).

Table 4 Isotherm parameters for three anionic dyes by the hydrogel PA-5. Model Parameter Anionic dyes AR 18 Langmuir qe (mg g?1) qm (mg g?1) b ? 10?2 (L mg?1) RL R2 L Freundlich KF ((mg g?1)(L mg?1)1/n) 1/n R2 F 342.54 358.42 14.28 0.02 0.9996 88.07 0.299 0.8908 AO 7 221.10 243.31 4.49 0.06 0.9994 35.60 0.37 0.9663 MO 185.24 202.02 4.41 0.06 0.9968 31.46 0.35 0.8997

PA-5 sample within the concentration range studied, suggesting homogeneous surfaces of the hydrogel composite and monolayer coverage of anionic dyes onto the hydrogel. The values of RL for all anionic dyes were below unit, close to zero, con?rming the favorable uptake of anionic dyes. As seen from Table 4, the value of 1/n was below one, suggesting a normal Langmuir isotherm [26].

4. Conclusions In this work, semi-IPN hydrogel composites were prepared via the copolymerization of PEG macromer with AAm monomer and

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