poly(vinyl alcohol) hydrogel beads: Preparation, characterization and application for adsorption of dye from aqueous solution

Bioresource Technology 105 (2012) 24–30

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Novel magnetic chitosan/poly(vinyl alcohol) hydrogel beads: Preparation, characterization and application for adsorption of dye from aqueous solution H.-Y. Zhu a,b, Y.-Q. Fu a, R. Jiang a,⇑, J. Yao a, L. Xiao b, G.-M. Zeng c a

College of Life Science, Taizhou University, Linhai 317000, Zhejiang, PR China Key Laboratory for Biomass-Resource Chemistry and Environmental Biotechnology of Hubei Province, Wuhan University, Wuhan 430072, Hubei, PR China c Key Laboratory of Environmental Biology and Pollution Control, Hunan University, Ministry of Education, Changsha 410082, Hunan, PR China b

a r t i c l e

i n f o

Article history: Received 27 July 2011 Received in revised form 12 November 2011 Accepted 16 November 2011 Available online 25 November 2011 Keywords: Chitosan Poly(vinyl alcohol) Magnetic adsorbent Adsorption Congo Red

a b s t r a c t Novel magnetic chitosan/poly(vinyl alcohol) hydrogel beads (m-CS/PVA HBs) were prepared by an instantaneous gelation method and characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and thermogravimetric analysis (TGA). Results of characterization indicated that m-CS/ PVA HBs have been prepared successfully without damaging the crystal structure of Fe3O4 and their saturation magnetization were about 21.96 emu g1. The adsorption capacity of Congo Red on the m-CS/PVA HBs was 470.1 mg g1. The adsorption was well described by pseudo-second-order kinetics and Langmuir equation. Positive value of enthalpy change ðDH Þ (13.32 kJ mol1) showed that the adsorption was endothermic and physical in nature. The values of Gibbs free energy change ðDG Þ were found to be 3.321 kJ mol1 at 298 K for m-CS/PVA HBs, indicating the spontaneity of Congo Red adsorption. Therefore, the m-CS/PVA HBs could be employed as a low-cost alternative to other adsorbents in the removal of dyes from aqueous solution. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Recently, many researchers have studied the feasibility of using low-cost biomass for the removal of various dyes, such as chitosan (Dotto and Pinto, 2011; Wang and Wang, 2008; Zhu et al., 2010a), cellulose (Annadurai et al., 2002; Luo and Zhang, 2009), Rhizopus oryzae (Das et al., 2006), and so on. Chitosan (CS), poly (1 ? 4)-2 amino-2-deoxy-b-D-glucan is usually obtained from waste biomass during seafood processing, mainly shells of crabs, shrimp, prawns, and krill (Crini, 2006). Various biomaterials based on chitosan have already been explored as excellent adsorbents for removal of all kinds of dyes from aqueous solutions since chitosan has three functional groups, i.e. two hydroxyl groups (–OH) and one amino group (–NH2), per glucosamine unit (Crini, 2006). However, pure chitosan materials have some obvious disadvantages such as poor chemical resistance, low mechanical strength and difficult recovery (Wang and Wang, 2008). In addition, the adsorption capacity for crosslinked chitosan was lower when compared with free chitosan, because of functional group (–NH2) being crosslinked (Shawky, 2009). Blending of two or more polymers has increasingly become an important technique for improving the cost-performance ratio of commercial products (Kim et al., 2003). Due to the characteristics ⇑ Corresponding author. Address: No. 605, Orient Avenue, Linhai City 317000, Zhejiang Province, PR China. Tel.: +86 158 6763 6396; fax: +86 0576 8513 7066. E-mail address: [email protected] (R. Jiang). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.11.057

of easy preparation, good biodegradability, excellent chemical resistance, and good mechanical properties, poly(vinyl alcohol) (PVA) has been used in many biomaterial applications (Park et al., 2001). Since chitosan contains high contents of amino and hydroxyl functional groups, chitosan may potentially be miscible with PVA because of the formation of hydrogen bonds (Chen et al., 2007; Wan Ngah et al., 2004). Chitosan blended with PVA has been reported to have good mechanical and chemical properties because of the specific intermolecular interactions between PVA and chitosan in the blends, thus extensively being studied in water treatment (Kumar et al., 2009; Nakano et al., 2007; Wan Ngah et al., 2004) and drug-controlled release (Wang et al., 2005). What’s more, blending of chitosan with PVA can create a three-dimensional network (Liang et al., 2009), which is benefit for dye adsorption. Magnetic separation have been one of the promising ways for an environmental purification technique because of producing no contaminants such as flocculants and having capability of treating large amount of wastewater within a short time (Zhu et al., 2010b; Rocher et al., 2008). From the viewpoints of environmental protection and resource utilization, it is very important and significant to develop novel magnetic recyclable biomaterials and explore their adsorption properties in order to expand the utilities as industrial biomaterials. Although magnetic chitosan and magnetic poly(vinyl alcohol) particles with a wide size distribution were obtained (Albornoz et al., 2004; Li et al., 2008; Xue and Sun, 2001; Zhi

H.-Y. Zhu et al. / Bioresource Technology 105 (2012) 24–30

et al., 2006; Zhu et al., 2010a), study on preparation, characterization and adsorption properties of magnetic chitosan/PVA hydrogel beads (m-CS/PVA HBs) for dye removal has not yet been studied. In the present study, the m-CS/PVA HBs were prepared by an instantaneous gelation method and characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and thermogravimetric analysis (TGA). The adsorption equilibrium of azo dye (Congo Red, CR) onto m-CS/PVA HBs from aqueous solution was investigated in detail. The Langmuir and Freundlich equations were used to fit the equilibrium isotherm. Thermodynamic parameters were also calculated. The adsorption rates were determined quantitatively and compared by the Lagergren-first-order and second-order kinetic models. This information will be useful for further applications of system design in the treatment of practical waste effluents.

25

Wide-angle X-ray diffraction (XRD) measurements were carried out using an XRD diffractometer (D8-Advance, Bruker). The patterns with the Cu Ka radiation (k = 0.15406 nm) at 40 kV and 30 mA were recorded in the region of 2h from 20° to 70°. The magnetization and hysteresis loop were measured at room temperature (293 K) with a HH-15 model vibrating sample magnetometer (VSM) (Nanjing University, China) and its measurement range was ±10.0 kOe. Thermogravimetry (TG) was performed using a Setaram Setsys 16 TG/DTA/DSC (France) under a nitrogen atmosphere of 0.15 MPa from 273 to 1261 K with heating rate of 2 K min1. The mass of the samples was generally in the range of 18–25 mg. The size of magnetic nanoparticles were measured by a high-resolution transmission electron microscope (HRTEM; JEOL, JEM-2010) using an accelerating voltage of 300 kV. A copper grid coated with a holey carbon support film was used to prepare samples for the HRTEM observation.

2. Methods 2.4. Batch adsorption studies 2.1. Chemicals and materials The degree of deacetylation and a weight-average molecular weight of chitosan (Zhejiang Yuhuan Ocean Biology Co., China) are 91.5% and 210 kDa, respectively. Poly(vinyl alcohol) (98% hydrolyzed, average molecular weight 22,000) purchased from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China) was of analytical-reagent grade. Commercially available Congo Red (C32H22N6O6S2Na2, molecular weight 696.67) was purchased from Yongjia Fine Chemical Factory (Zhejiang, China) and used as received without further purification. A stock solution of CR (1000 mg L1) was prepared, which was diluted to desired concentrations. Other chemical agents used were all analytical grade and all solutions were prepared with double distilled water. 2.2. Preparation of magnetic Fe3O4 nanoparticles Fe3O4 nanoparticles were prepared by co-precipitating Fe2+ and Fe salts under hydrothermal conditions. 7.2 g of FeCl36H2O and 1.53 g of FeSO47H2O were dissolved in 44 mL of deoxidized distilled water. Chemical precipitation was achieved under vigorous stirring in a water bath at 313 K for 20 min by adding 40 mL of NH3H2O solution (28%) in the protection of N2 gas. The temperature of solution was adjusted and maintained at 333 K for another 5 h while adding another 6.6 mL of NH3H2O solution during the reaction. After reaction, Fe3O4 nanoparticles were magnetically separated from aqueous solution by an adscititious magnet at room temperature, and washed several times with water and ethanol, and finally dried in a vacuum oven at 343 K. 3+

Batch adsorption experiments were conducted and equilibrated using a model KYC-1102 air-temperature-controlled shaker (Ningbo Jiangnan Instrument Factory, China) at 100 rpm in 100 mL of beaker. Batch experiments were repeated at least twice to ensure accuracy of the obtained data. Typically, a 50 mL solution of known dye concentrations and 0.05 g of m-CS/PVA HBs were added into 100 mL glass flasks and then shook under 298 ± 0.2 K. At given time intervals, 4 mL of the suspensions were collected and m-CS/ PVA HBs were separated from aqueous solution using an adscititious magnet. The concentration of the residual CR was analyzed by a Varian Cary 50 UV–vis spectrophotometer (USA) and kmax 496.0 nm was monitored. Isotherm studies were conducted with a constant m-CS/PVA HBs dosage (1.4 g L1) and varying initial concentrations of CR in the range 50–800 mg L1. Adsorption kinetics experiments were carried out with a constant CR concentration and changing the dosage of adsorbents in the range of 0.6–1.8 g L1. The amount of CR adsorbed on m-CS/PVA HBs, qt (mg g1) was obtained as follows:

qt ¼

ðC 0  C t ÞV W

ð1Þ

where C0 and Ct are the initial CR concentration and CR phase concentrations at any time t (mg L1), respectively, V is the volume of solution (L) and W is the amount of adsorbent (g). 3. Results and discussion 3.1. Characterization of materials

2.3. Preparation and characterization of m-CS/PVA HBs The m-CS/PVA HBs with a weight ratio of W(PVA):W(chitosan):W(Fe3O4) = 1:1:1 were prepared by an instantaneous gelation method. Chitosan solution was prepared by dissolving 4 g of chitosan powder in 200 mL of 2% (v/v) aqueous acetic acid. 4 g of PVA powder was dissolved in 200 mL of double distilled water to form PVA solution under mechanical stirring at 343 ± 1 K. Then, PVA solution was mixed homogenously with chitosan solution and 4 g of Fe3O4 to form composite gel-forming solutions with vigorous stirring for 3.0 h at 303 K. The resulting mixed solution was dropped through a needle into a sodium hydroxide bath (1000 mL, 0.1 M), and the spherical hydrogel beads formed instantaneously. Magnetic hydrogel beads were obtained by washing with double distilled water for three times and preserved in an aqueous environment for future use. The out diameter of the resultant m-CS/PVA HBs was in the range of 2–3 mm.

3.1.1. XRD analysis The crystalline phase of m-CS/PVA HBs and Fe3O4 nanoparticles were analyzed by XRD. The main peaks of Fe3O4 nanoparticles were at 2h = 30.32°, 35.64°, 43.36°, 53.66°, 57.26° and 62.87°, which were corresponding to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystal planes of pure Fe3O4 with a spinel structure, respectively (Chuang and Chen, 2005; Li et al., 2008; Zhi et al., 2006). These peaks are consistent with the database in JCPDS file (PDF No. 65-3107). At the same time, it can be seen that the strong characteristic diffraction peaks of Fe3O4 ((2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0)) could be found in the pattern of m-CS/ PVA HBs. According to the Debye–Scherrer equation (Klug and Alexander, 1974), the naked Fe3O4 and Fe3O4 nanoparticles in mCS/PVA HBs had an average diameter of 8 nm, which was in accordance with the high-resolution transmission electron microscopy result (the figure of HRTEM is not shown). The diffraction peaks

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H.-Y. Zhu et al. / Bioresource Technology 105 (2012) 24–30

80

Magnetization (emu/g)

60 40 20 0 -20

b m-CS/PVA HBs Fe3O4

-40 -60

a

-80 -10000

-5000

0

5000

10000

Magnetic Field (Oe) Fig. 1. Magnetic hysteresis loop for (a) Fe3O4 and (b) m-CS/PVA HBs at 300 K (left) and magnetic recovery of m-CS/PVA HBs (right).

associated with chitosan and PVA were very weak indicating low crystallinity (Nakano et al., 2007). This result indicated that the m-CS/PVA HBs have been prepared successfully without damaging the crystal structure of Fe3O4 core.

3.1.3. TGA analysis Fig. 2 shows the TG/DSC curves of m-CS/PVA HBs in the temperature range from 273 to 1261 K at a heating rate of 2 K min1. It was found that weight losses of the m-CS/PVA HBs occurred at three different temperature ranges. The first stage ranged between 298 K (25 °C) and 423 K (150 °C) and showed about 3% loss in weight for m-CS/PVA HBs. This might be corresponding to the loss of adsorbed and bound water and the residue of acetic acid (Lewandowska, 2009; Tripathi et al., 2010). Since chitosan contains – NH2 and –OH functional groups, the hydrogen bonding force is

2

580.4786

80

0

70 -2 60

Heat flow (W/g)

90

Weight (%)

3.1.2. VSM analysis and magnetic recovery of adsorbents Fig. 1 (left) shows the hysteresis loops of Fe3O4 and m-CS/PVA HBs between ±10 kOe at 293 K. The specific saturation magnetizations (rs) were 65.53 emu g1 and 21.96 emu g1 for Fe3O4 and mCS/PVA HBs, respectively. Obviously, the rs value of m-CS/PVA HBs was lower than that of Fe3O4, which was caused by the low Fe3O4 content (33.3%). The similar result has been reported in magnetic Fe3O4/o-carboxymethyl chitosan composite (Zhu et al., 2008). However, the ratio of rs (m-CS/PVA HBs):rs (Fe3O4) was 1:3.02, which was almost consistent with the weight ratio (1:3.00) of Fe3O4 content in m-CS/PVA HBs. The result indicated further that magnetic Fe3O4 introduced in m-CS/PVA HBs kept their intrinsic magnetic properties and crystal structure. Therefore, the saturated magnetization (rs) of m-CS/PVA HBs could be expediently adjusted by changing additive dosage of Fe3O4. In addition, the saturation magnetization (21.96 emu g1) of the m-CS/PVA HBs was higher than or comparable to other magnetic bioadsorbents based on chitosan reported by other researchers (Zhi et al., 2006; Zhu et al., 2010a). At the same time, almost no remainance and coercivity was observed, indicating that Fe3O4 and m-CS/PVA HBs were superparamagnetic (Li et al., 2008; Lu et al., 2007). The superparamagnetic properties of both Fe3O4 and m-CS/PVA HBs might be due to the smaller size of Fe3O4 nanoparticles (Lu et al., 2007). As a result, the magnetic hydrogen beads with higher rs value could be easily removed from the aqueous solution with an addition of a permanent magnet, indicating the sensitive magnetic response. As shown in Fig. 1 (right), m-CS/PVA HBs can be completely attracted to the sidewall from the aqueous solution within about 10 s by applying a magnet 1 cm away, eventually leaving the colorless and clear supernatant.

100

50 -4

928.9558 40 400

600

800

1000

1200

Temperature (K) Fig. 2. DSC and TGA curves for m-CS/PVA HBs.

strongly formed among molecules (Chen et al., 2007). The second stage of weight loss started at 493 K (220 °C) and continued up to 763 K (490 °C) during which there was about 22% in m-CS/PVA HBs, which was associated with both the crystalline polymer fraction of PVA and the degradation and deacetylation of chitosan (Yang et al., 2004). The third weight loss at 883–1033 K (610– 760 °C) was due to the further degradation of chitosan. This process ascribed to a complex process which included not only the dehydration of the saccharide rings but also the decomposition of the acetylated and deacetylated units of chitosan (Chen et al., 2007). 3.2. Adsorption of CR on m-CS/PVA HBs 3.2.1. Effect of pH on Congo Red adsorption The pH of solution is one of the most important parameters affecting the adsorption process (Zhu et al., 2010b). CR dye is slightly soluble in water with a pH value of 2.0 (Wang and Wang, 2008), so the range of pH for CR adsorption test selected in this study was between 4.0 and 11.0. The dye solution was adsorbed for 10 h after pH adjustment, and thereafter the solution was scanning from 200 to 800 nm. The adsorption increased gradually from 45.1% to 91.1% with decrease in pH from 11 to 6.0, reached a maximum at pH 6.0. Further decrease in pH from 6.0 to 4.0 resulted in obvious decrease in adsorption (77.0%). In weakly acidic solution, more protons will be available, which resulted in the increasing electrostatic attraction between negatively charged dye anions

27

H.-Y. Zhu et al. / Bioresource Technology 105 (2012) 24–30 1.5

60

(b)

1.0

50

0.5

40

t/qt

log(qe-qt)

(a)

0.0 -1

1.8 gL -1 1.4 gL -1 1.0 gL -1 0.6 gL

-0.5

-1

1.8 gL -1 1.4 gL -1 1.0 gL -1 0.6 gL

30

20

10

-1.0

0

100

200

300

400

500

600

0

100

200

300

t (min)

400

500

t(min)

Fig. 3. Lagergren-first-order kinetic model (a) and pseudo-second-order kinetic model (b) for adsorption of CR on m-CS/PVA HBs at 298 K.

where k2 is the rate constant of pseudo-second-order adsorption (g mg1 min1). Values of k2 are obtained from plotting (t/qt) versus t. The intra-particle mass transfer diffusion model is expressed as (Periasamy et al., 1994):

and positively charged surface of m-CS/PVA HBs. However, at pH below 6.0, CR adsorption on the adsorbent decreased with the decrease of pH because of the fact that m-CS/PVA HBs have disbanded partly since only hydrogen bond between chitosan and PVA is involved at interaction. CR dye solution at pH 4.0 turned from transparent to appreciably turbid in the presence of m-CS/ PVA HBs. Base on the fact that natural pH of CR solution (20 mg L1) is about 5.6, it is suitable that m-CS/PVA HBs were used to adsorb the CR dye from aqueous solution.

qt ¼ kid t1=2 þ c

where c is the intercept (mg g1) and kid is the intra-particle diffusion rate constant mg g1 min1/2, which can be evaluated from the slope of the linear plot of qt versus t1/2. The linearized forms of two kinetic models for the sorption of CR onto m-CS/PVA HBs at 293 K are presented in Fig. 3 and the kinetic parameters (k1, k2, kid, qe) and correlation coefficients (R2) are summarized in Table 1. The values of the correlation coefficient (R2) for the pseudo-second-order model were P0.998 for all m-CS/PVA HBs dosages, and the adsorption capacities calculated by the model (q2e,cal) were also closer to those determined by experiments (qe,exp). These results indicated that it was feasible for the applicability of pseudo-second-order kinetic model to describe the adsorption process of CR on m-CS/PVA HBs. The pseudo-second-order model is based on the assumption that the rate-determining step may be a chemical sorption involving valence forces through sharing or exchange of electrons between adsorbent and sorbate (Bayramoglu et al., 2009). In fact, chitosan has three functional groups, i.e. two hydroxyl groups and one amino group per glucosamine unit (Crini, 2006; Zhu et al., 2010a). Therefore, the dye could be adsorbed by interaction between the CR dye molecules and three functional groups of chitosan in m-CS/PVA HBs. In many cases, the pseudosecond-order equation correlated well to the adsorption of metal ions onto chitosan/PVA materials (Kumar et al., 2009; Wan Ngah et al., 2004). In addition, the values of the pseudo-second-order rate constants (k2) were found to increase from 2.59  104 to 10.74  104 g mg1 min1 as the adsorbent dosage increased from 0.6 to 1.8 g L1, indicating that it was favorable for CR adsorption onto m-CS/PVA HBs when the dosage of adsorbent increased.

3.2.2. Adsorption kinetics The kinetic parameters are helpful for the prediction of adsorption rate, which gives important information for the efficiency of adsorption. To analyze the adsorption rate of CR on m-CS/PVA HBs, two kinetic models such as Lagergren-first-order and pseudo-second-order kinetic models were used to understand the dynamics of the adsorption process. The linear forms of Lagergren-first-order kinetic model (Eq. (2)) (Weber et al., 1963; Periasamy and Namasivayam, 1994), pseudo-second-order kinetic model (Eq. (3)) (Kumar et al., 2009; Zhu et al., 2010a) and intra-particle mass transfer diffusion model (Eq. (4)) (Periasamy et al., 1994) were evaluated based on the experimental data. The Lagergren-first-order model is expressed as (Weber et al., 1963; Periasamy and Namasivayam, 1994):

logðqe  qt Þ ¼ log qe 

k1 t 2:303

ð2Þ

where qe and qt are the amount of CR adsorbed (mg g1) on the adsorbents at the equilibrium and at time t, respectively; k1 is the rate constant of adsorption (L min1). Values of k1 are calculated from the plots of log(qe  qt) versus t. The pseudo-second-order kinetic model is expressed as (Kumar et al., 2009; Zhu et al., 2010a):

t 1 1 ¼ þ t qt k2 q2e qe

ð4Þ

ð3Þ

Table 1 Kinetic parameters of CR adsorption on m-CS/PVA HBs at different adsorbent dosages. Adsorbent dosage (g L1)

1.8 1.4 1.0 0.6

qe,exp (mg g1)

9.97 12.80 17.82 29.56

Lagergren-first-order kinetic model

Pseudo-second-order kinetic model

Intra-particle mass transfer diffusion model

q1e,cal (mg g1)

k1  103 (min1)

R2

q2e,cal (mg g1)

k2  104 (g mg1 min1)

R2

k2 (mg g1 min1/2)

c (mg g1)

R2

10.59 12.85 16.23 27.04

7.74 6.82 4.95 4.95

0.983 0.992 0.998 0.997

10.12 12.82 17.65 29.52

10.74 7.65 4.64 2.59

0.999 0.999 0.998 1.000

0.34 0.44 0.62 1.05

1.94 2.14 2.03 2.95

0.889 0.910 0.942 0.934

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H.-Y. Zhu et al. / Bioresource Technology 105 (2012) 24–30 700

30 -1

1.8 gL -1 1.4 gL -1 1.0 gL -1 0.6 gL

25

500

400

-1

qe (mg g )

qt (mg g-1)

20

600

15

experimental data

300

Langmuir model Freundlich model

10 200

5 100

0 5

10

15

t

0.5

20

0

25

0

(min )

40

60

80

100

120

140

160

180

-1

Ce (mg L )

Fig. 4. Plots of intra-particle diffusion model for CR adsorption of CR onto m-CS/ PVA HBs at 298 K.

In order to assess the nature of the diffusion process, the adsorption of dyes onto m-CS/PVA HBs was made to calculate the pore diffusion coefficients. It is important to predict the ratelimiting step in an adsorption process to understand the mechanism of the sorption. If the regression of qt versus t1/2 is linear and passes through the origin, then intra-particle diffusion is the sole rate-limiting step (Bayramoglu et al., 2009). The R2 values (Fig. 4 and Table 1) obtained were much lower (R2 < 0.942) compared with those obtained from pseudo-second-order kinetic model. Therefore, the deviation of straight lines from the experimental indicated that intra-particle transport was not the rate-limiting step. 3.2.3. Adsorption isotherms An adsorption isotherm provides a relationship between the solute concentration in the solution and the amount of dye adsorbed on the solid phase when the two phases are at equilibrium. The adsorption of CR onto m-CS/PVA HBs was carried out in the CR concentrations ranging from 50 to 800 mg L1. The adsorption isotherm experimental data were fitted by two common adsorption models: Langmuir (Langmuir, 1918), Freundlich (Freundlich, 1932). The Langmuir (Eq. (5)) and Freundlich (Eq. (6)) isotherms are represented by the following equations:

qe ¼

20

-1

Ce ð1=qm K L Þ þ ðC e =qe Þ

ð5Þ

qe ¼ K F ln C e1=n

ð6Þ

Fig. 5. Isotherm for adsorption of CR on m-CS/PVA HBs (conditions: V = 50 mL, m = 0.7 g, contact time = 36 h, agitation speed = 100 rpm, at 298 K).

Freundlich models were 0.994 and 0.933, respectively. In addition, the qm value for the adsorption of CR by m-CS/PVA HBs was 467.3 mg g1, which agreed well with the experimental value qe,exp (470.1 mg g1). Obviously, the Langmuir isotherm, compared with the Freundlich isotherm, was more suitable to describe the adsorption of CR on m-CS/PVA HBs. The Langmuir isotherm is based on the assumption that a point of valence exists on the surface of the adsorbent and that each of these sites is capable of adsorbing one molecule (Vijayakumar et al., 2009). Further, the ‘‘Equilibrium parameter’’ or ‘‘Langmuir parameter’’ (RL) obtained from Langmuir adsorption isotherm model is used to predict the favorability of adsorption process. The RL was calculated from the following equation using Langmuir KL parameter:

RL ¼

1 1 þ K LC0

ð7Þ

where KL is the Langmuir constant (L mg1) and C0 is the initial concentration (mg L1). Isotherm is considered to be unfavorable (RL > 1.0), linear (RL = 1.0), favorable (1 > RL > 0) or irreversible (RL = 0) depending on the value of RL (Bayramoglu et al., 2009). The RL values were in the ranges of 0.026–0.211 for 50–800 mg L1 of initial CR concentrations, indicating that the adsorption of CR on m-CS/PVA HBs was a favorable and useful process for CR removal from aqueous solution. In addition, the value of RL decreased with the increase of initial CR concentration, which could be attributed to an increase in the driving force of the concentration gradient with the increase in the initial CR concentration.

1

where Ce is the equilibrium concentration (mg L ), qm is the monolayer adsorption capacity (mg g1) and KL is the constant related to the free adsorption energy (Langmuir constant, L mg1), n is the heterogeneity factor and KF is the Freundlich constant (L g1). The theoretical parameters of adsorption isotherms along with regression coefficients (R2) are listed in Table 2. Fig. 5 shows the fitted equilibrium data in Langmuir and Freundlich models for CR adsorption. The correlation coefficients (R2) of Langmuir and

Table 2 Langmuir and Freundlich isotherm parameters and correlation coefficients for the adsorption of CR in aqueous solutions onto m-CS/PVA HBs at 298 K. Langmuir isotherm constants

Freundlich isotherm constants

qm (mg g1)

KL

R2

KF (mg1(1/n) L1/n g1)

n

R2

467.3

0.05353

0.994

28.34

1.64

0.933

3.2.4. Thermodynamic parameters The thermodynamic parameters such as Gibbs free energy change ðDG Þ, standard enthalpy change ðDH Þ and standard entropy change ðDS Þ were also studied to understand better the effect of temperature on the adsorption (Lian et al., 2009). Adsorption experiments were conducted at 298, 308, 318 and 328 K to investigate the effect of temperature, with 800 mg L1 of initial CR concentration and 1.4 g L1 of adsorbent dosage:

DG ¼ RT ln K L ln K L ¼ 

DH  DS þ RT R

ð8Þ ð9Þ

where DG (kJ mol1), DH (kJ mol1) and DS (J mol1 K1) are changes of Gibbs free energy, enthalpy and entropy, respectively; R is the universal gas constant (8.314 J mol1 K1) and T is the

29

H.-Y. Zhu et al. / Bioresource Technology 105 (2012) 24–30 Table 3 Thermodynamic parameters for the adsorption of CR in aqueous solutions onto m-CS/ PVA HBs. T (K) Thermodynamic parameters KL (L mg1) 4G° (kJ mol1) 4H° (kJ mol1) 4S° (J mol1 K1) R2 298 308 318 328

3.82 3.31 2.80 2.32

–3.321 –3.071 –2.730 –2.297

13.32

–33.42

0.992

absolute temperature in Kelvin; KL is ratio of concentration of CR on adsorbent at equilibrium (qe) to the remaining concentration of the dye in solution at equilibrium (Ce). The plot of ln KL as a function of 1/T yielded a straight line from which DH and DS are calculated from the slope and intercept, respectively. The values of Gibbs free energy ðDG Þ, standard enthalpy change ðDH Þ and standard entropy change ðDS Þ are presented in Table 3. The overall free energy changes during the adsorption process at 298, 308, 318 and 328 K were negative, corresponding to a spontaneous and thermodynamically favorable process of adsorption of CR onto m-CS/PVA HBs. As the temperature increased from 298 to 328 K, the DG values decreased, indicating less driving force and hence resulting in lesser adsorption capacity at higher temperatures. The positive value of +13.32 kJ mol1 of the enthalpy change ðDH Þ indicated that the adsorption is endothermic in nature. 3.2.5. Reusability of m-CS/PVA HBs The economic feasibility of using adsorbent based on biopolymer to remove dye from aqueous solution relies on its regeneration ability during multiple adsorption/desorption cycles. With a view to the fact that the m-CS/PVA HBs could be dissolved and disbanded partly in strong acid (pH < 5), it is unsuited that 0.1 M HNO3 or HCl solution were used as desorption agents to investigate the reusability of the m-CS/PVA HBs. In this study, desorption and regeneration studies were carried out by using 0.1 M NaCl solution. After four cycles, the CR adsorption capacity on m-CS/PVA HBs decreases from 91.1% to 86.2% (shown in Fig. 6). This behavior indicates that the m-CS/PVA HBs can be used successfully four times after regeneration for the CR adsorption from aqueous solution.

Table 4 qm values for the adsorption of CR on different adsorbents. Adsorbents

qm (mg g1)

References

m-CS/PVA HBs Ca-bentonite Chitosan/SDS beads Chitosan/organomontmorillonite Chitosan/CNTs N,O-carboxymethyl-chitosan Bentonite

470.1 85.29 223.25 290.8 450.4 330.62 158.7

This study Lian et al. (2009) Chatterjee et al. (2009) Wang and Wang (2007) Chatterjee et al. (2010) Wang and Wang (2008) Bulut et al. (2008)

HBs showed the comparable adsorption capacity with respect to other adsorbents, revealing that m-CS/PVA HBs was suitable and promising for the removal of CR from aqueous solutions since it has a relatively high adsorption capacity. 4. Conclusions The characterization of m-CS/PVA HBs indicated that Fe3O4 have been introduced successfully in m-CS/PVA HBs without damaging the crystal structure of Fe3O4. The saturation magnetization (21.96 emu g1) of the m-CS/PVA HBs was higher than or comparable to other magnetic bioadsorbents. The adsorption of CR onto mCS/PVA HBs agreed very well with the pseudo-second-order kinetic model. The adsorption processes are spontaneous and endothermic in nature. The adsorption capacity (470.1 mg g1) of m-CS/PVA HBs was higher than those of other absorbents. This work will be helpful in the development of magnetic bioadsorbents with high adsorption capacity and convenient recovery. Acknowledgements This study was support financially by the National Natural Science Foundation for Young Scholar of China (No. 21007044 and No. 21106091). The authors thank two anonymous reviewers for their valuable and helpful comments and suggestions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2011.11.057.

3.3. Comparison with other adsorbents References The adsorption capacity (qm value) for CR on m-CS/PVA HBs are comparable with those of the dyes onto other low-cost adsorbents as shown in Table 4. It can be seen from the table that m-CS/PVA 100

adsorption percent (%) Desorption percent (%)

%

90

80

70

60 1

2

Use time

3

4

Fig. 6. The performance of m-CS/PVA HBs by multiple cycles of regeneration.

Albornoz, C., Sileo, E.E., Jacoboa, S.E., 2004. Magnetic polymers of maghemite (c-Fe2O3) and polyvinyl alcohol. Physica B 354, 149–153. Annadurai, G., Juang, R.S., Lee, D.J., 2002. Use of cellulose-based wastes for adsorption of dyes from aqueous solutions. J. Hazard. Mater. 92, 263–274. Bayramoglu, G., Altintas, B., Arica, M.Y., 2009. Adsorption kinetics and thermodynamic parameters of cationic dyes from aqueous solutions by using a new strong cation-exchange resin. Chem. Eng. J. 152, 339–346. _ 2008. Equilibrium and kinetic data and process Bulut, E., Özacar, M., S ß engil, I., design for adsorption of Congo Red onto bentonite. J. Hazard. Mater. 154, 613– 622. Chatterjee, S., Lee, D.S., Lee, M.W., Woo, S.H., 2009. Congo Red adsorption from aqueous solutions by using chitosan hydrogel beads impregnated with nonionic or anionic surfactant. Bioresour. Technol. 100, 3862–3868. Chatterjee, S., Lee, M.W., Woo, S.H., 2010. Adsorption of Congo Red by chitosan hydrogel beads impregnated with carbon nanotubes. Bioresour. Technol. 101, 1800–1806. Chen, C.H., Wang, F.Y., Mao, C.F., Yang, C.H., 2007. Studies of chitosan. I. Preparation and characterization of chitosan/poly(vinyl alcohol) blend films. J. Appl. Polym. Sci. 105, 1086–1092. Chuang, Y.C., Chen, D.H., 2005. Preparation and adsorption properties of monodisperse chitosan-bound Fe3O4 magnetic nanoparticles for removal of Cu(II) ions. J. Colloid Interf. Sci. 238, 446–451. Crini, G., 2006. Non-conventional low-cost adsorbents for dye removal: a review. Bioresour. Technol. 97, 1061–1085. Das, S.K., Bhowal, J., Das, A.R., Guha, A.K., 2006. Adsorption behavior of rhodamine B on Rhizopus oryzae biomass. Langmuir 22, 7265–7272. Dotto, G.L., Pinto, L.A.A., 2011. Adsorption of food dyes onto chitosan: optimization process and kinetic. Carbohydr. Polym. 84, 231–238.

30

H.-Y. Zhu et al. / Bioresource Technology 105 (2012) 24–30

Freundlich, H., 1932. Of the adsorption of gases. Section II. Kinetics and energetics of gas adsorption. Introductory paper to section II. Trans. Faraday Soc. 28, 195– 201. Kim, S.J., Park, S.J., Kim, S.I., 2003. Swelling behavior of interpenetrating polymer network hydrogels composed of poly(vinyl alcohol) and chitosan. React. Funct. Polym. 55, 53–59. Klug, H.P., Alexander, L.E., 1974. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials. Wiley, New York, p. 618. Kumar, K., Tripathi, B.P., Shahi, V.K., 2009. Crosslinked chitosan/polyvinyl alcohol blend beads for removal and recovery of Cd(II) from wastewater. J. Hazard. Mater. 172, 1041–1048. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361–1403. Lewandowska, K., 2009. Miscibility and thermal stability of poly(vinyl alcohol)/ chitosan mixture. Thermochim. Acta 493, 42–48. Li, G.Y., Jiang, Y.R., Huang, K.L., Ding, P., Jie, Chen., 2008. Preparation and properties of magnetic Fe3O4–chitosan nanoparticles. J. Alloys Compd. 466, 451–456. Lian, L., Guo, L., Guo, C., 2009. Adsorption of Congo Red from aqueous solutions onto Ca-bentonite. J. Hazard. Mater. 161, 126–131. Liang, S., Liu, L., Huang, Q., Yam, K.L., 2009. Preparation of single or double-network chitosan/poly(vinyl alcohol) gel films through selectively cross-linking method. Carbohydr. Polym. 77, 718–724. Lu, A.H., Salabas, E.L., Schüth, F., 2007. Magnetic nanoparticles: synthesis, protection, functionalization, and application. Angew. Chem., Int. Ed. 46, 1222–1244. Luo, X., Zhang, L., 2009. High effective adsorption of organic dyes on magnetic cellulose beads entrapping activate carbon. J. Hazard. Mater. 171, 340–347. Nakano, Y., Bin, Y., Bando, M., Nakashima, T., Okuno, T., Kurosu, H., Matsuo, M., 2007. Structure and mechanical properties of chitosan/poly(vinyl alcohol) blend films. Macromol. Symp. 258, 63–81. Park, J.S., Park, J.W., Ruckenstein, E., 2001. Thermal and dynamic mechanical analysis of PVA/MC blend hydrogels. Polymer 42, 4271–4280. Periasamy, K., Namasivayam, C., 1994. Process development for removal and recovery of cadmium from wastewater by a low cost adsorbent: adsorption rates and equilibrium studies. Ind. Eng. Chem. Res. 33, 317–320. Rocher, V., Siaugue, J.M., Cabuil, V., Bee, A., 2008. Removal of organic dyes by magnetic alginate beads. Water Res. 42, 1290–1298.

Shawky, H.A., 2009. Synthesis of ion-imprinting chitosan/PVA crosslinked membrane for selective removal of Ag(I). J. Appl. Polym. Sci. 114, 2608–2615. Tripathi, S., Mehrotra, G.K., Dutta, P.K., 2010. Preparation and physicochemical evaluation of chitosan/poly(vinyl alcohol)/pectin ternary film for foodpackaging applications. Carbohydr. Polym. 79, 711–716. Vijayakumar, G., Dharmendirakumar, M., Renganathan, S., Sivanesan, S., Baskar, G., Elango, K.P., 2009. Removal of Congo Red from aqueous solutions by perlite. Clean Soil Air Water 37, 355–364. Wan Ngah, W.S., Kamari, A., Koay, Y.J., 2004. Equilibrium and kinetics studies of adsorption of copper (II) on chitosan and chitosan/PVA beads. Int. J. Biol. Macromol. 34, 155–161. Wang, L., Wang, A., 2007. Removal of Congo Red from aqueous solution using a chitosan/organomontmorillonite nanocomposite. J. Chem. Technol. Biotechnol. 82, 711–720. Wang, L., Wang, A., 2008. Adsorption properties of Congo Red from aqueous solution onto N,O-carboxymethyl-chitosan. Bioresour. Technol. 99, 1403–1408. Wang, Q., Du, Y.M., Fan, L.H., 2005. Properties of chitosan/poly(vinyl alcohol) films for drug-controlled release. J. Appl. Polym. Sci. 96, 808–813. Weber Jr., W.J., Morris, J.C., 1963. Kinetics of adsorption on carbon from solution. J. Sanit. Eng. Div. 89, 31–60. Xue, B., Sun, Y., 2001. Protein adsorption equilibria and kinetics to a poly(vinyl alcohol)-based magnetic affinity support. J. Chromatogr. A 921, 109–119. Yang, J.M., Su, W.Y., Leu, T.L., Yang, M.C., 2004. Evaluation of chitosan/PVA blended hydrogel membrances. J. Membr. Sci. 236, 39–51. Zhi, Jia., Wang, Y., Lu, Y., Ma, J., Luo, G., 2006. In situ preparation of magnetic chitosan/Fe3O4 composite nanoparticles in tiny pools of water-in-oil microemulsion. React. Funct. Polym. 66, 1552–1558. Zhu, A., Yuan, L., Liao, T., 2008. Suspension of Fe3O4 nanoparticles stabilized by chitosan and o-carboxymethylchitosan. Int. J. Pharm. 350, 361–368. Zhu, H.Y., Jiang, R., Xiao, L., Li, W., 2010a. A novel magnetically separable c-Fe2O3/ crosslinked chitosan adsorbent: preparation, characterization and adsorption application for removal of hazardous azo dye. J. Hazard. Mater. 179, 251–257. Zhu, H.Y., Jiang, R., Xiao, L., Zeng, G.M., 2010b. Preparation, characterization, adsorption kinetics and thermodynamics of novel magnetic chitosan enwrapping nanosized c-Fe2O3 and multiwalled carbon nanotubes with enhanced adsorption properties for methyl orange. Bioresour. Technol. 101, 5063–5069.