Study of adsorption of cationic dye on magnetic kappa ...

Journal of Environmental Chemical Engineering 2 (2014) 1578–1587

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Study of adsorption of cationic dye on magnetic kappa-carrageenan/ PVA nanocomposite hydrogels Gholam Reza Mahdavinia a,*, Abdolhossein Massoudi b, Ali Baghban b, Ebrahim Shokri a a b

Department of Chemistry, Faculty of Science, Payame Noor University, P.O. Box 19395-3697, Tehran, Iran Department of Chemistry, Faculty of Science, University of Maragheh, P.O. Box 55181-83111, Maragheh, Iran

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 January 2014 Accepted 28 May 2014

In this study, magnetic nanocomposite hydrogels were synthesized by incorporation of kappacarrageenan, polyvinyl alcohol (PVA), and magnetic Fe3O4 nanoparticles. Magnetite nanoparticles were prepared in the presence of a mixture of kappa-carrageenan/PVA (CaraPVA) via in situ co-precipitation method. For preparation of hydrogels, the synthesized magnetite-polymers were crosslinked with freezing–thawing technique and subsequently with K+ solution. The structure of magnetic hydrogels was characterized by transmittance electron microscopy (TEM), scanning electron microscopy (SEM), Xray diffraction (XRD), thermogravimetric analysis (TGA), and vibrating sample magnetometer (VSM). The adsorption of cationic dye crystal violet (CV) from aqueous solution by magnetic hydrogels was investigated via a batch adsorption system on the subject of contact time, pH of the initial dye solution, temperature, ion strength, and initial dye concentration. The results showed that while the removal of dye decreased by the incorporation of magnetite particles, removal efficiency was improved by increasing the ratio of kappa-carrageenan. Dye adsorption capacity of hydrogels decreased with increasing the ion strength of dye solution. The effect of pH of the initial dye solution revealed that while the adsorption capacity of hydrogels was not significantly changed in the pH range 2–10, removal efficiency was reduced at highly basic media. According to thermodynamic parameters, the adsorption of CV on magnetic hydrogels occurred spontaneously. The equilibrium adsorption data were analyzed with non-linear Langmuir and Freundlich models. It was found that the equilibrium process was followed the Langmuir model well. ß 2014 Elsevier Ltd. All rights reserved.

Keywords: Polyvinyl alcohol kappa-Carrageenan Magnetic Adsorption Kinetic

Introduction Industrial sources such as textile and paper printing produce large quantity of wastewater containing dyes. Toxic dyes are type of pollutants which can damage the environment and thus, they should be removed from wastewater before discharge them into the environment. Some techniques such as chemical precipitation, filtration, electrochemical treatment, reverse osmosis, and adsorption have been developed to remove dyes from water [1,2]. Among them, removal of dye from aqueous solutions has been developed by adsorption process due to easy recovery and reusability, low cost, and simplicity of adsorbents [3]. In this case, activated carbon [4], clay minerals [5], nanocomposites [6], and hydrogels [7] have been widely evaluated as adsorbent to adsorb dyes from

* Corresponding author. Tel.: +98 4212276060; fax: +98 4212276060. E-mail addresses: [email protected], [email protected] (G.R. Mahdavinia). http://dx.doi.org/10.1016/j.jece.2014.05.020 2213-3437/ß 2014 Elsevier Ltd. All rights reserved.

wastewater. The hydrogels especially composed of renewable and sustainable sources have been used for treatment of colored water [8,9]. Hydrogels are three-dimensional crosslinked polymers containing hydrophilic groups which enable them to absorb water. Hydrogels can be classified into ionic and non-ionic. The ionic types comprise anionic (–CO2, –SO3) or cationic pendants (–NR3+) [10]. The presence of these ionic groups in the hydrogels opens potential area of application particularly removal of pollutants from wastewaters. The hydrogels composed of sodium alginate with carboxylate groups (–CO2) [6], chitosan with pendants amine (–NH2) [11], carboxymethyl cellulose with anionic carboxylate [12], and kappa-carrageenan with anionic sulfate groups (–OSO3) [13] have been synthesized and evaluated as adsorbent to remove cationic dyes from water. Recently, due to non-toxicity, biodegradability, and biocompatibility of kappa-carrageenan biopolymer there are considerable attentions to provide new carrageenan-based materials. The gelling of kappa-carrageenan can be occurred physically (in the presence of K+ cation) and chemically (non-toxic genipin) [14,15].

G.R. Mahdavinia et al. / Journal of Environmental Chemical Engineering 2 (2014) 1578–1587

The adsorption of cationic dye onto carrageenan-based hydrogels has been reported [13]. In addition to biopolymers, non-ionic and water soluble polyvinyl alcohol (PVA) has been attracted the attention of scientist. Because of non-toxicity, biocompatibility, and physical gelation ability, the PVA has been widely applied in the field of wastewater treatment [16]. The chemical and physical methods can be applied to obtain PVA-based hydrogels. Among the physical methods, freezing–thawing has gained much attention in recent years owing to the simplicity and deleting of toxic crosslinkers [17]. However, the applications of PVA-based hydrogels have been limited in many fields due to the low degree of swelling and lack of ionic pendants. Introducing an ionic biopolymer into PVA hydrogels is an efficient method to improve the ionic behavior of PVA hydrogels. Sodium alginate [18], kappa-carrageenan [19], and chitosan [20] are ionic biopolymers with active functional groups that have been applied to the synthesis of PVA-based hydrogels. The PVA-based hydrogels have been used to remove cationic dyes [18,20]. Unlike the traditional adsorbents, recovery of the magnetic adsorbents from solutions can be easily achieved by an external magnetic field [21]. Magnetic adsorbents have been synthesized and applied to remove pollutant from aqueous solutions [20]. In this study, we synthesized the magnetic Fe3O4 nanoparticles through in situ co-precipitation of iron salts in the presence of a mixture of kappa-carrageenan and PVA polymers. The structure of magnetic nanocomposite hydrogels was characterized by SEM, TEM, TGA, XRD, and VSM techniques. Then, the obtained magnetic nanocomposite hydrogels were examined for the removal of cationic dye crystal violet from water. Finally, the isotherm and thermodynamics of adsorption process as well as the effect of salinity and pH on adsorption were also investigated.

into the solution. When the pH of solution was reached around 10, a dark solution was appeared indicating the formation of magnetic nanoparticles. The pH of solution was adjusted at 10 and allowed to stir at 70 8C for 1 h. The crosslinking of magnetic polymer solution was done by freezing–thawing method and subsequent with K+ solution. At first, the obtained solution containing PVA, kappa-carrageenan, and magnetic nanoparticles was cooled to ambient temperature and the products were kept frozen for overnight. Then, the frozen hydrogels were thawed at ambient temperature (18 8C) for 5 h. This procedure was repeated for 4 times. After freezing–thawing step, the hydrogels were immersed into 0.5 M of KCl solution for 30 min. Finally, for purification of the crosslinked magnetic hydrogels, samples were immersed into excess distilled water for overnight. The hydrogels were cut into 0.5 0.5 mm discs with 0.4 mm thickness and dried at 40 8C for constant weight. It may be noted that because of strong interactions between PVA chains, dissolving or disintegration of hydrogels was not occurred during purification [17]. Dye adsorption measurements Adsorption of CV dye on nanocomposites was carried out by immersing the 0.1 g of nanocomposites into dye solution (25 mL, 25 mg L1 of CV). All adsorption experiments were examined by a batch method on a shaker with a constant speed at 120 rpm. All the adsorption experiments were done at ambient temperature (18 8C). To study the adsorption kinetics, at specified time intervals, the amount of adsorbed CV was evaluated using a UV spectrometer at lmax = 595 nm. The solutions were centrifuged (at 3000 rpm for 10 min) before measurements. The content of adsorbed dye was calculated using Eq. (1):

Materials and methods qt ¼ Materials kappa-Carrageenan was obtained from Condinson Co. (Denmark). Polyvinyl alcohol (MW 89,000–98,000; degree of hydrolysis 99%) was purchased from Aldrich Chemicals (United States). All other chemicals were analytical grade and used without any purification. Synthesis of magnetic hydrogels The magnetic nanocomposite hydrogels were prepared via in situ co-precipitation of iron salts in the presence of binary mixture of PVA and kappa-carrageenan. Table 1 indicates the different hydrogels synthesized by the required content of initial materials. In brief, 1 g of PVA was poured in 20 mL of distilled water and the temperature was adjusted at 80 8C and stirred until PVA was completely dissolved. The kappa-carrageenan solution was prepared separately by dissolving of 1 g of this biopolymer in 25 mL of distilled water at 70 8C. Two solutions were mixed together and the temperature was adjusted at 70 8C. Then, the desired contents of FeSO47H2O and FeCl36H2O salts were dissolved in 5 mL of water and were added into polymer solution. The solution was stirred until to be clear. To obtain the magnetic nanoparticles in the presence of polymers, the 3 M NH3 solution was slowly dropped

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ðC 0 C t Þ V m

(1)

where, C0 is the initial CV concentration (mg L1); Ct is the remaining dye concentration in the solution at time t; V is the volume of dye solution used (L); and m (g) is the weight of nanocomposite. Adsorption isotherm was carried out by immersing of 0.1 g of nanocomposite into 25 mL of dye solutions with 10, 25, 75, 100, 150, and 200 mg L1 of CV at ambient temperature (18 8C) for 24 h. The equilibrium adsorption capacity of nanocomposites, qe (mg g1), was determined using Eq. (1). At this equation, the Ct and the qt will be replaced with equilibrium concentration of dye in the solution (Ce) and equilibrium adsorption capacity (qe), respectively. Also, to investigate the effect of pH on adsorption, the pH of initial dye solution was adjusted by 0.1 M HCl or 0.1 M NaOH solutions. Desorption studies Desorption study was carried out via bath method. The desorption solutions in this study were ethanol (96%, V/V), water/ethanol (50/50, V/V), 0.5 M KCl in water, 0.5 M KCl in water/ethanol (50/50, V/V), and 0.2 M acetic acid solutions. The dye loaded mCaraPVA2 nanocomposite was transferred into distilled water for 1 h to remove un-desorbed dye. Then, the sample was immersed into desorption solution and stirred on a

Table 1 The required amount of materials for synthesis of magnetic CaraPVA nanocomposite hydrogels; degree of swelling (DS) of hydrogels.

CaraPVA mCaraPVA1 mCaraPVA2 mCaraPVA3

PVA (g)

kappa-Carrageenan (g)

FeSO47H2O (g)

FeCl36H2O (g)

DS (g/g)

1 1 0.8 0.6

1 1 1.2 1.4

0 0.28 0.28 0.28

0 0.4 0.4 0.4

25.5 20.5 26.7 28.2

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shaker (120 rpm) for 24 h. Desorption content was calculated according to the calibration curve for each solution.

Results and discussion Synthesis of magnetic nanocomposite

Point of zero charge The points of zero charge (pHpzc) of hydrogels were determined according to the literature [22,23]. In these series of experiments, 20 mL of 0.05 M of NaCl solution was poured into several beakers and the pH of NaCl solutions were adjusted from 1 to 10 by adding either 0.1 M of HCl or 0.1 M of NaOH solutions. After setting the pH, the volume of the solutions was adjusted to 30 mL by adding of 0.05 M of NaCl solution. The initial pH (pHi) of the solutions was exactly recorded and the solutions were transferred in Erlenmeyer flasks. Then, 50 mg of hydrogels were immersed into solutions and were shaken on a shaker (90 rpm, ambient temperature) for 48 h. Then, the solutions containing hydrogels were centrifuged (4500 rpm) for 15 min and the pH of supernatant solution was recorded (pHf). The points of zero charge were found from plot of DpH (pHi pHf) versus pHi. Instrumental analysis Dried nanocomposites were coated with a thin layer of gold and imaged in a scanning electron microscopy (SEM) instrument (Vega, Tescan). One-dimensional, wide angle X-ray diffraction (XRD) patterns were obtained by using a Siemens D-500 X-ray diffractometer with wavelength, l = 1.54 A˚ (Cu-Ka), at a tube voltage of 35 kV, and tube current of 30 mA. Transmission electron microscopy (TEM) micrographs were recorded with a Philips CM10 operating at 60 kV tension. The magnetic properties of the beads were studied with a vibrating sample magnetometer (VSM) (model 7400, Lakeshore Company, USA). A thermal analyzer (Mettler Toledo, USA) was used for thermogravimetric analysis (TGA) under nitrogen atmosphere. The heating rate was 5 8C/min.

Magnetic nanocomposite hydrogels composed of PVA, kappacarrageenan, and magnetic Fe3O4 nanoparticles were synthesized successfully via in situ co-precipitation of iron salts in the presence of a mixture of PVA and kappa-carrageenan. In general, the iron salts were poured into polymer solution and the magnetic nanoparticles were obtained by slowly adding of ammonia solution. By adjusting the pH of solution at 10, the magnetic nanoparticles were formed with the dark color. The crosslinking of magnetic polymers was done in two steps. First, the freezing– thawing method was applied to crosslink the PVA component [17]. After freezing–thawing for 4 times, the hydrogels were transferred into KCl solution for crosslinking of the kappa-carrageenan component. The anionic sulfate groups on kappa-carrageenan backbones can be interacted electrostatically with K+ cations which led to crosslink of kappa-carrageenan component [24]. A simple scheme indicating the synthesis of magnetic nanocomposite hydrogels was shown in Fig. 1. The surface morphology of nanocomposite hydrogels was studied by SEM technique and shown in Fig. 2. The SEM micrographs depicted that the surface micro-structure of hydrogels is affected by the composition of matrix. According to Fig. 2a, a smooth surface was obtained for magnetic-free CaraPVA hydrogel. As the magnetic nanoparticles were introduced into hydrogel, the surface morphology of hydrogel was changed, consequently an unsmooth surface with bulge sections was observed (Fig. 2b, mCaraPVA2). The presence of magnetic Fe3O4 nanoparticles in hydrogels was confirmed by XRD patterns. The XRD pattern of mCaraPVA2 was shown in Fig. 3a. The distinct peaks at 2u about 30.2, 35.5, 43.5, 53.4, 57.4, and 63.58 are appeared and these peak positions are in agreement with the characteristic peaks of Fe3O4

Fig. 1. A simple scheme for preparation of magnetic kappa-carrageenan/PVA nanocomposite hydrogels.


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loss (93%) can be attributed to chemical degradation originating from bond scission in PVA backbones [25]. Unlike the PVA, the weight loss of native kappa-carrageenan happened in three different stages. The first stage in the range of 20–184 8C, peaking at 92.5 8C, is associated with the loss of water (11.2 wt%). The second stage occurred in the range of 184–250 8C, with the maximum decomposition rate at 201 8C. Weight loss of this stage was 11 wt%. The main mass loss was observed in the range of 251– 438 8C with maximum decomposition rate at 321 8C and the weight loss of 29.8 wt%. The TGA traces of magnetic-free (CaraPVA) and magnetic hydrogel (mCaraPVA1) were between native PVA and kappa-carrageenan with three different decomposition stages. The results of this study revealed that the magnetic nanoparticles caused an increase in thermal stability of hydrogel. In the second stage of decomposition of hydrogels, the maximum decomposition rate of CaraPVA and mCaraPVA1 obtained at 231 8C and 283 8C indicated high thermal stability of magnetic hydrogel due to the presence of magnetite nanoparticles [26]. Compared with CaraPVA, in the third stage, the weight loss of mCaraPVA1 was occurred with slower slope showing high thermal stability of magnetic hydrogel. The percent residual weight of mCaraPVA1 was approximately 8% higher than non-magnetic hydrogel. This value could be attributed to the percent of synthesized magnetic nanoparticles in hydrogel matrix. Effect of contact time on adsorption

Fig. 2. SEM micrographs of non-magnetic CaraPVA (a) and magnetic mCaraPVA2 hydrogel (b).

confirming the formation of Fe3O4 in hydrogel matrix [5]. The dispersion of nanoparticles was investigated by TEM and the TEM image of mCaraPVA2 was shown in Fig. 3b. According to this figure, the magnetic nanoparticles in matrix of hydrogel have cluster-like structure and the size of nanoparticles obtained lower than 50 nm. The hysteresis loop of mCaraPVA2 was investigated using VSM technique between 9 kOe at 298 K. The result was depicted in Fig. 3c. Based on this figure, the saturation magnetization of mCaraPVA2 hydrogels was obtained 3.4 emu g1. The obtained saturation magnetization of hydrogels was sufficient to remove them from aqueous solution by a permanent magnet. TGA thermograms of neat PVA, neat kappa-carrageenan, CaraPVA hydrogel, and magnetic mCaraPVA1 hydrogel were studied and the results were shown in Fig. 4. Despite the high thermal stability of neat PVA, kappa-carrageenan showed low thermal resistance. It indicated that the main weight loss of neat PVA took place at one stage ranged from 315 to 456 8C. This mass

Removal of dye by hydrogels as a function of magnetic nanoparticles and weight ratio of kappa-carrageenan/PVA was investigated. The results were shown in Fig. 5a. Based on this figure, the adsorption of dye was affected by the magnetic nanoparticles as well as the weight ratio of PVA/carrageenan. The dye adsorption capacity of CaraPVA was obtained 15 mg g1. As the magnetic nanoparticles were incorporated in the composition of hydrogel (mCaraPVA1), adsorption capacity was found to be 13.5 mg g1. The corresponding decrement in dye adsorption capacity can be attributed to: (a) the decrease in weight ratio of kappa-carrageenan with anionic centers in composition of hydrogel [9] and (b) the degree of swelling (DS) of mCaraPVA1 which was lower than the CaraPVA (Table 1). The decrease in the swelling can result in a reduction in the surface of adsorbent and the availability of more anion centers [9]. When the ratio of carrageenan was increased, an enhancement in dye adsorption capacity of adsorbents for dye was observed. The adsorption capacity of mCaraPVA2 and mCaraPVA3 for CV was 17.2 and 20.5 mg g1, respectively. In this work, the amount of magnetic nanoparticles remained constant. So, the increase in dye adsorption can be attributed to the increase in weight ratio of carrageenan and thereby the increase in anionic sulfate centers for adsorption of cationic CV. Moreover, according to Table 1, the increase in carrageenan component caused an enhancement in swelling capacity and due to the high degree of swelling an increase in the surface of adsorbent occurs. The adsorption of adsorbate on the adsorbent can take place through several steps. These steps may be considered as film diffusion, pore diffusion, surface diffusion, and adsorption on the pore surface [27]. In fact, under sufficient speed of stirring, intraparticle diffusion/transport process is the rate-limiting step of adsorption kinetic. The possibility of intra-particle diffusion is expressed according to Eq. (2) [27]: qt ¼ kid t 0:5 þ C i

(2)

where, qt is the amount of dye adsorbed on adsorbent at time t; Ci (mg g1) and kid (mg g1 min0.5) are the intercept and intraparticle diffusion rate constant, respectively.

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Fig. 3. XRD pattern of magnetic mCaraPVA2 (a), TEM image of magnetic mCaraPVA2 (b), and hysteresis loops of mCaraPVA2 at 298 K (c).

According to Eq. (2), by plotting qt versus t0.5, a straight line suggests that the intra-particle diffusion is the rate limiting step. As can be seen in Fig. 5b, the diffusion plot of CV onto hydrogels seems multi-linear and it contains three linear parts. The linear segments did not pass through the origin which showed that the intra-particle diffusion is not the only rate limiting step. The first linear segments indicated that the mass transfer controlling may be due to boundary layer effect [28]. Also, the steep slope of first stage can be attributed to high concentration of dye in solution. In second stage, the adsorption of CV on hydrogels occurred slowly which revealed that the diffusion of adsorbate through macropores of hydrogel and internal adsorption of dye on adsorbents [29]. At this stage, the intra-particle diffusion is rate

Fig. 4. TGA thermograms of neat kappa-carrageenan, neat PVA, CaraPVA and mCaraPVA1 hydrogels.

Fig. 5. Amount of adsorbed dye on magnetic hydrogels versus time (a) and plot of qt against t0.5 according to intra-particle diffusion model (b).

G.R. Mahdavinia et al. / Journal of Environmental Chemical Engineering 2 (2014) 1578–1587 Table 2 Constant parameters from intra-particle diffusion model. Intra-particle diffusion constants of stage i (kid, mg g1 min0.5)

CaraPVA mCaraPVA1 mCaraPVA2 mCaraPVA3

Thickness of boundary layer at stage i (Ci)

kd1

kd2

kd3

C1

C2

C3

1.68 1.29 1.81 2.97

1.08 0.81 1.23 0.82

0.24 0.21 0.13 0.29

0 0 0 0

2.07 2.04 2.75 9.5

10.17 9.5 14.7 14.8

controlling step. Also, the gentle slope of third step showed the occurring of saturation of active centers on adsorbents. The Ci and kid were shown in Table 2. The order of kid and Ci were k1d > k2d > k3d and C1 < C2 < C3, respectively. The corresponding observation can be attributed to the gradual decrease of CV concentration in aqueous solution [30]. Effect of pH and salinity on adsorption The pH of the initial dye solution is an important factor in adsorption process. This behavior arises from the nature of the active centers on the adsorbents [31]. In order to investigate the adsorption of CV dye on adsorbents, the pH of the initial dye solution was changed between 2 and 12. The effect of pH ranging from 2 to 12 on the adsorption of CV on CaraPVA and mCaraPVA3 adsorbents was studied and shown in Fig. 6a. According to the obtained results, the adsorption capacity of hydrogels for CV was relatively invariant in the pH range of 2–10. This behavior may be originated from the pKa of anionic sulfate groups on kappacarrageenan. According to the literature, the pKa of these anionic sulfates is around 2 and its ionization occurs above this value [32,33]. This behavior of the corresponding hydrogels was similar to our previous work about the adsorption of CV dye on

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nanocomposite hydrogels composed of kappa-carrageenan and alginate biopolymers [6]. For obtaining the reliability of this behavior of hydrogels, the pHpzc of corresponding adsorbents was determined and found about 1.41 and 1.71 for CaraPVA and mCaraPVA3, respectively (Fig. 6b). When the pH of dye solutions is higher than that of pHpzc, the hydrogels comprise a negative surface charge in aqueous solutions and electrostatic interactions between cationic dye and negative center on the surface of adsorbents occurs [34]. But at lower pH value than pHpzc, the positive surface charge on adsorbents restricts the adsorption of cationic dye and thereby the decrease of adsorption capacity of samples. In fact, the pHpzc values of hydrogels corroborate the effect of pH of initial dye solution on adsorption of CV on hydrogels. At pH 12, a reduction in dye affinity for hydrogels was seen which may be attributed to the screening effect of the counterions (Na+) restricts the approaching of cationic dye on sulfate groups [35]. In addition to pH, the presence of salt in dye solutions can affect the adsorption process. In this section of our work, we studied the effect of NaCl concentration on CV adsorption on CaraPVA and mCaraPVA3 hydrogels. The NaCl concentration was varied from 0.01 to 0.5 M which were presented in Fig. 6c. The adsorption capacity of both hydrogels was gradually decreased by increasing NaCl concentration. Most of the dyes contain hydrophobic structure and the solubility of these dyes is reduced in the presence of salt, thus an enhancement in dye adsorption is observed [35]. This reduction in the solubility of dyes is originated from increasing in polarity of the solution. In contrast, among the adsorbents with ionic pendants, often a reduction in dye adsorption is obtained [36]. The corresponding decrement in the dye adsorption capacity of nanocomposite hydrogels can be attributed to the neutralization or screening of anionic sulfate (– OSO3) groups on kappa-carrageenan by Na+ cations [35,37]. This effect can lead to a reduction in electrostatic interactions between cationic dye and anionic sulfate groups on adsorbent.

Fig. 6. Effect of pH on the adsorption of CV on CaraPVA and mCaraPVA3 hydrogels (a), point zero charges of CaraPVA and mCaraPVA3 hydrogels (b), and effect of NaCl concentration on the adsorption of CV on CaraPVA and mCaraPVA3 hydrogels (c).


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Table 3 Constants parameters of isotherm models for adsorption of CV on hydrogels. Freundlich model 1

n (g L CaraPVA mCaraPVA1 mCaraPVA2 mCaraPVA3

2.48 1.94 2.17 2.63

)

Langmuir model 1

kF (mg g 9.04 4.11 6.4 12.86

1 1/n

)(L mg

)

x

2

16.6 4.22 7.2 74

r

2

0.94 0.98 0.94 0.88

Adsorption isotherm The adsorption isotherm expresses the correlation between the content of the adsorbed dye on adsorbent and the remained dye concentration at equilibrium time [38]. In this study, the experimental data from adsorption isotherms were fitted to non-linear Langmuir and Freundlich models to describe the adsorption process. Langmuir model describes monolayer adsorption of adsorbate on specific homogeneous sites within the adsorbent. Non-linear Langmuir model is expressed by Eq. (3) [39]: qe ¼

qm K L C e 1 þ kL C e

(3)

where, Ce is the equilibrium dye concentration in the solution (mg L1) at equilibrium; KL is the Langmuir adsorption constant related to the energy of adsorption (L mg1); and qm is the maximum adsorption capacity (mg g1). Dimensionless constant

1

qm (mg g

)

55 52 60.7 78.2

qm,exp 1

KL (L mg 0.019 0.015 0.021 0.037

)

2

2

x

r

RL

4.5 1.99 1 2

0.98 0.99 0.999 0.98

0.84 0.86 0.82 0.73

55.2 49.2 57 74.1

adsorption parameter RL, as an important characteristic of Langmuir model, is expressed as follows [40]: 1 RL ¼ (4) 1 þ K LCo where KL is the Langmuir constant (L mg1) and Co is the initial concentration of dye. The RL indicating type of isotherm is varied: unfavorable for RL > 1; adsorption is linear condition for RL = 1; favorable for 0 < RL < 1; irreversible conditions for RL = 0 [40]. Unlike the Langmuir model, in the Freundlich model, the adsorption of adsorbate occurs on a heterogeneous surface by multilayer sorption. Non-linear Freundlich model is defined as follows [39]: qe ¼ kF C e 1=n

(5)

kF is the equilibrium adsorption coefficient where, (mg g1)(L mg1)1/n, and 1/n is the empirical constant. In fact,

Fig. 7. Comparison between the experimental and modeled isotherms plots for adsorption of CV dye on CaraPVA (a), mCaraPVA1 (b), mCaraPVA2 (c), and mCaraPVA3 (d).


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the n value depicts the favorability of adsorption process and kF is related to the adsorption capacity of adsorbent. The nonlinear fitting of data was performed by using Origin 8 software. The validity of models was estimated by regression coefficient (r2, Eq. (6)) and chi square test (x2, Eq. (7)) that obtained from the analysis of variance (ANOVA) in origin. A well-fitting occurs when r2 is close to unity and x2 is the lowest [41]. r 2 ¼ Pn

ðqi;meas q¯ i;cal Þ2

i¼1

x2 ¼

(6)

ðqi;meas q¯ i;cal Þ2 þ ðqi;meas qi;cal Þ2

n X ðqi;cal qi;meas Þ2

(7)

qi;meas

i¼1

where, qi,meas and qi,cal are the experimental and calculated amount of dye adsorption. The q¯ cal indicates the average of q calculated theoretically. The adsorption constant parameters, r2 and x2, obtained after fitting the adsorption isotherm data to Langmuir and Freundlich models were summarized in Table 3. Fig. 7a–d displays curves to compare the modeled adsorption isotherms with the practical data. According to Fig. 7, it was observed that the adsorption of dye on hydrogels followed the Langmuir model better than that of the Freundlich model. According to the data from Table 3, the values of regression coefficient r2 (close to unity, r2 > 0.98) and chi square test x2 (low value) confirmed well-fitting of experimental data to Langmuir model. Additionally, the maximum adsorption capacity of hydrogels for dye obtained from Langmuir model was relatively in agreement with experimental data. This fitting revealed that a monolayer adsorption of dye takes place on hydrogel adsorbents. According to Langmuir model, maximum dye adsorption capacities of hydrogels for CV were obtained 55, 52, 60.7, and 78.2 mg g1 for CaraPVA, mCaraPVA1, mCaraPVA2, and mCaraPVA3, respectively. Also, the values of RL for hydrogels were calculated from Langmuir model and were found between zero and 1 indicating favorable adsorption system. Thermodynamic study Thermodynamic parameters should be considered as important factors in the design of adsorption process. It is necessary to specify the change of thermodynamic parameters to estimate the feasibility and mechanism of adsorption process [38]. Thermodynamic parameters including standard Gibbs free energy (DG, kJ mol1), enthalpy change (DH, kJ mol1), and entropy change (DS, J K1 mol1) can be used for the estimation of the feasibility and mechanism of adsorption process and are calculated according to the following equations [42]: ln K c ¼

DH RT

þ

Fig. 8. Plot of ln Kc versus 1/T for mCaraPVA3.

According to Fig. 8, the DH and DS were obtained 28.121 kJ mol1 and 86.78 J K1 mol1 (Table 4). While the negative value of DH indicates the exothermic nature of adsorption process, the negative value of enthalpy shows that during the adsorption of dye onto the nanocomposite a decreased randomness at the solid– solution interface is occurred [38]. The Gibbs free energy for the adsorption of CV on mCaraPVA3 was obtained 3.39, 2.52, and 1.65 kJ mol1 at 285, 295, and 305 K, respectively. The negative values of DG show a spontaneous adsorption process. Also, an enhancement in DG values was observed with increasing the temperature from 285 to 305 K indicating a decrease in dye adsorption on mCaraPVA3. The mechanism of adsorption of dye on nanocomposites could be estimated from the values of DG and DH [31,43]. The values of DG ranging from 20 to 0 kJ mol1 indicate that the physisorption is dominated; but chemisorption occurred for a range of 80 to 400 kJ mol1. By considering the data of Table 4, it is concluded that in this study the mechanism of adsorption process is physisorption. In addition, the values of DH lower than 20 kJ mol1 1 depicts that the physisorption interactions such as Van der Waals are dominated. The values of DH ranging from 20 to 80 kJ mol1 indicate that the physisorption interaction such as electrostatic leads to adsorption of adsorbate on adsorbent. The chemisorption interaction occurs when the values of DH are between 80 and 450 kJ mol1. The enthalpy value for the adsorption of CV on magnetic nanocomposite was obtained 28.121 kJ mol1. According to the DH value for the adsorption process, the positive CV dye molecules adsorb electrostatically on the anionic centers of magnetic nanocomposite [31]. Regeneration studies

DS

(8)

R

DG ¼ DH T DS

(9) 1

where, the equilibrium constant Kc (L g ) was calculated by multiplying the Langmuir constants qm and KL (Kc = qm KL) [42]. R is the universal gas constant (8.314 J mol1 K1); and T is the absolute temperature (K). DH and DS were calculated from slope and intercept of linear plot of ln Kc versus 1/T, respectively (Fig. 8).

In order to cost-effectiveness of adsorbents, desorption of adsorbed dye should be considered as an important factor to reuse of adsorbents. Therefore, by using different solutions, the regeneration of magnetic hydrogel mCaraPVA2 was evaluated. Ethanol (96%, V/V), 0.2 M of acetic acid solution, 0.5 M of KCl solution, 50/50 (V/V) of ethanol/water mixture, and 0.5 M of KCl in 50/50 (V/V) of ethanol/water mixture were used for desorbing of dye from hydrogels. Desorption efficiencies by various solutions

Table 4 Thermodynamic parameters for adsorption of CV on mCaraPVA3 magnetic hydrogel. T (K)

KL (L mg1)

qm (mg g1)

Kc (L g1)

DG (kJ mol1)

DH (kJ mol1)

DS (J K1 mol1)

285 295 305

0.0487 0.04 0.026

79 77.3 70

3.85 3.092 1.82

3.39 2.52 1.65

28.121

86.78

1586


magnetic nanocomposite hydrogels was characterized by SEM, TEM, VSM, TGA, and XRD techniques. Adsorption investigation was performed as a function of temperature, the contact time, the ion strength, the pH of initial dye solution, and the initial dye concentration. A decrease in dye adsorption on hydrogels was observed by introduction of magnetite nanoparticles into hydrogels. This reduction was improved by increasing the ratio of kappacarrageenan in composition of hydrogels. According to intraparticle diffusion model, the adsorption of dye on hydrogels was not happened only through intra-particle diffusion. The negative values of Gibbs free energy DG revealed spontaneous occurring of adsorption of dye on hydrogels. In addition, the magnitudes of DG and DH values were in agreement with physisorption process especially electrostatic interaction between anionic centers of hydrogels and cationic dye. The adsorption of dye on hydrogels was not affected by changing the pH of initial dye solution ranging from 2 to 10. The values of pHpzc of non-magnetic and magnetic hydrogels were obtained lower than 2 indicating the presence of anionic centers on adsorbents at pH values ranged 2–10. The adsorption capacity of hydrogels for dye seemed to decrease with increasing the ion strength of dye solution. This behavior may be attributed to the screening effect of anionic sulfates on carrageenan by Na+ cations. Equilibrium adsorption data followed Langmuir isotherm model and the values of RL of hydrogels indicated favorable adsorption system. Finally, the desorption experiments were performed by using different solutions and the 0.5 M of KCl in ethanol/water (50/50, V/V) found to be suitable. References

Fig. 9. Desorption efficiency of dye from mCaraPVA2 by various solutions (a) and adsorption–desorption behavior of CV for mCaraPVA2 using 0.5 M KCl in water/ alcohol (b).

were illustrated in Fig. 9a. First of all, the desorbing of adsorbed dye was examined by 0.2 M acetic acid solution and desorption efficiency was low (