Magnetic chitosan composite for adsorption of cationic and anionic dyes in aqueous solution

Magnetic chitosan composite for adsorption of cationic and anionic dyes in aqueous solution

G Model JIEC-2403; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx Contents lists available at ScienceDirect Jour...

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G Model

JIEC-2403; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Magnetic chitosan composite for adsorption of cationic and anionic dyes in aqueous solution Dong-Wan Cho a, Byong-Hun Jeon b,*, Chul-Min Chon c, Franklin W. Schwartz d, Yoojin Jeong a, Hocheol Song a,** a

Department of Environment and Energy, Sejong University, Seoul 143-747, South Korea Department of Natural Resources and Environmental Engineering, Hanyang University, 2226 Wangsimni-ro, Seongdong-gu, Seoul 133-791, South Korea Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea d School of Earth Sciences, The Ohio State University, Columbia, OH 43210, USA b c

A R T I C L E I N F O

Article history: Received 24 December 2014 Received in revised form 6 January 2015 Accepted 31 January 2015 Available online xxx Keywords: Methylene blue Methyl orange Chitosan Clay Magnetite

A B S T R A C T

A magnetic composite material composed of nano-magnetite (NMT), heulandite (HE), and cross-linked chitosan was prepared and used as an adsorbent for methylene blue (MB) and methyl orange (MO). The composite was characterized for the morphology, magnetic and surface properties. The optimal mass ratio of chitosan:HE:NMT for the best removal of both dyes was determined to be 1:1:0.33. The adsorption of MB and MO followed the pseudo-second order kinetics, and the maximum adsorption capacities were 45.1 and 149.2 mg g1 at pH 5.5, respectively. The adsorption of MB increased with the pH increase, while MO adsorption showed an opposite trend. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction Many industries such as textile, leather, food processing, dyeing, and cosmetics generate massive amount of wastewater, and dye chemicals represent one of the most prominent contaminants contained in the wastewater effluents [1]. Surface- and groundwaters contaminated with dyes are easily recognizable due to their high colorant visibility even in very small quantities. The presence of dye in the water bodies increases the chemical oxygen demand as well as adversely influences the metabolic functions of phytoplankton and aquatic plants by interfering with photosynthesis [2]. Industrial wastewater treatments largely depend on microbialmediated treatments processes, but they occasionally suffer from limited efficiencies in dyes removal due to low biodegradability of dye materials, and inhibition effect of high salt concentration of wastewaters [3]. Alternatively, numerous technologies have been applied to control dye contamination, including adsorption [4,5],

* Corresponding author. Tel.: +82 2 2220 2242; fax: +82 2 2281 7769. ** Corresponding author. Tel.: +82 2 3408 3232; fax: +82 2 3408 4320. E-mail addresses: [email protected] (B.-H. Jeon), [email protected] (H. Song).

oxidation [6,7], membrane [8], biological treatment [9], and electro-coagulation [10]. Ozone-assisted chemical oxidation has been successfully applied for dyes removal without causing sludge production, but it suffers from short lifetime of ozone and high operation cost [11]. Oxidation utilizing Fenton’s reagent is also shown to be effective in decolorizing both soluble and insoluble dyes, but it generates excessive amounts of sludge during the process [12]. Membrane filtration has disadvantages such as high capital cost and frequent pore clogging. Adsorption method, despite its problems associated with adsorbent disposal and postcontamination by used adsorbents, is considered to be an effective and simple method relatively free from concerns of generating unwanted byproducts, and many recent investigations have focused on the development of low cost and high efficient adsorbents to treat dyes [13–15]. Recently, chitosan-based adsorbents have attracted particular interests due to their environmental-friendly properties, high availability and versatility as a treatment medium [16,17]. Chitosan is a polymeric substance derived from deacetylation of chitin, a major component of skeletons of crustaceans. It has a large number of hydroxyl and amino groups branched from polymeric backbones that are held tight by hydrogen bonding by those functional groups. The amino groups undergo protonation reaction

http://dx.doi.org/10.1016/j.jiec.2015.01.023 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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and it loses structural integrity to form a gel-like solution in acidic condition. To overcome such an application limitation in acidic condition, cross-linking agents including epichlorohydrin, tripolyphosphate (TPP), and glutaraldehyde have been utilized to prevent liquidation of chitosan [18–20]. Among those cross-linkers, TPP is more preferred than others because it has been frequently used as an additive material for structural reinforcement of solids with no toxic effect [21]. It bridges adjacent chitosan polymers by interaction with protonated amines and encapsulates into a bead form [22]. In addition, the TPP-based chitosan beads were further tailored to impart desired functionality. For example, Zhu et al. [20] prepared g-Fe2O3/carbon nanotubes/chitosan composite and demonstrated its adsorption capacity for methyl orange and separation properties. A composite of chitosan/g-Fe2O3/fly-ashcenospheres was also evaluated for its capacity for the adsorption of bisphenol-A and 2,4,6-trichlorophenol [23]. An adsorbent capable of removing a wide range of contaminants has a great merit because many wastewaters contain multiple contaminants with distinctive behaviors. Chitosan is an attractive material to be developed into such a multi-functional adsorbent by imparting desired functionality during material preparation. For example, in our previous study, a composite composed of chitosan, clay, nano-magnetite composite was prepared and used for adsorption of cationic (Cu(II)) and anionic metals (As(V)) [24]. Sikder et al. [25] also synthesized the iron nanoparticles-entrapped chitosan composite for adsorption of Cr(VI) and Cu(II). The objective of this study was to evaluate the utility of the chitosan/clay/nano magnetite composite for removal of representative dye pollutants, methylene blue (MB) and methyl orange (MO). A composite material composted of chitosan, nanomagnetite (NMT), and clay (heulandite) using TPP as a crosslinking agent was prepared. Heulandite (HE) is a zeolite-type mineral possessing sheet-like structure, and therefore it is expected to provide good anchoring sites for NMT and chitosan. The composite was characterized by various instrumental analyses and a series of experiments were performed to demonstrate synthesis optimization, adsorption kinetics, isotherms, thermodynamics, and effect of pH. Materials and methods Materials Chitosan (75–85% degree of deacetylation and 190,000– 310,000 g mol1 viscosity molecular weight) was obtained from Sigma–Aldrich (USA). Heulandite was obtained from Donghae Chemical Co., South Korea, and pulverized and sieved through a 100-mesh screen before use. NMT (<50 nm), sodium tripolyphosphate (STPP), MB, MO, acetic acid (99%), sodium hydroxide, and hydrochloric acid were purchased from Sigma–Aldrich (USA), and used as received. Preparation of chitosan/heulandite/Fe3O4 composites Chitosan solution was prepared by dissolving chitosan (2 g) in 100 mL acetic acid (2%). STPP salt (13.3 g) was dissolved in 1 L of distilled deionized water (DDW) for preparation of STPP solution, followed by control of the solution pH to 4 with 1 N HCl. Desired amounts of NMT and HE were added to the chitosan solution and mixed under ultrasonic stirring for 30 min. The mixture solution was transferred to a 10 mL syringe, and then added dropwise to the STPP solution to form bead composites. The composites were cured in the solution for 12 h, washed several times with DDW to remove TPP, and dried at 50 8C for 24 h. The dried composite particles were pulverized and passed through a 100-mesh sieve (<150 mm size)

for use in experiments. The composite material prepared was referred to as chitosan/clay/Fe3O4 (CCM). Characterization of the composites Magnetic susceptibility of CCM was measured using a Bartington MS2 magnetic susceptibility meter (UK) with a sensor which has an internal diameter of 36 mm and accepts granular or liquid samples in vol. 10 cm3 sample pots and 100 drill cores. The experiments of pH titration were carried out for determining the point of zero charge (pHPZC) of CCM adsorbent. The titration was performed in three electrolyte solutions (DDW, 0.01 M NaCl, and 0.1 M NaCl) and 0.5 g of the solid was added to 50 mL of electrolyte solution and then titrated with 0.1 M NaOH. Field-emission scanning electron microscope coupled with energy dispersive spectroscopy (FE-SEM/EDS) and Fourier transform infrared spectroscopy (FTIR) were measured using a JSM-7000F FE-SEM analyzer (JEOL, Germany) and an ALPHA-P analyzer (Bruker, USA), respectively. Adsorption experiments Dye adsorption experiments were performed using 25 mL glass vials in duplicate (Fisher Scientific, USA). Stock solutions of MB and MO (1000 mg L1) were prepared by dissolving 1 g of MB and MO in 1 L DDW, respectively. To carry out adsorption kinetics experiments, adsorbent (0.1 g) was added to 20 mL solutions of 80.2 mg L1 MB and 49.7 mg L1 MO in the vials, and was mixed at 150 rpm at room temperature (23  2 8C). At given time intervals, the suspensions were collected and filtered with 0.45 mm filter (Whatman, USA). Adsorption isotherm experiments were conducted in the MB and MO concentrations of 17–549 mg L1 and 12–377 mg L1, respectively. The concentrations of MB and MO in aqueous solution were measured using a UV-vis spectrophotometer (Hach DR/4000, USA) at wavelength 665 and 465 nm, respectively. The solution pH was measured using a pH meter (Horiba, Ltd. Kyoto, Japan). The effect of temperature on MB and MO removal by CCM was conducted at varying temperature in the range of 25–45 8C under the same condition as adsorption isotherm experiments. MB and MO adsorption at varying pH was also investigated in the pH ranges between 3 and 9. The reactors contained 0.1 g CCM and 20 mL of 59.3 mg L1 MB or 63.2 mg L1 MO, and were reacted for 24 h at room temperature. The initial pH of the reactors were adjusted by using 0.1 N HCl or 0.1 N NaOH. Results and discussion Optimal ratio of chitosan:NMT:HE in the composite Various mass ratios of chitosan:NMT:HE were used in the composites synthesis and the resulting composites were evaluated in adsorption experiments to find the optimal mass ratio giving the most efficient dye removal. Two separate sets of CCM composites were prepared. First, the mass ratio of HE:chitosan was varied from 1:0.13 to 1:4 at constant NMT mass (HE:NMT = 1:0.33). Second, the mass ratio of HE:NMT was varied from 1:0.33 to 1:4 at constant chitosan mass (HE:chitosan = 1:1). Also as a reference, HE and NMT were separately evaluated for their adsorption capacities for MB and MO. Adsorption experiments were carried out for 24 h with 5 g L1 adsorbent dose and 137.7 mg L1 MB or 56.6 mg L1 MO at pH 5.5. The results showed HE had a great adsorption capacity for MB, giving almost complete removal of MB, but exhibited negligible adsorption capacity for MO (Fig. 1(a)). Heulandite-group minerals are reported to have strong adsorption capacities for cationic species due to their negatively charged interlayer cavities that

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50 Adsorbed amount_MB Adsorbed amount_MO RE (%)_MB RE (%)_MO

qe (mg g-1)

40

75

30 50 20

Removal Efficiency (%)

100

25 10

0

0 HE

1:0.13

1:0.4

1:1

1:2

1:4

(a) Adsorbed amount_MB Adsorbed amount_MO RE (%)_MB RE (%)_MO

qe (mg g-1)

40

100

75

30 50 20 25

Removal Efficiency (%)

50

10

0

0 1:0.33

1:1

1:2

1:4

NMT

(b) Fig. 1. Removal efficiencies (%) and adsorbed amount (mg g1) of MB and MO by composites with (a) varying ratio of chitosan to HE (HE:NMT = 1:0.33) at initial concentrations of 137.7 mg L1 MB and 56.6 mg L1 MO, and (b) varying ratio of NMT to HE (HE:chitosan = 1:1) at initial concentrations of 140 mg L1 MB and 58 mg L1 MO (adsorbent dose = 5 g L1, contact time = 24 h).

3

initially filled with exchangeable cations such as Ca2+, Na+, and K+ [4,26]. No such exchange reactions could occur for anionic MO, as evinced by almost negligible removal of MO. The adsorption efficiency of MO increased from 55.1 to 85.9% as the mass ratio of chitosan increased from 0.13 to 4 (Fig. 1(a)). Adsorption of MO on CCM is presumed to occur via electrostatic attraction between anionic functional group of MO and protonated amine groups of chitosan [27]. Therefore, it appears the major adsorption sites for MO were provided by chitosan. On the contrary, the removal efficiency of MB decreased by 54% with the same mass change of chitosan, which could be attributable to the decrease of HE mass in the composite. Fig. 1(b) shows the results of MB and MO removal at varying mass of NMT relative to HE at constant chitosan mass (HE:chitosan = 1:1). Adsorption by only NMT (5 g L1) showed very low adsorption of MB (0.5%) and MO (6.7%). When NMT mass ratio increased from 0.33 to 4, MB adsorption decreased by almost 50%, while MO adsorption decreased by less than 15%. Considering the little adsorption capacity of NMT and the high affinity of MB to HE, the decrease of MB adsorption is likely due to the decrease of relative HE mass in the composite. Evaluating the overall results of MB and MO adsorption at varying mass of composite constituents, the optimal mass ratio of HE:chitosan:NMT for adsorption of MO and MB was determined to be 1:1:0.33. Characterization of adsorbent Fig. 2 presents the FE-SEM images of CCM composite particles. The CCM had heterogeneous and uneven surface, and showed clusters of layered nano-sized particles, presumably NMT, on the surface. The EDS spectrum of CCM showed appearance of P (11 wt%) and relatively high content of Fe and C (31 and 23 wt%, respectively), all of which were not detectable in elemental analysis of HE (Table S1 and Fig. S1). This indicates presence of phosphate groups of TPP and iron bearing minerals on the surface of CCM. Fig. 3 shows the FTIR spectra of HE, fresh CCM, and reacted CCM after MB and MO adsorption. The FTIR spectra of zeolite-type minerals exhibit band peaks arising from Si–O–Si (1200 to 950 cm1) and Si–O–Al (420 to 500 cm1) vibrations, and the peaks from O–H stretching of water molecules (3440 cm1) and molecular vibration of water (1640 cm1) [28]. HE spectrum shows the strongest Si–O–Si stretching vibration at 1007 cm1, the

Fig. 2. Field-emission scanning electron microscope (FE-SEM) image of CCM.

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as compared to the value reported for mesoporous alumina (1.8  103 mol g1) [31]. The measured magnetic susceptibility was 2.1  104 m3 kg1, and this indicates CCM is readily separable from water by applying small magnetic force to the medium [32]. Adsorption kinetics and isotherm The adsorption kinetics for MB and MO by 1 g L1 of the CCM composite are shown in Fig. 4, along with the kinetics obtained for HE. The rate and extent of MB adsorption on HE was very high, giving 82% uptake in 20 min. CCM composite showed much slower adsorption kinetics for MB, presumably due to the reduced availability of HE in the composite. In other words, the presence of NMT clusters and chitosan is likely to mask the binding sites or inhibit diffusion of MB into pores in HE. Meanwhile, adsorption of MO was significantly higher on CCM than HE, giving 88% MO removal in the first 10 min. This rapid adsorption process may be attributed to the interaction between MO molecules and protonated amine groups on chitosan. Moreover, it appears the adsorption sites for MO were relatively accessible from outer interface, which probably resulted in the rapid adsorption [33]. The adsorption kinetics data of MB and MO on CCM were analyzed using the linearized form of pseudo-first-order and the pseudo-second-order kinetics models. A linear form of pseudofirst-order kinetics can be expressed as:

bending vibration of water at 1640 cm1, and O–H stretching bend at 3620 cm1. The shift of O–H band of water is attributed to the association of the hydroxyl with the cationic components within the structure. The FTIR spectrum of chitosan is characterized by several amine and amide bands in the 1800–1200 cm1 range of wavenumbers [29]. The band at 1632 cm1 of CCM spectrum (i.e. amide I band) is assigned to the absorption C5 5O group of chitosan. The amide II band due to N–H bending appears at 1535 cm1 and it is overlapped by amine bands. The CCM spectrum shows amide and amine bands at 1631 and 1536 cm1, respectively, indicating the presence of chitosan in the composite. These bands decreased to some extent after the adsorption of MB and MO, probably due to the suppression of those functional groups by adsorbed dye molecules. In addition, both CCM and the reacted CCM showed a band peak at 2881 cm1, attributable to the stretching of –CH2 in chitosan. The adsorption band of magnetite was not observed in this study because the range of scanning wavenumbers (600–3900 cm1) did not cover the region of the corresponding band peak of magnetite that centered around 570 cm1 [30]. The result of pH titration for PZC determination CCM is shown in Fig. S2(a). The addition of base solution in different concentrations of NaCl produced a series of curves which share a common intersection point at pH 5.7, at which the surface charge becomes neutral. To investigate the surface charge density of CCM, the moles of surface charge per CCM weight (Q) were calculated using the following equations [31]. þ

Q PZC ¼





c þ ð½H   ½OH ÞV M

c þ ð½Hþ   ½OH ÞV  Q PZC M

logðqe  qt Þ ¼ log qe 

k1 t 2:303

(3)

where qt (mg g1) is the amount adsorbed at time t (min), and k1 is the rate constant of pseudo first-order adsorption (min1). The values of log (qe  qt) were calculated from the kinetic data. The pseudo-second-order model based on the adsorption equilibrium capacity can be expressed as: t 1 t ¼ þ qt k2 q2e qe

(4)

where k2 (g mg1 min1) is the rate constant of pseudo-second order adsorption. The kinetics parameters obtained by fitting the data of MB and MO adsorption on CCM are given in Table 1. The results showed pseudo-second-order kinetics gave better agreement with the data 120 MB_HE MO_HE MB_CCM MO_CCM

90

qe (mg g-1)

Fig. 3. Fourier transform infrared (FTIR) spectra of CCM.

60

(1) 30

(2)

where c is moles of added base; V is the liquid volume (L); M is the mass of CCM; and QPZC is a reference point for addition of acid or base. The variation of surface charge with values of pHPZC in solution is given in Fig. S2(b). The magnitude of surface charge per solid mass indicates the sorption capacity of material for ions. The observed difference of the surface charge at pHPZC between 1 and 1 (8.1  103 mol g1) suggests a great capability of CCM for ions

0 0

200

400

600

Time (min) Fig. 4. Adsorption kinetics of MB and MO by HE and CCM (initial conc. of MB and MO: 80.2 and 49.7 mg L1; adsorbent dose: 1 g L1; initial pH = 5.5).

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Table 1 Kinetic parameters for the removal of MB and MO by CCM. qe(exp) (mg g1)

MB MO

32.0 22.3

Pseudo first-order kinetics model

Pseudo second-order kinetics model

Weber and Morris intra-particle diffusion model

k1 (L min1)

qe(cal) (mg g1)

R2

k2 (g mg1 min1)

qe(cal) (mg g1)

R2

kid 1 (mg g1 min0.5)

kid 2 (mg g1 min0.5)

R2id 1

R2id 2

0.0159 0.0111

23.3 13.6

0.9719 0.8718

0.0016 0.1170

33.2 21.7

0.9938 0.9998

4.2460 5.0340

1.3912 0.1427

0.9651 0.9513

0.9819 0.9173

qe(exp) = experimental adsorption capacity (mg g1); qe(cal) = calculated adsorption capacity (mg g1).

and closer qe(cal) values to the experimental qe(exp) values (32.0 mg g1 for MB, 22.3 mg g1 for MO). Accordance with pseudo-second-order kinetics suggests the adsorption process is governed by chemisorption [34], consistent with the suggested modes of MB and MO adsorption that involved ion exchange or specific interaction with surface functional groups. The rate of an adsorbate sorption on porous sorbents is often controlled by mass transport through solid–liquid interface or within the particle. Intra-particle diffusion model by Weber and Morris is frequently invoked to assess the diffusion mechanism of adsorption processes [35]: qt ¼ kid t 0:5

(5)

1 1 þ bC 0

(7)

where C0 (mg L1) is the initial MB and MO concentrations. The RL values indicate whether the adsorption is unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0) [38]. RL values at all the tested MB and MO concentrations fell in the range of 0–1, which indicates that the adsorption of MB and MO on CCM is favorable. Freundlich isotherm assumes that adsorbent surface sites have a spectrum of active sites with different binding energies.

0.5

) is the intra-particle diffusion rate where kid (mg g min constant. The sorption process is controlled solely by intra-particle diffusion as long as Weber and Morris plot of qt vs. t0.5 gives a single straight line passing through the origin, but otherwise it is controlled by both interfacial and intra-particle diffusion [36]. In the adsorption of MB and MO, both kinetics data were linearly fitted by two distinctive lines of qt vs. t0.5 with none of them passing through the origin. This suggests adsorption kinetics were controlled by both diffusion processes simultaneously. The values of intra-particle rate constant, kid 1 for initial phase and kid 2 for the later phase obtained from the slope of linear plots are given in Table 1. The higher values of kid 1 than kid 2 values suggest the adsorption of dyes in the initial phase mainly occurred on external binding sites on the adsorbent where diffusion processes are relatively rapid. When external active sites are occupied, dye molecules seek to enter into the pores of the adsorbent where diffusion of the molecules becomes much more difficult. As a result, the adsorption gradually slows down and shifts to second phase of adsorption that mainly involves adsorption within the interior of pores. The adsorption isotherm experiments gave qe values of 45.1 and 149.2 mg g1 for MB and MO adsorption at their highest initial concentrations of 549 and 377 mg L1, respectively (Fig. 5). The maximum adsorption capacities of chitosan based adsorbents for MB and MO adsorption reported in the literature are presented in Table 2. Compared to those values, the CCM composite showed higher adsorption capacity for MO removal, but it was less effective for MB adsorption. The equilibrium data were fitted to the linear form of Langmuir and Freundlich isotherm models. The Langmuir model assumes adsorption occurs at specific homogeneous sites with equal adsorption energy and is expressed as follows: 1 1 1 ¼ þ qe bC e qm qm

RL ¼

1=n

qe ¼ kf Ce

(8)

where kf is a constant taken as an indicator of adsorption capacity to (mg g1)(L mg1)1/n and n is a constant indicative of the adsorption intensity. The Langmuir and Freundlich constants for the adsorption of MB and MO on CCM are given in Table 3. The Freundlich constants (n) for MB and MO are within the range of 1 and 10, suggesting the adsorption of MB and MO on the adsorbents is favorable [39]. The correlation coefficients (R2) of Freundlich model for MB and MO are higher than those of Langmuir model, indicating the adsorption on the surface of CCM was a multilayer sorption resulting from heterogeneous distribution of cross-linked chitosan on the surface [40]. Thermodynamics of adsorption and pH effect The thermodynamic parameters including change in free energy change (DG8), enthalpy change (DH8) and entropy change (DS8) were determined to evaluate the thermodynamics of

160 MB MO

120 qe (mg g-1)

1

constant, separation factor or equilibrium parameter, RL, was calculated to characterize the isotherms [37]:

80

40 (6)

where Ce is the equilibrium concentration (mg L1) and the Langmuir constants qm (mg g1) represent the maximum monolayer adsorption capacity and b (L mg1) is the Langmuir constant related to energy of adsorption. The values of monolayer capacity (qm), Langmuir constant (b) were obtained from the intercept and slope of these plots, respectively. In addition, a dimensionless

0 0

200

400 Ce (mg L-1)

600

Fig. 5. Adsorption isotherm of MB and MO by CCM (conc. range of MB and MO: 17–549 and 12–377 mg L1; adsorbent dose: 1 g L1, contact time = 24 h).

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Table 2 Comparison of adsorption capacity of CCM with different adsorbent materials. Reference

Adsorption capacity (mg g1)

Adsorbent

MB Magnetic chitosan/graphene oxide composite N-Benzyl-O-carboxymethylchitosan magnetic nanoparticles Chitosan-g-poly (acrylic acid)/vermiculite hydrogel composites Pyromellitic dianhydride (PMDA) grafted chitosan microspheres Magnetic chitosan enwrapping nanosized g-Fe2O3 and multi-walled carbon nanotubes Magnetic maghemite/chitosan nanocomposite films Chitosan/alumina composite CCM

MO

180.8 278.2 1682 935 – –

– – – – 66.1 28.9 32.7

45.1

149.3

Fan et al. [14] Debrassi et al. [42] Liu et al. [43] Xing et al. [44] Zhu et al. [20] Jiang et al. [45] Zhang et al. [46] This study

Table 3 Langmuir and Freundlich constants for the adsorption of MB and MO on CCM.

MB MO

qm (mg g1)

b (L mg1)

RL

R2

kf (mg g1)(L mg1)1/n

n

R2

45.1 149.3

0.1897 0.0072

0.02–0.23 0.09–0.79

0.9066 0.9366

0.9202 19.6698

6.0496 1.0022

0.9904 0.9654

Table 4 Thermodynamic parameters for the adsorption of MB and MO on CCM. Temperature (8C)

DG8 (kJ mol1)

DS8 (J mol K1)

DH8 (kJ mol1)

6

K

MB

51.0 48.0 45.2

25 35 45

1.69  10 1.50  106 1.25  106

35.5 36.4 37.1

89.0 89.0 88.3

9.0

MO

166.7 158.7 156.2

25 35 45

3.56  105 3.26  105 3.13  105

31.7 32.5 33.5

84.2 84.2 84.5

6.6

adsorption of MB and MO on CCM: 

DG ¼ RT ln K ln

  K 2 DH 1 1 ¼  T1 T2 K1 R

D G ¼ D H   T D S 

(9)

(10)

effect on the adsorption of cationic dye. On the other hand, the decrease in adsorption of MB at low pH values may be attributed to the development of positive surface charge which created unfavorable electrostatic condition for cationic dye. For anionic MO, more adsorption occurred at lower pH conditions where amine groups of chitosan become protonated to interact with the anionic functional groups of MO [41]. Considering the strong dependency in adsorption

(11)

100

where R is the universal gas constant (8.314 J (mol K)1), T is the temperature in Kelvin and K is the equilibrium constant calculated by the following equation. (12)

The values of these parameters are given in Table 4. The calculated DH8 (MB: 9.0; MO: 6.6 kJ mol1) were negative values, indicating adsorption processes were exothermic. In addition, positive values of DS8 (MB: 89.0, 89.0, and 88.3; MO: 84.2, 84.2, and 84.5 J (mol K)1) indicate the increased randomness at the solid-solution interface during the fixation of adsorbates on the active sites of the adsorbent. The negative values of DG8 indicate the reactions were thermodynamically favorable and spontaneous processes. The decrease of adsorption capacities for MB and MO with increasing temperature may be attributed to the increasing energy state of dye molecules, which causes the molecules to come out from the solid to the liquid [38]. The effect of initial solution pH on the adsorption of MB and MO is given in Fig. 6. The adsorption of MB gradually increased with increasing pH, giving its highest removal at pH 9. The increase of pH led to the increase of negative charge density on the adsorbent due to association of OH ions with the surface and had a favorable

75 qe (mg g-1)

K ¼ b  55:5

100

Adsorbed amount_MB Adsorbed amount_MO RE(%)_MB RE(%)_MO

75

50

50

25

25

0

Removal Efficiency (%)

qm (mg g1)

0 3

4

5

6 pH

7

8

9

Fig. 6. Effect of pH on adsorbed amount (mg g1) and removal efficiencies (%) of MB and MO by CCM (initial concentration of MB and MO: 59.3 and 63.2 mg L1; adsorbent dose: 1 g L1, contact time = 24 h).

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of both ions on CCM composite, the primary adsorption mechanism would be electrostatic attraction, which is consistent with evidence for non-specific adsorption in FT-IR data. Conclusions A magnetic composite material was prepared by entrapping crosslinked chitosan and nano-magnetite on the clay surface for adsorption of cationic and anionic dyes. The optimized mass ratio of chitosan, HE, and NMT was determined to be 1:1:0.33. The composite showed good adsorption capability, giving maximum uptakes of 45.1 and 149.2 mg g1 for MB and MO, respectively. Pseudo-second order kinetics was a better model to describe the adsorption behavior for both dyes. The adsorption data for both MB and MO showed good agreement with the Freundlich isotherm model. Thermodynamic study indicated the adsorption of both dyes is exothermic and thermodynamically favorable process. The adsorption of MB increased proportionally with the increase in solution pH (3–9), while MO adsorption showed an opposite pH dependence. In overall, the composite material developed in this study could be effectively utilized as a multi-functional adsorbent for both cationic and anionic contaminants with a practical merit of magnetic property that enables facile separation after use in treatment units. Acknowledgments This work was supported by Korea Research Foundation (Ministry of Education, NRF-2014R1A1A2054607) and Mid-career Researchers Program (the National Research Foundation of Korea, 2013069183). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jiec.2015.01.023. References [1] [2] [3] [4] [5] [6] [7]

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