Removal of fluoride from aqueous solution using Zr(IV) immobilized cross-linked chitosan

International Journal of Biological Macromolecules 77 (2015) 15–23

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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac

Removal of fluoride from aqueous solution using Zr(IV) immobilized cross-linked chitosan Qian Liu a , Lujie Zhang a , Bingchao Yang b , Ruihua Huang a,∗ a b

College of Science, Northwest A&F University, Yangling, Shaanxi 712100, China Xi’an Institute of Geology and Mineral Resource, Xi’an, Shaanxi 710054, China

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 11 December 2014 Received in revised form 28 February 2015 Accepted 2 March 2015 Available online 16 March 2015 Keywords: Zr-CCS Kinetics Adsorption

In the present paper, zirconium immobilized cross-linked chitosan (Zr-CCS) was reported for the adsorption of fluoride. Zr-CCS was synthesized by methods of membrane-forming and subsequent cross-linking reaction. Zr-CCS was characterized by FTIR, XRD, and SEM technologies. Batch adsorption experiments were performed to evaluate the adsorption capacity of Zr-CCS toward fluoride. The adsorption of fluoride onto the Zr-CCS favored at low pH values, and reduced in the presence of other co-anions. The adsorption equilibrium data had a good agreement with the Langmuir isotherm model, and the maximum adsorption capacity was calculated as 48.26 mg/g for fluoride at 303 K and natural pH (6.0). Thermodynamic parameters indicate that the nature of fluoride adsorption was spontaneous and endothermic. The adsorption mechanism of fluoride onto the Zr-CCS was controlled by chemical ion-exchange and electrostatic attraction between Zr-CCS and fluoride. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Fluoride is a critical component for dental and bone health of mammals and found to be beneficial for the mineralization of hard tissues in the human body [1], but excessive intake of fluoride through food and drink may cause diseases such as skeletal fluorosis, mottling of teeth, deformation of bones in children and adults or neurological damage [2]. Fluoride contamination is commonly found in a wide range of industrial wastewaters produced from aluminum and steel production, metal finishing and electroplating, glass and semiconductor manufacturing, and fertilizer operation [3]. The effluents of these industries have higher fluoride concentrations than natural waters, ranging from ten to thousands of mg/L [4]. It is estimated that more than 200 million people worldwide rely on drinking water with fluoride concentrations that exceed the WHO guideline of 1.5 mg/L [5]. Therefore, decreasing the concentration of fluoride in drink water or its removal in industrial effluent is a topic of extensive research. Various techniques of defluoridation including reverse osmosis [6], nanofiltration [7], electrodialysis and electrolysis [8,9], membrane processes [10], ion-exchange [11], and adsorption [12], have been applied to reduce the excess fluoride. Among these methods, adsorption is widely used and offers satisfactory results and seems to be a more attractive method for

∗ Corresponding author. Tel.: +86 029 87092226. E-mail address: [email protected] (R. Huang). http://dx.doi.org/10.1016/j.ijbiomac.2015.03.008 0141-8130/© 2015 Elsevier B.V. All rights reserved.

the removal of fluoride due to low cost, simplicity of design and operation. Chitosan (CS) contains many functional groups, hydroxyl ( OH) and amino ( NH2 ), which are responsible for the reactivity of this polymer as an excellent natural adsorbent and give chitosan its powerful adsorptive capacity [13,14]. Besides, chitosan has been regarded as a useful material for various purposes such as the treatment of wastewater, ion-exchanger and functional matrixes due to the highly reactive amino and hydroxyl groups [15]. In recent years, chitosan based metal particles composites such as using metals [16], metal oxides [17], and bimetals [18] have been studied increasingly as an alternative adsorbent in water treatment. Viswanathan and Meenakshi [19] prepared Zr(IV) loaded carboxylated chitosan beads (Zr-CCB) for selective fluoride sorption, and the monolayer adsorption capacity of 13.69 mg/g was obtained. Liu et al. [20] prepared a novel, bio-based Zr(IV) impregnated dithiocarbamate modified chitosan beads (Zr-DMCB) for the adsorption of fluoride from aqueous solutions, and the maximum adsorption capacity calculated from Langmuir isotherm equation was 7.78 mg/g. In both literatures, cross-linked chitosan beads were treated with ZrOCl2 ·8H2 O solution for the improvement of its adsorption capacity toward fluoride. However, the adsorption capacity was not very high due to the limited loading amount of Zr(IV) onto chitosan beads. In this study, Zr(IV) immobilized cross-linked chitosan was prepared by membraneforming and subsequent cross-linked, and more zirconium (IV) would be introduced into chitosan matrixes. This bio-adsorbent

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was characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform-infrared spectrometer (FTIR). The adsorption capacity of fluoride from aqueous solution in bath adsorption experiments was evaluated. Besides, the kinetics and equilibrium isotherms models were employed to describe the adsorption process.

2. Materials and methods 2.1. Materials Chitosan (CS) was supplied by Sinopharm Group Chemical Reagent Limited Company (China) with a degree of deacetylation of 90% and average molecular weight of 54,000 g/mol. Zirconium oxychloride octahydrate (analytical grade) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Glutaraldehyde was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (China). All other reagents used in this study, including NaF, NaOH, HCl, and HAc, were of analytical grade. The concentration of fluoride in the solutions was determined using Leici acidity meter with fluoride ion selective electrode PF-1 (Leici, Shanghai). The measurement of pH was done with the same instrument with pH electrode.

2.2. Preparation of Zr(IV) immobilized cross-linked chitosan (Zr-CCS)

Fig. 1. FTIR spectra of (a) CS, (b) Zr-CCS and (c) fluoride-adsorbed Zr-CCS.

at time t (qt , mg/g) and at equilibrium (qe , mg/g) were calculated by using the following equations: R = 100 ×

Zr-CCS was prepared as follows: 2.0 g chitosan powders were added into 100 mL 2% (v/v) HAc solution and mixed until chitosan dissolved completely. 10 mL 0.31 mol/L ZrOCl2 ·8H2 O solution was added into chitosan solution drop by drop and stirred for 5 h. After the above solution was de-bubbled, the solution was uniformly coated in petri-dishes, and allowed to stay at 60 ◦ C in an oven to form membranes. Subsequently, these membranes were cross-linked at 60 ◦ C for 5 h with 2.5 wt% glutaraldehyde which was dissolved in anhydrous ethanol. Finally, the cross-linked membranes were washed with distilled water to remove any free glutaraldehyde, dried at 60 ◦ C, and ground to obtain particles of 100-mesh size, which were used for adsorption studies. Zr(IV) immobilized cross-linked chitosan was referred to as Zr-CCS.

C0 − Ct C0

(1)

qe = (C0 − Ce ) ×

V M

(2)

qt = (C0 − Ct ) ×

V M

(3)

where C0 , Ct and Ce are the concentrations of fluoride at the initial state, at time t, and at equilibrium (mg/L), respectively; V is the volume of fluoride aqueous solution (L) and M is the mass of ZrCCS (g). All experiments were conducted at 303 K unless otherwise stated. 3. Results and discussion 3.1. Characterization of Zr-CCS

2.3. Characterization of Zr-CCS FTIR spectra of the samples were obtained using FTIR spectrometer (Shimadzu 4100) to confirm the presence of functional groups. Surface morphology of the Zr-CCS before and after adsorption of fluoride was determined by field emission scanning electronic microscope (FE-SEM) (Hitachi S4800). X-ray diffraction (XRD) profiles of the powdered samples were performed using a Shimadzu XD3A diffractometer equipped with a monochromatic Cu Ka source operating at 40 kV and 30 mA. The diffraction patterns were recorded from 3◦ to 50◦ with a scan rate of 0.02◦ /s.

2.4. Adsorption studies The adsorption of fluoride onto the Zr-CCS was carried out at batch scale by adding a certain amount of adsorbent into 50 mL of fluoride solution. The flasks containing the mixture of sorbate and adsorbent were placed onto a thermostated shaker, and shaken at 200 rpm. When the pre-determined time attained, the adsorbent was separated by filtration. The concentration of fluoride in the filtrate was measured by method of fluoride selective electrode. The removal efficiency (R, %) and the amounts of fluoride adsorbed

FTIR is a useful tool to identify functional groups in a molecule, as each specific chemical bond often has a unique peak and can obtain structural and bond information. The FTIR spectra of CS, Zr-CCS and fluoride-adsorbed Zr-CCS were shown in Fig. 1. The major bands for chitosan (Fig. 1a) could be assigned as follows: 1658 cm−1 ( NH2 bending vibration), 1380 cm−1 ( CH symmetric bending vibration), 1080 cm−1 (C O stretching vibration) and 1260 cm−1 ( OH stretching vibration). In Fig. 1b, the bending vibration of NH2 at 1648 cm−1 with a shift of 10 cm−1 as compared to CS (1658 cm−1 ). This shift suggested that Zr(IV) has been interacted with NH2 group present in CS. The peak corresponding to OH stretching vibration shifted from 1260 to 1252 cm−1 in Zr-CCS. This may be due to the interaction of OH groups of CS with the Zr(IV). Furthermore, new peaks at 653 cm−1 corresponding to asymmetric Zr N vibration [21] and 546 cm−1 attributed to the stretching vibration of Zr(IV) ions [20] were observed. These results indicated Zr(IV) ions have been bended onto amino and hydroxyl groups of CS. The FTIR spectrum of F− -adsorbed Zr-CCS (Fig. 1c) showed some differences from the one of Zr-CCS. A new peak at 472 cm−1 attributed to Zr F vibration appeared [22], while the absorption peak around 546 cm−1 attributed to the stretching vibration of Zr(IV) ions disappeared, which indicated that Zr(IV) ions participated in

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Fig. 2. SEM images of (a) Zr-CCS and (b) fluoride-adsorbed Zr-CCS.

the adsorption process [23]. The peak at 3449 cm−1 in fluorideadsorbed broadened slightly. Therefore, a conclusion was obtained that fluoride ions were adsorbed by the Zr-CCS successively. The SEM images of the Zr-CCS before and after adsorption of fluoride were shown in Fig. 2. The SEM image of the Zr-CCS showed the surface was compact. The surface of the Zr-CCS after fluoride adsorption (Fig. 2b) has been changed and found to be rough and loose. And some tiny interspaces were present. The XRD spectra of CS, Zr-CCS and fluoride-adsorbed Zr-CCS were shown in Fig. 3. CS (Fig. 3a) showed two well defined peaks at 2 = 10.2◦ , 19.6◦ . These peaks were corresponded to a crystalline structure of CS. XRD spectrum of Zr-CCS was shown in Fig. 3b. Obviously, the position, intensity and nature of the peak varied in contrast to CS. The peak at 2 = 19.6◦ in CS was shifted to 26.1◦ with extraordinary broadness in the peak after CS was immobilized with Zr(IV). This observation may be explained by the strong absorption of X-ray by Zr(IV) in contrast to oxygen, carbon and other elements in CS. Thus, the results clearly confirmed the incorporation of Zr(IV) into the CS. The peak at 2 = 26.1◦ in Zr-CCS was shifted to 24.6◦ after fluoride adsorption as shown in Fig. 3c. These results were consistent with the ones obtained by Zhang et al. [21] when investigating the adsorption of Cr(VI) onto zirconium cross-linked chitosan composite. 3.2. Effect of adsorbent dosage on adsorption The effect of adsorbent dosage on the adsorption of fluoride was studied by varying the dosage from 0.03 to 0.25 g with fixed contact time of 40 min by keeping initial fluoride concentration at 100 mg/L. From Fig. 4, the removal efficiency increased with

Fig. 3. XRD patterns of (a) CS, (b) Zr-CCS and (c) fluoride-adsorbed Zr-CCS.

an increase in adsorbent dosage up to 0.15 g. It was found that the increase in adsorbent dosage beyond 0.15 g did not affect the removal efficiency significantly. 90% removal of fluoride was found in the dosage range of 0.15–0.25 g. This may be presumed that the availability of active sites on the Zr-CCS was increasing with increasing dosages. However, the adsorbent-adsorbent interactions were more dominating than adsorbate–adsorbent interactions due to increasing adsorbent dosage, and the availability of active sites on the Zr-CCS was decreasing at higher adsorbent dosages [24], resulting in a slight increase in removal efficiency at higher dosages. Since a relatively high removal efficiency was observed with 0.15 g of Zr-CCS, the other experiments were performed at 0.15 g. 3.3. Effect of pH on adsorption The pH was monitored in the range of 3–11 using 0.1 M HCl and NaOH solutions at 0.15 g of adsorbent dosage with fixed contact time of 40 min by keeping 100 mg/L of initial fluoride concentration. Fig. 5 showed the removal efficiency of Zr-CCS toward fluoride as a function of initial pH value. The maximum removal efficiency was recorded at pH 1.0, whereas it slightly decreased, thereafter, a sharp decrease was observed at pH 9.0. The effect of initial pH value on fluoride adsorption can be explained by interaction between the adsorbent surface and the adsorbate. The values of pH affect the surface properties of the adsorbent. The potential binding sites in chitosan are amine and hydroxyl groups. At lower pH values, the

Fig. 4. Effect of adsorbent dosage on adsorption.

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Fig. 5. Effect of pH value on adsorption.

surface of the adsorbent might be affected due to the protonation of amine groups in chitosan which would increase the adsorbate interaction with functional groups in the Zr-CCS by greater attractive forces, and thereby, improve the adsorption of fluoride onto the Zr-CCS. In the pH range of 3–9, high removal was still observed. This may result from the strong affinity of Zr(IV) ions in the Zr-CCS for fluoride anions. Above pH 9, the reduction of removal efficiency can originate from the competition of hydroxyl ions in the solutions with fluoride anions for adsorption sites. 3.4. Effect of the presence of co-anions on adsorption In the practical application, the fluoride contaminated water or aqueous solution always contains several other anions, viz., Cl− , SO4 2− , CO3 2− and PO4 3− , which can compete with fluoride in the adsorption process. In order to evaluate the effect of interfering ions, the adsorption studies were carried out at 303 K in the presence of co-anions with the concentrations of these ions of 0.05 M and 0.1 M by keeping 100 mg/L of initial fluoride concentration. The effect of co-anions on the removal efficiency of fluoride was shown in Fig. 6. The presence of these ions presented negative effect on the removal efficiency of fluoride. The order of the removal efficiency followed the decreasing order: Cl− > SO4 2− > CO3 2− > PO4 3− . A decrease in the adsorption of fluoride in the presence of Cl− and SO4 2− ions may due to the competition among the anions for the sites on the adsorbent surfaces. However, the presence of CO3 2− and PO4 3− ions resulted in increased pH values of fluoride solution. From the above experiments on the effect of pH (Section 3.3), it was observed that the adsorption of fluoride decreased in alkaline pH as also explained. Therefore, the presence of co-anions decreased in the removal efficiency of fluoride mainly due to the increasing pH value of aqueous solution and the competing effect of these co-anions.

Fig. 6. Effect of the presence of co-anions on adsorption.

removal percentage decreased by increasing initial concentration of fluoride. This effect could be explained by the fact that at low fluoride concentrations, the adsorbent/adsorbate ratios were big, and thus the available sites were enough for the adsorption of fluoride, leading to high removal efficiencies. However, as the fluoride concentration increased, the adsorbent/adsorbate ratios decreased and fewer sites were used for adsorption, and so the removal efficiency reduced.

3.5. Effect of initial fluoride concentration and contact time on adsorption The effect of initial fluoride concentration and contact time on adsorption was studied using initial fluoride solutions with concentrations of 50, 100 and 200 mg/L at natural pH value and 303 K. The adsorption of fluoride has been investigated as a function of time in the range of 5–120 min and illustrated in Fig. 7. It is found that the adsorbent showed strong affinity for fluoride anions, and the adsorption reached equilibrium in a short time. Besides, the

Fig. 7. Effect of initial fluoride concentration and contact time on adsorption.

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3.6. Adsorption kinetics To investigate the adsorption mechanism of Zr-CCS toward fluoride, the pseudo-first-order and pseudo-second-order kinetic models have been used at different concentrations. The pseudofirst-order kinetic model, also known as the Lagergren kinetic equation is widely employed to understand the kinetic behavior of the adsorption reactions. A simple pseudo-first-order kinetic model is given in Eq. (4) [25]: 1 k1 1 = + qt qe t qe

(4)

where qe and qt are the amounts of fluoride adsorbed (mg/g) at equilibrium and at time t (min), k1 is the pseudo-first-order rate constant (min−1 ). The k1 and qe were calculated from the slope and intercept of plots of 1/qt versus 1/t, respectively (Fig. 8a). The pseudo-second-order model is based on the assumption that the rate limiting step may be chemisorption which involves valence forces by sharing or electron exchange between the adsorbent and the adsorbate. The pseudo-second-order kinetic model is given in Eq. (5) [25]: t 1 1 = + t qt qe k2 q2e

(5)

where k2 is the equilibrium rate constant of pseudo-second-order model. The values of qe (1/slope) and adsorption rate constant k2 (slope2 /intercept) can be obtained by plotting t/qt versus t (Fig. 8b). These parameters and correlation coefficients (R2 ) for the pseudofirst-order and pseudo-second-order kinetic models were listed in Table 1. Though the calculated qe values from the pseudo-firstorder and pseudo-second-order kinetic models agreed very well with the experimental data, the correlation coefficients (R2 ) for the pseudo-second-order model obtained were greater than 0.999 at all concentrations, indicating that the pseudo-second-order model better described the adsorption process of fluoride onto the ZrCCS. The similar phenomena were also observed in the adsorption of fluoride by La(III)–Al(III) loaded scoria [26] and carboxymethyl cellulose loaded with zirconium [22].

3.7. Adsorption isotherm Adsorption isotherm study was performed at three different temperatures (303 K, 313 K, and 323 K) and three different pH values (3.5, 6.0, and 9.5) by varying fluoride concentrations from 20 mg/L to 200 mg/L. The removal of fluoride in equilibrium adsorption process can be described by several isotherms based on a set of assumptions related to the homogeneity of adsorbents, the type of coverage, and the possibility of interaction between the adsorbent species. The adsorption isotherms were investigated using three equilibrium models namely the Langmuir, Freundlich and D–R isotherm models. The Langmuir model assumes that the homogenous adsorption occurs on a monolayer surface coverage, and no interaction exists between the adsorbed species. The linearized Langmuir equation is expressed in Eq. (6): Ce 1 Ce + = Q qe Qb

(6)

where qe (mg/g) is the amount of fluoride adsorbed (mg/g) at equilibrium, Ce (mg/L) is the final concentration at equilibrium, Q (mg/g) is the maximum adsorption at monolayer coverage and b (L/mg) is the constant related to the extent of adsorption. From the linear plots of Ce /qe against Ce (Fig. 9a and a ), Q and b values can be calculated from the slope and intercept, respectively.

19

Freundlich isotherm assumes a heterogeneous adsorption surface and active sites with different energy. The linearized Freundlich model is expressed in Eq. (7): log qe = log Kf +

1 log Ce n

(7)

where Kf [(mg/g)(L/mg)1/n ] and n are Freundlich constants related to adsorption capacity and heterogeneity factor, respectively. Kf and n values can be calculated from the intercept and slope of the linear plot between log Ce and log qe (Fig. 9b and b ). The isotherm constants and correlation coefficients (R2 ) for linear Langmuir and Freundlich equations were given in Table 2. Based on the R2 values, the Langmuir isotherm model better fitted showed R2 values than the Freundlich model. This implies the monolayer distribution of fluoride onto homogeneous active sites on the surface of the Zr-CCS. Besides, the maximum adsorption capacities of Zr-CCS for fluoride were estimated to be 48.26–48.12 mg/g in the range of 303–323 K. It suggested that in this temperature range, increasing temperature only had a slight effect on the adsorption of fluoride onto Zr-CCS. However, the increased pH values reduced the adsorption capacity of adsorbent toward fluoride. This result was consistent with the one mentioned in Section 3.3. In this study, the adsorption capacity of Zr-CCS toward fluoride was compared to Zirconium(IV)-based chitosan composites. From Table 3, it was found that the studied Zr-CCS allowed a high adsorption capacity as compared with some adsorbents reported in these literatures [19,20,27–30]. This may be due to more Zr(IV) ions effectively immobilized in CS before the accomplishment of the cross-king reaction between chitosan and glutaraldehyde during the preparation of this adsorbent. To determine the type of adsorption for the removal of fluoride from aqueous solution by whether it is chemical or physical, the adsorption processes were studied by analyzing the equilibrium data using D–R isotherm model. The linear form of D–R isotherm model is presented by the following equation: ln qe = ln qm − ˇε2

(8)

where qe (mg/g) is the amount of fluoride adsorbed (mg/g) at equilibrium, Ce (mg/L) is the fluoride concentration at equilibrium, qm is the maximum adsorption capacity (mg/g), ˇ is the activity coefficient related to mean adsorption energy (mol2 /J2 ) and ε is the Polanyi potential ε = RT ln(1 + 1/Ce ). From the linear plots of ln qe against ε2 (Fig. 9c and c ), qm and ˇ values can be calculated from the slope and intercept, respectively. The isotherm constants and correlation coefficients (R2 ) for D–R isotherm model were presented in Table 2. The D–R isotherm model well fitted the equilibrium data due to high R2 values. The adsorption mean free energy E (kJ/mol) gives information about nature of adsorption, i.e. physical or chemical. The mean sorption energy E (kJ/mol) is expressed as follow: E=

1



(9)

2ˇ

The adsorption process is chemical in nature when E values lie between 8 and 16 kJ/mol. Adsorption is physical in nature when it is below 8 kJ/mol. In this study, this parameter was calculated as 12 kJ/mol or so for the Zr-CCS. The adsorption of fluoride onto the Zr-CCS occurred due to chemical ion-exchange mechanism, since the adsorption energies lied within 8–16 kJ/mol. Adsorption of fluoride onto biomimetically synthesized nano zirconium chitosan composite and an anion exchange AFN membrane were also believed to occur due to chemical ion-exchange mechanism [27,31].

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Fig. 8. Pseudo-first-order model (a) and pseudo-second-order model (b) of fluoride onto Zr-CCS.

Table 1 Comparison between pseudo-first-order and pseudo-second-order parameters on fluoride adsorption. Parameters

Pseudo-first-order kinetic model

50 mg/L 100 mg/L 200 mg/L

qe (exp)

Pseudo-second-order kinetic model

2

k1

qe (cal)

R

0.1624 0.0681 0.0121

15.92 30.67 46.36

0.9678 0.5614 0.0658

15.89 30.58 47.03

k2

qe (cal)

R2

0.4031 1.3697 0.0847

15.92 30.97 46.66

1.000 1.000 0.999

Table 2 Adsorption isotherm parameters of fluoride onto Zr-CCB. Isotherms

Parameter

6.0

9.5

Langmuir

b Q R2

0.305 48.26 0.9986

0.328 48.17 0.9988

0.334 48.12 0.9988

0.221 53.42 0.9950

0.305 48.26 0.9986

0.237 46.88 0.9975

Freundlich

Kf 1/n R2

12.57 0.3537 0.9884

12.83 0.3512 0.9868

12.97 0.3482 0.9870

12.45 0.3746 0.9950

12.57 0.3537 0.9884

11.85 0.3413 0.9936

D–R

qm ˇ E R2

98.74 3.262E−9 12.38 0.9977

98.74 3.262E−9 12.38 0.9977

85.37 3.099E−9 12.70 0.9941

Temperature 303 K

pH 313 K

323 K

107.7 3.277E−9 12.35 0.9938

114.9 3.259E−9 12.39 0.9869

3.8. Thermodynamic treatment of the adsorption process

ln KL =

Thermodynamic parameters associated with the adsorption, viz., standard free energy change (G ), standard enthalpy change (H ) and standard entropy change (S ) were calculated as follows: G = −RT ln KL

3.5

(10)

104.0 3.331E−9 12.25 0.9929

H  S  − R RT

(11)

where KL (L/mol) is the Langmuir constant, R is the universal gas constant, 8.314 J/mol/K, T is the absolute temperature in K. The values of H and S can be obtained from the slope and intercept of a van’t Hoff plot of ln KL versus 1/T. The calculated values of thermodynamic parameters were shown in Table 4. The negative values of G indicate the

Table 3 Comparison of adsorption capacity of some zirconium based adsorbents for fluoride removal. Adsorbent

pH/Temperature/K

Adsorption capacity/mg/g

References

Zr(IV) entrapped chitosan polymeric matrix Zr(IV) impregnated dithiocarbamate modified chitosan beads Biomimetically synthesized nano zirconium chitosan composite Zirconium(IV) doped chitosan bio-composite Chitosan supported zirconium(IV) tungsto-phosphate composite Zr(IV) encapsulated silica gel/chitosan bio-composite Zr(IV) loaded cross-linked chitosan composite

7/303 7/303 7/303 6–6.9/303 3/323 7/303 6.0/303

13.69 7.78 96 0.98 9.901 3.464 48.26

[19] [20] [27] [28] [29] [30] This study

Q. Liu et al. / International Journal of Biological Macromolecules 77 (2015) 15–23

1.4 1.2

1.6

(a)

303 K 313 K 323 K

pH=3.5 pH=6.0 pH=9.5

1.4

21

(a')

1.2

1.0

1.0

ce/qe

ce/qe

0.8 0.6

0.6

0.4

0.4

0.2 0.0

0.8

0.2

0

10

20

30

40

50

60

0.0

70

0

10

20

30

40

60

70

1.8

1.8

303 K 313 K 323 K

1.6

(b')

pH=3.5 pH=6.0 pH=9.5

(b) 1.6

1.4

logqe

1.4

logqe

50

ce

ce

1.2

1.2

1.0

1.0

0.8

0.8

-1.0

-0.5

0.0

0.5

1.0

1.5

-1.0

2.0

-0.5

0.0

0.5

4.0

303 K 313 K 323 K

(c)

1.0

1.5

2.0

logce

logce

(c')

pH=3.5 pH=6.0 pH=9.5

4.0

3.5

3.5 3.0

lnqe

lnqe

3.0 2.5

2.5

2.0

1.5

2.0

2x10

8

3x10

8

4x10

8

5x10

8

6x10

8

7x10

8

8x10

8

9x10

8

1.5

2x10

8

3x10

8

4x10

8

2

5x10

8

6x10

8

7x10

8

8x10

8

9x10

8

2

e

e

Fig. 9. Adsorption isotherm models of fluoride onto Zr-CCS at different temperatures (a, b, and c) and different pH values (a , b , and c ).

spontaneous nature of fluoride adsorption onto the Zr-CCS. The positive value of H (3.72 kJ/mol) indicated that the adsorption process was endothermic. The positive value of S showed the increased randomness at the solid/solution interface during fluoride adsorption, and suggested good affinity of the Zr-CCS for fluoride.

3.9. Adsorption mechanism During the preparation process of Zr-CCS, ZrOCl2 ·8H2 O was added into chitosan solution prior to the cross-linking reaction between chitosan and glutaraldehyde. The reactive amino and hydroxyl groups of chitosan have formed a chelated complex with

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Fig. 10. A schematic reaction between Zr-CCS composite and fluoride.

Table 4 Thermodynamic parameters for the adsorption of fluoride onto Zr-CCS. T/K

G /kJ/mol

H //kJ/mol

S /kJ/(mol K)

303 313 323

−21.8 −22.7 −23.5

3.72

0.0843

Zr4+ ions, as shown the results of FTIR spectra (Section 3.1). The possible mechanism of fluoride removal by the Zr-CCS was shown in Fig. 10. Zr-CCS removed fluoride by means of chemical ionexchange. This was confirmed by the bonding between Zr F in the FTIR spectra of fluoride-adsorbed Zr-CCS (Fig. 1c). Simultaneously, Zr-CCS can remove fluoride due to the electrostatic attraction of the positively charged Zr(IV) surface in Zr-CCS and the negatively charged fluoride ions. Thus, Zr-CCS removed fluoride by chemical ion-exchange and electrostatic attraction with a relatively high adsorption capacity. 4. Conclusions The present study clearly shows that the Zr-CCS was an efficient adsorbent for the removal of fluoride from aqueous solution. This study investigated these factors affecting fluoride adsorption onto Zr-CCS, including adsorbent dosage, pH value of fluoride solution, initial fluoride concentrations, and contact time. The adsorption of fluoride onto the Zr-CCS favored at low pH values. The presence of other anions decreased the adsorption of fluoride from aqueous solution. Langmuir isotherm mode fitted the equilibrium data well, and the maximum adsorption capacity was calculated as 48.26 mg/g for fluoride at 303 K and natural pH (6.0). The values of thermodynamic parameters confirmed the spontaneous and endothermic nature of the fluoride adsorption. The pseudo-second

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