Synthesis of a novel Ce(III)-incorporated cross-linked chitosan and its effective removal of fluoride from aqueous solution

JOURNAL OF RARE EARTHS, Vol. 34, No. 10, Oct. 2016, P. 1053

Synthesis of a novel Ce(III)-incorporated cross-linked chitosan and its effective removal of fluoride from aqueous solution LI Jinfang (李金芳), LIU Qian (刘 倩), HUANG Ruihua (黄瑞华)*, WANG Guodong (王国栋) (College of Science, Northwest A&F University, Yangling 712100, China) Received 8 March 2016; revised 23 May 2016

Abstract: A novel Ce(III)-incorporated cross-linked chitosan (Ce-CCS) was prepared and used for the removal of fluoride from aqueous solution. The structure and morphology of Ce-CCS were measured by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electronic microscopy (SEM) and energy dispersive X-ray analyzer (EDAX) techniques. The factors affecting the fluoride adsorption such as adsorbent dosage, initial fluoride concentration, pH, coexisting anions and contact time were investigated. Increasing adsorbent dosage enhanced the removal towards fluoride while increasing initial fluoride concentration reduced the removal towards fluoride. The optimal pH value for fluoride adsorption was 3 or so. The presence of coexisting anions weakened the adsorption of fluoride, and the decreasing order of the removal towards fluoride was PO43–>CO32–>SO42–>Cl–. The adsorption data were described by Freundlich isotherm model and the pseudo-second order kinetic model. The incorporation of Ce(III) enhanced the adsorption capacity of CCS for fluoride ions, the adsorption capacity at equilibrium (qe) of Ce-CCS increased by 5.0 mg/g or so as compared with the one of CCS at the same temperature tested. The exhausted Ce-CCS could regenerate with 0.1 mol/L HCl solution. Keywords: Ce(III)-incorporated cross-linked chitosan; fluoride; adsorption; kinetics; isotherms; rare earths

Fluoride in drinking water may be beneficial or detrimental depending on its concentration. Excessive fluoride intake can affect the metabolism of elements in human body and lead to dental and bone fluorosis[1], and the recommendation of the World Health Organization regarding safe amount of fluoride is 1.5 mg/L in drinking water[2]. Therefore, it was necessary to reduce the discharge of wastewaters containing excessive fluoride. At present, some methods such as precipitation[3], ion exchange[4], adsorption[5], electrochemical methods[6] and nanofiltration[7] were applied for the defluoridation. Among these methods, adsorption has attracted attention due to simple design, low cost and land required. Numerous studies have been focused on the application of natural materials, including chitosan[8], citrus limonum (lemon) leaves[9], lateritic soils[10], bone char[11], and Kanuma mud[12], in the removal of fluoride from aqueous solutions. Recently, considerable work has been conducted in developing new adsorbents loaded with metal ions, for instance, Zr(IV) impregnated zirconium cellulose[13], especially with rare earth metals such as La(III) loaded a granular ceramic adsorbent[14], and La(III)Al(III) loaded scoria[15], for the purpose of defluoridation. Chitosan is a biopolymer produced from the deacetylation of chitin, and it has been extensively studied for the removal of heavy metals due to the strong binding capacity of amine and hydroxyl groups in chitosan for

metal ions[16]. Only a few reports were available about its adsorption for anions[17,18]. To enhance its adsorption capacity for anions, i.e., fluoride, chitosan was modified by composition with metals as well as protonation with acids. In our previous work, we reported that the adsorption capacity of the protonated cross-linked chitosan particles was 8.10 mg/g, which was obtained from the Langmuir model[19]. Although the adsorption capacity of the protonated cross-linked chitosan increased as compared with the one of natural chitosan, it was expected to further enhance the adsorption capacity of chitosan. Meanwhile, some acid effluent was generated during the preparation of the protonated cross-linked chitosan. Cerium oxide is one of the most abundant and cheapest rare earth oxides, possesses the resistance to acid and base, and does not elute during the removal of harmful substances in water. The incorporation of Ce(III) in chitosan can enhance the binding for fluoride due to the possible electrostatic attraction and strong Lewis acid-base interaction between fluorides ions and Ce(III). At present, there are few reports about the preparation and application of Ce(III) incorporated cross-linked chitosan. In the present study, to enhance the adsorption capacity of chitosan for fluoride, a Ce(III) incorporated crosslinked chitosan (Ce-CCS) was prepared. The effects of Ce-CCS dosage, initial fluoride concentration, the presence of other anions and pH value of fluoride solution on

* Corresponding author: HUANG Ruihua (E-mail: [email protected]; Tel.: +86-29-87092226) DOI: 10.1016/S1002-0721(16)60134-5

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fluoride adsorption were investigated in detail. The adsorption behavior of Ce-CCS was also evaluated using adsorption kinetic and isotherm models. Furthermore, FTIR, XRD and SEM with EDAX techniques were used to elucidate the adsorption mechanism of fluoride on the Ce-CCS.

patterns of the samples were performed using a Shimadzu XD3A diffractometer equipped with a mono chromatic Cu Kα source operating at 40 kV and 30 mA. The diffraction patterns were recorded from 5º to 55º with a scan rate of 0.02 (º)/s. Surface morphology of the samples was determined by a field emission scanning electronic microscope (FE-SEM) (Hitachi S4800). Elemental spectra were obtained using an energy dispersive X-ray analyzer (EDAX) which allowed a qualitative detection and localization of elements present in the particles.

1 Materials and methods 1.1 Materials Chitosan was supplied by Sinopharm Group Chemical Reagent Limited Company (China) with a degree of deacetylation of 90% and average molecular weight of 105 g/mol. CeNO3·6H2O (analytical grade) was purchased from Shanghai AiBi Chemistry Preparation Co., Ltd. (China). Epichlorohydrin was purchased from Tianjin BoDi Chemical Co., Ltd. (China). Working solution of fluoride was prepared by dissolving NaF (supplied by Tianjin BoDi Chemical Co., Ltd., China) in deionized water. All other reagents used in this study, including NaOH, HCl, HAc, NaCl, Na2SO4, Na2CO3 and Na3PO4 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. 1.2 Preparation of Ce(III) incorporated cross-linked chitosan (Ce-CCS) 2.0 g chitosan powders were added into 100 mL 2 vol.% acetic acid solution and mixed until chitosan dissolved completely. 10 mL 0.23 mol/L CeNO3·6H2O solution was added into chitosan solution drop by drop and stirred for 5 h. After the above solution was de-bubbled, it was uniformly coated in petri-dishes, and allowed to stay at 60 ºC in an oven to form membranes. Subsequently, these membranes were soaked in 0.1 mol/L sodium hydroxide solution to separate from these culture vessels. The membranes were washed with distilled water to neutral pH and dried at 60 ºC. After immersing in an aqueous solution of 2.5% epichlorohydrin at 60 ºC for 23 h, these membranes were washed with distilled water to remove any free epichlorohydrin, dried at 60 ºC, and ground. Ce(III) incorporated cross-linked chitosan (CeCCS) particles with size of 100 mesh were obtained, and used for adsorption studies. The cross-linked chitosan (CCS) was prepared as described above without the addition of CeNO3·6H2O.

1.4 Adsorption studies To investigate the adsorption behavior of the Ce-CCS for fluoride ions, the batch experiments were conducted by adding 0.15 g of Ce-CCS dosage in conical flasks containing 50 mL fluoride solutions. The conical flasks were placed in a shaker and shaken at 200 r/min and 298 K for 40 min. The Ce-CCS was removed by filtration. The concentration of fluoride in the filtrate was measured by fluoride selective electrodes. Adjustments for pH value were done using 0.1 mol/L HCl or 0.1 mol/L NaOH solutions. The adsorption kinetic studies were conducted at different fluoride concentrations (100, 200, and 300 mg/L) and different dosages (0.12, 0.15 and 0.18 g). A certain amount of Ce-CCS was added in conical flasks containing 50 mL fluoride solutions. The flasks were shaken for time varying from 5 to 120 min. At the end of each adsorption period, a flask was taken out, and the adsorbent was removed by filtration. The concentrations of the residual fluoride in the filtrates were then determined. Adsorption isotherm studies were performed by varying initial fluoride concentrations from 20 to 300 mg/L at different temperatures (303, 313 and 323 K). 0.15 g of CeCCS was added in conical flasks containing 50 mL fluoride solutions. The flasks were shaken for 60 min. After equilibrium, these flask were taken out, and the adsorbent was removed by filtration, and the final concentrations of fluoride in the filtrates were analyzed similarly. All experiments were carried out twice and the adsorbed concentrations given were the means of duplicated experimental results. The experimental error was below 4%, the average data were reported. The removal (R, %) and the amounts of fluoride adsorbed at equilibrium (qe, mg/g) and at time t (qt, mg/g) were calculated by using the following equations: C − Ct (1) R = 100 × 0 C0

1.3 Characterization of Ce(III) incorporated crosslinked chitosan

qe = (C0 − Ce ) ×

FTIR spectra of the samples were obtained using a FTIR spectrometer (Shimadzu 4100) to confirm the presence of functional groups. X-ray diffraction (XRD)

qt = (C0 − Ct ) ×

V M V

M

(2) (3)

where C0, Ct and Ce are the concentrations of fluoride at

LI Jinfang et al., Synthesis of a novel Ce(III)-incorporated cross-linked chitosan and its effective removal of …

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 the adsorbent (g). 1.5 Desorption study The desorption experiments were conducted as follows: 0.15 g of Ce-CCS was added to fluoride solutions with different initial concentrations ranging from 20 to 200 mg/L. After performing the equilibrium adsorption, the fluoride-loaded Ce-CCS was obtained by filtration. In the desorption process, the spent adsorbent was immersed in 50 mL of 0.1 mol/L HCl, NaOH and NaCl solutions as eluents and shaken for 12 h at 200 r/min and 298 K. The desorbed samples were separated by filtration, washed with deionized water and dried in an oven. Following that, the above regenerated adsorbent was reused for next-cycle adsorption experiments.

2 Results and discussion 2.1 Characterization of Ce-CCS Fig. 1 illustrates the FTIR spectra of CCS, Ce-CCS and fluoride-adsorbed Ce-CCS. In the spectrum of CCS, the major bands for chitosan can be assigned as follows: 3422 cm−1 (–OH and –NH2 stretching vibrations), 2926 cm−1 (–CH stretching vibration in –CH and –CH2), 1652 cm−1 (–NH2 bending vibration), 1379 cm−1 (–CH symmetric bending vibrations in –CHOH–), 1066 and 1028 cm−1 (–CO stretching vibration in –COH) [17]. After incorporating Ce(III), the intensity of –NH2 bending vibration at 1652 cm–1 was increased obviously, which implied that the interaction occurred between Ce(III) and amine groups. In the spectrum of the fluoride-adsorbed Ce-CCS, the peak corresponding to –OH and –NH2 stretching vibrations was shifted from 3422 to 3446 cm–1. Besides, the peak of –NH2 bending vibration was shifted from 1652 to 1656 cm–1. These results indicated that the –OH and –NH2 groups were involved in the adsorption of fluoride onto the Ce-CCS.

Fig. 1 FTIR spectra of CCS (1), Ce-CCS (2) and fluoride-adsorbed Ce-CCS (3)

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The X-ray diffraction (XRD) patterns of CCS, CeCCS and fluoride-adsorbed Ce-CCS are shown in Fig. 2. Fig. 2(1) presents a peak at 19º attributed to a crystalline structure of chitosan. Three well defined peaks at 2θ=29.4º, 33º and 47º can be observed in Fig. 2(2), indicating the presence of crystalline CeO2[20], which may be formed by alteration of Ce3+ due to the heated treatment mentioned in Section 1.2. Besides, the peak corresponding to the crystalline structure of chitosan almost disappeared due to the incorporation of Ce(III). However, there were no marked changes for most of the peaks as shown in Fig. 2(3), indicating that there are no obvious changes in crystal structure after adsorption of fluoride.  SEM images of the particles before and after fluoride adsorption of Ce-CCS are shown in Fig. 3. The changes in the SEM morphology of the Ce-CCS before and after fluoride adsorption indicated the occurrence of fluoride adsorption onto the Ce-CCS. This was further supported by EDAX analysis. The EDAX spectra of Ce-CCS show that the elements of C, N, O and Ce were present on the surface of Ce-CCS (Fig. 3(a′)). In the EDAX spectra of fluoride-adsorbed Ce-CCS (Fig. 3(b′)), a new fluoride peak was observed, confirming the fluoride adsorption onto the Ce-CCS. 2.2 Effect of adsorbent dosage on adsorption To examine the effect of Ce-CCS dosage on fluoride adsorption, the experiments were conducted with a fixed time of 40 min, 20 mg/L as initial fluoride concentration at natural pH and 298 K. The adsorbent dosage was varied from 0.01 to 0.18 g and it was observed that the Ce-CCS dosage significantly influenced the extent of fluoride adsorption (Fig. 4). The removal was 11% with 0.01 g of Ce-CCS, while it was greatly increased to 94.4% for 0.15 g of Ce-CCS. However, a further increase in Ce-CCS dosage had slight effect on the removal, indicating the saturation of adsorption sites. Besides, increasing the Ce-CCS dosage reduced the adsorption capacity. This trend may be explained like this. When the Ce-CCS dosage was low, the active sites of adsorbent

Fig. 2 XRD patterns of CCS (1), Ce-CCS (2) and fluoride-adsorbed Ce-CCS (3)

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Fig. 3 SEM images and EDAX spectra of Ce-CCS before (a and a′) and after (b and b′) adsorption of fluoride

adsorption sites of Ce-CCS surfaces to the initial number of fluoride anions was relatively higher, thus higher removal was observed. With a further increase in fluoride concentration, an increasing number of fluoride anions will fiercely compete for the constant available adsorption sites, as a result, more fluoride anions were left unadsorbed in the solution due to the saturation of binding sites[22], resulting in the decreasing removal.    2.4 Effect of pH on adsorption

Fig. 4 Effect of adsorbent dosage on adsorption

can be efficiently occupied by fluoride anions, so the calculated adsorption capacity was high due to the small mass of adsorbent[21]. Taking into consideration of the removal and adsorption capacity, the optimum dosage of Ce-CCS was chosen as 0.15 g for further experiments.

Generally, pH is an important factor affecting the nature of the adsorbent as well as the adsorbate. For this effect, the initial pH values were changed from 1 to 12, and the fluoride solutions (20 and 50 mg/L) were tested. The results are shown in Fig. 6. The pH value had no considerable effect on fluoride adsorption in the pH

2.3 Effect of initial fluoride concentration on adsorption To determine the effect of initial fluoride concentration on fluoride adsorption, the experiments were carried out with initial fluoride concentrations ranging from 20 to 300 mg/L at 298 K. Fig. 5 shows that the removal decreased rapidly with the increase of initial fluoride concentration. The maximum removal was obtained at 94.4% by using 20 mg/L of fluoride concentration. At a lower fluoride concentration, the ratio of the available

Fig. 5 Effect of initial fluoride concentration on adsorption

LI Jinfang et al., Synthesis of a novel Ce(III)-incorporated cross-linked chitosan and its effective removal of …

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relative with the increasing pH which is caused by the addition of CO32– and PO43–. As mentioned in Section 2.4, the Ce-CCS showed the decreasing removal when increasing pH value beyond 9. 2.6 Adsorption kinetics

Fig. 6 Effect of pH on adsorption

range of 3–9, and the optimal pH value was observed at 3. The removal was 97.0% and 69.7% for 20 and 50 mg/L fluoride solutions at pH 3, respectively. When the pH value was lower than 3 or higher than 9, the removal decreased. Under strong acidic conditions, the low removal can be attributed to the formation of weak hydrofluoric acid. Under alkaline conditions, the decreased removal may be due to the competition of hydroxyl ions with fluoride ions for adsorption sites[23]. 2.5 Effect of coexisting anions on adsorption Chloride, sulfate, carbonate and phosphate ions are typically present in fluoride-contaminated water and compete with fluoride for active adsorption sites. The effect of coexisting anions was investigated with 0.15 g of Ce-CCS dosage and 20 mg/L of fluoride concentrations. The concentrations of coexisting anions were kept at 0.05 and 0.10 mol/L. As shown in Fig. 7, the presence of these anions weakened the fluoride adsorption on the Ce-CCS, which resulted from the competition between fluoride and these ions such as chloride, carbonate, sulfate and phosphate for the sites on the adsorbent surfaces, especially for high valence anions such as phosphate. The decreasing order of the removal towards fluoride was PO43–>CO32–>SO42–>Cl–. Besides, for both PO43–and CO32–, a considerable decrease in removal may be

Fig. 7 Effect of co-existing anions on adsorption

The kinetic behaviors were studied at three different initial fluoride concentrations (100, 200 and 300 mg/L) and at three different dosages (0.12, 0.15 and 0.18 g). The adsorption of fluoride was investigated as a function of time in the range of 0–120 min and is illustrated in Fig. 8. It is found that the adsorption was rapid initially and then slowed down gradually until equilibrium was reached, beyond which there was no further adsorption. Rapid adsorption at initial contact time can be attributed to the availability of a large amount of vacant sites on the surface of Ce-CCS and the strong binding of Ce-CCS for fluoride ions. However, after a lapse of time, the slow rate of adsorption was possibly due to slow pore diffusion of the dye into the bulk adsorbent. For the sake of the attainment of equilibrium by Ce-CCS, an equilibrium time of 60 min was considered to be optimum in further experiments. To investigate the kinetic behaviors of Ce-CCS for fluoride, the pseudo-first-order and pseudo-second-order kinetic models[24] have been used to predict the kinetic data. The pseudo-first-order kinetic model, also known as the Lagergren kinetic equation is given in Eq. (4):

Fig. 8 Effect of contact time on adsorption by Ce-CCS (a) Different fluoride concentrations; (b) Different fluoride dosages

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1 qt

=

JOURNAL OF RARE EARTHS, Vol. 34, No. 10, Oct. 2016

k1 qe t

+

1

(4)

qe

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. 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): t 1 1 = + t (5) 2 qt k 2 q e qe where k2 is the equilibrium rate constant of pseudo-second-order model ((g/(mg·min)). The values of qe (1/slope) and adsorption rate constant k2 (slope2/intercept) can be obtained by plotting t/qt versus t. The kinetic parameters, correlation coefficients (R2) and the experimental adsorption capacities (qe,exp) at equilibrium by the Ce-CCS are listed in Table 1. The pseudo-first-order equation showed very low correlation coefficients. The pseudo-second-order kinetic model was in good agreement with the experimental data for CeCCS, as given by the high R2 values (R2>0.999), and the calculated qe,cal values agreed very well with the experimental ones (qe,exp). These results imply that the studied adsorption system followed the pseudo-second-order kinetic model for Ce-CCS and the chemisorption was the rate-limiting step of fluoride adsorption. It was likely that the adsorption process involved the formation of hydrogen bond through sharing the electrons between fluoride ions and the binding sites of the adsorbents[25].

increased with increasing the initial fluoride concentration in the aqueous solution until it approached a platform. This trend can be explained as follows: the mass of adsorbent was constant in these experiments, more mass of fluoride was put into the system with increasing the initial fluoride concentration in the aqueous solution, the interaction of fluoride with Ce-CCS and CCS also increased accordingly, thus a higher initial concentration of fluoride ions may enhance the adsorption capacity. These data were analyzed using the isotherm models viz., Langmuir, Freundlich and Dubinin- Radushkevich (D-R) models. The Langmuir isotherm describes a homogenous adsorption occurring on a monolayer surface without net interaction between the adsorbed species[26]. The linearized Langmuir equation is given as: Ce 1 Ce = + (6) qe Qb Q

where Q is the maximum amount of adsorption with the complete monolayer coverage on the adsorbent surface (mg/g) and b is the Langmuir constant, which is related to the energy of adsorption (L/mg). From the linear plots of Ce/qe against Ce, Q and b values can be calculated from the slope and intercept, respectively. Freundlich isotherm describes the multilayer coverage of adsorbate over a heterogeneous adsorbent surface where the interaction of adjacent adsorbed molecules exists[27]. The linearized Freundlich equation is expressed as:

2.7 Adsorption isotherms The isotherm deals with the relationship between the equilibrium amount of solute on the adsorbent and the solute concentration in solutions. Adsorption isotherm study was performed at three different temperatures (303, 313 and 323 K) by varying fluoride concentrations from 20 to 250 mg/L. The equilibrium adsorption data by Ce-CCS and CCS are presented in Fig. 9. It is clear that the adsorption capacity of Ce-CCS and CCS for fluoride

Fig. 9 Plots of qe versus Ce

Table 1 Kinetic parameters of the adsorption of fluoride on Ce-CCS Adsorbent

Ce-CCS

Concentration/

Adsorbent

(mg/L)

dosage/g

100

0.15

200

0.15

qe (exp)

Pseudo-first-order

Pseudo-second-order 2

R2

qe(cal)

k1

R

qe (cal)

k2

17.92

17.77

0.197

0.6189

18.03

0.091

1.000

24.74

24.38

0.733

0.8617

25.10

0.024

0.9998

300

0.15

30.89

31.08

0.642

0.8056

31.09

0.057

0.9999

100

0.12

20.86

20.92

0.271

0.8056

20.92

0.200

1.000

100

0.15

17.92

17.77

0.197

0.6189

18.03

0.091

1.000

100

0.18

16.33

16.28

0.943

0.9579

16.52

0.044

0.9999

LI Jinfang et al., Synthesis of a novel Ce(III)-incorporated cross-linked chitosan and its effective removal of …

lg qe = lg K f +

1

(7)

lg Ce

n where Kf [(mg/g)(L/mg)1/n] and n are Freundlich constants related to the adsorption capacity and heterogeneity factor, respectively. Kf and n values can be calculated from the intercept and slope of the linear plot between lgCe and lgqe. To determine the type of adsorption for the removal of fluoride from aqueous solution by whether it is chemical or physical, the adsorption process was also studied by analyzing the equilibrium data using the DubininRadushkevich (D-R) isotherm model[28]. The equation can be expressed as: (8) lnqe=lnqm–βε2 where qe and qm are described as the amount of adsorbed fluoride ions at equilibrium and the D-R adsorption capacity (mg/g), respectively. The β parameter is constant with a dimension of energy, ε is the Polanyi potential and can be calculated as the following equation: (9) ε=RTln(1+1/Ce) where Ce is the equilibrium concentration of fluoride in aqueous solution (mol/L), T is the temperature (K), and R is the gas constant (8.314 J/(mol·K)). In addition, the value of E can give important information about the adsorption mechanism. When 1 mol of ions are transferred, E=1–8 kJ/mol corresponds to physical adsorption, E=8 and 16 kJ/mol suggests that the adsorption is chemical in nature. The value of the mean free energy E (kJ/mol) of adsorption can be calculated from the value of β: 1 (10) E= 2β

The various constants of the three models were calculated and are given in Table 2. The values of 1/n that lay between 0 and 1 confirm the favorable adsorption by both Ce-CCS and CCS. The adsorption data by the Ce-CCS were better described by Freundlich models based on higher R2 values, suggesting that the multilayer adsorption was involved in the adsorption of fluoride onto the Ce-CCS. On the other hand, the adsorption data by CCS were better described by the Langmuir isotherm model, indicating that the adsorption of fluoride onto the CCS was attributed to a monolayer adsorption. Besides, for the D-R isotherm model, the mean E values were

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about 13.50 kJ/mol for Ce-CCS and 9.0 kJ/mol for CCS (Table 2), which suggested that the fluoride adsorption processes by both Ce-CCS and CCS were mainly preceded chemically. Besides, the Ce-CCS allowed higher adsorption capacity than CCS in this study. This increase may be due to the effective immobilization of Ce(III) ions in chitosan during the preparation of this adsorbent. 2.8 Desorption and regeneration

Any adsorbent is economically viable if the adsorbent can be regenerated and reused. The exhausted Ce-CCS was regenerated using HCl, NaOH and NaCl solutions. All the regeneration experiments were carried out at 298 K. The results (Fig. 10) show that HCl solution was a better eluent as compared with NaOH and NaCl solutions. After the exhausted Ce-CCS was regenerated with 0.1 mol/L HCl solution, this adsorbent still allowed high removal towards fluoride. This was similar to the reports using other chitosan-containing adsorbents[17,29]. 2.9 Adsorption mechanism

During the preparation process of Ce-CCS, CeNO3·6H2O was added into chitosan solution prior to the cross-linking reaction between chitosan and epichlorohydrin. The reactive amino and hydroxyl groups of chitosan had formed a chelated complex with Ce3+ ions. The possible mechanisms of fluoride adsorption by the CCS and Ce-CCS were advised as follows. For CCS, CCS removed fluoride ions by means of hydrogen bonding.

Fig. 10 Regeneration of Ce-CCS on fluoride adsorption

Table 2 Adsorption isotherm parameters of fluoride onto Ce-CCS and CCS Adsorbent

Temperature/ K 303

Ce-CCS

CCS

Langmuir b 0.047

Q 33.43

Freundlich 2

R

0.9896

Kf 1.21

1/n 0.288

D-R 2

R

0.9933

qm 45.43

E

R2

–9

13.30

0.9788

–9

β 2.829×10

313

0.054

30.43

0.9923

6.67

0.274

0.9941

41.54

2.709×10

13.59

0.9816

323

0.057

27.31

0.9926

6.42

0.261

0.9942

37.14

2.620×10–9

13.81

0.9835

–9

303

0.028

28.29

0.9793

1.85

0.512

0.9704

53.40

6.10×10

9.06

0.9651

313

0.025

26.64

0.9874

1.62

0.519

0.9723

67.88

6.28×10–9

8.92

0.9842

72.46

–9

9.03

0.9823

323

0.034

20.20

0.9747

1.50

0.491

0.9465

6.13×10

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Moreover CCS may be considered as hard acid as it possesses H+ ion and hence it prefers to bind with hard base, most preferably with high electronegative and low polarisable F− ion[17]. For Ce-CCS, fluoride ions were trapped from the solution due to the electrostatic attraction and strong Lewis acid-base interaction[30]. Simultaneously, Ce(III) modified cross-linked chitosan (Ce-CCS) may form a complex with fluoride due to the introduction of Ce(III). Therefore, the Ce-CCS showed a better adsorption for fluoride ions than the CCS, as mentioned in Section 2.7.

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3 Conclusions Ce-CCS was synthesized successfully for defluoridation by Ce(III) incorporated cross-linked chitosan. The defluoridation capacity of Ce-CCS was altered in pH, adsorbent dosage, contact time, initial fluoride concentration and co-existing anions. The optimal pH value of defluoridation by Ce-CCS occurred at pH 3 or so. CeCCS was a heterogeneous material with infinite amount of active sites for defluoridation. The adsorption followed the pseudo-second order kinetic model and was controlled by chemical process. The adsorption mechanisms included the electrostatic attraction, strong Lewis acid-base interaction and complexation between Ce-CCS and fluoride. In addition, the Ce-CCS adsorbent was regenerated with 0.1 mol/L HCl solution effectively. These results indicated that the Ce-CCS could be effectively employed as a promising defluoridating agent.

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