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Removal of F− from aqueous solution using Zr(IV) impregnated dithiocarbamate modified chitosan beads
Chemical Engineering Journal 228 (2013) 224–231
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Removal of F from aqueous solution using Zr(IV) impregnated dithiocarbamate modified chitosan beads Bingjie Liu a,⇑, Dongfeng Wang a,⇑, Guangli Yu b, Xianghong Meng a a b
Laboratory of Food Chemistry and Nutrition, College of Food Science and Engineering, Ocean University of China, Qingdao 266003, PR China School of Medicine and Pharmacy, Ocean University of China, Qingdao 266003, PR China
h i g h l i g h t s
Zr(IV) impregnated dithiocarbamate modified chitosan beads were prepared to adsorb F . The adsorption selectivity was higher than that of dithiocarbamate chitosan beads. The maximum adsorption capacity was 4.58 mg/g at pH 7 and 30 °C. The kinetic data was fit with a pseudo-second order equation and Langmuir isotherms model.
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 25 April 2013 Accepted 28 April 2013 Available online 7 May 2013 Keywords: Dithiocarbamate Chitosan beads Zr(IV) F Removal
a b s t r a c t A novel, bio-based Zr(IV) impregnated dithiocarbamate modified chitosan beads (Zr-DMCB) was successfully synthesized for the adsorption of F from aqueous solutions. Batch adsorption experiments were performed to evaluate the adsorption conditions, selectivity and reusability. The results showed that the saturation adsorption capacity was 4.58 mg/g at pH 7.0, 30 °C for 40 min. The adsorption kinetic data was fitted with pseudo-second-order model. Adsorption process could be well described by Langmuir isotherm model and the maximum adsorption capacity calculated from Langmuir isotherm equation was 7.78 mg/g. The adsorption capacity of Zr-DMCB for F was significantly affected by co-anions. The adsorption mechanism of Zr-DMCB for F was characterized by FTIR, SEM and EDX analysis. The results above indicated that Zr-DMCB is a very promising biosorbent for the removal of F from aqueous solutions. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Fluorion (F) is one of the most abundant anions present in groundwater worldwide and creates a major problem in safe drinking water supply [1]. Contamination of F is a worldwide problem, especially in Africa, Asia and United states [2]. Fluorosis caused by excess intake of F, is a slow, progressive disorder, and known to affect predominantly the skeletal systems, teeth, the structure and function of the skeletal muscle, brain and spinal cord [3]. F in drinking water may be beneficial or detrimental depending on its concentration and total amount ingested. Drinking water is the main source for F ingestion to human body therefore it is necessary to remove the redundant of F. The World Health Organization has specified the tolerance limit for F content in drinking water as 1.5 mg/L [4] while the tolerance limit is 1.0 mg/L in China. Several methods have been suggested to remove excessive F from water such as chemical precipitation [5,6], ion exchange [7], ⇑ Corresponding authors. Tel.: +86 532 82031575; fax: +86 532 82032093. E-mail addresses: [email protected] (B. Liu), [email protected] (D. Wang). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.04.099
nanofiltration/reverse osmosis [8–10], electrolysis [11] and adsorption [12–14]. Among the above reported methods, adsorption seems to be the most attractive technique for F removal while the other methods have the shortcomings such as high operational and maintenance costs, secondary pollution (generation of toxic sludge, etc.) and complicated procedure involved in the treatment [15]. Nowadays, considerable work has been conducted in developing new adsorbents loaded with metal ions for the purpose of F removal, such as Fe(III) [16], Ti(IV) [17], Zr(IV) [18,19], Ce(III) [20] and La(III) [21,22]. In the present work, Zr(IV) has been incorporated by impregnation method on the surface of chitosan beads. The advantage of using chitosan for Zr(IV) impregnation over other supports like cellulose, activated carbon and alumina is that chitosan contains amino and hydroxyl groups, which have ability to be modified for binding more metal ions. As Zr(IV) is highly electropositive, it shows high affinity for highly electronegative F. Chitosan is a cationic biopolymer of 2-glucosamine and N-acetyl-2-glucosamine with excellent properties such as biocompatiability, biodegradability, non-toxicity and adsorption properties [23]. Chitosan is obtained by deacetylation of chitin and it contains
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reactive amino groups, which selectively binds to all transition metal ions and not bind to alkali and alkaline metal ions and it has been extensively studied for metal ions adsorption [24,25]. However, the adsorption capacity of chitosan beads was minimal, which indicated that the hydroxyl and amine groups of chitosan have not been effectively involved in F removal [26]. Hence, in order to exploit these groups of chitosan, chemical modifications were carried out. In this study, chitosan beads were modified with dithiocarbamate to maximized the adsorption capacity for Zr(IV). For the metal of concern, Zr(IV) is primarily considered due to its strong affinity toward F anions, environmental safe and low cost. The aim of this work is to evaluate the removal of F from aqueous solution in batch adsorption experiments. Langmuir isotherm model and pseudo-second-order kinetic model were employed to evaluate the sorption process.
2.4. Batch adsorption experiments Batch adsorption experiments were carried out in 50 mL Erlenmeyer flasks. The amount of Zr-DMCB was 0.1 g and volume of 20 mg/L F aqueous solution was maintained at 25.0 mL. The pH of aqueous solution was adjusted by adding 1.0 mol/L NaOH or 1.0 mol/L HCl aqueous solution and measured with a pH meter (Delta 320, Mettler-toledo). The batch experiments were carried out at a constant temperature for 12 h. After adsorption equilibration, the adsorbent was separated by filtration. The concentration of F in the filtrate was measured by the method of F selective electrode. The adsorption capacity of Zr-DMCB for F was calculated using the following equation:
qe ¼ 2. Materials and methods 2.1. Materials Chitosan (degree of deacetylation 92.2%, MW 5.0 105 Da) was purchased from Shandong Hecreat marine bio-tech Co., Ltd. (Qingdao, China). Acetic acid, sodium NaF, liquid paraffin, glutaraldehyde, dithiocarbamate, ZrOCl28H2O and all the other reagents used in this experiment were of analytical grade or better and used as received without any further purification. Deionized distilled water (18.2 MX, Millipore) was used throughout the experiments.
2.2. Preparation of Zr(IV) loaded dithiocarbamate modified chitosan beads (Zr-DMCB) Chitosan beads (CBs) were crosslinked with glutaraldehyde in our previously reported literature to increase the stability [27]. In order to effectively utilize the hydroxyl groups of CB, they were modified using dithiocarbamate (DTC) to make use of reactive hydroxyl groups of chitosan beads for F adsorption, these groups were chelated with Zr(IV) as follows. The wet CB was treated with DTC aqueous solution to convert hydroxyl groups of chitosan to sulfur-carboxyl groups (DMCB). The DMCB was washed with distilled water to pH 7.0, dried at room temperature. Then, DMCB was treated with 5% ZrOCl28H2O solution for 24 h and washed distilled water to pH 7.0, dried at room temperature till to get constant weight, abbreviation as Zr-DMCB. The dried Zr-DMCB was used for adsorption studies.
Ci Ce V W
ð1Þ
where qe is the adsorption capacity of the beads (mg/g); Ci and Ce are the concentrations of F in the initial and equilibrium solution (mg/L), respectively; V is the volume of F aqueous solution (L) and W is the mass of dry beads (g). 2.5. Desorption study The desorption studies are very important since the economic success of the desorption process depends on the regeneration of adsorbent. In the desorption study, several solvents/solutions were tried to regenerate the biosorbents. Out of the desorption solutions, 0.5 mol/L NaOH aqueous solutions were found to be effective in desorbing F from the F loaded Zr-DMCB. The beads was regenerated using 0.5 mol/L NaOH aqueous solutions, the procedure was repeated for many times until F could not be detected in the filtrate. Then, Zr-DMCB was washed thoroughly with deionized distilled water to pH 7.0. The regenerated Zr-DMCB was reused in the following adsorption experiments and the procedure was repeated for 5 times by using the same Zr-DMCB. 3. Results and discussion 3.1. FTIR analysis The FTIR spectra of Zr-DMCB before and after F adsorption were recorded and presented in Fig. 1. The broad peaks around 3400 cm1 are attributed to the stretching vibration of –NH2 groups [30]. The bands at 2922 and 2850 cm1 are attributed to the stretching vibration of –CH3 and –CH2– (Fig. 1a). The intensity of the band at 3442.02 cm1 was substantially decreased after
2.3. Characterization of Zr-DMCB The FTIR spectra of Zr-DMCB before and after adsorption of F were recorded using Nexus 470 FTIR Spectrometer with a resolution of 4 cm1. The dried sorbent, Zr-DMCB before and after adsorption of F was ground into powder. For each type of the powder, 1 mg of the powder was blended with 100 mg of IR-grade KBr in an agate mortar and pressed into a tablet. The spectra of the tablets were scanned within the spectral range of 400–4000 cm1 [28]. Photographs were taken using a scanning electron microscope (SEM) (Hitachi S-4800) with combined energy dispersive X-ray analyzer at a voltage of 10 keV to analyze the surface structure and morphology of the adsorbents. SEM allowed the identification of any interesting structural features on the adsorbent surface with EDX used to determine the elemental composition of the surface before and after F binding [29].
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Fig. 1. FTIR spectra of Zr-DMCB before (a) and after and (b) F adsorption.
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F binding and shifted to higher wavenumber 3447.67 cm1, which indicated that –NH2 groups participated in the adsorption process (Fig. 1b). The weak absorption peak around 2600 cm1 duo to the stretching vibration of –SH groups disappeared, which confirmed that –SH groups participated in the adsorption process (Scheme 1b). The absorption peak around 1458.56 cm1 is attributed to the stretching vibration of –S = C– groups, the absorption peak disappeared after F adsorption, which indicated that –
S = C– groups participated in the adsorption process [31]. The absorption peak around 564.91 cm1 is attributed to the stretching vibration of Zr(IV) ions, the absorption peaks disappeared after F adsorption, which indicated that Zr(IV) ions participated in the adsorption process [32]. This result confirmed that –NH2, –SH, – S = C– groups and Zr(IV) of Zr-DMCB were involved in F binding process (Scheme 1c). It is essential to take into account the correlations between Zr(IV) and F. Based on the above analysis, it was
Scheme 1. Proposed reaction mechanism for preparing Zr(IV) impregnated dithiocarbamate modified chitosan beads.
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concluded that –NH2, –SH, –C = S– groups and Zr(IV) ions anticipated in the adsorption of Zr-DMCB for F from aqueous solutions. 3.2. SEM-EDX analysis The SEM micrographs of Zr-DMCB were presented in Fig. 2. The images illustrated that the surface of Zr-DMCB particles was spherical (Fig. 2A), smooth (Fig. 2B), marked with holes (Fig. 2C) and tiny interspaced structures (Fig. 2D), which increased the contact area and improved ions adsorption capacity [33]. The energy dispersive X-ray (EDX) analysis spectrum of Zr-DMCB is shown in Fig. 3A and B. Comparing the EDX spectrum before and after adsorption, there was an obvious content of F in the EDX spectrum after adsorption (Fig. 3B), which demonstrated that F was adsorbed onto Zr-DMCB [34]. 3.3. Effect of pH on F adsorption The effect of pH on F sorption was studied at five different pH levels of 3.0, 5.0, 7.0, 9.0 and 11.0 by keeping other parameters like contact time, temperature and initial F concentration the same. Fig. 4 explained the adsorption capacity of Zr-DMCB as a function of pH and the maximum adsorption capacity was recorded at pH 7.0. It appeared that the adsorption capacity of the sorbent was slightly influenced by pH of aqueous solution, the differences were significant and hence it can be concluded that there was pH dependence on adsorption capacity of Zr-DMCB. The reason could be the surface of the sorbent acquired negative charge above its pHzpc values and hence above pH 3.18 electrostatic attraction between the sorbent and F can be ruled out and complexation played a dominant role. For further experiments, pH 7.0 was fixed as the optimal pH in the following experiments. 3.4. Effect of temperature on F adsorption The effect of temperature on F sorption was studied at five different temperature levels at 20, 30, 40, 50 and 60 °C by keeping
Fig. 3. EDX micrographs of Zr-DMCB: (A) before adsorption and (B) after adsorption.
other parameters like contact time, pH and initial F concentration the same. Fig. 5 indicates that the adsorption capacity of Zr-DMCB for F increased as the temperature was raised from 20 to 30 °C, which was similar with DMCB. It is apparent that at the lower temperature (<30 °C), the adsorption of F onto Zr-DMCB or the formation of Zr-DMCB with F complexes is favored. However, when the temperature exceeded 30 °C, a higher temperature favored desorption or dechelation, which indicating that 30 °C was the optimal temperature for the adsorption of F onto Zr-DMCB. Hence, 30 °C
Fig. 2. SEM micrographs of Zr-DMCB: (A) SEM image (130), (B) SEM image (180), (C) SEM image (400) and (D) SEM image (4000).
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Fig. 4. Effect of pH on adsorption capacity of Zr-DMCB for F.
Fig. 6. Effect of co-anions on adsorption capacity of Zr-DMCB for F.
competing effect of anions as well as the decrease of active Zr(IV) sites available for adsorption. 3.6. Effect of contact time on F adsorption
Fig. 5. Effect of temperature on adsorption capacity of Zr-DMCB for F.
was chosen as the appropriate temperature in the following experiments.
3.5. Effect of the presence of co-anions In the practical application, the F contaminated water or aqueous solution always contains several other anions, which can compete with F in the adsorption process. In order to evaluate the effect of interfering ions, the adsorption studies were carried out in the presence of 0.1 mol/L salt solutions of chloride, sulfate, nitrate, bicarbonate, and dihydricphosphate, separately. The effect of anions on the removal of F was shown in Fig. 6. From the above results, it was observed that, Cl ; SO2 4 ; NO3 ; HCO3 and H2 PO4 ions showed negative effect on the removal of F . It may be the change of pH as well as the competing effect of these co-anions. The pH of F solution were 5.51, 5.67, 5.43, 8.34, 4.59, respec tively for Cl, SO2 4 , NO3 , HCO3 and H2 PO4 , while the pH of single F solution was 6.51 without addition any salt/anions. This indicated that the addition of salt resulted in decreased pH of F solution. From the above experiments on the effect of pH (Section 3.3), it was observed that the adsorption of F decreased in alkaline pH as also explained. Another observation was found that in the absence of anions, Zr(IV) release was a very negligible amount (below detection limit), this indicated that the active Zr(IV) ions available at alkaline pH would be relatively less for adsorption of F. The overall effect therefore decreased in the removal of F from water mainly due to the decreased pH of aqueous solution and
Under the conditions of 0.1 g Zr-DMCB, pH 7.0, 30 °C and 25 mL 20 mg/L F, the adsorption experiments were carried out for contact time ranging from 10 to 60 min. The results are shown in Fig. 7. It was clear that the adsorption capacity of F onto Zr-DMCB increased with an increasing of contact time and reached adsorption equilibrium within 60 min. Kinetics of adsorption of F consisted of two stages: an initial rapid stage where adsorption was fast and contributed significantly to equilibrium uptake, and a slower second stage whose contribution to the total metal adsorption sites was relatively small. The first stage is interpreted to be the instantaneous adsorption stage or external surface adsorption and availability of more numbers of adsorption sites and smaller mass transfer resistance on the surface with the continuation of adsorption. The second stage is interpreted to be the gradual adsorption stage where intraparticle diffusion controls the adsorption rate until finally the metal uptake reaches equilibrium [35]. After 40 min, the change of adsorption capacity for F was not significant. As a consequence, 40 min was considered as the adsorption equilibrium time in the following experiments. In this study, the experimental data, obtained from the studies of contact time at 30 °C was analyzed by two different adsorption kinetic models such as the pseudo-first-order model [36] and pseudo-second-order model [37].
Fig. 7. Effect of contact time on adsorption capacity of Zr-DMCB for F.
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B. Liu et al. / Chemical Engineering Journal 228 (2013) 224–231 Table 1 The values of kinetic parameters for pseudo-first-order equation and pseudo-second-order equation together with correlation coefficients. Adsorbent
Pseudo-first-order model
Pseudo-second-order model 1
R21
Regression equation
K2 (g/mg h)
qe (mg/g)
R22
t qt t qt
¼ 0:1851t þ 1:7603
0.0195
5.40
0.995
¼ 0:5141t þ 18:692
0.0141
1.95
0.973
Regression equation
q1,cal (mg/g)
K1 (min
Zr-DMCB
log(q1qt) = 0.993–0.064t
9.84
0.1474
0.969
DMCB
log(q1qt) = 0.015–0.019t
1.04
0.0438
0.937
)
The linear form of pseudo-first-order kinetic model is given as:
logðq1 qt Þ ¼ log q1
k1 t 2:303
ð2Þ
where q1 and qt are the adsorbed F amounts per unit mass of adsorbent at equilibrium and at any time t (mg/g), and k1 is the pseudo-first-order adsorption kinetic constant (min1). The calculated values k1, q1 and correlation coefficients (R21 ) of typical pseudo-first-order model at 30 °C are listed in Table 1. The adsorption kinetics of F onto Zr-DMCB was analyzed on the basis of pseudo-second-order kinetic model, which is expressed as Eq. (3):
t 1 t ¼ þ qt K 2 q2e qe
ð3Þ
where t is the contact time (h), qt and qe are the amount of F adsorbed at an arbitrary time t and at equilibrium (mg/g), respectively, and K2 is the rate constant (h g/mg).
From the data of Fig. 7, plots of t/qt versus t for the adsorption of F are obtained, as shown in Fig. 8. The pseudo-first-order and pseudo-second-order kinetic model rate constants for adsorption of F were summarized in Table 1. From Table 1, the calculated R21 from pseudo-first-order kinetic model is much lower than that of pseudo-second-order kinetic model. This result indicated that the adsorption of F onto Zr-DMCB was better described by pseudo-second-order kinetic model than pseudo-first-order kinetic model. The pseudo-second-order kinetic model assumed that chemical adsorption may be the rate-limiting step [38]. 3.7. Effect of concentration on F adsorption The effect of the initial F concentration on the adsorption capacity of Zr-DMCB was studied by contacting 0.1 g of Zr-DMCB with 25 mL F aqueous solution at 30 °C and initial solution pH 7.0 using a range of initial F concentration (2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 mg/L). The results are shown in Fig. 9. It is clear that the adsorption capacity of Zr-DMCB for F increased with increasing the initial F concentration in the aqueous solution. On one hand, this is because of more mass of F is put into the system with increasing the initial F concentration in the aqueous solution, however, the mass of adsorbent is constant. On the other hand, because of the higher mobility of F in the diluted solutions, the interaction of F with Zr-DMCB also increased [39]. This result was consistent with the result reported by Han et al. in their study on adsorption of F from aqueous solution [40]. The adsorption isotherm is the most important information, which indicates how the adsorbent molecules distribute between the liquid and the solid phase when the adsorption process reaches an equilibrium state [41]. The Langmuir adsorption isotherm model and Freundlich adsorption isotherm model are often used to describe equilibrium adsorption isotherms, which are based on several assumptions, including homogeneous surface, localized adsorption on the surface and solo molecule accommodated active sites. The Langmuir isotherm equation may be written as Eq. (4) [42]:
Fig. 8. Plots for pseudo-second-order kinetic model.
Fig. 9. Effect of F concentration on adsorption capacity of Zr-DMCB for F.
Fig. 10. Langmuir adsorption isotherm.
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Table 2 Langmuir isotherm and Freundlich isotherm parameters for the adsorption of F onto Zr-DMCB. Temperature (°C)
20 30 40
Langmuir isotherm model
Freundlich isotherm model 2
qs (mg/g)
Kb (L/mg)
R
RL
KF ((mg/g) (mg/L)1/n)
1/n
R2
7.2674 7.7821 7.5131
0.2484 0.3712 0.2628
0.9929 0.9902 0.9971
0.6681 0.5739 0.6555
2.0370 4.6238 2.3878
0.724 0.719 0.737
0.894 0.889 0.965
Ce 1 Ce ¼ þ qe K b qs qs
ð4Þ
where qe is the adsorption capacity of the beads (mg/g) at equilibrium, Ce is the concentration of F in the equilibrium solution (mg/L) and qs is the Langmuir constant, which is equal to the monolayer adsorption capacity (mg/g). The parameter Kb is the Langmuir sorption equilibrium constant (L/mg) related to the free energy of adsorption. The Freundlich isotherm equation may be written as Eq. (5) [43]:
ln qe ¼ ln K F þ
1 ln C e n
ð5Þ
where qe is the adsorbed F amount per unit mass of adsorbent at equilibrium (mg/g); KF is the Freundlich constant ((mg/g) (mg/L)1/n), n is the Freundlich constant related to adsorption intensity (dimensionless). The Langmuir isotherm model for adsorption of F onto Zr-DMCB is presented in Fig. 10. The straight lines of plots indicated that the adsorption processes of all cases could be well described by Langmuir isotherm model. The parameters and correlation coefficients calculated from corresponding models were given in Table 2, which indicated that Langmuir isotherm model was more suitable for assessing the adsorption of F onto Zr-DMCB and the adsorption of Zr-DMCB was a complex process containing physical adsorption and chemical adsorption. The adsorption process of F onto Zr-DMCB could be considered as monolayer adsorption. The essential characteristics of Langmuir isotherm model can be expressed in terms of dimensionless constant separation factor RL, which is used to predict if an adsorption system is ‘unfavorable’, ‘favorable’, ‘irreversible’ or ‘linear’. The separation factor RL is defined as Eq. (6) [44]:
RL ¼
1 1 þ K bC0
ð6Þ
where RL indicates the favorability and the capacity of the adsorbent/adsorbate system, C0 is the initial F concentration (mg/L) and Kb is the Langmuir adsorption equilibrium constant (L/mmol).
The RL value classified as RL > 1, 0 < RL < 1, RL = 0 and RL = 1 suggested that the adsorption was unfavorable, favorable, irreversible and linear, respectively [45]. The values of RL for sorption of Zr-DMCB for F at 20, 30, 40 °C are all between 0 and 1, which indicated that the uptake of F onto Zr-DMCB was favorable. The mechanisms of ion adsorption on porous adsorbents may involve three steps: (1) diffusion of F to the external surface of Zr-DMCB; (2) diffusion of F into the pores of Zr-DMCB and (3) adsorption of F on the internal surface of Zr-DMCB.
3.8. Desorption and reuse studies When considering the cost of the sorbent, it is necessary to consider the efficiency as well as the preservation of adsorption capacity of the biomass [46]. In our work, several solvents/solutions were tried in order to regenerate the biosorbent. Among them, 0.5 mol/L NaOH was proved to be a relatively effective eluent for desorption of F from Zr-DMCB. The reusability of Zr-DMCB for F adsorption was shown in Fig. 11. It was revealed that the adsorption capacity of Zr-DMCB for F decreased slightly from 4.56 to 4.03 mg/g with increasing the times of reuse. Zr-DMCB could be reused for at least 5 times with about 11.6% regeneration loss, which indicated that Zr-DMCB had a good reusability in the application.
4. Conclusions In this study, the adsorption of Zr-DMCB for F from aqueous solution was examined. The –NH2, –SH, –S = C– groups and Zr(IV) functional groups of Zr-DMCB were involved in the complexation with F; The surface of Zr-DMCB particles was spherical, smooth, marked with holes and tiny interspaced structures; Zr(IV) was involved in Zr-DMCB, and F element was also involved in Zr-DMCB after adsorption with F in aqueous solution. The adsorption capacity of Zr-DMCB for F was dependent on initial pH and temperature. The saturation adsorption capacity of Zr-DMCB was obtained at pH 7.0, 30 °C for 40 min. The adsorption process followed Langmuir adsorption isotherm model for over all concentration range studied and the maximum adsorption capacity of Zr-DMCB for F was 7.78 mg/g. The adsorption of F onto Zr-DMCB followed pseudo-second-order kinetic model. The adsorbent Zr-DMCB can be used effectively for at least 5 times of reuse cycles with only 11.6% loss.
Acknowledgements
Fig. 11. Relationship between times of reuse and adsorption capacity.
This work was financially supported by the Ocean Public Welfare Scientific Research Special Appropriation Project (201005020-6), National Science and Technology Plan Project of China (2012AA101607-2), Fundamental Research Funds for the Central Universities and Program for Changjiang Scholars and Innovative Research Team in University (IRT 1188). We are grateful to thank the editors and anonymous reviewers for their positive comments and suggestions to the manuscript.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Removal of F from aqueous solution using Zr(IV) impregnated dithiocarbamate modified chitosan beads Bingjie Liu a,⇑, Dongfeng Wang a,⇑, Guangli Yu b, Xianghong Meng a a b
Laboratory of Food Chemistry and Nutrition, College of Food Science and Engineering, Ocean University of China, Qingdao 266003, PR China School of Medicine and Pharmacy, Ocean University of China, Qingdao 266003, PR China
h i g h l i g h t s
Zr(IV) impregnated dithiocarbamate modified chitosan beads were prepared to adsorb F . The adsorption selectivity was higher than that of dithiocarbamate chitosan beads. The maximum adsorption capacity was 4.58 mg/g at pH 7 and 30 °C. The kinetic data was fit with a pseudo-second order equation and Langmuir isotherms model.
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 25 April 2013 Accepted 28 April 2013 Available online 7 May 2013 Keywords: Dithiocarbamate Chitosan beads Zr(IV) F Removal
a b s t r a c t A novel, bio-based Zr(IV) impregnated dithiocarbamate modified chitosan beads (Zr-DMCB) was successfully synthesized for the adsorption of F from aqueous solutions. Batch adsorption experiments were performed to evaluate the adsorption conditions, selectivity and reusability. The results showed that the saturation adsorption capacity was 4.58 mg/g at pH 7.0, 30 °C for 40 min. The adsorption kinetic data was fitted with pseudo-second-order model. Adsorption process could be well described by Langmuir isotherm model and the maximum adsorption capacity calculated from Langmuir isotherm equation was 7.78 mg/g. The adsorption capacity of Zr-DMCB for F was significantly affected by co-anions. The adsorption mechanism of Zr-DMCB for F was characterized by FTIR, SEM and EDX analysis. The results above indicated that Zr-DMCB is a very promising biosorbent for the removal of F from aqueous solutions. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Fluorion (F) is one of the most abundant anions present in groundwater worldwide and creates a major problem in safe drinking water supply [1]. Contamination of F is a worldwide problem, especially in Africa, Asia and United states [2]. Fluorosis caused by excess intake of F, is a slow, progressive disorder, and known to affect predominantly the skeletal systems, teeth, the structure and function of the skeletal muscle, brain and spinal cord [3]. F in drinking water may be beneficial or detrimental depending on its concentration and total amount ingested. Drinking water is the main source for F ingestion to human body therefore it is necessary to remove the redundant of F. The World Health Organization has specified the tolerance limit for F content in drinking water as 1.5 mg/L [4] while the tolerance limit is 1.0 mg/L in China. Several methods have been suggested to remove excessive F from water such as chemical precipitation [5,6], ion exchange [7], ⇑ Corresponding authors. Tel.: +86 532 82031575; fax: +86 532 82032093. E-mail addresses: [email protected] (B. Liu), [email protected] (D. Wang). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.04.099
nanofiltration/reverse osmosis [8–10], electrolysis [11] and adsorption [12–14]. Among the above reported methods, adsorption seems to be the most attractive technique for F removal while the other methods have the shortcomings such as high operational and maintenance costs, secondary pollution (generation of toxic sludge, etc.) and complicated procedure involved in the treatment [15]. Nowadays, considerable work has been conducted in developing new adsorbents loaded with metal ions for the purpose of F removal, such as Fe(III) [16], Ti(IV) [17], Zr(IV) [18,19], Ce(III) [20] and La(III) [21,22]. In the present work, Zr(IV) has been incorporated by impregnation method on the surface of chitosan beads. The advantage of using chitosan for Zr(IV) impregnation over other supports like cellulose, activated carbon and alumina is that chitosan contains amino and hydroxyl groups, which have ability to be modified for binding more metal ions. As Zr(IV) is highly electropositive, it shows high affinity for highly electronegative F. Chitosan is a cationic biopolymer of 2-glucosamine and N-acetyl-2-glucosamine with excellent properties such as biocompatiability, biodegradability, non-toxicity and adsorption properties [23]. Chitosan is obtained by deacetylation of chitin and it contains
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reactive amino groups, which selectively binds to all transition metal ions and not bind to alkali and alkaline metal ions and it has been extensively studied for metal ions adsorption [24,25]. However, the adsorption capacity of chitosan beads was minimal, which indicated that the hydroxyl and amine groups of chitosan have not been effectively involved in F removal [26]. Hence, in order to exploit these groups of chitosan, chemical modifications were carried out. In this study, chitosan beads were modified with dithiocarbamate to maximized the adsorption capacity for Zr(IV). For the metal of concern, Zr(IV) is primarily considered due to its strong affinity toward F anions, environmental safe and low cost. The aim of this work is to evaluate the removal of F from aqueous solution in batch adsorption experiments. Langmuir isotherm model and pseudo-second-order kinetic model were employed to evaluate the sorption process.
2.4. Batch adsorption experiments Batch adsorption experiments were carried out in 50 mL Erlenmeyer flasks. The amount of Zr-DMCB was 0.1 g and volume of 20 mg/L F aqueous solution was maintained at 25.0 mL. The pH of aqueous solution was adjusted by adding 1.0 mol/L NaOH or 1.0 mol/L HCl aqueous solution and measured with a pH meter (Delta 320, Mettler-toledo). The batch experiments were carried out at a constant temperature for 12 h. After adsorption equilibration, the adsorbent was separated by filtration. The concentration of F in the filtrate was measured by the method of F selective electrode. The adsorption capacity of Zr-DMCB for F was calculated using the following equation:
qe ¼ 2. Materials and methods 2.1. Materials Chitosan (degree of deacetylation 92.2%, MW 5.0 105 Da) was purchased from Shandong Hecreat marine bio-tech Co., Ltd. (Qingdao, China). Acetic acid, sodium NaF, liquid paraffin, glutaraldehyde, dithiocarbamate, ZrOCl28H2O and all the other reagents used in this experiment were of analytical grade or better and used as received without any further purification. Deionized distilled water (18.2 MX, Millipore) was used throughout the experiments.
2.2. Preparation of Zr(IV) loaded dithiocarbamate modified chitosan beads (Zr-DMCB) Chitosan beads (CBs) were crosslinked with glutaraldehyde in our previously reported literature to increase the stability [27]. In order to effectively utilize the hydroxyl groups of CB, they were modified using dithiocarbamate (DTC) to make use of reactive hydroxyl groups of chitosan beads for F adsorption, these groups were chelated with Zr(IV) as follows. The wet CB was treated with DTC aqueous solution to convert hydroxyl groups of chitosan to sulfur-carboxyl groups (DMCB). The DMCB was washed with distilled water to pH 7.0, dried at room temperature. Then, DMCB was treated with 5% ZrOCl28H2O solution for 24 h and washed distilled water to pH 7.0, dried at room temperature till to get constant weight, abbreviation as Zr-DMCB. The dried Zr-DMCB was used for adsorption studies.
Ci Ce V W
ð1Þ
where qe is the adsorption capacity of the beads (mg/g); Ci and Ce are the concentrations of F in the initial and equilibrium solution (mg/L), respectively; V is the volume of F aqueous solution (L) and W is the mass of dry beads (g). 2.5. Desorption study The desorption studies are very important since the economic success of the desorption process depends on the regeneration of adsorbent. In the desorption study, several solvents/solutions were tried to regenerate the biosorbents. Out of the desorption solutions, 0.5 mol/L NaOH aqueous solutions were found to be effective in desorbing F from the F loaded Zr-DMCB. The beads was regenerated using 0.5 mol/L NaOH aqueous solutions, the procedure was repeated for many times until F could not be detected in the filtrate. Then, Zr-DMCB was washed thoroughly with deionized distilled water to pH 7.0. The regenerated Zr-DMCB was reused in the following adsorption experiments and the procedure was repeated for 5 times by using the same Zr-DMCB. 3. Results and discussion 3.1. FTIR analysis The FTIR spectra of Zr-DMCB before and after F adsorption were recorded and presented in Fig. 1. The broad peaks around 3400 cm1 are attributed to the stretching vibration of –NH2 groups [30]. The bands at 2922 and 2850 cm1 are attributed to the stretching vibration of –CH3 and –CH2– (Fig. 1a). The intensity of the band at 3442.02 cm1 was substantially decreased after
2.3. Characterization of Zr-DMCB The FTIR spectra of Zr-DMCB before and after adsorption of F were recorded using Nexus 470 FTIR Spectrometer with a resolution of 4 cm1. The dried sorbent, Zr-DMCB before and after adsorption of F was ground into powder. For each type of the powder, 1 mg of the powder was blended with 100 mg of IR-grade KBr in an agate mortar and pressed into a tablet. The spectra of the tablets were scanned within the spectral range of 400–4000 cm1 [28]. Photographs were taken using a scanning electron microscope (SEM) (Hitachi S-4800) with combined energy dispersive X-ray analyzer at a voltage of 10 keV to analyze the surface structure and morphology of the adsorbents. SEM allowed the identification of any interesting structural features on the adsorbent surface with EDX used to determine the elemental composition of the surface before and after F binding [29].
225
Fig. 1. FTIR spectra of Zr-DMCB before (a) and after and (b) F adsorption.
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F binding and shifted to higher wavenumber 3447.67 cm1, which indicated that –NH2 groups participated in the adsorption process (Fig. 1b). The weak absorption peak around 2600 cm1 duo to the stretching vibration of –SH groups disappeared, which confirmed that –SH groups participated in the adsorption process (Scheme 1b). The absorption peak around 1458.56 cm1 is attributed to the stretching vibration of –S = C– groups, the absorption peak disappeared after F adsorption, which indicated that –
S = C– groups participated in the adsorption process [31]. The absorption peak around 564.91 cm1 is attributed to the stretching vibration of Zr(IV) ions, the absorption peaks disappeared after F adsorption, which indicated that Zr(IV) ions participated in the adsorption process [32]. This result confirmed that –NH2, –SH, – S = C– groups and Zr(IV) of Zr-DMCB were involved in F binding process (Scheme 1c). It is essential to take into account the correlations between Zr(IV) and F. Based on the above analysis, it was
Scheme 1. Proposed reaction mechanism for preparing Zr(IV) impregnated dithiocarbamate modified chitosan beads.
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concluded that –NH2, –SH, –C = S– groups and Zr(IV) ions anticipated in the adsorption of Zr-DMCB for F from aqueous solutions. 3.2. SEM-EDX analysis The SEM micrographs of Zr-DMCB were presented in Fig. 2. The images illustrated that the surface of Zr-DMCB particles was spherical (Fig. 2A), smooth (Fig. 2B), marked with holes (Fig. 2C) and tiny interspaced structures (Fig. 2D), which increased the contact area and improved ions adsorption capacity [33]. The energy dispersive X-ray (EDX) analysis spectrum of Zr-DMCB is shown in Fig. 3A and B. Comparing the EDX spectrum before and after adsorption, there was an obvious content of F in the EDX spectrum after adsorption (Fig. 3B), which demonstrated that F was adsorbed onto Zr-DMCB [34]. 3.3. Effect of pH on F adsorption The effect of pH on F sorption was studied at five different pH levels of 3.0, 5.0, 7.0, 9.0 and 11.0 by keeping other parameters like contact time, temperature and initial F concentration the same. Fig. 4 explained the adsorption capacity of Zr-DMCB as a function of pH and the maximum adsorption capacity was recorded at pH 7.0. It appeared that the adsorption capacity of the sorbent was slightly influenced by pH of aqueous solution, the differences were significant and hence it can be concluded that there was pH dependence on adsorption capacity of Zr-DMCB. The reason could be the surface of the sorbent acquired negative charge above its pHzpc values and hence above pH 3.18 electrostatic attraction between the sorbent and F can be ruled out and complexation played a dominant role. For further experiments, pH 7.0 was fixed as the optimal pH in the following experiments. 3.4. Effect of temperature on F adsorption The effect of temperature on F sorption was studied at five different temperature levels at 20, 30, 40, 50 and 60 °C by keeping
Fig. 3. EDX micrographs of Zr-DMCB: (A) before adsorption and (B) after adsorption.
other parameters like contact time, pH and initial F concentration the same. Fig. 5 indicates that the adsorption capacity of Zr-DMCB for F increased as the temperature was raised from 20 to 30 °C, which was similar with DMCB. It is apparent that at the lower temperature (<30 °C), the adsorption of F onto Zr-DMCB or the formation of Zr-DMCB with F complexes is favored. However, when the temperature exceeded 30 °C, a higher temperature favored desorption or dechelation, which indicating that 30 °C was the optimal temperature for the adsorption of F onto Zr-DMCB. Hence, 30 °C
Fig. 2. SEM micrographs of Zr-DMCB: (A) SEM image (130), (B) SEM image (180), (C) SEM image (400) and (D) SEM image (4000).
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Fig. 4. Effect of pH on adsorption capacity of Zr-DMCB for F.
Fig. 6. Effect of co-anions on adsorption capacity of Zr-DMCB for F.
competing effect of anions as well as the decrease of active Zr(IV) sites available for adsorption. 3.6. Effect of contact time on F adsorption
Fig. 5. Effect of temperature on adsorption capacity of Zr-DMCB for F.
was chosen as the appropriate temperature in the following experiments.
3.5. Effect of the presence of co-anions In the practical application, the F contaminated water or aqueous solution always contains several other anions, which can compete with F in the adsorption process. In order to evaluate the effect of interfering ions, the adsorption studies were carried out in the presence of 0.1 mol/L salt solutions of chloride, sulfate, nitrate, bicarbonate, and dihydricphosphate, separately. The effect of anions on the removal of F was shown in Fig. 6. From the above results, it was observed that, Cl ; SO2 4 ; NO3 ; HCO3 and H2 PO4 ions showed negative effect on the removal of F . It may be the change of pH as well as the competing effect of these co-anions. The pH of F solution were 5.51, 5.67, 5.43, 8.34, 4.59, respec tively for Cl, SO2 4 , NO3 , HCO3 and H2 PO4 , while the pH of single F solution was 6.51 without addition any salt/anions. This indicated that the addition of salt resulted in decreased pH of F solution. From the above experiments on the effect of pH (Section 3.3), it was observed that the adsorption of F decreased in alkaline pH as also explained. Another observation was found that in the absence of anions, Zr(IV) release was a very negligible amount (below detection limit), this indicated that the active Zr(IV) ions available at alkaline pH would be relatively less for adsorption of F. The overall effect therefore decreased in the removal of F from water mainly due to the decreased pH of aqueous solution and
Under the conditions of 0.1 g Zr-DMCB, pH 7.0, 30 °C and 25 mL 20 mg/L F, the adsorption experiments were carried out for contact time ranging from 10 to 60 min. The results are shown in Fig. 7. It was clear that the adsorption capacity of F onto Zr-DMCB increased with an increasing of contact time and reached adsorption equilibrium within 60 min. Kinetics of adsorption of F consisted of two stages: an initial rapid stage where adsorption was fast and contributed significantly to equilibrium uptake, and a slower second stage whose contribution to the total metal adsorption sites was relatively small. The first stage is interpreted to be the instantaneous adsorption stage or external surface adsorption and availability of more numbers of adsorption sites and smaller mass transfer resistance on the surface with the continuation of adsorption. The second stage is interpreted to be the gradual adsorption stage where intraparticle diffusion controls the adsorption rate until finally the metal uptake reaches equilibrium [35]. After 40 min, the change of adsorption capacity for F was not significant. As a consequence, 40 min was considered as the adsorption equilibrium time in the following experiments. In this study, the experimental data, obtained from the studies of contact time at 30 °C was analyzed by two different adsorption kinetic models such as the pseudo-first-order model [36] and pseudo-second-order model [37].
Fig. 7. Effect of contact time on adsorption capacity of Zr-DMCB for F.
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B. Liu et al. / Chemical Engineering Journal 228 (2013) 224–231 Table 1 The values of kinetic parameters for pseudo-first-order equation and pseudo-second-order equation together with correlation coefficients. Adsorbent
Pseudo-first-order model
Pseudo-second-order model 1
R21
Regression equation
K2 (g/mg h)
qe (mg/g)
R22
t qt t qt
¼ 0:1851t þ 1:7603
0.0195
5.40
0.995
¼ 0:5141t þ 18:692
0.0141
1.95
0.973
Regression equation
q1,cal (mg/g)
K1 (min
Zr-DMCB
log(q1qt) = 0.993–0.064t
9.84
0.1474
0.969
DMCB
log(q1qt) = 0.015–0.019t
1.04
0.0438
0.937
)
The linear form of pseudo-first-order kinetic model is given as:
logðq1 qt Þ ¼ log q1
k1 t 2:303
ð2Þ
where q1 and qt are the adsorbed F amounts per unit mass of adsorbent at equilibrium and at any time t (mg/g), and k1 is the pseudo-first-order adsorption kinetic constant (min1). The calculated values k1, q1 and correlation coefficients (R21 ) of typical pseudo-first-order model at 30 °C are listed in Table 1. The adsorption kinetics of F onto Zr-DMCB was analyzed on the basis of pseudo-second-order kinetic model, which is expressed as Eq. (3):
t 1 t ¼ þ qt K 2 q2e qe
ð3Þ
where t is the contact time (h), qt and qe are the amount of F adsorbed at an arbitrary time t and at equilibrium (mg/g), respectively, and K2 is the rate constant (h g/mg).
From the data of Fig. 7, plots of t/qt versus t for the adsorption of F are obtained, as shown in Fig. 8. The pseudo-first-order and pseudo-second-order kinetic model rate constants for adsorption of F were summarized in Table 1. From Table 1, the calculated R21 from pseudo-first-order kinetic model is much lower than that of pseudo-second-order kinetic model. This result indicated that the adsorption of F onto Zr-DMCB was better described by pseudo-second-order kinetic model than pseudo-first-order kinetic model. The pseudo-second-order kinetic model assumed that chemical adsorption may be the rate-limiting step [38]. 3.7. Effect of concentration on F adsorption The effect of the initial F concentration on the adsorption capacity of Zr-DMCB was studied by contacting 0.1 g of Zr-DMCB with 25 mL F aqueous solution at 30 °C and initial solution pH 7.0 using a range of initial F concentration (2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 mg/L). The results are shown in Fig. 9. It is clear that the adsorption capacity of Zr-DMCB for F increased with increasing the initial F concentration in the aqueous solution. On one hand, this is because of more mass of F is put into the system with increasing the initial F concentration in the aqueous solution, however, the mass of adsorbent is constant. On the other hand, because of the higher mobility of F in the diluted solutions, the interaction of F with Zr-DMCB also increased [39]. This result was consistent with the result reported by Han et al. in their study on adsorption of F from aqueous solution [40]. The adsorption isotherm is the most important information, which indicates how the adsorbent molecules distribute between the liquid and the solid phase when the adsorption process reaches an equilibrium state [41]. The Langmuir adsorption isotherm model and Freundlich adsorption isotherm model are often used to describe equilibrium adsorption isotherms, which are based on several assumptions, including homogeneous surface, localized adsorption on the surface and solo molecule accommodated active sites. The Langmuir isotherm equation may be written as Eq. (4) [42]:
Fig. 8. Plots for pseudo-second-order kinetic model.
Fig. 9. Effect of F concentration on adsorption capacity of Zr-DMCB for F.
Fig. 10. Langmuir adsorption isotherm.
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Table 2 Langmuir isotherm and Freundlich isotherm parameters for the adsorption of F onto Zr-DMCB. Temperature (°C)
20 30 40
Langmuir isotherm model
Freundlich isotherm model 2
qs (mg/g)
Kb (L/mg)
R
RL
KF ((mg/g) (mg/L)1/n)
1/n
R2
7.2674 7.7821 7.5131
0.2484 0.3712 0.2628
0.9929 0.9902 0.9971
0.6681 0.5739 0.6555
2.0370 4.6238 2.3878
0.724 0.719 0.737
0.894 0.889 0.965
Ce 1 Ce ¼ þ qe K b qs qs
ð4Þ
where qe is the adsorption capacity of the beads (mg/g) at equilibrium, Ce is the concentration of F in the equilibrium solution (mg/L) and qs is the Langmuir constant, which is equal to the monolayer adsorption capacity (mg/g). The parameter Kb is the Langmuir sorption equilibrium constant (L/mg) related to the free energy of adsorption. The Freundlich isotherm equation may be written as Eq. (5) [43]:
ln qe ¼ ln K F þ
1 ln C e n
ð5Þ
where qe is the adsorbed F amount per unit mass of adsorbent at equilibrium (mg/g); KF is the Freundlich constant ((mg/g) (mg/L)1/n), n is the Freundlich constant related to adsorption intensity (dimensionless). The Langmuir isotherm model for adsorption of F onto Zr-DMCB is presented in Fig. 10. The straight lines of plots indicated that the adsorption processes of all cases could be well described by Langmuir isotherm model. The parameters and correlation coefficients calculated from corresponding models were given in Table 2, which indicated that Langmuir isotherm model was more suitable for assessing the adsorption of F onto Zr-DMCB and the adsorption of Zr-DMCB was a complex process containing physical adsorption and chemical adsorption. The adsorption process of F onto Zr-DMCB could be considered as monolayer adsorption. The essential characteristics of Langmuir isotherm model can be expressed in terms of dimensionless constant separation factor RL, which is used to predict if an adsorption system is ‘unfavorable’, ‘favorable’, ‘irreversible’ or ‘linear’. The separation factor RL is defined as Eq. (6) [44]:
RL ¼
1 1 þ K bC0
ð6Þ
where RL indicates the favorability and the capacity of the adsorbent/adsorbate system, C0 is the initial F concentration (mg/L) and Kb is the Langmuir adsorption equilibrium constant (L/mmol).
The RL value classified as RL > 1, 0 < RL < 1, RL = 0 and RL = 1 suggested that the adsorption was unfavorable, favorable, irreversible and linear, respectively [45]. The values of RL for sorption of Zr-DMCB for F at 20, 30, 40 °C are all between 0 and 1, which indicated that the uptake of F onto Zr-DMCB was favorable. The mechanisms of ion adsorption on porous adsorbents may involve three steps: (1) diffusion of F to the external surface of Zr-DMCB; (2) diffusion of F into the pores of Zr-DMCB and (3) adsorption of F on the internal surface of Zr-DMCB.
3.8. Desorption and reuse studies When considering the cost of the sorbent, it is necessary to consider the efficiency as well as the preservation of adsorption capacity of the biomass [46]. In our work, several solvents/solutions were tried in order to regenerate the biosorbent. Among them, 0.5 mol/L NaOH was proved to be a relatively effective eluent for desorption of F from Zr-DMCB. The reusability of Zr-DMCB for F adsorption was shown in Fig. 11. It was revealed that the adsorption capacity of Zr-DMCB for F decreased slightly from 4.56 to 4.03 mg/g with increasing the times of reuse. Zr-DMCB could be reused for at least 5 times with about 11.6% regeneration loss, which indicated that Zr-DMCB had a good reusability in the application.
4. Conclusions In this study, the adsorption of Zr-DMCB for F from aqueous solution was examined. The –NH2, –SH, –S = C– groups and Zr(IV) functional groups of Zr-DMCB were involved in the complexation with F; The surface of Zr-DMCB particles was spherical, smooth, marked with holes and tiny interspaced structures; Zr(IV) was involved in Zr-DMCB, and F element was also involved in Zr-DMCB after adsorption with F in aqueous solution. The adsorption capacity of Zr-DMCB for F was dependent on initial pH and temperature. The saturation adsorption capacity of Zr-DMCB was obtained at pH 7.0, 30 °C for 40 min. The adsorption process followed Langmuir adsorption isotherm model for over all concentration range studied and the maximum adsorption capacity of Zr-DMCB for F was 7.78 mg/g. The adsorption of F onto Zr-DMCB followed pseudo-second-order kinetic model. The adsorbent Zr-DMCB can be used effectively for at least 5 times of reuse cycles with only 11.6% loss.
Acknowledgements
Fig. 11. Relationship between times of reuse and adsorption capacity.
This work was financially supported by the Ocean Public Welfare Scientific Research Special Appropriation Project (201005020-6), National Science and Technology Plan Project of China (2012AA101607-2), Fundamental Research Funds for the Central Universities and Program for Changjiang Scholars and Innovative Research Team in University (IRT 1188). We are grateful to thank the editors and anonymous reviewers for their positive comments and suggestions to the manuscript.
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