Adsorption of cesium on copper hexacyanoferrate–PAN composite ion exchanger from aqueous solution

Chemical Engineering Journal 172 (2011) 572–580

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Adsorption of cesium on copper hexacyanoferrate–PAN composite ion exchanger from aqueous solution A. Nilchi a,∗ , R. Saberi a , M. Moradi b , H. Azizpour b , R. Zarghami b a b

NSTRI, P.O. Box 11365-8486, Tehran, Iran Multiphase Systems Research Lab., Oil and Gas Centre of Excellence, School of Chemical Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 23 March 2011 Received in revised form 7 June 2011 Accepted 8 June 2011 Keywords: Cesium Ion exchanger Adsorption Copper hexacyanoferrate Polyacrylonitrile

a b s t r a c t In this study, copper hexacyanoferrate–polyacrylonitrile composite (CHCF–PAN) was prepared and used as an ion exchanger for the separation of cesium from aqueous solution. Various characterization methods including XRD, FT-IR, TG–DSC, SEM, BET and XRF were utilized for the evaluation of the synthesized ion exchanger. In order to obtain the optimum conditions for the adsorption, the influence of pH of initial aqueous solution, contact time, solution temperature and presence of the interfering cations on the distribution coefficient of cesium onto CHCF–PAN sorbent were studied. Furthermore, adsorption thermodynamic parameters namely the standard enthalpy, entropy, and Gibbs free energy were calculated and it was found that the ion exchange reaction is an endothermic and spontaneous process. Langmuir, Freundlich, Dubinin–Radushkevich (D–R) and Temkin isotherm models were fitted to the obtained experimental sorption data and it was observed that the sorption of cesium on the synthesized sorbent was better represented by the Freundlich model. Finally, in order to calculate the dynamic adsorption capacity of the synthesized ion exchanger, the column experiments were also investigated. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The removal of pollutants from industrial wastewaters has recently become one of the most important processes because of which its importance is becoming more profound with increasing industrial activities. Cesium is one of these pollutants that its separation from aqueous solution is mostly needed. The amount control of cesium isotopes, particularly 137 Cs and 135 Cs, in liquid wastes has become an issue of great concern because of their destructive effects on the environment. They are potentially dangerous to human health and also to the environment, because the high solubility of cesium can cause its migration through groundwater to the biosphere. Furthermore, they can be easily incorporated in terrestrial and aquatic organisms because they are chemically similar to potassium [1]. In case of exposure to ingestion route, Cs metal is confidently adsorbed to the body and can be easily distributed throughout the soft tissues of body. Thyroid cancer is one of the terrible consequences of 137 Cs adsorption via the contaminated food and water [2]. There are a lot of methods such as precipitation, liquid–liquid extraction, ion exchange, and chromatography, which can be used for wastewater treatment. Among these methods, ion exchange

∗ Corresponding author. Tel.: +98 21 88221128; fax: +98 21 82062539. E-mail address: [email protected] (A. Nilchi). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.06.011

process has recently attracted a lot of attentions for removal of cesium from waste streams because of its convenience, efficiency and selectivity. Use of various materials has been previously reported for this technique [3–7]. It was found that among organic and inorganic ion exchangers, the inorganic type has several superior qualities required for the treatment of waste streams compared to organic resins. These are their higher thermal stability and good compatibility with the final waste forms. Several inorganic ion exchangers such as zeolites, sodium titanates, silicotitanates, metal oxides, and hexacyanoferrates are in use for the treatment of wastes. However, the slow mass-transfer rate in column operation has been the impeding factor for extensive applications [8]. For solving this problem, composite adsorbents/ion exchangers have been widely studied for treatment of liquid wastes. The composite ion exchangers present improved qualities with respect to those of pure inorganic exchangers, such as; better selectivity for the capture of some ions, increased mechanical and chemical resistance, more regular form of the grains, smaller solubility in water than the respective inorganic compound, and better kinetics of exchange relatively to the pure inorganic exchangers. The composite ion exchangers are generally obtained by implantation of inorganic into the wide range of organic materials during the polymerization process [9]. In composite ion exchangers, inorganic materials are active components and organic materials are simply inert binders.

A. Nilchi et al. / Chemical Engineering Journal 172 (2011) 572–580

Among hexacyanoferrate (II) complexes of transition metals as the active components, which are very useful for the removal of Cs from water streams, copper (II) hexacyanoferrate (II) is often selected as the agent in practical analysis because it can be readily prepared in a granular or powdery form while most of the other hexacyanoferrates are usually obtained in a colloidal or gelatinous form and are better soluble in water. Some researchers have used copper hexacyanoferrate impregnated to various materials for the separation of cesium from aqueous solutions. Milonji et al. [10] used copper hexacyanoferrate/polymer/silica composites for the cesium sorption from aqueous solution in batch and dynamic conditions. Koichi [11] prepared copper hexacyanoferrate/amberlite XAD-7 composite ion exchanger using a trioctylalkylammonium chloride as a mediating agent for the cesium separation from waste solutions. As noted above, copper hexacyanoferrate supported on various materials has been previously prepared for the cesium sorption process but this is almost for the first time that polyacrylonitrile (PAN) is used as a matrix material for copper hexacyanoferrate (CHCF) ion exchanger in order to separate cesium from liquid wastes. Polyacrylonitrile has been proposed as a universal binding polymer for practically any inorganic ion exchanger. The use of PAN-based organic binding polymer has a number of advantages provided by the relatively easy modification of its physico-chemical properties (hydrophilicity, porosity, mechanical strength). The ion exchange kinetics and sorption capacity of such composite ion exchangers are not influenced by the PAN binding polymer. The contents of the active component in the composite exchanger can be varied over a broad range, depending on the planned application [12]. Several authors published papers on the use of PAN as a binding polymer for different inorganic cation exchangers [13–16]. Sodium titanosilicate [8], zeolite [9], TiO, MgO and ZrO [12] were also supported on PAN and used successfully for removal of cesium from different aqueous solutions. In this study, the synthesized copper(II) hexacyanoferrate(II)–PAN (CHCF–PAN) composite adsorbent is used for the removal of cesium from aqueous solution and many factors such as adsorption kinetics, isotherm models, as well as the influence of pH, interfering cations, and temperature on adsorption are investigated. In order to specify the type of ion exchange reaction, thermodynamic parameters such as the standard enthalpy, entropy, and Gibbs free energy were also determined. In addition, the columnar studies were carried out to obtain the breakthrough curve of cesium on the synthesized adsorbent. Finally, in order to regenerate the used adsorbent, desorption studies were carried out. 2. Experimental 2.1. Reagents and apparatus All the reagents used in the present study were of analytical grade. Potassium hexacyanoferrate (II), copper nitrate, dimethylsulfoxide (DMSO) were prepared from Merck and polyacrylonitrile was obtained from Aldrich. Stock solution was prepared by dissolving CsCl in distilled water. Various apparatuses were used to characterize the synthesized CHCF–PAN ion exchanger. An 1800 PW Philips diffractometer with Cu K␣ beam was applied for determining the X-ray diffractometry of the adsorbent. The X-ray source was a rotating anode operating at 40 kV and 30 mA with a copper target and the data were collected between 5◦ and 70◦ in 2; scanning electron microscope (SEM) imaging was performed by means of a Philips XL30; the infrared spectra was recorded using a Brucker-Vector 22 spectrophotometer; Brunauer–Emmett–Teller (BET) specific surface area was determined through nitrogen adsorption isotherms using Quantachrome NOVA 2200e system; thermogravimetry–differential scanning calorimetry (TGA–DSC) was carried out on heating the

573

sample up to 800 ◦ C at a heating rate of 10 ◦ C/min in the Argon atmosphere using a DuPont model 951; CHN analysis was performed using an Elementar-Vario ELIII, CHN elemental analyzer; the amounts of iron and copper were determined by the X-ray Fluorescence Unit (Oxford ED2000) and finally the pH measurement was made with a Schott pH-meter model CG841. 2.2. Preparation of CHCF–PAN composite sorbent The obtained CHCF–PAN ion exchanger was prepared in two steps. For preparation of copper hexacyanoferrate (II), 0.25 mol/L potassium hexacyanoferrate (II) solution was mixed with 0.75 mol/L copper nitrate solution with the volume ratio of 1:1 at 50 ◦ C and 500 rpm. A brown sediment was obtained after 2 days by filtering and washing with distilled water. The precipitate was dried at 70 ◦ C in an oven and then powdered by a mill and sieved to size of 224–400 ␮m. In the second step for preparation of the composite adsorbent, 5 g of copper hexacyanoferrate (II) powder was mixed with 20 mL of dimethylsulfoxide in a three-necked flask and stirred at 50 ◦ C for 2 h. Then, a mixture of 5 g of polyacrylonitrile, 55 mL of dimethylsulfoxide, and a few drops of Tween-80 as a surfactant for increasing the solving rate of PAN in DMSO solvent was stirred for 3 h and then added to the previously obtained solution. A homogeneous solution of composite dope was finally prepared by stirring this solution in the three-necked flask at 50 ◦ C for 3 h. The dissolved air in the composite dope was removed by vacuum pump, and the air-free composite dope was passed through inside a dual nozzle while the compressed air was ejected through the outside annulus of the dual nozzle to adjust the size of the composite granules. To synthesize spherical and homogeneous composite granules, the composite dope and distilled water were continuously stirred by magnetic stirrers in different vessels. A little amount of a surfactant was added to the distilled water for decreasing the water adhesion to obtain spherical granules. The composite granules were ejected through the dual nozzle and then dropped in distilled water, which was used as a gelation agent. Finally, the granules were washed using demineralized water and dried at 50 ◦ C for 2 days. 2.3. Cesium ion exchange experiments Preliminary batch experiments with the prepared CHCF–PAN composite adsorbent and 10−4 mol/L initial cesium ion concentration were conducted to investigate the effects of solution pH, temperature, contact time between the adsorbent and the aqueous solution, and interfering cations on the ion exchange process. NaOH and HCl solutions were used to adjust the pH values, monitored with a digital pH meter. All experiments were carried out at 25 ◦ C except for the temperature dependence studies where temperature was varied from 25 to 65 ◦ C. Concentration of Cs+ ions in the aqueous solution was determined by atomic absorption spectrophotometer (PerkinElmer, Model 432, FRG). The distribution coefficient (Kd ) of cesium on CHCF–PAN composite adsorbent was measured as a function of contact time, from 5 to 120 min time intervals. At the end of each time interval, the solution was filtered for phase separation and the concentration of supernatant solutions was measured. Then distribution coefficient (Kd ) of adsorbed cesium on the composite adsorbent was calculated using the following equation: Kd =

C0 − Ce V × (mL/g) Ce m

(1)

where C0 and Ce are the initial and final Cs+ ion concentration in the aliquots, respectively. V (mL) is the solution volume and m (g) is the mass of sorbent. For evaluating the influence of Na+ , K+ , Ca2+

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(200)

1 0.9

479 589 649

873

0.4 485

1059

1246 1340

594

1401 1451 1606

2244

2851 2919

3244

3420

3596

(a)

0.5

25

35

45

55

0.1

65

2θ (degree)

0.3 0.2

2096

15

Transmittance

0.7

1713

2096

Intensity

862

1241

3265 3421

(440)

(a)

(b)

5

0.8

0.6

(424)

(420)

(400)

(c)

1615

(220)

(b)

0 4000

3600

3200

2800

2400

2000

1600

1200

800

400

-1

Wavenumber (cm )

Fig. 1. XRD patterns of (a) PAN, (b) CHCF, and (c) CHCF–PAN composite.

Fig. 2. FT-IR spectra of (a) CHCF–PAN composite and (b) CHCF.

Mg2+

3. Results and discussions 3.1. Characterization

stretching vibration of (C≡N). Furthermore, the bending vibration of water molecules in both materials is specified by the peak at 1615 cm−1 and finally the absorption peaks at 3421 and 3265 cm−1 are attributed to the stretching vibration of interstitial water. A comparison between the IR spectrum of PAN and CHCF–PAN can be seen in Fig. 3. The sharp peaks at 2939 and 2253 cm−1 for PAN spectrum and corresponding values of 2919 and 2244 cm−1 for CHCF–PAN composite spectrum can be attributed to the saturated –CH stretch and nitrile group in both materials, respectively. Furthermore, the similar bending vibration of –CH2 group in CHCF–PAN and PAN is characterized by the sharp and strong absorption peak at 1452 cm−1 . The weak peaks in the broad region of 3000–3650 cm−1 (3239, 3516, and 3623 cm−1 for PAN and the corresponding values of 3244, 3420, and 3596 cm−1 for CHCF–PAN) are related to the stretching vibration of interstitial water. Finally, the stretching vibration of C–O in PAN and CHCF–PAN structures is observed at 1070 cm−1 .

3.1.1. X-ray diffraction (XRD) analysis By analyzing the XRD patterns of the synthesized materials (Fig. 1): (a) polyacrylonitrile, (b) CHCF, and (c) CHCF–PAN, it was observed that the XRD data of the synthesized CHCF agreed well with those previously reported [17]. It was also observed that there are no diffraction peaks in the XRD pattern of polyacrylonitrile, indicating its amorphous structure. Since the 2 values at the peak points of CHCF–PAN composite sorbent and CHCF are alike, it can be deduced that their crystalline structures are very similar. As shown in Fig. 1, the peaks at 2 values of 17.7◦ , 25.1◦ , 35.9◦ , 40.4◦ , 44.5◦ , and 51.1◦ are attributed to the Miller indexes of (2 0 0), (2 2 0), (4 0 0), (4 2 0), (4 2 4), and (4 4 0) of the diffraction planes [18], respectively, indicating the crystalline structure of CHCF–PAN and CHCF materials.

759 871 605

0.9

534

0.8

0.6

1452

2939

2253

1355

0.7

1071

1249

1623

3239

1723

2868

(b) 3516 3623

0.5 649

873

0.4 485

594

1401 1451

1606

1059

1246 1340

1713

3244

3420

2244

2851 2919

(a) 3596

0.3 0.2

2096

3.1.2. Fourier transformed infrared (FT-IR) spectrometry IR spectra of the synthesized CHCF–PAN composite and CHCF are shown in Fig. 2. It can be concluded from the comparison of these spectra that there are some similar absorption peaks in both spectra. The presence of carbon-metal bond in the structure of CHCF–PAN and CHCF is determined by the absorption peaks (479 and 589 cm−1 ) in the region of 450–600 cm−1 . The sharp and strong peaks at 2096 cm−1 in CHCF and CHCF–PAN spectra are due to the

1

0.1 0

4000

3600

3200

2800

2400

2000

1600

1200

800

-1

Wavenumber (cm ) Fig. 3. FT-IR spectra of (a) CHCF–PAN composite and (b) PAN.

400

Transmittance

and cations on the distribution coefficient (Kd ) of cesium, the same experimental conditions were applied. In order to conduct the adsorption isotherm experiments, 0.1 g of the adsorbent was mixed with 10 mL samples of different initial concentration of Cs+ solution and then the resulting mixture was shaken at the temperature of 25 ◦ C and pH 9.0 (±0.1) in order to achieve an equilibrium. The application of the prepared composite sorbent was evaluated in continuous column systems after obtaining the optimum conditions for adsorption of Cs+ ions on this ion exchanger sorbent in batch process. Breakthrough studies were also carried out by passing the aqueous solution into an ion exchange column with radius of 0.8 cm and length of 60 cm, packed with 0.2 g of the sorbent. They were passed into the column at the flow rate of 0.66 mL/min. At various time intervals, the effluent was collected and Cs+ ion concentration of the aliquot was measured.

A. Nilchi et al. / Chemical Engineering Journal 172 (2011) 572–580

575

Fig. 5. Scanning electron microscopic (SEM) photograph of CHCF.

Fig. 4. Scanning electron microscopic (SEM) photographs of CHCF–PAN composite adsorbent cross-section.

3.1.4. Surface area measurements In order to measure the specific surface area of the composite, which is a sign of adsorption efficiency of the sorbent, the nitrogen adsorption–desorption curve was obtained and the BET surface area of copper hexacyanoferrate–polyacrylonitrile composite was found to be 73.58 m2 /g. 3.1.5. Thermogravimetry–differential scanning calorimetry In order to study the stability of the synthesized composite against thermal conditions, the TGA–DSC thermal analysis of CHCF–PAN composite was carried out in the temperature range of 25 and 800 ◦ C (Fig. 7). It was found from this analysis that the weight reduction up to 200 ◦ C is due to elimination of free and interstitial water molecules and the structural collapse of PAN contained in the composite is occurred from 200 to 480 ◦ C. The last step of CHCF–PAN decomposition results in releasing of hydrogen cyanide and ammonia gases from the ion exchanger structure. As it can be

Fig. 6. Scanning electron microscopic (SEM) photograph of PAN.

observed from these results, the prepared composite is stable up to 200 ◦ C. 3.1.6. X-ray fluorescence spectroscopy (XRF) and CHN elemental analysis A CHN elemental analyzer was used to measure the amounts of carbon, hydrogen, and nitrogen existed in the composite and the amount of copper and iron metals was also determined by an XRF spectroscopy unit. The obtained results are shown in Table 1. 90 80 Weight loss %

3.1.3. Scanning electron microscopy The morphological structure of the prepared CHCF–PAN composite was obtained by using scanning electron microscopy (Fig. 4). SEM photographs were obtained by accelerating electrons within a range of 1–30 KeV. It was found from the SEM images of the composite that the internal structure of the composite granules (like PAN granules) is very porous. In addition, the pores size of the granules in their inner part was larger than that near the surface. The even dispersion of the active material, CHCF, throughout the binding matrix of the composite is a sign of being an effective ion exchanger for cesium adsorption from aqueous solution. The porous structure of PAN and CHCF can also be seen from their SEM images (Figs. 5 and 6).

70 60 50 40 30 20 10 0 0

100

200

300

400

500

600

700

0

Temperature ( C) Fig. 7. TGA–DSC curve of the synthesized composite.

800

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A. Nilchi et al. / Chemical Engineering Journal 172 (2011) 572–580

Table 1 Analytical results of the synthesized composite elements. Weight (%)

Fe Cu C H N

5.10 5.80 39.06 4.42 19.80

1600

Kd (mL/g)

Element

2000

800

3.2. Kinetic studies 3.2.1. Effect of contact time In order to determine the influence of contact time between CHCF–PAN ion exchanger and aqueous solution on cesium adsorption, variations of distribution coefficient (Kd ) of cesium versus time were plotted, as it can be seen in Fig. 8. It was observed that the adsorption of Cs+ ions from aqueous solution using the composite adsorbent is continuously increased with time increase until reaching equilibrium between two phases after 280 min. Therefore, this obtained equilibrium time was selected for the next adsorption experiments. 3.2.2. Effect of pH It was found that pH is a determining factor in the adsorption of Cs+ ions on CHCF–PAN composite ion exchanger. To determine the optimal condition at which cesium ions are effectively adsorbed on the prepared adsorbent, the adsorption experiments were carried out at different pH values ranging from 1.0 to 9.0. As it can be seen in Fig. 9, the adsorption process is continuously improved by increasing pH value from acidic to alkali medium. This may be attributed to the completion behavior between hydrogen and cesium ions for adsorption on the synthesized composite ion exchanger. It was also observed that the maximum amount of distribution coefficient of cesium was obtained at pH 9.0, and thus all sorption experiments in the present study were carried out at initial pH 9.0. 3.2.3. Effect of interfering ions The influence of interfering cations (Na+ , K+ , Ca2+ and Mg2+ ) on the adsorption of Cs+ cation by the ion exchanger were investigated in the presence of 10−4 mol/L of the interfering cations in the form of their nitrate, at 298 K. As shown in Table 2, the distribution coefficient of cesium on the prepared CHCF–PAN composite was substantially decreased in the existence of interfering ions. The obvious influences of Na+ and K+ ions on the adsorption of Cs+ can

2000

1600

K d (mL/g)

1200

1200

400 0 0

1

2

3

4

5

6

7

8

9

10

pH Fig. 9. Variation of distribution coefficient of cesium with pH of the solution.

Table 2 The effect of interfering cations in the solution on the distribution coefficient of cesium ions on CHCF–PAN. Interfering cation

Kd (mL/g)

None Na+ K+ Ca2+ Mg2+

1673 229 167 535 319

be explained as to their similar chemical characteristics which exist among K, Na and Cs, due to being in the same group of the periodic table. Meanwhile, the influences of Ca2+ and Mg2+ cations on the adsorption might be involved in other process except ion exchange, such as complexation or nonspecific surface adsorption [19]. 3.2.4. Effect of temperature In order to investigate the effect of other parameters on the adsorption of cesium, the influence of solution temperature on the distribution coefficient of cesium was examined, while other effective parameters were kept constant (contact time of 280 min, 10−4 mol/L Cs+ concentration, and pH 9.0). The results observed is shown in Table 3 which indicate that the distribution coefficient of cesium is considerably increased by enhancing the solution temperature from 298 to 338 K in steps of 10 K. This enhancing of adsorption can be attributed to the fact that at higher temperatures, cations move faster. This could be due to the fact that the specific or electrostatic interactions become weaker and the ions become smaller, since solvation is reduced [20]. Based on this explanation, it is obvious that the adsorption of Cs+ ions on the adsorbent is an endothermic process. 3.2.5. Adsorption thermodynamic parameters In order to obtain the thermodynamic nature of the sorption process, several adsorption thermodynamic parameters including standard enthalpy (H◦ ), standard entropy (S◦ ), and standard Gibbs free energy (G◦ ) were determined. The amounts of H◦ and S◦ were calculated from the slope and intercept of the straight line

800

400

0 0

100

200

300

400

500

Time (min) Fig. 8. Variation of distribution coefficient of cesium with contact time.

Table 3 The effect of solution temperature on the distribution coefficient of cesium ions on CHCF–PAN. Temperature (K) Kd (mL/g)

298 1673

308 1820

318 1927

328 2056

338 2109

A. Nilchi et al. / Chemical Engineering Journal 172 (2011) 572–580

Table 5 The parameters of Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Temkin isotherms for CHCF–PAN composite at 298 K.

7.7 7.65

Isotherms Langmuir

Q◦ (mmol/g) b (L/mmol) R2

0.192 0.044 0.991

Freundlich

KF (mmol/g) nf R2

0.084 1.16 0.998

Dubinin–Radushkevich (D–R)

qmax (mmol/g)  K (mol2 /KJ2 ) E (KJ/mol) R2

0.462 0.64 0.884 0.885

Temkin

b (KJ/mol) A (dm3 /g) R2

ln Kd

7.6 7.55 7.5 7.45 7.4 0.0029

0.003

0.0031

0.0032

0.0033

0.0034

-1

1/T (k ) Fig. 10. The effect of solution temperature on the distribution coefficient of cesium ions on CHCF–PAN composite.

obtained from plotting ln Kd values versus reciprocal temperature (Fig. 10), respectively, and using the following equation [1]: ln Kd =

H ◦ S ◦ − R RT

(2)

After obtaining H◦ and S◦ values of the adsorption, G◦ of the adsorption process at each temperature was calculated from the well-known equation: G◦ = H ◦ − T S ◦

(3)

As it can be seen in Table 4, the obtained negative amounts of G◦ at different temperatures and the positive amount of H◦ reveal that the ion exchanger process is an endothermic and spontaneous sorption reaction. Furthermore, the positive value of S◦ and thus decreasing value of G◦ with increasing the temperature, indicate that cesium adsorption on the sorbent is more spontaneous at higher temperatures. 3.3. Sorption isotherms Sorption equilibrium is usually described by an isotherm equation whose parameters express the surface properties and affinity of the sorbent, at a fixed temperature and pH. An adsorption isotherm describes the relationship between the amount of adsorbate on the adsorbent and the concentration of dissolved adsorbate in the liquid at equilibrium [21]. The Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Temkin isotherms are common kinds of several isotherm equations that were tested to fit the obtained sorption data. 3.3.1. Langmuir isotherm The Langmuir adsorption model assumes that molecules are adsorbed at a fixed number of well-defined sites, each of which Table 4 Thermodynamic parameters for cesium adsorption on the synthesized CHCF–PAN. H◦ (kJ/mol) S◦ (kJ/K mol) Temp (K) 298 308 318 328 338

577

3.9248 0.07833 −G◦ (kJ/mol) 18.4176 19.2009 19.9842 20.7675 21.5508

11.210 1.2 0.962

can only hold one molecule and no trans-migration of adsorbate in the plane of the surface. These sites are also assumed to be energetically equivalent and distant to each other, so that there are no interactions between molecules adsorbed to adjacent sites. The linear form of the Langmuir isotherm is represented by the following equation [22]: Ce 1 Ce = ◦ + ◦ qe Q Q b

(4)

where Q◦ (mmol/g) is the maximum adsorption at monolayer, Ce (mmol/L) is the equilibrium concentration of Cs+ ions, qe (mmol/g) is the adsorption amount of Cs+ ions per unit weight of adsorbent at equilibrium, and b (L/mmol) is the Langmuir constant related to the affinity of binding sites, which is a measure of the energy of adsorption. With the slope and intercept of the linearized plot of Ce /qe versus Ce , Q◦ and b can be calculated. The amounts of these parameters are presented in Table 5. 3.3.2. Freundlich isotherm Freundlich isotherm is an empirical equation that encompasses the heterogeneity of sites and the exponential distribution of sites and their energies. The sorption data have been analyzed using the logarithmic form of the Freundlich isotherm as shown below [21]: ln qe = ln KF +

1 ln Ce nf

(5)

where KF and nf are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. When ln qe is plotted against ln Ce , a straight line with slope 1/nf and intercept ln KF is obtained. From these obtained values, the values of nf and KF are calculated and presented in Table 5. The experimental adsorption isotherm of the prepared composite is represented by Fig. 11. Figs. 12 and 13 are the schematic representation of the Langmuir and Freundlich isotherm models fitted for the cesium adsorption on the prepared composite, respectively. All of the required parameters for determination of these two models were calculated from this plots and summarized in Table 5. As shown in Table 5, the good fit of experimental data with Langmuir and Freundlich isotherm models and high correlation coefficient (R2 ) obtained for these plots (0.991 and 0.998 for Langmuir and Freundlich models, respectively) indicates the validity of these models to the ion exchange data. But Freundlich equation shows better results than Langmuir model because of higher correlation coefficient. 3.3.3. Dubinin–Radushkevich (D–R) isotherm Another popular isotherm model, which is more general than the Langmuir and Freundlich models, is Dubinin–Radushkevich

578

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0.7 0.6

qe (mmol/g)

0.5 0.4 0.3 0.2 0.1 0 0

2

4

6

8

10

Ce (mmol/L) Fig. 11. Adsorption isotherm of cesium on CHCF–PAN adsorbent at 298 K.

Fig. 14. Dubinin–Radushkevich (D–R) adsorption isotherm of cesium on CHCF–PAN at 298 K.

(D–R) isotherm. The non-linear and linear forms of D–R isotherm are given below, respectively [23]:

Ce/qe (g/L)

qe = qmax e−K

 ε2

(6)  2

17

ln qe = ln qmax − K ε

16

where qe is the amount of metal (mmol/g) adsorbed per unit mass of ion exchanger, qmax is the D–R sorption capacity (mmol/g), K is the constant related to the adsorption energy (mol2 /kJ2 ), and ε is the Polanyi potential that is defined as follows:

15

ε = RT ln

14 13 12 0

2

4

6

8

10

12

Ce (mmol/L) Fig. 12. Langmuir adsorption isotherm of cesium on CHCF–PAN at 298 K.

(7)

1 + 1

(8)

Ce

where Ce is the solution concentration at equilibrium (mol/L), R is the gas constant (8.314 J/mol K), and T is the absolute temperature of the aqueous solution (K). The amounts of D–R parameters (qmax and K ) can be easily derived from linear regression analysis using plotting ln qe versus ε2 (Fig. 14). The obtained parameters of D–R isotherm are presented in Table 5. The mean sorption energy E (kJ/mol), defined as the free energy change when one mole of ion is transferred to the surface of the solid from infinity in the solution, is calculated according to the following equation [23]: 1 E= √ 2K 

(9)

The value of E obtained from the slope of the D–R plot is given in Table 5. As it can be seen from Table 5, the correlation coefficient obtained for this plot (R2 = 0.885) indicates that D–R isotherm does not fit the experimental sorption data well. 3.3.4. Temkin isotherm Temkin isotherm equation, which considers the effects of the heat of adsorption that decreases linearly with coverage of the adsorbate and adsorbent interactions, has been used in the linear form as follows [24]: qe =

Fig. 13. Freundlich adsorption isotherm of cesium on CHCF–PAN at 298 K.

RT RT ln A + ln(Ce ) b b

(10)

where the values of b (KJ/mol) and A are obtained from the slope and intercept of linear plot of qe versus ln Ce (Fig. 15) and shown in Table 5. As it can be seen from Table 5, Temkin isotherm shows

Table 6 Characteristics of water sample. TDS (mg/L)

EC (␮S/Cm)

PH

K+ (mg/L)

Na+ (mg/L)

Mg2+ (mg/L)

Ca2+ (mg/L)

200

34

8.36

1.17

10.35

10.56

50.2

A. Nilchi et al. / Chemical Engineering Journal 172 (2011) 572–580

579

0.7 0.6

qe(mmol/g )

0.5 0.4 0.3 0.2 0.1 0 0

0.5

1

1.5

2

2.5

lnC e(mmol/L) Fig. 15. Temkin adsorption isotherm of cesium on CHCF–PAN at 298 K. Fig. 16. Breakthrough curve of cesium on CHCF–PAN composite.

(R2

a relatively high correlation coefficient = 0.962) for the cesium sorption system from aqueous solution. Finally, it must be noted that the applicability of all the studied isotherm models to the cesium sorption system from aqueous waste shows that both monolayer sorption and heterogeneous energetic distribution of active sites on the sorbent surface are possible [25]. From the obtained correlation coefficient (R2 ) values for these models, it was evident that the isotherm models applicability for cesium removal using the prepared CHCF–PAN composite follows from the order: Frendlich > Langmuir >Temkin > Dubinin–Radushkevich (D–R) isotherm. 3.4. Column studies Batch experimental data are often difficult to apply directly to the fixed bed sorption column because isotherms are unable to give accurate data for scale up since a flow in the column is not at equilibrium. In order to study the dynamic behavior of the sorption of Cs+ ions from aqueous solution by the prepared ion exchanger, fixed bed column sorption experiments were carried out. The fixed bed column operation allows more efficient utilization of the sorptive capacity than the batch process. The shape of the breakthrough curve and the time for the breakthrough appearance are the predominant factors for determining the operation and the dynamic response of the sorption column. The general position of the breakthrough curve along the volume/time axis depends on the capacity of the column with respect to bed height, the feed concentration and flow rate [26]. In dynamic experiments, 0.2 g of CHCF–PAN adsorbent was placed in a glass tube with 0.8 cm diameter and 60 cm length. Then 200 mL of the aqueous solution containing Cs+ ions together with tracer of 50 ppm concentration was passed through the tube. The flow rate of effluent was adjusted at 0.66 mL/min by a peristaltic pump. Cesium breakthrough curve was obtained by plotting the percent of breakthrough C/C0 × 100 versus the number of bed volumes (BV), where C0 is the initial Cs concentration and C is the Cs concentration in the column effluent. Fig. 16 shows cesium breakthrough curve on CHCF–PAN composite ion exchanger. Aqueous solutions were made using water samples taken from river near capital city of Iran, Tehran. Table 6 shows the analytical results of the water sample. It is noteworthy that this river is seasonal, and during different seasons has little or no water. To obtain an estimate of cesium dynamic adsorption capacity, a second order kinetic equation was fitted to the data sets to acquire a relationship for Cs effluent concentration as a function of throughput volume. This equation for Cs concentration was then substituted into the following relationship for dynamic capacity (DC) [8,27]:



DC =

0

(C0 − C)d M

(11)

Table 7 Adsorption dynamic capacity in columnar experiment on CHCF–PAN adsorbent. Flow rate (bed volume/h)

Dynamic capacity at 5% breakthrough (practical)

Dynamic capacity at 100% breakthrough (total)

Efficiency of adsorbent column (%)

15

7.31 mg Cs/g CHCF–PAN

11.46 mg Cs/g CHCF–PAN

63.78

where  is volume at specified breakthrough, and M is mass of CHCF–PAN sorbent (g dry). Dynamic capacity estimates of the prepared adsorbent were then obtained by evaluating the integral numerically with upper-limit volume values corresponding to approximately 5% and 100% Cs breakthrough. These values at a flow rate of 15 BV/h were found to be 7.31 and 11.46 mg Cs/g CHCF–PAN, respectively (Table 7). As it can be seen in Table 7, the efficiency of adsorption column (E) [8], which is determined by the following formula, was obtained to be 63.78% E=

area 1 × 100 area 1 + area 2

(12)

where area 1 is the area above the curve from C/C0 = 0% till C/C0 = 5%, and area 2 is that from C/C0 = 5% to C/C0 = 100%. 3.5. Desorption of cesium ions The desorption process was carried out by passing 200 mL of 6 mol/L nitric acid solution as an eluent through a glass column including 0.2 g of the sorbent which was loaded by an aqueous solution with 10−4 mol/L concentration of Cs+ ions. The eluent was pumped through the desorption column by a peristaltic pump at a flow rate of 0.66 mL/min at 25 ◦ C. Fig. 17 shows the elution curve

Fig. 17. Desorption curve of cesium from CHCF–PAN composite with 6 mol/L nitric acid solution.

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A. Nilchi et al. / Chemical Engineering Journal 172 (2011) 572–580

of cesium from CHCF–PAN composite. As it can be seen, about 45% of the adsorbed Cs was desorbed by 30 mL of the eluent. 4. Conclusion As the studies showed, the synthesized composite adsorbent had porous crystalline structure with a surface area of 73.58 m2 /g and was thermally stable up to 200 ◦ C. It was found that the adsorption of cesium on CHCF–PAN sorbent was considerably depended on solution pH, contact time, solution temperature and as these important variables increase, the distribution coefficient of cesium on the sorbent increases. On the other hand, presence of interfering cations has a hindrance effect on the separation of cesium from aqueous solution. The Freundlich model showed a better correspondence with the obtained equilibrium data than the other sorption isotherms studied (Langmuir, Dubinin–Radushkevich (D–R) and Temkin isotherm) over the entire concentration range studied. Based on the values obtained from some adsorption thermodynamic parameters such as G◦ , H◦ and S◦ , it was found that cesium adsorption on CHCF–PAN ion exchanger is an endothermic and spontaneous ion exchange reaction. Beside the adsorption efficiency of CHCF–PAN composite in batch adsorption systems, it was successfully used for the separation of cesium from aqueous solution in fixed bed columns. The calculated dynamic capacities for cesium were 7.31 and 11.46 mg Cs/g sorbent, for 15 bed volumes per hour flow at 5 and 100% Cs breakthrough, respectively. In addition, the efficiency of adsorption column was 63.78%. Finally, the cesium desorption from the adsorbent was successfully carried out by nitric acid solution. References [1] R. Cortés-Martínez, M.T. Olguín, M. Solache-Ríos, Cesium sorption by clinoptilolite-rich tuffs in batch and fixed-bed systems, Desalination 258 (2010) 164–170. [2] T. Sangvanich, V. Sukwarotwat, R.J. Wiacek, R.M. Grudzien, G.E. Fryxell, R.S. Addleman, C. Timchalk, W. Yantasee, Selective capture of cesium and thallium from natural waters and simulated wastes with copper ferrocyanide functionalized mesoporous silica, J. Hazard. Mat. 182 (2010) 225–231. [3] M.R. El-Naggar, A.M. El-Kamash, M.I. El-Dessouky, A.K. Ghonaim, Two-step method for preparation of NaA–X zeolite blend from fly ash for removal of cesium ions, J. Hazard. Mater. 154 (2008) 963–972. [4] Y. Park, Y.C. Lee, W.S. Shina, S.J. Choi, Removal of cobalt, strontium and cesium from radioactive laundry wastewater, J. Chem. Eng. 162 (2010) 685–695. [5] E. Bascetin, H. Haznedaroglu, A.Y. Erkol, The adsorption behavior of cesium on silica gel, Appl. Radiat. Isot. 59 (2003) 5–9. [6] G. Gürbo˘ga, H. Tel, Y. Altas, Sorption studies of cesium on TiO2 –SiO2 mixed gel spheres, Sep. Purif. Technol. 47 (2006) 96–104. [7] K.C. Song, H.K. Lee, H. Moon, K.J. Lee, Simultaneous removal of the radiotoxic nuclides 137 Cs and 129 I from aqueous solution, Sep. Purif. Technol. 12 (1997) 215–227.

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