Enhancement of the anionic dye adsorption capacity of clinoptilolite by Fe3+-grafting

Journal of Hazardous Materials 267 (2014) 1–8

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Enhancement of the anionic dye adsorption capacity of clinoptilolite by Fe3+ -grafting Murat Akgül ∗ Department of Chemistry, Hacettepe University, Beytepe, 06800 Ankara, Turkey

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Iron-grafted clinoptilolite was found to have higher adsorption capacity than that of raw clinoptilolite. • Iron-grafted clinoptilolite can be used as an efficient adsorbent for the removal of congo red at near-neutral pH. • pH dependency of adsorption process is different for raw and modified clinoptilolites.

a r t i c l e

i n f o

Article history: Received 19 September 2013 Accepted 13 December 2013 Available online 27 December 2013 Keywords: Clinoptilolite Dye Adsorption Zeolite Modification

a b s t r a c t In this paper, a batch system was applied to study the adsorption behavior of congo red (CR) on raw and modified clinoptilolites. Raw clinoptilolite (Raw-CL) was treated with Fe(NO3 )3 in ethanol to obtain its iron-grafted form (Fe-CL). Adsorbents were characterized by X-ray diffraction (XRD), Fourier transforminfrared (FT-IR), energy dispersive X-ray spectroscopy (EDX), thermogravimetric/differential thermal analysis (TG/DTA), zeta-potential measurement and N2 gas adsorption–desorption techniques. Effects of the experimental parameters (initial pH, dye concentration, temperature and adsorption time) were investigated to find optimum conditions that result in highest adsorption capacity for CR removal. The obtained results suggest that the solution pH appears to be a key factor of the CR adsorption process. The maximum dye adsorption was achieved with Fe-CL adsorbent at pH ∼6.3 and the corresponding adsorption capacity was found to be 36.7 mg/g, which is higher than that of its raw counterpart (16.9 mg/g). A significant decrease in CR removal was given by Fe-CL between pH 7 and 11 opposite to Raw-CL which has nearly constant qe in the same pH range. The Fe3+ -grafting increased the zeta potential of raw clinoptilolite, leading to a higher adsorption capacity compared to that of unfunctionalized adsorbent. Also, temperature change was found to have a significant effect on the adsorption process. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Azo dyes comprise approximately one-half of all textile dyes used today, and a significant amount of the total world production of these dyes is estimated to be released into the environment without proper treatment. The effluent waters discharged from different industries such as textile, leather tanning, paper, and plastics are

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usually polluted by dyes [1]. The presence of dyes in water reduces light penetration which can in turn modify the photosynthetic activity. Also, many dyes or their metabolites have toxic effects; carcinogenic, mutagenic and teratogenic effects on aquatic life and humans [2,3]. Hence, several physical, chemical, and biological methods, such as membrane separation, coagulation, adsorption, chemical oxidation, membrane separation and aerobic or anaerobic treatment, have been developed to remove dyes and other colored contaminants from wastewaters [4]. Among the treatment options applied, adsorption is one of the most effective treatment methods. The removal of colorants from industrial wastewaters is an important application of the adsorption process using suitable

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M. Akgül / Journal of Hazardous Materials 267 (2014) 1–8

adsorbents. Many different types of adsorbents such as activated carbon [5], zeolites [6], biomass [7] and clays [8] are effective in removing color from aqueous effluents and the most commonly used one is activated carbon. But it is rather expensive to run a system including activated carbon as adsorbent [9]. Therefore, many researchers have focused on the low-cost natural substances and made attempts to develop facile and economical methods for the removal of dyes [1,10,11]. In recent years, natural zeolites have been widely used to control the pollution due to their ion-exchange capability, low price and availability around the world in many deposites. These properties make zeolites particularly useful for wastewater treatment [12,13]. The natural zeolite, clinoptilolite is one of the world’s most abundantly occurring and used zeolitic mineral which is a member of the heulandite group of natural zeolites. Its three-dimensional crystal structure contains two-dimensional channels which embody some ion exchangeable cations such as Na+ , K+ , Ca2+ and Mg2+ . These cations can be exchanged with organic and inorganic cations. The basic structure of the clinoptilolite crystal contains three types of channels with the following approximate dimensions: channels A (10-member rings, free diameters 0.44 nm × 0.72 nm) and B (8-member rings, free diameters 0.41 nm × 0.47 nm) are parallel to each other and to the c-axis of the unit cell, while C channels (8-member rings, free diameters 0.40 nm × 0.55 nm) run along the a-axis intersecting both A and B channels [14]. As is well-known, through surface functionalization, the properties of the zeolite’s external surface can be tailored for specific applications. These modifications can change physicochemical properties of the zeolites and the resulting modified zeolite can be used for a variety of applications such as ion-exchange, adsorption, catalysis, etc. [15,16]. In this study, raw clinoptilolite was modified via Fe3+ -grafting to prepare a functionalized zeolite material that has enhanced adsorptive properties for CR in aqueous solution. The dye adsorption capacity of the resulting modified zeolite was evaluated to determine the adsorption characteristics for CR which is a benzidine-based anionic diazo dye. CR is found in industrial waste effluents generated from textile, printing, dyeing, paper and plastic industries [17] and these effluents must be treated before being discharged into the environment.

2. Experimental 2.1. Materials and modifications Natural zeolite, clinoptilolite, sample was taken from the deposits in Bigadic, Turkey. Zeolite sample was crushed and sieved into a diameter of <150 ␮m. Subsequently, the sample was washed with distilled water to remove the surface dust and dried in an oven at 393 K. In order to obtain modified form, raw clinoptilolite was treated with Fe(NO3 )3 in ethanol at 333 K. The treatment was carried out by adding 1.5 g zeolite into a Fe(NO3 )3 (0.62 g) solution in ethanol (30 mL). The mixture was treated in an oil bath at 333 K for 6 h. Then, the zeolite sample was separated by filtration, washed with ethanol to remove excess salt and dried at 353 K for 24 h. The resulting Fe3+ grafted sample is designated as Fe-CL. The negatively charged dye, congo red (1-naphthalenesulfonic acid, 3,3 -(4,4 -biphenylenebis(azo)) bis (4-amino-) disodium salt, C32 H22 N6 O6 S2 Na2 ; FW: 696.7 g/mol, max : 497 nm)) was used as received. This dye was chosen as a model organic dye for evaluating the adsorption and desorption behavior of the sorbent. Its chemical structure is given in Fig. 1. Stock solutions of CR were prepared by dissolving the dye in distilled water and their concentrations were measured by UV–vis spectrophotometer.

Fig. 1. Structure of congo red.

2.2. Characterization XRD, FT-IR, EDX, TG/DTA, zeta-potential measurement and N2 gas adsorption methods were used for the characterization of the zeolite samples. In order to know the effects of Fe3+ -grafting on the chemical composition of natural clinoptilolite, zeolite samples were analyzed by EDX (Oxford Instruments, X-Supreme). FT-IR spectra were recorded on a Perkin Elmer precisely Spectrum One FT-IR spectrometer in the range of 4000–450 cm−1 . Powder X-ray diffraction patterns of the samples were recorded on a Rigaku ˚ D/Max 2200 PC diffractometer using Cu K␣ radiation (k = 1.542 A) in the 2–70◦ (2) range at a scan speed of 2◦ min−1 and a time constant of 1 s. The zeolite specimens were classified according to recognized zeolitic types from the diffractograms obtained. Nitrogen adsorption–desorption isotherms of the samples were determined at 77 K using a Quadrasorb SI Automated Surface Area and Pore Size Analyzer apparatus following the BET procedure. Thermal analysis of the samples was performed on a Shimadzu DTG-60H Simultaneous DTA-TG apparatus from 303 to 1173 K with a heating rate of 10 K min−1 . The surface zeta potential was measured in the Malvern Zetasizer Nano instrument. Five milligrams of powder sample was dispersed in 10 mL of deionized distilled water and a 5 mL suspension was measured in zeta potential cells. 2.3. Batch adsorption experiments CR adsorption was measured in batch experiments by adding a fixed amount of sorbent (0.015 g) into a definite volume (5 mL) of different initial concentrations (20–240 mg/L) of dye solution. Initial pH values of the solutions were adjusted with dilute (0.1 M) HCl or NaOH by using a pH meter. The mixture was stirred on a magnetic stirrer for a certain adsorption time. After centrifugation, zeolite sorbents were separated from the supernatant and the liquid was analyzed for the remaining dye. CR in the supernatant samples was quantified using a double beam UV–vis spectrophotometer (Varian, UV-VIS Spectrophotometer, Cary 100) by measuring absorbance at max of 497 nm. Effects of the experimental parameters; initial dye concentration, adsorption time, pH and temperature were investigated to optimize the adsorption process. To obtain adsorption equilibrium isotherm data, adsorption experiments were performed using a fixed sorbent/liquid ratio and varied concentrations of CR solutions. The adsorption kinetics study was carried out to determine the time required for the adsorption equilibrium to be reached. The effect of temperature on CR uptake was investigated under isothermal conditions within a temperature range between 298 and 348 K. Additionally, influence of the initial pH was observed by adjusting the pH value of the dye solutions between 4 and 11. In this study, adsorption experiments were conducted in duplicate and the negative controls (with no sorbent) were simultaneously carried out to determine any possible adsorption by the container. Amount of the dye adsorbed onto the zeolite at

M. Akgül / Journal of Hazardous Materials 267 (2014) 1–8

3

Table 1 Chemical composition of the raw and Fe3+ -grafted clinoptilolites. Zeolite

Composition (%)

Raw-CL Fe-CL

Si

Al

Na

K

Ca

Mg

Fe

Ti

69.3 67.1

10.7 8.8

0.6 0.4

3.8 3.5

4.2 2.6

2.6 2.3

0.3 3.1

∼0.1 ∼0.1

equilibrium, qe (mg/g), was calculated by the mass balance equation (Eq. (1)) given below; qe =

(Ci − Cf ) × V m

Table 2 Physical properties of the zeolite samples. Zeolite

Surface area (m2 /g)

Total pore volume (cc/g)

Mean pore diameter (Å)

Raw-CL Fe-CL

21.1 24.6

0.068 0.064

39.0 36.2

(1)

where qe (mg/g) is the mass of dye adsorbed per unit mass of clinoptilolite at time t (min), Ci (ppm) is the initial dye concentration, Cf (ppm) is the dye concentration at time t, and V is the adsorption volume and m is the amount of clinoptilolite in the solution. 2.4. Desorption studies Investigation on the desorption of dye from the adsorbent is necessary to better understand the mechanism of CR adsorption onto raw and modified clinoptilolites. For batch desorption study, the adsorbent utilized for the adsorption was separated from the dye solution by centrifugation. The dye-loaded adsorbent was washed gently with water to remove any unadsorbed dye. Then, the spent adsorbent was agitated with 30 mL of distilled water, adjusted to different pH values (3.4, 7.2 and 12.2) for 24 h. The final CR concentration in the desorption medium was determined using UV–vis spectrophotometer as described earlier. The desorption efficiency was defined as the ratio between the amount of CR desorbed and the amount of CR adsorbed on adsorbent. 3. Results and discussion 3.1. Characterization Prior to using in the adsorption studies, detailed characterization of the zeolite samples were carried out using powder XRD, N2 adsorption–desorption, TG/DTA, FT-IR, zeta-potential measurement and EDX analysis. Powder X-ray diffraction analysis was performed to investigate the structural changes in clinoptilolite upon Fe3+ -grafting. As shown in Fig. 2, the XRD pattern for Raw-CL exhibits three peaks (2 = 10.06, 22.66 and 30.38), which are characteristics of the clinoptilolite [18]. The peak positions remained almost unchanged after Fe3+ -grafting, although a slight decrease in peak intensities was noticed which might be attributed to the distortion of mesoporous channels caused by the collapse of pore structure during the grafting process [19]. As is well known, Fe(III)

Fig. 2. XRD pattern of the clinoptilolite samples; (a) raw and (b) Fe3+ -grafted clinoptilolite samples.

easily hydrolyzes to form (hydr)oxides which are known to gradually transfer to crystalline iron (III) oxides [20]. But, no diffraction line corresponding to iron oxide crystallites was observed in the XRD pattern for Fe-CL (Fig. 2). So, it can be concluded that oxide formation did not take place for our sample eventhough the preparation procedure involved drying at 353 K for 24 h [21]. It could be that the grafted iron is mostly in a coordinated form with the functional groups of raw clinoptilolite, not in polymeric iron (hydr)oxide form, so the formation of crystalline iron (III) oxides was prohibited (Scheme 1) [22]. The changes in the surface chemistry as a consequence of the modification process were monitored by EDX-ray spectroscopy and the results provided clear evidence for Fe3+ -grafting onto the clinoptilolite (Table 1). During the grafting process, Fe3+ ions displaced exchangeable cations on the clinoptilolite resulting in a modified zeolite having 3.1% Fe and lower Na, K, Ca and Mg contents as compared to those of Raw-CL [14]. The physical properties derived from N2 adsorption–desorption data are summarized in Table 2. The BET surface area was increased from 21.1 to 24.6 m2 /g following the grafting of Fe3+ probably due to the formation of surface cracks and defects as a result of the collapse of pore structure during Fe3+ -grafting in ethanol at 333 K. The decreased pore volume and mean pore diameter of the Fe3+ -grafted zeolite comparing to the raw clinoptilolite suggest an increase in the microporosity. As it is well-known, solvent extraction can result an increase in the microporosity which can be ascribed to different reasons, such as the opening of side pockets, removal of some impurities occluded within the clinoptilolite pores [19,23]. However, both materials still show a type IV adsorption isotherm (not shown here), which is typical for mesoporous materials. Hence, it can be concluded that the ordered mesoporous structure is maintained after surface modification. FT-IR analysis was conducted to obtain a better insight into the surface characteristics of the raw and modified zeolites and the spectra are shown in Fig. 3. The characteristic infrared signals for Si O Al asymmetrical stretching (1034 cm−1 ), Si O bending (469 cm−1 ), O Si O bending (606 cm−1 ), Si O asymmetric stretching (1207 cm−1 ) and O Si O symmetric stretching (796 cm−1 ) are present in both Raw-CL and Fe-CL. Furthermore, the peaks at 524 and 734 cm−1 corresponding to “pore opening” vibration and to symmetric stretching of free SiO4 , were also appeared. The FT-IR

Scheme 1. Fe3+ -grafting in Fe(NO3 )3 -alcohol system. (M: Na+ , K+ , Ca2+ or Mg2+ ).

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M. Akgül / Journal of Hazardous Materials 267 (2014) 1–8

Fig. 3. FT-IR-spectra of (a) Raw-CL and (b) Fe-CL zeolite samples.

−H+

−H+

+H+

+H+

SiOH+  SiOH  SiO− 2

(2)

The zeta potential for a zeolite sample represents the surface charge which varies with pH as shown in Eq. (2) [31]. The zeta potential profiles of raw and modified zeolites against pH are given in Fig. 5. In the case of Fe-CL, there is a sharper pH-dependent increase in the zeta potential relative to the parent zeolite (RawCL) probably due to the incorporation of Fe3+ in zeolites. The zeta potential measurements reveal that the Fe-CL acquires positive

Fig. 4. (a) Differential thermal and (b) thermogravimetric analysis results of (i) Fe3+ grafted and (ii) raw clinoptilolite zeolites.

charge between pH 3–8 although its raw counterpart has a negative charge in the entire pH range. For Fe-CL, the zeta potential increases from −25.9 mV at pH = ∼11 to +48.6 mV at pH = ∼3. This charge reversal may be explained on the basis of the charge neutralization via specific adsorption of Fe3+ ions onto the clinoptilolite. The specific adsorption can be defined as the attraction of Fe3+ ions to the adsorbent surface by chemical forces in addition to electrostatic forces as shown in Scheme 1 [32]. Also, the formation of additional hydroxyl groups (Fe-OH), which are protonated to form Fe-OH2 + , is another factor for increasing zeta potential with decreasing pH. It was reported in previous studies that the adsorption of ions and the formation of structural OH groups can cause changes in the surface charge of zeolites [33].

50 40

Zeta potential (mV)

spectra also exhibit typical bands of the bridging OH groups in ≡Al OH Si≡ groups at 3615 cm−1 and the vibration of the bonds O H· · ·O at 3450 cm−1 [24]. FT-IR measurements indicated that some of the aforementioned bands such as 3400 and 1600 cm−1 still appear but exhibit increased intensity for Fe-CL. Since the absorption band at 3400 cm−1 arises from stretching vibrations of O H bonds, the above changes suggest an increment in the hydroxyl groups, which means the formation of additional Brønsted acid sites (new hydroxyl groups) by the incorporation of Fe3+ into the clinoptilolite (Scheme 1) [25]. In addition, there is a broadening of the absorption band at 1000–1100 cm−1 (T O T framework vibrations) due to the incorporation of Fe3+ into the zeolite structure. Furthermore, no peaks corresponding to the typical frequencies for Fe O (490, 599, 1370 and 1580 cm−1 ) can be observed probably due to the overlap of these peaks with the characteristic peaks of zeolite [26–28]. The TG/DTA curves of the Raw-CL and Fe-CL samples are given in Fig. 4. The TG/DTA thermograms for the samples indicate that endothermic changes were occurred in the temperature range of 298–473 K due to loss of water placed in super cages of the zeolitic structure while losses at higher temperatures (between 573 and 1073 K) are related to the dehydroxylation by condensation of silanols [29]. The DTA spectra of modified zeolite sample (Fe-CL) shows an exothermic change in the temperature range of 900–1173 K, which lie within the endothermic area of the raw clinoptilolite. This DTA exothermal change can be interpreted as a result of the recrystallization of the amorphous phase formed after the zeolite breakdown. As is well-known, zeolites containing monovalent alkali cations are known to be more stable than those containing divalent and trivalent cations. The larger ions (monovalent) are suggested to prevent the zeolite structure from collapsing by keeping the rings open, whereas smaller cations (di- and trivalent) are too small to maintain expansion of the channels. As it is given in Table 1, alkaline and earth-alkaline metal ion contents of Raw-CL are higher than those of Fe-CL. Therefore, it is not surprising for Raw-CL to be more stable than Fe-CL at temperatures higher than 900 K [30].

30

Raw-CL Fe-CL

20 10 0 2 -10

4

6

8

pH

-20 -30

Fig. 5. Variation of the zeta potential with pH.

10

12

M. Akgül / Journal of Hazardous Materials 267 (2014) 1–8 40

35

Raw-CL Fe-CL

Raw-CL Fe-CL

35

CR adsorbed (mg/g)

30

CR adsorbed (mg/g)

5

25

20

15

10

30

25

20

15

10 3

4

5

5 0

50

100

150

200

7

8

9

10

11

12

pH

250

Initial concentration of CR (ppm)

6

Fig. 7. Effect of pH on CR adsorption (dye concentration = 200 ppm, adsorption time = 0.5 h).

Fig. 6. Variation of the adsorbed amount of CR as a function of initial dye concentration (adsorption time = 0.5 h, pH 6.5, temperature = 303 K).

3.2. Influence of variables on CR adsorption 3.2.1. Effect of the initial dye concentration Fig. 6 shows the curves for the adsorption of CR on raw and modified clinoptilolites using different initial dye concentrations. For both zeolite samples, amount of the dye adsorbed increases with the initial dye concentration up to an equilibrium reached (maximum adsorption capacity) which means the saturation of the active sites available for the adsorption. The qe values increased with the increase of initial CR concentrations up to the equilibrium concentration values. This is a result of the increase in the driving force for the transfer of dye molecules from bulk solution to the particle surface. Under the same conditions, if the number of CR molecules in the unit volume increases, active sites of the adsorbents were surrounded by much more CR ions and the process of adsorption would carry out more efficiently. When the initial dye concentration was increased beyond the equilibrium concentration, the removal efficiency decreased due to a greater competition of CR ions themselves for a fixed surface adsorbing sites. It is demonstrated in Fig. 6 that Fe3+ -grafting obviously increased CR adsorption resulting in a material with higher adsorption capacity than Raw-CL. The maximum adsorption capacities (qmax at room temperature) of Raw-CL and Fe-CL were found as 16.9 and 36.7 mg/g, respectively. The reason for the difference can be attributed to the marked change in the surface charge as a result of Fe3+ -grafting process, as can be seen in Fig. 5. With the modification, the surface charge of the natural clinoptilolite, Raw-CL, was changed from anionic to cationic state, i.e. zeta potential was changed from −12.4 mV to +48.6 mV at pH ∼3. Therefore, attractive forces between the surface of adsorbent and CR can be expected to increase after Fe3+ -grafting. As seen in Table 2, grafting process also resulted an increase in the surface area, which will increase the amount of accessible active sites for adsorption [34]. 3.2.2. Effect of pH As is known, adsorption capacity strongly depends on the properties of the adsorbent surface and adsorbate structure which are known to be effected by pH [35]. Hence, to evaluate the influence of pH on the removal abilities of zeolite samples, adsorption studies were carried out in the pH range of 4–11 and the results showed that the adsorption behavior of CR was different for both adsorbents. As was seen in Fig. 5, different zeta potential plots were recorded for Raw-CL and Fe-CL, an indication of the dissimilarity in the surface properties of these two adsorbents. It is therefore not surprising that

the adsorbents display different adsorption behaviors as shown in Fig. 7. For Fe-CL, adsorption capacity increased from pH 4 to 6 and then, the dye removal was remarkably decreased beyond pH 6, attaining nearly 10.6 mg/g adsorption capacity at pH ∼11. At lower pHs, the presence of high concentrations of H+ can increase the positive charge on the surface of Fe-CL by protonating the negatively charged sites (M O− ) and hydroxyl groups (M OH) and forming M OH2 + (M: Si or Fe). Hence, the hindrance to diffusion of dye ions decreases and a considerably high electrostatic attraction exists between the positively charged surface of the adsorbent and negatively charged dye molecules, resulting a high adsorption capacity for CR [36]. The large reduction in dye adsorption at highly basic conditions can be explained on the basis of the increasing electrostatic repulsion between the anionic dye adsorbate species and negatively charged adsorbent surfaces. Also, the adsorption at alkaline pH can be affected by the presence of excess OH− ions competing with the dye anions for the adsorption sites. Similarly from Fig. 7, it was found that the amount of dye adsorbed on Raw-CL increased from 11.7 mg/g to 14.9 mg/g with the change in solution pH from 4 to 6 after which the extent of adsorption remained nearly constant in the pH-range of 7–11. Although the negatively charged surface sites on Raw-CL do not favor the adsorption of anionic dye due to electrostatic repulsion, significant amount of dye adsorption onto Raw-CL still occurred as the pH of the solution increases from 7 to 11. This suggests that a mechanism other than electrostatic attraction may be operative for the adsorption of CR on Raw-CL (Scheme 2) [37,38]. As reported earlier, two different mechanisms can be proposed for dye adsorption: (a) electrostatic interaction between the protonated functional groups of adsorbent and acidic dye and (b) the chemical interaction between the adsorbate and adsorbent [39]. At low pHs (<6.0), protonation of the functional groups of CR occurs and leads to lower qe values for both adsorbents at pH ∼4 compared to those at pH ∼6 probably due to the lower electrostatic attraction between less negatively charged dye and the adsorbent [40]. −H

−H

−H

+H

+H

+H

H3 L  H2 L  HL2−  L3−

(3)

The structural change of a dye molecule (L) with pH is shown in Eq. (3). In the adsorption studies of dyes on zeolites, the more of L3− , HL2− and Si OH2 + the species are, the higher the adsorption quantity qe is. At lower pH values, more dye molecules will exist as H2 L− or H3 L, and the adsorbent surface will exist as Si OH2 + ; at higher pH values, the dye molecule exists more as HL2− or L3− and the adsorbent surface exists as Si O− . Neither of these conditions is in favor of the electrostatic interaction between CR and the adsorbent

6

M. Akgül / Journal of Hazardous Materials 267 (2014) 1–8 40

Raw-CL Fe-CL

35

CR adsorbed (mg/g)

30 25 20 15 10 5 0 0

10

20

30

40

50

60

Adsorption time (min.) Fig. 8. Dependency of the adsorbed amount of CR on adsorption time (dye concentration = 200 ppm, pH 6.5).

surface [41]. Therefore, the removal percentage increased with the increasing pH at first, and then reached the maximum adsorption ability and subsequently decreased with increasing pH. Table 3 presents a summary of the adsorption capacities of the various adsorbents that have been investigated. Adsorption capacity of Fe-CL is comparable or much greater than those of adsorbents listed in Table 3. But, it is a few times lower than the adsorption capacities achieved for chitosan [42], organo-attapulgite [47], commercial activated carbon [5] and coal-based mesoporous activated carbons [48]. A major drawback of most of these adsorbents is the low pH’s required for their optimal adsorption capacity. The requirement of such low pH values may be an impediment to practical applications of these technologies. Therefore, expanding the pH range of maximum adsorption to neutral pH is vital to facilitate the dye adsorption from industrial wastewaters. Our results showed that improved CR adsorption can be obtained at higher pH (∼6.3) by grafting Fe3+ onto the clinoptilolite. 3.2.3. Effect of time As seen in Fig. 8, the adsorption intensity increases quickly with the increase of adsorption time and this increase is more intense for Fe-CL compared to its raw counterpart (Raw-CL). The adsorption of CR is initially a rapid process that reaches the equilibrium adsorption capacity in ∼30 min. This is the first step of sorption occurring at the external surface of the zeolite where there is a high number of adsorption sites at the beginning of sorption. The second step occurs at a slower rate, since at that stage the diffusion within the clinoptilolite framework openings becomes also Table 3 Adsorption capacities of various adsorbents. Adsorbent

qe (mg/g)

Reference

Chitosan Waste orange peal Waste red mud Aspergillus niger biomass Granulated activated carbon Chitosan/montmorillonite Na-Bentonite Organo-attapulgite Commercial activated carbon Coal-based mesoporous activated carbons Fe-CL

92.6 22.4 4.1 14.2 13.8 54.5 35.8 189.4 493.8 52–189 ∼37

[42] [43] [44] [7] [7] [45] [46] [47] [5] [48] This study

3.2.4. Effect of temperature The effect of temperature on adsorption capacity was investigated by measuring the adsorption isotherms at various temperatures between 298 and 348 K and the results are presented in Fig. 9. It is clearly seen that with the increase of temperature from 298 to 333 K, the adsorption capacity of CR increases from 32.7 to 56.0 mg/g. This observation reveals that CR removal is controlled by a slightly endothermic process in which the mobility of large dye molecules increases with temperature. The increase of qe with an increase in temperature is also an indication of higher penetration of dye molecules into the pores at higher temperatures resulting increased number of available sites for adsorption [50]. The decrease in the adsorption capacity beyond 333 K may be associated with the relative increase in the tendency of dye molecules 70

60

CR adsorbed (mg/g)

Scheme 2. Interaction between CR molecule and clinoptilolite.

important [13]. After an adsorption time of 30 min, the solid phase loading of CR does not increase significantly. Hence, an adsorption time of around 30 min can be said to be sufficient to reach the equilibrium concentration. The short adsorption equilibrium time, coupled with a high removal indicates the high affinity of clinoptilolite adsorbents, especially Fe-CL, for CR. As was reported previously, due to the aggregation of dye ions in aqueous solution, the adsorption of dyes usually takes place in the mesopores and the presence of mesopores was reported to be as important as the surface charge of the adsorbent [49]. So, the adsorption of CR can be said to be fast and facilitated by the presence of mesopores in the clinoptilolite structure.

50

40

30 20

10 0 300

310

320

330

340

350

Temperature (K) Fig. 9. Effect of the temperature on CR adsorption (dye concentration = 200 ppm, pH 6.5, adsorption time = 0.5 h).

M. Akgül / Journal of Hazardous Materials 267 (2014) 1–8 Table 4 Desorption of CR from raw and modified clinoptilolites. Zeolite

pH

Desorption (%)

Fe-CL

12.2 7.2 3.4

82.3 9.5 –

Raw-CL

12.2 7.2 3.4

26.1 4.7 –

to escape from the solid phase to the solution with increasing temperature [51]. 3.3. Desorption studies As it is seen in Table 4, desorption occurs to a larger extent in the alkaline medium as compared with neutral and acidic medium. Desorption tests in aqueous solution at pH 12.2 showed that maximum dye releasing values of 82.3% and 26.1% were achieved for Fe-CL and Raw-CL, respectively. These results substantiates the mechanism of adsorption mentioned in Part 3.2.2, indicating that ion exchange is probably the major mode of adsorption process for Fe-CL. Low desorption percentage, 26.1%, suggests that the adsorption of CR onto Raw-CL occurred significantly via a chemisorption mechanism [37,38]. Because, Raw-CL adsorbs significant amounts of dye though the electrostatic attraction does not favor the adsorption within the pH range studied. 4. Conclusion The results presented in this work demonstrated that the external zeolite surface of clinoptilolite can be tailored through Fe3+ -grafting in order to increase the adsorption capacity for CR. The highest adsorption capacity obtained with modified clinoptilolite can be attributed to the charge reversal on the surface and the formation of new hydroxyl groups by the incorporation of Fe3+ . The results also indicated that several factors such as pH, adsorption time, dye concentration, and temperature have a significant effect on the adsorption process. The improved CR adsorption was observed to take place at near-neutral pH on Fe3+ -grafted clinoptilolite. This is indicative of the possibility to employ the clinoptilolite as alternative low-cost adsorbents for the adsorption of CR from wastewaters with near-neutral pH. Also, compared to the conventional adsorbents used, Fe-CL was found to be very promising from an industrial viewpoint due to its high capacity, local availability and low cost. References [1] V.S. Mane, I.D. Mall, V.C. Srivastava, Use of bagasse fly ash as an adsorbent for the removal of brilliant green dye from aqueous solution, Dyes Pigm. 73 (2007) 269–278. [2] K.R. Ramakrishna, T. Viraraghavan, Dye removal using low cost adsorbents, Water Sci. Technol. 36 (1997) 189–196. [3] O.J. Hao, H. Kim, P.C. Chiang, Decolorization of wastewater, Crit. Rev. Environ. Sci. Technol. 30 (2000) 449–505. [4] Y.M. Slokar, A.M. Le Marechal, Methods of decoloration of textile wastewaters, Dyes Pigm. 37 (1998) 335–356. [5] K. Nagarethinam, M. Mariappan, Adsorption of congo red on various activated carbons, Water Air Soil Pollut. 138 (2002) 289–305. [6] O. Ozdemir, B. Armagan, M. Turan, M. Celik, Comparison of the adsorption characteristics of azo-reactive dyes on mesoporous minerals, Dyes Pigm. 62 (2004) 49–60. [7] Y. Fu, T. Viraraghavan, Removal of congo red from an aqueous solution by fungus Aspergillus niger, Adv. Environ. Res. 7 (2002) 239–247. [8] M.M. Kamel, B.M. Youssef, M.M. Kamel, Adsorption of anionic dyes by kaolinites, Dyes Pigm. 15 (1991) 175–182. [9] R. Sanghi, B. Bhattacharya, Review on decolorisation of aqueous dye solutions by low cost adsorbents, Color. Technol. 118 (2002) 256–269.

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