Cr

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Synthesis, Characterization, and Adsorption Properties of Fe/Cr-Pillared Bentonites Fatma Tomul* Department of Science Teaching, Education Faculty, Mehmet Akif Ersoy University, 15100 Bahc-elievler-Burdur, Turkey ABSTRACT: Bentonites pillared with iron, iron/chromium with different Fe/Cr molar ratios, and chromium were synthesized and tested as adsorbents of cadmium. The pillared bentonites were characterized by using scanning electron microscopy with energy dispersive system (SEM-EDS), powder X-ray diffraction (XRD), N2-adsorption/desorption, and Fourier-transformed infrared spectroscopy (FTIR). The thermal stability of the samples was studied with thermogravimetry (TG). Adsorption experiments were conducted by varying pH, contact time, and temperature. The content of the pillaring solution was the most important parameter that influenced the physicochemical, textural, and acidic properties of the pillared bentonites. The results revealed that the adsorption capacity of the materials increased with an increase in pH and contact time. The results also indicated that the equilibrium adsorption data of Cd2þ onto Cr-pillared bentonite best fit the RedlichPeterson model, whereas adsorption onto Fe-pillared bentonite and Fe/Cr-pillared bentonite with Fe/Cr molar ratio of 5:5 correlated with the first-order model. Adsorption kinetics results showed that the adsorption followed the pseudo-second-order kinetic model. Thermodynamic studies suggested that the adsorption of Cd2þ onto Fe-pillared bentonite was an endothermic and spontaneous process.

1. INTRODUCTION Natural clays can be modified with single and/or mixed metal oxides to obtain metal-pillared clays. Mixed metal oxide-pillared clays have been evaluated for a variety of different applications due to their excellent physicochemical, catalytic, and adsorptive properties. Various studies have been reported in the literature related to the synthesis of clay pillared with mixed metal oxides, such as Al/Cr, Al/Fe, Al/Zr, and Al/Cu, by different methods.15 However, studies related to the synthesis of clays pillared with mixed-transition metal oxides are still limited.610 Chromium and iron are interesting for industrial applications because of their different oxidation states and magnetic properties.11,12 The disadvantage of Fe-PILCs is the resulting small basal spacing, which could have an adverse impact on their surface areas and pore properties and thus reduce their reaction activities.10 Cr-PILCs are not thermally stable at temperatures higher than 200 °C.11,13 The thermal, adsorptive, and/or catalytic properties of PILCs containing single-metal oxides can be enhanced by the addition of a second metal.13 For this reason, the physicochemical, catalytic, and adsorptive characteristics of iron/chromium mixed metal oxide-pillared clays should be analyzed. Cadmium is a highly toxic metal that is used in the production of alloys, ceramics, and pigments. Moreover, cadmium can be found in the wastewater of metal plating, dye, and battery production plants.1416 Inhalation of a large amount of cadmium causes serious damage to the lungs and may result in death. In addition, regular exposure to small amounts of cadmium leads to metal accumulation in the kidneys, which impairs kidney function. Furthermore, cadmium causes hypertension, bone lesions, liver insufficiency, and cancer in humans.14,1719 According to the Environmental Protection Agency (EPA), the concentration of cadmium in wastewater must be less than 5 ppb (5 μg dm3).14 For this reason, the removal of cadmium from water and wastewater is crucial for the environment and thus for human health. r 2011 American Chemical Society

Adsorption is the preferred technique used for the removal of toxic metals from wastes that contain low concentrations of metal. The chemical and structural characteristics of intercalated/ pillared clays obtained by modifications of polyhydroxy metal cations with natural clays can have potential applications as adsorbents and catalysts.20 For example, pillared clay adsorbents have been used to remove Cd2þ from aqueous solutions.16,19,21,22 In this study, bentonites pillared with iron, iron/chromium with different Fe/Cr molar ratios, and chromium were synthesized from Hancili Green Bentonite and tested as adsorbents of cadmium. SEM, EDS, XRD, N2-adsorption/desorption, FTIR, and TGA analyses were conducted to characterize the adsorbents. Cd2þ removal was evaluated as a function of pH, contact time, and temperature. In addition, the kinetic of adsorption was evaluated, and the data were fit to equilibrium isotherm models to determine the mechanism of adsorption and relevant ion exchange constants.

2. MATERIALS AND METHODS Bentonite from the Hancili region of Central Anatolia, Turkey (Hancili Green Bentonite, HGB), was used to synthesize Fe-, Fe/Cr-, and Cr-pillared bentonites. To obtain a homogeneous distribution of polycations, the host clay was saturated with Ca2þ. Hancili Green Bentonite used in the synthesis of pillared clays contained approximately 11% quartz and 14% feldspar impurities.5 2.1. Synthesis of Fe-, Fe/Cr-, and Cr-Pillared Bentonite. The Fe-pillared bentonite was prepared by using the procedure Received: October 11, 2010 Accepted: May 8, 2011 Revised: May 7, 2011 Published: May 08, 2011 7228

dx.doi.org/10.1021/ie102073v | Ind. Eng. Chem. Res. 2011, 50, 7228–7240

Industrial & Engineering Chemistry Research described by Belver et al.,23 while the Fe/Cr- and Cr-pillared bentonites were obtained according to the method reported by Mata et al.11 with some modifications. Fe-pillaring solution was prepared by slowly (1 cm3 min1) adding 0.4 M NaOH solution to 0.4 M FeCl3 3 6H2O solution under vigorous stirring to obtain an OH/Fe3þ molar ratio of 2.0. The solution was kept at room temperature for 20 h (final pH = 1.71). Then this pillaring solution was added to a previously prepared suspension of bentonite in water with a ratio of 10 mmol Fe3þ/g bentonite. The mixture was kept at room temperature for 24 h, filtered with suction filtration, and washed with distilled water until chloride ions were completely removed. The materials were dried at room temperature and calcined at 250 °C for 3 h. The Fe-pillared bentonite was labeled as Fe-PB. Cr-pillaring solution was prepared by dropwise addition of 0.4 M NaOH solution to 0.4 M CrCl3 3 6H2O solution under vigorous stirring until the OH/ Cr3þ molar ratio reached the value of 2. The solution was kept at room temperature for 72 h (final pH = 3.04). Fe/Cr-pillaring solutions were prepared by slowly adding 0.4 M NaOH solution to 0.4 M Fe/Cr solutions containing different amounts of iron and chromium cations with ratios of 9:1, 5:5, and 1:9 under vigorous stirring to obtain an OH/(Fe3þ þ Cr3þ) molar ratio of 2.0. The solutions were aged for 72 h (final pH = 1.75, 3.15, and 3.10, respectively). The Fe/Cr- and Cr-pillaring solutions were added to the suspension of bentonite (2 wt %) to obtain 10 mmol (FeþCr)/g bentonite. The resulting suspensions were kept at room temperature for 72 h. Filtration, drying, and calcination were performed as described in the synthesis of Fe-pillared bentonite. The Cr-pillared bentonite was labeled as Cr-PB, whereas Fe/Cr-pillared bentonites were labeled as Fe/ Cr0.9-PB, Fe/Cr0.5-PB, and Fe/Cr0.1-PB according to metal/ total metal ratio content. 2.2. Characterization Studies. Scanning electron microscopy (SEM) microphotographs were obtained with powdered samples on a Philips XL-30S FEG instrument. The chemical composition of pillared bentonites was determined with a Philips XL30S FEG energy dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) patterns were obtained with a Philips PW 3040 diffractometer using Cu KR radiation (40 kV, 40 mA) between 2θ values of 1° and 70°. The textural properties of the materials were determined by nitrogen adsorption/desorption at the temperature of liquid nitrogen with a Quantochrome Autosorp 1C instrument. Before performing the measurements, the materials were degassed under vacuum for 5 h at 250 °C. Specific BET surface area (SBET) values were calculated with 0.05 < P/P0 < 0.30. The t-plot method was used to determine external surface area (Sext) and the micropore volume (Vμ, t). The total pore volume (Vt) was estimated from the adsorption data at a P/P0 value of ∼0.97. The BarrettJoynerHalenda (BJH) method was applied to the adsorption data for P/P0 values above 0.20 to determine the mesopore surface area (SBJH) and the mesopore volume (VBJH) for pores in the 250 nm range. The micropore and mesopore volume distribution as a function of pore size was calculated by HorvathKawazoe (HK) and BarrettJoynerHalenda (BJH) methods, respectively. Micropore volume (Vμ, HK) values were calculated by HorvathKawazoe (HK) method, which was applied for micropore size distribution.24 All IR measurements were performed on a Perkin-Elmer BXFTIR spectrometer with a resolution of 4 cm1 in transmission mode at room temperature. Types and distribution of the acidic sites within the structure were determined by collecting the

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spectra of the samples after saturating them with ammonia by exposure to ammonia steam for a week. Thermogravimetric (TG) analysis was performed by heating the samples in a N2 flow at a rate of 20 mL/min using a PerkinElmer Diamond TG/DTA thermal analyzer with a heating rate of 10 °C min1. 2.3. Cadmium Adsorption Experiments. Cadmium adsorption experiments were conducted to evaluate the adsorption of Cd2þ on Fe-PB, Fe/Cr0.5-PB, and Cr-PB. Adsorption experiments were conducted in a batch system at 120 rpm with a fixed amount of adsorbent (2 g adsorbent dm3 solution). In each experiment, the materials were combined with 50 cm3 of Cd2þ solution. Solutions with concentration ranges between 0.178 and 5.34 mmol Cd2þ dm3 were prepared from an 8.89 mmol dm3 stock solution of 3CdSO4 3 8H2O. The efficiency of cadmium adsorption was evaluated as a function of pH, contact time, and temperature. Specifically, the pH was varied from 3.0 to 9.0, the contact time varied from 5 to 1440 min, and the temperature ranged from 25 to 40 °C. These experiments were conducted in a solution containing 0.445 mmol of Cd2þ dm3. Solution pH was adjusted to the desired value by adding 0.1 M HCl and 0.1 M NaOH. Preliminary kinetic experiments demonstrated that equilibrium was attained within 4 h. After this period, the solutions were filtered and the concentration of Cd2þ in solution was evaluated by ICP-OES (Perkin-Elmer Optima 5300DV Model). The content of the metal ions (Fe3þ, Cr3þ) that remained dissolved during the adsorption was determined by atomic absorption spectroscopy (Perkin-Elmer AA800 Model AAS). All of the experiments were conducted in duplicate, and the average values were used in the calculations.

3. RESULTS AND DISCUSSIONS 3.1. Characterization Studies. 3.1.1. Scanning Electron Microscopy with Energy Dispersive System. SEM micrographs of natural

and pillared bentonites are provided in Figure 1. The results indicated that the tightly packed structure of natural bentonite became more porous after the material had been pillared with a solution of Fe, Fe/Cr, and Cr. Moreover, pillaring resulted in the formation of flower-like micro- and mesopores. This porous and flower-like structure may arise from the change of the surface charge of the particles as a result of pillaring.25 The chemical compositions of the natural and pillared bentonites are presented in Table 1. Pillaring of the natural bentonite by Fe-, Fe/Cr-, and Cr-pillaring solutions resulted in an increase of Fe2O3 and/or Cr2O3 contents with replacement of the interlayer cations, such as calcium and sodium. This increase in iron and/or chromium contents, with a corresponding decrease in the number of exchangeable cations, indicates the successive replacement of the interlayer cations with iron, iron/chromium, and chromium species. The relative amounts of SiO2 and Al2O3 after pillaring remained almost constant in all of the pillared samples. Furthermore, from EDS analysis results, it was observed that, by changing the Fe/Cr ratio in the pillaring solution, the Fe/ Cr ratio in pillared clay could be efficiently changed; the success of the replacement of iron and chromium in the structure was increased by pillaring with a solution containing equal amounts of iron and chromium (Table 1). 3.1.2. X-ray Diffraction Patterns. The X-ray diffraction pattern of calcined samples at 250 °C is presented in Figure 2, and their basal spacing values (d001) with textural characteristics are 7229

dx.doi.org/10.1021/ie102073v |Ind. Eng. Chem. Res. 2011, 50, 7228–7240

Industrial & Engineering Chemistry Research

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Figure 1. SEM microphotographs of (a) HGB, (b) Fe-PB, (c) Fe/Cr0.9PB, (d) Fe/Cr0.5-PB, (e) Fe/Cr0.1-PB and (f) Cr-PB samples.

Table 1. Chemical Properties of HGB and Pillared Bentonites metal oxides % m/m sample code SiO Al O Fe O MgO CaO Na O K O Cr O 2 2 3 2 3 2 2 2 3 HGB(5)

66.95 18.38

6.11

2.75

1.80

2.81

1.19

0.00

Fe-PB

47.88 14.93

32.30

2.61

0.00

1.13

1.15

0.00

Fe/Cr0.9-PB

51.28 15.96

25.43

2.67

0.00

1.77

0.94

1.93

Fe/Cr0.5-PB

44.91 13.32

26.14

1.92

0.00

0.78

1.13 11.80

Fe/Cr0.1-PB Cr-PB

55.33 15.78 51.50 15.57

8.58 6.45

2.65 2.26

0.00 0.00

1.98 1.60

0.97 14.70 1.11 21.51

summarized in Table 2. The data show that d001 ordering in pillared clays depends on the chemical composition of pillaring solution. The natural layered aluminosilicate shows 1.24 nm of basal spacing at room temperature, but after intercalation of the natural layered aluminosilicate by Fe- and Cr-solutions and calcination at 250 °C, basal spacing increases to 1.34 nm (2θ =

6.58) and 1.79 nm (2θ = 4.94) for Fe-PB and Cr-PB, respectively. The basal spacing values of mixed pillared bentonites increase from 1.41 nm (2θ = 6.28) to 1.86 nm (2θ = 4.74) with the increase of Cr content. These findings are compatible with the results in the literature.6,2629 3.1.3. Textural Analysis. The nitrogen adsorption/desorption isotherms of all the pillared bentonites are shown in Figure 3, and the structural parameters calculated from these isotherms are given in Table 2. According to the BDDT classification system, all the isotherms were characterized as type II. Moreover, the isotherms possessed an H4 hysteresis loop, indicating that open slit-shaped pores were formed between parallel layers.24 The width of the hysteresis loop produced by the materials was highly variable. Cr-PB produced the narrowest hysteresis loop, whereas the widest hysteresis loop was observed for Fe/Cr0.5-PB. A narrow hysteresis loop indicates a limited number of mesopores, and a large hysteresis loop suggests that the material possesses a large number of mesopores. It was found that the values of SBET for Fe-PB and Cr-PB samples are 157 and 138 m2 g1, respectively. For Fe/Cr-PB 7230



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samples with different Fe/Cr molar ratios, it was observed that the values of SBET increased with increasing chromium content in the pillaring solutions, which confirmed the trend observed in the XRD patterns. The same behavior was observed for external surface area (Sext) values (Table 2). These results could be explained by the formation of larger Fe/Cr pillars with increasing chromium content in the pillaring solution containing both the cations, as well as by pore structure formation due to an increase in surface area caused by the distribution of pillars in the interlayer region. Moreover, with increasing chromium content of Fe/Cr-pillared bentonites, the surface area values increased. The decrease of the surface area value of the pillared bentonite containing only chromium cation could be related to the number of pillars entering into the interlayer region for this sample. For the samples that were pillared by the solutions containing both

cations, an increase in surface area with increasing chromium content is consistent with the increase in the basal spacing and in the diameters of mesopores (Table 3). The total pore volumes (Vt) in Fe-PB and Cr-PB samples were determined to be 0.186 and 0.148 cm3 g1, respectively. In Fe/ Cr-pillared bentonites with different ratios of Fe/Cr, this volume considerably increased with increasing Cr content in the pillaring solution and reached a value of 0.242 cm3 g1. The high values of total pore volume indicate that most of the total pores were formed from mesopores. The t-plot micropore volume (Vμ,t) values for Fe-PB and Cr-PB samples were determined to be 0.012 and 0.028 cm3 g1, respectively. These values represent almost 6% and 19% of total pore volumes, respectively. Low chromium content in the pillaring solution with different ratios of Fe/Cr caused a decrease in total pore volume and an increase in micropore volume. Further increase in chromium content in Fe/Cr pillaring solution resulted in an increase in both total and micropore volumes. The volume values of micropore determined with the HK method are mostly greater than the values derived from the t-plot. VBJH values contributed in a very significant way to the total pore volume of the pillared bentonites, representing

Figure 3. Nitrogen adsorption/desorption isotherms of pillared bentonite samples.

Figure 2. X-ray diffraction patterns of pillared bentonite samples.

Table 2. Textural Properties and Basal Spacing (d001) Values of Pillared Samples surface area (m2 g1) sample

pore volume (cm3 g1) SBJH

Vt

Vm, t

SBET

Sext

Fe-PB

157

115

135

0.186

0.012

0.175

0.098

4.73

2.23

0.437

1.34

Fe/Cr0.9-PB

139

89

108

0.156

0.016

0.140

0.084

4.48

2.23

0.437

1.41

Fe/Cr0.5-PB Fe/Cr0.1-PB

176 202

111 151

117 173

0.218 0.242

0.028 0.014

0.177 0.215

0.105 0.124

4.14 4.53

2.24 2.50

0.437 0.437

1.46 1.86

Cr-PB

138

75

87

0.148

0.028

0.121

0.083

4.29

3.48

0.637

1.79

7231

VBJH

pore diameter (nm) Vμ,HK

dort

dBJH

dμ,HK

d001 (nm)


3.69 103 0.966 2.25 7.05 10 0.934 3.06 0.264 1.08 10 0.990 2.75 0.423

6.46 103 0.974 1.66 0.536

0.370 1.34

3.43

0.943

0.991

9.14 10

4

1.17 10 0.953 1.000 1.80 1.14 3.81 10 0.847 2.65 0.347

3 3

1.17 10 0.953 1.80 0.633

6.45 103 0.986 1.76 0.667 8.82 103 0.983 0.980 2.28 1.71

2 2

3.24 102 0.932 2.75 0.451 7.56 103 0.984 2.13 0.773

2

KFO β RR R2 KF χ2

n

0.667

0.525

0.391

Fe-PB

Fe/Cr0.5-PB

Cr-PB

Freundlich Langmuir

R2 KL qm qm, exp mmol g

1

Table 3. Langmuir, Freundlich, RedlichPeterson, and First-Order Model Parameters

χ2

KRP

RedlichPeterson

R2

χ2

qm

First-Order

R2

χ2


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between 81 and 94% of the total pore volume in the pillared bentonites (Table 2). Thus, it can be concluded that all of the samples are predominantly mesoporous, which is desirable for the liquid phase adsorption of metal ions. The HK-micropore and the BJH-adsorption mesopore size distribution curves of the samples are shown in Figure 4, and average pore dimensions obtained from the maxima (dHK and dBJH) are presented in Table 2. All the pillared bentonites studied displayed different micropore and mesopore size distributions. These results are also supported by the different volume values of Vμ, HK and VBJH. The average micropore and mesopore diameters were determined to be 0.437 and 0.637 nm for Fe-PB and 2.23 and 3.48 nm for Cr-PB. However, the average micropore diameters of Fe/Cr-pillared bentonites are not affected by Fe/Cr ratio; mesopore diameters, on the contrary, increase with increasing chromium content, as do the values of mesopore volume. This behavior indicates that the distribution of Fe/Crpillars with high chromium content entering into the interlayer region forms pores at the mesopore dimension. 3.1.4. Fourier-Transformed Infrared Spectroscopy. FTIR spectra of the natural and pillared bentonites are shown in Figure 5a. The wavenumbers and assignments of main vibration peaks were attributed according to the literature.30,31 Natural bentonite shows bands at 3631 and 3446 cm1 in the OH stretching region. These two bands were attributed to the OH stretching vibration of the structural hydroxyl groups in the clay and the water molecules present in the interlayer, respectively. After pillaring, these bands shifted to lower wavenumbers, and their intensities decreased. It was observed that the band at 3446 cm1 broadened due to the introduction of additional OH groups, which is interpreted as an effect of pillaring. The decrease in the intensity of the band arises from the dehydration and dehydroxylation steps during pillaring.32,33 The intensity of the bending vibration of water molecules occurring at 1638 cm1 decreased and shifted to 16361629 cm1 after pillaring. Pillaring process replaces a large amount of interlayer cations that generally exist in hydrated form, and this replacement decreases the intensity of OH peaks. Pillared clays have a lower amount of adsorbed water due to their nonswelling nature. Thus, as a result of pillaring, the intensity of the band around 1638 cm1 decreases. The band at 1038 cm1 corresponding to SiOSi stretching vibrations shifted to 1044 cm1. The intensities of SiO bending and AlO stretching vibrations occurring at 468 and 524 cm1, respectively, decreased and shifted to higher wavenumbers in the pillared bentonites. Thus, FTIR spectra are indicative of effective pillaring. Pillared materials adsorb ammonia, which can be followed by FTIR to analyze the surface acidity in pillared bentonites. The adsorption of ammonia on all of the samples (Figure 5b) resulted in the appearance of a band at 1400 cm1. This band is assigned to the asymmetric bending modes of ammonium ions formed by proton transfer from the surface hydroxyl groups to ammonia molecules.34,35 The intensity of the band at 1400 cm1 increased due to pillaring, and this increase became more dominant for mixed-pillared bentonites, compared with Fe-PB and Cr-PB. It was found that, after ammonia adsorption, there was an increase in water bending vibration band intensity for all of the samples. If we compare the intensity of this band for all the clay samples in Figure 5b, it becomes clear that pillaring has a pronounced effect on the intensity of this band. The presence of an intense band in the pillared clay indicates that the acidity of the pillared materials possibly increased because of the formation of protons through 7232



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Figure 4. Micropore and mesopore size distribution of pillared bentonite samples using HK and BJH methods.

the dissociation of water molecules present in the first coordination sphere.36 This increase in acidity in Fe/Cr-pillared samples is greater than that in Fe-PB and Cr-PB. The acidity further increases if the chromium content in the pillaring solution is raised. These results can be explained by the presence of different species in the interlayer region during the pillaring process. It was observed that, after ammonia desorption at 150 and 350 °C, the intensity of both the peaks at 1636 and 1400 cm1 decreased for Fe/Cr0.1-PB and Cr-PB; however, for Fe-PB, the peak intensity at 1636 cm1 decreased after desorption at 150 °C but increased after desorption at 350 °C (Figure 6). The differences in FTIR results could be explained by the differences in the strength of acidic sites. 3.1.5. Thermogravimetric Analysis. The TG curves of the airdried bentonites pillared with Fe, Fe/Cr (with Fe/Cr ratio of 1:9 and 9:1), and Cr-pillaring solutions are shown in Figure 7. For all of the pillared bentonites, there was a mass loss below 150 °C, which can be attributed to the removal of physically adsorbed water. The mass loss of Fe-PB (6.3%) in this temperature range is less than that of other three samples and follows the order Fe/ Cr0.1-PB (8.5%) < Cr-PB (9.4%) < Fe/Cr0.9-PB (9.5%). The Fe-PB and Fe/Cr0.9-PB show two additional steps of mass loss between 150 and 650 °C. The first step of mass losses (5.5% and 5.0%) in the temperature range 150500 °C is attributed to the loss of water and some hydroxyls from the iron/ironchromium pillars in the interlayer region. The second step of mass losses (0.5 and 0.7%) in the range 500650 °C is ascribed to the dehydroxylation of the pillars and the clay structure. A slightly different pattern is observed for Cr-PB and Fe/Cr0.1-PB with Fe/Cr ratio of 1:9 samples. For these samples, after initial

desorption of physically adsorbed water, a more continuous decrease in mass, with values of 5.61% and 6.39%, is noticed up to approximately 500 °C. The mass loss of Fe/Cr0.1-PB at 500650 °C is higher than that of the other samples, and the increased losses result from the decomposition of this sample. The differences observed on the TG curves could be explained by the variation in the dehydration and dehydroxylation process. The variations in the dehydration and dehydroxylation process could be attributable to the presence of different types of iron, iron/chromium, and chromium in the interlayer and the external surfaces and due to different hydration energies. These results are also in accordance with the results of FTIR spectra. 3.2. Cd2þ Adsorption Studies. 3.2.1. Effect of Initial pH. The pH of the Cd2þ solution is an important factor that controls the adsorption capacity of adsorbent. Specifically, the pH affects the surface charge of the adsorbent, the ionization of cadmium, and the dissociation of functional groups on the active sites of the adsorbent.37 The effect of solution pH on the adsorption capacity of pillared bentonites was investigated in the pH range of 39, and the results are shown in Figure 8. In Figure 8, as the pH of the solution increases, the adsorption capacities of the adsorbents also increase. For the Fe-PB, Fe/Cr0.5-PB, and Cr-PB adsorbents, the maximum amount of adsorbed Cd2þ was observed at pH 9. Fe-PB exhibited the highest adsorption, followed by Cr-PB and Fe/Cr0.5-PB. As the pH of the solution changes, the ionic charge of the medium and the surface properties of the adsorbent also change. In strongly acidic medium, the concentration of Hþ on the surface of the adsorbent is high. Therefore, Cd2þ must compete with Hþ for active sites on the adsorbent. Alternatively, as the pH of the solution decreases, the density of positive charge 7233



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Figure 5. FTIR spectra of pillared bentonite samples (A) before and (B) after ammonia adsorption.

on the surface of the adsorbent decreases, and the density of negative charge (due to OH ions) increases. As a result, the probability of Cd2þ adsorption increases. However, depending on the increase in pH, there is a possibility for the formation of different hydroxo-cadmium ions through the hydrolysis of Cd2þ ions and subsequent precipitation of Cd2þ ions in the form of hydroxide. In the present study, cadmium adsorption on pillared bentonite samples showed a gradual increase in the pH range of 3.09.0 with no indication of precipitation of cadmium hydroxide. In previous studies, the adsorption of cadmium could be safely performed up to pH 10.17 Gupta and Bhattacharyya38 performed adsorption of Cd2þ on kaolinite and montmorillonite up to pH 10.0 with no indication of precipitation of Cd(II)-hydroxide. Maximum adsorption capacity of montmorillonite modified with polyoxozirconium and tetrabutylammonium was achieved at pH 9.0.14

3.2.2. Adsorption Isotherms. Adsorption isotherms describe how adsorbates interact with adsorbents, and hence they are critical in optimizing the use of adsorbents. Therefore, the correlation of experimental data with adsorption model equations is essential to the practical design and operation of adsorption systems. To optimize the design of a sorption system to remove Cd2þ from wastewater, it is important to establish the most appropriate correlation for the equilibrium curves. In this study, the experimental equilibrium adsorption data were fitted to the nonlinear forms of Langmuir, Freundlich, Redlich Peterson, and first-order adsorption isotherm models. These models are widely used because they can describe experimental results in a wide range of concentrations.16,19,25 The equilibrium data over the concentration range of 0.1785.34 mmol dm3 at 25 °C were used. 7234



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Figure 6. FTIR spectra of some pillared bentonite samples after ammonia desorption at different temperatures.

Figure 8. Effect of pH on the adsorption of Cd2þ by some pillared bentonite samples (initial Cd2þ concentration: 0.445 mmol dm3, adsorbent concentration: 2 g dm3, temperature: 25 °C).

over the surface. However, the RedlichPeterson isotherm can be applied to both homogeneous and heterogeneous systems. The first-order equation39 is an asymptotic growth curve and can be used for adsorption from aqueous solution. The above models could be expressed as: Figure 7. TG curves of some pillared bentonite samples.

K L q m Ce 1 þ K L Ce

ð1Þ

Freundlich : ne ¼ KF Ce 1=n

ð2Þ

Langmuir : ne ¼

The Langmuir isotherm assumes that the adsorption takes place at specific homogeneous sites within the adsorbent. The Freundlich isotherm is valid for a heterogeneous adsorbent surface with a nonuniform distribution of heat of adsorption 7235



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Figure 9. Comparison of the Langmuir, Freundlich, RedlichPeterson, and first-order isotherms for Cd2þ adsorption onto Fe-PB, Fe/Cr0.5-PB, and Cr-PB.

Redlich Peterson : ne ¼

KR C e 1 þ a R Ce β

First-Order : ne ¼ qm ð1 eKFO Ce Þ

ð3Þ ð4Þ

In these equations, Ce (mmol L1) is the equilibrium concentration of Cd2þ in the solution, ne (mmol g1) is the amount of Cd2þ adsorbed at equilibrium, qm (mmol g1) is the theoretical maximum monolayer adsorption capacity, KL (dm3 mmol1) is a Langmuir constant representing the affinity of the adsorbent for the solute, KF (dm3 g1) and n are the indicators of the adsorption capacity and adsorption intensity, respectively, KR (dm3 g1), aR (dm3 mmol1), and β are the RedlichPeterson isotherm variables, and KFO is the first-order model constant. Model variables were determined using nonlinear-regression analysis. To evaluate the suitability of experimental data to the isotherms, the correlation coefficient (R2) and the sum of squared error (χ2) were determined for each model. Isotherm parameters obtained with the nonlinear method are presented Table 3. The results obtained with Langmuir isotherm showed different KL values for Fe-PB, Fe/Cr0.5-PB, and Cr-PB adsorbents, indicating significant differences in the retention intensity toward cadmium ions. According to qm parameters, cadmium adsorption capacity followed the order of Fe-PB > Fe/Cr0.5-PB > Cr-PB (Table 2). The experimental adsorption capacity of the adsorbents also followed the same trend. In addition, solubilization studies of pillars containing iron, iron/chromium with Fe/Cr ratio of 5:5, and chromium at 25 °C indicated that the concentrations of the dissolved metals were 0.09 ( 0.02 ppm Fe, 0.20 ( 0.04 ppm Fe and 0.10 ( 0.02 ppm Cr, and 0.57 ( 0.03 ppm Cr, respectively. These results show that iron pillars are more stable than iron/chromium and chromium pillars. As given in Table 2, the BET surface area values of Fe-PB and Cr-PB are lower than that of Fe/Cr0.5-PB. Calculated qm values indicate that surface area is not a crucial factor for the adsorption capacity of pillared bentonites. In the literature, it is stated that the pore structure and surface chemistry of an adsorbent have a significant effect on

adsorption process, and the distribution of pores affects the efficiency and selectivity of the adsorption.40 The difference in adsorption capacities for Fe-PB, Fe/Cr0.5-PB, and Cr-PB adsorbents could be explained by different distributions of pores (Figure 4). The Freundlich isotherm parameter n gives an indication of the favorability of adsorption. Values of n > 1 indicate favorable adsorption conditions. In the present study, the values of n obtained for all the three adsorbents are greater than unity, which indicates that adsorption intensity is favorable over the entire range of concentrations studied. KF values were in the order FePB > Fe/Cr0.5-PB > Cr-PB, and this order is imputable to the different adsorption affinities of adsorbent surfaces (Table 3). The RedlichPeterson equation is a combination of the Langmuir and Freundlich models. It approaches the Freundlich model at high concentration and is in accordance with the low concentration limit of the Langmuir equation. Examination of the data showed that the RedlichPeterson isotherm was an appropriate description of the data for Cd2þ adsorption over the concentration ranges studied. The constant β was close to 1, which indicates that the isotherms were approaching the Langmuir form. These observations indicated that the monolayer adsorption of Cd2þ on three pillared adsorbents was predominant. The first-order model results indicated that Fe-PB possessed the highest maximum adsorption capacity, whereas the lowest capacity was observed with Cr-PB. This observation was similar to the results of other models. Specifically, the maximum adsorption capacity of Cr-PB obtained from the first-order model was lower than the experimental adsorption capacity. Moreover, the theoretical adsorption capacity of Fe/Cr0.5-PB was higher than the experimental capacity. However, the theoretical and experimental results of Fe-PB were similar. Similar KFO values were obtained for the Fe-PB and Fe/Cr0.5-PB adsorbents. The experimental and predicted adsorption data are shown in Figure 9. The first-order model appears to be the better-fitting model for Cd2þ adsorption onto Fe-PB and Fe/Cr0.5-PB because of its highest R2 and lowest χ2 values. These results indicated that cadmium adsorption onto these adsorbents has 7236


Industrial & Engineering Chemistry Research consistent behavior with an increase in Cd2þ concentration. The other isotherms had lower values of R2 and higher values of χ2. For data from Cd2þ adsorption onto Cr-PB, the RedlichPeterson model seems to have the best fit with the highest R2 and the lowest χ2 values. However, adsorption equilibrium data were also compatible with the Langmuir model. The value of β derived from the RedlichPeterson model was close to unity, which indicated that the results of both models were similar. It can be said that the adsorption of Cd2þ is onto the homogeneous sites on the surface of Cr-PB. Compared with the other adsorbents, the highest correlation to the Freundlich model was observed with Cr-PB. 3.2.3. Effect of Contact Time and Adsorption Kinetics. The effect of contact time on the adsorption of Cd2þ by pillared bentonites is shown in Figure 10. Cd2þ adsorption increased with time and attained equilibrium within 240 min. After 240 min of contact, no obvious variation in Cd2þ adsorption was found.

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Based on these results, 240 min was taken as the equilibrium time in batch adsorption experiments. The adsorption is higher in the beginning due to the greater number of reaction sites available for the adsorption of Cd2þ. After equilibrium is attained, the surface of the adsorbent is saturated with Cd2þ, and an increase in contact time does not affect adsorption. It was also found from Figure 10 that the amount of Cd2þ adsorbed by Fe-PB was higher than that by the other two samples. Kinetic models have been developed to elucidate the mechanism and rate of adsorption. The mechanism of adsorption depends on the physical and/or chemical characteristics of the adsorbent. To determine the mechanism of cadmium adsorption onto Fe-PB, Fe/Cr0.5-PB, and Cr-PB, pseudo-first-order,41 pseudo-second-order,42 intraparticle diffusion, and Elovich43 kinetic models were evaluated. These models are provided in the following equations: Pseudo-first-order : nt ¼ ne ð1 ek1 t Þ Pseudo-second-order : nt ¼ k2

ne 2 t 1 þ k2 ne t

Intraparticle diffusion : nt ¼ ki t 0:5 Elovich : nt ¼

Figure 10. Effect of contact time on the adsorption of Cd2þ onto FePB, Fe/Cr0.5-PB, and Cr-PB (initial Cd2þ concentration: 0.445 mmol dm3, adsorbent concentration: 2 g dm3, temperature: 25 °C).

lnðRβÞ lnðtÞ þ β β

ð5Þ ð6Þ ð7Þ ð8Þ

In these equations, nt is the concentration of Cd2þ on the adsorbent at a given time, and ne is the equilibrium concentration of Cd2þ. Moreover, k1 (min1), k2 (g mmol1 min1), and ki (mmol g1 min1/2) are the rate constants of pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, respectively. In the Elovich equation, R (mmol g1 min1) is a parameter related to the initial rate of adsorption, and β (g mmol1) is related to the surface coverage. First-order, second-order, intraparticle diffusion, and Elovich kinetic models of Fe-PB, Fe/Cr0.5-PB, and Cr-PB are shown in Figure 11. Model parameters obtained from nonlinear regression

Figure 11. Kinetics of the adsorption of Cd2þ onto Fe-PB, Fe/Cr0.5-PB, and Cr-PB (initial Cd2þ concentration: 0.445 mmol dm3, adsorbent concentration: 2 g dm3, temperature: 25 °C). 7237


2.71 103 0.770 49.8 3.66 10 0.689 0.041

Kd

ΔG

ΔH

ΔS

t (°C)

(dm3 g1)

(kJ mol1)

(kJ mol1)

(kJ mol1 K1)

25

1.29

0.63

10.9

0.04

30

1.33

0.73

35

1.46

0.96

40

1.58

1.20

25

0.65

1.08

16.9

0.05

30

0.69

0.92

35

0.79

0.60

40

0.89

0.31

25

0.79

0.59

6.1

0.02

30

0.89

0.30

35

0.97

0.08

40

1.17

0.40

Fe-PB

0.117

3.46 103 0.717 50.6 0.601 0.043

4.89 10

Table 5. Distribution Coefficients and Thermodynamic Parameters for the Adsorption of Cd2þ on Pillared Bentonite Samples at Different Temperatures (Initial Concentration of Cd2þ: 0.445 mmol dm3, Adsorbent Concentration: 2 g dm3)

3

0.150

2.05 103 0.899 35.1

3

0.872 0.050

2.60 103

0.070

χ2 R2 χ2 R2 ki

1.51 10 0.122

3.72

0.987

4

8.49 10

4.89 10 0.958 0.115 0.130

0.252

4.68 0.124 3.72 10 0.970 0.118 0.128

0.295

4

0.993

5

6.98 104 0.966 1.03 0.162 1.60 103 0.921 0.151 0.159

0.103

4

R2 k2 ne χ2 R2

Cr-PB

Fe/Cr0.5-PB

Fe-PB

pseudo-second order pseudo-first order

k1 ne sample

ne-exp. mmolg

1

Table 4. Kinetic Parameters for the Adsorption of Cd2þ on Pillared Bentonite Samples at 25°C

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Fe/Cr0.5-PB

Cr-PB

χ2

intraparticle diffusion

R

β

Elovich


analysis are also shown in Table 4. For all of the materials, the lowest R2 and highest χ2 were obtained with the Elovich and intraparticle diffusion models; thus, these models did not fit the experimental data. Both pseudo-first-order and pseudo-second-order models provided high R2 and low χ2 values for all of the materials; however, the highest R2 and lowest χ2 values were obtained with the pseudo-second-order model. Thus, the adsorption of Cd2þ on the studied materials best fit the pseudo-second-order kinetic model. Moreover, the theoretical value of ne was obtained from model equalities using nonlinear regression analysis, and the results were similar to the experimental values, further supporting the conclusion that the experimental data correlated best with the pseudo-second-order kinetic model. This result implies that chemisorption may play an important role in the adsorption of Cd2þ onto Fe-PB, Fe/Cr0.5-PB, and Cr-PB. The pseudo-second-order rate constant (k2) of the adsorbents (Table 4) indicated that highest adsorption rates were observed with Fe/ Cr0.5-PB, followed by Fe-PB and Cr-PB. 3.2.4. Effect of Temperature. The amounts of Cd2þ adsorbed by Fe-PB, Fe/Cr0.5-PB, and Cr-PB were measured between 25 and 40 °C. Thermodynamic parameters, such as the Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) of adsorption, were determined using the following equations:17,37 ΔS ΔH R RT

ð9Þ

ΔG ¼ ΔH TΔS

ð10Þ

ln Kd ¼

where R (8.314 J/mol K) is the gas constant, T (K) is the absolute temperature, and Kd (dm3 g1) is the distribution coefficient of the adsorbate defined as ne/Ce. By plotting a graph of lnKd versus 1/T (figure not shown), ΔH and ΔS were determined from the slope and intercept of the plot, respectively. The thermodynamic 7238


Industrial & Engineering Chemistry Research parameters of Cd2þ adsorption are provided in Table 5. The positive value of ΔH was indicative of the endothermic nature of the process of adsorption of Cd2þ. The positive value of ΔS illustrates increased randomness at the solidsolution interface indicating strong affinity of the adsorbent for Cd2þ ions.17 Sen Gupta and Bhattacharyya14 reported positive values of ΔH and ΔS for the adsorption of Cd2þ onto clay-based adsorbents. However, Sharma15 observed negative values for ΔH and ΔS for the adsorption of Cd2þ onto clay. The ΔG value was negative over the entire temperature range only for Fe-PB. In contrast, most of the ΔG values at various temperatures were positive for Fe/Cr0.5-PB and Cr-PB. The ΔG value was negative only for Cr-PB at 40 °C. These results indicated that the adsorption of Cd2þ onto Fe-PB was thermodynamically feasible and could occur spontaneously. A positive ΔG indicated that the adsorptive forces were not strong enough to shift the reaction to the right. Thus, the adsorption of Cd2þ onto the surface of Fe/Cr0.5-PB and Cr-PB could not occur spontaneously at low temperatures. However, as the temperature of adsorption increased, the amount of Cd2þ adsorbed onto Fe/ Cr0.5-PB and Cr-PB increased, and ΔG of adsorption decreased. Overall, these results suggested that the adsorption of Cd2þ was favorable at higher temperatures. Similarly, Unuabonah et al.44 found that the adsorption of Cd2þ onto sodium tetraboratemodified kaolinite clay displayed a positive ΔG.

4. CONCLUSION Bentonites pillared with iron, with iron/chromium at 9:1, 5:5, and 1:9 iron/chromium molar ratios, and with chromium were prepared, and the adsorption behavior of Cd2þ onto some of the samples was investigated by batch adsorption. X-ray diffraction analysis confirmed that pillaring was successful. Chemical analysis confirmed the incorporation of Fe3þ and Cr3þ species into the pillared bentonites. The results of characterization study performed for synthesized adsorbents indicated that natural clay samples’ surface, textural, and acidity characteristics of pillaring solution could effectively be modified depending on chemical composition. It has been shown that the textural properties of Fe, Fe/Cr-, and Cr-containing clays can be controlled by the Fe/Cr ratio of Fe/Cr-containing pillaring solution and the nature of polyoxocation. The increase in chromium content leads to the increase in total surface area, total pore volume, and basal spacing values of Fe/Cr-pillared bentonites. Moreover, it was seen that the surface acidities of the pillared bentonites also changed depending on the nature of the pillaring agents and Fe/Cr ratios. Furthermore, the results of this study indicated that the adsorption process was affected by experimental conditions, such as pH, contact time, and temperature. Moreover, the amount of Cd2þ adsorbed onto Fe-PB was greater than the amount adsorbed onto Fe/Cr0.5-PB and Cr-PB, and the maximum adsorption capacity of Fe-PB was 0.667 mmol/g. The low leaching of iron species from Fe-PB suggests that the iron species were strongly fixed to the clay mineral layers. Whereas the adsorption isotherms of Cr-PB could be described by the RedlichPeterson model, the first-order model best described the isotherms obtained from Fe-PB and Fe/Cr0.5PB. Moreover, the adsorption of Cd2þ ions onto the studied materials followed a pseudo-second-order kinetic model. The Gibbs free energy of Cd2þ adsorption onto Fe-PB was negative; thus, the adsorption processes could occur spontaneously. However, in the adsorption of Cd2þ by Fe/Cr0.5-PB

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and Cr-PB, the Gibbs free energy of the process was positive, and the adsorption processes could only occur at high temperatures. The results of this study indicated that Fe-PB, Fe/Cr0.5-PB, and Cr-PB could be used as effective adsorbents for the removal of Cd2þ from aqueous solutions.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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