Adsorption of methylene blue onto sonicated sepiolite from aqueous solutions

Adsorption of methylene blue onto sonicated sepiolite from aqueous solutions

Ultrasonics Sonochemistry 17 (2010) 250–257 Contents lists available at ScienceDirect Ultrasonics Sonochemistry journal homepage: www.elsevier.com/l...

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Ultrasonics Sonochemistry 17 (2010) 250–257

Contents lists available at ScienceDirect

Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch

Adsorption of methylene blue onto sonicated sepiolite from aqueous solutions _ Ilknur Küncek, Savasß Sß ener * Department of Environmental Engineering, University of Mersin, 33343 Mersin, Turkey

a r t i c l e

i n f o

Article history: Received 17 February 2009 Received in revised form 12 May 2009 Accepted 14 May 2009 Available online 20 May 2009 Keywords: Ultrasonic treatment Sepiolite Dye removal Adsorption Isotherm

a b s t r a c t The aim of the present study is to enhance the methylene blue (MB) adsorption of sepiolite by ultrasonic treatment. The natural sepiolite was pretreated by sonication to improve the surface characteristics and enhance the dye uptake capacity. Sonication process resulted in a significant increase in the specific surface area (SSA) of sepiolite. The FTIR spectrum of the sonicated sepiolite indicates that the tetrahedral sheet is probably distorted after sonication process. The effect of various parameters such as sonication, pH, initial dye concentration and temperature on dye adsorption has been investigated. The adsorbed amount of MB on sepiolite increased after sonication as well as with increasing pH and temperature. The experimental data were evaluated by applying the pseudo-first- and second-order, and the intraparticle diffusion adsorption kinetic models. Adsorption process of MB onto sepiolite followed the pseudo-second-order rate expression. The experimental data were analyzed by Langmuir and Freundlich isotherms, and found that the isotherm data were reasonably well correlated by Langmuir isotherm. Maximum monolayer adsorption capacity of sepiolite for MB increased from 79.37 to 128.21 mg/g after the sonication. Various thermodynamic parameters, such as DG0, DH0 and DS0 were calculated. The thermodynamics of MB/sepiolite system indicated spontaneous and endothermic nature of the process. Adsorption measurements showed that the process was very fast and physical in nature. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The residual dyes in the wastewater of textile industries, even at very low concentrations, are common water pollutants. Their presence in water is highly visible and undesirable and may significantly affect photo-synthetic activity in aquatic life due to reduced light penetration. Adsorption is an effective process for the removal of dyes from waste effluents. Currently, activated carbon is the most common adsorbent because of its higher adsorption capacity. However, because of its relatively high cost, in recent years, there has been an increasing interest in utilizing non-conventional adsorbents such as clay minerals as alternative low-cost adsorbents for the removal of dyes from aqueous solution. Sepiolite is a natural clay mineral with a unit cell formula of magnesium hydrosilicate Si12Mg8O30(OH)4(OH2)4  8H2O [1]. It is structurally formed by blocks and channels extending in the fibre direction (c-axis). Each structural block is composed of two inverted tetrahedral silica sheets and a central magnesium octahedral sheet. In the inner blocks, all corners of the silica tetrahedral are connected to adjacent blocks, but in the outer blocks, some of the corners are Si atoms bound to hydroxyls (Si–OH) [2,3]. This unique fibrous structure with interior channels and its high surface area allows penetration of organic and inorganic ions into the

* Corresponding author. Tel.: +90 3243610001/7089; fax: +90 3243610032. E-mail address: [email protected] (S. Sßener). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.05.012

structure of sepiolite [4,5]. Some investigators have studied adsorptive properties of sepiolite for removing cationic dyes onto sepiolite [6–13]. Surface reactivity of the clay minerals can be enhanced with particle-size reduction, which traditionally has been produced by grinding (either wet or dry). The dry grinding process of clay minerals has been previously studied by Cicel and Kranz [14], PerezRodriguez et al. [15] and Stepkowska et al. [16]. The mechanical treatment by dry grinding results in particle size reduction (delamination and lateral size reduction), folding and gliding of layers, as well as, aggregation of the newly formed particles into spherical particles [17,18]. These treatments increase the specific surface area (SSA) of clay minerals but, at the same time, they produce undesirable effects, such as the degradation of the crystal structure [19]. The progressive grinding produces significant microstructural modification (reaglomeration process) that results in amorphization of the material. Initially, grinding produces an increase of the specific surface area. However, when grinding proceeds, it results in a decrease of the surface area [20]. Very recently, it has been reported the use of sonication, as an alternative to grinding for particle size reduction of some silicates and clay minerals. The sonication produces significant delamination and lateral size reduction while the crystalline structure is retained. Micron and submicron-sized clay minerals with narrow particle size distribution have been prepared from natural macroscopic samples such as micas [21,22], kaolinite [20,23], vermiculite [24], pyrophyllite [25] and talc [26].

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Notations maximum monolayer adsorption capacity (mg/g) adsorption capacity in equilibrium (mg/g) amount of adsorption at time t (mg/g) universal gas constant (8.314 J/mol K) dimensionless separation factor linear correlation coefficient entropy change of adsorption (J/mol K) absolute temperature (K) time (min) volume of solution (L) mass of adsorbent (g)

0.30% Na2O, 0.03% TiO2 and 19.46% loss on ignition. The XRD examination was performed using a Philips PW3710 X-ray powder diffractometer employing Cu Ka radiation. The X-ray diffraction pattern (see Fig. 1) indicates that this structure belongs to sepiolite. A thiazin group cationic MB (Merck grade, C.I. 52015) was used as model adsorbate. The concentrations of the dye in the solution after equilibrium adsorption was determined with Shimadzu brand UV-160 UV-visible spectrophotometer by measuring absorbance at kmax of 664 nm. 2.2. Experimental procedure Ultrasonic treatment was performed in order to break up aggregates in the suspension by using a Sonix ultrasonic processor of 750 W output with a 20 kHz converter and a solid titanium probe of 13 mm (1/2 inches) diameter. The probe tip was dipped into a cylindrical cell of 10 cm internal diameter, where 10 g of samples were mixed with 200 mL of freshly deionised water. The particle size distributions of the sepiolite samples before and after sonication were determined by laser light scattering method (Mastersizer 2000, Malvern). Measurements were performed in diluted aqueous dispersions. The SSA of sepiolite samples before and after sonication were determined using the BET method (N2 adsorption–desorption at 77 K) with a Quantachrome Autosorb-1 Analyzer. FTIR spectra of natural, sonicated and MB adsorbed sonicated sepiolite samples in the wave number range of 650–4000 cm1 were obtained by using Perkin Elmer Spectrum 100 FTIR spectrophotometer. Single stage batch adsorption tests were conducted using a IKA RT5 model magnetic stirrer operated at 400 rpm. A suspension

d=1.5083

d=2.2560

The lump size samples of a-sepiolite obtained from Eskisßehir, Turkey were used as an adsorbent in this study. The sepiolite sample was treated as follows: the lump size sample was subjected to crushing; the fine grained suspension containing 20 g/L sepiolite was mechanically stirred for 24 h; after waiting for about 5 min the supernatant suspension was filtered through filter paper. After mechanical cleaning of the sample, solid impurities were sieved to obtain 850 lm (20 mesh, ASTM) size fraction. Then, it was dried at 105 °C for 2 h, and used in further experiments. The sample was analyzed for its chemical composition and found to contain 52.41% SiO2, 25.53% MgO, 0.35% CaO, 0.62% Al2O3, 0.76% Fe2O3, 0.54% K2O,

d=7.4179

2.1. Materials

Relative intensity

2. Materials and methods

d=2.5368 d=2.4092

Ultrasound represents mechanical waves, i.e. a variation of pressure or density with frequencies above the human hearing threshold (ca. 18 kHz). As it is not perceived, high sound intensities are feasible, where non-linear phenomena like acoustic cavitation occur [27]. The ultrasonic energy has the capacity to produce crack propagation from within the particle to its outer surface, producing an efficient fracture. Due to high sound intensities, the tensile stress of the liquid exceeds. Little gas bubbles are formed during the expansion cycle of the sound wave and grow over one or several cycles. After having reached a critical size, they collapse. During the collapse, a lot of energy is released inducing extreme thermodynamic conditions of several thousand Kelvin and a few hundred bars in the vicinity of the imploding bubble [28,29]. These extreme thermodynamic conditions induce different mechanical effects. Cavitational collapse of bubbles on solid surfaces leads to micro jet and shock-wave impacts on the surface of the solids, together with interparticle collisions which can result in particle size reduction [30]. The aim of the present study is to enhance the methylene blue (MB) adsorption capacity of sepiolite by ultrasonic treatment. The natural sepiolite was pretreated by sonication to improve the surface characteristics and enhance the dye uptake capacity. The effect of various parameters such as sonication, pH, initial dye concentration and temperature on dye adsorption has been investigated. Batch kinetic experiments were performed to provide appropriate equilibrium times. The experimental data were evaluated by applying the pseudo-first- and second-order, and the intraparticle diffusion adsorption kinetic models. The Langmuir and Freundlich isotherm models were tested for their applicability. The thermodynamic parameters were evaluated to define mechanisms of the process. FTIR spectra of natural sepiolite, the sonicated sepiolite and the dye adsorbed sonicated sepiolite were also investigated.

Q0 qe qt R RL r2 DS0 T t V W

d=4.9892 d=4.4372 d=4.2661 d=3.7055 d=3.3024 d=3.1638 d=2.9900

Kads KF KL N

initial dye concentration in solution (mg/L) equilibrium dye concentration (mg/L) Gibbs free energy of adsorption (kJ/mol) enthalpy change of adsorption (kJ/mol) rate constant of pseudo-first-order model (1/min) rate constant of pseudo-second-order model (g/mg min) rate parameter of intraparticle diffusion model (mg/g min0.5) sorption equilibrium constant Freundlich constant ((mg/g)(L/mg)1/n) Langmuir constant (L/mg) Freundlich constant

d=11.9964

C0 Ce DG0 DH0 k1 k2 ki

0 10

20

40 30 2 theta (o)

50

Fig. 1. The XRD pattern of sepiolite.

60

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containing 0.25 g of adsorbent sample was contacted with 250 mL aqueous solution of dye at a known initial concentration in a flask that was immersed in a thermostated water bath to maintain constant working temperature. An aliquot of the solution (10 mL) was withdrawn at predetermined time intervals, and was centrifuged at 5600 rpm for 15 min in order to remove any adsorbent particles. The residual dye concentration in the filtrate was subsequently determined by colorimetric method with Shimadzu brand UV160 UV-visible spectrophotometer. The adsorption tests were continued until the equilibrium concentration was reached. The experiments were carried out by varying the concentration of dye solutions from 25 to 400 mg/L. The inherent pH of the sepiolite suspension without adding dye was measured as 8.3. The pH of the solution was adjusted by addition of a NaOH or HCl (0.1 N) solution under continuous stirring. A WTW 340i Model pH meter was used for measurement of the pH of solutions. The obtained data from the adsorption tests were then used to calculate the adsorption capacity, qe (mg/g), of the adsorbent by a mass–balance relationship (Eq. (1)), which represents the amount of adsorbed dye per amount of dry adsorbent:

qe ¼

ðC o  C e ÞV W

ð1Þ

where C0 and Ce are the initial and equilibrium concentrations of dye in solution (mg/L), respectively, V is the volume of the solution (L), and W is the weight of the dry adsorbent used (g). Finally, the adsorption capacity, qe, was plotted against equilibrium concentration, Ce. All of the experiments were duplicated to check the reproducibility of data and the average value was taken. Several kinetic models are available to understand the behavior of the adsorbent and also to examine the controlling mechanism of the adsorption process and to test the experimental data. In order to investigate the mechanism of process and potential rate controlling steps such as mass transfer and chemical reaction, the experimental kinetic data for the uptake of dye were modeled by the pseudo-first-order by Lagergren [31] and the pseudo-second-order by Ho [32], and the intraparticle diffusion equations by Weber and Morris [33], given in Eqs. (2)–(4), respectively.

logðqe  qt Þ ¼ logðqe Þ 

k1 t 2:303

ð2Þ

t 1 1 ¼ þ t qt k2 q2e qe

ð3Þ

qt ¼ ki t 0:5 þ C

ð4Þ

qt and qe are the amount of dye adsorbed (mg/g) at contact time t (min), C is the intercept (mg/g) and at equilibrium, k1, k2 and ki are the rate constants of pseudo-first-order, pseudo-second-order and intraparticle diffusion models, respectively. The adsorption isotherm indicates how the adsorption molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. Two wellknown equilibrium models: Freundlich [34] and Langmuir [35] were applied for the analysis of adsorption data obtained at different initial dye concentrations and temperatures, keeping the adsorbent concentration constant at 1 g/L. The applicability of the isotherm equations is compared by judging the correlation coefficients, r2. The Freundlich adsorption isotherm is expressed by the following equations obtained on the assumption that the multi layer adsorption takes place on a heterogeneous adsorbent surface.

log qe ¼ log K F þ

1 log C e n

The Langmuir isotherm is represented by the following equations:

Ce 1 1 ¼ þ Ce qe Q 0 K L Q 0

ð6Þ

where qe is solid phase adsorbate concentration at equilibrium (mg/ g), Ce is aqueous phase adsorbate concentration at equilibrium (mg/ L), Q0 (mg/g) is the maximum amount of adsorbate per unit weight of adsorbent to form a complete monolayer on the surface, KL is Langmuir isotherm constant (L/mg) related to the affinity of the adsorption sites. The shape of isotherm was used to predict whether a sorption system is ‘favorable’ or ‘unfavorable’ in batch processes [36]. According to Hall et al. [37], the essential features of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter RL, which is defined by the following relationship:

RL ¼

1 1 þ K LCo

ð7Þ

where RL is a dimensionless separation factor, C0 is initial concentration (mg/L) and KL is Langmuir constant (L/mg). The parameter RL indicates the shape of the isotherm accordingly: The value of RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). Thermodynamic parameters such as Gibbs free energy (DG0), enthalpy (DH0), and entropy (DS0) change of adsorption can be evaluated by using following equations [38]. The adsorption equilibrium constants, Kads, were used in following equation to determine the DG0 of adsorption process at different temperatures.

DG0 ¼ RT ln K ads

ð8Þ

where R universal gas constant (8.314 J/mol K). The Kads may also be expressed in terms of the DH0 (kJ/mol) and DS0 (J/mol K) as a function of temperature:

ln K ads ¼ 

DH 0 DS0 þ RT R

ð9Þ

The values of DH0 and DS0 can be calculated from the slope and intercept of a van’t Hoff linear plot of ln Kads vs. 1/T. 3. Results and discussion 3.1. Effect of sonication on particle size distribution and specific surface area of sepiolite Fig. 2 shows the particle size distribution of the sepiolite samples in volume fraction of particle before and after sonication for 5 h. The untreated sepiolite sample has a very broad particle size distribution in the range of micron-sized particles. The mean particle size of this distribution is 29.7 lm. As seen in the figure (Fig. 2), there is significant particle size reduction after a sonication period of 5 h, although particle size distribution shows a bimodal particle size distribution. The volume fraction of the fine-sized particles is predominant as compared with that of the larger particles. The mean particle size drops after 5 h sonication to 3.8 lm. The effect of sonication on the SSA of sepiolite was also investigated. The decrease in the particle size with sonication results a significant increase in the SSA. Thus, the SSA of sepiolite rises from 322 m2/g to 487 m2/g after 5 h of sonication.

ð5Þ

where KF and n are the Freundlich constants which represent the adsorption capacity and adsorption intensity of the sorbent, respectively.

3.2. Fourier transform infrared analysis FTIR spectra of natural sepiolite, the sonicated sepiolite and the MB adsorbed sonicated sepiolite are illustrated in Fig. 3. In the FTIR

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0.05

sheet is probably distorted after sonication process. In the FTIR spectrum of the sonicated sepiolite (as seen in Fig. 3c), the disappeared stretching SiO band at 1057 cm1 indicate interactions of the cations of MB and the silanol groups of the sonicated sepiolite.

Natural sepiolite Sonicated sepiolite (5 hrs)

0.04

3.3. Adsorption equilibrium 0.03

0.02

0.01

0.00 0.1

1

10 Diameter (µm)

100

1000

Fig. 2. Particle size distribution of the sepiolite sample and after sonication for 5 h.

3685

a 3267

3573

1668

3350

1211

855 937

1060

688

b %T

3686

1008 3267

3568

1666

3351

1210

855

1057

3685

c

3570

938

690

1014 967 982 3267 1668

3353

855

1211

690

1014 966 981

4000

3.3.1. Effect of sonication on adsorption of MB onto sepiolite The equilibrium adsorption of MB onto sepiolite before and after sonication was studied by measuring the isotherm of adsorption. Fig. 4 represents the quantity of MB uptake against the equilibrium concentration of MB in the solution, and it corresponds to the equilibrium distribution of MB ions between the aqueous and solid phases when the concentration increases. From the graph of this isotherm, the maximum uptake of MB removal by the untreated and the sonicated sepiolite samples was determined as 75.18 and 124.36 mg/g, respectively. The adsorption of dye increases after the sonication. The increase in the MB uptake can be explained by the creation of new adsorbing sites resulting from the decrease in particle size and the increase in SSA of sepiolite after sonication.

3000

2000

1500

1000

650

cm-1 Fig. 3. FTIR spectra of (a) natural sepiolite, (b) the sonicated sepiolite and (c) MB adsorbed on the sonicated sepiolite.

spectrum of natural sepiolite (spectrum (a) in Fig. 3), bands in the 4000–3000 cm1 range corresponding to the vibrations of the Mg– OH group. The band at 3685 cm1 corresponds to stretching vibrations of OH groups attached to octahedral Mg ions located in the interior blocks of sepiolite [39,40]. The band at 3570 cm1 indicates coordinated water molecules weakly hydrogen bonded to the SiO surface and the bands at 3350 cm1 and 1668 cm1 correspond to the OH stretching, representing the zeolitic water in the channels and bound water coordinated to magnesium in the octahedral sheet, respectively [41]. The band at 1668 cm1 developed due to probably the hydroxyl bending vibrations again reflects the presence of bound water [42]. Bands in the 1250–800 cm1 range are observed as a result of the SiO coordination vibrations. The wide band centered with a deep band at 1008 cm1, which is actually composed of four more different bands at 1211, 1060, 937 and 855 cm1 represent the stretching of SiO in the Si–O–Si groups of the tetrahedral sheet [43–45]. The band at 688 cm1 represents the bending vibration of Mg–OH [46]. In the FTIR spectrum of the sonicated sepiolite (as seen in Fig. 3b), the stretching sharp band at 1008 cm1 disappeared and was replaced by a broad band with a series of peaks at 1014, 982 and 967 cm1. It indicates that the symmetry of the tetrahedral

3.3.2. Effect of pH on adsorption of MB onto sepiolite pH is one of the most important factors which controls the adsorption capacity of dye on clay surfaces. Change of pH affects the adsorptive process through dissociation of functional groups on the adsorbent surface active sites. For clay minerals the potential determining ions are H+ and OH and complex ions formed by bonding with H+ and OH. Adsorption process usually depends on the electrokinetic properties of clay minerals which determines with H+/OH amount. In order to study the effect of pH on the adsorption capacity of MB onto the sonicated sepiolite, experiments were performed using various initial solution pH values (pH 3–10). Fig. 4 also indicates that adsorption of MB onto sepiolite shows the pH-dependent adsorption mechanism. The removal of MB increased from 94.36 mg/g to 133.35 mg/g with increase in pH from 3 to 10. Sepiolite exhibited maximum removals of MB dye, at the highest pH tested (pH 10). This can be explained by considering the surface charge of sepiolite. In the previous studies, the zeta potential of sepiolite suspensions as a function of pH had been investigated. Sepiolite has an isoelectrical point at pH 6.3 [40], 7.4 [46] and 6.6 [47], above which it exhibits negative zeta potential values.

160

140

120

100

qe (mg/g)

Volume fraction

253

80

60 sonicated sepiolite at pH 3 sonicated sepiolite at pH 5 sonicated sepiolite at pH 7 sonicated sepiolite at pH 8.3 sonicated sepiolite at pH 10 untreated sepiolite at pH 8.3

40

20

0

0

50

100

150

200

250

300

350

Ce (mg/L) Fig. 4. The effect sonication and solution pH on the adsorption of MB dye onto sepiolite. (the solid–liquid ratio: 1 g/L, temperature: 25 °C, agitation rate 400 rpm).

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As a result, as the pH of the system increases and the number of negatively charged sites increase due to deprotonation of the surface. A negatively charged surface site on the sepiolite favors the adsorption of MB cations due to electrostatic attraction. Lower adsorption of MB at acidic pH is probably due to the presence of excess H+ ions competing with the dye cations for the adsorption sites [4].

3.3.4. Effect of temperature on adsorption of MB onto sepiolite In order to study the effect of temperature on the adsorption of MB dye on the sonicated sepiolite, a series of experiments were conducted at 25, 40 and 50 °C at pH 8.3. As seen in Fig. 6, the equilibrium adsorption capacity of MB has increased with increasing temperature from 124.36 mg/g at 25 °C to 131.14 mg/g at 40 °C and to 135.42 mg/g at 50 °C, indicating that the dye adsorption on the adsorbent was favored at higher temperatures. This indicated that the adsorption of MB onto sepiolite was controlled by an endothermic process. The increase of adsorption capacity with increasing temperature may be due to an increase in the chemical potential of the dye molecules to penetrate to the surface of sepi-

140

120

q t (mg/g)

100

80

60

40

20

0 0

10 Co=25 mg/L Co=200 mg/L

20

30 t (min) Co=50 mg/L Co=300 mg/L

40

50

140 120 100 qe (mg/g)

3.3.3. Effect of contact time and initial dye concentration on adsorption of MB onto sepiolite The initial concentration provides an important driving force to overcome all mass transfer resistance of ions between the aqueous and solid phases. A series of experiments were performed at different initial MB concentrations, viz., 25, 50, 75, 100, 200, 300, and 400 mg/L. Fig. 5 shows the effect of initial concentration on the adsorption kinetics of MB onto sonicated sepiolite at pH 8.3 and 25 °C. The equilibrium adsorption capacity increased with increasing initial MB concentration, due to the increase in the number of ions competing for the available binding sites in the surface of sepiolite. The kinetics of adsorption represented a shape characterized by a strong increase of the capacity within the first few minutes of contact and gradually tailed off thereafter. This indicated that equilibrium time was independent of concentration. The figure also showed that the contact time to reach the equilibrium was about 60 min.

160

80 60 25°C

40

40°C 50°C

20 0 0

50

100

150

200

250

300

Ce (mg/L) Fig. 6. The effect of temperature on the adsorption of MB dye onto the sonicated sepiolite. (the solid–liquid ratio: 1 g/L, pH 8.3, agitation rate 400 rpm).

olite, and thereby suggesting the possibility of bonding between the MB ions and the functional groups on the adsorption sites of sepiolite. 3.4. Adsorption kinetics modeling and mechanism The adsorption data was analyzed in terms of pseudo-first-order and pseudo-second-order kinetic models. Adsorption rate constants were summarized in Table 1. The plots of log (qeqt) versus t (Fig. 7a) gave straight lines with slope of k1/2.303 and intercept log qe. The values of regression coefficient for pseudo-first-order model (Table 1) indicated that the adsorption kinetics of MB on sonicated sepiolite was not diffusion controlled. The plots of t/qt versus t (Fig. 7b) gave straight lines with slope of 1/qe and intercept 1=k2 q2e . The values of regression coefficient for pseudo-second-order model were nearly unity (>0.9952) for all initial dye concentrations studied. The calculated qe values were very close to that of experimentally obtained qe. Thus the adsorption of MB on sonicated sepiolite was explained by the pseudo-second-order kinetic model. The intraparticular diffusion model was also tested to identify the diffusion mechanism and the results are tabulated in Table 2. The intraparticle diffusion model presents multi-linearity (Fig. 8), indicating that two steps take place. The sharper first-stage portion is attributed to the diffusion of adsorbate through the solution to the external surface of adsorbent or the boundary layer diffusion of solute molecules [48]. The second portion describes the gradual adsorption stage, where intra-particle diffusion rate is rate-limiting [49]. During these two stages, MB ions were slowly transported via intraparticular diffusion in the particles and were finally retained in the pores. From the figure, since the linear portion of the first initial stage does not pass through the origin, there is an initial boundary layer resistance between adsorbent and adsorbate. This deviation from the origin is proportional to the boundary layer thickness, which gives an insight into the tendency of the dye ions to adsorb to the adsorbent or remain in solution [50].

60

3.5. Adsorption isotherms Co=75 mg/L Co=400 mg/L

Co=100 mg/L

Fig. 5. The effect of contact time and initial dye concentration on the adsorption of MB dye onto the sonicated sepiolite. (the solid–liquid ratio: 1 g/L, pH: 8.3, temperature: 25 °C, agitation rate 400 rpm).

Table 3 indicates the results of Langmuir and Freundlich isotherm analyses calculated for adsorption of MB dye onto sepiolite from aqueous solutions before and after sonication at different pH and temperatures. The applicability of the Freundlich isotherm is

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Table 1 Pseudo-first order and pseudo-second order kinetic model parameters for adsorption of MB onto the sonicated sepiolite at 25 °C. Parameters

qe,exp (mg/g)

Pseudo-first-order model

Co (mg/L) 25 50 75 100 200 300 400

21.79 45.04 68.64 91.39 115.97 121.22 124.36

Pseudo-second-order model 1

qe,cal (mg/g)

k1 (min

10.63 24.87 45.76 64.82 92.45 89.14 95.06

4.25x103 7.41x103 7.25x103 6.71x103 7.49x103 7.68x103 7.44x103

)

r2

qe,cal (mg/g)

k2 (g/mg min)

r2

0.8556 0.9024 0.9636 0.9826 0.9670 0.9629 0.9038

22.52 46.05 69.22 91.19 116.65 120.33 125.09

1.39x103 1.84x103 1.83x103 2.60x103 1.52x103 1.59x103 2.01x103

0.9952 0.9986 0.9979 0.9996 0.9992 0.9994 0.9998

140

2.5

(a)

Co=25 mg/L

120

Co=50 mg/L

2

Co=75 mg/L

100

Co=100 mg/L Co=200 mg/L

1.5

qt (mg/g)

Co=300 mg/L

log(qe-qt)

Co=400 mg/L

1

80

60

0.5 40

0 0

10

20

30

40

50

60

70

20

-0.5

0

2

3

-1

4

5

6

7

8

t0.5 (min 0.5)

t (min) Co=25 mg/L

Co=50 mg/L

Co= 75 mg/L

Co=200 mg/L

Co=300 mg/L

Co=400 mg/L

Co=100 mg/L

3.0 Fig. 8. Intraparticle diffusion model for the adsorption of MB onto the sonicated sepiolite at different initial concentrations. (the solid–liquid ratio: 1 g/L, pH: 8.3, temperature: 25 °C, agitation rate 400 rpm).

(b) 2.5

t/qt (min.g/mg)

2.0

analyzed by plotting log (qe) versus log (Ce), but data are not found in good agreement with the correlation coefficients less than 0.8947. The extremely high correlation coefficients (r2 > 0.9959) in all experimental conditions studied suggest that the adsorption of MB onto both natural and the sonicated sepiolite closely follow a Langmuir isotherm. According to the model, the adsorption of MB molecules is limited with monolayer coverage and the surface of sepiolite is relatively homogeneous in terms of functional groups. Once a MB molecule occupies a site on the sepiolite surface, no further adsorption of MB molecule can take place at that site. Theoretically, sepiolite has a finite capacity for adsorbing MB molecules. The rate of adsorption of MB to the sepiolite surface is proportional to a driving force, which is the difference between concentrations in the solution and the bare surface area [51]. The Langmuir monolayer capacity Q0 of natural sepiolite and the sonicated sepiolite for

1.5

1.0

0.5

0.0 0

10

20

30

40

50

60

70

t (min) Fig. 7. Pseudo-first and second-order kinetic models for the adsorption of MB onto the sonicated sepiolite at different initial concentrations. (the solid–liquid ratio: 1 g/L, pH 8.3, temperature: 25 °C, agitation rate 400 rpm).

Table 2 Intraparticle diffusion kinetic model parameters for adsorption of MB onto the sonicated sepiolite at 25oC. Parameters

Intraparticle diffusion model

Co (mg/L)

C1 (mg/g)

ki,1 (mg/g min0.5)

r2

C2 (mg/g)

ki,2 (mg/g min0.5)

r2

25 50 75 100 200 300 400

4.24 9.05 13.53 18.46 21.55 21.62 25.17

1.38 2.34 3.39 4.10 5.38 8.04 6.26

0.9527 0.9700 0.9712 0.9780 0.9727 0.9654 0.9525

16.21 35.36 57.00 72.69 81.75 95.82 103.44

0.80 1.29 1.74 2.70 5.04 3.52 3.39

0.9511 0.9656 0.9604 0.9710 0.9768 0.9480 0.9449

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Table 3 Isotherm constants for MB adsorption onto natural and the sonicated sepiolites. Adsorbent

Parameters

Langmuir isotherm

o

Natural sepiolite Sonicated sepiolite

*

0

Freundlich isotherm

Temp. ( C)

pH

Q (mg/g)

KL (L/mg)

RL*

25 25 25 25 25 25 40 50

8.3 3.0 5.0 7.0 8.3 10.0 8.3 8.3

79.37 105.26 116.28 123.46 128.21 135.14 135.02 138.89

0.065 0.034 0.043 0.064 0.116 0.159 0.135 0.148

0.380.037 0.540.068 0.480.055 0.380.038 0.260.021 0.200.015 0.230.018 0.210.017

2

r

0.9982 0.9978 0.9959 0.9956 0.9988 0.9994 0.9997 0.9994

KF (mg/g) (L/mg)1/n

1/n

r2

5.47 11.16 14.32 20.78 29.75 34.75 35.34 36.05

2.31 2.43 2.56 2.78 2.85 2.97 3.52 4.07

0.8079 0.8340 0.8205 0.7844 0.8947 0.8101 0.7466 0.7767

The RL range indicates the values calculated for minimum and maximum Ce values.

Table 4 Comparison of maximum adsorption capacity of various cationic dyes by some adsorbents. Adsorbent

Adsorbate

Adsorption capacity (mg/g)

References

Natural sepiolite Natural sepiolite Natural sepiolite Natural sepiolite Base activated sepiolite Natural sepiolite Natural sepiolite Perlite Pyrophyllite Zeolite Acid treated diatomite Activated carbon Natural sepiolite Sonicated sepiolite

Methylene blue Reactive dyes Basic blue 41 Crystal violet Crystal violet Astrazon blue FGRL (mixture of basic blue 159 and basic blue 3) Basic red 46 Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue

5287 109169 8 58 131 155209 94106 58227 71 57 127 303345 79 128

[6] [7] [8] [10] [10] [12] [13] [52] [53] [54] [55] [56] This work This work

Table 5 Thermodynamic parameters for adsorption of MB onto the sonicated sepiolite for 2550 °C.

25 40 50

Thermodynamical parameters KL (L/mg)

Kads (L/mol)  104

DG0 (kJ/mol)

DH0 (kJ/mol)

DS0 (J/mol K)

0.116 0.135 0.148

3.710 4.318 4.734

26.068 27.774 28.909

2.648

102.47

MB dye at pH 8.3 and 25 °C were found as 79.37 and 128.21 mg/g, respectively. The values of RL in all experimental conditions studied are presented in Table 3. The RL values calculated indicate that adsorption of MB dye on sonicated sepiolite is favorable (0 < RL < 1) within the experimental conditions studied. The RL values indicate that the adsorption is more favorable for the higher initial MB concentrations than for the lower ones. The maximum adsorption of MB onto sepiolite before and after sonication was compared with some cationic dyes and MB removal by various adsorbents is presented in Table 4. Compared with some data in literature, sepiolite after sonication process studied in this work, has relatively higher adsorption capacity for removing MB.

10.8

10.75

10.7

ln (Kads )

T (oC)

10.65

y = -0.9385x+ 13.671 2

R =1

10.6

3.6. Thermodynamic parameters In this study, the thermodynamic parameters during the adsorption process were calculated for 25–50 °C using the Langmuir isotherm, i.e., by replacing the equilibrium constant, Kads from Eqs. (8) and (9) by the Langmuir isotherm constant, KL (L/mol) and are given in Table 5. The increase in values of Kads with rise in temperature indicated the endothermic nature of the process. A van’t Hoff plot of ln Kads vs. 1/T (Fig. 9) was found to be linear.

10.55

10.5 3.05

3.1

3.15

3.2

3.25

3.3

3.35

3.4

1/T (K-1)x103 Fig. 9. A plot of ln Kads against 1/T for MB sorption by the sonicated sepiolite.

_ Küncek, S. Sßener / Ultrasonics Sonochemistry 17 (2010) 250–257 I.

The negative values of DG0 (Table 5) indicate the feasibility of the process and the spontaneous nature of adsorption with higher negative value reflecting a more energetically favorable sorption. The value of DG0 became more negative with increasing temperature. This showed that an increase in temperature was favorable for the removal process. Generally, the change of DG0 for physisorption is between 20 and 0 kJ mol1, but chemisorption is a range of 80 to 400 kJ/mol [57]. The results obtained are 26.068 kJ mol1 at 25 °C, 27.774 kJ mol1 at 40 °C and 28.909 kJ mol1 at 50 °C. It should be noted that since the values of DG0 are slightly more than 20 kJ/mol, the interaction between MB dye and sepiolite surface may be weak chemical bonding. The positive value of the enthalpy change, DH0 (2.648 kJ/mol), indicated that the adsorption reaction was endothermic. The positive DS0 value (102.47 J/mol K) showed the increasing randomness at the solid/liquid interface during the adsorption of MB dye ions on the sonicated sepiolite. 4. Conclusion This study investigates the effect of ultrasonic treatment on the adsorption capacity of sepiolite for removing methylene blue. Sonication process resulted in a significant increase in the specific surface area (SSA) of sepiolite. The study presented revealed that the specific surface area of sepiolite significantly increased from 322 to 487 m2/g after 5 h of sonication. The ultrasonic pretreatment of sepiolite also improved the adsorption of MB so that maximum monolayer adsorption capacity increased from 79.37 to 128.21 mg/ g after the sonication. The experimental data fit well with Langmuir isotherm model. Adsorption process of MB onto sepiolite followed the pseudo-second-order rate expression. The thermodynamics parameters, such as DG0, DH0 and DS0, indicated that the adsorption process of MB onto sepiolite is spontaneous and endothermic nature which is favored at higher temperatures and occurs by both physical adsorption and weak chemical interactions. References [1] K. Brauner, A. Preisinger, Tschermaks Mineralogische Petrographische Mitteilungen 6 (1956) 120–140. [2] M. Shirvani, H. Shariatmadari, M. Kalbasi, F. Nourbakhsh, B. Najafi, Colloid Surf. A 287 (1–3) (2006) 182–190. [3] M. Alkan, Ö. Demirbasß, M. Dog˘an, Micropor. Mesopor. Mat. 101 (2007) 388– 396. [4] A. Özcan, E.M. Öncü, A.S. Özcan, Colloid Surf. A 277 (1–3) (2006) 90–97. [5] E. Sabah, J. Colloid Interface Sci. 310 (1) (2007) 1–7. [6] G. Rytwo, D. Tropp, C. Serban, Appl. Clay Sci. 20 (86) (2002) 273–282. [7] O. Özdemir, B. Armagan, M. Turan, M.S. Çelik, Dyes Pigments 62 (2004) 49–60. [8] M. Dog˘an, M. Alkan, Ö. Demirbasß, Y. Özdemir, C. Özmetin, Chem. Eng. J. 124 (1– 3) (2006) 89–101. [9] Y. Özdemir, M. Dog˘an, M. Alkan, Micropor. Mesopor. Mat. 96 (1–3) (2006) 419– 427. [10] E. Eren, B. Afsin, Dyes Pigments 73 (2) (2007) 162–167. [11] M. Dog˘an, Y. Özdemir, M. Alkan, Dyes Pigments 75 (3) (2007) 701–713. [12] B. Karagozoglu, M. Tasdemir, E. Demirbas, M. Kobya, J. Hazard. Mat. 147 (1–2) (2007) 297–306. [13] S.C.R. Santos, R.A.R. Boaventura, Appl. Clay Sci. 42 (2008) 137–145. [14] B. Cicel, G. Kranz, Clay Miner. 16 (1981) 151–162.

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