Kinetics, isotherm and thermodynamic studies of adsorption of Acid Blue 193 from aqueous solutions onto natural sepiolite

Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) 90–97

Kinetics, isotherm and thermodynamic studies of adsorption of Acid Blue 193 from aqueous solutions onto natural sepiolite ∗ , E. Mine Onc¨ ¨ ¨ u, A. Safa Ozcan ¨ Adnan Ozcan Department of Chemistry, Faculty of Science, Anadolu University, 26470 Eski¸sehir, Turkey Received 26 July 2005; received in revised form 29 October 2005; accepted 5 November 2005 Available online 15 December 2005

Abstract In the present study, natural sepiolite was used as an adsorbent for the investigation of the adsorption kinetics, isotherms and thermodynamic parameters of an acid dye (Acid Blue 193, AB193) from aqueous solution at various pHs, temperatures and concentrations. Two simplified kinetic models, first-order and pseudo-second-order, were used to predict the adsorption rate constants. It was found that the kinetics of the adsorption of AB193 onto natural sepiolite at different operating conditions was the best described by the pseudo-second-order model. The rate parameters of the intraparticle diffusion model for adsorption were also evaluated and compared to identify the adsorption mechanisms. Adsorption isotherms and equilibrium adsorption capacities were determined by the fittings of the experimental data to three well-known isotherm models including Langmuir, Freundlich and Dubinin-Radushkevich (D-R). The results showed that the D-R model appears to fit the adsorption better than other adsorption models for the adsorption of AB193 onto natural sepiolite. The equilibrium constants were used to calculate thermodynamic parameters, such as the change of free energy, enthalpy and entropy. © 2005 Elsevier B.V. All rights reserved. Keywords: Sepiolite; Adsorption; Acid dye; Kinetics; Isotherm

1. Introduction Synthetic dyes are being gradually used in the textile, paper, cosmetics, pharmaceutical and food industries. The removal of color from textile effluents, which are toxic, to pollute water and to cause severe damage human beings, is a major problem because of the difficulty in treating such wastewaters by conventional waste treatment methods, such as coagulation, chemical oxidation, membrane filtration, solvent extraction, chemical precipitation, osmosis, etc. These methods have not been very successful since dyes are stable to light, oxidizing agents, high capital cost and operational costs or secondary sludge disposal problem and aerobic digestion. Adsorption has been proved to be an excellent way to treat textile waste effluents, offering significant advantages like the cheapest, easy availability,

∗

Corresponding author. Tel.: +90 222 3350580x5815; fax: +90 222 3204910. ¨ E-mail addresses: [email protected] (A. Ozcan), ¨ u), [email protected] (A.S. Ozcan). ¨ [email protected] (E.M. Onc¨ 0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.11.017

most profitable, easy of operation and most efficient, over may conventional methods especially from an economical and environmental point of view [1–4]. Activated carbon is widely used as an adsorbent due to its high surface area, high adsorption capacity, but it is relatively high price, which limits their usage [5]. For this reason, many researchers have investigated low-cost, locally available, biodegradable substitutes made from natural sources to remove dyes from wastewater [3,6–10]. Clays, such as sepiolite [11,12], zeolite [13], montmorillonite [14], smectite [15] and bentonite [5,16,17] are being considered as alternative low-cost adsorbents. Sepiolite is a fibrous hydrated magnesium silicate and a natural clay mineral with a unit cell formula (Si12 )(Mg8 )(O30 ) (OH)4 (OH)2 ·8H2 O and a general structure formed by an alternation of blocks and tunnels that grow up in the fibre direction. Each block consists of two tetrahedral silica sheets enclosing a central magnesia sheet. However, the silica sheets are discontinued and inversion of these silica sheets gives rise to tunnels in the structure. These characteristics of sepiolite make it a powerful

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adsorbent for organic dye molecules. In addition, some isomorphic substitutions in the tetrahedral sheets of the lattice of the sepiolite, such as Al3+ instead of Si4+ form negatively adsorption sites. Such sites are occupied by exchangeable cations that compensate for the electrical charge [18,19]. Several studies were achieved using natural sepiolite as an adsorbent including catalyst support, wastewater treatment, solid wastes, reducing the toxic effect of some heavy metals and pesticides [20]. Most literature on dye removal is related to cationic dyes. To our knowledge, compared with cationic dyes, only little information exists on the use of natural sepiolite, as an adsorbent for the removal of anionic dyes and also needs to research. The water-soluble anionic dyes are commonly used to dye fabrics like wool, nylon and silk. Due to the weak interactions between the negatively charged surface in clays and anionic dyes, a few studies on the adsorption of acid dyes have been carried out using sepiolite as an adsorbent [11,12,21], but none of them has investigated the kinetics, isotherms and thermodynamics of adsorption of Acid Blue 193 (AB193) onto natural sepiolite. The study of adsorption equilibrium, isotherms and kinetics is essential in supplying the basic information required for the design and operation of adsorption equipments for wastewater treatment. Various models have been put forward to describe or predict the adsorption kinetics. The resistance models, such as the first-order, pseudo-second-order and intraparticle diffusion models and the adsorption isotherms including Langmuir, Freundlich and Dubinin-Radushkevich (D-R) provide a detailed description of the adsorption. The objective of this work is to study the adsorption of Acid Blue 193 (AB193) from aqueous solutions onto natural sepiolite. The effects of temperature, pH, contact time and concentration were examined and the kinetic and the thermodynamic data were also evaluated. 2. Materials and methods 2.1. Materials A commercial textile dye AB193 (Isolan Dark Blue 2-SGL; C.I. 15707) was obtained from Dystar, Turkey and used without further purification. The chemical structure of AB193 is illustrated in Fig. 1. The adsorbent used in this work was provided from Dolsan, Eskis¸ehir-Turkey. It was crushed, ground, sieved through a 63-␮m size sieve and samples collected from under the sieve and dried in an oven at 110 ◦ C for 2 h before use.

Fig. 1. The chemical structure of AB193.

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Natural sepiolite was characterized with respect to its cationexchange capacity (CEC) and surface area by the methylene blue method [22,23] and they were found as 299 mmol kg−1 and 234.3 m2 g−1 , respectively. 2.2. Material characterization The chemical analysis of natural sepiolite was conducted using an energy dispersive X-ray spectrometer (EDX-LINK ISIS 300) attached to a scanning electron microscope (SEM-Cam Scan S4). The crystalline phases present in sepiolite were determined via X-ray diffractometry (XRD-Rigaku Rint 2000) using Cu K␣ radiation. 2.3. Adsorption experiments All pH experiments were conducted by mixing 50 ml of a 3.5 × 10−4 mol dm−3 aqueous dye solution with 0.05 g of natural sepiolite at 25 ◦ C and at various pH values in the ranges 1–11. The solution pH was carefully adjusted by adding a small amount of HCl or NaOH solution and measured using a pH meter (Fisher Accumet AB15), while the dye solutions contained in 100 ml erlenmeyer flasks closed with parafilm to avoid evaporation were stirred using a mechanical magnetic stirrer. Once the optimum pH had been attained, kinetic studies were conducted at this pH value for increasing periods of time, until no more dye was removed from the aqueous phase and equilibrium had been achieved. After such time (60 min), the samples were filtered to remove any organic or inorganic precipitates formed under acidic or basic conditions and the equilibrium concentrations ascertained by spectrophotometer (Shimadzu UV-2101PC) at the respective λmax value, which is 609 nm for AB193. The amount of the dye adsorbed onto natural sepiolite was determined by the difference between the initial and remaining concentrations of dye solution. In order to study the adsorption isotherms for 120 min to allow attainment of equilibrium at a constant temperature of 20 ◦ C and kinetics for time intervals at various temperatures a 0.05 g of natural sepiolite were kept in contact with 50 ml of dye solution of various concentrations. 3. Results and discussion 3.1. Chemical composition of sepiolite The chemical composition of natural sepiolite obtained by using EDX analysis given in Table 1 indicates the presence of silica and magnesium oxide as major constituents along with traces of aluminium, potassium, sodium, iron and titanium oxides in the form of impurities. XRD results combined with EDX analysis show that most of the magnesium is in the form of sepiolite and calcium and some of magnesium are in the form of dolomite. XRD also showed the presence of free quartz in natural sepiolite. It is thus expected that the adsorbate species will be removed mainly by SiO2 and MgO.

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Table 1 Chemical composition of natural sepiolite obtained by EDX analysis Constituents

wt.%

SiO2 MgO CaO Al2 O3 K2 O Na2 O Fe2 O3 TiO2 Loss on ignition

51.17 25.50 7.52 1.04 0.80 0.54 0.40 0.05 12.98

3.2. Effect of pH Natural sepiolite has proved to be an effective adsorbent for the removal of acid dye, AB193, via adsorption from aqueous solution, as can be seen from the data recorded in Fig. 2. It was observed that the adsorption is highly dependent on pH of the solution which affects the surface charge of the adsorbent and the degree of ionization of adsorbate. The maximum removal of AB193 was higher in acidic pH value (ca. 1.5) than in the alkaline pH (Fig. 2). This can be explained as follows: At low pH, natural sepiolite surface is closely associated with the hydronium (H3 O+ ) ions. The surface of natural sepiolite becomes positively charged, thereby increasing electrostatic attractions between negatively charged dye anions and positively charged adsorption sites and causing an increase in the dye adsorption. With the gradual increase in the pH of the solution, a decrease in the positive charge on the oxide or solution interface has been observed and the adsorbent surface appears negatively charged due to deprotonation of the adsorbent surface. The removal of dye decreases at higher pH values may be due to the abundance of OH− ions and because of electrostatic repulsion between the negatively charged surface of adsorbent and the anionic acid dye molecules. There are also no exchangeable anions on the outer surface of the adsorbent at higher pH values due to the presence of excess OH− ions competing with dye anions for adsorption sites and a result of the adsorption decreases [24].

Fig. 2. Effect of pH for the adsorption of AB193 onto natural sepiolite at 20 ◦ C.

Fig. 3. Effect of initial dye concentration for the adsorption of AB193 onto natural sepiolite at 20 ◦ C.

3.3. Adsorption kinetic considerations The influence of the initial concentration of AB193 in the solutions on the rate of adsorption onto natural sepiolite was investigated at the pH value of 1.5. As shown in Fig. 3, when the initial dye concentration was increased from 2.5 × 10−4 to 5.0 × 10−4 mol dm−3 ; the adsorption capacity of dye increased from 1.12 × 10−4 to 2.12 × 10−4 mol g−1 . This indicates that the initial dye concentrations play an important role in the adsorption capacities of AB193 onto natural sepiolite. The influence of contact time on the amount of AB193 adsorbed was investigated at various temperatures as shown in Fig. 4. It is seen that the amount of adsorption increased with increasing the contact time. Maximum adsorption was observed after 60 min, beyond which there was almost no further increase in the adsorption. This was therefore fixed as the equilibrium contact time. The equilibrium adsorption capacity of AB193 onto natural sepiolite was found to decrease with increasing temperature, decreasing from 1.50 × 10−4 mol g−1 at 20 ◦ C to 1.27 × 10−4 mol g−1 at 40 ◦ C indicating that the dye adsorption on the adsorbent was favored at lower temperatures. This result

Fig. 4. Effect of contact time for the adsorption of AB193 onto natural sepiolite at various temperatures.

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Fig. 5. Pseudo-second-order kinetic plots for the adsorption of AB193 onto natural sepiolite at various temperatures.

may be due to an increase in the mobility of AB193 ions with an increase in the temperature in the solution and due to a weakening in the van der Waals forces of attraction between AB193 and natural sepiolite [25,26]. This effect suggests that an explanation of the adsorption mechanism associated with the removal of AB193 onto natural sepiolite involves a physical process. Three kinetic models, i.e. the first-order equation, the pseudosecond-order equation and an intraparticle diffusion equation, were considered for interpreting the experimental data: The first-order rate expression [27] is given as:

 1 1 k1 1 = (1) + qt q1 t q1 The pseudo-second-order kinetic model [28] is expressed as: t 1 1 = + t 2 qt q2 k2 q 2

(2)

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order kinetic model. A similar phenomenon has been observed in the adsorption of acid dyes by acid-activated bentonite [5], surfactant-modified bentonite [16,17], Acid Red 57 and Acid Blue 294 onto sepiolite [11] and Acid Red 57 onto surfactantmodified sepiolite [12]. The pseudo-second-order rate constants indicate a steady increase from 2.69 × 103 to 6.00 × 103 dm3 mol−1 min−1 with an increase in the solution temperatures from 20 to 40 ◦ C (Table 2). It may be concluded that the adsorption of AB193 onto natural sepiolite follows a physisorption mechanism, increasing temperature generally increases the rate of approach to equilibrium, but decreases the equilibrium adsorption capacity. The first-order and pseudo-second-order models cannot identify the diffusion mechanism and the kinetic results were then subjected to analyze by the intraparticle diffusion model [29] for diffusion mechanism. If this does occur, then the plot of uptake, qt , versus square root of time, t1/2 , should be linear and if it passes through the origin then intraparticle diffusion will be the sole rate-limiting process [11,12,27,30,31]. In the present study, it was found that the plots of qt versus t1/2 exhibited an initial linear portion followed by a plateau which occurred after 60 min for natural sepiolite (figure not shown). The initial curved portion of the plots seems to be due to boundary layer adsorption and the linear portion to intraparticle diffusion, with the plateau corresponding to equilibrium. However, neither plot passed through the origin. This indicates that although intraparticle diffusion was involved in the adsorption process, it was not the rate-controlling step. Values of the intraparticle diffusion constant, kp , were obtained from the slopes of the linear portions of the plots and are listed in Table 2. The correlation coefficients for the intraparticle diffusion model (rp2 ) were 0.700 and 0.990. These values indicate that the adsorption of AB193 onto natural sepiolite may be followed by an intraparticle diffusion up to 60 min.

The intraparticle diffusion equation [29] can be written by following:

3.4. Adsorption isotherms

qt = kp t 1/2 + C

The adsorption data were analyzed to see whether the isotherm obeyed the Langmuir [32], Freundlich [33] and Dubinin-Radushkevich (D-R) [34] isotherm models equations. Langmuir equation: 
1 1 1 1 = + (4) qe qmax qmax KL Ce

(3)

where q1 and qt are the amounts of dye adsorbed on adsorbent at equilibrium and at various times t (mol g−1 ); k1 , the rate constant of the first-order model for the adsorption process (min−1 ); q2 , the maximum adsorption capacity (mol g−1 ) for the pseudo-second-order adsorption; k2 , the rate constant of the pseudo-second-order model for the adsorption process (g mol−1 min−1 ); C, the intercept; and kp , the intraparticle diffusion rate constant (mol g−1 min−1/2 ). The straight-line plots of 1/qt versus 1/t for the first-order reaction (figure not shown) and t/qt against t for the pseudo-second-order reaction (Fig. 5) for the adsorption of AB193 onto natural sepiolite have also been tested to obtain the rate parameters. The kinetic parameters of AB193 under different conditions were calculated from these plots and are given in Table 2. The correlation coefficients (r12 ), for the first-order kinetic model are between 0.752 and 0.994 and the correlation coefficients (r22 ), for the pseudo-second-order kinetic model are 0.999. It is probable, therefore, that this adsorption system is not a first-order reaction, it fits the pseudo-second-

Freundlich equation: ln qe = ln KF +

1 ln Ce n

(5)

D-R equation: ln qe = ln qm − βε2

(6)

where qe is the equilibrium dye concentration on the adsorbent (mol g−1 ); Ce , the equilibrium dye concentration in solution (mol dm−3 ); qmax , the monolayer capacity of the adsorbent (mol g−1 ); KL , the Langmuir constant (dm3 mol−1 ) and related to the free energy of adsorption; KF , the Freundlich constant (dm3 g−1 ); n (dimensionless), the heterogeneity factor

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Table 2 Kinetic parameters for the adsorption of AB193 onto natural sepiolite at various temperatures t (◦ C)

First-order C0

(mol dm−3 )

Pseudo-second-order k1

(min−1 )

q1

(mol g−1 )

r12

k2

(dm3

mol−1

min−1 )

Intraparticle diffusion q2

(mol g−1 )

r22

kp (mol g−1 min−1/2 )

C (mol g−1 )

rp2

20

2.5 × 10−4 3.0 × 10−4 3.5 × 10−4 4.0 × 10−4 4.5 × 10−4 5.0 × 10−4

2.308 2.446 2.018 2.890 3.051 3.922

1.13 × 10−4 1.25 × 10−4 1.48 × 10−4 1.60 × 10−4 1.88 × 10−4 2.18 × 10−4

0.752 0.876 0.942 0.943 0.975 0.958

4.49 × 103 3.32 × 103 2.69 × 103 2.38 × 103 1.80 × 103 1.18 × 103

1.13 × 10−4 1.25 × 10−4 1.50 × 10−4 1.59 × 10−4 1.87 × 10−4 2.18 × 10−4

0.999 0.999 0.999 0.999 0.999 0.999

4.62 × 10−6 4.42 × 10−6 3.94 × 10−6 6.59 × 10−6 7.01 × 10−6 10.1 × 10−6

7.73 × 10−5 8.91 × 10−5 1.14 × 10−4 1.06 × 10−4 1.27 × 10−4 1.31 × 10−4

0.868 0.761 0.933 0.990 0.919 0.888

30 35 40

3.5 × 10−4 3.5 × 10−4 3.5 × 10−4

1.142 1.385 1.338

1.41 × 10−4 1.33 × 10−4 1.28 × 10−4

0.714 0.994 0.961

3.46 × 103 5.40 × 103 6.00 × 103

1.43 × 10−4 1.33 × 10−4 1.27 × 10−4

0.999 0.999 0.999

2.47 × 10−6 2.78 × 10−6 2.78 × 10−6

1.20 × 10−4 1.11 × 10−4 1.06 × 10−4

0.700 0.876 0.856

which has a lower value for more heterogeneous surfaces; β, a constant related to the mean free energy of adsorption per mole of the adsorbate (mol2 kJ−2 ); qm , the theoretical saturation capacity; and ε, the Polanyi potential, which is equal to RT ln(1 + (1/Ce )), where R (J mol−1 K−1 ) is the gas constant and T (K) is the absolute temperature. Hence by plotting ln qe versus ε2 it is possible to obtain the value of qm (mol g−1 ) from the intercept, and the value of β from the slope. Fig. 6 indicates the D-R isotherm for AB193 adsorption onto natural sepiolite. The Langmuir, Freundlich and D-R parameters for the adsorption of AB193 onto natural sepiolite being listed in Table 3. The fit of the data for AB193 adsorption onto natural sepiolite suggests that the D-R model gave slightly closer fittings than those of Langmuir and Freundlich models, as is obvious from a comparison of the r2 in Table 3. The constant β gives an idea about the mean free energy E (kJ mol−1 ) of adsorption per molecule of the adsorbate when it is transferred to the surface of the solid from infinity in the solution and can be calculated using the relationship [35–37]: E=

1 (2β)1/2

(7)

This parameter gives information about adsorption mechanism is ion-exchange or physical adsorption. The magnitude of E is between 8 and 16 kJ mol−1 , the adsorption process follows

by ion-exchange [38], while for the values of E < 8 kJ mol−1 , the adsorption process is of a physical nature [39]. The numerical value of adsorption of the mean free energy is 7.110 kJ mol−1 (Table 3) corresponds to a physisorption and the predominance of van der Waals forces. The effect of isotherm shape has been discussed [40] with a view to predict whether an adsorption system is favorable or unfavorable. The essential feature of the Langmuir isotherm can be expressed by means of ‘RL ’, a dimensionless constant referred to as separation factor or equilibrium parameter RL is calculated using the following equation. RL =

1 1 + K L C0

(8)

where C0 is the highest initial dye concentration (mol dm−3 ). The values of RL calculated as above equation are incorporated in Table 3. As the RL values lie between 0 and 1, the on-going adsorption process is favorable [40,41]. Further, the RL value for AB193 onto natural sepiolite at 20 ◦ C is 0.609 and therefore, its adsorption is favorable. 3.5. Thermodynamic parameters In any adsorption process, both energy and entropy considerations must be taken into account in order to determine what Table 3 Isotherm constants for the adsorption of AB193 onto natural sepiolite at 20 ◦ C

Fig. 6. D-R plot for the adsorption of AB193 onto natural sepiolite at 20 ◦ C.

Langmuir qmax (mol g−1 ) KL (dm3 mol−1 ) RL rL2

7.12 × 10−4 1.28 × 103 0.609 0.939

Freundlich n KF (dm3 g−1 ) rF2

0.893 3.990 0.938

D-R qmax (mol g−1 ) β (mol2 kJ−2 ) 2 rD-R E (kJ mol−1 )

1.90 × 10−3 9.89 × 10−3 0.999 7.110

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Table 4 Thermodynamic parameters calculated with the q2 values of pseudo-second-order model for AB193 onto natural sepiolite t (◦ C) 20 30 35 40

Ea (kJ mol−1 )

KC

32.41

0.746 0.690 0.615 0.570

r2

G◦ (kJ mol−1 )

H◦ (kJ mol−1 )

S◦ (J K−1 mol−1 )

0.940

0.714 0.934 1.245 1.464

−10.35

−37.59

process will occur spontaneously. Values of thermodynamic parameters are the actual indicators for practical application of a process. The amount of dye adsorbed at equilibrium at different temperatures is 20, 30, 35 and 40 ◦ C, have been examined to obtain thermodynamic parameters for the adsorption system. The pseudo-second-order rate constant of dye adsorption is expressed as a function of temperature by the following Arrhenius type relationship: Ea ln k2 = ln A − RT

(9)

where Ea is the Arrhenius activation energy of adsorption; A, the Arrhenius factor; R, the gas constant and is equal to 8.314 J mol−1 K−1 and T is the operated temperature. When ln k2 is plotted versus 1/T (Fig. 7), a straight line with slope −Ea /R is obtained. The magnitude of activation energy gives an idea about the type of adsorption which is mainly physical or chemical. Low activation energies (5–40 kJ mol−1 ) are characteristics for physisorption, while higher activation energies (40–800 kJ mol−1 ) suggest chemisorption [42]. The result obtained is +32.41 kJ mol−1 with the correlation coefficient (r2 ) of 0.927 (Table 4) for the adsorption of AB193 onto natural sepiolite, indicating that the adsorption has a low potential barrier and corresponding to a physisorption and this result was also consistent with the value of the mean free energy E (7.110 kJ mol−1 ) from D-R isotherm model, which was also indicated a physical adsorption. The other thermodynamic parameters, change in the free energy (G◦ ), enthalpy (H◦ ), and entropy (S◦ ), were deter-

Fig. 7. Arrhenius plot for the adsorption of AB193 onto natural sepiolite.

mined by using following equations: KC =

CA CS

G◦ = −RT ln KC ln KC =

S ◦ H ◦ − R RT

(10) (11) (12)

where KC is the equilibrium constant; CA , the amount of dye adsorbed on the adsorbent of the solution at equilibrium (mol dm−3 ); CS , the equilibrium concentration of the dye in the solution (mol dm−3 ). The q2 of the pseudo-second-order model from Table 2 was used to obtain CA and CS . T is the solution temperature (K) and R, the gas constant. H◦ and S◦ were calculated the slope and intercept of van’t Hoff plot of ln KC versus 1/T (see Fig. 8). The results are given in Table 4. Generally, the change in free energy for physisorption is between −20 and 0 kJ mol−1 , but chemisorption is a range of −80 to −400 kJ mol−1 [43]. The results obtained are +0.714 kJ mol−1 at 20 ◦ C, +0.934 kJ mol−1 at 30 ◦ C, +1.245 kJ mol−1 at 35 ◦ C and +1.464 kJ mol−1 at 40 ◦ C (see Table 4), these indicated that the adsorption reaction was not a spontaneous one and that the system gained energy from an external source. The small negative value of the change in enthalpy (−10.35 kJ mol−1 ) indicate that the adsorption is physical in nature involving weak forces of attraction and is also exothermic, thereby demonstrating that the process is stable energetically. At the same time, the low value of H◦ implies that there was loose bonding between the adsorbate molecules and the adsorbent surface [44].

Fig. 8. Plot of ln KC vs. 1/T for estimation of thermodynamic parameters for the adsorption of AB193 onto natural sepiolite.

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The negative entropy change (S◦ ) value (−37.59 J K−1 mol−1 ) corresponds to a decrease in the degree of freedom of the adsorbed species. 4. Conclusions This study investigated the equilibrium and the dynamics of the adsorption of an anionic dye, which is namely Acid Blue 193, AB193, onto natural sepiolite. The adsorption was found to be strongly dependent on pH, contact time and temperature. A maximum of 1.90 × 10−3 mol g−1 from D-R equation for AB193 removal could be achieved at pH 1.5 and 20 ◦ C. The adsorption of AB193 onto natural sepiolite was exothermic in nature with the dye removal capacity decreasing with increasing temperature due to increasing mobility of the dye molecules. The pseudo-second-order kinetic model agrees very well with the dynamic behavior for the adsorption of AB193 onto natural sepiolite under different temperatures. However, the evidence is provided that the adsorption of dye onto natural sepiolite is a complex process, so it cannot be sufficiently described by a single kinetic model throughout the whole process. For example, intraparticle diffusion (up to 60 min) played a significant role, but it was not the main rate determining step during the adsorption. The experimental data fitted well to the D-R adsorption isotherm. The activation energy of adsorption can be evaluated using the pseudo-second-order rate constants. The positive value of Ea (+32.41 kJ mol−1 ) confirms the nature of physisorption of AB193 onto natural sepiolite. The enthalpy change (H◦ ) for the adsorption process was −10.35 kJ mol−1 , which did not indicate very strong chemical forces between the adsorbed dye molecules and natural sepiolite. The G◦ values were positive therefore the adsorption was not spontaneous and the negative value of S◦ suggests a decreased randomness at the solid/solution interface and no significant changes occur in the internal structure of the adsorbent through the adsorption of AB193 onto natural sepiolite. References [1] K. Ravikumar, B. Deebika, K. Balu, Decolourization of aqueous dye solutions by a novel adsorbent: application of statistical designs and surface plots for the optimization and regression analysis, J. Hazard. Mater. 122 (2005) 75–83. [2] S.J. Allen, Q. Gan, R. Matthews, P.A. Johnson, Kinetic modeling of the adsorption of basic dyes by kudzu, J. Colloid Interface Sci. 286 (2005) 101–109. [3] A. Mittal, L. Krishnan, V.K. Gupta, Removal and recovery of malachite green from wastewater using an agricultural waste material, de-oiled soya, Sep. Purif. Technol. 43 (2005) 125–133. [4] M. Arami, N.Y. Limaee, N.M. Mahmoodi, N.S. Tabrizi, Removal of dyes from colored textile wastewater by orange peel adsorbent: equilibrium and kinetic studies, J. Colloid Interface Sci. 288 (2005) 371–376. ¨ ¨ [5] A.S. Ozcan, A. Ozcan, Adsorption of acid dyes from aqueous solutions onto acid-activated bentonite, J. Colloid Interface Sci. 276 (2004) 39–46. [6] V.K. Gupta, I. Ali, Adsorbents for water treatment: low cost alternatives to carbon, in: A.T. Hubbard (Ed.), Encyclopaedia of Surface and Colloid Science, Marcel Dekker, New York, 2003, pp. 1–34. [7] V.K. Gupta, Suhas, I. Ali, V.K. Saini, Removal of rhodamine B, fast green, and methylene blue from wastewater using red mud, an aluminum industry waste, Ind. Eng. Chem. Res. 43 (2004) 1740–1747.

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