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colloids05-Adsorption of Acid Blue 193 from aqueous solutions


Colloids and Surfaces A: Physicochem. Eng. Aspects 266 (2005) 73–81

Adsorption of Acid Blue 193 from aqueous solutions onto BTMA-bentonite
? , Bilge Erdem, Adnan Ozcan ¨ ¨ A. Safa Ozcan
Anadolu University, Faculty of Science, Department of Chemistry, Yunusemre Campus, 26470 Eski? sehir, Turkey Received 11 January 2005; received in revised form 31 May 2005; accepted 1 June 2005

Abstract The adsorption of Acid Blue 193 (AB193) onto benzyltrimethylammonium (BTMA)-bentonite was investigated in aqueous solution in a batch system with respect to contact time, pH and temperature. The surface modi?cation of BTMA-bentonite was examined using the FTIR technique. The pseudo-?rst-order, pseudo-second-order kinetic models and the intraparticle diffusion model were used to describe the kinetic data and the rate constants were evaluated. The experimental data ?tted very well the pseudo-second-order kinetic model and also followed the intraparticle diffusion model up to 60 min, whereas diffusion is not only the rate controlling step. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) adsorption models were applied to describe the equilibrium isotherms and the isotherm constants were also determined. The Langmuir, Freundlich and D–R models agree with experimental data well. The change of free energy, enthalpy and entropy of adsorption were also evaluated for the adsorption of AB193 onto BTMA-bentonite. The results show that BTMA-bentonite could be employed as low-cost material for the removal of acid dyes from ef?uents. ? 2005 Elsevier B.V. All rights reserved.
Keywords: Bentonite; Adsorption; Acid dye; Surfactant; Kinetics

1. Introduction Colored dyes are important water pollutants which are generally present in the ef?uents of the textile and other industries. The high level of production and extensive use of dyes generates colored wastewater which produces toxicological and technical problems and environmental pollution. Some dyes, for instance, are reported to cause allergy, dermatitis, skin irritation, cancer and mutation in human. Thus, the removal of color dyes from wastewater before they are contacted with unpolluted natural water bodies is important. Although several traditional chemical and biological processes exist for dye removal, the application of these techniques has been restricted due to the essentially non? Corresponding author. Tel.: +90 222 3350580/5781; fax: +90 222 3204910. ¨ E-mail addresses: asozcan@anadolu.edu.tr (A.S. Ozcan), ¨ bilgee@anadolu.edu.tr (B. Erdem), aozcan@anadolu.edu.tr (A. Ozcan).

biodegradable nature of dyes, which are stable to light and oxidation [1–3]. Adsorption is one of the effective methods to remove colored textile contaminants from wastewaters. Adsorption phenomenon in solution systems plays a vital role in many areas of practical environmental technology, which are mainly in water and wastewater treatment due to several advantages such as high ef?ciency, simple operation and easy recovery/reuse of adsorbent [4–6]. Even though the most promising adsorbent for adsorption is activated carbon, which has a high surface area and a high adsorption capacity, it is very expensive, has high operation costs and there is a need for regeneration after each adsorption cycle [7–9]. Therefore, there is a growing need to ?nd low cost and ef?cient, locally available materials for the removal of dyes. Some clays such as sepiolite [10], kaolinite [11], montmorillonite [12], smectite [13], bentonite [3,14] and alunite [15] have been investigated for this purpose. These kinds of clays have a variety of surface and structural properties, high chemical stability, high

0927-7757/$ – see front matter ? 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.06.001

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speci?c surface area and high adsorption capacity and hence they can be used to remove dye from ef?uents. The water-soluble anionic dyes are used to dye fabrics, such as wool, nylon and silk. Because of 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 bentonite as an adsorbent [3,16–18], but none of them has investigated adsorption of Acid Blue 193 (AB193) dye onto BTMA-bentonite. In addition, this kind of clay is mainly used as an emulsifying agent for aspaltic and resinous substances, as an adhesive agent in horticultural sprays and insecticides, in concrete mixtures, and as a plasticizer in ceramic materials. It is also used in re?ning oils and fats, drilling mud, foundry sands, in some detergents, cosmetics, pharmaceuticals, thickeners and extenders for paints, coating and ?lling of paper [19,20]. Bentonite is natural clay which contains montmorillonite. The inner layer is consisted of an octahedral sheet situated between two tetrahedral sheets. Substitutions within the lattice structure of trivalent aluminum for quadrivalent silicon in the tetrahedral sheet and of ions of lower valence, especially magnesium, for trivalent aluminum in the octahedral sheet result in unbalanced charges in the structural units of clays. The above factors generally cause a good adsorbent for the removal of dye in aqueous solutions [3,21]. The surface properties of bentonite may be greatly modi?ed with a surfactant by simple ion-exchange reactions to lead van der Waals interaction between organic surfactant cations and adsorbate. The modi?cation of clay surface with surfactant is called as organoclay to cause to transform organophobic to strongly organophilic and therefore the adsorption capacity increases [22]. This kind of surfactantmodi?ed organobentonite has been used extensively for a wide variety of environmental applications [23]. The characteristics of the adsorption behavior are usually understood by means of both equilibrium isotherm and adsorption kinetics. The adsorption isotherm is also an inevitable tool for the theoretical evaluation and interpretation of thermodynamic parameters including changes in Gibbs free energy, entropy and enthalpy. In this type of study, the construction of an adsorption isotherm plays an important role in understanding the adsorption mechanism. For adsorption kinetics, temporal variations of the amount of adsorption are measured and thus the obtained experimental data are used to develop a proper kinetic model [5,6]. The present paper is to investigate the possibility of BTMA-bentonite as an adsorbent for removal of an anionic acid dye, which is, namely Acid Blue 193, from aqueous solution by adsorption method. The adsorption capacity of AB193 onto BTMA-bentonite was carried out using various kinetic models. The experimental data were ?tted into Langmuir, Freundlich and Dubinin–Radushkevich (D–R) equations to determine which isotherm gives the best correlation to experimental data. The calculated thermodynamic parameters from the Langmuir isotherm constant were also used to explain the nature of adsorption.

Fig. 1. Chemical structure of AB193.

2. Materials and methods 2.1. Materials A commercial textile dye AB193 (Isolan Dark Blue 2SGL) was obtained from Dystar, Turkey, and used without further puri?cation. The chemical structure of AB193 is depicted in Fig. 1. Bentonite was provided from C ? anakkale, Turkey. It was crushed, ground, sieved through a 63-?m sieve and dried at 110 ? C in an oven for 2 h prior to use. The determined cation exchange capacity (CEC) and the surface area of the natural bentonite by the methylene blue method [24] were 980 mmol kg?1 and 767 m2 g?1 , respectively. 2.2. Material characterization The chemical analysis of natural bentonite was determined by using an energy dispersive X-ray spectrometer (EDXLINK ISIS 300) attached to a scanning electron microscope (SEM-Cam Scan S4). FTIR spectra for bentonite and BTMA-bentonite were recorded (KBr) on a Jasco FT/IR-300E Model Fourier transform infrared spectrometer to con?rm the surface modi?cation. 2.3. Preparation of BTMA-bentonite Bentonite was ?rstly washed with deionized water by several times and then the Na+ -exchanged form of clay was prepared by stirring samples for 24 h with 1 N NaCl. This was followed by several washings with distilled water and ?ltered to remove the excess NaCl and other exchangeable cations from the clay. The clay was then resuspended and ?ltered until a negative chloride test was obtained with 0.1 M AgNO3 . Twenty grams of the Na-saturated clay was dispensed in 0.5 dm?3 of distilled water. Benzyltrimethylammonium (BTMA) chloride was used as a surfactant. BTMA-bentonite was prepared by adding quantities of the respective chloride salts equal to twice the cation exchange capacity of the bentonite and stirring for 24 h. The clay was then washed with distilled water until free of salts and a negative chloride test had been obtained with 0.1 M AgNO3 and was used for the adsorption studies [25].

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The elemental analysis (Vario EL III Elemental Analyzer, Hanau, Germany) of BTMA-bentonite was carried out to determine C/N ratio in BTMA-bentonite. 2.4. Adsorption experiments The pH experiments were carried out by 50 ml of a 250 mg dm?3 dye solutions with 0.01 g of BTMA-bentonite and the pH was carefully adjusted between 1 and 11 with adding a small amount of dilute HCl or NaOH solution using a pH meter (Fisher Accumet AB15). The dye solutions were stirred using a mechanical magnetic stirrer in a 100 ml erlenmeyer sealed with para?lm to avoid evaporation. The optimum pH was then determined as 1.5 and used throughout all adsorption experiments, which were conducted at various time intervals, the initial concentrations (100–250 mg dm?3 ) and temperatures (20, 30 and 40 ? C), to determine the adsorption equilibrium time and the maximum removal of dye. The solutions were ?ltered and then subjected to quantitative analyses. The equilibrium concentrations of each solution were measured by spectrophotometer (Shimadzu UV-2101PC) at the λmax value, which is 609 nm for AB193. The amount of the dye adsorbed onto BTMA-bentonite surface was determined by the difference between the initial and remaining concentrations of dye solution. The adsorption of AB193 onto BTMA-bentonite was also evaluated at constant temperatures of 20, 30 and 40 ? C for the adsorption isotherms.

Fig. 2. FTIR spectra of: (a) natural bentonite (—) and (b) BTMA-bentonite (- - -).

The ratio of C/N for BTMA-bentonite from elemental analysis results is 8.95 and the calculated value of C/N ratio for BTMA is 8.57 and the percentage of BTMA onto clay is 11.13. These results con?rm that the intercalation of BTMA molecules between the bentonite layers occurs, and these results are also consistent with FTIR results. 3.2. FTIR analysis FTIR spectra of natural bentonite and BTMA-bentonite are illustrated in Fig. 2. There is a group of absorption peaks between 3427 and 3626 cm?1 , which is due to stretching band of the OH groups and bending bands at 914 and 890 cm?1 . The band at 1637 cm?1 also corresponds to the OH deformation of water to observe natural bentonite and BTMA-bentonite, but the peak intensity of BTMA-bentonite is lower than natural bentonite. This may be acceptable evidence for the surface modi?cation occurring on bentonite. For the band at 3037 cm?1 , which is characteristic of aromatic C H bonds of BTMA (surfactant) and is generally rather low intensity and occurs just to the left of a normal saturated C H band, and the bands between 1450 and 1600 cm?1 , characteristic peaks are assigned to the stretching vibrations of aromatic ring double bond. In addition, the bands at 690–900 cm?1 range are due to C H out-ofplane bending of aromatic surfactant. These bands were only observed for BTMA-bentonite and these could be an acceptable evidence for the modi?cation. The bands at 2854, 2927 and 2962 cm?1 were only observed for BTMA-bentonite. They can be assigned to the symmetric and asymmetric stretching vibrations of the methyl and methylene groups and their bending vibrations are between 1380 and 1475 cm?1 [26] supporting the intercalation of surfactant (BTMA) molecules between the silica layers, but these stretching and bending bands are not observed in the natural bentonite. The stretching band of the Si–O was observed at 1041 cm?1 . The bands at 519 and 467 cm?1 for natural bentonite and BTMA-bentonite were from Si–O–Al

3. Results and discussion 3.1. Chemical composition of bentonite The chemical composition of bentonite obtained by using EDX analysis, given in Table 1, indicates the presence of silica and alumina as major constituents along with traces of sodium, potassium, iron, magnesium, calcium and titanium oxides in the form of impurities. XRD results combined with EDX analysis show that most of the aluminum is in the form of bentonite. XRD also indicated the presence of free quartz in bentonite. It is, thus, expected that the adsorbate species will be removed mainly by SiO2 and Al2 O3 .
Table 1 Chemical composition of bentonite Constituents SiO2 Al2 O3 K2 O CaO MgO Fe2 O3 TiO2 Na2 O Loss of ignition wt% 70.75 16.18 2.12 1.62 1.25 0.70 0.18 0.11 6.63

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Fig. 3. pH effect for the adsorption of AB193 onto BTMA-bentonite at 20 ? C.

(where Al is an octahedral cation) and bending vibrations, respectively. 3.3. Effect of pH Fig. 3 indicates the effect of pH on the removal of dye (AB193) onto BTMA-bentonite from aqueous solution. 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 and speciation of adsorbate. At lower pH, more protons will be available, thereby increasing electrostatic attractions between negatively charged dye anions and positively charged adsorption sites and causing an increase in dye adsorption [3]. The high adsorption capacity is due to the strong electrostatic interaction between the –N+ (CH3 )3 of BTMA-bentonite and dye anions. As can be seen (Fig. 3), the maximum AB193 removal was observed at acidic pH 1.5. When the pH of the solution is increased, the positive charge on the oxide or solution interface decreases and the adsorbent surface appears negatively charged. On the contrary, a lower adsorption at higher pH values may be due to the abundance of OH? ions and because of ionic repulsion between the negatively charged surface and the anionic dye molecules. There are also no exchangeable anions on the outer surface of the adsorbent at higher pH values and consequently the adsorption decreases [27]. 3.4. Effect of initial dye concentration, contact time and temperature The in?uence of the initial concentrations of AB193 in the solutions on the rate of adsorption onto BTMA-bentonite was investigated between 100 and 250 mg dm?3 at the initial pH value of 1.5. As shown in Fig. 4, when the initial dye concentrations were increased from 100 to 250 mg dm?3 , the adsorption capacity of dye increased from 252.8 to 505.3 mg g?1 at 20 ? C, from 234.7 to 480.4 mg g?1 at 30 ? C and from 209.5 to

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

472.4 mg g?1 at 40 ? C. These indicate that the initial dye concentrations play an important role in the adsorption capacities of AB193 on the BTMA-bentonite. The effect of contact time on the amount of AB193, adsorbed onto BTMA-bentonite at various temperatures (Fig. 4), was also investigated at the range of initial dye concentration of 100–250 mg dm?3 . When the equilibrium time was increased, the amount of adsorption was not increased. The rate of removal of AB193 onto BTMA-bentonite by adsorption was rapid initially and then slows down gradually until it attained an equilibrium beyond which there was no signi?cant increase in the rate of removal. Maximum adsorption capacity of AB193 onto BTMA-bentonite was observed at 60 min; it can be said that beyond this, there is almost no

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further increase in the adsorption and it is thus ?xed as the equilibrium contact time. The equilibrium adsorption capacity of AB193 onto BTMA-bentonite was also affected by temperature and decreased with increasing temperature from 20 to 40 ? C which indicates that the adsorption of AB193 onto BTMAmodi?ed adsorbent surface was favored at lower temperatures and it is controlled by an exothermic process. This is partly due to a weakening of the attractive forces [28,29] between AB193 and BTMA-bentonite. Based on the above results, it implies that physical adsorption mechanism may play a vital role in this system. Before and after equilibrium time, the adsorption capacity shows different trends at various temperatures. In general, below the equilibrium time, an increase in the temperature leads to an increase in dye adsorption rate, which shows a kinetically controlling process. After the equilibrium attained, the uptake decreases with increasing temperature indicating that the adsorption of AB193 onto BTMA-bentonite from aqueous solution is controlled by an exothermic process.

where q1 and qt are the amounts of the dye adsorbed at equilibrium and at time t (mg g?1 ) and k1 is the pseudo-?rst-order rate constant (min?1 ), applied to the adsorption of AB193. Values of k1 calculated from the slope of the plots of 1/qt versus 1/t are shown in Table 2 (?gure not shown). It was found that the correlation coef?cients for the pseudo-?rstorder model are lower than the pseudo-second-order model. This indicates that the adsorption of AB193 onto BTMAbentonite does not follow pseudo-?rst-order kinetics. The pseudo-second-order kinetic model [30] is expressed as: t 1 1 = + t, 2 qt q k2 q 2 2

(2)

3.5. Kinetics of adsorption The kinetics of adsorption is one of the most important characteristics in de?ning the ef?ciency of adsorption. Various kinetic models have been proposed by different research groups where the adsorption has been treated as a pseudo?rst-order [30], a pseudo-second-order [30] and intraparticle diffusion [31]. The pseudo-?rst-order kinetic model equation is: 1 k1 1 = + , qt q1 t q1 (1)

where q2 is the maximum adsorption capacity (mg g?1 ) for the pseudo-second-order adsorption and k2 is the equilibrium rate constant of pseudo-second-order adsorption (g mg?1 min?1 ). Values of k2 and q2 were calculated from the plot of t/qt against t (Fig. 5). All of kinetic data of AB193 under different conditions were calculated from plots and are given in Table 2. The correlation coef?cients for the second-order kinetic plots at all the studied concentrations were generally above 0.999 (Table 2) and the calculated q2 values also agree with experimental q2 values. These results imply that the adsorption system studied obeys to the secondorder-kinetic model, a similar phenomenon which we have observed in the adsorption of acid dyes by acid-activated bentonite [14] and sepiolite [10]. The pseudo-?rst-order and pseudo-second-order kinetic models cannot identify the diffusion mechanism and the kinetic results were then analyzed by using the intraparticle diffusion model. The intraparticle diffusion equation [31]

Table 2 Kinetic parameters for the adsorption of AB193 at various temperatures t (? C) 20 Co (mg dm?3 ) 100 150 175 200 225 250 100 150 175 200 225 250 100 150 175 200 225 250 k1 (min?1 ) 2.024 3.425 3.170 3.540 3.300 3.003 2.573 2.585 1.751 3.016 2.631 2.997 1.724 2.678 2.643 4.922 2.803 3.002 q1 (mg g?1 ) 257.7 353.6 394.2 438.6 488.5 515.5 237.6 330.7 364.4 422.8 469.3 495.1 211.6 316.9 360.9 435.5 460.4 487.1
2 r1

k2 (×104 ) (g mg?1 min?1 ) 12.85 8.702 7.090 6.829 6.334 5.911 16.22 10.18 8.837 7.500 6.986 6.905 17.32 11.82 9.551 8.339 8.326 7.229

q2 (mg g?1 ) 260.7 352.6 398.3 437.4 478.5 518.9 237.6 333.6 377.6 423.3 473.4 494.5 216.3 316.7 363.0 415.1 458.8 485.8

2 r2

kp (mg g?1 min?1/2 ) 7.681 16.23 16.74 20.28 21.51 19.09 7.965 10.69 7.987 15.54 14.84 19.21 5.714 11.10 12.33 21.72 17.40 20.08

C (mg g?1 ) 195.72 222.51 257.45 273.57 316.99 355.49 171.81 240.52 294.52 292.29 342.30 336.21 165.73 225.55 258.20 250.91 318.90 324.82

2 rp

0.936 0.960 0.915 0.955 0.926 0.918 0.990 0.895 0.804 0.924 0.860 0.977 0.666 0.961 0.882 0.856 0.964 0.938

0.999 0.999 0.999 0.999 0.992 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999

0.945 0.948 0.914 0.921 0.826 0.819 0.869 0.779 0.795 0.777 0.722 0.923 0.751 0.836 0.793 0.686 0.884 0.899

30

40

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els may control the rate of adsorption, all of which may be operating simultaneously. The slope of linear portion from the ?gure can be used to derive values for the rate parameter, kp , for the intraparticle diffusion, given in Table 2. The cor2 ) for the intraparticle diffusion model relation coef?cients (rp are also lower than the pseudo-second-order model but this model indicates that the adsorption of AB193 onto BTMAbentonite may be followed by an intraparticle diffusion model up to 60 min. The values of intercept give an idea about the boundary layer thickness such as the larger the intercept, the greater is the boundary layer effect. The pseudo-second-order rate constants for AB193 onto BTMA-bentonite indicate a steady increase with temperature. The values of rate constants were found to increase at all the studied concentrations for BTMA-bentonite with an increase in the solution temperatures from 20 to 40 ? C. It may be concluded that the adsorption of AB193 onto BTMAbentonite follows a physisorption mechanism, increasing temperature generally increases the rate of approach to equilibrium, but decreases the equilibrium adsorption capacity [30]. 3.6. Adsorption isotherms The equilibrium adsorption isotherm is one of the most important data to understand the mechanism of the adsorption systems. Several isotherm equations are available and three important isotherms are selected in this study, which are, namely the Langmuir [34], Freundlich [35] and Dubinin–Radushkevich [36] isotherms. The Langmuir adsorption isotherm assumes that adsorption takes place at speci?c homogenous sites within the adsorbent and has found successful application in many adsorption processes of monolayer adsorption. The linear form of the Langmuir isotherm equation is represented by the following equation: 1 1 = + qe qmax
Fig. 5. Pseudo-second-order kinetic plots for the adsorption of AB193 onto BTMA-bentonite at various concentrations and temperatures.

1 qmax KL

1 , Ce

(4)

can be written by following: qt = kp t 1/2 + C, (3)

where C is the intercept and kp is the intraparticle diffusion rate constant (mg g?1 min?1/2 ). According to this model, the plot of uptake, qt , versus the square root of time, t1/2 (?gure not shown), should be linear if intraparticle diffusion is involved in the adsorption process and if these lines pass through the origin, then intraparticle diffusion is the rate controlling step [7,32,33]. When the plots do not pass through the origin, this is indicative of some degree of boundary layer control and this further shows that the intraparticle diffusion is not the only rate-limiting step, but also other kinetic mod-

where qe is the equilibrium dye concentration of the adsorbent (mol g?1 ), Ce the equilibrium dye concentration in the solution (mol dm?3 ), qmax the monolayer adsorption capacity of the adsorbent (mol g?1 ) and KL is the Langmuir adsorption constant (dm3 mol?1 ) and related to the free energy of adsorption. The plots of 1/qe versus 1/Ce for the adsorption of AB193 onto BTMA-bentonite (Fig. 6) give a straight line 1 1 of slope qmax KL and intercept qmax . The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems. A linear form of the Freundlich equation is ln qe = ln KF + 1 ln Ce , n (5)

where KF (dm3 g?1 ) and n are Freundlich adsorption isotherm constants, being indicative of the extent of the adsorption and the degree of nonlinearity between solution concentration and

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Fig. 6. Langmuir plots for the adsorption of AB193 onto BTMA-bentonite at various temperatures.

Fig. 8. Dubinin–Radushkevich plots for the adsorption of AB193 onto BTMA-bentonite at various temperatures.

where β is a constant related to the mean free energy of adsorption per mole of the adsorbate (mol2 J?2 ), qm the theoretical saturation capacity and ε is the Polanyi potential, 1 which is equal to RT ln(1 + C ), where R (J mol?1 K?1 ) is e 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. 8 indicates the D–R isotherm for AB193 adsorption onto BTMA-bentonite. 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 in?nity in the solution and can be calculated using the relationship [38–40]:
Fig. 7. Freundlich plots for the adsorption of AB193 onto BTMA-bentonite at various temperatures.

E=

1 (2β)1/2

(7)

adsorption, respectively. The plots of ln qe versus ln Ce for the adsorption of AB193 onto BTMA-bentonite (Fig. 7) were employed to generate the intercept value of KF and the slope of 1/n. The D–R isotherm is more general than the Langmuir isotherm because it does not assume a homogeneous surface or constant adsorption potential. It was applied to distinguish between the physical and chemical adsorptions of dye [37]. The linear form of D–R isotherm equation [36] is ln qe = ln qm ? βε2 (6)

This parameter gives information whether adsorption mechanism is ion-exchange or physical adsorption. If the magnitude of E is between 8 and 16 kJ mol?1 , the adsorption process follows by ion-exchange [41], while for the values of E < 8 kJ mol?1 , the adsorption process is of a physical nature [42]. The numerical value of adsorption of the mean free energies between 8.975 and 8.177 kJ mol?1 corresponds to a boundary of a physisorption and the predominance of van der Waals forces. The Langmuir, Freundlich and D–R parameters for the adsorption of AB193 are listed in Table 3. It is evident from these data that the surface of BTMA-bentonite is made up

Table 3 Adsorption isotherm constants for the adsorption of AB193 onto BTMA-bentonite t (? C) Langmuir qmax (mol g?1 ) 20 30 40 2.228 2.693 6.039 (×103 ) KL (dm3 mol?1 )
2 rL

Freundlich RL 0.348 0.447 0.717 KF (dm3 g?1 ) n 1.598 1.431 1.177
2 rF

Dubinin–Radushkevich (D–R) qm (×102 ) (mol g?1 ) 1.223 1.566 2.774 β (×103 ) (mol2 kJ?2 ) 6.208 6.510 7.478
2 rD –R

E (kJ mol?1 ) 8.975 8.764 8.177

3112.21 2060.37 656.62

0.994 0.996 0.997

0.173 0.298 0.974

0.999 0.994 0.990

0.998 0.950 0.993

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of homogeneous and heterogeneous adsorption patches. In other words, all of isotherm models ?t very well when the r2 values are compared (Table 3). The Langmuir model correlation coef?cients are in a range of 0.994–0.997, the Freundlich model correlation coef?cients are between 0.990 and 0.999 and the D–R model correlation coef?cients are in a range of 0.950–0.998. The effect of isotherm shape has been discussed [43] with a view to predicting 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 Co (dm3 mol?1 ) (8)
Fig. 9. Plot of ln KL vs. 1/T for estimation of thermodynamic parameters.

where KL is the Langmuir constant and Co is the highest initial dye concentration (mol dm?3 ). The values of RL calculated from above equation are incorporated in Table 3. As the RL values lie between 0 and 1, the on-going adsorption process is favorable [43,44]. Further, the RL values for AB193 are between 0 and 1 and therefore, its adsorption is favorable. One of the Freundlich constants KF indicates the adsorption capacity of the adsorbent. The other Freundlich constant n is a measure of the deviation from linearity of the adsorption. If a value for n is equal to unity, the adsorption is linear. If a value for n is below unity, this implies that adsorption process is chemical, but if a value for n is above unity, adsorption is favorable, a physical process. The highest value of n at equilibrium, 1.598 at 20 ? C, represents favorable adsorption at low temperature, and therefore this would seem to suggest that physical, which is referred the adsorption bond becomes weak [45] and conducted with van der Waals forces, rather than chemical adsorption is dominant when it is used for adsorbing AB193. 3.7. Thermodynamic parameters Because KL is equilibrium constant, its dependence with temperature can be used to estimate thermodynamic parameters, such as change in the standard free energy ( Go ), enthalpy ( Ho ) and entropy ( So ) associated to the adsorption process and were determined by using following equations and represented in Table 4: Go = ?RT ln KL , (9)

ln KL = ?

Go Ho So =? + , RT RT R

(10)

The plot of ln KL as a function of 1/T (Fig. 9) yields a straight line from which Ho and So were calculated from the slope and intercept, respectively. The results are given in Table 4. Generally, the change of free energy for physisorption is between ?20 and 0 kJ mol?1 , but chemisorption is a range of ?80 to ?400 kJ mol?1 [46]. The overall standard free energy change during the adsorption process was negative for the experimental range of temperatures (see Table 4), corresponding to a spontaneous physical process of AB193 adsorption and that the system does not gain energy from an external source. When the temperature decreases from 40 to 20 ? C, the magnitude of standard free energy change shifts to high negative value (from 16.89 to 19.60 kJ mol?1 ) suggesting that the adsorption was rapid and more spontaneous at low temperature [47]. The negative value of the standard enthalpy change (?59.06 kJ mol?1 ) indicates that the adsorption is physical in nature involving weak forces of attraction and is also exothermic, thereby demonstrating that the process is stable energetically [48]. The negative standard entropy change ( So ) value (?133.56 J mol?1 K?1 ) corresponds to a decrease in the degree of freedom of the adsorbed species.

4. Conclusion From the foregoing experiments, the reasonable concludes that BTMA-bentonite is an effective adsorbent for removing AB193 from aqueous solution, and it can be represented as a suitable adsorbent and environmentally clean utilization of wastewater. The kinetics of adsorption of AB193 onto BTMAbentonite is studied on the basis of the pseudo-?rst-order, pseudo-second-order and intraparticle kinetic models under several different initial dye concentrations, temperatures

Table 4 Thermodynamic parameters calculated from the Langmuir isotherm data for the adsorption of AB193 onto BTMA-bentonite t (? C) 20 30 40 Go (kJ mol?1 ) ?19.60 ?19.23 ?16.89 Ho (kJ mol?1 ) ?59.06 So (J K?1 mol?1 ) ?133.56

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and pH. A pseudo-second-order and intraparticle diffusion kinetic models have been developed to predict the rate constants of adsorption and equilibrium adsorption capacities. The high adsorption capacity of BTMA-bentonite in acidic solutions (pH around 1.5) is due to the strong electrostatic interactions between its adsorption site and dye anion. The adsorption isotherms of AB193 onto BTMAbentonite were analyzed according to Langmuir, Freundlich and D–R models. All of three-isotherm models for AB193 adsorption were ?tted very well with the experimental equilibrium data. The mechanism of the dye–BTMA-bentonite interaction is thus likely to be very complicated involving a wide range of sites differing in a number of aspects including energy considerations. The mean energy of adsorption is found from the D–R isotherm. The small positive value of E (kJ mol?1 ) indicates a low potential barrier and con?rms the nature of physical adsorption of AB193 onto BTMA-bentonite. The enthalpy change ( Ho ) for the adsorption process was indicative of the exothermic nature of adsorption and a physical adsorption. The Go values were negative; therefore, the adsorption was spontaneous and the negative value of So suggests a decreased randomness at the solid/solution interface and no signi?cant changes occur in the internal structure of the adsorbent through the adsorption of AB193 onto BTMA-bentonite.

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