Adsorption of acid dyes from aqueous solutions onto acid-activated bentonite

Adsorption of acid dyes from aqueous solutions onto acid-activated bentonite

Journal of Colloid and Interface Science 276 (2004) 39–46 www.elsevier.com/locate/jcis Adsorption of acid dyes from aqueous solutions onto acid-activ...

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Journal of Colloid and Interface Science 276 (2004) 39–46 www.elsevier.com/locate/jcis

Adsorption of acid dyes from aqueous solutions onto acid-activated bentonite A. Safa Özcan and Adnan Özcan ∗ Department of Chemistry, Faculty of Science, Anadolu University, 26470 Eski¸sehir, Turkey Received 24 December 2003; accepted 18 March 2004 Available online 6 May 2004

Abstract The adsorption of two dyes, namely, Acid Red 57 (AR57) and Acid Blue 294 (AB294), onto acid-activated bentonite in aqueous solution was studied in a batch system with respect to contact time, pH, and temperature. Acidic pH was favorable for the adsorption of these dyes. The surface characterization of acid-activated bentonite was performed using the FTIR technique. The pseudo-first-order and pseudo-secondorder kinetic models and the intraparticle diffusion model were used to describe the kinetic data and the rate constants were evaluated. The dynamic data fitted the pseudo-second-order kinetic model well and also followed the intraparticle diffusion model up to 90 min, but diffusion is not the only rate controlling step. The Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms and the isotherm constants were determined. The Freundlich model agrees very well with experimental data. The activation energies of adsorption were also evaluated for the adsorption of AR57 and AB294 onto activated bentonite.  2004 Elsevier Inc. All rights reserved. Keywords: Adsorption; Acid dyes; Kinetics; Clays; Activated bentonite

1. Introduction Industrial effluents are one of the major causes of environmental pollution because effluents discharged from dyeing industries are highly colored [1]. Due to their chemical structure, dyes are resistant to light, many chemicals, oxidizing agents, and heat and are biologically nondegradable and therefore difficult to decolorize once released into the aquatic environment [2]. Disposal of this colored water into receiving waters can be toxic to aquatic life. It may be mutagenic and carcinogenic and can cause severe damage to human beings, such as dysfunction of the kidneys, reproductive system, liver, and brain and central nervous system [1]. Wastewater containing dyes from the textile industry is very difficult to treat using conventional wastewater treatment methods, which are coagulation, ultrafiltration, ozonation, oxidation, sedimentation, reverse osmosis, flotation, precipitation, etc., due to economic considerations. Adsorption has gained favor in recent years due to proven efficiency in the removal of pollutants from effluents to stable forms for the above conventional treatment methods [3]. * Corresponding author. Fax: +90-222-3353616.

E-mail address: [email protected] (A. Özcan). 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.03.043

Wastewater-contained molecules are attached to the surface of the adsorbent and this interaction may be expressed in terms of both adsorptive characteristics and physical properties [4]. The successful removal of acid dyes with activated carbon [5,6] and with sepiolite [7] has been carried out by several researchers. Activated carbon is a widely used adsorbent due to its high adsorption capacity, high surface area, microporous structure, and high degree of surface reactivity, but there are some problems with its use. It is expensive and regeneration using solution procedures requires a small additional effluent, while regeneration results in a 10–15% loss of adsorbent and its uptake capacity and therefore this adds to the operational costs. This led to a search for cheaper, easily obtainable materials for the adsorption of dye [8–12]. Clays such as sepiolite, zeolite, montmorillonite, smectite, and bentonite are being considered as alternative low-cost adsorbents. The wide usefulness of clays is a result of their specific surface area, high chemical and mechanical stability, and variety of surface and structural properties. The chemical nature and pore structure usually determine the sorption ability of clays [13]. Bentonite, which is predominantly montmorillonite clay, is characterized by one Al octahedral sheet placed between

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two Si tetrahedral sheets. The isomorphous substitution of Al3+ for Si4+ in the tetrahedral layer and Mg2+ for Al3+ in the octahedral layer results in a net negative surface charge on the bentonite. This charge imbalance is offset by exchangeable cations (Na+ and Ca2+ , etc.) at the bentonite surface. The layered structure of the clay expands after wetting. Na+ and Ca2+ are strongly hydrated in the presence of water, resulting in a hydrophilic environment at the bentonite surface [14]. The specific surface area and surface acidity of the clay samples can be greatly increased by acid activation, and therefore clays that are acid-activated in nature can serve as effective decolorizing agents for various industries. However, acid-activated bentonite use for the adsorption of AR57 and AB294 in aqueous solution has not been documented. A limited study was performed with basic and acidic dyes on activated clay [15–18]. Whether the results are also valid for other basic or acidic dyes needs further investigation. Equilibrium data are known as adsorption isotherms, Langmuir, Freundlich, etc., are the main requirements for the design of adsorption systems. Obtaining equilibrium data for a specific sorbate/sorbent system can be carried out experimentally, with a time-consuming procedure that is incompatible with the growing need for adsorption systems design [19]. In this research, the ability of acid-activated bentonite to remove acid dyes, namely Acid Red 57 (AR57) and Acid Blue 294 (AB294), by adsorption, has been studied. The adsorption capacities of dyes were also examined using the adsorption isotherm technique. The kinetic and thermodynamic parameters were also calculated to determine rate constants and adsorption mechanism. The experimental data were fitted into Langmuir and Freundlich equations to determine which isotherm gives the best correlation to experimental data.

2. Materials and methods 2.1. Materials Commercial grade dyes AR57 (Nylosan Red EBL) and AB294 (Nylosan Blue EBL) were supplied by ClairentSwitzerland and used without further purification. The adsorbent used in this study was a Turkish bentonite and was provided from Çanakkale. It was crushed, ground, and sieved through a 63-µm sieve and samples were collected from under the sieve and dried at 120 ◦ C in an oven for 2 h prior to use. Bentonite cation exchange capacity (CEC) characterized by the methylene blue method [20] was 837.5 mmol kg−1 and the surface area was 665.4 m2 g−1 . 2.2. Material characterization The chemical analysis of bentonite was determined using an energy-dispersive X-ray spectrometer (EDX-LINK ISIS

300) attached to a scanning electron microscope (SEM-Cam Scan S4). FTIR spectra were obtained (KBr) on a Jasco Model FT/IR-300E Fourier transform infrared spectrometer to observe surface activation. 2.3. Preparation of activated bentonite Fifteen grams of natural bentonite were activated by refluxing with 200 ml 50/50 (v/v) H2 SO4 at 60 ◦ C for 2 h in a round-bottom flask. The suspension was cooled in air and filtered off and then washed several times with double-distilled water and dried in an oven at 120 ◦ C for 2 h prior to use. 2.4. Adsorption studies Adsorption experiments were evaluated in batch equilibrium mode. A 0.1-g sample of acid-activated bentonite with 50 ml aqueous solution of a 1 g L−1 dye solution at various pHs (1–11) reacted for 60 min was used for the pH studies. Experiments were carried out at 25 ◦ C unless otherwise stated. The solution pH was adjusted by adding a small amount of HCl or NaOH. The optimum pH was determined and used through all adsorption processes. Experiments were conducted for various time intervals to determine when adsorption equilibrium was reached and the maximum removal of dye was attained. The solution was then filtered through a Whatmann (Number 40) filter paper to remove any organic or inorganic precipitates formed under acidic or basic conditions and the filtrates were subjected to quantitative analyses. The equilibrium concentration of each solution was determined at the wavelengths of UV-maximum (λmax ) at 512.5 and 605.5 nm for AR57 and AB294, respectively, through the use of a UV–visible spectrophotometer (Shimadzu UV-2101PC). Adsorption experiments were also carried out to obtain isotherms at different temperatures. This was done by maintaining the water bath with a magnetic stirrer at 20, 40, and 60 ◦ C. In this group of experiments 50 ml of dye solution was used at 0.1, 0.2, 0.4, 0.6, 0.8, and 1 g L−1 concentrations for 90 min to allow attainment of equilibrium at constant temperatures.

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

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Table 1 Chemical composition of natural bentonite Constituents

Percentage by weight

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

70.8 16.2 2.12 1.62 1.25 0.70 0.18 0.11 6.63

in bentonite. It is thus expected that the adsorbate species will be removed mainly by SiO2 and/or Al2 O3 . 3.2. FTIR analysis In order to obtain complementary evidence for the intercalation of hydrogen ions into the silicate lattice, FTIR spectra were recorded in the region of 400–4000 cm−1 . FTIR spectroscopy is very sensitive to modification of the clay structure upon acid treatment (see Fig. 1). As protons penetrate into the clay layers and attack the OH groups, the resulting dehydroxylation connected with successive dissolution of the central atoms can be readily followed by changes in the characteristic absorption bands, attributed to vibrations of OH groups and/or octahedral cations. There is a group of absorption peaks between 3446 and 3629 cm−1 , which is due to stretching bands of the OH groups, and the band at 1640 cm−1 also corresponds to the OH deformation of water to observe natural bentonite, and acid-activated bentonite, but the peak intensities of acid-activated bentonite are lower than that of natural bentonite. This may be acceptable evidence for acid activation occurring on bentonite. A gradual transformation of the tetrahedral sheet to a three-dimensional framework of protonated amorphous silica can be observed in the region of the stretching vibrations

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of Si–O groups. The 1041 cm−1 component of the Si–O stretching band, assigned to the Si–O vibrations within the layer, decreased with increasing extent of dissolution, while that at 1092 cm−1 , which is due to the vibrations of the three-dimensional reaction product, increased. The bands at 519 and 466 cm−1 were from the Si–O–Al (where Al is an octahedral cation) and bending vibrations, respectively, in the untreated bentonite. After acid treatment the Si–O–Al vibration was only observed at 474 cm−1 ; the bending vibration of Si–O–Si was not observed. It is the most sensitive indication of the remains of the layers after extensive acid dissolution [21–25]. 3.3. The effect of pH on the adsorption process The pH value of the solution was an important controlling parameter in the adsorption process, as can be seen from Fig. 2. It shows that the adsorption capacity of acid dyes AR57 and AB294 onto acid-activated bentonite increases significantly with decreasing pH. The maximum removals of AR57 and AB294 for contact time 60 min were carried out at pH 2. Although the molecular structure of AB294 is not known, the values of λmax for AR57 (512.5 nm) and AB294 (605.5 nm) may indicate that conjugation at AB294 is higher than at AR57. In addition, lower adsorption capacity was observed for AB294, which is an anthraquinone dye; this may due to the lower polarity of AB294 than of AR57, which is a monoazo dye. At strongly acidic pHs, a significantly high electrostatic attraction exists between the positively charged surface of the adsorbent and acid (anionic) dyes. As the pH of the system increases, the number of negatively charged sites increases and the number of positively charged sites decreases. A negatively charged surface site on the adsorbent does not favor the adsorption of dye anions, due to the electrostatic repulsion. Also, lower adsorption of acid dyes at alkaline pH is due to the presence of excess hydroxyl ions competing with the dye anions for the adsorption sites.

Fig. 1. FTIR spectra of (a) bentonite and (b) acid-activated bentonite.

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Fig. 2. Effect of pH on adsorption of AR57 and AB294 onto acid-activated bentonite for 60 min contact time.

Fig. 3. Effect of contact time on AR57 and AB294 by acid-activated bentonite at pH 2 and various temperatures.

3.4. Adsorption dynamics The effect of contact time on the amount of acid dyes, AR57 and AB294, adsorbed onto acid-activated bentonite (Fig. 3) was investigated at the optimum initial concentration of dyes. From the figure, when the equilibrium time was increased, the amount of adsorption was also increased. The maximum adsorption of acid dyes onto acid-activated bentonite was observed at 90 min, beyond which there is almost no further increase in the adsorption, and it is thus fixed as the optimum contact time. The equilibrium adsorption capacity of AR57 on activated bentonite was favored at lower temperature. This may show that adsorption of AR57 onto activated bentonite is physical. An increase in the temperature leads to a decrease in the initial adsorption rate, but the adsorption capacities at 120 min are almost the same at 40 and 60 ◦ C. Below and above equilibrium time, the adsorption capacity shows different trends at various temperatures. Below equilibrium time, an increase in the temperature leads to an increase in

Fig. 4. Second-order kinetic plots for the adsorption of AR57 and AB294 onto acid-activated bentonite at pH 2 and various temperatures.

the dye adsorption rate dq/dt and q, which indicates a kinetically controlling process. After equilibrium, the uptake decreases with increasing temperature, indicating that the adsorption of AR57 onto activated bentonite is controlled by an exothermic process [26]. The equilibrium adsorption capacity of AB294 on activated bentonite was not affected by temperature drastically. In this case, an increase in temperature decreases the adsorption capacity slightly at 40 ◦ C and increases the adsorption capacity at 60 ◦ C, almost the same as at 20 ◦ C. This effect may be due to the fact that at higher temperatures an increase in active sites occurs due to bond rupture. In order to optimize the design of an adsorption system to remove the dye, it is important to establish the most appropriate correlations for the equilibrium data for each system. In this respect, several kinetic models including the pseudo-first-order equation [12], pseudo-second-order equation [27], and intraparticle diffusion model [28] were tested here to find out adsorption mechanism,    1 1 k1 1 + , = (1) qt q1 t q1 t 1 1 (2) = + t, 2 qt q k2 q2 2 qt = kp t 1/2 + C,

(3) (mg g−1 )

where qt is the amount of dye adsorbed at various times t, q1 is the maximum adsorption capacity (mg g−1 ) for pseudo-first-order adsorption, k1 is the pseudo-firstorder rate constant for the adsorption process (min−1 ), q2 is the maximum adsorption capacity (mg g−1 ) for the pseudo-second-order adsorption, k2 is the rate constant of pseudo-second-order adsorption (g mg−1 min−1 ), C is the intercept, and kp is the intraparticle diffusion rate constant (mg g−1 min−1/2 ). The straight-line plots of 1/qt vs 1/t for the pseudo-first-order reaction and t/qt vs t for the secondorder reaction (Fig. 4) for adsorption of acid dyes onto acid-activated bentonite have also been tested to obtain the

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Table 2 Kinetic parameters for the adsorption of AR57 and AB294 at various temperatures T (◦ C)

k1 (min−1 )

q1 (mg g−1 )

r12

k2 (g mg−1 min−1 )

q2 (mg g−1 )

r22

kp (mg g−1 min−1/2 )

C (mg g−1 )

rp2

AR57

20 40 60

14.1 9.56 3.00

430.0 400.7 377.4

0.961 0.932 0.929

2.23 × 10−4 5.59 × 10−4 9.35 × 10−4

416.3 376.7 377.2

0.999 0.999 0.999

28.1 21.8 9.68

127.3 179.4 282.0

0.899 0.827 0.963

AB294

20 40 60

23.1 15.6 6.94

134.9 96.6 114.7

0.963 0.969 0.974

7.53 × 10−4 1.10 × 10−3 1.55 × 10−3

119.1 90.9 113.3

0.998 0.999 0.999

10.9 6.90 5.37

16.0 23.9 60.4

0.931 0.916 0.960

rate parameters. The k1 , k2 , q1 , q2 , and correlation coefficients r12 and r22 of dyes under different conditions were calculated from these plots and are given in Table 2. As can be seen from Table 2, the correlation coefficients (r12 ) for the pseudo-first-order kinetic model are between 0.929 and 0.974 and the correlation coefficients (r22 ), for the pseudosecond-order kinetic model are between 0.998 and 0.999. These results show that the adsorption system obeys the pseudo-second-order kinetic model. Adsorption kinetics are usually controlled by different mechanisms, of which the most limiting are the diffusion mechanisms, including the initial curved portion, attributed to rapid external diffusion or boundary layer diffusion and surface adsorption, and the linear portion, a gradual adsorption stage due to intraparticle diffusion, followed by a plateau to equilibrium where the intraparticle diffusion starts to decrease due to the low concentration in solution as well as fewer available adsorption sites [29]. The rate-limiting step may be due to intraparticle diffusion. If the intraparticle diffusion is involved in the adsorption process, then a plot of the square root of time (t 1/2 ) vs the uptake (qt ) would result in a linear relationship, and the particle diffusion would be the controlling step if this line passed through the origin [9,30]. When the plots do not pass through the origin, this is indicative of some degree of boundary layer control and this further show that the intraparticle diffusion is not the only rate-controlling step, but also other processes may control the rate of adsorption, all of which may be operating simultaneously. The slope of the linear portion from the figure can be used to derive values for the rate parameter, kp , for the intraparticle diffusion, given in Table 2. The correlation coefficients (rp2 ) for the intraparticle diffusion model are between 0.827 and 0.963. This indicates that the adsorption of AR57 and AB294 onto acid-activated bentonite can be followed by an intraparticle diffusion model after 90 min. The values of the intercept give an idea about the boundary layer thickness: the larger the intercept, the greater is the boundary layer effect. For most systems reported in the literature, there is some evidence of boundary layer resistance in the initial stages of the adsorption process. The boundary layer resistance is affected by the rate of adsorption and increase in contact time, which reduce the resistance and increases the mobility of dye during adsorption. The boundary layer diffusion depends on sev-

eral parameters, including the external surface area of the adsorbent, which is mainly controlled by the particle size, the shape and density of the particles, the concentration of the solution, and the agitation velocity. Intraparticle diffusion is usually controlled by the porosity of the particle. The greater the particle size, the greater the contribution of the intraparticle diffusion resistance in the control of the sorption kinetics of low-porous materials [12,31]. The values of the sorption capacity at equilibrium, q2 , are decreased from 416.3 to 377.2 mg g−1 for AR57 and from 119.1 to 113.3 mg g−1 for AB294 when the temperature is increased from 20 to 60 ◦ C (Table 2). This suggests that a low temperature favors acid dye removal by adsorption onto acid-activated bentonite. Even though the values of q2 for the sorption of AR57 and AB294 onto acid-activated bentonite decrease with increased temperature, the values of the rate constant, k2 , were found to increase from 2.23 × 10−4 to 9.35 × 10−4 g mg−1 min−1 for the sorption of AR57 and from 7.53 × 10−4 to 1.55 × 10−3 g mg−1 min−1 for the sorption of AB294 for an increase in the solution temperature from 20 to 60 ◦ C. 3.5. Adsorption isotherms In order to optimize the design of an adsorption system to remove acid dyes from effluents, it is important to establish the most appropriate correlation for the equilibrium curves. In this respect, the equilibrium experimental data for adsorbed AR57 and AB294 on acid-activated bentonite were compared using two isotherm equations namely, Langmuir and Freundlich. 3.5.1. Langmuir isotherm The Langmuir adsorption, which is the monolayer adsorption, depends on the assumption that the intermolecular forces decrease rapidly with distance, and consequently predicts the existence of monolayer coverage of the adsorbate at the outer surface of the adsorbent. The isotherm equation further assumes that adsorption occurs at specific homogeneous sites within the adsorbent. It is then assumed that once a dye molecule occupies a site, no further adsorption can take place at that site. Furthermore, the Langmuir equation is based on the assumption of a structurally homogeneous adsorbent, where all sorption sites are identical and

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Table 3 Langmuir and Freundlich isotherm constants for the adsorption of AR57 and AB294 Dye

Langmuir

AR57 AB294

Fig. 5. Freundlich adsorption isotherms of AR57 and AB294 onto acid-activated bentonite at pH 2 for 90 min contact time.

energetically equivalent. Theoretically, the sorbent has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further sorption can occur. The saturated or monolayer capacity can be represented as the known Langmuir equation, 1 Ce Ce = + , qe qmax KL qmax

(4)

where qe is the equilibrium dye concentration on the adsorbent (mg g−1 ), Ce is the equilibrium dye concentration in solution (mg dm−3 ), qmax is the monolayer capacity of the adsorbent (mg g−1 ), and KL is the Langmuir adsorption constant (dm3 mg−1 ) [32]. Therefore, a plot of Ce /qe vs Ce gives a straight line of slope 1/qmax and intercept 1/(qmax KL ). The Langmuir equation is applicable to homogeneous sorption, where the sorption of each sorbate molecule onto the surface has equal to sorption activation energy. 3.5.2. Freundlich isotherm The Freundlich equation [33] is an empirical equation employed to describe heterogeneous systems, characterized by the heterogeneity factor 1/n, describes reversible adsorption, and is not restricted to the formation of the monolayer, 1/n qe = KF Ce ,

(5)

where qe is the equilibrium dye concentration on adsorbent (mg g−1 ), Ce is the equilibrium dye concentration in solution (mg dm−3 ), KF is Freundlich constant (dm3 g−1 ), and 1/n is the heterogeneity factor. A linear form of the Freundlich expression can be obtained by taking logarithms of the equation 1 ln Ce . (6) n Therefore, the plots of ln qe vs ln Ce for the adsorption of AR57 and AB294 onto acid-activated bentonite (Fig. 5) were employed to generate the intercept value of KF and the ln qe = ln KF +

Freundlich

qmax (mg g−1 )

KL (dm3 mg−1 )

641.9 117.8

2.46 × 10−3 7.16 × 10−3

2 rL

n

KF (dm3 g−1 )

rF2

0.870 0.952

1.27 3.16

3.18 12.2

0.981 0.973

slope of 1/n. The values of the Langmuir constant (KL ) and the monolayer capacity of adsorbent (qmax ), Freundlich constants (KF ) and (n), the correlation coefficients for Langmuir (rL2 ) and for Freundlich (rF2 ) are given in Table 3. It can be seen that the Freundlich model yields a much better fit than the Langmuir model when the r 2 values are compared in Table 3. This suggests that the boundary layer thickness is increased. The Freundlich constant KF indicates the sorption capacity of the sorbent. From Table 3, the values of KF are 3.18 and 12.2 for AR57 and AB294, respectively. Furthermore, the value of n at equilibrium is 1.27 and 3.16 for AR57 and AB294, respectively. It is noted that the values of n are bigger than 1, reflecting the favorable adsorption. These results indicate that the acid-activated bentonite has a very strong adsorption capacity for anionic AR57 and AB294 dyes in the solution. 3.6. Thermodynamic parameters The amount of dye adsorbed at equilibrium at different temperatures is 20, 40, and 60 ◦ C. It has been utilized to evaluate the thermodynamic parameters for the sorption system. The activation energy, Ea , was obtained from an Arrhenius plot [16]. The other activation parameters, activation enthalpy change (H ∗ ), activation free energy change (G∗ ), and activation entropy change (S ∗ ), are also calculated by using the following equations [16,34] and represented in Table 4, ln k2 = ln A −

Ea , RT

kB T ∗ K , h G∗ = −RT ln K ∗ , k2 =



(7) (8) (9)

H = Ea − RT , (10) ∗ − G∗ H , S ∗ = (11) T where Ea is the Arrhenius activation energy, A is the Arrhenius factor, kB and h are Boltzmann’s and Planck’s constants, respectively, R is the gas constant, and K ∗ is the equilibrium constant at temperature T . A linear plot of ln k2 vs 1/T for the adsorption of AR57 and AB294 onto acidactivated bentonite was constructed to generate the activation energy from the slope. The results obtained are 29.3 and 14.6 kJ mol−1 for AR57 and AB294, respectively. Those

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Table 4 Thermodynamic parameters calculated with the second-order rate constant Dyes

T (◦ C)

AR57

20 40 60

AB294

20 40 60

Ea (kJ mol−1 ) 29.3

r2

14.6

0.999

0.984

experimental calculated low activation energies are characteristic for a physical adsorption (5–40 kJ mol−1 ), while higher activation energies (40–800 kJ mol−1 ) suggest chemical adsorption [35]. The chemical (chemisorption) or physical adsorption (physisorption) mechanism are often an important indicator to describe the type of interactions between dye molecule and bentonite. The adsorption capacity (q2 ) for AR57 and AB294 onto acid-activated bentonite (from Fig. 4), which decreases with increasing temperature, may be concluded to be a physisorption, otherwise a chemisorption. The value of G∗ and S ∗ can be calculated only if the molar mass of the dye is known. Therefore G∗ and S ∗ can not be calculated for AB294. They are only calculated for AR57, which is also commercial dye, the molar mass of this dye is 526 g mol−1 . As can be seen from Table 4, the positive values of Ea , H ∗ , and G∗ indicate the presence of an energy barrier in the adsorption process. The positive values for these parameters are quite common because the activated complex in the transition state is in an excited form. The values of H ∗ and Ea are nearly the same for AR57 and AB294, which confirms that the adsorption of AR57 and AB294 takes place by the cation-exchange mechanism. Entropy of activation can be regarded as a measure of the “saddle point of energy” over which reactant molecules must pass as activated complexes. Thus S ∗ conveys whether a particular reaction proceeds faster or slower than another individual reaction [36]. The negative values of S ∗ suggest decreased randomness at the solid/solution interface and no significant changes occur in the internal structure of the adsorbent through the adsorption of AR57 on acid-activated bentonite [37].

G∗ (kJ mol−1 ) 71.8 74.4 78.0

H ∗ (kJ mol−1 ) 26.8 26.7 26.5

S ∗ (J K−1 mol−1 ) −153.4 −152.7 −154.5

– – –

12.2 12.0 11.8

– – –

for the acid-activated bentonite surface that they are completely adsorbed from dilute solution. The surface charge on the adsorbent and the solution of pH play a significant role in influencing the capacity of an adsorbent toward acid dyes, AR57 and AB294 ions. Having an excess of positive charge on their surfaces, acid-activated bentonite has a greater capacity to adsorb acid dyes. With respect to the suitability of the pseudo-first-order and pseudo-second-order kinetic models for AR57 and AB294 sorption onto acid-activated bentonite, it has been represented that the adsorption kinetics of these dyes corresponding to treated clay material obey preferably the pseudo-second-order kinetics which provide the best correlation of the data in most cases. However, the evidence is provided that the sorption of dye onto acid-activated bentonite is a complex process, so it cannot be adequately described by a single kinetic model throughout the whole process. In this manner, for instance, intraparticle diffusion played a significant role, but it was not the main rate determining step during the adsorption. By comparing the correlation coefficients determined for each linear transformation of isotherm analysis, the Freundlich isotherm model, which fit the experimental data reasonably well, was found to provide the best prediction for the sorption of AR57 and AB294. The activation energy of sorption can be evaluated with the pseudo-second-order rate constants. The positive value of Ea confirms the nature of physical adsorption of AR57 and AB294 onto acid-activated bentonite. The enthalpies of adsorption (H ∗ ) at 20 ◦ C were obtained 26.8 and 12.2 kJ mol−1 for AR57 and AB294, respectively. References

4. Conclusion Bentonite has proven to be a promising material for the removal of contaminants from wastewater. Not only is bentonite abundant, but also it is really an efficient and economic natural adsorbent of pollutants including dyes, oil, and heavy metals. The capacity of acid-activated bentonite to adsorb AR57 and AB294 dyes normally used in the textile industry is very high (416.3 mg g−1 for AR57 and 119.1 mg g−1 for AB294) at 20 ◦ C. It could be successfully applied to clean the wastewaters of the dyeing industry. The dye molecules, AR57 and AB294 acid dyes, have such high affinity

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