Chemical Engineering Journal 200–202 (2012) 59–67
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Anionic dye removal from aqueous solutions using modiﬁed zeolite: Adsorption kinetics and isotherm studies Erol Alver, Aysßegül Ü. Metin ⇑ Department of Chemistry, Faculty of Science and Arts, Kırıkkale University, Yahsßihan 71450, Kırıkkale, Turkey
h i g h l i g h t s " Zeolite is a natural, abundant and environmental friendly material. " Zeolite has low cost and high ion exchange capacity. " Modiﬁcation of zeolite with hexamethylenediamine can be easily performed. " The modiﬁed zeolite demonstrated a superior dye removal performance according to natural one. " Removal efﬁciency of modiﬁed zeolite for both dyes is independent from solution of pH.
a r t i c l e
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Article history: Received 8 March 2012 Received in revised form 8 June 2012 Accepted 11 June 2012 Available online 21 June 2012 Keywords: Zeolite Modiﬁcation Anionic dye Isotherm Kinetic
a b s t r a c t The natural zeolite was modiﬁed with hexamethylenediamine (HMDA) and used as adsorbent to remove anionic dyes, namely Reactive Red 239 (RR-239) and Reactive Blue 250 (RB-250). And, the adsorption equilibrium and kinetic studies of anionic dyes were carried out. In presented work, the origin of the zeolite (Heulandite) used was in Turkey. The adsorption of reactive dyes on modiﬁed zeolite (HMDA-Z) was investigated by batch adsorption experiments. The effects of pH, temperature, sorbent dosage and the initial dyes concentrations were investigated. While the increase in temperature resulted in a higher RB-250 loading per unit weight of the modiﬁed zeolite, adsorption capacity of modiﬁed zeolite did not constitute a noticeable change for RR-239. As an additional factor effecting the removal of reactive dyes, the effects of competitive ions such as nitrate, sulfate and chloride were investigated. The adsorption results indicate that the natural zeolite had a limited adsorption capacity for reactive dyes but is substantially improved upon modifying its surfaces with HMDA. The isotherm data of both investigated dyes were analyzed by the Langmuir, Freundlich, and Redlich–Peterson isotherm model according to temperature. The most appropriate model for the equilibrium process of both dyes was the Freundlich model. The kinetic studies indicated that the adsorption of reactive dyes followed the pseudo-second-order kinetic. Thermodynamic calculations showed that the adsorption of both investigated dyes was a spontaneous and endothermic process for RB-250 and an exothermic process for RR-239. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Dyes are signiﬁcant pollutants causing environmental and health problems . Most of them are toxic and even carcinogenic, and this constitutes a serious hazard for humans and aquatic animals . A large number of dyes are commercially available and used in many industries, such as textile, printing, paper and plastics [3,4]. Dyes can be classiﬁed as anionic (direct, acid and reactive dyes), cationic (basic dyes) and non ionic (disperse dyes) . However, reactive azo dyes constitute over 50% of all textile ⇑ Corresponding author. Tel.: +90 318 357 42 42/4083; fax: +90 318 357 24 60. E-mail addresses: [email protected]
(E. Alver), [email protected]
(A.Ü. Metin). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.038
dyes used in the industry, and they are characterized by the existence of nitrogen–nitrogen double bonds ([email protected]
) [6,7]. Removal of many reactive azo dyes from textile wastewater is difﬁcult because of their highly solubility in water, complex structure and synthetic origin [8,9]. In the removal of dyes from wastewater mostly physical, chemical and biological methods such as ﬂocculation, coagulation, precipitation, adsorption, membrane ﬁltration, electrochemical techniques, ozonation and fungal decolorization have been used [10,11]. However, none of the methods described above have been completely successful in removing color from wastewater. In comparison with other techniques, adsorption is probably one of the simplest, low-cost and effective physical processes for the removal of dyes from wastewater [11–13]. Actived carbon is among the most effective adsorbent for the adsorption
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process because it has a high surface area and excellent sorption capacity, but it has a relatively high cost, low selectivity and regeneration problems [7,14,15]. Therefore, other inexpensive and effective synthetic and natural adsorbents, or their modiﬁed products, have been tested; for example, bentonite (montmorillonite) [13,16,17], sepiolite  and zeolite [12,19]. Natural zeolites are highly porous hydrated alumina silicate materials having three dimensional crystal structures. There are more than 40 natural zeolites identiﬁed in the world . Natural zeolites have been used quite widely in many ﬁelds for building stone, cement, pozzolan, lightweight aggregate, oil spill cleanup, paper ﬁller and desiccants and for gas and liquid separations . They have also been intensively studied for their applicability in the removing of pollutants, such as heavy metal ions, ammonium, inorganic anions, phenols, pesticides and dyes in water [20,21]. High ion-exchange capacity and relatively high speciﬁc surface areas, and more importantly, their relatively low cost make zeolites attractive adsorbents . However, the adsorption of anionic reactive dyes using natural zeolite is very limited due to the surface of the zeolite and the dye molecules having negative charges [15,20,22]. Until now, in order to increase the adsorption capacity of anionic dyes from wastewater using zeolite, its surface has been modiﬁed with various agents, such as CTAB (cetyltrimethylamoniumbromide), HTAB (hexadecyltrimethylammonium bromide) . Reactive Red 239 (RR-239) and Reactive Blue 250 (RB-250) have been widely used in textile industry. There are a lot of reports in the literature, including different methods such as biodegradation , photocatalysis  and adsorption [8,15,23] for the removal of RR-239 from wastewater. Nevertheless, no study has been found in the literature for the removal of RB-250 from wastewater. The aim of the present study is to investigate an alternative method of zeolite modiﬁcation for removal of anionic dyes from wastewater. The hexamethylenediamine (HMDA) was used for modiﬁcation of natural zeolite. It was thought that the zeolite surface can be cover with HMDA molecules during modiﬁcation reaction and so that the repulsion between negatively charged zeolite and anionic dye molecules can be prevent and the afﬁnity of zeolite to anionic dyes improve. The removal studies of reactive dyes with modiﬁed zeolite were carried out by batch adsorption experiments. The inﬂuence of various important parameters, such as pH, temperature, time, adsorbent amount and initial dye concentration was investigated.
Langmuir, Freundlich, and Redlich–Peterson isotherm models were examined in terms of their appropriateness for the experimental data obtained. Pseudo ﬁrst-order, second-order, Bangham’s equation and intra-particle diffusion kinetic models were used to evaluate the mechanism of adsorption. The thermodynamic parameters, such as DGo, DHo, and DSo were calculated. 2. Experimental 2.1. Chemicals and reagents The textile reactive dyes Reactive Red 239 (RR-239) and Reactive Blue 250 (RB-250) were purchased from DyStar Co. (Germany) and used without further puriﬁcation. Some of the important physicochemical properties of the investigated dyes are presented in Table 1. All other chemicals and reagents used were of analytical grade. 2.2. Preparation of HMDA-zeolite The zeolite (minerals of Heulandite type) sample was selected as an adsorbent for its relatively mild surface area and exceptionally high selective ion-exchange capacity and was provided by Eti Mine Management in the Bigadiç region of Turkey. Before modiﬁcation reaction, zeolite was dried to remove moisture. The dried zeolite material was sieved using a molecular sieve and the <125 lm size of fraction was used in further reactions. Natural zeolite particles were modiﬁed according to following modiﬁcation method . The natural zeolite particles were reacted with hexamethylenediamine (HMDA) solution (1% w/v) at 65 °C in a reactor containing 10 g zeolite particles and were stirred magnetically for 5 h. After this reaction, the solid phase was separated by ﬁltration then washed several times with distilled water and dried at room temperature for 24 h. The effect of HMDA concentration on removal efﬁciency of modiﬁed zeolite was also investigated. For this purpose, HMDA concentration was changed between range of 1– 10% in reaction medium. 2.3. Analysis and measurements The chemical analysis of natural zeolite was performed by using an X-ray spectrometer (Rigaku, D/MAX-2200, Japan).
Table 1 Basic properties of investigated dyes. Dye
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The FTIR spectra of modiﬁed zeolite was obtained using a Bruker Vartex 70 V model Fourier Transform Infrared Spectrometer to observe the surface functional groups. Elemental analyses were performed to determine the amount of nitrogen (N) in HMDA-zeolite. For this purpose, elemental analyses of natural zeolite and HMDA modiﬁed zeolite were carried out in a helium atmosphere by using a Vario MICRO Cube Elemental Analyzer. The maximum absorbance wavelengths of RR-239 and RB-250 reactive dyes were determined with a UV–vis spectrometer (Table 1) and all the absorbance measurements of reactive dyes were made with a UV–vis spectrometer (Labormed Inc., USA).
layer (mg/g), a constant related to adsorption capacity, and KL is the constant related to the afﬁnity of the binding sites and energy of adsorption (L mg1). A dimensionless constant separation factor (RL) of Langmuir isotherm was used to determine the favorability of the adsorption process. RL is deﬁned as Eq. (4); the values of RL indicate the type of isotherm to be irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1).
2.4. Dye removal experiments
where KF is the Freundlich constant related to the adsorption capacity of adsorbent. Generally, the adsorption capacity of an adsorbent for a given adsorbate enhances with an increase in KF; 1/n is the Freundlich exponent related to surface heterogeneity.
Removal experiments of reactive azo dyes (RR-239 and RB-250) with modiﬁed zeolite from solutions by adsorption were carried out individually for each of the reactive azo dyes in a batch system. Each adsorption experiment consisted of preparing 10 ml of dye solution with the desired initial concentration and pH by diluting the stock dye solutions with distilled water and transferring them into an Erlenmeyer ﬂask on the magnetic stirrer. The solution pH is adjusted in the range of 2–12 by adding 0.1 N HCl or 0.1 N NaOH solutions according to requirement. A known mass of modiﬁed zeolite was then added to the solution and stirred magnetically at 150 rpm until reaching equilibrium. The effect of the adsorption parameters such as ionic strength, sorbent dosage, contact time on removal efﬁciency of zeolite was investigated. The experimental conditions of each parameter are given as: The effect of ionic strength on the removal efﬁciency of both dyes onto adsorbent was discussed over the NaCl, Na2SO4 and NaNO3 concentration range from 0.0 to 1 mol/L. The effect of the contact time was studied at 20 °C with an initial dye concentration of 50 mg/L. The effect of the adsorbent amount was studied by varying it in the range of 0.05–1.0 g with the initial dye concentration of 50 mg/L at 20 °C. The amount of each dye adsorbed by the modiﬁed zeolite and the percentage removal of them were calculated using the following Eqs. (1) and (2), respectively.
q ¼ ðC 0 C f ÞV=m
Dye removal efficiencyð%Þ ¼ ðC 0 C f =C 0 Þ 100
where, q is adsorption capacity in mg/g, C0 and Cf are the respective dye concentrations of initial and ﬁnal after treatment for certain period of time (mg/L), V is the volume of each dye solution in mL and m is the total amount of modiﬁed zeolite in g. 2.5. Adsorption isotherms Adsorption isotherm experiments were conducted by equilibrating modiﬁed zeolite in Erlenmeyer ﬂasks containing 10 mL of dye solutions of varying initial dye concentrations (25–500 mg/ L). The mixture was stirred magnetically at 150 rpm until reaching equilibrium by varying the temperature range of 20–60 °C. After equilibrium, samples were centrifuged and analyzed for the residual dye concentrations. The amount of each dye adsorbed by the modiﬁed zeolite were calculated using Eq. (1). The equilibrium of adsorption was evaluated by using the following isotherm models. Langmuir isotherm model [25,26]:
1=qe ¼ 1=qmax þ ð1=qmax K L Þ1=C e
where Ce is the equilibrium concentration (mg/L), qe the amount of adsorbed dye molecule (mg/g), qmax the qe for a complete mono-
RL ¼ 1=1 þ K L C
Freundlich isotherm model [25–27]:
qe ¼ K F C 1=n e
Redlich–Peterson isotherm model [28,29]:
qe ¼ K R C R =ð1 þ aR C be Þ 1
where KR (L g ) and aR (L mg ) are the R–P constants: b is the exponent which has a value between 0 and 1 and Ce the equilibrium liquid phase concentration (mg L1). The R–P isotherm incorporates three parameters and can be applied either in homogenous or heterogeneous systems. Eq. (6) can be converted into a linear form by taking the logarithms of both sides as:
lnðK R C e =qe 1Þ ¼ ln aR þ b ln C e
A minimization procedure was adopted to solve Eq. (7) by maximizing the correlation coefﬁcient between the predicted values of qe from Eq. (7) and the experimental data using the solver add-in function of MS excel . 2.6. Adsorption kinetics Adsorption kinetic experiments were conducted in Erlenmeyer ﬂasks containing dye solution (50 mg/L) and modiﬁed zeolite of 5 g/L sorbent. The ﬂasks were stirred magnetically at 150 rpm under constant temperature 20 °C. The samples were taken at predetermined time intervals, centrifuged and analyzed for the residual dye concentrations. The kinetic data were analyzed using pseudo-ﬁrst-order, pseudo-second-order and intra particle diffusion models and Bangham’s equation. Pseudo-ﬁrst-order model :
logðqe qt Þ ¼ log qe kf =2:303t
where qt is the amount of adsorbate adsorbed at time t (mg/g), qe the adsorption capacity at equilibrium (mg/g), kf the pseudo-ﬁrstorder rate constant (min1), and t the contact time (min).The values of the adsorption rate constant, kf, for both reactive dyes were determined from the plot of log(qe – qt) against t. The pseudo-second-order model is represented as :
t=qt ¼ 1=k2 q2e þ t=qe
where k2 is the pseudo second-order rate constant (g mg1 min1). The initial adsorption rate h0 (mg g1 min1) at t = 0 is deﬁned as follows :
h0 ¼ k2 q2e
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qe is obtained from the slope of t/qt versus t and h is obtained from the intercept. Since qe is known from the slope, k2 can be determined from the value of h0. The intra-particle diffusion model :
qt ¼ kt t 1=2 þ C
where kt is the intra-particle diffusion rate constant (mg g min ) and C is the intercept. The value of C relates to the thickness of the boundary layer. The larger C implies the greater effect of the boundary layer . Bangham’s equation :
logðC 0 =C 0 qt mÞ ¼ logðk0 m=2:303VÞ þ a logðtÞ
where V is the volume of the solution (mL), and a (<1) and k0 are constants. 3. Results and discussion 3.1. Properties of natural and modiﬁed zeolite In this study, zeolite was used in the removal of reactive dyes. It is known that natural zeolite has a negatively charged surface, this causes a relatively low adsorption capacity for reactive azo dyes with a negative sulfonate group due to repulsion occurring between the dye and the zeolite surface [35,36]. To reduce the negative charge of the surface, the zeolite was modiﬁed with HMDA. It was thought that the zeolite surface was covered with positive charges with HMDA modiﬁcation and increased interaction between dye molecules and the zeolite surfaces. Efﬁciency of modiﬁed zeolite was tested by adsorption of RR-239 and RB-250 and dye removal efﬁciency increased from 5% to 97% for RR-239 and from 6% to 99% for RB-250 with modiﬁcation (see Fig. 1). Natural zeolite has the following properties: purity of 96.2%, density of 2.14 g/mL, and suspension pH of 7.5–7.8. A cation exchange capacity (CEC) was 0.11 meg/g was determined using methylene blue method . The chemical composition of natural zeolite was as follows (%): SiO2: 69.2, Al2O3: 10.81, CaO: 2.98, K2O: 2.78, MgO: 1.25, Fe2O3: 1.18, Na2O: 0.367, TiO2: 0.08 and P2O5: 0.021. This result showed that the major components of zeolite were silica and alumina and the impurities were titanium, iron, sodium, potassium, calcium and magnesium oxides. The XRD result (not shown here) was compatible with that of the chemical analysis. As a result, the zeolite consisted of mainly silica and alumina.
Fig. 1. Effect of initial solution pH on removal efﬁciency of RR-239 and RB-250 dyes onto zeolite and modiﬁed zeolite; initial dye concentration: 50 mg/L; temperature: 20 °C.
The effect of HMDA concentration on dye removal efﬁciency of zeolite was also investigated. The modiﬁcation experiments were performed at different HMDA concentrations (1%, 5% and 10%). Dye removal efﬁciency of zeolite was increased for both dyes upon increasing the HMDA concentration and the adsorption capacity of zeolite was not changed between 5% and 10%, and reached a plateau value. For this purpose, the 5% HMDA modiﬁcation was used in further experiments. The elemental analysis results provided direct evidence that natural zeolite could be modiﬁed in the presence of HMDA . The ratio of C/N for HMDA-zeolite was 2.57 and the calculated value of the C/N ratio for HMDA was 2.80. The percentage of the modifying agent onto zeolite was 5.21. According to these results, it can be said that the modiﬁcation of zeolite with HMDA was achieved. The FT-IR spectra of modiﬁed zeolite in the range 400– 4000 cm1 was obtained. Modiﬁed zeolite had many peaks around 470 cm1 and 1100 cm1 which were due to bending vibrations and the symmetric stretching vibration of SiAOASi bonds. The bands at 850 cm1 and 1079 cm1 belonged to SiAO-terminal vibration and asymmetric stretching vibration of the SiAOASi bridge band. The peak around 1600 cm1 can be assigned to the scissoring vibration of ANH2 and the absorption bands around 2850 and 2900 cm1 which were characteristic symmetric and asymmetric ACH2 vibrations of HMDA, respectively. 3.2. Dye removal studies 3.2.1. Effect of system parameters on removal efﬁciency of reactive azo dyes 18.104.22.168. Effect of pH. As is known, the pH of a solution is an important parameter affecting the adsorption process because it affects both aqueous charge distribution and the surface binding site of the sorbent . Therefore, in this study, in order to determine the effect of the initial solution pH on two reactive azo dyes on natural zeolite and modiﬁed zeolite (HMDA-Z) was investigated in the range of 2–12, while initial dye concentration (50 mg/L) and temperature (20 °C) were kept constant. The results are shown in Fig. 1. As can be seen in this ﬁgure, HMDA-Z particles have more effective dye removal capacity than natural zeolite for both investigated dyes. Also, it was observed that the pH had no signiﬁcant effect over a wide pH range 2–10, on removal of RR-239 and RB250, on modiﬁed zeolite. Removal efﬁciency of RR-239 and RB250 on modiﬁed zeolite remained above 97% and 99% in this pH range, respectively. The constant removal efﬁciency of zeolite for both investigated dyes over the pH range 2–10 was an indication that the ion-exchange mechanism is formed between the moieties of dissociation for dyes and HMDA modiﬁed zeolite was not only mechanism for dye removal in this system. Other interaction mechanisms may be play a role. The zeolite that is not covered by HMDA can also interact with dye molecules via hydrogen bonding and van der Waals interactions . The large reduction in dye removal efﬁciency at highly basic conditions (pH >10) can attributed to ionic repulsion as a result of the presence of OH ions between deprotonated dye molecules and negatively charged support. A similar result was obtained and reported by another researcher . According to obtained results, pH 7 was chosen as the optimum pH for removal of both dyes for further experiments. Netpradit et al.  reported that two structural factors have an important effect on the removal of anionic dyes by adsorption; (i) the amount of SO 3 groups is an attractive force with positive charges; (ii) position of SO 3 and NH2 groups in the same benzenic chain reduces the electrostatic interaction between anionic dyes and positive charges. In this situation, as in Table 1, both the anionic dyes have approximately the same amount of SO 3 , so their pH proﬁles resemble each other.
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Fig. 2. Effect of contact time on the dye removal efﬁciency onto modiﬁed zeolite; initial dye concentration: 50 mg/L; pH of dye solution: 7; temperature: 20 °C.
Fig. 3. Effect of modiﬁed zeolite dosage on dye removal efﬁciency from single dye solutions; initial dye concentration: 50 mg/L; pH of dye solution: 7; temperature: 20 °C.
22.214.171.124. Effect of contact time. The effect of contact time on the dye removal efﬁciency of HMDA-Z is shown in Fig. 2. It was seen from Fig. 2 that adsorption increased quickly at the beginning. After 30 min, adsorption of reactive dyes nearly reached equilibrium. From this result, it can be said that kinetics of removal efﬁciency of both investigated dyes were formed in two phases: ﬁrst, an initial rapid phase where the adsorption of dye molecules was fast and instantaneous, and the second phase was a slow stage in which the contribution to the total reactive dyes removal efﬁciency was relatively small, and ﬁnally the removal of both reactive dyes reached equilibrium . 126.96.36.199. Effect of sorbent dosage. The effect of adsorbent dosage on the removal of selected azo dyes was evaluated by varying the dosage of the modiﬁed zeolite, which ranged from 0.05 g to 1 g, by keeping constant initial dye concentration at 50.0 mg/L. As seen in Fig. 3, removal efﬁciencies of the reactive dyes increased by increasing the amount of adsorbent. It was observed that more than 93% adsorption was reached by using 0.25 g of the sorbent and there was no signiﬁcant change in dye removal efﬁciency. This may be due to the availability of more adsorbent surface for the azo dyes to be adsorbed. 188.8.131.52. Effect of electrolytes. Textile industries usually use large amounts of salts for the dyeing of fabric [42,43]. Therefore, it is
Fig. 4. Effect of electrolyte concentrations in RR-239 dye solution on the removal efﬁciency of modiﬁed zeolite; initial dye concentration: 50 mg/L; pH of dye solution: 7.0; sorbent dosage: 0.5 g; temperature: 20 °C.
Fig. 5. Effect of electrolyte concentrations in RB-250 dye solution on the removal efﬁciency of modiﬁed zeolite; initial dye concentration: 50 mg/L; pH of dye solution: 7.0; sorbent dosage: 0.5 g; temperature: 20 °C.
useful to investigate the effect of different anions on the dye removal efﬁciency of modiﬁed zeolite. Figs. 4 and 5 show the effect 2 of Cl, NO 3 and SO4 on removal of RR-239 and RB-250 by modiﬁed zeolite, respectively. When concentration of electrolyte increased in the dye solution from 0 to 1 M, the dye removal efﬁciency of modiﬁed zeolite decreased. The blocking active sites can be explained by support to the surface in the form of adding electrolytes. SO2 4 , NO3 , Cl ions may interfere with electrostatic interaction be tween SO3 ions on dye molecules and the positive charge of HMDA-Z surface. The order of reduction was SO2 4 > NO3 > Cl for 2 RR-239, and NO > SO > Cl for RB-250. The dye removal efﬁ3 4 ciency was affected more by SO2 4 and NO3 ions than Cl because these ions are more negative than Cl and the removal of large dye molecules by adsorption mainly depend on the electrical interaction. These ions compete with the SO 3 groups of dye molecules for interaction with the support surface . However, the maximum reduction of the removal efﬁciency was 17% (presence of SO2 4 ) for RR-239 and 3% (presence of NO3 ) for RB-250. It may be said that the HMDA-Z is a usefull sorbent in removing reactive dyes in the presence of these anions. 184.108.40.206. Effect of initial dye concentration. The effect of the initial concentration of the RR-239 and RB-250 dyes on removal efﬁciency was investigated at a different concentration range of 25–500 mg/L on HMDA-Z under previously determined optimum
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Fig. 6. Effect of initial dye concentration on removal efﬁciency of modiﬁed zeolite; pH of solution pH of solution: 7.0; sorbent dosage: 0.5 g; temperature: 20 °C.
conditions. The results are showed in Fig. 6. According to Fig. 6, RB250 removal decreased from around 97% at concentration of 25 mg/L to 57% when the concentration was increased 500 mg/L. However, the removal efﬁciency of RR-239 dye remained constant at greater than 99% over the whole range of the investigated dye concentrations. It indicates that its adsorption is independent from the initial concentration in the studied range. Furthermore, modiﬁed zeolite has greater afﬁnity for RR-239 than RB-250. This is probably due to the steric hinderance between RB-250 dye molecules because of its larger molecular size . 3.2.2. Adsorption isotherms Adsorption isotherms are important for describing the adsorption mechanism for the interaction of dye molecules on the adsorbent surface. A variety of adsorption isotherm models are in use, some of which have a theoretical base and some being solely of an empirical nature . The equilibrium data for the removal of reactive dyes using HMDA-Z into various isotherm models results in a suitable model that can be used for the design of an adsorption process. In the present study, various isotherm equations, like those of Langmuir , Freundlich  and Redlich–Peterson  were applied to describe the dye molecules-modiﬁed zeolite system. The adsorption constants of RR-239 and RB-250 reactive dyes onto modiﬁed zeolite were calculated according to these adsorption models and listed in Table 2. According to Table 2, all values of R2 were bigger than 0.9 for both dyes. Thus, all isotherm models Table 2 Isotherm model parameters for the removal of RR-239 and RB-250 by modiﬁed zeolite. (pH: 7.0, sorbent dosage: 0.05 g, initial dye concentration: 25–500 mg/L). RR-239
Langmuir qm (mg/g) qe,exp (mg/g) KL 103 R2 RL
33.0 28.57 3.22 0.9886 0.383
31.05 27.77 3.37 0.9829 0.373
29.67 28.57 3.39 0.9717 0.371
20.63 17.63 5.63 0.9881 0.262
47.39 38.46 4.45 0.9865 0.305
58.5 40.56 3.64 0.9781 0.354
Freundlich KF 102 1/n R2
17.66 0.8188 0.9975
17.42 0.8162 0.9959
14.95 0.8431 0.9939
34.85 0.6411 0.9902
55.82 0.6904 0.9927
52.72 0.7196 0.9962
Redlich–Peterson 0.2 KR aR 1.076 b 0.1953 R2 0.9458
0.2 1.186 0.1761 0.9271
0.2 1.276 0.1664 0.9336
0.2 0.5699 0.3603 0.9682
0.2 0.3543 0.3117 0.9659
0.2 0.3799 0.2804 0.9172
were seen to be appropriate for the experimental data. This result showed that the adsorption process might be a heterogeneous adsorption. The Langmuir constants (KL and qm) can be evaluated from the intercept and the slope of the linear plot of experimental data of 1/qe versus 1/Ce at 293, 313 and 333 K. Langmuir constant, qm, represents the monolayer saturation at equilibrium when the surface is fully covered with dye molecules and assists in the explanation of adsorption performance. The changes of maximum predictable adsorption capacity of RR-239 and RB-250 on modiﬁed zeolite are shown in Table 2. As seen, the alteration values of qm for RR239 removal were negligible with temperature rise and compatible with the values of qe,exp at all investigated temperatures; however, the values of qm for RB-250 adsorption increased with temperature rise and it changed quite considerably from the values of qe,exp at all studied temperatures. This distinctness may be due to steric hindrance caused by the size of the dyes . The other Langmuir constant KL, indicates the afﬁnity for the binding of reactive dyes. A high KL value indicates a high afﬁnity. The values of dimensionless parameter RL, which is a measure of adsorption favorability were calculated according to Eq. (4) and are given in Table 2. The RL values were found in the range of 0 < RL < 1 which conﬁrmed a favorable adsorption process for RR-239 and RB-250 removal using modiﬁed zeolite. It is apparent that modiﬁed zeolite is a suitable sorbent for removal of the RR-239 and RB-250 dyes from aqueous solution with the conditions used in this study. Freundlich model applies to adsorption on heterogeneous surfaces with the interaction between the adsorbed molecules . The KF and n values can be evaluated from the intercept and the slope of the linear plot of experimental data of lnqe versus ln Ce at 293, 313 and 333 K. For RB-250, all measured values of KF showed an easy uptake to high adsorptive capacity of modiﬁed zeolite and signiﬁcant differences in adsorption capacities along with increasing temperature. The values of 1/n imply the type of isotherm and can be classiﬁed as irreversible (1/n = 0), favorable (0 < 1/n < 1) and unfavorable (1/n > 1) [25–27]. The obtained values of 1/n (0.1 < 1/n < 1) showed the favorable nature of both investigated dyes and the heterogeneity of the adsorbent sites at all temperatures studied. However, it was seen from Table 2 that the RR-239 showed no speciﬁc trend of heterogeneity with the rise in temperature. In addition, the n values of both dyes were lower when temperatures were higher, meaning that the slope (1/n) of the Freundlich plots was steeper and the capacity of sorbent was more susceptible to the dye concentration change. Furthermore, Baocheng et al. reported that according to the classiﬁcation of Giles et al., adsorption isotherm follows L-type curves (n > 1) which indicates that the adsorption ﬁrstly occurs on the high energy sites of sorbents followed by the low energy sites . Relevant adsorption parameters were also calculated according to Redlich–Peterson isotherm at different temperatures. Redlich– Peterson isotherm contains three parameters and is an improvement over the Langmuir and Freundlich isotherms. It can be applied either in homogenous or heterogeneous systems [28,29]. A minimization procedure was adopted to solve R–P isotherm equation, the relative parameters were obtained according to the intercept and slope from the plots between ln(KRCe/qe1) versus ln Ce at 293, 313 and 333 K . Examination of the data showed that the R–P isotherms were appropriate descriptions of the data for both dyes adsorption over the concentration range studied. The R–P constant b, normally changes between 0 and 1, indicating favorable adsorption. As can be seen from Table 2, values of b were in this range for both dyes. It is also noted that constant b is near to 0, and these indicate the isotherm is approaching the Freundlich form. The values of the R–P constants were converted into the values of the Freundlich constants. KR/aR approximated the Freundlich constant KF. As a result,
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Table 3 Calculated kinetic parameters for pseudo ﬁrst-order, second-order and Weber Morris intra particle models for the removal of RR-239 and RB-250 using modiﬁed zeolite as an adsorbent. (T: 293 K, C: 50 ppm; sorbent dosage: 0.5 g). Pseudo-ﬁrst-order
h0 (mg g1 min1)
k2 (g mg1 min1)
the Freundlich and Redlich–Peterson adsorption models seem to provide the best ﬁt with the experimental data and predict values for the removal of RR-239 and RB-250 anionic dyes on modiﬁed zeolite. 3.2.3. Adsorption kinetics The adsorption rate is an important parameter used to image the adsorption process. Many applications, such as wastewater treatment and dye removal need a rapid adsorption rate and short contact time. In order to investigate the removal of RR-239 and RB250 using modiﬁed zeolite, pseudo-ﬁrst-order, pseudo-second-order, Bangham’s equation and intra-particle diffusion kinetic models were used. The constants of all kinetic models were calculated and listed in Table 3. As seen in Table 3, the correlation coefﬁcients (R2) obtained from pseudo second-order model were found to be higher than 0.999 for both investigated dyes, which were larger than those of the pseudo ﬁrst-order model. Despite the correlation coefﬁcients for the ﬁrst-order kinetic model obtained at 293, 313 and 333 K were quite high (>0.90), the calculated qe values did not give admissible values, thus the removal of both investigated dyes on modiﬁed zeolite did not ﬁt this equation. On the other hand, the results revealed that the pseudo-second-order model was the best for describing the kinetics of RR-239 and RB-250 adsorbed on modiﬁed zeolite (Fig. 7), the calculated values of qe (qe,cal) obtained from the pseudo-second-order model perfectly agreed with the experimental values of qe (qe,exp). The results indicated that the adsorption of investigated reactive dyes from aqueous solution on modiﬁed zeolite followed the pseudo second-order model well. The adsorbate transport from the solution to the surface of the adsorbent occurs in several steps. This phenomenon may be controlled by one or more steps such as ﬁlm or external diffusion, pore diffusion, surface diffusion and adsorption on the pore surface, or a combination of more than one step through the adsorption process. The probability of intra-particle diffusion resistance affecting adsorption process was explored by using the intra-particle diffusion model [45,46]. The value C in Eq. (11) is a constant that gives an idea about the thickness of the boundary layer, larger the value the greater is the boundary effect . If the plot of q versus t0.5 gives a straight line, the adsorption process is controlled by intraparticle diffusion only. However, if the data exhibit multi-linear plots, then two or more steps inﬂuence the sorption process . It seen in Fig. 8 that there were three separate regions for adsorption of both dyes. The ﬁrst shaper section is attributed to the surface adsorption, so it called external surface adsorption, the second section describes the gradual adsorption, and the third section is expressed as the ﬁnal equilibrium stage where adsorption becomes very slow and stable, approaching maximum adsorption value . It has been reported by various researchers that the plots do not pass through the origin, this indicates that the intraparticle diffusion is not only rate controlling step, also some other processes may control the rate of adsorption [49,50]. Kinetic data can further be used to check whether pore diffusion is the only rate-controlling step in the adsorption system using Bangham’s equation, given in Eq. (12). If the experimental data are compatible with this equation, the adsorption kinetics are lim-
Fig. 7. Pseudo-second-order kinetic plot for the removal of RB-250 and RR-239 by modﬁed zeolite; T: 293 K; Co: 50 mg/L.
Fig. 8. Weber–Morris intra-particle diffusion plot for the removal of RR-239 and RB-250 by HMDA-zeolite; T: 293 K, Co: 50 mg/L.
ited to the pore diffusion. The double logarithmic plot (not shown here), according to Eq. (12), did not yield satisfactory linear curves for the removal of both investigated dyes by the modiﬁed zeolite. However, kinetic parameters and the correlation coefﬁcients obtained by the Bangham’s equation are given in Table 3 and the experimental data were seen poor in terms of the appropriateness for the model. This situation indicates that the diffusion of adsorbate into pores of the sorbent is not only the rate-controlling step. The intraparticle diffusion and Bangham’s equation results suggest that the adsorption of both dyes probably occurs through surface exchange reactions (i.e. ion exchange) until functional sites of sorbent are completely ﬁlled; after that dye molecules diffuse into the modiﬁed zeolite and other interactions may take place.
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3.2.4. Effect of temperature on dye removal and determination of thermodynamic parameters Temperature is a signiﬁcant parameter effecting adsorption capacity of sorbents and transport/kinetic process of the dye adsorption. Therefore, the thermodynamic parameters of process such as enthalpy (DH), entropy (DS) and free energy (DG) of dye adsorption on modiﬁed zeolite were calculated by using following equations : o
DG ¼ RT ln K
DGo ¼ DHo T DSo
Combining the above two equations, we get:
ln K ¼ DHo =RT þ DSo =R
where R is universal gas constant (8.314 Jmol1 K1), T is the temperature (K) and K is the equilibrium constant. The adsorption of RR-239 and RB-250 on modiﬁed zeolite was studied at temperatures of 293, 313 and 333 K. The maximum adsorption capacities of HMDA-zeolite for RR-239 and RB-250 were obtained as 28.57 and 17.63 mg/g at 293 K, respectively. The adsorption capacity of modiﬁed zeolite for RB-250 increased to 40.56 mg/g when the temperature was increased from 293 K to 333 K, showing that the adsorption had endothermic nature. However, the effect of temperature on the removal behavior of modiﬁed zeolite for RR-239 was different from RB-250. It was observed that the effect of the temperature for the removal of it was not particularly important. When the temperature was increased, the adsorption capacity of the modiﬁed zeolite was approximately constant. This result indicates that the adsorption of RR-239 is favorable at low temperature, and the adsorption bond becomes weak and conducted with van der Waals forces. The standart enthalpy changes (DHo) were determined as 0.809 and 17.220 kJ/mol for RR-239 and RB-250, respectively by the slope of the linear Van’t Hoff plot, as lnK versus (1/T), using Eq. (15). Adsorption process can be classiﬁed as physical adsorption and chemisorption by the magnitude of the enthalpy change. It is accepted that if magnitude of enthalpy change is less 84 kJ/ mol, adsorption is physical. However chemisorption takes place range from 84 to 420 kJ/mol . The negative value of DHo showes that RR-239 adsorption is exhothermic and the value of DHo implies that the nature of adsorption is also physical involving weak interaction forces such as van der Waals. On the other hand, the adsorption of RB-250 had a positive DHo value that conﬁrmed the endothermic nature of the overall sorption process. The adsorption behavior of molecules in the solid–liquid system was determined by a combination of two processes : (i) the desorption of the molecules of solvent previously adsorbed, and (ii) the adsorption of adsorbate species . The RB-250 dye molecules for adsorption on HMDA-Z surface had to replace more than one water molecule due to its moleculer size. This phenomena might have occured in the endothermicity of the adsorption process. The magnitude of entalphy change (+17.220 kJ/mol) indicates that the adsorption is phsical in nature and the primary interaction mechanism of the adsorption process is ion exchange occurred between dye and SiAOA interactions . This is agreement with what was found in the adsorption kinetics where we found that adsorption occurs via surface exchange reactions. The negative values of (10.21, 10.85, 11.48 kJ/mol) DGo indicated the spontaneous nature of adsorption for RR-239 at 20, 40 and 60 °C. and the adsorption of RB-250, which also had negative values of DGo (8.679, 11.301 and 12.170 kJ/mol), indicated the feasibility and spontaneity of the adsorption process at the same reaction temperatures, respectively . The positive value of DSo (0.0321, 0.0892 kJ/mol K for RR-239 and RB-250, respec-
tively) showed the increasing randomness at the solid–solution interface during the adsorption process . Also, a positive DSo value corresponded to an increase in the degree of freedom of the adsorbed species . 4. Conclusion Natural zeolite has a limited dye removal capacity for anionic dyes. Nevertheless, the presented study showed that the dye removal capacity of zeolite was substantially improved upon modifying its surface with HMDA. The effect of system parameters, such as pH, temperature and salt concentration, on dye removal efﬁciency of modiﬁed zeolite was investigated. The removal efﬁciency of modiﬁed zeolite for both dyes was independent of solution of pH; having no signiﬁcant effect over a wide pH range 2–10. Adsorption isotherm, adsorption kinetic and adsorption thermodynamics were studied for both dyes. Equilibrium adsorption data were best represented by the Freundlich and Redlich–Peterson isotherm models. The values of the coefﬁcient R2 obtained from pseudo secondorder model were higher than 0.999 for both dyes; thus, the adsorption process obeyed the pseudo second-order model for the entire adsorption period. It is important to achieve the rate at which both dyes are adsorbed on modiﬁed zeolite, which is useful in design of a ﬁxed-bed adsorption columns for the removal of anionic dye from wastewater. Thermodynamic calculations for both dyes indicated that the adsorption was spontaneous. The values of enthalpy change for both the investigated dyes were less than 84 kJ/mol, this indicated clearly that the adsorption process on modiﬁed zeolite was physical. References  S. Wang, E. Ariyanto, Competitive adsorption of malachite green and Pb ions on _ natural zeolite, J. Colloid Interf. Sci. 314 (2007) 25–31.  M. Qiu, C. Qian, J. X, J. Wu, G. Wang, Studies on the adsorption of dyes into clinoptilolite, Desalination 243 (2009) 286–292.  X.S. Wang, Y. Zhou, Y. Jiang, C. Sun, The removal of basic dyes from aqueous solutions using agricultural by-products, J. Hazard. Mater. 157 (2008) 374–385.  R.O. Crist?vão, A.P.M. Tavares, L.A. Ferreira, J.M. Loureiro, R.A.R. Boaventura, E.A. Macedo, Modeling the discoloration of a mixture of reactive textile dyes by commercial laccase, Bioresour. Technol. 100 (2009) 1094–1099.  I.D. Mall, V.C. Srivastava, N.K. Agarval, Removal of orange-G and methyl violet by adsorption onto bagasse ﬂy ash – kinetic study and equilibrium isoterm analyses, Dyes Pigments 69 (2006) 128–136.  H.L. Liu, Y.R. Chiou, Optimal decolorization efﬁciency of Reactive Red 239 by UV/TiO2 photocatalytic process coupled with response surface methodology, Chem. Eng. J. 112 (2005) 173–179.  D.D. Asouhidou, K.S. Triantafyllidis, N.K. Lazaridis, K.A. Matis, S.S. Kim, T.J. Pinnavaia, Sorption of reactive dyes from aqueous solution by ordered hexagonal and disordered mesoporous carbons, Micropor. Mesopor. Mater. 117 (2009) 257–267.  O. Özdemir, B. Armagan, M. Turan, M.S. Celik, Comparison of the adsorption characteristics of azo-reactive dyes on mezoporous minerals, Dyes Pigments 62 (2004) 49–60.  O. Crist?vão, A.P.M. Tavares, A.S. Ribeiro, J.M. Loureiro, R.A.R. Boaventura, E.A. Macedo, Kinetic modelling and simulation of laccase catalyzed degradation of reactive textile dyes, Bioresour. Technol. 99 (2008) 4768–4774.  B.H. Hameed, A.A. Ahmad, N. Aziz, Isotherms, kinetics and thermodynamics of acid dye adsorption on activated palm ash, Chem. Eng. J. 133 (2007) 195–203.  S. Wang, Z.H. Zhu, Characterisation and environmental application of an Australian natural zeolite for basic dye removal from aqueous solution, J. Hazard. Mater. B136 (2006) 946–952.  T. Sismanoglu, Y. Kismir, S. Karakus, Single and binary adsorption of reactive dyes from aqueous solutions onto clinoptilolite, J. Hazard. Mater. 184 (2010) 164–169.  Ö. Gök, A.S. Özcan, A. Özcan, Adsorption behavior of a textile dye of Reactive Blue 19 from aqueous solutions onto modiﬁed bentonite, Appl. Surf. Sci. 256 (2010) 5439–5443.  X. Jin, M. Jiang, X. Shan, Z. Pei, Z. Chen, Adsorption of methylene blue and orange II onto unmodiﬁed and surfactant-modiﬁed zeolite, J. Colloid Interf. Sci. 328 (2008) 243–247.  D. Karadag, M. Turan, E. Akgul, S. Tok, A. Faki, Adsorption equilibrium and kinetics of Reactive Black 5 and Reactive Red 239 in aqueous solution onto surfactant-modiﬁed zeolite, J. Chem. Eng. Data 52 (2007) 1615–1620.
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