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KAVEH Arzani, BEHDAD Ghaderi Ashtiani, AMIRHOSSEIN Haji Aboutorab Kashi. Equilibrium and Kinetic Adsorption Study of the Removal of Orange-G Dye Using Carbon Mesoporous Material. Journal of Inorganic Materials, 2012, 27(6): 660-666  
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KAVEH Arzani, male, PhD. E-mail:arzani_kaveh@yahoo.com

修回日期:2012-02-13
文章编号:1000-324X(2012)06-0660-07
Equilibrium and Kinetic Adsorption Study of the Removal of Orange-G Dye Using Carbon Mesoporous Material
KAVEH Arzani, BEHDAD Ghaderi Ashtiani, AMIRHOSSEIN Haji Aboutorab Kashi
Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran 14515/775, Iran
Abstract

Mesoporous carbon CMK-3 was synthesized by using SBA-15 silica mesoporous as hard template and characterized through nitrogen adsorption/desorption and low angle X-ray diffraction. As-prepared material with large pores and high surface area was used to remove Orange G dye from aqueous solution. Adsorption experiments were carried out as batch studies at variety of contact times, pH, initial dye concentrations, temperatures and salt concentrations. Langmuir and Freundlich isotherm models were employed to simulate the equilibrium data of anionic dye. It was found that the equilibrium data were well represented by the Langmuir isotherm, yielding maximum monolayer adsorption capacity of 189 mg/g. Experimental data were analyzed using pseudo-first order and pseudo-second order kinetic models and obtained results indicated that kinetics followed a pseudo-second order equation.

Keyword: CMK-3; Orange G; adsorption isotherm; kinetic

Amongst the numerous environmental pollutants, dyes are a large and important class. They are widely used in different kinds of industries including textile, leather, cosmetics, paper, printing, plastic, pharmaceutical, and food[ 1, 2]. These dyes are inevitably left in the industrial wastes and consequently discharged mostly in surface water resources. The discharged wastes containing dyes even in small amounts could cause dangerous byproducts from oxidation, hydrolysis, or other chemical reactions in the wastewater phase. These are noxious to microorganisms, aquatic life and human beings and cause serious destruction to the earth ecosystems[ 1, 2, 3]. Therefore, developing a sustainable method of effluent management for the dyeing industry has long been an important issue for environmental protection.

Regarding the dye stability to light and oxidizing agents as well as resistance to biodegradation, the usefulness of some traditional methods for treatment of dye-containing sewage is limited. The processes of flocculation and coagulation with metallic compounds produce sludge and increase concentration of metallic contaminants[ 2, 3, 4]. As an alternative method, the liquid-phase adsorption process can be used as an effective technique in the treatment of dye-containing waste effluents[ 5, 6, 7, 8, 9, 10].

A wide variety of adsorbents, have been tested for the removal of dyes from aqueous solutions including activated carbon[ 5, 6, 7, 8, 9], zeolite[ 10], perlite[ 11], chitin[ 12], lemon peel[ 13], etc. Among various materials and methods, adsorption using activated carbon is a common and popular method. This method has considerable potential for treatment of wastewater due to its low process costs and relatively efficient dye removal. However, adsorption of dye using activated carbon has some drawbacks such as slow adsorption kinetics and low adsorption capacity of bulky adsorbates due to the microporous nature of activated carbon. Therefore, it is of interest to develop effective adsorbents with short contact times for the removal of toxic matter.

There is an increasing demand for mesoporous materials as adsorbent[ 14, 15, 16], catalyst support[ 17, 18, 19], gas separation[ 20, 21], energy storage[ 22] and drug delivery[ 23]. As mesoporous carbon, CMK-3 is a fascinating material and exhibits a high surface area, a large pore volume, narrow pore size distribution, and well-ordered mesoporous structure[ 24, 25]. These unique properties make CMK-3 a good candidate for adsorption of hazardous materials.

The main objective of the present work is to investigate the potential of CMK-3 as carbon mesoporous material for the removal of Orange G dye from aqueous solutions. Orange G (O.G.), (1,3-Naphthalenedisulfonic acid, 7-hyd-roxy- 8-(phenylazo)-, disodium salt) is a typical water-soluble anionic dye and has toxic effects for living organisms[ 26]. The effect of contact time, pH, initial concentration, temperature and electrolyte on adsorption characteristics of CMK-3 was studied. The experimental data obtained from the equilibrium studies were fitted to Langmuir, Freundlich adsorption models. In addition, kinetic studies were also carried out to determine the characters of adsorption process.

1 Materials and methods
1.1 Materials

Triblock copolymer P123 (EO20PO70EO20, EO=ethylene oxide, PO = propylene oxide, 5800) was supplied from Aldrich Co., and tetraethyl orthosilicate (TEOS, Si (OCH2CH4)4), sucrose, Orange G, NaCl, NaOH, HCl and ethanol were purchased from Merck (Germany).

1.2 Preparation of adsorbent

Mesoporous carbon material CMK-3 was prepared by using mesoporous silica material SBA-15 as hard template and sucrose as carbon source. SBA-15 was synthesized as reported by Zhao, et al[ 27]. In a typical synthesis, 2 g of P123 was dissolved in 75 mL 2 mol/L HCl solution at 40◦C and 4.16 g of tetraethyl orthosilicate was then added. After the solution was magnetically stirred at 40℃ for 24 h, the mixture was transferred to an autoclave, which was kept at 100℃ for 48 h under static condition. The resulting material was recovered by filtration and washing with distilled water. Subsequently, the sample was calcined at 550℃ in air for 6 h to remove the organic template P123. Thus, SBA-15 was obtained.

Mesoporous carbon, CMK-3, was prepared according to the process described by Lee, et al[ 28]. 1 g SBA-15 was added to 5 mL aqueous solution containing 1.25 g sucrose and 0.14 g H2SO4. The resulting sludge was heated in an oven at 100℃ for 6 h and then 160℃ for another 6 h. In order to obtain fully polymerized and carbonized sucrose inside the pores of the silica template, 5 mL aqueous solution containing 0.8 g sucrose and 0.09 g H2SO4 were added again and the mixture was subjected to the thermal treatment described above. Then, it was carbonized in an argon flow at 900℃ for 6 h at a heating rate of 5 ℃/min. Finally, the mesoporous carbon (CMK-3) was obtained by removing the silica matrix using a 4 mol/L NaOH solution (50vol% ethanol-50vol% H2O) at room temperature followed by filtration, washing, and drying at 120℃ for 4 h.

1.3 Characterization of adsorbent

The textural properties of adsorbent were determined by using the nitrogen sorption technique. The nitrogen adsorption-desorption isotherms were measured at -196℃ using an ASAP 2000 analyses. Prior to the measurement, sample degassed for 5 h at 70℃. The specific surface area was calculated according to the BET (Brunauer-Emmet and Teller) model[ 29] while the pore size and pore volume were calculated using the Barrett-Joyner-Halenda (BJH) formula[ 30] based on the desorption branch of the isotherm. The mesoporous structure of the sample was investigated using X-ray powder diffraction (XRD). The XRD patterns were obtained at room temperature on a Bruker D8 diffractometer with a CuKα (0.15406 nm) radiation source in the 2 θ range from 0.5°to 5°, with a step size of 0.02°.

1.4 Adsorption studies

Batch experiments were carried out to evaluate the effect of contact time, initial dye concentration, solution pH, temperature and electrolyte for the removal of O.G. dye on CMK-3 adsorbent from aqueous solutions. In All experiments except for the initial concentration, 50 mg CMK-3 was added to 25 mL water solution of O.G. with a concentration of 1000 mg/L. After stirring on a shaker for predetermined time intervals, the solution was treated with centrifugation for solid-liquid separation. The residual concentration of dye solution was determined using a calibration curve prepared at the corresponding maximum wavelength (480 nm) using a UV-visible spectrometer (Unicol Instrument Co., Ltd.). The amount of adsorbed dye, Q (mg/g), was calculated by

(1)

Where C0 and Ce are the initial and equilibrium concentrations (mg/L) respectively, V is the volume of dye solution (L) and m is the weight (g) of CMK-3 adsorbent. The dye removal efficiencies under different conditions were calculated from the difference between the initial (without adsorbent) and equilibrium concentrations of the solution.

The effect of pH on dye removal was studied over a pH range of 2 to 9. The initial pH of the dye solution was adjusted by the addition of 1 mol/L solution of HCl or NaOH. The concentration of Orange G dye solution ranged from 50-1000 mg/L to investigate the adsorption isotherms. The sorption studies were also carried out at different temperatures (30℃, 40℃, 50℃ and 60℃) to determine the effect of temperature in the removal of Orange G from aqueous media.

2 Result and discussion
2.1 Characterization of adsorbent

As shown in Fig. 1, the N2adsorption/desorption isotherm of CMK-3 is a type IV curve of mesoporous materials with a steep hysteresis loop. The sharp rise at relative pressure ( P/ P0) of about 0.4 indicates the existence of mesopores with narrow pore size distribution.According to BET method, the specific surface area ( SBET) and pore volume are estimated to be 918 m2/g and 0.7557 m3/g, respectively. The CMK-3 obtained from SBA-15 possesses pores with average diameter of 3.64 nm.

Fig. 1 Nitrogen adsorption-desorption isotherm of CMK-3

The low angle X-ray diffraction patterns of the SBA-15 and CMK-3 are shown in Fig. 2. For SBA-15, three well-resolved peaks with 2 θat 0.82°, 1.46° and 1.7°, indexed as (100), (110) and (200) reflections associated with p6mm hexagonal symmetry, were observed, indicative of the well-ordered mesoporous structure of SBA-15. The XRD patterns of CMK-3 also gave a strong (100) peak and weak (110) and (200) reflection peaks similar to the p6mm hexagonal symmetry of the SBA-15 template. This indicates that CMK-3 is a perfect replica of SBA-15[ 24, 28].

Fig. 2 Low-angle XRD patterns of SAB-15 and CMK-3

2.2 Study of important parameters on the adsorption of Orange G dye by CMK-3 adsorbent

2.2.1 Effect of contact time

The effect of contact time for the adsorption of Orange G dye on CMK-3 was investigated for a period of 2 h for initial dye concentration of 1000 mg/L. As seen in Fig. 3, it is evident that time has significant influence on the adsorption of dye. It can be seen that the adsorption of Orange G dye was quite rapid in the first 30 min, then gradually increased with the prolongation of contact time. After 60 min of contact time, no obvious variation in adsorbed dye was observed. Based on these results, 60 min was taken as the equilibrium time in batch adsorption experiments.

Fig. 3 Effect of contact time on removal of O.G. dye by CMK-3 (Initial dye concentration: 1000 mg/L, CMK-3 dosage: 50 mg/25 mL)

2.2.2 Effect of pH

To study the influence of solution pH on the adsorption capacities of CMK-3 for Orange G dye at equilibrium conditions, experiments were carried out using various initial pHs varying from 2 to 9. As shown in Fig. 4, the amount of adsorption of solute increases as the pH is decreasing. When the pH is changed from 2 to 9, the adsorption will decrease from 270 mg/g to 181 mg/g. Higher adsorptions at lower pH values could be well explained by protonation properties of the adsorbent. At low pH, i.e., higher hydrogen ion concentration, the negative charges at the surface of internal pores are neutralized and some more new adsorption sites are developed as a result the surface provides a positive charge for anionic Orange G dye to get adsorbed. A similar type of behavior is also reported for the adsorption of the dye at different adsorbents[ 31].

Fig. 4 Effect of pH on removal of O.G. dye by CMK-3 (Initial dye concentration: 1000 mg/L, CMK-3 dosage: 50 mg/25 mL)

2.2.3 Effect of initial dye concentration

The adsorption experiments were carried out in initial dye concentration range of 50-1000 mg/L. Figure 5 shows the effect of the initial concentration on the amount of adsorption and dye removal efficiency. It was observed that dye removal efficiency reached 98% at lower concentration (5×10-5), then decreased to 37% at higher concentration (1×10-4). Dye removal efficiency was higher for low initial concentration because of availability of unoccupied binding sites on the adsorbents. Percent color removal decreased with increasing dye concentration because of nearly complete coverage of the binding sites at high dye concentration. In contrast, the amount of dye adsorbed was found to increase with increasing initial concentration of dyes. When the dye concentration was increased, the sorption capacity at equilibrium increased from 24.07 to 183.94 mg/g with an increase in the initial concentration from 50 to 1000 mg/L. The initial dye concentration provides the necessary driving force to overcome the resistance to the mass transfer of dye molecules between the aqueous phase and the solid phase. The increase in initial dye concentration also enhances the interaction between O.G. and adsorbent. Therefore, an increase in initial concentration of O.G. enhances the adsorption uptake of O.G. This is due to the increase in the driving force of the concentration gradient, as an increase in the initial dye concentration.

Fig. 5 Effect of initial dye concentration on removal of O.G. dye by CMK-3

2.2.4 Effect of electrolyte (sodium chloride) concentration

Wastewaters that contain dyes commonly include significant quantities of salts, thus the effect of electrolyte on O.G. removal needs to be investigated. Figure 6 illustrates the effect of NaCl on the adsorption of dye at an initial O.G. concentration of 1000 mg/L at 25℃ and solution pH. It can be seen that adsorption increased with increasing NaCl concentration. The presence of electrolyte may have two opposite effects. Firstly, it may cause the neutralization of surface charge of adsorbent while competing with dye for surface adsorption. With the increasing ionic strength, the adsorption capacity decreases due to screening of the surface charges. Secondly, the presence of large quantities of salts in the solution decreases the dissociation of dye molecules to their ionic forms and hence dye molecules as dominant species in the solution extract to carbon phase so rapidly because the extraction of molecular species to an organic adsorbent is much easier than its ionic species. The second effect seems to be dominant in this case and causes higher degree of dye adsorption on mesoporous adsorbent[ 32].

Fig. 6 Effect of electrolyte on removal of O.G. dye by CMK-3 (Initial dye concentration: 1000 mg/L, CMK-3: dosage 50 mg/25 mL)

2.2.5 Effect of temperature on the adsorption

Figure 7 shows the maximum adsorption capacity of O.G. on the prepared material v s temperature. It was found that the adsorption capacity decreased from 181.28 to 164.93 mg/g with increase in temperature from 30 to 60◦C, indicating the exothermic nature of the adsorption reaction. It can be explained that as temperature increased, the physical bonding between the dye molecules and the active sites of the adsorbent weakened. In addition, the solubility and dissociation of O.G. dye also increased and solute–adsorbent interactions decreased. Therefore the solute was more difficult to adsorb at higher temperature.

Fig. 7 Maximum adsorption capacity of O.G. dye on the CMK-3 v s temperatures

2.3 Adsorption isotherms

For solid-liquid system, the equilibrium of sorption is one of important physico-chemical aspects in description of adsorption behavior. In this work, two well-known models of Langmuir and Freundlich isotherm are evaluated.

The Langmuir isotherm assumes monolayer adsorption

onto the surface containing a finite number of adsorption sites with no transmigration of the adsorbent in the plane of the surface[ 33]. This model is the most widely used two-parameter equation, generally expressed in the form by the following equation:

(2)

Where Ce is the equilibrium concentration of the adsorbate (mg/L), Qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), Kl the Langmuir adsorption constant (L/mg), and Qm is the theoretical maximum adsorption capacity (mg/g).

The essential characteristic of Langmuir equation can be expressed in terms of a dimensionless separation factor RL, which is defined as:

(3)

Where kLis the Langmuir isotherm constant (L/mg) and C0 is the initial dye concentration (mg/L). The RL value indicates the type of the isotherm to be either favorable (0< RL<1), unfavorable ( RL>1), linear ( RL=1) or irreversible ( RL=0).

The Freundlich isotherm on the other hand takes heterogeneous systems into account and is not restricted to the formation of the monolayer[ 34]. The well-known logarithmic form of the Freundlich isotherm is given by the following equation:

(4)

Where KF (L/mg) and n are Freundlich constants. Kf is define as an adsorption or distribution coefficient representing the amount of solute adsorbed on an adsorbent for a unit equilibrium concentration while n giving an indication of how favorable the adsorption process. The slope of 1/ n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero. A value for 1/ n below one indicates a normal Langmuir isotherm while 1/ nabove one is indicative of cooperative adsorption[ 35].

Langmuir and Freundlich isotherms for the Orange G, CMK-3 system are shown in Fig. 8. Isotherm parameters and the Correlation coefficients, R2, were calculated and summarized in Table 1. The Langmuir isotherm with correlation coefficient of 0.9991 represents a better fit of experimental data than Freundlich model with correlation coefficient of 0.9263. It indicates that monolayer adsorption of O.G. dye takes place on the homogeneous surface of carbon mesoporous adsorbent. The amount of computed maximum monolayer capacity for removal of O.G. from Langmuir model found to be 189 mg/g. Moreover, the values of the dimensionless constant RL (0.019-0.275) indicate that the adsorption is favorable and rather irreversible. As seen from Table 1, the value of 1/ n is also found to be between 0 and 1, indicating the high adsorption intensity.

Fig. 8 Langmuir isotherms (a) and Freundlich isotherm (b) for adsorption of O.G. on CMK-3

Table 1 Langmuir and Freundlich constants for the adsorption of O.G. on CMK-3
2.4 Adsorption kinetics

It is important to be able to predict the rate at which a solute is removed from aqueous solutions in order to design an adsorption treatment plant. To evaluate adsorption kinetics, two common models were applied to the experimental data obtained at adsorption processes. These are pseudo-first order[ 36] and pseudo-second order[ 37] kinetic models as shown in Eqs. (5) and (6), respectively:

(5)

(6)

Where k1 (1/min) and k2 (g/(mg•min)) are the the adsorption rate constants of pseudo first and second-order adsorptions, respectively; Qe and Q t are the amounts of dyes adsorbed at equilibrium and time ( t ,mg/g), respectively.

The adsorption kinetic plots are shown in Fig. 9 and all

the kinetic parameters determined are listed in Table 2. Since calculated correlation coefficients are closer to unity for pseudo second-order kinetics model than the pseudo first-order kinetic model (0.9999 vs 0.7117), therefore the present adsorption system follows predominantly the second-order rate model.

Fig. 9 Kinetic models for adsorption of O.G. on CMK-3(a) Pseudo-first order; (b) Pseudo-second-order

Table 2 Pseudo-first order and pseudo-second-order models constants for the adsorption of O.G. on CMK-3
3 Conclusions

Ordered mesoporous carbon (CMK-3) was successfully prepared using silica SBA-15 as hard template. N2 adsorption and XRD results demonstrate that the synthesized material is a good replica of SBA-15 and its specific surface area and average pore diameter found to be 918 m2/g and 3.64 nm, respectively. Batch experiment studies showed that adsorption of Orange G dye on mesoporous carbon surface is fast, as maximum removal take place within 60 min of contact time and removal efficiency of dye is improved in acidic solutions. The adsorption of dye increased with increasing initial dye concentration and salt concentration and the maximum adsorption capacity decreased with increasing temperature. Fitting equilibrium data to Langmuir and Freundlich isotherms showed that Langmuir model was more suitable to describe the Orange G adsorption with maximum monolayer adsorption capacity of 189 mg/g. The adsorption kinetics was found to follow closely the pseudo-second-order kinetic model.

Acknowledgement

The authors thank the research council of Science and Research campus of Islamic Azad University for the financial support.

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