Removal of methylene blue from aqueous solution using nanoporous SBA-3

Desalination 261 (2010) 61–66

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Removal of methylene blue from aqueous solution using nanoporous SBA-3 Mansoor Anbia ⁎, Saba Asl Hariri Research Laboratory of Nanoporous Materials, Faculty of Chemistry, Iran University of Science and Technology, Narmak, Tehran 16846, Iran

a r t i c l e

i n f o

Article history: Received 17 March 2010 Received in revised form 15 May 2010 Accepted 18 May 2010 Available online 16 June 2010 Keywords: Methylene blue Dye Mesoporous silica SBA-3 Adsorption Langmuir

a b s t r a c t The adsorption behavior of methylene blue from aqueous systems onto mesoporous SBA-3 has been studied. Batch experiments were carried out to measure the adsorption as a function of contact time, initial concentration (50–150 mg L− 1), pH (4–12), and temperature (303, 313 and 323 K). The equilibrium of the process was achieved within 1 h. The sorption of methylene blue on the mesoporous silica SBA-3 slightly increases with increasing pH, and temperature, indicating an endothermic process. Adsorption isotherms were fitted with the Langmuir, Freundlich and Temkin models. The kinetic data were analyzed using pseudofirst-order and pseudo-second-order models. The adsorption kinetics of methylene blue on mesoporous SBA3 matched well with pseudo-second order kinetic model. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Many industries use dyes and pigments to color their products. At present more than 9000 types of dyes are incorporated in the color index and the biggest consumers of these dyes are textile, tannery, paper and pulp industry, cosmetics, plastics, coffee pulping, pharmaceuticals, food processing, electroplating and distilleries. The discharge of colored wastewater from these industries into natural streams has caused significant environmental [1] problems such as increasing the toxicity and chemical oxygen demand (COD) of the effluents, as well as reducing the light penetration; the latter has a derogatory effect on photosynthetic phenomena [2]. Though methylene blue (MB) is employed in some medical uses in large quantities, it can also be widely used in coloring paper, dyeing cottons, wools, coating paper of stocks, etc. Although MB is not strongly hazardous, it can cause some harmful effects. Acute exposure to MB causes increased heart rate, vomiting, shock, Heinz body formation, cyanosis, jaundice and quadriplegia and tissue necrosis in humans [3,4]. Various techniques have been employed for the removal of dyes from wastewaters. Due to low biodegradability of dyes, a conventional biological treatment process is not very effective. Dye containing wastewaters are usually treated by physical or chemical processes. The conventional methods for treating dyes containing wastewaters are coagulation and flocculation [5], oxidation or ozonation [6,7], membrane separation [8] and adsorption [9–18]. All these methods have different capabilities of color removal, capital costs, and

⁎ Corresponding author. Tel.: + 98 21 77240516; fax: + 98 21 77491204. E-mail address: [email protected] (M. Anbia). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.05.030

operating rates. Among these processes, adsorption has been found to be superior to other techniques in wastewater treatment in terms of initial cost, simplicity of design, ease of operation, and insensitivity to toxic substances. The removal of MB blue from wastewater streams is considered to be an important application of adsorption process using suitable adsorbents. Highly functional porous materials with high surface areas are generally used for such applications due to their excellent removal efficiency [19]. Nowadays, the discovery of ordered mesoporous materials has produced a worldwide resurgence of synthesis, characterization, and application of these materials [20]. A family of highly ordered mesoporous silica structures (SBA acronym), with pores between 2 and 30 nm, has been synthesized by the use of cationic alkylammonium surfactants [21], non-ionic alkyl poly (ethyleneoxide) oligomeric surfactants; and poly (alkene oxide) block copolymers [22]. SBA-3 materials are prepared with cetyltrimethylammonium cations, the mesopores are formed by a hexagonal array of parallel channels separated by silica walls (P6mm space group) [23]. Mesoporous silica structures SBA-3 have large adsorption capacity, excellent selectivity and improved powder recoverability. Recently, ordered mesoporous silica materials have been used to adsorb basic dyes from wastewater streams and are proven to be an effective adsorbent for the removal of these compounds [24,25]. It is well known that SBA-3 has a negative charge density due to the presence of Si–O and Si–OH groups, which adsorb positively charged dyes. Moreover, the enormous surface area of mesoporous SBA-3 adsorbent provides great capacity for MB adsorption. In this study, the interaction mechanism of MB dye with mesoporous silica SBA-3 has been investigated. Operating parameters such as pH, initial dye concentration and temperature have also been studied.

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2. Experimental

following equation:

2.1. Materials

qe = ðCo – Ce Þ V = W

The reactants used in this study were tetraethylorthosilicate (TEOS) as a silica source, cetyltrimethylammonium bromide (CTAB) as a surfactant, hydrochloric acid (HCl), and MB as an adsorbate (Methylene blue (C16H18N3SCl) is a basic blue dyestuff with CI Classification Number 52015). The chemical structure of the dye is shown in Fig. 1. All chemicals were of analytical grade and were obtained from E. Merck (GERMANY).

where Co and Ce (mg L− 1) are the liquid phase concentrations of dye at initial and equilibrium stages, respectively, V (L) is the volume of the solution and W (g) is the mass of adsorbent used.

2.2. Preparation of SBA-3 The procedure for SBA-3 synthesis was followed from literature [26]. One gram of CTAB and 15 mL of HCl (37%) were dissolved in 47 mL of deionized water. TEOS (4.45 mL) was then added dropwise to the acidic CTAB solution while it was stirred at 400 rpm and 30 °C for 1 h. The resultant white precipitate (as-synthesized SBA-3) was filtered without washing and dried at 100 °C overnight. The assynthesized SBA-3 powder was calcined at 550 °C in air for 5 h prior to characterization. The heating rate to 550 °C was 1 °C min− 1.

ð1:aÞ

2.5. Adsorption kinetics of MB For the measurement of time resolved uptake of MB onto the SBA3 samples, 50 mg of SBA-3 was added into a flask containing 250 mL of MB solution with an initial concentration of 120 mg L− 1 and was stirred continuously at 303–323 K. The concentration of residual MB in the solution was monitored by the same spectrophotometer and the adsorption capacity qt was calculated by applying the following equation: qt = ð Co – Ct Þ V = W

ð1:bÞ

The amount of adsorbed dye was calculated by qt is the adsorption capacity at time t (mg g− 1) and Ct is the concentration of MB in the solution at time t (mg L− 1).

2.3. Characterization of SBA-3 3. Results and discussion X-ray powder diffraction patterns were recorded on a Philips 1830 diffractometer using Cu Kα radiation. The diffractograms were recorded in the 2θ range of 0.8–10 with a 2θ size step of 0.01° and a time step of 1 s. Adsorption–desorption isotherms of the synthesized samples were measured at 77 K on Micromeritics model ASAP 2010 sorptometer to determine average pore diameter. Pore-size distributions were calculated by the Barrett–Joyner–Halenda (BJH) method, while surface area of the samples was measured by Brunauer– Emmet–Teller (BET) method. Scanning electron microscope (SEM) images of these solids were obtained with a Philips XL30 instrument after gold metallization. The Fourier transform infrared spectra for the mesoporous SBA-3 sample were measured on a DIGILAB FTS 7000 instrument under attenuated total reflection (ATR) mode using a diamond module. 2.4. Adsorption studies Adsorption experiments were carried out in 100-mL glassstoppered round-bottom flasks immersed in a thermostatic shaker bath. 5 mg of mesoporous silica SBA-3 sample was mixed with 25 mL of the aqueous solutions of various initial concentrations (50, 80, 100, 120, and 150 mg L− 1) of methylene blue. The flasks with their contents were shaken for the different adsorption times (20, 40, 60, 80, and 100 min) at the temperatures of 303, 313 and 323 K and neutral pH. The effect of pH was investigated at 303 K. The initial pH (4, 6, 8, 10, and 12) of the solutions was adjusted using concentrated HCl and NaOH solutions at the end of adsorption interval; the supernatant was centrifuged for 2 min at 3750 min− 1 and 400 rpm. The concentration of MB in the supernatant solution before and after the adsorption was determined with a 1.0 cm light path quartz cells using a Shimadzu 1201 UV spectrophotometer (Shimadzu Co., Kyoto, Japan) at a maximum wavelength (λmax) of 666 nm. The amount of adsorbed dye at equilibrium qe (mg g− 1) was calculated from the

Fig. 1. Chemical structure of methylene blue dye.

3.1. Characterization of SBA-3 The XRD patterns recorded in Fig. 2, display three peaks at 2θ = 2.94°, 4.97° and 5.71°, which are typical (100), (110) and (200) reflections of one-dimensional hexagonal (P6m) mesostructures. The nitrogen adsorption isotherm is recorded in Fig. 3. Its general feature appears to be a type IV isotherm. The adsorption step between 0.20 and 0.30 P/P0 is due to the capillary condensation in the mesopores of SBA-3. Further, its initial part shows a high “knee” (e.g., the adsorbed volume of liquid N2 reaches 0.20 cm3 g− 1 at 0.001 P/P0). This implies the presence of micropores in SBA-3 [27]. In combination with both observations, the isotherm (Fig. 3) represents a superposition of type I and type IV isotherms, as stated in the literature [28]. The BET surface area and pore volume are 1423 m2 g− 1 and 0.93 cm3 g− 1, respectively. The SEM picture of SBA sample is shown in Fig. 4. The figure shows the size and shape of the crystals. The FTIR spectrum as shown in Fig. 5, presents a broad band in the range of 3700.43 and 3016.30 cm− 1 which can be attributed to the framework Si–OH group interaction with the defect sites and adsorbed water molecules. Various C–H stretching vibrations are due to the presence of the organic surfactant molecules, which appeared in the as-synthesized sample at 2925 and 2840 cm− 1, and disappeared after the removal of the surfactant. The characteristic band for ammonium ion can be seen from 1405.57 to 1529.90 cm− 1. The stretching vibrations of Si–O–Si and Si–OH, which appeared in the

Fig. 2. XRD pattern of SBA-3.

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Fig. 5. FTIR spectra of SBA-3 (a) after calcination (b) before calcination.

increased with an increase in contact time and then reached equilibrium. Fig. 3. Adsorption isotherms of nitrogen at 77 K on SBA-3.

region 1000–1400 cm− 1, became much narrower in the region 1000– 1360 cm− 1 after surfactant removal. 3.2. Effect of various parameters on the MB adsorption 3.2.1. Effects of initial dye concentration and contact time The initial concentration provides an important driving force to overcome all mass transfer resistance of the dye between the aqueous and solid phases. The effect of initial dye concentration and contact time on the removal rate of MB by the mesoporous silica SBA-3 is shown in Fig. 6. As shown, the amount adsorbed increases with increasing initial dye concentration, so the removal of dye depends on the concentration of the dye. Moreover, the amount adsorbed

3.2.2. Effect of solution pH on MB adsorption MB is a cationic dye, which exists in the aqueous solution in the form of positively charged ions. As a charged species, the degree of its adsorption onto the adsorbent surface is primarily influenced by the surface charge on the adsorbent, which in turn is influenced by the solution pH [28]. Fig. 7 shows the effect of pH on the SBA-3 adsorption capacity of MB [2]. It was found that the adsorption capacity increased with increasing pH values in the studied range. Lower adsorption capacity at lower pH values was probably due to the presence of excess H+ ions competing with the dye cations for adsorption sites. At higher pH values (8–10) the amount of dye adsorption remained almost constant. The surface of SBA-3 contains a large number of active sites, and the solute (dye) uptake can thus be related to the active sites and also to the chemistry of the solute in the solution. At higher pH, the surface of SBA-3 particles may become negatively charged, which enhances the positively charged MB cations through electrostatic forces of attraction [2]. 3.2.3. Effect of temperature The effect of temperature on the adsorption of methylene blue onto mesoporous silica SBA-3 is shown in Fig. 8. The results revealed that the adsorption capacity increase with increasing temperature from 303 to 323 K, showing that this process is endothermic. The temperature has two major effects on the adsorption process. Increasing temperature is known to increase the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle, owing to the decrease in the viscosity of the solution. In addition, changing temperature will change the equilibrium capacity of the adsorbent for a particular adsorbate. The increasing temperature may increase the tendency of deaggregation and so the uptake of the monomers of MB. In order to gain an insight into the mechanism involved in the adsorption process, thermodynamic parameters for the present system were calculated. The adsorption free energy (ΔGo), adsorption

Fig. 4. SEM microphotographs of SBA-3 (a) 10 µm, (b) 5 µm.

Fig. 6. Effect of initial concentration over the MB removal on SBA-3 (stirring speed = 150 (rpm), adsorbent dosage = 0.2 g L− 1, room temperature = 30 °C).

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M. Anbia, S.A. Hariri / Desalination 261 (2010) 61–66 Table. 1 Thermodynamic quantities obtained from the Langmuir model for SBA-3/methylene blue adsorption system.

Fig. 7. Effect of pH on MB removal over SBA-3 as adsorbent (adsorbent dose = 0.2 g L− 1, agitation speed = 150 rpm, room temperature = 30 °C).

enthalpy (ΔHo) and adsorption entropy (ΔSo) from the Langmuir isotherms at different temperatures were calculated using the following thermodynamic functions: δ ln K ΔHads = δT RT 2 ln Kc =

o −ΔHads ΔSo + : RT R

ð2Þ

ð3Þ

The slope and intercept of the van't Hoff plot is equal to _ΔH/R and ΔS /R, respectively where R is the universal gas constant (8.314 J/ (mol K)); T is the absolute temperature (K) [29,30]. The change in standard free energy (ΔGo) of adsorption was calculated from the following equation: o

o

ΔG = −RT ln K

ð4Þ

where R is gas constant, K the equilibrium constant obtained from Langmuir equations and T is temperature in Kelvin. Thermodynamic parameters obtained are summarized in Table 1. It is noted that all ΔGo values listed in Table 1 are negative. This suggests that the adsorption process is spontaneous with high preference of methylene blue for activated carbon. As seen from Table 1, the positive value of adsorption enthalpy shows that the adsorption process is endothermic. 3.2.4. Adsorption isotherms In order to indicate the adsorption behavior and to estimate the adsorption capacity, adsorption isotherms were studied. The analysis of the isotherm data through fitting with different isotherm models is

Fig. 8. The variation of the amount adsorbed vs. equilibrium concentration at various temperatures.

Temperature (K)

Thermodynamics quantities ΔH (kJ/mol)

ΔS (kJ/mol K)

ΔG (kJ/mol)

303 313 323

5.81 5.81 5.81

0.032 0.032 0.032

− 4.7 − 5.7 − 6.9

Equilibrium constant 7.25 9.2 11.2

an important step in finding the suitable model that can be used for design purpose [31]. The isotherm data were fitted to the Langmuir, Freundlich and Temkin isotherms. Langmuir isotherm [32] is represented by the following linear equation: Ce = qe = 1 = qm b + ð1 = qm ÞCe

ð5Þ

where Ce (mg/L) is the equilibrium concentration, qe (mg/g) the amount of adsorbate adsorbed per unit mass of adsorbate, and qm and b are Langmuir constants related to adsorption capacity and rate of adsorption, respectively (Fig. 9). The Langmuir constants b and qm were calculated from this isotherm and their values are listed in Table 2. The Freundlich model can take the following form [33]: ln qe = ln Kf + ð1 = nÞ ln Ce

ð6Þ

where Kf is the Freundlich constant (mg/g (mg/L)n) and 1/n is the heterogeneity factor. The slope 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 [34]. The plot of lnqe versus lnCe, (Fig. 10), gives straight lines with slope 1/n. Accordingly, Freundlich constants (Kf and n) were calculated and recorded in Table 2. Temkin isotherm [34] is represented by the following equation: qe = B ln Kt + B ln Ce :

ð7Þ

The adsorption data were analyzed according to Eq. (7). A plot of qe versus lnCe , Fig. 11, enables the determination of the isotherm constants Kt and B. Kt is the equilibrium binding constant (L mg− 1) corresponding to the maximum binding energy and constant B is related to the heat of adsorption. The values of the parameters are given in Table 2, indicating that the Langmuir isotherm model yielded the best fit with the highest R2 value (0.99) compared to the Temkin and Freundlich model. 3.2.5. Adsorption kinetics The pseudo-first-order kinetic model has been widely used to predict the kinetics of dye adsorption. A linear form of pseudo-first

Fig. 9. Linear form of the Langmuir isotherm for MB adsorption on SBA-3.

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Table. 2 Langmuir, Freundlich and Temkin constants for adsorption of MB on SBA-3. Isotherms

Parameters

Data

Langmuir

qm(mg/g) b (L/mg) R2 KF (mg/g (L/mg)1/n) N R2 Kt B R2

285.7 0.075 0.99 10.26 3.70 0.97 0.88 57.68 0.98

Freundlich

Temkin

order model was described by Lagergren [35]: log ðqe – qt Þ = log qe –ðk1 = 2:303Þt

Fig. 11. Linear form of the Temkin isotherm for MB adsorption on SBA-3.

ð8Þ

where qe and qt are the adsorption capacities at equilibrium and at time t, respectively (mg g− 1); k1 is the rate constant of pseudo-first order adsorption (L min− 1). The pseudo-second order rate equation of McKay can be represented in the following form [36]: 2

t = qt = 1 = k2 qe + ð1 = qe Þt

ð9Þ

where the equilibrium adsorption capacity qe, and the pseudosecond-order constants k2 (g (mg min)− 1) can be determined experimentally from the slope and intercept of the plot t/qt versus t, Fig. 12. The model fits the kinetic data very well with R2 ≥ 0.99, which is better than pseudo-first, order kinetic (Table 3). However, the obtained curves ( not shown ) obtained by plotting log(qe−qt) versus t did not show a straight-line during the whole adsorption process, indicating that pseudo-first order kinetics could not be used to describe the adsorption behavior of MB onto the mesoporous silica SBA-3. These results suggest that the adsorption of MB on SBA-3 may be best described by the pseudo-second-order kinetic model at all temperatures with high correlation coefficients.

Fig. 12. Pseudo-second-order kinetic plots for MB adsorption on SBA-3 at different temperatures.

Table. 3 Kinetic parameters for MB removal on SBA-3 at three different temperatures. Temperature (K)

303 313 323

Pseudo-first kinetic

Pseudo-second-order kinetic

qe (mg g− 1)

k1 (L min− 1)

R2

qe (mg g− 1)

k2 (g(mg.min)− 1)

R2

200.4 234.4 284.1

2.3 × 10− 4 4.6 × 10− 4 9.2 × 10− 4

0.97 0.90 0.85

192.2 238.0 312.5

4.0 × 10− 2 2.9 × 10− 2 1.9 × 10− 2

0.99 0.99 0.99

4. Conclusions Mesoporous silica SBA-3 with abundant pores and high surface area (over 1435 m2 g− 1) are shown to be effective in removing MB from aqueous solutions. The Langmuir, and Temkin adsorption isotherm models were applied to the adsorption data of MB onto SBA-3 at 303 K. The Langmuir isotherm was the best model to describe the experimental data. Temperature affected the adsorption capacity and kinetics as well as the equilibrium of the adsorption process. The adsorption kinetics of MB onto SBA-3 is well described by a pseudo-second order kinetic model.

Fig. 10. Linear form of the Freundlich isotherm for MB adsorption on SBA-3.

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