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Original article
10 (
2_suppl
); S2160-S2169
doi:
10.1016/j.arabjc.2013.07.049

Adsorption of yellow dye on calcined or uncalcined Al-MCM-41 mesoporous materials

, , ,
 Laboratoire de Chimie des Matériaux L.C.M, Université d’Oran, BP 1524, El-Mnaouer, Algeria

⁎Corresponding author. Tel.: +213 771663458. bbouhdjer@voila.fr (Bouhadjar Boukoussa) bbouhdjer@yahoo.fr (Bouhadjar Boukoussa)

Received: , Accepted: ,
© The Author(s) 2013
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Yellow dye (YD) adsorption was carried out on some mesoporous materials such as calcined and uncalcined Al-MCM-41. These two matrices were synthesized by variation of Si/Al molar ratio (20, 40 and 80) under thermal conditions. The obtained materials were characterized by various methods: XRD diffraction (XRD), nitrogen adsorption at 77 K, Fourier transformed infrared spectroscopy (FTIR) and energy dispersive spectroscopy by X-rays (EDX). Elimination of YD as a function of both pH and shaking, contacting time, and reaction temperature was studied. The uncalcined Al-MCM-41 had the highest adsorption capacities with adsorption rate of about 92%. Both the uncalcined and calcined materials which contain a very little amount of aluminum seem to have a high affinity toward YD molecules.

Keywords

Mesoporous materials
Al MCM-41
Adsorption
Dye
Wastewater
1

1 Introduction

Porous solids are of scientific and technological interest because of their ability to interact with atoms, ions and molecules (Davis, 2002). Since the discovery of M41S silica in 1992, mesoporous materials have attracted intense interest due to their large specific surface areas, well-defined pore structures, inert framework, non toxicity and high biocompatibility (Beck et al., 1992; Kresge et al., 1992; Cicuéndez et al., 2013; Zhao et al., 2005). All these properties allow mesoporous materials to be used in a wide range of applications, such as catalysis (Lou et al., 2013; Kazemian et al., 2013; Bachari et al., 2016, 2017; Bellahouel et al., 2011), adsorption (Russo et al., 2012; Wu et al., 2012; Yang et al., 2013; Nguyen et al., 2008;), separation (LaeeqKhan et al., 2013; Galve et al., 2013; Kim et al., 2008), energy (Matsuura et al., 2010; Sun et al., 2007) and drug delivery systems (Moritz and Łaniecki, 2012; Chen et al., 2013; Xu et al., 2013; Du et al., 2012). Due to the interesting properties particularly the high specific surface areas, thermal and hydrothermal stabilities (Jiang et al., 2011; Russo et al., 2007; Xia and Mokaya, 2003) and ordered mesoporous nature with uniform pore diameters, the hexagonal MCM-41 materials have confirmed their high affinity toward organic compounds. For these reasons, some laboratories have investigated both the catalysis (Tayebee and Ghadamgahi, 2017; Alrouh et al., 2017) and adsorption processes (Benhamou et al., 2013; Li et al., 2013; Kamarudin and Alias, 2013) by using suitable adsorbent.

In another way, it could be possible to modulate these properties so as to suit those of the adsorbate (Zhao et al., 2000). Recent researches related to the adsorption of organic molecules describe the use of MCM-41 which still contains the surfactant. Because, the existence of surfactant cationic entity in the MCM-41 caused modifications both in surface chemistry and porosity of the adsorbent, which in turn affected their behavior as adsorbents. The new surfactant-MCM-41 materials were used as adsorbent in aqueous solutions to remove phenol and o-chlorophenol (Mangrulkar et al., 2008), toluene and cumene (Huang et al., 2007) and aniline (Yang et al., 2011). On the other hand, the presence of aluminum enhances the hydrothermal stability of mesoporous materials (Russo et al., 2007).

In this study, we present firstly, our obtained results on the characterization of both the uncalcined and calcined surfactant Al-MCM-41. In the second part, we present all results concerning the adsorption of yellow dye on these two mesoporous materials. The extent of adsorption was investigated as a function of solution pH, shaking speed, contact time and reaction temperature. At our knowledge, there is no study about adsorption of the YD on Al-MCM41 type materials.

2

2 Materials and methods

2.1

2.1 Reagents

All chemicals were of analytical grade and used without further purification. All solutions were prepared with deionised water. All glassware were cleaned several times and rinsed with deionized water. The YD stock solution was prepared from JadeChem, China.

2.2

2.2 Al-MCM-41 sorbents

Uncalcined and calcined Al-MCM-41 molecular sieves with various Si/Al molar ratios (20, 40 and 80) were prepared under hydrothermal synthesis conditions using cetyltrimethylammonium bromide (CTAB) as a structure-directing template, and tetraethyl orthosilicate (TEOS) and sodium aluminate as the silica and aluminum source respectively.

The gel composition used is as following:

2 SiO2: 0.24 CTAB: 0.5 NaOH: x Al2O3: 3 EtOH: 200 H2O (x = 0.0125, 0.025 and 0.05 corresponding to the Si/Al molar ratio = 80, 40 and 20). We have first prepared two solutions, the first contained 2.91 g of CTAB (98%, Alfa Aesar), 110 ml distilled water and 6 ml of ethanol (99.5%, Riedel-de-Haën), and was stirred for 15 min at 308 K. The second solution containing 0.66 g of NaOH (98% Sigma–Aldrich), 10 ml distilled water and a variable mass (according to the Si/Al ratios) of sodium aluminate (54% Al2O3, 41% Na2O, 5% H2O, Aldrich), was stirred for 10 min. The second solution with 7.4 ml of TEOS (98%, Aldrich), was added dropwise in the first solutions. After ageing at 308 K for 3 h, the obtained homogeneous gel was transferred to an autoclave and hydrothermally treated under autogenous pressure at 423 K for 10 h. The final obtained gel was then removed from the oven and cooled to room temperature. After filtration and washing several times with deionized water, the obtained solid was dried at 333 K for at least 24 h, ground for homogenization, and sheltered from light. The powder was then calcined in air at 823 K for 12 h to remove template.

2.3

2.3 Solid characterization

The obtained Al-MCM-41 sorbents (with different Si/Al ratios) were characterized using different methods. Crystallization state of both prepared mesoporous materials, in the case of the optimization of the Si/Al molar ratios was determined by XRD powder diffraction patterns on a Bruker AXS D-8 diffractometer with Cu-Kα radiation. The diffractograms were recorded in the 2θ range of 0–10° in the steps of 0.010° with a count time of 15 s at each point. Textural measurements such as specific surface areas, pore volume and pore size distribution were obtained according to the Brunauer–Emett–Teller theory and by the B.J.H method using a Gemini 2375 V5.01 porosimeter from Micromeritics Corporation, GA. The samples were outgassed at 623 K and 10−5 Torr overnight prior to the adsorption experiments. Infrared spectra (FTIR) were obtained using KBr pellet technique (Sigma–Aldrich, ⩾99.0%) (1 mg of solid with 150 mg of KBr) with JASCO (4200) instrument. All spectra were recorded between 400 and 4000 cm−1. Energy dispersive X-ray analysis (EDX) jointed to a XL-30 scanning electron microscope was used to calculate Si/Al molar ratio of the two studied materials.

2.4

2.4 Adsorption studies

YD (C.I. Acid Yellow 49, CAS no.: 12239-15-5., P.M = 426.27 g/mol, chemical formula = C16H13Cl2N5O3S, λmax = 370 nm) (Fig. 1) was purchased from (JadeChem, China). Stock solution of YD was prepared by dissolving the appropriate amount of solute in distilled water at room temperature, shaken for 24 h, and then filtered through Sartorius membrane (0.45 μm). The YD concentration was chosen to be representative of the amount rejected in industrial wastewaters. All experiments were carried out in duplicate and each point on the curve was the average value of two data sets. YD concentrations were determined from the linear graph established by standard concentration plot in the range of 5–80 mg L−1 and the absorbance at λmax = 370 nm.

Molecular structure of YD.
Figure 1 Molecular structure of YD.

2.4.1

2.4.1 Adsorption kinetics

Experiments were performed at 296 K separately on both calcined and uncalcined Al-MCM-41. The dye’s pH is close to 6 and the experiments were conducted without adjusting the pH of solutions. Kinetic studies were conducted in 250 mL brown flasks at room temperature by shaking 100 mg of sorbent (MCM-41) with 100 mL of YD solution (50 mg L−1) at 900 rpm on a mechanical shaker (variomag mono). The samples were withdrawn from the shaker at intervals from 15 min to 24 h, filtered and the supernatants were analyzed by spectrophotometry (Thermo Electron corporation UV spectrophotometer Nicolet evolution e100).

Kinetic models, i.e. the pseudo-first order, the pseudo-second order and intraparticle diffusion models were considered for the purpose of interpreting experimental data.

The pseudo-first-order model was suggested by Lagergren for the sorption of solid/liquid systems and can be expressed in integrated and linear form using the following equation (Yao and Xu, 2011)

(1)
log ( Qe - Qt ) Qe = - k 2.303 t where Qe and Qt are the amounts of YD adsorbed at equilibrium at time t (h), respectively, and k is the rate constant of adsorption. The values of rate constant k, and correlation coefficient r2 are calculated from the plots of log [(Qe − Qt)/Qe] versus t.

The second-order kinetic model was expressed as follows (Hui et al., 2005):

(2)
t Qt = 1 K Qe + 1 Qe t Equilibrium adsorption capacities Qe and second-order constants K could be determined experimentally from the slope and intercept of plot t/Qt versus t.

The adsorption rate of YD was calculated by the following equation:

(3)
ζ = C o - C i C o × 100 % ζ:% Removal dye, Co: initial concentration (Co = 50 mg/L), Ci: final concentration determined by UV.

2.4.2

2.4.2 Effect of some parameters on YD adsorption

For each isotherm, 100 mL aliquots of YD solution at a fixed concentration (50 mg L−1) were placed in contact with 0.1 g of MCM-41. The brown flasks were closed and shaken overnight at room temperature (296 K) on a mechanical shaker (Variomag mono), set at 900 rpm for over 20 h. The kinetic studies (done first) using equilibration times ranging from 15 min to 24 h have shown that the adsorption of YD on MCM-41 reached pseudo-equilibrium within 0.5 h which indicates that the adsorption process can be very fast and the adsorption rates are generally greater than the desorption rates. The effects of pH (2–10), shaking (200–900 rpm), temperature range (24–80 °C) and presence of surfactant template were investigated. The pH medium was adjusted by adding a few drops of dilute NaOH or HCl. pH values were controlled by a pH meter (Hanna pH/mV/ORP Bench Meter).

3

3 Results and discussion

3.1

3.1 Characterization of sorbent

All obtained XRD patterns (Fig. 2) and FTIR spectra (Fig. 3) of the uncalcined and calcined adsorbent Al-MCM-41 showed that the synthetized solids have well-defined mesoporous structures of MCM-41 type in accordance with results obtained previously by some authors (Beck et al., 1992; Chen et al., 1993a,b). Samples present crystallographic patterns characteristic of the mesoporous solid aluminosilicate Al-MCM-41 based on the following small reflection angles (1 0 0), (1 1 0), and (2 0 0) crystal planes (Fig. 2). The hexagonal orientation of Al-MCM-41 is drastically affected by the amount of aluminum present, producing alterations in the porous structure of the solid without changing its primary structure (Mokaya and Jones, 1997). The registered XRD patterns of the calcined solids showed that these later are better structurated than before calcination this is due to the liberation of porosity following the degradation of the surfactant molecules present inside the pores. The textural measurement values are given in Table 1. BET protocol gave a SSA for Al-MCM-41(80) of 1448 m2 g−1, for Al-MCM-41(40) of 1177 and 1108 m2 g−1 for Al-MCM-41(20), in agreement with earlier data (Vaschetto et al., 2013; Ajaikumar and Pandurangan, 2008).The average pore diameters are in the range 35–37 Å higher than those obtained for some MCM-41 type materials (Ajaikumar and Pandurangan, 2008).

X-ray diffraction patterns of Al-MCM-41 at different Si/Al ratios (Si/Al = 80, 40 and 20), (A): calcined and (B): uncalcined Al-MCM-41.
Figure 2 X-ray diffraction patterns of Al-MCM-41 at different Si/Al ratios (Si/Al = 80, 40 and 20), (A): calcined and (B): uncalcined Al-MCM-41.
FTIR spectra of Al-MCM-41 at different ratios, (A): calcined Al-MCM-41 and (B) uncalcined Al-MCM-41.
Figure 3 FTIR spectra of Al-MCM-41 at different ratios, (A): calcined Al-MCM-41 and (B) uncalcined Al-MCM-41.
Table 1 Physical characteristics of Al-MCM-41 (20), Al-MCM-41 (40) and Al-MCM-41 (80).
Adsorbent Calcined Surface area (m2/g) DBJH (Å) Pore volume (cm3/g)
Si/Al Si% (EDX) Al% (EDX) Si/Al (EDX) d100 (Å) ao (Å)
Al-MCM-41(80) 80 98.91 1.09 90.74 41.24 47.61 1448 35.69 1.097
Al-MCM-41(40) 40 97.87 2.13 45.94 36.77 42.45 1177 37.54 0.927
Al-MCM-41(20) 20 96.74 3.26 29.67 36.77 42.45 1108 37.61 0.808

The FT-IR spectra of MCM-41 samples were used to determine the frequency change in the functionality on the surface of the used mesoporous materials.

An example of the obtained FTIR spectra of the calcined and uncalcined samples (for Si/Al = 40) is given in Fig. 3(A and B), respectively. The presence of absorption bands around 2927 and 2852 cm−1 for the uncalcined materials corresponds to asymmetric and symmetric CH2 vibrations of the surfactant molecules. Appearance of a large band at 3500 cm−1 is due to O–H stretching of surface hydroxyl groups, bridged hydroxyl groups and adsorbed water molecules. We observed the appearance of bands between 1626 and 1638 cm−1 due to deformation vibrations of adsorbed molecules. The bands between 500 and 1200 cm−1 are assigned to framework vibration, those around 1247 and 1072 cm−1 are attributed to the asymmetric stretching of T–O–T groups and are also observed around 800 and 544 cm−1. The bands at 460 cm−1 are due to the bending mode of T–O–T groups. These special features resemble with those reported by previous workers (Zhao et al., 1996; Biz and Occelli, 1998).

Aluminum incorporation into the structure of MCM-41 results from an isomorphous substitution mechanism. Quantitative data for the estimation of aluminum incorporation were obtained by EDX analysis of silicon and aluminum. According to the chemical composition used for the synthesis of the Al-MCM-41, we expected a nominal value of Si/Al molar ratio = 80, 40 and 20. However, the real Si/Al ratio in the uncalcined Al-MCM-41 was a little higher indicating that amount of aluminum incorporated in the structure was a little less than the expected one (Table 1).

3.2

3.2 Effect of various parameters on adsorption

3.2.1

3.2.1 Adsorption kinetics

Obtained results of Kinetic studies are shown in Fig. 4(A and B). As shown in these curves, a rapid YD adsorption occurred during the first minutes, indicating a high affinity between dye molecules and the Al-MCM-41 surface. This affinity increased with increasing Si/Al ratio both of uncalcined and calcined Al-MCM-41.

Kinetics of YD adsorption on Al-MCM-41 at different Si/Al ratios. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, pH = 6 and 900 tours/min, (A): calcined and (B): uncalcined.
Figure 4 Kinetics of YD adsorption on Al-MCM-41 at different Si/Al ratios. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, pH = 6 and 900 tours/min, (A): calcined and (B): uncalcined.

This can be related to the highest BET specific surface areas of the Al-MCM-41 (80) compared to Al-MCM-41 (40 and 20). After approximately 5 h, the adsorbed amount of about 95% was stable suggesting a gradual equilibrium, possibly due to the intra particle diffusion of the YD molecules. Therefore a duration of 5 h was chosen for the following experiments concerning both the uncalcined and calcined Al-MCM-41. The uptake amount of YD on uncalcined Al-MCM-41 (80) was lesser compared to the other matrix with the other Si/Al molar ratios (40 and 20). This is probably due to the destructuration (Fig. 5) of Al-MCM-41 (80) after the adsorption of YD as evidenced by the XRD pattern. On the contrary the structures of the other uncalcined samples (Si/Al (20 and 40)) and those of the calcined one were retained.

X-ray diffraction patterns of Al-MCM-41 at different Si/Al ratios (Si/Al = 80, 40 and 20) after adsorption of the dye. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, Time 24 h and pH = 6, (A): calcined and (B): uncalcined.
Figure 5 X-ray diffraction patterns of Al-MCM-41 at different Si/Al ratios (Si/Al = 80, 40 and 20) after adsorption of the dye. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, Time 24 h and pH = 6, (A): calcined and (B): uncalcined.

The process of YD removal from aqueous phase by each used adsorbent could be modelized by pseudo-first-order kinetics (reversible or irreversible) or pseudo-second-order kinetics.

The correlation coefficients for calcined Al-MCM41 and uncalcined Al-MCM41 are higher than 0.78, (Table 2), which indicates that the adsorption of yellow dye is not an ideal pseudo-first-order reaction. Kinetic data were further examined with the pseudo-second-order kinetic model. If pseudo-second-order kinetics was applicable, the plot of t/Qt versus t should show a linear relationship.

Table 2 Kinetic parameters for the adsorption of yellow dye onto calcined Al-MCM41 and uncalcined Al-MCM41.
Sample Qe Pseudo-first-order kinetic model Pseudo-second-order kinetic model
k (L/h) r2 Qexp K (g/mg h) r2
Al-MCM-41(80) calcined 33.00 1.5066 0.9392 37.453 0.1257 0.9998
Al-MCM-41(40) calcined 30.94 1.3892 0.9433 31.181 0.1705 0.9999
Al-MCM-41(20) calcined 28.55 0.8769 0.9987 29.429 0.1174 0.9999
Al-MCM-41(80) uncalcined 44 2.206 0.7923 44.563 0.0457 0.9999
Al-MCM-41(40) uncalcined 46 1.3793 0.9031 47.771 0.0561 0.9998
Al-MCM-41(20) uncalcined 40.98 0.9060 0.9350 41.186 0.0924 0.9999

The obtained values of K, Qe values and the correlation coefficients r2, were calculated and are listed in Table 2.

It can be seen from Table 2 that the correlation coefficient (R2) varies in the order: pseudo-second-order > pseudo-first-order under all experimental conditions, which indicates that the pseudo-second order model is the most suitable in describing the adsorption kinetics of YD on the two used Al-MCM-41.

3.2.2

3.2.2 Effect of shaking speed on the adsorption procedure

The effect of shaking speed on adsorption capacities of the used adsorbents was studied by varying the agitation speed from 200 to 900 rpm, while keeping constant the dye concentration, adsorbents’ dosage, pH, temperature and contact time. As it can be seen in Fig. 6(A and B), the adsorption capacities of YD generally increased with increasing shaking speed from 200 to 900 rpm. These results could be associated to the fact that the increasing shaking speed improved the diffusion of dye into the pores of the adsorbent. This also indicated that a shaking rate in the range 700–900 rpm was sufficient to assure that the maximum sites of adsorption existing in the pores of mesoporous adsorbents were made readily available for dye adsorption. For convenience, shaking speed of 900 rpm was selected.

Effect of shaking speed on the adsorption. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, pH = 6. (A): Calcined Al-MCM-41 and (B): uncalcined Al-MCM-41.
Figure 6 Effect of shaking speed on the adsorption. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, pH = 6. (A): Calcined Al-MCM-41 and (B): uncalcined Al-MCM-41.

3.2.3

3.2.3 Effect of temperature on YD adsorption

The adsorption of yellow dye was carried out at the temperature range 24–80 °C and the results are illustrated in Fig. 7(A and B). As shown in these curves, the adsorption of YD decreased from about 72% to 60% when the temperature increased from 24 to 80 °C with the uncalcined Al-MCM-41(80). In these conditions, the increase in temperature seems to promote a gradual decrease of the electrostatic interaction forces between the dye molecules and the Al-MCM-41 surfaces. Then, the decrease in adsorption capacities of Al-MCM41 with temperature indicated an exothermal process.

Effect of adsorption temperature. Experimental conditions: concentration 50 mg/l, adsorbent 0.1 g, pH = 6, and 900 tours/min. (A): Calcined Al-MCM-41 and (B): uncalcined Al-MCM-41.
Figure 7 Effect of adsorption temperature. Experimental conditions: concentration 50 mg/l, adsorbent 0.1 g, pH = 6, and 900 tours/min. (A): Calcined Al-MCM-41 and (B): uncalcined Al-MCM-41.

3.2.4

3.2.4 Effect of pH on YD adsorption

The pH of aqueous medium is an important factor that may modify the YD adsorption. The chemical characteristics of both adsorbent and adsorbate could be varied with pH. The pH of the solution affects the degree of ionization and speciation of various dyes which subsequently leads to a change in the reaction kinetics and equilibrium characteristics of the adsorption process. To study the influence of the pH on the adsorption capacity of Al-MCM-41 materials, experiments were investigated using the pH varying from 2 to 10.

The experimental results for the YD adsorption both on the calcined and uncalcined Al-MCM-41 are shown in Fig. 8. These figures clearly show that the removal rate of YD was markedly enhanced at higher pH values. As shown in Fig. 8(B), the adsorption capacities increased with increasing pH for the three studied Si/Al molar ratios particularly with 80% of Si/Al ratio which increased from 31 at pH 2 to 92% at pH 10. The obvious decrease in the uptake of YD at lower pH values might be attributed to the lower stability of the uncalcined Al-MCM-41 structure as proposed earlier by some authors (Zhao et al., 1996). For the calcined Al-MCM-41 materials, the adsorption capacities were lesser compared to those obtained with the uncalcined samples. For the same Si/Al ratio (80%), the adsorption capacities increased from 23% at pH 2 to 67% at pH 10 (Fig. 8(A)).

Effect of pH, experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, 900 tours/min. (A): Calcined Al-MCM-41 and (B): uncalcined Al-MCM-41.
Figure 8 Effect of pH, experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, 900 tours/min. (A): Calcined Al-MCM-41 and (B): uncalcined Al-MCM-41.

Mangrulkar et al., 2008 when studying the adsorption of phenol and o-chlorophenol by mesoporous MCM-41, found that uncalcined MCM-41 shows significant adsorption for PhOH and o-CP as compared to calcined MCM-41, and they claimed that this may be because of the hydrophobicity created by surfactant template in the MCM-41.

It must be noted that the surface of the adsorbent changes its polarization according to the value of the pH of the solution and to the isoelectric point (IEP) of the solid (Poncelet et al., 1991). Thus, the pH influences at the same time both the surface state of the adsorbent; either the carbon or modified FSM-16, and the ionization state of ionizable organic molecules. At pHs lower than IEP, of, e.g., uncalcined Al-MCM-41 (∼5) (Grzechulska and Morawski, 2002; Huang et al., 2007; Benhamou et al.,2013), the surface becomes positively charged and it is the opposite for pHs higher than IEP; accordingly the equilibria could be: pH < IEP : ≡ Si - OH + H + ⇔ modified / Si - OH 2 + pH > IEP : ≡ Si - OH + OH - ⇔ modified / Si - O - + H 2 O. The YD that has several functional groups such as SO 3 - , Cl− and NH2 results in weak adsorption at low pH. At pH < IEP, the surface is positively charged and many anions are competing but the formation of ammonium ions by protonation of NH2 groups seems to be dominant leading to electrostatic repulsion between the positively charged sorbent and the NH 3 + .

At pH > IEP, the sorbent has negative surfaces thus it interacts strongly with the cationic YD via electrostatic attractions.

3.3

3.3 Mechanism of dye sorption

The YD adsorption on each calcined and uncalcined Al-MCM-41 was studied in order to understand the influence of surfactant template on the adsorption behavior. As it can be seen in Fig. 4(A and B), the adsorption of YD on uncalcined Al-MCM-41 was significantly higher than that obtained on calcined Al-MCM-41 because the nature of the surface chemistry of these two mesoporous materials was different. In another way, the presence of cetyltrimethylammonium bromide (CTAB) chain modified the porosity of the adsorbent, which in turn affects the sorption behavior of the adsorbent and also increased the hydrophobicity of these materials. Fig. 9 shows an adsorption mechanism in order to reflect the interactions between yellow dye and calcined or uncalcined Al-MCM-41 containing the surfactant.

Scheme of YD adsorption on Al-MCM-41: (A) calcined and (B) uncalcined Al-MCM-41.
Figure 9 Scheme of YD adsorption on Al-MCM-41: (A) calcined and (B) uncalcined Al-MCM-41.

Due to the complex structure of YD molecules, there were multiple possible interactions among dye molecules and adsorbents. The aromatic cycles could interact with the alkyl part of CTAB by hydrophobic interactions; the azo groups could interact with the silanol groups present on the surface of the mesoporous silica by hydrogen bonding; the sulfonic acid groups could interact also with the positive polar heads of surfactants by electrostatic interactions. The lower adsorption capacity of “calcined Al-MCM-41” is due generally to the Van der Waals interactions present among the molecules of dyes and the walls of the mesopores (Fig. 9(A)). It was previously shown (Zhou et al., 2011) that surfactant-containing mesoporous materials were characterized by low specific surface areas. In these conditions, the latter may limit the accessible interfacial areas and restrict diffusion within the surfactant phase of the materials. The YD retention was mostly presumed to occur on the surface covered by surfactant molecules. It is due to filling of the mesopores of the Al-MCM-41 sorbents by the used long chain of alkyl. As shown in Fig. 9, the obtained results suggest that the mechanisms of YD adsorption both on the uncalcined and calcined Al-MCM-41 were different. Two colors are obtained after adsorption,light yellow and orange colors were formed with the calcined Al-MCM-41 and uncalcined Al-MCM-41, respectively.

These results suggest also that electrostatic force could play more important role in YD adsorption than hydrogen bonding or Van-der-waals interactions. To better understand the mechanism of the YD adsorption onto Al-MCM-41, we characterized by FTIR the different solids after YD (Fig. 10(A and B)). The sharp peak at 3746 cm−1 of Al-MCM-41 is due to OH-stretching of non-hydrogen-bonded Si–OH. Broad tail having a peak near 3482–3300 cm−1 is due to hydrogen bonded Si–OH. Shifts of these peaks indicated interactions of the acid dyes with the silanol groups (Rytwo et al., 2002; Ghouti et al., 2003). After adsorption of YD both on Al-MCM-41 calcined and uncalcined at different Si/Al ratios (Fig. 10(A and B)) shifting of the O–H peak at 795–815 cm−1 was observed. Similar effects could be observed for main Si–OH vibration at 3445 cm−1, which shifted to 3469 cm−1 in the case of calcined Al-MCM-41 at different Si/Al ratios.

FTIR spectra of Al-MCM-41 at different Si/Al ratios after the adsorption of the dye. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, Time 24 h, pH = 6, (A): calcined and (B):uncalcined.
Figure 10 FTIR spectra of Al-MCM-41 at different Si/Al ratios after the adsorption of the dye. Experimental conditions: temperature 297 K, concentration 50 mg/l, adsorbent 0.1 g, Time 24 h, pH = 6, (A): calcined and (B):uncalcined.

From Fig. 11, we confirm that Al-MCM-41 solids have always an affinity for the YD molecules. In the case of non regenerated Al-MCM-41 with different Si/Al ratios, we observed a relationship between Si/Al ratio and YD adsorption. The high aluminum content decreased over the silanol group (Jentys et al., 1999), that is why there was a slight decrease of the dye adsorption (number of interactions between the dye and Al-MCM-41 is low when Si/Al is lower). After regeneration of Al-MCM-41 there is a slight decrease of the adsorption of the dye, explained by a decrease in the Si–OH groups that have been dehydrated during calcination.

Yellow dye adsorption before regeneration and after regeneration of calcined Al-MCM-41. Experimental conditions: temperature 297 K, Co = 50 mg/l, adsorbent 0.1 g, Time 24 h, pH = 6 and 900 tours/min.
Figure 11 Yellow dye adsorption before regeneration and after regeneration of calcined Al-MCM-41. Experimental conditions: temperature 297 K, Co = 50 mg/l, adsorbent 0.1 g, Time 24 h, pH = 6 and 900 tours/min.

4

4 Conclusion

It is shown in this work that the Al-MCM-41 mesoporous materials are effective adsorbents for the removal of acidic dyes such as yellow dye from an aqueous solution. The obtained results indicate that the surfactant templates played an important role in the adsorption process due to their strong hydrophobic properties. It was shown that the amount of YD uptake increased with increasing agitation speed, adsorption time, temperature and pH of the solution. The percentage YD removal was higher in alkali solution and lower in acid solution. A relationship seems to exist between the Si/Al ratio and Si-OH hydroxyl groups. Thus, more the Si/Al ratio is higher more the yellow dye adsorption was important. The adsorption kinetic data were well described with the pseudo-second-order kinetic model. Finally, the calcined Al-MCM-41 adsorbent was easily regenerated after yellow dye adsorption but XRD patterns indicated a significant decrease of the crystallinity. The aromatic cycles could interact with the alkyl part of CTAB by hydrophobic interactions which result in a significant adsorption capacity of uncalcined Al MCM 41. Calcined adsorbent was characterized by a weak affinity due to the Van der Waals interactions between the molecules of YD and the walls of the mesopores.

Acknowledgements

The authors thank Dr Françoise Quignard, ICGM-FRANCE, for fruitful discussions, Pr Aicha Derdour Director of Laboratory of organic synthesis for the analyses by UV and FTIR and also the technician Miss Hanaa Hidour for analysis.

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