Synthesis and characterization of MCM-49/MCM-41 composite molecular sieve: an effective adsorbent for chromate from water

Abstract

In this work a porous material, an MCM-49/MCM-41 composite molecular sieve, was synthesized via a microwave assisted hydrothermal process and characterized to observe its optimal properties for heavy metal chromate removal. The adsorption properties of chromate onto MCM-49/MCM-41 and the factors that influence its adsorption, such as chromate concentration and contact time, were investigated. It was found that MCM-49/MCM-41 is micro/mesoporous with a surface area of 372 m² g⁻¹ and average pore size of 2.9 nm. The adsorption capacity of MCM-49/MCM-41 was as high as 4.10 mg g⁻¹ for chromate at a temperature of 298 K, when the initial concentration of chromate was 50 mg L⁻¹. The adsorption isotherm data of chromate fit well with both Freundlich and Langmuir adsorption isotherm models and the adsorption kinetics was well fitted using a pseudo-second order kinetic model. Thermodynamic parameters such as the standard Gibbs free energy (ΔG), enthalpy change (ΔH) and standard entropy change (ΔS) were also evaluated. Thermodynamic analysis indicates that the adsorption of chromate onto MCM-49/MCM-41 is a spontaneous and exothermic process. These results suggest that MCM-49/MCM-41 could serve as a promising adsorbent for potential applications in the removal of chromate from wastewater.

---

1. Introduction

Water is an important resource in the existence and development of human beings. However, rapid industrialization causes the negative consequence of environmental pollution, whereby water is badly polluted by the discharge of heavy metals in the processes of mining, electroplating, metal finishing, metal plating and also in the manufacturing process of battery, pigment, paint, and dyestuff.¹ The non-ignorable environmental problems caused by excessive metal ions threaten living organisms including the health of human beings, and it is becoming increasingly difficult to provide water of a suitable quality for all application purposes.² Chromate, an example of a carcinogenic heavy metal, is very common in diverse industrial processes involving metals, such as electroplating, leather tanning, wood preservation and the chemical industry.^3,4 The entry of Cr into the environment has been reported due to leakage, poor storage or unsafe disposal practices from such industrial processes.³ Chromate is carcinogenic as well as toxic to the kidney and liver, and usually causes gastric damage. Therefore, the removal of large amounts of chromate ions from aqueous solutions to limit their discharge is very important concerning the prevention of environment pollution and has been an area of focus for research.

In recent years, various techniques for the removal of highly toxic ions from aqueous solutions have been developed such as solvent extraction, chemical precipitation, ion exchange and reverse osmosis.⁵ All these methods have their own limitations with respect to complexity, cost, and efficiency.⁶ On the other hand, an adsorption technique is considered to be an economical, efficient, and promising technology for the removal of heavy metal ions.⁷ In fact, adsorption is the adhesion of atoms or molecules from a solution onto the surface of a highly porous material.⁸ Porous materials with superstructures and outstanding capacities, including activated carbon (AC),⁹ zeolites,¹⁰ biomaterials,¹¹ etc., have been reported to be capable of the successful adsorption and removal of heavy metal ions (Cr,^12–14 As,¹⁵ Cd,^14,16–18 Zn,^12,18 Cu,^12,17 Mn,¹⁸ Pb,¹⁹ etc.) from aqueous solutions by many groups. As a porous material, composite molecular sieves, especially micro/mesoporous composites, have been paid much attention in the catalytic domain due to the fact they not only combine the advantages of the two distinct kinds of molecular sieve, but also induce the formation of special properties.^20,21 It is found that composite molecular sieves usually exhibit better adsorption properties during catalytic processes. Therefore it is speculated that if used correctly composite molecular sieves could also be used as adsorbents of heavy metals. MCM-41 is a well-known mesoporous molecular sieve and many MCM-41 based micro/mesoporous composite molecular sieves have been synthesized and applied in different areas.^22–24 However, the synthesis of these composite molecular sieves usually takes a long time which will limit their effective application in adsorption.

Therefore, in this paper we synthesized a MCM-49/MCM-41 micro/mesoporous composite molecular sieve by a microwave assisted hydrothermal method, which shortened the synthesis time by many folds. The morphology and structure of the synthesized MCM-49/MCM-41 were analyzed by XRD, SEM and N₂ adsorption–desorption measurements. This little reported micro/mesoporous composite molecular sieve was also applied as a novel adsorbent for the removal of chromate ions from aqueous solutions. The adsorption kinetics and thermodynamics of chromate onto MCM-49/MCM-41 was systematically investigated in detail. Based on the results related to adsorption behaviour, the adsorption was fitted well by the Langmuir and Freundlich equations. Meanwhile, thermodynamic analysis indicates that the adsorption process was spontaneous and exothermic in nature. The results of this study can be used as guidelines to assess the application of MCM-49/MCM-41 in the adsorption of heavy metal ions.

2. Experimental section

2.1 Materials

The materials and equipment used in this study include the following: hexamethyleneimine (HMI, Kermel Chemreagent Co., Ltd., Tianjin, China), sodium aluminate (Kermel Chemreagent Co., Ltd., Tianjin, China), silica sol (Guolian Technology Co., Ltd., Jiangyin, China), cetyltrimethylammonium bromide (Kermel Chemreagent Co., Ltd., Tianjin, China), tetraethyl orthosilicate (Kermel Chemreagent Co., Ltd., Tianjin, China) and ammonia solution (Kermel Chemreagent Co., Ltd., Tianjin, China). All of the other chemicals and solvents in this work were commercially available and were used as received without further purification.

2.2 Synthesis of MCM-49/MCM-41 composite molecular sieves

MCM-49 zeolites were obtained by a reported procedure.²⁵ To synthesize MCM-49/MCM-41, a sol–gel of MCM-41 was firstly prepared according to a published procedure.²⁶ While stirring, a certain amount of MCM-49 zeolite was added to the colloidal precursor of MCM-41. The mixture was then stirred for a few minutes, followed by hydrothermal crystallization at 100 °C for 15 min assisted by microwave conditions of 700 W and 40 bar. After cooling to room temperature, the resulting solid product was filtered, washed thoroughly with deionized water and dried at 120 °C. Composite samples were then obtained by calcining the product at 540 °C for 5 h in air with a heating rate of 10 °C per min from room temperature to 540 °C. By varying the weight of MCM-49 added to the MCM-41 gel, several MCM-49/MCM-41 composites with different weight percentages of MCM-49 to MCM-41 gel were prepared, denoted as MCM-49/MCM-41(x%), where x represents the theoretical weight percentage of MCM-49 to MCM-41 gel.

2.3 Characterization

X-ray diffraction (XRD) experiments were carried out using a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 0.15405 nm). The morphology of the samples was observed using a MX2600FE scanning electron microscope (SEM). The N₂ adsorption–desorption of samples was conducted on a Micromeritics ASAP2020 surface area and pore size analyzer at −196 °C (liquid nitrogen temperature) using accompanying software from Micromeritics. Surface area was calculated using a conventional BET method, and the pore size diameters were calculated by using a BJH method. The concentration of the heavy metal ions in solution was analyzed by flame atomic absorption spectrometry (AAS) (Jena ZEEnit 700) with a standard calibration curve.

2.4 Adsorption experiments

Adsorption experiments were carried out by a batch method by varying contact time, adsorbate concentration and temperature. For the batch method, several individual flasks with a 100 mL solution containing a fixed 1.0 g MCM-49/MCM-41 loading, and an initial CrO₄²⁻ concentration (10 to 90 mg L⁻¹) at constant pH (6–7) were stirred at 298 K for a period time ranging from 20 to 120 min. Adsorption isotherms were obtained at different temperatures (298, 313 and 333 K). After the adsorption process, the concentration of the metal ions in the residual solution in all groups was analyzed using flame atomic absorption spectrometry (AAS) (Jena ZEEnit 700) with a standard calibration curve. The adsorption degree (percentage removal) and adsorption capacity (Q) of the adsorbent was calculated using the following equations, respectively.where C₀ and C_e (mg dm⁻³) are the initial and equilibrium concentrations of the CrO₄²⁻ solutions, respectively, V (L) is the volume of the chromate solution subjected to adsorption and m (g) is the weight of the sorbent.

2.5 Desorption study

A desorption study was carried out by loading MCM-49/MCM-41 (1 g) into a set of 100 mL solutions with a chromate concentration of 50 mg L⁻¹. It was equilibrated for 24 hours at temperature 298 K. The equilibrium concentration was measured and the chromate-adsorbed MCM-49/MCM-41 was separated from the solution by a filter cloth. It was then transferred to a 100 mL aqueous solution and the pH was varied from 2 to 12. Desorption experiments were conducted at a temperature of 298 K with a shaking time of 10 h. The residual chromate concentration was measured and the percentage of desorbed chromate was calculated according to the equation:where, D is the desorbed percentage, C_des is the desorbed concentration and C_ads is the adsorbed concentration.

3. Results and discussion

3.1 MCM-49/MCM-41 characterization

The crystal phase, morphology, pore size distribution and surface area of MCM-49/MCM-41 prepared by a microwave assisted hydrothermal method were analyzed by XRD, SEM and N₂ adsorption–desorption measurements.

Fig. 1 shows the XRD patterns of the MCM-49/MCM-41 with weight percentages of MCM-49 to MCM-41 gel from 1.0 to 5.0%. It can be observed from Fig. 1a that the mesostructure is well formed when the weight percentage of MCM-49 to MCM-41 gel is lower than 3.0%. With an increase in the amount of MCM-49 in the MCM-41 gel, the peak intensity of MCM-41 decreases significantly. When the weight percentage of MCM-49 is increased to 5.0%, the characteristic peaks corresponding to MCM-41 can no longer be detected by XRD. This is ascribed to the fact that adding too much MCM-49 to MCM-41 gel may suppress the crystallization of MCM-41. As shown in Fig. 1b, the characteristic peak position of MCM-49 shows no obvious change with an increasing weight percentage of MCM-49 to MCM-41 gel and the MCM-49 peak intensity increases slightly with an increasing amount of MCM-49 in the composite. The diffraction peaks of samples are consistent with those of MCM-49 reported in the literature.^25,27 It could be concluded from the XRD analysis that this microwave assisted hydrothermal method could successfully synthesize a MCM-49/MCM-41 micro/mesoporous composite molecular sieve in a short time. Furthermore, when the weight percentage of MCM-49 to MCM-41 gel is 2.0%, both the mesostructural component of MCM-41 and the microstructural component of MCM-49 are well formed. Therefore, the MCM-49/MCM-41 composite molecular sieve with a MCM-49 to MCM-41 gel percent ratio of 2.0% was selected for further investigation.

Fig. 1 X-ray diffraction of MCM-49/MCM-41(x%) composite molecular sieves; (a) low angle and (b) high angle.

Fig. 2 shows the SEM images of MCM-49/MCM-41, MCM-49 and MCM-41. MCM-41 shows irregular crystal morphology (Fig. 2a), whereas MCM-49 shows numerous lamellar crystals (Fig. 2b). The MCM-49/MCM-41 composite molecular sieve shows a unique aggregated crystal-like morphology, which is clearly different from those of the parent MCM-41 and MCM-49 (Fig. 2c). Furthermore, it could be found that the mesostructure of MCM-41 was formed over the MCM-49 structure.

Fig. 2 SEM images of samples (a) MCM-41, (b) MCM-49 and (c) MCM-49/MCM-41.

The N₂ adsorption–desorption isotherm of MCM-49/MCM-41 is exhibited in Fig. 3, and the corresponding surface area and pore size parameters of MCM-49/MCM-41 are summarized in Table 1. As shown in Fig. 3, the N₂ adsorption–desorption isotherm of MCM-49/MCM-41 was similar to type I in the low pressure range (p/p₀ < 0.05), which is typical for microporous zeolites, and the adsorbed amount of N₂ was below 100 cm³ g⁻¹, which was the value of the filling volume of micropores. However, the N₂ adsorption–desorption isotherms of MCM-49/MCM-41 were similar to those of type IV in the pressure range (p/p₀ > 0.05), typical of mesoporous molecular sieves, and this indicates the existence of mesopores. In the pressure range (p/p₀ < 0.40), the adsorbed amount of N₂ increased linearly with pressure due to the monolayer adsorption of N₂ on the walls of the pores, while in the range of 0.40 < p/p₀ < 0.95, a jump in the adsorbed amount appeared because N₂ began filling the mesopores. Multilayer adsorption of N₂ in the mesopores occurred when p/p₀ became higher. Also, the MCM-49/MCM-41 composite molecular sieves have a higher specific surface area (372 m² g⁻¹) and larger pore volume (0.68 cm³ g⁻¹) than that of pure MCM-49. These results may be attributed to the simultaneous existence of MCM-49 and MCM-41 nanoparticles in MCM-49/MCM-41 composite molecular sieves. These results demonstrate that it is feasible to adsorb heavy metal with MCM-49/MCM-41 porous materials.

Fig. 3 N₂ adsorption–desorption isotherms of MCM-49/MCM-41, MCM-41 and MCM-49.

Table 1 Physical properties of pure molecular sieves and MCM-49/MCM-41 composite

Samples	BET surface area (m^2 g^−1)	Pore size (nm)	Pore volume (cm^3 g^−1)
MCM-41	718	3.6	0.76
MCM-49	291	0.4	0.58
HMCM-41/MCM-49	372	2.9	0.68

---

3.2 Kinetics of adsorption

In order to verify the feasibility of MCM-49/MCM-41 for adsorbing heavy metal ions, the application of MCM-49/MCM-41 porous material in removal of chromate from aqueous solution was investigated in detail. Firstly, adsorption kinetics was investigated to evaluate the effect of adsorption time and initial concentration on the adsorption process, and then a kinetic model of adsorption was found.

3.2.1 Effect of absorption time. The effect of adsorption time on the adsorption process of metal ions was essential to determine and optimize the experimental conditions. The adsorption processes as a function of time were studied from adsorption experiments of chromate onto MCM-49/MCM-41 at an initial chromate concentration of 50 mg L⁻¹ and a temperature of 298 K. The results shown in Fig. 4 demonstrate that the adsorption of chromate onto MCM-49/MCM-41 is rather quick and after 80 min, complete adsorption equilibrium is obtained. No further adsorption quantity was obtained by placing adsorbent samples in contact with CrO₄²⁻ solutions for a longer adsorption time.

Fig. 4 Adsorption of chromate on MCM-49/MCM-41 at different adsorption times with an initial chromate concentration of 50 mg L⁻¹ and a temperature of 298 K.

3.2.2 Effect of initial concentration of chromate. The effect of the initial concentration of CrO₄²⁻ on its adsorption by MCM-49/MCM-41 was studied under an adsorption time of 80 min and a temperature of 298 K. The results are shown in Fig. 5. The curves in Fig. 5 indicate that the removal percentage decreases while the adsorption capacity increases upon increasing the initial concentration from 10 to 90 mg L⁻¹. This observation is probably due to the fact that, at low concentration, metals are adsorbed by specific sites, while with increasing the metal concentration, specific sites are saturated and exchange sites are filled.²⁸ Therefore, the percentage removal gradually drops with an increase of initial concentration and there is only 74% removal at an initial concentration of 90 mg L⁻¹. At the same time, the adsorption capacity increases with increasing the initial concentration of the metal ion.

Fig. 5 Adsorption of chromate on MCM-49/MCM-41 with different initial chromate concentrations under an adsorption time of 80 min and a temperature of 298 K.

3.2.3 Kinetics models of adsorption. In order to further investigate the adsorption kinetics of chromate onto MCM-49/MCM-41, Lagergren pseudo-first order and pseudo-second order kinetic models, two of the most widely used models for the adsorption of a solute or metal ion from a liquid solution, were applied to fit the present experimental data (Table 2) and to describe the adsorption behaviour.

Table 2 Adsorption kinetic experimental data of chromate onto MCM-49/MCM-41 (298 K)

C0 (mg L^−1)	t (min)	Ct (mg L^−1)	Qt (mg g^−1)	Qe (mg g^−1)
10	30	1.37	0.86	0.91
	40	1.12	0.89
	60	1.04	0.90
	80	1.01	0.90
20	30	6.93	1.31	1.74
	40	4.58	1.54
	60	3.77	1.62
	80	3.15	1.69
50	30	25.27	2.47	4.10
	40	15.16	3.48
	60	11.79	3.82
	80	9.90	4.01
70	30	39.95	3.00	5.46
	40	28.68	4.13
	60	21.66	4.83
	80	17.14	5.29
90	30	49.52	4.05	6.66
	40	35.83	5.42
	60	29.24	6.08
	80	25.07	6.49

---

The Lagergren equation is as follows:²⁹

where Q_e (mg g⁻¹) and Q_t (mg g⁻¹) are the amount of the metal ions adsorbed at equilibrium and at time (t), respectively, and k₁ is the rate constant of adsorption (min⁻¹). Fig. 6 shows the Lagergren pseudo-first order model of CrO₄²⁻ adsorption on MCM-49/MCM-41 and the calculated results of the kinetic parameters k₁, Q_e and the correlation coefficient (R²) are listed in Table 3. The pseudo-first order kinetic model gives a low correlation coefficient (<0.99) for chromate as shown in Table 3. Also, the calculated Q_e is not in agreement with the experimental data. This finding suggests that the adsorption of chromate does not follow the pseudo-first order kinetic model.

Fig. 6 Lagergren pseudo-first order kinetic plots for the adsorption of chromate onto MCM-49/MCM-41.

Table 3 Lagergren pseudo-first order kinetic parameters for adsorption of chromate onto MCM-49/MCM-41

C0 (mg L^−1)	k1 (min^−1)	Qe (mg g^−1) (calc.)	R^2	Qe (mg g^−1) (exp.)
10	0.06	0.87	0.91	0.91
20	0.04	1.51	0.98	1.74
50	0.05	3.62	0.98	4.10
70	0.04	4.83	0.95	5.46
90	0.05	5.72	0.97	6.66

---

The pseudo-second order rate expression is given by the following equation:³⁰

where k₂ (g mg⁻¹ min⁻¹) is the rate constant of the pseudo-second order equation. The results of the kinetic plot of pseudo-second order are shown in Fig. 7 and the calculated results of the kinetic parameters k₂, Q_e and the correlation coefficient (R²) are listed in Table 4. The pseudo-second order rate expression gives a high correlation coefficient (>0.99) for chromate as shown in Table 4. Also, the calculated Q_e values agree very well with the experimental data which indicates that the pseudo-second order kinetic model would be more suitable to analyze the adsorption behavior of chromate using MCM-49/MCM-41 as adsorbent.

Fig. 7 Lagergren pseudo-second order kinetic plots for the adsorption of chromate onto MCM-49/MCM-41.

Table 4 Lagergren pseudo-second order kinetic parameters for adsorption of chromate onto MCM-49/MCM-41

C0 (mg L^−1)	k2 (mg g^−1 min^−1)	Qe (mg g^−1) (calc.)	R^2	Qe (mg g^−1) (exp.)
10	1.13	0.91	0.99	0.91
20	0.22	1.70	0.99	1.74
50	0.14	3.94	0.99	4.10
70	0.07	5.06	0.99	5.46
90	0.07	6.30	0.99	6.66

---

3.3 Adsorption isotherm

The equilibrium adsorption isotherm is also important to describe the adsorption behavior of chromate. In order to evaluate the maximum metal adsorption capacity of MCM-49/MCM-41 and the effect of chromate concentration on adsorption, Langmuir and Freundlich isotherm models were adopted to further explain the adsorption behaviour of metal ions. The Freundlich isotherm model is based on adsorption on a heterogeneous or non-uniform surface and used to describe adsorption from solution as a multilayer sorption, while the Langmuir isotherm model assumes a monolayer coverage of sorption, and that each molecule adsorbed onto the surface has equal sorption activation.³¹ In this study, the data collected were fitted by the above two commonly used models.

Freundlich equation:³²

where k_f (mg g⁻¹) and n are Freundlich isotherm constant.

Langmuir equation:³³

where Q_m (mg g⁻¹) is the maximum uptake capacity and k_l is the Langmuir constant which relates to the affinity between metal ions and adsorbent (L mg⁻¹).

Fig. 8 illustrates the linearized Freundlich and Langmuir adsorption isotherms for the adsorption of chromate onto MCM-49/MCM-41 at different temperatures (298 K, 313 K and 333 K) with initial concentrations of adsorption solution ranging from 10 to 90 mg L⁻¹ and a fixed sorbate–sorbent contact time of 80 min. The Langmuir and Freundlich constants and correlative fitting parameters and their correlation coefficients (R²) as evaluated from the isotherms are listed in Table 5. The Freundlich isotherm constant, k_f, which represents the adsorption capacity for metal ions, was determined by the intercept and slope of a plot of log Q_e versus log C_e (Fig. 8). The Freundlich constant k_f increases with temperature, which indicates that the adsorption process is endothermic. The values of k_f calculated using the Freundlich model illustrate that MCM-49/MCM-41 has a high affinity toward metal ions. Also, the values of the adsorption intensities, n, of metal ions were larger than 1, which indicates the favorable adsorption of metal ions onto the adsorbent.³⁶ The Langmuir constant k_l and the maximum uptake capacity Q_m were calculated using the slope and intercept of plots of C_e/Q_e versus C_e at different temperatures. By a comparison of the Langmuir constants k_l, which also represents the adsorption capacity of the porous material for metal ions, it can be concluded that MCM-49/MCM-41 has a greater mass capacity for CrO₄²⁻ at a higher temperature.³⁴ Furthermore, the correlation coefficients (R²) were close to 1. On the basis of these considerations, the overall adsorption process, well fitted by Freundlich isotherm and Langmuir isotherm equations, indicates that the porous material will be highly effective in removing trace amounts of metal ions from aqueous solution.

Fig. 8 Linearized (a) Freundlich and (b) Langmuir adsorption isotherms for the adsorption of chromate, where C_e is equilibrium concentration and Q_e is the metal uptake capacity per unit adsorbent at equilibrium.

Table 5 Langmuir and Freundlich equation parameters of the adsorption of chromate onto MCM-49/MCM-41 at different temperature

Isotherm	Temperature (K)	Parameters		R^2
		kf (mg g^−1)	n
Freundlich isotherm	298	1.1220	9.6021	0.9876
	313	1.2882	2.2728	0.9920
	333	1.4125	2.3976	0.9938

---

Isotherm	Temperature (K)	Parameters		R^2
		kl (L mg^−1)	Qm (mg g^−1)
Langmuir isotherm	298	0.1116	10.56	0.9895
	313	0.1125	11.24	0.9918
	333	0.1184	16.89	0.9936

---

3.4 Adsorption thermodynamics

In order to obtain the thermodynamic parameters, i.e. heat of enthalpy, ΔH entropy change, ΔS for the adsorption of chromate onto MCM-49/MCM-41, the adsorption experiments were carried out at 298, 313 and 333 K with a constant adsorption concentration of 50 mg L⁻¹. The values of ΔH and ΔS of the process can be determined from the slope and intercept of the linear regression of log k_f vs. 1/T (data from the Freundlich model, Fig. 9) according to the following equation.³⁵

Fig. 9 A plot of log K_a vs. 1/T using the Freundlich model.

The ΔH and ΔS values for the adsorption of chromate onto MCM-49/MCM-41 were found to be 17.08 kJ mol⁻¹ and 60.64 J mol⁻¹ K⁻¹, respectively. The Gibbs free energy change (ΔG) for the adsorption of chromate onto MCM-49/MCM-41 was calculated using the equation:

ΔG = ΔH − TΔS (9)

It is found to be −0.99, −1.90 and −3.11 kJ K⁻¹ mol⁻¹ at the temperatures of 298, 313 and 333 K, respectively. The negative ΔG values indicate that the adsorption of chromate onto MCM-49/MCM-41 was thermodynamically feasible and spontaneous, and the degree of spontaneity of the reaction increased in their absolute values with increasing temperature. This result suggests that a high temperature was favored for the adsorption of heavy metal ions on MCM-49/MCM-41. The positive value of ΔH confirms the endothermic character of the adsorption process. The positive ΔS reflects the affinity of MCM-49/MCM-41 toward chromate ions.

Fig. 10 shows the adsorption capacity Q and adsorption degree (%) of the adsorption process as a function of different temperatures from 298 K to 333 K. It is clearly seen that with increasing temperature, Q and adsorption degree (%) (Fig. 10) increase accordingly, which is consistent with the analysis of thermodynamic parameters.

Fig. 10 Adsorption capacity Q and adsorption degree (%) of the adsorption process as a function of different temperatures from 298 K to 333 K.

3.5 Effect of coexisting ions

To determine the adsorption selectivity of MCM-49/MCM-41 for chromate, the Q_e of respective ions adsorbed on MCM-49/MCM-41 from a mixed solution of Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺ with the same concentration (50 mg L⁻¹) were measured. As shown in Fig. 11, the adsorption capacity of MCM-49/MCM-41 toward Ni²⁺, Cu²⁺, Cd²⁺, Pb²⁺ and chromate reaches 1.2, 1.6, 1.3, 1.8 and 4.0 mg g⁻¹, respectively. It is obvious that the absorption capacity of MCM-49/MCM-41 toward chromate is higher than that toward other heavy metal ions, indicating a high selectivity toward chromate. These results further suggest that MCM-49/MCM-41 is a promising adsorbent for removing chromate from wastewater containing heavy metal ions.

Fig. 11 Effect of competing ions on the adsorption capacity of MCM-49/MCM-41. (The concentration Ni²⁺, Cu²⁺, Cd²⁺, Pb²⁺ and chromate are all maintained at 50 mg L⁻¹ with an initial pH of 6.0 at 298 K).

3.6 Mechanism of chromate adsorption on MCM-49/MCM-41

The adsorption mechanisms of metal ions onto nanomaterials such as MCM-49/MCM-41 are not yet fully understood. The type of metal–adsorbent interaction depends on the chemistry of metal ions, the solution pH and the chemical and physical nature of nanosorbent. Various adsorption mechanisms are possible including physical adsorption, surface complexation, ion exchange, electrostatic interaction and hard/soft acid base interaction. For example, at moderate pH the chromate group of CrO₄²⁻ will adsorb onto the MCM-49/MCM-41 composite as follows:

ROH + CrO₄²⁻ → RO–CrO₃–OR + H₂O

where R is the molecular framework of MCM-49/MCM-41. Part of the adsorbed chromate was firmly kept in the pore channels of MCM-49/MCM-41 composite and the other part may be reduced to Cr³⁺, which may further adsorbed by chelating with the negatively charged ion groups in MCM-49/MCM-41. Therefore, the adsorption of chromate onto the MCM-49/MCM-41 composite occurs mainly by the method of physical adsorption, surface complexation and chelation binding.

3.7 Desorption study and performance of the regenerated MCM-49/MCM-41

The stability and reusability of an adsorbent are important from environmental and economic points of view. The variation in desorption percentage with pH is shown in Fig. 12a. It is observed that maximum desorption occurs at acidic pH. Since the surface charge of an adsorbent can be modified by changing the pH of a solution, or hydrogen ions themselves strongly compete with adsorbates, pH is one of the most important parameters affecting the metal adsorption process. At lower pH, the concentration of H⁺ is high in the solution and they compete for adsorption sites,³⁶ and the desorbed percentage is 85% at pH 2. However, binding of adsorbed chromate on MCM-49/MCM-41 surface is stronger at a high pH due to electrostatic attraction, resulting in lower desorption of chromate from MCM-49/MCM-41 at higher pH; desorption becomes 1.5% at pH 12. Therefore, MCM-49/MCM-41 could be easily regenerated at low pH conditions.

Fig. 12 (a) Desorption study of chromate from MCM-49/MCM-41. (b) Adsorption/desorption cycles of chromate on the MCM-49/MCM-41 composite.

The adsorption/desorption regeneration cycles of MCM-49/MCM-41 were tested three times in this work and the results are shown in Fig. 12b. It can be seen from Fig. 12b that the amount of adsorbed or desorbed chromate almost remains constant over three cycles. A decrease of desorption of no more than 5% for each cycles indicates that the adsorbent is a good potential material for chromate adsorption, even though it is reused for a few times.

3.8 Comparison of MCM-49/MCM-41 with MCM-49 and MCM-41 adsorbents

The value of adsorption capacity Q is of importance to identify which adsorbent shows the highest adsorption capacity and is useful in scale-up considerations. The direct comparison of adsorbent capacity of MCM-49/MCM-41 with other sorbents is difficult due to the varying experimental conditions employed in those studies; however, as shown in Table 6, the MCM-49/MCM-41 porous material in this study possesses reasonable adsorption capacity in comparison with other adsorbents.

Table 6 The adsorption capacities of various adsorbents for chromate

Adsorbent	Q/mg g^−1	Reference
Brown coal	0.68	37
Fly ash impregnated with aluminum	1.80	38
AC derived from coconut shells	1.38	39
Lignite	3.80	40
MCM-49/MCM-41 porous material	4.10	This work

---

4. Conclusions

In the presented study a micro/mesoporous material, an MCM-49/MCM-41 composite molecular sieve, was successfully synthesized in a short time via a microwave assisted hydrothermal process and was characterized by XRD, SEM and N₂ adsorption–desorption measurements. When it was applied as an adsorbent of heavy metals in water, the MCM-49/MCM-41 composite molecular sieve exhibited an adsorption capacity as high as 4.10 mg g⁻¹ for chromate when the initial concentration of chromate was 50 mg L⁻¹, at a temperature of 298 K. The adsorption isotherm data of chromate fit well with both the Freundlich and Langmuir adsorption isotherm models and the adsorption kinetics was well fitted using a pseudo-second order kinetic model. Thermodynamic analysis indicates that the adsorption of chromate onto MCM-49/MCM-41 was a spontaneous and exothermic process. These results suggest that MCM-49/MCM-41 could serve as a promising adsorbent for potential applications in the removal of chromate from wastewater.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21203058 and 51307046), the National Key Technology Support Program of China (No. 2013BAE04B03), the Science Foundation for Youths of Heilongjiang Province of China (Grant No. OC2016013), the Foundation of Educational Commission of Heilongjiang Province of China (No. 12531579) and the Innovative Talents Program of Heilongjiang University of Science and Technology (Q20130202).

Notes and references

1. M. Yusuf, F. M. Elfghi, S. A. Zaidi, E. C. Abdullah and M. A. Khan, RSC Adv., 2015, 5, 50392–50420.
2. F. Fu and Q. Wang, J. Environ. Manage., 2011, 92, 407–418.
3. S. R. Chowdhury and E. K. Yanful, J. Environ. Manage., 2010, 91, 2238–2247.
4. T. Ren, P. He, W. Niu, Y. Wu, L. Ai and X. Gou, Environ. Sci. Pollut. Res., 2013, 20, 155–162.
5. P. Z. Ray and H. J. Shipley, RSC Adv., 2015, 5, 29885–29907.
6. Y. Liu, L. Xu, J. Liu, X. Liu, C. Chen, G. Li and Y. Meng, Chem. Eng. J., 2016, 285, 698–708.
7. C. Y. Cao, Z. M. Cui, C. Q. Chen, W. G. Song and W. Cai, J. Phys. Chem. C, 2010, 114, 9865–9870.
8. Y. Jeong, M. Fan, S. Singh, C. L. Chuang, B. Saha and J. H. Leeuwen, Chem. Eng. Process., 2007, 46, 1030–1039.
9. L. Largitte, T. Brudey, T. Tant, P. C. Dumesnil and P. Lodewyckx, Microporous Mesoporous Mater., 2016, 219, 265–275.
10. M. Karatas, J. Hazard. Mater., 2012, 199–200, 383–389.
11. A. E. Ofomaja, E. B. Naidoo and S. J. Modise, J. Environ. Manage., 2010, 91, 1674–1685.
12. I. Sargın, M. t. Kaya, G. Arslan, T. Baran and T. Ceter, Bioresour. Technol., 2015, 177, 1–7.
13. H. Fida, S. Guo and G. K. Zhang, J. Colloid Interface Sci., 2015, 442, 30–38.
14. A. Djukic, U. Jovanovic, T. Tuvic, V. Andric, J. G. Novakovic, N. Ivanovic and L. Matovic, Ceram. Int., 2013, 39, 7173–7178.
15. L. S. Zhong, J. S. Hu, L. J. Wan and W. G. Song, Chem. Commun., 2008, 10, 1184–1186.
16. Y. H. Ni, K. M. Liao and J. Li, CrystEngComm, 2010, 12, 1568–1575.
17. M. Ghaedi, M. Montazerozohori, M. Sajedi, M. Roosta, M. N. Jahromi and A. Asghari, J. Ind. Eng. Chem., 2013, 19, 1781–1787.
18. J. Oliva, J. D. Pablo, J. L. Cortina, J. Cama and C. Ayora, J. Hazard. Mater., 2010, 184, 364–374.
19. N. N. Zhang, H. X. Qiu, Y. M. Si, W. Wei and J. P. Gao, Carbon, 2011, 49, 827–837.
20. W. Yu, P. Yuan, D. Liu, L. Deng, W. Yuan, B. Tao, H. Cheng and F. Chen, J. Hazard. Mater., 2015, 285, 173–181.
21. Q. Zhang, C. Li, S. Xu and H. Shan, J. Porous Mater., 2013, 20, 171–176.
22. H. Li, S. He, K. Ma, Q. Wu, Q. Jiao and K. Sun, Appl. Catal., A, 2013, 450, 152–159.
23. Q. Tang, H. Xu, Y. Zheng, J. Wang, H. Li and J. Zhang, Appl. Catal., A, 2012, 413–414, 36–42.
24. Y. Li, W. Zhang, X. Wang, Y. Zhang and T. Dou, J. Porous Mater., 2015, 15, 133–138.
25. J. M. Bennett, C. D. Chang and S. L. Lawton, US Pat., 5236757, 1993.
26. W. L. Huang, B. J. Liu, F. M. Sun, Z. H. Zhang and X. J. Bao, Microporous Mesoporous Mater., 2006, 94, 254–260.
27. M. K. Rubin and P. Chu, US Pat., 4954325, 1990.
28. N. Li, L. Zhang, Y. Chen, Y. Tian and H. Wang, J. Hazard. Mater., 2011, 189, 265–272.
29. S. Lagergren, K. Sven. Vetenskapsakad. Handl., 1898, 24, 1–39.
30. Y. S. Ho and G. McKay, Process Biochem., 1999, 34, 451–465.
31. M. Iqbal, A. Saeed and I. Kalim, Sep. Sci. Technol., 2009, 44, 3770–3791.
32. C. H. Qin, R. Wang and W. Ma, Chem. Eng. J., 2010, 156, 540–545.
33. A. Saeed, M. W. Akhter and M. Iqbal, Sep. Purif. Technol., 2005, 45, 25–31.
34. M. A. Tofighy and T. Mohammadi, J. Hazard. Mater., 2011, 185, 140–147.
35. H. K. Boparai, M. Joseph and D. M. O’Carroll, J. Hazard. Mater., 2011, 186, 458–465.
36. H. Boujaady, M. Mourabet, M. Ziatni and A. Taitai, J. Assoc. Arab Univ. Basic Appl. Sci., 2014, 16, 64–73.
37. G. Arselan and E. Pehlivan, Bioresour. Technol., 2007, 98, 2836–2845.
38. S. S. Banarjee, M. V. Joshi and R. V. Jayaram, Sep. Sci. Technol., 2004, 39, 1611–1629.
39. D. Mohan, K. P. Singh and V. K. Singh, Ind. Eng. Chem. Res., 2005, 44, 1027–1042.
40. L. Wei, L. Xie and L. Liu, J. Heilongjiang Univ. Sci. Technol., 2011, 21, 89–92.

---