Fabrication of poly(methyl methacrylate)/silica KIT-6 nanocomposites via in situ polymerization approach and their application for removal of Cu²⁺ from aqueous solution

Abstract

In this study, three types of novel mesoporous silica nanocomposites (called MSNCs) based on poly(methyl methacrylate) and modified mesoporous silica KIT-6 were prepared. The chemical, structural and textural properties of MSNCs were characterized by several methods including FT-IR spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Experimental results showed that MSNCs have many active sites on their surface and this feature has a promising future for targeting applications like adsorption. Accordingly, its application for adsorption of Cu(II) from aqueous solution was examined. The adsorption behavior of Cu(II) onto pure KIT-6, modified KIT-6 (called m-KIT-6) and three types of MSNCs was investigated by kinetic and isotherm studies. It was observed that the adsorption equilibrium is short (15 and 30 min for m-KIT-6 and pure KIT-6, respectively and 60–90 min for MSNCs). Adsorption kinetics was fitted by a pseudo second order kinetic equation. Also, the maximum adsorptive capacities of the three types of MSNCs were 79.14%, 91.20% and 81.72%, respectively. And finally, the resulting adsorption isotherms of the MSNCs as well as of pure KIT-6 and m-KIT-6 were found to best fit the Langmuir isotherm model.

---

1. Introduction

Based on IUPAC, porous materials are classified according to the pore diameter in three main categories including micro- (<2 nm), meso- (2–50 nm) and macroporous (>50 nm).¹ In the past decade, ordered mesoporous materials like carbon and silica materials with two dimensional (2D) structures, such as FDU, MCM and SBA, have been utilized in the areas of energy storage, drug delivery and biology, catalysis and adsorption, sensing, and sample preparation applications.^2–7 These widespread applications of mesoporous materials as mentioned is because of their unique properties, such as extremely large surface areas and large pore volumes, tunable micro-texture, functionalizable surface and chemical stability for recyclability. Nowadays, one of the most widely used applications of mesoporous materials is their use in the nanocomposite (NC) industry as nanofillers.⁸ Hence, the global market of NCs containing nano-sized filler materials has been expanding rapidly owing to their marvelous electronic, optical, mechanical and barrier properties in comparison to those of the virgin materials.⁹ As the size of polymer molecules is much smaller than the mesopore size (2–50 nm), so the penetration of the polymer chain into the mesopore space during the preparation of NCs is easily possible to form intimate composites.

KIT-6, with a three-dimensional (3D) cubic Ia3d symmetric structure and porous networks, possess high specific surface area, large pore volume, high hydrothermal stability and large readily tunable pores with thick pore walls.^10–12 KIT-6 with a 3D mesoporous structure has advantages over mesoporous materials having one-dimensional (1D) or two-dimensional (2D) arrays of pores, mainly because mesoporous materials with a 3D pore structure provide more adsorption sites for modification and functionalization and they are more resistant to pore blocking and so have a good mass transfer of the reactant molecules through the channels.¹³ Interconnected silica supports also significantly enhance the surface activity of the support for modification and linking between support and polymer in order to prepare nanocomposites. In previous studies the superiority of KIT-6 as cubic 3D mesoporous material has been reported, for instance in catalysis, gas adsorption and separation, sensing, electrodes and supercapacitors, as well as in drug delivery and cancer therapy applications.^10,14–20

Poly(methyl methacrylate) (PMMA) has been widely utilized as a versatile polymer due to its unique advantages, such as excellent optical transparency, good solvent resistance, low density, low cost, chemical resistance, good flexibility, and good physicomechanical properties. Because of these properties, PMMA has been widely used in the medicine, optical and supercapacitor, automobile, hydrogen storage material and construction industries.^21–26 However, PMMA has some drawbacks including a brittle texture, low thermal and mechanical stability and insufficient surface hardness. It has been reported that the addition of mesoporous materials, such as mesoporous carbon or silica, to the polymer matrix like PMMA results in an increase in the performance of the polymer and its thermal and mechanical behavior.^27,28 Since mesoporous silica is difficult to disperse evenly in a polymer matrix, because of aggregation, for a homogeneous dispersion of an inorganic moiety into a polymer matrix, the introduction of an organic group as a coupling agent into mesoporous silica is a useful method.²⁹

The presence of heavy metal ions in the environment, such as in water and soil, is a significant concern because of their harmful effects.^30,31 With the increasing demand for economic large-scale water treatment applications, the development of novel, low-cost, stable and efficient sorbents, therefore, is of great significance.^32,33 Recently, polymer nanocomposites (NCs) have received more and more attention for the removal of heavy metal ions from contaminated waste water.³⁴ Therefore, in this study we attempted to use novel hybrids of polymer and mesoporous silica to separate heavy metal ions from aqueous solution. At first, several novel mesoporous silica nanocomposites (MSNCs) were prepared by an in situ polymerization method. PMMA as a polymer matrix was selected due to its low cost, low toxicity, chemical resistance and excellent chemical properties. Silica KIT-6 was chosen as the inorganic filler because of its highly interconnected and interpenetrating system of channels and, accordingly, good accessibility for modification and compatibility with the polymers like PMMA. m-KIT-6 was prepared through the reaction between 3-mercaptopropyl-trimethoxysilane with hydroxyl groups of the surface mesoporous KIT-6. Methyl methacrylate (MMA) monomer infiltrated into the pores of the 3D structure of m-KIT-6 and different MSNCs were synthesized by in situ polymerization. The structure and morphology of the resulting hybrid compounds were characterized by different methods such as Fourier transform-infrared (FT-IR) spectroscopy, X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), field emission-scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Then, the application of the MSNCs for adsorption of Cu(II) from aqueous solution was examined. The contact time as a significant parameter of the removal of Cu(II) ions and the adsorption mechanism were investigated.

2. Experimental

2.1. Materials

3-Mercaptopropyl-trimethoxysilane, copper(II) nitrate trihydrate [Cu(NO₃)₂·3H₂O], tetraethyl orthosilicate (TEOS), methacrylate (MMA), benzoyl peroxide (BP), HCl and NaOH were purchased from Merck Chemical Co. and used without further purification. Triblock copolymer Pluronic P123 (average M_n = 5800, (EO)₂₀(PO)₇₀(EO)₂₀) was purchased from Aldrich Chemical Inc.

2.2. Instruments

FT-IR spectra were recorded with a Jasco-680 (Japan) spectrometer from 400 to 4000 cm⁻¹, using a KBr pellet technique by making 60 scans at 4 cm⁻¹ resolution. The small angle powder XRD data were measured by an XRD (Bruker Nanostar U) with a Cu K_α radiation (λ = 0.1542 nm) at 45 kV and 100 mA. The diffraction patterns were collected between 2θ 0.7° and 8° at a scanning rate of 0.05° min⁻¹. The scanning speed was 0.02° s⁻¹. TGAs of the samples were carried out in a nitrogen atmosphere by heating at a rate of 20 °C min⁻¹ from room temperature to 800 °C using the STA503 TA instrument. The morphology of the nanostructure materials was examined by FE-SEM (HITACHI, S-4160) and TEM (Philips CM 120, Netherlands). For SEM preparation, the powdered sample was dispersed in H₂O, and then the sediment was dried at room temperature before gold coating. For TEM, the powders were dispersed in isopropanol and a drop of this suspension was put on a carbon coated nickel grid. The sono-chemical reaction was carried out on a MISONIX ultrasonic liquid processor, XL-2000 SERIES (Raleigh, North Carolina, USA). Ultrasound was at a wave of frequency 2.25 × 10⁴ Hz and power of 100 W. The concentrations of the metal ions in the solution were measured by the use of a flame atomic absorption spectrophotometer (PerkinElmer 2380-Waltham).

2.3. Preparation of ordered mesoporous silica KIT-6

Mesoporous silica KIT-6 was prepared according to a previous report.³⁵ In a typical synthesis, 2.0 g of triblock copolymer P123 (EO₂₀PO₇₀EO₂₀) was dissolved in a solution with 7.0 g of HCl (36 wt%) and 60.0 g of deionized water under stirring at room temperature. Then, 2.0 g of n-butanol was added to the solution and, after 1 h, 4.0 g of tetraethoxysilane was added. The mixture was heated at 35 °C and stirred for 24 h. The mixture was then treated in an autoclave at 140 °C for 24 h. The product was centrifuged at 14 240 relative centrifugal force (RCF; g-force) for 20 min, washed with deionized water, and dried at 60 °C. Finally, the product was calcined in air at 500 °C for 3 h to remove the surfactant, and pure mesoporous silica KIT-6 was obtained.

2.4. Modification of KIT-6 with 3-mercaptopropyl-trimethoxysilane

A silane coupling agent was used as a surface modifier for KIT-6 by the following procedure: KIT-6 was dried at 105 °C for 3 h to remove the absorbed water. 40 μL of 3-mercaptopropyl-trimethoxysilane and 0.4 g of KIT-6 were added into 25 mL of ethanol. They were mixed at room temperature under stirring, refluxed for 24 h, and then subjected to ultrasonic irradiation for 2 h. Finally, the suspension was collected by centrifuge (3560 RCF) and washed 3 times with 10 mL ethanol to remove unreacted silane coupling agent and then dried at 60 °C for 24 h to obtain the modified KIT-6 (m-KIT-6).

2.5. In situ synthesis of mesoporous silica nanocomposites (MSNCs)

MSNCs with m-KIT-6 contents of 1, 2, and 3 wt%, denoted as MSNC 1%, MSNC 2%, MSNC 3%, respectively, were prepared using an in situ method. For the preparation of MSNCs, different amounts of m-KIT-6 were added to 10 mL of dry toluene and sonicated in an ultrasonic bath for 1 h. Then MMA was added to the solution of the m-KIT-6 and the mixture was stirred to obtain a stable suspension and sonicated for 1 h. The BP initiator (1 wt%) was added into the above solution and it was refluxed for 6 h. Polymerization was carried out under isothermal conditions. The solution was then poured into a clean glass, and the solvent was evaporated under a nitrogen atmosphere to form PMMA and m-KIT-6 hybrids.

2.6. Adsorption studies: copper removal over pure KIT-6, m-KIT-6 and MSNCs

An essential parameter of heavy metal adsorption from wastewater, determining whether or not the use of a given sorbent is possible for an economical approach is the contact time. So it is obvious that the time needed to reach the maximum capacity for capturing the hazardous material that needs to be separated is an important investigation for heavy metal adsorption research. In this study, the adsorption behavior of Cu(II) from aqueous solution by the MSNCs as well as by pure KIT-6 and m-KIT-6 was investigated and kinetic and isotherm parameters were assessed. A stock solution of 1000 mg L⁻¹ Cu(II) was prepared by dissolving a weighed amount of analytical grade Cu(NO₃)₂·3H₂O (>99.0% purity) in deionized water, and the other solutions were prepared by dilution. Samples (10 mg) of the three types of MSNCs, pure KIT-6 and m-KIT-6 were added into 10 mL of 10 mg L⁻¹ Cu(II) solution in a plastic container. The pH of the solution was adjusted to 5.5 with 1 M HCl and 0.1 M NaOH. The containers were placed in a shaker at a speed of 150 rpm and the contact time was varied from 1 to 255 min until equilibrium was reached. The adsorbent was separated by centrifugation (3560 RCF, 10 min) and filtered through a 0.45 μm filter membrane. The residual Cu(II) concentrations were detected by a flame atomic absorption spectrometer. The percentage of removed metal ions in the solution was computed using the following equation:where C_i and C_e refer to the initial and equilibrium concentrations (mg L⁻¹) of Cu(II) in the metal solution, respectively.

3. Results and discussion

3.1. Preparation of PMMA/m-KIT-6 NCs

Scheme 1 shows the overall strategy applied for the preparation of MSNCs by the in situ synthesis approach. Possibly the most remarkable feature of ordered mesoporous KIT-6 is its ability to link with other active sites of molecules due to the abundant active hydroxyl groups on the interior and exterior surfaces of its 3D framework. In addition to the exterior surface modification of KIT-6, the penetration of silane coupling agents into the 3D interconnected pores of the KIT-6 and the interior surface modification provided a high active surface area with accessible functional groups for the following steps. MMA can be easily permeated into the 3D pores of m-KIT-6. So, before the start of the polymerization in the presence of an initiator, monomers are present in and out of the 3D pores of m-KIT-6. Accordingly, it is possible that polymerization takes place after adding the initiator both inside and outside of the pores. The interaction between carbonyl groups of the polymer moiety and –OH and/or –SH on the surface of the modified mesoporous silica moiety during the polymerization procedure led to the formation of polymer chains through the 3D interconnected network of KIT-6. Hence, the advantage of this polymerization method compared to other methods is the higher percentage polymerization in the interior pore space.

Scheme 1 Surface modification of mesoporous KIT-6 and preparation of PMMA/m-KIT-6 hybrid materials.

3.2. Characterization techniques

3.2.1. FT-IR study. Fig. 1 shows the FT-IR spectra of pristine KIT-6 and silane-modified KIT-6 (m-KIT-6). For neat KIT-6, a broad peak at 3200–3600 cm⁻¹ was attributed to the O–H stretching of the silanol groups of the surface and the remaining adsorbed water molecules, while the peak at 1600–1630 cm⁻¹ corresponds to the bending mode of O–H of water. A broad and strong band observed between 1100 and 1250 cm⁻¹ corresponds to the siloxane groups (Si–O–Si), and the stretching band of the Si–O bonds at 806 cm⁻¹ was also recorded. In the FT-IR spectrum of the m-KIT-6, characteristic bands corresponding to the organic part of the silane coupling agent attached to the m-KIT-6, appearing at 2952 and 2894 cm⁻¹, were assigned to the C–H stretching vibration of the CH₂ of the propyl group and the band at 1470 cm⁻¹ was assigned to the bending vibration of the C–H. This spectrum also showed a peak at 2552 cm⁻¹ that was characteristic of the SH of the coupling agent (Fig. 1). Thus, the above spectrum indicated the reaction of the coupling agent with KIT-6.

Fig. 1 FT-IR spectra of the pure KIT-6 and m-KIT-6.

The FT-IR spectra recorded for the pure PMMA and composite samples are depicted in Fig. 2. For PMMA, the absorption bands around 2997 and 2951 cm⁻¹ corresponded to C–H asymmetric stretching in CH₃ and CH₂, respectively. The vibrational band at 2842 cm⁻¹ was from the C–H symmetric stretching in CH₃. The characteristic band that corresponds to C=O stretching was observed at 1732 cm⁻¹ for the pure PMMA. The vibrations due to the deformation modes of CH₃ groups appeared at 1486, 1450 and 1388 cm⁻¹. Bands at 1245 and 1273 cm⁻¹ corresponded to C–O stretching modes. As shown in PMMA/m-KIT-6 NCs (MSNCs) spectra, the band around 1732 cm⁻¹, which corresponds to carbonyl bond of PMMA, is shifted to lower wave numbers (1718 cm⁻¹) (red shift). The band at 1193 cm⁻¹, corresponding to CH₃ wagging, and the two bands at 1145 cm⁻¹ and 1058 cm⁻¹ were due to CH₃ twisting. The vibration modes due to C–C stretching appeared at 988 cm⁻¹ and 965 cm⁻¹. The peaks at 912 cm⁻¹ and 840 cm⁻¹ were assigned to CH₂ rocking and the peak at 750 cm⁻¹ was due to the C–O out of plane bending.³⁶ In the FT-IR spectra of the PMMA/m-KIT-6 NCs, there were some new multiple absorption bands persisting in the region 400–600 cm⁻¹ as well as at 1070 cm⁻¹, mainly from the Si–O bands of m-KIT-6. Overall, these results indicated that there was a good interaction between the m-KIT-6 and PMMA matrix. Moreover, the peak intensity of the carbonyl bond in the spectrum of MSNCs is lower than that of pure PMMA. This may be due to the interaction between the mercapto groups of modified KIT-6 and the C=O groups of PMMA (Fig. 2). This means that the double bond CO stretches become weak through hydrogen bonding to hydroxyl and mercapto groups. Thus, it is confirmed that hydrogen bonding between mercapto groups of modified KIT-6 and PMMA molecules exists on the surface of mesoporous KIT-6. As a result, these analyses show the formation of the PMMA/m-KIT-6 NCs.

Fig. 2 FT-IR spectra of the neat PMMA and NCs of PMMA with different amounts of m-KIT-6.

3.2.2. X-ray diffraction. The low-angle XRD patterns of the unmodified KIT-6 and m-KIT-6 are shown in Fig. 3. The XRD patterns of these samples indicated a sharp diffraction peak at around 2θ = 0.84–0.93° and two weak reflections at 2θ = 1.34 and 1.68°. As shown in Fig. 3, they can be indexed as the (2 1 0), (2 2 0) and (4 2 0) reflections of a 3-D cubic structure with Ia3d space group symmetry, respectively.³⁷ The diffraction peaks shifted a little to lower 2θ angles and decreased progressively in intensity by modifications, which indicates that the ordered mesoporous structures were actually affected by the introduction of the organic substrate. Also, the XRD patterns of PMMA/m-KIT-6 NCs with different m-KIT-6 (2 and 3 wt%) are shown in Fig. 3. In the MCNC with 2% m-KIT-6, the XRD peaks of the m-KIT-6 can still be observed in the hybrid material which confirmed that ordered structures of KIT-6 have little aggregation and may be uniformly dispersed throughout the polymer matrix, but in the MSNC 3% the peak of the m-KIT-6 disappeared.

Fig. 3 XRD patterns of the KIT-6, m-KIT-6, and NC of PMMA with 2 and 3 wt% of m-KIT-6.

3.2.3. Morphology (FE-SEM and TEM). Fig. 4a and b show the morphological information of the pristine KIT-6 and m-KIT-6, respectively. As shown in these figures, after modification the surface morphology of KIT-6 has little change and is smoother than pristine KIT-6. To investigate the dispersion of the m-KIT-6 within the polymer matrix, the surface morphology of the PMMA with m-KIT-6 contents of 1, 2 and 3 wt% was investigated by FE-SEM. The results show that mesoporous m-KIT-6 entered the PMMA matrix (Fig. 4c–e). For the 1 and 2 wt% NCs (Fig. 4c and d), the filler distribution is apparently homogeneous due to the good adhesion between the inorganic and organic phases. In addition, when the amount of m-KIT-6 reached 3 wt%, the agglomerated particles were observed (Fig. 4e).

Fig. 4 FE-SEM images of pristine KIT-6 (a), m-KIT-6 (b) and NCs of PMMA with 1 (c), 2 (d) and 3 wt% (e) of m-KIT-6.

The morphology of modified mesoporous KIT-6 and the distribution of the m-KIT-6 nanoparticles in the PMMA matrix were studied by TEM observations. The typical TEM micrographs clearly show that the m-KIT-6 has a well ordered cubic Ia3d pore array structure with alternative pore channels. Pore size is approximately 8 nm based on the TEM image (Fig. 5a and b). After polymerization of MMA in the presence of 2 wt% of m-KIT-6, shrinkage of pore channels was observed in the mesoporous KIT-6. According to the TEM images, MSNC with 2% of m-KIT-6 showed an indistinct edge and a less disordered mesoporous structure due to the chain entanglement of PMMA on the surface of the m-KIT-6 and some aggregation could also be observed in the TEM images of this compound (Fig. 5c and d).

Fig. 5 TEM images of m-KIT-6 (a, b) and MSNC with 2 wt% of m-KIT-6 (c, d).

3.2.4. Thermal properties. The thermal behaviors of pristine KIT-6 and m-KIT-6 are shown in Fig. 6. By comparison of the TGA thermograms of modified KIT-6 with unmodified one, the results provide an indication of how the organic modifier effects the decomposition of the mesoporous material. Unmodified KIT-6 had a very small weight loss, with only 2 wt% around 600 °C.^10,12 After the functionalization of KIT-6 with the silane coupling agent, the number of decomposition stages increased. The first decomposition stage, owing to the removal of water or silane coupling agent that physically absorbed at the surface of the m-KIT-6, occurred at 100–250 °C. The second stage weight loss is from 280–450 °C, which may be attributed to the decomposition of the silane coupling agent. These results indicated the successful functionalization of the KIT-6.

Fig. 6 TGA thermograms of the KIT-6 and m-KIT-6.

In order to examine the thermal stability, thermal analyses were carried out on PMMA and NCs of PMMA with 1, 2 and 3 wt% of m-KIT-6. The TGA thermograms of the pure PMMA and the composites are illustrated in Fig. 7. The thermal stability of these nanocomposites is higher than that of pure PMMA; the TGA data showing the temperature at which 10% degradation occurs, T_0.1, the temperature at which 50% degradation occurs, T_0.5, and the amount of material which is not volatile at 800 °C, char, are shown in Table 1. The temperature at which 10% degradation occurs is a measure of the onset temperature of the degradation and this is increased for all MSNC materials. Likewise, the temperature at which 50% degradation occurs is some measure of thermal stability, and this is 40 °C higher for all three MSNCs. The initial step of the degradation was attributed to the presence of weak links in the polymer chain and occurs at temperatures in the range of 170–200 °C. The amount of this first step is decreased in the presence of the KIT-6 and it occurs at a higher temperature in the range of 200–250 °C. Another degradation step occurs at about 200–400 °C for pristine PMMA, and for NC materials with different amounts of m-KIT-6 it is around 320–500 °C (Fig. 7). Overall, the resulting MSNCs had better thermal stability, as compared to PMMA, due to the good thermal stability of the m-KIT-6. This improvement could be due to a good interaction between the polymer matrix and mesoporous materials as well as the inherently good thermal stability of the mesoporous KIT-6.

Fig. 7 TGA thermograms of the PMMA and NCs of PMMA with different amounts of m-KIT-6.

Table 1 Thermal properties of the PMMA and NC with different amount of m-KIT-6

Samples	Decomposition temperature (°C)		Char^c (%)
	T0.1^a (°C)	T0.5^b (°C)
a Temperature at which 10% degradation occurs.b Temperature at which 50% degradation occurs.c The amount of material which is not volatile at 800 °C.
PMMA	186	367	2
MSNC 1%	315	401	5
MSNC 2%	332	428	7
MSNC 3%	345	456	8

---

3.3. Adsorption equilibrium

The lack of biodegradability of heavy metals leads to accumulation of these materials in living organisms. Copper ions at doses higher than the allowed amounts are known as a toxic contaminant. The accumulation of Cu(II) in the organs of human body can cause serious problems such as liver, heart, kidney, and pancreas damage and gastrointestinal disturbance.³⁸ An essential parameter of heavy metal adsorption from wastewater determining whether or not the use of a given sorbent is possible for an economical approach is the contact time. So it is obvious that the time needed to reach the maximum capacity for capturing the hazardous material that needs to be separated is an important investigation for heavy metal adsorption research. In this study, pure KIT-6, m-KIT-6 and MSNCs were used for the removal of Cu(II) from an aqueous solution. In the case of MSNCs the free hydroxyl and mercapto groups of the modified KIT-6 and the carbonyl groups available on the polymer matrix provided a good platform for the absorption process (Scheme 2).

Scheme 2 Adsorption of Cu²⁺ by MMA/m-KIT-6 hybrid materials.

3.3.1. The effect of contact time. Fig. 8a and b show that the adsorption rates of Cu(II) ions by the pure KIT-6 and m-KIT-6 increase rapidly until the contact time reaches 15 and 30 min, respectively. Also, the adsorption rates of Cu(II) by the MSNCs increase continuously until the contact time reaches 60, 90 and 90 min for MSNCs 1%, MSNCs 2% and MSNCs 3%, respectively, and leveled off, until the Cu(II) removal attained equilibrium. Beyond these limits there was no considerable increase in the removal percentage, suggesting that all the adsorption sites have been occupied. The fast adsorption of Cu(II) ions at the initial stages by the three types of MSNCs, pure KIT-6 and m-KIT-6 can be interpreted by the availability of uncovered surface and active sites of mesoporous silica and NCs. Within 255 min of the contact time, the maximum adsorptive capacities of three types of MSNCs of 79.14%, 91.20%, and 81.72% were attained. Also, the adsorption of two species of adsorbent, pure and modified KIT-6, was higher than 97% through the optimum time (Fig. 8).

Fig. 8 The effect of contact time on Cu(II) adsorption by pure KIT-6 and m-KIT-6 (a) and MSNCs with different m-KIT (b) (contact time = 255 min, pH = 5.5, agitation speed = 150 rpm, initial Cu(II) concentration = 10 mg L⁻¹, and adsorbent dosage = 10 mg), intra-particle diffusion plot for the removal of Cu(II) by pure KIT-6 and m-KIT-6 (c) and MSNCs with different m-KIT (d).

3.3.2. Adsorption kinetics. The amounts of metal adsorbed per unit mass of adsorbent at equilibrium (q_e, mg g⁻¹) and at any time (q_t, mg g⁻¹) (adsorption capacity) were calculated according to:

q_e = (C_i − C_e)V/m (2)

q_t = (C_i − C_t)V/m (3)

where C_t (mg L⁻¹) is the metal concentrations at the equilibrium and at any time t, V (L) is the volume of the solution, and m (g) is the amount of sorbent. Sorbent performance was frequently evaluated with the distribution coefficient (K_d) (in mL g⁻¹).³⁹ K_d was calculated according to eqn (4) and the results are summarized in Table 2:

K_d = [(C_i − C_e)/C_e] × (V/m) (4)

Table 2 Adsorption quality and distribution coefficient parameters for Cu(II) solution (10 mg L⁻¹, pH = 5.5) on three types of MSNCs

Sorbent	Content solution (mg L^−1)		Removal efficiency%	Cu(II) adsorbed on sorbent		Kd (mL g^−1)
	Initial (Ci)	Final (Ce)		(mg g^−1)	(μmol g^−1)
Pure KIT-6	10	0.19	98.08	9.81	154.37	51.63 × 10^3
m-KIT-6	10	0.10	99.00	9.90	155.80	99.00 × 10^3
MSNCs 1%	10	2.26	77.41	7.74	121.80	3.42 × 10^3
MSNCs 2%	10	0.96	90.34	9.03	142.10	9.42 × 10^3
MSNCs 3%	10	1.83	81.72	8.17	128.57	4.46 × 10^3

---

The values of K_d for pure KIT-6, m-KIT-6 and MSNCs are shown in Fig. 9.

Fig. 9 Distribution coefficient (K_d) of pure KIT-6, m-KIT-6 and MSNCs with different m-KIT.

To investigate the changes in sorption with time and quantifying an appropriate mechanism of adsorption kinetic models, pseudo-second-order, Elovich, and intra-particle diffusion equations are used to interpret the experimental data.^40–42 The pseudo-second-order equation may be represented in a linear form as:

and

α = k_adq_e² (6)

where α is the initial sorption rate (mg g⁻¹ min⁻¹) and k_ad is the pseudo-second-order rate constant of adsorption (g mg⁻¹ min⁻¹). The slope and intercept of the plot of t/q_t versus t are used to calculate k_ad and q_e,cal (Fig. 10).

Fig. 10 Pseudo-second-order kinetics plots for the adsorption of Cu(II).

The Elovich kinetic equation is generally expressed by the following equation:

where β is the desorption constant (mg g⁻¹ min⁻¹).

Intra-particle diffusion describes the movement of species from the bulk of the solution to the solid phase. It is given by Weber and Morris:

where k_intra is the intra-particle diffusion rate constant (mg g⁻¹ min⁻¹), which may be taken as a rate factor, and C is a constant. The intra-particle diffusion plots exhibited a multi-linear correlation which revealed that two or more steps occurred during the adsorption process (Fig. 8c and d and Table 2). The first step can be attributed to the external surface adsorption (intra-particle diffusion), and the second one is the pore diffusion.^43,44

The corresponding three kinetic parameters from these three models are illustrated in Table 3. According to the obtained results, it was found that the pseudo second order equation provides larger correlation coefficient values than the Elovich model when using the pure KIT-6 and m-KIT-6 as sorbents. For MSNCs, the pseudo-second order model represented the better approximation of correlation coefficient values (R₁² > 99%) than those of the Evolich (R₂²) and intra-particle diffusion (R₃²) models for the three types of MSNCs. Also, the calculated q_e values (q_e,cal) from the pseudo-second-order model were close to the experimental q_e values (q_e,exp), suggesting that the chemical adsorption can be well described with the pseudo-second-order kinetic model for pure KIT-6, m-KIT-6 and MSNCs. These data expressed that the adsorption is very fast, probably controlled by chemical adsorption which might involve the valency forces through the sharing or exchange of electrons between Cu(II) ions and adsorbents.^44,45

Table 3 Comparison of the kinetic parameters for the three types of MSNCs in Cu(II) adsorption

Sorbent	qe,exp (mg g^−1)	α (mg g^−1 min^−1)	Pseudo second order			Elovich kinetic		Intra-particle diffusion
			qe,cal (mg g^−1)	kad (g mg^−1 min^−1) × 10^−3	R1^2	β (mg g^−1 min^−1)	R2^2	kintra (mg g^−1 min^−1)	C	R3^2
Pure KIT-6	9.81	9.75	9.99	97.69	0.9993	0.809	0.8108	0.6791	6.3136	0.6139
m-KIT-6	9.90	16.79	9.96	169.25	0.9998	1.372	0.8935	0.4155	7.6994	0.7285
MSNCs 1%	7.74	1.12	8.26	16.38	0.9978	0.886	0.7272	0.2132	5.1630	0.5423
MSNCs 2%	9.03	1.23	9.47	13.73	0.9992	0.894	0.8440	0.2200	6.1779	0.6817
MSNCs 3%	8.17	0.91	8.60	12.31	0.9980	0.812	0.8021	0.2392	5.0329	0.6323

---

3.3.3. Adsorption isotherm. Adsorption behavior and adsorption capacity of Cu(II) ions onto pure Kit-6, m-KIT-6 and MSNCs can be described by an adsorption isotherm (Fig. 11a). Aqueous solutions of Cu(II) with different initial concentrations were prepared for investigating adsorption isotherms at a constant pH and optimum times at room temperature. Among different adsorption isotherm models we selected the Langmuir⁴⁶ and Freundlich⁴⁷ models, which are widely used in experimental works, for describing the adsorption behavior of Cu(II) ions. The Langmuir isotherm model is applied to describe the monolayer adsorption of an adsorbent onto a structurally homogeneous surface with a finite number of identical sites. The adsorption process is completed when these identical sites are occupied by adsorbents. Mathematically, the linearized form of the Langmuir model can be written in the following form:

C_e/q_e = 1/K_Lq_max + C_e/q_max (9)

where C_e is equilibrium concentration of the metal ions (mg L⁻¹) and q_e is the solid phase equilibrium concentration. q_max is the maximum sorption capacity (mg g⁻¹), and K_L is a constant related to the binding energy of the sorption system (L mg⁻¹). K_L and q_max can be obtained from the intercept and slope of the linear plots when C_e/q_e results in a fairly straight line.

Fig. 11 Isotherm equilibrium (a), Langmuir plot for Cu(II) on the pure KIT-6, m-KIT-6 and MSNCs, pH = 5.5 (b), Freundlich plot for Cu(II) on the pure KIT-6, m-KIT-6 and MSNCs, pH = 5.5 (c).

Unlike the Langmuir isotherm, the Freundlich isotherm model is derived to describe the multilayer adsorption of an adsorbate onto a heterogeneous surface of an adsorbent. The linear form of the Freundlich isotherm model can be represented by the logarithmic eqn (10):

log q_e = log K_f + 1/n log C_e (10)

where n and K_f are the Freundlich isotherm constants. K_f (mg g⁻¹) (mg L⁻¹)^1/n and n relate to the adsorption capacity and intensity of a given adsorbent, respectively.

The values of the constants in both models are obtained from the slope and the position (Fig. 11b and c). Table 4 shows the results of the fit and of the constants of both models for three adsorbents including pure KIT-6, m-KIT-6 and MSNCs 2%. The values of n for pure KIT-6, m-KIT-6 and MSNCs 2% were 1.33, 1.29 and 1.95, respectively. The values between 1 and 10 for n in the adsorption process are favorable according to Slejko.⁴⁸ All the correlation coefficients and parameters obtained for the isotherm models from Table 4 reveal that the Langmuir isotherm is the best model to demonstrate the adsorption of Cu(II) onto pure KIT-6, m-KIT-6 and MSNCs.

Table 4 Adsorption equilibrium constant for Langmuir and Freundlich isotherm equations

Sorbents	Langmuir isotherm equation			Freundlich isotherm equation
	qmax (mg g^−1)	KL (L mg^−1)	R^2	Kf (mg g^−1)	1/n	R^2
Pure KIT-6	76.92	0.163	0.9987	9.497	0.7492	0.9860
m-KIT-6	102.04	0.134	0.9989	10.824	0.7774	0.9874
MSNCs 2%	24.45	0.583	0.9990	6.977	0.5134	0.9489

---

4. Conclusions

Novel mesoporous silica nanocomposites (MSNCs) were prepared by in situ polymerization of MMA and modified KIT-6 as a filler. The m-KIT-6 was prepared through the reaction between 3-mercaptopropyl-trimethoxysilane with hydroxyl groups of the surface mesoporous KIT-6. The chemical, structural and textural properties of the MSNCs were characterized by several methods. Due to the good interaction between the polymer matrix and mesoporous KIT-6 as well as the inherently good thermal stability of the mesoporous KIT-6, TGA results show an improvement in the thermal properties of the MSNCs in comparison to that of the neat PMMA. Then, the application of the MSNCs for the adsorption of Cu(II) from an aqueous solution was examined. As a comparison between the three types of MSNCs, it can easily be observed that MSNCs 2% exhibits higher adsorption capacities than MSNC 1% and MSNC 3%. In addition, higher K_d values represent a more effective sorbent for the removal of heavy metals and so MSNC 2% has a more effective adsorption capacity in comparison to those of MSNC 1% and MSNC 3%. Also, it was observed that the time necessary for the MSNCs to reach equilibrium is short (60–90 min) when compared to the other sorbents, which in general conditions only achieve an equilibrium state after several hours of exposure. In addition, the equilibrium data for Cu(II) ions confirm that the Langmuir isotherm is the best model for interpreting the adsorption mechanism according to the R² values of four adsorbents.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was partially funded by the Research Affairs Division of Isfahan University of Technology (IUT) and Iran Nanotechnology Initiative Council (INIC).

References

1. U. Ciesla and F. Schüth, Microporous Mesoporous Mater., 1999, 27, 131–149.
2. C. Liu, F. Li, L.-P. Ma and H.-M. Cheng, Adv. Mater., 2010, 22, E28–E62.
3. M. Vallet-Regí, F. Balas and D. Arcos, Angew. Chem., Int. Ed., 2007, 46, 7548–7558.
4. M. Hartmann, Chem. Mater., 2005, 17, 4577–4593.
5. Z. Wu and D. Zhao, Chem. Commun., 2011, 47, 3332.
6. M. Dinari, G. Mohammadnezhad and A. Nabiyan, Colloid Polym. Sci., 2015, 293, 1569–1575.
7. M. Laghaei, M. Sadeghi, B. Ghalei and M. Dinari, Prog. Org. Coat., 2016, 90, 163–170.
8. H. Zou, S. Wu and J. Shen, Chem. Rev., 2008, 108, 3893–3957.
9. I. A. Rahman and V. Padavettan, J. Nanomater., 2012, 2012, 1–15.
10. J. Wang, Y. Li, Z. Zhang and Z. Hao, J. Mater. Chem. A, 2015, 3, 8650–8658.
11. J. KukáShon, S. SungáKong, J. ManáKim, C. HyunáKo, Y. YunáLee, S. HeeáHwang and J. AháYoon, Chem. Commun., 2009, 650–652.
12. M. Dinari, G. Mohammadnezhad and A. Nabiyan, J. Appl. Polym. Sci., 2016, 133, 43098–43103.
13. K. Soni, B. S. Rana, A. K. Sinha, A. Bhaumik, M. Nandi, M. Kumar and G. M. Dhar, Appl. Catal., B, 2009, 90, 55–63.
14. R. J. Kalbasi and N. Mosaddegh, J. Inorg. Organomet. Polym. Mater., 2012, 22, 404–414.
15. R. Kishor and A. K. Ghoshal, Chem. Eng. J., 2015, 262, 882–890.
16. H. Zhao, S. Liu, R. Wang and T. Zhang, Mater. Lett., 2015, 147, 54–57.
17. F. Li, N. van der Laak, S.-W. Ting and K.-Y. Chan, Electrochim. Acta, 2010, 55, 2817–2823.
18. D. Zhang, L. Zheng, Y. Ma, L. Lei, Q. Li, Y. Li, H. Luo, H. Feng and Y. Hao, ACS Appl. Mater. Interfaces, 2014, 6, 2657–2665.
19. L. Ma'mani, S. Nikzad, H. Kheiri-manjili, S. al-Musawi, M. Saeedi, S. Askarlou, A. Foroumadi and A. Shafiee, Eur. J. Med. Chem., 2014, 83, 646–654.
20. J. Ceballos-Torres, P. Virag, M. Cenariu, S. Prashar, M. Fajardo, E. Fischer-Fodor and S. Gómez-Ruiz, Chem.–Eur. J., 2014, 20, 10811–10828.
21. W. Sun, L. Li, E. A. Stefanescu, M. R. Kessler and N. Bowler, J. Non-Cryst. Solids, 2015, 410, 43–50.
22. J. Hong, S. Lee, J. Seo, S. Pyo, J. Kim and T. Lee, ACS Appl. Mater. Interfaces, 2015, 7, 3554–3561.
23. Z. Fan, F. Gong, S. T. Nguyen and H. M. Duong, Carbon, 2015, 81, 396–404.
24. W. F. Mousa, M. Kobayashi, S. Shinzato, M. Kamimura, M. Neo, S. Yoshihara and T. Nakamura, Biomaterials, 2000, 21, 2137–2146.
25. N. B. Trung, T. V. Tam, D. K. Dang, K. F. Babu, E. J. Kim, J. Kim and W. M. Choi, Chem. Eng. J., 2015, 264, 603–609.
26. V. M. Cuijpers, J. Jaroszewicz, S. Anil, A. Al Farraj Aldosari, X. F. Walboomers and J. A. Jansen, Clin. Oral Implants Res., 2014, 25, 359–365.
27. S. Kango, S. Kalia, A. Celli, J. Njuguna, Y. Habibi and R. Kumar, Prog. Polym. Sci., 2013, 38, 1232–1261.
28. G. Mohammadnezhad, M. Dinari, R. Soltani and Z. Bozorgmehr, Appl. Surf. Sci., 2015, 346, 182–188.
29. J. Jiao, L. Wang, P. Lv, P. Liu and Y. Cai, Mater. Lett., 2013, 109, 158–162.
30. J.-X. Yu, L.-Y. Wang, R.-A. Chi, Y.-F. Zhang, Z.-G. Xu and J. Guo, Appl. Surf. Sci., 2013, 268, 163–170.
31. A. R. Keshtkar, M. Irani and M. A. Moosavian, J. Taiwan Inst. Chem. Eng., 2013, 44, 279–286.
32. S. Egodawatte, A. Datt, E. A. Burns and S. C. Larsen, Langmuir, 2015, 31, 7553–7562.
33. L. Wang, C. Cheng, S. Tapas, J. Lei, M. Matsuoka, J. Zhang and F. Zhang, J. Mater. Chem. A, 2015, 3, 13357–13364.
34. B. Samiey, C.-H. Cheng and J. Wu, Materials, 2014, 7, 673–726.
35. L. Qian, Y. Ren, T. Liu, D. Pan, H. Wang and G. Chen, Chem. Eng. J., 2012, 213, 186–194.
36. Z. Jia, Z. Wang, C. Xu, J. Liang, B. Wei, D. Wu and S. Zhu, Mater. Sci. Eng., A, 1999, 271, 395–400.
37. A. Ramanathan, B. Subramaniam, D. Badloe, U. Hanefeld and R. Maheswari, J. Porous Mater., 2012, 19, 961–968.
38. T. Aman, A. A. Kazi, M. U. Sabri and Q. Bano, Colloids Surf., B, 2008, 63, 116–121.
39. W. Yantasee, C. L. Warner, T. Sangvanich, R. S. Addleman, T. G. Carter, R. J. Wiacek, G. E. Fryxell, C. Timchalk and M. G. Warner, Environ. Sci. Technol., 2007, 41, 5114–5119.
40. B. M. W. P. K. Amarasinghe and R. A. Williams, Chem. Eng. J., 2007, 132, 299–309.
41. S. Chien and W. Clayton, Soil Sci. Soc. Am. J., 1980, 44, 265–268.
42. C. Namasivayam and K. Ranganathan, Water Res., 1995, 29, 1737–1744.
43. T. A. Saleh, Desalin. Water Treat., 2015, 1–15.
44. Q. Yuan, Y. Chi, N. Yu, Y. Zhao, W. Yan, X. Li and B. Dong, Mater. Res. Bull., 2014, 49, 279–284.
45. S. Wang, K. Wang, C. Dai, H. Shi and J. Li, Chem. Eng. J., 2015, 262, 897–903.
46. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361–1403.
47. H. Freundlich, J. Phys. Chem., 1906, 57, e470.
48. F. L. Slejko, Adsorption technology. A step-by-step approach to process evaluation and application, Dekker New York, Basel, 1985.

---