Kinetics and Thermodynamic Study on Novel ModifiedâMesoporous

Article pubs.acs.org/jced

Kinetics and Thermodynamic Study on Novel Modified−Mesoporous Silica MCM-41/Polymer Matrix Nanocomposites: Effective Adsorbents for Trace CrVI Removal Mohammad Dinari,* Roozbeh Soltani, and Gholamhossein Mohammadnezhad* Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran S Supporting Information *

ABSTRACT: It is no longer news that the world is in the midst of a freshwater crisis, with the future health of both humans and the environment being threatened by heavy metal pollution. Accordingly, attempts at removing the toxic heavy metals from water are attracting considerable attention around the world. As a result, for the first time, amine-modified MCM-41/polymer matrix nanocomposites (m-MCM-41/ PMNCs) have been applied to remove hexavalent chromium species (CrVI) from aqueous solution. Poly(methyl methacrylate), poly(vinyl alcohol), nylon-6, and polystyrene, which are environmentally benign and economically accessible polymer matrices, were used for fabricating four different types of m-MCM-41/PMNCs. Owing to the abundance of chemically active functional groups on the surface of these m-MCM-41/PMNCs, excellent adsorption performance was observed. Maximum metal removal was observed at low pH (2−3) and with the efficiency of CrVI removal being in the range of 61.78−85.71%. The kinetic modeling and equilibrium isotherm data were analyzed, and the results showed that various adsorption mechanisms, depending on the type of polymers, may occur. Also, the computed thermodynamic parameters, i.e., ΔG°, ΔH°, and ΔS°, suggested that the adsorption of CrVI onto the m-MCM-41/PMNCs is an endothermic, spontaneous, and physical adsorption process.

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INTRODUCTION With increasing global freshwater scarcity due to population growth, rapid water pollution, and human socioeconomic development, it is no surprise that policy makers and water managers all around the world are beginning to pay greater attention to this issue. The past 3 decades have seen an increase in awareness over water demand management as an environmental policy.1,2 Along this line, industrial release of heavy metal ions into the environment has come to be identified as one of the major sources of water pollution. This is regarded as being particularly troublesome and dangerous, as these metal ions often become concentrated throughout the food chain, a consequence of their nonbiodegradability and while being prone to bioaccumulation. Among the major toxic metal ion pollutants, CrVI (i.e., in its forms HCrO4−, CrO42− and Cr2O72−) is considered as being one of the more hazardous due to its mobility and extreme solubility in industrial effluents. This poses severe health challenges as it is known to be potentially mutagenic, carcinogenic, and teratogenic toward man as well as animals.3−5 Discharge from industrial processes such as steel production, cement industries, leather tanning, electroplating, photographic materials, nuclear power plant coolant water, and the manufacture of dyes, etc., have been implicated as the major sources of CrVI found in ground surface water.6,7 To this end, © 2017 American Chemical Society

the federal drinking water standard recommended by the Environmental Protection Agency (EPA), USA is a maximum allowable level of 100 ppb for total chromium, below which no adverse health effects are likely to occur.8 However, chromium is an essential mineral required by the human body (a daily dose of 0.1−0.3 mg L−1) and can be found in drinks and various foods.9,10 The biochemical behavior of its species hugely depends on environmental pH and its oxidation state. For example, while hexavalent chromium is extremely toxic to mankind, animals, and plants, trivalent chromium (CrIII) is an essential element in animal and plant metabolism.4,11,12 In the environment, chromium exists in a variety of oxidation states ranging from +2 to +6, with the +5, + 4, and +1 states being unstable. However, due to its redox ability, chromium commonly occurs in two oxidation states, + 3 and +6. CrII is unstable as it is easily oxidized to CrIII in air. The relationship between CrIII and CrVI is demonstrated by the following equation: Cr2O7 2 − + 14H+ + 6e− → 2Cr III + 7H 2O

1.33 eV (1)

Received: February 21, 2017 Accepted: June 12, 2017 Published: June 26, 2017 2316

DOI: 10.1021/acs.jced.7b00197 J. Chem. Eng. Data 2017, 62, 2316−2329

Journal of Chemical & Engineering Data

Article

Equation 1 reflects that CrVI has a strong oxidation potential of 1.33 eV. As a consequence of its stronger oxidizing power and higher membrane penetration, hexavalent chromium in its soluble form, is spontaneously reduced within living organisms whereas oxidation of trivalent chromium never occurs under these conditions. For instance, in the cardiovascular system, the hexavalent state undergoes swift reduction to its trivalent state. Therefore, once CrVI has easily passed through the membrane of the blood cell, it is reduced and the resulting CrIII becomes linked to hemoglobin making it incapable of leaving the erythrocyte.13,14 It is thus safe to state that only the hexavalent and trivalent forms of chromium are critical for mankind and animal health owing to the large differences in their biological and chemical properties. Because of the high toxicity of heavy metals, especially CrVI, in effluents, considerable efforts are correspondingly being made to advance new remediation strategies for treatment of these toxic metal ions. Various treatment strategies have been adopted to remove heavy metals, some of which include chemical chelation precipitation, membrane filtration, electrochemical methods, ion exchange, and adsorption.15,16 Of the aforementioned methodologies, the adsorption method stands out as one of the most widely used conventional methods for economic and effective water treatment.17 It is in fact considered to be one of the attractive and advantageous approaches in remediation processes among other methods. This is due to the availability of various adsorbents, flexibility in their design and operation, and the simplicity, the high efficiency, and the reversibility when coupled with the desorption process.18−20 In addition, it offers the advantage of being a less expensive procedure while preventing formation of sludge when compared with other methods. As an important family of adsorbents, polymer based nanocomposites (NCs) and inorganic filler are attracting increasing attention because of their considerable physicochemical properties. Some of the properties which make them a marvelous choice include their relatively high external surface areas, chemically modifiable surfaces, low toxicities, high adsorption kinetics, and easy handling in comparison with powder-type adsorbents.21,22 It should also be noted that most polymer matrices applied are cheap and readily available. Recently, utilization of mesoporous silica materials as nanofillers has been on the receiving end of increased interest in the area of polymer science technology. One of these is MCM-41, a well-known mesoporous silica which possesses the significant advantages of chemical stability and large and easily accessible pores coupled with an extremely high surface area full of hydroxyl active groups within its framework.23,24 With its highly regular hexagonal arrays of uniform-sized channels, it is more commonly known and has been employed as a molecular sieve while belonging to the M41S family.24 By virtue of its unique and functionalizable surface, it has found application in the fields of drug delivery and cancer therapy, separation technology, catalyst, polymerization science, and adsorption.23−27 Poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA), nylon-6 (N6), and polystyrene (PS) are the most famous and common polymers. They are known to possess chemically active functional groups such as CO, O−H, N−H, phenyl, and -OCH3, which are capable of establishing π bonding as well as hydrogen bonding. Effective and sustainable interactions between the abundant surface hydroxyl groups of mesoporous silica and these chemically active

functional groups are therefore expected to lead to the formation of an NC with improved physicochemical properties.28−31 Also, it has been proven that by adding inorganic fillers such as mesoporous silica materials to the polymer matrix, certain deficiencies found in polymers such as their brittle texture, low thermal, and mechanical stability, etc., can be ameliorated.29 Often, the introduction of silane coupling agents such as (3-aminopropyl)trimethoxysilane into a mesoporous silica framework plays the important role of forming more strong interactions between the polymer matrix and the filler. As a result, polymer matrix nanocomposites (PMNCs) with chemically active surface sites on both polymer and mesoporous silica filler are being considered for use in heavy metal adsorption. This is due to their possession of effective electrostatic interactions as well as excellent chelating properties. We have previously prepared mesoporous KIT-6/PMMA and boehmite/ PMMA NCs via an in situ polymerization approach for utilization in CuII removal. PMMA as a polymer matrix has been shown to display acceptable adsorption characteristics when incorporated within polymer NCs.30,31 To the best of our knowledge, just a few studies have been performed on the preparation of m-MCM-41/PMNCs with PMMA, PVA, N6, and PS as polymer matrices.32−35 Li et al. and Cao et al.,36,37 prepared amine-modified MCM-41 (m-MCM-41) for CrVI removal from aqueous solution via adsorption mechanism, but no study has been carried out until now on the removal of heavy metal ions by m-MCM-41/ PMNCs. This present study is focused on preparing an aminemodified MCM-41/polymer matrix NC, rich in surface active functional groups, which are capable of extracting hexavalent chromium species. It is supposed that, using m-MCM-41/ PMNCs with previously mentioned polymer matrices as cheap adsorbents, they may offer viable and economical alternatives with none of the side problems resulting from sludge formation. The effect of the initial pH of the solution on the adsorption behavior of CrVI was evaluated. Furthermore, various equilibrium isotherm models as well as adsorption kinetic models were studied, and possible mechanisms of adsorption were also investigated. Moreover, thermodynamic parameters such as free energy (ΔG°), entropy (ΔS°), and enthalpy (ΔH°) were investigated to fit the CrVI adsorption data. The morphological, textural, and surface features of m-MCM-41/PMNCs were monitored using Fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) analyses.

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MATERIALS AND METHODS Chemicals. All chemicals in this study were of analytical grade. Toluene (99%), formic acid (99%), potassium dichromate (K2Cr2O7, 99%), PMMA (Mw = 350,000 g mol−1), general-purpose polystyrene (PS) pellet, with Mw = 150,000 g mol−1, PVA (99% hydrolysis, weight-average molecular weight = 72,000 g mol−1), (3-aminopropyl)trimethoxysilane, HCl, and NaOH were commercially obtained from Merck Chemical Co. Commercial grade UBE nylon P1011F (Nylon-6; commercial name, Polyamide 6) was procured from UBE America (USA). The density of Nylon-6 is 1.09−1.19 g cm−3, while the melting point is around 498 K. Methods. First, MCM-41 mesoporous silica was synthesized according to previous reports.24,27 In a typical syntheses procedure, 5.22 g of C16TAB as the template and 30 mL of 10% TEOS as the silica source in 12.25 mL of (TMA)OH were 2317



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Figure 1. Schematic illustration of the formation of the m-MCM-41/PMNCs: (step 1) modification of pristine MCM-41 with silane coupling agent to form m-MCM-41; (step 2) preparation of m-MCM-41/polymer nanocomposites.

addition of the m-MCM-41 solutions to the related polymer matrix solution followed by refluxing for 6 h. In all cases, excellent dispersion of the m-MCM-41 in solvent−polymer matrix was observed after which sonication was carried out for a further 2 h. The solutions were then stirred at 298 K under ambient pressure to promote the slow evaporation of the solvents until they became highly viscous. Finally, these were then poured out onto clean glass, after which the rest of the solvents were allowed to evaporate under a constant flow of nitrogen gas. Further treatment under suitable heat (no greater than 333 K) and vacuum led to the formation of dry and opaque samples of the three NCs consisting of m-MCM-41 and the mentioned polymers (PMMA, N6, and PS, respectively). To synthesize m-MCM/PVA NCs; 0.04 g of m-MCM-41 was dispersed in DDW while being stirred followed by ultrasonication for 1 h. Simultaneously, the PVA solution was prepared by dissolving 2.0 g of commercial PVA in 30 mL of DDW at 363 K under continuous stirring for at least 30 min. The two solutions were then combined, and the mixture stirred for 5 h at 353 K with subsequent ultrasonication for 30 min. The resulting clean homogeneous solution was poured onto a glass plate and allowed to dry in an oven at 298 K for 24 h. Characterization. Scanning electron micrographs of samples were recorded using a HITACHI, S-4160 scanning electron microscope. FT-IR spectra were recorded at room temperature with a Jasco-680 (Japan) spectrometer in transmission mode, using KBr pellets. Measurements were taken by performing 60 scans at a resolution of 4 cm−1 from 400 to 4000 cm−1. The sonochemical reactions were performed on a MISONIX ultrasonic liquid processor, XL-2000 SERIES (Raleigh, NC, USA) with an ultrasound power of 100 W and frequency of 22.5 kHz. Solution-phase analysis of CrVI ions was performed using a flame atomic absorption spectrophotometer (FAAS; PerkinElmer 2380-Waltham) equipped with a CrVI hollow cathode lamp and an air−acetylene burner. Adsorption Experiment. Stock solution of Cr VI (1000 mg L−1) was prepared by dissolving K2Cr2O7 in DDW.

dissolved in water at room temperature to form a supersaturated solution and stirred until hydrolysis of the TEOS was complete. The obtained gel was subsequently transferred into a flask and heated at 363 K for 24 h under hydrothermal conditions. The resultant white precipitate was then filtered, washed with double distilled water (DDW), and dried at 333 K for 12 h. This as-synthesized powder was finally calcined at 900 K for 5 h. The amine-modified MCM-41 (m-MCM-41) was fabricated by the following method. MCM-41 was dried at 423 K for 5 h to remove the absorbed water. A 0.4 g amount of this compound was then dispersed in ethanol (20 mL) by sonicating for 45 min at room temperature. This was then followed by injecting (3-aminopropyl) trimethoxysilane (50 μL) to the MCM-41-ethanol suspension. The suspension was refluxed for 24 h and then subjected to ultrasonication for 2 h. Finally, the suspension was centrifuged, filtered, and washed with ethanol to remove any unreacted silane coupling agent, followed by drying overnight in an oven (343 K) to obtain m-MCM-41. The m-MCM-41/PMNCs were fabricated by a mixture of polymer matrix solution and suspension of the mesoporous silica in the same solvents. A scheme of the synthetic procedure used in preparation of m-MCM-41/PMNCs is represented in Figure 1. Four types of m-MCM-41/PMNCs including m-MCM-41/nylon-6 (m-MCM/N6), m-MCM-41/poly(vinyl alcohol) (m-MCM/PVA), m-MCM-41/poly(methyl methacrylate) (m-MCM/PMMA), and m-MCM-41/polystyrene (m-MCM/PS) were easily fabricated using a solution method by treating 2 wt % m-MCM-41 and polymer matrix according to our previously reported procedure.38 For fabrication of m-MCM/PMMA, m-MCM/N6, and m-MCM/PS, 2.0 g of the polymers were added to 10 mL of toluene, formic acid, and toluene under a nitrogen atmosphere, respectively, followed by stirring at 323 K until the solutions became clear (after about 3 h). Solutions of m-MCM-41 were then made by adding 2 wt % of it into the same solvent mixture and sonicating for 2 h. This step was succeeded by the dropwise 2318



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Figure 2. FT-IR spectra of (A1) pure MCM-41, (A2) m-MCM-41, (B1) N6, (B2) m-MCM/N6, (C1) PVA, (C2) m-MCM/PVA, (D1) PMMA, (D2) m-MCM/PMMA, (E1) PS, and (E2) m-MCM/PS NCs.

of the metal ions in the aqueous phase (mg L−1), respectively. The adsorption capacity of m-MCM-41/PMNCs was calculated using the following relation: Q = (C0 − Ce)V/W, where Q is the amount of metal ions adsorbed onto unit mass of the m-MCM-41/PMNCs (mg g−1); V, the volume of the aqueous phase (L), and W, the dry weight of the adsorbent (g). Adsorption kinetics experiments were conducted at room temperature and at optimum pH by adding 10 mg of adsorbent into 10 mL of 10 mg L−1 CrVI solution in polyethylene test tubes. After shaking (180 rpm) at room temperature, 5 mL of supernatant was taken out, at specific time intervals, and then centrifuged (6000 rpm) and filtered, followed by determination of the residual CrVI concentration also by FAAS. The distribution coefficient (Kd (mL g−1)), an additional vital parameter,

The stock solution was then further diluted with DDW to obtain the working solutions of desired concentrations. Prior to adding the adsorbents, pHs of the solutions were adjusted to values between 1 and 8 using 0.01 M NaOH/0.01 M HCl. For the studies of the adsorption isotherms, 10 mL portions of the CrVI solution with initial concentrations ranging between 1.5 and 40 mg L−1 were placed into a polyethylene test tube. The adsorbents (0.01 g) were added and shaken for 4 h (180 rpm) at room temperature. Then, the adsorbents were separated using centrifugation (6000 rpm) after which the supernatants were then removed using a Whatman filter. The CrVI concentration was analyzed by using a FAAS. The uptake is calculated using the equation of 100(C0 − Ce)/C0, where C0 and Ce are the initial and equilibrium concentrations 2319



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is defined by the equation Kd = (V[(C0 − Ce)/Ce])/W. After 4 h contact time, there was no longer any change in the concentration of CrVI ions in solution suggesting that the saturation limit of the matrix had been attained. All four cases reveal good CrVI removal efficiency, with 85.7, 76.4, 74.6, and 61.8% uptake achieved for m-MCM/PMMA, m-MCM/PVA, m-MCM/N6, and m-MCM/PS, respectively. All reported data are obtained from the arithmetic mean of three separate determinations, and the relative errors of the data related to the calculations of the atomic concentrations were all within ±5%.

Spectra D1 and D2 of Figure 2 show the FT-IR spectra of PMMA and m-MCM/PMMA NC. In the spectra of PMMA (Figure 2D1), the bands at 1732 and 1249 cm−1 are attributed to the vibrations of the CO and C−O groups respectively, belonging to the ester. The bands at 2995 and 2950 cm−1 are due to the stretching vibration of C−H. Two bands at 1486 and 1450 cm−1 originate from the O−CH3 bending vibrations while the strong band at 1148 cm−1 is associated with the C−O stretching mode. In the FT-IR spectrum of m-MCM/PMMA, new absorption bands are seen in the range of 400−8000 cm−1, confirming the presence of m-MCM-41 in the hybrid NC (Figure 2D2). In the FT-IR spectrum of PS (Figure 2E1), the bands at 3000−3100 cm−1 represent the aromatic C−H stretching vibrations while the symmetric and asymmetric stretching vibrations of CH2 are seen at 2920 and 2849 cm−1, respectively. Bands at 1600, 1580, and 1491 cm−1 can be assigned to stretching vibrations of the benzene ring whereas the bands at 753 and 697 cm−1 are attributed to the C−H out-of-plane bending vibration of the benzene ring. In the FT-IR spectrum of m-MCM/PS (Figure 2E2), the resulting hybrid materials not only exhibit characteristic neat PS bands but also have the distinctive peaks for modified MCM-41. These data altogether confirm the formation of the m-MCM-41/PMNCs. The XRD patterns for MCM-41, m-MCM-41, m-MCM/N6, and m-MCM/PMMA NCs are presented in Figure 3. Three

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RESULTS AND DISCUSSION Characterization of m-MCM/41 and m-MCM-41/ PMNCs. Figure 2 shows the FT-IR spectra of the pure MCM-41 and m-MCM-41. For pure MCM-41 (Figure 2A1), the broad band around 3500 cm−1 may be attributed to the surface silanols and adsorbed water molecules. The bands observed at 1235 and 1073 cm−1 are characteristics of the asymmetric Si−O−Si vibration. Another characteristic band is the symmetric Si−O−Si stretching observed at 785 cm−1, while the band at 456 cm−1 can be assigned to the of Si−O−Si bending mode. For m-MCM-41 (Figure 2A1), a new band could be seen around 2963 cm−1 which is typical of the CH stretch of methylene groups. The bands at 851 and 760 cm−1 are attributed to the Si−C stretching vibrations which are absent in the spectrum of pure MCM-41. Also, there is a broad peak at the 3100−3500 cm−1 regions attributed to the hydroxyl and amine groups present on the surface of MCM-41 as well as the silane coupling groups. This is suggestive of the successful grafting of the silane coupling groups onto the surface of MCM-41 which can in turn increase its hydrophobicity thereby facilitating the threading of the organic polymer into its channel. The FT-IR spectra of nylon-6 and m-MCM/N6 NCs are shown in Figure 2B1,B2. For the neat polymer (Figure 2B1), the characteristic bands seen at 3300 and 3070 cm−1 are due to the asymmetric and symmetric stretching vibrations of the N−H bond. The bands at 2877 and 2927 cm−1 are attributed to the C−H stretching vibrations of the nylon-6 chains. The stretching vibrations of the carbonyl groups (CO) of the amide can be seen at 1642 cm−1, whereas the bending vibration of the N−H are observed at 1545 cm−1. For m-MCM/N6, the positions of characteristic bands of the neat polymer are changed and new bands corresponding to the m-MCM-41 were seen as shown in Figure 2B2. The broadening of the amide band is thought to be due to the hydrogen bonding between the amide group of the polymer and hydroxyl and amine group on the m-MCM-41 in the resulting NC. In the FT-IR spectra of PVA displayed in Figure 2C1, a broad absorption bands is seen around 3100−3500 cm−1 which is attributed to the presence of hydroxyl groups of the PVA as well as those of adsorbed water. The CH and CH2 groups likewise show stretching vibrations at 2942 cm−1 with the peak at 1420 cm−1 being due to the vibration of the CH2 bonds in PVA molecules. The band observed at 1713 cm−1 can be assigned to the carbonyl functional groups of the residual acetate groups (1−2%) which may result from the formation of PVA via hydrolysis of poly(vinyl acetate) (Figure 2C1). The incorporation of m-MCM-41 in PVA led to slight changes in the intensities of the absorption bands as well as the appearance of new absorption bands in the range of 400−800 cm−1 attributed to the Si−C stretching modes as shown in Figure 2C2.

Figure 3. XRD patterns of the (a) pure MCM-41, (b) m-MCM-41, (c) m-MCM/N6, and (d) m-MCM/PMMA NCs.

diffraction peaks are seen in the XRD pattern of MCM-41 which could be indexed as (100), (110), and (200) reflections, respectively, and these are characteristic of the long-range ordered hexagonal MCM-41 mesoporous phase. For m-MCM-41, the diffraction peaks show gradual decrease in their intensities upon modification, indicating that the introduction of the organic groups had an effect on the pore structure (Figure 3b). From the XRD patterns of m-MCM/N6 and m-MCM/PMMA NCs, the peaks at higher angles cannot be observed, suggestive of a slight change in the ordered structure of m-MCM-41 (Figure 3c,d). Figure 4 displays FE-SEM images of the MCM-41 and m-MCM-41 nanoparticles. According to the FE-SEM images, MCM-41 nanoparticles are spherical (Figure 4a). Although the particles sizes are mainly in the range of 50−80 nm, slightly larger particles can also be observed (around 90 nm). After modification, the m-MCM-41 nanoparticles retain their 2320



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Figure 4. FE-SEM images of (a) pure MCM-41 and (b) m-MCM-41.

shapes despite the decrease in the sizes of the particles (Figure 4b). The FE-SEM images of the m-MCM/N6, m-MCM/PVA, m-MCM/PMMA, and m-MCM/PS are presented in Figure 5.

Figure 6. FE-SEM images and EDX spectrum of the m-MCM-41/N6 NC after adsorption process.

Figure 5. FE-SEM images of (a) m-MCM/N6, (b) m-MCM/PVA, (c) m-MCM/PMMA, and (d) m-MCM/PS NCs.

The surface morphologies of the m-MCM/N6 and m-MCM/ PVA show the difference from those of m-MCM/PMMA and m-MCM/PS. In the case of N6 and PVA, the obtained NCs have porous structures which could be due to the possible hydrogen bonding interaction resulting from the high proliferation of NH and -OH groups in their structures. For m-MCM/ PMMA, MCM-41 nanoparticles are both well and evenly distributed and are visible in the polymer matrix (Figure 5c). But, in the case of m-MCM/PS, some aggregation can also be observed, stemming from the weak interactions between the filler and polymer matrix due to the absence of polar functional groups (Figure 5d). The surface morphology and EDX spectrum of the m-MCM-41/N6 NC after the adsorption process are presented in Figure 6. The EDX spectrum clearly confirms the presence of Cr in the NC. The TEM technique was used to probe the porosity as well as internal pore structure of the nanoparticles. Figure 7 portrays

Figure 7. TEM images of (a, b) pure m-MCM and (c, d) m-MCM/ N6 NCs.

the TEM images of the pristine MCM-41 nanoparticles and their modified forms. From these images, the porous structure with regular hexagonal channels of the synthesized MCM nanoparticles can be clearly perceived. This distinct partitioning of ordered and nonordered areas is evident throughout the TEM study of this sample. Also, the TEM images of m-MCM/ N6 with different magnifications are presented in Figure 7c,d. The TEM images show that the m-MCM particles are relatively well dispersed in the hybrids. Furthermore, an indistinct edge and a less disordered mesoporous structure are seen which are due to the chain entanglement of polymer on the surface of the m-MCM-41 while some aggregation could also be observed in the TEM images of this compound. 2321



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Table 1. Kinetic, Isotherm, and Thermodynamic Data and Related Reletive Standard Deviations (RSDs) for m-MCM/PMMA kinetic

isotherm

thermodynamic

time/min Ce/(mg L−1) % removal Q/(mg L−1) RSD C0/(mg L−1) Ce/(mg L−1) % removal Q/(mg L−1) RSD T/K ln Kd RSD

20 6.29 37.14 3.71 9.62 1.50 0.10 93.33 1.40 3.57 298 8.70 1.36

40 4.32 56.79 5.68 9.43 5.00 0.50 90.00 4.5 4.44 308 8.88 1.21

60 3.07 69.29 6.93 5.15 10.00 1.43 85.70 8.57 7.00 318 9.10 0.99

80 2.18 78.21 7.82 4.57 18.00 4.00 77.78 14.00 5.36

100 1.82 81.79 8.18 2.18 25.00 7.30 70.80 17.7 3.95

120 1.64 83.57 8.36 4.27 32.00 12.50 60.94 19.50 2.56

140 1.43 85.71 8.57 2.50 40.00 20.00 50.00 20.00 2.50

160 1.40 85.95 8.60 5.84

180 1.43 85.71 8.57 5.83

200 1.45 85.48 8.55 4.21

220 1.43 85.71 8.57 2.50

240 1.40 85.95 8.60 5.84

Figure 8. Equilibrium isotherm and various isotherm modes for the removal of CrVI by m-MCM-41/PMNCs at 298 K and optimum pH with adsorbent dosage of 0.01 g: (a) equilibrium isotherm, (b) Langmuir model, (c) values of RL, (d) Freundlich model, (e) D−R model, and (f) Temkin model.

Adsorption Behavior of m-MCM-41/PMNCs for CrIV. Adsorption Isotherms. To obtain valuable insight into the surface features while exploring the dominant adsorption behavior, the linear form of four isotherm models were applied. The first of these is the Freundlich which assumes that nonideal and reversible adsorption occurs on multilayer heterogeneous surfaces with a nonuniform distribution of the heat of adsorption in addition to the occurrence of interaction between the adsorbed species. Langmuir was the second model and assumes that the maximum adsorption corresponds to a saturated monolayer of a finite number of definite localized sites on the adsorbent surface without any interaction between adsorbed molecules. Furthermore, the energy of adsorption is assumed to be independent of the adsorbed quantity, with homogeneous adsorption occurring with no migration of adsorbate species on the uniform surface. The third model used is the Dubinin− Redushkevich (D−R) which assumes that physical or chemical adsorption of molecules on porous solids occurs on both homogeneous and heterogeneous surfaces. The Temkin model

which was the fourth assumes that the heat of adsorption of the heterogeneous system decreases linearly rather than logarithmically with increasing adsorption quantity due to the adsorbent−adsorbate interactions and was suggestive of uniform distribution of the adsorption binding energy39−41 Equilibrium data and related relative standard deviations (RSD) for m-MCM/PMMA and the other PMNCs are presented in Table 1 and Supporting Information Table S1, respectively. The adsorption isotherms of CrVI on m-MCM-41/PMNCs are depicted in Figure 8 and Table 2 wherein the adsorption capacities under initial CrVI concentration ranging from 1.5 to 40 mg L−1 corresponding to residual concentrations of 0.1−25 mg L−1 were analyzed. Previous literature reports (Table 2) had delved into the utilization of MCM-41 and modified MCM-41 for CrVI adsorption,36,37,42−46 but to the best of our knowledge, this work is the first attempt to investigate the CrVI adsorption capability of MCM-41/PMNCs. When compared with other types of adsorbent such as MCM-41 or the modified MCM-41 as presented in Table 2, the m-MCM-41/PMNCs 2322



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Table 2. Comparison of Adsorption Capacities of CrVI Obtained by Different MCM-41 and Modified MCM-41 Adsorbents exptl conditions adsorbents

Qm/(mg g−1)

pH

T/K

t/min

analytical technique

NH2-MCM-41 TiO2-MCM-41 magMCM-41 N-MCM-41 MCM-41 NH2-MCM-41 AP-MCM-41 m-MCM/PMMA m-MCM/PVA m-MCM/N6 m-MCM/PS

7.52 38.48 105 52.9 10 20 40 20 18.5 17.9 15

3.5 5.6 2.0 3−4 8.5 2.0 3.0 2.0 2.0 2.0 3.0

313 323 298 298 298 298 298 298 298 298 298

120 80 10 5 1440 240 120 140 180 180 200

UV UV/vis ICP-AES ICP UV/vis/near-IR FAAS UV−vis FAAS FAAS FAAS FAAS

ref 32 40 39 41 38 31 37 this this this this

work work work work

Table 3. Adsorption Isotherm Parameters for Adsorption of CrVI onto m-MCM-41/PMNCs at 298 K and Optimum pHa models

param

m-MCM/PMMA

m-MCM/PVA

m-MCM/N6

m-MCM/PS

Qm/(mg g−1)

22.0264

21.5517

20.5338

17.0648

KL/(L mg−1) R2

0.5206 0.9983

0.2409 0.9945

0.2382 0.9904

0.1894 0.9677

KF/(mg g−1)

5.7796

3.8264

3.7128

3.0283

n R2

1.9635 0.9581

1.7544 0.9773

1.8073 0.9809

1.9662 0.9982

Qm/(mg g−1)

18.9452

15.9037

15.0869

11.3815

−BD × 10−8/(mol2 kJ−2) ED/(kJ mol−1) R2

9.2655 2.3230 0.9793

12.132 2.0301 0.9523

11.8484 2.0542 0.9523

10.0590 2.2295 0.8954

KT/(L g−1)

9.4778

4.3872

4.4883

4.3356

bT/(kJ mol−1) BT R2

0.6362 3.8944 0.9703

0.6435 3.8502 0.9629

0.6839 3.6223 0.9622

0.8754 2.8301 0.9222

Langmuir

Ce C 1 = e + Q Qm Q mKL

Freundlich

log Q =

1 log Ce + log KF n

Dubinin−Radushkevich (D-R)

log Q = log Q m

⎛ 1⎞ − 2BDR T log⎜1 + ⎟ Ce ⎠ ⎝ 2 2

Temkin

Q=

RT RT ln K T + ln Ce bT bT

a In the Langmuir model, Qm, Ce, and KL represent the maximum adsorption capacity of m-MCM-41/PMNCs, equilibrium concentration of adsorbate, and Langmuir constant related to the adsorption capacity, respectively. KF is the Freundlich constant related to the rate of adsorption, and n means an empirical parameter relating the favorability of the sorption process in Freundlich model. BD, R, and T indicate a constant related to adsorption energy, gas constant (kJ mol−1 K−1), and temperature (K), respectively. In the Temkin model, bT and KT represent Temkin constants related to heat of adsorption and isotherm equilibrium binding, respectively; also, BT is the Temkin parameter related to the heat of adsorption.

clearly exhibit promising CrVI adsorption capacity. The Qm values for the m-MCM-41/PMNCs are seen to be higher than about half of those reported for other adsorbents. And while the equilibrium times might seem slightly poorer compared with other adsorbent types, they all still performed well despite operating under milder conditions. By implication, it can be said that these developed NCs can adequately function as effective adsorbents for CrVI removal from aqueous media.

More importantly, the polymer matrices used for preparation of m-MCM-41/PMNCs are less expensive while being more readily available than the pristine MCM-41 and modified MCM-41 adsorbents. The content of m-MCM-41 in the as-prepared adsorbents is just 2%. Also, the as-prepared m-MCM-41/PMNCs occur in their plastic forms and are thus able to tolerate variable conditions. A further advantage the NCs have over powder adsorbents is the absence of any problems 2323



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Figure 9. (a) Time profile for the adsorption of CrVI onto m-MCM-41/PMNCs. (b) Effect of solution pH values on CrVI removal percentage by m-MCM-41/PMNC. Adsorption assays were carried out for 3 h using 10 mL of 10 mg L−1 CrVI solution and 0.01 g of adsorbents.

Temkin constant bT related to the heat of adsorption (BT = RT/bT shown in Table 3 are in good agreement with the concept of physisorption phenomenon. It has been found that bT values lower than 20 kJ mol−1 are characteristic of electrostatic interactions, i.e., the physisorption.48,50 The values of bT were found to be 0.6362, 0.6435, 0.6839, and 0.8754 kJ mol−1 for m-MCM/PMMA, m-MCM/PVA, m-MCM/N6, and m-MCM/PS, respectively, also supporting a mechanism by physisorption. Adsorption Kinetics. To obtain an idea as to the nature of the kinetics of the adsorption process, the mass transfer and adsorbate removal rate in a continuous process were investigated. This in turn can provide valuable insight into the factors which control the mechanism of adsorption at the solid−liquid phase interface. Hence, for the purpose of interpretation and quantifying the changes in uptake with time and the mechanism of CrVI removal by the prepared m-MCM-41/PMNCs, various kinetic models were investigated and compared in this study: the pseudo-first-order kinetic model, which is the most reliable and suitable only for the initial fast response; the pseudo-secondorder model, which assumes that the adsorption rate depends on the number of active sites on the adsorbent surface as a ratecontrolling step; the Elovich model, which assumes that activated sorption is dominant on heterogeneous surfaces and that the adsorption rate will decrease over time, due to the increment in adsorbent surface coverage; and intraparticle diffusion, which assumes that internal diffusion of adsorbate into the surface of the adsorbent is related to the mass transfer resistance and determines the adsorption rate of the liquid system. These four models were investigated and compared.39,51 Kinetic data and related relative standard deviations (RSDs) for m-MCM/PMMA and the other PMNCs are presented in Table 1 and Table S1, respectively. Figure 9a displays the capacity of CrVI adsorption into m-MCM-41/PMNCs as a function of their equilibrium times. The equilibrium times of CrVI adsorbed on m-MCM/ PMMA, m-MCM/PVA, m-MCM/N6, and m-MCM/PS are all within 140, 180, 160, and 180 min, respectively. Also, the corresponding graphical interpretation of the kinetic data for each system after linear fitting is depicted in Figure 10, while the calculated results after linear fitting are presented in Table 4. To evaluate the conformity of the models with experimental results, the correlation (R2, close or equal to 1) values were introduced with higher R2 values indicating a more applicable model to the kinetics of CrVI adsorption.

resulting from sludge formation typically associated with the latter. Table 3 includes all corresponding experimental parameters in four-type isotherm models. By comparing the linear regression coefficients (R2), it was found that the Langmuir model employed to describe the homogeneous system with monolayer coverage fitted well for three m-MCM-41/PMNCs, namely, m-MCM/PMMA, m-MCM/PVA, and m-MCM/N6. This may be attributed to the homogeneous distribution of equivalent and identical active sites on their composites and mesopores. Moreover, no significant interactions seemed to interfere with the adsorbed species on adsorbent. The R2 values for m-MCM/ PMMA, m-MCM/PVA, and m-MCM/N6 were 0.9983, 0.9945, and 0.9904, respectively. For m-MCM/PS, the Freundlich model represented the better fit of experimental data (R2 > 99) compared with the other isotherm models, thus implying the occurrence of multilayer adsorption onto the heterogeneous surface. The favorability of the adsorption can be confirmed by the parameters RL, ED, BT and n,47−50 which represent the separation factor, mean free energy of adsorption, Temkin parameter related to the heat of adsorption, and Freundlich adsorption intensity constant, respectively. The dimensionless RL parameter is defined by the equation of RL = 1/[1 + (KLC)], which may be introduced into the Langmuir model to give a measure of the a reliability of the adsorption. Values of the RL express the type of isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (1 > RL > 0), or irreversible (RL = 0).48,49 For the CrVI adsorption onto m-MCM-41/PMNCs, the values of RL (Figure 8c) are seen to vary between 0 and 1, depending on the initial concentrations, strongly suggestive of favorable adsorption. The n and KF parameters, are also greater than unity, thus corroborating the occurrence of favorable adsorption.31,47 The mean free energy of adsorption, ED (kJ mol−1), can also be calculated from the D-R isotherm constant BD using the equation E D = (1/ −2BD ). The magnitude of ED provides important information about both the physical and chemical natures of the adsorption process. For ED < 8 kJmol−1 physisorption tends to be the dominant adsorption process, whereas for values in the range ED = 8−16 kJ mol−1, the predominant process is usually chemisorption.49,50 The ED values derived from the D-R model are 2.32, 2.03, 2.05, and 2.23 kJ mol−1 for m-MCM/PMMA, m-MCM/PVA, m-MCM-N6, and m-MCM/PS, respectively, confirming that physisorption was indeed the adsorption process. Also, the values of the 2324



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Figure 10. Kinetic adsorption models: (a) pseudo-first-order kinetic, (b) pseudo-second-order kinetic, (c) Elovich, and (d) intraparticle diffusion models. The dosage of adsorbents was 0.01 g into 10 mL of 10 mg L−1 CrVI at 298 K and optimum pH.

changes the solution chemistry of both the adsorbate and the chemical properties of the adsorbent. In aqueous media, chromium can be found both as its trivalent cation CrIII and CrVI oxyanion forms. At low pH conditions, the most significant forms of CrVI are Cr2O72−, HCrO4−, CrO42− and Cr3O102−. Among these oxyanions, HCrO4− is the predominant anionic form at pH 2−5, but as the pH increases, it transforms to the other forms such as to forms of CrO42− and Cr2O72−.53−55 At pH greater than 5.5, the dominant species becomes CrO42− in aqueous phase.56 Consequently, CrVI ions removal is largely pH-dependent with its adsorption being favored under acidic conditions, which was similarly observed in this work. The effect of the solution pH on the CrVI removal by m-MCM-41/PMNCs was monitored in the pH range 1.0−8.0 using 0.01 g of the adsorbent and 10 mL of solution containing CrVI ions (10 mg L−1) at 298 K. As shown in Figure 9b, the maximal adsorption is attained at pH = 2.0 for m-MCM/PMMA, m-MCM/PVA, and m-MCM/N6 and at pH = 3.0 for m-MCM/PS, respectively. Therefore, these pH values were chosen as optimal conditions for subsequent experiments. An obvious fact was the CrVI adsorption decrease in CrVI adsorption from 85.7 to 1.4, from 77.1 to 0.7, from 75.0 to 1.4, and from 62.1 to 19.2 for m-MCM/PMMA, m-MCM/PVA, m-MCM/N6, and m-MCM/PS, respectively, as pH values increased from 2 to 8. The inverse effect of increasing solution pH on the CrVI ion adsorption by m-MCM41/PMNCs could be explained as follows (Figure 11). As already stated there is a plethora of functional polar groups such as hydroxyl, ketone, amine, and phenyl on the surface of the m-MCM-41/PMNCs (including amine-modified mesoporous silica and carbonaceous polymers such as PMMA, PVA, N6, and PS). At low pH, the aforementioned functional groups would be protonated to form - OH2+, -COH+, -NH2+, -NH3+, and -phH+ which should greatly aid the adsorption of CrVI

According to Table 4, pseudo-second-order model provides excellent values of R2 for the adsorption of CrVI on m-MCM/ PVA, m-MCM/N6, and m-MCM/PS (all greater than 99%), implying that the mechanism of adsorption for these three is well in accordance with the pseudo-second-order model. The abundance of active functional groups on their surface as well as the regular structure of these NCs are factors thought to be responsible for such behavior. For m-MCM/PMMA NC, considering R2 (0.9967) in Table 4, the experimental results fit the pseudo-first-order model better than the pseudo-secondorder, Elovich, and intraparticle diffusion models. This observation supports the adsorption of the hexavalent chromium species onto the surface of the active m-MCM/PMMA NC being a physical process.52 In the case of intraparticle diffusion model, a linear regression of plots which passes through the origin (C = 0) hints at the intraparticle diffusion being the sole rate-determining step. When this is not the situation, it can be explained that other kinetic models are also somehow involved and intraparticle diffusion is not the sole rate-limiting step.7,49,51 The occurrence of two linear portions in the plots of Qt vs t0.5 (Figure 10d) were indications that the adsorption of CrVI by m-MCM-41/PMNCs occurred via two steps. The first linear portion can be easily attributed to rapid adsorption of CrVI due to boundary layer diffusion (instantaneous adsorption step or external surface adsorption in macropores). The second linear portion on the other hand may be due to gradual adsorption where intraparticle diffusion phenomenon is the rate-controlling step (diffusion of CrVI throughout the porous surface of the m-MCM-41/PMNCs).7,41,50 The kdif,1 values were higher than kdif,2 values for adsorption of CrVI into the four m-MCM-41/PMNCs, signifying that the external surface adsorption could be faster than the diffusion in mesopores. Effect of pH. The pH of the aqueous solution is known to effectively alter the surface charge on the adsorbent, which 2325



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Table 4. Kinetic Models and Statistical Parameters Obtained from Different Linear Models at 298 K and Optimal pHa models

parameter

m-MCM/PMMA

m-MCM/PVA

m-MCM/N6

m-MCM/PS

pseudo-first-order

log(Q − Q t ) = log Q −

k1 t 2.303

Qexp/(mg g−1)

8.5714

7.6428

7.4643

6.1786

Kd/(mL g−1) k1/min−1 Q = Qcal/(mg g−1) R2

6000.000 0.0320 9.9954 0.9967

3242.424 0.0209 6.9167 0.9659

2943.662 0.0175 2.6539 0.7921

1616.822 0.0188 7.5370 0.9714

k2/(g mg−1 min−1)

0.0025

0.0031

0.0088

0.0014

Q = Qcal/(mg g−1) h/(mg g−1 min−1) R2

10.9769 0.3018 0.9963

9.0992 0.2555 0.9990

8.0064 0.5641 0.9964

8.7184 0.1085 0.9963

β/(g mg−1)

0.3894

0.5106

0.7284

0.4968

R2

0.9812

0.9937

0.8428

0.9934

Kdif,1/(mg g−1 min−1)

0.9218

0.6084

0.8772

0.6344

C1/(mg g−1) R12 Kdif,2/(mg g−1 min−1) C2/(mg g−1) R22

−0.2984 0.9940 0.2140 6.0300 0.9941

0.7985 0.9871 0.2377 4.4274 0.8839

0.1970 0.9955 0.0715 6.3883 0.7734

−0.1718 0.9958 0.3033 1.9237 0.9913

pseudo-second-order

t 1 t = + Qt Q k1Q 2

Elovich

Qt =

1 1 ln(hβ) + ln t β β

intraparticle diffusion

Q t = kdif t 0.5 + C

a

Q is the adsorption capacity at equilibrium, Qt denotes the solid-phase loading of CrVI in the m-MCM-41/PMNCs at any time t, k1 is the rate constant of the pseudo-first-order kinetic model. k2 and h are the rate constant of adsorption and the initial adsorption rate in the pseudo-secondorder kinetic model; h = k1Q2. In the Elovich kinetic model, α is the initial adsorption rate and β is the desorption constant. kdif means the intraparticle diffusion rate constant, and C provides information proportional to the thickness of the boundary layer. All parameters calculated in the boundary conditions of Qt = 0 at t = 0 and Qt = Qt at t = t.

Figure 11. Proposed processes for CrVI anion adsorption by positive active sites onto surface of m-MCM/PMMA under acidic conditions.

faciliates the CrVI redox reactions in both solid and aqueous media due to high redox potential (1.3 V at standard state)55 as shown in eq 1. During the adsorption, adsorbed CrVI anions were partially reduced to CrIII by groups such as -OH, -CO,

through electrostatic attraction (eqs 2−7). On the contrary, with increase in pH, this electrostatic attraction is less due to a decrease in the likelihood of protonating these functional groups. Other reported literature have shown that acidic environments 2326



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Table 5. Thermodynamic Parameters for CrVI Adsorption onto m-MCM-41/PMNCs at Different Temperatures ΔG°/(kJ mol−1) adsorbent

298 K

308 K

318 K

ΔH°/(kJ mol−1)

ΔS°/(J K−1 mol−1)

m-MCM/PMMA m-MCM/PVA m-MCM/N6 m-MCM/PS

21.55 20.03 19.79 18.30

22.75 21.24 20.70 19.11

24.07 22.29 21.70 19.98

15.96 13.81 8.64 6.69

125.82 113.62 95.58 83.85

-NH-, -NH2, and -C−H on the m-MCM/41/PMNCs.12,54 In this regard, m-MCM-41/PMNCs with the lavish distribution of these reducing functional groups on their surfaces exhibit good affinity toward oxyanions of CrVI, especially CrO4−, which they then convert to CrIII (eqs 8−11). According to the previous reports and the recently obtained results, the suggested adsorption mechanism is as follows (-NH2 and -OH as a representative):6,54 HCrO4 − + H3N+R → HCrO4 − ··· H3N+R

(2)

CrO4 2 − + H3N+R → CrO4 2 − ··· H3N+R

(3)

Cr2O7 2 − + H3N+R → Cr2O7 2 − ··· H3N+R

(4)

HCrO4 − + H 2O+ − → HCrO4 − ··· H 2O+−

(5)

CrO4 2 − + H 2O+ − → CrO4 2 − ··· H 2O+−

(6)

Cr2O7 2 − + H 2O+ − → Cr2O7 2 − ··· H 2O+−

(7)

Cr VI + RC−H + H 2O → RC−OH + Cr III + H+

(8)

Cr VI + RC−OH + H 2O → RCO + Cr III + H+

(9)

Cr VI + RCO + H 2O → RCOOH + Cr III + H+

(10)

Figure 12. Thermodynamic curves at 298, 308, and 318 K for fourtype m-MCM-41/PMNCs. Adsorption assays were carried out for 24 h using 10 mL of 10 mg L−1 CrVI solution and 0.01 g of adsorbents.

with temperature indicated the endothermic nature of the CrVI adsorption onto m-MCM-41/PMNCs. Furthermore, because these values are in the range of 2.1−20.9 kJ mol−1 which corresponds to a physical adsorption process, the adsorption of CrVI on m-MCM-41/PMNCs can be conclusively stated to have taken place by physisorption.57 This is in good agreement with the kinetic results interpretations (D-R and Temkin models data). ΔS° can characterize the extent of randomness of the system at elevated temperature and, when measured as a thermodynamic factor, could be used to evaluate ion exchange at the solid-phase interface as well as the structural change at active sites during the adsorption process. The positive value of ΔS° supports the endothermic nature of the adsorption interaction of CrVI anion with m-MCM-41/PMNCs which is enhanced by the randomness at the solid−liquid interface in the course of the adsorption process.52,57

Cr VI + RCOOH + H 2O → RC− H + Cr III + H+ + CO2 (11)

Thermodynamic Studies. In order to determine the orientation and feasibility of physicochemical reactions, the thermodynamic parameters such as standard Gibbs free energy (ΔG°), standard entropy change (ΔS°), and standard enthalpy change (ΔH°) associated with the adsorption of CrVI onto m-MCM-41/PMNCs can be calculated from the temperaturedependent adsorption isotherm using the following equations: ΔG° = −RT ln Kd

ln Kd = − Kd =

Q Ce

ΔH ° ΔS° + RT R

■

(12)

CONCLUSION In conclusion, four types of m-MCM-41/PMNCs were fabricated via solution polymerization as effective adsorbents to capture hexavalent chromium from aqueous solution. The conditions such as pH, contact time, and initial metal concentration were optimized, and the detection/adsorption process was carried out accordingly. The results indicated that the removal of target CrVI was fast and effective, which suggested the excellent adsorptive affinity of m-MCM-41/PMNCs toward the anionic form of chromium. The adsorption of CrVI at low pH was maximal due to the protonation of the abundant functional groups on m-MCM41/PMNCs such as -OH, -NH2, -NH-, and CO with optimal adsorption performance being observed at pH 2−3. Several kinetic models were applied and best fits achieved with the pseudo-first-order and pseudo-second-order models for adsorption of CrVI by m-MCM-41/PMNCs. The intraparticle diffusion kinetic model also suggested that the adsorption process occurred

(13)

(14)

The values of ΔH° and ΔS° can be calculated from the slope and intercept, respectively, of the plot of ln Kd vs 1/T. ln Kd values and related relative standard deviations (RSDs) for m-MCM/PMMA and the other PMNCs are presented in Table 1 and Table S1, respectively. The obtained thermodynamic parameters are given in Table 5 and Figure 12. The thermodynamic analysis was performed at 298, 308, and 318 K. The value of Gibbs free energy became more negative with the increase of temperature from 298 to 318 K, which suggested the spontaneous nature of the adsorption of CrVI onto all four adsorbents at higher temperatures. In addition, the positive values of ΔH° change 2327



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in two distinct steps. The maximum adsorption capacities for the adsorption of CrVI from aqueous media were found to be 20.0, 18.5, 17.9, and 15.0 mg g−1 by m-MCM/PMMA, m-MCM/PVA, m-MCM/N6, and m-MCM/PS, respectively, at 298 K. Four equilibrium isotherm models as well as four adsorption kinetic models were used to explain the obtained adsorption data. The Langmuir model fitted well for m-MCM/PMMA, m-MCM/PVA, and m-MCM/N6 with high correlation values while for m-MCM/PS the Freundlich model is fitted well. The thermodynamic parameters including ΔG°, ΔH°, and ΔS° were calculated, and the obtained positive values of ΔH° and ΔS° indicated that the adsorption of CrVI on four types of PMNCs is spontaneous with an endothermic nature. Meanwhile, the isotherms indicated that the Langmuir and Freundlich models showed better fit of experimental data (R2 > 99) than the other isotherm models for adsorption CrVI onto m-MCM-41/PMNCs. Their low cost in addition to not presenting any problems resulting from sludge formation implies that the m-MCM-41/ PMNCs as compared to pristine silica adsorbents such as m-MCM-41/PMNC are promising candidates for removal of CrVI from aqueous media.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00197. Kinetic, isotherm, and thermodynamic data and relative standard deviations (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +98-31-3391-3270. FAX: +98-31-3391-2350. E-mail: [email protected]; [email protected] (M.D.). *Tel.: +98-31-3391-3270. FAX: +98-31-3391-2350. E-mail: [email protected]; [email protected] (G.M.). ORCID

Mohammad Dinari: 0000-0001-5291-7142 Gholamhossein Mohammadnezhad: 0000-0003-1765-8063 Funding

We express our gratitude to the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, for partial financial support. Notes

The authors declare no competing financial interest.

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REFERENCES

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