Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTBE removal

CLAY-04081; No of Pages 9 Applied Clay Science xxx (2016) xxx–xxx

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Research paper

Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTBE removal Ajlan Abbas a, Abdelazeem Sh. Sallam a, Adel R.A. Usman a,b, Mohammad I. Al-Wabel a,⁎ a b

Soil Science Department, College of Food & Agricultural Sciences, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia Department of Soils and Water, Faculty of Agriculture, Assiut University, Assiut 71526, Egypt

a r t i c l e

i n f o

Article history: Received 17 May 2016 Received in revised form 14 November 2016 Accepted 27 November 2016 Available online xxxx Keywords: Adsorption Organic cations Alkyl chains Organic contaminants Aqueous solutions

a b s t r a c t This study introduces organoclay-based submicron- and nanosized particles prepared from montmorillonite (SWy2) and naturally occurring clay deposits using sequential steps of wet ball milling, ultrasonication and sedimentation followed by an ion exchange process using two surfactants, long alkyl chain HDTMA + (hexadecyltrimethylammonium bromide) and short alkyl chain TMA + (tetramethylammonium bromide). The resultant organoclay-based nanoparticles were characterized using X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DTA) and Fourier-transform infrared (FTIR) spectroscopy. The adsorption of MTBE (methyl tert-butyl ether) by organo-montmorillonite with different surfactant loadings (0.5–2.0 CEC) in aqueous solution was also investigated. The results of XRD analysis for organoclay-based nanoparticles showed that the value of d (001) increased with increasing alkyl chain length and surfactant loading. FTIR results showed that the functional groups of the organoclays changed due to the intercalation of the long alkyl chain surfactant HDTMA+ between clay layers. The removal of MTBE is highly dependent upon surfactant type and loading. The results indicated that the organo-montmorillonite-based nanoparticles prepared from HDTMA+ have a higher affinity for MTBE in aqueous solution than the organo-montmorillonite prepared from TMA+. Therefore, long alkyl chain HDTMA+ can be considered to be the most suitable organo-modifier for enhancing the adsorption characteristics of montmorillonite for MTBE removal. © 2016 Published by Elsevier B.V.

1. Introduction Modification of naturally occurring clay minerals by the intercalation and surface grafting of various types of organic compounds to form organoclays has been shown to be very important for improving the adsorption characteristics of clay minerals (He et al., 2014; Xu et al., 2014; Cabrera et al., 2008; Lee et al., 2011). Therefore, a significant amount of research has focused on preparing organoclay or clay-based nanocomposites (Zhu et al., 2014; Gamoudi et al., 2015; Okada et al., 2014; He et al., 2010; Xie et al., 2001). The intercalation of organic cations into clay minerals can modify the surface properties of the clay structure through the attachment of different functional groups, resulting in hydrophobic properties and positively charged surfaces, and subsequently can enhance the adsorption characteristics of clay minerals for the removal of organic contaminants (Zhu et al., 2015; Zhu et al., 2014; Okada et al., 2014; He et al., 2010; Ray and Okamoto, 2003; Ishii et al., 2005; Xie et al., 2001). A number of studies have

⁎ Corresponding author. E-mail address: [email protected] (M.I. Al-Wabel).

suggested that the characteristics of organoclays are highly dependent on the alkyl chain length of the organic cations, surfactant packing density, clay charge density, and clay mineral type (Zhu et al., 2014; Zhu et al., 2007; He et al., 2006; Klapyta et al., 2001; Yui et al., 2002). The semi-volatile organic compound methyl tertiary-butyl ether (MTBE) is considered to be a potential environmental pollutant found in many environments, such as surface water, groundwater and soil (Zadaka-Amir et al., 2012). Because it is a carcinogen, MTBE has been banned in many parts of the world (Aivalioti et al., 2010). MTBE is resistant to biodegradation and chemical oxidation, has a relatively low adsorption efficiency, and is highly soluble in aqueous solution, making its removal from and decontamination of the environment difficult. The adsorption of MTBE onto porous sorbents such as zeolites and activated carbon has been shown in several reports (Li et al., 2003; Hung and Lin, 2006; Anderson, 2000; Ji et al., 2009). To our knowledge, however, MTBE adsorption onto organoclays has rarely been investigated, and the effects of the intercalation of HDTMA+ (long alkyl chain surfactant) into clay minerals compared to the intercalation of TMA+ (short alkyl chain surfactant) on MTBE adsorption have not been reported. Therefore, the aim of this study was to synthesize organoclay-based nanoparticles prepared from montmorillonite (SWy2) or naturally occurring clay deposits and two surfactants, long alkyl chain HDTMA +

http://dx.doi.org/10.1016/j.clay.2016.11.028 0169-1317/© 2016 Published by Elsevier B.V.

Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028

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and short alkyl chain TMA+. The resultant organoclay-based nanoparticles were characterized using XRD, TG-DTA, and FTIR analysis. The adsorption capacity of the prepared organo-montmorillonite-based nanoparticles with different surfactant loadings to remove methyl tert-butyl ether (MTBE) from aqueous solution was also investigated.

2. Materials and methods 2.1. Clay samples The local clay enriched deposits used in this study were located in the central (Al-Kharj Mahawes) and western (Khulais) regions of Saudi Arabia. The clay samples collected from Al-Kharj Mahawes and Khulais are denoted as SMH and SKH, respectively. The clay mineral samples of montmorillonite (SWy2) were purchased from the Clay Minerals Society (West Lafayette, IN).

2.2. The separation of clay fractions (b2 μ) The clay mineral samples of SWy2 and the collected clay-rich samples were purified to remove carbonates, organic matter, iron oxides and manganese oxides. Specifically, the samples were treated with 1 M NaOAc (pH = 5) to remove carbonates, followed by treatment with H2O2 (30%) to remove organic matter, and then the samples were treated with citrate-bicarbonate-dithionite (CBD) to remove Fe and Mn oxides. After treatment, the samples were suspended in distilled water, and the clay was purified following the sedimentation method (Jackson, 1979). 2.3. Preparation of submicron and nanoscale clay Clay particles are mainly found in the form of aggregates of different sizes. To obtain clay particles under 100 nm in size, the following method that includes three sequential operations was used: 1) dry or wet

Fig. 1. SEM images for montmorillonite (SWy2 (a): original; NSWy2 (b): nanoscale clay), local clay from Khulais (SKH (c): original; NSKH (d): nanoscale clay), and local clay from Al-Kharj Mahawes (SMH (e): original; NSMH (f): nanoscale clay).

Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028

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mechanical ball milling, 2) ultrasound and 3) sedimentation treatment. First, dry or wet ball milling treatment was carried out by high energy ball milling (Pulverisette 7 premium line, FRITSCH, Germany). Specifically, for the wet ball milling treatment, 30 mL of 5% Calgon solution (sodium hexametaphosphate) was added to a 5 g clay sample, followed by ball milling at 600 rpm for 1–4 h. Then, 50 mL of 5% Calgon solution and 100 mL of water were added to 50 mL of the clay suspension collected from the previous step, and the mixture was ultrasonicated (500 W, 20 kHz) for different times ranging from 1 to 4 h. Finally, the

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ultrasonicated mixture was subjected to sedimentation for different times ranging from 0 to 72 h. The resultant samples are denoted NSWy2 for montmorillonite and NSKH and NSMH for the local clay samples. Table S1 shows the particle size distribution and surface area of the clay particles of the montmorillonite and local clay samples obtained from the different treatments. Among the various treatments, wet ball milling combined with ultrasonication and sedimentation resulted in the smallest sized particles and the highest specific surface areas (Table S1). The cation exchange capacity (CEC) values for SWy2, 3.35

3.35

13.90

4.47

13.90

4.47

3.10 SWy2

3.10 SWy2

12.24 12.24

13.85 22.89 NSWy2

NSWy2

13.85

35.90 ONSWy2-TMA (0.5 CEC)

ONSWy2-HDTMA (0.5 CEC)

ONSWy2-TMA (1.0 CEC)

ONSWy2-HDTMA (1.0 CEC)

35.34

13.90

ONSWy2-HDTMA (2.0 CEC) ONSWy2-TMA (2.0 CEC) 10

20

2-Theta (o)

30

40

10

20

30

40

2-Theta (o)

Fig. 2. XRD patterns of montmorillonite SWy2: original montmorillonite; NSWy2: nanoscale montmorillonite; ONSWy2-TMA: nanoscale montmorillonite with short alkyl chain surfactant (TMA+) loadings of 0.5, 1.0 and 2.0 CEC; ONSWy-HDTMA (0.5 CEC): nanoscale montmorillonite with long alkyl chain surfactant (HDTMA+) loadings of 0.5, 1.0 and 2.0 CEC.

Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028

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SKH, and SMH are 75.6, 73.9 and 53.8 cmolc kg−1, respectively, while the corresponding values for the synthesized submicron and nanoscale clays are 261, 84.7 and 66.85 cmolc kg−1 for NSWy2, NSKH, and NSMH, respectively (Table S2). The SEM images confirmed the presence of many small and aggregated particles in the resultant samples of nanoscale clays (Fig. 1). 2.4. Preparation of organoclay-based nanoparticles The organoclay-based nanoparticles were prepared by an ion exchange reaction between the clay samples and an organic cationic surfactant, either long alkyl chain HDTMA + (hexadecyltrimethylammonium bromide) or short alkyl chain TMA+ (tetramethylammonium bromide), according to He et al. (2010). Specifically, the organic cationic surfactant solutions were prepared by heating at 80 °C for 0.5 h. Then, the submicron and nanoscale clay samples were slowly added to the organic cationic surfactant solution. The loading of TMA+ and HDTMA+ surfactant was equivalent to 0.5, 1.0, and 2.0 times the cation exchange capacity (CEC) for NSWy2. However, for local clay samples, only long alkyl chain HDTMA+ was used, and its loading was equivalent only to 2.0 CEC. The organoclay-based

nanoparticles were then removed from solution by centrifugation and rinsed several times with water before being dried at 60 °C. The organoclay-based nanoparticles obtained from montmorillonite modified with long alkyl chain HDTMA + are denoted ONSWy2-HDTMA (0.5 CEC), ONSWy2-HDTMA (1.0 CEC) and ONSWy2-HDTMA (2.0 CEC). In addition, those obtained from organo-montmorillonite modified with short alkyl chain TMA + are denoted ONSWy2-TMA (0.5 CEC), ONSWy2-TMA (1.0 CEC) and ONSWy2-TMA (2.0 CEC). Meanwhile, the organoclay-based nanoparticles from local clay samples are denoted ONSKH-HDTMA (2.0 CEC) and ONSMH-HDTMA (2.0 CEC). 2.5. Analysis techniques The particle size distributions and surface areas of the clay particles were determined using a particle-size analyzer (Hydro 2000s, Malvern Mastersizer, UK). CECs of the original and synthesized submicron and nanosized clay particles were measured by sodium saturation (1 N sodium acetate at pH 7), according to Sumner and Miller (1996). The mineralogy of the original, synthesized clay and organoclay nanoparticles was investigated using X-ray diffraction (XRD-7000 Shimadzu, Japan). The thermal characteristics based on weight change of the original,

Fig. 3. XRD patterns for (a) local clay from Khulais (SKH: original; NSKH: nanoscale clay; ONSKH-HDTMA (2.0 CEC): organoclay-based nanoparticles with a loading of 2.0 CEC) and (b) local clay from Al-Kharj Mahawes (SMH: original; NSMH: nanoscale clay; ONSMH-HDTMA (2.0 CEC): organoclay-based nanoparticles with a loading of 2.0 CEC).

Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028

A. Abbas et al. / Applied Clay Science xxx (2016) xxx–xxx

synthesized clay and organoclay were studied using thermogravimetric-differential thermal analysis (TG-DTA; DTG-60H, Shimadzu, Japan). The surface functional groups of the original, synthesized clay and organoclay were determined using FTIR spectroscopy (TENSOR Series FT-IR Spectrometer, Bruker, Germany). Additionally, scanning electron microscope (SEM; FEI, Inspect S50, Netherlands) was utilized to identify the morphological changes in the surface structure of the nanoscale clays. 2.6. Adsorption experiments Samples of a known amount (0.1 g) of organoclay (ONSWy2-TMA and ONSWy2-HDTMA) with different surfactant loadings (0.5, 1.0 and 2 CEC) were shaken at 250 rpm for 24 h in 50.0 mL Teflon centrifuge tubes at room temperature with 25 mL of solution containing different MTBE concentrations (1–30 mg L−1). Then, the suspensions were centrifuged, and the concentration of MTBE in the supernatant was analyzed by GC-MS (Agilent 7890 A GC coupled to a 220 MS ion trap) after separation using the purge and trap technique (EPA, 2004; Rosell et al., 2006). MTBE was separated on a capillary column using helium as the carrier gas. The amount (qe) of MTBE adsorbed by the various adsorbents was determined using the following equation:

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higher than those in the original montmorillonite sample. In addition, increasing the HDTMA + loading sharply increased the value of d (001) in the organoclays prepared from SWy2 and NSWy2. In accordance with previous studies (He et al., 2010; Zhu et al., 2003; Li and Ishida, 2003; Pálková et al., 2007), the basal spacing of the montmorillonite sample increased with increased surfactant loading. The effect of alkyl chain length on the intercalation of the alkyl organic cations between the clay layers has been investigated previously (He et al., 2010; Zhu et al., 2007). Similar to our findings, they reported that the basal spacing of organo-montmorillonites increased with an increase in the alkyl chain length of the surfactant. The large value of d (001) due to the addition of the long alkyl chain surfactant (HDTMA+) can perhaps be attributed to the intercalation of organic cations between the layers of clay nanoparticles as a result of the reduction of the particle size into the nanoscale, which subsequently increases the CEC and plays an

qe ¼ VðCo −Ce Þ=m where qe is the MTBE concentration on the adsorbents (mg g− 1) at equilibrium, Ce is the MTBE concentration in solution (mg L−1) at equilibrium, Co is the initial MTBE concentration in solution (mg L−1), V is the volume of the initial MTBE solution (L), and m is the mass of the adsorbent (g). Sorption isotherm parameters were determined by least squares analysis using the Linear, Langmuir, Freundlich, and Temkin isotherms described by the following equations (Ho et al., 2005; Foo and Hameed, 2010): • • • •

Linear model: qe = KL Ce Langmuir model: Ce/qe = 1 ∕ (Kaqm) + Ce ∕ qm Freundlich model: log qe = log Kf + (1 ∕ n) log Ce Temkin model: qe = a + B ln Ce

where Ce is the equilibrium solution phase concentration (mg L−1), qe is the equilibrium solid phase concentration (mg g−1), qm is the Langmuir isotherm sorption capacity (mg g−1), Ka is the enthalpy related sorption constant (L mg−1), n is the sorption intensity constant, Kf is the sorption capacity constant, KL is the linear distribution coefficient computed from the slope of the linear isotherm (L g−1), and a and B are constants related to the energy and capacity of adsorption, respectively. 3. Results and discussion 3.1. Characterization results The XRD patterns showed that montmorillonite (SWy2) is enriched with Na+ by the signals at d = 13.9 Å and 4.47 Å (Fig. 2). In addition, quartz, shown at d = 3.35 Å and 4.27 Å, and the local clay deposits samples (SKH and SMH) are dominated by smectite (Fig. 3). The semi-quantitative estimation of the mineralogical composition indicated that the local clay samples of Khulais were composed of 79% smectite, 7% kaolinite, 8% feldspars and 6% quartz. Meanwhile, the local clay samples of AlKharj Mahawes were composed of 45% smectite, 28% kaolinite, 5% feldspars and 22% quartz. Compared to the values of d (100) in the original montmorillonite, there are no changes in the value of d (100) following the preparation of organo-montmorillonite with the short alkyl chain surfactant (TMA+) (Fig. 2). In contrast, the values of d (001) in organo-montmorillonite prepared with the long alkyl chain surfactant (HDTMA+) are

Fig. 4. The FTIR spectra of montmorillonite (a) SWy2: original montmorillonite; (b) NSWy2: nanoscale montmorillonite; and (c) ONSWy-HDTMA (2.0 CEC): organoclaybased nanoparticles with a long alkyl chain surfactant (HDTMA+) loading of 2.0 CEC.

Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028

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important role in the positioning of cations between the clay layers (Slade and Gates, 2004). The X-ray diffraction analysis also showed an increase in d (001) spacing in organoclays prepared from local clay deposits and HDTMA+ (Fig. 3). Overall, the value of d (001) decreased in the following order: (ONSWy2) N (ONSKH) N (ONSMH). This sequence corresponds with the CEC of the clays. Moreover, the higher intensity and greater expansion of the d (001) basal distance of montmorillonite (SWy2) compared to local clays enriched in smectite (mainly Ca-montmorillonite) may be because Na-montmorillonite can expand more than Ca-montmorillonite (Tabak et al., 2007). The difference in the expandability may be explained by the effect of the co-clay minerals (such as kaolinite) in local clays, leading to a decrease in the intercalation efficiency.

Generally, the d (001) spacing values are N22.0 for ONSWy2HDTMA (2.0 CEC) and ONSKH-HDTMA (2.0 CEC), indicating the formation of a paraffin complex. It has been suggested that the structural configuration of alkyl chains may be as follows: monolayer (at 13.7 Å) b bilayer (at 17.7 Å) b pseudo-trimolecular (at 21.7 Å) b paraffin complex (N 22.0 Å) (Xi et al., 2004). When the loading of HDTMA + in smectite is higher than 1.0 CEC, HDTMA + can be retained through hydrophobic bonds, resulting in an increased interlayer spacing (Williams-Daryn and Thomas, 2002). The TG-DTA peak numbers of the prepared organoclays varied from the original and nanoscale clays (Tables S3 and S4; Figs. S1, S2 and S3). Compared to the original clay and clay-based nanoparticles, the mass loss from the organoclay-based nanoparticles at the second stage at 184–480 °C is mainly due to the decomposition of the organic cationic

Fig. 5. The FTIR spectra of local clays from Khulais (a) SKH: original; (b) NSKH: nanoscale clay; and (c) ONSKH-HDTMA (2.0 CEC): organoclay-based nanoparticles with a loading of 2.0 CEC and Al-Kharj Mahawes (d) SMH: original; (e) NSMH: nanoscale clay; and (f) ONSMH-HDTMA (2.0 CEC): organoclay-based nanoparticles with a loading of 2.0 CEC.

Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028

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successful intercalation of organic cations between the clay layers. This is in line with the results obtained from XRD and TG-DTA. 3.2. Adsorption isotherms and removal efficiency

Fig. 6. Equilibrium sorption isotherms for MTBE adsorption onto the investigated sorbents (ONSWy2-TMA with short alkyl chain surfactant (TMA+) loadings of 0.5, 1.0 and 2.0 CEC; ONSWy2-HDTMA: organo-montmorillonite-based nanoparticles with long alkyl chain surfactant (HDTMA+) loadings of 0.5, 1.0 and 2.0 CEC).

surfactants (Pálková et al., 2007; Xi et al., 2004). It has been previously reported that a mass loss at 150–500 °C is due to the decomposition of organic cations (Pálková et al., 2007; Marras et al., 2007). It was observed that no DTG peak appeared for ONSWy2-HDTMA+ at the third stage. However, for organoclays prepared from local clays and HDTMA +, the mass loss at 494.52 °C and 552 °C for ONSKH-HDTMA and at 489.10 °C and 685.89 °C for ONSMH-HDTMA corresponds to continuous organic cation decomposition and oxidation. At the fourth stage, the peaks centered at 644.96 °C and 710.04 °C for ONSWy2-HDTMA suggest the oxidation of the remaining of organic substances. The FTIR spectra indicated that the functional groups of the organoclays changed due to the intercalation of long alkyl chain HDTMA+ between the clay layers (Figs. 4 and 5 and Tables S5, S6 and S7). Compared to the original clays and clay-based nanoparticles, the peaks appearing at approximately 1477 cm− 1, 2850 cm− 1 and 2920 cm−1 in the organoclays prepared from clay-based nanoparticles and HDTMA+ confirm the presence of the active groups in the cationic aliphatic long alkyl chain surfactant (HDTMA +) and suggests the

The adsorption isotherms and removal efficiency of MTBE by organo-montmorillonite-based nanoparticles (ONSWy2-TMA and ONSWy2-HDTMA) with different surfactant loadings of 0.5, 1.0 and 2.0 CEC were compared in this study. The equilibrium data isotherm analysis for MTBE adsorption onto the surfactant-modified clays is shown in Fig. 6. The experimental results showed that the removal of MTBE is highly dependent on the surfactant type and loading (Figs. 6 and 7). Sorption isotherms indicated that MTBE has a higher affinity for adsorption onto organoclays when HDTMA+ (long alkyl chain surfactant) was intercalated within the clay compared with organoclay prepared with the incorporation of short alkyl chain surfactant (TMA +). A 100% removal of MTBE was obtained by the long alkyl chain organoclays for all initial concentrations, indicating the high adsorption affinity between the MTBE and the sorbent (H type) and emphasizing the affinity of the long alkyl chain organoclays for MTBE. The results of the XRD analysis showed that the increased value of d (001) was largely due to the addition HDTMA+ (long alkyl chain surfactant) and that the d spacing did not increase for the montmorillonite modified with the short alkyl chain surfactant (TMA+). In this context, it has been reported that the higher sorption of organic contaminants by clays loaded with HDTMA + is attributed to the large basal spacing being able to accumulate a higher quantity of solute molecules (Jaynes and Boyd, 1991). In addition, the higher sorption capacity of the cationic surfactant with the larger chain length can be explained by the higher hydrophobicity of the longer alkyl chain, indicating that hydrophobic interactions are important in the adsorption process (Liao and Xie, 2004). In general, the enhancement in the adsorption capacity of organo-montmorillonite prepared with long alkyl chain surfactant can be explained by the increased number of adsorption sites on the organoclay and the increased interaction affinity between the contaminant and the organoclay (Zhu et al., 2014). However, the percent removal of MTBE by short alkyl chain organoclay was affected by surfactant loading and initial concentration (Fig. 7). An increased surfactant loading of TMA+ in the organoclay resulted in an increase in the efficiency of removing MTBE. The positive

Fig. 7. Effect of initial MTBE concentration and surfactant loading on MTBE removal (ONSWy2-TMA: organo-montmorillonite-based nanoparticles with short alkyl chain surfactant (TMA+) loadings of 0.5, 1.0 and 2.0 CEC; ONSWy2-HDTMA: organo-montmorillonite-based nanoparticles with long alkyl chain surfactant (HDTMA+) loadings of 0.5, 1.0 and 2.0 CEC).

Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028

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Table 1 Linear, Langmuir, Freundlich and Temkin parameters for MTBE sorption onto the ONSWy2-TMA. Sorbents ONSWy2-TMA (0.5 CEC) ONSWy2-TMA (1.0 CEC) ONSWy2-TMA (2.0 CEC)

Linear KL

r2

Langmuir qm

Ka

r2

Freundlich n Kf

r2

Temkin a

B

r2

0.112 0.149 0.218

0.969 0.994 0.992

−1.20 4.66 7.43

0.032 0.074 0.051

0.912 0.818 0.725

0.78 1.37 1.22

0.983 0.995 0.991

−0.203 0.520 0.606

0.605 0.646 0.838

0.701 0.842 0.833

0.037 0.336 0.361

ONSWy2-TMA (0.5 CEC): organoclay-based nanoparticles from montmorillonite with short chain alkyl (TMA) loading level of 0.5 CEC; ONSWy2-TMA (1.0 CEC): organoclay-based nanoparticles from montmorillonite with short chain alkyl (TMA) loading level of 1.0 CEC; ONSWy2-TMA (2.0 CEC): organoclay-based nanoparticles from montmorillonite with short chain alkyl (TMA) loading level of 2.0 CEC.

correlation between the level of cationic surfactant loading and the adsorption of organic contaminants has been well reported in the literature (Dentel et al., 1998). The percentage removal of MTBE by organo(TMA)-clay with higher surfactant loadings of 1.0 and 2.0 CEC decreased as the initial concentration increased (Fig. 7). The higher adsorption at lower initial concentration may be due to the sufficient number of binding sites, resulting in the high adsorption affinity between MTBE and the organo(TMA)-clay. This may overcome the initial MTBE concentration. At higher initial MTBE concentration, however, the availability of free adsorption sites for adsorbing MTBE may decrease and prevent further MTBE adsorption onto the adsorption surfaces. In contrast, the percentage removal of MTBE by the organo(TMA)-clay with the lowest surfactant loading of 0.5 CEC increased with increasing initial MTBE concentration. For MTBE adsorption onto the organo(TMA)-clay, the isotherms appear to be linear with respect to the initial MTBE concentration. The equilibrium data for MTBE adsorption onto the organo(TMA)clay with surfactant loadings of 0.5–2.0 CEC were analyzed by the linear regression of various isotherm model equations including Linear, Langmuir, Freundlich, and Temkin. The computed related parameters from the values of the slopes and intercepts of the respective linear plots are tabulated in Table 1. The adsorption isotherm of MTBE on the organo(TMA)-clay was in a good agreement with the Linear (r2 = 0.969–994) and Freundlich (r2 = 0.983–0.995) isotherm models, which correlated better than the Langmuir (r2 = 0.725–0.912) and Temkin (r2 = 0.701–0.842) models. Therefore, the sorption of MTBE onto organo(TMA)-clay occurs through a partitioning mechanism, as indicated by the Linear isotherm model (Zhu et al., 2014). This is consistent with the previous work of Dentel et al. (1998) on the uptake of organic contaminants by organoclays. According to the Linear isotherm model, the linear distribution coefficient (KL) for the adsorption of MTBE onto organo(TMA)-clay tended to increase with increasing surfactant loading and was 0.112, 0.149 and 0.218 L kg− 1 for 0.5, 1.0 and 2.0 CEC, respectively. In the Freundlich isotherm, it is evident from the higher correlation coefficients that MBTE adsorption is a multilayer adsorption process onto the heterogeneous surfaces of organo(TMA)-clay that have different adsorption energies (Vidal et al., 2012). Several other researchers have shown that the sorption of organic contaminants with different adsorbents could be well described by the Freundlich model (Su et al., 2010; Aivalioti et al., 2012). The calculated Freundlich adsorption constant (Kf) increased with increased surfactant loading and amounted to 0.037, 0.336 and 0.361 L mg−1 for 0.5, 1.0 and 2.0 CEC, respectively. The importance of the n values obtained from the Freundlich model gives an indication of the adsorption favorability. Values of n higher than unity suggest that adsorption is favorable. It has been suggested that the adsorption bond between sorbent and sorbate could be relatively strong when the n value obtained from the Freundlich model is higher than unity (Nourmoradi et al., 2013; Koyuncu et al., 2011). In the current study, the Freundlich constant n was lower than unity for the adsorption of MTBE onto the organo(TMA)-montmorillonite with the lowest surfactant loading of 0.5 CEC (Table 1), indicating that the adsorption of MTBE onto organo(TMA)-montmorillonite is not favorable. However, the values of n were higher than unity for the adsorption of MTBE onto the organo(TMA)-montmorillonite with higher surfactant loadings of 01.0 and 2.0 CEC, suggesting that MTBE

adsorption is favorable. Therefore, MTBE adsorption onto the short alkyl chain organoclay with higher surfactant loadings of 1.0 and 2.0 CEC has relatively stronger bonds than that with lower surfactant loading (0.5 CEC), and MTBE adsorption is suitable when a higher surfactant loading is used for the short alkyl chain organoclay. Generally, our findings agree with previous conclusions that the surfactant-modified adsorbents resulted in high affinity and adsorption efficiency in the removal of organic contaminants and volatile and aromatic organic compounds from aqueous solution and water (Nourmoradi et al., 2013; Sharmasarkar et al., 2000; Zadaka-Amir et al., 2012; Witthuhn et al., 2006). The results also indicated that the intercalation of HDTMA + (long alkyl chain surfactant) into nanoscale montmorillonite can modify the surface properties of the clay structure through the attachment of different functional groups, resulting in hydrophobic properties and a positively charged surface. Therefore, treatment with HDTMA + enhanced the adsorption characteristics of montmorillonite for the removal of MTBE. 4. Conclusions Organoclay-based nanoparticles were prepared by the synthesis of clay nanoparticles followed by an ion exchange process using organic cationic surfactants. The results of XRD, TG-DTA and FTIR analysis indicated the incorporation of organic cations between the clay layers. The value of d (001) increased due to the addition HDTMA + (long alkyl chain surfactant) compared to TMA + (short alkyl chain surfactant). The results also suggest that more organic cations penetrated into the layers of Na-montmorillonite (SWy2) than into those of the local clays. Additionally, long alkyl chain HDTMA+ can be considered to be the most suitable organo-modifier for enhancing the adsorption characteristics of montmorillonite for the removal of MTBE, and the resultant organoclays had the highest extent of intercalation. It can be concluded that future studies should be carried out and focused on using the prepared organoclay-based nanoparticles for environmental applications. Acknowledgments The authors extend their appreciation to the Deanship of Scientific Research, King Saud University for funding this work through the international research group project IRG-14-14. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.clay.2016.11.028. References Aivalioti, M., Vamvasakis, I., Gidarakos, E., 2010. BTEX and MTBE adsorption onto raw and thermally modified diatomite. J. Hazard. Mater. 178, 136–143. Aivalioti, M., Pothoulaki, D., Papoulias, P., Gidarakos, E., 2012. Removal of BTEX, MTBE and TAME from aqueous solutions by adsorption onto raw and thermally treated lignite. J. Hazard. Mater. 207-208, 136–146. Anderson, M.A., 2000. Removal of MTBE and other organic contaminants from water by sorption to high silica zeolites. Environ. Sci. Technol. 34, 725–727.

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Please cite this article as: Abbas, A., et al., Organoclay-based nanoparticles from montmorillonite and natural clay deposits: Synthesis, characteristics, and application for MTB..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.11.028