Bentonite-decorated calix [4] arene: A new, promising hybrid material for heavy-metal removal

Applied Clay Science 161 (2018) 15–22

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Research paper

Bentonite-decorated calix [4] arene: A new, promising hybrid material for heavy-metal removal ⁎

T

⁎

Khouloud Jlassia, , Rym Abidib, Memia Bennac, Mohamed M. Chehimid, , Peter Kasaka, ⁎ Igor Krupae, a

Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar Laboratory of Applied Chemistry and Natural Substances Resources and Environment (LACReSNE), Faculty of Sciences at Bizerte, 7021 Zarzouna-Bizerte, Tunisia c ISSTE, Higher Institute of Environmental Science and Technology, 2050 Borj Cedria, Tunisia d Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086, CNRS, F-75013 Paris, France e QAPCO Polymer Chair, Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar b

A R T I C LE I N FO

A B S T R A C T

Keywords: Bentonite Calixarene Surfactant New hybrid adsorptive materials Cd (II) and Zn(II) extraction

There is global concern about the contamination of ground, river, and tap waters as well as soil contamination with heavy metal ions; these chemical species are known to not degrade and to cause severe health problems if ingested by humans and animals. Such environmental and health concerns necessitate the development of ultrasensitive sensors and high-capacity adsorbents. This study demonstrates for the first time the potential of organophilic bentonite combined with tetra(2-pyridylmethyl)amide calix [4] arene as a high-performance hybrid material for the removal of toxic heavy metals. After consecutive synthesis steps, the modified bentonites were thoroughly characterized by FT-IR, XRD, UV spectroscopy, and TEM. In particular, the XRD analysis showed strong supporting evidence for intercalation in the clay following each modification step. The salient feature of the newly prepared hybrid material is its high extraction capacity for Cd(II) and Zn(II) metals, as determined by atomic absorption spectrometry and UV spectrometry. Different preparation methods, with respect to the quantity of the added cationic surfactant, were investigated to determine the optimal conditions for synthesis. The extraction percentage for the as-prepared hybrid material was measured to be as high as 97.4% and 94.2% for Cd(II) and Zn(II), respectively.

1. Introduction Heavy metals can cause serious environmental problems due to their abundant use and toxicity. Indeed, high amounts of heavy metals are used in many industries, including tanneries, electroplating, pesticides, phosphate fertilizers, mining, and batteries(Halim et al., 2003; Liu et al., 2018). Once introduced into the environment, heavy metal pollution of both surface and ground waters poses a serious threat because of the metal motility and solubility, which can represent serious risks to the environment and human health (Balasubramanian et al., 2009; Chen et al., 2018). Among all heavy metal water pollutants, cadmium and zinc heavy metals have received increased attention because of their hazardous effect on humans. Cadmium may cause kidney damage and renal disorder (Godt et al., 2006), while zinc can cause abnormal pregnancy and retardation of children's growth(Galbeiro et al., 2014). To remove zinc and cadmium from aqueous solutions, several methods and techniques have been proposed(Uddin, 2017); ⁎

chemical precipitation, evaporation, adsorption, ion exchange, electrochemical treatment, and membrane filtration technologies(Uddin, 2017). Among all these techniques, adsorption is considered as the most efficient and most cost-effective method to remove low metal concentrations(Khan et al., 2014; Rao et al., 2015). Many types of adsorbent materials have been tested for zinc and cadmium ion uptake, such as clay(Hamid et al., 2017; Refaey et al., 2017), carbon nanotubes (Tofighy and Mohammadi, 2012), calixarenes(Sliwa and Deska, 2008), polymers(Liu et al., 2017), and zeolites (Adinehvand et al., 2016). Bentonite can be used as advantageous absorbent for water treatment(Mo et al., 2017), compared to commercial adsorbents; because of their abundance(Bergaya and Lagaly, 2006; Bouazizi et al., 2016), low cost(Hamid et al., 2017) and high cation exchange capacity(HellerKallai, 2006), which is responsible for the hydration and swelling of the absorbent(Sidhoum et al., 2013). The uptake of trace heavy metals by bentonite requires complex adsorption mechanisms involving direct bonding between metal cations and the surface of the clay, surface

Corresponding authors. E-mail addresses: [email protected] (K. Jlassi), [email protected] (M.M. Chehimi), [email protected] (I. Krupa).

https://doi.org/10.1016/j.clay.2018.04.005 Received 12 December 2017; Received in revised form 3 April 2018; Accepted 4 April 2018 0169-1317/ © 2018 Published by Elsevier B.V.

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bentonite was estimated to be 736 m2/g, consisting of external and internal surfaces with BET areas of 113 and 623 m2/g, respectively (Jlassi et al., 2013). To prepare organophilic bentonites, the surfactant benzyldodecyl dimethyl ammonium chloride (C14) was used as provided by Sigma Aldrich (purity, 99%). The critical micellar concentration (cmc) obtained from the conductimetry measurements was 2.07 × 10−3 M. The dimensions of C14 in relation to the length of the chemical CeC and CeN bonds as well as the C-C-C and C-N-C angles have been previously determined from crystallographic data (Rodier et al., 1995). These are listed in Table (see SI1) (Othmani-Assmann et al., 2007).

complexation, and ion exchange(Churchman et al., 2006). In several studies, pretreatment(de Paiva et al., 2008) was necessary to enhance the adsorption of bentonite and to improve the metals uptake(Behera et al., 2016). Such pretreatment facilitates the reaction between clay and calixarene organic macromolecules. Although pretreatment methods can be carried out using a variety of techniques, such as grafting of organosilanes(Jlassi et al., 2013) and diazonium salt treatments(Jlassi et al., 2014), cation exchange is the most commonly implemented method(Slabaugh, 1954). Calixarenes, which are cyclic oligomers, are especially important in separation chemistry, and they are commonly prepared by oligomerization of phenol and formaldehyde (Baldini et al., 2007). The popularity of these compounds is attributed to their simple and large-scale synthesis, amenability to simple chemical tailoring and modification, excellent capability to form complexes between host and guest molecules(Notestein et al., 2006; Troian-Gautier et al., 2016; Zhang et al., 2016), and multi-center bonding with guest molecules. Furthermore, different types of inorganic and organic molecules can be bound specifically and selectively to calixarenes(Ludwig, 2000; Vögtle and Weber, 2012). In recent years, numerous studies have been conducted using calixarenes and their derivatives as potential extractants of metal ions in liquid–liquid extraction processes and transport via liquid membranes(Ohto, 2010). Functionalization at the level of the phenolic OH by amide functions opens up more interesting applications(Maya et al., 2017). Indeed, (Hamdi et al., 2001) reported a new synthesis route for tetra (2-pyridylmethyl) amide calix [4] arene, and they demonstrated the use of this material for cation complexation(McKervey et al., 1996). Moreover, the compound contains an additional cavity delineated by the attachment of the four pyridine moieties via the amido functions that is capable of complexing cations via the nitrogen atoms. Despite the progress in the preparation of calixarene molecules and organophilic clays as separate materials, organophilic clay-decorated calix [4] arene heterostructures have not been previously prepared. In this report, organophilic bentonite-decorated tetra(2-pyridylmethyl)amide calix [4] arene hybrid materials are prepared by simple cation exchange method using the benzyltetradecyl dimethyl ammonium (C14) cationic surfactant to increase the bentonite interlayer space for intercalation of the calixarene organic species. Organophilic bentonite-calixarene hybrids were characterized by FTIR, UV, TEM, and XRD to investigate their surface morphology, chemical composition, and crystalline structure. The extraction performances of the prepared hybrid materials were evaluated using Zn(II) and Cd(II) heavy metals. Surprisingly, to the best of the authors's knowledge, there is no report describing the preparation of an organophilic bentonite-decorated tetra(2-pyridylmethyl)amide calix [4] arene hybrid material. Such a material is expected to be highly important for many applications, including the removal of Zn(II) and Cd(II) heavy metals.

2.2. Synthesis of tetra(2-pyridylmethyl)amide calix [4] arene (calix) Tetra (2-pyridylmethyl)amide calix [4] arene (calix) was prepared by reacting tetrakis(carboxymethoxy)calix[4]arene with an excess of 2(aminomethyl)pyridine in a refluxing 1:1 mixture of methanol–toluene for 10 and 8 days(Hamdi et al., 2001). The as prepared calixarene is tetra-functionalized, and its height (H) was calculated from the crystallographic data based on the lengths of the CeC and CeN bonds, the molecular diameter of the pyridine ring, and the C-C-O and C-N-C angles (Fig. 1). 2.3. Preparation of the organophilic bentonite An aqueous solution of C14 surfactant (0.3, 0.7 and 1 CEC) was added dropwise to an aqueous dispersion of bentonite (1 g dissolved in 100 mL of deionized water). The mixture was stirred for 24 h at room temperature. The resulting dispersion was centrifuged and then washed several times with deionized water to remove any unreacted surfactant molecules. Finally, the resulting product was dried at 70 °C for 48 h. Hereafter, purified Bent is denoted purified Bent, and the organomodified bentonites are abbreviated as OxBent (x stands for the CEC fraction). 2.4. Preparation of the organophilic bentonite calixarene In a typical procedure, BP and BPxCEC samples (approximately 4 g in weight) were dispersed in 50 mL of acetonitrile and mechanically stirred (12 h, room temperature). Then, calixarene solutions (10−3 M, 0.3 CEC) were prepared and added dropwise to the bentonite dispersions using a micropipette. The mixture was kept under mechanical stirring for 5 h, centrifuged (9000 rpm, 45 min), and washed several times with acetonitrile to remove unreacted calixarene molecules. The resulting material was recovered and dried at 40 °C. The supernatants were collected to quantify the amount of reacted calixarene. In the following sections, the resulting products are denoted OxBent-Calix, where x stands for the fraction of CEC (0.3, 0.7, or 1.0).

2. Experimental 2.1. Materials Tetra(2-pyridylmethyl)amide calix [4] arene was prepared following previous method(Hamdi et al., 2001), the benzyltetradecyl dimethyl ammonium surfactant was purchased from Sigma Aldrich and used without further purification. The organic solvents used were of analytical grade; deionized (DI) water was used for washing and solution preparation. The raw bentonite was extracted from the Gafsa-Metloui basin, Tunisia, and purified according to standard procedures(Bergaya and Lagaly, 2013). The cationic exchange capacity (CEC) of the used bentonite was 101.86 meq/100 g calcined clay, as determined from the adsorption isotherm of methylene blue (Brindley and Thompson, 1970). The total Brunauer-Emmett-Teller (BET) surface area of the as-used

Fig. 1. Structure of tetra(2-pyridylmethyl)amide calix [4] arene (calix) including the calculated height. 16

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their net positive charge is balanced by the chloride ions (counter ion of ammonium) not adsorbed on the surface (Tahani et al., 1999). Hybrid bentonite calixarene nanocomposites were prepared using two consecutive steps involving an ammonium salt C14 modification, resulting in three organophilic bentonites: O3Bent, O7Bent, and O10Bent; the latter was prepared by reaction with the previously prepared tetra 2-pyridylmethyl amide calix [4] arene, leading to the synthesis of a new generation of hybrid materials described as: O3BentCalix, O7Bent-Calix, and O10Bent-Calix. For comparison purposes, the same hybrid materials were prepared using purified raw bentonite to verify the role of the interface chemistry of the surfactant (C14) chains intercalated inside the bentonite layers on the synthesis.

2.5. Characterization Initial, modified, and pristine samples were examined by an X-ray diffraction (XRD) spectrometer (model X'Pert PRO) with Co Kα (1.789 Å) radiation to determine the basal distances for the pure, organophilic, and calix-modified bentonite. Samples were scanned under a diffraction angle 2Ө ranging from 4 to 70 degrees with a scanning rate of 2.0 min−1. The basal distance was determined from the diffraction peaks using the Bragg equation (n ʎ = 2d sin Ө). FTIR spectra of KBr compressed pellets were recorded with a Nicolet Magna 860 FTIR (Thermo-Electron) spectrometer in the 400 and 4000 cm−1 wavenumber range with a resolution of 4 cm−1 over 50 scans. Thermal degradation was studied using a thermogravimetric analyzer (Perkin Elmer Pyris) by heating samples from ambient temperature to 800 ̊C at a rate of 10 ̊C min−1 under oxygen atmosphere. The quantification of the amount of non-adsorbed surfactant C14 and calixarene, was carried out using UV–Vis spectrophotometry (Perkin Elmer UV/Vis spectrometer Lambda 11) after each modification step. The supernatants collected after centrifugation were quantified by UV irradiation at a wavelength of 265 nm and 238 nm for C14 and calixarene, respectively (see SI2, SI3 § SI4). The Cd (II) and Zn (II) concentrations in solution were determined by atomic absorption UV–Vis spectrometry at a wavelength of 355 nm.

3.2. Characterization of the organophilic clays The prepared organophilic bentonites, O3Bent, O7Bent, and O10Bent, were characterized by FTIR, XRD, TEM and UV (see SI2) and TGA (SI5, SI6). 3.2.1. Infrared spectroscopy The FTIR spectra of pristine and organophilic bentonite are shown in (Fig. 3). This technique gives a qualitative idea about the adsorption process, and, moreover, it provides accurate information about conformational changes in the alkyl chains within the interlayer space (Othmani-Assmann et al., 2007). For the raw, pristine, purified Bent (BP), the stretching mode of Si-O-Si is identified at 1032 cm−1. The vibrational bands for valence and deformation of the OH of the hydration waters in the clay are at 3450 and 1633 cm−1, respectively. Infrared bands centred at 3619 and 910 cm−1 confirm the dominant presence of dioctahedral smectite with [Al, Al-OH] stretching and bending bands, respectively (Ayari et al., 2005). The presence of kaolinite is confirmed by the Al-O-H valence vibration bands at 3696 cm −1 and 692 cm−1. The Si-O-Mg deformation vibration bands at 463 cm−1 show the existence of isomorphic substitution of aluminum by magnesium in the octahedral sheet. After treatment with the C14 surfactant, the absorption bands for the organoclays at 2925 and 2854 cm−1 appear, which are associated with the antisymmetric and symmetric –CH2 stretching vibrations, respectively. The intensity of these bands gradually increases with the concentration of the surfactant present in the interlayer space. The intensities strongly depend on the alkyl chain density within the interlayer space.

3. Results and discussion 3.1. Approach for synthesizing organophilic bentonite calixarene through a cation exchange reaction (Fig. 2) illustrates the sequential steps for making the organophilic bentonite using benzyldimethyldodecylammonium chloride surfactant (C14). Upon the cation exchange reaction, the resulting organophilic bentonite will be used as a platform to immobilize the calixarenes molecules. As the size of the organic cations is larger than that of the interlayer cations originally present in the clay, the basal distance increases with the rate of organophilization, which can exceed 1 CEC of bentonite. Two adsorption mechanisms characterize this concentration domain: up to 1 CEC, adsorption occurs by ion exchange between the surfactant cations and sodium exchangeable cations in stoichiometric proportions (Klapyta et al., 2001). This exchange is almost complete, and the sodium ions expelled from the interlayer spaces are found in solution;

Fig. 2. Schematic illustration of the preparation of the organophilic Bentonite using benzyl dimethyldodecyl ammonium chloride C14 as the surfactant. 17

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40

(c)

d001 (Å)

35 30

(b)

25

(a)

20 15 10

0

0.5 C14 (CEC)

1

1.5

Fig. 5. Schematic drawings of alkylammonium ion configuration. Bentonite layers are indicated by the rectangles: (a) monolayer arrangement of alkyl chains; (b) beginning of a paraffin-like arrangement of the alkyl chains; (c) paraffin arrangement of the alkyl chains.

For all the treated bentonite samples, the concentration of C14 is lower than 1 CEC. This interval of concentration corresponds to a total adsorption of the added C14 surfactant via a mechanism that is essentially controlled by electrostatic interactions between the organic cations and the negative interlayer sites. The anhydrous bentonite layer thickness is 9.5 Å, and the minimum height of the surfactant in the basal spacing is 6 Å (maximum is 10 Å), which implies the formation of a lateral monolayer of the C14 surfactant with alkyl chains parallel to the bentonite layer (Fig. 5a) (Chen et al., 2005). For low concentrations (0.3 CEC), only 30% of the possible cationic exchange of sodium cations by organic cations is reached. Thus, because the bentonite layers are flexible, the basal distances must vary between 12.5 Å (with a monolayer of hydrated Na+ cations) and approximately 16 Å (for a monolayer of surfactant)(Chen et al., 2005). This explains the obtained value of 15.4 Å (Fig. 5b). However, for the highest concentration (0.7 CEC), the mentioned increase in the d spacing of approximately 9 Å implies the beginning of a paraffin-like structure. In the first, the surfactants are flat with alkyl chains parallel to the silicate layer (Lee and Kim, 2002) in their maximum height configuration (Fig. 5a). In the second type of arrangement, the alkyl chains are nearly flat but slightly tilted with respect to the silicate layer (Fig. 5b). The FTIR spectra recorded for samples in this interval of concentration (Fig. 3) show that the alkyl chains adopt a lefthanded conformation, which indicates that the second arrangement is more probable. Indeed, for coverage rates lower than 1 CEC, the surfactant chains lie separately and parallel to the bentonite layer, which leads to weak hydrophobic attractions between the alkyl chains. Moreover, the hydrocarbon tails are expected to show different types of interactions with the bentonite surface, i.e., van der Waals attractions between the carbon atoms of the alkyl chains and the siloxane group of the clay surface and hydrophobic-hydrophilic repulsions. The presence of a small amount of water adsorbed on the clay surfaces makes the repulsive interactions stronger, which can dominate the hydrocarbonclay surface interactions (Li and Ishida, 2003). The organophilisation efficiency was estimated to ~99% as deduced from UV spectrophotometry at the wavelength λ = 265 nm. The results are summarized in the table Supporting Information (SI2). The organophilization mechanism is most likely a cation exchange since the amount of alkylammonium does not exceed the CEC of the clay.

Fig. 3. Infrared spectra of the purified, organophilic bentonites and the surfactant C14.

The wavenumbers for the absorption bands corresponding to the –CH2 stretching of the alkyl chain are extremely sensitive to conformational changes in the alkyl chains in the interlayer space (Madejová et al., 2011). For the pure surfactant C14, these bands are present at 2850 and 2921 cm−1, indicating that the alkyl chains are in an essentially all-trans conformation. However, the appearance of a left-handed conformation shifts these wavenumbers to higher values based on the quantity of disordered chains. In this study, at relatively low concentrations (BP03 CEC), these wavenumbers appear at 2853 and 2925 cm−1, which prove the existence of a left-handed conformation in the interlayer space; these wavenumbers shift to slightly higher values for the two other samples, BP07 CEC and BP01 CEC, but never reach the values found in the pure C14 salt.

3.2.2. The X-ray diffraction (XRD) According to the Bragg equation, the d(001) value can be determined from the diffraction angle (2Ө), d(001) value of the pure bentonite (BP) was determined to be equal to 1.26 nm. It is noteworthy that after the pretreatment with C14, the (001) reflection shifts to lower angles (clearly mentioned in the inset of (Fig. 4)), indicating intercalation of C14 surfactant within bentonite interlayer space, with increasing basal distance from 1.26 nm for BP pristine to 1.54, 2.98, and 3.96 nm for O3BENT, BP07CEE, and O10BENT, respectively.

3.3. Characterization of the as-prepared organophilic bentonite-calix The as-prepared organophilic bentonite-calixarene hybrid samples O3Bent-Calix, O7Bent-Calix, and O10Bent-Calix were characterized by UV (see SI3§ SI4), FTIR, XRD, TGA (SI5 and SI6), and TEM. The FTIR spectra of the pristine and hybrid materials are shown in (Fig. 6). From the FTIR spectra, the characteristic peaks of the pristine clay as well as the characteristic peaks for the adsorbed calixarene were

Fig. 4. XRD patterns for the as-prepared pure and organophilic (03, 07, and 1 CEC) bentonite samples. 18

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1.38 to 1.5, 2.8, and 3.68 nm for BP-Calix, O3Bent-calix, O7Bent-calix, and O10Bent-Calix, respectively, indicating the intercalation of the calixarene inside the interlayer spaces of all bentonite samples measured. Indeed, the ions of the clay mineral were exchanged with the ammonium cations of the C14 ammonium salts, which enabled the calixarene to easily penetrate into the interlayer space of the bentonite. The same average intercalation level was determined from the analysis of the TEM images. TEM images for the selected BP-calix and O10BENT-calix samples are shown in (Fig. 8a) and (Fig. 8b). Both samples showed a perfect layered structure. Unmodified BP-calix showed a more compact structure compared to O10Bent-calix, and the average of interlayer spaces of the two selected samples match the distances determined by XRD (see previous section). The organomodification efficiency was estimated also using the same UV spectrophotometry at the wavelength λ = 238 nm. The results are summarized in the table Supporting Information (SI4). The TGA curves of BP, O3Bent, O7Bent, O10Bent, O3Bent-Calix, O7Bent-Calix and O10Bent-Calix samples are presented in (SI 5). The corresponding amount of organic moieties (surfactant, calixarene) was deduced and summarized in table (SI6). For the purified Bent sample a mass loss of 9% at the temperature range of 40–140 °C was noted; it is explained by the removal of interlayer water which surrounds Na+ molecules, moreover a 5.5% mass loss appeared at 350–800 °C witch correspond to desorption of structural OH groups of clay. Regarding the organophilic bentonite samples, the highest amount of mass loss was observed at the temperature range of 200–650 °C, which is related to the degradation of surfactant and calixarene as organic compounds. The degradation is described in two-step process (two shoulders): First step is related to degradation of organic compounds bonded to the surfaces via van der Waals or hydrophobic tail interactions, it was found to be (1.7 and 11.2% for O7Bent and O7Bent-Calix respectively); and the second step is related to degradation of organic compounds electrostatically or covalently bonded to bentonite surfaces, and it correspond to (8.8 and 13.8% for O7Bent and O7Bent-Calix respectively)(Calderon et al., 2008). The organophilic bentonites revealed improved thermal stability compared to pristine with onset temperature of decomposition close to 300 °C. All the previously discussed results in this work demonstrate a successful interaction between the bentonite and the C14 surfactant first; via cation exchange reaction, then with the prepared calixarene. However, the exchange mechanism between organophilic bentonites and the used calixarene is not simple to predict because it is influenced by several parameters. Indeed the calixarene could interact with the surface of organophilic bentonites through several mechanisms:

Fig. 6. FTIR spectra for the calixarene adsorbed onto pristine and organophilic clays.

recorded. The valence vibration bands of the CH2 groups are at 2956, 2923, and 2856 cm−1; the deformation and vibration bands of the phenyl CeH are at 1596 and 1570 cm −1, respectively; the valencevibrating band of the aromatic carbon is at 1478 cm−1; the CeN valence vibration band is at 1437 cm −1; the band attributed to the methyl groups at 1361 cm−1; and the NeH deformation vibration band of the amide is approximatively at 1550 cm−1. This band appears at approximately 1537 cm−1 in the calixarene spectrum, suggesting the formation of pyridinium ions after adsorption onto the bentonite, which would support the hypothesis of acid-base interactions(Mittal and Anderson, 1991). From this information, it very likely that the organic moiety (surfactant and calixarene) is intercalated into the bentonite interlayer space. The FTIR spectra recorded for the organophilic clays treated with the calixarene show that the adsorption of the calixarene on the different pretreated bentonite surfaces does not occur in the same manner. From the XRD patterns measured for pure and organophilic bentonite modified with the calixarene (Fig. 7), it can be mentioned that the 001 reflection shift to lower angles as the quantity of the added C14 surfactant increases. The average interlayer space was estimated to be

⁎ Acid-base interactions (Mittal and Anderson, 1991). ⁎ Cation exchange reaction: Many studies focusing on the binding ability of calixarene, suggested that calixarenes carrying coordinating groups, such as amide, can be Protonated (Kříž et al., 2008) and in our case can replace Na+ present in purified Bent. ⁎ Chemical bonding: The organophilic treatment using C14 as surfactant transferred, the hydrophilic Bent into the organic phase and could made the calixarene bind to the oranophilic silicate by surface-bonded polymeric coating(Li and Qu, 2007; Métivier et al., 2005; Yuskova et al., 2013). ⁎ Chemical bonding: The organophilic treatment using C14 as surfactant transferred, the hydrophilic bentonite into the organic phase and could made the calixarene bind to the oranophilic silicate by surface-bonded polymeric coating(Li and Qu, 2007; Yuskova et al., 2013). ⁎ Physical interaction such as van der Waals(Thompson et al., 2011), interactions can occur between surfactant alkyl chains and calixarene, however extensive washing of the resulting product was performed in order to remove the physisorbed species. Fig. 7. XRD of calixarene adsorbed onto pure and organophilic bentonites. 19

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Fig. 8. TEM images of (a) Purified Bent-Calix and (b) O10Bent-Calix.

Fig. 9. Trends of the extraction efficiency percentages for cadmium and zinc using: (a) purified and organophilic bentonites. (b) Cadmium extraction using the extractants purified Bent, Calix, O3Bent, O7Bent, O10Bent, O3Bent-Calix, O7Bent-Calix, and O10Bent-Calix for, (c) Zinc extraction using the same extractants.

O3Bent-Calix, O7Bent-Calix, O10Bent-Calix, and pure Calix were added to a metal picrate solution (25 mL, 2.5 10−4 M). The dispersions was then mechanically stirred (60 min, 300 rpm) before being centrifuged (9000 rpm, 20 min). To quantify the concentration of metal picrate remaining in the aqueous phase, the supernatants were recovered and then analyzed using UV–Vis spectrometry at a wavelength of 355 nm Fig. 9. The extraction efficiency as a percentage (%E) was obtained using the following formula: %E = 100(A0 – A)/A0, where A0 is the absorbance of the blank (without the calixarene). From (Fig. 9a) one can see that the purified and organophilic bentonites have a better affinity for cadmium than zinc. The extraction efficiency percentage increases as the quantity of added surfactant increases until reaching 07CEC; after that point, a decrease in the extraction efficiency was noted. The two histograms in (Fig. 9b) and (Fig. 9c) show the extraction efficiency percentage for the two metals studied using the purified Bent, pure calixarene, organophilic and modified bentonite with calixarene.

The dominant electrostatic and/or covalent nature of calixarene attachment to bentonite demonstrated by the fact that its calixarene content is not changed by extensive washing with protic solvents such as methanol and water.

3.4. Metal extraction experiments Several previous studies have shown the ability of organophilic clays to adsorb heavy metals from aqueous solution(Bakr et al., 2016), others have shown the use of calixarenes for the same purpose(Hamdi et al., 2001). In this work, the extraction of heavy cations such as Cd(II) and Zn(II) from aqueous solutions was conducted using purified, organophilic-C14 and calixarene-modified bentonite, in order to identify synergic effect (via metal extraction) that can demonstrate a direct application of the new hybrid materials. The metal extraction experiments based on metal picrates were conducted according to the procedure reported in the literature (Frensdorff, 1971). Diluted solutions containing respectively 0.3 g of 20

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In (Fig. 9b), the highest extraction efficiency percentage (E = 97.5%) and a slight synergy in the extraction capacity were found only for the O10BENT-Calix sample; the same synergic effect was reported for zinc extraction (E = 81%) using the O10Bent-Calix sample (Fig. 9b). The synergic effect can be explained as follows; the calixarene present on the bentonite surfaces can complex sodium cations present in the interlayer space of the purified Bent; this can occur in all the samples treated with a quantity of surfactant lower than the CEC of the clay (03 and 07 CEC of added C14). From the 1 CEC of added (C14), almost all of the sodium cations present in the purified clay were exchanged with C14 consequently, in this case, calixarene was only engaged with the zinc or cadmium present in the aqueous solution, which leads to the detailed synergic effect in the metal extraction for the O10Bent-Calix sample.

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4. Conclusions Beyond the applied aspect of this study, it has been demonstrated for the first time the preparation of a new hybrid material that uses bentonite as low cost and natural resource as active and functional platform to immobilize the tetra(2-pyridylmethyl)amide calix [4] are ne molecule in order to prepare new hybrid materials of interest. The different organo-bentonites were obtained first by dispersing C14 surfactant equivalent to (0.3, 0.7 and 1 CEC), then with calixarene solutions (0.3 CEC). A range of complementary tools (namely FTIR, XRD, UV, TGA and TEM) permitted to track the effective intercalation of C14 first, and the post-modification with calixarene. Finally the prepared hybrid material was tested for cadmium and zinc heavy metals removal from aqueous solution. More efficient extraction of cadmium with 97.45% was observed compared to the zinc. A synergic effect was reported only when all the sodium cations present in bentonite were exchanged with the organic surfactant molecules. This combination of organic and inorganic low-cost starting materials provides a new generation of hybrid material with excellent extraction properties in aqueous solutions and promising potentials for use in environmental and biological applications. Acknowledgements This paper was made possible by the NPRP award [8-878-1-172] from Qatar National Research Fund (a member of the Qatar foundation). The statements made herein are solely the responsibility of the Authors. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2018.04.005. References Adinehvand, J., Shokuhi Rad, A., Tehrani, A.S., 2016. Acid-treated zeolite (clinoptilolite) and its potential to zinc removal from water sample. Int. J. Environ. Sci. Technol. 13, 2705–2712. Ayari, F., Srasra, E., Trabelsi-Ayadi, M., 2005. Characterization of bentonitic clays and their use as adsorbent. Desalination 185, 391–397. Bakr, A.A., Betiha, M.A., Mady, A.H., Menoufy, M.F., Dessouky, S.M., 2016. Removal of manganese ions from their aqueous solutions by organophilic montmorillonite (OMMT). Desalin. Water Treat. 57, 19519–19528. Balasubramanian, R., Perumal, S., Vijayaraghavan, K., 2009. Equilibrium isotherm studies for the multicomponent adsorption of lead, zinc, and cadmium onto Indonesian peat. Ind. Eng. Chem. Res. 48, 2093–2099. Baldini, L., Casnati, A., Sansone, F., Ungaro, R., 2007. Calixarene-based multivalent ligands. Chem. Soc. Rev. 36, 254–266. Behera, S.K., Kalyani, G., Amrita, B., Park, H.-S., 2016. Response surface optimization of pH and coagulant dosage for pharmaceutical wastewater pretreatment using alum and bentonite. Desalin. Water Treat. 57, 6863–6874. Bergaya, F., Lagaly, G., 2006. Chapter 1 General Introduction: Clays, Clay Minerals, and Clay Science. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Developments in Clay Science. Elsevier, pp. 1–18.

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