Silylation of leached-vermiculites following reaction with imidazole and copper sorption behavior

Journal of Hazardous Materials 306 (2016) 406–418

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Silylation of leached-vermiculites following reaction with imidazole and copper sorption behavior Saloana S.G. Santos a , Mariana B.B. Pereira a , Ramon K.S. Almeida b , Antônio G. Souza a , Maria G. Fonseca a,∗ , M. Jaber c a

Chemistry Department of Paraíba Federal University, João Pessoa, Paraíba, Brazil Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, São Paulo, Brazil c Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 8220, Laboratoire d’archéologie moléculaire et structurale (LAMS), Boîte courrier 225, 4 place Jussieu, 75005 Paris, France b

h i g h l i g h t s • • • •

Silylated vermiculites reacted covalently with imidazole. Modified vermiculites adsorbed copper from aqueous solution. Copper retention in all solids occurred at rapid time of 80 min. Higher organic content on the solid improved the copper adsorption.

a r t i c l e

i n f o

Article history: Received 3 June 2015 Received in revised form 10 September 2015 Accepted 21 November 2015 Available online 2 December 2015 Keywords: Vermiculite Silylated clay Hybrid material Copper sorption

a b s t r a c t Organically modified vermiculites were synthesized by previous silylation of three lixiviated vermiculites, V0.3Cl, V0.5Cl and V0.8Cl, under anhydrous conditions following reaction with imidazole (Im), which acted as chelating agent for copper retention. Elemental analysis, X-ray diffraction, infrared spectroscopy, scanning electronic microscopy, transmission electron microscopy, 29 Si and 13 C NMR and nitrogen adsorption/desorption measurements were used to characterize pristine, lixiviated and organofunctionalized solids. X-ray photoelectron spectroscopy (XPS) was used to evaluate the surface after copper sorption. Parameters such as contact time, pH and initial cation concentration for the adsorption of Cu(II) ions were investigated. The adsorption equilibrium data were fitted using the Langmuir isotherm model and the monolayer adsorption capacities were 2.38, 2.52 and 2.69 mmol g−1 for V0.5Cl-Im, V0.3ClIm and V0.8Cl-Im, respectively, at pH 6.0 and 298 K for a time reaction of 80 min. The sorption rates were described by pseudo-second-order kinetics. The chloropropyl imidazole vermiculites are promising adsorbents for the rapid removal of Cu(II) ions from aqueous solution. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The accumulation of heavy metals in aquatic bodies and soil is a serious environmental and human health issue because of their toxicity, non-biodegradable nature and accumulation in the food chain [1,2]. Among the heavy metals, copper is one of the most common polluting metal ions in industrial effluents. There are various sources of introducing copper in the environment including industrial and domestic wastes, agricultural practices, copper mine drainage, copper-based pesticides, and antifouling paints [3]. Many

∗ Corresponding author. E-mail address: [email protected] (M.G. Fonseca). http://dx.doi.org/10.1016/j.jhazmat.2015.11.042 0304-3894/© 2015 Elsevier B.V. All rights reserved.

works have reported on its toxicity on the organism: nausea, vomiting, diarrhea, respiratory difficulties, liver and kidney failure [4–6]. Precipitation, coagulation/flotation, sedimentation, flotation, filtration, membrane processes, electrochemical techniques, biological process, chemical reactions, adsorption and ion exchange have been practiced to remove copper ions from environment [7]. In the recent years, novel adsorbents have been developed, and clay minerals have received special attention because of their affinity for heavy metal ions, which makes them potential sorbents for the removal of pollutants [8–13]. Clay minerals are also widely available, inexpensive, biocompatible and environmentally friendly. Clay minerals are a subset of the family of layered oxides (or oxyhydroxides) that can be classified in three different categories according to the electrical charge: neutral layers, negatively

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Fig. 1. XRD patterns for sodium and the organically modified vermiculites.

charged layers and positively charged layers. Herein, we are interested in the negatively charged layers listed as phyllosilicates and called cationic clays [14]. Phyllosilicates are layered materials where each layer of the structure is consisting of the assembly of two or three sheets, which are either tetrahedral or octahedral [15–17]. Each tetrahedron consist of a cation, T (Si4+ , Al3+ and Fe3+ ), coordinated to four oxygen atoms, and linked to adjacent tetrahedra by sharing three corners. Octahedral sheets, abbreviated as “O,” consist of edge-sharing [MO4 (OH)2 ] units, where M can be either a trivalent (such as Al3+ ), a divalent (such as Mg2+ ), or a monovalent (Li+ ) ion; the central site of the octahedron may also be vacant. All these fundamental structural elements are arranged to form a hexagonal (or pseudohexagonal) network in each sheet. According to the number of sheets constituting a layer, the phyllosilicates can be classified into 1:1 or 2:1 phyllosilicates. The second criteria is the occupancy of the

octahedral sheet where we can distinguish between the trioctahedral character when all the octahedral sites are filled with a divalent cation and dioctahedral character when 2 of 3 octahedral sites are occupied by a trivalent cation. Other criterion for the classification exist (layer charge) and are well reported in the Handbook of Clay Science [14]. In this paper we are interested in vermiculite which is a typical 2:1 representative layered silicate. Vermiculite is composed of octahedral alumina or magnesia sheets sandwiched between two tetrahedral silicate sheets. Substitutions of high-charge cations by lower charge ones in the tetrahedral and/or octahedral sheets generate a deficit of positive charges in the layers, which are therefore negatively charged. These later are counter balanced by magnesium cations located in the interlayer preserving in this way the electroneutrality of the structure [14]. The cation exchange capacity of vermiculite is usually 90–150 meq per 100 g of solid. Because of its structural

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Fig. 2. SEM images of (a) sodium vermiculite, (b) leached vermiculite, V0.5, (c) and (d) V0.5Cl and (e) and (f) V0.8Cl.

arrangements, vermiculite is an ideal carrier material for loading copper ions and preparing antimicrobial materials [18–22]. A vermiculite-copper ion antibacterial material with up to ∼6 wt% of Cu was developed at Michigan Tech [23]. Therefore, vermiculite has been used to remove different heavy metals cations from aqueous solutions [8]. However, problems due to release of cations are often encountered in non modified clay minerals. Therefore, chemical modifications of the later through silylation have been developed [24]. Silylating agents highly react with hydroxyl groups located on the edges of the clay minerals. The silylating process involves the formation of covalent bonds between silanols and organosilanes groups [24]. After the modification, the surface properties of the clay minerals and their adsorption capacities are modified. Silylated clay minerals have wide applications and have proved to be efficient adsorbents in the treatment of the waste and toxic effluents [25–27], in catalysis [28,29], as electrochemical sensors [30], as drug carriers [31], as enzyme loads [32,33] and for polymer/clay nanocomposite synthesis [34]. Acid lixiviation is used to increase the number of silanol groups on the clay surface [35]. In this study, leached vermiculites were used as a support for the silylating agents. Therefore, many Si OH groups on clay mineral surface can be obtained by acid treatment, which can influence interactions with other species, such as a silylating agent. Several studies have demonstrated that imidazole-modified silica gel removed heavy metals as Hg(II) and Cr(VI) from

aqueous solutions [36,37]. For example, silica-based adsorbent was prepared by gamma-radiation grafting vinyl imidazole onto chloropropyl silica [36]. The vinyl imidazole propyl silica had selectivity for Hg(II) removal in a binary Hg(II)/Cl− solution. The Hg(II) capacity reached 356 mg g−1 in HgCl2 /HNO3 solution at room temperature at pH 5.0. In another study [37], propyl imidazole silica was prepared and applied as adsorbent to Cr(VI) and the maximum adsorption capacity was 47.79 mg g−1 at pH 2.0 with initial Cr(VI) concentration of 150 mg L−1 . The goal of this study is to describe the silylation with chloropropyltrimethoxysilane of three lixiviated vermiculites following reaction with imidazole. The effectiveness of the modified vermiculites for the removal of copper cations from aqueous solution was subsequently determined. 2. Experimental 2.1. Materials and chemicals Vermiculite (V) sample from Santa Luzia (Paraiba, Brazil) was used as the precursor material. Specific surface area and the cation exchange capacity (CEC) were determined in previous work [38,39]. BET surface area was 13 m2 g−1 . All chemicals were of analytical reagent grade. For the modification of vermiculite, the following reagents were used

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washed in deionized water. The solid was finally dried at 333 K under vacuum. 2.3. Synthesis of leached vermiculites, silylation and reaction with imidazole The lixiviation of vermiculite was described previously [38]. The process of leaching consisted of reacting 200.0 g of sodium vermiculite with 500.0 mL of nitric acid solution at three concentrations, 0.3, 0.5 and 0.8 mol L−1 . This suspension was refluxed under mechanical stirring for 4 h at 353 K, yielding 70.0 g leached solid named V0.3, V0.8 and V1.0. The final product was activated by heating the sample at 423 K for 48 h under vacuum. The organofunctionalization consisted of resuspending 5.0 g of the activated solid in 100.0 mL of xylene while a mixture of 5.0 mL of the silane dissolved in xylene was added. The suspension was refluxed with mechanical stirring under a N2 flux at 373 Kfor 48 h. After cooling, the product was filtered and washed with xylene and ethanol. The solid was dried in vacuum at 353 K for 24 h. The obtained products were named V0.3Cl, V0.8Cl and V1.0Cl. The degree of functionalization was determined from the amount of chloro. In the next stage, 3.0 g of each solid,V0.3Cl, V0.8Cl and V1.0Cl, were resuspended in 50.0 mL of DMF, and 5.0 mL imidazole (Im) was added to the suspension. The system was refluxed with mechanical stirring at 333 K for 4 h. The final product was filtered and washed with the solvents DMF, ethanol and water. Then, the solid was dried at 373 K for 24 h. To compare the efficiency of the grafting on lixiviated and pristine samples, sodium vermiculite was reacted with silane under the same conditions as a control reaction. 2.4. Adsorption For the non-equilibrium adsorption measurements, samples of approximately 50 mg solid were resuspended in 25.0 mL of 0.01 mol L−1 Cu(NO3 )2 aqueous solution. The system was maintained in a thermostated bath with stirring for 5–120 min at 298 K. After equilibrium was established, each solid was separated by centrifugation and aliquots of the supernatant were carefully removed. The unreacted amounts of cation were then determined using a GBC model 908A atomic absorption spectrophotometer. The amount of copper adsorbed (q) onto the solid was calculated from the concentrations in the solutions before and after adsorption by Eq. (1): q=

Fig. 3. TEM images of silylated vermiculites (a) V0.3Cl, (b) V0.5Cl and (c) V0.8Cl.

chloropropyltrimethoxysilane (Aldrich), imidazole (Aldrich), nitric acid (Carlo Erba), sodium chloride (Carlo Erba), xylene (Vetec), dimethylformamide (DMF) (Aldrich) and ethanol (Merck).

2.2. Pre-treatment of the surface The vermiculite (V) was initially saturated by ion exchange with excess sodium to eliminate the original cations from the interlayer region as described previously [38,39]. Briefly, the pristine vermiculite was initially subjected to ion exchange, using excess sodium to eliminate the original exchangeable cations (as K+ , Ca2+ , Mg2+ ). Therefore, 100.0 g of clay mineral reacted with 500.0 mL of 1.0 mol L−1 sodium chloride for 48 h at 323 K. The sodium vermiculite was separated via centrifugation and then exhaustively

(Ci − Ce ) × V m

(1)

where Ci and Ce are the initial and equilibrium concentrations of the cation solution (mol L−1 ), respectively, V is the volume of the solution (mL), and m is the mass of the solid (g). The preliminary experiments demonstrated that equilibrium was established in 80 min. Therefore, the time of 80 min was selected for all equilibrium tests. The effect of pH was determined by evaluating the adsorption of the cation over a pH range of 2–8 at 298 K adjusted with 0.1 mol L−1 NaOH or HCl solution using a Digimed model DM-22 pH meter equipped with a combined pH electrode. The effect of pH on the removal of the cation was investigated by mixing 25.0 mL salt solution with 0.05 g solids in a solution with an initial copper concentration of 0.01 mol L−1 at 298 K for 80 min. The pH was adjusted by adding HCl/NaOH solution dropwise. The effect of the copper concentration was studied by measuring the sorption isotherms by the method reported previously [18,19]. 0.05 g of each sample were mixed with 25.0 mL copper solutions ranging between 1 and 10 mmol L−1 and stirred for 80 min at 298 K and pH 6.0 to reach equilibrium. The same procedure was adopted

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to determine the retention of copper on VNa+ under similar conditions, and the adsorption was lower.

2.5. Characterizations The X-ray diffraction (XRD) patterns were obtained using a Shimadzu XD3A model diffractometer equipped with a monochromatic CuK␣ source operating at 40 kV and 30 mA. The diffraction patterns were recorded from 1.4 to 50◦ with a scan rate of 0.67◦ s−1 . The quantities of carbon, nitrogen and hydrogen were determined using a PerkinElmer PE-2400 micro elemental analyzer. The FTIR spectra of the samples dispersed in KBr disks were recorded using a Bomem spectrometer (MB series) over a range of 4000–400 cm−1 at a resolution of 4 cm−1 and with 30 scans for each run. The surface area was determined using the BET method with a FlowsorbII 300 Micromeritics analyzer. Nuclear magnetic resonance spectra of solid samples were performed at room temperature on a Bruker 500 MHz Avance spectrometer with a probe using a 4 mm zirconium rotor. The 13 C and 29 Si cross polarization/magic-angle spinning (CP/MAS) spectra were obtained at frequencies of 75.47 and 59.6 MHz respectively. The experimental conditions were: spinning rate of 10 kHz with a pulse repetition and contact time of 3 s and 3 ms, respectively, for 13 C and rotation frequency of 15 kHz and an acquisition time 3 ms for 29 Si. Scanning electron microscopy was performed using a JEOL JSTM-300 microscope. To examine these nonconducting materials, the samples were coated with a conducting layer of gold and carbon by sputter coating (Plasma Science Inc.). Transmission Electron Microscopy (TEM) measurements of the samples were performed on a JEOL100CX microscope. Samples in the form of bulk powders were suspended in ethanol and then deposited on specific grids (400 mesh copper grids covered with an ultrathin carbon membrane of 2–3 nm thickness). XPS analyses were performed on all samples using a PHOIBOS 100 X-ray photoelectron spectrometer from SPECS GmbH (Berlin, Germany) with a MgK␣ X-ray source (h = 1253.6 eV) operating at >10–10 torr or less and an electron beam power of 300 W (12.5 kV and 24 mA). High resolution XPS conditions were selected (“Fixed Analyser Transmission” analysis mode, a 7 × 20 mm entrance slit), leading to a resolution of 0.1 eV for the binding energies. Spectra were carried out with a 20 eV pass energy for the survey scan and 10 eV pass energy for Cu2p, Cl2p, O1s, Si2p and N1s regions. After data collection, the binding energies were calibrated with respect to the O1s peak at 532.5 eV corresponding to the oxygen atoms bond to silicon. A takeoff angle of 90◦ from the surface was employed for each sample. Element peak intensities were corrected by Scofield factors. The spectra were fitted using the Casa XPS software (Casa Software Ltd., UK) and applying a Gaussian/Lorentzian ratio, G/L equal to 70/30.

2.6. Cation exchange capacity (CEC) determination The experimental cation exchange capacity [40] was measured by method reported previously [41] by using ammonium chlorite buffered at pH 7.0. In brief, 3.0 g sample of clay mineral was suspended in 250.0 mL of a 1.0 mol L−1 NH4 Cl solution and stirred for 24 h. This procedure was repeated two more times, and, finally, the solid was washed several times with deionized water and after being dried under vacuum at 313 K. The ammonium-saturated vermiculite was subjected to nitrogen elemental analysis, which was performed on an instrument from Perkin Elmer, model PE 2400.

2.7. pHPZC measurement The pHPZC (point of zero charge) of the modified vermiculites was determined using the solid addition method [42] and the procedure adopted previously [43]. Briefly, a fixed amount of the sorbent (50 mg) was suspended in 50.0 mL of 0.001 mol L−1 NaCl solutions at various pHs and stirred for 24 h in a shaking bath at 298 K. The initial pH (pHi ) of each solution was adjusted using HCl or NaOH 0.10 mol L−1 . The suspensions were then shaken for 24 h, and the final pH of the supernatant for each flask was measured. The variation of pH values (pH = pHf − pHi ) was plotted against the pHi and the point of intersection of the resulting curve at which pH = 0 was the pHPZC value. This measurement gives an indication of pH where the negative and positive charges on solid were balanced.

3. Results and discussion 3.1. Characterization of solids Lixiviated vermiculites were obtained as described in detail elsewhere [38], and the samples were used as support to silylation. Briefly, the chemical composition data of the native and leached vermiculites is represented in Table 1. Based on these results, the cell composition for pristine vermiculite was [(Si5.98 Al2.02 )(Mg4.17 Al0.15 Fe0.77 Ti0.16 䊐0.75 )O20 (OH)4 ]Ca0.37 Na0.97 K0.54 in which the Fe3+ is equal to the total iron content based on wet chemical analysis and 䊐 represents vacancy sites. The experimental CEC for the sodium vermiculite was 0.6 mmol g−1 . This value is lower than the theoretical value obtained from cell composition (2.25 mmol g−1 ) and those generally reported for vermiculites, but this is most likely because of the purity of the sample. The cation exchange capacity of typical vermiculite is between 0.9 and 1.5 mmol g−1 solid [44]. The SiO2 content increased linearly after treatment with 0.3–0.80 mol L−1 nitric acid because of the lixiviation of magnesium, aluminum and iron, as indicated by decreasing amounts of these elements. The increase in SiO2 results in the larger silanol populations in the lixiviated solids, which favors silylation. A good indication of lixiviation is the data of specific surface area (SSA) obtained from N2 adsorption/desorption measurements. An increase in the SSA was observed during the acid treatments in which the values of 31, 94 and 131 m2 g−1 were obtained for the samples V0.3, V0.5 and V0.8, respectively. These results contrasted with the SSA of the VNa+ (13 m2 g−1 ). The amount of immobilized silylating agent on the solids was determined from the Cl content (Table 2). The elemental analysis suggested that 5.34, 8.04 and 8.17% chloro were present in the silylated vermiculites, indicating that it immobilized 1.5, 2.26 and 2.30 mmol chloropropyl moieties per gram V0.3, V0.5 and V0.8, respectively. These results suggested a high grafting degree for the lixiviated samples, which can be associated to the high population of silanol groups on the leached samples. Successful silylation of clay mineral surfaces depends strongly on the reactivity of clay mineral surfaces, including all internal surfaces, external surfaces and broken edges [24,45]. Indeed, 29 Si NMR spectra showed a higher silanol content as acid concentrations were increased [38]. The data were in agreement with the values of the SSA and the lower content of chloro in the pristine vermiculite (VNa+ ) (0.5%, 012 mmol g−1 ). The observed high immobilization degrees of the lixiviated vermiculites were compared with those observed to others clay minerals, i.e, silylated montmorrillonite, bentonite, palygorskite and halloysite [46–50]. Considering that the VxCl-Im contains two nitrogen atoms, the degrees of immobilization of imidazole were 0.6; 0.6 and 0.7 mmol

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(c)

(b) Intensity (a.u.)

Intensity (a.u.)

(b)

(a)

(a)

(a) V0.3 (b) V0.3Cl (c) V 0.3Cl -Im

(a) V (b) VCl 4000

3500

3000

2500

2000

1500

1000

500

4000

3500

3000

(c)

1500

1000

500

1000

500

(c)

Intensity (a.u.)

(b)

Intensity (a.u.)

2000

Wavenumber (cm )

Wavenumber (cm )

(a)

(b) (a)

(a) V0.5 (b) V0.5Cl (c) V 0.5Cl-Im 4000

2500

-1

-1

3500

3000

(a) V0.8 (b) V0.8Cl (c) V08Cl-Im 2500

2000

1500

1000

4000

500

3500

3000

2500

2000

1500 -1

-1

Wavenumber (cm )

Wavenumber (cm )

Fig. 4. Infrared spectra for sodium and the organically modified vermiculites.

Table 1 Chemical analysis and fire loss of precursor vermiculite and leached vermiculites. Solid

SiO2 (%)

Al2 O3 (%)

Fe2 O3 (%)

TiO2 (%)

CaO(%)

MgO(%)

Na2 O(%)

K2 O(%)

Fire loss (%)

V V0.3 V0.5 V0.8

40.08 43.47 44.33 46.14

12.35 11.48 11.13 9.85

6.83 6.85 6.80 6.36

1.43 1.51 1.62 1.82

2.32 2.37 1.55 0.58

18.74 16.04 15.31 13.70

3.37 1.05 0.42 0.45

2.86 2.45 2.65 2.84

11.85 14.62 15.99 18.21

Table 2 Elemental analysis (in percentages and mmol g−1 ) of chloro (Cl) for silylated solids and carbon (C), nitrogen (N) and hydrogen (H) for the vermiculites containing imidazole. Solid

VNa+ Cl V0.3Cl V0.5Cl V0.8Cl V0.3Cl-Im V0.5Cl-Im V0.8Cl-Im

Cl

C −1

(%)

(mmol g

0.50 5.34 8.04 8.17 nd nd nd

0.14 1.50 2.26 2.30

)

(%) – – – – 3.11 3.56 3.66

N −1

(mmol g

2.59 2.92 3.05

)

(%) – – – – 1.66 1.63 1.94

H −1

(mmol g

1.18 1.17 1.38

)

(%) – – – – 0.76 2.07 2.29

nd: not determined.

per gram of solid onto the silylated surfacesof V0.3Cl, V0.5Cl and V0.8Cl, respectively. These values are lower than the chloro quantity in mmol g−1 indicating that a larger population of unreacted

chloropropyl can be present onto the final surface. Therefore, despite good silane immobilization, the pendant groups of the organic moieties of the silane were not completely available for

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reaction with the imidazole molecule possibly because of steric hindrance. The XRD of the VNa+ show the typical reflection of vermiculite at 6.2◦ that resulted in a basal spacing of 1.42 nm assigned to 002 reflection (Fig. 1). The attributions were based on previous studies with Brazilian vermiculites [38,51–53]. Broader reflections at 12.2 (d = 0.71 nm), 18.07 (d = 0.49 nm), 24.2 (d = 0.37 nm), 29.1 (d = 0.30 nm) 31.2 (d = 0.29), 32.2 (d = 0.28 nm), 36.3 (d = 0.25 nm), 43.5 (d = 0.21 nm) were also observed. The occurrences of biotite/vermiculite and chlorite/vermiculite with regular interstratifications were verified due the weak reflection at 10.72◦ (d = 0.824 nm) [54] and medium reflection at 25.3◦ (d = 0.35 nm), respectively. The reflection at 7.15◦ is attributed to one water layer and the reflection at 3.91◦ (d = 2.56 nm) indicated the occurrence of an interstratification phase [55] and the peak at 26.0◦ was attributed to an impurity of quartz. After acid activation a change in the crystal structure of the vermiculites occurred as demonstrated by broadening and decreasing the characteristic peaks of the pattern (Fig. 1). The intensity of all the basal reflections decreased after acid treatment suggesting that both the octahedral and tetrahedral sheets were modified. Similar behavior was observed for lixiviated vermiculites [56–59]. XRD patterns of the organically modified samples exhibit the same reflections than the activated vermiculites. The difference lies on the splitting of the (0 0 2) reflection in two components for both silylated samples. For V0.3Cl and V05Cl, the reflections occurred at 6.5◦ and 7.3◦ which corresponded to interplanar distances of 1.36 and 1.21 nm while for V0.8Cl one reflection at 7.5◦ (d = 1.18 nm) was observed. The d002 value do not shift after reacting with the organic moities suggesting that the later are not intercalated in the interlayer space of the vermiculites. The presence of the second component at 7.6◦ (V0.3Cl-Im) and 7.8◦ (V0.5Cl-Im) and corresponding to an interplanar distances of 1.16 and 1.13 nm indicate that the imidazole groups are located on the surface. The SEM images of the sodium and lixiviated vermiculites are shown in Fig. 2. The pristine vermiculite (VNa+ ) showed typical thick aggregation with a plate-like morphology formed from the stacking of the silicate layers [59]. After leaching, the layered appearance of the solids remained but a relative decrease in the particle size was observed resulting in solids with higher surface area as discussed previously. This same morphology was maintained after organofunctionalization (Fig. 2f). TEM micrographs (Fig. 3) of the V-Na+ show a thickness of the layer in addition of the interlayer distance of 1.36 nm (Fig. 1). The values measured for V0.3Cl, V0.5Cl and V0.8Cl hybrid images are about 1.2–1.3 nm. The sample V0.8Cl presented some amorphous regions indicating that the acid treatment can alter the surface at high concentrations. It can be suggested that the organic moities are not intercalated but adsorbed on the surface of the vermiculite as attested by XRD. The infrared spectra of the solids before and after organofunctionalization of the vermiculite are shown in Fig. 4. The primary infrared absorption bands for VNa+ are located at 3450, 997, 814 and 680 cm−1 and are attributed to water stretching vibration, symmetric deformation of Si O Si and Si O Al groups, Al OH deformation, and Al O deformation, respectively [60,61]. The bending vibrations of the hydroxyl groups and the stretching and bending of the silicate and octahedral cationsare not significantly altered after lixiviation. The FTIR spectra of the lixiviated vermiculites show absorption bands at 1080, 810 and 460 cm−1 arising from the stretching and bending vibrations of Si O Si and Si O Al groups [60]. For the silylated vermiculites, new absorptions were verified at 2900 and 2850 cm−1 , which were attributed to asymmetric and symmetric C H stretching. The band at 1443 cm−1 was related

Fig. 5. i) 13 C and ii) 29 Si NMR spectra of silylated leached vermiculites of (a) V0.3Cl, (b) V0.5Cl and (c) V0.8Cl. *Side bands.

to CH2 deformation [62]. These characteristic absorptions were detected for clay minerals organofunctionalized with alcoxysilanes of different functionalities as amino, mercapto and epoxy groups [63–65]. The expected C Cl stretching band at 657 cm−1 can be overlaps with the vibrations of the groups related to theinorganic structure. The spectra of the VCl-Im solids were compared with the infrared absorptions of the free imidazole [66]. Typical absorptions of imidazole are observed in the modified vermiculite (V0.3Cl-Im) at 3162 cm−1 caused by N H stretching, 2950 cm−1 (C H asymmetric stretching of ring), 2865 cm−1 (C H symmetric stretching of ring), 1588 cm−1 (CC stretching), 1507 cm−1 (N H bending), 1486 cm−1 (ring stretching), and 1313 (C H bending). The other bands are unchanged in all the derived spectra.

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Fig. 6. Reactions on the vermiculite surface showing the silane grafting and the subsequent interaction with imidazole molecule (Im). Mn+ represents H+ or remained Na+ .

Table 3 Kinetic parameters, regression coefficients (R2 ) and Chi-square test (2 ) for the adsorption of copper onto modified vermiculites. Solid

Parameter

Model

V0.3Cl-Im

q qe k R2 2

Pseudo-first-order 0.89 0.897 0.051 0.543 1.605

Pseudo-second-order 0.732 0.643 0.083 0.968 0.051

Intraparticle diffusion C 0.276 kdi 0.038 R2 0.844

V0.5Cl-Im

q qe k R2 2

0.192 0.191 0.045 0.711 0.647

0.336 0.324 0.623 0.993 0.045

C 0.281 kdi 0.004 R2 0.357 2 1.043

V0.8Cl-Im

q qe k R2 2

0.428 0.407 0.025 0.924 0.064

0.587 0.457 0.049 0.984 0.011

C −0.042 kdi 0.054 R2 0.845

CP/MAS 13 C NMR was used to confirm the presence of organic functional groups on the leached vermiculites after silylation and

the spectra are shown in Fig. 5i. Three well-defined peaks were detected at 10.6; 27.2 and 48.4 ppm which they were assigned to

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a) b)

-1

qt (mmol g )

0.6

c)

0.4

0.2

equation, pseudo-second order equation, and intraparticle diffusion, were used. The linear form of the pseudo-first-order rate equation in integrated form is as follows [70] log(qe − qt ) = logqe −

0

20

40

60

80

100

120

t (min) Fig. 7. The effect of adsorption time of the divalent copper from aqueous solution on the modified vermiculites (a) V0.3Cl-Im, (b) V0.8Cl-Im and (c) V0.5Cl-Im.

carbons C1, C2 and C3 of the chloropropyl moieties, respectively [67]. These data confirmed the silylation of the leached vermiculites without structural alteration in the carbon atoms of the organic molecule. CP/MAS 29 Si NMR (Fig. 5ii) suggested the grafting of silane on surface of the vermiculite. Typical signals at −55 and −67 ppm were attributed to T2 and T3 species, which are related to the number of oxygens bound to silyl groups, R Si* (OSi )2 X, where X could be OH or OCH3 groups (T2 ), and R Si* (OSi )3 (T3 ) [68]. The presence of Tn species were not detected in the 29 Si NMR of the lixiviated solids and the spectra presented a series of typical silanol and siloxane signals corresponding to Q2, Si*(OH)2 (OSi )2 at −86 ppm, Q3 , Si*(OH)(OSi )3 at −92 ppm and Q4 species, Si*(OSi )4 at −110 ppm [38]. The structural analysis techniques XRD, 13 C and 29 Si NMR and infrared spectroscopy, revealed chemical modification of the lixiviated vermiculites and the covalently organofunctionalization of the clay mineral surface without significant incorporation of the silylating agent on interlayer spacing, primarily because of the moderate expansive behavior of vermiculite. The reactions involved in the chemical modifications are the initial substitution of the hydrogen in the hydroxyl groups onto the surface by the methoxyl of the silylant agent resulting in the incorporation of Si(CH2 )3 Cl onto the surface. Subsequently, the imidazole molecule was covalently linked onto the surface as shown in Fig. 6.

(2)

where qe and qt are the amounts of dye adsorbed (mg g−1 or mol g−1 ) at equilibrium and at time t (min), respectively, and k1 (min−1 ) is the adsorption rate constant of first-order adsorption. The experimental data were also used in the pseudo-second order kinetic model given by Eq. (3) [71]. t 1 t = + 2 qt q k2 × qe e

0.0

k1 × t 2.303

(3)

here k2 (g mg−1 min−1 or g mmol−1 min−1 ) is the rate constant of pseudo-second-order chemisorptions, and qe and qt are defined in Eq. (3). Under the established conditions, the rate constants k1 and k2 and the theoretical equilibrium sorption capacities qe were calculated from the slopes and intercepts of the linear plots of the pseudo-first-order and pseudo-second-order kinetic models. The fitting of the data to these two models at 298 K is shown in Supplementary material 1, and the results are summarized in Table 3. In the pseudo-second-order model, the R2 values are larger, and the qe value is consistent with the calculated experiment values (qeexp ), indicating a good fit of the adsorption process to this model. These results are consistent with other studies for copper adsorption on vermiculite under conditions different from those adopted here [72]. The Webber’s pore-diffusion model was the third adopted model [73] which is defined as: qt = kid t 0.5

(4)

where kid (mg g−1 min−1/2 or mmol g−1 min−1/2 ) is the intraparticle diffusion constant. The plot of qt versus t0.5 represents the different stages of adsorption controlled by the film diffusion during the faster stages and by particle diffusion at later stages [74].The intraparticle diffusion model shows that the adsorption processes are not linear over the entire time range and that the processes occur in three stages as shown in Supplementary material 1. Therefore, the linearity was evaluated separately. The second linear plots did not pass through the origin, which indicates that intraparticle diffusion is involved in the adsorption process but is not the only rate controlling step. 3.3. Effect of pH

3.2. Adsorption The organically modified vermiculites have potential sorption properties because of the imidazole groups, which have a chelating characteristic that favors their coordination with metal cations, such as copper at the solid/liquid interface. Therefore, these modified solids as adsorbents for divalent copper ions from aqueous solutions were evaluated. First, the time necessary for the sorption equilibrium to occur was determined. The effect of contact time at a fixed temperature on the retention of copper onto the VCl-Im surfaces is shown in Fig. 7. Saturation was achieved in 80 min for all solids under the given test conditions with a copper concentration of 0.01 mol L−1 . Therefore, 80 min was used for all subsequent experiments. This time contrasted with the regular equilibrium observed for the removal of copper by pure vermiculite [18,19], which extended times (above 48 h) were observed for copper retention [18,19,69]. A kinetic study of adsorption is important because it provides information about the efficiency and possibility of the expansion of the process. In this study three kinetic models, pseudo-first order

pH is an important parameter that impacts the adsorption process because influences the adsorption sites on surface and the solution chemistry of the heavy metals through hydrolysis, complexation by organic and/or inorganic ligands, redox reactions, precipitation and availability of heavy metals [75]. At pH < 4.0 at 298 K with copper concentration of 0.01 mol L−1 without control of the ionic strength, divalent copper is the dominant species in solution (>99.9%). At the same conditions of concentration and temperature and at pH 5.0, copper ions are the dominant species (99.4%) and 0.6% of hydroxo complexes can be formed, i.e., Cu2 (OH)2 2+ , CuOH+ and Cu2 OH3+ . At pH 6.0, copper species are present as follow: 72% of Cu2+ , 20% of Cu2 (OH)2 2+ , 6.45% of Cu3 (OH)4 2+ , 1.55% of CuOH+ and 0.26% of Cu2 OH3+ . At pH 8.0, we have most of the copper species present under Cu3 (OH)4 2+ (93,5%) form, with small amount of the other species: 0.34% of Cu2+ , 5.16% of Cu2 (OH)2 2+ , 0.83% and 0.14% are CuOH+ and Cu(OH)2(aq) , respectively. Since the speciation of the functional groups of the adsorbent is pH dependent, the measurements of the pHPCZ were done and shown in Supplementary information (material 2). In our case, the

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415

a) b)

2.5

8.0

2.0

c)

-1

qe (mmol g )

q (mmol/g)

6.0 4.0 c)

2.0

1.5 1.0 0.5

b) a) 0.0

0.0

0

3.0

4.0

5.0

6.0

7.0

8.0

pH Fig. 8. The effect of the pH on the sorption of copper on the modified vermiculites (a) V0.3Cl-Im, (b) V0.5Cl-Im and (c) V0.8Cl-Im at 298 K.

point of zero charge occurred in the pH range of 4.0–6.0. The pHPZC occurred at near of pH 6.0 for V0.3Cl-Im and V0.8Cl-Im, and at 4.0 and 6.0 for V0.5Cl-Im. These values illustrated the heterogeneity of the sites on the surface. Comparing with vermiculite, it is established that pure vermiculite shown negative zeta potential in pH range of 1.0–10.0 [64]. For the other hand, free imidazole has amphoteric character whereas pKa is 6.95 [76]. The effect of pH on the sorption of copper on the three modified vermiculites is shown in Fig. 8. The lowest copper sorption occurred in acidic medium (pH ≤ 3), whereas the highest copper removal was obtained at pH 8.0, which is primarily attributed to precipitation. Other studies have reported the same behavior using natural vermiculite [18,19,69]. The adsorption of metals on the active sites of clay minerals results in the formation of outer and inner sphere complexes [77–79]. Outer-sphere complexes are formed through electrostatic interactions between metal cations and the negatively

1

2

3

4

5

6

7

-3

Ce (mmol dm ) Fig. 10. Sorption isotherms of copper from aqueous solution on modified vermiculites (a) V0.8Cl-Im, (b) V0.3Cl-Im and (c) V0.5Cl-Im at 298 K and pH 6.0.

charged centers. Therefore, metal binding occurs via ion exchange at the planar sites of the adsorbent [69]. During the earlier adsorption stages, outer-sphere complexes are formed at the external surface sites [8]. As the metal concentration increases, metal ions are forced into the internal surface sites forming inner-sphere complexes. Outer-sphere complexation is whereas rapid and reversible, while inner-sphere complexation is slower and may be irreversible [8]. For organically modified vermiculites, other adsorption mechanism can be dominant as the formation of coordination compound between the organic moieties and the cations. It is demonstrated that in acid conditions (i.e., pH < 3) most of the silanol and aluminol groups on edges are protonated and other aspect is the presence of H3 O+ ions which compete for the edges sites, toward which they have a higher affinity than metal ions [69,80]. For the modified solids, the imidazole is protonated. At these conditions, the copper retention was very low. At pH 6.0, the

Fig. 9. The proposed mechanism for adsorption of copper on modified vermiculites. The cationic hidroxo copper species were not considered. Mn+ represents H+ or remained Na+ .

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Table 4 Parameters obtained from the Langmuir and Freundlich sorption isotherms of copper for three modified vermiculites. Model

Parameter

Solids

−1

Langmuir

qmax (mmol g KL (L mol−1 ) R2

Freundlich

n KF R2

)

V0.3Cl-Im

V0.5Cl-Im

V0.8Cl-Im

2.52 1.59 0.986

2.38 0.84 0.995

2.69 0.79 0.998

4.02 1.59 0.659

3.36 1.19 0.956

4.35 1.62 0.924

c) b) a)

0

200

400

600

800

1000

1200

BE (eV)

Cu 2p

c) b) a)

930

940

950

960

970

980

BE (EV) Fig. 11. XPS spectra of total and Cu elements for the solids (a) V0.3Cl-Im, (b) V0.5ClIm and (c) V0.8Cl-Im after copper sorption (initial concentration of copper solution was 0.01 mol L−1 ).

sites are deprotonated and the negative surface can adsorb cations by electrostatic interaction or by covalent bonding with imidazole derivative. Based on this, the mechanism of copper adsorption involves: a) the complexation with the imidazole on surface or interlayer spacing, which is the preponderant effect (Fig. 9), but other two probable mechanisms can occur b) cation exchange at the planar sites due the presence unreacted silanol groups, which was

detected in 29 Si NMR (Fig. 5), resulting from the interactions with the negative permanent charge (outer-sphere complexes) and c) complexes between the deprotonated silanol (Si O− ) and aluminol (AlO− ) groups at the edges of the clay mineral. Although the maximum removal caused to adsorption was higher than 8.0, this also includes precipitation that occurs at basic pH. Therefore, pH 6.0 was established as the optimum pH for the sorption experiments. 3.4. Effect of the initial copper concentration The equilibrium sorption isotherm is fundamentally important in the design of sorption systems [81]. Therefore, a series of different concentrations varied from 10−3 to 10−2 mol L−1 were used to investigate the influence of initial copper concentrations on adsorption. The adsorption isotherms of all the modified vermiculites (Fig. 10) showed similar behavior over a wide range of copper concentration. The sorption capacity of the modified vermiculite for copper cations was obtained from the sorption isotherms (Fig. 10), and the copper sorption order is V0.5Cl-Im < V0.3Cl-Im < V0.8Cl-Im, which the values were 1.98, 2.25 and 2.38 mmol g−1 . The adsorption was higher for solid with higher organic content (V0.8Cl-Im) but the other two solids had the same immobilization quantity of imidazole. In others words, this means that the complexation of copper with imidazole can be the preponderant effect on sorption but it is not the only effect as it was discussed previously. Compared to the pure vermiculite, the sorption capability of modified vermiculites copper(II) was improved. Previous study demonstrated that pure vermiculite has copper sorption capacity of 0.84 mmol g−1 [18]. The adsorption equilibrium data for a single component can be interpreted by the Langmuir and Freundlich isotherms, which are represented mathematically in the linear forms as follows [82,83]: Ce Ce 1 = + qe qmax KL × qmax

Langmuir equation (5)

1 logCe n

Freundlich equation (6)

logqe = logkF +

In these equations, Ce is the concentration of the copper in solution (mol L−1 ) at equilibrium with the adsorbed copper, qe is the amount of the adsorbed copper (mol g−1 ) at the solid/liquid interface, qmax is the monolayer capacity of the adsorbent (mol g−1 ), KL is the Langmuir adsorption constant (L mol−1 ), KF and 1/n are empirical parameters, kF is the adsorption constant [mol g−1 (L g−1 )−1/n ] related to the bonding energy and 1/n is associated with the surface heterogeneity. The parameter n is a measure of the deviation from linearity of the adsorption, which indicates the degree of nonlinearity between the solution concentration and the adsorption. The linear Langmuir and Freundlich plots for the sorption of copper(II) onto imidazole modified vermiculites were obtained by plotting the Ce /qe versus Ce (Supplementary material 3) and the logqe versus logCe (Supplementary material 1), respectively. The isotherm constants and correlation coefficients (R2 ) were calculated and listed in Table 4. The adsorption data were properly fitted by the Langmuir equation, and Table 4 summarizes the adsorption parameters (qmax and KL ). The applicability of Langmuir isotherm suggests the monolayer coverage of the copper on the surface of the modified vermiculites. From the slope of the graph Ce /qe versus Ce , the maximum adsorption capacity (qmax ) of each modified vermiculites was calculated and is consistent with experimental values shown in Fig. 10. The qmax value denotes the monolayer capacity and the KL represents the Langmuir bonding term, a constant related to the adsorption/desorption energy. According to the KL , copper adsorption was more favorable following the order of V0.5Cl-Im < V0.3ClIm < V0.8Cl-Im (Table 4).

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XPS was used to available the copper species on surface of modified solids. The attributions of copper species were based on previous studies with imidaloze copper complexes [84] and copper adsorbed on polystyrene-supported chitosan [85]. The XPS spectra of Cu(2p) for three imidazole vermiculites with adsorbed copper (Fig. 11) show two peaks with binding energy (BE) of 935 eV (Cu 2p3/ 2 ) and 955 eV (Cu 2p1/2). The first peak (935 eV) may related to imidazole-copper complexes [84]. Peak at binding energy of 935 eV is accompanied by a satellite peak at 943 eV, and peak at 955 eV by 963 eV. No photo reduction effect seems to be observed. The Cu(2p) line shape indicates that Cu is present only as Cu(II) ions as the shake up satellites present at binding energies of 942.7 and 962.0 eV are characteristic of the unfilled d orbitals. A symmetrical band in the O 1s region (Supplementary material 4) at 532.6 eV is due to singlebonded oxygen from the clay lattice. The peak shows shifts to lower energy values (531.7 and 532 eV) suggesting a change in the atoms environment as a consequence of the grafting reaction. The Cu/Cl and Cu/N atomic ratios are consistent with the elemental analysis values obtained from the bulk material. 4. Conclusions The silylation of clay minerals raises several questions. Previous works reported on the importance of lixiviation to increase the reactivity of the surface in the presence of organosilanes. Indeed the amount of silanol is more important for samples that were submitted to intense acid attack and will allow a higher amount of grafted organosilane in the clay mineral. New organo-modified vermiculites were prepared by the initial silylation of the inorganic matrices and subsequent reaction with imidazole. The multitechniques approach usually used in the field of materials science allow the determination of organic amount present in the samples in our case, their location and their interaction with the surface. Indeed, the XRD patterns do not exhibit any increasing in the d002 indicating that there is no doubt concerning the grafting on the surface of these organosilanes. Moreover, on the 29 Si NMR spectra, the presence of the Tn sites is a proof of the grafting. The addition of imidazole on the grafted vermiculites induced the covalent bonding of the later via a covalent bonding between the atom 3 of the imidazole ring and the carbon of the organosilane. IR spectroscopy shed the light on the presence of the different bands attributed to the imidazole groups. The adsorption of copper was then undertaken and compared to pure vermiculite, the sorption capability of the modified vermiculite for copper(II) was improved. The sorption of Cu2+ onto the three modified vermiculites was fitted to the Langmuir model, indicating monolayer sorption. The sorption kinetics was described by a pseudo-second-order model. Acknowledgement The authors greatly appreciate the financial support provided by the CNPq (Grant Number 307637/2013-1). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2015.11. 042. References [1] N.X. Wang, X.Y. Zhang, J. Wu, L. Xiao, Y. Yin, A.J. Miao, R. Ji, L.Y. Yang, Effects of microcystin-LR on the metal bioaccumulation and toxicity in Chlamydomonas reinhardtii, Water Res. 46 (2012) 369–377. [2] T. Ozaki, T. Kimura, T. Ohnuki, Z. Yoshida, A. Francis, Association mechanisms of Europium(III) and Curium(III) with Chlorella vulgaris, J. Environ. Toxicol. Chem. 22 (2003) 2800–2805.

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