Green synthesis of the copper nanoparticles supported on bentonite and investigation of its catalytic activity

Accepted Manuscript Green synthesis of the copper nanoparticles supported on bentonite and investigation of its catalytic activity Zahra Issaabadi, Mahmoud Nasrollahzadeh, S. Mohammad Sajadi PII:

S0959-6526(16)31729-2

DOI:

10.1016/j.jclepro.2016.10.109

Reference:

JCLP 8300

To appear in:

Journal of Cleaner Production

Received Date: 17 August 2016 Revised Date:

6 October 2016

Accepted Date: 21 October 2016

Please cite this article as: Issaabadi Z, Nasrollahzadeh M, Sajadi SM, Green synthesis of the copper nanoparticles supported on bentonite and investigation of its catalytic activity, Journal of Cleaner Production (2016), doi: 10.1016/j.jclepro.2016.10.109. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Graphical Abstract Green synthesis of the copper nanoparticles supported on bentonite and investigation of its catalytic activity

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Zahra Issaabadi, Mahmoud Nasrollahzadeh* and S. Mohammad Sajadi

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Green synthesis of the copper nanoparticles supported on bentonite and investigation of its catalytic activity Zahra Issaabadi,a Mahmoud Nasrollahzadeha,b,* and S. Mohammad Sajadic Department of Chemistry, Faculty of Science, University of Qom, Qom 37185-359, Iran b

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Center of Environmental Researches, University of Qom, Qom, Iran

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Department of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government,

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Iraq

ABSTRACT

In this study, we reported a cost effective and environment friendly technique for the synthesis of the

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copper nanoparticles supported on bentonite (bentonite/Cu NPs) using Thymus vulgaris L. leaf extract as a mild, renewable and non-toxic reducing agent and efficient stabilizer without adding any surfactants. The catalytic performance of the catalyst was examined for the degradation of methylene blue (MB) and Congo red (CR) in aqueous medium at room temperature using sodium borohydride (NaBH4) as the source of hydrogen, which indicated that the composite had an excellent catalytic activity, convenient reusability and

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long-term stability for the reduction of organic dyes.

1. Introduction

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Keywords: Bentonite/Cu NPs, NaBH4; Green synthesis; Reduction; Methylene blue; Congo red

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Dyes are the major effluents from various industries such as paper, plastic, leather, food, and textiles that cause significant pollution (Dia et al., 2009; Spain, 1995). Most of these dyes are toxic, non-biodegradable and persist in the environment and have a potential toxicity toward humans, animals and plants. Among them, MB and CR are usually toxic and have carcinogenic and mutagenic effects towards the biosphere. Therefore, the degradation of these pollutants from the environment is an important challenge in ecological systems, due to their toxicity and carcinogenic properties.

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Corresponding author. Tel.: +98 25 32850953; Fax: +98 25 32103595.

E-mail address: [email protected] (M. Nasrollahzadeh).

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There are several methods in the literature such as chemical reduction, photodegradation, reverse osmosis and coagulation for the safe disposal of these dyes (Wilhelm and Stephan, 2007; Bastaki, 2004; Shi et al., 2007). Among them, the chemical reduction of organic dyes using a strong reducing agent in the

(Jana et al., 1999; Sen et al., 2013; Dasog et al., 2011).

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presence of noble metals such as Pt, Au, Ag and Cu is one of the famous removal methods in this field

Nowadays, metal nanoparticles (MNPs) have been developed for the treatment of wastewaters (Kundu et al., 2003; Kundu et al., 2009; Sahay et al., 2012). However, from the perspective of catalysis, such

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nanoparticles are expensive, scarce and usually need suitable support to prevent their aggregation. Among MNPs, Cu is several orders of magnitude cheaper than noble metals, which favours its wider applications

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in the fields of catalysis (Nemanashi and Meijboom, 2013; Zhang et al., 2013; Sahiner et al, 2012). However, agglomeration is of the Cu NPs inevitable. Also, separation and reuse of catalysts based on Cu NPs with a smaller size are difficult, which limits their practical applications. Recently, our research group reported the green synthesis of MNPs supported on the various supports such as eggshell (Nasrollahzadeh et al, 2016a), Fe3O4 (Nasrollahzadeh and Sajadi, 2016), TiO2 (Atarod et al, 2016a), zeolite (Hatamifard et al, 2016), graphene oxide (Atarod et al, 2016b) and CuO (Nasrollahzadeh, 2016) to decrease MNPs

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agglomeration and overcome the drawbacks concerning stability and recovery of MNPs. To date, MNPs have been successfully prepared by chemical and physical methods which are extremely expensive and also involve use of hazardous reagents, toxic organic solvents and high pressure and

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environmental and biological risks that limit their usage in medical applications (Komarneni et al., 2002). Therefore, it is still a challenge in the development of environmentally friendly, sustainable, simple, rapid,

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easy to control and energy-efficient methods for the preparation of MNPs with suitable catalytic activity. Compared to the chemical and physical protocols, environmentally benign production methods of MNPs are very attractive (Tavakoli et al., 2015; Tavakoli et al., 2013; Mandizadeh et al., 2015; Tavakoli et al., 2014; Tavakoli and Salavati-Niasari, 2014; Tavakoli et al., 2016). In the last years, biosynthesis of NPs have been received considerable attention due to the growing need to develop clean, nontoxic chemicals, environmentally benign solvents and renewable materials (Nasrollahzadeh et al, 2016a; Nasrollahzadeh and Sajadi, 2016; Atarod et al, 2016a; Hatamifard et al, 2016; Atarod et al, 2016b; Nasrollahzadeh, 2016). In recent years, the development of efficient green chemistry methods employing natural reducing, capping,

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and stabilizing agents to prepare MNPs with desired morphology and size have become a major focus of researchers. The green method employing plant extracts have drawn attention as a simple and viable alternative to chemical procedures and physical methods. Biological methods can be used to synthesize

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MNPs without the use of any harsh, toxic and expensive chemical substances. However, there are only a few reports on the biosynthesis of Cu NPs using plant extracts without using any toxic or hazardous reagent, additives/promoters, high pressure and energy and organic solvent or harmful reducing and capping agents (Nasrollahzadeh et al, 2016a; Nasrollahzadeh and Sajadi, 2016; Atarod et al, 2016a;

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Hatamifard et al, 2016; Atarod et al, 2016b; Nasrollahzadeh, 2016; Salavati-Niasari et al, 2013; Ansari and Salavati-Niasari, 2016a; Ansari et al, 2016b; Gholami et al, 2016; Mandizadeh et al, 2014).

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Quite recently, one of the researchers of our group has published preparation of bentonite/Cu NPs using Thymus vulgaris L. leaf extract and its application as a new and reusable heterogeneous catalyst for the synthesis of 1-substituted 1H-1,2,3,4-tetrazoles and reduction of 4-nitrophenol (Rostami-Vartooni et al., 2015). From this investigation we found that the Thymus vulgaris L. leaf extract mediated biosynthesis process can be cost effective and also alternate for conventional chemical and physical processes for the synthesis of Cu NPs.

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Based on the above considerations, herein, we present an efficient and simple method for the catalytic degradation of several organic dyes, including MB and CR by NaBH4 as reducing agent in an aqueous solution and in the presence of bentonite/Cu NPs as a heterogeneous catalyst, which can be recycled for

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2. Experimental

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five successive cycles of the reduction reaction.

2.1. Instruments and reagents All reagents were purchased from the Merck and Aldrich chemical companies and used without further purification. IR (KBr) spectra were recorded on a Perkin-Elmer 781 spectrophotometer. Melting points were taken in open capillary tubes with a BUCHI 510 melting point apparatus and were uncorrected. X-ray diffraction (XRD) measurements were performed with a Philips powder diffractometer type PW 1373 goniometer. It was equipped with a graphite monochromator crystal. The X-ray wavelength was 1.5405 A˚ and the diffraction patterns were recorded in the 2θ range (10-80) with scanning speed of 2º/min.

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Morphology and particle dispersion was investigated by scanning electron microscopy (SEM) (Cam scan MV2300). The chemical composition of the prepared nanostructures was measured by EDS (Energy Dispersive X-ray Spectroscopy) performed in SEM. EDS (S3700N) was utilized for chemical analysis of

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prepared nanostructures. The shape and size of the bentonite/Cu NPs were identified by transmission electron microscope (TEM) using a Philips EM208 microscope operating at an accelerating voltage of 90 kV.

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2.2. Preparation of Thymus vulgaris L. leaf extract

Thymus vulgaris L. leaf extract was prepared according to our recent work (Nasrollahzadeh et al, 2016b)

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with some modifications. Briefly, to achieve a suitable concentration of antioxidant phenolics (e.g; flavonoids and glycosides) as highly soluble compounds in water, 100 g of dried powdered leaf of Thymus vulgaris L. was added to 500 mL double distilled water and refluxed on a magnetic heating stirrer at 70 °C for 60 min. Then after cooling to room temperature the obtained extract was centrifuged at 7000 rpm, then filtered and filtrate was kept in the refrigerator for further use.

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2.3. Preparation of the Cu NPs using the aqueous extract of the leaves of Thymus vulgaris L. 50 mL aqueous extract of the leaves of Thymus vulgaris L. was added to 50 mL 0.003 M solution of the CuSO4·5H2O at 60 °C with shaking vigorously under Ar atmosphere. After 5 min the dark color was

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appeared in the mixture which indicated the formation of Cu NPs. The colloidal dispersion of nanoparticles was then filtered and centrifuged at 7000 rpm for 30 min. Finally, the obtained precipitation washed with

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absolute alcohol to remove possible impurities and kept in Ar atmosphere to protect any deformation and decomposition processes.

2.4. Preparation of the bentonite/Cu NPs using the aqueous extract of the leaves of Thymus vulgaris L. The bentonite/Cu NPs were prepared according to the green reported procedure (Rostami-Vartooni et al, 2015). For the synthesis of the bentonite/Cu NPs, 100 mL of 0.2 M CuSO4·5H2O solution was added dropwise to a well-mixed solution of the plant extract and 10 g of natural bentonite with constant stirring at

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80 °C for 4 h. The solid product was separated by filtration and then washed with deionized water and ethanol several times and dried at 100 °C for 2 h.

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2.5. CR catalytic degradation in the presence of the bentonite/Cu NPs 25 mL of CR aqueous solution (1.44 × 10-5 M) was mixed with 10.0 mg of the bentonite/Cu NPs and stirred. Then 25 mL of freshly prepared aqueous NaBH4 solution (5.3 × 10-3 M) was added into the above mixture and stirred for 5 min and the color of the mixture vanished gradually, indicating the reduction of

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the CR. The progress of the reaction was monitored by UV-Vis spectrometer at regular intervals. The characteristic absorption of the CR at λmax = 493 nm was selected as monitoring the catalytic reduction

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process. After completion of the catalytic reaction, the catalyst was separated from the catalytic system by centrifugation, washed with deionized water and ethanol, dried and used as it is for further reactions.

2.6. MB catalytic degradation in the presence of the bentonite/Cu NPs

In a typical experiment, 10.0 mg of the bentonite/Cu NPs was mixed with the MB aqueous solution (3.1 × 10-5 M, 25 mL) at room temperature. Then freshly prepared aqueous NaBH4 solution (5.3 × 10-3 M, 25 mL)

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was rapidly injected into the above mixture under stirring. The catalytic activity of the bentonite/Cu NPs was evaluated by monitoring the variation in optical absorption at the wavelength of the absorption maximum (λmax = 663 nm) of the MB with UV-Vis absorption spectra. After completion of the catalytic

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reaction, the catalyst was separated from the reaction mixture by centrifugation, washed with deionized

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water and ethanol and then dried for the next cycle.

3. Results and discussion

The basic concept of this process is to utilize the Thymus vulgaris L. leaf extract as reducing agent and efficient stabilizer to convert Cu2+ ions to Cu NPs and their immobilization on the surface of bentonite. The biomolecules found in the Thymus vulgaris L. leaf extract induce the reduction of Cu2+ ions from CuSO4·5H2O to copper nanoparticles (Scheme 1). Phenolics and other chemicals present in the leaf extract are not only cogently reduce copper salts but also provide excellent tenacity against agglomeration. Flavonoids can directly scavenge molecular species of active oxygen. Antioxidant action of flavonoids

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resides mainly in their ability to donate electrons or hydrogen atoms. Phenolic compounds possess hydroxyl and ketonic groups which are able to bind to metals and show chelate effect. To investigate the functional groups of the Thymus vulgaris L. leaf extract, UV-Vis and FT-IR studies were carried out and

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the spectra are shown in Fig. 1 and 2 (Rostami-Vartooni et al., 2015; Nasrollahzadeh et al, 2016b).

Scheme 1.

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The UV spectrum of the Thymus vulgaris L. leaf extract (Fig. 1) shows specified signals at λmax 342 nm (bond Ι) and 285 nm (bond ΙΙ) due to the cinnamoyl and benzoyl systems of flavonoid nuclei, respectively (Nasrollahzadeh et al, 2016b). These absorbent bonds are related to the π → π* transitions and indicate the

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presence of polyphenolics as antioxidant source for green synthesis of nanoparticles.

Fig. 1. FT-IR analysis was carried out to identify the possible biomolecules responsible for the reduction of the Cu2+ ions and capping of the bioreduced copper nanoparticles synthesized by Thymus vulgaris L. leaf extract. Fig. 2 represents the FT-IR spectrum of the Thymus vulgaris L. leaf extract and shows peaks at

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3500 to 3000, 1750 and 1680, 1490, 1396 and 1250 cm-1 related to the free OH in molecule and OH group forming hydrogen bonds, carbonyl group (C=O), stretching C=C aromatic ring and C-OH stretching vibrations, respectively (Nasrollahzadeh et al, 2016b). It indicates that the presence of the flavonoid and

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other phenolic compounds inside the extract are mainly responsible for the reduction of Cu2+ ions.

Fig. 2.

The progression of the reaction, formation and stability of Cu nanoparticles were controlled by UV-Vis spectroscopy (Fig. 3). The color of the CuII solution immediately changed into dark indicating reduction of

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CuII to Cuo and formation of Cu NPs as characterized by UV-Vis spectrum. According to the UV-Vis results, the synthesized Cu NPs by this method are quite stable and can be kept under inert atmosphere for 20 days. Of course the oxidizing process and converting Cu NPs to CuO and Cu2O can be occurred under

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ambient conditions but after synthesis the mentioned NPs to prevent decomposition and deformation

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processes we used inert atmosphere of Ar to conservation.

Fig. 3.

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It was observed that use of the Thymus vulgaris L. leaf extract makes a fast and convenient method for the preparation of the bentonite/Cu NPs and can reduce Cu2+ ions into copper nanoparticles without using any harsh conditions. In this work, the bentonite/Cu NPs were characterized by using FESEM, EDS, TEM

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and XRD. The XRD patterns of the bentonite and bentonite/Cu NPs are shown in Fig. 4. From XRD pattern of the natural bentonite, we can conclude that quartz and montmorillonite are the major constituents of used bentonite. For bentonite sample, it is found that the diffraction peak at angle of 19.84, 35.22, and 61.84°

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corresponds to (020), (130), and (060) planes of montmorillonite, respectively. The XRD peaks located at 2θ = 26.81 and 36.25° were indexed to (101) and (110) planes of quartz, respectively. The other peaks of

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bentonite are due to impurities corresponding to crystobalite, feldspar and illite and consistent with earlier reports on the bentonite (Caglar et al, 2009). No characteristic peak for Cu was observed at 2θ = 47.7° (200) and 72.9° (220) in the pattern of bentonite/Cu NPs, indicating that the Cu NPs were highly dispersed

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on the bentonite support.

Fig. 4. The bentonite was further characterized by SEM (Fig. 5). It can be seen that the bentonite exhibit a sheet like structure.

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Fig. 5.

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The morphology of the bentonite/Cu NPs was studied by FESEM analysis (Fig. 6). The FESEM image showed sheeted structure of montmorillonite. The nano particles were observed in the FESEM of

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bentonite/Cu NPs. It can be seen that the particles are formed in various sizes. The Cu NPs can be observed

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on both the surface of bentonite and interlayer spaces.

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Fig. 6.

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The elemental composition of the bentonite/Cu NPs was determined by energy dispersive X-ray spectroscopy (EDS) analysis (Fig. 7). Furthermore, EDS analysis indicates that Cu is present on the surface of the bentonite along with the other elements of bentonite such Al and Si. The peaks of C, Al, O, Si, Mg and Cu elements were observed, indicating bentonite/Cu NPs is formed.

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SiKα

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AlKα

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MgKα

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Fig. 7. The size and shape of the products were examined by TEM. Fig. 8 shows the TEM image of the bentonite/Cu NPs. The immobilized Cu NPs with spherical morphology on the surface of bentonite were observed in the TEM images (Fig. 8). It can be seen that the Cu NPs are formed in various sizes. However,

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an aggregation of Cu particles was observed. The spherical particles also have a diameter in the range of

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23-94 nm. The TEM images indicated the presence of Cu NPs with average size around 56 nm.

Fig. 8.

The catalytic reductions of MB and CR with an excess amount of NaBH4 were chosen as model reactions to evaluate the catalytic activity of the bentonite/Cu NPs (Scheme 2). It was demonstrated that the

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reduction reaction did not proceed in the absence of the bentonite/Cu NPs. The catalytic reduction of dyes was monitored by UV-Vis absorption spectroscopy of the reaction mixture after the addition of the bentonite/Cu NPs. The progress of the reduction of CR and MB is evident in Fig. 9 in the form of decrease in the intensity of the absorbance maximum at λmax = 493 nm (characteristic absorption band of CR) and at

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λmax = 663 nm (characteristic absorption band of MB) with time. The characteristic peaks for the MB and CR disappeared completely after 40 s and 5 min, respectively and the whole peak almost disappeared and

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the color became transparent, indicating the completion of the reaction.

As a promising process, the application of metal NPs as catalyst in dyes reduction process caused to electron transformation from the BH4- donor to the acceptor (dyes) through the diffusion of both factors to the surface of nanocatalyst which the hydrides generated by the BH4- attack to dye molecules and reduce and discolor the dye (El-Sheikh et al., 2013; Deng et al., 2007). Deng et al. (Deng et al., 2007) believe that the metal NPs play as electron storage during electron transfer process. Furthermore, the negatively charged metal nanoparticles probably act as nanoelectrodes with negative potential.

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Scheme 2.

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Fig. 9.

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A comparison of the catalytic activities of various catalysts for the reduction of MB is given in Table 1. It is clearly evident from the Table 1 that the bentonite/Cu NPs outperformed other catalysts that are reported in the literature. The reduction reaction was carried out in shorter time in the presence of the

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bentonite/Cu NPs. On the other hand, the bentonite/Cu NPs was prepared via a simple, green and ecofriendly method using aqueous extract of the leaves of Thymus vulgaris without using any harmful reducing or surfactant template.

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Table 1 A comparison of catalytic activities of different catalysts with the bentonite/Cu NPs for the reduction of MB. Entry Catalyst Time Ref. 1 Ag NPs on silica spheres 7.5 min Jiang et al. (2005) 2 copper nanocrystals 200 s Zhang et al. (2014a) 3 Aucore-PANIshell 5 min Dutt et al. (2015) 4 Porous Cu microspheres 8 min Zhang et al. (2014b) 5 Au/Fe3O4@C 10 min Gan et al. (2013) 6 SiNWAs-Cu 10 min Yang et al. (2014) 7 Bentonite/Cu NPs 40 s This work

Separation and reusability are of great importance for the practical applications of catalysts. The reusability of the bentonite/Cu NPs was evaluated for the reduction of CR and MB. The UV-visible

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absorption spectra show that the activity of the bentonite/Cu NPs in reduction of CR or MB remain unchanged after use of 5 times in catalytic reaction. But in the sixth cycle, the bentonite/Cu NPs still exhibited excellent catalytic performance and the CR and MB could be completely reduced within 7 min

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and 70 s, respectively. The decrease in catalytic efficiency may be a result of the loss of Cu NPs from the catalyst during recycling. To address this possibility, the catalyst was analyzed by Inductively Coupled

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Plasma Atomic Emission Spectroscopy (ICP-AES) analysis after five cycles. It was shown that less than 0.1% of the total amount of the original copper species was lost into solution during the course of a reaction. It was further confirmed by FT-IR analysis of the recycled catalyst (Fig. 10). Once the reaction was completed, the bentonite/Cu NPs catalyst was removed from the reaction mixture by centrifugation and washed with deionized water and ethanol and then dried for the next cycle.

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Fig. 10.

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4. Conclusions

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This study developed a rapid, eco-friendly and convenient green method for the synthesis of the bentonite/Cu NPs using aqueous extract of the leaves of Thymus vulgaris as a reducing and stabilizing agent without using any harmful reducing or surfactant template. This green chemistry approach using plants toward the synthesis of bentonite/Cu NPs has many advantages such as, ease with which the process

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can be scaled up, economic viability, etc. The as-prepared bentonite/Cu NPs exhibits excellent catalytic activity for CR and MB degradation in the presence of NaBH4 in water at room temperature. The catalyst can be easily separated from the catalytic system and reused several times without appreciable loss in its

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catalytic activity.

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Acknowledgements

We gratefully acknowledge the Iranian Nano Council and the University of Qom for the support of this work.

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Scheme captions: Scheme 1. Mechanism for the formation of the Cu NPs.

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Scheme 2. Mechanism of the catalytic reduction and degradation of MB and CR with bentonite/Cu NPs.

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Figure captions: Fig. 1. UV-Vis spectrum of the Thymus vulgaris L. leaf extract. Fig. 2. FT-IR spectrum of the Thymus vulgaris L. leaf extract.

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Fig. 3. UV-Vis spectrum of the Cu NPs. Fig. 4. XRD pattern of the bentonite/Cu NPs. Fig. 5. SEM images of the bentonite. Fig. 6. FESEM images of the bentonite/Cu NPs.

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Fig. 7. EDS spectrum of the bentonite/Cu NPs. Fig. 8. TEM image of the bentonite/Cu NPs.

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Fig. 9. UV-Visible spectra of the CR (a) and MB (b) aqueous solution reduced by NaBH4 in the presence of the bentonite/Cu NPs.

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Fig. 10. FT-IR spectrum of the recycled bentonite/Cu NPs.

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Highlights: •

Facile synthesis of the bentonite/Cu NPs using aqueous extract of the leaves of Thymus vulgaris.

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Catalytic efficiency of synthesized bentonite/Cu NPs in the reduction/degradation of different

The bentonite/Cu NPs have been successfully recovered and reused.

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organic dyes.

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