Efficient Reduction of hexavalent chromium and 4-nitrophenol using Ag NPs/Zeolite 13X nanocomposite as a green and retrievable Catalyst

INTRODUCTION 
Water pollution, caused by the improper discharge of urban and industrial waste, toxic pollutants and improper management of solid waste, can have serious impacts on human health. Nitrophenol compounds are refractory pollutants in wastewater that are widely used in a variety of industries including ceramics, cosmetics, textiles, paper, and enterprises that manufacture explosives.  Nitrophenol compounds, such as 4NP, have been demonstrated to be capable of damaging the central nervous system, liver, kidneys, and blood in humans and animals. Hexavalent chromium (Cr (VI)) is one of the toxic heavy metals with carcinogenic and mutagenic activity generated in industrial processes such as tanned metal electroplating and pigment manufacturing [1-4]
Traditional wastewater treatment methods, such as adsorption and chemical coagulation, which are two common techniques for wastewater treatment, only transfer pollutants from the liquid to the solid phase. This can cause secondary contamination and requires additional treatment  [5].
One of the most effective methods for removing toxic, bio-refractory, chemically stable and carcinogenic pollutants such as 4-NP in wastewater is the reduction of these compounds in the presence of metal nanoparticles (MNPs) and NaBH4. Thus, an effective method for 4-NP reduction using metal nanoparticles have recently received a lot of attention due to its cost-effectiveness [4-9].
It is well-known that silver NPs exhibit unique physical, chemical, thermodynamic and optical properties, which make them useful in many fields such as catalysis, gas sensors, batteries, solar energy conversion tools, high-temperature superconductors, biosensing, antibacterial, antiviral and antifungal activities, drug delivery, etc. [10-15]. In addition, silver nanoparticles have a high surface-to-volume ratio that can dramatically increase the interaction between reactants and catalysts. Heterogeneous catalysts are also increasingly used in the form of the M NPs due to their higher existing catalyst surface area. Additionally, heterogeneous nanocatalysts can easily separate products from the reaction mixture to form recyclable catalysts [16-18]. However, the agglomeration of the M NPs is a major problem, which can be overcome using ideal supports. In order to prevent the agglomeration of the M NPs and overcome the problems related to their stability, separation, and recovery, several inorganic compounds including graphene oxide, various types of zeolites, TiO2 and Fe3O4 have been applied as M NPs supports [19-23]. However, to date, there has been no report about Zeolite 13X as support. Thus, the present study is reporting the Zeolite 13X due to its good chemical and thermal stability, low cost, low toxicity and excellent optical properties. 
Several physical and chemical techniques are applied for the synthesis of the M NPs. However, these methods have some disadvantages such as long reaction times, high temperature, use of expensive, hazardous and toxic agents or stabilizers, environmental pollution caused by the utilization of organic solvents, and low product yields [24-29]. 
Recently green synthesis of the MNPs has received much attention owing to several advantages such as simple methodology, cost-effectiveness, environmental friendliness, very moderate reaction conditions, being organic solvent-free, easy to work up and suitably scaled up [16]. It is worth noting that, plant extracts act both as reducing and capping agents in the synthesis of nanoparticles [15, 16, 30-36]. Several plants have been successfully used for the rapid and efficient synthesis of NPs from extracellular Ag [16, 19, 35]. To date, the biosynthesis of NPs from Ag using the Tragopogon graminifolius extract has not been reported.
The family Trragopogon graminifolius DC. Compositae is well known in traditional Iranian medicine (TIM) as “Sheng” or “Lachiatotis”. Its aerial parts, including leaves and stems, are widely used as greens and various local products in western Iran. It has been used to treat wounds, bleeding, and various digestive and liver disorders in TIM. The hydroalcoholic extract of the roots and aerial parts of the plant exhibits powerful antioxidant activity and is rich in polyphenolic compounds. It can also be used as an important source for the biosorption of metal ions and the production of nanoparticles [37-39]. 
In this study an environmentally friendly, non-toxic and clean method is used for the first time for the green synthesis of the Ag NPs/Zeolite 13X nanocomposite using Tragopogon graminifolius in the absence of any stabilizer or surfactant (Scheme 1). The Ag NPs/Zeolite 13X nanocomposite was utilized for the reduction of 4-NP and Cr (VI) in water at room temperature. 

EXPERIMENTAL METHOD
Instruments and reagents
All chemical reagents were purchased from the Merck and Aldrich Chemical Companies. All materials were of commercial reagent grade. A Nicolet 370 FT/IR spectrometer (Thermo Nicolet, USA) using pressed KBr pellets recorded FT-IR spectra. To record UV-visible spectra in the wavelength range of 200-700 nm, a Shimadzu UV-2500 double-beam spectrophotometer was used. X-ray diffraction (XRD) measurements were carried out using a Philips model X’Pert Pro diffractometer type PW 1373 goniometer (Cu Kα = 1.5406 A˚).  The scanning rate was 2º/min in the 2θ range from 10 to 90˚. Particle dispersion morphology were investigated by scanning electron microscopy (SEM) (Cam scan MV2300). EDS (Energy Dispersive X-ray Spectroscopy) performed in SEM were used for measuring the chemical composition of the prepared nanostructures. Transmission electron microscope (TEM) using a Philips EM208 microscope operating at an accelerating voltage of 90 kV were used for identifying the size and shape of the prepared Ag NPs.

Preparation of Tragopogon graminifolius extract
15 g of dried, powdered Tragopogon graminifolius were added to 100 mL of 75% (V/V) ethanol solution at 70°C over a period of 30 min. After that, the mixture was allowed to cool to room temperature. Then, the extract of Tragopogon graminifolius was centrifuged at 6500 rpm and the supernatant was separated by filtration.

Preparation of the Ag NPs using Tragopogon graminifolius extract
For green synthesis of the Ag NPs, 0.02 g AgNO3 was dissolved in aqueous media. Then, the prepared aqueous solution of AgNO3 was added dropwise to 50 mL of Tragopogon graminifolius extract under constant stirring at 80 °C by changing the color of the mixture. The reduction of silver ions (Ag+) to silver (Ago) completed in about 30 min, as monitored by UV-Vis and FT-IR spectra of the reaction mixture. The color of the reaction mixture gradually changed in 15 min at 80 °C, indicating the formation of silver nanoparticles. Finally, the colored solution of silver nanoparticles was then centrifuged at 6500 rpm for 30 min for complete separation. 

Preparation of the Ag NPs/ Zeolite 13X
0.5 g of Zeolite 13X powder was dispersed in 50 mL of Tragopogon graminifolius extract under constant stirring at ambient temperature for 15 min. Afterward, 20 mL of a 0.03 M aqueous solution of AgNO3 was added dropwise to the prepared mixture and the reaction mixture was heated under traditional reflux conditions for 2 h at 80 °C. The reaction mixture was allowed to cool; the formed precipitate was filtered and collected over a round dish. Finally, it was washed several times with distilled water and once with ethanol and air, dried at 50 °C for 24 h in an oven, and then characterized. 

Reduction of 4-NP 
A typical 25 mL of 2.5 mM aqueous solution of the 4-NP and 3.0 mg of the Ag NPs/Zeolite 13X nanocomposite were mixed at room temperature under constant stirring for 2 min. Then, 25 mL of the freshly prepared NaBH4 solution (0.25 M) was added to the contents of the baker and the reaction process was monitored using UV-Vis spectrophotometer and through recording λmax changes at 400 nm. When the color of the solution disappeared, the catalyst was separated from the reaction mixture, washed, dried and then reused for the next run.
Reduction of Cr (VI) 
Typically, various amounts of Ag NPs/Zeolite 13X nanocomposite and 25 mL of 3.4 mM aqueous solution of Cr (VI) were added to 1.0 mL formic acid solution (88%) at room temperature under constant stirring. The Cr (VI) concentration was then determined using an indirect UV–visible spectrophotometric method based on the reaction of Cr (VI) and diphenylcarbazide, which forms a red–violet colored complex. Finally, the absorbance of the colored complex was measured in a double beam spectrophotometer at 540 nm wavelength [40].

RESULTS AND DISCUSSION
Tragopogon graminifolius extract and green synthesized Ag NPs were characterized by UV-Vis and FT-IR analysis. Fig. 2 (a) shows the UV two bonds of the extract at 292 nm and 335 nm due to the cinnamoyl and benzoyl systems, respectively. In fact, they are concerned with the π → π* transitions of polyphenolics [15, 16]. The surface plasmon resonance (SPR) signal at about 430 nm, in the UV-Vis spectrum of biosynthesized Ag NPs after changing the color of the solution demonstrates the formation of Ag NPs (Fig. 2(b)) [41-43].
The FT-IR of the Tragopogon graminifolius extract (Fig.3 (a)) shows signals around 3450, 1725, 1596 and 1386 to 1066 cm-1 indicating the OH, carbonyl group (C=O), C=C aromatic ring and C-OH, C-C and C-H vibrations, respectively [4, 41]. Owing to the presence of these functional groups in the structure of polyphenolics, which exist in Tragopogon graminifolius extract, it can be concluded that the phenolics in the extract are probably responsible for the reduction of Ag+ and formation of the corresponding Ag. According to the results obtained from the FT-IR study of green synthesized Ag NPs using Tragopogon graminifolius extract (Fig. 3(b)), the positions of observed peaks are almost similar to those of the corresponding peak in the FT-IR spectrum of Tragopogon graminifolius extract. These signals clearly confirm the presence of plant phytochemicals on the surface of Ag NPs and their effects on the protection and stability of NPs.  
As Fig.4 shows, the strong peak observed at around 1066 cm-1 in the spectra of the Zeolite 13X and Ag NPs/Zeolite 13X nanocomposite corresponds to the stretching vibration of -Si-O-Si- bond overlaps with the stretching vibration of -Al-O- and -Al-O-Si- bonds.  The stretching modes, located in the region of 620-740 cm-1 which can be associated with the tetrahedral atoms, are sensitive to the Si-Al composition of the framework and may shift to lower frequencies as the number of tetrahedral aluminum atoms increases. The bands related to pseudo-lattice vibrations of structural units are observed in the range of 500-700 cm-1 [35, 44] in the FT-IR spectra of the zeolitic structure. 
The FT-IR analysis of Ag NPs/Zeolite 13X nanocomposite (Fig. 4 (b)) shows that no changes occur in the functional groups following the Ag NP immobilization on Zeolite 13X compared with Zeolite 13X. The antioxidant action of flavonoids resides mainly in their ability to donate electrons or hydrogen atoms (Scheme 2). In this work, the Ag(I) ions were reduced to Ag(0) metallic particles by flavonoids and phenolics present in Tragopogon graminifolius extract according to the following mechanism. Flavonoids and other phenolics present in the extract are not only facilitating the formation of pure Ag NPs by the reduction of the Ag(I) to Ag(0) but also provide excellent tenacity against agglomeration.
Moreover, Fig. 5 represents the XRD pattern of the Ag NPs/ Zeolite 13X. On the basis of the above results, the diffraction peaks at 38.73°, 44.26°, 64.59°, and 78.12° are consistent with the metallic silver particles and well-matched with JCPDS card of Ag (NO. 04-0783), which exhibits face-centered cubic (FCC) structure of the metallic silver NPs immobilized on the surface of Zeolite 13X without the formation of impurities such as silver oxide (Ag2O) [4, 42]. 
Scanning electron microscopy (SEM) analysis was employed in order to determine the size and morphologies of both Zeolite 13X and Ag NPs/ Zeolite 13X. Fig. 6 shows a typical FE-SEM image of the samples. These images indicate that the Ag NPs and Zeolite 13X manifest spherical morphology with diameters of less than 20 nm with very narrow diameter distributions. From SEM images, it is clear that Ag NPs are deposited on the surface of Zeolite 13X without being incorporated in Zeolite 13X.
The elemental composition of the Ag NPs/Zeolite 13X nanocomposite was also characterized by EDS spectrum (Fig. 7). Fig. 7 confirms the presence of Na, Al, O, Si and Ag elements in the Ag NPs/ Zeolite 13X. Moreover, Fig. 8 illustrates the elemental mapping images, which demonstrate that Ag NPs are dispersed on the surface of the Ag NPs/ Zeolite 13X.
Transmission electron microscopy (TEM) was carried out in order to perform a more detailed study of the morphology and size of the Ag NPs/Zeolite 13X nanocomposite sample. Fig. 9 shows the TEM image of typical Ag NPs/ Zeolite 13X, in which Ag nanoparticles appear as dark dots over the surface of Zeolite 13X with average sizes of less than 20 nm.        
According to the obtained results, we propose a structure for the nanocomposite in which the Ag NPs have been embedded in the glass matrix (Scheme 3). Zeolite 13X has some regular holes as well, and an appropriate interaction can be occurred between the empty orbital of Si with P-orbital electron pairs of the metal or among electron pairs of oxygen with P-empty orbital of the metal. 

Evaluation of catalytic activity of the Ag NPs/ Zeolite 13X nanocomposite through the reduction of 4-NP
In this study, the catalytic performance of the Ag NPs/Zeolite 13X nanocomposite was evaluated through the reduction of 4-NP in the presence of excess amounts of NaBH4. According to UV-Vis spectrum, the aqueous solution of 4-NP exhibits an intense absorption about at 315 nm in neutral or acidic media. After the addition of NaBH4, a red shift of the peak of 4-NP from 315 to 400 nm was observed and the color of the solution altered from light yellow to deep yellow due to the formation of 4-nitrophenolate ions under strong basic conditions. The effects of NaBH4 and catalyst concentrations and various amounts of catalyst and NaBH4 on the catalytic reduction of 4-NP to 4-AP were studied, and the results are shown in Table 1. In the absence of the catalyst, the λmax of 4-nitrophenolate remains unchanged even after 150 min. To study the effect of support and Ag NPs on the efficiency of the reduction process, the catalytic activity of the Ag NPs/Zeolite 13X nanocomposite in the reduction of 4-NP was compared separately with untreated zeolite 13X and Ag NPs. No reduction reaction was observed with zeolite 13X after 100 min, which indicates the main role of Ag NPs in the reduction process. In addition, the importance of the support was clearly identified by the longer reaction time achieved with bare Ag NPs which was attributed to the agglomeration of the Ag NPs during the reaction (Table 1). It seems that when Ag NPs/ Zeolite 13X is used, the BH4¯ ion and 4-NP are both absorbed on the catalyst surface. This can facilitate electron transfer from BH4¯ to 4-NP, leading to 4-AP production. In other words, faster electron transfer can occur on the surface of the catalyst, which causes a faster reaction. When Ag NPs/Zeolite 13X nanocomposite was used as a catalyst the λmax at 400 nm gradually decreased and disappeared; the solution became clear in the presence of the catalyst and a new peak appeared at about 295 nm for 4-aminophenol (4-AP). This indicates that 4-NP had converted to 4-AP with increased reaction time [1, 16, 42] (Fig. 10). According to Table 1, the catalytic efficacy increased with an increase in the amount of NaBH4 and catalyst. This means lower reduction times were observed for higher NaBH4 and Ag NPs/ Zeolite 13Xamounts. The best result was obtained using 250 mM (100 equivalents) of NaBH4 and 10.0 mg of catalyst. The reduction process was monitored by using UV-Vis measurements at room temperature, and the results are shown in Fig. 10. 
According to the literature, as shown in Scheme 4, the mechanism of the reduction of 4-NP consists of two steps: (1) adsorption of NaBH4 and 4-NP onto the surface of the Ag NPs/Zeolite 13X nanocomposite and (2) electron transfer from BH4− to 4-NP through catalyst-mediated reactions and then desorption of 4-AP from the catalyst surface.
The catalytic performance of the Ag NPs/Zeolite 13X nanocomposite is compared with other reported catalysts and the comparative results are given in Table 2. According to these results, it can be concluded that Zeolite 13X as a support could be employed successfully to decrease Ag NPs agglomeration and to increase the catalyst surface area.

Evaluation of catalytic activity of the Ag NPs/ Zeolite 13X nanocomposite through the reduction of Cr (VI)
The Ag NPs/Zeolite 13X nanocomposite was also used to reduce Cr (VI) in the presence of formic acid. The progress of the reduction reaction was monitored using an indirect UV–visible spectrophotometric method based on the reaction of the remained Cr (VI) and diphenylcarbazid, which forms a red–violet colored complex. Finally, the absorbance of the colored complex was measured in a double-beam spectrophotometer at 540 nm wavelength (Fig. 11) [40]. 
It is well known that formic acid with potent reducing features in the presence of the nanocatalyst can be easily decomposed to CO2 and H2 without the production of intermediate materials [1, 41]. Reduction of Cr (VI) to Cr (III) is accomplished through hydrogen transfer (Scheme 5). As can be seen in Fig. 12, during 10 min, the progress in the reduction of Cr (VI) to Cr (III) is about 97 percent. Optimum conditions included 10.0 mg Ag NPs/Zeolite 13X nanocomposite and 2.0 mL of formic acid. 

Catalyst reusability
Obviously, the level of recyclability is an important factor affecting the practical applications of heterogeneous catalysts, especially for commercial and industrial applications. In order to evaluate the reusability of the Ag NPs/ Zeolite 13X, at the end of the reaction, the catalyst was separated from the reaction mixture by brief centrifugation and multiple washings with distilled water followed by drying and reusing in the next cycle. The obtained results showed that the catalyst was reused at least in 6 consecutive cycles for the reduction of 4-nitrophenol without significant loss of catalytic activity, as monitored by UV-Vis spectroscopy. Little decrease of efficiency was observed after the 7th cycle. These results demonstrate the high stability and catalytic activity of the catalyst under the operating conditions. In addition, the catalytic efficiency of the catalyst remained almost constant up to five cycles of operation and the time required for 100% reduction of Cr (VI) was found to be approximately the same up to the 6th cycle.

CONCLUSIONS
In this article, a rapid, convenient, biocompatible and efficient method is developed for the preparation of the Ag NPs and Ag NPs/ Zeolite 13X nanocomposite by Tragopogon graminifolius extract as reducing agent without any stabilizer or surfactant. Moreover, using Tragopogon graminifolius extract, as a bioreducing agent provides a clean, cheap, non-toxic and environmentally benign precursor for the synthesis of the nanocatalyst without using toxic and hazardous reagents and produces non-toxic and less dangerous by-products. SEM, TEM, XRD, EDS, FT-IR and UV-Vis spectroscopic techniques were employed to characterize the synthetic Ag NPs/ Zeolite 13X. Additionally, the green synthesized nanocatalyst exhibited excellent catalytic performance for the reduction of 4-NP and Cr (VI) in water under environmental conditions. The biosynthesized catalyst was stable and easily recycled and reused at least 6 times for 4NP and 5 times for Cr (VI) without sensible loss in its catalytic efficiency.

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