Elsevier

Journal of Cleaner Production

Volume 170, 1 January 2018, Pages 1536-1543
Journal of Cleaner Production

Green synthesis of the silver nanoparticles mediated by Thymbra spicata extract and its application as a heterogeneous and recyclable nanocatalyst for catalytic reduction of a variety of dyes in water

https://doi.org/10.1016/j.jclepro.2017.09.265Get rights and content

Abstract

The present study investigates the green synthesis of Ag nanoparticles (Ag NPs) via the mediation of the extract of Thymbra spicata leaves. In this synthesis, no stabilizers or surfactants were utilized. Field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), and UV–Vis were applied to characterize the green Ag NPs synthesized. It was observed that the Ag NPs/Thymbra is a useful nanocatalyst which can decrease different dyes, including 4-nitrophenol (4-NP), Rhodamin B (RhB), and Methylene blue (MB) in the presence of NaBH4 in water at room temperature. With the help of UV–Vis spectroscopy, catalysis reactions were examined. Furthermore, these reactions followed the pseudo-first order rate equation.

Introduction

Researchers have been deeply interested in nanoparticles owing to their large surface-to-volume ratio and small size, resulting in variations in their physical and chemical characteristics. The noble metal nanoparticles such as Ag, Au, Pd, and Pt are the focus of attention today (Yokoyama and Welchons, 2007). Silver nanoparticles (Ag nanoparticles) are not an exception to this fact and have been extensively applied in diverse fields, including optics, bio-sensing, plasmonics, catalysis, electronics, antimicrobials, and medicine (cancer therapy) (Mohapatra et al., 2015, Manikandan et al., 2015, Jyoti et al., 2016, Park et al., 2016, Nakkala et al., 2015, Azizi et al., 2016, Shaham et al., 2017). In general, the method of preparing Ag NPs includes reducing silver ions in the solution or at high temperatures in gaseous environments (Cheng et al., 2013). The reducing agents, however, could raise toxic pollutants and biological dangers in the environment (Cheng et al., 2013, He et al., 2003). Efficient green methods have been applied thus far to synthesize NPs by utilizing bacteria, plants, yeasts, viruses, and fungi (Mohapatra et al., 2015, Nayak et al., 2015, Mata et al., 2015, Muniyappan and Nagarajan, 2014). The green synthesis of Ag NPs has been recently reported, where plant extracts such as fenugreek leaf extract, Daucus carota extract, Dioscorea bulbifera tuber extract, and Citrus lemon extract are the reducing agents (Medina-Ramirez et al., 2009, Singh et al., 2011, Umadevi et al., 2012, Ghosh et al., 2012, Prathna et al., 2011). Moreover, studies have shown that leaf extracts of neem (Shankar et al., 2004), Hibiscus cannabinus (Bindhu and Umadevi, 2013), tamarind (Ankamwar et al., 2005), Murraya koenigii (Philip et al., 2011), Parthenium leaf extract (Parashar et al., 2009), Rosa rugosa (Dubey et al., 2010), Hibiscus rosa sinensis (Philip, 2010), Nelumbo nucifera (Santhoshkumar et al., 2011), and oak fruit bark extract (Veisi et al., 2016a, Veisi et al., 2016b) are effective extracts in the biosynthesis of Ag NPs. Advantages green synthesis of Ag NPs are an economical, eco-friendly, mild condition and simple method for preparing metal nanoparticles. No surfactant, capping agent, and/or template are used in these methods. The disadvantages of this method include the longer synthesis time and the larger size of nanoparticles.

Organic dyes are broadly utilized in industry, particularly textiles. Mutagen and carcinogen constitute the majority of synthetic textile dyes and are considered to be two of the most hazardous pollutants worldwide (Banat et al., 1996, Martínez-Huitle and Brillas, 2009, Vidhu and Philip, 2014). Sunlight is absorbed and reflected by the color content in dyes in polluted water, thus hampering photosynthesis and development of aqua species (Karimi et al., 2012, Sannino et al., 2013). There are various types of dyes such as azo, metal complex, acidic, basic, and anthraquinone (Padhi, 2012). The above-mentioned organic pollutants can cause many diseases both in humans and animals, including blood disorder, skin irritation, kidney and liver damage, and central nervous system poisoning. Due to their great stability, these compounds cannot be easily degraded to non-toxic products (Lefebvre and Moletta, 2006, Mohmood et al., 2013). Despite the fact that dye effluents have been largely de-colorized by conventional physical and chemical methods, including ozonation, adsorption, and precipitation, these techniques face such limitations as high cost, the creation of dangerous products, and strict energy needs (Manu and Chaudhari, 2002, Patel and Suresh, 2006, Devi et al., 2009). It is worth mentioning that environmentally friendly processes play an important role in reducing colorless organic pollutants catalytically. Environmental researchers have been meeting tremendous challenges of late, one of which is assessing novel technologies to expand these environmentally friendly processes and to employ harmless, cost-effective, and efficient methods to eliminate contaminants from industrial wastewater. As a result, in this regard, nanotechnology has recently been introduced as a new technology.

In the present study, Thymbra spicata (Fig. 1) was selected to synthesize Ag NPs because it is a great source of polyphenolic compounds and flavonoidic groups (with the main contents: thymol, carvacrol, α-pinene, myrcene, α-phellandrene, α-terpinene, γ-terpinen, ρ-cymene, and β-caryophyllene) (Dorman and Deans, 2000) and functions as a reducing and stabilizing agent rather than a hazardous or capping one. Subsequent to our previous study (Veisi et al., 2016a, Veisi et al., 2016b), the current research seeks to describe the green synthesis of Ag NPs mediated by Thymbra spicata leaf extract and its role as a recyclable and heterogeneous catalyst to reduce organic dyes.

Section snippets

Preparation of Thymbra spicata leaf extract

One hundred grams of Thymbra spicata dried leaves were ground and refluxed at 70°C with 500 mL of sterile distilled water for 2 h. Afterward, the mixture was cooled down to room temperature. The aqueous extract of Thymbra spicata leaves was centrifuged at 6500 rpm. Moreover, the supernatant was separated by filtration and was kept in a refrigerator at 4°C for later investigations.

Green synthesis of Ag NPs

The Ag NPs were typically prepared by steadily adding 20 mL of AgNO3 (1 mM) aqueous solution to 30 mL of the plant

Results of catalyst characterization

As the UV spectrum of Thymbra spicata extract demonstrates (Fig. 2), the bonds around 278 and 335 nm are attributed to the π → π* transitions, being assigned to the presence of polyphenolics as the antioxidant employed for the green synthesis of nanoparticles (Veisi et al., 2016a, Veisi et al., 2016b). The Thymbra spicata leaf extract was added to the AgNO3 solution, leading to reduction and formation of Ag NPs and visual color change from light yellow to reddish brown. Thus the conversion

Conclusion

Green synthesis of silver nanoparticles in a single step using Thymbra spicata leaf extract was successfully developed. Due to the existing major problems in the physical and chemical methods for producing NPs, there is a need to easy, low-cost, and non-toxic procedures. FTIR analysis revealed that the leaf extract constituents (phytochemicals) have the dual performance of reducing agents, as well as stabilizers. SEM and TEM images verified the production of well-dispersed and spherical shape

Acknowledgements

We gratefully acknowledge from the Iranian Nano Council and University of Payame Noor for the support of this work.

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