Elsevier

Journal of Environmental Management

Volume 236, 15 April 2019, Pages 481-489
Journal of Environmental Management

Research article
Degradation of methyl orange on Fe/Ag nanoparticles immobilized on polyacrylonitrile nanofibers using EDTA chelating agents

https://doi.org/10.1016/j.jenvman.2019.02.023Get rights and content

Highlights

  • Electrospun polyacrylonitrile nanofibers (PAN NFs) were surface modified with EDTA.

  • Nano Fe and Ag were immobilized on PAN NFs through chelation and chemical reduction.

  • Ag/Fe-EDTA-EDA-PAN NFs can remove dyes from synthetic water samples.

  • Catalytic hydrogenation of methyl orange model dyes confirmed with HPLCMS.

  • Ag/Fe-EDTA-EDA-PAN NFs can be reused for five cycles without efficiency loss.

Abstract

Bimetallic nanoparticles are effective for the removal of organic pollutants from environmental water samples through catalytic degradation reactions. Hence, this work reports on the preparation of Fe/Ag bimetallic nanoparticles immobilized on electrospun polyacrylonitrile nanofibers (PAN NFs) pre-functionalized with EDTA and ethylenediamine (EDA) chelating agents. Characterization techniques included attenuated total reflectance coupled to Fourier transform infrared spectrometer (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The liquid chromatography coupled to a mass spectrometer (HPLC-MS) was used to investigate the degradation by-products. The impregnation of EDTA-EDA chelating agents imparted changes on the pristine PAN NFs as evidenced by increased nanofiber's average diameter and surface chemistry. The zero valent Fe and Ag NPs were successfully immobilized on PAN NFs and their catalytic activity was tested for the degradation of azo dyes. Results showed efficient decolourization of methyl orange dye molecules from synthetic water samples after four (4) cycles of reuse (e.g. >96% removal efficiency). The hydrogenation of methyl orange was found to be the removal mechanism due to the presence of hydrogenated methyl orange by-products in the treated water samples. Therefore, the fabricated nanocomposites exhibit potential application for the remediation of textile wastewater.

Introduction

The release of coloured wastewater to the ecosystem is among the major water pollution challenges worldwide. These effluents are complex in nature and mostly composed of organic matter, metals, dyes, dyestuffs, pigments, surfactants, detergents (Verma et al., 2012; Savin and Butnaru, 2008). The azo dyes, characteristic of one or more azo bonds (–Ndouble bondN–), constitute the major portion (over 50%) of the total dyes currently in use (Sun et al., 2009). Their concern is related to lower biodegradability and environmental persistence (Verma et al., 2012). Individual techniques such as conventional biological wastewater treatment are incapable of meeting the standards for colour and nitrogen when applied for the removal of dyes from dye-bearing wastewater (Szpyrkowicz et al., 2001).

The role of metallic iron nanoparticles for the remediation of water pollution has been extended for the treatment of coloured wastewater (Liu et al., 2013; Fan et al., 2009; Shu et al., 2007). These studies are motivated by the high reactivity of metallic iron nanoparticles (ZVI NPs). However, the oxide layer covering ZVI NPs formed before or during the wastewater treatment constitute the major drawback for this technique (Xu et al., 2005). Hence, ZVI NPs are usually capped or blended with Ni, Pd or Ag NPs forming bimetallic composites (Xu et al., 2005; Kim and Carraway, 2003; Schrick et al., 2002). The basic principle of bimetallic systems relies on the capability of the central metal (i.e. Fe or Zn) to generate hydrogen (H2) through the reduction of water molecules while the coupling metal (i.e. Ni, Pd, or Ag) work as a catalyst (Xu et al., 2005; Kim and Carraway, 2003; Schrick et al., 2002; Xu and Zhang, 2000). Effective removal of azo dyes including methyl orange, methyl blue, orange IV, acid black 24, and direct black G were reported in the open literature (Sikhwivhilu and Moutloali, 2015; Wang et al., 2015; Liu et al., 2013; Ma et al., 2013; Fan et al., 2009; Shu et al., 2007). A sequence of adsorption, reduction, catalysis and flocculation was indicated as the removal mechanism of methyl orange on ZVI NPs (Fan et al., 2009). The major drawback of this process is related either to the agglomeration of nanocatalysts, lowering the catalytic activity or to challenges involving their separation from the treated effluents.

Prevention of bimetallic composites' agglomeration is usually achieved through their immobilization on support materials (Sikhwivhilu and Moutloali, 2015; Xu et al., 2005). This explains the wide use of polymeric membranes for surface impregnation of bimetallic nanoparticles used for the remediation of chlorinated organic compounds (Sikhwivhilu and Moutloali, 2015; Meyer and Bhattacharyya, 2007; Xu et al., 2005). However, very limited attention has been paid for chemical immobilization of nanocatalysts on organic polymeric materials. Instead, many studies are based on physical mixture of nanocatalysts' precursors with polymer solutions prior to membrane casting or electrospinning. Hence, this work is devoted to immobilize powdery nanocatalysts on the surface of polyacrylonitrile nanofibers (PAN) through their chelation reactions with the ethylenediaminetetraacetic acid (EDTA) and ethylenediamine (EDA) chelating couple. The use of PAN supports is related to its characteristic properties that include high mechanical strength, chemical resistance to common solvents, thermal stability and suitability to be transformed into nanofibers through electrospinning (Nataraj et al., 2012; Botes and Cloete, 2010; Chen et al., 2010). Most importantly, under specific conditions, the surface nitrile groups in PAN nanofibers can be partially hydrolyzed and be involved in chemical reactions for the immobilization of chelating agents of the class of aminopolycarboxylates such as the ethylenediaminetetraacetic acid (EDTA) (Chaúque et al., 2016). The use of nanofibrous supports in catalytic applications is advantageous compared to powdery catalysts owned to similar chemical properties, comparable surface areas and easy separation from the treated effluents through filtration. This work is the first of its kind providing insights on the application of EDTA-EDA couple cross-linked to immobilize Fe/Ag bimetallic nanoparticles on nanofibrous organic supports. The fabricated composite nanofibers were applied for the degradation of methyl orange model dye from synthetic water solutions.

Section snippets

Reagents

The ethylenediamine (EDA), ethylenediaminetetraacetic acid (EDTA), triethylamine, pyridine, acetic anhydride, diethyl ether, dimethylformamide (DMF), methyl orange, AgNO3, FeSO4.7H2O, NaBH4, NaOH and HCl were purchased from Sigma Aldrich (Gauteng, South Africa). All reagents were analytical grade and used as-received without further purification. Polyacrylonitrile fibres (composed of 93% acrylonitrile, 6% methyl acrylate and 1% itaconic acid) were sourced from BlueStar Fibres Company Limited

Surface characterization of nanofibers

The fact that chemical functional groups absorb specific infrared (IR) energy makes the FT-IR an analytical tool of choice to investigate the surface chemistry of materials. However, some functional groups do absorb IR energy at similar wavelengths making their identification difficult if occurring simultaneously in the same material. This is the case of functional groups such as Cdouble bondC, Cdouble bondN and Ndouble bondN absorbing IR energy around 1600 cm−1 (Fatiadi, 1967). The FT-IR characteristics of pristine and

Conclusions

In summary, PAN NFs were prepared through electrospinning and surface modified with EDTA-EDA chelating agents couple. The amine, nitrile and carboxylic groups on the surface of EDTA-EDA-PAN NFs were used to successfully immobilize Fe/Ag NPs as evidenced by the maximum adsorption capacity of 73.8 and 104.0 mg g−1 for Fe2+ and Ag+, respectively. The surface impregnated Fe/Ag bimetallic composites exhibited suitable catalytic properties against methyl orange from synthetic water samples for the

Acknowledgements

The study was supported by the Water Research Commission (WRC) of South Africa (Contract #K5/2386). We also acknowledge the insightful comments of Analytical Research Group members of the Department of Applied Chemistry at the University of Johannesburg, RSA.

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