Research articleDegradation of methyl orange on Fe/Ag nanoparticles immobilized on polyacrylonitrile nanofibers using EDTA chelating agents
Graphical abstract
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 (–NN–), 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 CC, CN and NN 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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