Effect of cold plasma pre-treatment on photocatalytic activity of 3D fabric loaded with nano-photocatalysts: Response surface methodology
Graphical abstract
Introduction
Wastewater of textile industries which contains conventional dyes because of the toxicity and tendency to cause eutrophication is categorized as one of the major sources of environmental contamination [1]. Among a variety of common dyes, azoic dyes comprise the most extensive class of dyes which are utilized for textile dyeing and printing because of their desirable fastness and brilliant hues [2].
However, conventional techniques are not efficacious enough to treat wastewater containing azoic reactive dyes because of their high water solubility and strong stability [3]. Therefore, a new generation of techniques has been developed for the treatment of azoic-dye-containing wastewater [4], [5], [6].
Amidst those techniques, advanced oxidation processes (AOPs) are established as one of the most promising approaches by virtue of their efficacy. They are able to oxidize intractable organic contaminants to CO2, H2O and mineral acids, preventing the secondary pollution formation [7].
AOPs incorporate several techniques, ozonation, Fenton, photo-Fenton oxidation, Fenton-like and photocatalytic processes, to name but a few. Generally, heterogeneous photocatalytic processes via a combination of nano-photocatalysts (nphs) and UV light have attracted a lot of attention as an effective, environmentally friendly, sustainable and affordable technique for a variety of chemical transformations [8], [9], [10], [11], [12], [13], [14], [15]. In addition, heterogeneous photocatalytic processes, either alone or in combination with other treatments, have been found efficacious in treatment of highly loaded industrial wastewater [16].
Nphs-based photocatalytic processes involve two typical approaches, suspension of nphs in a solution and immobilization of nphs on a surface. Among these mentioned methods, the immobilization method is usually preferred, inasmuch as separation and recycling of the nphs from the treated wastewater before discharge are excluded [17]. There have been a few reports on loading of nphs on textile materials such as electrospun fibrous mats, natural and synthetic fabrics [18], [19], [20], [21], [22]. Recently, Ghoreishian et al., reported the loading of nphs on 3D fabrics (spacer fabrics) for the photocatalytic processes [23] .
Spacer fabrics defined as a 3D knitted fabric with two external surfaces connected to each other through pile yarns [24]. Mass transferring in both middle parts and surfaces of the spacer fabric and slower passing rate through the fabric, which provides more time for wastewater to be in contact with the nphs, are some benefits of this structure compared to the conventional textiles [25], [26].
Spacer fabrics are produced mainly from poly (ethylene terephthalate) (PET). In order to modify the surface of PET fibers, increase the surface free energy and consequently enhance the adhesion of nphs, several surface modification techniques such as chemical, thermal, mechanical and electrical treatments can be employed.
Recently, pre-treatment of textiles with non-thermal plasma technologies (cold plasma discharge (CPD)) have been increasingly utilized in order to enhance hydrophilicity and adhesive properties of the fabrics [27], [28]. CPD is an environmentally-friendly technique which can induce new polar functional groups to the fiber surface facilitating anchoring of nphs while the properties of the bulk material are preserved [29], [30], [31], [32], [33], [34], [35], [36], [37], [38].
In this study, the effect of CPD pre-treatment on improvement of the photo-degradation of Reactive Orange 16 (RO16) by spacer fabrics loaded with ZnO:TiO2 nphs was studied. Thereupon, response surface methodology (RSM) with a Box–Behnken design (BBD) was employed to find a suitable approximating function determining influence of the main process variables (CPD treatment time, dye concentration and irradiation time) on the photo-decolorization efficiency. Furthermore, the kinetics of the photocatalytic decolorization were investigated.
Section snippets
Chemicals
Spacer fabric (both surface and beneath strings of 150 den, a monofilament connector string of 30 den, a weight of 255 g/m2 and a thickness of 9 mm) was provided from Bonyad Fiber Production Co., Iran. Nano-powder of ZnO and TiO2 (Degussa P-25) were purchased from Sigma-Aldrich, USA and Evonik, Germany, respectively. RO16 was acquired from Alvan Sabet Co., Iran. Table 1 shows the structure and characteristics of RO16. Citric acid monohydrate (CA, C6H8O7.H2O) as a cross-linking agent, H2O2 (30%
SEM images
SEM images of the untreated and CPD-treated spacers are shown in Fig. 2. Some PET granules and impurities are seen on the untreated spacer fabric (Fig. 2a) while, a CPD treatment removed the uneven portions due to the fact that plasma species bombard the fiber surface (Fig. 2b) [55].
Also, CPD treatment in a greater length of time (5 min) created micro-cracks and ripple like patterns oriented in the fiber axis (Fig. 2c). In fact, the highly reactive and energetic plasma species, created by
Conclusions
The effect of CPD pre-treatment on the enhanced photo-decolorization of spacer fabric loaded with ZnO:TiO2 nphs was studied. SEM images and XRD spectra verified successful loading of the nphs on the spacer fabric. RSM with the BBD model was employed to study the effects of three process variables on the efficiency of RO16 photo-decolorization. Based on the results, optimum conditions of CPD treatment time, dye concentration and irradiation time were 4 min, 40 mg/L and 60 min, respectively. Albeit
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