Ultrasound-assisted degradation of organic dyes over magnetic CoFe2O4@ZnS core-shell nanocomposite
Introduction
Hazardous colorful wastewater realized from industries, particularly textile industry, is one of the major causes of the pollution of the water resources [1], [2]. The release of the synthetic dyes into the aqueous environment causes serious problems because of their strong color and low biodegradability [3], [4]. The removal of these pollutants from the contaminated aqueous solutions is of great importance in the environmental processes [5]. Thus, there is currently a considerable interest in developing effective processes for degradation of these types of organic pollutants [6].
In recent years, sonochemical degradation has been introduced as a possible method for wastewater treatment, and its successful application has been reported for the degradation of halogenated hydrocarbons [7], pesticide components [8] and dyes[9]. During this process, ultrasound waves lead to a quick growth and collapse of bubbles within the solution, which results in an extremely high temperature and pressure in the bubbles. High temperature near the bubbles causes thermal dissociation of water and generates hydroxyl radicals (OH) as a strong oxidant for non-selective oxidation of organic pollutants [10]. However, ultrasound alone is not efficient for the degradation of the target organic pollutant because of the need for longer time and higher amount of energy for an effective degradation. To overcome to this problem, catalysts which are active under ultrasonic irradiation, namely sonocatalyst, can be used for degrading organic pollutants in the aqueous phase. The generation of OH will be accelerated during the ultrasonic irradiation in the presence of an insoluble sonocatalyst [11], [12]. In this context, some metal oxide nanostructures such as Fe3O4, ZnO, and MgO have been used as sonocatalysts for the degradation of organic dyes [13], [14], [15].
Photoactive semiconductor based nanomaterials with high surface area and long life-span have been widely used for degradation of organic pollutants in water [16], [17]. Among them, ZnS (band gap energy: 3.6–3.8 eV) has served as one of the most efficient photocatalysts for the degradation of dyes, p-nitrophenol, halogenated benzene derivatives, reduction of heavy metals and water-splitting for H2 evolution [18], [19], [20], [21], [22], [23]. ZnS is a good photocatalyst due to the rapid generation of electron–hole (e−/h+) pairs by photoexcitation and high negative reduction potentials of the excited electrons. However, it has a serious drawback of the easy recombination of e−/h+ pairs, which in turn reduces the catalytic activity significantly [24], [25]. Moreover, the suspended ZnS photocatalyst suffers noticeable limitations such as the difficulty in separation, recovery, recycling and high cost in large scale production [26]. On the other hand, ZnS nanoparticles tend to agglomerate in aqueous solution and hinder the light penetration. For overcome to these drawbacks, ZnS was coupled with various magnetic semiconducting material. For example, Du et al. synthesized Fe3O4/ZnSe/ZnS magnetic fluorescent bifunctional nanocomposites by depositing heterogeneous semiconductor on magnetic nanoparticles [27]. Song et al. reported the MWNTs/ZnS/Fe3O4 nanocomposite with situ chemical precipitation method for the photocatalytic degradation of methylene blue solution with higher activity compared to pure ZnS nanoparticles [28]. Indeed, the sonocatalytic efficiency depends highly on the type of the catalyst. Then, the development of novel sonocatalysts is important to further understand the sonocatalytic mechanism and promote the sonocatalysis applications.
In this paper, we report the use of magnetic CoFe2O4@ZnS core-shell nanocomposite as a novel sonocatalyst for efficient H2O2-assisted sonodegradation of organic dyes in aqueous solutions. The sonocatalyst was prepared by a facile one-step hydrothermal decomposition of zinc(II) diethanoldithiocarbamate (Zn(DEDTC)2), as an air-stable, easily obtained single-source molecular precursor, over magnetic CoFe2O4 nanoparticles. Various spectroscopic techniques were used to characterize the CoFe2O4@ZnS nanocomposite. Thereafter, the sonocatalytic degradation of methylene blue (MB), Rhodamine B (RhB) and methyl orange (MO) organic dyes over the CoFe2O4@ZnS nanocomposite and effects of various operational parameters (e.g. H2O2 quantity, the catalyst amount, and initial dye concentration) on their degradation process were evaluated. Furthermore, the activity of CoFe2O4@ZnS nanocomposite was compared with those of pure ZnS and CoFe2O4 under similar conditions. To the best of our knowledge and based on the literature review, there is no report on the use of a magnetic core-shell nanocomposite such as CoFe2O4@ZnS for efficient and rapid sonodegradation of organic dyes, especially in the presence an environmental-friendly oxidizing agent (H2O2).
Section snippets
Materials
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%), diethanolamine (C4H11NO2, 98%), carbon sulfide (CS2, 98%), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 99%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%), hydrogen peroxide (H2O2, 30%), methyl orange (MO, C14H14N3NaO3S), methylene blue (MB, C16H18ClN3S), and Rhodamine B (RhB, C28H31ClN2O3) were purchased from Merck chemical Company and used as received.
Characterization techniques
X-ray powder diffractometer (XRD, XPertPro Panalytical, Holland) with Cu kα radiation (40 kV,
Characterization of the CoFe2O4@ZnS nanocomposite
The Powder XRD measurement was done for ZnS, CoFe2O4 and CoFe2O4@ZnS nanostructures to characterize the phase and crystallization (Fig. 1). Fig. 1a illustrates a typical XRD pattern of CoFe2O4 nanoparticles and all diffraction peaks can be indexed to the spinel cubic structure of CoFe2O4 (JCPDS card: 01-1121). From Fig. 1b, it can be seen that cubic ZnS nanoparticles (JCPDS card: 77-2100) [30] have been prepared and the broadening of diffraction peaks is in accordance with their nanocrystalline
Conclusions
Magnetic CoFe2O4@ZnS nanocomposite, with average crystallite size of about 18 nm, was successfully synthesized by a simple hydrothermal reaction, and its sonocatalytic activity was evaluated through degradation of organic dyes in the presence of H2O2. Magnetic measurements revealed that CoFe2O4@ZnS nanocomposite showed a ferromagnetic behaviour that could be easily separated from the aqueous solutions. The effect of different operational parameters including the catalyst dosage, initial dye
Acknowledgement
The authors gratefully acknowledge the Lorestan University and Iran Nanotechnology Initiative Council (INIC) for their support.
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