Highly efficient photo-Fenton degradation of methyl orange facilitated by slow light effect and hierarchical porous structure of Fe2O3-SiO2 photonic crystals
Graphical abstract
Introduction
Leftover organic dyes dissolved in water are considered refractory industrial waste, solutions of which are intensely colored, highly toxic and exhibit poor biodegradability [1]. With the increasing use of synthetic dyes in a variety of industries, the treatment of organic dye waste water before their release has been promoted as one way to protect the environment from the detrimental effects of these dyes [2], [3]. However, since conventional biological wastewater treatment processes are not efficient at removing most of these dyes [4], [5], the treatment of dye wastewater has proven rather difficult. The development of cost-effective methods to remove dyes from wastewater is urgently needed.
Advanced oxidation processes (AOPs) are considered promising methods for the treatment of toxic organic pollutants in industrial wastewater [6], [7], [8], [9]. Among a variety of AOPs, Fenton’s reagent has attracted much attention due to its strong oxidation potential, mild reaction conditions and easy operation. Even more promising, light irradiation can further enhance the catalytic activity of the system, a reaction called the photo-Fenton process. Unfortunately, the drawbacks associated with the homogeneous Fenton or photo-Fenton process, namely easy deactivation by ion-complexing agents like phosphate anions and the formation of large amounts of iron-containing sludge, limit the application of this reaction [10], [11], [12]. Therefore, many efforts have been made to incorporate heterogeneously photo-Fenton catalysts into various supports, such as crushed brick, activated alumina, zeolite, silica, graphene oxide, etc [13], [14], [15], [16], [17]. However, the resulting supported catalysts are more or less unsatisfactory in terms of their relatively low efficiencies of hydrogen peroxide utilization [18], or having low specific surface area [19]. Moreover, little has been reported on improving the photo-Fenton catalytic activity by enhancing light-harvesting efficiency, a property determined by the nanostructure of the catalyst.
Recently, photonic crystals have become an attractive topic in part because of their potential applications [20], [21]. Photonic crystals (PCs) with ordered macroporous structures give rise to the photonic stop band for certain frequencies of light [22]. Photons near photonic stop band edges can be slowed down (termed slow photons). If the energy of slow photons overlaps with the absorbance of the material or dye interest, its absorption can be enhanced as a result of the increased effective optical path length [22], [23]. Therefore, a photo-Fenton catalyst with a PC structure can promote light harvesting for the photo-Fenton reaction and is expected to enhance catalytic activity. Furthermore, incorporating a mesoporous structure into PC framework would effectively increase the specific surface area. Hence, a PC containing a mesoporous framework with 3D ordered macropores is desirable for photo-Fenton catalysis, because creating such a structure will combine high surface area with efficient light harvesting.
Herein, we describe the preparation of a hierarchical macro-mesoporous Fe2O3-SiO2 photonic crystal (MM-Fe-Si-PC) using a double-templated synthesis combining colloidal crystals and an amphiphilic triblock copolymer. The structure of the catalyst was characterized in detail by SEM, TEM, XRD and so on. The photo-Fenton catalytic activity was evaluated by the degradation of methyl orange (MO), a typical azo dye, under visible light irradiation. MO was chosen, because its light absorption spectrum overlaps with the slow-light-effect region of the photonic crystal structure. The photo-Fenton experiments demonstrate that, at a concentration of H2O2 lower than 0.4 mM, the MM-Fe-Si-PC exhibits remarkable catalytic activity which is better than that of the corresponding mesoporous or macroporous material as well as the commercial Fe2O3 or the homogeneous Fenton system. The photo-Fenton catalytic mechanism is discussed. Based on a dye-sensitization mechanism, both the excellent adsorption of MO onto the hierarchical macro-mesoporous structure and the enhanced light harvesting from the slow-light effect contribute to the efficient use of H2O2 and the high catalytic activity. In addition, the photo-Fenton experiments using MM-Fe-Si-PC with different slow-light-effect region for degradation of different dyes demonstrated that the catalyst can be efficient for degradation of a variety of dyes by deliberately tuning the slow-light-effect region to overlap with the light absorption of the dyes. Finally, the stability and reusability of the catalyst were investigated.
Section snippets
Catalyst synthesis
Monodispersed polystyrene (PS) spheres and the PS colloidal crystals were synthesized as described previously [24], [25], [26]. For the preparation of MM-Fe-Si-PC, 2.2 mL tetraethylorthosilicate (TEOS) and 2.5 mg acetylacetone (AcAc) were mixed together for 30 min. At the same time, 1.0 g F127, 0.1 mL HCl (2 M) and 0.8 mL deionized water were dissolved in 16 mL of ethanol at 40 °C. After stirring at 25 °C for 1 h, 0.27 g (0.54 g, or 0.81 g) FeCl3·6H2O was added, and then the mixture was
Fabrication and structural properties of MM-Fe-Si-PC
To obtain the macro-mesoporous Fe2O3-SiO2 photonic crystals (MM-Fe-Si-PC), monodispersed polystyrene spheres (PS) were used as the template for the assembly of colloidal crystals with 3D-ordered macropores, while the amphiphilic triblock copolymer F127 acted as the template for the mesopores. Fig. 1a shows the uniformly sized and closely arranged PS colloidal crystals. A simple soaking method was used to allow the infiltration of the mixed aqueous solution of silica precursor TEOS, iron
Conclusions
Fe2O3-SiO2 photonic crystals with hierarchically structured macro-mesopores have been developed for treating dye-contaminated wastewater by a photo-Fenton mechanism. A remarkably high and stable photo-Fenton performance for removal of MO was achieved using a very low concentration of H2O2 under visible light illumination. The efficient utilization of H2O2 and the high catalytic activity of MM-Fe-Si-PC can be ascribed to the high accessible surface area of hierarchical macro-mesoporous structure
Acknowledgements
This work was financially supported by National Nature Science Foundation of China (21777044, 21407049, 21237003 and 21377038), China Postdoctoral Science Foundation (2015T80409), the Science and Technology Commission of Shanghai Municipality (16JC1401400), and the Science and Technology Commission of Jiangsu Province (BC2015135).
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