Singlet oxygen mediated Fe2+/peroxymonosulfate photo-Fenton-like reaction driven by inverse opal WO3 with enhanced photogenerated charges
Graphical abstract
Introduction
With the development of economy and population, Fenton process with a history of more than 100 years also has been constantly optimized and enriched [1], [2], [3]. Since Anipsitakis and Dionysiou firstly applied the Co2+/PMS (Peroxymonosulfate) system to the field of water treatment in 2003, the Fenton-like system based on peroxymonosulfate (PMS) has attracted attention owing to the better oxidation performance and stability of PMS than H2O2 [4], [5]. Although the Co2+/PMS system exhibits the best Fenton-like activity, cobalt has a potential carcinogenic risk and will endanger human health. The low-toxicity iron has become an excellent cobalt alternative for PMS activation. However, replacing Co with Fe to activate the PMS molecule only obtains about half the activity of degrading pollutants [4]. Meanwhile, a large amount of Fe2+ needs to be added due to the slow regeneration rate of Fe2+ ions [6]. Over the past few years, various auxiliary methods on Fe2+/PMS Fenton-like system have been reported. Unfortunately, most studies focus on homogeneous Fenton-like catalysis and suffer from the problems, such as huge energy consumption (UV/PMS/Fe2+, ultrasound/PMS/Fe2+) [5], [7] and slow Fe3+/Fe2+ cycle rate [8] .Recently, it has been reported humic acid (HA) used as a cocatalyst for direct reduction of Fe3+ used in efficient degradation of benzoic acid by Fe2+/PMS process. However, HA is a toxic compound and residual HA will cause secondary pollution [9]. Therefore, there is an urgent need for designing more environmentally friendly co-catalysts to improve the performance of Fe2+/PMS processes. Alternatively, choosing inorganic catalysts as cocatalyst over visible-light Fenton-like system will be able to make up for the above shortcomings. WO3 is known as a visible-light-responsive photocatalyst possessing reducing capability under the visible-light irradiation [10], [11]. By modifying WO3 materials to improve the photocatalytic characteristics, we can intuitively explore the effect of the enhanced photogenerated charges on the Fe2+/PMS system and the mechanism of the activity improvement.
As a radical predominant reaction, PMS activation can produce a variety of strong reactive oxidizing species (ROS), such as sulfate radical (SO4−), hydroxyl radical (OH), superoxide radical (O2−), singlet oxygen (1O2), etc [12], [13]. These ROS can rapidly attack pollutant molecules and complete the contaminant mineralization. In this regard, approaches to maximize the reactive oxidizing radical production are highly desirable for enhancing the catalytic performance. Such as, Guo et al. detected the ROS intensity of Fe3O4@C/PB + PMS system at different reaction times, and demonstrated the catalytic activity will enhance with the increase of ROS concentration [14]. As we all know, the generation of ROS is considered to involve electrons, so increasing the rate of electron generation will probably increase the ROS production [15]. For example, Zhou et al. reported that C60 fullerenol modified Fe(III) can efficiently activate PMS under visible light irradiation because fullerenol as the electron donor greatly accelerated the generation of ROS [16]. Thus, based on the previous experience of our group in the design of photocatalysts [17], [18], [19], the inverse opal structure WO3 catalyst with excellent photo-generated charge capacity was designed as cocatalyst to improve the Fe2+/PMS Fenton-like activity under visible-light irradiation. On the other hand, although the reactive oxygen species generated from PMS activation are used for environmental remediation, different ROS can be transformed into each other in a Fenton-like process, which makes the whole reaction process very intricate [20]. It has been reported that Haber–Weiss reaction can convert O2− to OH [21], then OH can be converted to 1O2 [22], but the direct conversion from O2− or SO4− to 1O2 in the PMS Fenton-like system has been hardly reported. To confirm whether 1O2 is transformation from O2−, it is an essential need to minimize OH in the Fenton-like reaction. Interestingly, in the IO WO3 co-catalytic photo-Fenton-like system we designed, OH was detected as absent, which provided evidence for the direct conversion of 1O2 from O2−. In this process, 1O2 is selectively generated, which has a certain application prospect in organic synthesis, medical treatment, cell repair et al. [23], [24], [25].
In this work, we synthesized the inverse opal structure WO3 by the template method. It is used as an outstanding co-catalyst, which can extremely enhance the efficiency of Fe2+/PMS photo-Fenton-like process and Fe2+ regeneration for the remediation of rhodamine B. In this system, the high concentration of 1O2 presence in the solution, mostly stemmed from the conversion of O2− and SO4− radicals under the photo-Fenton-like process, was proven to be the main ROS for pollutant degradation. In addition, the IO WO3 cocatalyst can promote the reduction of Fe3+ to Fe2+ ions to improve decomposition of PMS molecule. As for inverse opal structured of WO3, the interconnected periodic macroporous system with large specific surface areas would help to improve mass transfer and light harvest in the photo-Fenton-like process. Importantly, IO WO3 with improved hydrophilic surface exhibits a very recyclable and stable co-catalytic activity for the contaminant remediation, which highlight its huge application in environmental potential.
Section snippets
Preparation of inverse opal WO3 (IO WO3) and Bulk WO3 materials
The IO WO3 was prepared using a saturated ammonium metatungstate (NH4)6H2W12O40 (AMT) solution as a precursor. In order to prepare high-quality IO WO3, the impregnation method was used in the infiltration process. The prepared PS colloidal crystal (SI, Text) was soaked in a methanol bath, and then the PS colloidal crystal was taken out of the methanol bath and immersed in the (NH4)6H2W12O40 solution bath, which including 4.75 g of AMT, 5 mL of H2O, and 2.5 mL of methanol. During vacuum
Morphology and chemical characteristics
The IO WO3 was synthesized by polystyrene sphere (PS) crystal template method. Monodispersed PS spheres were used to form ordered PS opals photonic crystal (Fig. S2). Then, the ammonium metatungstatea solution was impregnated into the void of the PS opals photonic crystal template. After calcinations to remove the PS templates, the IO WO3 was obtained. Fig. 1a and b present the FESEM images of IO WO3 sample, exhibiting that 3D ordered and periodic macroporous inverse opal structures are formed,
Conclusion
At present, the researches on Fe2+/PMS photo-Fenton-like system are almost focused on the homogeneous activation by UV light, but not on the heterogeneous photo-Fenton-like with visible-light. Designing a heterogeneous Fe2+/PMS photo-Fenton-like system with recyclable cocatalyst that can work efficiently under visible-light will greatly broaden its environmental application and save operating costs. Herein, the heterogeneous IO WO3 co-catalytic photo-Fenton-like system we designed can complete
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Key Research and Development Program (2016YFA0204200), the National Natural Science Foundation of China (21777044, 22076046), the Science and Technology Commission of Shanghai Municipality (19ZR1472400, 19230711300, 20DZ2250400), and the Fundamental Research Funds for the Central Universities (222201818014 and 50321022017009).
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