Efficient photocatalytic debromination of 2,2ʹ,4,4ʹ-tetrabromodiphenyl ether by Ag-loaded CdS particles under visible light
Introduction
In recent years, polybrominated diphenyl ethers (PBDEs) have been widely employed in textiles, electronics, furniture, and petroleum products due to their excellent stabilities and flame-retardant properties (de Wit, 2002; Alaee et al., 2003; Hale et al., 2002; Sjödin et al., 2003; Ma et al., 2012). Nowadays, congeners of PBDEs are commonly detected in various media due to their high stabilities and resulting bioaccumulation (Shaw et al., 2014), with human blood (Wang et al., 2013), breast milk (Sudaryanto et al., 2008), indoor air and ash (Wit et al., 2010), and sediments (Mai et al., 2005) being typical examples. Moreover, PBDEs are endocrine disruptors, in addition to exhibiting neurotoxicity and reproductive toxicity (Costa and Giordano, 2007; Waaijers et al., 2013). It is therefore necessary to establish efficient techniques for the removal of PBDEs.
In this context, reductive debromination techniques based on direct photolysis, electrochemistry, microbial degradation, bimetallic systems, and photocatalysis have been employed in the degradation of PBDE (Santos et al., 2016; Wu et al., 2016a). However, the efficiencies of the direct photolysis (Söderstrom et al., 2004; Wei et al., 2013), electrochemical (Su et al., 2012), and microbial degradation (Huang et al., 2012; Rayne et al., 2003) methods are low. In addition, although the use of bimetallic systems is an efficient reductive debromination method (Luo et al., 2012; Pirhashemi et al., 2018), deactivation is facile (Fang et al., 2011). Photocatalysis is therefore an ideal choice, and so this technique has drawn significant attention in recent years. Indeed, a series of TiO2-based materials have been employed for the degradation of BDE209 or BDE47. More specifically, the rapid photocatalytic debromination of BDE47 has been achieved using Ag/TiO2 (Lei et al., 2015), Pt/TiO2 (Li et al., 2014), Cu/TiO2 (Lv et al., 2016), CuO/TiO2 (Ming et al., 2016), and Cu2O/TiO2 (Ming et al., 2016). However, UV irradiation is required in all such systems due the wide band gap (i.e., 3.2 eV) of TiO2 (Zinatloo-AjabshirMortazavi-Derazkola and Salavati-Niasari, 2017). In contrast, few studies have been carried out regarding the degradation of PBDEs under visible light (Su et al., 2012). Due to the abundance of visible light in sunlight (unlike in the case of ultraviolet light), the preparation of a visible light-driven catalyst is therefore necessary.
As an example, CdS is a narrow bandgap material that can be driven by visible light, and has been widely applied in photocatalytic processes (Wang et al., 2017), such as in the efficient photooxidation or photoreduction of 4-nitrophenol under visible light irradiation (Hernández-Gordillo et al., 2014). However, the rapid rate of electron-hole recombination of this material has restricted its wider application. Thus, to enhance separation of the electron-hole pair, and improve the photocatalytic activity of CdS, noble metal deposition (Wang et al., 2017; Han et al., 2014), graphene modification, the incorporation of a MoS2 composite (Jo and Selvam, 2017; Wu et al., 2016b), and heterojunction modification (Cui et al., 2014; Zinatloo-Ajabshir and Salavati-Niasari, 2017; Zinatloo-Ajabshir et al., 2018) have been reported. In addition, the removal efficiencies of methyl orange and salicylic acid by Bi/CdS were enhanced compared with that of CdS (Wang et al., 2017), while the combination of CdS with reduced graphene oxide and graphitic carbon nitride significantly improved the H2 generation activity via the Z-scheme electron transfer pathway (Jo and Selvam, 2017). These CdS-based materials can therefore be considered efficient materials for application in photocatalytic reduction processes under visible light.
We therefore wished to examine the use of a visible-light driven Ag-loaded CdS photocatalyst (Ag/CdS) for the reduction of BDE47. Initially, CdS will be prepared by a hydrothermal synthetic route. This will be followed by the reduction of AgNO3 to Ag0 and deposition on the CdS surface via a photodeposition method. Finally, the removal experiments of BDE47 will be conducted under visible light using the prepared Ag/CdS photocatalyst.
Section snippets
Synthesis of Ag/CdS
The CdS photocatalyst was prepared using a precipitation method (Vaquero et al., 2016). More specifically, cadmium acetate dihydrate (Cd(CH3COO)2·2H2O, 2.77 g, >99.5%) and thiourea (NH2CSNH2, 2.38 g, >99.0%) were dissolved in ethlenediamine (100 mL) and ultrapure water (5 mL) in a Teflon-lined stainless-steel autoclave. The autoclave was subsequently heated in an oven at 120 °C for 12 h, then allowed to cool to 25 °C. The resulting yellow product was washed with ethanol and deionized water
Phase structure and morphology
The XRD patterns of the prepared CdS and Ag/CdS samples are shown in Fig. 1. CdS was identified by the presence of diffraction peaks at scattering angles (2θ) of 24.8, 26.5, 28.2, 36.6, 43.7, 47.9, and 51.9 that were indexed to the (100), (002), (101), (102), (110), (103), and (112) planes of CdS (JCPDS No. 77–2306). Interestingly, upon increasing the Ag content of Ag/CdS, the color of the material changed from light yellow to dark green, despite the Ag photodeposition process not being
Conclusions
We herein reported the successful preparation of highly active and visible light-driven Ag-loaded CdS (Ag/CdS) photocatalysts via a hydrothermal and photodeposition route. The various photocatalysts bearing different Ag loadings were then applied in the efficient debromination of 2,2ʹ,4,4ʹ-tetrabromodiphenyl ether (BDE47) under visible light irradiation (i.e., 300 W Xe lamp irradiation with a 420 nm cutoff filter). Optimization of the Ag loading, the methanol/water ratio, and the organic
Acknowledgements
This work was supported by the National High-Tech Research and Development Program (No. 2013AA06A305) and the Science & Technology Office of Guangzhou (No. 201607010276).
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