Improved debromination of polybrominated diphenyl ethers by bimetallic iron–silver nanoparticles coupled with microwave energy
Highlights
► The Fe–Ag nanoparticles with a core–shell structure were successfully prepared. ► A highly efficient technology for debromination of PBDEs by Fe–Ag/MW was investigated. ► The effect of bromine's number on the stability against reduction of PBDEs was explored. ► The role of MW energy and Ag in the reactivity of the Fe–Ag/MW system was demonstrated. ► The possible degradation pathways of BDE-209 and BDE-47 were proposed.
Introduction
Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardants in various industrial products to reduce their flammability (Alaee et al., 2003). Although the use of PBDEs has been limited in selective products, their global demand is still high, especially in the case of deca-BDE (BDE-209). In recent years, PBDEs have been detected frequently in sediments, sewage sludge, marine organisms, food samples, and mammals (including humans) (de Wit, 2002, Ikonomou et al., 2002, Mai et al., 2005). They can impair liver enzyme activity and act as endocrine disruptors on neurodevelopmental processes (Darnerud et al., 2001, Kierkegaard et al., 1999, Meerts et al., 2000). Thus, the demand for treatment of PBDEs in contaminated environmental systems has increased worldwide.
PBDEs are highly resistant to oxidative degradation. Previous studies primarily focused on their degradation via reductive debromination, such as biodegradation (He et al., 2006, Stapleton et al., 2006) and photodecomposition (Ahn et al., 2006, Söderström et al., 2004). These two approaches could be both important to debrominate PBDEs. However, biotic debromination was found to be inefficient, and further treatment strategies were needed to achieve complete degradation and mineralization. Photocatalytic technology can debrominate PBDEs rapidly, but light absorption and light scattering severely reduced the photodegradation efficiency in environmental systems. A few reports are available on debromination by zero valent iron (ZVI) (Keum and Li, 2005, Li et al., 2007, Shih and Tai, 2010), bimetallic nano-particles such as Ni/Fe (Fang et al., 2011) and hydrothermal treatment (Nose et al., 2007). It is believed that debromination of PBDEs by ZVI or an iron-based bimetal has a high potential for remediation of PBDEs in cleaning up the polluted environment (Keum and Li, 2005, McDowall, 2005).
Because the reactivity of ZVI or a bimetal towards chlorinated and brominated compounds is generally low and the nano-scale metal particles tend to aggregate rapidly and lose their chemical reactivity, various modification approaches have been explored to increase catalytic efficiency (Chang et al., 2011, Nowack, 2008). These include the coupling with some techniques, such as ultra-sonic (Luo et al., 2010) and microwave irradiation (Jou, 2008). Microwave (MW) irradiation has been successfully applied to the decomposition and dechlorination of organic substances (Coss and Cha, 2000, Jou and Tai, 1999, Wada et al., 2000). As a result of the selective absorption of MW energy by polar molecules or polar transition states formed during the course of reactions, an MW-assisted technique proved to be effective in increasing reaction rates and enhancing the yield of degradation products (Bram et al., 1990, Jones et al., 2002, Kingston and Jassie, 1988, Wada et al., 2000). Liu et al. (2008) adopted granular activated carbon (GAC) as an MW absorbing material to increase the system temperature, which could provide a better reaction condition for iron powder to fully display its role as a reductant. The adsorption of MW energy by ZVI particles could induce electronic vibration and friction among molecules, and lead to generation of thermal energy thus raising the iron molecular temperature to help degradation of chemical compounds (Jou, 2008).
This study focused on the reductive debromination of PBDEs by ZVI and Fe–Ag bimetallic nano-particles coupling with MW irradiation. It is expected to enhance reaction efficiency and provide an innovative and cost-effective MW treatment system using metal nano-particles. The effects of MW and Ag on the debromination efficiency were investigated. On the basis of identifying intermediates and final products, degradation pathways were proposed. The positional preference of bromine elimination was also discussed.
Section snippets
Materials
BDE-209 (99%) and BDE-47 (98.5%) were obtained from China Chem Co., Ltd. (Ningbo, China). A mixture of 39 PBDE congeners was obtained from AccuStandard, Inc. (New Haven, CT), and individual congeners (BDE-196 and ‐197) were purchased from Cambridge Isotope Laboratory, Inc. (Andover, MA). AgCl was from Nanjing Chemical Co., Ltd. (Nanjing, China). HPLC-grade methanol, toluene, tetrahydrofuran (THF) and n-hexane were obtained from Tedia Company, Inc. (Fairfield, OH, USA) and used without further
Characterization of bimetallic nano-particles
The structure and morphology of the Fe–Ag bimetallic nano-particles were characterized by a number of analytical techniques, including BET, XRD, TEM and XPS. Specific surface area of the synthesized Fe–Ag nano-particles is 72 m2 g− 1, while the commercially available iron powder used has a smaller surface area of 50 m2 g− 1. As shown in Fig. 1, the three characteristic peaks at 44.66° (main peak), 64.34° and 38.06° in XRD spectrum correspond to Fe-110, Fe-200 and Ag-111 diffraction peaks,
Conclusions
Fe–Ag bimetallic nanoparticles synthesized and characterized with coupling to MW can degrade BDE-209 and BDE-47 through stepwise debromination. The dehalogenation efficiency by Fe–Ag/MW was about 97% and 78% for BDE-209 and BDE-47 in 8 min, while that of Fe0/MW was 94% and 56% for BDE-209 and BDE-47 in 8 min, respectively. In addition, the observed Pseudo First-order rate constants of BDE‐209 degradation by Fe–Ag/MW and Fe0/MW were calculated to be 0.61 and 0.45 min− 1, and that of BDE‐47 was 0.19
Acknowledgements
The authors greatly acknowledge the National Natural Science Foundation of China (50938004), National Major Project of Science & Technology Ministry of China (nos. 2009CB421604 and 2012ZX07529-003) and the Foundation of Furong Scholar project of Hunan Province for financial support, and an honorary professorship (J-DG).
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