Debromination Of Polybrominated Diphenyl Ethers(PBDEs)by Pd/ZVZ:Influence Factors And Mechanism

As organic flame retardants,the yield of polybrominated biphenyl ether(PBDEs)is largest in the world.They have been widely used in textiles,building materials and electronic products.PBDEs are hard to be degraded in the environment,so they have been detected in the soil,atmosphere and water,and even detected in the arctic area.PBDEs will bring great environmental risks to humans and wildlifes with their hepatotoxicity,embryonic toxicity and reproductive toxicity.Thus,many technologies have been used to treat these pollutants,such as photodegradation,chemical degradation and biodegradation.In this study,palladized zerovalent zinc(Pd/ZVZ)was used to degrade PBDEs,which was safe and effective.We compared the properties of ZVZ and of Pd/ZVZ,and explored the advantages of Pd/ZVZ in practical treatment.In order to explore the optimal conditions,we selected the influence factors including,Pd loading rate,dosage of Pd/ZVZ,p H,aging time and bromine number on the benzene rings.BDE-47,which is high toxic and ubiquitous in the environment,was selected as the target pollutant to explore the mechanism of PBDEs degradation by Pd/ZVZ.It could provide predicted descriptor for accumulation of intermediates,and reduce the environmental risks in the degradation process.The experimental results and conclusions are as follows:(1)In this study,Pd/ZVZ particles were prepared by direct displacement method.The particle sizes of ZVZ and Pd/ZVZ were characterized by scanning electron microscopy(SEM),and their diameters were in the range of 5-13 m.Accroding to the element content pattern with element distribution pattern of Energy Dispersive Spectroscopy(EDS),concluded that the Pd was loaded on the ZVZ surface successfully,and Pd/ZVZ obtained the expected loading rate.According to BET calculation,the specific surface area of Pd/ZVZ was 7.869 m~2/g.(2)Compared with ZVZ,the Pd loaded improved the degradation rate significantly.All of the Pd loading(0.5%,1%and 2%wt)could degrade at least 97%of BDE-47 within 2h.Considering the cost and degradation rate,the optimal Pd loading was 1%wt.The optimal dosage of Pd/ZVZ was 20 mg.With the aging time increase,the degradation rate of bde-47 was unchanged,and all of these materials could degrade over 95%of BDE-47 within 2 h.(3)In the ZVZ system,the degradation rate decreased with the p H increase,while in the Pd/ZVZ system,the degradation rate of BDE-47 hardly changed.It proved that the performance of Pd/ZVZ material would not influence by p H(p H=3,5,7).In ZVZ system,the degradation rate decreased with the bromine atom decreased,and as for the dibromobiphenyl ether,it was hard to be further degraded.In contrast,in Pd/ZVZ system,the debromination rate was not affected by the number of bromine substitution.Therefore,during the degradation process of BDE-47,lots of intermediates,especially BDE-15,were accumulated in the ZVZ system.While there were almost no intermediates in the Pd/ZVZ system,and BDE-47 was completely debrominated to DE rapidly.(4)In the pure water,debromination pathway of PBDEs by ZVZ followed e-transfer.When Pd loaded on the ZVZ successfully.the prior products of PBDEs by Pd/ZVZ changed.Compared with the previous researches,the mechanism of PBDEs degradation follows the H-transfer,and with the increasing Pd loading,the H-transfer mechanism gradually dominantes the system.However,when we explored the PBDEs degradation by Pd/ZVZ in low p H condition,the mechanism was different with any previous know mechanism and the Sigmatropic shift of bromine appeared.