Reductive Degradation And Debromination Mechanism Of Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers(PBDEs),as a group of the most important flame retardants,have been widely used in plastics,textiles,and electronic devices.As additive flame retardants,PBDEs in these products will inevitably escape to the outside environment during the production,using and recycling of the products,causing environmental pollution.With the disclosures of environmental persistence,biotoxicity and bioaccumulation of PBDEs,the research on PBDEs elimination has received great attention.At present time,reductive debromination of PBDEs is the main strategy.However,the accumulation of lower-brominated intermediates with higher toxicity cannot be avoided in most reduction systems,which results in a common acccpted conclusion that the lower-bromined PBDEs are more difficult to be reduced than higher-brominated ones.It is necessary to elaborate the chemical properties and some related laws of PBDEs analogues in more detail to develop methods for completely debromination of PBDEs.Besides,the study on mechanisms of PBDEs reductive debromination is undoubtedly of great significance to the selection of the corresponding reduction systems.Therefore,this dissertation focuses on the main mechanisms of reductive debromination of PBDEs for developing efficient reductive debromination systems.The main contents were summarized as follows.In the first chapter,the development and application,environmental and biological exposure,physiological toxicity of PBDEs and their degradation methods were systematically reviewed.The main problems in the process of reductive debromination of PBDEs were pointed out,and the research purpose and main research contents of this dissertation were provided.In the second chapter,the Br-rich ring effect was studied in the process of electron transfer reductive debromination of PBDEs.The photocatalytic debromination of 10PBDEs analogues were carried out with Cu O/Ti O₂ nanocomposites as the photocatalyst in anaerobic methanol,and their relative pseudo-first-order constants(k _R)were obtained based on the pseudo first-order reaction rate constant(k)of BDE209.It was found that the k _R of 2,4,6-tribromodiphenyl ether(BDE30)is 11 times of 2,2',4,4'-tetrabromodiphenyl ether(BDE47),the k _R of 2,3,4,5,6-pentabromodiphenyl ether(BDE116)is 350 times of2,2',4,4',5-pentabromodiphenyl ether(BDE99).These data indicated that the reductive debromination rate of PBDEs is not related to its total Br number(N).Combined with the density functional theory(DFT)calculation,it was further found that the Br-rich phenyl ring(m ring)of a PBDE molecule was more susceptible in an electron transfer debromination process,and the reaction rate was positively correlated with the Br number of m ring.In addition,when the Br number substituted on m ring decreased from 5 to 1,the reaction rate sharply decreased by 7 orders of magnitude.It suggested that it was extremely difficult for the complete debromination of PBDEs when their debromination was governed by electron transfer.Based on the above results,this chapter proposed the mechanism of reductive debromination of PBDEs in an electron transfer process:the debromination rate of PBDEs is mainly determined by the number of bromine atoms on the Br-rich ring,with the decrease of Br atoms on the Br-rich ring,the debromination rate of PBDEs decreases sharply;the debromination process of BDE209 mainly follows the rockchair type step-by-step debromination mode of“A-B-A-B ring”.In the third chapter,the specific activation of Pt on C-Br bonds of PBDEs in methanol solution was studied.In the atmosphere of H₂,the rapid and complete debromination of BDE47 was achieved.The mechanism of catalytic activation of Pt on C-Br bond was proposed for H₂ assisted reduction debromination.The photocatalytic debromination of 11 PBDEs were carried out with Pt/Ti O₂ nanocomposites as the photocatalyst in anaerobic methanol.Their pseudo first-order reaction rate constants(k)were measured.It was found that the k of PBDEs had no clear correlation with their total Br number(N)or the Br number on m ring(m),and the k of PBDEs with different N were similar.The difference of k between decabromodiphenyl ether(BDE209,k=0.48 h^-1)and monobromodiphenyl ether(BDE1,k=0.10 h^-1;BDE3,k=0.14 h^-1)was only 5 times and less.PBDEs could be reduced rapidly to completely debrominated phenylether(DF)in methanol with Pt/Ti O₂ or Pt as catalyst under H₂ bubbling condition,but the surface of Pt catalyst was easily passivated by excessive H₂.The rapid and complete debromination of PBDEs could be realized by bubbling H₂ interval.The isotope labeling experiments showed that the H atom of methanol hydroxyl participated in the debromination processes of PBDEs.In addition,the introduction of inorganic acid can greatly enhance the debromination rate of PBDEs,while the addition of alkali showed obvious inhibition.Furthermore,combined with DFT calculation,the following reaction mechanism was proposed:the C-Br bond of PBDEs was firstly weakened on the surface of Pt,when the introduced H₂ reached the surface of Pt,it was decomposed into H atom,and providing electrons to reduce PBDEs,which made the C-Br bond broken and generated PBDEs residue with certain negative charge,which combined with the hydroxyl H of methanol to generate debromination products.In the fourth chapter,the specific activation of C-Br bond of PBDEs by Pd was studied.Pd has more excellent catalytic performance than Pt.In Pd/Ti O₂ photocatalysis system,PBDEs with different N or m values also had similar debromination rates,but the reaction rate of PBDEs increased 400 times compared with Pt/TiO₂ photocatalysis system.When H₂ was used as reducing agent,Pd could catalyze the rapid and complete debromination of PBDEs,but there was a large amount of Pd dissolution after the reaction.In addition,only H₂ was involved in the reduction of PBDEs.The mechanism of the reaction was also different from that of Pt-H₂ system:H₂ was decomposed into actived H atom on the surface of Pd particle,and Pd reacted with C-Br bond of benzene ring by oxidation addition.The actived H on the surface of Pd then attacked C atom,generating debroming product.Then,the remaining Br atoms on the surface of Pd were reduced to bromine ions(dispersed in solution)by excessive H atoms or Pd.Pd as catalyst only can be recycled for many times under the condition of excessive H₂.This system has very high application value for the degradation of PBDEs.In the fifth chapter,the main research results of this dissertation were summarized,forming the conclusion of this thesis.The main innovation points of this dissertation were point out.We also put forward the corresponding prospects for the degradation research of PBDEs according to the accumulated experience in the research.