| As widely applied brominated flame retardants(BFRs),polybrominated diphenyl ethers(PBDEs)greatly decrease the fire risks in daily life due to their excellent flame retardant ability.However,they also bring extremely high environmental risks to human health and ecological system due to their persistence and toxicity.Photodegradation is one of the most important transformation processes for PBDEs in the environment.The highly toxic polybrominated dibenzofurans(PBDFs)is one of the major products in photodegradation.However,the mechanism involved in the transformation process has not been fully understood.For PBDEs removal strategies,photocatalytic debromination process based on metal modified Titanium dioxide(M/Ti O2)is one of the most promising approaches,which can degrade PBDEs to their final debromination products at high degradation rate under sunlight.However,the role that metallic catalyst remains to be investigated.In this study,18 lower PBDEs(number of Br?4)were selected as target pollutants.The mechanisms of PBDE debromination and their conversion to PBDFs under UV light were systematically investigated.In addition,the degradation mechanism of 2,2',4,4'-tetrabromodiphenyl ether(BDE-47)in those photocatalytic systems were investigated,the merits and demerits of different catalysts were compared.The main findings of this study were:(1)BDE-47,BDE-29,BDE-25,BDE-21 and their potential debromination products were selected as target pollutants,their overall debromination pathways in methanol under UV condition were investigated.The results indicated the debromination pathways of PBDEs do not necessarily follow the general rule of thumb proposed in the literature.The degradation kinetics of BDE-47 increased with increased methanol/water ratio,which was attributed to that the degradation mechanisms gradually changed from debromination to undebromination process.(2)By comparing photodegradation products generated from PBDEs with different Br numbers and positions,the results suggested that PBDEs without ortho-bromine cannot transform into n BDF(n?1,n means the number of bromine substituted on the phenyl ring)or(n-1)BDF(n?2),but it can firstly debrominate to DE,which then transform into DF under UV condition.PBDEs with one ortho-bromine can transform in to correspondent(n-1)BDFs via elimination of ortho-bromine and ortho-hydrogen on the other ring.The solvent isotope experiment combined with density function theory(DFT)calculations suggests that PBDE molecule with at least on one ortho-bromine receives a photon to form PBDE excited state,which form an(n-1)BDE radical and a bromine atom.The(n-1)BDE radical can form(n-1)BDF via intramolecular reaction.(3)Four metal modified(Pd,Pt,Cu and Ag)Ti O2 photocatalytic materials were synthesized.Debromination pathways and mechanism of BDE-47 in these photocatalytic systems were investigated.The results suggest the degradation rates of BDE-47 were improved greatly in metal modified Ti O2 compared with that in pristine Ti O2 system.BDE-47preferentially debrominate para-bromine to generate BDE-17 in Pd/Ti O2 and Pt/Ti O2 systems,while it preferentially debrominate ortho-bromine to generate BDE-28 in Ag/Ti O2 and Cu/Ti O2systems.In Pd/Ti O2-H2 and Pt/Ti O2-H2 systems,the degradation rate of BDE-47 increased with increased metal loading amount,and BDE-47 preferentially debrominate para-bromine to generate BDE-17,suggesting that in Pd/Ti O2 and Pt/Ti O2 photocatalytic systems,BDE-47degradation was based on H-atom transfer mechanism,while in Ag/Ti O2 and Cu/Ti O2,BDE-47 degradation was based on electron transfer mechanism.(4)Four metal modified(Pd,Pt,Cu and Ag)Ti O2 photocatalytic materials can only catalyze BDE-47 degradation in methanol under anaerobic condition.For Ag/Ti O2 and Cu/Ti O2systems,oxygen can capture the electron form superoxide radical,which were quenched by methanol.Therefore,BDE-47 cannot be degraded in these two systems.For Pd/Ti O2 and Pt/Ti O2 systems,we found that oxygen can also inhibit the degradation of BDE-47 in Pd/Ti O2-H2 and Pt/Ti O2-H2 systems,indicating oxygen can capture active H-atom as well.In Pd/Ti O2and Pt/Ti O2 systems,BDE-47 can be fully debrominated to form DE,while in Ag/Ti O2 and Cu/Ti O2 systems,BDE-47 can only be debrominated into di BDE.Except for debromination products,DF was also generated in Pd/Ti O2 and Pt/Ti O2 systems but not found in Ag/Ti O2 and Cu/Ti O2 systems.In addition,the ability to continuously degrade PBDEs for Pd/Ti O2 and Pt/Ti O2 is greater than Ag/Ti O2 and Cu/Ti O2.All these differences were attributed to the different mechanisms.In addition to PBDEs,the degradation of 2,4,5-tribromodiphenyl(PBB-29)catalyzed by Pd/Ti O2 and Ag/Ti O2 was investigated,results suggested that the mechanisms in these systems are the same to that in case of BDE-47,i.e.,the degradation of PBB-29 was dominated by H-atom transfer in Pd/Ti O2system,and dominated by electron transfer in Ag/Ti O2system.(5)209 PBDEs molecules were optimized by Gaussian 16.The LUMO of each PBDEs was visualized,and the relationship between their LUMO and the structural pattern were discussed.The LUMO of PBDEs in their excited states optimized by TD-B3LYP/6-31G(d)can well correlated with the actual debromination pathways of PBDEs under UV light,suggesting their good ability for predicting their photochemical debromination pathways.The findings of this study greatly improved our understanding on PBDEs photochemical behaviors and is helpful for assessing their environmental risk in environment.In addition,the elucidated mechanism for M/Ti O2 system and their comparison result provided the theoretical and experimental basis for degradating PBDEs. |
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