Research On The Degradation Of 2,2',4,4'-tetrabromodiphenyl Ether (BDE-47) Based On Zero-valent Zinc

Polybrominated Diphenyl Ethers?PBDEs?,as a kind of bromine-based flame retardants,are extensively applied in various industrial and consumer products,such as textiles,plastics,wire insulation,and automobiles.Due to the featuers of being hydrophobic,difficult to remove by biodegradation,and easliy achieving high levels of bioaccumulation in the environment,PBDEs have caused various serious environmental problems.2,2',4,4'-tetrabromodiphenyl ether?BDE-47?,being one of the homologues of PBDEs,has become an persistent organic pollutant because of its higher toxicity,persistence and bioaccumulation.In recent years,with the increasing content of PBDEs in the environment,research on related degradation technologies has become unusually critical.The current treatment technologies are primarily focused on the two areas:photodegradation and microbial degradation.However,these methods are found to be high-cost,invalid and time-consuming and even can not degrade completely low brominated products.Therefore,we are eager to develop a technology that can rapidly,efficiently and fully removed polybrominated diphenyl ethers.Zero valent metal?ZVM?is widely used in the degradation of halogenated organic compounds due to its accessible and efficient dehalogenation advantages.However,for the field of dehalogenation reduction,most of studies are only focused on the application of zero valent iron?ZVI?materials and ignore the high activity of zero valent zinc?ZVZ?in the removal of PBDEs.Compared with zero-valent iron(E^?(Fe²⁺/Fe⁰)=-0.4402 V),zero-valent zinc(E^?(Zn²⁺/Zn⁰)=-0.7628 V)has more reactive sites and higher redox potential value,which is inclined to bring about a rapid reduction reaction.Here we investigated the probable use of zinc powders to the debromination of BDE-47.Zinc powders were directly added to reduce BDE-47 in the presence of nitrogen and mixture solvent of methanol and water.The optimal reaction conditions were as followes:[Zn⁰]=0.8g/L,methanol:water=50:50?v/v?,pH=4.0,T=15°C.with the satisfaction of these conditions,BDE-47?10 mg/L?reached a reduction efficiency of 95.0%in 90 reaction-minute.The effectiveness of a two-step reduction/subsequent oxidation treatment of BDE-47,consisting of Zinc-bound surfactant reduction and Fenton-like oxidation,was also investigated in this study.On the first debromination step of 60 min,the experiments of BDE-47?10 mg/L?debromination were carried out after adding a certain amount of cationic hexadecyltrimethylammonium bromide?CTAB?and zinc powder.The optimal reduction conditions were as follows:[Zn⁰]=0.3 g/L,pH=4.0,[CTAB]=0.05 g/L.On the sequent oxidation step of 120 min,the debromination products were attacked by hydroxyl radicals?·OH?generated after the adding of H₂O₂ and Fe²⁺,and further decomposed and mineralized to achieve complete removal of BDE-47.Thus,surfactant-assisted zinc reduction and subsequent Fenton method was proven to efficiently remove BDE-47?10 mg/L?,for that96.1%reduction efficiency and 98.2%oxidation efficiency were obtained.To verify the degradation mechanism,some analytical techniques including HPLC,LC-MS/MS and GC-MS were utilized to detected intermediate products during the entire process.Vitamin B₁₂ possesses the special electronic and spatial effects,so that it is potential for dehalogenation of halogenated organic compounds when combined with solid electron donor.Thereby,a method on the synergistic effect of zinc powder with vitamin B₁₂ for degrading toxic and recalcitrant BDE-47 was conducted.In this study,the debromination experiments of BDE-47 were managed at room temperature?25°C?and also investigated by the dosage of vitamin B₁₂,the dosage of zinc powder,the pH value and the solvent ratio.The optimal degradation conditions were as follows:[VB₁₂]=0.6 mM,[Zn⁰]=0.8 g/L,pH=3.0,methanol:water=40:60?v/v?.Meanwhile,we further explored the reaction mechanism of BDE-47 degradation by coordinated catalytic reduction of ZVZ/VB₁₂ system.VB₁₂ used as a catalytic reagent still stayed a stable content,which was mainly attributed to that strong nucleophilic effect can greatly lead to a prompt electron-directed transfer.Therefore,under the optimal reaction conditions,BDE-47?10 mg/L?reached a reduction efficiency of 94.4%in60 reaction-minute.The debrominated intermediate products were detected by HPLC,LC-MS/MS analytical techniques to infer the degradation pathway.