The Transformation Process And Mechanism Of Typical Polybrominated Diphenyl Ether-heavy Metals In The Iron Sulfide/microbacterium Systems

Environmental hazards caused by improper dismantling of electronic waste have been a long history.In particular,the typical brominated flame retardants polybrominated diphenyl ethers(PBDEs)and heavy metals combined pollution have brought great risks to the ecological environment and human health.To date,many studies focused on the remediation of single PBDEs or heavy metal pollution,but few reports followed with interest on the efficient simultaneous remediation of PBDEs-heavy metal pollution.Therefore,this thesis take typical PBDEs-heavy metal composite pollutants as the research object,use iron sulfide modified nano zero-valent(S-nZVI)as the remediation material,and make use of Microbacterium Y2(MY2)as an experimental strain to construct S-nZVI-MY2 coupled degradation systems.Furthermore,the transformation mechanism of PBDEs-heavy metal composite pollutants in the degradation system was systematically studied.This work has carried out the following four aspects of work.(1)The reductive degradation of decabromodiphenyl ether(BDE-209)and electron transfer mechanism in S-nZVI system were explored.The results showed that the degradation kinetic constant k_obsof BDE-209 in S-nZVI system was 58.3 and 7.1 times larger than that of the k_obs in S^2-and nZVI systems,respectively.In S-nZVI system,BDE-209 could be quickly transformed into lower brominated diphenyl ethers,and BDE-3 was the detected end product of the reaction.Solvent isotope kinetics experiments showed that directly accepting electron transfer from the iron sulfide shell on S-nZVI surface was the main removal mechanism of BDE-209,while the indirect active·H reduction reaction was not the debromination degradation mechanism.Compared with nZVI,the iron sulfide coated on S-nZVI surface could not only greatly reduce the corrosion rate of nZVI by water,inhibit the dissolution rate of Fe(II),and avoid unnecessary electron loss,but also could accelerate the electrons transformation efficiency from nZVI core to the iron sulfide layer,promote the enrichment of BDE-209 on iron sulfide shell surface,and accept electrons to undergo a fast reductive debromination reaction.(2)Clarified the competitive reaction process and synchronous transformation mechanism of BDE-209 and Pb(II)in S-nZVI system.In the low-dose S-nZVI system,since the electrostatic adsorption between S-nZVI and Pb(II)was stronger than that of S-nZVI and BDE-209,S-nZVI could preferentially react with Pb(II)to generate Pb⁰Pb O and a small amount of Pb S on S-nZVI surface through adsorption,co-precipitation and reductive reactions.It was the Pb⁰ active site formed on S-nZVI surface formed a galvanic effect with Fe⁰ that accelerated the transfer rate of electrons from the Fe⁰ core to BDE-209,thus promoting the reductive process of BDE-209.In addition,sulfur modification could enhance the hydrophobicity of nZVI,inhibit the passivation and corrosion process of water and dissolved oxygen,and prolong the storage time of the material.(3)The adsorption/reductive transformation mechanism of typical heavy metals Ni(II),Cd(II),Pb(II)and Cr(VI)in S-nZVI system and the effect of the co-existing metal ions on removing PBDEs(BDE-209 and BDE-47)by S-nZVI were analyzed.The results showed that the adsorption process and the reduction reaction dominated the removal of Cr(VI),while the adsorption and co-precipitation reaction were the main transformation mechanisms of Cd(II).In addition,co-precipitation and reductive transformation dominated the removal process of Ni(II)and Pb(II)in S-nZVI system.As the formation of Fe-Cr hydroxide hindered the electron transfer process from S-nZVI to PBDEs,the co-existence of Cr(VI)ions significantly inhibited the removal efficiency of BDE-209 and BDE-47 by S-nZVI.However,the coexisting Cd(II),Ni(II)and Pb(II)ions could significantly promote the conversion rate of BDE-209 and BDE-47 by S-nZVI.Cd(II)ions enhanced the transformation of PBDEs in S-nZVI system through the de-passivation process of the iron oxide shell on S-nZVI surface,but the de-passivation process had little effect on PBDEs degradation in Ni(II)/S-nZVI and Pb(II)/S-nZVI systems.The Pb⁰/Fe⁰ galvanic effect generated in the Pb(II)/S-nZVI system significantly improved the conversion kinetic constant k_obs of BDE-209 and BDE-47.However,for the Ni(II)/S-nZVI system,in addition to promoting the degradation of PBDEs through the galvanic effect,the atomic hydrogen(·H)generated by the reaction of the Ni⁰/Fe⁰ bimetallic played a more critical role in the reductive degradation of low brominated diphenyl ether of BDE-47.(4)The S-nZVI-MY2 coupling system was constructed,and the migration process and transformation mechanism of BDE-209 and Cr(VI)at the interface of S-nZVI and MY2 were studied.In S-nZVI-MY2 coupling system,both BDE-209 and Cr(VI)showed higher conversion efficiency.Cr(VI)was almost completely converted within1 day,and the conversion efficiency of BDE-209 was as high as 60.5%in 7 days,which was much higher than the single MY2 or S-nZVI system.The toxicity of Cr(VI)to MY2 was much higher than that of S-nZVI and BDE-209.The co-existence of Cr(VI)ions would significantly inhibit the biodegradability of MY2.The reaction of S-nZVI and Cr(VI)greatly reduced the reactive oxygen species(ROS),especially·O^2-and H₂O₂,thus inhibiting the toxic effect of Cr(VI)on MY2 and providing a beneficial effect on biodegradation of BDE-209.In this work,typical PBDEs and heavy metals combined pollutants were selected as the research objects,which broke through the deficiencies of existing studies that only focused on single POPs or single heavy metal pollution remediation.Building S-nZVI/MY2 coupling system overcame the insufficient of zero-valent iron and bioremediation system that could not remove the PBDEs and heavy metal ions combined pollution simultaneously.The migration process and transformation mechanism of PBDEs and heavy metal ions at the S-nZVI/MY2 interface were discussed in depth,which would provide theoretical basis and strong support for the development and application of POPs and heavy metal combined pollution removal technology at the e-waste disposal site.