Enhanced Microbial Degradation Of Tetrabromobisphenol A By Electrode And Nano Palladium Iron And The Analysis Of Function Mechanism

Tetrabromobisphenol A(TBBPA),as one of representative brominated flame retardants,has posed great harm to human health and the ecological environment due to the persistence,bioaccumulation,biotoxicity and potential carcinogenicity.In aquatic ecosystems,TBBPA tends to be deposited in the sediment under anoxic/anaerobic condition.However,the microbial degradation of TBBPA under anaerobic condition always suffers from low efficiency and the accumulation of toxic metabolites(BPA).The bioelectrochemical systems and iron-based nanomaterials are two representatives biostimulation methods through applying electron donor/accepter which has been applied in the bioremediation of persistent pollutants in recent years.However,the feasibility of them in the enhanced biodegradation and detoxification of TBBPA,and the related mechanism of microbial succession and microbial ecological network associations are remained unknown.Herein,the influence of TBBPA degrading performances and routes under the conditions with bioelectrochemical anode system and nano Pd/Fe were investigated.And the microbial activation effect and the function mechanism of the function bacteria by these two biostimulation methods were revealed.Besides,the interacting mechanism among the core function bacteria were revealed.It will provide important basis for developing efficient and stabilized technology for the enhanced bioremediation of persistent organic pollutants.With the inoculum of the sediment enrichment,the TBBPA degrading rate was obviously increased by the acclimation of bioanode(2.4 times higher than the open circuit),and the generation of toxic product BPA was depressed.In the bioanode,the first step of TBBPA degradation could be separated in two main pathways,one is the reductive debromination with bromophenol A(BPA)as final product,the other one is the hydrolytic debromination.And the products of hydrolytic debromination was further degraded through methylation and ring opening,finally the products of ring opened could be further degraded and possibly mineralized.The TBBPA degrading efficiency and routes were similar between opened bioanode and bioanode,while the transformation proportion of reductive debromination increased a little(from 5.1% to 8.3%),and the degradation efficiency of ring opened products decreased obviously.Indicating that the sufficient electron donor in bioanode could remove the barrier for the further mineralization of TBBPA metabolites.Cyclic voltammetry analysis showed that the reducing peak of TBBPA in bioanode shifted positive(+220 m V)apparently,and meanwhile a weak oxidation peak arose,which indicated that the biofilm on the electrode distinctly increased the electrochemically catalytic activity.The reductive debromination and hydrolytic debromination of TBBPA were closely related to the lactate fermentation and carbon source transformation.Function bacteria in the plankton dominated in the reductive debromination and hydrolytic debromination.Besides,the generation of toxic product BPA was depressed due to the inactivation of reductive dehalogenators by the addition of electron accepter in bioanode.The activity of electroactive bacteria and the potential TBBPA degrading bacteria(e.g.Geobacter,Acinetobacter and Holophaga)was stimulated,and the relative abundance of them was distinctly increased and become predominated after the weak electrostimulation,they dominated in the TBBPA hydrolytic debromination,ring opening and further mineralization.The addition of Pd/Fe nanoparticles distinctly increased the TBBPA degradation efficiency,and the toxic metabolites BPA was further degraded and possibly mineralized.After the nano Pd/Fe(optimized dosage 0.412 g/L,and Pd loading 0.5 wt%)was introduced into the sediment solution,3.7 times of higher k(degradation rate constant)was observed in sediment fed with nano Pd/Fe when compared to the sediment solution.Reductive debromination was the only pathway in sediment solution with BPA as final product.But BPA could be further degraded through β scission and oxidative hydrolysis with the generation of 4-(allene)phenol and 2,2-bis(4-hydroxyphenyl)propanoic acid,and they could be further degraded and mineralized in sediment fed with nano Pd/Fe.The addition of nano Pd/Fe distinctly improved the generation H2 and restricted the methanogenesis,providing sufficient electron donor for the reductive debromination of TBBPA.And it obviously stimulated the activity of potential reductive dehalogenators(e.g.Dehalobacter,Desulfuromonas).Meanwhile,the ferric oxide on the surface of Pd/Fe nanoparticles provided enough electron accepter for microbial degradation of BPA,activating the aromatics degraders(e.g.Cryptanaerobacter,Citrobacter,Bacillus).And the relative abundance of the function bacteria increased and became dominated in the system,which played an important role in the further degradation and mineralization of BPA.The biostimulation methods through addition of electron accepter(bioanode)and electron donor(nano Pd/Fe)obviously optimized the microbial structure,they created livable environment(enough electron donor/accepter or appropriate redox potential,etc.)for the function bacteria,breaking the bottleneck during the TBBPA degradation.The biostimulation distinctly lowered the complexity of the microbial ecological network,higher the modularity and the niche differentiation.In the influence of enough electron accepter,the number of nodes which represented potential function bacteria(including reductive dehalogenators,hydrolytic dehalogenators and aromatics degraders)and electroactive bacteria(e.g.Geobacter,Desulfovibrio)increased distinctly in the core OTU of modules,and they shared more positive interaction.The number of nodes which represented potential reductive dehalogenators(e.g.Desulfuromonas)and BPA degraders(e.g.Citrobacter,Bacillus and Cryptanaerobacter)increased obviously in the core OTU of modules in the influence of electron donor,and the directly positive interactions were also increased.After the influence of the two biostimulation methods,the interactions among function bacteria were more tightened.The phenomenon of cross-feeding properties or shared similar niches was crucial for the enhanced TBBPA degradation and detoxification.Basing on the analysis of TBBPA degradation mechanism of the two biostimulation methods above,the related bioremediation mode was raised and the application potential was investigated.