Study On The Stable Carbon Isotope Fractionation Characteristics Of Polybrominated Diphenyl Ethers During The Anaerobic Micro-Degradation In Wetland Sediments

Polybrominated diphenyl ethers（PBDEs）are typical brominated flame retardants,which have been widely detected in the environment.PBDEs have environmental persistence,bioaccumulation and biotoxicity,potentially harmful to the environment and human body.Therefore,the removal of PBDEs from environmental media has become one of the hot issues in environmental research in recent years.Sediments are one of the important concentrations of PBDEs released into the environment,especially coastal wetlands located between land and sea.Micro-degradation is an important way of natural degradation of PBDEs in environmental media,mainly including anaerobic and aerobic degradation.Because the sediment of coastal wetland is in anaerobic state all year round,the micro-degradation of pollutants in the sediment is mainly anaerobic degradation.Determination of concentration changes of target pollutants and products and identification of possible transformed（intermediate）products are conventional methods to study the anaerobic micro-degradation of PBDEs.However,this method cannot eliminate the problem that physical processes such as adsorption,volatilization and dilution of substances in the real natural environment will affect the concentration of PBDEs.Therefore,the change of concentration of the target substance and product does not really represent the degradation transformation.Based on this,this study aimed to explore the fractionation characteristics of stable carbon isotopes during the in-situ anaerobic micro-degradation of PBDEs in coastal wetland sediments,so as to accurately characterize the anaerobic micro-degradation process of PBDEs.The research conclusions are as follows:（1）Firstly,the anaerobic degradation processes of BDE-47 and BDE-153 in surface and subsurface sediments were investigated.It was found that the degradation degree of PBDEs was affected by the depth of sediment,and the degradation degree of BDE-153 in surface sediments was higher than that of BDE-47.In addition,the degradation rate of BDE-47 and BDE-153 cultured alone was higher than that of the combined culture.The results showed that the anaerobic degradation of BDE-47 and BDE-153 was mutually inhibited under the combined culture condition.On the contrary,the degradation degree of BDE-47 in subsurface sediments was higher than that of BDE-153,and the degradation rate of corresponding composite culture was higher than that of single culture.The differences in degradation degree and rate between surface and subsurface layers of BDE-47 and BDE-153 were mainly attributed to the differences in community composition and relative abundance of degrading bacteria.（2）Micro-degradation products and potential degradation pathways of PBDEs in surface and subsurface sediments were analyzed.In 150 days,the degradation products of BDE-47were tribrominated diphenyl ethers and dibrominated diphenyl ethers,while the degradation products of BDE-153 were penta and tetrabrominated diphenyl ethers.The para-bromine was removed preferentially in both of them,followed by intersite and para-bromine.During 150 to180 days of culture,the degradation product of BDE-153,tetrabromodiphenyl ether,continued to degrade to form tribromodiphenyl ether.In conclusion,gradual debromination is the main path of anaerobic micro-degradation of PBDEs,and the debromination capacity of different replacement sites is in the order of para position>meta position>ortho position.（3）The fractionation characteristics of stable carbon isotopes during the anaerobic degradation of surface sediments BDE-47 and BDE-153 were investigated,combined with the Apparent kinetic isotope effect（AKIE）calculation and the identification of degradation products.The mechanism of PBDEs anaerobic microbial debromination was analyzed.Anaerobic micro-degradation resulted in the fractionation of stable carbon isotopes of BDE-47and BDE-153,during which the heavy isotope ¹³C was continuously enriched.The change degree ofδ¹³C in combined culture was smaller than that in single culture,and the degradation degree and rate of BDE-47 and BDE-153 in combined culture were smaller than that in single culture,confirming that there was mutual inhibition of micro-degradation in BDE-47 and BDE-153 culture.The AKIE_C values of BDE-47 and BDE-153 were the same under the combined culture condition,indicating that the mechanism of anaerobic microbial debromination was similar.（4）The dynamic changes of microbial community composition and abundance during the anaerobic degradation of PBDEs in surface and subsurface sediments were studied.The species and abundance of degraded bacteria are affected by the depth of sediment.In surface sediments,BDE-47 and BDE-153 share the same Dehalococcoides and Acinetobacter,and are enriched accordingly under the combined culture condition.It was further confirmed that BDE-47 and BDE-153 had potential competition for dehalogenated bacteria in the combined culture condition.The main degrading bacteria in subsurface sediments are Paeisporosarcina,Gp7 and Marinobacter.Different from surface dehalogenation bacteria,the metabolic level of combined culture dehalogenation bacteria is higher than that of single culture.The metabolic level of the dehalogenated bacteria cultured separately by BDE-47 was higher than that of BDE-153,so the metabolic function indirectly verified the degradation ability of the bacteria.