Anaerobic Microbial Degradation Of Persistent Organic Pollutants In Sediments Based On Multi-element Compound-specific Stable Isotope Analysis

Polychlorinated biphenyls(PCBs)and polybrominated diphenyl ethers(PBDEs)are two classic groups of persistent organic pollutants.Although their production and use have been banned by various countries,a mass of PCBs and PBDEs had been released into the environment and eventually become enriched in sediment,which would sustainedly threat ecology and human health via bioaccumulation and trophic transfer.Therefore,in situ microbial degradation,as the main natural remediation way of these pollutants in sediments,have always been one of the important research topics in the field of environmental sciences.However,given the complexity of the field environment and the limitation of research methods,there are many difficulties in this study.Up to now,most studies have explored in situ microbial degradation of PCBs and PBDEs based on pollutant composition analysis,which cannot accurately distinguish microbial degradation from physical processes,such as diffusion and migration.Aiming at the above research problems and difficulties,the current study firstly built a compound-specific chlorine/bromine isotope analysis(Cl-/Br-CSIA)for PCBs and PBDEs using gas chromatography-quadrupole mass spectrometry(GC-q MS).Then,a multiple-line-of-evidence approach,including C-/Cl-/Br-CSIA,qualitative and quantitative analysis,16S r RNA gene tag sequencing,and q-PCR,was used to comprehensively investigate the in situ microbial degradation of PCBs and PBDEs in sediments.Finally,the anaerobic degradation process and mechanism of PCBs and PBDEs by indigenous functional bacteria in sediments were thoroughly studied via the enrichment and subculture of dehalogenated functional microorganisms combined with the degradation experiment of pollutants.The main results are as follows:(1)A Cl-/Br-CSIA for PCBs and PBDEs based on GC-q MS was developed successfully by systematically optimizing the computational scheme,concentration gradient,dwell time,electric voltage,electric current,and ionization energy.For PCB congeners,the molecular ion method for a concentration of 0.5?10.0 ppm,a dwell time of 20?100 ms,a relative EM voltage of 200 V,an electric current of 34?A,and an ionization energy of 70 e V was determined as the most suitable scheme,which obtained standard deviations(SDs)of chlorine isotope ratios ranging from 0.00008 to 0.00068.As for the PBDE congeners,the lowest SDs,ranging from 0.00050 to 0.00172,were determined using the top four ion method with a concentration of 5?10 ppm and dwell time of 20?50 ms.This newly developed Cl-/Br-CSIA method successfully determined the chlorine/bromine isotope ratios of PCBs/PBDEs in technical mixtures and traced the chlorine and bromine isotope enrichment factors of PCBs and PBDEs in photodegradation experiments,suggesting that it is a promising tool for assessing the sources and transformation processes of PCBs and PBDEs in the environment.(2)Based on the quantitative analysis of PCBs and PBDEs and the microbial composition analysis in the field sediment cores from three sampling sites,the following results were found:the different distribution patterns of PCBs concentrations along with the sediment core at the three sampling sites implied that the PCBs sources at site 3 were different from those at site 1 and 2,while the PBDEs concentrations showed the similar vertical trends at the three sampling sites.The significant variations in chlorine per biphenyl(CPB)and enantiomeric fraction(EF)of chiral PCBs were observed in three sediment cores,which suggested the occurrence of PCBs microbial degradation;although PBDEs were dominated by BDE209,its PMF results indicated that there was a source of debromination degradation(factor 5)besides the sources from e-waste dismantling.Moreover,the ratio of factor 5 to the total PBDEs(factor 5/TBDE)generally increased along with the sediment depths.A variety of potential PBDEs dehalogenating bacteria,including Dehalococcoides,Dehalobacter and Sulfurospirillum,were widely detected from field sediments using high-throughput Mi Seq sequencing of 16S r RNA gene amplicons.The CPB had a significant(p<0.01)negative correlation with Log the abundance of Dehalococcoides/total molar concentration of PCBs(Log Dhc/TPCB)along the cores;for PBDEs,the factor5/TBDE and the relative abundance of putative dehalogenating bacteria generally showed the synchronous variations along the cores,which further demonstrated that PCBs and PBDEs possibly occurred in situ microbial degradation with different degree in field sediments.(3)The current study further detected the carbon,chlorine,and bromine isotope compositions of PCB and PBDE congeners(?¹³C,³⁷Cl/³⁵Cl or ⁸¹Br/⁷⁹Br)in the three sediment cores.Some PBDE congeners(BDE28,37,47,49,99,and 153)and PCB congeners(PCB18,31/28,44,49,52,64,87,91,97,101,105,107/149,110,118,153/132,and so on)showed significant ¹³C and ⁸¹Br(or ³⁷Cl)enrichments or depletions from the surface to the bottom layers,which firstly provided the official evidences for in situ microbial degradation of PBDEs and PCBs based on kinetic isotope effect.The variations of?¹³C and ⁸¹Br/⁷⁹Br values of PBDEs showed significant correlations with the factor 5/TBDE and the relative abundance of potential dehalogenating bacteria,demonstrated that these congeners underwent significant microbial degradation in the sediment cores over time.In addition,the differential variation trends of carbon,chlorine,and bromine isotope compositions for the same PCB or PBDE congener in different sediment cores or the different PCB and PBDEs in the same sediment core suggested that the microbial degradation of PCBs and PBDEs was congeners-specific and sites-specific.(4)The in vivo experiments with native functional microorganism showed that PCB132,PCB174,BDE47,and BDE99 were significantly degraded.The main degradation pathways of PCBs were meta-dechlorination,while PBDEs degraded mainly via ortho-dechlorination.The enantiomeric fraction(EF)of substrates(PCB132and 174)and chiral products(PCB91,135,and 149)were significantly larger than 0.5,demonstrated that there might be chiral inversion from substrates to products during the degradation of chiral PCBs.In addition,PCB132,PCB174,and BDE47 occurred significant carbon isotope fractionation with carbon isotope enrichment factors of–0.8‰,–1.1‰,and–1.2‰,respectively,under QY-2-degradation.However,there was no significant carbon isotope fractionation for BDE99,which might be caused by the low degradation degree(35%).Moreover,the chlorine isotope fractionation for PCB132 and 174 and bromine isotope fractionation for BDE47 and 99 were not observed.These were inconsistent with the chlorine and bromine isotope fractionation of some PCBs and PBDEs observed in the field sediments.This discrepancy might be caused by the change of environmental factors,such as dehalogenated microbial community structures and the types of pollutants,which could lead to different degradation processes of PCBs and PBDEs.In this study,Cl-/Br-CSIA methods for PCBs and PBDEs based on GC-q MS were successfully established,which reduced the detection threshold of Cl-/Br-CSIA and expanded its application prospect.Aiming at the microbial degradation of PCBs and PBDEs,this study is the first to clarify in situ microbial degradation of PCBs and PBDEs in field sediments by combining field monitoring with laboratory research from the perspective of geochemistry and microbial ecology.The results have important scientific significance for the official assessment of the natural remediation potential of PCBs and PBDEs in sediments.At the same time,this study also provides a successful case for monitoring the sources and fate of organic pollutants in a complex environment.