Distribution And Microbial Dechlorination Enhancement Of Polychlorinated Biphenyls In Freshwater Sediments

Polychlorinated biphenyls（PCBs）are typical persistent organic pollutants,commonly adsorbed in lake sediments and pose threat to aquatic ecological environment.The fate and transport of PCBs in sediments,as well as the in situ remediation of PCB-contanminated sites are of great concern.In the past three decades,anaerobic microbial dechlorination has attracted extensive attention due to its low cost,less environmental impacts and applicability to large-scale in situ remediation.However,under natural conditions,PCBs natural attenuation catalyzed by anaerobic microorganisms proceeded slowly,and detoxification was relatively limited.Therefore,it is of great significance to investigate the enhancement of PCBs microbial dechlorination.In this study,two sediment cores were collected from northern Fuxian Lake,a typical deep freshwater lake.²¹⁰Pb and ¹³⁷Cs isotope dating was conducted.Also,congener specific PCBs analyses were conducted in each sediment section.The sedimentary records of PCBs in Fuxian Lake were carefully studied.Moreover,potential environmental risks posed by PCBs were also assessed.Given that PCB dechlorination potential in Taihu Lake sediments had been proved in our previous study,surficial sediments collected from Taihu Lake were used to further explore the enhancement approaches of PCBs microbial dechlorination.Aroclor 1254,known as the most toxic and commonly used commercial PCB mixture,was chose as the targeted parents,and 10%（dry weight basis）of humin or 3%（dry weight basis）of biochar were amended into microcosms respectively to investigate their effects on the extent,rate and pathways of PCBs microbial dechlorination.Finally,the shifts of microbial community,putative dechlorinating microorganisms and reductive dehalogenase genes were investigated through high-throughput sequencing and q PCR techniques.The important findings of this study are as follows:（1）A total of 76 PCB congeners were observed in Fuxian Lake sediments,with the total concentrations ranging from 5.55 to 31.47 ng/g.Compared to the sediments of typical domestic and overseas waters,PCBs in Fuxian Lake sediments was at a medium-low level.And PCBs showed scarcely no adverse impacts on exposed organisms due to the low ecological risk.However,PCBs concentrations showed an increasing trend,suggesting new inputs of PCBs to Fuxian Lake.（2）Both humin and biochar could accelerate PCB dechlorination.At Week 24,the total PCB mass concentrations decreased by 11.2%,36.9%and 16.4%in PCBs control group,biochar-amendment group and humin-amendment group,respectively.The enhancement by biochar was more significant than that by humin,attributing to the faciliated dechlorination of relatively stable intermediate PCBs.Ortho-dechlorination,a rare dechlorination pattern in nature,was observed in all PCBs-spiked groups.Meanwhile,the amendment with humin did not change dechlorination preference,while biochar preferrably enhanced ortho-and unflanked meta-dechlorination.（3）In PCBs control group and humin-amendment group,dioxin-like PCB congeners in Aroclor 1254 were dechlorinated more rapidly,and toxicity equivalent quotients（TEQs）decreased from 913.05±9.00 pg/g at Week 0 to 21.05±4.45 pg/g and 43.47±13.48 pg/g at Week24,accouting for 95%of the TEQs in the parent Aroclor 1254.Meanwhile,amended biochar tended to retard the degradation of dioxin-like PCBs,only 72%of TEQs was removed.（4）Both humin and biochar could change the microbial communities in microcosms.Biochar showed more significant effects.The ralative abundance of Proteobacteria in biochar-amendment group was higher than that in PCBs control group and humin-amendment group,indicating that biochar supplementation enriched Proteobacteria.Whereas,the relative abundance of Chloroflexi in PCBs control group and humin-amendment group increased,while the relative abundance of this phylum in biochar-amendment group was lower than that in these two groups,which was even lower than the no PCBs control.These suggested that biochar amendment could not favor Chloroflexi enrichment in microcosms.In no PCBs control,the relative abundance of Dehalococcoides showed no significant increase within the 36-week incubation.However,Dehalococcoides were selectively enriched in PCBs control group and humin-amendment group,indicating that Dehalococcoides may play an important role in PCB dechlorination in Taihu Lake sediments.Notably,the amendment with biochar did not induced the growth of Dehalococcoides,indicating that Dehalococcoides might not be critical to PCB dechlorination in the presence of biochar,while Geobacter was likely to play a significant role in PCB dechlorination in the microcosms amended with biochar.Three typical reductive dehalogenase genes,pcb A4,pcb A5 and rdh 12,increased by 2-3 orders of magnitude in PCBs control groups and humin-amendment group after PCB dechlorination took place,illustrating that their corresponding reductive dehalogenase might play a crucial role in PCB dechlorination in Taihu Lake sediments.The jump of these three genes could be used as a biomarker for the initiation and potential of PCB dechlorination.However,these three reductive dehalogenase genes showed no significant increase along with PCB dechlorination in biochar-amendment group,indicating that PCB dechlorination in biochar amended sediments might be regulated by other dehalogenase genes.