Mechanisms Of Nitrate-reducing Strengthening Microbial Fuel Cells To Remove The Industrial-and Agricultural-source Organic Pollutions In The River Sediment

Owing to the rapid growth of the population and the quick development of the economy,plenty of organic pollutions were produced and released into environment voluntarily or involuntarily.The industrial source emission（e.g.,petroleum hydrocarbons,which was released by the transportation and chemical industry）and agricultural source emission（e.g.,estrogen,which was relessed by the livestock and poultry breeding）are the two main source of the organic pollutions.Once in the environment,sediment tend to be the main“reservoir pool”and“secondary pollution source”of the orgonic pollutions,and the biodegradation were the main pathway for the attenuation of organic pollutions.In the sedimentral condition,O₂,NO₃^-,Fe³⁺,SO₄^2-,HCO₃^-could be acted as as terminal electron acceptors（TEAs）in the biodegradation of organic pollutions.Meanwhile,the complex of the organic pollutions was another challenge of understanding the biodegradation mechanism.Thus,the degradation mechanisms of organic pollution in the sediment were still poorly understand.Therefore,the development and application of microbial remediation technology for organic pollution in sediment were restricted.Hence,the objectives of this study were（1）the biodegradation mechanisms of organic pollution（i.e.,petroleum hydrocarbons and estrogen）were investigated in terms of biodegradation behaviors of organic pollutions and driving mechanisms of microbial communities;（2）to explore the bioremediation technology of nitrate-reducing strengthening microbial fuel cells,basing on the driving mechanisms of microbial communities in the biodegradation of organic pollution.These studies would contribute to understand the degradation mechanisms of organic pollutions in the sediment and be useful in establishing a strategy for bioremediation.The main results and conclusions are as follows:（1）In the present study,the biodegradation behaviors of petroleum hydrocarbons under various reducing conditions were investigated.n-Alkanes and polycyclic aromatic hydrocarbons（PAHs）were degraded with NO₃^-,Fe³⁺,SO₄^2-,or HCO₃^-as terminal electron acceptors（TEAs）,which link to four typical reducing conditions（i.e.,nitrate-reducing,ferric-reducing,sulfate-reducing and methanogenic conditions,respectively）in sediment.The fastest degradation rates were achieved under sulfate-reducing conditions with half-lives of 49.51 days for n-alkanes and 58.74 days for PAHs.For short-chain n-alkanes and low-molecular weight（LMW）PAHs,relatively higher removal efficiencies were achieved under nitrate-and ferric-reducing conditions.The degradation of long-chain n-alkanes and high-molecular weight（HMW）PAHs coupled to methanogenesis was the most favored as compared with other reducing conditions.Carboxylation was found to be the principle mechanism for regulating n-alkane degradation coupled to denitrification,while the activation of n-alkanes by the addition of fumarate was the principle mechanism for the n-alkane degradation under sulfate-reducing conditions.The anaerobic metabolism of n-alkanes may not proceed via fumarate addition or carboxylation under ferric-reducing and methanogenic conditions.Illumina Hi Seq sequencing revealed dissimilar structures of the microbial communities under various reducing conditions.It is hypothesized that the utilization of different TEAs for n-alkane and PAH degradation resulted in distinct microbial community structures,which were highly correlated with the varied degradation behaviors of petroleum hydrocarbons in sediment.The current results may provide reference value on better understanding the biodegradation behaviors of n-alkanes and PAHs in association with the induced microbial communities in sedimentary environments under the four typical reducing conditions.（2）The widespread detection of 17β-estradiol（E2）in the environment has become an emerging concern worldwide due to its endocrine disrupting effects.This work focuses on the aerobic and anaerobic biodegradations of E2 in various sedimentary environments with different availabilities of electron acceptors,including O2,NO3-,Fe3+,SO42-,or HCO3-.The highest removal efficiency（98.9%）and shortest degradation kinetics of E2（t1/2=5.0d）were achieved under aerobic condition,followed by nitrate-reducing,ferric-reducing,sulfate-reducing and methanogenic conditions.We propose four different degradation pathways of E2 based on the metabolites identified under various redox conditions.Although most of E2 was effectively removed under aerobic condition,the potential environmental risk still needs to be considered due to the residual estrogenic activity induced by estrone（E1）formation.The endocrine-disrupting activities,as indicated by estradiol equivalent（EEQ）values,were related to E2 removal efficiency,degradation rate and metabolite formation.We further analyzed the succession of bacterial community compositions and functions using Illumina Hi Seq sequencing and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States（PICRUSt）.The findings herein evidenced that bacterial community compositions and metabolic functions associated with different redox conditions impact the biodegradation of E2 and its endocrine-disrupting activity.This knowledge will be useful in predicting the environmental fates of estrogenic hormones in various sedimentary environments and aid in establishing appropriate strategies for eliminating potential environmental risks.（3）In order to compare the biodegradation of Phenanthrene（PHE）under the nitrate-reducing and ferric-reducing conditions,stable isotope probing（SIP）technology was used.The biodegradation efficiency under nitrate-reducing condition was higher than ferric-reducing condition.Six degradation bacteria were only identified under nitrate-reducing condition,including OTU13（MN473241,Sphingobium）,OTU2（MN473240,Desulfitobacterium）,OTU9（MN473243,Sinobacteraceae）,OTU6（MN473242,Oxalobacteraceae）,OTU3（MN473239,Azoarcus）,OTU4（MN473238,Anaerolineaceae）.Two degradation bacteria were only identified under ferric-reducing condition,including OTU12（MN473244,Desulfatiglans）,OTU15（MN473245,Deltaproteobacteria）.The OTU1（MN473237,Deltaproteobacteria）was identified as degradation bacteria under both nitrate-reducing and ferric-reducing conditions.The differences of the species and abundance of degradation bacteria were the main factors that leading to the difference of degradation efficiency.（4）The study investigates a bioremediation process of polycyclic aromatic hydrocarbons（PAHs）removal and odour mitigation combined with energy harvesting.Sediment microbial fuel cells（SMFCs）were constructed with the addition of nitrate in the sediment to remove acid-volatile sulphide（AVS）.With the combined nitrate-SMFC treatment,over 90%of the AVS is removed from the sediment in 6 weeks of the SMFC operation and a maximum of 94%of AVS removal efficiency is reached at week 10.The highest removal efficiencies of phenanthrene,pyrene and benzo[a]pyrene are 93%,80%and69%,respectively.The maximum voltage attained for the combined nitrate-SMFC treatment was 341 m V.The 16S r RNA gene analysis revealed that the autotrophic denitrifiers Thiobacillus are the dominant genus.In electricity generation,both sulphide-oxidation and PAH-oxidation are the possible pathways.Besides,the addition of nitrate stimulates the growth of Pseudomonas which is responsible for the electricity generation and direct biodegradation of the PAHs,indicating the synergistic effect.The developed bioremediation process demonstrated the potential in in-situ bioremediation process utilizing SMFC combined with nitrate stimulated bioremediation.Nonetheless,further optimization is required for the voltage output.In conclusion,this study focused on biodegradation of industrial-and agricultural-source organic pollutions in the river sedimenteral conditions and and driving mechanisms of microbial communities.Furthermore,we explored the bioremediation technology of nitrate-reducing strengthening microbial fuel cells basing on the driving mechanisms of microbial communities in the biodegradation of organic pollution.These studies would contribute to understand the degradation mechanisms of organic pollutions in the sediment and be useful in establishing a strategy for bioremediation.