| In recent years,the rapid development of society and industrialization has led to the emergence of various environmental pollution problems.Polycyclic aromatic hydrocarbons(PAHs)are a class of persistent organic pollutants that pollute the natural environment and endanger the health of organisms.Some studies have found that the methanogenic degradation of PAHs with CO2 as electron acceptor is suitable for biological removal of PAHs in anaerobic environments such as deep soils,flooding soils and sediments.However,the biodegradation process of PAHs in methanogenic environments is complex,and there are still some problems including slow removal,low efficiency,unclear microbial characteristics and mechanism of action.To address these problems,the complex oil pollution and characteristic pollutant of PAHs-phenanthrene(PHE)were selected as the research object.This study investigated the response of methanogenic system to long-term PAHs pollution and the mechanisms associated with biochar-enhanced anaerobic microbial communities to remediate PAHs pollution.This could provide the scientific basis for the application of biochar to the anaerobic bioremediation of PAHs-polluted environment.Moreover,it would be an important reference for the development of in-situ microbial remediation technology for PAHs-polluted soils.The main results are as follows:1.The study of long-term PHE contaminated anaerobic methanogenic system showed that the biodegradation of PHE was limited with a maximum removal efficiency of 61.1±6.6%.PHE was gradually adsorbed by sludge and final PHE concentration accumulated to 12.53 mg/g TS.This process weakened the ability of UASB reactor to treat COD,NH4+-N,and volatile fatty acids(VFAs)in the wastewater.Meanwhile,PHE inhibited the growth and proliferation of some fermenting bacteria in Thermotogae,Latescibacteria and Bacteroidetes,as well as acetoclastic Methanosaeta.The microbial extracellular polymeric substances(EPS)secretion and ATPase activity increased significantly to self-protect against PHE toxicity.The abundances of functional bacteria involved in PHE degradation(Syntrophic acetogenic bacteria,toluene-degrading consortium)were improved.The changes in relative abundance of potential methanogenic functional enzymes and the results of methanogenic metabolism together verified that PHE adversely affected acetoclastic methanogenesis.2.The potential of biochar used to enhance the biodegradation of PAHs in complex petroleum-polluted and PHE-polluted methanogenic environments was explored.(1)The optimal dose of biochar in petroleum-polluted environment was 0.6 g/g vs oily sludge(5.6 g/L),which significantly enhanced the efficiency of hydrolysis,acidogenesis,acetogenesis,and methanogenesis.Electroactive fermentation bacteria(Clostridium and Bacteroides)and acetoclastic Methanosaeta were enriched.This accelerated direct interspecies electron transfer(DIET)and changed microbial community structure.Most importantly,biochar stimulated the expression of most functional genes in the Xenobiotics biodegradation&metabolism pathway.Pollutants such as long-chain alkanes,PAHs,phenols and benzenes were effectively degraded in the group with adding moderate biochar(0.6 g/g VS oil sludge),among which intermediate metabolites of recalcitrant PAHs(NAP,PHE)were obviously increased.(2)In PHE-polluted environment,biochar was able to maintain system stability,strengthen microbial metabolic capacity and enhance the conversion of co-metabolic carbon sources to methane through improving microbial electron transfer system(ETS)and enzymes activities and stimulating higher proteins content in microbial EPS.Biochar enabled a rapid recovery of system imbalances caused by elevated organic loads,and the biodegradation efficiency of PHE increased from 47.8%to 85.1%.In this process,dominant microbial community for PHE degradation evolved to Firmicutes and Bacteroidota,and facilitated DIET between electrogenic bacteria(Bacteroides,f_Geobacteraceae,syntrophic VFAs-oxidizing bacteria,etc.)and Methanosaeta.The significant enrichment of Bacteroides and Methanolinea on the surface of biochar promoted the removal of PHE adsorbed on biochar.3.The PHE-polluted flooded soil environment was constructed to further explore the feasibility of application of the above study.With adding anaerobic sludge as exogenous microbial community,the removal efficiency of PHE in soil increased from 55.7%to 72.0%.The adsorption and immobilization of biochar limited the PHE removal.However,the combination of biochar and anaerobic sludge enhanced the anaerobic biodegradation of PHE,and the highest removal efficiency of PHE was achieved with the value of 81.0%.With the addition of anaerobic sludge,the predominant microbial communities related to PHE biodegradation evolved to Chloroflexi and Proteobacteria.Besides,methanogen(Euryarchaeota)was introduced to soil,which enriched the metabolic pathways of carbon cycle in anaerobic system.When biochar was added to anaerobic environment composed of soil and sludge,the succession of some microbial communities was similar to sludge environment.For example,the relative abundances of Bacteroides and Methanolinea were improved.Moreover,the DIET process between electroactive bacteria(Bacteroides,f_Geobacteraceae)and acetoclastic Methanosaeta.Therefore,the anaerobic biodegradation of PHE was strengthened,indicating that biochar and anaerobic microbial communities(methanogenic environments)have great potential for application in the remediation of PAHscontaminated soils. |
