Study On The Biodegradation And Biogeochemical Response Of Benzene Pollution In Groundwater Of A Petrochemical Industrial Park In Northeast China

Groundwater pollution has the characteristics of concealment,lag and difficulty in treatment,posing a serious threat to human health and ecological environment.Biodegradation technology is widely used in in-situ groundwater organic pollution control strategies due to its low cost and environmental friendliness.The biodegradation process of groundwater organic pollutants is a comprehensive expression of hydrogeological conditions,pollutant conditions,main geochemical indicators and microbial activities.There is a complex feedback mechanism between different biogeochemical components.The interaction and multiple responses between groundwater microbial degradation and geochemical indicators are scientific questions that need to be studied in depth.Therefore,it is of great theoretical and practical significance to investigate the biodegradation of characteristic organic pollutants in groundwater and their biogeochemical response.The study is based on the key R&D project of the Ministry of Science and Technology,“Mechanisms of pollution formation and multi-phase distribution characteristics of chemical sites with optimally controlled organic pollutants”,a petrochemical industrial park in northeast China is selected to study the biodegradation of benzene pollution in groundwater and its biogeochemical response.The biodegradation of benzene in groundwater is identified through a combination of geochemistry,isotopic techniques and microbial composition.The biodegradation pattern of benzene is analysed through indoor physical simulation experiments,complemented by isotopic fractionation models and microbial characterisation parameters,to elucidate the structure,dynamics and diversity of indigenous microbial communities and their tolerance mechanisms to benzene under different pollution conditions.The mechanism of benzene degradation by indigenous microorganisms under the influence of geochemistry interactions is revealed,and the multiple response relationships between biodegradation and geochemistry indicators are clarified.A multi-component reactive solute transport and biodegradation coupling model is developed to finely characterise the biodegradation of benzene in groundwater in the study area.The main conclusions obtained in this thesis are as follows:（1）Dynamic monitoring of multiple indicators in the study field and multivariate statistical methods were used to analyse groundwater chemical indicators,isotopes and microorganisms indicative of benzene degradation,to identify the biodegradation of benzene and to quantify the contribution of different electron acceptors.The concentration of benzene in groundwater in the study area showed a significant decreasing trend with a mean decay rate of 0.00558 day^-1.The changes of DO,ORP,NO₃^-,SO₄^2-and HCO₃^-contents were consistent with the spatial and temporal evolution of benzene biodegradation.The relative contributions of each electron acceptor were NO₃^-（44.01%）,DO（29.44%）and SO₄^2-（26.55%）.The difference inδ¹³C values between different regions of groundwater is greater than 2‰,the more modern carbon nature of ¹⁴C,and the presence of indigenous microbial genera that degrade benzene in groundwater,many evidences reveal that benzene concentration changes are closely related to biodegradation.This part of the study provides practical support for subsequent studies on the biodegradation pattern of benzene and the mechanism of the influence of the interaction between indigenous microorganisms and geochemistry on benzene degradation.（2）Indoor static and dynamic biodegradation experiments,complemented by isotope fractionation models,were designed to clarify the biodegradation pattern of benzene in groundwater and the tolerance level of indigenous microorganisms to different benzene concentrations.Indigenous benzene-degrading microorganisms were domesticated from groundwater in the study area,and their indoor growth environment was set up with groundwater conditions,and the microorganisms reached saturation at 163h.The biodegradation efficiency of benzene was 63.66%,68.26%,69.59%and 67.23%at 10mg/L,20mg/L,30mg/L and 50mg/L,respectively,with different concentration gradients,and the indigenous microorganisms were well tolerated.The microbial degradation was consistent with the first-order kinetic equation,with degradation rate constants ranging from 0.0081 h^-1-0.136h^-1.And the stable carbon isotope enrichment coefficients for benzene degradation ranging from-4.5‰to-1.1‰.The migration degradation pattern of benzene under hydrodynamic influence showed that adsorption played a dominant role in the 0-8h stage and biodegradation played a dominant role after 8h,and the contribution of microbial degradation was 26.63%,33.10%and 26.66%at 20h,48h and 72h,respectively.The enrichment coefficient of benzene degradation under hydrodynamic influence was-4.6‰.The study provided a basis for further clarification of microbial succession characteristics.（3）The structure,dynamics,diversity and key microorganisms of indigenous microbial communities under different contamination conditions were explored by macrogenome sequencing techniques,community screening mechanisms for benzene degradation by indigenous microorganisms were identified and community succession characteristics of benzene were clarified.Benzene contamination caused significant changes in microbial community abundance and diversity,with Pseudomonas（22.91%vs 25.33%）,Klebsiella（0.002%vs 15.04%）,Arthrobacter（1.91%vs 13.16%）,Azoarcus（3.91%vs 10.30%）,Acinetobacter（0.005%vs8.98%）,Desulfovibrio（0.19%vs 3.01%）and Delftia（0.02%vs 2.18%）genera were enriched and Stenotrophomonas（4.19%vs 2.39%）spp.were enriched and Stenotrophomonas（4.19%vs 2.39%）spp.abundance decreased.The effect of different benzene concentrations（10 mg/L,20mg/L,30 mg/L and 50 mg/L）on microbial growth did not show a linear relationship,but rather higher concentrations inhibited their growth.The microbial community degraded benzene mainly through complex co-metabolic mechanisms,and in groundwater contaminated with benzene at 50 mg/L..（4）Multivariate statistical analysis revealed the influence of indigenous microbial-water chemical interactions on benzene degradation and clarified the multiple response relationships between biodegradation and biogeochemical indicators.The effect of benzene on the microbial community was related to its own concentration,and various electron acceptors could influence benzene degradation efficiency by regulating the ratio of benzene degrading bacteria,and the indirect effect of electron acceptors was greater than the direct effect of benzene on the microbial community.The DO,ORP,p H,CON,TDS,NO₃^-,SO₄^2-,HCO₃^-,Ca²⁺,Mg²⁺,Na⁺and C1^-all changed to some extent during microbial degradation.The response of DO,ORP,p H,CON,TDS,NO₃^-,SO₄^2-,HCO₃^-,Ca²⁺,Mg²⁺,Na⁺and C1^-to the degradation rate during degradation was established to indicate the changes in the groundwater environment caused by biodegradation.Among them,DO,NO₃^-and SO₄^2-had a greater influence on the characteristics of groundwater microbial communities,and NO₃^-had the greatest influence.The Pseudomonas,Arthrobacter,Azoarcus,Acinetobacter,and Desulfovibrio were positively correlated with NO₃^-and SO₄^2-,indicating that several genera mainly used NO₃^-and SO₄^2-to participate in benzene degradation.Response surface and neural network prediction models were introduced to characterise the interaction between NO₃^-and SO₄^2-on benzene degradation efficiency,showing that benzene degradation rates were not influenced by a single water chemistry factor and were higher in the coexistence of NO₃^-and SO₄^2-.In a high NO₃^-level environment,the degradation rate was more sensitive to changes in SO₄^2-,and the response surface and neural network models predicted optimal degradation rates of 75.42%and 73.65%,respectively.（5）A coupled simulation model of benzene migration,dynamic dissolution,adsorption and biodegradation in groundwater in the study area was developed to finely characterise the biodegradation process of benzene.Benzene migrates horizontally along groundwater flow,with an overall decreasing trend in concentration as the contaminant diffuses due to sorption and biodegradation.Biodegradation enhanced benzene dissolution by 514%,resulting in a stronger dissolution drive due to biodegradation reducing the concentration of benzene in the aqueous phase.The presence of biodegradation decreases the migration distance of contaminants,with anaerobic degradation of 77%of benzene in the dissolved phase and aerobic degradation of 17%,revealing a mixed biodegradation mechanism for benzene in groundwater in the study area,clarifying the amount of multi-electron receptor contribution,and providing theoretical support for the development of reasonable and effective remediation tools.