Effect Of Btex And Chlorinated Hydrocarbons On Fe(?) Bio-Reduction

Iron(Fe)is the fourth most abundant element in the earth's crust.The biological reduction effect of Fe is widespread in the underground environment.It is not only related to the cycle of elements such as carbon(C),nitrogen(N),and sulfur(S),but also directly affects the migration and transformation of pollutants.Due to improper operations during mining,production,storage,and disposal,the pollution of organic pollutants in the underground environment is widespread.Therefore,in the underground environment containing organic pollutants,the coexistence of Fe(III)-reducing microorganisms and these organic pollutants is widespread.At present,there have been a lot of researches on the degradation of organic pollutants under Fe(III)reduction,but the effect of organic pollutants on the biological reduction of Fe(III)has not attracted attention.BTEX and chlorinated hydrocarbons are two typical organic pollutants commonly found in underground environments.Due to their teratogenic,carcinogenic and mutagenic properties,a large number of toxicological studies have been carried out on humans and animals at this stage,and this toxicity may also affect Fe(III)-reducing microorganisms,thereby affecting the biological reduction of Fe(III).In addition,due to the special physical and chemical properties of BTEX and chlorinated hydrocarbons,such as hydrophobicity and the non-localized ?-orbital properties of BTEX,they may potentially interact with the outer membrane of cells and interfere with their functions.Therefore,organic pollutants such as BTEX and chlorinated hydrocarbons may have an important impact on the survival of Fe(III)-reducing microorganisms and the biological reduction of Fe(III).However,the law and mechanism of this effect need further verification and in-depth research.In the actual compound organic polluted underground environment,the influence of compound organic pollutants on the reduction and transformation of Fe forms is more complicated.Therefore,the research contents of this thesis include:(1)the effect and mechanism of benzene on the survival of model Fe(III)-reducing bacteria and its Fe(III)-reducing function;(2)the law and mechanism of the influence of four different BTEX and chlorinated hydrocarbon on Fe(III)bioreduction;(3)The influence and mechanism of compound chlorinated hydrocarbon on Fe(III)reduction in actual sediment environment.This article mainly obtains the following three aspects of the results and conclusions:(1)The mechanism of the effect of benzene on the bioreduction of Fe(III)In order to study the impact of organic pollutants on the survival of Fe(III)-reducing bacteria in the environment and the Fe(III)-reducing function,this study first selected benzene as the representative organic pollutant,and the ferrihydrite reduction mediated by the Fe(III)-reducing bacteria Shewanella oneidensis MR-1(MR-1)was used as the model system.Monitoring the reduction of Fe(III)in the system and the changes in the concentration and morphology of MR-1,combined with the changes in benzene concentration,the secretion of electronic shuttles and the study of membrane permeability,to explore the mechanism.The results show that,benzene has little effect on the growth,cell morphology,and integrity of Fe(III)-reducing bacteria MR-1,but it promotes the reduction of microbial Fe(III).When the benzene concentration was 3.8 ?M,the promotion effect on microbial Fe(III)reduction was the greatest.The Fe(II)produced by the microbial reduction of Fe(III)within 60 h is twice that of the absence of benzene.The Fe(II)-O content on the mineral surface increased by 4.73% after the reduction experiment.The promotion of microbial Fe(III)reduction is attributed to the increase in cell membrane permeability induced by benzene,which promotes extracellular electron transfer and the secretion and release of flavin mononucleotide(FMN)as an electron shuttle or cofactor.On the other hand,the accumulation of too many benzene molecules on the cell surface will weaken the benzene effect on cell membrane permeability,resulting in less FMN secretion.Therefore,under a higher concentration of benzene,the promoting effect of iron reduction is weakened.(2)The law and mechanism of the influence of BTEX and chlorinated hydrocarbons on Fe(III)bioreductionIn order to explore the law and mechanism of the influence of BTEX and chlorinated hydrocarbons on Fe(III)bioreduction,four organic pollutants with different physical and chemical characteristics are represented by benzene,toluene,carbon tetrachloride,and tetrachloroethylene,and MR-1 mediated ferrihydrite reduction is used as a model system.Combining the reduction kinetics of Fe(III)in the system with the physical and chemical characteristics of the pollutants to analyze the correlation,and obtains the influence rule.At the same time,it combines the changes of MR-1 cell membrane functional groups,membrane potential and the membrane permeability to explore the mechanism.The results show that,in addition to benzene,toluene and tetrachloroethylene also promote the Fe(III)bioreduction.Under the influence of the only degraded carbon tetrachloride among the four pollutants,the Fe(III)bioreduction rate was the fastest in the first 24 hours,but as the concentration of degraded carbon tetrachloride decreased,its reduction and dechlorination competed for electron greater than the promotion effect,slow down the Fe(III)bioreduction rate.The dielectric constant value of organic pollutants has a negative correlation with the intensity of promoting microbial iron reduction.When the dielectric constant is lower than 2.61,it can promote microbial iron reduction.Further observation of the iron-reducing bacteria MR-1 cultured with organic pollutants by Fourier infrared spectroscopy showed that the deprotonation intensity of the organic pollutants to the phosphorus-related groups on the MR-1 cell membrane gradually decreased: carbon tetrachloride>benzene>toluene >perchloroethylene.The stronger the deprotonation of phosphorus-related groups,the lower the surface potential of MR-1.The lower surface potential can enhance the permeability of the cell membrane,thereby stimulating the faster release of the electron shuttle/cofactor flavin mononucleotide FMN,thereby accelerating the biological reduction of iron.(3)The mechanism of chlorinated hydrocarbons on Fe(III)reduction in anaerobic sediment environmentIn order to further explore the influence of organic pollutants in the actual underground anaerobic environment on Fe(III)reduction,this study simulates the sediment system of the actual underground environment and selects two types of complex chlorinated hydrocarbons with different Fe forms in the south(Wuhan)and the north(Tianjin)of China contaminated sediments are used as the research substrate,using trans-1,2-dichloroethylene,tetrachloroethylene and tetrachloromethane,three kinds of chlorinated hydrocarbons with a dielectric constant of less than 2.61,and seven other kinds of chlorinated hydrocarbons(vinyl chloride,trichloroethylene,dichloromethane,chloroform,1,2-dichloroethane,1,1,2-trichloroethane and1,1,2,2-tetrachloroethane)are used as composite pollutants,by measuring the reduction of Fe,the evolution of Fe-bearing mineral types,the degradation of chlorinated hydrocarbons,the change of microbial community and the influence of electron donor hydrogen,the influence mechanism of compound chlorinated hydrocarbons on Fe(III)reduction in the actual underground environment was explored.According to the conclusion of Chapter 2,chlorinated hydrocarbons with a dielectric constant smaller than 2.61 can promote the biological reduction of Fe(III).However,in the two sediment systems of this study,after 37 days of cultivation,the promotion effect of chlorinated hydrocarbons with a dielectric constant smaller than2.61 was not obvious.Instead,the addition of chlorinated hydrocarbons consumed part of the Fe(II)in the original sediments.This aspect is because chlorinated hydrocarbons such as carbon tetrachloride,1,1,2,2-tetrachloroethane and other chlorinated hydrocarbons chemically react with Fe(II)in the sediments,resulting in chemical degradation intermediate products,such as carbon disulfide,trichloroethylene,consume Fe(II)produced by the reduction of easily reducible iron(hydrogen)oxides(ferririte,lepidocrocite,etc.)with a lower crystallinity and convert them into more crystalline reducible iron(hydrogen)oxides.On the other hand,the results of the microbial community showed that after the addition of chlorinated hydrocarbons,the relative abundance of Fe(III)-reducing bacteria decreased and the Fe(III)-reducing ability of the community weakened.At the same time,the relative abundance of Fe(II)-oxidizing bacteria increased,the sediment redox environment changed,which may degrade chlorinated hydrocarbons in the process of microbial Fe(II)oxidation and consume Fe(II).The innovations of this paper are:(1)reveal the influence mechanism of BTEX and chlorinated hydrocarbons on microbial iron reduction,(2)establish a quantitative relationship between the dielectric constant of organic pollutants and their promotion of Fe(III)bioreduction.