Microbially-mediated Transformation Of Chlorobenzene In Low Permeability Silty Clay Medium

DNAPL(Dense Non-Aqueous Phase Liquid)can through the aeration zone under the gravity,and migrate in low-permeability media.This present,the research of DNAPLs in low-permeability media has become a scientific hotspot,however most of them focus on adsorption and percolation behavior,but few on degradation and transformation.This article takes chlorobenzene as the representative of DNAPLs to discuss its transformation behavior in low-permeability media,which is of great significance to improve the behavior and fate of DNAPL,and establish a suitable repair measures.One chlorobenzene pollutant site was selected to study the pollution characteristics and physical and chemical properties of low-permeability clay layers(Chapter 2),the degradation characteristics of chlorobenzene by indigenous microorganisms(Chapter 3),and the transformation of chlorobenzene in low-permeability clay media(Chapters 4and 5).Chapter 2 was to find out the pollution concentration and spatial distribution characteristics of chlorobenzene compounds,analyze the microbial community and quantity in the soil by correlation analysis,research the characteristics of low permeability clay media in terms of basic physical properties,chemical composition and mineral composition,particle composition analysis and permeability.Chapter 3 was to isolate microorganisms from soil samples severely affected by chlorobenzene pollutants in the study area,observe and identify microbial strains.The single factor analysis method was used to certain the most suitable growth conditions of the bacteria,the growth characters under the most suitable conditions,and the kinetic equation of the bacteria.An anaerobic environment was used to simulate low infiltrate the reducing conditions of the clay medium,and study the conversion under the reducing conditions.Chapter 4 takes the undisturbed soil samples to approximate the actual low-permeability clay medium.Bacterias injected into the undisturbed soil to obtain the conversion rate of chlorobenzene,infer the low-permeability environment in the study area and simulate the soil environment in the study area.Bacterias are mixed into the disturbed soil to study the conversion of chlorobenzene and influencing factors.Chapter 5 takes iron and manganese oxides as the representatives of variable valence metal to study their adsorption and redox capacity for chlorobenzene.Iron and manganese oxides are put into the microbial degradation chlorobenzene system to study the microbial degradation of chlorobenzene.The results and conclusions are as follows:(1)The pollutants are chlorobenzene compounds in the study area.In the horizontal distribution,the pollutant concentration is related to the process function zoning and the ground elevation.In the vertical distribution,the pollutants can migrate downward through the surface backfill soil layer and gathers in irregular waves in the longitudinal depth.The low permeability clay medium is weakly alkaline,with low organic matter content(0.03%?2.24%,average value 0.61%),low water content(14.5%?23.7%,average value 18.5%),low clay mineral content(20%?30%),low clay particles content(10.3%?11.9%),and the ability to adsorb and intercept organic pollutants is weak.(2)The potential chlorobenzene-degrading bacteria in the low-permeability medium is Microbacterium paraoxydans,which can take chlorobenzene as the only carbon source in aerobic metabolism.The total degradation rate of chlorobenzene is 64.3%(7 days),but in an anaerobic environment,the bacteria cannot completely degraded.the degradation of chlorobenzene by Microbacterium paraoxydans is inhibited and the inhibitory effect of chlorobenzene on the bacteria is dominant.The kinetic model of chlorobenzene degradation under aerobic conditions can be modeled by Haldane,the reaction kinetics equation is:v=17.1/(1+100/S+S/7940000).Under anaerobic conditions,the degradation of chlorobenzene is restricted by enzymes and other factors.and the first-order reaction has limitations.(3)The conversion rate of Microbacterium paraoxydans to chlorobenzene is extremely low in soil media.An initial concentration of 100 mg/kg of chlorobenzene cannot be completely degraded whether in undisturbed soil or disturbed soil media.After the modified pseudo first-order reaction kinetic equations are fitted,the maximum degradation rate of chlorobenzene in the undisturbed soil medium and the disturbed soil medium is 20.5%and 20.1%,respectively,which is higher than that of the bacteria in the water.The maximum degradation rate of chlorobenzene under oxygen conditions(6.3%).Appropriate temperature,sufficient moisture and appropriate exogenous organic nutrients can increase the conversion of bacteria in the soil medium,and the higher initial concentration of chlorobenzene and the accumulation of metabolites concentration will inhibit the bacteria(4)Iron and manganese oxides mainly exhibit physical adsorption to chlorobenzene under neutral pH conditions.They can indirectly participate in the conversion of chlorobenzene by promoting or inhibiting the conversion of chlorobenzene by microorganisms.Fe₂O₃powder can accelerate the conversion rate of chlorobenzene and increase the conversion rate of chlorobenzene.MnO₂inhibit the conversion of chlorobenzene obviously at the beginning of the experiment.As the conversion process progresses,the inhibition gradually weakens.The low-permeability clay medium in the study area is a reducing environment.Variable metal ions(such as Fe²⁺and Fe³⁺)can provide energy by participating in the electron transfer of the chlorobenzene converting enzyme.The conversion process of chlorobenzene mediated by microorganisms is unidirectional.With the downward percolation of chlorobenzene in the low-permeability clay medium,the conversion of chlorobenzene shows a long continuous reaction equilibrium conversion process.In-situ bioremediation technology is not applicable in the study area.