Indigenous Microbial Community-Based Remediation For Trichloroethylene And Nitrate Mixed Contamination In Groundwater

In recent years,the development of urbanization and the acceleration of industrialization resulted in an increasing attention to environmental problems.Contamination of pollutants with different properties existed in groundwater has become urgent problems to be solved.Among those problems,TCE/NO₃^--N mixed contamination has been considered as a representative one.However,few studies were developed for the remediation technology that could remove TCE and NO₃^--N simultaneously.In this study,the performance of indigenous microbial community from potentially contaminated sites on the anaerobic removal of TCE/NO₃^--N mixed contamination in groundwater was investigated,and the traditional microbial immobilization technology has been developed to construct a bioreactor for long-term stable operation.The result would provide an experience for TCE/NO₃^--N bioremediation in groundwater.In this study,a suspended microbial community was extracted and acclimated through the soil samples that collected from the potentially contaminated site in China.Powdered activated carbon?PAC?and maifanite?MF?were added as active substances in the system.The effects of different pH,temperature,the dosage of active substance and the concentration of target contaminants on microorganism performance were investigated.Central composite design?CCD?and response surface methodology?RSM?were used to analyze the interaction between TCE and NO₃^--N within the system.The results indicated that TCE and NO₃^--N didn't show inhibition to microbial activities,while the pH and temperature showed a significant effect.Besides,the optimal conditions for TCE/NO₃^--N removal obtained by RSM were pH8.01,temperature 20.18?,PAC dosage 4.04%and the concentration of TCE was3.87 mg/L in the PAC addictive system;pH 8.06,temperature 23.07?,MF dosage4.15%and the TCE concentration was 2.93 mg/L in the MF addictive system.Then,based on the optimal conditions,the immobilized pellets were further prepared by using this suspended microbial community,and the effects of different TCE concentrations on the immobilized microbial community were studied.It was found that the microbial community showed better adaptation to TCE after immobilization.The TCE/NO₃^--N removal rate remained stable when the TCE concentration increased from 1.25 to 7.50 mg/L.Through the analysis of the microbial enzyme dynamics and the model fitting,it was found that the enzyme reaction in the PAC and MF addictive systems were followed the enzyme-catalyzed reaction model.And there was no inhibition showed to the enzyme activities in the presence of TCE/NO₃^--N contaminations.In addition,this study also used the real polluted groundwater to verify the performance of the immobilized pellets.As a result,the pellets could achieve the desired effect when the real groundwater was treated,and the removal rate was higher than that of the synthetic groundwater experiment.MF could gradually dissolve a variety of trace elements to promote the enzyme of microorganisms,consequently improving their removal performance.Furthermore,a plug flow reactor filled with immobilized pellets was constructed.According to the experiments,it was obtained that the adsorption capacity of TCE was 0.040 mg/g and NO₃^--N was 0.020 mg/g when using PAC immobilized pellets;the adsorption capacity of TCE was 0.052 mg/g and NO₃^--N was 0.024 mg/g when using MF immobilized pellets.It was proved that a majority of degradation was dominated by biological removal and the tiny minority was contributed by the adsorption of PAC or MF.In addition,in the immobilized systems with PAC and MF,the effect of changing the hydraulic retention time?HRT?on TCE removal was greater than that on NO₃^--N removal,while the MF immobilized system showed a more stable removal efficiency for TCE/NO₃^--N mixed contamination.The total reaction of microbial community using different nitrogen sources was calculated,and the stoichiometric relationship between substrates and products in the TCE/NO₃^--N mixed contamination was established.Through the analysis of the structure and diversity of the indigenous microbial community,it was indicated that the increase of the reaction time and the change of the external conditions showed influence on its structure.The additional MF could significantly increase the community diversity and the abundance,by means of promoting the microbial growth in the long-term-enrichment and forming a more stable microbial community structure.In this study,based on the indigenous microbial community,the microbial immobilized pellets with PAC and MF were developed and a bioreactor with the long-term stable operation was constructed.The obtained result suggests a theoretical foundation for the remediation of TCE/NO₃^--N mixed contamination in groundwater and supplied reasonable suggestion for the field scale application.