EVO?Emulsified Vegetable Oil?-Mg?OH?₂ Enhanced Remediation Mechanism Of TCE Contaminated Groundwater

Trichloroethene?TCE?is a common groundwater pollutant used as dry cleaning and metal degreasing agents in many industrial processes.As a priority pollutant,TCE has a serious threat to the human health and environmental safety.Therefore,the remediation of TCE has great importance and significance.Emulsified vegetable oil combined with colloidal Mg?OH?₂?EVO-Mg?OH?₂?can slowly release electron donors and OH^-into groundwater,which could provide carbon source and suitable living conditions for microbes.Therefore,it has a good application prospect for enhanced bioremediation of TCE contaminated groundwater by injecting EVO-Mg?OH?₂.EVO-Mg?OH?₂ emulsion with uniform drop size and desirable stability was prepared.EVO-Mg?OH?₂ was injected into simulated sand columns to evaluate its migration properties in different porous media or different injection volumes,and the influence of EVO-Mg?OH?₂ injection on the migration of TCE.The TCE degradation system was built by addition of EVO-Mg?OH?₂ and river sand as organic substrate and microbial source,respectively.The system was conducted to investigate the enhanced reductive dechlorination mechanism and dynamic process under different EVO:Mg?OH?₂ ratios.EVO-Mg?OH?₂ reaction zone was built in the column to evaluate the changing process of related indicators and microbial community structure and diversity,analysing the mechanisms of TCE biodegradation.This research laid a theoretical foundation for the application of EVO-Mg?OH?₂ reactive zone enhanced remediation TCE contaminated groundwater.The EVO-Mg?OH?₂ was successfully prepared to analyse the related properties.The results showed that the prepared EVO-Mg?OH?₂ was a heterogeneous mixture,and had a good stability and dispersibility with no significant difference in particle size(D₅₀=0.45?m).The EVO-Mg?OH?₂ was able to be transported effectively in different porous media,and the migration rate of EVO was greater than colloidal Mg?OH?₂.However,The EVO and colloidal Mg?OH?₂ were synchronously transported through the column systems.The retention of EVO-Mg?OH?₂ was significantly greater in fine sand,and the permeability only declined 9.6%in the fine sand after the injection of EVO-Mg?OH?₂.The injection volumes had an obvious effect on the migration of EVO-Mg?OH?₂.When the injection volumes were greater than one pore volume,the retention of EVO was proportional to the injection volumes,but the colloidal Mg?OH?₂ was not.EVO-Mg?OH?₂ reduced the gravity deposition process of colloidal Mg?OH?₂.The injection of EVO-Mg?OH?₂ significantly enhanced the transport of TCE in the column systems.EVO-Mg?OH?₂ with different EVO:Mg?OH?₂ ratios enhanced reduction dechlorination to evaluate the changing process of related indicators and reaction.The results showed that the addition of EVO-Mg?OH?₂ significantly facilitated the dechlorination of TCE,and the degradation process could be divided into three phases:physical adsorption phase,slow decline phase and rapid drop phase.The degradation process was accompanied by dissimilation iron reduction and methanogenic reaction,but there was no obvious effect on the dechlorination process.TCE could be almost completely degraded to DCE when the EVO:Mg?OH?₂ ratio was 1:1.The ratio of colloidal Mg?OH?₂ in EVO-Mg?OH?₂ determined the buffering ability to pH,and then influenced the utilization efficiency of electron donors and the degradation efficiency of TCE.The reduction rate of TCE and the formation rate of DCE decreased with the decreased of TCE concentration during the reaction.When enhanced degradation of40 mg/L TCE by addition of 15 mL EVO-Mg?OH?₂,the EVO consumption rate was proportional to the degradation rate of TCE,and colloidal Mg?OH?₂ could play a role for 42 days.EVO-Mg?OH?₂ reaction zone was built to evaluate the changing process of related indicators and microbial community structure and diversity.The results showed that compared with the EVO reaction zone,the EVO-Mg?OH?₂ reaction zone could enhance the dechlorination efficiency by buffering pH.And complex geochemical reactions occurred in the reaction zone,including dissimilation iron reduction,methanogenic reaction,and so on.In the early stage of the establishment of EVO-Mg?OH?₂ reaction zone,the concentration of TCE decreased with the increase of the distance from the pollution source.With the increase of time,the concentration of TCE was observed to decline along the flow direction due to migration and diffusion.In addition,the higher concentration of TCE delayed and inhibited the degradation process,and the biochemical effect was not obvious in the pollution source region due to the concentration of TCE.The total number of species and the diversity of microbial community increased at first and then decreased during the degradation process in the middle and lower reaches of the EVO-Mg?OH?₂ reaction zone,and the total number of species in the upper reaches of the reaction zone gradually increased.But the total number of species and the shannon value were always lower>middle>upper reaches.The microbial community in the lower reaches of the EVO-Mg?OH?₂ reaction zone was abundant and the structure was relatively uniform,and the process of community evolution was not obvious.The dominant bacteria in the middle reaches of the reaction zone were Pseudomonas,Acinetobacter and Marseilles in the initial stage,and then Clostridium sensu stricto and Afipia evolved into dominant bacteria.At the same time,the dechlorination genera of Geobacter,Dehalobacter,Desulfuromonas and Desulfitobacterium were gradually formed in the middle and lower reaches of the reaction zone,and TCE could be reduced to DCE or VC.Besides,the Methanothrix and Methanobacteriu were also gradually formed.However,the dominant bacteria in the upper reaches of the reaction zone were always Pseudomonas and Acinetobacter in the process of biological evolution due to the high concentration of TCE.