| With the development of the industry,nitrobenzene,as important industrial materials,is widely applied in the organic synthesis industries of aniline dyes,organic solvent,drugs and so on.Due to inappropriate disposal during the usage,storage,and transportation processes,nitrobenzene is already responsible for quite serious pollution in many groundwater systems.Nitrobenzene is one kind of DNAPL pollutants and has greater density.After entering to the groundwater,nitrobenzene will sink to the bottom and remains stable for a long time,and cause long-time water pollution by continuously releasing.Nitrobenzene has a serious threat to the water safety and human health for it is known as highly toxic,mutagenic,carcinogenic and persistence organic contaminant.So,to seek a economic and effective remediation technology and to analyze its efficiency,process and mechanism is urgent desire for the nitrobenzene polluted site,and also has a good theoretical and practical significance.In situ biological reactive zone is a good option for the long-term remediation of contaminated plume for the small disturbance on the underground environment,simple engineering operation,low cost and no secondary pollution.Due to the low biodegradability of nitrobenzene,the remediation efficiency and period of this technology were limited in the contaminated site.The coupling relationship between microbial dissimilatory iron reduction and nitrobenzene reduction provides a new way for the application of in situ biological reactive zone in the nitrobenzene contaminated site.The selection of additional organic matter is a crucial for in situ biological reactive zone technology,which is based on the microbial dissimilatory iron reduction.Emulsified vegetable oil,as a kind of slow-release material,can slowly release electrons for a long time in the underground environment.Furthermore,there is several advantages-good migration ability,easy formation of zone,low cost,high miscibility with organic pollutants.So,emulsified vegetable oil is an ideal organic matter for the application of in situ biological reactive zone remediation technology in nitrobenzene contaminated site.To stimulate microbial activity,Y-ESO(Yeast-Emulsified Soybean Oil)was prepared based on the traditional emulsified vegetable oil.An indigenous microorganism dissimilatory iron reduction coupled to nitrobenzene degradation with addition of Y-ESO system was built.In this study,the effective component for nitrobenzene degradation was identified,and the reaction processes and mechanism in the system were studied.Moreover,the dynamics was derived from Michaelis-Menten model,and the nitrobenzene degradation dynamic model was then modified based on the significant factor.Finally,the Y-ESO enhanced in situ biological reactive zone was established in a 3-D simulator,to monitor the reactive zone formation and development evolution,evaluate the nitrobenzene remediation efficiency,analysing the mechanisms of nitrobenzene remediation in the Y-ESO enhanced in situ biological reactive zone.This research laid a theoretical foundation for the application of in situ biological reactive zone technology with addition of emulsified vegetable oil in nitrobenzene contaminated site.Composite reagent Y-ESO was prepared in the lab,and its performance was analyzed.The emulsified vegetable oil showed good emulsification effect,uniform particle size,high stability and migration ability with 2%of Tween-80,10%of soybean oil.The migration ability of Y-ESO in aquifer medium was mainly affected by groundwater flow velocity,since the migration velocity and maximum relative concentration of Y-ESO would increase with the groundwater velocity.The Y-ESO retention in aquifer medium mainly influenced by medium size,and the smaller the medium diameter,the greater the retention.The mechanism of indigenous microorganism dissimilatory iron reduction coupled to nitrobenzne degradation with addition of Y-ESO was studied based on the material change and reaction processes in the system.Results showed that the additional Y-ESO served as electron donor through mitochondrial P-oxidation and made iron easier utilized by indigenous microorganisms and thus promoted dissimilatory iron reduction.The readily used Fe(?)served as electron acceptor and was reduced to Fe(?),and then the microbially generated Fe(?)reduced nitrobenzene to aniline,also,part of Fe(?)reprecipitated and mineralized with Sn2-" species attaching to the surface of aquifer medium.Nitrobenzene degradation in the system was mainly caused by the generated Fe(?),and the degradation process can be divided into three phases:physical adsorption between Y-ESO and nitribenzene;slow decline phase and rapid drop phase.Four factors including Y—ESO,total hardness(Ca2+/Mg2+),NO3-,SO42-concentration,and the grain size of aquifer media were investigated on nitrobenzene degradation dynamics.Studies showed that the generation of biological secondary Fe(?)is the limiting step in the process of indigenous microorganism dissimilatory iron reduction coupled to nitrobenzene degradation.The Y-ESO dosage,when the mass ratio of Y-ESO and nitrobenzene is above 91:1,can provide high nitrobenzene degradation rate and can save cost.The effects of the four factors on nitrobenzene degradation rate were sorted as follows:grain size of aquifer media ? NV3-? SO42-?Ca2+/Mg2+.A modified nitrobenzene degradation dynamics model coupled to dissimilatory iron reduction was derived,and based on the modified model,the remediation rate and period in an actual nitrobenzene polluted site can be calculated.A 3-D simulator was designed to simulate the remediation process of nitrobenzene polluted aquifer by establishing Y-ESO enhanced in situ biological reactive zone.In this study,Y-ESO was pumped through a single screened well to simulate 1/2 of the flow-field in the downstream based on the curtain concept.After 29 days,obvious biological reactive zone was formed in the simulator,and the retention of Y-ESO mainly located near the injection well.CH3COO-,generated by Y-ESO oxidation,was evenly distributed in the reactive zone.In the biological reactive zone,the addition of Y-ESO can form and maintain long-time anaerobic reductive environment,and can also generate a wide range of iron reduction zone,which provide favorable conditions for the nitrobenzene degradation.Nitrobenzene of 100 mg/L concentration was flowed into the simulator continuously to simulate the nitrobenzene plume.The process of nitrobenzene migration and transformation in the in situ biological reaction zone can be divided into two obvious stages:acclimation period during 0?50 days,nitrobenzene plume kept moving forward,with decrease of the reactive zone range and active component concentrations;after 50 days the reactive zone entered stable period,as the interception ability of the reactive zone increased,the concentration and range of nitrobenzene in the contaminated area decreased,and nitrobenzene plume stop pushing forward.The results clearly showed that nitrobenzene plume will not affect the reaction in the reactive zone,Y-ESO enhanced in situ biological reactive zone can remain stable for a long time in aquifer.In the stable period,the biological reactive zone only existed within screened depth of injection well,so the injection well should be screened throughout depth of the remediation zone during construction.Microbial community structure change in the simulator was studied,Y-ESO and nitrobenzene both have acclimated effect on the indigenous microbes,Y-ESO played a main role,the diversity of microbial community decreased with the increase of Y-ESO concentration,while at the same time,the degree of acclimation increased.After acclimation,many functional microorganisms were observed in the simulator:in the upstream of reactive zone,there were a large number of Pseudomonas;and Clostridium sensu stricto proliferated in the reactive zone center,Pseudomonas and Clostridium sensu stricto often have roles in the nitrobenzene degradation;while the dominant population in the downstream of the reactive zone were Arthrobacter,Geobacter and Acinetobacter,among them,Arthrobacter and Geobacter belong to dissimilatory iron reducing bacteria,while Acinetobacter and Arthrobacter have the ability to degrade aniline.The mechanisms of nitrobenzene degradation in the Y-ESO enhanced in situ biological reactive zone were analyzed.Part of the entered nitrobenzene was degraded by aerobic pseudomonas in the upstream,the rest continued flowing to the zone center;and Clostridium sensu stricto in the zone center could directly mineralize nitrobenzene into CO2,in this process Fe(?)served as electron acceptor and generated a large amount of Fe(?),and then the microbial generated Fe(?)reduced nitrobenzene to aniline;in the downstream of reactive zone,Arthrobacter and Geobacter undertook dissimilatory iron reduction and generated Fe(?),high levels of Fe(?)could reduce most nitrobenzene into aniline,the generated aniline was degradated into succinic acid and acetyl CoA by Acinetobacter and Arthrobacter.85.24%of the entered nitrobenzene was degradated by the synergistic effect based dissimilatory iron reduction,so the process of dissimilatory iron reduction coupled to nitrobenzene degradation was the main degradation pathway in this study.The nitrobenzene remediation efficiency in the Y-ESO enhanced in situ biological reactive zone was assessed.Nitrobenzene remediation efficiency:by the 135th day,the total amount of nitrobenzene flowed into the tank was 178400 mg,and 3.57%of them were flowed out from the tank,3.39%were adsorbed on the surface of medium,79.31%were reduced to aniline and 13.73%were degraded by microbes directly;the amount of aniline production was 141489 mg,within them,the biodegradation rate reached 90.21%.Permeability in the aquifer medium:with the addition of nitrobenzene,the permeability coefficient of aquifer medium decreased obviously,but the steady value of permeability coefficient was in the same order of magnitude with the initial value.Economic performance:the remediation cost of 1 m3 aquifer with a single injection well is 1220.5 RMB.In conclusion,Y-ESO enhanced in situ biological reactive zone technology has good economic performance,can easily form a range of reactive zone and effectively remediate nitrobenzene plume for a long time in aquifer,and would not obviously affects the permeability coefficient of midiem.So,Y-ESO enhanced in situ biological reactive zone technology has good application prospects in the remediation of nitrobenzene polluted aquifer. |
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