Mechanism And Simulation Of Multiphase Migration And Transformation Of The Typical Organic Pollutant In The Chemical Industry Park In Northeast China

In case of accidental leakage,the organic pollutants produced in the chemical industry park will infiltrate into the underground environment.These pollutants have a wide variety and are highly toxic and difficult to degrade,threatening people's life,health and property safety.With the process of volatilization,adsorption and dissolution,the organic pollutants form a multiphase multicomponent system.This system is controlled by multiple factors,such as geological and hydrogeology conditions and environmental factors.The transport and transformation of organic pollutants in the underground environment are extremely complicated.Therefore,it is of great theoretical and practical significance to systematically study the law of multiphase migration and transformation of organic pollutants in the underground environment of the chemical industry park.This study is based on the national key research and development project"formation mechanism and multiphase distribution characteristics of optimal control of organic pollutants in chemical sites".The research was conducted in order of field investigation,indoor experiment,numerical simulation and practical application.This paper chose a chemical industry park in Northeast China as the research area.Firstly,by data collection and field investigation,we analyzed the pollution characteristics of the research area,established the typical organic pollutant identification system,and defined the typical organic pollutant;Then batch experiments of volatilization and adsorption were carried out.Based on batch experiments,the migration and transformation behavior of the typical organic pollutant was analyzed and the model parameters of volatilization and adsorption were determined.According to laboratory soil column experiments,a laboratory scale multi-phase migration and transformation model of the typical organic pollutant in the unsaturated zone was constructed.This model was verified by the data of column experiments,the model parameters were determined and the sensitivity of model parametrs was analyzed;Finally,the model was adjusted according to the actual situation of the study area to simulate the multiphase migration and transformation process of the typical organic pollutant in the study area.The space-time evolution process of the typical organic pollutant in the study area was analyzed,and the multiphase migration and transformation mechanism was explored.The contents and achievements of this paper are as follows:(1)By the pollution data of organic pollutants collected and monitored in study area,the characteristic of the organic pollutions in 2009–2019 was systematically analyzed.From 2009 to 2019,it was found that the concentration of benzene,toluene and nitrobenzene decreased and the concentration of phenol and benzo[a]pyrene increased initially and then decreased.The pollution of benzene and nitrobenzene in the first six years is very serious,and the pollution decreased significantly in the last 4 years.Except for benzo[a]pyrene,the pollution center of other organic pollutants was around G1.The statistical characteristics and correlation analysis of five kinds of organic pollutants in the study area were carried out.Benzene,toluene and nitrobenzene were positively correlated,and it indicated a same pollution source.The identification system of the typical organic pollutant in the study area was established by combining the hazardous results of different pollutants with the corresponding pollution indicators such as detection rate,over standard rate and times of over standard.According to the results of comprehensive scoring,the typical organic pollutant in the study area was determined as benzene.(2)Considering the characteristics of strong volatilization and easy adsorption of the typical organic pollutant,three soils of silty loam,loam and sandy loam in the study area were selected and then batch experiments on volatilization and adsorption of benzene were designed and carried out.The model parameters of volatilization and adsorption were determined by batch experiments.The volatilization and adsorption kinetic curves of benzene in three soils conform to the first and pseudo second order kinetics model,and the adsorption thermodynamic curves conform to Linear adsorption model.The average volatilization rate of benzene was in the order of silty loam,loam and sandy loam,and the adsorption capacity of benzene was in the order of loam,silty loam and sandy loam.It indicated that the volatilization process of benzene was mainly affected by the particle size of soils,and the adsorption process is affected by the organic matter and mineral components of soils.According to the literature and the actual conditions of the study area,the experiments on different factors affecting the volatilization and adsorption of benzene were carried out.Based on the experimental results,the volatilization and adsorption behavior of benzene under different factor conditions were further clarified,and the main influencing factor was determined by correlation analysis.The results showed that the average volatilization rate of benzene increased with the increase of temperature,water content and the speed of wind.Wind speed had the greatest influence among three factors.The adsorption capacity of silty loam and loam increased with the increase of p H and the content of organic matter,while the adsorption capacity of sandy loam decreased with the increase of p H and was not affected by the change of organic matter content.The results showed that initial concentration and organic matter content had greater influence on the adsorption process of benzene than other influencing factors.In order to break the limitation of traditional adsorption model and calculate adsorption capacity of benzene under different influence factors,BP(Back Propagation)neural network model was applied to approximate the relationship between different influencing factors and adsorption capacity.The results showed that BP neural network model can accurately simulate adsorption capacity under different influencing factors,providing a new research idea for solving similar problems.(3)A two-dimensional laboratory device was designed to carry out the experiments of migration of NAPL phase and rainfall leaching in media of silty loam,loam and sandy loam.In the column experiments,the law of NAPL phase migration,volatilization and dissolution of benzene in the vadvate zone was analyzed by multiphase monitoring means.Benzene mainly migrated horizontally in three soils,which showed benzene remained in top of media after leaching.With the increase of time,the migration area of NAPL phase increased while the average saturation of NAPL phase decreased.The migration rate of NAPL phase increased with the increase of the particle size of soils.In three soils,the concentration of gas phase initially increased with time,then decreased and finally tended to be stable.The concentration of gas phase was in the order of sandy loam,loam and silty loam,which means the concentration of gas phase was negatively correlated with particle size.During the duration of the experiment,the concentration of liquid phase all showed an increasing trend.The concentration of liquid phase was in the order of silty loam,loam and sandy loam,which means the concentration of liquid phase was positively correlated with particle size.Based on laboratory experiments,a laboratory-scale conceptual model was established to determine the mathematical model considering the process of fluid migration,volatilization,dissolution and adsorption.COMSOL Multiphysics software was applied to solve the model.The sensitivity of parameters of the multiphase flow migration and transformation model was analyzed by local sensitivity method.The measured values of the area of the migration area of NAPL phase,the concentration of volatilization and the concentration of dissolution fitted the simulated values well.It showed that the numerical model established in this study could be successfully applied to the multiphase migration and transformation process of the typical organic pollutant.Based on the simulation results,the multiphase distribution mechanism of benzene was analyzed.In the migration stage of NAPL phase,most of benzene was adsorbed and intercepted by the soil in the vadose zone,and a little of benzene was volatilized.The smaller particle size was,the less volatilization,and the more NAPL phase remained in the soil.During the leaching process,part of the benzene in the soil was dissolved with the migration of water flow.And the decrease of particle size,caused the higher content of liquid phase in the soils and the lower residual NAPL phase.The results of the sensitivity analysis showed that,8)(the empirical parameter of model),residual saturation and porosity were the three most sensitive parameters to the migration area of NAPL phase,8),9)₁(the mass transfer rate of the volatilization process)and porosity were the three most sensitive parameters to the concentration of gas phase,density,adsorption coefficient and disperty were the three most sensitive parameters to the concentration of liquid phase.Combined with the laboratory experiments and numerical models,this conclusion revealed the mechanism of the multiphase distribution of benzene in the underground environment with different media and provided the theoretical basis for the subsequent application of the model in the research area.(4)According to the actual conditions of the study area,the above model was adjusted and the long-term monitoring data were used to verify the model.Then the multiphase migration and transformation process of benzene in the multilayer soils in the study area was simulated to analyze the pollution distribution and the multiphase formation mechanism in vadose zone and groundwater.The results indicated that this model can simulate the migration and transformation process of benzene well.NAPL,gas and liquild phases were mainly horizontal migration in the study area.The range of benzene in gas phase was larger than that in liquild and NAPL phases.With the increase of time,the contaminated area of NAPL phase increased at the initial stage and decreases slightly at the later stage,the contaminated area of gas phase increased repidly to spread to the entire the profile and the contaminated area of liquild phase increased continuously.The concentration of liquild phase in groundwater increased firstly and then decreased with time.Meanwhile,the concentration of liquild phase decreased with the increase of distance from the pollution center in the direction of groundwater flow.Most of the benzene in the study area was distributed in NAPL phase(95.8%),followed in gas phase(3%),and the least in liquild phase(1.19%).This conclusion clarified the multiphase formation mechanism of the typical organic pollutant in the study area,provided theoretical support for the reasonable and effective remediation methods,and guided for the study of similar pollution problems.