Research On Performance Of Ferric Activated Persulfate Oxidation For Remediation Of Organochlorine Contaminated Soil

In recent years,the in-situ advanced oxidation technology with Iron divalent(Fe²⁺)activated persulfate as the remediation agent has been widely used in the remediation of organochlorine contaminated sites.Considering the complexity of organochlorine contaminated sites and the diversity of pollutant types,it is of great significance to put forward relatively efficient oxidation remediation system parameters to improve the performance of oxidation remediation.Secondly,the present research of the changes of soil properties after oxidation remediation are mainly concentrated in the aspects of soil organic matter and microorganisms,the changes of its physical and mechanical properties,the migration and diffusion of oxidation agent in soil and its influence radius are rarely involved.With the financial supports of national key research and development plan:the research and development of efficient and rapid oxidation technology system for medium and high concentration pesticide contaminated sites,,considering the concentration of 1,2-dichlorobenzene pollutants in the organochlorine contaminated site,this dissertation put forward relatively better proportioning parameters of Fe²⁺activated persulfate oxidation system and soil environmental conditions through indoor small-scale test,defined the changes and causes of physical and mechanical properties of soil after Fe²⁺activated persulfate oxidation remediation through macro geotechnical test and micro test analysis,and explored the migration and diffusion characteristics of in-situ injection oxidant agent in soil layer through numerical simulation,in order to provide a theoretical basis for the remediation and reuse of organochlorine contaminated sites and the design of construction parameters of in-situ injection remediation technology.The main conclusions are as follows:（1）The Fe²⁺activated persulfate can degrade 1,2-dichlorobenzene pollutants in organic contaminated soil effectively.The removal efficiency of 1,2-dichlorobenzene is affected by the persulfate content,Fe²⁺/persulfate ratio parameters,initial p H and water soil ratio in the oxidation remediation system.The residual concentration of 1,2-dichlorobenzene decreases with the increase of persulfate content,and the removal rate increases.When the content is 10%and 15%,the residual concentration of 1,2-dichlorobenzene is 68.04mg/kg and 63.59mg/kg respectively,which is far lower than the screening value of the risk control standard of construction land,and the removal rate reaches more than 90%.The residual concentration of 1,2-dichlorobenzene first decreased and then increased with the increase of Fe²⁺/persulfate ratio,and the removal rate first increased and then decreased.When the ratio of Fe²⁺/persulfate was 1:4,the removal rate of 1,2-dichlorobenzene was the highest,92.07%.Compared with alkaline environment,acidic environment and weak acidic environment are more conducive to the degradation of 1,2-dichlorobenzene by Fe²⁺activated persulfate.When the initial p H is 3.2,4.5 and 6.7,the residual concentrations of 1,2-dichlorobenzene are 40.09mg/kg,44.64mg/kg and 68.04mg/kg respectively;The removal rates were 95.33%,94.8%and 92.07%respectively.When the initial p H was 10.2 and 11.3,the removal rates of 1,2-dichlorobenzene were only 46.01%and 42.39%.Compared with low water soil ratio,high water soil ratio improves the degradation efficiency of 1,2-dichlorobenzene by Fe²⁺activated persulfate.When the water soil ratio is 2:10 And 3:10,the removal rate of 1,2-dichlorobenzene is significantly higher than that when the water soil ratio is 1:10.（2）The degradation of 1,2-dichlorobenzene in organic contaminated soil by Fe²⁺activated persulfate conform with the first-order reaction kinetics,and the reaction rate increased first and then decreased with the increase of persulfate content.The reaction rate of persulfate oxidation system with 10%persulfate content was the fastest,which was 0.10608h^-1,under the test conditions of the ratio of Fe²⁺/persulfate is 1:4,the initial p H is 6.7 and the ratio of water to soil is 2:10.（3）The macro geotechnical test results show that compared with 1,2-dichlorobenzene contaminated soil,the content of fine particles in oxidized soil increases and the soil particles become finer.The liquid limit of oxidized soil increases with the increase of persulfate content,while the plastic limit decreases and the plastic capacity of soil improve.The unconfined compressive strength of oxidized soil is between 1,2-dichlorobenzene contaminated soil and uncontaminated soil,the results shows that as the activated persulfate content and curing time increases,it first increases and then decreases.The unconfined compressive strength of activated persulfate oxidized soil with 10%content and 15 days curing time is the largest,which can reach 313.22 k Pa.The initial void ratio,compressibility and permeability of oxidized soil first decrease and then increase with the increase of persulfate content.The triaxial undrained shear failure of oxidized soil is plastic failure.The maximum deviatoric stress at shear failure first increases and then decreases with the increase of activated persulfate content,and the maximum deviatoric stress at 15%content is even lower than that of 1,2-dichlorobenzene.（4）XRD test shows that the addition of Fe²⁺activated persulfate caused the change of mineral composition in 1,2-dichlorobenzene contaminated soil.The newly formed substances in oxidized soil was mainly sodium sulfate,calcium sulfate and a small amount of iron hydroxide,and the types of newly formed substances did not change with different dosage.SEM test indicates that the newly formed sulfate crystals will adhere to the surface of soil particles and fill the pores between soil particles,while 15%persulfate will significantly increase the pores in oxidized soil,and the overall structure will be damaged into small aggregates and loose inside.（5）The numerical simulation results show that the migration and diffusion of in-situ injected oxidant agent in low permeability silty clay layer is mainly dominated by the change of local groundwater flow field caused by injection pressure and injection flow.The convection of groundwater and the gravity migration of solute itself have little influence,while the migration and diffusion in medium permeability and high permeability soil layer are affected by the superposition of pressure diffusion and groundwater convection.The influence range of migration and diffusion increases with the increase of soil permeability.The maximum transverse diffusion influence radius of oxidant in silty clay layer is 0.69m and 0.47m in longitudinal direction,under the condition of injection pressure of 0.2MPa,the injection flow rate of 3.5l/d,and the injection time of 10 days,that transverse diffusion in clayey silt layer is 1.52m,0.79m in longitudinal direction,transverse diffusion in silty sand layer,1.2m in longitudinal direction.（6）The migration and diffusion of oxidant agent in silty clay layer are affected by groundwater head,injection pressure,oxidant concentration and other factors.Groundwater head and injection pressure affect the migration and diffusion of oxidant agent by changing the local distribution of groundwater flow field in the soil layer.The change of oxidant concentration will not cause the change of migration and diffusion range,but only affect the concentration distribution of oxidants within the migration and diffusion range.