Effects Of Additives On Coke-contaminated Soil Properties And Thermal Desorption Behavior

Recently,the problem of soil pollu tion has become incr easingly prominent,and organic pollutants in soil have also received widespread attention.Volatile and semi-volatile organic contaminated soil can be effectively remediated by thermal desorption technology as a fast and effective soil repair method.In the present study,fundamental research of coke soil and some chlorine-containing soil was conducted.In order to reduce the concentration of soil pollutants after thermal desorption,improve soil quality after thermal desorption,control dioxin generation during thermal desorption,and reduce the cost of remediation,basic research on thermal desorption characteristics of typical organic contaminated soils was conducted,and dioxin formation characteristics during thermal desorption were also studied.The conclusions of this article are as follows:As the heating temperature increased,the desorption efficiency first increased significantly and then stabilized.Under the heating temperature of 300℃,the total PAHs concentration decreased from 151.56 mg·kg^-1 to 12.88 mg·kg^-1,and the total TEQ value decreased from 17.2 ppm to 2.04 ppm,and the thermal desorption efficiency reached 91.7%.At 350℃,the total PAHs concentration decreased to approximately 4.2 mg·kg^-1,and the thermal desorption efficiency increased to 96.8%.As the temperature continued to increase,the PAHs concentration remained at approximately 3 mg·kg^-1,and the thermal desorption efficiency also remained at98%.The thermal desorption efficiency increased in mixed soil with eggshell or plant ash compared with the blank soil at the same temperature.The thermal desorption efficiency of the mixed soil with 2.5%eggshell increased from 91.7%to 94.1%.When the mass fraction increased to 10%,the thermal desorption efficiency reached the maximum,which was 96.0%.The thermal desorption efficiency of the soil mixed with 2.5%by mass of plant ash increased from 91.7%to 95.3%.At the same time,the thermal desorption efficiency reached the maximum at 96.6%when 15%by mass of plant ash was added.Excessive additives will reduce soil thermal conductivity and increase residual PAHs concentration.Regardless of whether the curve was fitted by the first-order kinetic equation or the second-order kinetic equation,the addition of eggshell or plant ash increased the thermal desorption rate of PAHs.It could be further considered that the two additives effectively changed the soil properties and improved soil thermal desorption efficiency during the thermal desorption.The soil will agglomerate during thermal desorption,and large particles break into small particles during heating.As the thermal desorption temperature increased,the particle size tended to increase,then decrease,and then increase,and the values of TOC,CEC,EC,and n all began to decrease.Especially for TOC and CEC,when the temperature increased to 500℃,they decreased for 60%.At the same time,the addition of eggshell and plant ash could alleviate the deterioration in soil properties with increasing temperature.At all test temperatures（300⁴50℃）,the thermal desorption treatment significantly increased the dioxin（PCDD/Fs）content in the gas phase.As the temperature increased,the amount of PCDD/Fs produced and the total TEQ increased at index type.There was an optimal temperature for the production of dioxin in solid phase soil.Compared with the amount of dioxin produced in the gas phase,solid phase dioxin only accounts for a small part.The addition of Fe₂O₃ promoted the formation of PCDD/Fs.The higher the temperature was,the more obvious the effect would be,and the generation of low-chlorinated PCDD/Fs increased.With the increase of the added mass fraction,the concentration of PCDD/Fs in the exhaust gas also gradually increased.