Study On Migration And Transformation Of Toluene Vapor In Silty Sand

The problem of vapor intrusion?VI?caused by volatile organic compounds?VOCs?which migrate to the surface or indoors through vadose zone has been paid more and more attention.Toluene was used as the research object chemical to carry out soil column experiment.To study the law of diffusion,adsorption and biodegradation of pure toluene in silty sand soil under unsteady-state condition.Furthermore,under steady-state conditions,the law of diffusion and biodegradation of pure toluene and dissolved?saturated solution?toluene vapor in silty sand media were studied.Biodegradation was validated by measuring the volume fraction of O₂/CO₂,the number of microorganisms and the bioinformatics analysis.following were the main conclusions.Migration of toluene vapor in soil column under unsteady-state conditions?1?under unsteady-state condition,the volatile concentration of 10 ml pure toluene was 45.64g/m3,and the diffusion time of toluene vapor penetrating 70 centimeter soil column was 24 hours and the maximum penetration concentration was 4.86g/m3.After 10 days,the toluene vapor reached a short steady-state balance in soil column;in 12³0 days,the toluene vapor concentration decreased at a constant rate;in 30⁶0days,The descent rate of toluene vapor concentration in soil column accelerated.The volume fraction of O₂ in the soil column decreased by 8%,and the volume fraction of CO₂ increased by 6%;after 60 days,Toluene was basically completely degraded.?2?Adding toluene while adding 10 ml of methanol,had a certain effect on promoting the biodegradation.Migration of toluene vapor in soil column under steady-state conditions?1?Under steady-state conditions,5ml of pure toluene volatilization,the source concentration maintained at 20g/m3,the diffusion time of toluene vapor penetrating 120 centimeter soil column was 36 hours and the maximum penetration concentration was 2.03g/m3.After 14 days,the toluene vapor in soil column reached a short steady-state balance,in 14¹9days,would be in a short steady-state balance.The total height of 150 centimeter soil column,from the bottom to the top,respectively,at 15,40,60,80,100,120,140 centimeter set 7 gas sampling ports.For the soil column 1 filled with untreated soil,in 19²8days,the concentration of toluene vapor in each sampling port was decreased and reached the lowest value on the 28 th day,the lowest penetration value reached to 0.81g/m3,in 28⁵0days,the concentration of toluene vapor increased gradually.The O₂ volume fraction in the column highest decreased by 5.8% and the CO₂ volume fraction highest increased by 5.4%.For the soil column 2 filled with additional N/P soil,in 19³5days,the concentration of toluene vapor in each sampling port was decreased and reached the lowest value on the 35 th day,the lowest penetration value reaches 0.66g/m3;in 35⁵0days,the concentration of toluene vapor increased gradually.The O₂ volume fraction in the column highest decreased by 6.2% and the CO₂ volume fraction highest increased by 6.1%.?2?Under steady-state conditions,dissolved toluene volatilization,the source concentration maintained at 1g/m3,in the entire experimental period,toluene vapor did not penetrate the soil column.After 30 days,the toluene vapor in soil column reached a short steady-state balance.The total height of 150 centimeter soil column,from the bottom to the top,respectively,at 15,40,60,80,100,120,140 centimeter set 7 gas sampling ports.For the soil column 4,in 1²0days,the toluene vapor concentration has been rising,at sampling ports 2,3,4,toluene vapor concentration were Respectively reached 0.39g/m3,0.14g/m3 and 0.067g/m3.After 22 days,toluene vapor concentration began to decline,toluene vapor was not detected at sampling ports 4.Furthermore,Sampling ports 2,3 were not detected at 30 d and 25 d respectively.Similarly,at sampling port 5,6,7,toluene vapor was not detected from the beginning to the end of the experiment.In summary,for high concentration of toluene vapor,covering a certain thickness clean soil had a certain retarding effect,after a period of time,would occur biodegradation,but as time passed,biodegradation would slowly weaken,because the microbial activity would be inhibited and microorganisms would be poisoned by continuous high concentration of toluene vapor.an appropriately amount of N/P addition had a certain effect on promoting biodegradation;for low concentration of toluene vapor,not consider the effect of biodegradation,relying on the effect of soil adsorption and diffusion,covering a certain thickness clean soil could also have a good retarding effect,but in the long process of vapor migration,biodegradation also played a important role.