The Effect Of Shallow Temperature Variation On The Vertical Migration And Transformation Of Petroleum Hydrocarbons In Loess

Petroleum is an important source of energy and high quality chemical raw material,and is also an important strategic material for the national economy and people's livelihood.In recent years,the rapid development of petroleum industry in the loess plateau of northwest China has brought great economic benefits,but it inevitably brings many environmental problems.One of the most outstanding and urgent problems is the pollution of soil and water in the loess area.Oil pollutants that scattered on the earth surface not only pollute the air environment through evaporation,but affect the physical and chemical properties of soil and bring adverse effects on the physiological ecology of plants and soil microorganisms,and pose a long-term threat to groundwater.The main way to pollute soil environment of petroleum hydrocarbon in the northwest loess plateau is the infiltration of oil and oily wastewater.There are obvious temperature variations in the shallow loess because of the poor thermal conductivity of loess,and the temperature change is closely related to the migration and transformation process of organic pollutants.Based on the relevant research status,this paper studies the factors of temperature variation,and discusses the law of migration and transformation of petroleum hydrocarbon in loess medium under the influence of temperature variation through the indoor adsorption,desorption and volatilization experiment and dynamic migration and degradation experiment.This paper confirms different vertical stratification temperature of seasons in the study area of the loess will affect the migration behavior of petroleum hydrocarbon contaminants.The spatial distribution of petroleum hydrocarbon pollutants in the contaminated loess in the study area was determined by measuring the total hydrocarbon content of the oil pollution in the study area.The loess medium has a strong adsorption capacity for oil pollutants,and more than 90% of the oil remains in the soil layer above 10 cm.The temperature difference in the shallow surface affects the viscosity of crude oil,which leads to different volatile amounts of petroleum hydrocarbons.The volatilization process of petroleum hydrocarbons in loess media conforms to the parabola model,with the increase of temperature,the growing rate of the volatile rate coefficient increased.The results of indoor static adsorption/desorption experiments show that the adsorption/desorption process of the loess medium in the study area conforms to the quasi-second-order kinetics model.The adsorption effect of loess on petroleum hydrocarbon pollutants is the exothermic process.With the increase of temperature,the equilibrium adsorption amount of loess to petroleum hydrocarbon decreases.Under the condition of normal temperature(25 ?)in the study area of the loess medium,theoretical maximum adsorption capacity petroleum hydrocarbon is 1111.11 mg/kg.The influence of temperature change on migration process of petroleum hydrocarbon depends not only on the temperature difference,but also on the temperature value of the cold and hot end of loess and whether it is leached or not.In the process of oily wastewater infiltration and water leaching loess in crude oil,the existence of the temperature gradient caused the loess moisture movement,and promoted the migration behavior of petroleum hydrocarbons,and at the same time the adsorption/desorption of petroleum hydrocarbon ability also vary with different temperature change.In the season of high temperature,the more obvious the temperature difference,the greater the migration depth and migration rate of petroleum hydrocarbon in loess.In July,the average migration rate of petroleum hydrocarbons within 40 cm below the ground reached 5.17cm/h,compared with 4.17cm/h in loess without temperature change.In addition,the shallow temperature variation affects the degradation rate of petroleum hydrocarbons in loess: In July,August,June and April,The temperature difference of loess decreases gradually and the degradation rate of petroleum hydrocarbon also decreases;Below 13.3 cm depth in July(Difference in temperature of 15 ?)petroleum hydrocarbon degradation rate up to 29%,in April(Difference in temperature of 3 ?)degradation rate is only 3%;The degradation rates were 11.76% and 1.67% below 26.7 cm depth under the ground in two months.Under long-term leaching,about 2.4 percent of petroleum hydrocarbons continue to migrate deep into loess,which may threaten the safety of shallow groundwater.The results can provide a basis for the underground water and soil control and optimization and pollution prediction of related numerical simulation models in the study area.