Pollutant Removal During The Thermally Enhanced Soil Vapor Extraction Process

In recent years, with the rapid development of petroleum and chemical industry, problems caused by petroleum hydrocarbon retention of soil pollution has been increasingly serious, restoration of petroleum hydrocarbon pollution is just around the corner. Soil Vapor Extraction(SVE) is an in-situ remediation technology widely usedin the removal of volatile and semi volatile organic compounds in the soil, and the thermally enhanced SVE can overcome the limitation of mass transfer in the traditional SVE technology, significantly improving the removal efficiency of organic pollutants.In fact, thermal desorption of contaminants from the soil is the process that solutes diffuse among the soil articles under heating and air-blowing conditions, which is similar to gas-solid chromatographic separation course. Based on this, we developed the Soil Column Gas Chromatography (SCGC) method to study the desorption kinetics and thermodynamic mechanism of organic pollutants. The thermal desorption behaviors of four typical hydrocarbonswere measured. For each solute, capacity factor (k’) and lag factor (Tf) were determined as the index of thermal desorption rate and efficiency, respectively. It was found that, when the soil was heated above the hydrocarbon’s boiling point, the rapid desorption was achieved, and the desorption velocities of fast desorption the fractions were inversely proportional to their boiling points. At the temperatures substantially below the hydrocarbon’s boiling point, the desorption velocities of slow desorption fractions were inversely proportional to their values of log Kow. The activation energy of adsorption of the hydrocarbons was positively correlated with soil organic matter content. In order to overcome the activation energy barriers, the higher temperature is required for soil with the higher organic content. In addition, the increase of carrier gas flow rate brought about the significant reduction in the apparent activation energy of adsorption for the fraction of slow desorption.Through the one-dimensional column experiment, tridecane, tetradecane, pentadecane and hexadecane were used for pollutants and we studied the influence of different heating temperatures, moisture contents, organic contents and organic matter properties on the removal rate of organic contaminants. Results showed that the higher the heating temperature, the lower the soil retention rate of the organic matters. When the temperature was increased to 180℃, most contaminants have been removed. By continuing to raise the temperature, the change of pollutants removal rate was not obvious. The soil moisture had a two-way effect on the removal efficiency of organic pollutants and the removal rate was controlled by the temperature. As the soil organic matter increases, the soil residual rate went up. When the soil temperature continued to rise, its influence on pollutant removal rate became the dominant factor compared with the organic matter content. According to the order of the boiling points, organic matter removal effects gradually strengthened.