Study On Transportation Principle Of Gasoline In Soil

Soil vapor extraction (SVE) is a widely accepted and cost-effective technique that is used to remediate unsaturated soil contaminated with volatile organic compounds (VOCs). SVE is a simple process in which a vacuum induces advective airflow to enhance the volatilization of entrapped VOCs.VOCs'leakage usually happens in unsaturated zone. VOCs existing formats are non aqueous phase liquids,interface phase and vaporous phase. NAPL will gradually disappear with time elapsing. Capillary and wetting will enlarge the contaminated soil zone when NAPL is existing, VOCs vapor will transfer into soil porosity constantly.Transportation principle and mechanism, tailing mechanism of VOCs existing phase, non vapor and vapor VOCs when liquid VOCs goes into soil was studied.Laboratory Test: liquid and vapor transportation principle and absorption mechanism1. Liquid gasoline transportation: Quantitative liquid gasoline was added into the soil column. The relationship between square of liquid gasoline running distance vs. elapsed time is a liner one; it shows that the capillary and wetting exist between soil particle and liquid gasoline. The Washburn equation can be used to describe the running distance vs. elapsed time.2. Vaporous gasoline diffusion: Vaporous gasoline source in stable condition used to test vaporous gasoline diffuse in air and soil column. The results shows that diffusion velocity was affected by the soil water content, soil particle size, and soil density. The VOCs concentration is declining from the vaporous source. Concentration equilibrium will be built with the time elapsing. The relationship of vapor running distance vs. elapsed time can be described with Einstein-Smoluchowski equation.3. Gasoline absorption: Two methods were applied, for the direct method, adding quantitative gasoline into the column directly; the indirectly method, a stable gasoline vaporous source was applied and connected with the soil column. Venting the two columns after 10 days equilibrium, the venting shows that the concentration in the directly polluted column is declining slowly, and the concentration can be recovered after venting was stopped. It shows that the VOCs concentration comes from NAPL. For the indirectly polluted column, the concentration decline fast, and the concentration can't be recovered after stop venting. The concentration came from the soil porosity. It also shows the low concentration comes from the desorption from the interface, which is one reason for low concentration in late venting period.4. Tailing: Sorption happened when the liquid gasoline impregnation into soil part. And the contaminated zone will be enlarged with the time elapsed. The wetting is a kind of interface sorption. The enthalpy will be low and the surface tension will be changed when the sorption happened between the soil and the gasoline. The quantitative gasoline was estimated based on the soil surface that was measured by BET. The sorpted VOCs vapor pressure will be low and the same for concentration based on adsorption theory and the law of partial pressure Field verification: liquid and vapor gasoline transporting principle5. Liquid principle: The concentration and components at different depth were measured. The results show that for the same depth and soil components, the gasoline components are similar and concentrations are closer. The evaporation affects the gasoline components close to ground surface; it contains less short carbon chain components.6. Gas phase principle: the pressure field, concentration field and variation of the concentration in field were measured. The results show the plot has permeability and it fits for SVE. TVOC concentration is declining from the polluted center to the ground surface. GC results show that the components are same at the same depth; the components are different at different depth.. Conclusions:1.Impregnation is the main transportation for NAPL in unsaturated soil and diffusion is the transportation for vapor. The VOCs contaminated zone should be defined as three zones: capillary zone, wetting zone and diffusion zone.2. Non gas phase VOCs should include NAPL and the new interface phase formed by wetting and vapor absorption.3. Wetting and vapor absorbing will reduce VOCs enthalpy, causing lowering of VOCs components vapor pressure, resulting in tailing.