Experimental Study On Multiphase Flow Related To Remediation Of LNAPL-Polluted Soil By Surfactant Flushing

In recent years, soil and groundwater contamination caused by leaks and accidental spill of Light Non-Aqueous Phase Liquids (LNAPLs) are paying more attention and becoming a concern of current subsurface environmental protection. After spilling into subsurface soils, a LNAPL migrate downwards accompanying with mutual flow of immiscible air and water and two phase or three phase of water, air and LNAPL exist together and affect each other to constitute a multiphase flow system (LNAPL also refers to as oil phase). Surfactant flushing (SF) technology is a remediation method of removing organic liquids from the subsurface environment in situ. Surfactants enhance removal of LNAPLs from porous media due to two aspects: solubilization caused by micelle and mobilization caused by the reduction in NAPL-water interfacial tension that occurs when surfactant molecules partition to NAPL surfaces.Based on the knowledge of Environmental Engineering, Hydrokinetics of porous media and Hydrogeology, the paper mainly investigated the effect of interfacial tension reduction caused by surfactant on capillary pressure- saturation relations of two-phase system and the mechanism of multiphase flow and pollutant transport in remediation of LNAPL-polluted soil by surfactant flushing after selecting appropriate surfactant which can effectively remove a given LNAPL from polluted soil.A surfactant that can effectively remove a given LNAPL from polluted soil was selected at first. The four preliminary selected chemical surfactants were investigated water solubility enhancement of diesel, removal efficiency from diesel-polluted soil and soil column. According to the results, Triton X-100 showed higher capability to removal oil and was selected to employ in the next multiphase flow experiments.Capillary pressure and saturation are two important parameters in study on multiphase flow in porous media. Influence of the surfactant on the relationships between capillary pressure and saturation in two-phase system was studied. A simple and reliable setup was employed to obtain relationships in sand and soil samples of water-air and water-oil system that contained or didn't contain surfactant Triton X-100. Experimental results showed that capillary pressure of two-phase system in the presence of surfactant was lower than system employing pure water under the same saturation of the wetting phase; the reduction was proportion to the decrease in interfacial tension caused by the surfactant. According to the results, relationships between capillary pressure and saturation in two-phase system of surfactant can be estimated by scaled to the relations in two-phase system of pure water.The physical model tests simulating LNAPL spills and surfactant flushing were carried out in two-dimensional plexiglass tanks. The oil front movement during spill and flushing process was traced and recorded at appropriate intervals with the method of dying LNAPL and taking colorful photos. A data acquisition system comprised of pressure transducer, cruising-check instrument and computer was set up to real-time monitor the pressure of every phase. The pressure values of oil, water and air phase collected by every pressure transducer were gained during the process of spilling and surfactant flushing. Pressure contour charts of every phase based on those values were drawn to analysis pressure trend in two-dimensional space of the whole process of surfactant flushing. During the flushing process, a depression of the capillary fringe in the whole width of the tank because the air-entry pressure decreased in proportion to the decrease in surface tension caused by the surfactant was found. In addition, surfactant-induced drainage of the vadose zone because surfactant solute transport in depressing capillary zone led to the expansion of unsaturated zone.Results from the study are useful to remove oil pollutant from contaminated soil or groundwater by surfactant flushing.