| Ground water contamination by Leaking Underground Storage Tanks (LUSTs) containing gasoline is one of the most pervasive environmental problems in the USA. The leaked gasoline can be partially removed by conventional technologies such as pump and treat, but the unrecoverable gasoline which remains as the residual saturation in the vadose zone, may act as a long term source of contamination for ground water and soil. In this research, the objective was to develop a technique to remove the residual gasoline and its regulated components from the vadose zone to a level below the maximum contaminant level (MCL) prescribed by EPA for drinking water standards. This technique is based on surfactant flushing in the soil in the presence of a direct current electrical field. The application of electricity in soil causes various electrokinetic phenomena among which electroosmosis and electrophoresis were expected to be the major processes which were effective in mobilizing nonpolar gasoline components. The effects of electrophoresis and solubilization of surfactant micelles together with the property of surface tension reduction by monomeric surfactant molecules were found to be significant in mobilization and removal of gasoline. Electroosmosis without surfactant seemed to have very little effect in the removal of gasoline. A set of column experiments was performed with two types of soils, kaolinite clay and a gasoline contaminated soil obtained from a site in Laramie, WY. The surfactant used was anionic sodium dodecyl sulfate (SDS) which was applied at its critical micelle concentration. The result shows that 99.9% of the gasoline can be removed from kaolinite and 40% of the gasoline can be removed from field soil. In the field soil, SDS adsorption was found to be very high which probably reduced the efficiency of the process. The high organic content and the presence of various metal cations in the field soil could be the main reason why the SDS became adsorbed by the soil. Further research is needed to alleviate three main problems found in applying the technology: high adsorption of surfactant by soils, development of pH gradients and presence of air in the system with surfactant, water and soil. |
