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UBC Theses and Dissertations

MTBE gas phase diffusion in subsurface soil : influence of soil water, clay and organic content Baradaran Ghavami, Seyedmohammad


Volatile organic compounds (VOCs) have adverse effects on human health upon short- and long-term exposure, causing allergy and asthma in adults and respiratory problems and immune system disorders in children. The most important mechanisms responsible for VOC migration and deposition in soil include diffusion, advection, adsorption, biodegradation and chemical reaction. MTBE (Methyl Tertiary-Butyl Ether), a very common VOC, detected in groundwater near landfills and hazardous waste dumps, can cause cancer in humans. Due to its specific physical and chemical properties such as high volatility, high water solubility, not much adsorptivity on soil particle and biodegradability, it is very mobile in the environment. The objectives of this research were to design and build a diffusivity apparatus, determine gas-phase diffusion of MTBE in soil, and investigate major factors affecting the effective diffusivity of MTBE such as particle size distribution, soil water and organic carbon content. Previously, sorbents and reservoir-based soil columns have been used to determine soil diffusion. A novel apparatus was designed which can overcome the limitations of conventional designs. Special features of the design include a three-segment body design stainless steel column and a very accurate humidity adjustment system, with humidity sensors at the inflow and outflow, giving the ability to determine the effective diffusivity of MTBE in soil more accurately and easily. Soil samples in this study were at 0 to 80% saturation, with 0 to 15% clay content (Kaolinite) and 0 to 15% organic content. They were compacted in a developed stainless steel one-flow reservoir-based column to a dry density of 1.6 to 1.7 g/cm³. Effective diffusivities were calculated based on Fick’s first law. Overall MTBE effective diffusivities ranged from 0.0004 to 0.003 cm²/s. The results demonstrate that higher water content of soil resulted in lower effective diffusivities. Increasing clay content of soil resulted in longer equilibrium time and lower effective diffusivity values. Variations of soil particle size had less effect on the effective diffusivity than changes in water content. It was also discovered that the organic content of soil has a significant capacity for adsorbing MTBE.

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