The fate of volatile organic compounds (VOCs) moving as vapors in the subsurface is dependent on their interaction with the soil. Adsorption of VOC vapors is greatly influenced by soil texture and soil‐water content. The effects of differences in texture and soil‐water content on vapor partition coefficients for trichloroethylene (TCE) were examined. Batch experiments were conducted for a variety of soils and at different soil‐water contents, w, to determine the relationship between the vapor/solid partition coefficient, KD′, and w. In dry soils, KD′ was nonlinearly related to soil‐water content because, in that range, water molecules compete with VOC molecules for adsorption sites on the soil surface. Under wet conditions, KD′ became linearly related to water content according to Henry's law, indicating that adsorbed water molecules were acting as a solvent for VOC molecules. In general, KD′ under oven‐dry conditions did not relate well to total specific surface area of soils, most likely because VOC molecules adsorb only on the outside surfaces of soil particles (due to their nonpolarity), rather than the total surface area present. In the dry range, adsorption was dominated by soils with high specific areas (i.e., high clay content), while soils with higher organic carbon content manifested higher adsorption amounts in the wet moisture range. A one‐parameter, exponential model well described the log KD′‐w curve in the nonlinear region. The model parameter, a, was found to be highly dependent on the specific surface area of the soil. The proposed KD′‐(w) model incorporated in conventional VOC transport models seems promising for analyzing the effects of VOC vapor adsorption on VOC subsurface transport.
Knowledge of the relationship between DP/D0 (diffusion coefficient in soil divided by diffusion coefficient in free air) and the volumetric soil‐air content, ɛ, is important when modeling gaseous movement of volatile organic compounds (VOCs) in soils. The effective diffusion (i.e., diffusion and retardation) of trichloroethylene (TCE), toluene and freon in Yolo silt loam (fine‐silty, mixed, nonacid, thermic Typic Xerorthent) were measured in a two‐chamber diffusion apparatus. The experiments were conducted on packed soil cores over a range of water contents. Vapor retardation factors were calculated from soil parameters and equilibrium partition coefficients. Partition coefficients were measured in batch experiments. It was found that for water contents higher than four molecular layers of water surface coverage, solid/vapor partition coefficients, KD′, were consistent with values predicted by Henry's Law constants (KH), and aqueous/solid partition coefficients, KD. For less than four molecular layers of water, sorption increased by orders of magnitude. The vapor retardation factors, along with the measured effective diffusion, allowed a calculation of diffusion coefficients (DP) for the investigated species by using the analytical solution to diffusion in a two‐chamber apparatus. Values of the ratio DP/D0 were generally higher than the values predicted by the Millington‐Quirk equation, and lower than the values predicted by the Penman equation. Compared with the nonreactive tracer freon, DP/D0 values for TCE and toluene agreed very well for higher water contents. Values obtained for air‐dry soil, however, were under‐predicted. The experimental work for determination of the effective diffusion of reactive tracers can, therefore, for sufficiently high water contents be limited to the determination of Dp/D0‐ɛ relations for a nonreactive tracer and measurement of KD, KD′, and KH values for the reactive tracers.
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