THE EFFECT OF MOISTURE AND SOIL TEXTURE ON THE ADSORPTION OF ORGANIC VAPORS

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 co ntent. The effects of differences in texture and soil-water content on vapor partition coefficients for trichloroethylene (TCE) were examine d. Batch experiments were conducted for a variety of soils and at diff erent soil-water contents, w, to determine the relationship between th e vapor/solid partition coefficient, K-D', and w, In dry soils, K-D' w as nonlinearly related to soil-water content because, in that range, w ater molecules compete with VOC molecules for adsorption sites on the soil surface. Under wet conditions, K-D' became linearly related to wa ter content according to Henry's law, indicating that adsorbed water m olecules were acting as a solvent for VOC molecules. In general, K-D' under oven-dry conditions did not relate well to total specific surfac e 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 adso rption amounts in the wet moisture range. A one-parameter, exponential model well described the log K-D'-w curve in the nonlinear region. Th e model parameter, alpha, was found to be highly dependent on the spec ific surface area of the soil. The proposed K-D'(w) model incorporated in conventional VOC transport models seems promising for analyzing th e effects of VOC vapor adsorption on VOC subsurface transport.