Contaminant vapor adsorption at the gas-water interface in soils

There is evidence to suggest that the gas-water interface serves as an important retention domain for volatile organic compounds (VOCs) in vadose-zone soil. Moreover, vapor adsorption at the gas-water interface may represent the dominant retention mechanism under certain conditions. In general, vapor-phase interfacial adsorption is most significant for low organic matter soils at intermediate water contents. Among nonpolar compounds, those with low saturated vapor pressure have the greatest tendency for interfacial adsorption, as represented by higher interfacial sorption coefficients, K-IA. Although polar compounds may have greater tendency to adsorb at the interface than nonpolar compounds, the high aqueous solubility of polar compounds may limit the relative importance of interfacial sorption to total contaminant retention. The magnitude of interfacial retention is controlled by the specific interfacial area, A(IA), as well as by K-IA. Validated methods for measuring A(IA) are currently lacking. However, three promising methods for measuring A(IA) in soils have been proposed. Preliminary results indicate that the three methods are complimentary in terms of the type of information derived, as well as their applicability for different water content ranges and varying scales (e.g., laboratory vs field).