Critical Review
Contaminant Vapor Adsorption at the Gas-Water Interface in Soils M O L L Y S . C O S T A N Z A † A N D M A R K L . B R U S S E A U * ,†,‡ Soil, Water, and Environmental Science Department and Hydrology and Water Resources Department, 429 Shantz, Building 38, University of Arizona, Tucson, Arizona 85721
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, KIA. 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, AIA, as well as by KIA. Validated methods for measuring AIA are currently lacking. However, three promising methods for measuring AIA 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).
Introduction Each phase in an unsaturated system may contribute to the total retention of gas-phase volatile organic compounds (VOCs). Phases present may include the solid mineral phase, organic matter, bulk water, and gas phase. These four phases represent the possibility for four retention mechanisms: sorption to mineral surfaces, association with organic matter, dissolution into the bulk water phase, and volatilization into the gas phase. Many studies of gas-phase VOC transport have been conducted to evaluate the relative contribution of these retention processes to the total retention. It is generally found that soil organic matter serves as the primary sorption domain in high water content systems, while sorption to mineral surfaces dominates at very low water contents (e.g.,
