Adsorption of organic vapors to air-dry soils: Model predictions and experimental validation

Soil/air equilibrium partitioning has an important impact on the environmental distribution and fate of many organic chemicals. Modeling approaches that cover this process commonly assume that sorption in soil only occurs in soil organic matter. However, many researchers have already shown that this is not even correct for nonpolar compounds in air-dry soils. Here, we extend the existing data set on sorption in air-dry soils by using a large and very diverse set of organic compounds covering many different functional groups for two standard soils and for relative humidity between 50 and 90%. The experimental data presented here as well as those from the literature are then used to examine two different modeling approaches: one that only considers absorption in soil organic matter and one that also considers adsorption to mineral surfaces. The results clearly show that sorption in air-dry soil cannot be explained when adsorption to mineral surfaces is ignored. Only the model that considers both sorbing phases, organic matter and mineral surfaces, gives good agreement for all experimental data. Our model for predicting adsorption to mineral surfaces does not require any further fitting with experimental data; thus, it can readily be incorporated into existing fate models in order to improve the description of the soil/air partition process.