The Air/Surface Adsorption Equilibrium of Organic Compounds Under Ambient Conditions

Abstract

The environmental fate of volatile and semivolatile organic pollutants strongly depends on their partitioning between the gas phase and condensed phases. This partitioning includes absorption in bulk phases like water or organic matter and adsorption on ambient surfaces. Often, the latter process is neglected due to the limited knowledge of adsorption as compared to absorption. The first part of this work discusses how the adsorption equilibrium can be assessed as a function of surface and adsorbate properties, temperature, and relative humidity. It is shown that the traditional approach of correlating adsorption constants with the volatility of the considered compounds or with their octanol/air partitioning does not provide a reasonable understanding of the experimental data. Instead, a model is needed that is based on a simple description of the relevant intermolecular interactions. The second part of this work analyses the relative importance of adsorption to surfaces for the overall sorption of airborne organic compounds in the environment. This discussion is focused on gas/particle partitioning and on the exchange between soil surfaces and the atmosphere.

Keywords:

• fate-modeling
• gas/particle partitioning
• mineral surfaces
• relative humidity
• volatilization
• water surface

Notes

*For the following discussion of sorption between air and other phases it is essential to distinguish between adsorption and absorption: adsorption is the partitioning between a bulk phase—here the gas phase—and an interface—here the surface of a solid or liquid medium. Absorption is the partitioning between a bulk phase and another liquid or “solid” bulk phase.

*In case of highly charged surfaces, interactions between the surface charge and uncharged organic molecules also have to be taken in account. However, these interactions can be neglected for surface potentials typically found in the natural environment (−15 to −60 mV).

†In contrast, chemisorption involves the formation of a covalent bond between the adsorbate and the surface and is reversible only at high temperatures.

^aDepending on the pretreatment of the fibers, significantly different values have been reported in the literature.

*The scale of van Oss50 implies that the water molecules in the surface layer of a bulk water phase have the same capability to interact by EDA interactions with their horizontal neighbors as with other molecules across the interface. However, EDA interactions strongly depend on the orientation of molecules. A surface water molecule that is in an ideal orientation for EDA interactions with its neighboring water molecules in the surface plane cannot at the same time be in an ideal orientation for EDA interactions across the interface. Hence, the EDA component of the water surface tension cannot be a correct measure for the EDA interaction capability of a water surface across the interface. For the same reason (i.e., the variable orientation of surface molecules in liquid surfaces depending on the position of the strongest EDA interaction partner) the measurement of EDA surface parameters with bipolar liquids as suggested by van Oss50 may not lead to a consistent scale.

*Below 90% rh, absorption in the adsorbed water film is negligible. However, at rh > 90% the adsorbed water film becomes so thick that absorption may dominate the overall sorption behavior of a compound. In this case, an increase of the overall sorption constant with increasing humidity can be observed for some compounds at rh > 90.%.9, 21, 22, 86, 87