Adsorption of CO2 and N2 on Soil Organic Matter:  Nature of Porosity, Surface Area, and Diffusion Mechanisms

The surface area of soil organic matter (SOM) is a crucial parameter for the interpretation of sorption mechanisms of organic contaminants. The surface area of three SOM samples was studied by using CO2 and N2 gas adsorption, revealing that SOM is a microporous material with a high surface area of 94−174 m2 g-1. The ethylene glycol monoethyl ether (EGME) retention technique has major drawbacks for application to SOM samples, as liquid EGME changes the SOM solid phase density. Nitrogen (N2) is subject to molecular sieving at 77 K due to activated diffusion in micropores. CO2 is not limited by activated diffusion since higher experimental temperatures are applied (273 K). About 95−99% of the SOM surface area is formed by micropores with maximum restrictions of approximately 0.5 nm. Results suggest that the diffusion coefficient of CO2 is influenced by the cross-linking density of the matrix and that the microporous structure is not strongly affected by hydration of the sample. On the basis of pore dimensions, configurational diffusion is proposed as the primary transport mechanism of nonionic organic contaminants in SOM micropores.