Characterization of the porous structure of soils:: adsorption of nitrogen (77 K) and carbon dioxide (273 K), and mercury porosimetry

Accessibility of adsorbates to internal active sites of soils depends mainly on the porous structure of the material. We aimed to determine the distribution of pore sizes in soils with varied physico-chemical properties, by combining adsorption of gases and mercury porosimetry. Microporosity was studied by physical adsorption of N-2 at 77 K and CO2 at 273 K; mercury intrusion porosimetry allowed us to evaluate the macroporosity; and mesoporosity was determined by capillary condensation of N-2 and mercury porosimetry. The soils investigated were essentially macroporous, with volumes between 0.33 and 0.73 cm(3) g(-1) the maxima in the differential pore-size distribution were in the range 1500-4000 nm. Volumes of meso- or micropores were always less than 10% of macropore volumes. Calculations based on the theory of Dubinin and the alpha(s)-method (for N-2 at 77 K) provided, generally, coincident results. In a soil containing much organic matter, N-2 adsorption was only one-ninth that of CO2 adsorption, showing that N2 adsorption into the narrow micropores of organic matter was kinetically restricted. When accessibility to micropores was not restricted, the total volume of micropores could be deduced from NZ adsorption, whereas CO2 measured exclusively the narrowest microporosity.