The Microporous Structure of Organic and Mineral Soil Materials

Microporous properties of soil materials are considered important to the physical sequestration processes of contaminants and the influence on risk assessment for chemicals in the environment. We studied the microporous properties of five organic soil materials and two agricultural topsoils and their size separates. The methods used were gas adsorption with N2, CO2, C2H6, C2H4, and C2H2, as well as retention of the polar liquid EGME (ethylene-glycol-dimethyl-ether). Measured surface areas and micropore volumes determined with gas adsorption were strongly dependent on the molecular dimensions of the adsorbent, revealing that dried organic and mineral soil materials behave like rigid structures in the micropore range. A precipitated humic acid behaved differently, revealing a more open and/or homogeneous micropore structure. The EGME-derived surface areas of organic soil materials had a magnitude similar to the CO2-derived surface areas. The microporous structure of the more rigid soil organic matter (SOM) materials does not seem to be affected strongly by EGME penetration. The SOM rigidity seems to be positively correlated to SOM polarity and negatively correlated to sorption coefficients of naphthalene under wet conditions. The surface areas of mineral soils and size-separates measured with CO2 and EGME were of comparable size in the absence of swelling clay minerals.