Analogy between Allophanic Soils and Synthetic Gels : Use of the Supercritical Drying to Preserve the Andosols Porous Features(Soil and Non-Crystalline Clays,Proceedings of the 13^<th> International Clay Conference)

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Allophanic soils exhibit higher organic carbon and nitrogen content than the one measured in other clayey soils. So, there is a need to understand the links between the sequestration mechanism of C and N in these soils, and other soil physical parameters. Allophanes aggregates have physical features very close to that of synthetic gels, a fractal geometry, a bulk density close to 0.5g/cm^3 and develop a high specific surface area. Moreover, allophanic soils behave as gels upon drying and exhibit an important irreversible shrinkage which affects the soil physical features. Because of this analogy between allophanes and synthetic gels we propose to control the drying step by CO_2 supercritical drying procedure (SD) and we present new results on the preservation of the porous features of allophanic soils by SD. The textural properties such as specific surface area and mean pore size are systematically higher for the supercritical dried samples compared to the classical dried samples, indicating the preserving effect of the SD which shows the interest of this drying method to this natural and complex "gels". We also discuss the C and N content of these soils and possible correlation with the pore volume features measured after SD. We thus show the clear effect of the true specific surface area on the C and N content of the allophanic soils.